Nonquadratic gauge fixing and global gauge invariance in the effective action

The possibility of having a gauge fixing term in the effective Lagrangian that is not a quadratic expression has been explored in spin-two theories so as to have a propagator that is both traceless and transverse. We first show how this same approach can be used in spontaneously broken gauge theories as an alternate to the 't Hooft gauge fixing which avoids terms quadratic in the scalar fields. This ""nonquadratic"" gauge fixing in the effective action results in two complex fermionic and one real bosonic ghost field. A global gauge invariance involving a fermionic gauge parameter, analogous to the usual Becchi-Rouet-Stora-Tyutin invariance, is present in this effective action.

Multimode Vibrational Couplings in Resonant Positron Annihilation

The mechanisms for multimode vibrational couplings in resonant positron annihilation are not well understood. We show that these resonances can arise from positron-induced distortions of the potential energy surface (target response to the positron field). Though these distortions can transfer energy into single- and multiquantum vibrations, they have so far been disregarded as a pathway to resonant annihilation. We also compare the existing annihilation theories and show that the currently accepted model can be cast as a special case of the Feshbach annihilation theory.

Supergraph techniques for D=3, N=1 broken supersymmetric theories

We enlarge the usual D = 3 N = 1 supergraph techniques to include the case of (explicitly or spontaneously) broken supersymmetric gauge theories. To illustrate the utility of these techniques, we calculate the two-loop effective potential of the SQED(3) by using the tadpole and the vacuum bubble methods. In these methods, to investigate the possibility of supersymmetry breaking, the superfields must be shifted by theta(alpha) dependent classical superfields (vacuum expectation values), what implies in the explicit breakdown of supersymmetry in the intermediate steps of the calculation. Nevertheless, after studying the minimum of the resulting effective potential, we find that supersymmetry is conserved, while gauge symmetry is dynamically broken, with a mass generated for the gauge superfield.

Perturbations of black p-branes

We consider black p-brane solutions of the low-energy string action, computing scalar perturbations. Using standard methods, we derive the wave equations obeyed by the perturbations and treat them analytically and numerically. We have found that tensorial perturbations obtained via a gauge-invariant formalism leads to the same results as scalar perturbations. No instability has been found. Asymptotically, these solutions typically reduce to a AdSd((p+2)) x Sd((8-p)) space which, in the framework of Maldacena's conjecture, can be regarded as a gravitational dual to a conformal field theory defined in a (p+1)-dimensional flat space-time. The results presented open the possibility of a better understanding the AdS/CFT correspondence, as originally formulated in terms of the relation among brane structures and gauge theories.

Aetherlike Lorentz-breaking actions

We show that CPT-even aetherlike Lorentz-breaking actions, for the scalar and electromagnetic fields, are generated via their appropriate Lorentz-breaking coupling to spinor fields, in three, four, and five space-time dimensions. Besides, we also show that aetherlike terms for the spinor field can be generated as a consequence of the same couplings. We discuss the dispersion relations in the theories with aetherlike Lorentz-breaking terms and find the tree-level effective (Breit) potential for fermion scattering and the one-loop effective potential corresponding to the action of the scalar field.

Effective actions at finite temperature

This is a more detailed version of our recent paper where we proposed, from first principles, a direct method for evaluating the exact fermion propagator in the presence of a general background field at finite temperature. This can, in turn, be used to determine the finite temperature effective action for the system. As applications, we discuss the complete one loop finite temperature effective actions for 0+1 dimensional QED as well as for the Schwinger model in detail. These effective actions, which are derived in the real time (closed time path) formalism, generate systematically all the Feynman amplitudes calculated in thermal perturbation theory and also show that the retarded (advanced) amplitudes vanish in these theories. Various other aspects of the problem are also discussed in detail.

Field theory model for dark matter and dark energy in interaction

We propose a field theory model for dark energy and dark matter in interaction. Comparing the classical solutions of the field equations with the observations of the CMB shift parameter, baryonic acoustic oscillations, lookback time, and the Gold supernovae sample, we observe a possible interaction between dark sectors with energy decay from dark energy into dark matter. The observed interaction provides an alleviation to the coincidence problem.

A Goldstone theorem in thermal relativistic quantum field theory

We prove a Goldstone theorem in thermal relativistic quantum field theory, which relates spontaneous symmetry breaking to the rate of spacelike decay of the two-point function. The critical rate of fall-off coincides with that of the massless free scalar field theory. Related results and open problems are briefly discussed. (C) 2011 American Institute of Physics. [doi:10.1063/1.3526961]

Magnetic-field-induced transition in a wide parabolic well superimposed with a superlattice

We study a Al(x)Ga(x-1)As parabolic quantum well (PQW) with GaAs/Al(x)Ga(x-1)As square superlattice. The magnetotransport in PQW with intentionally disordered short-period superlattice reveals a surprising transition from electrons distribution over whole parabolic well to independent-layer states with unequal density. The transition occurs in the perpendicular magnetic field at Landau filling factor v approximate to 3 and is signaled by the appearance of the strong and developing fractional quantum Hall (FQH) states and by the enhanced slope of the Hall resistance. We attribute the transition to the possible electron localization in the x-y plane inside the lateral wells, and formation of the FQH states in the central well of the superlattice, driven by electron-electron interaction.

Resistively detected NMR of the nu=1 quantum Hall state: A tilted magnetic field study

Previous resistively detected NMR (RDNMR) studies on the nu approximate to 1 quantum Hall state have reported a ""dispersionlike"" line shape and extremely short nuclear-spin-lattice relaxation times, observations which have been attributed to the formation of a skyrme lattice. Here we examine the evolution of the RDNMR line shape and nuclear-spin relaxation for Zeeman: Coulomb energy ratios ranging from 0.012 to 0.036. According to theory, suppression of the skyrme crystal, along with the associated Goldstone mode nuclear-spin-relaxation mechanism, is expected at the upper end of this range. However, we find that the anomalous line shape persists at high Zeeman energy, and only a modest decrease in the RDNMR-detected nuclear-spin-relaxation rate is observed.

In-field Mossbauer study of disordered surface spins in core/shell ferrite nanoparticles

Magnetization and Mossbauer spectroscopy measurements are performed at low temperature under high field, on nanoparticles with a nickel ferrite core and a maghemite shell. These nanoparticles present finite size and surface effects, together with exchange anisotropy. High field magnetization brings the evidences of a monodomain ordered core and surface spins freezing in disorder at low temperature. Mossbauer spectra at 4.2 K present an extra contribution from the disordered surface which is field dependent. Field and size dependences of this latter show a progressive spin alignment along the ferrite core which is size dependent. The weak surface pinning condition of the nanoparticles confirms that the spin disorder is localized in the external shell. The underfield decrease in the mean canting angle in the superficial shell is then directly related to the unidirectional exchange anisotropy through the interface between the ordered core and the disordered shell. The obtained anisotropy field H(Ea) scales as the inverse of the nanoparticle diameter, validating its interfacial origin. The associated anisotropy constant K(Ea) equals 2.5 x 10(-4) J/m(2). (C) 2009 American Institute qf Physics. [doi: 10.1063/1.3245326]

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.

Magnetoconfined levels in a parabolic quantum dot: An analytical solution of a three-dimensional Fock-Darwin problem in a tilted magnetic field

The energy spectrum of an electron confined in a quantum dot (QD) with a three-dimensional anisotropic parabolic potential in a tilted magnetic field was found analytically. The theory describes exactly the mixing of in-plane and out-of-plane motions of an electron caused by a tilted magnetic field, which could be seen, for example, in the level anticrossing. For charged QDs in a tilted magnetic field we predict three strong resonant lines in the far-infrared-absorption spectra.

Critical Properties at the Field-Induced Bose-Einstein Condensation in NiCl(2)-4SC(NH(2))(2)

We report new magnetization measurements on the spin-gap compound NiCl(2)-4SC(NH(2))(2) at the low-field boundary of the magnetic field-induced ordering. The critical density of the magnetization is analyzed in terms of a Bose-Einstein condensation of bosonic quasiparticles. The analysis of the magnetization at the transition leads to the conclusion for the preservation of the U(1) symmetry, as required for Bose-Einstein condensation. The experimental data are well described by quantum Monte Carlo simulations.

Interlayer interference in double wells in a tilted magnetic field

Magnetotransport measurements on bilayer electron systems reveal repeated reentrance of the resistance minima at filling factors nu=4N+1 and nu=4N+3, where N is the Landau index number, in the tilted magnetic field. At high filling factors, the Shubnikov-de Haas oscillations exhibit beating effects at certain tilt angles. We attribute such behavior to oscillations of the tunneling gap due to Aharonov-Bohm interference effect between cyclotron orbits in different layers. The interplay between quantum and quasiclassical regimes is established.