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Thomas P. Devereaux

Publications and source records attributed to Thomas P. Devereaux.

At least 19 recordsLinked to original sources
arXiv2026

Transition-Metal-Oxygen Covalency Controls Ligand-Hole Stability in Li-Rich Cathode

Lithium-rich cathodes boast a large specific capacity due to the presence of excess lithium per transition metal. However, making use of that excess Li drives the cathodes to unusually high formal oxidation states, often triggering unwanted side reactions, typically signaled by gas release and commonly associated with oxygen oxidation. By varying the transition metal (TM) chemistry in Li-rich materials, cyclability can vary drastically, with some materials suffering rapid capacity fade while others are able to maintain large and stable voltage windows. Such observations indicate a crucial role for both TM and O activities, but the details remain elusive. Here, we model core-level spectroscopy to understand differences in the charge compensation mechanism between different Li2MO3 (M = Mn, Ru, Ir) cathode materials. We find that ligand holes are important throughout the series, but their stability depends strongly on the transition-metal-oxygen electronic structure. In Li2MnO3, the ground state is already strongly negative-charge-transfer-like, so further delithiation rapidly increases ligand-hole character and promotes instability. Moving from Mn to Ru and Ir increases TM-O hybridization while reducing the tendency toward an extreme negative-charge-transfer state, allowing additional holes to remain delocalized over the covalent TM-O network over a wider range of charge. These results identify TM-O covalency, rather than the absence of oxygen-hole character, as a key factor controlling reversible high-valent redox and suggest a route for stabilizing high-energy-density cathodes without invoking reversible molecular oxygen formation.

cond-mat.mtrl-sci↗
arXiv2026

Framework for an AI-Ready ARPES Community Database

Condensed matter stands to gain profoundly from widespread adoption of artificial intelligence (AI). However, its current means of data organization impedes this endeavor. As a premier measure of electronic structure underlying material properties, Angle-Resolved Photoemission Spectroscopy (ARPES) has accumulated over four decades of spectra, yet the community's publication-centered workflow preserves only a highly selected fraction of its experimental record. A shared data ecosystem could transform these fragmented measurements and their contexts into a persistent, collective scientific memory, creating immediate value for reproducibility and reuse while laying the foundation for AI-enabled discovery. In this Perspective, we report outcomes of a recent community workshop and propose a practical path toward such an ecosystem. Its benefits unfold along three timescales. In the near term, standardized, searchable, and citable datasets strengthen analysis pipelines independently of AI. In the intermediate future, community-validated tools operating across laboratories could enable broader reuse and increasingly closed-loop automated experiments. In the long term, a diverse corpus of measured electronic spectra could support foundation models for quantum matter. Looking forward, a diverse corpus of measured electronic spectra could support foundation models for quantum matter and new discoveries. We discuss lessons from existing scientific databases, the current state of AI in ARPES, as well as areas of community convergence on data organization, access, credit, governance, and a staged pilot program.

cond-mat.mtrl-sci↗
arXiv2026

Bad metallicity in the semi-quantum regime of the Hubbard model

Bad metals exhibit approximately $T$-linear dc resistivity beyond the Ioffe--Regel limit. That this behavior occurs in systems with radically different ground states suggests it is a generic manifestation of strong local correlations. We test this hypothesis in the infinite-$U$ Hubbard model with small hole densities using exact diagonalization to compute thermodynamic and transport properties of finite clusters. Upon cooling, we find an intermediate temperature range, an electronic analogue of the ``semi-quantum regime'' of liquid helium, in which quantum effects produce a roughly $T$-independent compressibility, yet the resistivity is $T$-linear and exceeds the Ioffe--Regel limit. Entry into this regime is accompanied by the formation of quasi-local ferromagnetic ``spin cages'' around doped holes, regions that facilitate local quantum motion embedded in a fluctuating spin background, analogous to the transient crystalline cages thought to control incoherent transport in semi-quantum liquid helium. Remarkably, despite the simplicity of the model, the bad metal behavior found here resembles that seen in various material platforms.

cond-mat.str-el↗
arXiv2026

Robust Strange Metallicity across Attractive and Repulsive Hubbard Models

Using determinant quantum Monte Carlo simulations, we compare charge transport in the two-dimensional attractive and repulsive Hubbard models at strong coupling, $|U|/t=6$. Although the interaction with opposite signs generates qualitatively different low-energy spin, charge, and pairing correlations, both exhibit approximately linear-in-temperature resistivity over a broad intermediate- and high-temperature regime. At asymptotically high temperature this common behavior follows from the moment expansion of the conductivity, whose leading contribution is even in $U$. More strikingly, the similarity persists to temperatures well below $|U|$, where strong interaction-dependent correlations have already developed, and also irrespective of whether resistivity crosses the MIR limit. Using the Nernst--Einstein relation, we find that the common linear-in-temperature resistivity is primarily associated with an approximately Curie-like charge compressibility and weakly temperature-dependent diffusivity. The two models separate only at lower temperatures, where the attractive model develops a pronounced feature in the charge diffusivity correlated with signatures of pair formation. These results show that linear-in-temperature incoherent transport in the Hubbard model can be remarkably insensitive to the microscopic nature of the low-energy correlations. Our results suggest that transport in the incoherent regime appears insensitive to what the system will ultimately become at low temperature; the distinction between competing low-energy states becomes visible only when their characteristic correlations acquire sufficiently long spatial or temporal coherence.

cond-mat.str-el↗
arXiv2026

Selective coupling of high-order phonons in La2-xSrxCuO4

In crystals and molecules with strong coupling between the charge carriers and atomic vibrations, high order harmonics of the vibrational excitations can be observed with intensity following the Franck-Condon envelope. Here, we uncover a new regime of electron-phonon coupling in La2-xSrxCuO4 using resonant inelastic X-ray scattering. In the undoped compound, we find sharp peaks at approximately 85$\pm$5 meV, 180$\pm$7 meV, and 330$\pm$5 meV with no observable dispersion, and an absence of appreciable intensity from other modes observed in Raman spectroscopy in this energy range. The 180 meV and 330 meV excitations have energies approximately consistent with two-phonon and four-phonon in-plane Cu-O bond stretching mode, suggesting that the resonantly excited valence electrons strongly couple to this mode. These excitations exhibit significant doping dependence, eventually becoming unresolvable at 10% hole doping, where the low energy charge excitations are dominated by dispersive plasmons. The observation of selective coupling to even-ordered phonons and stark contrast with the Raman spectra indicate anomalous electron-phonon coupling beyond the Franck-Condon picture. One intriguing possibility is the existence of locally paired quasiparticles in insulating cuprates.

cond-mat.str-el↗
arXiv2026

Probing the pseudogap and beyond: Examining single-particle properties of the hole- and electron-doped Hubbard model

We compute high-resolution angle-resolved photoemission spectroscopy of the Hubbard model using the unbiased determinant quantum Monte Carlo algorithm, revealing an asymmetry between electron and hole doping. Electron doping exhibits more coherent quasiparticles and stronger antiferromagnetic correlations compared to hole doping. At low doping, a nodal-antinodal dichotomy on the Fermi surface is observed, similar to cuprate experiments. The dichotomy reflects the momentum dependence of the Mott gap, as manifested in both the spectral function and the self-energy. For hole doping, we observe a transition towards the pseudogap, without signature of pocket formation. The simulated nuclear magnetic resonance pseudogap temperatures do not necessarily agree with the temperature determined by spectroscopy. These findings collectively suggest the pseudogap is a smooth crossover driven by strong correlations.

cond-mat.str-el↗
arXiv2026

Collective Magnetic Excitations in a Photo-excited Electron-doped Cuprate Superconductor

Elucidating the microscopic behavior of cuprates under ultrafast photoexcitation offers critical insights into their highly correlated out-of-equilibrium states. Although quasiparticle dynamics have been investigated extensively, the behavior of collective magnetic excitations remains comparatively unexplored. Here, we use time-resolved resonant inelastic X-ray scattering (trRIXS) at the Cu $L_3$-edge to track the collective magnetic excitations (paramagnons) in an optimally electron-doped cuprate driven out-of-equilibrium by a femtosecond pump laser pulse. Upon pumping, we observed an anti-Stokes signal associated with paramagnon generation, which modifies the paramagnon dispersion near the zone center, although the bandwidth remained unchanged. Moreover, the spectral weight exhibits a momentum-dependent variation across the Brillouin zone. The light-driven boost of the paramagnon population and the resulting spectral-weight transfer could provide new leverage to manipulate the properties of cuprates.

cond-mat.mtrl-sci↗
arXiv2026

Interlayer Five-Spin Polaron in Superconducting Bilayer Nickelates

The discovery of high-$T_c$ superconductivity in Ruddlesden-Popper nickelates has sparked substantial effort towards understanding unconventional electronic states beyond a traditional cuprate-like $d^9$ configurational ground state. An understanding of the interplay between magnetic ground states and multi-orbital physics is key for establishing a microscopic mechanism for superconductivity. In the bilayer nickelates, spin density wave (SDW) order is a prominent feature in the non-superconducting regime, yet its relation to superconducting pairing remains an open question. Here, we use resonant x-ray scattering to examine the existence of SDW order in superconducting bilayer nickelate thin films La$_2$PrNi$_2$O$_7$ (LPNO). Comparing superconducting and oxygen-deficient LPNO thin films, we find that superconductivity occurs in SDW-free, oxygen-stoichiometric regions, whereas oxygen-deficiency promotes SDW order, indicating phase segregation of SDW and superconductivity. Furthermore, Ni-$L_3$ and O-$K$ edge spectroscopy reveals distinct electronic structures - particularly along the $c$-axis - between the two regions. Our results identify oxygen stoichiometry as a key parameter controlling interlayer coupling and thus the electronic structure of bilayer nickelates. In concert with theory, we propose that a ligand hole primarily resides at the inter-bilayer apical oxygen, forming a robust interlayer five-spin polaron state, which serves as the ground state for superconducting bilayer nickelates.

cond-mat.str-el↗
arXiv2026

Hall conductivity reveals the nature of quantum coherence in strongly correlated metals

Linear-in-temperature resistivity is a hallmark for strange metallic transport, and appears universally in many strongly correlated electron systems. However, the focus on the longitudinal channel often overshadows the profound microscopic insights contained within the transverse response. Here, we utilize numerically exact determinantal quantum Monte Carlo simulations of the doped Hubbard model in a magnetic field to calculate longitudinal and transverse transport. We demonstrate that while the resistivity is robustly $T$-linear across parameter sets, the Hall response is highly sensitive to particle-hole asymmetry, Fermi surface topology, and many-body correlation effects. Specifically, the combination of these effects determine a crossover scale in which the system becomes quantum-coherent, and is reflected in the Hall conductivity. Our results demonstrate that while the $T$-linearity in resistivity appears universal, the Hall response reveals a crossover from semi-classical to quantum-coherent transport otherwise masked in the longitudinal channel.

cond-mat.str-el↗
arXiv2026

Spectroscopic evidence for a molecular orbital Kondo insulator

A Kondo insulator (KI) is a prototypical example of a highly entangled phase of matter, where many-body interactions between local moments and delocalized electrons engender the non-magnetic insulating ground state. Conventionally, the local moments arise from atomic multiplet states with a narrow bandwidth, limiting Kondo coherence to low temperatures. Here, we realize a new paradigm for constructing the KI state with hybridized molecular orbitals in FeSb2. Resonant inelastic X-ray scattering (RIXS) at the Fe L-edge reveals distinct signatures of band-like continuum states and localized states. Comparisons with first-principles calculations establish a mixed-configuration ground state with hybridized Fe d-Sb p molecular orbitals as basis states. By systematically investigating the RIXS momentum, temperature, and doping dependences, we find propagating collective modes commensurate with many-body charge and spin excitations. Our results pave the way for understanding the emerging class of unconventional d electron insulators and engineering high temperature Kondo many-body states.

cond-mat.str-el↗
arXiv2026

Strongly-coupled hybrid lattice-plasmons in layered cuprates

Metallic systems with delocalized valence electrons host collective charge density oscillations known as plasmons. On the other hand, conventional insulators do not have free electrons and the low energy charge degrees of freedom are pinned to the ions. The fate of the collective charge excitations in the intermediate regime is an outstanding question. This problem is especially important for strongly correlated systems such as the layered cuprates, where unconventional superconductivity and other emergent phenomena arise from valence electrons on the border between Mott localization and itinerancy. Using resonant inelastic X-ray scattering, we track this evolution in the prototypical electron-doped cuprate Nd2-xCexCuO4. We find a continuous transformation of the low-energy charge response: from an acoustic plasmon in the metallic regime, to a gapped hybrid mode at intermediate doping, and finally to a nearly dispersionless 139 meV excitation at half filling. Remarkably, the 139 meV excitation has approximately twice the energy of the oxygen breathing phonon responsible for the dispersion kink observed in angle-resolved photoemission spectroscopy, and is consistent with a putative 2-phonon excitation observed in Raman spectroscopy. These results establish a unified picture of collective charge excitations across the phase diagram of electron-doped cuprates, showing that such modes persist across the Mott transition via strong coupling to lattice degrees of freedom and revealing a missing link in the charge dynamics of carrier doped Mott insulators.

cond-mat.str-el↗
arXiv2026

Robust fluctuating intertwined charge stripes in the Emery model

The single-band Hubbard model is one of the most extensively studied models in condensed matter physics, giving rise to intertwined spin and charge stripes that coexist with, or lie in the vicinity of, superconductivity in the phase diagram. However, whether the low energy physics of the single-band Hubbard model is fully equivalent to the multi-band (multi-orbital) Emery model remains an unsettled question. While the intertwined stripes and nematicity have been studied in the single-band Hubbard model, a comprehensive picture in the Emery model is lacking. In this paper, we focus on the less investigated intertwined charge stripes using complementary density matrix renormalization group (DMRG) and determinant quantum Monte Carlo (DQMC) techniques. Our ground state DMRG confirms the presence of the oxygen-centered charge stripes at a reduced amplitude in the Emery model parameter regime widely used in the study of intertwined stripes. Close analysis of the oxygen orbital structure of the static charge correlation function from DQMC reveals the charge stripe pattern in real-space, showcasing stronger charge density modulation on $p$-orbitals pointing along ``the rivers of charge'', consistent with DMRG. For the parameter set with the largest fermion signs, we managed to reach a temperature where the system first demonstrated tendencies to form purely unidirectional spin and charge stripes, and the $B_{1g}$ component becomes dominant in the bond-charge nematic susceptibility. This observation correlates with the doping dependence of the kinetic energy anisotropy, suggesting a close relation between the nematicity and charge stripes in the Emery model.

cond-mat.str-el↗
arXiv2026

Biaxial Strain Control of Helimagnetism via Chemical Expansion in Thin Film SrFeO3

We demonstrate control of helimagnetic order in biaxially strained SrFeO3 thin films using neutron diffraction and resonant soft x-ray scattering. SrFeO3, a negative charge-transfer oxide, exhibits a complex magnetic phase diagram that includes multi-q spin structures. Tensile epitaxial strain produces a pronounced shortening of the helimagnetic ordering length and a tilting of the magnetic ordering vector. We interpret this behavior in terms of chemical expansion: lattice dilation under tensile strain lowers the energetic cost of oxygen vacancies, leading to an expanded unit cell that modifies Fe-O hybridization and enhances superexchange relative to double exchange. These results reveal how epitaxial strain can indirectly tune helimagnetism through defect-driven chemical expansion, highlighting the strong coupling between lattice, chemistry, and magnetic order in transition-metal oxides. Our findings establish chemical expansion as an effective mechanism for engineering complex magnetic textures in oxide thin films, with implications for spintronic, magnonic, and quantum information applications.

cond-mat.mtrl-sci↗
arXiv2026

Microscopic theory for electron-phonon coupling in twisted bilayer graphene

The origin of superconductivity in twisted bilayer graphene -- whether phonon-driven or electron-driven -- remains unresolved, in part due to the absence of a quantitative and efficient model for electron-phonon coupling (EPC). In this work, we develop a first-principles-based microscopic theory to calculate EPC in twisted bilayer graphene for arbitrary twist angles without requiring a periodic moiré supercell. Our approach combines a momentum-space continuum model for both electronic and phononic structures with a generalized Eliashberg-McMillan theory beyond the adiabatic approximation. Using this framework, we find that the EPC is strongly enhanced near the magic angle. The superconducting transition temperature induced by low-energy phonons peaks at $1.1^\circ$ around 1 K, and remains finite for a range of angles both below and above the magic angles. We predict that superconductivity persists up to $\sim 1.4^\circ$, where superconductivity has been recently observed despite the dispersive electronic bands. Beyond a large density of states, we identify a key condition for strong EPC: resonance between the electronic bandwidth and the dominant phonon frequencies. We also show that the EPC strength of a specific phonon corresponds to the modification of the moiré potential. In particular, we identify several $Γ$-phonon branches that contribute most significantly to the EPC, which are experimentally detectable via Raman spectroscopy.

cond-mat.mes-hall↗
arXiv2026

Enhanced superconducting correlations in the Emery model and its connections to strange metallic transport and normal state coherence

Numerical evidence for superconductivity in the single-band Hubbard model is elusive or ambiguous despite extensive study, raising the question of whether the single-band Hubbard model is a faithful low energy effective model for cuprates, and whether explicitly including the oxygen ions will recover the properties necessary for a superconducting transition. Here we show, by using numerically exact determinant quantum Monte Carlo (DQMC) simulations of the doped Emery model, that while the single-band model exhibits strikingly T-linear resistivity, the three-band model crosses the resistivity of the single-band model from above, indicating a crossover to a more metallic transport regime. The enhanced conductivity is mainly contributed by a steep increase in the diffusivity of the three-band model at the crossover, suggesting that three-band transport is more coherent than single-band transport at lower temperatures. Below the same crossover temperature in the three-band model, the pair-field susceptibility increases more steeply than at higher temperatures or when compared to the single-band model. This suggests a possible connection between superconductivity and coherent transport, and further implies that coherent transport might be necessary for a model to capture the high-temperature superconductivity observed in hole-doped cuprates.

cond-mat.str-el↗
arXiv2025

Competition between charge-density-wave and superconducting orders on eight-leg square Hubbard cylinders

The issue of whether $d$-wave superconductivity (SC) occurs in the square-lattice Hubbard model with $U$ of order of the bandwidth has been one of the most debated issues to emerge from the study of high temperature SC. Here, we report variational results on eight-leg cylinders with next-nearest-neighbor hopping in the range $-0.5 t \leq t'\leq 0.25 t$ with $U = 8t$ and $12t$ and doped hole concentrations $δ=1/12$ and $1/8$. For $t'\leq 0$, the ground-state appears to be a charge-density wave (CDW) of one sort or another with SC correlations that are extremely short-ranged. In contrast, in some cases, the local magnetic order has a correlation length greater than half the cylinder width - suggestive that magnetic order might also arise in the 2D limit. For $t'>0$, our results depend more strongly on boundary conditions (periodic vs antiperiodic), making it still harder to correctly guess whether SC or CDW correlations dominate in the 2D limit. These results were obtained employing matrix-product states with bond dimensions large enough that energy differences as small as $10^{-3}t$ per site can be resolved.

cond-mat.str-el↗
arXiv2025

Effects of the Next-Nearest-Neighbor Hopping on the Low-Dimensional Hubbard Model: Ferromagnetism, Antiferromagnetism, and Superconductivity

The Hubbard model has attracted considerable interest due to its prototypical role in describing strongly interacting electronic systems, such as high-critical-temperature superconductors as well as many novel quantum materials. By introducing next-nearest-neighbor (NNN) hoppings to the Hubbard model, the phase diagram becomes richer, and fascinating phenomena arise in both, one-dimensional chains and square lattices, such as: antiferromagnetism (AFM), ferromagnetism (FM), superconductivity (SC), as well as charge orders, among others. Moreover, NNN hoppings play a fundamental role in understanding effects of doping on magnetism and pairing orders in strongly interacting regimes. In this article, we review the recent progress in understanding the different competing phases of this model in one and two dimensions from a computational perspective. We comment on the pressing technical challenges, illustrate the controversial results concerning the emergence of the SC phase, and conclude with our perspectives on future explorations.

cond-mat.str-el↗
arXiv2025

Finite-temperature signatures of underlying superconductivity in the electron-doped Hubbard model

We perform numerically exact determinant quantum Monte Carlo simulations of the Hubbard model and analyze pairing tendencies by evaluating correlation functions at the imaginary-time midpoint ($τ=β/2$), which suppresses high-frequency weight and emphasizes low-energy physics. Using this diagnostic, we identify clear finite-temperature signatures of underlying $d$-wave superconductivity for electron doping, while finding no clear indication upon cooling for hole doping. Our analysis enables direct comparison with ground-state DMRG, revealing consistent real-space pairing patterns. These results provide a practical route to bridge the gap between finite-temperature and ground-state numerically exact simulations of the Hubbard model despite the fermion sign problem.

cond-mat.supr-con↗