Sohrab Ismail‐Beigi

The mechanism behind superconductivity suppression induced by Pr substitutions in YBa 2 Cu 3 O 7− δ (YBCO) has been a mystery since its discovery: in spite of being isovalent to Y 3+ with a small magnetic moment, it is the only rare-earth element that has a dramatic impact on YBCO’s superconducting properties. Using angle-resolved photoemission spectroscopy (ARPES) and DFT+ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline" overflow="scroll"> <mml:mi>U</mml:mi> </mml:math> calculations, we uncover how Pr substitution modifies the low-energy electronic structure of YBCO. Contrary to the prevailing Fehrenbacher–Rice (FR) and Liechtenstein–Mazin (LM) models, the low-energy electronic structure contains no signature of any f -electron hybridization or additional f -state Fermi surface sheets. Yet, strong electron doping is observed primarily on the antibonding Fermi surface. Meanwhile, we reveal major electronic structure modifications to Cu-derived states with increasing Pr substitution: a pronounced CuO 2 bilayer decoupling and enhanced hopping along the CuO chain, implying indirect electron-release pathways beyond simple 4 f state ionization. Our results challenge the long-standing FR/LM mechanism, and establish Pr substituted YBCO as a potential platform for exploring correlation-driven phenomena in coupled 1D–2D systems.

Since 2019, eighteen NSF Research Traineeship (NRT) awards in quantum information science and engineering (QISE) and adjacent fields have been funded, constituting the largest NSF-coordinated investment in graduate QISE training in the United States. Synthesizing lessons from our programs, we work through the central tensions that every QISE graduate program must negotiate: between depth in a home discipline and breadth across the field, between structured instruction and open-ended experiential and hands-on learning, and between training individual specialists and cultivating teams that collectively cover all areas of QISE. We describe the structural and pedagogical innovations the NRT programs have developed in response, assess what is working and what remains unresolved, and sketch 12 open problems the community will need to address as QISE graduate education scales beyond the well-resourced research universities where it has up till now been mainly concentrated. Eight concrete recommendations follow: (1) adopt the startup model of team-based training as an organizing philosophy; (2) invest immediately in sensing and communication curriculum development; (3) build student agency into program governance, not just activities; (4) establish structural mechanisms for industrial engagement rather than depending on goodwill; (5) design for sustainability from year one; (6) develop graduate-level textbooks spanning all three QISE pillars: computing, sensing, and communications; (7) establish shared outcome assessment instruments across programs; and (8) develop structured mechanisms for faculty professional development in QISE.

Since 2019, eighteen NSF Research Traineeship (NRT) awards in quantum information science and engineering (QISE) and adjacent fields have been funded, constituting the largest NSF-coordinated investment in graduate QISE training in the United States. Synthesizing lessons from our programs, we work through the central tensions that every QISE graduate program must negotiate: between depth in a home discipline and breadth across the field, between structured instruction and open-ended experiential and hands-on learning, and between training individual specialists and cultivating teams that collectively cover all areas of QISE. We describe the structural and pedagogical innovations the NRT programs have developed in response, assess what is working and what remains unresolved, and sketch 12 open problems the community will need to address as QISE graduate education scales beyond the well-resourced research universities where it has up till now been mainly concentrated. Eight concrete recommendations follow: (1) adopt the startup model of team-based training as an organizing philosophy; (2) invest immediately in sensing and communication curriculum development; (3) build student agency into program governance, not just activities; (4) establish structural mechanisms for industrial engagement rather than depending on goodwill; (5) design for sustainability from year one; (6) develop graduate-level textbooks spanning all three QISE pillars: computing, sensing, and communications; (7) establish shared outcome assessment instruments across programs; and (8) develop structured mechanisms for faculty professional development in QISE.

The idea of these notes is to look at the origin of localized Hubbard-like models for electron correlation effects and see what one can do exactly (very little) and then more approximately within a mean-field approximation such as DFT, Hartree-Fock or the like. One of the other aims is to present background information as well as some derivations and explanations of the various forms and outcomes that are used in DFT+U calculations. These are meant to be graduate-level pedagogical readings that we have found useful in my research group.

The idea of these notes is to look at the origin of localized Hubbard-like models for electron correlation effects and see what one can do exactly (very little) and then more approximately within a mean-field approximation such as DFT, Hartree-Fock or the like. One of the other aims is to present background information as well as some derivations and explanations of the various forms and outcomes that are used in DFT+U calculations. These are meant to be graduate-level pedagogical readings that we have found useful in my research group.

High temperature superconductivity is typically associated with strong coupling and a large superconducting gap, yet these characteristics have not been demonstrated in the nickelates. Here, we provide experimental evidence that Eu substitution in the spacer layer of Nd1-xEuxNiO2 (NENO) thin films enhances the superconducting gap, driving the system toward a strong-coupling regime. This is accompanied by a magnetic-exchange-driven magnetic-field-enhanced superconductivity. We investigate the upper critical magnetic field, Hc2, and the superconducting gap of superconducting NENO thin films with x = 0.2 to 0.35. Magnetoresistance measurements reveal magnetic-field-enhanced superconductivity in NENO films. We interpret this phenomenon as a result of an interaction between magnetic Eu ions and superconducting states in the Ni dx2-y2 orbital. The upper critical magnetic field strongly violates the weak-coupling Pauli limit. Infrared spectroscopy confirms a large gap-to-Tc ratio $$2\Delta /{k}_{B}{T}_{{\rm{c}}}\simeq 5-6$$, indicating a stronger coupling pairing mechanism in NENO relative to the Sr-doped NdNiO2. The substitution of Eu in the rare-earth layer causes pronounced modifications of the superconducting gap and magnetic interactions in Nd-based nickelates, opening new pathways to engineer high-Tc superconductivity in infinite-layer nickelates. The authors provide experimental evidence that Eu substitution in the spacer layer of Nd1-xEuxNiO2 thin films enhances the superconducting gap, driving the system toward a strong-coupling regime. The Eu substitution also introduces exchange coupling between Eu 4f magnetic moments and Ni 3dx²−y² electrons, leading to magnetic-field-enhanced “re-entrant” superconductivity.

High temperature superconductivity is typically associated with strong coupling and a large superconducting gap, yet these characteristics have not been demonstrated in the nickelates. Here, we provide experimental evidence that Eu substitution in the spacer layer of Nd_1-xEu_xNiO₂ (NENO) thin films enhances the superconducting gap, driving the system toward a strong-coupling regime. This is accompanied by a magnetic-exchange-driven magnetic-field-enhanced superconductivity. We investigate the upper critical magnetic field, H_c2, and the superconducting gap of superconducting NENO thin films with x = 0.2 to 0.35. Magnetoresistance measurements reveal magnetic-field-enhanced superconductivity in NENO films. We interpret this phenomenon as a result of an interaction between magnetic Eu ions and superconducting states in the Ni d_x2-y2 orbital. The upper critical magnetic field strongly violates the weak-coupling Pauli limit. Infrared spectroscopy confirms a large gap-to-T_c ratio <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mn>2</mml:mn><mml:mi>Δ</mml:mi><mml:mo>/</mml:mo><mml:msub><mml:mrow><mml:mi>k</mml:mi></mml:mrow><mml:mrow><mml:mi>B</mml:mi></mml:mrow></mml:msub><mml:msub><mml:mrow><mml:mi>T</mml:mi></mml:mrow><mml:mrow><mml:mi>c</mml:mi></mml:mrow></mml:msub><mml:mo>≃</mml:mo><mml:mn>5</mml:mn><mml:mo>-</mml:mo><mml:mn>6</mml:mn></mml:math>, indicating a stronger coupling pairing mechanism in NENO relative to the Sr-doped NdNiO₂. The substitution of Eu in the rare-earth layer causes pronounced modifications of the superconducting gap and magnetic interactions in Nd-based nickelates, opening new pathways to engineer high-T_c superconductivity in infinite-layer nickelates.

The mechanism behind superconductivity suppression induced by Pr substitutions in YBa$_2$Cu$_3$O$_{7-δ}$ (YBCO) has been a mystery since its discovery: in spite of being isovalent to Y$^{3+}$ with a small magnetic moment, it is the only rare-earth element that has a dramatic impact on YBCO's superconducting properties. Using angle-resolved photoemission spectroscopy (ARPES) and DFT+$U$ calculations, we uncover how Pr substitution modifies the low-energy electronic structure of YBCO. Contrary to the prevailing Fehrenbacher-Rice (FR) and Liechtenstein-Mazin (LM) models, the low energy electronic structure contains no signature of any $f$-electron hybridization or new states. Yet, strong electron doping is observed primarily on the antibonding Fermi surface. Meanwhile, we reveal major electronic structure modifications to Cu-derived states with increasing Pr substitution: a pronounced CuO$_2$ bilayer decoupling and an enhanced CuO chain hopping, implying indirect electron-release pathways beyond simple 4$f$ state ionization. Our results challenge the long-standing FR/LM mechanism and establish Pr substituted YBCO as a potential platform for exploring correlation-driven phenomena in coupled 1D-2D systems.

Dimensionality and electronic correlations are crucial elements of many quantum material properties. An example is the change of the electronic structure accompanied by the loss of quasiparticles when a metal is reduced from three dimensions to a lower dimension, where the Coulomb interaction between carriers becomes poorly screened. Here, using angle-resolved photoemission spectroscopy, we report an orbital-selective decoherence of spectral density in the perovskite nickelate <a:math xmlns:a="http://www.w3.org/1998/Math/MathML"> <a:mrow> <a:mi>LaNi</a:mi> <a:msub> <a:mi mathvariant="normal">O</a:mi> <a:mn>3</a:mn> </a:msub> </a:mrow> </a:math> toward the monolayer limit. The spectral weight of the <c:math xmlns:c="http://www.w3.org/1998/Math/MathML"> <c:msub> <c:mi>d</c:mi> <c:mrow> <c:mi>z</c:mi> <c:mn>2</c:mn> </c:mrow> </c:msub> </c:math> band vanishes much faster than that of the <d:math xmlns:d="http://www.w3.org/1998/Math/MathML"> <d:msub> <d:mi>d</d:mi> <d:mrow> <d:mi>x</d:mi> <d:mn>2</d:mn> <d:mo>−</d:mo> <d:mi>y</d:mi> <d:mn>2</d:mn> </d:mrow> </d:msub> </d:math> band as the thickness of the <e:math xmlns:e="http://www.w3.org/1998/Math/MathML"> <e:mrow> <e:mi>LaNi</e:mi> <e:msub> <e:mi mathvariant="normal">O</e:mi> <e:mn>3</e:mn> </e:msub> </e:mrow> </e:math> layer is decreased to a single unit cell, indicating a stronger correlation effect for the former upon dimensional confinement. Dynamical mean-field theory calculations show an orbital-selective Mott transition largely due to the localization of <g:math xmlns:g="http://www.w3.org/1998/Math/MathML"> <g:msub> <g:mi>d</g:mi> <g:mrow> <g:mi>z</g:mi> <g:mn>2</g:mn> </g:mrow> </g:msub> </g:math> electrons along the <h:math xmlns:h="http://www.w3.org/1998/Math/MathML"> <h:mi>c</h:mi> </h:math> axis in the monolayer limit. This orbital-selective correlation effect underpins many macroscopic properties of nickelates, such as metal-to-insulator transition and superconductivity, where most theories are built upon a <i:math xmlns:i="http://www.w3.org/1998/Math/MathML"> <i:mrow> <i:msub> <i:mi>d</i:mi> <i:mrow> <i:mi>x</i:mi> <i:mn>2</i:mn> <i:mo>−</i:mo> <i:mi>y</i:mi> <i:mn>2</i:mn> </i:mrow> </i:msub> <i:mtext>–</i:mtext> <i:msub> <i:mi>d</i:mi> <i:mrow> <i:mi>z</i:mi> <i:mn>2</i:mn> </i:mrow> </i:msub> </i:mrow> </i:math> two-band model.

We investigate the microscopic mechanisms behind the stabilization of three-dimensional (3D) charge order by Pr doping in YBa$_2$Cu$_3$O$_7$ (YBCO7). Density-functional-theory calculations locate the lowest-energy Pr superlattices for both Ba- and Y-site substitution. In the Ba-site case, the smaller Pr ion pulls the surrounding atoms inward. This breathing-mode distortion pins charge-stripe walls to the Pr columns and forces them to align along the $c$ axis. Y-site Pr is larger than the host ion, produces an outward distortion, and fails to pin the stripes. Coarse-grained Monte-Carlo simulations show that the stripe correlation length rises in step with the structural correlation length of the Pr dopant as observed in prior experiments. We thus identify Ba-site substitution and dopant-induced lattice pinning as the key mechanism behind 3D charge order in Pr-doped YBCO7. This approach provides quantitative guidelines for engineering electronic orders through targeted ionic substitution.

We quantitatively identify the multiple distinct microscopic mechanisms contributing to effective interlayer couplings (EICs) by performing first-principle calculations for two prototype superconducting cuprate families, pristine and doped ${\mathrm{Bi}}_{2}{\mathrm{Sr}}_{2}{\mathrm{CaCuO}}_{2}{\mathrm{O}}_{8+x}$ and ${\mathrm{Pr}}_{x}{\mathrm{Y}}_{1\ensuremath{-}x}{\mathrm{Ba}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7}$. The major mechanisms are mediated by interlayer oxygen ${p}_{\ensuremath{\sigma}}\text{\ensuremath{-}}{p}_{\ensuremath{\sigma}}$ and ${p}_{z}\text{\ensuremath{-}}{p}_{z}$ hoppings as well as interlayer copper ${d}_{3{z}^{2}\ensuremath{-}{r}^{2}}$-oxygen ${p}_{\ensuremath{\sigma}}$ hoppings. Furthermore, we show how EICs are closely related to structural distortions such as layer bucklings and bond length changes. This allows us to provide analytical formulas that permit direct estimation of the key interatomic hoppings and the EICs based only on the crystal structure. Finally, we benchmark our method on ${\mathrm{YBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7}$ to estimate the strength and anisotropy of the EIC.

Wannier Wave Function Probe Angle-resolved photoemission spectroscopy (ARPES) has been a widely adopted technique to investigate surface and shallow interface electron energy-momentum dispersion. The cover picture of article 2400427 by Charles H. Ahn and co-workers, proposes a new way of using ARPES to reconstruct the electron wave function on crystalline surfaces, via the dipole-transition matrix element effect and spectral sum rule.

Materials-realistic microscopic theoretical descriptions of copper-based superconductors are challenging due to their complex crystal structures combined with strong electron interactions. Here, we demonstrate how density functional theory can accurately describe key structural, electronic, and magnetic properties of the normal state of the prototypical cuprate <a:math xmlns:a="http://www.w3.org/1998/Math/MathML" display="inline"> <a:mrow> <a:msub> <a:mrow> <a:mi>Bi</a:mi> </a:mrow> <a:mrow> <a:mn>2</a:mn> </a:mrow> </a:msub> </a:mrow> <a:mrow> <a:msub> <a:mrow> <a:mi>Sr</a:mi> </a:mrow> <a:mrow> <a:mn>2</a:mn> </a:mrow> </a:msub> </a:mrow> <a:mrow> <a:msub> <a:mrow> <a:mi>CaCu</a:mi> </a:mrow> <a:mrow> <a:mn>2</a:mn> </a:mrow> </a:msub> </a:mrow> <a:mrow> <a:msub> <a:mrow> <a:mi mathvariant="normal">O</a:mi> </a:mrow> <a:mrow> <a:mn>8</a:mn> <a:mo>+</a:mo> <a:mi>x</a:mi> </a:mrow> </a:msub> </a:mrow> </a:math> (Bi-2212). We emphasize the importance of accounting for energy-lowering structural distortions, which then allows us to (a) accurately describe the insulating antiferromagnetic (AFM) ground state of the undoped parent compound (in contrast to the metallic state predicted by previous studies); (b) identify numerous low-energy competing spin and charge stripe orders in the hole-overdoped material nearly degenerate in energy with the AFM ordered state, indicating strong spin fluctuations; (c) predict the lowest-energy hole-doped crystal structure including its long-range structural distortions and oxygen dopant positions that match high-resolution scanning transmission electron microscopy measurements; and (d) describe electronic bands near the Fermi energy with flat antinodal dispersions and Fermi surfaces that are in agreement with angle-resolved photoemission spectroscopy (ARPES) measurements and provide a clear explanation for the structural origins of the so-called “shadow bands.” We also show how one must go beyond band theory and include fully dynamic spin fluctuations via a many-body approach when aiming to make quantitative predictions to measure the ARPES spectra in the overdoped material. Finally, regarding spatial inhomogeneity, we show that the local structure at the <d:math xmlns:d="http://www.w3.org/1998/Math/MathML" display="inline"> <d:mrow> <d:msub> <d:mrow> <d:mi>CuO</d:mi> </d:mrow> <d:mrow> <d:mn>2</d:mn> </d:mrow> </d:msub> </d:mrow> </d:math> layer, rather than dopant electrostatic effects, modulates the local charge-transfer gaps, local correlation strengths, and by extension the local superconducting gaps. Published by the American Physical Society 2024

We quantitatively identify the multiple distinct microscopic mechanisms contributing to effective interlayer couplings (EICs) by performing first-principle calculations for two prototype superconducting cuprate families, pristine and doped Bi$_2$Sr$_2$CaCuO$_2$O$_{8+x}$ and Pr$_{x}$Y$_{1-x}$Ba$_2$Cu$_3$O$_7$. The major mechanisms are mediated by interlayer oxygen $p_σ$-$p_σ$ and $p_z$-$p_z$ hoppings as well as interlayer copper $d_{z^2}$-oxygen $p_σ$ hoppings. Furthermore, we show how EICs are closely related to structural distortions such as layer bucklings and bond length changes. This allows us to provide analytical formulae that permit direct estimation of the key interatomic hoppings and the EICs based only on the crystal structure. Finally, we benchmark our method on YBa$_2$Cu$_3$O$_7$ to estimate the strength and anisotropy of the EIC.

Oxygen vacancies and their correlation with the nanomagnetism and electronic structure are crucial for applications in dilute magnetic semiconductors design applications. Here, we report on cobalt single atom-incorporated titanium dioxide (TiO 2 ) monodispersed nanoparticles synthesized using a thermodynamic redistribution strategy. Using advanced synchrotron-based X-ray techniques and simulations, we find trivalent titanium is absent, indicating trivalent cations do not influence ferromagnetic (FM) stability. Density functional theory calculations show that the FM stability between Co 2+ ions is very weak. However, electron doping from additional oxygen vacancies can significantly enhance this FM stability, which explains the observed room-temperature ferromagnetism. Moreover, our calculations illustrate enhanced FM interactions between Co Ti + V O complexes with additional oxygen vacancies. This study explores the electronic structure and room-temperature ferromagnetism using monodispersed nanocrystallites with single-atom-incorporated TiO 2 nanostructures. The strategies described herein offer promise in revealing magnetism in other single-atom-incorporated nanostructures.

Abstract Angle‐resolved photoemission spectroscopy (ARPES) has been a widely adopted technique in the studies of quantum materials. The surface sensitivity of photoelectric effect also makes it a powerful tool to investigate surface and shallow interface phenomena. While an overwhelming majority of its use focuses on extracting the eigenenergy of the electron Bloch states in momentum space, attempts to extract information of the wave function via ARPES has been limited to molecular systems. In this perspective, it is proposed and advocated use ARPES to investigate and unravel wave function properties, as opposed to only the electron energy‐momentum dispersion relation, in crystalline solids and their interfaces. This can help enhance the rapidly growing development of material properties based on the spatial and geometric properties of the electronic wave functions.

Two-dimensional electron gas (2DEG) states at oxide interfaces between two ferroic materials have been fertile ground to realize controllable multiferroicity. Here, we investigate the 2DEG states at the interface of ferroelectric $\mathrm{BaTi}{\mathrm{O}}_{3}$ and a magnetic layer of iron using angle-resolved photoemission spectroscopy. Orbital-selective charge transfer occurs on the surprisingly robust 2DEG. Based on first-principles calculations, we show how the interfacial hybridization can give rise to the unexpected charge transfer in the magnetic 2DEG. Our study reveals a close interplay on a 2DEG between magnetic and ferroelectric interfaces, which sheds light on future design principles of multiferroic 2DEG states.

Materials-realistic microscopic theoretical descriptions of copper-based superconductors are challenging due to their complex crystal structures combined with strong electron interactions. Here, we demonstrate how density functional theory can accurately describe key structural, electronic, and magnetic properties of the normal state of the prototypical cuprate Bi$_2$Sr$_2$CaCu$_2$O$_{8+x}$ (Bi-2212). We emphasize the importance of accounting for energy-lowering structural distortions, which then allows us to: (a) accurately describe the insulating antiferromagnetic (AFM) ground state of the undoped parent compound (in contrast to the metallic state predicted by previous {\it ab initio} studies); (b) identify numerous low-energy competing spin and charge stripe orders in the hole-overdoped material nearly degenerate in energy with the AFM ordered state, indicating strong spin fluctuations; (c) predict the lowest-energy hole-doped crystal structure including its long-range structural distortions and oxygen dopant positions that match high-resolution scanning transmission electron microscopy (STEM) measurements; and (d) describe electronic bands near the Fermi energy with flat antinodal dispersions and Fermi surfaces that in agreement with angle-resolved photoemission spectroscopy (ARPES) measurements and provide a clear explanation for the structural origins of the so-called ``shadow bands''. We also show how one must go beyond band theory and include fully dynamic spin fluctuations via a many-body approach when aiming to make quantitative predictions to measure the ARPES spectra in the overdoped material. Finally, regarding spatial inhomogeneity, we show that the local structure at the CuO$_2$ layer, rather than dopant electrostatic effects, modulates the local charge-transfer gaps, local correlation strengths, and by extension the local superconducting gaps.

Abstract Identifying environmentally inert, ferromagnetic two-dimensional (2D) materials with high Curie temperatures ( T c ) down to the single layer limit has been an obstacle to fundamental studies of 2D magnetism and application of 2D heterostructures to spin-polarized devices. To address this challenge, the growth, structure and magnetic properties of a 2D Cr-silicate single layer on Pt(111) was investigated experimentally and theoretically. The layer was grown by sequentially depositing SiO and Cr followed by annealing in O 2 . Scanning tunneling microscopy (STM), low-energy electron diffraction (LEED), and low energy electron microscopy all indicated a well-ordered layer that uniformly covered the surface, with STM and LEED indicating that the silicate relaxed to its favored lattice constant. Further experimental characterizations demonstrated that the Cr was nominally 3+ but with a lower electron density than typical trivalent Cr compounds. Comparison with theory identified a Cr 2 Si 2 O 9 structure that resembles a single layer of a dehydrogenated dioctahedral silicate. Magnetic circular dichroism in x-ray absorption spectroscopy revealed a ferromagnetically ordered state up to at least 80 K. Theoretical analysis revealed that the Cr in a dehydrogenated Cr-silicate/Pt(111) is more oxidized than Cr in freestanding Cr 2 Si 2 O 9 H 4 layers. This greater oxidation was found to enhance ferromagnetic coupling and suggests that the magnetism may be tuned by doping. The 2D Cr-silicate is the first member of a broad series of possible layered first-row transition metal silicates with magnetic order; thus, this paper introduces a new platform for investigating 2D ferromagnetism and the development of magnetoelectronic and spintronic devices by stacking 2D atomic layers.

We reveal in this study the fundamental low-energy landscape in the ferrimagnetic ${\mathrm{Sr}}_{2}\mathrm{CrRe}{\mathrm{O}}_{6}$ double perovskite and describe the underlying mechanisms responsible for the three low-energy excitations below 1.4 eV. Based on resonant inelastic x-ray scattering and magnetic dynamics calculations, and experiments collected from both ${\mathrm{Sr}}_{2}\mathrm{CrRe}{\mathrm{O}}_{6}$ powders and epitaxially strained thin films, we reveal a strong competition between spin-orbit coupling, Hund's coupling, and the strain-induced tetragonal crystal field. We also demonstrate that a spin-flip process is at the origin of the lowest excitation at 200 meV, and we bring insights into the predicted presence of orbital ordering in this material. We study the nature of the magnons through a combination of ab initio and spin-wave theory calculations, and show that two nondegenerate magnon bands exist and are dominated either by rhenium or chromium spins. The rhenium band is found to be flat at about 200 meV (\ifmmode\pm\else\textpm\fi{}25 meV) through X-L-W-U high-symmetry points and is dispersive toward \ensuremath{\Gamma}.

Efficient and accurate computational methods for dealing with interacting electron problems on a lattice are of broad interest to the condensed matter community. For interacting Hubbard models, we introduce a cluster slave-particle approach that provides significant computational savings with high accuracy for total energies, site occupancies, and interaction energies. Compared to exact benchmarks using density matrix renormalization group for $d\text{\ensuremath{-}}p$ Hubbard models, our approach delivers accurate results using two to three orders of magnitude lower computational cost. Our method is based on a slave-particle decomposition with an improved description of particle hoppings, and a density matrix expansion method where the interacting lattice slave-particle problem is turned into a set of overlapping real-space clusters which are solved self-consistently with appropriate physical matching constraints at shared lattice sites between clusters.

Abstract Interlayer excitons in solid‐state systems have emerged as candidates for realizing novel platforms ranging from excitonic transistors and optical qubits to exciton condensates. Interlayer excitons have been discovered in 2D transition metal dichalcogenides, with large exciton binding energies and the ability to form various van der Waals heterostructures. Here, an oxide system consisting of a single unit cell of Mg 2 TiO 4 on MgO (001) is proposed as a platform for hosting interlayer excitons. Using a combination of density functional theory (DFT) calculations, molecular beam epitaxy growth, and in situ crystal truncation rod measurements, it is shown that the Mg 2 TiO 4 ‐MgO interface can be precisely controlled to yield an internal electric field suitable for hosting interlayer excitons. The atoms in the polar Mg 2 TiO 4 layers are observed to be displaced to reduce polarity at the interface with the non‐polar MgO (001) surface. Such polarity‐driven atomic displacements strongly affect electrostatics of the film and the interface, resulting in localization of filled and empty band‐edge states in different layers of the Mg 2 TiO 4 film. The DFT calculations suggest that the electronic structure is favorable for localization of photoexcited electrons in the bottom layer and holes in the top layer, which may bind to form interlayer exciton states.