The possible existence and properties of different polytypes of graphene monoxide (GmO)—two-dimensional crystalline monolayers composed of equal numbers of C and O atoms—are investigated using density functional calculations. Structural parameters, electronic, and mechanical properties of the different phases, including the previously reported $\alpha$ and $\beta$ phases, are determined and compared. It is suggested that multiple phases of GmO can coexist in the same composite, but developing a synthesis process for single-phase GmO may be experimentally challenging. The phases exhibit band structures ranging from insulating to metallic. From an analysis of the calculated elastic moduli, it is concluded that all the GmO structures are softer than graphene and are likely to change their lattice parameters if present in a composite with graphene.

Exploring topological phases in interacting systems is a challenging task. We investigate many-body topological physics of interacting fermions in an extended Su-Schrieffer-Heeger (SSH) model, which extends the two sublattices of the SSH model into four sublattices and thus is dubbed the SSH4 model, based on the density-matrix renormalization-group numerical method. The interaction-driven phase transition from the topological insulator phase to the charge density wave (CDW) phase can be identified by analyzing the variations of the entanglement spectrum, entanglement entropies, energy gaps, CDW order parameter, and fidelity. We map the global phase diagram of the many-body ground state, which contains nontrivial topological insulator, trivial insulator, and CDW phases, respectively. In contrast to the interacting SSH model, in which the phase transitions to the CDW phase are argued to be first-order phase transitions, the phase transitions between the CDW phase and topologically trivial or nontrivial phases are shown to be continuous phase transitions. Finally, we also show the phase diagram of the interacting spinful SSH4 model, where the attractive (repulsive) on-site spin interaction amplifies (suppresses) the CDW phase. The models analyzed here can be implemented with ultracold atoms on optical superlattices.

${\mathrm{FePS}}_3$ is a layered magnetic van der Waals compound that undergoes a Mott insulator-metal transition under applied pressure. The transition has an associated change in the crystal symmetry and magnetic structure. Understanding the underlying physics of these transitions requires a detailed understanding of the crystal structure as a function of pressure. Two conflicting models have previously been proposed for the evolution of the structure with pressure. To settle the disagreement, we present a study of the pressure-dependent crystal structures using both single-crystal and powder x-ray diffraction measurements. We show unambiguously that the highest-pressure transition involves a collapse of the interplanar spacing, along with an increase in symmetry from a monoclinic to a trigonal space group, to the exclusion of other models. Our collected results are crucial for understanding high-pressure behavior in these materials and demonstrate a clear and complete methodology for exploring complex two-dimensional material structures under pressure.

We probe the bulk geometry of the Wegner Wilson Flow (WWF) in the context of many-body localization by addressing efficiency and bulk entanglement growth measures through approximating upper bounds on the boundary entanglement entropy. We connect these upper bounds to the Fubini-Study metric and clarify how a central quantity, the information fluctuation complexity, distinguishes bulk unitary rotation from entanglement production. We also give a short new proof of the small incremental entangling theorem in the absence of ancillas, achieving a dimensionindependent, universal factor of $c=2$.

We show how to enhance the detection sensitivity of magnon Kerr nonlinearity (MKN) in cavity magnonics. The considered cavity-magnon system consists of a three-dimensional microwave cavity containing two yttrium iron garnet (YIG) spheres, where the two magnon modes (one has the MKN, while the other is linear) in YIG spheres are simultaneously coupled to microwave photons. To obtain the effective gain of the cavity mode, we feed two input fields into the cavity. By choosing appropriate parameters, the coherent perfect absorption of the two input fields occurs, and the cavity-magnon system can be described by an effective non-Hermitian Hamiltonian. Under the pseudo-Hermitian conditions, the effective Hamiltonian can host the third-order exceptional point (EP3), where the three eigenvalues of the Hamiltonian coalesce into one. When the magnon frequency shift ${\mathrm{\Delta }}_K$ induced by the MKN is much smaller than the linewidths $\mathrm{\Gamma }$ of the peaks in the transmission spectrum of the cavity (i.e., ${\mathrm{\Delta }}_K\ll \mathrm{\Gamma }$), the magnon frequency shift can be amplified by the EP3, which can be probed via the output spectrum of the cavity. The scheme we present provides an alternative approach to measure the MKN in the region ${\mathrm{\Delta }}_K\ll \mathrm{\Gamma }$ and has potential applications in designing low-power nonlinear devices based on the MKN.

Control of dimensionality is a powerful tool to unlock hidden electronic and magnetic phases. In particular, reduced dimensionality in strongly correlated oxides leads to an intriguing “dead-layer” behavior that a transition from (ferromagnetic) metal to (antiferromagnetic) insulator occurs as the thickness approaches the two-dimensional limit. However, the origin of such transition has been a subject of debate that both the intrinsic dimensionality effect and the extrinsic disorder effect are proposed to be the driving force. Here, we reveal a transition from ferromagnetic metal with perpendicular magnetic anisotropy to antiferromagnetic insulator with in-plane magnetic anisotropy in ${\mathrm{SrRuO}}_3$ epitaxial films down to the monolayer limit. Experimentally, we demonstrate that ${\mathrm{SrRuO}}_3$ films below 3 unit cells become magnetic insulators and the spin easy axis changes to the in-plane ⟨110⟩ directions. First-principles calculations reveal that the interplay of the orbital-selective quantum confinement on Ru $4d$ orbitals and the oxygen octahedral rotation drives the ultrathin films from ferromagnetic metal to antiferromagnetic insulator, reorienting Ru spins from the perpendicular to the ⟨110⟩ directions. Our findings demonstrate how reduced dimensionality can tailor the magnetic state and provide significant advances in one of the debated topics in complex oxide heterostructures that dimensionality effect alone can be the driving force of dead-layer phenomenon in two-dimensional systems.

The relaxation of excitonic polaron and the transport properties in two-dimensional monolayers of transition metal dichalcogenides (TMDCs) such as $\mathrm{Mo}{\mathrm{S}}_2, \mathrm{Mo}{\mathrm{Se}}_2, \mathrm{W}{\mathrm{Se}}_2$, and $\mathrm{W}{\mathrm{S}}_2$ are investigated under the influence of a magnetic barrier and an electric field using the relaxation-time approximation and the Kubo formula. We find that the presence of magnetic barrier strengthens the electron-hole interaction and stabilizes the exciton-polaron while the electric field alters this stability. In addition, exciton-polaron is more relaxed as the electric field increases but due to the magnetic barrier it quickly returns into its equilibrium. Moreover, the electrical conductivity of TMDCs is favored by the electric field and a barrier of high magnetic lengths. $\mathrm{Mo}{\mathrm{Se}}_2$ is the compound that presents the highest relaxation time and electrical conductivity. The result indicates that the electrical conductivity grows when the system is relaxed. The thermoelectric power of TMDCs falls when the electric field increases, whereas it does not present a monotonic behavior in the magnetic barrier. It globally decreases for weak values of the magnetic length and enhances for high values. The highest thermoelectricity is obtained in $\mathrm{Mo}{\mathrm{Se}}_2$. A high optical conductivity is observed in TMDCs. The result shows that optical transitions rise as the magnetic strength of the barrier increases, but the electric field presents an opposite effect. The probability of absorb energy $\hslash \omega$ by the exciton-polaron steps up when the magnetic length and electric field increase. The highest value of optical conductivity and oscillator strength is observed for $\mathrm{Mo}{\mathrm{S}}_2$. We demonstrate that the magnetic barrier and electric field are suitable parameters which can be used to improve the performance of TMDCs materials.

A key problem in ion-solid interaction is the lack of experimental access to the dynamics of the processes. While it is clear that the mechanisms of interaction and sputtering depend on the kinetic and potential energy (sum of ionization energies) of the projectile, the importance and interplay of the various interaction mechanisms are unknown. Here, we have irradiated substrate-supported (Au, ${\mathrm{SiO}}_2$) monolayers of ${\mathrm{MoS}}_{\text{2}}$ with highly charged xenon ions (HCIs; charge state: $17+$ to $40+$), extracted the emitted neutral postionized Mo particles in a time-of-flight mass spectrometer, and determined their velocity distributions. We find two main contributions, one at high velocities and a second at lower velocities, and assign them to kinetic and potential effects, respectively. We show that for slow HCIs (5 keV) the interaction mechanisms leading to particle emission by electronic excitation and momentum transfer, respectively, are independent of each other, which is consistent with our atomistic simulations. Our data suggest that the predominant mechanism for potential sputtering is related to electron-phonon coupling, while nonthermal processes do not play a significant role. We anticipate that our work will be a starting point for further experiments and simulations to better understand the interplay of processes arising from $E_{\text{pot}}$ and $E_{\text{kin}}$.

The present study aims to model the optical response of plasmonic metasurfaces made of a periodic arrangement of metallic particles with arbitrary shape and subwavelength dimensions. By combining homogenization with quasistatic plasmonic eigenmode expansion, the metasurface is replaced by a zero-thickness interface associated with frequency-dependent effective susceptibilities. The resulting discontinuities of the fields are responsible for strong interaction with the incoming light at the resonances when the complex permittivity of the metal passes close to the real permittivity of an eigenmode. Our modeling provides a physical picture of resonances in plasmonic metasurfaces, and it allows for a huge decrease in the numerical cost of their computations. In addition, comparisons with direct numerics in two dimensions evidence its predictive force at any incidence, particle shape, and arrangement.

Opening a band gap and realizing static valley control have been long sought in graphene-based two-dimensional (2D) materials. Motivated by the recent success in synthesizing 2D materials passivated by Si-N layers, here, we propose two new graphene-based materials, 2D ${\mathrm{C}}_2\mathrm{SiN}$ and CSiN, via first-principles calculations. Monolayer ${\mathrm{C}}_2\mathrm{SiN}$ is metallic and realizes superconductivity at low temperatures. Monolayer CSiN enjoys excellent stability and a mechanical property. It is a semiconductor with a ternary valley structure for electron carriers. Distinct from existing valleytronic platforms, these valleys can be controlled by applied uniaxial strain. The valley polarization of carriers further manifests as a pronounced change in the anisotropic conductivity, which can be detected in simple electric measurement. The strong interaction effects also lead to large exciton binding energy and enhance the optical absorption in the ultraviolet range. Our paper opens a new route to achieve superconductivity, ternary valley structure, and a semiconductor with enhanced optical absorption in 2D materials.

We report a nanoscale infrared (IR) imaging and spectroscopy study of hot-electron plasmons in graphene, which are excited by the sharp metallic probe of the scattering-type scanning near-field optical microscope (s-SNOM) illuminated with a mid-IR femtosecond pulsed laser. We found the average electron temperature $\left(T_e\right)$ can reach as high as $\sim 1400\mathrm{K}$ within the pulse duration, which can be controlled by tuning the laser power. With s-SNOM, we monitored both the plasmon interference fringes and the hybrid plasmon-phonon resonances of graphene. When graphene is heavily doped, a higher $T_e$ leads to a smaller plasmon wavelength and a weaker plasmon-phonon resonance intensity. At the charge neutrality point, on the other hand, the plasmon-phonon resonance intensity is enhanced when $T_e$ increases. With quantitative modeling and theoretical analysis, we concluded that the observed plasmonic responses of hot electrons are governed by the temperature dependencies of chemical potential, electron scattering, and thermal carrier generation. The competition of these factors leads to distinct $T_e$ dependence of graphene plasmons at various doping levels.

We present a theory of superconductivity in magic-angle twisted bilayer graphene and analyze the superconducting phase diagram in the presence of the magnetic field. Namely, we consider a model of a granular array hosting localized states, which are hybridized via the delocalized fermions in the intergrain regions. We study a strong-coupling situation when the interactions lead to an incoherent state with preformed Cooper pairs inside the grains. The Andreev scattering among different grains manifests itself through the global phase-coherent superconducting state at lower temperatures. We demonstrate that a new phase transition between the preformed Cooper pairing state and the Larkin-Ovchinnikov-Fulde-Ferrell state might be induced by the spin pair-breaking effect of in-plane magnetic field. The upper critical magnetic field is shown to be enhanced in the strong-coupling case.

Electron beams in scanning transmission electron microscopy (STEM) exert forces and torques on study samples, with magnitudes that allow the controlled manipulation of nanoparticles (a technique called electron tweezers). Related theoretical research has mostly focused on the study of forces and linear momentum transfers from swift electrons (like those used in STEM) to nanoparticles. However, theoretical research on the rotational aspects of the interaction would benefit not only the development of electron tweezers but also other fields within electron microscopy such as electron vortices studies. Starting from a classical-electrodynamics description, we present a theoretical model, alongside an efficient numerical methodology, to calculate the angular momentum transfer from a STEM swift electron to a spherical nanoparticle. We show simulations of angular momentum transfers to aluminum, gold, and bismuth nanoparticles of different sizes. We found that the transferred angular momentum is always perpendicular to the system's plane of symmetry, displaying a constant direction for all the cases considered. In the simulations, the angular momentum transfer increased with the radius of the nanoparticle but decreased as the speed of the electron or the impact parameter increased. Also, the electric contribution to the angular momentum transfer dominated the magnetic one, being comparable only for high electron speeds (greater than 90% of the speed of light). Moreover, for nanoparticles with 1 nm radius of the studied materials, it was found that the small-particle approximation (in which the nanoparticles are modeled as electric point dipoles) is valid and accurate to compute the angular momentum transfer as long as the impact parameter is greater than four times the nanoparticle's radius and that the electron's speed exceeds 50% of the speed of light. We believe that these findings contribute to the understanding of rotational aspects present in STEM experiments, and might be useful for further developments in electron tweezers and other electron microscopy related techniques.

Recent theoretical studies have suggested that Kitaev physics and such effects as formation of a mysterious spin-liquid state can be expected not only in $\alpha \text{−}{\mathrm{RuCl}}_3$ and iridates, but also in conventional $3d$ transition metal compounds. Using dc and ac magnetometry, thermodynamic, and ${}^{23}\mathrm{Na}$ nuclear magnetic resonance (NMR) measurements we studied such a candidate material ${\mathrm{Na}}_3{\mathrm{Co}}_2{\mathrm{SbO}}_6$. A full phase diagram of ${\mathrm{Na}}_3{\mathrm{Co}}_2{\mathrm{SbO}}_6$ in a wide range of magnetic fields and temperatures is presented. The results demonstrate transformation of the antiferromagnetic structure under the external magnetic field, gradual development of the saturation phase, as well as evidence of gapped behavior in certain parts of the phase diagram.

A class of pyrochlore oxides, $A_2{\mathrm{Mo}}_2{\mathrm{O}}_7$ $(A=\mathrm{Ho},\mathrm{Y},\mathrm{Dy},\mathrm{Tb})$, with magnetic ions on corner-sharing tetrahedra is known to exhibit spin glass transitions without an appreciable amount of quenched disorder. Recently, a disorder-free theoretical model for such a system has been proposed which takes into account not only spins but also lattice distortions as dynamical variables [Mitsumoto, Hotta, and Yoshino, Phys. Rev. Lett. 124, 087201 (2020)]. In this paper, we develop and analyze an exactly solvable disorder-free mean-field model which is a higher-dimensional counterpart of the model. We find the system exhibits a complex free-energy landscape accompanying replica symmetry breaking through the spin-lattice coupling.