Directional ordering of fluctuations in a two-dimensional compass model

In the Mott insulating phase of the transition metal oxides, the effective orbital-orbital interaction is directional both in orbital space and in real space. We discuss a classical realization of directional coupling in two dimensions. Despite extensive degeneracy of the ground state, the model exhibits partial orbital ordering in the form of directional ordering of fluctuations at low temperatures stabilized by an entropy gap. Transition to the disordered phase is shown to be in the Ising universality class through exact mapping and multicanonical Monte Carlo simulations.     

A derivation for the energy dependence of the density of band tail states in disordered materials

Variational arguments are employed to present the first explicit derivation of the behavior of band tail states and localization length over most of the tail region of a random alloy. Exponential tails are shown to exist near the mobility edges, in conformity with the observation in disordered systems. The character of the states forming the band tails is discussed and, for random alloys, the localization length is shown to exhibit a minimum as a function of energy between band edge and mobility edge.     

Ten-fold spin-orbital degeneracy in the true d-electron Hubbard model of the Mott metal-insulator transition

The ten-fold spin-orbital degeneracy of true d-electrons is included in the non-spin-orbitally degenerate Hubbard model treatment theory of the Mott metal-insulator transition of Rice and Brinkman, and Gutzwiller. The Rice-Brinkman and Kohn phase diagrams of the Mott metal-insulator transition are shown to shift due to spin-orbital degeneracy of true d-electrons so as to increase the available phase space in the temperature-“interaction” Mott phase diagram for the formation of the antiferromagnetic insulator, superlattice metal or superlattice insulator states in these phase diagrams.     

Hall-effect and metal-nonmetal transition in copper-argon and lead-argon systems

Summary Hall-effect measurements have been made to find out the type of transition from metal to nonmetal in copper-argon and lead-argon films prepared at helium temperature. The Hall measurements indicate that the transition from metal to nonmetal is a new-type transition. Such a metal-nonmetal transition in metal-argon systems may be attributed to the combined effect of electron correlation intermediated by phase transition. To speed up publication, the authors of this paper have agreed to not receive the proofs for correction.     

Enhanced Coulomb and exchange interaction crystal fields in the modified Zener model of magnons in metallic ferromagnets

The Bartel treatment of magnons in metallic ferromagnets within the modified Zener model of itinerant-localized (s-electron or hole) exchange is generalized to include ten-fold d-electron (hole) spin-orbital degeneracy. As such one can ab initio include enhanced intra-cell Coulomb repulsive crystal fields and exchange attractive crystal fields. This treatment includes itinerant-itinerant (d-electron or hole) Coulomb and exchange interactions as well as itinerant-localized (d-electron or hole) interactions; Hund's rule coupling is included automatically. Also, ab initio the ferromagnet specificity is included via the d-bandfilling and d-bandwidth inherent in the degenerate theory; the possibility of finite temperatures is also included. Acoustic and optic ferromagnon branches are recalculated and corrected and the position of the Stoner particle-hole continuum boundary is recalculated at zero and finite temperatures.     

Electronic structure,pairwise interactions and ordering energies in binary f.c.c. transition metal alloys

Effective pair interactions for binary f.c.c. transition metal alloys have been computed using a generalized perturbation method starting from the completely disordered state. Their properties, as a function of interatomic distance, band filling, disorder and concentration are discussed. The ordering energy computed from the pair interactions is succesfully compared, for the Cu₃Autype structure, to the energy derived from the band structure determined by the recursion method.     

Non-Hermitian topological Anderson insulators

Non-Hermitian systems can exhibit exotic topological and localization properties.Here we elucidate the non-Hermitian effects on disordered topological systems using a nonreciprocal disordered Su-Schrieffer-Heeger model.We show that the non-Hermiticity can enhance the topological phase against disorders by increasing bulk gaps.Moreover,we uncover a topological phase which emerges under both moderate non-Hermiticity and disorders,and is characterized by localized insulating bulk states with a disorder-averaged winding number and zero-energy edge modes.Such topological phases induced by the combination of non-Hermiticity and disorders are dubbed non-Hermitian topological Anderson insulators.We reveal that the system has unique non-monotonous localization behavior and the topological transition is accompanied by an Anderson transition.These properties are general in other non-Hermitian models.     

Critical thermodynamics of the two-dimensional +/-J Ising spin glass

We compute the exact partition function of 2d Ising spin glasses with binary couplings. In these systems, the ground state is highly degenerate and is separated from the first excited state by a gap of size 4J. Nevertheless, we find that the low temperature specific heat density scales as exp(-2J/T), corresponding to an "effective" gap of size 2J; in addition, an associated crossover length scale grows as exp(J/T). We justify these scalings via the degeneracy of the low lying excitations and by the way low energy domain walls proliferate in this model.     

Orbital liquid in three-dimensional mott insulator: LaTiO3

We present a theory of spin and orbital states in Mott insulator LaTiO3. The spin-orbital superexchange interaction between d(1)(t(2g)) ions in cubic crystal suffers from a pathological degeneracy of orbital states at the classical level. Quantum effects remove this degeneracy and result in the formation of the coherent ground state, in which the orbital moment of t(2g) level is fully quenched. We find a finite gap for orbital excitations. Such a disordered state of local degrees of freedom on unfrustrated, simple cubic lattice is highly unusual. Orbital liquid state naturally explains observed anomalies of LaTiO3.     

Uniaxial-stress effects on electronic structures of nanographite ribbons

The uniaxial-stress effects on the low-energy electronic properties of nanographite ribbons are studied by the tight-binding model. The dependence on the strain, the edge structure, the ribbon width, and the stacking sequence is strong. The strain could induce the alternation of energy dispersions, the destruction of state degeneracy, the variation of energy gap, the semiconductor–metal transition, and the change of special structures in density of states. The effects of strain are important for the AB- and AA-stacked armchair ribbons. However, they are negligible for the AB- and AA-stacked zigzag ribbons. Armchair ribbons could exhibit the semiconductor–metal transition. Such transition is mainly determined by the strain and the ribbon–ribbon interactions.     

The electronic structure of some transition metal alloys

Thermal neutron scattering experiments have provided detailed information on the distributions of magnetic moment in a number of disordered ferromagnet binary alloys. The general features of these distributions together with saturation magnetization data are discussed and compared with various simple theories. Attention is focused on dilute alloy systems. After an introduction the paper is divided into four sections, the first of which deals with alloys which tend to follow the Slater-Pauling curve. Here a simple Thomas-Fermi treatment due to Friedel suggests that magnetic moment changes, largely confined to the minor constituent (solute) sites, should occur with a sign dependent on the nature of the density of states at the Fermi level in the pure major constituent (solvent). Comparison with experiment shows qualitative agreement except in the case of Fe-based alloys containing transition element solutes from the right of Fe in the periodic table. This discrepancy is examined and an explanation put forward. The next section outlines a discussion of the electronic structure of alloys of transition elements with non-transition metal solutes. The view is taken that the electronic configuration of a solute atom is roughly similar to the configuration found in the pure non-transition metal: it follows that no partially filled d orbitals are expected at solute sites. Use of a simple Thomas-Fermi model based on this assumption indicates that some of the electric screening associated with a non-transition metal solute takes place in the surrounding transition metal slovent. Additional electrons introduced in this way into the solvent occupy mainly d states and cause a reduction in magnetic properties. This reduction together with the total loss of d-state effects from the solute sites themselves can account qualitatively for the changes observed in Ni, Pd and Fe-based alloys with non-transition elements. The fourth section deals with the transition metal alloys which show marked departures from Slater-Pauling behaviour, e.g. NiCr. An explanation for these alloys has been provided by Friedel's bound impurity state model and the mechanism suggested by Comly, Holden and Low to account for the similarity in shape of the magnetic disturbances observed in different systems. The final section discusses ferromagnetic alloys of PdFe and PdCo. The giant moments associated with the Fe and Co solutes result from a widespread polarization of the Pd solvent contiguous to the solute atoms. This polarization can be interpreted with the use of a non-local exchange-enhanced susceptibility function for the Pd host. With increasing solute content this function becomes modified to an extent dependent on the shift of d holes from one spin direction to the other, i.e. on the mean polarization of the Pd.     

Study of the single-particle energy spectrum of disordered systems: III. The two-band case

A previous investigation of the energy spectrum of strongly disordered systems with the help of the recursion method is generalized to the two-band case. The real atomic structure of the considered systems is taken into consideration. The presence of short-range order within the system is shown to be the reason for the existence of extended states and of mobility edges. Typical fields of application are doped crystalline and amorphous Silicon as well as Germanium. For the two-band case a number of results is obtained: Mobility edges and their physical interpretation, localized and extended states as well as the structural conditions for their coexistence, analytical representation of the local density of states and of the state vectors, resonances and the general form of the total density of states (TDS) in the complete energy region. The spectral strength as function of the energy is shown to be discontinuous at the mobility edges. The TDS has a square root behaviour on both sides of the mobility edges what leads to a pseudogap of vanishing width at these points. This edge behaviour is shown to be related with the divergency the localization length of a state has as the state energy tends to the mobility edges. Model calculations are carried out.     

Electronic structure and stability of binary transition metal compounds

It is shown that the ordered structures occurring in binary substitutional transition metal alloys on simple lattices (f.c.c., b.c.c.) can systematically be obtained from the electronic structure of the disordered alloys. Generalized structural maps are introduced; their physical meaning and their relation to the phenomenological structural maps used previously by other authors are discussed.     

Extended potentials in the CPA: A model calculation

We show how the usual Coherent Potential Approximation for disordered substitutional alloys may be generalized to situations where the atomic potentials are not confined to one site. We present numerical results for the simplest one-dimensional case, and discuss the density of states and the localization of electrons in the impurity band.     

Energy Spectrum and Electron Density of States in 3D-Transition Metals and Alloys

Russian Physics Journal - Based on a generalized two-band Hamiltonian, the paper studies the formation of magnetic moment, energy spectrum and d-electron density of states of transition metals. It...     

Isomer shift in binary iron alloys

The isomer shift of binary iron alloys is related to the itineracy of the valence electrons. The problem of a volume correction in disordered alloys is discussed and further avoided by using data of intermetallic compounds. From a large selection of possible parameters a linear combination of the electronegativityφ ^* and the electron densityn _ws in the Wigner-Seitz cell seems to be a good choice for fitting the data. The problem that these parameters are of minor importance for the cohesion in transition metal alloys is discussed.     

ON LOCALIZATION IN DISORDERED ELECTRON SYSTEMS

The field theory of Anderson localization of disordered electron systems is formulated in the framework of closed time path Green's functions (CTPGF). The properties of the mobility edge are studied by the renormalization group method and the density of states is found to be zero at the mobility edge.     

On the possible mechanism of structural transition in cuprates

A possible mechanism of tetragonal to orthorhombic transition in high-T_c cuprates based on the removal of orbital degeneracy of p states in the CuO₂ cell by electron lattice interaction is proposed. Spontaneous distortion creates a finite energy gap or a pseudogap in the density of states depending on the relative strength of the next-near and nearest neighbour hopping strengths. The gap is a function of electron density and vanishes beyond the structural transition temperature. The growth of the gap leads to a metal semiconductor transition as temperature decreases with attendant stripe and orbital ordering. The phase diagram for the distorted phase is examined in detail in the parameter space.     

Complex dynamics induced by strong confinement – From tracer diffusion in strongly heterogeneous media to glassy relaxation of dense fluids in narrow slits

We provide an overview of recent advances of the complex dynamics of particles in strong confinements. The first paradigm is the Lorentz model where tracers explore a quenched disordered host structure. Such systems naturally occur as limiting cases of binary glass-forming systems if the dynamics of one component is much faster than the other. For a certain critical density of the host structure the tracers undergo a localization transition which constitutes a critical phenomenon. A series of predictions in the vicinity of the transition have been elaborated and tested versus computer simulations. Analytical progress is achieved for small obstacle densities. The second paradigm is a dense strongly interacting liquid confined to a narrow slab. Then the glass transition depends nonmonotonically on the separation of the plates due to an interplay of local packing and layering. Very small slab widths allow to address certain features of the statics and dynamics analytically.     

Electronic localization in disordered systems

A brief review is given of the current understanding of the electronic structure, transport properties and the nature of the electronic states in disordered systems. A simple explanation for the observed exponential behaviour in the density of states (Urbach tails) based on short-range Gaussian fluctuations is presented. The theory of Anderson localization in a disordered system is reviewed. Basic concepts, and the physics underlying the effects of weak localization, are discussed. The scaling as well as the self-consistent theory of localization are briefly reviewed. It is then argued that the problem of localization in a random potential within the so-called ladder approximation is formally equivalent to the problem of finding a bound state in a shallow potential well. Therefore all states are exponentially localized in d=1 and d=2. The fractal nature of the states is also discussed. Scaling properties in highly anisotropic systems are also discussed. A brief presentation of the recently observed metal-to-insulator transition in dequals;2 is given and, finally, a few remarks about interaction effects in disordered systems are presented.