Structural Changes under Pressure and the Superconductivity of Organic Superconductors

有机导体和超导体的特点是它们经历了各种类型的结构变化，这些变化对电子性质有显着的影响。本研究的目的是通过X射线衍射仪直接观察单晶在低温加压下的结构变化和散射，从结构的角度研究超导电性等电学性质。为了观察有机单晶在低温下的弱散射X射线，我们设计了一种特殊类型的金刚石顶锤高压盒和具有独特结构的测角仪。在10K下，在0.25kbar的较大样品空间内，我们成功地获得了高达5kbar的高压。在没有压力室几何限制的情况下，可以观察到衍射角为&lt；正负&gt；20ﾟ的X射线。我们用上述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。我们得到了以下结果：(1)我们从实验上证实了超结构存在于临界压力0.4kbar以下的低压范围内。(2)描述超结构的波矢的大小在110K以下有所减小。(3)波矢的变化速度随压力的降低而减小，在常压下，变化需要40h才能完成。(4)改变波矢后的样品的超导临界温度为2K，明显高于未改变的1-1.5K的超导临界温度，从而发现了超结构的波矢与超导临界温度之间的关系。我们将从微观上研究波矢的变化带来了什么，以及它在超导电性中所起的作用。

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.