arXiv subjects

G. Finkelstein

Publications and source records attributed to G. Finkelstein.

At least 19 recordsLinked to original sources

arXiv2021

One-Dimensional Edge Contact to Encapsulated MoS2 with a Superconductor

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 Kelvin, which has ramifications for future fabrication recipes.

cond-mat.mes-hall↗

arXiv2020

Zero-bias crossings and peculiar Shapiro maps in graphene Josephson junctions

The AC Josephson effect manifests itself in the form of "Shapiro steps" of quantized voltage in Josephson junctions subject to RF radiation. This effect presents an early example of a driven-dissipative quantum phenomenon and is presently utilized in primary voltage standards. Shapiro steps have also become one of the standard tools to probe junctions made in a variety of novel materials. Here, we study Shapiro steps in a widely tunable graphene-based Josephson junction. We investigate the variety of patterns that can be obtained in this well-understood system depending on the carrier density, temperature, RF frequency, and magnetic field. Although the patterns of Shapiro steps can change drastically when just one parameter is varied, the overall trends can be understood and the behaviors straightforwardly simulated. The resulting understanding may help in interpreting similar measurements in more complex materials.

cond-mat.mes-hall↗

arXiv2019

2$\Phi_{0}$-periodic magnetic interference in ballistic graphene Josephson junctions

We investigate supercurrent interference patterns measured as a function of magnetic field in ballistic graphene Josephson junctions. At high doping, the expected $\Phi_{0}$-periodic "Fraunhofer" pattern is observed, indicating a uniform current distribution. Close to the Dirac point, we find anomalous interference patterns with an apparent 2$\Phi_{0}$ periodicity, similar to that predicted for topological Andreev bound states carrying a charge of $e$ instead of $2e$. This feature persists with increasing temperature, ruling out a non-sinusoidal current-phase relationship. It also persists in junctions in which sharp vacuum edges are eliminated. Our results indicate that the observed behavior may originate from an intrinsic property of ballistic graphene Josephson junctions, though the exact mechanism remains unclear.

cond-mat.mes-hall↗

arXiv2019

Chiral Quasiparticle Tunneling Between Quantum Hall Edges in Proximity with a Superconductor

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 (QH) 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. Moreover, 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 counter-propagating quantum Hall edge states along opposite superconducting contacts.

cond-mat.mes-hall↗

arXiv2019

Sub-Kelvin Lateral Thermal Transport in Diffusive Graphene

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.

cond-mat.mes-hall↗

arXiv2016

Nonequilibrium Quantum Critical Steady State: Transport Through a Dissipative Resonant Level

Nonequilibrium properties of correlated quantum matter are being intensively investigated because of the rich interplay between external driving and the many-body correlations. Of particular interest is the nonequilibrium behavior near a quantum critical point (QCP), where the system is delicately balanced between different ground states. We present both an analytical calculation of the nonequilibrium steady-state current in a critical system and experimental results to which the theory is compared. The system is a quantum dot coupled to resistive leads: a spinless resonant level interacting with an ohmic dissipative environment. A two channel Kondo-like QCP occurs when the level is on resonance and symmetrically coupled to the leads, conditions achieved by fine-tuning using electrostatic gates. We calculate and measure the nonlinear current as a function of bias ($I$-$V$ curve) at the critical values of the gate voltages corresponding to the QCP. The quantitative agreement between the experimental data and the theory, with no fitting parameter, is excellent. As our system is fully accessible to both theory and experiment, it provides an ideal setting for addressing nonequilibrium phenomena in correlated quantum matter.

cond-mat.str-el↗

arXiv2014

SERS Plasmonic Enhancement using DNA Origami-based Complex Metallic Nanostructures

DNA origami is a novel self-assembly technique allowing one to form various 2D shapes and position matter with nanometer accuracy. It has been used to coordinate placement of nanoscale objects, both organic and inorganic; to make molecular motors and walkers; and to create optically active nanostructures. Here we use DNA origami templates to engineer Surfaced Enhanced Raman Scattering (SERS) substrates. Specifically, gold nanoparticles were selectively attached to the corners of rectangular origami and subsequently enlarged via solution-based metal deposition. The resulting assemblies were designed to form "hot spots" of enhanced electromagnetic field between the nanoparticles. We observed a significant enhancement of the Raman signal from molecules covalently attached to the assemblies, as compared to control nanoparticle samples which lack inter-particle hot spots. Our method opens up the prospects of using DNA origami to rationally engineer and assemble plasmonic structures for molecular spectroscopy.

cond-mat.mtrl-sci↗

arXiv2012

Phonon bottleneck in graphene-based Josephson junctions at millikelvin temperatures

We examine the nature of the transitions between the normal and the superconducting branches of 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 ($T \lesssim 1$K). The relationship between the applied power and the electron temperature was found to be $P\propto T^3$, which we argue is consistent with cooling due to electron-phonon interactions.

cond-mat.mes-hall↗

arXiv2012

Observation of Majorana Quantum Critical Behavior in a Resonant Level Coupled to a Dissipative Environment

We investigate experimentally an exotic state of electronic matter obtained by fine-tuning to a quantum critical point (QCP), realized in a spin-polarized resonant level coupled to strongly dissipative electrodes. Several transport scaling laws near and far from equilibrium are measured, and then accounted for theoretically. Our analysis reveals a splitting of the resonant level into two quasi-independent Majorana modes, one strongly hybridized to the leads, and the other tightly bound to the quantum dot. Residual interactions involving these Majorana fermions result in the observation of a striking quasi-linear non-Fermi liquid scattering rate at the QCP. Our devices constitute a viable alternative to topological superconductors as a platform for studying strong correlation effects within Majorana physics.

cond-mat.mes-hall↗

arXiv2011

Pb-graphene-Pb Josephson junctions: characterization in B field

We fabricate superconductor-graphene-superconductor Josephson junctions with superconducting regions made of lead (Pb). The critical current through grapehene may be modulated by external magnetic field; the resulting Fraunhofer interference pattern shows several periods of oscillations, indicating 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.

cond-mat.mes-hall↗

arXiv2011

Phase diffusion in graphene-based Josephson junctions

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 $\sim 2$ K, at which point we can detect a small, but non-zero, 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 electromagnetic environment and dissipation in our samples.

cond-mat.mes-hall↗

arXiv2010

Resonant Tunneling in a Dissipative Environment

We measure tunneling through a single quantum level in a carbon nanotube quantum dot connected to resistive metal leads. For the electrons tunneling to/from the nanotube, the leads serve as a dissipative environment, which suppresses the tunneling rate. In the regime of sequential tunneling, the height of the single-electron conductance peaks increases as the temperature is lowered, although it scales more weekly than the conventional 1/T. In the resonant tunneling regime (temperature smaller than the level width), the peak width approaches saturation, while the peak height starts to decrease. Overall, the peak height shows a non-monotonic temperature dependence. We associate this unusual behavior with the transition from the sequential to the resonant tunneling through a single quantum level in a dissipative environment.

cond-mat.mes-hall↗

arXiv2010

Two-Stage Kondo Effect and Kondo Box Level Spectroscopy in a Carbon Nanotube

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 non-monotonic 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.

cond-mat.mes-hall↗

arXiv2007

Four-Probe Measurements of Carbon Nanotubes with Narrow Metal Contacts

We find that electrons in single-wall carbon nanotubes may propagate substantial distances (tens of nanometers) under the metal contacts. We perform four-probe transport measurements of the nanotube conductance and observe significant deviations from the standard Kirchhoff's circuit rules. Most noticeably, injecting current between two neighboring contacts on one end of the nanotube, induces a non-zero voltage difference between two contacts on the other end.

cond-mat.mes-hall↗

arXiv2007

Evolution of SU(4) Transport Regimes in Carbon Nanotube Quantum Dots

We study the evolution of conductance regimes in carbon nanotubes with doubly degenerate orbitals (``shells'') by controlling the contact transparency within the same sample. For sufficiently open contacts, Kondo behavior is observed for 1, 2, and 3 electrons in the topmost shell. As the contacts are opened more, the sample enters the ``mixed valence'' regime, where different charge states are strongly hybridized by electron tunneling. Here, the conductance as a function of gate voltage shows pronounced modulations with a period of four electrons, and all single-electron features are washed away at low temperature. We successfully describe this behavior by a simple formula with no fitting parameters. Finally, we find a surprisingly small energy scale that controls the temperature evolution of conductance and the tunneling density of states in the mixed valence regime.

cond-mat.mes-hall↗

arXiv2007

SU(4) and SU(2) Kondo Effects in Carbon Nanotube Quantum Dots

We study the SU(4) Kondo effect in carbon nanotube quantum dots, where doubly degenerate orbitals form 4-electron ``shells''. The SU(4) Kondo behavior is investigated for one, two and three electrons in the topmost shell. While the Kondo state of two electrons is quenched by magnetic field, in case of an odd number of electrons two types of SU(2) Kondo effect may survive. Namely, the spin SU(2) state is realized in the magnetic field parallel to the nanotube (inducing primarily orbital splitting). Application of the perpendicular field (inducing Zeeman splitting) results in the orbital SU(2) Kondo effect.

cond-mat.mes-hall↗

arXiv2006

Persistent Orbital Degeneracy in Carbon Nanotubes

The quantum-mechanical orbitals in carbon nanotubes are doubly degenerate over a large number of states in the Coulomb blockade regime. We argue that this experimental observation indicates that electrons are reflected without mode mixing at the nanotube-metal contacts. Two electrons occupying a pair of degenerate orbitals (a ``shell'') are found to form a triplet state starting from zero magnetic field. Finally, we observe unexpected low-energy excitations at complete filling of a four-electron shell.

cond-mat.mes-hall↗

arXiv2002

Modeling Subsurface Charge Accumulation Images of a Quantum Hall Liquid

Subsurface Charge Accumulation imaging is a cryogenic scanning probe technique that has recently been used to spatially probe incompressible strips formed in a two-dimensional electron system (2DES) at high magnetic fields. In this paper, we present detailed numerical modeling of these data. At a basic level, the method produces results that agree well with the predictions of models based on simple circuit elements. Moreover, the modeling method is sufficiently advanced to simulate the spatially resolved measurements. By comparing directly the simulations to the experimentally measured data, we can extract quantitatively local electronic features of the 2DES. In particular, we deduce the electron density of states inside the incompressible strips and electrical resistance across them.

cond-mat.mes-hall↗