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Register a new userEvidence for Oxygen Holes due to d-p Rehybridization in Thermoelectric Sr_{1-x}Rh_{2}O_{4}
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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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We present the electronic structure of Sr_{1-(x+y)}La_{x+y}Ti_{1-x}Cr_{x}O_{3} investigated by high-resolution photoemission spectroscopy. In the vicinity of Fermi level, it was found that the electronic structure were composed of a Cr 3d local state with the t_{2g}^{3} configuration and a Ti 3d itinerant state. The energy levels of these Cr and Ti 3d states are well interpreted by the difference of the charge-transfer energy of both ions. The spectral weight of the Cr 3d state is completely proportional to the spin concentration x irrespective of the carrier concentration y, indicating that the spin density can be controlled by x as desired. In contrast, the spectral weight of the Ti 3d state is not proportional to y, depending on the amount of Cr doping.
We present a study of the evolution of magnetism from the quantum critical system YbRh2Si2 to the stable trivalent Yb system YbCo2Si2. Single crystals of Yb(Rh_(1-x)Co_x)2Si2 were grown for 0 < x < 1 and studied by means of magnetic susceptibility, electrical resistivity, and specific heat measurements, as well as photoemission spectroscopy. The results evidence a complex magnetic phase diagram, with a non-monotonic evolution of T_N and two successive transitions for some compositions resulting in two tricritical points. The strong similarity with the phase diagram of YbRh2Si2 under pressure indicates that Co substitution basically corresponds to the application of positive chemical pressure. Analysis of the data proves a strong reduction of the Kondo temperature T_K with increasing Co content, T_K becoming smaller than T_N for x ~ 0.5, implying a strong localization of the 4f electrons. Furthermore, low-temperature susceptibility data confirm a competition between ferromagnetic and antiferromagnetic exchange. The series Yb(Rh_(1-x)Co_x)2Si2 provides an excellent experimental opportunity to gain a deeper understanding of the magnetism at the quantum critical point in the vicinity of YbRh2Si2 where the antiferromagnetic phase disappears (T_N=>0).
The specific heat of (Na,Ca)Co_{2}O_{4} is measured at low-temperatures to determine the magnitude of the electronic specific-heat coefficient gamma, in an attempt to gain an insight into the origin of the unusually large thermoelectric power of this compound. It is found that gamma is as large as 48 mJ/molK^2, which is an order of magnitude larger than gamma of simple metals. This indicates that (Na,Ca)Co_{2}O_{4} is a strongly-correlated electron system, where the strong correlation probably comes from the low-dimensionality and the frustrated spin structure. We discuss how the large thermopower and its dependence on Ca doping can be understood with the strong electron correlations.
We investigate the effect of Ni${text -}$substitution on the crystalline structure and the critical behavior of $Nd_{0.6}Sr_{0.4}Mn_{1-x}Ni_{x}O_{3}$ (0.00 $leq$ x $leq$ 0.20) perovskite. X${text -}$ray diffraction patterns revealed that the major phase in all samples is the orthorhombic structure with space group $textit{Pnma}$. Rietveld refinement revealed a linear reduction in the lattice parameters along with monotonic reduction in the O2${text -}$Mn${text -}$O2 angel with increasing Ni concentration. The modified Arrott plots and the Kouvel${text -}$Fisher method have been used to analyze the magnetization isotherms near the paramagnetic to ferromagnetic (PM${text -}$FM) phase transition. The obtained critical exponents ($beta$, $gamma$ and $delta$) revealed that the Ni${text -}$free sample is consistent with 3D${text -}$Heisenberg like behavior. However, upon Ni${text -}$substitution, the critical exponents exhibit a mean field like behavior. The reliability of the obtained critical exponent ($beta$, $gamma$ and $delta$) values have been confirmed by the universal scaling behavior of the isothermal magnetization near the transition temperature.
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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