arXiv subjects

Harley D. Scammell

Publications and source records attributed to Harley D. Scammell.

At least 19 recordsLinked to original sources

arXiv2026

Distinct lattice and charge excitations in AV3Sb5 kagome superconductors

The kagome superconductor family AV3Sb5 (A=Cs, Rb, K) provides a rich platform for exploring diverse electronic symmetry breaking phenomena, including superconductivity and various forms of density wave orders. Although these compounds share the identical lattice structure in the normal state, they exhibit distinct forms of symmetry breaking upon entering the charge density wave (CDW) phase, and the microscopic origin of which remain elusive. Here, we investigate the lattice and charge degrees of freedom in AV3Sb5 using angle-resolved polarized Raman spectroscopy. Our comprehensive polarization-resolved measurements reveal that the lifting of the twofold-degeneracy of the E2g phonon mode in the CDW phase-previously reported only in CsV3Sb5 with a 3 GHz splitting-also appears ubiquitously in the other two compounds. In contrast, the collective CDW excitations exhibit markedly different polarization dependences depending on the alkali-metal species. These distinct behaviors in the lattice and charge channels provide crucial insight into the enigmatic material-dependent symmetry breaking phenomena that appear in the CDW phase. Furthermore, our experiments, together with first-principles calculations and an effective Hamiltonian model, shed light on the nature of the charge order structure in AV3Sb5 kagome superconductors.

cond-mat.str-el↗

arXiv2025

Correlations between superconducting and resistive anisotropies

There are multiple possible origins of transport anisotropies in metals and superconductors. For instance, rotational symmetry can be spontaneously broken in the normal state as a result of electronic nematic order inducing anisotropies in an otherwise $s$-wave superconducting phase. Another possibility is that the dominant source of rotational symmetry breaking is the superconductor itself and its vestiges that may survive in the normal state. We here theoretically analyze the correlations of transport anisotropies in the normal and the corresponding superconducting phase for different scenarios of broken symmetry, either coming solely from the normal state, solely from the superconductor and its vestiges in the metallic regimes, or from both simultaneously. We further include both zero-momentum and finite-momentum pairing; we develop a theory of vestigial order for the latter, characterized by broken rotational and translational symmetry. Our findings reveal that the relative transport anisotropies in the normal and superconducting phases sensitively depend on the scenario, including the form of vestigial order and, in some cases, the parity of the superconducting order parameter. As such, measuring the directional dependence of the critical current and resistivity can provide strong constraints on the origin of rotational symmetry breaking. We demonstrate our findings in minimal models relevant to twisted multilayer graphene, rhombohedral graphene, and twisted transition metal dichalcogenides.

cond-mat.supr-con↗

arXiv2025

Theory of unconventional magnetism in a Cu-based kagome metal

Kagome metals have established a new arena for correlated electron physics. To date, the predominant experimental evidence centers around unconventional charge order, nematicity, and superconductivity, while magnetic fluctuations due to electronic interactions, i.e., beyond local atomic magnetism, have largely been elusive. We find the challenge of locating the appropriate parameter regime for such exotic order to center around two aspects. First, the correlations implied by low-energy orbitals have to be sufficiently large to yield a dominance of magnetic fluctuations and weak to retain an itinerant parent state. Second, the kinematic kagome profile at the Fermi level demands an efficient mitigation of sublattice interference causing the suppression of magnetic fluctuations descending from electronic on-site repulsion. We elucidate our methodology by analyzing the potential copper-based kagome compound CsCu$_3$Cl$_5$: From ab initio design and many-body analysis, we develop a model framework of realistic Cu-based kagome materials the simulations of which reveal unconventional magnetic order in a kagome metal.

cond-mat.str-el↗

arXiv2025

Group theory method for extracting order parameters from scanning tunneling microscopy data

Scanning tunneling microscopy (STM) is a powerful local probe of correlated electronic states. Here we present a group theoretical framework for the analysis of STM data, filtering STM images into components which provide a real space mapping of the local symmetry properties of the underlying density of states. Using this formalism, we show that certain kinds of symmetry breaking are impossible to resolve in the first Brillouin zone, due to symmetry restrictions we term ``Bragg peak extinctions'' in analogy with related ideas in x-ray crystallography. We show extinct patterns of symmetry breaking can be resolved using sub-unit cell structure, and develop methodological details for the accurate extraction of this symmetry information. We illustrate our results on synthetic STM data for $2\times 2$ charge density waves on the kagome lattice, and on topographic data for kagome metal ScV$_6$Sn$_6$. Our results provide a powerful method for extracting symmetry insights from STM data, and provide constraints on when and how certain ground states are experimentally observable.

cond-mat.str-el↗

arXiv2025

Spin-orbit crossover and the origin of magnetic torque in kagome metals

Recent experiments on the kagome metal CsV$_3$Sb$_5$ reveal a curious phase transition-like feature: a nematic magnetic torque response that abruptly sets in at $T_τ\approx 130$K, above the known charge density wave transition at $T_\text{CDW} \approx 100$K. Counterintuitively, elastoresistance measurements--a standard probe of nematicity--show no corresponding signal, ruling out a nematic phase transition and placing strong constraints on possible explanations. Beyond nematicity, the torque is paramagnetic for in-plane magnetic field, while above a critical out-of-plane field, an in-plane magnetisation appears, accompanied by hysteresis. We show that this combination of features cannot be accounted for by charge density waves or intraband magnetic order. Instead, we propose that interband ordering--via a symmetry-allowed interband spin-orbit coupling and a time-reversal and spatial symmetry-breaking interband order parameter--together with a background strain field, consistent with typical experimental conditions, provides a natural explanation; in our picture, the behaviour at $T_τ$ is understood as a crossover in the symmetry-allowed interband spin-orbit coupling strength. Our theory accounts for the nematic magnetic torque, hysteresis, and the transition-like onset at $T_τ$, while also making testable predictions, including strain-induced magnetisation. In doing so, it challenges the prevailing view of the normal state.

cond-mat.str-el↗

arXiv2025

Exciton condensation from level repulsion: application to bilayer graphene

Exciton condensation in semiconductors and semimetals has long been predicted but remains elusive. In a semiconductor, condensation occurs when the exciton binding energy matches the band gap. This binding energy results from a balance between Coulomb attraction, which enhances it, and kinetic energy, which suppresses it. However, reducing kinetic energy typically increases screening, weakening Coulomb attraction. Empirically, in most candidate materials, the binding energy remains below the band gap, with few external parameters capable of altering this balance. Here, we propose an in-plane electric field as a control parameter. This field induces hybridisation between even- and odd-parity excitons, and the resulting level repulsion effectively enhances binding energy. We argue that this mechanism is generic to excitons in semiconductors and illustrate it with a model of biased bilayer graphene. Bilayer graphene is chosen since it has a tunable band gap, making it an excitonic condensate candidate and moreover, the Zener tunnelling rate contains, in addition to the usual exponential decay, a non-standard oscillating component -- thanks to details of the electron dispersion. Analogous to quantum oscillations, we propose that Fourier spectrum of the current-voltage data allows for a novel test of exciton condensation. Finally, we show that the is a large excitonic gap to critical temperature ratio -- a clear prediction for STM studies.

cond-mat.str-el↗

arXiv2025

Theory of excitonic order in kagome metals ScV$_6$Sn$_6$ and LuNb$_6$Sn$_6$

We argue that kagome metals can feature an excitonic condensate of unconventional nature. Studying the recently discovered variants ScV$_6$Sn$_6$ and LuNb$_6$Sn$_6$ we identify electron and hole pockets due to a pair of van Hove singularities (vHS) close to the Fermi level, with an approximate spectral particle-hole symmetry. A significant fraction of the Fermi level density of states away from the vHS is removed by the onset of high temperature charge density wave order, and makes the bands more two-dimensional, setting the stage for the formation of excitons. We develop a two-orbital minimal tight-binding model of these materials which captures these features along with the sublattice support of the wavefunctions, and find $s$- or $d$-wave excitons depending on interaction parameters -- the latter of which exhibits either charge nematicity or time-reversal symmetry breaking (TRSB) depending on strain, offering an explanation of recent STM and transport experiments. The presence of particle- and hole-type vHS, and the associated excitonic resonance, may be a common thread to understanding nematicity and TRSB in kagome metals.

cond-mat.str-el↗

arXiv2025

Moiré $M$-valley bilayers: quasi-one-dimensional physics, unconventional spin textures and twisted van Hove singularities

Motivated by the discovery of quasi-two-dimensional kagome metals AV$_3$Sb$_5$, we consider the theory of twisted bilayers in which the Fermi surface is near the $M$-point. Surprisingly, unlike twisted bilayers of graphene or transition metal dichalcogenides, the moiré potential is quasi-one-dimensional: at each $M$-valley, the potential flattens the dispersion strongly along one direction, and weakly along the perpendicular direction. The combination of spin-orbit coupling and twist-induced broken inversion symmetry results in a similarly anisotropic `$\textit{moiré-Rashba}$' potential, which spin-splits the dispersion into coexisting two-dimensional and quasi-one-dimensional bands. We discuss novel aspects of the interplay between mixed dimensionality and spin textures in this platform. First, an applied electric field produces spin polarisation which can be tuned by doping, suggesting potential spintronics applications. Secondly, an in-plane magnetic field momentum- and spin-polarises the Fermi surfaces, producing unconventional spin density waves. Thirdly, in the small-twist-angle limit, the large density of states due to a twisted van Hove singularity near $M$ results in a dense energy spectrum. Our results demonstrate a new variation of moiré bandstructure engineering, instigating the study of spin-textured one-dimensional physics in moiré materials.

cond-mat.str-el↗

arXiv2025

Vestigial Order from an Excitonic Mother State in Kagome Superconductors $A$V$_3$Sb$_5$

Alongside high-temperature charge order and superconductivity, kagome metals exhibit signatures of time-reversal symmetry breaking and nematicity which appear to depend strongly on external perturbations such as strain and magnetic fields, posing a fundamental challenge for conceptual reconciliation. We develop a theory of vestigial order descending from an excitonic mother state in $A$V$_3$Sb$_5$ ($A$=K,Rb,Cs), which develops around $T_* \approx 40$ K. The application of external fields stabilises a subset of the phase-melted order parameter manifold, referred to as a vestigial state, producing a symmetry-breaking response which depends on the applied probe. Our theory reproduces the observations of piezomagnetism, electric magnetic chiral anisotropy, absence of Kerr rotation, unusual elastoresistance response, and superconducting diode effect. Our proposed excitonic mother state accounts for probe-dependent symmetry breaking patterns without fine-tuning, and predicts additional signatures accessible through optical spectroscopy.

cond-mat.str-el↗

arXiv2025

Theory of Band Gap Reduction Due to Conduction Electrons in 2D TMDs: Imaginary Frequency Formalism

Two Dimensional (2D) Transition Metal Dichalcogenides (TMDs) possess a large direct band gap which has been experimentally observed to shrink with increasing charge carrier density (doping). The effect has been the subject of theoretical study in recent years using various approaches and approximations. In this work we develop the theory of bandgap renormalization based on Feynman diagrammatic technique in the imaginary frequency formalism. We consider dynamical screening from conduction band electrons using the random phase approximation (RPA), as well as screening from a metallic gate. While our theory is general for any 2D semiconductor, to be specific we consider MoS$_2$ and WSe$_2$ and compare with available experimental data. In both cases we calculate large band gap renormalization that reaches several hundred meV at relatively low carrier density. This is in good agreement with experimental data.

cond-mat.mes-hall↗

arXiv2024

Excitons in Atomically Thin TMD in Electric and Magnetic Fields

The magnetic field dependence of photoabsorption provides direct insights into the band structure of semiconductors. It is perhaps surprising that there is a large discrepancy between electron, hole, and reduced mass reported in the recent literature. Motivated by this puzzle we reconsider excitonic magneto-absorption and find that the commonly employed perturbative approach, namely for computing the diamagnetic shift, is inadequate to account for the parameter ranges considered in existing data. In particular, we develop the theory for strong magnetic field and, upon analysis of the data, arrive at the set of exciton parameters different to what has been estimated perturbatively in the literature. Only s-wave excitons are visible in photoluminescence as the spectral weight of p-wave states is too small, this limits the amount of information that can be extracted about the underlying band structure. To overcome this, we propose to study p-wave states by mixing them with s-wave states by external in-plane electric field and show that a moderate DC electric field would provide sufficient mixing to brighten p-wave states. We calculate energies of the p-wave states including the effects of valley-orbital splitting and the orbital Zeeman shift, and show that this provides direct information on the electron-hole mass asymmetry.

cond-mat.mes-hall↗

arXiv2024

Anomalous magnetic flux via junction twist-angle in a triplet-superconducting transmon qubit

Superconducting transmon qubits with strong anharmonicity and insensitivity to offset charge are highly desirable for low-error implementation. In this work we propose a c-axis junction, comprising triplet superconductors, and set at a relative twist angle. Invoking spin-orbit coupling and spin polarization, which are known to occur in the material platform of choice, we examine the resulting transmon Hamiltonian. This junction allows for direct control of the single and double Cooper pair tunneling strength, and most remarkably, an anomalous magnetic flux -- i.e. a phase offset equivalent to magnetic flux, yet in zero magnetic field. Having control over these three parameters -- single and double pair tunneling and anomalous flux -- allows for optimal design of the transmon qubit. Interestingly, in this architecture, the anomalous flux is determined by the twist angle of the junction, thereby offering a novel zero-field functionality. Our key results rely on symmetry arguments, for concreteness we demonstrate the implementation of our concept using a model of moiré graphene-based c-axis junctions.

cond-mat.mes-hall↗

arXiv2023

Displacement-field-tunable superconductivity in an inversion-symmetric twisted van der Waals heterostructure

We investigate the superconducting properties of inversion-symmetric twisted trilayer graphene by considering different parent states, including spin-singlet, triplet, and SO(4) degenerate states, with or without nodal points. By placing transition metal dichalcogenide layers above and below twisted trilayer graphene, spin-orbit coupling is induced in TTLG and, due to inversion symmetry, the spin-orbit coupling does not spin-split the bands. The application of a displacement field ($D_0$) breaks the inversion symmetry and creates spin-splitting. We analyze the evolution of the superconducting order parameters in response to the combined spin-orbit coupling and $D_0$-induced spin-splitting. Utilizing symmetry analysis combined with both a direct numerical evaluation and a complementary analytical study of the gap equation, we provide a comprehensive understanding of the influence of spin-orbit coupling and $D_0$ on superconductivity. These results contribute to a better understanding of the superconducting order in twisted trilayer graphene.

cond-mat.mes-hall↗

arXiv2023

Quadratic Dirac fermions and the competition of ordered states in twisted bilayer graphene

Magic-angle twisted bilayer graphene (TBG) exhibits a captivating phase diagram as a function of doping, featuring superconductivity and a variety of insulating and magnetic states. The bands host Dirac fermions with a reduced Fermi velocity; experiments have shown that the Dirac dispersion reappears near integer fillings of the moiré unit cell -- referred to as the $\textit{Dirac revival}$ phenomenon. The reduced velocity of these Dirac states leads us to propose a scenario in which the Dirac fermions possess an approximately quadratic dispersion. The quadratic momentum dependence and particle-hole degeneracy at the Dirac point results in a logarithmic enhancement of interaction effects, which does not appear for a linear dispersion. The resulting non-trivial renormalisation group (RG) flow naturally produces the qualitative phase diagram as a function of doping -- with nematic and insulating states near integer fillings, which give way to superconducting states past a critical relative doping. The RG method further produces different results to strong-coupling Hartree-Fock treatments: producing T-IVC insulating states for repulsive interactions, explaining the results of very recent STM experiments, alongside nodal $A_2$ superconductivity near half-filling, whose properties explain puzzles in tunnelling studies of the superconducting state. The model explains a diverse range of additional experimental observations, unifying many aspects of the phase diagram of TBG.

cond-mat.str-el↗

arXiv2023

Chiral excitonic order from twofold van Hove singularities in kagome metals

Recent experiments on kagome metals AV$_3$Sb$_5$ (A=K,Rb,Cs) [M. Kang et al., Nat. Phys. 18, 301-308 (2022)] identify twofold van Hove singularities (TvHS) with opposite concavity near the Fermi energy, generating two approximately hexagonal Fermi surfaces -- one electron-like and the other hole-like. Here we propose that a TvHS generates a novel time-reversal symmetry breaking excitonic order -- arising due to bound pairs of electrons and holes located at opposite concavity van Hove singularities. We introduce a minimal model for the TvHS and investigate interaction induced many-body instabilities via the perturbative renormalisation group technique and a free energy analysis. Specialising to parameters appropriate for the kagome metals AV$_3$Sb$_5$, we construct a phase diagram comprising chiral excitons, charge density wave and a region of coexistence. We propose this as an explanation of a diverse range of experimental observations in AV$_3$Sb$_5$. Notably, the chiral excitonic state gives rise to a quantum anomalous Hall conductance, providing an appealing interpretation of the observed anomalous Hall effect in kagome metals. Possible alternative realisations of the TvHS mechanism in bilayer materials are also discussed. We suggest that TvHS open up interesting possibilities for correlated phases, enriching the set of competing ground states to include excitonic order.

cond-mat.supr-con↗

arXiv2023

Exciton condensation in biased bilayer graphene

We consider suspened bilayer graphene under applied perpendicular electric bias field that is known to generate a single particle gap $2Δ$ and a related electric polarization ${\cal P}$. We argue that the bias also drives a quantum phase transition from band insulator to superfluid exciton condensate. The transition occurs when the exciton binding energy exceeds the band gap $2Δ$. We predict the critical bias (converted to band gap), $Δ_c\approx 60$ meV, below which the excitons condense. The critical temperature, $T_c(Δ)$, is maximum at $Δ\approx 25$ meV, $T_c^\text{max}\approx 115$ K, decreasing significantly at smaller $Δ$ due to thermal screening. Entering the condensate phase, the superfluid transition is accompanied by a cusp in the electric polarization ${\cal P}(Δ)$ at $Δ\toΔ_c$, which provides a striking testable signature. Additionally, we find that the condensate prefers to form a pair density wave.

cond-mat.mes-hall↗

arXiv2022

Tunable superconductivity and Möbius Fermi surfaces in an inversion-symmetric twisted van der Waals heterostructure

We study theoretically a moiré superlattice geometry consisting of mirror-symmetric twisted trilayer graphene surrounded by identical transition metal dichalcogenide layers. We show that this setup allows to switch on/off and control the spin-orbit splitting of the Fermi surfaces via application of a perpendicular displacement field $D_0$, and explore two manifestations of this control: first, we compute the evolution of superconducting pairing with $D_0$; this features a complex admixture of singlet and triplet pairing and, depending on the pairing state in the parent trilayer system, phase transitions between competing superconducting phases. Second, we reveal that, with application of $D_0$, the spin-orbit-induced spin textures exhibit vortices which lead to "Möbius fermi surfaces'' in the interior of the Brillouin zone: diabatic electron trajectories, which are predicted to dominate quantum oscillation experiments, require encircling the $Γ$ point twice, making their Möbius nature directly observable. We further show that the superconducting order parameter inherits the unconventional, Möbius spin textures. Our findings suggest that this system provides a promising experimental avenue for studying systematically the impact of spin-orbit coupling on the multitude of topological and correlated phases in near-magic-angle twisted trilayer graphene.

cond-mat.mes-hall↗

arXiv2022

Theory of zero-field superconducting diode effect in twisted trilayer graphene

In a recent experiment [Lin et al., arXiv:2112.07841], the superconducting phase hosted by a heterostructure of mirror-symmetric twisted trilayer graphene and WSe$_2$ was shown to exhibit significantly different critical currents in opposite directions in the absence of external magnetic fields. We here develop a microscopic theory and analyze necessary conditions for this zero-field superconducting diode effect. Taking into account the spin-orbit coupling induced in trilayer graphene via the proximity effect, we classify the pairing instabilities and normal-state orders and derive which combinations are consistent with the observed diode effect, in particular, its field trainability. We perform explicit calculations of the diode effect in several different models, including the full continuum model for the system, and illuminate the relation between the diode effect and finite-momentum pairing. Our theory also provides a natural explanation of the observed sign change of the current asymmetry with doping, which can be related to an approximate chiral symmetry of the system, and of the enhanced transverse resistance above the superconducting transition. Our findings not only elucidate the rich physics of trilayer graphene on WSe$_2$, but also establish a means to distinguish between various candidate interaction-induced orders in spin-orbit-coupled graphene moiré systems, and could therefore serve as a guide for future experiments as well.

cond-mat.mes-hall↗