Incorporating Fe atoms into the lattice is shown to significantly alter electronic and magnetic properties of $\mathrm{Ti}{\mathrm{O}}_2$ nanotubes synthesized by electrochemical anodization of Ti-Fe alloy sheets. The effects of Fe incorporation on the nanotube morphology, crystallinity, crystal structure, magnetic behavior and electronic structure were investigated with crystallographic and magnetic probes, including synchrotron-based spectroscopy. Results indicate that the iron cations predominately adopt the $\mathrm{F}{\mathrm{e}}^{3+}$ configuration, leading to a large increase of the electronic density of states at the Fermi energy. This increase is anticipated to provide enhanced catalytic action, for instance, in the degradation of water and of air pollutants. These results provide insight for tailoring the functionality of these nanostructures for energy-related applications.

The propagation of light in Weyl semimetal films is analyzed. The magnetic family of these materials is known by anomalous Hall effect, which, being enhanced by the large Berry curvature, allows one to create strong gyrotropic and nonreciprocity effects without external magnetic field. The existence of nonreciprocal waveguide electromagnetic modes in ferromagnetic Weyl semimetal films in the Voigt configuration is predicted. Thanks to the strong dielectric response caused by the gapless Weyl spectrum and the large Berry curvature, ferromagnetic Weyl semimetals combine the best waveguide properties of magnetic dielectrics or semiconductors with strong anomalous Hall effect in ferromagnets. The magnitude of the nonreciprocity depends both on the internal Weyl semimetal properties, the separation of Weyl nodes, and the external factor, the optical contrast between the media surrounding the film. By tuning the Fermi level in Weyl semimetals, one can vary the operation frequencies of the waveguide modes in THz and mid-IR ranges. Our findings pave the way to the design of compact, tunable, and effective nonreciprocal optical elements.

Multiple experiments have observed a sharp transition in the band structure of ${\mathrm{LaAlO}}_3/{\mathrm{SrTiO}}_3$ (001) interfaces as a function of applied gate voltage. This Lifshitz transition, between a single occupied band at low electron density and multiple occupied bands at high density, is remarkable for its abruptness. In this work, we propose a mechanism by which such a transition might happen. We show via numerical modeling that the simultaneous coupling of the dielectric polarization to the interfacial strain (“electrostrictive coupling”) and strain gradient (“flexoelectric coupling”) generates a thin polarized layer whose direction reverses at a critical density. The Lifshitz transition occurs concomitantly with the polarization reversal and is first-order at $T=0$. A secondary Lifshitz transition, in which electrons spread out into semiclassical tails, occurs at a higher density.

Analogs of the elementary particles, Dirac fermions in condensed matter, have received extensive attention for both scientific interest and device applications. In this Rapid Communication, we study the Dirac semimetals (DSMs) in the magnetic space groups (MSGs), and find a category of DSMs in centrosymmetric type-IV MSGs, where the Dirac points (DPs) are protected by inversion symmetry, nonsymmorphic time-reversal symmetry, and suitable rotation symmetry. Moreover, we propose the interlayer antiferromagnetic (AFM) material ${\mathrm{EuCd}}_2{\mathrm{As}}_2$ as a promising candidate hosting only one pair of such DPs at the Fermi level. Many exotic topological states, such as the AFM triple-point semimetal, an AFM topological insulator exhibiting the half-quantum Hall effect, can be derived from such AFM DSMs by breaking certain symmetry. Our results extend the range of DSMs, and provide a platform to study the topological phase transition and the exotic properties of AFM topological states.

We present a valence transition model for electron- and hole-doped cuprates, within which there occurs a discrete jump in ionicity ${\mathrm{Cu}}^{2+}\to {\mathrm{Cu}}^{1+}$ in both families upon doping, at or near optimal doping in the conventionally prepared electron-doped compounds and at the pseudogap phase transition in the hole-doped materials. In thin films of the ${\mathrm{T}}^{\prime }$ compounds, the valence transition has occurred already in the undoped state. The phenomenology of the valence transition is closely related to that of the neutral-to-ionic transition in mixed-stack organic charge-transfer solids. Doped cuprates have negative charge-transfer gaps, just as rare-earth nickelates and ${\mathrm{BaBiO}}_3$. The unusually high ionization energy of the closed shell ${\mathrm{Cu}}^{1+}$ ion, taken together with the doping-driven reduction in three-dimensional Madelung energy and gain in two-dimensional delocalization energy in the negative charge transfer gap state drives the transition in the cuprates. The combined effects of strong correlations and small $d-p$ electron hoppings ensure that the systems behave as effective 1/2-filled Cu band with the closed shell electronically inactive ${\mathrm{O}}^{2-}$ ions in the undoped state, and as correlated two-dimensional geometrically frustrated 1/4-filled oxygen hole band, now with electronically inactive closed-shell ${\mathrm{Cu}}^{1+}$ ions, in the doped state. The model thus gives microscopic justification for the two-fluid models suggested by many authors. The theory gives the simplest yet most comprehensive understanding of experiments in the normal states. The robust commensurate antiferromagnetism in the conventional $T^{\prime }$ crystals, the strong role of oxygen deficiency in driving superconductivity and charge carrier sign corresponding to holes at optimal doping are all manifestations of the same quantum state. In the hole-doped pseudogapped state, there occurs a biaxial commensurate period 4 charge density wave state consisting of ${\mathrm{O}}^{1-}-{\mathrm{Cu}}^{1+}-{\mathrm{O}}^{1-}$ spin singlets that coexists with broken rotational ${\mathrm{C}}_4$ symmetry due to intraunit cell oxygen inequivalence. Finite domains of this broken symmetry state will exhibit two-dimensional chirality and the polar Kerr effect. Superconductivity within the model results from a destabilization of the 1/4-filled band paired Wigner crystal [Phys. Rev. B 93, 165110 (2016) and Phys. Rev. B 93, 205111 (2016)]. We posit that a similar valence transition, ${\mathrm{Ir}}^{4+}\to {\mathrm{Ir}}^{3+}$, occurs upon electron doping ${\mathrm{Sr}}_2{\mathrm{IrO}}_4$. We make testable experimental predictions in cuprates including superoxygenated ${\mathrm{La}}_2{\mathrm{CuO}}_{4+\delta }$ and iridates. Finally, as indirect evidence for the valence bond theory of superconductivity proposed here, we note that there exist an unusually large number of unconventional superconductors that exhibit superconductivity proximate to exotic charge ordered states, whose band fillings are universally 1/4 or 3/4, exactly where the paired Wigner crystal is most stable.

Disorder derived from defects or local strain in monolayer transition-metal dichalcogenides (TMDs) can lead to a dramatic change in the physical behavior of the interband excitations, producing inhomogeneous spectral broadening and localization leading to radiative lifetime increase. In this study, we have modeled the surface disorder of a monolayer TMD sample through a randomized potential in the layer plane. We show that this model, applied to a monolayer of ${\mathrm{WSe}}_2$, allows us to simulate the spectra of localized exciton states as well as their radiative lifetime. In this context, we give an in depth study of the influence of the disorder potential parameters on the optical properties of these defects through energies, density of states, oscillator strengths, photoluminescence (PL) spectroscopy, and radiative lifetime at low temperature (4 K). We demonstrate that localized excitons have a longer emission time than free excitons, in the range of tens of picoseconds or more, the radiative decay time depending strongly on the disorder parameter and dielectric environment. Finally, in order to prove the validity of our model, we compare it to available experimental results of the literature.

Equilibrium fluctuations of dislocations are central to the plastic response of metals and alloys because they control the attempt frequency of thermally activated events. We analyze here atomic-scale simulations of thermally vibrating dislocations with the help of an analytical model and show that thermal fluctuations intimately involve both long-range elasticity and short-range core effects. In addition, equilibrium fluctuations of edge and screw dislocations in aluminum are used to derive quantitative parameters that characterize their energetics and dynamics and we discuss how large-scale models such as dislocation dynamics can be parametrized based on these results. In particular, we show that the core parameters found here through fluctuations are transferable and can be used to predict dislocation bow-out under an applied stress.

We report first principles calculations of the structural parameters and phonon dispersion of the tetragonal noncentrosymmetric heavy fermion compound ${\mathrm{CeAuAl}}_3$. Taking into account weak magnetoelastic interactions of the rare-earth (RE) ions with the spectrum of phonons, we obtain an analytical expression for the hybridization of quadrupole excitations and phonons from the poles of the one-phonon Green function. In the paramagnetic phase, we predict the formation of mixed modes that may be observed by inelastic neutron scattering. Our results show that magnetoelastic interactions, albeit being moderate, play an important role in ${\mathrm{CeAuAl}}_3$. This suggests that magnetoelastic interactions may be equally important in a wide range of related compounds.

Magnetic exchange coupling behavior was investigated in MnBi/FeCo bilayer system at the hard/soft magnetic interface. We performed a combined study of cross-sectional high resolution transmission electron microscopy (HR-TEM), DFT calculations, and micromagnetic simulations to elucidate effect of interface structure on exchange coupling. Exchange spring MnBi/${\mathrm{Fe}}_x{\mathrm{Co}}_{1-x}(x=0.65$ and 0.35) bilayers with various thicknesses of the soft magnetic layer were deposited in a dc magnetron sputtering unit from alloy targets. According to magnetic measurements, using a Co-rich layer leads to a more coherent exchange coupling with optimum soft layer thickness of about 1 nm. Our DFT calculations predicted formation of a polycrystalline FeCo layer with coexisting crystalline and disordered (110) phases. The indexed FFTs from HR-TEM images confirmed a crystalline FeCo(110) layer, with slight misorientation in some areas, and a disordered region close to the interface which deteriorates interface exchange coupling. Moreover, our micromagnetic simulations showed how the thickness of the FeCo layer and the interface roughness both control the effectiveness of exchange coupling in MnBi/FeCo bilayer.

We consider a vortex line in the $B$ phase of superfluid ${}^3\mathrm{He}$ under uniformly precessing magnetization. The magnetization exerts torque on the vortex, causing its order parameter to oscillate. These oscillations generate spin waves, which is analogous to an oscillating charge generating electromagnetic radiation. The spin waves carry energy, causing dissipation in the system. Solving the equations of spin dynamics, we calculate the energy dissipation caused by spin-wave radiation for arbitrary tipping angles of the magnetization and directions of the magnetic field, and for both vortex types of ${}^3\mathrm{He}$-B. For the double-core vortex we also consider the anisotropy of the radiation and the dependence of the dissipation on twisting of the half cores. The radiated energy is compared with experiments in the midtemperature range $T\sim 0.5T_c$. The dependence of the calculated dissipation on several parameters is in good agreement with the experiments. Combined with numerically calculated vortex structure, the radiation theory produces the order of magnitude of the experimental dissipation. The agreement with the experiments indicates that spin-wave radiation is the dominant dissipation mechanism for vortices in superfluid ${}^3\mathrm{He}$-B in the midtemperature range.

Topological metamaterials have invaded the mechanical world, demonstrating acoustic cloaking and waveguiding at finite frequencies and variable, tunable elastic response at zero frequency. Zero-frequency topological states have previously relied on the Maxwell condition, namely, that the system has equal numbers of degrees of freedom and constraints. Here, we show that otherwise rigid periodic mechanical structures are described by a map with a nontrivial topological degree (a generalization of the winding number introduced by Kane and Lubensky) that creates, directs, and protects modes on their boundaries. We introduce a model system consisting of rigid quadrilaterals connected via free hinges at their corners in a checkerboard pattern. This bulk structure generates a topological linear deformation mode exponentially localized in one corner, as investigated numerically and via an experimental prototype. Unlike the Maxwell lattices, these structures select a single desired mode, which controls variable stiffness and mechanical amplification that can be incorporated into devices at any scale.

The compounds $A_2\mathrm{C}{\mathrm{u}}_3\mathrm{O}{\left(\mathrm{S}{\mathrm{O}}_4\right)}_3\left(A=\mathrm{Na},\mathrm{K}\right)$ are characterized by copper hexamers which are weakly coupled along the $b$ axis to realize one-dimensional antiferromagnetic chains below $T_{\mathrm{N}}\approx 3\mathrm{K}$, whereas the interchain interactions along the $a$ and $c$ axes are negligible. We investigated the energy-level splittings of the copper hexamers by inelastic neutron scattering below and above $T_{\mathrm{N}}$. The eight lowest-lying hexamer states could be unambiguously assigned and parametrized in terms of a Heisenberg exchange Hamiltonian, providing direct experimental evidence for an $S=1$ triplet ground-state associated with the copper hexamers. Therefore, the compounds $A_2\mathrm{C}{\mathrm{u}}_3\mathrm{O}{\left(\mathrm{S}{\mathrm{O}}_4\right)}_3$ serve as cluster-based spin-1 antiferromagnets to support Haldane's conjecture that a gap appears in the excitation spectrum below $T_{\mathrm{N}}$, which was verified by inelastic neutron scattering.

We report on unconventional perfectly shaped, fully asymmetric $\sim {90}^{\circ }$ Néel domain walls in multilayered films of the diluted ferromagnetic semiconductor (Ga,Mn)(As,P) with a stepwise variation of P doping. Our results contradict micromagnetic calculations, which favor symmetric domain walls due to crystallographic anisotropy and stray field energy. We demonstrate that both the puzzling uniaxial in-plane anisotropy in the tetragonal multilayered film and the asymmetry of the domain walls could result from Dzyaloshinskii-Moriya interactions that are enhanced by the multiple sharp interfaces between the layers and from anisotropic nonrelativistic exchange coupling. Our finding shows that digital variations of composition during the molecular beam epitaxy can be used to tune the anisotropy and chirality of magnetic multilayers.

We show that a weak periodic drive removes the accidental degeneracy in the ground state of the XY antiferromagnet on the triangular lattice in a uniform static magnetic field. The underlying mechanism involves adding a small periodically modulated component to the magnetic field, which influences finite-frequency modes to generate an effective potential for the accidental pseudo-Goldstone mode. This selection can be arranged to compete with the degeneracy lifting via the thermal order by disorder mechanism. This yields a nonequilibrium phase transition as the relative strength of the two mechanisms is tuned by varying temperature. This proposal may be amenable to experimental realization, in particular, as applying the field is noninvasive, and no complex bath engineering is required.

Making use of generalized Eliashberg equations, we describe the Altshuler-Aronov (AA) effect and superconductivity on equal footing. We derive explicit expressions for the Coulomb pseudopotential in 3D, taking into account also the anomalous diffusion. We present a full numerical solution for two normal-state and two anomalous self-energies. In the normal state, we amend the known results for the purely electronic AA effect; with electron-phonon coupling turned on, we find additional anomalies in the density of states close to the phonon energy. We study how the critical temperature and density of states of strongly disordered 3D superconductors change with normal-state resistivity. We find that the type of transition from the superconducting to the insulating state depends on the strength of electron-phonon coupling: at weak coupling, there exists an intermediate normal state, whereas at strong coupling the transition is direct.

Crossed Andreev reflection (CAR) is an intriguing effect that occurs in a normal-superconductor-normal junction. In CAR, an incoming electron from one terminal is coherently scattered as an outgoing hole into the other terminal. Here, we reveal that there exists a transverse spatial shift in CAR, i.e., the plane of CAR for the outgoing hole may have a sizable transverse shift from the plane of incidence for the incoming electron. We explicitly demonstrate the effect in a model system based on Weyl semimetals. We further show that the effect is quite general and exists when the terminals have sizable spin-orbit coupling. In addition, we find that the corresponding shift in the elastic cotunneling process shows different behaviors, and it vanishes when the two terminals are identical. Based on these findings, we suggest possible experimental setups for detecting the effect, which may also offer an alternative method for probing CAR.

We study the coherent dynamics of localized excitons in 100 periods of 2.5-nm-thick (In,Ga)N/GaN quantum wells with 7.5% indium concentration, measured with spectroscopic resolution through two-pulse and three-pulse photon echoes at the temperature of 1.5 K. A long-lived coherent exciton dynamics is observed in the (In,Ga)N quantum wells: When the laser photon energy is tuned across the 43-meV-wide inhomogeneously broadened resonance line, the coherence time $T_2$ varies between 45 and 255 ps, increasing with stronger exciton localization. The corresponding narrow homogeneous linewidths ranging from 5.2 to $29\mu \mathrm{eV}$ as well as the relatively weak exciton-phonon interaction $\left(0.7\mu \mathrm{eV}/\mathrm{K}\right)$ confirm a strong, quantum-dot-like exciton localization in a static disordered potential inside the (In,Ga)N quantum well layers.

We study parametrically driven quantum oscillators and show that, even for weak coupling between the oscillators, they can exhibit various many-body states with broken time-translation symmetry. In the quantum-coherent regime, the symmetry breaking occurs via a nonequilibrium quantum phase transition. For dissipative oscillators, the main effect of the weak coupling is to make the switching rate of an oscillator between its period-2 states dependent on the states of other oscillators. This allows mapping the oscillators onto a system of coupled spins. For identical oscillators, the stationary state can be mapped on that of the Ising model with an effective temperature $\propto \hslash$, for low temperature. If the oscillators are different and are away from the bifurcation parameter values where the period-2 states emerge, the stationary state corresponds to having a microscopic current in the spin system. Close to the bifurcation point the coupling cannot be considered weak and the system maps onto coupled overdamped Brownian particles performing quantum diffusion in a potential landscape.

Gray tin, also known as $\alpha$-Sn, can be turned into a three-dimensional topological insulator (3D-TI) by strain and finite-size effects. Such room-temperature 3D-TI is peculiarly interesting for spintronics due to the spin-momentum locking along the Dirac cone (linear dispersion) of the surface states. Angle-resolved photoemission spectroscopy (ARPES) has been used to investigate the dispersion close to the Fermi level in thin (001)-oriented epitaxially strained films of $\alpha$-Sn for different film thicknesses as well as for different capping layers (Al, ${\mathrm{AlO}}_x$, and MgO). Indeed a proper capping layer is necessary to be able to use $\alpha$-Sn surface states for spintronic applications. In contrast with free surfaces or surfaces coated with Ag, coating the $\alpha$-Sn surface with Al or ${\mathrm{AlO}}_x$ leads to a drop in the Fermi level below the Dirac point, and an important consequence for electronic transport is the presence of bulk states at the Fermi level. $\alpha$-Sn films coated by ${\mathrm{AlO}}_x$ are studied by electrical magnetotransport: Despite magnetotransport properties of the bulk electronic states of the ${\mathrm{\Gamma }}_8$ band playing an important role as suggested by ab initio calculations, there is clear evidence of surface states revealed by Shubnikov–de Haas oscillations corresponding to the ARPES observation.

The discovery of different phases as a result of correlations, especially in low-dimensional materials, has been always an exciting and fundamental subject of research. Recent experiments on twisted bilayer graphene have revealed reentrant unconventional superconductivity as a function of doping as well as a Mott-like insulating phase when the two layers are twisted with respect to each other at certain “magic” angles for doping, corresponding to two particles per moiré unit cell. In this paper, we propose a microscopic model that takes into account interactions and the Van Hove singularities in the density of states of twisted bilayer graphene at doping corresponding to one particle ($\nu =1$) per moiré unit cell and study how superconductivity emerges. We identify the possible symmetry of the order parameter as $s^{\pm }$, while, if the intervalley coupling is negligible, the symmetry is $s^{++}$. In addition, we find and characterize the insulating region of the system as a region with a uniform charge instability where there is coexistence of the metallic and insulating phases.