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1.	Brenning, Henrik, 1972, et al. (author) A single electron transistor on an atomic force microscope probe 2006 In: Nano Letters. ; 6:5, s. 937-941 Journal article (peer-reviewed)
2.	Brenning, Henrik, 1972, et al. (author) An ultrasensitive radio-frequency single-electron transistor working up to 4.2K 2006 In: Journal of Applied Physics. ; 100:11, s. 114321- Journal article (peer-reviewed)abstract
3.	Tzalenchuk, Alexander, et al. (author) Engineering and metrology of epitaxial graphene 2011 In: Solid State Communications. - : Elsevier. - 0038-1098 .- 1879-2766. ; 151:16, s. 1094-1099 Journal article (peer-reviewed)abstract ere we review the concepts and technologies, in particular photochemical gating, which contributed to the recent progress in quantum Hall resistance metrology based on large scale epitaxial graphene on silicon carbide.
4.	Adamyan, Astghik, 1984, et al. (author) Kinetic inductance as a microwave circuit design variable by multilayer fabrication 2015 In: Superconductor Science and Technology. - : IOP Publishing. - 0953-2048 .- 1361-6668. ; 28:8 Journal article (peer-reviewed)abstract We report on the development of a reliable NbN/Al/Nb/NbN multilayer fabrication technique for combining design elements with and without kinetic inductance in superconducting microwave circuits. As a proof-of-concept we demonstrate the application of the proposed technique to build a slow microwave propagation line matched to 50 Ω terminals. Fabrication details along with the design and measurements are discussed. At 8 GHz the presented device operates as a dc controllable full-turn phase shifter. We suggest that by exploiting the kinetic inductance as a design variable one can greatly improve operation parameters for a variety of standard microwave designs such as step-impedance filters and resonators.
5.	Adamyan, Astghik, 1984, et al. (author) Superconducting microwave parametric amplifier based on a quasi-fractal slow propagation line 2016 In: Journal of Applied Physics. - : AIP Publishing. - 0021-8979 .- 1089-7550. ; 119:8 Journal article (peer-reviewed)abstract Quantum limited amplifiers are sought after for a wide range of applications within quantum technologies and sensing. One promising candidate is the travelling wave parametric amplifier which exploits the non-linear kinetic inductance of a superconducting transmission line. This type of microwave amplifier promises to deliver a high gain, a quantum limited noise performance over several GHz bandwidth, and a high dynamic range. However, practical realizations of this type of device have so far been limited by fabrication defects, since the length of the superconducting transmission line required for achieving substantial parametric gain is on the order of similar to 1m. Here, we report on a design for a microwave traveling wave amplifier based on a slow propagation line comprising a central strip with high kinetic inductance and quasi-fractal line-to-ground capacitors. Due to an enhanced per unit length inductance (73 nH cm(-1)) and capacitance (15 pF cm(-1)), the line has a microwave propagation velocity as low as 9.8 x 10(8) cm s(-1). This translates into parametric gain up to 0.5 dB cm(-1) and a total gain of 6 dB for just a similar to 10 cm long transmission line. Moreover, the flexibility of the presented design allows balancing the line inductance and capacitance in order to keep the characteristic impedance close to 50 Omega and to suppress standing waves, both factors being essential in order to implement a practical parametric amplifier in the microwave domain.
6.	Adamyan, Astghik, 1984, et al. (author) Tunable superconducting microstrip resonators 2016 In: Applied Physics Letters. - : AIP Publishing. - 0003-6951 .- 1077-3118. ; 108:17 Journal article (peer-reviewed)abstract We report on a simple yet versatile design for a tunable superconducting microstrip resonator. Niobium nitride is employed as the superconducting material and aluminum oxide, produced by atomic layer deposition, as the dielectric layer. We show that the high quality of the dielectric material allows to reach the internal quality factors in the order of Q(i) similar to 10(4) in the single photon regime. Q(i) rapidly increases with the number of photons in the resonator N and exceeds 10(5) for N similar to 10 - 50. A straightforward modification of the basic microstrip design allows to pass a current bias through the strip and to control its kinetic inductance. We achieve a frequency tuning delta f = 62 MHz around f(0) = 2.4 GHz for a fundamental mode and delta f = 164MHz for a third harmonic. This translates into a tuning parameter Q(i)delta f/f(0) = 150. The presented design can be incorporated into essentially any superconducting circuitry operating at temperatures below 2.5K.
7.	Ahlers, F.J., et al. (author) The EMRP project GraphOhm- Towards quantum resistance metrology based on graphene 2014 In: CPEM Digest (Conference on Precision Electromagnetic Measurements). - 0589-1485. - 9781479952052 ; , s. 548-549, s. 548-549 Conference paper (peer-reviewed)abstract A new joint research project (JRP) integrating metrology institutes and universities from nine countries is aimed at realization of a new generation of standards for quantum resistance metrology. The project exploits graphene's properties to simplify operation of standards without compromising the unprecedented precision delivered by semiconductor quantum Hall devices. Higher operating temperatures (above 4.2 K, and up to 8 K) and together with lower magnetic fields (below 5 T, and potentially down to 2 T) will lead to a significantly improved and cost-saving dissemination of intrinsically referenced resistance standards to all end-users relying on electrical measurements.
8.	Alexander-Webber, J. A., et al. (author) Giant quantum Hall plateaus generated by charge transfer in epitaxial graphene 2016 In: Scientific Reports. - : Springer Science and Business Media LLC. - 2045-2322 .- 2045-2322. ; 6 Journal article (peer-reviewed)abstract Epitaxial graphene has proven itself to be the best candidate for quantum electrical resistance standards due to its wide quantum Hall plateaus with exceptionally high breakdown currents. However one key underlying mechanism, a magnetic field dependent charge transfer process, is yet to be fully understood. Here we report measurements of the quantum Hall effect in epitaxial graphene showing the widest quantum Hall plateau observed to date extending over 50 T, attributed to an almost linear increase in carrier density with magnetic field. This behaviour is strong evidence for field dependent charge transfer from charge reservoirs with exceptionally high densities of states in close proximity to the graphene. Using a realistic framework of broadened Landau levels we model the densities of donor states and predict the field dependence of charge transfer in excellent agreement with experimental results, thus providing a guide towards engineering epitaxial graphene for applications such as quantum metrology.
9.	Alexander-Webber, J. A., et al. (author) Phase Space for the Breakdown of the Quantum Hall Effect in Epitaxial Graphene 2013 In: Physical Review Letters. - : American Physical Society. - 0031-9007 .- 1079-7114. ; 111:9, s. e096601- Journal article (peer-reviewed)abstract We report the phase space defined by the quantum Hall effect breakdown in polymer gated epitaxial graphene on SiC (SiC/G) as a function of temperature, current, carrier density, and magnetic fields up to 30 T. At 2 K, breakdown currents (Ic) almost 2 orders of magnitude greater than in GaAs devices are observed. The phase boundary of the dissipationless state (ρxx=0) shows a [1-(T/Tc)2] dependence and persists up to Tc>45 K at 29 T. With magnetic field Ic was found to increase ∝B3/2 and Tc∝B2. As the Fermi energy approaches the Dirac point, the ν=2 quantized Hall plateau appears continuously from fields as low as 1 T up to at least 19 T due to a strong magnetic field dependence of the carrier density.
10.	Andzane, J., et al. (author) Application of Tuning Fork Sensors for In-situ Studies of Dynamic Force Interactions Inside Scanning and Transmission Electron Microscopes 2012 In: Medziagotyra. - : Kaunas University of Technology (KTU). - 1392-1320. ; 18:2, s. 197-201 Journal article (peer-reviewed)abstract Mechanical properties of nanoscale contacts have been probed in-situ by specially developed force sensor based on a quartz tuning fork resonator (TF). Additional control is provided by observation of process in scanning electron microscope (SEM) and transmission electron microscope (TEM). A piezoelectric manipulator allows precise positioning of atomic force microscope (AFM) probe in contact with another electrode and recording of the IF oscillation amplitude and phase while simultaneously visualizing the contact area in electron microscope. Electrostatic control of interaction between the electrodes is demonstrated during observation of the experiment in SEM. In the TEM system the TF sensor operated in shear force mode: Use of TEM allowed for direct control of separation between electrodes. New opportunities for in situ studies of nanomechanical systems using these instruments are discussed.
11.	Andzane, J., et al. (author) Effect of graphene substrate type on formation of Bi 2 Se 3 nanoplates 2019 In: Scientific Reports. - : Springer Science and Business Media LLC. - 2045-2322 .- 2045-2322. ; 9:1 Journal article (peer-reviewed)abstract Knowledge of nucleation and further growth of Bi 2 Se 3 nanoplates on different substrates is crucial for obtaining ultrathin nanostructures and films of this material by physical vapour deposition technique. In this work, Bi 2 Se 3 nanoplates were deposited under the same experimental conditions on different types of graphene substrates (as-transferred and post-annealed chemical vapour deposition grown monolayer graphene, monolayer graphene grown on silicon carbide substrate). Dimensions of the nanoplates deposited on graphene substrates were compared with the dimensions of the nanoplates deposited on mechanically exfoliated mica and highly ordered pyrolytic graphite flakes used as reference substrates. The influence of different graphene substrates on nucleation and further lateral and vertical growth of the Bi 2 Se 3 nanoplates is analysed. Possibility to obtain ultrathin Bi 2 Se 3 thin films on these substrates is evaluated. Between the substrates considered in this work, graphene grown on silicon carbide is found to be the most promising substrate for obtaining of 1–5 nm thick Bi 2 Se 3 films.
12.	Baker, A M R, et al. (author) Energy loss rates of hot Dirac fermions in epitaxial, exfoliated, and CVD graphene 2013 In: Physical Review B. Condensed Matter and Materials Physics. - : American Physical Society. - 1098-0121 .- 1550-235X .- 2469-9950 .- 2469-9969. ; 87:4, s. 045414- Journal article (peer-reviewed)abstract Energy loss rates for hot carriers in graphene have been measured using graphene produced by epitaxial growth on SiC, exfoliation, and chemical vapor deposition (CVD). It is shown that the temperature dependence of the energy loss rates measured with high-field damped Shubnikov-de Haas oscillations and the temperature dependence of the weak localization peak close to zero field correlate well, with the high-field measurements understating the energy loss rates by similar to 40% compared to the low-field results. The energy loss rates for all graphene samples follow a universal scaling of T-e(4) at low temperatures and depend weakly on carrier density proportional to n(-1/2), evidence for enhancement of the energy loss rate due to disorder in CVD samples.
13.	Baker, A M R, et al. (author) Weak localization scattering lengths in epitaxial, and CVD graphene 2012 In: Physical Review B. Condensed Matter and Materials Physics. - : American Physical Society. - 1098-0121 .- 1550-235X .- 2469-9950 .- 2469-9969. ; 86:23, s. 235441- Journal article (peer-reviewed)abstract Weak localization in graphene is studied as a function of carrier density in the range from 1 x 10(11) cm(-2) to 1.43 x 10(13) cm(-2) using devices produced by epitaxial growth onto SiC and CVD growth on thin metal film. The magnetic field dependent weak localization is found to be well fitted by theory, which is then used to analyze the dependence of the scattering lengths L-phi, L-i, and L-* on carrier density. We find no significant carrier dependence for L-phi, a weak decrease for L-i with increasing carrier density just beyond a large standard error, and a n(-1/4) dependence for L-*. We demonstrate that currents as low as 0.01 nA are required in smaller devices to avoid hot-electron artifacts in measurements of the quantum corrections to conductivity. DOI: 10.1103/PhysRevB.86.235441
14.	Bergvall, Anders, 1981, et al. (author) Graphene nanogap for gate-tunable quantum-coherent single-molecule electronics 2011 In: Physical Review B - Condensed Matter and Materials Physics. - 2469-9950 .- 2469-9969. ; 84:15 Journal article (peer-reviewed)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.
15.	Broman, S. L., et al. (author) Dihydroazulene Photoswitch Operating in Sequential Tunneling Regime: Synthesis and Single-Molecule Junction Studies 2012 In: Advanced Functional Materials. - : Wiley. - 1616-3028 .- 1616-301X. ; 22:20, s. 4249-4258 Journal article (peer-reviewed)abstract Molecular switches play a central role for the development of molecular electronics. In this work it is demonstrated that the reproducibility and robustness of a single-molecule dihydroazulene (DHA)/vinylheptafulvene (VHF) switch can be remarkably enhanced if the switching kernel is weakly coupled to electrodes so that the electron transport goes by sequential tunneling. To assure weak coupling, the DHA switching kernel is modified by incorporating p-MeSC6H4 end-groups. Molecules are prepared by Suzuki cross-couplings on suitable halogenated derivatives of DHA. The synthesis presents an expansion of our previously reported brominationeliminationcross-coupling protocol for functionalization of the DHA core. For all new derivatives the kinetics of DHA/VHF transition has been thoroughly studied in solution. The kinetics reveals the effect of sulfur end-groups on the thermal ring-closure of VHF. One derivative, incorporating a p-MeSC6H4 anchoring group in one end, has been placed in a silver nanogap. Conductance measurements justify that transport through both DHA (high resistivity) and VHF (low resistivity) forms goes by sequential tunneling. The switching is fairly reversible and reenterable; after more than 20 ON-OFF switchings, both DHA and VHF forms are still recognizable, albeit noticeably different from the original states.
16.	Burnett, J., et al. (author) PC2: Identifying noise processes in superconducting resonators 2013 In: 2013 IEEE 14th InternationalSuperconductive Electronics Conference, ISEC 2013. - 9781467363716 ; , s. Art. no. 6604284- Conference paper (peer-reviewed)abstract Extensive studies of dielectric loss due to two level fluctuators (TLFs) in superconducting resonators have provided routes for low loss resonators. The research is motivated not only by the use of resonators as detectors and in quantum information processing, but more generally due to TLFs being a source of noise and decoherence in all quantum devices. In this work a frequency locked loop was used to measure frequency fluctuations at timescales in excess of 104 seconds, thereby accurately probing the TLF induced low- frequency noise of the resonator. Our measurement method lead to very high statistical confidence even for very long timescales, and here we can therefore present results explicitly identifying power dependent flicker frequency noise (S = 1/fa where a=1) persisting down to the mHz level.
17.	Cherednichenko, Serguei, 1970, et al. (author) Quantum transport at Dirac point enables graphene for terahertz heterodyne astronomy 2019 In: ISSTT 2019 - 30th International Symposium on Space Terahertz Technology, Proceedings Book. - : International Symposium on Space Terahertz Technology. ; , s. 236- Conference paper (peer-reviewed)
18.	Christensen, M. A., et al. (author) Mixed valence radical cations and intermolecular complexes derived from indenofluorene-extended tetrathiafulvalenes 2014 In: Journal of Materials Chemistry C. - : Royal Society of Chemistry (RSC). - 2050-7526 .- 2050-7534. ; 2:48, s. 10428-10438 Journal article (peer-reviewed)abstract Engineering of mixed-valence (MV) radical cations and intermolecular complexes based on pi-extended tetrathiafulvalenes (TTFs) is central for the development of organic conductors. On another front, redox-controlled dimerization of radical cations has recently been recognized as an important tool in supramolecular chemistry. Here we show that pi-extended TTFs based on the indenofluorene core, prepared by Horner-Wadsworth-Emmons reactions, undergo reversible and stepwise one-electron oxidations and that the detectable, intermediate radical cation forms remarkably strong intermolecular MV ([neutral.cation]) and pi-dimer ([cation.cation]) complexes with near-infrared radical cation absorptions. The radical cation itself seems to be a so-called Class III MV species in the Robin-Day classification. The formation of MV dimers was corroborated by ESR spectroelectrochemical studies, revealing two slightly different ESR signals upon oxidation, one assigned to the MV dimer and the other to the cation monomer. Crystals of the radical cation with different anions (PF6- , BF4-, and TaF6-) were grown by electrocrystallization. Conductance studies revealed that the salts behave as semiconductors with the hexafluorotantalate salt exhibiting the highest conductance. Using a custom-built ESR spectrometer with sub-femtomole sensitivity, the magnetic properties of one crystal were investigated. While the spin-to-spin interaction between radical cations was negligible, a high cooperativity coupling to the microwave field was observed - as a result of an exceptionally narrow spin line width and high spin density. This could have great potential for applications in quantum computation where crystalline spin ensembles are exploited for their long coherence times.
19.	Christie, Michael, 1946, et al. (author) Becoming a doctoral student. Why students decide to do a PhD 2006 In: Shifting Perspectives in Engineering Education / publ. Chalmers Strategic Effort on Learning and Teaching (C-SELT), Chalmers University of Technology, Göteborg. ; , s. 320-331 Book chapter (other academic/artistic)
20.	Chua, C., et al. (author) Observation of Coulomb blockade in nanostructured epitaxial bilayer graphene on SiC 2017 In: Carbon. - : Elsevier BV. - 0008-6223 .- 1873-3891. ; 119, s. 426-430 Journal article (peer-reviewed)abstract We study electron transport in nanostructures patterned in bilayer graphene patches grown epitaxially on SiC as a function of doping, magnetic field, and temperature. Away from charge neutrality transport is only weakly modulated by changes in carrier concentration induced by a local side-gate. At low n-type doping close to charge neutrality, electron transport resembles that in exfoliated graphene nanoribbons and is well described by tunnelling of single electrons through a network of Coulomb-blockaded islands. Under the influence of an external magnetic field, Coulomb blockade resonances fluctuate around an average energy and the gap shrinks as a function of magnetic field. At charge neutrality, however, conduction is less insensitive to external magnetic fields. In this regime we also observe a stronger suppression of the conductance below T*, which we interpret as a sign of broken interlayer symmetry or strong fluctuations in the edge/potential disorder.
21.	Chua, C., et al. (author) Quantum Hall Effect and Quantum Point Contact in Bilayer-Patched Epitaxial Graphene 2014 In: Nano Letters. - : American Chemical Society (ACS). - 1530-6992 .- 1530-6984. ; 14:6, s. 3369-3373 Journal article (peer-reviewed)abstract We study an epitaxial graphene monolayer with bilayer inclusions via magnetotransport measurements and scanning gate microscopy at low temperatures. We find that bilayer inclusions can be metallic or insulating depending on the initial and gated carrier density. The metallic bilayers act as equipotential shorts for edge currents, while closely spaced insulating bilayers guide the flow of electrons in the monolayer constriction, which was locally gated using a scanning gate probe.
22.	Danilov, Andrey, 1961, et al. (author) Electron Transfer Dynamics of Bistable Single-Molecule Junctions 2006 In: Nano Letters. ; 6:10, s. 2184-2190 Journal article (peer-reviewed)
23.	Danilov, Andrey, 1961, et al. (author) Electronic transport in single molecule junctions: Control of the molecule-electrode coupling through intramolecular tunneling barriers 2008 In: Nano Letters. - : American Chemical Society (ACS). - 1530-6992 .- 1530-6984. ; 8:1, s. 1-5 Journal article (peer-reviewed)abstract We report on single molecule electron transport measurements of two oligophenylenevinylene (OPV3) derivatives placed in a nanogap between gold (Au) or lead (Pb) electrodes in a field effect transistor device. Both derivatives contain thiol end groups that allow chemical binding to the electrodes. One derivative has additional methylene groups separating the thiols from the delocalized pi-electron system. The insertion of methylene groups changes the open state conductance by 3-4 orders of magnitude and changes the transport mechanism from a coherent regime with finite zero-bias conductance to sequential tunneling and Coulomb blockade behavior.
24.	Danilov, Andrey, 1961, et al. (author) Nanoelectromechanical switch operating by tunneling of an entire C-60 molecule 2008 In: Nano Letters. - 1530-6992 .- 1530-6984. ; 8:8, s. 2393-2398 Journal article (peer-reviewed)abstract We present a solid state single molecule electronic device where switching between two states with different conductance happens predominantly by tunneling of an entire C-60 molecule. This conclusion is based on a novel statistical analysis of similar to 10(5) switching events. The analysis yields (i) the relative contribution of tunneling, current induced heating and thermal fluctuations to the switching mechanism, (ii) the voltage dependent energy barrier (similar to 100-200 meV) separating the two states of the switch and (iii) the switching attempt frequency, omega(0) corresponding to a 2.8 meV mode, which is most likely rotational.
25.	Danilov, Andrey, 1961, et al. (author) Single molecular devices with fullerenes and oligophenylenevinylene (OPV) derivatives 2005 In: IEEE INTERNATIONAL ELECTRON DEVICES MEETING 2005, TECHNICAL DIGEST. - 078039268X ; , s. 481-484 Conference paper (peer-reviewed)abstract We have studied low temperature electron transport in three terminal single molecule transistors. Devices with OPV revealed a relation between transistor characteristics and position of molecular redox potentials strongly influenced by proximity to the metal. Fullerene based devices demonstrate negative differential resistance and indicate strong coupling of fullerene to gold.
26.	Danilov, Andrey, 1961, et al. (author) Strong electronic coupling between single C60 molecules and gold electrodes prepared by quench condensation at 4 K. A single molecule three terminal device study 2006 In: Faraday Discussions. ; 131, s. 337- Journal article (peer-reviewed)
27.	Danilov, Andrey, 1961, et al. (author) Switching Mechanisms in Molecular Switches 2016 In: Handbook of Single-Molecule Electronics; ed. by Kasper Moth-Poulsen. - 9789814463393 ; , s. 263-298 Book chapter (other academic/artistic)abstract Molecular electronics aims at using tailor-built molecules as active device elements to achieve the desired electronic functionality. Molecular-based electronic switches appear to be the most promising candidates for compact memory arrays with low power consumption. In the past decade, detailed investigations have been performed on a great variety of molecular switches, including mechanically interlocked switches, conformational switches, and redox-active molecules. In this chapter, we will review basic switching mechanisms in molecular switches: thermal fluctuations, current-induced excitations, and quantum tunneling. We will demonstrate how the quantitative information allowing to judge between those different switching mechanisms can be extracted from the data measured on single-molecule devices. We will also discuss how the intrinsic switching properties may be affected when the molecule is bridged to electrodes, and how to distinguish whether the switching happens in molecular kernel or at the molecule-to-electrode interface.
28.	de Graaf, Sebastian Erik, 1986, et al. (author) A near-field scanning microwave microscope based on a superconducting resonator for low power measurements 2013 In: Review of Scientific Instruments. - : AIP Publishing. - 1089-7623 .- 0034-6748. ; 84:2, s. 023706- Conference paper (peer-reviewed)abstract We report on the design and performance of a cryogenic (300 mK) near-field scanning microwave microscope. It uses a microwave resonator as the near-field sensor, operating at a frequency of 6 GHz and microwave probing amplitudes down to 100 μV, approaching low enough photon population (N ∼ 1000) of the resonator such that coherent quantum manipulation becomes feasible. The resonator is made out of a miniaturized distributed fractal superconducting circuit that is integrated with the probing tip, micromachined to be compact enough such that it can be mounted directly on a quartz tuning-fork, and used for parallel operation as an atomic force microscope (AFM). The resonator is magnetically coupled to a transmission line for readout, and to achieve enhanced sensitivity we employ a Pound-Drever-Hall measurement scheme to lock to the resonance frequency. We achieve a well localized near-field around the tip such that the microwave resolution is comparable to the AFM resolution, and a capacitive sensitivity down to 6.4 × 10−20 F/rtHz, limited by mechanical noise. We believe that the results presented here are a significant step towards probing quantum systems at the nanoscale using near-field scanning microwave microscopy.
29.	de Graaf, Sebastian Erik, 1986, et al. (author) Accurate Real-Time Monitoring of Quality Factor and Center Frequency of Superconducting Resonators 2014 In: IEEE Transactions on Applied Superconductivity. - : Institute of Electrical and Electronics Engineers (IEEE). - 1558-2515 .- 1051-8223. ; 24:6 Journal article (peer-reviewed)abstract In this paper, we track both center frequency and quality factor of a superconducting microwave resonator with very high precision using a modified Pound-Drever-Hall frequency locking scheme. The quality factor measurements can be done with high enough bandwidth to study different processes in a superconducting resonator in real time. With this added functionality, the frequency readout remains unaffected, with typical accuracy around 10-30 Hz/root Hz, whereas the cavity loss rate can be determined to within 0.1% on a millisecond time scale (34 dB/root Hz). As we demonstrate, this can be particularly useful for electron spin resonance measurements, where we achieve a minimum number of detectable spins coupled to the resonator N = 5 . 10(5) spins/root Hz, based on quality factor measurements. It also enables the study of fast dynamic processes, for example, vortex dynamics, in these devices on a short time scale.
30.	de Graaf, Sebastian Erik, 1986, et al. (author) Charge Qubit Coupled to an Intense Microwave Electromagnetic Field in a Superconducting Nb Device: Evidence for Photon-Assisted Quasiparticle Tunneling 2013 In: Physical Review Letters. - 1079-7114 .- 0031-9007. ; 111:13, s. Art. no. 137002- Journal article (peer-reviewed)abstract We study a superconducting charge qubit coupled to an intensive electromagnetic field and probe changes in the resonance frequency of the formed dressed states. At large driving strengths, exceeding the qubit energy-level splitting, this reveals the well known Landau-Zener-Stückelberg interference structure of a longitudinally driven two-level system. For even stronger drives, we observe a significant change in the Landau-Zener-Stückelberg pattern and contrast. We attribute this to photon-assisted quasiparticle tunneling in the qubit. This results in the recovery of the qubit parity, eliminating effects of quasiparticle poisoning, and leads to an enhanced interferometric response. The interference pattern becomes robust to quasiparticle poisoning and has a good potential for accurate charge sensing.
31.	de Graaf, Sebastian Erik, 1986, et al. (author) Coherent interaction with two-level fluctuators using near field scanning microwave microscopy 2015 In: Scientific Reports. - : Springer Science and Business Media LLC. - 2045-2322 .- 2045-2322. ; 5 Journal article (peer-reviewed)abstract Near field Scanning Microwave Microscopy (NSMM) is a scanning probe technique that non-invasively can obtain material properties on the nano-scale at microwave frequencies. While focus has been on developing room-temperature systems it was recently shown that this technique can potentially reach the quantum regime, opening up for applications in materials science and device characterization in solid state quantum information processing. In this paper we theoretically investigate this new regime of NSMM. Specifically we show that interaction between a resonant NSMM probe and certain types of two-level systems become possible when the NSMM probe operates in the (sub-) single photon regime, and we expect a high signal-to-noise ratio if operated under the right conditions. This would allow to detect single atomic material defects with energy splittings in the GHz range with nano-scale resolution, provided that individual defects in the material under study are well enough separated. We estimate that this condition is fulfilled for materials with loss tangents below tan delta similar to 10(-3) which holds for materials used in today's quantum circuits and devices where typically tan delta
32.	de Graaf, Sebastian Erik, 1986, et al. (author) Direct Identification of Dilute Surface Spins on Al2 O3: Origin of Flux Noise in Quantum Circuits 2017 In: Physical Review Letters. - 1079-7114 .- 0031-9007. ; 118:5, s. 057703- Journal article (peer-reviewed)abstract An on-chip electron spin resonance technique is applied to reveal the nature and origin of surface spins on Al2O3. We measure a spin density of 2.2×1017 spins/m2, attributed to physisorbed atomic hydrogen and S=1/2 electron spin states on the surface. This is direct evidence for the nature of spins responsible for flux noise in quantum circuits, which has been an issue of interest for several decades. Our findings open up a new approach to the identification and controlled reduction of paramagnetic sources of noise and decoherence in superconducting quantum devices.
33.	de Graaf, Sebastian Erik, 1986, et al. (author) Galvanically split superconducting microwave resonators for introducing internal voltage bias 2014 In: Applied Physics Letters. - : AIP Publishing. - 0003-6951 .- 1077-3118. ; 104:5, s. art no 052601 - Journal article (peer-reviewed)abstract We present the design and performance of high-Q superconducting niobium nitride microwave resonators intended for use in hybrid quantum systems, coupling spin degrees of freedom to the cavity mode, both magnetically and electrically. We demonstrate a solution that allows to introduce static electric fields in the resonator without compromising the microwave performance. Quality factors above 10(5) remain unchanged in strong applied static electric fields above 10 MV/m and magnetic fields up to similar to 400 mT. By design, the configuration of the dc field matches that of the microwave field, especially advantageous for experiments on electrostatically controlled spin systems.
34.	de Graaf, Sebastian Erik, 1986, et al. (author) Magnetic field resilient superconducting fractal resonators for coupling to free spins 2012 In: Journal of Applied Physics. - : AIP Publishing. - 0021-8979 .- 1089-7550. ; 112:12, s. 123905- Journal article (peer-reviewed)abstract We demonstrate a planar superconducting microwave resonator intended for use in applications requiring strong magnetic fields and high quality factors. In perpendicular magnetic fields of 20 mT, the niobium resonators maintain a quality factor above 25 000 over a wide range of applied powers, down to single photon population. In parallel field, the same quality factor is observed above 160 mT, the field required for coupling to free spins at a typical operating frequency of 5 GHz. We attribute the increased performance to the current branching in the fractal design. We demonstrate that our device can be used for spectroscopy by measuring the dissipation from a pico-mole of molecular spins.
35.	de Graaf, Sebastian Erik, 1986, et al. (author) Quantifying dynamics and interactions of individual spurious low-energy fluctuators in superconducting circuits 2021 In: Physical Review B. - 2469-9969 .- 2469-9950. ; 103:17 Journal article (peer-reviewed)abstract Understanding the nature and dynamics of material defects in superconducting circuits is of paramount importance for improving qubit coherence and parameter stability and much needed for implementing large-scale quantum computing. Here we present measurements on individual highly coherent environmental two-level systems (TLS). We trace the spectral diffusion of specific TLS and demonstrate that it originates from the TLS coupling to a small number of low energy incoherent fluctuators. From the analysis of these fluctuations, we access the relevant parameters of low energy fluctuators: Dipole moments, switching energies, and, more importantly, interaction energies. Our approach opens up the possibility of deducing the macroscopic observables in amorphous glassy media from direct measurements of local fluctuator dynamics at the microscopic level- A route towards substantiating commonly accepted, but so far phenomenological, models for the decohering environment.
36.	de Graaf, Sebastian Erik, 1986, et al. (author) Scaling of self-stimulated spin echoes 2024 In: Applied Physics Letters. - 0003-6951 .- 1077-3118. ; 124:2 Journal article (peer-reviewed)abstract Self-stimulated echoes have recently been reported in the high cooperativity and inhomogeneous coupling regime of spin ensembles with superconducting resonators. In this work, we study their relative amplitudes using echo-silencing made possible by a fast frequency tunable resonator. The highly anisotropic spin linewidth of Er3+ electron spins in the CaWO4 crystal also allows to study the dependence on spin-resonator ensemble cooperativity. It is demonstrated that self-stimulated echoes primarily result from a combination of two large control pulses and the echo preceding it.
37.	de Graaf, Sebastian Erik, 1986, et al. (author) Suppression of low-frequency charge noise in superconducting resonators by surface spin desorption 2018 In: Nature Communications. - : Springer Science and Business Media LLC. - 2041-1723 .- 2041-1723. ; 9:1 Journal article (peer-reviewed)abstract Noise and decoherence due to spurious two-level systems located at material interfaces are long-standing issues for solid-state quantum devices. Efforts to mitigate the effects of two-level systems have been hampered by a lack of knowledge about their chemical and physical nature. Here, by combining dielectric loss, frequency noise and on-chip electron spin resonance measurements in superconducting resonators, we demonstrate that desorption of surface spins is accompanied by an almost tenfold reduction in the charge-induced frequency noise in the resonators. These measurements provide experimental evid ence that simultaneously reveals the chemical signatures of adsorbed magnetic moments and highlights their role in generating charge noise in solid-state quantum devices.
38.	de Graaf, S. E., et al. (author) Two-level systems in superconducting quantum devices due to trapped quasiparticles 2020 In: Science advances. - : American Association for the Advancement of Science (AAAS). - 2375-2548. ; 6:51 Journal article (peer-reviewed)abstract A major issue for the implementation of large-scale superconducting quantum circuits is the interaction with interfacial two-level system (TLS) defects that lead to qubit parameter fluctuations and relaxation. Another major challenge comes from nonequilibrium quasiparticles (QPs) that result in qubit relaxation and dephasing. Here, we reveal a previously unexplored decoherence mechanism in the form of a new type of TLS originating from trapped QPs, which can induce qubit relaxation. Using spectral, temporal, thermal, and magnetic field mapping of TLS-induced fluctuations in frequency tunable resonators, we identify a highly coherent subset of the general TLS population with a low reconfiguration temperature similar to 300 mK and a nonuniform density of states. These properties can be understood if the TLS are formed by QPs trapped in shallow subgap states formed by spatial fluctutations of the superconducting order parameter. This implies that even very rare QP bursts will affect coherence over exponentially long time scales.
39.	Dell'Anna, M., et al. (author) THz Spectroscopy Using Low Temperature Mesoscopic Devices 2012 In: Journal of Low Temperature Physics. - : Springer Science and Business Media LLC. - 0022-2291 .- 1573-7357. ; 167:3-4, s. 467-472 Journal article (peer-reviewed)abstract The prototype of a THz spectroscopic camera based on low temperature mesoscopic devices is presented. The core of this system is an array of Quantum-Dots coupled to Quantum Point Contact sensors. Readout electronics is based on Time Domain Multiplexing combined with Lock-in technique. Results show that such system can reach the sensitivity needed to detect THz emission of materials in a fully passive way.
40.	Drexler, C., et al. (author) Magnetic quantum ratchet effect in graphene 2013 In: Nature Nanotechnology. - 1748-3387 .- 1748-3395. ; 8:2, s. 104-107 Journal article (peer-reviewed)abstract A periodically driven system with spatial asymmetry can exhibit a directed motion facilitated by thermal or quantum fluctuations(1). This so-called ratchet effect(2) has fascinating ramifications in engineering and natural sciences(3-18). Graphene(19) is nominally a symmetric system. Driven by a periodic electric field, no directed electric current should flow. However, if the graphene has lost its spatial symmetry due to its substrate or adatoms, an electronic ratchet motion can arise. We report an experimental demonstration of such an electronic ratchet in graphene layers, proving the underlying spatial asymmetry. The orbital asymmetry of the Dirac fermions is induced by an in-plane magnetic field, whereas the periodic driving comes from terahertz radiation. The resulting magnetic quantum ratchet transforms the a.c. power into a d.c. current, extracting work from the out-of-equilibrium electrons driven by undirected periodic forces. The observation of ratchet transport in this purest possible two-dimensional system indicates that the orbital effects may appear and be substantial in other two-dimensional crystals such as boron nitride, molybdenum dichalcogenides and related heterostructures. The measurable orbital effects in the presence of an in-plane magnetic field provide strong evidence for the existence of structure inversion asymmetry in graphene.
41.	Drexler, C., et al. (author) Reststrahlen Band assisted photocurrents in graphene 2013 In: International Conference on Infrared, Millimeter, and Terahertz Waves, IRMMW-THz. - 2162-2027 .- 2162-2035. - 9781467347174 Conference paper (peer-reviewed)abstract We report on the experimental and theoretical study of the Reststrahlen Band assisted photocurrents in epitaxial grown graphene on SiC. We show that excitation of graphene with infrared radiation results in a dc current. We demonstrate that photocurrent in response to linearly polarized radiation exhibit a resonance enhancement in the frequency range of the Reststrahlen Band of the SiC substrate. By contrast the photocurrent excited by circularly polarized radiation is suppressed in the same spectral range. The developed theory is in agreement with the data and reveals a strong influence of the Reststrahl Band on the high frequency transport in graphene.
42.	Drexler, C., et al. (author) Terahertz radiation induced edge currents in graphene 2011 In: RMMW-THz 2011 - 36th International Conference on Infrared, Millimeter, and Terahertz Waves. - 9781457705090 Conference paper (peer-reviewed)abstract We report on the observation of the terahertz radiation induced edge photogalvanic effect. The directed net electric current is generated in single layer graphene by the irradiation of the samples' edges with linearly or circularly polarized terahertz laser radiation at normal incidence. We show that the directed net electric current stems from the sample edges, which reduce locally the symmetry and result in an asymmetric scattering of carriers driven by the radiation field.
43.	Eless, V, et al. (author) Phase coherence and energy relaxation in epitaxial graphene under microwave radiation 2013 In: Applied Physics Letters. - : American Institute of Physics (AIP). - 0003-6951 .- 1077-3118. ; 103:9 Journal article (peer-reviewed)abstract We have performed low-temperature magnetotransport measurements on monolayer epitaxial graphene under microwave radiation and extracted the radiation-induced effective temperatures, energy relaxation, and the dephasing times. We established that the response of the graphene sample is entirely bolometric at least up to 170 GHz. Dynamic dephasing, i.e., the time-reversal symmetry breaking effect of the ac electromagnetic field rather than mediated by heating, may become significant in the terahertz frequency range and in samples with longer phase coherence time.
44.	Fabbri, Filippo, et al. (author) Silicene nanosheets intercalated in slightly defective epitaxial graphene on a 4H-SiC(0001) substrate 2022 In: Surfaces and Interfaces. - : Elsevier BV. - 2468-0230. ; 33 Journal article (peer-reviewed)abstract In the last years, epitaxial graphene (epi-Gr) demonstrated to be an excellent substrate for the synthesis of epitaxial or intercalated two dimensional (2D) materials. Among 2D materials, silicene has been for a long time a dream for the scientific community, for its importance both from the fundamental and the application point of view. Despite the theoretical prediction of silicene energetic viability, experimentally the substrate proved to play a fundamental role in the Si atom adsorption process leading, in case of metal substrates, to a mixed phase formation and, for van der Waals chemical inert substrates, to Si atom intercalation even at room temperature. Such an intercalation has been associated to the presence of surface defects. Very recently it has been shown that hundreds of nanometer area quasi-free standing silicene can be grown on top of an almost ideal epi-Gr layer synthesized on 6H-SiC substrate. In the present paper, using scanning tunneling microscopy and Raman analysis, we demonstrate that a non-ideal (slightly defective) epi-Gr network obtained by thermal decomposition of Si-terminated 4H-SiC(0001) enables the Si atom penetration forming intercalated silicene nanosheets at room temperature, thus opening a path towards controlled intercalation of silicon atoms through epi-Gr and formation of silicene nanosheets for future applications in nanotechnology.
45.	Ganichev, S.D., et al. (author) Magnetic quantum ratchet effect in graphene 2013 In: International Conference on Infrared, Millimeter, and Terahertz Waves, IRMMW-THz. - 2162-2027 .- 2162-2035. - 9781467347174 Conference paper (peer-reviewed)abstract We report on the observation of magnetic quantum ratchet (MQR) effect induced by electric field of terahertz radiation in single-layer graphene samples subjected to an inplane magnetic field. We show that the dc electric current stems from the orbital asymmetry of the Dirac fermions induced by an in-plane magnetic field, while the periodic driving comes from terahertz radiation. A microscopic theory of the observed effect is developed being in a good qualitative agreement with the experiment. The observation of the ratchet transport in the purest possible two-dimensional system indicates that the orbital effects may appear and be substantial in other 2D crystals, such as boron nitride, molybdenum dichalcogenides, and related heterostructures. The measurable orbital effects in the presence of an in-plane magnetic field give strong evidence for the existence of structure inversion asymmetry in graphene.
46.	Ganichev, S.D., et al. (author) Photon helicity driven currents in graphene 2010 In: IRMMW-THz 2010 - 35th International Conference on Infrared, Millimeter, and Terahertz Waves, Conference Guide. - 9781424466573 Conference paper (other academic/artistic)abstract We report on the observation of photon helicity driven currents in graphene. We demonstrate that by illuminating unbiased monolayer graphene samples with terahertz (THz) laser radiation at room temperature under oblique and normal incidence causes directed electric currents. This includes currents which are solely driven by the light's helicity.
47.	Geaney, S., et al. (author) Near-Field Scanning Microwave Microscopy in the Single Photon Regime 2019 In: Scientific Reports. - : Springer Science and Business Media LLC. - 2045-2322 .- 2045-2322. ; 9:1 Journal article (peer-reviewed)abstract The microwave properties of nano-scale structures are important in a wide variety of applications in quantum technology. Here we describe a low-power cryogenic near-field scanning microwave microscope (NSMM) which maintains nano-scale dielectric contrast down to the single microwave photon regime, up to 109 times lower power than in typical NSMMs. We discuss the remaining challenges towards developing nano-scale NSMM for quantum coherent interaction with two-level systems as an enabling tool for the development of quantum technologies in the microwave regime.
48.	Geskin, V., et al. (author) Bianthrone at a Metal Surface: Conductance Switching with a Bistable Molecule Made Feasible by Image Charge Effects 2015 In: AIP Conference Proceedings. - : AIP Publishing LLC. - 1551-7616 .- 0094-243X. ; 1642, s. 469-472 Conference paper (peer-reviewed)abstract Bianthrone is a sterically hindered compound that exists in the form of two non-planar isomers. Our experimental study of single-molecule junctions with bianthrone reveals persistent switching of electric conductance at low temperatures, which can be reasonably associated to molecular isomerization events. Temperature dependence of the switching rate allows for an estimate of the activation energy of the process, on the order of 35-90 meV. Quantum-chemical calculations of the potential surface of neutral bianthrone and its anion, including identification of transition states, yields the isolated molecule isomerization barriers too high vs. the previous estimate, though in perfect agreement with previous experimental studies in solution. Nevertheless, we show that the attraction of the anion in the vicinity of the metal surface by its image charge can significantly alter the energetic landscape, in particular, by reducing the barrier to the values compatible with the observed switching behavior.
49.	Gustafsson, David, 1982, et al. (author) Fully gapped superconductivity in a nanometresize YBa2Cu3O7-delta island enhanced by a magnetic field 2013 In: Nature Nanotechnology. - : Springer Science and Business Media LLC. - 1748-3387 .- 1748-3395. ; 8:1, s. 25-30 Journal article (peer-reviewed)abstract The symmetry of Cooper pairs is central to constructing a superconducting state. The demonstration of a d(x2-y2)-wave order parameter with nodes represented a breakthrough for high critical temperature superconductors (HTSs)(1,2). However, despite this fundamental discovery, the origin of superconductivity remains elusive, raising the question of whether something is missing from the global picture. Deviations from d(x2-y2)-wave symmetry(3,4), such as an imaginary admixture d(x2-y2) + is (or id(xy)), predict a ground state with unconventional properties exhibiting a full superconducting gap and time reversal symmetry breaking(5). The existence of such a state, until now highly controversial(6-10), can be proved by highly sensitive measurements of the excitation spectrum. Here, we present a spectroscopic technique based on an HTS nanoscale device that allows an unprecedented energy resolution thanks to Coulomb blockade effects, a regime practically inaccessible in these materials previously. We find that the energy required to add an extra electron depends on the parity (odd/even) of the excess electrons on the island and increases with magnetic field. This is inconsistent with a pure d(x2-y2)-wave symmetry and demonstrates a complex order parameter component that needs to be incorporated into any theoretical model of HTS.
50.	He, Hans, et al. (author) Accurate graphene quantum Hall arrays for the new International System of Units 2022 In: Nature Communications. - : Nature Research. - 2041-1723. ; 13:1 Journal article (peer-reviewed)abstract Graphene quantum Hall effect (QHE) resistance standards have the potential to provide superior realizations of three key units in the new International System of Units (SI): the ohm, the ampere, and the kilogram (Kibble Balance). However, these prospects require different resistance values than practically achievable in single graphene devices (~12.9 kΩ), and they need bias currents two orders of magnitude higher than typical breakdown currents IC ~ 100 μA. Here we present experiments on quantization accuracy of a 236-element quantum Hall array (QHA), demonstrating RK/236 ≈ 109 Ω with 0.2 part-per-billion (nΩ/Ω) accuracy with IC ≥ 5 mA (~1 nΩ/Ω accuracy for IC = 8.5 mA), using epitaxial graphene on silicon carbide (epigraphene). The array accuracy, comparable to the most precise universality tests of QHE, together with the scalability and reliability of this approach, pave the road for wider use of graphene in the new SI and beyond. © 2022, The Author(s).

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