|Budget Amount *help
¥2,200,000 (Direct Cost : ¥2,200,000)
Fiscal Year 1996 : ¥1,000,000 (Direct Cost : ¥1,000,000)
Fiscal Year 1995 : ¥1,200,000 (Direct Cost : ¥1,200,000)
The electrical resistivity, thermoelectric power, and specific heat of Nb_3Te_4 inserted with Hg : Hg_xNb_3Te_4 were measured from 0.5K to 300K.The superconductivity and superconducting upper critical field H_<c2> were also measured. Hg insertion suppresses the charge density wave transition at about 80K The anomaly of the resistivity at about 30K is also suppressed by Hg insertion. However, the anomaly of the thermoelectric power at 30K is not greatly affected by Hg insertion. These results are discussed in terms of the modification of the shape of the Fermi surfaces. The residual resistivity ratio is not affected by Hg insertion. The thermoelectric power is negative indicating a dominant transport by electron at concentrations of x<0.3. While, for concentrations of x<0.3 the thermoelectric power is positive near superconducting transition temperature.
With addition of Hg, the superconducting transition temperature is enhanced from 1.9K to 5.4K.The superconducting upper critical field
H_<c2> is proportional to the temperature difference from the transition temperature. The ratio H_<c2*>/H_<c2*> of the parallel and the perpendicular to the c axis is 4.5 for concentrations of X<0.3. In contrast, the ratio increases remarkably at about x=0.3 and is about 40 for concentrations of x>0.3.
For Nb_3Te_4, although it becomes superconducting at about 2K in the electrical-resistivity measurement, an excess of specific heat due to the superconducting transition was not found down to about 0.5K.However, the excess of specific heat appears by Hg insertion and increases with increasing concentration x. The coefficient of the electronic specific heat is enhanced from 18.7 (mJ/molK^2) for x=0 to 146 (mJ/molK^2) for x=0.4. This is due to a change in dominant transport from electrons to holes.
(1) Nb_3Te_4については超伝導のに伴う比熱の増加は観測されないが,Hgを挿入したx=0.2, 0.4, 0.7の試料では抵抗から求めた超伝導の転移温度とほぼ同じ温度で,幅の広い比熱の増加が観測されることを初めて見い出した。このことは体積に占める超伝導の割合が増したことを示している。このように超伝導が少しでもbulk的になるのは,空洞が部分的にせよ埋められたために,c軸に垂直方向の結合が強められ,多様なジョセフン結合が可能になったことによるのであろう。