Charge Order-DisorderCharge Order-Disorder Phase Phase Transition Detected By EPRTransition Detected By EPR

in α'-(BEDT-TTF)in α'-(BEDT-TTF)22IBrIBr22

11Roman Morgunov, Roman Morgunov, 11Alexei Dmitriev, Alexei Dmitriev, 11Alisa Chernenkaya,Alisa Chernenkaya, 22Kaoru Yamamoto, Kaoru Yamamoto, 22Kyuya YakushiKyuya Yakushi

11Institute of Problem of Chemical Physics, Institute of Problem of Chemical Physics, Chernogolovka, RussiaChernogolovka, Russia

22Institute for Molecular Sciences, Okazaki, JapanInstitute for Molecular Sciences, Okazaki, Japan

Crystal structure and charge localization in Crystal structure and charge localization in α'-(BEDT-TTF)α'-(BEDT-TTF)22IBrIBr22

Yagubskii E., Shegolev I., Shibaeva R. et al. Yagubskii E., Shegolev I., Shibaeva R. et al. JETP letters, 1985JETP letters, 1985

Temperature dependenciesTemperature dependencies

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

Resume of temperature dependencies Resume of temperature dependencies studyingstudying

No structural transitions!No structural transitions!

ESR spectra of theESR spectra of the α'-(BEDT-TTF)α'-(BEDT-TTF)22IBrIBr2 2 single single

crystalcrystal

3200 3400 3600-0.2

-0.1

0.0

0.1

0.2 2

H (Oe)

dI/

dH

(ar

b. u

nit

s)

1e

Hp

H

HHH

22)3/10(

Lowest linewidth at 3 K is 0.8 Oe might be explainedby Anderson-Weiss theory:

Нр2 = 5.1(gВ 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 JgВ (S(S + 1))1/2 = 42 kOe is exchange

interaction energy expressed in field units.

Lorenz lineshape doesn’t change under Lorenz lineshape doesn’t change under localization at 208 Klocalization at 208 K

3000 3300 3600 3900-4

-2

0

2

4

dP

/dH

, arb

. un

its

H, Oe

T = 215 K

3000 3300 3600 3900

-2

0

2

dP

/dH

, arb

. un

its

H, Oe

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)2IBr2α'-(BEDT-TTF)2IBr2

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

0 30 60 90 120 150 180

2,002

2,004

2,006

2,008

2,010

100 K

220 K190 Kg

-va

lue

(deg)

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.

2sin2cos

2sin22cos22

HII

HobsH

gIIg

obsg

a

Hdc

Hrf

c

b-a

(110)

(001) b

Angular dependencies of Angular dependencies of the linewidth the linewidth above and below Tc = 208 Kabove and below Tc = 208 K

0 30 60 90 120 150 18050

60

70

80

90

100

110

220 K

190 K

H (

Oe)

(deg)

100 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 parametersSharp 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.

0 100 200 3000

30

60

90

T (K)

H (O

e)

2,0055

2,0056

2,0057

2,0058

g-value

180 200 220 24050

60

70

80

T (K)

H (O

e)

2,0055

2,0056

2,0057

2,0058

g-value

DC and AC magnetic susceptibilities DC and AC magnetic susceptibilities

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

0 100 200 300

0

4

8

12Deviationfrom SQUID

Antiferro-

Curie

T (K)

(ar

b. u

nit

s)

Pauli

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., 2009Yue Y., et al., J.Phys.Soc.Jap., 2009

SQUID data

Competition of ESR and hopping rate Competition of ESR and hopping rate frequenciesfrequencies

ESR frequencyESR frequency 10 10 HGzHGz

ESR frequencyESR frequency 10 10 HGzHGz

ConclusionsConclusions::

• 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 discussionWe are grateful to Prof. E.Yagubskii for fruitful discussion