86 research outputs found
Far-infrared spectroscopy of spin excitations and Dzyaloshinskii-Moriya interactions in a Shastry-Sutherland compound SrCu(BO)$_2
We have studied spin excitation spectra in the Shastry-Sutherland model
compound SrCu(BO) in magnetic fields using far-infrared Fourier
spectroscopy. The transitions from the ground singlet state to the triplet
state at 24 cm and to several bound triplet states are induced by the
electric field component of the far-infrared light. To explain the light
absorption in the spin system we invoke a dynamic Dzyaloshinskii-Moriya (DM)
mechanism where light couples to a phonon mode, allowing the DM interaction.
Two optical phonons couple light to the singlet to triplet transition in
SrCu(BO). One is -polarized and creates an intra-dimer dynamic
DM along the c axis. The other is -polarized and creates an intra-dimer
dynamic DM interaction, it is in the plane and perpendicular to the
dimer axis. Singlet levels at 21.5 and 28.6 cm anti-cross with the first
triplet as is seen in far-infrared spectra. We used a cluster of two dimers
with a periodic boundary condition to perform a model calculation with scaled
intra- and inter-dimer exchange interactions. Two static DM interactions are
sufficient to describe the observed triplet state spectra. The static
inter-dimer DM in the c-direction cm splits the triplet state
sub-levels in zero field [C\'{e}pas et al., Phys. Rev. Lett. \textbf{87},
167205 (2001)]. The static intra-dimer DM in the plane (perpendicular to
the dimer axis) cm, allowed by the buckling of CuBO
planes, couples the triplet state to the 28.6 cm singlet as is seen from
the avoided crossing.Comment: 12 pages with 7 figures, some references correcte
Unidirectional terahertz light absorption in the pyroelectric ferrimagnet CaBaCo4O7
Spin excitations were studied by absorption spectroscopy in CaBaCo4O7 which
is a type-I multiferroic compound with the largest magnetic-order induced
ferroelectric polarization ({\Delta}P=17mC/m2) reported, so far. We observed
two optical magnon branches: a solely electric dipole allowed one and a mixed
magnetoelectric resonance. The entangled magnetization and polarization
dynamics of the magnetoelectric resonance gives rise to unidirectional light
absorption, i.e. that magnon mode absorbs the electromagnetic radiation for one
propagation direction but not for the opposite direction. Our systematic study
of the magnetic field and temperature dependence of magnon modes provides
information about the energies and symmetries of spin excitations, which is
required to develop a microscopic spin model of CaBaCo4O7.Comment: 5 pages, 4 figure
Are Foreign-Owned Firms Different? Comparison of Employment Volatility and Elasticity of Labour Demand
This paper analyses differences in employment volatility in foreign-owned and domestic companies using firm-level data from 24 European countries. The presence of foreign-owned companies may lead to higher employment volatility because subsidiaries of multinational companies react more sensitively to changes in labour demand in host countries or because they are more exposed to external shocks. We assess the conditional employment volatility of firms with foreign and domestic owners using propensity score matching and find that it is higher in foreign-owned firms in about half of the countries that our study covers. In addition, we explore how and why labour demand elasticity differs between these two groups of companies. Our estimations indicate that labour demand can be either more or less elastic in subsidiaries of foreign-owned multinationals than in domestic enterprises, depending on the institutional environments of their home and host countries. When FDI originates from a region with a more flexible institutional environment then the elasticity of labour demand is smaller in absolute value in foreign-owned firms. In the opposite case the elasticity of labour demand is higher. A potential explanation for this empirical finding is that it is easier for multinational companies to substitute between factor inputs and therefore they have more flexibility than domestic firms in choosing which channels of adjustment to use
Electron Spin Resonance of SrCu2(BO3)2 at High Magnetic Field
We calculate the electron spin resonance (ESR) spectra of the
quasi-two-dimensional dimer spin liquid SrCu2(BO3)2 as a function of magnetic
field B. Using the standard Lanczos method, we solve a Shastry-Sutherland
Hamiltonian with additional Dzyaloshinsky-Moriya (DM) terms which are crucial
to explain different qualitative aspects of the ESR spectra. In particular, a
nearest-neighbor DM interaction with a non-zero D_z component is required to
explain the low frequency ESR lines for B || c. This suggests that crystal
symmetry is lowered at low temperatures due to a structural phase transition.Comment: 4 pages, 4 b&w figure
Theory of phonon-assisted "forbidden" optical transitions in spin-gapped systems
We consider the absorption of light with emission of one S(tot)=1 magnetic
excitation in systems with a spin gap induced by quantum fluctuations. We argue
that an electric dipole transition is allowed on the condition that a virtual
phonon instantaneously breaks the inversion symmetry. We derive an effective
operator for the transition and argue that the proposed theory explains the
polarized experiments in CuGeO(3) and SrCu(2)[BO(3)](2).Comment: 9 pages, 4 figure
Optical spectroscopy of superconducting Ba{0.55}K{0.45}Fe2As2: evidence for strong coupling to low energy bosons
Optical spectroscopy on single crystals of the new iron arsenide
superconductor Ba{0.55}K{0.45}Fe2As2 shows that the infrared spectrum consists
of two major components: a strong metallic Drude band and a well separated mid
infrared absorption centered at 0.7 eV. It is difficult to separate the two
components unambiguously but several fits of Lorentzian peaks suggest a model
with a Drude peak having a plasma frequency of 1.8 to 2.1 eV and a midinfrared
peak with a plasma frequency of 2.5 eV. In contrast to the cuprate
superconductors the scattering rate obtained from the extended Drude model
saturates at 150 meV as compared to 500 meV for a typical cuprate. Detailed
analysis of the frequency dependent scattering rate shows that the charge
carriers interact with broad bosonic spectrum with a peak at 25 meV and a
coupling constant lambda =approx 2 at low temperature. As the temperature
increases this coupling weakens to lambda=0.6 at ambient temperature. This
suggests a bosonic spectrum that is similar to what is seen in the lower Tc
cuprates.Comment: 4 pages, 4 figure
Magnetic Excitations and Continuum of a Field-Induced Quantum Spin Liquid in -RuCl
We report on terahertz spectroscopy of quantum spin dynamics in
-RuCl, a system proximate to the Kitaev honeycomb model, as a
function of temperature and magnetic field. An extended magnetic continuum
develops below the structural phase transition at K. With the onset
of a long-range magnetic order at K, spectral weight is transferred to
a well-defined magnetic excitation at meV, which is
accompanied by a higher-energy band at meV. Both
excitations soften in magnetic field, signaling a quantum phase transition at
T where we find a broad continuum dominating the dynamical response.
Above , the long-range order is suppressed, and on top of the continuum,
various emergent magnetic excitations evolve. These excitations follow clear
selection rules and exhibit distinct field dependencies, characterizing the
dynamical properties of the field-induced quantum spin liquid
Optical signature of sub-gap absorption in the superconducting state of Ba(Fe,Co)2As2
The optical conductivity of Ba(FeCo)As shows a
clear signature of the superconducting gap, but a simple -wave description
fails in accounting for the low frequency response. This task is achieved by
introducing an extra Drude peak in the superconducting state representing
sub-gap absorption, other than thermally broken pairs. This extra peak and the
coexisting -wave response respect the total sum rule indicating a common
origin for the carriers. We discuss the possible origins for this absorption as
(i) quasiparticles due to pair-breaking from interband impurity scattering in a
two band gap symmetry model, which includes (ii) the possible
existence of impurity levels within an isotropic gap model; or (iii) an
indication that one of the bands is highly anisotropic.Comment: 5 pages, 4 figure
Spin-stretching modes in anisotropic magnets: spin-wave excitations in the multiferroic Ba2CoGe2O7
We studied spin excitations of the multiferroic Ba2CoGe2O7 in high magnetic
fields up to 33 T. In the electron spin resonance and far infrared absorption
spectra we found several spin excitations beyond the two conventional magnon
modes expected for such a two-sublattice antiferromagnet. We show that a
multi-boson spin-wave theory can capture these unconventional modes, that
include spin-stretching modes associated with an oscillating magnetic dipole
(or only quadrupole) moment. The lack of the inversion symmetry allows these
modes to become electric dipole active. We expect that the spin-stretching
modes can be generally observed in inelastic neutron scattering and light
absorption experiments in a broad class of ordered S > 1/2 spin systems with
strong single-ion anisotropy and/or non-centrosymmetric lattice structure.Comment: 5+4 pages, 3 figures, supplement added, manuscript revise
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