Session G08: Metal-Insulator Phase Transition

A theory for colors of strongly correlated electronic systems

Many strongly correlated transition metal insulators are colored, even though they have band gaps much larger than the highest energy photons from the visible light. An adequate explanation for the color requires a theoretical approach able to compute subgap excitons in periodic crystals, reliably and without free parameters—a formidable challenge. The literature often fails to disentangle two important factors: what makes excitons form and what makes them optically bright. We pick two archetypal cases as examples: NiO with green color and MnF2 with pink color, and employ two kinds of ab initio many body Green’s function theories; the first, a perturbative theory based on low-order extensions of the GW approximation, is able to explain the color in NiO, while the same theory is unable to explain why MnF2 is pink. We show its color originates from higher order spin-flip transitions that modify the optical response, which is contained in dynamical mean-field theory (DMFT). We show that symmetry lowering mechanisms may determine how ‘bright’ these excitons are, but they are not fundamental to their existence.   

Tensile nano regions as nucleation centers across the Mott transition in La1/3Sr2/3FeO3

First-order phase transitions are ubiquitous in quantum materials and often take place as nucleation growth processes, from regions that are perceived as defects in the material. The detailed nature of the defects and their relation with the phase transition are generally not well understood. We investigated the evolution of lattice across the Mott transition in perovskite La_1/3Sr_2/3FeO₃ using coherent nano diffraction. Using an unsupervised K-means clustering method, we revealed the existence of nano regions with tensile strains compared to the average atomic structure. These regions serve as nucleation centers and grow beyond the percolation threshold across the phase transition. We further performed first-principles calculations and confirmed the insulating nature of these nano regions. Our results establish a connection among the atomic lattice structure, nanoscale strain heterogeneity and macroscopic material properties, and provide a pathway toward rational material synthesis and control via strain engineering.

    

Interaction-driven Mott Transition with a non-degenerate Ground State in the Orbital Hatsugai-Kohmoto model

Recent work on the Mott transition has focused on the exactly solvable model of Hatsugai/Kohmoto (HK) which is strictly local in momentum space. We explore an extension of this model in which orbital degrees of freedom are incorporated. We find dramatically different behaviour in the interaction-driven Mott transition in this model compared to the band HK model. First, unlike the band HK model in which the interaction must exceed a critical value for the insulating state to obtain, we find an insulating state for all interaction strengths in agreement with state-of-the-art cluster calculations. Second, we find that the density of states deviates strongly from single-site dynamical mean field theory (DMFT) calculations on the Hubbard model (which show the spectral weight at zero frequency contains a coherence quasiparticle peak). We characterize the Mott transition by computing the compressibility, entropy and heat capacity, displaying novel non-Fermi liquid behaviour driven by strong correlations. We conclude that the orbital version of HK offers an exactly solvable model of the Mott problem in agreement with more complex cluster methods but with minimal computational cost.   

Quantum Cluster Embedding DMFT Description of Electron Localization in Disordered and Strongly Correlated Systems

Dynamical Mean Field Theory (DMFT) and its quantum cluster embedding schemes—DCA (Dynamical Cluster Approximation), CDMFT (Cluster DMFT), and TMDCA (Typical Medium DCA)—serve as robust computational tools to address the critical non-local spatial correlations posed by interaction and disorder-induced phase transitions. In this study, we use these cluster embedding DMFT methods to both two and three-dimensional Anderson and Hubbard models. Our findings underscore pronounced non-local, long-range correlation effects around localizations, emphasizing the important role of non-local descriptions in capturing metal-insulator transitions in such systems. Furthermore, we conduct a comparative benchmark between various quantum cluster embedding DMFT techniques, accentuating their precision in delineating metal-to-insulator phase transitions within the Hubbard and Anderson model frameworks.   

Reentrant behavior of metal-insulator transition in AgCoxNi1-xO2 (0.33 ≤ x ≤ 0.66) delafossite : DFT+U and Quantum Monte Carlo Study

 As the only semimetallic d¹⁰-based delafossite, AgNiO₂ has received a great deal of attention due to both its unique semimetallicity and its antiferromagnetism in the NiO₂ layer that is coupled with a lattice distortion. In contrast, other delafossites such as AgCoO₂ are insulating. Here we study how the electronic structure of AgCo_xNi_1-xO₂ alloys vary with Ni/Co concentration, in order to investigate the electronic properties and phase stability of the intermetallics. In order to treat strong on-site Coulomb interactions accurately, in this study we use Quantum Monte Carlo (QMC) simulations to obtain accurate estimates for the electronic and magnetic properties of AgNiO₂. By comparison to Density Functional Theory (DFT) results, we show that strong electron correlations in NiO₂ layer are critical to account for. We show that Co doping on the magnetic Ni sites results in a metal-insulator transition near x ~ 0.33, which is consistent with the experimental result. In addition, we observe the reentrance to the metallic phase on high Co concentration near x  ~ 0.66.   

Evidence of electronic structural change at metal-insulator transition in the perovskite NaOsO3

NaOsO₃ exhibits an unusual metal-insulator transition (MIT), that has been touted as Slater-type, though there are observations that do not entirely conform to this typing. Consistent with a Slater transition, the cell volume and the crystal symmetry show no change on cooling through the transition, but both a and b axes in this Pbnm orthorhombic perovskite show a clear kink and the splitting between them enlarges slightly. Since atomic positions, determined by fitting the intensity of state-of-the-art neutron diffraction data, exhibit no anomalies within the measurement uncertainty, the origin of the a-b splitting and the kink in the data have not yet been elucidated. Understanding the origin of these subtle structural changes across the transition temperature may help provide understanding about the electronic behavior.  In this study, we connect local structural changes to changes in the lattice parameters through (1) well-established structural modelling and (2) density functional theory (DFT) calculations. This approach successfully reveals a subtle, but significant change of the O-Os-O bond angle on cooling through T_MIT. A Madelung energy calculation suggests that this bond angle change signals electron localization in the t_2g orbital complex and this is verified by DFT which shows an electronic transition from itinerant to localized electronic behavior along with the bond angle shift. This detailed structural study adds an important, though heretofore overlooked, element in the microscopic picture of the metal-insulator transition in NaOsO₃.    

Hund correlation near the Mott metal-insulator transition: NiS2 under pressure

Strong correlation effects due to Hund coupling J in multi-band systems have been intensively studied in so-called Hund’s metal. The Hund correlation effects are usually contrasted with the Mott correlation effects, as Hund’s metal exhibits a correlated metallic behavior while being far from the Mott insulating limit. Recently, however, the Hund correlation effects were revealed to be essential also in an archetypal Mott material, NiS_2-xSe_x, near the Mott metal-insulator transition. A kink structure inside the quasiparticle bands is observed from the angle-resolved photoemission spectroscopy experiment, and the role of Hund’s coupling in producing the kink was theoretically demonstrated from the density functional theory plus the dynamical mean-field theory study.

To unveil the underlying physics of the Hund correlation near the Mott metal-insulator transition of a half-filled multi-band system, we studied the pressure-induced metallic state of NiS₂ using first principles DFT+DMFT methods. Our main results consist of 1) definitizing the Hund versus Mott physics by investigating the local impurity properties, 2) finding a new interplay of correlations produced by Hund physics together with Mott physics, which is exhibited as the kink scaled by J times Z, and 3) the prediction of observable Hund trace in optical conductivity as the non-Drude-like tail and the non-monotonic temperature evolution of optical spectral weight.   

Local lattice distortions and metal-insulator transition in Nd0.5Sr0.5MnO3 perovskite manganite

 High energy temperature-dependent x-ray diffraction coupled with the Atomic Pair Distribution function was employed to reveal the structure model of Nd_0.5Sr_0.5MnO₃ manganite. The system exhibits substantial disorder effects leading to local lattice distortions due to the coexistence of Nd⁺³ and Sr⁺² ions at the same A-atomic site. Surprisingly, the high symmetry model was inadequate to explain local lattice distortions even observed at room temperature. To address this, we explored that low symmetry Monoclinic model S.G. P2₁/m was able to explain the distortions in the present sample. Our results show evidence of charge, orbital ordering CO/OO around 140 K corresponding to metal-insulator transition T_MI followed by a second-order insulator-metal transition T_IM around 261 K. Additionally, the effect of the external magnetic field was examined on our sample which initially was in an antiferromagnetic state AFM, therefore magnetic field was strong enough to melt CO/OO state, rendering system to ferromagnetic state FM state, coinciding with metamagnetic phase transition. I will address how a present work displays a complex interplay between spin, charge, and lattice degree of freedom, and a low symmetry model with constraints was able to explain local lattice instabilities which makes this work unique and interesting. Additionally, how the experiments done in a magnetic field were able to melt the charge ordering/orbital ordering state exhibiting metamagnetic phase transition.   

Electronic correlation across the metal-insulator transition in single crystals of β-Na0.33V2O5

The Wadsley-type vanadium bronze β-Na_0.33V₂O₅ exhibits a thermally driven metal-insulator transition (MIT) due to charge ordering at T_CO = 136 K. This type of correlated behavior is typically observed in these β(β’)-vanadium bronzes and are induced by the local lattice distortions and electrical interactions due to the intercalated dopants as they sit within the tunnel structure formed by chains of V₂O₅. The evolution of the thermally driven MIT in single crystals of β-Na_0.33V₂O₅ using electrical transport and noise spectroscopy measurements is studied to elucidate the charge dynamics across this transition. We observe a crossover from an Arrhenius-type conductivity above T_CO to a Mott variable range hopping. This crossover can alternately be observed in the power spectral density (PSD) of the residual resistance fluctuations as a gradual change in the noise magnitude as charges localize and order in the insulating state. The slope of the PSD also deviates significantly signifying a mixing of phases as we transition from a charge disordered to an ordered state. Vanadium bronzes prove to be fertile playgrounds to understand tunable correlated electron behavior in nanoscale systems.   

Signature of Hund-Correlation in Non-Spin-Freezing Metallic Regime Studied by Dynamical Mean Field Theory : Hundness in Global Phase Diagram

 Hund-correlated systems show unique feature in contrast to Hubbard-correlated systems. Previously well studied signature of Hund interaction exists only in the spin-freezing regime, namely Hund-metal, in which spin Kondo scale and orbital Kondo scale differs by order. However, the signature of Hund interaction can also exist in different phases in the global phase diagram. Experimentally, the discrepancy between the bandwidth renormalization observed by angle-resolved photoemission spectroscopy (ARPES) and heat capacity measurement in transition metal compounds arises. This can be explained by the unique feature of Hund interaction, that strong bandwidth renormalization occurs only in low-energy scale while overall conduction band doesn’t get strongly renormalized. Our study based on Dynamical Mean Field Theory resulted that this feature can be observed at any electron fillings with non-spin-freezing regime. In contrast to Hund-Metal, this can be observed at any electron occupation including half-filled systems. In this work, we suggest a new parameter that measures the degree of Hund-correlation signature in the non-spin-freezing correlated metal phases, namely the ‘Hund-Fermi-liquid-ness’. This work suggests that we can contrast the signature of the Hund correlation to the signature of Hubbard correlation and measure the Hundness in the global phase diagram for any multi-orbital strongly correlated systems including non-spin-freezing correlated metal phase.   

Suppression of metal-to-insulator transition and stabilization of superconductivity by pressure in Re3Ge7

The effect of pressure on the low-temperature states of the Re₃Ge₇ is investigated by both electrical resistance and magnetization measurements. At ambient pressure, the temperature dependent resistance of Re₃Ge₇ behaves quasi-linearly from room temperature to above 60 K, then undergoes a two-step metal-to-insulator transitions (MIT) at temperatures T₁ and T₂ which may be related to a structural phase transition or occurrence of charge density wave (CDW) ordering. Upon applying pressure, a two-step (T₁, T₂) MITs split into 3 steps (T₁, T₂ and T₃) above 1 GPa, and all traces of MITs are fully suppressed at ~8 GPa. Subsequently, the SC shows a bulk nature at and above 12.2 GPa, where the SC T_c and H_c2 (onset) reach the maximum of T_c (onset) ~5.9 K and H_c2 (0 K) ~1.8 Tesla. Our results not only present the observation of SC under high pressure in Re₃Ge₇ but also provide a rich phase diagram associating the interplay between SC and different competing electronic states in the potentially topologically nontrivial system Re₃Ge₇.   

Proposed experiment to find the Sordi transition in doped κ-BEDT systems

Doping a Mott insulator triggers fascinating physics, such as the Sordi transition, a first-order transition between a pseudogap and a correlated Fermi liquid [1]. Although it is said that this transition could explain some of the most interesting features of cuprates, its origin remains unclear, and it has yet to be observed experimentally in 2D correlated systems. Recent work has dispelled the idea that the transition is an artifact of the specific numerical techniques used, as well as the fact that it is linked to magnetic transitions [2], but experimental confirmation is required to verify the existence of the Sordi transition. Because the Mott transition is inaccessible in cuprates, it’s necessary to search for it in other such as layered organic superconductors. Based upon results from dynamical cluster approximation on the triangular lattice Hubbard model, we propose a variety of experimental measurements for Hg-doped κ-BEDT layered organic superconductors to identify signatures of this elusive transition.

 

References

[1]. G. Sordi et al. Phys. Rev. B, 84 :075161, 2011.

[2]. P.-O. Downey et al. arXiv:2307.11190, 2023.

[3]. H. Oike et al. Nat. Comm. 8, 756, 2017.

 

    

Angle-dependent electronic properties of TiZn16 under low temperatures and high magnetic field

Titanium and Zinc forms a family of crystals that range from Ti_2­Zn to TiZn₁₆. Although the synthesis and physical properties of these crystals have been documented, their electronic properties have not been mapped in detail. Here, we explore the electronic structure of TiZn₁₆, the most Zinc heavy member. Under a strong magnetic field at cryogenic temperatures, we make three observations. First, TiZn₁₆ behaves as a conductor when the magnetic field is parallel to the applied current, but transitions to an insulator when the magnetic field is rotated to perpendicular to the applied current. Second, quantum oscillations persist through this transition, existing even when the material is in an insulating state. Third, the resistance oscillates as the angle between the magnetic field and applied current, implying some symmetry enhanced by the magnetic field.   

Physics-Driven Modeling of the Metal-Insulator Transition Temperature in W-Doped VO2 through Symbolic Regression

Vanadium dioxide (VO₂), a correlated semiconductor, can exhibit a metal-insulator transition (MIT). With suitable choices of dopants on either the cation or anion site, it is possible to tune the IMT temperature by several tens of degrees and control the hysteresis. A challenge in modelling the effects of control parameters, such as doping concentration, type of dopants, etc, on the IMT is the complexity associated with experimental procedures. The scarcity of experimental data hinders the development of modern Artificial Intelligence (AI)/Machine Learning (ML) models, with only a few empirical linear models available. A promising approach for bridging the gap between physical reasoning and data-driven methodologies is the Symbolic Regression (SR) approach. SR can identify nonlinear analytical expressions connecting target properties to the key input parameters, even with relatively small datasets. In this work, we develop SR models to capture the trends in the IMT in VO₂ affected by different dopant control parameters. We use a comprehensive set of experimental data from a wide range of literature sources reported over the past two decades. Our study reveals a dual nature of the IMT transition for different Tungsten (W) doping concentrations likely due to electronic (low doping) versus structural interactions (dominant at high doping densities). Our models can further uncover mechanistic insights on controlling phase transitions useful for realizing low-power Mott electronic devices.   

Understanding the giant resistivity peak and negative magnetoresistance in EuCd2P2

Colossal negative magnetoresistance (CMR) has been observed in various Eu-based Zintl compounds, a outstanding example is EuCd₂P₂. Several different mechanisms such as spin fluctuations [1], strong spin-carrier interactions [2], BKT transition [3] and band reconstruction [4] were predicted to be involved in producing CMR in EuCd₂P₂. Here we report observation of two different kinds of high temperature resistivity behaviors, an insulating trend and a bad metallic trend in different samples of EuCd₂P₂. Samples which display an insulating trend show very large resistivity peak happening at a lower temperatures compared to bad metallic samples. We present the heat capacity, magnetoresistance and carrier concentration data comparing these two samples. We also report a very large non-linear Hall effect and an intriguing non-linear transport behavior in this compound. Our results can shed light on the microscopic mechanism of CMR in EuCd₂P₂.