arXiv subjects

V. G. Kogan

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

97 records · Page 6Linked to original sources

arXiv2001

Elastic moduli of vortex lattices within nonlocal London model

Vortex lattice (VL) elastic response is analyzed within nonlocal London model. The squash modulus turns zero at the field $H_{\Box}$ where VL undergoes square-to-rhombus structural transition. In the field region $H>H_{\Box}$ where the square VL is stable, the rotation modulus turns zero at a field $H_r$ indicating instability of the square VL with respect to rotations. The shear modulus depends on the shear direction; the dependence is strong in the vicinity of $H_{\Box}$ where the square VL is soft with respect to the shear along [110]. The $H$ dependences of elastic moduli are evaluated for LuNi$_2$B$_2$C.

cond-mat.supr-con↗

arXiv2001

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

We have 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*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 MgB_2 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 MgB_2 is an anisotropic superconductor with H_c2^{ab} / H_{c2}^{c} \approx 6--9.

cond-mat.supr-con↗

arXiv2001

Field evolution of vortex lattice in LuNi_2B_2C seen by decoration in fields up to 1.5 kOe

Evolution of the vortex lattice, the rhombus-square transition included, for LuNi_2B_2C in the field along the c crystal axis, is tracked by the decoration technique pushed up to the record high (for this method) field of 1480 Oe. Decoration images are analyzed with the help of the Fourier transform, which indicates disordered structures in small fields of a few Oe. In fields H < 200 Oe the coexisting domains of different structures are observed. A technique based on the Fourier transform is employed to see the domains separately. The transition to the square lattice is recorded near 900 Oe. The results are in agreement with predictions of the nonlocal London theory.

cond-mat.supr-con↗

arXiv2001

Temperature Dependence of the Flux Line Lattice Transition into Square Symmetry in Superconducting LuNi$_2$B$_2$C

We have investigated the temperature dependence of the H || c flux line lattice structural phase transition from square to hexagonal symmetry, in the tetragonal superconductor LuNi_2B_2C (T_c = 16.6 K). At temperatures below 10 K the transition onset field, H_2(T), is only weakly temperature dependent. Above 10 K, H_2(T) rises sharply, bending away from the upper critical field. This contradicts theoretical predictions of H_2(T) merging with the upper critical field, and suggests that just below the H_c2(T)-curve the flux line lattice might be hexagonal.

cond-mat.supr-con↗

arXiv2001

Direct measurement of single fluxoid dynamics in superconducting rings

We have measured the dynamics of individual magnetic fluxoids entering and leaving thin film, photolithographically patterned rings of underdoped high-temperature superconductor BSCCO using a variable sample temperature scanning SQUID microscope. These measurements can be understood within a phenomenological model in which the fluxoid number changes by thermal activation of a Pearl vortex in the ring wall. We place upper limits on the "vison" binding energy in these samples from these measurements.

cond-mat.supr-con↗

arXiv2000

Nature of 45 degree vortex lattice reorientation in tetragonal superconductors

The transformation of the vortex lattice in a tetragonal superconductor which consists of its 45 degree reorientation relative to the crystal axes is studied using the nonlocal London model. It is shown that the reorientation occurs as two successive second order (continuous) phase transitions. The transition magnetic fields are calculated for a range of parameters relevant for borocarbide superconductors in which the reorientation has been observed.

cond-mat.supr-con↗

arXiv1998

Magnetic field of an in-plane vortex outside a layered superconductor

We present the solution to London's equations for the magnetic fields of a vortex oriented parallel to the plane, and normal to a crystal face, of a layered superconductor. These expressions account for flux spreading at the superconducting surface, which can change the apparent size of the vortex along the planes by as much as 30%. We compare these expressions with experimental results.

cond-mat.supr-con↗