Quantum Criticality Between Metal and Charge Order

• Applicant: Professor Dr. Martin Dressel
• Co-Applicant: Dr. Bojana Hamzic
• Subject Area: Experimental Condensed Matter Physics
• Term: from 2013 to 2017
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 204566485

• Final Report Year: 2018

Final Report Abstract

The aim of the project was to study quantum critical points that may occur at the boundary between a charge-ordered insulator and a metal upon tuning the effective correlations. While commonly quantum fluctuations are investigated in magnetically ordered systems, here we focused on the charge degrees of freedom. We wanted to investigate the transport properties, i.e. dc-resistivity and Hall effect, of two-dimensional organic conductors in the vicinity of charge order to see how the coherence scale characteristic of Fermi liquid behavior of the homogeneous metal vanishes at the onset of charge order. It is predicted that a strong effective mass enhancement reminiscent of heavy fermion behavior indicates the possible destruction of quasiparticles at the quantum-critical points. We provide substantial indications for a non-trivial interplay between charge order and superconductivity in quarter-filled (BEDT-TTF)2X. Future studies may address this issue on related materials, such as deuterated compounds, and extend experiments down to lower temperatures to probe deep inside the superconducting state. In addition, we elucidate the origin of charge order and its mutual interaction with the anions. To that end, we suggest that such a situation is common rather than serendipity in the charge-transfer salts. Comprehensive structural studies down to cryogenic temperatures are a convenient route towards a better understanding of the anion-BEDT-TTF coupling strength and its influence on the conduction properties. Another prospective direction is dielectric spectroscopy as it probes the polarizability of the material and, thus, delivers important information on the charge-disproportionate states. Also, a careful analysis of the anionic vibrations and those of the ethylene endgroups may provide detailed information.

Publications

• Charge Order Fluctuations and Superconductivity in Two-Dimensional Organic Metals. Physical Review B 89, 174503 (2014)
  Girlando, Alberto; Masino, Matteo; Schlueter, John A.; Drichko, Natalia; Kaiser, Stefan & Dressel, Martin
  
• Metallic state and charge-order metal-insulator transition in the quasi-twodimensional conductor κ-(BEDT-TTF)2Hg(SCN)2Cl Physical Review B 89, 075133 (2014)
  Drichko, Natalia; Beyer, Rebecca; Rose, Eva; Dressel, Martin; Schlueter, John A.; Turunova, S. A.; Zhilyaeva, E. I. & Lyubovskaya, R. N.
  
• Ferroelectricity in Molecular Solids: A Review of Electrodynamic Properties. Reports on Progress in Physics 78, 096501 (2015)
  Tomić, S. & Dressel, M.
  
• Electrically-induced phase transition in α-(BEDT-TTF)2I3: Indications for Diraclike hot charge carriers. Physical Review B 93, 245133 (2016)
  Peterseim, T.; Ivek, T.; Schweitzer, D. & Dressel, M.
  
• Pressure dependence of the metal-insulator transition in α-(BEDT-TTF)2 Hg(SCN)2Cl: optical and transport studies. Journal of Physics: Condensed Matter 29, 055601 (2017)
  Löhle, A.; Rose, E.; Singh, S.; Beyer, R.; Tafra, E.; Ivek, T.; Zhilyaeva, E. I.; Lyubovskaya, R. N. & Dressel, M.
  
• Pressure-dependent optical investigations of α-(BEDT-TTF)2I3: tuning charge order and narrow gap towards a Dirac semimetal. Physical Review B 93, 195116 (2016)
  Beyer, R.; Dengl, A.; Peterseim, T.; Wackerow, S.; Ivek, T.; Pronin, A. V.; Schweitzer, D. & Dressel, M.
  
• Metal-Insulator Transition in the Dimerized Organic Conductor κ-(BEDT-TTF)2 Hg(SCN)2Br. Physical Review B 96, 085116 (2017)
  Ivek, Tomislav; Beyer, Rebecca; Badalov, Sabuhi; Čulo, Matija; Tomić, Silvia; Schlueter, John A.; Zhilyaeva, Elena I.; Lyubovskaya, Rimma N. & Dressel, Martin
  
• Semimetallic and charge-ordered α-(BEDT-TTF)2I3: on the role of disorder in dc transport and dielectric properties. Physical Review B 96, 075141 (2017)
  Ivek, Tomislav; Čulo, Matija; Kuveždić, Marko; Tutiš, Eduard; Basletić, Mario; Mihaljević, Branimir; Tafra, Emil; Tomić, Silvia; Löhle, Anja; Dressel, Martin; Schweitzer, Dieter & Korin-Hamzić, Bojana
  
• The metal-insulator transition in the organic conductor κ′′-(BEDT-TTF)2Hg(SCN)2Cl. Journal of Chemical Physics 147, 064503 (2017)
  Li, Weiwu; Rose, Eva; Tran, Minh Vu; Hübner, Ralph; Łapiński, Andrzej; Świetlik, Roman; Torunova, Svetlana A.; Zhilyaeva, Elena I.; Lyubovskaya, Rimma N. & Dressel, Martin