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We studied the temperature dependences of the optical spectra for optimally and underdoped Bi$_2$Sr$_2$Ca$_2$Cu$_3$O$_{10+z}$ single crystals. Similarly to the other cuprates' cases, a gap-like conductivity suppression was observed with reducing the temperature from above $T_{\mathrm{c}}$, creating a peak in the conductivity spectrum. The conductivity peak energy was insensitive to the doping level, namely $T_{\mathrm{c}}$, which suggests that this gap is not a superconducting gap but is related to the pseudogap. Comparing the data of various mono-, double-, and triple-layer cuprates, we found a clear correlation between the optimal $T_{\mathrm{c}}$ of each material and the pseudogap-related conductivity peak energy., Comment: 12 pages, 5 figures

The observance of vortex and anti-vortex effects in bulk crystals can prove the existence of phase-disordered superconductivity in the bulk. To gain insights into the mechanisms that govern superconducting transition in copper oxide high-transition temperature ($T_c$) superconductors, this study investigated the transport properties of underdoped Bi$_2$Sr$_2$Ca$_2$Cu$_3$O$_{10+\delta}$ (Bi-2223) bulk single crystals.The $I$-$V$ characteristics results and the typical tailing behavior owing to the temperature dependence of in-plane resistivity ($\rho_{ab}$) were consistent with the Kosterlitz-Thouless (KT) transition characteristics. Thus, with increasing temperature, copper oxide high-$T_c$ superconductors transitioned to their normal state owing to destruction of their phase correlations, although a finite Cooper pair density was prevalent at $T_c$. Further, magnetization measurements were performed to determine the temperature dependence of the irreversible magnetic field $B_{irr}$. Consequently, the mechanism governing the KT transition-like superconducting transition in this bulk system was elucidated. These results support the extreme strong-coupling models for high-$T_c$ superconductivity in cuprates., Comment: 14 pages, 11 figures, revised according to the referees'comments

Knowledge of the doping (x)-temperature (T) phase diagram of Fe-based superconductors is crucial in understanding the mechanism of high transition temperature superconductivity. Here, we measured the Se doping dependence of the magnetic susceptibility for Te-annealed FeTe$_{1-x}$Se$_x$. Two characteristic temperatures $T^*_{\chi}$ and $T^{**}_{\chi}$ were observed in the temperature dependence of magnetic susceptibility, where the magnetic susceptibility decreased below T$^{**}_{\chi}$ and tended to increase with further decreasing temperature below T$^*_{\chi}$. The decrease in magnetic susceptibility below T$^{**}_{\chi}$ is considered to be due to the opening of the pseudogap. Whereas the increase in magnetic susceptibility below T$^*_{\chi}$ is attributed to the increase in the carrier concentration of electrons. Furthermore, we found the large superconducting fluctuation, which may be related to BCS-BEC crossover., Comment: 6 pages, 3 figures. to be published in JPS Conference Proceedings

Recently, the superconductors' community has witnessed an unsettled debate regarding whether iron-based superconductors, in particular FeSe and FeSe$_{1-x}$S$_x$, are in the Bardeen-Cooper-Shrieffer (BCS) - Bose-Einstein condensation (BEC) crossover regime. Nonetheless, one particular system, FeTe$_{1-x}$Se$_x$, has been less investigated in this regard owing to the screening of its intrinsic superconducting properties by the inevitable iron excess. Herein, the onset temperatures for superconducting fluctuations ($T_{scf}$) are investigated by measuring the magnetoresistance (MR) of Te-annealed, high-quality FeTe$_{1-x}$Se$_x$ ($x$ = 0.1, 0.2, 0.3, and 0.4) single crystals. The results reveal very high $T_{scf}$ values for these crystals. Particularly for $x$ = 0.4, $T_{scf}$ reaches approximately 40 K, which is 2.7 times larger than $T_c$. This indicates that the superconductivity of the FeTe$_{1-x}$Se$_x$ system is well within the BCS-BEC crossover regime., Comment: 6 pages, 3 figures, and 1 table. to be published in JPS Conference Proceedings

For an infinite word $x$, Bugeaud and Kim introduced a quantity $\mathrm{rep}(x)$ called the exponent of repetition of $x$. We prove that $\mathrm{rep}(x) = \mathrm{rep}(y)$ holds for a Sturmian word $x$ and every suffix $y$ of $x$. Let $c$ be the characteristic Sturmian word of slope $\theta$. When $\theta$ is a quadratic irrational, a formula of $\mathrm{rep}(c)$ is given. This formula shows that $\mathrm{rep}(c) = \mathrm{rep}(c')$ if $c'$ is the characteristic Sturmian word whose slope is a quadratic irrational equivalent to $\theta$., Comment: The results of this paper are not new

Although phase diagrams can be leveraged to investigate high transition temperature (high-$T_c$) superconductivity, the issue has not been discussed thoroughly. In this study, we elucidate the phase diagram of the overdoped side of high-$T_c$ cuprates via systematic anisotropic transport measurements for Pb-doped Bi-2212 single crystals. We demonstrate that the characteristic temperatures of the "weak" pseudogap opening and electronic coherence cross each other at a critical doping level, while those of the "strong" pseudogap merges into that of superconducting fluctuations above the critical doping level. Our results indicate the importance of Mottness in high-$T_c$ superconductivity., Comment: 8 pages, 9 figures; The title is changed

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., Comment: 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.

By measuring second occurring times of factors of an infinite word $x$, Bugeaud and Kim introduced a new quantity ${\rm rep}(x)$ called the exponent of repetition of $x$. It was proved by Bugeaud and Kim that $1 \leq {\rm rep}(x) \leq r_{\max} = \sqrt{10} - 3/2$ if $x$ is a Sturmian words. In this paper, we determine the value $r_1$ such that there is no Sturmian word $x$ satisfying $r_1 < {\rm rep}(x) < r_{\max}$ and $r_1$ is an accumulation point of the set of ${\rm rep}(x)$ when $x$ runs over the Sturmian words., Comment: 31 pages

In this study, we conducted various magnetotransport measurements on Fe$_{1+y}$Te$_{1-x}$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 opening of the pseudogap occur 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. A comparison of these results with those of the as-grown (non-superconducting) samples implies that the coherent state accompanied by the pseudogap is needed for the occurrence of superconductivity in this system., Comment: 9 pages, 7 figures

We report the pressure effect in Bi2Sr2Ca2Cu3O10+{\delta} (Bi-2223) single crystal with a small amount of intergrowth of Bi2Sr2CaCu2O8+{\delta} (Bi-2212). Their superconducting transition temperatures Tcs showed a domelike shape as a function of pressure, which showed a good agreement with the general relation between the carrier concentration and Tc. Our experimental results indicate that high pressure can induce effective carrier doping into the multilayered high-Tc cuprate superconductor, Comment: 8 pages including 5 figures

To elucidate the pseudogap phase diagram including the overdoped state of high transition temperature (high-$T_c$) cuprates, we must understand the origin of the positive out-of-plane magnetoconductivity (MC) observed in these compounds. For this purpose, the out-of-plane resistivity $\rho_c(T,H)$ of an overdoped Bi$_{1.6}$Pb$_{0.4}$Sr$_{2}$CaCu$_{1.96}$Fe$_{0.04}$O$_{8+\delta}$ (Bi-2212) single crystal is measured under pulsed magnetic fields up to 60 T. We show that the superconductive density-of-states (DOS) depletion effect, in addition to the pseudogap effect, clearly appears below the superconductive fluctuation regime, and the contribution becomes dominant in the superconducting state., Comment: 6 pages, 5 figures

We analyzed the in-plane and out-of-plane magnetoresistance (MR) for overdoped Bi$_{1.6}$Pb$_{0.4}$Sr$_{2}$CaCu$_{2}$O$_{8+\delta}$ (Bi-2212) single crystals using superconductive fluctuation theory, which considers the density-of-states (DOS) contribution in layered superconductors with the conventional s-wave pairing state. The out-of-plane results are well reproduced by the theory, implying that the large, negative out-of-plane MR as well as the sharp increase in the zero-field out-of-plane resistivity $\rho_{c}$ near the superconducting transition temperature $T_c$ originate from the superconductive DOS fluctuation effect. On the other hand, the in-plane results are better reproduced without the DOS contribution (i.e., using only the Aslamazov-Larkin (AL) contribution), which may be explained in terms of the d-wave superconductivity of the layered superconductors., Comment: 4 pages, 3 figures, 27th International Symposium on Superconductivity, ISS 2014, to appear in Physics Procedia

To investigate the origin of the enhanced $T_c$ ($\approx$ 110 K) of the trilayer cuprate superconductor Bi$_{2}$Sr$_{2}$Ca$_{2}$Cu$_{3}$O$_{10+\delta}$ (Bi-2223), we have performed systematic magnetoresistance (MR) measurements on this superconductor, as well as on the bilayer superconductor, Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+\delta}$ (Bi-2212). The in-plane coherence length, $\xi_{ab}$, and the specific-heat jump, ${\Delta}C$, have been estimated using the theory of renormalized superconductive fluctuations, and the doping dependence of these parameters has been qualitatively explained using the Fermi arc approach. A detailed comparison of the superconducting parameters with $T_c$ for these compounds suggests that an additional superconducting condensation energy exists, due to an increase in the number of stacking CuO$_{2}$ planes in a unit cell., Comment: 17 pages, 5 figures, to appear in JPSJ

To investigate the relationship between the pseudogap and superconductivity, we measured both the in-plane ($\rho_{ab}$) and out-of-plane ($\rho_c$) resistivity for oxygen-controlled Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+\delta}$ single crystals subject to magnetic fields (parallel to the $c$ axis) of up to 17.5 T. The onset temperature for the superconductive fluctuation, $T_{scf}$, is determined by the large positive in-plane magnetoresistance (MR) and negative out-of-plane MR observed near $T_c$, whereas the pseudogap opening temperature $T^*$ is determined by the semiconductive upturn of the zero-field $\rho_c$. $T_{scf}$ was found to scale roughly as $T_c$, with a decreasing temperature interval between them upon doping. On the other hand, $T^*$ starts out much higher than $T_{scf}$ but decreases monotonically upon doping; finally, at a heavily overdoped state, it is not observed above $T_{scf}$. These results imply that the pseudogap is not a simple precursor of superconductivity, but that further study is needed to determine whether or not $T^*$ exists below $T_{scf}$ in the heavily overdoped state., Comment: 17 pages, 7 figures, to appear in JPSJ

In this study, the effects of annealing under tellurium vapor have been investigated for Fe$_{1.03}$Te$_{0.8}$Se$_{0.2}$ single crystals. The as-grown crystal is not superconducting. After annealing it at 400 $^\circ$C for 250 h in tellurium (Te) vapor, it shows metallic resistivity temperature dependence below 150 K and a sharp superconducting transition at 13 K. The sample also shows a full magnetic shielding effect at low temperatures. The improvement in superconductivity has been attributed to the removal of excess Fe, which is inevitably incorporated in the as-grown crystals, by the annealing., Comment: 9 pages, 6 figures

The characteristic pseudogap temperature T* in Bi2Sr2CaCu2O8 system has been systematically evaluated as a function of doping, especially focusing on its overdoped region, by measuring the out-of-plane resistivity under the magnetic fields. Overdoped samples have been prepared by annealing TSFZ-grown Bi2Sr2CaCu2O8 single crystals under the high oxygen pressures (990 kgf/cm2). At a zero field, the out-of-plane resistivity showed a metallic behavior down to Tc (= 62 K), while under the magnetic fields of over 3 T,it showed typical upturn behavior from around 65 K upon decreasing temperature. This result suggests that the pseudogap and superconductivity are different phenomena., Comment: 2 pages, 2 figures. Final version accepted for the Proceedings of the M2S-IX Conference (Tokyo, September 2009)