| 1. |
- Josefsson, Martin, et al.
(författare)
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A quantum-dot heat engine operating close to the thermodynamic efficiency limits
- 2018
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Ingår i: Nature Nanotechnology. - : Springer Science and Business Media LLC. - 1748-3387 .- 1748-3395. ; 13:10, s. 920-924
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Tidskriftsartikel (refereegranskat)abstract
- Cyclical heat engines are a paradigm of classical thermodynamics, but are impractical for miniaturization because they rely on moving parts. A more recent concept is particle-exchange (PE) heat engines, which uses energy filtering to control a thermally driven particle flow between two heat reservoirs1,2. As they do not require moving parts and can be realized in solid-state materials, they are suitable for low-power applications and miniaturization. It was predicted that PE engines could reach the same thermodynamically ideal efficiency limits as those accessible to cyclical engines3–6, but this prediction has not been verified experimentally. Here, we demonstrate a PE heat engine based on a quantum dot (QD) embedded into a semiconductor nanowire. We directly measure the engine’s steady-state electric power output and combine it with the calculated electronic heat flow to determine the electronic efficiency η. We find that at the maximum power conditions, η is in agreement with the Curzon–Ahlborn efficiency6–9 and that the overall maximum η is in excess of 70% of the Carnot efficiency while maintaining a finite power output. Our results demonstrate that thermoelectric power conversion can, in principle, be achieved close to the thermodynamic limits, with direct relevance for future hot-carrier photovoltaics10, on-chip coolers or energy harvesters for quantum technologies.
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| 2. |
- Josefsson, Martin, et al.
(författare)
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Double quantum-dot engine fueled by entanglement between electron spins
- 2020
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Ingår i: Physical Review B. - 2469-9950. ; 101:8
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Tidskriftsartikel (refereegranskat)abstract
- The laws of thermodynamics allow work extraction from a single heat bath provided that the entropy decrease of the bath is compensated for by another part of the system. We propose a thermodynamic quantum engine that exploits this principle and consists of two electrons on a double quantum dot (QD). The engine is fueled by providing it with singlet spin states, where the electron spins on different QDs are maximally entangled, and its operation involves only changing the tunnel coupling between the QDs. Work can be extracted since the entropy of an entangled singlet is lower than that of a thermal (mixed) state, although they look identical when measuring on a single QD. We show that the engine is an optimal thermodynamic engine in the long-time limit. In addition, we include a microscopic description of the bath and analyze the engine's finite-time performance using experimentally relevant parameters.
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| 3. |
- Josefsson, Martin, et al.
(författare)
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Optimal power and efficiency of single quantum dot heat engines : Theory and experiment
- 2019
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Ingår i: Physical Review B. - 2469-9950. ; 99:23
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Tidskriftsartikel (refereegranskat)abstract
- Quantum dots (QDs) can serve as near perfect energy filters and are therefore of significant interest for the study of thermoelectric energy conversion close to thermodynamic efficiency limits. Indeed, recent experiments in [Nat. Nano. 13, 920 (2018)1748-338710.1038/s41565-018-0200-5] realized a QD heat engine with performance near these limits and in excellent agreement with theoretical predictions. However, these experiments also highlighted a need for more theory to help guide and understand the practical optimization of QD heat engines, in particular regarding the role of tunnel couplings on the performance at maximum power and efficiency for QDs that couple seemingly weakly to electronic reservoirs. Furthermore, these experiments also highlighted the critical role of the external load when optimizing the performance of a QD heat engine in practice. To provide further insight into the operation of these engines we use the Anderson impurity model together with a Master equation approach to perform power and efficiency calculations up to co-tunneling order. This is combined with additional thermoelectric experiments on a QD embedded in a nanowire where the power is measured using two methods. We use the measurements to present an experimental procedure for efficiently finding the external load RP which should be connected to the engine to optimize power output. Our theoretical estimates of RP show good agreement with the experimental results, and we show that second order tunneling processes and nonlinear effects have little impact close to maximum power, allowing us to derive a simple analytic expression for RP. In contrast, we find that the electron contribution to the thermoelectric efficiency is significantly reduced by second order tunneling processes, even for rather weak tunnel couplings.
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| 4. |
- Majidi, Danial, et al.
(författare)
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Quantum Confinement Suppressing Electronic Heat Flow below the Wiedemann–Franz Law
- 2022
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Ingår i: Nano Letters. - : American Chemical Society (ACS). - 1530-6984 .- 1530-6992. ; 22:2, s. 630-635
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Tidskriftsartikel (refereegranskat)abstract
- The Wiedemann–Franz law states that the charge conductance and the electronic contribution to the heat conductance are proportional. This sets stringent constraints on efficiency bounds for thermoelectric applications, which seek a large charge conduction in response to a small heat flow. We present experiments based on a quantum dot formed inside a semiconducting InAs nanowire transistor, in which the heat conduction can be tuned significantly below the Wiedemann–Franz prediction. Comparison with scattering theory shows that this is caused by quantum confinement and the resulting energy-selective transport properties of the quantum dot. Our results open up perspectives for tailoring independently the heat and electrical conduction properties in semiconductor nanostructures.
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| 5. |
- Svilans, Artis, et al.
(författare)
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Thermoelectric Characterization of the Kondo Resonance in Nanowire Quantum Dots
- 2018
-
Ingår i: Physical Review Letters. - 0031-9007. ; 121:20
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Tidskriftsartikel (refereegranskat)abstract
- We experimentally verify hitherto untested theoretical predictions about the thermoelectric properties of Kondo correlated quantum dots (QDs). The specific conditions required for this study are obtained by using QDs epitaxially grown in nanowires, combined with a recently developed method for controlling and measuring temperature differences at the nanoscale. This makes it possible to obtain data of very high quality both below and above the Kondo temperature, and allows a quantitative comparison with theoretical predictions. Specifically, we verify that Kondo correlations can induce a polarity change of the thermoelectric current, which can be reversed either by increasing the temperature or by applying a magnetic field.
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| 6. |
- Aasen, David, et al.
(författare)
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Milestones toward Majorana-based quantum computing
- 2016
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Ingår i: Physical Review X. - 2160-3308. ; 6:3
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Tidskriftsartikel (refereegranskat)abstract
- We introduce a scheme for preparation, manipulation, and read out of Majorana zero modes in semiconducting wires with mesoscopic superconducting islands. Our approach synthesizes recent advances in materials growth with tools commonly used in quantum-dot experiments, including gate control of tunnel barriers and Coulomb effects, charge sensing, and charge pumping. We outline a sequence of milestones interpolating between zero-mode detection and quantum computing that includes (1) detection of fusion rules for non-Abelian anyons using either proximal charge sensors or pumped current, (2) validation of a prototype topological qubit, and (3) demonstration of non-Abelian statistics by braiding in a branched geometry. The first two milestones require only a single wire with two islands, and additionally enable sensitive measurements of the system's excitation gap, quasiparticle poisoning rates, residual Majorana zero-mode splittings, and topological-qubit coherence times. These pre-braiding experiments can be adapted to other manipulation and read out schemes as well.
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| 7. |
- Awoga, Oladunjoye A., et al.
(författare)
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Mitigating disorder-induced zero-energy states in weakly coupled superconductor-semiconductor hybrid systems
- 2023
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Ingår i: Physical Review B. - : American Physical Society (APS). - 2469-9950 .- 2469-9969. ; 107:18
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Tidskriftsartikel (refereegranskat)abstract
- Disorder has appeared as one of the main mechanisms to induce topologically trivial zero-energy states in superconductor-semiconductor systems, thereby challenging the detection of topological superconductivity and Majorana bound states. Here, we demonstrate that, for disorder in any part of the system, the formation of disorder-induced trivial zero-energy states can, to a large extent, be mitigated by keeping the coupling between the semiconductor and superconductor weak. The only exception is strong disorder in the semiconductor, where instead the strong-coupling regime is somewhat more robust against disorder. Furthermore, we find that the topo-logical phase in this weak-coupling regime is robust against disorder, with a large and well-defined topological gap which is highly beneficial for topological protection. Our work shows the advantages and disadvantages of weak and strong couplings under disorder, important for designing superconductor-semiconductor hybrid structures.
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| 8. |
- Debbarma, Rousan, et al.
(författare)
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Effects of Parity and Symmetry on the Aharonov-Bohm Phase of a Quantum Ring
- 2021
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Ingår i: Nano Letters. - : American Chemical Society (ACS). - 1530-6984 .- 1530-6992.
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Tidskriftsartikel (refereegranskat)abstract
- We experimentally investigate the properties of one-dimensional quantum rings that form near the surface of nanowire quantum dots. In agreement with theoretical predictions, we observe the appearance of forbidden gaps in the evolution of states in a magnetic field as the symmetry of a quantum ring is reduced. For a twofold symmetry, our experiments confirm that orbital states are grouped pairwise. Here, a π-phase shift can be introduced in the Aharonov-Bohm relation by controlling the relative orbital parity using an electric field. Studying rings with higher symmetry, we note exceptionally large orbital contributions to the effective g-factor (up to 300), which are many times higher than those previously reported. These findings show that the properties of a phase-coherent system can be significantly altered by the nanostructure symmetry and its interplay with wave function parity.
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| 9. |
- Debbarma, Rousan, et al.
(författare)
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Josephson Junction π-0 Transition Induced by Orbital Hybridization in a Double Quantum Dot
- 2023
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Ingår i: Physical Review Letters. - 0031-9007. ; 131:25
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Tidskriftsartikel (refereegranskat)abstract
- In this Letter, we manipulate the phase shift of a Josephson junction using a parallel double quantum dot (QD). By employing a superconducting quantum interference device, we determine how orbital hybridization and detuning affect the current-phase relation in the Coulomb blockade regime. For weak hybridization between the QDs, we find π junction characteristics if at least one QD has an unpaired electron. Notably the critical current is higher when both QDs have an odd electron occupation. By increasing the inter-QD hybridization the critical current is reduced, until eventually a π-0 transition occurs. A similar transition appears when detuning the QD levels at finite hybridization. Based on a zero-bandwidth model, we argue that both cases of phase-shift transitions can be understood considering an increased weight of states with a double occupancy in the ground state and with the Cooper pair transport dominated by local Andreev reflection.
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| 10. |
- Escribano, Samuel D., et al.
(författare)
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Semiconductor-ferromagnet-superconductor planar heterostructures for 1D topological superconductivity
- 2022
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Ingår i: npj Quantum Materials. - : Springer Science and Business Media LLC. - 2397-4648. ; 7:1
-
Tidskriftsartikel (refereegranskat)abstract
- Hybrid structures of semiconducting (SM) nanowires, epitaxially grown superconductors (SC), and ferromagnetic-insulator (FI) layers have been explored experimentally and theoretically as alternative platforms for topological superconductivity at zero magnetic field. Here, we analyze a tripartite SM/FI/SC heterostructure but realized in a planar stacking geometry, where the thin FI layer acts as a spin-polarized barrier between the SM and the SC. We optimize the system’s geometrical parameters using microscopic simulations, finding the range of FI thicknesses for which the hybrid system can be tuned into the topological regime. Within this range, and thanks to the vertical confinement provided by the stacking geometry, trivial and topological phases alternate regularly as the external gate is varied, displaying a hard topological gap that can reach half of the SC one. This is a significant improvement compared to setups using hexagonal nanowires, which show erratic topological regions with typically smaller and softer gaps. Our proposal provides a magnetic field-free planar design for quasi-one-dimensional topological superconductivity with attractive properties for experimental control and scalability.
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| 11. |
- Fahlvik Svensson, Sofia, et al.
(författare)
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Using Polymer Electrolyte Gates to Set-and-Freeze Threshold Voltage and Local Potential in Nanowire-based Devices and Thermoelectrics
- 2015
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Ingår i: Advanced Functional Materials. - : Wiley. - 1616-3028 .- 1616-301X. ; 25:2, s. 255-262
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Tidskriftsartikel (refereegranskat)abstract
- The strongly temperature-dependent ionic mobility in polymer electrolytes is used to freeze in specific ionic charge environments around a nanowire using a local wrap-gate geometry. This makes it possible to set both the threshold voltage for a conventional doped substrate gate and the local disorder potential at temperatures below 220 K. These are characterized in detail by combining conductance and thermovoltage measurements with modeling. The results demonstrate that local polymer electrolyte gates are compatible with nanowire thermoelectrics, where they offer the advantage of a very low thermal conductivity, and hold great potential towards setting the optimal operating point for solid-state cooling applications.
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| 12. |
- Ganjipour, Bahram, et al.
(författare)
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Transport studies of electron-hole and spin-orbit interaction in GaSb/InAsSb core-shell nanowire quantum dots
- 2015
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Ingår i: Physical Review B (Condensed Matter and Materials Physics). - 1098-0121. ; 91:16
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Tidskriftsartikel (refereegranskat)abstract
- We report low-temperature transport studies of parallel double quantum dots formed in GaSb/InAsSb core-shell nanowires. At negative gate voltages, regular patterns of Coulomb diamonds are observed in the charge stability diagrams, which we ascribe to single-hole tunneling through a quantum dot in the GaSb core. As the gate voltage increases, the measured charge stability diagram indicates the appearance of an additional quantum dot, which we suggest is an electron quantum dot formed in the InAsSb shell. We find that an electron-hole interaction induces shifts of transport resonances in the source-drain voltage from which an average electron-hole interaction strength of 2.9 +/- 0.3 meV is extracted. We also carry out magnetotransport measurements of a hole quantum dot in the GaSb core and extract level-dependent g factors and a spin-orbit interaction.
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| 13. |
- Geier, Max, et al.
(författare)
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Fermion-parity qubit in a proximitized double quantum dot
- 2024
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Ingår i: Physical Review Research. - 2643-1564. ; 6:2
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Tidskriftsartikel (refereegranskat)abstract
- Bound states in quantum dots coupled to superconductors can be in a coherent superposition of states with different electron number but with the same fermion parity. Electrostatic gating can tune this superposition to a sweet spot, where the quantum dot has the same mean electric charge independent of its electron-number parity. Here, we propose to encode quantum information in the local fermion parity of two tunnel-coupled quantum dots embedded in a Josephson junction. At the sweet spot, the qubit states have zero charge dipole moment. This protects the qubit from dephasing due to charge noise acting on the potential of each dot, as well as fluctuations of the (weak) interdot tunneling. At weak interdot tunneling, relaxation is suppressed because of disjoint qubit states. However, for strong interdot tunneling the system is protected against noise affecting each quantum dot separately (energy-level noise, dot-superconductor tunneling fluctuations, and hyperfine interactions). Finally, we describe initialization and readout as well as single-qubit and two-qubit gates by pulsing gate voltages.
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| 14. |
- Gluschke, Jan-Göran, et al.
(författare)
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Characterization of Ambipolar GaSb/InAs Core-Shell Nanowires by Thermovoltage Measurements.
- 2015
-
Ingår i: ACS Nano. - : American Chemical Society (ACS). - 1936-086X .- 1936-0851. ; 9:7, s. 7033-7040
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Tidskriftsartikel (refereegranskat)abstract
- In semiconductor heterostructures with a type II band alignment, such as GaSb-InAs, conduction can be tuned from electron- to hole-dominated using an electrostatic gate. However, traditional conductance measurements give no direct information on the carrier type, and thus limit the ability to distinguish transport effects originating from the two materials. Here, we employ thermovoltage measurements to GaSb/InAs core-shell nanowires, and reliably identify the dominant carrier type at room temperature as well as in the quantum transport regime at 4.2 K, even in cases where the conductance measurement does not allow for such a distinction. In addition, we show that theoretical modeling using the conductance data as input can reproduce the measured thermovoltage under the assumption that electron and hole states shift differently in energy with the applied gate voltage.
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| 15. |
- Gooth, Johannes, et al.
(författare)
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Thermoelectric performance of classical topological insulator nanowires
- 2015
-
Ingår i: Semiconductor Science and Technology. - : IOP Publishing. - 0268-1242 .- 1361-6641. ; 30:1
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Tidskriftsartikel (refereegranskat)abstract
- There is currently substantial effort being invested into creating efficient thermoelectric (TE) nanowires based on topological insulator (TI) chalcogenide-type materials. A key premise of these efforts is the assumption that the generally good TE properties that these materials exhibit in bulk form will translate into similarly good or even better TE performance of the same materials in nanowire form. Here, we calculate TE performance of TI nanowires based on Bi2Te3, Sb2Te3 and Bi2Se3 as a function of diameter and Fermi level. We show that the TE performance of TI nanowires does not derive from the properties of the bulk material in a straightforward way. For all investigated systems the competition between surface states and bulk channel causes a significant modification of the TE transport coefficients if the diameter is reduced into the sub 10 mu m range. Key aspects are that the surface and bulk states are optimized at different Fermi levels or have different polarity as well as the high surface to volume ratio of the nanowires. This limits the maximum TE performance of TI nanowires and thus their application in efficient TE devices.
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| 16. |
- Haupt, F., et al.
(författare)
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Heat, molecular vibrations, and adiabatic driving in non-equilibrium transport through interacting quantum dots
- 2013
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Ingår i: Physica Status Solidi. B: Basic Research. - : Wiley. - 0370-1972. ; 250:11, s. 2315-2329
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Tidskriftsartikel (refereegranskat)abstract
- In this article we review aspects of charge and heat transport in interacting quantum dots and molecular junctions under stationary and time-dependent non-equilibrium conditions due to finite electrical and thermal bias. In particular, we discuss how a discrete level spectrum can be beneficial for thermoelectric applications, and investigate the detrimental effects of molecular vibrations on the efficiency of a molecular quantum dot as an energy converter. In addition, we consider the effects of a slow time-dependent modulation of applied voltages on the transport properties of a quantum dot and show how this can be used as a spectroscopic tool complementary to standard dc-measurements. Finally, we combine time-dependent driving with thermoelectrics in a double-quantum dot system -a nanoscale analog of a cyclic heat engine -and discuss its operation and the main limitations to its performance.
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| 17. |
- Hell, Michael, et al.
(författare)
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Coupling and braiding Majorana bound states in networks defined in proximate two-dimensional electron gases
- 2017
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Ingår i: Physical Review B. - 2469-9950. ; 96:3
-
Tidskriftsartikel (refereegranskat)abstract
- Two-dimensional electron gases with strong spin-orbit coupling covered by a superconducting layer offer a flexible and potentially scalable platform for Majorana networks. We predict Majorana bound states (MBSs) to appear for experimentally achievable parameters and realistic gate potentials in two designs: either underneath a narrow stripe of a superconducting layer (S stripes) or where a narrow stripe has been removed from a uniform layer (N stripes). The coupling of the MBSs can be tuned for both types in a wide range (<1neV to >10μeV) using gates placed adjacent to the stripes. For both types, we numerically compute the local density of states for two parallel Majorana-stripe ends as well as Majorana trijunctions formed in a tuning-fork geometry. The MBS coupling between parallel Majorana stripes can be suppressed below 1 neV for potential barriers in the meV range for separations of about 200 nm. We further show that the MBS couplings in a trijunction can be gate controlled in a range similar to the intrastripe coupling while maintaining a sizable gap to the excited states (tens of μeV). Altogether, this suggests that braiding can carried out on a time scale of 10-100 ns.
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| 18. |
- Hell, Michael, et al.
(författare)
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Distinguishing Majorana bound states from localized Andreev bound states by interferometry
- 2018
-
Ingår i: Physical Review B. - 2469-9950. ; 97:16
-
Tidskriftsartikel (refereegranskat)abstract
- Experimental evidence for Majorana bound states (MBSs) is so far mainly based on the robustness of a zero-bias conductance peak. However, similar features can also arise due to Andreev bound states (ABSs) localized at the end of an island. We show that these two scenarios can be distinguished by an interferometry experiment based on embedding a Coulomb-blockaded island into an Aharonov-Bohm ring. For two ABSs, when the ground state is nearly degenerate, cotunneling can change the state of the island, and interference is suppressed. By contrast, for two MBSs the ground state is nondegenerate, and cotunneling has to preserve the island state, which leads to h/e-periodic conductance oscillations with magnetic flux. Such interference setups can be realized with semiconducting nanowires or two-dimensional electron gases with proximity-induced superconductivity and may also be a useful spectroscopic tool for parity-flip mechanisms.
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| 19. |
- Hell, M., et al.
(författare)
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Spin resonance without spin splitting
- 2015
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Ingår i: Physical Review B (Condensed Matter and Materials Physics). - 1098-0121. ; 91:19
-
Tidskriftsartikel (refereegranskat)abstract
- We predict that a single-level quantum dot without discernible splitting of its spin states develops a spin-precession resonance in charge transport when embedded into a spin valve. The resonance occurs in the generic situation of Coulomb blockaded transport with ferromagnetic leads whose polarizations deviate from perfect antiparallel alignment. The resonance appears when electrically tuning the interaction-induced exchange field perpendicular to one of the polarizations-a simple condition relying on vectors in contrast to usual resonance conditions associated with energy splittings. The spin resonance can be detected by stationary dI/dV spectroscopy and by oscillations in the time-averaged current using a gate-pulsing scheme. The generic noncollinearity of the ferromagnets and junction asymmetry allow for an all-electric determination of the spin-injection asymmetry, the anisotropy of spin relaxation, and the magnitude of the exchange field. We also investigate the impact of a nearby superconductor on the resonance position. Our simplistic model turns out to be generic for a broad class of coherent few-level quantum systems.
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| 20. |
- Hell, Michael, et al.
(författare)
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Time scales for Majorana manipulation using Coulomb blockade in gate-controlled superconducting nanowires
- 2016
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Ingår i: Physical Review B. - 1098-0121. ; 94:3
-
Tidskriftsartikel (refereegranskat)abstract
- We numerically compute the low-energy spectrum of a gate-controlled superconducting topological nanowire segmented into two islands, each Josephson coupled to a bulk superconductor. This device may host two pairs of Majorana bound states and could provide a platform for testing Majorana fusion rules. We analyze the crossover between (i) a charge-dominated regime utilizable for initialization and readout of Majorana bound states, (ii) a single-island regime for dominating interisland Majorana coupling, (iii) a Josephson-plasmon regime for large coupling to the bulk superconductors, and (iv) a regime of four Majorana bound states allowing for topologically protected Majorana manipulations. From the energy spectrum, we derive conservative estimates for the time scales of a fusion-rule testing protocol proposed recently (D. Aasen, arXiv:1511.05153). We also analyze the steps needed for basic Majorana braiding operations in branched nanowire structures.
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| 21. |
- Hell, Michael, et al.
(författare)
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Two-Dimensional Platform for Networks of Majorana Bound States
- 2017
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Ingår i: Physical Review Letters. - 0031-9007. ; 118:10
-
Tidskriftsartikel (refereegranskat)abstract
- We model theoretically a two-dimensional electron gas (2DEG) covered by a superconductor and demonstrate that topological superconducting channels are formed when stripes of the superconducting layer are removed. As a consequence, Majorana bound states (MBSs) are created at the ends of the stripes. We calculate the topological invariant and energy gap of a single stripe, using realistic values for an InAs 2DEG proximitized by an epitaxial Al layer. We show that the topological gap is enhanced when the structure is made asymmetric. This can be achieved either by imposing a phase difference (by driving a supercurrent or using a magnetic-flux loop) over the strip or by replacing one superconductor by a metallic gate. Both strategies also enable control over the MBS splitting, thereby facilitating braiding and readout schemes based on controlled fusion of MBSs. Finally, we outline how a network of Majorana stripes can be designed.
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| 22. |
- Huang, Guangyao, et al.
(författare)
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Tunnel spectroscopy of Majorana bound states in topological superconductor/quantum dot Josephson junctions
- 2014
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Ingår i: Physical Review B (Condensed Matter and Materials Physics). - 1098-0121. ; 90:21
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Tidskriftsartikel (refereegranskat)abstract
- We theoretically investigate electronic transport through a junction where a quantum dot (QD) is tunnel coupled on both sides to semiconductor nanowires with strong spin-orbit interaction and proximity-induced superconductivity. The results are presented as stability diagrams, i.e., the differential conductance as a function of the bias voltage applied across the junction and the gate voltage used to control the electrostatic potential on the QD. A small applied magnetic field splits and modifies the resonances due to the Zeeman splitting of the QD level. Above a critical field strength, Majorana bound states (MBS) appear at the interfaces between the two superconducting nanowires and the QD, resulting in a qualitative change of the entire stability diagram, suggesting this setup as a promising platform to identify MBS. Our calculations are based on a nonequilibrium Green's function description and is exact when Coulomb interactions on the QD can be neglected. In addition, we develop a simple pictorial view of the involved transport processes, which is equivalent to a description in terms of multiple Andreev reflections, but provides an alternative way to understand the role of the QD level in enhancing transport for certain gate and bias voltages. We believe that this description will be useful in future studies of interacting QDs coupled to superconducting leads (with or without MBS), where it can be used to develop a perturbation expansion in the tunnel coupling.
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| 23. |
- Karbaschi, Hossein, et al.
(författare)
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Nonlinear thermoelectric efficiency of superlattice-structured nanowires
- 2016
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Ingår i: Physical Review B. - 1098-0121. ; 94:11
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Tidskriftsartikel (refereegranskat)abstract
- We theoretically investigate nonlinear ballistic thermoelectric transport in a superlattice-structured nanowire. By a special choice of nonuniform widths of the superlattice barriers - analogous to antireflection coating in optical systems - it is possible to achieve a transmission which comes close to a square profile as a function of energy. We calculate the low-temperature output power and power-conversion efficiency of a thermoelectric generator based on such a structure and show that the efficiency remains high also when operating at a significant power. To provide guidelines for experiments, we study how the results depend on the nanowire radius, the number of barriers, and on random imperfections in barrier width and separation. Our results indicate that high efficiencies can indeed be achieved with today's capabilities in epitaxial nanowire growth.
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| 24. |
- Kirsanskas, Gediminas, et al.
(författare)
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Designing pi-stacked molecular structures to control heat transport through molecular junctions
- 2014
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Ingår i: Applied Physics Letters. - : AIP Publishing. - 0003-6951 .- 1077-3118. ; 105:23
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Tidskriftsartikel (refereegranskat)abstract
- We propose and analyze a way of using pi stacking to design molecular junctions that either enhance or suppress a phononic heat current, but at the same time remain conductors for an electric current. Such functionality is highly desirable in thermoelectric energy converters, as well as in other electronic components where heat dissipation should be minimized or maximized. We suggest a molecular design consisting of two masses coupled to each other with one mass coupled to each lead. By having a small coupling (spring constant) between the masses, it is possible to either reduce or perhaps more surprisingly enhance the phonon conductance. We investigate a simple model system to identify optimal parameter regimes and then use first principle calculations to extract model parameters for a number of specific molecular realizations, confirming that our proposal can indeed be realized using standard molecular building blocks. (C) 2014 AIP Publishing LLC.
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| 25. |
- Kirsanskas, Gediminas, et al.
(författare)
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QmeQ 1.0 : An open-source Python package for calculations of transport through quantum dot devices
- 2017
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Ingår i: Computer Physics Communications. - : Elsevier BV. - 0010-4655. ; 221, s. 317-342
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Tidskriftsartikel (refereegranskat)abstract
- QmeQ is an open-source Python package for numerical modeling of transport through quantum dot devices with strong electron-electron interactions using various approximate master equation approaches. The package provides a framework for calculating stationary particle or energy currents driven by differences in chemical potentials or temperatures between the leads which are tunnel coupled to the quantum dots. The electronic structures of the quantum dots are described by their single-particle states and the Coulomb matrix elements between the states. When transport is treated perturbatively to lowest order in the tunneling couplings, the possible approaches are Pauli (classical), first-order Redfield, and first-order von Neumann master equations, and a particular form of the Lindblad equation. When all processes involving two-particle excitations in the leads are of interest, the second-order von Neumann approach can be applied. All these approaches are implemented in QmeQ. We here give an overview of the basic structure of the package, give examples of transport calculations, and outline the range of applicability of the different approximate approaches. Program summary: Program Title: QmeQ Program Files doi: http://dx.doi.org/10.17632/8687mrhgg9.1 Licensing provisions: BSD 2-Clause. Programming language: Python External libraries: NumPy, SciPy, Cython Nature of problem: Calculation of stationary state currents through quantum dots tunnel coupled to leads. Solution method: Exact diagonalization of the quantum dot Hamiltonian for a given set of single particle states and Coulomb matrix elements. Numerical solution of the stationary-state master equation for a given approximate approach. Restrictions: Depending on the approximate approach the temperature needs to be sufficiently large compared to the coupling strength for the approach to be valid.
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| 26. |
- Li, Tommy, et al.
(författare)
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Four-Majorana qubit with charge readout : Dynamics and decoherence
- 2018
-
Ingår i: Physical Review B. - 2469-9950. ; 98:20
-
Tidskriftsartikel (refereegranskat)abstract
- We present a theoretical analysis of a Majorana-based qubit consisting of two topological superconducting islands connected via a Josephson junction. The qubit is operated by electrostatic gates which control the coupling of two of the four Majorana zero modes. At the end of the operation, readout is performed in the charge basis. Even though the operations are not topologically protected, the proposed experiment can potentially shed light on the coherence of the parity degree of freedom in Majorana devices and serve as a first step towards topological Majorana qubits. We discuss in detail the charge-stability diagram and its use for characterizing the parameters of the devices, including the overlap of the Majorana edge states. We describe the multilevel spectral properties of the system and present a detailed study of its controlled coherent oscillations, as well as decoherence resulting from coupling to a non-Markovian environment. In particular, we study a gate-controlled protocol where conversion between Coulomb-blockade and transmon regimes generates coherent oscillations of the qubit state due to the overlap of Majorana modes. We show that, in addition to fluctuations of the Majorana coupling, considerable measurement errors may be accumulated during the conversion intervals when electrostatic fluctuations in the superconducting islands are present. These results are also relevant for several proposed implementations of topological qubits which rely on readout based on charge detection.
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| 27. |
- Li, Zeng Zhao, et al.
(författare)
-
Quantum interference in transport through almost symmetric double quantum dots
- 2019
-
Ingår i: Physical Review B. - 2469-9950. ; 99:12
-
Tidskriftsartikel (refereegranskat)abstract
- We theoretically investigate transport signatures of quantum interference in highly symmetric double quantum dots in a parallel geometry and demonstrate that extremely weak symmetry-breaking effects can have a dramatic influence on the current. Our calculations are based on a master equation in which quantum interference enters as nondiagonal elements of the density matrix of the double quantum dots. We also show that many results have a physically intuitive meaning when recasting our equations as Bloch-like equations for a pseudospin associated with the dot occupation. In the perfectly symmetric configuration with equal tunnel couplings and orbital energies of both dots, there is no unique stationary-state density matrix. Interestingly, however, adding arbitrarily small symmetry-breaking terms to the tunnel couplings or orbital energies stabilizes a stationary state either with or without quantum interference, depending on the competition between these two perturbations. The different solutions can correspond to very different current levels. Therefore, if the orbital energies and/or tunnel couplings are controlled by, e.g., electrostatic gating, the double quantum dot can act as an exceptionally sensitive electric switch.
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| 28. |
- Maiani, Andrea, et al.
(författare)
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Nonsinusoidal current-phase relations in semiconductor-superconductor- ferromagnetic insulator devices
- 2023
-
Ingår i: Physical Review B. - 2469-9950. ; 107:24
-
Tidskriftsartikel (refereegranskat)abstract
- Coherent tunneling processes of multiple Cooper pairs across a Josephson junction give rise to high harmonics in the current phase relation. In this work, we propose and study Josephson junctions based on semiconductor-superconductor-ferromagnetic insulator heterostructures to engineer nonsinusoidal current-phase relations. The gate-tunability of the charge carriers' density in the semiconductor, together with the adjustable magnetization of the ferromagnetic insulator, provides control over the content of the supercurrent harmonics. At finite exchange field, hybrid junctions can undergo a 0 - π phase transition, resulting in a supercurrent reversal. Close to the transition, single-pair tunneling is suppressed and the current-phase relation is dominated by the second-harmonic, indicating transport primarily by pairs of Cooper pairs. Finally, we demonstrate that noncollinear magnetization or spin-orbit coupling in the leads and the junction can lead to a gate-tunable Josephson diode effect with efficiencies of up to ∼30%.
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| 29. |
- Matern, Stephanie, et al.
(författare)
-
Metastability and quantum coherence assisted sensing in interacting parallel quantum dots
- 2023
-
Ingår i: Physical Review B. - 2469-9950. ; 107:12
-
Tidskriftsartikel (refereegranskat)abstract
- We study the transient dynamics subject to quantum coherence effects of two interacting parallel quantum dots weakly coupled to macroscopic leads. The stationary particle current of this quantum system is sensitive to perturbations much smaller than any other energy scale, specifically compared to the system-lead coupling and the temperature. We show that this is due to the presence of a parity-like symmetry in the dynamics, as a consequence of which two distinct stationary states arise. In the presence of small perturbations breaking this symmetry, the system exhibits metastability with two metastable phases that can be approximated by a combination of states corresponding to stationary states in the unperturbed limit. Furthermore, the long-time dynamics can be described as classical dynamics between those phases, leading to a unique stationary state. In particular, the competition of those two metastable phases explains the sensitive behavior of the stationary current towards small perturbations. We show that this behavior bears the potential of utilizing the parallel dots as a charge sensor, which makes use of quantum coherence effects to achieve a signal to noise ratio that is not limited by the temperature. As a consequence, the parallel dots outperform an analogous single-dot charge sensor for a wide range of temperatures.
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| 30. |
- Miao, Ruijiao, et al.
(författare)
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Influence of Quantum Interference on the Thermoelectric Properties of Molecular Junctions
- 2018
-
Ingår i: Nano Letters. - : American Chemical Society (ACS). - 1530-6984 .- 1530-6992. ; 18:9, s. 5666-5672
-
Tidskriftsartikel (refereegranskat)abstract
- Molecular junctions offer unique opportunities for controlling charge transport on the atomic scale and for studying energy conversion. For example, quantum interference effects in molecular junctions have been proposed as an avenue for highly efficient thermoelectric power conversion at room temperature. Toward this goal, we investigated the effect of quantum interference on the thermoelectric properties of molecular junctions. Specifically, we employed oligo(phenylene ethynylene) (OPE) derivatives with a para-connected central phenyl ring (para-OPE3) and meta-connected central ring (meta-OPE3), which both covalently bind to gold via sulfur anchoring atoms located at their ends. In agreement with predictions from ab initio modeling, our experiments on both single molecules and monolayers show that meta-OPE3 junctions, which are expected to exhibit destructive interference effects, yield a higher thermopower (with ∼20 μV/K) compared with para-OPE3 (with ∼10 μV/K). Our results show that quantum interference effects can indeed be employed to enhance the thermoelectric properties of molecular junctions.
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| 31. |
- Misiorny, M., et al.
(författare)
-
Probing transverse magnetic anisotropy by electronic transport through a single-molecule magnet
- 2015
-
Ingår i: Physical Review B (Condensed Matter and Materials Physics). - 1098-0121. ; 91:3
-
Tidskriftsartikel (refereegranskat)abstract
- By means of electronic transport, we study the transverse magnetic anisotropy of an individual Fe-4 single-molecule magnet (SMM) embedded in a three-terminal junction. In particular, we determine in situ the transverse anisotropy of the molecule from the pronounced intensity modulations of the linear conductance, which are observed as a function of applied magnetic field. The proposed technique works at temperatures exceeding the energy scale of the tunnel splittings of the SMM. We deduce that the transverse anisotropy for a single Fe-4 molecule captured in a junction is substantially larger than the bulk value.
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| 32. |
- Nilsson, Malin, et al.
(författare)
-
Electron-hole interactions in coupled InAs-GaSb quantum dots based on nanowire crystal phase templates
- 2016
-
Ingår i: Physical Review B (Condensed Matter and Materials Physics). - 1098-0121. ; 94:11
-
Tidskriftsartikel (refereegranskat)abstract
- We report growth and characterization of a coupled quantum dot structure that utilizes nanowire templates for selective epitaxy of radial heterostructures. The starting point is a zinc blende InAs nanowire with thin segments of wurtzite structure. These segments have dual roles: they act as tunnel barriers for electron transport in the InAs core, and they also locally suppress growth of a GaSb shell, resulting in coaxial InAs-GaSb quantum dots with integrated electrical probes. The parallel quantum dot structure hosts spatially separated electrons and holes that interact due to the type-II broken gap of InAs-GaSb heterojunctions. The Coulomb blockade in the electron and hole transport is studied, and periodic interactions of electrons and holes are observed and can be reproduced by modeling. Distorted Coulomb diamonds indicate voltage-induced ground-state transitions, possibly a result of changes in the spatial distribution of holes in the thin GaSb shell.
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| 33. |
- Nilsson, Malin, et al.
(författare)
-
Single-electron transport in InAs nanowire quantum dots formed by crystal phase engineering
- 2016
-
Ingår i: Physical Review B (Condensed Matter and Materials Physics). - 1098-0121. ; 93:19
-
Tidskriftsartikel (refereegranskat)abstract
- We report electrical characterization of quantum dots formed by introducing pairs of thin wurtzite (WZ) segments in zinc blende (ZB) InAs nanowires. Regular Coulomb oscillations are observed over a wide gate voltage span, indicating that WZ segments create significant barriers for electron transport. We find a direct correlation of transport properties with quantum dot length and corresponding growth time of the enclosed ZB segment. The correlation is made possible by using a method to extract lengths of nanowire crystal phase segments directly from scanning electron microscopy images, and with support from transmission electron microscope images of typical nanowires. From experiments on controlled filling of nearly empty dots with electrons, up to the point where Coulomb oscillations can no longer be resolved, we estimate a lower bound for the ZB-WZ conduction-band offset of 95 meV.
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| 34. |
- Nilsson, Malin, et al.
(författare)
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Tuning the Two-Electron Hybridization and Spin States in Parallel-Coupled InAs Quantum Dots
- 2018
-
Ingår i: Physical Review Letters. - 0031-9007. ; 121:15
-
Tidskriftsartikel (refereegranskat)abstract
- We study spin transport in the one- and two-electron regimes of parallel-coupled double quantum dots (DQDs). The DQDs are formed in InAs nanowires by a combination of crystal-phase engineering and electrostatic gating, with an interdot tunnel coupling (t) tunable by one order of magnitude. Large single-particle energy separations (up to 10 meV) and |g∗| factors (∼10) enable detailed studies of the B-field-induced transition from a singlet-to-triplet ground state as a function of t. In particular, we investigate how the magnitude of the spin-orbit-induced singlet-triplet anticrossing depends on t. For cases of strong coupling, we find values of 230 μeV for the anticrossing using excited-state spectroscopy. Experimental results are reproduced by calculations based on rate equations and a DQD model including a single orbital in each dot.
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| 35. |
- Nitsch, Maximilian, et al.
(författare)
-
Interference and parity blockade in transport through a Majorana box
- 2022
-
Ingår i: Physical Review B. - 2469-9950. ; 106:20
-
Tidskriftsartikel (refereegranskat)abstract
- A Majorana box - two topological superconducting nanowires coupled via a trivial superconductor - is a building block in devices aiming to demonstrate non-Abelian physics, as well as for topological quantum computer architectures. We theoretically investigate charge transport through a Majorana box and show that current can be blocked when two Majoranas couple to the same lead, fixing their parity. In direct analogy to a Pauli spin blockade in spin qubits, this parity blockade can be used for fast and high-fidelity qubit initialization and readout, as well as for current-based measurements of decoherence times. Furthermore, we demonstrate that transport can distinguish between a clean Majorana box and a disordered box with additional unwanted Majorana or Andreev bound states.
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| 36. |
- Nitsch, Maximilian, et al.
(författare)
-
Transport-based fusion that distinguishes between Majorana and Andreev bound states
- 2024
-
Ingår i: Physical Review B. - 2469-9950. ; 109:16
-
Tidskriftsartikel (refereegranskat)abstract
- It has proven difficult to distinguish between topological Majorana bound states and nontopological Andreev bound states and to measure the unique properties of the former. In this work, we aim to alleviate this problem by proposing and theoretically analyzing a different type of fusion protocol based on transport measurements in a Majorana box coupled to normal leads. The protocol is based on switching between different nanowire pairs being tunnel coupled to one of the leads. For a Majorana system, this leads to switching between different states associated with parity blockade. The charge being transmitted at each switch provides a measurement of the Majorana fusion rules. Importantly, the result is different for a system with nontopological Andreev bound states. The proposed protocol only requires measuring a DC current combined with fast gate control of the tunnel couplings.
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| 37. |
- Pedersen, Kim G. L., et al.
(författare)
-
Quantum interference in off-resonant transport through single molecules
- 2014
-
Ingår i: Physical Review B (Condensed Matter and Materials Physics). - 1098-0121. ; 90:12
-
Tidskriftsartikel (refereegranskat)abstract
- We provide a simple set of rules for predicting interference effects in off-resonant transport through single molecule junctions. These effects fall into two classes, showing, respectively, an odd or an even number of nodes in the linear conductance within a given molecular charge state, and we demonstrate how to decide the interference class directly from the contacting geometry. For neutral alternant hydrocarbons, we employ the Coulson-Rushbrooke-McLachlan pairing theorem to show that the interference class is decided simply by tunneling on and off the molecule from same or different sublattices. More generally, we investigate a range of smaller molecules by means of exact diagonalization combined with a perturbative treatment of the molecule-lead tunnel coupling. While these results generally agree well with GW calculations, they are shown to be at odds with simpler mean-field treatments. For molecules with spin-degenerate ground states, we show that for most junctions interference causes no transmission nodes, but we argue that it may lead to a nonstandard gate dependence of the zero-bias Kondo resonance.
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| 38. |
- Potts, Heidi, et al.
(författare)
-
Selective tuning of spin-orbital Kondo contributions in parallel-coupled quantum dots
- 2020
-
Ingår i: Physical Review B. - 2469-9950. ; 101:11
-
Tidskriftsartikel (refereegranskat)abstract
- We use cotunneling spectroscopy to investigate spin, orbital, and spin-orbital Kondo transport in a strongly confined system of InAs double quantum dots that are parallel coupled to source and drain. In the one-electron transport regime, the higher-symmetry spin-orbital Kondo effect manifests at orbital degeneracy and no external magnetic field. We then proceed to show that the individual Kondo contributions can be isolated and studied separately: either by orbital detuning in the case of spin Kondo transport or by spin splitting in the case of orbital Kondo transport. By varying the interdot tunnel coupling, we show that lifting of the spin degeneracy is key to confirming the presence of an orbital degeneracy and to detecting a small orbital hybridization gap. Finally, in the two-electron regime, we show that the presence of a spin-triplet ground state results in spin Kondo transport at zero magnetic field.
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| 39. |
- Potts, Heidi, et al.
(författare)
-
Symmetry-controlled singlet-triplet transition in a double-barrier quantum ring
- 2021
-
Ingår i: Physical Review B. - 2469-9950. ; 104:8
-
Tidskriftsartikel (refereegranskat)abstract
- We engineer a system of two strongly confined quantum dots to gain reproducible electrostatic control of the even-electron spin at zero magnetic field. Coupling the dots in a tight ring-shaped potential with two tunnel barriers, we demonstrate that an electric field can switch the electron ground state between a singlet and a triplet configuration. Comparing our experimental cotunneling spectroscopy data to a full many-body treatment of interacting electrons in a double-barrier quantum ring, we find excellent agreement in the evolution of many-body states with electric and magnetic fields. The calculations show that the singlet-triplet energy crossover, not found in conventionally coupled quantum dots, is made possible by the ring-shaped geometry of the confining potential.
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| 40. |
- Prawiroatmodjo, Guenevere E.D.K., et al.
(författare)
-
Transport and excitations in a negative-U quantum dot at the LaAlO3/SrTiO3 interface
- 2017
-
Ingår i: Nature Communications. - : Springer Science and Business Media LLC. - 2041-1723. ; 8:1
-
Tidskriftsartikel (refereegranskat)abstract
- In a solid-state host, attractive electron-electron interactions can lead to the formation of local electron pairs which play an important role in the understanding of prominent phenomena such as high T c superconductivity and the pseudogap phase. Recently, evidence of a paired ground state without superconductivity was demonstrated at the level of single electrons in quantum dots at the interface of LaAlO3 and SrTiO3. Here, we present a detailed study of the excitation spectrum and transport processes of a gate-defined LaAlO3/SrTiO3 quantum dot exhibiting pairing at low temperatures. For weak tunneling, the spectrum agrees with calculations based on the Anderson model with a negative effective charging energy U, and exhibits an energy gap corresponding to the Zeeman energy of the magnetic pair-breaking field. In contrast, for strong coupling, low-bias conductance is enhanced with a characteristic dependence on temperature, magnetic field and chemical potential consistent with the charge Kondo effect.
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| 41. |
- Samuelson, William, et al.
(författare)
-
Minimal quantum dot based Kitaev chain with only local superconducting proximity effect
- 2024
-
Ingår i: Physical Review B. - 2469-9950. ; 109:3
-
Tidskriftsartikel (refereegranskat)abstract
- The possibility to engineer a Kitaev chain in quantum dots coupled via superconductors has recently emerged as a promising path toward topological superconductivity and possibly non-Abelian physics. Here we show that it is possible to avoid some of the main experimental hurdles on this path by using only local proximity effect on each quantum dot in a geometry that resembles a two-dot version of the proposal in Fulga et al. [New J. Phys. 15, 045020 (2013)]. There is no need for narrow superconducting couplers, additional Andreev bound states, or spatially varying magnetic fields; it suffices with spin-orbit interaction and a constant magnetic field in combination with control of the superconducting phase to tune the relative strengths of elastic cotunneling and an effective crossed-Andreev-reflection-like process generated by higher-order tunneling. We use a realistic spinful, interacting model and show that high-quality Majorana bound states can be generated already in a double quantum dot.
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| 42. |
- Schulenborg, Jens, et al.
(författare)
-
Detecting Majorana modes by readout of poisoning-induced parity flips
- 2023
-
Ingår i: Physical Review B. - 2469-9950. ; 107:12
-
Tidskriftsartikel (refereegranskat)abstract
- Reading out the parity degree of freedom of Majorana bound states is key to demonstrating their non-Abelian exchange properties. Here, we present a low-energy model describing localized edge states in a two-arm device. We study parity-to-charge conversion based on coupling the superconductor bound states to a quantum dot whose charge is read out by a sensor. The dynamics of the system, including the readout device, is analyzed in full using a quantum-jump approach. We show how the resulting signal and signal-to-noise ratio differentiates between local Majorana and Andreev bound states.
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| 43. |
- Schulenborg, Jens, et al.
(författare)
-
Multilevel effects in quantum dot based parity-to-charge conversion of Majorana box qubits
- 2021
-
Ingår i: Physical Review B. - 2469-9950. ; 103:24
-
Tidskriftsartikel (refereegranskat)abstract
- Quantum dot based parity-to-charge conversion is a promising method for reading out quantum information encoded nonlocally into pairs of Majorana zero modes. To obtain a sizable parity-to-charge visibility, it is crucial to tune the relative phase of the tunnel couplings between the dot and the Majorana modes appropriately. However, in the presence of multiple quasidegenerate dot orbitals, it is in general not experimentally feasible to tune all couplings individually. This paper shows that such configurations could make it difficult to avoid a destructive multiorbital interference effect that substantially reduces the readout visibility. We analyze this effect using a Lindblad quantum master equation. This exposes how the experimentally relevant system parameters enhance or suppress the visibility when strong charging energy, measurement dissipation, and, most importantly, multiorbital interference is accounted for. In particular, we find that an intermediate-time readout could mitigate some of the interference-related visibility reductions affecting the stationary limit.
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| 44. |
- Seoane Souto, Ruben, et al.
(författare)
-
Timescales for charge transfer based operations on Majorana systems
- 2020
-
Ingår i: Physical Review B - Condensed Matter and Materials Physics. - 1550-235X.
-
Tidskriftsartikel (refereegranskat)abstract
- In this Rapid Communication we analyze the efficiency of operations based on transferring charge from a quantum dot (QD) to two coupled topological superconductors, which can be used for performing non-Abelian operations on Majorana bound states (MBSs). We develop a method which allows us to describe the full time evolution of the system as the QD energy is manipulated. Using a full counting statistics analysis, we set bounds to the operation timescales. The lower bound depends on the superconducting phase difference due to a partial decoupling of the different MBS parity sectors, while the upper bound is set by the tunneling of quasiparticles to the MBSs. Using realistic parameters, we find the existence of a regime where the operation can be carried out with a fidelity close to unity. Finally, we propose the use of a two-operation protocol to quantify the effect of the dephasing and accumulated dynamical phases, demonstrating their absence for certain superconducting phase differences.
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| 45. |
- Seoane Souto, Rubén, et al.
(författare)
-
Tuning the Josephson diode response with an ac current
- 2024
-
Ingår i: Physical Review Research. - 2643-1564. ; 6:2
-
Tidskriftsartikel (refereegranskat)abstract
- Josephson diodes are superconducting elements that show an asymmetry in the critical current depending on the direction of the current. Here, we theoretically explore how an alternating current bias can tune the response of such a diode. We show that for slow driving there is always a regime where the system can only carry zero-voltage dc current in one direction, thus effectively behaving as an ideal Josephson diode. Under fast driving, the diode efficiency is also tunable, although the ideal regime cannot be reached in this case. We also investigate the residual dissipation due to the time-dependent current bias and show that it remains small. All our conclusions are solely based on the critical current asymmetry of the junction, and are thus compatible with any Josephson diode.
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| 46. |
- Souto, Rubén Seoane, et al.
(författare)
-
Fusion rules in a Majorana single-charge transistor
- 2022
-
Ingår i: SciPost Physics. - 2542-4653. ; 12:5
-
Tidskriftsartikel (refereegranskat)abstract
- A demonstration of the theoretically predicted non-abelian properties of Majorana bound states (MBSs) would constitute a definite proof of a topological superconducting phase. Alongside the nontrivial braiding statistics, the fusion rules are fundamental properties of all non-abelian anyons. In this work, we propose and theoretically analyze a way to demonstrate MBS fusion rules in a Majorana single-charge transistor. Our proposal reduces both the number of operations and the device complexity compared to previous designs. Furthermore, we show that the fusion protocol can be adapted to pump a quantized amount of charge in each cycle, providing a straightforward method to detect fusion rules through a DC current measurement. We analyze the protocol numerically and analytically and show that the required operational timescales and expected measurement signals are within experimental capabilities in various superconductor-semiconductor hybrid platforms.
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| 47. |
- Souto, Rubén Seoane, et al.
(författare)
-
Josephson Diode Effect in Supercurrent Interferometers
- 2022
-
Ingår i: Physical Review Letters. - 0031-9007. ; 129:26
-
Tidskriftsartikel (refereegranskat)abstract
- A Josephson diode is a nonreciprocal circuit element that supports a larger dissipationless supercurrent in one direction than in the other. In this Letter, we propose a class of Josephson diodes based on supercurrent interferometers composed of Andreev bound state Josephson junctions or interacting quantum dot Josephson junctions, which are not diodes themselves but possess nonsinusoidal current-phase relations. We show that such Josephson diodes have several important advantages, like being electrically tunable and requiring only time-reversal breaking by a magnetic flux. We also show that our diodes have a characteristic ac response, revealed by the Shapiro steps. Even the simplest realization of our Josephson diode paradigm that relies on only two junctions can achieve efficiencies of up to ∼40% and, interestingly, far greater efficiencies are achievable by concatenating interferometer loops. We hope that our Letter will stimulate the search for highly tunable Josephson diode effects in Josephson devices based semiconductor-superconductor hybrids, 2d materials, and topological insulators, where nonsinusoidal current-phase relations were recently observed.
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| 48. |
- Souto, Rubén Seoane, et al.
(författare)
-
Multiterminal transport spectroscopy of subgap states in Coulomb-blockaded superconductors
- 2022
-
Ingår i: Physical Review B. - 2469-9950. ; 106:23
-
Tidskriftsartikel (refereegranskat)abstract
- Subgap states are responsible for the low-bias transport features of hybrid superconducting-semiconducting devices. Here we analyze the local and nonlocal differential conductance of Coulomb-blockaded multiterminal superconducting islands that host subgap states with different spatial structures. The emerging patterns of their transport spectroscopy are used to characterize the possible topological nature of these devices and offer the possibility of controlling their transport properties. We develop a next-to-leading order master equation to describe the multiterminal transport in superconductors with both strong Coulomb interactions and multiple subgap states, coupled with metallic leads. We show that the nonlocal differential conductance characterizes the spatial extension of the subgap states and signals the presence of degenerate bound states with a finite support on different parts of the device. Additionally, it displays sharp sign changes as a function of the induced charge of the superconductor, signaling energy crossings among its lowest excited states.
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| 49. |
- Souto, Rubén Seoane, et al.
(författare)
-
Probing Majorana localization in minimal Kitaev chains through a quantum dot
- 2023
-
Ingår i: Physical Review Research. - 2643-1564. ; 5:4
-
Tidskriftsartikel (refereegranskat)abstract
- Artificial Kitaev chains, formed by quantum dots coupled via superconductors, have emerged as a promising platform for realizing Majorana bound states. Even a minimal Kitaev chain (a quantum dot-superconductor-quantum dot setup) can host Majorana states at discrete sweet spots. However, unambiguously identifying Majorana sweet spots in such a system is still challenging. In this work, we propose an additional dot coupled to one side of the chain as a tool to identify good sweet spots in minimal Kitaev chains. When the two Majorana states in the chain overlap, the extra dot couples to both and thus splits an even-odd ground-state degeneracy when its level is on resonance. In contrast, a ground-state degeneracy will persist for well-separated Majorana states. This difference can be used to identify points in parameter space with spatially separated Majorana states, using tunneling spectroscopy measurements. We perform a systematic analysis of different relevant situations. We show that the additional dot can help distinguish between Majorana sweet spots and other trivial zero-energy crossings. We also characterize the different conductance patterns, which can serve as a guide for future experiments aiming to study Majorana states in minimal Kitaev chains.
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| 50. |
- Svilans, Artis, et al.
(författare)
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Experiments on the thermoelectric properties of quantum dots
- 2016
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Ingår i: Comptes Rendus. Physique. - : Elsevier BV. - 1631-0705. ; 17:10, s. 1096-1108
-
Tidskriftsartikel (refereegranskat)abstract
- Quantum dots (QDs) are good model systems for fundamental studies of mesoscopic transport phenomena using thermoelectric effects because of their small size, electrostatically tunable properties and thermoelectric response characteristics that are very sensitive to small thermal biases. Here we provide a review of experimental studies on thermoelectric properties of single QDs realized in two-dimensional electron gases, single-walled carbon nanotubes and semiconductor nanowires. A key requirement for such experiments is to have some methods for nanoscale thermal biasing at one's disposal. We briefly review the main techniques used in the field, namely, heating of the QD contacts, side heating and top heating, and touch upon their relative advantages. The thermoelectric response of a QD as a function of gate potential has a characteristic oscillatory behavior with the same period as is observed for conductance peaks. Much of the existing literature focuses on the agreement between experiments and theory, particularly for amplitude and line-shape of the thermovoltage Vth. A general observation is that the widely used single-electron tunneling approximation for QDs has limited success in reproducing measured Vth. Landauer-type calculations are often found to describe measurement results better, despite the large electron–electron interactions in QDs. More recently, nonlinear thermoelectric effects have moved into the focus of attention, and we offer a brief overview of the experiments done so far. We conclude by discussing open questions and avenues for future work, including the role of asymmetries in tunnel- and capacitive couplings in the thermoelectric behavior of QDs.
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