Session S44: Metal-Insulator Phase Transition II

Live

We study the interplay between several control mechanisms on the emerging functionalities of complex oxide thin films and heterostructures composed in early transition metal oxides using a combination of density functional theory (DFT) and dynamical mean-field theory (DMFT).
We discuss factors, such as e.g. lattice missmatch-induced epitaxial strain, dimensional confinement, and substrate effects, which can lead to modifications of structural as well as electronic properties. This can result in a metal-insulator transition in the correlated metal CaVO3, where we address the effect of a substrate in the simulation and draw a conceptual comparison between the two typical substrate materials, SrTiO3 and LaAlO3, and how they affect the CaVO3 thin film properties. We demonstrate that in the first case the influence of the interface is marginal beyond the effect of epitaxial strain, such that the substrate need not necessarily be included in the simulations. For LaAlO3, on the other hand, we show that different choices of the simulation cell can lead to strong modifications of the materials properties as a result of the associated electrostatic boundary conditions. We demonstrate how this can give rise to an enlarged spectrum of thin film properties via multilayer engineering.   

Live

Since the “vanadium trimers” in layered LiVO₂ has been proposed by Goodenough in 1963, many experimental studies have been carried out to clarify the realization of the trimers. Our comprehensive structural studies using synchrotron X-ray diffraction experiments clarified that vanadium trimers are randomly aligned along the c-axis direction in LiVO₂, while the long-range ordering of vanadium trimers along the c-axis direction appears in LiVS₂, which is a sulfide analog of LiVO₂. In this presentation, we will explain that the presence/absence of such a periodicity in the trimer patterns is brought by the difference in their inherent stacking structures. We also present that the trimerization of LiVO₂ without a long-range ordering along the c-axis direction, can be identified using pair distribution function (PDF) analysis based on appropriate structural models.   

Live

It was recently reported that a continuous electric current is a powerful control parameter to trigger changes in the electronic structure and metal–insulator transitions (MITs) in Ca₂RuO₄. However, the spatial evolution of the MIT and the implications of the unavoidable Joule heating have not been clarified yet, often hindered by the difficulty to assess the local sample temperature. We present infrared thermal imaging measurements performed on single-crystal Ca₂RuO₄ while controlling the MIT by electric current. The change in emissivity at the phase transition allows us to monitor the gradual formation and expansion of metallic phase upon increasing current. Our local temperature measurements indicate that, within our experimental resolution, the MIT always occurs at the same local transition temperatures, irrespectively if driven by temperature or by current. Our results highlight the importance of local heating, phase coexistence, and microscale inhomogeneity when studying strongly correlated materials under the flow of electric current.   

Live

CuIr₂S₄ is a strongly correlated material wherein intricate entanglement of electronic degrees of freedoms makes it sensitive to external stimuli resulting in an emergent phenomenon such as metal-insulator transition at T_c ~ 231 K. Below T_c, a hysteretic resistive transition from an insulating to a metallic phase can be induced by sweeping electric field across the sample. The nature of the switching changes from an abrupt jump in current-voltage characteristics away from T_c to step-like jumps near T_c. The electrically-driven transition is studied using electrical transport, resistance noise spectroscopy, and a nonequilibrium free-energy model, where effective temperature of charge carriers is defined as a function of local electric field. Normalized power spectral density (PSD) of resistance fluctuations show distinct features at temperatures far away and near T_c, and the behavior of normalized PSD is reproduced well in theoretical simulation. Spontaneous segregation of phases near T_c and their evolution in response to electrical bias appear to have an impact on charge transport near the transition. The noise behavior observed in CuIr₂S₄ can be seen in other materials and it points towards a common origin to phase separation near transition in strongly correlated materials.   

Live

In systems near phase transitions, macroscopic properties often follow algebraic scaling laws, determined by the dimensionality and symmetries of the system. The emergence of such universal scaling implies that microscopic details are irrelevant. In my talk, I will discuss the scaling properties of the metal-insulator transition at the LaAlO₃/SrTiO₃ interface. Using scanning SQUID, we map the current flow at the interface close to the gate tunable transition. I will show that coupling between structural and electronic degrees of freedom modifies the filamentary, preventing a fractal with the expected universal dimension from forming. Our results open the door for engineering electronic transitions at the nanoscale.   

Live

One of the main bottlenecks in the discovery of new thermally-driven metal-insulator transition (MIT) materials is the lack of a clear database of MIT compounds, and their relevant features. We have built a database of MIT and stoichiometrically related materials, and trained a machine learning model designed to classify whether a material is an MIT material or not [1]. Our easily interpretable model allows us to identify new features, such as the Average Deviation of the Covalent Radius and its interplay with the Range Mendeleev Number, as well as others. We also built an online pipeline where one can upload their own structures, and obtain a prediction on whether the material is a metal, an insulator, or an MIT material and tested it on previously identified materials [2].

[1] https://arxiv.org/abs/2010.13306
[2] https://arxiv.org/abs/2004.07365   

Live

The interplay between spin-orbital coupling and structural inversion symmetry breaking in solid states has attracted a lot of interest due to non-trivial electronic and magnetic ground states. Such phenomena are typically observed in compounds of heavy elements in the transitional metal section where large atomic number gives rise to strong spin-orbital coupling. A previous study [1] has reported magnetic anisotropy driven by reconfiguration of electronic properties near Fermi surface at spin reorientation temperature. Here we demonstrate for the oxide compound Ca₃Ru₂O₇ strong correlation among phononic properties, electron reconstruction, and spin reorientation due to spin-orbital coupling and strong electron-phonon correlation. Specifically, strong renormalization of two phonon modes observed experimentally and from the first principle. Our result suggests Raman spectroscopy is a sensitive tool to fermiology changes in strong correlated systems.

[1] Markovic, I. et al. P Natl Acad Sci USA 117, 15524-15529 (2020)   

Live

The Verwey transition in magnetite (Fe₃O₄), the first metal-insulator transition ever observed, is accompanied by charge ordering, orbital ordering, and a structural rearrangement. Due to the intricate interplay of these degrees of freedom, a complete understanding of the microscopic mechanism of this transition to the exotic low-temperature phase has not yet been reached. Recently, the charge-ordered structure was established as a network of small polarons extending across three sites, termed trimerons. However, the dynamics of the Verwey transition from an electronic point of view remains not understood, as no collective excitations of this trimeron order have been observed to date. In this talk, I will present our discovery of spectroscopic signatures of the low-energy electronic modes of the trimeron network using terahertz light. By exciting these collective modes coherently with an ultrashort laser pulse, we unveil their softening towards the transition temperature. These findings represent the first observation of collective modes of any sort displaying a critical softening in magnetite and thus shed light on the long-sought cooperative mechanism responsible for the Verwey transition.   

Live

We numerically solve the Hubbard model on the Bethe lattice with finite coordination number z=3, and determine its T=0 phase diagram. For this purpose, we introduce and develop the `variational uniform tree state' (VUTS) algorithm, a tensor network (TN) algorithm which generalizes the variational uniform matrix product state algorithm to tree TNs. Our results reveal an AFM insulating phase and a PM metallic phase, separated by a first-order doping-driven metal-insulator transition (MIT). We show that the metallic state is a Fermi liquid (FL) with coherent quasiparticle (qp) excitations for all values of the interaction strength U, and we obtain the finite qp weight Z from the occupation function of a generalized "momentum" variable. We find that Z decreases with increasing U, ultimately saturating to a non-zero, doping-dependent value. Our work demonstrates that TN calculations on tree lattices, and the VUTS algorithm in particular, are a platform for obtaining controlled results for phenomena absent in 1D, such as FLs, while avoiding computational difficulties associated with TNs in 2D. We envision that future studies could observe non-FLs, interaction-driven MIT, and doped spin liquids using this platform.   

Live

The capabilities of surface probe experiments are rapidly expanding, providing views on quantum materials at unprecedented length and time scales. Many such materials display intricate pattern formation in the electronic properties on the observable surface. This rich spatial information contains information about interactions, dimensionality, and disorder.[1] A well-tuned machine learning framework can decipher this information with minimal effort from the user.[2] We show the effectiveness of our deep learning framework on simulations of statistical models. We then use our machine learning model to analyze experimental data from an optical microscope[3] on a vanadium dioxide film as it goes through the insulator-metal transition.
[1] B. Phillabaum, et al. Nat Commun 3, 915(2012).
[2] L. Burzawa, et al. Phys. Rev. Materials 3, 033805(2019).
[3] A. Zimmers, et al. Phys. Rev. Lett. 110, 056601(2013).   

Live

Metal-to-insulator phase transitions (MIT) in 2D Systems is a controversial topic in condensed matter physics due to the fact that it challenges the prediction made by Abrahams et al., which states that two-dimensional disordered electronic systems must be insulating (Phys. Rev. Lett. 42, 673 (1979)). Recent studies^1,2 have indicated a possible MIT in few-layered transition metal dichalcogenide such as MoS₂, ReS₂ and CuIn₇Se₁₁³, but the mechanism leading to the MIT still remains enigmatic. We will present a possible MIT in few-layered As-doped black phosphorus from conductivity measurements in field effect transistors measured from 300 K to 2 K reveal an insulating-to-metallic-like phase transition when the carrier density is tuned via an applied gate voltage. The nature of the phase transition will be discussed with existing theoretical model. ¹Nat Commun. 2018; 9: 2052, ² Nano Lett. 2015, 15, 12, 8377, ³ACS Nano 2019, 13, 11, 13413   

Live

The main challenge in applications of rare-earth-based new materials is understanding the complexities that arise from the highly localized f-electrons that impact the properties of these materials significantly. We will present the first principles-based DFT+U study of structural, magnetic, electronic, and optical properties of Ca(La_1-x Ce_x)₂S₄ (0 ≤ x ≤ 1) solid solution. We employed a site-selection technique based on local magnetic moment to form Ce doped Ca(La_1-x Ce_x)₂S₄ solid solutions. The incorporation of f-electrons via Ce doping modifies properties of the parent compound, CaLa₂S₄. As the occupation of f-electron increases, magnetic phase transition, as well as semiconductor-to-metal transitions, have been observed. We will also explain the origin of the experimentally observed color in these solid solutions. Finally, we will emphasize the role of Ca in stabilizing these solid solutions. Our result agrees well with the available experimental results.   

Live

Tuning of the metal-to-insulator transition in perovskite transition metal oxides is important to control physical phenomena such as magnetism, ferroelectricity, and superconductivity. The tuning by atomic size effects is mediated by long range elastic degrees of freedom that lead to anisotropic frustrated interactions, which vary according to their ability to be accommodated in the solid by octahedral rotations [1]. Treated at the mean-field level, these effects suppress the phase transitions due to thermal fluctuations. When randomness is included, we explore whether the combination of long-range coupling and frustration can lead to a glassy phase where the local distortions become frozen.

[1] G. G. Guzman-Verri, R. T. Brierley, and P. B. Littlewood, Cooperative elastic fluctuations provide tuning of the metal-insulator transition, Nature 576, 429 (2019).   

Live

A group of praseodymium-containing cobaltites are known to undergo 1st order metal-insulator transitions that are correlated to valence shifts that involve both Pr and Co cations. We have measured the dispersion of acoustic phonons in a single crystal of (Pr_0.85Y_0.15)_0.7Ca_0.3CoO_3-δ in both the metallic and insulating states and have observed differences which arise at the phase transition temperature. We interpret changes in the mode energies in terms of changes in the elastic constants. Anomalous damping of the phonons is observed near the phase transition. Elastic diffuse scattering, symptomatic of precursory behavior in the metallic phase is observed.   

On Demand

We studied some aspects of the two-dimensional metal-insulator transition in the disordered case using the Hubbard model through the Statistical Dynamical Mean-Field Theory. We obtained the spinodal lines at which metal and insulator cease to be meta-stable. We also studied the spatial fluctuations of local quantities, such as the self-energy and the local Green's function for different disorders and temperature values. We showed the appearance of metallic "bubbles" within the insulator and vice-versa. We performed an analysis of finite-size effects and showed that the first-order transition is smeared in the thermodynamic limit. We analyzed transport properties by mapping to a random classical resistor network and calculating both the average current and its distribution across the Mott metal-insulator transition. We studied the domain wall's behavior, which forms between the metal and the insulator in the clean case, by employing a one-dimensional chain model connected to two reservoirs, one metallic at the left side and another insulating at the right side, each attached to one of the chain's ends. We then obtained the domain wall width as a function of temperature and interactions.