Using a combination of infrared ellipsometry, time-domain terahertz spectroscopy, and far-infrared reflectometry, we have obtained the $ac$-plane complex dielectric function of monoclinic ($C2/c$) ${\mathrm{Ta}}_2{\mathrm{NiSe}}_5$ and orthorhombic ($Cmcm$) ${\mathrm{Ta}}_2{\mathrm{NiS}}_5$ single crystals. The identified dipole-active phonon modes polarized along the $a$ and $c$ axes are in good agreement with density functional theory (DFT) calculations. With increasing temperature the $a$-axis phonon modes of ${\mathrm{Ta}}_2{\mathrm{NiSe}}_5$ become poorly discernible, as they are superimposed on the electronic background, which gradually fills the energy gap near the monoclinic-to-orthorhombic phase transition temperature $T_c=326\mathrm{K}$. In ${\mathrm{Ta}}_2{\mathrm{NiS}}_5$, which does not exhibit such a structural transition and remains orthorhombic down to low temperatures, the $a$-axis phonon modes are superimposed on a persistent broad electronic mode centered near 16 meV. We attribute this difference to strongly overlapping exciton-phonon complexes in ${\mathrm{Ta}}_2{\mathrm{NiSe}}_5$, as opposed to isolated instances of the same in ${\mathrm{Ta}}_2{\mathrm{NiS}}_5$, and find this to be in good agreement with an excitonic insulator state below $T_c$ in the former, compared to the absence of one in the latter.

The origin of the anisotropy in physical quantities related to a symmetry-broken (nematic) electronic state is still very much debated in high-temperature superconductors. FeSe at ambient pressure undergoes a structural, tetragonal-to-orthorhombic phase transition at $T_s\simeq 90$ K without any magnetic ordering on further cooling, which leads to an ideal electronic nematicity. Our unprecedented optical results provide evidence that the low-energy excitation spectrum in the nematic phase is shaped by an important interplay of the anisotropic Drude weight and scattering rate. In the zero-frequency limit though, the temperature dependence of the anisotropic scattering rate plays the dominant role and, combined with the nematic order parameter as evinced from the high energy optical response, accounts for the anisotropic dc resistivity. This favors the scattering by anisotropic spin fluctuations as the prominent candidate in governing the properties of the nematic phase.

Using a real-time path integral approach we develop an algorithm to calculate multitime correlation functions of open few-level quantum systems that is applicable to highly nonequilibrium dynamics. The calculational scheme fully keeps the non-Markovian memory introduced by the pure-dephasing type coupling to a continuum of oscillators. Furthermore, we discuss how to deal consistently with the simultaneous presence of non-Markovian and Markovian system reservoir interactions. We apply the method to a crucial test case, namely the evaluation of emission spectra of a laser-driven two-level quantum dot coupled to a continuum of longitudinal acoustic phonons, which give rise to non-Markovian dynamics. Here, we also account for the coupling to a photonic environment, which models radiative decay and can be treated as a Markovian bath. The phonon side bands are found on the correct side of the zero phonon line in our calculation, in contrast to known results where the quantum regression theorem is applied naively to non-Markovian dynamics. Combining our algorithm with a recently improved iteration scheme for performing the required sum over paths we demonstrate the numerical feasibility of our approach to systems with more than two levels. Results are shown for the second-order photonic two-time correlation function of a quantum dot-cavity system with seven states on the Jaynes-Cummings ladder taken into account.

Theoretically challenging, the understanding of the dynamical response in quantum antiferromagnets is of great interest, in particular for both inelastic neutron scattering (INS) and nuclear magnetic resonance (NMR) experiments. In such a context, we theoretically address this question for quasi-one-dimensional quantum magnets, e.g., weakly coupled spin chains for which many compounds are available in nature. In this class of systems, the dimensional crossover between a three-dimensional ordered regime at low temperature towards one-dimensional physics at higher temperature is a nontrivial issue, notably difficult concerning dynamical properties. Here we present a comprehensive theoretical study based on both analytical calculations (bosonization + random phase and self-consistent harmonic approximations) and numerical simulations (quantum Monte Carlo + stochastic analytic continuation) which allows us to describe the full temperature crossover for the NMR relaxation rate $1/T_1$, from one-dimensional Tomonaga-Luttinger liquid physics to the three-dimensional ordered regime, as a function of interchain couplings. The dynamical structure factor, directly probing the INS intensity, is also computed in the different regimes.

A static electric field generates circulating currents at the surfaces of a magnetoelectric insulator. The anomalous Hall part of the surface conductivity tensor describing such bound currents can change by multiples of $e^2/h$ depending on the insulating surface preparation, and a bulk calculation does not fix its quantized part. To resolve this ambiguity, we develop a formalism for calculating the full surface anomalous Hall conductivity in a slab geometry. We identify a Berry-curvature term, closely related to the expression for the bulk anomalous Hall conductivity, whose value can change by quantized amounts by adjusting the surface Hamiltonian. In addition, the surface anomalous Hall conductivity contains a nongeometric part that does not depend on the surface preparation.

Gapless Dirac fermions appear as quasiparticle excitations in various condensed-matter systems. They feature quantum critical points with critical behavior in the 2+1-dimensional Gross-Neveu universality class. The precise determination of their critical exponents defines a prime benchmark for complementary theoretical approaches, such as lattice simulations, the renormalization group, and the conformal bootstrap. Despite promising recent developments in each of these methods, however, no satisfactory consensus on the fermionic critical exponents has been achieved, so far. Here, we perform a comprehensive analysis of the Ising Gross-Neveu universality classes based on the recently achieved four-loop perturbative calculations. We combine the perturbative series in $4-\epsilon$ spacetime dimensions with the one for the purely fermionic Gross-Neveu model in $2+\epsilon$ dimensions by employing polynomial interpolation as well as two-sided Padé approximants. Further, we provide predictions for the critical exponents exploring various resummation techniques following the strategies developed for the three-dimensional scalar $O\left(n\right)$ universality classes. We give an exhaustive appraisal of the current situation of Gross-Neveu universality by comparison to other methods. For a large enough number of spinor components $N\ge 8$ as well as for the case of emergent supersymmetry $N=1$, we find our renormalization group estimates to be in excellent agreement with the conformal bootstrap, building a strong case for the validity of these values. For intermediate $N$ as well as in comparison with recent Monte Carlo results, deviations are found and critically discussed.

We investigate the effects of metal dopants on ${\mathrm{BiFeO}}_3$ (BFO) by first-principles calculations. Substitutional doping in oxide materials is often complicated by the formation of defects that interfere with, dominate, or otherwise change the effects of the introduced dopant. As a result, extracting correct conclusions and working principles experimentally requires extensive characterization of the material properties, which is not easily accessible. We solve this problem by an extensive model study of the changes that are introduced in the crystal, electronic, and magnetic structure of BFO, focusing on substitutional doping in an otherwise ideal crystal. We examine a large number of candidate elements. From our results, trends can be established within rows and groups of the periodic table. We predict the preferred doping site (Bi or Fe substitution) and oxidation state for each dopant and provide an in-depth understanding of the structural and electronic changes that are introduced upon doping. From this, we are able to divide the periodic table into direct $p$ dopants, $n$ dopants, and isovalent cases. For the latter, understanding the valence configuration and the band structure of the doped systems enables us to distinguish between isovalent dopants that can enable $p$-type, $n$-type, or no doping. A comparison of the resulting acceptor and donor states provides insight into the performance of such dopants and, together with defect formation energies, enables ranking all candidates and identification of optimal dopants.

Motivated by experiments on the carrier-doped bilayer iridate (${\mathrm{Sr}}_{1-x}{\mathrm{La}}_x{)}_3{\mathrm{Ir}}_2{\mathrm{O}}_7$, we study the dynamics of a single doped electron in a bilayer magnet in the presence of spin-orbit coupling, taking into account the spatially staggered rotation of ${\mathrm{IrO}}_6$ octahedra. We employ an effective single-orbital bilayer $t-J$ model, concentrating on the quantum paramagnetic phase near the magnetic quantum critical point. We determine the carrier dispersion using a combination of self-consistent Born and bond-operator techniques. Extrapolating to finite small carrier density we find that, for experimentally relevant parameters, the combination of octahedral rotation and spin-orbit coupling induces a band folding which results in a Fermi surface of small double electron pockets, in striking agreement with experimental observations. We also determine the influence of spin-orbit coupling on the location of the quantum critical point in the undoped case, and discuss aspects of the global phase diagram of doped bilayer Mott insulators.

The Berry curvature of a Bloch band can be interpreted as a local magnetic field in reciprocal space. This analogy can be extended by defining an electric field analog in reciprocal space which arises from the time-dependent Berry connection. We explore the term in the semiclassical equation of motion that gives rise to this phenomenon, and show that it can lead to anomalous drift in wave-packet motion. A similar effect arises from changes in the band population due to periodic driving, where the resulting drift depends on the nature of the drive and can be expressed in terms of a shift vector. Finally, these effects can be combined to build a pump with a net anomalous drift during a cyclic evolution in momentum space.

In the quest for miniaturizing magnetic devices, the effects of boundaries and surfaces become increasingly important. Here we show how the recently predicted boundary-induced Dzyaloshinskii-Moriya interaction (DMI) affects the magnetization of ferromagnetic films with a $C_{\infty v}$ symmetry and a perpendicular magnetic anisotropy. For an otherwise uniformly magnetized film, we find a surface twist when the magnetization in the bulk is canted by an in-plane external field. This twist at the surfaces caused by the boundary-induced DMI differs from the common canting caused by internal DMI observed at the edges of a chiral magnet. Furthermore, we find that the surface twist due to the boundary-induced DMI strongly affects the width of the domain wall at the surfaces. We also find that the skyrmion radius increases in the depth of the film, with the average size of the skyrmion increasing with boundary-induced DMI. This increase suggests that the boundary-induced DMI contributes to the stability of the skyrmion.

We experimentally study the radii of excitons in hBN-encapsulated ${\mathrm{WS}}_2$ monolayers by means of magneto-optical reflectance spectroscopy at cryogenic temperatures in magnetic fields up to 29 T. We observe field-induced energy shifts of the exciton ground and excited states due to valley Zeeman and diamagnetic effects. We find the $g$ factor of the first excited state of $-4.2\pm 0.1$ to be essentially equal to that of the ground state of $-4.35\pm 0.1$. From diamagnetic shifts, we determine the root mean square radii of the excitons. The radius of the first excited state is found to be 5–8 nm and that of the ground state around 2 nm. Our results further confirm the Wannier-Mott nature of the exciton quasiparticles in monolayer semiconductors and the assignment of the optical resonances in absorption-type measurements. They also provide additional support for the applicability of the effective mass hydrogenlike models in these systems.

The spin Seebeck effect (SSE) is a phenomenon where a spin current is generated from thermal excitations in magnetic materials. It is believed that magnetic long-range order (LRO) is not required for SSE, while short-ranged order (SRO) plays an important role. However, a definitive experimental demonstration of the connection between the SSE and SRO has been missing. Here, we show that the SSE is able to probe a specific SRO in a model geometrically frustrated magnet ${\mathrm{Gd}}_3{\mathrm{Ga}}_5{\mathrm{O}}_{12}$ (GGG). A field-induced SRO in GGG is detected by the SSE in the temperature range 2–5 K. The origin of the SRO is verified by comparing the SSE data directly to existing neutron scattering measurements at much lower temperatures ($T<0.9$ K), where field-induced LRO exists. Our theoretical calculations of the magnetic structure further confirm the anisotropic field dependence of the SRO in GGG. These findings establish that the SSE can serve as an effective probe of SRO in geometrically frustrated magnets.

The flexoelectric effect refers to polarization induced in an insulator when a strain gradient is applied. We have developed a first-principles methodology based on density-functional perturbation theory to calculate the elements of the bulk clamped-ion flexoelectric tensor. In order to determine the transverse and shear components directly from a unit-cell calculation, we calculate the current density induced by the adiabatic atomic displacements of a long-wavelength acoustic phonon. Previous implementations based on the charge-density response required supercells to capture these components. At the heart of our approach is the development of an expression for the current-density response to a generic long-wavelength phonon perturbation that is valid for the case of nonlocal pseudopotentials. We benchmark our methodology on simple systems of isolated noble gas atoms, and apply it to calculate the clamped-ion flexoelectric constants for a variety of technologically important cubic oxides.

We propose a lattice-scale two-band generalized Hubbard model as a caricature of the electronic structure of twisted bilayer graphene. Various possible broken-symmetry phases can arise, including a nematic phase (which is a form of orbital ferromagnet) and an orbital-triplet spin-singlet superconducting phase. Concerning the mechanism of superconductivity, we propose an analogy with superconductivity in alkali-doped ${\mathrm{C}}_{60}$ in which a violation of Hund's first rule plays a central role.

A remarkable feature of the band structure of bilayer graphene at small twist angle is the appearance of isolated bands near neutrality, whose bandwidth can be reduced at certain magic angles (e.g., $\theta \sim 1.{05}^{\circ }$). In this regime, correlated insulating states and superconductivity have been experimentally observed. A microscopic description of these phenomena requires an understanding of universal aspects of the band structure, which we discuss here. First, we point out the importance of emergent symmetries, such as valley conservation, which are excellent symmetries in the limit of small twist angles and dictate qualitative features of the band structure. These have sometimes been overlooked when discussing commensurate approximants to the band structure, which we also review here, and solidify their connection with the continuum theory which incorporates all emergent symmetries. Finally, we discuss obstructions to writing tight-binding models of just the isolated bands, and in particular a symmetry-based diagnostic of these obstructions, as well as relations to band topology and strategies for resolving the obstruction. Especially, we construct a four-band model where the two lower isolated bands realize all identified Wannier obstructions of the single-valley nearly flat bands of twisted bilayer graphene.

The link between macroscopic electric polarizations and microscopic charge densities, a fundamental problem of condensed matter physics, has been addressed by quantum theory involving geometric quantum phases for calculating polarization differences. While such work has provided quantitative insight into stationary electric polarizations of crystalline ferroelectrics, polarization dynamics driven by elementary electronic and/or lattice excitations are still far from understood. X-ray diffraction with a femtosecond time resolution allows for mapping of charge and lattice motions on their intrinsic time scales and provides time-dependent maps of electron density. We present a combined theoretical and experimental approach which allows for determining time-dependent macroscopic polarization changes from transient microscopic charge density maps. In an extension of existing theory, the key steps consist of deriving a microscopic current density from the charge density maps and calculating the macroscopic polarization change from this current density. Using femtosecond x-ray diffraction data for crystalline $({\mathrm{NH}}_4{)}_2{\mathrm{SO}}_4$ and ${\mathrm{KH}}_2{\mathrm{PO}}_4$, prototype materials displaying ferro- and paraelectric phases, we determine the ultrafast macroscopic polarization dynamics induced by coherent phonon motions. The results establish femtosecond x-ray diffraction as a key method for grasping transient polarization changes and for validating theoretical predictions for a wide range of polar and ferroelectric materials.

By performing pressure simulations within density functional theory for the family of iron-based superconductors $AeA{\mathrm{Fe}}_4{\mathrm{As}}_4$ with $Ae=\mathrm{Ca}$, Sr, Ba and $A=\mathrm{K}$, Rb, Cs we predict in these systems the appearance of two consecutive half-collapsed tetragonal transitions at pressures $P_{c_1}$ and $P_{c_2}$, which have a different character in terms of their effect on the electronic structure. We find that, similar to previous studies for ${\mathrm{CaKFe}}_4{\mathrm{As}}_4$, spin-vortex magnetic fluctuations on the Fe sublattice play a key role for an accurate structure prediction in these materials at zero pressure. We identify clear trends of critical pressures and discuss the relevance of the collapsed phases in connection to magnetism and superconductivity. Finally, the intriguing cases of ${\mathrm{EuRbFe}}_4{\mathrm{As}}_4$ and ${\mathrm{EuCsFe}}_4{\mathrm{As}}_4$, where Eu magnetism coexists with superconductivity, are discussed as well in the context of half-collapsed phases.

An open resonator fabricated in a two-dimensional electron gas is used to explore the transition from strongly invasive scanning gate microscopy to the perturbative regime of weak tip-induced potentials. With the help of numerical simulations that faithfully reproduce the main experimental findings, we quantify the extent of the perturbative regime in which the tip-induced conductance change is unambiguously determined by properties of the unperturbed system. The correspondence between the experimental and numerical results is established by analyzing the characteristic length scale and the amplitude modulation of the conductance change. In the perturbative regime, the former is shown to assume a disorder-dependent maximum value, while the latter linearly increases with the strength of a weak tip potential.

Spin superfluidity, i.e., coherent spin transport mediated by topologically stable textures, is limited by parasitic anisotropies rooted in relativistic interactions and spatial inhomogeneities. Since structural disorder in amorphous magnets can average out the effect of these undesired couplings, we propose this class of materials as platforms for superfluid spin transport. We establish nonlinear equations describing the hydrodynamics of spin in insulating amorphous magnets, where the currents are defined in terms of coherent rotations of a noncollinear texture. Our theory includes dissipation and nonequilibrium torques at the interface with metallic reservoirs. This framework allows us to determine different regimes of coherent dynamics and their salient features in nonlocal magnetotransport measurements. Our work paves the way for future studies on macroscopic spin dynamics in materials with frustrated interactions.

We study the field theory for the $\mathrm{SU}\left(N_c\right)$ symmetric antiferromagnetic quantum critical metal with a one-dimensional Fermi surface embedded in general space dimensions between two and three. The asymptotically exact solution valid in this dimensional range provides an interpolation between the perturbative solution obtained from the $\epsilon$ expansion near three dimensions and the nonperturbative solution in two dimensions. We show that critical exponents are smooth functions of the space dimension. However, physical observables exhibit subtle crossovers that make it hard to access subleading scaling behaviors in two dimensions from the low-energy solution obtained above two dimensions. These crossovers give rise to noncommutativities, where the low-energy limit does not commute with the limits in which the physical dimensions are approached.