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A. M. Goldman

Publications and source records attributed to A. M. Goldman.

At least 19 recordsLinked to original sources
arXiv2015

Hopping Conduction via Ionic Liquid Induced Silicon Surface States

In order to clarify the physics of the gating of solids by ionic liquids (ILs) we have gated lightly doped $p$-Si, which is so well studied that it can be called the "hydrogen atom of solid state physics" and can be used as a test bed for ionic liquids. We explore the case where the concentration of induced holes at the Si surface is below $10^{12}\text{cm}^{-2}$, hundreds of times smaller than record values. We find that in this case an excess negative ion binds a hole on the interface between the IL and Si becoming a surface acceptor. We study the surface conductance of holes hopping between such nearest neighbor acceptors. Analyzing the acceptor concentration dependence of this conductivity, we find that the localization length of a hole is in reasonable agreement with our direct variational calculation of its binding energy. The observed hopping conductivity resembles that of well studied $\text{Na}^{+}$ implanted Si MOSFETs.

cond-mat.mes-hall↗
arXiv2015

Homes Scaling in Ionic Liquid Gated La$_{2}$CuO$_{4+x}$ Thin Films

Finding more efficient ways of exploring the doping phase diagrams of high temperature superconductors as well as probing the fundamental properties of these materials are essential ingredients for driving the discovery of new materials. We use a doping technique involving gating with ionic liquids to systematically and continuously tune the T$_{c}$ of superconducting La$_{2}$CuO$_{4+x}$ thin films. We probe both the transport properties and the penetration depth of these samples and find that Homes scaling $λ^{-2}\proptoσT_{c}$ is obeyed, consistent with these materials being in the dirty limit. This result is independent of the precise mechanism for the gating process as all of the parameters of the scaling relationship are determined by direct measurements on the films.

cond-mat.supr-con↗
arXiv2015

Metallic State of Low Mobility Silicon at High Carrier density induced by an Ionic Liquid

High mobility and dilute two-dimensional electron systems exhibit metallic behavior down to the lowest experimental temperatures. In studies of ionic liquid gated insulating silicon, we have observed transitions to a metallic state in low mobility samples at much higher areal carrier densities than found for samples of high mobility. We have also observed a mobility peak in metallic samples as the carrier density was increased beyond $10^{13} \text{cm}^{-2}$.

cond-mat.mes-hall↗
arXiv2013

Electrostatic Tuning of the Properties of Disordered Indium Oxide Films near the Superconductor-Insulator Transition

The evolution with carrier concentration of the electrical properties of amorphous indium oxide (InO) thin films has been studied using electronic double layer transistor configurations. Carrier variations of up to 7 X 10^(14) carriers/cm^2 were achieved using an ionic liquid as a gate dielectric. The superconductor-insulator transition was traversed and the magnitude and position of the large magnetoresistance peak found in the insulating regime were modified. The systematic variation of the magnetoresistance peak with charge concentration was found to be qualitatively consistent with a simulation based on a model involving granularity.

cond-mat.supr-con↗
arXiv2011

Phase diagram of electrostatically doped Strontium Titanate (SrTiO3)

Electric double layer transistor configurations have been employed to electrostatically dope single crystals of insulating SrTiO_{3}. Here we report on the results of such doping over broad ranges of temperature and carrier concentration employing an ionic liquid as the gate dielectric. The surprising results are, with increasing carrier concentration, an apparent carrier-density dependent conductor-insulator transition, a regime of anomalous Hall effect, suggesting magnetic ordering, and finally the appearance of superconductivity. The possible appearance of magnetic order near the boundary between the insulating and superconducting regimes is reminiscent of effects associated with quantum critical behavior in some complex compounds.

cond-mat.supr-con↗
arXiv2011

Magnetic Field Tuned Quantum Phase Transition in the Insulating Regime of Ultrathin Amorphous Bi Films

A surprisingly strong variation of resistance with perpendicular magnetic field, and a peak in the resistance vs. field, R(B) has been found in insulating films of a sequence of homogeneous, quench-condensed films of amorphous Bi undergoing a thickness-tuned superconductor-insulator transition. Isotherms of magnetoresistance, rather than resistance, vs. field were found to cross at a well-defined magnetic field higher than the field corresponding to the peak in R(B). For all values of B, R(T) was found to obey an Arrhenius form. At the crossover magnetic field the prefactor became equal to the quantum resistance of electron pairs, h/4e^2, and the activation energy returned to its zero field value. These observations suggest that the crossover is the signature of a quantum phase transition between two distinct insulating ground states, tuned by magnetic field.

cond-mat.supr-con↗
arXiv2010

The Stabilization of Superconductivity by Magnetic Field in Out-of-Equilibrium Nanowires

A systematic study has been carried out on the previously reported "magnetic-field-induced superconductivity" of Zn nanowires. By varying parameters such as magnetic field orientation and wire length, the results provide evidence that the phenomenon is a nonequilibrium effect associated with the boundary electrodes. They also suggest there are two length scales involved, the superconducting coherence length and quasiparticle relaxation length. As wire lengths approach either of these length scales, the effect weakens. We demonstrate that it is appropriate to consider the effect to be a stabilization of superconductivity, that has been suppressed by an applied current.

cond-mat.mes-hall↗
arXiv2010

Indirect Magnetic-Field-Tuned Superconductor-Insulator Transitions and Weak Localization of Bosons of Quasi-Two Dimensional Metal Films

Magnetic field and electrostatically tuned superconductor-insulator (SI) transitions of ultrathin metal films with levels of disorder that place them near the disorder-tuned SI transition appear to be direct, continuous quantum phase transitions. When films with lower levels of disorder are subjected to a perpendicular magnetic field, instead of a direct transition, a mixed superconductor-nonsuperconductor regime emerges at the lowest temperatures. The zero temperature limit of the resistance is either insulating or superconducting, depending upon the value of the field, suggesting that the behavior in this limit is governed by percolation physics. At high fields and low temperatures, in the nominally insulating regime, the resistance rather than the conductance is found to be a logarithmic function of temperature corresponding to predicitons for the weak localization of bosons.

cond-mat.supr-con↗
arXiv2009

Magnetic Field Induced Superconductivity in Out-of-Equilibrium Nanowires

Four-terminal resistance measurements have been carried out on Zn nanowires formed using electron-beam lithography. When driven resistive by current, these wires re-enter the superconducting state upon application of small magnetic fields. The data are qualitatively different from those of previous experiments on superconducting nanowires, which revealed either negative magnetoresistance near T_{c} or magnetic field enhanced critical currents. We suggest that our observations are associated with the damping of phase slip processes by the enhancement of dissipation by the quasiparticle conductance channel resulting from the application of a magnetic field.

cond-mat.mes-hall↗
arXiv2008

Thickness dependence of the exchange bias in epitaxial manganite bilayers

Exchange bias has been studied in a series of La2/3Ca1/3MnO3 / La1/3Ca2/3MnO3 bilayers grown on (001) SrTiO3 substrates by ozone-assisted molecular beam epitaxy. The high crystalline quality of the samples and interfaces has been verified using high-resolution X-ray diffractometry and Z-contrast scanning transmission electron microscopy with electron energy loss spectroscopy. The dependence of exchange bias on the thickness of the antiferromagnetic layer has been investigated. A critical value for the onset of the hysteresis loop shift has been determined. An antiferromagnetic anisotropy constant has been obtained by fitting the results to the generalized Meiklejohn-Bean model.

cond-mat.str-el↗
arXiv2007

Evidence of Spatially Inhomogeous Pairing on the Insulating Side of a Disorder-Tuned Superconductor-Insulator Transition

Measurements of transport properties of amorphous insulating indium oxide thin films have been interpreted as evidence of the presence of superconducting islands on the insulating side of a disorder-tuned superconductor-insulator transition. Although the films are not granular, the behavior is similar to that observed in granular films. The results support theoretical models in which the destruction of superconductivity by disorder produces spatially inhomogenous pairing with a spectral gap.

cond-mat.supr-con↗
arXiv2007

Signatures of Random Matrix Theory in the Discrete Energy Spectra of Subnanosize Metallic Clusters

Lead clusters deposited on Si(111) substrates have been studied at low temperatures using scanning tunneling microscopy and spectroscopy. The current-voltage characteristics exhibit current peaks that are irregularly spaced and varied in height. The statistics of the distribution of peak heights and spacings are in agreement with random matrix theory for several clusters. The distributions have also been studied as a function of cluster shape.

cond-mat.mes-hall↗
arXiv2006

Hot Electron Effects in the 2D Superconductor-Insulator Transition

The parallel magnetic field tuned two-dimensional superconductor-insulator transition has been investigated in ultrathin films of amorphous Bi. The resistance is found to be independent of temperature on both sides of the transition below approximately 120 mK. Several observations suggest that this regime is not intrinsically "metallic" but results from the failure of the films' electrons to cool. The onset of this temperature-independent regime can be moved to higher temperatures by either increasing the measuring current or the level of electromagnetic noise. Temperature scaling is successful above 120 mK. Electric field scaling can be mapped onto temperature scaling by relating the electric fields to elevated electron temperatures. These results cast doubt on the existence of an intrinsic metallic regime and on the independent determination of the correlation length and dynamical critical exponents obtained by combining the results of electric field and temperature scaling.

cond-mat.supr-con↗
arXiv2005

Electrostatic- and Parallel Magnetic Field- Tuned Two Dimensional Superconductor-Insulator Transitions

The 2D superconductor-insulator transition in disordered ultrathin amorphous bismuth films has been tuned both by electrostatic electron doping using the electric field effect and by the application of parallel magnetic fields. Electrostatic doping was carried out in both zero and nonzero magnetic fields, and magnetic tuning was conducted at multiple strengths of electrostatically induced superconductivity. The transitions were analyzed using finite size scaling with critical exponent products nu*z = 0.65-0.7. The parallel critical magnetic field increased with electron transfer as (dn_c-dn)^0.33, where dn is the electron transfer and dn_c is its critical value, and the critical resistance decreased linearly with dn. However at lower temperatures, in the insulating regime, the resistance became larger than expected from extrapolation of its temperature dependence at higher temperatures, and scaling failed. These observations imply that although the electrostatic- and parallel magnetic field- tuned superconductor-insulator transitions would appear to belong to the same universality class and to be delineated by a robust phase boundary that can be crossed either by tuning electron density or magnetic field, in the case of the field-tuned transition at the lowest temperatures, some different type of physical behavior turns on in the insulating regime.

cond-mat.supr-con↗
arXiv2005

Electrostatic Tuning of the Superconductor-Insulator Transition in Two Dimensions

Superconductivity has been induced in insulating ultra-thin films of amorphous bismuth using the electric field effect. The screening of electron-electron interaction was found to increase with electron concentration in a manner correlated with the tendency towards superconductivity. This does not preclude an increase in the density of states being important in the development of superconductivity. The superconductor-insulator transition appears to belong to the universality class of the three dimensional XY model.

cond-mat.supr-con↗
arXiv2005

Observation of discrete energy levels in a quantum confined system

Low temperature scanning tunneling microscope images and spectroscopic data have been obtained on subnanometer size Pb clusters fabricated using the technique of buffer layer assisted growth. Discrete energy levels were resolved in current-voltage characteristics as current peaks rather than current steps. Distributions of peak voltage spacings and peak current heights were consistent with Wigner-Dyson and Porter-Thomas distributions respectively, suggesting the relevance of random matrix theory to the description of the electronic eigenstates of the clusters. The observation of peaks rather than steps in the current-voltage characteristics is attributed to a resonant tunneling process involving the discrete energy levels of the cluster, the tip, and the states at the interface between the cluster and the substrate surface.

cond-mat.other↗
arXiv2004

Low-Temperature Glassy Response of Ultrathin Manganite Films to Electric and Magnetic Fields

The glassy response of thin films of La0.8Ca0.2MnO3 to external magnetic and gated electrostatic fields in a field-effect geometry has been studied at low temperatures. A hierarchical response with irreversible memory effects, non-ergodic time evolution, aging and annealing behavior of the resistance suggest that the dynamics are governed by strain relaxation for both electronic and magnetic perturbations. Cross-coupling of charge, spin, and strain have been exploited to tune the coercivity of an ultrathin manganite film by electrostatic gating.

cond-mat.str-el↗