arXiv subjects

Yu. G. Pogorelov

Publications and source records attributed to Yu. G. Pogorelov.

At least 19 recordsLinked to original sources

arXiv2014

Dynamic exchange via spin currents in acoustic and optical modes of ferromagnetic resonance in spin-valve structures

Two ferromagnetic layers magnetically decoupled by a thick normal metal spacer layer can be, nevertheless, dynamically coupled via spin currents emitted by the spin-pump and absorbed through the spin-torque effects at the neighboring interfaces. A decrease of damping in both layers due to a partial compensation of the angular momentum leakage in each layer was previously observed at the coincidence of the two ferromagnetic resonances. In case of non-zero magnetic coupling, such a dynamic exchange will depend on the mutual precession of the magnetic moments in the layers. A difference in the linewidth of the resonance peaks is expected for the acoustic and optical regimes of precession. However, the interlayer coupling hybridizes the resonance responses of the layers and therefore can also change their linewidths. The interplay between the two mechanisms has never been considered before. In the present work, the joint influence of the hybridization and non-local damping on the linewidth has been studied in weakly coupled NiFe/CoFe/Cu/CoFe/MnIr spin-valve multilayers. It has been found that the dynamic exchange by spin currents is different in the optical and acoustic modes, and this difference is dependent on the interlayer coupling strength. In contrast to the acoustic precession mode, the dynamic exchange in the optical mode works as an additional damping source. A simulation in the framework of the Landau-Lifshitz-Gilbert formalism for two ferromagnetic layers coupled magnetically and by spin currents has been done to separate the effects of the non-local damping from the resonance modes hybridization. In our samples both mechanisms bring about linewidth changes of the same order of magnitude, but lead to a distinctly different angular behavior. The obtained results are relevant for a broad class of coupled magnetic multilayers with ballistic regime of the spin transport.

cond-mat.mes-hall↗

arXiv2010

Superconducting junctions from non-superconducting doped CuO$_2$ layers

The theoretical approach proposed recently for description of redistribution of electronic charge in multilayered selectively doped systems is modified for a system with finite number of layers. A special attention is payed to the case of a finite heterostructure made of copper-oxide layers which are all non-superconducting (including non-conducting) because of doping levels being beyond the well-known characteristic interval for superconductivity. Specific finite structures and doping configurations are proposed to obtain atomically thin superconducting heterojunctions of different compositions.

cond-mat.supr-con↗

arXiv2008

Modulated electronic configurations in selectively doped multilayered nanostructures

A simple theoretical model is proposed to describe the recent experimental results on formation of induced superconducting state and anomalous tunneling characteristics in selectively doped multilayered nanostructures based on La$_2$CuO$_4$ perovskite. In particular, it is shown that the structure composed from the nominally non-superconducting (undoped and overdoped) layers turns to be superconducting with superconductivity confined to narrow regions near the interfaces, in agreement with the experimental observations.

cond-mat.supr-con↗

arXiv2007

Quantum effects in atomically perfect specular spin valve structures

A simple tight-binding theoretical model is proposed for spin dependent, current-in-plane transport in highly coherent spin valve structures under specularity conditions. Using quantum-mechanically coherent and spatially quantized Fermi states in the considered multilayered system, a system of partial Boltzmann kinetic equations is built for relevant subbands to yield the expressions for conductance in parallel or antiparallel spin valve states and thus for the magneto-conductance. It is shown that specularity favors the magnetoresistance to reach its theoretical maximum for this structure close to 100%. This result is practically independent of the model parameters, in particular it does not even need that lifetimes of majority and minority carriers be different (as necessary for the quasiclassical regimes). The main MR effect in the considered limit is due to the transformation of coherent quantum states, induced by the relative rotation of magnetization in the FM layers. Numerical calculation based on the specific Boltzmann equation with an account of spin-dependent specular reflection at the interfaces is also performed for a typical choice of material parameters.

cond-mat.mtrl-sci↗

arXiv2006

Impurity clusters and localization of nodal quasiparticles in \emph{d}-wave superconductors

The long disputed issue of the limiting value of quasiparticle density of states $\r(0) = \r (\e \to 0)$ in a \emph{d-}wave superconductor with impurities (\emph{vs} its linear vanishing, $\r_0(\e) \propto |\e|/\D$, near the nodal point $\e = 0$ in a pure system with the gap parameter $\D$) is discussed. Using the technique of group expansions of Green functions in complexes of interacting impurities, it is shown that finite $\r(0)$ value is possible if the (finite) impurity perturbation $V$ is spin-dependent (magnetic). The found value has a power law dependence on the impurity concentration $c$: $\r(0) \propto \r_N c^{n}$, where $\r_N$ is the normal metal density of states and $n \geq 2$ is the least number of impurities in a complex that can localize nodal quasiparticle. This result essentially differs from the known predictions of self-consistent approximation: $\r(0) \propto \r_N \sqrt {c/\r_N\D}$ (for the unitary limit $V \to \infty$) or $\r(0) \propto (\D/cV^2)\exp(-\D/cV^2\r_N)$ (for the Born limit $|V|\r_N \ll 1$). We predict also existence of a narrow interval (mobility gap) around the Fermi energy, where all the states are localized on proper impurity clusters, leading to exponential suppression of low-temperature kinetics.

cond-mat.supr-con↗

arXiv2006

Origin of four-fold anisotropy in square lattices of circular ferromagnetic dots

We discuss the four-fold anisotropy of in-plane ferromagnetic resonance (FMR) field $H_r$, found in a square lattice of circular Permalloy dots when the interdot distance $a$ gets comparable to the dot diameter $d$. The minimum $H_r$, along the lattice $<11>$ axes, and the maximum, along the $<10>$ axes, differ by $\sim$ 50 Oe at $a/d$ = 1.1. This anisotropy, not expected in uniformly magnetized dots, is explained by a non-uniform magnetization $\bm(\br)$ in a dot in response to dipolar forces in the patterned magnetic structure. It is well described by an iterative solution of a continuous variational procedure.

cond-mat.mtrl-sci↗

arXiv2006

Anomalous impurity resonance in graphene

A Green function analysis has been developed for quasiparticle spectrum and localized states of a 2D graphene sheet in presence of different types of substitutional disorder, including vacancies. The anomalous character of impurity effects in this system is demonstrated, compared to those in well known doped semiconductors, and explained in terms of conical singularities in the band spectrum of pure graphene. The criteria for appearance of localized states on clusters of impurity scatterers and for qualitative restructuring of band spectrum are established and a phase diagram in variables ``disorder'' \emph{vs} ``electron density'' is proposed.

cond-mat.dis-nn↗

arXiv2004

Magnetic tuning of tunnel conductivity

Using the simplest two-subband Stoner model, it is shown that the variation of the Fermi energy under applied magnetic field is inverse proportional to the spontaneous magnetization and hence most pronounced close to the critical Stoner condition, that is to the quantum critical point of ferromagnetic transition. The perspectives of this result for magnetic tuning of tunnel conductivity in spintronics devices is discussed.

cond-mat.str-el↗

arXiv2004

A possible scenario of metallization in boron doped diamond CB$_x$

Possibility for collectivization of acceptor states in a semiconductor, converting it to metal, is discussed within the scope of Anderson s-d hybride model. This model is generalized for multicomponent band structure and composite acceptor states, localized on pairs of neighbor dopants (impurity "dumbbells"), in order to describe boron doped diamond CB$_x$. The resulting parameters of band structure, in particular, position of the Fermi level, are compared to the recent experimental data on metallized and superconducting CB$_x$.

cond-mat.dis-nn↗

arXiv2004

Effects of extended impurity perturbation in d-wave superconductor

We describe the effects of electronic perturbation distributed on nearest neighbor sites to the impurity center in a planar \textit{d}-wave superconductor, in approximation of circular Fermi surface. Alike the behavior previously reported for point-like perturbation and square Fermi surface, the quasiparticle density of states $ρ(ε)$ can display a resonance inside the gap (and very weak features from low symmetry representations of non-local perturbation) and asymptotically vanishes at $ε\to 0$ as $ρ\simε/\ln^2ε$. The local suppression of SC order parameter in this model is found to be somewhat weaker than for an equivalent point-like (non-magnetic) perturbation and much weaker than for a spin-dependent (extended) perturbation.

cond-mat.supr-con↗

arXiv2004

Exact Solution of Ising Model on a Small-World Network

We present an exact solution of a one-dimensional Ising chain with both nearest neighbor and random long-range interactions. Not surprisingly, the solution confirms the mean field character of the transition. This solution also predicts the finite-size scaling that we observe in numerical simulations.

cond-mat.stat-mech↗

arXiv2003

Group expansions for impurities in superconductors

A new method is proposed for practical calculation of the effective interaction between impurity scatterers in superconductors, based on algebraic properties of related Nambu matrices for Green functions. In particular, we show that the density of states within the s-wave gap can have a non-zero contribution (impossible either in Born and in T-matrix approximation) from non-magnetic impurities with concentration $c \ll 1$, beginning from $\sim c^{3}$ order.

cond-mat.supr-con↗

arXiv2003

Adiabatic theory of boundary friction and stick-slip processes

An adiabatic approach is developed for the problem of boundary friction between two atomically smooth and incommensurate solid surfaces, separated by a monolayer of lubricant atoms. This method permits to consider very slow macroscopic motion of the parts in contact, separately from fast thermic motions of individual atoms. A characteristic ''stick-slip'' behavior of the tangential force on the contact is obtained within a simple 1D model, relevant for the tip and sample system in friction force microscopy (FFM). This behavior reflects the specific mechanism of stress energy accumulation, through formation of long-living metastable states (defects) within the monoatomic lubricant layer, and their subsequent collapse with energy conversion into heat. This is similar to the dislocation mechanism of irreversible deformation in bulk solids. The peculiar feature predicted by the present theory is the twofold periodicity of ''stick-slip'' spikes with relative displacement: the shorter period $aδ$ (where $a$ is the tip lattice periodicity and $δ$ the relative tip-sample lattice mismatch) relates to defect skips by one elementary cell, and the longer period $% a(1-δ)$ relates to defect annihilation or nucleation at the boundaries of contact area.

cond-mat.mtrl-sci↗

arXiv2003

Boundary Friction on Molecular Lubricants: Rolling Mode?

A theoretical model is proposed for low temperature friction between two smooth rigid solid surfaces separated by lubricant molecules, admitting their deformations and rotations. Appearance of different modes of energy dissipation (by ''rocking'' or ''rolling'' of lubricants) at slow relative displacement of the surfaces is shown to be accompanied by the stick-and-slip features and reveals a non-monotonic (mean) friction force {\it vs} external load

cond-mat.mtrl-sci↗

arXiv2003

Nodal quasiparticles in doped d-wave superconductors: self-consistent T-matrix approach

A comparative analysis has been done of the formerly established two self-consistent solutions for the density of quasiparticle states in doped d-wave superconductors, having strikingly different and disputed behavior near the Fermi energy. One of them (1) remains finite in this limit, while the other (2) tends to zero. To resolve this discrepancy, the known Ioffe-Regel criterion for band states, widely used for doped semiconductors, was applied to these solutions. It is shown that both them are valid in limited and different energy regions, where the corresponding quasiparticles are weakly damped. In particular, density of states of nodal quasiparticles near the Fermi level is provided by the (2) solution, while the (1) only applies far enough from this level.

cond-mat.supr-con↗

arXiv2003

Fluctuating order parameter in doped cuprate superconductors

We discuss static fluctuations of the d-wave superconducting order parameter $Δ$ in CuO$_2$ planes, due to quasiparticle scattering by charged dopants. The analysis of two-particle anomalous Green functions at $T = 0$ permits to estimate the mean-square fluctuation $δ^2 = <Δ^2> - <Δ>^2$, averaged in random dopant configurations, to the lowest order in doping level $c$. Since $Δ$ is found to saturate with growing doping level while $δ$ remains to grow, this can explain the collapse of $T_c$ at overdoping. Also we consider the spatial correlations $<Δ(0)Δ({\bf R})>$ for order parameter in different points of the plane.

cond-mat.supr-con↗

arXiv2002

Transition from non Fermi Liquid Behavior to Landau Fermi Liquid Behavior Induced by Magnetic Fields

We show that a strongly correlated Fermi system with the fermion condensate, which exhibits strong deviations from Landau Fermi liquid behavior, is driven into the Landau Fermi liquid by applying a small magnetic field $B$ at temperature T=0. This field-induced Landau Fermi liquid behavior provides the constancy of the Kadowaki-Woods ratio. A reentrance into the strongly correlated regime is observed if the magnetic field $B$ decreases to zero, then the effective mass $M^*$ diverges as $M^*\propto 1/\sqrt{B}$. At finite temperatures, the strongly correlated regime is restored at some temperature $T^*\propto\sqrt{B}$. This behavior is of general form and takes place in both three dimensional and two dimensional strongly correlated systems. We demonstrate that the observed $1/\sqrt{B}$ divergence of the effective mass and other specific features of heavy-fermion metals are accounted for by our consideration.

cond-mat.str-el↗

arXiv2002

Fermions under strong repulsion: from multiconnected Fermi surfaces to Fermi condensation

We consider a system of fermions with mass m and model repulsive interaction U(q) = g/\sqrt{q^2 + q_0^2}, where q is the momentum transfer, q_0 the screening constant, and g > 0 the coupling constant. It is shown that at g > g_cr > 3π^2/m the system ground state is changed from fully occupied Fermi sphere to multiconnected Fermi sphere (MFS), consisting in N fully occupied spherical layers (icebergs) separated by empty spacers. An effective description is developed for such states at N >> 1, showing their tendency at N \to \infty to the Fermi condensate (FC) state known for the model U(q) = g/q [Khodel, Shaginyan, JETP Lett., 51, 553, 1990]. At finite temperatures, a crossover is predicted from the Fermi liquid behavior of MFS with effective mass m^* \sim m N/ln(N) to non-Fermi-liquid behavior with m^* \propto 1/T at T > T^* \sim T_F ln(N)/N.

cond-mat.str-el↗