The phase stability of icosahedral boron subselenide ${\mathrm{B}}_{12}{\left({\mathrm{B}}_{1-x}{\mathrm{Se}}_x\right)}_2$, where $0.5\le x\le 1$, is explored using a first-principles cluster expansion. The results shows that, instead of a continuous solid solution, ${\mathrm{B}}_{12}{\left({\mathrm{B}}_{1-x}{\mathrm{Se}}_x\right)}_2$ is thermodynamically stable as an individual line compound at the composition of ${\mathrm{B}}_{9.5}\mathrm{Se}$. The ground-state configuration of ${\mathrm{B}}_{9.5}\mathrm{Se}$ is represented by a mixture of ${\mathrm{B}}_{12}$(Se-Se), ${\mathrm{B}}_{12}$(B-Se), and ${\mathrm{B}}_{12}$(Se-B) with a ratio of 1:1:1, where they form a periodic $ABCABC\cdots$ stacking sequence of ${\mathrm{B}}_{12}$(Se-Se), ${\mathrm{B}}_{12}$(B-Se), and ${\mathrm{B}}_{12}$(Se-B) layers along the $c$ axis of the hexagonal conventional unit cell. The structural and electronic properties of the ground-state ${\mathrm{B}}_{9.5}\mathrm{Se}$ are also derived and discussed. By comparing the derived ground-state properties of ${\mathrm{B}}_{9.5}\mathrm{Se}$ to the existing experimental data of boron subselenide ${\mathrm{B}}_{\sim 13}\mathrm{Se}$, I proposed that the as-synthesized boron subselenide ${\mathrm{B}}_{\sim 13}\mathrm{Se}$, as reported in the literature, has the actual composition of ${\mathrm{B}}_{9.5}\mathrm{Se}$.

Bulk FeSe is superconducting with a critical temperature $T_c\cong 8$ K and ${\mathrm{SrTiO}}_3$ is insulating in nature, yet high-temperature superconductivity has been reported at the interface between a single-layer FeSe and ${\mathrm{SrTiO}}_3$. Angle-resolved photoemission spectroscopy and scanning tunneling microscopy measurements observe a gap opening at the Fermi surface below $\approx 60$ K. Elucidating the microscopic properties and understanding the pairing mechanism of single-layer FeSe is of utmost importance as it is a basic building block of iron-based superconductors. Here, we use the low-energy muon spin rotation/relaxation technique to detect and quantify the supercarrier density and determine the gap symmetry in FeSe grown on ${\mathrm{SrTiO}}_3$ (100). Measurements in applied field show a temperature-dependent broadening of the field distribution below $\sim 60$ K, reflecting the superconducting transition and formation of a vortex state. Zero-field measurements rule out the presence of magnetism of static or fluctuating origin. From the inhomogeneous field distribution, we determine an effective sheet supercarrier density $n_s^{2\text{D}}\simeq 6\times {10}^{14}{\mathrm{cm}}^{-2}$ at $T\to 0$ K, which is a factor of 4 larger than expected from ARPES measurements of the excess electron count per Fe of 1 monolayer FeSe. The temperature dependence of the superfluid density $n_s\left(T\right)$ can be well described down to $\sim 10$ K by simple $s$-wave BCS, indicating a rather clean superconducting phase with a gap of 10.2(1.1) meV. The result is a clear indication of the gradual formation of a two-dimensional vortex lattice existing over the entire large FeSe/STO interface and provides unambiguous evidence for robust superconductivity below 60 K in ultrathin FeSe.

Line nodes in the superconducting gap are known to be a source of Majorana flat bands (MFBs) on a surface. Here, we extend this relation to all symmetry-protected line nodes where an additional constraint arising from crystal symmetry destabilizes or hides the existence of MFBs. By establishing a one-to-one correspondence between group theoretical and topological classifications, we are able to classify the possible line-node-induced MFBs, including cases with (magnetic) nonsymmorphic space groups. Our theoretical analysis reveals MFBs in antiferromagnetic superconductors.

We report on the stabilization of the topological bimeron structures in confined geometries. The Monte Carlo simulations for a ferromagnet with a strong Dzyaloshinskii-Moriya interaction revealed the formation of a mixed skyrmion-bimeron phase at finite temperatures. The vacancy grid created in the spin lattice drastically changes the picture of the observed spin configurations and allows one to choose between the formation of a pure bimeron and skyrmion lattice. We found that the rhombic plaquette provides a natural environment for stabilization of the bimeron structures. Such a rhombic geometry can protect the topological state even in the absence of the magnetic field.

${\mathrm{Mn}}_2\mathrm{Au}$ is an important antiferromagnetic (AF) material for spintronics applications. Due to its very high Néel temperature of about 1500 K, some of the basic properties are difficult to explore, such as the AF susceptibility and the exchange constants. Experimental determination of these parameters is further hampered in thin films by the unavoidable presence of uncompensated and quasiloose spins on antisites and at interfaces. Using x-ray magnetic circular dichroism (XMCD), we measured induced perpendicular spin and orbital moments for a ${\mathrm{Mn}}_2\mathrm{Au}$(001) film in fields up to $\pm 8$ T. By performing these measurements at a low temperature of 7 K and at room temperature (RT), we were able to separate the loose spin contribution from the susceptibility of AF coupled spins. The value of the AF exchange constant obtained with this method for a 10-nm-thick ${\mathrm{Mn}}_2\mathrm{Au}$(001) film is $(22\pm 5)\mathrm{meV}$.

${\mathrm{C}}_2\mathrm{N}$ holey two-dimensional crystals, or ${\mathrm{C}}_2\mathrm{N}$-h2D, a recently synthesized carbon nitride layered material, show promising properties for electronic devices, highly selective molecular filters, and supercapacitors. Few studies have investigated the stacking order in ${\mathrm{C}}_2\mathrm{N}$-h2D, which is fundamental to determine its optical activity and plays an important role in its band gap and in the diffusion barrier for ions and molecules through its structure. In this work, we investigate the phonon stability of several bulk ${\mathrm{C}}_2\mathrm{N}$-h2D polytypes by using first-principles calculations. Among the polytypes addressed, only one does not display phonon instabilities and is expected to be observed in equilibrium. The electronic structure evolution of dynamically stable ${\mathrm{C}}_2\mathrm{N}$-h2D from monolayer to bilayer and to bulk is unveiled. The direct band gap at $\mathrm{\Gamma }$ can be decreased by 34% from monolayer to bulk, offering opportunities for tuning it in optoelectronics. In addition, the effective masses of both carriers become smaller as the number of layers increases, and their anisotropy along in-plane directions displayed in the monolayer is reduced, which suggest that the carrier mobility may be tuned as well. These effects are then explained according to the interaction of the orbitals in neighboring layers. The results presented here shed light on the geometry and electronic structure of an emerging layered material due to its specific stacking and increasing number of layers and suggest new perspectives for applications in optoelectronics.

Heavy fermions represent an archetypal example of strongly correlated electron systems which, due to entanglement among different interactions, often exhibit exotic and fascinating physics involving Kondo screening, magnetism, and unconventional superconductivity. Here we report a comprehensive study on the transport and thermodynamic properties of a cerium-based heavy-fermion compound ${\mathrm{CeBi}}_2$ which undergoes an antiferromagnetic transition at $T_N\sim 3.3\mathrm{K}$. Its high-temperature paramagnetic state is characterized by an enhanced heat capacity with Sommerfeld coefficient $\gamma$ over $200\mathrm{mJ}{\mathrm{mol}}^{-1}{\mathrm{K}}^{-2}$. The magnetization in the magnetically ordered state features a metamagnetic transition. Remarkably, a large negative magnetoresistance associated with the magnetism was observed in a wide temperature and field-angle range. Collectively, ${\mathrm{CeBi}}_2$ may serve as an intriguing system to study the interplay between the $f$ electrons and the itinerant Fermi sea.

We propose and model Floquet topological polaritons in semiconductor microcavities, using the interference of frequency-detuned coherent fields to provide a time-periodic potential. For arbitrarily weak field strength, where the Floquet frequency is larger than the relevant bandwidth of the system, a Chern insulator is obtained. As the field strength is increased, a topological phase transition is observed with an unpaired Dirac cone proclaiming the anomalous Floquet topological insulator. As the relevant bandwidth increases even further, an exotic Chern insulator with flatband is observed with unpaired Dirac cone at the second critical point. Considering the polariton spin degree of freedom, we find that the choice of field polarization allows oppositely polarized polaritons to either copropagate or counterpropagate in chiral edge states.

Radiation pressure, electrostriction, and photothermal forces have been investigated to evidence backaction, nonlinearities, and quantum phenomena in cavity optomechanics. We show here through a detailed study of the relative intensity of the cavity mechanical modes observed when exciting with pulsed lasers close to the GaAs optical gap that optoelectronic forces involving real carrier excitation and deformation potential interaction are the strongest mechanism of light-to-sound transduction in semiconductor GaAs/AlAs distributed Bragg reflector optomechanical resonators. We demonstrate that the ultrafast spatial redistribution of the photoexcited carriers in microcavities with massive GaAs spacers leads to an enhanced coupling to the fundamental 20-GHz vertically polarized mechanical breathing mode. The carrier diffusion along the growth axis of the device can be enhanced by increasing the laser power, or limited by embedding GaAs quantum wells in the cavity spacer, a strategy used here to prove and engineer the optoelectronic forces in phonon generation with real carriers. The wavelength dependence of the observed phenomena provide further proof of the role of optoelectronic forces. The optical forces associated with the different intervening mechanisms and their relevance for dynamical backaction in optomechanics are evaluated using finite-element methods. The results presented open the path to the study of hitherto seldom investigated dynamical backaction in optomechanical solid-state resonators in the presence of optoelectronic forces.

Molecular hydrogen in a scanning tunneling microscope (STM) junction has been found to enhance the lateral spatial resolution of the STM imaging, referred to as scanning tunneling hydrogen microscopy (STHM). Here we report atomic resolution imaging of 2- and 3-monolayer (ML) thick ZnO layers epitaxially grown on Ag(111) using STHM. The enhanced resolution can be obtained at a relatively large tip to surface distance and resolves a more defective structure exhibiting dislocation defects for 3-ML-thick ZnO than for 2 ML. In order to elucidate the enhanced imaging mechanism, the electric and mechanical properties of the hydrogen molecular junction (HMJ) are investigated by a combination of STM and atomic force microscopy. It is found that the HMJ shows multiple kinklike features in the tip to surface distance dependence of the conductance and frequency shift curves, which are absent in a hydrogen-free junction. Based on a simple modeling, we propose that the junction contains several hydrogen molecules and sequential squeezing of the molecules out of the junction results in the kinklike features in the conductance and frequency shift curves. The model also qualitatively reproduces the enhanced resolution image of the ZnO films.

The structure and growth of ultrathin Bi(110) islands were investigated on a Si(111)$\sqrt{3}\times \sqrt{3}\text{−}B$ substrate by scanning tunneling microscopy and scanning tunneling spectroscopy (STS). Both even- and odd-layer-height islands nucleated on a one-monolayer-thick wetting layer. The islands preferred the even layer heights over the odd layer heights with an area ratio of 3:1. A weak, long-range corrugation was observed to overlap on the atomic arrangement at the top of the islands. The average distance between the peaks of the corrugation oscillated in accordance with the alternation of even and odd layer heights. Nucleation of single- and double-layer terraces occurred on the islands with even layer heights but not on those with odd layer heights. The unit cell of the single-layer terrace was aligned with that of the underlying even-layer-height island. The inequality in the height preference and the height-dependent oscillation of the corrugation suggested that the even- and odd-layer-height islands possessed different structures. The dominance and stability against terrace nucleation of the even-layer-height islands were consistent with the theoretically predicted stability of the paired layer-stacked black-phosphorus (BP)-like structure for ultrathin Bi(110) films. The alignment of the unit cell at the terrace on the island and STS spectra suggested a BP-like/bulklike/BP-like sandwich structure for the odd-layer-height Bi(110) islands.

Topological nodal-line semimetals (TNLSMs) are characterized by symmetry-protected band crossings extending along one-dimensional lines in momentum space. The nodal lines exhibit a variety of possible configurations, such as nodal ring, nodal link, nodal chain, and nodal knot. Here, using angle-resolved photoemission spectroscopy, we observe nodal rings on the orthogonal $k_z=0$ and $k_x=0$ planes of the Brillouin zone in $\mathrm{Ti}{\mathrm{B}}_2$. The nodal rings connect with each other on the intersecting line Γ-K of the orthogonal planes forming a remarkable nodal-chain structure. Furthermore, we observe surface states (SSs) on the (001) cleaved surface, which are consistent with the calculated SSs considering the contribution from both Ti and B terminations. The calculated SSs have novel Dirac-cone-like band structures, which are distinct from the usual drumhead SSs with a single flatband proposed in other TNLSMs.

Laser-induced modification of transparent solids by intense femtosecond laser pulses allows fast integration of nanophotonic and nanofluidic devices with controlled optical properties. Experimental observations suggest that the local and dynamic nature of the interactions between light and the transient plasma plays an important role during fabrication. Current analytical models neglect these aspects and offer limited coverage of nanograting formation on dielectric surfaces. In this paper, we present a self-consistent dynamic treatment of the plasma buildup and its interaction with light within a three-dimensional electromagnetic framework. The main finding of this work is that local light-plasma interactions are responsible for the reorientation of laser-induced periodic plasma patterns with respect to the incident light polarization, when a certain energy density threshold is reached. Plasma reconfiguration occurs within a single laser pulse, on a femtosecond time scale. Moreover, we show that the reconfigured subwavelength plasma structures actually grow into the bulk of the sample, which agrees with the experimental observations of self-organized volume nanogratings. We find that mode coupling of the incident and transversely scattered light with the periodic plasma structures is sufficient to initiate the growth and self-organization of the pattern inside the medium with a characteristic half-wavelength periodicity.

We analyze emergent quantum multicriticality for strongly interacting, massless Dirac fermions in two spatial dimensions ($d=2$) within the framework of Gross-Neveu-Yukawa models, by considering the competing order parameters that give rise to fully gapped (insulating or superconducting) ground states. We focus only on those competing orders which can be rotated into each other by generators of an exact or emergent chiral symmetry of massless Dirac fermions, and break $\text{O}\left(S_1\right)$ and $\text{O}\left(S_2\right)$ symmetries in the ordered phase. Performing a renormalization-group analysis by using the $\epsilon =(3-d)$ expansion scheme, we show that all the coupling constants in the critical hyperplane flow toward a new attractive fixed point, supporting an enlarged $\text{O}(S_1+S_2)$ chiral symmetry. Such a fixed point acts as an exotic quantum multicritical point (MCP), governing the continuous semimetal-insulator as well as insulator-insulator (for example, antiferromagnet to valence bond solid) quantum phase transitions. In comparison with the lower symmetric semimetal-insulator quantum critical points, possessing either $\text{O}\left(S_1\right)$ or $\text{O}\left(S_2\right)$ chiral symmetry, the MCP displays enhanced correlation length exponents, and anomalous scaling dimensions for both fermionic and bosonic fields. We discuss the scaling properties of the ratio of bosonic and fermionic masses, and the increased dc resistivity at the MCP. By computing the scaling dimensions of different local fermion bilinears in the particle-hole channel, we establish that most of the four fermion operators or generalized density-density correlation functions display faster power-law decays at the MCP compared to the free fermion and lower symmetric itinerant quantum critical points. Possible generalization of this scenario to higher-dimensional Dirac fermions is also outlined.

We investigate the effects of crystal lattice vibrations on the dispersion of plasmons. The loss function of the homogeneous electron gas (HEG) in two and three dimensions is evaluated numerically in the presence of electronic coupling to an optical phonon mode. Our calculations are based on many-body perturbation theory for the dielectric function as formulated by the Hedin-Baym equations in the Fan-Migdal approximation. The coupling to phonons broadens the spectral signatures of plasmons in the electron-energy loss spectrum (EELS) and it induces the decay of plasmons on timescales shorter than 1 ps. Our results further reveal the formation of a kink in the plasmon dispersion of the two-dimensional HEG, which marks the onset of plasmon-phonon scattering. Overall, these features constitute a fingerprint of plasmon-phonon coupling in EELS of simple metals. It is shown that these effects may be accounted for by resorting to a simplified treatment of the electron-phonon interaction which is amenable to first-principles calculations.

We investigate nonequilibrium excitations and charge transport in charge-neutral graphene driven with dc electric field by using the nonequilibrium Green's-function technique. Due to the vanishing Fermi surface, electrons are subject to nontrivial nonequilibrium excitations such as highly anisotropic momentum distribution of electron-hole pairs, an analog of the Schwinger effect. We show that the electron-hole excitations, initiated by the Landau-Zener tunneling with a superlinear $IV$ relation $I\propto E^{3/2}$, reaches a steady state dominated by the dissipation due to optical phonons, resulting in a marginally sublinear $IV$ with $I\propto E$, in agreement with recent experiments. The linear $IV$ starts to show the sign of current saturation as the graphene is doped away from the Dirac point, and recovers the semiclassical relation for the saturated velocity. We give a detailed discussion on the nonequilibrium charge creation and the relation between the electron-phonon scattering rate and the electric field in the steady-state limit. We explain how the apparent Ohmic $IV$ is recovered near the Dirac point. We propose a mechanism where the peculiar nonequilibrium electron-hole creation can be utilized in a infrared device.

We study the Floquet phase diagram of two-dimensional Dirac materials such as graphene and the one-dimensional (1D) spin-$\frac{1}{2}XY$ model in a transverse field in the presence of periodic time-varying terms in their Hamiltonians in the low drive frequency ($\omega$) regime where standard $1/\omega$ perturbative expansions fail. For graphene, such periodic time-dependent terms are generated via the application of external radiation of amplitude $A_0$ and time period $T=2\pi /\omega$, while for the 1D $XY$ model, they result from a two-rate drive protocol with a time-dependent magnetic field and nearest-neighbor couplings between the spins. Using the adiabatic-impulse method, whose predictions agree almost exactly with the corresponding numerical results in the low-frequency regime, we provide several semianalytic criteria for the occurrence of changes in the topology of the phase bands (eigenstates of the evolution operator $U$) of such systems. For irradiated graphene, we point out the role of the symmetries of the instantaneous Hamiltonian $H\left(t\right)$ and the evolution operator $U$ behind such topology changes. Our analysis reveals that at low frequencies, topology changes of irradiated graphene phase bands may also happen at $t=T/3\text{and}2T/3$ (apart from $t=T$) showing the necessity of analyzing the phase bands of the system for obtaining its phase diagrams. We chart out the phase diagrams at $t=T/3,2T/3,\mathrm{and}T$, where such topology changes occur, as a function of $A_0$ and $T$ using exact numerics, and compare them with the prediction of the adiabatic-impulse method. We show that several characteristics of these phase diagrams can be analytically understood from results obtained using the adiabatic-impulse method and point out the crucial contribution of the high-symmetry points in the graphene Brillouin zone to these diagrams. We study the modes that can appear at the edges of a finite-width strip of graphene and show that the change in the number of such modes agrees with the change in the Chern number of bulk graphene as we go across a phase band crossing. Finally, we study the 1D $XY$ model with a two-rate driving protocol. After studying the symmetries of the system, we use the adiabatic-impulse method and exact numerics to study its phase band crossing which occurs at $t=T/2$ and $k=\pi /2$. We also study the end modes generated by such a drive and show that there can be anomalous modes whose Floquet eigenvalues are not equal to $\pm 1$. We suggest experiments to test our theory.

The effect of pressure on the structural, vibrational, and electronic properties of Mg-doped Cr bearing spinel $\mathrm{C}{\mathrm{o}}_{0.5}\mathrm{M}{\mathrm{g}}_{0.5}\mathrm{C}{\mathrm{r}}_2{\mathrm{O}}_4$ was studied up to 55 GPa at room-temperature using x-ray diffraction, Raman spectroscopy, electrical transport measurements, and ab initio calculations. We found that the ambient-pressure phase is cubic (spinel-type, $Fd\overline{3}m$) and underwent a pressure-induced structural transition to a tetragonal phase (space group $I\overline{4}m2$) above 28 GPa. The ab initio calculation confirmed this first-order phase transition. The resistivity of the sample decreased at low pressures with the existence of a low-pressure (LP) phase and started to increase with the emergence of a high-pressure (HP) phase. The temperature dependent resistivity experiments at different pressures illustrated the wide band gap semiconducting nature of both the LP and HP phases with different activation energies, suggesting a semiconductor-semiconductor transition at HP. No evidence of chemical decomposition or a semiconductor-metal transition was observed in our studies.

Transition metal-pnictide compounds have received attention for their tendency to combine magnetism and unconventional superconductivity. Binary CoAs lies on the border of paramagnetism and the more complex behavior seen in isostructural CrAs, MnP, FeAs, and FeP. Here we report the properties of CoAs single crystals grown with two distinct techniques along with density functional theory calculations of its electronic structure and magnetic ground state. While all indications are that CoAs is paramagnetic, both experiment and theory suggest proximity to a ferromagnetic instability. Quantum oscillations are seen in torque measurements up to 31.5 T and support the calculated paramagnetic Fermiology.