arXiv subjects

A. Bezryadin

Publications and source records attributed to A. Bezryadin.

At least 19 recordsLinked to original sources

arXiv2020

Large energy storage efficiency of the dielectric layer of graphene nanocapacitors

Electric capacitors are commonly used in electronic circuits for short-term storage of small amounts of energy. It is desirable however to use capacitors to store much larger energy amounts to replace rechargeable batteries. Unfortunately, the existing capacitors cannot store a sufficient energy to be able to replace common electrochemical energy storage systems. Here we examine energy storage capabilities of graphene nanocapacitors, which are try-layers involving an Al film, Al2O3 dielectric layer, and a single layer of carbon atoms, i.e., graphene. This is a purely electronic capacitor and therefore it can function in a wide temperature interval. The capacitor shows a high dielectric breakdown electric field strength, of the order of 1000 kV/mm (i.e., 1GV/m), which is much larger than the table value of the Al2O3 dielectric strength. The corresponding energy density is 10 to 100 times larger than the energy density of a common electrolytic capacitor. Moreover, we discover that the amount of charge stored in the dielectric layer can be equal or can even exceed the amount of charge stored on the capacitor plates. The dielectric discharge current follows a power-law time dependence. We suggest a model to explain this behavior.

cond-mat.mes-hall↗

arXiv2020

Reversed Photoeffect in Transparent Graphene Nanocapacitors

Electronic properties of ultrathin dielectric films consistently attract much attention since they play important roles in various electronic devices, such as field effect transistors and memory elements. Insulating properties of the gate oxide in transistors represent the key factor limiting Moore's law. The dielectric strength of the insulating film limits how much energy can be stored in nanocapacitors. The origin of the electric current in the nanometer-scale insulating barrier remains unexplained. Here we present an optically transparent Al-Al2O3-graphene nanocapacitor suitable for studying electronic transport in calibrated nanoscale dielectric films under high electric fields and with light exposure. The controllable flow of photons provides an additional powerful probe helping to resolve the puzzle of the electric conductivity in these high-quality insulating films. The dielectric alumina, Al2O3, is deposited by atomic layer deposition technology. With this device we observe a photon-assisted field emission effect, in which the effective barrier height is reduced by a quantity equal to the photon energy. Our main finding is a reversed photoeffect. Namely, at sufficiently high bias voltages the current through the dielectric film decreases as the light intensity increases. Moreover, higher photon energies correlate with stronger decreases of the current. To explain this reversed photoeffect, we present a qualitative model based on a conjecture that electrons leak into the dielectric and form charged sandpile-like branching patterns, which facilitate transport, and which can be dispersed by light.

cond-mat.mes-hall↗

arXiv2016

Quasi-1-Dimensional Superconductivity in Highly Disordered NbN Nanowires

The topic of superconductivity in strongly disordered materials has attracted a significant attention. In particular vivid debates are related to the subject of intrinsic spatial inhomogeneity responsible for non-BCS relation between the superconducting gap and the pairing potential. Here we report experimental study of electron transport properties of narrow NbN nanowires with effective cross sections of the order of the debated inhomogeneity scales. We find that conventional models based on phase slip concept provide reasonable fits for the shape of the R(T) transition curve. Temperature dependence of the critical current follows the text-book Ginzburg-Landau prediction for quasi-one-dimensional superconducting channel Ic~(1-T/Tc)^3/2. Hence, one may conclude that the intrinsic electronic inhomogeneity either does not exist in our structures, or, if exist, does not affect their resistive state properties.

cond-mat.supr-con↗

arXiv2014

Dark counts initiated by macroscopic quantum tunneling in NbN superconducting photon detectors

We perform measurements of the switching current distributions of three w = 120 nm wide, 4 nm thick NbN superconducting strips which are used for single-photon detectors. These strips are much wider than the diameter the vortex cores, so they are classified as quasi-two-dimensional (quasi-2D). We discover evidence of macroscopic quantum tunneling by observing the saturation of the standard deviation of the switching distributions at temperatures around 2 K. We analyze our results using the Kurkijarvi-Garg model and find that the escape temperature also saturates at low temperatures, confirming that at sufficiently low temperatures, macroscopic quantum tunneling is possible in quasi-2D strips and can contribute to dark counts observed in single photon detectors.

cond-mat.supr-con↗

arXiv2013

Universal features of counting statistics of thermal and quantum phase slips in superconducting nanocircuits

We perform measurements of phase-slip-induced switching current events on different types of superconducting weak links and systematically study statistical properties of the switching current distributions. We employ two types of devices in which a weak link is formed either by a superconducting nanowire or by a graphene flake subject to proximity effect. We demonstrate that, independently on the nature of the weak link, higher moments of the distribution take universal values. In particular, the third moment (skewness) of the distribution is close to -1 both in thermal and quantum regimes. The fourth moment (kurtosis) also takes a universal value close to 5. The discovered universality of skewness and kurtosis is confirmed by an analytical model. Our numerical analysis shows that introduction of extraneous noise into the system leads to significant deviations from the universal values. We suggest to use the discovered universality of higher moments as a robust tool for checking against undesirable effects on noise in various types of measurements.

cond-mat.supr-con↗

arXiv2012

Quantitative analysis of quantum phase slips in superconducting MoGe nanowires revealed by switching-current statistics

We measure quantum and thermal phase-slip rates using the standard deviation of the switching current in superconducting nanowires at high bias current. Our rigorous quantitative analysis provides firm evidence for the presence of quantum phase slips (QPS) in homogeneous nanowires. We observe that as temperature is lowered, thermal fluctuations freeze at a characteristic crossover temperature Tq, below which the dispersion of the switching current saturates to a constant value, indicating the presence of QPS. The scaling of the crossover temperature Tq with the critical temperature Tc is linear, which is consistent with the theory of macroscopic quantum tunneling. We can convert the wires from the initial amorphous phase to a single crystal phase, in situ, by applying calibrated voltage pulses. This technique allows us to probe directly the effects of the wire resistance, critical temperature and morphology on thermal and quantum phase slips.

cond-mat.supr-con↗

arXiv2011

Distribution of supercurrent switching in graphene under proximity effect

We study the stochastic nature of switching current in hysteretic current-voltage characteristics of superconductor-graphene-superconductor (SGS) junctions. We find that the dispersion of the switching current distribution scales with temperature as a power law with the power close to 1/3. This observation is in sharp contrast with the known Josephson junction behavior where switching current dispersion is expected to scale with the power 2/3. We propose an explanation using a generalized version of Kurkijarvi's theory for the flux stability in rf-SQUID and attribute this anomalous effect to the temperature dependence of the critical current which persists down to low temperatures.

cond-mat.supr-con↗

arXiv2011

Microwave Enhancement of Phase Slip Rate in Quasi One-Dimensional Superconducting Nanowires

We study current-voltage (V-I) characteristics of short superconducting nanowires of length ~ 100 nm exposed to microwave radiation of frequencies between 100 MHz and 15 GHz. The radiation causes a decrease of the average switching current of the wire. This suppression of the switching current is modeled assuming that there is one-to-one correspondence between Little's phase slips and the experimentally observed switching events. At some critical power P* of the radiation a dissipative dynamic superconducting state occurs as an extra step on the V-I curve. It is identified as a phase slip center (PSC). With the dependence of the switching currents and the standard deviations observed at the transitions (i) from a constant supercurrent state to a normal state and (ii) from a constant superconducting state to a PSC state, we conclude that both of the two types of the switching events are triggered by the same microscopic event, namely a single Little's phase slip. We show that the Skocpol-Beasely-Tinkham model is not applicable to our microwave-driven phase slip centers, since it leads to an unphysical small estimated value of the size of the dissipative core of the PSC. Through the analysis of the witching current distributions at a sufficiently low temperature, we also present evidence that the quantum phase slip play a role in switching events under microwaves.

cond-mat.supr-con↗

arXiv2011

Observation of Little-Parks oscillations at temperatures much lower than the critical temperature using a GHz resonator

It is demonstrated that a thin-film Fabry-Perot superconducting resonator can be used to reveal the Little-Parks (LP) effect even at temperatures much lower than the critical temperature. A pair of parallel nanowires is incorporated into the resonator at the point of a supercurrent antinode. As magnetic field is ramped, the Meissner current develops, changing the kinetic inductance of the wires and, correspondingly, the resonance frequency of the resonator, which can be detected. The LP oscillation are revealed as a periodic set of distorted parabolas observed in the transmission of the resonator and corresponding to the states of the wire loop having different vorticities. We also report a direct observation of single and double phase slip events and their statistical analysis.

cond-mat.supr-con↗

arXiv2008

Multiple-Retrapping Process in High-$T_c$ Intrinsic Josephson Junctions

We report measurements of switching-current distribution (SWCD) from a phase-diffusion branch to a quasiparticle-tunneling branch as a function of temperature in a cuprate-based intrinsic Josephson junction. Contrary to the thermal-activation model, the width of the SWCD increases with decreasing temperature, down to 1.5 K. Based on the multiple-retrapping model, we quantitatively demonstrate that the quality factor of the junction in the phase-diffusion regime determines the observed temperature dependence of the SWCD.

cond-mat.supr-con↗

arXiv2008

Determination of the Superconductor-Insulator Phase Diagram for One-Dimensional Wires

We establish the superconductor-insulator phase diagram for quasi-one dimensional wires by measuring a large set of MoGe nanowires. This diagram is consistent with the Chakravarty-Schmid-Bulgadaev phase boundary, namely with the critical resistance being equal to R_Q = h/4e^2. We find that transport properties of insulating nanowires exhibit a weak Coulomb blockade behavior.

cond-mat.supr-con↗

arXiv2008

Zero-crossing Shapiro steps in focused-ion-beam-tailored high-$T_c$ superconducting microstructures

Microwave response of S-shaped Bi$_2$Sr$_2$CaCu2O$_{8+x}$ (Bi-2212) micron-scale samples, in which the supercurrent was forced to flow perpendicular to the crystal layers, was investigated. A treatment with a focused ion beam allowed us to reduce the plasma frequency down to $f_p$$\sim$5 GHz at $T$=0.3 K in naturally stacked Josephson junctions in a crystal. We observed Shapiro steps at frequencies as low as $\sim$5 GHz. Well-developed zero-crossing Shapiro steps were observed at frequencies as low as $\sim$10 GHz. They appeared as constant-voltage plateaus with a non-zero voltage occurring at zero bias current. We confirmed that zero-crossing Shapiro steps in the Bi-2212 stacked junctions can be observed when the irradiated frequency is sufficiently larger than $f_p$. The observed high-order fractional steps in the microwave responses indicate that the interlayer-coupled Bi-2212 Josephson junctions have nonsinusoidal current-phase relation. Based on the temperature dependence of the steps we also showed that the finite slope of the steps is due to the enhancement of the phase diffusion effect.

cond-mat.supr-con↗

arXiv2006

Dichotomy in short superconducting nanowires: thermal phase slippage vs. Coulomb blockade

Quasi-one-dimensional superconductors or nanowires exhibit a transition into a nonsuperconducting regime, as their diameter shrinks. We present measurements on ultrashort nanowires (~40-190 nm long) in the vicinity of this quantum transition. Properties of all wires in the superconducting phase, even those close to the transition, can be explained in terms of thermally activated phase slips. The behavior of nanowires in the nonsuperconducting phase agrees with the theories of the Coulomb blockade of coherent transport through mesoscopic normal metal conductors. Thus it is concluded that the quantum transition occurs between two phases: a "true superconducting phase" and an "insulating phase". No intermediate, "metallic" phase was found.

cond-mat.supr-con↗

arXiv2006

Magnetic field enhancement of superconductivity in ultra-narrow wires

We study the effect of an applied magnetic field on sub-10nm wide MoGe and Nb superconducting wires. We find that magnetic fields can enhance the critical supercurrent at low temperatures, and does so more strongly for narrower wires. We conjecture that magnetic moments are present, but their pair-breaking effect, active at lower magnetic fields, is suppressed by higher fields. The corresponding microscopic theory, which we have developed, quantitatively explains all experimental observations, and suggests that magnetic moments have formed on the wire surfaces.

cond-mat.supr-con↗

arXiv2005

Electronic mean free path in as-produced and purified single-wall carbon nanotubes

The effect of purification on room temperature electronic transport properties of single-wall carbon nanotubes (SWNT) was studied by submerging samples into liquid mercury. The conductance plots of purified SWNTs showed plateaus, indicating weak dependence of the electrical resistance on the length of the tube connecting the electrodes, providing evidence of quasi-ballistic conduction in SWNTs. The electronic mean free path of the purified SWNTs reached a few microns, which is longer than that of the as-produced SWNTs, and which is consistent with the calculation based on the scattering by acoustic phonons.

cond-mat.mtrl-sci↗

arXiv2004

Influence of high magnetic fields on superconducting transition of one-dimensional Nb and MoGe nanowires

The effects of strong magnetic field on superconducting Nb and MoGe nanowires with diameter $\sim10$ nm have been studied. We have found that the Langer-Ambegaokar-McCumber-Halperin (LAMH) theory of thermally activated phase slips is applicable in a wide range of magnetic fields and describes well the temperature dependence of the wire resistance, over eleven orders of magnitude. The field dependence of the critical temperature, $T_{c}$, extracted from the LAMH fits is in good quantitative agreement with the theory of pair-breaking perturbations that takes into account both spin and orbital contributions. The extracted spin-orbit scattering time agrees with an estimate $τ_{so}\simeq τ(\hbar c/ Ze^{2})^{4}$, where $τ$ is the elastic scattering time and $Z$ is the atomic number.

cond-mat.supr-con↗

arXiv2004

Phase slips in superconducting films with constrictions

A system of two coplanar superconducting films seamlessly connected by a bridge is studied. We observe two distinct resistive transitions as the temperature is reduced. The first one, occurring in the films, shows some properties of the Berezinskii-Kosterlitz-Thouless (BKT) transition. The second apparent transition (which is in fact a crossover) is related to freezing out of thermally activated phase slips (TAPS) localized on the bridge. We also propose a powerful indirect experimental method allowing an extraction of the sample's zero-bias resistance from high-current-bias measurements. Using direct and indirect measurements, we determined the resistance $R(T)$ of the bridges within a range of {\em eleven orders of magnitude}. Over such broad range, the resistance follows a simple relation $R(T)=R_N \text{exp} [-(c/t)(1-t)^{3/2}]$, where $c=ΔF(0) / kT_c$ is the normalized free energy of a phase slip at zero temperature, $t=T/T_c$ is normalized temperature, and $R_N$ is the normal resistance of the bridge.

cond-mat.supr-con↗

arXiv2004

The effect of morphology on the superconductor-insulator transition in 1-D nanowires

We study the effect of morphology on the low temperature behavior of superconducting nanowires of length $\approx$100 nm. A well-defined superconductor-insulator transition (SIT) is observed only in homogenous wires, in which case the transition occurs when the normal resistance is close to $h/4e^2$. Inhomogeneous wires, on the other hand, exhibit a mixed behavior, such that signatures of the superconducting and insulating regimes can be observed in the same sample. The resistance versus temperature curves of inhomogeneous wires show multiple steps, each corresponding to a weak link constriction (WLC) present in the wire. Similarly, each WLC generates a differential resistance peak when the bias current reaches the critical current of the WLC. Due to the presence of WLCs an inhomogeneous wire splits into a sequence of weakly interacting segments where each segment can act as a superconductor or as an insulator. Thus the entire wire then shows a mixed behavior.

cond-mat.supr-con↗