arXiv subjects

Alexandru B. Georgescu

Publications and source records attributed to Alexandru B. Georgescu.

At least 19 recordsLinked to original sources

arXiv2026

A Structural Map of Potential Correlated Electron Molecular Orbital Materials

Correlated electron molecular orbital (CEMO) materials host emergent electronic states built from molecular orbitals localized over clusters of transition metal ions in a solid, yet have historically been discovered sporadically and generally been treated as isolated case studies. Here, we present criteria and tools to classify and analyze these materials, provide a database of identified materials with cluster-specific symmetry analysis, including cluster point groups, effective cluster sublattice dimensionality. Our electronic structure analysis shows that narrow bands in cluster materials can arise either from relatively isolated cluster molecular orbitals or from a mixed mechanism in which cluster localization coexists with frustrated-lattice flat-band topology. We discuss how CEMO properties do not arise from metal clusters in a solid alone: rather they depend on electron count, symmetry, frustration and correlation strength. By integrating cluster classification with flat-band lattice topology and electrochemistry-relevant information, we provide further relevant information to multiple scientific communities. Applying this approach in a high-throughput screen of 34,548 compounds yields potential 2,627 stable or metastable CEMO compounds with isolated cluster motifs. The accompanying open dataset, Cluster Finder software, and interactive web platform enable systematic exploration of cluster-containing electronic solid state materials and establish a starting point for discovering a new class of correlated quantum materials with a wide array of properties.

cond-mat.mtrl-sci↗

arXiv2025

Accelerating Discovery of Metal-Insulator Transition Compounds Using Physics-Informed Machine Learning

Metal-insulator transition (MIT) materials are a useful platform for emerging microelectronic, optoelectronic, and neuromorphic devices, but their discovery is hindered by the high computational cost of electronic structure modeling, the complexity of underlying mechanisms, and the challenges of experimental validation. Here, we present a physics-informed machine learning framework that accelerates the discovery of thermally driven MIT materials. Using a trained classifier, we screen a crystal structure database to identify promising candidates for higher fidelity simulations. We focus on Ca$_2$Fe$_3$O$_8$, CaCo$_2$O$_4$, and CaMn$_2$O$_4$, and use density functional theory (DFT) to determine their electronic and magnetic ground states and assess their microscopic MIT mechanisms. We further apply machine learning regression models to estimate their transition temperatures and employ synthesis prediction tools to identify likely precursors and reaction routes. This integrated approach reduces the time and effort required to identify, understand, and synthesize new MIT materials, providing a generalizable pathway for accelerating correlated quantum materials discovery.

cond-mat.mtrl-sci↗

arXiv2025

In-Plane Ni-O-Ni Bond Angles as Structural Fingerprints of Superconductivity in Layered Nickelates: Effects of Pressure, Strain, Layering, and Correlations

We investigate the structural and electronic conditions conducive to superconductivity in layered nickelates using density functional theory with Hubbard corrections (DFT+$U$). For both the bilayer and 1-3 polymorphs of La$_3$Ni$_2$O$_7$, we find that the in-plane Ni-O-Ni bond angles under pressure strongly correlate with the experimentally observed superconducting transition temperature ($T_c$) dome, and may serve as a reasonable proxy. Under compressive strain, the bond angles straighten, peaking near 2\% strain-consistent with experimental reports of superconductivity in strained bilayer thin films. However, the bond angles at this strain are more bent than those achieved under hydrostatic pressure, correlating with a lower $T_c$. We show that increasing the number of NiO$_2$ layers, as in La$_4$Ni$_3$O$_{10}$, or substituting heavier rare-earth elements (e.g., Pr) raises the pressure required to reach the structural configuration associated with superconductivity. Our results indicate that these systems require higher external pressure to achieve in-plane bond straightening. Varying the on-site Coulomb interaction $U$ reveals that stronger electronic correlations delay the structural transition and favor high-spin states. This suggests that moderate correlation strength may be optimal for superconductivity, with stronger correlation preventing the formation of favorable bond geometries. Electronic structure analysis shows that the Ni $e_g$ orbitals dominate near the Fermi level and shift downward with pressure, enhancing Ni-O hybridization. These results highlight how pressure and strain tune structural features that may be essential for engineering high-$T_c$ phases in nickelate superconductors.

cond-mat.supr-con↗

arXiv2025

Symmetry-Driven Trimer Formation in Kagome Correlated Electron Materials

Correlated electron materials with molecular orbital states extending over transition metal clusters can host multiferroicity, spin frustration, and unconventional insulating phases. However, the fundamental criteria that govern cluster formation and stability remain unclear. Here, we identify a symmetry, correlation, and electron filling driven criteria that stabilize triangular metal trimers in materials displaying transition metal kagome patterns. Using density functional theory and chemical bonding analysis, we show that trimer formation emerges when 6 to 8 electrons occupy molecular orbitals derived from transition metal d-states, achieving near complete filling of bonding states while avoiding antibonding occupation, and correlations are of intermediate strength. This principle explains the stability of Nb$_3$X$_8$ (X = Cl, Br, I), and more broadly, our findings offer a general design rule to obtain quantum materials with quantum states extended across transition metal clusters.

cond-mat.mtrl-sci↗

arXiv2025

Emerging Microelectronic Materials by Design: Navigating Combinatorial Design Space with Scarce and Dispersed Data

The increasing demands of sustainable energy, electronics, and biomedical applications call for next-generation functional materials with unprecedented properties. Of particular interest are emerging materials that display exceptional physical properties, making them promising candidates in energy-efficient microelectronic devices. As the conventional Edisonian approach becomes significantly outpaced by growing societal needs, emerging computational modeling and machine learning (ML) methods are employed for the rational design of materials. However, the complex physical mechanisms, cost of first-principles calculations, and the dispersity and scarcity of data pose challenges to both physics-based and data-driven materials modeling. Moreover, the combinatorial composition-structure design space is high-dimensional and often disjoint, making design optimization nontrivial. In this Account, we review a team effort toward establishing a framework that integrates data-driven and physics-based methods to address these challenges and accelerate materials design. We begin by presenting our integrated materials design framework and its three components in a general context. We then provide an example of applying this materials design framework to metal-insulator transition (MIT) materials, a specific type of emerging materials with practical importance in next-generation memory technologies. We identify multiple new materials which may display this property and propose pathways for their synthesis. Finally, we identify some outstanding challenges in data-driven materials design, such as materials data quality issues and property-performance mismatch. We seek to raise awareness of these overlooked issues hindering materials design, thus stimulating efforts toward developing methods to mitigate the gaps.

cond-mat.mtrl-sci↗

arXiv2025

Why Charge Added Using Transition Metals To Some Insulators -- Including LK-99 -- Localizes and Does Not Yield a Metal

While adding charge to semiconductors via dopants is a well-established method for tuning electronic properties, we demonstrate that introducing transition metal impurities into certain insulators can lead to localized charge, assisted by a Jahn-Teller distortion. This leads to isolated charge, and an insulating material as opposed to emergent states - including superconductivity. We focus on Cu impurities added to Pb$_{10}$(PO$_4$)$_6$O ('LK-99'), replacing 10% of Cu ions, as discussed in recent literature. Our calculations show that the material remains a wide bandgap insulator with isolated, S=1/2 localized charges on the Cu ions-similar to color centers-even within standard DFT, without the need for electron correlation corrections to the Cu d-orbitals. Superconductivity is excluded by known mechanisms that require the material to be metallic. We resolve previously observed inconsistencies between density functional theory results and experimental findings related to doping site energetics, crystal structure, and transparency. We find that Cu doping either Pb site leads to CuO$_4$ coordination and a similar unit cell volume contraction. Engineering materials with dopant sites that have different local symmetries can induce non-relativistic spin splitting-often referred to as altermagnetism. However, in the case of localized charges, this may enable spins to be individually controlled.

cond-mat.str-el↗

arXiv2024

Electronic Bound States in the Continuum in a 2D Metal

Bound states in the continuum (BICs) are quantum states that remain localized despite existing within a continuum of extended, delocalized states. They defy conventional wave theories and could be instrumental for quantum technologies that rely on the precise control of quantum states. While optical BICs have been realized in photonic systems, achieving electronic bound states in a metallic background remains an ongoing challenge. Here, we observe two defect states that remain localized within the metallic continuum of Pd5AlI2, a two-dimensional van der Waals metal. The emergence of these states is a manifestation of the hopping interference in the Pd5AlI2 lattice. This interference results in a (quasi) flat band and spatially localized eigenstates that are orthogonal to the metallic continuum thus avoiding hybridization with extended states.

cond-mat.mtrl-sci↗

arXiv2024

Tight-Binding Models for Lone Pair, Heteroanionic Solids, and Application to Layered Oxyhalides

We provide a methodology to understand materials with complex bonding patterns, and apply it to the example of heteroanionic and lone pair materials. We build a tight-binding model based on Wannier functions fitted on density functional theory results, followed by enforcing symmetry on the atomic orbital basis set, and finally connecting and disconnecting sets of orbitals from the tight-binding model to understand their individual contribution to the resulting materials properties. We apply this methodology to complex materials, namely BiOCl and Bi$_2$YO$_4$Cl - part of a broader class of materials investigated for their applications in photocatalysis and photoluminescence. Our methodology can be generalized and applied to a wide variety of other materials, including halide perovskites, and multiferroic materials. This methodology allows us to isolate the origin of key electronic features in these materials, including the role of the Bi lone pair-anion bonding interaction - key to photoluminescence in many materials. Finally, we investigate the role of the crystal structure, Chlorine and Oxygen orbital energy levels and bonding in determining the photostability of bismuth oxyhalides. Our methodology allows us to understand the functionality of complex materials in an intuitive and qualitative manner.

cond-mat.mtrl-sci↗

arXiv2024

Frustrated hopping from orbital decoration of a primitive two-dimensional lattice

Materials hosting flat electronic bands are a central focus of condensed matter physics as promising venues for novel electronic ground states. Two-dimensional (2D) geometrically frustrated lattices such as the kagome, dice, and Lieb lattices are attractive targets in this direction, anticipated to realize perfectly flat bands. Synthesizing these special structures, however, poses a formidable challenge, exemplified by the absence of solid-state materials realizing the dice and Lieb lattices. An alternative route leverages atomic orbitals to create the characteristic electron hopping of geometrically frustrated lattices. This strategy promises to expand the list of candidate materials to simpler structures, but is yet to be demonstrated experimentally. Here, we report the realization of frustrated hopping in the van der Waals (vdW) intermetallic Pd$_5$AlI$_2$, emerging from orbital decoration of a primitive square lattice. Using angle-resolved photoemission spectroscopy and quantum oscillations measurements, we demonstrate that the band structure of Pd$_5$AlI$_2$ includes linear Dirac-like bands intersected at their crossing point by a flat band, essential characteristics of frustrated hopping in the Lieb and dice lattices. Moreover, Pd$_5$AlI$_2$ is exceptionally stable, with the unusual bulk band structure and metallicity persisting in ambient conditions down to the monolayer limit. Our ability to realize an electronic structure characteristic of geometrically frustrated lattices establishes orbital decoration of primitive lattices as a new approach towards electronic structures that remain elusive to prevailing lattice-centric searches.

cond-mat.str-el↗

arXiv2024

Engineering the Magnetic Transition Temperatures and the Rare Earth Exchange Interaction in Oxide Heterostructures

The properties of functional oxide heterostructures are strongly influenced by the physics governing their interfaces. Modern deposition techniques allow us to accurately engineer the interface physics through the growth of atomically precise heterostructures. This enables minute control over the electronic, magnetic, and structural characteristics. Here, we investigate the magnetic properties of tailor-made superlattices employing the ferromagnetic and insulating double perovskites RE$_2$NiMnO$_6$ (RE = La, Nd), featuring distinct Curie temperatures. Adjusting the superlattice periodicity at the unit cell level allows us to engineer their magnetic phase diagram. Large periodicity superlattices conserve the individual para- to ferromagnetic transitions of the La$_2$NiMnO$_6$ and Nd$_2$NiMnO$_6$ parent compounds. As the superlattice periodicity is reduced, the Curie temperatures of the superlattice constituents converge and, finally, collapse into one single transition for the lowest period samples. This is a consequence of the magnetic order parameter propagating across the superlattice interfaces, as supported by a minimal Landau theory model. Further, we find that the Nd-Ni/Mn exchange interaction can be enhanced by the superlattice interfaces. This leads to a field-induced reversal of the Nd magnetic moments, as confirmed by synchrotron X-ray magnetic circular dichroism measurements and supported by first-principles calculations. Our work demonstrates how superlattice engineering can be employed to fine-tune the magnetic properties in oxide heterostructures and broadens our understanding of magnetic interfacial effects.

cond-mat.mtrl-sci↗

arXiv2024

Non-relativistic spin splitting in compensated magnets that are not altermagnets

The non-relativistic spin-splitting (NRSS) of electronic bands in "altermagnets" has sparked renewed interest in antiferromagnets (AFMs) that have no net magnetization. However, altermagnets with collinear and compensated magnetism are not the only type of NRSS AFMs. In this study, we identify the symmetry conditions and characteristic signatures of a distinct group of NRSS AFMs that go beyond the description of altermagnets. These compounds exhibit a broken spin-degeneracy among the spin-polarized bands at the $Γ$ point in the absence of spin-orbit coupling (SOC). We use density functional theory calculations to validate these models in ternary magnetic nitrides, specifically MnXN$_2$ (X = Si, Ge, Sn), and their cation ordered variants. By removing the previous NRSS constraint on $Γ$, these compounds may facilitate the generation of spin currents without cancellation arising from the alternating spin polarizations. Our findings expand the scope of NRSS eligible materials.

cond-mat.mtrl-sci↗

arXiv2023

Canted Antiferromagnetism in Polar MnSiN$_2$ with High Néel Temperature

MnSiN$_2$ is a transition metal nitride with Mn and Si ions displaying an ordered distribution on the cation sites of a distorted wurtzite-derived structure. The Mn$^{2+}$ ions reside on a 3D diamond-like covalent network with strong superexchange pathways. We simulate its electronic structure and find that the N anions in MnSiN$_2$ act as $σ$- and $π$-donors, which serve to enhance the N-mediated superexchange, leading to the high Néel ordering temperature of $T_N$ = 443 K. Polycrystalline samples of MnSiN$_2$ were prepared to reexamine the magnetic structure and resolve previously reported discrepancies. An additional magnetic canting transition is observed at $T_\mathrm{cant}$ = 433 K and the precise canted ground state magnetic structure has been resolved using a combination of DFT calculations and powder neutron diffraction. The calculations favor a $G$-type antiferromagnetic spin order with lowering to $Pc^\prime$. Irreducible representation analysis of the magnetic Bragg peaks supports the lowering of the magnetic symmetry. The computed model includes a 10$^\circ$ rotation of the magnetic spins away from the crystallographic $c$-axis consistent with measured powder neutron diffraction data modeling and a small canting of 0.6$^\circ$.

cond-mat.mtrl-sci↗

arXiv2021

Trigonal Symmetry Breaking and its Electronic Effects in Two-Dimensional Dihalides and Trihalides

We study the consequences of the approximately trigonal ($D_{3d}$) point symmetry of the transition metal (M) site in two-dimensional van der Waals MX$_2$ dihalides and MX$_3$ trihalides. The trigonal symmetry leads to a 2-2-1 orbital splitting of the transition metal $d$ shell, which may be tuned by the interlayer distance, and changes in the ligand-ligand bond lengths. Orbital order coupled to various lower symmetry lattice modes may lift the remaining orbital degeneracies, and we explain how these may support unique electronic states using ZrI$_2$ and CuCl$_2$ as examples, and offer a brief overview of possible electronic configurations in this class of materials. By building and analysing Wannier models adapted to the appropriate symmetry we examine how the interplay among trigonal symmetry, electronic correlation effects, and $p$-$d$ orbital charge transfer leads to insulating, orbitally polarized magnetic and/or orbital-selective Mott states. Our work establishes a rigorous framework to understand, control, and tune the electronic states in low-dimensional correlated halides. Our analysis shows that trigonal symmetry and its breaking is a key feature of the 2D halides that needs to be accounted for in search of novel electronic states in materials ranging from CrI$_3$ to $α$-RuCl$_3$.

cond-mat.str-el↗

arXiv2021

Database, Features, and Machine Learning Model to Identify Thermally Driven Metal-Insulator Transition Compounds

Metal-insulator transition (MIT) compounds are materials that may exhibit insulating or metallic behavior, depending on the physical conditions, and are of immense fundamental interest owing to their potential applications in emerging microelectronics. There is a dearth of thermally-driven MIT materials, however, which makes delineating these compounds from those that are exclusively insulating or metallic challenging. Here we report a material database comprising temperature-controlled MITs (and metals and insulators with similar chemical composition and stoichiometries to the MIT compounds) from high quality experimental literature, built through a combination of materials-domain knowledge and natural language processing. We featurize the dataset using compositional, structural, and energetic descriptors, including two MIT relevant energy scales, an estimated Hubbard interaction and the charge transfer energy, as well as the structure-bond-stress metric referred to as the global-instability index (GII). We then perform supervised classification, constructing three electronic-state classifiers: metal vs non-metal (M), insulator vs non-insulator (I), and MIT vs non-MIT (T). We identify two important descriptors that separate metals, insulators, and MIT materials in a 2D feature space: the average deviation of the covalent radius and the range of the Mendeleev number. We further elaborate on other important features (GII and Ewald energy), and examine how they affect classification of binary vanadium and titanium oxides. We discuss the relationship of these atomic features to the physical interactions underlying MITs in the rare-earth nickelate family. Last, we implement an online version of the classifiers, enabling quick probabilistic class predictions by uploading a crystallographic structure file.

cond-mat.mtrl-sci↗

arXiv2021

Energy Landscape analysis of metal-insulator transitions: theory and application to Ca$_2$RuO$_4$, $R$NiO$_3$ and their heterostructures

We present a general methodology that enables the disentanglement of the electronic and lattice contributions to the metal-insulator transition by building an energy landscape from numerical solutions of the equation of state. The methodology works with any electronic structure method that provides electronic expectation values at given atomic positions. Applying the theory to rare-earth perovskite nickelates ($R$NiO$_3$) and Ruddlesden-Popper calcium ruthenates (Ca$_2$RuO$_4$) in bulk, heterostructure and epitaxially strained thin film forms using equation of state results from density functional plus dynamical mean field calculations we show that the electron-lattice coupling is an essential driver of the transition from the metallic to the insulating state in these materials.

cond-mat.str-el↗

arXiv2020

Boson Slave Solver (BoSS) v1.1

Accurate and computationally efficient modeling of systems of interacting electrons is an outstanding problem in theoretical and computational materials science. For materials where strong electronic interactions are primarily of a localized character and act within a subspace of localized quantum states on separate atomic sites (e.g., in transition metal and rare-earth compounds), their electronic behaviors are typically described by the Hubbard model and its extensions. In this work, we describe BoSS (Boson Slave Solver), a software implementation of the slave-boson method appropriate for describing a variety of extended Hubbard models, namely $p-d$ models that include both the interacting atomic sites ("$d$" states) and non-interacting or ligand sites ("$p$" states). We provide a theoretical background, a description of the equations solved by BoSS, an overview of the algorithms used, the key input/output and control variables of the software program, and tutorial examples of its use featuring band renormalization in SrVO$_3$, Ni $3d$ multiplet structure in LaNiO$_3$, and the relation between the formation of magnetic moments and insulating behavior in SmNiO$_3$. BoSS interfaces directly with popular electronic structure codes: it can read the output of the Wannier90 software package which postprocesses results from workhorse electronic structure software such as Quantum Espresso or VASP.

cond-mat.str-el↗

arXiv2020

Effect of strain on magnetic and orbital ordering of LaSrCrO$_3$/LaSrMnO$_3$ heterostructures

We investigate the effect of strain and film thickness on the orbital and magnetic properties of LaSrCrO$_3$ (LSCO)/LaSrMnO$_3$ (LSMO) heterostructures using bulk magnetometry, soft X-ray magnetic spectroscopy, first-principles density functional theory, high-resolution electron microscopy and X-ray diffraction. We observe an anti-parallel ordering of the magnetic moments between the ferromagnetic LSMO layers and the LSCO spacers leading to a strain-independent ferromagnetic ground state of the LSCO/LSMO heterostructures for LSMO layers as thin as 2 unit cells. As the LSMO thickness is increased, a net ferromagnetic state is maintained, however, the average magnetic moment per Mn is found to be dependent on the magnitude of the substrate-induced strain. The differences in the magnetic responses are related to preferential occupation of the Mn $x^2-y^2$ (in-plane) d-orbitals for tensile strain and $3z^2-r^2$ (out-of-plane) orbitals under compressive strain leading to competing ferromagnetic and anti-ferromagnetic exchange interactions within the LSMO layers. These results underscore the relative contributions of orbital, structural and spin degree of freedom and their tunability in atomically-thin crystalline complex oxide layers.

cond-mat.mtrl-sci↗

arXiv2020

Observation of high-temperature quantum anomalous Hall regime in intrinsic MnBi$_2$Te$_4$/Bi$_2$Te$_3$ superlattice

The quantum anomalous Hall effect is a fundamental transport response of a topologically non-trivial system in zero magnetic field. Its physical origin relies on the intrinsically inverted electronic band structure and ferromagnetism, and its most consequential manifestation is the dissipation-free flow of chiral charge currents at the edges that can potentially transform future quantum electronics. Here we report a previously unknown Berry-curvature-driven anomalous Hall regime ('Q-window') at above-Kelvin temperatures in the magnetic topological bulk crystals where through growth Mn ions self-organize into a period-ordered MnBi$_2$Te$_4$/Bi$_2$Te$_3$ superlattice. Robust ferromagnetism of the MnBi$_2$Te$_4$ monolayers opens a large surface gap, and anomalous Hall conductance reaches an $e^2/h$ quantization plateau when the Fermi level is tuned into this gap within a Q-window in which the anomalous Hall conductance from the bulk is to a high precision zero. The quantization in this new regime is not obstructed by the bulk conduction channels and thus should be present in a broad family of topological magnets.

cond-mat.mtrl-sci↗