Showing papers on "Mott transition published in 2000"

Electrical switching and Mott transition in VO2

Abstract: In this paper the problem of the Mott metal-insulator transition in vanadium dioxide driven by an external electric field is considered. Delay time (td) measurements have shown that the experimental value of td is almost three orders of magnitude lower than the theoretical value, calculated in a simple electrothermal model. This suggests that under non-equilibrium conditions (in high electric fields) electron correlation effects contribute to the development of the insulator to metal transition. The extra-carrier injection from Si into VO2 was carried out in the structures Si-SiO2-VO2 on p-type silicon with ρ = 0.1 Ω cm and a SiO2 thickness 70 nm. It has been shown that the metal-insulator transition in VO2 can be initiated by injection, i.e. by the increase of the electron density. The value of the critical density was found to be of the order of the electron density in VO2 in the semiconducting phase, approximately 1018-1019 cm-3. This confirms that the metal-insulator transition in VO2 is the purely electronic Mott-Hubbard transition.

Electronic structure of Mott insulators studied by inelastic X-ray scattering

Abstract: The electronic structure of Mott insulators continues to be a major unsolved problem in physics despite more than 50 years of research. Well-developed momentum-resolved spectroscopies such as photoemission or neutron scattering cannot probe the full Mott gap. High-resolution resonant inelastic x-ray scattering revealed dispersive charge excitations across the Mott gap in a high–critical temperature parent cuprate (Ca2CuO2Cl2), shedding light on the anisotropy of the Mott gap. These charge excitations across the Mott gap can be described within the framework of the Hubbard model.

Landau theory of the finite temperature mott transition

Abstract: In the context of the dynamical mean-field theory of the Hubbard model, we identify microscopically an order parameter for the finite temperature Mott end point. We derive a Landau functional of the order parameter. We then use the order parameter theory to elucidate the singular behavior of various physical quantities which are experimentally accessible.

Switching of magnetic coupling by a structural symmetry change near the Mott transition in Ca 2 − x Sr x RuO 4

Abstract: We studied the structural, magnetic, and transport properties of the quasi-two-dimensional Mott transition system ${\mathrm{Ca}}_{2\ensuremath{-}x}{\mathrm{Sr}}_{x}{\mathrm{RuO}}_{4}.$ In the vicinity of the metal-nonmetal (M-NM) transition at $x\ensuremath{\simeq}0.2,$ we found a structural transition accompanied by a structural symmetry change with the instability point at ${x}_{\mathrm{c}}\ensuremath{\simeq}0.5.$ The critical change across the structural transition in the temperature dependence of the susceptibility indicates a crossover of the metallic state, most likely from the nearly antiferromagnetic state next to the M-NM transition to the nearly ferromagnetic state around ${x}_{\mathrm{c}}.$ The latter evolves into the spin-triplet superconductor ${\mathrm{Sr}}_{2}{\mathrm{RuO}}_{4}$ $(x=2)$ with enhanced paramagnetism. We argue that the competition between the structural instability and the ferromagnetism results in such a structural symmetry change with orbital degeneracy lifting, which induces the switching of magnetic coupling. In addition, a changeover from metallic to nonmetallic behavior was observed across the structural transition in the out-of-plane resistivity, which reveals highly anisotropic transport due to the quasi-two-dimensional electronic structure.

Conductive Copper Salts of 2,5-Disubstituted N,N′-Dicyanobenzoquinonediimines (DCNQIs): Structural and Physical Properties

Abstract: Structural and physical properties of the Cu salts of a series of π-acceptors N,N′-dicyanobenzoquinonediimines (DCNQIs) are described. The most notable feature of this system is that 3d electrons in Cu interact with pπ electrons in DCNQI near the Fermi level. This unique feature has provided a lot of interesting solid state properties: the Mott transition triggered by the Peierls transition, the pressure-induced metal-insulator transition, the metal-insulator-metal (reentrant) transition, the three-dimensional Fermi surface, the anomalous isotope effects, the antiferromagnetic transition, the weak ferromagnetism, and the electron mass enhancement. The aim of this account is to give an overview of this unique pπ-d system.

Effect of long range coulomb interactions on the mott transition

Abstract: We reconsider the Mott transition problem in the presence of long range Coulomb interactions. Using an extended dynamical mean field theory (DMFT) that sums an important class of diagrams absent in ordinary DMFT, we show that in the presence of Coulomb interactions, the zero temperature Mott transition is, as envisioned by Mott, discontinuous in two and three dimensions.

Dielectric breakdown of one-dimensional Mott insulators Sr 2 CuO 3 and SrCuO 2

Abstract: Dielectric breakdown phenomena accompanied by characteristic time delay and subsequent negative differential resistance effect have been observed for typical one-dimensional (1D) Mott insulators, ${\mathrm{Sr}}_{2}{\mathrm{CuO}}_{3}$ and ${\mathrm{SrCuO}}_{2}.$ In the both compounds, the threshold electric-field for the breakdown, defined as a field above which the differential resistance becomes negative, has been found to increase exponentially with decreasing temperature. This result is similar to the case of charge-density-wave depinning, and indicates the collective nature of charge dynamics in these 1D systems. The electric-field dependence of the delay time is analyzed in terms of a phenomenological model.

Room-temperature electronic phase transitions in the continuous phase diagrams of perovskite manganites

Abstract: Highly correlated electronic systems—such as transition-metal oxides that are doped Mott insulators—are complex systems which exhibit puzzling phenomena, including high-temperature superconductivity and colossal magnetoresistivity Recent studies1,2,3 suggest that in such systems collective electronic phenomena are important, arising from long-range Coulomb interactions and magnetic effects The qualitative behaviour of these systems is strongly dependent on charge filling (the level of doping) and the lattice constant Here we report a time-efficient and systematic experimental approach for studying the phase diagrams of condensed-matter systems It involves the continuous mapping of the physical properties of epitaxial thin films of perovskite manganites (a class of doped Mott insulator) as their composition is varied We discover evidence that suggests the presence of phase boundaries of electronic origin at room temperature

Spin in a fluctuating field: The Bose(+Fermi) Kondo models

Abstract: I consider models with an impurity spin coupled to a fluctuating Gaussian field with or without additional Kondo coupling of the conventional sort. In the case of isotropic fluctuations, the renormalization-group flows for these models have controlled fixed points when the autocorrelation of the Gaussian field $h(t),$ $〈\mathrm{Th}(t)h(0)〉\ensuremath{\sim}{1/t}^{2\ensuremath{-}\ensuremath{\epsilon}}$ with small positive $\ensuremath{\epsilon}.$ In the absence of any additional Kondo coupling, I get power-law decay of spin correlators, $〈\mathrm{TS}(t)S(0)〉\ensuremath{\sim}{1/t}^{\ensuremath{\epsilon}}.$ For negative $\ensuremath{\epsilon},$ the spin autocorrelation is constant in long-time limit. The results agree with calculations in Schwinger Boson mean-field theory. In presence of a Kondo coupling to itinerant electrons, the model shows a phase transition from a Kondo phase to a field fluctuation dominated phase. These models are good starting points for understanding behavior of impurities in a system near a zero-temperature magnetic transition. They are also useful for understanding the dynamical local mean-field theory of Kondo lattice with Heisenberg (spin-glass-type) magnetic interactions as well as for understanding spin-fluid solutions near Mott transition in $t\ensuremath{-}J$ model.

Many-body renormalization of semiconductor quantum wire excitons: absorption, gain, binding, and unbinding.

Abstract: We consider theoretically the formation and stability of quasi-one-dimensional many-body excitons in GaAs quantum wire structures under external photoexcitation conditions by solving the dynamically screened Bethe-Salpeter equation for realistic Coulomb interaction. In agreement with several recent experimental findings the calculated excitonic peak shows weak carrier-density dependence up to (and even above) the Mott transition density, nc approximately 3 x 10(5) cm(-1). Above nc we find considerable optical gain demonstrating compellingly the possibility of a one-dimensional quantum wire laser operation.

Calculation of photoemission spectra of the doped Mott insulator \(\) using LDA+DMFT(QMC)

Abstract: The spectral properties of La1–xSrxTiO3, a doped Mott insulator with strong Coulomb correlations, are calculated with the ab initio computational scheme LDA+DMFT(QMC). It starts from the non-interacting electronic band structure as calculated by the local density approximation (LDA), and introduces the missing correlations by the dynamical mean-field theory (DMFT), using numerically exact quantum Monte-Carlo (QMC) techniques to solve the resulting self-consistent multi-band single-impurity problem. The results of the LDA+DMFT(QMC) approach for the photoemission spectra of La1–xSrxTiO3 are in good agreement with experiment and represent a considerable qualitative and quantitative improvement on standard LDA calculations.

Similarities between the hubbard and periodic anderson models at finite temperatures

Abstract: The single band Hubbard and the two band periodic Anderson Hamiltonians have traditionally been applied to rather different physical problems-the Mott transition and itinerant magnetism, and Kondo singlet formation and scattering off localized magnetic states, respectively. In this paper, we compare the magnetic and charge correlations, and spectral functions, of the two systems. We show quantitatively that they exhibit remarkably similar behavior, including a nearly identical topology of the finite temperature phase diagrams at half filling. We address potential implications of this for theories of the rare earth "volume collapse" transition.

The alpha-particle in nuclear matter

Abstract: Among the light nuclear clusters the alpha-particle is by far the strongest bound system and therefore expected to play a significant role in the dynamics of nuclei and the phases of nuclear matter. To systematically study the properties of the alpha-particle we have derived an effective four-body equation of the Alt-Grassberger-Sandhas (AGS) type that includes the dominant medium effects, i.e. self energy corrections and Pauli-blocking in a consistent way. The equation is solved utilizing the energy dependent pole expansion for the sub system amplitudes. We find that the Mott transition of an alpha-particle at rest differs from that expected from perturbation theory and occurs at approximately 1/10 of nuclear matter densities.

Extension of the Brinkman-Rice picture and the Mott transition

Abstract: In order to explain the metal-Mott-insulator transition, the Brinkman-Rice (BR) picture is extended. In the case of less than one as well as one electron per atom, the on-site Coulomb repulsion is given by U = κρ 2 U c by averaging the electron charge per atom over all atomic sites, where κ is the correlation strength of U, p is the band filling factor, and U c is the critical on-site Coulomb energy. The effective mass of a quasiparticle is found to be m*/m = 1/1-x 2 p 4 for 0 < κρ 2 < 1 and seems to follow the heat capacity data of Sr 1-x La x TiO 3 and YBa 2 Cu 3 O 7-δ at κ = 1 and 0 < κρ 2 < 1. The Mott transition of the first order occurs at κρ 2 = 1 and a band-type metal-insulator transition takes place at κρ 2 = 0. This Mott transition is compared with that in the d = oo Hubbard model.

Variational Scheme for the Mott Transition

Abstract: The Hubbard model is studied at half filling, using two complementary variational wave functions, the Gutzwiller ansatz for the metallic phase at small values of the interaction parameter U and its analog for the insulating phase at large values of U. The metallic phase is characterized by the Drude weight, which exhibits a jump at the critical point Uc. In the insulating phase the system behaves as a collection of dipoles which increase both in number and in size as U gets smaller. The two wave functions are able to describe the two asymptotic regimes (small and large values of U, respectively), but they can no longer be trusted in the region of the Mott transition (U≈Uc). More powerful methods are needed to study, for instance, the divergence of the electric susceptibility for U→Uc.

Semiconductor–Metal Transition in Many‐Valley Semiconductors

Abstract: The common conclusion that the Thomas-Fermi approximation for screening leading to a poor estimate of the critical donor concentration for a metal-insulator transition in many-valley semiconductors is critically examined. The many-body effects are introduced in a simple way, partly in the kinetic energy term as a mass renormalization in the spirit of the Fermi liquid theory and partly in the potential energy term when random distribution of impurities leading to Anderson localization is considered. In the absence of localization we obtain a*N c 1/3 = 0.36v -1/3 for the value of Mott constant, where v refers to the number of equivalent conduction band minima and a * is the orbit size of the donor electron in the semiconductor. When an impurity distribution is included we lose a simple expression as given above; instead we get an expression for the energy containing two parameters, the mass in the kinetic energy and the distribution parameter a in the potential energy. The mass is fixed choosing the ionization energy of an isocoric impurity in the low doping regime, and the value of a is fixed demanding the vanishing of the donor binding energy. Thus we are able to account for the variation of the binding energy with the donor concentration for several donors in Si and Ge. Our results are compared with the existing data, and the agreement is good.

Extension of the Brinkman-Rice picture and the Mott transition

Abstract: In order to explain the metal-Mott-insulator transition, the Brinkman-Rice (BR) picture is extended. In the case of less than one as well as one electron per atom, the on-site Coulomb repulsion is given by U={kappa}{rho}^2U_c by averaging the electron charge per atom over all atomic sites, where {kappa} is the correlation strength of U, {rho} is the band filling factor, and U_c is the critical on-site Coulomb energy. The effective mass of a quasiparticle is found to be m*/m=1/{1-{kappa}^2{rho}^4} for 0<{kappa}{rho}^2<1 and seems to follow the heat capacity data of Sr_{1-x}La_xTiO_3 and YBa_2Cu_3O_{7-delta} at {kappa}=1 and 0<{kappa}{rho}^2<1. The Mott transition of the first order occurs at {kappa}{rho}^2=1 and a band-type metal-insulator transition takes place at {kappa}{rho}^2=0. This Mott transition is compared with that in the d=infinity Hubbard model. The effective mass for 2D-DOS instead of the vHs can be used for the mechanism of high T_c superconductivity.

The Mott Transition Field Effect Transistor: A Nanodevice?

Abstract: A field effect transistor device (FET), consisting of a nonlinear Mott Insulator channel material, and a high dielectric-constant gate oxide, is explored as a nanoscale device. Experimental functionality of a large scale prototype (5 μm channel length) has been demonstrated. The underlying physics of the device is analyzed and modeled using a time-dependent Hartree approach. Timing estimates suggest a relatively short switching time.

Born effective charge reversal and metallic threshold state at a band insulator-Mott insulator transition

Abstract: We study the quantum phase transition between a band (“ionic”) insulator and a Mott-Hubbard insulator, realized at a critical value in a bipartite Hubbard model with two inequivalent sites, whose on-site energies differ by an offset . The study is carried out both in D=1 and D=2 (square and honeycomb lattices), using exact Lanczos diagonalization, finite-size scaling, and Berry's phase calculations of the polarization. The Born effective charge jump from positive infinity to negative infinity previously discovered in D=1 by Resta and Sorella is confirmed to be directly connected with the transition from the band insulator to the Mott insulating state, in agreement with recent work of Ortiz et al. In addition, symmetry is analysed, and the transition is found to be associated with a reversal of inversion symmetry in the ground state, of magnetic origin. We also study the D=1 excitation spectrum by Lanczos diagonalization and finite-size scaling. Not only the spin gap closes at the transition, consistent with the magnetic nature of the Mott state, but also the charge gap closes, so that the intermediate state between the two insulators appears to be metallic. This finding, rationalized within Hartree-Fock as due to a sign change of the effective on-site energy offset for the minority spin electrons, underlines the profound difference between the two insulators. The band-to-Mott insulator transition is also studied and found in the same model in D=2. There too we find an associated, although weaker, polarization anomaly, with some differences between square and honeycomb lattices. The honeycomb lattice, which does not possess an inversion symmetry, is used to demonstrate the possibility of an inverted piezoelectric effect in this kind of ionic Mott insulator.

Mott transition of the f-electron system in the periodic Anderson model with nearest neighbor hybridization

Abstract: We show analytically that, under certain assumptions, the periodic Anderson model and the Hubbard model become equivalent within the dynamical mean field theory for quasiparticle weight Z → 0. A scaling relation is derived which is validated numerically using the numerical renormalization group at zero temperature and quantum Monte Carlo simulations at finite temperatures. Our results show that the ƒ-electrons of the half-filled periodic Anderson model with nearest neighbor hybridization get localized at a finite critical interaction strength U c, also at zero temperature. This transition is equivalent to the Mott-transition in the Hubbard model.

Low-field magnetic anisotropy in Mott-insulating ferromagnet Y1−xCaxTiO3 (x⩽0.1)

Abstract: YTiO3 has been known as a Mott-insulating ferromagnet with the Curie temperature Tc=30 K. We have measured the magnetization of single crystals of Y1−xCaxTiO3 ( x=0, 0.01, 0.02, 0.05, 0.10 ) grown by a floating-zone method. In YTiO3, a metamagnetic-like transition was observed for B||a and B||c (easy axis) at 1.5 and 7.5 mT, respectively, only after zero-field cooling. For B||b (hard axis), however, no metamagnetic behavior appears. From the magnetization measurements of alloys, it is found that the anisotropy in magnetization for B||a and B||c vanishes with increasing x up to 0.1.

First order valence transition in YbInCu_4 in the (B,T) - plane

Abstract: The puzzling properties of the first order phase transition in YbInCu$_4$ and its alloys in the wide range of magnetic fields and temperatures are perfectly described in terms of a simple entropy transition for free Yb ions. In particular, it turns out that the transition line in the $(B,T)$-plane is very close to the elliptic shape, as it has been observed experimentally. Similar calculations are done, and the experiments are proposed for the $(\gamma{-}\alpha)$ phase transition in Ce in Megagauss fields. We speculate, that in case of YbInCu$_4$ the first order transition is a Mott transition between a higher temperature phase in which localized moments are stabilized by the entropy terms in the free energy, and a band-like non-magnetic ground state of the $f$-electrons.

First-order valence transition in YbInCu4 in the (B,T)-plane

Abstract: The puzzling properties of the first-order phase transition in YbInCu4 and its alloys in the wide range of magnetic fields and temperatures are perfectly described in terms of a simple entropy transition for free Yb ions. In particular, it turns out that the transition line in the (B,T)-plane is very close to the elliptic shape, as it has been observed experimentally. Similar calculations are done, and the experiments are proposed for the (γ-α) phase transition in Ce in Megagauss fields. We speculate, that in the case of YbInCu4 the first-order transition is a Mott transition between a higher temperature phase in which localized moments are stabilized by the entropy terms in the free energy, and a band-like non-magnetic ground state of the f-electrons.

Representative Conducting Oxides

Abstract: In this chapter, various representative oxides will be discussed in detail to present useful ideas on their electronic transport phenomena. They are representative by virtue of the following characteristic features: ReO3 (Sect. 4.1): The structure is simple cubic and it shows the highest conductivity in the normal oxides. The conduction band is a simple de — O2p type. SnO2 and TiO2 (Sect. 4.2): SnO2 is sometimes called a transparent metal and it is a broad s — p band semiconductor. TiO2 has the same lattice structure but its electron-phonon interaction is large and it is often disputed whether the electrons form large polarons or localized small polarons. LiTi2O4 and LiV2O4 (Sect. 4.3): LiTi2O4 may be considered as a heavily doped TiO2. When the polarons condence in such a substance with a strong electron-phonon interaction, superconductivity appears, and until the discovery of Cu-oxides, its critical temperature of 13.7 K was the highest among the oxides. In metallic LiV2O4, a localized moment appears, in contrast to LiTi2O4. WO3 and M x W03 (Sect.4.4): The carriers may be large polarons in WO3. They are heavily doped in M x WO3 where the M ions distribute randomly and there a metal-insulator transition occurs at certain carrier concentrations. Percolation theory will be useful here. M x V2O5and MMMoO3 (Sect. 4.5): These are low dimensional substances. The former is quasi-one dimensional and the carriers may be small polarons. A bipolaron state has been reported. Mo-bronzes form various low dimensional lattices and charge density waves, CDWs, have been observed. NiO (Sect. 4.6): NiO is an insulator while the simple Hartree—Fock mean field theory predicts that it should be metallic. In this material, the localized nature of the electrons is strong due to the strong electron correlation and many investigations have been carried out to elucidate a “hopping” conduction. However, the nature of the electrons is not yet clear. V2O3 (Sect. 4.7): This shows two metal-insulator transitions. The higher temperature one may be the Mott transition with the metallic phase at the lower temperature side. Below the lower transition temperature, the crystal becomes antiferromagnetic and insulating, accompanied by lattice distortion. Fe3O4 (Sect. 4.8): This is ferrimagnetic below 860 K and shows a transition at 123 K with a jump in the electrical conductivity, which was ascribed to the order-disorder transition of Fe2+ and Fe3+. Many results have been accumulated on the nature of correlated polarons in this fluctuating-valence-material. EuO (Sect. 4.9): This is a ferromagnetic NaC1 type oxide. The reduced material shows the metallic conductivity below the Curie temperature and the conductivity jump there is order 1013. The MIT is due to the magnetic interaction between the localized 4 f magnetic moments and the propagating electrons. High T c Cu-Oxides (Sect. 4.10): These are d γ conductors whereas most of the metallic oxides are de conductors. CuO2 planes constitute a multilayer structure and the superconducting transition temperature increases with the layer number at least up to four and is higher than 120 K.

Impact of magnetic frustration on the Mott transition within a slave-boson mean-field theory

Abstract: We investigate the paramagnetic-metal-to-antiferromagnetic-metal and antiferromagnetic-metal-to- antiferromagnetic-insulator transitions using a slave-boson mean-field theory. To this effect, we discuss the ground state of the half-filled Hubbard model as a function of t'/t and correlation strength U, where t and t' are the hopping amplitudes between nearest and next-nearest neighbors, respectively. The metal-insulator transition at a critical U_{MIT} is of second order for small levels of magnetic frustration, t'/t 0.06. The insulator is always antiferromagnetically ordered, while the metal exhibits a second-order transition from a paramagnetic to an antiferromagnetic state up to t'/t=0.14, as U is increased. We also contrast these findings with what we obtain in Hartree-Fock approximation.

Structural and electronic relationships between the lanthanide and actinide elements

Abstract: The similarity and difference between the solid state properties of the 4f and 5f transition metals are pointed out. The heavier 5f elements show properties which have direct correspondence to the early 4f transition metals, suggesting a localized behaviour of the 5f electrons for those metals. On the other hand, the fact that Pu metal has a 30% lower volume than its neighbour heavier element, Am, suggests a tremendous difference in the properties of the 5f electrons for this element relative to the heavier actinides. This change in behaviour between Pu and Am can be viewed as a Mott transition within the 5f shell as a function of the atomic number Z. On the metallic 5f side of the Mott transition (i.e., early actinides), the elements show most unusual crystal structures, the common feature being their low symmetry. An analogous behaviour for the lanthanides is found in cerium metal under compression, where structures typical for the light actinides have been observed experimentally. A generalized phase diagram for the actinides is shown to contain features comparable to the individual phase diagram of Ce metal. The crystal structure behaviour of the lanthanides and heavier actinides is determined by the number of 5d (or 6d) electrons in the metallic state, since for these elements the f electrons are localized and nonbonding. For the earlier actinide metals electronic structure calculations - where the 5f orbitals are treated as part of the valence bands - account very well for the observed ground state crystal structures. The distorted structures can be understood as Peierls distortions away from the symmetric bcc structure and originate from strongly bonding 5f electrons occupying relatively narrow 5f states. High pressure is an extremely useful experimental tool to demonstrate the interrelationship between the lanthanides and the actinides.

Coexistence of solutions in dynamical mean-field theory of the Mott transition

Abstract: In this paper, I discuss the finite-temperature metal-insulator transition of the paramagnetic Hubbard model within dynamical mean-field theory. I show that coexisting solutions, the hallmark of such a transition, can be obtained in a consistent way both from Quantum Monte Carlo (QMC) simulations and from the Exact Diagonalization method. I pay special attention to discretization errors within QMC. These errors explain why it is difficult to obtain the solutions by QMC close to the boundaries of the coexistence region.

Contact Phenomena and Mott Transition in Carbon Nanotubes

Abstract: We describe the transfer of electric charge in junctionsbetween a metal and carbon nanotube as well as betweenmetallic and semiconducting carbon nanotubes. The long rangeCoulomb interaction drastically modifies the charge transferphenomena in one-dimensional nanotube systems compared toconventional semiconductor heterostructures. Being broughtinto a contact with a metal, conducting nanotube accumulateselectric charge whose density decays slowly with the distancefrom the junction. The length of the Schottky barrier innanotube heterojunctions varies from the distances of theorder of the nanotube radius (nanometers) to the distances ofthe order of the nanotube length (microns) depending on adoping strength. The Schottky barrier height shows pronouncedasymmetry under the forward and reverse bias. This results inrectifying behavior of heterojunctions, in agreement withrecent experimental observations by Z. Yao et al. andP. McEuen et al. Finally, we discuss observability of recentlypredicted Mott insulating phase in metallic carbon nanotubes.