We argue that the formation of Yu-Shiba-Rusinov excitations in proximitized thin films is largely mediated by a type of Andreev-bound state named after de Gennes and Saint-James. This is shown by studying an experimentally motivated model and computing the overlap of the wave functions of these two subgap states. We find the overlap stays close to unity even as the system moves away from weak coupling across the parity-changing quantum phase transition. Based on this observation, we introduce a single-site model of the bound state coupled to a quantum spin. The adequacy of this description is assessed by reintroducing the coupling to the continuum as a weak perturbation and studying its scaling flow using Anderson's poor man's scaling.

Pursuit of superconductivity in light-element systems at ambient pressure is of great experimental and theoretical interest. In this work, we combine a machine learning (ML) method with first-principles calculations to efficiently search for the energetically favorable ternary Ca-B-C compounds. Three layered borocarbides (stable $\mathrm{CaB}{\mathrm{C}}_5$ and metastable ${\mathrm{Ca}}_2\mathrm{B}{\mathrm{C}}_{11}$ and $\mathrm{Ca}{\mathrm{B}}_3{\mathrm{C}}_3$) are predicted to be phonon-mediated superconductors at ambient pressure. The stable $\mathrm{CaB}{\mathrm{C}}_5$ and the low-energy metastable ${\mathrm{Ca}}_2\mathrm{B}{\mathrm{C}}_{11}$ (with formation energy only 9.5 meV/atom above the convex hull) have a superconducting $T_c$ of 5.2 and 8.9 K, respectively. While the hexagonal $\mathrm{Ca}{\mathrm{B}}_3{\mathrm{C}}_3$ possesses a $T_c$ of 26.1 K, it is metastable with formation energy of 153 meV/atom above the convex hull. The ML-guided approach opens up a way for greatly accelerating the discovery of new high-$T_c$ superconductors.

A large class of type-I fracton models, including the X-cube model, have been found to be fixed points of the foliated renormalization group (RG). The system size of such foliated models can be changed by adding or removing decoupled layers of 2D topological states and continuous deformation of the Hamiltonian. In this paper, we study a closely related model—the Ising cage-net model—and find that this model is not foliated in the same sense. In fact, we point out certain unnatural restrictions in the foliated RG, and find that removing these restrictions leads to a generalized foliated RG under which the Ising cage-net model is a fixed point, and which includes the original foliated RG as a special case. The Ising cage-net model thus gives a prototypical example of the generalized foliated RG, and its system size can be changed either by condensing/uncondensing bosonic planon excitations near a 2D plane or through a linear-depth quantum circuit in the same plane. We show that these two apparently different RG procedures are closely related, as they lead to the same gapped boundary when implemented in part of a plane. Finally, we briefly discuss the implications for foliated fracton phases, whose universal properties will need to be reexamined in light of the generalized foliated RG.

We develop a model which incorporates both intra- and intervalley scatterings to master equation, to explore exciton valley coherence in monolayer ${\mathrm{WS}}_2$ subjected to magnetic field. For linearly polarized (LP) excitation accompanied with an initial coherence, our determined valley dynamics manifests the coherence decay being faster than the exciton population relaxation and agrees with experimental data by Hao et al. [Nat. Phys. 12, 677 (2016)]. Further, we reveal that magnetic field may quench the electron-hole (e-h) exchange-induced pure dephasing—a crucial decoherence source—as a result of lifting of valley degeneracy, allowing one to magnetically regulate valley coherence. In particular, at low temperatures for which the exciton-phonon (ex-ph) interaction is weak, we find that the coherence time is expected to attain ${\tau }_{\mathcal{C}}\sim 1$ ps, facilitating full control of qubits based on the valley pseudospin. For dark excitons, we demonstrate an emerging coherence even in the absence of initial coherent state, which has a long coherence time ($\sim 15$ ps) at low temperature. Our work provides an insight into tunable valley coherence and coherent valley control based on dark excitons.

The photoluminescence (PL) spectra of monolayer (1L) semiconductors feature peaks ascribed to different charge-carrier complexes. We perform diffusion quantum Monte Carlo simulations of the binding energies of these complexes and examine their response to electric and magnetic fields. We focus on quintons (charged biexcitons), since they are the largest free charge-carrier complexes in undoped and low doping transition-metal dichalcogenides (TMDs). We examine the accuracy of the Rytova-Keldysh interaction potential between charges by comparing the binding energies (BEs) of charge-carrier complexes in 1L-TMDs with results obtained using ab initio interaction potentials. Magnetic fields $<8\mathrm{T}$ change BEs by $\sim 0.2\mathrm{meV}{\mathrm{T}}^{-1}$, in agreement with experiments, with BE variations of different complexes being very similar. Our results will help identify charge complexes in the PL spectra of 1L semiconductors.

The magic-angle phenomenon of twisted bilayer graphene (tBG), i.e, the nontrivial topological flat bands with vanishing Fermi velocity at half filling, has aroused prominent attention on superconductivity, correlated insulators, orbital magnetism, etc. Several efforts have been made to unravel the generation mechanism of the magic-angle phenomenon in tBG. Herein, we show that the hybridizations between the conduction band (CB) and valence band (VB) from different monolayers are critically responsible for the flat bands with band inversion as a signature of the magic-angle phenomenon. The proposed new mechanism for the magic-angle phenomenon is verified by the reversion of irreducible representations of energy bands at $\mathrm{\Gamma }$ point. We also discuss the effects of VB-VB and CB-CB hybridizations on the band structures of tBG. The tight-binding results indicate that the VB-VB and CB-CB hybridizations from different layers play the role of the moiré potential in real space and reduce the bandwidth in the framework of nearly-free electron model. Our conclusions can also give the explanation on the absence of the magic-angle phenomenon even with bandwidth reductions in twisted bilayer ${\mathrm{MoS}}_2$ and BN.

Hourglass nodal loops have garnered significant attention due to their unique band dispersion; yet their existence in two-dimensional (2D) solid materials has been scarcely studied, particularly in spinless phonon systems. While recent solutions have been proposed for spin systems, none have been proposed for spinless 2D phonon systems. In this paper, we perform a thorough search of all 80 layer groups (LGs) and identify only three LGs capable of hosting hourglass nodal loop phonons. Using symmetry analysis, we discover the minimum symmetry requirement for hourglass nodal loop phonons. In addition to a horizontal glide mirror and the time-reversal symmetry, which are necessary for spinful systems, the existence of hourglass nodal loops in 2D spinless systems requires a fourfold rotation symmetry, which differs from that in 2D spinful systems. With the presence of the fourfold rotation symmetry, 2D hourglass nodal loop phonons always appear with a quadratic Weyl point located at the center of this loop. Using first-principles calculations, we identify a realistic material that hosts the hourglass nodal loop to demonstrate our symmetry analysis. Furthermore, we demonstrate that the nodal loop can transform into a half-hourglass-like dispersion while still persisting when uniaxial strain is applied to break the fourfold rotation symmetry. Simultaneously, the quadratic nodal point splits into two linear Weyl points with opposite chirality. Our work highlights the differences between identifying hourglass nodal loops in spinless and spinful symmetry requirements and offers an effective approach to studying materials with hourglass nodal loops in spinless systems.

We investigate the origin of the translational symmetry breaking in epitaxially grown single-layer graphene. Despite the surface morphology of graphene films influenced by the presence of mutually parallel SiC surface terraces, the far-infrared magnetoplasmon absorption is almost independent of the angle between the probing light polarization and the orientation of terraces. Based on a detailed analysis of the plasmon absorption line shape and its behavior in the magnetic field, supported by confocal Raman mapping and atomic force microscopy, we explain this discrepancy by spontaneously formed graphene microflakes. We further support our conclusions using data collected on artificially created graphene nanoribbons: we recognize similar plasmon origin in artificial ribbons and naturally formed grains. An unexpectedly large plasmon resonance redshift was observed in nanoribbons. In a hydrogen-intercalated sample (which does not contain the buffer), this redshift is quantitatively considered by a plasmon-plasmon interaction. This redshift is due to an interplay between the plasmon-plasmon coupling and the Coulomb screening by the buffer-induced interface states in nonintercalated samples featuring a buffer layer. This model determines the density of interface states in good agreement with experimentally reported values.

In recent years, the formation of exciton-polaritons in semiconductor materials has attracted major interest because of their exotic optical properties, allowing the study of many interesting physical phenomena, such as superfluids and Bose-Einstein condensation. Here, we demonstrate a double strong coupling regime in a perovskite and tungsten disulfide $\left(\mathrm{W}{\mathrm{S}}_2\right)$ monolayer using single and dual quasibound states in the continuum (QBIC). Except for supporting exciton resonance, bi(phenethylammonium)tetraiodoplumbate perovskite thin film is patterned as a grating to act as a single symmetry-protected bound state in the continuum (BIC) or dual BICs by harnessing its high index. A $\mathrm{W}{\mathrm{S}}_2$ monolayer is sandwiched inside the perovskite nanowire so that a single BIC can be strongly coupled to both exciton resonances of perovskite and $\mathrm{W}{\mathrm{S}}_2$ monolayers with a Rabi splitting of 380 and 42.2 meV. We found that the $Q$ factor and thickness of perovskite film play a central role in governing the coupling strength. In addition, we demonstrate that double strong coupling can be satisfied by using dual QBICs, where each QBIC is individually coupled to the exciton of either a perovskite or a $\mathrm{W}{\mathrm{S}}_2$ monolayer. The design approach achieves a Rabi splitting as high as 353.6 and 31.80 meV in the perovskite and the $\mathrm{W}{\mathrm{S}}_2$ monolayer, respectively. This is a demonstration of a double strong coupling regime using a single structure based on perovskite and $\mathrm{W}{\mathrm{S}}_2$ monolayers. The realization of double strong coupling paves the way for the emergence of polaritonic devices with advanced functionalities, such as dual wavelength polariton nanolasers.

The nitrogen-vacancy (NV) center in diamond has enabled studies of nanoscale nuclear magnetic resonance (NMR) and electron paramagnetic resonance with high sensitivity in small sample volumes. Most NV-detected NMR (NV-NMR) experiments are performed at low magnetic fields. While low fields are useful in many applications, high-field NV-NMR with fine spectral resolution, high signal sensitivity, and the capability to observe a wider range of nuclei is advantageous for surface detection, microfluidic, and condensed matter studies aimed at probing micro- and nanoscale features. However, only a handful of experiments above 1 T were reported. Herein, we report ${}^{13}\mathrm{C}$ NV-NMR spectroscopy at 4.2 T, where the NV Larmor frequency is 115 GHz. Using an electron-nuclear double resonance technique, we successfully detect NV-NMR of two diamond samples. The analysis of the NMR linewidth based on the dipolar broadening theory of Van Vleck shows that the observed linewidths from sample 1 are consistent with the intrinsic NMR linewidth of the sample. For sample 2 we find a narrower linewidth of 44 ppm. This work paves the way for new applications of nanoscale NV-NMR.

Bilayer graphene exhibits a rich phase diagram in the quantum Hall regime, arising from a multitude of internal degrees of freedom, including spin, valley, and orbital indices. The variety of fractional quantum Hall states between filling factors $1<\nu \le 2$ suggests, among other things, a quantum phase transition between valley-unpolarized and polarized states at a perpendicular electric-field $D^{*}$. We find that the behavior of $D^{*}$ with $\nu$ changes markedly as $B$ is reduced. At $\nu =2, D^{*}$ may even vanish when $B$ is sufficiently small. We present a theoretical model for lattice-scale interactions, which explains these observations; surprisingly, both repulsive and attractive components in the interactions are required. Within this model, we analyze the nature of the $\nu =2$ state as a function of the magnetic and electric fields and predict that valley coherence may emerge for $D\sim D^{*}$ in the high-$B$ regime. This suggests the system supports Kekulé bond ordering, which could, in principle, be verified via scanning tunneling measurements.

We consider the time-dependent transverse field Ising chain with time-periodic perturbations. Without perturbations, this model is one of the famous models that obeys the scaling in the adiabatic limit predicted by the quantum Kibble-Zurek mechanism (QKZM). However, it is known that when oscillations are added to the system, the nonperturbative contribution becomes larger and the scaling may break down even if the perturbation is small. Therefore, we analytically analyze the density of defects in the model and discuss how much the oscillations affect the scaling. As a result, although the nonperturbative contribution does not become zero in the adiabatic limit, the scaling does not change from the prediction of the QKZM. This indicates that the QKZM is robust to the perturbations.

We numerically examine the transport of skyrmions driven over weak random quenched disorder using a modified Thiele approach that includes the thermal softening of skyrmion pairwise interactions introduced by Wang et al. [Phys. Rev. Appl. 18, 044024 (2022)]. The depinning transition is elastic at low temperatures but becomes plastic with a reduced threshold at higher temperatures due to the competition between thermal creep and thermal softening, indicating a temperature-induced order to disorder transition into a glass state. The resulting nonmonotonic critical depinning forces and crossing of the velocity-force curves are similar to what is observed in the peak effect for type-II superconducting vortex lattices, where the softening of vortex-vortex interactions with temperature leads to an order-disorder transition. For low skyrmion densities the peak effect is absent since the system is always in a disordered state. We map the dynamical phase transition as a function of temperature, density, drive, and materials parameters, and show that the signatures are similar to those of the superconducting peak effect. Our results are consistent with recent experiments.

Two triangular lattice antiferromagnets ${\mathrm{Sr}}_3\mathrm{Co}{(\mathrm{Nb},\mathrm{Ta})}_2{\mathrm{O}}_9$ with an effective $j_{\mathrm{eff}}=1/2$ of ${\mathrm{Co}}^{2+}$ are synthesized and their magnetic properties are investigated via magnetization and heat capacity measurements. The leading in-plane antiferromagnetic exchange coupling is estimated to be $J/k_{\mathrm{B}}\simeq 4.7$ and 5.8 K, respectively. Both compounds feature two-step magnetic transitions at low temperatures [($T_{\mathrm{N}1}\simeq 1.47\mathrm{K}$ and $T_{\mathrm{N}2}\simeq 1.22\mathrm{K}$) and ($T_{\mathrm{N}1}\simeq 0.88\mathrm{K}$ and $T_{\mathrm{N}2}\simeq 0.67\mathrm{K}$), respectively], driven by weak easy-axis anisotropy. Under magnetic field, ${\mathrm{Sr}}_3{\mathrm{CoNb}}_2{\mathrm{O}}_9$ evinces a plateau at $1/3$ magnetization. Interestingly, the high-field magnetization of ${\mathrm{Sr}}_3{\mathrm{CoTa}}_2{\mathrm{O}}_9$ reveals an exotic regime (between $H_{\mathrm{S}1}$ and $H_{\mathrm{S}2}$) below the fully polarized state in which the heat capacity at low temperatures is governed by a power law ($C_{\mathrm{p}}\propto T^{\alpha }$) with a reduced exponent $\alpha \simeq 2$. These results demonstrate an unusual field-induced state with gapless excitations in the strongly frustrated magnet ${\mathrm{Sr}}_3{\mathrm{CoTa}}_2{\mathrm{O}}_9$. The complete $T\text{−}H$ phase diagram is discussed for both compounds.

Cavity optomechanics, a promising platform to investigate macroscopic quantum effects, has been widely used to study nonreciprocal entanglement with the Sagnac effect. Here, we propose an alternative way to realize nonreciprocal entanglement among magnons, photons, and phonons in a hybrid cavity-magnon optomechanics, where the magnon Kerr effect is used. We show that the Kerr effect gives rise to a magnon frequency shift and an additional two-magnon effect. Both of them can be tuned from positive to negative via tuning the magnetic field direction, leading to nonreciprocity. By tuning system parameters such as magnon frequency detuning or the coefficient of the two-magnon effect, bipartite and tripartite entanglements can be nonreciprocally enhanced. By further studying the defined bidirectional contrast ratio, we find that nonreciprocity in our system can be switched on and off, and can be engineered by the bath temperature. Our proposal not only provides a potential path to demonstrate nonreciprocal entanglement with the magnon Kerr effect, but also opens a direction to engineer and design diverse nonreciprocal devices in hybrid cavity-magnon optomechanics with nonlinear effects.

In this study, we successfully synthesized (${p\text{−}\mathrm{Py}\text{−}\mathrm{V}\text{−}p\text{−}\mathrm{F})}_2\left[\mathrm{Ni}{\left(\mathrm{hfac}\right)}_2\right]$, a verdazyl-based complex. Molecular orbital calculations revealed the formation of a two-dimensional spin lattice composed of spin $\frac{1}{2}$ and spin 1. The magnetization curve displayed a $\frac{1}{2}$ plateau representing full polarization of spin 1. We confirm that the increase in the magnetization curve after the $\frac{1}{2}$ plateau phase largely reflects the exchange couplings forming the effective spin-$\frac{1}{2}$ ladder composed of radicals. In the low-field and low-temperature regions, the magnetic properties are primarily attributed to spin 1 with effective interactions forming a spin-1 antiferromagnetic chain. Additionally, an anomalous change in the field derivative of the magnetization curve was observed near the plateau onset.

At low temperature and zero applied magnetic field, besides the equilibrium helical state, monoaxial chiral helimagnets have a continuum of helical states differing by the wave number of the modulation. The wave number of these states in units of the equilibrium state wave number is denoted here by $p$, and accordingly the corresponding states are called the $p$ states. In this work we study in detail the metastability of the $p$ states. The application of an external magnetic field in the direction of the chiral axis has a double effect: On the one hand, it introduces a conical deformation of the $p$ states, and, on the other hand, it destabilizes some of them, shrinking the range of $p$ in which the $p$ states are metastable. If a polarized current is applied along the chiral axis, then the $p$ states reach a steady moving state with a constant velocity proportional to the current intensity. Besides this dynamical effect, the polarized current also induces a conical deformation and reduces the range of stability of the $p$ states. The stability diagram in the plane applied field–applied current intensity has interesting features that, among other things, permits the manipulation of $p$ states by a combination of applied fields and currents. These features can be exploited to devise processes to switch between $p$ states. In particular there are $p$ states with negative $p$, opening the possibility to helicity switching. The theoretical feasibility of such processes, crucial from the point of view of applications, is shown by micromagnetic simulations. Analogous $p$ states exists in cubic chiral helimagnets and therefore similar effects are expected in those systems.

External magnetic fields conventionally suppress superconductivity, both by orbital and paramagnetic effects. A recent experiment has shown that, in a Bernal stacked bilayer graphene system, the opposite occurs—a finite critical magnetic field is necessary to observe superconducting features occurring in the vicinity of a magnetic phase transition. We propose an extraordinary electronic-correlation-driven mechanism by which this anomalous superconductivity manifests. Specifically, the electrons tend to avoid band occupations near high density of states regions due to their mutual repulsion. Considering the nature of spontaneous symmetry breaking involved, we dub this avoidance Stoner blockade. We show how a magnetic field softens this blockade, allowing weak superconductivity to take place, consistent with experimental findings. Our principle prediction is that a small reduction of the Coulomb repulsion would result in sizable superconductivity gains, both in achieving higher critical temperatures and expanding the superconducting regime. Within the theory we present, magnetic field and spin-orbit coupling of the Ising type have a similar effect on the Bernal stacked bilayer graphene system, elucidating the emergence of superconductivity when the system is proximitized to a ${\mathrm{WSe}}_2$ substrate. We further demonstrate in this paper the sensitivity of superconductivity to disorder in the proposed scenario. We find that a disorder that does not violate Anderson's theorem may still induce a reduction of $T_c$ through its effect on the density of states, establishing the delicate nature of the Bernal bilayer graphene superconductor.