1,018 research outputs found
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
TiOCl, an orbital-ordered system?
We present first principles density functional calculations and downfolding
studies of the electronic and magnetic properties of the layered quantum spin
system
TiOCl. We discuss explicitely the nature of the exchange pathes and attempt
to clarify the concept of orbital ordering in this material. An analysis of the
electronic structure of slightly distorted structures according to the phononic
modes allowed in this material suggests that this system is subject to large
orbital fluctuations driven by the electron-phonon coupling. Based on these
results, we propose a microscopic explanation of the behavior of TiOCl near the
phase transition to a spin-gapped system.Comment: Some figures are compressed, for higher quality please contact the
author
Bulk antiferromagnetism in single crystals
Susceptibility, specific heat, and muon spin rotation measurements on
high-quality single crystals of have revealed bulk
antiferromagnetism with N\'{e}el temperature K and an
ordered moment perpendicular to the layers. The magnetic order
encompasses nearly 100% of the crystal volume. The susceptibility exhibits a
broad peak around 30 K, characteristic of two-dimensional antiferromagnetic
fluctuations. The in-plane resistivity is metallic at high temperatures and
exhibits a minimum at .Comment: published versio
Low-Lying Magnetic Excitation of the Shastry-Sutherland Model
By using perturbation calculation and numerical diagonalization, low-energy
spin dynamics of the Shastry-Sutherland model is investigated paying particular
attention to the two-particle coherent motion. In addition to spin-singlet- and
triplet bound states, we find novel branches of coherent motion of a bound
quintet pair, which are usually unstable because of repulsion. Unusual
dispersion observed in neutron-scattering measurements are explained by the
present theory. The importance of the effects of phonon is also pointed out.Comment: 4 pages, 6 figures combined into 1 PSfile. Final version to appear in
Phys.Rev.Lett. (Jan. 2000
Anomalous electronic Raman scattering in Na_xCoO_2 H_2O
Raman scattering experiments on Na_{x}CoO_2 yH_2O single crystals show a
broad electronic continuum with a pronounced peak around 100 cm-1 and a cutoff
at approximately 560 cm-1over a wide range of doping levels. The electronic
Raman spectra in superconducting and non-superconducting samples are similar at
room temperature, but evolve in markedly different ways with decreasing
temperature. For superconducting samples, the low-energy spectral weight is
depleted upon cooling below T* sim 150K, indicating a opening of a pseudogap
that is not present in non-superconducting materials. Weak additional phonon
modes observed below T* suggest that the pseudogap is associated with charge
ordering.Comment: 5 pages, 4 figures, for further information see www.peter-lemmens.d
Comparative investigation of the coupled-tetrahedra quantum spin systems Cu2Te2O5X2, X=Cl, Br and Cu4Te5O12Cl4
We present a comparative study of the coupled-tetrahedra quantum spin systems
Cu2Te2O5X2, X=Cl, Br (Cu-2252(X)) and the newly synthesized Cu4Te5O12Cl4
(Cu-45124(Cl)) based on ab initio Density Functional Theory calculations. The
magnetic behavior of Cu-45124(Cl) with a phase transition to an ordered state
at a lower critical temperature T=13.6K than in Cu-2252(Cl) (T=18K) can
be well understood in terms of the modified interaction paths. We identify the
relevant structural changes between the two systems and discuss the
hypothetical behavior of the not yet synthesized Cu-45124(Br) with an ab initio
relaxed structure using Car-Parrinello Molecular Dynamics.Comment: 2 pages, 1 figure; submitted to Proceedings of M2S-HTSC VIII, Dresden
200
Momentum distribution of confined bosons: temperature dependence
The momentum distribution function of a parabolically confined gas of bosons
with harmonic interparticle interactions is derived. In the Bose-Einstein
condensation region, this momentum distribution substantially deviates from a
Maxwell-Boltzmann distribution. It is argued that the determination of the
temperature of the boson gas from the Bose-Einstein momentum distribution
function is more appropriate than the currently used fitting to the high
momentum tail of the Maxwell-Boltzmann distribution.Comment: 5 REVTEX pages + 2 postscript figures. Accepted in Phys. Rev.
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
Magnetism of a tetrahedral cluster spin-chain
We discuss the magnetic properties of a dimerized and completely frustrated
tetrahedral spin-1/2 chain. Using a combination of exact diagonalization and
bond-operator theory the quantum phase diagram is shown to incorporate a
singlet-product, a dimer, and a Haldane phase. In addition we consider one-,
and two-triplet excitations in the dimer phase and evaluate the magnetic Raman
cross section which is found to be strongly renormalized by the presence of a
two-triplet bound state. The link to a novel tellurate materials is clarified.Comment: 8 pages, 8 figure
The Pairing Mechanism in HTSC investigated by Electronic Raman Scattering
By means of electronic Raman scattering we investigated the symmetry of the
energy gap Delta(k), its temperature dependence and its variation with doping
of well characterized Bi2Sr2CaCu2O8+delta single crystals. The oxygen content
delta was varied between the under- and the overdoped regime by subsequently
annealing the same single crystal in Ar and O2, respectively. The symmetry
analysis of the polarized electronic Raman scattering is consistent with a
d_x^2-y^2-wave symmetry of the energy gap in both regimes. The gap ratio
2Delta_max/k_BT_c and its temperature dependence changes with doping similar to
predictions of theories based on paramagnon coupling.Comment: 3 pages, LaTeX, 2 ps figures available on request to
[email protected]
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