We revisit the holon-doublon binding problem in two-dimensional (2D) photoexcited Mott insulators. Low-energy photoexcited states in Mott insulators are described as a pair state of a doublon and a holon. The most basic question is whether its bound state is formed in the lowest-energy state, and negative and positive responses have been discussed in the past. In this study we begin with the 2D Hubbard model, and transform it into the first effective model, which is based on the $t/U$ expansion, with $U$ and $t$ being the Hubbard $U$ and the electron hopping energy, respectively. We find that quantitative reliability is assured for $U/t\gtrsim 10$. Furthermore, we transform it into a second effective model that selects essential states in the low-energy region. In both effective models we distinguish two magnetic terms, namely, the spin-exchange interaction and the three-site transfer, and parametrize the two terms with the parameters $J_{\mathrm{ex}}$ and $J_{3\mathrm{site}}$. By changing the parameters apart from the restriction given by the Hubbard model, any positive $J_{\mathrm{ex}}$ value with $J_{3\mathrm{site}}=0$ yields a finite amount of binding, whereas a finite value of $J_{3\mathrm{site}}$ suppresses the binding significantly, still leaving the Hubbard case of $U=10t$ in the vicinity of the bound-unbound boundary.

Energy levels in an organic semiconductor are mainly determined by the molecular orbital energies of the constituent molecules. Recent studies, however, have revealed that the energy levels can be altered as much as 1 eV by the molecular orientation in the film or the molecular mixing ratio in the binary film, owing to the intermolecular electrostatic interaction. Because of the long-range nature of Coulomb interaction, theory predicts that the electrostatic energy should depend on the sample shape. In this article, we examine the coverage-dependent energy levels of zinc phthalocyanine and per-fluorinated zinc phthalocyanine in the submonolayer region with ultraviolet photoelectron spectroscopy (UPS) and low-energy inverse photoelectron spectroscopy (LEIPS). Using the procedure we reported previously, we separately evaluated the electronic polarization energy and electrostatic energy as a function of coverage. Unlike the electronic polarization, which contributes only as much as 10 meV, the electrostatic energy contributes as much as 120 meV to the coverage-dependent energy shift. We conclude that the shift in energy levels by changing the coverage is attributed to the sample shape-dependent energy level, owing to the long-range nature of the charge–permanent quadrupole interaction.

A new photoelectric phenomenon, the in-plane photoelectric (IPPE) effect, has been recently discovered at terahertz (THz) frequencies in a $\mathrm{Ga}\mathrm{As}/{\mathrm{Al}}_x{\mathrm{Ga}}_{1-x}\mathrm{As}$ heterostructure with a two-dimensional (2D) electron gas [W. Michailow et al., Sci. Adv. 8, eabi8398 (2022)]. In contrast to the conventional PE phenomena, the IPPE effect is observed at normal incidence of radiation, the height of the in-plane potential step, which electrons overcome after absorption of a THz photon, is electrically tunable by gate voltages, and the effect is maximal at a negative electron “work function,” when the Fermi energy lies above the potential barrier. Based on the discovered phenomenon, efficient detection of THz radiation has been demonstrated. In this work we present a detailed theory of the IPPE effect providing analytical results for the THz wave generated photocurrent, the quantum efficiency, and the internal responsivity of the detector, in dependence on the frequency, the gate voltages, and the geometrical parameters of the detector. The calculations are performed for macroscopically wide samples at zero temperature. Results of the theory are applicable to any semiconductor systems with 2D electron gases, including III-V structures, silicon-based field effect transistors, and the novel 2D layered, graphene-related materials.

We study the Landau level (LL) structure and magneto-optical responses in the Weyl semimetal (WSM) quantum well in the presence of a non-uniform magnetic field (NUMF). The number of LLs in the NUMF is found to be finite, and the LL structure of the system is significantly affected by the inhomogeneity of the magnetic field. The added electric field significantly affects the separation of the two Weyl nodes in momentum space. We investigate the role of the inhomogeneity of the magnetic field, temperature, electron density, and electric field, as well as the doping level on the optical response properties. Both longitudinal and Hall susceptibilities in the $(x,y)$ plane and the longitudinal susceptibility in the $z$ direction display a series of peaks whose height and spacing decrease with the increase of the LL index. At $T\ne 0$, thermal excitation triggers new transitions, which are forbidden by the Pauli blocked at $T=0$. The optical response spectra are different when the chemical potential lies in different energy regions, which are strongly dependent on the inhomogeneity of the magnetic field, electron density, and electric field. In this paper, we provide a possible way to control the optical response spectrum in WSM materials by changing these parameters.

The charge degrees of freedom in several different organic charge transfer salts display slow or glassy dynamics. To gain insight into this behavior, we obtain the low-energy theory for an extended dimer Hubbard model, considering the occupations of sites on neighboring dimers. We take a classical limit of the resulting effective model of coupled spins and dimers and study it using classical Monte Carlo simulations. We find that frustration induced by intradimer and interdimer interactions leads to glassiness in the charge degrees of freedom in the absence of ordering of the spin degrees of freedom. Our results may have relevance to experimental observations of relaxor ferroelectric behavior in the dynamics of organic charge transfer salts.

We present a quantum impurity solver based on a pseudoparticle framework, which combines diagrammatic resummations for a three-point vertex with diagrammatic Monte Carlo sampling of a four-point vertex. This recently proposed approach (A. J. Kim et al., arXiv:2112.15549) is generalized here to fermionic impurity problems and we discuss the technical details of the implementation, including the time-stepping approach, the Monte Carlo updates, and the routines for checking the two-particle irreducibility of the four-point vertex. We also explain how the vertex information can be efficiently stored using a Dubiner basis representation. The convergence properties of the algorithm are demonstrated with applications to exactly solvable impurity models and dynamical mean field theory simulations of the single-orbital Hubbard model. It is furthermore shown that the algorithm can handle a two-orbital problem with off-diagonal hybridizations, which would cause a severe sign problem in standard hybridization-expansion Monte Carlo simulations. Since the vertex-based algorithm successfully handles sign-oscillating integrals in equilibrium and samples only connected diagrams, it may be a promising approach for real-time simulations.

Electronic and transport properties of bilayer heterostructure under light irradiation are of fundamental interest to improve functionality of optoelectronic devices. We theoretically study the modification of transport properties of bilayer graphene and bilayer heterostructures under a time-periodic external light field. The bulk electronic and transport properties are studied in a Landauer-type configuration by using the nonequilibrium Green's function formalism. To illustrate the behavior of the differential conductance of a bilayer contact under light illumination, we consider tight-binding models of bilayer graphene and graphene/hexagonal boron-nitride heterostructures. The nonadiabatic driving induces sidebands of the original band structure and opening of gaps in the quasienergy spectrum. In transport properties, the gap openings are manifested in a suppression of the differential conductance. In addition to suppression, an external light field induces an enhancement of the differential conductance if photoexcited electrons tunnel into or out of a Van Hove singularity.

We present first-principles results on the structural, electronic, and magnetic properties of a new family of two-dimensional antiferromagnetic (AFM) manganese chalcogenides, namely, monolayer $\mathrm{Mn}X$ and Janus $X\mathrm{Mn}Y$ ($X,Y=\mathrm{S}$, Se, Te), among which monolayer MnSe was recently synthesized in experiments [Aapro et al., ACS Nano 15, 13794 (2021)]. By carrying out calculations of the phonon dispersion and ab initio molecular dynamics simulations, we first confirmed that these systems, characterized by an unconventional strongly-coupled-bilayer atomic structure [consisting of Mn atoms buckled to chalcogens forming top and bottom ferromagnetic (FM) planes with antiparallel spin orientation], are dynamically and thermally stable. The analysis of the magnetic properties shows that these materials have robust AFM order, retaining a much lower energy than the FM state even under strain. Our electronic structure calculations reveal that pristine $\mathrm{Mn}X$ and their Janus counterparts are indirect-gap semiconductors, covering a wide energy range and displaying tunable band gaps by the application of biaxial tensile and compressive strain. Interestingly, owing to the absence of inversion and time-reversal symmetry, and the presence of an asymmetrical potential in the out-of-plane direction, Janus $X\mathrm{Mn}Y$ become spin-split gapped systems, presenting a rich physics yet to be explored. Our findings provide insights into this physics and highlight the potential for these two-dimensional manganese chalcogenides in AFM spintronics.

The crystal structure of ${\mathrm{BC}}_2\mathrm{N}$ and the origin of its superhardness remain under constant debate, hindering its development. Herein, by evaluating the x-ray diffraction pattern, the thermodynamic stability at normal and high pressures of a series of ${\mathrm{BC}}_2\mathrm{N}$ candidates, the (111) $\mathrm{B}{\mathrm{C}}_2{\mathrm{N}}_{2\times 2}$ superlattice (labeled $\mathrm{R}2u-\mathrm{B}{\mathrm{C}}_2\mathrm{N}$) is identified as the realistic crystal structure of the experimentally synthesized ${\mathrm{BC}}_2\mathrm{N}$. We further reveal that the strain-induced Friedel-like oscillations dominates the preferable slip systems of $\mathrm{R}2u-\mathrm{B}{\mathrm{C}}_2\mathrm{N}$ by drastically weakening the heterogenous bonds across the slip plane and thus leads to its ultralow dislocation slip resistance, which originates from the metallization triggered by the reduction in energy separation between bonding and antibonding interactions of the softened bonds. Our results rule out $\mathrm{R}2u-\mathrm{B}{\mathrm{C}}_2\mathrm{N}$ as the intrinsic superhard material surpassing $c\text{−}\mathrm{BN}$, whereas the experimentally determined extreme hardness can be attributed to the nanocrystalline grains glued by interfacial amorphous carbon which provides a strong barrier for plastic deformation. These findings provide a view of the longstanding issue of the possible structure of experimentally observed ${\mathrm{BC}}_2\mathrm{N}$, and establish a mechanism underlying the strain-driven electronic instability of superlattice structures, providing guidance towards rational design of superhard materials.

We investigate the electrostrictive response across a ferroelectric phase transition from first-principles calculations and show that $M$, the field-induced electrostrictive tensor, controlling the amplitude of the electric-field induced strain, can be made arbitrarily large through strain engineering. We take as a case study the epitaxial strain-induced transition from para- to ferroelectricity of ${\mathrm{KTaO}}_3$. We show that the magnitude of the field-induced electrostriction diverges with the permittivity at the transition, hence exhibiting giant responses through a calculation of both the $M$ and $Q$ electrostrictive tensors. We explain the origin of this giant electrostrictive response in ${\mathrm{KTaO}}_3$ using a microscopic decomposition of the electrostriction coefficients, and use this understanding to propose design rules for the development of future giant electrostrictors for electromechanical applications. Finally, we introduce a further means to calculate electrostriction, specific to ferroelectrics, and not yet utilized in the literature.

Encoded in the transient x-ray absorption (XAS) and magnetic circular (MCD) response functions resides a wealth of information of the microscopic processes of ultrafast demagnetization. Employing state-of-the-art first-principles dynamical simulations we show that the experimentally observed energy shift of the $L_3$ XAS peak in Ni, and the absence of a corresponding shift in the dichroic MCD response, can be explained in terms of laser-induced changes in band occupation. Strikingly, we predict that for the same ultrashort pump pulse applied to Co the opposite effect will occur: a substantial shift upward in energy of the MCD peaks will be accompanied by very small change in the position of XAS peaks, a fact we relate to the reduced $d$-band filling of Co that allows a greater energetic range above the Fermi energy into which charge can be excited. We also carefully elucidate the dependence of this effect on pump pulse parameters. These findings (i) establish an electronic origin for early-time peak shifts in transient XAS and MCD spectroscopy and (ii) illustrate the rich information that may be extracted from transient response functions of the underlying dynamical system.

The hybridization of light and matter excitations in the form of polaritons has enabled major advances in understanding and controlling optical nonlinearities. Entering the quantum regime of strong interactions between individual photons has, however, remained challenging since the strength of achievable polariton interactions is typically limited by the available interactions in the material. Here, we investigate collisions between dark-state polaritons in three-level systems and discover a resonant process that yields effective interactions, which are much larger than the underlying interaction between their matter constituents. We systematically investigate the underlying mechanism and demonstrate a substantial enhancement of polariton interactions by several orders of magnitude. This suggests a promising approach to quantum nonlinear optics in a range of physical settings, from atomic gases to excitons in semiconductors and two-dimensional bilayer materials.

We demonstrate the scattering of laser light from two InAs quantum dots coupled to a photonic crystal waveguide, which is achieved by strain-tuning the optical transitions of the dots into mutual resonance. By performing measurements of the intensity and photon statistics of transmitted laser light before and after tuning the dots into resonance, we show that the nonlinearity is enhanced by scattering laser light from two dots. In addition to providing a means of manipulating few-photon optical nonlinearities, our approach establishes opportunities for multiemitter quantum optics in a solid-state platform.

Analytical derivations and first-principles calculations are performed to investigate the electro-optic (EO) response of the two-dimensional ferroelectric SnS in its monolayer form. In addition to derive the formula for EO coefficients for two-dimensional systems, large linear and nonlinear electro-optic coefficients are discovered, along with a strongly nonlinear electro-optic response as a function of electric field. The origin of these electro-optic responses is further found and involves a few specific phonon modes, in general, and the evolution of their frequencies and atomic characteristics under electric fields, in particular. Large linear and nonlinear electro-optic coefficients can be found in other two-dimensional ferroelectrics, paving the way for potential optical device applications.

Unconventional topological phonons with unusual properties in crystalline solids have attracted increasing attention. By performing symmetry arguments and low-energy effective Hamiltonian, we confirm the coexistence of charge-2 Dirac and Weyl phonons at high-symmetry points and high-symmetry lines by analyzing all possible space groups. We reveal that such a particular system only emerges in chiral space groups in the presence of time-reversal symmetry. Unlike conventional Weyl pairs with opposite chirality that host one surface arc, there are two surface arcs by connecting one charge-2 Dirac point with topological charge = $\pm 2$ and two Weyl points with topological charge = $\mp 1$. Due to the constraints of symmetry arguments and projection direction in chiral crystals, the surface arcs are long enough to span over the whole surface Brillouin zone. We take ${\mathrm{Na}}_2{\mathrm{Zn}}_2{\mathrm{O}}_3$ as an example to confirm our predication, and two branches of surface states across the whole surface Brillouin zone are clearly visible. Our work not only presents exotic quasiparticles but also provides an idea for realizing long nontrivial surface arcs.

The cooperation between time-periodic driving fields and non-Hermitian effects could endow systems with distinctive spectral and transport properties. In this paper, we uncover an intriguing class of non-Hermitian Floquet matter in one-dimensional quasicrystals, which is characterized by the emergence of multiple driving-induced $\mathcal{PT}$-symmetry breaking/restoring transitions, mobility edges, and reentrant localization transitions. These findings are demonstrated by investigating the spectra, level statistics, inverse participation ratios, and wave-packet dynamics of a periodically quenched nonreciprocal Harper model. Our results not only unveil the richness of localization phenomena in driven non-Hermitian quasicrystals but also highlight the advantage of the Floquet approach in generating unique types of nonequilibrium phases in open systems.

How superconductivity emerges in the vicinity of an antiferromagnetic insulating state is a long-standing issue of strong correlation physics. We study the transition from an antiferromagnetic insulator to a superconductor by hole doping based on a bilayer generalization of a Hubbard model. The projector quantum Monte Carlo simulations are employed, which are sign problem free both at and away from half filling. An anisotropic Ising antiferromagnetic Mott insulating phase occurs at half filling, which is weakened by hole doping. Below a critical doping value, antiferromagnetism coexists with the singlet superconductivity, which is a pairing across each rung with an extended $s$-wave symmetry. As doping further increases, the antiferromagnetic order vanishes, leaving only a superconducting phase. These results provide important information on how superconductivity appears upon doping the parent Mott-insulating state.