Pressure-induced Mott Transition Research Articles

Correlated electron physics near a site-selective pressure-induced Mott transition in α-LiFe5O8

The Mott insulator-to-metal transition (IMT) driven by electron correlations has been among the main research topics in materials science over the past decades. The complex interplay between electronic and lattice degrees of freedom leads to various transition scenarios. Of particular interest may be the case of a transition involving the formation of complex phases comprising regions that differ significantly in their physical properties within the same material. Here, we present the results that advance the understanding of the IMT phenomenon, offering the documentation of a pure site-selective mechanism that is not complicated by any structural and spin transformation. Combining XRD, resistivity, Mössbauer and Raman spectroscopy measurements, we provide evidence for a pure pressure-induced Mott transition in α-LiFe5O8, characterized by site-selective delocalization of electrons, leading to the formation, above ~65 GPa, of a site-selective Mott phase consisting of metallic and insulating sublattices. We note that the electron delocalization in the partially disordered octahedral sublattice cannot be understood purely in terms of a Mott transition, the Anderson-Mott transition picture seems more adequate.

Pressure-induced transition from a Mott insulator to a ferromagnetic Weyl metal in La2O3Fe2Se2

The insulator-metal transition in Mott insulators, known as the Mott transition, is usually accompanied with various novel quantum phenomena, such as unconventional superconductivity, non-Fermi liquid behavior and colossal magnetoresistance. Here, based on high-pressure electrical transport and XRD measurements, and first-principles calculations, we find that a unique pressure-induced Mott transition from an antiferromagnetic Mott insulator to a ferromagnetic Weyl metal in the iron oxychalcogenide La2O3Fe2Se2 occurs around 37 GPa without structural phase transition. Our theoretical calculations reveal that such an insulator-metal transition is mainly due to the enlarged bandwidth and diminishing of electron correlation at high pressure, fitting well with the experimental data. Moreover, the high-pressure ferromagnetic Weyl metallic phase possesses attractive electronic band structures with six pairs of Weyl points close to the Fermi level, and its topological property can be easily manipulated by the magnetic field. The emergence of Weyl fermions in La2O3Fe2Se2 at high pressure may bridge the gap between nontrivial band topology and Mott insulating states. Our findings not only realize ferromagnetic Weyl fermions associated with the Mott transition, but also suggest pressure as an effective controlling parameter to tune the emergent phenomena in correlated electron systems.

Emergence of quantum critical charge and spin-state fluctuations near the pressure-induced Mott transition in MnO, FeO, CoO, and NiO

We perform a comprehensive theoretical study of the pressure-induced evolution of the electronic structure, magnetic state, and phase stability of the late transition metal monoxides MnO, FeO, CoO, and NiO using a fully charge self-consistent $\mathrm{DFT}+\mathrm{dynamical}$ mean-field theory method. Our results reveal that the pressure-induced Mott insulator-to-metal phase transition in MnO-NiO is accompanied by a simultaneous collapse of local magnetic moments and lattice volume, implying a complex interplay between chemical bonding and electronic correlations. We compute the pressure-induced evolution of relative weights of the different valence states and spin-state configurations. Employing the concept of fluctuating valence in a correlated solid, we demonstrate that in MnO, FeO, and CoO a Mott insulator-metal transition and collapse of the local moments is accompanied by a sharp crossover of the spin-state and valence configurations. Our microscopic explanation of the magnetic collapse differs from the accepted picture and points out a remarkable dynamical coexistence (frustration) of the high-, intermediate-, and low-spin states. In particular, in MnO, the magnetic collapse is found to be driven by the appearance of the intermediate-spin state (IS), competing with the low-spin (LS) state; in FeO, we observe a conventional high-spin to low-spin (HS-LS) crossover. Most interestingly, in CoO, we obtain a remarkable (dynamical) coexistence of the HS and LS states, i.e., a HS-LS frustration, up to high pressure. Our results demonstrate the importance of quantum fluctuations of the valence and spin states for the understanding of quantum criticality of the Mott transitions.

Contrasting Pressure-Induced Metallization Processes in Layered Perovskites, α-Sr_{2}MO_{4} (M=V, Cr).

Electric resistivity, magnetic susceptibility, and x-ray diffraction measurements under high pressure are performed in both α-Sr_{2}VO_{4} and α-Sr_{2}CrO_{4}, which are carefully prepared with regard to their stoichiometry. These measurements reveal contrasting and peculiar metallization processes of these compounds with increasing pressure. In contrast to a previously reported one in a V compound, we find two kinds of pressure-induced metallic states at low- (T<50 K) and high-temperature (T>100 K) regions. The high-temperature one seems to emerge beyond the pressure-induced Mott transition. The low-temperature one might imply a topological nature of the V compound, which is expected in the spin-orbit coupled 3d^{1} state that arises from their degenerated d_{zx} and d_{yz} orbits.

Inverse pressure-induced Mott transition in TiPO4

TiPO$_4$ shows interesting structural and magnetic properties as temperature and pressure are varied, such as a spin-Peierls phase transition and the development of incommensurate modulations of the lattice. Recently, high pressure experiments for TiPO$_4$ reported two new structural phases appearing at high pressures, the so-called phases IV and V [M. Bykov et al., Angew. Chem. Int. Ed. 55, 15053]. The latter was shown to include the first example of 5-fold O-coordinated P-atoms in an inorganic phosphate compound. In this work we characterize the electronic structure and other physical properties of these new phases by means of ab-initio calculations, and investigate the structural transition. We find that the appearance of phases IV and V coincides with a collapse of the Mott insulating gap and quenching of magnetism in phase III as pressure is applied. Remarkably, our calculations show that in the high pressure phase V, these features reappear, leading to an antiferromagnetic Mott insulating phase, with robust local moments.

Effects of Disorder on the Pressure-Induced Mott Transition in κ-(BEDT-TTF)2Cu[N(CN)2]Cl

We present a study of the influence of disorder on the Mott metal-insulator transition for the organic charge-transfer salt κ -(BEDT-TTF) 2 Cu[N(CN) 2 ]Cl. To this end, disorder was introduced into the system in a controlled way by exposing the single crystals to X-ray irradiation. The crystals were then fine-tuned across the Mott transition by the application of continuously controllable He-gas pressure at low temperatures. Measurements of the thermal expansion and resistance show that the first-order character of the Mott transition prevails for low irradiation doses achieved by irradiation times up to 100 h. For these crystals with a moderate degree of disorder, we find a first-order transition line which ends in a second-order critical endpoint, akin to the pristine crystals. Compared to the latter, however, we observe a significant reduction of both, the critical pressure p c and the critical temperature T c . This result is consistent with the theoretically-predicted formation of a soft Coulomb gap in the presence of strong correlations and small disorder. Furthermore, we demonstrate, similar to the observation for the pristine sample, that the Mott transition after 50 h of irradiation is accompanied by sizable lattice effects, the critical behavior of which can be well described by mean-field theory. Our results demonstrate that the character of the Mott transition remains essentially unchanged at a low disorder level. However, after an irradiation time of 150 h, no clear signatures of a discontinuous metal-insulator transition could be revealed anymore. These results suggest that, above a certain disorder level, the metal-insulator transition becomes a smeared first-order transition with some residual hysteresis.

Erratum: Pressure-Induced Mott Transition Followed by a 24-K Superconducting Phase in BaFe_{2}S_{3} [Phys. Rev. Lett. 115, 246402 (2015)

This corrects the article DOI: 10.1103/PhysRevLett.115.246402.

Pressure-induced Mott transition in an organic superconductor with a finite doping level.

We report the pressure study of a doped organic superconductor with a Hall coefficient and conductivity measurements. We find that maximally enhanced superconductivity and a marginal-Fermi liquid appear around a certain pressure where mobile carriers increase critically, suggesting a possible quantum phase transition between strongly and weakly correlated regimes. This observation points to the presence of a criticality in Mottness for a doped Mott insulator with tunable correlation.

Electronic and magnetic structure of neutral radical FBBO

The fluorinated oxobenzo-bridged bisdithiazolyl radical FBBO was recently observed to undergo a pressure-induced Mott insulator-to-metal transition, suggesting a novel organic system for studying Mott physics. This report describes the electronic structure of this material in relation to the observed magnetic response at low pressures. Through analysis of antiferromagnetic resonance measurements, we identify a layered antiferromagnetic ordered phase below ${T}_{N}=13$ K at ambient pressure, which requires strong ferromagnetic coupling between nearest neighbours. The origin of such coupling is elucidated from both molecular and solid-state electronic-structure calculations, which suggest a minimal two-orbital model with strong Hund's-rule coupling. This layered phase is partially frustrated by a second-nearest-neighbor antiferromagnetic coupling, which drives a magnetic phase transition at elevated pressure. On the basis of the two-orbital model, we suggest the pressure-induced Mott transition to proceed via rehybridization of the frontier molecular orbitals, resulting in a half-filled insulator to quarter-filled metal crossover.

Crystallographic transitions related to magnetic and electronic phenomena in TM compounds under high pressure

Structural aspects of magnetic/electronic transitions in strongly correlated systems in a regime of very high static density are the main issues of this article. To achieve this objective, we have carried out series of X-ray diffraction (XRD) measurements up to 120 GPa using diamond anvil cells (DACs) to probe structural features specifically related to pressure-induced (PI) magnetic/electronic phenomena in transition metal (TM) compounds. The types of phenomena are the Mott transition (MT), high-spin (HS) to low-spin (LS) transition, valence transformations, the quenching of the orbital term, and Verwey transition. In all these cases, the electronic transition may induce or be a consequence of structural alterations. These studies provide essential information concerning: (i) structural alterations attributed to the valence transformation in some TM compounds; (ii) mechanisms and precursors responsible for the PI MT; features of the structural transformation specifically attributed to the MT for different types of the electronic transitions (Mott–Hubbard and Charge-Transfer); (iii) the mechanism of the PI degradation of the magnetic state due to HS–LS transition and their influence on the structural properties of material; (iv) mechanism of Verwey transition in magnetite; (v) structure of new phases at high pressure. As example cases we present recent results in some Fe-oxides, halides, and sulfides.

Reentrant Mott transition from a Fermi liquid to a spin-gapped insulator in an organicspin-1/2 triangular-lattice antiferromagnet

The pressure-induced Mott transition is investigated in a quasi-two-dimensional molecular conductor,EtMe3P[Pd(dmit)2]2 (P 21/m) with a triangular lattice, which shows a valence bond solid state at ambient pressure.Near the critical pressure of the Mott boundary, an insulator-to-metal transition occurs at18 K, followed by a first-order reentrant metal-to-insulator transition below 12 K. Finally, asuperconducting transition from the insulating state is observed at 5.2 K. The reentrantmetal–insulator transition temperature is lowered by application of magnetic field. Theresult indicates the presence of a spin gap diminished by the Zeeman energyin the reentrant insulating phase neighbouring on the superconducting phase.

Unconventional critical behaviour in a quasi-two-dimensional organic conductor

Changing the interactions between particles in an ensemble--by varying the temperature or pressure, for example--can lead to phase transitions whose critical behaviour depends on the collective nature of the many-body system. Despite the diversity of ingredients, which include atoms, molecules, electrons and their spins, the collective behaviour can be grouped into several families (called 'universality classes') represented by canonical spin models. One kind of transition, the Mott transition, occurs when the repulsive Coulomb interaction between electrons is increased, causing wave-like electrons to behave as particles. In two dimensions, the attractive behaviour responsible for the superconductivity in high-transition temperature copper oxide and organic compounds appears near the Mott transition, but the universality class to which two-dimensional, repulsive electronic systems belongs remains unknown. Here we present an observation of the critical phenomena at the pressure-induced Mott transition in a quasi-two-dimensional organic conductor using conductance measurements as a probe. We find that the Mott transition in two dimensions is not consistent with known universality classes, as the observed collective behaviour has previously not been seen. This peculiarity must be involved in any emergent behaviour near the Mott transition in two dimensions.

Neutron Scattering Study on Metal-Insulator Transitions of BaCoS2Induced by External Pressure and by Ni-Doping

Magnetic excitation spectra of BaCo 1- x Ni x S 2 , which has a layered structure and exhibits Mott metal-insulator (M-I) transition with varying x and/or with increasing external pressure, have been measured and analyzed. The random phase treatment has been found to be able to basically describe the observed dynamical magnetic behavior except in the temperature region where the intrinsic magnetic correlation length is larger than the average separation of Ni atoms. The characteristic energy of the spin fluctuation in two kinds of metallic phases induced by the Ni-doping and by the external pressure p are rather differrent. These results indicate that the pressure-induced Mott transition is mainly driven by the increase of the transfer energy, while the doping-induced transition is driven by the deviation of the electron number of each relevant band from unity.

Pressure-induced Mott transition in doped semiconductors

Application of a hydrostatic pressure upon a suitably doped, direct-gap semiconductor may provoke the occurrence of a finite activation energy. Calculation, in the case of InSb, shows this effect to be sizeable.