Extension of random-phase approximation preserving energy weighted sum rules: an application to a 3-level Lipkin model

A limitation common to all extensions of random-phase approximation including only particle-hole configurations is that they violate to some extent the energy weighted sum rules. Considering one such extension, the improved RPA (IRPA), already used to study the electronic properties of metallic clusters, we show how it can be generalized in order to eliminate this drawback. This is achieved by enlarging the configuration space, including also elementary excitations corresponding to the annihilation of a particle (hole) and the creation of another particle (hole) on the correlated ground state. The approach is tested within a solvable 3-level model.Comment: 2 figure

Effect of the Pauli principle on photoelectron spin transport in p+p^+ GaAs

In p+ GaAs thin films, the effect of photoelectron degeneracy on spin transport is investigated theoretically and experimentally by imaging the spin polarization profile as a function of distance from a tightly-focussed light excitation spot. Under degeneracy of the electron gas (high concentration, low temperature), a dip at the center of the polarization profile appears with a polarization maximum at a distance of about $2 \; \mu m$ from the center. This counterintuitive result reveals that photoelectron diffusion depends on spin, as a direct consequence of the Pauli principle. This causes a concentration dependence of the spin stiffness while the spin dependence of the mobility is found to be weak in doped material. The various effects which can modify spin transport in a degenerate electron gas under local laser excitation are considered. A comparison of the data with a numerical solution of the coupled diffusion equations reveals that ambipolar coupling with holes increases the steady-state photo-electron density at the excitation spot and therefore the amplitude of the degeneracy-induced polarization dip. Thermoelectric currrents are predicted to depend on spin under degeneracy (spin Soret currents), but these currents are negligible except at very high excitation power where they play a relatively small role. Coulomb spin drag and bandgap renormalization are negligible due to electrostatic screening by the hole gas

A proposal for a scalable universal bosonic simulator using individually trapped ions

We describe a possible architecture to implement a universal bosonic simulator (UBS) using trapped ions. Single ions are confined in individual traps, and their motional states represent the bosonic modes. Single-mode linear operators, nonlinear phase-shifts, and linear beam splitters can be realized by precisely controlling the trapping potentials. All the processes in a bosonic simulation, except the initialization and the readout, can be conducted beyond the Lamb-Dicke regime. Aspects of our proposal can also be applied to split adiabatically a pair of ions in a single trap

An exactly solvable model of a superconducting to rotational phase transition

We consider a many-fermion model which exhibits a transition from a superconducting to a rotational phase with variation of a parameter in its Hamiltonian. The model has analytical solutions in its two limits due to the presence of dynamical symmetries. However, the symmetries are basically incompatible with one another; no simple solution exists in intermediate situations. Exact (numerical) solutions are possible and enable one to study the behavior of competing but incompatible symmetries and the phase transitions that result in a semirealistic situation. The results are remarkably simple and shed light on the nature of phase transitions.Comment: 11 pages including 1 figur

Interregional migration efficiency in adjusting regional labour markets in Chile

The objective of the article is to re-review the interregional migration process in Chile according to Aroca & Hewings (2002), using up-dated data of 1992 and 2002 from CENSO of population and housing in a probability model probit. Additionally, analyse the efficiency of the interregional migration process in terms of equalising regional salaries and unemployment rates. The results show that signal labour markets are less important for explaining the probability to migrate related to others regional characteristics and the migration process is inefficient to adjust regional markets. The results are consistent with previous findings.Migración Intrarregional; Empleo; Desempleo; Salarios; Utilidad

Spin dependent photoelectron tunnelling from GaAs into magnetic Cobalt

The spin dependence of the photoelectron tunnel current from free standing GaAs films into out-of- plane magnetized Cobalt films is demonstrated. The measured spin asymmetry (A) resulting from a change in light helicity, reaches +/- 6% around zero applied tunnel bias and drops to +/- 2% at a bias of -1.6 V applied to the GaAs. This decrease is a result of the drop in the photoelectron spin polarization that results from a reduction in the GaAs surface recombination velocity. The sign of A changes with that of the Cobalt magnetization direction. In contrast, on a (nonmagnetic) Gold film A ~ 0%

Analytically solvable potentials for γ\gamma-unstable nuclei

An analytical solution of the collective Bohr equation with a Coulomb-like and a Kratzer-like $\gamma-$unstable potential in quadrupole deformation space is presented. Eigenvalues and eigenfunctions are given in closed form and transition rates are calculated for the two cases. The corresponding SO(2,1)$\times$SO(5) algebraic structure is discussed.Comment: 9 pages, 4 figures in one .ps fil

Experimental Determination of the Lorenz Number in Cu0.01Bi2Te2.7Se0.3 and Bi0.88Sb0.12

Nanostructuring has been shown to be an effective approach to reduce the lattice thermal conductivity and improve the thermoelectric figure of merit. Because the experimentally measured thermal conductivity includes contributions from both carriers and phonons, separating out the phonon contribution has been difficult and is mostly based on estimating the electronic contributions using the Wiedemann-Franz law. In this paper, an experimental method to directly measure electronic contributions to the thermal conductivity is presented and applied to Cu0.01Bi2Te2.7Se0.3, [Cu0.01Bi2Te2.7Se0.3]0.98Ni0.02, and Bi0.88Sb0.12. By measuring the thermal conductivity under magnetic field, electronic contributions to thermal conductivity can be extracted, leading to knowledge of the Lorenz number in thermoelectric materials

A machine learning potential for hexagonal boron nitride applied to thermally and mechanically induced rippling

We introduce an interatomic potential for hexagonal boron nitride (hBN) based on the Gaussian approximation potential (GAP) machine learning methodology. The potential is based on a training set of configurations collected from density functional theory (DFT) simulations and is capable of treating bulk and multilayer hBN as well as nanotubes of arbitrary chirality. The developed force field faithfully reproduces the potential energy surface predicted by DFT while improving the efficiency by several orders of magnitude. We test our potential by comparing formation energies, geometrical properties, phonon dispersion spectra, and mechanical properties with respect to benchmark DFT calculations and experiments. In addition, we use our model and a recently developed graphene-GAP to analyze and compare thermally and mechanically induced rippling in large scale two-dimensional (2D) hBN and graphene. Both materials show almost identical scaling behavior with an exponent of η ≈ 0.85 for the height fluctuations agreeing well with the theory of flexible membranes. On the basis of its lower resistance to bending, however, hBN experiences slightly larger out-of-plane deviations both at zero and finite applied external strain. Upon compression, a phase transition from incoherent ripple motion to soliton-ripples is observed for both materials. Our potential is freely available online at [http://www.libatoms.org]

Collective states of the odd-mass nuclei within the framework of the Interacting Vector Boson Model

A supersymmetric extension of the dynamical symmetry group $Sp^{B}(12,R)$ of the Interacting Vector Boson Model (IVBM), to the orthosymplectic group $OSp(2\Omega/12,R)$ is developed in order to incorporate fermion degrees of freedom into the nuclear dynamics and to encompass the treatment of odd mass nuclei. The bosonic sector of the supergroup is used to describe the complex collective spectra of the neighboring even-even nuclei and is considered as a core structure of the odd nucleus. The fermionic sector is represented by the fermion spin group $SO^{F}(2\Omega)\supset SU^{F}(2)$. The so obtained, new exactly solvable limiting case is applied for the description of the nuclear collective spectra of odd mass nuclei. The theoretical predictions for different collective bands in three odd mass nuclei, namely $^{157}Gd$, $^{173}Yb$ and $^{163}Dy$ from rare earth region are compared with the experiment. The $B(E2)$ transition probabilities for the $^{157}Gd$ and $^{163}Dy$ between the states of the ground band are also studied. The important role of the symplectic structure of the model for the proper reproduction of the $B(E2)$ behavior is revealed. The obtained results reveal the applicability of the models extension.Comment: 18 pages, 8 figure