Charge density waves and bond order waves in a quarter filled extended Hubbard ladder

We investigate the phase diagram of a quarter filled Hubbard ladder with nearest-neighbor Coulomb repulsion using bosonization and renormalization group approach. Focusing on the strong-repulsion regime, we discuss the effect of an interchain exchange interaction J and interchain repulsion V on the possible ground states of the system and charge order configurations. Since the spin excitations always possess a gap, we find competing bond-order wave and charge density wave phases as possible ground states of the ladder model. We discuss the elementary excitations in these various phases and point an analogy between the excitations on some of these phases and those of a Kondo-Heisenberg insulator. We also study the order of the quantum phase transitions between the different ground states of the system. We obtain second order transitions in the Ising or SU(2)₂ universality class or first order transitions. We map the complete phase diagram in the J –V plane by integrating perturbative renormalization-group equations. Finally, we discuss the effect of doping away from half-filling and the effect of an applied magnetic field.     

Ab initio study of rumpled relaxation and core-level shift of barium titanate surface

The rumpled relaxation and the core-level shift of full-relaxed BaTiO₃ (0 0 1) surface have been investigated by first-principles calculation. Based on the work function and the electric-field gradient, the right size of vacuum and the slab have been evaluated. The large displacements of ions deviated from their crystalline sites to lead to the formation of the surface rumples have been found. Some fully occupied surface oxygen p states at the top M point of the valance band and the empty surface titanium d states at the edge of the bulk conduction band are observed on the TiO₂-terminated surface. In contrast, on the BaO-terminated surface, two different core levels of the Ba 5p states shifted about 1.29 eV are induced by the bulk perovskite Ba atoms and the relaxation of surface Ba atoms, respectively. Our calculations are consistent with the experimental data.     

Frequency-dependent fluctuational conductivity above in anisotropic superconductors: effects of a short wavelength cutoff

We discuss the excess conductivity at nonzero frequencies in a superconductor above T_c within the Gaussian approximation. We focus the attention on the temperature range not too close to T_c: within a time-dependent Ginzburg-Landau formulation, we phenomenologically introduce a short wavelength cutoff (of the order of the inverse coherence length) in the fluctuational spectrum to suppress high momentum modes. We treat the general cases of thin wires, anisotropic thin films and anisotropic bulk samples. We obtain in all cases explicit expressions for the finite frequency fluctuational conductivity. The dc case directly follows. Close to T_c the cutoff has no effect, and the known results for Gaussian fluctuations are recovered. Above T_c, and already for ε = ln(T/T _c) > 10^-2, we find strong suppression of the paraconductivity as compared to the Gaussian prediction, in particular in the real part of the paraconductivity. At high ε the cutoff effects are dominant. We discuss our results in comparison with data on high-T_c superconductors. Received 19 March 2002 Published online 25 June 2002     

Ro-vibrational states of H+2. Variational calculations

The non-relativistic variational calculation of a complete set of ro-vibrational states in the H⁺₂ molecular ion supported by the ground 1sσ adiabatic potential is presented. It includes both bound states and resonances located above the n = 1 threshold. In the latter case we also evaluate a predissociation width of a state wherever it is significant. Relativistic and radiative corrections are discussed and effective adiabatic potentials of these corrections are included as supplementary files.     

Exact propagator of the Fokker-Planck equation with a space-dependent diffusion coefficient and a time-dependent mean-reverting force

We have investigated the algebraic structure of the Fokker-Planck equation with a variable diffusion coefficient and a time-dependent mean-reverting force. Such a model could be useful to study the general problem of a Brownian walker with a space-dependent diffusion coefficient. We also show that this model is related to the Fokker-Planck equation with a constant diffusion coefficient and a time-dependent anharmonic potential of the form V(x, t) = ?a(t)x ² + b ln x, which has been widely applied to model different physical and biological phenomena, e.g. the study of neuron models and stochastic resonance in monostable nonlinear oscillators. Using the Lie algebraic approach we have derived the exact diffusion propagators for the Fokker-Planck equations associated with different boundary conditions, namely (i) the case of a single absorbing barrier, and (ii) the case of two absorbing barriers. These exact diffusion propagators enable us to study the time evolution of the corresponding stochastic systems. Received 23 October 2001 and Received in final form 24 December 2001     

X-ray photoemission studies on InSb clusters

The changes in core-level electronic structure of In_1−xSb_x nano-clusters have been studied using X-ray photoelectron spectroscopy. Though, clusters with mean composition InSb show shallow and deep core-level binding energies similar to those of bulk InSb, the Sb and In rich versions show a negative BE shift for the excess element and a positive shift for the minority element. The observed BE shifts have been explained considering a core and surface shell model for the structure of the clusters and possible surface atom core-level shifts.     

Anisotropy of electron structure at InAs(1 1 1) surfaces by laser pump-and-probe photoemission spectroscopy

The electronic structure and the electron dynamics of the clean InAs(1 1 1)A 2 × 2 and the InAs(1 1 1)B 1 × 1 surfaces have been studied by laser pump-and-probe photoemission spectroscopy. Normally unpopulated electron states above the valence band maximum (VBM) are filled on the InAs(1 1 1)A surface due to the conduction band pinning above the Fermi level (E_F). Accompanied by the downward band banding alignment, a charge accumulation layer is confined to the surface region creating a two dimensional electron gas (2DEG). The decay of the photoexcited carriers above the conduction band minimum (CBM) is originated by bulk states affected by the presence of the surface. No occupied states were found on the InAs(1 1 1)B 1 × 1 surface. This fact is suggested to be due to the surface stabilisation by the charge removal from the surface into the bulk. The weak photoemission intensity above the VBM on the (1 1 1)B surface is attributed to electron states trapped by surface defects. The fast decay of the photoexcited electron states on the (1 1 1)A and the (1 1 1)B surfaces was found to be τ_1 1 1 A ? 5 ps and τ_1 1 1 B ?  4 ps, respectively. We suggest the diffusion of the hot electrons into the bulk is the decay mechanism.     

First-principles study of the (0 0 1) surface of cubic SrZrO3

Density functional calculations have been used to investigate the (0 0 1) surface of cubic SrZrO₃ with both SrO and ZrO₂ termination. Surface structure and electronic structure have been obtained. The SrO surface is found to be similar to its counterpart in SrTiO₃, while there are marked differences between the ZrO₂ and TiO₂ terminations in SrZrO₃ and SrTiO₃, respectively, concerning surface relaxation and rumpling. For the ZrO₂-terminated surface of SrZrO₃, the covalency of the interaction between the outmost Zr and the O beneath is enhanced as a result of their bond contraction. The band gap reduction and the presence of the surface states are also discussed in relation with the behavior of the electrostatic potential.     

MJ and collision energy dependent attenuation of Rg (3P2) by CF3Br

The total attenuation cross-section of Rg (³P₂) (Rg = Ar, Kr) by the collision with CF₃Br is measured as a function of the magnetic sub level M_J of Rg (³P₂) and the collision energy. For Ar (³P₂), the attenuation process indicates a M_J dependence, in particular, the cross-section of the M_J = 0 state is lower compared with that for other states. On the other hand, Kr (³P₂) shows no M_J dependent attenuation.     

First-principles study on the electronic structures of iron phthalocyanine monolayer

The electronic band structure and magnetic properties of iron phthalocyanine (FePc) monolayer were investigated by using the first-principles all-electron full-potential linearized augmented plane wave energy band method. It is found that the ferromagnetic FePc monolayer is energetically more stable than the paramagnetic one. The exchange interaction, which splits the majority and minority bands, influences strongly on the electronic structure near the Fermi level (E_F). Magnetic moment of the central Fe atom is calculated to 1.95 μ_B. The range of the positive polarization of Fe site is larger in the out-of-plane than in the in-plane direction. The FePc ligand remains paramagnetic. The presence of states at E_F indicates the metallic character of FePc monolayer both for the paramagnetic and ferromagnetic states. However, the large density of states at E_F of the majority spins in the ferromagnetic state is expected to cause a phase transition to insulating antiferromagnetic state from the metallic ferromagnetic one.     

Multi-qubit stabilizer and cluster entanglement witnesses

One of the problems concerning entanglement witnesses (EWs) is the construction of them by a given set of operators. Here several multi-qubit EWs called stabilizer EWs are constructed by using the stabilizer operators of some given multi-qubit states such as GHZ, cluster and exceptional states. The general approach to manipulate the multi-qubit stabilizer EWs by exact(approximate) linear programming (LP) method is described and it is shown that the Clifford group play a crucial role in finding the hyper-planes encircling the feasible region. The optimality, decomposability and non-decomposability of constructed stabilizer EWs are discussed.     

Ground state properties of an asymmetric Hubbard model for unbalanced ultracold fermionic quantum gases

In order to describe unbalanced ultracold fermionic quantum gases on optical lattices in a harmonic trap, we investigate an attractive (U < 0) asymmetric (t_↑≠t_↓) Hubbard model with a Zeeman-like magnetic field. In view of the model's spatial inhomogeneity, we focus in this paper on the solution at Hartree-Fock level. The Hartree-Fock Hamiltonian is diagonalized with particular emphasis on superfluid phases. For the special case of spin-independent hopping we analytically determine the number of solutions of the resulting self-consistency equations and the nature of the possible ground states at weak coupling. We present the phase diagram of the homogeneous system and numerical results for unbalanced Fermi-mixtures obtained within the local density approximation. In particular, we find a fascinating shell structure, involving normal and superfluid phases. For the general case of spin-dependent hopping we calculate the density of states and the possible superfluid phases in the ground state. In particular, we find a new magnetized superfluid phase.     

Oxygen K-edge in vanadium oxides: simulations and experiments

Band-structure (BS) calculations of the density of states (DOS) using the full potential augmented plane waves code WIEN97 were performed on the four single-valence vanadium oxides VO, V₂O₃, VO₂ and V₂O₅. The DOS are discussed with respect to the distortions of the VO₆ octahedra, the oxidation states of vanadium and the orbital hybridisations of oxygen atoms. The simulated oxygen K-edge fine structures (ELNES) calculated with the TELNES program were compared with experimental results obtained by electron energy-loss spectrometry (EELS), showing good agreement. We show that changes in the fine structures of the investigated vanadium oxides mainly result from changes in the O-p DOS and not from the shift of the DOS according to a rigid band model. Received 17 December 2001 / Received in final form 19 June 2002 Published online 13 August 2002     

Antiferromagnetic order and phase transitions in GdS as studied with X-ray resonance-exchange scattering

We report on X-ray resonance exchange and neutron scattering of metallic GdS. At the L_II and L _III absorption edges of Gd, resonance enhancements of more than two orders of magnitude over the non-resonant magnetic scattering are observed. Polarisation analysis proves that these enhancements are due to dipolar transitions from the 2p to the 5d states. The branching ratio between the L_II and L _III edges of 2.5 suggests a polarisation of the 5d electrons in the ground state. The antiferromagnetic order is of type II in the fcc lattice. Single crystal diffraction of hot neutrons suggests that the spin direction lies within the (111) planes with a value for the sublattice magnetisation of 6.51(3) . The critical exponent for the sublattice magnetisation has a value of β = 0.38(2) in agreement with a pure Heisenberg model. Above T _N, a sharp component persists in the critical diffuse scattering. Lattice distortions give indications for two additional low-temperature phase transitions at about 49 K and 32 K. We argue that these transitions are not connected to spin reorientations and discuss the possible influence of fourth-order exchange interactions. Received 19 November 1999 and Received in final form 12 December 2000     

“W = 0” pairing in Cu-O clusters and in the plane

The Cu-O plane and the clusters that possess the same C_4v symmetry around a Cu ion have 2-hole eigenstates of the kinetic energy with vanishing on-site repulsion (W=0 pairs). Cluster calculations by exact diagonalisation show that these are the quasiparticles that lead to a paired ground state, and have superconducting flux-quantisation properties. Here, we extend the theory to the full plane, and show that the W=0 quasiparticles are again the natural explanation of superconducting flux-quantisation. Moreover, by a new approach which is exact in principle, we calculate the effective interaction between two holes added to the ground state of the repulsive three-band Hubbard model. To explain how a noninteracting electron gas becomes a superconductor when switching the local Coulomb interaction, we obtain a closed-form analytic expression including the effects of all virtual transitions to 4-body intermediate states (exchange of an electron-hole pair). Our scheme is ready to include other interactions which are not considered in the Hubbard model but may be important. In the plane, the W=0 pairs have ¹ B ₂ and ¹ A ₂ symmetry. The effective interaction in these channels is attractive and leads to a Cooper-like instability of the Fermi liquid, while it is repulsive for triplet pairs. From , we derive an integral equation for the pair eigenfunction; the binding energy of the pairs is in the range of tens of meV. However, our symmetry-based method is far more general than the model. Received 18 December 1998     

Effect of electron-phonon interaction on surface states in a ternary mixed crystal

A variational theory is proposed to study the surface states of electrons in a semi-infinite ternary mixed crystal, by taking the effect of electron-surface optical (SO) phonon interaction into account. The energy and the wave function of the electronic surface-states are calculated. The numerical results of the energies of the surface states of the polarons and the self-trapping energies are obtained as functions of the composition x and surface potential V₀ for several ternary mixed crystal materials. The results show that the electron-phonon interaction lowers the surface-state levels with the energies from several to scores of meV. It is also found that the self-trapping energy of the surface polaron has a minimum at some middle value of the composition x. It is indicated that the electron-phonon coupling effect can not be neglected. Received 4 January 1999 and Received in final form 7 January 2000     

On the evolution of the ground state in the system U x La 1 - x S: Polarized neutron diffraction and X-ray magnetic circular dichroism study

In the U_xLa_1-xS system there is an abrupt loss of the long-range ferromagnetic ordering found in pure US at a critical concentration x _c ∼ 0.57, which is far above the percolation limit. As the magnetic ground state in such a system can be strongly affected by small variations of the 5f localization, we have investigated a set of samples with different x by polarized neutron diffraction and X-ray magnetic circular dichroism (XMCD). The neutron results are consistent with early measurements performed on pure US. Even at the lowest U content (x = 0.15, below x _c ) the shape of the induced form factor (f ( Q )) is comparable with that found for x = 1 and is well reproduced by either a U⁴⁺ or a U³⁺ state. The ratio between the orbital and the effective spin moments in the XMCD measurements confirms this result, but the evolution of the shape at the M₅ edge suggests an abrupt change in the distribution of the electrons (holes) in the 5 f density of states around x _c . Received 31 January 2001     

Stripe orders driven by long-range Coulomb forces in the 2D-Hubbard model

Within the single band 2D-Hubbard model treated by means of a strong-coupling approach based on a cumulant expansion and a nonstandard diagrammatic technique, we discuss the existence of critical charge fluctuations that could give rise to an instability towards a phase separation (PS). It turns out that such instability exists and evolves into an incommensurate charge density wave (ICDW) when long-range Coulomb forces are taken into account. We find a stripe phase with a crossover from diagonal to vertical stripes at increasing doping in the range 0.01 ?δ? 0.2 and increasing Coulomb potential U, similarly to recent NMR experiments on La _{2 - x} Sr _x CuO ₄. Received 20 November 2000