Session Q22: Metal-Insulator Phase Transitions II

Fast pulsed measurements of the electric-field-driven metal-insulator transition in magnetite

Magnetite, Fe3O4, is an example of a strongly correlated material in which strong electron-electron interactions lead to unusual magneto-electronic properties. ~In particular, it undergoes a first-order phase transition on cooling through TV$\sim $122K in bulk, in which a structural transition is accompanied by a significant drop in electrical conductivity. ~Recent electronic transport measurements have shown an electric-field driven breakdown of the insulating state in large aspect-ratio nanogaps fabricated on magnetite thin-films. ~The mechanism of this breakdown is of great interest in understanding the Verwey transition, and probing the intrinsic speed of the breakdown may further constrain possible mechanisms. ~We investigate the kinetics of this nonequilibrium transition by employing a high-speed pulse generator to apply voltages approaching the nanosecond time scale that exceed the critical switching value, and measuring the transmitted pulse via a high-speed oscilloscope. ~A significant change in transmission is observed for pulses that exceed the critical amplitude necessary to initiate the transition. ~Our initial results include an evaluation of the material response as a function of temperature and amplitude of the applied voltage.   

Magnetic field dependence of the nonequilibrium metal-insulator transition in magnetite nanostructures

At low temperatures magnetite undergoes a Verwey transition from a comparatively conducting state to a strongly correlated, ordered, more insulating state, the detailed nature of which remains under active debate. Recent experiments using nanostructures based on epitaxial magnetite films have shown that an applied dc electric field can lead to a nonequilibrium transition out of the insulating state. The kinetics of this nonequilibrium transition are nontrivial, with switching taking place over a distribution of applied voltages in a particular device at a given temperature below the Verwey transition. An externally applied magnetic field is observed to alter the kinetics of the nonequilibrium transition as the magnetization of the magnetite film is coerced out of plane. We present this data and discuss what it implies about the nature of the ordered, insulating ground state.   

Mesoscopic transport in ultrathin films of La$_{0.67}$Ca$_{0.33}$MnO$_3$

We investigate the electrical transport in mesoscopic structures of La$_{0.67}$Ca$_{0.33}$MnO$_3$ in the regime of the metal-insulator transition by fabricating microbridges from strained and unstrained thin films. We measure current-voltage characteristics as function of temperature and in high magnetic fields and with varying film thickness. For strained films, in warming from the metallic to the insulating state, we find non-linear effects in the steep part of the transition characterized by a differential resistance with a strong peak around zero applied current, and saturating at higher currents after resistance drops up to 60 $\%$. We propose that this nonlinear behavior is associated with melting of the insulating state by injecting charge carriers, signalling the occurrence of an intervening phase which involves the formation of short range polaron correlations.   

In-situ TEM Analysis and Transport in Manganites La$_{5/8-y}$Pr$_{y}$Ca$_{3/8}$MnO$_{3}$ Exhibiting Phase Separation below Metal-Insulator Transition

Epitaxial films of doped La$_{5/8-y}$Pr$_{y}$Ca$_{3/8}$MnO$_{3}$ (LPCMO: y = 0.275-0.375) manganites were examined by \textit{in-situ} Lorentz microscopy and other TEM methods below the metal-insulator transition point T$_{MI}\sim $164 K. Such films are known for colossal magneto-resistance effect (CMR). Clear evidences were obtained for mesoscale two-phase separation process involving antiferromagnetic charge-ordered (AFM/CO) and ferromagnetic (FM) phases, coexisting below T$_{MI}$ in LPCMO films. The first-order CO-FM phase transition is accompanied by partial magnetic melting of the CO phase at CO/FM interfaces thereby creating charge-disordered spin-glass metastates. In contrast, FM phase shows specific ``zig-zag'' magnetic domains coupled with dense (101) crystal twins. This allows refining relations for charge-orbital and spin-ordering vectors in films. Transport resistance data show that T$_{MI}$ point is decreased with Pr$_{y}$ growth in LPCMO. On cooling films below T$_{MI}$ their resistance drops by several orders in magnitude. The observed M-I transition shows striking linear relation for log-conductance curve versus FM fraction measured by TEM, which does not follow typical percolation equations, suggesting that percolation transport model in manganites needs further revisions.   

First Order CMR Transitions and Spin-Charge Dynamics Above the Curie Temperature in the Two-Orbital Model for Manganites

We study the two $e_g$-orbital model including Jahn-Teller lattice distortions and the superexchange interaction using exact diagonalization Monte Carlo techniques at various dopings, $x$. We report the presence of first order CMR transitions at the Curie temperature ($T_{\rm C}$) for doping $x=0.25$ for the clean system and with weak disorder [1], in qualitative agreement with several experiments. We also discuss spin and charge dynamics as a function of Monte Carlo time above $T_{\rm C}$, addressing the properties that characterize the exotic CMR state. It is observed that in CMR regimes there are various quasidegenerate spin states that may play an important role in this phenomenon [2].\\[4pt] [1] C. Sen {\it et al.}, Phys. Rev. Lett. {\bf 105}, 097203 (2010).\\[0pt] [2] Shuhua Liang {\it et al.}, preprint, submitted to PRB. See also Hotta {\it et al.}, Phys. Rev. Lett. {\bf 86}, 4922 (2001).   

Spin frustration effect near the Mott transition in the pyrochlore-type molybdates

Spin frustration and nontrivial spin structures by antiferromagnetic spins on a frustrated lattice have been widely studied such as a spin ice system with the pyrochlore structure Ho$_{2}$Ti$_{2}$O$_{7}$. However, spin frustration effects on conduction electrons have been less studied so far. Here, we have investigated spin frustration effects near the insulator-metal (Mott) transition in pyrochlore molybdates, where a paramagnetic diffuse metal state with antiferromagnetic spins is characteristic. Hole carriers were introduced by doping of Cd ions into the spin glass (Mott) insulator Y$_{2}$Mo$_{2}$O$_{7}$. The insulator to metal transition occurs at around $x = 0.1$ in (Y$_{1-x}$Cd$_{x}$)$_{2}$Mo$_{2}$O$_{7}$ with magnetically spin glass ground state. With increasing in the hole concentration, the spin glass transition disappears at around $x = 0.30$ and the resistivity shows almost no temperature dependence. Such a paramagnetic diffusive metallic character has been widely observed in R$_{2}$Mo$_{2}$O$_{7}$ (R = rare-earth ion) under high pressures. We have also measured the heat capacity in the system and found the anomalous enhancement of effective electron mass at around the transition from the spin glass metal to the paramagnetic metal phase.   

Mott transition in multi-orbital Hubbard models for iron pnictides

The bad-metal behavior of the iron pnictides has motivated a theoretical description in terms of a proximity to Mott localization. Since the parent compounds of the iron pnictides contain an even number of 3d-electrons per Fe, it is important to determine whether a Mott transition robustly exists and the nature of the possible Mott insulating phases. We address these issues in multi-orbital Hubbard models for the parent iron pnictides using a slave-spin approach. We show a metal-to-Mott-insulator transition in $xz$ and $yz$ orbitals generally exists in these models [1]. The nature of the metal-to-insulator transition may be strongly affected by the Hund's couplings. For certain values of Hund's couplings, we show there is a orbitally selective metal-to-insulator transition: the transition to a Mott insulator in the $xz$ and $yz$ orbitals takes place at the same critical coupling as the transition to either an orbitally polarized insulator or a band insulator in the other orbitals. Implications for the electronic and magnetic properties of the iron pnictides are discussed. \\[4pt] [1] R. Yu and Q. Si, arXiv:1006.2337.   

Transport and spectra in the half--filled Hubbard model

We study the issues of scaling and universality in spectral and transport properties of the infinite dimensional particle--hole symmetric Hubbard model within dynamical mean field theory. We have used and reformulated the iterated perturbation theory approach to avoid problems such as analytic continuation of Matsubara frequency quantities or calculating multi-dimensional integrals, while taking full account of the very sharp structures in the Green's functions that arise close to the Mott transitions. We find a ``coherence peak'' in the dc resistivity of the metallic regime, which appears to be a universal feature occurring at a temperature roughly equal to the low energy scale of the system and agrees qualitatively well with the pressure dependent dc resistivity experiments on Selenium doped NiS$_2$. Resistivity hysteresis across the Mott transition is found and a direct comparison of the thermal hysteresis observed in V$_2$O$_3$ with our theoretical results yields a value of the hopping integral, which we find to be in the range estimated through first-principle methods. Finally, a systematic study of optical conductivity is carried out and the changes in absorption as a result of varying interaction strength and temperature are identified.   

Spectral function near the Mott transition in the one-dimensional Hubbard model

Spectral properties near the Mott transition are investigated in the one-dimensional Hubbard model. The single-particle spectral function is calculated using the dynamical density-matrix renormalization group method, and the dominant modes are identified using the Bethe ansatz. Characteristic features near the Mott transition, such as the pseudogap, hole-pocket behavior, spectral-weight transfer, and upper Hubbard band, are explained in a unified manner in terms of spinons, holons, antiholons, and doublons. From the insulating side, the Mott transition is characterized by the emergence of a gapless mode whose dispersion relation extends up to the order of hopping (spin exchange) in the weak (strong) interaction regime caused by infinitesimal doping. From the metallic side, the transition is characterized as a loss of charge character from the mode having both spin and charge characters, while the spin excitation remains gapless and dispersing. These features cannot be explained in either the rigid-band picture or the Fermi liquid theory. I expect that generic features near Mott transitions can be deduced from the present results.   

Localized states in Mott insulator $\kappa$-(BEDT-TTF)$_2$Cu[N(CN)$_{2}$]Cl probed by photoluminescence

We present the luminescence spectra of a low-temperature Mott insulator (T$_c$=35 K) $\kappa$-(BEDT-TTF)$_2$Cu[N(CN)$_{2}$]Cl and a metal with a superconductor transition $\kappa$-(BE\-DT\--TTF)$_2$\-Cu\-[N\-(CN)$_{2}$]Br in the temperature range between 300 and 10 K. In the Mott insulating state we observe an appearance of a luminescence at 1.95 eV, which corresponds to a LUMO-HOMO transition in a BEDT-TTF molecule. This luminescence is quenched both in the higher-temperature semiconducting state of $\kappa$-(BE\-DT\--TTF)$_2$\-Cu\-[N\-(CN)$_{2}$]Cl and metallic $\kappa$-(BE\-DT\--TTF)$_2$\-Cu\-[N\-(CN)$_{2}$]Br. To our knowledge, it is the first observation of luminescence driven by electronic correlations. This observation gives an evidence of the local character of BEDT-TTF energy levels in the Mott insulating state, in contrast to the band-character in the metallic and semiconducting states.   

Quantum electric dipoles in spin liquid dimer Mott insulator

We present an effective dipolar-spin model based on the strong coupling analysis, which may explain the possible origin of the ``spin liquid Mott insulator'' of the organic triangular lattice system, kappa-ET$_{2}$Cu$_{2}$ (CN)$_{ 3}$, whose gapless spin liquid state had been discussed in the context of geometrical frustration of exchange coupling, $J$, between spins on dimer orbitals. We find out that another degrees of freedom within the insulator, quantum electric-dipoles on dimers, interacts with each other by the Coulomb interaction and brings about a significant suppression of$ J$ through the dipolar-spin coupling.   

Numerical study on the ionic Hubbard model in one and two dimensions

We investigate the quantum phase transition of an ionic Hubbard model in one and two dimensions. There are three parameters in the ionic Hubbard model, one is the hopping term $t$, the other is the Coulomb term $U$ between local spin-up and spin-down electrons, and another is the band energy $\Delta$ which corresponds to the difference in local chemical potentials of bipartite lattice. Via exact diagonalization and quantum Monte Carlo simulations, we obtained the phase boundary of Mott insulator, metal, and band insulator. We measure the ground state energy and the energy gap between the ground-state and the first excited-state energy, and also measure the order parameters such as Drude weight and double occupancy in the three phases to understand the nature of three phases.   

Spectral properties of the one-dimensional Hubbard model: cluster dynamical mean-field approaches

We investigate static and dynamic properties of the one-dimensional Hubbard model using cluster extensions of the dynamical mean-field theory. It is shown that the two different extensions, the cellular dynamical mean-field theory and the dynamic cluster approximation, yield the ground-state properties which are qualitatively in good agreement with each other. We compare the results with the Bethe ansatz results to check the accuracy of the calculation with finite sizes of clusters. We also analyze the spectral properties of the model with the focus on the spin-charge separation and discuss the dependency on the cluster size in the two approaches.   

Temperature Effects on Metal-Insulator Transitions in the Ionic Hubbard Model

The ionic Hubbard model is known to exhibit interesting transitions between metallic and insulating phases at zero temperature. We investigate finite-temperature phase transitions in the model. For an extensive finite-temperature study, we apply a dynamical mean- field theory with continuous-time quantum Monte Carlo method employed as an impurity solver. We examine how the transitions at zero temperature evolve as the temperature increases. We also discuss the effects of temperature on the nature of transitions and other properties of the system.