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Register a new userOptical properties of the charge-density-wave polychalcogenide compounds $R_2$Te$_5$ ($R$=Nd, Sm and Gd)
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We investigate the rare-earth polychalcogenide $R_2$Te$_5$ ($R$=Nd, Sm and Gd) charge-density-wave (CDW) compounds by optical methods. From the absorption spectrum we extract the excitation energy of the CDW gap and estimate the fraction of the Fermi surface which is gapped by the formation of the CDW condensate. In analogy to previous findings on the related $R$Te$_n$ (n=2 and 3) families, we establish the progressive closing of the CDW gap and the moderate enhancement of the metallic component upon chemically compressing the lattice.
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Diffraction measurements performed via transmission electron microscopy and high resolution X-ray scattering reveal two distinct charge density wave transitions in Gd$_2$Te$_5$ at $T_{c1}$ = 410(3) and $T_{c2}$ = 532(3) K, associated with the textit{on}-axis incommensurate lattice modulation and textit{off}-axis commensurate lattice modulation respectively. Analysis of the temperature dependence of the order parameters indicates a non-vanishing coupling between these two distinct CDW states.
The antiferromagnetic transition is investigated in the rare-earth (R) tritelluride RTe3 family of charge density wave (CDW) compounds via specific heat, magnetization and resistivity measurements. Observation of the opening of a superzone gap in the resistivity of DyTe3 indicates that additional nesting of the reconstructed Fermi surface in the CDW state plays an important role in determining the magnetic structure.
The intermetallic compound LaAgSb2 displays two charge-density-wave (CDW) transitions, which were detected with measurements of electrical resistivity (rho), magnetic susceptibility, and X-ray scattering; the upper transition takes place at T1 approx. 210 K, and it is accompanied by a large anomaly in rho(T), whereas the lower transition is marked by a much more subtle anomaly at T2 approx. 185 K. We studied the effect of hydrostatic pressure (P) on the formation of the upper CDW state in pure and doped La1-xRxAgSb2 (R = Ce, Nd) compounds, by means of measurements of rho(T) for P < 23 kbar. We found that the hydrostatic pressure, as well as the chemical pressure introduced by the partial substitution of the smaller Ce and Nd ions for La, result in the suppression of the CDW ground state, e.g. the reduction of the ordering temperature T1. The values of dT1/dP are approx. 2-4 times higher for the Ce-doped samples as compared to pure LaAgSb2, or even La0.75Nd0.25AgSb2 Nd-doped with a comparable T1 (P=0). This increased sensitivity to pressure may be due to increasing Ce- hybridization under pressure. The magnetic ordering temperature of the cerium-doped compounds is also reduced by pressure, and the high pressure behavior of the Ce-doped samples is dominated by Kondo impurity scattering.
Versatile features of impurity doping effects on perovskite manganites, $R_{0.6}$Sr$_{0.4}$MnO$_{3}$, have been investigated with varying the doing species as well as the $R$-dependent one-electron bandwidth. In ferromagnetic-metallic manganites ($R$=La, Nd, and Sm), a few percent of Fe substitution dramatically decreases the ferromagnetic transition temperature, leading to a spin glass insulating state with short-range charge-orbital correlation. For each $R$ species, the phase diagram as a function of Fe concentration is closely similar to that for $R_{0.6}$Sr$_{0.4}$MnO$_{3}$ obtained by decreasing the ionic radius of $R$ site, indicating that Fe doping in the phase-competing region weakens the ferromagnetic double-exchange interaction, relatively to the charge-orbital ordering instability. We have also found a contrastive impact of Cr (or Ru) doping on a spin-glass insulating manganite ($R$=Gd). There, the impurity-induced ferromagnetic magnetization is observed at low temperatures as a consequence of the collapse of the inherent short-range charge-orbital ordering, while Fe doping plays only a minor role. The observed opposite nature of impurity doping may be attributed to the difference in magnitude of the antiferromagnetic interaction between the doped ions.
The La and Ce di-tellurides LaTe$_2$ and CeTe$_2$ are deep in the charge-density-wave (CDW) ground state even at 300 K. We have collected their electrodynamic response over a broad spectral range from the far infrared up to the ultraviolet. We establish the energy scale of the single particle excitation across the CDW gap. Moreover, we find that the CDW collective state gaps a very large portion of the Fermi surface. Similarly to the related rare earth tri-tellurides, we envisage that interactions and Umklapp processes play a role in the onset of the CDW broken symmetry ground state.
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