We investigate a highly nonlocal generalization of the Lindhard function, given by the jellium-with-gap model. We find a band-gap-dependent gradient expansion of the kinetic energy, which performs noticeably well for large atoms. Using the static linear response theory and the simplest semilocal model for the local band gap, we derive a nonempirical generalized gradient approximation (GGA) of the kinetic energy. This GGA kinetic-energy functional is remarkably accurate for the description of weakly interacting molecular systems within the subsystem formulation of density functional theory.

Interacting resonators can lead to strong nonlinearities but the details can be complicated to predict. In this work, we study the nonlinearities introduced by two nested microcavities that interact with nitrogen vacancy centers in a diamond waveguide. Each cavity has a differently designed resonance: one in the green and one in the infrared. The magnetic-field dependence of the nitrogen vacancy center absorption rates on the recently observed infrared transitions allows us to propose a scalable on-chip magnetometer that combines high magnetic-field sensitivity and micrometer spatial resolution. By investigating the system behaviors over several intrinsic and extrinsic parameters, we explain the main nonlinearities induced by the NV centers and enhanced by the cavities. We finally show that the cavities can improve the magnetic-field sensitivity by up to two orders of magnitude.

We introduce a new approach for the correlation energy of one- and two-valley two-dimensional electron gas (2DEG) systems. Our approach is based on an interpolation between two limits, a random phase approximation at high densities and a classical approach at low densities which gives excellent agreement with available Quantum Monte Carlo (QMC) calculations. The two-valley 2DEG model is introduced to describe the electron correlations in monolayer transition metal dichalcogenides (TMDs). We study the zero-temperature transition from a Fermi liquid to a quantum Wigner crystal phase in monolayer TMDs. Consistent with QMC, we find that electrons crystallize at $r_s=31$ in one-valley 2DEG. For two valleys, we predict Wigner crystallization at $r_s=30$, implying that valley degeneracy has little effect on the critical $r_s$, in contrast to an earlier claim.

We propose a concept of chiral photonic limiters utilizing topologically protected localized midgap defect states in a photonic waveguide. The chiral symmetry alleviates the effects of structural imperfections and guarantees a high level of resonant transmission for low intensity radiation. At high intensity, the light-induced absorption can suppress the localized modes, along with the resonant transmission. In this case the entire photonic structure becomes highly reflective within a broad frequency range, thus increasing dramatically the damage threshold of the limiter. Here, we demonstrate experimentally the loss-induced reflection principle of operation which is at the heart of reflective photonic limiters using a waveguide consisting of coupled dielectric microwave resonators.

Using first-principles calculation methods, we study the possibility of realizing a quantum anomalous Hall effect in graphene from stable $3d$ atomic adsorption via a charge-compensated $n-p$ codoping scheme. As concrete examples, we show that long-range ferromagnetism can be established by codoping $3d$ transition metal and boron atoms, but only the Ni codopants can open up a global bulk gap to harbor the quantum anomalous Hall effect. Our estimated ferromagnetic Curie transition temperature can reach over 10 K for various codoping concentrations.

We report thickness-tuned electrical transport in highly anisotropic three-dimensional Dirac semimetal $\mathrm{ZrT}{\mathrm{e}}_5$ nanosheets with thickness down to 10 nm. We find that the resistivity peak temperature $T^{*}$ can be significantly tuned by the nanosheet thickness. When the thickness is reduced from 160 to 40 nm, $T^{*}$ reduces systematically from 145 to 100 K. However, with the thickness further reducing to 10 nm, $T^{*}$ shifts up to a higher temperature. From our analysis, the system transitions from a topological semimetal with two types of carriers to a single band with conventional hole carriers when the thickness is less than 40 nm. Furthermore, by tracking the thickness dependence of the carrier density, we find that the Fermi level shifts continuously downward from the conduction band to the valence band with decreasing the thickness. Our experiment reveals a thickness-tuned transition of band topology in $\mathrm{ZrT}{\mathrm{e}}_5$ nanosheets which may be helpful for the understanding of the contrast observations in this material.

The existence of polyexcitons, the $N$-body complexes of excitons for $N>2$ in three-dimensional bulk systems, has been controversial for more than 40 years since its first theoretical suggestion. We investigated the stability of fundamental excitonic complexes in diamond numerically with the stochastic variational method and an explicitly correlated Gaussian basis. The electron-hole many-body system is described by an effective mass Hamiltonian. Our model includes the effective mass anisotropy and multiple valley and band degrees of freedom. We show that the excitons, trions, biexcitons, charged biexcitons, and triexcitons are stable in diamond. Numerical calculations reproduce from 81% to 86% of the experimentally reported binding energies for neutral bound states.

We perform ab initio molecular dynamics simulations to study the femtosecond laser induced desorption of ${\mathrm{H}}_2,{\mathrm{D}}_2$, and HD from a H:D-saturated Ru(0001) surface. To this aim we have extended the ab initio molecular dynamics with electronic friction (AIMDEF) scheme to include a random force that is a function of a time-dependent electronic temperature. The latter characterizes the action of the ultrashort laser pulse according to a two temperature model. This allows us to perform multidimensional, hot-electron driven reaction dynamics and investigate the dependence of the desorption yields on the relative H:D isotope concentration on the surface. Our AIMDEF simulations show that the desorption process takes place in the presence of a heated adsorbate system that clearly influences the desorption dynamics. The heating of the adsorbate system is more (less) pronounced the larger is the concentration of the lighter (heavier) isotope. As a result, we conclude that the presence of H on the surface favors the desorption of molecules, whereas the presence of D hampers it, in agreement with previous experimental observations in which the phenomenon of “dynamical promotion” of a surface reaction had been postulated [Denzler et al., Phys. Rev. Lett. 91, 226102 (2003)].

We report a paradigm of ferrielectricity in a single-phase crystal, tris-sarcosine calcium chloride [TSCC; ${\left(\mathrm{C}{\mathrm{H}}_3\mathrm{NHC}{\mathrm{H}}_2\mathrm{COOH}\right)}_3\mathrm{CaC}{\mathrm{l}}_2]$. Ferrielectricity is well known in smectic liquid crystals but almost unknown in true crystalline solids. C. F. Pulvari [Ferrielectricity, Phys. Rev. 120, 1670 (1960)] reported it in 1960 in mixtures of ferroelectrics and antiferroelectrics (AFEs), but only at high fields. Tris-sarcosine calcium chloride exhibits a second-order displacive phase transition near $T_{\mathrm{c}}=130\mathrm{K}$ that can be lowered to a quantum critical point at 0 K via Br or I substitution and phases previously predicted to be AFE at high pressure and low temperatures. Unusually, the size of the primitive unit cell does not increase in the so-called AFE phase. We measure hysteresis loops and polarization below $T=64\mathrm{K}$ and provide clear Raman evidence for this paraelectric-ferrielectric transition.

We theoretically study the magnetoresistance (MR) of two-dimensional massless Dirac electrons as found on the surface of three-dimensional topological insulators (TIs) that are capped by a ferromagnetic insulator (FI). We calculate charge and spin transport by Kubo and Boltzmann theories, taking into account the ladder-vertex correction and the in-scattering due to normal and magnetic disorder. The induced exchange splitting is found to generate an electric conductivity that depends on the magnetization orientation, but its form is very different from both the anisotropic and the spin Hall MR. The in-plane MR vanishes identically for nonmagnetic disorder, while out-of-plane magnetizations cause a large MR ratio. On the other hand, we do find an in-plane MR and planar Hall effect in the presence of magnetic disorder aligned with the FI magnetization. Our results may help us understand recent transport measurements on $\mathrm{TI}|\mathrm{FI}$ systems.

We studied the properties of domain walls (DWs) of the Neodymium magnet, ${\mathrm{Nd}}_2{\mathrm{Fe}}_{14}\mathrm{B}$. Applying an atomistic model, in which the magnetic moments of all atoms and exchange interactions were determined by a first-principles calculation (Korringa-Kohn-Rostoker Green's function method), we performed a Monte Carlo simulation for two types of DW, i.e., moving along the $a$ axis and along the $c$ axis, which are classified into a Bloch-type wall and a Neel-type wall, respectively. We found that the shapes of the DWs of both types are described well by those derived from the continuum model used in micromagnetics. We show that the estimated DW widths are very close to the experimentally evaluated ones. Furthermore, we discovered that the width of the latter type is smaller than that of the former type. We also investigated the temperature dependence of the DW width and found that at higher temperatures it becomes larger and the magnitude of the magnetization becomes smaller, which agrees with experimental observations.

We report the evolution of the Zeeman-mediated superconducting phase diagram (PD) in ultrathin crystalline Al films. Parallel critical field measurements, down to 50 mK, were made across the superconducting tricritical point of films ranging in thickness from 7 to 30 ML. The resulting phase boundaries were compared with the quasiclassical theory of a Zeeman-mediated transition between a homogeneous BCS condensate and a spin-polarized Fermi liquid. Films thicker than $\sim 20\mathrm{ML}$ showed good agreement with theory, but thinner films exhibited an anomalous PD that cannot be reconciled within a homogeneous BCS framework.

An analytical theory is suggested for carrier recombination in organic bulk heterojunction (BHJ) structures taking into account the spatial separation of recombining species. In BHJ systems, electrons and holes are confined to two different material phases, and the nongeminate recombination can happen only at the interfaces between the phases. The carrier concentration in the vicinity of the interface can essentially differ from the average concentration in the bulk. Conditions are considered at which this difference can lead to deviations of the apparent recombination order from the value $\delta =2$ that is generally expected for bimolecular reactions. The theory is universal in the sense that it is based solely on the inhomogeneous distributions of the recombining species. It can therefore be applied not only to organic semiconductors but to any system in which recombining species are confined to different material phases and can meet only at the interfaces between these phases.

We report that synthetic antiferromagnets (SAFs) can be efficiently switched by spin-orbit torques (SOTs) and the switching scheme does not obey the usual SOT switching rule. We show that both the positive and negative spin Hall angle (SHA)-like switching can be observed in Pt/SAF structures with only positive SHA, depending on the strength of applied in-plane fields. A switching mechanism directly arising from the asymmetric domain expansion is proposed to explain the anomalous switching behaviors. Contrary to the macrospin-based switching model where the SOT switching direction is determined by the sign of SHA, this switching mechanism suggests that the SOT switching direction is dominated by the field-modulated domain wall motion and can be reversed even with the same sign of SHA. The presence of this switching mechanism is further confirmed by the domain wall motion measurements. The anomalous switching behaviors provide important insights for understanding SOT switching mechanisms and also offer novel features for applications.

Spin current generated by the spin Hall effect in a heavy metal that would diffuse up and down to adjacent ferromagnetic layers and exert torque on their magnetization is called spin-orbit torque. Antiferromagnetically coupled trilayers, namely, the so-called synthetic antiferromagnets usually are employed to serve as the pinned layer of spintronic devices based on spin valves and magnetic tunnel junctions to reduce the stray field and/or increase the pinning field. Here we investigate the spin-orbit torque in a MgO/CoFeB/Ta/CoFeB/MgO perpendicularly magnetized multilayer with interlayer antiferromagnetic coupling. It is found that the magnetization of two CoFeB layers can be switched between two antiparallel states simultaneously. This observation is replicated by the theoretical calculations by solving the Stoner-Wohlfarth model and the Landau-Lifshitz-Gilbert equation. Our findings combine spin-orbit torque and interlayer coupling, which might advance the magnetic memories with a low stray field and low power consumption.

In magnets with noncollinear spin configuration the expectation value of the conventionally defined spin current operator contains a contribution which renormalizes an external magnetic field and hence affects only the precessional motion of the spin polarization. This term, which has been named angular spin current by Sun and Xie [Phys. Rev. B 72, 245305 (2005)], does not describe the translational motion of magnetic moments. We give a prescription for how to separate these two types of spin transport and show that the translational movement of the spin is always polarized along the direction of the local magnetization. We also show that at vanishing temperature the classical magnetic order parameter in magnetic insulators cannot carry a translational spin current and elucidate how this affects the interpretation of spin supercurrents.

We combined magneto-infrared spectroscopy and first-principles calculations to unravel the role of spin-phonon coupling in the vicinity of the magnetic field-driven phase transitions in two chemically similar $S=1/2$ Heisenberg antiferromagnets, ${\mathrm{CuF}}_2\left({\mathrm{H}}_2\mathrm{O}\right){}_2$(3-Clpy) and $[\mathrm{Cu}{\left(\mathrm{pyz}\right)}_2({\mathrm{HF}}_2\left)\right]{\mathrm{PF}}_6$. This comparison resolves questions about the conditions under which the lattice participates in magnetic-field-driven transitions and, at the same time, provides a way to predict how the lattice is likely to support microscopic spin rearrangements.

We investigate the effects related to vortex-core deformations when vortices approach each other. As a result of these vortex-core deformations, the vortex-vortex interaction effectively acquires an attractive component leading to a variety of vortex patterns typical for systems with nonmonotonic repulsive-attractive interaction, such as stripes and labyrinths. The core deformations are anisotropic and can induce frustration in the vortex-vortex interaction. In turn, this frustration has an impact on the resulting vortex patterns, which are analyzed in the presence of additional random pinning, as a function of the pinning strength. This analysis can be applicable to vortices in multiband superconductors or to vortices in Bose-Einstein condensates.