Charge Order-Disorder Phase Transition Detected By EPR in α'-(BEDT-TTF) 2 IBr 2

1. Charge Order-DisorderPhase Transition Detected By EPRin α'-(BEDT-TTF)2IBr2 1Roman Morgunov, 1Alexei Dmitriev, 1Alisa Chernenkaya, 2Kaoru Yamamoto, 2Kyuya Yakushi 1Institute of Problem of Chemical Physics, Chernogolovka, Russia 2Institute for Molecular Sciences, Okazaki, Japan

2. Crystal structure and charge localization in α'-(BEDT-TTF)2IBr2 Yagubskii E., Shegolev I., Shibaeva R. et al. JETP letters, 1985

3. Temperature dependencies Yue Y., Nakano C., Yamamoto K. et al., J.Phys.Soc.Jap., 2009

4. Resume of temperature dependencies studying No structural transitions!

5. ESR spectra of theα'-(BEDT-TTF)2IBr2 single crystal Lowest linewidth at 3 K is 0.8 Oe might be explained by Anderson-Weiss theory: Нр2 = 5.1(gВ n)2S(S + 1) = 1893 Oe is dipole-dipole interaction between dimer spins, НН is a contribution of spin-relaxation processes to linewidth, He = 2.83 JgВ (S(S + 1))1/2 = 42 kOe is exchange interaction energy expressed in field units.

6. Lorenz lineshape doesn’t change under localization at 208 K Journal of Experimental and Theoretical Physics, 2006, Vol. 102, No. 1, pp. 121–130. (http://scitation.aip.org/jetp/). α'-(BEDT-TTF)2IBr2

7. Angular dependencies of g-factor above and below Tc = 208 K Angular dependencies of the g-value below CODT (at temperatures T = 100 K (white dots), and T = 190 K (grey dots)) and above CODT (at temperature T = 220 K (black dots)). The solid line is the approximation by Eq. The insert shows orientations of rf and dc magnetic fields and the direction of the sample rotation. At Θ = 0 the dc magnetic field lies in the ab plane.

8. Angular dependencies of the linewidth above and below Tc = 208 K Angular dependencies of the linewidth ΔH below CODT (at temperatures T = 100 K (white dots), and T = 190 K (grey dots)) and above CODT (at temperature T = 220 K (black dots)). The solid line is the approximation by Eq.. The insert shows orientations of rf and dc magnetic fields and the direction of the sample rotation. At Θ = 00 the dc magnetic field lies in the ab plane.

9. Sharp jumps of the EPR parameters Temperature dependencies of the linewidth ΔH (white dots) and the g-value (black dots). Arrows mark the charge order-disorder phase transition. Solid lines are guides for the eye.

10. DC and AC magnetic susceptibilities SQUID data one-dimensional alternating Heisenberg model S = 1/2, J1 = 106 K and α = J2/J1 = 0.35 Yue Y., et al., J.Phys.Soc.Jap., 2009 Difference of the DC and AC susceptibilities indicates dynamics of the charge carriers hopping

11. Competition of ESR and hopping rate frequencies ESR frequency 10 HGz ESR frequency 10 HGz

12. Conclusions: • Charge ordering in α'-(BEDT-TTF)2IBr2 is accompanied by sharp changes of the EPR parameters: integral intensity, g- factor and linewidth H. This fact corresponds to spin transition in the subsystem of the localized charge carriers. • Exchange interaction between charge carriers provides narrowing of the EPR line as well as rapid decrease of magnetic susceptibility caused by cooling of the crystal below 50 K. • Difference between static and dynamic magnetic measurements indicates hopping of the charge carriers. We are grateful to Prof. E.Yagubskii for fruitful discussion