290 research outputs found
Recent results on energy relaxation in disordered charge and spin density waves
We briefly review different approaches used recently to describe collective
effects in the strong pinning model of disordered charge and spin density
waves, in connection with the CRTBT very low temperature heat relaxation
experiments.Comment: 4 pages, invited talk at ECRYS-200
Slow relaxation experiments in disordered charge and spin density waves: collective dynamics of randomly distributed solitons
We show that the dynamics of disordered charge density waves (CDWs) and spin
density waves (SDWs) is a collective phenomenon. The very low temperature
specific heat relaxation experiments are characterized by: (i) ``interrupted''
ageing (meaning that there is a maximal relaxation time); and (ii) a broad
power-law spectrum of relaxation times which is the signature of a collective
phenomenon. We propose a random energy model that can reproduce these two
observations and from which it is possible to obtain an estimate of the glass
cross-over temperature (typically mK). The broad
relaxation time spectrum can also be obtained from the solutions of two
microscopic models involving randomly distributed solitons. The collective
behavior is similar to domain growth dynamics in the presence of disorder and
can be described by the dynamical renormalization group that was proposed
recently for the one dimensional random field Ising model [D.S. Fisher, P. Le
Doussal and C. Monthus, Phys. Rev. Lett. {\bf 80}, 3539 (1998)]. The typical
relaxation time scales like . The
glass cross-over temperature related to correlations among solitons is
equal to the average energy barrier and scales like . is the concentration of defects, the correlation length of
the CDW or SDW and the charge or spin gap.Comment: 20 pages, 16 figure
Evidence of 1D behaviour of He confined within carbon-nanotube bundles
We present the first low-temperature thermodynamic investigation of the
controlled physisorption of He gas in carbon single-wall nanotube (SWNT)
samples. The vibrational specific heat measured between 100 mK and 6 K
demonstrates an extreme sensitivity to outgassing conditions. For bundles with
a few number of NTs the extra contribution to the specific heat, C,
originating from adsorbed He at very low density displays 1D behavior,
typical for He atoms localized within linear channels as grooves and
interstitials, for the first time evidenced. For larger bundles, C
recovers the 2D behaviour akin to the case of He films on planar
substrates (grafoil).Comment: 4 pages, 3 figures, submitted to Phys. Rev. Let
Critical behavior of the 3-state Potts model on Sierpinski carpet
We study the critical behavior of the 3-state Potts model, where the spins
are located at the centers of the occupied squares of the deterministic
Sierpinski carpet. A finite-size scaling analysis is performed from Monte Carlo
simulations, for a Hausdorff dimension . The phase
transition is shown to be a second order one. The maxima of the susceptibility
of the order parameter follow a power law in a very reliable way, which enables
us to calculate the ratio of the exponents . We find that the
scaling corrections affect the behavior of most of the thermodynamical
quantities. However, the sequence of intersection points extracted from the
Binder's cumulant provides bounds for the critical temperature. We are able to
give the bounds for the exponent as well as for the ratio of the
exponents , which are compatible with the results calculated from
the hyperscaling relation.Comment: 13 pages, 4 figure
Dynamics of the hysteretic voltage-induced torsional strain in tantalum trisulfide
We have studied how the hysteretic voltage-induced torsional strain,
associated with charge-density-wave depinning, in orthorhombic tantalum
trisulfide depends on square-wave and triangle-wave voltages of different
frequencies and amplitudes. The strains are measured by placing the sample,
with a wire glued to the center as a transducer, in a radio frequency cavity
and measuring the modulated response of the cavity. From the triangle waves, we
map out the time dependence of the hysteresis loops, and find that the
hysteresis loops broaden for waves with periods less than 30 seconds. The
square-wave response shows that the dynamic response to positive and negative
voltages can be quite different. The overall frequency dependence is
relaxational, but with multiple relaxation times which typically decrease with
increasing voltage. The detailed dynamic response is very sample dependent,
suggesting that it depends in detail on interactions of the CDW with sample
defects.Comment: 13 pages, 6 figures, to be published in Journal of Physics: Cond.
Mat
Fractional power-law susceptibility and specific heat in low temperature insulating state of o-TaS_{3}
Measurements of the magnetic susceptibility and its anisotropy in the
quasi-one-dimensional system o-TaS_{3} in its low-T charge density wave (CDW)
ground state are reported. Both sets of data reveal below 40 K an extra
paramagnetic contribution obeying a power-law temperature dependence
\chi(T)=AT^{-0.7}. The fact that the extra term measured previously in specific
heat in zero field, ascribed to low-energy CDW excitations, also follows a
power law C_{LEE}(0,T)=CT^{0.3}, strongly revives the case of random exchange
spin chains. Introduced impurities (0.5% Nb) only increase the amplitude C, but
do not change essentially the exponent. Within the two-level system (TLS)
model, we estimate from the amplitudes A and C that there is one TLS with a
spin s=1/2 localized on the chain at the lattice site per cca 900 Ta atoms. We
discuss the possibility that it is the charge frozen within a soliton-network
below the glass transition T_{g}~40 K determined recently in this system.Comment: 7 pages, 3 figures, submitted to Europhysics Letter
Microwave dielectric study of spin-Peierls and charge ordering transitions in (TMTTF)PF salts
We report a study of the 16.5 GHz dielectric function of hydrogenated and
deuterated organic salts (TMTTF)PF. The temperature behavior of the
dielectric function is consistent with short-range polar order whose relaxation
time decreases rapidly below the charge ordering temperature. If this
transition has more a relaxor character in the hydrogenated salt, charge
ordering is strengthened in the deuterated one where the transition temperature
has increased by more than thirty percent. Anomalies in the dielectric function
are also observed in the spin-Peierls ground state revealing some intricate
lattice effects in a temperature range where both phases coexist. The variation
of the spin-Peierls ordering temperature under magnetic field appears to follow
a mean-field prediction despite the presence of spin-Peierls fluctuations over
a very wide temperature range in the charge ordered state of these salts.Comment: 7 pages, 6 figure
Alternativni modeli za složeno opuštanje niskoenergijskih pobuđenja u sustavima s valovima gustoće
We apply the Palmer, Stein, Abrahams and Anderson (PSAA) model of hierarchically constrained dynamics for glassy relaxation to the complex thermal relaxation at very low temperatures in density wave systems. Alternatively, we simulate various experimental conditions in a simple, intuitive model of an electrical RC line and find some relations with the PSAA parameters.Primjenjujemo model Palmera, Steina, Abrahamsa i Andersona (PSAA) za hijerarhijski zapriječenu dinamiku opuštanja u staklima na kompleksno opuštanje topline na vrlo niskim temperaturama u sistemima s valovima gustoće. Jednako tako, pomoću jednostavnog modela slijeda električnih RC (otpor – kapacitet) krugova oponašamo različite eksperimentalne uvjete i nalazimo neke odnose s dobivenim PSAA parametrima
Electronic Instability in a Zero-Gap Semiconductor: The Charge-DensityWave in (TaSe4)(2)I
We report a comprehensive study of the paradigmatic quasi-1D compound (TaSe4)(2)I performed by means of angle-resolved photoemission spectroscopy (ARPES) and first-principles electronic structure calculations. We find it to be a zero-gap semiconductor in the nondistorted structure, with non-negligible interchain coupling. Theory and experiment support a Peierls-like scenario for the charge-density wave formation below T-CDW = 263 K, where the incommensurability is a direct consequence of the finite interchain coupling. The formation of small polarons, strongly suggested by the ARPES data, explains the puzzling semiconductor-to-semiconductor transition observed in transport at T-CDW.open114sciescopu
Equilibrium Low Temperature Heat Capacity of the Spin Density Wave compound (TMTTF)2 Br: effect of a Magnetic Field
We have investigated the effect of the magnetic field (B) on the very
low-temperature equilibrium heat capacity ceq of the quasi-1 D organic compound
(TMTTF)2Br, characterized by a commensurate Spin Density Wave (SDW) ground
state. Below 1K, ceq is dominated by a Schottky-like contribution, very
sensitive to the experimental time scale, a property that we have previously
measured in numerous DW compounds. Under applied field (in the range 0.2- 7 T),
the equilibrium dynamics, and hence ceq extracted from the time constant,
increases enormously. For B = 2-3 T, ceq varies like B2, in agreement with a
magnetic Zeeman coupling. Another specific property, common to other
Charge/Spin density wave (DW) compounds, is the occurrence of metastable
branches in ceq, induced at very low temperature by the field exceeding a
critical value. These effects are discussed within a generalization to SDWs in
a magnetic field of the available Larkin-Ovchinnikov local model of strong
pinning. A limitation of the model when compared to experiments is pointed out.Comment: 10 pages, 11 figure
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