We have investigated the structural, magnetic, thermodynamic, and charge-transport properties of $\mathrm{M}{\mathrm{n}}_{1/3}\mathrm{Nb}{\mathrm{S}}_2$ single crystals through x-ray and neutron diffraction, magnetization, specific heat, magnetoresistance, and Hall-effect measurements. $\mathrm{M}{\mathrm{n}}_{1/3}\mathrm{Nb}{\mathrm{S}}_2$ displays a magnetic transition at $T_C\sim 45\mathrm{K}$ with highly anisotropic behavior expected for a hexagonal-structured material. Below $T_C$, neutron diffraction reveals increased scattering near the structural Bragg peaks having a wider Q dependence along the $c$ axis than the nuclear Bragg peaks. This indicates either a short-range ferromagnetic (FM) order with a domain size of ∼250 nm along the $c$ axis or a possible magnetic modulation with a large pitch length. The expectation of a significant Dzyaloshinskii-Moriya interaction in this chiral-structured magnet, along with the helical state discovered in isostructural $\mathrm{C}{\mathrm{r}}_{1/3}\mathrm{Nb}{\mathrm{S}}_2$, suggest either a long period helical state with $q\sim 0.0025{\AA }^{-1}$, or FM regions separated by magnetic solitons, may be responsible for the apparent small size of the FM domains. Here, the domain length along the $c$ axis is substantially larger than the pitch length of 48 nm found for the helimagnetic state in $\mathrm{C}{\mathrm{r}}_{1/3}\mathrm{Nb}{\mathrm{S}}_2$. Specific-heat-capacity measurements confirm a second-order magnetic phase transition with a substantial magnetic contribution that persists to low temperature. The low-temperature specific-heat capacity is consistent with a large density of low-lying magnetic excitations that are likely associated with topologically interesting magnetic modes. Changes to the magnetoresistance, the magnetization, and the magnetic neutron diffraction, which become more apparent below 20 K, imply a modification in the character of the magnetic ordering corresponding to the magnetic contribution to the specific-heat capacity. These observations signify a more complex magnetic structure both at zero and finite fields for $\mathrm{M}{\mathrm{n}}_{1/3}\mathrm{Nb}{\mathrm{S}}_2$ than for the well-investigated $\mathrm{C}{\mathrm{r}}_{1/3}\mathrm{Nb}{\mathrm{S}}_2$.

Dirac semimetals (DSM) and Weyl semimetal (WSM) fall under the generic class of three-dimensional (3D) solids, which follow the relativistic energy-momentum relation ${\epsilon }_{\mathbf{k}}=\hslash v_F\left|\mathbf{k}\right|$ at low energies. Such a linear dispersion when regularized on a lattice can lead to remarkable properties such as the anomalous Hall effect, the presence of Fermi surface arcs, positive longitudinal magnetoconductance, and dynamic chiral magnetic effect. The last two properties arise due to the manifestation of chiral anomaly in these semimetals, which refers to the nonconservation of chiral charge in the presence of electromagnetic gauge fields. Here we propose the planar Nernst effect, or transverse thermopower, as another consequence of chiral anomaly, which should occur in both Dirac and Weyl semimetals. We analytically calculate the planar Nernst coefficient for DSMs (type I and type II) and also WSMs (type I and type II) using a quasiclassical Boltzmann formalism. The planar Nernst effect manifests in a configuration when the applied temperature gradient, magnetic field, and the measured voltage are all coplanar and is of distinct origin when compared to the anomalous and conventional Nernst effects. Our findings, specifically a 3D map of the planar Nernst coefficient in type-I Dirac semimetals (${\mathrm{Na}}_3\mathrm{Bi}$, ${\mathrm{Cd}}_3{\mathrm{As}}_2$, etc.) and type-II DSM (${\mathrm{PdTe}}_2$, VAI3, etc.), can be verified experimentally by an in situ 3D double-axis rotation extracting the full $4\pi$ solid angular dependence of the Nernst coefficient.

A two-particle self-consistency is rarely part of mean-field theories. It is, however, essential for avoiding spurious critical transitions and unphysical behavior. We present a general scheme for constructing analytically controllable approximations with self-consistent equations for the two-particle vertices based on the parquet equations. We explain in detail how to reduce the full set of parquet equations so as not to miss quantum criticality in strong coupling. We further introduce a decoupling of convolutions of the dynamical variables in the Bethe-Salpeter equations to make them analytically solvable. We connect the self-energy with the two-particle vertices to satisfy the Ward identity and the Schwinger-Dyson equation and discuss the role of the one-particle self-consistency in making the approximations reliable in the whole spectrum of the input parameters. Finally, we demonstrate the general construction on the simplest static approximation that we apply to the Kondo behavior of the single-impurity Anderson model.

The physical properties of the organic conductor $\kappa \text{−}\left(\mathrm{BEDT}\text{−}{\mathrm{TTF})}_2{\mathrm{Cu}[\mathrm{N}\left(\mathrm{CN}\right)}_2\right]\mathrm{I}$ $\left(\kappa \text{−}\mathrm{I}\right)$ are inconsistent with the universal phase diagram of isostructural $\kappa$-Br and $\kappa$-Cl. To systematically understand $\kappa$-type salts, we measured and compared ${}^{13}\mathrm{C}$ NMR spectra of $\kappa$-I, $\kappa$-Br, and $\kappa$-Cl. The $\kappa$-I spectra manifested abrupt line broadening below 40 K due to the development of electronic inhomogeneity and an increase in the spin-lattice relaxation rate divided by temperature $1/T_{1}T$ down to the lowest temperature, suggesting an enhanced antiferromagnetic (AF) fluctuation. These results imply that AF fluctuation without long-range order in $\kappa$-I is induced by electronic inhomogeneity, which disturbs the development of coherency. Consequently, $\kappa$-I is an insulator with AF fluctuation at low temperature similar to x-ray-irradiated $\kappa$-Cl. We suggest that not only pressure but also disorder is a key parameter in the physical properties of $\kappa$-type salts.

We study the injection current response tensor (also known as circular photogalvanic effect or ballistic current) in ferrolectric monolayer GeS, GeSe, SnS, and SnSe. We find that the injection current is perpendicular to the spontaneous in-plane polarization and could reach peak (bulk) values of the order of ${10}^{10}\mathrm{A}{\mathrm{V}}^{-2}{\mathrm{s}}^{-1}$ in the visible spectrum. The magnitude of the injection current is the largest reported in the literature to date for a two dimensional material. To rationalize the large injection current, we correlate the injection current spectrum with the joint density of states, electric polarization, strain, etc. We find that various factors such as anisotropy, in-plane polarization and wave function delocalization are important in determining the injection current tensor in these materials. We also find that compression along the polar axis can increase the injection current (or change its sign), and hence strain can be an effective control knob for their nonlinear optical response. Conversely, the injection current can be a sensitive probe of the crystal structure.

Weyl semimetals are known to host massless surface states called Fermi arcs. These Fermi arcs are the manifestation of the bulk-boundary correspondence in topological matter and thus are analogous to the topological chiral surface states of topological insulators. It has been shown that the latter, depending on the smoothness of the surface, host massive Volkov-Pankratov states that coexist with the chiral ones. Here, we investigate these VP states in the framework of Weyl semimetals, namely their density of states and magneto-optical response. We find the selection rules corresponding to optical transitions which lead to anisotropic responses to external fields. In the presence of a magnetic field parallel to the interface, the selection rules and hence the poles of the response functions are mixed.

We present atomistic calculations on structural and electronic properties of twisted double bilayer graphene (TDBG) consisting of two sets of rotationally misaligned Bernal-stacked bilayer graphene. Obtained equilibrium atomic structures exhibit in-plane strains and the modulation of the interlayer distances at the rotationally mismatched interface layers. We find that the electronic structure of TDBG can have an intrinsic band gap at the charge neutral point for a large range of the twist angle $\theta$. Near $\theta =1.{25}^{\circ }$, the intrinsic band gap disappears and TDBG hosts flat bands at the Fermi level that are energetically well separated from higher and lower energy bands. We also show that the flat bands are easily tunable by applying vertical electric fields, and extremely narrow bandwidths less than 10 meV can be achieved for the electron-side flat bands in a wide range of the twist angle. Our results serve as a theoretical guide for exploring emergent correlated electron physics in this versatile moiré superlattice system.

A hybrid theory which combines configuration interaction and Green's function approaches is proposed here to treat charged and neutral excitations. The theory is parameter free and reduces to the well-known Bethe-Salpeter equation (BSE) in the case of excitons. However, unlike the BSE, the theory can be applied to calculate any excitation beyond the exciton. As this type of computation is generally time consuming, we show that in the case of Wannier-type excitations, the localization in reciprocal space can be used to reduce the required computation load. We apply our approach to excitons and trions in ${\mathrm{WS}}_2$ and ${\mathrm{MoS}}_2$ transition metal dichalcogenides monolayers and obtain optical spectra, binding energies, and dark-bright exciton splitting in good agreement with experimental measurements. Moreover, in the case of ${\mathrm{WS}}_2$, we have found that the negative trion peak shows a fine structure splitting (FSS) of 15 meV which corresponds to the FSS for an isolated monolayer.

The emergence of flat bands in twisted bilayer graphene leads to an enhancement of interaction effects, and thus to insulating and superconducting phases at low temperatures, even though the exact mechanism is still widely debated. The position and splitting of the flat bands is also very sensitive to the residual interactions. Moreover, the low-energy bands of twisted graphene bilayers show a rich structure of singularities in the density of states, van Hove singularities, which can enhance further the role of interactions. We study the effect of the long-range interactions on the band structure and the van Hove singularities of the low-energy bands of twisted graphene bilayers. Reasonable values of the long-range electrostatic interaction lead to a band dispersion with a significant dependence on the filling. The change of the shape and position of the bands with electronic filling implies that the van Hove singularities remain close to the Fermi energy for a broad range of fillings. This result can be described as an effective pinning of the Fermi energy at the singularity. The sensitivity of the band structure to screening by the environment may open new ways of manipulating the system.

Doping twisted bilayer graphene away from charge neutrality leads to an enormous buildup of charge inhomogeneities within each moiré unit cell. Here, we show, using unbiased real-space self-consistent Hartree calculations on a relaxed lattice, that Coulomb interactions smoothen this charge imbalance by changing the occupation of earlier identified “ring” orbitals in the AB/BA region and “center” orbitals at the AA region. For hole doping, this implies an increase of the energy of the states at the $\mathrm{\Gamma }$ point, leading to a further flattening of the flat bands and a pinning of the Van Hove singularity at the Fermi level. The charge smoothening will affect the subtle competition between different possible correlated phases.

We design an efficient and balanced approach that captures major effects of collective electronic fluctuations in strongly correlated fermionic systems using a simple diagrammatic expansion on a basis of dynamical mean-field theory. For this aim we perform a partial bosonization of collective fermionic fluctuations in leading channels of instability. We show that a simultaneous account for different bosonic channels can be done in a consistent way that allows to avoid the famous Fierz ambiguity problem. The present method significantly improves a description of an effective screened interaction $W$ in both charge and spin channels, and has a great potential for application to realistic $GW$-like calculations for magnetic materials.

Atomlike defect levels in silicon carbide (SiC) polytypes have been proposed and proven to be an excellent platform for various quantum technology applications. Single-photon emitters, coherent control at room temperature, and temperature and magnetic field sensing at the nanoscale have already been demonstrated for the case of vacancy and divacancy defects in SiC and more recently proposed for negatively charged NV centers. NV centers, which allow a better control of their generation, offer in addition a further shift of the spectral range in the near infrared, i.e., in the $O$- and $S$-band telecom range. We demonstrate here that the association of high resolution optical spectroscopy and electron paramagnetic resonance spectroscopy combined with first-principles calculations allow the identification of the microscopic structure of the six distinct NV centers in $6H-\mathrm{SiC}$ and the assignment of their associated zero-phonon photoluminescence lines. Time resolved photo-EPR measurements at $T=4\mathrm{K}$ show that NV centers in $6H-\mathrm{SiC}$ present spin lattice relaxation times of several seconds. These excellent qubit properties should enable their application and implementation in quantum information devices.

The valley transport properties of a superlattice of out-of-plane Gaussian deformations are calculated using a Green's function and a machine learning approach. Our results show that periodicity significantly improves the valley filter capabilities of a single Gaussian deformation; these manifest themselves in the conductance as a sequence by valley filter plateaus. We establish that the physical effect behind the observed valley notch filter is the coupling between counterpropagating transverse modes; the complex relationship between the design parameters of the superlattice and the valley filter effect make it difficult to estimate in advance the valley filter potentialities of a given superlattice. With this in mind, we show that a deep neural network can be trained to predict valley polarization with a precision similar to the Green's function but with much less computational effort.

Skyrmions are spatially localized magnetic swirls which carry a nonzero integer topological charge. We study crystals of skyrmions in a two-dimensional ferromagnet model with chiral interactions induced by the presence of broken inversion symmetry. We show that nonlinear quartic mode-coupling terms allowed by symmetry enhance the Zeeman-field range over which the skyrmion-crystal phase remains stable. Furthermore, it leads to a significant dependence of the lattice constant of this spin crystal over this wide field range. Conduction electrons coupled to such a tunable spin crystal are shown to experience a Berry-flux density which varies with the Zeeman field. Such tunable skyrmion crystals provide a distinct realization of a Berry-Hofstadter butterfly, resulting in a phenomenon we term “topological quantum oscillations” in the electronic density of states and associated observables.

We report direct detection of charge tunneling between a quantum dot and a superconducting island through radio-frequency gate sensing. We are able to resolve spin-dependent quasiparticle tunneling as well as two-particle tunneling involving Cooper pairs. The quantum dot can act as an RF-only sensor to characterize the superconductor addition spectrum, enabling us to access subgap states without transport. Our results provide guidance for future dispersive parity measurements of Majorana modes, which can be realized by detecting the parity-dependent tunneling between dots and islands.

By performing a series of thermodynamic measurements in an applied magnetic field $H_{\mathrm{ext}}$, we investigated the effects of Eu substitution on the Pr sites in the filled skutterudite compound ${\mathrm{Pr}}_{1-x}{\mathrm{Eu}}_x{\mathrm{Pt}}_4{\mathrm{Ge}}_{12}$ ($0\le x\le 1$). A heat capacity Schottky anomaly is present over the whole doping range. For the samples with $x>0.5$, the temperature of the maximum, $T_{max}$, shifts to lower temperature with increasing $H_{\mathrm{ext}}$. We argue that this behavior reflects the antiferromagnetic (AFM) ordering of the Eu moments, as the AFM transition is suppressed by $H_{\mathrm{ext}}$. The Schottky anomaly in the samples with $x\le 0.5$ shifts to higher temperatures with increasing magnetic field, signaling the presence of an internal magnetic field due to short-range AFM correlations induced by magnetic moments of neighboring Eu sites. In low $H_{\mathrm{ext}}$, the Schottky gaps show a nonlinear relationship with $H_{\mathrm{ext}}$ as the magnetic moments become weakly magnetized. In high $H_{\mathrm{ext}}$, the magnetic moments of Eu sites become completely aligned with $H_{\mathrm{ext}}$. Thus, increasing $H_{\mathrm{ext}}$ does not further increase the magnetization; hence the Schottky gaps increase linearly with $H_{\mathrm{ext}}$.

The compression of liquid nickel and cobalt has been investigated up to 102 and 83 GPa, respectively, by x-ray absorption spectroscopy at the Ni and Co $K$ edges, using laser-heated diamond anvil cell. A shortening of about 8%–10% of the metal bond distances is observed in the pressure range considered and the experimental results are confirmed by theoretical calculations. Ab initio quantum molecular dynamics calculations have been performed, providing a model pair distribution function for cobalt, used as a starting model in the extended x-ray absorption fine structure analysis. The specific volume of the liquid has also been estimated and compared to calculations in the literature. This work sets the basis for x-ray absorption spectroscopy studies of liquid metals under extreme conditions with potentially important implications for the understanding of Earth and planetary interiors.

Ferroelectrics have been rarely considered as suitable candidates for the design of agile devices in the booming field of millimeter wavelengths and terahertz applications. Here a specifically designed free-space quasioptical system to determine the dielectric tunability and losses of a barium titanate single crystal at different temperatures is described. The values obtained are promising to establish ferroelectrics in the inventory of functional materials relevant at millimeter wavelengths. The potential for application of Landau's theory and a discussion of the role of phonon modes on the tunability is presented.

The paper studies the existence of surface acoustic waves in half-infinite one-dimensional piezoelectric phononic crystals consisting of perfectly bonded layers, which are arranged so that the unit cell is symmetric, i.e., is invariant with respect to inversion about its midplane. An example is a bilayered structure with exterior layer being half thinner than the interior layers of the same material. The layers may be generally anisotropic. The maximum possible number of surface acoustoelectric waves referred to a fixed wave number and a given full stop band is established for different types of electric boundary conditions at the mechanically free or clamped surface. In particular, it is proved that the phononic crystal-vacuum interface can support two surface waves in any full stop band. The same statement holds true in the case of a metallized surface of the crystal. This number is greater than that in a purely elastic case. In the presence of crystallographic symmetry, which decouples the sagittally and horizontally polarized surface waves, their separate admissible numbers are obtained. It is shown that the propagation along the normal to the surface is a special case, where the maximum number of surface waves is less than that along oblique directions.