Superconductor-to-insulator transition and transport properties of underdoped YBa2Cu3O(y) crystals

The carrier-concentration-driven superconductor-to-insulator (SI) transition as well as transport properties in underdoped YBa2Cu3O(y) twinned crystals is studied. The SI transition takes place at y approximately 6.3, carrier concentration n(SI)H approximately 3x10(20) cm(-3), anisotropy rho(c)/rho(ab) approximately 10(3), and the threshold resistivity rho(SI)ab approximately 0.8 mOmega cm which corresponds to a critical sheet resistance h/4e2 approximately 6.5 kOmega per CuO2 bilayer. The evolution of a carrier, nH infiniti y - 6.2, is clearly observed in the underdoped region. The resistivity and Hall coefficient abruptly acquire strong temperature dependence at y approximately 6.5 indicating a radical change in the electronic state.     

Multiple photonic responses in films of organic-based magnetic semiconductor V(TCNE)x, x approximately 2

Concomitant photoinduced magnetic and electrical phenomena are reported for the organic-based magnetic semiconductor V(TCNE)x (x approximately 2; TCNE=tetracyanoethylene; magnetic ordering temperature Tc approximately 400 K). Upon optical excitation (457.9 nm), the system can be trapped in a thermally reversible photoexcited state, which exhibits reduced magnetic susceptibility and increased conductivity with a simultaneous change in IR absorption spectrum. The multiple photonic effects in V(TCNE)x are proposed to originate from structural changes induced by internal excitation in (TCNE)- anions, which lead to relaxation to a long-lived metastable state.     

Systematic (63)Cu NQR and (89)Y NMR study of spin dynamics in Y(1-z)Ca(z)Ba(2)Cu(3)O(y) across the superconductor-insulator boundary

We demonstrate that the spin dynamics in underdoped Y(1-z)Ca(z)Ba(2)Cu(3)O(y) for y approximately equal to 6.0 exhibit qualitatively the same behavior to underdoped La(2-x)Sr(x)CuO(4) for an equal amount of hole concentration p = z/2 = x< or =0.11. However, a spin gap appears as more holes are doped into the CuO(2) plane by increasing the oxygen concentration to y approximately equal to 6.5 for a fixed value of Ca concentration z. Our results also suggest that Ca doping causes disorder effects that enhance the low frequency spin fluctuations.     

Study of the e(g) orbitals in the bilayer manganite La(2--2x)Sr(1+2x)Mn(2)O(7) by using magnetic Compton-profile measurement.

From the magnetic Compton-profile (MCP) measurement, we have directly differentiated for the first time the populations in two e(g)-type orbitals ( x(2) - y(2) and 3z(2) - r(2)) in a manganite. The experimental MCP's along the [001] direction for La(2--2x)Sr(1+2x)Mn(2)O(7) at x = 0.35 and 0.42 are fitted by the theoretical profiles obtained from the (MnO(6))(8-) ab initio calculations. The calculation confirms that the MCP clearly detects the oxygen hybridization in the e(g) orbitals. The e(g) state is dominated by the x(2) - y(2)-type orbital with almost constant population, while the population in the 3z(2) - r(2)-type orbital decreases with increasing the hole concentration x.     

Photoinduced magnetism and random magnetic anisotropy in organic-based magnetic semiconductor V(TCNE){x} films, for x approximately 2

The V(TCNE){x}, x approximately 2 is an organic-based amorphous ferrimagnet, whose magnetic behavior is significantly affected in the low field regime by the random magnetic anisotropy. It was determined that this material has thermally reversible persistent change in both magnetization and conductivity driven by the optical excitation. Here, we report results of a ferrimagnetic resonance study of the photoinduced magnetism in V(TCNE){x} film. Upon optical excitation (lambda approximately 457.9 nm), the ferrimagnetic resonance spectra display substantial changes in their linewidths and line shifts, which reflect a substantial increase in the random magnetic anistropy. The results reflect the role of magnetic anisotropy in disordered magnets and suggest a novel mechanism of photoinduced magnetism in V(TCNE){x} induced by the increased structural disorder in the system.     

Epitaxial growth and band alignment of (Gd(x)La(1-x))(2)O(3) films on n-GaAs (001)

Herein we demonstrate the epitaxial stabilization of single-crystalline (Gd(x)La(1-x))(2)O(3) films on n-GaAs (001) with a controlled lattice match. (Gd(x)La(1-x))(2)O(3) films have an in-plane epitaxial relationship with a twofold rotation on GaAs (001). Spectroscopic characterization by photoemission and absorption confirms that the band gap of (Gd(x)La(1-x))(2)O(3) film is approximately approximately 5.8eV. However, the conduction band offset is increased by the unpinned Fermi level of the n-GaAs in the (Gd(x)La(1-x))(2)O(3) film (x=0.97). The correlation of the crystalline property and the interfacial band offset by the electrical properties, as probed by capacitance and leakage current measurements, is also discussed.     

Surface reconstructions of TiO2(110) driven by suboxides

Scanning tunneling microscopy and density functional theory are used to develop a new structural model for surface reconstructions driven by Ti interstitials on TiO2(110). Ti interstitials form the edge- or face-sharing octahedra that serve as building blocks for (1 x 1) reconstruction. Thus, contrary to conventional wisdom, the 1 x 1 periodicity is insufficient to establish the correct surface stoichiometry. Furthermore, in our structural and compositional model the reversible oxidation or reduction between (1 x 1) and (1 x 2) is entirely achieved by transfer of the added rows.     

Nonequilibrium steady states in sheared binary fluids

We simulate by lattice Boltzmann the steady shearing of a binary fluid mixture undergoing phase separation with full hydrodynamics in two dimensions. Contrary to some theoretical scenarios, a dynamical steady state is attained with finite domain lengths L(x,y) in the directions (x,y) of velocity and velocity gradient. Apparent scaling exponents are estimated as Lx approximately gamma (-2/3) and Ly approximately gamma(-3/4). We discuss the relative roles of diffusivity and hydrodynamics in attaining steady state.     

Vortex state in NaxCoO2.yH2O: px+/-ipy-wave versus d(x2-y2)+/-id xy-wave pairing

Based on an effective Hamiltonian specified in the triangular lattice with possible p(x)+/-ip(y)- or dx(2)(-y(2))+/-id(xy)-wave pairing, which has close relevance to the newly discovered Na0.35CoO2.yH(2)O, the electronic structure of the vortex state is studied by solving the Bogoliubov-de Gennes equations. It is found that p(x)+/-ip(y) wave is favored for the electron doping as the hopping integral t<0. The lowest-lying vortex bound states are found to have, respectively, zero and positive energies for p(x)+/-ip(y)- and dx(2)(-y(2))+/-id(xy)-wave superconductors, whose vortex structures exhibit the intriguing sixfold symmetry. In the presence of strong on-site repulsion, the antiferromagnetic order and local ferromagnetic moment are induced around the vortex cores for the former and the latter, respectively, both of which cause the splitting of the local density of states peaks due to the lifting of spin degeneracy.     

Nanocrystalline versus microcrystalline Li(2)O:B(2)O3 composites: anomalous ionic conductivities and percolation theory

We study ionic transport in nano- and microcrystalline (1-x)Li(2)O:xB(2)O3 composites using standard impedance spectroscopy. In the nanocrystalline samples (average grain size of about 20 nm), the ionic conductivity sigma(dc) increases with increasing content x of B2O3 up to a maximum at x approximately 0.5. Above x approximately 0.92, sigma(dc) vanishes. By contrast, in the microcrystalline samples (grain size about 10 &mgr;m), sigma(dc) decreases monotonically with x and vanishes above x approximately 0. 55. We can explain this strikingly different behavior by a percolation model that assumes an enhanced conductivity at the interfaces between insulating and conducting phases in both materials and explicitly takes into account the different grain sizes.     

Exotic d-wave superconducting state of strongly hole-doped K(x)Ba(1-x)Fe2As2

We investigate the superconducting phase in the K(x)Ba(1-x)Fe2As2 122 compounds from moderate to strong hole-doping regimes. Using the functional renormalization group, we show that, while the system develops a nodeless anisotropic s(±) order parameter in the moderately doped regime, gapping out the electron pockets at strong hole doping drives the system into a nodal (cos k(x) + cos k(y))(cos k(x) - cos k(y)) d-wave superconducting state. This is in accordance with recent experimental evidence from measurements on KFe2As2 which observe a nodal order parameter in the extreme doping regime. The magnetic instability is strongly suppressed.     

Ab initio study of the electronic and magnetic properties of Sr(2-x)La(x)Fe(1+y/2)Mo(1-y/2)O6 double perovskites

Using the first-principles full potential linearized augmented plane wave method, the electronic structure of Sr(2-x)La(x)Fe(1+y/2)Mo(1-y/2)O6 (SLFMO) double-perovskite systems is investigated for x = 1/2 and 1 and for y = +1 and -1. Substituting Sr atoms by La atoms allows one to tune the electrons added into the minority spin band and enhances the half-metal feature--according to the rigid band shift model--even if the magnetization decreases with increasing La concentration. By taking into account the chemical disorder on the Fe and Mo sites, resulting from the introduction of La as shown experimentally, it is shown (i) that a supplemental magnetization reduction occurs because the magnetic moment on the Fe antisite is opposite to the one on the Fe regular sites and (ii) that the half-metal feature is preserved in SLFMO for x = 1/2 and 1 contrary to the case for SFMO. Finally, because surface or interface Fe deficiency should have a more limited impact on the spin polarization than for SFMO, SLFMO/SrTiO3 multilayers are investigated in order to confirm this prediction by determining the spin polarization at the interface, which is found to remain high.     

红色荧光粉Na_zCa_(1-x-2y-z)Bi_yMoO_4∶Eu_(x+y)~(3+)发光研究

采用高温固相法制备系列红色荧光粉NazCa1-x-2y-zBiyMoO4∶Eux3++y(y,z=0,x=0.24,0.26,0.30,0.34,0.38;x=0.30,y=0.01,0.02,0.03,0.04,0.05,0.06,0.07,z=0;x=0.30,y=0.04,z=0.38)。用X射线粉末衍射(XRD)法测试了所制样品晶相结构。采用荧光光谱仪对样品的发光性能进行了表征,结果表明:当Eu3+单掺杂量浓度x=0.30时,荧光粉(Ca0.70MoO4∶Eu03.+30)的发光强度最强;当Eu3+-Bi3+共掺杂量浓度y=0.03时,电荷迁移带(CTB)强度达到最强,而对于Eu3+特征发射峰,当共掺杂浓度y<0.03时,位于393 nm处的激发峰强度比464 nm强,共掺浓度y>0.03时,464 nm峰比393 nm峰强,共掺浓度为y=0.04时,393和464 nm处两峰位置强度都达到最强。作为电荷补尝剂的Na2CO3掺入上述荧光粉中后,荧光粉激发和发射强度明显地增强。结果表明,通过调节Bi3+/Eu3+掺杂比例可以改变位于近紫外光393 nm和蓝光区464 nm处激发光相对强度。     

Electronic structure of charge- and spin-controlled Sr(1-(x+y))La(x+y)Ti(1-x)Cr(x)O3

We present the electronic structure of Sr(1-(x+y))La(x+y)Ti(1-x)Cr(x)O3 investigated by high-resolution photoemission spectroscopy. In the vicinity of the Fermi level, it was found that the electronic structure was composed of a Cr 3d local state with the t(2g)3 configuration and a Ti 3d itinerant state. The energy levels of these Cr and Ti 3d states are well interpreted by the difference of the charge-transfer energy of both ions. The spectral weight of the Cr 3d state is completely proportional to the spin concentration x irrespective of the carrier concentration y, indicating that the spin density can be controlled by x as desired. In contrast, the spectral weight of the Ti 3d state is not proportional to y, depending on the amount of Cr doping.     

Phase fluctuations and the pseudogap in YBa(2)Cu(3)O (x)

The thermodynamics of the superconducting transition is studied as a function of doping using high-resolution expansivity data of YBa(2)Cu(3)O (x) single crystals and Monte Carlo simulations of the anisotropic 3D- XY model. We directly show that T(c) of underdoped YBa(2)Cu(3)O (x) is strongly suppressed from its mean-field value (T(MF)(c)) by phase fluctuations of the superconducting order parameter. For overdoped YBa(2)Cu(3)O (x) fluctuation effects are greatly reduced and T(c) approximately T(MF)(c). We find that T(MF)(c) exhibits a similar doping dependence as the pseudogap energy, naturally suggesting that the pseudogap arises from phase-incoherent Cooper pairing.     

Angle-resolved photoemission study of the cobalt oxide superconductor Na(x)CoO(2) x yH(2)O: observation of the Fermi surface

The cobalt oxide superconductor Na(x)CoO(2) x yH(2)O is studied by angle-resolved photoemission spectroscopy. We report the Fermi surface (FS) topology and electronic structure near the Fermi level (E(F)) in the normal state of Na(x)CoO(2) x yH(2)O. Our result indicates the presence of the hexagonal FS centered at the Gamma point, while the small pocket FSs along Gamma-K direction are absent, similar to Na(x)CoO(2). The top of the e(g)(') band, which is expected in band calculations to form the small pocket FSs, extends to within approximately 30 meV below E(F), closer to E(F) than in Na(x)CoO(2). We discuss its possible role in superconductivity, comparing with other experimental and theoretical results.     

Relation between the superconducting gap energy and the two-magnon raman peak energy in Bi(2)Sr(2)Ca(1-x)Y(x)Cu(2)O(8+delta)

The relation between the electronic excitation and the magnetic excitation for the superconductivity in Bi(2)Sr(2)Ca(1-x)Y(x)Cu(2)O(8+delta) was investigated by wide-energy Raman spectroscopy. In the underdoping region the B(1g) scattering intensity is depleted below the two-magnon peak energy due to the charge-spin interactions. The depleted region decreases according to the decrease of the two-magnon peak energy, as the carrier concentration increases. This two-magnon peak energy also determines the B(1g) superconducting gap energy as 2Delta approximately alphaPlanck's over 2piomega(two-magnon) approximately J(eff) (alpha = 0.34-0.41) from under to overdoping hole concentration.     

d x 2 - y 2 superconductivity and the Hubbard model

The numerical studies of d x 2 - y 2 -wave pairing in the two-dimensional (2D) and the 2-leg Hubbard models are reviewed. For this purpose, the results obtained from the determinantal Quantum Monte Carlo and the Density-Matrix Renormalization-Group calculations are presented. These are calculations which were motivated by the discovery of the high- T c cuprates. In this review, the emphasis is placed on the microscopic many-body processes which are responsible for the d x 2 - y 2 -wave pairing correlations observed in the 2D and the 2-leg Hubbard models. In order to gain insight into these processes, the results on the effective pairing interaction as well as the magnetic, density and the single-particle excitations will be reviewed. In addition, comparisons will be made with the other numerical approaches to the Hubbard model and the numerical results on the t - J model. The results reviewed here indicate that an effective pairing interaction which is repulsive at ( ~ , ~ ) momentum transfer, and enhanced single-particle spectral weight near the ( ~ ,0) and (0, ~ ) points of the Brillouin zone, create optimum conditions for d x 2 - y 2 -wave pairing. These are two effects which act to enhance the d x 2 - y 2 -wave pairing correlations in the Hubbard model. Finding additional ways is an active research problem.     

Inversion of two-band superconductivity at the critical electron doping of (Mg, Al)B2

Electron energy-loss spectroscopy (EELS) was combined with heat capacity measurements to probe changes of electronic structure and superconductivity in Mg(1-x)Al(x)B(2). A simultaneous decrease of EELS intensity from sigma-band hole states and the magnitude of the sigma gap was observed with increasing x, thus verifying that band filling results in the loss of strong superconductivity. These quantities extrapolated to zero at x approximately 0.33 as inferred from the unit cell volume. However, superconductivity was not quenched completely, but persisted with T(c) < 7 K up to about x approximately 55. Only the pi band had detectable density of states for 0.33 < or =x < or = 0.55, implying an inversion of the two-band hierarchy of MgB(2) in that regime. Since pi-band superconductivity is active in other materials such as intercalated graphite, implications for new materials with high T(c) are discussed.