76 research outputs found
Superconductive Phonon Anomalies in High- Cuprates
We consider the effects on phonon dynamics of spin-lattice coupling within
the slave-boson mean-field treatment of the extended - model. With no
additional assumptions the theory is found to give a semi-quantitative account
of the frequency and linewidth anomalies observed by Raman and neutron
scattering for the 340 phonon mode in at the
superconducting transition. We discuss the applicability of the model to phonon
modes of different symmetries, and report a connection to spin-gap features
observed in underdoped YBCO. The results suggest the possibility of a unified
understanding of the anomalies in transport, magnetic and lattice properties.Comment: heavily revised version of previous paper, including systematic
treatment of effect of term, coupling constant derivation and calculation
of phonon linewidth broadening. Revised Figs. 3 and 4 still only available by
sending fax # to [email protected]
On the presence of mid-gap states in CaV4O9
Using exact diagonalizations of finite clusters with up to 32 sites, we study
the model on the 1/5 depleted square lattice. Spin-spin correlation
functions are consistent with plaquette order in the spin gap phase which
exists for intermediate values of . Besides, we show that singlet
states will be present in the singlet-triplet gap if is not too small
(). We argue that this property should play a central
role in determining the exchange integrals in Comment: 4 pages, 5 postscript figure
Effect of Quantum Fluctuations on Magnetic Ordering in CaVO
We present a theoretical model for CaVO: the -depleted square
spin- Heisenberg model which includes both the nearest-neighbor coupling
() and the next-nearest-neighbor coupling (), where and are
antiferromagnetic. Recent experiments of the neutron diffraction by Harashina
et.al. report the magnetic ordering at low temperatures, which may be called as
a stripe phase. It is shown that the observed spin structure is not stable in
the classical theory. By employing the modified spin wave theory, we show that
the stripe phase is stabilized by the quantum fluctuations for .
In CaVO, the coupling constants are estimated as by
comparing the theoretical and experimental results.Comment: submitted to J. Phys. Soc. Jp
Single-Particle Pseudogap in Two-Dimensional Electron Systems
We investigate pseudogap phenomena in the 2D electron system.
Based on the mode-mode coupling theory of antiferromagnetic (AFM) and
-wave superconducting (SC) fluctuations, single-particle
dynamics is analyzed. For the parameter values of underdoped cuprates,
pseudogap structure grows in the single-particle spectral weight
around the wave vector and below the pseudo-spin-gap
temperature \TPG signaled by the reduction of dynamical spin correlations in
qualitative agreement with the experimental data. The calculated results for
the overdoped cuprates also reproduce the absence of the pseudogap in the
experiments. We also discuss limitations of our weak-coupling approach.Comment: 6 pages with 4 figures, submitted to J. Phys. Soc. Jp
Strongly Correlated Electrons and Neutron Scattering
Various aspects of close and interesting relationship between
antiferromagnetism and singlet ground states are introduced for which neutron
scattering have been playing vital roles. The special emphasis is on the
disorder-induced antiferromagnetism in spin-Peierls systems, which can be
viewed as a nucleation process of classical magnetic order in the background of
singlet state whose origin is purely quantum-mechanical. It is then pointed out
that similar features will be found in Ce_xCu_2Si_2 and high T_c cuprates.
Finally the possible charge ordering process in NaV_2O_5 is discussed which
leads to the quenching of localized spins.Comment: 8 pages, 3 figures, Proceedings of 7th ISSP International Symposium
"Frontiers in Neutron Scattering Research" (To appear in J. Phys. Chem.
Solids (1999)
Exchange interactions and magnetic properties of the layered vanadates CaV2O5, MgV2O5, CaV3O7 and CaV4O9
We have performed ab-initio calculations of exchange couplings in the layered
vanadates CaV2O5, MgV2O5, CaV3O7 and CaV4O9. The uniform susceptibility of the
Heisenberg model with these exchange couplings is then calculated by quantum
Monte Carlo method; it agrees well with the experimental measurements. Based on
our results we naturally explain the unusual magnetic properties of these
materials, especially the huge difference in spin gap between CaV2O5 and
MgV2O5, the unusual long range order in CaV3O7 and the "plaquette resonating
valence bond (RVB)" spin gap in CaV4O9
Electronic Specific Heat of La_{2-x}Sr_{x}CuO_{4}: Pseudogap Formation and Reduction of the Superconducting Condensation Energy
To examine the so-called small pseudogap and the superconducting (SC)
condensation energy U(0), the electronic specific heat Cel was measured on
La_{2-x}Sr_{x}CuO_{4} up to ~120K. In samples with doping level p (=x) less
than ~0.2, small pseudogap behavior appears in the \gamma (=Cel/T) vs. T curve
around the mean-field critical temperature for a d-wave superconductor Tco
(=2*\Delta_{0}/(4~5)k_B), where \Delta_{0} is the maximum gap at T<<Tc. The
condensation energy U(0) is largely reduced in the pseudogap regime (p< ~0.2).
The reduction of U(0) can be well reproduced by introducing an effective SC
energy scale \Delta_{eff}=\beta*p*\Delta_{0} (\beta=4.5) instead of \Delta_{0}.
The effective SC energy scale is discussed in relation to the coherent pairing
gap formed over the nodal Fermi arc.Comment: 8page
suppression in co-doped striped cuprates
We propose a model that explains the reduction of due to the pinning of
stripes by planar impurity co-doping in cuprates. A geometrical argument about
the planar fraction of carriers affected by stripe pinning leads to a a linear
suppression as a function of impurity concentration . The critical
value for the vanishing of superconductivity is shown to scale like
in the under-doped regime and becomes universal in the optimally- and
over-doped regimes. Our theory agrees very well with the experimental data in
single- and bi-layer cuprates co-doped with Zn, Li, Co, etc...Comment: 4 pages, 4 figure
Effect of anisotropic impurity scattering in superconductors
We discuss the weak-coupling BCS theory of a superconductor with the
impurities, accounting for their anisotropic momentum-dependent potential. The
impurity scattering process is considered in the t-matrix approximation and its
influence on the superconducting critical temperature is studied in the Born
and unitary limit for a d- and (d+s)-wave superconductors. We observe a
significant dependence of the pair-breaking strength on the symmetry of the
scattering potential and classify the impurity potentials according to their
ability to alter T_c. A good agreement with the experimental data for Zn doping
and oxygen irradiation in the overdoped cuprates is found.Comment: 31 pages, RevTex, 15 PostScript figure
Impurity Effects on the Superconducting Transition Temperature of NazCoO2.yH2O
Superconducting transition temperature Tc of NazCo1-xMxO2.yH2O has been
studied for M=Ir and Ga, which can substitute for Co. It has been found that Tc
is suppressed by the M-atom doping. The decreasing rates |dTc/dx| are of the
order of 1 K/% and too small to be explained by the pair breaking mechanism for
the anisotropic order parameters by non-magnetic impurities. Brief arguments
are given on possible origins of the Tc suppression.Comment: 10 pages, 14 figures,submitted to J. Phys. Soc. Jp
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