326 research outputs found
Magnetic susceptibility of the frustrated spinels ZnCr2O4, MgCr2O4 and CdCr2O4
We analyzed the magnetic susceptibilities of several Cr spinels using two
recent models for the geometrically frustrated pyrochlore lattice, the Quantum
Tetrahedral Mean Field model and a Generalized Constant Coupling model. Both
models can describe the experimental data for ACr2 O4 (with A = Zn, Mg, and Cd)
satisfactorily, with the former yielding a somewhat better agreement with
experiment for A = Zn, Mg. The obtained exchange constants for nearest and
next-nearest neighbors are discussed.Comment: 4 pages, 1 figure, 1 table, conferenc
Spin-phonon coupling in antiferromagnetic chromium spinels
The temperature dependence of eigenfrequencies and intensities of the IR
active modes has been investigated for the antiferromagnetic chromium spinel
compounds CdCr2O4, ZnCr2O4, ZnCr2S4, ZnCr2Se4, and HgCr2S4 by IR spectroscopy
for temperatures from 5 K to 300 K. At the transition into the magnetically
ordered phases, and driven by spin-phonon coupling, most compounds reveal
significant splittings of the phonon modes. This is true for geometrically
frustrated CdCr2O4, and ZnCr2O4, for bond frustrated ZnCr2S4 and for ZnCr2Se4,
which also is bond frustrated, but dominated by ferromagnetic exchange. The
pattern of splitting is different for the different compounds and crucially
depends on the nature of frustration and of the resulting spin order. HgCr2S4,
which is almost ferromagnetic, exhibits no splitting of the eigenfrequencies,
but shows significant shifts due to ferromagnetic spin fluctuations.Comment: 15 pages, 6 figure
Broadband dielectric response of CaCu3Ti4O12: From dc to the electronic transition regime
We report on phonon properties and electronic transitions in CaCu3Ti4O12, a
material which reveals a colossal dielectric constant at room temperature
without any ferroelectric transition. The results of far- and mid-infrared
measurements are compared to those obtained by broadband dielectric and
millimeter-wave spectroscopy on the same single crystal. The unusual
temperature dependence of phonon eigenfrequencies, dampings and ionic plasma
frequencies of low lying phonon modes are analyzed and discussed in detail.
Electronic excitations below 4 eV are identified as transitions between full
and empty hybridized oxygen-copper bands and between oxygen-copper and
unoccupied Ti 3d bands. The unusually small band gap determined from the
dc-conductivity (~200 meV) compares well with the optical results.Comment: 7 pages, 8 figure
Polar phonons and spin-phonon coupling in HgCr2S4 and CdCr2S4
Polar phonons of HgCr2S4 and CdCr2S4 are studied by far-infrared spectroscopy
as a function of temperature and external magnetic field. Eigenfrequencies,
damping constants, effective plasma frequencies and Lyddane-Sachs-Teller
relations, and effective charges are determined. Ferromagnetic CdCr2S4 and
antiferromagnetic HgCr2S4 behave rather similar. Both compounds are dominated
by ferromagnetic exchange and although HgCr2S4 is an antiferromagnet, no phonon
splitting can be observed at the magnetic phase transition. Temperature and
magnetic field dependence of the eigenfrequencies show no anomalies indicating
displacive polar soft mode behavior. However, significant effects are detected
in the temperature dependence of the plasma frequencies indicating changes in
the nature of the bonds and significant charge transfer. In HgCr2S4 we provide
experimental evidence that the magnetic field dependence of specific polar
modes reveal shifts exactly correlated with the magnetization showing
significant magneto-dielectric effects even at infrared frequencies.Comment: 8 pages, 8 figure
Optical phonons, spin correlations, and spin-phonon coupling in the frustrated pyrochlore magnets CdCr2O4 and ZnCr2O4
We report on infrared, Raman, magnetic susceptibility, and specific heat
measurements on CdCr2O4 and ZnCr2O4 single crystals. We estimate the
nearest-neighbor and next-nearest neighbor exchange constants from the magnetic
susceptibility and extract the spin-spin correlation functions obtained from
the magnetic susceptibility and the magnetic contribution to the specific heat.
By comparing with the frequency shift of the infrared optical phonons above TN
, we derive estimates for the spin-phonon coupling constants in these systems.
The observation of phonon modes which are both Raman and infrared active
suggest the loss of inversion symmetry below the Neel temperature in CdCr2O4 in
agreement with theoretical predictions by Chern and coworkers [Phys. Rev. B 74,
060405 (2006)]. In ZnCr2O4 several new modes appear below TN, but no phonon
modes could be detected which are both Raman and infrared active indicating the
conservation of inversion symmetry in the low temperature phase.Comment: 11 pages, 13 figure
Exciton-magnon transitions in the frustrated chromium antiferromagnets CuCrO2, alpha-CaCr2O4, CdCr2O4, and ZnCr2O4
We report on optical transmission spectroscopy of the Cr-based frustrated
triangular antiferromagnets CuCrO2 and alpha-CaCr2O4, and the spinels CdCr2O4
and ZnCr2O4 in the near-infrared to visible-light frequency range. We explore
the possibility to search for spin correlations far above the magnetic ordering
temperature and for anomalies in the magnon lifetime in the magnetically
ordered state by probing exciton-magnon sidebands of the spin-forbidden
crystal-field transitions of the Cr3+ ions (spin S = 3/2). In CuCrO2 and
alpha-CaCr2O4 the appearance of fine structures below T_N is assigned to magnon
sidebands by comparison with neutron scattering results. The temperature
dependence of the line width of the most intense sidebands in both compounds
can be described by an Arrhenius law. For CuCrO2 the sideband associated with
the 4A2 -> 2T2 transition can be observed even above T_N. Its line width does
not show a kink at the magnetic ordering temperature and can alternatively be
described by a Z2 vortex scenario proposed previously for similar materials.
The exciton-magnon features in alpha-CaCr2O4 are more complex due to the
orthorhombic distortion. While for CdCr2O4 magnon sidebands are identified
below T_N and one sideband excitation is found to persist across the magnetic
ordering transition, only a weak fine structure related to magnetic ordering
has been observed in ZnCr2O4.Comment: 14 pages, 10 figures, all comments are welcome and appreciated,
accepted for publication in PR
Spectral origin of the colossal magnetodielectric effect in multiferroic DyMn2O5
The origin of the colossal magnetodielectric effect in DyMn2O5 [1] has been
an outstanding question in multiferroics. Here, we report the activation of the
electric dipole mode at 4-5 cm-1 in an applied magnetic field which fully
accounts for the CMD effect. We examine two alternative explanations of this
mode: an electromagnon and transitions between f-electron levels of Dy3+ ions.
The experimental and theoretical evidence supports the electromagnon origin of
the CMD effect.Comment: 5 pages, 4 figures, submitted to PR
Optical properties of ZnCr2Se4 - Spin-phonon coupling and electronic d-d-like excitations
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 transition of the Cr3+ ions. The
unexpected strength of these forbidden onsite d-d transitions is attributed to
a considerable hybridization of the selenium p with the chromium d orbitals.Comment: 8 pages, 6 figure
Magnetic and superconducting transitions in BaKFeAs studied by specific heat
We report on specific heat measurements in BaKFeAs
(). For the underdoped sample with both the spin-density-wave
transition at K and the superconducting transition at 23 K can be
identified. The electronic contribution to the specific heat in the
superconducting state for concentrations in the vicinity of optimal doping
can be well described by a full single-gap within the BCS limit.Comment: 5 pages, 4 figures, 2 table
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