5,028 research outputs found
Ground states with cluster structures in a frustrated Heisenberg chain
We examine the ground state of a Heisenberg model with arbitrary spin S on a
one-dimensional lattice composed of diamond-shaped units. A unit includes two
types of antiferromagnetic exchange interactions which frustrate each other.
The system undergoes phase changes when the ratio between the
exchange parameters varies. In some phases, strong frustration leads to larger
local structures or clusters of spins than a dimer. We prove for arbitrary S
that there exists a phase with four-spin cluster states, which was previously
found numerically for a special value of in the S=1/2 case. For S=1/2
we show that there are three ground state phases and determine their
boundaries.Comment: 4 pages, uses revtex.sty, 2 figures available on request from
[email protected], to be published in J. Phys.: Cond. Mat
Spin Gap of Two-Dimensional Antiferromagnet Representing CaVO
We examined a two-dimensional Heisenberg model with two kinds of exchange
energies, and . This model describes localized spins at vanadium
ions in a layer of CaVO, for which a spin gap is found by a recent
experiment. Comparing the high temperature expansion of the magnetic
susceptibility to experimental data, we determined the exchange energies as
610 K and 150 K. By the numerical diagonalization we
estimated the spin gap as 120 K, which consists
with the experimental value 107 K. Frustration by finite enhances the
spin gap.Comment: 12 pages of LaTex, 4 figures availavule upon reques
Comment on "c-axis Josephson tunneling in -wave superconductors''
This comment points out that the recent paper by Maki and Haas [Phys. Rev. B
{\bf 67}, 020510 (2003)] is completely wrong.Comment: 1 page, submittted to Phys. Rev.
Measurement schemes for the spin quadratures on an ensemble of atoms
We consider how to measure collective spin states of an atomic ensemble based
on the recent multi-pass approaches for quantum interface between light and
atoms. We find that a scheme with two passages of a light pulse through the
atomic ensemble is efficient to implement the homodyne tomography of the spin
state. Thereby, we propose to utilize optical pulses as a phase-shifter that
rotates the quadrature of the spins. This method substantially simplifies the
geometry of experimental schemes.Comment: 4pages 2 figure
Tomonaga-Luttinger Liquid in a Quasi-One-Dimensional S=1 Antiferromagnet Observed by the Specific Heat
Specific heat experiments on single crystals of the S=1 quasi-one-dimensional
bond-alternating antiferromagnet Ni(C_9H_24N_4)(NO_2)ClO_4, alias NTENP, have
been performed in magnetic fields applied both parallel and perpendicular to
the spin chains. We have found for the parallel field configuration that the
magnetic specific heat (C_mag) is proportional to temperature (T) above a
critical field H_c, at which the energy gap vanishes, in a temperature region
above that of the long-range ordered state. The ratio C_mag/T increases as the
magnetic field approaches H_c from above. The data are in good quantitative
agreement with the prediction of the c=1 conformal field theory in conjunction
with the velocity of the excitations calculated by a numerical diagonalization,
providing a conclusive evidence for a Tomonaga-Luttinger liquid.Comment: 4 pages, 4 postscript figure
Specific heat of the spin-dimer antiferromagnet BaMnO in high magnetic fields
We have measured the specific heat of the coupled spin-dimer antiferromagnet
BaMnO to 50 mK in temperature and to 29 T in the magnetic field.
The experiment extends to the midpoint of the field region (25.9 T 32.3 T) of the magnetization plateau at 1/2 of the saturation
magnetization, and reveals the presence of three ordered phases in the field
region between that of the magnetization plateau and the low-field spin-liquid
region. The exponent of the phase boundary with the thermally disordered region
is smaller than the theoretical value based on the Bose-Einstein condensation
of spin triplets. At zero field and 29 T, the specific-heat data show gapped
behaviors characteristic of spin liquids. The zero-field data indicate that the
gapped triplet excitations form two levels whose energies differ by nearly a
factor of two. At least the lower level is well localized. The data at 29 T
reveal that the low-lying excitations at the magnetization plateau are weakly
delocalized.Comment: 6 pages, 5 figures, revised versio
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