Thermodynamic Picture of Dimer-Mott Organic Superconductors Revealed by Heat Capacity Measurements with External and Chemical Pressure Control
Thermodynamic Picture of Dimer-Mott Organic Superconductors Revealed by Heat Capacity Measurements with External and Chemical Pressure Control
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DOI：
10.3390/cryst8040143
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发表时间：
2018-03
期刊：
影响因子：
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通讯作者：
Y. Nakazawa;S. Imajo;Y. Matsumura;S. Yamashita;H. Akutsu
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文献类型：
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作者：
Y. Nakazawa;S. Imajo;Y. Matsumura;S. Yamashita;H. Akutsu
本文综述并讨论了具有（BEDT-TTF）2X组成的二聚Mott型分子筛化合物的热力学性质，其中BEDT-TTF为双（亚乙基二硫代）四硫富瓦烯，X分别为抗衡阴离子。我们主要集中在κ型结构中出现的特征，其中d波的相位出现取决于库仑排斥U和带宽W，这是可调的外部和化学压力。首先，我们报告了高压交流量热技术和实验系统的构建，以测量单晶样品的分子基化合物，以获得低温热力学参数。使用极小的电阻芯片作为加热器和温度计，可通过四端检测，以足够的灵敏度准确检测样品部分的温度及其振荡。通过对κ-ε的交流热容与温度关系的分析，（BEDT-TTF）2Cu（NCS）2在外部压力下，我们讨论了在不同的外压p下，热反常峰形在相变温度Tc处的变化。9.1 K时具有较强的耦合特性，随着压力增加到0.45 GPa，随着相变温度的降低，耦合逐渐减弱。这一特征与基于κ-（BEDT-TTF）2X化合物和其他典型的二聚体-Mott 2：1化合物的热异常的化学压力效应的系统论证进行了比较。最后，讨论了化学压力对正常态电子热容系数γ的影响，该系数是通过施加高于Hc 2的磁场得到的，而剩余的γ* 由于节点结构周围的感生电子态密度而保持在π态。从整体的参数与化学和外部的压力，我们建议，交叉的电子状态内的电子相的发生和电子对的耦合强度从电子相关区域附近的金属-绝缘体边界的带图片区域的变化。
This article reviews and discusses the thermodynamic properties of dimer-Mott-type molecular superconductive compounds with (BEDT-TTF)2X composition, where BEDT-TTF is bis(ethylenedithio)tetrathiafulvalene and X denotes counter-anions, respectively. We focus mainly on the features occurring in the κ-type structure in which the d-wave superconductive phase appears depending on the Coulomb repulsion U and the bandwidth W, which is tunable by external and chemical pressures. First, we report the high-pressure ac (alternating current) calorimetry technique and experimental system constructed to measure single-crystal samples of molecule-based compounds to derive low-temperature thermodynamic parameters. Using extremely small resistance chips as a heater and a thermometer allows four-terminal detection of an accurate temperature and its oscillation in the sample part with sufficient sensitivity. From the analyses of the temperature dependence of the ac heat capacity of κ-(BEDT-TTF)2Cu(NCS)2 under external pressures, we discuss the changes in the peak shape of the thermal anomaly at the superconductive transition temperature Tc at various external pressures p. The rather sharp peak in CpT−1 at Tc = 9.1 K with a strong coupling character at ambient pressure is gradually reduced to weaker coupling as the pressure increases to 0.45 GPa concomitant with suppression of the transition temperature. This feature is compared with the systematic argument of the chemical–pressure effect on the basis of thermal anomalies around the superconductive transition of κ-(BEDT-TTF)2X compounds and other previously studied typical dimer-Mott 2:1 compounds. Finally, the discussion is extended to the chemical pressure effect on the normal state electronic heat capacity coefficient γ obtained by applying magnetic fields higher than Hc2 and the residual γ*, which remains in the superconductive state due to the induced electron density of states around the node structure. From the overall arguments with respect to both chemical and external pressures, we suggest that a crossover of the electronic state inside the superconductive phase occurs and the coupling strength of electron pairs varies from the electron correlation region near the metal-insulator boundary to the band picture region.