13 research outputs found
Doping-dependent study of the periodic Anderson model in three dimensions
We study a simple model for -electron systems, the three-dimensional
periodic Anderson model, in which localized states hybridize with
neighboring states. The states have a strong on-site repulsion which
suppresses the double occupancy and can lead to the formation of a Mott-Hubbard
insulator. When the hybridization between the and states increases, the
effects of these strong electron correlations gradually diminish, giving rise
to interesting phenomena on the way. We use the exact quantum Monte-Carlo,
approximate diagrammatic fluctuation-exchange approximation, and mean-field
Hartree-Fock methods to calculate the local moment, entropy, antiferromagnetic
structure factor, singlet-correlator, and internal energy as a function of the
hybridization for various dopings. Finally, we discuss the relevance of
this work to the volume-collapse phenomenon experimentally observed in
f-electron systems.Comment: 12 pages, 8 figure
Screened-interaction expansion for the Hubbard model and determination of the quantum Monte Carlo Fermi surface
We develop a systematic self-consistent perturbative expansion for the self
energy of Hubbard-like models. The interaction lines in the Feynman diagrams
are dynamically screened by the charge fluctuations in the system. Although the
formal expansion is exact-assuming that the model under the study is
perturbative-only if diagrams to all orders are included, it is shown that for
large-on-site-Coulomb-repulsion-U systems weak-coupling expansions to a few
orders may already converge. We show that the screened interaction for the
large-U system can be vanishingly small at a certain intermediate electron
filling; and it is found that our approximation for the imaginary part of the
one-particle self energy agrees well with the QMC results in the low energy
scales at this particular filling. But, the usefulness of the approximation is
hindered by the fact that it has the incorrect filling dependence when the
filling deviates from this value. We also calculate the exact QMC Fermi
surfaces for the two-dimensional (2-D) Hubbard model for several fillings. Our
results near half filling show extreme violation of the concepts of the band
theory; in fact, instead of growing, Fermi surface vanishes when doped toward
the half-filled Mott-Hubbard insulator. Sufficiently away from half filling,
noninteracting-like Fermi surfaces are recovered. These results combined with
the Luttinger theorem might show that diagrammatic expansions for the
nearly-half-filled Hubbard model are unlikely to be possible; however, the
nonperturbative part of the solution seems to be less important as the filling
gradually moves away from one half. Results for the 2-D one-band Hubbard model
for several hole dopings are presented. Implications of this study for the
high-temperature superconductors are also discussed.Comment: 11 pages, 12 eps figures embedded, REVTeX, submitted to Phys. Rev. B;
(v2) minor revisions, scheduled for publication on November 1
A planar extrapolation of the correlation problem that permits pairing
It was observed previously that an SU(N) extension of the Hubbard model is
dominated, at large N, by planar diagrams in the sense of 't Hooft, but the
possibility of superconducting pairing got lost in this extrapolation. To allow
for this possibility, we replace SU(N) by U(N,q), the unitary group in a vector
space of quaternions. At the level of the free energy, the difference between
the SU(N)and U(N,q) extrapolations appears only to first nonleading order in N.Comment: 8 pages, 2 figure
Fluctuation Exchange Analysis of Superconductivity in the Standard Three-Band CuO2 Model
The fluctuation exchange, or FLEX, approximation for interacting electrons is
applied to study instabilities in the standard three-band model for CuO2 layers
in the high-temperature superconductors. Both intra-orbital and near-neigbor
Coulomb interactions are retained. The filling dependence of the d(x2-y2)
transition temperature is studied in both the "hole-doped" and "electron-doped"
regimes using parameters derived from constrained-occupancy density-functional
theory for La2CuO4. The agreement with experiment on the overdoped hole side of
the phase diagram is remarkably good, i.e., transitions emerge in the 40 K
range with no free parameters. In addition the importance of the "orbital
antiferromagnetic," or flux phase, charge density channel is emphasized for an
understanding of the underdoped regime.Comment: REVTex and PostScript, 31 pages, 26 figures; to appear in Phys. Rev.
B (1998); only revised EPS figures 3, 4, 6a, 6b, 6c, 7 and 8 to correct
disappearance of some labels due to technical problem
A crib-shaped triplet pairing gap function for an orthogonal pair of quasi-one dimensional Fermi surfaces in SrRuO
The competition between spin-triplet and singlet pairings is studied
theoretically for the tight-binding - bands in SrRuO,
which arise from two sets of quasi-one dimensional Fermi surfaces. Using
multiband FLEX approximation, where we incorporate an anisotropy in the spin
fluctuations as suggested from experiments, we show that (i) the triplet can
dominate over the singlet (which turns out to be extended s), and (ii) the
triplet gap function optimized in the Eliashberg equation has an unusual, very
non-sinusoidal form, whose time-reversal-broken combination exhibits a
crib-shaped amplitude with dips.Comment: 5 pages, RevTeX, to appear in Phys.Rev.B (Rapid Communications
Possible Ordered States in the 2D Extended Hubbard Model
Possible ordered states in the 2D extended Hubbard model with on-site (U>0)
and nearest-neighbor (V) interaction are examined near half filling, with
emphasis on the effect of finite V. First, the phase diagram at absolute zero
is determined in the mean field approximation. For , a state where
d_{x^{2}-y^{2}}-wave superconductivity (dSC), commensurate spin-density-wave
(SDW) and -triplet pair coexist is seen to be stabilized. Here, the
importance of -triplet pair on the coexistence of dSC and SDW is
indicated. This coexistent state is hampered by the phase separation (PS),
which is generally expected to occur in the presence of finite-range attractive
interaction, but survives. For V>0, a state where commensurate
charge-density-wave (CDW), SDW and ferromagnetism (FM) coexist is seen to be
stabilized. Here, the importance of FM on the coexistence of CDW and SDW is
indicated. Next, in order to examine the effects of fluctuation on each mean
field ordered state, the renormalization group method for the special case that
the Fermi level lies just on the saddle points, (,0) and (0,), is
applied. The crucial difference from the mean field result is that
superconductivity can arise even for U>0 and , where the
superconducting gap symmetry is d_{x^{2}-y^{2}}-wave for U>4V and s-wave for
U<4V. Finally, the possibilities that the mean field coexistent states survive
in the presence of fluctuation are discussed.Comment: 12 pages, 19 figures included, revised versio
Theory of Spin Fluctuation-Induced Superconductivity Based on a d-p Model. II. -Superconducting State-
The superconducting state of a two-dimensional d-p model is studied from the
spin fluctuation point of view by using a strong coupling theory. The
fluctuation exchange (FLEX) approximatoin is employed to calculate the spin
fluctuations and the superconducting gap functions self-consistently in the
optimal- and over-doped regions of hole concentration. The gap function has a
symmetry of d_{x^2 - y^2} type and develops below the transition temperature
T_c more rapidly than in the BCS model. Its saturation value at the maximum is
about 10 T_c. When the spin fluctuation-induced superconductivity is well
stabilized at low temperatures in the optimal regime, the imaginary part of the
antiferromagnetic spin susceptibility shows a very sharp resonance peak
reminiscent of the 41 meV peak observed in the neutron scattering experiment on
YBCO. The one-particle spectral density around k=(pi,0) shows sharp
quasi-particle peaks followed by dip and hump structures bearing resemblance to
the features observed in the angle-resolved photoemission experiment. With
increasing doping concentration these features gradually disappear.Comment: 13 pages(LaTeX), 20 eps figure
Comparison of elongation amount and bond strength between different soft lining materials and PMMA denture base material [Farkli yumusak astar malzemelerinin uzama miktarlarinin ve PMMA kaideye baglanma dayaniminin karsilastirilmasi]
Objectives: The bond strength and elongation amount of different soft lining materials were evaluated in the current study. Material and Methods: Following the flasking of wax specimens in dental stone, wax removal and processing of polymethylmetacrylate (Meliodent) denture material, 3 different soft relining materials (Molloplast-B, Mollosil Plus, Vertex Soft) were placed within the space prepared in the center of polymethylmetacrylate (PMMA) specimens (n=8). Using a crosshead speed of 5mm/min, tensile test was continued until failure was occurred. Mean tensile strength (MPa) and mean elongation (mm) was measured. Data was analyzed using one-way ANOVA and Tukey multiple comparison test (?=0.05). Results: Significant differences were found among the tensile bond strength results (p<0.05). The highest bond strength value was observed for acrylic base soft lining material (1.6 (0.2) MPa), while the lowest value was observed for autopolymerized silicon based material (0.9 (0.1) MPa). A significant difference was not found between the heat polymerized acrylic and the silicon based soft lining materials. All three materials showed similar values in terms of elongation amounts (p<0.05). The elongation amounts were 6 (2.1) mm for acrylic based soft lining material, 7.1 (1.8) mm for autopolymerized silicon soft lining material and 4.8 (2.3) mm for heat polymerized silicon material. Conclusions: No significant difference was found between the heat polymerized acrylic and the silicone based soft lining materials. Autopolymerizing silicone based soft lining material has the lowest bond strength value. No significant difference was found for elongation amounts