Pressure induced lattice instability and phase separation in the cuprates

High pressure structural studies using a synchrotron source and Raman measurements on various cuprates reveal several structural modifications. The data have shown strong deviations from the normal equation of state at characteristic pressures, hysteresis, and the appearance of additional peaks that can be attributed to a new phase. The combined data of synchrotron angle-dispersive experiments with the optical measurements indicate that at some critical pressures, at least for certain compounds, non-linear effects are observed together with phase separation that affect the distribution of the carriers and the transition temperature. The comparison of the data with those induced by an internal pressure by an atomic substitution indicates that the effect is related to the existence of carriers within the CuO₂ superconducting planes.     

Phase diagram of superconductivity and antiferromagnetism in high-Tc hole-doped cuprates

A phase diagram of superconductivity (SC) and antiferromagnetism (AFM) for hole-doped cuprate superconductors in presence of chemical potential (μ) by using a model Hamiltonian is reported here. The Hamiltonian of the system is a mean field one and has been solved by writing equations of motion for the single particle Green functions. The expressions for appropriate single particle correlation function are derived. It is assumed that SC arises due to BCS pairing mechanism and AFM order is simulated by staggered magnetic field in lattices of Cu–O planes. The expressions for SC order parameter, AFM order parameter and dopant concentration are calculated analytically by using Green function technique of D.N. Zubarev. The value of SC gap (z), AFM gap (h) and chemical potential (μ) are solved self consistently for different dopant concentrations (x) by changing model parameters. It is found that a disordered phase appears after antiferromagnetism is destroyed in the range of very small doping. On further increase of the doping, the SC critical temperature first increases, attains a maximum value (?39 K) and then decreases which agrees well with experimental observations for hole-doped cuprates. Our theoretical findings suggest that the AFM coupling plays the vital role of the glue for the Cooper pairs.     

On the existence of percolative phase separation in high-T c cuprates

The phase separation behavior of superconducting cuprate systems is studied. Using experimental data obtained from electrical resistivity and susceptibility measurements on La₂CuO₄₊ we demonstrate that the phase separation is of electronic and percolative nature. In addition, the experiments clearly prove the coexistence of bulk superconductivity and long-range antiferromagnetic (afm) order.     

Phase separation in supersolids

We study quantum phase transitions in the ground state of the two dimensional hard-core boson Hubbard Hamiltonian. Recent work on this and related models has suggested "supersolid" phases with simultaneous diagonal and off-diagonal long range order. We show numerically that, contrary to the generally held belief, the most commonly discussed "checkerboard" supersolid is thermodynamically unstable. Furthermore, this supersolid cannot be stabilized by next-near-neighbor interaction. We obtain the correct phase diagram using the Maxwell construction. We demonstrate that the "striped" supersolid is thermodynamically stable and is separated from the superfluid phase by a continuous phase transition.     

The NQR picture of electronic phase separation in the lanthanum–barium cuprates

We report the results of Cu and La NQR investigations of the spin and charge separation in La_2−xBa_xCuO₄ compounds. For Ba doping inducing the low-temperature tetragonal phase our Cu–NQR measurements revealed three inequivalent Cu sites in CuO₂ planes. In terms of the stripe-phase picture we assign them to the charged stripe, to its neighborhood and to the AF correlated regions free of holes correspondingly. The last site corresponds to the ordered Cu magnetic moment of 0.29μ_B.     

Phase separation in a chaotic flow

The phase separation between two immiscible liquids advected by a bidimensional velocity field is investigated numerically by solving the corresponding Cahn-Hilliard equation. We study how the spinodal decomposition process depends on the presence-or absence-of Lagrangian chaos. A fully chaotic flow, in particular, limits the growth of domains, and for unequal volume fractions of the liquids, a characteristic exponential distribution of droplet sizes is obtained. The limiting domain size results from a balance between chaotic mixing and spinodal decomposition, measured in terms of Lyapunov exponent and diffusivity constant, respectively.     

Phase separation dynamics in small-world media

We present results of a numerical study of phase separation dynamics of a two-dimensional quenched system which has a small-world topology, both with and without the conserved order parameter. We examine how the domain coarsening changes with the density of long-range connections (shortcuts). It is demonstrated that the shortcuts have directly opposite effects, i.e., a speeding up of the coarsening process of reaching a fully uniform state, and a freezing of the system in a disordered metastable state.     

Phase separation in asymmetrical fermion superfluids

Motivated by recent developments on cold atom traps and high density QCD we consider fermionic systems composed of two particle species with different densities. We argue that a mixed phase composed of normal and superfluid components is the energetically favored ground state. We suggest how this phase separation can be used as a probe of fermion superfluidity in atomic traps.     

Impurity state electroabsorption studies in oxygen-doped GaAs

Electroabsorption measurements at 300 K made within the band gap on high resistivity GaAs:O are reported. For a crystal with the Fermi level at 0.54 eV, major structure has been detected at 0.52, 0.57, 0.67, 0.75, 0.87, 0.93, 1.03, 1.12, 1.25, 1.36, and 1.39 eV. The 0.67 eV peak appears only in the second harmonic spectrum, indicating no change in permanent dipole for this transition. Polarized light, lock-in amplifier phase angle, and applied field variation data are presented.     

Phase separation of binary systems

In this paper, three physical predictions on the phase separation of binary systems are derived based on a dynamic transition theory developed recently by the authors. First, the order of phase transitions is precisely determined by the sign of a nondimensional parameter K such that if K>0, the transition is first order with latent heat and if K<0, the transition is second order. Here the parameter K is defined in terms of the coefficients in the quadratic and cubic nonlinear terms of the Cahn-Hilliard equation and the typical length scale of the container. Second, a phase diagram is derived, characterizing the order of phase transitions, and leading in particular to a prediction that there is only a second-order transition for molar fraction near 1/2. This is different from the prediction made by the classical phase diagram. Third, a TL-phase diagram is derived, characterizing the regions of both homogeneous and separation phases and their transitions.     

Phase separation of intercalation electrodes

Lithium ion cells are widely used for portable equipment because of their high energy density. These cells employ lithium intercalation materials as their anode and cathode active materials. Lithium intercalation materials sometimes show phase separation as a function of the amount of inserted Li, and the cell voltage becomes a constant at the two-phase region. The thermodynamic criterion of the phase separation of binary mixtures is already known using Gibbs free energy. The criterion of the phase separation was applied to that of lithium intercalation materials. First, the phase separation of spinel LiMn₂O₄ and LiNiO₂ was studied, with the assumption that the cathode materials do not deform during Li insertion and extraction, are completely ionic, and only the Coulomb potential is effective in terms of changing the internal energy. Our calculated phase separation was in agreement with the experiments at 0<x<0.5 for Li_xMn₂O₄ and 0<x<0.25 for Li_xNiO₂. Our calculated voltage of Li_xMn₂O₄ was very high compared with the experimental value, because of a very low value of the Coulomb potential at the lithium tetrahedral site in Li_xMn₂O₄. Therefore, it seems that LiMn₂O₄ is not a perfect ionic crystal. Next, the phase separation of graphite was studied, considering a work (w_a) to expand the layer when Li is inserted. When w_a is small, it is expected that the phase separation is the co-existence of the Li-poor phase and the Li-rich phase. However, if w_a is large, it is expected that the two-phases are a phase without Li and a phase filled with Li completely. From these considerations, graphite seems to have a large w_a. Paper presented at the 8th EuroConference on Ionics, Carvoeiro, Algarve, Portugal, Sept. 16–22, 2001.     

Phase separation dynamics in driven diffusive systems

We study phase separation dynamics in a driven diffusive system. Our simulations are based on the Cahn-Hilliard equation with an additional flux term due to an external field. We study the dynamical scaling parallel and perpendicular to the field. A crossover is observed from isotropic domains at early times to extremely anisotropic domains at later times. We find that the inverse interfacial density (an isotropic measure of the domain size) increases ast , with =1/3, from early times independent of the field strength, even though we do not observe dynamical scaling during these times. Our results indicate that a growth exponent =1/3 may be more universal than previously expected. We analyze the dynamics in terms of surface driven instabilities and one-dimensional solitary waves.     

Phase separation in Sr doped BiMnO_3

Phase separation in Sr doped BiMnO3 （Bil_xSrxMnO3, x = 0.4-0.6） was studied by means of temperature-dependent high-resolution neutron powder diffraction （NPD）, high resolution X-ray powder diffraction （XRD）, and physical property measurements. All the experiments indicate that a phase separation occurs at the temperature coinciding with the reported charge ordering temperature （Tco） in the literature. Below the reported TCO, both the phases resulting from the phase separation crystallize in the orthorhombically distorted perovskite structure with space group Imma. At lower temperature, these two phases order in the CE-type antiferromagnetic structure and the A-type antiferromagnetic structure, respectively. However, a scrutiny of the high-resolution NPD and XRD data at different temperatures and the electron diffraction exper- iment at 300 K did not manifest any evidence of a long-range charge ordering （CO） in our investigated samples, suggesting that the anomalies of physical properties such as magnetization, electric transport, and lattice parameters at the TCO might be caused by the phase separation rather than by a CO transition.     

Phase separation in systems with charge ordering

A simple model of charge ordering is considered. It is explicitly shown that at any deviation from half-filling (n≠1/2), the system is unstable with respect to the phase separation into the charge ordered regions with n=1/2 and the metallic regions with a smaller electron or hole density. A possible structure of this phase-separated state (metallic droplets in a charge ordered matrix) is discussed. The model is extended to account for the strong Hund-rule onsite coupling and the weaker intersite antiferromagnetic exchange. The analysis of this extended model allows us to determine the magnetic structure of the phase-separated state and to reveal the characteristic features of the manganites and other substances with charge ordering.     

掺杂锰基氧化物中的相分离和渗流效应

目前，相分离仍是锰基氧化物超大磁电阻材料研究的热点，渗流效应假设已广泛用于解释其电输运特性．作者用变温磁力显微镜首次在La0.33Pr0.34Ca0.33MnO3薄膜中直接观察到了渗流过程，微观上证明了渗流效应假设的正确性．实验发现，降温过程中电阻率的陡降是铁磁金属相的渗流效应引起的，升温过程中电阻率的上升，则是由导电路径上磁畴的磁化强度随温度的升高而降低引起的，而导电路径一直存在．微观的磁回滞和宏观的电阻回滞相吻合．当然要定量解释锰氧化物中的超大磁电阻效应还需要做大量的理论和实验工作．