We study the structure of the threading edge dislocations, or “elbows,” which are an essential component of the well-known herringbone reconstruction of the (111) surface of Au. Previous work had shown that these dislocations can be stabilized by long-range elastic relaxations into the bulk. However, the validity of the harmonic spring model that had been used to estimate the energies of the dislocations is uncertain. To enable a more refined model of the dislocation energetics, we have imaged the atomic structure of these dislocations using scanning tunneling microscopy. We find that the harmonic spring model does not adequately reproduce the observed structure. We are able to reproduce the structure, however, with a two-dimensional Frenkel-Kontorova (FK) model that uses a pairwise Morse potential to describe the interactions between the top layer Au atoms on a rigid substrate. The parameters of the potential were obtained by fitting the energy of uniaxially compressed phases, or “stripes”, computed with density functional theory, as a function of surface Au density. Within this model, the formation of the threading dislocations remains unfavorable. However, the large forces on the substrate atoms near the threading-dislocation cores, render the assumption of a completely rigid substrate questionable. Indeed, if the FK parameters are modified to account for the relaxation of just one more atomic layer, threading dislocations can, in principle, become favorable, even without bulk elastic relaxations. Additional evidence for a small elbow energy is that our computed change in the Au(111) surface stress tensor caused by the $\left(\sqrt{3}\times 22\right)$ reconstruction is considerably smaller than previous estimates.

We provide a self-consistent extension of the Lorentz reciprocity theorem and the Poynting theorem for media possessing electric and magnetic dipolar and quadrupolar responses related to electric and magnetic fields and field gradients thus corresponding to weak spatial dispersion. Using these two theorems, we respectively deduce the conditions of reciprocity and gainlessness and losslessness that apply to the various tensors mediating the interactions of these multipole moments and the associated fields and field gradients. We expect that these conditions will play an essential role in developing advanced metamaterial modeling techniques that include quadrupolar and spatially dispersive responses.

The target of valleytronics developments is to manipulate the valley degree of freedom and utilize it in microelectronics as charge and spin degrees of freedom. Based on first-principles calculations, we demonstrate that $M{\mathrm{Br}}_2$ $(M=\mathrm{Ru},\mathrm{Os})$ monolayers are intrinsically ferrovalley materials with large valley polarization up to 530 meV. Compressive strain can induce phase transitions in the materials from ferrovalley insulators to complete valley-polarized metals, called half-valley metals, in analogy to the concept of half metals in spintronics. With the increase of the strain, the materials become Chern insulators, whose edge states are chiral-spin-valley locking. The phase transition is caused by sequent band inversions of the $d_{xy}/d_{x^2-y^2}$ and $d_{z^2}$ orbitals at $K-$ and $K+$ valleys, analyzed based on a strained $\mathit{k}·\mathit{p}$ model. Our work provides a pathway for carrying out low-dissipation electronics devices with complete spin and valley polarizations.

We study polarons in the one-dimensional Bond-Peierls electron-phonon model in which phonons on bonds of a lattice modulate the hopping of electrons between lattice sites and contrast the results to those known for the breathing-mode Peierls problem. By inspecting the atomic limit, we show that polaronic dressing and mass enhancement of Bond-Peierls polarons depend on the momentum dependence of the phonons. For dispersionless phonons, Bond-Peierls polarons are perfectly localized in the atomic limit, unlike their breathing-mode counterparts because the carrier creates a string of phonon excitations that can only be annihilated via processes that retrace the carrier to its original site. However, inclusion of phonon dispersion leads to a nondivergent polaron mass even in the atomic limit and depending on the form of the phonon dispersion may lead to a transition to a non-zero-momentum ground state akin to that found in the breathing-mode Peierls model.

Upconversion (UC) in a molecular system is a process in which excitons produced by a multiple absorption of low-energy photons at long wavelengths undergo fusion to produce high energy excitons that consequently recombine to emit anti-Stokes shifted photons. Molecular systems for UC typically require a sensitizer. However, recent experiments show that UC in rubrene occurs even without the presence of the sensitizer. In this system, intermediate states are assumed to absorb photons at near-infrared wavelengths, which either absorb additional photons to populate the emissive singlet state or undergo fusion to generate triplets. The triplets can again undergo fusion to populate the excited singlet state. The final emission is around 600 nm. These models have been tested against the intensity dependence of the UC emission. Here, we have measured the kinetics of UC in rubrene by using intensity modulated photoexcitation at 800 nm to better understand the underlying mechanism. The models of UC that have been proposed so far do not agree with our measurements. Our results show that the yield of UC lags behind excitation significantly, indicating that triplet states are directly excited from the ground state, and their fusion, which depends on the population, becomes prominent after a certain build up time. While the intermediate states could form dynamically after the UC has been initiated and enhance the process, further sensitive absorption measurements are necessary to understand the role of the intermediate states in the process. Our results are important in finding new routes to enhance UC in pristine organic semiconductors for applications in photovoltaics, lasers, bioimaging, optical devices, and lighting.

This Letter reports the first experimental demonstration of progressive chaos development through four-wave mixing of spin wave amplified harmonics in magnonic active-ring oscillators. We find that the route to chaos represents a sequence of quasiperiodic regimes of oscillations and ultimately yields formation of a 3D torus in the phase space. The transition from the quasiperiodic oscillations to chaos proceeds through progressive spectrum stochastization during which the amplified harmonics gradually disappear.

Unconventional superconductivity is commonly linked to electronic pairing mechanisms, since it is believed that the conventional electron-phonon interaction (EPI) cannot cause sign-changing superconducting gap symmetries. Here, we show that this common understanding needs to be revised when one considers a more elaborate theory of electron-phonon superconductivity beyond standard approximations. We self-consistently solve the full-bandwidth, anisotropic Eliashberg equations including vertex corrections beyond Migdal's approximation assuming the usual isotropic EPI for cuprate, Fe-based, and heavy-fermion superconductors with nested Fermi surfaces. In the case of the high-$T_c$ cuprates we find a $d$-wave order parameter, as well as a nematic state upon increased doping. For Fe-based superconductors, we obtain $s_{\pm }$ gap symmetry, while for heavy-fermion ${\mathrm{CeCoIn}}_5$ we find unconventional $d$-wave pairing. These results provide a proof of concept that EPI cannot be excluded as a mediator of unconventional and of high-$T_c$ superconductivity.

The topological Anderson and Mott insulators are two phases that have so far been separately and widely explored beyond topological band insulators. Here we combine the two seemingly different topological phases into a system of spin-1/2 interacting fermionic atoms in a disordered optical lattice. We find that the topological Anderson and Mott insulators in the noninteracting and clean limits can be adiabatically connected without gap closing in the phase diagram of our model. Lying between the two phases, we uncover a disordered correlated topological insulator, which is induced from a trivial band insulator by the combination of disorder and interaction, as the generalization of topological Anderson insulators to the many-body interacting regime. The phase diagram is determined by computing various topological properties and confirmed by unsupervised and automated machine learning. We develop an approach to provide a unified and clear description of topological phase transitions driven by interaction and disorder. The topological phases can be detected from disorder-/interaction-induced edge excitations and charge pumping in optical lattices.

Layered transition-metal dichalcogenides (TMDs) constitute an emerging class of materials that provide researchers a platform to realize fundamental studies and to design promising optoelectronic devices. While defects are an almost unavoidable part of TMDs, they bring additional interesting properties absent in defect-free layers. Moreover, the controlled introduction of defects in TMDs makes it possible to tailor the electromagnetic properties of the materials. Here we report defect-induced properties of single-layer $\mathrm{Pd}{\mathrm{Se}}_2$ and demonstrate the emergence of magnetism at the nanoscale. Our first-principle calculations indicate that Se vacancies create in-gap defect states, which can be responsible for trapping of carriers. The complex square $V_{\mathrm{Pd}+4\mathrm{Se}}$ vacancy induces spin-polarized states with a total local magnetic moment of $2{\mu }_{\mathrm{B}}$ per defect, making it possible to introduce magnetization into $\mathrm{Pd}{\mathrm{Se}}_2$ and therefore expand the family of two-dimensional (2D) magnets. The defect formation energies are much lower compared to many other TMD materials that can explain the presence of a large number of Se defects after mechanical exfoliation of $\mathrm{Pd}{\mathrm{Se}}_2$ layers, while the central location of the Pd atoms preserves them from exfoliation-induced defect formation. The negatively charged vacancies are prone to form and in many cases demonstrate spin-polarized states. The small diffusion barrier of ${\mathrm{V}}_{\mathrm{Se}}$ in 2D $\mathrm{Pd}{\mathrm{Se}}_2$ leads to an easy room-temperature migration, while ${\mathrm{V}}_{\mathrm{Pd}}$ demonstrates a high diffusion barrier preventing its spontaneous migration. As a guide for experimentalists, we simulate and characterize scanning tunneling microscope images in valence and conduction states and estimate the electron-beam energy for a controllable production of various defect patterns. These intriguing findings make $\mathrm{Pd}{\mathrm{Se}}_2$ an ideal platform to study defect-induced phenomena.

Non-Hermitian systems with parity-time $\left(\mathcal{P}\mathcal{T}\right)$ symmetry have attracted increasing interest due to their intriguing properties and associated applications. Inspired by that, we propose here an acoustic anti-parity-time $\left(\mathrm{A}\text{−}\mathcal{P}\mathcal{T}\right)$ symmetric structure with metamaterials featuring balanced positive and negative indices, together with equal gain/loss. According to the derived scattering matrix, we demonstrate its extraordinary scattering properties. For example, the spontaneous phase transition of the scattering matrix is observed, by simply modulating the frequency, gain/loss, or geometric width. In the absence of gain/loss, the $\mathrm{A}\text{−}\mathcal{P}\mathcal{T}$ symmetric structure degrades into a pair of complementary media, resulting in bidirectional total transmission. At the zero and pole of the scattering matrix, the structure behaves as an acoustic coherent perfect absorber and equivalent laser, respectively, which can perfectly absorb two-port coherent incident waves and radiate two coherent output waves with extensive and equal amplitudes. Different from that in the $\mathcal{P}\mathcal{T}$ symmetric structure, these three effects based on the $\mathrm{A}\text{−}\mathcal{P}\mathcal{T}$ symmetric structure are all achieved in the symmetric phase of the scattering matrix, resulting in the symmetric intensity distributions. Besides, the coherent perfect absorption and lasing modes of the $\mathrm{A}\text{−}\mathcal{P}\mathcal{T}$ symmetric structure are continuous and symmetric in the parameter space, which may facilitate further experimental realizations. The proposed $\mathrm{A}\text{−}\mathcal{P}\mathcal{T}$ symmetric structure provides an alternative method to demonstrate the physics of the non-Hermitian Hamiltonian, and may offer an alternative approach to design acoustic functional devices such as absorbers, sensors, and amplifiers.

The quantum wetting transition in the one-dimensional transverse-field Ising model with random bonds is studied. Opposite boundary fields are applied at the two ends of the Ising chain. The interface between two domains with oppositely oriented magnetization is localized near the boundary or in the middle of the chain. The wetting transition refers to the phase transition of localization and delocalization of the interface. The tuning parameter $h_L$ is the boundary field at one end, e.g., the left end. At the transition point $h_L=h_w$, the wetting transition occurs. First, we study the wetting transition with one defect bond. It is a first-order transition, and the interface jumps from the left end of the chain to the defect bond at the transition point. Second, we study the wetting transition with two defect bonds and find that the weaker defect bond dominates the phase transition. The wetting transition is still first order, and the interface is localized at the left end of the chain or at the weaker defect bond. Lastly, the random bond case is studied. The random bonds have a rectangular distribution. The wetting transition is still first order. The finite-size effects are studied. The statistics of the transition points and the energy gaps are obtained. We study lattices with sizes $N=100$, 150, 200, 250, 300, and 350. The deviation of the phase transition points $h_w$ does not decrease as the lattice size increases. It is argued that the transition point is sample dependent, i.e., the variation in the transition point $h_w$ does not approach zero, even at the thermodynamic limit.

X-ray magnetic circular dichroism (XMCD) signals are usually absent in antiferromagnets. In this paper, we report the observation of spontaneous XMCD spectra originating from the inverse triangular spin structure, or the polarization of the cluster magnetic octupole, in the chiral antiferromagnet ${\mathrm{Mn}}_3\mathrm{Sn}$ thin film. The result is consistent with the recent theoretical predictions that the inverse triangular spin structure can give rise to finite XMCD signals in the absence of net magnetization [J. Phys. Soc. Jpn 89, 083703 (2020) and Phys. Rev. Lett. 126, 157402 (2021)].

The crystal structure and magnetic property of the single crystalline hexagonal rare-earth indium oxides ${\mathrm{GdInO}}_3$ have been studied by combing experiments and model calculations. The two inequivalent ${\mathrm{Gd}}^{3+}$ ions form the centered honeycomb lattice, which consists of honeycomb and triangular sublattices. The dc magnetic susceptibility and specific heat measurements suggest two antiferromagnetic phase transitions at $T_{\text{N1}}=2.3\mathrm{K}$ and $T_{\text{N2}}=1.02\mathrm{K}$. An inflection point is observed in the isothermal magnetization curve, which can be an indication of an up-up-down phase with a 1/3 magnetization plateau, further supported by our theoretical calculation. We also observe a large magnetic entropy change originated from the magnetic frustration in ${\mathrm{GdInO}}_3$. By considering a classical spin Hamiltonian, we establish the ground state phase diagram, which suggests that ${\mathrm{GdInO}}_3$ has a weak easy-axis anisotropy and is close to the equilateral triangular-lattice system. The theoretical ground-state phase diagram may be used as a reference in NMR, ESR, or $\mu \mathrm{SR}$ experiments in future.

We demonstrate robust ergodicity breaking in interacting many-body systems in arbitrary Euclidian dimension based on disorder-free many-body localization. Translationally invariant fine-tuned single-particle lattice Hamiltonians can host dispersionless (flat) bands only. Suitable short-range many-body interactions enforce complete suppression of particle transport due to local constraints and lead to ergodicity breaking termed many-body flat-band localization. However, heat might still flow between spatially locked charges. We demonstrate that heat transport is completely suppressed in one dimension. In higher dimensions we establish a universal bound on the filling fraction below which the heat transport is suppressed. The bound is based on the mapping to a classical percolation problem. Above the bound, the heat transport in percolation clusters is additionally affected by emerging bulk disorder and edge scattering induced by local constraints, which work in favor of arresting the heat flow and might keep the ergodicity breaking above the universal bound. We discuss explicit examples in one and two dimensions.

Spin-orbit coupling (SOC), which is inherent in a Dirac particle that moves under the influence of electromagnetic fields, manifests itself in a variety of physical systems, including nonrelativistic ones. For instance, it plays an essential role in spintronics developed in the past few decades, particularly by controlling spin current generation and relaxation. In the present work, by using an extended Caldeira-Leggett model, we elucidate how the interplay between spin relaxation and momentum dissipation of an open system of a single spin-$\frac{1}{2}$ particle with Rashba-type SOC is induced by the interactions with a spinless, three-dimensional environment. Staring from the path-integral formulation for the reduced density matrix of the system, we have derived a set of coupled nonlinear equations that consists of a quasiclassical Langevin equation for the momentum with a frictional term and a spin precession equation. The spin precesses around the effective magnetic field generated by both the SOC and the frictional term. It is found from analytical and numerical solutions to these equations that a spin torque effect included in the effective magnetic field causes a spin relaxation and that the spin and momentum orientations after a long time evolution are largely controlled by the Rashba coupling strength. Such a spin relaxation mechanism is qualitatively different from, e.g., the one encountered in semiconductors in which essentially no momentum dissipation occurs due to the Pauli blocking.

The effective field theory approach simplifies the perturbative computation of the ground-state energy of the diluted gas of repulsively interacting fermions allowing in the case of the unpolarized system to easily rederive the classic results up to the ${\left(k_{\mathrm{F}}{a}_0\right)}^2$ order (where $k_{\mathrm{F}}$ is the system's Fermi momentum and $a_0$ the $s$-wave scattering length) and (with more labor) to extend it up to the order ${\left(k_{\mathrm{F}}{a}_0\right)}^4$. The analogous expansion of the ground-state energy of the polarized gas of spin $1/2$ fermions is known only up to the $k_{\mathrm{F}}{a}_0$ order (where $k_{\mathrm{F}}$ stands for $k_{\mathrm{F}\uparrow }$ or $k_{\mathrm{F}\downarrow }$); the order ${\left(k_{\mathrm{F}}{a}_0\right)}^2$ contribution has been computed (analytically) only using a specific (hard-core type) interaction potential. Here we show that the effective field theory method also allows to easily obtain the order ${\left(k_{\mathrm{F}}{a}_0\right)}^2$ correction to the ground state of the polarized gas in a way applicable to all repulsive interactions.

Spin susceptibility and low-frequency dynamics of uranium $5f$ electrons have been investigated by nuclear magnetic resonance (NMR) on the ${}^{14}\mathrm{N}$ nuclei in paramagnetic and magnetically ordered phases for single crystalline and polycrystalline samples of uranium mononitride (UN). NMR spectra, shifts of the ${}^{14}\mathrm{N}$ NMR lines, and the spin-lattice relaxation times $T_1$ have been obtained in the temperature range $T=10–760\mathrm{K}$ in magnetic field $B=92.8\mathrm{kOe}$. It is shown that in the UN paramagnetic phase, temperature dependence of the ${}^{14}\mathrm{N}$ NMR line shift is proportional to the spin susceptibility of the uranium $5f$ electrons. Joint analysis of NMR and magnetic susceptibility data allows us to determine temperature dependence of spin fluctuation energy ${\mathrm{\Gamma }}_{\mathrm{nmr}}\left(T\right)$ of the uranium $5f$ electrons and to demonstrate that its temperature variation is close to $\mathrm{\Gamma }\left(T\right)\propto T^{0.5}$ dependence which is characteristic of the concentrated Kondo systems above the coherent state formation temperature. In the magnetically ordered UN phase the ${}^{14}\mathrm{N}$ NMR spectra consist of several lines that can be explained in terms of the model of type $I$ antiferromagnetic order corresponding to $1k$ structure in the presence of magnetic domains.

We examine the magneto-optical response of chiral multifold fermions. Specifically, we show that they are ideal candidates for observing the helical magnetic effect (HME) previously predicted for simple Weyl fermions. Unlike Weyl fermions, the HME is present in multifold fermions even in the simplest case where the low-energy dispersion is linear and spherically symmetric. In this ideal case, we derive an analytical expression for the HME and find it is proportional to the circular photogalvanic effect; for realistic parameters and accounting for the geometry of the setup, the HME photocurrent could be roughly the same order of magnitude as the circular photogalvanic effect observed in multifold fermions. Additional nonlinear and symmetry-breaking terms will ruin the quantization but not hurt the observation of the HME.

We report a local probe investigation of the frustrated antiferromagnet ${\mathrm{Pr}}_3{\mathrm{BWO}}_9$ with a distorted kagome lattice. The absence of magnetic order or spin-freezing is indicated by a spectral analysis down to 0.3 K and by specific heat measurements down to 0.09 K. The Knight shifts show an upturn behavior with the sample cooling down, which is further suppressed by the external field. For the spin dynamics, gapped spin excitation is observed from the temperature dependence of spin-lattice relaxation rates, with the gap size proportional to the applied magnetic field intensity. Comparatively, an unexpected sharp peak is observed in the nuclear spin-spin relaxation rate data at $T^{*}\sim 4\text{–}5$ K. These results indicate an unconventional persistent fluctuating paramagnetic ground state with antiferromagnetic collective spin excitations in the strongly frustrated spin system.

Propagating surface plasmons (PSPs) provide an important platform for designing various photoelectric devices such as nanometer-confined ultrafast electron sources. Here, we investigate the nanoscale photoemission from a focused PSP hot spot using time-of-flight photoemission electron microscopy. It is found that the induced photoelectrons from the focused PSP exhibit an obvious feature of the above threshold photoemission process, while the photoelectron spectra from a local surface plasmon (LSP) under the same illumination condition exhibit a characteristic of thermally assisted multiphoton photoemission. In addition, results show that the measured photoelectron yield from the focused PSP region is nearly one order of magnitude higher than that from the LSP region, and we found this is contrary to the calculation results that the yield from the LSP is much higher than that from the focused PSP when the local field intensity and plasmon lifetime of the two plasmonic modes are included. The large discrepancy in the photoelectron energy spectra, as well as in the measured and calculated photoelectron intensity between the focused PSP and LSP, can be attributed to the existence of different decay channels between the two plasmonic modes. This paper provides a deep understanding of the photoemission mechanism induced by plasmon effects, and it shows that the focused PSP is promising as a robust nanoscale electron source which can supply energetic and higher flux electron emission.