arXiv subjects

V. D. Kochev

Publications and source records attributed to V. D. Kochev.

4 recordsLinked to original sources

arXiv2024

A method for measuring deviation from Lambert diffuse scattering law of ultracold neutrons on material walls

Modeling the motion of ultracold neutrons (UCNs) is crucial for assessing their losses, accurately measuring their lifetime, and describing other experiments. In material traps, it is necessary to account not only for specular but also for diffuse elastic reflection of UCNs from the trap walls. Typically, the Lambert cosine law is used to describe the angular distribution of diffusely scattered neutrons. However, this law lacks a rigorous theoretical derivation and is often violated. In our work, we propose an experiment to measure the deviation of the angular distribution of UCNs during diffuse scattering from the Lambert law. This deviation can be determined by the difference in the number of neutrons exiting through the central and end windows of a long narrow UCN trap. Monte Carlo simulations corresponding to a possible experiment have been performed, demonstrating a significant effect for different trap geometries.

physics.ins-det↗

arXiv2024

Numerical Calculation of Electric Field Enhancement in Neutron Traps with Rough Walls Coated with Superfluid Helium

A film of liquid helium on the surface of material traps for ultracold neutrons protects the neutrons from being absorbed by the trap walls. By using surface roughness and an electrostatic field, it is possible to maintain a helium film of sufficient thickness throughout the height of the trap. The field distribution near the tip of such wall roughness of the trap was calculated, and the effect of this field on holding the helium film was estimated.

physics.ins-det↗

arXiv2023

Inhomogeneous superconductivity onset in FeSe studied by transport properties

Heterogeneous superconductivity onset is a common phenomenon in high-$T_c$ superconductors of both the cuprate and iron-based families. It is manifested by a fairly wide transition from the metallic to zero-resistance state. Usually, in these strongly anisotropic materials, superconductivity (SC) first appears as isolated domains. This leads to anisotropic excess conductivity above $T_c$, and the transport measurements provide valuable information about the SC domain structure deep within the sample. In bulk samples, this anisotropic SC onset gives an approximate average shape of SC grains, while in thin samples it also indicates the average size of SC grains. In this work, both interlayer and intralayer resistivity are measured as a function of temperature in FeSe samples of various thickness. To measure the interlayer resistivity, FeSe mesa structures oriented across the layers were fabricated using FIB. As the sample thickness decreases, a significant increase in superconducting transition temperature $T_c$ is observed : $T_c$ raises from 8 K in bulk material to 12 K in microbridges of thickness $\sim 40$ nm. We apply analytical and numerical calculations to analyze these and earlier data and find the aspect ratio and size of the SC domains in FeSe consistent with our resistivity and diamagnetic response measurements. We propose a simple and fairly accurate method for estimating the aspect ratio of SC domains from $T_c$ anisotropy in samples of various small thickness. The relationship between nematic and superconducting domains in FeSe is discussed. We also generalize the analytical formulas for conductivity in heterogeneous anisotropic superconductors to the case of elongated SC domains of two perpendicular orientations with equal volume fractions, corresponding to the nematic domain structure in various Fe-based superconductors.

cond-mat.supr-con↗

arXiv2022

Peculiar effect of sample size in layered superconductors

We discuss an analytical model to calculate the superconducting volume ratio. Apart from this, our model can also predict the shape of embedded superconducting domains. We applied our model to calculate the superconducting volume ratios and shape of domains in (TMTSF)$_2$PF$_6$, (TMTSF)$_2$ClO$_4$, YBa$_2$Cu$_4$O$_8$, $β$-(BEDT)TTF$_2$I$_3$ and FeSe. Usually in layered superconductors resistivity drops anisotropically. Our analysis also explains that, this behaviour is due to flat or needle shape of the superconducting samples.

cond-mat.supr-con↗