5,116 research outputs found
Chain breaks and the susceptibility of Sr_2Cu_{1-x}Pd_xO_{3+\delta} and other doped quasi one-dimensional antiferromagnets
We study the magnetic susceptibility of one-dimensional S=1/2
antiferromagnets containing non-magnetic impurities which cut the chain into
finite segments. For the susceptibility of long anisotropic Heisenberg
chain-segments with open boundaries we derive a parameter-free result at low
temperatures using field theory methods and the Bethe Ansatz. The analytical
result is verified by comparing with Quantum-Monte-Carlo calculations. We then
show that the partitioning of the chain into finite segments can explain the
Curie-like contribution observed in recent experiments on
Sr_2Cu_{1-x}Pd_xO_{3+\delta}. Possible additional paramagnetic impurities seem
to play only a minor role.Comment: 4 pages, 3 figures, final versio
Quantum Disordered Ground States in Frustrated Antiferromagnets with Multiple Ring Exchange Interactions
We present a certain class of two-dimensional frustrated quantum Heisenberg
spin systems with multiple ring exchange interactions which are rigorously
demonstrated to have quantum disordered ground states without magnetic
long-range order. The systems considered in this paper are s=1/2
antiferromagnets on a honeycomb and square lattices, and an s=1 antiferromagnet
on a triangular lattice. We find that for a particular set of parameter values,
the ground state is a short-range resonating valence bond state or a valence
bond crystal state. It is shown that these systems are closely related to the
quantum dimer model introduced by Rokhsar and Kivelson as an effective
low-energy theory for valence bond states.Comment: 6 pages, 4 figure
Magnetohydrodynamic Simulations of A Rotating Massive Star Collapsing to A Black Hole
We perform two-dimensional, axisymmetric, magnetohydrodynamic simulations of
the collapse of a rotating star of 40 Msun and in the light of the collapsar
model of gamma-ray burst. Considering two distributions of angular momentum, up
to \sim 10^{17} cm^2/s, and the uniform vertical magnetic field, we investigate
the formation of an accretion disk around a black hole and the jet production
near the hole. After material reaches to the black hole with the high angular
momentum, the disk is formed inside a surface of weak shock. The disk becomes
in a quasi-steady state for stars whose magnetic field is less than 10^{10} G
before the collapse. We find that the jet can be driven by the magnetic fields
even if the central core does not rotate as rapidly as previously assumed and
outer layers of the star has sufficiently high angular momentum. The magnetic
fields are chiefly amplified inside the disk due to the compression and the
wrapping of the field. The fields inside the disk propagate to the polar region
along the inner boundary near the black hole through the Alfv{\'e}n wave, and
eventually drive the jet. The quasi-steady disk is not an advection-dominated
disk but a neutrino cooling-dominated one. Mass accretion rates in the disks
are greater than 0.01 Msun/sec with large fluctuations. The disk is transparent
for neutrinos. The dense part of the disk, which locates near the hole, emits
neutrino efficiently at a constant rate of < 8 \times 10^{51} erg/s. The
neutrino luminosity is much smaller than those from supernovae after the
neutrino burst.Comment: 42 pages, accepted for publication in the Astrophysical Journal. A
paper with higher-resolution figures available at
http://www.ec.knct.ac.jp/~fujimoto/collapsar/mhd-color.pd
All-order evaluation of weak measurements: --- The cases of an operator which satisfies the property ---
Some exact formulae of the expectation values and probability densities in a
weak measurement for an operator which satisfies the property are derived. These formulae include all-order effects of the unitary
evolution due to the von-Neumann interaction. These are valid not only in the
weak measurement regime but also in the strong measurement regime and tell us
the connection between these two regime. Using these formulae, arguments of the
optimization of the signal amplification and the signal to noise ratio are
developed in two typical experimental setups.Comment: 17 pages, 10 figures (v1); Fig.3 and some typos are corrected (v2);
Comments and references are added and some typos are corrected (v3
Spin fluctuations and superconductivity in noncentrosymmetric heavy fermion systems CeRhSi and CeIrSi
We study the normal and the superconducting properties in noncentrosymmetric
heavy fermion superconductors CeRhSi and CeIrSi. For the normal state,
we show that experimentally observed linear temperature dependence of the
resistivity is understood through the antiferromagnetic spin fluctuations near
the quantum critical point (QCP) in three dimensions. For the superconducting
state, we derive a general formula to calculate the upper critical field
, with which we can treat the Pauli and the orbital depairing effect on
an equal footing. The strong coupling effect for general electronic structures
is also taken into account. We show that the experimentally observed features
in , the huge value up to 30(T), the downward
curvatures, and the strong pressure dependence, are naturally understood as an
interplay of the Rashba spin-orbit interaction due to the lack of inversion
symmetry and the spin fluctuations near the QCP. The large anisotropy between
and is explained in terms of
the spin-orbit interaction. Furthermore, a possible realization of the
Fulde-Ferrell- Larkin-Ovchinnikov state for is studied. We
also examine effects of spin-flip scattering processes in the pairing
interaction and those of the applied magnetic field on the spin fluctuations.
We find that the above mentioned results are robust against these effects. The
consistency of our results strongly supports the scenario that the
superconductivity in CeRhSi and CeIrSi is mediated by the spin
fluctuations near the QCP.Comment: 21pages, 13figures, to be published in Phys. Rev.
Kondo Problem and Related One-Dimensional Quantum Systems: Bethe Ansatz Solution and Boundary Conformal Field Theory
We review some exact results on Kondo impurity systems derived from
Bethe-ansatz solutions and boundary conformal field theory with particular
emphasis on universal aspects of the phenomenon. The finite-size spectra
characterizing the low-energy fixed point are computed from the Bethe-ansatz
solutions of various models related to the Kondo problem. Using the finite-size
scaling argument, we investigate their exact critical properties. We also
discuss that a universal relation between the Kondo effect and the impurity
effect in one-dimensional quantum systems usefully expedites our understanding
of these different phenomena.Comment: 6 pages, no figure
Geometrical, electronic and magnetic properties of NaCoO from first principles
We report a first-principles projector augmented wave (PAW) study on
NaCoO. With the sodium ion ordered insulating phase being
identified in experiments, pure density functional calculations fail to predict
an insulating ground state, which indicates that Na ordering alone can not
produce accompanying Co charge ordering, if additional correlation is not
properly considered. At this level of theory, the most stable phase presents
ferromagnetic ordering within the CoO layer and antiferromagnetic coupling
between these layers. When the on-site Coulomb interaction for Co 3d orbitals
is included by an additional Hubbard parameter , charge ordered insulating
ground state can be obtained. The effect of on-site interaction magnitude on
electronic structure is studied. At a moderate value of (4.0 eV for
example), the ground state is antiferromagnetic, with a Co magnetic
moment about 1.0 and a magnetic energy of 0.12 eV/Co. The
rehybridization process is also studied in the DFT+U point of view.Comment: 21 pages, 7 figure
Explosions inside Ejecta and Most Luminous Supernovae
The extremely luminous supernova SN2006gy is explained in the same way as
other SNIIn events: light is produced by a radiative shock propagating in a
dense circumstellar envelope formed by a previous weak explosion. The problems
in the theory and observations of multiple-explosion SNe IIn are briefly
reviewed.Comment: 9 pages, 6 figures, LateX aipproc.cls. A bit more details and color
added to Fig.3. The 10th International Symposium on Origin of Matter and
Evolution of Galaxies (OMEG07), Sapporo, Japan, December 200
Disc-Jet coupling in the LMXB 4U1636-53 from INTEGRAL
We report on the spectral analysis results of the neutron star, atoll type,
low mass X-ray Binary 4U1636-53 observed by INTEGRAL and BeppoSAX satellites.
Spectral behavior in three different epochs corresponding to three different
spectral states has been deeply investigated. Two data set spectra show a
continuum well described by one or two soft blackbody plus a Comptonized
components with changes in the Comptonizing electrons and black body
temperature and the accretion rates, which are typical of the spectral
transitions from high to low state. In one occasion INTEGRAL spectrum shows,
for first time in this source, a hard tail dominating the emission above 30
keV. The total spectrum is fitted as the sum of a Comptonized component similar
to soft state and a power-law component (Gamma=2.76), indicating the presence
of a non thermal electron distribution of velocities. In this case, a
comparison with hard tails detected in soft states from neutron stars systems
and some black hole binaries suggests that a similar mechanism could originate
these components in both cases.Comment: 6 pages, 4 figures, 2 tables. accepted Ap
- …