Thomas Devereaux
VerifiedStanford University · Materials Science and Engineering
Active 1989–2026
About
Thomas Devereaux is a Professor of Photon Science and of Materials Science and Engineering at Stanford University. He is also a Senior Fellow at the Precourt Institute for Energy. His professional titles indicate a focus on photon science and materials science and engineering, suggesting his research and academic contributions lie at the intersection of these fields. The page lists him among other faculty members in the Materials Science and Engineering department, highlighting his role within the Stanford academic community and his involvement with the Precourt Institute for Energy, which is dedicated to advancing energy science and technology.
Research topics
- Crystallography
- Chemistry
- Computer Science
- Materials science
- Physical chemistry
- Geometry
- Physics
- Mathematics
- Computational chemistry
- Inorganic chemistry
Selected publications
Vacuum-free spectroscopy reveals metastable oxygen redox intermediates in oxygen-redox cathodes
Research Square · 2026-02-10
preprintOpen accessMicroscopic theory for electron-phonon coupling in twisted bilayer graphene
Physical review. B./Physical review. B · 2026-01-07
preprintOpen accessSenior authorThe 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.
Biaxial Strain Control of Helimagnetism via Chemical Expansion in Thin Film SrFeO3
ArXiv.org · 2026-02-10
articleOpen accessWe 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.
Biaxial Strain Control of Helimagnetism via Chemical Expansion in Thin Film SrFeO3
Open MIND · 2026-02-10
preprintWe 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.
Robust Paramagnon and Acoustic Plasmon in a Photo-excited Electron-doped Cuprate Superconductor
ArXiv.org · 2025-11-27
preprintOpen accessCharacterizing the spin and charge degrees of freedom in high-temperature superconducting cuprates under non-equilibrium conditions provides new insights into their electronic correlations. However, their collective dynamics have been largely unexplored due to experimental challenges. Here, we use time-resolved resonant inelastic X-ray scattering (trRIXS) at the Cu $L_3$-edge to simultaneously track the collective spin (paramagnon) and charge (acoustic plasmon) dynamics 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, though the bandwidth remained unchanged, suggesting no significant alteration to spin exchange interactions. Simultaneously, in the charge sector, the acoustic plasmon's energy and spectral weight decreased, suggesting a light-induced redistribution of charge carriers. The variations of both the paramagnon and the plasmon were locked in time, demonstrating a robust intertwining between the spin and charge degrees of freedom on a femtosecond timescale, even in this non-equilibrium state.
Physical Review Letters · 2025-04-08 · 5 citations
articleOpen accessRecent photoemission experiments on the quasi-one-dimensional Ba-based cuprates suggest that doped holes experience an attractive potential not captured using the simple Hubbard model. This observation has garnered significant attention due to its potential relevance to Cooper pair formation in high-T_{c} cuprate superconductors. To scrutinize this assertion, we examined signatures of such an attractive potential in doped 1D cuprates Ba_{2}CuO_{3+δ} by measuring the dispersion of the 2-spinon excitations using Cu L_{3}-edge resonant inelastic x-ray scattering (RIXS). Upon doping, the 2-spinon excitations appear to weaken, with a shift of the minimal position corresponding to the nesting vector of the Fermi points, q_{F}. Notably, we find that the energy scale of the 2-spinons near the Brillouin zone boundary is substantially softened compared to that predicted by the Hubbard model in one dimension. Such a discrepancy implies missing ingredients, which lends support for the presence of an additional attractive potential between holes.
Detection of chiral spin fluctuations driven by frustration in Mott insulators
Physical review. B./Physical review. B · 2025-05-12 · 4 citations
articleOpen accessSenior authorTopologically ordered states, such as chiral spin liquids, have been proposed as candidates that host fraction alized excitations. However, detecting chiral character or proximity to these nontrivial states remains a challenge. Resonant Raman scattering can be a powerful tool for detecting chiral fluctuations, as the ${A}_{2g}$ channel probes excitations with broken time-reversal symmetry and local chiral order. Here, we use exact diagonalization to characterize the resonant ${A}_{2g}$ channel, alongside two-magnon scattering in ${B}_{1g}$ and ${E}_{g}$ channels, for the Hubbard model on lattices with increasing levels of geometric spin frustration, where tuning the incident energy near the Mott gap reveals strong chiral spin excitation intensity. Increased spin frustration in the Mott insulator results in an overall softening of the Raman ${A}_{2g}$ response, indicating a tendency toward low energy chiral-chiral fluctuations in Mott insulators with magnetic frustration and proximity to chiral spin liquid states that can potentially be tuned by external perturbations.
Physical Review X · 2025-10-29
articleOpen accessTwisted bilayer <a:math xmlns:a="http://www.w3.org/1998/Math/MathML" display="inline"> <a:mrow> <a:msub> <a:mrow> <a:mi>MoTe</a:mi> </a:mrow> <a:mn>2</a:mn> </a:msub> </a:mrow> </a:math> ( <c:math xmlns:c="http://www.w3.org/1998/Math/MathML" display="inline"> <c:mrow> <c:msub> <c:mrow> <c:mi>tMoTe</c:mi> </c:mrow> <c:mn>2</c:mn> </c:msub> </c:mrow> </c:math> ) is an emergent platform for exploring exotic quantum phases driven by the interplay between nontrivial band topology and strong electron correlations. Direct experimental access to its momentum-resolved electronic structure is essential for uncovering the microscopic origins of the correlated topological phases therein. Here, we report angle-resolved photoemission spectroscopy measurements of <e:math xmlns:e="http://www.w3.org/1998/Math/MathML" display="inline"> <e:mrow> <e:msub> <e:mrow> <e:mi>tMoTe</e:mi> </e:mrow> <e:mn>2</e:mn> </e:msub> </e:mrow> </e:math> , revealing pronounced twist-angle-dependent band reconstruction shaped by orbital character, interlayer coupling, and moiré potential modulation. Density functional theory captures the qualitative evolution, yet underestimates key energy scales across twist angles, highlighting the importance of electronic correlations. Notably, the hole effective mass at the <g:math xmlns:g="http://www.w3.org/1998/Math/MathML" display="inline"> <g:mrow> <g:mi>K</g:mi> </g:mrow> </g:math> point exhibits a nonmonotonic dependence on twist angle, peaking near 2°, consistent with band flattening at the magic angle predicted by continuum models. Via electrostatic gating and surface dosing, we further visualize the evolution of electronic structure versus doping, enabling direct observation of the conduction band minimum and confirm <i:math xmlns:i="http://www.w3.org/1998/Math/MathML" display="inline"> <i:mrow> <i:msub> <i:mrow> <i:mi>tMoTe</i:mi> </i:mrow> <i:mn>2</i:mn> </i:msub> </i:mrow> </i:math> as a direct band gap semiconductor. These results establish a spectroscopic foundation for modeling and engineering emergent quantum phases in this moiré platform.
Negative Charge Transfer: Ground State Precursor towards High Energy Batteries
ArXiv.org · 2025-09-24
preprintOpen accessSenior authorModern energy applications, especially electric vehicles, demand high energy batteries. However, despite decades of intensive efforts, the highest energy density and commercially viable batteries are still based on LiCoO2, the very first generation of cathode materials. The technical bottleneck is the stability of oxide-based cathodes at high operating voltages. The fundamental puzzle is that we actually never understood the redox mechanism of LiCoO2. Conventional wisdom generally defines redox to be centered on cations at low voltages, and on anions, i.e. oxygen, at high voltages by forming oxidized chemical states like O2 or peroxo-species. Here, through in-situ and ex-situ spectroscopy coupled with theoretical calculations, we show that high-energy layered cathodes, represented by LiCoO2 and LiNiO2, operate through enhancement of negative charge transfer (NCT) ground states upon charging throughout the whole voltage range - i.e., NCT evolution itself is the intrinsic redox mechanism regardless of voltage ranges. NCT inherently engages high covalency and oxygen holes, leading to optimized performance without conventional redox centers in LiCoO2. The level of NCT, i.e., number of ligand holes, naturally explains many seemingly controversial results. The redefinition of redox mechanism reveals the pathway toward viable high energy battery electrodes.
Direct Observation of the Lindhard Continuum using Resonant Inelastic X-ray Scattering
ArXiv.org · 2025-09-12
preprintOpen accessUnderstanding the excitations of quantum materials is essential for unraveling how their microscopic constituents interact. Among these, particle-hole excitations form a particularly important class, as they govern fundamental processes such as screening, dissipation, and transport. In metals, the continuum of electron-hole excitations is described by the Lindhard function. Although central to the theory of Fermi liquids, the corresponding Lindhard continuum has remained experimentally elusive. Here, we report its direct observation in the weakly correlated metal MgB$_{2}$ using ultra-soft resonant inelastic X-ray scattering (RIXS). We resolve a linearly dispersing excitation with velocity comparable to the Fermi velocity and find quantitative agreement with simulations of the non-interacting charge susceptibility. A detailed analysis and decomposition of the simulations reveal the intra-band origin of this low-energy excitation, confirming it as the Lindhard continuum. Our results establish ultra-soft RIXS as a momentum-resolved probe of the fermiology in metals and call for deeper investigations of continuum features in RIXS and related spectroscopy of other materials beyond MgB$_{2}$.
Frequent coauthors
- 654 shared
Brian Moritz
Stanford University
- 639 shared
Zhi‐Xun Shen
Stanford University
- 432 shared
Dong-Hui Lu
Stanford Synchrotron Radiation Lightsource
- 410 shared
Makoto Hashimoto
SLAC National Accelerator Laboratory
- 305 shared
Su-Di Chen
Kavli Energy NanoScience Institute
- 291 shared
Yu He
Yale University
- 288 shared
Jun-Feng He
- 286 shared
Hiroshi Eisaki
National Institute of Advanced Industrial Science and Technology
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