**Kosei Hino, Tetsuya Nomoto, Satoshi Yamashita and Yasuhiro Nakazawa \***

Department of Chemistry, Graduate School of Science, Osaka University, Machikaneyama 1-1, Toyonaka, Osaka 560-0043, Japan; hino.kosei.ma@m-ep.co.jp (K.H.); nomotot15@chem.sci.osaka-u.ac.jp (T.N.); sayamash@chem.sci.osaka-u.ac.jp (S.Y.)

**\*** Correspondence: nakazawa@chem.sci.osaka-u.ac.jp; Tel.: +81-6-6850-5396

Received: 30 October 2020; Accepted: 18 November 2020; Published: 21 November 2020

**Abstract:** Heat capacity measurements of θ-(BEDT-TTF)2CsZn(SCN)4 in its non-equilibrium electronic states induced by applying electric currents and voltages were performed by a modified relaxation calorimetry technique. We developed a single crystal heat capacity measurements system by which the Joule heating produced in samples by external currents and voltages can be balanced with the cooling power to make a non-equilibrium steady state. Although temperature versus time profiles in the relaxation process in calorimetry can be obtained as exponential curves as in the usual relaxation technique, we found that the change of resistivity that occurs during the heating and relaxation process should be taken into account in analyzing the data. By correcting this factor in the analyses, we succeeded in evaluating absolute values of *Cp*(*I*) and *Cp*(*V*) in these non-equilibrium states. The experiments up to 150 μA and the constant voltage of 20 mV do not induce visible change in the structure of the Boson peak in *CpT*−<sup>3</sup> vs. *T* suggestive of the glassy ground state of phonons. Although the suppression of the short-range fluctuations of the charge density has been reported, it does not seriously affect the glassy phonons in this current range.

**Keywords:** charge glass; heat capacity; electric current; electric voltage; Boson peak
