We revisit the problem of local moment formation in graphene due to chemisorption of individual atomic hydrogen or other analogous ${sp}^3$ covalent functionalizations. We describe graphene with the single-orbital Hubbard model, so that the H chemisorption is equivalent to a vacancy in the honeycomb lattice. To circumvent artifacts related to periodic unit cells, we use either huge simulation cells of up to $8\times {10}^5$ sites, or an embedding scheme that allows the modeling of a single vacancy in an otherwise pristine infinite honeycomb lattice. We find three results that stress the anomalous nature of the magnetic moment $\left(m\right)$ in this system. First, in the noninteracting $\left(U=0\right)$ zero-temperature $\left(T=0\right)$ case, the $m\left(B\right)$ is a continuous smooth curve with divergent susceptibility, different from the stepwise constant function found for single unpaired spins in a gapped system. Second, for $U=0$ and $T>0$, the linear susceptibility follows a power law $\propto T^{-\alpha }$ with an exponent of $\alpha =0.77$ different from the conventional Curie law. For $U>0$, in the mean-field approximation, the integrated moment is smaller than $m=1{\mu }_B$, in contrast with results using periodic unit cells. These three results highlight the fact that the magnetic response of the local moment induced by ${sp}^3$ functionalizations in graphene is different from that of local moments in gapped systems, for which the magnetic moment is quantized and follows a Curie law, and from Pauli paramagnetism in conductors, for which linear susceptibility can be defined at $T=0$.

The algebraic structure of representation theory naturally arises from 2D fixed-point tensor network states, and conceptually formulates the pattern of long-range entanglement realized in such states. In 3D, the same underlying structure is also shared by Turaev-Viro state-sum topological quantum field theory (TQFT). We show that a 2D fixed-point tensor network state arises naturally on the boundary of the 3D manifold on which the TQFT is defined, and the fact that exactly the same information is needed to construct either the tensor network or the TQFT is made explicit in a form of holography. Furthermore, the entanglement of the fixed-point states leads to an emergence of pregeometry in the 3D TQFT bulk. We further extend these ideas to the case where an additional global on-site unitary symmetry is imposed on the tensor network states.

${\mathrm{BaBiO}}_3$ is a well-known example of a 3D charge density wave (CDW) compound, in which the CDW behavior is induced by charge disproportionation at the Bi site. At ambient pressure, this compound is a charge-ordered insulator, but little is known about its high-pressure behavior. In this work, we study from first principles the high-pressure phase diagram of ${\mathrm{BaBiO}}_3$ using phonon mode analysis and evolutionary crystal structure prediction. We show that charge disproportionation is very robust in this compound and persists up to 100 GPa. This causes the system to remain insulating up to the highest pressure we studied.

In this paper, we tackle the complexity of coexisting disorder and Coulomb electron-electron interactions (CEEIs) in solids by addressing a strongly disordered system with intricate CEEIs and a screening that changes both with charge carrier doping level $Q$ and temperature $T$. We report on an experimental comparative study of the $T$ dependencies of the electrical conductivity $\sigma$ and magnetic susceptibility $\chi$ of polyaniline pellets doped with dodecylbenzenesulfonic acid over a wide range. This material is special within the class of doped polyaniline by exhibiting in the electronic transport a crossover between a low-$T$ variable range hopping (VRH) and a high-$T$ nearest-neighbor hopping (NNH) well below room temperature. Moreover, there is evidence of a soft Coulomb gap ${\mathrm{\Delta }}_{\mathrm{C}}$ in the disorder band, which implies the existence of a long-range CEEI. Simultaneously, there is an onsite CEEI manifested as a Hubbard gap $U$ and originating in the electronic structure of doped polyaniline, which consists of localized electron states with dynamically varying occupancy. Therefore, our samples represent an Anderson-Mott insulator in which long-range and short-range CEEIs coexist. The main result of the study is the presence of a crossover between low- and high-$T$ regimes not only in $\sigma \left(T\right)$ but also in $\chi \left(T\right)$, the crossover temperature $T^{*}$ being essentially the same for both observables over the entire doping range. The relatively large electron localization length along the polymer chains results in $U$ being small, between 12 and 20 meV for the high and low $Q$, respectively. Therefore, the thermal energy at $T^{*}$ is sufficiently large to lead to an effective closing of the Hubbard gap and the consequent appearance of NNH in the electronic transport within the disorder band. ${\mathrm{\Delta }}_{\mathrm{C}}$ is considerably larger than $U$, decreasing from 190 to 30 meV as $Q$ increases, and plays the role of an activation energy in the NNH.

Phonon-mediated thermal conductivity, which is of great technological relevance, arises due fundamentally to anharmonic scattering from interatomic potentials. Despite its prevalence, accurate first-principles calculations of thermal conductivity remain challenging, primarily due to the high computational cost of anharmonic interatomic force constant (IFC) calculations. Meanwhile, the related anharmonic phenomenon of thermal expansion is much more tractable, being computable from the Grüneisen parameters associated with phonon frequency shifts due to crystal deformations. In this work, we propose an approach for computing the largest cubic IFCs from the Grüneisen parameter data. This allows an approximate determination of the thermal conductivity via a much less expensive route. The key insight is that although the Grüneisen parameters cannot possibly contain all the information on the cubic IFCs, being derivable from spatially uniform deformations, they can still unambiguously and accurately determine the largest and most physically relevant ones. By fitting the anisotropic Grüneisen parameter data along judiciously designed deformations, we can deduce (i.e., reverse-engineer) the dominant cubic IFCs and estimate three-phonon scattering amplitudes. We illustrate our approach by explicitly computing the largest cubic IFCs and thermal conductivity of graphene, especially for its out-of-plane (flexural) modes that exhibit anomalously large anharmonic shifts and thermal conductivity contributions. Our calculations on graphene not only exhibit reasonable agreement with established density-functional theory results, but they also present a pedagogical opportunity for introducing an elegant analytic treatment of the Grüneisen parameters of generic two-band models. Our approach can be readily extended to more complicated crystalline materials with nontrivial anharmonic lattice effects.

In solid state conductors, linear response to a steady electric field is normally dominated by Bloch state occupation number changes that are correlated with group velocity and lead to a steady state current. Recently it has been realized that, for a number of important physical observables, the most important response even in conductors can be electric-field induced coherence between Bloch states in different bands, such as that responsible for screening in dielectrics. Examples include the anomalous and spin-Hall effects, spin torques in magnetic conductors, and the minimum conductivity and chiral anomaly in Weyl and Dirac semimetals. In this paper we present a general quantum kinetic theory of linear response to an electric field which can be applied to solids with arbitrarily complicated band structures and includes the interband coherence response and the Bloch-state repopulation responses on an equal footing. One of the principal aims of our work is to enable extensive transport theory applications using computational packages constructed in terms of maximally localized Wannier functions. To this end we provide a complete correspondence between the Bloch and Wannier formulations of our theory. The formalism is based on density-matrix equations of motion, on a Born approximation treatment of disorder, and on an expansion in scattering rate to leading nontrivial order. Our use of a Born approximation omits some physical effects and represents a compromise between comprehensiveness and practicality. The quasiparticle bands are treated in a completely general manner that allows for arbitrary forms of the spin-orbit interaction and for the broken time reversal symmetry of magnetic conductors. We demonstrate that the interband response in conductors consists primarily of two terms: an intrinsic contribution due to the entire Fermi sea that captures, among other effects, the Berry curvature contribution to wave-packet dynamics, and an anomalous contribution caused by scattering that is sensitive to the presence of the Fermi surface. To demonstrate the rich physics captured by our theory, we explicitly solve for some electric-field response properties of simple model systems that are known to be dominated by interband coherence contributions. At the same time we discuss an extensive list of complicated problems that cannot be solved analytically. Our goal is to stimulate progress in computational transport theory for electrons in crystals.

We performed a first-principles molecular dynamics (FPMD) simulation of the interfacial reactions between a ${\mathrm{LiCoO}}_2$ electrode and a liquid ethylene carbonate (EC) electrolyte. For configurations during the FPMD simulation, we also performed first-principles Co K-edge x-ray absorption near-edge structure (XANES) simulations, which can properly reproduce the bulk and surface spectra of ${\mathrm{LiCoO}}_2$. We observed strong absorption of an EC molecule on the ${\mathrm{LiCoO}}_2$ {110} surface, involving ring opening of the molecule, bond formation between oxygen atoms in the molecule and surface Co ions, and emission of one surface Li ion, while all the surface Co ions remain ${\mathrm{Co}}^{3+}$. The surface Co ions having the bond with an oxygen atom in the molecule showed remarkable changes in simulated K-edge spectra which are similar to those of the in situ observation under electrolyte soaking [D. Takamatsu et al., Angew. Chem., Int. Ed. 51, 11597 (2012)]. Thus, the local environmental changes of surface Co ions due to the reactions with an EC molecule can explain the experimental spectrum changes.

We present a combined experimental and theoretical analysis of a Te rich interface layer which represents a template for chalcogenide-based van der Waals epitaxy on Si(111). On a clean Si(111)-$\left(1\times 1\right)$ surface, we find Te to form a Te/Si(111)-$\left(1\times 1\right)$ reconstruction to saturate the substrate bonds. A problem arising is that such an interface layer can potentially be highly conductive, undermining the applicability of the on-top grown films in electric devices. We perform here a detailed structural analysis of the pristine Te termination and present direct measurements of its electrical conductivity by in situ distance-dependent four-probe measurements. The experimental results are analyzed with respect to density functional theory calculations and the implications of the interface termination with respect to the electrical conductivity of chalcogenide-based topological insulator thin films are discussed. In detail, we find a Te/Si(111)-$\left(1\times 1\right)$ interface conductivity of ${\sigma }_{2\mathrm{D}}^{\mathrm{Te}}=2.6\left(5\right)\times {10}^{-7}\mathrm{S}/\square$, which is small compared to the typical conductivity of topological surface states.

Hexagonal tin diselenide $\left(6\mathrm{Hb}-{\mathrm{SnSe}}_2\right)$, a two-dimensional (2D) layered metal dichalcogenide from the IVA and VIA groups, has recently drawn numerous attention in 2D nano-optoelectronics. In this paper, we investigate characteristic lattice dynamics of mechanically exfoliated mono- and few-layer $6\mathrm{Hb}-{\mathrm{SnSe}}_2$ samples by Raman spectroscopy. Bulk ${\mathrm{SnSe}}_2$ has all four Raman active modes of low-frequency shear $E_g^2$ and layer-breathing $A_{1g}^2$ modes, and high-frequency intralayer vibrational $E_g^1$ and $A_{1g}^1$ modes observed around 18.9, 33.6, 107.9, and $182.1\mathrm{c}{\mathrm{m}}^{-1}$, respectively. From polarized Raman measurements, we find that $E_g^1$ mode intensity is independent of polarization configuration and increases linearly with layer number, which provides an effective approach to determine sample thickness. From low-temperature Raman measurements, $E_g^1$ and $A_{1g}^1$ mode temperature coefficients of one-layer, three-layer, and bulk ${\mathrm{SnSe}}_2$ are around $-0.018$ and $-0.014\mathrm{c}{\mathrm{m}}^{-1}/\mathrm{K}$, whereas they have almost zero values for low-frequency $E_g^2$ and $A_{1g}^2$ modes of bulk ${\mathrm{SnSe}}_2$ due to different thermal responses of intralayer and interlayer vibrations. Using multiple excitation laser lines of 488, 514.5, 568, 647, and 785 nm, $E_g^1$ and $A_{1g}^1$ mode intensities of bulk ${\mathrm{SnSe}}_2$ have a similar trend with weak maxima around 2.41 eV. Our work provides valuable information about ${\mathrm{SnSe}}_2$ lattice vibrations for further fundamental research and potential applications in 2D devices such as thermoelectric and infrared light detectors.

We develop a theoretical formalism for collectively responding point scatterers where the radiating electromagnetic fields from each emitter are considered in the electric dipole, magnetic dipole, and electric quadrupole approximation. The contributions of the electric quadrupole moment to electromagnetically-mediated interactions between the scatterers are derived in detail for a system where each scatterer represents a linear $RLC$ circuit resonator, representing common metamaterial resonators in radiofrequency, microwave, and optical regimes. The resulting theory includes a closed set of equations for an ensemble of discrete resonators that are radiatively coupled to each other by propagating electromagnetic fields, incorporating potentially strong interactions and recurrent scattering processes. The effective model is illustrated and tested for examples of pairs of interacting point electric dipoles, where each pair can be qualitatively replaced by a model point emitter with different multipole radiation moments.

We investigate the charge transport in nanocrystal (NC) films using field effect transistors (FETs) of silicon NCs. By studying films with various thicknesses in the dark and under illumination with photons with different penetration depths (UV and red light), we are able to predictably change the spatial distribution of charge carriers across the films' profile. The experimental data are compared with photoinduced charge carrier generation rates computed using finite-difference time-domain (FDTD) simulations complemented with optical measurements. This enables us to understand the optoelectronic properties of NC films and the depth profile dependence of the charge transport properties. From electrical measurements, we extract the total (bulk) photoinduced charge carrier densities ($n_{\text{photo}}$) and the photoinduced charge carrier densities in the FETs channel ($n_{\text{photo}}^{*}$). We observe that the values of $n_{\text{photo}}$ and their dependence on film thickness are similar for UV and red light illumination, whereas a significant difference is observed for the values of $n_{\text{photo}}^{*}$. The dependencies of $n_{\text{photo}}$ and $n_{\text{photo}}^{*}$ on film thickness and illumination wavelength are compared with data from FDTD simulations. Combining experimental data and simulation results, we find that charge carriers in the top rough surface of the films cannot contribute to the macroscopic charge transport. Moreover, we conclude that below the top rough surface of NC films, the efficiency of charge transport, including the charge carrier mobility, is homogeneous across the film thickness. Our work shows that the use of NC films as photoactive layers in applications requiring harvesting of strongly absorbed photons such as photodetectors and photovoltaics demands a very rigorous control over the films' roughness.

We report on the temperature-dependent magnetospectroscopy of two HgTe/CdHgTe quantum wells below and above the critical well thickness $d_c$. Our results, obtained in magnetic fields up to 16 T and s temperature range from 2 to 150 K, clearly indicate a change in the band-gap energy with temperature. A quantum well wider than $d_c$ evidences a temperature-driven transition from topological insulator to semiconductor phases. At a critical temperature of 90 K, the merging of inter- and intraband transitions in weak magnetic fields clearly specifies the formation of a gapless state, revealing the appearance of single-valley massless Dirac fermions with a velocity of $5.6\times {10}^5\mathrm{m}{\mathrm{s}}^{-1}$. For both quantum wells, the energies extracted from the experimental data are in good agreement with calculations on the basis of the eight-band Kane Hamiltonian with temperature-dependent parameters.

We report infrared magnetospectroscopy studies on thin crystals of an emerging Dirac material ${\mathrm{ZrTe}}_5$ near the intrinsic limit. The observed structure of the Landau-level transitions and zero-field infrared absorption indicate a two-dimensional Dirac-like electronic structure, similar to that in graphene but with a small relativistic mass corresponding to a 9.4-meV energy gap. Measurements with circularly polarized light reveal a significant electron-hole asymmetry, which leads to splitting of the Landau-level transitions at high magnetic fields. Our model, based on the Bernevig-Hughes-Zhang effective Hamiltonian, quantitatively explains all observed transitions, determining the values of the Fermi velocity, Dirac mass (or gap), electron-hole asymmetry, and electron and hole $g$ factors.

Topological nodal line semimetals are characterized by the crossing of the conduction and valence bands along one or more closed loops in the Brillouin zone. Usually, these loops are either isolated or touch each other at some highly symmetric points. Here, we introduce a different kind of nodal line semimetal, that contains a pair of linked nodal loops. A concrete two-band model was constructed, which supports a pair of nodal lines with a double-helix structure, which can be further twisted into a Hopf link because of the periodicity of the Brillouin zone. The nodal lines are stabilized by the combined spatial inversion $\mathcal{P}$ and time reversal $\mathcal{T}$ symmetry; the individual $\mathcal{P}$ and $\mathcal{T}$ symmetries must be broken. The band exhibits nontrivial topology that each nodal loop carries a $\pi$ Berry flux. Surface flatbands emerge at the open boundary and are exactly encircled by the projection of the nodal lines on the surface Brillouin zone. The experimental implementation of our model using cold atoms in optical lattices is discussed.

In topological semimetals, the valance band and conduction band meet at zero-dimensional nodal points or one-dimensional nodal rings, which are protected by band topology and symmetries. In this Rapid Communication, we introduce “nodal-link semimetals”, which host linked nodal rings in the Brillouin zone. We put forward a general recipe based on the Hopf map for constructing models of nodal-link semimetals. The consequences of nodal ring linking in the Landau levels and Floquet properties are investigated.

We investigate the effects of strong correlation on the surface state of a topological insulator (TI). We argue that electrons in the regime of crossover from weak antilocalization to weak localization are strongly correlated, and calculate the magnetotransport coefficients of TIs using the gauge-gravity principle. Then, we examine the magnetoconductivity (MC) formula and find excellent agreement with the data of chrome-doped ${\mathrm{Bi}}_2{\mathrm{Te}}_3$ in the crossover regime. We also find that the cusplike peak in MC at low doping is absent, which is natural since quasiparticles disappear due to the strong correlation.

We propose a type of Hopf semimetal indexed by a pair of numbers $(p,q)$, where the Hopf number is given by $pq$. The Fermi surface is given by a preimage of the Hopf map, which consists of loops nontrivially linked for a nonzero Hopf number. The Fermi surface forms a torus link, whose examples are a Hopf link indexed by $(1,1)$, Solomon's knot $(2,1)$, a double Hopf link $(2,2)$, and a double trefoil knot $(3,2)$. We may choose $p$ or $q$ to be a half integer, where the Fermi surface is a torus knot, such as a trefoil knot $(3/2,1)$. It is even possible to make the Hopf number an arbitrary rational number, where a semimetal whose Fermi surface forms open strings is generated.