Searcharxiv⌕ Search

arXiv subjects

V. G. Kogan

Publications and source records attributed to V. G. Kogan.

At least 73 records · Page 4Linked to original sources
arXiv2010

Scaling relations in anisotropic superconductors with strong pair-breaking

Following the work of Abrikosov and Gor'kov on the pair-breaking effects, one can derive the temperature dependencies of the electronic specific heat $C_s/T=γ^\prime+μT^2$ (with the jump at the superconducting transition $ΔC \propto T_c^3$) for materials with zero Fermi surface average of the order parameter $<Δ>=0$ (e.g. d-wave) or for those with $<Δ> \ll Δ_{max}$ (e.g., $\pm s$ of iron-pnictides) in the presence of strong pair-breaking. Moreover, the London penetration depth satisfies $λ^{-2}=λ_0^{-2}(1-T^2/T_c^2)$ (or $λ-λ_0=βT^2 $ at low temperatures) and the slope of the upper critical field near $T_c$ is $H_{c2}^\prime \propto T_c$. Remarkably simple relations between these at first sight unrelated quantities take place: $μλ_0^2 T_c^3/|H_{c2}^\prime|=3ϕ_0/8π^2$ and $ΔC β^2 T_c^4/|H_{c2,c}^\prime| = ϕ_0/16π^2 $ are universal constants. The prediction is checked on two samples of Ba(Fe$_{1-x}$Co$_{x}$)$_2$As$_2$ and on CeCoIn$_5$ for which the data needed are available.

cond-mat.supr-con↗
arXiv2009

London penetration depth and strong pair-breaking in iron-based superconductors

The low temperature variation of the London penetration depth for a number of iron-pnictide and iron-chalcogenide superconductors is nearly quadratic, $Δλ(T) = βT^n$ with $n\approx 2$. The coefficient in this dependence shows a robust scaling, $β\propto 1/T_c^3$ across different families of these materials. We associate the scaling with a strong pair-breaking. The same mechanism have recently been suggested to explain the scalings of the specific heat jump, $ΔC \propto T_c^3$, and of the slopes of the upper critical field, $dH_{c2}/dT\propto T_c$ in these materials. This suggests that thermodynamic and electromagnetic properties of the iron-based superconductors can be described within a strong pair-breaking scenario.

cond-mat.supr-con↗
arXiv2009

Three-dimensional nodal superconducting gap in single crystals Ba(Fe$_{1-x}$Ni$_x$)$_2$As$_2$

The London penetration depth, $λ$, is directly related to the density, $n_{s}$, of the Cooper pairs ($λ^{2}\propto 1/n_{s}$) and its variation with temperature provides valuable insight into the pairing mechanism. Here we study the evolution with doping of the temperature dependence of the in-plane ($λ_{ab}$) and out-of-plane ($λ_{c}$) penetration depths in single crystals of electron-doped Ba(Fe$_{1-x}$Ni$_x$)$_2$As$_2$. As is the case for other pnictides, $λ(T) \sim T^n$ over the whole doping range and this behavior extends down to at least $T=T_c/100$, setting a very small upper limit on the gap minimum. Furthermore, in the overdoped regime: 1) the exponent $n$ becomes substantially smaller than 2, which is incompatible with the models that explain power-law behavior to be due to scattering; 2) the exponent $n$ becomes anisotropic, with $λ_{c}(T)$ showing a clear $T$-linear behavior over a large temperature interval. These findings suggest that in the overdoped regime the superconducting gap in iron-based pnictide superconductors develops nodal structure, which unlike in the cuprates, cannot be understood within a two-dimensional picture.

cond-mat.supr-con↗
arXiv2009

Pair-breaking in iron-pnictides

The puzzling features of the slopes of the upper critical field at the critical temperature $T_c$, $H^\prime_{c2}(T_c)\propto T_c$, and of the specific heat jump $ΔC\propto T_c^3$ of iron-pnictides are interpreted as caused by a strong pair-breaking.

cond-mat.supr-con↗
arXiv2009

Flux quanta driven by high-density currents in low-impurity V3Si and LuNi2B2C: free flux flow and flux-core size effect

High density direct currents (DC) are used to drive flux quanta via the Lorentz force towards a highly ordered "free flux flow" (FFF) dynamic state, made possible by the weak-pinning environment of high-quality, single-crystal samples of two low-Tc superconducting compounds, V3Si and LuNi2B2C. We report the effect of the magnetic field-dependent fluxon core size on flux flow resistivity rho_f. Much progress has been made in minimizing the technical challenges associated with the use of high currents. Attainment of a FFF phase is indicated by the saturation at highest currents of flux-flow dissipation levels that are well below the normal state resistance and have field-dependent values. The field dependence of the corresponding rho_f is shown to be consistent with a prediction based on a model for the decrease of flux core size at higher fields in weak-coupling BCS s-wave materials.

cond-mat.supr-con↗
arXiv2009

Superfluid density and specific heat within self-consistent scheme for two-band superconductor

The two gaps in a two-band clean s-wave superconductor are evaluated self-consistently within the quasiclassical Eilenberger weak-coupling formalism with two in-band and one inter-band pairing potentials. Superfluid density, free energy and specific heat are given in the form amenable for fitting the experimental data. Well-known two-band MgB$_2$ and V$_3$Si superconductors are used to test the developed approach. The pairing potentials obtained from the fit of the superfluid density data in MgB$_2$ crystal were used to calculate temperature-dependent specific heat, $C(T)$. The calculated $C(T)$ compares very well with the experimental data. Advantages and validity of this, what we call the "$γ$-model", are discussed and compared with the commonly used empirical (and \textit{not self-consistent}) "$α$-model". Correlation between the sign of the inter-band coupling and the signs of the two order parameters is discussed. Suppression of the critical temperature by the inter-band scattering is evaluated and shown to be severe for the inter-band repulsion as compared to the attraction. The data on a strong $T_c$ suppression in MgB$_2$ crystals by impurities suggest that the order parameters on two effective bands of this material may have opposite signs, i.e., may have the $s_{\pm}$ structure similar to the current proposals in iron-based pnictide superconductors.

cond-mat.supr-con↗
arXiv2009

Non-exponential London penetration depth in RFeAsO$_{0.9}$F$_{0.1}$ (R=La,Nd) single crystals

The superconducting penetration depth, $λ(T)$, has been measured in RFeAsO$_{0.9}$F$_{0.1}$ (R=La,Nd) single crystals (R-1111). In Nd-1111, we find an upturn in $λ(T)$ upon cooling and attribute it to the paramagnetism of the Nd ions, similar to the case of the electron-doped cuprate Nd-Ce-Cu-O. After the correction for paramagnetism, the London penetration depth variation is found to follow a power-law behavior, $Δλ_L(T)\propto T^{2}$ at low temperatures. The same $T^2$ variation of $λ(T)$ was found in non-magnetic La-1111 crystals. Analysis of the superfluid density and of penetration depth anisotropy over the full temperature range is consistent with two-gap superconductivity. Based on this and on our previous work, we conclude that both the RFeAsO (1111) and BaFe$_2$As$_2$ (122) families of pnictide superconductors exhibit unconventional two-gap superconductivity.

cond-mat.supr-con↗
arXiv2009

Non-exponential London penetration depth in Ba$_{1-}$K$_{x}$Fe$_{2}$As$_{2}$ single crystals

We have studied the in- and out-of-plane magnetic penetration depths in the hole- doped iron based superconductor Ba$_{1-x}$K$_{x}$Fe$_{2}$As$_{2}$ ($T_{c}\approx$ 30K). The study was performed on single crystals grown from different fluxes and we find that the results are nearly the same. The in-plane London penetration depth $λ_{ab}$ does not show exponential saturation at low temperature, as would be expected from a fully gapped superconductor. Instead, $λ_{ab}(T)$ shows a power-law behavior, $λ\propto T^{n}$ ($n\approx 2$), down to $T\approx 0.02 T_{c}$, similar to the electron doped Ba(Fe$_{1-x}$Co$_{x}$)$_{2}$As$_{2}$. The penetration depth anisotropy $γ_λ=λ_{c}(T)/λ_{ab}(T)$ increases upon cooling, opposite to the trend observed in the anisotropy of the upper critical field, $γ_ξ=H_{c2}^{\perp c}(0)/H_{c2}^{\parallel c}(0)$. These are universal characteristics of both the electron and hole doped 122 systems, suggesting unconventional superconductivity. The behavior of the in-plane superfluid density $ρ_{ab}(T)$ is discussed in light of existing theoretical models proposed for the iron pnictides superconductors.

cond-mat.supr-con↗
arXiv2009

Anisotropic London Penetration Depth and Superfluid Density in Single Crystals of Iron-based Pnictide Superconductors

In- and out-of-plane magnetic penetration depths were measured in three iron-based pnictide superconducting systems. All studied samples of both 122 systems show a robust power-law behavior, $λ(T) T^n$, with the sample-dependent exponent n=2-2.5, which is indicative of unconventional pairing. This scenario could be possible either through scattering in a $s_{\pm }$ state or due to nodes in the superconducting gap. In the Nd-1111 system, the interpretation of data may be obscured by the magnetism of rare-earth ions. The overall anisotropy of the pnictide superconductors is small. The 1111 system is about two times more anisotropic than the 122 system. Our data and analysis suggest that the iron-based pnictides are complex superconductors in which a multiband three-dimensional electronic structure and strong magnetic fluctuations play important roles.

cond-mat.supr-con↗
arXiv2008

Anisotropy of the Optimally-Doped Iron Pnictide Superconductor Ba(Fe0.926Co0.074)2As2

Anisotropies of electrical resistivity, upper critical field, London penetration depth and critical currents have been measured in single crystals of the optimally doped iron pnictide superconductor Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$, $x$=0.074 and $T_c \sim$23 K. The normal state resistivity anisotropy was obtained by employing both the Montgomery technique and direct measurements on samples cut along principal crystallographic directions. The ratio $γ_ρ = ρ_c /ρ_a$ is about 4$\pm$1 just above $T_c$ and becomes half of that at room temperature. The anisotropy of the upper critical field, $γ_{H} = H_{c2,ab} /H_{c2,c} $, as determined from specific heat measurements close to $T_c$, is in the range of 2.1 to 2.6, depending on the criterion used. A comparable low anisotropy of the London penetration depth, $γ_λ=λ_{c}/λ_{ab}$, was recorded from TDR measurements and found to persist deep into the superconducting state. An anisotropy of comparable magnitude was also found in the critical currents, $γ_j=j_{c,ab}/j_{c,c}$, as determined from both direct transport measurements ($\sim$1.5) and from the analysis of the magnetization data ($\sim$3). Overall, our results show that iron pnictide superconductors manifest anisotropies consistent with essentially three-dimensional intermetallic compound and bear little resemblance to cuprates.

cond-mat.supr-con↗
arXiv2008

Vortex phase diagram of Ba(Fe0.93Co0.07)2As2 single crystals

Detailed measurements of the global and local electromagnetic properties of Ba(Fe$_{0.93}$Co$_{0.07}$)$_{2}$As$_{2}$ single crystals are reported. Analysis of the irreversible magnetic response provides strong evidence for similar vortex physics in this Fe-based pnictide superconductor to the high-$T_{c}$ cuprates, such as Y-Ba-Cu-O or Nd-Ce-Cu-O. In particular, we have found a nonmonotonic "fishtail" magnetization in $M(H,T=const)$ loops and its signature is also present in $M(H=const,T)$ scans. The supercurrent density is evaluated by using several techniques, including direct transport measurements. At 5 K we estimate its value to be a moderate $j \approx 2.6 \pm 0.2 \times 10^5$ A/cm$^2$. Analysis of the magnetic relaxation is consistent with the collective pinning and creep models (weak pinning and fast creep) and suggests a crossover from the collective to the plastic creep regime in fields exceeding the value corresponding to the maximum in fishtail magnetization.

cond-mat.supr-con↗
arXiv2008

Magnetic field of Josephson vortices outside superconductors

We consider the structure of Josephson vortices approaching the junction boundary with vacuum in large area Josephson junctions with the Josephson length $λ_J$ large relative to the London penetration depth $λ_L$. Using the stability argument for one-dimentional solitons with respect to 2D perturbations, it is shown that on the scale $λ_J$ the Josephson vortices do not spread near the boundary in the direction of the junction. %, which is in a striking difference with behavior of Abrikosov vortices exiting superconductors. The field distribution in vacuum due to the Josephson vortex is evaluated, the information needed for the Scanning SQUID Microscopy.

cond-mat.supr-con↗
arXiv2008

Josephson junctions in narrow thin-film strips

We study the field dependence of the maximum supercurrent in narrow edge-type thin-film Josephson junctions. It is assumed that the junction extends across thin-film strip of width W that is much less than the Pearl length; the film thickness is much less than the London penetration depth. We calculate the maximum supercurrent within nonlocal Josephson electrodynamics, which takes into account the stray fields affecting tunneling currents. In the case when W is much less than the thin-film Josephson length, the phase difference along the junction depends only on the junction geometry and the applied field, but is independent of the Josephson critical current density, i.e., it is universal. Zeros of the maximum supercurrent are equidistant only in large fields (unlike the case of junctions with bulk banks); they are spaced by a field that is much smaller than the one of bulk junctions. Peaks of the maximum supercurrent decrease inversely proportional to the square root of the applied field, i.e., slower than 1/H for the bulk.

cond-mat.supr-con↗
arXiv2007

Superfluid density in gapless superconductor CeCoIn5

Temperature dependence of the London penetration depth $λ$ measured in single crystals of CeCoIn$_5$ is interpreted as caused by a strong pair-breaking scattering that makes the superconductivity in this compound gapless. For a gapless d-wave superconductor, we derive $λ=λ_0/\sqrt{1-(T/T_c)^2}$ caused by combined effect of magnetic and non-magnetic scattering in excellent agreement with the data in the full temperature range and with the gapless s-wave case of Abrikosov and Gor'kov. We also obtain the slope of the upper critical field at $T_c$ that compares well with the measured slope.

cond-mat.supr-con↗
arXiv2007

Radio-frequency magnetic response of vortex lattices undergoing structural transformations

We report a clear anomaly in the rf response of a tunnel-diode resonator measured in borocarbide crystals in dc fields along the $c-$axis. We associate the anomaly in the dynamic magnetic susceptibility, $χ$, with a structural transition in the vortex lattice. $χ$ is sensitive to the Campbell penetration depth $λ_{C}$ of the rf perturbation into the mixed state. $λ_{C}$ depends on the vortex lattice elastic moduli, which in turn depend on the vortex lattice structure. The high-field transition line is clearly mapped and shown to behave in agreement with theory.

cond-mat.supr-con↗
arXiv2006

Interaction of vortices in thin superconducting films and Berezinskii-Kosterlitz-Thouless transition

The precondition for the BKT transition in thin superconducting films, the logarithmic intervortex interaction, is satisfied at distances short relative to $Λ=2λ^2/d$, $λ$ is the London penetration depth of the bulk material and $d$ is the film thickness. For this reason, the search for the transition has been conducted in samples of the size $L<Λ$. It is argued below that film edges turn the interaction into near exponential (short-range) thus making the BKT transition impossible. If however the substrate is superconducting and separated from the film by an insulated layer, the logarithmic intervortex interaction is recovered and the BKT transition should be observable.

cond-mat.supr-con↗