The 10th QLC young colloquium

Date & Time : Friday, May 26, 2023.　16：00～
Speakers：Shusaku IMAJO（University of Tokyo）,  Syu OMURA（Nagoya Institute of Technology）
Place：online using “Zoom”

*If you wish to join this seminar, please register at this site by noon on the day of the event.

１）16:00～16:30
Speaker：Shusaku IMAJO（ISSP, University of Tokyo）
Title：An impact of ultrahigh magnetic field on the Pomeranchuk effect
Abstract：
　Helium (He) at ambient pressure is the only element that does not become solid at extremely low temperatures, making it the unique element in which quantum effects are most strongly manifested. The stable isotopes are ³He and ⁴He. Since ⁴He atom is a boson with I=0 nuclear spin, Bose-Einstein condensation occurs at 2.17 K, resulting in superfluidity. On the other hand, ³He behaves as a Fermi liquid at sufficiently low temperatures because ³He atom is a fermion with nuclear spin of I=1/2. Helium can be solidified under high pressure. In the case of ³He, it becomes a paramagnetic solid because of its nuclear spin I=1/2, and magnetic entropy of Rln2 persists even at low temperatures of several tens of mK. Below ~0.3 K, solid ³He has larger entropy than liquid ³He, which is a Fermi liquid, and solidification leads to negative latent heat. Given the Clausius-Clapeyron equation, the slope of the melting curve becomes negative, and a strange region, where a liquid becomes a solid upon heating, appears. The phenomena are called the Pomeranchuk effect [1] and are known as one of the representative peculiarities of ³He.
　As mentioned above, the origin of the Pomeranchuk effect is the entropy difference between the Fermi liquid state and the paramagnetic solid state. This means that the application of a magnetic field must have an influence on the Pomeranchuk effect through the polarization of nuclear spins. However, the nuclear magneton is extremely small, about 1/1840 of the electron Bohr magneton, and therefore, an ultra-high magnetic field is required to investigate the magnetic field effect. In fact, only tiny changes have been observed in a magnetic field of about 10 T [2].
　Recently, our laboratory succeeded in generating an ultrahigh magnetic field of 1200 T [3], and advances in measurement technology enable us to perform various measurements even in ultrahigh magnetic fields of several hundred T. Nevertheless, these experiments are challenging because of the preciousness of ³He element and extreme conditions required for the experiments.
　Here, we focused on electron spins, not nuclear spins, and searched for materials that exhibit the Pomeranchuk effect in the liquid-solid transition of electrons, namely the metal-insulator transition. In this study, we found that a coordination polymer exhibits the Pomeranchuk effect due to an hybridization of π and d electrons. In this talk, we will discuss the technical background of the experiments in magnetic fields above 100 T and preliminary results of the experiments.

[1] R. C. Richardson, Rev. Mod. Phys. 69, 683 (1997).
[2] C. C. Kranenburg, S. A. J. Wiegers, P. G. van der Haar, R. Jochemsen, and G. Frossati, Jpn. J. Appl. Phys. 26, 2133 (1987).
[3] D. Nakamura, A. Ikeda, H. Sawabe, Y. H. Matsuda, and S. Takeyama, Rev. Sci. Instrum. 89, 095106 (2018).

２）16:30-17:00　
Speaker：Syu OMURA（Nagoya Institute of Technology）
Title：Ultrashort-pulse-induced nonlinear charge dynamics in κ-(BEDT-TTF)₂X
Abstract：
　κ-(BEDT-TTF)₂X is known to exhibit various phases, such as dimer-Mott insulator, superconductor and metallic states, by substituting X [1]. There is interest in the ultrafast change and control [2] of such physical properties induced by light and/or electric fields.
　Recently, induced radiation has been observed in κ-(BEDT-TTF)₂X irradiated with 6 fs pulse in higher energy region than its linear absorption band [3]. It has been shown that this induced radiation is due to non-linear intra-dimer charge oscillation [3.4].
　In order to investigate the origin of the nonlinear charge oscillation, we numerically calculated the dynamics induced by an ultrashort pulse using the extended Hubbard model. The singular value decomposition (SVD) of the numerical solution provides quantum states that contribute to the dynamics. As a result of the SVD, we found that even higher energy states contribute to the dynamics than the nonlinear component obtained from the Fourier transformation of the intra-dimer charge oscillation. This suggests that the coupling between multiphoton excited states and one-photon excited states plays a role in the nonlinear charge oscillation.
　By calculating the entanglement entropy of the quantum states obtained from the above calculation, we discovered an anomaly which implies a quantum scar [5]. We will also discuss these peculiar quantum states in this presentation.

[1] K. Kanoda, J. Phys. Soc. Jpn. 75, 051007 (2006).
[2] S. Ohmura et al., Phys. Rev. B 104, 134302 (2021).
[3] Y. Kawakami et al., Nat. Photon. 12, 474 (2018).
[4] K. Yonemitsu, J. Phys. Soc. Jpn. 87, 044708 (2018).
[5] K. Pakrouski, Phys. Rev. Res. 3, 043156 (2021).

３）17:00-
Free discussion with speakers (free participation)

Committee Chair：Hiroshi WATANABE（Ritsumeikan University）, Jun TOKIMOTO (Tokyo University of Science)