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V. G. Kogan

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

At least 91 records · Page 5Linked to original sources
arXiv2006

Magnetic field induced orientation of superconducting MgB$_2$ crystallites determined by X-ray diffraction

X-ray diffraction studies of fine polycrystalline samples of MgB$_2$ in the superconducting state reveal that crystals orient with their \emph{c}-axis in a plane normal to the direction of the applied magnetic field. The MgB$_2$ samples were thoroughly ground to obtain average grain size 5 - 10 $μ$m in order to increase the population of free single crystal grains in the powder. By monitoring Bragg reflections in a plane normal to an applied magnetic field we find that the powder is textured with significantly stronger (\emph{0,0,l}) reflections in comparison to (\emph{h,k,0}), which remain essentially unchanged. The orientation of the crystals with the \emph{ab}-plane parallel to the magnetic field at all temperatures below $T_c$ demonstrates that the sign of the torque under magnetic field does not alter, in disagreement with current theoretical predictions.

cond-mat.supr-con↗
arXiv2006

Effect of field dependent core size on reversible magnetization of high-$κ$ superconductors

The field dependence of the vortex core size $ξ(B)$ is incorporated in the London model, in order to describe reversible magnetization $M(B,T)$ for a number of materials with large Ginzburg-Landau parameter $κ$. The dependence $ξ(B)$ is directly related to deviations in $M(\ln B)$ from linear behavior prescribed by the standard London model. A simple method to extract $ξ(B)$ from the magnetization data is proposed. For most materials examined, $ξ(B)$ so obtained decreases with increasing field and is in qualitative agreement both with behavior extracted from $μ$SR and small angle neutron scattering data and with that predicted theoretically.

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arXiv2006

Measuring the Penetration Depth Anisotropy in MgB$_2$ using Small-Angle Neutron Scattering

Using small-angle neutron scattering we have measured the misalignment between an applied field of 4 kOe and the flux-line lattice in MgB$_2$, as the field is rotated away from the c axis by an angle $θ$. The measurements, performed at 4.9 K, showed the vortices canting towards the c axis for all field orientations. Using a two-band/two-gap model to calculate the magnetization we are able to fit our results yielding a penetration depth anisotropy, $\glam = 1.1 \pm 0.1$.

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arXiv2005

Dissipation in ultra-thin current-carrying superconducting bridges; evidence for quantum tunneling of Pearl vortices

We have made current-voltage (IV) measurements of artificially layered high-$T_c$ thin-film bridges. Scanning SQUID microscopy of these films provides values for the Pearl lengths $Λ$ that exceed the bridge width, and shows that the current distributions are uniform across the bridges. At high temperatures and high currents the voltages follow the power law $V \propto I^n$, with $n=Φ_0^2/8π^2Λk_B T+1$, and at high temperatures and low-currents the resistance is exponential in temperature, in good agreement with the predictions for thermally activated vortex motion. At low temperatures, the IV's are better fit by $\ln V$ linear in $I^{-2}$. This is expected if the low temperature dissipation is dominated by quantum tunneling of Pearl vortices.

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arXiv2005

Anomalous Paramagnetic Effects in the Mixed State of LuNi2B2C

Anomalous paramagnetic effects in dc magnetization were observed in the mixed state of LuNi2B2C, unlike any reported previously. It appears as a kink-like feature for H > 30 kOe and becomes more prominent with increasing field. A specific heat jump at the corresponding temperature suggests that the anomaly is due to a true bulk transition. A magnetic flux transition from a square to an hexagonal lattice is consistent with the anomaly.

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arXiv2004

On the field dependence of the vortex core size

We argue that in clean high-$κ$ type II superconductors, the low temperature vortex core size (defined as the coherence length $ξ$) in high fields should decrease with increasing applied field in qualitative agreement with experimental data. Calculations are done for the Fermi sphere and cylinder (with the field parallel to the cylinder axis). The results for clean materials at T=0 can be represented as $ξ(H)/ξ(H_{c2}) = U (H/H_{c2})$ with $U$ being an universal function.

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arXiv2003

Penetration depth anisotropy in two-band superconductors

The anisotropy of the London penetration depth is evaluated for two-band superconductors with arbitrary inter- and intra-band scattering times. If one of the bands is clean and the other is dirty in the absence of inter-band scattering, the anisotropy is dominated by the Fermi surface of the clean band and is weakly temperature dependent. The inter-band scattering also suppress the temperature dependence of the anisotropy.

cond-mat.supr-con↗
arXiv2003

Properties of mesoscopic superconducting thin-film rings. London approach

Superconducting thin-film rings smaller than the film penetration depth (the Pearl length) are considered. The current distribution, magnetic moment, and thermodynamic potential ${\cal F}(H,N,v)$ for a flat, washer-shaped annular ring in a uniform applied field $H$ perpendicular to the film are solved analytically within the London approach for a state with winding number $N$ and a vortex at radius $v$ between the inner and outer radii.

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arXiv2003

Meissner response of anisotropic superconductors

The response field of a half-space anisotropic superconductor is evaluated for an arbitrary weak external field source. Example sources of a point magnetic moment and a circular current are considered in detail. For the penetration depth $λ\ll L$ with $L$ being any other relevant distance (the source size, or the distance between the source and the superconductor), the major contribution to the response is the $λ$ independent field of the source image. It is shown that the absolute value of $λ$ cannot be extracted from the response field with a better accuracy than that for the source position. Similar problems are considered for thin films.

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arXiv2003

Fluxoid dynamics in superconducting thin film rings

We have measured the dynamics of individual magnetic fluxoids entering and leaving photolithographically patterned thin film rings of the underdoped high-temperature superconductor Bi$_2$Sr$_2$CaCu$_2$O$_{8+δ}$, using a variable sample temperature scanning SQUID microscope. These results can be qualitatively described using a model in which the fluxoid number changes by thermally activated nucleation of a Pearl vortex in, and transport of the Pearl vortex across, the ring wall.

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arXiv2002

Anisotropy parameters of superconducting MgB$_2$

Data on macroscopic superconducting anisotropy of MgB$_2$ are reviewed. The data are described within a weak coupling two-gaps anisotropic s-wave model of superconductivity. The calculated ratio of the upper critical fields $γ_H=H_{c2,ab}/H_{c2,c}$ increases with decreasing temperature in agreement with available data, whereas the calculated ratio of London penetration depths $γ_λ=λ_c/λ_{ab}$ decreases to reach $\approx 1.1$ at T=0. Possible macroscopic consequences of $γ_λ\neγ_H$ are discussed.

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arXiv2002

Current in narrow channels of anisotropic superconductors

We argue that in channels cut out of anisotropic single crystal superconductors and narrow on the scale of London penetration depth, the persistent current must cause the transverse phase difference provided the current does not point in any of the principal crystal directions. The difference is proportional to the current value and depends on the anisotropy parameter, on the current direction relative to the crystal, and on the transverse channel dimension. An experimental set up to measure the transverse phase is proposed.

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arXiv2002

Free energy and torque for superconductors with different anisotropies of H_{c2} and lambda

The free energy is evaluated for a uniaxial superconductor with the anisotropy of the upper critical field, gamma_H = H_{c2,ab}/H_{c2,c}, different from the anisotropy of the penetration depth gamma_{lambda} = lambda_c/lambda_{ab}. With increasing difference between gamma_H and gamma_{lambda}, the equilibrium orientation of the crystal relative to the applied field may shift from theta = pi/2 (theta is the angle between the field and the c axis) to lower angles and reach theta = 0 for large enough gamma_H. These effects are expected to take place in MgB_2.

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arXiv2002

Effects of La substitution on superconducting state of CeCoIn5

We report effects of La substitution on superconducting state of heavy fermion superconductor CeCoIn5, as seen in transport and magnetization measurements. As opposed to the case of conventional superconductors, pair breaking by nonmagnetic La results in depression of Tc and indicates strong gap anisotropy. Upper critical field Hc2 values decrease with increased La concentration, but the critical field anisotropy, gamma=Hc2(a)/Hc2(c), does not change in the Ce_{1-x}La_xCoIn5 (x=0-0.15). The electronic system is in the clean limit for all values of x.

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arXiv2002

Macroscopic anisotropy in superconductors with anisotropic gaps

It is shown within the weak-coupling model that the macroscopic superconducting anisotropy for materials with the gap varying on the Fermi surface cannot be characterized by a single number, unlike the case of clean materials with isotropic gaps. For clean uniaxial materials, the anisotropy parameter $γ(T)$ defined as the ratio of London penetration depths, $λ_c/λ_{ab}$, is evaluated for all $T$'s. Within the two-gap model of MgB$_2$, $γ(T)$ is an increasing function of $T$.

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arXiv2001

Determination of superconducting anisotropy from magnetization data on random powders as applied to LuNi$_2$B$_2$C, YNi$_2$B$_2$C and MgB$_2$

The recently discovered intermetallic superconductor MgB2 appears to have a highly anisotopic upper critical field with Hc2(max)/Hc2(min} = γ> 5. In order to determine the temperature dependence of both Hc2(max) and Hc2(min) we propose a method of extracting the superconducting anisotropy from the magnetization M(H,T) of randomly oriented powder samples. The method is based on two features in dM/dT the onset of diamagnetism at Tc(max), that is commonly associated with Hc2, and a kink in dM/dT at a lower temperature Tc(min). Results for LuNi2B2C and YNi2B2C powders are in agreement with anisotropic Hc2 obtained from magneto-transport measurements on single crystals. Using this method on four different types of MgB2 powder samples we are able to determine Hc2(max)(T) and Hc2(min)(T) with γ\approx 6.

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arXiv2001

Effect of fluctuations on vortex lattice structural transitions in superconductors

The rhombic-to-square transition field $H_{\Box}(T)$ for cubic and tetragonal materials in fields along [001] is evaluated using the nonlocal London theory with account of thermal vortex fluctuations. Unlike extended Ginzburg-Landau models, our approach shows that the line $H_{\Box}(T)$ and the upper critical field $H_{c2}(T)$ do not cross due to strong fluctuations near $H_{c2}(T)$ which suppress the square anisotropy induced by the nonlocality. In increasing fields, this causes re-entrance of the rhombic structure in agreement with recent neutron scattering data on borocarbides.

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