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Pressure dependence of magnetic and superconducting transitions in sodium cobalt oxides NaxCoO2  [PDF]
Y. V. Sushko,O. B. Korneta,S. O. Leontsev,R. Jin,B. C. Sales,D. Mandrus
Physics , 2005,
Abstract: The results of DC magnetization measurements under hydrostatic (helium-gas) pressure are reported for an ambient pressure superconductor Na0.35CoO2.1.4D2O and its precursor compound, the gamma-phase Na0.75CoO2 that is known to combine a metallic conductivity with an unusual magnetic state below ~22K. The obtained data allowed us to present for the first time the pressure dependence of the magnetic transition in a metallic sodium cobaltate system. This dependence appears to be positive, with the magnetic transition rapidly shifting towards higher temperatures when an applied pressure increases. We ascribe the observed effect to the pressure-induced enhancement of the out-of-plane antiferromagnetic coupling mediated by localized spins interactions (of either superexchange or RKKY type), the scenario consistent with the A-type antiferromagnetic state suggested by recent neutron-scattering data. As for the pressure effect on the superconductivity in Na0.35CoO2.1.4D2O, our measurements established negative and linear for the entire pressure range from 1 bar to 8.3 kbar pressure dependence of Tc, the behavior quite different from the reported by previous workers strong non-linearity of the Tc (P) dependence. (Dated September 12, 2005) PACS numbers: 74.62.Fj, 74.70.-b, 75.20. En, 75.50 Ee, 75.30 Kz.
Nonlinear Insulator in Complex Oxides  [PDF]
Z. Q. Liu,D. P. Leusink,W. M. Lü,X. Wang,X. P. Yang,K. Gopinadhan,A. Annadi,S. Dhar,Y. P. Feng,H. B. Su,G. Xiong,T. Venkatesan,Ariando
Physics , 2010, DOI: 10.1103/PhysRevB.84.165106
Abstract: The insulating state is one of the most basic electronic phases in condensed matter. This state is characterised by an energy gap for electronic excitations that makes an insulator electrically inert at low energy. However, for complex oxides, the very concept of an insulator must be re-examined. Complex oxides behave differently from conventional insulators such as SiO2, on which the entire semiconductor industry is based, because of the presence of multiple defect levels within their band gap. As the semiconductor industry is moving to such oxides for high-dielectric (high-k) materials, we need to truly understand the insulating properties of these oxides under various electric field excitations. Here we report a new class of material called nonlinear insulators that exhibits a reversible electric-field-induced metal-insulator transition. We demonstrate this behaviour for an insulating LaAlO3 thin film in a metal/LaAlO3/Nb-SrTiO3 heterostructure. Reproducible transitions were observed between a low-resistance metallic state and a high-resistance non-metallic state when applying suitable voltages. Our experimental results exclude the possibility that diffusion of the metal electrodes or oxygen vacancies into the LaAlO3 layer is occurring. Instead, the phenomenon is attributed to the formation of a quasi-conduction band (QCB) in the defect states of LaAlO3 that forms a continuum state with the conduction band of the Nb-SrTiO3. Once this continuum (metallic) state is formed, the state remains stable even when the voltage bias is turned off. An opposing voltage is required to deplete the charges from the defect states. Our ability to manipulate and control these defect states and, thus, the nonlinear insulating properties of complex oxides will open up a new path to develop novel devices.
Nonadiabatic transitions in electrostatically trapped ammonia molecules  [PDF]
Moritz Kirste,Boris Saratakov,Melanie Schnell,Gerard Meijer
Physics , 2008, DOI: 10.1103/PhysRevA.79.051401
Abstract: Nonadiabatic transitions are known to be major loss channels for atoms in magnetic traps, but have thus far not been experimentally reported upon for trapped molecules. We have observed and quantified losses due to nonadiabatic transitions for three isotopologues of ammonia in electrostatic traps, by comparing the trapping times in traps with a zero and a non-zero electric field at the center. Nonadiabatic transitions are seen to dominate the overall loss rate even for samples at relatively high temperatures of 10-50 mK.
Dynamical ordering induced by preferential transitions in Planar Arrays of Superheated Superconducting granules  [PDF]
A. Penaranda,L. Ramirez-Piscina
Physics , 2003, DOI: 10.1016/S0038-1098(03)00352-1
Abstract: We perform simulations of planar arrays of superheated superconducting granules (PASS) under an external magnetic field, analyzing transitions undergone by the system when the external field is slowly increased from zero. We observe, for high concentrations, the existence of an interval of external fields for which no transitions are induced. This effect is analogous to a "hot border zone" identified in the response of Superheated Superconducting Granule detectors. We explain such behaviour as produced by a geometrical ordering dynamically induced in the system by transitions in preferential sites due to diamagnetic interactions.
Electrostatically-Driven Granular Media: Phase Transitions and Coarsening  [PDF]
I. S. Aranson,D. Blair,V. A. Kalatsky,G. W. Crabtree,W. -K. Kwok,V. M. Vinokur,U. Welp
Physics , 1999, DOI: 10.1103/PhysRevLett.84.3306
Abstract: We report the experimental and theoretical study of electrostatically driven granular material. We show that the charged granular medium undergoes a hysteretic first order phase transition from the immobile condensed state (granular solid) to a fluidized dilated state (granular gas) with a changing applied electric field. In addition we observe a spontaneous precipitation of dense clusters from the gas phase and subsequent coarsening - coagulation of these clusters. Molecular dynamics simulations shows qualitative agreement with experimental results.
Polarons in Complex Oxides and Molecular Nanowires  [PDF]
A. S. Alexandrov
Physics , 2003,
Abstract: There is a growing understanding that transport properties of complex oxides and individual molecules are dominated by polaron physics. In superconducting oxides the long-range Froehlich and short-range Jahn-Teller electron-phonon interactions bind carriers into real space pairs - small bipolarons with surprisingly low mass but sufficient binding energy, while the long-range Coulomb repulsion keeps bipolarons apart preventing their clustering. The bipolaron theory numerically explains high Tc values without any fitting parameters and describes other key features of the cuprates. The same approach provides a new insite into the theory of transport through molecular nanowires and quantum dots (MQD). Attractive polaron-polaron correlations lead to a "switching" phenomenon in the current-voltage characteristics of MQD. The degenerate MQD with strong electron-vibron coupling has two stable current states (a volatile memory), which might be useful in molecular electronics.
Multiple fluxoid transitions in mesoscopic superconducting rings  [PDF]
Hendrik Bluhm,Nicholas C. Koshnick,Martin E. Huber,Kathryn A. Moler
Physics , 2007,
Abstract: The authors report magnetic measurements of fluxoid transitions in mesoscopic, superconducting aluminum rings. The transitions are induced by applying a flux to the ring so that the induced supercurrent approaches the critical current. In a temperature range near $T_c$, only a single fluxoid enters or leaves at a time, leading to a final state above the ground state. Upon lowering the temperature, several fluxoids enter or leave at once, and the final state approaches the ground state, which can be reached below approximately 0.5 $T_c$. A model based on the widely used time dependent Ginzburg-Landau theory for gapless superconductors can only explain the data if unphysical parameters are used. Heating and quasiparticle diffusion may be important for a quantitative understanding of this experiment, which could provide a model system for studying the nonlinear dynamics of superconductors far from equilibrium.
On the Problem of Metal-Insulator Transitions in Vanadium Oxides  [PDF]
A. L. Pergament,G. B. Stefanovich,N. A. Kuldin,A. A. Velichko
ISRN Condensed Matter Physics , 2013, DOI: 10.1155/2013/960627
Abstract: The problem of metal-insulator transition is considered. It is shown that the Mott criterion is applicable not only to heavily doped semiconductors but also to many other materials, including some transition-metal compounds, such as vanadium oxides (particularly, VO2 and V2O3). The low-temperature transition (“paramagnetic metal—antiferromagnetic insulator”) in vanadium sesquioxide is described on the basis of this concept in terms of an intervening phase, between metal and insulator states, with divergent dielectric constant and effective charge carrier mass. Recent communications concerning a possible “metal-insulator transition” in vanadium pentoxide are also discussed. 1. Introduction Strongly correlated transition metal oxides exhibiting metal-insulator transitions (MIT) are currently considered as basic functional materials of oxide electronics [1]. That is why the theoretical models describing MITs are of such importance. Vanadium oxide Magneli phases, forming the homologous series ( ), may be considered as intermediate structures between the end members VO2 ( ) and V2O3 ( ). With one exception (V7O13), the vanadium Magneli phases exhibit MITs [2] as functions of temperature—see Figure 1 and [1–10]. Figure 1: Resistivity versus reciprocal temperature for vanadium oxides. In recent years, a number of papers [4–6] have appeared concerning a “metal-insulator transition” in V2O5 [3]. Here, we do not intend to dispute scientific aspects of these works, and the object of our discussion will be different. The fact is that the term “metal-insulator transition”, particularly with respect to vanadium oxides (and not only to these—generally to a variety of transition metal oxides and related materials [7]), has long been used in the scientific literature to describe quite a certain class of phenomena and experimental facts. Thus, in this paper we will comment on the aforecited references [4–6], with a more detailed discussion than that in our brief note [3], in view of the problem of MITs in vanadium oxides. Also, the MITs in V2O3 are discussed, and an attempt is made to describe them in terms of a model based on the Mott criterion concept, developing some ideas of our previous work [7]. However, first, in Section 2.1, we have to acquaint a reader with the main general points of discussion and recall some results obtained earlier for vanadium dioxide and other related materials. 2. Mott Metal-Insulator Transitions in Vanadium Oxides 2.1. Strongly Correlated Systems and Mott Transition At present, there is no unified approach to interpret the electronic
Berry phases in superconducting transitions  [PDF]
A. A. Aligia
Physics , 1998, DOI: 10.1209/epl/i1999-00181-4
Abstract: I generalize the concept of Berry's geometrical phase for quasicyclic Hamiltonians to the case in which the ground state evolves adiabatically to an excited state after one cycle, but returns to the ground state after an integer number of cycles. This allows to extend the charge Berry phase gamma_c related to the macroscopic polarization, to many-body systems with fractional number of particles per site. Under certain conditions, gamma_c and the spin Berry phase gamma_s jump in pi at the boundary of superconducting phases. In the extended Hubbard chain with on-site attraction U and nearest-neighbor interaction V at quarter filling, the transitions detected agree very well with exact results in two limits solved by the Bethe ansatz, and with previous numerical studies. In chains with spin SU(2) symmetry, gamma_s jumps when a spin gap opens.

Chen Nianyi,Wen Yuankai,

金属学报 , 1979,
Abstract: Factors concerning with the stability of binary complex oxides has been stu-died. For complex oxides with ionic bonds,the main factors determining the heatof formation of such oxides appear to be the relevant ionic radius and the va-lency of the component elements. For oxides with partial covalency, however, thescreening parameter proposed in this paper may also be important in addition tothe factors mentioned above.
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