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A M Bratkovsky

Publications and source records attributed to A M Bratkovsky.

6 recordsLinked to original sources
PubMed2006

Phase coexistence and resistivity near the ferromagnetic transition of manganites.

Pairing of oxygen holes into heavy bipolarons in the paramagnetic phase and their magnetic pair breaking in the ferromagnetic phase (the so-called current-carrier density collapse) has accounted for the first-order ferromagnetic-phase transition, colossal magnetoresistance, isotope effect, and pseudogap in doped manganites. Here we propose an explanation of the phase coexistence and describe the magnetization and resistivity of manganites near the ferromagnetic transition in the framework of the current-carrier density collapse. The present quantitative description of resistivity is obtained without any fitting parameters, by using the experimental resistivities far away from the transition and the experimental magnetization, and is essentially model-independent.

Journal Article↗
PubMed2005

Smearing of phase transition due to a surface effect or a bulk inhomogeneity in ferroelectric nanostructures.

The boundary conditions, customarily used in the Landau-type approach to ferroelectric thin films and nanostructures, have to be modified to take into account that a surface of a ferroelectric is a defect of a field type. The surface (interface) field is coupled to a normal component of polarization and, as a result, the second order phase transitions are generally suppressed and anomalies in response are washed out, as observed experimentally.

Journal Article↗
PubMed2004

High-frequency spin-valve effect in a ferromagnet-semiconductor-ferromagnet structure based on precession of the injected spins.

A new mechanism of magnetoresistance, based on tunneling emission of spin-polarized electrons from ferromagnets (FM) into semiconductors (S) and precession of electron spin in the semiconductor layer under external magnetic field, is described. The FM-S-FM structure is considered, which includes very thin heavily doped (delta-doped) layers at FM-S interfaces. At certain parameters the structure is highly sensitive at room temperature to variations of the field with frequencies up to 100 GHz. The current oscillates with the field, and its relative amplitude is determined only by the spin polarizations of FM-S junctions.

Journal Article↗
PubMed2002

Geometrical factors in conductance through molecular films.

We have studied the conductance of molecular films as a function of orientation of the molecules in the film with respect to electrodes. Depending on the angle between the axis of the molecules and the normal to the electrode, the conductance may change by more than an order of magnitude. This is a consequence of the strong directional character of p orbitals that determine the conductance through the conjugated molecules. We illustrate this general result on an exactly solvable model and also present the calculations for two different experimentally studied molecular films sandwiched between gold electrodes.

Benzene↗
PubMed2001

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

The first analytical study of phase transitions and domain structure in ferroelastic-ferroelectric epitaxial thin films is presented for an exactly solvable model. The emerging domain structure with domains of equal width (which may be exponentially large) remains stable irrespective of the film thickness. Shifts of the domain walls, unexpectedly, contribute nothing (or insignificantly) to the dielectric response of the film. Generally, the motion of the domain walls results in about the same contribution to the response as the one that comes from a standard bulk term. Therefore, no particular softening of the dielectric response is expected to occur due to the motion of domain walls.

Journal Article↗