389 research outputs found
Superconductivity and Spin Fluctuations in the Electron-Doped Infinitely-Layered High Tc Superconductor SrLaCuO (Tc=42K)
This paper describes the first 63-Cu NMR study of an electron-doped
infinitely-layered high Tc superconductor SrLaCuO (Tc=42K). The
spin dynamics in the normal state above Tc exhibits qualitatively the same
behavior as some hole-doped materials with significantly enhanced spin
fluctuations. Below Tc, we observed no signature of a Hebel-Slichter coherence
peak, suggesting an unconventional nature of the symmetry of the
superconducting order parameter.Comment: Invited Paper to SNS-95 Conference (Spectroscopies on Novel
Superconductors 1995 at Stanford). Also presented at Aspen Winter Conference
on Superconductivity and Grenoble M^2S-HTSC in 199
Local dynamics and thermal activation in the transverse-field Ising chain
There has been considerable recent progress in identifying candidate
materials for the transverse-field Ising chain (TFIC), a paradigmatic model for
quantum criticality. Here, we study the local spin dynamical structure factor
of different spin components in the quantum disordered region of the TFIC. We
show that the low-frequency local dynamics of the spins in the Ising- and
transverse-field directions have strikingly distinctive temperature
dependencies. This leads to the thermal-activation gap for the secular term of
the NMR relaxation rate to be half of that for the
relaxation rate. Our findings reveal a new surprise in the nonzero-temperature
dynamics of the venerable TFIC model and uncover a means to evince the material
realization of the TFIC universality.Comment: 17 pages, 5 figure
Epitaxial growth of FeSeTe thin films on CaF substrates with high critical current density
In-situ epitaxial growth of FeSeTe thin films is demonstrated
on a non-oxide substrate CaF. Structural analysis reveals that compressive
stress is moderately added to 36-nm thick FeSeTe, which pushes
up the critical temperature above 15 K, showing higher values than that of bulk
crystals. Critical current density at = 4.5 K reaches 5.9 x 10
Acm at = 10 T, and 4.2 x 10 Acm at = 14 T.
These results indicate that fluoride substrates have high potential for the
growth of iron-based superconductors in comparison with popular oxide
substrates.Comment: 9 pages, 3 figures, to be published in Applied Physics Express 4,
053101 (2011
Field-induced phase transitions in a Kondo insulator
We study the magnetic-field effect on a Kondo insulator by exploiting the
periodic Anderson model with the Zeeman term. The analysis using dynamical mean
field theory combined with quantum Monte Carlo simulations determines the
detailed phase diagram at finite temperatures. At low temperatures, the
magnetic field drives the Kondo insulator to a transverse antiferromagnetic
phase, which further enters a polarized metallic phase at higher fields. The
antiferromagnetic transition temperature takes a maximum when the Zeeman
energy is nearly equal to the quasi-particle gap. In the paramagnetic phase
above , we find that the electron mass gets largest around the field where
the quasi-particle gap is closed. It is also shown that the induced moment of
conduction electrons changes its direction from antiparallel to parallel to the
field.Comment: 7 pages, 6 figure
Spin susceptibility, phase diagram, and quantum criticality in the electron-doped high Tc Superconductor Ba[Fe(1-x)Co(x)]2As2
We report a systematic investigation of Ba[Fe(1-x)Co(x)]2As2 based on
transport and 75-As NMR measurements, and establish the electronic phase
diagram. We demonstrate that doping progressively suppresses the uniform spin
susceptibility and low frequency spin fluctuations. The optimum superconducting
phase emerges at x_c~0.08 when the tendency toward spin ordering completely
diminishes. Our findings point toward the presence of a quantum critical point
near x_c between the SDW (spin density wave) and superconducting phases.Comment: 5 Figure
Fabrication of FeSe1-x superconducting films with bulk properties
We have fabricated high-quality FeSe1-x superconducting films with a bulk Tc
of 11-12 K on different substrates, Al2O3(0001), SrTiO3(100), MgO(100), and
LaAlO3(100), by using a pulsed laser deposition technique. All the films were
grown at a high substrate temperature of 610 oC, and were preferentially
oriented along the (101) direction, the latter being to be a key to fabricating
of FeSe1-x superconducting thin films with high Tc. According to the energy
dispersive spectroscopy data, the Fe:Se composition ratio was 1:0.90+-0.02. The
FeSe1-x film grown on a SrTiO3 substrate showed the best quality with a high
upper critical magnetic field [Hc2(0)] of 56 T
NMR and Neutron Scattering Experiments on the Cuprate Superconductors: A Critical Re-Examination
We show that it is possible to reconcile NMR and neutron scattering
experiments on both LSCO and YBCO, by making use of the Millis-Monien-Pines
mean field phenomenological expression for the dynamic spin-spin response
function, and reexamining the standard Shastry-Mila-Rice hyperfine Hamiltonian
for NMR experiments. The recent neutron scattering results of Aeppli et al on
LSCO (x=14%) are shown to agree quantitatively with the NMR measurements of
and the magnetic scaling behavior proposed by Barzykin and Pines.
The reconciliation of the relaxation rates with the degree of
incommensuration in the spin fluctuation spectrum seen in neutron experiments
is achieved by introducing a new transferred hyperfine coupling between
oxygen nuclei and their next nearest neighbor spins; this leads to a
near-perfect cancellation of the influence of the incommensurate spin
fluctuation peaks on the oxygen relaxation rates of LSCO. The inclusion of the
new term also leads to a natural explanation, within the one-component
model, the different temperature dependence of the anisotropic oxygen
relaxation rates for different field orientations, recently observed by
Martindale . The measured significant decrease with doping of the
anisotropy ratio, in LSCO system, from
for to for LSCO (x=15%) is made compatible with the
doping dependence of the shift in the incommensurate spin fluctuation peaks
measured in neutron experiments, by suitable choices of the direct and
transferred hyperfine coupling constants and B.Comment: 24 pages in RevTex, 9 figures include
Superconductivity of FeSe0.5Te0.5 Thin Films Grown by Pulsed Laser Deposition
FeSe0.5Te0.5 thin films with PbO-type structure are successfully grown on
MgO(100) and LaSrAlO4(001) substrates from FeSe0.5Te0.5 or FeSe0.5Te0.75
polycrystalline targets by pulsed-laser deposition. The film deposited on the
MgO substrate (film thickness ~ 55 nm) shows superconductivity at 10.6 K
(onset) and 9.2 K (zero resistivity). On the other hand, the film deposited on
the LaSrAlO4 substrate (film thickness ~ 250 nm) exhibits superconductivity at
5.4 K (onset) and 2.7 K (zero resistivity). This suggests the strong influence
of substrate materials and/or the c-axis length to superconducting properties
of FeSe0.5Te0.5 thin films.Comment: 24 pages, 5 figure
Magnetic Properties of Ab initio Model for Iron-Based Superconductors LaFeAsO
By using variational Monte Carlo method, we examine an effective low-energy
model for LaFeAsO derived from an ab initio downfolding scheme. We show that
quantum and many-body fluctuations near a quantum critical point largely reduce
the antiferromagnetic (AF) ordered moment and the model not only quantitatively
reproduces the small ordered moment in LaFeAsO, but also explains the diverse
dependence on LaFePO, BaFe2As2 and FeTe. We also find that LaFeAsO is under
large orbital fluctuations, sandwiched by the AF Mott insulator and weakly
correlated metals. The orbital fluctuations and Dirac-cone dispersion hold keys
for the diverse magnetic properties.Comment: 4 pages, 4 figure
d_{x^2-y^2}-Wave Pairing Fluctuations and Pseudo Spin Gap in Two-Dimensional Electron Systems
Pseudogap phenomena of high-T_c cuprates are examined. In terms of AFM
(antiferromagnetic) and dSC (d_{x^2-y^2}-wave superconducting) auxiliary fields
introduced to integrate out the fermions, the effective action for 2D electron
systems with AFM and dSC fluctuations is considered. By the self-consistent
renormalization (SCR), the NMR relaxation rate T_1^{-1}, the spin correlation
length \xi_\sigma and the pairing correlation length \xi_d are calculated. From
this calculation, a mechanism of the pseudogap formation emerges as the region
of dominant d-wave short-range order (SRO) over AFM-SRO. When damping for the
AFM fluctuation strongly depends on the dSC correlation length through the
formation of precursor singlets around (\pi,0) and (0,\pi) points in the
momentum space, the pseudogap appears in a region of the normal state
characterized by decreasing 1/T_1T and increasing AFM correlation length with
decrease in temperature. This reproduces a characteristic feature of the
pseudogap phenomena in many underdoped cuprates. When the damping becomes
insensitive to the dSC correlation length, the pseudogap region shrinks as in
the overdoped cuprates.Comment: 13 pages with 5 figures, submitted to J. Phys. Soc. Jpn.; figure
inclusion correcte
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