A. M. Bratkovsky

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.

Gold Nanofingers for Molecule Trapping and Detection

Here we demonstrate a molecular trap structure that can be formed to capture analyte molecules in solution for detection and identification. The structure is based on gold-coated nanoscale polymer fingers made by nanoimprinting technique. The nanofingers are flexible and their tips can be brought together to trap molecules, while at the same time the gold-coated fingertips form a reliable Raman hot spot for molecule detection and identification based on surface enhanced Raman spectroscopy (SERS). The molecule self-limiting gap size control between fingertips ensures ultimate SERS enhancement for sensitive molecule detection. Furthermore, these type of structures, resulting from top-down meeting self-assembly, can be generalized for other applications, such as plasmonics, meta-materials, and other nanophotonic systems.

Hot-Spot Engineering in Polygonal Nanofinger Assemblies for Surface Enhanced Raman Spectroscopy

Multiparticle assemblies of nanoscale structures are the fundamental building blocks for powerful plasmonic devices. Here we show the controlled formation of polygonal metal nanostructure assemblies, including digon, trigon, tetragon, pentagon, and hexagon arrays, which were formed on top of predefined flexible polymer pillars that undergo self-coalescence, analogous to finger closing, with the aid of microcapillary forces. This hybrid approach of combining top-down fabrication with self-assembly enables the formation of complex nanoplasmonic structures with sub-nanometer gaps between gold nanoparticles. On comparison of the polygon-shaped assemblies, the symmetry dependence of the nanoplasmonic structures was determined for application to surface enhanced Raman spectroscopy (SERS), with the pentagonal assembly having the largest Raman enhancement for the tested molecules. Electromagnetic simulations of the polygonal structures were performed to visualize the field enhancements of the hot spots so as to guide the rational design of optimal SERS structures.

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.

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

A 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 current perpendicular to planes geometry in the limit of large coherence length. The magnetic materials considered are Co and Ni, with various nonmagnetic materials from the 3d ,4 d , 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 nonmagnetic metal ~i! form an almost perfect match with one of spin subbands of the magnetic metal ~as in Cu/Co spin valves!, ~ii! have almost pure sp character at the Fermi level ~e.g., Ag!, and ~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 nonmagnetic metal and one of spin subbands of the magnetic metal, and ~iii! strong interband scattering in one of spin subbands of a magnetic metal. The present results show that GMR oscillates with variation of the thickness of either nonmagnetic or magnetic layers, as observed experimentally. @S0163-1829~99!03118-5#

Current rectification by molecules with asymmetric tunneling barriers

A simple experimentally accessible realization of current rectification by molecules (molecular films) bridging metal electrodes is described. It is based on the spatial asymmetry of the molecule and requires only one resonant conducting molecular level

Assisted tunneling in ferromagnetic junctions and half-metallic oxides

(\ensuremath{\pi}

Orientational dependence of current through molecular films

orbital). The rectification, which is due to the asymmetric coupling of the level to the electrodes by tunnel barriers, is largely independent of the work function difference between the two electrodes. Results of extensive numerical studies of the family of suggested molecular rectifiers

Depolarizing field and “real” hysteresis loops in nanometer-scale ferroelectric films

\mathrm{HS}\ensuremath{-}({\mathrm{CH}}_{2}{)}_{m}\ensuremath{-}{\mathrm{C}}_{6}{\mathrm{H}}_{4}\ensuremath{-}({\mathrm{CH}}_{2}{)}_{n}\ensuremath{-}\mathrm{SH}

Memory effect in a molecular quantum dot with strong electron-vibron interaction

are presented. The highest rectification ratio