We theoretically study the optical generation of dc spin current (i.e., a spin-current solar cell) in ordered antiferromagnetic and ferrimagnetic insulators, motivated by a recent study on the laser-driven spinon spin current in noncentrosymmetric quantum spin chains [H. Ishizuka and M. Sato, Phys. Rev. Lett. 122, 197702 (2019)]. Using a nonlinear response theory for magnons, we analyze the dc spin current generated by a linearly polarized electromagnetic wave (typically, terahertz or gigahertz waves). Considering noncentrosymmetric two-sublattice magnets as an example, we find a finite dc spin-current conductivity at $T=0$, where no thermally excited magnons exist; this is in contrast to the case of the spinon spin current, in which the optical transition of the Fermi degenerate spinons plays an essential role. We find that the dc spin-current conductivity is insensitive to the Gilbert damping, i.e., it may be viewed as a shift current carried by bosonic particles (magnons). Our estimate shows that an electric-field intensity of $E\sim {10}^4\text{–}{10}^6$ V/cm is sufficient for an observable spin current. Our theory indicates that the linearly polarized electromagnetic wave generally produces a dc spin current in noncentrosymmetric magnetic insulators.

We study spontaneous emission of two-level quantum emitter agglomerates embedded in transparent oxides. The two levels correspond to the splitting between vacancy impurity states $F,F^{+}$, and $V^{-}$ color centers, whereas correlations are due to narrow bands in transition-metal oxides with ${\mathrm{ABO}}_3$ perovskite structures. For these systems, we put forward a mechanism to control light emission in which micrometer-size vacancy-rich regions play the role of cavity resonators for the emitted radiation with normal frequencies mode volumes, and emission linewidths determined by their size and material properties. We argue that a physical scenario in which the emitter's natural frequency and linewidth can be fine-tuned via electronic correlations or temperature explains unusual features in the experimental photoluminescence spectra of Ar-irradiated ${\mathrm{SrTiO}}_3$. By unveiling the role of electronic correlations on spontaneous emission in transparent oxides, we hope our findings help set the basis for the design of controllable engineered solid-state single-photon sources.

We systematically investigate the recombination mechanism of photogenerated charge carriers in bulk $\mathrm{CsPbB}{\mathrm{r}}_3$ by means of time-resolved photoluminescence (TR-PL) spectroscopy at low temperature and various laser excitation powers. A dynamic recombination model is proposed to describe the TR-PL that predicts the time-dependent exciton and free-charge populations. It provides a clear representation of competing mono- and bimolecular recombination processes. A decrease in carrier lifetime with increasing laser intensity was observed that was attributed to exciton-exciton scattering. A bimolecular recombination coefficient of $\sim {10}^{-7}\mathrm{c}{\mathrm{m}}^3/\mathrm{s}$ was obtained for exciton recombination. As the concentration of photoexcited carriers increases, stronger exciton-exciton annihilation occurs. The exciton-exciton annihilation rate for $\mathrm{CsPbB}{\mathrm{r}}_3$ is $3.63\times {10}^{-7}\mathrm{c}{\mathrm{m}}^3{\mathrm{s}}^{-1}$ at 10-mW laser power. Notably, the exciton-exciton annihilation rate in bulk material is comparable to that obtained for photoexcited $\mathrm{CsPbB}{\mathrm{r}}_3$ nanoscale quantum dots.

${\mathrm{PbTaSe}}_2$ is a noncentrosymmetric superconductor having topological Dirac surface states as shown by recent experiments. Here, we report calculations on bulk and few-layer ${\mathrm{PbTaSe}}_2$ using the first-principles anisotropic Migdal-Eliashberg theory, to explore the nature of the superconducting gap and the surface-bulk proximity effect in this stoichiometric system. In bulk, the multiband Pb $p$/Ta $d$ Fermi surface exhibits a single anisotropic full superconducting gap, with larger gap values appearing on in-plane orbitals that can couple effectively to the in-plane phonons. A Pb-terminated few-layer ${\mathrm{PbTaSe}}_2$ is found to still have a single-anisotropic-gap feature, but its superconducting gap is evidently reduced relative to the bulk case. Comparison between our theoretical results and experimental tunneling data suggests a significant proximity-enhanced superconductivity on the realistic surface of ${\mathrm{PbTaSe}}_2$. The present findings would apply to other same-structured $AB{\mathrm{Se}}_2$ ($A$ = Pb, Sn; $B$ = Ta, Nb) and are crucial for studying topological superconductors and Majorana fermions in promising topological metals with intrinsic full-gap superconductivity.

In this work we construct the first-principles theory for the near-field resonances arising in the system of two different coupled nanoparticles excited by the external field. The obtained expressions are valid for an arbitrary external field; the detailed analysis is performed for the case of a fast electron passing by. The energy of electron is arbitrary, so the results are valid both for relativistic and nonrelativistic electrons. We have obtained rigorous analytical solution for the radiation field and for distribution of radiation over angles and frequencies. The effect of interaction between the single particles manifests itself very distinctly in conditions of resonance, resulting in a spectral shift of the radiation maximum, significant increase of its altitude, and, interestingly, in clear oscillations in radiation intensity not only when the particles are very close to each other, but even when the distance between the particles exceeds considerably their sizes. It attests to the strong influence of interaction on the radiation processes in a system of coupling particles.

The recent ability to manipulate graphene-like structures at the atomic scale is opening up new challenges in electronics and photonics, with a key focus on the bulk and edges of the systems, which generate peculiar and interfering charge density oscillations, quantized as plasmons. These quasiparticles are here scrutinized in planar periodic distributions (two-dimensional arrays) of parallel and atomically wide graphene nanoribbons. Time-dependent density functional theory is used, with a specifically developed adjustment on the random phase approximation, suitable for two-dimensional materials. Several extrinsic conditions (for doped or gated nanoribbon arrays) are simulated to characterize the propagation and interplay of the bulk and edge plasmons, at far infrared to visible energies, and over a broad range of momentum transfers. The main technological interest is on the bulk mode, which is dominant and propagates undamped, at energies below the band gap of the intrinsic systems. On the other hand, the edge mode is always well defined at energies above the band gap, and highly dependent on the band gap value, though it decays via electron-hole excitations between the first valence and conduction bands. Particular attention is paid to the interaction or overlap region of the two plasmons, explaining its sensitivity to induced Fermi level shifting, transferred momentum, ribbon type, and geometry, with the inclusion of many-body, GW-like effects. More importantly, the lower-terahertz behavior of the bulk plasmon is explored, highlighting the limits of available non–ab initio approaches, suitable for stripes of graphene being tenths of nanometers wide. Then, an effective model is derived from the ab initio framework, which reasonably accounts for the two-plasmon response of the studied, ultranarrow nanoribbon systems, at small momentum transfers. The range of applicability of the same derivation procedure may be extended to more complex nanoribbon heterostructures available for synthesis, which emphasizes the need for an ab initio guide to a reliable design of nanoplasmonic devices.

In this work, we provide an effective model to evaluate the one-electron dipole matrix elements governing optical excitations and the photoemission process of single-layer (SL) and bilayer (BL) transition metal dichalcogenides. By utilizing a $\mathit{k}·\mathit{p}$ Hamiltonian, we calculate the photoemission intensity as observed in angle-resolved photoemission from the valence bands around the $\overline{K}$ valley of ${\mathrm{MoS}}_2$. In SL ${\mathrm{MoS}}_2$, we find a significant masking of intensity outside the first Brillouin zone, which originates from an in-plane interference effect between photoelectrons emitted from the Mo $d$ orbitals. In BL ${\mathrm{MoS}}_2$, an additional interlayer interference effect leads to a distinctive modulation of intensity with photon energy. Finally, we use the semiconductor Bloch equations to model the optical excitation in a time- and angle-resolved pump-probe photoemission experiment. We find that the momentum dependence of an optically excited population in the conduction band leads to an observable dichroism in both SL and BL ${\mathrm{MoS}}_2$.

Twisted bilayer graphene (tBLG) has recently emerged as a new platform for studying electron correlations, the strength of which can be controlled via the twist angle. Here, we study the effect of internal screening on electron-electron interactions in undoped tBLG. Using the random-phase approximation, we find that the dielectric response of tBLG drastically increases near the magic angle and is highly twist-angle dependent. As a consequence of the abrupt change of the Fermi velocity as a function of wave vectors, the screened interaction in real space exhibits attractive regions for certain twist angles near the magic angle. Attractive interactions can induce charge-density waves and superconductivity and therefore our findings could be relevant to understand the microscopic origins of the recently observed strong correlation phenomena in undoped tBLG. The resulting screened Hubbard parameters are strongly reduced and exhibit a nonlinear dependence on the twist angle. We also carry out calculations with the constrained random-phase approximation and parametrize a twist-angle dependent Keldysh model for the resulting effective interaction.

Motivated by recent experimental progress in the realization of hybrid structures with a topologically superconducting nanowire coupled to a quantum dot, viewed through the lens of the emerging field of correlated Majorana fermions, we introduce a class of interacting Majorana-Anderson impurity models which admit an exact solution for a wide range of parameters, including on-site repulsive interactions of arbitrary strength. The model is solved by mapping it via the ${\mathbb{Z}}_2$ slave-spin method to a noninteracting resonant level model for auxiliary Majorana degrees of freedom. The resulting gauge constraint is eliminated by exploiting the transformation properties of the Hamiltonian under a special local particle-hole transformation. For a spin-polarized Kitaev chain coupled to a quantum dot, we obtain exact expressions for the dot spectral functions at both zero and finite temperature. We study how the interaction strength and localization length of the end Majorana zero mode affect the physical properties of the dot, such as the quasiparticle weight, double occupancy, and odd-frequency pairing correlations, as well as the local electronic density of states in the superconducting chain.

It is well known that the conductivity of disordered metals is suppressed in the limit of low frequencies and temperatures by quantum corrections. Although predicted by theory to exist up to much higher energies, such corrections have so far been experimentally proven only for $\lesssim 80$ meV. Here, by a combination of transport and optical studies, we demonstrate that the quantum corrections are present in the strongly disordered conductor MoC up to at least $\sim 4$ eV, thereby extending the experimental window where such corrections were found by a factor of 50. The knowledge of both the real and imaginary parts of conductivity enables us to identify the microscopic parameters of the conduction electron fluid. We find that the conduction electron density of strongly disordered MoC is surprisingly high and we argue that this should be considered a generic property of metals on the verge of a disorder-induced localization transition.

In solid state photoemission experiments it is possible to extract information about the symmetry and orbital character of the electronic wave functions via the photoemission selection rules that shape the measured intensity. This approach can be expanded in a pump-probe experiment where the intensity contains additional information about interband excitations induced by an ultrafast laser pulse with tunable polarization. Here, we find an unexpected strong linear dichroism effect (up to 42.4%) in the conduction band of bilayer ${\mathrm{MoS}}_2$, when measuring energy- and momentum-resolved snapshots of excited electrons by time- and angle-resolved photoemission spectroscopy. We model the polarization-dependent photoemission intensity in the transiently populated conduction band using the semiconductor Bloch equations. Our theoretical analysis reveals a strongly anisotropic momentum dependence of the optical excitations due to intralayer single-particle hopping, which explains the observed linear dichroism.

We perform exact diagonalization studies for fractional quantum Hall states at filling factor $4/5$ in a bilayer system, on a torus with various aspect ratios and angles. We find that in the absence of tunneling, two weakly coupled $2/5$ layers undergo a phase transition into an interlayer-correlated regime, which is also Abelian with the fivefold degeneracy on the torus. In the limit of zero layer separation, this phase becomes a singlet in the pseudospin variable describing the layer degree of freedom. By studying the Chern-number matrix, we show that the $K$ matrix describing the interlayer-correlated regime requires a matrix dimension larger than two and this regime is in particular not described by a Halperin state. A detailed analysis of possible $4\times 4K$ matrices having the requisite symmetries and quantum numbers shows that there is only one equivalence class of such matrices. A model wave function representing this universality class is constructed. The role of separate particle number conservation in both layers is discussed, and it is argued that this additional symmetry allows for the further distinction of two different symmetry-protected Abelian phases in the interlayer correlated regime. Interlayer tunneling breaks this symmetry and can drive the system into a single-layer regime when strong enough. A qualitative phase diagram in the tunneling-layer separation parameter space is proposed based on our numerical results.

Variational representations of quantum states abound and have successfully been used to guess ground-state properties of quantum many-body systems. Some are based on partial physical insight (Jastrow, Gutzwiller-projected, and fractional quantum Hall states, for instance), and others operate as a black box that may contain information about the underlying structure of entanglement and correlations (tensor networks, neural networks) and offer the advantage of a large set of variational parameters that can be efficiently optimized. However, using variational approaches to study excited states and, in particular, calculating the excitation spectrum, remains a challenge. We present a variational method to calculate the dynamical properties and spectral functions of quantum many-body systems in the frequency domain, where the Green's function of the problem is encoded in the form of a restricted Boltzmann machine (RBM). We introduce a natural gradient descent approach to solve linear systems of equations and use Monte Carlo to obtain the dynamical correlation function. In addition, we propose a strategy to regularize the results that improves the accuracy dramatically. As an illustration, we study the dynamical spin structure factor of the one dimensional $J_1-J_2$ Heisenberg model. The method is general and can be extended to other variational forms.

It is often assumed that the self-assembly of organic molecules on a noble-metal surface results in a structure that is in thermodynamic equilibrium. Here, using scanning tunnel microscopy, we observe instead a highly ordered metastable striped monolayer phase of zinc tetraphenylporphyrin, self-assembled at 300 K on Ag(100). The usually reported stable square phase is found only after higher-temperature annealing. We use a statistical mechanical model to reveal a possible molecular mechanism for this process, in which the competition of molecule and substrate interactions at the growth front leads to the growth of a kinetically trapped inhomogeneous ordered structure. Our proposed mechanism rests only on simple features of molecular geometry and interactions, and the resulting principles could be used to promote particular outcomes of growth in other examples of molecular assembly at surfaces.

We consider the helical edge state of a quantum spin Hall insulator, connected to leads and irradiated by a monochromatic and circularly polarized electromagnetic wave. The photocurrent carried by a single helical edge state is studied as a function of the characteristics of the electromagnetic radiation and electronic doping. We focus on the effect of the distance between the leads on the photocurrent. We also investigate the differential photoconductance of the helical edge state in the presence of a small voltage bias between the leads.

We predict the stability of a graphenelike silicene sheet on one monolayer of aluminum oxide. We find that the honeycomb buckled structure of silicene is not broken upon interaction with one monolayer of ${\mathrm{Al}}_2{\mathrm{O}}_3$ in the kagome geometry. As a consequence, the electronic band structure shows unperturbed cones with massless Dirac fermions embedded into ${\mathrm{Al}}_2{\mathrm{O}}_3$-derived bands. The heterostructure conserves the topological character of the freestanding silicene. The optical properties of the silicene/${\mathrm{Al}}_2{\mathrm{O}}_3$ bilayers clearly maintain the characteristic infrared universal limit of $\pi \alpha$. A quantized absorbance $N·\pi \alpha$ (with $N$ silicene layers) also occurs for stacking of multilayers.

A Chern insulator or a quantum anomalous Hall state is a topological state with integer Hall conductivity but in absence of the Landau level. It had been well established on various two-dimensional lattices with periodic structure. Here, we report similar Chern insulators that can also be realized on a quasicrystal with fivefold rotational symmetry. Providing the staggered flux through plaquettes, we propose two types of quasicrystalline Chern insulators. Their topological characterizations are well identified by the robustness of edge states, nonzero real-space Chern number, and quantized conductance. We further find the failure of integer conductivity but with the quantized Chern number at some special energies. Our study, therefore, provides a new opportunity to searching topological materials in aperiodic systems.

The transverse magneto-optical Kerr effect (TMOKE) in magnetite-based magnetoplasmonic crystals is studied experimentally and theoretically. We analyze angle-resolved TMOKE spectra from two types of structures where noble metallic stripes are incorporated inside a thin magnetite film or located on top of a homogeneous film. A multiple-wide-band enhancement of the TMOKE signal in transmission is demonstrated. The complex dielectric permittivity and gyration are experimentally determined using the ellipsometry technique as well as Faraday rotation and ellipticity measurements. The obtained parameters are used in rigorous coupled-wave analysis (RCWA) calculations for studying the optical resonances. Our RCWA calculations of transmittance and TMOKE are in good agreement with the experimental data. The role of guiding and plasmonic modes in the TMOKE enhancement is revealed. We demonstrate that the TMOKE provides rich information about the studied optical resonances.

Swirling topological spin configurations, known as magnetic skyrmions, are known to be stabilized by different mechanisms. In particular, in chiral ferromagnets with Dzyaloshinskii-Moriya interaction (DMI), they appear in a topological crystalline phase, termed the skyrmion crystal phase (SkX), for appropriate values of the external magnetic field. A similar phenomenon is present in the antiferromagnetic case for the Heisenberg triangular antiferromagnet (HTAF) with DMI. Here, the most striking feature is that the emergent topological phase consists of three SkX interpenetrated sublattices. On the other hand, the pure HTAF, being described by an SO(3) order parameter, can host ${\mathbb{Z}}_2$ vortices. This raises the fundamental question of whether both nontrivial structures are related. In this Rapid Communication we unravel a hidden connection between both topological entities by studying the HTAF with anisotropic DMI. To this end, we combine an effective field theory description, the Luttinger-Tisza approximation, and Monte Carlo simulations. We show that even a slight anisotropy in the DMI proves to be the key ingredient to deform the interpenetrated SkX structure and reveal a ${\mathbb{Z}}_2$-vortex crystal.