687 research outputs found
Evolution of optical properties of chromium spinels CdCrO, HgCrS, and ZnCrSe under high pressure
We report pressure-dependent reflection and transmission measurements on
ZnCrSe, HgCrS, and CdCrO single crystals at room
temperature over a broad spectral range 200-24000 cm. The pressure
dependence of the phonon modes and the high-frequency electronic excitations
indicates that all three compounds undergo a pressure-induced structural phase
transition with the critical pressure 15 GPa, 12 GPa, and 10 GPa for
CdCrO, HgCrS, and ZnCrSe, respectively. The
eigenfrequencies of the electronic transitions are very close to the expected
values for chromium crystal-field transitions. In the case of the chalcogenides
pressure induces a red shift of the electronic excitation which indicates a
strong hybridization of the Cr d-bands with the chalcogenide bands.Comment: Accepted for publication in Phys. Rev.
Spinon Confinement in the One-Dimensional Ising-Like Antiferromagnet SrCo2V2O8
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 Schr\"odinger equation with a
linear confinement potential imposed by weak interchain couplings, the
hierarchy of the confined spinons can be fully described.Comment: 4 pages, 3 figure
Phonon anomalies and charge dynamics in Fe_{1-x}Cu_{x}Cr_{2}S_{4} single crystals
A detailed investigation of phonon excitations and charge carrier dynamics in
single crystals of Fe_{1-x}Cu_{x}Cr_{2}S_{4} (x = 0, 0.2, 0.4, 0.5) has been
performed by using infrared spectroscopy. In FeCr_{2}S_{4} the phonon
eigenmodes are strongly affected by the onset of magnetic order. Despite
enhanced screening effects, a continuous evolution of the phonon excitations
can be observed in the doped compounds with x = 0.2 (metallic) and x = 0.4, 0.5
(bad metals), but the effect of magnetic ordering on the phonons is strongly
reduced compared to x = 0. The Drude-like charge-carrier contribution to the
optical conductivity in the doped samples indicates that the colossal
magneto-resistance effect results from the suppression of spin-disorder
scattering.Comment: 8 pages, 6 figure
The Orbital Order Parameter in La0.95Sr0.05MnO3 probed by Electron Spin Resonance
The temperature dependence of the electron-spin resonance linewidth in
La0.95Sr0.05MnO3 has been determined and analyzed in the paramagnetic regime
across the orbital ordering transition. From the temperature dependence and the
anisotropy of linewidth and -value the orbital order can be unambiguously
determined via the mixing angle of the wave functions of the -doublet. The linewidth shows a similar evolution with temperature as
resonant x-ray scattering results
Spin fluctuations in the quasi-two dimensional Heisenberg ferromagnet GdI_2 studied by Electron Spin Resonance
The spin dynamics of GdI_2 have been investigated by ESR spectroscopy. The
temperature dependences of the resonance field and ESR intensity are well
described by the model for the spin susceptibility proposed by Eremin et al.
[Phys. Rev. B 64, 064425 (2001)]. The temperature dependence of the resonance
linewidth shows a maximum similar to the electrical resistance and is discussed
in terms of scattering processes between conduction electrons and localized
spins.Comment: to be published in PR
Anomalous optical phonons in FeTe pnictides: spin state, magnetic order, and lattice anharmonicity
Polarized Raman-scattering spectra of non-superconducting, single-crystalline
FeTe are investigated as function of temperature. We have found a relation
between the magnitude of ordered magnetic moments and the linewidth of A1g
phonons at low temperatures. This relation is attributed to the intermediate
spin state (S=1) and the orbital degeneracy of the Fe ions. Spin-phonon
coupling constants have been estimated based on microscopic modeling using
density-functional theory and analysis of the local spin density. Our
observations show the importance of orbital degrees of freedom for the Fe-based
superconductors with large ordered magnetic moments, while small magnetic
moment of Fe ions in some iron pnictides reflects the low spin state of Fe ions
in those systems.Comment: 17 pages, 3 figure
Chain Formation by Spin Pentamers in eta-Na9V14O35
The nature of the gapped ground state in the quasi-one-dimensional compound
eta-Na9V14O35 cannot easily be understood, if one takes into account the odd
number of spins on each structural element. Combining the results of specific
heat, susceptibility and electron spin resonance measurements we show that
eta-Na9V14O35 exhibits a novel ground state where multi-spin objects build up a
linear chain. These objects - pentamers - consist of five antiferromagnetically
arranged spins with effective spin 1/2. Their spatial extent results in an
exchange constant along the chain direction comparable to the one in the
high-temperature state.Comment: 6 pages, 5 figure
Structural and magnetic dimers in the spin-gapped system CuTe2O5
We investigated the magnetic properties of the system CuTe2O5 by
susceptibility and electron spin resonance measurements. The anisotropy of the
effective g-factors and the ESR linewidth indicates that the anticipated
structural dimer does not correspond to the singlet-forming magnetic dimer.
Moreover, the spin susceptibility of CuTe2O5 can only be described by taking
into account interdimer interactions of the same order of magnitude than the
intradimer coupling. Analyzing the exchange couplings in the system we identify
the strongest magnetic coupling between two Cu ions to be mediated by
super-super exchange interaction via a bridging Te ligand, while the
superexchange coupling between the Cu ions of the structural dimer only results
in the second strongest coupling
Spin dynamics in the low-dimensional magnet TiOCl
We present detailed ESR investigations on single crystals of the
low-dimensional quantum magnet TiOCl. The anisotropy of the g-factor indicates
a stable orbital configuration below room temperature, and allows to estimate
the energy of the first excited state as 0.3(1) eV ruling out a possible
degeneracy of the orbital ground state. Moreover, we discuss the possible spin
relaxation mechanisms in TiOCl and analyze the angular and temperature
dependence of the linewidth up to 250 K in terms of anisotropic exchange
interactions. Towards higher temperatures an exponential increase of the
linewidth is observed, indicating an additional relaxation mechanism.Comment: 10 pages, 8 figures; accepted for publication in Phys. Rev.
Evidence for Jahn-Teller distortions at the antiferromagnetic transition in LaTiO
LaTiO is known as Mott-insulator which orders antiferromagnetically at
K. We report on results of thermal expansion and temperature
dependent x-ray diffraction together with measurements of the heat capacity,
electrical transport measurements, and optical spectroscopy in untwinned single
crystals. At significant structural changes appear, which are
volume conserving. Concomitant anomalies are also observed in the
dc-resistivity, in bulk modulus, and optical reflectivity spectra. We interpret
these experimental observations as evidence of orbital order.Comment: 4 pages, 4 figures; published in Phys. Rev. Lett. 91, 066403 (2003
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