The magnetic structure of superconducting Eu(Fe${}_{0.82}$Co${}_{0.18}$)${}_2$As${}_2$ is unambiguously determined by single-crystal neutron diffraction. A long-range ferromagnetic order of the Eu${}^{2+}$ moments along the $c$ direction is revealed below the magnetic phase transition temperature $T_C=17$ K. In addition, the antiferromagnetism of the Fe${}^{2+}$ moments still survives and the tetragonal-to-orthorhombic structural phase transition is also observed, although the transition temperatures of the Fe spin-density-wave (SDW) order and the structural phase transition are significantly suppressed to $T_N=70$ K and $T_S=90$ K, respectively, compared to the parent compound EuFe${}_2$As${}_2$. We present microscopic evidence for the coexistence of the Eu ferromagnetism and the Fe SDW in the superconducting crystal. The superconductivity competes with the Fe SDW in Eu(Fe${}_{0.82}$Co${}_{0.18}$)${}_2$As${}_2$. Moreover, the comparison between Eu(Fe${}_{1-x}$Co${}_x$)${}_2$As${}_2$ and Ba(Fe${}_{1-x}$Co${}_x$)${}_2$As${}_2$ indicates a considerable influence of the rare-earth element Eu on the magnetism of the Fe sublattice.

We report an experimental observation of terahertz Cherenkov radiation from a moving magnetic moment produced in terbium gallium garnet by a circularly polarized femtosecond laser pulse via the inverse Faraday effect. Contrary to some existing theoretical predictions, the polarity of the observed radiation unambiguously demonstrates the paramagnetic, rather than diamagnetic, nature of the ultrafast inverse Faraday effect. From measurements of the radiation field, the Verdet constant in the subpicosecond regime is $\sim$3–10 times smaller than its table quasistatic value.

We study a one-dimensional model of interacting bosons on a lattice with two flat bands. Regular condensation is suppressed due to the absence of a well-defined minimum in the single-particle spectrum. We find that interactions stabilize a number of nontrivial phases such as a pair (quasi)condensate, a supersolid at incommensurable fillings, and valence bond crystals at commensurability. We support our analytical calculations with numerical simulations using the density matrix renormalization group technique. Implications for cold atoms are discussed.

We investigate Floquet topological insulators in the presence of spatially modulated light. We extend on previous work to show that light can be used to generate and control localized modes in the bulk of these systems. We provide examples of bulk modes generated through modulation of different properties of the light, such as its phase, polarization, and frequency. We show that these effects may be realized in a variety of systems, including a zincblende and honeycomb models, and also provide a generalization of these results to three-dimensional systems.

The layered FeTe${}_2$O${}_5$Cl compound was studied by specific-heat, muon-spin relaxation, nuclear magnetic resonance, and dielectric as well as neutron and synchrotron x-ray diffraction measurements, and the results were compared to isostructural FeTe${}_2$O${}_5$Br. We find that the low-temperature ordered state, similarly as in FeTe${}_2$O${}_5$Br, is multiferroic: the elliptical amplitude-modulated magnetic cycloid and the electric polarization simultaneously develop below 11 K. However, compared to FeTe${}_2$O${}_5$Br, the magnetic elliptical envelope rotates by 75(4)${}^{\circ }$ and the orientation of the electric polarization is much more sensitive to the applied electric field. We propose that the observed differences between the two isostructural compounds arise from geometric frustration, which enhances the effects of otherwise subtle Fe${}^{3+}$ ($S=\frac{5}{2}$) magnetic anisotropies. Finally, x-ray diffraction results imply that, on the microscopic scale, the magnetoelectric coupling is driven by shifts of the O${}_1$ atoms, as a response to the polarization of the Te${}^{4+}$ lone-pair electrons involved in the Fe-O-Te-O-Fe exchange bridges.

We theoretically study the equilibrium and dynamic properties of nanoscale magnetic tunnel junctions (MTJs) and magnetic wires, in which an electric field controls the magnetic anisotropy through spin-orbit coupling. By performing micromagnetic simulations, we construct a rich phase diagram and find that, in particular, the equilibrium magnetic textures can be tuned between Néel and Bloch domain walls in an elliptical MTJ. Furthermore, we develop a phenomenological model of a quasi-one-dimensional domain wall confined by a parabolic potential and show that, near the Néel-to-Bloch-wall transition, a pulsed electric field induces precessional domain-wall motion which can be used to reverse the chirality of a Néel wall and even depin it. This domain-wall motion controlled by electric fields, in lieu of applied current, may provide a model for ultralow-power domain-wall memory and logic devices.

Using first-principles methods, we study theoretically the properties of an individual $\left\{{\text{Fe}}_4\right\}$ single-molecule magnet (SMM) attached to metallic leads in a single-electron transistor geometry. We show that the conductive leads do not affect the spin ordering and magnetic anisotropy of the neutral SMM. On the other hand, the leads have a strong effect on the anisotropy of the charged states of the molecule, which are probed in Coulomb blockade transport. Furthermore, we demonstrate that an external electric potential, modeling a gate electrode, can be used to manipulate the magnetic properties of the system. For a charged molecule, by localizing the extra charge with the gate voltage closer to the magnetic core, the anisotropy magnitude and spin ordering converges to the values found for the isolated $\left\{{\text{Fe}}_4\right\}$ SMM. We compare these findings with the results of recent quantum transport experiments in three-terminal devices.

We used angle-resolved photoemission spectroscopy to make direct measurements of the electronic structure of the untwinned uniaxial state of CaFe${}_2$As${}_2$, the parent compound of an iron-based superconductor. The very small photon beam size, combined with the relatively large single-domain area on the crystal surfaces, allowed us to obtain the intrinsic symmetry-broken dispersions and Fermi surface (FS) geometries along the orthogonal Fe-Fe bond directions without any mechanical or magnetic detwinning processes. Comparing the optimized local density approximation calculations, an orbital-dependent band shifting is introduced to obtain better agreement, which is consistent with the development of orbital ordering. More interestingly, unidirectional straight and flat FS segments are observed near the zone center, which indicates the existence of a unidirectional charge density wave order. Our results indicate strong electronic anisotropy in CaFe${}_2$As${}_2$ and put strong constraints on theories for the iron-pnictide system.

We have performed angle-resolved photoemission spectroscopy (ARPES) on the (111) surface of the topological crystalline insulator SnTe. Distinct from a pair of Dirac-cone surface states across the $\overline{X}$ point of the surface Brillouin zone on the (001) surface, we revealed two types of Dirac-cone surface states each centered at the $\overline{\Gamma }$ and $\overline{M}$ points, which originate from the bulk-band inversion at the $L$ points. We also found that the energy location of the Dirac point and the Dirac velocity are different from each other. This ARPES experiment demonstrates the surface states on different crystal faces of a topological material, and it elucidates how mirror-symmetry-protected Dirac cones of a topological crystalline insulator show up on surfaces with different symmetries.

We studied the possibility of exciton condensation in Mott insulating bilayers. In these strongly correlated systems, an exciton is the bound state of a double occupied and empty site. In the strong coupling limit, the exciton acts as a hard-core boson. Its physics is captured by the exciton $t$-$J$ model, containing an effective $XXZ$ model describing the exciton dynamics only. Using numerical simulations and analytical mean-field theory, we constructed the ground-state phase diagram. Three homogeneous phases can be distinguished: the antiferromagnet, the exciton checkerboard crystal, and the exciton superfluid. For most model parameters, however, we predict macroscopic phase separation between these phases. The exciton superfluid exists only for large exciton hopping energy. Additionally, we studied the collective modes and susceptibilities of the three phases. In the superfluid phase, we find the striking feature that the bandwidth of the spin-triplet excitations, potentially detectable by resonant inelastic x-ray scattering (RIXS), is proportional to the superfluid density. The superfluid phase mode is visible in the charge susceptibility, measurable by RIXS or electron energy loss spectroscopy (EELS).

Ab initio calculations have been performed on all solid phases of U metal and U-Zr alloy, the basis of a promising metallic fuel for fast nuclear reactors. Based on generalized gradient approximation, both density functional theory (DFT) in its standard form and the so-called DFT plus Hubbard $U$ (DFT+$U$) modification are evaluated. The evolution of calculated energetics, volume, magnetic moments, electronic structure, and $f$-orbital occupation as functions of the effective Hubbard $U$ parameter, $U_{\mathrm{eff}}$, is carefully examined at $U_{\mathrm{eff}}$ from 0 to 4 eV. DFT is found to overestimate energetics, underestimate volume, downward shift some $f$ bands near Fermi level and overestimate $f$-orbital occupation against existing experimental and/or computational data. The error is ∼0.07 eV/atom in terms of enthalpy, which affects phase stability modeling for δ(U,Zr) and γ(U,Zr). DFT+$U$ at $U_{\mathrm{eff}}=1-1.5$ eV offers clear improvement on these calculated properties (∼0.05 eV/atom in terms of enthalpy) and in general still neither promotes ordered magnetic moments nor opens unphysical band gaps, which occur at higher $U_{\mathrm{eff}}$ values. The empirical $U_{\mathrm{eff}}$ values of 1–1.5 eV are close to but smaller than the theoretical estimations of 1.9–2.3 eV that we obtain from the linear response approach. $U_{\mathrm{eff}}$ is found to vary only slightly (≤0.24 eV) between different phases and at different compositions of U and U-Zr; thus, a single $U_{\mathrm{eff}}=1.24$ eV, which is the statistical optimal from energetic fitting, is suggested for both U and U-Zr. Besides correlation, the relativistic effect of spin-orbit coupling (SOC) is also systematically explored. SOC is found to lower energy, increase volume, and split the 5$f$ shell above Fermi level and reduce $f$-orbital occupation. The effect predominates in the unoccupied states and is very small on all these calculated ground state properties (∼0.02 eV/atom in terms of enthalpy).

The carbon vacancy ($V_C$) has been suggested by different studies to be involved in the $Z_1$/$Z_2$ defect-a carrier lifetime killer in SiC. However, the correlation between the $Z_1$/$Z_2$ deep level with $V_C$ is not possible since only the negative carbon vacancy ($V_{\mathrm{C}}^{-}$) at the hexagonal site, $V_{\mathrm{C}}^{-}$($h$), with unclear negative-$U$ behaviors was identified by electron paramagnetic resonance (EPR). Using freestanding $n$-type 4$H$-SiC epilayers irradiated with low energy (250 keV) electrons at room temperature to introduce mainly $V_C$ and defects in the C sublattice, we observed the strong EPR signals of $V_{\mathrm{C}}^{-}$($h$) and another $S$ = 1/2 center. Electron paramagnetic resonance experiments show a negative-$U$ behavior of the two centers and their similar symmetry lowering from $C_{3v}$ to $C_{1h}$ at low temperatures. Comparing the Si and C ligand hyperfine constants observed by EPR and first principles calculations, the new center is identified as $V_{\mathrm{C}}^{-}$($k$). The negative-$U$ behavior is further confirmed by large scale density functional theory supercell calculations using different charge correction schemes. The results support the identification of the lifetime limiting $Z_1$/$Z_2$ defect to be related to acceptor states of the carbon vacancy.

We perform polarization resolved spectroscopy of two- and one-dimensional microcavity-polariton condensates, which are formed by exciting the system in the optical parametric oscillator configuration. We observe polarization inversion for linearly polarized pumping parallel to the wire in both the 1D and 2D systems. As the polarization plane of the pump is rotated, the degree of linear polarization of the 2D system oscillates between orthogonal polarizations with the same period as that of the pump. However, the 1D system switches abruptly between two states of high degree of linear polarization with half the period. Two complementary models, based on semiclassical Boltzmann kinetic equations and the Gross-Pitaevskii equation, respectively, obtain an excellent agreement with the experimental results, providing a deep insight into the mechanisms responsible for the polarization switching.

When a heterostructure is created at the center of a photonic crystal line-defect cavity to form a nanocavity, the photonic band gap contains several high-quality ($Q$) factor resonant modes. We have studied the optical properties of these modes to examine their applicability to Raman silicon lasers, which require two high-$Q$ resonant modes with a frequency spacing of 15.6 THz. Our experimental and numerical analyses reveal four types of resonant modes. We demonstrate that pairing the resonant mode originating from the first-order propagation mode with that arising from the second-order propagation mode is the most promising approach toward the realization of Raman silicon lasers.

We discuss the contribution of edge scattering to the conductance of graphene nanoribbons and nanoflakes. Using different possible types of the boundary conditions for the electron wave function at the edge, we found dependences of the momentum relaxation time and conductance on the geometric sizes and on the carrier density. We also consider the case of ballistic nanoribbon and nanodisk, for which the edge scattering is the main mechanism of momentum relaxation.

Circular dichroism in angle-resolved photoemission spectroscopy (CD-ARPES) has been recently proposed as a sensitive probe of either the spin texture or the orbital momentum texture of helical Dirac fermions at the surface of three-dimensional topological insulators. Recent results obtained on Bi${}_2$Te${}_3$ point rather towards a final state effect. In this Rapid Communication, we study the prototypical topological insulator Bi${}_2$Se${}_3$ by CD-ARPES as a function of the photon energy and as a function of thickness. Analysis of the photon energy dependence of CD-ARPES, as well as the existence of a large dichroic signal from massive Dirac fermion states in ulthathin films, confirm that the dichroism is predominantly a final state effect.

We have studied the resistance of $1T$-CrSe${}_2$, as the Cr atoms are substituted by V or Ti. The V replacement leads to a logarithmic increase in the resistance as the temperature is lowered, proportional to the V concentration. While this behavior is consistent with the Kondo effect, the weak dependence of the resistance with magnetic field and the fact that the system has antiferromagnetic order, rule out a Kondo effect due to spin degeneracy. In contrast to the case of V, Ti substitution does not increase the logarithmic term while application of pressure destroys it. Calculations of the electronic structure within the framework of density functional theory, maximally localized Wannier functions, and many-body calculations in a cluster containing a Cr or V atom and its six nearest-neighbor Se atoms, helped to reveal the existence of an orbital Kondo effect due to orbital degeneracy in the V substitutional impurities.

We examine the low-energy behavior of a double quantum dot in a regime where spin and pseudospin excitations are degenerate. The individual quantum dots are described by Anderson impurity models with an on-site interaction $U$ which are capacitively coupled by an interdot interaction $U_{12}<U$. The low-energy response functions are expressed in terms of renormalized parameters, which can be deduced from an analysis of the fixed point in a numerical renormalization group calculation. At the point where the spin and pseudospin degrees of freedom become degenerate, the free quasiparticle excitations have a phase shift of $\pi /4$ and a 4-fold degeneracy. We find, however, when the quasiparticle interactions are included, that the low-energy effective model has SU(4) symmetry only in the special case $U_{12}=U$ unless both $U$ and $U_{12}$ are greater than $D$, the half bandwidth of the conduction electron bath. We show that the gate voltage dependence of the temperature-dependent differential conductance observed in recent experiments can be described by a quasiparticle density of states with temperature-dependent renormalized parameters.

We present first-principles many-body calculations of the dielectric constant, quasiparticle band structure, and optical absorption spectrum of monolayer MoS${}_2$ using a supercell approach. As the separation between the periodically repeated layers is increased, the dielectric function of the layer develops a strong $q$ dependence around $q=0$. This implies that denser $k$-point grids are required to converge the band gap and exciton binding energies when large supercells are used. In the limit of infinite layer separation, here obtained using a truncated Coulomb interaction, a $45\times 45$ $k$-point grid is needed to converge the $G_0{W}_0$ band gap and exciton energy to within 0.1 eV. We provide an extensive comparison with previous studies and explain agreement and variations in the results. It is demonstrated that too coarse $k$-point sampling and the interactions between the repeated layers have opposite effects on the band gap and exciton energy, leading to a fortuitous error cancellation in the previously published results.