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

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

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

Resume of temperature dependencies studying No structural transitions!

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

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

Angular dependencies of g-factor above and below T c = 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.

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.

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.

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

Competition of ESR and hopping rate frequencies ESR frequency 10 HGz

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