Session R45: Strong Correlation in Metals: Mott Transition

The metal-insulator transition in vanadium dioxide explored by optical methods

The nature of the metal-insulator transition (MIT) in vanadium dioxide (VO$_{2})$ remains a matter of debate. At the center of the debate is the relative importance of electron-electron correlations to the MIT. We study the MIT with spectroscopic ellipsometry on VO$_{2}$ films grown on sapphire substrates, and determine the optical constants in the insulating and metallic states from 50 meV to 5.5 eV. The changes in the optical constants with temperature in the metallic state have been tracked from 360 K up to 500 K. The redistribution of spectral weight in the real part of the optical conductivity as a result of the MIT will be discussed with emphasis on the changes to the interband transitions. This work has been supported by Department of Energy Grant No.DE-FG03-00ER45799.   

First-order metal-insulator transition not accompanied by the structural phase transition observed in VO$_2$-based devices

An abrupt first-order metal-insulator transition (MIT) is observed during the application of a switching pulse voltage to VO$_2$-based two-terminal devices. When the abrupt MIT occurs, the structural phase transition (SPT) is investigated by a micro- Raman spectroscopy and a micro-XRD. The result shows that the MIT is not accompanied with the structural phase transition (SPT); the abrupt MIT is prior to the SPT. Moreover, any switching pulse over a threshold voltage of 7.1 V for the MIT enabled the device material to transform efficiently from an insulator to a metal. The measured delay time from the source switching pulse to an induced MIT pulse is an order of 20 nsec which is much less than a delay time of about one msec deduced by thermal model. This indicates that the first-order MIT does not occur due to thermal. We think this MIT is the Mott transition. (Reference: New J. Phys. 6 (1994) 52 (www.njp.org), Appl. Phys. Lett. 86 (2005) 242101, Physica B 369 (2005. December) xxxx)   

Terahertz spectroscopy of the metal insulator transition in vanadium dioxide

We employ terahertz spectroscopy to study the metal-insulator phase transition in vanadium dioxide (VO$_2$ ). We measure the terahertz frequency conductivity in the metallic phase that has a real conductivity of 1000 $\Omega^{-1}~\mathrm{cm}^{-1}$ and a negligible imaginary conductivity. The observed conductivity dynamics are consistent with a photoinduced transition in spatially inhomogeneous regions of the film, followed by a thermally driven transition to the maximum conductivity.   

The role of dimers in the metal-insulator transitions in VO${_2}$ and Ti${_2}$O${_3}$

The nature of the metal-insulator-transition (MIT) in VO$_2$ has been subject of debate for a long time. Recently the scenario of an orbital assisted MIT has been proposed on the basis of the dramatic change of the orbital occupation as observed by soft-X-ray absorption spectroscopy measurements.\footnote{M.W. Haverkort {\it et al.}, PRL {\bf 95}, 196404 (2005).} We present the results of our bulk sensitive photoemission study on the MIT in VO${_2}$ using high quality single crystals. We observe a huge transfer of spectral weight across the MIT, supporting recent LDA+DMFT cluster calculations.\footnote{S. Biermann {\it et al.}, PRL {\bf 94}, 026404 (2005).} To investigate the possible role of dimers herein, we also studied the insulating phase of Ti${_2}$O${_3}$. Here we clearly observe a double peak structure which can be taken as a characteristic for the presence of dimers.   

Orbital-assisted metal-insulator transition in VO$_{2}$

We found direct experimental evidence for an orbital switching in the V $3d$ states across the metal-insulator transition in VO$_{2}$. We have used soft-x-ray absorption spectroscopy at the V $L_{2,3}$ edges as a sensitive local probe, and have determined quantitatively the orbital polarizations. These results strongly suggest that, in going from the metallic to the insulating state, the orbital occupation changes in a manner that charge fluctuations and effective band widths are reduced, that the system becomes more 1-dimensional and more susceptible to a Peierls-like transition, and that the required massive orbital switching can only be made if the system is close to a Mott insulating regime.   

X-ray Absorption Fine Structure Study of the Metal-Insulator Transition in Cr Doped V$_{2}$O$_{3}$

We have applied the polarized x-ray absorption fine structure (XAFS) spectroscopy to study the series of (V$_{1-x}$Cr$_{x})_{2}$O$_{3}$ where x ranges from 0 to 0.052. At room temperature, the metal-insulator transition (MIT) between paramagnetic metal 1 (PM-1) and paramagnetic insulator (PI) phases occurs as $x$ exceeds 0.01. At elevated temperatures, the second PM phase (PM 2) sets in the metallic samples ($x \quad <$ 0.01). We have obtained V K-edge and Cr K-edge spectra for the samples in the PM1, PM2 and PI phases at different concentrations (from 0.00365 to 0.0052) and temperatures (from 300 K to 673 K). V K-edge X-ray absorption near-edge structure (XANES) data show systematic changes across varies phase boundaries, both temperature and dopant-driven. Polarized Cr K-edge and V K-edge XANES and extended XAFS (EXAFS) data demonstrate, for the first time, that Cr enters the V$_{2}$O$_{3}$ lattice substitutionally, at all concentrations. Contrary to the Cr K-edge, the changes in the V K-edge XANES across the metal-insulator transition are strongly anisotropic. X-ray diffraction (XRD) and temperature dependent XANES and EXAFS results of Cr and V data will be discussed in terms of the possible percolative nature of the MIT in this system.   

Nonequilibrium dynamical mean-field theory near the Mott transition

We exactly calculate the non-equilibrium properties of the Falicov-Kimball model in an external electric field by using a recently developed non-equilibrium dynamical mean-field theory formalism [1,2]. In particular, we analyze the time-dependence of the electrical current in cases with different values of the on-site Coulomb interaction, when the system is in the metallic and in the insulating phases. The most interesting properties take place when the interaction energy is close to the metal-insulator transition value. The dynamics of the Bloch oscillations of the electrical current and the long time steady state nonlinear response are analyzed in detail. [1] J.K.Freericks, V.M.Turkowski, V. Zlatic, To be published in ``Proceedings of the 2005 Users Group Conference, IEEE Computer Society.''; cond-mat/0511061; [2] V.M. Turkowski, J.K. Freericks, Submitted to Phys. Rev. B; cond-mat/0511063.   

Symmetry of Photoexcited States in Two-Dimensional Mott Insulators

The charge gap in Mott insulators is a consequence of strong electron correlation represented by large on-site Coulomb interaction. The correlation induces novel phenomena in terms of the interplay of charge and spin degrees of freedom. In this study, we examine symmetry of photoexcited states with two photoinduced carriers in two-dimensional Mott insulators by applying the numerically exact diagonalization method to finite- size clusters of a half-filled Hubbard model in the strong- coupling limit [1]. The symmetry of minimum-energy bound state is found to be $s$-wave, which is different from a $d_{x^2-y^2}$ wave of a two-hole pair in doped Mott insulators. We demonstrate that the difference is originated from an exchange of fermions due to the motion of a doubly occupied site. Correspondingly large-shift Raman scattering across the Mott gap exhibits a minimum-energy excitation in the A$_1$ ($s$-wave) channel. We discuss implications of the results for the Raman scattering and other optical experiments. [1] T. Tohyama, cond-mat/0508519.   

GW+DMFT Electronic Structure Calculations in Mott Insulators: Application to NiO

We discuss the nature of Mott insulator ground state in transition metal oxides using newly developed GW+DMFT approach[1], which permits to join advantages of model (Hubbard-like) and density functional (LDA) calculations. The gap in LDA calculated antiferromagnetic state is usually much smaller than in experiment. Pure GW calculations increase the gap[2],but also cannot reproduce the paramagnetic insulating state. In GW+DMFT approach, without any fitting parameters, we calculate electronic structures of both antiferromagnetic and paramagnetic insulating states. We discuss relative position of d-metal and p-oxygen states,problems of renormalization of both electronic states near Fermi level and effective interaction at small energies, changes in static interaction under development of the gap in the paramagnetic state, effect of nearly filled bands on the value of self-energy correlation contribution. [1] N.E.Zein, S.Savrasov, G.Kotliar cond-mat/0511064 [2] S.V.Faleev, M.van Schilfgaarde, T.Kotani Phys.Rev.Lett. 93, 126406(2004)   

Dynamical Mean Field Study of the Mott Transition on the Triangular Lattice Employing the Quantum Monte Carlo Technique

We present results on the Mott transition on the triangular lattice with nearest neighbor hopping employing the Dynamical Mean Field Theory (DMFT) and the Quantum Monte Carlo (QMC). DMFT and QMC can yield the critical value of electron-electron interaction U as a function of temperature T for the metal-insulator transition. By using the Maximum Entropy (MaxEnt) method, we calculate the Density of States (DOS) for a variety of U and T values. The formation of an Abrikosov-Suhl resonance peak in the intermediate coupling regime and a Mott gap in the strong coupling regime is observed. In addition, we discuss the behavior of the magnetic moment as a function of U and T.   

First principles many-body approach to electron gas

We describe a first principles approach of constructing low energy, effective lattice model for the electron gas in a continuum of 2 and 3 spatial dimensions. The effects of the higher energy, unoccupied states are taken into account through virtue particle-hole excitations which screen the effective interactions at low energies. Using this effective model we obtain the correct order of magnitudes of the average inter- electron distance ($R_s$) at which the metal-insulator transition happens in 2d and 3d.   

Effects of interorbital hopping on orbital fluctuations and metal-insulator transitions.

We study the effects of inter-orbital hopping on orbital fluctuations and Mott-Hubbard metal-insulator transition (MIT) in the two-orbital Hubbard model within the extended linearized dynamical mean-field theory. By mapping the model onto an effective model with different bandwidths through the canonical transformation, we find that at half filling, the increases of the inter-orbital Coulomb interaction U$'$ and the Hund's coupling $J$ drive the MIT, and the critical $J_c$ for MIT increases with the lift of the inter-orbital hopping integral $t_{ab}$. Meanwhile at quarter filling and in the strong correlation regime, the system without $t_{ab}$ exhibits MIT with the decreasing of $J$, and favors the orbital liquid ground state. However, the system transits from metal to insulator with the increasing of t$_{ab}$, accompanied with the rising of the orbital order parameter. These results show the important role of the inter-orbital hopping in the orbital fluctuation and orbital ordering.