Jorn Venderbos

• PhD
• PostDoc Position at University of Pennsylvania

Magnetic states with zero magnetization but non-relativistic spin splitting are outstanding candidates for the next generation of spintronic devices. Their electron-volt (eV) scale spin splitting, ultrafast spin dynamics and nearly vanishing stray fields make them particularly promising for several applications. A variety of such magnetic states wi...

The electronic spectra of altermagnets are a fertile ground for nontrivial topology due to the unique interplay between time-reversal and crystalline symmetries. This is reflected in the unconventional Zeeman splitting between bands of opposite spins, which emerges in the absence of spin-orbit coupling (SOC) and displays nodes along high-symmetry d...

1$T$-TaS$_2$ is the only insulating transition-metal dichalcogenide (TMD) with an odd number of electrons per unit cell. This insulating state is non-magnetic, making it a potential spin-liquid candidate. The unusual electronic behavior arises from a naturally occurring nearly flat mini-band, where the properties of the strongly correlated states a...

Magnetic frustration can lead to peculiar magnetic orderings that break a discrete symmetry of the lattice in addition to the fundamental magnetic symmetries (i.e., spin rotation invariance and time-reversal symmetry). In this work, we focus on frustrated quantum magnets and study the nature of the quantum phase transition between a paramagnet and...

In this work, we present a theory to reconcile conflicting experimental claims regarding the charge density wave (CDW) state in TiSe$_2$, including whether there is a single or multiple CDW transitions and the occasional observation of rotation symmetry breaking. Using a $\boldsymbol{k}\cdot\boldsymbol{p}$ model coupled to the CDW order parameter,...

Layered van der Waals magnets have attracted much recent attention as a promising and versatile platform for exploring intrinsic two-dimensional magnetism. Within this broader class, the transition metal phosphorous trichalcogenides $M$P$X_3$ stand out as particularly interesting, as they provide a realization of honeycomb lattice magnetism and are...

Graphene moiré superlattices display electronic flat bands. At integer fillings of these flat bands, energy gaps due to strong electron–electron interactions are generally observed. However, the presence of other correlation-driven phases in twisted graphitic systems at non-integer fillings is unclear. Here, we report the existence of three-fold ro...

We introduce a two-dimensional electronic insulator that possesses a toric-code topological order enriched by translation symmetry. This state can be realized from disordering a weak topological superconductor by double-vortex condensation. It is termed the toric-code insulator, whose anyonic excitations consist of a charge-e chargon, a neutral fer...

We introduce a two-dimensional electronic insulator that possesses a toric code topological order enriched by translation symmetry. This state can be realized from disordering a weak topological superconductor by double-vortex condensation. It is termed the toric code insulator, whose anyonic excitations consist of a charge-$e$ chargon, a neutral f...

Topological Dirac semimetals are a class of semimetals that host symmetry-protected Dirac points near the Fermi level, which arise due to a band inversion of the conduction and valence bands. In this work, we study the less explored class of noncentrosymmetric topological Dirac semimetals in three dimensions. We identify the noncentrosymmetric crys...

Graphene-based moiré systems have attracted considerable interest in recent years as they display a remarkable variety of correlated phenomena. Besides insulating and superconducting phases in the vicinity of integer fillings of the moiré unit cell, there is growing evidence for electronic nematic order both in twisted bilayer graphene and twisted...

Graphene-based moir\'{e} systems have attracted considerable interest in recent years as they display a remarkable variety of correlated phenomena. Besides insulating and superconducting phases in the vicinity of integer fillings of the moir\'{e} unit cell, there is growing evidence for electronic nematic order both in twisted bilayer graphene and...

Topological Dirac semimetals are a class of semimetals that host symmetry-protected Dirac points near the Fermi level, which arise due to a band inversion of the conduction and valence bands. In this work, we study the less explored class of \emph{noncentrosymmetric} topological Dirac semimetals in three dimensions. We identify the noncentrosymmetr...

Nematic order is the breaking of rotational symmetry in the presence of translational invariance. While originally defined in the context of liquid crystals, the concept of nematic order has arisen in crystalline matter with discrete rotational symmetry, most prominently in the tetragonal Fe-based superconductors where the parent state is four-fold...

Graphene moir\'e superlattices display electronic flat bands. At integer fillings of these flat bands, energy gaps due to strong electron-electron interactions are generally observed. However, the presence of other correlation-driven phases in twisted graphitic systems at non-integer fillings is unclear. Here, we report scanning tunneling microscop...

Motivated by recent reports of nematic order in twisted bilayer graphene (TBG), we investigate the impact of the triangular moiré superlattice degrees of freedom on nematicity. In TBG, the nematic order parameter is not Ising like, as in tetragonal crystals, but has a three-state Potts character related to the threefold rotational symmetry ( C 3 z...

Chiral topological metals are materials that can quantize the circular photogalvanic effect (CPGE), a universal photocurrent that is generated by circularly polarized light. However, to determine quantization, a precise knowledge of the linear and non-linear optical responses are necessary. In this work we report a broad theoretical and experimenta...

We report the optical conductivity in the linear-response regime of the chiral topological semimetal CoSi, predicted to host elusive topological quasiparticles known as multifold fermions. We find that the optical response is separated into several distinct regions as a function of frequency, each dominated by different types of quasiparticles. The...

Despite the growing interest in topological materials, the difficulty of experimentally synthesizing and integrating them with other materials has been one of the main barriers restricting access to their unique properties. Recent advances in synthesizing metastable phases of crystalline materials can help to overcome this barrier and offer new pla...

Motivated by recent reports of nematic order in twisted bilayer graphene (TBG), we investigate the impact of the triangular moir\'e superlattice degrees of freedom on nematicity. In TBG, the nematic order parameter is not Ising-like, as it is the case in tetragonal crystals, but has a 3-state Potts character related to the threefold rotational symm...

Despite the growing interest in topological materials, the difficulty of experimentally synthesizing and integrating them with other materials has been one of the main barriers restricting access to their unique properties. Recent advances in synthesizing metastable phases of crystalline materials can help to overcome this barrier and offer new pla...

We review recent theoretical progress in the understanding and prediction of novel topological semimetals. Topological semimetals define a class of gapless electronic phases exhibiting topologically stable crossings of energy bands. Different types of topological semimetals can be distinguished on the basis of the degeneracy of the band crossings,...

Nematic superconductors possess unconventional superconducting order parameters that spontaneously break rotational symmetry of the underlying crystal. In this work we propose a mechanism for nematic superconductivity stabilized by strong density wave fluctuations in two dimensions. While the weak-coupling theory finds the fully gapped chiral state...

We study a special class of topological phase transitions in two dimensions described by the inversion of bands with relative angular momentum higher than 1. A band inversion of this kind, which is protected by rotation symmetry, separates the trivial insulator from a Chern insulating phase with higher Chern number, and thus generalizes the quantum...

The recent observation of superconductivity in proximity to an insulating phase in twisted bilayer graphene (TBG) at small “magic” twist angles has been linked to the existence of nearly flat bands, which make TBG a fresh playground to investigate the interplay between correlations and superconductivity. The low-energy narrow bands were shown to be...

We review recent theoretical progress in the understanding and prediction of novel topological semimetals. Topological semimetals define a class of gapless electronic phases exhibiting topologically stable crossings of energy bands. Different types of topological semimetals can be distinguished based on the degeneracy of the band crossings, their c...

Nematic superconductors possess unconventional superconducting order parameters that spontaneously break rotational symmetry of the underlying crystal. In this work we propose a mechanism for nematic superconductivity stabilized by strong density wave fluctuations in two dimensions. While the weak-coupling theory finds the fully gapped chiral state...

We study a special class of topological phase transitions in two dimensions described by the inversion of bands with relative angular momentum higher than 1. A band inversion of this kind, which is protected by rotation symmetry, separates the trivial insulator from a Chern insulating phase with higher Chern number, and thus generalizes the quantum...

We argue that a correlated fluid of electrons and holes can exhibit a fractional quantum Hall effect at zero magnetic field analogous to the Laughlin state at filling 1/m. We introduce a variant of the Laughlin wave function for electrons and holes and show that for m=1 it is the exact ground state of a free fermion model that describes px+ipy exci...

The recent observation of superconductivity in proximity to an insulating phase in twisted bilayer graphene (TBG) at small `magic' twist angles has been linked to the existence of nearly-flat bands, which make TBG a fresh playground to investigate the interplay between correlations and superconductivity. The low-energy narrow bands were shown to be...

We propose that the noncentrosymmetric LiGaGe-type hexagonal $ABC$ crystal SrHgPb realizes a new type of topological semimetal that hosts both Dirac and Weyl points in momentum space. The symmetry-protected Dirac points arise due to a band inversion and are located on the sixfold rotation $z$-axis, whereas the six pairs of Weyl points related by si...

We study the topological properties of superconductors with paired $j=\frac{3}{2}$ quasiparticles. Higher spin Fermi surfaces can arise, for instance, in strongly spin-orbit coupled band-inverted semimetals. Examples include the Bi-based half-Heusler materials, which have recently been established as low-temperature and low-carrier density supercon...

We study the topological properties of superconductors with paired $j=\frac{3}{2}$ quasiparticles. Higher spin Fermi surfaces can arise, for instance, in strongly spin-orbit coupled band-inverted semimetals. Examples include the Bi-based half-Heusler materials, which have recently been established as low-temperature and low-carrier density supercon...

Motivated by the experimental detection of superconductivity in the low-carrier density half-Heusler compound YPtBi, we study the pairing instabilities of three-dimensional strongly spin-orbit coupled semimetals with a quadratic band touching point. In these semimetals the electronic structure at the Fermi energy is described by spin j=3/2 quasipar...

Motivated by the experimental detection of superconductivity in the low-carrier density half-Heusler compound YPtBi, we study the pairing instabilities of three-dimensional strongly spin-orbit coupled semimetals with a quadratic band touching point. In these semimetals the electronic structure at the Fermi energy is described by spin j=3/2 quasipar...

DOI:https://doi.org/10.1103/PhysRevB.95.179902

Through a systematic symmetry and topology analysis we establish that three-dimensional chiral superconductors with strong spin-orbit coupling and odd-parity pairing generically host low-energy nodal quasiparticles that are spin-non-degenerate and realize Majorana fermions in three dimensions. By examining all types of chiral Cooper pairs with tota...

Through a systematic symmetry and topology analysis we establish that three-dimensional chiral superconductors with strong spin-orbit coupling and odd-parity pairing generically host low-energy nodal quasiparticles that are spin-non-degenerate and realize Majorana fermions in three dimensions. By examining all types of chiral Cooper pairs with tota...

Topological Dirac and Weyl semimetals have an energy spectrum that hosts Weyl nodes appearing in pairs of opposite chirality. Topological stability is ensured when the nodes are separated in momentum space and unique spectral and transport properties follow. In this work we study the effect of a space dependent Weyl node separation, which we interp...

Topological Dirac and Weyl semimetals have an energy spectrum that hosts Weyl nodes appearing in pairs of opposite chirality. Topological stability is ensured when the nodes are separated in momentum space and unique spectral and transport properties follow. In this work we study the effect of a space dependent Weyl node separation, which we interp...

Recent nuclear magnetic resonance and specific heat measurements have provided concurring evidence of spontaneously broken rotational symmetry in the superconducting state of the doped topological insulator Cu$_x$Bi$_2$Se$_3$. This suggests that the pairing symmetry corresponds to a two-dimensional representation of the $D_{3d}$ crystal point group...

Recent nuclear magnetic resonance and specific heat measurements have provided concurring evidence of spontaneously broken rotational symmetry in the superconducting state of the doped topological insulator Cu$_x$Bi$_2$Se$_3$. This suggests that the pairing symmetry corresponds to a two-dimensional representation of the $D_{3d}$ crystal point group...

We consider a system of spinless fermions on the honeycomb lattice with substrate-induced modulated electrostatic potentials tripling the unit cell. The resulting non-Abelian ${\cal SU} (2)$ gauge fields act cooperatively to realize a quadratic band crossing point (QBCP). This QBCP can be gapped out by an additional intra-unit-cell potential modula...

We consider a system of spinless fermions on the honeycomb lattice with substrate-induced modulated electrostatic potentials tripling the unit cell. The resulting non-Abelian SU(2) gauge fields act cooperatively to realize a quadratic band crossing point (QBCP). Using a combination of mean-field theory and renormalization group techniques, we show...

We study hexagonal spin-channel ("triplet") density waves with commensurate $M$-point propagation vectors. We first show that the three $Q=M$ components of the singlet charge density and charge-current density waves can be mapped to multi-component $Q=0$ nonzero angular momentum order in three dimensions ($3D$) with cubic crystal symmetry. This one...

In this work we introduce a symmetry classification for electronic density waves which break translational symmetry due to commensurate wave vector modulations. The symmetry classification builds on the concept of extended point groups: symmetry groups which contain, in addition to the lattice point group, translations that do not map the enlarged...

Motivated by the recent experiment indicating that superconductivity in the doped topological insulator Cu$_x$Bi$_2$Se$_3$ has an odd-parity pairing symmetry with rotational symmetry breaking, we study the general class of odd-parity superconductors with two-component order parameters in trigonal and hexagonal crystal systems. In the presence of st...

Motivated by the recent experiment indicating that superconductivity in the doped topological insulator Cu$_x$Bi$_2$Se$_3$ has an odd-parity pairing symmetry with rotational symmetry breaking, we study the general class of odd-parity superconductors with two-component order parameters in trigonal and hexagonal crystal systems. In the presence of st...

Both topological crystalline insulators surfaces and graphene host multi-valley massless Dirac fermions which are not pinned to a high-symmetry point of the Brillouin zone. Strain couples to the low-energy electrons as a time-reversal invariant gauge field, leading to the formation of pseudo-Landau levels (PLL). Here we study periodic pseudo-magnet...

Both topological crystalline insulators surfaces and graphene host multi-valley massless Dirac fermions which are not pinned to a high-symmetry point of the Brillouin zone. Strain couples to the low-energy electrons as a time-reversal invariant gauge field, leading to the formation of pseudo-Landau levels (PLL). Here we study periodic pseudo-magnet...

We present a physical scenario in which both Fermi arcs and two-dimensional gapless Dirac states coexist as boundary modes at the same two-dimensional surface. This situation is realized in topological insulator-Weyl semimetal interfaces in spite of explicit time reversal symmetry breaking. Based on a heuristic topological index, we predict that th...

We consider the Kondo-lattice model on the kagome lattice and study its weak-coupling instabilities at band filling fractions for which the Fermi surface has singularities. These singularites include Dirac points, quadratic Fermi points in contact with a flat band, and Van Hove saddle points. By combining a controlled analytical approach with large...

Whereas the concept of topological band-structures was developed originally for insulators with a bulk bandgap, it has become increasingly clear that the prime consequences of a non-trivial topology -- spin-momentum locking of surface states -- can also be encountered in gapless systems. Concentrating on the paradigmatic example of mercury chalcoge...

In this work we study the magnetic phase diagram of classical spins which interact with itinerant electrons on a checkerboard lattice, a lattice that constitutes a two-dimensional equivalent of the three-dimensional spin-ice pyrochlore lattice. We explore both the strong coupling and weak coupling limit and find a rich ground state phase diagram as...

The opportunity for the formation of fractional quantum-Hall (FQH) states in 3-orbital Hubbard and Kondo lattice models on the triangular lattice without an external magnetic field has been recently demonstrated [1,2]. With this as motivation, an effective interacting spinless-fermion model, which is designed to capture the essential relevant physi...

We establish that the interplay of itinerant fermions with localized magnetic moments on a checkerboard lattice leads to magnetic flux-phases. For weak itineracy the flux-phase is coplanar and the electronic dispersion takes the shape of graphene-like Dirac fermions. Stronger itineracy drives the formation of a non-coplanar, chiral flux-phase, in w...

We discuss the low-energy limit of three-orbital Kondo-lattice and Hubbard models describing $t_{2g}$ orbitals on a triangular lattice near half-filling. We analyze how very flat bands with non-trivial topological character, a Chern number C=1, arise both in the limit of infinite on-site interactions as well as in more realistic regimes. Exact diag...

For topologically nontrivial and very narrow bands, Coulomb repulsion between electrons has been predicted to give rise to a spontaneous fractional quantum-Hall (FQH) state in the absence of magnetic fields. Here we show that strongly correlated electrons in a t(2g)-orbital system on a triangular lattice self-organize into a spin-chiral magnetic or...

In recent years, there has been immense research interest in frustrated magnets with metallic character, such as the pyrochlores R2Mo2O7, where R denotes a rare-earth element (Science 291, 2573 (2001)). The frustration in magnetic sector in such systems can have interesting consequences for the electronic properties. More interestingly, the electro...

We investigated the theory of the interplay of itinerant electrons and localized magnetic moments on the frustrated checkerboard lattice as function of the super-exchange interaction between the localized moments and the band filling of fermions. We find that at half filling a very robust magnetic ``flux'' phase is lowest in energy. The ordering of...

For topologically nontrivial and very narrow bands, Coulomb repulsion between electrons has been predicted to give rise to a spontaneous fractional quantum-Hall (FQH) state in absence of magnetic fields. Here we show that strongly correlated electrons in a t_2g-orbital system on a triangular lattice self-organize into a spin-chiral magnetic orderin...

DOI:https://doi.org/10.1103/PhysRevLett.107.139902

The Fractional Quantum Hall (FQH) effect has been predicted to occur in absence of magnetic fields and at high temperature in lattice systems that have flat bands with non-zero Chern number. We demonstrate that the presence of orbital degrees of freedom in frustrated lattice systems leads to a narrowing of topologically nontrivial bands. This robus...

Using a hybrid method based on fermionic diagonalization and classical Monte Carlo techniques, we investigate the interplay between itinerant and localized spins, with competing double- and superexchange interactions, on a honeycomb lattice. For moderate superexchange, a geometrically frustrated triangular lattice of hexagons forms spontaneously. F...

The Fractional Quantum Hall (FQH) effect has been predicted to occur in absence of magnetic fields and at high temperature in lattice systems that have flat bands with non-zero Chern number. We demonstrate that the presence of orbital degrees of freedom in frustrated lattice systems leads to a narrowing of topologically nontrivial bands. This robus...

A DFT-based investigation of rhombohedral (ABC)-type graphene stacks in finite static electric fields is presented. Electronic band structures and field-induced charge densities are compared with related literature data as well as with own results on (AB) stacks. It is found, that the undoped AB-bilayer has a tiny Fermi line consisting of one elect...

Using a hybrid method based on fermionic diagonalization and classical Monte Carlo, we investigate the interplay between itinerant and localized spins, with competing double- and super-exchange interactions, on a honeycomb lattice. For moderate superexchange, a geometrically frustrated triangular lattice of hexagons forms spontaneously. For slightl...

We calculate the statistical distribution P2(I2) of the speckle pattern produced by a photon pair current I2 transmitted through a random medium, and compare it with the single-photon speckle distribution P1(I1). We show that the purity of a two-photon density matrix can be directly extracted from the first two moments of P1 and P2. A one-to-one re...

We calculate the statistical distribution P_2(I_2) of the speckle pattern produced by a photon pair current I_2 transmitted through a random medium, and compare with the single-photon speckle distribution P_1(I_1). We show that the purity Tr rho^2 of a two-photon density matrix rho can be directly extracted from the first two moments of P_1 and P_2...

Abstract This thesis presents a theory of the statistical properties of two-photon speckle, the random in- terference pattern observed in coincidence detection measurements,of photon pairs transmitted through disordered media. Ordinary speckle, the familiar collection of bright and dark spots observed on an image screen when illuminated by randomly...