534 research outputs found
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
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
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
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
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
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
Phase transitions on the surface of a carbon nanotube
A suspended carbon nanotube can act as a nanoscale resonator with remarkable
electromechanical properties and the ability to detect adsorption on its
surface at the level of single atoms. Understanding adsorption on nanotubes and
other graphitic materials is key to many sensing and storage applications. Here
we show that nanotube resonators offer a powerful new means of investigating
fundamental aspects of adsorption on carbon, including the collective behaviour
of adsorbed matter and its coupling to the substrate electrons. By monitoring
the vibrational resonance frequency in the presence of noble gases, we observe
the formation of monolayers on the cylindrical surface and phase transitions
within these monolayers, and simultaneous modification of the electrical
conductance. The monolayer observations also demonstrate the possibility of
studying the fundamental behaviour of matter in cylindrical geometry.Comment: Unpublished; 7 pages with 4 figures plus 3 pages of supplementary
materia
Damping and decoherence of a nanomechanical resonator due to a few two level systems
We consider a quantum model of a nanomechanical flexing beam resonator
interacting with a bath comprising a few damped tunneling two level systems
(TLS's). In contrast with a resonator interacting bilinearly with an ohmic free
oscillator bath (modeling clamping loss, for example), the mechanical resonator
damping is amplitude dependent, while the decoherence of quantum superpositions
of mechanical position states depends only weakly on their spatial separation
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