Interplay of the electric dipoles and lattice degree of freedom in the triangular lattice Mott insulator κ-(BEDT-TTF)2Cu2(CN)3
e-mail: hmori@issp.u-tokyo.ac.jpLanguage in Speech : English
Molecular BEDT-TTF-based Mott insulators are unique S=1/2 triangular lattice antiferromagnets, where the electrons, which carry spin, can also produce an on-site electric dipole moment[1].
The interplay of the charge and spin degrees of freedom in κ-phase BEDT-TTF based salts is studied already for about a decade. κ-(BEDT-TTF)2Cu2(CN)3 is the most studied material of thisgroup, which was the first proposed triangular S=1/2 spin liquid candidate[2.3], but recently a low-temperature singlet state is suggested by some measurements[4]. There are still unsolved questions: Does κ-(BEDT-TTF)2Cu2(CN)3 develop charge disproportionation? How does it influence the spin degree of freedom? Can we control charge and spin degrees of freedom and their coupling by external stimuli?
Raman scattering spectroscopy is known to be a practical tool to study these problems, allowing to probe in one measurement the charge degrees of freedom by charge sensitive vibrations of ET molecule, magnetic excitations, and lattice phonons.
In our recent experiments we followed the behavior of lattice modes as well as charge sensitive vibrations in κ-(BEDT-TTF)2Cu2(CN)3 down to low temperatures. The analysis of the line shape of the charge sensitive molecular vibration shows a Gaussian line shape with broadening on cooling, pointing on the developing fluctuating charge disproportionation with disordered charge below approximately 40 K. Lattice modes respond to this change in the charge degree of freedom on BEDT-TTF molecule by an increase of intensity, which is a function of polarizability. Below approximately 20 K lattice modes associated with the BEDT-TTF vibrations signal the new changes: intensity starts to drop, an extra mode becomes active at 37 cm-1 in (b,c) scattering channel, in (c,c) channel BEDT-TTF lattice modes broaden considerably. We discuss how the observation suggest charge freezing and scattering of phonons on charge or spin-charge excitations.
1. N. Hassan, S. Cunningham, M. Mourigal, E. I. Zhilyaeva, S. A. Torunova, R. N. Lyubovskaya, J. A. Schlueter, and N. Drichko, Science, 360, 1101 (2018).
2. Y. Shimizu, K. Miyagawa, K. Kanoda, M. Maesato, and G. Saito, Phys. Rev. Lett, 91, 107001(2003).
3. S. Yamashita, Y. Nakazawa, M. Oguni, Y. Oshima, H. Nojiri, Y. Shimizu, K. Miyagawa, and K. Kanoda, Nature Physics 4, 459 (2005).
4. B. Miksch, A. Pustogow, M. J. Rahim, A. A. Bardin, K. Kanoda, J. A. Schlueter, R. Hubner, M. Scheffler, and M. Dressel, Science 372, 276 (2021).
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