64 research outputs found
Thermodynamic Properties of the Spin-1/2 Antiferromagnetic ladder Cu2(C2H12N2)2Cl4 under Magnetic Field
Specific heat () measurements in the spin-1/2
Cu(CHN)Cl system under a magnetic field up to
are reported and compared to the results of numerical calculations
based on the 2-leg antiferromagnetic Heisenberg ladder. While the temperature
dependences of both the susceptibility and the low field specific heat are
accurately reproduced by this model, deviations are observed below the critical
field at which the spin gap closes. In this Quantum High Field phase,
the contribution of the low-energy quantum fluctuations are stronger than in
the Heisenberg ladder model. We argue that this enhancement can be attributed
to dynamical lattice fluctuations. Finally, we show that such a Heisenberg
ladder, for , is unstable, when coupled to the 3D lattice, against a
lattice distortion. These results provide an alternative explanation for the
observed low temperature ( -- ) phase (previously
interpreted as a 3D magnetic ordering) as a new type of incommensurate gapped
state.Comment: Minor changes, list of authors complete
Field-Induced Magnetic Order in Quantum Spin Liquids
We study magnetic field-induced three-dimensional ordering transitions in
low-dimensional quantum spin liquids, such as weakly coupled, antiferromagnetic
spin-1/2 Heisenberg dimers and ladders. Using stochastic series expansion
quantum Monte Carlo simulations, thermodynamic response functions are obtained
down to ultra-low temperatures. We extract the critical scaling exponents which
dictate the power-law dependence of the transition temperature on the applied
magnetic field. These are compared with recent experiments on candidate
materials and with predictions for the Bose-Einstein condensation of magnons
obtained in mean-field theory.Comment: RevTex, 4 pages with 5 figure
Aging in a topological spin glass
We have examined the nonconventional spin glass phase of the 2-dimensional
kagome antiferromagnet (H_3 O) Fe_3 (SO_4)_2 (OH)_6 by means of ac and dc
magnetic measurements. The frequency dependence of the ac susceptibility peak
is characteristic of a critical slowing down at Tg ~ 18K. At fixed temperature
below Tg, aging effects are found which obey the same scaling law as in spin
glasses or polymers. However, in clear contrast with conventional spin glasses,
aging is remarkably insensitive to temperature changes. This particular type of
dynamics is discussed in relation with theoretical predictions for highly
frustrated non-disordered systems.Comment: 4 pages, 4 figure
Low-temperature specific heat and thermal conductivity of glycerol
We have measured the thermal conductivity of glassy glycerol between 1.5 K
and 100 K, as well as the specific heat of both glassy and crystalline phases
of glycerol between 0.5 K and 25 K. We discuss both low-temperature properties
of this typical molecular glass in terms of the soft-potential model. Our
finding of an excellent agreement between its predictions and experimental data
for these two independent measurements constitutes a robust proof of the
capabilities of the soft-potential model to account for the low-temperature
properties of glasses in a wide temperature range.Comment: 4 pages, 3 figures. To be published in Phys. Rev. B (2002
The Magnetic Spin Ladder (C_{5}H_{12}N)_{2}CuBr_{4}: High Field Magnetization and Scaling Near Quantum Criticality
The magnetization, T, 0.7 K K), from single
crystals and powder samples of (CHN)CuBr has been used
to identify this system as an Heisenberg two-leg ladder in the strong
coupling limit, K and K, with T and T. An inflection point in K) at
half-saturation, , is described by an effective \emph{XXZ} chain. The
data exhibit universal scaling behavior in the vicinity of and
, indicating the system is near a quantum critical point.Comment: 4 pages, 4 figure
Investigation of thermal and magnetic properties of defects in a spin-gap compound NaV2O5
The specific heat, magnetic susceptibility and ESR signals of a Na-deficient
vanadate Na_xV_2O_5 (x=1.00 - 0.90) were studied in the temperature range 0.07
- 10 K, well below the transition point to a spin-gap state. The contribution
of defects provided by sodium vacancies to the specific heat was observed. It
has a low temperature part which does not tend to zero till at least 0.3 K and
a high temperature power-like tail appears above 2 K. Such dependence may
correspond to the existence of local modes and correlations between defects in
V-O layers. The magnetic measurements and ESR data reveal S=1/2 degrees of
freedom for the defects, with their effective number increasing in temperature
and under magnetic field. The latter results in the nonsaturating magnetization
at low temperature. No long-range magnetic ordering in the system of defects
was found. A model for the defects based on electron jumps near vacancies is
proposed to explain the observed effects. The concept of a frustrated
two-dimensional correlated magnet induced by the defects is considered to be
responsible for the absence of magnetic ordering.Comment: 6 pages, 8 figure
Magnetic properties of a new molecular-based spin-ladder system: (5IAP)2CuBr4*2H2O
We have synthesized and characterized a new spin-1/2 Heisenberg
antiferromagnetic ladder: bis 5-iodo-2-aminopyridinium tetrabromocuprate(II)
dihydrate. X-ray diffraction studies show the structure of the compound to
consist of well isolated stacked ladders and the interaction between the Cu(2+)
atoms to be due to direct Br...Br contacts. Magnetic susceptibility and
magnetization studies show the compound to be in the strong-coupling limit,
with the interaction along the rungs (J' ~ 13 K) much greater than the
interaction along the rails (J ~ 1 K). Magnetic critical fields are observed
near 8.3 T and 10.4 T, respectively, establishing the existence of the energy
gap.Comment: 10 pages, 4 figures, submitted to Phys. Rev. B Figure 4 did not
print. *.eps files replaced with figures.ps fil
Specific heat of an S=1/2 Heisenberg ladder compound Cu(CHN)Cl under magnetic fields
Specific heat measurements down to 0.5 K have been performed on a single
crystal sample of a spin-ladder like compound
Cu(CHN)Cl under magnetic fields up to 12
T. The temperature dependence of the observed data in a magnetic field below 6
T is well reproduced by numerical results calculated for the S=1/2 two-leg
ladder with /=5. In the gapless region above 7 T
(), the agreement between experiment and calculation is good above
about 2 K and a sharp and a round peak were observed below 2 K in a magnetic
field around 10 T, but the numerical data show only a round peak, the magnitude
of which is smaller than that of the observed one. The origin of the sharp peak
and the difference between the experimental and numerical round peak are
discussed.Comment: 14 pages, 11 figures, Submitted to PR
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