8,544 research outputs found
Marginal Fermi liquid analysis of 300 K reflectance of Bi2Sr2CaCu2O8+x
We use 300 K reflectance data to investigate the normal-state electrodynamics
of the high temperature superconductor BiSrCaCuO
over a wide range of doping levels. The data show that at this temperature the
free carriers are coupled to a continuous spectrum of fluctuations. Assuming
the Marginal Fermi Liquid (MFL) form as a first approximation for the
fluctuation spectrum, the doping-dependent coupling constant can
be estimated directly from the slope of the reflectance spectrum. We find that
decreases smoothly with the hole doping level, from underdoped
samples with ( K) where to overdoped
samples with , ( K) where . An analysis of
the intercept and curvature of the reflectance spectrum shows deviations from
the MFL spectrum symmetrically placed at the optimal doping point . The
Kubo formula for the conductivity gives a better fit to the experiments with
the MFL spectrum up to 2000 cm and with an additional Drude component or
an additional Lorentz component up to 7000 cm. By comparing three
different model fits we conclude that the MFL channel is necessary for a good
fit to the reflectance data. Finally, we note that the monotonic variation of
the reflectance slope with doping provides us with an independent measure of
the doping level for the Bi-2212 system.Comment: 11 pages, 11 figure
History and results of the Riga dynamo experiments
On 11 November 1999, a self-exciting magnetic eigenfield was detected for the
first time in the Riga liquid sodium dynamo experiment. We report on the long
history leading to this event, and on the subsequent experimental campaigns
which provided a wealth of data on the kinematic and the saturated regime of
this dynamo. The present state of the theoretical understanding of both regimes
is delineated, and some comparisons with other laboratory dynamo experiments
are made.Comment: 8 pages, 5 figure, accepted for publication in Comptes Rendus
Physiqu
Direct observation of a Fermi liquid-like normal state in an iron-pnictide superconductor
There are two prerequisites for understanding high-temperature (high-T)
superconductivity: identifying the pairing interaction and a correct
description of the normal state from which superconductivity emerges. The
nature of the normal state of iron-pnictide superconductors, and the role
played by correlations arising from partially screened interactions, are still
under debate. Here we show that the normal state of carefully annealed
electron-doped BaFeCoAs at low temperatures has all the
hallmark properties of a local Fermi liquid, with a more incoherent state
emerging at elevated temperatures, an identification made possible using
bulk-sensitive optical spectroscopy with high frequency and temperature
resolution. The frequency dependent scattering rate extracted from the optical
conductivity deviates from the expected scaling
with
1.47 rather than = 2, indicative of the presence of residual elastic
resonant scattering. Excellent agreement between the experimental results and
theoretical modeling allows us to extract the characteristic Fermi liquid scale
1700 K. Our results show that the electron-doped iron-pnictides
should be regarded as weakly correlated Fermi liquids with a weak mass
enhancement resulting from residual electron-electron scattering from thermally
excited quasi-particles.Comment: 6+9pages, 3+9 figures To be published in Scientific Report
Three-dimensional MHD flow and heat transfer in a channel with internal obstacle
The magnetohydrodynamic flow and heat transfer of a liquid metal in a channel past a circular cylinder with walls of non-uniform conductivity were investigated. The applied magnetic field was transversal to the forced flow (x-direction) and coplanar with the obstacle, featuring non-null components in both the z- and y-directions. Moreover, the cylinder was displaced by the duct centreline toward the bottom wall and its surface was at uniform temperature, so that a ΔT was present between the obstacle and the fluid at the inlet. Non-uniform thickness for the duct-bounding walls is considered which leads to the promotion of jets close to the less-conductive surfaces. The flow features and heat transfer for this case were numerically investigated for different values of the Reynolds number (20 ≤ Re ≤ 80) and Hartmann number (0 ≤ Ha ≤ 100). Their effects on the flow features, pressure drop and heat transfer are analysed and discussed in detail in the present paper. The additional pressure drop introduced by the cylinder presence is found to be independent by Re and decreasing with Ha. Enhanced heat transfer is observed for an increasing Ha with NuMHD/Nu = 1.25. at Ha = 100 due to the augmented mass flow rate in the bottom sub-channel
Log-periodic drift oscillations in self-similar billiards
We study a particle moving at unit speed in a self-similar Lorentz billiard
channel; the latter consists of an infinite sequence of cells which are
identical in shape but growing exponentially in size, from left to right. We
present numerical computation of the drift term in this system and establish
the logarithmic periodicity of the corrections to the average drift
Energy oscillations and a possible route to chaos in a modified Riga dynamo
Starting from the present version of the Riga dynamo experiment with its
rotating magnetic eigenfield dominated by a single frequency we ask for those
modifications of this set-up that would allow for a non-trivial magnetic field
behaviour in the saturation regime. Assuming an increased ratio of azimuthal to
axial flow velocity, we obtain energy oscillations with a frequency below the
eigenfrequency of the magnetic field. These new oscillations are identified as
magneto-inertial waves that result from a slight imbalance of Lorentz and
inertial forces. Increasing the azimuthal velocity further, or increasing the
total magnetic Reynolds number, we find transitions to a chaotic behaviour of
the dynamo.Comment: 8 pages, 8 figures, submitted to Astronomische Nachrichte
Characterisation of HTSC ceramics from their resistive transition
The resistivity vs. temperature relation in bulk ceramic HTSC under
self-field conditions as well as in weak external magnetic fields is modelled
by local Lorentz force induced fluxon motion with temperature dependent
pinning. A pinning force density and two viscous drag coefficients in
intergrain and intragrain regions, respectively, can be used as characteristic
parameters describing the temperature, current, and external field dependences
of the sample resistance.Comment: 12 pages, LaTeX2e, 6 figures (epsfig), to be published in Supercond.
Sci. and Techno
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