Exciton spectroscopy of hexagonal boron nitride using non-resonant x-ray Raman scattering

We report non-resonant x-ray Raman scattering (XRS) measurements from hexagonal boron nitride for transferred momentum from 2 to 9 $\mathrm{\AA}^{-1}$ along directions both in and out of the basal plane. A symmetry-based argument, together with real-space full multiple scattering calculations of the projected density of states in the spherical harmonics basis, reveals that a strong pre-edge feature is a dominantly $Y_{10}$-type Frenkel exciton with no other \textit{s}-, \textit{p}-, or \textit{d}- components. This conclusion is supported by a second, independent calculation of the \textbf{q}-dependent XRS cross-section based on the Bethe-Salpeter equation

The local electronic structure of alpha-Li3N

New theoretical and experimental investigation of the occupied and unoccupied local electronic density of states (DOS) are reported for alpha-Li3N. Band structure and density functional theory calculations confirm the absence of covalent bonding character. However, real-space full-multiple-scattering (RSFMS) calculations of the occupied local DOS finds less extreme nominal valences than have previously been proposed. Nonresonant inelastic x-ray scattering (NRIXS), RSFMS calculations, and calculations based on the Bethe-Salpeter equation are used to characterize the unoccupied electronic final states local to both the Li and N sites. There is good agreement between experiment and theory. Throughout the Li 1s near-edge region, both experiment and theory find strong similarities in the s- and p-type components of the unoccupied local final density of states projected onto an orbital angular momentum basis (l-DOS). An unexpected, significant correspondence exists between the near-edge spectra for the Li 1s and N 1s initial states. We argue that both spectra are sampling essentially the same final density of states due to the combination of long core-hole lifetimes, long photoelectron lifetimes, and the fact that orbital angular momentum is the same for all relevant initial states. Such considerations may be generically applicable for low atomic number compounds.Comment: 34 pages, 7 figures, 1 tabl

Magnetic field symmetry of pump currents of adiabatically driven mesoscopic structures

We examine the scattering properties of a slowly and periodically driven mesoscopic sample using the Floquet function approach. One might expect that at sufficiently low driving frequencies it is only the frozen scattering matrix which is important. The frozen scattering matrix reflects the properties of the sample at a given instant of time. Indeed many aspects of adiabatic scattering can be described in terms of the frozen scattering matrix. However, we demonstrate that the Floquet scattering matrix, to first order in the driving frequency, is determined by an additional matrix which reflects the fact that the scatterer is time-dependent. This low frequency irreducible part of the Floquet matrix has symmetry properties with respect to time and/or a magnetic field direction reversal opposite to that of the frozen scattering matrix. We investigate the quantum rectification properties of a pump which additionally is subject to an external dc voltage. We split the dc current flowing through the pump into several parts with well defined properties with respect to a magnetic field and/or an applied voltage inversion.Comment: 13 pages, 4 figure

Electronic structure of the strongly hybridized ferromagnet CeFe2

We report on results from high-energy spectroscopic measurements on CeFe2, a system of particular interest due to its anomalous ferromagnetism with an unusually low Curie temperature and small magnetization compared to the other rare earth-iron Laves phase compounds. Our experimental results indicate very strong hybridization of the Ce 4f states with the delocalized band states, mainly the Fe 3d states. In the interpretation and analysis of our measured spectra, we have made use of two different theoretical approaches: The first one is based on the Anderson impurity model, with surface contributions explicitly taken into account. The second method consists of band-structure calculations for bulk CeFe2. The analysis based on the Anderson impurity model gives calculated spectra in good agreement with the whole range of measured spectra, and reveals that the Ce 4f -- Fe 3d hybridization is considerably reduced at the surface, resulting in even stronger hybridization in the bulk than previously thought. The band-structure calculations are ab initio full-potential linear muffin-tin orbital calculations within the local-spin-density approximation of the density functional. The Ce 4f electrons were treated as itinerant band electrons. Interestingly, the Ce 4f partial density of states obtained from the band-structure calculations also agree well with the experimental spectra concerning both the 4f peak position and the 4f bandwidth, if the surface effects are properly taken into account. In addition, results, notably the partial spin magnetic moments, from the band-structure calculations are discussed in some detail and compared to experimental findings and earlier calculations.Comment: 10 pages, 8 figures, to appear in Phys. Rev. B in December 200

Multiple Core-Hole Coherence in X-Ray Four-Wave-Mixing Spectroscopies

Correlation-function expressions are derived for the coherent nonlinear response of molecules to three resonant ultrafast pulses in the x-ray regime. The ability to create two-core-hole states with controlled attosecond timing in four-wave-mixing and pump probe techniques should open up new windows into the response of valence electrons, which are not available from incoherent x-ray Raman and fluorescence techniques. Closed expressions for the necessary four-point correlation functions are derived for the electron-boson model by using the second order cumulant expansion to describe the fluctuating potentials. The information obtained from multidimensional nonlinear techniques could be used to test and refine this model, and establish an anharmonic oscillator picture for electronic excitations

Model Exact Low-Lying States and Spin Dynamics in Ferric Wheels; Fe6_6 to Fe12_{12}

Using an efficient numerical scheme that exploits spatial symmetries and spin-parity, we have obtained the exact low-lying eigenstates of exchange Hamiltonians for ferric wheels up to Fe$_{12}$. The largest calculation involves the Fe$_{12}$ ring which spans a Hilbert space dimension of about 145 million for M$_s$=0 subspace. Our calculated gaps from the singlet ground state to the excited triplet state agrees well with the experimentally measured values. Study of the static structure factor shows that the ground state is spontaneously dimerized for ferric wheels. Spin states of ferric wheels can be viewed as quantized states of a rigid rotor with the gap between the ground and the first excited state defining the inverse of moment of inertia. We have studied the quantum dynamics of Fe$_{10}$ as a representative of ferric wheels. We use the low-lying states of Fe$_{10}$ to solve exactly the time-dependent Schr\"odinger equation and find the magnetization of the molecule in the presence of an alternating magnetic field at zero temperature. We observe a nontrivial oscillation of magnetization which is dependent on the amplitude of the {\it ac} field. We have also studied the torque response of Fe$_{12}$ as a function of magnetic field, which clearly shows spin-state crossover.Comment: Revtex, 24 pages, 8 eps figure

Floquet states and persistent currents transitions in a mesoscopic ring

We consider the effect of an oscillating potential on the single-particle spectrum and the time-averaged persistent current of a one-dimensional phase-coherent mesoscopic ring with a magnetic flux. We show that in a ring with an even number of spinless electrons the oscillating potential has a strong effect on the persistent current when the excited side bands are close to the eigen levels of a pure ring. Resonant enhancement of side bands of the Floquet state generates a sign change of the persistent current.Comment: 2 figure

Radio continuum observations of Class I protostellar disks in Taurus: constraining the greybody tail at centimetre wavelengths

We present deep 1.8 cm (16 GHz) radio continuum imaging of seven young stellar objects in the Taurus molecular cloud. These objects have previously been extensively studied in the sub-mm to NIR range and their SEDs modelled to provide reliable physical and geometrical parametres.We use this new data to constrain the properties of the long-wavelength tail of the greybody spectrum, which is expected to be dominated by emission from large dust grains in the protostellar disk. We find spectra consistent with the opacity indices expected for such a population, with an average opacity index of beta = 0.26+/-0.22 indicating grain growth within the disks. We use spectra fitted jointly to radio and sub-mm data to separate the contributions from thermal dust and radio emission at 1.8 cm and derive disk masses directly from the cm-wave dust contribution. We find that disk masses derived from these flux densities under assumptions consistent with the literature are systematically higher than those calculated from sub-mm data, and meet the criteria for giant planet formation in a number of cases.Comment: submitted MNRA

Solvable three-state model of a driven double-well potential and coherent destruction of tunneling

A simple model for a particle in a double well is derived from discretizing its configuration space. The model contains as many free parameters as the original system and it respects all the existing symmetries. In the presence of an external periodic force both the continuous system and the discrete model are shown to possess a generalized time-reversal symmetry in addition to the known generalized parity. The impact of the driving force on the spectrum of the Floquet operator is studied. In particular, the occurrence of degenerate quasienergies causing coherent destruction of tunneling is discussed—to a large extent analytically—for arbitrary driving frequencies and barrier heights

Tunneling control and localization for Bose-Einstein condensates in a frequency modulated optical lattice

The similarity between matter waves in periodic potential and solid-state physics processes has triggered the interest in quantum simulation using Bose-Fermi ultracold gases in optical lattices. The present work evidences the similarity between electrons moving under the application of oscillating electromagnetic fields and matter waves experiencing an optical lattice modulated by a frequency difference, equivalent to a spatially shaken periodic potential. We demonstrate that the tunneling properties of a Bose-Einstein condensate in shaken periodic potentials can be precisely controlled. We take additional crucial steps towards future applications of this method by proving that the strong shaking of the optical lattice preserves the coherence of the matter wavefunction and that the shaking parameters can be changed adiabatically, even in the presence of interactions. We induce reversibly the quantum phase transition to the Mott insulator in a driven periodic potential.Comment: Laser Physics (in press