1,721,110 research outputs found
Real space and Q space analysis of local disorder induced by Al doping in the SmBa2Cu3O6+d superconductor
No full textThe cation substitutions for copper in RBa2Cu3O6+d (R = Y, Ln; 0 < d < 1) high Tc superconductors are widely studied in order to understand their physical properties [1]. In particular Al seems to be a suitable dopant because: i) it is a non-magnetic ion; ii) it substitutes for copper only on Cu1 (0,0,0) site, far from the superconducting planes; iii) its valence state is fixed, making simple the calculation of the total hole concentration; iiii) it is possible to vary the medium range structure of the superconductor throughout an aluminium-clustering process [2]. However, Aluminium doping introduces local disorder in the structure because of its different valence (+3 instead of +1/+2) and local environment (tetrahedric instead of square planar) in respect to copper. In order to fully understand the effect of Al doping on the physical properties of RBa2Cu3O6+d superconductors the nature of this disorder has to be defined. To do so we have investigated some SmBa2Cu3-xAlxO6+d samples with different Al concentration (x=0, 0.15, 0.33), annealed either in oxidising or in reducing conditions. The long and short range structure of the samples have been determined through the parallel real pace (PDF) and Q space (Rietveld) analysis of XRPD patterns obtained at the ID31 beamline of ESRF facility. References
[1] See, for instance, M.S. Skakle, Mater. Sci. Eng., R23, 1 (1998) and references therein
[2] M. Scavini, M. Daldosso, S. Cappelli, C. Oliva, M. Brunelli, C. Ferrero, A. Lascialfari, Europhys. Lett., 76(3), (2006) 443-44
Melting of cooperative Jahn-Teller distortion in KMg0.2Cu0.8F3
No full textKCuF3 is a Mott-Hubbard insulator with a distorted perovskite structure (space group I4/mcm). The structural distortion is due to orbital ordering (OO) associated with cooperative Jahn-Teller effect (JT) [1]. and corresponds to an alternate occupation of Cu-3dy2-z2 and Cu-3dx2-z2 hole states on Cu(3d9) ion [2]. The orbital configuration results in quasi one-dimensional magnetic properties. Nearest-neighbour superexchange (NN-SE) interactions are strong and antiferromagnetic (AF) along the c axis and, for T>TN=38 K, weak and ferromagnetic in the ab plane. Debate is open in the literature on the actual driving force (i.e. either OO or JT) of the structural distortion and of the related electronic and magnetic properties. We have recently found experimental evidence of the ideal situation in which OO is melted while the JT distortion is still present:, in fact OO is expected to be very sensitive to slight changes in the electronic structure. Electron paramagnetic resonance investigations revealed melting of OO at room temperature in the KCu1-xMgxF3 system for x=0.1 [3]. We presents here a Synchrotron Radiation X-ray powder diffraction (XRPD) study in a sample with composition KCu0.8Mg0.2F3, which at room temperature is isostructural with KCuF3, a prototypical system for studying Orbital Order (OO). This sample can be considered a realisation of the ideal situation in which OO is melted while the cooperative JT distortion is still present. The melting of the cooperative JT distortion is observed in this system for T~600 K. This result is discussed in the framework of the different energy scales for OO and cooperative JT distortion.
[1] L.F. Feiner, A.M. Oleś, J. Zaanen, Phys. Rev. Lett., 1997, 78, 2799. [2] R. Caciuffo, L. Paolasini, A. Sollier, P. Ghigna, E. Pavarini, J. van den Brink, M. Altarelli Phys.Rev., 2002, B65, 174425
[3] C. Oliva, M. Scavini, S. Cappelli, C. Bottalo, C. Mazzoli, P. Ghigna, J.Phys.Chem., 2007, B111, 597
Disorder in oxides
No full textIn this paper the intimate correlation among structure, defect thermodynamics and physical properties in oxides is briefly reviewed through few examples concerning high Tc superconductors and ionic conductors. In some cases the desired physical properties appear only when disorder is introduced in the structures, like in neodymium cuprate and doped ceria. Disorder can be even expressly introduced in order to tune the physical properties already present in the undoped compound, which is the case of Al-doped REBa2Cu3O6+d. It will be demonstrated that not only the point defects and the local distortions induced by them, but also the correlation lengths of disorder are to be taken into account in order to correctly understand the physical properties of interesting materials
Direct-space analysis of the electronic structure of the YBa2Cu3O6 and YBa2Cu3O7 crystals
No full textThe electronic structure of the YBa2Cu3O6 and YBa2Cu3O7 crystals is investigated by means of fully periodic Restricted Hartree-Fock LCAO pseudopotential calculations. In order to give a detailed description of the bonding and atomic properties of the materials studied we selected the Quantum Theory of Atoms in Molecules approach, and complementary information is obtained from the analysis of the Electron Localisation Function in specific cell regions. The main features of the YBa2Cu3O6 and YBa2Cu3O7 crystals electron densities are detailed and compared with available experimental data. As the superconducting transition requires oxidation of the YBa2Cu3O6 crystal, we focus on the electronic rearrangements related to the inclusion of oxygen. Cu-O planes, probably responsible for the superconducting transition, exhibit peculiar electronic properties
Strongly correlated perovskite oxides: interplay of structure and physical properties
No full textIn recent years, a great interest has been devoted to strongly correlated systems containing
perovskite building blocks. These systems exhibit an intricate interplay between system degrees
of freedom giving rise to very fascinating and interesting physical properties. In particular, the
interplay between spin, charge, orbital and lattice degrees of freedom can induce
magnetoresistance, magnetodielectric effects paving the way for future applications in sensors,
data storage and spintronics.
I will describe the application of X-ray diffraction techniques to study the interplay between the
structure and physical properties of bulk systems based of perovskite structure. I will present
examples to show how these techniques can be used to investigate the coupling between structural
phase transition and magnetic, metal to insulator transition or to provide evidence of mismatch
between the local and the average crystallographic structures
An EPR investigtion on Pseudogap in SmBa2Cu3O6+d superconductor
No full textThe energy gap is the energy needed to break a Cooper pair into two free electrons and it is one of the defining characteristics of a superconductor. However, in the mid 1990s a similar gap (called pseudogap) has been discovered at the temperature T* > Tc in the so-called underdoped materials, i.e. with carrier concentration not high enough to give the best superconducting properties to the oxide [1, 2]. Many theories have been proposed on pseudogap and superconductivity mechanism, among which that of preformed pairing. Following this theory, electrons would bind into pairs at T*> Tc, but would remain unable to superconduct because they move incoherently, being unable to establish “phase coherence” with all the other pairs. Phase coherence and superconduction would occur only at T = Tc. Therefore, the spin pairing at T* could be only the first step towards superconductivity. However, other researchers have claimed the presence of a pseudogap in a nonsuperconducting material such as a strontium doped lanthanum manganite, which became ferromagnetic at enough low temperature and displayed colossal magnetoresistance [3]. Therefore, pseudogap formation at T = T* could be a general phenomenon not necessarily involving superconductivity at any T < T*. Indeed, in the present investigation we attribute a broadening at T* @ 180 K of the exchange-narrowed Lorentzian-shaped EPR line of SmBa2Cu3-xAlxO6.98 (x = 0.33) to pseudogap formation in this non-superconducting material. An analogous linebroadening is observed with the x = 0.15 sample characterised by Tc @ 47 K [4] but not with the x = 0 sample, which is the best-performing superconductor of this series, having Tc @ 90 K. References
[1] T.Morinari, Superconductor Science and Technology, 16 (2003) 624.
[2] M.Vershinin, S.Misra, S.Ono, Y.Abe, Y.Ando, A.Yazdani, Science, 303 (2004) 1995. [3] N.Mannella, W.L.Yang, X.J.Zhou, H.Zheng, J.F.Mitchell, J.Zaanen, T.P.Devereaux, N.Nagaosa, Z.Hussain, Z.-X.Shen, Nature, 438 (2005) 474.
[4] M.Scavini, M.Daldosso, S.Cappelli, C.Oliva, M.Brunelli, C.Ferrero, A.Lascialfari, Europhysics Lett., 76 (2006) 443
Germanium K edge in GeO2 polymorphs. Correlation between local coordination and electronic structure of germanium
No full textThe electronic properties of two room Temperature persistent phases of germanium dioxide have been studied by means of
experimental and theoretical techniques. We collected the Ge-K edge XANES spectra of these materials at the GILDA beamline of
ESRF. The Density Of States of the two crystal phases, obtained from fully periodic Hartree-Fock and Density Functional calculations,
is taken as the reference term to rationalise and assign the manifolds of the XANES spectra. Although this scheme requires a number of
severe approximations, we obtained a good overall agreement between experiment and theory. The topological analysis of the
theoretical electron density distribution in the crystals gave further information regarding the electronic properties of germanium
dioxide
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