Heavy-fermion or Kondo lattice materials are considered to be typical strongly correlated systems, for which mean-field approximations have shown that the Coulomb interaction increases the effective mass and narrows the band gap. In this paper, to clarify interaction effects on the electronic excitation, the spectral function of the Kondo lattice and periodic Anderson models is studied around Kondo insulators in the strong-coupling regime, by using the non-Abelian dynamical density-matrix renormalization group method and perturbation theory. Upon doping a Kondo insulator, an electronic mode emerges in the Kondo insulating gap, exhibiting the momentum-shifted magnetic dispersion relation. Although the ground-state properties are similar to those of a doped band insulator, the emergence of the electronic mode reflecting spin-charge separation of the Kondo insulator is a crucial interaction effect that allows us to regard the Kondo-insulator–metal transition as a type of Mott transition. In addition, electronic modes emerge even in the high-energy regime by doping in the periodic Anderson model. These strong-correlation effects have not been expected in conventional mean-field approximations and would bring a different perspective on heavy-fermion or Kondo lattice systems.

The strong-coupling phase diagram of magic-angle twisted bilayer graphene (TBG) predicts a series of exact one-particle charge $\pm 1$ gapped excitations on top of the integer-filled ferromagnetic ground states. Finite-size exact diagonalization studies showed that these are the lowest charge $\pm 1$ excitations in the system (for 10 nm screening length), with the exception of charge $+1$ at filling $-1$ in the chiral limit. In the current paper we show that this “trion bound state,” a 3-particle, charge 1 excitation of the insulating ferromagnetic ground state of the projected Hamiltonian of TBG, is the lowest charge $+1$ overall excitation at $\nu =-1$, and also for some large ($\approx 20$ nm) screening lengths at $\nu =-2$ in the chiral limit and with very small binding energy. At other fillings, we show that trion bound states do exist, but only for momentum ranges that do not cover the entire moiré Brillouin zone. The trion bound states (at different momenta) exist for finite parameter range $w_0/w_1$ but they all disappear in the continuum far below the realistic values of $w_0/w_1=0.8$. We find the conditions for the existence of the trion bound state, a good variational wave function for it, and investigate its behavior for different screening lengths, at all integer fillings, on both the electron and hole sides.

We implement the $GW$ space-time method at finite temperatures, in which the Green's function $G$ and the screened Coulomb interaction $W$ are represented in the real space on a suitable mesh and in imaginary time in terms of Chebyshev polynomials, paying particular attention to controlling systematic errors of the representation. Having validated the technique by the canonical application to silicon and germanium, we apply it to the calculation of band gaps in hexagonal solid hydrogen with the bare Green's function obtained from density functional approximation and the interaction screened within the random phase approximation. The results, obtained from the asymptotic decay of the full Green's function without resorting to analytic continuation, suggest that the solid hydrogen above 150 GPa cannot adopt an orientationally ordered hexagonal-closed-pack structure due to its metallic behavior. The demonstrated ability of the method to store the full $G$ and $W$ functions in memory with sufficient accuracy is crucial for its subsequent extensions to include higher orders of the diagrammatic series by means of diagrammatic Monte Carlo algorithms.

Single crystals of ${\mathrm{Ce}}_2{\mathrm{Re}}_3{\mathrm{Si}}_5$ and ${\mathrm{Pr}}_2{\mathrm{Re}}_3{\mathrm{Si}}_5$ have been grown by the Czochralski method in a tetra-arc furnace. Powder x-ray diffraction confirmed that these compounds crystallize in the ${\mathrm{U}}_2{\mathrm{Mn}}_3{\mathrm{Si}}_5$-type tetragonal crystal structure with space group $P4/mnc$ (no. 128). The anisotropic physical properties have been studied comprehensively by measuring the magnetic susceptibility, isothermal magnetization, electrical transport, and specific heat. The low value of magnetic susceptibility together with no magnetic transition down to 2 K and the observation of the signature of the Kondo feature in the photoemission spectra provide evidence that the Ce ions are in the intermediate valence state in ${\mathrm{Ce}}_2{\mathrm{Re}}_3{\mathrm{Si}}_5$. On the other hand, ${\mathrm{Pr}}_2{\mathrm{Re}}_3{\mathrm{Si}}_5$ revealed a magnetic ordering at 9 K. The sharp drop in the magnetic susceptibility and a spin-flip-like metamagnetic transition for $H\parallel \left[001\right]$ in the magnetization plot of ${\mathrm{Pr}}_2{\mathrm{Re}}_3{\mathrm{Si}}_5$ suggest an Ising-type antiferromagnetic ordering. Based on magnetic susceptibility and isothermal magnetization data, a detailed crystal electric field (CEF) analysis shows that the degenerate $\mathit{J}=4$ Hund's rule derived ground state of a ${\mathrm{Pr}}^{3+}$ ion splits into nine singlets with an overall splitting of 1179 K. The magnetic ordering in ${\mathrm{Pr}}_2{\mathrm{Re}}_3{\mathrm{Si}}_5$ is due to the exchange-generated admixture of the lowest-lying CEF energy levels. Heat capacity data reveal a sharp peak at 9 K, which confirms the bulk nature of the magnetic ordering in ${\mathrm{Pr}}_2{\mathrm{Re}}_3{\mathrm{Si}}_5$.

Topological band insulators and (semi-)metals can arise out of atomic insulators when the hopping strength between electrons increases. Such topological phases are separated from the atomic insulator by a bulk gap closing. In this paper, we show that in many (magnetic) space groups, the crystals with certain Wyckoff positions and atomic orbitals being occupied must be semimetals in the atomic limit, e.g., the hopping strength between electrons is infinitesimal but not vanishing, which then are termed atomic semimetals (ASMs). We derive a sufficient condition for realizing ASMs in both spinless and spinful systems. Remarkably, with both symmetries and electron fillings of system preserved, increasing the hopping strength between electrons may transform an ASM into an insulator, and the induced insulators inevitably are topologically nontrivial. Particularly, using silicon as an example, we show the ASM criterion can discover the obstructed atomic insulators (OAIs) that are marked as trivial insulators on the topological quantum chemistry website. Our paper not only establishes an efficient way to identify and design topologically nontrivial insulators, but also predicts that the group-IV elemental semiconductors are ideal candidate materials for OAI.

Understanding electronic properties and charge transport in organic semiconductors is important for improving organic electronic materials and devices. Here we investigate the impact of electronic band occupation, charge-carrier concentration, and symmetry of phonon modes on the electron mobility in naphthalene crystals for various temperatures. Our theoretical approach is based on the description of the electron-phonon coupling (EPC), where the coupling to low-frequency modes is treated by an effective vibrational disorder potential with local and nonlocal contributions and the coupling to high-frequency modes is included by a polaron treatment. Surprisingly, the coupling to high-frequency modes leads to an increase in the mobility in presence of the low-frequency modes, which is explained by localization and band occupation effects that further depend on the carrier density. A symmetry analysis sheds additional light on the energy dependence of the EPC, which is important to describe transport properties as a function of charge density and temperature. We also find that coupling to low-frequency phonons together with band occupation effects can lead to a vanishing slope of the mobility versus temperature that is known from experiments.

Non-Hermiticity gives rise to unique topological phases that have no counterparts in Hermitian systems. Such intrinsic non-Hermitian topological phases appear even in one dimension while no topological phases appear in one-dimensional Hermitian systems. Despite the recent considerable interest, the intrinsic non-Hermitian topological phases have been mainly investigated in noninteracting systems described by band theory. It has been unclear whether they survive or reduce in the presence of many-body interactions. Here, we demonstrate that the intrinsic non-Hermitian topological phases in one dimension survive even in the presence of many-body interactions. We formulate a many-body topological invariant by the winding of the complex-valued many-body spectrum in terms of a $\mathrm{U}\left(1\right)$ gauge field (magnetic flux). As an illustrative example, we investigate the interacting Hatano-Nelson model and find a unique topological phase and skin effect induced by many-body interactions.

Quantum information technologies have triggered enormous interest in single-photon emitters (SPEs), such as local defects in wide-band-gap semiconductors. Recently, visible single-photon emission was observed in carbon-doped hexagonal boron nitride (h-BN), yet the structure giving rise to the emission has not been unambiguously identified. Further progress in the rational design of SPEs requires a deep insight into all the ground-state properties, the electronic structure of the defects in question, and their relationship with the optical properties. To this end, we apply state-of-the-art computational methods to evaluate the stability and optical properties of defects that can be responsible for the above-mentioned experimental findings. For our analysis, we select seven possible C-based defects: (i) C substitutes B $\left({\mathrm{C}}_{\mathrm{B}}\right)$, (ii) C substitutes N $\left({\mathrm{C}}_{\mathrm{N}}\right)$, (iii) C substitutes N and nearest B and N are interchanged $({\mathrm{C}}_{\mathrm{N}}{\mathrm{N}}_{\mathrm{B}})$, (iv) C substitutes N and there is a nearby B vacancy $\left(V_{\mathrm{B}}{\mathrm{C}}_{\mathrm{N}}\right)$, and (v) three different configurations of $V_{\mathrm{N}}{\mathrm{C}}_{\mathrm{B}}$ (C substitutes B and a nearby N vacancy). We perform calculations of the electronic structure using the linear response approach ($GW$ method), followed by finding the solution to the Bethe-Salpeter equation to obtain the excitation spectra of dynamically stable defects. Our density-functional-theory-based calculations of the optimized geometries and their Γ-point phonon frequencies reveal that $V_{\mathrm{B}}{\mathrm{C}}_{\mathrm{N}}$ is dynamically unstable and undergoes a transformation to one corrugated $V_{\mathrm{N}}{\mathrm{C}}_{\mathrm{B}}$ structure. Likewise, the flat $V_{\mathrm{N}}{\mathrm{C}}_{\mathrm{B}}$ (with $C_{2\mathrm{v}}$ symmetry) lies on a very shallow and anharmonic minimum that would also relax toward the corrugated $V_{\mathrm{N}}{\mathrm{C}}_{\mathrm{B}}$ structure. Clearly, our results demonstrate how important is to attest to the dynamical stability of any proposed structure. Upon comparing our results of the considered defects with the reported experimental SPE spectra, we show that only the ${\mathrm{C}}_{\mathrm{N}}$ defect has an excitation spectrum fitting the conditions for the observed single-photon emission. Importantly, this defect is found to be the most thermodynamically stable among those under consideration.

We study the effects caused by Rashba and Dresselhaus spin-orbit coupling over the thermoelectric transport properties of a single-electron transistor, viz., a quantum dot connected to one-dimensional leads. Using linear response theory and employing the numerical renormalization group method, we calculate the thermopower, electrical and thermal conductances, dimensionless thermoelectric figure of merit, and study the Wiedemann-Franz law, showing their temperature maps. In the Kondo regime, the effect of spin-orbit coupling on those properties appears as a downward renormalization of the Kondo scale. We show that the thermoelectric transport properties, in the presence of spin-orbit coupling, obey the expected universality of the Kondo strong coupling fixed point. In addition, our results show a notable increase in the thermoelectric figure of merit, caused by the spin-orbit coupling in the one-dimensional quantum dot leads. We also provide simulations of the thermocurrent through a quantum dot in good agreement with experimental results.

The rare-earth compounds $R_2{\mathrm{O}}_2\mathrm{Bi}$ $(R=\mathrm{Tb}, \mathrm{Dy}, \mathrm{Er}, \mathrm{Lu}, \mathrm{Y})$ are newly discovered superconductors in the vicinity of a rare-earth magnetic long-range order. In this work, we determine the magnetic order of the parent compound ${\mathrm{Dy}}_2{\mathrm{O}}_2\mathrm{Te}$ by neutron scattering as the $A$-type antiferromagnetic structure below the Néel temperature $T_N=9.7\mathrm{K}$. The large staggered magnetic moment 9.4(1) ${\mu }_B$ per Dy at $T=3.5\mathrm{K}$ lies in the basal $ab$ plane. In a magnetic field, anomalous magnetic properties including the bifurcation between zero-field- and field-cooling magnetization, a butterfly-shaped magnetic hysteresis, and slow magnetic relaxation emerge, which are related to the field-induced metamagnetic transitions in ${\mathrm{Dy}}_2{\mathrm{O}}_2\mathrm{Te}$. Our experimental findings could stimulate further research on the relation between antiferromagnetism and superconductivity in these rare-earth compounds.

$\left[\mathrm{Ni}\left({\mathrm{HF}}_2\right){\left(3\text{−}\mathrm{Clpyradine}\right)}_4\right]\mathrm{B}{\mathrm{F}}_4$ (NBCT) is a one-dimensional, $S=1$ spin-chain material that shows no long-range magnetic order down to thermometer temperatures of 0.1 K. Previous ambient pressure inelastic neutron scattering experiments identified NBCT to be in the large-$D$ quantum paramagnetic phase of the $D/J$ phase diagram, where $D$ is the axial single-ion anisotropy and $J$ is the intrachain superexchange. Here, we extend the previous experiments to a hydrostatic pressure of 0.9 GPa. By comparing to density matrix renormalization group calculations, we find $D/J$ increases from 1.5 to 3.2 as pressure increases from 0 GPa to 0.9 GPa, which pushes the system further into the large-$D$ phase.

We address the concomitant metal-insulator transition (MIT) and antiferromagnetic ordering in the novel pyrochlore iridate ${\mathrm{Eu}}_2{\mathrm{Ir}}_2{\mathrm{O}}_7$ by combining x-ray absorption spectroscopy, x-ray and neutron diffractions, and density functional theory (DFT)-based calculations. The temperature dependent powder x-ray diffraction clearly rules out any change in the lattice symmetry below the MIT, nevertheless a clear anomaly in the Ir-O-Ir bond angle and Ir-O bond length is evident at the onset of MIT. From the x-ray absorption near edge structure (XANES) spectroscopic study of Ir-$L_3$ and $L_2$ edges, the effective spin-orbit coupling is found to be intermediate, at least quite far from the strong atomic spin-orbit coupling limit. Powder neutron diffraction measurement is in line with an all-in-all-out magnetic structure of the Ir-tetrahedra in this compound, which is quite common among rare-earth pyrochlore iridates. The sharp change in the Ir-O-Ir bond angle around the MIT possibly arises from the exchange striction mechanism, which favors an enhanced electron correlation via weakening of Ir-Ir orbital overlap and an insulating phase below $T_{MI}$. The theoretical calculations indicate an insulating state for shorter bond angle validating the experimental observation. Our DFT calculations show a possibility of intriguing topological phase below a critical value of the Ir-O distance, which is shorter than the experimentally observed bond length. Therefore, a topological state may be realized in bulk ${\mathrm{Eu}}_2{\mathrm{Ir}}_2{\mathrm{O}}_7$ sample if the Ir-O bond length can be reduced by the application of sufficient external pressure.

The origin of the strong piezoelectric phenomenon in $\mathrm{Pb}{\mathrm{Zr}}_{1-x}{\mathrm{Ti}}_x{\mathrm{O}}_3$ (PZT) perovskites still suffers from a lack of complete understanding. It concerns the distinction between the intrinsic and extrinsic mechanisms that govern PZT's piezo activity. These two mechanisms have been investigated in single crystal $\mathrm{Pb}{\mathrm{Zr}}_{0.54}{\mathrm{Ti}}_{0.46}{\mathrm{O}}_3$ at the morphotropic phase boundary. After poling in a DC electric field, the piezoelectric properties were examined on the same crystal by observing piezoelectric resonances to determine the piezoelectric coefficient $d_{31}$ and measuring quasistatic deformation to determine the coefficient $d_{33}$. The domain populations were investigated during and after poling in a DC electric field. These populations were also investigated as a function of DC fields for strengths similar to those used to measure quasistatic piezoelectric properties for a poled crystal. The experiments indicate that the intrinsic origin of the enhancement of the piezoelectric properties is connected with a change in the population of domains with monoclinic symmetry, in which there is an easy polarization rotation under the action of the electric field.

We construct a class of lattice Hamiltonians that exhibit fractional Hall conductivity. These Hamiltonians, while not being exactly solvable, can be controllably solved in their low-energy sectors, through a combination of perturbative and exact techniques. Our construction demonstrates a systematic way to circumvent the Kapustin-Fidkowski no-go theorem and is generalizable.

Hexagonal CoSn is a newly discovered frustrated kagome metal. It shows close-to-textbook flat bands and orbital-selective Dirac fermions, which are largely associated with its strongly correlated Co $3d$ orbitals. Because correlated electronic states are easily regulated by external conditions (such as chemical doping, pressure, and temperature), the fate of these kagome-derived electronic bands upon temperature becomes an interesting and unsolved question. In this work, we try to study the temperature-dependent electronic structures of hexagonal CoSn by means of the density functional theory in conjunction with the embedded dynamical mean-field theory. We find that the Co $3d$ electrons are in close proximity to Mott insulating states at ambient condition. Special attention is devoted to the evolution of Co $3d$ electronic states with respect to temperature. At least six different temperatures (or energy scales), namely $T^{*}$, $T_{\text{FL}}$, $T_{\text{S1}}$ (and $T_{\text{S2}}$), $T_{\text{SF}}$, and $\overline{T}$, are figured out. They are related to stabilization of the “pseudogap” state, emergence of the non-Fermi-liquid phase, onset (and completeness) of the spin plateau, occurrence of the spin-frozen phase, and beginning of the orbital freezing transition, respectively.

A heterostructure consisting of a topological insulator (TI) nanowire brought into proximity with a superconducting layer provides a promising route to achieve topological superconductivity and associated Majorana bound states. Here we study the effects caused by such a coupling between a thin layer of an $s$-wave superconductor and a TI nanowire. We show that there is a distinct phenomenology arising from the metallization of states in the TI nanowire by the superconductor. In the strong coupling limit, required to induce a large superconducting pairing potential, we find that metallization results in a shift of the TI nanowire sub-bands $(\sim 20 \mathrm{meV})$ as well as it leads to a small reduction in the size of the sub-band gap opened by a magnetic field applied parallel to the nanowire axis. Surprisingly, we find that metallization effects in TI nanowires can also be beneficial. Most notably, coupling to the superconductor induces a potential in the portion of the TI nanowire close to the interface with the superconductor; this breaks inversion symmetry and at finite momentum lifts the spin degeneracy of states within a sub-band. As such coupling to a superconductor can create or enhance the sub-band splitting that is key to achieving topological superconductivity. This is in stark contrast with semiconductors, where it has been shown that metallization effects always reduce the equivalent sub-band splitting caused by spin-orbit coupling. We also find that in certain geometries metallization effects can reduce the critical magnetic required to enter the topological phase. We conclude that, unlike in semiconductors, the metallization effects that occur in TI nanowires can be relatively easily mitigated, for instance, by modifying the geometry of the attached superconductor or by compensation of the TI material.

Magnetic topological insulators $\left({\mathrm{MnBi}}_2{\mathrm{Te}}_4\right){\left({\mathrm{Bi}}_2{\mathrm{Te}}_3\right)}_n$ ($n=0,1,2,3$) are promising to realize exotic topological states such as the quantum anomalous Hall effect (QAHE) and axion insulator (AI), where the ${\mathrm{Bi}}_2{\mathrm{Te}}_3$ layer introduces versatility to engineer electronic and magnetic properties. However, whether surface states on the ${\mathrm{Bi}}_2{\mathrm{Te}}_3$ terminated facet are gapless or gapped is debated, and its consequences in thin-film properties are rarely discussed. In this work, we find that the ${\mathrm{Bi}}_2{\mathrm{Te}}_3$ terminated facets are gapless for $n\ge 1$ compounds by calculations. Although the surface ${\mathrm{Bi}}_2{\mathrm{Te}}_3$ (one layer or more) and underlying ${\mathrm{MnBi}}_2{\mathrm{Te}}_4$ layers hybridize and give rise to a gap, such a hybridization gap may overlap with bulk valence bands, leading to a gapless surface after all. Such a metallic surface poses a fundamental challenge to realize QAHE or AI, which requires an insulating gap in thin films with at least one ${\mathrm{Bi}}_2{\mathrm{Te}}_3$ surface. In theory, the insulating phase can still be realized in a film if both surfaces are ${\mathrm{MnBi}}_2{\mathrm{Te}}_4$ layers. Otherwise, it requires that the film thickness be less than $10–20\mathrm{nm}$ to push down bulk valence bands via the size effect. Our work paves the way to understand surface states and design bulk-insulating quantum devices in magnetic topological materials.

Embedding molecular symmetries into machine learning models is key for efficient learning of chemico-physical scalar properties, but little evidence on how to extend the same strategy to tensorial quantities exists. Here we formulate a scalable equivariant machine learning model based on local atomic environment descriptors. We apply it to a series of molecules and show that accurate predictions can be achieved for a comprehensive list of dielectric and magnetic tensorial properties of different ranks. These results show that equivariant models are a promising platform to extend the scope of machine learning in materials modeling.

The B20-type cobalt germanide CoGe was investigated by measuring the specific heat, resistivity, and ${}^{59}\mathrm{Co}$ nuclear magnetic resonance (NMR). We observed a phase transition at $T_Q=13.7$ K, evidenced by a very narrow peak of the specific heat and sharp changes of the nuclear spin-spin ($T_2^{-1}$) and spin-lattice ($T_1^{-1}$) relaxation rates. The fact that the entropy release is extremely small and the Knight shift is almost independent of temperature down to low temperatures as anticipated in a paramagnetic metal indicates that the $T_Q$ transition is of nonmagnetic origin. In addition, we detected a crossover scale $T_0\sim 30$ K below which the resistivity and the NMR linewidth increase, and $T_1^{-1}$ is progressively distributed in space, that is, a static and dynamical spatial inhomogeneity develops. While the order parameter for the $T_Q$ transition remains an open question, a group-theoretical analysis suggests that the finite electric quadrupole density arising from the low local site symmetry at cobalt sites could drive the crystal symmetry lowering from the $P{2}_{1}3$ symmetry that is commensurate to the $R3$ symmetry with an incommensurate wave vector, which fairly well accounts for the $T_Q$ transition. The quadrupole-order-driven commensurate-incommensurate phase transition may be another remarkable phenomenon arising from the structural chirality inherent in the noncentrosymmetric B20 family.