Doping-induced variations of the Fermi level in calcium-containing Y-based HTSC and their influence on the critical temperature

Thermopower and Nernst coefficient in the Y0.85Ca0.15Ba2−xLaxCu3Oysystem: experimental results and joint quantitative analysis

“…The observed modification of the F(x) dependence in samples with the additional Ca→Y substitution is due to the presence of additional states in the band. As shown in [41], the calciuminduced peak is located at energies corresponding to the Fermi level position in the Y 0.85 Ca 0.15 Ba 2 Cu 3 O y sample. As a result, when doped by lanthanum (that should lead to a rise in the number of electrons in the band [40]) the effect of the Fermi level pinning inside the Ca-induced peak in the DOS function is realized, with this effect being more pronounced at low La contents [41].…”

“…As shown in [41], the calciuminduced peak is located at energies corresponding to the Fermi level position in the Y 0.85 Ca 0.15 Ba 2 Cu 3 O y sample. As a result, when doped by lanthanum (that should lead to a rise in the number of electrons in the band [40]) the effect of the Fermi level pinning inside the Ca-induced peak in the DOS function is realized, with this effect being more pronounced at low La contents [41]. Note, that this effect can also be considered as the reason for the observed S(T=300 K) constancy at x=0-0.1 (see figure 4).…”

“…Second, and more important, the range of lanthanum content where a drop of T c is not observable becomes wider. The latter is most probably related to the effect of the Fermi level pinning inside the calcium-induced peak in the DOS function resulting in an unchanged value of the DOSs at the Fermi level [41].…”

“…Note that when discussing this issue, it is more appropriate to use the C≡W σ /W D value instead of the W σ one. According to our previous results [4,14,27,28,32,37,[39][40][41], the main mechanism responsible for the band transformation under doping in different HTSC systems is the Anderson localization of states at the band edges caused by a disorder in oxygen and/or cation sublattices. The variation of the above ratio gives information on the change in this localization degree, namely, the lower the C value the stronger the state localization.…”

“…It is necessary to stress that the observed variations in the energy spectrum parameters make it possible to explain qualitatively the critical temperature dependence on the lanthanum content in the Y 0.85 Ca 0.15 Ba 2−x La x Cu 3 O y system (see figure 1). The variation of T c in doped HTSC was earlier shown to be due to the change in the D(E F ) value induced by doping, which is determined, in its turn, by a variation of the bandwidth and, to a smaller extent, by a Fermi level shift [4,14,41]. In the studied systems, the W D value changes very slightly at low La contents and the Fermi level in this doping range is pinned inside the calcium peak.…”