Mott-Hubbard Insulator Research Articles

Prediction of metal-free Stoner and Mott-Hubbard magnetism in triangulene-based two-dimensional polymers.

Ferromagnetism and antiferromagnetism require robust long-range magnetic ordering, which typically involves strongly interacting spins localized at transition metal atoms. However, in metal-free systems, the spin orbitals are largely delocalized, and weak coupling between the spins in the lattice hampers long-range ordering. Metal-free magnetism is of fundamental interest to physical sciences, unlocking unprecedented dimensions for strongly correlated materials and biocompatible magnets. Here, we present a strategy to achieve strong coupling between spin centers of planar radical monomers in π-conjugated two-dimensional (2D) polymers and rationally control the orderings. If the π-states in these triangulene-based 2D polymers are half-occupied, then we predict that they are antiferromagnetic Mott-Hubbard insulators. Incorporating a boron or nitrogen heteroatom per monomer results in Stoner ferromagnetism and half-metallicity, with the Fermi level located at spin-polarized Dirac points. An unprecedented antiferromagnetic half-semiconductor is observed in a binary boron-nitrogen-centered 2D polymer. Our findings pioneer Stoner and Mott-Hubbard magnetism emerging in the electronic π-system of crystalline-conjugated 2D polymers.

Mott interactions driven insulating behaviour in ruthenium based double perovskites: A2SmRuO6 (A = Ba & Sr)

Double perovskites (DP’s) are important for understanding the diverse physical properties arising from strongly correlated interactions. According to experimental observations, the DP’s A2SmRuO6 (A = Ba & Sr) are insulators; however, first-principles calculations using GGA approximations show that they are metallic. We observed a band gap (Eg) opening of 1.3 eV for Ba2SmRuO6 (BSRO) and 1.4 eV for Sr2SmRuO6 (SSRO) upon the insertion of Hubbard parameter (U) in the theoretical computations for accounting the electron-electron interactions. Further investigations using core-level spectroscopy reveal Eg of 1.05 eV for BSRO and 0.85 eV for SSRO. Furthermore, the calculations for Δ (charge transfer energy), U (coulomb repulsion energy), and W (bandwidth) from the combined spectra show U < Δ and U/W > 1.15, which confirms these compounds be classified as Mott-Hubbard insulators. These results expand our understanding of the strongly correlated compounds around the Fermi level, which might have applications in optoelectronics.

Fingerprints of Mott–Hubbard physics in optical spectra of antiferromagnetic LaTiO3

Magnetic properties of Mott–Hubbard insulators are determined by superexchange interactions mediated by the high-spin (HS) and low-spin (LS) intersite d–d charge excitations, which can be associated with the HS- and LS-Hubbard subbands in optical experiments. To explore the Mott–Hubbard physics in orthorhombic LaTiO3 crystal exhibiting the G-type antiferromagnetic order at the Néel temperature TN = 146 K, we present a comprehensive spectroscopic ellipsometry study in the spectral range 0.5–5.6 eV at temperatures 10 ⩽ T ⩽ 300 K. We found that the complex dielectric function spectra of LaTiO3 crystal are almost featureless, nearly isotropic, and weakly temperature dependent in the range of the intersite d–d optical transitions. Nonetheless, analyzing the difference spectra below the TN, we have identified the LS-state d1d1⇌d2d0 excitations at ∼3.7 and ∼5.15 eV and estimated values of the on-site Coulomb repulsion U ∼ 4.2 eV and Hund’s exchange constant JH ∼ 0.5 eV, which define the energy of the HS-state d1d1⇌d2d0 excitation at ∼2.7 eV. In addition, we discovered that the pronounced lowest-energy 1.3 eV optical band displays the critical intensity behavior and anomalous broadening with decreasing temperature below the TN. The discovered properties indicate that the 1.3 eV band in LaTiO3 can be associated with a Mott–Hubbard exciton.

Thermoelectric Power and Hall Effect in Correlated Metals and Doped Mott–Hubbard Insulators: DMFT Approximation

We present comparative theoretical investigation of thermoelectric power and Hall effect in the Hubbard model for correlated metal and Mott insulator (considered as prototype cuprate superconductor) for different concentrations of current carriers. Analysis is performed within standard DMFT approximation. For Mott insulator we consider the typical case of partial filling of the lower Hubbard band (hole doping). We calculate the dependence of thermopower on doping level and determine the critical concentration of carriers corresponding to sign change of thermopower. An anomalous dependence of thermopower on temperature is obtained significantly different from linear temperature dependence typical for the usual metals. The role of disorder scattering is analyzed on qualitative level. The comparison with similar studies of the Hall effect shows, that breaking of electron-hole symmetry leads to the appearance of the relatively large interval of band-fillings (close to the half-filling) where thermopower and Hall effects have different signs. We propose a certain scheme allowing to determine the number of carriers from ARPES data and perform semi-quantitative estimate of both thermopower and Hall coefficient using the usual DFT calculations of electronic spectrum.

Thermoelectric Power and Hall Effect in Correlated Metals and Doped Mott–Hubbard Insulators: DMFT Approximation

Thermoelectric Power and Hall Effect in Correlated Metals and Doped Mott–Hubbard Insulators: DMFT Approximation

Hall Effect in Doped Mott–Hubbard Insulator

We present theoretical analysis of Hall effect in doped Mott–Hubbard insulator, considered as a prototype of cuprate superconductor. We consider the standard Hubbard model within DMFT approximation. As a typical case we consider the partially filled (hole doping) lower Hubbard band. We calculate the doping dependence of both the Hall coefficient and Hall number and determine the value of carrier concentration, where Hall effect changes its sign. We obtain a significant dependence of Hall effect parameters on temperature. Disorder effects are taken into account in a qualitative way. We also perform a comparison of our theoretical results with some known experiments on doping dependence of Hall number in the normal state of YBCO and Nd-LSCO, demonstrating rather satisfactory agreement of theory and experiment. Thus the doping dependence of Hall effect parameters obtained within Hubbard model can be considered as an alternative to a popular model of the quantum critical point.

Insights into the Fe3+ Doping Effects on the Structure and Electron Distribution of Cr2O3 Nanoparticles.

Herein, we carefully investigated the Fe3+ doping effects on the structure and electron distribution of Cr2O3 nanoparticles using X-ray diffraction analysis (XRD), maximum entropy method (MEM), and density functional theory (DFT) calculations. We showed that increasing the Fe doping induces an enlargement in the axial ratio of c/a, which is associated with an anisotropic expansion of the unit cell. We found that as Fe3+ replaces Cr in the Cr2O3 lattice, it caused a higher interaction between the metal 3d states and the oxygen 2p states, which led to a slight increase in the Cr/Fe-O1 bond length followed by an opposite effect for the Cr/Fe-O2 bonds. Our results also suggest that the excitations characterize a well-localized bandgap region from occupied Cr d to unoccupied Fe d states. The Cr2O3 and Fe-doped Cr2O3 nanoparticles behave as Mott-Hubbard insulators due to their band gap being in the d-d gap, and Cr 3d orbitals dominate the conduction band. These findings suggest that the magnitude and the character of the electronic density near the O atom bonds in Cr2O3 nanoparticles are modulated by the Cr-Cr distances until its stabilization at the induced quasi-equilibrium of the Cr2O3 lattice when the Fe3+ doping values reaches the saturation level range.

Hall Effect in Doped Mott–Hubbard Insulator

We present theoretical analysis of Hall effect in doped Mott-Hubbard insulator, considered as a prototype of cuprate superconductor. We consider the standard Hubbard model within DMFT approximation. As a typical case we consider the partially filled (hole doping) lower Hubbard band. We calculate the doping dependence of both the Hall coefficient and Hall number and determine the value of carrier concentration, where Hall effect changes its sign. We obtain a significant dependence of Hall effect parameters on temperature. Disorder effects are taken into account in a qualitative way.We also perform a comparison of our theoretical results with some known experiments on doping dependence of Hall number in the normal state of YBCO and Nd-LSCO, demonstrating rather satisfactory agreement of theory and experiment. Thus the doping dependence of Hall effect parameters obtained within Hubbard model can be considered as an alternative to a popular model of the quantum critical point.

Study on the decisive factor for metal-insulator transitions in a LaVO3 Mott-Hubbard insulator.

The decisive physical parameters on electrical conduction in a LaVO3 Mott-Hubbard system are systematically investigated by analyzing pure, Ca-, and Sr-doped samples. The Rietveld refinement of the X-ray diffraction data indicates that a drastic change occurs along the c-axis to reduce the octahedral tilt thereby relaxing the distortion for the doped compounds, in contrast to an insignificant change in the in-plane distortion. From electrical, optical, and photoemission measurements, both Ca and Sr-doping in LaVO3 induce insulator to metal transitions under a similar hole carrier concentration as suppressing the Mott-gap excitation. Fitting results on temperature-dependent resistivity based on various conduction models indicate that the most localized conduction behavior takes place for the highly distorted pure LaVO3, while disordered Fermi liquid behavior starts to appear for moderately distorted Ca-doped LaVO3. The least distorted Sr-doped LaVO3 exhibits fully delocalized conduction governed by a non-Fermi-liquid-like behavior in the whole temperature range. Our analysis indicates that the difference in the transport mechanism arises from the differing degree of hybridization of the V 3d and O 2p states in the pure and doped systems, strongly associated with the structural distortion.

Optical signature for distinguishing between Mott-Hubbard, intermediate, and charge-transfer insulators

Determining the nature of band gaps in transition-metal compounds remains challenging. We present a first-principles study on electronic and optical properties of CoO using hybrid functional pseudopotentials. We show that optical absorption spectrum can provide a clear fingerprint to distinguish between Mott-Hubbard, intermediate, and charge-transfer insulators. This discrimination is reflected by the qualitative difference in peak satellites due to unique interplay between $d\text{\ensuremath{-}}d$ and $p\text{\ensuremath{-}}d$ excitations, thus allowing identification from experimental data alone, unlike the existing methods that require additional theoretical interpretation. We conclude that the CoO is an intermediate rather than a Mott-Hubbard insulator as is initially believed.

Spectroscopic comprehension of Mott-Hubbard insulator to negative charge transfer metal transition in LaNixV1−xO3 thin films

The room temperature (300 K) electronic structure of pulsed laser deposited LaNi_{x}V_{1-x}O_{3} thin films have been demonstrated. The substitution of early-transition metal (TM) V in LaVO_{3} thin films with late-TM Ni leads to the decreasing in out-of-plane lattice parameter. Doping of Ni does not alter the formal valence state of Ni and V in LaNi_{x}V_{1-x}O_{3} thin films, divulging the absence of carrier doping into the system. The valence band spectrum is observed to comprise of incoherent structure owing to the localized V 3d band along with the coherent structure at Fermi level. With increase in Ni concentration, the weight of the coherent feature increases, which divulges its origin to the Ni 3d-O 2p hybridized band. The shift of Ni 3d-O 2p hybridized band towards higher energy in Ni doped LaVO_{3} films compared to the LaNiO_{3} film endorses the modification in ligand to metal charge transfer (CT) energy. The Ni doping in Mott-Hubbard insulator LaVO_{3} leads to the closure of Mott-Hubbard gap by building of spectral weight that provides the delocalized electrons for conduction. A transition from bandwidth control Mott-Hubbard insulator LaVO_{3} to negative CT metallicity character in LaNiO_{3} film is observed. The study reveals that unlike in Mott-Hubbard insulators where the strong Coulomb interaction between the 3d electrons decides the electronic structure of the system, CT energy can deliver an additional degree of freedom to optimize material properties in Ni doped LaVO_{3} films.

Nonequilibrium dynamics of α-RuCl3 - a time-resolved magneto-optical spectroscopy study.

We present time-resolved magneto-optical spectroscopy on the magnetic Mott-Hubbard-insulating Kitaev spin liquid candidate α-RuCl3 to investigate the nonequilibrium dynamics of its antiferromagnetically ordered zigzag groundstate after photoexcitation. A systematic study of the transient magnetic linear dichroism under different experimental conditions (temperature, external magnetic field, photoexcitation density) gives direct access to the dynamical interplay of charge excitations with the zigzag ordered state on ultrashort time scales. We observe a rather slow initial demagnetization (few to 10s of ps) followed by a long-lived non-thermal antiferromagnetic spin-disordered state (100-1000s of ps), which can be understood in terms of holons and doublons disordering the antiferromagnetic background after photoexcitation. Varying temperature and fluence in the presence of an external magnetic field reveals two distinct photoinduced dynamics associated with the zigzag and quantum paramagnetic disordered phases. The photo-induced non-thermal spin-disordered state shows universal compressed-exponential recovery dynamics related to the growth and propagation of zigzag domains on nanosecond time scales, which is interpreted within the framework of the Fatuzzo-Labrune model for magnetization reversal. The study of nonequilibrium states in strongly correlated materials is a relatively unexplored topic, but our results are expected to be extendable to a large class of Mott-Hubbard insulator materials with strong spin-orbit coupling.

Electronic structure and magnetic properties of the effective spin Jeff=12 two-dimensional triangular lattice K3Yb(VO4)2

We report the structural, magnetic, specific heat, and electronic structure studies of the material ${\mathrm{K}}_{3}\mathrm{Yb}{({\mathrm{VO}}_{4})}_{2}$, which has two-dimensional triangular layers constituted by rare-earth magnetic ${\mathrm{Yb}}^{3+}$ ions. Magnetic susceptibility data show the absence of magnetic long-range order down to $0.5\phantom{\rule{0.16em}{0ex}}\mathrm{K}$. No bifurcation is observed between zero-field-cooled and field-cooled magnetic susceptibility data, ruling out the possibility of spin-glassiness down to $0.5\phantom{\rule{0.16em}{0ex}}\mathrm{K}$. From the fit to magnetic susceptibility data with Curie-Weiss law in the low-temperature region, the observed Curie-Weiss temperature $(\ensuremath{\theta}{}_{\mathrm{CW}})$ is about $\ensuremath{-}1\phantom{\rule{0.16em}{0ex}}\mathrm{K}$, implying an antiferromagnetic coupling between the ${\mathrm{Yb}}^{3+}$ ions. Magnetic field-dependent specific heat fits well with two-level Schottky behavior. The analysis of magnetization and specific heat data confirms that the ${\mathrm{Yb}}^{3+}$ ion hosts the effective spin ${J}_{\mathrm{eff}}=1/2$ state. To provide a microscopic understanding of the ground state nature of the titled material, we carried out state-of-the-art first-principles calculations based on density functional theory $+$ Hubbard U and density functional theory $+$ dynamical mean-field theory approaches. Our calculations reveal that the system belongs to the novel class of spin-orbit driven Mott Hubbard insulators and possesses large in-plane magnetocrystalline anisotropy.

Extremely correlated superconductors

Superconductivity in the t-J model is studied by extending the recently introduced extremely correlated fermi liquid theory. Exact equations for the Greens functions are obtained by generalizing Gor’kov’s equations to include extremely strong local repulsion between electrons of opposite spin. These equations are expanded in a parameter λ representing the fraction of double occupancy, and the lowest order equations are further simplified near Tc, resulting in an approximate integral equation for the superconducting gap. The condition for Tc is studied using a model spectral function embodying a reduced quasiparticle weight Z near half-filling, yielding an approximate analytical formula for Tc. This formula is evaluated using parameters representative of single layer High-Tc systems. In a narrow range of electron densities that is necessarily separated from the Mott–Hubbard insulator at half filling, we find a typical Tc∼102 K.

One-particle spectral functions of the one-dimensional Fermionic Hubbard model with one fermion per site at zero and finite magnetic fields

Here the line shape of the up- and down-spin one-particle spectral functions at and in the (k,energy)-plane's vicinity of their cusp singularities is studied for the Mott-Hubbard insulator described by the 1D Hubbard model with one fermion per site at zero and finite magnetic fields. At zero field they can be accessed in terms of electrons by photoemission experiments. At finite field, such functions and corresponding interplay of correlations and magnetic-field effects refer to an involved non-perturbative many-particle problem that is poorly understood. The Mott-Hubbard gap that separates the addition and removal spectral functions is calculated for all spin densities and interaction values. The qualitative differences in the one-particle properties of the Mott-Hubbard insulator and corresponding doped insulator are also investigated. The relation of our theoretical results and predictions to both condensed-matter and ultracold spin-1/2 atom systems is discussed.

Piezomagnetic switching and complex phase equilibria in uranium dioxide

Actinide materials exhibit strong spin–lattice coupling and electronic correlations, and are predicted to host new emerging ground states. One example is piezomagnetism and magneto-elastic memory effect in the antiferromagnetic Mott-Hubbard insulator uranium dioxide, though its microscopic nature is under debate. Here, we report X-ray diffraction studies of oriented uranium dioxide crystals under strong pulsed magnetic fields. In the antiferromagnetic state a [888] Bragg diffraction peak follows the bulk magnetostriction that expands under magnetic fields. Upon reversal of the field the expansion turns to contraction, before the [888] peak follows the switching effect and piezomagnetic ‘butterfly’ behaviour, characteristic of two structures connected by time reversal symmetry. An unexpected splitting of the [888] peak is observed, indicating the simultaneous presence of time-reversed domains of the 3-k structure and a complex magnetic-field-induced evolution of the microstructure. These findings open the door for a microscopic understanding of the piezomagnetism and magnetic coupling across strong magneto-elastic interactions.

Moiré quantum chemistry: Charge transfer in transition metal dichalcogenide superlattices

Transition metal dichalcogenide (TMD) bilayers have recently emerged as a robust and tunable moir\'e system for studying and designing correlated electron physics. In this work, by combining large-scale first principle calculation and continuum model approach, we provide an electronic structure theory that maps long-period heterobilayer TMD superlattices onto diatomic crystals with cations and anions. We find that the interplay between moir\'e potential and Coulomb interaction leads to filling-dependent charge transfer between MM and MX regions several nanometers apart. We show that the insulating state at half-filling found in recent experiments on WSe$_2$/WS$_2$ is a charge-transfer insulator rather than a Mott-Hubbard insulator. Our work reveals the richness of simplicity in moir\'e quantum chemistry.

Magnetoelastic coupling to coherent acoustic phonon modes in the ferrimagnetic insulator GdTiO3

In this work we investigate single crystal $\mathrm{Gd}\mathrm{Ti}{\mathrm{O}}_{3}$, a promising candidate material for Floquet engineering and magnetic control, using ultrafast optical pump-probe reflectivity and magneto-optical Kerr spectroscopy. $\mathrm{Gd}\mathrm{Ti}{\mathrm{O}}_{3}$ is a Mott-Hubbard insulator with a ferrimagnetic and orbitally ordered ground state (${T}_{C}$ = 32 K). We observe multiple signatures of the magnetic phase transition in the photoinduced reflectivity signal, in response to above band-gap 660-nm excitation. Magnetic dynamics measured via Kerr spectroscopy reveal optical perturbation of the ferrimagnetic order on spin-lattice coupling timescales, highlighting the competition between the ${\mathrm{Gd}}^{3+}$ and ${\mathrm{Ti}}^{3+}$ magnetic sub-lattices. Furthermore, a strong coherent oscillation is present in the reflection and Kerr dynamics, attributable to an acoustic strain wave launched by the pump pulse. The amplitude of this acoustic mode is highly dependent on the magnetic order of the system, growing sharply in magnitude at ${T}_{C}$, indicative of strong magneto-elastic coupling. The driving mechanism, involving strain-induced modification of the magnetic exchange interaction, implies an indirect method of coupling light to the magnetic degrees of freedom and emphasizes the potential of $\mathrm{Gd}\mathrm{Ti}{\mathrm{O}}_{3}$ as a tunable quantum material.

Mott-Hubbard gaps and their doping-induced collapse in strongly correlated pyrochlore ruthenates

We investigate the variation of charge dynamics in the course of the filling-control metal-insulator transitions (MITs) for pyrochlore ${R}_{2}{\mathrm{Ru}}_{2}{\mathrm{O}}_{7}$ ($R$ being rare-earth ions). With widely changing of $R$ ions from Lu to Pr, the antiferromagnetic transition temperature systematically increases while the magnitude of the charge gap decreases toward zero at the hypothetical bandwidth-control Mott transition. It is attributable to the reduction of effective electron correlation in the Mott-Hubbard insulator regime. Doping holes give rise to the filling-control MITs from antiferromagnetic insulators to paramagnetic metals accompanied by the collapse of the Mott-Hubbard gaps. The doping-induced spectral weight transfer of the optical conductivity from the above-gap region to the in-gap region is nearly proportional to the doping level, whose rate is enhanced with the reduction of the electron correlation, in accord with the standard Mott criticality.

Oblique incidence infrared reflectance spectroscopy of phonons in cubic MgO, MnO, and NiO

The infrared (IR) reflectivity of the cubic metal oxides MgO, MnO, and NiO has been measured at room temperature using the technique of oblique incidence. The use of this technique at three angles of incidence provides multiple sets of spectra (including both s- and p-polarizations) to analyze when compared to the standard normal incidence case, which has been used extensively in the past for these compounds. It is shown that the transverse optic (TO) and longitudinal optic (LO) mode phonon parameters can be determined with greater accuracy by using the factorized model for the fits, as compared with the popular classical model used previously, and by fitting the derivative of the reflectivity. Our results for the phonon mode parameters are similar to those found earlier, as could be expected, but are generally more precise. An analysis of the difference in frequency of the TO mode in antiferromagnetic NiO at room temperature for the two polarizations of reflected light revealed a TO mode splitting of about 4 cm−1, with the p-polarized light TO mode having the higher frequency: This small splitting is in agreement with theoretical predictions. This more precise IR method for revealing the phonon mode behavior in such magnetically ordered cubic metal oxides, which are prototypes of strongly correlated electronic systems and have been found to be Mott-Hubbard insulators, may be readily applied to any similar antiferromagnetic system.