Next Article in Journal
Unprecedented Hexanuclear Cobalt(II) Nonsymmetrical Salamo-Based Coordination Compound: Synthesis, Crystal Structure, and Photophysical Properties
Next Article in Special Issue
(BEDT-TTF)2Cu2(CN)3 Spin Liquid: Beyond the Average Structure
Previous Article in Journal / Special Issue
β”-(CNB-EDT-TTF)4BF4; Anion Disorder Effects in Bilayer Molecular Metals
Open AccessReview

Thermodynamic Picture of Dimer-Mott Organic Superconductors Revealed by Heat Capacity Measurements with External and Chemical Pressure Control

Department of Chemistry, Graduate School of Science, Osaka University, Machikaneyama 1-1, Toyonaka, Osaka 560-0043, Japan
*
Author to whom correspondence should be addressed.
Crystals 2018, 8(4), 143; https://doi.org/10.3390/cryst8040143
Received: 24 February 2018 / Revised: 18 March 2018 / Accepted: 19 March 2018 / Published: 21 March 2018
(This article belongs to the Special Issue Advances in Organic Conductors and Superconductors)
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. View Full-Text
Keywords: organic superconductor; dimer-Mott system; heat capacity; electron correlations; d-wave; pressure organic superconductor; dimer-Mott system; heat capacity; electron correlations; d-wave; pressure
Show Figures

Figure 1

MDPI and ACS Style

Nakazawa, Y.; Imajo, S.; Matsumura, Y.; Yamashita, S.; Akutsu, H. Thermodynamic Picture of Dimer-Mott Organic Superconductors Revealed by Heat Capacity Measurements with External and Chemical Pressure Control. Crystals 2018, 8, 143.

Show more citation formats Show less citations formats
Note that from the first issue of 2016, MDPI journals use article numbers instead of page numbers. See further details here.

Article Access Map by Country/Region

1
Search more from Scilit
 
Search
Back to TopTop