208 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
Relaxation time spectrum of low-energy excitations in one- and two-dimensional materials with charge or spin density waves
The long-time thermal relaxation of (TMTTF)Br, SrCuO
and SrCaCuO single crystals at temperatures below 1 K
and magnetic field up to 10 T is investigated. The data allow us to determine
the relaxation time spectrum of the low energy excitations caused by the
charge-density wave (CDW) or spin-density wave (SDW). The relaxation time is
mainly determined by a thermal activated process for all investigated
materials. The maximum relaxation time increases with increasing magnetic
field. The distribution of barrier heights corresponds to one or two Gaussian
functions. The doping of SrCaCuO with Ca leads to
a drastic shift of the relaxation time spectrum to longer time. The maximum
relaxation time changes from 50 s (x = 0) to 3000 s (x = 12) at 0.1 K and 10 T.
The observed thermal relaxation at x=12 clearly indicates the formation of the
SDW ground state at low temperatures
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Heat capacity signature of frustrated trimerons in magnetite
Recently it has been proposed that the long-range electronic order formed by trimerons in magnetite should be frustrated due to the great degeneracy of arrangements linking trimerons. This result has important consequences as charge ordering from the condensed minority band electrons leads to a complex 3D antiferro orbital order pattern. Further more, the corner sharing tetrahedra structure of spinel B-sites supports frustration for antiferromagnetic alignments. Therefore frustration due to competing interactions will itself induce disorder and very likely frustration in the spin orientations. Here we present very low temperature specific heat data that show two deviations to the magnons and phonons contributions, that we analyze in terms of Schottky-type anomalies. The first one is associated with the thermal activation across both ferroelastic twin and ferromagnetic anti-phase domains. The second Schottky-type anomaly displays an inverse (1/H) field dependence which is a direct indication of the disordered glassy network with macroscopically degenerated singular ground states. © 2020, The Author(s)
Interplay between phase defects and spin polarization in the specific heat of the spin density wave compound (TMTTF)_2Br in a magnetic field
Equilibrium heat relaxation experiments provide evidence that the ground
state of the commensurate spin density wave (SDW) compound (TMTTF)Br after
the application of a sufficient magnetic field is different from the
conventional ground state. The experiments are interpreted on the basis of the
local model of strong pinning as the deconfinement of soliton-antisoliton pairs
triggered by the Zeeman coupling to spin degrees of freedom, resulting in a
magnetic field induced density wave glass for the spin carrying phase
configuration.Comment: 4 pages, 5 figure
Low energy excitations in crystalline perovskite oxides: Evidence from noise experiments
In this paper we report measurements of 1/f noise in a crystalline metallic
oxide with perovskite structure down to 4.2K. The results show existence of
localized excitations with average activation energy 70-80 meV which
produce peak in the noise at T 35-40K. In addition, it shows clear
evidence of tunnelling type two-level-systems (as in glasses) which show up in
noise measurements below 30K.Comment: 11 pages, 4 figures, to appear in Phys Rev B, vol 58, 1st Dec issu
Magnetic excitations and effects of magnetic fields on the spin-Peierls transition in CuGeO
We analyze the magnetic excitations of a spin-1/2 antiferromagnetic
Heisenberg model with alternating nearest neighbor interactions and uniform
second neighbor interactions recently proposed to describe the spin-Peierls
transition in CuGeO. We show that there is good agreement between the
calculated excitation dispersion relation and the experimental one. We have
also shown that this model reproduces satisfactorily the experimental results
for the magnetization vs. magnetic field curve and its saturation value. The
model proposed also reproduces qualitatively some features of the magnetic
phase diagram of this compound and the overall behavior of the magnetic
specific heat in the presence of applied magnetic fields.Comment: 12 pages Revtex v2.0 + 4 figures postscripts include
Phenomenological model of elastic distortions near the spin-Peierls transition in
A phenomenological model of the Landau type forms the basis for a study of
elastic distortions near the spin-Peierls transition in . The
atomic displacements proposed by Hirota {\it et al.} [Phys. Rev. Lett. {\bf
73}, 736 (1994)] are accounted for by the model which includes linear coupling
between and distortions. displacements are seen to be responsible
for anomalies in the elastic properties {\it at} , whereas incipient
distortions give rise to temperature dependence below . A discussion of
possible critical behavior is also made.Comment: 1 figure available upon reques
Study of the magnetic susceptibility in the spin-Peierls system CuGeO
We study numerically, using a one-dimensional Heisenberg model, the
spin-Peierls transition in the linear Cu spin-1/2 chains in the
inorganic compound CuGeO which has been recently observed experimentally.
We suggest that the magnetic susceptibility, the temperature dependence of the
spin gap and the spin-Peierls transition temperature of this material can be
reasonably described by including nearest and next nearest neighbor
antiferromagnetic interactions along the chain. We estimate that the nearest
neighbor exchange parameter J is approximately , and that the next
nearest neighbor exchange parameter is approximately .Comment: 14 pages, Revtex v2.0, 4 figures available upon reques
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