Dynamical Lorentz symmetry breaking in 3D and charge fractionalization

We analyze the breaking of Lorentz invariance in a 3D model of fermion fields self-coupled through four-fermion interactions. The low-energy limit of the theory contains various submodels which are similar to those used in the study of graphene or in the description of irrational charge fractionalization.

Crossover between distinct mechanisms of microwave photoresistance in bilayer systems

We report on temperature-dependent magnetoresistance measurements in balanced double quantum wells exposed to microwave irradiation for various frequencies. We have found that the resistance oscillations are described by the microwave-induced modification of electron distribution function limited by inelastic scattering (inelastic mechanism), up to a temperature of T*similar or equal to 4 K. With increasing temperature, a strong deviation of the oscillation amplitudes from the behavior predicted by this mechanism is observed, presumably indicating a crossover to another mechanism of microwave photoresistance, with similar frequency dependence. Our analysis shows that this deviation cannot be fully understood in terms of contribution from the mechanisms discussed in theory.

Magnetoresistance oscillations in multilayer systems: Triple quantum wells

Magnetoresistance of two-dimensional electron systems with several occupied subbands oscillates owing to periodic modulation of the probability of intersubband transitions by the quantizing magnetic field. In addition to previous investigations of these magnetointersubband (MIS) oscillations in two-subband systems, we report on both experimental and theoretical studies of such a phenomenon in three-subband systems realized in triple quantum wells. We show that the presence of more than two subbands leads to a qualitatively different MIS oscillation picture, described as a superposition of several oscillating contributions. Under a continuous microwave irradiation, the magnetoresistance of triple-well systems exhibits an interference of MIS oscillations and microwave-induced resistance oscillations. The theory explaining these phenomena is presented in the general form, valid for an arbitrary number of subbands. A comparison of theory and experiment allows us to extract temperature dependence of quantum lifetime of electrons and to confirm the applicability of the inelastic mechanism of microwave photoresistance for the description of magnetotransport in multilayer systems.

Emergent and reentrant fractional quantum Hall effect in trilayer systems in a tilted magnetic field

Magnetotransport measurements in triple-layer electron systems with high carrier density reveal fractional quantum Hall effect at total filling factors nu>2. With an in-plane magnetic field we are able to control the suppression of interlayer tunneling which causes a collapse of the integer quantum Hall plateaus at nu=2 and nu=4, and an emergence of fractional quantum Hall states with increasing tilt angles. The nu=4 state is replaced by three fractional quantum Hall states with denominator 3. The state nu=7/3 demonstrates reentrant behavior and the emergent state at nu=12/5 has a nonmonotonic behavior with increasing in-plane field. We attribute the observed fractional quantum Hall plateaus to correlated states in a trilayer system.

High-order fractional microwave-induced resistance oscillations in two-dimensional systems

We report on the observation of microwave-induced resistance oscillations associated with the fractional ratio n/m of the microwave irradiation frequency to the cyclotron frequency for m up to 8 in a two-dimensional electron system with high electron density. The features are quenched at high microwave frequencies independent of the fractional order m. We analyze temperature, power, and frequency dependencies of the magnetoresistance oscillations and discuss them in connection with existing theories.

Adiabatic intramolecular movements for water systems

An effective treatment of the intramolecular degrees of freedom is presented for water, where these modes are decoupled from the intermolecular ones, ""adiabatically"" allowing these coordinates to be positioned at their local minimum of the potential energy surface. We perform ab initio Monte Carlo simulations with the configurational energies obtained via density functional theory. We study a water dimer as a prototype system, and even in this simple case the intramolecular relaxations are very important to properly describe properties such as the dipole moment. We show that rigid simulations do not correctly sample the phase space, resulting in an average dipole moment smaller than the one obtained with the adiabatic model, which is closer to the experimental result. (c) 2008 American Institute of Physics.

Microwave-induced Hall resistance in bilayer electron systems

The influence of microwave irradiation on dissipative and Hall resistance in high-quality bilayer electron systems is investigated experimentally. We observe a deviation from odd symmetry under magnetic-field reversal in the microwave-induced Hall resistance boolean AND R(xy), whereas the dissipative resistance boolean AND R(xx) obeys even symmetry. Studies of Delta R(xy) as a function of the microwave electric field and polarization exhibit a strong and nontrivial power and polarization dependence. The obtained results are discussed in connection to existing theoretical models of microwave-induced photoconductivity.

Barrier distributions for weakly bound systems

We discuss the use of reduced fusion cross sections in the derivation of fusion barrier distributions. We show that the elimination of static effects associated with system sizes and optical potentials obtained by the recently introduced fusion functions can be extended to barrier distributions. This can be a useful tool for systematic studies of breakup coupling effects in fusion processes.

Elastic scattering and total reaction cross sections for the (8)B, (7)Be, and (6)Li+(12)C systems

Angular distributions for the elastic scattering of (8)B, (7)Be, and (6)Li on a (12)C target have been measured at E(lab) = 25.8, 18.8, and 12.3 MeV, respectively. The analyses of these angular distributions have been performed in terms of the optical model using Woods-Saxon and double-folding type potentials. The effect of breakup in the elastic scattering of (8)B + (12)C is investigated by performing coupled-channels calculations with the continuum discretized coupled-channel method and cluster-model folding potentials. Total reaction cross sections were deduced from the elastic-scattering analysis and compared with published data on elastic scattering of other weakly and tightly bound projectiles on (12)C, as a function of energy. With the exception of (4)He and (16)O, the data can be described using a universal function for the reduced cross sections.

Decoherence-free evolution of time-dependent superposition states of two-level systems and thermal effects

In this paper we detail some results advanced in a recent letter [Prado et al., Phys. Rev. Lett. 102, 073008 (2009).] showing how to engineer reservoirs for two-level systems at absolute zero by means of a time-dependent master equation leading to a nonstationary superposition equilibrium state. We also present a general recipe showing how to build nonadiabatic coherent evolutions of a fermionic system interacting with a bosonic mode and investigate the influence of thermal reservoirs at finite temperature on the fidelity of the protected superposition state. Our analytical results are supported by numerical analysis of the full Hamiltonian model.

Conservation laws, integrability, and transport in one-dimensional quantum systems

In integrable one-dimensional quantum systems an infinite set of local conserved quantities exists which can prevent a current from decaying completely. For cases like the spin current in the XXZ model at zero magnetic field or the charge current in the attractive Hubbard model at half filling, however, the current operator does not have overlap with any of the local conserved quantities. We show that in these situations transport at finite temperatures is dominated by a diffusive contribution with the Drude weight being either small or even zero. For the XXZ model we discuss in detail the relation between our results, the phenomenological theory of spin diffusion, and measurements of the spin-lattice relaxation rate in spin chain compounds. Furthermore, we study the Haldane-Shastry model where a conserved spin current exists.

Nonadiabatic Coherent Evolution of Two-Level Systems under Spontaneous Decay

In this Letter we extend current perspectives in engineering reservoirs by producing a time-dependent master equation leading to a nonstationary superposition equilibrium state that can be nonadiabatically controlled by the system-reservoir parameters. Working with an ion trapped inside a nonideal cavity, we first engineer effective interactions, which allow us to achieve two classes of decoherence-free evolution of superpositions of the ground and excited ionic levels: those with a time-dependent azimuthal or polar angle. As an application, we generalize the purpose of an earlier study [Phys. Rev. Lett. 96, 150403 (2006)], showing how to observe the geometric phases acquired by the protected nonstationary states even under nonadiabatic evolution.

Finite-size corrections to entanglement in quantum critical systems

We analyze the finite-size corrections to entanglement in quantum critical systems. By using conformal symmetry and density functional theory, we discuss the structure of the finite-size contributions to a general measure of ground state entanglement, which are ruled by the central charge of the underlying conformal field theory. More generally, we show that all conformal towers formed by an infinite number of excited states (as the size of the system L -> infinity) exhibit a unique pattern of entanglement, which differ only at leading order (1/L)(2). In this case, entanglement is also shown to obey a universal structure, given by the anomalous dimensions of the primary operators of the theory. As an illustration, we discuss the behavior of pairwise entanglement for the eigenspectrum of the spin-1/2 XXZ chain with an arbitrary length L for both periodic and twisted boundary conditions.

Universal and nonuniversal contributions to block-block entanglement in many-fermion systems

We calculate the entanglement entropy of blocks of size x embedded in a larger system of size L, by means of a combination of analytical and numerical techniques. The complete entanglement entropy in this case is a sum of three terms. One is a universal x- and L-dependent term, first predicted by Calabrese and Cardy, the second is a nonuniversal term arising from the thermodynamic limit, and the third is a finite size correction. We give an explicit expression for the second, nonuniversal, term for the one-dimensional Hubbard model, and numerically assess the importance of all three contributions by comparing to the entropy obtained from fully numerical diagonalization of the many-body Hamiltonian. We find that finite-size corrections are very small. The universal Calabrese-Cardy term is equally small for small blocks, but becomes larger for x > 1. In all investigated situations, however, the by far dominating contribution is the nonuniversal term stemming from the thermodynamic limit.

Entanglement in spatially inhomogeneous many-fermion systems

We investigate entanglement of strongly interacting fermions in spatially inhomogeneous environments. To quantify entanglement in the presence of spatial inhomogeneity, we propose a local-density approximation (LDA) to the entanglement entropy, and a nested LDA scheme to evaluate the entanglement entropy on inhomogeneous density profiles. These ideas are applied to models of electrons in superlattice structures with different modulation patterns, electrons in a metallic wire in the presence of impurities, and phase-separated states in harmonically confined many-fermion systems, such as electrons in quantum dots and atoms in optical traps. We find that the entanglement entropy of inhomogeneous systems is strikingly different from that of homogeneous systems.