16,949 research outputs found
Hydrostatic and uniaxial pressure dependence of superconducting transition temperature of KFe2As2 single crystals
We present heat capacity, c-axis thermal expansion and pressure dependent,
low field, temperature dependent magnetization for pressures up to ~ 12 kbar,
data for KFe2As2 single crystals. Tc decreases under pressure with dTc/dP ~
-0.10 K/kbar. The inferred uniaxial, c-axis, pressure derivative is positive,
dTc/dpc ~ 0.11 K/kbar. The data are analyzed in comparison with those for
overdoped Fe-based superconductors. Arguments are presented that
superconductivity in KFe2As2 may be different from the other overdoped,
Fe-based materials in the 122 family
Rates of Neutrino Absorption on Nucleons and the Reverse Processes in Strong Magnetic Fields
The rates of electron neutrino capture on neutron, electron anti-neutrino
capture on proton, and their reverse processes are important for understanding
the production of heavy elements in the supernova environment above the
protoneutron star. Observations and theoretical considerations suggest that
some protoneutron stars may be born with strong magnetic fields. We develop a
numerical method to calculate the above rates in supernova environments with
magnetic fields up to 10^16 G. This method is accurate to the order of one over
nucleon mass. We show that our results have the correct behavior in the limit
of high neutrino energy or small magnetic field. Based on comparison of our
results with various approximations, we recommend efficient estimates of the
above rates for use in models of supernova nucleosynthesis in the presence of
strong magnetic fields.Comment: 21 pages, 4 figures. Some explaination and references are added in
the second versio
Bounds for state-dependent quantum cloning
Due to the no-cloning theorem, the unknown quantum state can only be cloned
approximately or exactly with some probability. There are two types of cloners:
universal and state-dependent cloner. The optimal universal cloner has been
found and could be viewed as a special state-dependent quantum cloner which has
no information about the states. In this paper, we investigate the
state-dependent cloning when the state-set contains more than two states. We
get some bounds of the global fidelity for these processes. This method is not
dependent on the number of the states contained in the state-set. It is also
independent of the numbers of copying.Comment: 13 pages, 1 figure, to appear in Phys. Rev.
Ferromagnetically coupled dimers on the distorted Shastry-Sutherland lattice: Application to (CuCl)LaNb2O7
A recent study [Tassel {\it et al.}, Phys. Rev. Lett. {\bf 105}, 167205
(2010)] has proposed a remarkable spin model for (CuCl)LaNb2O7, in which dimers
are ferromagnetically coupled to each other on the distorted Shastry-Sutherland
lattice. In this model, the intra-dimer exchange coupling J>0 is
antiferromagnetic, while the inter-dimer exchange couplings are ferromagnetic
and take different values, J_x,J_y<0, in the two bond directions. Anticipating
that the highly frustrated character of this model may lead to a wide range of
behaviors in (CuCl)LaNb2O7 and related compounds, we theoretically investigate
the ground state phase diagram of this model in detail using the following
three approaches: a strong-coupling expansion for small J_x and J_y, exact
diagonalization for finite clusters, and a Schwinger boson mean field theory.
When |J_x|, |J_y| <~ J, the system stays in a dimer singlet phase with a finite
spin gap. This state is adiabatically connected to the decoupled-dimer limit
J_x=J_y=0. We show that the magnetization process of this phase depends
crucially on the spatial anisotropy of the inter-dimer couplings. The
magnetization shows a jump or a smooth increase for weak and strong anisotropy,
respectively, after the spin gap closes at a certain magnetic field. When |J_x|
or |J_y| >~ J, quantum phase transitions to various magnetically ordered phases
(ferromagnetic, collinear stripe, and spiral) occur. The Schwinger boson
analysis demonstrates that quantum fluctuations split the classical degeneracy
of different spiral ground states. Implications for (CuCl)LaNb2O7 and related
compounds are discussed in light of our theoretical results and existing
experimental data.Comment: 21 pages, 20 figure
Nonequilibrium quantum criticality in open electronic systems
A theory is presented of quantum criticality in open (coupled to reservoirs)
itinerant electron magnets, with nonequilibrium drive provided by current flow
across the system. Both departures from equilibrium at conventional
(equilibrium) quantum critical points and the physics of phase transitions
induced by the nonequilibrium drive are treated. Nonequilibrium-induced phase
transitions are found to have the same leading critical behavior as
conventional thermal phase transitions.Comment: 5 pages, 1 figur
Finding cool subdwarfs using a V-J reduced proper-motion diagram: Stellar parameters for 91 candidates
We present the results of a search for cool subdwarfs for which our
candidates were drawn from a V-J reduced proper-motion diagram constructed by
Salim & Gould (2002). Kinematic (U, V, and W) and self-consistent stellar
parameters (Teff, log g, [Fe/H], and V_t) are derived for 91 candidate
subdwarfs based on high resolution spectra. The observed stars span 3900K <
Teff < 6200K and -2.63 < [Fe/H] < 0.25 including only 3 giants (log g < 4.0).
Of the sample, 77 stars have MgH lines present in their spectra. With more than
56% of our candidate subdwarfs having [Fe/H] < -1.5, we show that the V-J
reduced proper-motion diagram readily identifies metal-poor stars.Comment: PASP (in press
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