We investigate the effects of lithium intercalation in twisted bilayers of graphene, using first-principles electronic structure calculations. To model this system we employ commensurate supercells that correspond to twist angles of $7.{34}^{\circ }$ and $2.{45}^{\circ }$. From the energetics of lithium absorption we demonstrate that for low Li concentration the intercalants cluster in the AA regions with double the density of a uniform distribution. The charge donated by the Li atoms to the graphene layers results in modifications to the band structure that can be qualitatively captured using a continuum model with modified interlayer couplings in a region of parameter space that has yet to be explored either experimentally or theoretically. Thus, the combination of intercalation and twisted layers simultaneously provides the means for spatial control over material properties and an additional knob with which to tune moiré physics in twisted bilayers of graphene, with potential applications ranging from energy storage and conversion to quantum information.

We consider states bound at the flip of the electric field in buckled silicene. Along the electric flip lines a topological confinement is formed with the orientation of the charge current and the resulting magnetic dipole moment determined by the valley index. We compare the topological confinement to the trivial one that is due to a local reduction of the vertical electric field but without energy gap inversion. For the latter the valley does not protect the orientation of the magnetic dipole moment from inversion by external magnetic field. We demonstrate that the topologically confined states can couple and form extended bonding or antibonding orbitals with the energy splitting influenced by the geometry and the external magnetic field.

Solid-state single photon sources with Fourier transform (FT) limited lines are among the most crucial constituents of photonic quantum technologies and have been accordingly the focus of intensive research over the last several decades. However, so far, solid-state systems have only exhibited FT limited lines at cryogenic temperatures due to strong interactions with the thermal bath of lattice phonons. In this Rapid Communication, we report a solid-state source that exhibits FT limited lines measured in photoluminescence excitation (sub-100-MHz linewidths) from 3 to 300 K. The studied source is a color center in the two-dimensional hexagonal boron nitride and we propose that the center's decoupling from phonons is a fundamental consequence of the material's low dimensionality. While the center's luminescence lines exhibit spectral diffusion, we identify the likely source of the diffusion and propose to mitigate it via dynamic spectral tuning. The discovery of FT limited lines at room temperature, which once the spectral diffusion is controlled, will also yield FT limited emission. Our work motivates a significant advance towards room-temperature photonic quantum technologies and a different research direction in the remarkable fundamental properties of two-dimensional materials.

The coexistence of the strongly correlated $3d$ transition metal (TM) atom and the strong spin-orbit coupling of the $5d$ TM atom potentially hosts exotic physical phenomena. In the present work, we have studied the magnetism resulting from such a coexistence in ${\mathrm{La}}_2{\mathrm{CoIrO}}_6$ (LCIO), a representative of bulk $3d\text{−}5d$ double perovskites. In order to gain further insight into the effect of $\mathrm{Co}\text{−}d$ states on the $\mathrm{Ir}\text{−}d$ states, comparisons are carried out with the isostructural ${\mathrm{La}}_2{\mathrm{ZnIrO}}_6$, where the nonmagnetic Zn atom replaces the Co atom. An in-depth analysis of the magnetic states in the framework of first-principles calculations, using Landau theory and magnetic multipole analysis, shows that the magnetism at the two constituent TM atoms in LCIO is driven by two different magnetic order parameters, viz., the spin moment as the primary order parameter responsible for the broken time-reversal state in Co and the higher-order multipole: the triakontadipole for the Ir magnetic state. A tight-binding analysis with the $\mathrm{Ir}\text{−}{t}_{2g}$ orbitals further indicates that the $\mathrm{Ir}\text{−}d$ states are hardly affected by the $\mathrm{Co}\text{−}d$ states, in agreement with the multipole analysis. The computed hierarchy of the relevant multipoles in the present work can be probed in neutron diffraction measurements, motivating further experiments in this direction.

According to the Landau criterion, a phase transition should be first order when cubic terms of order parameters are allowed in its effective Ginzburg-Landau free energy. Recently, it was shown by renormalization-group (RG) analysis that continuous transition can happen at putatively first-order $Z_3$ transitions in two-dimensional (2D) Dirac semimetals and such non-Landau phase transitions were dubbed “fermion-induced quantum critical points” (FIQCPs) [Z.-X. Li, Y.-F. Jiang, S.-K. Jian, and H. Yao, Nat. Commun. 8, 314 (2017)]. The RG analysis, controlled by the $1/N$ expansion with $N$ the number of flavors of four-component Dirac fermions, shows that the FIQCP occurs for $N\ge N_c$. Previous quantum Monte Carlo simulations of a microscopic model of $\mathrm{SU}(N$) fermions on the honeycomb lattice showed that the FIQCP occurs at the transition between Dirac semimetals and Kekule valence bond solids for $N\ge 2$. However, the precise value of the lower bound $N_c$ has not been established. Especially, the case of $N=1$ has not been explored by studying microscopic models so far. Here, by introducing a generalized $\mathrm{SU}(N$) fermion model with $N=1$ (namely, spinless fermions on the honeycomb lattice), we perform large-scale sign-problem-free Majorana quantum Monte Carlo simulations and find convincing evidence of the FIQCP for $N=1$. Consequently, our results suggest that the FIQCP can occur in 2D Dirac semimetals for all positive integers $N\ge 1$.

Wave function scars refer to localized complex patterns of enhanced wave function probability distributions in a quantum system. Existing experimental studies of wave function scars concentrate nearly exclusively on nonrelativistic quantum systems. Here we present a combined experimental and theoretical study of a relativistic quantum cavity system realized by etching a graphene sheet. The conductance of the graphene cavity has been measured as a function of the back gate voltage (or the Fermi energy) and the magnetic field applied perpendicularly to the graphene sheet, and characteristic conductance contour patterns are observed at low temperatures. In particular, two types of high-conductance contour lines, i.e., straight and paraboliclike high-conductance contour lines, are found in the measurements. The theoretical calculations are performed within the framework of the tight-binding approach and Green's function formalism. Characteristic high-conductance contour features similar to those in the experiments are found in the calculations. Specifically, the equally spaced, parallel, straight high-conductance contour lines signify the persistence of relativistic quantum scars. The wave functions calculated at points selected along such a straight conductance contour line are found to be dominated by a chain of scars of high-probability distributions arranged as a necklace following the shape of the cavity, and the current density distributions calculated at these point are dominated by an overall vortex in the cavity. These characteristics are found to be insensitive to increasing magnetic field. However, the wave function probability distributions and the current density distributions calculated at points selected along a paraboliclike contour line show a clear dependence on increasing magnetic field, and the current density distributions at these points are characterized by the complex formation of several localized vortices in the cavity. Our work brings insight into quantum chaos in relativistic particle systems and should greatly stimulate experimental and theoretical efforts in this still emerging field.

We report microwave-power-driven strong magnon-magnon coupling in $\mathrm{N}{\mathrm{i}}_{80}\mathrm{F}{\mathrm{e}}_{20}$ nanocross array with large anticrossing gaps up to 1.03 GHz. We also observe microwave-driven large nonlinear shift in ferromagnetic resonance (FMR) frequency with its sign dependent on the strength of bias magnetic field. A drastic enhancement of inter-nanocross-dynamic dipolar interaction results in the anticrossing, while variation in internal spin texture leads to the nonlinear FMR shift. The tunable coupling strength and nonlinearity by microwave power ushers externally controlled nonlinear magnonic devices.

We study the dynamics of domain walls (DWs) in a metallic, ferromagnetic nanowire, focusing on inertial effects on the DW due to interaction with a conduction electron bath. We develop a Keldysh collective coordinate technique to describe the effect of conduction electrons on rigid magnetic structures. The effective Lagrangian and Langevin equations of motion for a DW are derived microscopically, including the full response kernel which is nonlocal in time. The DW dynamics is described by two collective degrees of freedom: position and tilt angle. The coupled Langevin equations therefore involve two correlated noise sources, leading to a generalized fluctuation-dissipation theorem (FDT). The DW response kernel due to electrons contains two parts: one related to dissipation via FDT and another reactive part. We prove that the latter term leads to a mass for both degrees of freedom, even though the intrinsic bare mass is zero. The electron-induced mass is present even in a clean system without pinning or specifically engineered potentials. The resulting equations of motion contain rich dynamical solutions and point toward a way to control domain wall motion in metals via the electronic system properties. We discuss two observable consequences of the mass, hysteresis in the DW dynamics, and resonant response to ac current.

Period tripling in driven quantum oscillators reveals unique features absent for linear and parametric drive, but generic for all higher-order resonances. Here we focus at zero temperature on the relaxation dynamics towards a stationary state starting initially from a domain around a classical fixed point in phase space. Beyond a certain threshold for the driving strength, the long-time dynamics is governed by a single time constant that sets the rate for switching between different states with broken time translation symmetry. By analyzing the lowest eigenvalues of the corresponding time evolution generator for the dissipative dynamics, we find that near the threshold the gap between these eigenvalues nearly closes. The closing becomes complete for a vanishing quantum parameter. We demonstrate that this behavior, reminiscent of a quantum phase transition, is associated with a transition from a stationary state which is localized in phase space to a delocalized one. We further show that switching between domains of classical fixed points happens via quantum activation, however, with rates that differ from those obtained by a standard semiclassical treatment. As period tripling has been explored with superconducting circuits mainly in the quasiclassical regime recently, our findings may trigger new activities towards the deep quantum realm.

In a closed system, the total number of particles is fixed. We ask how much does this conservation law restricts the amount of entanglement that can be created. We derive a tight upper bound on the bipartite entanglement entropy in closed systems, and find what a maximally entangled state looks like in such a system. Finally, we illustrate numerically on an isolated system of one-dimensional fermionic gas, that the upper bound can be reached during its unitary evolution, when starting in a pure state that emulates a thermal state with high enough temperature. These results are in accordance with current experiments measuring Rényi-2 entanglement entropy, all of which employ a particle-conserving Hamiltonian, where our bound acts as a loose bound, and will become especially important for bounding the amount of entanglement that can be spontaneously created, once a direct measurement of entanglement entropy becomes feasible.

In this paper, we investigate the enhancement of the antiferromagnetic damping in the sintered NiO-HM (where HM is heavy metal) granular systems having NiO/HM interfaces, where $\mathrm{HM}=\mathrm{Pt}$ or Pd. Under the assumption of the spin pumping model, we derive the effective interfacial damping conductance $g_{\mathrm{eff}}$, the parameter which characterizes the upper-bound estimate of the spin pumping effect, to be $12\pm 1\mathrm{n}{\mathrm{m}}^{-2}$ and $5\pm 1\mathrm{n}{\mathrm{m}}^{-2}$ for the NiO/Pt and the NiO/Pd interfaces, respectively, at room temperature. $g_{\mathrm{eff}}$ experimentally derived in this study are an important milestone in antiferromagnetic spintronics, giving a guideline for various spin current transfer and spin interaction phenomena with antiferromagnets where the spin mixing conductance is involved.

The factors that affect the thermal conductivity of semiconductors is a topic of great scientific interest, especially in relation to thermoelectrics. Key developments have been the concept of the phonon-glass-electron-crystal (PGEC) and the related idea of rattling to achieve this. We use first-principles phonon and thermal conductivity calculations to explore the concept of rattling for stoichiometric-ordered half-Heusler compounds. These compounds can be regarded as filled zinc blende materials, and the filling atom could be viewed as a rattler if it is weakly bound. We use two simple metrics, one related to the frequency and the other to bond frustration and anharmonicity. We find that both measures correlate with thermal conductivity. This suggests that both may be useful in screening materials for low thermal conductivity.

In this work, the DC Josephson effect is investigated for a superconductor-graphene-superconductor junction in both short- and long-junction regimes. The electric transport properties are calculated while taking into account the contribution of the discrete and continuous energy spectrum. In our approach, the phase dependence of the critical current is calculated at arbitrary temperature and doping level, which generalizes previous results. We show that critical current $I_c$ and skewness $S$ exhibit critical points as a function of graphene doping $E_F$, which can be explained by Klein resonances in graphene. We give a general characterization of $S$ vs $I_c$ curves while fixing temperature or doping level. When the temperature dependence of $I_c$ is analyzed, we find differences with respect to conventional Josephson junctions, given that there is a relevant doping effect. In the long-junction regime with $E_F$ far away from the Dirac point, the $I_c$ vs $T$ curve may exhibit an exponential decay law, which has been measured recently. We report the temperature dependence of $S$ in the whole range of temperature, and our approach allows us to account for skewness suppression in the vicinity of the Dirac point, which is in agreement with recent experiments. We mention some effects which can be attained in Josephson junctions with well-defined edges and for transparency values below unity of the graphene-superconductor interfaces.

Scanning tunneling microscopy (STM) and angle-resolved photoemission spectroscopy (ARPES) observations combined with the density functional theory (DFT) calculations were used to elucidate atomic arrangement of Au/Si(111) reconstructions forming at Au coverage around one monolayer. They include the Si(111)$6\times 6$-Au with ordered domain walls, the Si(111)$\alpha -\sqrt{3}\times \sqrt{3}$-Au and the Si(111)$\beta -\sqrt{3}\times \sqrt{3}$-Au surfaces with disordered domain walls of various density, and the Si(111)$2\sqrt{21}\times 2\sqrt{21}$-Au surface which structure is relevant to those of other reconstructions, albeit cannot be ascribed solely to the domain walls. Using detailed comparison of the STM and ARPES data with the results of DFT calculations, we explicitly proved the atomic model of Si(111)$6\times 6$-Au surface proposed by Grozea et al. [Surf. Sci. 418, 32 (1998)]. This model provided us a hint for constructing an atomic model of domain walls which demonstrates a proper coincidence between experimental and simulated STM images. The model contains five Au atoms per $2\times \sqrt{3}$ unit cell, i.e., 1.25 monolayers of Au. For the $\alpha -\sqrt{3}\times \sqrt{3}$-Au and $\beta -\sqrt{3}\times \sqrt{3}$-Au surfaces, this finding allows an overall description of their atomic arrangement and, hence, accurate determination of their Au coverages. Knowledge on the atomic arrangement of the Si(111)$6\times 6$-Au surface and domain walls also allowed us to construct an atomic model of the Si(111)$2\sqrt{21}\times 2\sqrt{21}$-Au surface. The obtained data were unified in the refined formation phase diagram for the Au/Si(111) system.

We study the coupling of colloidal thick-shell CdSe/CdS nanocrystals with amorphous gold films (AFs) and crystalline gold platelets (CPs) at 4 K. Time resolved experiments show that the photoluminescence lifetime increases from 2.4 ns (AF) to 5.1 ns (CP) due to the reduction of Joule losses. The comparison of the experimental results with numerical predictions shows that the radiative quantum efficiency is at least twice higher ($\ge 20\%$) for the crystalline structure.

We study theoretically the electronic structure of topological nodal-line semimetals. We show that, in the presence of a gap-opening spatially dependent mass term that forms a domain wall, an in-gap charged localized mode emerges at the domain wall. It turns out that such a domain wall is realized by head-to-head (or tail-to-tail) bulk electric polarizations. The dispersion of the localized mode evolves from gapless to gapped as the bulk band gap increases, which means that its conductivity is tunable. The localized mode has a topological origin, i.e., a topological confinement is realized, which is understood by a semiclassical topological number defined in semiclassical momentum-real space. In contrast to previous studies, the origin of the charged domain wall in this study is purely electronic, i.e., due to the band topology. Moreover, this study demonstrates a topological confinement at the interface between two insulators without bulk topological numbers. We discuss a possible experimental realization of the stable, electrically tunable charged domain wall.

The band structures of CrN are calculated using the quasiparticle self-consistent $GW$ approach in both the low-temperature antiferromagnetic (AFM) [$110{\mathrm{]}}_{2}Pmna$ orthorhombic phase and in a hypothetical ferromagnetic phase representing the paramagnetic cubic state in a saturating magnetic field. A gap of about 1 eV is found in the AFM state, while the ferromagnetic band structure is found to be half metallic. Another hypothetical AFM-1 structure [$001{\mathrm{]}}_1$ is also considered and gives a smaller band gap. The orbital nature of the bands is studied and reveals a gap between Cr-$d$ states, indicating a strongly correlated behavior. Optical dielectric functions are calculated from the interband transitions and compared to experimental data.

We present a generic grand-canonical theory for the Peierls transition in atomic wires deposited on semiconducting substrates such as In/Si(111) using a mean-field solution of the one-dimensional Su-Schrieffer-Heeger model. We show that this simple low-energy effective model for atomic wires can explain naturally the occurrence of a first-order Peierls transition between a uniform metallic phase at high temperature and a dimerized insulating phase at low temperature as well as the existence of a metastable uniform state below the critical temperature.

The hybrid improper ferroelectricity is due to the trilinear coupling between three symmetric phonon modes, and the free energy of the system can be described as $F_{\mathrm{tri}}\sim \alpha Q_1{Q}_2{Q}_P$, where $\alpha$ is the coupling constant and $Q_1$ and $Q_2$ are the primary order parameters coupled to the polar mode $Q_P$. In the case of $Pnma$ symmetry, the $Q_1$ and $Q_2$ modes are rotation, $Q_{\mathrm{Rot}}(a^0{a}^0{c}^{+}$), and tilt, $Q_{\mathrm{Tilt}}(a^{-}{\mathrm{a}}^{-}{\mathrm{c}}^0$), of ${\mathrm{BO}}_6$ octahedra, respectively. Here, we perform density functional theory (DFT) calculations along with machine learning (ML) to investigate the effects of biaxial strain on the ferroelectric and magnetic properties for hybrid improper ferroelectric $({\mathrm{LaFeO}}_3{)}_1/({\mathrm{LnFeO}}_3{)}_1$ superlattices (SLs), where Ln represents Ce, Nd, Sm, Gd, Dy, Y, Tm, and Lu. We have investigated how the polarization, magnetization, and coupling coefficients are modified subject to the external strain. The shift in energy minima in the energy landscape, which is a function of both $Q_{\mathrm{Rot}}$ and $Q_{\mathrm{Tilt}}$, indicates that the ferroelectric switching can be modulated upon imposing strain. Strain can change the polarization up to $16\%$ with respect to the zero-strain value. In the case of magnetization, we observe a change in the easy axis leading to a change in the magnetic configuration from $G_x{A}_y{F}_z$ to $F_x{C}_y{G}_z$ upon tensile strain. It is impractical to compute the strain response for all possible $({\mathrm{LnFeO}}_3{)}_1/({\mathrm{Ln}}^{\prime }{\mathrm{FeO}}_3{)}_1$ combinations. To overcome this limitation we perform ML with DFT calculated data within a subset of all the combinations. Using machine learning, we predict the change in polarization upon strain for tolerance $\tau$ ranging from 0.844 to 0.902, which covers all the possible $({\mathrm{LnFeO}}_3{)}_1/({\mathrm{Ln}}^{\prime }{\mathrm{FeO}}_3{)}_1$ SL combinations. The prediction by ML is in excellent agreement with the DFT calculations.

In magnetic trilayer systems, spin pumping is generally addressed as a reciprocal mechanism characterized by one unique spin-mixing conductance common to both interfaces. However, this assumption is questionable in cases where different types of interfaces are present. Here, we present a general theory for analyzing spin pumping in cases with more than one unique interface and where the magnetic coupling is allowed to be noncollinear. The theory is applied to analyze layer-resolved ferromagnetic resonance experiments on the trilayer system ${\mathrm{Ni}}_{80}{\mathrm{Fe}}_{20}/{\mathrm{Ru}/\mathrm{Fe}}_{49}{\mathrm{Co}}_{49}{\mathrm{V}}_2$ where the Ru spacer thickness is varied to tune the indirect exchange coupling. It is demonstrated that the equation of motion of macrospins driven by spin pumping need to be modified in case of noncollinear coupling. Our analysis also shows that the spin pumping in trilayer systems with dissimilar magnetic layers, in general, is nonreciprocal.