Magnetic instability in strongly correlated superconductors

Recently, a new phenomenological Hamiltonian has been proposed to describe the superconducting cuprates. This so-called Gossamer Hamiltonian is an apt model for a superconductor with strong on-site Coulomb repulsion between the electrons. It is shown that at half-filling the Gossamer superconductor with strong repulsion is unstable toward an antiferromagnetic insulator. The superconducting state undergoes a quantum phase transition to an antiferromagnetic insulator as one increases the on-site Coulomb repulsion. Near the transition the Gossamer superconductor becomes spectroscopically indistinguishable from the insulator.     

Spin-spin correlations in the insulating and metallic phases of the Mott system V 2 O 3

The magnetic response in V ₂ O ₃ has been investigated using polarised neutron scattering with polarisation analysis. Measurements were carried out at three temperatures corresponding to the antiferromagnetic insulating ground state, the metallic phase and the high temperature metallic phase. At the first order metal insulator transition there is a dramatic change in the magnetic response with the metallic and high temperature metallic phases being characterised by ferromagnetic spatial correlations of the paramagnetic response. The establishment of ferromagnetic correlations at the metal insulator transition accounts for the abrupt jump in the uniform susceptibility. It is proposed that the differentiation of the V-V distances across the edges of VO ₆ octahedra is of critical importance for the change in electronic conductivity but also for the establishment of the spatial correlations. The gradual high temperature evolution of the conductivity then occurs by the reduction in the vanadium d overlap brought about by thermal expansion. The first order reduction in atomic volume which occurs on the establishment of the metallic phase results from an instability of the vanadium local moment arising from the change in electronic structure. Received 7 April 1999     

Insulator–metal transition in Mn-doped NaNbO3 induced by chemical and thermal treatment

The transition between insulator and metal conductor states, induced by oxygen non-stoichiometry, was studied for NaNbO₃ : Mn crystals. Conditions for an optimal reduction were determined on the basis of TGA tests. The temperature dependencies of the resistance measured on the macroscale showed that the transition from thermally activated to metallic features depends on the level of oxygen deficiency. The LC-AFM measurement exhibited non-homogeneous electric resistance on the nanoscale. We ascribed the local insulator–metal transition to changeover in the electronic state of the Nb ions occurring in filaments. The Mn dopant stabilised the induced oxygen non-stoichiometry and the metallic conduction down to room temperature.     

Possible ground states of rare-earth nickelates (RNiO3, R = La,Nd, Pr): a mean-field study

The ground states of rare-earth nickelates (RNiO₃) are antiferromagnetic insulators except LaNiO₃, which is a strongly correlated metal. Rare-earth nickelates (R ≠ La) undergo a sharp transition from a high-temperature paramagnetic metal to a low-temperature antiferromagnetic insulator at finite temperature T_MI that increases with the increase in atomic number of the rare-earth ions. The magnetic and resistive transitions are coupled for NdNiO₃ and PrNiO₃, but independent for the other members of the series. Whether the antiferromagnetic and insulating ground states of nickelates are due to charge ordering, or orbital ordering, it is a matter of current dispute. The present paper intends to explain the difference in the observed ground states of RNiO₃ compounds, by calculating their band structure and density of states. The experimentally observed ground states of nickelates have been explained on the basis of mean-field calculation and the correlation effect is incorporated in the dynamical mean-field theory thereof.     

Mott-Hubbard transition and antiferromagnetism on the honeycomb lattice

The Hubbard model is investigated for a halffilled honeycomb lattice, using a variational method. Two trial wave functions are introduced, the Gutzwiller wave function, well suited for describing the “metallic” phase at small U and a complementary wave function for the insulating regime at large values of U. The comparison of the two variational ground states at the mean-field level yields a Mott transition at U _c /t ≈ 5:3. In addition, a variational Monte Carlo calculation is performed in order to locate the instability of the “metallic” wave function with respect to antiferromagnetism. The critical value U _m/t ≈ 3:7 obtained in this way is considered to be a lower bound for the true critical point for antiferromagnetism, whereas there are good arguments that the mean-field value U _c/t ≈ 5:3 represents an upper bound for the Mott transition. Therefore the “metal”- insulator transition for the honeycomb lattice may indeed be simultaneously driven by the antiferromagnetic instability and the Mott phenomenon.     

Low temperature properties of the Kondo insulator FeSi

In this paper we study the low temperature (T) properties of the Kondo insulator FeSi within the X-boson approach. We show that the ground state of the FeSi is metallic and highly correlated with a large effective mass; the low temperature contributions to the specific heat and the resistivity are of the Fermi-liquid type. The low temperature properties are governed by a reentrant transition into a metallic state, that occurs when the chemical potential crosses the gap and enters the conduction band, generating a metallic ground state. The movement of the chemical potential is due to the strong correlations present in the system. We consider the low temperature regime of the Kondo insulator FeSi, where the hybridization gap is completely open. In this situation we identify the two characteristic temperatures: the coherence temperature T₀ and the Kondo temperature T_KL. In the range T < T₀, we identify a regime characterized by the formation of coherent states and Fermi-liquid behavior of the low temperature properties; in the range T_KL > T > T₀, we identify a regime characterized by an activation energy. Within the X-boson approach we study those low temperature regimes although we do not try to adjust parameters to recover the experimental energy scales.     

Gossamer superconductivity near antiferromagnetic Mott insulator in layered organic conductors

Layered organic superconductors are on the verge of the Mott insulator. We use the Gutzwiller variational method to study a two-dimensional Hubbard model including a spin exchange coupling term as a minimal model for the compounds. The ground state is found to be a Gossamer superconductor at small on-site Coulomb repulsion U and an antiferromagnetic Mott insulator at large U, separated by a first order phase transition. Our theory is qualitatively consistent with major experiments reported in organic superconductors.     

Suppression of Jahn-Teller distortion by chromium and magnesium doping in spinel LiMn2O4: A first-principles study using GGA and GGA+U

The effect of doping spinel LiMn₂O₄ with chromium and magnesium has been studied using the first-principles spin density functional theory (DFT) within generalized gradient approximation (GGA ) and GGA+U. We find that GGA and GGA+U give different ground states for pristine LiMn₂O₄ and same ground state for doped systems. For LiMn₂O₄, the body-centered tetragonal phase was found to be the ground-state structure using GGA and face-centered orthorhombic using GGA+U, while for LiM_0.5Mn_1.5O₄ (MCr or Mg) it was base-centered monoclinic and for LiMMnO₄ (MCr or Mg) it was body-centered orthorhombic in both GGA and GGA+U. We find that GGA predicts the pristine LiMn₂O₄ to be metallic while GGA+U predicts it to be insulating, which is in accordance with the experimental observations. For doped spinels, GGA predicts the ground state to be half metallic while GGA+U predicts it to be insulating or metallic depending on the doping concentration. GGA+U predicts insulator-metal-insulator transition as a function of doping in case of Cr and in case of Mg the ground state is found to go from insulating to a half metallic state as a function of doping. Analysis of the charge density and the density of states (DOS) suggest a charge transfer from the dopants to the neighboring oxygen atoms and manganese atoms. We have calculated the Jahn-Teller active mode displacement Q₃ for doped compounds using GGA and GGA+U. The bond lengths calculated from GGA+U are found to be in better agreement with experimental bond lengths. Based on the bond lengths of metal and oxygen, we have also estimated the average oxidation states of the dopants.     

Effect of substrate strain on lattice structure,electrical resistivity,and optical conductivity of Nd0.5Sr0.5MnO3 thin films grown on SrTiO3

In this study, we investigated the lattice structure, electrical resistivity, and optical conductivity of Nd_0.5Sr_0.5MnO₃ thin films grown on SrTiO₃ (001) and SrTiO₃ (011) substrates. The thin film on SrTiO₃ (001) experiences isotropic tensile strain and shows characteristics of the semiconducting ground state. On the other hand, the thin film on SrTiO₃ (011) experiences anisotropic tensile strain, which means that one of the two in-plane lattice axes is fixed by the substrate lattice and the other axis is relaxed. The thin film shows the insulator–metal phase transition at 220 K and characteristics of the charge-ordered insulating ground state below 150 K. By comparing the single crystal data of the lattice along with the resistivity and optical conductivity, we suggest that the substrate strain affects the electronic structure as well as the carrier dynamics of the Nd_0.5Sr_0.5MnO₃ thin films. We propose the possible ground states formed in the thin films.     

Phase separation induced by oxygen isotope substitution in manganites of the Sm<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>Sr<Subscript>x</Subscript>MnO<Subscript>3</Subscript> system

The effect of ¹⁶O → ¹⁸O isotope substitution on the electrical and magnetic properties of the manganite system Sm_1?xSr_xMnO₃ has been studied. It is shown that oxygen isotope substitution brings about a substantial change in the phase diagram in the intermediate region 0.4<x<0.6 between the ferromagnetic metal and antiferromagnetic insulator regions and induces phase separation and transformation of the ground metallic into insulating state for x=0.475 and 0.5. The specific features of the metal-insulator transitions in the Sm-Sr system and the nature of the low-temperature phase are discussed.     

Band effects on the conductivity for a two-band Hubbard model

The band effects on the conductivity of a one-dimensional two-band Hubbard model is studied based on the ground state energy analysis. It is found that the system with filling factor one is a metal at zero temperature if the on-site interaction U is smaller than a critical value U_c, and is an insulator if U is larger than U_c. The value of metal-insulator transition point U_c is obtained. This result is different from that of 1D single-band Hubbard model where the quantum phase transition point U_c=0. Therefore, the orbital degree of freedom plays an essential role in the states of matter.     

Microwave properties of Nd0.5Sr0.5MnO3: a key role of the (x2−y2)-orbital effects

Transmittance of the colossal magnetoresistive compound Nd_0.5Sr_0.5MnO₃ showing metal–insulator phase transition has been studied by means of the submm- and mm-wavelength band spectroscopy. An unusually high transparency of the material provided direct evidence for the significant suppression of the coherent Drude-weight in the ferromagnetic metallic state. Melting of the A-type antiferromagnetic states has been found to be responsible for a considerable increase in the microwave transmission, which was observed at the transition from the insulating to the metallic phase induced by magnetic field or temperature. This investigation confirmed a dominant role of the (x²−y²)-orbital degree of freedom in the low-energy optical properties of Nd_0.5Sr_0.5MnO₃ and other doped manganites with planar (x²−y²)-orbital order, as predicted theoretically. The results are discussed in terms of the orbital-liquid concept.     

Insulator-superconductor-metal transitions induced by spin fluctuations in the paramagnetic phase of doped insulators

AbstractThe band structure of cuprates as a doped 2D insulator is modeled assuming that the excess charge carriers are associated with the corresponding substitution atoms, and the phase diagram of the paramagnetic states as a function of the degree x of doping at zero temperature is studied. The Hamiltonian contains electronic correlations on impurity orbitals and hybridization between them and the initial band states of the insulator. It is shown that the change in the electronic structure of a doped compound includes the formation of impurity bands of distributed and localized electronic states in the initial insulator gap. It is established that in the case of one excess electron per substitution atom the spin fluctuations (1) give rise to an insulator state of the doped compound for x < x _thr, 1, (2) lead to a superconducting state for x _thr, 1 < x < x _thr, 2, and (3) decay as x > x _thr, 2 increases further, and the doped compound transforms into a paramagnetic state of a “poor” metal with a high density of localized electronic states at the Fermi level.     

Competition between crystal field splitting and Hund’s rule coupling in two-orbital magnetic metal-insulator transitions

Competition between crystal field splitting and Hund’s rule coupling in magnetic metal-insulator transitions of half-filled two-orbital Hubbard model is investigated by multi-orbital slave-boson mean field theory. We show that with the increase of Coulomb interaction, the system firstly transits from a paramagnetic (PM) metal to a Néel antiferromagnetic (AFM) Mott insulator, or to a nonmagnetic orbital insulator, depending on the competition of crystal field splitting and the Hund’s rule coupling. The AFM Mott insulating, PM metallic and orbital insulating phases are not, partially and fully orbital polarized, respectively. For a small J _H and a finite crystal field, the orbital insulator is robust. These results demonstrate that large crystal field splitting favors the formation of the orbital insulating phase, while large Hund’s rule coupling tends to destroy it, driving the low-spin to high-spin transition.     

Topological insulator nanostructures: Materials synthesis, Raman spectroscopy, and transport properties

Nanostructured topological insulator materials such as ultrathin films, nanoplates, nanowires, and nanoribbons are attracting much attention for fundamental research as well as potential applications in low-energy dissipation electronics, spintronics, thermoelectrics, magnetoelectrics, and quantum computing due to their extremely large surface-to-volume ratios and exotic metallic edge/surface states. Layered Bi₂Se₃ and Bi₂Te₃ serve as reference topological insulator materials with a large nontrivial bulk gap up to 0.3 eV (equivalent to 3600 K) and simple single-Dirac-cone surface states. In this mini-review, we present an overview of recent advances in nanostructured topological insulator Bi₂Se₃ and Bi₃Te₃ from the viewpoints of controlled synthesis and physical properties. We summarize our recent achievements in the vapor-phase synthesis and structural characterization of nanostructured topological insulator Bi₂Se₃ and Bi₂Te₃, such as nanoribbons and ultrathin nanoplates.We also demonstrate the evolution of Raman spectra with the number of few-layer topological insulators, as well as the transport measurements that have succeeded in accessing the surface conductance and surface state manipulations in the device of topological insulator nanostructures.     

The role of orbital ordering in the formation of electron structure in undoped LaMnO3 manganites in the regime of strong electron correlations

The electron structure of undoped LaMnO₃ and slightly doped La_1?x Sr_xMnO₃ manganites has been calculated within the framework of a generalized tight binding method with explicit allowance for strong intraatomic electron correlations. According to the results of these calculations, the ground state in orbitally disordered undoped LaMnO₃ ferromagnets would be metallic despite the Mott-Hubbard correlation gap in the spectrum of quasiparticles. Owing to the orbital ordering, the insulating state is stabilized in both antiferromagnetic and paramagnetic phases. In-gap states of a polaron nature with a spectral weight proportional to the dopant concentration have been found near the top of the valence band in La_1?x Sr_xMnO₃. As the doping level increases, a metal state appears in the ferromagnetic phase, which has a metallic character for one spin subband and an insulating character for the other subband (representing the so-called half-metallic state).     

Insulating surface layer on single crystal K $\mathsf{_{3}}\mathsf{C}\mathsf{_{60}}$

Using angle-dependent photoemission spectra of core and valence levels we show that metallic, single crystal K³C₆₀ is terminated by an insulating or weakly-conducting surface layer. We attribute this to the effects of strong intermolecular correlations combined with the average surface charge state. Several controversies on the electronic structure are thereby resolved.Received: 16 July 2004, Published online: 5 November 2004PACS: 71.20.Tx Fullerenes and related materials; intercalation compounds - 73.20.-r Electron states at surfaces and interfaces - 79.60.Bm Clean metal, semiconductor, and insulator surfaces     

Symmetrical metallic and magnetic edge states of nanoribbon from semiconductive monolayer PtS2

Transition metal dichalcogenides (TMD) MoS₂ or graphene could be designed to metallic nanoribbons, which always have only one edge show metallic properties due to symmetric protection. In present work, a nanoribbon with two parallel metallic and magnetic edges was designed from a noble TMD PtS₂ by employing first-principles calculations based on density functional theory (DFT). Edge energy, bonding charge density, band structure, density of states (DOS) and simulated scanning tunneling microscopy (STM) of four possible edge states of monolayer semiconductive PtS₂ were systematically studied. Detailed calculations show that only Pt-terminated edge state among four edge states was relatively stable, metallic and magnetic. Those metallic and magnetic properties mainly contributed from 5d orbits of Pt atoms located at edges. What's more, two of those central symmetric edges coexist in one zigzag nanoribbon, which providing two atomic metallic wires thus may have promising application for the realization of quantum effects, such as Aharanov–Bohm effect and atomic power transmission lines in single nanoribbon.     

Activation conduction in metallic island films

The differential conductivity of metallic island films of Ti, Co, W, and FeNi is investigated in the vicinity of liquid nitrogen temperatures. It is found that the temperature dependence of the conductivity of metallic island films in the insulator phase varies in accordance with the activation law σ ∝T ⁿ exp(?E/kT). It is shown that the power of temperature in the preexponential factor varies from n = 2 to 1 upon an increase in the film thickness. In thicker films, in which a transition from the insulator to the metal conductivity phase takes place, the temperature dependence of the conductivity increases in proportion to temperature. The mechanism of conduction in metallic island films is discussed.