Luttinger liquid superlattices

We calculate the correlation functions and the DC conductivity of Luttinger liquid superlattices, modeled by a repeated pattern of interacting and free Luttinger liquids. In a specific realization, where the interacting subsystem is a Hubbard chain, the system exhibits a rich phase diagram with four different phases: two metals and two compressible insulators. In general, we find that the effective low energy description amalgamates features of both types of liquids in proportion to their spatial extent, suggesting the interesting possibility of `engineered' Luttinger liquids.Comment: RevTeX, 5 pages, 3 figure

Phase Diagram of Multilayer Magnetic Structures

Multilayer "ferromagnet-layered antiferromagnet" (Fe/Cr) structures frustrated due to the roughness of layer interfaces are studied by numerical modeling methods. The "thickness-roughness" phase diagrams for the case of thin ferromagnetic film on the surface of bulk antiferromagnet and for two ferromagnetic layers separated by an antiferromagnetic interlayer are obtained and the order parameter distributions for all phases are found. The phase transitions nature in such systems is considered. The range of applicability for the "magnetic proximity model" proposed by Slonczewski is evaluated.Comment: 8 pages, 8 figure

Enhancement of the Josephson current by an exchange field in superconductor-ferromagnet structures

We calculate the dc Josephson current for two superconductor-ferromagnet (S/F) bilayers separated by a thin insulating film. It is demonstrated that the critical Josephson current $I_{c}$ in the junction strongly depends on the relative orientation of the effective exchange field $h$ of the bilayers. We found that in the case of an antiparallel orientation, $I_{c}$ increases at low temperatures with increasing $h$ and at zero temperature has a singularity when $h$ equals the superconducting gap $\Delta$. This striking behavior contrasts suppression of the critical current by the magnetic moments aligned in parallel and is an interesting new effect of the interplay between superconductors and ferromagnets.Comment: to be published in PR

Surface Half-Metallicity of CrAs in the Zinc-Blende Structure

The development of new techniques such as the molecular beam epitaxy have enabled the growth of thin films of materials presenting novel properties. Recently it was made possible to grow a CrAs thin-film in the zinc-blende structure. In this contribution, the full-potential screened KKR method is used to study the electronic and magnetic properties of bulk CrAs in this novel phase as well as the Cr and As terminated (001) surfaces. Bulk CrAs is found to be half-ferromagnetic for all three GaAs, AlAs and InAs experimental lattice constants with a total spin magnetic moment of 3 $\mu_B$. The Cr-terminated surface retains the half-ferromagnetic character of the bulk, while in the case of the As-termination the surface states destroy the gap in the minority-spin band.Comment: 4 pages, 2 figures, new text, new titl

Exchange Field Induced Magnetoresistance in Colossal Magnetoresistance Manganites

The effect of an exchange field on electrical transport in thin films of metallic ferromagnetic manganites has been investigated. The exchange field was induced both by direct exchange coupling in a ferromagnet/antiferromagnet multilayer and by indirect exchange interaction in a ferromagnet/paramagnet superlattice. The electrical resistance of the manganite layers was found to be determined by the absolute value of the vector sum of the effective exchange field and the external magnetic field.Comment: 5 pages, 4 figure

Spin Diode Based on Fe/MgO Double Tunnel Junction

We demonstrate a spin diode consisting of a semiconductor free nano-scale Fe/MgO-based double tunnel junction. The device exhibits a near perfect spin-valve effect combined with a strong diode effect. The mechanism consistent with our data is resonant tunneling through discrete states in the middle ferromagnetic layer sandwiched by tunnel barriers of different spin-dependent transparency. The observed magneto-resistance is record high, ~4000%, essentially making the structure an on/off spin-switch. This, combined with the strong diode effect, ~100, offers a new device that should be promising for such technologies as magnetic random access memory and re-programmable logic.Comment: 14 page

Field-driven femtosecond magnetization dynamics induced by ultrastrong coupling to THz transients

Controlling ultrafast magnetization dynamics by a femtosecond laser is attracting interest both in fundamental science and industry because of the potential to achieve magnetic domain switching at ever advanced speed. Here we report experiments illustrating the ultrastrong and fully coherent light-matter coupling of a high-field single-cycle THz transient to the magnetization vector in a ferromagnetic thin film. We could visualize magnetization dynamics which occur on a timescale of the THz laser cycle and two orders of magnitude faster than the natural precession response of electrons to an external magnetic field, given by the Larmor frequency. We show that for one particular scattering geometry the strong coherent optical coupling can be described within the framework of a renormalized Landau Lifshitz equation. In addition to fundamentally new insights to ultrafast magnetization dynamics the coherent interaction allows for retrieving the complex time-frequency magnetic properties and points out new opportunities in data storage technology towards significantly higher storage speed.Comment: 25 page

Mesoscopic mechanism of exchange interaction in magnetic multilayers

We discuss a mesoscopic mechanism of exchange interaction in ferromagnet-normal metal-ferromagnet multilayers. We show that in the case when the metal's thickness is larger than the electron mean free path, the relative orientation of magnetizations in the ferromagnets is perpendicular. The exchange energy between ferromagnets decays with the metal thickness as a power law

Tenfold Magnetoconductance in a Non-Magnetic Metal Film

We present magnetoconductance (MC) measurements of homogeneously disordered Be films whose zero field sheet conductance (G) is described by the Efros-Shklovskii hopping law $G(T)=(2e^2/h)\exp{-(T_o/T)^{1/2}}$. The low field MC of the films is negative with G decreasing 200% below 1 T. In contrast the MC above 1 T is strongly positive. At 8 T, G increases 1000% in perpendicular field and 500% in parallel field. In the simpler parallel case, we observe {\em field enhanced} variable range hopping characterized by an attenuation of $T_o$ via the Zeeman interaction.Comment: 9 pages including 5 figure

Ground state properties of ferromagnetic metal/conjugated polymer interfaces

We theoretically investigate the ground state properties of ferromagnetic metal/conjugated polymer interfaces. The work is partially motivated by recent experiments in which injection of spin polarized electrons from ferromagnetic contacts into thin films of conjugated polymers was reported. We use a one-dimensional nondegenerate Su-Schrieffer-Heeger (SSH) Hamiltonian to describe the conjugated polymer and one-dimensional tight-binding models to describe the ferromagnetic metal. We consider both a model for a conventional ferromagnetic metal, in which there are no explicit structural degrees of freedom, and a model for a half-metallic ferromagnetic colossal magnetoresistance (CMR) oxide which has explicit structural degrees of freedom. The Fermi energy of the magnetic metallic contact is adjusted to control the degree of electron transfer into the polymer. We investigate electron charge and spin transfer from the ferromagnetic metal to the organic polymer, and structural relaxation near the interface. Bipolarons are the lowest energy charge state in the bulk polymer for the nondegenerate SSH model Hamiltonian. As a result electrons (or holes) transferred into the bulk of the polymer form spinless bipolarons. However, there can be spin density in the polymer localized near the interface.Comment: 7 figure