76 research outputs found
Superconductivity in SrFe_(2-x)Co_xAs_2: Internal Doping of the Iron Arsenide Layers
In the electron doped compounds SrFe_(2-x)Co_xAs_2 superconductivity with T_c
up to 20 K is observed for 0.2 < x < 0.4. Results of structure determination,
magnetic susceptibility, electrical resistivity, and specific heat are
reported. The observation of bulk superconductivity in all thermodynamic
properties -- despite strong disorder in the Fe-As layer -- favors an itinerant
picture in contrast to the cuprates and renders a p- or d-wave scenario
unlikely. DFT calculations find that the substitution of Fe by Co (x > 0.3)
leads to the suppression of the magnetic ordering present in SrFe_2As_2 due to
a rigid down-shift of the Fe-3d_(x^2-y^2) related band edge in the density of
states.Comment: 5 pages, 3 figure
Mixed-State Specific Heat of the Type-II Superconductor Nb0.77Zr0.23 in Magnetic Fields up to B c2
In order to document the behavior of the mean-field mixed-state specific heat of an isotropic. strongly type-II superconductor (i.e., with a large value of the Ginzburg parameterk), and to provide a basis for comparison with high-temperature superconductors, we measured the specific heatC of the alloy Nb0.77Zr0.23 withT c = 10.8K, B c2 (0) = 7.9T, in magnetic fieldsB = 0, 0.2, 1.0, 12, 2.0, 2.4. 3.0, 3.3. 4.0. 4.4, 4.8, 5.2, 6.0. 6.6, 7.2 and 10 T. The values of the upper critical fieldB c2 ( T), thermodynamic critical fieldB c (T), Ginzburg parameterk(T), and coefficient γ(B) = limT0(C(T. B)/T) are derived from the specific heat data and found to be in agreement with the GLAG theory in the dirty limit. The behavior of the mixed-state specific heat is analyzed in terms ofC el /T,∂(C el /T)/∂B, and∂(C el /T)/∂T vs. Tcurves, whereC el is the electronic contribution to the specific hea
Diodes with Breakdown Voltages Enhanced by the Metal-Insulator Transition of LaAlO-SrTiO Interfaces
Using the metal-insulator transition that takes place as a function of
carrier density at the LaAlO-SrTiO interface, oxide diodes have been
fabricated with room-temperature breakdown voltages of up to 200 V. With
applied voltage, the capacitance of the diodes changes by a factor of 150. The
diodes are robust and operate at temperatures up to 270 C
CHARACTERIZATION OF THE 105 K SUPERCONDUCTOR Tl2Ba2CaCu208 ("2212")
International audienceHomogeneous Tl1.8Ba2CaCu208 ceramics were synthesized by a novel route, starting from Tl2Ba205 precursors and using high gas pressures. This method allows tight control of thallium losses, resulting in dense, large-grained samples with sharp superconducting transitions above 105 K. Results of a characterization by X-ray diffraction, optical micrographs and micro-probe analysis are presented, together with selected physical properties
Direct evidence for ferroelectric polar distortion in ultrathin lead titanate perovskite films
X-ray photoelectron diffraction is used to directly probe the intra-cell
polar atomic distortion and tetragonality associated with ferroelectricity in
ultrathin epitaxial PbTiO3 films. Our measurements, combined with ab-initio
calculations, unambiguously demonstrate non-centro-symmetry in films a few unit
cells thick, imply that films as thin as 3 unit cells still preserve a
ferroelectric polar distortion, and also show that there is no thick
paraelectric dead layer at the surface
Reversible magnetization below Tc in high-quality superconducting ceramics
International audienceWe have investigated the reversible magnetization below Tc in high-quality YBa2Cu307_d (Y-123), YBa2Cu4Os (Y-124), Y2Ba4Cu7O15+x (Y-247) and Bi2Sr2CaCu2O8+x (Bi-2212), Tl2Ba2Cu106+d (Tl-2201) and Tl2Ba2CalCu2O8+d (T1-2212) ceramics. Except for the stoichiometric Y-124 phase, the oxygen concentration was optimized in order to obtain the highest value of the critical temperature for which the normal-state susceptibility becomes temperature independent. Using the simple London model, we are able to fit the reversible magnetization M(T, H) outside the region near Tc with good accuracy for the nearly three-dimensional YBaCuO phases. For the very anisotropic BiSrCaCuO and TlBaCaCuO phases, we have to include an additional term to take into account the fluctuations of vortices. An important result is that Y-123 exhibits a critical field clearly higher than those of the BiSrCaCuO or TlBaCaCuO phases. We obtain for the Y-123 phase a slope at Tc μodHC2,C/dT = -4.3 T/K and an extrapolated μoHC2,C (0) = 280 T
X-ray photoelectron diffraction study of ultrathin PbTiO3 films
Abstract.: Full hemispherical X-ray photoelectron diffraction (XPD) experiments have been performed to investigate at the atomic level ultrathin epitaxial c-axis oriented PbTiO3 (PTO) films grown on Nb-doped SrTiO3 substrates. Comparison between experiment and theory allows us to identify a preferential ferroelectric polarization state in a 60 Ã… -thick PTO film. Multiple scattering theory based on a cluster-model [ Phys. Rev. B , 075404 (2001)] is used to simulate the experiment
Scaling of the Equilibrium Magnetization in the Mixed State of Type-II Superconductors
We discuss the analysis of mixed-state magnetization data of type-II
superconductors using a recently developed scaling procedure. It is based on
the fact that, if the Ginzburg-Landau parameter kappa does not depend on
temperature, the magnetic susceptibility is a universal function of H/H_c2(T),
leading to a simple relation between magnetizations at different temperatures.
Although this scaling procedure does not provide absolute values of the upper
critical fieldH_c2(T), its temperature variation can be established rather
accurately. This provides an opportunity to validate theoretical models that
are usually employed for the evaluation of H_c2(T) from equilibrium
magnetization data. In the second part of the paper we apply this scaling
procedure for a discussion of the notorious first order phase transition in the
mixed state of high temperature superconductors. Our analysis, based on
experimental magnetization data available in the literature, shows that the
shift of the magnetization accross the transition may adopt either sign,
depending on the particular chosen sample. We argue that this observation is
inconsistent with the interpretation that this transition always represents the
melting transition of the vortex lattice.Comment: 18 pages, 12 figure
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