2,990 research outputs found
NMR imaging of the soliton lattice profile in the spin-Peierls compound CuGeO_3
In the spin-Peierls compound CuGeO, the commensurate-incommensurate
transition concerning the modulation of atomic position and the local
spin-polarization is fully monitored at T=0 by the application of an external
magnetic field () above a threshold value 13 Tesla. The
solitonic profile of the spin-polarization, as well as its absolute magnitude,
has been precisely imaged from NMR lineshapes obtained for
varying from 0.0015 to 2. This offers a unique possibility
to test quantitatively the various numerical and analytical methods developed
to solve a generic Hamiltonian in 1-D physics, namely strongly interacting
fermions in presence of electron-phonon coupling at arbitrary band filling.Comment: 3 pages, 4 eps figures, RevTeX, submitted to Physical Review Lette
Mixing of magnetic and phononic excitations in incommensurate Spin-Peierls systems
We analyze the excitation spectra of a spin-phonon coupled chain in the
presence of a soliton. This is taken as a microscopic model of a Spin-Peierls
material placed in a high magnetic field. We show, by using a semiclassical
approximation in the bosonized representation of the spins that a trapped
magnetic state obtained in the adiabatic approximation is destroyed by
dynamical phonons. Low energy states are phonons trapped by the soliton. When
the magnetic gap is smaller than the phonon frequencies the only low energy
state is a mixed magneto-phonon state with the energy of the gap. We emphasize
that our results are relevant for the Raman spectra of the inorganic
Spin-Peierls material CuGeO.Comment: 5 pages, latex, 2 figures embedded in the tex
Charge Order Driven spin-Peierls Transition in NaV2O5
We conclude from 23Na and 51V NMR measurements in NaxV2O5(x=0.996) a charge
ordering transition starting at T=37 K and preceding the lattice distortion and
the formation of a spin gap Delta=106 K at Tc=34.7 K. Above Tc, only a single
Na site is observed in agreement with the Pmmn space group of this first
1/4-filled ladder system. Below Tc=34.7 K, this line evolves into eight
distinct 23Na quadrupolar split lines, which evidences a lattice distortion
with, at least, a doubling of the unit cell in the (a,b) plane. A model for
this unique transition implying both charge density wave and spin-Peierls order
is discussed.Comment: 4 pages, 3 figures, submitted to Phys. Rev. Let
Modulated Phases in Spin-Peierls Systems
Lattice modulations in the high magnetic field phase and close to impurities
in spin-Peierls systems are considered and compared to experiment. Necessary
extensions of existing theories are proposed. The influence of zero-point
fluctuations on magnetic amplitudes is shown.Comment: 10 pages, 4 figures included, to appear in Advances in Solid State
Physics/Festkoerperprobleme Spring Conference 1999 of the DP
The electronic structure and the phases of BaVS3
BaVS3 is a moderately correlated d-electron system with a rich phase diagram.
To construct the corresponding minimal electronic model, one has to decide
which d-states are occupied, and to which extent. The ARPES experiment
presented here shows that the behavior of BaVS3 is governed by the coexistence
of wide-band (A_1g) and narrow-band (twofold degenerate E) d-electrons. We
sketch a lattice fermion model which may serve as a minimal model of BaVS3.
This serves foremost for the understanding of the metal-insulator in pure BaVS3
and its absence in some related compounds. The nature of the low temperature
magnetic order differs for several systems which may be described in terms of
the same electron model. We describe several recent experiments which give
information about magnetic order at high pressures. In particular, we discuss
field-induced insulator-to-metal transition at slightly subcritical pressures,
and an evidence for magnetic order in the high-pressure metallic phase. The
phase diagram of Sr-doped BaVS3 is also discussed. The complexity of the phases
of BaVS3 arises from the fact that it is simultaneously unstable against
several kinds of instabilities.Comment: Presented at the International Conference on Magnetism 2006 (Kyoto),
6 pages, 9 figure
Ab initio evaluation of the charge-ordering in
We report {\it ab initio} calculations of the charge ordering in
using large configurations interaction methods on
embedded fragments. Our major result is that the electrons of the
bridging oxygen of the rungs present a very strong magnetic character and
should thus be explicitly considered in any relevant effective model. The most
striking consequence of this result is that the spin and charge ordering differ
substantially, as differ the experimental results depending on whether they are
sensitive to the spin or charge density.Comment: 4 page
Ultrafast Atomic Diffusion Inducing a Reversible (2√3×2√3)R30°↔(√3×√3)R30° Transition on Sn/Si(111)∶B
Dynamical phase transitions are a challenge to identify experimentally and describe theoretically. Here, we study a new reconstruction of Sn on silicon and observe a reversible transition where the surface unit cell divides its area by a factor of 4 at 250 °C. This phase transition is explained by the 24-fold degeneracy of the ground state and a novel diffusive mechanism, where four Sn atoms arranged in a snakelike cluster wiggle at the surface exploring collectively the different quantum mechanical ground states.This work was supported by the French Agence Nationale de la Recherche (ANR) under Contract SurMott, No. NT-09-618999, and by Spanish Ministerio de EconomÃa y Competitividad, Project No. MAT2014-59966-R
Zigzag Charge Ordering in alpha'-NaV2O5
23Na NMR spectrum measurements in alpha'-NaV2O5 with a single- crystalline
sample are reported. In the charge-ordered phase, the number of inequivalent Na
sites observed is more than that expected from the low-temperature structures
of space group Fmm2 reported so far. This disagreement indicates that the real
structure including both atomic displacement and charge disproportionation is
of lower symmetry. It is suggested that zigzag ordering is the most probable.
The temperature variation of the NMR spectra near the transition temperature is
incompatible with that of second-order transitions. It is thus concluded that
the charge ordering transition is first-order.Comment: 4 pages, 5 eps figures, submitted to J. Phys. Soc. Jp
Thermal Conductivity of the Spin Peierls Compound CuGeO_3
The thermal conductivity of the Spin-Peierls (SP) compound CuGeO_3 was
measured in magnetic fields up to 16 T. Above the SP transition, the heat
transport due to spin excitations causes a peak at around 22 K, while below the
transition the spin excitations rapidly diminish and the heat transport is
dominated by phonons; however, the main scattering process of the phonons is
with spin excitations, which demonstrates itself in an unusual peak in the
thermal conductivity at about 5.5 K. This low-temperature peak is strongly
suppressed with magnetic fields in excess of 12.5 T.Comment: 6 pages, including 2 postscript figure
Competition and coexistence of bond and charge orders in (TMTTF)2AsF6
(TMTTF)2AsF6 undergoes two phase transitions upon cooling from 300 K. At
Tco=103 K a charge-ordering (CO) occurs, and at Tsp(B=9 T)=11 K the material
undergoes a spin-Peierls (SP) transition. Within the intermediate, CO phase,
the charge disproportionation ratio is found to be at least 3:1 from carbon-13
NMR 1/T1 measurements on spin-labeled samples. Above Tsp, up to about 3Tsp,
1/T1 is independent of temperature, indicative of low-dimensional magnetic
correlations. With the application of about 0.15 GPa pressure, Tsp increases
substantially, while Tco is rapidly suppressed, demonstrating that the two
orders are competing. The experiments are compared to results obtained from
calculations on the 1D extended Peierls-Hubbard model.Comment: 4 pages, 5 figure
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