arXiv subjects

V. L. Pokrovsky

Publications and source records attributed to V. L. Pokrovsky.

At least 37 records · Page 2Linked to original sources

arXiv2003

From Bubble to Skyrmion: Dynamic Transformation Mediated by a Strong Magnetic Tip

Skyrmions in thin metallic ferromagnetic films are stable due to competition between the RKKY interaction and uniaxial magnetic anisotropy. We mimic the RKKY interaction by the next-nearest-neighbors ferromagnetic and antiferromagnetic exchange interactions. We demonstrate analytically and numerically dissipative transformation of a bubble created by a strong cylindrical magnetic tip into a stable Skyrmion.

cond-mat.mtrl-sci↗

arXiv2003

Fast noise in the Landau-Zener theory

We study the influence of a fast noise on Landau-Zener transitions. We demonstrate that a fast colored noise much weaker than the conventional white noise can produce transitions itself or can change substantially the Landau-Zener transition probabilities. In the limit of fast colored or strong white noise we derive asymptotically exact formulae for transition probabilities and study the time evolution of a spin coupled to the noise and a sweeping magnetic field.

cond-mat.mes-hall↗

arXiv2002

Ising magnets with mobile defects

Motivated by recent experiments on cuprates with low-dimensional magnetic interactions, a new class of two-dimensional Ising models with short-range interactions and mobile defects is introduced and studied. The non-magnetic defects form lines, which, as temperature increases, first meander and then become unstable. Using Monte Carlo simulations and analytical low- and high-temperature considerations, the instability of the defect stripes is monitored for various microscopic and thermodynamic quantities in detail for a minimal model, assuming some of the couplings to be indefinitely strong. The robustness of the findings against weakening the interactions is discussed as well.

cond-mat↗

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.

cond-mat.supr-con↗

arXiv2001

Landau-Zener transitions in a linear chain

We present an exact asymptotic solution for electron transition amplitudes in an infinite linear chain driven by an external homogeneous time-dependent electric field. This solution extends the Landau-Zener theory for the case of infinite number of states in discrete spectrum. In addition to transition amplitudes we calculate an effective diffusion constant.

cond-mat.mes-hall↗

arXiv2000

On the generalized Landau-Zener problem

During the adiabatic time evolution levels crossing violates the adiabaticity and makes transitions between levels possible. Conventionally only two energy levels cross simultaneously. The transition probabilities for this case were found by Landau and Zener. However, the multilevel crossing happens systematically rather than occasionally if the Hamiltonian possesses a special symmetry. The simplest physical realization of the multilevel crossing are the Zeeman multiplet in a varying magnetic field and an electron in one-dimensional chain driven by the time-dependent electric field. We present asymptotics of the transition amplitudes for these kinds of the multilevel crossing. They are based on an exact solution for a model n-state, time-dependent Schrödinger equation.

cond-mat.str-el↗

arXiv1999

Vortex Plasma in a Superconducting Film with Magnetic Dots

We consider a superconducting film, placed upon a magnetic dot array. Magnetic moments of the dots are normal to the film and randomly oriented. We determine how the concentration of the vortices in the film depends on the magnetic moment of a dot at low temperatures. The concentration of the vortices, bound to the dots, is proportional to the density of the dots and depends on the magnetization of a dot in a step-like way. The concentration of the unbound vortices oscillates about a value, proportional to the magnetic moment of the dots. The period of the oscillations is equal to the width of a step in the concentration of the bound vortices.

cond-mat.supr-con↗

arXiv1998

Large Spins in External Fields

Spectra and magnetic properties of large spins $J$, placed into a crystal electric field (CEF) of an arbitrary symmetry point group, are shown to change drastically when $J$ changes by 1/2 or 1. At a fixed field symmetry and configuration of its $N$ extrema situated at $p$-fold symmetry axis, physical characteristics of the spin depend periodically on $J$ with the period equal to $p$. The problem of the spectrum and eigenstates of the large spin $J$ is equivalent to analogous problem for a scalar charged particle confined to a sphere $S^2$ and placed into magnetic field of the monopole with the charge $J$. This analogy as well as strong difference between close values of $J$ stems from the Berry's phase occurring in the problem. For energies close to the extrema of the CEF, the problem can be formulated as Harper's equation on the sphere. The $2J+1$-dimensional space of states is splitted into smaller multiplets of classically degenerated states. These multiplets in turn are splitted into submultiplets of states transforming according to specific irreducible representations of the symmetry group determined by $J$ and $p$. We classify possible configurations and corresponding spectra. Experimental realizations of large spins in a symmetric environment are proposed and physical effects observable in these systems are analyzed.

quant-ph↗

arXiv1998

Skyrmion in a real magnetic film

Skyrmions are the magnetic defects in an ultrathin magnetic film, similar to the bubble domains in the thicker films. Even weak uniaxial anisotropy determines its radius unambiguously. We consider the dynamics of the skyrmion decay. We show that the discreteness of the lattice in an isotropic 2D magnet leads to a slow rotation of the local magnetization in the skyrmion and, provided a small dissipation, to decay of the skyrmion. The radius of such a skyrmion as a function of time is calculated. We prove that uniaxial anisotropy stabilizes the skyrmion and study the relaxation process.

cond-mat↗

arXiv1998

Large moments in external fields of cubic group symmetry

We predict that large moments $J$, placed into a crystal field with the cubic point symmetry group, differ by their spectrum and magnetic properties. E. g., properties of the odd-integer moments are different from those of the even-integer. The effect is due to Berry's phases gained by the moment, when it tunnels between minima of the external field. Two cases of the group $O$ are classified, namely, 6- and 8-fold coordinations. The spectrum and degeneration of energy levels depend on a remainder $\{J/n\}$, where the divisor $n=4$ and 3 for 6-fold and 8-fold coordination respectively. %High symmetry results in a finite magnetic moment for half-integer %and some integer moments, for example odd $J$ at 6-fold coordination. Large moments in the cubic environment can be realized by diluted alloys ${R}_{1-x}{R}_{x}'$Sb, where R=Lu, La, and R$'$=Tb, Dy, Ho, Er.

quant-ph↗

arXiv1997

Berry's phase for large spins in external fields

It is shown that even for large spins $J$ the fundamental difference between integer and half-integer spins persists. In a quasi-classical description this difference enters via Berry's connection. This general phenomenon is derived and illustrated for large spins confined to a plane by crystalline electric fields. Physical realizations are rare-earth Nickel Borocarbides. Magnetic moments for half-integer spin (Dy$^{3+}$, $J=15/2$) and magnetic susceptibilities for integer spin (Ho$^{3+}$, $J=8$) are calculated. Experiments are proposed to furnish evidence for the predicted fundamental difference.

quant-ph↗

arXiv1997

The microscopic model for the magnetic subsystem in HoNi2B2C

We demonstrate that the system of localized magnetic moments in HoNi$_2$B$_2$C can be described by the 4-positional clock model. This model, at a proper choice of the coupling constants, yields several metamagnetic phases in magnetic field at zero temperature in full agreement with the experimental phase diagram. The model incorporates couplings between not nearest neighbors in the direction perpendicular to the ferromagnetic planes. The same model leads to a c-modulated magnetic phase near the Curie temperature. The theoretical value of the modulation wave-vector agrees surprisingly well with that observed by the neutron diffraction experiment without new adjustable parameters.

cond-mat↗

arXiv1996

Long Wavelength Anomalous Diffusion Mode in the 2D XY Dipole Magnet

In 2D XY ferromagnet the dipole force induces a strong interaction between spin-waves in the long-wavelength limit. The major effect of this interaction is the transformation of a propagating spin-wave into a diffusion mode. We study the anomalous dynamics of such diffusion modes. We find that the Janssen-De Dominics functional, which governs this dynamics, approaches the non-Gaussian fixed-point. A spin-wave propagates by an anomalous anisotropic diffusion with the dispersion relation: $iω{\sim}k_{y}^{Δ_y}$ and $iω{\sim}k_{x}^{Δ_x}$, where ${Δ_y}=47/27$ and ${Δ_x}=47/36$. The low-frequency response to the external magnetic field is found.

cond-mat↗

arXiv1996

Hall-Effect for Neutral Atoms

It is shown that polarizable neutral systems can drift in crossed magnetic and electric fileds. The drift velocity is perpendicular to both fields, but contrary to the drif t velocity of a charged particle, it exists only, if fields vary in space or in time. We develop an adiabatic theory of this phenomenon and analyze conditions of its experimental observation. The most proper objects for the observation of this effect are Rydberg atoms. It can be applied for the separation of excited atoms.

atom-ph↗

arXiv1996

Spin Diffusion in 2D XY Ferromagnet with Dipolar Interaction

In the ordered phase of 2D XY ferromagnet the dipolar interaction between spins induces a strong, relevant interaction between spin-waves. We study quasi-excitations of the interacting spin-wave 'liquid' in the long wavelength limit. We employ the Janssen-De-Dominicis method for classical Langevin equation to find the transformation of the spin-wave excitation into a new soft-mode excitation in the intermediate range of wavelengths; and into an anomalous anisotropic diffusion mode excitation at long wavelengths. The dissipation of a spin-wave at short wavelengths is found to be highly anisotropic.

cond-mat↗

arXiv1995

Magnetic Structure of Rare-Earth Nickel Boride-Carbides

We present a theory of the magnetic rare-earth nickel boride-carbides (RENi$_2$B$_2$C, RE=Ho, Er, Tm). Large total angular momentum allows one to employ semiclassical approximation and reduce the problem to the four-positional clock model. The latter gives a proper phase diagram in magnetic field-magnetic moment plane for varying directions of in-plane magnetic field. The theory explains recent experimental observations. PACS: 74.72.Ny, 75.30.Cr, 75.30.Kz, 75.10.Dg

cond-mat↗

arXiv1995

Energy Gap Induced by Impurity Scattering: New Phase Transition in Anisotropic Superconductors

It is shown that layered superconductors are subjected to a phase transition at zero temperature provided the order parameter (OP) reverses its sign on the Fermi-surface but its angular average is finite. The transition is regulated by an elastic impurity scattering rate $1/τ$. The excitation energy spectrum, being gapless at the low level of scattering, develops a gap as soon as the scattering rate exceeds some critical value of $1/τ_\star$.

cond-mat↗

arXiv1994

Orthorhombic distortions may reconcile all the experimental data on YBCO

A lot of controversy appeared recently in measurements of different properties of High-$T_c$-superconductor YBCO. A part of data supports d-wave hypothesis whereas other one contradicts it. We suggest to reconcile visibly controversial experimental data by an assumption that orthorombicity is not small for electronic properties of YBCO, and naturally mixes s- and d-pairing. We examine available experiments to find a proportion of such a mixture. We find in particular that the reconcilation is plausible if the Fermi-surface in YBCO is square-shaped.

cond-mat↗