Search

Among the Fe-based superconductors, Fe$_{1+y}$Te$_{1-x}$Se$_{x}$ is unique in that its crystal structure is the simplest and the electron correlation level is the strongest, and thus it is important to investigate the doping($x$)-temperature ($T$) phase diagram of this system. However, inevitably incorporated excess Fe currently prevents the establishment of the true phase diagram. We overcome the aforementioned significant problem via developing a new annealing method termed as "Te-annealing" wherein single crystals are annealed under Te vapor. Specifically, we conducted various magnetotransport measurements on Te-annealed superconducting Fe$_{1+y}$Te$_{1-x}$Se$_{x}$. We observed that crossover from the incoherent to the coherent electronic state and opening of the pseudogap occurs at high temperatures ($\approx$ 150 K for $x$ = 0.2). This is accompanied by a more substantial pseudogap and the emergence of a phase with a multi-band nature at lower temperatures (below $\approx$ 50 K for $x$ = 0.2) before superconductivity sets in. Based on the results, the third type electronic phase diagram in Fe-based high-$T_c$ superconductors is revealed., 10 pages, 5 figures, Proceedings of the International conference for ECSN-2019, Odessa, Ukraine

Manganites exhibit various types of electronic phenomena, and these electronic characteristics can be controlled by carrier doping. Herein, we report the dielectric and magnetic properties of electron-doped manganite Ca$_{1-x}$Sr$_x$Mn$_{0.85}$Sb$_{0.15}$O$_3$ ($x$ = 0, 0.1, 0.2, and 0.3). The temperature dependence of the real part of the dielectric constant exhibits a broad and large peak just below the kink temperature of magnetization and a sharp decrease at lower temperatures, accompanied by an anomaly of the imaginary part. Furthermore, isovalent Sr substitution enhances the temperature of the dielectric peak by more than 50 K. Interestingly, the dielectric peak exhibits a negative magnetic-field effect. For all measured samples, the low-temperature variation of the dielectric constant can be qualitatively explained based on the Maxwell-Wagner (MW) model that describes a system composed of grain boundaries and semiconducting grains. However, the observed peak and its negative magneto-capacitance effect at high temperatures cannot be reproduced by a combination of the MW model and magnetoresistance effect. The dielectric peak strongly indicates polaronic relaxation in the present system. These results suggest that polarons form clusters with a dipole ordering and magneto-electric coupling, which might be consistently understood by the charge-ordering scenario.

In this paper, we conducted various magnetotransport measurements on ${\mathrm{Fe}}_{1+y}{\mathrm{Te}}_{1\ensuremath{-}x}{\mathrm{Se}}_{x}$ single crystals from which excess iron was sufficiently removed. Our results revealed that crossover from the incoherent to the coherent electronic state and the opening of the pseudogap occur at high temperatures ($\ensuremath{\approx}150\phantom{\rule{0.28em}{0ex}}\mathrm{K}$ for $x=0.2$). This is accompanied by a more substantial pseudogap and the emergence of a phase with a multiband nature at lower temperatures (below $\ensuremath{\approx}50\phantom{\rule{0.28em}{0ex}}\mathrm{K}$ for $x=0.2$) before superconductivity sets in. A comparison of these results with those of the as-grown (nonsuperconducting) samples implies that the coherent state accompanied by the pseudogap is needed for the occurrence of superconductivity in this system.

We report the Raman scattering measurements on the triple-layer ${\mathrm{Bi}}_{2}{\mathrm{Sr}}_{2}{\mathrm{Ca}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{10+\mathrm{z}}$ (Bi2223) crystals of four different doping levels from slightly overdoped to strongly underdoped regimes. We observed a double pair breaking peak in the antinodal ${B}_{1\mathrm{g}}$ configuration that we attribute to the two antinodal gaps opening on the outer and inner $\mathrm{Cu}{\mathrm{O}}_{2}$ plane (OP and IP) band, respectively. The doping dependence of the pair breaking peak energy was investigated. Considering the difference in doping level between the IP and OP, all the ${B}_{1\mathrm{g}}$ pair breaking peak energies for OP and IP were found to align on a single line as a function of doping, which is consistent with the previous results on the double and monolayer cuprates. Within our experimental accuracy the IP and OP peaks start to appear almost at the same temperature. These findings suggest some sort of interaction between the layers. The observed gap energy is very large, not scaling with ${T}_{\mathrm{c}}$.