Structural Changes under Pressure and the Superconductivity of Organic Superconductors

有机导体和超导体的特征是它们经历各种类型的结构变化，这些结构变化显著地影响电子性质。本研究的目的是从结构的观点出发，通过X射线衍射和低温压力下单晶的散射直接观察结构的变化，研究电子性质，如超导电性。为了观察低温下有机单晶的弱散射X射线，我们设计了一种特殊类型的金刚石压砧高压室和一种结构独特的测角仪。我们成功地在0.25立方毫米的相对较大的样品空间中获得了10 K时高达5千巴的高压。用<plus-minus>这种装置测量了有机超导体bate-（BEDT-TTF）_2I_3超结构的X射线卫星反射，得到以下结果：（1）实验上证实了超结构存在于临界压力0.4kbar以下的低压区的推测。(2)然而，我们发现，在低于约110 K描述的超结构的波矢量的幅度减少。(3)波矢的变化速度随压力的减小而减小，在常压下，波矢的变化需要40 h才能完成。(4)波矢改变后样品的超导临界温度为2K，明显高于波矢未改变时样品的超导临界温度1-1.5K，从而发现超结构的波矢与超导临界温度之间存在一定的相关性。我们将研究波矢变化在微观上所带来的结果，以及它在超导电性中所扮演的角色。

It is characteristic of organic conductors and superconductors that they undergo various types of structural changes which affects electronic properties remarkably. The purpose of the present study is to investigate electronic properties such as the superconductivity from structural points of view through direct observation of structural changes by x-ray diffraction and scattering of single crystals under pressure at low temperature.In order to observe weak scattering x-rays from organic single crystals at low temperature, we designed a diamond anvil high-pressure cell of special type and a goniometer having a unique structure. We succeeded to obtain high- pressures as high as 5 kbar at 10K in a relatively large sample space of 0,25mm^3. X-ray with diffraction angles of <plus-minus>20ﾟ could be observed without the geometrical restriction of the pressure cell.We measured x-rays satellite reflections from the superstructure of the organic superconductor bate-(BEDT-TTF)_2I_3 using the above apparatuses.The followings are the results we obtained: (1) We verified experimentally the speculation that the superstructure is present in the low pressure range below the critical pressure 0.4kbar. (2) We found, however, a decrease in the magnitude of the wave vector below about 110K describing the superstructure.(3) The changing speed of the wave vector decreases with decreasing pressure.At ambient, pressure the change needs 40 h to complete. (4) Samples after the wave-vector change have the superconducting critical temperature of 2K, which is obviously higher than that, 1-1.5K, without the change.Thus we found a correlation between the wave-vector of the superstructure and the superconducting critical temperature. We will investigate what is brought about microscopically by the wave-vector change and what roles it plays in the superconductivity.