The pyrochlore-type iridium oxide Eu${}_2$Ir${}_2$O${}_7$ exhibits a metal-insulator transition at 120 K, accompanied by magnetic ordering. We used x-ray diffraction measurements with photon energies near the iridium $L_3$ absorption edge to analyze a single crystal to investigate the arrangement of Ir${}^{4+}$ magnetic moments. Resonant magnetic scattering at $\left(4n+200\right)$ was observed in the insulating phase, providing direct evidence of long-range ordering of Ir${}^{4+}$ magnetic moments with a propagation vector of $\mathit{q}=\left(000\right)$. Our single-crystal structure analysis revealed that the lattice retains its face-centered-cubic structure across the metal-insulator transition, indicating all-in-all-out magnetic order, where all the magnetic moments on the four vertices of each Ir tetrahedron point inward or outward.

In this Rapid Communication we show that Rashba-based topological superconductor nanowires, where the spin-orbit coupling may change its sign, support three topological phases protected by chiral symmetry. When a superconducting phase gradient is applied over the interface of the two nontrivial phases, the Andreev spectrum is qualitatively phase shifted by $\pi$ compared to usual Majorana weak links. The topological $\pi$ junction has the striking property of exhibiting a maximum supercurrent in the vicinity of vanishing phase difference. Qualitative features of the junction are robust against disorder and magnetic fields violating chiral symmetry. The studied system could be realized by local gating of the wire or by an appropriate stacking of permanent magnets in synthetic Rashba systems.

We performed an optical spectroscopy measurement on single crystals of Na${}_2$Ti${}_2$Sb${}_2$O, which is a parent compound for the newly discovered Ti-based superconductors. The study reveals a significant spectral change across the phase transition at 114 K and formation of a density wave type energy gap. The opening of the gap removes most of the free carrier spectral weight and causes a dramatic reduction in the carrier scattering rate. The ratio of $2\Delta /k_B{T}_s\approx 14$ is considerably larger than the mean-field value based on the weak-coupling BCS theory. The study also reveals a weak correlation effect in the titanium oxypnictides.

We construct a phenomenological $Z_2$ lattice gauge theory to describe the spin-liquid state of the $S=\frac{1}{2}$ kagome Heisenberg antiferromagnet in the sector with zero total spin. The model is a natural generalization of the Misguich-Serban-Pasquier Hamiltonian with added interactions that are strongly constrained by lattice symmetries. We are able to reproduce qualitatively many of the characteristic features observed in recent numerical studies. We also make connections to the previous works along similar lines.

We present lateral spin-valve measurements where spin pumping serves as the spin current injection mechanism. The first electrode is excited at ferromagnetic resonance and the voltage drop between the interconnecting nonmagnetic channel and the second electrode with static magnetization is detected. We find a voltage difference of 10 nV between parallel and antiparallel magnetization alignment of the electrodes caused by spin-dependent differences in the chemical potential. The experimental value is in good agreement with a theoretical estimation of 12 nV. Our interpretation of the voltage signals is supported by simultaneous broadband-ferromagnetic resonance measurements.

Topological insulator quantum wells with induced attractive interactions between electrons are candidate systems for the realization of vortex lattice states and incompressible quantum vortex liquids with fractional excitations. These phases depend on the formation of low-energy spin-triplet Cooper pairs in the quantum well, under the combined influence of the superconducting proximity effect and Rashba spin-orbit coupling. We analyze, using the Landau-Ginzburg framework, the competition between different pairing channels stimulated by the proximity effect and argue that correlated phases of triplets are likely accessible by tuning the gate voltage. We perturbatively calculate the triplet pairing instability and show that it occurs at finite momenta. This yields a spatially inhomogeneous superconducting state, which is likely a vortex lattice of spin supercurrents. We discuss the phase diagram of topological insulator quantum wells tunable by the gate voltage.

The temperature dependence of the thermal expansion coefficient $\alpha$ has been measured in URu${}_2$Si${}_2$ under uniaxial pressure $\sigma$ along the [100] and [110] directions up to $\sim 0.6$ GPa. For both cases, the hidden-order phase transition temperature $T_0$ increases with increasing uniaxial pressure. An anomaly $\Delta \alpha$ of the thermal expansion coefficient at $T_0$ appears to be enhanced under a small uniaxial pressure ($<0.03$ GPa). An antiferromagnetically ordered phase appears at $T_X$ above the critical pressure ${\sigma }_X\sim 0.25$ GPa for uniaxial pressure along the [100] direction, whereas no antiferromagnetic transition is detected up to $\sim 0.6$ GPa for uniaxial pressure along the [110] direction, indicating that the critical pressure is beyond $\sim 0.6$ GPa for the [110] case. In the previously reported hydrostatic case, an antiferromagnetic phase transition appears above the critical pressure $P_X\sim 0.5$ GPa. The difference in critical pressure between ${\sigma }_X$ and $P_X$ can be understood in terms of the pressure dependence of the U-U distance in the basal (001) plane.

We discuss the electronic structure, lattice dynamics, and electron-phonon interaction of the newly discovered superconductor LaO${}_{0.5}$F${}_{0.5}$BiS${}_2$ using density-functional-based calculations. A strong Fermi surface nesting at $\mathbf{k}=(\pi ,\pi ,0)$ suggests a proximity to charge-density-wave instability and leads to imaginary harmonic phonons at this $\mathbf{k}$ point associated with in-plane displacements of S atoms. Total energy analysis resolves only a shallow double-well potential well preventing the appearance of static long-range order. Both harmonic and anharmonic contributions to electron-phonon coupling are evaluated and give a total coupling constant $\lambda \simeq 0.85$, prompting this material to be a conventional superconductor contrary to structurally similar FeAs materials.

The origins of the commonly observed green emission (GE) from ZnO nanostructures remain highly controversial despite extensive studies over the past few decades. Herein, through a comprehensive ultrafast optical spectroscopy study, new insights into its origin and the charge trapping dynamics at the GE centers in ZnO nanowires prepared by the vapor transport method are gained. Transient absorption spectroscopy (TAS) revealed a sub-band-gap absorption bleaching band arising from the state filling of the electrons in the conduction band and holes trapped in the GE centers. The GE originates from the recombination between the electrons in the conduction band and/or shallow donor levels and the holes trapped at the GE centers (which are located at ∼0.88 eV above the valence band). Importantly, an ultrafast excitonic Auger-type hole trapping process to the GE centers occurring in a subpicosecond time scale was also uncovered by TAS—shedding new light on the mechanism behind the fast and efficient charge trapping of photoexcited carriers. The knowledge gained is crucial for the development of ZnO-based optoelectronic devices.

We present atomistic theory of emission from dark excitons in self-assembled quantum dots. Using the atomistic tight-binding model coupled with the configuration-interaction approach, we calculate the emission spectrum of a dark exciton confined in a single, self-assembled quantum dot. We demonstrate that the dark exciton state is optically active and that it emits photons in the plane of the dot, polarized along its vertical axis. This finite radiative lifetime is due to the mixing of hole subbands and can be tuned over several orders of magnitude by engineering the size and shape of the quantum dot.

Quantum wires subject to the combined action of spin-orbit and Zeeman coupling in the presence of $s$-wave pairing potentials (superconducting proximity effect in semiconductors or superfluidity in cold atoms) are one of the most promising systems for the developing of topological phases hosting Majorana fermions. The breaking of time-reversal symmetry is essential for the appearance of unpaired Majorana fermions. By implementing a time-dependent spin rotation, we show that the standard magnetostatic model maps into a nonmagnetic one where the breaking of time-reversal symmetry is guaranteed by a periodical change of the spin-orbit coupling axis as a function of time. This suggests the possibility of developing the topological superconducting state of matter driven by external forces in the absence of magnetic fields and magnetic elements. From a practical viewpoint, the scheme avoids the disadvantages of conjugating magnetism and superconductivity, even though the need of a high-frequency driving of spin-orbit coupling may represent a technological challenge. We describe the basic properties of this Floquet system by showing that finite samples host unpaired Majorana fermions at their edges despite the fact that the bulk Floquet quasienergies are gapless and that the Hamiltonian at each instant of time preserves time-reversal symmetry. Remarkably, we identify the mean energy of the Floquet states as a topological indicator. We additionally show that the localized Floquet Majorana fermions are robust under local perturbations. Our results are supported by complementary numerical Floquet simulations.

The thermal conductivities of graphene and graphite are computed within the framework of Callaway's effective relaxation time theory. Analytical expressions derived by Nihira and Iwata for phonon dispersion relations and vibrational density of states are employed, based on the semicontinuum model proposed by Komatsu and Nagamiya. The conductivity of graphene is predicted to be higher than the in-plane conductivity of graphite for all temperatures. Incorporation of the ${}^{13}$C isotope can be expected to produce significant reduction in the conductivity of graphene in the temperature range 50–300 K. In the presence of tensile strain on graphene, the specific heat increases, but the conductivity can decrease or increase depending on the level of the purity and temperature of the sample.

Recently a new high-mobility Dirac material, trilayer graphene, was realized experimentally. The band structure of $ABA$-stacked trilayer graphene consists of a monolayer-like and a bilayer-like pair of bands. Here we study electronic properties of $ABA$-stacked trilayer graphene biased by a perpendicular electric field. We find that the combination of the bias and trigonal warping gives rise to a set of new Dirac points: In each valley, seven species of Dirac fermions with small masses of order of a few meV emerge. The positions and masses of the emergent Dirac fermions are tunable by bias, and one group of Dirac fermions becomes massless at a certain bias value. Therefore, in contrast to bilayer graphene, the conductivity at the neutrality point is expected to show nonmonotonic behavior, becoming of the order of a few $e^2/h$ when some Dirac masses vanish. Further, we analyze the evolution of the Landau level spectrum as a function of bias. The emergence of new Dirac points in the band structure translates into new threefold-degenerate groups of Landau levels. This leads to an anomalous quantum Hall effect, in which some quantum Hall steps have a height of $3e^2/h$. At an intermediate bias, the degeneracies of all Landau levels get lifted, and in this regime all quantum Hall plateaus are spaced by $e^2/h$. Finally, we show that the pattern of Landau level crossings is very sensitive to certain band structure parameters, and can therefore provide a useful tool for determining their precise values.

In this paper, we present a systematic theoretical investigation of the phthalocyanine based dimer MnPc${}^{\delta +}$/F${}_{16}$CoPc${}^{\delta -}$ within density functional theory framework. For all considered stacking geometries, a charge transfer from MnPc to F${}_{16}$CoPc is observed which is therefore identified as an intrinsic property of the molecular pair. In addition, a ferromagnetic coupling of the molecular magnetic moments within the dimer is always observed due to 90${}^{\circ }$ superexchange and direct-exchange contributions. The comparison of the calculated absorption spectra of the different stacking arrangements to the experimentally recorded electron energy-loss spectrum strongly indicates the $\beta$ stacking to be preferred over all others. Only for this geometry an experimentally observed excitation around 0.6 eV is reproduced by the calculation.

Pressure-dependent transport measurements of Ir${}_{1-x}$Pt${}_x$Te${}_2$ are reported. With increasing pressure, the structural phase transition at high temperatures is enhanced while its superconducting transition at low temperatures is suppressed. These pressure effects make Ir${}_{1-x}$Pt${}_x$Te${}_2$ distinct from other studied $\mathit{TX}$${}_2$ systems exhibiting a charge density wave (CDW) state, in which pressure usually suppresses the CDW state and enhances the superconducting state. The results reveal that the emergence of superconductivity competes with the stabilization of the low-temperature monoclinic phase in Ir${}_{1-x}$Pt${}_x$Te${}_2$.

Muon-spin relaxation (${\mu }^{+}$SR) is a sensitive probe of magnetism, but its utility can be severely limited by the lack of knowledge of the muon implantation site and the extent to which the muon perturbs its host. We demonstrate systematically that these problems can be addressed accurately using electronic-structure calculations. We show that diamagnetic muons introduce significant short-ranged distortions in ionic insulators that would lead to systematic errors on magnetic moments determined by ${\mu }^{+}$SR, and quantify these. The F-$\mu$-F complex formed by muons in many fluorides can be understood as an exotic molecule-in-a-crystal defect with a zero-point energy larger than that of any naturally occurring triatomic molecule.

We report on variable temperature magnetic force microscopy studies of a strain-enabled multiferroic EuTiO${}_3$ film epitaxially grown on a (110)-oriented DyScO${}_3$ substrate. Our temperature- and magnetic-field-dependent studies clearly reveal an inhomogeneous magnetic state with the coexistence of ferromagnetic and nonferromagnetic states at low magnetic fields, which provides a microscopic origin of the anomalous missing moment in previous studies [Lee et al., Nature (London) 466, 954 (2010)]. The spins of the nonferromagnetic phase can be aligned by modest magnetic fields (>1.5 T). The observed magnetic inhomogeneity probably originates from the coexistence of nearly degenerate magnetic ground states.

We observe optical third harmonic generation from graphene and few-layer graphite flakes produced by exfoliation. The emission scales with the cube of the intensity of the incident near-infrared femtosecond pulses and has a wavelength that is one-third of the incident wavelength, both consistent with third harmonic generation. We extract an effective third-order susceptibility for graphene that is on the order of 10${}^{-16}$ m${}^2$/V${}^2$, which is comparable to that for materials that are resonantly excited, but larger than for materials that are transparent at the fundamental and third harmonic wavelengths.

We report the temperature dependence of magnetic hyperfine field, $B_{\mathrm{hf}}$, for ${}^{111}$Cd impurity in nanocrystalline CePd${}_3$ measured using the time differential perturbed angular correlation technique. With a reduction of particle size, we find progressive increase in the hyperfine field of Cd which shows Curie-Weiss-like temperature dependence for nanoparticles smaller than 60 nm in size. Supplemented by bulk susceptibility and isothermal magnetization measurements, our data show microscopic evidence for $4f$ localization in nanoparticles of correlated electron system CePd${}_3$.

The first few low-lying spin states of alternant polycyclic aromatic hydrocarbon (PAH) molecules of several shapes showing defect states induced by contour hydrogenation have been studied both by ab initio methods and by a precise numerical solution of Pariser-Parr-Pople (PPP) interacting model. In accordance with Lieb's theorem, the ground state shows a spin multiplicity equal to one for balanced molecules, and it gets larger values for imbalanced molecules (that is, when the number of $\pi$ electrons on both subsets is not equal). Furthermore, we find a systematic decrease of the singlet-triplet splitting as a function of the distance between defects, regardless of whether the ground state is singlet or triplet. For example, a splitting smaller than 0.001 eV is obtained for a medium size C${}_{46}$H${}_{28}$ PAH molecule (di-hydrogenated [11]phenacene) showing a singlet ground state. We conclude that $\pi$ electrons unbound by lattice defects tend to remain localized and unpaired even when long-range Coulomb interaction is taken into account. Therefore they show a biradical character (polyradical character for more than two defects) and should be studied as two or more local doublets. The implications for electron transport are potentially important since these unpaired electrons can trap traveling electrons or simply flip their spin at a very small energy cost.