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V G Kogan

Publications and source records attributed to V G Kogan.

8 recordsLinked to original sources
PubMed2003

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 idea on how to measure the transverse phase is proposed.

Journal Article↗
PubMed2002

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,a)/H(c2,c), different from the anisotropy of the penetration depth gamma(lambda)=lambda(c)/lambda(a). 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 MgB2.

Journal Article↗
PubMed2001

Effect of fluctuations on vortex lattice structural transitions in superconductors.

The rhombic-to-square transition field H(square)(T) for cubic and tetragonal materials in fields along [001] is evaluated using the nonlocal London theory with the account of thermal vortex fluctuations. Unlike extended Ginzburg-Landau models, our approach shows that the line H(square)(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, fluctuations cause a reentrance of the rhombic vortex lattice, in agreement with recent neutron scattering data on borocarbides.

Journal Article↗
PubMed2001

Elastic moduli of vortex lattices within nonlocal London model.

Vortex lattice (VL) elastic response is analyzed within the nonlocal London model which holds for high-kappa clean superconductors. The squash modulus vanishes at the field H( square) where VL undergoes a square-to-rhombus transition. For H>H( square), where the square VL is stable, the rotation modulus turns zero at H = H(r), indicating VL instability to rotations. The shear modulus depends on the shear direction; the dependence is strong in the vicinity of H( square) where the square VL is soft with respect to the shear along [110]. The H dependences of the moduli are evaluated for LuNi(2)B(2)C.

Journal Article↗
PubMed2001

Anisotropy of superconducting MgB2 as seen in electron spin resonance and magnetization data.

We observed the conduction electron spin resonance (CESR) in fine powders of MgB2 both in the superconducting and normal states. The Pauli susceptibility is chi(s) = 2.0 x 10(-5) emu/mole in the temperature range of 450 to 600 K. The spin relaxation rate has an anomalous temperature dependence. The CESR measured below T(c) at several frequencies suggests that MgB2 is a strongly anisotropic superconductor with the upper critical field, H(c2), ranging between 2 and 16 T. The high-field reversible magnetization data of a randomly oriented powder sample are well described assuming that MgB2 is an anisotropic superconductor with H(ab)(c2)/H(c)(c2) approximately 6-9.

Journal Article↗
PubMed2001

Temperature dependence of the flux line lattice transition into square symmetry in superconducting LuNi2B2C.

We have investigated the temperature dependence of the H parallel to c flux line lattice structural phase transition from square to hexagonal symmetry, in the tetragonal superconductor LuNi2B2C ( T(c) = 16.6 K). At temperatures below 10 K the transition onset field, H2(T), is only weakly temperature dependent. Above 10 K, H2(T) rises sharply, bending away from the upper critical field. This contradicts theoretical predictions of H2(T) merging with the upper critical field and suggests that just below the H(c2)(T) curve the flux line lattice might be hexagonal.

Journal Article↗
PubMed1979

An immunofluorescence diagnostic test for avian infectious laryngotracheitis.

An antiserum to a recent field isolate of infectious laryngotracheitis virus was conjugated with fluorescein isothiocyanate and used to detect viral antigen to infected chorioallantoic membranes and trachael epithelium by the direct fluorescent antibody technique. In experimentally infected birds, viral antigen was detected with the fluorescent antibody technique from 2 to 14 days post-inoculation but histological evidence of tracheitis was only observed from day three to day ten. The fluorescent antibody test detected 22 of 23 histologically confirmed cases of ILT and was more accurate than virus isolation when used in the diagnosis of respiratory disease from field outbreaks. It was concluded that the speed, accuracy and sensitivity of the fluorescent antibody technique make it a useful tool in the diagnosis of infectious laryngotracheitis.

Animals↗