Far-infrared spectroscopy of spin excitations and Dzyaloshinskii-Moriya interactions in a Shastry-Sutherland compound SrCu2_2(BO3_3)$_2

We have studied spin excitation spectra in the Shastry-Sutherland model compound SrCu$_2$(BO$_3$)$_2$ in magnetic fields using far-infrared Fourier spectroscopy. The transitions from the ground singlet state to the triplet state at 24 cm$^{-1}$ 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$_2$(BO$_3$)$_2$. One is $a$-polarized and creates an intra-dimer dynamic DM along the c axis. The other is $c$-polarized and creates an intra-dimer dynamic DM interaction, it is in the $(ab)$ plane and perpendicular to the dimer axis. Singlet levels at 21.5 and 28.6 cm$^{-1}$ 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 $d_1=0.7$ cm$^{-1}$ 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 $(ab)$ plane (perpendicular to the dimer axis) $d_2=1.8$ cm$^{-1}$, allowed by the buckling of CuBO$_3$ planes, couples the triplet state to the 28.6 cm$^{-1}$ 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 α\alpha-RuCl3_3

We report on terahertz spectroscopy of quantum spin dynamics in $\alpha$-RuCl$_3$, 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 $T_{s2}=62$K. With the onset of a long-range magnetic order at $T_N=6.5$K, spectral weight is transferred to a well-defined magnetic excitation at $\hbar \omega_1 = 2.48$meV, which is accompanied by a higher-energy band at $\hbar \omega_2 = 6.48$meV. Both excitations soften in magnetic field, signaling a quantum phase transition at $B_c=7$T where we find a broad continuum dominating the dynamical response. Above $B_c$, 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(Fe$_{0.92}$Co$_{0.08}$)$_2$As$_2$ shows a clear signature of the superconducting gap, but a simple $s$-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 $s$-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 $s_{\pm}$ 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