2,064 research outputs found
Magnetic Coherence in Cuprate Superconductors
Recent inelastic neutron scattering (INS) experiments on
LaSrCuO observed a {\it magnetic coherence effect}, i.e.,
strong frequency and momentum dependent changes of the spin susceptibility,
, in the superconducting phase. We show that this effect is a direct
consequence of changes in the damping of incommensurate antiferromagnetic spin
fluctuations due to the appearance of a d-wave gap in the fermionic spectrum.
Our theoretical results provide a quantitative explanation for the weak
momentum dependence of the observed spin-gap. Moreover, we predict {\bf (a)} a
Fermi surface in LaSrCuO which is closed around up
to optimal doping, and {\bf (b)} similar changes in for all cuprates
with an incommensurate magnetic response.Comment: 5 pages, 4 figures, Fig.3 is in colo
Space efficient opposed-anvil high-pressure cell and its application to optical and NMR measurements up to 9 GPa
We have developed a new type of opposed-anvil high pressure cell with
substantially improved space efficiency. The clamp cell and the gasket are made
of non-magnetic Ni-Cr-Al alloy. Non-magnetic tungsten carbide (NMWC) is used
for the anvils. The assembled cell with the dimension \phi 29mm \times 41mm is
capable of generating pressure up to 9 GPa over a relatively large volume of 7
mm3. Our cell is particularly suitable for those experiments which require
large sample space to achieve good signal-to-noise ratio, such as the nuclear
magnetic resonance (NMR) experiment. Argon is used as the pressure transmitting
medium to obtain good hydrostaticity. The pressure was calibrated in situ by
measuring the fluorescence from ruby through a transparent moissanite (6H-SiC)
window. We have measured the pressure and temperature dependences of the 63Cu
nuclear-quadrupole-resonance (NQR) frequency of Cu2O, the in-plane Knight shift
of metallic tin, and the Knight shift of platinum. These quantities can be used
as reliable manometers to determine the pressure values in situ during the
NMR/NQR experiments up to 9 GPa.Comment: 9 pages, 5 figures, 3 tables, accepted for publication in J. Phys.
Soc. Jp
Coherent `ab' and `c' transport theory of high- cuprates
We propose a microscopic theory of the `'-axis and in-plane transport of
copper oxides based on the bipolaron theory and the Boltzmann kinetics. The
fundamental relationship between the anisotropy and the spin susceptibility is
derived, . The
temperature and doping dependence of the in-plane, and
out-of-plane, resistivity and the spin susceptibility,
are found in a remarkable agreement with the experimental data in underdoped,
optimally and overdoped for the entire temperature
regime from up to . The normal state gap is explained and its
doping and temperature dependence is clarified.Comment: 12 pages, Latex, 3 figures available upon reques
Neutron scattering in a d_{x^2-y^2}-wave superconductor with strong impurity scattering and Coulomb correlations
We calculate the spin susceptibility at and below T_c for a d_{x^2-y^2}-wave
superconductor with resonant impurity scattering and Coulomb correlations. Both
the impurity scattering and the Coulomb correlations act to maintain peaks in
the spin susceptibility, as a function of momentum, at the Brillouin zone edge.
These peaks would otherwise be suppressed by the superconducting gap. The
predicted amount of suppression of the spin susceptibility in the
superconducting state compared to the normal state is in qualitative agreement
with results from recent magnetic neutron scattering experiments on
La_{1.86}Sr_{0.14}CuO_4 for momentum values at the zone edge and along the zone
diagonal. The predicted peak widths in the superconducting state, however, are
narrower than those in the normal state, a narrowing which has not been
observed experimentally.Comment: 24 pages (12 tarred-compressed-uuencoded Postscript figures), REVTeX
3.0 with epsf macros, UCSBTH-94-1
C-axis resistivity and high Tc superconductivity
Recently we had proposed a mechanism for the normal-state C-axis resistivity
of the high-T layered cuprates that involved blocking of the
single-particle tunneling between the weakly coupled planes by strong
intra-planar electron-electron scattering. This gave a C-axis resistivity that
tracks the ab-plane T-linear resistivity, as observed in the high-temperature
limit. In this work this mechanism is examined further for its implication for
the ground-state energy and superconductivity of the layered cuprates. It is
now argued that, unlike the single-particle tunneling, the tunneling of a
boson-like pair between the planes prepared in the BCS-type coherent trial
state remains unblocked inasmuch as the latter is by construction an eigenstate
of the pair annihilation operator. The resulting pair-delocalization along the
C-axis offers energetically a comparative advantage to the paired-up trial
state, and, thus stabilizes superconductivity. In this scheme the strongly
correlated nature of the layered system enters only through the blocking
effect, namely that a given electron is effectively repeatedly monitored
(intra-planarly scattered) by the other electrons acting as an environment, on
a time-scale shorter than the inter-planar tunneling time. Possible
relationship to other inter-layer pairing mechanisms proposed by several
workers in the field is also briefly discussed.Comment: typos in equations corrected, contents unchange
Two-dimensional incommensurate magnetic fluctuations in Sr(RuTi)O
We investigate the imaginary part of the wave vector dependent dynamic spin
susceptibility in Sr(RuTi)O as a function of
temperature using neutron scattering. At T=5 K, two-dimensional incommensurate
(IC) magnetic fluctuations are clearly observed around
up to approximately 60 meV energy transfer.
We find that the IC excitations disperse to ridges around the
point. Below 50 K, the energy and temperature dependent excitations are well
described by the phenomenological response function for a Fermi liquid system
with a characteristic energy of 4.0(1) meV. Although the wave vector dependence
of the IC magnetic fluctuations in Sr(RuTi)O is
similar to that in the Fermi liquid state of the parent compound,
SrRuO, the magnetic fluctuations are clearly suppressed by the
Ti-doping.Comment: 5 pages, 4 figure
Pseudogap behavior of nuclear spin relaxation in high Tc superconductors in terms of phase separation
We analyze anew experiments on the NMR in cuprates and find an important
information on their phase separation and its stripe character hidden in the
dependence of on degree of doping. In a broad class of materials
is the sum of two terms: the temperature independent one
attributed to ``incommensurate'' stripes that occur at external doping, and an
``universal'' temperature dependent term ascribed to moving metallic and AF
sub-phases. We argue that the frustrated first order phase transition in a
broad temperature interval bears a dynamical character.Comment: 5 pages, 3 figures; some comments and references added; accepted for
publication in JETP Letter
Model of C-Axis Resistivity of High-\Tc Cuprates
We propose a simple model which accounts for the major features and
systematics of experiments on the -axis resistivity, , for \lsco,
\ybco and \bsco . We argue that the -axis resistivity can be separated
into contributions from in-plane dephasing and the -axis ``barrier''
scattering processes, with the low temperature semiconductor-like behavior of
arising from the suppression of the in-plane density of states
measured by in-plane magnetic Knight shift experiments. We report on
predictions for in impurity-doped \ybco materials.Comment: 10 pages + figures, also see March Meeting J13.1
One-loop approximation for the Heisenberg antiferromagnet
We use the diagram technique for spin operators to calculate Green's
functions and observables of the spin-1/2 quantum Heisenberg antiferromagnet on
a square lattice. The first corrections to the self-energy and interaction are
taken into account in the chain diagrams. The approximation reproduces main
results of Takahashi's modified spin-wave theory [Phys. Rev. B 40, 2494 (1989)]
and is applicable in a wider temperature range. The energy per spin calculated
in this approximation is in good agreement with the Monte Carlo and
small-cluster exact-diagonalization calculations in the range 0 <= T < 1.2J
where J is the exchange constant. For the static uniform susceptibility the
agreement is good for T < 0.6J and becomes somewhat worse for higher
temperatures. Nevertheless the approximation is able to reproduce the maximum
in the temperature dependence of the susceptibility near T = 0.9J.Comment: 15 pages, 6 ps figure
Magnetic Field and Pressure Phase Diagrams of Uranium Heavy-Fermion Compound UZn
We have performed magnetization measurements at high magnetic fields of up to
53 T on single crystals of a uranium heavy-fermion compound UZn
grown by the Bridgman method. In the antiferromagnetic state below the N\'{e}el
temperature = 9.7 K, a metamagnetic transition is found at
32 T for the field along the [110] direction (-axis). The
magnetic phase diagram for the field along the [110] direction is
given. The magnetization curve shows a nonlinear increase at 35
T in the paramagnetic state above up to a characteristic
temperature where the magnetic susceptibility or
electrical resistivity shows a maximum value. This metamagnetic behavior of the
magnetization at is discussed in comparison with the metamagnetic
magnetism of the heavy-fermion superconductors UPt, URuSi, and
UPdAl. We have also carried out high-pressure resistivity measurement
on UZn using a diamond anvil cell up to 8.7 GPa. Noble gas argon was
used as a pressure-transmitting medium to ensure a good hydrostatic
environment. The N\'{e}el temperature is almost
pressure-independent up to 4.7 GPa and starts to increase in the
higher-pressure region. The pressure dependences of the coefficient of the
term in the electrical resistivity , the antiferromagnetic gap
, and the characteristic temperature are
discussed. It is found that the effect of pressure on the electronic states in
UZn is weak compared with those in the other heavy fermion
compounds
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