1,390 research outputs found
Multiple Superconducting Gaps, Anisotropic Spin Fluctuations and Spin-Orbit Coupling in Iron-Pnictides
This article reviews the NMR and NQR studies on iron-based high-temperature
superconductors by the IOP/Okayama group. It was found that the electron pairs
in the superconducting state are in the spin-singlet state with multiple
fully-opened energy gaps. The antiferromagnetic spin fluctuations in the normal
state are found to be closely correlated with the superconductivity. Also the
antiferromagnetic spin fluctuations are anisotropic in the spin space, which is
different from the case in copper oxide superconductors. This anisotropy
originates from the spin-orbit coupling and is an important reflection of the
multiple-bands nature of this new class of superconductors.Comment: 20 pages, 16 figure
Hydration-induced anisotropic spin fluctuations in Na_{x}CoO_{2}\cdot1.3H_{2}O superconductor
We report ^{59}Co NMR studies in single crystals of cobalt oxide
superconductor Na_{0.42}CoO_{2}\cdot1.3H_{2}O (T_c=4.25K) and its parent
compound Na_{0.42}CoO_{2}. We find that both the magnitude and the temperature
(T) dependence of the Knight shifts are identical in the two compounds above
T_c. The spin-lattice relaxation rate (1/T_1) is also identical above T_0
\sim60 K for both compounds. Below T_0, the unhydrated sample is found to be a
non-correlated metal that well conforms to Fermi liquid theory, while spin
fluctuations develop in the superconductor. These results indicate that water
intercalation does not change the density of states but its primary role is to
bring about spin fluctuations. Our result shows that, in the hydrated
superconducting compound, the in-plane spin fluctuation around finite wave
vector is much stronger than that along the c-axis, which indicates that the
spin correlation is quasi-two-dimensional.Comment: 4 pages, 5 figure
Spin Current and Current-Induced Spin Transfer Torque in Ferromagnet-Quantum Dot-Ferromagnet Coupled Systems
Based on Keldysh's nonequilibrium Green function method, the spin-dependent
transport properties in a ferromagnet-quantum dot (QD)-ferromagnet coupled
system are investigated. It is shown the spin current shows quite different
characteristics from its electrical counterpart, and by changing the relative
orientation of both magnetizations, it can change its magnitude even sign. The
current-induced spin transfer torque (CISTT) is uncovered to be greatly
enhanced when the bias voltage meets with the discrete levels of the QD at
resonant positions. The relationship between the CISTT, the electrical current
and the spin current is also addressed.Comment: 21 pages, 8 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
Antiferromagnetic Spin Fluctuation above the Superconducting Dome and the Full-Gaps Superconducting State in LaFeAsO1-xFx Revealed by 75As-Nuclear Quadrupole Resonance
We report a systematic study by 75As nuclear-quadrupole resonance in
LaFeAsO1-xFx. The antiferromagnetic spin fluctuation (AFSF) found above the
magnetic ordering temperature TN = 58 K for x = 0.03 persists in the regime
0.04 < x < 0.08 where superconductivity sets in. A dome-shaped x-dependence of
the superconducting transition temperature Tc is found, with the highest Tc =
27 K at x = 0.06 which is realized under significant AFSF. With increasing x
further, the AFSF decreases, and so does Tc. These features resemble closely
the cuprates La2-xSrxCuO4. In x = 0.06, the spin-lattice relaxation rate (1/T1)
below Tc decreases exponentially down to 0.13 Tc, which unambiguously indicates
that the energy gaps are fully-opened. The temperature variation of 1/T1 below
Tc is rendered nonexponential for other x by impurity scattering.Comment: 5 pages, 5 figures, more references adde
Fermi-liquid ground state in n-type copper-oxide superconductor Pr0.91Ce0.09LaCuO4-y
We report nuclear magnetic resonance studies on the low-doped n-type
copper-oxide Pr_{0.91}LaCe_{0.09}CuO_{4-y} (T_c=24 K) in the superconducting
state and in the normal state uncovered by the application of a strong magnetic
field. We find that when the superconductivity is removed, the underlying
ground state is the Fermi liquid state. This result is at variance with that
inferred from previous thermal conductivity measurement and contrast with that
in p-type copper-oxides with a similar doping level where high-T_c
superconductivity sets in within the pseudogap phase. The data in the
superconducting state are consistent with the line-nodes gap model.Comment: version to appear in Phys. Rev. Let
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