arXiv subjects

Nadya Mason

Publications and source records attributed to Nadya Mason.

At least 37 records · Page 2Linked to original sources

arXiv2017

Observation of domain wall motion in a polycrystalline vortex lattice

We present measurements showing how disorder determines the dynamics of a vortex lattice. Using superconductor-normal-superconductor (SNS) arrays placed in finite magnetic fields, disorder is introduced by shifting the field away from values where the vortex lattice is commensurate with the SNS array. By applying a current to drive vortex motion, we observe a two-step transition at incommensurate fields: first from pinned vortices to lattice defect motion, and then from individual defect to bulk vortex lattice flow. This behavior is consistent with a polycrystalline vortex structure where defects form on the edges of crystalline domains. This demonstrates that a disordered, interacting vortex system can favor interface physics rather than a quasiperiodic regime.

cond-mat.supr-con↗

arXiv2016

Observation of non-sinusoidal current-phase relation in graphene Josephson junctions

We present direct measurements of the current-phase relation for lateral Josephson junctions with a graphene barrier, obtained by a phase-sensitive SQUID interferometry technique. We find that the current-phase relation is forward skewed with respect to the commonly observed sinusoidal behavior for short junctions in the quasi-ballistic transport regime, consistent with predictions for the behavior of Dirac fermions in a Josephson junction. The skewness increases with critical current and decreases sharply with increasing temperature.

cond-mat.mes-hall↗

arXiv2016

Rare-Region Onset of Superconductivity in Granular Systems

The critical behavior of disordered systems-from metals (1) to magnets (2) and superconductors (3)-is often dominated by the behavior of rare regions of a correlated phase, which control the inception and dynamics of the phase transition. Yet, despite significant theoretical (3,4,5) and experimental (6,7,8,9) interest, there has been little direct evidence of the presence of these regions, or of their role in initiating transitions. Here, we provide direct evidence for rare-region effects at the onset of superconductivity in granular superconducting islands. By considering the strong diameter-dependence of the transition, as well as observations of large fluctuations in the transition temperature as island diameters decrease, we are able to show that superconducting order first appears in unusually large grains- i.e. rare regions- within each island and, due to proximity coupling, spreads to other grains. This work thus provides a quantitative, local understanding of the onset of correlated order in strongly disordered systems.

cond-mat.supr-con↗

arXiv2016

History-dependent dissipative vortex dynamics in superconducting arrays

We perform current(I)-voltage(V) measurements on low resistance superconductor-normal-superconductor arrays in finite magnetic fields, focusing on the dilute vortex population regime. We observe significant deviations from predicted behavior, notably the absence of a differential resistance peak near the vortex de-pinning current, and a broad linear I-V region with an extrapolated I intercept equal to the de-pinning current. Comparing these results to an overdamped molecular vortex model, we find that this behavior can be explained by the presence of a history dependent dissipative force. This approach has not been considered previously, yet is crucial for obtaining a correct description of the vortex dynamics in superconducting arrays.

cond-mat.supr-con↗

arXiv2016

Single Gate P-N Junctions in Graphene-Ferroelectric Devices

Graphene's linear dispersion relation and the attendant implications for bipolar electronics applications have motivated a range of experimental efforts aimed at producing p-n junctions in graphene. Here we report electrical transport measurements of graphene p-n junctions formed via simple modifications to a PbZr$_{0.2}$Ti$_{0.8}$O$_3$ substrate, combined with a self-assembled layer of ambient environmental dopants. We show that the substrate configuration controls the local doping region, and that the p-n junction behavior can be controlled with a single gate. Finally, we show that the ferroelectric substrate induces a hysteresis in the environmental doping which can be utilized to activate and deactivate the doping, yielding an `on-demand' p-n junction in graphene controlled by a single, universal backgate.

cond-mat.mtrl-sci↗

arXiv2015

Kondo-like zero-bias conductance anomaly in a three-dimensional topological insulator nanowire

Zero bias anomalies in topological nanowires have recently captured significant attention, as they are possible signatures of Majorana modes. Yet there are many other possible origins of zero bias peaks in nanowires, for example, weak localization, Andreev bound states, or the Kondo effect. Here, we discuss observations of differential conductance peaks at zero bias voltage in nonsuperconducting electronic transport through a 3D topological insulator nanowire. The zero bias conductance peaks show logarithmic temperature dependence and often linear splitting with magnetic fields, both of which are signatures of the Kondo effect in quantum dots. Here, we characterize the zero bias peaks and discuss their origin.

cond-mat.mes-hall↗

arXiv2015

Mechanical Control of Graphene on Engineered Pyramidal Strain Arrays

Strain can tune desirable electronic behavior in graphene, but there has been limited progress in controlling strain in graphene devices. In this paper, we study the mechanical response of graphene on substrates patterned with arrays of mesoscale pyramids. Using atomic force microscopy, we demonstrate that the morphology of graphene can be controlled from conformal to suspended depending on the arrangement of pyramids and the aspect ratio of the array. Non-uniform strains in graphene suspended across pyramids are revealed by Raman spectroscopy and supported by atomistic modeling, which also indicates strong pseudomagnetic fields in the graphene. Our results suggest that incorporating mesoscale pyramids in graphene devices is a viable route to achieving strain-engineering of graphene.

cond-mat.mes-hall↗

arXiv2014

Reversible mechanical and electrical properties of ripped graphene

We examine the mechanical properties of graphene devices stretched on flexible elastomer substrates. Using atomic force microscopy, transport measurements, and mechanics simulations, we show that micro-rips form in the graphene during the initial application of tensile strain; however subsequent applications of the same tensile strain elastically open and close the existing rips. Correspondingly, while the initial tensile strain degrades the devices' transport properties, subsequent strain-relaxation cycles affect transport only moderately, and in a largely reversible fashion, yielding robust electrical transport even after partial mechanical failure.

cond-mat.mes-hall↗

arXiv2014

Aharonov-Bohm Oscillations in a Quasi-Ballistic 3D Topological Insulator Nanowire

In three-dimensional topological insulators (3D TI) nanowires, transport occurs via gapless surface states where the spin is fixed perpendicular to the momentum[1-6]. Carriers encircling the surface thus acquire a πBerry phase, which is predicted to open up a gap in the lowest-energy 1D surface subband. Inserting a magnetic flux (Φ) of h/2e through the nanowire should cancel the Berry phase and restore the gapless 1D mode[7-8]. However, this signature has been missing in transport experiments reported to date[9-11]. Here, we report measurements of mechanically-exfoliated 3D TI nanowires which exhibit Aharonov-Bohm oscillations consistent with topological surface transport. The use of low-doped, quasi-ballistic devices allows us to observe a minimum conductance at Φ = 0 and a maximum conductance reaching e^2/h at Φ = h/2e near the lowest subband (i.e. the Dirac point), as well as the carrier density dependence of the transport.

cond-mat.mes-hall↗

arXiv2013

From Coulomb Blockade to Resonant Transmission in a MoS2 Nanoribbon

We have measured a nanoribbon of MoS2 at low temperature, and observed the evolution of the system from a regime of multiple small quantum dots in series to one where the entire nanoribbon acts as a single quantum dot. At higher Fermi energies, resonant transmission through disorder-induced potential wells is evident. Our findings shed light on the length scale of quasi-ballistic transport in the material.

cond-mat.mes-hall↗

arXiv2013

Spatial Dependence of Electron Interactions in Carbon Nanotubes

We report measurements of the spatial dependence of the electron energy distribution in carbon nanotubes, from which electron interactions are determined. Using nonequilibrium tunneling spectroscopy with multiple superconducting probes, we characterize electron transport as ballistic or diffusive, and interactions as elastic or inelastic. We find that transport in nanotubes is generally diffusive, caused by elastic scattering from a few defects. However, local inelastic scattering can be tuned `on' or `off' with a gate voltage.

cond-mat.mes-hall↗

arXiv2012

Tunneling Spectroscopy of Graphene using Planar Pb Probes

We show that evaporating lead (Pb) directly on graphene can create high-yield, high-quality tunnel probes, and we demonstrate high magnetic field/low temperature spectroscopy using these probes. Comparisons of Pb, Al and Ti/Au probes shows that after oxidation a well-formed self-limited tunnel barrier is created only between the Pb and the graphene. Tunneling spectroscopy using the Pb probes manifests energy-dependent features such as scattering resonances and localization behavior, and can thus be used to probe the microscopic electronics of graphene.

cond-mat.mes-hall↗

arXiv2012

Symmetry Protected Josephson Supercurrents in Three-Dimensional Topological Insulators

Coupling the surface state of a topological insulator (TI) to an s-wave superconductor is predicted to produce the long-sought Majorana quasiparticle excitations. However, superconductivity has not been measured in surface states when the bulk charge carriers are fully depleted, i.e., in the true topological regime that is relevant for investigating Majorana modes. Here, we report measurements of DC Josephson effects in TI-superconductor junctions as the chemical potential is moved from the bulk bands into the band gap, or through the true topological regime characterized by the presence of only surface currents. We examine the relative behavior of the system at different bulk/surface ratios, determining the effects of strong bulk/surface mixing, disorder, and magnetic field. We compare our results to 3D quantum transport simulations to conclude that the supercurrent is largely carried by surface states, due to the inherent topology of the bands, and that it is robust against disorder.

cond-mat.mes-hall↗

arXiv2012

Fate of global superconductivity in arrays of long SNS junctions

Normal-metal films overlaid with arrays of superconducting islands undergo Berezinskii-Kosterlitz-Thouless (BKT) superconducting transitions at a temperature $T_{BKT}$. We present measurements of T$_{BKT}$ for arrays of mesoscopic Nb islands patterned on Au films for a range of island spacings $d$. We show that $T_{BKT} \sim 1/d^2$, and explain this dependence in terms of the quasiclassical prediction that the Thouless energy, rather than the superconducting gap, governs the inter-island coupling. We also find two deviations from the quasiclassical theory: (i) $T_{BKT}$ is sensitive to island height, because the islands are mesoscopic; and (ii) for widely spaced islands the transition appears to lead, not to a superconducting state, but to a finite-resistance "metallic" one.

cond-mat.supr-con↗

arXiv2011

Observation of a Snap-Through Instability in Graphene

We examine the competition between adhesive and bending energies for few-layered graphene samples placed on rigid, microscale-corrugated substrates. Using atomic force microscopy, we show that the graphene undergoes a sharp "snap-through" transition as a function of layer thickness, where the material transitions between conforming to the substrate and lying flat on top of the substrate. By utilizing the critical snap-through thickness in an elasticity model for the FLG's bending, we extract a value for graphene-surface adhesion energy that is larger than expected for van der Waals forces.

cond-mat.mes-hall↗

arXiv2011

Approaching Zero-Temperature Metallic States in Mesoscopic Superconductor-Normal-Superconductor Arrays

Systems of superconducting islands placed on normal metal films offer tunable realizations of two-dimensional (2D) superconductivity; they can thus elucidate open questions regarding the nature of 2D superconductors and competing states. In particular, island systems have been predicted to exhibit zero-temperature metallic states. Although evidence exists for such metallic states in some 2D systems, their character is not well understood: the conventional theory of metals cannot explain them, and their properties are difficult to tune. Here, we characterize the superconducting transitions in mesoscopic island-array systems as a function of island thickness and spacing. We observe two transitions in the progression to superconductivity; both transition temperatures exhibit unexpectedly strong depression for widely spaced islands. These depressions are consistent with the system approaching zero-temperature metallic states. The nature of the transitions and the state between them is explained using a phenomenological model involving the stabilization of superconductivity on each island via a weak coupling to and feedback from its neighbors.

cond-mat.supr-con↗

arXiv2010

Transport Through Andreev Bound States in a Graphene Quantum Dot

Andreev reflection-where an electron in a normal metal backscatters off a superconductor into a hole-forms the basis of low energy transport through superconducting junctions. Andreev reflection in confined regions gives rise to discrete Andreev bound states (ABS), which can carry a supercurrent and have recently been proposed as the basis of qubits [1-3]. Although signatures of Andreev reflection and bound states in conductance have been widely reported [4], it has been difficult to directly probe individual ABS. Here, we report transport measurements of sharp, gate-tunable ABS formed in a superconductor-quantum dot (QD)-normal system, which incorporates graphene. The QD exists in the graphene under the superconducting contact, due to a work-function mismatch [5, 6]. The ABS form when the discrete QD levels are proximity coupled to the superconducting contact. Due to the low density of states of graphene and the sensitivity of the QD levels to an applied gate voltage, the ABS spectra are narrow, can be tuned to zero energy via gate voltage, and show a striking pattern in transport measurements.

cond-mat.supr-con↗

arXiv2010

Negative and Positive Magnetoresistance in Bilayer Graphene: Effects of Weak Localization and Charge Inhomogeneity

We report measurements of magnetoresistance in bilayer graphene as a function of gate voltage (carrier density) and temperature. We examine multiple contributions to the magnetoresistance, including those of weak localization (WL), universal conductance fluctuations (UCF), and inhomogeneous charge transport. A clear WL signal is evident at all measured gate voltages (in the hole doped regime) and temperature ranges (from 0.25 K to 4.3 K), and the phase coherence length extracted from WL data does not saturate at low temperatures. The WL data is fit to demonstrate that electron-electron Nyquist scattering is the major source of phase decoherence. A decrease in UCF amplitude with increasing gate voltage and temperature is shown to be consistent with a corresponding decrease in the phase coherence length. In addition, a weak positive magnetoresistance at higher magnetic fields is observed, and attributed to inhomogeneous charge transport.

cond-mat.mtrl-sci↗