Zero-phonon emission and magnetic polaron parameters in EuTe

A phonon structure in the photoluminescence of EuTe was discovered, with a well-defined zero-phonon emission line (ZPL). The ZPL redshifts linearly with the intensity of applied magnetic field, indicating spin relaxation of the photoexcited electron, and saturates at a lower magnetic field than the optical absorption bandgap, which is attributed to formation of magnetic polarons. From the difference in these saturation fields, the zero-field polaron binding energy and radius are estimated to be 43 meV and 3.2 (in units of the EuTe lattice parameter), respectively. (C) 2011 American Institute of Physics. [doi:10.1063/1.3634030]

Spin-Induced Optical Second Harmonic Generation in the Centrosymmetric Magnetic Semiconductors EuTe and EuSe

Spectroscopy of the centrosymmetric magnetic semiconductors EuTe and EuSe reveals spin-induced optical second harmonic generation (SHG) in the band gap vicinity at 2.1-2.4 eV. The magnetic field and temperature dependence demonstrates that the SHG arises from the bulk of the materials due to a novel type of nonlinear optical susceptibility caused by the magnetic dipole contribution combined with spontaneous or induced magnetization. This spin-induced susceptibility opens access to a wide class of centrosymmetric systems by harmonics generation spectroscopy.

Character of magnetic excitations in a quasi-one-dimensional antiferromagnet near the quantum critical points: Impact on magnetoacoustic properties

We report results of magnetoacoustic studies in the quantum spin-chain magnet NiCl(2)-4SC(NH(2))(2) (DTN) having a field-induced ordered antiferromagnetic (AF) phase. In the vicinity of the quantum critical points (QCPs) the acoustic c(33) mode manifests a pronounced softening accompanied by energy dissipation of the sound wave. The acoustic anomalies are traced up to T > T(N), where the thermodynamic properties are determined by fermionic magnetic excitations, the ""hallmark"" of one-dimensional (1D) spin chains. On the other hand, as established in earlier studies, the AF phase in DTN is governed by bosonic magnetic excitations. Our results suggest the presence of a crossover from a 1D fermionic to a three-dimensional bosonic character of the magnetic excitations in DTN in the vicinity of the QCPs.

Magnetic ordering of EuTe/PbTe multilayers determined by x-ray resonant diffraction

In this work we use resonant x-ray diffraction combined with polarization analysis of the diffracted beam to study the magnetic ordering in EuTe/PbTe multilayers. The presence of satellites at the (1/2 1/2 1/2) magnetic reflection of a 50 /repetition EuTe/PbTe superlattice demonstrated the existence of magnetic correlations among the alternated EuTe layers. The behavior of the satellites intensity as T increases toward the Neel temperature T(N) indicates that these correlations persist nearly up to T(N) and suggests the preferential decrease of the magnetic order parameter of external monolayers of each EuTe layer within the superlattice. (C) 2008 American Institute of Physics.

Modeling the dichroic absorption band edge and light-induced magnetism in EuTe

The band-edge optical absorption in EuTe is studied in the framework of the 5d conduction band atomic model. Both relaxed antiferromagnetic order, and ferromagnetic order induced by an external magnetic field, were analyzed. For ferromagnetic arrangement, the absorption is characterized by a hugely dichroic doublet of narrow lines. In the antiferromagnetic order, the spectrum is blueshifted, becomes much broader and weaker, and dichroism is suppressed. These results are in excellent qualitative and quantitative agreement with experimental observations on EuTe and EuSe published by us previously [Phys. Rev. B 72, 155337 (2005)]. The possibility of inducing ferromagnetic order by illuminating the material at photon energies resonant with the band gap is also discussed.

Finite-temperature Casimir effect for graphene

We adopt the Dirac model for quasiparticles in graphene and calculate the finite-temperature Casimir interaction between a suspended graphene layer and a parallel conducting surface. We find that at high temperature, the Casimir interaction in such system is just one-half of that for two ideal conductors separated by the same distance. In this limit, a single graphene layer behaves exactly as a Drude metal. In particular, the contribution of the TE mode is suppressed, while the contribution of the TM mode saturates at the ideal-metal value. The behavior of the Casimir interaction for intermediate temperatures and separations accessible in experiments is studied in some detail. We also find an interesting interplay between two fundamental constants of graphene physics: the fine-structure constant and the Fermi velocity.

Elementary Excitations at Magnetic Surfaces and Their Spin Dependence

The elementary surface excitations are studied by spin-polarized electron energy loss spectroscopy on a prototype oxide surface [an oxygen passivated Fe(001)-p(1 x 1) surface], where the various excitations coexist. For the first time, the surface phonons and magnons are measured simultaneously and are distinguished based on their different spin nature. The dispersion relation of all excitations is probed over the entire Brillouin zone. The different phonon modes observed in our experiment are described by means of ab initio calculations.

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.

Dynamical Conductivity at the Dirty Superconductor-Metal Quantum Phase Transition

We study the transport properties of ultrathin disordered nanowires in the neighborhood of the superconductor-metal quantum phase transition. To this end we combine numerical calculations with analytical strong-disorder renormalization group results. The quantum critical conductivity at zero temperature diverges logarithmically as a function of frequency. In the metallic phase, it obeys activated scaling associated with an infinite-randomness quantum critical point. We extend the scaling theory to higher dimensions and discuss implications for experiments.

Spin-charge coupling in quantum wires at zero magnetic field

We discuss an approximation for the dynamic charge response of nonlinear spin-1/2 Luttinger liquids in the limit of small momentum. Besides accounting for the broadening of the charge peak due to two-holon excitations, the nonlinearity of the dispersion gives rise to a two-spinon peak, which at zero temperature has an asymmetric line shape. At finite temperature the spin peak is broadened by diffusion. As an application, we discuss the density and temperature dependence of the Coulomb drag resistivity due to long-wavelength scattering between quantum wires.

Effect of long cyclic exchanges on the magnetic properties of bcc (3)He

Using path-integral Monte Carlo calculations, we have calculated ring exchange frequencies in the bcc phase of solid (3)He for densities from melting to the highest stable density. We evaluate 42 different exchange frequencies from two atoms up to eight atoms and find their Gruneisen exponents. Using a fit to these frequencies, we calculate the contribution to the Curie-Weiss temperature, Theta(CW), and upper critical magnetic field, B(c2), for even longer exchanges using a lattice Monte Carlo procedure. We find that contributions from seven-and eight-particle exchanges make a significant contribution to Theta(CW) and B(c2) at melting density. Comparison with experimental data is given.

Artificial molecular quantum rings under magnetic field influence

The ground states of a few electrons confined in two vertically coupled quantum rings in the presence of an external magnetic field are studied systematically within the current spin-density functional theory. Electron-electron interactions combined with inter-ring tunneling affect the electronic structure and the persistent current. For small values of the external magnetic field, we recover the zero magnetic field molecular quantum ring ground state configurations. Increasing the magnetic field many angular momentum, spin, and isospin transitions are predicted to occur in the ground state. We show that these transitions follow certain rules, which are governed by the parity of the number of electrons, the single-particle picture, Hund's rules, and many-body effects. (C) 2009 American Institute of Physics. [doi:10.1063/1.3223360]

Effect of Mn concentration and atomic structure on the magnetic properties of Ge thin films

This work reports on the magnetic properties of Ge(100-x)Mn(x) (x=0-24 at. %) films prepared by cosputtering a Ge+Mn target and submitted to cumulative thermal annealing treatments up to 500 degrees C. Both as-deposited and annealed films were investigated by means of compositional analysis, Raman scattering spectroscopy, magnetic force microscopy, superconducting quantum interference device magnetometry, and electrical resistivity measurements. All as-deposited films (either pure or containing Mn) exhibit an amorphous structure, which changes to crystalline as the annealing treatments are performed at increasing temperatures. In fact, the magnetic properties of the present Ge(100-x)Mn(x) films are very sensitive to the Mn content and whether their atomic structure is amorphous or crystalline. More specifically: whereas the amorphous Ge(100-x)Mn(x) films (with high x) present a characteristic spin glass behavior at low temperature; after crystallization, the films (with moderate Mn contents) are ferromagnetic at room temperature. Moreover, the magnetic behavior of the films scales with their Mn concentration and tends to be more pronounced after crystallization. Finally, the semiconducting behavior of the films, experienced by previous optical studies, was confirmed through electrical measurements, which also indicate the dependence of the resistivity with the atomic composition of the films. (C) 2010 American Institute of Physics. [doi: 10.1063/1.3520661]

Energy spectra for quantum wires and two-dimensional electron gases in magnetic fields with Rashba and Dresselhaus spin-orbit interactions

We introduce an analytical approximation scheme to diagonalize parabolically confined two-dimensional (2D) electron systems with both the Rashba and Dresselhaus spin-orbit interactions. The starting point of our perturbative expansion is a zeroth-order Hamiltonian for an electron confined in a quantum wire with an effective spin-orbit induced magnetic field along the wire, obtained by properly rotating the usual spin-orbit Hamiltonian. We find that the spin-orbit-related transverse coupling terms can be recast into two parts W and V, which couple crossing and noncrossing adjacent transverse modes, respectively. Interestingly, the zeroth-order Hamiltonian together with W can be solved exactly, as it maps onto the Jaynes-Cummings model of quantum optics. We treat the V coupling by performing a Schrieffer-Wolff transformation. This allows us to obtain an effective Hamiltonian to third order in the coupling strength k(R)l of V, which can be straightforwardly diagonalized via an additional unitary transformation. We also apply our approach to other types of effective parabolic confinement, e. g., 2D electrons in a perpendicular magnetic field. To demonstrate the usefulness of our approximate eigensolutions, we obtain analytical expressions for the nth Landau-level g(n) factors in the presence of both Rashba and Dresselhaus couplings. For small values of the bulk g factors, we find that spin-orbit effects cancel out entirely for particular values of the spin-orbit couplings. By solving simple transcendental equations we also obtain the band minima of a Rashba-coupled quantum wire as a function of an external magnetic field. These can be used to describe Shubnikov-de Haas oscillations. This procedure makes it easier to extract the strength of the spin-orbit interaction in these systems via proper fitting of the data.

Structural and magnetic properties of the oxyborate Co(5)Ti(O(2)BO(3))(2)

We present an extensive study of the oxyborate material Co(5)Ti(O(2)BO(3))(2) using x-ray, magnetic, and thermodynamic measurements. This material belongs to a family of oxyborates known as ludwigites which presents low-dimensional subunits in the form of three leg ladders in its structure. Differently from previously investigated ludwigites the present material does not show long-range magnetic order although it goes into a spin-glass state at low temperatures. The different techniques employed in this paper allow for a characterization of the structure, the nature of the low-energy excitations and the magnetic anisotropy of this system. Its unique magnetic behavior is discussed and compared with those of other magnetic ludwigites.