Browsing by Author "Finkelstein, G"

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Open Access
Anomalous periodicity of magnetic interference patterns in encapsulated graphene Josephson junctions
(Physical Review Research) Ke, CT; Draelos, AW; Seredinski, A; Wei, MT; Li, H; Hernandez-Rivera, M; Watanabe, K; Taniguchi, T; Yamamoto, M; Tarucha, S; Bomze, Y; Borzenets, IV; Amet, F; Finkelstein, G
Open Access
Ballistic Graphene Josephson Junctions from the Short to the Long Junction Regimes.
(Physical review letters, 2016-12-02) Borzenets, IV; Amet, F; Ke, CT; Draelos, AW; Wei, MT; Seredinski, A; Watanabe, K; Taniguchi, T; Bomze, Y; Yamamoto, M; Tarucha, S; Finkelstein, G
We investigate the critical current I_{C} of ballistic Josephson junctions made of encapsulated graphene-boron-nitride heterostructures. We observe a crossover from the short to the long junction regimes as the length of the device increases. In long ballistic junctions, I_{C} is found to scale as ∝exp(-k_{B}T/δE). The extracted energies δE are independent of the carrier density and proportional to the level spacing of the ballistic cavity. As T→0 the critical current of a long (or short) junction saturates at a level determined by the product of δE (or Δ) and the number of the junction's transversal modes.
Open Access
Chiral quasiparticle tunneling between quantum Hall edges in proximity with a superconductor
(Physical Review B, 2019-09-10) Wei, MT; Draelos, AW; Seredinski, A; Ke, CT; Li, H; Mehta, Y; Watanabe, K; Taniguchi, T; Yamamoto, M; Tarucha, S; Finkelstein, G; Amet, F; Borzenets, IV
© 2019 American Physical Society. We study a two-terminal graphene Josephson junction with contacts shaped to form a narrow constriction, less than 100nm in length. The contacts are made from type-II superconducting contacts and able to withstand magnetic fields high enough to reach the quantum Hall regime in graphene. In this regime, the device conductance is determined by edge states, plus the contribution from the constricted region. In particular, the constriction area can support supercurrents up to fields of ∼2.5T. Additionally, enhanced conductance is observed through a wide range of magnetic fields and gate voltages. This additional conductance and the appearance of supercurrent is attributed to the tunneling between counterpropagating quantum Hall edge states along opposite superconducting contacts.
Open Access
Connecting the nanodots: programmable nanofabrication of fused metal shapes on DNA templates.
(Nano letters, 2011-08) Pilo-Pais, M; Goldberg, S; Samano, E; Labean, TH; Finkelstein, G
We present a novel method for producing complex metallic nanostructures of programmable design. DNA origami templates, modified to have DNA binding sites with a uniquely coded sequence, were adsorbed onto silicon dioxide substrates. Gold nanoparticles functionalized with the cDNA sequence were then attached. These seed nanoparticles were later enlarged, and even fused, by electroless deposition of silver. Using this method, we constructed a variety of metallic structures, including rings, pairs of bars, and H shapes.
Open Access
Investigation of Supercurrent in the Quantum Hall Regime in Graphene Josephson Junctions
(Journal of Low Temperature Physics, 2018-06-01) Draelos, A; Wei, MT; Seredinski, A; Ke, C; Watanabe, K; Taniguchi, T; Yamamoto, M; Tarucha, S; Borzenets, I; Amet, F; Finkelstein, G
© 2018, Springer Science+Business Media, LLC, part of Springer Nature. In this study, we examine multiple encapsulated graphene Josephson junctions to determine which mechanisms may be responsible for the supercurrent observed in the quantum Hall (QH) regime. Rectangular junctions with various widths and lengths were studied to identify which parameters affect the occurrence of QH supercurrent. We also studied additional samples where the graphene region is extended beyond the contacts on one side, making that edge of the mesa significantly longer than the opposite edge. This is done in order to distinguish two potential mechanisms: (a) supercurrents independently flowing along both non-contacted edges of graphene mesa, and (b) opposite sides of the mesa being coupled by hybrid electron–hole modes flowing along the superconductor/graphene boundary. The supercurrent appears suppressed in extended junctions, suggesting the latter mechanism.
Open Access
Nonequilibrium quantum transport through a dissipative resonant level
(Physical Review B - Condensed Matter and Materials Physics, 2013-06-21) Chung, CH; Le Hur, K; Finkelstein, G; Vojta, M; Wölfle, P
The resonant-level model represents a paradigmatic quantum system which serves as a basis for many other quantum impurity models. We provide a comprehensive analysis of the nonequilibrium transport near a quantum phase transition in a spinless dissipative resonant-level model, extending earlier work. A detailed derivation of a rigorous mapping of our system onto an effective Kondo model is presented. A controlled energy-dependent renormalization-group approach is applied to compute the nonequilibrium current in the presence of a finite bias voltage V. In the linear-response regime V→0, the system exhibits as a function of the dissipative strength a localized-delocalized quantum transition of the Kosterlitz-Thouless (KT) type. We address fundamental issues of the nonequilibrium transport near the quantum phase transition: Does the bias voltage play the same role as temperature to smear out the transition? What is the scaling of the nonequilibrium conductance near the transition? At finite temperatures, we show that the conductance follows the equilibrium scaling for VT. We furthermore provide different signatures of the transition in the finite-frequency current noise and ac conductance via a recently developed functional renormalization group (FRG) approach. The generalization of our analysis to nonequilibrium transport through a resonant level coupled to two chiral Luttinger liquid leads, generated by fractional quantum Hall edge states, is discussed. Our work on the dissipative resonant level has direct relevance to experiments on a quantum dot coupled to a resistive environment, such as H. Mebrahtu,. © 2013 American Physical Society.
Open Access
Observation of majorana quantum critical behaviour in a resonant level coupled to a dissipative environment
(Nature Physics, 2013-01-01) Mebrahtu, HT; Borzenets, IV; Zheng, H; Bomze, YV; Smirnov, AI; Florens, S; Baranger, HU; Finkelstein, G
A quantum phase transition is an abrupt change between two distinct ground states of a many-body system, driven by an external parameter. In the vicinity of the quantum critical point (QCP) where the transition occurs, a new phase may emerge that is determined by quantum fluctuations and is very different from either phase. In particular, a conducting system may exhibit non-Fermi-liquid behaviour. Although this scenario is well established theoretically, controllable experimental realizations are rare. Here, we experimentally investigate the nature of the QCP in a simple nanoscale system - a spin-polarized resonant level coupled to dissipative contacts. We fine-tune the system to the QCP, realized exactly on-resonance and when the coupling between the level and the two contacts is symmetric. Several anomalous transport scaling laws are demonstrated, including a striking non-Fermi-liquid scattering rate at the QCP, indicating fractionalization of the resonant level into two Majorana quasiparticles. © 2013 Macmillan Publishers Limited.
Open Access
Observation of Majorana quantum critical behaviour in a resonant level coupled to a dissipative environment
(Nature Physics, 2013) Mebrahtu, HT; Borzenets, IV; Zheng, H; Bomze, YV; Smirnov, AI; Florens, S; Baranger, HU; Finkelstein, G
A quantum phase transition is an abrupt change between two distinct ground states of a many-body system, driven by an external parameter. In the vicinity of the quantum critical point (QCP) where the transition occurs, a new phase may emerge that is determined by quantum fluctuations and is very different from either phase. In particular, a conducting system may exhibit non-Fermi-liquid behaviour. Although this scenario is well established theoretically, controllable experimental realizations are rare. Here, we experimentally investigate the nature of the QCP in a simple nanoscale system-a spin-polarized resonant level coupled to dissipative contacts. We fine-tune the system to the QCP, realized exactly on-resonance and when the coupling between the level and the two contacts is symmetric. Several anomalous transport scaling laws are demonstrated, including a striking non-Fermi-liquid scattering rate at the QCP, indicating fractionalization of the resonant level into two Majorana quasiparticles.
Open Access
One-dimensional edge contact to encapsulated MoS2 with a superconductor
(AIP Advances, 2021-04-01) Seredinski, A; Arnault, EG; Costa, VZ; Zhao, L; Larson, TFQ; Watanabe, K; Taniguchi, T; Amet, F; Newaz, AKM; Finkelstein, G
Establishing ohmic contact to van der Waals semiconductors such as MoS2 is crucial to unlocking their full potential in next-generation electronic devices. Encapsulation of few layer MoS2 with hBN preserves the material’s electronic properties but makes electrical contacts more challenging. Progress toward high quality edge contact to encapsulated MoS2 has been recently reported. Here, we evaluate a contact methodology using sputtered MoRe, a type II superconductor with a relatively high critical field and temperature commonly used to induce superconductivity in graphene. We find that the contact transparency is poor and that the devices do not support a measurable supercurrent down to 3 K, which has ramifications for future fabrication recipes.
Open Access
Pb-Graphene-Pb josephson junctions: Characterization in magnetic field
(IEEE Transactions on Applied Superconductivity, 2012-06-14) Borzenets, IV; Coskun, UC; Mebrahtu, H; Finkelstein, G
We fabricate superconductor-graphene-superconductor Josephson junctions with superconducting regions made of lead (Pb). The critical current through graphene may be modulated by the external magnetic field; the resulting Fraunhofer interference pattern shows several periods of oscillations, suggesting that the junction is uniform. Deviations from the perfect Fraunhofer pattern are observed, and their cause is explained by a simulation that takes into account the sample design. © 2002-2011 IEEE.
Open Access
Phase diffusion in graphene-based Josephson junctions.
(Physical review letters, 2011-09-21) Borzenets, IV; Coskun, UC; Jones, SJ; Finkelstein, G
We report on graphene-based Josephson junctions with contacts made from lead. The high transition temperature of this superconductor allows us to observe the supercurrent branch at temperatures up to ∼2 K, at which point we can detect a small, but nonzero, resistance. We attribute this resistance to the phase diffusion mechanism, which has not been yet identified in graphene. By measuring the resistance as a function of temperature and gate voltage, we can further characterize the nature of the electromagnetic environment and dissipation in our samples.
Open Access
Phonon bottleneck in graphene-based Josephson junctions at millikelvin temperatures.
(Physical review letters, 2013-07-09) Borzenets, IV; Coskun, UC; Mebrahtu, HT; Bomze, Yu V; Smirnov, AI; Finkelstein, G
We examine the nature of the transitions between the normal and superconducting branches in superconductor-graphene-superconductor Josephson junctions. We attribute the hysteresis between the switching (superconducting to normal) and retrapping (normal to superconducting) transitions to electron overheating. In particular, we demonstrate that the retrapping current corresponds to the critical current at an elevated temperature, where the heating is caused by the retrapping current itself. The superconducting gap in the leads suppresses the hot electron outflow, allowing us to further study electron thermalization by phonons at low temperatures (T≲1 K). The relationship between the applied power and the electron temperature was found to be P∝T3, which we argue is consistent with cooling due to electron-phonon interactions.
Open Access
Retrapping current, self-heating, and hysteretic current-voltage characteristics in ultranarrow superconducting aluminum nanowires
(Physical Review B - Condensed Matter and Materials Physics, 2011-11-08) Li, P; Wu, PM; Bomze, Y; Borzenets, IV; Finkelstein, G; Chang, AM
Hysteretic I-V (current-voltage) curves are studied in narrow Al nanowires. The nanowires have a cross section as small as 50 nm2. We focus on the retrapping current in a down-sweep of the current, at which a nanowire re-enters the superconducting state from a normal state. The retrapping current is found to be significantly smaller than the switching current at which the nanowire switches into the normal state from a superconducting state during a current up-sweep. For wires of different lengths, we analyze the heat removal due to various processes, including electron and phonon processes. For a short wire 1.5μm in length, electronic thermal conduction is effective; for longer wires 10μm in length, phonon conduction becomes important. We demonstrate that the measured retrapping current as a function of temperature can be quantitatively accounted for by the self-heating occurring in the normal portions of the nanowires to better than 20% accuracy. For the phonon processes, the extracted thermal conduction parameters support the notion of a reduced phase-space below three dimensions, consistent with the phonon thermal wavelength having exceeded the lateral dimensions at temperatures below ∼1.3 K. Nevertheless, surprisingly the best fit was achieved with a functional form corresponding to three-dimensional phonons, albeit requiring parameters far exceeding known values in the literature. © 2011 American Physical Society.
Open Access
Subkelvin lateral thermal transport in diffusive graphene
(Physical Review B, 2019-03-29) Draelos, AW; Silverman, A; Eniwaye, B; Arnault, EG; Ke, CT; Wei, MT; Vlassiouk, I; Borzenets, IV; Amet, F; Finkelstein, G
© 2019 American Physical Society. In this work, we report on hot carrier diffusion in graphene across large enough length scales that the carriers are not thermalized across the crystal. The carriers are injected into graphene at one site and their thermal transport is studied as a function of applied power and distance from the heating source, up to tens of micrometers away. Superconducting contacts prevent out-diffusion of hot carriers to isolate the electron-phonon coupling as the sole channel for thermal relaxation. As local thermometers, we use the amplitude of the universal conductance fluctuations, which varies monotonically as a function of temperature. By measuring the electron temperature simultaneously along the length we observe a thermal gradient which results from the competition between electron-phonon cooling and lateral heat flow.
Open Access
Superconductivity: When Andreev meets Hall
(Nature Physics, 2017-07-01) Finkelstein, G; Amet, F
Open Access
Supercurrent in Graphene Josephson Junctions with Narrow Trenches in the Quantum Hall Regime
(MRS Advances, 2018-01-01) Seredinski, A; Draelos, A; Wei, MT; Ke, CT; Fleming, T; Mehta, Y; Mancil, E; Li, H; Taniguchi, T; Watanabe, K; Tarucha, S; Yamamoto, M; Borzenets, IV; Amet, F; Finkelstein, G
© 2018 Materials Research Society. Coupling superconductors to quantum Hall edge states is the subject of intense investigation as part of the ongoing search for non-abelian excitations. Our group has previously observed supercurrents of hundreds of picoamperes in graphene Josephson junctions in the quantum Hall regime. One of the explanations of this phenomenon involves the coupling of an electron edge state on one side of the junction to a hole edge state on the opposite side. In our previous samples, these states are separated by several microns. Here, a narrow trench perpendicular to the contacts creates counterpropagating quantum Hall edge channels tens of nanometres from each other. Transport measurements demonstrate a change in the low-field Fraunhofer interference pattern for trench devices and show a supercurrent in both trench and reference junctions in the quantum Hall regime. The trench junctions show no enhancement of quantum Hall supercurrent and an unexpected supercurrent periodicity with applied field, suggesting the need for further optimization of device parameters.
Open Access
Supercurrent in the quantum Hall regime.
(Science (New York, N.Y.), 2016-05) Amet, F; Ke, CT; Borzenets, IV; Wang, J; Watanabe, K; Taniguchi, T; Deacon, RS; Yamamoto, M; Bomze, Y; Tarucha, S; Finkelstein, G
A promising route for creating topological states and excitations is to combine superconductivity and the quantum Hall (QH) effect. Despite this potential, signatures of superconductivity in the QH regime remain scarce, and a superconducting current through a QH weak link has been challenging to observe. We demonstrate the existence of a distinct supercurrent mechanism in encapsulated graphene samples contacted by superconducting electrodes, in magnetic fields as high as 2 tesla. The observation of a supercurrent in the QH regime marks an important step in the quest for exotic topological excitations, such as Majorana fermions and parafermions, which may find applications in fault-tolerant quantum computing.
Open Access
Surface-enhanced Raman scattering plasmonic enhancement using DNA origami-based complex metallic nanostructures.
(Nano letters, 2014-01) Pilo-Pais, M; Watson, A; Demers, S; LaBean, TH; Finkelstein, G
DNA origami is a novel self-assembly technique allowing one to form various two-dimensional shapes and position matter with nanometer accuracy. We use DNA origami templates to engineer surface-enhanced Raman scattering substrates. Specifically, gold nanoparticles were selectively placed on the corners of rectangular origami and subsequently enlarged via solution-based metal deposition. The resulting assemblies exhibit "hot spots" of enhanced electromagnetic field between the nanoparticles. We observed a significant Raman signal enhancement from molecules covalently attached to the assemblies, as compared to control nanoparticle samples that lack interparticle hot spots. Furthermore, Raman molecules are used to map out the hot spots' distribution, as they are burned when experiencing a threshold electric field. Our method opens up the prospects of using DNA origami to rationally engineer and assemble plasmonic structures for molecular spectroscopy.
Open Access
Tunable quantum phase transitions in a resonant level coupled to two dissipative baths
(Physical Review B - Condensed Matter and Materials Physics, 2014-02-18) Liu, DE; Zheng, H; Finkelstein, G; Baranger, HU
We study tunneling through a resonant level connected to two dissipative bosonic baths: one is the resistive environment of the source and drain leads, while the second comes from coupling to potential fluctuations on a resistive gate. We show that several quantum phase transitions (QPT) occur in such a model, transitions which emulate those found in interacting systems such as Luttinger liquids or Kondo systems. We first use bosonization to map this dissipative resonant level model to a resonant level in a Luttinger liquid, one with, curiously, two interaction parameters. Drawing on methods for analyzing Luttinger liquids at both weak and strong coupling, we obtain the phase diagram. For strong dissipation, a Berezinsky-Kosterlitz-Thouless QPT separates strong-coupling and weak-coupling (charge localized) phases. In the source-drain symmetric case, all relevant backscattering processes disappear at strong coupling, leading to perfect transmission at zero temperature. In fact, a QPT occurs as a function of the coupling asymmetry or energy of the resonant level: the two phases are (i) the system is cut into two disconnected pieces (zero transmission), or (ii) the system is a single connected piece with perfect transmission, except for a disconnected fractional degree of freedom. The latter arises from the competition between the two fermionic leads (source and drain), as in the two-channel Kondo effect. © 2014 American Physical Society.
Open Access
Two-stage Kondo effect and Kondo-box level spectroscopy in a carbon nanotube
(Physical Review B - Condensed Matter and Materials Physics, 2010-10-18) Bomze, Y; Borzenets, I; Mebrahtu, H; Makarovski, A; Baranger, HU; Finkelstein, G
The concept of the "Kondo box" describes a single spin, antiferromagnetically coupled to a quantum dot with a finite level spacing. Here, a Kondo box is formed in a carbon nanotube interacting with a localized electron. We investigate the spins of its first few eigenstates and compare them to a recent theory. In an "open" Kondo-box, strongly coupled to the leads, we observe a nonmonotonic temperature dependence of the nanotube conductance, which results from a competition between the Kondo-box singlet and the "conventional" Kondo state that couples the nanotube to the leads. © 2010 The American Physical Society.