366 research outputs found
Theory of phonon-assisted "forbidden" optical transitions in spin-gapped systems
We consider the absorption of light with emission of one S(tot)=1 magnetic
excitation in systems with a spin gap induced by quantum fluctuations. We argue
that an electric dipole transition is allowed on the condition that a virtual
phonon instantaneously breaks the inversion symmetry. We derive an effective
operator for the transition and argue that the proposed theory explains the
polarized experiments in CuGeO(3) and SrCu(2)[BO(3)](2).Comment: 9 pages, 4 figure
Stability of NbsSn Wires with CuNb Reinforcing Stabilizer on Cryocooled Superconducting Magnet
Magnetic structures of RbCuCl_3 in a transverse field
A recent high-field magnetization experiment found a phase transition of
unknown character in the layered, frustrated antiferromagnet RbCuCl_3, in a
transverse field (in the layers). Motivated by these results, we have examined
the magnetic structures predicted by a model of RbCuCl_3, using the classical
approximation. At small fields, we obtain the structure already known to be
optimal, an incommensurate (IC) spiral with wave vector q in the layers. At
higher fields, we find a staircase of long-period commensurate (C) phases
(separated initially by the low-field IC phase), then two narrow IC phases,
then a fourth IC phase (also with intermediate C phases), and finally the
ferromagnetically aligned phase at the saturation field H_S. The
three-sublattice C states familiar from the theory of the triangular
antiferromagnet are never optimal. The C phases and the two intermediate IC
phases were previously unknown in this context. The magnetization is
discontinuous at a field \approx 0.4H_S, in qualitative agreement with
experiment, though we find much fine structure not reported.Comment: 9 pages, 8 figure
Practical Use of a Liquid Helium-Free Superconducting Magnet(Magnet Technology)
A cryocooler-cooled 4.6 T superconducting magnet with a 38 mm room temperature bore, which consists of a low-T_c Nb_3Sn coil and high-T_c Bi_2Sr_2Ca_2Cu3O_ current leads, has been working in vacuum for about 18000 cooling hours without trouble, It is found that the high-T_c current leads can hold excellent superconducting properties for a long enough time to be practically used. As a next step, we have succeeded in the construction of a l0.7 T-52 mm room temperature bore and a 5.7 T-220 mm room temperature bore liquid helium-free superconducting magnet
High Field Magnetization and Spin Reorientation in Sm_2(Fe_<1-x>Al_x)_<17> and Nd_2(Fe_<1-x>Al_x)_<17> Single Crystals(Research in High Magnetic Fields)
High field magnetization measurements have been performed for single crystals of R_2(Fe_Al_x)_ with R=Sm and Nd. The magnetization along the c-axis has been found to show a small spontaneous component, indicating that the total magnetization vector is not within the c-plane but in the plane spanned by the b- and c-axes. The angle between the magnetization direction and the b-axis increases linearly with increasing x in the Sm system. The results are analyzed on the basis of the crystalline electric field theory
ESR investigation on the Breather mode and the Spinon-Breather dynamical crossover in Cu Benzoate
A new elementary-excitation, the so called "breather excitation", is observed
directly by millimeter-submillimeter wave electron spin resonance (ESR) in the
Heisenberg quantum spin-chain Cu benzoate, in which a field-induced gap is
found recently by specific heat and neutron scattering measurements. Distinct
anomalies were found in line width and in resonance field around the "dynamical
crossover" regime between the gap-less spinon-regime and the gapped
breather-regime. When the temperature becomes sufficiently lower than the
energy gap, a new ESR-line with very narrow line-width is found, which is the
manifestation of the breather excitation. The non-linear field dependence of
the resonance field agrees well with the theoretical formula of the first
breather-excitation proposed by Oshikawa and Affleck. The present work
establishes experimentally for the first time that a sine-Gordon model is
applicable to explain spin dynamics in a S=1/2 Heisenberg spin chain subjected
to staggered field even in high fields.Comment: Revtex, 4 pages, 4 figures, submitted to Phys. Rev. Let
Magnetotransport properties of (Ga,Mn)As investigated at low temperature and high magnetic field
Magnetotransport properties of ferromagnetic semiconductor (Ga,Mn)As have
been investigated. Measurements at low temperature (50 mK) and high magnetic
field (<= 27 T) have been employed in order to determine the hole concentration
p = 3.5x10^20 cm ^-3 of a metallic (Ga0.947Mn0.053)As layer. The analysis of
the temperature and magnetic field dependencies of the resistivity in the
paramagnetic region was performed with the use of the above value of p, which
gave the magnitude of p-d exchange energy |N0beta | ~ 1.5 eV.Comment: PDF file, 8 pages, 4 figure
Quantum Fluctuation-Induced Phase Transition in S=1/2 XY-like Heisenberg Antiferromagnets on the Triangular Lattice
The selection of the ground state among nearly degenerate states due to
quantum fluctuations is studied for the S=1/2 XY-like Heisenberg
antiferromagnets on the triangular lattice in the magnetic field applied along
the hard axis, which was first pointed out by Nikuni and Shiba. We find that
the selected ground state sensitively depends on the degree of the anisotropy
and the magnitude of the magnetic field. This dependence is similar to that in
the corresponding classical model at finite temperatures where various types of
field induced phases appear due to the entropy effect. It is also found that
the similarity of the selected states in the classical and quantum models are
not the case in a two-leg ladder lattice, although the lattice consists of
triangles locally and the ground state of this lattice in the classical case is
the same as that of the triangular lattice.Comment: 15 pages, 35 figure
Magnetic Phase Diagrams with Possible Field-induced Antiferroquadrupolar Order in TbBC
Magnetic phase diagrams of a tetragonal antiferromagnet TbBC were
clarified by temperature and field dependence of magnetization. It is
noticeable that the N{\'e}el temperature in TbBC is anomalously
enhanced with magnetic fields, in particular the enhancement reaches 13.5 K for
the direction at 10 T. The magnetization processes as well as the
phase diagrams are well interpreted assuming that there appear field-induced
antiferroquadrupolar ordered phases in TbBC. The phase diagrams of the
AFQ compounds in RBC are systematically understood in terms of the
competition with AFQ and AFM interactions.Comment: 4 pages, 4 figures, RevTeX
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