arXiv subjects

A. Sergeev

Publications and source records attributed to A. Sergeev.

30 records · Page 2Linked to original sources

arXiv2009

Thermomagnetic Effects in Vortex Liquid: Transport Entropy Revisited

Traditionally the Nernst and Ettingshausen effects in the vortex liquid are described in terms of the "transport entropy" of vortices, S_d. According to current theories, the main contribution to S_d is originated from the electromagnetic free energy, F^{em}, which includes kinetic and magnetic energy of superconducting currents circulating around vortex cores. However, this concept contradicts the London postulate, according to which a supercurrent consists of macroscopic number of particles in a single quantum state and does not transfer any entropy. Here we resolve this contradiction and show that the transport entropy is just ordinary thermodynamic entropy transferred by cores. Only in this form the theory becomes simultaneously consistent with the London postulate and the Onsager principle. The revised theory explains measured temperature dependence S_d. The linear increase of S_d at low temperatures is determined by the entropy of electrons in the core, then S_d reaches a maximum at roughly T_c/2 and then vanishes due to increase of the background entropy.

cond-mat.supr-con↗

arXiv2007

The pulsation modes of the pre-white dwarf PG 1159-035

PG 1159-035, a pre-white dwarf with T_eff=140,000 K, is the prototype of both two classes: the PG1159 spectroscopic class and the DOV pulsating class. Previous studies of PG 1159-035 photometric data obtained with the Whole Earth Telescope (WET) showed a rich frequency spectrum allowing the identification of 122 pulsation modes. In this work, we used all available WET photometric data from 1983, 1985, 1989, 1993 and 2002 to identify the pulsation periods and identified 76 additional pulsation modes, increasing to 198 the number of known pulsation modes in PG 1159-035, the largest number of modes detected in any star besides the Sun. From the period spacing we estimated a mass M = 0.59 +/- 0.02 solar masses for PG 1159-035, with the uncertainty dominated by the models, not the observation. Deviations in the regular period spacing suggest that some of the pulsation modes are trapped, even though the star is a pre-white dwarf and the gravitational settling is ongoing. The position of the transition zone that causes the mode trapping was calculated at r_c = 0.83 +/- 0.05 stellar radius. From the multiplet splitting, we calculated the rotational period P_rot = 1.3920 +/- 0.0008 days and an upper limit for the magnetic field, B < 2000 G. The total power of the pulsation modes at the stellar surface changed less than 30% for l=1 modes and less than 50% for l=2 modes. We find no evidence of linear combinations between the 198 pulsation mode frequencies. PG 1159-035 models have not significative convection zones, supporting the hypothesis that nonlinearity arises in the convection zones in cooler pulsating white dwarf stars.

astro-ph↗

arXiv2007

Monitoring of FR Cnc Flaring Activity

Being excited by the detection of the first ever-observed optical flare in FR Cnc, we decided to continue photometrical monitoring of this object. The observations were carried out at Crimean Astrophysical Observatory (Crimea, Ukraine; CrAO - hereafter) and at the Terskol Observatory (Russia, Northern Caucasus). The obtained lightcurves are presented and discussed. No distinguishable flares were detected that could imply that flares on FR Cnc are very rare event.

astro-ph↗

arXiv2007

One-dimensional resistive states in quasi-two-dimensional superconductors

We investigate competition between one- and two-dimensional topological excitations - phase slips and vortices - in formation of resistive states in quasi-two-dimensional superconductors in a wide temperature range below the mean-field transition temperature $T_{C0}$. The widths $w$ = 100 nm of our ultrathin NbN samples is substantially larger than the Ginzburg-Landau coherence length $ξ$ = 4nm and the fluctuation resistivity above $T_{C0}$ has a two-dimensional character. However, our data shows that the resistivity below $T_{C0}$ is produced by one-dimensional excitations, - thermally activated phase slip strips (PSSs) overlapping the sample cross-section. We also determine the scaling phase diagram, which shows that even in wider samples the PSS contribution dominates over vortices in a substantial region of current/temperature variations. Measuring the resistivity within seven orders of magnitude, we find that the quantum phase slips can only be essential below this level.

cond-mat.supr-con↗

arXiv2007

How Magnetic Field Enters Heat Current: Application to Fluctuation Nernst Effect

A problem of the definition of the heat transported in thermomagnetic phenomena has been well realized in the late sixties, but not solved up to date. Ignoring this problem, numerous recent theories grossly overestimate the thermomagnetic coefficients in strongly interacting systems. Here we develop a gauge-invariant microscopic approach, which shows that the heat transfer should include the energy of the interaction between electrons and a magnetic field. We also demonstrate that the surface currents induced by the magnetic field transfer the charge in the Nernst effect, but do not transfer the heat in the Ettingshausen effect. Only with these two modifications of the theory, the physically measurable thermomagnetic coefficients satisfy the Onsager relation. We critically revised the Gaussian fluctuation model above the superconducting transition and show that the gauge invariance uniquely relates thermomagnetic phenomena in the Fermi liquid with the particle-hole asymmetry.

cond-mat.supr-con↗

arXiv2006

On the heat current in the magnetic field: Nernst-Ettingshausen effect above the superconducting transition

For maintaining gauge invariance in a magnetic field, the heat current operator should include the magnetic term. Taking this term into account, we revised calculations of the Nernst-Ettingshausen effect above the superconducting transition. We found that the fluctuations of the modulus of the order parameter do not change the particle-hole asymmetry (PHA) of the thermomagnetic effects. As in the normal state, the thermomagnetic effects in the fluctuation region are proportional to the square of PHA and, therefore, small. Large Nernst effect observed in the high-$T_c$ cuprates requires vortex-like excitations due to the phase fluctuations, which are beyond the Gaussian-fluctuation theory.

cond-mat.supr-con↗

arXiv2005

Deformation Electron-Phonon Coupling in Disordered Semiconductors and Nanostructures

We study the electron-phonon relaxation (dephasing) rate in disordered semiconductors and low-dimensional structures. The relaxation is determined by the interference of electron scattering via the deformation potential and elastic electron scattering from impurities and defects. We have found that in contrast to the destructive interference in metals, which results in the Pippard ineffectiveness condition for the electron-phonon interaction, the interference in semiconducting structures substantially enhances the effective electron-phonon coupling. The obtained results provide an explanation to energy relaxation in silicon structures.

cond-mat.mes-hall↗

arXiv2003

Effects of Electron-Electron and Electron-Phonon Interactions in Weakly Disordered Conductors and Heterostuctures

We investigate quantum corrections to the conductivity due to the interference of electron-electron (electron-phonon) scattering and elastic electron scattering in weakly disordered conductors. The electron-electron interaction results in a negative $T^2 \ln T$-correction in a 3D conductor. In a quasi-two-dimensional conductor, $d < v_F/T$ ($d$ is the thickness, $v_F$ is the Fermi velocity), with 3D electron spectrum this correction is linear in temperature and differs from that for 2D electrons (G. Zala et. al., Phys. Rev.B {\bf 64}, 214204 (2001)) by a numerical factor. In a quasi-one-dimensional conductor, temperature-dependent correction is proportional to $T^2$. The electron interaction via exchange of virtual phonons also gives $T^2$-correction. The contribution of thermal phonons interacting with electrons via the screened deformation potential results in $T^4$-term and via unscreened deformation potential results in $T^2$-term. The interference contributions dominate over pure electron-phonon scattering in a wide temperature range, which extends with increasing disorder.

cond-mat.mes-hall↗

arXiv2003

Gravitational lensing by globular clusters and Arp objects

We try to explain quasar-galaxy associations by gravitational lensing by globular clusters, located in the halos of foreground galaxies. We propose observational test for verification of this hypothesis. We processed SUPERCOSMOS sky survey and found overdensities of star-like sources with zero proper motions in the vicinities of foreground galaxies from CfA3 catalog. We show mean effect for galaxies with different redshifts. Two effects can explain observational data - these are lensing by globular clusters and lensing by dwarf galaxies. We made CCD 3-color photometry with 2.0-1.2 meter telescopes to select extremely lensed objects around several galaxies for spectroscopic observations.

astro-ph↗

arXiv2000

Static Charge Coupling of Intrinsic Josephson Junction

A microscopic theory for the coupling of intrinsic Josephson oscillations due to charge fluctuations on the quasi two-dimensional superconducting layers is presented. Thereby in close analogy to the normal state the effect of the scalar potential on the transport current is taken into account consistently. The dispersion of collective modes is derived and an estimate of the coupling constant is given. It is shown that the correct treatment of the quasiparticle current is essential in order to get the correct position of Shapiro steps. In this case the influence of the coupling on dc-properties like the $I-V$-curve is negligible.

cond-mat.supr-con↗

arXiv1999

Weak localization effect on thermomagnetic phenomena

The quantum transport equation (QTE) is extended to study weak localization (WL) effects on galvanomagnetic and thermomagnetic phenomena. QTE has many advantages over the linear response method (LRM): (i) particle-hole asymmetry which is necessary for the Hall effect is taken into account by the nonequilibrium distribution function, while LRM requires expansion near the Fermi surface, (ii) when calculating response to the temperature gradient, the problem of WL correction to the heat current operator is avoided, (iii) magnetic field is directly introduced to QTE, while the LRM deals with the vector potential and and special attention should be paid to maintain gauge invariance, e.g. when calculating the Nernst effect the heat current operator should be modified to include the external magnetic field. We reproduce in a very compact form known results for the conductivity, the Hall and the thermoelectric effects and then we study our main problem, WL correction to the Nernst coefficient (transverse thermopower).

cond-mat.dis-nn↗

arXiv1998

Coupling of intrinsic Josephson oscillations in layered superconductors by charge fluctuations

The coupling of Josephson oscillations in layered superconductors is studied with help of a tunneling Hamiltonian formalism. The general form of the current density across the barriers between the superconducting layers is derived. The induced charge fluctuations on the superconducting layers lead to a coupling of the Josephson oscillations in different junctions. A simplified set of equations is then used to study the non-linear dynamics of the system. In particular the influence of the coupling on the current-voltage characteristics is investigated and upper limits for the coupling strength are estimated from a comparison with experiments on cuprate superconductors.

cond-mat.supr-con↗