The correspondence between curved space-times and inhomogeneous dielectrics has been recently explored as a powerful tool to understand and manipulate novel electromagnetic behaviors in complex media. Here, we present a theoretical investigation on the optics of radially anisotropic material from a geometric viewpoint. Within the framework of transformation optics, we show that the optical medium with radial anisotropy is equivalent to a disclination geometry, which is a line topological defect carrying singular curvature. By introducing a geometric parameter characterizing the global topology of the disclination space, we systematically analyze the effective geometry and the topological charge associated with two typical radial anisotropies consisting of concentric multilayers or symmetric slices in both elliptical and hyperbolical regions. It is shown that elliptical and hyperbolic radial anisotropies give rise to optical Riemannian and pseudo-Riemannian geometries, respectively. Moreover, we investigate the wave optics as well as the semiclassical ray dynamics of light in the metamaterials at optical wavelengths from the perspective of coordinate transformation. It is found that the singularity acts on the light with an attractive or repulsive inverse cube force, depending on the topological charge. Our theory provides a simple and unified framework for light in optical media of various radial anisotropies and may shed new light on the dynamics of classical and quantum waves in topological nontrivial space.

We report the existence of a two-dimensional electron system (2DES) at the $\left(001\right)$ surface of ${\mathrm{CaTiO}}_3$. Using angle-resolved photoemission spectroscopy, we find a hybridization between the $d_{xz}$ and $d_{yz}$ orbitals, not observed in the 2DESs at the surfaces of other $A{\mathrm{TiO}}_3$ perovskites, e.g., ${\mathrm{SrTiO}}_3$ or ${\mathrm{BaTiO}}_3$. Based on a comparison of the 2DES properties in these three materials, we show how the electronic structure of the 2DES (bandwidth, orbital energy order, and electron density) is coupled to different typical lattice distortions in perovskites. The orbital hybridization in orthorhombic ${\mathrm{CaTiO}}_3$ results from the rotation of the oxygen octahedra, which can also occur at the interface of oxide heterostructures to compensate strain. More generally, the control of the orbital energy order in 2DES by choosing different $A$-site cations in perovskites offers a gateway toward 2DESs in oxide heterostructures beyond ${\mathrm{SrTiO}}_3$.

We propose a simple scheme to construct a model whose Fermi surface is comprised of crossing-line nodes. The Hamiltonian consists of a normal hopping term and an additional term which is odd under the mirror reflection. The line nodes appear along the mirror-invariant planes, where each line node carries the quantized Berry magnetic flux. We explicitly construct a model with the $N$-fold rotational symmetry, where the $2N$ line nodes merge at the north and south poles. When we apply photoirradiation along the $k_z$ axis, there emerge point nodes carrying the monopole charge $\pm N$ at these poles, while all the line nodes disappear. In this model, photoirradiation induces a topological phase transition from a crossing-line nodal semimetal to a multiple-Weyl semimetal, where the surface state turns from a drumhead state into a Fermi-arc state.

Weyl points with monopole charge $\pm 1$ have been extensively studied; however, real materials of multi-Weyl points, whose monopole charges are higher than 1, have yet to be found. In this Rapid Communication, we show that nodal-line semimetals with nontrivial line connectivity provide natural platforms for realizing Floquet multi-Weyl points. In particular, we show that driving crossing nodal lines by circularly polarized light generates double-Weyl points. Furthermore, we show that monopole combination and annihilation can be observed in crossing-nodal-line semimetals and nodal-chain semimetals. These proposals can be experimentally verified in pump-probe angle-resolved photoemission spectroscopy.

High-$T_{\mathrm{c}}$ pairing mechanism absent of hole pockets in heavily electron-doped FeSe is one of the key unsolved problems in Fe-based superconductors. Here, this problem is attacked by focusing on the higher-order many-body effects neglected in conventional Migdal-Eliashberg formalism. We uncover two significant many-body effects for high-$T_{\mathrm{c}}$ superconductivity. (i) Due to the “vertex correction”, the dressed multiorbital Coulomb interaction acquires prominent orbital dependence for low-energy electrons. The dressed Coulomb interaction not only induces the orbital fluctuations, but also magnifies the electron-boson coupling constant. Therefore, moderate orbital fluctuations give strong attractive pairing interaction. (ii) The “multi-fluctuation-exchange pairing process” causes large interpocket attractive force, which is as important as the usual single-fluctuation-exchange process. Due to these two significant effects dropped in the Migdal-Eliashberg formalism, the anisotropic $s_{++}$-wave state in heavily electron-doped FeSe is satisfactorily explained. The proposed “inter-electron-pocket pairing mechanism” will enlarge $T_{\mathrm{c}}$ in other Fe-based superconductors.

Among the most interesting predictions in two-dimensional materials with a Dirac cone is the existence of the zeroth Landau level (LL), equally filled by electrons and holes with opposite chirality. The gapless edge states with helical spin structure emerge from Zeeman splitting at the LL filling factor $\nu =0$ gapped quantum Hall (QH) state. We present observations of a giant, nonlocal, four-terminal transport in zero-gap HgTe quantum wells at the $\nu =0$ QH state. Our experiment clearly demonstrates the existence of the robust helical edge state in a system with single-valley Dirac cone materials.

In the presence of strong magnetic fields the electronic band structure of graphene drastically changes. The Dirac cone collapses into discrete nonequidistant Landau levels, which can be externally tuned by changing the magnetic field. In contrast to conventional materials, specific Landau levels are selectively addressable using circularly polarized light. Exploiting these unique properties, we propose the design of a tunable laser operating in the technologically promising terahertz spectral range. To uncover the many-particle physics behind the emission of light, we perform a fully quantum mechanical investigation of the nonequilibrium dynamics of electrons, phonons, and photons in optically pumped Landau-quantized graphene embedded in a high-quality optical cavity. The microscopic insights gained allow us to predict optimal experimental conditions to realize a technologically promising terahertz laser.

The effect of disorder on lattice vibrational modes has been a topic of interest for several decades. In this work, we employ a Green's function based approach, namely, the dynamical cluster approximation (DCA), to investigate phonons in mass-disordered systems. Detailed benchmarks with previous exact calculations are used to validate the method in a wide parameter space. An extension of the method, namely, the typical medium DCA (TMDCA), is used to study Anderson localization of phonons in three dimensions. We show that, for binary isotopic disorder, lighter impurities induce localized modes beyond the bandwidth of the host system, while heavier impurities lead to a partial localization of the low-frequency acoustic modes. For a uniform (box) distribution of masses, the physical spectrum is shown to develop long tails comprising mostly localized modes. The mobility edge separating extended and localized modes, obtained through the TMDCA, agrees well with results from the transfer matrix method. A reentrance behavior of the mobility edge with increasing disorder is found that is similar to, but somewhat more pronounced than, the behavior in disordered electronic systems. Our work establishes a computational approach, which recovers the thermodynamic limit, is versatile and computationally inexpensive, to investigate lattice vibrations in disordered lattice systems.

A detailed neutron diffraction study on $\mathrm{NdMn}{\mathrm{O}}_3$ infers that the low temperature transition at 15 K is due to the ordering of Nd sublattice moment with a $(0,-F_y,0)$ type spin arrangement. Interestingly, the ordering of the Nd sublattice drives a reorientation (by ${180}^{\circ }$) of the net ferromagnetic moment of the Mn sublattice along the $b$ axis. Such a Mn spin reorientation from $(A_x,F_y,0)$ (with an antiferromagnetic ordering temperature of 73 K) to $(A_x,-F_y,0)$ at 15 K, explains the magnetization reversal phenomenon present in this perovskite compound at 15 K. Moreover at 15 K, significant crystallographic structural distortions in terms of temperature variations of lattice parameters and bond angles are found. A sign change in the temperature variation of magnetic entropy is also found at 15 K. The present study signifies the role of rare-earth (Nd) moment ordering in tuning various physical properties, such as magnetocaloric and magnetoelastic of the larger size $(>0.912\AA ) R$ ion based $R\mathrm{Mn}{\mathrm{O}}_3$ compounds.

Lattice and magnetic dynamics of ${\mathrm{NiWO}}_4$ single crystals were studied with the use of polarized Raman spectroscopy in a wide temperature range of 10–300 K including the antiferromagnetic ordering temperature $T_N=62$ K. Static magnetic measurements were used for characterizing the single crystals. All Raman-active phonons predicted by the group theory were observed and characterized. Magnetic symmetry analysis was used to determine possible magnetic space groups for ${\mathrm{NiWO}}_4$ which can be also applied to any other isostructural crystal with the same magnetic propagation vector $\mathbf{k}=(1/2,0,0)$. Although the magnetic structure of ${\mathrm{NiWO}}_4$ is relatively simple, a rich set of narrow and broad magnetic excitations with different polarization properties and temperature behavior in the very broad frequency range of 10–200 ${\mathrm{cm}}^{-1}$ was observed, with some modes surviving at temperatures much higher than $T_N$ up to 220 K. Part of the magnetic excitations were identified as acoustic and optical spin-wave branches which allowed us to construct exchange structure and estimate exchange and anisotropy constants with the use of linear spin-wave theory.

We theoretically study the dynamics of skyrmion crystals in electrically insulating chiral magnets subjected to current-induced spin torques by adjacent metallic layers. We develop an elasticity theory that accounts for the gyrotropic force engendered by the nontrivial topology of the spin texture, tensions at the boundaries due to the exchange of linear and spin angular momentum with the metallic reservoirs, and dissipation in the bulk of the film. A steady translation of the skyrmion crystal is triggered by the current-induced tensions and subsequently sustained by dissipative forces, generating an electromotive force on itinerant spins in the metals. This phenomenon should be revealed as a negative drag in an open two-terminal geometry, or equivalently, as a positive magnetoresistance when the terminals are connected in parallel. We propose nonlocal transport measurements with these salient features as a tool to characterize the phase diagram of insulating chiral magnets.

We model changes in the conductivity of flexible composite films stressed by bending. By treating stress as a perturbation of the effective medium conductivity, we obtain an expression of the piezoresistance as a function of four material parameters. The model correctly predicts resistance spikes and their recovery under the action of viscoelastic forces, in good agreement with experimental observations over stress cycles. The theory may be used to design composite materials for high-sensitivity touch sensors.

The multilevel system ${}^{55}\mathrm{Mn}^{2+}$ is used to generate two pseudoharmonic level systems, as representations of the same electronic sextuplet at different nuclear spin projections. The systems are coupled using a forbidden nuclear transition induced by the crystalline anisotropy. We demonstrate Rabi oscillations between the two representations in conditions similar to two coupled pseudoharmonic quantum oscillators. Rabi oscillations are performed at a detuned pumping frequency which matches the energy difference between electronuclear states of different oscillators. We measure a coupling stronger than the decoherence rate to indicate the possibility of fast information exchange between the systems.

We compare theoretical results for electron spin resonance (ESR) properties of the Heisenberg-Ising Hamiltonian with ESR experiments on the quasi-one-dimensional magnet $\mathrm{Cu}{\left(\mathrm{py}\right)}_2{\mathrm{Br}}_2$ (CPB). Our measurements were performed over a wide frequency and temperature range giving insight into the spin dynamics, spin structure, and magnetic anisotropy of this compound. By analyzing the angular dependence of ESR parameters (resonance shift and linewidth) at room temperature, we show that the two weakly coupled inequivalent spin-chain types inside the compound are well described by Heisenberg-Ising chains with their magnetic anisotropy axes perpendicular to the chain direction and almost perpendicular to each other. We further determine the full $g$ tensor from these data. In addition, the angular dependence of the linewidth at high temperatures gives us access to the exponent of the algebraic decay of a dynamical correlation function of the isotropic Heisenberg chain. From the temperature dependence of static susceptibilities, we extract the strength of the exchange coupling ($J/k_B=52.0\text{K}$) and the anisotropy parameter ($\delta \approx -0.02$) of the model Hamiltonian. An independent compatible value of $\delta$ is obtained by comparing the exact prediction for the resonance shift at low temperatures with high-frequency ESR data recorded at $4\text{K}$. The spin structure in the ordered state implied by the two (almost) perpendicular anisotropy axes is in accordance with the propagation vector determined from neutron scattering experiments. In addition to undoped samples, we study the impact of partial substitution of Br by Cl ions on spin dynamics. From the dependence of the ESR linewidth on the doping level, we infer an effective decoupling of the anisotropic component $J\delta$ from the isotropic exchange $J$ in these systems.

We theoretically investigate the spin-wave (magnon) excitations in a classical antiferromagnetic spin chain with easy-axis anisotropy. We obtain a Dirac-like equation by linearizing the Landau-Lifshitz-Gilbert equation in this antiferromagnetic system, in contrast to the ferromagnetic system in which a Schrödinger-type equation is derived. The Hamiltonian operator in the Dirac-like equation is a pseudo-Hermitian. We compute and demonstrate relativistic Zitterbewegung (trembling motion) in the antiferromagnetic spin chain by measuring the expectation values of the wave-packet position.

We have studied the proximity effect in thin-film heterostructures composed of the superconducting Pb (S) and of the ferromagnetic Heusler alloy ${\mathrm{Co}}_2{\mathrm{Cr}}_{1-x}{\mathrm{Fe}}_x\mathrm{Al}$ (F) with different degrees of the spin polarization of the conduction band. A thin Cu interlayer is inserted in between the S and F layers to ensure the stability of the S/F interface. The analysis of the dependences of the superconducting transition temperatures in such heterostructures on the thickness of the Pb or of the Heusler alloy using the theory by Fominov et al. [Ya. V. Fominov, N. M. Chtchelkatchev, and A. A. Golubov, Phys. Rev. B 66, 014507 (2002)] shows that, with increasing the exchange splitting of the conduction band of a ferromagnet and, respectively, of the degree of the spin polarization, the probability of transmission of the superconducting Cooper pairs through the S/F interface decreases. We conclude that the spin imbalance plays a key role in the processes taking place at the interface between a superconductor and a ferromagnet with spin-polarized conduction electrons.

How to make the metallicity compatible with a polar structure for forming a “polar metal” has been an interesting and important topic since the polar structure was discovered in metallic $\mathrm{LiOs}{\mathrm{O}}_3$. Here, we present robust polar instability under electrostatic doping in tetragonal $\mathrm{SnTi}{\mathrm{O}}_3$ based on our first-principles calculations. The mechanism for polar distortion surviving free carriers is investigated from the “weak-coupling” perspective. The contributions of different interactions between two polar modes in a “ferroelectric” phase transition are also studied. We found that the short-range interaction contributes the most for lowering the total energy during the ferroelectricphase transition. Moreover, the dipole-dipole interaction between two polar modes of oxygen atoms provides the largest energy gain among local interactions, which cannot be screened out by doping. We propose that the promising candidates for noncentrosymmetric metals obtained by doping are ferroelectrics with bonding states responsible for polar distortions away from the Fermi level. We believe that this mechanism sheds light on a method to obtain noncentrosymmetric metals from a large amount of ferroelectric perovskite oxides by doping.

The use of ab initio methods for accurate simulations of electronic, phononic, and electron-phonon properties of molecular materials such as organic crystals is a challenge that is often tackled stepwise based on molecular properties calculated in gas phase and perturbatively treated parameters relevant for solid phases. In contrast, in this work we report a full first-principles description of such properties for the prototypical rubrene crystals. More specifically, we determine a Holstein-Peierls–type Hamiltonian for rubrene, including local and nonlocal electron-phonon couplings. Thereby, a recipe for circumventing the issue of numerical inaccuracies with low-frequency phonons is presented. In addition, we study the phenyl group motion with a molecular dynamics approach.