arXiv subjects

S. U. Piatrusha

Publications and source records attributed to S. U. Piatrusha.

9 recordsLinked to original sources

arXiv2022

Heat-mode excitation in a proximity superconductor

Mesoscopic superconductivity deals with various quasiparticle excitation modes, only one of them -- the charge-mode -- being directly accessible for conductance measurements due to the imbalance in populations of quasi-electron and quasihole excitation branches. Other modes carrying heat or even spin, valley etc. currents populate the branches equally and are charge-neutral, which makes them much harder to control. This noticeable gap in the experimental studies of mesoscopic non-equilibrium superconductivity can be filled by going beyond the conventional DC transport measurements and exploiting spontaneous current fluctuations. Here, we perform such an experiment and investigate the transport of heat in an open hybrid device based on a superconductor proximitized InAs nanowire. Using shot noise measurements, we investigate sub-gap Andreev heat guiding along the superconducting interface and fully characterize it in terms of the thermal conductance on the order of $G_\mathrm{th}\sim e^2/h$, tunable by a back gate voltage. Understanding of the heat-mode also uncovers its implicit signatures in the non-local charge transport. Our experiments open a direct pathway to probe generic charge-neutral excitations in superconducting hybrids.

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arXiv2021

Charge-neutral nonlocal response in superconductor-InAs nanowire hybrid devices

Nonlocal quasiparticle transport in normal-superconductor-normal (NSN) hybrid structures probes sub-gap states in the proximity region and is especially attractive in the context of Majorana research. Conductance measurement provides only partial information about nonlocal response composed from both electron-like and hole-like quasiparticle excitations. In this work, we show how a nonlocal shot noise measurement delivers a missing puzzle piece in NSN InAs nanowire-based devices. We demonstrate that in a trivial superconducting phase quasiparticle response is practically charge-neutral, dominated by the heat transport component with a thermal conductance being on the order of conductance quantum. This is qualitatively explained by numerous Andreev reflections of a diffusing quasiparticle, that makes its charge completely uncertain. Consistently, strong fluctuations and sign reversal are observed in the sub-gap nonlocal conductance, including occasional Andreev rectification signals. Our results prove conductance and noise as complementary measurements to characterize quasiparticle transport in superconducting proximity devices.

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arXiv2020

Spatial and energy resolution of electronic states by shot noise

Shot noise measurements are widely used for the characterization of nonequilibrium configurations in electronic conductors. The recently introduced quantum tomography approach was implemented for the studies of electronic wavefunctions of few-electron excitations created by periodic voltage pulses in phase-coherent ballistic conductors based on the high-quality GaAs two-dimensional electron gas. Still relying on the manifestation of Fermi correlations in noise, we focus on the simpler and more general approach beneficial for the local measurements of energy distribution (ED) in electronic systems with arbitrary excitations with well-defined energies and random phases. Using biased diffusive metallic wire as a testbed, we demonstrate the power of this approach and extract the well-known double-step ED from the shot noise of a weakly coupled tunnel junction. Our experiment paves the way for the local measurements of generic nonequilibrium configurations applicable to virtually any conductor.

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arXiv2019

Universal bottleneck for thermal relaxation in disordered metallic films

We study the heat relaxation in current biased metallic films in the regime of strong electron-phonon coupling. A thermal gradient in the direction normal to the film is predicted, with a spatial temperature profile determined by the temperature-dependent heat conduction. In the case of strong phonon scattering the heat conduction occurs predominantly via the electronic system and the profile is parabolic. This regime leads to the linear dependence of the noise temperature as a function of voltage bias, in spite of the fact that all the dimensions of the film are large compared to the electron-phonon relaxation length. This is in stark contrast to the conventional scenario of relaxation limited by the electron-phonon scattering rate. A preliminary experimental study of a 200 nm thick NbN film indicates the relevance of our model for materials used in superconducting nanowire single-photon detectors.

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arXiv2019

Topological protection brought to light by the time-reversal symmetry breaking

Recent topological band theory distinguishes electronic band insulators with respect to various symmetries and topological invariants, most commonly, the time reversal symmetry and the $\rm Z_2$ invariant. The interface of two topologically distinct insulators hosts a unique class of electronic states -- the helical states, which shortcut the gapped bulk and exhibit spin-momentum locking. The magic and so far elusive property of the helical electrons, known as topological protection, prevents them from coherent backscattering as long as the underlying symmetry is preserved. Here we present an experiment which brings to light the strength of topological protection in one-dimensional helical edge states of a $\rm Z_2$ quantum spin-Hall insulator in HgTe. At low temperatures, we observe the dramatic impact of a tiny magnetic field, which results in an exponential increase of the resistance accompanied by giant mesoscopic fluctuations and a gap opening. This textbook Anderson localization scenario emerges only upon the time-reversal symmetry breaking, bringing the first direct evidence of the topological protection strength in helical edge states.

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arXiv2018

Noise insights into electronic transport

Typical experimental measurement is set up as a study of the system's response to a stationary external excitation. This approach considers any random fluctuation of the signal as spurious contribution which is to be eliminated via time-averaging or, equivalently, bandwidth reduction. Beyond that lies a conceptually different paradigm -- the measurement of the system's spontaneous fluctuations. The goal of this overview article is to demonstrate how current noise measurements bring insight into hidden features of electronic transport in various mesoscopic conductors, ranging from 2D topological insulators to individual carbon nanotubes.

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arXiv2017

Edge States in Lateral p-n Junctions in Inverted Band HgTe Quantum Wells

We investigate lateral p-n junctions, electrostatically defined in 14 nm-wide HgTe-based quantum wells (QWs) with inverted band structure. The p-n junctions resistances are close to $h/2e^2$, consistent with some previous experiments on 8-10 nm QWs, and the current-voltage characteristics are highly linear, indicating the transport via ballistic helical edge states. Shot noise measurements are performed in order to further verify the underlying transport mechanism. We discuss the role of unknown inelastic relaxation rates in the leads and in the edge channels for the correct interpretation of the noise data. Although the interpretation in favor of the helical edge states seems more consistent, a definite conclusion can not be drawn based on the present experiment. Our approach looks promising for the study of short quasi-ballistic edges in topological insulators (TIs) in suitable geometry.

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arXiv2017

Proximity effect and interface transparency in Al/InAs-nanowire/Al diffusive junctions

We investigate the proximity effect in InAs nanowire (NW) junctions with superconducting contacts made of Al. The carrier density in InAs is tuned by means of the back gate voltage $V_g$. At high positive $V_g$ the devices feature transport signatures characteristic of diffusive junctions with highly transparent interfaces - sizable excess current, re-entrant resistance effect and proximity gap values ($Δ_N$) close to the Al gap ($Δ_0$). At decreasing $V_g$, we observe a reduction of the proximity gap down to $Δ_N\approxΔ_0/2$ at NW conductances $\sim2e^2/h$, which is interpreted in terms of carrier density dependent reduction of the Al/InAs interface transparency. We demonstrate that the experimental behavior of $Δ_N$ is closely reproduced by a model with shallow potential barrier at the Al/InAs interface.

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arXiv2017

Measuring electron energy distribution by current fluctuations

A recent concept of local noise sensor is extended to measure the energy resolved electronic energy distribution $f(\varepsilon)$ at a given location inside a non-equilibrium normal metal interconnect. A quantitative analysis of $f(\varepsilon)$ is complicated because of a nonlinear differential resistance of the noise sensor, represented by a diffusive InAs nanowire. Nevertheless, by comparing the non-equilibrium results with reference equilibrium measurements, we conclude that $f(\varepsilon)$ is indistinguishable from the Fermi distribution.

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