arXiv subjects

A. Glatz

Publications and source records attributed to A. Glatz.

At least 37 records · Page 2Linked to original sources

arXiv2012

Pseudogap and zero-bias anomaly due to fluctuation suppression of quasiparticle tunneling

We study the effect of superconducting fluctuations on the tunnel current-voltage characteristics of disordered superconducting films placed in a perpendicular magnetic field, $H$, in the whole $H$-$T$ phase diagram outside the superconducting region. This tunnel-current is experimentally accessible by STM measurements. In the domain of temperatures $T\geq T_{c0}$ and relatively weak fields $H\ll H_{c2}(0)$ we reproduce existing results for the zero-voltage tunneling conductance, but also discover an important nonlinear contribution, which appears due to dynamic fluctuation modes and results in the formation of a strong zero-bias anomaly (ZBA) on the scale $eV\sim k_{\mathrm{B}}(T-T_{c0})$. At large voltages ($eV\sim k_{\mathrm{B}}T_{c0}$) these modes, together with the contribution from static fluctuations, form a pseudogap maximum. At low temperatures, with magnetic field values near $H_{c2}(0)$, fluctuations acquire quantum character and the general picture of the voltage dependent tunneling conductance resembles that one close to $T_{c0}$, where the role of temperature and magnetic field are exchanged. In particular, a gap-like structure appears with maximum at $eV_{\max}\sim Δ_{\mathrm{BCS}}$ and a sharp ZBA on the scale $eV\sim Δ_{\mathrm{BCS}}(H/H_{c2}(0)-1)$. The complete expression for the tunneling current at arbitrary fields and temperatures can be evaluated only numerically, which is presented in detail.

cond-mat.supr-con↗

arXiv2012

Interplay of charge and heat transport in a nano-junction in the out-of-equilibrium cotunneling regime

We study the charge transport and the heat transfer through a nano-junction composed of a small metallic grain weakly coupled to two metallic leads. We focus on the cotunneling regime out-of-equilibrium, where the bias voltage and the temperature gradient between the leads strongly drive electron and phonon degrees of freedom in the grain that in turn have a strong feedback on the transport through the grain. We derive and solve coupled kinetic equations for electron and phonon degrees of freedom in the grain. We obtain the heat fluxes between cotunneling electrons, bosonic electron-hole excitations in the grain, and phonons, and self-consistently find the current-voltage characteristics. We demonstrate that the transport in the nano-junction is very sensitive to the spectrum of the bosonic modes in the grain.

cond-mat.mes-hall↗

arXiv2012

Transport characteristics of nanojunctions far-from-equilibrium

We study the tunneling transport through a nanojunction in the far-from-equilibrium regime at relatively low temperatures. We show that the current-voltage characteristics is significantly modified as compared to the usual quasi-equilibrium result by lifting the suppression due to the Coulomb blockade. These effects are important in realistic nanojunctions. We study the high-impedance case in detail to explain the underlying physics and construct a more realistic theoretical model for the case of a metallic junction taking into account dynamic Coulomb interaction. This dynamic screening further reduces the effect of the Coulomb blockade.

cond-mat.mes-hall↗

arXiv2011

Nucleation of Spontaneous Vortices in Trapped Fermi Gases Undergoing a BCS-BEC Crossover

We study the spontaneous formation of vortices during the superfluid condensation in a trapped fermionic gas subjected to a rapid thermal quench via evaporative cooling. Our work is based on the numerical solution of the time dependent crossover Ginzburg-Landau equation coupled to the heat diffusion equation. We quantify the evolution of condensate density and vortex length as a function of a crossover phase parameter from BCS to BEC. The more interesting phenomena occur somewhat nearer to the BEC regime and should be experimentally observable; during the propagation of the cold front, the increase in condensate density leads to the formation of supercurrents towards the center of the condensate as well as possible condensate volume oscillations.

cond-mat.quant-gas↗

arXiv2011

Localization-Delocalization Transition and Current Fractalization

We develop an analytical theory of the localization-delocalization transition for a disordered Bose system, focusing on a Cooper-pair insulator. We consider a chain of small superconducting granules coupled via Josephson links and show that the low-temperature tunnelling transport of Cooper pairs is mediated by a self-generated environment of dipole excitations comprised of the same particles as the tunnelling charge carriers in accord with the early notion by Fleishman, Licciardello, and Anderson. We derive an analytical expression for the current-voltage characteristic and find that at temperatures, T, below the the charging energy of a single junction, E_c, the dc transport is completely locked by Coulomb blockade effect at all voltages except for a discrete set of resonant ones. At T>E_c the combined action of disorder and temperature unlocks the charge transport, since the environment excitation spectrum becomes quasi-continuous according to a Landau-Hopf-like scenario of turbulence, and the conductivity acquires an Arrhenius-like thermal activation form. The transition from the localized to delocalized behaviour occurs at T=E_c which corresponds to the onset of turbulence in the spectral flow of environmental excitations with Reynolds number Re=(k_B T/E_c)=1. The proposed theory breaks ground for a quantitative description of dynamic and quantum phase transitions in a wealth of physical systems ranging from cold atoms in optical lattices, through disordered films and wires to granular and nanopatterned materials.

cond-mat.mes-hall↗

arXiv2011

Superconducting phase transitions in ultrathin TiN films

We investigate transition to the superconducting state in the ultrathin titanium nitride films on approach to superconductor-insulator transition. Building on the complete account of quantum contributions to conductivity, we demonstrate that the resistance of thin superconducting films exhibits a non-monotonic temperature behaviour due to the competition between weak localization, electron-electron interaction, and superconducting fluctuations. We show that superconducting fluctuations give rise to an appreciable decrease in the resistance even at temperatures well exceeding the superconducting transition temperature, T_c, with this decrease being dominated by the Maki-Thompson process.The transition to a global phase-coherent superconducting state occurs via the Berezinskii-Kosterlitz-Thouless (BKT) transition, which we observe both by power-law behaviour in current-voltage characteristics and by flux flow transport in the magnetic field. The ratio T_{BKT}/T_c follows the universal Beasley-Mooij-Orlando relation. Our results call for revisiting the past data on superconducting transition in thin disordered films.

cond-mat.supr-con↗

arXiv2011

Giant Quantum Freezing of Tunnel Junctions mediated by Environments

We investigate the quantum heat exchange between a nanojunction and a many-body or electromagnetic environment far from equilibrium. It is shown that the two-temperature energy emission-absorption mechanism gives rise to a giant heat flow between the junction and the environment. We obtain analytical results for the heat flow in an idealized high impedance environment and perform numerical calculations for the general case of interacting electrons and discuss the giant freezing and heating effects in the junction under typical experimental conditions.

cond-mat.mes-hall↗

arXiv2010

Fluctuoscopy of Disordered Two-Dimensional Superconductors

We revise the long studied problem of fluctuation conductivity (FC) in disordered two-dimensional superconductors placed in a perpendicular magnetic field by finally deriving the complete solution in the temperature-magnetic field phase diagram. The obtained expressions allow both to perform straightforward (numerical) calculation of the FC surface $δσ_{xx}^{(\mathrm{tot})}(T,H)$ and to get asymptotic expressions in all its qualitatively different domains. This surface becomes in particular non-trivial at low temperatures, where it is trough-shaped with $% δσ_{xx}^{(\mathrm{tot})}(T,H)<0$. In this region, close to the quantum phase transition, $δσ_{xx}^{(\mathrm{tot})}(T,H=\mathrm{const})$ is non-monotonic, in agreement with experimental findings. We reanalyzed and present comparisons to several experimental measurements. Based on our results we derive a qualitative picture of superconducting fluctuations close to $H_{\mathrm{c2}}(0) $ and T=0 where fluctuation Cooper pairs rotate with cyclotron frequency $ω_{c}\simΔ_{\mathrm{BCS}}^{-1}$ and Larmor radius $\sim ξ_{\mathrm{BCS}}$, forming some kind of quantum liquid with long coherence length $ξ_{\mathrm{QF}}\ggξ_{\mathrm{BCS}}$ and slow relaxation ($τ_{\mathrm{QF}}\gg\hbarΔ_{\mathrm{BCS}}^{-1}$).

cond-mat.supr-con↗

arXiv2010

Reply to Comment "Density of states and critical behavior of the three-dimensional Coulomb Glass"

We reply to the Comment by Mobius and Richter [arXiv:0908.3092, Phys. Rev. Lett. 105, 039701 (2010)] on "Density of States and Critical Behavior of the Coulomb Glass" [arXiv:0805.4640, Phys. Rev. Lett. 102, 067205 (2009)] and address the issues raised with our results on the density of states. In addition, we correct our statements about the random displacement version of the Coulomb glass model where the Wigner crystal is not as robust to disorder as stated. Still, our main result of a lack of a finite-temperature transition in the Coulomb glass remains unchallenged.

cond-mat.dis-nn↗

arXiv2010

Quantum Fluctuations and Dynamic Clustering of Fluctuating Cooper Pairs

We derive exact expressions for the fluctuation conductivity in two dimensional superconductors as a function of temperature and magnetic field in the whole fluctuation region above the upper critical field H_{c2}(T). Focusing on the vicinity of the quantum phase transition near zero temperature, we propose that as the magnetic field approaches the line near H_{c2}(0) from above, a peculiar dynamic state consisting of clusters of coherently rotating fluctuation Cooper-pairs forms and estimate the characteristic size and lifetime of such clusters. We find the zero temperature magnetic field dependence of the the transverse magnetoconductivity above H_{c2}(0) in layered superconductors.

cond-mat.supr-con↗

arXiv2010

Synchronized Andreev Transmission in Chains of SNS Junctions

We construct a nonequilibrium theory for the charge transfer through a diffusive array of alternating normal (N) and superconducting (S) islands comprising an SNSNS junction, with the size of the central S-island being smaller than the energy relaxation length. We demonstrate that in the nonequilibrium regime the central island acts as Andreev retransmitter with the Andreev conversions at both NS interfaces of the central island correlated via over-the-gap transmission and Andreev reflection. This results in a synchronized transmission at certain resonant voltages which can be experimentally observed as a sequence of spikes in the differential conductivity.

cond-mat.mes-hall↗

arXiv2010

Heating Effects in a Chain of Quantum Dots

We study heating effects in a chain of weakly coupled grains due to electron-hole pair creation. The main mechanism for the latter at low temperatures is due to inelastic electron cotunneling processes in the array. We develop a quantitative kinetic theory for these systems and calculate the array temperature profile as a function of grain parameters, bias voltage or current, and time and show that for nanoscale size grains the heating effects are pronounced and easily measurable in experiments. In the low- and high-voltage limits we solve the stationary heat-flux equation analytically. We demonstrate the over-heating hysteresis in the large-current or voltage regimes. In addition we consider the influence of a substrate on the system which acts as a heat sink. We show that nano dot chains can be used as highly sensitive thermometers over a broad range of temperatures.

cond-mat.mes-hall↗

arXiv2010

Synchronized Andreev Transmission in SNS Junction Arrays

cond-mat.mes-hall↗

arXiv2010

Model for Dynamic Self-Assembled Magnetic Surface Structures

We propose a first-principles model for self-assembled magnetic surface structures on the water-air interface reported in earlier experiments \cite{snezhko2,snezhko4}. The model is based on the Navier-Stokes equation for liquids in shallow water approximation coupled to Newton equations for interacting magnetic particles suspended on the water-air interface. The model reproduces most of the observed phenomenology, including spontaneous formation of magnetic snake-like structures, generation of large-scale vortex flows, complex ferromagnetic-antiferromagnetic ordering of the snake, and self-propulsion of bead-snake hybrids. The model provides valuable insights into self-organization phenomena in a broad range of non-equilibrium magnetic and electrostatic systems with competing interactions.

cond-mat.soft↗

arXiv2010

Statistics of subgap states in $s_\pm$ superconductors

There is strong support in favor of an unusual $s_{\pm}$ superconducting state in the new iron-based superconductors, in which the gap parameter has opposite signs in different bands. In this case scattering between different bands by impurities has a pair-breaking effect and introduces states inside the gap. We studied the statistics of disorder-induced subgap states in $s_{\pm}$ superconductors due to collective effects of impurities. Numerically solving the two-band Bogolyubov equations, we explored the behavior of the density of states and localization length. We located the mobility edge separating the localized and delocalized states for the 3D case and the crossover between the weak and strong localization regimes for the 2D case. We found that the widely used self-consistent T-matrix approximation is not very accurate in describing subgap states.

cond-mat.supr-con↗

arXiv2009

Thermoelectric performance of weakly coupled granular materials

We study thermoelectric properties of inhomogeneous nanogranular materials for weak tunneling conductance between the grains, g_t < 1. We calculate the thermopower and figure of merit taking into account the shift of the chemical potential and the asymmetry of the density of states in the vicinity of the Fermi surface. We show that the weak coupling between the grains leads to a high thermopower and low thermal conductivity resulting in relatively high values of the figure of merit on the order of one. We estimate the temperature at which the figure of merit has its maximum value for two- and three-dimensional samples. Our results are applicable for many emerging materials, including artificially self-assembled nanoparticle arrays.

cond-mat.mes-hall↗

arXiv2008

Giant Magnetoresistance in Nanogranular Magnets

We study the giant magnetoresistance of nanogranular magnets in the presence of an external magnetic field and finite temperature. We show that the magnetization of arrays of nanogranular magnets has hysteretic behaviour at low temperatures leading to a double peak in the magnetoresistance which coalesces at high temperatures into a single peak. We numerically calculate the magnetization of magnetic domains and the motion of domain walls in this system using a combined mean-field approach and a model for an elastic membrane moving in a random medium, respectively. From the obtained results, we calculate the electric resistivity as a function of magnetic field and temperature. Our findings show excellent agreement with various experimental data.

cond-mat.str-el↗

arXiv2007

Electron Transport in Nanogranular Ferromagnets

We study electronic transport properties of ferromagnetic nanoparticle arrays and nanodomain materials near the Curie temperature in the limit of weak coupling between the grains. We calculate the conductivity in the Ohmic and non-Ohmic regimes and estimate the magnetoresistance jump in the resistivity at the transition temperature. The results are applicable for many emerging materials, including artificially self-assembled nanoparticle arrays and a certain class of manganites, where localization effects within the clusters can be neglected.

cond-mat.str-el↗