Pressure-induced Metal-insulator Transition Research Articles

Crystal structure, properties and pressure-induced insulator-metal transition in layered kagome chalcogenides

Layered materials with kagome lattice have attracted a lot of attention due to the presence of nontrivial topological bands and correlated electronic states with tunability. In this work, we investigate a unique van der Waals (vdW) material system, A 2 M 3 X 4 (A = K, Rb, Cs; M = Ni, Pd; X = S, Se), where transition metal kagome lattices, chalcogen honeycomb lattices and alkali metal triangular lattices coexist simultaneously. A notable feature of this material is that each Ni/Pd atom is positioned in the center of four chalcogen atoms, forming a local square-planar environment. This crystal field environment results in a low spin state S= 0 of Ni2+/Pd2+. A systematic study of the crystal growth, crystal structure, magnetic and transport properties of two representative compounds, Rb2Ni3S4 and Cs2Ni3Se4, has been carried out on powder and single crystal samples. Both compounds exhibit nonmagnetic p-type semiconducting behavior, closely related to the particular chemical environment of Ni2+ ions and the alkali metal intercalated vdW structure. Additionally, Cs2Ni3Se4 undergoes an insulator-metal transition (IMT) in transport measurements under pressure up to 87.1 GPa without any structural phase transition, while Rb2Ni3S4 shows the tendency to be metalized.

High-pressure evolution of the magnetic order in LaMnO3

A relationship between evolution of the long-range magnetic order, crystal structure, and lattice distortions in $\mathrm{LaMn}{\mathrm{O}}_{3}$ was studied by a combination of neutron diffraction and Raman spectroscopy at high pressures up to 39 and 50 GPa, respectively, covering the temperature range 5 to 290 K. The Raman spectra reveal a gradual structural phase transformation evolving in the pressure range of 4 to 17 GPa, caused by a modification of the Jahn-Teller (JT) lattice distortions from the static cooperative character to the local one. A presence of residual JT-distorted regions associated with the initial phase is detected up to 32 GPa, where the insulator-metal transition occurs. At higher pressure, the local JT distortions also vanish completely at further compression up to 50 GPa. In the neutron diffraction data, a strong suppression of the A-type antiferromagnetic (AFM) phase is observed over the pressure region of the phase transformation. This is accompanied by a noticeable reduction in magnitude of the ${Q}_{2}$ and ${Q}_{3}$ JT local modes. The effective ordered magnetic manganese moment is reduced about twice at pressures up to 14 GPa. At higher pressures up to 30 GPa, residual regions of the A-type AFM phase coexist with the magnetically disordered phase. In the range 30 to 39 GPa, i.e., during the pressure-induced insulator-metal transition, these regions disappear and, finally, the magnetically disordered metallic phase becomes the only ground state. The possible models of the insulator-metal transition in $\mathrm{LaMn}{\mathrm{O}}_{3}$ are analyzed.

Pressure-Induced Intermetallic Charge Transfer and Semiconductor-Metal Transition in Two-Dimensional AgRuO 3

Pressure-Induced Intermetallic Charge Transfer and Semiconductor-Metal Transition in Two-Dimensional AgRuO ₃

Pressure-induced concomitant topological and metal-insulator quantum phase transitions in Ce3Pd3Bi4

The electronic property and magnetic susceptibility of Ce3Pd3Bi4 were systemically investigated from 18 to 290 K for varying values of cell volume using dynamic mean-field theory coupled with density functional theory. By extrapolating to zero temperature, the ground state of Ce3Pd3Bi4 at ambient pressure is found to be a correlated semimetal due to insufficient hybridization. Upon applying pressure, the hybridization strength increases and a crossover to the Kondo insulator is observed at finite temperatures. The characteristic temperature signaling the formation of Kondo singlet, as well as the characteristic temperature associated with f-electron delocalization–localization change, simultaneously vanishes around a critical volume of 0.992 V0, suggesting that such metal–insulator transition is possibly associated with a quantum critical point. Finally, Wilson’s loop calculations indicate that the Kondo insulating side is topologically trivial, thus a topological transition also occurs across the quantum critical point.

Pressure-induced metal–insulator transition in oxygen-deficient LiNbO3-type ferroelectrics

Hydrostatic pressure and oxygen vacancies usually have deleterious effects on ferroelectric materials because both tend to reduce their polarization. In this work we use first-principles calculations to study an important class of ferroelectric materials—LiNbO3-type ferroelectrics (LiNbO3 as the prototype), and find that in oxygen-deficient LiNbO3−δ , hydrostatic pressure induces an unexpected metal–insulator transition between 8 and 9 GPa. Our calculations also find that strong polar displacements persist in both metallic and insulating oxygen-deficient LiNbO3−δ and the size of polar displacements is comparable to pristine LiNbO3 under the same pressure. These properties are distinct from widely used perovskite ferroelectric oxide BaTiO3, whose polarization is quickly suppressed by hydrostatic pressure and/or oxygen vacancies. The anomalous pressure-driven metal–insulator transition in oxygen-deficient LiNbO3−δ arises from the change of an oxygen vacancy defect state. Hydrostatic pressure increases the polar displacements of oxygen-deficient LiNbO3−δ , which reduces the band width of the defect state and eventually turns it into an in-gap state. In the insulating phase, the in-gap state is further pushed away from the conduction band edge under hydrostatic pressure, which increases the fundamental gap. Our work shows that for LiNbO3-type strong ferroelectrics, oxygen vacancies and hydrostatic pressure combined can lead to new phenomena and potential functions, in contrast to the harmful effects occurring to perovskite ferroelectric oxides such as BaTiO3.

Pressure-Induced Insulator–Metal Transition in Silicon Telluride from First-Principles Calculations

Silicon telluride (Si2Te3) is a two-dimensional semiconductor with unique structural properties due to the size contrast between Si and Te atoms. A recent experiment shows that the material turns metallic under hydrostatic pressure, while the lattice structure of the metallic phase remains to be identified. In this paper, we propose two metallic phases, M1 and M2, of Si2Te3 using the evolution algorithm and first-principles density functional theory (DFT) calculations. Unlike the presence of Si-Si dimers in the semiconducting (SC) phase, both M1 and M2 phases have individual Si atoms, which play important roles in the metallicity. Analysis of structural properties, electronic properties, dynamical as well as thermal stability is performed. The energies of these new structures are compared with the SC phase under the subsequent hydrostatic pressure up to 12 GPa. The results show that M1 and M2 phases have lower energies under high pressure, thus elucidating the appearance of the metallic phase of Si2Te3. In addition, the external pressure causes the SC phase to have an indirect-direct-indirect bandgap transition. Analysis of Raman spectra of the SC phase at a different pressure shows the shifting of the major Raman peaks, and finally disappearing confirms the phase transition. The results are in good agreement with the experimental observations. The understanding of the insulator-metal phase transition increases the potential usefulness of the material system.

Synthesis of an Antiferromagnet Ba7Fe6S14 with One-dimensional Chains Constituted by FeS4 Tetrahedra

Compounds with chain structures have received great attention due to their unique quasi-one-dimensional charge transport channel which usually leads to striking physical phenomena. Here, we report the synthesis and detailed physical properties of Ba7Fe6S14 single crystals whose crystalline structure is featured by one-dimensional chains constituted by units of FeS4 tetrahedra. Susceptibility measurements reveal three magnetic phase transitions located at T = 10.5 K, 38 K, and 121 K. Further heat capacity measurements suggest a second-order phase transition for the transition at 121 K. At ambient pressure, the compound is an insulator with a large thermal activation energy around 0.19 eV. Application of pressure can gradually suppress the insulating behavior, and pressure-induced insulator-metal transitions can be achieved for P ≥ 21.3 GPa. No superconductivity was observed with pressure up to 47 GPa. Nevertheless, the unique chain-like structure of Ba7Fe6S14 provides chances for comparative studies with the quasi-one-dimensional spin-ladder iron-based superconductor BaFe2S3 for exploring the relation among crystalline dimensionality, magnetism, and superconductivity.

Pressure-induced orbital-selective metal from the Mott insulator BaFe2Se3

A general understanding of the mechanism underlying the pressure-induced Mott insulator-metal transition in strongly correlated materials is still lacking. Here we explore the pressure-induced electronic reconstruction in BaFe 2 Se 3 , a potential two-leg ladder system for unconventional (non-BCS) superconductivity. We stress the importance of multiorbital Coulomb interactions in concert with first-principles band-structure calculations for a consistent understanding of its intrinsic Mott-Hubbard insulating state both at ambient and under pressure. We elucidate the nature of pressure-induced insulator-metal transition seen in experiment, showing that it is driven by bandwidth broadening under pressure. We reveal an orbital-selective electronic state where Mott localized and itinerant electrons coexist in compressed BaFe 2 Se 3 , which incorporates orbital-resolved scattering rates and renormalization factors hidden in the normal state at high pressures.

Magnetic phase boundary of BaVS3 clarified with high-pressure μ+SR

The magnetic nature of the quasi-one-dimensional BaVS3 has been studied as a function of temperature down to 0.25 K and pressure up to 1.97 GPa on a powder sample using the positive muon spin rotation and relaxation (mu(+) SR) technique. At ambient pressure, BaVS3 enters an incommensurate antiferromagnetic ordered state below the Neel temperature (T-N)31 K. T-N is almost constant as the pressure (p) increases from ambient pressure to 1.4 GPa, then T-N decreases rapidly for p > 1.4 GPa, and finally disappears at p similar to 1.8 GPa, above which a metallic phase is stabilized. Hence, T-N is found to be equivalent to the pressure-induced metal-insulator transition temperature (T-MI) at p > 1.4 GPa.

Structural study of the pressure-induced metal-insulator transition in LiV2O4

At ambient pressure the spinel $\mathrm{Li}{\mathrm{V}}_{2}{\mathrm{O}}_{4}$ is a metal with a heavy fermion ground state. However, under applied pressure a transition to an insulating, nonmagnetic state occurs. Powder x-ray diffraction has been used to study structural changes associated with this transition, and reveals that the ambient-pressure $Fd\overline{3}m$ spinel structure distorts to a monoclinic cell of $C2/m,C2$, or $Cm$ symmetry above 11 GPa at low temperatures. The changes of structure and properties provide evidence for orbital molecule formation in the cubic phase at high pressures and low temperatures that leads to long-range orbital molecule order in the monoclinic phase.

Pressure-induced phase transitions and superconductivity in a quasi–1-dimensional topological crystalline insulator α-Bi4Br4

Great progress has been achieved in the research field of topological states of matter during the past decade. Recently, a quasi-1-dimensional bismuth bromide, Bi4Br4, has been predicted to be a rotational symmetry-protected topological crystalline insulator; it would also exhibit more exotic topological properties under pressure. Here, we report a thorough study of phase transitions and superconductivity in a quasihydrostatically pressurized α-Bi4Br4 crystal by performing detailed measurements of electrical resistance, alternating current magnetic susceptibility, and in situ high-pressure single-crystal X-ray diffraction together with first principles calculations. We find a pressure-induced insulator-metal transition between ∼3.0 and 3.8 GPa where valence and conduction bands cross the Fermi level to form a set of small pockets of holes and electrons. With further increase of pressure, 2 superconductive transitions emerge. One shows a sharp resistance drop to 0 near 6.8 K at 3.8 GPa; the transition temperature gradually lowers with increasing pressure and completely vanishes above 12.0 GPa. Another transition sets in around 9.0 K at 5.5 GPa and persists up to the highest pressure of 45.0 GPa studied in this work. Intriguingly, we find that the first superconducting phase might coexist with a nontrivial rotational symmetry-protected topology in the pressure range of ∼3.8 to 4.3 GPa; the second one is associated with a structural phase transition from monoclinic C2/m to triclinic P-1 symmetry.

Pressure-induced metal-insulator transition in twisted bilayer graphene

Recent experiments [arXiv: 1808.07865] on twisted bilayer graphene (TBLG) show that under hydrostatic pressure, an insulating state at quarter-filling of the moir\'e superlattice (i.e., one charge per supercell) emerges, in sharp contrast with the previous ambient pressure measurements of Cao et al. where the quarter--filling state (QFS) is a metal [Nature 556, 43 & 80 (2018)]. In fact, the insulating state at the other commensurate fillings of two and three charges per supercell is also enhanced under applied pressure. Based on realistic computations of the band structure for TBLG which show that the bandwidth first shrinks and then expands with increasing hydrostatic pressure, we compute the ratio of the potential to the kinetic energy, $r_s$. We find an experimentally relevant window of pressure for which $r_s$ crosses the threshold for a triangular Wigner crystal, thereby corroborating our previous work [Nano Lett. (2018)] that the insulating states in TBLG are due to Wigner rather than Mott physics. A key prediction of this work is that the window for the onset of the hierarchy of Wigner states that obtains at commensurate fillings is dome-shaped as a function of the applied pressure, which can be probed experimentally. Theoretically, we find a peak for crystallization around $1.5$ GPa relative to the experimental optimal pressure of $1.33$ GPa for the observation of the insulating states. Consequently, TBLG provides a new platform for the exploration of Wigner physics and its relationship with superconductivity.

Metal-Insulator Transition of Solid Hydrogen by the Antisymmetric Shadow Wave Function

Abstract We revisit the pressure-induced molecular-atomic metal-insulator transition of solid hydrogen by means of variational quantum Monte Carlo simulations based on the antisymmetric shadow wave function. For the purpose of facilitating the study of the electronic structure of large-scale fermionic systems, the shadow wave function formalism is extended by a series of technical advancements as implemented in our HswfQMC code. Among others, these improvements include a revised optimization method for the employed shadow wave function and an enhanced treatment of periodic systems with long-range interactions. It is found that the superior accuracy of the antisymmetric shadow wave function results in a significantly increased transition pressure with respect to previous theoretical estimates.

Pressure-induced insulator-metal transition in Ca2Ru0.92Fe0.08O4 investigated by infrared microspectroscopy

We investigated pressure-induced insulator-metal transition in Ca2Ru0.92Fe0.08O4 by using infrared microspectroscopy. As the pressure is increased up to 1.7 GPa, we observed a large increment of the reflectivity in the entire mid-infrared range. Accompanied by such a clear signature of the insulator-metal transition, we found an evidence of the structural transition from the frequency shift of the Ru-O stretch phonon mode which is attributed to the shortening of the in-plane Ru-O bond length. When we compared these pressure-dependent changes with the corresponding temperature-dependent results, we found that the pressure-induced metallic state has a higher reflectivity as well as the higher phonon frequency. Indeed, it turns out that the pressure-induced metallic state of Ca2Ru0.92Fe0.08O4 looks very similar with the metallic state of Sr-substituted Ca2RuO4 not only in the reflectivity level but also in the phonon frequency. This suggests that the electronic properties are closely related to the structural degree of freedom, and the pressure can be a useful parameter to induce the transitions from the Mott-insulator to the metal and further to the superconductor as observed for Sr2RuO4.

Metallization of solid molecular hydrogen in two dimensions: Mott-Hubbard-type transition

We analyze the pressure-induced metal-insulator transition in a two-dimensional vertical stack of $H_2$ molecules in x-y plane, and show that it represents a striking example of the Mott-Hubbard-type transition. Our combined exact diagonalization approach, formulated and solved in the second quantization formalism, includes also simultaneous ab initio readjustment of the single-particle wave functions, contained in the model microscopic parameters. The system is studied as a function of applied side force (generalized pressure), both in the $H_2$-molecular and $H$-quasiatomic states. Extended Hubbard model is taken at the start, together with longer-range electron-electron interactions incorporated into the scheme. The stacked molecular plane transforms discontinuously into a (quasi)atomic state under the applied force via a two-step transition: the first between molecular insulating phases and the second from the molecular to the quasiatomic metallic phase. No quasiatomic insulating phase occurs. All the transitions are accompanied by an abrupt changes of the bond length and the intermolecular distance (lattice parameter), as well as by discontinuous changes of the principal electronic properties, which are characteristic of the Mott-Hubbard transition here associated with the jumps of the predetermined equilibrium lattice parameter and the effective bond length. The phase transition can be interpreted in terms of the solid hydrogen metallization under pressure exerted by e.g., the substrate covered with a monomolecular $H_2$ film of the vertically stacked molecules. Both the Mott and Hubbard criteria at the insulator to metal transition are discussed.

Pressure induced insulator–metal transition and giant negative piezoresistance in Pr0.6Ca0.4Mn0.96Al0.04O3 polycrystal

The effect of external hydrostatic pressure (P) on the magnetization (M) and resistivity (ρ) properties of charge-orbital (CO) ordered-insulating phase-separated manganite Pr0.6Ca0.4Mn0.96Al0.04O3 system is reported here. At ambient P, CO ordering transition and spin-canting in the AFM are observed at 223K and 55K respectively in M(T) and ρ(T) measurements. Application of P increases simultaneously the magnitude of magnetization (M) and transition temperature, and weakens the CO ordering in M(T) measurements up to 0.98GPa. During ρ(T) measurements, P induces an insulator–metallic transition (TIM) at 1.02GPa, and further increase of P up to 2.84GPa leads to increase of TIM (dTIM/dP =21.6K/GPa). ρ at TIM is reduced about three orders of magnitude at 2.84GPa, and leads to the giant negative piezoresistance (~98%). These results are analyzed separately in two temperature regions i.e., below and above TIM by power function equation and small polaronic hopping model respectively. It is understood from these analyses that the application of P suppresses the Jahn–Teller distortions, electron–electron and electron–magnon scattering factors, and induces the insulator–metal transition in Pr0.6Ca0.4Mn0.96Al0.04O3 system.

Structural transition in the magnetoelectricZnCr2Se4spinel under pressure

Τhe magnetoelectric $\mathrm{ZnC}{\mathrm{r}}_{2}\mathrm{S}{\mathrm{e}}_{4}$ spinel, with space group $Fd\overline{3}m$, undergoes a reversible first-order structural transition initiating at 17 GPa, as revealed by our high-pressure x-ray diffraction studies at room temperature. We tentatively assign the high-pressure modification to an $A\mathrm{M}{\mathrm{o}}_{2}{\mathrm{S}}_{4}$-type phase, a distorted variant of the monoclinic $\mathrm{C}{\mathrm{r}}_{3}{\mathrm{S}}_{4}$ structure. Furthermore, our Raman investigation provides evidence for a pressure-induced insulator-metal transition. Our density functional theory calculations successfully reproduce the structural transition. They indicate significant band gap and magnetic moment reduction accompanying the pressure-induced structural modification. We discuss our findings in conjunction with the available high-pressure results on other Cr-based chalcogenide spinels.

Percolative metal-insulator transition inLaMnO3

We show that the pressure-induced metal-insulator transition (MIT) in ${\mathrm{LaMnO}}_{3}$ is fundamentally different from the Mott-Hubbard transition and is percolative in nature, with the measured resistivity obeying the percolation scaling laws. Using the Gutzwiller method to treat correlation effects in a model Hamiltonian that includes both Coulomb and Jahn-Teller interactions, we show, one, that the MIT is driven by a competition between electronic correlation and the electron-lattice interaction, an issue that has been long debated, and two, that with compressed volume, the system has a tendency towards phase separation into insulating and metallic regions, consisting, respectively, of Jahn-Teller distorted and undistorted octahedra. This tendency manifests itself in a mixed phase of intermixed insulating and metallic regions in the experiment. Conduction in the mixed phase occurs by percolation and the MIT occurs when the metallic volume fraction, steadily increasing with pressure, exceeds the percolation threshold ${v}_{c}\ensuremath{\approx}0.29$. Measured high-pressure resistivity follows the percolation scaling laws quite well, and the temperature dependence follows the Efros-Shklovskii variable-range hopping behavior for granular materials.

Pressure-induced structural changes and insulator-metal transition in layered bismuth triiodide, BiI3: a combined experimental and theoretical study

Noting that BiI3 and the well-known topological insulator (TI) Bi2Se3 have the same high symmetry parent structures, and that it is desirable to find a wide-band gap TI, we determine here the effects of pressure on the structure, phonons and electronic properties of rhombohedral BiI3. We report a pressure-induced insulator-metal transition near 1.5 GPa, using high pressure electrical resistivity and Raman measurements. X-ray diffraction studies, as a function of pressure, reveal a structural peculiarity of the BiI3 crystal, with a drastic drop in c/a ratio at 1.5 GPa, and a structural phase transition from rhombohedral to monoclinic structure at 8.8 GPa. Interestingly, the metallic phase, at relatively low pressures, exhibits minimal resistivity at low temperatures, similar to that in Bi2Se3. We corroborate these findings with first-principles calculations and suggest that the drop in the resistivity of BiI3 in the 1–3 GPa range of pressure arises possibly from the appearance of an intermediate crystal phase with a lower band-gap and hexagonal crystal structure. Calculated Born effective charges reveal the presence of metallic states in the structural vicinity of rhombohedral BiI3. Changes in the topology of the electronic bands of BiI3 with pressure, and a sharp decrease in the c/a ratio below 2 GPa, are shown to give rise to changes in the slope of phonon frequencies near that pressure.

Pressure-induced metal-insulator transition of the mott insulator Ba2IrO4

The electrical resistivity of single crystals of the spin-orbit Mott insulator Ba2IrO4 has been measured at pressures up to 30 GPa and at temperatures from 100 mK to 300 K. Ba2IrO4 shows a metal-insulator transition at around Pc = 24 GPa, though it does not show superconductivity down to 100 mK. The low-temperature resistivity in the metallic state does not obey a conventional Fermi-liquid description. This suggests that carriers are incoherently scattered by antiferromagnetic quantum spin fluctuations. The critical exponent δ for the metal-insulator transition is about 1.6, indicating that Ba2IrO4 is located near the boundary between a Mott and an Anderson insulator. This means that even in a single crystal, the effect of crystallographic disorder should not be ignored.