Quantum Tunneling in Half-Integer Spin Systems

Motivated by the experimental observations of resonant tunnelings in the systems with half-integer spin, such as V$_{15}$ and Mn$_4$, we study the mechanism of adiabatic change of the magnetization in systems with the time-reversal symmetry. Within the time-reversal symmetric models, effects of several types of perturbations are investigated. Although tunneling between the ground states is suppressed in a simple Kramers doublet, we show that the nonadiabatic transition governed by the Landau-Zener-St\"uckelberg mechanism occurs in many cases due to the additional degeneracy of the ground state. We also found more general cases where LZS mechanism can not be applied directly even the system shows a kind of adiabatic change of the magnetization

Ground-State Decay Rate for the Zener Breakdown in Band and Mott Insulators

Non-linear transport of electrons in strong electric fields, as typified by dielectric breakdown, is re-formulated in terms of the ground-state decay rate originally studied by Schwinger in non-linear QED. We discuss the effect of electron interaction on Zener tunneling by comparing the dielectric breakdown of the band insulator and the Mott insulator, where the latter is studied by the time-dependent density-matrix renormalization group (DMRG). The relation with the Berry's phase theory of polarization is also established.Comment: 5 pages 2 figures, revised text, version to appear in Phys. Rev. Let

Electronic states in a magnetic quantum-dot molecule: phase transitions and spontaneous symmetry breaking

We show that a double quantum-dot system made of diluted magnetic semiconductor behaves unlike usual molecules. In a semiconductor double quantum dot or in a diatomic molecule, the ground state of a single carrier is described by a symmetric orbital. In a magnetic material molecule, new ground states with broken symmetry can appear due the competition between the tunnelling and magnetic polaron energy. With decreasing temperature, the ground state changes from the normal symmetric state to a state with spontaneously broken symmetry. Interestingly, the symmetry of a magnetic molecule is recovered at very low temperatures. A magnetic double quantum dot with broken-symmetry phases can be used a voltage-controlled nanoscale memory cell.Comment: 4 pages, 5 figure

Lattice and superexchange effects in doped CMR manganites

We report on the influence of the lattice degrees of freedom on charge, orbital and spin correlations in colossal magnetoresistance (CMR) manganites. For the weakly doped compounds we demonstrate that the electron-phonon coupling promotes the trapping of charge carriers, the disappearance of the orbital polaron pattern and the breakdown of ferromagnetism at the CMR transition. The role of different superexchange interactions is explored.Comment: 2 pages, 1 figure, submitted to ICM 200

Magnetic and Transport Properties of (La,Sr)MnO3_3

Magnetic and transport properties of the perovskite-type $3d$ transition-metal oxide (La,Sr)MnO$_3$ are theoretically studied using the double-exchange model in infinite dimension. Magnetoresistance properties as well as the magnetic transition temperatures are in good agreement with the experimental data.Comment: 5 pages, LaTeX, including 2 PS figures. To be published in Proc. International Symposium 'Frontiers of High Tc Superconductivity' (Oct. 1995, Morioka, Japan

Role of Correlation and Exchange for Quasi-particle Spectra of Magnetic and Diluted Magnetic Semiconductors

Theoretical foundation and application of the generalized spin-fermion (sp-d) exchange lattice model to magnetic and diluted magnetic semiconductors are discussed. The capabilities of the model to describe spin quasi-particle spectra are investigated. The main emphasis is made on the dynamic behavior of two interacting subsystems, the localized spins and spin density of itinerant carriers. A nonperturbative many-body approach, the Irreducible Green Functions (IGF) method, is used to describe the quasi-particle dynamics. Scattering states are investigated and three branches of magnetic excitations are calculated in the regime, characteristic of a magnetic semiconductor. For a simplified version of the model (Kondo lattice model) we study the spectra of quasi-particle excitations with special attention given to diluted magnetic semiconductors. For this, to include the effects of disorder, modified mean fields are determined self-consistently. The role of the Coulomb correlation and exchange is clarified by comparing of both the cases.Comment: 34 page

Different origin of the ferromagnetic order in (Ga,Mn)As and (Ga,Mn)N

The mechanism for the ferromagnetic order of (Ga,Mn)As and (Ga,Mn)N is extensively studied over a vast range of Mn concentrations. We calculate the electronic structures of these materials using density functional theory in both the local spin density approximation and the LDA+U scheme, that we have now implemented in the code SIESTA. For (Ga,Mn)As, the LDA+U approach leads to a hole mediated picture of the ferromagnetism, with an exchange constant $N\beta$ =~ -2.8 eV. This is smaller than that obtained with LSDA, which overestimates the exchange coupling between Mn ions and the As $p$ holes. In contrast, the ferromagnetism in wurtzite (Ga,Mn)N is caused by the double-exchange mechanism, since a hole of strong $d$ character is found at the Fermi level in both the LSDA and the LDA+U approaches. In this case the coupling between the Mn ions decays rapidly with the Mn-Mn separation. This suggests a two phases picture of the ferromagnetic order in (Ga,Mn)N, with a robust ferromagnetic phase at large Mn concentration coexisting with a diluted weak ferromagnetic phase.Comment: 12 pages, 11 figure

MICROWAVE-INDUCED RESONANT REFLECTION AND LOCALIZATION OF BALLISTIC ELECTRONS IN QUANTUM MICROCHANNELS

We show that electron transport in a ballistic microchannel supporting both propagating and reflected modes can be completely blocked by applying a microwave electromagnetic field. The effect is due to resonant reflection caused by multiple coherent electron-photon scattering involving at least two spatially localized scattering centers in the channel. With many such scattering centers present the conductance is shown to have an irregular dependence on bias voltage, gate voltage and frequency with irregularily spaced dips corresponding to resonant reflection. When averaged over bias, gate voltage or frequency the conductance will decay exponentially with channel length in full analogy with the localization of 1D electrons caused by impurity scattering.Comment: 4 pages, latex, 1 figure available on reques

Y(Ni, Mn)O3 epitaxial thin films prepared by pulsed laser deposition

High–quality epitaxial YNixMn1-xO3 thin films have been successfully grown on SrTiO3 (100) (STO) by pulsed laser deposition. X-ray diffraction studies showed that the films deposited on STO are fully c-axis oriented and exhibit in-plane alignment. The magnetic transition temperatures (Tc) of the films (both x=0.33 and 0.5) are equivalent to the values of the corresponding bulk samples. However, when x=0.5, the films show magnetic properties quite different from those of bulk samples. This difference may be caused by the structure distortion in these films.Comment: 14 pages, 4 figures. to be publishe