arXiv subjects

A. M. Bratkovsky

Publications and source records attributed to A. M. Bratkovsky.

72 records · Page 4Linked to original sources

arXiv2000

Easy collective polarization switching in ferroelectrics

The actual mechanism of polarization switching in ferroelectrics remains a puzzle for many decades, since the usually estimated barrier for nucleation and growth is insurmountable ("paradox of the coercive field"). To analyze the mechanisms of the nucleation we consider the exactly solvable case of a ferroelectric film with a "dead" layer at the interface with electrodes. The classical nucleation is easier in this case but still impossible, since the calculated barrier is huge. We have found that the {\em interaction} between the nuclei is, however, long range, hence one has to study an {\em ensemble} of the nuclei. We show that there are the ensembles of small (embryonic) nuclei that grow {\em without the barrier}. We submit that the interaction between nuclei is the key point for solving the paradox.

cond-mat.stat-mech↗

arXiv2000

Phase transitions, stability, and dielectric response of the domain structure in ferroelectric-ferroelastic thin films

We present the first analytical study of phase transitions in ferroelastic-ferroelectric epitaxial thin films on exactly solvable model. The emerging domain structure with domains of equal width (which may be exponentially large on a "soft" substrate) always remains stable irrespective of the film thickness. The dielectric response of an epitaxial film is {\em smaller} than that of a free film, in striking contrast with assertions in the literature.

cond-mat.stat-mech↗

arXiv2000

Semi-classical theory of magnetic quantum oscillations in a two-dimensional multiband canonical Fermi liquid

The semi-classical Lifshitz-Kosevich (LK) description of quantum oscillations is extended to a multiband two-dimensional Fermi liquid with a constant number of electrons. The amplitudes of novel oscillations with combination frequencies, recently predicted and observed experimentally, are analytically derived and compared with the single-band amplitudes. The combination amplitudes decay with temperature exponentially faster than the standard harmonics, and this provides a valuable tool for their experimental identification.

cond-mat.str-el↗

arXiv2000

Ferroelectric Phase Transitions in Films with Depletion Charge

We consider ferroelectric phase transitions in both short-circuited and biased ferroelectric-semiconductor films with a space (depletion) charge which leads to some unusual behavior. It is shown that in the presence of the charge the polarization separates into `switchable' and `non-switchable' parts. The electric field, appearing due to the space charge, does not wash out the phase transition, which remains second order but takes place at somewhat reduced temperature. At the same time, it leads to a suppression of the ferroelectricity in a near-electrode layer. This conclusion is valid for materials with both second and first order phase transitions in pure bulk samples. Influence of the depletion charge on thermodynamic coercive field reduces mainly to the lowering of the phase transition temperature, and its effect is negligible. The depletion charge can, however, facilitate an appearance of the domain structure which would be detrimental for device performance (fatigue). We discuss some issues of conceptual character, which are generally known but were overlooked in previous works. The present results have general implications for small systems with depletion charge.

cond-mat.stat-mech↗

arXiv2000

Abrupt appearance of the domain pattern and fatigue of thin ferroelectric films

We study the domain structure in ferroelectric thin films with a `passive' layer (material with damaged ferroelectric properties) at the interface between the film and electrodes within a continuous medium approximation. An abrupt transition from a monodomain to a polydomain state has been found with the increase of the `passive' layer thickness $d$. The domain width changes very quickly at the transition (exponentially with $d^{-2}$). We have estimated the dielectric response $dP/dE$ (the slope of the hysteresis loop) in the `fatigued' multidomain state and found that it is in agreement with experiment, assuming realistic parameters of the layer. We derive a simple universal relation for the dielectric response, which scales as $1/d$, involving only the properties of the passive layer. This relation qualitatively reproduces the evolution of the hysteresis loop in fatigued samples and it could be tested with controlled experiments. It is expected that the coercive field should increase with decreasing lateral size of the film. We believe that specific properties of the domain structure under bias voltage in ferroelectrics with a passive layer can resolve the long-standing `paradox of the coercive field'.

cond-mat.stat-mech↗

arXiv1999

The Essential Interactions in Oxides and Spectral Weight Transfer in Doped Manganites

We calculate the value of the Fröhlich electron-phonon interaction in manganites, cuprates, and some other charge-transfer insulators and show that this interaction is much stronger than any relevant magnetic interaction. A polaron shift due to the Fröhlich interaction, which is about 1 eV, suggests that carriers in those systems are small (bi)polarons at all temperatures and doping levels, in agreement with the oxygen isotope effect and other data. An opposite conclusion, recently suggested in the literature, is shown to be incorrect. The frequency and temperature dependence of the optical conductivity of ferromagnetic manganites is explained within the framework of the bipolaron theory.

cond-mat.str-el↗

arXiv1999

Colossal magnetooptical conductivity in doped manganites

We show that the current carrier density collapse in doped manganites, which results from bipolaron formation in the paramagnetic phase, leads to a colossal change of the optical conductivity in an external magnetic field at temperatures close to the ferromagnetic transition. As with the colossal magnetoresistance (CMR) itself, the corresponding magnetooptical effect is explained by the dissociation of localized bipolarons into mobile polarons owing to the exchange interaction with the localized Mn spins in the ferromagnetic phase. The effect is positive at low frequencies and negative in the high-frequency region. The present results agree with available experimental observations.

cond-mat.str-el↗

arXiv1999

Theory of optical conductivity in doped manganites

The frequency and temperature dependence of the optical conductivity of ferromagnetic manganites is explained within the framework of the bipolaron theory. As these materials are cooled below the Curie temperature, the colossal magnetoressitance (CMR) is accompanied by a massive transfer of the spectral weight of the optical conductivity to lower frequencies. As with the CMR itself, this change in the optical conductivity is explained by the dissociation of bipolarons into small polarons by exchange interaction with the localized Mn spins during the transition to the low temperature ferromagnetic phase.

cond-mat.str-el↗

arXiv1998

Theory of Colossal Magnetoresistance in Doped Manganites

The exchange interaction of polaronic carriers with localized spins leads to a ferromagnetic/paramagnetic transition in doped charge-transfer insulators with strong electron-phonon coupling. The relative strength of the exchange and electron-phonon interactions determines whether the transition is first or second order. A giant drop in the number of current carriers during the transition, which is a consequence of local bound pair (bipolaron) formation in the paramagnetic phase, is extremely sensitive to an external magnetic field. Below the critical temperature of the transition, $T_c$, the binding of the polarons into immobile pairs competes with the ferromagnetic exchange between polarons and the localized spins on Mn ions, which tends to align the polaron moments and, therefore, breaks up those pairs. The number of carriers abruptly increases below $T_c$ leading to a sudden drop in resistivity. We show that the carrier density collapse describes the colossal magnetoresistance of doped manganites close to the transition. Below $T_c$, transport occurs by polaronic tunneling, whereas at high temperatures the transport is by hopping processes. The transition is accompanied by a spike in the specific heat, as experimentally observed. The gap feature in tunneling spectroscopy is related to the bipolaron binding energy, which depends on the ion mass. This dependence explains the giant isotope effect of the magnetization and resistivity upon substitution of $^{16}$O by $^{18}$O. It is shown also that the localization of polaronic carriers by disorder {\em cannot} explain the observed huge sensitivity of the transport properties to the magnetic field in doped manganites.

cond-mat.str-el↗

arXiv1998

Carrier Density Collapse and Colossal Magnetoresistance in Doped Manganites

A novel ferromagnetic transition, accompanied by carrier density collapse, is found in doped charge-transfer insulators with strong electron-phonon coupling. The transition is driven by an exchange interaction of polaronic carriers with localized spins; the strength of the interaction determines whether the transition is first or second order. A giant drop in the number of current carriers during the transition, which is a consequence of bound pairs formation in the paramagnetic phase close to the transition, is extremely sensitive to an external magnetic field. This carrier density collapse describes the resistivity peak and the colossal magnetoresistance of doped manganites.

cond-mat.str-el↗

arXiv1998

General Green's function formalism for transport calculations with spd-Hamiltonians and giant magnetoresistance in Co and Ni based magnetic multilayers

A novel, general Green's function technique for elastic spin-dependent transport calculations is presented, which (i) scales linearly with system size and (ii) allows straightforward application to general tight-binding Hamiltonians (spd in the present work). The method is applied to studies of conductance and giant magnetoresistance (GMR) of magnetic multilayers in CPP (current perpendicular to planes) geometry in the limit of large coherence length. The magnetic materials considered are Co and Ni, with various non-magnetic materials from the 3d, 4d, and 5d transition metal series. Realistic tight-binding models for them have been constructed with the use of density functional calculations. We have identified three qualitatively different cases which depend on whether or not the bands (densities of states) of a non-magnetic metal (i) form an almost perfect match with one of spin sub-bands of the magnetic metal (as in Cu/Co spin valves); (ii) have almost pure sp character at the Fermi level (e.g. Ag); (iii) have almost pure d character at the Fermi energy (e.g. Pd, Pt). The key parameters which give rise to a large GMR ratio turn out to be (i) a strong spin polarization of the magnetic metal, (ii) a large energy offset between the conduction band of the non-magnetic metal and one of spin sub-bands of the magnetic metal, and (iii) strong interband scattering in one of spin sub-bands of a magnetic metal. The present results show that GMR oscillates with variation of the thickness of either non-magnetic or magnetic layers, as observed experimentally.

cond-mat.mes-hall↗

arXiv1998

Assisted Tunneling in Ferromagnetic Junctions and Half-Metallic Oxides

Different mechanisms of spin-dependent tunneling are analyzed with respect to their role in tunnel magnetoresistance (TMR). Microscopic calculation within a realistic model shows that direct tunneling in iron group systems leads to about a 30% change in resistance, which is close but lower than experimentally observed values. The larger observed values of the tunnel magnetoresistance (TMR) might be a result of tunneling involving surface polarized states. It is found that tunneling via resonant defect states in the barrier radically decreases the TMR by order of magnitude. One-magnon emission is shown to reduce the TMR, whereas phonons increase the effect. The inclusion of both magnons and phonons reasonably explains an unusual bias dependence of the TMR. The model presented here is applied qualitatively to half-metallics with 100% spin polarization, where one-magnon processes are suppressed and the change in resistance in the absence of spin-mixing on impurities may be arbitrarily large. Even in the case of imperfect magnetic configurations, the resistance change can be a few 1000 percent. Examples of half-metallic systems are CrO2/TiO2 and CrO2/RuO2.

cond-mat↗

arXiv1998

Impurities and Inelastic Processes in Magnetic Tunnel Junctions

We have studied tunnel magnetoresistance (TMR) in junctions with 3d ferromagnetic electrodes. Previously we predicted that defects in the barrier would result in reduced effective polarization P of the impurity assisted current. This is confirmed experimentally in the present work: introductions of defects into the barrier drastically decreases the TMR. The degradation of magnetoresistance with bias has also been studied and shows universal features, attributed to effects of tunneling assisted by magnons and phonons, whose different role is described. Details of the bias dependence of the TMR depend on preparation procedures and well described by the model which includes assisted tunneling. Non-linear features, seen at low biases, are related to excitation of bulk modes by tunneling electrons. The analysis of factors resulting in fall-off of the TMR with bias is presented.

cond-mat↗

arXiv1998

Conductance Oscillations in Transition Metal Superlattices

We present a numerical study of conductance oscillations of transition metal multilayers as a function of layer thickness. Using a material-specific tight-binding model, we show that for disorder-free layers with random thicknesses but clean interfaces, long-period oscillations in the conductance can occur, which are reminiscent of those found in structures exhibiting GMR. Using a heuristic effective mass model, we argue that these oscillations arise from beating between the Fermi wavevector and a class of wavevectors characteristic of the superlattice structure.

cond-mat.mes-hall↗

arXiv1996

New Fundamental dHvA Frequency in Canonical Low-Dimensional Fermi Liquids

We show that a new fundamental period $P_{f}$ of dHvA oscillations, which appears along with other ``forbidden'' combination frequencies in a multi-band canonical Fermi-liquid, is very robust with respect to a finite smearing of Landau levels and a background of non-quantized states. We analyse the possibility of measuring small Fermi surface pockets with the use of the ``forbidden'' frequencies.

cond-mat↗

arXiv1996

Step-induced unusual magnetic properties of ultrathin Co/Cu films: ab initio study

We have performed ab initio studies to elucidate the unusual magnetic behavior recently observed in epitaxial Co films upon absorption of submonolayers of Cu and other materials. We find that a submonolayer amount of Cu on a stepped Co/Cu (100) film changes dramatically the electronic and magnetic structure of the system. The effect is mainly due to hybridization of Co and Cu $d$-electrons when copper forms a ``wire'' next to a Co step at the surface. As a result, a non-collinear arrangement of magnetic moments (switching of the easy axis) is promoted. [PACS 75.70.Ak,75.70.-i]

cond-mat↗

arXiv1995

Non-collinear magnetism in Al-Mn topologically disordered systems

We have performed the first ab-initio calculations of a possible complex non-collinear magnetic structure in aluminium-rich Al-Mn liquids within the real-space tight-binding LMTO method. In our previous work we predicted the existence of large magnetic moments in Al-Mn liquids [A.M. Bratkovsky, A.V. Smirnov, D. N. Manh, and A. Pasturel, \prb {\bf 52}, 3056 (1995)] which has been very recently confirmed experimentally. Our present calculations show that there is a strong tendency for the moments on Mn to have a non-collinear (random) order retaining their large value of about 3~$μ_B$. The d-electrons on Mn demonstrate a pronounced non-rigid band behaviour which cannot be reproduced within a simple Stoner picture. The origin of the magnetism in these systems is a topological disorder which drives the moments formation and frustrates their directions in the liquid phase.

cond-mat↗

arXiv1995

The de Haas-van Alphen effect in canonical and grand canonical multiband Fermi liquid

A qualitatively different character of dHvA oscillations has been found in a multiband (quasi)two dimensional Fermi liquid with a fixed fermion density $n_{e}$ (canonical ensemble) compared with an open system where the chemical potential $μ$ is kept fixed (grand canonical ensemble). A new fundamental period $P_{f}$ appears when $n_{e}$ is fixed, a damping of the Landau levels is relatively small and a background density of states is negligible. $P_{f}$ is determined by the total density rather than by the partial densities of carriers in different bands: $P_{f}=1/(2n_{e}ϕ)$ for spin-split Landau levels and $P_{f}=1/(n_{e}ϕ)$ in the case of spin degenerate levels where $ϕ$ is the flux quantum.

mtrl-th↗