43,334 research outputs found
Spin singlet pairing in the superconducting state of NaxCoO2\cdot1.3H2O: evidence from a ^{59}Co Knight shift in a single crystal
We report a ^{59}Co Knight shift measurement in a single crystal of the
cobalt oxide superconductor Na_{x}CoO_2\cdot1.3H_2O (T_c=4.25 K). We find that
the shift due to the spin susceptibility, K^s, is substantially large and
anisotropic, with the spin shift along the a-axis K^s_a being two times that
along the c-axis K^s_c. The shift decreases with decreasing temperature (T)
down to T\sim100 K, then becomes a constant until superconductivity sets in.
Both K^s_a and K^s_c decrease below T_c. Our results indicate unambiguously
that the electron pairing in the superconducting state is in the spin singlet
form.Comment: 4 pages, 5 figure
Robust formulation of Wick's theorem for computing matrix elements between Hartree-Fock-Bogoliubov wavefunctions
Numerical difficulties associated with computing matrix elements of operators
between Hartree-Fock-Bogoliubov (HFB) wavefunctions have plagued the
development of HFB-based many-body theories for decades. The problem arises
from divisions by zero in the standard formulation of the nonorthogonal Wick's
theorem in the limit of vanishing HFB overlap. In this paper, we present a
robust formulation of Wick's theorem that stays well-behaved regardless of
whether the HFB states are orthogonal or not. This new formulation ensures
cancellation between the zeros of the overlap and the poles of the Pfaffian,
which appears naturally in fermionic systems. Our formula explicitly eliminates
self-interaction, which otherwise causes additional numerical challenges. A
computationally efficient version of our formalism enables robust
symmetry-projected HFB calculations with the same computational cost as
mean-field theories. Moreover, we avoid potentially diverging normalization
factors by introducing a robust normalization procedure. The resulting
formalism treats even and odd number of particles on equal footing and reduces
to Hartree-Fock as a natural limit. As proof of concept, we present a
numerically stable and accurate solution to a Jordan-Wigner-transformed
Hamiltonian, whose singularities motivated the present work. Our robust
formulation of Wick's theorem is a most promising development for methods using
quasiparticle vacuum states
New molecular candidates: X(1910), X(2200), and X(2350)
Assuming the newly observed resonant structures X(1910), X(2200), and X(2350)
as , , and molecular states respectively,
we compute their mass values in the framework of QCD sum rules. The numerical
results are for state,
for state, and for state, which
coincide with the experimental values of X(1910), X(2200), and X(2350),
respectively. This supports the statement that X(1910), X(2200), and X(2350)
could be , , and molecular candidates
respectively.Comment: 9 pages, 9 eps figures; the name of X(2000) changed to X(1910)
according to the updated data of experiments; more references and discussions
added; accepted for publication in PRD. arXiv admin note: substantial text
overlap with arXiv:1211.2277, arXiv:1201.341
Extended calculations of energy levels, radiative properties, , hyperfine interaction constants, and Land\'e -factors for nitrogen-like \mbox{Ge XXVI}
Employing two state-of-the-art methods, multiconfiguration
Dirac--Hartree--Fock and second-order many-body perturbation theory, highly
accurate calculations are performed for the lowest 272 fine-structure levels
arising from the , , , ~(), (), and ()
configurations in nitrogen-like Ge XXVI. Complete and consistent atomic data,
including excitation energies, lifetimes, wavelengths, hyperfine structures,
Land\'e -factors, and E1, E2, M1, M2 line strengths, oscillator
strengths, and transition rates among these 272 levels are provided.
Comparisons are made between the present two data sets, as well as with other
available experimental and theoretical values. The present data are accurate
enough for identification and deblending of emission lines involving the
levels, and are also useful for modeling and diagnosing fusion plasmas
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