Topological phases have become an enabling role in exploiting new applications of nonlinear optics in recent years. Here we theoretically propose a valley photonic crystal resonator emulating topologically protected dissipative Kerr soliton combs. It is shown that topological resonator modes can be observed in the resonator. Moreover, we also simulate the dynamic evolution of the topological resonator with the injection of a continuous-wave pump laser. We find that the topological optical frequency combs evolve from Turing rolls to chaotic states, and eventually into single soliton states. More importantly, such dissipative Kerr soliton combs generated in the resonator are inborn topologically protected, showing robustness against sharp bends and structural disorders. Our design supporting topologically protected dissipative Kerr soliton combs could be implemented experimentally in on-chip nanofabricated photonic devices.

The correlation between the photoluminescence properties and excited state dynamics of perylene $\left({\mathrm{C}}_{20}{\mathrm{H}}_{12}\right)$ formed on a graphite (0001) substrate was investigated at the monolayer limit. Time-resolved two-photon photoemission spectroscopy was used to evaluate the lifetime of the excited state on the order of nanoseconds. On the molecular monolayer films, this unexpectedly long lifetime at the interface is significantly different from those typically observed for adsorption-induced electronic states, where ultrafast decays on the order of femtoseconds are dominant with electron/hole scattering. On the graphite (0001) surface, the standing molecular orientation of perylene indicates that the excited states are electronically decoupled from the substrate, which results in the suppression of the ultrafast nonradiative decay. As a result, orange light luminescence (610 nm) is observed with visible strength, which is ascribed to the deexcitation from the excited state of the dimeric molecular arrangement at the interface. Understanding the photoluminescence properties at the organic/electrode interface could be a key to the realization of organic optoelectronic thin-film devices with only a few molecular layers.

We find a class of Floquet topological phases exhibiting gap-dependent topological classifications in quantum systems with a dynamical space-time symmetry and an antisymmetry. This is in contrast to all existing Floquet topological phases protected by static symmetries, where the topological classification across all quasienergy gaps is characterized by the same Abelian group. We demonstrate this gap-dependent classification phenomenon using the frequency-domain formulation of the time-dependent Hamiltonian. Moreover, we provide an interpretation of the resulting Floquet topological phases using a frequency lattice with a decoration represented by color degrees of freedom on the lattice vertices. These colors correspond to the coefficient $N$ of the group extension of the system symmetry group $G$ along the frequency lattice, given by $N=Z\times H^1[A,M]$. The distinct topological classifications that arise at different energy gaps in its quasienergy spectrum are described by the torsion product of the cohomology group $H^2[G,N]$ classifying the group extension.

By means of density functional theory and constrained random phase approximation we analyze the band structure of ${\mathrm{Pb}}_9\mathrm{Cu}{\left({\mathrm{PO}}_4\right)}_6\mathrm{O}$ (named LK-99). Our data show that the lead-phosphate apatite LK-99 in the proposed Cu-doped structure is a semiconductor with a predominant charge-transfer nature, a result incompatible with a superconducting behavior. In order to understand the interesting electronic and magnetic properties of this compound, it will be necessary to study the actual response to doping, the possibility of alternative structural and stoichiometric (dis)order, and to clarify the magnetic interactions as well as their impact on the electronic structure.

The strongly correlated two-dimensional electron liquid within ${\mathrm{SmTiO}}_3/{\mathrm{SrTiO}}_3/{\mathrm{SmTiO}}_3$ quantum well structures exhibits a pseudogap phase when the quantum well width is sufficiently narrow. Using low-temperature transport and optical experiments that drive the quantum-well system out of equilibrium, we find evidence of mobile, long-lived, negatively charged quasiparticles, consistent with the idea that the pseudogap phase arises from strongly bound electron pairs. These experimental results establish a connection between quantum confinement and enhanced electron pairing, which may provide a pathway to higher-temperature superconductors.

${\mathrm{CsYbSe}}_2$ has an ideal triangular-lattice geometry with pronounced two-dimensionality, pseudospin-1/2 nature, and the absence of structural disorder. These excellent characteristics favor a quantum spin-liquid realization in this material. In this work, we applied quasihydrostatic compression methods to explore the structural behaviors. Our study reveals that ${\mathrm{CsYbSe}}_2$ undergoes a structural transition around 24 GPa, accompanied by a large volume collapse of $\mathrm{\Delta }\mathrm{V}$ /$V{}_0\sim 13\%$. The ambient hexagonal structure with the space group $P{6}_3/mmc$is lowered to the tetragonal structure ($P4$/mmm) under high pressure. Meanwhile, the color of ${\mathrm{CsYbSe}}_2$ changes gradually from red to black before the transition. Dramatic pressure-induced changes are clarified by the electronic structure calculations from the first principles, which indicate that the initial insulating ground state turns metallic in a squeezed lattice. These findings highlight Yb-based dichalcogenide delafossites as an intriguing material to probe novel quantum effects under high pressure.

Collective modes and dynamics of dense disordered materials such as water are not well understood, but nonlinear optics provides a sensitive probe to study polar modes in these materials. Here we report the hyper-Raman (HRS) light scattering spectrum measured for liquid ${\mathrm{D}}_2\mathrm{O}$ decomposed into contributions from transverse and longitudinal dipolar modes and octupolar modes, using the polarization dependence of HRS. Transverse HRS is observed for orientation and stretching modes, while longitudinal HRS is observed for translation, libration, and bending modes. The HRS observations indicate molecular correlation at distances $>200$ nm for all modes of orientation, libration, and vibration. The rocking/wagging, twisting, and translation modes for ${\mathrm{D}}_2\mathrm{O}$ molecules in the hydrogen bonded network are distinguished. The LO-TO splitting is $28{\mathrm{cm}}^{-1}$ for the libration mode and $16{\mathrm{cm}}^{-1}$ for the translation mode, and the relaxation time for libration modes is about 80 fs. The long-range correlation of the orientation and stretching modes is explained as the result of the dipole-dipole orientation correlation in a dipolar fluid. The long-range correlation of the longitudinal polarized libration and bending modes needs further study.

We consider an impurity in a one-dimensional weakly interacting Bose gas and analytically calculate the density profile of the Bose gas. Within the mean-field approximation, by increasing the distance from the impurity, the Bose gas density saturates exponentially fast to its mean thermodynamic-limit value at distances beyond the healing length. The effect of quantum fluctuations drastically changes this behavior, leading to a power-law decay of the density deviation from the mean density. At distances longer than the healing length and shorter than a new length scale proportional to the impurity coupling strength, the power-law exponent is 2, while at the longest distances the corresponding exponent becomes 3. The latter crossover does not exist in two special cases. The first one is realized for infinitely strongly coupled impurity; then the density deviation always decays with the exponent 2. The second special case occurs when the new length scale is smaller than the healing length, i.e., at weak impurity coupling; then the density deviation always decays with the exponent 3. The obtained results are exact in the impurity coupling strength and account for the leading order in the interaction between the particles of the Bose gas.

The old problem of the supercurrent in a long ballistic SNS Josephson junction is interpreted using a simple picture of the fermion energy levels in the normal and superconducting regions. We argue that a recent paper on the topic by Sonin contains erroneous results.

This is the Reply to the Comment by E. Thuneberg refuting the paper by E. B. Sonin [Phys. Rev. B 104, 094517 (2021)] as incorrect. The criticism is based on a misunderstanding of the goal and the approach of the paper and does not provide any reasonable argument that it is incorrect.

Non-Hermitian topological phenomena have gained much interest among physicists in recent years. In this paper, we expound on the physics of dissipatively coupled Su-Schrieffer-Heeger (SSH) lattices, specifically in systems with bosonic and electrical constituents. In the context of electrical circuits, we demonstrate that a series of resistively coupled LCR circuits mimics the topology of a dissipatively coupled SSH model. In addition, we propose a scheme to construct dissipatively coupled SSH lattices involving a set of noninteracting bosonic oscillators weakly coupled to engineered reservoirs of modes possessing substantially small lifetimes when compared to other system timescales. Further, by activating the coherent coupling between bosonic oscillators, we elucidate the emergence of nonreciprocal dissipative coupling, which can be controlled by the phase of the coherent interaction strength precipitating in phase-dependent topological transitions and skin effect. Our analyses are generic, apropos of a large class of systems involving, for instance, optical and microwave settings, while the circuit implementation represents the most straightforward of them.

Both temperature $\left(T\right)$ increase and/or application of an external magnetic field $\left(H\right)$ are able to induce ferromagnetism in otherwise antiferromagnetic FeRh. We present a theory that allows us to predict $H\text{−}T$ phase diagrams for FeRh, with a special emphasis on the canted antiferromagnetic phase. Both cases of in-plane and out-of-plane magnetic anisotropy in FeRh films are studied numerically, and the results of the simulations are compared with the results obtained via magneto-transport studies done in magnetic fields up to 34 T and x-ray magnetic circular dichroism measured at the Rh $L_{2,3}$ and Fe $K$ absorption edges in magnetic fields up to 17 T. The comparisons suggest examples of the simulated $H\text{−}T$ phase diagrams that are most suitable to fit the case of FeRh.

The recently synthesized molecular compound $\left[{\mathrm{Ce}}_2^{\mathrm{III}}{\mathrm{Ce}}^{\mathrm{IV}}{\mathrm{Mn}}_8^{\mathrm{III}}{\mathrm{O}}_8{\left({\mathrm{O}}_2\mathrm{CPh}\right)}_{18}\right({\mathrm{HO}}_2\mathrm{CPh}\left)\right]({\mathrm{Ce}}_3{\mathrm{Mn}}_8)$ has gained much interest due to its structural resemblance to the repeating unit of perovskite manganites and containing rich physics. We investigate the electronic and magnetic properties of the molecular analog of perovskite ferrites $\left[{\mathrm{Ce}}_2^{\mathrm{III}}{\mathrm{Ce}}^{\mathrm{IV}}{\mathrm{Fe}}_8^{\mathrm{III}}{\mathrm{O}}_8{\left({\mathrm{O}}_2\mathrm{CPh}\right)}_{18}\left({\mathrm{HO}}_2\mathrm{CPh}\right)\right]$ $\left({\mathrm{Ce}}_3{\mathrm{Fe}}_8\right)$, another fascinating family of magnetic oxides, using first-principles methods. Our results have shown that compared to ${\mathrm{Mn}}^{\mathrm{III}}$, the inclusion of the $d_{x^2-y^2}$ orbital in ${\mathrm{Fe}}^{\mathrm{III}}$ can exhibit a strong antiferromagnetic (AF) Fe-O-Fe superexchange interaction and a pronounced asymmetric behavior of the Fe-Ce-Fe ferromagnetic (FM) interaction involving the central Ce-$f$ orbitals. The combination of pairwise ${\mathrm{Fe}}_2^{\mathrm{III}}$ FM and AF exchange interactions result in $C$-type AF spin vector alignments that are found within the 3-D perovskites but are different from ${\mathrm{Ce}}_3{\mathrm{Mn}}_8$. The Wannier orbital analysis indicates that the edge Ce-$f$ orbitals, though farther away from the Fermi level, can hybridize with the central Ce-$f$ orbitals near the Fermi level, giving rise to a FM interaction between the neighboring Fe ions with relatively large distance. These findings can contribute to the theory of magnetism in complex magnetic molecules involving mixed valence and both $d$ and $f$ electrons.

Superconductor-insulator transition (SIT) driven by disorder and transverse magnetic field has been investigated in ultrathin MoN films by means of transport measurements and scanning tunneling microscopy and spectroscopy. Upon decreasing thickness, the homogeneously disordered films show increasing sheet resistance $R_s$, shift of the superconducting transition $T_c$ to lower temperatures with the 3-nm MoN being the last superconducting film and thinner films already insulating. Fermionic scenario of SIT is evidenced by applicability of the Finkel'stein's model, by the fact that $T_c$ and the superconducting gap $\mathrm{\Delta }$ are coupled with a constant ratio, and by the spatial homogeneity of the superconducting and electronic characteristics. The logarithmic anomaly found in the tunneling spectra of the nonsuperconducting films is further enhanced in increased magnetic field due to the Zeeman spin effects driving the system deeper into the insulating state and pointing also to fermionic SIT.

We study magnetic and charge order in the electron-doped high-$T_c$ cuprates based on the one-band Hubbard model with onsite $U$ and nearest-neighbor $V$ interactions. To investigate the interplay between the orders, we employ the Kotliar-Ruckenstein slave-boson method and analyze fluctuations descending from an antiferromagnetic parent state. Our analysis reveals incommensurate charge order whose ordering vector matches the doping dependence of resonant inelastic x-ray scattering (RIXS) measurements in ${\mathrm{Nd}}_{2-x}{\mathrm{Ce}}_x{\mathrm{CuO}}_4$ (NCCO). From our calculations of paramagnon dispersion as well as dynamical charge and spin structure factors, we reproduce the characteristic features of the RIXS signal.

Nonlinear topological insulators have garnered substantial recent attention as they have both enabled the discovery of new physics due to interparticle interactions, and may have applications in photonic devices such as topological lasers and frequency combs. However, due to the local nature of nonlinearities, previous attempts to classify the topology of nonlinear systems have required significant approximations that must be tailored to individual systems. Here, we develop a general framework for classifying the topology of nonlinear materials in any discrete symmetry class and any physical dimension. Our approach is rooted in a numerical $K$-theoretic method called the spectral localizer, which leverages a real-space perspective of a system to define local topological markers and a local measure of topological protection. Our nonlinear spectral localizer framework yields a quantitative definition of topologically nontrivial nonlinear modes that are distinguished by the appearance of a topological interface surrounding the mode. Moreover, we show how the nonlinear spectral localizer can be used to understand a system's topological dynamics, i.e., the time evolution of nonlinearly induced topological domains within a system. We anticipate that this framework will enable the discovery and development of novel topological systems across a broad range of nonlinear materials.

Manipulation of the valley degree of freedom provides a paradigm in quantum information technology. Here, through first-principles calculations and model analysis, we demonstrate that monolayer ${\mathrm{Cr}}_2\mathrm{COOH}$ MXene is a promising candidate material for valleytronics applications. We reveal that ${\mathrm{Cr}}_2\mathrm{COOH}$ is a ferromagnetic semiconductor and harbors valley features. Due to the simultaneous breaking inversion symmetry and time-reversal symmetry, the valleys are polarized spontaneously. Moreover, the valley polarization is sizable in both the valence and conduction bands, benefiting the observation of the anomalous valley Hall effect. More remarkably, the valley splitting can be effectively tuned by the magnetization direction, strain, and ferroelectric substrate. More interestingly, the ferroelectric substrate ${\mathrm{Sc}}_2{\mathrm{CO}}_2$ can not only regulate the MAE, but also tune valley polarization state. Our findings offer a practical way for realizing highly tunable valleys by multiferroic couplings.

We theoretically study the Andreev reflection (AR) in AA-stacked bilayer graphene-superconductor junction. AABG has a linear gapless energy band with two shifted Dirac cones and the electronic states are described by the cone indices. The results indicate that the property of AR strongly depends on the cone degree of freedom. In the absence of the interlayer potential difference, only intracone AR and normal reflection (NR) could occur, and the intercone process is forbidden. By adjusting the potential, the intracone AR can be specular AR (SAR) in one cone and it is retro-AR (RAR) in the other cone. The existence of the interlayer potential difference would lead to the intercone scattering. As a result, double ARs and double NRs can take place between the two cones. The intercone SAR could happen in a broad potential region. Furthermore, the intercone retro-NR (RNR) could happen as well. The switch between SAR and RAR, and the switch between specular NR (SNR) and RNR can be achieved by regulating the potential. Therefore, different cone carriers can be separated spatially based on the RAR and SAR. The cone-dependent Andreev conductance may be separately measured near the critical values where RAR crosses over to SAR.

Quantum interference is studied in a three-band model of pseudospin-1 fermions in the $\alpha -{\mathcal{T}}_3$ lattice. We derive a general formula for magnetoconductivity that predicts a rich crossover between weak localization (WL) and weak antilocalization (WAL) in various scenarios. Recovering the known results for graphene ($\alpha =0$), we remarkably discover that WAL is notably enhanced when one deviates slightly from the graphene lattice, i.e., when $\alpha >0$, even though Berry's phase is no longer $\pi$; this is attributed to the presence of multiple Cooperon channels. Upon further increasing $\alpha$, a crossover to WL occurs that is maximal for the case of the Dice lattice ($\alpha =1$). Our work distinctly underscores the role of nontrivial band topology in the localization properties of a two-dimensional lattice.