We have measured the reflectivity of the optimally doped ${\mathrm{Ca}}_{8.5}{\mathrm{La}}_{1.5}\left({\mathrm{Pt}}_3{\mathrm{As}}_8\right)\left({\mathrm{Fe}}_{10}{\mathrm{As}}_{10}\right)$ single crystal $\left(T_c=32.8\mathrm{K}\right)$ over the broad frequency range from $40{\text{cm}}^{-1}$ to 12 $000{\text{cm}}^{-1}$ and for temperatures from 8 K to 300 K. The optical conductivity spectra of the low-frequency region $\left(<1000{\text{cm}}^{-1}\right)$ in the normal state (80 K $<T\le 300$ K) is well fitted with two Drude forms, which indicates the presence of multiple bands at the Fermi level. Decreasing temperature below 80 K, this low-frequency Drude spectra develops a pseudogap (PG) hump structure at around $100{\text{cm}}^{-1}$ and continuously evolves into the fully opened superconducting (SC) gap structure below $T_c$. Theoretical calculations of the optical conductivity with the preformed Cooper pair model provide an excellent description of the temperature evolution of the PG structure above $T_c$ into the SC gap structure below $T_c$. The extracted two SC gap sizes are ${\mathrm{\Delta }}_S=4.9$ $\text{meV}$ and ${\mathrm{\Delta }}_L=14.2$ $\text{meV}$, suggesting ${\mathrm{Ca}}_{8.5}{\mathrm{La}}_{1.5}\left({\mathrm{Pt}}_3{\mathrm{As}}_8\right)\left({\mathrm{Fe}}_{10}{\mathrm{As}}_{10}\right)$ as a multiple-gap superconductor with a mixed character of the weak-coupling and strong-coupling superconductivity.

We use the Chern-Simons (CS) fermion representation of $s=1/2$ spin operators to construct topological, long-range magnetically ordered states of interacting two-dimensional (2D) quantum spin models. We show that the fermion-fermion interactions mediated by the dynamic CS flux attachment may give rise to Cooper pairing of the fermions. Specifically, in an $XY$ model on the honeycomb lattice, this construction leads to a “CS superconductor”, which belongs to a topologically nontrivial in 2D symmetry class DIII, with particle-hole and time-reversal symmetries. It is shown that in the original spin language, this state corresponds to a symmetry protected topological state, which coexists with a magnetic long-range order. We discuss physical manifestations of the topological character of the corresponding state.

The dielectric and structural properties of the $S=1$ kagome antiferromagnet ${\mathrm{KV}}_3{\mathrm{Ge}}_2{\mathrm{O}}_9$ are examined. The low-temperature structure below 50 K is orthorhombic with a typical correlation length of 8 nm. While the high-temperature hexagonal phase can be considered $C$-centered orthorhombic, the $C$-centered symmetry is broken below 50 K. The low-temperature symmetry does not support the simplex solid state, which is the theoretically expected ground state for the $S=1$ kagome lattice. Above 60 K, incommensurate lattice modulation is observed. The lock-in transition suggests that the origin of the orthorhombic deformation is the development of a short-range magnetic ordering.

Domain wall propagation dynamics has been studied in nanostructured artificial kagome spin-ice structures. A stripline circuit has been used to provide localized pulsed magnetic fields within the artificial spin-ice (ASI) structure. This provides control of the system through electrically assisted domain wall nucleation events. Synchronization of the pulsed fields with additional global magnetic fields and the use of a focused magneto-optical Kerr effect magnetometer allows our experiments to probe the domain wall transit through an extended ASI structure. We find that the propagation distance depends on the driving field revealing field-driven properties of domain walls below their intrinsic nucleation field.

NiO is a room-temperature antiferromagnetic (AF) insulator with important applications in AF spintronics. Although it is considered a prototypical AF material with a simple magnetic structure with two sublattice spins aligned in easy planes and having small in-plane magnetic anisotropy, its critical behavior has not been studied in detail. Here we present an experimental investigation of the critical magnetic field for the transition from the AF to the spin-flop (SF) phase obtained with magnetization and susceptibility measurements. The measured temperature dependence of the AF-SF critical field can be quite well explained by the instability of the low-lying magnon mode with energy renormalized by four-magnon interactions.

We use the LDA+$U$ method to study the possibility of exciton condensation in perovskites of transition metals with the $d^6$ electronic configuration such as ${\mathrm{LaCoO}}_3$. For realistic interaction parameters we find several distinct solutions exhibiting a spin-triplet exciton condensate, which gives rise to a local spin density distribution while the ordered moments are vanishingly small. Rhombohedral distortion from the ideal cubic structure suppresses the ordered state, contrary to the spin-orbit coupling which enhances the excitonic condensation energy. We explain the trends observed in the numerical simulations with the help of a simplified strong-coupling model. Our results indicate that ${\mathrm{LaCoO}}_3$ is close to the excitonic instability and suggest ways how to achieve the exciton condensation.

In the presence of spin-orbit coupling, electron scattering off impurities depends on both spin and orbital angular momentum of electrons—spin-orbit scattering. Although some transport properties are subject to spin-orbit scattering, experimental techniques directly accessible to this effect are limited. Here we show that a signature of spin-orbit scattering manifests itself in quasiparticle interference (QPI) imaged by spectroscopic-imaging scanning tunneling microscopy. The experimental data of a polar semiconductor BiTeI are well reproduced by numerical simulations with the $T$-matrix formalism that include not only scalar scattering normally adopted but also spin-orbit scattering stronger than scalar scattering. To accelerate the simulations, we extend the standard efficient method of QPI calculation for momentum-independent scattering to be applicable even for spin-orbit scattering. We further identify a selection rule that makes spin-orbit scattering visible in the QPI pattern. These results demonstrate that spin-orbit scattering can exert predominant influence on QPI patterns and thus suggest that QPI measurement is available to detect spin-orbit scattering.

A fractional quasiparticle charge is a manifestation of strong interactions in the fractional quantum Hall effect. Nevertheless, shot noise of quasiparticles is well described by a formula, derived for noninteracting charges. We explain the success of that formula by proving that in the limits of strong and weak backscattering it holds irrespectively of microscopic details in weakly and strongly interacting systems alike. The derivation relies only on principles of statistical mechanics. We also derive an approximate model-independent formula for shot noise in the regime of intermediate backscattering. The equation is numerically close to the standard “noninteracting” fitting formula but suggests a different physical interpretation of the experimental results. We verify our theoretical predictions with a shot-noise experiment at the filling factor $3/5$.

We report in situ low-energy electron diffraction intensity versus voltage [LEED-$I$($V$)] studies of the surface atomic structure of epitaxially grown ${\left(001\right)}_{\text{pc}}$-oriented $\left(\text{pc}=\text{pseudocubic}\right)$ thin films of the correlated $3d$ transition-metal oxide ${\mathrm{LaNiO}}_3$. Our analysis indicates the presence of large out-of-plane bucklings of the topmost LaO layers but only minor bucklings of the topmost ${\mathrm{NiO}}_2$ layers, in close agreement with earlier surface x-ray diffraction data. In view of materials design approaches that have suggested using atomic-scale polar structural distortions to produce designer electronic structures in artificial nickelate heterostructures, we propose that the broken inversion symmetry inherent to the surface of rare-earth nickelate thin films offers a similar opportunity to study the coupling between atomic structure and electronic properties in this family of materials.

We report that a high electric field and current triggers the switching of multiple states in two-dimensional (2D) crystals of $1T\text{−}\mathrm{Ta}{\mathrm{S}}_2$, accompanying a metamorphosis of the electronic structure. We fabricated four-terminal devices of nanometer-thick crystals of $1T\text{−}\mathrm{Ta}{\mathrm{S}}_2$ with charge-density-wave (CDW) phases. By applying in-plane electric fields and concomitantly injecting currents, we realized nonvolatile switching among normal metals, Mott insulators, and thermally inaccessible semimetals. The field and current not only interact with the CDW but also generate Joule heat, and both effects contribute to the switching. The results indicate the potential existence of multiple electronic states accessible only in 2D crystals.

Compared to the previous report on $\mathrm{C}{\mathrm{u}}_3\mathrm{Bi}{\left(\mathrm{Se}{\mathrm{O}}_3\right)}_2{\mathrm{O}}_{2}X\left(X=\mathrm{Cl},\mathrm{Br}\right)$ [V. Gnezdilov et al., (arXiv:1604.04249)],where no dielectric anomaly was observed without magnetic field near $T_N\sim 25.6\mathrm{K}$, we further present the dielectric behaviors without and with magnetic field in $\mathrm{C}{\mathrm{u}}_3\mathrm{Bi}{\left(\mathrm{Se}{\mathrm{O}}_3\right)}_2{\mathrm{O}}_2\mathrm{Cl}$. At zero field $H=0$, an antiferromagnetic transition from magnetization and specific heat measurements is clearly established at $T_N$, while no dielectric anomaly was observed. Those results are similar to the previous report [V. Gnezdilov et al., (arXiv:1604.04249)]. Above the critical field $H_{\mathrm{c}}\sim 0.8\mathrm{T}$, a metamagnetic spin-flip transition from antiferromagnetic to ferrimagnetic order at $T\sim T_N$ is induced anisotropically only for $H\parallel c$. Meanwhile, a ferroelectric behavior from dielectric and pyroelectric current measurements is observed below $T\sim T_N$; then a corresponding type-II multiferroics emerges above $H_{\mathrm{c}}$. The key mechanism of the anisotropic spin-flip-induced multiferroicity in $\mathrm{C}{\mathrm{u}}_3\mathrm{Bi}{\left(\mathrm{Se}{\mathrm{O}}_3\right)}_2{\mathrm{O}}_2\mathrm{Cl}$ can be ascribed to the breaking of magnetic twofold symmetry in the $bc$ plane above $H_{\mathrm{c}}$.

Elastic strain is potentially an important approach in tuning the properties of the improperly multiferroic hexagonal ferrites, the details of which, however, have been elusive due to experimental difficulties. Employing the method of restrained thermal expansion, we have studied the effect of isothermal biaxial strain in the basal plane of $h-\mathrm{LuFe}{\mathrm{O}}_3$ (001) films. The results indicate that a compressive biaxial strain significantly enhances the $K_3$ structural distortion (the order parameter of the improper ferroelectricity), and the effect is larger at higher temperatures. The compressive biaxial strain and the enhanced $K_3$ structural distortion together cause an increase in the electric polarization and a reduction in the canting of the weak ferromagnetic moments in $h-\mathrm{LuFe}{\mathrm{O}}_3$, according to our first principles calculations. These findings are important for understanding the strain effect as well as the coupling between the lattice and the improper multiferroicity in $h-\mathrm{LuFe}{\mathrm{O}}_3$. The experimental elucidation of the strain effect in $h-\mathrm{LuFe}{\mathrm{O}}_3$ films also suggests that the restrained thermal expansion can be a viable method to unravel the strain effect in many other thin film materials.

The dielectric function of several amorphous phase-change materials has been determined by employing a combination of impedance spectroscopy (9 kHz–3 GHz) and optical spectroscopy from the far- $(20\mathrm{c}{\mathrm{m}}^{-1}$, 0.6 THz) to the near- $(12000\mathrm{c}{\mathrm{m}}^{-1}$, 360 THz) infrared, i.e., from the DC limit to the first interband transition. While phase-change materials undergo a change from covalent bonding to resonant bonding on crystallization, the amorphous and crystalline phases of ordinary chalcogenide semiconductors are both governed by virtually the same covalent bonds. Here, we study the dielectric properties of amorphous phase-change materials on the pseudobinary line between GeTe and $\mathrm{S}{\mathrm{b}}_2\mathrm{T}{\mathrm{e}}_3$. These data provide important insights into the charge transport and the nature of bonding in amorphous phase-change materials. No frequency dependence of permittivity and conductivity is discernible in the impedance spectroscopy measurements. Consequently, there are no dielectric relaxations. The frequency-independent conductivity is in line with charge transport via extended states. The static dielectric constant significantly exceeds the optical dielectric constant. This observation is corroborated by transmittance measurements in the far infrared, which show optical phonons. From the intensity of these phonon modes, a large Born effective charge is derived. Nevertheless, it is known that crystalline phase-change materials such as GeTe possess even significantly larger Born effective charges. Crystallization is hence accompanied by a huge increase in the Born effective charge, which reveals a significant change of bonding upon crystallization. In addition, a clear stoichiometry trend in the static dielectric constant along the pseudobinary line between GeTe and $\mathrm{S}{\mathrm{b}}_2\mathrm{T}{\mathrm{e}}_3$ has been identified.

Generalized stacking fault energies of nondilute binary alloys in the Ag-Au-Pd system are calculated using density functional theory and special quasirandom structures. Supercells containing 90 and 135 atoms are compared for direct calculations of the generalized stacking fault energy, and the axial interaction model is used to estimate the intrinsic stacking fault energy. The axial interaction model approximates the directly calculated energy to within 10% in most cases, but is sensitive to the particular structures used. Increasing the number of atoms used for direct calculations decreases the uncertainty of the calculated stacking fault energies in most cases, and we show that this uncertainty is related to certain correlations between pairs of adjacent layers within the supercell.

We address the question of the stability of the fractional quantum Hall effect in the presence of pseudomagnetic disorder generated by mechanical deformations of a graphene sheet. Neglecting the potential disorder and taking into account only strain-induced random pseudomagnetic fields, it is possible to write down a Laughlin-like trial ground-state wave function explicitly. Exploiting the Laughlin plasma analogy, we demonstrate that in the case of fluctuating pseudomagnetic fluxes of a relatively small amplitude, the fractional quantum Hall effect is always stable upon the deformations. By contrast, in the case of bubble-induced pseudomagnetic fields in graphene on a substrate (a small number of large fluxes) the disorder can be strong enough to cause a glass transition in the corresponding classical Coulomb plasma, resulting in the destruction of the fractional quantum Hall regime and in a quantum phase transition to a nonergodic state of the lowest Landau level.

A first-principles-based effective Hamiltonian technique is used to investigate the polarization switching mechanisms in two polymorphic phases of ${\mathrm{BiFeO}}_3$ having no defects. The switching mechanism is homogeneous for any switching field in the rhombohedral phase, while in the supertetragonal phase it changes from the classical nucleation and domain-wall motion to nucleation-limited switching with virtually no propagation, and then to homogeneous switching with increasing electric field. The first two inhomogeneous switching mechanisms of the supertetragonal phase of ${\mathrm{BiFeO}}_3$ are thus intrinsic in nature, and can be well described by the classical and nucleation-limited switching models, respectively. The reason behind their absence in the rhombohedral phase is also indicated. Moreover, the field-induced changes of switching mechanism within the supertetragonal phase are further elucidated from an energetic point of view.

Within the shoving model of the glass transition, the relaxation time and the viscosity are related to the local cage rigidity. This approach can be extended down to the atomic level in terms of the interatomic interaction or potential of mean force. We applied this approach to both real metallic glass formers and model Lennard-Jones glasses. The main outcome of this analysis is that in metallic glasses the thermal expansion contribution is mostly independent of composition and is uncorrelated with the interatomic repulsion: As a consequence, the fragility increases upon increasing the interatomic repulsion steepness. In the Lennard-Jones glasses, the scenario is opposite: Thermal expansion and interatomic repulsion contributions are strongly correlated, and the fragility decreases upon increasing the repulsion steepness. This framework allows one to tell apart systems where “soft atoms make strong glasses” from those where, instead, “soft atoms make fragile glasses.” Hence, it opens up the way for the rational, atomistic tuning of the fragility and viscosity of widely different glass-forming materials all the way from strong to fragile.

Strontium doping transforms manganites of type $\mathrm{L}{\mathrm{a}}_{1-x}\mathrm{S}{\mathrm{r}}_x\mathrm{Mn}{\mathrm{O}}_3$ from an insulating antiferromagnet $(x=0)$ to a metallic ferromagnet $(x>0.16)$ due to the induced charge carriers (holes). Neutron scattering experiments were employed to investigate the effect of Sr doping on a tailor-made compound of composition $\mathrm{L}{\mathrm{a}}_{0.7}\mathrm{S}{\mathrm{r}}_{0.3}\mathrm{M}{\mathrm{n}}_{0.1}\mathrm{T}{\mathrm{i}}_{0.3}\mathrm{G}{\mathrm{a}}_{0.6}{\mathrm{O}}_3$. By the simultaneous doping with $\mathrm{S}{\mathrm{r}}^{2+}$ and $\mathrm{T}{\mathrm{i}}^{4+}$ ions, the compound remains in the insulating state so that the magnetic interactions for large Sr doping can be studied in the absence of charge carriers. At $T_{\mathrm{C}}=215\mathrm{K}$, there is a first-order reconstructive phase transition from the trigonal $R-3c$ structure to the orthorhombic Pnma structure via an intermediate virtual configuration described by the common monoclinic subgroup $P{2}_1/c$. The magnetic excitations associated with $\mathrm{M}{\mathrm{n}}^{3+}$ dimers give evidence for two different nearest-neighbor ferromagnetic exchange interactions, in contrast to the undoped compound $\mathrm{LaM}{\mathrm{n}}_y{\mathrm{A}}_{1-y}{\mathrm{O}}_3$ where both ferromagnetic and antiferromagnetic interactions are present. The doping-induced changes of the exchange coupling originates from different Mn-O-Mn bond angles determined by neutron diffraction. The large fourth-nearest-neighbor interaction found for metallic manganites is absent in the insulating state. We argue that the Ruderman-Kittel-Kasuya-Yosida interaction reasonably accounts for all the exchange couplings derived from the spin-wave dispersion in metallic manganites.

The magnetic resonance properties of microsized monoaxial chiral crystals of ${\mathrm{CrNb}}_3{\mathrm{S}}_6$ are investigated. We observed that the resonance of the chiral soliton lattice is sensitive to the polarization of the driving microwave field. When the microwave field is parallel to the helical axis, the resonance is symmetric with regards to the magnetic field direction. In contrast, asymmetric field dependence emerges when the microwave field is perpendicular to the helical axis. The robustness of the chiral magnetic order, due to topological protection, allows tuning the resonance frequency in ways hardly accessible using nanopatterned films.