Structural Changes under Pressure and the Superconductivity of Organic Superconductors

有机导体和超导体的特点是发生各种类型的结构变化，对其电子性能有显著影响。本研究的目的是通过对单晶在低温压力下的x射线衍射和散射的结构变化的直接观察，从结构的角度来研究超导性等电子特性。为了在低温下观察有机单晶的弱散射x射线，我们设计了一种特殊类型的金刚石砧高压电池和一种结构独特的测角仪。我们成功地在0.25 mm^3的相对较大的样品空间中获得了10K时高达5 kbar的高压。在不受压力电池几何限制的情况下，可以观察到衍射角<正负bbb20的x射线。我们用上述仪器测量了有机超导体bate-(BEDT-TTF)_2I_3上层结构的x射线卫星反射。得到的结果如下：(1)实验验证了上层结构在低于0.4kbar临界压力的低压范围内存在的推测。(2)然而，我们发现，在描述上部结构的约110K以下，波矢量的震级有所下降。(3)波矢量的变化速度随压力的减小而减小。在环境压力下，这种变化需要40小时才能完成。(4)波矢量改变后样品的超导临界温度为2K，明显高于未改变时的1 ~ 1.5 k。因此，我们发现了超导临界温度与上层结构的波矢量之间的相关性。我们将研究波矢量的变化在微观上带来了什么，以及它在超导性中起什么作用。

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.