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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. - PowerPoint PPT Presentation
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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