EPJ manuscript No.
(will be inserted by the editor)
Metal-insulator transition in the low-dimensional organic
conductor (TMTSF)2FSO3 probed by infrared microspectroscopy
A. Pashkin1a, K. Thirunavukkuarasu1, Y.-L. Mathis2, W. Kang3, and C. A. Kuntscher1b
1 Experimentalphysik II, Universität Augsburg, 86159 Augsburg, Germany
2 Institute for Synchrotron Radiation, Forschungszentrum Karlsruhe, P.O. Box 3640, 76021 Karlsruhe, Germany
3 Department of Physics, Ewha Womans University, Seoul 120-750, Korea
Received: October 28, 2018
Abstract. We present measurements of the infrared response of the quasi-one-dimensional organic conduc-
tor (TMTSF)2FSO3 along (E‖a) and perpendicular (E‖b
′) to the stacking axis as a function of temper-
ature. Above the metal-insulator transition related to the anion ordering the optical conductivity spectra
show a Drude-like response. Below the transition an energy gap of about 1500 cm−1 (185 meV) opens,
leading to the corresponding charge transfer band in the optical conductivity spectra. The analysis of the
infrared-active vibrations gives evidence for the long-range crystal structure modulation below the transi-
tion temperature and for the short-range order fluctuations of the lattice modulation above the transition
temperature. Also we report about a new infrared mode at around 710 cm−1 with a peculiar temperature
behavior, which has so far not been observed in any other (TMTSF)2X salt showing a metal-insulator
transition. A qualitative model based on the coupling between the TMTSF molecule vibration and the
reorientation of electrical dipole moment of the FSO3 anion is proposed, in order to explain the anomalous
behavior of the new mode.
PACS. 71.30.+h Metal-insulator transitions and other electronic transitions – 74.70.Kn Organic super-
conductors
1 Introduction
The organic Bechgaard salts (TMTSF)2X consist of
stacks of planar TMTSF (tetramethyltetraselenafulva-
lene) molecules separated by anions (X = PF6, AsF6,
ClO4, Br, etc.). The charge transport in these systems
is restricted to the direction along the molecular stacks,
making the Bechgaard salts prime examples of one-
dimensional metals. However, on cooling down most of
them undergo a metal-insulator transition which prevents
the onset of a superconducting state [1]. In Bechgaard
salts with noncentrosymmetric anions such as ReO4, BF4
or FSO3 the metal-insulator transition is related to the
anion ordering [2]. It was furthermore demonstrated that
in some cases the metal-insulator transition can be sup-
pressed by the application of external pressure, leading to
a superconducting ground state [3].
The case of the anions X=FSO3 in this class of ma-
terials is particularly interesting, since these anions are
noncentrosymmetric and in addition possess a permanent
electrical dipole moment. The first study of the basic prop-
erties of (TMTSF)2FSO3 has been reported by Wudl et
al. in 1982 [4]. Further studies have shown that this com-
a email: oleksiy.pashkin@physik.uni-augsburg.de
b email: christine.kuntscher@physik.uni-augsburg.de
pound has the highest superconducting transition temper-
ature (2.5 K at 8.5 kbar) among the Bechgaard salts. It
was proposed that this is due to the interaction of the
conducting electrons with the FSO3 anion dipoles [5]. A
recent detailed study [6] revealed a very rich pressure-
temperature phase diagram of (TMTSF)2FSO3 with a va-
riety of different phases, which have not been completely
identified up to now. Furthermore, by magnetoresistance
measurements a two-dimensional electronic behavior was
found in (TMTSF)2FSO3 under a pressure of around 6.2
kbar [7].
The interaction of the FSO3 anions with each other via
long-range Coulomb forces and with the centrosymmetric
surrounding formed by the TMTSF cations tends to order
the anions below a certain temperature. The first-order
structural phase transition related to this anion ordering
occurs at around TMI=89 K in (TMTSF)2FSO3 at ambi-
ent pressure. The change of the crystal structure modifies
the electronic band structure: The effective half-filled con-
ducting band splits into one filled and one empty band
separated by an energy gap, leading to a sharp metal-
insulator transition [5]. The structural analysis suggested
a modulation of the crystal structure with wavevector q =
(1/2, 1/2, 1/2) below the phase transition, which implies
an antiferroelectric state [8,2]. The ordering of the FSO3
anions modulates the lattice resulting in a new unit cell
http://arxiv.org/abs/0704.0368v1
2 A. Pashkin et al.: Metal-insulator transition in (TMTSF)2FSO3 probed by infrared spectroscopy
of size 2a × 2b × 2c. Thus, there are eight formula units
of (TMTSF)2FSO3 per unit cell in the low temperature
phase. Correspondingly, one can expect a splitting of each
vibrational mode into up to eight components [9].
The ratio of the energy gap to the transition tem-
perature in (TMTSF)2FSO3 is ∼ 12.5 [4], which is ap-
preciably higher than the value 3.5 predicted by the
mean-field theory for the Peierls transition. Therefore,
the metal-insulator in the Bechgaard salts with non-
centrosymmetric anions was attributed to a special type of
Peierls instability which originates from the anion-electron
coupling [10].
In this work we present the results of a temperature-
dependent polarized infrared reflectivity study of
(TMTSF)2FSO3 single crystals in the far- and mid-
infrared frequency range, in order to characterize the
change of electronic and vibrational properties during
the metal-insulator transition at TMI=89 K. This is the
first infrared spectroscopic investigation of the compound
(TMTSF)2FSO3. Our results allow a direct determination
of the charge gap in the insulating state. Furthermore, we
determined and analyzed the behavior of the vibrational
modes during the metal-insulator transition, which can
clarify details of the dipolar ordering.
2 Experimental
(TMTSF)2FSO3 single crystals were grown by standard
electrochemical techniques from TMTSF molecules and
tetrabutylammonium-FSO3. The studied samples have a
needle-like shape, with a size of approximately 2 × 0.2 ×
0.1 mm3. The samples were mounted on a cold-finger Cry-
oVac Konti-Mikro cryostat. The actual measuring temper-
ature was controlled by a sensor attached in direct vicin-
ity of the sample. The measurements were performed at
the infrared beamline of the synchrotron radiation source
ANKA. The polarized infrared reflectivity was measured
in the range 150 - 10000 cm−1 using a Bruker IRscope
II microscope attached to a IFS66v/S spectrometer. The
frequency resolution was 1 cm−1 for all measured spec-
tra. Optically transparent TPX and KBr cryostat win-
dows were used for the measurements in the far- and mid-
infrared frequency range, respectively.
3 Results and discussion
3.1 Electronic properties
The reflectivity spectra of (TMTSF)2FSO3 above and be-
low the metal-insulator transition temperature of 89 K for
both polarizationsE‖a and E‖b′ (along and perpendicular
to the stacking axis, respectively) are shown in Fig. 1. The
reflectivity data in the spectral region at around 450 cm−1
are affected by the absorption features of the far-infrared
TPX cryostat window and are therefore not shown.
At 290 K the reflectivity of the sample along the
stacking axis E‖a demonstrates a typical Drude behavior
E || a
 290 K
 45 K
8000250
E || b'
Frequency (cm
 290 K
 45 K
 Energy (meV)
E || a
Fig. 1. Reflectivity spectra of (TMTSF)2FSO3 above and be-
low the metal-insulator transition for E‖a and E‖b′.
(growth up to 1 when frequency tends to zero). In con-
trast, at 45 K , i.e., below TMI , the reflectivity is almost
frequency independent below 1000 cm−1, which is typical
for an insulating state.
The interference fringes observed below 400 cm−1 in
the spectra for both polarizations are due to the partial
transparency of the sample in the insulating phase. Per-
pendicular to the stacking axis (E‖b′), the optical reflec-
tivity and conductivity is much lower than along the a
axis. Nevertheless, the observed changes during the metal-
insulator transition are similar to those of the E‖a direc-
tion. These results demonstrate the opening of an energy
gap at the Fermi level for both studied directions.
The dramatic effect of the temperature decrease on the
electronic properties of (TMTSF)2FSO3 are more directly
seen in the optical conductivity spectra. The E‖a optical
conductivity σ1(ω) of (TMTSF)2FSO3 in the insulating
(at 45 K) and conducting phase (at 290 K) obtained by
means of Kramers-Kronig analysis is shown in Fig. 2. The
dominating feature of the spectrum at 45 K is a strong
charge transfer band due to electronic transitions across
the gap. The arrow shows the band gap (1500 cm−1) ob-
tained from the published temperature-dependent dc re-
sistivity measurements [4]. Obviously, the agreement of
this value with the onset of the optical interband tran-
A. Pashkin et al.: Metal-insulator transition in (TMTSF)2FSO3 probed by infrared spectroscopy 3
1000 10000
100 1000
2D = 1500 cm
Frequency (cm
 290 K
 45 K
E || a
 Energy (meV)
Fig. 2. E‖a optical conductivity spectra of (TMTSF)2FSO3
above and below the metal-insulator transition at TMI= 89 K.
Hatched area depicts the Drude model fit of the high temper-
ature optical conductivity.
sition is very good. On the other hand, the optical con-
ductivity at room temperature is mostly dominated by
the Drude response of the free carriers. The corresponding
fit using the Drude model is shown as the hatched area
in Fig. 2. Obviously, the Drude model provides a good
description of the measured room-temperature spectrum
excluding the electron-molecular vibration (emv) antires-
onance modes. The plasma frequency ωp = 8660 cm
and the scattering rate Γ ≃ 1450 cm−1 obtained from the
fit agree well with the Drude model parameters reported
for other TMTSF salts [11]. The obtained value of the dc
conductivity, σdc ≃ 860 (Ωcm)
−1, is in reasonable agree-
ment with the dc and microwave conductivity values of
1600 and 300 (Ωcm)−1, respectively, reported by Wudl et
al. [4].
3.2 Vibrational modes
The TMTSF molecule with the point group symmetry
D2h has in total 72 local vibrational modes classified ac-
cording to the following representations [12]
ΓD2h = (12ag + 11b3g + 11b1u + 11b2u)
+(6b1g + 7b2g + 7au + 7b3u), (1)
where the vibrations in the first brackets are polarized in
the molecular plane (perpendicular to the stacking a axis)
and the vibrations in the second brackets are polarized out
of the plane (along the stacking axis a). The symmetric
(gerade) vibrations are Raman active and the asymmet-
ric (ungerade) vibrations are infrared active excluding the
au silent modes. Some of the totally symmetric ag Ra-
man modes are expected to appear in the infrared spectra
for E‖a due to efficient emv coupling in the modulated
stacking structure [11,13].
Table 1. The eigenfrequencies and assignment of some vi-
brational modes observed in (TMTSF)2FSO3 for E‖a at 45 K
below TMI . All numbers are in cm
45 K calculated frequency1 assignment
580 571 ν3(a1) FSO3
728 702 ν51(b2u)
902, 911, 915, 916 ν8(ag)
917, 924, 932
1020, 1031, 1036 1060 ν7(ag)
1067, 1072 1060 ν7(ag)
1362, 14502 1469 ν4(ag)
1354, 1364, 1369 1369 ν6(ag)
1373, 1379, 1385
1550, 1584, 1606 1596 ν3(ag)
1847, 1854, 1863 1863 ν3(ag) + ν11(ag)
The tetrahedral FSO3 anion has C3v point group sym-
metry which gives in total nine vibrational modes
ΓC3v = 3a1(z, x
2 + y2, z2) + 3e(x, y, x2 − y2, xy, yz, xz),
where e species correspond to the doublets. Thus, in the
infrared spectra one expects six modes, with the 3a1 and
3e modes being polarized along and perpendicular to the
polar axis of the anion, respectively.
In this section we want to concentrate on the changes
in the infrared phonon spectra for both polarizations
across the metal-insulator transition. For E‖a several ag
vibrations of the TMTSF molecules become infrared ac-
tive in the insulating phase. This is due to the effec-
tive emv coupling of these vibrations to the on-chain
charge transfer band in the structure modulated due to
the anion ordering. The list of the new modes observed
below the transition together with their tentative as-
signment is given in Table 1. Most of them are emv
coupled ag modes polarized in the molecular plane or
their combination as a triplet at around 1850 cm−1. The
ν4(ag) mode involving the central C=C bond stretching is
known to have especially strong emv coupling and there-
fore it appears as a strong antiresonance mode in the
optical conductivity spectrum. It should be pointed out
that the observed appearance of ag modes for E‖a in
the ordered phase is typical only for (TMTSF)2X com-
pounds with non-centrosymmetric anions. In comparison,
(TMTTF)2X salts possess a stronger stack dimerization,
resulting in the emv coupling of the ag modes already
in the disordered phase, and therefore the anion ordering
transition causes only a frequency shift and an intensity
change of the emv coupled modes [15].
4 A. Pashkin et al.: Metal-insulator transition in (TMTSF)2FSO3 probed by infrared spectroscopy
1264 1280 1296
560 570 580 590 600 1140 1150 1160
1358 1365 1372
Frequency (cm
 Frequency (cm
 Frequency (cm
  Frequency (cm
Fig. 3. Reflectivity spectra (shifted for clarity) of some
phonons which experience changes during the metal-insulator
transition at 89 K: (a) vibration polarized along the a axis;
(b)-(d) vibrations polarized along the b′ axis.
The ν3(a1) vibrational mode of the FSO3 anion at
580 cm−1 is observed for the whole studied temperature
range. However, the lineshape of this mode in the metallic
phase above TMI is inverted with respect to the insulating
phase [see Fig. 3(a)], since the background dielectric con-
stant is negative as expected for highly conducting metals
at low frequencies. Such a change is a clear evidence for
the suppression of the Drude conductivity in the insulat-
ing phase of (TMTSF)2FSO3.
The mode at 728 cm−1 observed for temperatures
below TMI is particularly interesting, since its intensity
gradually increases on temperature decrease (see Fig. 4).
A similar behavior is found for the polarization perpen-
dicular to the stacks, E‖b′. Moreover, above the transi-
tion temperature a strong asymmetric mode is seen at
710 cm−1. This mode shifts to lower frequencies and gets
stronger with increasing temperature. This mode has not
been observed in any other earlier study of the Bechgaard
salts. Therefore, it would be natural to assign it to a vibra-
tion of FSO3 anion. However, such an assignment would be
in contradiction to the experimental observations, since:
680 700 720 740 760
E || b'
E || a
200 K
200 K
  Frequency (cm
Fig. 4. Reflectivity spectra (shifted for clarity) of the vibration
at around 710 cm−1 at different temperatures for E||a and
E||b′.
(i) the ν5(e) and ν2(a1) vibrations of FSO3 located close
to the observed mode have frequencies which are by more
than 100 cm−1 higher or lower [14]; (ii) the intensity of
the anion vibration should not vanish at the order-disorder
transition point. Thus, one has to attribute the modes at
around 710 and 728 cm−1 to vibrations of the TMTSF
molecules. We suggest that both modes originate from the
ν51(b2u) in-plane vibration of the TMTSF molecule. Ac-
cording to the normal-coordinate analysis [12,16] its fre-
quency for a free TMTSF0.5+ cation is 702 cm−1.
The corresponding atomic movements involve stretch-
ing of the Se-C side bond and rocking of the adjacent
methyl group. For the b2u vibration the inversion symme-
try of the molecule is not preserved, causing its infrared-
activity for the polarization perpendicular to the stacks.
However, it is known that in (TMTSF)2X salts the dipole
moment corresponding to the ν51(b2u) vibration is very
small, and therefore this mode can hardly be detected
even for E‖b′ where it should have the strongest inten-
sity [16]. Nevertheless, in (TMTSF)2FSO3 this mode is
particularly strong even at room temperature. This find-
ing can be explained by the electrical dipole of the FSO3
anion pointing towards the Se-F bond. Similar to other
A. Pashkin et al.: Metal-insulator transition in (TMTSF)2FSO3 probed by infrared spectroscopy 5
Fig. 5. Schematic illustration of the ν51(b2u) vibration coupled
to the reorientation of the FSO3 electrical dipole moment. The
projection of the crystal structure on the b− c plane is shown.
Only the Se (large open circles) and C (small filled circles)
atoms of the TMTSF molecules are presented, together with
the displacements of the Se atoms. The grey filled circles be-
tween molecules denote the positions of the FSO3 anions, the
bold arrows show the two possible orientations of the anion
dipole moment (p1 and p2). Because of the symmetry prop-
erties of the b2u vibration the reorientation of the electrical
dipole moment leads to a change of polarization ∆p in the
perpendicular direction for any orientation of the anion dipole
moment.
non-centrosymmetric anions (ReO4, ClO4 etc.) the FSO3
anion has two possible symmetrically equivalent orienta-
tions for which the dipole moment points towards the Se
atoms of the neighboring TMTSF molecules. This situa-
tion is sketched in Fig. 5, where p1 and p2 are two possible
orientations of the FSO3 electrical dipole moment. During
the vibration the dipole moment of the anion follows the
position of the Se atom. Due to the symmetry properties
of the b2u vibration the nearest Se atoms on both sides
of the anion move in the same direction. Thus, for both
possible orientations of the dipole the b2u vibration results
in a change of the average polarization along the direction
of ∆p (Fig. 5).
The described coupling mechanism between the b2u
vibration and the dipole moment of the anion in
(TMTSF)2FSO3 should lead to a strong enhancement of
the infrared strength of the ν51(b2u) vibration for E‖b
since ∆p has the largest projection along this direction.
On the other hand, ∆p is perpendicular to the stacking
axis and the b2u mode should not appear for E‖a. This
is indeed observed in our experiment above the transition
temperature. Below the transition the long-range order of
the anion sublattice builds up. Then the anion dipole mo-
ment orientation is determined by the modulation of the
whole lattice and it is not dependent on the movement of
neighboring TMTSF molecules, i.e., the ν51(b2u) vibration
is decoupled from the FSO3 anions. Therefore, its inten-
sity should drop abruptly below TMI , in agreement with
our observations (see Fig. 4). Moreover, the observed de-
crease of the intensity of the coupled b2u mode at around
Table 2. The eigenfrequency, width (given in bracket), and
assignment of some vibrational modes observed for E‖b′ at
selected temperatures. All numbers are in cm−1.
95 K 80 K 45 K assignment
580 (1.3) 580 (0.9) 580 (0.8) ν3(a1) FSO3
710 (7.1) 728 728 (2.0) ν51(b2u)
1154 (5.0) 1150 (3.4) 1150 (3.3) ν48(b2u)
1157 (3.8) 1158 (2.5)
1280 (5.1) 1276 (4.2) 1276 (2.1) ν4(e) FSO3
1288 (16) 1286 (4.4) 1286 (1.7)
1363 (3.2) 1361 (2.8) 1361 (1.3) ν47(b2u)
1366 (4.3) 1367 (2.4) 1367 (1.8)
710 cm−1 at 95 K compared to higher temperatures can be
explained by taking into account short-range order fluc-
tuations above the transition, evidence for which is also
given below. Indeed, in the large enough dynamical regions
where the anions are ordered, the coupling is suppressed
and therefore the strength of the ν51(b2u) should decrease.
On cooling down below TMI a vibration appears again
at somewhat higher frequency (728 cm−1) for E‖b′ and
its strength gradually increases with decreasing temper-
ature. We suggest that this is the same ν51(b2u) vibra-
tion described above. Since it is decoupled from the anion
sublattice, its frequency is expected to increase abruptly
below the transition. The increase in strength for both
polarizations should be obviously related to the tempera-
ture dependence of the order parameter (i.e., the degree
of lattice modulation). One of the possible mechanisms
can be the emv coupling of the ν51(b2u) vibration to the
charge transfer bands along a and b′ directions. However,
the detailed picture of this emv coupling is not clear, since
the symmetry of b2u mode does not allow such kind of
coupling. One can speculate that the electric field of the
FSO3 dipoles in the ordered phase distorts the TMTSF
molecules making them non-centrosymmetric. Then the
emv coupling may become allowed for the b2u(ν51) mode.
Noticeable changes in the phonon mode spectra across
the metal-insulator transition are observed for E‖b′. The
list of the parameters of these modes at temperatures
above and below TMI is given in Table 2. An obvious split-
ting into two components is seen for the ν48(b2u) mode at
1154 cm−1 [see Figure 3(b)]. In addition, the damping of
the split components directly below TMI is lower than the
damping of the single component directly above the tran-
sition (see Table 2). This difference is probably related
to the precursor short-range order fluctuations above the
transition, which can induce a small splitting already in
the disordered phase. An evidence for such fluctuations
was found in x-ray diffuse scattering experiments [8,2].
This effect is even more clearly seen in the splitting of
two other modes: the doublet ν4(e) vibration of the FSO3
anion at 1280 cm−1 [Fig. 3(c)] and the ν47(b2u) mode at
around 1365 cm−1 [Fig. 3(d)]. For each of these modes
6 A. Pashkin et al.: Metal-insulator transition in (TMTSF)2FSO3 probed by infrared spectroscopy
above TMI one can resolve two weakly split components.
However, below TMI the splitting abruptly increases and
the damping decreases (see Table 2) indicating the onset
of long-range order. Since the described effect is observed
not only for the FSO3 anion vibration but also for two vi-
brations of the TMTSF cation, we can conclude that the
short-range order fluctuations involve the modulation of
the whole (TMTSF)2FSO3 lattice and not only the anion
sublattice.
4 Conclusion
We have performed an infrared spectroscopic study of
the metal-insulator transition in (TMTSF)2FSO3. The ob-
tained optical conductivity spectra for E‖a show a Drude-
like conductivity above the anion ordering temperature
and a charge transfer band formed below the transition.
The onset of this band is in agreement with the energy
gap value of 1500 cm−1 obtained from transport measure-
ments [4].
The analysis of the infrared-active vibrations leads to
the following conclusions: (i) the crystal structure modula-
tion below the metal-insulator transition leads to a strong
emv coupling of several ag vibrations which therefore be-
come infrared-active; (ii) short-range order fluctuations of
the FSO3 anions and the corresponding lattice modulation
exist above the transition temperature, as it is seen from
the splitting of some infrared-active modes for E‖b′; (iii)
a new infrared-active mode located at around 710 cm−1
with a peculiar temperature behavior is detected and as-
signed to the coupling between the b2u TMTSF molecule
vibration and the electrical dipole moment of the FSO3
anion. The latter feature has not been observed in any
other (TMTSF)2X salt showing a metal-insulator transi-
tion. This points out the important role of the electrical
dipole moment of the anion on the structural and dynam-
ical properties of the (TMTSF)2FSO3 salt.
5 Acknowledgements
We acknowledge the ANKA Angströmquelle Karlsruhe for
the provision of beamtime and thank M. Süpfle, D. Moss,
and B. Gasharova for technical assistance at the ANKA
IR beamline. The financial support of the DFG (Emmy
Noether-program) is acknowledged.
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	Introduction
	Experimental
	Results and discussion
	Conclusion
	Acknowledgements

  We present measurements of the infrared response of the quasi-one-dimensional
organic conductor (TMTSF)2$SO3 along (E||a) and perpendicular (E||b') to the
stacking axis as a function of temperature. Above the metal-insulator
transition related to the anion ordering the optical conductivity spectra show
a Drude-like response. Below the transition an energy gap of about 1500 cm-1
(185 meV) opens, leading to the corresponding charge transfer band in the
optical conductivity spectra. The analysis of the infrared-active vibrations
gives evidence for the long-range crystal structure modulation below the
transition temperature and for the short-range order fluctuations of the
lattice modulation above the transition temperature. Also we report about a new
infrared mode at around 710 cm-1 with a peculiar temperature behavior, which
has so far not been observed in any other (TMTSF)2X salt showing a
metal-insulator transition. A qualitative model based on the coupling between
the TMTSF molecule vibration and the reorientation of electrical dipole moment
of the FSO3 anion is proposed, in order to explain the anomalous behavior of
the new mode.

Introduction
The organic Bechgaard salts (TMTSF)2X consist of
stacks of planar TMTSF (tetramethyltetraselenafulva-
lene) molecules separated by anions (X = PF6, AsF6,
ClO4, Br, etc.). The charge transport in these systems
is restricted to the direction along the molecular stacks,
making the Bechgaard salts prime examples of one-
dimensional metals. However, on cooling down most of
them undergo a metal-insulator transition which prevents
the onset of a superconducting state [1]. In Bechgaard
salts with noncentrosymmetric anions such as ReO4, BF4
or FSO3 the metal-insulator transition is related to the
anion ordering [2]. It was furthermore demonstrated that
in some cases the metal-insulator transition can be sup-
pressed by the application of external pressure, leading to
a superconducting ground state [3].
The case of the anions X=FSO3 in this class of ma-
terials is particularly interesting, since these anions are
noncentrosymmetric and in addition possess a permanent
electrical dipole moment. The first study of the basic prop-
erties of (TMTSF)2FSO3 has been reported by Wudl et
al. in 1982 [4]. Further studies have shown that this com-
a email: oleksiy.pashkin@physik.uni-augsburg.de
b email: christine.kuntscher@physik.uni-augsburg.de
pound has the highest superconducting transition temper-
ature (2.5 K at 8.5 kbar) among the Bechgaard salts. It
was proposed that this is due to the interaction of the
conducting electrons with the FSO3 anion dipoles [5]. A
recent detailed study [6] revealed a very rich pressure-
temperature phase diagram of (TMTSF)2FSO3 with a va-
riety of different phases, which have not been completely
identified up to now. Furthermore, by magnetoresistance
measurements a two-dimensional electronic behavior was
found in (TMTSF)2FSO3 under a pressure of around 6.2
kbar [7].
The interaction of the FSO3 anions with each other via
long-range Coulomb forces and with the centrosymmetric
surrounding formed by the TMTSF cations tends to order
the anions below a certain temperature. The first-order
structural phase transition related to this anion ordering
occurs at around TMI=89 K in (TMTSF)2FSO3 at ambi-
ent pressure. The change of the crystal structure modifies
the electronic band structure: The effective half-filled con-
ducting band splits into one filled and one empty band
separated by an energy gap, leading to a sharp metal-
insulator transition [5]. The structural analysis suggested
a modulation of the crystal structure with wavevector q =
(1/2, 1/2, 1/2) below the phase transition, which implies
an antiferroelectric state [8,2]. The ordering of the FSO3
anions modulates the lattice resulting in a new unit cell
http://arxiv.org/abs/0704.0368v1
2 A. Pashkin et al.: Metal-insulator transition in (TMTSF)2FSO3 probed by infrared spectroscopy
of size 2a × 2b × 2c. Thus, there are eight formula units
of (TMTSF)2FSO3 per unit cell in the low temperature
phase. Correspondingly, one can expect a splitting of each
vibrational mode into up to eight components [9].
The ratio of the energy gap to the transition tem-
perature in (TMTSF)2FSO3 is ∼ 12.5 [4], which is ap-
preciably higher than the value 3.5 predicted by the
mean-field theory for the Peierls transition. Therefore,
the metal-insulator in the Bechgaard salts with non-
centrosymmetric anions was attributed to a special type of
Peierls instability which originates from the anion-electron
coupling [10].
In this work we present the results of a temperature-
dependent polarized infrared reflectivity study of
(TMTSF)2FSO3 single crystals in the far- and mid-
infrared frequency range, in order to characterize the
change of electronic and vibrational properties during
the metal-insulator transition at TMI=89 K. This is the
first infrared spectroscopic investigation of the compound
(TMTSF)2FSO3. Our results allow a direct determination
of the charge gap in the insulating state. Furthermore, we
determined and analyzed the behavior of the vibrational
modes during the metal-insulator transition, which can
clarify details of the dipolar ordering.
2 Experimental
(TMTSF)2FSO3 single crystals were grown by standard
electrochemical techniques from TMTSF molecules and
tetrabutylammonium-FSO3. The studied samples have a
needle-like shape, with a size of approximately 2 × 0.2 ×
0.1 mm3. The samples were mounted on a cold-finger Cry-
oVac Konti-Mikro cryostat. The actual measuring temper-
ature was controlled by a sensor attached in direct vicin-
ity of the sample. The measurements were performed at
the infrared beamline of the synchrotron radiation source
ANKA. The polarized infrared reflectivity was measured
in the range 150 - 10000 cm−1 using a Bruker IRscope
II microscope attached to a IFS66v/S spectrometer. The
frequency resolution was 1 cm−1 for all measured spec-
tra. Optically transparent TPX and KBr cryostat win-
dows were used for the measurements in the far- and mid-
infrared frequency range, respectively.
3 Results and discussion
3.1 Electronic properties
The reflectivity spectra of (TMTSF)2FSO3 above and be-
low the metal-insulator transition temperature of 89 K for
both polarizationsE‖a and E‖b′ (along and perpendicular
to the stacking axis, respectively) are shown in Fig. 1. The
reflectivity data in the spectral region at around 450 cm−1
are affected by the absorption features of the far-infrared
TPX cryostat window and are therefore not shown.
At 290 K the reflectivity of the sample along the
stacking axis E‖a demonstrates a typical Drude behavior
E || a
 290 K
 45 K
8000250
E || b'
Frequency (cm
 290 K
 45 K
 Energy (meV)
E || a
Fig. 1. Reflectivity spectra of (TMTSF)2FSO3 above and be-
low the metal-insulator transition for E‖a and E‖b′.
(growth up to 1 when frequency tends to zero). In con-
trast, at 45 K , i.e., below TMI , the reflectivity is almost
frequency independent below 1000 cm−1, which is typical
for an insulating state.
The interference fringes observed below 400 cm−1 in
the spectra for both polarizations are due to the partial
transparency of the sample in the insulating phase. Per-
pendicular to the stacking axis (E‖b′), the optical reflec-
tivity and conductivity is much lower than along the a
axis. Nevertheless, the observed changes during the metal-
insulator transition are similar to those of the E‖a direc-
tion. These results demonstrate the opening of an energy
gap at the Fermi level for both studied directions.
The dramatic effect of the temperature decrease on the
electronic properties of (TMTSF)2FSO3 are more directly
seen in the optical conductivity spectra. The E‖a optical
conductivity σ1(ω) of (TMTSF)2FSO3 in the insulating
(at 45 K) and conducting phase (at 290 K) obtained by
means of Kramers-Kronig analysis is shown in Fig. 2. The
dominating feature of the spectrum at 45 K is a strong
charge transfer band due to electronic transitions across
the gap. The arrow shows the band gap (1500 cm−1) ob-
tained from the published temperature-dependent dc re-
sistivity measurements [4]. Obviously, the agreement of
this value with the onset of the optical interband tran-
A. Pashkin et al.: Metal-insulator transition in (TMTSF)2FSO3 probed by infrared spectroscopy 3
1000 10000
100 1000
2D = 1500 cm
Frequency (cm
 290 K
 45 K
E || a
 Energy (meV)
Fig. 2. E‖a optical conductivity spectra of (TMTSF)2FSO3
above and below the metal-insulator transition at TMI= 89 K.
Hatched area depicts the Drude model fit of the high temper-
ature optical conductivity.
sition is very good. On the other hand, the optical con-
ductivity at room temperature is mostly dominated by
the Drude response of the free carriers. The corresponding
fit using the Drude model is shown as the hatched area
in Fig. 2. Obviously, the Drude model provides a good
description of the measured room-temperature spectrum
excluding the electron-molecular vibration (emv) antires-
onance modes. The plasma frequency ωp = 8660 cm
and the scattering rate Γ ≃ 1450 cm−1 obtained from the
fit agree well with the Drude model parameters reported
for other TMTSF salts [11]. The obtained value of the dc
conductivity, σdc ≃ 860 (Ωcm)
−1, is in reasonable agree-
ment with the dc and microwave conductivity values of
1600 and 300 (Ωcm)−1, respectively, reported by Wudl et
al. [4].
3.2 Vibrational modes
The TMTSF molecule with the point group symmetry
D2h has in total 72 local vibrational modes classified ac-
cording to the following representations [12]
ΓD2h = (12ag + 11b3g + 11b1u + 11b2u)
+(6b1g + 7b2g + 7au + 7b3u), (1)
where the vibrations in the first brackets are polarized in
the molecular plane (perpendicular to the stacking a axis)
and the vibrations in the second brackets are polarized out
of the plane (along the stacking axis a). The symmetric
(gerade) vibrations are Raman active and the asymmet-
ric (ungerade) vibrations are infrared active excluding the
au silent modes. Some of the totally symmetric ag Ra-
man modes are expected to appear in the infrared spectra
for E‖a due to efficient emv coupling in the modulated
stacking structure [11,13].
Table 1. The eigenfrequencies and assignment of some vi-
brational modes observed in (TMTSF)2FSO3 for E‖a at 45 K
below TMI . All numbers are in cm
45 K calculated frequency1 assignment
580 571 ν3(a1) FSO3
728 702 ν51(b2u)
902, 911, 915, 916 ν8(ag)
917, 924, 932
1020, 1031, 1036 1060 ν7(ag)
1067, 1072 1060 ν7(ag)
1362, 14502 1469 ν4(ag)
1354, 1364, 1369 1369 ν6(ag)
1373, 1379, 1385
1550, 1584, 1606 1596 ν3(ag)
1847, 1854, 1863 1863 ν3(ag) + ν11(ag)
The tetrahedral FSO3 anion has C3v point group sym-
metry which gives in total nine vibrational modes
ΓC3v = 3a1(z, x
2 + y2, z2) + 3e(x, y, x2 − y2, xy, yz, xz),
where e species correspond to the doublets. Thus, in the
infrared spectra one expects six modes, with the 3a1 and
3e modes being polarized along and perpendicular to the
polar axis of the anion, respectively.
In this section we want to concentrate on the changes
in the infrared phonon spectra for both polarizations
across the metal-insulator transition. For E‖a several ag
vibrations of the TMTSF molecules become infrared ac-
tive in the insulating phase. This is due to the effec-
tive emv coupling of these vibrations to the on-chain
charge transfer band in the structure modulated due to
the anion ordering. The list of the new modes observed
below the transition together with their tentative as-
signment is given in Table 1. Most of them are emv
coupled ag modes polarized in the molecular plane or
their combination as a triplet at around 1850 cm−1. The
ν4(ag) mode involving the central C=C bond stretching is
known to have especially strong emv coupling and there-
fore it appears as a strong antiresonance mode in the
optical conductivity spectrum. It should be pointed out
that the observed appearance of ag modes for E‖a in
the ordered phase is typical only for (TMTSF)2X com-
pounds with non-centrosymmetric anions. In comparison,
(TMTTF)2X salts possess a stronger stack dimerization,
resulting in the emv coupling of the ag modes already
in the disordered phase, and therefore the anion ordering
transition causes only a frequency shift and an intensity
change of the emv coupled modes [15].
4 A. Pashkin et al.: Metal-insulator transition in (TMTSF)2FSO3 probed by infrared spectroscopy
1264 1280 1296
560 570 580 590 600 1140 1150 1160
1358 1365 1372
Frequency (cm
 Frequency (cm
 Frequency (cm
  Frequency (cm
Fig. 3. Reflectivity spectra (shifted for clarity) of some
phonons which experience changes during the metal-insulator
transition at 89 K: (a) vibration polarized along the a axis;
(b)-(d) vibrations polarized along the b′ axis.
The ν3(a1) vibrational mode of the FSO3 anion at
580 cm−1 is observed for the whole studied temperature
range. However, the lineshape of this mode in the metallic
phase above TMI is inverted with respect to the insulating
phase [see Fig. 3(a)], since the background dielectric con-
stant is negative as expected for highly conducting metals
at low frequencies. Such a change is a clear evidence for
the suppression of the Drude conductivity in the insulat-
ing phase of (TMTSF)2FSO3.
The mode at 728 cm−1 observed for temperatures
below TMI is particularly interesting, since its intensity
gradually increases on temperature decrease (see Fig. 4).
A similar behavior is found for the polarization perpen-
dicular to the stacks, E‖b′. Moreover, above the transi-
tion temperature a strong asymmetric mode is seen at
710 cm−1. This mode shifts to lower frequencies and gets
stronger with increasing temperature. This mode has not
been observed in any other earlier study of the Bechgaard
salts. Therefore, it would be natural to assign it to a vibra-
tion of FSO3 anion. However, such an assignment would be
in contradiction to the experimental observations, since:
680 700 720 740 760
E || b'
E || a
200 K
200 K
  Frequency (cm
Fig. 4. Reflectivity spectra (shifted for clarity) of the vibration
at around 710 cm−1 at different temperatures for E||a and
E||b′.
(i) the ν5(e) and ν2(a1) vibrations of FSO3 located close
to the observed mode have frequencies which are by more
than 100 cm−1 higher or lower [14]; (ii) the intensity of
the anion vibration should not vanish at the order-disorder
transition point. Thus, one has to attribute the modes at
around 710 and 728 cm−1 to vibrations of the TMTSF
molecules. We suggest that both modes originate from the
ν51(b2u) in-plane vibration of the TMTSF molecule. Ac-
cording to the normal-coordinate analysis [12,16] its fre-
quency for a free TMTSF0.5+ cation is 702 cm−1.
The corresponding atomic movements involve stretch-
ing of the Se-C side bond and rocking of the adjacent
methyl group. For the b2u vibration the inversion symme-
try of the molecule is not preserved, causing its infrared-
activity for the polarization perpendicular to the stacks.
However, it is known that in (TMTSF)2X salts the dipole
moment corresponding to the ν51(b2u) vibration is very
small, and therefore this mode can hardly be detected
even for E‖b′ where it should have the strongest inten-
sity [16]. Nevertheless, in (TMTSF)2FSO3 this mode is
particularly strong even at room temperature. This find-
ing can be explained by the electrical dipole of the FSO3
anion pointing towards the Se-F bond. Similar to other
A. Pashkin et al.: Metal-insulator transition in (TMTSF)2FSO3 probed by infrared spectroscopy 5
Fig. 5. Schematic illustration of the ν51(b2u) vibration coupled
to the reorientation of the FSO3 electrical dipole moment. The
projection of the crystal structure on the b− c plane is shown.
Only the Se (large open circles) and C (small filled circles)
atoms of the TMTSF molecules are presented, together with
the displacements of the Se atoms. The grey filled circles be-
tween molecules denote the positions of the FSO3 anions, the
bold arrows show the two possible orientations of the anion
dipole moment (p1 and p2). Because of the symmetry prop-
erties of the b2u vibration the reorientation of the electrical
dipole moment leads to a change of polarization ∆p in the
perpendicular direction for any orientation of the anion dipole
moment.
non-centrosymmetric anions (ReO4, ClO4 etc.) the FSO3
anion has two possible symmetrically equivalent orienta-
tions for which the dipole moment points towards the Se
atoms of the neighboring TMTSF molecules. This situa-
tion is sketched in Fig. 5, where p1 and p2 are two possible
orientations of the FSO3 electrical dipole moment. During
the vibration the dipole moment of the anion follows the
position of the Se atom. Due to the symmetry properties
of the b2u vibration the nearest Se atoms on both sides
of the anion move in the same direction. Thus, for both
possible orientations of the dipole the b2u vibration results
in a change of the average polarization along the direction
of ∆p (Fig. 5).
The described coupling mechanism between the b2u
vibration and the dipole moment of the anion in
(TMTSF)2FSO3 should lead to a strong enhancement of
the infrared strength of the ν51(b2u) vibration for E‖b
since ∆p has the largest projection along this direction.
On the other hand, ∆p is perpendicular to the stacking
axis and the b2u mode should not appear for E‖a. This
is indeed observed in our experiment above the transition
temperature. Below the transition the long-range order of
the anion sublattice builds up. Then the anion dipole mo-
ment orientation is determined by the modulation of the
whole lattice and it is not dependent on the movement of
neighboring TMTSF molecules, i.e., the ν51(b2u) vibration
is decoupled from the FSO3 anions. Therefore, its inten-
sity should drop abruptly below TMI , in agreement with
our observations (see Fig. 4). Moreover, the observed de-
crease of the intensity of the coupled b2u mode at around
Table 2. The eigenfrequency, width (given in bracket), and
assignment of some vibrational modes observed for E‖b′ at
selected temperatures. All numbers are in cm−1.
95 K 80 K 45 K assignment
580 (1.3) 580 (0.9) 580 (0.8) ν3(a1) FSO3
710 (7.1) 728 728 (2.0) ν51(b2u)
1154 (5.0) 1150 (3.4) 1150 (3.3) ν48(b2u)
1157 (3.8) 1158 (2.5)
1280 (5.1) 1276 (4.2) 1276 (2.1) ν4(e) FSO3
1288 (16) 1286 (4.4) 1286 (1.7)
1363 (3.2) 1361 (2.8) 1361 (1.3) ν47(b2u)
1366 (4.3) 1367 (2.4) 1367 (1.8)
710 cm−1 at 95 K compared to higher temperatures can be
explained by taking into account short-range order fluc-
tuations above the transition, evidence for which is also
given below. Indeed, in the large enough dynamical regions
where the anions are ordered, the coupling is suppressed
and therefore the strength of the ν51(b2u) should decrease.
On cooling down below TMI a vibration appears again
at somewhat higher frequency (728 cm−1) for E‖b′ and
its strength gradually increases with decreasing temper-
ature. We suggest that this is the same ν51(b2u) vibra-
tion described above. Since it is decoupled from the anion
sublattice, its frequency is expected to increase abruptly
below the transition. The increase in strength for both
polarizations should be obviously related to the tempera-
ture dependence of the order parameter (i.e., the degree
of lattice modulation). One of the possible mechanisms
can be the emv coupling of the ν51(b2u) vibration to the
charge transfer bands along a and b′ directions. However,
the detailed picture of this emv coupling is not clear, since
the symmetry of b2u mode does not allow such kind of
coupling. One can speculate that the electric field of the
FSO3 dipoles in the ordered phase distorts the TMTSF
molecules making them non-centrosymmetric. Then the
emv coupling may become allowed for the b2u(ν51) mode.
Noticeable changes in the phonon mode spectra across
the metal-insulator transition are observed for E‖b′. The
list of the parameters of these modes at temperatures
above and below TMI is given in Table 2. An obvious split-
ting into two components is seen for the ν48(b2u) mode at
1154 cm−1 [see Figure 3(b)]. In addition, the damping of
the split components directly below TMI is lower than the
damping of the single component directly above the tran-
sition (see Table 2). This difference is probably related
to the precursor short-range order fluctuations above the
transition, which can induce a small splitting already in
the disordered phase. An evidence for such fluctuations
was found in x-ray diffuse scattering experiments [8,2].
This effect is even more clearly seen in the splitting of
two other modes: the doublet ν4(e) vibration of the FSO3
anion at 1280 cm−1 [Fig. 3(c)] and the ν47(b2u) mode at
around 1365 cm−1 [Fig. 3(d)]. For each of these modes
6 A. Pashkin et al.: Metal-insulator transition in (TMTSF)2FSO3 probed by infrared spectroscopy
above TMI one can resolve two weakly split components.
However, below TMI the splitting abruptly increases and
the damping decreases (see Table 2) indicating the onset
of long-range order. Since the described effect is observed
not only for the FSO3 anion vibration but also for two vi-
brations of the TMTSF cation, we can conclude that the
short-range order fluctuations involve the modulation of
the whole (TMTSF)2FSO3 lattice and not only the anion
sublattice.
4 Conclusion
We have performed an infrared spectroscopic study of
the metal-insulator transition in (TMTSF)2FSO3. The ob-
tained optical conductivity spectra for E‖a show a Drude-
like conductivity above the anion ordering temperature
and a charge transfer band formed below the transition.
The onset of this band is in agreement with the energy
gap value of 1500 cm−1 obtained from transport measure-
ments [4].
The analysis of the infrared-active vibrations leads to
the following conclusions: (i) the crystal structure modula-
tion below the metal-insulator transition leads to a strong
emv coupling of several ag vibrations which therefore be-
come infrared-active; (ii) short-range order fluctuations of
the FSO3 anions and the corresponding lattice modulation
exist above the transition temperature, as it is seen from
the splitting of some infrared-active modes for E‖b′; (iii)
a new infrared-active mode located at around 710 cm−1
with a peculiar temperature behavior is detected and as-
signed to the coupling between the b2u TMTSF molecule
vibration and the electrical dipole moment of the FSO3
anion. The latter feature has not been observed in any
other (TMTSF)2X salt showing a metal-insulator transi-
tion. This points out the important role of the electrical
dipole moment of the anion on the structural and dynam-
ical properties of the (TMTSF)2FSO3 salt.
5 Acknowledgements
We acknowledge the ANKA Angströmquelle Karlsruhe for
the provision of beamtime and thank M. Süpfle, D. Moss,
and B. Gasharova for technical assistance at the ANKA
IR beamline. The financial support of the DFG (Emmy
Noether-program) is acknowledged.
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	Introduction
	Experimental
	Results and discussion
	Conclusion
	Acknowledgements
