2,093 research outputs found
Magnetic Raman scattering of the ordered tetrahedral spin-1/2 clusters in Cu_2Te_2O_5(Br_(1-x)Cl_x)_2 compounds
Raman light-scattering experiments in the antiferromagnetic phase of the
Cu_2Te_2O_5(Br_(1-x)Cl_x)_2 compounds are analyzed in terms of a dimerized spin
model for the tetrahedral Cu-clusters. It is shown that the longitudinal
magnetic excitation in the pure Br system hybridizes with a localized singlet
excitation due to the presence of a Dzyaloshinskii-Moriya anisotropy term. The
drastic change of the magnetic scattering intensities observed when a
proportion of Br is replaced by Cl ions, is proposed to be caused by a change
of the magnetic order parameter. Instead of being parallel/antiparallel with
each other, the spins in the two pairs of spin-1/2 order perpendicular to each
other, when the composition x is larger than about 0.25.Comment: EPL, in pres
Approaching the ground state of the kagome antiferromagnet
Y{0.5}$Ca{0.5}BaCo4O7 contains kagome layers of Co ions, whose spins are
strongly coupled according to a Curie-Weiss temperature of -2200 K. At low
temperatures, T = 1.2 K, our diffuse neutron scattering study with polarization
analysis reveals characteristic spin correlations close to a predicted
two-dimensional coplanar ground state with staggered chirality. The absence of
three dimensional long-range AF order proves negligible coupling between the
kagome layers. The scattering intensities are consistent with high spin S=3/2
states of Co2+ in the kagome layers and low spin S=0 states for Co3+ ions at
interlayer sites. Our observations agree with previous Monte Carlo simulations
indicating a ground state of only short range chiral order.Comment: 4 pages, 4 figures, contact author: [email protected]
Low Energy Singlets in the Excitation Spectrum of the Spin Tetrahedra System Cu_2Te_2O_5Br_2
Low energy Raman scattering of the s=1/2 spin tetrahedra system
Cu_2Te_2O_5Br_2 is dominated by an excitation at 18 cm^{-1} corresponding to an
energy E_S=0.6\Delta, with \Delta the spin gap of the compound. For elevated
temperatures this mode shows a soft mode-like decrease in energy pointing to an
instability of the system. The isostructural reference system Cu_2Te_2O_5Cl_2
with a presumably larger inter-tetrahedra coupling does not show such a low
energy mode. Instead its excitation spectrum and thermodynamic properties are
compatible with long range Neel-ordering. We discuss the observed effects in
the context of quantum fluctuations and competing ground states.Comment: 5 pages, 2 figures, ISSP-Kashiwa 2001, Conference on Correlated
Electron
Electronic Raman scattering in Tl2Ba2CuO6+x: symmetry of the order parameter, oxygen doping effects, and normal state scattering
Single crystals of the optimally doped, moderately and strongly overdoped
high temperature superconductor Tl2Ba2CuO6+x (Tl-2201) with Tc=80, 56 and 30K,
respectively, have been investigated by polarized Raman scattering. By taking
the peak position of the B_1g component of electronic Raman scattering as
2Delta_0 we found that the reduced gap value (2Delta_0/k_BT_c) strongly
decreases with increasing doping. The behavior of the low frequency scattering
for the B_1g and B_2g scattering components is similar for optimally doped and
overdoped crystals and can be described by a w^3 - and w -law, respectively,
which is consistent with a d-wave symmetry of the order parameter. In contrast
to the optimally doped Tl-2201 in both, moderately and strongly overdoped
Tl-2201, the relative (compared to the B_1g) intensity of the A_1g scattering
component is suppressed. We suggest that the van Hove singularity is
responsible for the observed changes of Raman intensity and reduced gap value
with doping. Electronic Raman scattering in the normal state is discussed in
the context of the scattering from impurities and compared to the existing
infrared data. The scattering rate evaluated from the Raman measurements is
smaller for the overdoped samples, compared to the moderately overdoped
samples.Comment: 7 pages, 7 figure
Longitudinal magnon in the tetrahedral spin system Cu2Te2O5Br2 near quantum criticality
We present a comprehensive study of the coupled tetrahedra-compound
Cu2Te2O5Br2 by theory and experiments in external magnetic fields. We report
the observation of a longitudinal magnon in Raman scattering in the ordered
state close to quantum criticality. We show that the excited
tetrahedral-singlet sets the energy scale for the magnetic ordering temperature
T_N. This energy is determined experimentally. The ordering temperature T_N has
an inverse-log dependence on the coupling parameters near quantum criticality
Tuning the spin dynamics of kagome systems
Despite the conceptional importance of realizing spin liquids in solid states
only few compounds are known. On the other side the effect of lattice
distortions and anisotropies on the magnetic exchange topology and the
fluctuation spectrum are an interesting problem. We compare the excitation
spectra of the two s=1/2 kagome lattice compounds volborthite and vesignieite
using Raman scattering. We demonstrate that even small modifications of the
crystal structure may have a huge effect on the phonon spectrum and low
temperature properties.Comment: 3 pages, 2 figure
Anomalous frequency and intensity scaling of collective and local modes in a coupled spin tetrahedron system
We report on the magnetic excitation spectrum of the coupled spin tetrahedral
system CuTeOCl using Raman scattering on single
crystals. The transition to an ordered state at T=18.2 K evidenced
from thermodynamic data leads to the evolution of distinct low-energy magnetic
excitations superimposed by a broad maximum. These modes are ascribed to
magnons with different degree of localization and a two-magnon continuum. Two
of the modes develop a substantial energy shift with decreasing temperature
similar to the order parameter of other Neel ordered systems. The other two
modes show only a negligible temperature dependence and dissolve above the
ordering temperature in a continuum of excitations at finite energies. These
observations point to a delicate interplay of magnetic inter- and
intra-tetrahedra degrees of freedom and an importance of singlet fluctuations
in describing a spin dynamics.Comment: 7pages, 6figures, 1tabl
Interplay of electronic correlations and lattice instabilities in BaVS3
The quasi-one-dimensional metallic system BaVS3 with a metal-insulator
transition at T_MI=70 K shows large changes of the optical phonon spectrum, a
central peak, and an electronic Raman scattering continuum that evolve in a
three-step process. Motivated by the observation of a strongly fluctuating
precursor state at high temperatures and orbital ordering and a charge gap at
low temperatures we suggest a concerted action of the orbital, electronic, and
lattice subsystems dominated by electronic correlations.Comment: 4 pages, 4 figure
Energy-level ordering and ground-state quantum numbers for frustrated two-leg spin-1/2 ladder model
The Lieb-Mattis theorem about antiferromagnetic ordering of energy levels on
bipartite lattices is generalized to finite-size two-leg spin-1/2 ladder model
frustrated by diagonal interactions. For reflection-symmetric model with
site-dependent interactions we prove exactly that the lowest energies in
sectors with fixed total spin and reflection quantum numbers are monotone
increasing functions of total spin. The nondegeneracy of most levels is proved
also. We also establish the uniqueness and obtain the spin value of the
lowest-level multiplet in the whole sector formed by reflection-symmetric
(antisymmetric) states. For a wide range of coupling constants, we prove that
the ground state is a unique spin singlet. For other values of couplings, it
may be also a unique spin triplet or may consist of both multiplets. Similar
results have been obtained for the ladder with arbitrary boundary impurity
spin. Some partial results have also been obtained in the case of periodical
boundary conditions.Comment: 17 page
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