790 research outputs found
Comparison of the fluctuation influence on the resistive properties of the mixed state of BiSrCaCuO and of thin films of conventional superconductor
The resistive properties of layered HTSC BiSrCaCuO in the mixed state are
compared with those of thin films of conventional superconductors with weak
disorders (amorphous Nb_{1-x}0_{x} films) and with strong disorders
(Nb_{1-x}O_{x} films with small grain structure). The excess conductivity is
considered as a function of superconducting electron density and phase
coherence length. It is shown that the transition to the Abrikosov state
differs from the ideal case both in BiSrCaCuO and Nb_{1-x}O_{x} films, i.e. the
appearance of long-range phase coherence is continuous transition in both
cases. The quantitative difference between thin films with weak and strong
disorders is greater than the one between layered HTSC and conventional
superconductors, showing that the dimensionality of the system, rather than the
critical temperature, is the key factor ruling fluctuation effectsComment: 17 pages, 5 figure
Superconducting properties of [BaCuO_x]_2/[CaCuO_2]_n artificial structures with ultrathick CaCuO_2 blocks
The electrical transport properties of [BaCuO_x]_2/[CaCuO_2]_n
(CBCCO-2xn)underdoped high temperature superconducting superlattices grown by
Pulsed Laser Deposition have been investigated. Starting from the optimally
doped CBCCO-2x2 superlattice, having three CuO_2 planes and T_c around 80 K, we
have systematically increased the number n up to 15 moving toward the
underdoped region and hence decreasing T_c. For n>11 the artificial structures
are no longer superconducting, as expected, for a uniformly distributed charge
carriers density inside the conducting block layer. The sheet resistance of
such artificial structures (n nearly equal to 11) turns out to be quite
temperature independent and close to the 2D quantum resistance 26 kOhm. A
further increase of the number of CuO_2 planes results in an insulator-type
dependence of R(T) in the wide range of temperatures from room temperature to 1
K. The value of the sheet resistance separating the Superconducting and the
Insulating regimes supports the fermionic scenario of the
Superconductor-Insulator transition in these systems.Comment: 12 pages, 5 figures. Corresponding author: [email protected]
Presumptive Taxation, Markets, and Redistribution
In several Western countries, as well as in virtually all developing and transition
ones, the government’s ability to redistribute income in favour of the less well-off is severely
limited by the fact that certain groups of citizens can escape their tax obligations more
easily than others. In this paper, we focus our attention on one of the possible responses to
that problem, namely the recourse to presumptive taxation, whereby not income as such, but
a proxy for income, is selected as the tax base. To study this issue, we employ an occupational
choice model where an individual can either be a worker or an entrepreneur. We assume
that a conventional income tax is in place and that only entrepreneurs, who are at the top of
the income distribution, can partially avoid the income tax. In this setting, we show that presumptive
taxation based either on occupational choice or on the firms’ input costs can raise
the welfare of the workers, who are the poorest members of the society. This outcome is not
necessarily achieved, however, by taxing entrepreneurs: in a number of circumstances, presumptive
subsidies for the entrepreneurs are preferable to presumptive taxes, the reason being
that the latter may cause production inefficiency as well as increase tax avoidance costs
Effect of strain-induced electronic topological transitions on the superconducting properties of LaSrCuO thin films
We propose a Ginzburg-Landau phenomenological model for the dependence of the
critical temperature on microscopic strain in tetragonal high-Tc cuprates. Such
a model is in agreement with the experimental results for LSCO under epitaxial
strain, as well as with the hydrostatic pressure dependence of Tc in most
cuprates. In particular, a nonmonotonic dependence of Tc on hydrostatic
pressure, as well as on in-plane or apical microstrain, is derived. From a
microscopic point of view, such results can be understood as due to the
proximity to an electronic topological transition (ETT). In the case of LSCO,
we argue that such an ETT can be driven by a strain-induced modification of the
band structure, at constant hole content, at variance with a doping-induced
ETT, as is usually assumed.Comment: EPJB, to be publishe
Reflection High-Energy Electron Diffraction oscillations during epitaxial growth of artificially layered films of (BaCuOx)m /(CaCuO2)n
Pulsed Laser Deposition in molecular-beam epitaxy environment (Laser-MBE) has
been used to grow high quality BaCuOx/CaCuO2 superlattices. In situ Reflection
High Energy Electron Diffraction (RHEED) shows that the growth mechanism is
2-dimensional. Furthermore, weak but reproducible RHEED intensity oscillations
have been monitored during the growth. Ex-situ x-ray diffraction spectra
confirmed the growth rate deduced from RHEED oscillations. Such results
demonstrate that RHEED oscillations can be used, even for (BaCuOx)2/(CaCuO2)2
superlattices, for phase locking of the growth.Comment: 9 pages, 5 figures. Corresponding author: Dr. A. Tebano:
[email protected]
Analysis of charge transfer mechanism on (Ba1-xNdxCuO2+d)2/(CaCuO2)n superconducting superlattices by thermoelectric power measurements
We have investigated the charge transfer mechanism in artificial
superlattices by Seebeck effect measurements. Such a technique allows a precise
determination of the amount of charge transferred on each CuO2 plane. A
systematic characterization of thermoelectric power in (BaCuO2+d)2/(CaCuO2)n
and (Ba0.9Nd0.1CuO2+d)2/(CaCuO2)n superlattices demonstrates that electrical
charge distributes uniformly among the CuO2 planes in the Ca-block. The
differences observed in the Seebeck effect behavior between the Nd-doped and
undoped superlattices are ascribed to the different metallic character of the
Ba-block in the two cases. Finally, the special role of structural disorder in
superlattices with n=1 is pointed out by such analysis.Comment: subitted to PRB, 15 pages, 3 figure
Flux creep in Bi2Sr2CaCu2O8(sub +x) single crystals
The results of a magnetic study on a Bi2Sr2CaCu2O(8+x) single crystal are reported. Low field susceptibility (dc and ac), magnetization cycles and time dependent measurements were performed. With increasing the temperature the irreversible regime of the magnetization cycles is rapidly restricted to low fields, showing that the critical current J(sub c) becomes strongly field dependent well below T(sub c). At 2.4 K the critical current in zero field, determined from the remanent magnetization by using the Bean formula for the critical state, is J(sub c) = 2 10(exp 5) A/sq cm. The temperature dependence of J(sub c) is satisfactorily described by the phenomenological law J(sub c) = J(sub c) (0) (1 - T/T(sub c) (sup n), with n = 8. The time decay of the zero field cooled magnetization and of the remanent magnetization was studied at different temperatures for different magnetic fields. The time decay was found to be logarithmic in both cases, at least at low temperatures. At T = 4.2 K for a field of 10 kOe applied parallel to the c axis, the average pinning energy, determined by using the flux creep model, is U(sub o) = 0.010 eV
55Mn NMR and magnetization studies of La0.67Sr0.33MnO3 thin films
55Mn nuclear magnetic resonance and magnetization studies of the series of
La0.67Sr0.33MnO3 thin films have been performed at low temperature. Two
distinct lines were observed, at 322 MHz and 380 MHz, corresponding to two
different phases, the former located at the interface, with localized charges,
and the latter corresponding to the film bulk, with itinerant carriers (as it
was also found in Ca manganite films). The spin-echo amplitude was measured as
a function of a dc magnetic field applied either in the film plane or
perpendicular to it. The field dependence of both the main NMR signal intensity
and frequency shift is quite consistent with that calculated in a simple single
domain model. The best fit to the model shows that magnetization rotation
processes play a dominant role when the applied field exceeds the effective
anisotropy field. Distinctly different magnetic anisotropies are deduced from
the interface NMR signal.Comment: 7 pages, 8 figure
Evidence of orbital reconstruction at interfaces in La0.67Sr0.33MnO3 films
Electronic properties of transition metal oxides at interfaces are influenced
by strain, electric polarization and oxygen diffusion. Linear dichroism (LD)
x-ray absorption, diffraction, transport and magnetization on thin
La0.7Sr0.3MnO3 films, allow identification of a peculiar universal interface
effect. We report the LD signature of preferential 3d-eg(3z2-r2) occupation at
the interface, suppressing the double exchange mechanism. This surface orbital
reconstruction is opposite of that favored by residual strain and independent
of dipolar fields, chemical nature of the substrate and capping.Comment: 13 pages, 5 figure
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