We measured the thermopower S and the Hall coefficients RH of Bi2Sr2−zLazCuO 6+δ (BSLCO) single crystals in a wide doping range, in an effort to identify the actual hole concentrations per Cu, p, in this system. It is found that the "universal" relation between the room-temperature thermopower and Tc does not hold in the BSLCO system. Instead, comparison of the temperaturedependent RH data with other cuprate systems is used as a tool to identify the actual p value. To justify this approach, we compare normalized RH(T ) data of BSLCO, La2−xSrxCuO4 (LSCO), YBa2Cu3Oy, and Tl2Ba2CuO 6+δ , and demonstrate that the RH(T ) data of the LSCO system can be used as a template for the estimation of p. The resulting phase diagram of p vs Tc suggests that Tc is anomalously suppressed in the underdoped samples, becoming zero at around p ≃ 0.10, while the optimum Tc is achieved at p ≃ 0.16 as expected.PACS numbers: 74.25. Dw, 74.25.Fy, 74.72.Hs Determination of the actual carrier concentration in the high-T c cuprates is in general a difficult task. The La-214 system [La 2−x Sr x CuO 4 (LSCO) or La 2−x Ba x CuO 4 (LBCO)] is almost the only system where the carrier concentration is nearly unambiguously known; in this system, the hole concentrations per Cu, p, is identical to the x value, as long as the oxygen is stoichiometric. In the YBa 2 Cu 3 O y (YBCO) system, the hole concentration in the CuO 2 planes is ambiguous because of the existence of the imperfect CuO chains which absorb part of the doped holes. In other systems like Bi 2 Sr 2 CaCu 2 O 8+δ (Bi-2212), Bi 2 Sr 2 CuO 6+δ (Bi-2201), or Tl 2 Ba 2 CuO 6+δ (Tl-2201), the determination of the hole concentration is also ambiguous because Bi and Tl ions have mixed valencies [1].In ordinary metals or semiconductors, the Hall coefficient R H is often used for the determination of the carrier concentration. However, R H of the cuprates has not been considered to be a useful tool to determine p, because R H shows a rather strong temperature dependence. Moreover, it has been reported for LSCO that the magnitude of R H is several times smaller than that expected from the chemically-determined carrier concentration [2]. On the other hand, it has been proposed [3] that the magnitude of the thermopower at room temperature (290 K), denoted as S(290K), can be used for the determination of p, based on the observation that the plot of T c /T , so-called the "bell shape", the measurement of S(290K) yields an estimation of p as long as the two "universal" relations hold.The above mentioned relation between p and S(290K) has not been tested in Bi 2 Sr 2−z La z CuO 6+δ (BSLCO, or La-doped Bi2201), in which the carrier concentration can be changed over a wide range [6]. An increase in the La concentration z in this system leads to a smaller density of holes in the CuO 2 planes, and the optimum T c is achieved with z ≃ 0.4. In this paper, we report our systematic measurements of S(290K) and R H (T ) for a series of BSLCO single crystals, for which z is varied from 0.2 to 1.0. It is found tha...
The specific heat of (Na,Ca)Co2O4 is measured at low-temperatures to determine the magnitude of the electronic specific-heat coefficient γ, in an attempt to gain an insight into the origin of the unusually large thermoelectric power of this compound. It is found that γ is as large as ∼48 mJ/molK 2 , which is an order of magnitude larger than γ of simple metals. This indicates that (Na,Ca)Co2O4 is a strongly-correlated electron system, where the strong correlation probably comes from the lowdimensionality 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.PACS numbers: 65.40.+g, 71.27.+a, 71.20.Be Recently, coexistence of a large thermopower (∼100 µV/K at 300 K) and a low resistivity was found in a transition-metal oxide NaCo 2 O 4 [1], which made this compound an attractive candidate for thermoelectric (TE) applications. Normally, large thermopower is associated with materials with low carrier densities and the thermoelectric properties are optimized for systems with typical carrier concentration of 10 19 cm −3 [2]; on the other hand, NaCo 2 O 4 has two-orders-of-magnitude larger carrier density (∼10 21 cm −3 ) and yet shows a thermopower comparable to that of the usual low-carrierdensity TE materials [1]. The origin of the large thermopower in NaCo 2 O 4 is yet to be understood.In NaCo 2 O 4 , Co ion has a mixed valence between 3+ and 4+. Since NaCo 2 O 4 is a layered system with a triangular lattice and Co 4+ has spins [1], it is expected that the interplay between charges and spins is playing a major role in producing the peculiar electronic properties of this compound, as in the case of high-T c cuprates. In those systems where Coulomb interactions or spin fluctuations are important, it is often found that the electrons become strongly correlated and thus the simple band picture is not well applicable. In fact, magnetotransport studies of NaCo 2 O 4 found that the Hall coefficient has an opposite sign to the thermopower and is strongly temperature dependent [3], which suggests the presence of a strong correlation in this system. Therefore, to elucidate the origin of the large thermopower in NaCo 2 O 4 , it would be illuminating to determine the strength of the electron correlations in NaCo 2 O 4 by measuring the electronic specific heat.
The Technical Design for the COMET Phase-I experiment is presented in this paper. COMET is an experiment at J-PARC, Japan, which will search for neutrinoless conversion of muons into electrons in the field of an aluminum nucleus ($\mu$–$e$ conversion, $\mu^{-}N \rightarrow e^{-}N$); a lepton flavor-violating process. The experimental sensitivity goal for this process in the Phase-I experiment is $3.1\times10^{-15}$, or 90% upper limit of a branching ratio of $7\times 10^{-15}$, which is a factor of 100 improvement over the existing limit. The expected number of background events is 0.032. To achieve the target sensitivity and background level, the 3.2 kW 8 GeV proton beam from J-PARC will be used. Two types of detectors, CyDet and StrECAL, will be used for detecting the $\mu$–$e$ conversion events, and for measuring the beam-related background events in view of the Phase-II experiment, respectively. Results from simulation on signal and background estimations are also described.
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