Molecular dynamics simulations of oxide memory resistors (memristors).

Reversible bipolar nanoswitches that can be set and read electronically in a solid-state two-terminal device are very promising for applications. We have performed molecular dynamics simulations that mimic systems with oxygen vacancies interacting via realistic potentials and driven by an external bias voltage. The competing short- and long-range interactions among charged mobile vacancies lead to density fluctuations and short-range ordering, while illustrating some aspects of observed experimental behavior, such as memristor polarity inversion. The simulations show that the 'localized conductive filaments' and 'uniform push/pull' models for memristive switching are actually two extremes of the one stochastic mechanism.

Key pairing interaction in layered doped ionic insulators.

A controversial issue on whether the electron-phonon interaction (EPI) is crucial for high-temperature superconductivity or it is weak and inessential has remained one of the most challenging problems of contemporary condensed matter physics. We employ a continuum random phase approximation for the dielectric response function allowing for a self-consistent semianalytical evaluation of the EPI strength, electron-electron attractions, and the carrier mass renormalization in layered high-temperature superconductors. We show that the Fröhlich EPI with high-frequency optical phonons in doped ionic lattices is the key pairing interaction, which is beyond the BCS-Migdal-Eliashberg approximation in underdoped superconductors, and it remains a significant player in overdoped compounds.

Vortex polarization states in nanoscale ferroelectric arrays.

Two-dimensional arrays of ferroelectric lead zirconate titanate (PZT) nanodots were fabricated using pulsed laser deposition through ultrathin anodic aluminum oxide membrane stencil masks. The static distribution of polarization configurations was investigated using in- and out-of-plane piezoresponse force microscopy (PFM). The observed presence of an in-plane polarization component in nominally (001) oriented PZT suggests the existence of a significant deviation from the regular tetragonal structure that allows the formation of complex core-polarization states. Core-polarization states may indicate the presence of quasi-toroidal polarization ordering. The experimental results are compared with a theoretical model to determine the fingerprint of a vortex polarization state in PFM.

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.

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.

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.

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.

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.