We report the observation of highly anisotropic Dirac fermions in a Bi square net of SrMnBi(2), based on a first-principles calculation, angle-resolved photoemission spectroscopy, and quantum oscillations for high-quality single crystals. We found that the Dirac dispersion is generally induced in the (SrBi)(+) layer containing a double-sized Bi square net. In contrast to the commonly observed isotropic Dirac cone, the Dirac cone in SrMnBi(2) is highly anisotropic with a large momentum-dependent disparity of Fermi velocities of ~8. These findings demonstrate that a Bi square net, a common building block of various layered pnictides, provides a new platform that hosts highly anisotropic Dirac fermions.

Spin-orbit coupling results in technologically-crucial phenomena underlying magnetic devices like magnetic memories and energy-efficient motors. In heavy element materials, the strength of spin-orbit coupling becomes large to affect the overall electronic nature and induces novel states such as topological insulators and spin-orbit-integrated Mott states. Here we report an unprecedented charge-ordering cascade in IrTe 2 without the loss of metallicity, which involves localized spin-orbit Mott states with diamagnetic Ir 4 þ -Ir 4 þ dimers. The cascade in cooling, uncompensated in heating, consists of first order-type consecutive transitions from a pure Ir 3 þ phase to Ir 3 þ -Ir 4 þ charge-ordered phases, which originate from Ir 5d to Te 5p charge transfer involving anionic polymeric bond breaking. Considering that the system exhibits superconductivity with suppression of the charge order by doping, analogously to cuprates, these results provide a new electronic paradigm of localized charge-ordered states interacting with itinerant electrons through large spin-orbit coupling.

Excitonic insulator (EI) was proposed in 60's as a distinct insulating state originating from pure electronic interaction, but its material realization has been elusive with extremely few material candidates and with only limited evidence such as anomalies in transport properties, band dispersions, or optical transitions. We investigate the real-space electronic states of the low temperature phase in Ta2NiSe5 with an atomic resolution to clearly identify the quasiparticle energy gap together with the strong electron-hole band renormalization using scanning tunneling microscopy (STM) and spectroscopy (STS). These results are in good agreement with the EI transition scenario in Ta2NiSe5. Our spatially-resolved STS data and theoretical calculations reveal further the orbital inversion at band edges, which indicates the exciton condensation close to the Bardeen-Cooper-Schrieffer regime. arXiv:1811.01552v1 [cond-mat.mtrl-sci]

We report the valley-selective interlayer conduction of SrMnBi 2 under in-plane magnetic fields. The c-axis resistivity of SrMnBi 2 shows clear angular magnetoresistance oscillations indicating coherent interlayer conduction. Strong fourfold variation of the coherent peak in the c-axis resistivity reveals that the contribution of each Dirac valley is significantly modulated by the in-plane field orientation. This originates from anisotropic Dirac Fermi surfaces with strong disparity in the momentum-dependent interlayer coupling. Furthermore, we found a signature of broken valley symmetry at high magnetic fields. These findings demonstrate that a quasi-two-dimensional anisotropic Dirac system can host a valley-polarized interlayer current through magnetic valley control.

The effects of isovalent Ru substitution at the Fe sites of BaFe2−xRuxAs2 are investigated by measuring resistivity (ρ) and Hall coefficient(RH ) on high-quality single crystals in a wide range of doping (0 ≤ x ≤ 1.4). Ru substitution weakens the antiferromagnetic (AFM) order, inducing superconductivity for relatively high doping level of 0.4 ≤ x ≤ 0.9. Near the AFM phase boundary, the transport properties show non-Fermi-liquid-like behavior with a linear-temperature dependence of ρ and a strong temperature dependence of RH with a sign change. Upon higher doping, however, both ρ and RH recover conventional Fermi-liquid behavior. Strong doping dependence of RH together with a small magnetoresistance suggest that the anomalous transport properties can be explained in terms of anisotropic charge carrier scattering due to interband AFM fluctuations rather than a conventional multi-band scenario.PACS numbers: 74.70. Xa, 74.25.Dw, 74.40.Kb Unconventional superconductivity in the proximity of an antiferromagnetic (AFM) phase has been intensively studied on high-T c cuprates, heavy-fermion superconductors and the recently-discovered Fe-pnictides.1 In spite of subtle differences in their detailed properties, there is a growing body of evidence that they all exhibit a common phase diagram where inside a dome-shaped regime a superconducting phase appears as the AFM phase is suppressed by an external control parameter, such as doping or pressure. Even though the static AFM order is fully suppressed, AFM fluctuations survive and they can strongly modify the quasi-particle scattering spectrum, leading e. g. to the so-called non-Fermi-liquid-like transport properties. Clarifying the nature of the nonfermi-liquid behavior and understanding its relation to superconductivity are key issues for elucidating the unconventional superconductivity in Fe-pnitides.In Fe-pnictides, the AFM instability is closely related to the interband nesting between electron-and holeFermi surfaces (FS's).2-4 Degrading the nesting condition by introducing additional charge carriers or modifying the crystal structure suppress the AFM order and eventually induce superconductivity. So far, various types of chemical substitution have been employed in order to explore the phase diagram of Fe pnictides. In the so-called 122 pnictides, the substitution dependence of the electrical transport properties has been intensively studied e.g. for BaFe 2 As 2 with K-5 , Co-6-9 and P-substitution 10 at the Ba, Fe and As sites, respectively. Deviations from a T 2 -power-law dependence of the electrical resistivity (ρ) or the enhancement of the Hall coefficient (R H ) at low temperatures 8,10 have been commonly observed in various Fe-pnictides. These observations are usually considered as experimental indication for non-Fermi-liquid behavior. In Fe-pnictides, however, such deviations have also been ascribed to multi-band transport 8,9 , where a conventional description of different types of carriers is sufficient.In this respect, it is interesting to investigate how th...