TTF-CA - 1. Physikalisches Institut

COUPLING OF CHARGE AND SPIN ORDER
IN ORGANIC CHARGE TRANSFER SALTS
Martin Dressel
1. Physikalisches Institut, Universität Stuttgart, Germany
Outline
1.
2.
Introduction
organic materials, ferroelectrics
Mixed-Stack Compound TTF-CA
neutral-ionic transition
3.
Pressure-Temperature Phase Diagram
change of ionicity
4.
Dynamics of the Neutral-Ionic Transition
photo-induced phase transition,
random-walk model
5.
Summary
A.Dengl, R. Beyer, T. Ivek,
T. Peterseim, P. Haremski
1. Physikalisches Institut
Universität Stuttgart
M. Masino, A. Girlando
Universita Parma
4
Organic Conductors
basics
Organic solids
are in general insulators because the bonds are saturated,
the electronic bands are filled.
Universität Stuttgart
Requirements for electrical conductivity:
• overlap of orbitals:
band formation
• add or extract electrons:
- electrical field effect
- charge transfer salts
partially filled bands
5
Organic Conductors
charge-transfer salts
TTF-TCNQ
The structure consists of TTF and TCNQ stacks.
TTF is a strong electron donor,
TCNQ is an electron acceptor.
Morpurgo
6
Organic Conductors
charge-transfer salts
TTF-TCNQ
The structure consists of TTF and TCNQ stacks.
TTF is a strong electron donor,
TCNQ is an electron acceptor.
Along the segregated stacks the π-orbitals overlap
leading to one-dimensional conductivity.
7
Organic Conductors
charge-transfer salts
TTF-TCNQ
• The crystals are highly conducting
along the chains: σ|| = 5⋅102 (Ωcm)-1.
• The anisotropy approximate σ|| /σ⊥ = 100,
but grows upon cooling.
• At T = 53 K the material undergoes
a CDW transition.
D. Tanner, C.S. Jacobsen, et al. Phys. Rev. B 13, 3381 (1976)
M.J. Cohen et al., Phys. Rev. B 10, 1298 (1974), 13, 2254 (1976); 16, 688 (1977)
8
Ferroelectrics
basics
Ferroelectrics
spontaneous (permanent) electric polarization
Requirements for ferroelectricity:
• spontaneous dipole moment
that can be switched in an applied electric field
• loss of center symmetry
-q
d
+q
9
TTF-CA
charge-transfer salts
TTF-CA
Charge transfer compound with
single mixed stack:
donor:
TTF, DMTTF
acceptor:
QCl4 (CA), QBrCl3, QBr4, …
_
+
_
+
_
+
_
+
Universität Stuttgart
10
TTF-CA
neutral-ionic transition
TTF-CA
Charge transfer compound with
single mixed stack:
donor:
TTF, DMTTF
acceptor:
QCl4 (CA), QBrCl3, QBr4, …
Neutral-ionic transition
D0A0 → D+ρA-ρ
Ionicity
The antisymmetric CA mode ν10(b1u)
is a very sensitive local probe of the charge:
Dimerization
The appearance of the
totally symmetric CA mode ν5(ag)
indicates the loss of symmetry at low temperatures.
11
TTF-CA
neutral-ionic transition
TTF-CA
Charge transfer compound with
single mixed stack:
donor:
TTF, DMTTF
acceptor:
QCl4 (CA), QBrCl3, QBr4, …
Neutral-ionic transition
D0A0 → D+ρA-ρ
TTF-CA:
TNI = 81 K
TTF-QBrCl3: TNI = 66 K
TTF-BA:
TNI = 50 K
εmax = 500
εmax = 800
εmax = 20
Ferroelectric phase
K. Kobayashi et al. Phys. Rev. Lett. 108, 237601 (2012)
J.B. Torrance et al., Phys. Rev. Lett. 47, 1747 (1981)
S. Horiuchi et al. J. Phys. Soc. Jpn. 69, 1302 (2000)
13
TTF-CA
ferroelectricity
TTF-CA
electronic ferroelectricity of 6.3 µC/cm2
due to change of ionicity from 0.3e to 0.6e
plus a small dimerization.
• Switching effect,
• Curie behavior,
• etc.
K. Kobayashi et al. Phys. Rev. Lett. 108, 237601 (2012)
14
DMTTF-BA
quantum phase transition
DMTTF-BA
Suppression of neutral-ionic transition
by physical or chemical pressure
results in quantum phase transition
DMTTF-BA
S. Horiuchi et al. Science 299, 299 (2003)
15
TTF-CA
pressure dependence
TTF-CA
Pressure-dependent studies
of the molecular vibrations yield
that the charge per molecule changes with pressure.
At room temperature the neutral ionic transition
takes place at around 0.9 GPa = 9 kbar.
There seems to be a regime of coexistence
or some intermediate phase.
M. Masino, A. Girlando and A. Brillante, Phys. Rev. B 76, 064114 (2007)
16
Optical Measurements
experimental setup
Fourier transform infrared spectrometer
combined with IR microscope
Frequency range:
15 cm-1 – 25 000 cm-1
(2 meV – 3 eV)
Temperature range:
2 K ≤ T ≤ 300 K
Coherent-source THz spectrometer
Frequency range:
Temperature range:
0.3 K ≤ T ≤ 300 K
1 cm-1 – 48 cm-1
(0.1 meV – 6 eV)
Corbino microwave spectrometer
Frequency range:
Temperature range:
50 MHz GHz – 50 GHz
0.1 K ≤ T ≤ 300 K
Pin
Vector
network
analyzer
Sample holder
Coaxial cable
Spring
Corbino adapter
Sample
17
Optical Measurements
under hydrostatic pressure
Hydrostatic piston-clamp cell
1)
2)
3)
4)
Body made from Cu-Be
IR
Clamping nuts
Pistons applying and holding the pressure
Inset with optical access,
holding the diamond window and the sample
5) Teflon cap with pressure medium.
(1)
(2) (4)
(3)
(2)
(3)
(5)
(1)
Pressure
Temperature
Frequency
1 – 11 kbar
5 – 300 K
100 – 14000 cm-1
R. Beyer, N. Barisic, M. Dressel (2014)
18
TTF-CA
temperature and pressure dependence
TTF-CA
Pressure- and temperature dependent studies
of the molecular vibrations yield
a single charge per molecule.
No additional phase exists,
no regime of coexistence.
300
This work
IR (Phys. Rev. B 36, 3884–3887 (1987))
DC (Phys. Rev. B 35, 427–429 (1987))
280
NI-Transitiontemperature [K]
Upon applying pressure,
TNI shifts to higher temperatures
dTNI/dp = 25 K/kbar
and the ionicity becomes larger.
260
240
220
neutral
200
180
160
ionic
140
120
100
80
0
1
2
3
4
5
6
7
8
9
10
11
12
Pressure [kbar]
A. Dengl, R. Beyer, T. Peterseim, T. Ivek, G. Untereiner and M. Dressel, J. Chem. Phys. 140, 244511 (2014)
19
TTF-CA
Peierls-coupled mode
TTF-CA
Upon approaching TNI
• electrons become delocalized and
the effective Peierls coupling increases
• the lattice phonons become soft
• the dielectric constant increases
A. Girlando, M. Masino, A. Painelli, N. Drichko, M. Dressel, A. Brillante, R.G. DellaValle, E. Venturi, Phys. Rev. B 78, 045103 (2008)
M. Masino, A. Girlando, A. Brillante, R.G. DellaValle, E. Venturi, N. Drichko, M. Dressel, Chem. Phys. 325, 71 (2006)
20
TTF-CA
photoinduced phase transition
TTF-CA
Switching between neutral and ionic phase
• by temperature TNI = 81 K,
• by pressure pNI = 8.5 kbar and
• by light.
Cooperative charge transfer induced by laser pulses:
• switching from ionic to neutral phase,
switching from neutral to ionic phase;
• kick-off of domain walls depending on
• temperature,
• power and
• pulse duration.
D
A
D
A
D
A
D
ħω
D0A0 D+A- D0A0
D0A0 D0A0 D0A0
D0A0 D0A0 D0A0
D0A0 D0A0 D0A0
photo
switching
photo
switching
D+A- D+A- D+AD+A- D+A- D+A-
D+A- D+A- D+AD+A- D+A- D0A0
ħω
A
S. Koshihara et al. J. Phys. Chem. B. 103, 2592 (1999); H. Okamoto et al. Phys. Rev, B 70, 165202 (2004)
21
TTF-CA
photoinduced phase transition
TTF-CA
Switching between neutral and ionic phase
• by temperature TNI = 81 K,
• by pressure pNI = 8.5 kbar and
• by light.
Neutral-ionic domain walls motion explains:
• dielectric properties
• optical properties
• dc transport
• solitons: charge or spin
• polaron
D
A
D
A
D
A
D
ħω
D0A0 D+A- D0A0
D0A0 D0A0 D0A0
D0A0 D0A0 D0A0
D0A0 D0A0 D0A0
photo
switching
photo
switching
D+A- D+A- D+AD+A- D+A- D+A-
D+A- D+A- D+AD+A- D+A- D0A0
ħω
A
S. Koshihara et al. J. Phys. Chem. B. 103, 2592 (1999); H. Okamoto et al. Phys. Rev, B 70, 165202 (2004)
22
TTF-CA
photoinduced phase transition
TTF-CA
• What are the properties of the photoinduced state?
• How long does the metastable state exist?
• By which process does it relax back to the initial state?
Starting from the ionic state at T = 78 K,
we can switch the system locally to the neutral state
by a laser pulse:
• photon energy
ħω = 2.33 eV
• wavelength
λ = 532 nm
• pulse length
8 ns
• repetition rate
20 Hz
T. Petersheim, P. Haremski and M. Dressel (2014)
23
TTF-CA
photoinduced phase transition
TTF-CA
Switching between neutral and ionic phase
• by temperature TNI = 81 K,
• by pressure pNI = 8.5 kbar and
• by light.
The change in the optical reflectivity indicates
the transformation from the neutral back to the ionic phase.
ν = 1390 cm-1
T. Petersheim, P. Haremski and M. Dressel (2014)
24
TTF-CA
photoinduced phase transition
TTF-CA
The stretched-exponential time dependence
can be explained by a
one-dimensional random walk model
of photo-induced neutral-ionic domain walls (NIDW).
• The light pulse creates neutral domains
with number and size depending on
temperature and intensity.
• The NIDW diffuse left or right.
• They can eventually annihilate
• The survival probability S(t) of
a domain wall pair decays
in a stretched-exponential way:
T. Petersheim, P. Haremski and M. Dressel (2014)
25
TTF-CA
photoinduced phase transition
26
TTF-CA
photoinduced phase transition
temperature dependence
Temperature
TNI
Intensity
TTF-CA
The stretched-exponential time dependence
can be explained by a
one-dimensional random walk model
of photo-induced neutral-ionic domain walls (NIDW).
0K
intensity dependence
T. Petersheim, P. Haremski and M. Dressel (2014)
27
TTF-CA
domain wall motion
Neutral phase D0A0 D0A0 D0A0 D0A0 D0A0 D0A0 D0A0 D0A0 D0A0
TTF-CA
Neutral-ionic domain walls
• motion explains dielectric properties
• optical properties
• dc transport
• solitons: charge or spin
• polaron
D
A
D
A
D
A
D
Ionic phase
D+A- D+A- D+A- D+A- D+A- D+A- D+A- D+A- D+AD+ A-D+ A-D+ A-D+ A-D+ A-D+ A-D+ A-D+ A-D+ A-
NI domain wall D0A0 D0A0 D+A- D+A- D+A- D+A- D+A- D0A0 D0A0
type A
+ρ
-ρ
NI domain wall D0A0 D0A0 A-D+ A-D+ A-D+ A-D+ D0A0 D0A0 D0A0
type B
-ρ
+ρ
Spin-soliton
pair
D+A- D+ A-D+ A-D+ A-D+ A-D+ A- D+A- D+A- D+A-
Charged
soliton pair
D+ A-D+A0 D+A- D+A- D+A- D+A- D0 A-D+ A-D+ A
+ρ
-ρ
Polaron pair
D0A0 D+A- D+ A-D+ A0 D0A0 D0 A-D+ A- D+A- D0A0
+ρ
-ρ
A
H. Okamoto et al. Phys. Rev. B 43, 8224 (1991)
F. Kagawa et al., Phys. Rev. Lett. 104, 227602 (2010)
28
TTF-CA
domain wall motion
TTF-CA
Neutral-ionic domain walls
• motion explains dielectric properties
• optical properties
• dc transport
• solitons: charge or spin
• polaron
• reduction of domain walls
by external electric field
• spinless solitons enhance dielectric response
• but suppress dc conductivity
F. Kagawa et al., Phys. Rev. Lett. 104, 227602 (2010)
29
TTF-CA
domain wall motion
TTF-CA
Neutral-ionic domain walls
• motion explains dielectric properties
• optical properties
• dc transport
• solitons: charge or spin
• polaron
• reduction of domain walls
by external electric field
• spinless solitons enhance dielectric response
• but suppress dc conductivity
F. Kagawa et al., Phys. Rev. Lett. 104, 227602 (2010)
30
TTF-CA
domain wall motion
TTF-CA
Neutral-ionic domain walls
• motion explains dielectric properties
• optical properties
• dc transport
• solitons: charge or spin
• polaron
• reduction of domain walls
by external electric field
• ESR allows to detect spin solitons
• temperature independent
• well separated
Spin-soliton
pair
D+A- D+ A-D+ A-D+ A-D+ A-D+ A- D+A- D+A- D+A-
Charged
soliton pair
D+ A-D+A0 D+A- D+A- D+A- D+A- D0 A-D+ A-D+ A
+ρ
-ρ
F. Kagawa et al., Phys. Rev. Lett. 104, 227602 (2010)
31
TTF-CA
multiferroicity
TTF-CA
One-dimensional Heisenberg chain
with J = 1600 K
Ab initio calculations predict multiferroicity
• depending on the electron-phonon coupling α
• Coulomb repulsion U
G. Giovannetti et al. Phys. Rev. Lett. 103, 266401 (2009)
M. Filatov, Phys. Chem. Chem. Phys. 13, 144 (2011)
32
TTF-BA
multiferroicity
TTF-BA
• One-dimensional Heisenberg chain
that undergoes a spin-Peierls transition at 53 K.
• accompanied by a dielectric anomaly
F. Kagawa et al. Nature Physics 6, 169 (2010)
33
TTF-BA
multiferroicity
TTF-BA
• One-dimensional Heisenberg chain
that undergoes a spin-Peierls transition at 53 K.
• accompanied by a dielectric anomaly
with a magnetoelectic coupling
F. Kagawa et al. Nature Physics 6, 169 (2010)
34
TTF-BA
multiferroicity
TTF-BA
• One-dimensional Heisenberg chain
that undergoes a spin-Peierls transition at 53 K.
• accompanied by a dielectric anomaly
with a magnetoelectic coupling
F. Kagawa et al. Nature Physics 6, 169 (2010)
35
Mixed-Stack Compound
with supramolecular network
A.S. Tayi et al., Nature 488, 485 (2012)
36
Summary
Mixed-Stack Compound TTF-CA
• organic ferroelectrica
• change in ionicity and dimerization
• switching by light
• tuning by pressure

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