Sodium is a simple metal at ambient conditions, while it transits to an electride phase at pressures above ∼160 GPa along the room-temperature isotherm. We explore the thermal effects on the electronic properties of the $\mathit{hP}4$ phase of sodium along the $\rho =5.872\mathrm{g}/\mathrm{c}{\mathrm{m}}^3$ isochore. We quantitatively classify this phase as an insulator based on the criterion of nearsightedness of the one-particle density matrix. Ab initio calculations suggest that the band gap of this insulator decreases with increasing temperature along the isochore, primarily because of ionic distortions, culminating in an insulator-to-metal transition upon melting at $T_m\approx 2100\mathrm{K}$. This transition is accompanied by residual electronic localization (in $d$ orbitals) in the form of dynamic electron bubbles and a change in hybridization from p-d to s-p upon melting. This transition is explored by tracking the electronic and electro-optical properties along the isochore under consideration.

We describe the macroscopic behavior of superfluid ${}^3\mathrm{He}$ in the polar and the distorted $A$ and $B$ phases in an anisotropic aerogel. It turns out that the distorted $A$ phase shares many features with superfluid ${}^3\mathrm{He}-A$. Its order parameter contains the polar phase as well as the $A$ phase as a smooth limiting case. The former is in agreement with the fact that the phase transition polar to distorted $A$ is observed to be of second order. Due to the different structure of the order parameter, the unit vector $\widehat{l}$, which is characteristic of the superfluid ${}^3\mathrm{He}-A$ and the distorted $A$ phase, is not present in the polar phase. As new macroscopic variables compared to superfluid ${}^3\mathrm{He}$ we have for the superfluid distorted $A$ phase in an aerogel the strain field associated with the aerogel network. In addition, we have in superfluid ${}^3\mathrm{He}$ relative rotations between the anisotropic gel network and the $\widehat{l}$ vector as a macroscopic variable. As a result of these additional macroscopic variables we find new static and dynamic cross-coupling terms, which come as reversible (zero entropy production) as well as as irreversible contributions. The polar phase in an anisotropic aerogel is anisotropic in its elastic properties. We have as additional macroscopic variables relative rotations similar to those of a nematic liquid crystalline elastomer, since the preferred direction in the polar phase is even under time reversal and not odd as for the distorted $A$ phase. These additional features lead to static and dissipative cross-coupling terms, which cannot exist for a polar phase in an isotropic aerogel. For the distorted $B$ phase we find for the orbital part a macroscopic dynamic behavior which shares some features with that of the polar phase. In contrast to the polar phase, however, the distorted $B$ phase does not show relative rotations, since there is only one preferred direction. The preferred direction in the distorted $B$ phase leads to a preferred direction in spin space as well. As a consequence the spin waves become anisotropic and longitudinal and transverse spin waves become coupled, even in the absence of a magnetic field. We also discuss briefly the coupling to the spin degrees of freedom in the polar and the distorted $A$ phase.

We measure magnetic field dependence of the Hall angle in a metallic ferromagnetic nanomagnet where the adopted mechanisms of Hall effect predict linear plus a constant dependence on the external field originating from the ordinary and anomalous Hall effects, respectively. We suggest that the experimentally observed deviations from this dependence are caused by the inverse spin Hall effect (ISHE) and develop a phenomenological theory, which predicts a unique nonlinear dependence of the ISHE contribution on the external magnetic field. Perfect agreement between theory and experiment supports the considerable role of the ISHE in Hall transport in a ferromagnetic metal.

Cycling the $\alpha \leftrightarrow \epsilon$ transformation in polycrystalline Fe is investigated using in situ x-ray diffraction under quasihydrostatic conditions. The forward $\alpha \to \epsilon$ transformation starts at $14\pm 1$ GPa and completes at $18\pm 1$ GPa while the reverse $\epsilon \to \alpha$ transformation starts at $10.5\pm 0.5$ GPa and completes at $6\pm 1$ GPa. The anomalous evolution of $c/a$ ratios of $\epsilon$-Fe measured at the onset of the $\alpha \to \epsilon$ transition in earlier studies is not observed. Microstructural features are consistent with a Burgers path for the transformation. The forward $\alpha \to \epsilon$ transformation is sensitive to the average and intergranular stresses, inducing a strong variant selection with the $c$ axes of the new $\epsilon$ phase preferentially aligned perpendicular to compression and concentrated into one single orientation. Partial texture memory is observed as the sample returns to the $\alpha$ phase but, as transformation cycles go on, irreversible changes occur in the microstructures that are later fully induced by the cycles of phase transformations.

The cavity-mediated spin current between two ferrite samples has been reported by Bai et al., [Phys. Rev. Lett. 118, 217201 (2017)]. This experiment was done in the linear regime of the interaction in the presence of external drive. In the current paper, we develop a theory for the spin current in the nonlinear domain where the external drive is strong so that one needs to include the Kerr nonlinearity of the ferrite materials. In this manner, the nonlinear polaritons are created and one can reach both bistable and multistable behavior of the spin current. The system is driven into a far-from-equilibrium steady state that is determined by the details of the driving field and various interactions. We present a variety of steady-state results for the spin current. A spectroscopic detection of the nonlinear spin current is developed, revealing the key properties of the nonlinear polaritons. The transmission of a weak probe is used to obtain quantitative information on the multistable behavior of the spin current. The results and methods that we present are quite generic and can be used in many other contexts where cavities are used to transfer information from one system to another, e.g., two different molecular systems.

We study the behavior of the entropy of the pseudogap Bose-Fermi Kondo model within a dynamical large-$N$ limit, where $N$ is related to the symmetry group of the model. This model is a general quantum impurity model that describes a localized level coupled to a fermionic bath having a density of states that vanishes in a power-law fashion near the Fermi energy and to a bosonic bath possessing a power-law spectral density below a cutoff energy. As a function of the couplings to the baths, various quantum phase transitions can occur. We study how the impurity entropy changes across these zero-temperature transitions and compare our results with predictions based on the $g$-theorem. This is accomplished by an analysis of the leading and subleading scaling behaviors. Our analysis shows that the $g$-theorem does not apply to the pseudogap Bose-Fermi Kondo model at the large-$N$ level. This inapplicability originates from an anomalous contribution to the scaling function in the hydrodynamic regime where $k_{B}T>\hslash \omega$ which is absent in the quantum coherent regime, i.e., for $k_{B}T<\hslash \omega$. We also compare our results with those obtained for the Sachdev-Ye-Kitaev model.

Nonlinearity induced topological properties in nonlinear lattice systems are studied in both momentum space and real space. Experimentally realizable through the Kerr effect on photonic waveguide systems, our working model depicts onsite nonlinearity added to the Su-Schrieffer-Heeger (SSH) model plus a chiral-symmetry-breaking term. Under the periodic boundary condition, two of the nonlinear energy bands approach the energy bands of the chiral-symmetric SSH model as nonlinearity strength increases. Further, we account for a correction to the Zak phase and obtain a general expression for nonlinear Zak phases. For sufficiently strong nonlinearity, the sum of all nonlinear Zak phases (not the sum of all conventional Zak phases) is found to be quantized. In real space, it is discovered that there is a strong interplay between nonlinear solitons and the topologically protected edge states of the associated chiral-symmetric linear system. Nonlinearity can recover the degeneracy between two edge soliton states, albeit a chiral-symmetry-breaking term. We also reveal the topological origin of in-gap solitons even when the associated linear system is in the topological trivial regime. These momentum-space and real-space results have clearly demonstrated new topological features induced by nonlinearity, indicating that topological physics in nonlinear lattice systems is far richer than previously thought.

The antiferromagnetic topological insulator (AFM TI) and two-dimensional (2D) magnet constitute two of the most active fields for hosting prominent physical phenomena and significant advances in novel spintronic devices. However, the 2D AFM TI is scarce. Here, we propose a tight-binding model of the 2D AFM square lattice to demonstrate the emergence of the 2D AFM TI protected by a nonsymmorphic symmetry that is a combination of the twofold rotation symmetry and half-lattice translation. Moreover, based on first-principles calculations, we investigate in total 57 tetragonal antiferromagnets in the crystal structure of outstanding AFM CuMnAs to realize our proposal, and nine experimentally feasible candidates are predicted to be intrinsic 2D nonsymmorphic AFM TIs characterized by the nonzero topological invariant ${\mathbb{Z}}_2=1$ and gapless edge states. The presented results not only greatly extend the scope of magnetic topological states in two dimensions but also put forward potential applications in AFM spintronics.

Quantized Hall conductance without an external magnetic field, known as the quantum anomalous Hall effect (QAHE), may have important applications in dissipationless spintronics, yet to date, it has only been realized in magnetically doped topological insulators and at very low temperatures. Here we design a physically realistic system for realizing QAHE by expanding the recently discovered two-dimensional ferromagnetic insulators as a new class of candidate materials. Based on first-principles calculations, we predict that a ${\mathrm{CrMnI}}_6$ monolayer is energetically stable and can be readily exfoliated. This system is further shown to be a ferromagnetic insulator, with a transition temperature of $\sim 87\mathrm{K}$, higher than that of ${\mathrm{CrI}}_3$. Most strikingly, such a monolayer is characterized as an intrinsic QAHE system with a high Chern number of $C$ = 2, and the underlying mechanism for the nontrivial topology is attributed to the two inequivalent subset sites of the Cr and Mn atoms. The present study thus provides an ideal platform for realizing high-temperature QAHE beyond the prevailing materials class of magnetically doped topological insulators.

We report on a theoretical study of the electromagnetic response of an infinite two-dimensional electron system on a dielectric substrate. We find that electromagnetic radiation incident normal to the substrate surface excites two-dimensional electromagnetic plasma waves with a superluminal character of the spectrum. It is established that there is a strong hybridization between the plasma and the Fabry-Pérot light modes. In the presence of a finite magnetic field, the plasma resonance is shown to split into two modes, each corresponding to a particular direction of circular polarization. We discuss the implications of the results for terahertz detection technologies.

The coupling between electrons and phonons in solids plays a central role in describing many phenomena, including superconductivity and thermoelectric transport. Calculations of this coupling are exceedingly demanding as they necessitate integrations over both the electron and phonon momenta, both of which span the Brillouin zone of the crystal, independently. We present here an ab initio method for efficiently calculating electron-phonon mediated transport properties by dramatically accelerating the computation of the double integrals with a dual interpolation technique that combines maximally localized Wannier functions with symmetry-adapted plane waves. The performance gain in relation to the current state-of-the-art Wannier-Fourier interpolation is approximately $2n_s\times M$, where $n_s$ is the number of crystal symmetry operations and $M$, a number in the range $5–60$, governs the expansion in star functions. We demonstrate with several examples how our method performs some ab initio calculations involving electron-phonon interactions.

Strongly interacting topologically ordered many-body systems consisting of fermions or bosons can host exotic quasiparticles with anyonic statistics. This raises the question whether many-body systems of anyons can also form anyonic quasiparticles. Here we show that one can, indeed, construct many-anyon wave functions with anyonic quasiparticles. The braiding statistics of the emergent anyons are different from those of the original anyons. We investigate hole-type and particle-type anyonic quasiparticles in Abelian systems on a two-dimensional lattice and compute the density profiles and braiding properties of the emergent anyons by employing Monte Carlo simulations.

Hydrogenation of nitrogen (N) doped GaAs allows for reversible tuning of the band gap and the creation of site controlled quantum dots through the manipulation of $\mathrm{N}\text{−}n\mathrm{H}$ complexes, $\mathrm{N}\text{−}n\mathrm{H}$ complexes, wherein a nitrogen atom is surrounded by $n$ hydrogen (H) atoms. Here we employ cross-sectional scanning tunneling microscopy (X-STM) to study these complexes in the GaAs (110) surface at the atomic scale. In addition to that we performed density functional theory (DFT) calculations to determine the atomic properties of the $\mathrm{N}\text{−}n\mathrm{H}$ complexes. We argue that at or near the (110) GaAs surface two H atoms from $\mathrm{N}\text{−}n\mathrm{H}$ complexes dissociate as an ${\mathrm{H}}_2$ molecule. We observe multiple features related to the hydrogenation process, of which a subset is classified as N-1H complexes. These N-1H related features show an apparent reduction of the local density of states (LDOS), characteristic to N atoms in the GaAs (110) surface with an additional apparent localized enhancement of the LDOS located in one of three crystal directions. $\mathrm{N}\text{−}n\mathrm{H}$ features can be manipulated with the STM tip. Showing in one case a switching behavior between two mirror-symmetric states and in another case a removal of the localized enhancement of the LDOS. The disappearance of the bright contrast is most likely a signature of the removal of an H atom from the $\mathrm{N}\text{−}n\mathrm{H}$ complex.

Nonreciprocal effects in nanoelectronic devices offer unique possibilities for manipulating electron transport and engineering quantum electronic circuits for information processing purposes. However, a lack of rigorous theoretical tools is hindering this development. Here, we provide a general input-output description of nonreciprocal transport in solid-state quantum dot architectures, based on quantum optomechanical analogs. In particular, we break reciprocity between coherently coupled quantum dots by dissipation-engineering in which these (so-called) primary dots are mutually coupled to auxiliary, damped quantum dots. We illustrate the general framework in two representative multiterminal noninteracting models, which can be used as building blocks for larger circuits. Importantly, the identified optimal conditions for nonreciprocal behavior hold even in the presence of additional dissipative effects that result from local electron-phonon couplings. Besides the analysis of the scattering matrix, we show that a nonreciprocal coupling induces unidirectional electron flow in the resonant transport regime. Altogether, our analysis provides the formalism and working principles towards the realization of nonreciprocal nanoelectronic devices.

Magnetization, magnetic torque, neutron diffraction and NMR experiments are used to map out the $H-T$ phase diagram of the prototypical quasi-two-dimensional ferro-antiferromagnet ${\mathrm{Pb}}_2\mathrm{VO}{\left({\mathrm{PO}}_4\right)}_2$ in magnetic fields up to 27 T. When the field is applied perpendicularly to the axis of magnetic anisotropy, a new magnetic state emerges through a discontinuous transition and persists in a narrow field range just below saturation. The measured NMR spectra suggest a complex and possibly incommensurate magnetic order in that regime.

Reading the magnetic state of antiferromagnetic (AFM) thin films is key for AFM spintronic devices. We investigate the underlying physics behind the spin Hall magnetoresistance (SMR) of bilayers of platinum and insulating AFM hematite $\left(\alpha -{\mathrm{Fe}}_2{\mathrm{O}}_3\right)$ and find an SMR efficiency of up to 0.1%, comparable to ferromagnetic-based structures. To understand the observed complex SMR field dependence, we analyze the effect of misalignments of the magnetic axis that arise during growth of thin films, by electrical measurements and direct magnetic imaging, and find that a small deviation can result in significant signatures in the SMR response. This highlights the care that must be taken when interpreting SMR measurements on AFM spin textures.

By including a material-relevant off-diagonal interaction called the $\mathrm{\Gamma }$ term into the Kitaev model and introducing spatial anisotropy in the interaction strength on the honeycomb lattice, we obtain a series of nodal $Z_2$ quantum spin liquids (QSLs) from the parton approach. These QSLs share the same projective symmetry group and are characterized by certain numbers of symmetry-protected Majorana cones in their low-energy excitation spectrum. We illustrate that the physical properties of the QSLs are dependent on the information of the cones. Using the $\mathit{k}·\mathit{p}$ method, we analyze the chirality of every cone with respect to mass generating perturbations. Especially, for an applied external magnetic field, we provide the maximum-mass field orientation for every cone. Thus, for arbitrarily oriented weak magnetic fields, we can immediately read out the Chern number of the system and the properties of the resultant chiral spin liquids. The new gapless QSLs predicted in our phase diagrams are promising to be realized experimentally by exerting uniaxial pressure to tune the anisotropy of the interaction strength. We further show that all these QSLs can be distinguished by measurable quantities. Based on the study of these QSL phases, we conclude that a complete classification of nodal QSLs with certain symmetry should include not only the projective symmetry groups, but also the information of the cones, i.e., their total number, their chiralities, and the way in which they are symmetry related.

The seminal theoretical works of Berezinskii, Kosterlitz, and Thouless presented a paradigm for phase transitions in condensed matter that are driven by topological excitations. These transitions have been extensively studied in the context of two-dimensional XY models—coupled compasses—and have generated interest in the context of quantum simulation. Here, we use a circuit quantum-electrodynamics architecture to study the critical behavior of engineered XY models through their dynamical response. In particular, we examine not only the unfrustrated case but also the fully frustrated case which leads to enhanced degeneracy associated with the spin rotational $\left[\mathrm{U}\right(1\left)\right]$ and discrete chiral ($Z_2$) symmetries. The nature of the transition in the frustrated case has posed a challenge for theoretical studies while direct experimental probes remain elusive. Here we identify the transition temperatures for both the unfrustrated and fully frustrated XY models by probing a Josephson junction array close to equilibrium using weak microwave excitations and measuring the temperature dependence of the effective damping obtained from the complex reflection coefficient. We argue that our probing technique is primarily sensitive to the dynamics of the $\mathrm{U}\left(1\right)$ part.