521 research outputs found
High stability design for new centrifugal compressor
It is essential that high-performance centrifugal compressors be free of subsynchronous vibrations. A new high-performance centrifugal compressor has been developed by applying the latest rotordynamics knowledge and design techniques: (1) To improve the system damping, a specially designed oil film seal was developed. This seal attained a damping ratio three times that of the conventional design. The oil film seal contains a special damper ring in the seal cartridge. (2) To reduce the destabilizing effect of the labyrinth seal, a special swirl canceler (anti-swirl nozzle) was applied to the balance piston seal. (3) To confirm the system damping margin, the dynamic simulation rotor model test and the full load test applied the vibration exciting test in actual load conditions
Origin of the Weak Pseudo-gap Behaviors in Na_{0.35}CoO_2: Absence of Small Hole Pockets
We analyze the ``normal electronic states'' of Na_{0.35}CoO_2 based on the
effective d-p model with full d-orbital freedom using the fluctuation-exchange
(FLEX) approximation. They sensitively depend on the topology of the Fermi
surfaces, which changes as the crystalline electric splitting (CES) due to the
trigonal deformation. We succeed in reproducing the weak pseudo-gap behaviors
in the density of states (DOS) and in the uniform magnetic susceptibility below
300K, assuming that six small hole-pockets predicted by LDA band calculations
are absent. When they exist, on the contrary, then ``anti-pseudo-gap
behaviors'' should inevitably appear. Thus, the present study strongly supports
the absence of the small hole-pockets in Na_{0.35}CoO_2, as reported by recent
ARPES measurements. A large Fermi surface around the \Gamma-point would account
for the superconductivity in water-intercalated samples.Comment: 5pages, to appear in J. Phys. Soc. Jpn. Vol.74 (2005) No.
Electrical Conductivity of Fermi Liquids. I. Many-body Effect on the Drude Weight
On the basis of the Fermi liquid theory, we investigate the many-body effect
on the Drude weight. In a lattice system, the Drude weight is modified by
electron-electron interaction due to Umklapp processes, while it is not
renormalized in a Galilean invariant system. This is explained by showing that
the effective mass for is defined through the current, not
velocity, of quasiparticle. It is shown that the inequality is required
for the stability against the uniform shift of the Fermi surface. The result of
perturbation theory applied for the Hubbard model indicates that as a
function of the density is qualitatively modified around half filling
by Umklapp processes.Comment: 20 pages, 2 figures; J. Phys. Soc. Jpn. Vol.67, No.
Theory of Thermoelectric Power in High-Tc Superconductors
We present a microscopic theory for the thermoelectric power (TEP) in high-Tc
cuprates. Based on the general expression for the TEP, we perform the
calculation of the TEP for a square lattice Hubbard model including all the
vertex corrections necessary to satisfy the conservation laws. In the present
study, characteristic anomalous temperature and doping dependences of the TEP
in high-Tc cuprates, which have been a long-standing problem of high-Tc
cuprates, are well reproduced for both hole- and electron-doped systems, except
for the heavily under-doped case. According to the present analysis, the strong
momentum and energy dependences of the self-energy due to the strong
antiferromagnetic fluctuations play an essential role in reproducing
experimental anomalies of the TEP.Comment: 5 pages, 8 figures, to appear in J. Phys. Soc. Jpn. 70 (2001) No.10.
Figure 2 has been revise
Fermi Surface Study of Quasi-Two-Dimensional Organic Conductors by Magnetooptical Measurements
Magnetooptical measurements of several quasi-two-dimensional (q2D) organic
conductors, which have simple Fermi surface structure, have been performed by
using a cavity perturbation technique. Despite of the simple Fermi surface
structure, magnetooptical resonance results show a dramatic difference for each
sample. Cyclotron resonances (CR) were observed for q-(BEDT-TTF)2I3 and
(BEDT-TTF)3Br(pBIB), while periodic orbit resonances (POR) were observed for
(BEDT-TTF)2Br(DIA) and (BEDT-TTF)3Cl(DFBIB). The selection of the resonance
seems to correspond with the skin depth for each sample. The effective mass of
POR seems to have a mass enhancement due to the many-body effect, while
effective mass of CR is independent of the strength of the electron-electron
interaction. The scattering time deduced from each resonance's linewidth will
be also presented.Comment: 10 pages, 8 figures, to be published to J. Phys. Soc. Jpn Vol.72 No.1
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Cyclotron resonance in the layered perovskite superconductor Sr2RuO4
We report a detailed study of the magnetic-field-orientation dependence of
the millimetre-wave magnetoconductivity of the superconductor Sr2RuO4 We find
two harmonic series of cyclotron resonances. We assign the first, corresponding
to a quasiparticle mass of , where is the
free-electron mass, to the Fermi-surface section. We assign the second
series, which contains only odd harmonics, to cyclotron resonance of the
Fermi-surface section, yielding a quasiparticle mass of . A third, single cyclotron resonance, corresponding to a
quasiparticle mass of , is attributed to the
Fermi-surface section. In addition, we find a very strong absorption mode in
the presence of a magnetic field component parallel to the
quasi-two-dimensional planes of the sample. Its dependence on the orientation
of the magnetic field cannot be described in the context of conventional
cyclotron resonance, and the origin of this mode is not yet clear.Comment: Submitted to J. Phys. Cond. Ma
Cyclotron Resonance in the Layered Perovskite Superconductor Sr2RuO4
We have measured the cyclotron masses in Sr2RuO4 through the observation of
periodic-orbit-resonances - a magnetic resonance technique closely related to
cyclotron resonance. We obtain values for the alpha, beta and gamma Fermi
surfaces of (4.33+/-0.05)me, (5.81+/-0.03)me and (9.71+/-0.11)me respectively.
The appreciable differences between these results and those obtained from de
Haas- van Alphen measurements are attributable to strong electron-electron
interactions in this system. Our findings appear to be consistent with
predictions for a strongly interacting Fermi liquid; indeed, semi-quantitative
agreement is obtained for the electron pockets beta and gamma.Comment: 4 pages + 3 figure
Evolution of Hall coefficient in two-dimensional heavy fermion CeCoIn
We report on the pressure dependence of the Hall coefficient in
quasi-2D heavy fermion CeCoIn. At ambient pressure, below a temperature
associated with the emergence of non-Fermi liquid properties, is
anomalously enhanced. We found that the restoration of the Fermi liquid state
with applied pressure leads to a gradual suppression of this dramatic
enhancement. Moreover, the enhancement in was found to be confined to an
intermediate temperature window, where inelastic electron-electron scattering
is dominant. Our results strongly support the presence of cold and hot spots on
the Fermi surface probably due to anisotropic scattering by antiferromagnetic
fluctuations, which may also prove relevant for the debate on the anomalous
normal-state properties of high- cuprates.Comment: 9 pages, 5 fiqures, to be published in J. Phys. Soc. Jp
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