| 1. |
- Charpentier, Sophie, 1983, et al.
(författare)
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Hot spot formation in electron-doped PCCO nanobridges
- 2016
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Ingår i: Physical Review B. - 2469-9969 .- 2469-9950. ; 94:6
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Tidskriftsartikel (refereegranskat)abstract
- We have investigated the transport properties of optimally doped Pr2-xCexCuO4-δ (PCCO) nanobridges with width down to 100 nm. The critical current density of the nanobridges approaches the Ginzburg-Landau theoretical limit, which demonstrates nanostructures with properties close to the as-grown films. The current voltage characteristics are hysteretic with a sharp voltage switch, of the order of a few millivolts, that we interpret with the occurrence of a hot spot formation. The values of the retrapping current and the voltage switch obtained by modeling the heat transport in the nanobridges are very close to the experimental ones. This feature, together with the extremely short recombination times, make PCCO nanostructures attractive candidates for ultrafast single photon detectors.
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| 2. |
- Ferrari, A. C., et al.
(författare)
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Science and technology roadmap for graphene, related two-dimensional crystals, and hybrid systems
- 2015
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Ingår i: Nanoscale. - : Royal Society of Chemistry (RSC). - 2040-3372 .- 2040-3364. ; 7:11, s. 4598-4810
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Tidskriftsartikel (refereegranskat)abstract
- We present the science and technology roadmap for graphene, related two-dimensional crystals, and hybrid systems, targeting an evolution in technology, that might lead to impacts and benefits reaching into most areas of society. This roadmap was developed within the framework of the European Graphene Flagship and outlines the main targets and research areas as best understood at the start of this ambitious project. We provide an overview of the key aspects of graphene and related materials (GRMs), ranging from fundamental research challenges to a variety of applications in a large number of sectors, highlighting the steps necessary to take GRMs from a state of raw potential to a point where they might revolutionize multiple industries. We also define an extensive list of acronyms in an effort to standardize the nomenclature in this emerging field.
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| 3. |
- He, Hans, et al.
(författare)
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Highly efficient UV detection in a metal-semiconductor-metal detector with epigraphene
- 2022
-
Ingår i: Applied Physics Letters. - : American Institute of Physics Inc.. - 0003-6951 .- 1077-3118. ; 120:19
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Tidskriftsartikel (refereegranskat)abstract
- We show that epitaxial graphene on silicon carbide (epigraphene) grown at high temperatures (T >1850 °C) readily acts as material for implementing solar-blind ultraviolet (UV) detectors with outstanding performance. We present centimeter-sized epigraphene metal-semiconductor-metal (MSM) detectors with a peak external quantum efficiency of η ∼85% for wavelengths λ = 250-280 nm, corresponding to nearly 100% internal quantum efficiency when accounting for reflection losses. Zero bias operation is possible in asymmetric devices, with the responsivity to UV remaining as high as R = 134 mA/W, making this a self-powered detector. The low dark currents Io ∼50 fA translate into an estimated record high specific detectivity D = 3.5 × 1015 Jones. The performance that we demonstrate, together with material reproducibility, renders epigraphene technologically attractive to implement high-performance planar MSM devices with a low processing effort, including multi-pixel UV sensor arrays, suitable for a number of practical applications. © 2022 Author(s).
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| 4. |
- Bergvall, Anders, 1981, et al.
(författare)
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Basic Theory of Electron Transport Through Molecular Contacts
- 2016
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Ingår i: Handbook of Single-Molecule Electronics. - 9789814463393 ; , s. 31-78
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Bokkapitel (övrigt vetenskapligt/konstnärligt)abstract
- In this chapter, we will introduce a basic theoretical description of coherent electron transport through low-dimensional junctions and molecular devices. The description introduced is based on quantum transport theory using a tight-binding description of molecules and lead materials. We apply this theory in a few worked examples on junctions based on graphene and carbon nanotubes and on molecular-superconducting hybrid junctions.
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| 5. |
- Bergvall, Anders, 1981, et al.
(författare)
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Graphene nanogap for gate-tunable quantum-coherent single-molecule electronics
- 2011
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Ingår i: Physical Review B - Condensed Matter and Materials Physics. - 2469-9950 .- 2469-9969. ; 84:15
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Tidskriftsartikel (refereegranskat)abstract
- We present atomistic calculations of quantum coherent electron transport through fulleropyrrolidine terminated molecules bridging a graphene nanogap. We predict that three difficult problems in molecular electronics with single molecules can be solved by utilizing graphene contacts: (1) a back gate modulating the Fermi level in the graphene leads facilitates control of the device conductance in a transistor effect with high on-off current ratio; (2) the size mismatch between leads and molecule is avoided, in contrast to the traditional metal contacts; (3) as a consequence, distinct features in charge flow patterns throughout the device are directly detectable by scanning techniques. We show that moderate graphene edge disorder is unimportant for the transistor function.
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| 6. |
- Bergvall, Anders, 1981, et al.
(författare)
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Influence of 0001 tilt grain boundaries on the destruction of the quantum Hall effect in graphene
- 2015
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Ingår i: Physical Review B - Condensed Matter and Materials Physics. - 2469-9950 .- 2469-9969. ; 91:24
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Tidskriftsartikel (refereegranskat)abstract
- The half-integer quantum Hall effect (QHE) is often suppressed in graphene grown by chemical vapor deposition on metals. The reason behind the suppression is unclear, and we hypothesize that it might be connected to extended defects in the material. In this paper we present results for the quantum Hall effect in graphene with [0001] tilt grain boundaries connecting opposite sides of Hall bar devices. Such grain boundaries contain 5-7 ring complexes that host defect states that hybridize to form bands with varying degree of metallicity depending on grain-boundary defect density. In a magnetic field, edge states on opposite sides of the Hall bar can be connected by the defect states along the grain boundary. This destroys Hall resistance quantization and leads to nonzero longitudinal resistance. Anderson disorder can partly recover quantization, where current instead flows along returning paths along the grain boundary depending on defect density in the grain boundary and on disorder strength. Since grain sizes in graphene made by chemical vapor deposition are usually small, this may help explain why the quantum Hall effect is usually poorly developed in devices made of this material.
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| 7. |
- Bergvall, Anders, 1981, et al.
(författare)
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Spectral footprints of impurity scattering in graphene nanoribbons
- 2013
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Ingår i: Physical Review B - Condensed Matter and Materials Physics. - 2469-9950 .- 2469-9969. ; 87:20
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Tidskriftsartikel (refereegranskat)abstract
- We report a detailed investigation of the interplay between size quantization and local scattering centers in graphene nanoribbons, as seen in the local density of states. The spectral signatures, obtained after Fourier transformation of the local density of states, include characteristic peaks that can be related to the transverse modes of the nanoribbon. In armchair ribbons, the Fourier transformed density of states of one of the two inequivalent sublattices takes a form similar to that of a quantum channel in a two-dimensional electron gas, modified according to the differences in band structure. After addition of the second sublattice contribution, a characteristic modulation of the pattern due to superposition is obtained, similar to what has been obtained in spectra due to single impurity scattering in large-area graphene. We present analytic results for the electron propagator in armchair nanoribbons in the Dirac approximation, including a single scattering center within a T-matrix formulation. For comparison, we have extended the investigation with numerics obtained with an atomistic recursive Green's function approach. The spectral signatures of the atomistic approach include the effects of trigonal warping. The impurity induced oscillations in the local density of states are not decaying at large distance in few-mode nanoribbons.
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| 8. |
- Bylander, Jonas, 1978, et al.
(författare)
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Preface to the Proceedings of the LT28
- 2018
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Ingår i: Journal of Physics: Conference Series. - : IOP Publishing. - 1742-6588 .- 1742-6596. ; 969
-
Konferensbidrag (övrigt vetenskapligt/konstnärligt)
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| 9. |
- Chakraborty, Debmalya, et al.
(författare)
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Disorder-robust phase crystal in high-temperature superconductors stabilized by strong correlations
- 2022
-
Ingår i: npj Quantum Materials. - : Springer Science and Business Media LLC. - 2397-4648. ; 7:1
-
Tidskriftsartikel (refereegranskat)abstract
- The simultaneous interplay of strong electron–electron correlations, topological zero-energy states, and disorder is yet an unexplored territory but of immense interest due to their inevitable presence in many materials. Copper oxide high-temperature superconductors (cuprates) with pair breaking edges host a flat band of topological zero-energy states, making them an ideal playground where strong correlations, topology, and disorder are strongly intertwined. Here we show that this interplay in cuprates generates a fully gapped ‘phase crystal’ state that breaks both translational and time-reversal invariance, characterized by a modulation of the d-wave superconducting phase co-existing with a modulating extended s-wave superconducting order. In contrast to conventional wisdom, we find that this phase crystal state is remarkably robust to omnipresent disorder, but only in the presence of strong correlations, thus giving a clear route to its experimental realization.
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| 10. |
- Champel, Thierry, et al.
(författare)
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0-pi Transitions in a Superconductor/Chiral Ferromagnet/Superconductor Junction Induced by a Homogeneous Cycloidal Spiral
- 2008
-
Ingår i: Physical Review Letters. ; 100, s. 077003-
-
Tidskriftsartikel (refereegranskat)abstract
- We study the pi phase in a superconductor-ferromagnet-superconductor Josephson junction, with a ferromagnet showing a cycloidal spiral spin modulation with in-plane propagation vector. Our results reveal a high sensitivity of the junction to the spiral order and indicate the presence of 0-pi quantum phase transitions as function of the spiral wave vector. We find that the chiral magnetic order introduces chiral superconducting triplet pairs that strongly influence the physics in such Josephson junctions, with potential applications in nanoelectronics and spintronics.
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| 11. |
- Eschrig, Matthias, et al.
(författare)
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Triplet supercurrents in clean and disordered half-metallic ferromagnets
- 2008
-
Ingår i: Nature Physics. ; 4, s. 138-
-
Tidskriftsartikel (refereegranskat)abstract
- Interfaces between materials with differently ordered phases present unique opportunities to study fundamental problems in physics. One example is the interface between a singlet superconductor and a half-metallic ferromagnet, where Cooper pairing occurs between electrons with opposite spin on the superconducting side, whereas the other exhibits 100% spin polarization. The recent surprising observation of a supercurrent through half-metallic CrO2 therefore requires a mechanism for conversion between unpolarized and completely spin-polarized supercurrents. Here, we suggest a conversion mechanism based on electron spin precession together with triplet-pair rotation at interfaces with broken spin-rotation symmetry. In the diffusive limit (short mean free path), the triplet supercurrent is dominated by inter-related odd-frequency s-wave and even-frequency p-wave pairs. In the crossover to the ballistic limit, further symmetry components become relevant. The interface region exhibits a superconducting state of mixed-spin pairs with highly unusual symmetry properties that open up new perspectives for exotic Josephson devices.
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| 12. |
- Fogelström, Mikael, 1963, et al.
(författare)
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Large Thermoelectric Effects and Inelastic Scattering in Unconventional Superconductors
- 2006
-
Ingår i: AIP Conference Proceedings. ; 850, s. 567-
-
Tidskriftsartikel (refereegranskat)abstract
- The thermoelectric coefficient eta(T) in unconventional superconductors is enhanced below Tc by intermediate strength impurity scattering that is intrinsically particle-hole asymmetric. We compute eta(T) for a strong-coupling d-wave superconductor and investigate the effects of inelastic scattering originating from electron-boson interactions. We show that eta(T) is severely suppressed at temperatures just below Tc by a particle-hole symmetric inelastic scattering rate. At lower temperatures inelastic scattering is frozen out and eta(T) recovers and regains its large amplitude. In the limit T->0, we have eta(T) ~ eta_0 T + O[T^3] , where the slope eta_0 contains information about the Drude plasma frequency, the details of impurity scattering, and the change in effective mass by electron-boson interactions. In this limit eta(T) can be used as a probe, complementary to the universal heat and charge conductivities, in investigations of the nature of nodal quasiparticles.
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| 13. |
- Grein, R., et al.
(författare)
-
Inverse proximity effect and influence of disorder on triplet supercurrents in strongly spin-polarized ferromagnets
- 2013
-
Ingår i: Physical Review B - Condensed Matter and Materials Physics. - 2469-9950 .- 2469-9969. ; 88:5
-
Tidskriftsartikel (refereegranskat)abstract
- We discuss the Josephson effect in strongly spin-polarized ferromagnets where triplet correlations are induced by means of spin-active interface scattering, extending our earlier work [R. Grein, M. Eschrig, G. Metalidis, and G. Schon, Phys. Rev. Lett. 102, 227005 (2009)] by including impurity scattering in the ferromagnetic bulk and the inverse proximity effect in a fully self-consistent way. Our quasiclassical approach accounts for the differences of Fermi momenta and Fermi velocities between the two spin bands of the ferromagnet, and thereby overcomes an important shortcoming of previous work within the framework of Usadel theory. We show that nonmagnetic disorder in conjunction with spin-dependent Fermi velocities may induce a reversal of the spin current as a function of temperature.
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| 14. |
- Grein, R., et al.
(författare)
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Theory of superconductor-ferromagnet point-contact spectra: The case of strong spin polarization
- 2010
-
Ingår i: Physical Review B - Condensed Matter and Materials Physics. - 2469-9950 .- 2469-9969. ; 81:9, s. Art. no. 094508-
-
Tidskriftsartikel (refereegranskat)abstract
- We study the impact of spin-active scattering on Andreev spectra of point contacts between superconductors (SC) and strongly spin-polarized ferromagnets (FM) using recently derived boundary conditions for the quasiclassical theory of superconductivity. We describe the interface region by a microscopic model for the interface scattering matrix. Our model includes both spin filtering and spin mixing and is nonperturbative in both transmission and spin polarization. We emphasize the importance of spin-mixing caused by interface scattering, which has been shown to be crucial for the creation of exotic pairing correlations in such structures. We provide estimates for the magnitude of this effect in different scenarios and discuss its dependence on various physical parameters. Our main finding is that the shape of the interface potential has a tremendous impact on the magnitude of the spin-mixing effect. Thus, all previous calculations, being based on delta-function or box-shaped interface potentials, underestimate this effect gravely. As a consequence, we find that with realistic interface potentials the spin-mixing effect can easily be large enough to cause spin-polarized subgap Andreev bound states in SC/FM point contacts. In addition, we show that our theory generalizes earlier models based on the Blonder-Tinkham-Klapwijk approach.
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| 15. |
- Holmvall, Patric, 1988, et al.
(författare)
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Broken translational symmetry at edges of high-temperature superconductors
- 2018
-
Ingår i: Nature Communications. - : Springer Science and Business Media LLC. - 2041-1723 .- 2041-1723. ; 9:1
-
Tidskriftsartikel (refereegranskat)abstract
- Flat bands of zero-energy states at the edges of quantum materials have a topological origin. However, their presence is energetically unfavorable. If there is a mechanism to shift the band to finite energies, a phase transition can occur. Here we study high-temperature superconductors hosting flat bands of midgap Andreev surface states. In a second-order phase transition at roughly a fifth of the superconducting transition temperature, time-reversal symmetry and continuous translational symmetry along the edge are spontaneously broken. In an external magnetic field, only translational symmetry is broken. We identify the order parameter as the superfluid momentum p_s, that forms a planar vector field with defects, including edge sources and sinks. The critical points of the vector field satisfy a generalized Poincaré-Hopf theorem, relating the sum of Poincaré indices to the Euler characteristic of the system.
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| 16. |
- Holmvall, Patric, 1988, et al.
(författare)
-
Phase crystals
- 2020
-
Ingår i: Physical Review Research. - 2643-1564. ; 2:1
-
Tidskriftsartikel (refereegranskat)abstract
- Superconductivity owes its properties to the phase of the electron pair condensate that breaks the U(1) symmetry. In the most traditional ground state, the phase is uniform and rigid. The normal state can be unstable towards special inhomogeneous superconducting states: the Abrikosov vortex state and the Fulde-Ferrell-Larkin-Ovchinnikov state. Here we show that the phase-uniform superconducting state can go into a fundamentally different and more ordered nonuniform ground state, which we refer to as a phase crystal. This state breaks translational invariance through formation of a spatially periodic modulation of the phase, manifested by unusual superflow patterns and circulating currents, that also break time-reversal symmetry. We list the general conditions needed for realization of phase crystals. Using microscopic theory, we then derive an analytic expression for the superfluid density tensor for the case of a nonuniform environment in a semi-infinite superconductor. We demonstrate how the surface quasiparticle states enter the superfluid density and identify phase crystallization as the main player in several previous numerical observations in unconventional superconductors, and predict the existence of a similar phenomenon in superconductor-ferromagnetic structures. This analytic approach provides a unifying aspect for the exploration of boundary-induced quasiparticles and collective excitations in superconductors. More generally, we trace the origin of phase crystallization to nonlocal properties of the gradient energy, which implies the existence of similar pattern-forming instabilities in many other contexts.
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| 17. |
- Holmvall, Patric, 1988, et al.
(författare)
-
Spontaneous generation of fractional vortex-Antivortex pairs at single edges of high-Tc superconductors
- 2018
-
Ingår i: Journal of Physics: Conference Series. - : IOP Publishing. - 1742-6588 .- 1742-6596. ; 969:1
-
Konferensbidrag (refereegranskat)abstract
- Unconventional d-wave superconductors with pair-breaking edges are predicted to have ground states with spontaneously broken time-reversal and translational symmetries. We use the quasiclassical theory of superconductivity to demonstrate that such phases can exist at any single pair-breaking facet. This implies that a greater variety of systems, not necessarily mesoscopic in size, should be unstable to such symmetry breaking. The density of states averaged over the facet displays a broad peak centered at zero energy, which is consistent with experimental findings of a broad zero-bias conductance peak with a temperature-independent width at low temperatures.
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| 18. |
- Holmvall, Patric, 1988, et al.
(författare)
-
Spontaneous symmetry breaking at surfaces of d-wave superconductors: Influence of geometry and surface ruggedness
- 2019
-
Ingår i: Physical Review B. - 2469-9969 .- 2469-9950. ; 99:18
-
Tidskriftsartikel (refereegranskat)abstract
- Surfaces of d -wave superconductors may host a substantial density of zero-energy Andreev states. The zero-energy flat band appears due to a topological constraint, but comes with a cost in free energy. We have recently found that an adjustment of the surface states can drive a phase transition into a phase with finite superflow that breaks time-reversal symmetry and translational symmetry along the surface. The associated Doppler shifts of Andreev states to finite energies lower the free energy. Direct experimental verification of such a phase is still technically difficult and controversial, however. To aid further experimental efforts, we use the quasiclassical theory of superconductivity to investigate how the realization and the observability of such a phase are influenced by sample geometry and surface ruggedness. Phase diagrams are produced for relevant geometric parameters. In particular, critical sizes and shapes are identified, providing quantitative guidelines for sample fabrication in the experimental hunt for symmetry-breaking phases.
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| 19. |
- Holmvall, Patric, 1988, et al.
(författare)
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SuperConga: An open-source framework for mesoscopic superconductivity
- 2023
-
Ingår i: Applied Physics Reviews. - : AIP Publishing. - 1931-9401. ; 10:1
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Forskningsöversikt (refereegranskat)abstract
- We present SuperConga, an open-source framework for simulating equilibrium properties of unconventional and ballistic singlet superconductors, confined to two-dimensional (2D) mesoscopic grains in a perpendicular external magnetic field, at arbitrary low temperatures. It aims at being both fast and easy to use, enabling research without access to a computer cluster, and visualization in real-time with OpenGL. The core is written in C++ and CUDA, exploiting the embarrassingly parallel nature of the quasiclassical theory of superconductivity by utilizing the parallel computational power of modern graphics processing units. The framework self-consistently computes both the superconducting order-parameter and the induced vector potential and finds the current density, free energy, induced flux density, local density of states (LDOS), and the magnetic moment. A user-friendly Python frontend is provided, enabling simulation parameters to be defined via intuitive configuration files, or via the command-line interface, without requiring a deep understanding of implementation details. For example, complicated geometries can be created with relative ease. The framework ships with simple tools for analyzing and visualizing the results, including an interactive plotter for spectroscopy. An overview of the theory is presented, as well as examples showcasing the framework's capabilities and ease of use. The framework is free to download from https://gitlab.com/superconga/superconga, which also links to the extensive user manual, containing even more examples, tutorials, and guides. To demonstrate and benchmark SuperConga, we study the magnetostatics, thermodynamics, and spectroscopy of various phenomena. In particular, we study flux quantization in solenoids, vortex physics, surface Andreev bound-states, and a "phase crystal."We compare our numeric results with analytics and present experimental observables, e.g., the magnetic moment and LDOS, measurable with, for example, scanning probes, STM, and magnetometry.
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| 20. |
- Håkansson, Mikael, 1975, et al.
(författare)
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Spontaneously broken time-reversal symmetry in high-temperature superconductors
- 2015
-
Ingår i: Nature Physics. - : Springer Science and Business Media LLC. - 1745-2481 .- 1745-2473. ; 11:9, s. 755-760
-
Tidskriftsartikel (refereegranskat)abstract
- Conventional superconductors are strong diamagnets that, through the Meissner effect, expel magnetic fields. It would therefore be surprising if a superconducting ground state would support spontaneous magnetics fields. Such time-reversal symmetry-broken states have been proposed for the high-temperature superconductors, but their identification remains experimentally controversial. Here we show a route to a low-temperature superconducting state with broken time-reversal symmetry that may accommodate currently conflicting experiments. This state is characterized by an unusual vortex pattern in the form of a necklace of fractional vortices around the perimeter of the material, where neighbouring vortices have opposite current circulation. This vortex pattern is a result of a spectral rearrangement of current-carrying states near the edges.
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| 21. |
- Korniyenko, Yevgeniy, 1989, et al.
(författare)
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Nonlinear response of a ballistic graphene transistor with an ac-driven gate: High harmonic generation and terahertz detection
- 2016
-
Ingår i: Physical Review B. - 2469-9969 .- 2469-9950. ; 94:12
-
Tidskriftsartikel (refereegranskat)abstract
- We present results for time-dependent electron transport in a ballistic graphene field-effect transistor with an ac-driven gate. Nonlinear response to the ac drive is derived utilizing Floquet theory for scattering states in combination with Landauer-Buttiker theory for transport. We identify two regimes that can be useful for applications: (i) low and (ii) high doping of graphene under source and drain contacts, relative to the doping level in the graphene channel, which in an experiment can be varied by a back gate. In both regimes, inelastic scattering induced by the ac drive can excite quasibound states in the channel that leads to resonance promotion of higher-order sidebands. Already for weak to intermediate ac drive strength, this leads to a substantial change in the direct current between source and drain. For strong ac drive with frequency Omega, we compute the higher harmonics of frequencies n Omega (n integer) in the source-drain conductance. In regime (ii), we show that particular harmonics (for instance, n = 6) can be selectively enhanced by tuning the doping level in the channel or by tuning the drive strength. We propose that the device operated in the weak-drive regime can be used to detect THz radiation, while in the strong-drive regime, it can be used as a frequency multiplier.
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| 22. |
- Korniyenko, Yevgeniy, 1989, et al.
(författare)
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Resonant second-harmonic generation in a ballistic graphene transistor with an ac-driven gate
- 2016
-
Ingår i: Physical Review B. - 2469-9969 .- 2469-9950. ; 93:3
-
Tidskriftsartikel (refereegranskat)abstract
- We report a theoretical study of time-dependent transport in a ballistic graphene field effect transistor. We develop a model based on Floquet theory describing Dirac electron transmission through a harmonically driven potential barrier. Photon-assisted tunneling results in excitation of quasibound states at the barrier. Under resonance conditions, the excitation of the quasibound states leads to promotion of higher-order sidebands and, in particular, an enhanced second harmonic of the source-drain conductance. The resonances in the main transmission channel are of the Fano form, while they are of the Breit-Wigner form for sidebands. For weak ac drive strength Z(1), the dynamic Stark shift scales as Z(1)(4), while the resonance broadens as Z(1)(2). We discuss the possibility of utilizing the resonances in prospective ballistic high-frequency devices, in particular frequency doublers operating at high frequencies and low temperatures.
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| 23. |
- Korniyenko, Yevgeniy, 1989, et al.
(författare)
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Shot noise in a harmonically driven ballistic graphene transistor
- 2017
-
Ingår i: Physical Review B. - 2469-9969 .- 2469-9950. ; 95:16
-
Tidskriftsartikel (refereegranskat)abstract
- We study time-dependent electron transport and quantum noise in a ballistic graphene field effect transistor driven by an ac gate potential. The nonlinear response to the ac signal is computed through Floquet theory for scattering states and Landauer-Büttiker theory for charge current and its fluctuations. Photon-assisted excitation of a quasibound state in the top-gate barrier leads to resonances in transmission that strongly influence the noise properties. For strong doping of graphene under source and drain contacts, when electrons are transmitted through the channel via evanescent waves, the resonance leads to a substantial suppression of noise. The Fano factor is then reduced well below the pseudodiffusive value, F<1/3, also for strong ac drive. The good signal-to-noise ratio (small Fano factor) on resonance suggests that the device is a good candidate for high-frequency (THz) radiation detection. We show analytically that Klein tunneling (total suppression of back-reflection) persists for perpendicular incidence also when the barrier is driven harmonically. Although the transmission is inelastic and distributed among sideband energies, a sum rule leads to total suppression of shot noise.
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| 24. |
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| 25. |
- Löfwander, Tomas, 1972, et al.
(författare)
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Impurity scattering and Mott's formula in graphene
- 2007
-
Ingår i: Physical Review B - Condensed Matter and Materials Physics. - 2469-9950 .- 2469-9969. ; 76, s. 193401-
-
Tidskriftsartikel (refereegranskat)abstract
- We present calculations of the thermal and electric linear response in graphene, including disorder in the self-consistent t-matrix approximation. For strong impurity scattering, near the unitary limit, the formation of a band of impurity states near the Fermi level leads to that Mott’s relation holds at low temperature. For higher temperatures, there are strong deviations due to the linear density of states. The low-temperature thermopower is proportional to the inverse of the impurity potential and the inverse of the impurity density. Information about impurity scattering in graphene can be extracted from the thermopower, either measured directly or extracted via Mott’s relation from the electron-density dependence of the electric conductivity.
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| 26. |
- Löfwander, Tomas, 1972, et al.
(författare)
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Is CrO2 Fully Spin Polarized? Analysis of Andreev Spectra and Excess Current
- 2010
-
Ingår i: Physical Review Letters. - 1079-7114 .- 0031-9007. ; 105:20
-
Tidskriftsartikel (refereegranskat)abstract
- We report an extensive theoretical analysis of point-contact Andreev reflection data available in the literature on ferromagnetic CrO2. We find that the spectra can be well understood within a model of fully spin-polarized bands in CrO2 together with spin-active scattering at the contact. This is in contrast to analysis of the data within extended Blonder-Tinkham-Klapwijk models, which lead to a spin polarization varying between 50% and 100% depending on the transparency of the interface. We propose to utilize both the temperature dependence of the spectra and the excess current at voltages above the gap to resolve the spin polarization in CrO2 in a new generation of experiments.
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| 27. |
- Löfwander, Tomas, 1972, et al.
(författare)
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Large Thermoelectric Effects in Unconventional Superconductors.
- 2004
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Ingår i: Physical Review B. ; 70:2, s. 024515-
-
Tidskriftsartikel (refereegranskat)abstract
- We present analytic and numerical results for the thermoelectric effect in unconventional superconductors with a dilute random distribution of impurities, each scattering isotropically but with a phase shift intermediate between the Born and unitary limits. The thermoelectric response function has a linear temperature dependence at low temperatures, with a slope that depends on the impurity concentration and phase shift. Although the thermoelectric effect vanishes identically in the strict Born and unitary limits, even a small deviation of the phase shift from these limits leads to a large response, especially in clean systems. We also discuss possibilities of measuring counterflowing supercurrents in a SQUID-setup. The nonquantized thermoelectrically induced flux can easily be of the order of a percent of the flux quantum in clean systems at 4He temperatures.
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| 28. |
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| 29. |
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| 30. |
- Löfwander, Tomas, 1972, et al.
(författare)
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Phase diagrams of ferromagnet-superconductor multilayers with misaligned exchange fields
- 2007
-
Ingår i: Physical Review B. ; 75, s. 014512-
-
Tidskriftsartikel (refereegranskat)abstract
- We study the influence of misalignment of the ferromagnetic exchange field on the equilibrium properties of hybrid structures, composed of superconducting (S) and ferromagnetic (F) parts. In particular, we study numerically the superconducting critical temperature Tc in F-S-F trilayers and in F-S-F-S-F Josephson junctions as a function of the misalignment angle theta of the ferromagnetic magnetization. We discuss the corresponding phase diagrams for these hybrid structures. For the Josephson junctions, a transition between the zero-phase and the pi-phase ground states as a function of theta takes place under certain conditions. Within the quasiclassical Green's function technique in the diffusive limit, we introduce a fast and effective method for calculating Tc in such multilayer structures.
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| 31. |
- Löfwander, Tomas, 1972, et al.
(författare)
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Quantum Hall effect in graphene with twisted bilayer stripe defects
- 2013
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Ingår i: Physical Review B - Condensed Matter and Materials Physics. - 2469-9950 .- 2469-9969. ; 87:20
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Tidskriftsartikel (refereegranskat)abstract
- We analyze the quantum Hall effect in single layer graphene with bilayer stripe defects. Such defects are often encountered at steps in the substrate of graphene grown on silicon carbide. We show that AB or AA stacked bilayer stripes result in large Hall conductivity fluctuations that destroy the quantum Hall plateaux. The fluctuations are a result of the coupling of edge states at opposite edges through currents traversing the stripe. Upon rotation of the second layer with respect to the continuous monolayer (a twisted-bilayer stripe defect), such currents decouple from the extended edge states and develop into long-lived discrete quasibound states circulating around the perimeter of the stripe. Backscattering of edge modes then occurs only at precise resonant energies, and hence the quantum Hall plateaux are recovered as twist angle grows.
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| 32. |
- Löfwander, Tomas, 1972
(författare)
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Transport Properties of Superconducting d-wave junctions
- 2001
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- At surfaces and interfaces of d-wave superconductors, quasiparticle bound states with zero energy, midgap states (MGS), are necessarily formed as a consequence of the sign change of the d-wave order parameter upon 90° rotation. The MGS can never exist at s-wave superconductor surfaces and are therefore true fingerprints of unconventional superconductivity. In this thesis we have studied the basic properties of d-wave superconducting junctions, the central issue being the role of midgap states. Within a scattering theory of transport, we calculate both the dc Josephson current in phase-biased junctions and the current-voltage characteristics (IVC) and ac current components in voltage-biased junctions. We have studied the role of MGS in spontaneous time-reversal symmetry breaking at Josphson tunnel junctions of purely d-wave superconductors. For junction transparencies , where is the superconducting coherence length and is the penetration depth, the driving mechanism of the instability is the splitting of MGS by spontaneous establishment of a phase difference across the junction. For tunnel junctions with , the mechanism is instead self-induced Dopplershifts of MGS. For the particular orientation when a lobe points towards the junction at one side and a node at the other, we find that the odd ac components disapper, which results in a doubling of the Josephson frequency for this orientation. We present a detailed study of the IVC of voltage-biased junctions. For low transparency junctions we derive analytical results for the MGS resonances. We also elaborate on how intrinsic broadening of MGS may determine the nature of resonant tranport through MGS: either via single particle tunneling for large broadening or via pair tunneling for negligible broadening. We argue that it may be possible to distinguish between these types of transport by a measurement of either the low-voltage IVC of Josephson junctions or the zero-voltage zero-frequency shot noise in normal metal-superconductor junctions.
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| 33. |
- Persson, Daniel, 1987, et al.
(författare)
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Spectral properties of superconductors with ferromagnetically ordered magnetic impurities
- 2015
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Ingår i: Physical Review B - Condensed Matter and Materials Physics. - 2469-9950 .- 2469-9969. ; 92:24, s. 245430-
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Tidskriftsartikel (refereegranskat)abstract
- We present a comprehensive theoretical study of thermodynamic properties of superconductors with a dilute concentration of magnetic impurities, with focus on how the properties of the superconducting host change if the magnetic moments of the impurities order ferromagnetically. Scattering off the magnetic impurities leads to the formation of a band of Yu-Shiba-Rusinov states within the superconducting energy gap that drastically influences superconductivity. In the magnetically ordered system, the magnetization displays a sudden drop as a function of the impurity density or magnetic moment amplitude. The drop occurs as the spin-polarized impurity band crosses the Fermi level and is associated with a quantum phase transition first put forward by Sakurai for the single impurity case. Taking into account that the background magnetic field created by the ordered impurity moments enters as a Zeeman shift, we find that the superconducting phase transition changes from second order to first order for high enough impurity concentrations.
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| 34. |
- Persson, Daniel, 1987, et al.
(författare)
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Spin-polarized currents and noise in normal-metal/superconductor junctions with Yu-Shiba-Rusinov impurities
- 2016
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Ingår i: Physical Review B. - 2469-9969 .- 2469-9950. ; 94, s. 155424-
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Tidskriftsartikel (refereegranskat)abstract
- Conventional superconductors disordered by magnetic impurities demonstrate physical properties that are drastically different from their pristine counterparts. In our previous work [D. Persson et al., Phys. Rev. B 92, 245430 (2015)], we explored the spectral and thermodynamic properties of such systems for two extreme cases: completely random and ferromagnetically aligned impurity magnetic moments. Here we consider the transport properties of these systems and show that they have a potential to be used in superconducting spintronic devices. Each magnetic impurity contributes a Yu-Shiba-Rusinov (YSR) bound state to the spectrum, residing at subgap energies. Provided the YSR states form metallic bands, we demonstrate that the tunneling current carried by these states can be highly spin polarized when the impurities are ferromagnetically ordered. The spin polarization can be switched by tuning the bias voltage. Moreover, even when the impurity spins are completely uncorrelated, one can still achieve almost 100% spin polarization of the current, if the tunnel interface is spin active. We compute electric current and noise, varying parameters of the interface between tunneling and fully transparent regimes, and analyze the relative role of single-particle and Andreev reflection processes.
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| 35. |
- Seja, Kevin Marc, 1990, et al.
(författare)
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Finite element method for the quasiclassical theory of superconductivity
- 2022
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Ingår i: Physical Review B. - 2469-9969 .- 2469-9950. ; 106:14
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Tidskriftsartikel (refereegranskat)abstract
- The Eilenberger-Larkin-Ovchinnikov-Eliashberg quasiclassical theory of superconductivity is a powerful method enabling studies of a wide range of equilibrium and nonequilibrium phenomena in conventional and unconventional superconductors. We introduce here a finite element method, based on a discontinuous Galerkin approach, to self-consistently solve the underlying transport equations for general device geometries, arbitrary mean free path, and symmetry of the superconducting order parameter. We present results on (i) the influence of scalar impurity scattering on phase crystals in d-wave superconducting grains at low temperatures and (ii) the current flow and focusing in d-wave superconducting weak links, modeling recent experimental realizations of grooved high-temperature superconducting Dayem bridges. The high adaptability of this finite element method for quasiclassical theory paves the way for future investigations of superconducting devices and new physical phenomena in unconventional superconductors.
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| 36. |
- Seja, Kevin Marc, 1990, et al.
(författare)
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Quasiclassical theory of charge transport across mesoscopic normal-metal-superconducting heterostructures with current conservation
- 2021
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Ingår i: Physical Review B. - 2469-9969 .- 2469-9950. ; 104:10
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Tidskriftsartikel (refereegranskat)abstract
- We consider the steady-state nonequilibrium behavior of mesoscopic superconducting wires connected to normal-metal reservoirs. Going beyond the diffusive limit, we utilize the quasiclassical theory and perform a self-consistent calculation that guarantees current conservation through the entire system. Going from the ballistic to the diffusive limit, we investigate several crucial phenomena such as charge imbalance, momentumresolved nonequilbrium distributions, and the current-to-superflow conversion. Connecting to earlier results for the diffusive case, we find that superconductivity can break down at a critical bias voltage V-c. We find that V-c generally increases as the interface transparency is reduced, while the dependence on the mean-free path is nonmonotonous. We discuss the key differences of the ballistic and semiballistic regimes to the fully diffusive case.
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| 37. |
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| 38. |
- Seja, Kevin Marc, 1990, et al.
(författare)
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Thermopower and thermophase in a d -wave superconductor
- 2022
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Ingår i: Physical Review B. - 2469-9969 .- 2469-9950. ; 105:10
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Tidskriftsartikel (refereegranskat)abstract
- In an unconventional superconductor, the interplay of scattering off impurities and Andreev processes may lead to different scattering times for electronlike and holelike quasiparticles. Such electron-hole asymmetry appears when the impurity scattering phase shift is intermediate between the Born and unitary limits and leads to an expectation for large thermoelectric effects. Here, we examine the thermoelectric response of a d-wave superconductor connected to normal-metal reservoirs under a temperature bias using a fully self-consistent quasiclassical theory. The thermoelectrically induced quasiparticle current is cancelled by superflow in an open circuit setup, but at the cost of a charge imbalance induced at the contacts and extending across the structure. We investigate the resulting thermopower and thermophase and their dependencies on scattering phase shift, mean free path, and interface transparency. For crystal-axis orientations such that surface-bound zero-energy Andreev states are formed, the thermoelectric effect is reduced as a result of locally reduced electron-hole asymmetry. For a semiballistic superconductor with good contacts, we find thermopowers of order several μV/K, suggesting a thermovoltage measurement as a promising path to investigate thermoelectricity in unconventional superconductors.
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| 39. |
- Shelukhin, V, et al.
(författare)
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Observation of periodic pi-phase shifts in ferromagnet-superconductor multilayers
- 2006
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Ingår i: Physical Review B. ; 73, s. 174506-
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Tidskriftsartikel (refereegranskat)abstract
- We report complementary studies of the critical temperature and the critical current in ferromagnet (Ni) - superconductor (Nb) multilayers. The observed oscillatory behavior of both quantities upon variation of the thickness of the ferromagnetic layer is found to be in good agreement with theory. The length scale of oscillations is identical for both quantities and is set by the magnetic length corresponding to an exchange field of 200 meV in Ni. The consistency between the behavior of the two quantities provides strong evidence for periodic pi phase shifts in these devices.
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| 40. |
- Shevtsov, Oleksii, 1988, et al.
(författare)
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Quasiclassical Theory of Spin Imbalance in a Normal Metal-Superconductor Heterostructure with a Spin-Active Interface
- 2014
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Ingår i: Journal of Physics: Conference Series. - : IOP Publishing. - 1742-6588 .- 1742-6596. ; 568:2, s. 022044-
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Konferensbidrag (refereegranskat)abstract
- Non-equilibrium phenomena in superconductors have attracted much attention since the first experiments on charge imbalance in the early 1970's. Nowadays a new promising line of research lies at an intersection between superconductivity and spintronics. Here we develop a quasiclassical theory of a single junction between a normal metal and a superconductor with a spin-active interface at finite bias voltages. Due to spin-mixing and spin-filtering effects of the interface a non-equilibrium magnetization (or spin imbalance) is induced at the superconducting side of the junction, which relaxes to zero in the bulk. A peculiar feature of the system is the presence of interface-induced Andreev bound states, which influence the magnitude and the decay length of spin imbalance. Recent experiments on spin and charge density separation in superconducting wires required external magnetic field for observing a spin signal via non-local measurements. Here, we propose an alternative way to observe spin imbalance without applying magnetic field.
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| 41. |
- Shevtsov, Oleksii, 1988, et al.
(författare)
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Spin imbalance in hybrid superconducting structures with spin-active interfaces
- 2014
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Ingår i: Physical Review B - Condensed Matter and Materials Physics. - 2469-9950 .- 2469-9969. ; 90:8, s. 085432-
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Tidskriftsartikel (refereegranskat)abstract
- We consider a heterostructure consisting of a normal metal and a superconductor separated by a spin-active interface. At finite-bias voltages, spin-filtering and spin-mixing effects at the interface allow for an induced magnetization (spin imbalance) on the superconducting side of the junction, which relaxes to zero in the bulk. Such interfaces are also known to host a pair of in-gap Andreev bound states which were recently observed experimentally. We show that these states are responsible for the dominant contribution to the induced spin imbalance close to the interface. Motivated by recent experiments on spin-charge density separation in superconducting aluminum wires, we propose an alternative way to observe spin imbalance without applying an external magnetic field. We also suggest that the peculiar dependence of the spin imbalance on the applied bias voltage permits an indirect bound-state spectroscopy.
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| 42. |
- Sköldberg, Jonas, 1971, et al.
(författare)
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Spectrum of Andreev Bound States in a Molecule Embedded Inside a Microwave-Excited Superconducting Junction
- 2008
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Ingår i: Physical Review Letters. - 1079-7114 .- 0031-9007. ; 101:8, s. 087002-
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Tidskriftsartikel (refereegranskat)abstract
- Non-dissipative Josephson current through nanoscalesuperconducting constrictions is carried by spectroscopically sharpenergy states, so-called Andreev bound states. Althoughtheoretically predicted almost 40 years ago, no directspectroscopic evidence of these Andreev bound states exists to date.We propose a novel type of spectroscopy based on embedding a superconductingconstriction, formed by a single-level molecule junction, in a microwave QED cavity environment.In the electron-dressed cavity spectrum we find apolariton excitation at twice the Andreev bound state energy, and asuperconducting-phase dependent ac Stark shift of the cavityfrequency. Dispersive measurement of this frequency shift can beused for Andreev bound state spectroscopy.
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| 43. |
- Stadler, Pascal, 1986, et al.
(författare)
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Transport through vertical graphene contacts under intense laser fields
- 2020
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Ingår i: Physical Review Research. - 2643-1564. ; 2:2
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Tidskriftsartikel (refereegranskat)abstract
- We theoretically study the electronic and transport properties of two graphene layers vertically coupled by an insulating layer under the influence of a time-periodic external light field. The nonadiabatic driving induces excitations of electrons and a redistribution of the occupied states which are manifested in the opening of gaps in the quasienergy spectrum of graphene. When a voltage is applied between the top and bottom graphene layers, the photoinduced nonequilibrium occupation modifies the transport properties of the contact. We investigate the electronic and transport properties of the contact by using the nonequilibrium Green's function formalism. To illustrate the behavior of the differential conductance of the vertical contact under light illumination, we consider two cases. First, we assume that both the bottom and top layers consist of graphene and, second, we consider a finite mass term in the bottom layer. We obtain that the differential conductance is strongly suppressed due to opening of gaps in the quasienergy spectrum in graphene. Additionally, the conductance shows features corresponding to the tunneling of photoexcited electrons at energies of the Van Hove singularity for both the top and bottom layers. In the case of a finite mass term in the bottom layer, the differential conductance can be directly related to the tunneling of photoexcited electrons.
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| 44. |
- Tesch, J., et al.
(författare)
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Impurity scattering and size quantization effects in a single graphene nanoflake
- 2017
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Ingår i: Physical Review B. - 2469-9969 .- 2469-9950. ; 95:7
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Tidskriftsartikel (refereegranskat)abstract
- By using Fourier-transform scanning tunneling spectroscopy we measure the interference patterns produced by the impurity scattering of confined Dirac quasiparticles in epitaxial graphene nanoflakes. Upon comparison of the experimental results with tight-binding calculations of realistic model flakes, we show that the characteristic features observed in the Fourier-transformed local density of states are related to scattering between different transverse modes (subbands) of a graphene nanoflake and allow direct insight into the gapped electronic spectrum of graphene. We also observe a strong reduction of quasiparticle lifetime which is attributed to the interaction with the underlying substrate. In addition, we show that the distribution of the on-site energies at flower defects leads to an effectively broken pseudospin selection rule, where intravalley backscattering is allowed.
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| 45. |
- Wennerdal, Niclas, 1986, et al.
(författare)
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Breaking time-reversal and translational symmetry at edges of d -wave superconductors: Microscopic theory and comparison with quasiclassical theory
- 2020
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Ingår i: Physical Review Research. - 2643-1564. ; 2:4
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Tidskriftsartikel (refereegranskat)abstract
- We report results of a microscopic calculation of a second-order phase transition into a state-breaking time-reversal and translational invariance along pair-breaking edges of d-wave superconductors. By solving a tight-binding model through exact diagonalization with the Bogoliubov–de Gennes method, we find that such a state with current loops having a diameter of a few coherence lengths is energetically favorable below T∗ between 10%–20% of Tc of bulk superconductivity, depending on model parameters. This extends our previous studies of such a phase crystal within the quasiclassical theory of superconductivity, and shows that the instability is not qualitatively different when including a more realistic band structure and the fast oscillations on the scale of the Fermi wavelength. Effects of size quantization and Friedel oscillations are not detrimental. We also report on a comparison with quasiclassical theory with the Fermi surfaces extracted from the tight-binding models used in the microscopic calculation. There are quantitative differences in for instance the value of T∗ between the different models, but we can explain the predicted transition temperature within each model as due to the different spectral weights of zero-energy Andreev bound states and the resulting gain in free energy by breaking time-reversal and translational invariance below T∗.
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| 46. |
- Yates, K.A., et al.
(författare)
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Andreev spectroscopy of CrO2 thin films on TiO2 and Al2O3
- 2013
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Ingår i: Europhysics Letters. - : IOP Publishing. - 0295-5075 .- 1286-4854. ; 103:6
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Tidskriftsartikel (refereegranskat)abstract
- Here we analyse the spectroscopic information gathered at a number of single CrO2/Pb interfaces. We examine thin films requiring additional interfacial layers to generate long-range spin triplet proximity effect superconductivity (CrO2/TiO2) or not (CrO 2/Al2O3). We analyse the data using two theoretical models and explore the use of a parameter-free method to determine the agreement between the models and experimental observations, showing the necessary temperature range that would be required to make a definitive statement. The use of the excess current as a further tool to distinguish between models is also examined. The analysis of the spectra demonstrates that the temperature dependence of the normalised zero-bias conductance is independent of the substrate onto which the films are grown. This result has important implications for the engineering of interfaces required for the long-range spin triplet proximity effect. © Copyright EPLA, 2013.
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