2,496 research outputs found
Superconductivity of the Ternary Boride Li_2Pd_3B Probed by ^{11}B NMR
We report a ^{11}B NMR measurement on the recently discovered superconductor
Li_2Pd_3B. The nuclear spin lattice relaxation rate 1/T_1 shows a well-defined
coherence peak just below T_c (H=1.46 T)=5.7 K, and the spin susceptibility
measured by the Knight shift also decreases below T_c. These results indicate
that the superconductivity is of conventional nature, with an isotropic gap.
Our results also suggest that the -electrons of boron and the d-electrons of
palladium that hybridize with boron -electrons are primarily responsible for
the superconductivity.Comment: 4 pages, 5 figure
Theoretic Limits on the Equation of State Parameter of Quintessence
The value of scalar field coupled to gravity should be less than the Planck
scale in the consistent theory of quantum gravity. It provides a theoretic
constraint on the equation of state parameter for the quintessence. In some
cases our theoretic constraints are more stringent than the constraints from
the present experiments.Comment: 4 pages, 2 figures; minor corrections and refs adde
Weak gravity conjecture in the asymptotical dS and AdS background
The cosmological observations provide a strong evidence that there is a
positive cosmological constant in our universe and thus the spacetime is
asymptotical de Sitter space. The conjecture of gravity as the weakest force in
the asymptotical dS space leads to a lower bound on the U(1) gauge coupling
, or equivalently, the positive cosmological constant gets an upper bound
in order that the U(1) gauge theory can survive in four
dimensions. This result has a simple explanation in string theory, i.e. the
string scale should not be greater than the size of the
cosmic horizon. Our proposal in string theory can be generalized to U(N) gauge
theory and gives a guideline to the microscopic explanation of the de Sitter
entropy. The similar results are also obtained in the asymptotical anti-de
Sitter space.Comment: 4 pages; version for publication in JHEP (title changed
Superconducting state in the non-centrosymmetric Mg_{9.3}Ir_{19}B_{16.7} and Mg_{10.5}Ir_{19}B_{17.1} revealed by NMR
We report ^{11}B NMR measurements in non-centrosymmetric superconductors
Mg_{9.3}Ir_{19}B_{16.7} (T_c=5.8 K) and Mg_{10.5}Ir_{19}B_{17.1} (T_c=4.8 K).
The spin lattice relaxation rate and the Knight shift indicate that the Cooper
pairs are predominantly in the spin-singlet state with an isotropic gap.
However, Mg_{10.5}Ir_{19}B_{17.1} is found to have more defects and the spin
susceptibility remains finite even in the zero-temperature limit. We interpret
this result as that the defects enhance the spin-orbit coupling and bring about
more spin-triplet component.Comment: for a proper, high-resolution Fig.5, contact the corresponding autho
Measurement of electron correlations in LixCoO2 (x=0.0 - 0.35) using 59Co nuclear magnetic resonance and nuclear quadrupole resonance techniques
CoO2 is the parent compound for the superconductor NaxCoO2\cdot1.3H2O and was
widely believed to be a Mott insulator. We performed 59Co nuclear magnetic
resonance (NMR) and nuclear quadrupole resonance (NQR) studies on LixCoO2 (x =
0.35, 0.25, 0.12, and 0.0) to uncover the electronic state and spin
correlations in this series of compounds which was recently obtained through
electrochemical de-intercalation of Li from pristine LiCoO2. We find that
although the antiferromagnetic spin correlations systematically increase with
decreasing Li-content (x), the end member, CoO2 is a non-correlated metal that
well satisfies the Korringa relation for a Fermi liquid. Thus, CoO2 is not
simply located at the limit of x->0 for AxCoO2 (A = Li, Na) compounds. The
disappearance of the electron correlations in CoO2 is due to the three
dimensionality of the compound which is in contrast to the highly two
dimensional structure of AxCoO2.Comment: 4pages, 4figures, to be published in Phys.Rev.B. Rapid
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