Elementary Excitations in Solid and Liquid 4He at the Melting Pressure
Elementary Excitations in Solid and Liquid 4He at the Melting Pressure
复制标题
熔化压力下固体和液体 4He 的基本激发
DOI：
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发表时间：
2008
期刊：
影响因子：
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通讯作者：
V. Tsepelin
中科院分区：
文献类型：
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作者：
I. Todoshchenko;H. Alles;H. Junes;M. Manninen;A. Parshin;V. Tsepelin
摘要
Kim 和 Chan 最近在低于 0.2 K 的固体 4He 中发现了非经典转动惯量 (NCRI)，这重新激发了人们对超固体问题的极大兴趣，并启动了对固体 4He 性质的深入研究。 NCRI 的开始对应于超固体转变的直接证据是观察到固体 4He 的熵在转变温度以下相应下降。我们测量了在不同压力下生长的几种单晶在 0.01 至 0.45 K 温度范围内的超纯 4He 熔化压力，精度为 0.5 µbar。此外，还对恒体积液体 4He 中的压力进行了补充测量，这使我们能够从测量的熔化压力数据中消除压力表的温度相关特性的影响。通过对压力计随温度变化的灵敏度进行校正，4He 熔化压力在 320 mK 以下的变化遵循声子的纯 T4 定律，精度为 0.5 μbar，并且没有看到任何转变的迹象（Todoshchenko 等人，JETP Lett. 85:454, 2007）。这为低于 320 mK 的高质量 4He 晶体中可能存在的过量熵设定了 ∼5⋅10−8R 的上限。在较高温度下，已经观察到超流体 4He 中的旋转子的贡献。超流体 4He 的热膨胀系数在 0.01 至 0.7 K 范围内测量，精度为 ∼10−7 1/K，比之前的测量提高了两个数量级。转子对熔化压力和恒定体积液体压力的贡献是一致的，并且产生的转子间隙值为 6.8 K，这与其他方法获得的值非常接近。由于没有观察到弱相互作用的空位对 4He 熔化压力的贡献，因此可以将其活化能的下限设定为约 5.5 K。
Abstract
Recent discovery of a nonclassical rotational inertia (NCRI) in solid 4He below 0.2 K by Kim and Chan has revived great interest in the problem of supersolidity and initiated intensive study on the properties of solid 4He. A direct proof that the onset of NCRI corresponds to the supersolid transition would be the observation of a corresponding drop of the entropy of solid 4He below the transition temperature. We have measured the melting pressure of ultrapure 4He in the temperature range from 0.01 to 0.45 K with several single crystals grown at different pressures and with the accuracy of 0.5 μbar. In addition, supplementary measurements of the pressure in liquid 4He at constant volume have been performed, which allowed us to eliminate the contribution of the temperature-dependent properties of the pressure gauge from the measured melting pressure data. With the correction to the temperature-dependent sensitivity of the pressure gauge, the variation of the melting pressure of 4He below 320 mK obeys the pure T4 law due to phonons with the accuracy of 0.5 μbar, and no sign of the transition is seen (Todoshchenko et al. in JETP Lett. 85:454, 2007). This sets the upper limit of ∼5⋅10−8R for a possible excess entropy in high-quality 4He crystals below 320 mK. At higher temperatures the contribution from rotons in the superfluid 4He has been observed. The thermal expansion coefficient of the superfluid 4He has been measured in the range from 0.01 to 0.7 K with the accuracy of ∼10−7 1/K, or by two orders of magnitude better than in previous measurements. The roton contributions to the melting pressure and to the pressure in liquid at a constant volume are consistent and yield the value of 6.8 K for the roton gap, which is very close to the values obtained with other methods. As no contribution due to weakly interacting vacancies to the melting pressure of 4He has been observed, the lower limit of about 5.5 K for their activation energy can be set.