arXiv subjects

V. A. Stephanovich

Publications and source records attributed to V. A. Stephanovich.

At least 37 records · Page 2Linked to original sources

arXiv2015

Transversal spin freezing and re-entrant spin glass phase in chemically disordered Fe-contained double perovskite multiferroics

We propose experimental verification and theoretical explanation of magnetic anomalies in the complex Fe-contained double perovskite multiferroics like PbFe$_{1/2}$Nb$_{1/2}$O$_3$. The theoretical part is based on our model of coexistence of long-range magnetic order and spin glass in the above substances. In our model, the exchange interaction is anisotropic, coupling antiferromagnetically $z$ spin components of Fe$^{3+}$ ions. At the same time, the $xy$ components are coupled by much weaker exchange interaction of ferromagnetic sign. In the system with spatial disorder (half of corresponding lattice cites are occupied by spinless Nb$^{5+}$ ions) such frustrating interaction results in the fact that antiferromagnetic order is formed by $z$ projection of the spins, while their $xy$ components contribute to spin glass behaviour. Our theoretical findings are supported by the experimental evidence of coexistence of antiferromagnetic and spin glass phases in chemically disordered Fe-contained double perovskite multiferroics.

cond-mat.mtrl-sci↗

arXiv2015

Two-dimensional electron gas at the LaAlO$_3$/SrTiO$_3$ inteface with a potential barrier

We present a tight binding description of electronic properties of the interface between LaAlO$_3$ (LAO) and SrTiO$_3$ (STO). The description assumes LAO and STO perovskites as sets of atomic layers in the $x$-$y$ plane, which are weakly coupled by an interlayer hopping term along the $z$ axis. The interface is described by an additional potential, $U_0$, which simulates a planar defect. Physically, the interfacial potential can result from either a mechanical stress at the interface or other structural imperfections. We show that depending on the potential strength, charge carriers (electrons or holes) may form an energy band which is localized at the interface and is within the band gaps of the constituting materials (LAO and STO). Moreover, our description predicts a {\it valve effect} at a certain critical potential strength, $U_{0cr}$, when the interface potential works as a valve suppressing the interfacial conductivity. In other words, the interfacial electrons become dispersionless at $U_0= U_{0cr}$, and thus cannot propagate. This critical value separates the {\it quasielectron} ($U_0<$ $U_{0cr}$) and {\it quasihole} ($U_0>$ $U_{0cr}$) regimes of the interfacial conductivity.

cond-mat.str-el↗

arXiv2015

Macroscopic description of the two-dimensional LaAlO$_3$/SrTiO$_3$ interface

We propose a simple analytical model to explain possible appearance of the metallic conductivity in the two-dimensional (2D) LaAlO$_3$/SrTiO$_3$ interface. Our model considers the interface within a macroscopic approach which is usual to semiconductor heterojunctions and is based on drift-diffusion equations. The solution of these equations allows to obtain the positions of band edges as a function of distances from the interface. We show that for the 2D metallic conductivity to appear at the interface, the constituting substances should have the same type (either electronic or hole) of conductivity; in the opposite case the possible transition to metallic phase has a three-dimensional character. The results of our model calculations are in agreement with available experimental data.

cond-mat.mtrl-sci↗

arXiv2015

Heavy fermion spin liquid in herbertsmithite

We analyze recent heat capacity measurements in herbertsmithite $\rm ZnCu_3(OH)_6Cl_2$ single crystal samples subjected to strong magnetic fields. We show that the temperature dependence of specific heat $C_{mag}$ formed by quantum spin liquid at different magnetic fields $B$ resembles the electronic heat capacity $C_{el}$ of the HF metal $\rm YbRh_2Si_2$. We demonstrate that the spinon effective mass $M^*_{mag}\propto C_{mag}/T$ exhibits a scaling behavior like that of $C_{el}/T$. We also show that the recent measurements of $C_{mag}$ are compatible with those obtained on powder samples. These observations allow us to conclude that $\rm ZnCu_3(OH)_6Cl_2$ holds a stable strongly correlated quantum spin liquid, and a possible gap in the spectra of spinon excitations is absent even under the application of very high magnetic fields of 18 T.

cond-mat.str-el↗

arXiv2014

Comment on "Topological excitations and the dynamic structure factor of spin liquids on the kagome lattice" (Punk, M., Chowdhury, D. & Sachdev, S. Nature Physics 10, 289-293 (2014))

The authors of a recent paper evidently take the view that the whole of progress made toward a theoretical understanding of the physics of quantum spin liquids (QSL) is associated with models of the kind proposed and applied in their present work. As motivation for this work, they observe that in contrast to existing theoretical models of both gapped and gapless spin liquids, which give rise to sharp dispersive features in the dynamic structure factor, the measured dynamic structure factor reveals an excitation continuum that is remarkably flat as a function of frequency. They go on to assert that "so far, the only theoretical model for a spin liquid state on the kagome lattice which naturally gives rise to a flat excitation band at low energies consists of the $Z_2$ spin liquids". Here we point out that there already exists a different and demonstrably successful approach to the QSL problem that does naturally feature a flat band.

cond-mat.str-el↗

arXiv2014

Role of dimensionality in spontaneous magnon decay: easy-plane ferromagnet

We calculate magnon lifetime in an easy-plane ferromagnet on a tetragonal lattice in transverse magnetic field. At zero temperature magnons are unstable with respect to spontaneous decay into two other magnons. Varying ratio of intrachain to interchain exchanges in this model we consider the effect of dimensionality on spontaneous magnon decay. The strongest magnon damping is found in the quasi-one-dimensional case for momenta near the Brillouin zone boundary. The sign of a weak interchain coupling has a little effect on the magnon decay rate. The obtained theoretical results suggest possibility of experimental observation of spontaneous magnon decay in a quasi-one-dimensional ferromagnet CsNiF$_3$. We also find an interesting enhancement of the magnon decay rate for a three-dimensional ferromagnet. The effect is present only for the nearest-neighbor model and is related to effective dimensionality reduction in the two-magnon continuum.

cond-mat.str-el↗

arXiv2013

The exact solution of the diffusion trapping model of defect profiling with variable energy positrons

We report an exact analytical solution of so-called positron diffusion trapping model. This model have been widely used for the treatment of the experimental data for defect profiling of the adjoin surface layer using the variable energy positron (VEP) beam technique. Hovewer, up to now this model could be treated only numerically with so-called VEPFIT program. The explicit form of the solutions is obtained for the realistic cases when defect profile is described by a discreet step-like function and continuous exponential-like function. Our solutions allow to derive the analytical expressions for typical positron annihilation characteristics including the positron lifetime spectrum. Latter quantity could be measured using the pulsed, slow positron beam. Our analytical results are in good coincidence with both the VEPFIT numerics and experimental data. The presented solutions are easily generalizable for defect profiles of other shapes and can be well used for much more precise treatment of above experimental data.

cond-mat.mtrl-sci↗

arXiv2011

Identification of Strongly Correlated Spin Liquid in Herbertsmithite

Exotic quantum spin liquid (QSL) is formed with such hypothetic particles as fermionic spinons carrying spin 1/2 and no charge. Here we calculate its thermodynamic and relaxation properties. Our calculations unveil the fundamental properties of QSL, forming strongly correlated Fermi system located at a fermion condensation quantum phase transition. These are in a good agreement with experimental data and allow us to detect the behavior of QSL as that observed in heavy fermion metals. We predict that the thermal resistivity of QSL under the application of magnetic fields at fixed temperature demonstrates a very specific behavior. The key features of our findings are the presence of spin-charge separation and QSL formed with itinerant heavy spinons in herbertsmithite.

cond-mat.str-el↗

arXiv2011

Spontaneous magnon decays in planar ferromagnet

We predict that spin-waves in an easy-plane ferromagnet have a finite lifetime at zero temperature due to spontaneous decays. In zero field the damping is determined by three-magnon decay processes, whereas decays in the two-particle channel dominate in a transverse magnetic field. Explicit calculations of the magnon damping are performed in the framework of the spin-wave theory for the $XXZ$ square-lattice ferromagnet with an anisotropy parameter $λ<1$. In zero magnetic field the decays occur for $λ^*<λ<1$ with $λ^*\approx 1/7$. We also discuss possibility of experimental observation of the predicted effect in a number of ferromagnetic insulators.

cond-mat.str-el↗

arXiv2011

High magnetic fields thermodynamics of heavy fermion metal YbRh2Si2

We perform comprehensive theoretical analysis of high magnetic field behavior of the heavy-fermion (HF) compound YbRh2Si2. At low magnetic fields B, YbRh2Si2 has a quantum critical point related to the suppression of antiferromagnetic ordering at a critical magnetic field. Our calculations of the thermodynamic properties of YbRh2Si2 in wide magnetic field range allow us to straddle a possible metamagnetic transition region and probe the properties of both low-field HF liquid and high-field fully polarized one. Namely, high magnetic fields B\sim B^*\sim 10 T fully polarize corresponding quasiparticle band generating Landau Fermi liquid (LFL) state and suppressing HF (actually NFL) one, while at elevating temperatures both HF state and corresponding NFL properties are restored. Our calculations are in good agreement with experimental facts and show that the fermion condensation quantum phase transition is indeed responsible for the observed NFL behavior and quasiparticles survive both high temperatures and high magnetic fields.

cond-mat.str-el↗

arXiv2010

On the theory of domain switching kinetics in ferroelectric thin films

We investigate theoretically the polarization switching kinetics in ferroelectric thin films. In such substances, the domain walls are pinned by (usually dipole) defects, which are present also in ordered samples as technologically unavoidable impurities. This random interaction with dipole pinning centers results, in particular, in exponentially broad distribution of switching times. Under supposition of low pinning centers concentration, we derive the distribution function of switching times showing that it is not simply Lorentzian (as it was first suggested by Tagantsev et al [\prb, {\bf 66}, 214109 (2002)] ), but is a "square of Lorentzian", which is due to the vector nature of electric field. This improved formalism delivers a better description of available experimental data and elucidates the physical mechanism of domain switching times distribution.

cond-mat.mtrl-sci↗

arXiv2010

Tetragonal tungsten bronze compounds: relaxor vs mixed ferroelectric - dipole glass behavior

We demonstrate that recent experimental data (E. Castel et al J.Phys. Cond. Mat. {\bf 21} (2009), 452201) on tungsten bronze compound (TBC) Ba$_2$Pr$_x$Nd$_{1-x}$FeNb$_4$O$_{15}$ can be well explained in our model predicting a crossover from ferroelectric ($x=0$) to orientational (dipole) glass ($x=1$), rather then relaxor, behavior. We show, that since a "classical" perovskite relaxor like Pb(Mn$_{1/3}$ Nb$_{2/3}$)O$_3$ is never a ferroelectric, the presence of ferroelectric hysteresis loops in TBC shows that this substance actually transits from ferroelectric to orientational glass phase with $x$ growth. To describe the above crossover theoretically, we use the simple replica-symmetric solution for disordered Ising model.

cond-mat.mtrl-sci↗

arXiv2009

Quantum critical point in high-temperature superconductors

Recently, in high-T_c superconductors (HTSC), exciting measurements have been performed revealing their physics in superconducting and pseudogap states and in normal one induced by the application of magnetic field, when the transition from non-Fermi liquid to Landau Fermi liquid behavior occurs. We employ a theory, based on fermion condensation quantum phase transition which is able to explain facts obtained in the measurements. We also show, that in spite of very different microscopic nature of HTSC, heavy-fermion metals and 2D 3He, the physical properties of these three classes of substances are similar to each other.

cond-mat.supr-con↗

arXiv2009

Energy scales and magnetoresistance at a quantum critical point

The magnetoresistance (MR) of CeCoIn_5 is notably different from that in many conventional metals. We show that a pronounced crossover from negative to positive MR at elevated temperatures and fixed magnetic fields is determined by the scaling behavior of quasiparticle effective mass. At a quantum critical point (QCP) this dependence generates kinks (crossover points from fast to slow growth) in thermodynamic characteristics (like specific heat, magnetization etc) at some temperatures when a strongly correlated electron system transits from the magnetic field induced Landau Fermi liquid (LFL) regime to the non-Fermi liquid (NFL) one taking place at rising temperatures. We show that the above kink-like peculiarity separates two distinct energy scales in QCP vicinity - low temperature LFL scale and high temperature one related to NFL regime. Our comprehensive theoretical analysis of experimental data permits to reveal for the first time new MR and kinks scaling behavior as well as to identify the physical reasons for above energy scales.

cond-mat.str-el↗

arXiv2009

Magnetic-field-induced reentrance of Fermi-liquid behavior and spin-lattice relaxation rates in YbCu_{5-x}Au_x

A strong departure from Landau-Fermi liquid (LFL) behavior have been recently revealed in observed anomalies in both the magnetic susceptibility $χ$ and the muon and $\rm ^{63}Cu$ nuclear spin-lattice relaxation rates $1/T_1$ of ${\rm {YbCu_{5-x}Au_x}}$ ($x=0.6$). We show that the above anomalies along with magnetic-field-induced reentrance of LFL properties are indeed determined by the scaling behavior of the quasiparticle effective mass. We obtain the scaling behavior theoretically utilizing our approach based on fermion condensation quantum phase transition (FCQPT) notion. Our theoretical analysis of experimental data on the base of FCQPT approach permits not only to explain above two experimental facts in a unified manner, but to clarify the physical reasons for a scaling behavior of the longitudinal magnetoresistance in $\rm YbRh_2Si_2$.

cond-mat.str-el↗

arXiv2009

Stable topological textures in a classical 2D Heisenberg model

We show that stable localized topological soliton textures (skyrmions) with $π_2$ topological charge $ν\geq 1$ exist in a classical 2D Heisenberg model of a ferromagnet with uniaxial anisotropy. For this model the soliton exist only if the number of bound magnons exceeds some threshold value $N_{\rm cr}$ depending on $ν$ and the effective anisotropy constant $K_{\rm eff}$. We define soliton phase diagram as the dependence of threshold energies and bound magnons number on anisotropy constant. The phase boundary lines are monotonous for both $ν=1$ and $ν>2$, while the solitons with $ν=2$ reveal peculiar nonmonotonous behavior, determining the transition regime from low to high topological charges. In particular, the soliton energy per topological charge (topological energy density) achieves a minimum neither for $ν=1$ nor high charges, but rather for intermediate values $ν=2$ or $ν=3$.

cond-mat.stat-mech↗

arXiv2008

The influence of intergranular interaction on the magnetization of the ensemble of oriented Stoner-Wohlfarth nanoparticles

We consider the influence of interparticle interaction on the magnetization reversal in the oriented Stoner-Wohlfarth nanoparticles ensemble. To do so, we solve a kinetic equation for the relaxation of the overall ensemble magnetization to its equilibrium value in some effective mean field. Latter field consists of external magnetic field and interaction mean field proportional to the instantaneous value of above magnetization. We show that the interparticle interaction influences the temperature dependence of a coercive field. This influence manifests itself in the noticeable coercivity at $T>T_{b}$ ($T_{b}$ is so-called blocking temperature). The above interaction can also lead to a formation of the "superferromagnetic" state with correlated directions of particle magnetic moments at $T>T_{b}$. This state possesses coercivity if the overall magnetization has a component directed along the easy axis of each particle. We have shown that the coercive field in the "superferromagnetic" state does not depend on measuring time. This time influences both $T_{b}$ and the temperature dependence of coercive field at $T<T_{b}$. We corroborate our theoretical results by measurements on nanogranular films (CoFeB)$_{x}$-(SiO$_{2}$)$_{1-x}$ with concentration of ferromagnetic particles close, but below percolation threshold.

cond-mat.mes-hall↗

arXiv2008

Anomalous low-temperature magnetoelastic properties of nanogranular (CoFeB)$_{x}$-(SiO$_{2}$)$_{1-x}$

We report magnetostatic measurements for granulated films (CoFeB)$_{x}$-(SiO$_{2}$)$_{1-x}$ with fabrication induced intraplanar anisotropy. The measurements have been performed in the film plane in the wide temperature interval 4.5$÷$300 K. They demonstrate that above films have low-temperature anomaly below the percolation threshold for conductivity. The essence of the above peculiarity is that below 100 K the temperature dependence of coercive field for magnetization along easy direction deviates strongly from Neel-Brown law. At temperature lowering, the sharp increase of coercivity is observed, accompanied by the appearance of coercive field for magnetization along hard direction in the film plane. We establish that observed effect is related to the properties of individual ferromagnetic granules. The effect weakens as granules merge into conglomerates at $x$ higher then percolation threshold and disappears completely at $x>1$. We explain the above effect as a consequence of the difference in thermal expansion coefficients of granule and cover material. At temperature lowering this difference weakens the envelopment of an individual granule by the cover matrix material, thus permitting to realize the spontaneous magnetostriction of a granule. The latter induces an additional anisotropy with new easy axis of a granule magnetization along the external magnetic field direction. Our explanation is tested and corroborated by the ferromagnetic resonance measurements in the films at $T$ = 300 K and $T$ = 77 K.

cond-mat.mes-hall↗