Synchrotron radiation x-ray absorption and emission spectroscopy techniques, complemented by high-resolution transmission electron microscopy methods and density functional theory calculations, are employed to investigate the effect of Mn in ${\mathrm{Al}}_x{\mathrm{Ga}}_{1-x}\mathrm{N}:\mathrm{Mn}$ samples with an Al content up to 100%. The atomic and electronic structure of Mn is established together with its local environment and valence state. A dilute alloy without precipitation is obtained for ${\mathrm{Al}}_x{\mathrm{Ga}}_{1-x}\mathrm{N}:\mathrm{Mn}$ with Al concentrations up to 82%, and the surfactant role of Mn in the epitaxial process is confirmed.

We investigate the effects of dielectric environments on the transition levels of Re and Nb dopants in monolayer ${\mathrm{MoS}}_2$ through density functional theory calculations. The inherently weakly screened Coulomb interaction in free-standing monolayer ${\mathrm{MoS}}_2$ makes the dopant electrons and holes strongly bound, and the Re and Nb impurities are found to produce deep levels. It is shown that when the monolayer ${\mathrm{MoS}}_2$ is placed near high permittivity dielectrics the screened Coulomb interaction induces carrier delocalization with generating shallow levels.

We measure the dielectric confinement length scales of triplet excitons in organic semiconductors by jointly measuring their microwave-domain electric and magnetic susceptibilities. We apply this technique to characterize triplet excitons in two singlet fission materials with distinct solid-state packing and correlate the extracted localization length scales with the role of the excitonic environment. By using the magnetic susceptibility simultaneously determined through our experiments, we compare the independently extracted dielectric and spin-spin localization length scales, highlighting the role of local anisotropy on the properties of excitonic triplet states.

Skyrmions are topologically protected spin textures, characterized by a topological winding number $N$, that occur spontaneously in some magnetic materials. Recent experiments have demonstrated the capability to grow graphene on top Fe/Ir, a system that exhibits a two-dimensional skyrmion lattice. Here we show that a weak exchange coupling between the Dirac electrons in graphene and a two-dimensional skyrmion lattice with $N=\pm 1$ drives graphene into a quantum anomalous Hall phase, with a band gap in bulk, a Chern number $\mathcal{C}=2N$, and chiral edge states with perfect quantization of conductance $G=2N\frac{e^2}{h}$. Our findings imply that the topological properties of the skyrmion lattice can be imprinted in the Dirac electrons of graphene.

The different morphologies of Co films grown on either the clean Fe(001) surface and the oxygen-saturated Fe(001)-$p(1\times 1)\mathrm{O}$ substrate are investigated by means of scanning tunneling microscopy, Auger electron spectroscopy, and density functional theory. The considered Co coverage range extends beyond the thickness at which layer-by-layer growth is destabilized by plastic deformations induced by the relaxation of the strain accumulated in the film. Our findings indicate that the oxygen overlayer of the Fe(001)-$p(1\times 1)\mathrm{O}$ surface floats on top of the growing Co film and strongly influences both the Co nucleation process and the film structural evolution. The layer-dependent islands nucleation of Co films grown on clean Fe(001) substrates, recently associated with a thickness-dependent adatom mobility [A. Picone et al., Phys. Rev. Lett. 113, 046102 (2014)], is found to be suppressed by the oxygen overlayer. The latter also significantly delays the layer-by-layer instability with respect to the oxygen-free growth. Furthermore, the body-centered-tetragonal/hexagonal-close-packed transition is not observed in the case of Co/Fe(001)-$p(1\times 1)\mathrm{O}$ sample, replaced by the development of highly ordered surface undulations. These form a mesoscopic square pattern with the sides aligned to the $\mathrm{Fe}\langle 110\rangle$ directions, while the surface atomic structure retains the square $p(1\times 1)$ symmetry in registry with the substrate. Such undulations are likely generated by a highly ordered array of interfacial misfit dislocations running along the $\mathrm{Fe}\langle 110\rangle$ directions.

The effects of oxygen vacancy $\left(V_{\mathrm{O}}\right)$ and epitaxial strain on the ferroelectric (FE) and antiferrodistortive (AFD) properties of the [001]-oriented ${\mathrm{PbTiO}}_3/{\mathrm{SrTiO}}_3$ 1/1 superlattice are comprehensively studied using first-principles calculations. It is found that the oxygen vacancies form most easily when the superlattice in-plane lattice constant is between those of single-crystal ${\mathrm{PbTiO}}_3$ and ${\mathrm{SrTiO}}_3$. The polarization in the direction of the Ti-$V_{\mathrm{O}}$-Ti chain is remarkably reduced due to the $V_{\mathrm{O}}$-induced local tail-to-tail polarization patterns, and consequently, the $V_{\mathrm{O}}$ can pin the polarization to a certain direction. Moreover, the octahedral rotation or tilting around the direction of the Ti-$V_{\mathrm{O}}$-Ti chain is also suppressed while the rotations along the other two orthogonal directions are enhanced. In both perfect and oxygen-deficient superlattices, the ferroelectric phase has lower energy when the octahedra rotate around the polar axis. These results suggest a promising way to mediate the FE and AFD properties in oxygen-deficient superlattices.

We have explored metal-insulator phenomena for pyrochlore $R_2{\mathrm{Ir}}_2{\mathrm{O}}_7$, in which the electron correlation strength and magnetic configuration are systematically controlled by varying the average rare-earth ionic radius $(R={\mathrm{Nd}}_{1-x}{\mathrm{Pr}}_x$ and ${\mathrm{Sm}}_y{\mathrm{Nd}}_{1-y})$, external pressure, and magnetic field. Metal-insulator transitions in bulk are caused by increasing $x$ or tuning pressure, indicating that the effective correlation is responsible for the transition. The metallic state intervenes between the paramagnetic insulating and antiferromagnetically ordered insulating phases for $({\mathrm{Sm}}_y{\mathrm{Nd}}_{1-y}){}_2{\mathrm{Ir}}_2{\mathrm{O}}_7$ $(y=0.7–0.9)$, reminiscent of a first-order Mott transition. Furthermore, the metal-to-insulator crossover is observed (around $y=0.7$) for charge transport on magnetic domain walls in the insulating bulk. An application of magnetic field also drives metal-insulator transitions for $({\mathrm{Nd}}_{1-x}{\mathrm{Pr}}_x){}_2{\mathrm{Ir}}_2{\mathrm{O}}_7$, which is likely due to the modulation of Ir $5d$ band structures accompanied by a change in the spin textures. Abundant exotic states are revealed in the vicinity of metal-insulator transitions by finely tuning the physical parameters in $R_2{\mathrm{Ir}}_2{\mathrm{O}}_7$.

Making a combined use of bosonization and fermionization techniques, we build nonlocal transformations between dual fermion operators, describing junctions of strongly interacting spinful one-dimensional quantum wires. Our approach allows for trading strongly interacting (in the original coordinates) fermionic Hamiltonians for weakly interacting (in the dual coordinates) ones. It enables us to generalize to the strongly interacting regime the fermionic renormalization group approach to weakly interacting junctions. As a result, on one hand, we are able to pertinently complement the information about the phase diagram of the junction obtained within the bosonization approach; on the other hand, we map out the full crossover of the conductance tensors between any two fixed points in the phase diagram connected by a renormalization group trajectory.

In a quantizing magnetic field, the chiral two-dimensional electron gas in Landau level $N=0$ of bilayer graphene goes through a series of phase transitions at integer filling factors $\nu \in \left[-3,3\right]$ when the strength of an electric field applied perpendicularly to the layers is increased. At filling factor $\nu =3,$ the electron gas can be described by a simple two-level system where layer and spin degrees of freedom are frozen. The gas then behaves as an orbital quantum Hall ferromagnet. A Coulomb-induced Dzyaloshinskii-Moriya term in the orbital pseudospin Hamiltonian is responsible for a series of transitions first to a Wigner crystal state and then to a spiral state as the electric field is increased. Both states have a nontrivial orbital pseudospin texture. In this work, we study how the phase diagram at $\nu =3$ is modified by an electric field applied in the plane of the layers and then derive several experimental signatures of the uniform and nonuniform states in the phase diagram. In addition to the transport gap, we study the electromagnetic absorption and the Kerr rotation due to the excitations of the orbital pseudospin-wave modes in the broken-symmetry states.

We present a theory that describes the appearance of circular polarization of the photoluminescence (PL) in ferromagnet-semiconductor hybrid heterostructures due to the spin-dependent tunneling of photoexcited carriers from a quantum well into a magnetic layer. The theory succeeds in explaining the experimental data on time-resolved PL for heterostructures consisting of an InGaAs-based quantum well (QW) and a spatially separated Mn $\delta$ layer. We show that the circular polarization of the PL originates from the dynamic spin polarization of electrons due to spin-dependent leakage from the QW onto the Mn donor states split by the exchange field of the ferromagnetic Mn $\delta$ layer.

Nonlinear metasurfaces based on coupling a locally enhanced plasmonic response to intersubband transitions of $n$-doped multi-quantum-wells (MQWs) can provide second-order susceptibilities orders of magnitude larger than any other nonlinear flat structure measured so far. Here we present a comprehensive theory to characterize the electromagnetic response of nonlinear processes occurring in ultrathin MQW-based plasmonic metasurfaces, providing a homogeneous model that takes phase matching at the unit-cell level and the influence of saturation and losses into account. In addition, the limits imposed by saturation of the MQW transitions on the nonlinear response of these metasurfaces are analytically derived, revealing useful guidelines to design devices with enhanced performance. Our approach is first validated using experimental data and then applied to theoretically investigate novel designs able to achieve significant second-harmonic generation efficiency in the infrared frequency band.

It is found that for objects possessing small surface structures with differing radii of curvature the secondary electron emission (SEE) yield may be significantly higher than for objects with smooth surfaces of the same material. The effect is highly pronounced for surface structures of nanometer scale, often providing a more than $100\%$ increase of the SEE yield. The results also show that the SEE yield from surfaces with structure does not show a universal dependence on the energy of the primary, incident electrons as it is found for flat surfaces in experiments. We derive conditions for the applicability of the conventional formulation of SEE using the simplifying assumption of universal dependence. Our analysis provides a basis for studying low-energy electron emission from nanometer structured surfaces under a penetrating electron beam important in many technological applications.

We use atomistic simulations to study the resonant acoustic modes and compare different calculations of the acoustic mean-free path in amorphous systems with nanometric crystalline spherical inclusions. We show that the resonant acoustic properties are not a simple combination of the vibrations in the inclusions and in the amorphous matrix. The presence of the inclusion affects the transport properties mainly in the frequency range separating simple scattering from multiple scattering processes. However, propagation of acoustic wave packets is spatially heterogeneous and shows that the amorphous/crystalline interface acts as a low-energy pass filter slowing down the high kinetic energy motion whatever the vibration frequency. These heterogeneities cannot be caught by the mean-free path, but still they must play an important role in thermal transport, thus raising the question of the correct modeling of thermal transport in composite systems.

Antiferromagnetic ordering does not give rise to a measurable macroscopic symmetry-breaking order parameter such as the magnetization in a ferromagnet. An exception is the case of antiferromagnets with a vortexlike toroidal alignment of the magnetic moments in the unit cell because this gives rise to the formation of a spontaneous macroscopic toroidal moment which can be measured as a magnetoelectric effect. It is a long-standing question whether the toroidal moment is merely a side effect of the antiferromagnetic state or whether it can become the primary order parameter in a ferroic phase transition. Here we report the magnetoelectric properties of ${\mathrm{LiFeSi}}_2{\mathrm{O}}_6$ and show that they point to the role of the toroidal vector moment as the primary order parameter. Based on Landau theory we distinguish it from primary antiferromagnetic order. We thus justify the proposal that along with ferromagnets, ferroelectrics, and ferroelastics the ferrotoroidics constitute a new class of primary ferroics.

We consider the excitation of large amplitude gyrotropic vortex core precession in a permalloy nanodisk by the torques originating from the in-plane microwave current flowing along the interface of the permalloy/${\mathrm{Bi}}_2{\mathrm{Se}}_3$ heterostructures, in which the huge charge-to-spin conversion ratio is observed [A. Mellnik et al., Nature 511, 449 (2014)]. We consider analytically and by micromagnetic modeling the dependence of this excitation on the frequency and magnitude of the microwave current. The analogies of the vortex dynamics and the Landau phase-transition theory are demonstrated. These findings open the possibility to excite gyrotropic vortex motion with the current densities far lower than by any other means.

We study the nearest-neighbor $XXZ$ Heisenberg quantum antiferromagnet on the kagome lattice. Here we consider the effects of several perturbations: (a) a chirality term, (b) a Dzyaloshinski-Moriya term, and (c) a ring-exchange type term on the bowties of the kagome lattice, and inquire if they can support chiral spin liquids as ground states. The method used to study these Hamiltonians is a flux attachment transformation that maps the spins on the lattice to fermions coupled to a Chern-Simons gauge field on the kagome lattice. This transformation requires us to consistently define a Chern-Simons term on the kagome lattice. We find that the chirality term leads to a chiral spin liquid even in the absence of an uniform magnetic field, with an effective spin Hall conductance of ${\sigma }_{xy}^s=\frac{1}{2}$ in the regime of $XY$ anisotropy. The Dzyaloshinski-Moriya term also leads a similar chiral spin liquid but only when this term is not too strong. An external magnetic field also has the possibility of giving rise to additional plateaus which also behave like chiral spin liquids in the $XY$ regime. Finally, we consider the effects of a ring-exchange term and find that, provided its coupling constant is large enough, it may trigger a phase transition into a chiral spin liquid by the spontaneous breaking of time-reversal invariance.

Solid parahydrogen, which is known to have an exceptionally long vibrational coherence lifetime as a molecular solid, offers an ideal testbed to perform coherent control experiments in the condensed phase. Here we demonstrate the spatial manipulation and visualization of the relative phase of vibrational wave functions in solid parahydrogen. Spatial distribution of vibrational excitation is generated by femtosecond impulsive Raman excitation. It is shown that the imprinted initial phase can be manipulated by wave-front modulation of the excitation laser pulses with a spatial light modulator. An interferometric measurement is used to convert the spatial phase distribution of the vibrational wave functions to the amplitude distribution. We have confirmed that the spatial profile of the scattered anti-Stokes pulse reveals the spatial phase distribution of the wave functions. The read-and-write scheme demonstrated in this experiment is applicable to a broad range of Raman memory systems accessible by $\mathrm{\Lambda }$-type transitions.

Rare-earth hexaborides have attracted considerable attention recently in connection to a variety of correlated phenomena including heavy fermions, superconductivity, and low-temperature magnetic phases. Here, we present high-resolution angle-resolved photoemission spectroscopy studies of trivalent ${\mathrm{CeB}}_6$ and divalent ${\mathrm{BaB}}_6$ rare-earth hexaborides. We find that the Fermi surface electronic structure of ${\mathrm{CeB}}_6$ consists of large oval-shaped pockets around the $X$ points of the Brillouin zone, whereas the states around the zone center $\mathrm{\Gamma }$ point are strongly renormalized. Our first-principles calculations agree with our experimental results around the $X$ points but not around the $\mathrm{\Gamma }$ point, indicating areas of strong renormalization located near $\mathrm{\Gamma }$. The Ce quasiparticle states participate in the formation of hot spots at the Fermi surface, whereas the incoherent $f$ states hybridize and lead to the emergence of dispersive features absent in the non-$f$ counterpart ${\mathrm{BaB}}_6$. Our results provide an understanding of the electronic structure in rare-earth hexaborides, which will be useful in elucidating the nature of the exotic low-temperature phases in these materials.

We theoretically investigate the electronic and magnetic structure of vacancy bearing K-intercalated Fe chalcogenides ($\text{Ch}=\text{Se}$ and Te) using first-principles calculations. Motivated by experimental report on the compositions and suggested Fe valences of the parent insulating and superconducting phases on vacancy bearing K-intercalated Fe selenides, we consider three compositions, ${\mathrm{KFe}}_{1.5}{\mathrm{Ch}}_2, {\mathrm{K}}_{0.94}{\mathrm{Fe}}_{1.5}{\mathrm{Ch}}_2$, and ${\mathrm{KFe}}_{1.56}{\mathrm{Ch}}_2$. The electronic structure calculated on geometry optimized crystal structure of ${\mathrm{KFe}}_{1.56}{\mathrm{Se}}_2$ is found to be markedly different from the other two compositions due to formation of the ferromagnetic one-dimensional chain of Fe atoms in the case of ${\mathrm{KFe}}_{1.56}{\mathrm{Se}}_2$ as opposed to trimer geometry of Fe atoms in ${\mathrm{KFe}}_{1.5}{\mathrm{Se}}_2$ and ${\mathrm{K}}_{0.94}{\mathrm{Fe}}_{1.5}{\mathrm{Se}}_2$. This is expected to have an important bearing on the composition specific observation of superconducting properties in vacancy bearing K-intercalated Fe selenides. Interestingly, the Te compounds, which are yet to be synthesized, show very similar magnetic properties as in Se compounds, in marked contrast to 11 compounds, FeSe and FeTe. This hints that unlike FeTe, vacancy bearing K-intercalated Fe tellurides may be potential candidates for exploring superconductivity.