1,281 research outputs found
Reentrant Spin-Peierls Transition in Mg-Doped CuGeO_3
We report a synchrotron x-ray scattering study of the diluted spin-Peierls
(SP) material Cu_{1-x}Mg_xGeO_3. In a recent paper we have shown that the SP
dimerization attains long-range order only for x < x_c = 0.022(0.001). Here we
report that the SP transition is reentrant in the vicinity of the critical
concentration x_c. This is manifested by broadening of the SP dimerization
superlattice peaks below the reentrance temperature, T_r, which may mean either
the complete loss of the long-range SP order or the development of a
short-range ordered component within the long-range ordered SP state. Marked
hysteresis and very large relaxation times are found in the samples with Mg
concentrations in the vicinity of x_c. The reentrant transition is likely
related to the competing Neel transition which occurs at a temperature similar
to T_r. We argue that impurity-induced competing interchain interactions play
an essential role in these phenomena.Comment: 5 pages, 4 embedded eps figure
Metal-insulator transition in Ca_{1-x}Li_xPd_3O_4
Metal-insulator transition in Ca_{1-x}Li_xPd_3O_4 has been studied through
charge transport measurements. The resistivity, the Seebeck coefficient, and
the Hall coefficient are consistently explained in terms of a simple one-band
picture, where a hole with a moderately enhanced mass is itinerant
three-dimensionally. Contrary to the theoretical prediction [Phys. Rev. B62,
13426 (2000)], CaPd_3O_4 is unlikely to be an excitonic insulator, and holds a
finite carrier concentration down to 4.2 K. Thus the metal-insulator transition
in this system is basically driven by localization effects.Comment: RevTeX4 format, 4 pages, 5 eps figure
Spin-Vacancy-Induced Long-Range Order in a New Haldane-Gap Antiferromagnet
Magnetic susceptibility, high-field magnetization and inelastic neutron
scattering experiments are used to study the magnetic properties of a new S=1
quasi-1-dimensional antiferromagnet PbNi2V2O8. Inter-chain interactions are
shown to be almost, but not quite, strong enough to destroy the nonmagnetic
singlet ground state and the energy gap in the magnetic excitation spectrum.
Substituting nonmagnetic Mg (S=0) ions for Ni (S=1) induces a
magnetically ordered state at low temperatures. To our knowledge this is the
first observation of doping-induced long-range order in a Haldane-gap system.Comment: 5 pages including 4 figure
The first-order phase transition between dimerized-antiferromagnetic and uniform-antiferromagnetic phases in Cu_(1-x)M_xGeO_3
We have performed detailed magnetic susceptibility measurements as well as
synchrotron x-ray diffraction studies to determine the temperature vs
concentration ( - ) phase diagram of CuMgGeO. We
observe clear double peaks in the magnetic susceptibility implying two
antiferromagnetic (AF) transition temperatures in samples with Mg
concentrations in the range 0.0237 0.0271. We also observe a
drastic change in the inverse correlation length in this concentration range by
x-ray diffraction. The drastic change of the AF transition temperature as well
as the disappearance of the spin-Peierls (SP) phase have been clarified; these
results are consistent with a first-order phase transition between dimerized AF
(D-AF) and uniform AF (U-AF) phases as reported by T. Masuda {\it et al.}
\lbrack Phys. Rev. Lett. {\bf 80}, 4566 (1998)\rbrack. The - phase
diagram of CuZnGeO is similar to that of
CuMgGeO, which suggests that the present phase transition
is universal for CuGeO.Comment: 7 pages, 5 figures. submitted to PR
Separation of the magnetic phases at the N\'{e}el point in the diluted spin-Peierls magnet CuGeO3
The impurity induced antiferromagnetic ordering of the doped spin-Peierls
magnet Cu(1-x)Mg(x)GeO(3) was studied by ESR technique. Crystals with the Mg
concentration x<4% demonstrate a coexistence of paramagnetic and
antiferromagnetic ESR modes. This coexistence indicates the separation of a
macroscopically uniform sample in the paramagnetic and antiferromagnetic
phases. In the presence of the long-range spin-Peierls order (in a sample with
x=1.71%) the volume of the antiferromagnetic phase immediately below the
N\'{e}el point T_N is much smaller than the volume of the paramagnetic phase.
In the presence of the short-range spin-Peierls order (in samples with x=2.88%,
x= 3.2%) there are comparable volumes of paramagnetic and antiferromagnetic
phases at T=T_N. The fraction of the antiferromagnetic phase increases with
lowering temperature. In the absence of the spin-Peierls dimerization (at
x=4.57%)the whole sample exhibits the transition into the antiferromagnetic
state and there is no phase separation. The phase separation is explained by
the consideration of clusters of staggered magnetization located near impurity
atoms. In this model the areas occupied by coherently correlated spins expand
with decreasing temperature and the percolation of the ordered area through a
macroscopic distance occurs.Comment: 7pages, 10 figure
Orbital selectivity of the kink in the dispersion of Sr2RuO4
We present detailed energy dispersions near the Fermi level on the monolayer
perovskite ruthenate Sr2RuO4, determined by high-resolution angle-resolved
photoemission spectroscopy. An orbital selectivity of the kink in the
dispersion of Sr2RuO4 has been found: A kink for the Ru 4d_xy orbital is
clearly observed, but not for the Ru 4d_yz and 4d_zx ones. The result provides
insight into the origin of the kink.Comment: 5 pages, 4 figures. Accepted for publication in Phys. Rev.
Spin-phonon coupled modes in the incommensurate phases of doped CuGeO
The doping effect of the folded phonon mode at 98 cm was investigated
on the Si-doped CuGeO by magneto-optical measurements in far-infrared (FIR)
region under high magnetic field. The folded phonon mode at 98 cm
appears not only in the dimerized (D) phase but also in the
dimerized-anitiferromagnetic (DAF) phase on the doped CuGeO. The splitting
was observed in the incommensurate (IC) phase and the antiferromagnetically
ordered incommensurate (IAF) phase above . The split-off branches exhibit
different field dependence from that of the pure CuGeO in the vicinity of
, and the discrepancy in the IAF phase is larger than that in the IC
phase. It is caused by the interaction between the solitons and the impurities.Comment: 7 pages, 4 figures, resubmitted to Phys. Rev.
An Effective Theory for Midgap States in Doped Spin Ladder and Spin-Peierls Systems: Liouville Quantum Mechanics
In gapped spin ladder and spin-Peierls systems the introduction of disorder,
for example by doping, leads to the appearance of low energy midgap states. The
fact that these strongly correlated systems can be mapped onto one dimensional
noninteracting fermions provides a rare opportunity to explore systems which
have both strong interactions and disorder. In this paper we show that the
statistics of the zero energy midgap wave functions in these models can be
effectively described by Liouville Quantum Mechanics. This enables us to
calculate the disorder averaged N-point correlation functions of these states
(the explicit calculation is performed for N=2,3). We find that whilst these
midgap states are typically weakly correlated, their disorder averaged
correlation are power law. This discrepancy arises because the correlations are
not self-averaging and averages of the wave functions are dominated by
anomalously strongly correlated configurations.Comment: 13 page latex fil
Structural Critical Scattering Study of Mg-Doped CuGeO3
We report a synchrotron x-ray scattering study of the diluted spin-Peierls
(SP) material Cu_(1-x)Mg_xGeO_3. We find that for x>0 the temperature T_m at
which the spin gap is established is significantly higher than the temperature
T_s at which the SP dimerization attains long-range order. The latter is
observed only for xx_c the SP correlation length
quickly decreases with increasing x. We argue that impurity-induced competing
interactions play a central role in these phenomena.Comment: 5 pages, 4 embedded eps figures, to appear in PR
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