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 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 0.2

0.1

1 2



H

 ( 10 / 3 ) 

H p

2

H e

 

H H

2

0.0

-0.1

-0.2

3200 3400 H (Oe) 3600 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.

-2 0 2 0 -2 -4 2 4

Lorenz lineshape doesn’t change under localization at 208 K

T = 215 K 3000 3300 H, Oe 3600 3900 T = 200 K

Journal of Experimental and Theoretical Physics, 2006, Vol. 102, No. 1, pp. 121–130. ( http://scitation.aip.org/jetp/ ).

α'-(BEDT-TTF)2IBr2 3000 3300 H, Oe 3600 3900

Angular dependencies of g-factor above and below T c = 208 K

2,010

g obs

2  

H obs



g II

2 cos 2  

g

 2 sin 2  

H II

cos 2   

H

 sin 2 

2,008 2,006 2,004 2,002 0 30 60 90



(deg) 120 150 190 K 220 K 100 K 180

H rf a c

(110)

H dc 

(001)

b b-a

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

110 100 90 80 70 60 50 0 100 K 190 K 220 K 30 60 90



(deg) 120 150 180 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

90 60 30 0 0 100 T (K) 200 2,0058 2,0057 2,0056 300 2,0055 2,0058 80 2,0057 70 2,0056 60 50 180 200 T (K) 220 240 2,0055 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

12 Antiferro Deviation from SQUID Curie 8 Pauli 4 SQUID data 0 0 100 T (K) 200 300 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

Difference of the DC and AC susceptibilities indicates dynamics of the charge carriers hopping

Competition of ESR and hopping rate frequencies

ESR frequency 10 HGz 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