Session Y58: Computational Design, Understanding and Discovery of Novel Materials VI

Lattice instabilities and electronic correlations in laminated Cr based MBenes and MXenes

Transition metal carbides, nitrides and borides in the MAX and MAB phases are well-studied layered ceramic materials. Recently, their 2D layered structures (MXenes and MBenes) have attracted attention due to their possible applications in spintronics, quantum computing, and optoelectronics. In this talk, we will present a comparative first-principles (Density Functional Theory and Dynamical Mean Field Theory) study of the crystal and electronic structures of layered Cr-based MBene and MXene compunds. We will discuss how electronic correlation strength evolves with thickness and surface functionalization, as well as the possible crystal structural transitions.   

Modeling symmetric and defect-free carbon schwarzites into various zeolite templates

Recently, a process has been proposed for generating negatively-curved carbon schwarzites via zeolite-templating [1]. However, the proposed process leads to atomistic models which are not very symmetric and often rather defective. In the present work, an improved generation approach is developed, by imposing symmetry constraints, which systematically leads to defect-free, hence more stable, schwarzites. The stability of the newly predicted symmetric schwarzites is also compared to that of other carbon nanostructures (in particular carbon nanotubes — CNTs), which could also be accommodated within the same templates. Our results suggest that only a few of these (such as FAU, SBT and SBS) can fit schwarzites more stable than CNTs. Our predictions could help experimentalists in the crucial choice of the template for the challenging synthesis of schwarzites. Furthermore, being highly symmetric and stable phases, the models could also be synthesized by means of other experimental procedures. In this talk, I will present the results published in reference [2].

 

[1] E. Braun et al., Proc. Natl. Acad. Sci. 115 (2018) E8116–E8124

[2] E. Marazzi et al., Carbon 215 (2023) 118385   

Extraordinary Electromagnetic Properties of Disordered Stealthy Hyperuniform Layered Media

Disordered stealthy hyperuniform dielectric composites exhibit novel electromagnetic wave transport properties in two and three dimensions, but much less is known about such one-dimensional (1D) or layered media. From exact nonlocal strong-contrast expansions of the effective dynamic dielectric constant tensor that treat general three-dimensional two-phase composites with arbitrary structural symmetries, we extract an approximation formula suited for general layered media. This formula depends on the microstructure via the spectral density, and we apply it to estimate the effective dielectric constant for stealthy hyperuniform (SHU) variants. In particular, we predict that 1D SHU media are perfectly transparent (i.e., no Anderson localization, in principle) within finite wavenumber intervals. We validate that the resulting predictions are very accurate well beyond the long-wavelength regime by showing good agreement with the finite-difference time-domain simulations. The high predictive power of our formula implies that higher-order contributions are negligibly small. Indeed, we prove that such transparency intervals are exact through the next-order terms in the expansion. Thus, there can be no Anderson localization within the predicted perfect transparency interval in 1D SHU media in practice because the localization length (associated with only possibly negligibly small higher-order contributions) should be very large compared to any practically large sample size.   

Structure and mechanical properties of monolayer amorphous carbon and boron nitride

Amorphous materials exhibit characteristics that are not featured by crystals and their degree of disorder (DOD) may be tunable. Here, we report results on the mechanical properties of monolayer amorphous carbon (MAC) and monolayer amorphous boron nitride (maBN) with different DOD [1]. Pertinent structures are obtained by kinetic-Monte-Carlo simulations using DFT-based machine-learning potentials. An intuitive order parameter, namely the areal fraction F_x occupied by crystallites within the continuous random network (CRN), is proposed to quantify the DOD. We find that F_x captures the essence of the DOD: Samples with the same F_x but different sizes and distributions of crystallites have virtually identical radial distributions functions as well as bond-length and bond-angle distributions. Furthermore, the mechanical responses of MAC and maBN before fracture are solely determined by F_x and are insensitive to the sizes and specific arrangements of the crystallites. The behavior of cracks in the two materials is analyzed and found to mainly propagate in meandering paths in the CRN region and to be influenced by crystallites in distinct ways that toughen the material. These results may provide a universal toughening strategy for 2D materials.   

Magnetic Gold: First-principles investigation of stable Au2+ in a mixed-valence halide perovskite

Halide perovskites are attractive materials for solar energy conversion applications due to their strong light-matter interactions, structural tunability, and the relative ease with which they can be synthesized and processed. In this talk we present first-principles computational results on the perovskite Cs4AuIIAuIII2Cl12, an extended solid with putative Au2+ sites, which is stable under ambient conditions and was only recently synthesized for the first time. Using density functional theory, we compute the electronic structure and explain how its complex vacancy ordering leads to disproportionation into 2+ and 3+ Au sites. Alongside our computational results, we also rationalize the emergence of magnetism in this structure. Our data and theoretical arguments provide insights into the mechanism behind the novel oxidation state and emergent magnetic ordering. Additionally, this work further guides the design of next-generation halide perovskite-based functional materials.   

Machine Learning-Guided Discovery of Ce-based Ternary Intermetallics

Cerium-based intermetallics, as potential light rare earth element substitutes for permanent magnets, have drawn significant research attention. We present an integrated ML approach with first-principles calculations to efficiently explore low-energy ternary Ce-Co-Cu compounds. Our study reveals several structures are energetically as well as dynamically stable, along with a number of metastable ones. Notably, two Co-rich metastable compounds exhibit high magnetization, suggesting their potential as doped Ce2Co17-based permanent magnets.   

Emergence of Dirac half-semimetallic channels in graphene nanoribbon/hexagonal boron nitride heterojunctions

Materials hosting half-metallic phases are envisioned as active components in spintronic devices owing to their fully spin-polarized electrical currents, yet their deployment is limited by their scarcity. Here, using first-principles calculations and model Hamiltonians, we predict that recently fabricated heterojunctions of zigzag graphene nanoribbons embedded in two-dimensional hexagonal boron nitride are half-semimetallic, featuring Dirac points at the Fermi level. The Dirac half-semimetallicity originates from the transfer of ambipolar charges from hexagonal boron nitride to the embedded graphene nanoribbon. Upon charge doping, an antiferromagnetic-to-ferrimagnetic phase transition occurs in these heterojunctions, with the sign of the excess charge governing the spatial localization of the net magnetic moments. Our findings demonstrate that such heterojunctions realize one-dimensional conducting channels of spin-polarized Dirac fermions integrated into a two-dimensional insulator, thus holding promise for the development of carbon-based spintronics.   

The (very late) advent of Quantum Acoustics with application to strange metals

Quantum optics spans the gamut from photons to coherent states, including their classical limit, the electromagnetic field. Quantum acoustics should be parallel to quantum optics, with the phonon playing to role of photon (particles that can be counted), and the coherent states leading to classical fields (electromagnetic or sound waves) in both cases. However, it has not developed that way until recently (D. Kim et. al., references). This could have happened right after Hanbury-Brown Twiss in the late 1960's, when quantum optics was born, but did not. The coherent state limit demands a (perhaps unpopular) time dependent formulation, and has been disparaged as a kind of curiosity, with Ashcroft and Mermin warning in a short note that there is "no new physics" in the wave picture of lattice vibrations (chpt. 24). This statement is almost equivalent to saying that there is no new physics in the time dependent Schroedinger equation since we already have to the time independent Schroedinger equation. The point is that the approximations spun off the two pictures are very different and have completely different powers of computation and insight. That leads to new physics. Many of the strange metal mysteries, including Planckian resistivity, the Mott-Ioffe Regel bypass, and displaced Drude peaks, emerge from the quantum acoustics formulation. We now have a nonperturbative, coherent electron-phonon tool, including an interacting "electron wave-on-lattice wave" code, which is yielding a treasure trove of insight.   

The nature of the electro-optic response in tetragonal BaTiO3

Barium titanate, BaTiO₃ (BTO), has emerged as a promising electro-optic material with applications in silicon photonics. It boasts one of the largest known electro-optic coefficients; however, the origin of this giant electro-optic response has not been investigated in detail and is poorly understood. Here we report on a first principles study of the electro-optic or Pockels tensor in tetragonal P4mm BTO. We find good agreement with experiment if the P4mm structure is viewed as a dynamic average of four lower symmetry Cm structures. The large value of the Raman component of the EO coefficient is attributed to a low frequency and strong electron-phonon coupling of the lowest optical mode, and we trace the equally large piezoelectric contribution to the large components of the piezoelectric and elasto-optic tensors.   

Excitonic insulating phase in monolayer transition-metal dichalcogenides: An ab initio study

Excitonic insulator (EI) is a novel phase of matter characterized by a cooperative phenomenon in which electron-hole excitations within a system exhibit lower energy states in comparison to the conventional band ground state. Through GW plus Bethe-Salpeter equation (GW-BSE) calculations, several two-dimensional (2D) semiconducting materials have been proposed as EI candidates, as the binding energy between the excited electron and hole is larger than the bandgap. Nevertheless, the theoretical understanding of the EI phase has primarily relied on simplified two-band models with typically simplified analytical electron-hole Coulomb interaction kernel, containing several adjustable parameters such as electron and hole effective mass, and 2D polarizability. For real 2D systems, such modeling may be inadequate for obtaining a quantitative understanding of the novel physics underlying the EI state, especially in providing accurate results for comparison with experiments. Here, we develop a parameter-free ab initio framework on top of GW and GW-BSE calculated quasiparticle self-energies and electron-hole kernel matrix elements, to investigate the spectroscopic properties in the EI phase. We discuss results of the temperature-dependent electronic properties (e.g., single-particle excitation energies) and explore several experimental signatures (e.g., local density of states) of EI phase in monolayer 1T'-MoS₂.   

Decoding the Complex Short-Range Orders in Semiconductor GeSn Alloys: Insights from Accurate and Efficient Machine Learning-Based Atomistic Simulations

GeSn alloys have emerged as versatile materials with significant potential for electronic, photonic, and topological quantum applications, yet their structural properties remain elusive and challenging to determine. By employing extensive first-principles density functional theory (DFT) calculations and statistical sampling, our prior study unveiled the presence of short-range order (SRO) behaviors across the entire investigated composition range of GeSn alloys¹, and SRO was further predicted to substantially affect their electronic properties¹. Recent experiments using various characterization techniques have confirmed its existence^2,3, but important questions remain regarding the actual structures, spatial domain size and its distribution, and corresponding changes in the properties of SRO. To bridge the spatiotemporal scale gap and facilitate a direct comparison with advanced methods like atom probe tomography, we develop machine-learning interatomic potentials for these alloys. We show that the machine-learning interatomic potentials not only can accurately reproduce the results based on DFT calculations, but also enable an interesting discovery of subtle coexistence of two distinct types of SROs in GeSn alloys, similar to SiGeSn alloys⁴. Our results shed more light on the intricate SRO structural properties of Group IV alloys.

1. ACS Appl. Mater. Interfaces 12, 57245 (2020)

2. Small Methods 6, 2200029 (2022)

3. APL 122, 062103 (2023)

4. Communications Materials 3, 66 (2022)   

Using undulations to design novel functionality in 2D materials

Discovering functional quantum materials with desired properties and phenomena is tedious, often involving serendipitous discovery. In my talk, I will cover two examples where introducing undulations in 2D materials can be used as a recipe to induce quantum phases and phenomena. Firstly, I will show that introducing non-Euclidean deformation in 2D materials can create exotic electronic states. By employing elastic plate theory, density-functional, and coarse-grained tight-binding method, I will show that bi-periodic sinusoidal deformation of hBN creates pseudo-electric and magnetic fields with unexpected spatial dependence. [1] A combination of these fields leads to anisotropic confinement and 1D flat bands. The bandwidth of the flat bands can be changed by periodic undulations, which can drive the system to different strongly correlated regimes such as density waves, Luttinger liquid, and Mott insulator. In the second part, I will show how undulations in flat 2D materials break symmetry and create large effective electric fields. The origin of such large electric fields, its connection to flexoelectric voltage, and its implications in designing systems with large Rashba spin-splitting for spintronics-based applications will be presented.

[1] S. Gupta et. al., Nature Communications 13, 3103 (2022)   

A deconstructive analysis of charge-transfer and electrostatic field fluctuations to supplement first-principles modeling of disordered metals

High entropy alloys present a new class of disordered metals with hopeful applications in the next generation of materials and technology. However, much of the core physics underlying these and other, more generic forms of disordered matter remain the subject of ongoing inquiry. We thus present a minimal working model to describe random fluctuations in electronic charge and electrostatic "Madelung" field configurations in disordered metals. This work reveals both the nature and microscopic origins of these statistical qualities; it also suggests possible avenues for extending modern first-principles approaches to disorder that currently lack these features (e.g. conventional KKR-CPA). In our theory, disorder and interelectron Coulomb repulsion are incorporated in a standard perturbative manner, as is appropriate when simulating metallic alloys. The problem is then reformulated using a self-consistent linear response framework, which is capable of reproducing the same qualitative statistical trends obtained in more comprehensive treatments of disorder (e.g. LSMS) as we also show here. Our work may therefore bridge the gap between physical accuracy and computational affordability in first-principles disorder modeling and answer long-standing questions faced by the disordered materials community.   

Unlocking correlated electron physics through compensated metallicity

Van-Hove singularities (vHs), i.e. momentum-localized divergences of density of states, provide a fertile platform to elevate electronic correlation scales, that eventually drive symmetry breaking transitions upon lowering the temperature. However, in many material classes like Dirac semimetals, that display intricate topological features on the single particle level,  the energy spacing between vH and Fermi energy impedes the emergence of correlated phases stemming from electronic interactions. In this talk, we propose a charge compensation paradigm to unlock vH dominated physics at pristine filling and access the sought-after electronically driven phase transitions in systems like graphene: By depositing charges on an additional Fermi pocket with spherical symmetry around the Brillouin zone center, the vH singularity approaches the Fermi level. This triggers a plethora of topological phases inherited from the initial bandstructure, that we meticulously analyze within many-body calculations.