We find a class of topological superconductors which possess an emergent time-reversal symmetry that is present only after projecting to an effective low-dimensional model. We show that a topological phase in symmetry class DIII can be realized in a noninteracting system coupled to an $s$-wave superconductor only if the physical time-reversal symmetry of the system is broken, and we provide three general criteria that must be satisfied in order to have such a phase. We also provide an explicit model which realizes the class DIII topological superconductor in 1D. We show that, just as in time-reversal invariant topological superconductors, the topological phase is characterized by a Kramers pair of Majorana fermions that are protected by the emergent time-reversal symmetry.

Topological insulators represent unusual topological quantum states, typically with gapped bulk band structure but gapless surface Dirac fermions protected by time-reversal symmetry. Recently, a distinct kind of topological insulator resulting from nonsymmorphic crystalline symmetry was proposed in the KHgX ($X=\mathrm{As}$, Sb, Bi) compounds. Unlike regular topological crystalline insulators, the nonsymmorphic glide-reflection symmetry in KHgX guarantees the appearance of an exotic surface fermion with hourglass shape dispersion (where two pairs of branches switch their partners) residing on its (010) side surface, contrasting to the usual two-dimensional Dirac fermion form. Here, by using high-resolution angle-resolved photoemission spectroscopy, we systematically investigate the electronic structures of KHgSb on both (001) and (010) surfaces and reveal the unique in-gap surface states on the (010) surface with delicate dispersion consistent with the “hourglass Fermion” recently proposed. Our experiment strongly supports that KHgSb is a nonsymmorphic topological crystalline insulator with hourglass fermions, which serves as an important step to the discovery of unique topological quantum materials and exotic fermions protected by nonsymmorphic crystalline symmetry.

Scattering of obliquely incident electromagnetic waves from periodically space-time modulated slabs is investigated. It is shown that such structures operate as nonreciprocal harmonic generators and spatial-frequency filters. For oblique incidences, low-frequency harmonics are filtered out in the form of surface waves, while high-frequency harmonics are transmitted as space waves. In the quasisonic regime, where the velocity of the space-time modulation is close to the velocity of the electromagnetic waves in the background medium, the incident wave is strongly coupled to space-time harmonics in the forward direction, while in the backward direction it exhibits low coupling to other harmonics. This nonreciprocity is leveraged for the realization of an electromagnetic isolator in the quasisonic regime and is experimentally demonstrated at microwave frequencies.

We show that optically active coupled quantum dots embedded in a superconducting microwave cavity can be used to realize a fast quantum interface between photonic and transmon qubits. Single-photon absorption by a coupled quantum dot results in generation of a large electric dipole, which in turn ensures efficient coupling to the microwave cavity. Using cavity parameters achieved in prior experiments, we estimate that bidirectional microwave-optics conversion in nanosecond time scales with efficiencies approaching unity is experimentally feasible with current technology. We also outline a protocol for in-principle deterministic quantum state transfer from a time-bin photonic qubit to a transmon qubit. Recent advances in quantum-dot-based quantum photonics technologies indicate that the scheme we propose could play a central role in connecting quantum nodes incorporating cavity-coupled superconducting qubits.

When a charged particle moves close and parallel to the surface of a Babinet metasurface composed of metallic $C$-aperture resonators, strong electromagnetic radiation arises at resonant frequency. Here we systematically study the Smith-Purcell effects in Babinet metasurfaces with different periods. By tuning the period, the resonant (or working) frequency of the metasurface can vary from GHz to THz and infrared ranges. It is found that for working frequencies lower than 10 GHz, the ratio of absorption loss to input power is about 3.7%. Although the loss ratio increases with increasing working frequency, it remains as low as 11% (29%) at a working frequency of 10 THz (224 THz). Due to the existence of loss, a nonlinear relationship is also found between resonant frequency and the reciprocal of period. Our results suggest that Babinet metasurfaces could be a good candidate for fabricating efficient, compact THz and infrared free-electron light sources.

We report on the dynamics of the spin-1/2 quasi-one-dimensional frustrated magnet ${\mathrm{LiCuVO}}_4$ measured by nuclear spin relaxation in high magnetic fields 10–34 T, in which the ground state has spin-density-wave order. The spin fluctuations in the paramagnetic phase exhibit striking anisotropy with respect to the magnetic field. The transverse excitation spectrum probed by ${}^{51}\mathrm{V}$ nuclei has an excitation gap, which increases with field. On the other hand, the gapless longitudinal fluctuations sensed by ${}^7\mathrm{Li}$ nuclei grow with lowering temperature, but tend to be suppressed with increasing field. Such anisotropic spin dynamics and its field dependence agree with the theoretical predictions and are ascribed to the formation of bound magnon pairs, a remarkable consequence of the frustration between ferromagnetic nearest-neighbor and antiferromagnetic next-nearest-neighbor interactions.

Motivated by recent proposals of correlation induced insensitivity of $d$-wave superconductors to impurities, we develop a simple pairing theory for these systems for up to a moderate strength of disorder. Our description implements the key ideas of Anderson, originally proposed for disordered $s$-wave superconductors, but in addition takes care of the inherent strong electronic repulsion in these compounds, as well as disorder induced inhomogeneities. We first obtain the self-consistent one-particle states, which capture the effects of disorder exactly, and strong correlations using Gutzwiller approximation. These “normal states,” representing the interplay of strong correlations and disorder, when coupled through pairing attractions following the path of Bardeen-Cooper-Schrieffer (BCS), produce results nearly identical to those from a more sophisticated inhomogeneous Hartree-Fock-Bogoliubov analysis that takes care of strong electronic repulsions.

We explored the effects of isovalent doping on the evolution of structure, spin-density wave (SDW), and superconductivity (SC) in three series of Ba-doped $\mathrm{SrF}{\mathrm{e}}_2\mathrm{A}{\mathrm{s}}_{2-y}{\mathrm{P}}_y$ ($y=0.5,0.7,1.0$). For the $y=0.5$ series, the SDW and SC can coexist in the all-Ba-doped $\mathrm{SrF}{\mathrm{e}}_2\mathrm{A}{\mathrm{s}}_{1.5}{\mathrm{P}}_{0.5}$, while the SC first decreases and then saturates in the case of $y=0.7$. Besides, it is strikingly found that the SC of Ba-doped ${\mathrm{SrFe}}_2\mathrm{AsP}$ exhibits a dome-shaped $T_{\mathrm{c}}$, in which the anion height, $h_{\mathrm{pn}}$, approaching the optimal value of 1.308(2) Å and the $T_{\mathrm{c}}$ simultaneously increase to the maxima. The phase diagrams of SDW and SC as a function of $h_{\mathrm{pn}}$ and Pn-Fe-Pn angle are first constructed here. It is argued that the isovalent substitutions in FeAs-based superconductors could significantly alter SC and ground states equally as well as carrier doping. The theoretical calculations show that electronic band structure is strongly dependent on the parameter of $h_{\mathrm{pn}}$ without introducing carriers by doping. The transition from three-dimensional-like to two-dimensional electronic states and the nesting of the Fermi surface can be realized by changing the $h_{\mathrm{pn}}$.

Spin behaviors on kagome lattices have received increased attention because of their exotic magnetic states. Here, we report magnetic ordering in the $S=2$ kagome antiferromagnet $\mathrm{MgF}{\mathrm{e}}_3{\left(\mathrm{OH}\right)}_6\mathrm{C}{\mathrm{l}}_2$. The compound has been synthesized by selectively replacing the magnetic ions in the triangular planes of its parent compound of deformed pyrochlore $\mathrm{F}{\mathrm{e}}_2{\left(\mathrm{OH}\right)}_3\mathrm{Cl}$ with nonmagnetic Mg. Whereas $\mathrm{F}{\mathrm{e}}_2{\left(\mathrm{OH}\right)}_3\mathrm{Cl}$ showed antiferromagnetic order below $T_{\mathrm{N}}=8.5\mathrm{K}$ with persistent spin fluctuations, $\mathrm{MgF}{\mathrm{e}}_3{\left(\mathrm{OH}\right)}_6\mathrm{C}{\mathrm{l}}_2$ showed an antiferromagnetic transition at an enhanced $T_{\mathrm{N}}$ value of 9.9 K. Susceptibility, high-field magnetization, and neutron-diffraction studies suggest that $\mathrm{MgF}{\mathrm{e}}_3{\left(\mathrm{OH}\right)}_6\mathrm{C}{\mathrm{l}}_2$ is a Heisenberg-like spin system with a ${120}^{\circ }$ nearest-neighbor spin structure confined in the kagome plane and spin-vector chirality of $q=-1$ below $T_{\mathrm{N}}$. Muon spin-rotation/-relaxation studies demonstrate that the persisting spin fluctuations are suppressed relative to $\mathrm{F}{\mathrm{e}}_2{\left(\mathrm{OH}\right)}_3\mathrm{Cl}$. The present paper reports the experimental realization of an $S=2$ kagome antiferromagnet, highlighting a real system with a quasiclassical Heisenberg spin, which may also be a valuable reference system for quantum Heisenberg kagome antiferromagnets.

We demonstrate that the nonlinear spin-wave transport in two laterally parallel magnetic stripes exhibit the intensity-dependent power exchange between the adjacent spin-wave channels. By the means of Brillouin light scattering technique, we investigate collective nonlinear spin-wave dynamics in the presence of magnetodipolar coupling. The nonlinear intensity-dependent effect reveals itself in the spin-wave mode transformation and differential nonlinear spin-wave phase shift in each adjacent magnetic stripe. The proposed analytical theory, based on the coupled Ginzburg-Landau equations, predicts the geometry design involving the reduction of power requirement to the all-magnonic switching. A very good agreement between calculation and experiment was found. In addition, a micromagnetic and finite-element approach has been independently used to study the nonlinear behavior of spin waves in adjacent stripes and the nonlinear transformation of spatial profiles of spin-wave modes. Our results show that the proposed spin-wave coupling mechanism provides the basis for nonlinear magnonic circuits and opens the perspectives for all-magnonic computing architecture.

We study theoretically the topological surface states (TSSs) and the possible surface Andreev bound states (SABSs) of ${\mathrm{Cu}}_x{\mathrm{Bi}}_2{\mathrm{Se}}_3$, which is known to be a topological insulator at $x=0$. The superconductivity (SC) pairing of this compound is assumed to have broken spin-rotation symmetry, similar to that of the A-phase of ${}^3\mathrm{He}$ as suggested by recent nuclear-magnetic resonance experiments. For both spheroidal and corrugated cylindrical Fermi surfaces with the hexagonal warping terms, we show that the bulk SC gap is rather anisotropic; the minimum of the gap is negligibly small as compared to the maximum of the gap. This would make the fully gapped pairing effectively nodal. For a clean system, our results indicate the bulk of this compound to be a topological superconductor with the SABSs appearing inside the bulk SC gap. The zero-energy SABSs, which are Majorana fermions, together with the TSSs not gapped by the pairing, produce a zero-energy peak in the surface density of states (SDOS). The SABSs are expected to be stable against short-range nonmagnetic impurities, and the local SDOS is calculated around a nonmagnetic impurity. The relevance of our results to experiments is discussed.

The combination of the time-reversal-symmetric single-particle backscattering field (commonly known as Rashba spin-orbit coupling) and nonbackscattering electron interactions is generally expected to produce inelastic backscattering in one-dimensional helical electron liquids at the edge of two-dimensional topological insulators, as theoretically predicted in a number of works. An opposite conclusion of absent backscattering was reached in a recent work [H.-Y. Xie et al., Phys. Rev. Lett. 116, 086603 (2016)] for the “local” model of the backscattering field and interactions. Motivated to resolve this potential controversy, in the present work, we study backscattering effects employing fermionic perturbation theory and considering quite general forms of the backscattering field and electron interactions. We discover that backscattering effects are crucially sensitive to the locality properties of the backscattering field and electron interactions, to the symmetry of the latter, as well as to the presence or absence of the cutoff of the electron spectrum. We find that backscattering is indeed absent under the following assumptions: (i) local backscattering field; (ii.a) local or (ii.b) SU(2)-symmetric interactions; (iii) absent cutoff of the edge-state spectrum. However, violation of any of these conditions leads to backscattering. This also reconciles with the results based on the bosonization technique. We calculate the associated backscattering current, establish its low-bias scaling behavior, and predict a crossover between two different scaling regimes. The main implication of our findings is that backscattering of some magnitude is inevitable in a real system, although it could be quite suppressed for nearly local backscattering field and interactions.

The dynamical mean-field theory is used to study the three-orbital model of the $d^3$ compound ${\mathrm{SrRu}}_2{\mathrm{O}}_6$ both with and without explicitly including the O-$p$ states. Depending on the size of the Hund's coupling $J$, at low to intermediate temperatures we find solutions corresponding to Mott or correlated covalent insulators. The latter can explain the experimentally observed absence of Curie susceptibility in the paramagnetic phase. At high temperatures, a single phase with smoothly varying properties is observed. $d^3$ compound ${\mathrm{SrRu}}_2{\mathrm{O}}_6$ provides an ideal system to study the competition between the local moment physics and covalent bonding, since both effects are maximized, while SO interaction is found to play a minor role.

Recently, it was shown that quantum spin Hall insulator phase with a gap wide enough for practical applications can be realized in the ultrathin films constructed from two inversely stacked structural elements of trivial band insulator BiTeI. Here, we study the edge states in nanoribbons made of the ${\mathrm{Bi}}_2{\mathrm{Te}}_2{\mathrm{I}}_2$ sextuple layer (SL) and the electronic structure of the SL deposited on the natural BiTeI substrate as well as sandwiched between two thick BiTeI films. We show that the ${\mathrm{Bi}}_2{\mathrm{Te}}_2{\mathrm{I}}_2$ SL keeps nontrivial topological properties. We also demonstrate that ultrathin centrosymmetric films constructed in a similar manner from related material BiTeBr are trivial band insulators up to five-SL film thickness. In contrast to ${\mathrm{Bi}}_2{\mathrm{Te}}_2{\mathrm{I}}_2$ for which the stacking of nontrivial SLs in a three-dimensional (3D) limit results in a strong topological insulator (TI) phase, a 3D TI ${\mathrm{Bi}}_2{\mathrm{Te}}_2{\mathrm{Br}}_2$ is formed by trivial SLs. For the last material of the $\mathrm{BiTe}X$ ($X=\text{I}$,Br,Cl) series, BiTeCl, both 2D and 3D centrosymmetric phases are characterized by topologically trivial band structure.

We report results of simultaneous x-ray reflectivity and grazing incidence x-ray fluorescence measurements in combination with x-ray standing-wave-assisted depth-resolved near-edge x-ray absorption measurements to reveal new insights on chemical speciation of W in a ${\text{W-B}}_4\mathrm{C}$ multilayer structure. Interestingly, our results show the existence of various unusual electronic states for the W atoms especially those near to the surface and interface boundary of a thin film medium as compared to those of the bulk. These observations are found to be consistent with the results obtained using first-principles calculations. Unlike the conventional x-ray absorption measurements the present approach has an advantage that it permits the determination of the depth-resolved chemical nature of an element in the thin-layered materials at atomic length scale resolution.

At the interface between two-dimensional materials with different topologies, topologically protected one-dimensional states (also named zero-line modes) arise. Here, we focus on the quantum anomalous Hall-effect-based zero-line modes formed at the interface between regimes with different Chern numbers. We find that these zero-line modes are chiral and unilaterally conductive due to the breaking of time-reversal invariance. For a beam splitter consisting of two intersecting zero lines, the chirality ensures that a current can only be injected from two of the four terminals. Our numerical results further show that, in the absence of contact resistance, the (anti-)clockwise partitions of the currents from these two terminals are the same owing to the current conservation, which effectively simplifies the partition laws. We find that the partition is robust against the relative shift of Fermi energy but can be adjusted effectively by tuning the relative magnetization strengths at different regimes or relative angles between zero lines.

We investigate systematically the bulk and surface electronic structure of the candidate nodal-line semimetal CaAgAs by angle-resolved photoemission spectroscopy and density functional calculations. We observed a metallic, linear, non-$k_z$-dispersive surface band that coincides with the high-binding-energy part of the theoretical topological surface state, proving the topological nontriviality of the system. An overall downshift of the experimental Fermi level points to a rigid-band-like $p$ doping of the samples, due possibly to Ag vacancies in the as-grown crystals.

The low-energy behaviors of gapless double- and triple-Weyl fermions caused by the interplay of long-range Coulomb interaction and quenched disorder are studied by performing a renormalization group analysis. It is found that an arbitrarily weak disorder drives the double-Weyl semimetal to undergo a quantum phase transition into a compressible diffusive metal, independent of the disorder type and the Coulomb interaction strength. In contrast, the nature of the ground state of triple-Weyl fermion system relies sensitively on the specific disorder type in the noninteracting limit: The system is turned into a compressible diffusive metal state by an arbitrarily weak random scalar potential or $z$ component of random vector potential but exhibits stable critical behavior when there is only an $x$ or $y$ component of random vector potential. In case the triple-Weyl fermions couple to random scalar potential, the system becomes a diffusive metal in the weak interaction regime but remains a semimetal if Coulomb interaction is sufficiently strong. Interplay of Coulomb interaction and $x$, or $y$, component of random vector potential leads to a stable infrared fixed point that is likely to be characterized by critical behavior. When Coulomb interaction coexists with the $z$ component of random vector potential, the system flows to the interaction-dominated strong coupling regime, which might drive a Mott insulating transition. It is thus clear that double- and triple-Weyl fermions exhibit distinct low-energy behavior in response to interaction and disorder. The physical explanation of such distinction is discussed in detail. The role played by long-range Coulomb impurity in triple-Weyl semimetal is also considered. The main conclusion is that Coulomb impurity always drives the system to become a compressible diffusive metal, whereas Coulomb interaction tends to suppress the Coulomb impurity, rendering the robustness of the semimetal phase.

We study graphene quantum point contacts (QPCs) and imaging of the backscattering of the Fermi level wave function by potential introduced by a scanning probe. We consider both etched single-layer QPCs as well as the ones formed by bilayer patches deposited on the sides of the monolayer conducting channel. An atomistic tight-binding method is developed to effectively simulate an infinite graphene plane outside the QPCs using a computational box of a finite size. We demonstrate that in spite of the Klein phenomenon interference effects due to the backscattering at a circular $n-p$ junction induced by the probe potential are visible in spatial conductance maps.