6 research outputs found
Charge-ordering phase transition and order-disorder effects in the Raman spectra of NaV2O5
In the ac polarized Raman spectra of NaV2O5 we have found anomalous phonon
broadening, and an energy shift of the low-frequency mode as a function of the
temperature. These effects are related to the breaking of translational
symmetry, caused by electrical disorder that originates from the fluctuating
nature of the V {4.5+} valence state of vanadium. The structural correlation
length, obtained from comparisons between the measured and calculated Raman
scattering spectra, diverges at T< 5 K, indicating the existence of the
long-range charge order at very low temperatures, probably at T=0 K.Comment: 8 pages, 4 figures, new version, to appear in PR
Low energy excitations and dynamic Dzyaloshinskii-Moriya interaction in -NaVO studied by far infrared spectroscopy
We have studied far infrared transmission spectra of alpha'-NaV2O5 between 3
and 200cm-1 in polarizations of incident light parallel to a, b, and c
crystallographic axes in magnetic fields up to 33T. The triplet origin of an
excitation at 65.4cm-1 is revealed by splitting in the magnetic field. The
magnitude of the spin gap at low temperatures is found to be magnetic field
independent at least up to 33T. All other infrared-active transitions appearing
below Tc are ascribed to zone-folded phonons. Two different dynamic
Dzyaloshinskii-Moriya (DM) mechanisms have been discovered that contribute to
the oscillator strength of the otherwise forbidden singlet to triplet
transition. 1. The strongest singlet to triplet transition is an electric
dipole transition where the polarization of the incident light's electric field
is parallel to the ladder rungs, and is allowed by the dynamic DM interaction
created by a high frequency optical a-axis phonon. 2. In the incident light
polarization perpendicular to the ladder planes an enhancement of the singlet
to triplet transition is observed when the applied magnetic field shifts the
singlet to triplet resonance frequency to match the 68cm-1 c-axis phonon
energy. The origin of this mechanism is the dynamic DM interaction created by
the 68cm-1 c-axis optical phonon. The strength of the dynamic DM is calculated
for both mechanisms using the presented theory.Comment: 21 pages, 22 figures. Version 2 with replaced fig. 18 were labels had
been los
Using electronic structure changes to map the H-T phase diagram of alpha'-NaV2O5
We report polarized optical reflectance studies of \alpha'-NaV2O5 as a
function of temperature (4-45 K) and magnetic field (0-60 T). Rung directed
electronic structure changes, as measured by near-infrared reflectance ratios
\Delta R(H)=R(H)/R(H=0 T), are especially sensitive to the phase boundaries. We
employ these changes to map out an H-T phase diagram. Topological highlights
include the observation of two phase boundaries slightly below T_{SG}, enhanced
curvature of the 34 K phase boundary above 35 T, and, surprisingly, strong
hysteresis effects of both transitions with applied field.Comment: 4 pages, 3 figures, PRB accepte
A microscopic model for the structural transition and spin gap formation in alpha'-NaV2O5
We present a microscopic model for alpha'-NaV2O5. Using an extended Hubbard
model for the vanadium layers we derive an effective low-energy model
consisting of pseudospin Ising chains and Heisenberg chains coupled to each
other. We find a ``spin-Peierls-Ising'' phase transition which causes charge
ordering on every second ladder and superexchange alternation on the other
ladders. This transition can be identified with the first transition of the two
closeby transitions observed in experiment. Due to charge ordering the
effective coupling between the lattice and the superexchange is enhanced. This
is demonstrated within a Slater-Koster approximation. It leads to a second
instability with superexchange alternation on the charge-ordered ladders due to
an alternating shift of the O sites on the rungs of that ladder. We can explain
within our model the observed spin gap, the anomalous BCS ratio, and the
anomalous shift of the critical temperature of the first transition in a
magnetic field. To test the calculated superstructure we determine the
low-energy magnon dispersion and find agreement with experiment.Comment: 32 pages, 12 figures include