370 research outputs found
Multigap Superconductivity in YC: A C-NMR Study
We report on the superconducting (SC) properties of YC with a
relatively high transition temperature K investigated by
C nuclear-magnetic-resonance (NMR) measurements under a magnetic field.
The C Knight shift has revealed a significant decrease below , suggesting a spin-singlet superconductivity. From an analysis of the
temperature dependence of the nuclear spin-lattice relaxation rate in
the SC state, YC is demonstrated to be a multigap superconductor that
exhibits a large gap at the main band and a
small gap at other bands. These results have
revealed that YC is a unique multigap s-wave superconductor similar to
MgB.Comment: 4 pages, 5 figure
Evidence for Strong-coupling S-wave Superconductivity in MgB2 :11B NMR Study
We have investigated a gap structure in a newly-discovered superconductor,
MgB2 through the measurement of 11B nuclear spin-lattice relaxation rate,
^{11}(1/T_1). ^{11}(1/T_1) is proportional to the temperature (T) in the normal
state, and decreases exponentially in the superconducting (SC) state, revealing
a tiny coherence peak just below T_c. The T dependence of 1/T_1 in the SC state
can be accounted for by an s-wave SC model with a large gap size of 2\Delta
/k_BT_c \sim 5 which suggests to be in a strong-coupling regime.Comment: 2 pages with 1 figur
Physical properties of ferromagnetic-superconducting coexistent system
We studied the nuclear relaxation rate 1/T1 of a
ferromagnetic-superconducting system from the mean field model proposed in
Ref.14. This model predicts the existence of a set of gapless excitations in
the energy spectrum which will affect the properties studied here, such as the
density of states and, hence, 1/T1. The study of the temperature variation of
1/T1(for T<Tc) shows that the usual Hebel-Slichter peak exists, but will be
reduced because of the dominant role of the gapless fermions and the background
magnetic behavior. We have also presented the temperature dependence of
ultrasonic attenuation and the frequency dependence of electromagnetic
absorption within this model. We are successful in explaining certain
experimental results.Comment: 10 Pages, 9 figute
Linking satellite derived LAI patterns with subsoil heterogeneity using large-scale ground-based electromagnetic induction measurements
Patterns in crop development and yield are often directly related to lateral and vertical changes in soil texture
causing changes in available water and resource supply for plant growth, especially under dry conditions. Relict
geomorphologic features, such as old river channels covered by shallow sediments can challenge assumptions of
uniformity in precision agriculture, subsurface hydrology, and crop modeling. Hence a better detection of these
subsurface structures is of great interest. In this study, the origins of narrow and undulating leaf area index
(LAI) patterns showing better crop performance in large scale multi-temporal satellite imagery were for the
first time interpreted by proximal soil sensor data. A multi-receiver electromagnetic induction (EMI) sensor measuring
soil apparent electrical conductivity (ECa) for six depths of exploration (DOE) ranging from 0–0.25 to
0–1.9 m was used as reconnaissance soil survey tool in combination with selected electrical resistivity tomography
(ERT) transects, and ground truth texture data to investigate lateral and vertical changes of soil properties at
ten arable fields. The moderate to excellent spatial consistency (R2 0.19–0.82) of ECa patterns and LAI crop marks that indicate a higher water storage capacity as well as the increased correlations between large-offset ECa data
and the subsoil clay content and soil profile depth, implies that along this buried paleo-river structure the subsoil is mainly responsible for better crop development in drought periods. Furthermore, observed stagnant water in
the subsoil indicates that this paleo-river structure still plays an important role in subsurface hydrology. These
insights should be considered and implemented in local hydrological as well as crop models
Possible Multiple Gap Superconductivity with Line Nodes in Heavily Hole-Doped Superconductor KFe2As2 Studied by 75As-NQR and Specific Heat
We report the 75As nuclear quadrupole resonance (NQR) and specific heat
measurements of the heavily hole-doped superconductor KFe2As2 (Tc = 3.5 K). The
spin-lattice relaxation rate 1/T1 in the superconducting state exhibits quite
gradual temperature dependence with no coherence peak below Tc. The
quasi-particle specific heat C_QP/T shows small specific heat jump which is
about 30% of electronic specific heat coefficient just below Tc. In addition,
it suggests the existence of low-energy quasi-particle excitation at the lowest
measurement temperature T = 0.4 K \simeq Tc/10. These temperature dependence of
1/T1 and C_QP/T can be explained by multiple nodal superconducting gap scenario
rather than multiple fully-gapped s_\pm-wave one within simple gap analysis.Comment: 5 pages, 5 figures, to be published in J. Phys. Soc. Jpn. No.8 issue
(2009
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
Coherence effect in a two-band superconductor: Application to iron pnictides
From a theoretical point of view, we propose an experimental method to
determine the pairing symmetry of iron pnictides. We focus on two kinds of
pairing symmetries, and , which are strong candidates for the
pairing symmetry of iron pnictides. For each of these two symmetries, we
calculate both the density and spin response functions by using the two-band
BCS model within the one-loop approximation. As a result, a clear difference is
found between the - and -wave states in the temperature
dependence of the response functions at nesting vector , which connects
the hole and electron Fermi surfaces. We point out that this difference comes
from the coherence effect in the two-band superconductor. We suggest that the
pairing symmetry could be clarified by observing the temperature dependence of
both the density and spin structure factors at the nesting vector in
neutron scattering measurements.Comment: 15 pages, 7 figures, 1 tabl
Multiple superconducting gap and anisotropic spin fluctuations in iron arsenides: Comparison with nickel analog
We present extensive 75As NMR and NQR data on the superconducting arsenides
PrFeAs0.89F0.11 (Tc=45 K), LaFeAsO0.92F0.08 (Tc=27 K), LiFeAs (Tc = 17 K) and
Ba0.72K0.28Fe2As2 (Tc = 31.5 K) single crystal, and compare with the nickel
analog LaNiAsO0.9F0.1 (Tc=4.0 K) . In contrast to LaNiAsO0.9F0.1 where the
superconducting gap is shown to be isotropic, the spin lattice relaxation rate
1/T1 in the Fe-arsenides decreases below Tc with no coherence peak and shows a
step-wise variation at low temperatures. The Knight shift decreases below Tc
and shows a step-wise T variation as well. These results indicate spinsinglet
superconductivity with multiple gaps in the Fe-arsenides. The Fe
antiferromagnetic spin fluctuations are anisotropic and weaker compared to
underdoped copper-oxides or cobalt-oxide superconductors, while there is no
significant electron correlations in LaNiAsO0.9F0.1. We will discuss the
implications of these results and highlight the importance of the Fermi surface
topology.Comment: 6 pages, 11 figure
A topological Dirac insulator in a quantum spin Hall phase : Experimental observation of first strong topological insulator
When electrons are subject to a large external magnetic field, the
conventional charge quantum Hall effect \cite{Klitzing,Tsui} dictates that an
electronic excitation gap is generated in the sample bulk, but metallic
conduction is permitted at the boundary. Recent theoretical models suggest that
certain bulk insulators with large spin-orbit interactions may also naturally
support conducting topological boundary states in the extreme quantum limit,
which opens up the possibility for studying unusual quantum Hall-like phenomena
in zero external magnetic field. Bulk BiSb single crystals are
expected to be prime candidates for one such unusual Hall phase of matter known
as the topological insulator. The hallmark of a topological insulator is the
existence of metallic surface states that are higher dimensional analogues of
the edge states that characterize a spin Hall insulator. In addition to its
interesting boundary states, the bulk of BiSb is predicted to
exhibit three-dimensional Dirac particles, another topic of heightened current
interest. Here, using incident-photon-energy-modulated (IPEM-ARPES), we report
the first direct observation of massive Dirac particles in the bulk of
BiSb, locate the Kramers' points at the sample's boundary and
provide a comprehensive mapping of the topological Dirac insulator's gapless
surface modes. These findings taken together suggest that the observed surface
state on the boundary of the bulk insulator is a realization of the much sought
exotic "topological metal". They also suggest that this material has potential
application in developing next-generation quantum computing devices.Comment: 16 pages, 3 Figures. Submitted to NATURE on 25th November(2007
B NMR study of pure and lightly carbon doped MgB superconductors
We report a B NMR line shape and spin-lattice relaxation rate
() study of pure and lightly carbon doped MgBC for
, 0.02, and 0.04, in the vortex state and in magnetic field of 23.5 kOe.
We show that while pure MgB exhibits the magnetic field distribution from
superposition of the normal and the Abrikosov state, slight replacement of
boron with carbon unveils the magnetic field distribution of the pure Abrikosov
state. This indicates a considerable increase of with carbon doping
with respect to pure MgB. The spin-lattice relaxation rate
demonstrates clearly the presence of a coherence peak right below in pure
MgB, followed by a typical BCS decrease on cooling. However, at
temperatures lower than K strong deviation from the BCS behavior is
observed, probably from residual contribution of the vortex dynamics. In the
carbon doped systems both the coherence peak and the BCS temperature dependence
of weaken, an effect attributed to the gradual shrinking of the
hole cylinders of the Fermi surface with electron doping.Comment: 8 pages, 6 figures, submitted to Phys. Rev.
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