1,834 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
Branching ratios and CP asymmetries of decays in the pQCD approach
We calculate the branching ratios and CP violating asymmetries of the four B
\to K \etap decays in the perturbative QCD (pQCD) factorization approach.
Besides the full leading order contributions, the partial next-to-leading order
(NLO) contributions from the QCD vertex corrections, the quark loops, and the
chromo-magnetic penguins are also taken into account. The NLO pQCD predictions
for the CP-averaged branching ratios are , Br(B^\pm \to K^\pm \etar) \approx 51.0 \times 10^{-6},
, and Br(B^0 \to K^0 \etar)
\approx 50.3 \times 10^{-6}. The NLO contributions can provide a 70%
enhancement to the LO Br(B \to K \etar), but a 30% reduction to the LO , which play the key role in understanding the observed pattern of
branching ratios. The NLO pQCD predictions for the CP-violating asymmetries,
such as \acp^{dir} (K^0_S \etar) \sim 2.3% and \acp^{mix}(K^0_S \etar)\sim
63%, agree very well with currently available data. This means that the
deviation \Delta S=\acp^{mix}(K^0_S \etar) - \sin{2\beta} in pQCD approach is
also very small.Comment: 31 pages, 11 ps/eps figures, typos corrected. A little modificatio
Nodeless energy gaps of single-crystalline Ba0.68K0.32Fe2As2 as seen via 75As NMR
We report As nuclear magnetic resonance studies on a very clean
hole-doped single-crystal BaKFeAs ( K). The spin-lattice relaxation rate shows an exponential
decrease below down to ,
which indicates a fully opened energy gap. From the ratio , where and denote the crystal directions, we find that the
antiferromagnetic spin fluctuation is anisotropic in the spin space above
. The anisotropy decreases below and disappears at . We argue that the anisotropy stems from spin-orbit coupling
whose effect vanishes when spin-singlet electron pairs form with a nodeless
gap.Comment: 10 pages, 6 figure
Carrier-Concentration Dependence of the Pseudogap Ground State of Superconducting Bi2Sr2-xLaxCuO6+delta Revealed by 63,65Cu-Nuclear Magnetic Resonance in Very High Magnetic Fields
We report the results of the Knight shift by 63,65Cu-nuclear-magnetic
resonance (NMR) measurements on single-layered copper-oxide
Bi2Sr2-xLaxCuO6+delta conducted under very high magnetic fields up to 44 T. The
magnetic field suppresses superconductivity completely and the pseudogap ground
state is revealed. The 63Cu-NMR Knight shift shows that there remains a finite
density of states (DOS) at the Fermi level in the zero-temperature limit, which
indicates that the pseudogap ground state is a metallic state with a finite
volume of Fermi surface. The residual DOS in the pseudogap ground state
decreases with decreasing doping (increasing x) but remains quite large even at
the vicinity of the magnetically ordered phase of x > 0.8, which suggests that
the DOS plunges to zero upon approaching the Mott insulating phase.Comment: 4 pages, 5 figures, to appear in Phys. Rev. Let
Extraction of Plumes in Turbulent Thermal Convection
We present a scheme to extract information about plumes, a prominent coherent
structure in turbulent thermal convection, from simultaneous local velocity and
temperature measurements. Using this scheme, we study the temperature
dependence of the plume velocity and understand the results using the equations
of motion. We further obtain the average local heat flux in the vertical
direction at the cell center. Our result shows that heat is not mainly
transported through the central region but instead through the regions near the
sidewalls of the convection cell.Comment: 4 pages, 4 figures, submitted to Physical Review Letter
Magnetic Field Effect on the Pseudogap Temperature within Precursor Superconductivity
We determine the magnetic field dependence of the pseudogap closing
temperature T* within a precursor superconductivity scenario. Detailed
calculations with an anisotropic attractive Hubbard model account for a
recently determined experimental relation in BSCCO between the pseudogap
closing field and the pseudogap temperature at zero field, as well as for the
weak initial dependence of T* at low fields. Our results indicate that the
available experimental data are fully compatible with a superconducting origin
of the pseudogap in cuprate superconductors.Comment: 4 pages, 3 figure
Critical point and the nature of the pseudogap of single-layered copper-oxide BiSrLaCuO superconductors
We apply strong magnetic fields of H=28.5 \sim 43 T to suppress
superconductivity (SC) in the cuprates Bi_{2}Sr_{2-x}La_xCuO_{6+\delta}
(x=0.65, 0.40, 0.25, 0.15 and 0), and investigate the low temperature (T)
normal state by ^{63}Cu nuclear spin-lattice relaxation rate (1/T_1)
measurements. We find that the pseudogap (PG) phase persists deep inside the
overdoped region but terminates at x \sim 0.05 that corresponds to the hole
doping concentration of approximately 0.21. Beyond this critical point, the
normal state is a Fermi liquid characterized by the T_1T=const relation. A
comparison of the superconducting state with the H-induced normal state in the
x=0.40 (T_c = 32 K) sample indicates that there remains substantial part of the
Fermi surface even in the fully-developed PG state, which suggests that the PG
and SC are coexisting matters
Quantum transport theory for nanostructures with Rashba spin-orbital interaction
We report on a general theory for analyzing quantum transport through devices
in the Metal-QD-Metal configuration where QD is a quantum dot or the device
scattering region which contains Rashba spin-orbital and electron-electron
interactions. The metal leads may or may not be ferromagnetic, they are assumed
to weakly couple to the QD region. Our theory is formulated by second
quantizing the Rashba spin-orbital interaction in spectral space (instead of
real space), and quantum transport is then analyzed within the Keldysh
nonequilibrium Green's function formalism. The Rashba interaction causes two
main effects to the Hamiltonian: (i) it gives rise to an extra spin-dependent
phase factor in the coupling matrix elements between the leads and the QD; (ii)
it gives rise to an inter-level spin-flip term but forbids any intra-level
spin-flips. Our formalism provides a starting point for analyzing many quantum
transport issues where spin-orbital effects are important. As an example, we
investigate transport properties of a Aharnov-Bohm ring in which a QD having
Rashba spin-orbital and e-e interactions is located in one arm of the ring. A
substantial spin-polarized conductance or current emerges in this device due to
a combined effect of a magnetic flux and the Rashba interaction. The direction
and strength of the spin-polarization are shown to be controllable by both the
magnetic flux and a gate voltage.Comment: 12 pages, 8 figure
Coexistence of antiferromagnetic order and unconventional superconductivity in heavy fermion compounds CeRh_{1-x}Ir_xIn_5: nuclear quadrupole resonance studies
We present a systematic ^{115}In NQR study on the heavy fermion compounds
CeRh_{1-x}Ir_xIn_5 (x=0.25, 0.35, 0.45, 0.5, 0.55 and 0.75). The results
provide strong evidence for the microscopic coexistence of antiferromagnetic
(AF) order and superconductivity (SC) in the range of 0.35 \leq x \leq 0.55.
Specifically, for x=0.5, T_N is observed at 3 K with a subsequent onset of
superconductivity at T_c=0.9 K. T_c reaches a maximum (0.94 K) at x=0.45 where
T_N is found to be the highest (4.0 K). Detailed analysis of the measured
spectra indicate that the same electrons participate in both SC and AF order.
The nuclear spin-lattice relaxation rate 1/T_1 shows a broad peak at T_N and
follows a T^3 variation below T_c, the latter property indicating
unconventional SC as in CeIrIn_5 (T_c=0.4 K). We further find that, in the
coexistence region, the T^3 dependence of 1/T_1 is replaced by a T-linear
variation below T\sim 0.4 K, with the value \frac{(T_1)_{T_c}}{(T_1)_{low-T}}
increasing with decreasing x, likely due to low-lying magnetic excitations
associated with the coexisting magnetism.Comment: 20 pages, 14 figure
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