Using symmetry analysis and density functional theory calculations, we uncover the nature of the Dzyaloshinskii-Moriya interaction in monolayer ${\mathrm{Fe}}_3{\mathrm{GeTe}}_2$. We show that while such an interaction might result in small distortions of the magnetic texture in the short range, in the long range it favors in-plane Néel spin spirals along equivalent directions of the crystal structure. Whereas our results show that the observed Néel skyrmions cannot be explained by the Dzyaloshinskii-Moriya interaction at the monolayer level, they suggest that a canted magnetic texture shall arise at the boundary of ${\mathrm{Fe}}_3{\mathrm{GeTe}}_2$ nanoflakes or nanoribbons and, most interestingly, that homochiral planar magnetic textures could be stabilized.

We present a method for calculation of Raman modes of the quantum solid phase I hydrogen and deuterium. We use the mean-field assumption that the quantized excitations are localized on one molecule. This is done by explicit solution of the time-dependent Schroedinger equation in an angle-dependent potential, and direct calculation of the polarization. We show that in the free rotor limit, the ${\mathrm{H}}_2$ and ${\mathrm{D}}_2$ frequencies differ by a factor of 2, which evolves toward $\sqrt{2}$ as the modes acquire librational character due to stronger interactions. The ratio overshoots $\sqrt{2}$ if anharmonic terms weaken the harmonic potential. We also use density functional theory and molecular dynamics to calculate the $E_{2_g}$ optical phonon frequency and the Raman linewidths. The molecular dynamics shows that the molecules are not free rotors except at very low pressure and high temperature, and become like oscillators as phase II is approached. We fit the interaction strengths to experimental frequencies, but good agreement for intensities requires us to also include strong preferred orientation and stimulated Raman effects between $S_0$(1) and $S_0$(0) contributions. The experimental Raman spectrum for phase II cannot be reproduced, suggesting that the mean-field assumption is invalid in that case.

We describe an ab initio phonon Boltzmann transport equation (BTE) approach accounting for phonon-electron scattering in addition to the well-established phonon-phonon and isotope scatterings. The phonon BTE is linearized and can be exactly solved beyond the relaxation time approximation (RTA). We use this approach to study the lattice thermal conductivity (${\kappa }_{\text{ph}}$) of molybdenum (Mo). ${\kappa }_{\text{ph}}$ of Mo is found to possess several anomalous features: (1) like in another group VI element tungsten (W), ${\kappa }_{\text{ph}}$, with a large value of 37 W ${\mathrm{m}}^{-1}$ ${\mathrm{K}}^{-1}$ at room temperature, follows weak temperature dependence due to interplay between phonon-phonon (ph-ph), phonon-electron (ph-el), and phonon-isotope (isotope) scatterings; and (2) compared with W, though Mo is much lighter in mass, Mo has a smaller ${\kappa }_{\text{ph}}$. This is attributed to weaker interatomic bonding, larger isotope mixture, and larger density of states at Fermi level in Mo. In isotopically pure samples, ${\kappa }_{\text{ph}}$ increases from 37 to 48 W ${\mathrm{m}}^{-1}$ ${\mathrm{K}}^{-1}$ at room temperature. Considering the similarity of the phonon dispersion, our work suggests that chromium should also have a large ${\kappa }_{\text{ph}}$, which, rather than the complexity of the electronic band structure argued in the literature, accounts for the significant deviation of measured Lorenz number $L$ from the expected Sommerfeld value. The electrical conductivity ($\sigma$) and electronic thermal conductivity (${\kappa }_{\text{e}}$) of Mo are also calculated by using an ab initio electron BTE approach. $\sigma$ and the total thermal conductivity ($\kappa$) agree with the experimental data reasonably. These results demonstrate that the ab initio calculations can quantify the lattice and electronic contributions to $\kappa$. We also look into the cumulative $\sigma$ and ${\kappa }_{\text{ph}}$ with respect to electron and phonon mean free paths (MFPs), respectively, in order to reveal the size effect in Mo. The MFPs of electrons contributing to conductivity range from 5 to 22 nm, whereas the MFPs of phonons primarily distribute between 5 and 73 nm with more than 80% contribution to ${\kappa }_{\text{ph}}$. This suggests that a reduced Lorenz number can be observed in Mo nanostructures when the relevant size goes below $\sim 70$ nm.

We consider the spin-1/2 antiferromagnetic Heisenberg model on two one-dimensional frustrated lattices, a double-tetrahedral chain and an octahedral chain, with almost dispersionless (flat) lowest magnon band in a strong magnetic field. Using the localized-magnon picture, we construct an effective description of the one-dimensional chains with triangle and square traps within the strong-coupling approximation. We perform extensive exact diagonalization and density matrix renormalization group calculations to check the validity of the obtained effective Hamiltonians by comparison with the initial models with special focus on the magnetization and specific heat at high magnetic fields.

Bogoliubov Fermi surfaces (BFSs) are topologically protected regions of zero energy excitations in a superconductor whose dimension equals that of the underlying normal state Fermi surface. Examples of Hamiltonians exhibiting this “ultranodal” phase are known to preserve charge-conjugation ($C$) and parity ($P$) but break time-reversal ($T$). In this work, we provide examples of model Hamiltonians that do not necessarily preserve this symmetry pattern but have well-defined sign-changing Pfaffians yielding BFSs. While their topological character has not been recognized previously, some of the models we present have been extensively studied in prior literature. We further examine thermodynamic and electronic properties arising from the ultranodal state. In particular, we study the effect of a weak Zeeman field close to the topological transition and propose distinguishing features of BFSs using residual specific heat and tunneling conductance. Our calculation of the superfluid density in a toy multiband model indicates a window of interband pairing strength where BFSs are stable with a positive superfluid density. We also present additional signatures of BFSs in spin-polarized spectral weight and total magnetization measurements.

We theoretically study physical properties of the low-energy quasiparticle excitations at the vortex core in the full-gap superconducting state of the Kondo lattice coupled to compensated metals. Based on the mean-field description of the superconducting state, we numerically solve the Bogoliubov-de Gennes (BdG) equations for the tight-binding Hamiltonian. The isolated vortex is characterized by a length scale independent of the magnitude of the interaction and the energy level of the core bound state is the same order as the bulk gap. These properties are in strong contrast to the conventional $s$-wave superconductor. To gain further insights, we also consider the effective Hamiltonian in the continuous limit and construct the theoretical framework of the quasiclassical Green's function of conduction electrons. With the use of the Kramer-Pesch approximation, we analytically derive the spectral function describing the quasiparticle excitations which is consistent with the numerics. It has been revealed that the properties of the vortex bound state are closely connected to the characteristic odd frequency dependence of both the normal and anomalous self-energies which is proportional to the inverse of frequency.

We derive and study a microscopic spin Hamiltonian on a lattice for Rashba-coupled double exchange metals. The Hamiltonian consists of anisotropic interactions of the Dzyaloshinskii-Moriya and pseudodipolar form, in addition to the standard isotropic term. We validate the spin Hamiltonian by comparing results with those on the exact spin-fermion model, and present its phase diagram using large-scale Monte Carlo simulations. In addition to ferromagnetic, planar spiral, and flux states, the model hosts skyrmion crystal and classical spin-liquid states characterized, respectively, by multiple peaks and a diffuse ring pattern in the spin-structure factor. The filamentary domain-wall structures in the spin-liquid state are in remarkable agreement with experimental data on thin films of MnSi-type B20 metals and transition metals and their alloys.

For dielectric multilayered metamaterials, the effective-parameter representation is known to be insensitive to geometrical features occurring at deeply subwavelength scales. However, recent studies on periodic and aperiodically ordered geometries have shown the existence of certain critical parameter regimes where this conventional wisdom is upended, as the optical response of finite-size samples may depart considerably from the predictions of standard effective-medium theory. In these regimes, characterized by a mixed evanescent/propagating light transport, different classes of spatial (dis)order have been shown to induce distinctive effects in the optical response, in terms of anomalous transmission, localization, enhancement, absorption, and lasing. Here we further expand these examples by considering a quasiperiodic scenario based on a modified-Fibonacci geometry. Among the intriguing features of this model there is the presence of a scale parameter that controls the transition from perfectly periodic to quasiperiodic scenarios of different shades. Via an extensive parametric study, this allows us to identify the quasiperiodicity-induced anomalous effects, and to elucidate certain distinctive mechanisms and footprints. Our results hold potentially interesting implications for the optical probing of structural features at a resolution much smaller than the wavelength, and could also be leveraged to design novel types of absorbers and low-threshold lasers.

We present an accurate variational approach for calculating the ground and excited exciton states in CuCl, and provide detailed comparison with measured polariton energies of excited exciton states. Computed exciton energies and oscillator strengths allow us to reproduce the polariton dispersion of up to four exciton states with unprecedented accuracy. A reinterpretation of the observed $1s$-exciton binding energy shows that the actual Coulomb energy in the exciton ground state is more than 50% larger than the observed binding energy, with important consequences for calculation of exciton complexes such as the biexciton.

We present an approach to calculate the biexciton ground state including the electron-longitudinal-optical-phonon coupling and an accurate variational function to describe the Coulomb correlations in the biexciton. We apply this method to the long-standing problem of biexciton binding energy in CuCl and obtain a binding energy of 32.6 meV. We also discuss the effect of electron-hole ($e$-h) exchange interaction on the biexciton binding energy. Including correction due to $e$-h exchange, the theoretical binding energy is 28.8 meV, in good agreement with the experimental value of 32 meV. Details of the biexciton wavefunction are presented in the form of correlation functions with respect to two-particle separations, which show how the particles in the biexciton are distributed.

It is straightforward to calculate the conductance of a quantum device once all its scattering centers are fully specified. However, to do this in reverse, i.e., to find information about the composition of scatterers in a device from its conductance, is an elusive task. This is particularly more challenging in the presence of disorder. Here we propose a procedure in which valuable compositional information can be extracted from the seemingly noisy spectral conductance of a two-terminal disordered quantum device. In particular, we put forward an inversion methodology that can identify the nature and respective concentration of randomly-distributed impurities by analyzing energy-dependent conductance fingerprints. Results are shown for graphene nanoribbons as a case in point using both tight-binding and density functional theory simulations, indicating that this inversion technique is general, robust, and can be employed to extract structural and compositional information of disordered mesoscopic devices from standard conductance measurements.

Nanoparticle plasmon resonances can be controlled using external magnetic fields or internal magnetization. The modified resonance condition is complicated, however, by the induced anisotropy of the dielectric response. Working within the electrostatic regime, we formulate a simple eigenvalue problem for the magnetoplasmon resonances. Subsequently, a semianalytical perturbative solution valid for weak magnetic fields is obtained for the resonance shift. We apply the approach to Drude metal nanodisks and show that resonance shifts are generally smaller than for nanospheres and nanoellipsoids. Finally, energy-dependent effective masses in Dirac materials are shown to reduce shifts compared to free-electron-like materials.

We study the Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction between two magnetic impurities resided on the (001) plane of topological crystalline insulator (TCI) SnTe associated with four anisotropic Dirac cones protected by the mirror symmetry. The lattice Green's functions governing the correlation of both the sublattices and spins are deduced by developing a two-band effective model. We explore the RKKY Heisenberg interaction of the system established by the interference between four Dirac cones. Our key finding is that, independent of the position of magnetic impurities on the same sublattices or different ones, multiple wave vectors with different orders describe the RKKY coupling physics in TCIs. Further, both the undoped and doped spin susceptibility give rise to the same decay rates like graphene. Magnetic impurities on the same and different sublattices propose the ferromagnetic and antiferromagnetic base phase of the undoped system, respectively. Here we demonstrate that RKKY exchange in TCIs, undergoes a ferromagnetic-to-antiferromagnetic transition and vice versa in response to switching on/off the electron and hole doping. Analytic expressions of doped TCIs are obtained to draw the magnetic phase diagram of the distance between two magnetic impurities and the doping concentration, clarifying the contributions of short and long distances as well as low and high concentrations to the RKKY coupling. Our results may tailor the magnetic features of TCIs for further research and application.

Moiré patterns are known to confine electronic states in transition metal dichalcogenide bilayers, thus generalizing the notion of magic angles discovered in twisted bilayer graphene to semiconductors. Here, we present a revised Slater-Koster tight-binding model that facilitates reliable and systematic studies of such states in twisted bilayer ${\mathrm{MoS}}_2$ for the whole range of rotation angles $\theta$. We show that isolated bands appear at low energy for $\theta \lesssim 5^{\circ }–6^{\circ }$. Moreover, these bands become “flatbands,” characterized by a vanishing average velocity, for the smallest angles $\theta \lesssim 2^{\circ }$.

The spin-nematic phase is an intriguing state of matter that lacks usual long-range dipolar order, yet it exhibits higher multipolar order. This makes its detection extremely difficult and controversial. Recently, nuclear magnetic resonance (NMR) has been proposed as one of the most suitable techniques to confirm its existence. We report a ${}^{17}\mathrm{O}$ NMR observation of the reduction of the local magnetization in the polarized state of the frustrated spin-$\frac{1}{2}$ chain $\beta -{\mathrm{TeVO}}_4$, which was previously proposed to be a fingerprint of the spin-nematic behavior. However, our detailed study shows that the detected missing fraction of the magnetization, probed by NMR frequency shift, is thermally activated, thus undermining the presence of the spin-nematic phase in the investigated compound. This highlights the importance of careful considerations of temperature-dependent NMR shift that has been overlooked in previous studies of spin nematicity.

Despite extensive studies, the exact mechanism of magnetoelastic coupling in hexagonal rare-earth manganite $R{\mathrm{MnO}}_3$ still remains under debate. Using inelastic x-ray scattering, we have measured the lattice vibrations of hexagonal manganite $R{\mathrm{MnO}}_3$ over a wide momentum-energy space to identify clear magnetoelastic excitations, which are the long-sought-after direct evidence of the dynamic magnon-phonon coupling. Our calculations using a magnetoelastic coupling model of superexchange striction quantitatively reproduces the experimental observations. Based on these results, we present a broadly applicable method to analyze the mode and momentum dependence of the magnon-phonon coupling for noncollinear magnets.

The quantum anomalous Hall effect (QAHE) has been experimentally realized in the van der Waals layered topological insulator $\mathrm{Mn}{\mathrm{Bi}}_2{\mathrm{Te}}_4$. However, the antiferromagnetic order of $\mathrm{Mn}{\mathrm{Bi}}_2{\mathrm{Te}}_4$ restricts the experimental observation of the QAHE at zero magnetic field. Based on first-principle calculations, we find that stacking $\mathrm{Mn}{\mathrm{Bi}}_2{\mathrm{Te}}_4$ with $\mathrm{V}{\mathrm{Bi}}_2{\mathrm{Te}}_4$, another material from the $\mathrm{M}{\mathrm{Bi}}_2{\mathrm{Te}}_4$ family, can realize a ferromagnetic coupling between layers with an out-of-plane easy axis. The nontrivial topological phase exists in $\mathrm{Mn}{\mathrm{Bi}}_2{\mathrm{Te}}_4/\mathrm{V}{\mathrm{Bi}}_2{\mathrm{Te}}_4$ bilayers and multilayers. $\mathrm{Mn}{\mathrm{Bi}}_2{\mathrm{Te}}_4$ is the main source of QAHE, while $\mathrm{V}{\mathrm{Bi}}_2{\mathrm{Te}}_4$ boosts ferromagnetic coupling between the layers. Thus QAHE can be achieved at zero magnetic field in $\mathrm{Mn}{\mathrm{Bi}}_2{\mathrm{Te}}_4/\mathrm{V}{\mathrm{Bi}}_2{\mathrm{Te}}_4$ heterostructures, irrespective of odd and even layers. Our finding sheds light on strongly pursuing robust QAHE in van der Waals magnetic topological insulators.

We report quantitative microwave Faraday rotation measurements conducted with a high-mobility two-dimensional electron gas (2DEG) in a GaAs/AlGaAs semiconductor heterostructure. In a magnetic field, the Hall effect and the Faraday effect arise from the action of Lorentz force on electrons in the 2DEG. As with the Hall effect, a classical Faraday effect is observed at low magnetic field along with a quantized Faraday effect at high magnetic field. The high electron mobility of the 2DEG enables a giant single-pass Faraday rotation of ${\theta }_F^{\max }\simeq {45}^{\circ }(\simeq 0.8$ rad) to be achieved at a modest magnetic field of $B\simeq 100$ mT. In the quantum regime, we find that the Faraday rotation ${\theta }_F$ is quantized in units of ${\alpha }^{*}=2.80\left(4\right)\alpha$, where $\alpha \simeq 1/137$ is the fine-structure constant. The enhancement in rotation quantum ${\alpha }^{*}>\alpha$ is attributed to electromagnetic confinement within a waveguide structure.