It is well known that ternary zinc-blende semiconductors are always more stable in the chalcopyrite (CH) structure than the Cu-Au (CA) structure because the CH structure has a large Coulomb interaction and a reduced strain energy. Surprisingly, an experimental study showed that the $\mathrm{ZnFeS}{\mathrm{e}}_2$ alloy takes the CA order as the ground-state structure, which is consistent with our density functional theory calculations showing that the CA order has lower energy than the CH order for $\mathrm{ZnFeS}{\mathrm{e}}_2$. We reveal that the orbital degree of freedom of a high-spin $\mathrm{F}{\mathrm{e}}^{2+}$ ion $\left(d^6\right)$ in the tetrahedral crystal field plays a key role in stabilizing the CA order. First, the spin-minority $d$ electron of the $\mathrm{F}{\mathrm{e}}^{2+}$ ion tends to occupy the $d_{x^2-y^2}$-like orbital instead of the $d_{3z^2-r^2}$-like orbital because of its large negative Coulomb energy. Second, for a nearest-neighboring $\mathrm{F}{\mathrm{e}}^{2+}$ pair, two spin-minority $d$ electrons with occupied $d_{x^2-y^2}$-like orbitals on the plane containing the Fe-Fe bond have lower electronic kinetic energies. Both conditions can be satisfied in the CA ordered $\mathrm{ZnFeS}{\mathrm{e}}_2$ alloy, whereas there is an orbital frustration in the CH structure. Our results suggest that the orbital degree of freedom provides a new way to manipulate the structure and properties of alloys.

We report time-dependent calculations of attosecond and femtosecond forces imposed by a kilovolt swift electron during passage near a nanometer-sized metal particle. Contrary to expectations based on dielectric theory, which suggest that the forces should always be attractive, we find that for very close approaches, attosecond forces are repulsive, and are caused by interaction of the magnetic field of the relativistic electron with currents within even nominally nonmagnetic nanoparticles. These results suggest an explanation for the observation of both attractive and repulsive nanoparticle movement during the first use of Ångstrom-sized electron beams in electron microscopy.

We study one-dimensional optical lattices described by generalized Aubry-André models that include both commensurate and incommensurate modulations of the hopping amplitude. This brings together two interesting features of this class of systems: Anderson localization and the existence of topological edge states. We follow changes of the single-particle energy spectrum induced by variations of the system parameters, with focus on the survival of topological states in the localized regime.

Employing the $GW$ method, we discuss the electronic and topological properties of ${\mathrm{Bi}}_2{\mathrm{Te}}_3$ and ${\mathrm{Sb}}_2{\mathrm{Te}}_3$ thin films consisting of one to six quintuple layers (QLs). Although both bulk materials are three-dimensional topological insulators, the two-dimensional topological phases of their thin films differ. We find the nontrivial quantum spin Hall phase, together with a sizable band gap of 0.13 eV, for a ${\mathrm{Bi}}_2{\mathrm{Te}}_3$ film of 2 QL thickness, whereas the 2 QL ${\mathrm{Sb}}_2{\mathrm{Te}}_3$ film hosts a topologically trivial band structure. All our $GW$ results are in excellent agreement with experiments. This concerns the dispersions of the highest valence bands and lowest conduction bands around $\overline{\mathrm{\Gamma }}$, the band gaps of thin films, and, in particular, the dispersion of the topological surface state and its energetic position relative to the bulk bands. A crucial technical issue of our study is that we go beyond the “one-shot” $GW$ approach by diagonalizing the $GW$ Hamiltonian. This yields improved quasiparticle wave functions and band structures. The physical origin of the respective off-diagonal elements in the $GW$ Hamiltonian is analyzed in detail. Without the additional diagonalization, the $GW$ band structures are unphysical for many film thicknesses.

Sigmund's model of spatially resolved sputtering is the underpinning of many models of nanoscale pattern formation induced by ion bombardment. It is based on three assumptions: (i) the number of sputtered atoms is proportional to the nuclear energy deposition (NED) near the surface, (ii) the NED distribution is independent of the orientation and shape of the solid surface and is identical to the one in an infinite medium, and (iii) the NED distribution in an infinite medium can be approximated by a Gaussian. We test the validity of these assumptions using Monte Carlo simulations of He, Ar, and Xe impacts on Si at energies of 2, 20, and 200 keV with incidence angles from perpendicular to grazing. We find that for the more commonly-employed beam parameters (Ar and Xe ions at 2 and 20 keV and nongrazing incidence), the Sigmund model's predictions are within a factor of 2 of the Monte Carlo results for the total sputter yield and the first two moments of the spatially resolved sputter yield. This is partly due to a compensation of errors introduced by assumptions (i) and (ii). The Sigmund model, however, does not describe the skewness of the spatially resolved sputter yield, which is almost always significant. The approximation is much poorer for He ions and/or high energies (200 keV). All three of Sigmund's assumptions break down at grazing incidence angles. In all cases, we discuss the origin of the deviations from Sigmund's model.

The semiconducting two-dimensional transition metal dichalcogenides ${MX}_2$ show an abundance of one-dimensional metallic edges and grain boundaries. Standard techniques for calculating edge states typically model nanoribbons, and require the use of supercells. In this paper, we formulate a Green's function technique for calculating edge states of (semi-)infinite two-dimensional systems with a single well-defined edge or grain boundary. We express Green's functions in terms of Bloch matrices, constructed from the solutions of a quadratic eigenvalue equation. The technique can be applied to any localized basis representation of the Hamiltonian. Here, we use it to calculate edge states of ${MX}_2$ monolayers by means of tight-binding models. Aside from the basic zigzag and armchair edges, we study edges with a more general orientation, structurally modifed edges, and grain boundaries. A simple three-band model captures an important part of the edge electronic structures. An 11-band model comprising all valence orbitals of the $M$ and $X$ atoms is required to obtain all edge states with energies in the ${MX}_2$ band gap. Here, states of odd symmetry with respect to a mirror plane through the layer of $M$ atoms have a dangling-bond character, and tend to pin the Fermi level.

We analyze theoretically electronic transport through a core-shell nanowire in the presence of a transversal magnetic field. We calculate the conductance for a variable coupling between the nanowire and the attached leads and show how the snaking states, which are low-energy states localized along the lines of the vanishing radial component of the magnetic field, manifest their existence. In the strong-coupling regime they induce flux periodic, Aharonov-Bohm-like, conductance oscillations, which, by decreasing the coupling to the leads, evolve into well-resolved peaks. The flux periodic oscillations arise due to interference of the snaking states, which is a consequence of backscattering at either the contacts with leads or magnetic or potential barriers in the wire.

Coherent conversion of microwave and optical photons in the single quantum level can significantly expand our ability to process signals in various fields. Efficient up-conversion of a feeble signal in the microwave domain to the optical domain will lead to quantum-noise-limited microwave amplifiers. Coherent exchange between optical photons and microwave photons will also be a stepping stone to realize long-distance quantum communication. Here we demonstrate bidirectional and coherent conversion between microwave and light using collective spin excitations in a ferromagnet. The converter consists of two harmonic oscillator modes, a microwave cavity mode and a magnetostatic mode called the Kittel mode, where microwave photons and magnons in the respective modes are strongly coupled and hybridized. An itinerant microwave field and a traveling optical field can be coupled through the hybrid system, where the microwave field is coupled to the hybrid system through the cavity mode, while the optical field addresses the hybrid system through the Kittel mode via Faraday and inverse Faraday effects. The conversion efficiency is theoretically analyzed and experimentally evaluated. The possible schemes for improving the efficiency are also discussed.

An exchange-bias (EB) effect observed in single crystal $\mathrm{ErFe}{\mathrm{O}}_3$ compensated ferrimagnet, exhibiting the EB field $H_{\mathrm{EB}}$ increasing and diverging upon approaching compensation temperature $T_{\mathrm{comp}}=45\mathrm{K}$, and changing sign with crossing $T_{\mathrm{comp}}$, is reported. The EB sign may be changed to the opposite one by varying the field-cooling protocol, depending on whether $T_{\mathrm{comp}}$ is crossed with decreasing or increasing temperature. Namely, a different EB sign with the same $|H_{\mathrm{EB}}|$ and coercive field $H_{\mathrm{C}}$ values is obtained approaching a given $T$ with increasing and decreasing temperature and the $H_{\mathrm{EB}}\left(T\right)$ dependence completed in one way is a mirror image of that completed in another way.

Spin crossover in the ${\mathrm{Fe}(1,10-\text{phenanthroline})}_2{\left(\mathrm{NCS}\right)}_2$ (Fephen) molecule adsorbed on ferromagnetic cobalt (001) and (111) surfaces as well as a gold (001) surface has been studied using the projector augmented wave method in conjunction with the density functional theory within the generalized gradient approximation, including a Hubbard interaction $U$ at the iron site and a van der Waals weak interaction between the molecule and the surface. It is shown that the energy difference between the high-spin (HS) and the low-spin (LS) states and the energy barrier of Fephen/Co(111) are significantly reduced compared to those of the adsorbed molecule on Cu(001) and Au(001). This makes the switching of the molecule from the HS to the LS state much easier than when the molecule is adsorbed on a copper or gold substrate. It is also shown that an indirect exchange mechanism is responsible for the ferromagnetic coupling between the Fe center of Fephen and the cobalt substrate and that, in the LS state, the Fephen/Co interface remains magnetically active due to small induced magnetic moments in the NCS group and iron center. This magnetic coupling between the molecule and the substrate decreases in an oscillatory fashion as a function of the number of copper layers inserted between the molecule and the cobalt substrate, in a manner similar to that of the interlayer magnetic exchange coupling. Finally, scanning tunneling microscopy images at the Fermi level show that the separation between the phen lobes is larger in the HS state than in the LS state and the work functions of cobalt, gold, and copper substrates are strongly reduced upon adsorption of the Fephen molecule.

We have demonstrated a strain-induced metal-insulator transition for weak Jahn-Teller effects in a $t_{2g}$ electron system of $\mathrm{S}{\mathrm{m}}_{0.5}\mathrm{C}{\mathrm{a}}_{0.5}\mathrm{Ti}{\mathrm{O}}_3$, which has metallic ground states in the bulk. A clear variation of electronic properties in $\mathrm{S}{\mathrm{m}}_{0.5}\mathrm{C}{\mathrm{a}}_{0.5}\mathrm{Ti}{\mathrm{O}}_3$ thin films was investigated as a function of epitaxial stain by changing a substrate and the film thickness. Under strong biaxial tensile strain, metallic behaviors were completely eliminated, as evidenced by the temperature dependence of resistivity. The observed metal-insulator transition was consistent with the behavior of the Ti $3d$ density of states at the Fermi level seen in valence band hard x-ray photoemission spectra.

We exploit topological edge states in resonant photonic crystals to attain strongly localized resonances and demonstrate lasing in these modes upon optical excitation. The use of virtually lossless topologically isolated edge states may lead to a class of thresholdless lasers operating without inversion. One needs, however, to understand whether topological states may be coupled to external radiation and act as active cavities. We study a two-level topological insulator and show that self-induced transparency pulses can directly excite edge states. We simulate laser emission by a suitably designed topological cavity and show that it can emit tunable radiation. For a configuration of sites following the off-diagonal Aubry-André-Harper model, we solve the Maxwell-Bloch equations in the time domain and provide a first-principles confirmation of topological lasers. Our results open the road to a class of light emitters with topological protection for applications ranging from low-cost energetically effective integrated laser sources, also including silicon photonics, to strong-coupling devices for studying ultrafast quantum processes with engineered vacuum.

Cyclic Pancharatnam-Berry (PB) and adiabatic noncyclic geometric (ANG) effects are investigated in a single electron orbital system connected to two metal contacts with externally driven chemical potential and/or temperatures. The PB contribution doesn't affect the density matrix evolution, but has a quantitative effect on the statistics (fluctuations) of electron transfer. The ANG contribution, on the other hand, affects the net flux across the junction. Unlike the PB, the ANG contribution is nonzero when two parameters are identically driven. Closed analytical expressions are derived for the ANG contribution to the flux, and the PB contribution to the first two leading order fluctuations. Fluctuations can be modified by manipulating the relative phases of the drivings. Interestingly, we find that the fluctuations of the pumped charge do not satisfy the steady state fluctuation theorem in presence of nonzero geometric contribution, but can be recovered for a vanishing geometric contribution even in presence of the external driving.

We demonstrate experimentally giant spatial-dispersion-induced birefringence in metamaterials. The difference between the reflection coefficients for $(1,\overline{1},0)$- and $(0,0,1)$-polarized light reflected from the $[1,1,0]$ surface of a metamaterial reaches $78\%$. The magnitude of spatial-dispersion-induced birefringence in the transparency windows of the structure reaches $n_{1\overline{1}0}-n_{001}=-0.13$, which is at least three orders of magnitude larger than the typical values reported for natural crystals. Our results elucidate the important role of spatial dispersion effects in a wide class of metamaterials.

We propose an all-electrical experimental setup to detect and manipulate the amplitude of odd-frequency pairing in a double quantum dot. The odd-frequency pair amplitude is induced from the breakdown of orbital symmetry when Cooper pairs are injected in the double dot with electrons in different dots. When the dot levels are aligned with the Fermi energy, i.e., on resonance, nonlocal Andreev processes are directly connected to the presence of odd-frequency pairing. Therefore, their amplitude can be manipulated by tuning the level positions. The detection of nonlocal Andreev processes by conductance measurements contributes a direct proof of the existence of the odd-frequency pair amplitude and is available using current experimental techniques.

Obtaining highly spin-polarized currents in molecular junctions is crucial and important for nanoscale spintronics devices. Motivated by our recent symmetry-based theoretical argument for complete blocking of one spin conductance channel in model molecular junctions [A. Smogunov and Y. J. Dappe, Nano Lett. 15, 3552 (2015)], we explore the generality of the proposed mechanism and the degree of achieved spin-polarized current for realistic molecular junctions made of various ferromagnetic electrodes (Ni, Co, Fe) connected by different molecules (quaterthiophene or $p$-quaterphenyl). A simple analysis of the spin-resolved local density of states of a free electrode allowed us to identify the Fe(110) as the most optimal electrode, providing perfect spin filtering and high conductance at the same time. These results are confirmed by ab initio quantum transport calculations and are similar to those reported previously for model junctions. It is found, moreover, that the distortion of the $p$-quaterphenyl molecule plays an important role, reducing significantly the overall conductance.

There has been considerable interest in topological semimetals that exhibit extreme magnetoresistance (XMR). These have included materials lacking inversion symmetry such as TaAs, as well Dirac semimetals such as ${\mathrm{Cd}}_3{\mathrm{As}}_2$. However, it was reported recently that LaSb and LaBi also exhibit XMR, even though the rocksalt structure of these materials has inversion symmetry, and the band-structure calculations do not show a Dirac dispersion in the bulk. Here, we present magnetoresistance and specific-heat measurements on NdSb, which is isostructural with LaSb. NdSb has an antiferromagnetic ground state and, in analogy with the lanthanum monopnictides, is expected to be a topologically nontrivial semimetal. We show that NdSb has an XMR of $\sim {10}^4\%$, even within the antiferromagnetic state, illustrating that XMR can occur independently of the absence of time-reversal symmetry breaking in zero magnetic field. The persistence of XMR in a magnetic system offers the promise of new functionality when combining topological matter with electronic correlations. We also find that in an applied magnetic field below the Néel temperature there is a first-order transition, consistent with evidence from previous neutron scattering work.

We theoretically investigate a nanoscale mode-division multiplexing scheme based on parity-time- ($\mathcal{PT}$) symmetric coaxial plasmonic waveguides. Coaxial waveguides support paired degenerate modes corresponding to distinct orbital angular momentum states. $\mathcal{PT}$-symmetric inclusions of gain and loss break the degeneracy of the paired modes and create new hybrid modes without definite orbital angular momentum. This process can be made thresholdless by matching the mode order with the number of gain and loss sections within the coaxial ring. Using both a Hamiltonian formulation and degenerate perturbation theory, we show how the wave vectors and fields evolve with increased loss/gain and derive sufficient conditions for thresholdless transitions. As a multiplexing filter, this $\mathcal{PT}$-symmetric coaxial waveguide could help double density rates in on-chip nanophotonic networks.

Samarium orthoferrite $\left(\mathrm{SmFe}{\mathrm{O}}_3\right)$ has been the subject of debate on the existence or nonexistence of ferroelectric properties. It has a high spin-reorientation transition temperature $T_{\mathrm{SR}}$ (480 K). Detailed synchrotron x-ray diffraction and Raman spectroscopy dielectric investigations in the 300 to 600 K temperature range of $\mathrm{SmFe}{\mathrm{O}}_3$ nanoparticles $\left(\sim 65\mathrm{nm}\right)$ have been performed. Raman spectroscopy established the existence of strong spin-phonon coupling and magnetostriction in the system. Magnetic measurements showed a spin-reorientation transition region from 480 to 450 K similar to that of a single crystal. The dielectric constant confirmed a clear anomaly around $T_{\mathrm{SR}}$. The signature of unusual relaxor ferroelectric behavior in $\mathrm{SmFe}{\mathrm{O}}_3$ nanoparticles from 380 to 480 K was observed in the dielectric properties. The electron density plots obtained using Rietveld refinement of the synchrotron x-ray diffraction data showed the displacement of the Sm and Fe ions, which can play a key role in the anomalous behavior at $T_{\mathrm{SR}}$. The analysis indicated the presence of magnetoelectric coupling due to the exchange interaction between Sm and Fe spins in $\mathrm{SmFe}{\mathrm{O}}_3$ nanoparticles.

We study a quantum quench in a one-dimensional spinless fermion model (equivalent to the XXZ spin chain), where a magnetic flux is suddenly switched off. This quench is equivalent to imposing a pulse of electric field and therefore generates an initial particle current. This current is not a conserved quantity in the presence of a lattice and interactions, and we investigate numerically its time evolution after the quench, using the infinite time-evolving block decimation method. For repulsive interactions or large initial flux, we find oscillations that are governed by excitations deep inside the Fermi sea. At long times we observe that the current remains nonvanishing in the gapless cases, whereas it decays to zero in the gapped cases. Although the linear response theory (valid for a weak flux) predicts the same long-time limit of the current for repulsive and attractive interactions (relation with the zero-temperature Drude weight), larger nonlinearities are observed in the case of repulsive interactions compared with that of the attractive case.