Recent experiments on the iron-based superconductor FeSe in a high magnetic field have suggested the presence of both the fluctuation-induced vortex liquid regime and a Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) vortex lattice. To get a general picture of the magnetic phase diagram in type II superconductors with strong superconducting (SC) fluctuation and strong paramagnetic pair breaking (PPB) such as FeSe, the vortex lattice melting curve $H_m\left(T\right)$ is theoretically investigated in situations where a FFLO state is expected to occur. In general, PPB tends to narrow the vortex liquid regime intervening between $H_{c2}\left(T\right)$ and $H_m\left(T\right)$. In particular, the vortex liquid regime is found to rapidly shrink upon entering; by cooling, the temperature range in which the FFLO state with a periodic modulation parallel to the magnetic field is stable in mean-field theory. Based on the present results, the high-field SC phase diagrams of FeSe in the parallel and perpendicular field configurations are discussed.

The extreme frustration in the kagome antiferromagnet makes it possible to realize a large number of closely competing magnetic phases either at the classical or the quantum level. Motivated by recent neutron scattering study on the kagome antiferromagnet vesignieite ${\mathrm{BaCu}}_3{\mathrm{V}}_2{\mathrm{O}}_8{\left(\mathrm{OH}\right)}_2$, we have made systematic investigation of the phase diagram of the classical and quantum $J_1-J_3$ antiferromagnetic Heisenberg model defined on the kagome lattice (KAFHM). While it is shown previously that a large antiferromagnetic exchange between the third-neighboring spins can drive an emergent $q=4$ Potts order through the order-by-disorder mechanism, it is elusive what will happen in the region where the Luttinger-Tisza criteria fails to predict the classical ordering pattern. Through extensive Monte Carlo simulation, we find that such a region is characterized by competing emergent $q=3$ and $q=4$ Potts order, whose emergence are beyond the description of the conventional order-by-disorder mechanism. Our Schwinger boson mean-field calculation suggests the existence of the Potts-3 order in the ground state of quantum $J_1-J_3$ KAFHM in such an extremely frustrated region. The predicted Potts-3 state is found to take two different forms distinguished by their projective symmetry group character (PSG or quantum order), although they have exactly the same symmetry. The transition between these two quantum orders is found to occur at a $J_3$ value intriguingly close to the tricritical point in the Potts-3 phase of the classical phase diagram of the model. A comparison with the exact diagonalization result on a 36-site cluster shows that the intermediate region may indeed host a nematic spin liquid ground state featuring an anisotropic ring structure around the $\mathbf{q}=0$ point in its spinon dispersion relation. We find that such an extremely frustrated region should better be taken as the playground to study the rich competition between exotic emergent phases, rather than a burden on their theoretical analysis.

We study geometrical responses of magnons driven by a temperature gradient in frustrated spin systems. While Dzyaloshinskii-Moriya (DM) interactions are usually incorporated to obtain geometrically nontrivial magnon bands, here we investigate thermal Hall responses of magnons that do not rely on the DM interactions. Specifically, we focus on frustrated spin systems with sublattice degrees of freedom and show that a nonzero Berry curvature requires breaking of an effective $PT$ symmetry. According to this symmetry consideration, we study the $J_1\text{−}{J}_2\text{−}{J}_2^{\prime }$ Heisenberg models on a honeycomb lattice as a representative example and demonstrate that magnons in the spiral phase support the thermal Hall effect once we introduce a magnetic field and asymmetry between the two sublattices. We also show that driving the magnons by a temperature gradient induces spin current generation (i.e., magnon spin Nernst effect) in the $J_1\text{−}{J}_2\text{−}{J}_2^{\prime }$ Heisenberg models.

We propose the magnetic moiré surface states of topological insulators as a platform for time-reversal symmetry breaking flat Chern bands. We construct a generic continuum model of Dirac electrons with a single Dirac cone moving in a time-reversal symmetry breaking periodic potential containing minimal parameters. The Zeeman-type moiré potentials generically gap out the moiré surface Dirac cones and give rise to isolated flat Chern minibands with Chern number $\pm 1$. Interestingly, the bandwidth of such isolated flat minibands is insensitive to the twisting angle, which originates from the completely flat band of the $p$ orbitals on the honeycomb lattice. This result provides a promising platform for realizing time-reversal breaking correlated topological phases and circumvents two main experimental difficulties in correlated states with twisted graphene: that the bandwidth varies rapidly with angle as well as the moiré disorder. Furthermore, in a $C_6$ periodic potential, with the interplay of the scalar potential and the Zeeman moiré potential, the $\mathrm{\Gamma }$-valley moiré surface electrons simulate two-dimensional honeycomb lattice physics, leading to emergent moiré surface Dirac cones as well as the Haldane model.

The optical properties of weakly doped two-dimensional materials are dominated by strong exciton and trion absorption and luminescence features. In this paper, we examine the influence of moiré patterns in semiconductor heterobilayers on exciton and trion states in the limit of strong moiré modulation potentials, commenting on similarities and differences compared to the case of excitons and trions in semiconductor quantum dots. We discuss strategies for using optical properties as quantitative probes of moiré materials, and the prospects for exploiting moiré materials to design unique light emitters.

Circular dichroism in resonant elastic x-ray scattering (CD-REXS) has recently been observed in chiral structures composed of multiferroic materials as well as magnetic moments or electric polarization vectors. In order to comprehensively understand the experimental results of these previous studies, we present here in detail the analytical formulation of CD-REXS for one-dimensional helices composed of magnetic moments and electric polarization vectors, respectively. In particular, by comparing CD-REXS for a proper-screw-shaped Bloch-type helix and cycloid-shaped Néel-type helix, we found that CD-REXS for both magnetic and polar helices can discriminate between both types of helices. We also found that the x-ray polarization factor depending on the scattering geometry is a significant factor in determining the characteristics of CD-REXS for chiral structures. The results obtained from the detailed formulas can be intuitively understood using the concept of mirror reflection. In particular, in this way it can be understood that Bloch- and Néel-type helices correspond to truly chiral and achiral structures, respectively, and why CD-REXS is able to distinguish between these two types of helices.

Optical interfaces with arbitrary magnetic and electric surface conductivities can enable the design of photonic devices with new functionalities, but practical approaches to date are nonexistent. Regular interfaces, such as those with graphene, are optically interesting due to their tailorable electric surface conductivity. However, their magnetic surface conductivity is negligible since the magnetic response in natural materials is generally weak from the terahertz frequency onward. The quest for artificial magnetic response has recently triggered the development of magnetic metasurfaces that, however, can only provide the interface with a limited value of magnetic surface conductivity. Herein we find that vertical heterostructures based on regular nonmagnetic metasurfaces have a direct correspondence to an optical interface with both magnetic and electric surface conductivities, whose desired values can be structurally engineered. Moreover, we identify several unique photonic and plasmonic responses at optical interfaces with specific magnetic surface conductivities, including a polarization-insensitive Brewster effect and pure magnetic surface waves.

Dissipative dynamics of quantum systems can be classified topologically based on the correspondence between the Lindbladian in the Gorini-Kossakowski-Sudarshan-Lindblad equation and the non-Hermitian Hamiltonian in the Schrödinger equation. While general non-Hermitian Hamiltonians are classified into 38 symmetry classes, previous studies have shown that the Lindbladians are classified into 10 symmetry classes due to a physical constraint. In this work, however, we unveil a topological classification of Lindbladians based on shifted sublattice symmetry (SLS), which can increase the number of symmetry classes for the Lindbladians. We introduce shifted SLS so that the Lindbladian can retain this symmetry and take on the same role as SLS for the topological classification. For verification, we construct a model of a dissipative quantum system retaining shifted SLS and confirm the presence of edge states protected by shifted SLS. Moreover, the relationship between the presence of shifted SLS protected edge states and the dynamics of an observable quantity is also discussed.

We aim to illuminate how the microscopic properties of a metal surface map to its electric field noise characteristics. In our system, prolonged heat treatments of a metal film can induce a rise in the magnitude of the electric field noise generated by the surface of that film. We refer to this heat-induced rise in noise magnitude as a thermal transformation. The underlying physics of this thermal transformation process is explored through a series of heating, milling, and electron treatments performed on a single surface ion trap. Between these treatments, ${}^{40}\mathrm{Ca}^{+}$ ions trapped $70 \mu \mathrm{m}$ above the surface of the metal are used as detectors to monitor the electric field noise at frequencies close to 1 MHz. An Auger spectrometer is used to track changes in the composition of the contaminated metal surface. With these tools we investigate contaminant deposition, chemical reactions, and atomic restructuring as possible drivers of thermal transformations.

Surface-enhanced Raman scattering (SERS) is based on the ability of a surface substrate to increase Raman signals for sensing and imaging applications. The most widely used Au and Ag SERS substrates are primarily based on the electromagnetic mechanism (EM) with large enhancement factors (EFs), which are, however, limited by small gaps due to tunneling. Graphene has been explored as an alternative substrate for graphene-enhanced Raman scattering based on the chemical mechanism (CM). However, the limits of CM EFs in graphene-based substrates have not been well understood, especially as a function of tip-sample distance (TSD). Here we performed tip-enhanced Raman scattering of carbon nanotubes on Au and graphene-oxide (GO) hybrid substrates for different TSDs. We show evidence of quantum plasmonics with GO as a tunneling junction in an Au-GO-Au cavity with a 2-nm gap size. We demonstrate Raman signal enhancement by four orders of magnitude beyond the EM tunneling limit by switching to the CM regime for the resonant GO excitation at small TSD. Tip-induced GO-enhanced Raman scattering may be used to improve nanoimaging and biosensing on novel hybrid GO/Au substrates.

The magnetoelectric coupling of electrons in a three-dimensional solid can be effectively described by axion electrodynamics. Here we report the discovery of the fractional magnetoelectric effect in chiral anomalous semimetals of the three-dimensional massless Wilson fermions, which have linear dispersions at the energy crossing point and break the chiral symmetry at generic momenta. In the presence of electric and magnetic fields, the time-reversal and parity symmetry breaking give rise to the quarter-quantized topological magnetoelectric effect, which is directly related to the winding number 1/2 of the band structure and is only one-half of that for topological insulators. The fractional electromagnetic response can be revealed by the surface Hall conductance and extracted from the measurement of the topological Kerr and Faraday rotation. The transition-metal pentatelluride ${\mathrm{ZrTe}}_5$ with a strain-tunable band gap provides a potential platform to test the effect experimentally.

We study field theories with global dipole symmetries and gauge dipole symmetries. The famous Lifshitz theory is an example of a theory with a global dipole symmetry. We study in detail its $1+1\mathrm{D}$ version with a compact field. When this global symmetry is promoted to a $U\left(1\right)$ dipole gauge symmetry, the corresponding gauge field is a tensor gauge field. This theory is known to lead to fractons. To resolve various subtleties in the precise meaning of these global or gauge symmetries, we place these $1+1\mathrm{D}$ theories on a lattice and then take the continuum limit. Interestingly, the continuum limit is not unique. Different limits lead to different continuum theories, whose operators, defects, global symmetries, etc., are different. We also consider a lattice gauge theory with a ${\mathbb{Z}}_N$ dipole gauge group. Surprisingly, several physical observables, such as the ground state degeneracy and the mobility of defects, depend sensitively on the number of sites in the lattice. Our analysis forces us to think carefully about global symmetries that do not act on the standard Hilbert space of the theory, but only on the Hilbert space in the presence of defects. We refer to them as timelike global symmetries and discuss them in detail. These timelike global symmetries allow us to phrase the mobility restrictions of defects (including those of fractons) as a consequence of a global symmetry.

Barium stannate (${\mathrm{BaSnO}}_3$) is an emerging perovskite oxide with promising properties for many potential applications. One of the most outstanding properties is the high-room-temperature electron mobility. The extraordinary superior electronic transport performance has attracted great attention but the research is limited by semiempirical assumptions. In this work, the first-principles calculations are employed to study the electronic transport of ${\mathrm{BaSnO}}_3$ by solving the Boltzmann transport equation (BTE) with full information on electron, phonon, and electron-phonon interactions taken into account. The mode-resolved analyses of scattering rates show that the longitudinal optical (LO) phonons dominate the scattering processes not only at low but also at high carrier concentrations. As a consequence, the energy relaxation time approximation significantly underestimates the solution of BTE due to the divergent coupling coefficient of electrons and LO phonons. The drift and Hall mobilities are calculated in the framework of BTE in the presence of electric and magnetic fields. At low concentration limits, the room-temperature drift and Hall mobilities are 357 and $418{\mathrm{cm}}^2{\mathrm{V}}^{-1}{\mathrm{s}}^{-1}$, respectively, while the Hall factor is temperature dependent with values between 1.06 and 1.17 in the temperature range of 100 K to 800 K. In the doping system with carrier concentration above $1\times {10}^{19}{\mathrm{cm}}^{-3}$, the upper limit of drift mobility is $377{\mathrm{cm}}^2{\mathrm{V}}^{-1}{\mathrm{s}}^{-1}$ at room temperature, and the Hall factor becomes smaller than 1, which cannot be revealed by the formula under the isotropic parabolic assumption at all, indicating the importance of magnetic transport calculation from ab initio based BTE.

Spin-orbit coupling in thin HgTe quantum wells results in a relativistic-like electron band structure, making it a versatile solid state platform to observe and control nontrivial electrodynamic phenomena. Here we report an observation of universal terahertz (THz) transparency determined by fine-structure constant $\alpha \approx 1/137$ in 6.5-nm-thick HgTe layer, close to the critical thickness separating phases with topologically different electronic band structure. Using THz spectroscopy in a magnetic field we obtain direct evidence of asymmetric spin splitting of the Dirac cone. This particle-hole asymmetry facilitates optical control of edge spin currents in the quantum wells.

Twisted layers of atomically thin two-dimensional materials realize a broad range of quantum materials with engineered optical and transport phenomena arising from spin and valley degrees of freedom and strong electron correlations in hybridized interlayer bands. Here, we report on experimental and theoretical studies of ${\mathrm{WSe}}_2$ homobilayers obtained in two stable configurations of $2H$ (${60}^{\circ }$ twist) and $3R$ ($0^{\circ }$ twist) stackings by controlled chemical vapor synthesis of high-quality large-area crystals. Using optical absorption and photoluminescence (PL) spectroscopy at cryogenic temperatures, we uncover marked differences in the optical characteristics of $2H$ and $3R$ bilayer ${\mathrm{WSe}}_2$ which we explain on the basis of beyond-density functional theory calculations. Our results highlight the role of layer stacking for the spectral multiplicity of momentum-direct intralayer exciton transitions in absorption and relate the multiplicity of phonon sidebands in the PL to momentum-indirect excitons with different spin valley and layer character. Our comprehensive study generalizes to other layered homobilayer and heterobilayer semiconductor systems and highlights the role of crystal symmetry and stacking for interlayer hybrid states.

The thermal conductivity (κ) of the Weyl semimetal NbP was studied with the thermal gradient and magnetic field applied parallel to the [0 0 1] direction. At low temperatures κ($B$) exhibits large quantum oscillations with frequencies matching two of several determined from the Shubnikov–de Haas effect measured on the same sample with analogous electrical current and magnetic field orientation. Both frequencies found in κ($B$) originate from the electron pocket enclosing a pair of Weyl nodes. The amplitude of the oscillatory component of the thermal conductivity turns out to be two orders of magnitude larger than the corresponding value calculated from the electrical conductivity using the Wiedemann-Franz law. Analysis of possible sources of this discrepancy indicates the chiral zero sound effect as a potential cause of its appearance.

Gapped two-dimensional topological phases can feature ungappable edge states which are robust even in the absence of protecting symmetries. In this Letter we argue that a multipartite entanglement measure recently proposed in the context of holography, the Markov gap, provides a universal diagnostic of ungappable edge states. Defined as a difference of the reflected entropy and mutual information $h\left(A\text{:}B\right)=S_R\left(A\text{:}B\right)-I\left(A\text{:}B\right)$ between two parties, we argue that for $A,B$ being adjacent subregions in the bulk $h=\frac{c_{+}}{3}ln2$, where $c_{+}$ is the minimal total central charge of the boundary theory. As evidence, we prove that $h=0$ for string-net models and numerically verify that $h=\frac{\left|C\right|}{3}ln2$ for a Chern-$C$ insulator. Our Letter establishes a unique bulk entanglement criteria for the presence of a conformal field theory on the boundary.

We consider effects of spatial dispersion in noncentrosymmetric time-reversal invariant Weyl metals in the presence of a static magnetic field. In particular, we study currents that are linear in both the spatial derivatives of an applied electric field and the static magnetic field, which are responsible for the phenomenon of gyrotropic birefringence. We show that the chiral anomaly and the chiral magnetic effect make the leading contribution to this class of phenomena in metals. We apply the obtained results to the problem of electromagnetic wave transmission through a thin slab of a Weyl semimetal and show that the transmission coefficient contains a component that is odd in the applied static magnetic field. As such, it can be easily distinguished from conventional Ohmic magnetotransport effects, which are quadratic in the applied magnetic field. The relative magnitude of the effect can reach a few percent in Weyl materials subject to magnetic fields of 0.1 T, while the effect is several orders of magnitude smaller in metals without Berry monopoles. We conclude that the nonreciprocal optical and magnetotransport effects can be a robust probe of band topology in metals.

The traditional bulk-boundary correspondence assuring robust gapless modes at the edges and surfaces of insulating and nodal topological materials gets masked in non-Hermitian (NH) systems by the skin effect, manifesting an accumulation of a macroscopic number of states near such interfaces. Here we show that dislocation lattice defects are immune to such skin effect or at most display a weak skin effect (depending on its relative orientation with the Burgers vector), and as such they support robust topological modes in the bulk of a NH system, specifically when the parent Hermitian phase features band inversion at a finite momentum. However, the dislocation modes gradually lose their support at their core when the system approaches an exceptional point, and finally melt into the boundary of the system across the NH band gap closing. We explicitly demonstrate these findings for a two-dimensional NH Chern insulator, thereby establishing that dislocation lattice defects can be instrumental to experimentally probe pristine NH topology.