57 research outputs found
Microwave transport approach to the coherence of interchain hopping in (TMTSF)2PF6
We report a microwave study of the longitudinal and transverse transport
properties of the quasi-one-dimensional organic conductor (TMTSF)PF in
its normal phase. The contactless technique have provided a direct measurement
of the temperature profile of the resistivity along the {\bf b'} direction and
in magnetic fields up to 14 T. A characteristic energy scale ()
has been observed which delimits a transient regime from an insulating to a
metallic behavior. This anomalous profile is discussed in terms of the onset of
coherent transport properties along the {\bf b'} direction below 40 K. This is
also supported by the observation of a finite longitudinal and transverse
magnetoresitances only below 40 K, indicative of a two-dimensional regime.
Below , however, strong deviations with respect to a Fermi liquid behavior
are evidenced.Comment: 4 pages, 5 figures, submitted to Euro.Phys.J.
Flux pinning in (1111) iron-pnictide superconducting crystals
Local magnetic measurements are used to quantitatively characterize
heterogeneity and flux line pinning in PrFeAsO_1-y and NdFeAs(O,F)
superconducting single crystals. In spite of spatial fluctuations of the
critical current density on the macroscopic scale, it is shown that the major
contribution comes from collective pinning of vortex lines by microscopic
defects by the mean-free path fluctuation mechanism. The defect density
extracted from experiment corresponds to the dopant atom density, which means
that dopant atoms play an important role both in vortex pinning and in
quasiparticle scattering. In the studied underdoped PrFeAsO_1-y and NdFeAs(O,F)
crystals, there is a background of strong pinning, which we attribute to
spatial variations of the dopant atom density on the scale of a few dozen to
one hundred nm. These variations do not go beyond 5% - we therefore do not find
any evidence for coexistence of the superconducting and the antiferromagnetic
phase. The critical current density in sub-T fields is characterized by the
presence of a peak effect, the location of which in the (B,T)-plane is
consistent with an order-disorder transition of the vortex lattice.Comment: 12 pages, submitted to Phys Rev.
A Model Study of the Low-Energy Charge Dynamics of NaV_2O_5
An exact-diagonalization technique on small clusters is used to calculate the
dynamical density correlation functions of the dimerized t-J chain and coupled
anisotropic t-J ladders (trellis lattice) at quarter filling, i.e., the systems
regarded as a network of pairs (dimers or rungs) of sites coupled weakly via
the hopping and exchange interactions. We thereby demonstrate that the
intersite Coulomb repulsions between the pairs induce a low-energy collective
mode in the charge excitations of the systems where the internal charge degrees
of freedom of the pairs play an essential role. Implications to the electronic
states of NaV_2O_5, i.e., fluctuations of the valence state of V ions and phase
transition as a charge ordering, are discussed.Comment: 4 pages, 4 gif figures. Hardcopies of figures (or the entire
manuscript) can be obtained by e-mail request to [email protected]
Coexistence of charge density wave and spin-Peierls orders in quarter-filled quasi-one dimensional correlated electron systems
Charge and spin-Peierls instabilities in quarter-filled (n=1/2) compounds
consisting of coupled ladders and/or zig-zag chains are investigated. Hubbard
and t-J models including local Holstein and/or Peierls couplings to the lattice
are studied by numerical techniques. Next nearest neighbor hopping and magnetic
exchange, and short-range Coulomb interactions are also considered. We show
that, generically, these systems undergo instabilities towards the formation of
Charge Density Waves, Bond Order Waves and (generalized) spin-Peierls modulated
structures. Moderate electron-electron and electron-lattice couplings can lead
to a coexistence of these three types of orders. In the ladder, a zig-zag
pattern is stabilized by the Holstein coupling and the nearest-neighbor Coulomb
repulsion. In the case of an isolated chain, bond-centered and site-centered
2k_F and 4k_F modulations are induced by the local Holstein coupling. In
addition, we show that, in contrast to the ladders, a small charge ordering in
the chains, strongly enhances the spin-Peierls instability. Our results are
applied to the NaV_2O_5 compound (trellis lattice) and various phases with
coexisting charge disproportionation and spin-Peierls order are proposed and
discussed in the context of recent experiments. The role of the long-range
Coulomb potential is also outlined.Comment: 10 pages, Revtex, 10 encapsulated figure
Composition-, temperature- and pressure-induced transitions between high-pressure stabilized perovskite phases of the (1-x)BiFe0.5Sc0.5O3 - xLaFe0.5Sc0.5O3 series
Crystal structures of the high-pressure synthesized perovskite phases of the (1-x)BiFe0.5Sc0.5O3-xLaFe0.5Sc0.5O3 (0 = x ≤ 1) system and their temperature and pressure behaviours were studied using laboratory and synchrotron X-ray diffractions as well as neutron diffraction. At room temperature, the as-prepared phases with x ≤ 0.05 have an antipolar structure with the Pnma symmetry and with the √2ap × 4ap × 2√2ap superstructure (where ap is the pseudocubic perovskite unit-cell parameter). An incommensurately modulated phase with the Imma(00γ)s00 superspace group is observed for 0.10 = x ≤ 0.33, while a non-polar Pnma phase (√2ap × 2ap × √2ap) is stable when x ≥ 0.34. The antipolar Pnma phase in the as-prepared samples with composition corresponding to x = 0 transforms into the polar Ima2 one via irreversible annealing-caused transformation accompanied by a formation of a high-temperature intermediate polar R3c polymorph, while the antipolar Pnma phase in samples with x = 0.05 is stable until the decomposition temperature. In the solid solutions with 0.10 = x ≤ 0.33, increasing temperature was found to result in a reversible transformation of the Imma(00γ)s00 phase into a non-polar Pnma one. The transition temperature decreases with increasing x. A hydrostatic pressure of few GPa was also shown to induce a reversible Imma(00γ)s00 → Pnma transformation.publishe
Anomalous thermal conductivity of NaV2O5 as compared to conventional spin-Peierls system CuGeO3
A huge increase of thermal conductivity k is observed at the phase transition
in stoichiometric NaV2O5. This anomaly decreases and gradually disappears with
deviation from stoichiometry in Na(1-x}V2O5 (x = 0.01, 0.02, 0.03, and 0.04).
This behavior is compared with that of pure and Zn-doped CuGeO3 where only
modest kinks in the k(T) curves are observed at the spin-Peierls transition.
The change of k at critical temperature Tc could be partially attributed to the
opening of an energy gap in the magnetic excitation spectrum excluding the
scattering of thermal phonons on spin fluctuations. However, the reason for
such a strong anomaly in the k(T) may lie not only in the different energy
scales of CuGeO3 and NaV2O5, but also in the different character of the phase
transition in NaV2O5 which can have largely a structural origin, e.g. connected
with the charge ordering.Comment: PostScript 4 pages, 4 PostScript pictures. Submitted to Physical
Review Letter
Long-range modulation of a composite crystal in a five-dimensional superspace
Citation: Guerin, L., Mariette, C., Rabiller, P., Huard, M., Ravy, S., Fertey, P., . . . Toudic, B. (2015). Long-range modulation of a composite crystal in a five-dimensional superspace. Physical Review B, 91(18), 7. doi:10.1103/PhysRevB.91.184101The intergrowth crystal of n-tetracosane/urea presents a misfit parameter, defined by the ratio gamma = c(h)/c(g) (c(host)/c(guest)), that is very close to a commensurate value (gamma congruent to 1/3). High-resolution diffraction studies presented here reveal an aperiodic misfit parameter of gamma = 0.3369, which is found to be constant at all temperatures studied. A complex sequence of structural phases is reported. The high temperature phase (phase I) exists in the four-dimensional superspace group P6(1)22(00 gamma). At T-c1 = 179(1) K, a ferroelastic phase transition increases the dimension of the crystallographic superspace. This orthorhombic phase (phase II) is characterized by the five-dimensional (5D) superspace group C222(1)(00 gamma)(10 delta) with a modulation vector a(o)* + c(m)* = a(o)* + delta . c(h)*, in which the supplementary misfit parameter is delta = 0.025(1) in host reciprocal units. This corresponds to the appearance of a modulation of very long period (about 440 +/- 16 angstrom). At T-c2 = 163.0(5) K, a 5D to 5D phase transition leads to the crystallographic superspace group P2(1)2(1)2(1)(00 gamma)(00 delta) with a very similar value of delta. This phase transition reveals a significant hysteresis effect
High frequency ESR investigation on dynamical charge disproportionation and spin gap excitation in NaV_2O_5
A significant frequency dependence of the ESR line width is found in NaV_2O_5
between 34-100 K and the line width increases as the resonance frequency is
increased from 95 GHz to 760 GHz. The observed frequency dependence is
qualitatively explained in terms of the dynamical charge disproportionation.
The present results show the essential role of the internal charge degree of
freedom in a V-O-V bond. We have also proposed the existence of the
Dzyaloshinsky-Moriya interaction in the low temperature charge ordered phase
considering the breaking of the selection rule of ESR realized as the direct
observation of the spin gap excitation.Comment: 9 figures submitted to J. Phys.Soc. Jp
Spin-Peierls transition in NaV2O5 in high magnetic fields
We investigate the magnetic field dependence of the spin-Peierls transition
in NaVO in the field range 16T-30T. The transition temperature exhibits
a very weak variation with the field, suggesting a novel mechanism for the
formation of the spin-Peierls state. We argue that a charge ordering transition
accompanied by singlet formation is consistent with our observations.Comment: 4 pages, 3 figures, final version to appear in Phys. Rev. B (RC
Spin gap behavior and charge ordering in \alpha^{\prime}-NaV_2O_5 probed by light scattering
We present a detailed analysis of light scattering experiments performed on
the quarter-filled spin ladder compound -NaVO for
the temperature range 5 KT300 K. This system undergoes a phase
transition into a singlet ground state at T=34 K accompanied by the formation
of a super structure. For T34 K several new modes were detected. Three of
these modes are identified as magnetic bound states. Experimental evidence for
charge ordering on the V sites is detected as an anomalous shift and splitting
of a V-O vibration at 422 cm for temperatures above 34 K. The smooth and
crossover-like onset of this ordering at T= 80 K is accompanied by
pretransitional fluctuations both in magnetic and phononic Raman scattering. It
resembles the effect of stripe order on the super structure intensities in
LaNiO.Comment: 36 pages, 11 figures, accepted for publication in PRB (sept.99
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