arXiv subjects

Joseph J. Betouras

Publications and source records attributed to Joseph J. Betouras.

At least 19 recordsLinked to original sources

arXiv2026

Pressure and Proximity Tuned Twisted Bilayer Graphene

The coupling between layered atomically-thin materials mediated by van der Waals forces allows strong electronic correlations, unique topological signatures, and non-trivial spin textures, all of which play an important role in the creation of spin-, valley-, and orbitronic devices. Here, we demonstrate that twisted bilayer graphene encapsulated by transition metal dichalcogenides exhibits a generically non-radial spin texture yet hosts a purely collinear Edelstein effect despite this lack of radial symmetry. Moreover, we show how uniaxial pressure can be used to further tune the band structure and change the number of active Fermi surfaces without compromising the collinear response. Lastly, we illustrate how the quantum geometry changes in the encapsulated twisted graphene bilayer with larger pressures spreading the Berry curvature over large regions of the moiré Brillouin zone. These results illustrate how encapsulation and pressure can be used to drastically alter the topology and spin-charge interconversion processes of moiré heterostructures.

cond-mat.mes-hall↗

arXiv2026

Twisted Kagome Bilayers: High-Order Van Hove Singularities, Sublattice Interference, Magic Angles, and Possible Topology

We develop a low-energy continuum model to describe the moiré physics of heterostructures, which is a generalization of the celebrated Bistritzer-MacDonald (BM) method [R. Bistritzer and A. H. MacDonald, Proc. Natl. Acad. Sci. U.S.A. 108, 12233 (2011)]. We take as an example the moiré physics of electrons in twisted bilayer kagome metals near 1/3 filling where monolayer Dirac cones lie. We demonstrate the emergence of magic angles where significant local band flattening occurs as a high-order Van Hove singularity appears and find a momentum-dependent anti-unitary particle-hole symmetry potentially enabling stable topology. We, furthermore, show that while sublattice interference effects are present, their role is not as prominent as in monolayer kagome.

cond-mat.mes-hall↗

arXiv2026

Effects of high-order Van Hove singularities on exciton and trion energy dispersions

We investigate the effects of Van Hove singularities in the electronic band structure of two-dimensional semiconductors on the energy dispersion of excitons and positive trions. In particular, we study valence band energy dispersions which possess (i) a typical logarithmic Van Hove singularity, (ii) a line high-order Van Hove singularity (HOVHS) from a Mexican-hat dispersion or (iii) a point HOVHS such as a monkey saddle. We find that the density of states (DOS) of excitons and trions containing such singularities is dramatically enhanced and shows, in general, how the HOVHS in the valence band can strongly affect and be mirrored in the DOS of excitons and trions. This leads to new states that govern the optical properties of the system. In addition, we study a set of materials, InSe, GaSe and $α$-SnAs, from a class of materials in which the topmost valence band has an inverted Mexican-hat shape. The most favourable exciton occurs when the singularity is at the $Γ$-point, as in the example of monolayer $α$-SnAs, which hosts a HOVHS. Our work thus provides a pathway to engineer specific bound states in two-dimensional materials that host such singularities, thereby opening new avenues for potential applications.

cond-mat.mes-hall↗

arXiv2026

Magnetic field induced transition from nodeless to nodal superconductivity in $β$-PdBi$_{2}$

Recent tunneling measurements on $β$-PdBi$_2$ reported a magnetic field induced phase transition from a fully gapped to a nodal superconducting state. We develop a microscopic theory of superconductivity in this multi-band material, taking into account spin-orbit coupling and Zeeman splitting. We show that there are two attractive pairing channels in this system: an s-wave and a p-wave. At zero magnetic field, s-wave superconductivity wins. At a finite field, this channel becomes less favorable because Fermi surfaces split, and the gap symmetry changes to p-wave. We show that at a higher field, the excitation spectrum of a p-wave multi-band superconductor becomes gapless, with nodal points located in between the split Fermi surfaces. We argue that this behavior accounts for the experimentally observed field-induced nodal superconductivity in $β$-PdBi$_2$.

cond-mat.supr-con↗

arXiv2026

Triplet superconductivity supported by an X$_9$ high-order Van Hove singularity

We study a four-fold symmetric dispersion relation of a quantum material, which exhibits a single high-order Van Hove singularity of X$_9$ type at the Fermi energy. First, we analyze in detail its form, type and density of states when the energy dispersion is in its canonical form. Subsequently, we study the possibility of a superconducting state when Hubbard repulsive interactions are taken into account. By solving the gap equation, it is shown that triplet state superconductivity with power-law dependence of the critical temperature T$_c$ on the interaction strength can be formed when a single singularity is present in the Brillouin zone. We discuss the effects of fluctuations and provide an upper bound of a possible superconducting critical temperature for the ruthenate Sr$_3$Ru$_2$O$_7$ which has been shown to exhibit this type of singularity.

cond-mat.supr-con↗

arXiv2025

Anisotropic scattering rates in strain-tuned Sr$_2$RuO$_4$

Motivated by recent angle-resolved photoemission spectroscopy (ARPES) experiments, we analyze the temperature, frequency, and momentum dependence of the single-particle scattering rate in a model of the $γ$-band of Sr$_2$RuO$_4$ under strain, with particular emphasis on the behavior near the Lifshitz transition where the Fermi energy crosses a single Van Hove point. While the scattering rate is only moderately anisotropic at zero strain, we find that it becomes strongly anisotropic at the Lifshitz point. At the lowest energies, we recover the expected universal behavior: the scattering rate varies (ignoring logarithmic corrections) as $τ^{-1}\sim ω$ at the Van Hove point and as $τ^{-1}\sim ω^{3/2}$ away from it. At higher energies, however, corrections of order $ω^2$ become important in both regimes. We show that the experimentally observed behavior $τ^{-1} \sim ω^α$ with $α\approx 1.4(2)$ at the Van Hove point can be quantitatively explained by a superposition of linear and quadratic contributions to the scattering rate, which are comparable in magnitude at the intermediate energies probed by experiment, rather than in terms of a new universal power law. We further predict a distinctive anisotropy, strain dependence, and a non-monotonic frequency dependence of the scattering rate at a Lifshitz transition, all of which may be directly tested in experiments.

cond-mat.str-el↗

arXiv2025

Designing Topological High-Order Van Hove Singularities: Twisted Bilayer Kagomé

The interplay of high-order Van Hove singularities and topology plays a central role in determining the nature of the electronic correlations governing the phase of a system with unique signatures characterising their presence. Layered van der Waals heterostuctures are ideal systems for band engineering through the use of twisting and proximity effects. Here, we use symmetry to demonstrate how twisted Kagomé bilayers can host topological high-order Van Hove singularities. We study a commensurate system with a large twist angle and demonstrate how the initial choice of high-symmetry stacking order can greatly influence the electronic structure and topology of the system. We, furthermore, study the sublattice interference in the system. Our results illustrate the rich energy landscape of twisted Kagomé bilayers and unveil large Chern numbers (of order 10), establishing twisted bilayer Kagomé as a natural playground for probing the mixing of strong correlations and topology.

cond-mat.str-el↗

arXiv2025

Brightening dark excitons and trions in systems with a Mexican-hat energy dispersion: example of InSe

We investigate the properties of momentum-dark excitons and trions formed in two-dimensional (2D) materials that exhibit an inverted Mexican-hat-shaped dispersion relation, taking monolayer InSe as an example. We employ variational techniques to obtain the momentum-dark and bright ground-states (non-zero and zero quasiparticle momenta, respectively). These states are of particular interest due to their peaks in the quasiparticle density of states, the largest contribution comes from the momentum-dark ground state due to the presence of a van Hove singularity (VHS). These momentum-dark systems require a physical process to provide the necessary momentum to become bright. We study the brightening of this state due to coupling with phonons and compute the resulting photoluminescence spectrum. This work opens new avenues of research, such as exploiting dark excitons in solar cells and other semiconductor-based optoelectronic devices.

cond-mat.mes-hall↗

arXiv2025

On the engineering of higher-order Van Hove singularities in two dimensions

The properties of correlated electron materials are often intricately linked to Van Hove singularities (VHS) in the vicinity of the Fermi energy. The class of these VHS is of great importance, with higher-order ones -- with power-law divergence in the density of states -- leaving frequently distinct signatures in physical properties. We use a new theoretical method to detect and analyse higher-order VHS (HOVHS) in two-dimensional materials and apply it to the electronic structure of the surface layer of Sr$_2$RuO$_4$. We then constrain a low energy model of the VHS of the surface layer of Sr$_2$RuO$_4$ against angle-resolved photoemission spectroscopy and quasiparticle interference data to analyse the VHS near the Fermi level. We show how these VHS can be engineered into HOVHS.

cond-mat.str-el↗

arXiv2024

High-order Van Hove singularities and their connection to flat bands

The flattening of single-particle band structures plays an important role in the quest for novel quantum states of matter due to the crucial role of interactions. Recent advances in theory and experiment made it possible to construct and tune systems with nearly flat bands, ranging from graphene multilayers and moire' materials to kagome' metals and ruthenates. While theoretical models predict exactly flat bands under certain ideal conditions, evidence was provided that these systems host high-order Van Hove points, i.e., points of high local band flatness and power-law divergence in energy of the density of states. In this review, we examine recent developments in engineering and realising such weakly dispersive bands. We focus on high-order Van Hove singularities and explore their connection to exactly flat bands. We provide classification schemes and discuss interaction effects. We also review experimental evidence for high-order Van Hove singularities and point out future research directions.

cond-mat.str-el↗

arXiv2024

Reactive Hall and Edelstein effects in a tight-binding model with spin-orbit coupling

The reactive Hall constant R_H, described by reactive (nondissipative) conductivities, is analyzed within linear response theory in the presence of spin-orbit interaction. Within a two dimensional tight-binding model the effect of van Hove singularities is studied. Along the same line a formulation of the Edelstein constant is proposed and studied as a function of coupling parameters and fermion filling.

cond-mat.str-el↗

arXiv2023

Fate of density waves in the presence of a higher order van Hove singularity

Topological transitions in electronic band structures, resulting in van Hove singularities in the density of states, can considerably affect various types of orderings in quantum materials. Regular topological transitions (of neck formation or collapse) lead to a logarithmic divergence of the electronic density of states (DOS) as a function of energy in two-dimensions. In addition to the regular van Hove singularities, there are higher order van Hove singularities (HOVHS) with a power-law divergences in DOS. By employing renormalization group (RG) techniques, we study the fate of a spin-density wave phase formed by nested parts of the Fermi surface, when a HOVHS appears in parallel. We find that the phase formation can be boosted by the presence of the singularity, with the critical temperature increasing by orders of magnitude. We discuss possible applications of our findings to a range of quantum materials such as Sr$_3$Ru$_2$O$_7$, Sr$_2$RuO$_4$ and transition metal dichalcogenides.

cond-mat.str-el↗

arXiv2023

Multi-$Q$ magnetic phases from frustration and chiral interactions

We investigate the effect of Dzyaloshinskii-Moriya (DM) interactions in the planar pyrochlore (checkerboard) antiferromagnet, one of the paradigmatic models of spin frustration, and establish the classical phase diagram using a combination of analytical and numerical approaches. While anisotropic interactions generally tend to remove the frustration, here we show that a high degree of frustration survives in a large region of the phase diagram. In conjunction with the fixed handedness introduced by the DM anisotropy, this spawns a cascade of incommensurate and double-twisted multi-domain phases, consisting of spatially intertwined domains of the underlying competing phases of the isotropic frustrated point. The results underpin a novel mechanism of generating multi-Q phases in systems that combine high degree of frustration and chiral interactions.

cond-mat.str-el↗

arXiv2023

A practical method to detect, analyse and engineer higher order Van Hove singularities in multi-band Hamiltonians

We present a practical method to detect, diagnose and engineer higher order Van Hove singularities in multiband systems, with no restrictions on the number of bands and hopping terms. The method allows us to directly compute the Taylor expansion of the dispersion of any band at arbitrary points in momentum space, using a generalised extension of the Feynman Hellmann theorem, which we state and prove. Being fairly general in scope, it also allows us to incorporate and analyse the effect of tuning parameters on the low energy dispersions, which can greatly aid the engineering of higher order Van Hove singularities. A certain class of degenerate bands can be handled within this framework. We demonstrate the use of the method, by applying it to the Haldane model.

cond-mat.str-el↗

arXiv2022

Semiclassical transport with Berry curvature: Chambers formula and applications to systems with Fermi surface topological transitions

Starting with general semiclassical equations of motion for electrons in the presence of electric and magnetic fields, we extend the Chambers formula to include in addition to a magnetic field, time-dependent electric fields and bands with Berry curvature. We thereby compute the conductivity tensor $σ_{αβ}(B,ω)$ in the presence of magnetic field for bands in two (2D) and three (3D) dimensions with Berry curvature. We focus then on several applications to magnetotransport for metals with Fermi surface topological transitions in 2D. In particular, we consider a rectangular lattice and a model related to overdoped graphene, to investigate the signatures of different types of Fermi surface topological transitions in metals in the Hall coefficient, Hall conductivity $σ_{xy}$ and longitudinal conductivity $σ_{xx}$. The behavior of those quantities as a function of frequency, when the electric field is time dependent, is also investigated. As an example of non-zero Berry curvature, we study the magnetotransport of the Haldane model within this context. In addition, we provide the linear and nonlinear electric current formula to order $E^2$.

cond-mat.mes-hall↗

arXiv2022

Effect of disorder on density of states and conductivity in higher order van Hove singularities in two dimensional bands

We study systems with energy bands in two dimensions, hosting higher order Van Hove singularities (HOVHS) in the presence of disorder, using standard diagrammatic techniques for impurity averaging. In the clean limit, such singularities cause power-law divergence in the density of states (DOS), and this is expected to strongly affect electronic correlation. In order to analyse the signatures of these singularities in disordered systems, we employ various Born approximations, culminating in the self-consistent (non) Born approximation. Although the divergence of the DOS is smeared, we find that the shape of the DOS, as characterized by the power law tail and the universal ratio of prefactors, is retained slightly away from the singularity. This could help us to understand current and future experiments on materials that can be tuned to host HOVHS. The impurity induced smearing is calculated and analysed for several test cases of singularities. We also study the effects of impurities on electrical conductivity and determine the regimes where the quantitative features of the power law DOS manifest in the conductivity.

cond-mat.str-el↗

arXiv2021

Theory of surface-induced multiferroicity in magnetic materials, thin films and multilayers

We present a theoretical study of the onset of electric polarization close to a surface in magnetic materials and in thin films and multilayers. We consider two different paths that lead to the onset of multiferroic behavior at the boundary in materials that are bulk collinear ferromagnets or antiferromagnets. These two paths are distinguished by the presence or absence of a surface induced Dzyaloshinskii-Moriya interaction which can be taken into account through Lifshitz invariants in the free energy of the system. Experimental consequences are discussed in the light of the developed theory.

cond-mat.mtrl-sci↗

arXiv2019

Catastrophe theory classification of Fermi surface topological transitions in two dimensions

We classify all possible singularities in the electronic dispersion of two-dimensional systems that occur when the Fermi surface changes topology, using catastrophe theory. For systems with up to seven control parameters (i.e., pressure, strain, bias voltage, etc), the theory guarantees that the singularity belongs to to one of seventeen standard types. We show that at each of these singularities the density of states diverges as a power law, with a universal exponent characteristic of the particular catastrophe, and we provide its universal ratio of amplitudes of the prefactors of energies above and below the singularity. We further show that crystal symmetry restricts which types of catastrophes can occur at the points of high symmetry in the Brillouin zone. For each of the seventeen wallpaper groups in two-dimensions, we list which catastrophes are possible at each high symmetry point.

cond-mat.str-el↗