7,295 research outputs found
Field-Induced Ferromagnetic Order and Colossal Magnetoresistance in La_{1.2}Sr_{1.8}Mn_2O_7: a ^{139}La NMR study
In order to gain insights into the origin of colossal magneto-resistance
(CMR) in manganese oxides, we performed a ^{139}La NMR study in the
double-layered compound La_{1.2}Sr_{1.8}Mn_2O_7. We find that above the Curie
temperature T_C=126 K, applying a magnetic field induces a long-range
ferromagnetic order that persists up to T=330 K. The critical field at which
the induced magnetic moment is saturated coincides with the field at which the
CMR effect reaches to a maximum. Our results therefore indicate that the CMR
observed above T_C in this compound is due to the field-induced ferromagnetism
that produces a metallic state via the double exchange interaction
Quark Propagation in the Quark-Gluon Plasma
It has recently been suggested that the quark-gluon plasma formed in
heavy-ion collisions behaves as a nearly ideal fluid. That behavior may be
understood if the quark and antiquark mean-free- paths are very small in the
system, leading to a "sticky molasses" description of the plasma, as advocated
by the Stony Brook group. This behavior may be traced to the fact that there
are relatively low-energy resonance states in the plasma leading to
very large scattering lengths for the quarks. These resonances have been found
in lattice simulation of QCD using the maximum entropy method (MEM). We have
used a chiral quark model, which provides a simple representation of effects
due to instanton dynamics, to study the resonances obtained using the MEM
scheme. In the present work we use our model to study the optical potential of
a quark in the quark-gluon plasma and calculate the quark mean-free-path. Our
results represent a specific example of the dynamics of the plasma as described
by the Stony Brook group.Comment: 17 pages, 4 figures, revtex
The ground state of a mixture of two species of fermionic atoms in 1D optical lattice
In this paper, we investigate the ground state properties of a mixture of two
species of fermionic atoms in one-dimensional optical lattice, as described by
the asymmetric Hubbard model. The quantum phase transition from density wave to
phase separation is investigated by studying both the corresponding charge
order parameter and quantum entanglement. A rigorous proof that even for the
single hole doping case, the density wave is unstable to the phase separation
in the infinite U limit, is given. Therefore, our results are quite instructive
for both on-going experiments on strongly correlated cold-atomic systems and
traditional heavy fermion systems.Comment: 9 pages, 10 figures, extended versio
Deduction of Pure Spin Current from Spin Linear and Circular Photogalvanic Effect in Semiconductor Quantum Wells
We study the spin photogalvanic effect in two-dimensional electron system
with structure inversion asymmetry by means of the solution of semiconductor
optical Bloch equations. It is shown that a linearly polarized light may inject
a pure spin current in spin-splitting conduction bands due to Rashba spin-orbit
coupling, while a circularly polarized light may inject spin-dependent
photocurrent. We establish an explicit relation between the photocurrent by
oblique incidence of a circularly polarized light and the pure spin current by
normal incidence of a linearly polarized light such that we can deduce the
amplitude of spin current from the measured spin photocurrent experimentally.
This method may provide a source of spin current to study spin transport in
semiconductors quantitatively
Evidence for a full energy gap for nickel-pnictide LaNiAsO_{1-x}F_x superconductors by ^{75}As nuclear quadrupole resonance
We report systematic ^{75}As-NQR and ^{139}La-NMR studies on nickel-pnictide
superconductors LaNiAsO_{1-x}F_x (x=0, 0.06, 0.10 and 0.12). The spin lattice
relaxation rate 1/T_1 decreases below T_c with a well-defined coherence peak
and follows an exponential decay at low temperatures. This result indicates
that the superconducting gap is fully opened, and is strikingly different from
that observed in iron-pnictide analogs. In the normal state, 1/T_1T is constant
in the temperature range T_c \sim 4 K < T <10 K for all compounds and up to
T=250 K for x=0 and 0.06, which indicates weak electron correlations and is
also different from the iron analog. We argue that the differences between the
iron and nickel pnictides arise from the different electronic band structure.
Our results highlight the importance of the peculiar Fermi-surface topology in
iron-pnictides.Comment: 4 pages, 5 figure
Antiferromagnetic Spin Fluctuation above the Superconducting Dome and the Full-Gaps Superconducting State in LaFeAsO1-xFx Revealed by 75As-Nuclear Quadrupole Resonance
We report a systematic study by 75As nuclear-quadrupole resonance in
LaFeAsO1-xFx. The antiferromagnetic spin fluctuation (AFSF) found above the
magnetic ordering temperature TN = 58 K for x = 0.03 persists in the regime
0.04 < x < 0.08 where superconductivity sets in. A dome-shaped x-dependence of
the superconducting transition temperature Tc is found, with the highest Tc =
27 K at x = 0.06 which is realized under significant AFSF. With increasing x
further, the AFSF decreases, and so does Tc. These features resemble closely
the cuprates La2-xSrxCuO4. In x = 0.06, the spin-lattice relaxation rate (1/T1)
below Tc decreases exponentially down to 0.13 Tc, which unambiguously indicates
that the energy gaps are fully-opened. The temperature variation of 1/T1 below
Tc is rendered nonexponential for other x by impurity scattering.Comment: 5 pages, 5 figures, more references adde
Quantum correlations in the collective spin systems
Quantum and classical pairwise correlations in two typical collective spin
systems (i.e., the Dicke model and the Lipkin-Meshkov-Glick model) are
discussed. These correlations in the thermodynamical limit are obtained
analytically and in a finite-size system are calculated numerically. Large-size
scaling behavior for the quantum discord itself is observed, which has never
been reported in another critical system. A logarithmic diverging behavior for
the first derivative of the quantum discord is also found in both models, which
might be universal in the second-order quantum phase transition. It is
suggested that the pronounced maximum or minimum of first derivative of quantum
discord signifies the critical point. Comparisons between the quantum discord
and the scaled concurrence are performed. It is shown that the quantum discord
is very small in one phase and robust in the other phase, while the scaled
concurrence shows maximum at the critical point and decays rapidly when away
from the the critical point.Comment: 8 pages, 4 figure
Numerical simulations of a ballistic spin interferometer with the Rashba spin-orbital interaction
We numerically investigate the transport behavior of a quasi one-dimension
(1D) square loop device containing the Rashba spin-orbital interaction in the
presence of a magnetic flux. The conductance versus the magnetic field shows
the Al'tshuler-Aronov-Spivak (AAS) and Aharonov-Bohm (AB) oscillations. We
focus on the oscillatory amplitudes, and find that both of them are strongly
dependent on the spin precession angle (i.e. the strength of the spin-orbit
interaction) and exhibit no-periodic oscillations, which are well in agreement
with a recent experiment by Koga et al. [cond-mat/0504743(unpublished)].
However, our numerical results for the ideal 1D square loop device for the node
positions of the amplitudes of the AB and AAS oscillations are found to be of
some discrepancies comparing with quasi-1D square loop with a finite width. In
the presence of disorder and taking the disorder ensemble average, the AB
oscillation in the conductance will disappear, while the time-reversal
symmetric AAS oscillation still remains. Furthermore, the node positions of the
AAS oscillatory amplitude remains the same.Comment: 6 pages, 7 figure
Phase glass and zero-temperature phase transition in a randomly frustrated two-dimensional quantum rotor model
The ground state of the quantum rotor model in two dimensions with random
phase frustration is investigated. Extensive Monte Carlo simulations are
performed on the corresponding (2+1)-dimensional classical model under the
entropic sampling scheme. For weak quantum fluctuation, the system is found to
be in a phase glass phase characterized by a finite compressibility and a
finite value for the Edwards-Anderson order parameter, signifying long-ranged
phase rigidity in both spatial and imaginary time directions. Scaling
properties of the model near the transition to the gapped, Mott insulator state
with vanishing compressibility are analyzed. At the quantum critical point, the
dynamic exponent is greater than one. Correlation
length exponents in the spatial and imaginary time directions are given by
and , respectively, both assume values
greater than 0.6723 of the pure case. We speculate that the phase glass phase
is superconducting rather than metallic in the zero current limit.Comment: 14 pages, 4 figures, to appear in JSTA
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