arXiv subjects

A. Frydman

Publications and source records attributed to A. Frydman.

At least 19 recordsLinked to original sources

arXiv2023

Nernst Sign-Reversal in the Hexatic Vortex Phase of Weakly Disordered a-MoGe Thin Films

The hexatic phase is an intermediate stage in the melting process of a 2D crystal due to topological defects. Recently, this exotic phase was experimentally identified in the vortex lattice of 2D weakly disordered superconducting MoGe by scanning tunneling microscopic measurements. Here we study this vortex state by the Nernst effect, which is an effective and sensitive tool to detect vortex motion, especially in the superconducting fluctuation regime. We find a surprising Nernst sign reversal at the melting transition of the hexatic phase. We propose that they are a consequence of vortex dislocations in the hexatic state which diffuse preferably from the cold to hot.

cond-mat.supr-con↗

arXiv2022

Interdependent Superconducting Networks

Cascades are self-amplifying processes triggered by feedback mechanisms that may cause a substantial part of a macroscopic system to change its phase in response of a relatively small local event. The theoretical background for these phenomena is rich and interdisciplinary with interdependent networks providing a versatile "two-interactions" framework to study their multiscale evolution. Yet, physics experiments aimed at validating this ever-growing volume of predictions have remained elusive, hitherto hindered by the problem of identifying possible physical mechanisms realizing interdependent couplings. Here we develop and study the first experimental realization of an interdependent system as a multilayer network of two disordered superconductors separated by an insulating film. We show that Joule heating effects emerging at sufficiently large driving currents act as dependency links between the superconducting layers, igniting overheating cascades via adaptive back and forth electro-thermal feedbacks. Through theory and experiments, we unveil a rich phase diagram of mutual resistive transitions and cascading processes that physically realize and generalize interdependent percolation. The present work establishes the first physics laboratory bench for the manifestation of the theory of interdependent systems, enabling experimental studies to control and to further develop the multilayer phenomena of complex interdependent materials.

cond-mat.dis-nn↗

arXiv2019

Specific Heat Signature of the Berezinskii-Kosterlitz-Thouless Transition in Ultrathin Superconducting Films

The Berezinskii-Kosterlitz-Thouless (BKT) transition is expected to have a clear signature on the specific heat. The singularity at the transition temperature $T_{BKT}$ is predicted to be immeasurable, and a broad non-universal peak is expected at $T>T_{BKT}$. Up to date this has not been observed in two-dimensional superconductors. We use a unique highly sensitive technique to measure the specific heat of ultrathin Pb films. We find that thick films exhibit a specific heat jump at $T_C$ that is consistent with BCS theory. As the film thickness is reduced below the superconducting coherence length and the systems enters the 2D limit the specific heat reveals BKT-like behavior. We discuss these observations in the framework of the continuous BCS-BKT crossover as a function of film thickness.

cond-mat.supr-con↗

arXiv2015

Fractional Coulomb blockade in a coupling controlled metallic quantum dot

We use a novel technique to experimentally explore transport properties through a single metallic nanoparticle with variable coupling to electric leads. For strong dot-lead coupling the conductance is an oscillatory function of the gate voltage with periodicity determined by the charging energy, as expected. For weaker coupling we observe the appearance of additional multi-periodic oscillations of the conductance with the gate voltage. These harmonics correspond to a change of the charge on the dot by a fraction of an electron. This notion is supported by theoretical calculations based on dissipative action theory. Within this framework the multiple periodicity of the conductance oscillations arises due to non-pertubative instanton solutions.

cond-mat.mes-hall↗

arXiv2012

Zero bias anomaly in a two dimensional granular insulator

We compare tunneling density of states (TDOS) into two ultrathin Ag films, one uniform and one granular, for different degrees of disorder. The uniform film shows a crossover from Altshuler-Aronov (AA) zero bias anomaly to Efros Shklovskii (ES) like Coulomb gap as the disorder is increased. The granular film, on the other hand, exhibits AA behavior even deeply in the insulating regime. We analyze the data and find that granularity introduces a new regime for the TDOS. While the conductivity is dominated by hopping between clusters of grains and is thus insulating, the TDOS probes the properties of an individual cluster which is "metallic".

cond-mat.str-el↗

arXiv2011

A Superconducting Gap in an Insulator

We present tunneling spectroscopy and transport measurements on disordered indium oxide films that reveal the existence of a superconducting gap in an insulating state. Two films on both sides of the disorder induced superconductor to insulator transition (SIT) show the same energy gap scale at low temperatures. This energy gap persists up to relatively high magnetic fields and is observed across the magnetoresistance peak typical of disordered superconductors. The results provide useful information for understanding the nature of the insulating state in the disorder induced SIT.

cond-mat.supr-con↗

arXiv2009

Two gate-voltage periods in a metallic-nanoparticle based single-electron transistor

Systems of quantum dots (QD) connected to leads exhibit periodic conductance peaks as a function of gate voltage arising from the Coulomb blockade effect \cite{review1,review2,review3}. Much effort goes into minimizing the size of QDs and reaching the scale of single molecules \cite{molecular1,molecular2,molecular3} which could serve as nanoelectronic circuit components such as transistors. Connecting molecules or nanoparticles to external leads cannot be achieved by the traditional methods used in semiconductor based QDs, hence, controlling the coupling to nanoparticle QDs is a major technical challenge. Here we present a novel technique by which we can explore electric properties of a metallic nanoparticle while varying the coupling to leads. We find that the conductance through the nanoparticle is characterized by \emph{two} gate voltage periods. The relative strength of the periods depends both on the dot-lead coupling and on the source-drain voltage. These surprising findings may be a general property of strongly coupled metallic nanoparticles.

cond-mat.mes-hall↗

arXiv2009

Itinerant Ferromagnetism in the electronic localization limit

We present Hall effect, $R_{xy}(H)$, and magnetoresistance, $R_{xx}(H)$, measurements of ultrathin films of Ni, Co and Fe with thicknesses varying between 0.2-8 nm and resistances between 1 M$Ω$ - 100 $Ω.$ Both measurements show that films having resistance above a critical value, $R_{C}$, (thickness below a critical value, $d_{C}$) show no signs for ferromagnetism. Ferromagnetism appears only for films with $R<R_{C}$, where $R_{C}$ is material dependent. We raise the possibility that the reason for the absence of spontaneous magnetization is suppression of itinerant ferromagnetism by electronic disorder in the strong localization regime.

cond-mat.str-el↗

arXiv2008

Two-Fluid Behaviour at the Origin of the Resistivity Peak in Doped Manganites

We report a series of magnetic and transport measurements on high-quality single crystal samples of colossal magnetoresistive manganites, La_{0.7} Ca_{0.3} Mn O_3 and Pr_{0.7} Sr_{0.3} Mn O_3. 1 % Fe doping allows a Moessbauer spectroscopy study, which shows (i) unusual line broadening within the ferromagnetic phase and (ii) a coexistence of ferro- and paramagnetic contributions in a region, T_1 T_2. This shows that phase separation into metallic (ferromagnetic) and insulating (paramagnetic) phases cannot be generally responsible for the resistivity peak (and hence for the associated colossal magnetoresistance). Our results can be understood phenomenologically within the two-fluid approach, which also allows for a difference between T_C and T_{MI}. Our data indeed imply that while magnetic and transport properties of the manganites are closely interrelated, the two transitions at T_C and T_{MI} can be viewed as distinct phenomena.

cond-mat.str-el↗

arXiv2007

Inverse slow relaxation in granular hopping systems

We present experimental results that demonstrate a glassy behavior in the conductance of quench condensed insulating granular metals that is different from that observed in continuous disordered systems. Exciting the granular system by biasing the sample with a high electric field results in a slow conductance change both during the excitation and during the relaxation back to its steady state. The time scales for these processes are many orders of magnitudes longer than the typical hopping time. We find that, initially, this conductance change has an opposite sign to that observed in similar experiments performed on continuous films. Only after relatively long times this trend is reversed and the samples exhibit conventional behavior. We suggest that the granular systems exhibit a new glassy process related to charge redistribution among the grains. This process combines with the relaxation processes that are characteristic of disordered systems to give rise to a unique relaxation profile.

cond-mat.mes-hall↗

arXiv2007

Percolation model for the superconductor-insulator transition in granular films

We study the temperature dependence of the superconductor-insulator transition in granular superconductors. Empirically, these systems are characterized by very broad resistance tails, which depend exponentially on the temperature, and the normal state resistance. We model these systems by a two-dimensional random resistor percolation networks in which the resistance between two grains is governed either by Josephson junction coupling or by quasi particle tunneling. Our numerical simulations as well as an effective medium evaluation explain the experimental results over a wide range of temperatures and resistances. Using effective medium approximation we find an analytical expression for the effective resistance of the system and the value of the critical resistance separating conducting from insulating branches.

cond-mat.supr-con↗

arXiv2005

The signature of a double quantum-dot structure in the I-V characteristics of a complex system

We demonstrate that by carefully analyzing the temperature dependent characteristics of the I-V measurements for a given complex system it is possible to determine whether it is composed of a single, double or multiple quantum-dot structure. Our approach is based on the orthodox theory for a double-dot case and is capable of simulating I-V characteristics of systems with any resistance and capacitance values and for temperatures corresponding to thermal energies larger than the dot level spacing. We compare I-V characteristics of single-dot and double-dot systems and show that for a given measured I-V curve considering the possibility of a second dot is equivalent to decreasing the fit temperature. Thus, our method allows one to gain information about the structure of an experimental system based on an I-V measurement.

cond-mat.mes-hall↗

arXiv2005

The effect of a weak ferromagnetic matrix on a system of nanomagnetic particles

The study of system of magnetic nano-particle has received increasing attention recently both because of the novel physical concepts involved and also because of their vast potential for application. The influence of background material (the substrate coating) on magnetic properties of such systems is a relatively open topic and often a full understanding is missing. In the present work we discuss our experiments and interpretation for two systems: Ni nanoparticles coated with graphitic carbon and Ni nanoparticles coated with Au. While the latter system exibits behavior typical of superparamagnetic particle systems the former shows several puzzling results such as extremely high blocking temperature ($T_{B}$), very fast relaxation time well below $T_{B}$, temperature independent field-cooled magnetization and very small coercivity and remanent magnetization. We interpret these findings as being a result of weak ferromagnetism, characteristic of the graphitic carbon. This induces strong magnetic interactions between the Ni particles in the presence of small magnetic fields. Such systems give rise to a dramatic difference in blocking temperature between measurements performed at zero field and those performed at very small magnetic fields.

cond-mat.mtrl-sci↗

arXiv2005

Magnetoresistance of mesoscopic granular ferromagnets

We have performed magnetoresistance (MR) measurements of granular ferromagnets having lateral dimensions smaller than 0.5 $μ$m and containing a small number of grains (down to about 100). Compared to macroscopic samples, these granular samples exhibit unusually large saturation fields and MR amplitudes. In addition, the evolution of the magnetoresistance curve as the intergrain distance decreases is qualitatively different than that of large samples. We discuss these results and suggest that they reflect a transition from percolation to quasi single-channel dominated transport.

cond-mat.mes-hall↗

arXiv2004

Comment on "Memory Effects in an Interacting Magnetic Nanoparticle System" by Sun et. al, prl, 91, 167206 (2003)

In this Comment we report a phenomenon identical to that observed in ({Y. Sun, M. B. Salamon, K. Garnier and R. S. Averback, Phys. Rev. Lett. 91, 167206 (2003)}) for systems of NiFe{$_2$}O{$_4$} particles (mean size $\approx$ 3nm) embedded in a SiO{$_2$} matrix with two different interparticle spacings 4 nm (1) and 15 nm (2), which controls the strength of the dipolar interactions. Not only do we find the memory effect to be present in the non-interacting sample (2), indeed we find it to be {\em more} prominent than in the interacting case (1). We demonstrate that this effect can be simply attributed to a superposition of relaxation times of two sets of particles.

cond-mat.mtrl-sci↗

arXiv2003

Orange Peel coupling in granular ferromagnetic films

We present magnetoresistance (MR) measurements performed on magnetic tunnel junctions in which one of the electrodes is a granular ferromagnetic film. These junctions exhibit a zero field resistance dip. The dip magnitude depends on the size of the grains. We interpret these results as a consequence of the orange peel effect between the continuous ferromagnetic film and the magnetic grains. The coupling is found to be much stronger than that between continuous ferromagnetic layers.

cond-mat.str-el↗

arXiv2003

Proximity effect in ultrathin Pb/Ag multilayers within the Cooper limit

We report on transport and tunneling measurements performed on ultra-thin Pb/Ag (strong coupled superconductor/normal metal) multilayers evaporated by quench condensation. The critical temperature and energy gap of the heterostructures oscillate with addition of each layer, demonstrating the validity of the Cooper limit model in the case of multilayers. We observe excellent agreement with a simple theory for samples with layer thickness larger than 30Å. Samples with single layers thinner than 30Ådeviate from the Cooper limit theory. We suggest that this is due to the "inverse proximity effect" where the normal metal electrons improve screening in the superconducting ultrathin layer and thus enhance the critical temperature.

cond-mat.supr-con↗

arXiv2002

Magneto-transport properties of dilute granular ferromagnets

We present magnetoresistance (MR) measurements performed on quench condensed granular Ni thin films which are on the verge of electric continuity. In these systems the electric conductivity is believed to be governed by the resistance between a very small number of grains. The films exihibit sharp resistance jumps as a function of magnetic field. We interpret these findings as being the result of magneto-mechanical distortions that occour in single grains which act as bottlenecks in the dilute percolation network. The observed features provide a unique measure of magnetostriction effects in nano-grain structures as well as being able to shed light on some of the properties of regular granular magnetic films.

cond-mat.mes-hall↗