HAL Id: jpa-00247302
https://hal.archives-ouvertes.fr/jpa-00247302
Submitted on 1 Jan 1996
HAL is a multi-disciplinary open access archive for the deposit and dissemination of sci- entific research documents, whether they are pub- lished or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers.
L’archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d’enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.
Charge Density Wave Dynamics in Quasi-One Dimensional Molecular Conductors: a Comparative
Study of (Per)2M(mnt)2 with M = Au, Pt
E. Lopes, M. Matos, R. Henriques, M. Almeida, J. Dumas
To cite this version:
E. Lopes, M. Matos, R. Henriques, M. Almeida, J. Dumas. Charge Density Wave Dynamics in Quasi-One Dimensional Molecular Conductors: a Comparative Study of (Per)2M(mnt)2 with M = Au, Pt. Journal de Physique I, EDP Sciences, 1996, 6 (12), pp.2141-2149. �10.1051/jp1:1996210�. �jpa- 00247302�
"Charge Density Wave Dynamics in Quasi.One Dimensional Molecular Conductors:
a Comparative Study of (Per)21&I(mnt)2
with M
# Au, Pt
E-B- Lopes (~), M.J. Matos (~), R-T- Henriques (~), M. Almeida (~) and J. Dumas (~.*)
(~) Departamento de Quimica, Instituto Tecnolôgico e Nuclear, 2686 Sacavém Codex, Portugal (~) Instituto Supenor de Engenharia de Lisboa, 1900 Lisbon, Portugal
(~) Departamento Engenhana Quimica, Instituto Supenor Técnico, 1096 Lisbon Codex, Portugal
(~) Laboratoire d'Études des Propriétés Électroniques des Solides, CNRS, BP 166, 38042 Grenoble Cedex 9, France
(Received 14 May1996, revised 22 July 1996, accepted 20 August 1996)
PACS. il.45.Lr Charge-density-wave systems
PACS.72.15.Nj Collective modes (e,g., in one-dimensional conductors)
PACS.72.20.Ht High-field and non-hnear effects
Abstract. The molecular compounds (Per)2M(mnt)2 where Per
= perylene and mut
= ma-
IeomtriIedithioIate
are quasi-one dimensional conductors which undergo metal-to-semiconductor transitions towards a commensurate charge density wave (CDW) state, at Tc
= 8 K for M
= Pt
and Tc
= 12 K for M
= Au. Above
a threshold field, these molecular systems show nonlinear transport due to CDW motion as it is weII demonstrated in the
case of the Pt compound, where the Fourier analysis of the voltage at constant current clearly reveals
one fundamental frequency proportional to the excess CDW current and several harmomcs. A comparative study by pulsed
current methods of the transient CDW dynamics is reported for these compounds, showing
different threshold fields and variable degrees of CDW screening by the normal carriers. The temperature dependence of the threshold fields for both compounds is compared. The results
are discussed in relation with current mortels for CDW transport.
1. Introduction
Since its first discovery in NbSe3, the nonlinear transport associated with charge density wave (CDW) motion has been a very active research topic in low dimensional conductors and other
inorganic systems presenting this type of behavior, such as TaS3, (TaSe4)21, (NbSe4)io/31 or
the blue bronzes Ko.30Mo03, have been also studied in great detail dunng the past fifteen years il,2]. The ground state in these materials is characterised by a spatial modulation of the
charge density p(r), coupled to a penodic lattice distortion, that in a first approximation is described as p(r)
= pi cos(2kF + #), where pi is the amplitude, kF the Fermi wave vector and
# is the phase of the CDW. The nonlinear transport properties in these materials result frein
(*) Author for correspondence (e-mail: dumasfilepes.polycnrs-gre.fr)
© Les Editions de Physique 1996
2142 JOURNAL DE PHYSIQUE I N°12
the possibility of a coherent sliding motion of the CDW above a threshold electric field Et. At low fields, the CDW is pinned to the lattice, either due to impurites or lattice defects or to
commensurability effects. Above Et the CDW slides and its coherent motion is demonstrated by the observation of narrow band noise, with frequencies proportional to the excess current
carried by the CDW. The dynamics of the CDW motion results from the competition between the pinning potential and the elastic properties of the CDW il,2].
In organic molecular compounds, probably due to the generally poorer sample quality than
in the above mentioned inorganic systems, the observation of CDW nonlinear properties has been much more difficult. The clear observation of narrow band noise, usually viewed as the
signature of the collective density wave transport, has been restricted, to our knowledge, to the spin density wave (SDW) ground state, instead of the CDW state, as put into evidence recently in some Bechgaard salts. Due to the quite different nature of these CDW materials, large differences in their nonlinear transport properties are observed, making diilicult an overall comparison and a description of the key factors affecting the broad range of nonlinear transport phenomena.
We have recently reported that the isostructural molecular compounds (per)2M(mnt)2 where per = perylene and M
= Au, Pt show at T
= 4.2 Il nonlinear transport properties due to CDW motion, as demonstrated by the observation of narrow band noise, especially clear in the case of the Pt compounds [3,4]. These remarkable results were not only the first reports of nonlinear
transport in organic based CDW molecular systems, but they represented also the first case of
so closely related compounds with different transition temperatures in an isostructural family of compounds whose study can provide a unique chance of probing subtle changes in the nonlinear properties. In this paper, we report a comparative study of the CDW dynamics in the M
= Au
and Pt compounds performed by pulsed current measurements at variable temperature.
The title compounds are members of the more general family of closely related or even isostructural for some cases quasi-one dimensional compounds, the so-called a-phases of
(perylene)2M(mnt)2, including members with M
= Ni, Cu, Au, Pd, Pt, Fe and Co [5-7].
These matenals are highly anisotropic conductors at room temperature, with a room temper- ature conductivity along the chain axis b ajj(300 K) mJ
700 Q ~~ cm~~ for M
= Au and Pt,
and an anisotropy ajj lai estimated as 10~ from Montgomery four probe measurements. They undergo metal to semiconductor transitions at lower temperatures as shown in Figure 1 for the M
= Au and Pt compounds in which the transition is marked by a Sharp maximum in the slope d(log a)/d(1/T) at T
= 12 K and 8 Il respectively [8]. The one dimensional metallic
properties observed at higher temperatures are due to a 3/4 filled band of partially oxidised and closely stacked perylene molecules (Per)). The M(mnt)p anions also form stacks, but with
a larger intermolecular spacing and therefore without any significant contribution to the elec- trical conductivity. The M(mnt)p anions have however a major role in the magnetic properties
of these compounds. For some metals M, such as M
= Pt, the M(mnt)p anions form chains of locahzed S
= 1/2 spins which dominate their magnetic properties, while in others such as
M = Au the anions are diamagnetic. From trie companson of the behaviour of the different
members of this famiiy of compounds [6, 7], it becomes clear that the metal to semiconductor transitions are a consequence of the CDW state formed at low temperatures, associated with the tetramenzation of the perylene chains, as clearly seen by x-ray diffraction in the M
= Co,
Ni and Cu compounds [9,loi. In the cases of paramagnetic anions, as for M
= Pt, a dimer- ization of the Pt(mnt)2 also takes place at the metal to semiconductor transition [11], as if in these cases there was a spin-Peierls transition of the Pt(mnt)2 chains driven by the CDW
transition in the perylene conducting chains.
,~5
10~
. Per~Pt(mnt)~
+ Per~Au(mnt)~
RjR~~ 10
io"~
0 50 100 150 200 250
1000/T (K~)
Fig. l. Normalized electrical resistance, measured along the chain
axis b of (per)2M(mnt)2 com- pounds, M
= Au, Pt
as a function of reciprocal temperature. From reference [8].
2. Experimental Results
Single crystals of (perylene)2M(mnt)2 (M
= Au, Pt) have been obtained by electrocrystal-
lization using standard procedures for this family of compounds [3.4]. Crystals appear as long needles with typical size 4 x o.05 x o.025 mm~, ~v.ith the crystallographic b-axis as the
long direction. Four gold pads were evaporated around the needle, and gold wires 25 mm in diameter were attached to the evaporated areas with Pt paint. The current contacts covered the whole area of both ends of the needle. Typical contact resistance was mJ 20 Q for the Pt
compounds and 40 Q for the Au compounds at room temperature. Nonlinear conductivity
and noise measurements were best performed at 4.2 K by slowly immersing the sample into
liquid hehum, in order to avoid Joule heating, using de and pulse current methods. Pulsed
current measurements were also performed at variable temperatures with the sample in liqmd
helium or in gas inside a cryostat. In this latter case, the current pulse amplitude was hmited to somewhat lower values so that, by a systematic change of amplitude and repetition rates, it
was ensured that there were no Joule ieating effects. For the pulse current measurements, the computer controlled arrangement consists of a HP 2148 unipolar rectangular pulse generator,
Tektronix AM 502 differential amplifiers, and a Tektronix 2230 digital storage oscilloscope. A constant current was applied to the sample using a large resistance in series with the pulse generator. The noise voltage was detected at constant current with a Rhode-Schwarz spectrum analyzer.
Typical voltage-current characteristics obtained by current pulse technique are shown in
Figures 2a, b for Au and Pt compounds at T
= 4.2 K. Below Tc, a nonlinearity is always clearly observed above a well defined threshold voltage corresponding to a threshold field Et
which is at 4.2 K of the order of 460 mV cm~~ for M
= Au and 8.9 V cm~~ for M
= Pt. The
broad band noise (BBN) measured at 4.2 K in the frequency range loo Hz-10 kHz with an ac
voltmeter, is clearly correlated with the onset of the nonhneanty when the sample is driven
by a de current. The BBN shows a rapid use near threshold and decreases slowly above in
both compounds as shown in Figures 2a, b. In the whole range of temperature explored, the
voltage waveform shows a charactenstic change as the rectangular current pulse amplitude is
2144 JOURNAL DE PHYSIQUE I N°12
200 100 ~ "2
150 75
4
ç R
> ~ g j
~ ~~ ( $ ~w
>
2 0.4>
Per~Au(mnt)~ ~~ Per~Pflmnt)~
E~
O.ù
0 5 10 15 20 O.ù 0.5 1-o 1.5
1(mA) 1(ntA)
aj bj
Fig. 2. a) Left scale: Voltage.current characteristics obtained on a (per)2Au(mnt)2 sample by puise
current measurements (puise width 50 ms). Ilight scale: Iow frequency broda bond noise as a function
of de bios current. T
= 4.2 K. b) Same as in Figure 2a for
a (per)2Pt(mnt)2 sample (puise width 350
ms).
increased near Et, as illustrated in Figure 3a for different values of the voltage pulse height
at T
= 4.2 K. Below Et, the response is rectangular. Near threshold, the response shows
an overshoot at the leading edge of the pulse, and above Et, the decay time of the overshoot decreases as the current pulse amplitude increases. In some Pt samples, superimposed on the
transient response, asymetric voltage spikes of large amplitudes appear. The onset of these quasiperiodic oscillations corresponds to that of the BBN [4].
The overshoot is observed in both compounds at a threshold field Ec slightly below the threshold Et obtained on the I V curves from the measurement of the steady voltage at the end of the pulse. The amplitude of the overshoot as a function of time obeys a stretched
exponential law, as observed in a wide Mass of disordered materials:
à§[n c~ exp(-t/T)~ with 0 < fl < 1.
T represents a rough measurement of the average relaxation time and the exponent fl a measure of the width of the distribution. fl does not depend significantly on the applied current and
is found to be mJ o.9 for Au and Pt compounds. A single degree of freedom for the CDW is expected to give a purely exponential approach to equilibrium, 1-e- fl = 1. This result would indicate that the CDW is more rigid than that in the blue bronze Ko 30Mo03 where the exponent fl obtained at 77 K is of the order of o.5 [12]. T is found to be divergent as
the threshold is approached from above and is satisfactonly descnbed using a power law:
T c~ (E/Ec -1)~~ as shown in Figures 4a, b. From the fit to the data obtained at T
= 4.2 K
we find
~i = 2.72 in the Pt compounds and ~i = 2.80 in the Au compounds. The divergence
of T near threshold is simiiar to that found in the blue bronze [12] where an exponent ~ = 2 at 70 K is found and also suggests that the depinning can be viewed as a dynamical cntical
phenomenon, as predicted by several authors [13] where the threshold field plays the role of a transition temperature.
sa ioo
a b
o o
5Z o sa o
E sa
C
200
ioo
ioo
C d
o ~
0 50 0 50
Time(~s)
Fig. 3. Voltage waveform response at T = 4.2 K for different values of the current puise amplitude for a (per)2Au(mnt)2 sample; puise width 50 ms. a) V = 50 mV; b) V
= 90 mV; c) V
= 120 mV;
d) V =180 mV.
In the Au samples, the Fourier spectra of trie noise voltage at constant de bias current above threshold barely revealed a few frequencies not harmonically related [3j. In some Pt
compounds, the Fourier spectra show clearly one fundamental frequency and several harmonics,
as illustrated in Figure 5a. Figure 5b shows that these frequencies increase linearly with the
excess CDW current density with a slope Fi/JCDW
" 18 kHz A~~ cm~ for the fundamental
frequency Fi .These noise peaks reflect the penodic time dependence response of the CDW to
the de bias current. The observation, in Au samples, of several frequencies not harmonically
related, with weaker amplitudes than in the Pt compounds was interpreted as due to an
inhomonogeneous current distribution along the sample cross-section. Difficulties in achieving
a homogeneous current injection are not unexpected in view of the large anisotropy of these materials. Macroscopic inhomogeneities in CDW pinning may also play a role. Although
the CDW in M
= Au and Pt compounds is commensurate [la, llj we rather believe that, at least in the Au compound where the samples have a poorer quahty as inferred from a lack of reproducibility of the a(T) curves and broader resistive transitions, the dominant pinning
mechanism is by impurities or lattice structural defects. in this context, it is worth noting
that in trie Au compound, probably due to a higher impunty concentration, it shows at lower temperatures a higher conductivity and a smaller activation energy than in the Pt compound,
as shown in Figure 1.
2146 JOURNAL DE PHYSIQUE I N°12
0.6
Per~Au(mnt)~
Per~Pt(mnt)~
0.4
EW E
# #
0.2
o-i
"m
0.0 2
IE~ IE~
a) b)
Fig. 4. a) Characteristic time T as a function of normalized field at T
= 4.2 K for a (per)2Au(mnt)2 sample. The solid Iine is a fit to the data according to a power Iaw: T cc (E/Ec -1)~~ b) Same as
m Figure 4a for a (per)2Pt(mnt)2 sample.
1=0.685mA
400
1=0.627mA ~3
àÎ
C
*
W
'1 ~ °'~~~
z
«
E Fi
FrequÀliy
aj bj
Fig. 5. a) Voltage noise amplitude m arbitrary umts as a function of frequency for different values of the apphed de bios current at T
= 4.2 K. for a (per)2Pt(mnt)2 sample. (a)
= 0.52 mA; (b)
t = 0.59 mA: (c)
= 0.627 mA; (d)
= 0.685 mA. b) Frequency of the narrow bond noise as a
function of the
excess CDW current density at T
= 4.2 K.
3 ' 15
. Per~Pt(mnt)~
Per~Au(mnt)~
2
~
10
Ê
.
Ê
e ~
> a >
ut + à
5
£~~ . ~
~ é
+ $~
.
+ ~
o
0 2 4 6 8 4 6 8
Temperature (K) Temperature(K)
a) b)
Fig. 6. a) Temperature dependence of the threshold field Et for a (per)2Au(mnt)2 sample. b) Same
as in Figure fia for a (per)2Pt(mnt)2 sample.
In the classical model [2j of a rigid CDW moving in a periodic pinning potential tilted by the electric field [xi,Fi /JcDw
" 1/neÀ where n is the concentration of carriers condensed into the
CDW and the CDW wavelength. From the experimental value Fil JcDw " 18 kHz A~~ cm~~
and with
= 4b
= 16.8 À, corresponding to a tetramerization, we find n
= 2.1 x 10~~ cm~~,
close to the value obtained from the structural data n
= 3.2 x 10~~ cm~~. Taking into account the uncertainty in the determination of the sample cross-section we can conclude that the
CDW is depinned in nearly the entire sample. In the phase slip model [14], the removal of the accumulated phase difference along the sample is periodically repeated and, here also Fi /JcDw " 1/neÀ. The transient time evolution of the frequency and phase, not observed in the conventional Fourier analysis, have been investigated recently by means of the so-called
wavelet analysis of the real time voltage oscillations [15] supporting the picture of the phase slip model.
The temperature dependence of the threshold fields has been investigated by current pulse
measurements in the temperature interval 2 K-8 Il in the Au and 4 K-7 Il in the Pt compounds.
In this study, the higher temperature range is limited by the fast increase of the sample conductivity near Tc. Figures 6a, b shows that in both cases, Et increases rapidly at low temperatures. In the overlapping temperature range of our data, the threshold for the Pt
compound is always larger than the threshold of the Au one which show a higher transition temperature Tc = 12 K. The onset of the non-Ohmic conductivity at Et is found to become
more and more abrupt as the temperature is decreased, as illustrated in Figure 7 for a Au
compound. A possible contribution to a large threshold field in the Pt compounds is the dimenzation of the Pt(mnt)2 units below Tc [10,11] not observed in the Au compound, which may lead to a slightly enhanced commensurability pinning potential in this case.
Trie ongin of trie observed Et(T) dependence is not clear at the moment. It is worth
noting that trie observed increase of Et upon coohng below Tc is qualitatively similar to those
reported in other CDW systems such as NbSe3 [2] or SDW systems such as (TMTTF)2Br [16] or
2148 JOURNAL DE PHYSIQUE I N°12
4
+ +
2.98 K + .
Per~Aujmntj~ '
,
+
+ .
+
5 4.2K # * .
Jf .
% 2 ' + "
[ 7.68K 5.6 K : *
~
~
. .
Î .
+
'
' j +
'
,"
: *
+ .
o
. . K ioo
E(mvicm)
References
iii For a review, see: Physics and Chemistry of Low Dimensional Inorganic Conductors, NATO-ASI Series B, C. Schlenker~ M. Greenblatt, J. Dumas, S. Van Smaalen Eds., Vol.
354 (Plenum Pub.); Monceau P., in "Electronic Properties of Inorganic Quasi-One Di- mensional Materials, P. Monceau Ed. (D. Reidel, Dordrecht, 1985) p. 139.
[2] Grüner G., Density Wave in Solids (Addison-Wesley Pub., 1994).
[3] Lopes E-B-, Matos M.J., Henriques R-T-, Almeida M. and Dumas J., Eitrophys. Lett. 27
(1994) 241.
[4] Lopes E-B-, Matos M.J., Hennques R-T-, Almeida M. and Dumas J., Phys. Rev. B 52
(1995) 2237; Lopes E-B-, Matos M.J., Henriques R-T-, Almeida M. and Dumas J., Synth.
Metals 70 (1995) 126î; Alcàcer L., Novais H., Pedroso F., Flandrois S., Coulon C., Chas-
seau D. and Gautier J., Solid State Commitn. 35 (1980) 945.
[5j Almeida M., Gama V., Hennques R-T- and Alcàcer L., in "Organometallic Polymers with
Special Properties", R-M- Laine Ed. (Kluwer Academic Pub., 1992) p. 163.
[6] Gama V., Henriques R-T-, Bonfait G., Almeida M., Ravy S., Pouget J-P- and Alcàcer L.,
Moi. Cryst. and Liq. Cryt. 234 (1993) 171.
[7] Almeida M. and Hennques R-T-, in "Organic Conductive Molecules and Polymers", H-S- Nalwa Ed., Vol. 1, Chapter 12 (Wiley J. and Sons Publ.. 1996) in press.
[8] Bonfait G., Lopes E-B-, Matos M.J., Hennques R-T- and Almeida M., Solid State Com-
mitn. 80 (1991) 391.
[9] Gama V., Hennques R-T-, Almeida M. and Pouget J-P-, Synth. Metals 55-57 (1993) 1677.
[loi Gama V., Hennques R-T-, Almeida M., Bourbonnais C., Pouget J-P-, Jérome D.. Aubin- Senzier P. and Gocshy B., J. Phys. I France 3 (1993) 1235.
[iii Henriques R-T-, Alcàcer L., Pouget J-P- and Jérome D., J. Phys. C; Solid State Phys. 17
(1984) 5197.
[12j Arbaoui A., Dumas J., Lopes E-B- and Almeida M., Solid State Commitn. 81 (1992) 567j Dumas J. and Schlenker C., Int. J. Mod. Phys. B 7 (1993) 4045.
[13j Fisher D.S., Phys. Rev. B 31 (1985) 1396; Wang Z-Z- and Ong N-P-, Phys. Reu. Lett. 58
(1987) 2375.
[14] Ong N-P- and Maki K., Phys. Reu. B 32 (1985) 6582.
[15] Dumas J., Thirion N., Almeida M., Lopes E-B-, Matos M.J. and Henriques R-T-, J. Phys.
I France 5 (1995) 539.
[16] Tomic S., Biskup N., Dolanski Babic S. and Maki K., Eitrophys. Lett. 26 (1994) 295.
[17] Nagasawa M., Sambongi T. and Anzai H., Solid State Commitn. 98 (1996) 401.
[18] Tômic S., Cooper J-R-, Jérome D. and Bechgaard K.. Phys. Reu. Lett. 62 (1989) 462;
Tômic S., Cooper J-R-, Rang W., Jérome D. and Maki K., J. Phys. I France 51 (1991)
1603.
