Hubbard Insulator Research Articles

First-principles lattice dynamics, thermodynamics, and elasticity of Cr2O3

We present the calculation of the lattice dynamics of chromia (Cr2O3), a typical Mott–Hubbard insulator, employing the first-principles density functional theory plus U approach. We first report the phonon dispersions at the theoretical equilibrium volume. Then the phonon density-of-states is calculated as a function of volume. Finally, the atomic volume, heat capacity, linear thermal expansion coefficient, bulk modulus, Grüneisen constant, and elastic constants are calculated as functions of temperature.

Characterization of single crystal uranium-oxide thin films grown via reactive-gas magnetron sputtering on yttria-stabilized zirconia and sapphire

The microstructure and valence states of three single crystal thin film systems, UO2 on (11¯02) r-plane sapphire, UO2 on (001) yttria-stabilized zirconia, and U3O8 on (11¯02) r-plane sapphire, grown via reactive-gas magnetron sputtering are analyzed primarily with X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and ultraviolet photoelectron spectroscopy (UPS). XRD analysis indicates the growth of single crystal domains with varying degrees of mosaicity. XPS and UPS analyses yield U-4f, U-5f, O-1s, and O-2p electron binding energies consistent with reported bulk values. A change from p-type to n-type semiconductor behavior induced by preferential sputtering of oxygen during depth profile analysis was observed with both XPS and UPS. Trivalent cation impurities (Nd and Al) in UO2 lower the Fermi level, shifting the XPS spectral weight. This observation is consistent with hole-doping of a Mott–Hubbard insulator. The uranium oxide-(11¯02) sapphire system is unstable with respect to Al interdiffusion across the film–substrate interface at elevated temperature.

Spin crossovers in Mott–Hubbard insulators at high pressures

The high-pressure induced phase transitions initiated by electronic transition in 3d ions from the high-spin (HS) to the low-spin (LS) state (HS-LS spin-crossover) are considered. Behavior of the system with d6 electronic configuration is investigated in the ground state of zero temperature and critical pressure Pc. Magnetic properties of the Mott–Hubbard insulator (Mg1−xFex)O are studied in the vicinity of the quantum critical point (T=0, Pc). At the critical pressure of spin crossover Pc, the spin gap energy εS between HS and LS states is zero. The quantum spins fluctuations HS⇔LS do not require any energy, and the antiferromagnetism is destroyed in the quantum critical point by the first order transition.

Parity-Odd Atomic Multipoles in X-ray Scattering and Absorption

Scattering of x-rays enhanced by an electric dipole–electric quadrupole (E1–E2) event probe parity-odd atomic multipoles at the heart of many current theories of complex materials, e.g., Mott–Hubbard insulators and multiferroics. We achieve a complete decomposition of the E1–E2 scattering amplitude at spin–orbit split states in terms of atomic multipoles, albeit unfamiliar composite atomic operators built with electron position, spin and orbital variables. In consequence, one has clear insight as to the physical makeup of quantities that determine structure factors for Bragg diffraction enhanced by resonant ions that occupy sites devoid of inversion symmetry.

Structural and Electronic Properties of Li-Ion Battery Cathode Material FeF3

A new type of cathode materials for Li-ion batteries has been explored recently based on FeF3 in an attempt to raise the energy density and discharge voltage. In this work, the structural and electronic properties of cathode materials FeF3 for lithium ion batteries have been studied by the first-principles calculations within both the generalized gradient approximation (GGA) and GGA+U frameworks. Our results show that the antiferromagnetic configuration of FeF3 is more stable than the ferromagnetic one, which is consistent with experiments. An analysis of the electronic density of states shows that FeF3 is a Mott−Hubbard insulator with a large d−d type band gap. Additionally, a small spin polarization was found on F, consistent with a fluorin-mediated superexchange mechanism for the Fe−Fe magnetic interaction.

Electronic structures of quasi-one-dimensional ferrimagnetic insulator Ca 3Co 2O 6

The electronic structures of quasi-one-dimensional ferrimagnetic Ca 3Co 2O 6 are investigated using the generalized gradient approximation (GGA) as well as the GGA plus on-site Coulomb interaction ( GGA + U ) scheme. GGA + U calculations reveal that the interchain ferrimagnetic Ca 3Co 2O 6 is a Mott–Hubbard insulator rather than a metal given from GGA. In addition, we found an on-site U induced 3 z 2 − r 2 orbital ordering on Co pri sublattice which drives the intrachain ferromagnetic coupling along c-axis. Our findings suggest that strong electron-electron correlation plays an important role in Ca 3Co 2O 6.

Origin of valence and core excitations inLiFePO4 andFePO4

Electronic structures of LiFePO4 and FePO4 have been investigated using valence and core electron energy loss spectroscopy(EELS) supported by ab initio calculations. Valence electron energy loss spectra ofFePO4 are characterized by interband transitions found between 0 and 20 eV, which are not observed inLiFePO4. Spectra are fully analysed using band structure calculations and calculateddielectric functions. In particular, we show that interband transitions observed inFePO4 spectra originate from the states at the top of the valence band, which have mainlyoxygen p character. From core-loss EELS, it is observed that the O-K edge inFePO4 has apre-edge peak below the threshold of the main O-K edge. This pre-edge peak is not observed in the O-Kspectra of LiFePO4. The position of the pre-edge peak is determined by a charge transfer process, whichshifts the position of the iron 3d bands with respect to the conduction band. Theintensity of the pre-edge peak is also determined by the changes in the hybridizationof iron 3d and oxygen states as a result of extraction of lithium ions from theLiFePO4 lattice. We show that the extraction of lithium ions fromLiFePO4 results in large changes in the electronic structure, such thatFePO4 can be considered to be a charge transfer insulator whileLiFePO4 is a typical Mott–Hubbard insulator.

Gigantic stimulated Raman scattering in one-dimensional Mott–Hubbard insulators: A possible THz source

One-dimensional Mott–Hubbard insulators like Sr 2CuO 3, halogen-bridged Nickel chain compounds have orders-of-magnitude nonlinear optical properties compared to other one-dimensional organic or inorganic compounds. We show theoretically, that the stimulated Raman scattering susceptibility for such insulators could be order(s)-of-magnitude larger even compared to other nonlinear optical susceptibilities. The lowest two-photon state is at lower energy than the lowest one-photon state in some of these insulators. This leads to a potential for strong Stokes generation in the THz regime from these compounds. Our results and conclusions are based on exact numerical solution of finite size two-band extended Hubbard model.

Spectral properties of the Mott Hubbard insulator (Cr0.011V0.989)2O3 calculated by LDA+DMFT

Significant progress in the theoretical description of Mott-Hubbard metal-to-insulator transitions has been made in the last years, especially thanks to the LDA+DMFT approach (local density approximation + dynamical mean field theory). Obviously the main attention has been focused on the transition itself, as, for example, in the textbook case of the Cr-doped V2O3. As we discuss here, however, also the study of the insulating phase, characterized by the opening of a visible Mott-Hubbard gap in the spectral functions is far from being trivial: Strong-correlation effects make this phase strongly sensitive to small changes of external parameters, much more than one would expect for an insulator. In this situation, requiring a full consistency of the theoretical calculations with data from different spectroscopies may provide the most precise estimate for the local Coulomb interaction U in the LDA+DMFT approach.

Valence-bond states: Link models

An isotropic anti-ferromagnetic quantum state on a square lattice is characterized by symmetry arguments only. By construction, this quantum state is the result of an underlying valence bond structure without breaking any symmetry in the lattice or spin spaces. A detailed analysis of the correlations of the quantum state is given (using a mapping to a 2D classical statistical model and methods in field theory like mapping to the non-linear sigma model or bosonization techniques) as well as the results of numerical treatments (regarding exact diagonalization and variational methods). Finally, the physical relevance of the model is motivated. A comparison of the model to known anti-ferromagnetic Mott–Hubbard insulators is given by means of the two-point equal-time correlation function obtained (i) numerically from the suggested state and (ii) experimentally from neutron scattering on cuprates in the anti-ferromagnetic insulator phase.

Electronic and magnetic properties of the monoclinic phaseBiCrO3 from first-principles studies

Based on density functional theory, we systematically studied the electronicand magnetic properties of the real experimental structural phaseBiCrO3 with thespace group C 2/c. It is found that the ground state is a moderately correlated Mott–Hubbard insulator withG-type antiferromagnetic structure, which is in agreement with the experimental observations.The magnetism can be qualitatively understood in terms of the superexchange mechanism viaCr1(t2g)–O 2p–Cr2(t2g). Moreover, the total energies calculated for various magnetic orderings lead to an estimateof the magnetic interaction constants.

Magnetic and electronic properties of α-NaMnO2

The magnetic and electronic properties of α-NaMnO2 are investigated by performing the first-principles density functional calculations. A semiconducting ground state with high spin Mn is found. The magnetic coupling within a Mn layer is antiferromagnetic and frustrated, while the coupling between layers is very weak. Spatially anisotropic exchange is found within a Mn layer. The large Jahn–Teller distortion lowers the energy of d3r2−z2 orbital, leading to the stabilization of high spin Mn and ferromagnetic orbital ordering. Moreover, GGA+U studies show that α-NaMnO2 has the mixed characteristics of Mott–Hubbard insulators and charge transfer insulators.

Artificial charge modulation in perovskite vanadate superlattices

A series of epitaxial (LaVO 3) 6 m (SrVO 3) m superlattices having the same nominal composition as La 6/7Sr 1/7VO 3, a Mott–Hubbard insulator, was grown with pulsed-laser deposition on [001]-oriented SrTiO 3 substrates, and their superlattice period is varied. When m = 1, the insulating resistivity of bulk-like La 6/7Sr 1/7VO 3 is obtained; however, an increase in the periodicity ( m ≥ 2) results in metallic samples. Comparison of the superlattice periodicity with the coherence length of charge carriers in perovskite oxide heterostructures is used to understand these observations. A filling-controlled insulator-metal transition is induced by placing a single dopant layer of SrVO 3 within LaVO 3 layers of varying thickness.

Enhanced p-type conductivity and band gap narrowing in heavily Al doped NiO thin filmsdeposited by RF magnetron sputtering

Stoichiometric NiO, a Mott–Hubbard insulator at room temperature,shows p-type electrical conduction due to the introduction ofNi2+ vacancies(VNi′′) andself-doping of Ni3+ ions in the presence of excess oxygen. The electrical conductivity of this important materialis low and not sufficient for active device fabrication. Al doped NiO thin films were synthesizedby radio frequency (RF) magnetron sputtering on glass substrates at a substrate temperature of250 °C in anoxygen + argon atmosphere in order to enhance the p-type electrical conductivity. X-ray diffractionstudies confirmed the correct phase formation and also oriented growth of NiO thin films.Al doping was confirmed by x-ray photoelectron spectroscopic studies. The structural,electrical and optical properties of the films were investigated as a function of Al doping(0–4 wt%) in the target. The room temperature electrical conductivity increased from0.01–0.32 S cm −1 for 0–4% Al doping. With increasing Al doping, above the Mott critical carrier density,energy band gap shrinkage was observed. This was explained by the shift of the band edgesdue to the existence of exchange and correlation energies amongst the electron–electron andhole–hole systems and also by the interaction between the impurity quasi-particle system.

Phase Separation of Electrons Strongly Coupled with Phonons in Cuprates and Manganites

Recent advanced Monte Carlo simulations have not found superconductivity and phase separation in the Hubbard model with on-site repulsive electron–electron correlations. We argue that microscopic phase separations in cuprate superconductors and colossal magnetoresistance (CMR) manganites originate from a strong electron–phonon interaction (EPI) combined with unavoidable disorder. Attractive electron correlations, caused by an almost unretarded EPI, are sufficient to overcome the direct inter-site Coulomb repulsion in these charge-transfer Mott–Hubbard insulators, so that low energy physics is that of small polarons and small bipolarons (real-space electron (hole) pairs dressed by phonons). They form clusters localized by disorder below the mobility edge, but propagate as the Bloch states above the mobility edge. I identify the Frohlich finite-range EPI with optical phonons as the most essential for pairing and phase separation in superconducting layered cuprates. The pairing of oxygen holes into heavy bipolarons in the paramagnetic phase (current-carrier density collapse (CCDC)) explains also CMR of doped manganites due to magnetic break-up of bipolarons in the ferromagnetic phase. Here I briefly present an explanation of high- and low-resistance phase coexistence near the ferromagnetic transition as a mixture of polaronic ferromagnetic and bipolaronic paramagnetic domains due to unavoidable disorder in doped manganites.

Unconventional Pairs Glued by Conventional Phonons in Cuprate Superconductors

It has gone almost unquestioned that superexchange in the t−J (or Hubbard) model, and not phonons, is responsible for the unconventional (“d-wave”) pairing symmetry of cuprate superconductors. However, a number of advanced numerical studies have not found superconductivity in the Hubbard (or t−J) model. On the other hand, compelling experimental evidence for a strong electron–phonon interaction (EPI) has currently arrived. Here I briefly review some phonon-mediated unconventional pairing mechanisms. In particular the anisotropy of sound velocity makes the phonon-mediated attraction of electrons non-local in space providing unconventional Cooper pairs with a non-zero orbital momentum already in the framework of the conventional BCS theory with weak EPI. In the opposite limit of strong EPI rotational symmetry breaking appears as a result of a reduced Coulomb repulsion between unconventional bipolarons. Using the variational Monte Carlo method we have found that a relatively weak finite-range EPI induces a d-wave BCS state also in doped Mott–Hubbard insulators or strongly-correlated metals. These results tell us that poorly screened EPI with conventional phonons is responsible for the unconventional pairing in cuprate superconductors.

Full diagonal disorder in a strongly correlated system

We study the electronic properties of strongly correlated materials including a full diagonal disorder for site energies ε A ≠ ε B as well as for correlation strengths U A ≠ U B . Correlations are treated by the dynamical mean field theory with the non-crossing approximation as impurity solver and disorder is handled by the coherent potential approximation. For half-filled systems, various metal–insulator transitions are described by spectral analysis and a phase diagram is obtained. It shows two transitions from a band insulator to a metallic state and further to a Mott–Hubbard insulator by increasing the correlation strength of only one of the constituents. Simple and double occupancies are computed for each type of site. For the doped situation, we investigate the influence of disorder restricted to the correlation strengths ( ε A = ε B , U A ≠ U B ). We found that the site specific transfer of spectral weight is accompanied by dramatic changes of double occupancies when the metal–insulator transition occurs. Our approach gives a clear interpretation on the nature of the different bands composing the density of states and therefore on the nature of the insulating states.

Correlation effects in the electronic structure of theMn4molecular magnet

We present joint theoretical-experimental study of correlation effects in the electronic structure of ${(\mathrm{py}\mathrm{H})}_{3}[{\mathrm{Mn}}_{4}{\mathrm{O}}_{3}{\mathrm{Cl}}_{7}{(\mathrm{O}\mathrm{Ac})}_{3}]\ensuremath{\cdot}2\mathrm{Me}\mathrm{C}\mathrm{N}$ molecular magnet $({\mathrm{Mn}}_{4})$. Describing the many-body effects by cluster dynamical mean-field theory, we find that ${\mathrm{Mn}}_{4}$ is predominantly a Hubbard insulator with strong electron correlations. The calculated electron gap $(1.8\phantom{\rule{0.3em}{0ex}}\mathrm{eV})$ agrees well with the results of optical conductivity measurements, while other methods, which neglect many-body effects or treat them in a simplified manner, do not provide such an agreement. Strong electron correlations in ${\mathrm{Mn}}_{4}$ may have important implications for possible future applications.

Excitons in the One-Dimensional Hubbard Model: A Real-Time Study

We study the real-time dynamics of a hole and doubly occupied site pair, namely, a holon and a doublon, in a 1D Hubbard insulator with on-site and nearest-neighbor Coulomb repulsion. Our analysis shows that the pair is long-lived and the expected decay mechanism to underlying spin excitations is actually inefficient. For a nonzero intersite Coulomb repulsion, we observe that part of the wave function remains in a bound state. Our study also provides insight on the holon-doublon propagation in real space. Because of the one-dimensional nature of the problem, these particles move in opposite directions even in the absence of an applied electric field. The potential relevance of our results to solar cell applications is discussed.

2D multipartite valence bond states in quantum anti-ferromagnets

A quantum anti-ferromagnetic spin-1 model is characterised on a 2D lattice with the following requirements: (i) The Hamiltonian is made out of nearest neighbour interactions. (ii) It is homogeneous, translational and rotational invariant. (iii) The ground state is a real singlet state of SU(2) (non-chiral). (iv) It has a local spin-1 representation. Along the way to characterise the system, connections with classical statistical mechanics and integrable models are explored. Finally, the relevance of the model in the physics of low dimensional anti-ferromagnetic Mott–Hubbard insulators is discussed.