288 research outputs found
Downward shift of infrared conductivity spectral weight at the DDW transition: role of anisotropy
We consider the motion of conductivity spectral weight at a
finite-temperature phase transition at which density-wave (DDW)
order develops. We show that there is a shift of spectral weight to higher
frequencies if the quasiparticle lifetime is assumed to be isotropic, but a
shift to lower frequencies if the quasiparticle lifetime is assumed to be
anisotropic. We suggest that this is consistent with recent experiments on the
pseudogap phase of the cuprate superconductors and, therefore, conclude that
the observation of a downward shift in the spectral weight at the pseudogap
temperature does not militate against the DDW theory of the pseudogap.Comment: 8 pages, 7 figures. Added reference
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
Staphylinids (Coleoptera, Staphylinidae) of Ukrainian metropolises
During hundreds of years, in large cities man has been forming a specific urban environment with original species composition of insect communities, including the most diverse group of predatory beetles – Staphylinidae family. Kyiv, Kharkiv, Dnipro are the three most populated cities of Ukraine. In the urban cenoses of these cities, over 140 species from 66 genera of Staphylinidae have been recorded. The total of 69 species (43 genera) were recorded in Kyiv, 67 (39 genera) in Dnipro and 66 (37 genera) in Kharkiv. Among them, , eight species in the catalogue of Palearctic staphylinds had not been previously recorded for Ukraine: Arpedium quadrum Grav., Atheta laticeps Thomson, Medon apicalis Kraatz, Ocalea rivularis Müll., Philonthus salinus Kiesenwetter, Quedius invreae Gridelli, Tasgius pedator Grav. and Xantholinus gallicus Coiffait. By number, common species accounted for 29 in Dnipro, 21 in Kyiv and 19 in Kharkiv. In all the metropolises, two species (Staphylinus caesareus Cederhjelm and Drusilla canaliculata (F.)) were identified as eudominants. Dominants and subdominants comprised 18–25 species. Almost two thirds of the fauna of staphylinids of the cities was classified as rare species. The lowest faunistic similarity was seen between the staphylinids of the urban cenoses of Dnipro and those in Kyiv and Kharkiv (15.3% and 17.5%), similarity was higher for Staphylinidae of Kyiv and Kharkiv (36.0%). Similarity by common species of staphylinids for Kyiv and Kharkiv equaled 73.9%, 28.2% for Dnipro and Kyiv and 37.1% for Dnipro and Kharkiv. The article offers a review of species diversity, number and ecological structure (biotopic confinedness, hygrothermal preference) of typical species of staphylinids in different urban cenoses of Kharkiv. Differences in qualitative-quantitative and ecological structures of staphylinids are related to the diversity of the conditions in a particular urban cenoses. The commonest representatives of the family in the metropolises were polytopic mesophilous carnivorous species
Coherent vs incoherent pairing in 2D systems near magnetic instability
We study the superconductivity in 2D fermionic systems near antiferromagnetic
instability, assuming that the pairing is mediated by spin fluctuations. This
pairing involves fully incoherent fermions and diffusive spin excitations. We
show that the competition between fermionic incoherence and strong pairing
interaction yields the pairing instability temperature which
increases and saturates as the magnetic correlation length . We
argue that in this quantum-critical regime the pairing problem is qualitatively
different from the BCS one.Comment: 7 pages, 2 figure
Anomalous optical absorption in overdoped cuprates near the charge-ordering instability
We propose an interpretation for the hump observed in the optical
conductivity at or below a few hundreds of cm, in overdoped cuprates
like the electron-doped Nd_{2-x}Ce_xCuO_{4-y} at x\gtrsim 0.15 and the
hole-doped Bi_2Sr_2CuO_6 and La_{2-x}Sr_xCuO_4. This interpretation is based on
the direct excitation of charge collective modes, which become nearly critical
in the proximity to a charge-ordering instability. The nearly critical
character of these excitations entails a peculiar temperature dependence and a
pseudo-scaling form of the lineshapes, which are in agreement with the
experimental data.Comment: 5 pages, 3 figure
Electronic correlations in iron-pnictide superconductors and beyond; what can we learn from optics
The Coulomb repulsion, impeding electrons' motion, has an important impact on
the charge dynamics. It mainly causes a reduction of the effective metallic
Drude weight (proportional to the so-called optical kinetic energy),
encountered in the optical conductivity, with respect to the expectation within
the nearly-free electron limit (defining the so-called band kinetic energy), as
evinced from band-structure theory. In principle, the ratio between the optical
and band kinetic energy allows defining the degree of electronic correlations.
Through spectral weight arguments based on the excitation spectrum, we provide
an experimental tool, free from any theoretical or band-structure based
assumptions, in order to estimate the degree of electronic correlations in
several systems. We first address the novel iron-pnictide superconductors,
which serve to set the stage for our approach. We then revisit a large variety
of materials, ranging from superconductors, to Kondo-like systems as well as
materials close to the Mott-insulating state. As comparison we also tackle
materials, where the electron-phonon coupling dominates. We establish a direct
relationship between the strength of interaction and the resulting reduction of
the optical kinetic energy of the itinerant charge carriers
Infrared Properties of Electron Doped Cuprates: Tracking Normal State Gaps and Quantum Critical Behavior in Pr(2-x)Ce(x)CuO(4)
We report the temperature dependence of the infrared-visible conductivity of
Pr(2-x)Ce(x)CuO(4) thin films. When varying the doping from a
non-superconducting film (x = 0.11) to a superconducting overdoped film (x =
0.17), we observe, up to optimal doping (x = 0.15), a partial gap opening. A
model combining a spin density wave gap and a frequency and temperature
dependent self energy reproduces our data reasonably well. The magnitude of
this gap extrapolates to zero for x ~ 0.17 indicating the coexistence of
magnetism and superconductivity in this material and the existence of a quantum
critical point at this Ce concentration.Comment: 5 pages 6 figures include
Optical Response for the d-density wave model
We have calculated the optical conductivity and the Raman response for the
d-density wave model, proposed as a possible explanation for the pseudogap seen
in high Tc cuprates. The total optical spectral weight remains approximately
constant on opening of the pseudogap for fixed temperature. This occurs because
there is a transfer of weight from the Drude peak to interband transitions
across the pseudogap. The interband peak in the optical conductivity is
prominent but becomes progressively reduced with increasing temperature, with
impurity scattering, which distributes it over a larger energy range, and with
ineleastic scattering which can also shift its position, making it difficult to
have a direct determination of the value of the pseudogap. Corresponding
structure is seen in the optical scattering rate, but not necessarily at the
same energies as in the conductivity.Comment: 14 pages, 15 figures, final revised version published in PR
Sum rules and electrodynamics of high-Tc cuprates in the pseudogap state
We explore connections between the electronic density of states (DOS) in a
conducting system and the frequency dependence of the scattering rate
inferred from infrared spectroscopy. We show that changes in
the DOS upon the development of energy gaps can be reliably tracked through the
examination of the spectra using the sum rules discussed in
the text. Applying this analysis to the charge dynamics in high- cuprates
we found radically different trends in the evolution of the DOS in the
pseudogap state and in the superconducting state.Comment: 4 pages, 3 figure
Filling Control of the Mott Insulator Ca2RuO4
We have grown single crystals of electron doping system Ca2-xLaxRuO4 (0.00 <=
x <= 0.20) by a floating zone method. The first order metal/non-metal
transition and canted antiferromagnetic ordering occur for 0.00 < x < 0.15,
similar to those in the bandwidth controlled system Ca2-xSrxRuO4 (CSRO).
However, comparing with CSRO, we found a rather different metallic ground state
adjacent to the non-metallic ground state with canted antiferromagnetic order.
Instead of short-range antiferromagnetic correlation found in CSRO (0.20 <= x <
0.50), the metallic ground state of the present system is characterized by
strong ferromagnetic correlation.Comment: 8 pages, 8 figures (eps), submitted to J. Phys. Soc. Jp
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