One of the challenges in many-body physics is determining the effects of phonons on strongly correlated electrons. The difficulty arises from strong correlations at differing energy scales—for band metals, Migdal-Eliashberg theory accurately determines electron-phonon coupling effects due to the absence of vertex corrections, but strongly correlated electrons require a more complex description, and the standard Migdal-Eliashberg approach does not necessarily apply. In this work, we solve for the atomic limit Green's function of the Holstein-Hubbard model with both time-dependent electron-electron and electron-phonon couplings. We then examine the photoemission spectra of this model in and out of equilibrium. Next, we use similar methods to exactly solve an extended version of the Hatsugai-Komoto model and examine its behavior in and out of equilibrium. These calculations lead us to propose using the first moment of the photoemission spectra to signal non-equilibrium changes in electron-electron and electron-phonon couplings.

Gate induced carriers impact the many-body interactions in monolayer transition metal dichalcogenides (TMDs) by modifying the screened Coulomb potential and renormalizing the band gap, thus influencing the strong excitonic effects in these materials. Using the $GW$ approximation and a plasmon pole theory to model the carrier induced plasmons in the frequency-dependent part of the screening, we accurately calculate the band gap renormalization of the electron doped monolayer ${\mathrm{MoS}}_2$ and ${\mathrm{WS}}_2$. The excitonic states of the low doped systems are calculated by solving the Bethe-Salpeter equation. Our results clarify the competition between screening and band gap renormalization. An exact cancellation occurs between the reduced band gap and the exciton binding energy for doped monolayer ${\mathrm{WS}}_2$, in good agreement with previous experimental results. In contrast, the exciton energy of doped monolayer ${\mathrm{MoS}}_2$ blueshifts by tens of meV. We show the role of the electronic band structure of the monolayers in the calculated exciton energies. Our results could be generally expanded to other monolayer TMDs and are helpful for quantitatively engineering optoelectronic devices with desired features.

It is well known that impurities play a central role in the linear and nonlinear response of graphene at optical and terahertz frequencies. In this work, we calculate the bands and intraband dipole connection elements for nitrogen-doped monolayer graphene using a density functional tight-binding approach. Employing these results, we calculate the linear and nonlinear response of the doped graphene to terahertz pulses using a density-matrix approach in the length gauge. We present the results for the linear and nonlinear mobility as well as third-harmonic generation in graphene for adsorbed and substitutional nitrogen doping for a variety of doping densities. We show that the conduction bands are more parabolic in graphene structures with substitutional nitrogen doping than for those with adsorbed nitrogen. As a result, substitutional doping has a greater impact on the terahertz mobility and nonlinear response of graphene than adsorbed nitrogen does.

Two-dimensional honeycomb lattices beyond graphene, such as germanene, promise new physical properties such as quantum spin Hall effect. While there have been many claims of growth of germanene, the lack of precise structural characterization of the epitaxial layers synthesized hinders further research. The striped layer formed by Ge deposition on Ag(111) has been recently ascribed as a stretched germanene layer. Using surface x-ray diffraction and density-functional theory calculations, we demonstrate that it corresponds in fact to a ${\mathrm{Ag}}_2\mathrm{Ge}$ surface alloy with an atomic density 6.45% higher than the Ag(111) atomic density. The overall structure is formed by stripes associated with a face-centered cubic top-layer alignment, alternating with stripes associated with a hexagonal close-packed top-layer alignment, in great analogy with the $\left(22\times \sqrt{3}\right)$ Au(111) reconstruction.

We report on the observation of terahertz-radiation-induced edge photogalvanic currents in graphene, which are nonlinear in intensity. The increase of the radiation intensities up to MW/${\mathrm{cm}}^2$ results in a complex nonlinear intensity dependence of the photocurrent. The nonlinearity is controlled by the back gate voltage, temperature, and radiation frequency. A microscopic theory of the nonlinear edge photocurrent is developed. Comparison of the experimental data and theory demonstrates that the nonlinearity of the photocurrent is caused by the interplay of two mechanisms, i.e., by direct interband optical transitions and Drude-like absorption. Both photocurrents saturate at high intensities but have different intensity dependencies and saturation intensities. The total photocurrent shows a complex sign-alternating intensity dependence. The functional behavior of the saturation intensities and amplitudes of both kinds of photogalvanic currents, depending on gate voltages, temperature, radiation frequency, and polarization, is in good agreement with the developed theory.

We investigate a phase transition in an SU(3) symmetric spin-1 chain. To study this transition, we numerically diagonalize an SU(3) symmetric Hamiltonian combining the Uimin-Lai-Sutherland (ULS) model Hamiltonian with the Hamiltonian of the exact trimer ground state (${\mathbb{Z}}_3$ symmetry breaking). Our numerical results are discussed on the basis of the conformal field theory (CFT) and the renormalization group (RG). We then show the phase transition between a trimer liquid phase (massless, no long-range order) and a trimer phase (spontaneously ${\mathbb{Z}}_3$ symmetry breaking). Next, we find the central charge $c=2$ in the massless phase, and $c<2$ in the ${\mathbb{Z}}_3$ symmetry broken phase. From these results, we identify the universality class of the critical trimer liquid phase, as well as the phase boundary between these two phases.

We predict that two distinct strong spin filtering mechanisms in two-terminal geometries at zero magnetic field should be exhibited by bismuthene-silicon nanostructures with cowrie shell-like geometries. One of these is resonant spin filtering where a peak in resonant transmission of one spin species through the nanostructure coincides with an antiresonant transmission dip for the other spin species. The second is strong nonresonant spin filtering where nonresonant transmission of one spin species is much weaker than that of the other spin species. These mechanisms arise from localized electronic states associated with strong disorder and strong spin-orbit coupling in the bismuthene and differ fundamentally from edge state-related spin filtering and from the spin Hall effect. Our density functional theory (DFT)-based simulations suggest that such cowrie shell-like nanostructures can form by spontaneous folding of bismuthene-silicon bilayer domes with armchair edges.

The interplay of disorder and strong correlations in quantum many-body systems remains an open question. That is despite much progress made in recent years with ultracold atoms in optical lattices to better understand phenomena such as many-body localization or the effect of disorder on Mott metal-insulator transitions. Here, we utilize the numerical linked-cluster expansion technique, extended to treat disordered quantum lattice models, and study exact thermodynamic properties of the disordered Fermi-Hubbard model on the square and cubic geometries. We consider box distributions for the disorder in the onsite energy, the interaction strength, as well as the hopping amplitude and explore how energy, double occupancy, entropy, heat capacity, and magnetic correlations of the system in the thermodynamic limit evolve as the strength of disorder changes. We compare our findings with those obtained from determinant quantum Monte Carlo simulations and discuss the relevance of our results to experiments with cold fermionic atoms in optical lattices.

Titanium sapphire is one of the most important laser crystals suitable for widely tunable and ultrashort pulsed lasers with high gain and high power outputs, but its performance is limited by the residual infrared absorption at the operation wavelength region of the laser. Although studies have been made over decades in improving the laser performance and the solutions to eliminate this residual absorption, there still remain some controversies for the binding tendency and charge-transfer transition energy of Ti related pairs, which is supposed to be the culprit of this residual absorption. In this paper, we clarify that previously predicted strong binding tendencies in ${\mathrm{Ti}}^{3+}\text{−}{\mathrm{Ti}}^{3+}$ and ${\mathrm{Ti}}^{3+}\text{−}{\mathrm{Ti}}^{4+}$ pairs should be artificial and are blamed on the intrinsic delocalization error in general approximate density functionals. We show that such errors can be eliminated by Hubbard $U$ corrected generalized-gradient-approximation method with $4\le U\le 5\mathrm{eV}$ or hybrid density functionals such like HSE06 and PBE0, which approximately satisfy the generalized Koopmans' condition. Our calculations reveal that the equilibrium geometry and electronic structure of ${\mathrm{Ti}}^{3+}\text{−}{\mathrm{Ti}}^{4+}$ pairs can be ${\mathrm{Ti}}^{3+}\text{−}{\mathrm{Ti}}^{4+}$-type or ${\mathrm{Ti}}^{3.5+}\text{−}{\mathrm{Ti}}^{3.5+}$-type configurations with very small energy differences ($\lesssim 0.1\mathrm{eV}$), and both of them have residual infrared absorption, which are predicted to be about three orders of magnitude stronger in oscillator strength per defect than the pump absorption of ${\mathrm{Ti}}^{3+}$ dopants. Regarding ${\mathrm{Ti}}^{3+}\text{−}{\mathrm{Ti}}^{3+}$ pairs, it is shown that they do not contribute to the residual infrared absorption in the wavelength range of laser operation, but their charge-transfer transitions can explain the residual UV band at 270 nm and $E$ band at 400 nm in the absorption spectra of Ti:${\mathrm{Al}}_2{\mathrm{O}}_3$ crystals. Furthermore, the charge-transfer transition energies and Stokes shift of ${\mathrm{Ti}}^{4+}$ and ionization energies for ${\mathrm{Ti}}^{3+}$ are also well interpreted by our calculations. The calculation method developed together with the predicted optical properties forms the basis for exploration on eliminating the residual infrared absorption in titanium sapphire.

Eliminating the effect of residual infrared absorption in titanium sapphire remains a crucial task to fulfill, despite that this kind of laser crystal has been developed for decades. The ${\mathrm{Ti}}^{3+}\text{−}{\mathrm{Ti}}^{4+}$ (${\mathrm{Ti}}_{\mathrm{Al}}^0\text{−}{\mathrm{Ti}}_{\mathrm{Al}}^{1+}$) pair model is the most widely accepted explanation to this residual absorption, but theoretical analyses based on first-principles calculation have not yet depicted a clear picture for the variation of the ${\mathrm{Ti}}_{\mathrm{Al}}^0\text{−}{\mathrm{Ti}}_{\mathrm{Al}}^{1+}$ pair and a variety of other potentially important defects in titanium sapphire when fabricating conditions change. Here, we extend the work in [W. Jing, M. Liu, J. Wen, L. Ning, M. Yin, and C.-K. Duan, Phys. Rev. B 104, 165103 (2021)] about binding tendencies of ${\mathrm{Ti}}_{\mathrm{Al}}\text{−}{\mathrm{Ti}}_{\mathrm{Al}}$ pairs and optical transition properties to providing a comprehensive understanding of the formation and processing condition dependence of various potentially significant defects and complexes besides the (${\mathrm{Ti}}_{\mathrm{Al}}\text{−}{\mathrm{Ti}}_{\mathrm{Al}}{)}^{0,1+}$ pairs. Apart from the complexes composed of $V_{\mathrm{Al}}^{3-}$ and ${\mathrm{Ti}}_{\mathrm{Al}}^{0,1+}$ defects, two new significant negatively charged defects, ${\left(V_{\mathrm{Al}}\text{−}{\mathrm{Al}}_i\text{−}{V}_{\mathrm{Al}}\right)}^{3-}$ and ${\left(V_{\mathrm{Al}}\text{−}{\mathrm{Ti}}_i\text{−}{V}_{\mathrm{Al}}\right)}^{2-}$ are revealed. These two complexes are correspondingly an interstitial ${\mathrm{Al}}^{3+}$ and ${\mathrm{Ti}}^{4+}$ surrounded by two neighboring aluminium vacancies $V_{\mathrm{Al}}^{3-}$. We show that $V_{\mathrm{Al}}^{3-}$ plays the key role to stabilize ${\mathrm{Ti}}_{\mathrm{Al}}^0\text{−}{\mathrm{Ti}}_{\mathrm{Al}}^{1+}$ and form various stable complexes with ${\mathrm{Ti}}_{\mathrm{Al}}^{1+}$ and ${\mathrm{Ti}}_{\mathrm{Al}}^0\text{−}{\mathrm{Ti}}_{\mathrm{Al}}^{1+}$ in weak and moderate reductive atmospheres. Thus, besides annealing at strong reductive atmosphere at elevated temperatures, Al ion injection or annealing in Al vapor is a potential method to eliminate the harmful residual infrared absorption, which is pinpointed at the reduction of the concentration of $V_{\mathrm{Al}}^{3-}$ and its variant ${\left(V_{\mathrm{Al}}\text{−}{\mathrm{Al}}_i\text{−}{V}_{\mathrm{Al}}\right)}^{3-}$. While in extremely strong reductive atmosphere, isolated Ti substitution dominates over those complexes containing $V_{\mathrm{Al}}^{3-}$, but there still remains a tiny amount of ${\mathrm{Ti}}_{\mathrm{Al}}^0\text{−}{\mathrm{Ti}}_{\mathrm{Al}}^{1+}$ and ${\mathrm{Ti}}_{\mathrm{Al}}^{1+}$, which are charge compensators to balance the charge of trace amount ${\mathrm{Ti}}_{\mathrm{Al}}^{1-}$ further reduced from ${\mathrm{Ti}}_{\mathrm{Al}}^0$. And this effect can be further eliminated by lower temperature reductive-atmosphere annealing. In addition, we obtain a simple numerical expression to predict the achievable figure of merit when the concentration of ${\mathrm{Ti}}_{\mathrm{Al}}^{1+}$ is given. Formation energies for simple defects and binding energies for complexes obtained in this work may serve as the basis for simulations and design various quenching and annealing processes to further reduce harmful defect species in titanium sapphire.

We investigate the real-space spectral properties of strongly correlated multi-impurity arrays in the Kondo insulator regime. Employing a recently developed mapping onto an effective correlated cluster problem makes the problem accessible to the numerical renormalization group. The evolution of the spectrum as a function of cluster size and cluster site is studied. We applied the extended Lieb-Mattis theorem to predict whether the spectral function must vanish at the Fermi energy developing a true pseudogap or whether the spectral function remains finite at $\omega =0$. Our numerical renormalization group spectra confirm the predictions of the theorem and shows a metallic behavior at the surface of a cluster prevailing in arbitrary spatial dimensions. We present a conventional minimal extension of a particle-hole symmetric Anderson lattice model at $U=0$ that leads to a gapped bulk band but a surface band with mainly $f$-orbital character for weak and moderate hybridization strength. The change in the site-dependent spectra upon introducing a Kondo hole in the center of the cluster is presented as a function of the hole-orbital energy. In particular, the spectral signatures across the Kosterlitz-Thouless-type quantum phase transition from a singlet to a local moment fixed point are discussed.

We investigated Ru-doped single-crystal $5d$ iridates, ${\mathrm{Sr}}_2{\mathrm{Ir}}_{1-x}{\mathrm{Ru}}_x{\mathrm{O}}_4$, at three different doping concentrations ($x=0.01$, 0.07, and 0.10) using optical spectroscopy. The undoped pristine compound (${\mathrm{Sr}}_2{\mathrm{IrO}}_4$) is known as a novel $J_{\text{eff}}=1/2$ Mott insulator. Remarkably, the optical conductivity spectra of all three samples exhibited insulating behavior, although we observed weak Drude components in the optical conductivity spectra down to the lowest temperature of 30 K. The charge-carrier densities of the Ru-doped iridates estimated from the Drude components are significantly smaller than the expected values estimated from the nominal Ru-doping concentrations. Herein, we provide the temperature- and doping-dependent electronic structure evolution of Ru-doped iridates. We expect that our results will be useful for understanding the intriguing Ru-doping-dependent properties of $5d$ iridate ${\mathrm{Sr}}_2{\mathrm{IrO}}_4$.

We consider a three-dimensional topological insulator (TI) wire with a nonuniform chemical potential induced by gating across the cross section. This inhomogeneity in chemical potential lifts the degeneracy between two one-dimensional surface state subbands. A magnetic field applied along the wire, due to orbital effects, breaks time-reversal symmetry and lifts the Kramers degeneracy at zero momentum. If placed in proximity to an $s$-wave superconductor, the system can be brought into a topological phase at relatively weak magnetic fields. Majorana bound states (MBSs), localized at the ends of the TI wire, emerge and are present for an exceptionally large region of parameter space in realistic systems. Unlike in previous proposals, these MBSs occur without the requirement of a vortex in the superconducting pairing potential, which represents a significant simplification for experiments. Our results open a pathway to the realization of MBSs in present-day TI wire devices.

The Shockley surface state located at $\overline{\mathrm{Y}}$ on the $(1\times 2)$-reconstructed Au(110) surface is predicted to exhibit a Rashba-type spin splitting. Previous photoemission experiments searched for this splitting but it could not be resolved yet. In order to uncover a possible splitting, the unoccupied surface state on Au(110) is examined with spin- and angle-resolved inverse photoemission, whereas Na-covered Au(110) allows for investigation of the now occupied surface state by means of spin- and angle-resolved direct photoemission. Our data show clear spin splittings in the order of $100\mathrm{meV}$ with a sign reversal at $\overline{\mathrm{Y}}$ in the surface state's in-plane spin components which is characteristic for a Rashba-type behavior. Furthermore, we deduce an effective mass of $m^{*}=(0.27\pm 0.02)m_e$ and a Rashba parameter of ${\alpha }_R=(0.46\pm 0.04)\mathrm{eV}\AA$ from direct photoemission measurements.

A weak perpendicular magnetic field, $B$, breaks the chiral symmetry of each valley in the electron spectrum of graphene, preserving the overall chiral symmetry in the Brillouin zone. We explore the consequences of this symmetry breaking for the interaction effects in graphene. In particular, we demonstrate that the electron-electron interaction lifetime acquires an anomalous $B$ dependence. Also, the ballistic zero-bias anomaly, $\delta \nu \left(\omega \right)$, where $\omega$ is the energy measured from the Fermi level, emerges at a weak $B$ and has the form $\delta \nu \left(B\right)\sim B^2/{\omega }^2$. Temperature dependence of the magnetic-field corrections to the thermodynamic characteristics of graphene is also anomalous. We discuss experimental manifestations of the effects predicted. The microscopic origin of the $B$-field sensitivity is an extra phase acquired by the electron wave function resulting from the chirality-induced pseudospin precession.

In this Research Letter, we study the transient carrier dynamics and the nonlinear terahertz effects induced by an intense multicycle terahertz pulse in the $n$-doped semiconductor ${\mathrm{In}}_{0.57}{\mathrm{Ga}}_{0.43}\mathrm{As}$. For incident terahertz pulses with relatively high frequency $\ge 1$ THz, we reveal the generation of low- and high-frequency components in the transmitted intensity spectra. We study the time emission of these components at different frequencies using the Gábor transform, showing that the low- and high-frequency components are due to the nonparabolicity of the conduction band. Our results demonstrate that for a multicycle terahertz pulse, we generate a distorted biphasic truncated exponential current with the driving terahertz fields, leading to discrete high-order harmonic generation in the transmitted intensity spectra.

The magnetically frustrated manganese nitride antiperovskite family displays significant changes of entropy under hydrostatic pressure that can be useful for the emerging field of barocaloric cooling. Here we show that barocaloric properties of metallic antiperovskite Mn nitrides can be tailored for room-temperature application through quaternary alloying. We find an enhanced entropy change of $|\mathrm{\Delta }{S}_{\mathrm{t}}|=37\mathrm{J}{\mathrm{K}}^{-1}{\mathrm{kg}}^{-1}$ at the $T_t=300\mathrm{K}$ antiferromagnetic transition of quaternary ${\mathrm{Mn}}_3{\mathrm{Zn}}_{0.5}{\mathrm{In}}_{0.5}\mathrm{N}$ relative to the ternary end members. The pressure-driven barocaloric entropy change of ${\mathrm{Mn}}_3{\mathrm{Zn}}_{0.5}{\mathrm{In}}_{0.5}\mathrm{N}$ reaches $|\mathrm{\Delta }{S}_{\mathrm{BCE}}|=20\mathrm{J}{\mathrm{K}}^{-1}{\mathrm{kg}}^{-1}$ in 2.9 kbar. Our results open up a large phase space where compounds with improved barocaloric properties may be found.

A metal can be driven to an insulating phase through distinct mechanisms. A possible way is via the Coulomb interaction, which then defines the Mott metal-insulator transition (MIT). Another possibility is the MIT driven by disorder, the so-called Anderson MIT. Here we analyze interacting particles in disordered Hubbard chains—thus comprising the Mott-Anderson physics—by investigating the ground-state entanglement with density functional theory. The localization signature on entanglement is found to be a decreasing on the entanglement degree in comparison to the clean (without interaction and disorder) case, with local minima at certain critical densities. Individually, the Mott (Anderson) MIT has a single critical density whose minimum entanglement decreases as the interaction (disorder) enhances. While in the Mott MIT entanglement saturates at finite values, characterizing partial localization, in the Anderson MIT the system reaches full localization, with zero entanglement, for sufficiently strong disorder. In the combined Mott-Anderson MIT, we find three critical densities referring to local minima on entanglement. One of them is the same as for the Anderson MIT, but now the presence of interaction requires a stronger disorder potential to induce full localization. A second critical density is related to the Mott MIT, but due to disorder it is displaced by a factor proportional to the concentration of impurities. The third local minimum on entanglement is unique to the concomitant presence of disorder and interaction, found to be related to an effective density phenomenon, thus referred to as a Mott-like MIT. Since entanglement has been intrinsically connected to the magnetic susceptibility—a quantity promptly available in cold-atom experiments—our detailed numerical description might be useful for the experimental investigation of the Mott-Anderson MIT.

A theoretical model for the coherent control of longitudinal optical phonons in GaAs (001) by double-pulse excitation was derived using a simplified band model and the allowed and forbidden Raman scattering. The time evolution of the electron-phonon states was calculated with the density-matrix formalism and second-order perturbation. The amplitude of the longitudinal optical phonons controlled by the two pulses was obtained as a function of the delay between the pulses for several polarization conditions. For parallel-polarized pulses, electronic and phonon interference fringes were predicted, which were independent of the crystal orientation and the ratio between the allowed and forbidden Raman scattering intensities. For orthogonally polarized pulses, only phonon interference fringes were obtained at an angle of $\pi /4$ from the [100] direction. When one of the pulses was along the [100] direction, electronic interference fringes were induced by the allowed Raman scattering, although the pump pulses do not interfere themselves.

We investigate current-induced spin-orbit torque (SOT) in Co/Pt/oxide systems by varying a sort of the oxide layer. When the Pt thickness is less than ∼2 nm, Pt at the interface of the oxide layer is magnetically polarized owing to the magnetic proximity effect. In these systems, fieldlike (FL) SOT depends significantly on the adjacent oxide material, whereas dampinglike SOT is almost irrelevant to oxides. The FL SOT efficiency of the $\mathrm{Pt}/\mathrm{Hf}{\mathrm{O}}_2$ sample is 2.2 times greater than that of the Pt/MgO sample at the maximum. X-ray magnetic circular dichroism spectroscopy reveals that the anisotropy of the Pt orbital magnetic moment varies with the oxide material, suggesting that the modulation of the electronic structure at the Pt/oxide interface contributes to SOT enhancement.