We compute the electronic structure, spin and charge state of Fe ions, and the structural phase stability of paramagnetic ${\mathrm{CaFeO}}_3$ under pressure using a fully self-consistent in charge density $\mathrm{DFT}+$ dynamical mean-field theory method. We show that at ambient pressure ${\mathrm{CaFeO}}_3$ is a negative charge transfer insulator characterized by strong localization of the Fe $3d$ electrons. It crystallizes in the monoclinic $P{2}_1/n$ crystal structure with a cooperative breathing mode distortion of the lattice. While the Fe $3d$ Wannier occupations and local moments are consistent with robust charge disproportionation of Fe ions in the insulating $P{2}_1/n$ phase, the physical charge density difference around the structurally distinct Fe $A$ and Fe $B$ ions with the “contracted” and “expanded” oxygen octahedra, respectively, is rather weak, of $\sim 0.04$. This implies the importance of the Fe $3d$ and O $2p$ negative charge transfer and supports the formation of a bond-disproportionated state characterized by the Fe $A 3d^{5-\delta }{\underline{L}}^{2-\delta }$ and Fe $B 3d^5$ valence configurations with $\delta \ll 1$, in agreement with strong hybridization between the Fe $3d$ and O $2p$ states. This complex interplay between electronic correlations, strong covalency, and lattice effects, resulting in bond disproportionation, is in many ways reminiscent of the behavior of rare-earth nickelates, $R{\mathrm{NiO}}_3$ $(R=\text{rare earth})$. Upon compression, ${\mathrm{CaFeO}}_3$ undergoes the metal-to-insulator phase transition (MIT) which is accompanied by a structural transformation into the orthorhombic $Pbnm$ phase. The phase transition is accompanied by suppression of the cooperative breathing mode distortion of the lattice and, hence, results in the melting of bond disproportionation of the Fe ions. Our analysis suggests that the MIT transition is associated with orbital-dependent delocalization of the Fe $3d$ electrons and leads to a remarkable collapse of the local magnetic moments. Our results imply the crucial importance of the interplay of electronic correlations and structural effects to explain the properties of ${\mathrm{CaFeO}}_3$.

Detailed first-principles calculations have been carried out to study the stabilization, excitation and luminescence mechanisms of the ion ${\mathrm{Mn}}^{3+}$ in a series of $A_3{B}_2{B}_3^{\prime }{\mathrm{O}}_{12}$ garnet hosts. The formation energy shows that ${\mathrm{Mn}}^{3+}$ is dominant and is situated at the octahedral $B$ site. The excited states, excitation, and emission energies of ${\mathrm{Mn}}^{3+}$ have then been calculated. The calculated energy levels of ${\mathrm{Mn}}^{3+}$ confirm that the red emission is due to the ${}^5{\mathrm{T}}_2\to {}^5{\mathrm{E}}^{\prime }$ transition and the near-infrared (NIR) emission arises from the ${}^1{\mathrm{T}}_2\to {}^3{\mathrm{T}}_1$ transition. The populations of the ${}^5{\mathrm{T}}_2$ and ${}^1{\mathrm{T}}_2$ excited states and the corresponding radiative rates lead to the temperature dependence of the red to NIR emission. Furthermore, the adiabatic potential energy surfaces along the $A_{1g}$ and $E_g$ moeity modes of [${\mathrm{MnO}}_6$] have been calculated and fitted well in the harmonic approximation. The high activation energy for ${\mathrm{Mn}}^{3+}$ indicates a low nonradiative multiphonon relaxation rate of ${}^5{\mathrm{T}}_2$ to ${}^3{\mathrm{T}}_1$. Hence, the ionization process was considered, and we show that it is responsible for the luminescence quenching of ${\mathrm{Mn}}^{3+}$, so that the luminescence has rarely been reported experimentally. This work illustrates a well-designed approach based on the density-functional theory framework to predict the optical transition properties of the transition metal ion ${\mathrm{Mn}}^{3+}$ by calculating the structural distortions due to the Jahn-Teller effect, the optical transitions, quenching processes and the influence of pressure.

Topological systems are characterized by integer Chern invariants. In a continuous photonic system characterized by a local Drude model, the material response is ill-behaved at large wavenumbers, leading to noninteger Chern invariants and ambiguity in the existence of topological edge modes. This problem has been solved previously by introducing an ad hoc material model including a spatial cutoff material response, which leads to a finite Brillouin zone and integer invariants. In this work, we calculate Chern numbers in magnetized continuous plasma systems by considering the effect of nonlocality using a hydrodynamic (HD) Drude model. Then we argue that this model presents several advantages compared with the previous models, e.g., introducing physical response at large wavenumbers and integer Chern invariants with sum to zero without the need for an interpolated material response. Therefore the HD model forms a complete and self-consistent model, which resolves the Chern number issues in topological photonic continua.

We prove that a suitable correlation between system parameters can trigger topological phase transitions and flat bands in a multistrand Creutz ladder network when a staggered second neighbor interaction is included along the $x$ axis. An appropriate change of basis maps such a finite $N$-strand mesh into $N$ or $N-1$ decoupled Su-Schrieffer-Heeger chains, depending on $N$ even or odd. A simple intuitive method, using a real-space decimation scheme, turns out to be very powerful in analytically extracting the flat bands, explaining their degeneracy, or a lifting of same. Our results are analytically exact and may inspire experiments in photonics and ultracold atomic systems.

Fermionic quasiparticles in solids have attracted tremendous attention in the last decades. The conceptual framework of quasiparticles has recently been extended to bosonic systems, especially the phononic one. In this work we focus on a peculiar phononic topological category that features higher-order nodal points in two-dimensional (2D) systems. By searching through the total 80 layer groups, we find that the rotation symmetry (except the twofold one) combined with the time-reversal symmetry could support the presence of higher-order nodal points. We further find that the highest order of momentum in the 2D system is the second order, dubbed as quadratic nodal point (QNP). We show that the 2D QNP can be characterized by an integer topological invariant, demonstrated by the presence of edge states. Remarkably, this work reveals the mechanism for the band structure deforming, such as the tilting of the QNP cone and the warping of constant frequency surfaces. The work paves the way to study the higher-order nodal point in phononic systems, and several candidate materials are also provided.

We present a generalized mathematical model for predicting the transport properties of a quantum system exposed to a stationary magnetic field and a high-intensity electromagnetic field. The new formulation, which applies to two-dimensional (2D) dressed quantum Hall systems, is based on Landau quantization theory and the Floquet-Drude conductivity approach. We model our system as a two-dimensional electron gas (2DEG) that interacts with two external fields. To analyze the strong light coupling with the 2DEG, we employ the Floquet theory as a nonperturbative procedure. Moreover, the Floquet-Fermi golden rule is adopted to explore the impurity scattering effects on charge transport in disordered quantum Hall systems. We derive fully analytical expressions to describe longitudinal components in the conductivity tensor in dressed quantum Hall systems. Subsequently, we demonstrate that the conductivity characteristics of quantum Hall systems can be manipulated using strong external light. Our results align with well-established and experimentally verified theoretical descriptions for undressed systems while providing a more generalized analysis of the conductivity characteristics in quantum Hall systems. Thus our model can be applied to accurately interpret the usage of strong external radiation as a tool in nanoscale quantum devices.

Following previous works reporting an anomalous behavior of several zone-center optical phonons across the magnetic transition of square lattice iridates ${\mathrm{Sr}}_2{\mathrm{IrO}}_4$ and ${\mathrm{Sr}}_3{\mathrm{Ir}}_2{\mathrm{O}}_7$, we have investigated the lattice dynamics as a function of momentum in these materials by means of high-resolution inelastic x-ray scattering. The observed phonon energies and scattering intensities across the Brillouin zone are in excellent agreement with ab initio lattice dynamical calculations based on nonmagnetic density-functional-perturbation theory. Our results do not evidence any renormalization of the phonons at finite momentum across the magnetic transition of ${\mathrm{Sr}}_2{\mathrm{IrO}}_4$. The only anomalous behavior was detected for the in-plane polarized longitudinal-acoustic phonon branch in ${\mathrm{Sr}}_3{\mathrm{Ir}}_2{\mathrm{O}}_7$, which anomalously softens towards low temperatures and might be related to anisotropic negative thermal expansion in this compound. No anomalies related to potential charge ordering were observed.

We report the direct evidence of impacts of the Coulomb interaction in a prototypical Weyl semimetal, ${\mathrm{MoTe}}_2$, that alter its bare bands in a wide range of energy and momentum. Our quasiparticle interference patterns measured using scanning tunneling microscopy are shown to match the joint density of states of quasiparticle energy bands including momentum-dependent self-energy corrections, while electronic energy bands based on the other simpler local approximations of the Coulomb interaction fail to explain neither the correct number of quasiparticle pockets nor the shape of their dispersions observed in our spectrum. With this, we predict a transition between type-I and type-II Weyl fermions with doping and resolve its disparate quantum oscillation experiments, thus highlighting the critical roles of Coulomb interactions in layered Weyl semimetals.

Recent developments in fractional quantum Hall (FQH) physics highlight the importance of studying FQH phases of particles partially occupying energy bands that are not Landau levels. FQH phases in the regime of strong lattice effects, called fractional Chern insulators, provide one setting for such studies. As the strength of lattice effects vanishes, the bands of generic lattice models asymptotically approach Landau levels. In this paper, we construct lattice models for single-particle bands that are distinct from Landau levels even in this continuum limit. We describe how the distinction between such bands and Landau levels is quantified by band geometry over the magnetic Brillouin zone and reflected in the electromagnetic response. We analyze the localization-delocalization transition in one such model and compute a localization length exponent of 2.57(3). Moreover, we study interactions projected to these bands and find signatures of bosonic and fermionic Laughlin states. Most pertinently, our models allow us to isolate conditions for optimal band geometry and gain further insight into the stability of FQH phases on lattices.

Sliced-basis DMRG(sb-DMRG) is used to simulate a chain of hydrogen atoms and to construct low-energy effective Hubbard-like models. The downfolding procedure first involves a change of basis to a set of atom-centered Wannier functions constructed from the natural orbitals of the exact DMRG one-particle density matrix. The Wannier function model is then reduced to a fewer-parameter Hubbard-like model, whose parameters are determined by minimizing the expectation value of the Wannier Hamiltonian in the ground state of the Hubbard Hamiltonian. This indirect variational procedure not only yields compact and simple models for the hydrogen chain, but also allows us to explore the importance of constraints in the effective Hamiltonian, such as the restricting the range of the single-particle hopping and two-particle interactions, and to assess the reliability of more conventional downfolding. The entanglement entropy for a model's ground state, cut in the middle, is an important property determining the ability of DMRG and tensor networks to simulate the model, and we study its variation with the range of the interactions. Counterintuitively, we find that shorter ranged interactions often have larger entanglement.

Exotic tiling patterns of quasicrystals have motivated extensive studies of quantum phenomena such as critical states and phasons. Nevertheless, a systematic understanding of the Landau levels of quasicrystals in the presence of the magnetic field has not been established yet. One of the main obstacles is the complication of the quasiperiodic tilings without periodic length scales, thus it has been thought that the system cannot possess any universal features of the Landau levels. In this paper, contrary to these assertions, we develop a generic theory of the Landau levels for quasicrystals. Focusing on two-dimensional quasicrystals with rotational symmetries, we highlight that quasiperiodic tilings induce anomalous Landau levels where electrons are localized near the rotational symmetry centers. Interestingly, the localization length of these Landau levels has a universal dependence on $\mathbf{n}$ for quasicrystals with $\mathbf{n}$-fold rotational symmetry. Furthermore, macroscopically degenerate zero-energy Landau levels are present due to the chiral symmetry of the rhombic tilings. In this case, each Landau level forms an independent island where electrons are trapped at given fields, but with field control, the interference between the islands gives rise to an abrupt change in the local density of states. Our work provides a general scheme to understand the electron localization behavior of the Landau levels in quasicrystals.

For degenerately doped three-dimensional topological insulators (3D TIs), the interaction dynamics between Dirac fermions and collective charge oscillations (plasmons) are considerably complicated. In particular, the low-energy contribution near band inversion is largely obscure because of the bulk-surface interaction as well as the two-dimensional electron gas (2DEG). We have circumvented these issues by performing high-field terahertz (THz) pump-probe spectroscopy in bulk insulating ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ TI microribbon array structures. Plasmon blueshift of 11% was found to occur within 4 ps under 0.433 MV/cm THz excitation compared to the near-equilibrium state. Although the conventional optical pump or engineered microribbon TIs exhibits a similar plasmon blueshift, our observation is readily explained by the intrinsic plasmon behavior from the increased electronic temperature ($T_{\mathrm{e}}\cong 1430\mathrm{K}$) and reduced chemical potential ($\mu \cong 16\mathrm{meV}$) without considering extrinsic contributions. Our findings may apply to other classes of Dirac materials to investigate the coupled low-energy interactions with collective quasiparticles.

The search for new two-dimensional (2D) semiconductors with strong spin-orbit coupling, merging Rashba effect with valley physics, is essential for advancing the emerging fields of spintronics and valleytronics. $h\text{−}M\mathrm{N}$ $(M=\mathrm{Nb},\mathrm{Ta})$ monolayers are found to host valley physics together with Rashba effect. $h$-NbN (TaN) monolayers show Zeeman-type valley spin splitting of 32 (112) meV and 130 (406) meV at valence-band and conduction-band edges, respectively, based on density-functional theory calculations. One of the three degenerate valleys around the $K/K^{\prime }$ point in the first Brillouin zone (BZ) lies fully enclosed within the first BZ. Berry curvature $\sim 50\left(73\right){\AA }^2$ shown by h-NbN (TaN) monolayers is much higher than that in ${\mathrm{MoS}}_2$ monolayer $(\sim 11{\AA }^2)$, owing to the lower band gap and wave vector magnitude in $h$-NbN (TaN) monolayers. The Rashba energy splitting and Rashba constant induced by strong spin-orbit coupling (SOC) in $h$-NbN (TaN) monolayer is found to be 52 (74) meV and 2.9 (4.23) eV Å respectively, which are amongst the giant Rashba spin-splitting parameters observed so far in 2D materials. The degree of Zeeman-type valley spin splitting and Rashba-type spin splitting is substantially tunable via in-plane biaxial strain and out of-plane electric field. Higher SOC in $h$-TaN monolayer relative to $h$-NbN monolayer caused by the heavier Ta atoms gives rise to a higher Berry curvature, valley-, and Rashba spin splitting in the former as compared to the latter. Valleytronic and spintronic properties in the studied monolayers are found to be superior to that in $h\text{−}{\mathrm{MoS}}_2$ and Janus MoSSe monolayers and are therefore proposed for an effective coupling of spin and valley physics.

We study theoretically the effect of a rotating electric field on a diffusive nanowire and find an effect that is analogous to spin pumping, which refers to the generation of spin through a rotating magnetic field. The electron spin couples to the electric field because the particle motion induces an effective magnetic field in its rest frame. In a diffusive system the velocity of the particle, and therefore also its effective magnetic field, rapidly and randomly changes direction. Nevertheless, we demonstrate analytically and via a physical argument why the combination of the two effects described above produces a finite magnetization along the axis of rotation. This manifests as a measurable spin-voltage in the range of tens of microvolts.

Recent experimental developments in hydrogen-rich materials at high pressures have put this class of materials above others in the race toward room-temperature superconductivity. As it is the basis of all the materials in this class, the efforts to determine the properties of pure solid hydrogen at high pressures remain intense. Most notably, a recent experimental study of the metallization of hydrogen identified the crystal phase of the solid as the $C2\text{/}c\text{−}24$ molecular phase up to ∼425 GPa [Loubeyre et al., Nature (London) 577, 631 (2020)]. It is possible that the observed metallization is caused by band structure effects and not a structural phase transition, and the material remains in this crystal phase up to higher pressures [Dogan et al., J. Phys.: Condens. Matter 33, 03LT01 (2020)]. Therefore, it is of crucial importance to determine the superconducting properties of the $C2\text{/}c\text{−}24$ phase. Here, we employ a Wannier function-based dense $k$-point and $q$-point sampling to compute the electron-phonon coupling and superconducting properties of molecular hydrogen in the $C2\text{/}c\text{−}24$ phase. We find that the material has a high superconducting transition temperature of 242 K at 500 GPa. We also find that the transition temperature rapidly increases with pressure in the 400–500-GPa range.

Recently, the superconducting transition temperature $T_c=287\mathrm{K}$ has been experimentally obtained in a material composed of carbon, sulfur, and hydrogen under a high pressure of 267 GPa. The material structure is currently unknown, where carbon and sulfur were added at a molar ratio of $1:1$. Here, fixing the molar ratio of $\text{C}:\text{S}=1:1$, we studied several possible C-S-H structures, and found a stable structure ${\mathrm{C}}_2{\mathrm{S}}_2{\mathrm{H}}_4$ using first-principles calculations. ${\mathrm{C}}_2{\mathrm{S}}_2{\mathrm{H}}_4$ shows an insulator-to-metal transition and a superconducting ground state at a pressure of 64 GPa, and its $T_c$ can reach 16.5 K at 300 GPa. In addition, we found another stable structure of ${\mathrm{C}}_2{\mathrm{S}}_3{\mathrm{H}}_4$, whose $T_c$ is 47.4 K at 300 GPa. The calculations show that the added S atom in ${\mathrm{C}}_2{\mathrm{S}}_3{\mathrm{H}}_4$ breaks part of the C-H bonds in ${\mathrm{C}}_2{\mathrm{S}}_2{\mathrm{H}}_4$, makes the vibration of H atom occur at a lower frequency, and thus enhances the electron-phonon coupling and $T_c$. Our results suggest that a molar ratio of $\text{C}:\text{S}$ lower than $1:1$ in C-S-H systems may be favorable to enhance $T_c$. This can be useful to figure out the structure of a C-S-H material with room-temperature $T_c$ in a recent experiment.

We present a combined study of thermal expansion and resistance fluctuation spectroscopy measurements exploring the static and dynamic aspects of the charge-glass formation in the quasi-two-dimensional organic conductors $\theta \text{−}({\mathrm{BEDT-TTF})}_{2}M{M}^{\prime }{\left(\mathrm{SCN}\right)}_4$ with $M$ = Cs and $M^{\prime }$ = Co,Zn. In these materials, the emergence of a novel charge-glass state so far has been interpreted in purely electronic terms by considering the strong frustration of the Coulomb interactions on a triangular lattice. Contrary to this view, we provide comprehensive evidence for the involvement of a structural glasslike transition at $T_{\text{g}}\sim 90-100\mathrm{K}$. This glassy transition can be assigned to the freezing of structural conformations of the ethylene endgroups in the donor molecule with an activation energy of $E_{\mathrm{a}}\approx 0.32\mathrm{eV}$, and the concomitant slowing down of the charge-carrier dynamics is well described by a model of nonexponential kinetics. These findings disclose an important aspect of the phase diagram and calls for revisiting the present views of the glassy dynamics in the whole family of $\theta \text{−}({\mathrm{BEDT-TTF})}_{2}M{M}^{\prime }{\left(\mathrm{SCN}\right)}_4$. Our results suggest that the entanglement of slow structural and charge-cluster dynamics due to the intimate coupling of lattice and electronic degrees of freedom determine the charge-glass formation under geometric frustration.

The Shastry-Sutherland (SS) model as a canonical example of frustrated magnetism has been extensively studied. The conventional wisdom has been that the transition from the plaquette valence bond order to the Neel order is direct and potentially realizes a deconfined quantum critical point beyond the Ginzburg-Landau paradigm. This scenario, however, was challenged recently by improved numerics from density matrix renormalization group which offers evidence for a narrow gapless spin liquid between the two phases. Prompted by this controversy and to shed light on this intricate parameter regime from a fresh perspective, we report high-resolution functional renormalization group analysis of the generalized SS model. The flows of over 50 million running couplings provide a detailed picture for the evolution of spin correlations as the frequency/energy scale is dialed from the ultraviolet to the infrared to yield the zero-temperature phase diagram. The singlet dimer phase emerges as a fixed point, the Neel order is characterized by divergence in the vertex function, while the transition into and out of the plaquette order is accompanied by pronounced peaks in the plaquette susceptibility. The plaquette order is suppressed before the onset of the Neel order, lending evidence for a finite spin liquid region for $J_1/J_2\in (0.77,0.82)$, where the flow is continuous without any indication of divergence.

We report magnetophotoluminescence spectroscopy of gated ${\mathrm{MoS}}_2$ monolayers in high magnetic fields to 28 T. At $B=0$ T and electron density $n_s\sim {10}^{12} {\mathrm{cm}}^{-2}$, we observe three trion resonances that cannot be explained within a single-particle picture. Employing ab initio calculations that take into account three-particle correlation effects as well as local and nonlocal electron-hole exchange interaction, we identify those features as quantum superpositions of inter- and intravalley spin states. We experimentally investigate the mixed character of the trion wave function via the filling factor dependent valley Zeeman shift in positive and negative magnetic fields. Our results highlight the importance of exchange interactions for exciton physics in monolayer ${\mathrm{MoS}}_2$ and provide insights into the microscopic understanding of trion physics in two-dimensional multivalley semiconductors for low excess carrier densities.