The optical bound state in the continuum (BIC) is characterized by an extremely high-quality factor resulting in a drastic enhancement of light-matter interaction phenomena. We study the optical response of a one-dimensional photonic crystal slab with Kerr focusing nonlinearity in the vicinity of BICs analytically and numerically. We predict a strong nonlinear response including multistable behavior, self-tuning of BICs to the frequency of incident waves, and breaking of symmetry protected BICs. We show that all of these phenomena can be observed in silicon photonic structure at the pump power of several $\mu \mathrm{W}/{\mathrm{cm}}^2$. We also analyze the modulation instability of the obtained solutions and the effect of the finite size of the structure on the stability. Our findings have strong implications for nonlinear photonics and integrated optical circuits.

We present an application of the extended stochastic Liouville equation [G. M. G. McCaul et al., Phys. Rev. B 95, 125124 (2017)], which gives an exact solution for the reduced density matrix of an open system surrounded by a harmonic heat bath. This method considers the extended system (the open system and the bath) being thermally equilibrated prior to the action of a time-dependent perturbation, as opposed to the usual assumption that the system and the bath are initially partitioned. Our method presents an exact technique capable of accounting for arbitrary parameter regimes of the model. Here, we present our first numerical implementation of the method in the simplest case of a Caldeira-Leggett representation of the bath Hamiltonian, and apply it to a spin-boson system driven from coupled equilibrium. We observe significant behaviors in both the transient dynamics and asymptotic states of the reduced density matrix not present in the usual approximation.

We investigate the rectification of heat current carried by electrons through a double quantum dot (DQD) system under a temperature bias. The DQD can be realized by molecules such as suspended carbon nanotubes and be described by the Anderson-Holstein model in the presence of electron-phonon interaction. Strong electron-phonon interaction can lead to formation of polaronic states in which electronic states are dressed by a phonon cloud. Dressed tunneling approximation (DTA), which is nonperturbative in dealing with strong electron-phonon interaction, is employed to obtain the heat current expression. In DTA, self-energies are dressed by a phonon cloud operator and are temperature dependent. The temperature dependency of the imaginary part of the dressed retarded self-energy gives rise to the asymmetry of the system and is the necessary condition of thermal rectification. On top of this, one can either tune DQD effective energy levels such that $|{\overline{\epsilon }}_1|\ne |{\overline{\epsilon }}_2|$ or have asymmetric dot-lead couplings to achieve thermal rectification. We numerically find that increasing electron-phonon coupling and reducing interdot coupling can improve the thermal rectification effect, while the electronic heat current is reduced.

Spin-orbit torque (SOT) induced by spin Hall and interfacial effects in heavy-metal (HM)/ferromagnetic (FM) bilayers has recently been employed to switch the magnetization direction using in-plane current injection. In this paper, using the Keldysh Green's function approach and first-principles electronic structure calculations we determine the fieldlike (FL) and dampinglike (DL) components of the SOT for the HM/Co ($\text{HM}=\text{Pt},\text{Pd}$) bilayers. Our approach yields the angular dependence of both the FL- and DL-SOT on the magnetization direction without assuming a priori their angular form. Decomposition of the SOT into the Fermi sea and Fermi surface contributions reveals that the SOT is dominated by the latter. Due to the large lattice mismatch between the Co and the HM we have also determined the effect of tensile biaxial strain on both the FL- and DL-SOT components. The calculated dependence of FL- and DL-SOT on the HM thickness is overall in good agreement with experiment. The dependence of the SOT with the position of the Fermi level suggests that the DL-SOT is dominated by the spin Hall effect of the bulk HM.

We examine the current-induced dynamics of a skyrmion that is subject to both structural and bulk inversion asymmetry. There arises a hybrid type of Dzyaloshinskii-Moriya interaction (DMI) which is in the form of a mixture of interfacial and bulk DMIs. Examples include crystals with symmetry classes ${\mathrm{C}}_n$ as well as magnetic multilayers composed of a ferromagnet with a noncentrosymmetric crystal and a nonmagnet with strong spin-orbit coupling. As a striking result, we find that, in systems with a hybrid DMI, the spin-orbit-torque-induced skyrmion Hall angle is asymmetric for the two different skyrmion polarities $(\pm 1$ given by out-of-plane core magnetization), even allowing one of them to be tuned to zero. We propose several experimental ways to achieve the necessary straight skyrmion motion (with zero Hall angle) for racetrack memories, even without antiferromagnetic interactions or any interaction with another magnet. Our results can be understood within a simple picture by using a global spin rotation which maps the hybrid DMI model to an effective model containing purely interfacial DMI. The formalism directly reveals the effective spin torque and effective current that result in qualitatively different dynamics. Our work provides a way to utilize symmetry breaking to eliminate detrimental phenomena as hybrid DMI eliminates the skyrmion Hall angle.

We discuss a mechanism of real-space spin-triplet pairing, which is an alternative to the one due to quantum paramagnon excitations, and demonstrate its applicability to ${\mathrm{UGe}}_2$. Both the Hund's rule ferromagnetic exchange and interelectronic correlations contribute to the same extent to the equal-spin pairing, particularly in the regime where the weak-coupling solution does not provide any. The theoretical results, obtained within the orbitally degenerate Anderson lattice model, excellently match the observed phase diagram for ${\mathrm{UGe}}_2$ with the coexistent ferromagnetic (FM1) and superconducting ($A_1$-type) phases. Additionally, weak $A_2$- and $A$-type paired phases appear in very narrow regions near the metamaganetic ($\mathrm{FM}2\to \mathrm{FM}1$) and FM1 to paramagnetic first-order phase-transition borders, respectively. The values of magnetic moments in the FM2 and FM1 states are also reproduced correctly in a semiquantitative manner. The Hund's metal regime is also singled out as appearing near the FM1-FM2 boundary.

We report ${}^{29}\mathrm{Si}$ and ${}^{101}\mathrm{Ru}$ NMR measurements on high-quality, single-crystal ${\mathrm{URu}}_2{\mathrm{Si}}_2$ samples with a residual resistivity ratio $\mathrm{RRR}\sim 70$. Our results show that the Si and Ru sites exhibit fourfold electronic symmetry around the $c$ axis in the hidden-order state. A previously observed twofold contribution of Si NMR linewidth is concluded to be due to extrinsic magnetic centers. Since the U and Si sites are aligned along the $c$ axis, we conclude further that the electronic state shows fourfold symmetry around the U site below the hidden-order transition. From this observed local symmetry, possible space groups for the hidden-order state are $P4/nnc$ or $I4/m$, based on group theoretical considerations. Since the order vector is considered to be $Q=\left(001\right)$, the hidden-order state is then found to be $P4/nnc$ with rank 5 odd parity, i.e., electric dotriacontapolar order.

A widely used approximation to the exchange-correlation functional in density functional theory is the local density approximation (LDA), typically derived from the properties of the homogeneous electron gas (HEG). We previously introduced a set of alternative LDAs constructed from one-dimensional systems of one, two, and three electrons that resemble the HEG within a finite region. We now construct a HEG-based LDA appropriate for spinless electrons in one dimension and find that it is remarkably similar to the finite LDAs. As expected, all LDAs are inadequate in low-density systems where correlation is strong. However, exploring the small but significant differences between the functionals, we find that the finite LDAs give better densities and energies in high-density exchange-dominated systems, arising partly from a better description of the self-interaction correction.

We study the dynamics of systems quenched through topological quantum phase transitions and investigate the behavior of the bulk and edge excitations with various quench rates. Specifically, we consider the Haldane model and checkerboard model in slow quench processes with distinct band-touching structures leading to topology changes. The generation of bulk excitations is found to obey the power-law relation Kibble-Zurek and Landau-Zener theories predict. However, an anti-Kibble-Zurek behavior is observed in the edge excitations. The mechanism of excitation generation on edge states is revealed, which explains the anti-Kibble-Zurek behavior.

The recently emerging laminar transition metal dichalcogenides provide an unprecedented platform for exploring fascinating layer-dependent properties. Determining the dependence of exciton-phonon coupling (EPC) on dimensionality would set a foundation for these exotic thickness-dependent phenomena. Here we report the observation of layer-dependent EPC between the $A_{1g}\left(\mathrm{\Gamma }\right)$ phonon and A′ exciton in $\mathrm{WS}{\mathrm{e}}_2$ down to the monolayer limit. Our results uncover that the strength of EPC increases dramatically with a descent of layer thickness. Compared with their bulk counterparts, the strength of EPC in monolayer $\mathrm{WS}{\mathrm{e}}_2$ is enhanced by nearly an order of magnitude. Furthermore, our work demonstrates that the giant EPC in the monolayer plays a prominent role in the exotic interlayer EPC of $\mathrm{WS}{\mathrm{e}}_2$/boron nitride heterostructures. The gigantic EPC in the two-dimensional limit provides a firm basis for understanding and manipulating the peculiar phenomena and novel optoelectronic applications based on atomically thin $\mathrm{WS}{\mathrm{e}}_2$.

The thermoelectric properties of the surface states in three-dimensional topological insulator nanowires are studied. The Seebeck coefficients $S_c$ and the dimensionless thermoelectrical figure of merit $ZT$ are obtained by using the tight-binding Hamiltonian combined with the nonequilibrium Green's-function method. They are strongly dependent on the gate voltage and the longitudinal and perpendicular magnetic fields. By changing the gate voltage or magnetic fields, the values of $S_c$ and $ZT$ can be easily controlled. At zero magnetic field and zero gate voltage, or at large perpendicular magnetic field and nonzero gate voltage, $ZT$ has a large value. Owing to the electron-hole symmetry, $S_c$ is an odd function of the Fermi energy while $ZT$ is an even function regardless of the magnetic fields. $S_c$ and $ZT$ show peaks when the quantized transmission coefficient jumps from one plateau to another. The highest peak appears while the Fermi energy is near the Dirac point. At zero perpendicular magnetic field and zero gate voltage, the height of the $n\mathrm{th}$ peak of $S_C$ is $\frac{k_B}{e}\text{ln}2/\left(\right|n|+1/2)$ and $\frac{k_B}{e}\text{ln}2/\left|n\right|$ for the longitudinal magnetic flux ${\varphi }_{\parallel }=0$ and $\pi$, respectively. Finally, we also study the effect of disorder and find that $S_c$ and $ZT$ are robust against disorder. In particular, the large value of $ZT$ can survive even at strong disorder. These characteristics (that $ZT$ has a large value, is easily regulated, and is robust against the disorder) are very beneficial for the application of thermoelectricity.

Edge functionalization of graphene nanoribbons with nitrogen atoms for various adatom configurations at armchair and zigzag edges are investigated. We provide comprehensive information on the electronic and magnetic properties and investigate the stability of the various systems. Two types of rippling of the nanoribbons, namely edge and bulk rippling depending on the sign of edge stress induced at the edge, are found. They are found to play the decisive role for the stability of the structures. We also propose a type of edge decoration in which every third nitrogen adatom at the zigzag edges is replaced by an oxygen atom. In this way, the electron count is compatible with a full aromatic structure, leading to additional stability and a disappearance of magnetism that is usually associated with zigzag nanoribbons.

Attenuated total reflection spectra of planar metal-dielectric structures obtained in the Otto and Kretschmann configurations exhibit resonance dips due to excitation of surface-plasmon polaritons. Although the dips have asymmetric line shapes, the asymmetry has not received much attention and the Lorentzian symmetric line shape has commonly been used in the spectral analyses. We analyze the reflectivity spectra of two- and three-layer plasmonic systems based on the electromagnetic and coupled-mode theories, and demonstrate that the asymmetric line shapes can be regarded as the Fano line shapes. In the resonance region, the spectra calculated as Fano line shapes using our approximate expressions are in excellent agreement with those obtained by rigorous electromagnetic calculations. The present formulations of the reflectivity allow us to gain insight into physical origins of characteristic behavior of the plasmonic resonances.

We calculate the zero-magnetic-field spin splitting in quantum wells formed in topologically nontrivial bulk crystals. The momentum-dependent Rashba coefficient is expressed via parameters of the microscopic model of topological insulators. It is shown that the spin-split quantum well electron spectrum contains counterpropagating spin-locked modes. These modes need to be taken into account to distinguish between the contributions to spin polarization originating from helical surface fermions and quantum well states of a bulk crystal.

In the present paper we propose a mechanism of the structural instability with a periodic charge ordering in two-dimensional isotropic conductors with a closed Fermi surface which completely excludes the conventional nesting mechanism. We show that the structural instability in such conductors may arise as a topological reconstruction under which the initially closed Fermi surface is transformed into an open one. We have found that the order parameter of the charge ordering ground state may exceed one hundredth of the Fermi energy. Furthermore, this charge ordering is a quantum phase transition with respect to the dimensionless coupling constant $\lambda$ related to the mechanism that drives the band reconstruction (e.g., electron-phonon coupling), with the critical value given by ${\lambda }_c={(1+2/\pi )}^{-1}$. Preliminary estimations show that the suggested mechanism can be the origin of density waves observed in such materials as high-$T_c$ cuprates or graphite intercalates.

The 8-Pmmn borophene is one kind of new elemental monolayer, which hosts anisotropic and tilted massless Dirac fermions (MDF). The planar p-n junction (PNJ) structure as the basic component of various novel devices based on the monolayer material has attracted increasing attention. Here, we analytically study the transport properties of anisotropic and tilted MDF across 8-Pmmn borophene PNJ. Similar to the isotropic MDF across graphene junctions, perfect transmission exists but its direction departures the normal direction of borophene PNJ induced by the anisotropy and tilt, i.e., oblique Klein tunneling. The oblique Klein tunneling does not depend on the doping levels in $N$ and $P$ regions of PNJ as the normal Klein tunneling but depends on the junction direction. Furthermore, we analytically derive the special junction direction for the maximal difference between perfect transmission direction and the normal direction of PNJ and clearly distinguish the respective contribution of anisotropy and tilt underlying the oblique Klein tunneling. In light of the rapid advances of experimental technologies, we expect the oblique Klein tunneling to be observable in the near future.

We study the stability of multiple conducting edge states in a topological insulator against all multiparticle perturbations allowed by time-reversal symmetry. We model a system as a multichannel Luttinger liquid, where the number of channels equals the number of Kramers doublets at the edge. We show that in a clean system with $N$ Kramers doublets there always exist relevant perturbations (either of a superconducting or charge density wave character) which always open $N-1$ gaps. In the charge density wave regime, $N-1$ edge states get localized. The single remaining gapless mode describes the sliding of a “Wigner-crystal”-like structure. Disorder introduces multiparticle backscattering processes. While single-particle backscattering turns out to be irrelevant, the two-particle process may localize this gapless mode. Our main result is that an interacting system with $N$ Kramers doublets at the edge may be either a trivial insulator or a topological insulator for $N=1$ or 2, depending on the density-density repulsion parameters, whereas any higher number $N>2$ of doublets gets fully localized by disorder pinning, irrespective of the parity issue.

Recently, an air-stable layered semiconductor ${\mathrm{Bi}}_2{\mathrm{O}}_2\mathrm{Se}$ was discovered to exhibit an ultrahigh mobility in transistors fabricated with its thin layers. In this work, we explored the mechanism that induces the high mobility and distinguishes ${\mathrm{Bi}}_2{\mathrm{O}}_2\mathrm{Se}$ from other semiconductors. We found that the electron donor states lie above the lowest conduction band. Thus, electrons get spontaneously ionized from donor sites (e.g., Se vacancies) without involving the thermal activation, different from the donor ionization in conventional semiconductors. Consequently, the resistance decreases as reducing the temperature as observed in our measurement, which is similar to a metal but contrasts to a usual semiconductor. Furthermore, the electron conduction channels locate spatially away from ionized donor defects (Se vacancies) in different van der Waals layers. Such a spatial separation can strongly suppress the scattering caused by donor sites and subsequently increase the electron mobility, especially at the low temperature. We call this high-mobility mechanism self-modulation doping, i.e., the modulation doping spontaneously happening in a single-phase material without requiring a heterojunction. Our work paves a way to design high-mobility semiconductors with layered materials.

The production of new sensors, transducers, and electronic components can benefit from the possibility of altering the electronic transport of metal-semiconductor-metal devices. Two-dimensional (2D) materials are extremely appealing for those new technologies. The alteration of the electronic transport can be determined by several phenomena, such as the piezoelectric effect, piezoresistive effect, and modulation of the Schottky barrier. In particular, ${\mathrm{MoS}}_2$, among other transition-metal dichalcogenides, is predicted to show a transition from a semiconductor to metal under strain. In this Rapid Communication, we present measurements on the modulation of electronic transport on suspended few-layer ${\mathrm{MoS}}_2$ ribbons under uniaxial tensile strain. Experimentally observed changes in the two-terminal $I\text{−}V$ curves can be explained in terms of a band-gap closing in the semiconductor. A maximum gauge factor of 240 is achieved for a three-layer ribbon. We also report on the fabrication process that allows us to apply high strains to suspended ${\mathrm{MoS}}_2$ ribbons, paving the way for future studies on the effect of strain in 2D materials.

Configuration interaction cluster calculations have been performed to investigate the electronic structure of orthorhombic distorted ${\mathrm{SrVO}}_3$ compound, where a considerable tetragonal crystal-field splitting of larger than $180\text{meV}$ for $t_{2g}$ orbital levels is suggested for the good consistence between the simulated and the experimental spectral line shape. This supports the crystal-field splitting scenario for the metal-insulator transition mechanism. We further calculate V $L_{23}$-edge absorption spectra of vanadium oxides with similar ${\mathrm{VO}}_6$ octahedral environment for formal $d$ electrons varying from $3d^0$ to $3d^3$, where we obtain energy parameters representing the local ground-state properties and provide the theoretical guidance for identifying the chemical valence and the crystal-field strength of a particular vanadium oxide. Our simulated polarization-dependent spectra at different tetragonal crystal-field values reveal a nonlinear orbital-lattice interaction. The orbital polarization $P$ reflecting the electron occupations in different $d_{xy}$ and $d_{xz\left(yz\right)}$ orbital symmetries shows strong sensitivity to small tetragonal crystal field, indicating the strongly correlated electron effect and further addressing the efficiency of strain engineering in functional SVO-based thin films and heterostructures.