Annotation of the results obtained in 2021
Thermal conductance in nanowires with lithographic NSN contacts was measured using shot noise technique. The key result of the experiment is the analysis of nonlocal voltage and nonlocal noise data in terms of the average charge of the transmitted quasiparticle, the upper bound for which can be obtained from the nonlocal Fano factor (Fnl): <q_T> < 1/Fnl. Presented in this original form, the experimental data demonstrate gigantic values ​​of the nonlocal Fano factor, which corresponds to a value of <q_T> on the order of 1% of the elementary charge. In other words, the experiment unambiguously demonstrates the predominance of thermal over charge transport in the nonlocal response, which assigns the primary role to the thermal rather than the charge component and requires the development of new approaches, including in studies of the topological phase transition in Majorana structures. Published in Semiconductor Science and Technology https://iopscience.iop.org/article/10.1088/1361-6641/ac187b/meta An analytical calculation has been performed on the relationship between the spectral thermal conductance in NSN structures based on nanowires of two- and three-terminal geometry. In this part, the main results were strictly formalized, generalizing the well-known work of Nagaev and Buttiker [Phys Rev B 2001] on noise in diffusion NS contacts to the case of a finite density of states in a superconductor. About 20 years ago, when the first experiments on doubling of shot noise in NS contacts appeared, their interpretation often consisted of doubling the charge due to the correlated motion of an electron and a hole participating in the Andreev reflection. Nagaev and Buttiker just showed that this condition is not necessary, and noise doubling should be understood in the context of blocking by a superconductor exactly half of the heat flux, which leads to a strongly nonequilibrium distribution function of subgap quasiparticles at its boundary. Our work explicitly shows what consequences the final thermal conductivity of a superconductor has on the example of NSN structures with the proximity effect in nanowires of two- and three-terminal geometry. It is shown that in both cases there is a fundamental possibility of investigating the thermal conductance in a situation where the average charge response is identically zero or trivial. In addition, the role of classical effects of nonequilibrium superconductivity in a two-terminal geometry with floating superconducting island is discussed for the first time. The work was published in Physical Review B https://journals.aps.org/prb/abstract/10.1103/PhysRevB.104.125409 An analytical calculation of the influence of electron-electron collisions on the resistance of a two-dimensional electron gas in a semiconductor heterostructure with two populated transverse quantization subbands, that is, a system with a Galilean invariant spectrum, is carried out. The Fermi surface of such systems is doubly connected, but the effective masses are equal in both subbands, so any individual electron-electron collision cannot change the current. It is shown that these collisions affect the resistivity if the rates of electron-impurity scattering on two Fermi contours are different or if there is a sufficiently strong intersubband scattering on impurities. The reason is that two-particle collisions redistribute nonequilibrium electrons between the two circuits. As a result, the resistivity increases with temperature to a new final value. It is likely that the effect can be observed in state-of-the-art two-dimensional systems in GaAs, which are being studied in the context of magneto-intersubband resistance oscillations, in the region where the second subband of dimensional quantization starts filling. The preprint of the article is published on the website https://arxiv.org/abs/2109.06496

 

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Annotation of the results obtained in 2019
Here, understandably, we list only those results, which are currently published as a preprint of editorially approved for publication. 1. We investigated the samples of doped InAs nanowires, grown in the lab of Scuola Normale Superiore (Pisa, Italy) with normal metal based ohmics contacts. Here we set up a series of measurements of a thermal bias in a single nanowire, induced via a contact heating approach. This problem is addressed in detail, using the techniques of local and average noise thermometry, local noise spectroscopy (to demonstrate a quasi-equilibrium energy distribution, generated via the contact heating), as well as with the help of numeric and analytic calculations. We showed that a geometry of metallic strip with a constriction of the length below the electron-phonon relaxation length, is most perspective for the creation of known thermal gradient at the nanoscale. Analytically, we have shown that in the linear response regime (in terms of the Joule heat released in the strip) the thermal bias in question is determined by the only unknown parameter – the electron-phonon relaxation length. This length can be determined from a measurement of a thermoelectric (or analogous) response of the device as a function of the constriction length. As such, the suggested strategy of accurate thermal biasing is capable of the absolute calibration accuracy in the range of 10%, that can be useful in numerous applications in nanoobjects. The paper is under review and upload on the arXiv preprint server under the URL https://arxiv.org/abs/1812.06463 2. In relation to the problem of contact heating using a metallic strip at low temperatures, together with colleagues from MSPU, we have studied an unusual regime of thermal relaxation in a disordered metallic film, biased with a direct current. Such a regime could be observed provided two following conditions are fulfilled – (i) the film thickness by far exceeds the lengths of electron-phonon and phonon-electron scattering and (ii) electron heat conductance in the film is much higher than phonon heat conductance, that is possible in presence of very strong disorder, limiting the phonon mean-free path via Rayleigh scattering. In this case, a transverse temperature gradient is built in the film in the direction of the substrate, with a parabolic spatial profile. The average noise temperature of the film (of the length l and thickness d) is given by a universal expression for the Fano factor, which 2l/d>>1 times smaller than the standard shot noise in the regime pf hot electrons with suppressed electron-phonon relaxation [Nagaev1995, Kozub 1995]. With a suppport of present project we have set up an experiment on current noise measurement in a disordered NbN film, which is qualitatively consistent with a model prediction. The paper is accepted for publication in “JETP Letters” and published on the arXiv preprint server under the URL https://arxiv.org/abs/1912.03094

 

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Annotation of the results obtained in 2020
Two experimental papers are published, devoted to local thermometry and spectrocopy techniques and their application for quantitative description of a thermal bias and non-equilibrium electronic energy distributions in mesoscopic conductors. A sample with a series of ballistic constrictions in a two-dimensional electrpon gas in GaAs is processed, with constrictions oriented along one line at distance from 0.9 um to 3.1 um from each other. Non-local signals are measured at low temperature in the regime of non-linear response. Shot noise is measured. The analysis shows qualitative agreement with perturbative analytic and mnumerical calculations. Experiments on a comparison of noise and resistive thermometry applied for the detection of non-local heat signals in NSN nanowires with three terminals. It is showed that the resistive thermometry is qualitatively adequate, however is several times wrong in absolute value, that may be a result of electron dephasing and accompanying suppression of mesoscopic conductance fluctuations. Two series of devices from epitaxial Al/InAs nanowires with thick an thin shell are processed. Noise and transport measurement are performed, that can be qualified as successful preliminary. In all cases a superconducting proximity effect is demonstrated, and in the case of thin shell up to longitudinal magnetic fields of 1 T. Ohmic contacting to the superconducting shell failed, that motivates further studies of two-terminal nanowire devices with a superconducting island. Several theoretical calculations performed. Analytical calculation of e-e scattering effects in non-local response of ballistic constrictions is published in PRB. A sign change of the non-local response predicted in that paper is confirmed by a numerical study. Analytical calculation of the shot noise in two- and three- terminal NSN nanowires is performed, that allows to extract a spectral thermal conductance of the proximity region at sub-hap energies.

 

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