Using spin-resolved and angle-resolved photoemission spectroscopy and first-principles calculations, we have identified bulk band inversion and the spin-polarized surface state evolved from a weak topological insulator (TI) phase in van der Waals materials ${\mathrm{Nb}}_{3}X{\mathrm{Te}}_6$ $(X=\mathrm{Si}, \mathrm{Ge})$. The fingerprints of weak TIs homologically emerge with hourglass fermions as multinodal chains composed by the same pair of valence and conduction bands gapped by spin-orbit coupling. The unique topological state, with a pair of valence and conduction bands encoding both weak TI and hourglass semimetal nature, is essential and guaranteed by nonsymmorphic symmetry. It is distinct from TIs studied previously based on band inversions without symmetry protections.

Coupled quantum Hall edge channels show intriguing nontrivial modes, for example, charge and neutral modes at Landau level filling factors 2 and 2/3. We propose an appropriate and effective model with a Coulomb interaction and disorder-induced tunneling characterized by coupling capacitances and tunneling conductances, respectively. This model explains how the transport eigenmodes, within the interaction- and disorder-dominated regimes, change with the coupling capacitance, tunneling conductance, and measurement frequency. We propose frequency- and time-domain transport experiments, from which eigenmodes can be determined using this model.

Unlike their Hermitian counterparts, non-Hermitian (NH) systems may display an exponential sensitivity to boundary conditions and an extensive number of edge-localized states in systems with open boundaries, a phenomena dubbed the “non-Hermitian skin effect.” The NH skin effect is one of the primary challenges to defining a topological theory of NH Hamiltonians, as the sensitivity to boundary conditions invalidates the traditional bulk-boundary correspondence. The NH skin effect has recently been connected to the winding number, a topological invariant unique to NH systems. In this paper we extend the definition of the winding number to disordered NH systems by generalizing established results on disordered Hermitian topological insulators. Our real-space winding number is self-averaging, continuous as a function of the parameters in the problem, and remains quantized even in the presence of strong disorder. We verify that our real-space formula still predicts the NH skin effect, allowing for the possibility of predicting and observing the NH skin effect in strongly disordered NH systems. We use our theory to demonstrate the NH Anderson skin effect, in which a skin effect is developed as disorder is added to a clean system, as well as explain recent results in optical funnels.

We present a detailed study of the spin-wave dynamics in single Pt/Co/W and double Pt/Co/W/Co/Pt ferromagnetic layer systems. The dispersion of spin waves was measured by wave-vector-resolved Brillouin light scattering spectroscopy while the in-plane and out-of-plane magnetization curves were measured by alternating gradient field magnetometry. The interfacial Dzyaloshinskii-Moriya interaction induced nonreciprocal dispersion relation was demonstrated for both single and double ferromagnetic layers and explicated by numerical simulations and theoretical formulas. The results indicate the crucial role of the order of layers deposition on the magnetic parameters. A significant difference between the perpendicular magnetic anisotropy constant in double ferromagnetic layer systems conduces to the decline of the interlayer interactions and different dispersion relations for the spin-wave modes. Our study provides a significant contribution to the realization of the multifunctional nonreciprocal magnonic devices based on ultrathin ferromagnetic/heavy-metal layer systems.

We theoretically investigate spin-orbit torques in insulator/ferromagnet/normal-metal structures with a focus on interfacial spin-orbit coupling effect at an insulator/ferromagnet interface. Based on the spin drift-diffusion formalism generalized to consider transverse spin currents in a ferromagnet and the boundary condition to consider transverse spin currents leaving from a ferromagnet, we find that interfacial spin-orbit coupling at the insulator/ferromagnet interface contributes to dampinglike spin-orbit torque, which is important for current-driven magnetization dynamics, even when the interfacial spin-orbit coupling generates the only fieldlike component. We also calculate spin-orbit torques in a single ferromagnet sandwiched by two dissimilar insulators, which provides additional information about interfacial spin-orbit interaction at insulator/ferromagnet interfaces.

The pseudogap regime of the cuprate high-temperature superconductors is characterized by a variety of competing orders, the nature of which are still widely debated. Recent experiments have provided evidence for electron nematic order, in which the electron fluid breaks rotational symmetry while preserving translational invariance. Raman spectroscopy, with its ability to symmetry resolve low energy excitations, is a unique tool that can be used to assess nematic fluctuations and nematic ordering tendencies. Here we compare results from determinant quantum Monte Carlo simulations of the Hubbard model to experimental results from Raman spectroscopy in ${\mathrm{La}}_{2-x}{\mathrm{Sr}}_x{\mathrm{CuO}}_4$, which show a prominent increase in the $B_{1g}$ response around 10% hole doping as the temperature decreases, indicative of a rise in nematic fluctuations at low energy. Our results support a picture of nematic fluctuations with $B_{1g}$ symmetry occurring in underdoped cuprates, which may arise from melted stripes at elevated temperatures.

One-dimensional systems proximity coupled to a superconductor can be driven into a topological superconducting phase by an external magnetic field. Here, we investigate the effect of vortices created by the magnetic field in a type-II superconductor providing the proximity effect. We identify different ways in which the topological protection of Majorana modes can be compromised and discuss strategies to circumvent these detrimental effects. Our findings are also relevant to topological phases of proximitized quantum Hall edge states.

The occurrence of retarded (with glue) and unretarded (without glue) pairing is thoroughly discussed in cuprates. We analyze some aspects of this problem in the context of the $t\text{−}J\text{−}V$ model in a large-$N$ approximation. When $1/N$ renormalizations are neglected the mean-field result is recovered, where the unretarded $d$-wave superconducting pairing triggered by the spin-exchange interaction $J$ is obtained. However, the presence of a nonnegligible nearest-neighbors Coulomb interaction $V\left(\mathbf{q}\right)$ kills superconductivity. If the non-double-occupancy constraint and its fluctuations are considered, the situation changes drastically. In this case, $V\left(\mathbf{q}\right)$ is screened making $d$-wave superconductivity very robust. In addition, we show that the early proposal for the presence of an unretarded pairing contribution triggered by the spin-exchange interaction $J$ can be discussed in this context.

This paper presents a lattice model incorporating random fields and long-range interactions where a frustrated state emerges at a specific composition but is suppressed elsewhere. The model is motivated by perovskite solid solutions and explains the phase diagram in such materials including the morphotropic phase boundary (MPB) that plays a critical role in applications for its enhanced dielectric, piezoelectric, and optical properties. Further, the model also suggests the possibility of phenomena by exploiting MPBs.

In spin-transport experiments with spin currents propagating through an antiferromagnetic (AFM) material, the antiferromagnet is mainly treated as a passive spin conductor not generating nor adding any spin current to the system. The spin current transmissivity of the AFM NiO is affected by magnetic fluctuations, peaking at the Néel temperature and decreasing by lowering the temperature. To study the role of antiferromagnetism in local and nonlocal spin-transport experiments, we send spin currents through NiO of various thicknesses placed on ${\mathrm{Y}}_3{\mathrm{Fe}}_5{\mathrm{O}}_{12}$. The spin currents are injected either electrically or by thermal gradients and measured at a wide range of temperatures and magnetic field strengths. The transmissive role is reflected in the sign change of the local electrically injected signals and the decrease in signal strength of all other signals by lowering the temperature. The thermally generated signals, however, show an additional upturn below $100\mathrm{K}$ that is unaffected by an increased NiO thickness. A change in the thermal conductivity could affect these signals. The temperature and magnetic field dependence are similar to those for bulk NiO, indicating that NiO itself contributes to thermally induced spin currents.

Magnetism coupled with topological materials can offer a platform for realizing quite rich fascinating physical phenomena, including quantum anomalous Hall effect (QAHE), topological magnetoelectric effect, and giant anomalous Nernst effect. Recently, the discovery of the intrinsic magnetic topological insulator ${\mathrm{MnBi}}_2{\mathrm{Te}}_4$ has promoted the study of the physical phenomena of magnetic topological materials. Here, we grow single crystals of Sn-doped ${\mathrm{MnBi}}_2{\mathrm{Te}}_4$ in bulk with ratios of 25%, 33%, 50%, and 66% to probe the magnetic and electrical properties. We perform low-temperature magnetic and electrical transport measurements for Sn-doped ${\mathrm{MnBi}}_2{\mathrm{Te}}_4$ and find that the antiferromagnetic (AFM) character is clear in this material until the concentration rises to 66%, and mainly transport dominated carriers are n-type. Most of all, the transition points of ${\mathrm{MnBi}}_2{\mathrm{Te}}_4$ decrease upon Sn doping, providing an alternative approach to realize QAHE at lower magnetic field, which is essential for further application in AFM spintronics.

We investigate instabilities of the magnetic ground state in ferromagnetic metals that are induced by uniform electrical currents, and, in particular, go beyond previous analyses by including dipolar interactions. These instabilities arise from spin-transfer torques that lead to Doppler-shifted spin waves. For sufficiently large electrical currents, spin-wave excitations have negative energy with respect to the uniform magnetic ground state while remaining dynamically stable due to dissipative spin-transfer torques. Hence, the uniform magnetic ground state is energetically unstable but is not able to dynamically reach the new ground state. We estimate this to happen for current densities $j\gtrsim (1-D/D_c){10}^{13}\mathrm{A}/{\mathrm{m}}^2$ in typical thin-film experiments, with $D$ the Dzyaloshinskii-Moriya interaction constant, and $D_c$ the Dzyaloshinskii-Moriya interaction that is required for spontaneous formation of spirals or skyrmions. The critical current density can be made arbitrarily small for ultrathin-film thicknesses at the order of nanometers due to surface and interlayer effects. From an analog gravity perspective, the stable negative-energy states are an essential ingredient to implement event horizons for magnons—the quanta of spin waves—giving rise to, e.g., Hawking radiation, and can be used to amplify spin waves in a so-called black-hole laser.

Possible signatures of “Majorana end states” are discussed by combining information extracted from subgap Cooper pair transport and transport of heat by excited quasiparticles above the gap across a thermally biased Josephson junction setup formed out of a one-dimensional (1D) topological superconductor hosting Majorana end states. We show that the presence of Majorana end states results in two sets of testable relations: (i) the ratios between the various multiterminal thermal conductances is independent of the Josephson phase bias, and (ii) the phase independence of the ratios of “ratios of the phase derivative of multiterminal thermal conductance and the corresponding Josephson current”, in a three-terminal setup. We contrast out findings with a Josephson junction setup composed of a 1D topological superconductor hosting “Andreev type end states” and show that they violate the above-mentioned relations. We also discuss how the presence of nonequilibrium noise in thermal current influences our predictions.

We address the question of the degree of spatial nonlocality of the self-energy in the iron-based superconductors, a subject which is receiving considerable attention. Using LiFeAs as a prototypical example, we extract the self-energy from angular-resolved photoemission spectroscopy data. We use two distinct electronic structure references: density functional theory in the local density approximation and linearized quasiparticle self-consistent GW (LQSGW). We find that with the LQSGW reference, spatially local dynamical correlations provide a consistent description of the experimental data, and account for some surprising aspects of the data such as the substantial out-of-plane dispersion of the electron Fermi surface having dominant $xz/yz$ character. Hence, correlations effects can be separated into static nonlocal contributions well described by LQSGW and dynamical local contributions. Hall effect and resistivity data are shown to be consistent with this description.

We demonstrate spin-polarized jets in extended systems of ballistic exciton-polariton condensates in semiconductor microcavities using optical nonresonant excitation geometries. The structure of the spin jets is determined by the spatially patterned degree of circular polarization of the nonresonant excitation laser. The presence of the laser excitation, strong particle interactions, and spin relaxation leads to a tunable spin-dependent potential landscape for polaritons, with the appearance of intricate polarization patterns due to coherent matter-wave interference. Our work realizes polarization-structured coherent light sources in the absence of gauge fields.

High-flux photoelectron pulses with femto-nano spatiotemporal properties enhanced with plasmonic metallic nanostructures can provide possibilities for potential photoelectron-based devices. In this work, a large multiphoton photoemission enhancement in a single plasmonic Au-heptamer nanostructure, which supports the Fano resonance, is investigated using photoemission electron microscopy (PEEM). By exploiting the high spatial PEEM resolution and performing calculations, it is found that the photoemission electrons are mainly emitted from the junction area between the lower surface of the nanostructure and the indium tin oxide film on the substrate. It is also observed that photoemission is strongly affected by the Coulomb field. Specifically, the strong photoemission locates in the gap region with the Coulomb attraction rather than Coulomb repulsion, even though the latter exhibits a higher charge density. The measured photoelectron enhancement achieved by employing a Fano-resonant Au heptamer is supported by calculations, combining the photoemission effects of both near-field intensity in the lower surface and nanostructure absorption.

$A{\mathrm{Co}}_2{\mathrm{Se}}_2$ ($A$=K, Rb, Cs) is a homologue of the iron-based superconductor $A{\mathrm{Fe}}_2{\mathrm{Se}}_2$. From a comprehensive study of ${\mathrm{RbCo}}_2{\mathrm{Se}}_2$ via measurements of magnetization, transport, neutron diffraction, angle-resolved photoemission spectroscopy, and first-principles calculations, we identify a ferromagnetic order accompanied by an orbital-dependent spin splitting of the electronic dispersions. Furthermore, we identify the ordered moment to be dominated by a $d_{x^2-y^2}$ flat band near the Fermi level, which exhibits the largest spin splitting across the ferromagnetic transition, suggesting an itinerant origin of the ferromagnetism. In the broader context of the iron-based superconductors, we find this $d_{x^2-y^2}$ flat band to be a common feature in the band structures of both iron chalcogenides and iron pnictides, accessible via heavy electron doping.

Motivated by recent experiments searching for Majorana zero modes in tripartite semiconductor nanowires with epitaxial superconductor and ferromagnetic-insulator layers, we explore the emergence of topological superconductivity in such devices for paradigmatic arrangements of the three constituents. Accounting for the competition between magnetism and superconductivity, we treat superconductivity self-consistently and describe the electronic properties, including the superconducting and ferromagnetic proximity effects within a direct wave-function approach. We conclude that the most viable mechanism for topological superconductivity relies on a superconductor-semiconductor-ferromagnet arrangement of the constituents in which spin splitting and superconductivity are independently induced in the semiconductor by proximity and superconductivity is only weakly affected by the ferromagnetic insulator.

Herein, we investigate symmetry-protected toroidal dipole resonances and conditions of their excitation in a new type of electromagnetic metamaterials. These metamaterials are all-dielectric planar periodic arrays of dielectric disks disposed on a dielectric substrate. The elementary building blocks of the array are trimers which are distributed in hexagonal unit supercells. The highest geometrical symmetry of the unit supercell is $C_{6v}$. The analysis is fulfilled by using the representation theory of groups with application of the magnetic group theory, which is a new approach in solving such problems. We have shown that to get access to the toroidal supermodes of the array, the symmetry of the unit supercell must be broken twice: firstly, the $C_{3v}$ symmetry of the trimer, and secondly, the $C_{6v}$ symmetry of the unit supercell needs to be reduced. Selection rules for the symmetric and antisymmetric orders of the toroidal dipole moments in the arrays are defined. In particular, we have shown that with the reduction of the unit supercell symmetry to the $C_{2v}$ group, the array exhibits the toroidal dipole resonance with antitoroidic order. The arrays with the lower $C_s$ symmetry can provide resonances with both toroidic and antitoroidic orders. It is also shown that these arrays are always polarization sensitive. Full-wave simulations and experiments confirm the theoretical predictions. The suggested metamaterials can provide an enhanced light-matter interaction due to the spatially and temporally confined light in resonant systems with very high quality factors.