We discuss strategies for thermalization of the ground-state meta-generalized gradient approximation (meta-GGA) exchange-correlation (XC) functionals. A simple but accurate scheme is implemented via universal additive thermal correction to XC using a perturbativelike self-consistent approach. The additive correction with explicit temperature dependence is applied to the ground-state deorbitalized, strongly constrained, and appropriately normed (SCAN-L) meta-GGA XC leading to a thermal XC functional denoted here as T-SCAN-L. The thermal T-SCAN-L meta-GGA functional shows significant improvement in density functional theory calculation accuracy for warm dense matter by a factor of 3 to 10, achieving an accuracy of total pressure between a few tenths and $\sim 1$% when compared to traditional XC functionals, as demonstrated by the comparison to path-integral Monte Carlo simulations for helium equation of state. The T-SCAN-L calculations of dc conductivity of warm dense aluminum also give better agreement with experiments over other XC functionals such as PBE and SCAN-L.

The structural and vibrational behavior of $\mathrm{Mg}{\left({\mathrm{IO}}_3\right)}_2$ under compression has been investigated via a combination of high-pressure (HP) synchrotron x-ray diffraction (XRD), Raman scattering, and infrared spectroscopy experiments as well as first-principles ab initio calculations. In this paper, we reveal that $\mathrm{Mg}{\left({\mathrm{IO}}_3\right)}_2$ undergoes a pressure-induced phase transition between 7.5 and 9.7 GPa at ambient temperature from a monoclinic (space group $P{2}_1$) to a trigonal (space group P3) structure. $\mathrm{Mg}{\left({\mathrm{IO}}_3\right)}_2$ also exhibits the gradual formation of additional bonds between iodine and oxygen atoms in neighboring ${\mathrm{IO}}_3^{-}$ units with increasing pressure, thereby increasing the oxygen-iodine coordination from 3 to 6. The bond formation under compression is a consequence of the existence of lone electron pairs on the iodine cation. To accommodate the additional bonds, the I–O bonds within the original ${\left[{\mathrm{IO}}_3\right]}^{\text{–}}$ trigonal pyramids increase in length under increasing compression. The appearance of additional Raman modes at 7.7 GPa and infrared modes at 9.6 GPa supports the phase transition observed in XRD experiments. Interestingly, the lengthening of I–O bonds causes a softening of several Raman modes under compression. We provide the crystal structure of the HP phase, the pressure-volume equations of state for both low- and HP phases, and the symmetry assignment of the Raman- and infrared-active modes of both phases.

We study the properties of flat bands that appear in a heterostructure composed of strands of different widths of graphene armchair nanoribbons. One of the flat bands is reminiscent of the one that appears in pristine armchair nanoribbons and has its origin in a quantum mechanical destructive interference effect, dubbed “Wannier orbital states” by Lin et al. in Phys. Rev. B 79, 035405 (2009). The additional flat bands found in these heterostructures, some reasonably closer to the Fermi level, seem to be generated by a similar interference process. After doing a thorough tight-binding analysis of the band structures of the different kinds of heterostructures, focusing on the properties of the flat bands, we use density functional theory to study the possibility of magnetic ground states when placing, through doping, the Fermi energy close to the different flat bands. Our DFT results confirmed the expectation that these heterostructures, after being appropriately hole doped, develop a ferromagnetic ground state that seems to require, as in the case of pristine armchair nanoribbons, the presence of a dispersive band crossing the flat band. In addition, we found a remarkable agreement between the tight-binding and DFT results for the charge density distribution of the so-called Wannier orbital states.

Iron(IV) oxides are strongly correlated materials with negative charge-transfer energy (negative Δ), and exhibit peculiar electronic and magnetic properties such as topological helical spin structures in the metallic cubic perovskite ${\mathrm{SrFeO}}_3$. Here, the spin structure of the layered negative-Δ insulator ${\mathrm{Sr}}_2{\mathrm{FeO}}_4$ was studied by powder neutron diffraction in zero field and magnetic fields up to 6.5 T. Below $T_{\mathrm{N}}=56\mathrm{K}$, ${\mathrm{Sr}}_2{\mathrm{FeO}}_4$ adopts an elliptical cycloidal spin structure with modulated magnetic moments between 1.9 and 3.5 ${\mu }_{\mathrm{B}}$ and a propagation vector $\mathit{k}=\left(\tau ,\tau ,0\right)$ with $\tau =0.137$. With increasing magnetic field the spin structure undergoes a spin-flop transition near 5 T. Synchrotron ${}^{57}\mathrm{Fe}$-Mössbauer spectroscopy reveals that the spin spiral transforms to a ferromagnetic structure at pressures between 5 and 8 GPa, just in the pressure range where a Raman-active phonon nonintrinsic to the ${\mathrm{K}}_2{\mathrm{NiF}}_4$-type crystal structure vanishes. These results indicate an insulating ground state which is stabilized by a hidden structural distortion and differs from the charge disproportionation in other Fe(IV) oxides.

We present a comprehensive investigation of the magnetic properties of stage-1 graphite intercalated $\mathrm{Fe}{\mathrm{Cl}}_3$ using a combination of DC and AC magnetic susceptibility, thermoremanent magnetization, and field-dependent magnetization measurements. This van der Waals system, with a centrosymmetric honeycomb lattice, combines frustration and disorder, due to intercalation, and may be hosting topologically nontrivial magnetic phases. Our study identifies two magnetic phase transitions at $T_{f1}\approx 4.2$ K and at $T_{f2}\approx 2.7$ K. We find that the paramagnetic state, for $T>T_{f1}$, is dominated by short-range ferromagnetic correlations. These build up well above $T_{f1}$ and lead to a significant change in magnetic entropy, which reaches $\mathrm{\Delta }{S}_M^{Pk}=-5.52$ J ${\mathrm{kg}}^{-1}{\mathrm{K}}^{-1}$ at 7 T. Between $T_{f1}$ and $T_{f2}$, we observe slow spin dynamics characteristic of a cluster glasslike state, whereas for $T<T_{f2}$, our results indicate the onset of a low-temperature long-range ordered state. The analysis of the experimental results leads to a complex phase diagram, which may serve as a reference for future investigations searching for topological nontrivial phases in this system.

Under suitable conditions, some twisted graphene multilayers and transition-metal dichalcogenides become Chern insulators, exhibiting the anomalous quantum Hall effect and orbital magnetization due to spontaneous valley polarization. We study the interaction of a Chern insulator with circularly polarized light. The interaction energy contains an antisymmetric term that couples to the helicity of incident light. For a two-band Chern insulator, this term is expressed as an integral involving the Berry curvature of the system. Taking advantage of this interaction, we propose an experimental protocol for switching topological memory based on orbital magnetization by circularly polarized light. Moreover, two laser beams of opposite circular polarization can nucleate domains of opposite magnetization and thus produce an optically configurable domain wall carrying topologically protected chiral edge modes.

${\mathrm{Cs}}_2{\mathrm{Cu}}_3{\mathrm{SnF}}_{12}$ is a spin-1/2 antiferromagnet on a nearly uniform kagome lattice with an exchange interaction of $J=20.7$ meV. This compound undergoes magnetic ordering at $T_{\mathrm{N}}=20.2$ K with the $\mathit{q}=0$ structure and positive chirality, which is mainly caused by the large Dzyaloshinskii-Moriya interaction. ${\mathrm{Rb}}_2{\mathrm{Cu}}_3{\mathrm{SnF}}_{12}$ is a spin-1/2 antiferromagnet on a modified kagome lattice with a $2a\times 2a$ enlarged chemical unit cell at room temperature resulting in four kinds of nearest-neighbor exchange interaction, the average of which is $J_{\mathrm{avg}}=15.6$ meV. Its ground state is a pinwheel valence bond solid (VBS) with an excitation gap. Here, we show the structures of magnetic excitations in ${\mathrm{Cs}}_2{\mathrm{Cu}}_3{\mathrm{SnF}}_{12}$ and ${\mathrm{Rb}}_2{\mathrm{Cu}}_3{\mathrm{SnF}}_{12}$ investigated by inelastic neutron scattering in wide energy and momentum ranges. For ${\mathrm{Cs}}_2{\mathrm{Cu}}_3{\mathrm{SnF}}_{12}$, four single-magnon excitation modes were observed. Three low-energy modes are assigned to be transverse modes and the high-energy fourth mode is suggested to be an amplitude mode. It was confirmed that the energy of single-magnon excitations arising from the ${\mathrm{\Gamma }}^{\prime }$ point in the extended Brillouin zones is largely renormalized downwards. It was found that the broad excitation continuum without a marked structure spreads in a wide energy range from $0.15J$ to approximately $2.5J$ in contrast to the clearly structured excitation continuum observed in the spin-1/2 triangular-lattice Heisenberg antiferromagnet. These findings, as well as the results of the recent theories based on the fermionic approach of spinon excitations from the spin liquid ground state, strongly suggest spinon excitations as elementary excitations in ${\mathrm{Cs}}_2{\mathrm{Cu}}_3{\mathrm{SnF}}_{12}$. In ${\mathrm{Rb}}_2{\mathrm{Cu}}_3{\mathrm{SnF}}_{12}$, singlet-triplet excitations from the pinwheel VBS state and their ghost modes caused by the enlargement of the chemical unit cell were clearly confirmed. It was found that the excitation continuum is structured in the low-energy region approximately below $J_{\mathrm{avg}}$ and the almost structureless high-energy excitation continuum extends to approximately $2.6J_{\mathrm{avg}}$. The characteristics of the high-energy excitation continuum are common to both ${\mathrm{Cs}}_2{\mathrm{Cu}}_3{\mathrm{SnF}}_{12}$ and ${\mathrm{Rb}}_2{\mathrm{Cu}}_3{\mathrm{SnF}}_{12}$, irrespective of their ground states. The experimental results strongly suggest that the spin liquid component remains in the ground state as quantum fluctuations in ${\mathrm{Cs}}_2{\mathrm{Cu}}_3{\mathrm{SnF}}_{12}$ and ${\mathrm{Rb}}_2{\mathrm{Cu}}_3{\mathrm{SnF}}_{12}$.

We report the effect of pressure on the electrical transport properties of the four-layer-type bismuth chalcogenide semiconductors $\mathrm{La}\mathrm{O}\mathrm{Pb}\mathrm{Bi}{\mathrm{S}}_3$ and ${\mathrm{La}}_2{\mathrm{O}}_2{\mathrm{Bi}}_3\mathrm{Ag}{\mathrm{S}}_6$ and present the discovery of a pressure-induced superconductivity. In both compounds, the semiconductorlike behavior concomitant with an anomaly in the more insulating state is gradually suppressed with increasing pressure while the superconductivity develops. The obtained phase diagrams, in sharp contrast with the interplay between the conventional charge density waves and superconductivity, suggest that the enhancement of the superconductivity is due to the disorder near the localization threshold. These results indicate that pressure tuning of the local disorder in four-layer-type bismuth chalcogenides provides an attractive opportunity to study the interplay between disorder and superconductivity.

Motivated by recent experiments observing a large antidamping spin-orbit torque (SOT) on the surface of a three-dimensional topological insulator, we investigate the origin of the current-induced SOT beyond linear response theory. We find that a strong antidamping SOT arises from intraband transitions in the nonlinear response and does not require interband transitions as is the case in linear transport mechanisms. The joint effect of warping and an in-plane magnetization generates a nonlinear antidamping SOT which can exceed the intrinsic one by several orders of magnitude, depending on the warping parameter and the position of the Fermi energy, and exhibits a complex dependence on the azimuthal angle of the magnetization. This nonlinear SOT provides an alternative explanation of the observed giant SOT in recent experiments.

We have studied the interaction of water and graphene doped with nitrogen in different configurations, namely, graphitic and pyridinic nitrogen, by means of the van der Waals density functional. We found that the local nitrogen configuration plays a key role in determining the stable water configuration, while the dispersion force is responsible for the water adsorption. With the graphitic nitrogen, water prefers to orient with its oxygen toward the surface, whereas for the pyridinic nitrogen it prefers to orient with its hydrogens toward the surface, because nitrogen is positively and negatively charged for the former and the latter, respectively. Our results have great implications for the modeling of the interface between water and nitrogen-doped graphitic systems.

Angle-resolved photoelectron spectroscopy is an extremely powerful probe of materials to access the occupied electronic structure with energy and momentum resolution. However, it remains blind to those dynamic states above the Fermi level that determine technologically relevant transport properties. In this work we extend band structure mapping into the unoccupied states and across the entire Brillouin zone by using a state-of-the-art high repetition rate, extreme ultraviolet femtosecond light source to probe optically excited samples. The wide-ranging applicability and power of this approach are demonstrated by measurements on the two-dimensional semiconductor ${\mathrm{WSe}}_2$, where the energy-momentum dispersion of valence and conduction bands are observed in a single experiment. This provides a direct momentum-resolved view, not only on the complete out-of-equilibrium band gap but also on its renormalization induced by electronic screening. Our work establishes a benchmark for measuring the band structure of materials, with direct access to the energy-momentum dispersion of the excited-state spectral function.

We investigate theoretically the thermoelectric transport properties of a T-shaped double quantum dot side-coupled to a topological superconducting nanowire hosting Majorana zero-energy modes. The calculations are performed using the numerical renormalization group method focusing on the transport regime, where the system exhibits the two-stage Kondo effect. It is shown that the leakage of Majorana quasiparticles into the double dot system results in a half-suppression of the second stage of the Kondo effect, which is revealed through fractional values of the charge and heat conductances and gives rise to new resonances in the Seebeck coefficient. The heat conductance is found to satisfy a modified Wiedemann-Franz law. Finally, the interplay of Majorana-induced interference with strong electron correlations is discussed in the behavior of the spin Seebeck effect, which is a unique phenomenon of the considered setup.

Line-graph (LG) lattices are known for having flat bands (FBs) from the destructive interference of Bloch wave functions encoded in only lattice symmetry. Here, we develop a generic atomic/molecular $orbital$ design principle for FBs in non-LG lattices. Based on linear combination of atomic orbital theory, we demonstrate that the underlying wave-function symmetry of FBs in a LG lattice can be transformed into the atomic/molecular orbital symmetry in a non-LG lattice. We illustrate such orbital-designed topological FBs in three 2D non-LG, square, trigonal, and hexagonal lattices, where the designed orbitals faithfully reproduce the corresponding lattice symmetries of checkerboard, kagome, and diatomic-kagome lattices, respectively. Interestingly, systematic design of FBs with a high Chern number is also achieved based on the same principle. Fundamentally our theory enriches the FB physics; practically, it significantly expands the scope of FB materials, since most materials have multiple atomic/molecular orbitals at each lattice site, rather than a single $s$ orbital mandated in graph theory and generic lattice models.

The recent discovery of intrinsic magnetic topological insulator (TI) $\mathrm{Mn}{\mathrm{Bi}}_2{\mathrm{Te}}_4$ has inspired enormous research interest to explore emergent physics created by the interplay of magnetism and topology. Here we systematically investigated the influence of hydrostatic pressure on structural, magnetic, and topological electronic properties of the $\mathrm{Mn}{\mathrm{Bi}}_2{\mathrm{Te}}_4$ family of materials by first-principles calculations. Our results indicate that properties of these layered materials can be effectively tuned by pressure, leading to various kinds of magnetic and topological phase transitions. These include magnetic transitions from $A$-type antiferromagnetism to novel magnetic states, such as frustrated magnetism and intralayer ferromagnetism in conditions of antiferromagnetic intralayer coupling. Moreover, rich topological phase transitions can be driven by pressure in these materials, including trivial insulator to antiferromagnetic TI, type-I or type-II Weyl semimetals, or high-order TI phases. The findings call for in-depth experimental investigations of magnetic and topological physics in these intriguing material systems under high pressure.

We construct a constant depth quantum circuit that maps between Morita-equivalent string-net models. Due to its constant depth and unitarity, the circuit cannot alter the topological order, which demonstrates that Morita-equivalent string nets are in the same phase. The circuit is constructed from an invertible bimodule category connecting the two input fusion categories of the relevant string-net models, acting as a generalized Fourier transform for fusion categories. The circuit not only acts on the ground state subspace but also acts unitarily on the full Hilbert space when supplemented with ancillas.

We present a density functional theory analysis of the negatively charged nitrogen-vacancy (${\mathrm{NV}}^{-}$) defect complex in diamond located in the vicinity of (111)- or (100)-oriented surfaces with mixed (N,H)-terminations. We assess the stability and electronic properties of the ${\mathrm{NV}}^{-}$ center, and we study their dependence on the H:N ratio of the surface termination. The formation energy, the electronic density of states, the hyperfine structure, and the zero-field-splitting parameters of an ${\mathrm{NV}}^{-}$ center are analyzed as a function of its distance and orientation to the surface. We find stable ${\mathrm{NV}}^{-}$ centers with bulklike properties at distances of at least $\sim 8$ Å from the surface provided that the surface termination consists of at least 25% substitutional nitrogen atoms. The studied surface terminations have a minor effect on the ground-state properties, whereas the NV orientation has major effects. Our results indicate that axial NV centers near a flat 100% N-terminated (111) surface are the optimal choice for NV-based quantum sensing applications as they are the least influenced by the proximity of the surface.

We present a microscopic quantum theory of light-matter interaction in pristine sheets of two-dimensional semiconductors coupled to localized electromagnetic resonators such as optical nanocavities or plasmonic particles. The light-matter interaction breaks the translation symmetry of excitons in the two-dimensional lattice, and we find that this symmetry-breaking interaction leads to the formation of a localized exciton state, which mimics the spatial distribution of the electromagnetic field of the resonator. The localized exciton state is in turn coupled to an environment of residual exciton states. We quantify the influence of the environment and find that it is most pronounced for small lateral confinement length scales of the electromagnetic field in the resonator, and that environmental effects can be neglected if this length scale is sufficiently large. The microscopic theory provides a physically appealing derivation of the coupled-oscillator models widely used to model experiments on these types of systems, in which all observable quantities are directly derived from the material parameters and the properties of the resonant electromagnetic field. As a consistency check, we show that the theory recovers the results of semiclassical electromagnetic calculations and experimental measurements of the excitonic dielectric response in the linear excitation limit. The theory, however, is not limited to linear response, and in general describes nonlinear exciton-exciton interactions in the localized exciton state, thereby providing a powerful means of investigating the nonlinear optical response of such systems.

The hexagonal warping effect on transport properties and Goos-Hänchen (GH) lateral shift of electrons on the surface of a topological insulator with a potential barrier is investigated theoretically. Due to the warped Fermi surface for incident electron beams, we can expect two propagating transmitted beams corresponding to the occurrence of double refraction. The transmitted beams have spin orientations locked to their momenta so one of the spin directions rotates compared to the incident spin direction. Based on a low-energy Hamiltonian near the Dirac point and considering Gaussian beams, we derive expressions for calculating lateral shifts in the presence of warping effect. We study the dependence of transmission probabilities and GH shifts of transmitted beams on system parameters in detail by giving an explanation for the appearance of large peaks in the lateral shifts corresponding to their transmission peaks. It is shown that the separation between two transmitted beams through their different GH shifts can be as large as a few micrometers, which is large enough to be observed experimentally. Finally, we propose a method to measure the GH shift of electron beams based on the transverse magnetic focusing technique in which, by tuning an applied magnetic field, a detectable resonant path for electrons can be induced.

We study the influence of strong spin-orbit interaction on the formation of flat bands in relaxed twisted bilayer $\mathrm{W}{\mathrm{Se}}_2$. Flat bands, well separated in energy, emerge at the band edges for twist angles $\theta$ near $0^{\circ }$ and ${60}^{\circ }$. For $\theta$ near $0^{\circ }$, the interlayer hybridization together with a moiré potential determines the electronic structure. The bands near the valence band edge have nontrivial topology, with Chern numbers equal to $+1$ or $-1$. We propose that the nontrivial topology of the first band can be probed experimentally for twist angles less than a critical angle of $3.5{}^{\circ }$. For $\theta$ near ${60}^{\circ }$, the flattening of the bands arising from the $K$ point of the unit cell Brillouin zone is a result of atomic rearrangements in the individual layers. Our findings on the flat bands and the localization of their wave functions for both ranges of $\theta$ match well with recent experimental observations.