A Dirac-Fermi liquid (DFL), a doped system with Dirac spectrum, is an important example of a non-Galilean-invariant Fermi liquid (FL). Real-life realizations of a DFL include, e.g., doped graphene, surface states of three-dimensional (3D) topological insulators, and 3D Dirac/Weyl metals. We study the optical conductivity of a DFL arising from intraband electron-electron scattering. It is shown that the effective current relaxation rate behaves as $1/{\tau }_J\propto ({\omega }^2+4{\pi }^2{T}^2)(3{\omega }^2+8{\pi }^2{T}^2)$ for $max\{\omega ,T\}\ll \mu$, where $\mu$ is the chemical potential, with an additional logarithmic factor in two dimensions. In graphene, the quartic form of $1/{\tau }_J$ competes with a small FL-like term, $\propto {\omega }^2+4{\pi }^2{T}^2$, due to trigonal warping of the Fermi surface. We also calculated the dynamical charge susceptibility ${\chi }_{\mathrm{c}}(\mathbf{q},\omega )$ outside the particle-hole continua and to one-loop order in the dynamically screened Coulomb interaction. For a two-dimensional DFL, the imaginary part of ${\chi }_{\mathrm{c}}(\mathbf{q},\omega )$ scales as $q^2\omega ln\left|\omega \right|$ and $q^4/{\omega }^3$ for frequencies larger and smaller than the plasmon frequency at given $q$, respectively. The small-$q$ limit of $\mathrm{Im}{\chi }_{\mathrm{c}}(\mathbf{q},\omega )$ reproduces our result for the conductivity via the Einstein relation.

Magnetic topological semimetals have brought new insights into topological aspects because they nicely combine the band topology with intrinsic magnetic order. Comparing with ferromagnetic topological semimetals, antiferromagnetic (AFM) ones are even more special since they can break the time-reversal symmetry but show no net magnetic moment, and are highly expected for applications of topological AFM spintronics. Here, we report ${\mathrm{LiTi}}_2{\mathrm{O}}_4$ compound is an ideal AFM Weyl nodal line semimetal. It shows time-reversal breaking Weyl nodal lines in both the spin-up and spin-down channels. Each nodal line arises from the band in one single spin channel, thus is spin-polarized. The nodal lines locate quite near the Fermi level and do not coexist with other extraneous bands. The drumhead surface state of the nodal line can be clearly identified. The nodal lines are protected by the glide mirror symmetry and are robust against spin-orbit coupling. We also show that ${\mathrm{LiTi}}_2{\mathrm{O}}_4$ can transform into an AFM Weyl semimetal under an in-plane external magnetic field. Our results suggest ${\mathrm{LiTi}}_2{\mathrm{O}}_4$ can serve as an excellent platform to investigate AFM topological semimetals with attractive features.

Electron spins in edge or surface modes of topological insulators (TIs) with strong spin-orbit coupling cannot be directly manipulated with microwaves due to the locking of electron spin to its momentum. We show by contrast that a resistively detected nuclear magnetic resonance (RDNMR) based technique can be used to probe the helical nature of surface conducting states. In such experiments, one applies a radio frequency (RF) field to reorient nuclear spins that then couple to electronic spins by the hyperfine interaction. The spin of the boundary electrons can thereby be modulated, resulting in changes in conductance at nuclear resonance frequencies. Here, we demonstrate that the conductivity is sensitive to the direction of the applied magnetic field with respect to the helicity of the electrons. This dependence of the RDNMR signal on angle probes the nature of the conductive edge or surface states. In the case of 3D TI in the quantum Hall regime, we establish that the dominant mechanism responsible for the conductance change in a RDNMR experiment is based on the Overhauser field effect. Our findings indicate that the same physics underlying the use of RDNMR to probe TI states also enables us to use RF control of nuclear spins to coherently manipulate topologically protected states, which could be useful for a new generation of devices.

By using density functional theory (DFT) based first-principles calculations, electronic, vibrational, piezoelectric, and optical properties of monoclinic Janus single-layer ${\mathrm{Ga}}_2\mathrm{Te}X$ ($X$ = S or Se) are investigated. The dynamical, mechanical, and thermal stability of the proposed Janus single layers are verified by means of phonon bands, stiffness tensor, and quantum molecular dynamics simulations. The calculated vibrational spectrum reveals the either pure or coupled optical phonon branches arising from Ga-Te and Ga-$X$ atoms. In addition to the in-plane anisotropy, single-layer Janus ${\mathrm{Ga}}_2\mathrm{Te}X$ exhibits additional out-of-plane asymmetry, which leads to important consequences for its electronic and optical properties. Electronic band dispersions indicate the direct band-gap semiconducting nature of the constructed Janus structures with energy band gaps falling into visible spectrum. Moreover, while orientation-dependent linear-elastic properties of Janus single layers indicate their strong anisotropy, the calculated in-plane stiffness values reveal the ultrasoft nature of the structures. In addition, predicted piezoelectric coefficients show that while there is a strong in-plane anisotropy between piezoelectric constants along armchair (AC) and zigzag (ZZ) directions, there exists a tiny polarization along the out-of-plane direction as a result of the formation of Janus structure. The optical response to electromagnetic radiation has been also analyzed through density functional theory by considering the independent-particle approximation. Finally, the optical spectra of Janus ${\mathrm{Ga}}_2\mathrm{Te}X$ structures is investigated and it showed a shift from the ultraviolet region to the visible region. The fact that the spectrum is between these regions will allow it to be used in solar energy and many nanoelectronics applications. The predicted monoclinic single-layer Janus ${\mathrm{Ga}}_2\mathrm{Te}X$ are relevant for promising applications in optoelectronics, optical dichroism, and anisotropic nanoelasticity.

We theoretically show that two distinctive spin textures manifest themselves around saddle points in energy bands of monolayer ${\mathrm{NbSe}}_2$ under external gate potentials. While the density of states at all saddle points diverge logarithmically, ones at the zone boundaries display a windmill-shaped spin texture while the others unidirectional spin orientations. The disparate spin-resolved states are demonstrated to contribute an intrinsic spin Hall conductivity significantly while their characteristics differ from each other. Based on a minimal but essential tight-binding approximation reproducing first-principles computation results, we established distinct effective Rashba Hamiltonians for each saddle point, realizing the unique spin textures depending on their momentum. Energetic positions of the saddle points in a single layer ${\mathrm{NbSe}}_2$ are shown to be well controlled by a gate potential so that it could be a prototypical system to test a competition between various collective phenomena triggered by diverging density of states and their spin textures in low dimension.

Nonlinearities in lattices with topologically nontrivial band structures can give rise to topological solitons, whose properties differ from both conventional lattice solitons and linear topological boundary states. We show that a Su-Schrieffer-Heeger-type lattice with both nonlinearity and nonreciprocal non-Hermiticity hosts a novel oscillatory soliton, which we call a topological end breather. The end breather is strongly localized to a self-induced topological domain near the end of the lattice, in sharp contrast to the extended topological solitons previously found in one-dimensional lattices. Its stable oscillatory dynamics can be interpreted as a Rabi oscillation between two self-induced topological boundary states, emerging from a combination of chiral lattice symmetry and the non-Hermitian skin effect. This demonstrates that non-Hermitian effects can give rise to a wider variety of topological solitons than was previously known to exist.

We provide a unified interpretation of both paramagnon and plasmon modes in high-$T_c$ copper-oxides, and verify it quantitatively against available resonant inelastic $x$-ray scattering (RIXS) data across the hole-doped phase diagram. The three-dimensional extended Hubbard model, with included long-range Coulomb interactions and doping-independent microscopic parameters for both classes of quantum fluctuations, is used. Collective modes are studied using $\mathrm{VWF}+1/{\mathcal{N}}_f$ approach which extends variational wave function (VWF) scheme by means of an expansion in inverse number of fermionic flavors $(1/{\mathcal{N}}_f)$. We show that intense paramagnons persist along the antinodal line from the underdoped to the overdoped regime and undergo rapid overdamping in the nodal direction. Plasmons exhibit a three-dimensional character, with minimal energy corresponding to antiphase oscillations on neighboring ${\mathrm{CuO}}_2$ planes. The theoretical spin- and charge excitation energies reproduce semiquantitatively RIXS data for ${(\mathrm{Bi},\mathrm{Pb})}_2{(\mathrm{Sr},\mathrm{La})}_2{\mathrm{CuO}}_{6+\delta }$. The present $\mathrm{VWF}+1/{\mathcal{N}}_f$ analysis of dynamics and former VWF results for static quantities combine into a consistent description of the principal properties of hole-doped high-$T_c$ cuprates as strongly correlated systems.

Organic-inorganic halide perovskites have been intensively reinvestigated due to their applications, yet the optoelectronic function of the organic cation remains unclear. Through organic-selective resonant Auger electron spectroscopy measurements on well-defined single-crystal surfaces, we find evidence for electronic coupling in the unoccupied states between the organic and inorganic sublattices of the prototypical hybrid perovskite, which is contrary to the notion based on previous studies that the organic cation is electronically inert. The coupling is relevant for electron dynamics in the material and for understanding optoelectronic functionality.

By using first-principles calculations and symmetry analysis, we uncover that the rubidium-catena-telluridoaurate-family materials in space group (SG) 51 can exhibit ideal nodal-surface phonons separately in the three-dimensional Brillouin zone, characterized by nearly flat phonon bands in their phonon spectra. The nodal surfaces are protected both by the screw rotation symmetry and the time reversal symmetry. Moreover, our theoretical calculations demonstrate that the nodal surfaces are composed by twofold degenerate nodal points, which can form a straight singularity nodal line in the (001) surface, and that the (001) surface states display Dirac-type crossings and surface flat-band phononic states, confirming the topologically nontrivial nature of nodal-surface phonons. Our findings not only support that another class of nodal phononic states, i.e., one-node surface phonons, can exist in realistic materials in 230 SGs, but also put forward an effective way to search for nodal-surface phonons.

Current-induced spin-orbit torques (SOTs) in ferromagnet/nonmagnetic metal heterostructures open vast possibilities to design spintronic devices to store, process, and transmit information in a simple architecture. It is a central task to search for efficient SOT devices, and to quantify the magnitude as well as the symmetry of current-induced spin-orbit magnetic fields (SOFs). Here, we report an approach to determine the SOFs based on magnetization dynamics by means of time-resolved magneto-optic Kerr microscopy. A microwave current in a narrow Fe/GaAs (001) stripe generates an Oersted field as well as SOFs due to the reduced symmetry at the Fe/GaAs interface, and excites standing spin wave (SSW) modes because of the lateral confinement. Due to their different symmetries, the SOFs and the Oersted field generate distinctly different mode patterns. Thus, it is possible to determine the magnitude of the SOFs from an analysis of the shape of the SSW patterns. Specifically, this method, which is conceptually different from previous approaches based on line shape analysis, is phase independent and self-calibrated. It can be used to measure the current-induced SOFs in other material systems, e.g., ferromagnetic metal/nonmagnetic metal heterostructures.

We introduce three analytical and semianalytical tools that allow one to determine the topological character of impurity Shiba chains. The analytical methods are based on calculating the effective Green's function of an infinite embedded chain using the $T$-matrix formalism and describing the chain as a line impurity. We thus provide a solution to the longstanding size-effects problem affecting the only general alternative method, the numerical tight-binding analysis. As an example we consider a chain of magnetic impurities deposited on an $s$-wave superconducting substrate with Rashba spin-orbit and we calculate its topological phase diagram as a function of the magnetic impurity strength and the chemical potential. We find a perfect agreement between all our techniques and a numerical analysis.

There has been considerable interest in properties of condensed matter at high temperature, including nonequilibrium behavior and extreme conditions up to the warm dense matter regime. Such behavior is encountered, e.g., in experimental time-resolved x-ray absorption spectroscopy in the presence of intense laser fields. In an effort to simulate such behavior, we present an approach for calculations of high-temperature x-ray absorption spectra in arbitrary materials, using a generalization of the real-space Green's function formalism. The method is incorporated as an option in the core-level x-ray spectroscopy code FEFF10. To illustrate the approach, we present calculations for several materials together with comparisons to experiment and with other methods.

The spin-polarized bulk states in the centrosymmetric nodal-line semimetal HfSiS were observed by spin- and angle-resolved photoelectron spectroscopy. Combining with density functional theory calculations and the tight-binding model, the layer-locked spin states' nature of the observed $\mathrm{\Lambda }$-shaped bulk bands was revealed and the tight-binding model shows the importance of the next-nearest-neighbor term of spin-orbit coupling in the spin reversal of the bands along the $\overline{\mathrm{\Gamma }}\text{−}\overline{\mathit{M}}$ direction. Furthermore, the topological essence of attractive Dirac node arc states was confirmed.

A detailed investigation of the electronic structure in the neighborhood of Cd impurities in ${\mathrm{CeO}}_2$ has been performed by ab initio calculations to elucidate the interplay between the oxygen vacancies and electric quadrupole interactions. The quadrupole frequency related to the major component of the electric-field gradient (EFG) at impurities sites from its neighboring charge density as well as its symmetry were calculated by simulating oxygen vacancies at oxygen nearest neighbor of Cd. Results show a very good agreement with experimental hyperfine interactions measurements at the ${}^{111}\mathrm{Cd}$ nucleus replacing Ce at ${\mathrm{CeO}}_2$. A systematic mapping of oxygen vacancies in ${\mathrm{CeO}}_2$ supercells was proposed within the framework of density-functional theory using the wien2k code focusing on the electronic distribution in the vicinity of Cd impurities. Results show that the calculated values of EFG crucially depend on impurity-vacancy complex position and the striking axial symmetry observed when an oxygen monovancy is at the nearest neighborhood of Cd is explained by a rearrangement of its $p$ orbitals.

We study the effects of anisotropic interactions in the quantum Hall effect in the presence of a fourfold discrete rotational $\left(C_4\right)$ symmetry. Employing the density matrix renormalization group technique on an infinite cylinder geometry (iDMRG), we calculate the anisotropy response of the Laughlin state at $\nu =1/3$ and the composite-Fermi liquid (CFL) state at $\nu =1/2$. We find that the anisotropy transferred from the interaction potential to the $\nu =1/3$ state is stronger when compared with the complementary case of an anisotropic band. Further, the strength of anisotropy reduces as the interaction is made shorter ranged. Quite surprisingly, at $\nu =1/2$, the deformation in the CF Fermi surface changes sign as the interaction range is reduced. Our results imply that the short-distance and long-distance parts of the interaction potential have opposite effects on the CFL state in the presence of $C_4$-symmetric anisotropy.

In this paper, we analyze the transmission spectrum of a thin plate of cuprous oxide in the range of the absorption of the yellow exciton states with the coherent transfer matrix method. We demonstrate that, in contrast to the usual analysis using the Bouguer-Lambert law, a consistent quantitative description over the whole spectral range under consideration is possible. This leads to more accurate parameters not only for the Rydberg exciton states but also for the strengths of indirect transitions. Furthermore, the results have consequences for the interaction of Rydberg excitons with other systems, e.g., Rydberg states themselves or for the determination of the density of electron-hole pairs after optical excitation.

The quest to realize novel phases of matter with topological order is an important pursuit with implications for strongly correlated physics and quantum information. Utilizing ideas from state-of-the-art coherent control of artificial quantum systems such as superconducting circuits, we present a proposal to realize bosonic fractional quantum Hall physics on small lattices by creating nearly flat topological bands using staggered flux patterns. Fingerprints of fractionalization through charge pumping can be observed with nearly perfect quantization using as few as 24 lattice sites (two photons). We suggest an implementation using a finite lattice of superconducting qubits with cylindrical connectivity on both triangular and square lattices.

We have performed angle-resolved photoemission spectroscopy on epitaxial $\mathrm{V}{\mathrm{Te}}_2$ films to elucidate the relationship between the fermiology and charge density waves (CDWs). We found that a two-dimensional triangular pocket in 1 monolayer (ML) $\mathrm{V}{\mathrm{Te}}_2$ is converted to a strongly warped quasi-one-dimensional (1D) Fermi surface in the 6 ML counterpart, likely associated with the $1T$-to-$1T^{\prime }$ structural phase transition. We also revealed a metallic Fermi edge on the entire Fermi surface in 6 ML at low temperature distinct from anisotropic pseudogap in 1 ML, signifying a contrast behavior of CDW that is also supported by first-principles band-structure calculations. The present result points to the importance of simultaneously controlling the structural phase and fermiology to manipulate the CDW properties in ultrathin transition-metal dichalcogenides.

Dirac and Weyl points in condensed matters could resemble the behaviors of their counterparts, Dirac and Weyl fermions, in high-energy physics. However, some fermions have no analog in fundamental fermions. They are expected to render novel physical properties. In this work we study an important fermion in condensed matters which is threefold degenerate. Such a fermion has no counterpart in high-energy physics and is enforced by crystalline symmetry. We perform a systematic search over all 230 space groups with time-reversal symmetry but without spin-orbital coupling (SOC). We find that a triple degenerate point (TDP) intrinsically appears in a three-dimensional irreducible representation. Based on the order of their energy dispersion, TDPs can be classified into two types, namely linear and quadratic TDPs. Furthermore, the linear TDP can be classified into two categories depending on the definition of topological charge. To distinguish topologies of linear and quadratic TDPs, we study their Landau spectrum under a magnetic field. Notably, these TDPs present dramatically different properties. There exists two chiral Landau levels (LLs) for linear TDPs with nonzero topological charge, whereas no chiral LL appears for the quadratic TDPs. Given the novel physical properties of TDPs, we provide concrete materials to support our discoveries. Consequently, our work provides a platform to study the triple degenerate points in condensed matters.

We investigate metal-insulator transitions on an interacting two-dimensional Dirac fermion system using the determinant quantum Monte Carlo method. The interplay between Coulomb repulsion, disorder, and magnetic fields, drives the otherwise semimetallic regime to insulating phases exhibiting different characters. In particular, with the focus on the transport mechanisms, we uncover that their combination exhibits dichotomic effects. On the one hand, the critical Zeeman field $B_c$, responsible for triggering the band-insulating phase due to spin-polarization on the carriers, is largely reduced by the presence of the electronic interaction and quenched disorder. On the other hand, the insertion of a magnetic field induces a more effective localization of the fermions, facilitating the onset of Mott or Anderson insulating phases. Yet these occur at moderate values of $B$, and cannot be explained by the full spin-polarization of the electrons.