We report on the low-frequency optical excitations in the multiferroic ground state of polycrystalline FeCr2S4 in the frequency range 0.3-3 THz and their changes upon applying external magnetic fields up to 7 T. In the ground state below the orbital-ordering temperature T-OO = 9 K we observe the appearance of several new modes. By applying the external magnetic field parallel and perpendicular to the propagation direction of the THz radiation, we can identify the strongest absorptions to be of predominantly electric-dipole origin. We discuss these modes as the low-energy electronic excitations of the Fe2+ ions (3d(6), S = 2) in an tetrahedral S2- environment. The eigenfrequencies and relative intensities of these absorption lines are satisfactorily reproduced by our calculation assuming an effective exchange field of 12.8 cm(-1) at the Fe2+-ions sites. The direction of the exchange field is found to be slightly tilted out of the ab plane. With our approach we can also describe previously reported results from Mossbauer studies and the order of magnitude of the electric polarization induced by orbital and noncollinear spin ordering.

We report on the temperature evolution of the infrared-active optical phonons in FeCr2S4 investigated by Fourier-transform infrared spectroscopy. The eigen frequencies of the four triply degenerate infrared-active T-1u phonons of the room-temperature cubic spinel structure shift when entering into the ferrimagnetically ordered state below T-C = 165 K indicating strong spin-phonon coupling as reported earlier. A new mode at 200 cm(-1) emerges below a temperature T* similar or equal to 115 K and a splitting of the lowest-lying cubic phonon mode at about 120 cm(-1) appears below the temperature T-M approximate to 60 K associated with the onset of an incommensurate modulation of the magnetic structure. At the transition to the orbitally ordered and ferroelectric ground state at T-OO = 9 K two more modes emerge at 146 and 253 cm(-1) in very good agreement with the eigenfrequencies of two of the cubic Raman-active phonon modes reported by Choi et al. [J. Phys. Condens. Matter 19, 145260 (2007)]. These new modes are interpreted as signatures of a symmetry lowering with a loss of inversion symmetry to induce the multiferroic orbitally ordered ground state.

We report a high-resolution terahertz spectroscopic study of quantum spin dynamics in the antiferromagnetic Heisenberg-Ising spin-chain compound BaCo_{2}V_{2}O_{8} as a function of temperature and longitudinal magnetic field. Confined spinon excitations are observed in an antiferromagnetic phase below T_{N}≃5.5  K. In a field-induced gapless phase above B_{c}=3.8  T, we identify many-body string excitations as well as low-energy fractional psinon or antipsinon excitations by comparing to Bethe ansatz calculations. In the vicinity of B_{c}, the high-energy string excitations are found to have a dominant contribution to the spin dynamics as compared with the fractional excitations.

We report the first determination of the in-plane complex optical conductivity of 1111 high-T_{c} superconducting iron oxypnictide single crystals PrFeAs(O,F) and thin films SmFeAs(O,F) by means of conventional and microfocused infrared spectroscopy, ellipsometry, and time-domain THz transmission spectroscopy. A strong itinerant contribution is found to exhibit a dramatic difference in coherence between the crystal and the film. Using extensive temperature-dependent measurements of THz transmission, we identify a previously undetected 2.5-meV collective mode in the optical conductivity of SmFeAs(O,F), which is strongly suppressed at T_{c} and experiences an anomalous T-linear softening and narrowing below T^{*}≈110  K≫T_{c}. The suppression of the infrared absorption in the superconducting state reveals a large optical superconducting gap with a similar gap ratio 2Δ/k_{B}T_{c}≈7 in both materials, indicating strong pairing.

We report on terahertz time-domain spectroscopy on superconducting and metallic iron chalcogenides Rb_0.75Fe_1.6Se_2-zS_z. The superconducting transition is reduced from T_c= 32 K (z=0) to 22 K (z=1.0), and finally suppressed (z=1.4) by isoelectronic substitution of Se with S. Dielectric constant and optical conductivity exhibit a metal-to-insulator transition associated with an orbital-selective Mott phase. This orbital-selective Mott transition appears at higher temperature T_met with increasing sulfur content, identifying sulfur substitution as an efficient parameter to tune orbital-dependent correlation effects in iron-chalcogenide superconductors. The reduced correlations of the d_xy charge carriers can account for the suppression of the superconductivity and the pseudogap-like feature between T_c and T_met that was observed for z=0.

Here we present a detailed study of the terahertz and far-infrared response of the mixed perovskite manganite system Eu1-xHoxMnO3 for holmium concentrations x = 0.1 and 0.3. We compare the magnetic excitations of the four different magnetically ordered phases (A-type antiferromagnetic, sinusoidally modulated collinear, helical phases with spin planes perpendicular to the crystallographic a and c axes). The transition between the two latter phases goes hand in hand with a switching of the ferroelectric polarization from P parallel to a to P parallel to c. Special emphasis is paid to the temperature dependence of the excitations at this transition. We find a significant change of intensity indicating that the exchange-striction mechanism may not be the only mechanism to induce dipolar weight to spin-wave excitations. We also focus on excitations within the incommensurate collinear antiferromagnetic phase and find an excitation close to 40 cm(-1). A detailed analysis of optical weight gives a further unexpected result: In the multiferroic phase with P parallel to c all the spectral weight of the electromagnons comes from the lowest-phonon mode. However, for the phase with the polarization P parallel to a additional spectral weight must be transferred from higher frequencies.

We present a dielectric spectroscopy study of the polar dynamics linked to the orbitally driven ferroelectric transition in the Skyrmion host GaV4S8. By combining THz and MHz-GHz spectroscopy techniques, we succeed in detecting the relaxational dynamics arising from coupled orbital and polar fluctuations in this material and trace its temperature dependence in the paraelectric as well as in the ferroelectric phase. The relaxation time significantly increases when approaching the critical temperature from both sides of the transition. It is natural to assume that these polar fluctuations map the orbital dynamics at the Jahn-Teller transition. Because of the first-order character of the orbital-ordering transition, the relaxation time shows an enormous jump of about 5 orders of magnitude at the polar and structural phase transition.

In FeSc2S4 spin-orbital exchange competes with strong spin-orbit coupling, suppressing long-range spin and orbital order and, hence, this material represents one of the rare examples of a spin-orbital liquid ground state. Moreover, it is close to a quantum-critical point separating the ordered and disordered regimes. Using THz and FIR spectroscopy we study low-lying excitations in FeSc2S4 and provide clear evidence for a spin-orbiton, an excitation of strongly entangled spins and orbitals. It becomes particularly well pronounced upon cooling, when advancing deep into the quantum-critical regime. Moreover, indications of an underlying structureless excitation continuum are found, a possible signature of quantum criticality.

Theoretical models of the spin-orbital liquid (SOL) FeSc2S4 have predicted it to be in close proximity to a quantum critical point separating a spin-orbital liquid phase from a long-range ordered magnetic phase. Here, we examine the magnetic excitations of FeSc2S4 through time-domain terahertz spectroscopy under an applied magnetic field. At low temperatures an excitation emerges that we attribute to a singlet-triplet excitation from the SOL ground state. A threefold splitting of this excitation is observed as a function of applied magnetic field. As singlet-triplet excitations are typically not allowed in pure spin systems, our results demonstrate the entangled spin and orbital character of singlet ground and triplet excited states. Using experimentally obtained parameters we compare to existing theoretical models to determine FeSc2S4's proximity to the quantum critical point. In the context of these models, we estimate the characteristic length of the singlet correlations to be ξ/(a/2)≈8.2 (where a/2 is the nearest neighbor lattice constant), which establishes FeSc2S4 as a SOL with long-range entanglement.

For quasi-one-dimensional quantum spin systems theory predicts the occurrence of a confinement of spinon excitation due to interchain couplings. Here we investigate the system SrCo2V2O8, a realization of the weakly coupled Ising-like XXZ antiferromagnetic chains, by terahertz spectroscopy with and without applied magnetic field. At low temperatures a series of excitations is observed, which split in a Zeeman-like fashion in an applied magnetic field. These magnetic excitations are identified as the theoretically predicted spinon-pair excitations. Using a one-dimensional Schrodinger equation with a linear confinement potential imposed by weak interchain couplings, the hierarchy of the confined spinons can be fully described.

Understanding the origin of high-temperature superconductivity in copper- and iron-based materials is one of the outstanding tasks of current research in condensed matter physics. Even the normal metallic state of these materials exhibits unusual properties. Here we report on a hierarchy of temperatures T(c)<T(gap)<T(met) in superconducting Rb(1-x)Fe(2-y)Se(2) observed by THz spectroscopy (T(c)=critical temperature of the superconducting phase; T(gap)=temperature below which an excitation gap opens; T(met)=temperature below which a metallic optical response occurs). Above T(met)=90 K the material reveals semiconducting characteristics. Below T(met) a coherent metallic THz response emerges. This metal-to-insulator-type, orbital-selective transition is indicated by an isosbestic point in the temperature dependence of the optical conductivity and dielectric constant at THz frequencies. At T(gap)= 61 K, a gap opens in the THz regime and then the superconducting transition occurs at T(c)=32 K. This sequence of temperatures seems to reflect a corresponding hierarchy of the electronic correlations in different bands.

Zhe Wang,1,* D. Kamenskyi,2,† O. Cépas,3 M. Schmidt,1 D. L. Quintero-Castro,4 A. T. M. N. Islam,4 B. Lake,4,5 A. A. Aczel,6,7 H. A. Dabkowska,8 A. B. Dabkowski,8 G. M. Luke,6 Yuan Wan,9 A. Loidl,1 M. Ozerov,2 J. Wosnitza,2,10 S. A. Zvyagin,2 and J. Deisenhofer1 1Experimental Physics V, Center for Electronic Correlations and Magnetism, Institute of Physics, University of Augsburg, 86135 Augsburg, Germany 2Dresden High Magnetic Field Laboratory (HLD), Helmholtz-Zentrum Dresden-Rossendorf, 01328 Dresden, Germany 3Institut Néel, CNRS and Université Joseph Fourier, BP 166, 38042 Grenoble Cedex 9, France 4Helmholtz-Zentrum Berlin für Materialien und Energie, 14109 Berlin, Germany 5Institut für Festkörperphysik, Technische Universität Berlin, 10623 Berlin, Germany 6Department of Physics and Astronomy, McMaster University, Hamilton, Ontario, Canada L8S 4M1 7Quantum Condensed Matter Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, USA 8Brockhouse Institute for Materials Research, McMaster University, Hamilton, Ontario, Canada L8S 4M1 9Department of Physics and Astronomy, Johns Hopkins University, Baltimore, Maryland 21218, USA 10Institut für Festkörperphysik, Technische Universität Dresden, 01068 Dresden, Germany (Received 7 February 2014; revised manuscript received 8 April 2014; published 5 May 2014)

Zhe Wang, M. Schmidt, A. Günther, F. Mayr, Yuan Wan, S.-H. Lee, H. Ueda, Y. Ueda, A. Loidl, and J. Deisenhofer Experimental Physics V, Center for Electronic Correlations and Magnetism, Institute of Physics, University of Augsburg, D-86135 Augsburg, Germany Department of Physics and Astronomy, Johns Hopkins University, Baltimore, Maryland 21218, USA Department of Physics, University of Virginia, Charlottesville, Virginia 22904, USA Institute for Solid State Physics, University of Tokyo, Kashiwa 277-8581, Japan (Dated: May 1, 2014)

Multifrequency electron spin resonance (ESR) transmission spectra have been measured as a function of temperature and magnetic field on single crystals of the quasi-one-dimensional spin-1 chain compound SrNi2V2O8 in the gigahertz frequency range. Magnetic resonance modes above 50 K have been observed with an effective g factor of 2.24 at 100 K. Below 30 K, intratriplet excitations have been observed in the ESR spectra, which reveal the presence of single-ion anisotropy with D = -0.29 meV. DOI: 10.1103/PhysRevB.87.104405

We report a linear dependence of the phonon splitting Δω on the nondominant exchange coupling constant J(nd) in the antiferromagnetic transition-metal monoxides MnO, FeO, CoO, NiO, and in the frustrated antiferromagnetic oxide spinels CdCr(2)O(4), MgCr(2)O(4), and ZnCr(2)O(4). It directly confirms the theoretical prediction of an exchange-induced splitting of the zone-center optical phonon for the monoxides and explains the magnitude and the change of sign of the phonon splitting on changing the sign of the nondominant exchange also in the frustrated oxide spinels. The experimentally found linear relation [symbol:see text}Δω=βJ(nd)S(2) with slope β=3.7 describes the splitting for both systems and agrees with the observations in the antiferromagnets KCoF(3) and KNiF(3) with perovskite structure and negligible next-nearest neighbor coupling. The common behavior found for very different classes of cubic antiferromagnets suggests a universal dependence of the exchange-induced phonon splitting at the antiferromagnetic transition on the nondominant exchange coupling.

We report on the phonon spectrum of Ba3Cr2O8 determined by infrared spectroscopy, and on specific heat measurements across the Jahn-Teller transition in magnetic fields up to 9 T. Phonon modes split below the Jahn-Teller transition, which occurs at T_JT = 70 K as detected by specific heat measurements. The field-dependent specific heat data is analyzed in terms of the contributions from lattice, magnetic and orbital degrees of freedom. In contrast to the isostructural compound Sr3Cr2O8 our analysis does not indicate the existence of orbital fluctuations below the Jahn-Teller transition in Ba3Cr2O8.

We report on polarization dependent reflectivity measurements in KCuF3 in the far-infrared frequency regime. The observed IR active phonons at room temperature are in agreement with the expected modes for tetragonal symmetry. We observe a splitting of one mode already at 150 K and the appearance of a new mode in the vicinity of the Neel temperature. (C) 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

The magnetic susceptibility between 1.5 and 800 K, the infrared reflectivity at room temperature and the temperature dependence of the dielectric constant at mm-wavelengths are reported for single crystalline BiFeO3. A well developed anomaly in the magnetic susceptibility signals the onset of antiferromagnetic order close to 640 K. Beside this anomaly, no further indications of phase or spin-glass transitions are found in the susceptibility down to the lowest temperatures. From infrared reflectivity we were able to identify all 9 phonon modes which are expected to be infrared active within the ab plane of the crystal. The temperature dependence of the dielectric constant was measured contact free in the 100 GHz range yielding epsilon' = 54 at room temperature. The loss is substantial and strongly frequency dependent indicating the predominance of hopping conductivity.

We report on the optical response of the highly correlated transition-metal monoxides MnO, FeO, CoO and NiO in the far-infrared regime. The main focus is put on spin-phonon coupling effects, which are found to significantly influence the lattice dynamics in the magnetically ordered phase. Measurements have been performed in the ordered and paramagnetic state for temperatures up to 550 K. A clear splitting of the cubic mode accompanying the transition into long range magnetic order can be identified in MnO, CoO and NiO. In the case of FeO it is argued that an anisotropic phonon response seems to be very likely, though it could not be observed directly. The results are compared to recent experimental and theoretical studies in frustrated magnets, which predict the splitting of zone center phonon modes induced by a non-cubic spin-density distribution.

We studied the optical properties of antiferromagnetic ZnCr2Se4 by infrared spectroscopy up to 28,000 cm(-1) and for temperatures from 5 to 295 K. At the magnetic phase transition at 21 K, one of the four phonon modes reveals a clear splitting of 3 cm(-1) as a result of spin-phonon coupling, the other three optical eigenmodes only show shifts of the eigenfrequencies. The antiferromagnetic ordering and the concomitant splitting of the phonon mode can be suppressed in a magnetic field of 7 T. At higher energies we observed a broad excitation band which is dominated by a two-peak-structure at about 18,000 cm(-1) and 22,000 cm(-1), respectively. These energies are in good agreement with the expected spin-allowed crystal-field transitions of the Cr3+ ions. The unexpected strength of these transitions with d-d character is attributed to a considerable hybridization of the selenium p with the chromium d orbitals.