Showing papers on "Mott transition published in 2008"
TL;DR: In this article, the formation of a Mott insulator of a repulsively interacting two-component Fermi gas in an optical lattice has been studied, and it is identified by three features: a drastic suppression of doubly occupied lattice sites, a strong reduction of the compressibility inferred from the response of double occupancy to an increase in atom number, and the appearance of a gapped mode in the excitation spectrum.
Abstract: Strong interactions between electrons in a solid material can lead to surprising properties. A prime example is the Mott insulator, in which suppression of conductivity occurs as a result of interactions rather than a filled Bloch band. Proximity to the Mott insulating phase in fermionic systems is the origin of many intriguing phenomena in condensed matter physics, most notably high-temperature superconductivity. The Hubbard model, which encompasses the essential physics of the Mott insulator, also applies to quantum gases trapped in an optical lattice. It is therefore now possible to access this regime with tools developed in atomic physics. However, an atomic Mott insulator has so far been realized only with a gas of bosons, which lack the rich and peculiar nature of fermions. Here we report the formation of a Mott insulator of a repulsively interacting two-component Fermi gas in an optical lattice. It is identified by three features: a drastic suppression of doubly occupied lattice sites, a strong reduction of the compressibility inferred from the response of double occupancy to an increase in atom number, and the appearance of a gapped mode in the excitation spectrum. Direct control of the interaction strength allows us to compare the Mott insulating regime and the non-interacting regime without changing tunnel-coupling or confinement. Our results pave the way for further studies of the Mott insulator, including spin-ordering and ultimately the question of d-wave superfluidity.
761 citations
TL;DR: Renormalization group analysis shows that topological Mott phases displaying the quantum Hall and the quantum spin Hall effects are found for spinless and spin fermion models, respectively.
Abstract: We consider extended Hubbard models with repulsive interactions on a honeycomb lattice, and the transitions from the semimetal to Mott insulating phases at half-filling. Because of the frustrated nature of the second-neighbor interactions, topological Mott phases displaying the quantum Hall and the quantum spin Hall effects are found for spinless and spin fermion models, respectively. The mean-field phase diagram is presented and the fluctuations are treated within the random phase approximation. Renormalization group analysis shows that these states can be favored over the topologically trivial Mott insulating states.
455 citations
TL;DR: In this article, the Mott transition in the Hubbard model at half-filling using cluster dynamical mean field theory (DMFT) is investigated. But the authors show that the presence of short-range correlations on top of the on-site correlations does not change the order of the transition between the paramagnetic metal and the paramagnetworks, which remains first order.
Abstract: We address the nature of the Mott transition in the Hubbard model at half-filling using cluster dynamical mean field theory (DMFT). We compare cluster-DMFT results with those of single-site DMFT. We show that inclusion of the short-range correlations on top of the on-site correlations does not change the order of the transition between the paramagnetic metal and the paramagnetic Mott insulator, which remains first order. However, the short range correlations reduce substantially the critical U and modify the shape of the transition lines. Moreover, they lead to very different physical properties of the metallic and insulating phases near the transition point. Approaching the transition from the metallic side, we find an anomalous metallic state with very low coherence scale. The insulating state is characterized by the narrow Mott gap with pronounced peaks at the gap edge.
216 citations
TL;DR: In this paper, density functional theory calculations for the parent compound LaFeAsO of the recently discovered 26 K Fe-based superconductor were performed and it was shown that the ground state is an ordered antiferromagnet, with staggered moment of about 2.3
Abstract: We report density functional theory calculations for the parent compound LaFeAsO of the recently discovered 26 K Fe-based superconductor ${\text{LaFeAsO}}_{1\ensuremath{-}x}{\text{F}}_{x}$. We find that the ground state is an ordered antiferromagnet, with staggered moment of about $2.3\text{ }{\ensuremath{\mu}}_{B}$, on the border with the Mott insulating state. We fit the bands crossing the Fermi surface, derived from Fe and As, to a tight-binding Hamiltonian using maximally localized Wannier functions on $\text{Fe}\text{ }3d$ and $\text{As}\text{ }4p$ orbitals. The model Hamiltonian accurately describes the Fermi surface obtained via first-principles calculations. Due to the evident proximity of superconductivity to antiferromagnetism and the Mott transition, we suggest that the system may be an analog of the electron-doped cuprates, where antiferromagnetism and superconductivity coexist.
208 citations
TL;DR: Results are reported for the classic correlated insulator MnO that reproduce a simultaneous moment collapse, volume collapse and metallization transition near the observed pressure, and identify the mechanism as collapse of the magnetic moment due to an increase of crystal-field splitting, rather than to variation in the bandwidth.
Abstract: The precise mechanism of the insulator-to-metal transition in MnO has been unravelled by a computational approach that shows that the transition is a result of the simultaneous collapse of the magnetic moment.
159 citations
TL;DR: In this article, the authors studied the zero-temperature phase transition in two dimensions from a Fermi liquid to a paramagnetic Mott insulator with a spinon surface, and showed that the approach to the bandwidth controlled Mott transition from the metallic side is accompanied by a vanishing quasiparticle residue and a diverging effective mass.
Abstract: We study theoretically the zero-temperature phase transition in two dimensions from a Fermi liquid to a paramagnetic Mott insulator with a spinon Fermi surface. We show that the approach to the bandwidth-controlled Mott transition from the metallic side is accompanied by a vanishing quasiparticle residue and a diverging effective mass. The Landau parameters ${F}_{s}^{0},{F}_{a}^{0}$ also diverge. Right at the quantum critical point there is a sharply defined ``critical Fermi surface'' but no Landau quasiparticle. The critical point has a $T\text{ }\text{ln}\text{ }1/T$ specific heat and a nonzero $T=0$ resistivity. We predict an interesting universal resistivity jump in the residual resistivity at the critical point as the transition is approached from the metallic side. The crossovers out of the critical region are also studied. Remarkably the initial crossover out of criticality on the metallic side is to a marginal Fermi liquid metal. At much lower temperatures there is a further crossover into the Landau Fermi liquid. The ratio of the two crossover scales vanishes when approaching the critical point. Similar phenomena are found in the insulating side. The filling-controlled Mott transition is also studied. Implications for experiments on the layered triangular lattice organic material $\ensuremath{\kappa}\ensuremath{-}{(\text{ET})}_{2}{\text{Cu}}_{2}{(\text{CN})}_{3}$ are discussed.
159 citations
TL;DR: In this paper, the metal-insulator transition (MIT) and the structural phase transition (SPT) are simultaneously monitored by currentvoltage curve and diffraction measurements using a synchrotron micro-x-ray beam.
Abstract: In order to clarify whether ${\text{VO}}_{2}$ is a Mott insulator or a Peierls insulator, the metal-insulator transition (MIT) and the structural phase transition (SPT) are simultaneously monitored for ${\text{VO}}_{2}$ films by current-voltage curve and diffraction measurements using a synchrotron micro-x-ray beam. In the regime showing a metallic conductivity below the SPT temperature (approximately $70\text{ }\ifmmode^\circ\else\textdegree\fi{}\text{C}$), only the diffraction planes of the monoclinic structure are observed, while planes of the tetragonal structure are absent. This observation reveals the presence of a monoclinic and metal phase between the MIT and the SPT as a characteristic of a Mott insulator.
115 citations
TL;DR: Signals of the Mott phase in the density profile and in time-of-flight experiments are investigated and above a critical value of the on-site interaction, a Mott-insulating phase appears in the system.
Abstract: We study theoretically the Mott metal-insulator transition for a system of fermionic atoms confined in a three-dimensional optical lattice and a harmonic trap. We describe an inhomogeneous system of several thousand sites using an adaptation of dynamical mean-field theory solved efficiently with the numerical renormalization group method. Above a critical value of the on-site interaction, a Mott-insulating phase appears in the system. We investigate signatures of the Mott phase in the density profile and in time-of-flight experiments.
90 citations
TL;DR: Experimental evidence of a nonvolatile electric-pulse-induced insulator-to-metal transition and possible superconductivity in the Mott insulator GaTa4 Se8 is reported.
Abstract: Metal-insulator transitions (MIT) belong to a class of fascinating physical phenomena, which includes superconductivity, and colossal magnetoresistance (CMR), that are associated with drastic modifications of electrical resistance. In transition metal compounds, MIT are often related to the presence of strong electronic correlations that drive the system into a Mott insulator state. In these systems the MIT is usually tuned by electron doping or by applying an external pressure. However, it was noted recently that a Mott insulator should also be sensitive to other external perturbations such as an electric field. We report here the first experimental evidence of a non-volatile electric-pulse-induced insulator-to-metal transition and possible superconductivity in the Mott insulator GaTa4Se8. Our Scanning Tunneling Microscopy experiments show that this unconventional response of the system to short electric pulses arises from a nanometer scale Electronic Phase Separation (EPS) generated in the bulk material.
76 citations
TL;DR: It is demonstrated that the reentrant behavior is characteristic of the Mott transition with intermediate geometrical frustration and indeed consistent with recent experimental results of organic materials.
Abstract: We investigate the Hubbard model on the anisotropic triangular lattice by means of the cellular dynamical mean-field theory. The phase diagram determined in the Hubbard interaction versus temperature plane shows novel reentrant behavior in the Mott transition due to the competition between Fermi-liquid formation and magnetic correlations under geometrical frustration. We demonstrate that the reentrant behavior is characteristic of the Mott transition with intermediate geometrical frustration and indeed consistent with recent experimental results of organic materials.
71 citations
TL;DR: The superfluid to Mott insulator transition in cavity polariton arrays is analyzed using the variational cluster approach, taking into account quantum fluctuations exactly on finite length scales.
Abstract: The superfluid to Mott insulator transition in cavity polariton arrays is analyzed using the variational cluster approach, taking into account quantum fluctuations exactly on finite length scales. Phase diagrams in one and two dimensions exhibit important non-mean-field features. Single-particle excitation spectra in the Mott phase are dominated by particle and hole bands separated by a Mott gap. In contrast to Bose-Hubbard models, detuning allows for changing the nature of the bosonic particles from quasilocalized excitons to polaritons to weakly interacting photons. The Mott state with density one exists up to temperatures $T/g\ensuremath{\gtrsim}0.03$, implying experimentally accessible temperatures for realistic cavity couplings $g$.
TL;DR: In this article, a new model suggests that metal-insulator transitions in dilute 2D electron gases arise from strong correlations driven by non-local Coulomb interactions, providing an alternative view to that which considers disorder to be the over-riding influence.
Abstract: Evidence for metal–insulator transitions in dilute 2D electron gases has sparked controversy and debate. A new model suggests such behaviour could arise from strong correlations driven by non-local Coulomb interactions, providing an alternative view to that which considers disorder to be the over-riding influence.
TL;DR: This work studies the phase diagram of indirect excitons in coupled quantum wells and shows that the system undergoes a phase transition to an unbound electron-hole plasma and measures the exciton diamagnetism to show that the transition is associated with an abrupt increase in theexciton radius.
Abstract: In this work we study the phase diagram of indirect excitons in coupled quantum wells and show that the system undergoes a phase transition to an unbound electron-hole plasma. This transition is manifested as an abrupt change in the photoluminescence linewidth and peak energy at some critical power density and temperature. By measuring the exciton diamagnetism, we show that the transition is associated with an abrupt increase in the exciton radius. We find that the transition is stimulated by the presence of direct excitons in one of the wells and show that they serve as a catalyst of the transition.
Journal Article•
TL;DR: In this paper, the authors demonstrate that the underlying LDA band structure, buttressed by on-site interactions (U, J) treated within the dynamical DMFT ansatz, provides a realistic description of the Mott transition in MnO without input from experiment.
Abstract: Summary and Outlook These results demonstrate that the underlying LDA band structure, buttressed by on-site inter-actions (U, J) treated within the dynamical DMFT ansatz, provide a realistic description of theMott transition in MnO without input from experiment. This study finally allows a determinationof the mechanism of the transition, which could not be uncovered by experiment alone: the mag-netic moment collapse, volume collapse, and metal-insulator transitions occur simultaneously, butit is the increasing crystal field splitting (encroachment of the O 2− ion on the internal structure ofthe Mn ion) and not the increasing bandwidth that tips the balance.The current results illustrate success of the LDA+DMFT approach in describing a pressure-driven Mott transition in a strongly correlated insulator, joining the growing number of successesof this approach in other strongly correlated real materials. The Kondo volume collapse tran-sition in Ce 15,28 and other elemental lanthanides, 29 and the realistic modeling of parts of thecomplex phase diagram
TL;DR: The fingerprint of Mott physics in heavy electron systems with the appearance of surfaces in momentum space where the self-energy diverges is identified and experimental consequences of this scenario for photoemission, compressibility, optical conductivity, susceptibility, and specific heat are derived.
Abstract: We describe the $T=0$ quantum phase transition in heavy-fermion systems as an orbital-selective Mott transition (OSMT) using a cluster extension of dynamical mean-field theory. This transition is characterized by the emergence of a new intermediate energy scale corresponding to the opening of a pseudogap and the vanishing of the low-energy hybridization between light and heavy electrons. We identify the fingerprint of Mott physics in heavy electron systems with the appearance of surfaces in momentum space where the self-energy diverges and we derive experimental consequences of this scenario for photoemission, compressibility, optical conductivity, susceptibility, and specific heat.
TL;DR: In this paper, single-crystal VO2 nanowires were synthesized using atmospheric pressure and physical vapor deposition and outfitted with electrodes for current-voltage measurements, and the Mott insulator-to-metal transition temperatures of several VO2 wires with varying lateral dimensions were determined by measuring the voltage values at which the sharp current step, signaling that the occurrence of the insulator to metal or the reverse transitions had taken place.
Abstract: Single-crystal VO2 nanowires were synthesized using atmospheric-pressure and physical vapor deposition and outfitted with electrodes for current−voltage measurements. The Mott insulator-to-metal transition temperatures of several nanowires with varying lateral dimensions were determined by measuring the voltage values at which the sharp current step, signaling that the occurrence of the insulator-to-metal or the reverse transitions, had taken place. The observed Mott transition temperatures, which ranged between 62 and 70 °C for the nanowires measured, trended downward with decreasing nanowire width. We ascribe this to strong interactions between the nanowire and the underlying silica substrate. However, the scatter in the Mott-temperature versus nanowire width exceeded the experimental uncertainty in the values of the Mott temperature, indicating that other parameters also contribute to the precise value of the Mott transition temperature of nanostructured VO2.
TL;DR: The dynamics of charge carriers close to the Mott transition is explored theoretically and experimentally in the quasi-two-dimensional organic charge-transfer salt, kappa-(BEDT-TTF)_( 2)Cu[N(CN)_(2)]Br_(x)Cl_(1-x), with varying Br content.
Abstract: The dynamics of charge carriers close to the Mott transition is explored theoretically and experimentally in the quasi-two-dimensional organic charge-transfer salt, -(BEDT-TTF)2Cu[N(CN)2]BrxCl1-x, with varying Br content. The frequency dependence of the conductivity deviates significantly from simple Drude model behavior: there is a strong redistribution of spectral weight as the Mott transition is approached and with temperature. The effective mass of the quasiparticles increases considerably when coming close to the insulating phase. A dynamical mean-field-theory treatment of the relevant Hubbard model gives good quantitative description of experimental data.
TL;DR: A natural decomposition of the one-particle (photoemission) energy gap into two components is found in the underdoped regime, and the gap in the nodal regions, stemming from the anomalous self-energy, decreases with decreasing doping.
Abstract: We study the superconducting state of the hole-doped two-dimensional Hubbard model using cellular dynamical mean-field theory, with the Lanczos method as impurity solver. In the underdoped regime, we find a natural decomposition of the one-particle (photoemission) energy gap into two components. The gap in the nodal regions, stemming from the anomalous self-energy, decreases with decreasing doping. The antinodal gap has an additional contribution from the normal component of the self-energy, inherited from the normal-state pseudogap, and it increases as the Mott insulating phase is approached.
TL;DR: It is shown that in the presence of weak disorder the dimensionality of the system has crucial effects on the size of the critical region that is probed experimentally.
Abstract: We explain, in a consistent manner, the set of seemingly conflicting experiments on the finite temperature Mott critical point, and demonstrate that the Mott transition is in the Ising universality class. We show that, even though the thermodynamic behavior of the system near such critical point is described by an Ising order parameter, the global conductivity can depend on other singular observables and, in particular, on the energy density. Finally, we show that in the presence of weak disorder the dimensionality of the system has crucial effects on the size of the critical region that is probed experimentally.
TL;DR: In this article, the Pariser-Parr-Pople model was adopted to calculate the pump-probe signal in one-dimensional Mott insulators and showed that when the photoexcitation density is below about 10%, an extremely small portion of the energy eigenstates dominates the optical process and the spin-charge separation holds in these dominant energy states.
Abstract: We theoretically investigate the dynamics of insulator-metal transition induced by light-pulse excitation in one-dimensional Mott insulators. We adopt the Pariser--Parr--Pople model and the pump-probe signal is obtained by numerically calculating the time development of the state excited by light pulse. When the photoexcitation density is below about 10%, an extremely small portion of the energy eigenstates dominates the optical process and the spin-charge separation holds in these dominant energy eigenstates. As a result, the Mott gap and the short-range antiferromagnetic (AF) spin order are preserved and spin relaxation does not occur. When the density is above the value, a metallic state without the Mott gap is photogenerated and the magnitude of the short-range AF spin order is significantly reduced by photoexcitation. This is consistent with the experimentally observed photoinduced Mott transition. Furthermore, spin relaxation occurs in the metallic state. This photoinduced Mott transition is a manifestation of the spin-charge coupling in the intensely photoexcited states.
TL;DR: In this article, the authors examined the possibility of a Mott localization of the $f$ electrons for a finite value of the hybridization $V$ and found that they are in the same universality class as for the Kondo breakdown QCP, with the following notable features.
Abstract: Starting with an extended version of the Anderson lattice where the $f$ electrons are allowed a weak dispersion, we examine the possibility of a Mott localization of the $f$ electrons for a finite value of the hybridization $V$. We study the fluctuations at the quantum critical point (QCP) where the $f$ electrons localize. We find that they are in the same universality class as for the Kondo breakdown QCP, with the following notable features. The quantum critical regime sees the appearance of an additional energy scale separating two universality classes. In the low energy regime, the fluctuations are dominated by massless gauge modes, while in the intermediate energy regime, the fluctuations of the modulus of the order parameter are the most relevant ones. In the latter regime, electric transport simplifies drastically, leading to a quasilinear resistivity in three dimensional and anomalous exponents lower than $T$ in two dimensional. This rather unique feature of the quantum critical regime enables us to make experimentally testable predictions.
TL;DR: In this paper, the authors investigate a quarter-filled two-band Hubbard model involving a crystal-field splitting, which lifts the orbital degeneracy as well as interorbital hopping (interband hybridization).
Abstract: We investigate a quarter-filled two-band Hubbard model involving a crystal-field splitting, which lifts the orbital degeneracy as well as an interorbital hopping (interband hybridization). Both terms are relevant to the realistic description of correlated materials such as transition-metal oxides. The nature of the Mott metal-insulator transition is clarified and is found to depend on the magnitude of the crystal-field splitting. At large values of the splitting, a transition from a two-band to a one-band metal is first found as the on-site repulsion is increased and is followed by a Mott transition for the remaining band, which follows the single-band (Brinkman-Rice) scenario well documented previously within dynamical mean-field theory. At small values of the crystal-field splitting, a direct transition from a two-band metal to a Mott insulator with partial orbital polarization is found, which takes place simultaneously for both orbitals. This transition is characterized by a vanishing of the quasiparticle weight for the majority orbital but has a first-order character for the minority orbital. It is pointed out that finite-temperature effects may easily turn the metallic regime into a bad metal close to the orbital polarization transition in the metallic phase.
TL;DR: In this paper, the Burstein-Moss effect was used to explain the metal-like and metal-semiconductor transition in a degenerate semiconductor with varying substrate temperature.
Abstract: Al-doped ZnO films were grown on glass substrates by the pulsed-laser deposition technique with varying substrate temperatures. The optical band gap decreases from 3.64 to 3.46 eV as the substrate temperature increases from 350 to 450 °C, illustrating the increase in Al content in the context of a degenerate semiconductor, and can be explained in the framework of the Burstein–Moss effect. All films show optical transparency greater than 85%. Al:ZnO films show a metal–semiconductor transition to metal-like behavior as the substrate temperature increases from 350 to 450 °C. The observed metal-like and metal–semiconductor transitions are explained by taking into account the Mott phase transition and localization effects due to defects. The resistivity decreases from 896 to 470 µΩ cm as the substrate temperature increases from 350 to 450 °C. In addition, the competition between the thermally activated carriers and scattering effects of free carriers in a degenerate semiconductor can also explain the metal–semiconductor transition.
TL;DR: In this paper, the authors study the paramagnetic Anderson-Hubbard model using an extension of dynamical mean field theory (DMFT), known as statistical DMFT, that allows treating disorder and strong electronic correlations on equal footing.
Abstract: We study the paramagnetic Anderson-Hubbard model using an extension of dynamical mean field theory (DMFT), known as statistical DMFT, that allows us to treat disorder and strong electronic correlations on equal footing. An approximate nonlocal Green's function is found for individual disorder realizations and then configuration averaged. We apply this method to two-dimensional lattices with up to 1000 sites in the strong disorder limit, where an atomic-limit approximation is made for the self-energy. We investigate the scaling of the inverse participation ratio at quarter- and half-filling, and find a nonmonotonic dependence of the localization length on the interaction strength. For strong disorder, we do not find evidence for an insulator-metal transition, and the disorder potential becomes unscreened near the Mott transition. Furthermore, strong correlations suppress the Altshuler-Aronov density of states anomaly near half-filling.
TL;DR: A two-color lattice is used to break the homogeneous site occupation of an atomic Mott insulator of bosonic 87Rb and the appearance of additional correlation peaks evidences the redistribution of the atoms into a strongly inhomogeneous insulating state, in quantitative agreement with the predictions.
Abstract: We use a two-color lattice to break the homogeneous site occupation of an atomic Mott insulator of bosonic $^{87}\mathrm{Rb}$. We detect the disruption of the ordered Mott domains via noise correlation analysis of the atomic density distribution after time of flight. The appearance of additional correlation peaks evidences the redistribution of the atoms into a strongly inhomogeneous insulating state, in quantitative agreement with the predictions.
TL;DR: In this article, a cluster dynamical mean-field theory of the periodic Anderson model in three dimensions was developed, taking a cluster of two sites as a basic reference frame, showing the basic features of the Doniach phase diagram: a paramagnetic Fermi liquid state, an antiferromagnetic state, and a transition between them.
Abstract: We develop a cluster dynamical mean-field theory of the periodic Anderson model in three dimensions, taking a cluster of two sites as a basic reference frame. The mean-field theory displays the basic features of the Doniach phase diagram: a paramagnetic Fermi liquid state, an antiferromagnetic state, and a transition between them. In contrast with spin-density wave theories, the transition is accompanied by a large increase of the effective mass everywhere on the Fermi surface and a substantial change of the Fermi surface shape across the transition. To understand the nature and the origin of the phases near the transition, we investigate the paramagnetic solution underlying the antiferromagnetic state, and identify the transition as a point where the $f$ electrons decouple from the conduction electrons undergoing an orbitally selective Mott transition. This point turns out to be intimately related to the two-impurity Kondo model quantum critical point. In this regime, nonlocal correlations become important and result in significant changes in the photoemission spectra and the de Haas--van Alphen frequencies. The transition involves considerable $f$ spectral weight transfer from the Fermi level to its immediate vicinity, rather than to the Hubbard bands as in single-site dynamical mean-field theory.
TL;DR: In this paper, an electron correlation-driven superconductivity in an uncompensated boron doped diamond was shown to be strikingly close to an earlier observation of Anderson-Mott insulator to metal transition.
Abstract: Superconductivity in an uncompensated boron doped diamond, a very recent observation, is strikingly close to an earlier observation of Anderson-Mott insulator to metal transition, prompting us to suggest an electron correlation driven superconductivity in an impurity band. Random coulomb potential remove a three fold orbital degeneracy of boron acceptor states, resulting in an effective single, narrow, tight binding and half-filled band of holes. Singlet coupling between spins of neighboring neutral acceptors B 0 -B 0 is the seed of pairing. Across the insulator to metal transition, a small and equal fraction of charged B + and B - states (free carriers) get spontaneously generated and delocalize. Thereupon neutral singlets resonate and get charged resulting in a resonating valence bond (RVB) superconducting state.
TL;DR: It is found that the diffusion of unbound electrons and holes is ambipolar and is characterized by a large diffusion coefficient, similar to that found in p-i-n junctions.
Abstract: In this Letter, we study the diffusion properties of photoexcited carriers in coupled quantum wells around the Mott transition. We find that the diffusion of unbound electrons and holes is ambipolar and is characterized by a large diffusion coefficient, similar to that found in $p\mathrm{\text{\ensuremath{-}}}i\mathrm{\text{\ensuremath{-}}}n$ junctions. Correlation effects in the excitonic phase are found to significantly suppress the carriers' diffusion. We show that this difference in diffusion properties gives rise to the appearance of a photoluminescence ring pattern around the excitation spot at the Mott transition.
TL;DR: It is shown that the insulator-to-metal transition at Pc is driven by the widening of the bandwidth rather than structural changes or reduction of crystal field splittings and it shows a redistribution of the electronic occupation within the t2g bands.
Abstract: Using Car-Parrinello molecular dynamics we investigate the behavior of the low-dimensional multiorbital Mott insulator TiOCl under pressure. We show that the system undergoes two consecutive phase transitions, first at ${P}_{c}$ from a Mott-insulator to a metallic phase in the $ab$ plane with a strong Ti-Ti dimerization along $b$. At a pressure ${P}_{c}^{\ensuremath{'}}g{P}_{c}$ the dimerization disappears and the system behaves as a uniform metal. This second transition has not yet been reported experimentally. We show that the insulator-to-metal transition at ${P}_{c}$ is driven by the widening of the bandwidth rather than structural changes or reduction of crystal field splittings and it shows a redistribution of the electronic occupation within the ${t}_{2g}$ bands. Our computed pressure-dependent lattice parameters are consistent with experimental observations and the existing controversy on the change of crystal symmetry at high pressures is discussed.