arXiv subjects

A. F. Shevchun

Publications and source records attributed to A. F. Shevchun.

8 recordsLinked to original sources

arXiv2026

Local thermometry of NbSe$_2$ flake with delta-$T$ noise measurements

We perform transport and noise measurements for device consisting of a thin NbSe$_2$ flake laid onto the predefined gold electrodes and covered with a thin hBN flake. In the shot noise of a NbSe$_2$/Au tunnel junction (TJ), we identify Andreev reflection regime by demonstrating the effective charge doubling. Further, by creating temperature gradient across the TJ and measuring its delta-$T$ noise in the normal state, we extract electron-phonon scattering length in NbSe$_2$ and its $T$-dependence. The results of delta-$T$ noise measurements in the absence of a magnetic field when the flake is superconducting are in qualitative agreement with expectations. The introduced approach is promising for the study of nonequilibrium configurations in superconductors.

cond-mat.mes-hall↗

arXiv2018

Local impedance on a rough surface of a chiral $p$-wave superconductor

We develop a self-consistent approach for calculating the local impedance at a rough surface of a chiral $p$-wave superconductor. Using the quasiclassical Eilenberger-Larkin-Ovchinnikov formalism, we numerically find the pair potential, pairing functions, and the surface density of states taking into account diffusive electronic scattering at the surface. The obtained solutions are then employed for studying the local complex conductivity and surface impedance in the broad range of microwave frequencies (ranging from subgap to above-gap values). We identify anomalous features of the surface impedance caused by generation of odd-frequency superconductivity at the surface. The results are compared with experimental data for Sr$_2$RuO$_4$ and provide a microscopic explanation of the phenomenological two-fluid model suggested earlier to explain anomalous features of the microwave response in this material.

cond-mat.supr-con↗

arXiv2011

Normal state resistivity of Ba$_{1-x}$K$_x$Fe$_2$As$_2$: evidence for multiband strong-coupling behavior

We present theoretical analysis of the normal state resistivity in multiband superconductors in the framework of Eliashberg theory. The results are compared with measurements of the temperature dependence of normal state resistivity of high-purity Ba$_{0.68}$K$_{0.32}$Fe$_{2}$As$_{2}$ single crystals with the highest reported transition temperature $T_c$ = 38.5 K. The experimental data demonstrate strong deviations from the Bloch-Grüneisen behavior, namely the tendency to saturation of the resistivity at high temperatures. The observed behavior of the resistivity is explained within the two band scenario when the first band is strongly coupled and relatively clean, while the second band is weakly coupled and is characterized by much stronger impurity scattering.

cond-mat.supr-con↗

arXiv2009

Electromagnetic response of LaO_0.94F_0.06FeAs: AC susceptibility and microwave surface resistance

We discuss on the electromagnetic response of a polycrystalline sample of LaO_0.94F_0.06FeAs exposed to DC magnetic fields up to 10 kOe. The low- and high-frequency responses have been investigated by measuring the AC susceptibility at 100 kHz and the microwave surface resistance at 9.6 GHz. At low as well as high DC magnetic fields, the susceptibility strongly depends on the amplitude of the AC driving field, highlighting enhanced nonlinear effects. The field dependence of the AC susceptibility exhibits a magnetic hysteresis that can be justified considering the intragrain-field-penetration effects on the intergrain critical current density. The microwave surface resistance exhibits a clockwise magnetic hysteresis, which cannot be justified in the framework of the critical-state models of the Abrikosov-fluxon lattice; it may have the same origin as that detected in the susceptibility.

cond-mat.supr-con↗

arXiv2006

Observation of a Transition from BCS to HTSC-like Superconductivity in Ba_{1-x}K_xBiO_3 Single Crystals

A study of temperature dependences of the upper critical field B_{c2}(T) and surface impedance Z(T)=R(T)+iX(T) in Ba_{1-x}K_xBiO_3 single crystals that have transition temperatures in the range 6 < T_c < 32 K (roughly 0.6>x>0.4) reveals a transition from BCS to unusual type of superconductivity. B_{c2}(T) curves corresponding to the crystals that have T_c>20 K have positive curvature (like in some HTSC), and those of the crystals with T_c<15 K fall on the usual Werthamer-Helfand-Hohenberg curve. R(T) and X(T) dependences of the crystals with T_c~30 K and T_c~11 K are respectively linear (like in HTSC) and exponential (BCS) in the temperature range T << T_c. The experimental results are discussed in connection with the extended saddle point model by Abrikosov.

cond-mat.supr-con↗

arXiv2003

Pseudogap in the microwave response of YBa_2Cu_3O_{7-x}

The in-plane and out-of-plane surface impedance and microwave conductivity components of one and the same YBa_2Cu_3O_{7-x} (0.07\le x\le 0.47) single crystal are determined in the wide ranges of temperature T and carrier concentration p in CuO_2 planes. The following features of the superfluid density n_s(T,p)\proptoλ_{ab}^{-2}(T,p) are observed at T<Tc/2 and 0.078\le p\le 0.16: (i) n_s(0,p) depends linearly on p, (ii) the derivative |dn_s(T,p)/dT|_{T\to 0} depends on p slightly in the optimally and moderately doped regions (0.10<p\le 0.16); however, it rapidly increases with p further lowering and (iii) the latter finding is accompanied by the linear low-temperature dependence Δn_s(T)\propto(-T) changing to Δn_s(T)\propto(-\sqrt{T}). For optimum oxygen content the temperature dependence of the normalized imaginary part of the c-axis conductivity λ_c^2(0)/λ_c^2(T) is found to be strikingly similar to that of λ_{ab}^2(0)/λ_{ab}^2(T) and becomes more convex with p lowering. λ_c^{-2}(0,p) values are roughly proportional to the normal state conductivities σ_c(T_c,p) along the c-axis. All these properties can be treated in the framework of d-density wave order of pseudogap.

cond-mat.supr-con↗

arXiv2003

Superfluid density in the underdoped YBa_2Cu_3O_{7-x}: Evidence for d-density wave order of pseudogap

The investigation of the penetration depth λ_{ab}(T,p) in YBa_2Cu_3O_{7-x} crystals allowed to observe the following features of the superfluid density n_s(T,p)\propto λ_{ab}^{-2}(T,p) as a function of temperature T<Tc/2 and carrier concentration 0.078\le p\le 0.16 in CuO_2 planes: (i) n_s(0,p) depends linearly on p, (ii) the derivative |dn_s(T,p)/dT|_{T\to 0} depends on p slightly in the optimally and moderately doped regions (0.10<p\le 0.16); however, it rapidly increases with p further lowering and (iii) the latter finding is accompanied by the linear low-temperature dependence [-Δn_s(T)]\propto T changing to [-Δn_s(T)]\propto \sqrt{T}. All these peculiarities can be treated in the framework of d-density wave scenario of electronic processes in underdoped high-Tc materials.

cond-mat.supr-con↗

arXiv2002

Characteristic features of the temperature dependence of the surface impedance in polycrystalline MgB$_2$ samples

The real $R_s(T)$ and imaginary $X_s(T)$ parts of the surface impedance $Z_s(T)=R_s(T)+iX_s(T)$ in polycrystalline MgB$_2$ samples of different density with the critical temperature $T_c\approx 38$ K are measured at the frequency of 9.4 GHz and in the temperature range $5\le T<200$ K. The normal skin-effect condition $R_s(T)=X_s(T)$ at $T\ge T_c$ holds only for the samples of the highest density with roughness sizes not more than 0.1 $μ$m. For such samples extrapolation $T\to 0$ of the linear at $T<T_c/2$ temperature dependences $λ_L(T)=X_s(T)/ωμ_0$ and $R_s(T)$ results in values of the London penetration depth $λ_L(0)\approx 600$ Åand residual surface resistance $R_{res}\approx 0.8$ m$Ω$. In the entire temperature range the dependences $R_s(T)$ and $X_s(T)$ are well described by the modified two-fluid model.

cond-mat.supr-con↗