We investigate the electronic and magnetic properties of FeSe monolayer on the anisotropic ${\mathrm{SrTiO}}_3$ (110) surface. With compressive strain along the $\left[1\overline{1}0\right]$ direction from the substrate, the monolayer FeSe possesses a staggered bipartite iron lattice with a height difference around 0.06 Å along the out-of-plane direction. The staggering causes stronger magnetic frustration among the collinear, stagger-dimer, and stagger-trimer antiferromagnetic orders, and the strain elongates one electron and two hole pockets along the strain direction and the remaining hole pocket along the orthogonal direction. The strain-induced band splitting at $\mathrm{\Gamma }$ can also result in a band inversion to drive the system into a topologically nontrivial phase. The absence of strong superconducting suppression on the staggered lattice suggests that the superconducting pairings may be insensitive to the modification of interactions and hopping parameters between two Fe sublattices.

We spectroscopically investigated the energy gap of the correlated antiferromagnetic insulator ${\mathrm{LaMnPO}}_{1-x}{\mathrm{F}}_x$ ($x=0.0$ and 0.04) as a function of temperature and pressure, separately, in conjunction with many-body electronic structure calculations. These results show that the electronic structure in all measured regimes is well described by a model that includes both Mott-Hubbard interactions and Hund's rule coupling. Moreover, we find that by applying external pressure, thereby reducing the effective Mott-Hubbard interaction and Hund's coupling, the energy gap in ${\mathrm{LaMnPO}}_{1-x}{\mathrm{F}}_x$ can be fully closed, yielding a metallic state.

We report on the results of powder and single-crystal neutron diffraction to investigate the magnetic order in ${\mathrm{YbCo}}_2{\mathrm{Si}}_2$ below the Néel temperature $T_{\mathrm{N}}=1.7$ K in detail. Two different magnetically ordered phases can clearly be distinguished. At lowest temperatures a commensurate magnetic structure with a propagation vector ${\mathbf{k}}_1=\left(0.250.251\right)$ is found, while the intermediate phase ($T>0.9$ K) is characterized by an incommensurate magnetic structure with ${\mathbf{k}}_2=\left(0.250.0861\right)$. The magnetic structure in ${\mathrm{YbCo}}_2{\mathrm{Si}}_2$ is in marked contrast to all other known $R{\mathrm{Co}}_2{\mathrm{Si}}_2$ compounds ($R$ = rare earth element) likely due to some itineracy of the Yb $4f$ states being responsible for the magnetism.

We imaged the transport of current-induced spin coherence in a two-dimensional electron gas confined in a triple quantum well. Nonlocal Kerr rotation measurements, based on the optical resonant amplification of the electrically-induced polarization, revealed a large spatial variation of the electron $g$ factor and the efficient generation of a current-controlled spin-orbit field in a macroscopic Hall bar device. We observed coherence times in the nanoseconds range transported beyond half-millimeter distances in a direction transverse to the applied electric field. The measured long spin transport length can be explained by two material properties: large mean free path for charge diffusion in clean systems and enhanced spin-orbit coefficients in the triple well.

The existence of natural birefringence in x-ray reflection on graphene is demonstrated at energies spanning the carbon $1s$ absorption edge. This new x-ray effect has been discovered with precision measurements of the polarization-plane rotation and the polarization-ellipticity changes that occur upon reflection of linearly polarized synchrotron radiation on monolayer graphene. Extraordinarily large polarization-plane rotations of up to ${30}^{\circ }$, accompanied by a change from linearly to circularly polarized radiation have been measured for graphene on copper. Graphene on single crystalline cobalt, grown on tungsten, exhibits rotation values of up to ${17}^{\circ }$. Both graphene systems show resonantly enhanced effects at the ${\pi }^{*}$ and ${\sigma }^{*}$ energies. The results are referenced against those obtained for polycrystalline carbon and highly oriented pyrolytic graphite (HOPG), respectively. As expected, polycrystalline carbon shows negligible rotation, whereas a huge maximum rotation of ${140}^{\circ }$ has been observed for HOPG that may be considered a graphene multilayer system. HOPG is found to exhibit such large rotation values over a broad energy range, even well beyond the ${\pi }^{*}$ resonance energy due to the contributions of numerous graphene layers. To explain the origin of the observed natural birefringence of graphene, the Stokes parameters as well as the x-ray natural linear dichroism in reflection have been determined. It is shown that the birefringence directly results from the optical anisotropy related to the orthogonal alignment of ${\pi }^{*}$ and ${\sigma }^{*}$ bonds in the graphene layer. Our polarization analysis reveals a strong bonding of graphene on Co with a reduced ${\sigma }^{*}$ excitation energy and a strong tilt of $50\%$ of the $p_z$ orbitals towards diagonal orientation. In contrast, graphene on Cu is weakly bound with an orthogonal orientation of the $p_z$ orbitals. Exhibiting such a large natural birefringence that can be controlled through substrate choice, and because of excellent heat conductivity, graphene materials have a potential to be used as tunable x-ray phase shifting $\lambda /4$ or $\lambda /2$ plates in the design of future high-intensity light sources.

We discuss a fundamental theory of how to calculate the phonon-loss sector of the energy-loss spectrum for electrons scattering from crystalline solids. A correlated model for the atomic motion is used for calculating the vibrational modes. Spectra are calculated for crystalline silicon illuminated by a plane wave and by an atomic-scale focused coherent probe, in which case the spectra depend on probe position. These spectra are also affected by the size of the spectrometer aperture. The correlated model is contrasted with the Einstein model in which atoms in the specimen are assumed to vibrate independently. We also discuss how both the correlated and Einstein models relate to a classical view of the energy-loss process.

The pressure evolution of the local structure of ${\mathrm{Fe}}_2{\mathrm{O}}_3$ hematite has been determined by extended x-ray absorption fine structure up to $\sim 79$ GPa. Below the phase-transition pressure at $\sim 50$ GPa, no increasing of ${\mathrm{FeO}}_6$ octahedra distortion is observed as pressure is applied. Above the phase transition, an abrupt decrease of the nearest-neighbor Fe-O distance is observed concomitantly with a strong reduction in the ${\mathrm{FeO}}_6$ distortion. This information on the local structure, used as a test-bench for the different high-pressure forms proposed in the literature, suggests that the orthorhombic structure with space group $Aba2$, recently proposed by Bykova et al. [Nat. Commun. 7, 10661 (2016)], is the most probable, but puts into question the presence of the $P21/n$ form in the pressure range 54–67 GPa. Finally, the crossover from Fe high-spin to low-spin states with pressure increase has been monitored from the pre-edge region of the Fe $K$-edge absorption spectra. Its “simultaneous” comparison with the local structural changes allows us to conclude that it is the electronic transition that drives the structural transition and not vice versa.

We report the thermodynamic properties, magnetic ground state, and microscopic magnetic model of the spin-1 frustrated antiferromagnet ${\mathrm{Li}}_2{\mathrm{NiW}}_2{\mathrm{O}}_8$, showing successive transitions at $T_{\mathrm{N}1}\simeq 18$ K and $T_{\mathrm{N}2}\simeq 12.5$ K in zero field. Nuclear magnetic resonance and neutron diffraction reveal collinear and commensurate magnetic order with the propagation vector $\mathbf{k}=(\frac{1}{2},0,\frac{1}{2})$ below $T_{\mathrm{N}2}$. The ordered moment of $1.8{\mu }_B$ at 1.5 K is directed along $\left[0.89\right(9),-0.10(5),-0.49(6\left)\right]$ and matches the magnetic easy axis of spin-$1{\mathrm{Ni}}^{2+}$ ions, which is determined by the scissor-like distortion of the ${\mathrm{NiO}}_6$ octahedra. Incommensurate magnetic order, presumably of spin-density-wave type, is observed in the region between $T_{\mathrm{N}2}$ and $T_{\mathrm{N}1}$. Density-functional band-structure calculations put forward a three-dimensional spin lattice with spin-1 chains running along the $\left[01\overline{1}\right]$ direction and stacked on a spatially anisotropic triangular lattice in the $ab$ plane. We show that the collinear magnetic order in ${\mathrm{Li}}_2{\mathrm{NiW}}_2{\mathrm{O}}_8$ is incompatible with the triangular lattice geometry and thus driven by a pronounced easy-axis single-ion anisotropy of ${\mathrm{Ni}}^{2+}$.

We have investigated the transport properties of electrically and thermally excited spin currents in a lateral spin valve consisting of a spin injector and detector with a middle ferromagnetic wire by detecting the first- and second-harmonic voltages. The first-harmonic spin signal was significantly suppressed by the middle ferromagnetic wire because of the spin absorption effect. On the other hand, in the second-harmonic signal, a small signal related to the middle ferromagnetic wire was observed in addition to a conventional spin signal with a reduced magnitude. This indicates that the additional ferromagnetic wire acts not only as the spin absorber but also as another spin injector under thermal spin injection, because the heat current caused by direct spin injection propagates to the middle ferromagnetic wire and creates another temperature gradient. By using this effect, we show that the magnetization direction of a ferromagnetic nanodot embedded in a nonmagnetic Cu wire becomes measurable.

We investigate the electronic instabilities in a kagome lattice with Rashba spin-orbital coupling by the unbiased singular-mode functional renormalization group. At the parent $1/3$ filling, the normal state is a quantum spin Hall system. Since the bottom of the conduction band is near the van Hove singularity, the electron-doped system is highly susceptible to competing orders upon electron interactions. The topological nature of the parent system enriches the complexity and novelty of such orders. We find ${120}^{\circ }$-type intra-unit-cell antiferromagnetic order, $f$-wave superconductivity, and chiral $p$-wave superconductivity with increasing electron doping above the van Hove point. In both types of superconducting phases, there is a mixture of comparable spin singlet and triplet components because of the Rashba coupling. The chiral $p$-wave superconducting state is characterized by a Chern number $Z=1$, supporting a branch of Weyl fermion states on each edge. The model bares close relevance to the so-called $s{d}^2$ graphenes proposed recently.

We use transport and neutron diffraction to study the electronic phase diagram of ${\mathrm{NaFe}}_{1-x}{\mathrm{Co}}_x\mathrm{As}$. In the undoped state, NaFeAs exhibits a tetragonal-to-orthorhombic structural transition below $T_s$ followed by a collinear antiferromagnetic (AF) order below $T_N$. Upon codoping to form ${\mathrm{NaFe}}_{1-x}{\mathrm{Co}}_x\mathrm{As},T_s$ and $T_N$ are gradually suppressed, leading to optimal superconductivity near Co-doping $x=0.025$. While transport experiments on these materials reveal an anomalous behavior suggesting the presence of a quantum critical point (QCP) near optimal superconductivity, our neutron diffraction results indicate that commensurate AF order becomes transversely incommensurate with $T_N>T_c$ before vanishing abruptly at optimal superconductivity. These results are remarkably similar to electron-doping and isovalent-doping evolution of the AF order in ${\mathrm{BaFe}}_{2-x}{\mathrm{Ni}}_x{\mathrm{As}}_2$ and ${\mathrm{BaFe}}_2{(\mathrm{As}}_{1-x}{\mathrm{P}}_x{)}_2$, thus suggesting a universal behavior in the suppression of the magnetic order in iron pnictides as superconductivity is induced.

We predict that magnon motive force can lead to temperature dependent, nonlinear chiral damping in both conducting and insulating ferromagnets. We estimate that this damping can significantly influence the motion of skyrmions and domain walls at finite temperatures. We also find that in systems with low Gilbert damping moving chiral magnetic textures and resulting magnon motive forces can induce large spin and energy currents in the transverse direction.

Interatomic interactions and ordering in fcc Ni-rich Ni-Re alloys are studied by means of first-principles methods combined with statistical mechanics simulations based on the Ising Hamiltonian. First-principles calculations are employed to obtain effective chemical and strain-induced interactions, as well as ordering energies and enthalpies of formation of random and ordered Ni-Re alloys. Based on the nonmagnetic enthalpies of formation, we speculate that the type of ordering can be different in alloys with Re content less than 10 at.%. We demonstrate that effective chemical interactions in this system are quite sensitive to the alloy composition, atomic volume, and magnetic state. In statistical thermodynamic simulations, we have used renormalized interactions, which correctly reproduce ordering energies obtained in the direct total energy calculations. Monte Carlo simulations for ${\mathrm{Ni}}_{0.91}{\mathrm{Re}}_{0.09}$ alloy show that there exists a strong ordering tendency of the $\left(1\frac{1}{2}0\right)$ type leading to precipitation of the ${\mathrm{D}1}_a$ ordered structure at about 940 K. Our results for the atomic short-range order indicate, however, that the presently applied theory overestimates the strength of the ordering tendency compared to that observed in the experiment.

The zero-temperature conductivity $\sigma \left(0\right)$ of icosahedral $\left(i\right)$-AlPdRe has been found to be simply related to the resistance ratio $R={\rho }_{4.2\text{K}}/{\rho }_{295\text{K}}$ by a power law, $\sigma \left(0\right)\sim R^{-1.74}$, over four orders of magnitude in $\sigma \left(0\right)$. This relation includes metallic single grain samples, and polygrain samples of different morphologies which are metallic for small $R$ values, and insulatinglike at large $R$. Electronic transport properties of single grain $i$-AlPdRe samples are thus found to be on common ground with polygrain $i$-AlPdRe. The relation between $R$ and $\sigma \left(0\right)$ can be qualitatively understood from published band-structure calculations on quasicrystalline approximants.

We generate in-plane magnetoelastic waves in nickel films using the all-optical transient grating technique. When performed on amorphous glass substrates, two dominant magnetoelastic excitations can be resonantly driven by the underlying elastic distortions, the Rayleigh surface acoustic wave and the surface skimming longitudinal wave. An applied field, oriented in the sample plane, selectively tunes the coupling between magnetic precession and one of the elastic waves, thus demonstrating selective excitation of coexisting, large-amplitude magnetoelastic waves. Analytical calculations based on the Green's function approach corroborate the generation of multiple surface acoustic transients with disparate decay dynamics.

The nature of the magnetic structure in magnetic so-called MAX phases is a topic of some controversy. Here we present unpolarized neutron-diffraction data between 3.4 and 290.0 K and momentum transfer between $Q=0.0\text{and}1.1 {\AA }^{-1}$, as well as complementary x-ray-diffraction data on epitaxial thin films of the MAX phase material ${\mathrm{Mn}}_2\mathrm{GaC}$. This inherently layered material exhibits neutron-diffraction peaks consistent with long-ranged antiferromagnetic order with a periodicity of two structural unit cells. The magnetic structure is present throughout the measured temperature range. The results are in agreement with first-principles calculations of antiferromagnetic structures for this material where the Mn-C-Mn atomic trilayers are found to be ferromagnetically coupled internally but spin flipped or rotated across the Ga layers. The present findings have significant bearing on the discussion regarding the nature of the magnetic structure in magnetic MAX phases.

In the heavily hole-doped iron-based superconductors $A{\mathrm{Fe}}_2{\mathrm{As}}_2$ ($A=\text{K}$, Rb, and Cs), the electron effective mass increases rapidly with alkali-ion radius. To study the superconducting gap structure in this series, we measure the in-plane London penetration depth ${\lambda }_{ab}\left(T\right)$ in clean crystals of $A{\mathrm{Fe}}_2{\mathrm{As}}_2$ down to low temperature $T\sim 0.1$ K. In ${\mathrm{KFe}}_2{\mathrm{As}}_2$, the superfluid stiffness ${\rho }_s^{ab}\left(T\right)={\lambda }_{ab}^2\left(0\right)/{\lambda }_{ab}^2\left(T\right)$ at low temperatures can be accounted for by the strongly band-dependent multiple gaps reported recently. Although the ${\lambda }_{ab}\left(T\right)$ in all systems exhibits similar nonexponential temperature dependence indicating nodes or small minima in the gap, we find that the quasiparticle excitations at low temperatures show a systematic suppression with increasing alkali-ion radius. A possible origin of such evolution of low-energy excitations is discussed in terms of the momentum-dependent effect of enhanced quasiparticle mass near a quantum critical point.

There has been a growing interest in realizing topologically nontrivial states of matter in band insulators, where a quantum Hall effect can appear as an intrinsic property of the band structure. While ongoing progress is under way with a number of directions, the possibility of realizing novel interaction-generated topological phases, without the requirement of a nontrivial invariant encoded in single-particle wave function or band structure, can significantly extend the class of topological materials and is thus of great importance. Here, we show an interaction-driven topological phase emerging in an extended Bose-Hubbard model on a kagome lattice, where the noninteracting band structure is topological trivial with zero Berry curvature in the Brillouin zone. By means of an unbiased state-of-the-art density-matrix renormalization group technique, we identify that the ground state in a broad parameter region is equivalent to a bosonic fractional quantum Hall Laughlin state, based on the characterization of universal properties including ground-state degeneracy, edge excitations, and anyonic quasiparticle statistics. Our work paves a way to finding an interaction-induced topological phase at the phase boundary of conventionally ordered solid phases.

Recent findings of possible topological superconductivity in YPtBi motivated us to grow and investigate single crystals of this material. The compound was studied by means of ac magnetic susceptibility, electrical resistivity, Hall resistivity, and heat capacity measurements, performed in wide ranges of temperature and magnetic field. The superconductivity below the critical temperature $T_{\mathrm{c}}=0.97\mathrm{K}$ was clearly reflected in the magnetic and electronic transport data. The upper critical field $B_{\mathrm{c}2}$ was estimated at 1.84 T. However, no obvious anomaly near $T_{\mathrm{c}}$ was found in the temperature dependence of the heat capacity. In the normal state YPtBi appears semimetallic but its electrical conductance can be described by two components corresponding to parallel channels: metallic and semiconducting, with the latter becoming negligible at temperatures below 40 K. Shubnikov–de Haas oscillations were observed at temperatures below 10 K. Nontrivial Berry phase together with weak antilocalization effect strongly support the presence of Dirac fermions in YPtBi.