Electronic Phase Separation in Manganite/Insulator Interfaces

By using a realist microscopic model, we study the electric and magnetic properties of the interface between a half metallic manganite and an insulator. We find that the lack of carriers at the interface debilitates the double exchange mechanism, weakening the ferromagnetic coupling between the Mn ions. In this situation the ferromagnetic order of the Mn spins near the interface is unstable against antiferromagnetic CE correlations, and a separation between ferromagnetic/metallic and antiferromagnetic/insulator phases at the interfaces can occur. We obtain that the insertion of extra layers of undoped manganite at the interface introduces extra carriers which reinforce the double exchange mechanism and suppress antiferromagnetic instabilities.Comment: 8 pages, 7 figures include

A new Bloch period for interacting cold atoms in 1D optical lattices

The paper studies Bloch oscillations of ultracold atoms in optical lattice in the presence of atom-atom interaction. A new, interaction-induced Bloch period is identified. The analytical results are corroborated by realistic numerical calculations.Comment: revtex4, 4 pages, 4 figures, gzipped tar fil

Thermoelastic Damping in Micro- and Nano-Mechanical Systems

The importance of thermoelastic damping as a fundamental dissipation mechanism for small-scale mechanical resonators is evaluated in light of recent efforts to design high-Q micrometer- and nanometer-scale electro-mechanical systems (MEMS and NEMS). The equations of linear thermoelasticity are used to give a simple derivation for thermoelastic damping of small flexural vibrations in thin beams. It is shown that Zener's well-known approximation by a Lorentzian with a single thermal relaxation time slightly deviates from the exact expression.Comment: 10 pages. Submitted to Phys. Rev.

Bloch Oscillation under a Bichromatic Laser: Quasi-Miniband Formation, Collapse, and Dynamical Delocalization and Localization

A novel DC and AC driving configuration is proposed for semiconductor superlattices, in which the THz AC driving is provided by an intense bichromatic cw laser. The two components of the laser, usually in the visible light range, are near but not exactly resonant with interband Wannier-Stark transitions, and their frequency difference equals the Wannier-Stark ladder spacing. Multi-photon processes with the intermediate states in the conduction (valence) band cause dynamical delocalization and localization of valence (conduction) electrons, and the corresponding formation and collapse of the quasi-minibands.Comment: 4 pages, 3 figure

Dissipation in graphene and nanotube resonators

Different damping mechanisms in graphene nanoresonators are studied: charges in the substrate, ohmic losses in the substrate and the graphene sheet, breaking and healing of surface bonds (Velcro effect), two level systems, attachment losses, and thermoelastic losses. We find that, for realistic structures and contrary to semiconductor resonators, dissipation is dominated by ohmic losses in the graphene layer and metallic gate. An extension of this study to carbon nanotube-based resonators is presented.Comment: Published version with updated reference

Effect of magnetic state on the γ−α\gamma -\alpha transition in iron: First-principle calculations of the Bain transformation path

Energetics of the fcc ($\gamma$) - bcc ($\alpha$) lattice transformation by the Bain tetragonal deformation is calculated for both magnetically ordered and paramagnetic (disordered local moment) states of iron. The first-principle computational results manifest a relevance of the magnetic order in a scenario of the $\gamma$ - $\alpha$ transition and reveal a special role of the Curie temperature of $\alpha$-Fe, $T_C$, where a character of the transformation is changed. At a cooling down to the temperatures $T < T_C$ one can expect that the transformation is developed as a lattice instability whereas for $T > T_C$ it follows a standard mechanism of creation and growth of an embryo of the new phase. It explains a closeness of $T_C$ to the temperature of start of the martensitic transformation, $M_s$.Comment: 4 pages, 3 figures, submitted in Phys. Rev. Letter

Dynamics of quantum phase transition: exact solution in quantum Ising model

Quantum Ising model is an exactly solvable model of quantum phase transition. This paper gives an exact solution when the system is driven through the critical point at finite rate. The evolution goes through a series of Landau-Zener level anticrossings when pairs of quasiparticles with opposite pseudomomenta get excited with probability depending on the transition rate. Average density of defects excited in this way scales like a square root of the transition rate. This scaling is the same as the scaling obtained when the standard Kibble-Zurek mechanism of thermodynamic second order phase transitions is applied to the quantum phase transition in the Ising model.Comment: misprints corrected; version to appear in Phys.Rev.Let

Atom interferometry with trapped Fermi gases

We realize an interferometer with an atomic Fermi gas trapped in an optical lattice under the influence of gravity. The single-particle interference between the eigenstates of the lattice results in macroscopic Bloch oscillations of the sample. The absence of interactions between fermions allows a time-resolved study of many periods of the oscillations, leading to a sensitive determination of the acceleration of gravity. The experiment proves the superiorness of non interacting fermions with respect to bosons for precision interferometry, and offers a way for the measurement of forces with microscopic spatial resolution.Comment: 4 pages, 4 figure

Sufficiency Criterion for the Validity of the Adiabatic Approximation

We examine the quantitative condition which has been widely used as a criterion for the adiabatic approximation but was recently found insufficient. Our results indicate that the usual quantitative condition is sufficient for a special class of quantum mechanical systems. For general systems, it may not be sufficient, but it along with additional conditions is sufficient. The usual quantitative condition and the additional conditions constitute a general criterion for the validity of the adiabatic approximation, which is applicable to all $N-$dimensional quantum systems. Moreover, we illustrate the use of the general quantitative criterion in some physical models.Comment: 9 pages, no figure,appearing in PRL98(2007)15040

Quantum Annealing in the Transverse Ising Model

We introduce quantum fluctuations into the simulated annealing process of optimization problems, aiming at faster convergence to the optimal state. Quantum fluctuations cause transitions between states and thus play the same role as thermal fluctuations in the conventional approach. The idea is tested by the transverse Ising model, in which the transverse field is a function of time similar to the temperature in the conventional method. The goal is to find the ground state of the diagonal part of the Hamiltonian with high accuracy as quickly as possible. We have solved the time-dependent Schr\"odinger equation numerically for small size systems with various exchange interactions. Comparison with the results of the corresponding classical (thermal) method reveals that the quantum annealing leads to the ground state with much larger probability in almost all cases if we use the same annealing schedule.Comment: 15 pages, RevTeX, 8 figure