Showing papers on "Mott transition published in 1991"

VAN HOVE SINGULARITIES AND HIGH-Tc SUPERCONDUCTIVITY: A REVIEW

Abstract: This review describes a Fermi liquid picture of high-Tc superconductivity. A density-of-states (dos) peak associated with the CuO2-plane van Hove singularity causes a peak in Tc as a function of hole doping. Strong correlation effects drive a Mott transition at half filling. For intermediate doping, the electronic system is unstable against phase separation, with one phase near the insulating state, the other near the Tc peak. The large dos leads to competition between superconductivity and structural instability, in analogy with the A15 compounds. The superconductivity appears to be driven by strong electron-phonon coupling, enhanced by fluctuation effects.

Variational wave functions and the Mott transition.

Abstract: On montre qu'une grande classe des fonctions d'onde de Gutzwiller generalisees, que l'on croyait anterieurement etre utiles a l'etude de la transition metal-isolant de Mott, sont toujours metalliques dans ce sens que la reponse a un champ electrique inclut une fonction δ a la frequence zero (correspondant a une acceleration libre des porteurs dans un champ-dc). On montre egalement que la modification de ces fonctions d'onde pour renforcer la liaison des sites inoccupes et doublement occupes est insuffisante pour produire le comportement isolant. Deduction d'un critere que doit satisfaire une fonction d'onde isolante

Magnetism in rare-earth metals and rare-earth intermetallic compounds

Abstract: Some of our recent local spin density electronic structure calculations for a number of ferromagnetic rare-earth systems are reviewed. A simplified model of the level densities for rare-earth (R) transition metal (M) intermetallic compounds, RmMn, is used to describe in a simple way the main features of their basic electronic structure. Explicit calculations for LuFe2 and RFe2 (R = Gd-Yb) systems are presented, where a method to treat simultaneously the localized 4f and the conduction electron spin magnetism is introduced. Thereby it becomes possible to calculate the KRM exchange coupling constant. This method is also used to study theoretically the permanent magnet material Nd2Fe14B. The electronic structure of the anomolous ferromagnets CeFe2 and CeCo5 is discussed and an induced 4f itinerant magnetism is predicted. The γ-α transition in cerium metal is considered, and result from calculations including orbital polarization are presented, where a volume collapse of 10% is obtained. On one side of the transition the 4f electrons are calculated to be essentially non-bonding (localized) and on the other side they are found to contribute to the metallic bonding and this difference in behaviour gives rise to the volume collapse. Recent calculations by Wills, Eriksson and Boring (Ref. [5]) for the crystal structure changes in cerium metal under high pressure are discussed. Their successful results imply an itinerant picture for the 4f electrons in α-cerium. Consequently this strongly supports the view that the γ-α phase transformation is caused by a Mott transition of the 4f electrons.

Pressure-induced Mott transition in transition-metal iodides (invited)

Abstract: Many of the transition‐metal (TM) compounds, because of exchange and correlation interactions within the narrow and poorly overlapping d bands, become antiferromagnetic insulators, the Mott insulators (MI). The properties of the MI and their gradual transition into the noncorrelated metallic state (the Mott transition) are of crucial importance for the elucidation of high‐temperature superconducting materials features in particular and to magnetism in general. The transition of the MI into a metal can be achieved either by doping or by high pressure. The first method is definitely inappropriate for studying the nature of the Mott transition; for the narrow‐band materials the electronic and structural disorder inherent in doping has a strongly perturbing effect. To yield the definitive data on the Mott transition, high‐pressure work on well‐characterized materials should be sought. The present studies provide for the first time extensive information on the Mott–Hubbard gap closure induced by high pressure....

On the theory of the mott transition in the paramagnetic phase

Abstract: A critical consideration of some approaches to the metal-insulator transition problem, starting from the many-electron representation, is carried out within the framework of the Hubbard and classical s-d models in the far-paramagnetic region. The analytical properties of the corresponding one-electron Green functions are discussed, the importance of terms of sufficiently high orders in l/z being demonstrated. The total energy, electronic specific heat and corrections to the local moment are calculated. The Hubbard-III approximation in the Hubbard model (but not in the s-d model) is shown to lead to difficulties when calculating thermodynamic properties.

Nonmetallic behavior of Cs/GaAs(110)

Abstract: Adsorption of cesium on a GaAs(110) surface is studied, as a function of coverage, using the first-principles linearized augmented-plane-waves-calculation method and a jellium-slab model. For submonolayer coverage, cesium electrons are polarized to the interface by the surface states and surface resonances and do not undergo a Mott transition up to one monolayer. Compared with alkali-metal atoms on metal surfaces, the chemisorption bond for Cs/GaAs(110) goes into a much deeper region of the substrate due to less screening in the semiconductor, leading to substantial electron transfer to the substrate. The coverage dependence of the work function is also obtained and is in accordance with experimental measurements. Furthermore, our results support the idea that metal-induced gap states are responsible for the Fermi-level pinning. Comparison of various properties with Cs/W(100) are made and the essential differences between the semiconductor and metal surfaces are discussed.

Mass enhancement close to a Mott transition

Abstract: Etude de la solution de Gutzwiller du modele de Hubbard a bande a demi-remplie On identifie l'exposant dynamique implicite dans cette solution On relie les exposants caracterisant le comportement critique de la masse effective, susceptibilite et compressibilite pres de la transition metal-isolant a ceux d'une transition de phase continue a temperature nulle

Orbitals, correlation and valencies in high-Tc superconductors

Abstract: A survey of certain properties of high- T c superconductors is given in connection with details of their electronic structure such as the kind of orbitals involved and the degree of correlation. Special attention is given to the properties of cuprates at high doping level. The question of the existence of a ‘Mott transition’ at high electron or hole concentration is addressed. We also discuss physical factors (d-p Coulomb interaction, orbital mixing) leading to the partial occupation of the copper d z 2 orbital. In particular, we show that in the localized picture ( x 2 − y 2 ) and z 2 levels in La 2- x Sr x CuO 4 may cross at x ≈ 0.4. This crossing may be responsible for a marked change of many properties at this doping. The possible role of the z 2 electrons in pairing is discussed in connection with some recent experiments.

Study of Γ, L, and X donor states in Si‐doped AlGaAs by pressure dependent Hall measurements

Abstract: Using variable pressure for tuning the band structure and variable infrared light pulse intensity for tuning the number of Si‐related shallow donor metastable states, we have demonstrated that these states are effective mass‐like states linked to each of the conduction band Γ, L, and X minimum. The evolution of their binding energies with their increasing density is shown to decrease and the Mott transition for the Γ‐like states has been also observed.

Coexistence of Mott and Peierls Instabilities in Quasi-One-Dimensional Organic Conductors

Abstract: The quasi-one-dimensional conductors have so far being studied for a long period (see Review [1]). Also these systems are now of interest for both theoreticians and experimentators (see Review [2]). This interest, on the one hand, is due to advances in synthesis of polyacetylene (PA) polydiacetylene (PDA), organic crystalline conductors based on6 molecular donors and acceptor of electron [2]. On the other hand, one-dimensional (1-d) conductors are nontrivial systems. Thus, 1-d metal is unstable to the transition in semi-conducting state. As a result, the simple 1-d metal with half-filled conduction band becomes the Mott semi-conductor or Peierls semi-conductor [1, 2]. The Peierls transition leads to dimerization - or bond length alternation - of the uniform 1-d lattice and semi-conducting energy gap is proportional to the dimerization amplitude. The Mott transition is a result of electron correlation and energy gap in the Mott semi-conductor vanishes with decreasing electron-electron interaction strength (see Refs. [1, 2] and references there-in). The semi-conductor of the Mott and Peierls type possesses some properties of interest. The Mott semi-conductors axe characterized by antiferromagnetic structures [2, 3]. In the Peierls semi-conductors the kink-type excitations are possible [4, 5].

Mott transition in an exactly solvable k.s.s.h. model

Abstract: The solution of the K.S.S.H.-like model shown to be exactly solvable in any number of dimensions, for a particular choice of the coupling constant describing the hopping process amplitude, both for finite size and in the thermodynamic limit, is discussed in detail. The analysis of the zero-temperature phase space in d = 2 shows that the model exhibits a transition in the number of doubly occupied sites order parameter, which at half-filling coincides with the Mott transition found for the Hubbard model in the Gutzwiller approximation.

The pressure-induced Mott Transition in transition-metal iodides

Abstract: Many of the Transition Metal (TM) compounds, because of exchange and correlation interactions within the narrow and poorly overlapping d-bands, become antiferromagnetic insulators, the Mott Insulators (MI). The properties of the MI and their gradual transition into the non-correlated metallic state (the Mott Transition) are of crucial important for the elucidation of HTS materials features in particular and to magnetism in general. The transition of the MI into a metal can be achieved either by doping or by high pressure. To yield the definitive data on the Mott transition high pressure work on well characterized materials should be sought. The present studies provide for the first time extensive information on the Mott-Hubbard gap closure induced by high pressure. High pressure studies using Diamond Anvil Cells were conducted in several (TM)I{sub 2} compounds. They all have layered structures and other antiferromagnetically at ambient pressure. {sup 129}I Moessbauer Spectroscopy (MS) was used to study the properties of the (TM){sup 2+} sublattice magnetization as a function of pressure and temperature, and X-ray diffraction was used to look for possible crystallographic transitions and to obtain the equation of state. Results show that the high pressure transition at P{sub c} from a magnetic to amore » non-magnetic state is not accompanied by crystallographic changes. Previous studies{sup 1} with NiI{sub 2} have confirmed the presence of a metallic state at P > P{sub c}. Inherent to the pressure behavior of the magnetic state is the gradual increase of T{sub N} in all cases and a slight increase in the TM{sup 2+} moments with pressure increase. The collapse of the magnetic state is abrupt for some cases (NiI{sub 2}) and gradual for others (CoI{sub 2}), indicative of different band-overlap mechanisms. 2 refs., 5 figs.« less

The Application of the Mössbauer Effect for Probing Electronic Properties of the Pressure-Induced Mott Transition

Abstract: The problem of the Mott insulator1 and its transition into a metallic state (the Mott transition) is considered to be one of the most serious challenges to the prevailing concepts of solid state physics. At present it remains an unsolved problem. The subject of Mott insulators began in 1937 when De Boer and Verwey2 presented their experimental results on the electrical conductivity of transition-metal (TM) oxides (the oxides of Ni, Co, Mn and Fe). The fact that the majority of these oxides were insulators did not fit the conventional Bloch-Wilson band picture. Assuming the compounds were highly ionic would imply partially filled 3d bands and therefore be metallic! In discussion that followed Peierls suggested that the Coulomb repulsion was responsible for the 3d-electron localization. The TM-oxides such as NiO, CoO and MnO are classic examples of Mott insulators (MI). The phenomenological aspects of a MI can be described as follows: It is an antiferromagnetic insulator whose local moments persist unchanged above TN.

The Van Hove Singularity and High-Tc Superconductivity: The Role of (Nanoscopic) Disorder

Abstract: In any two-dimensional band structure, there must be a saddle-point at which the bands intersect the Brillouin zone boundary and the conduction crosses over from hole-like to electron-like. At this van-Hove singularity (vHs), the density-of-states (dos) has a logarithmic singularity. It has often been suggested that this enhanced dos is responsible for high-T c superconductivity. Theories of the CuO2plane vHs can be separated into two groups: in the earlier theories1-3 the vHs is assumed to occur exactly at half-filling of the band (square Fermi surface); in this case, there is direct competition with the Mott (or charge transfer insulator) transition, and the optimum T c occurs away from the vHs. In the other group of theories4-8 the vHs is assumed to occur away from half-filling, as found in band-structure calculations, and hence well separated from the Mott transition. The optimum T c occurs exactly at the hole filling appropriate to the vHs. Extensive reviews of these vHs models have recently appeared3,8. It is the latter category of vHs models which is the topic of this paper.