1,962 research outputs found
Interference of a first-order transition with the formation of a spin-Peierls state in alpha'-NaV2O5?
We present results of high-resolution thermal-expansion and specific-heat
measurements on single crystalline alpha'-NaV2O5. We find clear evidence for
two almost degenerate phase transitions associated with the formation of the
dimerized state around 33K: A sharp first-order transition at T1=(33+-0.1)K
slightly below the onset of a second-order transition at T2onset around
(34+-0.1)K. The latter is accompanied by pronounced spontaneous strains. Our
results are consistent with a structural transformation at T1 induced by the
incipient spin-Peierls (SP) order parameter above T2=TSP.Comment: 5 pages, 7 figure
Substitution effects on spin fluctuations in the spin-Peierls compound CuGeO_3
Using Raman scattering we studied the effect of substitutions on 1D spin
fluctuations in CuGeO_3 observed as a spinon continuum in frustration induced
exchange scattering. For temperatures below the spin-Peierls transition
(T_{SP}=14K) the intensity of this continuum at 120-500 cm^{-1} is
exponentially suppressed and transferred into a 3D two-magnon density of
states. Besides a spin-Peierls gap-induced mode at 30 cm^{-1} and additional
modes at 105 and 370 cm^{-1} are observed. Substitution of Zn on the Cu-site
and Si on the Ge-site of CuGeO_3 quenches easily the spin-Peierls state.
Consequently a suppression of the spin-Peierls gap observable below T_{SP}=14K
as well as a change of the temperature dependence of the spinon continuum are
observed. These effects are discussed in the context of a dimensional crossover
of this compound below T_{SP} and strong spin-lattice interaction.Comment: 9 pages, 2 eps figures include
Magnetic Bound States in Dimerized Quantum Spin Systems
Magnetic bound states are a general phenomenon in low dimensional
antiferromagnets with gapped singlet states. Using Raman scattering on three
compounds as dedicated examples we show how exchange topology, dimensionality,
defects and thermal fluctuations influence the properties and the spectral
weight of these states.Comment: 3 pages, 1 figure, proceedings of the SCES'98, Paris, to be published
in Physica
The J_1-J_2 model revisited : Phenomenology of CuGeO_3
We present a mean field solution of the antiferromagnetic Heisenberg chain
with nearest (J_1) and next to nearest neighbor (J_2) interactions. This
solution provides a way to estimate the effects of frustration. We calculate
the temperature-dependent spin-wave velocity, v_s(T) and discuss the
possibility to determine the magnitude of frustration J_2/J_1 present in quasi
1D compounds from measurements of v_s(T). We compute the thermodynamic
susceptibility at finite temperatures and compare it with the observed
susceptibility of the spin-Peierls compound CuGeO_3. We also use the method to
study the two-magnon Raman continuum observed in CuGeO_3 above the spin-Peierls
transition.Comment: Phys. Rev.
Effects of in-chain and off-chain substitutions on spin fluctuations in the spin-Peierls compound CuGeO_3
The effect of in-chain and off-chain substitutions on 1D spin fluctuations in
the spin-Peierls compound CuGeO_3 has been studied using Raman scattering in
order to understand the interplay between defect induced states, enhanced
spin-spin correlations and the ground state of low dimensional systems.
In-chain and off-chain substitutions quench the spin-Peierls state and induce
3D antiferromagnetic order at T\leq 5 K. Consequently a suppression of a 1D
gap-induced mode as well as a constant intensity of a spinon continuum are
observed at low temperatures. A 3D two-magnon density of states now gradually
extends to higher temperatures T\leq 60K compared with pure CuGeO_3. This
effect is more pronounced in the case of off-chain substitutions (Si) for which
a N\'eel state occurs over a larger substitution range, starting at very low
concentrations. Besides, additional low energy excitations are induced. These
effects, i.e. the shift of a dimensional crossover to higher temperatures are
due to an enhancement of the spin-spin correlations induced by a small amount
of substitutions. The results are compared with recent Monte Carlo studies on
substituted spin ladders, pointing to a similar instability of coupled,
dimerized spin chains and spin ladders upon substitution.Comment: 14 pages, 6 eps figures, to be published in PR
Magnetic Resonance in the Spin-Peierls compound
We present results from magnetic resonance measurements for 75-350 GHz in
'-NaVO. The temperature dependence of the integrated
intensity indicates that we observe transitions in the excited state. A
quantitative description gives resonances in the triplet state at high symmetry
points of the excitation spectrum of this Spin-Peierls compound. This energy
has the same temperature dependence as the Spin-Peierls gap. Similarities and
differences with the other inorganic compound CuGeO are discussed.Comment: 2 pages, REVTEX, 3 figures. to be published in Phys.Rev.
Investigation of the Spin-Peierls transition in CuGeO_3 by Raman scattering
Raman experiments on the spin-Peierls compound CuGeO and the substituted
(Cu,Zn)GeO and Cu(Ge,Ga)O compounds were
performed in order to investigate the response of specific magnetic excitations
of the one-dimensional spin-1/2 chain to spin anisotropies and
substitution-induced disorder. In pure CuGeO, in addition to normal phonon
scattering which is not affected at all by the spin-Peierls transition, four
types of magnetic scattering features were observed. Below T=14 K a
singlet-triplet excitation at 30 cm, two-magnon scattering from 30 to
227 cm and folded phonon modes at 369 and 819 cm were identified.
They were assigned by their temperature dependence and lineshape. For
temperatures between the spin-Peierls transition T and approximately 100
K a broad intensity maximum centered at 300 cm is observed.Comment: 7 pages, LaTex2e, including 3 figures (eps) to be published in
Physica B (1996
NaV_2O_5 as a quarter-filled ladder compound
A new X-ray diffraction study of the one-dimensional spin-Peierls compound
\alpha-NaV_2O_5 reveals a centrosymmetric (Pmmn) crystal structure with one
type of V site, contrary to the previously postulated non-centrosymmetric
P2_1mn structure with two types of V sites (V^{+4} and V^{+5}). Density
functional calculations indicate that NaV_2O_5 is a quarter-filled ladder
compound with the spins carried by V-O-V molecular orbitals on the rungs of the
ladder. Estimates of the charge-transfer gap and the exchange coupling agree
well with experiment and explain the insulating behavior of NaV_2O_5 and its
magnetic properties.Comment: Final version for PRL, value of U correcte
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