Chiral excitonic order from twofold van Hove singularities in kagome metals.

Recent experiments on kagome metals AV3Sb5 (A=K,Rb,Cs) 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 AV3Sb5, 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 AV3Sb5. 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.

Formation of Artificial Fermi Surfaces with a Triangular Superlattice on a Conventional Two-Dimensional Electron Gas.

Imposing an external periodic electrostatic potential to the electrons confined in a quantum well makes it possible to engineer synthetic two-dimensional band structures, with electronic properties different from those in the host semiconductor. Here we report the fabrication and study of a tunable triangular artificial lattice on a GaAs/AlGaAs heterostructure where it is possible to transform from the original GaAs band structure and a circular Fermi surface to a new band structure with multiple artificial Fermi surfaces simply by altering a gate bias. For weak electrostatic modulation magnetotransport measurements reveal multiple quantum oscillations and commensurability oscillations due to the electron scattering from the artificial lattice. Increasing the strength of the modulation reveals new commensurability oscillations of the electrons from the artificial Fermi surface scattering from the triangular artificial lattice. These results show that low disorder gate-tunable lateral superlattices can be used to form artificial two-dimensional crystals with designer electronic properties.

Nonlinear Quantum Electrodynamics in Dirac Materials.

Classical electromagnetism is linear. However, fields can polarize the vacuum Dirac sea, causing quantum nonlinear electromagnetic phenomena, e.g., scattering and splitting of photons, that occur only in very strong fields found in neutron stars or heavy ion colliders. We show that strong nonlinearity arises in Dirac materials at much lower fields ∼1 T, allowing us to explore the nonperturbative, extremely high field limit of quantum electrodynamics in solids. We explain recent experiments in a unified framework and predict a new class of nonlinear magnetoelectric effects, including a magnetic enhancement of dielectric constant of insulators and a strong electric modulation of magnetization. We propose experiments and discuss the applications in novel materials.

Near-Field Excited Archimedean-like Tiling Patterns in Phonon-Polaritonic Crystals.

Phonon-polaritons (PhPs) arise from the strong coupling of photons to optical phonons. They offer light confinement and harnessing below the diffraction limit for applications including sensing, imaging, superlensing, and photonics-based communications. However, structures consisting of both suspended and supported hyperbolic materials on periodic dielectric substrates are yet to be explored. Here we investigate phonon-polaritonic crystals (PPCs) that incorporate hyperbolic hexagonal boron nitride (hBN) to a silicon-based photonic crystal. By using the near-field excitation in scattering-type scanning near-field optical microscopy (s-SNOM), we resolved two types of repetitive local field distribution patterns resembling the Archimedean-like tiling on hBN-based PPCs, i.e., dipolar-like field distributions and highly dispersive PhP interference patterns. We demonstrate the tunability of PPC band structures by varying the thickness of hyperbolic materials, supported by numerical simulations. Lastly, we conducted scattering-type nanoIR spectroscopy to confirm the interaction of hBN with photonic crystals. The introduced PPCs will provide the base for fabricating essential subdiffraction components of advanced optical systems in the mid-IR range.

Enhancement Mechanism of the Electron g Factor in Quantum Point Contacts.

The electron g factor measured in a quantum point contact by source-drain bias spectroscopy is significantly larger than its value in a two-dimensional electron gas. This enhancement, established experimentally in numerous studies, is an outstanding puzzle. In the present work we explain the mechanism of this enhancement in a theory accounting for the electron-electron interactions. We show that the effect relies crucially on the nonequilibrium nature of the spectroscopy at finite bias.

Origin of the phonon Hall effect in rare-earth garnets.

The phonon Hall effect has been observed in the paramagnetic insulator Tb3Gd5O12. A magnetic field applied perpendicularly to a heat current induces a temperature gradient that is perpendicular to both the field and the current. We show that this effect is due to resonant skew scattering of phonons from the crystal field states of superstoichiometric Tb(3+) ions. This scattering originates from the coupling between the quadrupole moment of Tb(3+) ions and the lattice strain. The estimated magnitude of the effect is consistent with experimental observations at T∼5 K and can be significantly enhanced by increasing temperature.

Fermi surface reconstruction by dynamic magnetic fluctuations.

We demonstrate that nearly critical quantum magnetic fluctuations in strongly correlated electron systems can change the Fermi surface topology and also lead to spin charge separation in two dimensions. To demonstrate these effects, we consider a small number of holes injected into the bilayer antiferromagnet. The system has a quantum critical point (QCP) which separates magnetically ordered and disordered phases. We demonstrate that in the physically interesting regime, there is a magnetically driven Lifshitz point (LP) inside the magnetically disordered phase. At the LP, the topology of the hole Fermi surface is changed. We also demonstrate that in this regime, the hole spin and charge necessarily separate when approaching the QCP. The considered model sheds light on generic problems concerning the physics of the cuprates.

Thermodynamics of a gas of deconfined bosonic spinons in two dimensions.

We consider the quantum phase transition between a Néel antiferromagnet and a valence-bond solid (VBS) in a two-dimensional system of S = 1/2 spins. Assuming that the excitations of the critical ground state are linearly dispersing deconfined spinons obeying Bose statistics, we derive expressions for the specific heat C and the magnetic susceptibility χ at low temperature T in terms of a correlation length ξ(T). Comparing with quantum Monte Carlo results for the J-Q model, which is a candidate for a deconfined Néel-VBS transition, we obtain an almost perfect consistency between C, χ, and ξ. The corresponding expressions for magnon (triplet) excitations are not internally consistent, however, lending strong support for spinon excitations in the J-Q model.

Screening of coulomb impurities in graphene.

We calculate exactly the vacuum polarization charge density in the field of a subcritical Coulomb impurity, Z|e|/r, in graphene. Our analysis is based on the exact electron Green's function, obtained by using the operator method, and leads to results that are exact in the parameter Zalpha, where alpha is the "fine-structure constant" of graphene. Taking into account also electron-electron interactions in the Hartree approximation, we solve the problem self-consistently in the subcritical regime, where the impurity has an effective charge Z(eff), determined by the localized induced charge. We find that an impurity with bare charge Z=1 remains subcritical, Z(eff)alpha<1/2, for any alpha, while impurities with Z=2, 3 and higher can become supercritical at certain values of alpha.

Structure of the spin-glass state of La2-xSrxCuO4: the spiral theory.

Starting from the t-J model, we derive the effective field theory describing the spin dynamics in insulating La(2-x)Sr(x)CuO(4), x approximately < 0.055, at low temperature. The theory results in a disordered spiral ground state, in which the staggered component of the copper spins is confined in a plane determined by the spin anisotropies. The static spin structure factor obtained in our calculations is in perfect agreement with neutron scattering data over the whole range of doping in both, the Néel and the spin-glass phase. We show that topological defects (spin vortex-antivortex pairs) are an intrinsic property of the disordered spiral ground state.

Theory of incommensurate magnetic correlations across the insulator-superconductor transition of underdoped La2-xSrxCuO4.

The main feature in the elastic neutron scattering of La2-xSrxCuO4 is the existence of incommensurate peaks with positions that jump from 45 degrees to 0 degrees at 5% doping. We show that the spiral state of the t-t(')-t('')-J model with realistic parameters describes these data perfectly. We explain why in the insulator the peak is at 45 degrees while it switches to 0 degrees precisely at the insulator-metal transition. The calculated positions of the peaks are in agreement with the data in both phases.