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Register a new userEvidence for Undoped Weyl Semimetal Charge Transport in $Y_{2}Ir_{2}O_{7}$
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Weyl fermions scattering from a random Coulomb potential are predicted to exhibit resistivity versus temperature $rho space alpha space T^{-4}$ in a single particle model. Here we show that, in closed environment-grown polycrystalline samples of $Y_{2}Ir_{2}O_{7}$, $rho = rho_{0} T^{-4}$ over four orders of magnitude in $rho$. While the measured prefactor, $rho_{0}$, is obtained from the model using reasonable materials parameters, the $T^{-4}$ behavior extends far beyond the models range of applicability. In particular, the behavior extends into the low-temperature, high-resistivity region where the Ioffe-Regel parameter, $k_{T} ell ll 2pi$. Strong on-site Coulomb correlations, instrumental for predicting a Weyl semimetal state in $Y_{2}Ir_{2}O_{7}$, are the possible origin of such bad Weyl semimetal behavior.
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We investigated the electronic structures of the bandwidth-controlled ruthenates, Y$_{2}$Ru$_{2}$O$_{7}$, CaRuO$_{3}$, SrRuO$_{3}$, and Bi$_{2}$Ru$% _{2}$O$_{7}$, by optical conductivity analysis in a wide energy region of 5 meV $sim $ 12 eV. We could assign optical transitions from the systematic changes of the spectra and by comparison with the O 1$s$ x-ray absorption data. We estimated some physical parameters, such as the on-site Coulomb repulsion energy and the crystal-field splitting energy. These parameters show that the 4$d$ orbitals should be more extended than 3$d$ ones. These results are also discussed in terms of the Mott-Hubbard model.
We report detailed magneto-transport measurements on single crystals of the magnetic Weyl semi-metal Co$_{3}$Sn$_{2}$S$_{2}$. Recently a large anomalous Hall effect and chiral anomaly have been observed in this material which have been suggested to be related to the large Berry curvature between the Weyl points (Liu et al., Nature Physics (2018).). Another effect expected to result from the topological band structure of magnetic Weyl materials is the planar Hall effect (PHE). In this work we report observation of this intrinsic effect in single crystals of Co$_{3}$Sn$_{2}$S$_{2}$. Crucially, the PHE is observed for temperature $T leq 74$~K which is much smaller than the ferromagnetic ordering temperature $T_c = 175$~K@. Together with the large anomalous Hall conductivity, this further demonstrates the Topological character of Co$_3$Sn$_2$S$_2$.
While pyrochlore iridate thin films are theoretically predicted to possess a variety of emergent topological properties, experimental verification of these predictions can be obstructed by the challenge in thin film growth. Here we report on the pulsed laser deposition and characterization of thin films of a representative pyrochlore compound Bi2Ir2O7. The films were epitaxially grown on yttria-stabilized zirconia substrates and have lattice constants that are a few percent larger than that of the bulk single crystals. The film composition shows a strong dependence on the oxygen partial pressure. Density-functional-theory calculations indicate the existence of Bi_Ir antisite defects, qualitatively consistent with the high Bi: Ir ratio found in the films. Both Ir and Bi have oxidation states that are lower than their nominal values, suggesting the existence of oxygen deficiency. The iridate thin films show a variety of intriguing transport characteristics, including multiple charge carriers, logarithmic dependence of resistance on temperature, antilocalization corrections to conductance due to spin-orbit interactions, and linear positive magnetoresistance.
We have used rotational anisotropic polarized Raman spectroscopy to study the symmetries, the temperature and the doping dependence of the charge ordered state in metallic $(Sr_{1-x}La_{x})_{3}Ir_{2}O_{7}$. Although the Raman probe size is greater than the charge ordering length, we establish that the charge ordering breaks the fourfold rotational symmetry of the underlying tetragonal crystal lattice into twofold, as well as the translational symmetry, and forms short-range domains with $90^{circ}$ rotated charge order wave vectors, as soon as the charge order sets in below $T_{CO} = sim$ 200K and across the doping-induced insulator metal transition. We observe that this charge order mode frequency remains nearly constant over a wide temperature range and up to the highest doping level. These above features are highly reminiscent of the ubiquitous unidirectional charge order in underdoped high-$T_C$ copper-oxide-based superconductors (cuprates). We further resolve that the charge order damping rate diverges when approaching $T_{CO}$ from below and increases significantly as increasing the La doping level, which resembles the behaviors for a disorder-interrupted ordered phase and has not been observed for the charge order in cuprates.
Soft-x-ray photoemission and absorption spectroscopies are employed to investigate the electronic structures of Sr_{1-x}Rh_{2}O_{4}. Similarly to the layered cobaltates such as Na_{1-x}CoO_{2}, a valence-band satellite feature (VBS) occurs at higher binding energy to the O 2p band. We find that the VBS resonates at the O 1s edge. Additionally, core absorption shows clear x dependence in the O 1s edge rather than in the Rh 3p edge. These results indicate that the holes in the initial state mainly have O 2p character presumably due to d-p rehybridizations affected by Sr^{2+} vacancy potentials. The resultant inhomogenous charge texture may have impact on the TE transport properties at low x.
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