Antiferromagnetic (AFM) metals have attracted tremendous interest for memory applications due to their expected fast response dynamics in the terahertz frequency regime. Spin dynamics such as AFM resonance and the magnon relaxation rate have been measured using the linear magneto-optical Kerr effect (MOKE) with coherent spin precession induced by laser pumping. Polarized electromagnetic radiation is a promising alternative for probing the response in canted AFM systems. Hence, in this paper, we use first-principles simulations to study the magneto-optical response of AFM $M_2\mathrm{As} (M=\mathrm{Cr},\mathrm{Mn},\text{and} \mathrm{Fe})$ under external magnetic fields. We devise a computational scheme to compute the magnetic susceptibility from total-energy changes using constraints on magnetic-moment tilting. Our predictions of the spectral dependence of polar magneto-optical Kerr rotation and ellipticity allow us to attribute these effects to breaking of the magnetic symmetry. We show that tilting of magnetic moments affects the exchange interaction, while the spin-orbit interaction remains unaffected. In this paper, we provide an understanding of the polar MOKE on a band structure level and underscore the importance of the magnetic susceptibility when searching for materials with large magneto-optical response.

Surfaces of nodal noncentrosymmetric superconductors can host flat bands of Majorana modes, which provide a promising platform for quantum computation if one can find methods for manipulating localized Majorana wave packets. We study the fate of such flat bands when part of the surface is subjected to an exchange field induced by a ferromagnetic insulator. We use exact diagonalization to find the eigenstates and eigenenergies of the Bogoliubov–de Gennes Hamiltonian of a model system, for which an exchange field is applied along a strip on the surface of a slab. We consider different orientations of the strip and the applied field. If the spin polarization of the field-free system along the field direction is sufficiently large, perturbation theory predicts that energies of states which are mostly localized on the exchange-field strip are shifted away from zero energy by an amount proportional to the field strength. On the other hand, energies corresponding to states localized on the field-free strip are only weakly affected by the field. Exact diagonalization confirms this. Moreover, we discuss a setup with a small exchange field applied to the previously field-free strip with the goal of introducing a linear dispersion. By switching this dispersion on and off, a wave packet could be moved in a certain direction. We find that in our model system, a linear dispersion can indeed be achieved. The qualitative features of this dispersion can be predicted from the momentum-dependent spin polarization of the field-free surface.

Recent experiments with ultracold fermionic atoms in optical lattices have provided a tuneable and clean realization of the attractive Hubbard model (AHM). In view of this, several physical properties may be thoroughly studied across the crossover between weak [Bardeen-Cooper-Schrieffer, (BCS)] and strong [Bose-Einstein condensation (BEC)] couplings. Here we report on extensive determinant Quantum Monte Carlo (DQMC) studies of the AHM on a square lattice from which several different quantities have been calculated and should be useful as a road map to experiments. We have obtained a detailed phase diagram for the critical superconducting temperature $T_c$ in terms of the band filling $\langle n\rangle$ and interaction strength $U$ from which we pinpoint a somewhat wide region $\left|U\right|/t\approx 5\pm 1$ ($t$ is the hopping amplitude) and $\langle n\rangle \approx 0.79\pm 0.09$ leading to a maximum $T_c\approx 0.16t$. Two additional temperature scales, namely, pairing $T_p$ and degeneracy $T_d$ have been highlighted: The former sets the scale for pair formation (believed to be closely related to the scale for the gap of spin excitations in cuprates), whereas the latter sets the scale for dominant quantum effects. Our DQMC data for the distribution of doubly occupied sites for the momentum distribution function, and for the quasiparticle weight show distinctive features on both sides of the BCS-BEC crossover, being also suggestive of an underlying crossover between Fermi- and non-Fermi liquid behaviors.

In this paper, the effects of the external transverse magnetic field $B$ (perpendicular to the surface of the layers) on electronic and excitonic properties are studied in AA-stacked bilayer graphene (BLG). The effects of the Coulomb interactions and excitonic pairing have been taken into account and analyzed in detail within the bilayer Hubbard model. Both half-filling and partial-filling regimes have been taken into account and the magnetic field dependence of a series of physical parameters was found. It is shown that the difference between the average electron concentrations in the layers vanishes at some critical value of magnetic field $B_c$ and the chemical potential is calculated numerically above and below that value. The role of the Coulomb interactions on the average carrier concentrations in the layers has been analyzed and the excitonic order parameters have been calculated for different spin orientations. We found a possibility for the particle population inversion between the layers when varying the external magnetic field. The calculated electronic band structure in AA-BLG shows the presence of a metal-semiconductor transition, governed by the strength of the applied magnetic field or the interlayer interaction potential. We show that for high magnetic fields, the band gap is approaches the typical values of the gaps in the usual semiconductors. It is demonstrated that, at some parameter regimes, AA-BLG behaves like a spin-valve device by permitting the electron transport with only one spin direction. All calculations have been performed at the zero-temperature limit.

Recently, a model based on a combined ab initio/time-differential perturbed $\gamma \text{−}\gamma$ angular correlation (TDPAC) study in the ${}^{111}\mathrm{In}\left(\to {}^{111}\mathrm{Cd}\right)$-doped ${\mathrm{SnO}}_2$ semiconductor was proposed to enlighten the origin of the dynamic hyperfine interactions (HFIs) observed in oxides after the electron-capture (EC) decay of the probe nucleus of the ${}^{111}\mathrm{In}$ parent. It was demonstrated that both the EC as well as the acceptor level introduced by the Cd impurity can contribute to the variation of the electric-field gradient (EFG) at the Cd site (responsible for this phenomenon) due to charge fluctuations of the electronic holes created [Darriba et al., J. Phys. Chem. C 122, 17423 (2018)]. To disentangle both effects, we performed here TDPAC reversible experiments in ${}^{111}\mathrm{In}\left(\to {}^{111}\mathrm{Cd}\right)$-doped SnO in the temperature range 20–900 K and a complete electronic structure density functional theory study of the Cd-doped SnO system as a function of the charge state $\left(q\right)$ of the impurity. The isovalent ${}^{111}\mathrm{Cd}$ probe impurity in this isocationic oxide was selected to demonstrate that the acceptor character of the probe is not a necessary condition to observe this electronic phenomenon when the EC is already present. A majority fraction of probes sensed a time-dependent HFI (HFI1) for all temperatures, and the unexpected minority always sensed a static interaction (HFI2). Comparing results from these experiments, the ab initio calculations of the EFG tensor and defect formation energies, HFI1 and HFI2, were assigned to ${}^{111}\mathrm{Cd}$ probes localized at substitutional Sn sites free of defects [with the valence band (VB) completely filled, $q=0$] and with an electronic hole trapped $(q=1+)$, respectively. In addition, we show that the systems with charge state between these values have very similar formation energies at the top of the VB and different predicted EFGs, giving support from first principles to the existence of random fluctuations between different EFGs, necessary in the construction of the on-off perturbation factor used. We demonstrate that all the probes fill their electronic holes before the TDPAC time window except the outermost one, giving a quantification of the number of electronic holes involved in the production of the dynamic effect, only one in SnO. The probes related to HFI2 reach their stable electronic configuration $(q=1+)$ before the emission of the first γ ray $\left({\gamma }_1\right)$ of the γ-γ cascade, while those related to HFI1 fill the last hole after the emission of ${\gamma }_1$, producing the dynamic interaction. Enlighted by these results and reviewing TDPAC experiments from the literature using probes with different nuclear decay characteristics, we concluded that, in the absence of EC decay of the probe nucleus of the parent, any probe impurity introducing acceptor or donor states (in semiconductors and insulators) whose ionization changes the EFG should observe a time-dependent HFI provided the lifetime of the nuclear levels that feed the ${\gamma }_1$ level and this level itself is shorter than the lifetime of the recovery process (i.e., the lifetime of the electronic holes or the ionized donor electrons).

Contact aging, i.e., the strengthening of a tribological contact with time, is a phenomenon that is commonly attributed to an increase of the effective contact area between multiasperity surfaces based on contact mechanical processes. Only recently, atomic force microscopy experiments and simulations on single silica nanojunctions have also demonstrated logarithmic aging with contact time for single-asperity contacts and the effects were attributed to a gradual formation of chemical bonds in the contact, thereby suggesting an atomistic mechanism of “contact aging.” Generally, this atomic bond-formation process is considered as a shear-assisted thermally activated process, where both temperature and loading conditions are expected to influence the aging behavior. A direct link between aging and temperature has recently been substantiated by temperature-dependent aging experiments and complementary molecular dynamics simulations, but the potential influence of shear stress on single-asperity aging still remains largely elusive. To analyze the role of shear stress, we now apply a simple and semianalytical model for thermally activated bond formation, which explicitly considers the reduction of energy barriers by a shear-stress-related Eyring term. Based on this model, characteristic fingerprints of mechanical shear can be identified in our temperature-dependent contact aging experiments, including, for instance, the counterintuitive decrease of aging effects with increasing temperature. Our results thereby hint at a path to introduce this form of qualitative contact aging to the phenomenological rate and state friction, where the local aging effects might be closely linked to the shear-stress distribution within multiasperity contacts.

Double local and double nonlocal Andreev reflections in a nodal-line semimetal-superconductor-nodal-line semimetal junction were investigated. We find that double Andreev reflections (the specular Andreev reflection and the retro-Andreev reflection), double normal reflections (the specular normal reflection and the retronormal reflection), double electron transmissions (the specular electron transmission and the normal electron transmission), and double crossed Andreev reflections (the specular crossed Andreev reflection and the crossed Andreev reflection) exist simultaneously when an electron incidents from the semimetal side. The transport properties of the scattering process depend on the incident angle, incident energy, and barrier height. It is found that a finite incident angle and a finite barrier height are necessary conditions for the appearance of these local and nonlocal Andreev reflections. By adjusting these parameters, the reflections and transmissions can be controlled.

The structure of the ground-state exciton of Cd(Se,Te) quantum dots embedded in ZnTe matrix is studied experimentally using photoluminescence spectroscopy and theoretically using $\mathbf{k}·\mathbf{p}$ and configuration interaction methods. The experiments reveal a considerable reduction of fine-structure splitting energy of the exciton with an increase of Se content in the dots. That effect is interpreted by theoretical calculations to originate due to the transition from spatially direct (type-I) to indirect (type-II) transition between electrons and holes in the dot induced by an increase of Se. The trends predicted by the theory match those of the experimental results very well. The theory identifies that the main mechanism causing elevated fine-structure energy, in particular in type-I dots, is due to the multipole expansion of the exchange interaction. Moreover, the theory reveals that for Se contents in the dot $>0.3$, there also exists a peculiar type of confinement showing signatures of both type I and type II and which exhibits extraordinary properties, such as an almost purely light hole character of exciton and toroidal shapes of hole states.

The screening of Coulomb interaction controls many-body physics in carbon nanotubes, as it tunes the range and strength of the force that acts on charge carriers and binds electron-hole pairs into excitons. In doped tubes, the effective Coulomb interaction drives the competition between Luttinger liquid and Wigner crystal, whereas in undoped narrow-gap tubes it dictates the Mott or excitonic nature of the correlated insulator observed at low temperature. Here, by computing the dielectric function of selected narrow- and zero-gap tubes from first principles, we show that the standard effective-mass model of screening systematically underestimates the interaction strength at long wavelength, hence missing the binding of low-energy excitons. The reason is that the model critically lacks the full three-dimensional topology of the tube, being adapted from graphene theory. As ab inito calculations are limited to small tubes, we develop a two-band model dielectric function based on the plane-wave expansion of Bloch states and the exact truncated Coulomb cutoff technique. We demonstrate that our—computationally cheap—approach provides the correct screening for narrow-gap tubes of any size and chirality. A striking result is that the screened interaction remains long-ranged even in gapless tubes, as an effect of the microscopic local fields generated by the electrons moving on the curved tube surface. As an application, we show that the effective electron-electron force that is felt at distances relevant to quantum transport experiments is super Coulombic.

Recent approximate analytical work has suggested that, at certain values of the external pump, the optical parametric oscillator (OPO) regime of microcavity polaritons may provide a long-sought realization of Kardar-Parisi-Zhang (KPZ) physics in 2D. Here, by solving the full microscopic model numerically using the truncated Wigner method, we prove that this predicted KPZ phase for OPO is robust against the appearance of vortices or other effects. For those pump strengths, spatial correlations in the direction perpendicular to the pump, and the distribution of phase fluctuations, match closely to the forms characteristic of the KPZ universality. This strongly indicates the viability of observing KPZ behavior in future polariton OPO experiments.

Nanodiamonds containing negatively charged nitrogen vacancy centers (${\text{NV}}^{-}$) have applications as localized sensors in biological materials and have been proposed as a platform to probe the macroscopic limits of spatial superposition and the quantum nature of gravity. A key requirement for these applications is to obtain nanodiamonds containing ${\text{NV}}^{-}$ with long spin coherence times. Using milling to fabricate nanodiamonds processes the full 3D volume of the bulk material at once, unlike etching pillars, but has, up to now, limited ${\text{NV}}^{-}$ spin coherence times. Here, we use natural isotopic abundance nanodiamonds produced by ${\text{Si}}_3{\text{N}}_4$ ball milling of chemical vapor deposition grown bulk diamond with an average single substitutional nitrogen concentration of $121\text{ppb}$. We show that the electron spin coherence times of ${\text{NV}}^{-}$ centers in these nanodiamonds can exceed 400 $\mu \text{s}$ at room temperature with dynamical decoupling. Scanning electron microscopy provides images of the specific nanodiamonds containing ${\text{NV}}^{-}$ for which a spin coherence time was measured.

In this work, we explore interesting consequences arising from the coupling between a clean non-Hermitian chain with skin localization and a delocalized chain of the same length under various boundary conditions (BCs). We reveal that in the ladder with weak rung coupling, the non-Hermitian skin localization could induce a pseudo mobility edge (ME) in the complex energy plane, which separates states with extended and localized profiles yet allows unidirectional transport of signals. We also demonstrate the gradual takeover of the non-Hermitian skin effect in the entire system with the increase of the rung coupling under conventional open BC (OBC). When taking OBC for the non-Hermitian chain and periodic BC (PBC) for the other, it is discovered that a quantized winding number defined under PBC could characterize the transition from the pseudo-ME to the trivial extended phases, establishing a “bulk-defect correspondence” in our quasi-1D non-Hermitian system. This work hence unveils more subtle properties of non-Hermitian skin effects and sheds light on the topological nature of the non-Hermitian localized modes in the proximity to systems with dissimilar localization properties.

In this work, we study many-body “steady states” that arise in the non-Hermitian generalization of the noninteracting Su-Schrieffer-Heeger model at a finite density of fermions. We find that the hitherto known phase diagrams for this system, derived from the single-particle gap closings, in fact correspond to distinct nonequilibrium phases, which either carry finite currents or are dynamical insulators where particles are entrapped. Each of these have distinct quasiparticle excitations and steady-state correlations and entanglement properties. Looking at finite-sized systems, we further modulate the boundary to uncover the topological features in such steady states, particularly the emergence of leaky boundary modes. Using a variety of analytical and numerical methods, we develop a theoretical understanding of the various phases and their transitions, and we uncover the rich interplay of nonequilibrium many-body physics, quantum entanglement, and topology in a simple looking yet rich model system.

Magnetotransport properties of single- and double-rhombohedral trilayer graphene (TLG) are studied. Double TLG is obtained by folding single TLG, yielding a twist angle of 4°. Raman spectroscopic measurements confirm ABC stacking order and the presence of interlayer coupling. The Landau fan diagram exhibits independently superimposed features of each constituent TLG, indicating weak interlayer coupling. As a result, we show that the current framework for twisted systems can be extended to TLG.

Graphene deposited on top of a copper(111) substrate may develop a Y-shaped Kekulé bond texture (Kekulé-Y), locking the momentum with the valley degree of freedom of its Dirac fermions. Consequently, the valley degeneracy of its band structure is broken, generating an energy dispersion with two nested Dirac cones with different Fermi velocities. This work investigates the dynamics of electronic wave packets in the Kekulé-Y superlattice with strength ${\mathrm{\Delta }}_0$. We show that, as a result of the valley-momentum coupling, a valley-driven oscillatory motion of the wave packets (Zitterbewegung) could appear, but with a drastically reduced attenuation rate and lower frequency (proportional to ${\mathrm{\Delta }}_0$) when compared to the Zitterbewegung effect associated with pristine graphene. Furthermore, we justify the presence of these Zitterbewegung frequencies in terms of the Berry connection matrix and a discrete symmetry present in the system. These results make Kekulé-Y graphene a compelling candidate for experimental observation of the Zitterbewegung phenomenon in a two-dimensional system.

A non-Hermitian skin effect was observed in one-dimensional systems with short-range chiral interactions. A long-range chiral interaction mediated by traveling waves also favors the accumulation of energy, but has not yet exhibited non-Hermitian topology. Here, we find that the strong interference brought by the wave propagation is detrimental for accumulation. By the suppression of interference via the damping of traveling waves, we predict the non-Hermitian skin effect of magnetic excitation in a periodic array of magnetic nanowires that are coupled chirally via spin waves of a thin magnetic substrate. The local excitation of a wire at one edge by weak microwaves of magnitude $\sim \mu \mathrm{T}$ leads to a considerable spin-wave amplitude at the other edge, i.e., a remarkable functionality useful for the sensitive, nonlocal, and nonreciprocal detection of microwaves.

A large intrinsic anomalous Hall effect (AHE) originating in the Berry curvature has attracted growing attention for potential applications. The recently proposed magnetic Weyl semimetal ${\mathrm{EuCd}}_2{\mathrm{Sb}}_2$ provides an excellent platform for controlling the intrinsic AHE because it only hosts a Weyl-point-related band structure near the Fermi energy. Here, we report the fabrication of ${\mathrm{EuCd}}_2{\mathrm{Sb}}_2$ single-crystalline films and control of their anomalous Hall effect by a film technique. As also analyzed by first-principles calculations of energy-dependent intrinsic anomalous Hall conductivity, the obtained anomalous Hall effect shows a sharp peak as a function of carrier density, demonstrating a clear energy dependence of the intrinsic AHE.

In the presence of an external magnetic field the Fermi-liquid state supports oscillatory spin modes known as Silin modes. We predict the existence of the generalized Silin modes in a multivalley system, monolayer graphene. A gauge- and Berry-gauge-invariant kinetic equation for a multivalley Fermi liquid is developed and applied to the case of graphene with extrinsic spin-orbit coupling (SOC). The interplay of SOC and Berry curvature allows for the excitation of generalized Silin modes in the spin and valley-staggered-spin channels via an AC electric field. The resonant contributions from these modes to the optical conductivity are calculated.

Using ab initio simulations based on density functional theory, we have analyzed the crystal structure and thermodynamic stability of ${\mathrm{Mg}}_x{\mathrm{Fe}}_{1-x}\mathrm{O}$ ferropericlase, showing how the $P\text{−}T$ phase diagram associated with the $B1\text{−}B2$ phase transition of pure MgO is influenced by the presence of iron substitutional alloys. We find that a small concentration of Fe atoms contribute to an increase of the transition pressure at fixed temperature, extending the stability of $B1$ crystalline structure. Moreover, we find a significant nonhomogeneous distribution of the iron atoms between the two phases at low temperatures, with strong partitioning in the $B1$ phase, an interesting phenomena that could lead to important dynamic consequences. Finally, we analyze the effect of the iron impurities on the volume thermal expansion.

The phonon renormalization across the semiconductor-to-metal crossover in FeSi is investigated by inelastic neutron scattering combined with ab initio lattice dynamical calculations. A significant part of reciprocal space with a particular focus on the $110-001$ scattering plane is mapped by the time-of-flight inelastic neutron scattering data taken below and above the crossover. Individual momentum values are investigated in more detail as a function of temperature. The data reveal that the anomalous phonon softening upon metallization is not exclusive to the high symmetry $R$ and $\mathrm{\Gamma }$ points. Several other phonon modes around the $R$ point as well as the phonon modes at the $M$ and $X$ points of the Brillouin zone exhibit anomalous phonon softening with magnitudes comparable to that observed at the $R$ point. The momentum dependence of the phonon softening is reproduced by the lattice dynamical calculation based on the density functional perturbation theory. We discuss our findings with respect to the nature of the semiconductor-to-metal crossover in FeSi, for which different microscopic origins have been proposed, i.e., lattice thermal disorder and electronic correlation effects.