Nonohmic electrical transport in the PeierlsMott state of

1
Non-ohmic electrical transport in the
Peierls-Mott state of deuterated copper-DCNQI
systems
T. Vuletic1, M.Pinteric1,2, M. Loncaric1,
S. Tomic1, J. U. von Schütz3 contact e-mail
tvuletic_at_ifs.hr 1 Institute of Physics, Zagreb,
Croatia 2 Faculty of Civil Engineering,
University of Maribor, Slovenia 3
3.Physikalisches Institut, Universität Stuttgart,
Germany
d8 system Cu2,5(CD3)2-DCNQI2
h8/d6 7030 system Cu(2,5(CH3)2)0.70(2,5(CD3)
2)0.30-DCNQI2
Since 1987 an important amount of research has
been devoted to a novel class of charge-transfer
salts R1R2-DCNQI2Cu where DCNQI stands for
dicyanoquinonediimine and R1, R2 CH3, CH3O, Cl,
Br, I etc.), due to the possibility to control
easily, by varying external (pressure, magnetic
field) and internal (isotope substitution,
doping) parameters, unique physical properties
inside an extremely rich phase diagram 1,2,3.
Compared with conventional organic metals, the
novel electronic states have emerged associated
with the hybridization between p-orbitals of
DCNQI molecule and d-orbitals of Cu ions
4. We have used DC electrical transport
measurements in order to investigate N3 CDW in
the Peierls-Mott insulating state of the fully
deuterated d8 system and the partially
deuterated h8/d6 7030 system. We correlate
observed features of the electric conduction
below and above threshold field with the
temperature evolution of the N3 CDW order as
detected by the low-frequency dielectric
measurements 5.
h8/d6
Single-particle transport The observed
resistance Rxx vs. temperature T curves have
reproduced the previous ones given in the
literature 6. For both systems resistance is
normalized to metallic state values just above
TC1warm. Due to deuteration two systems are
representative in different temperature ranges,
while the general behavior is similar.
Experimental Technique The measurements
were performed on single crystals with varying
lengths of about 2 mm and cross sections of
typically 0.01 mm2. All measured samples
exhibited qualitatively the same behaviour. The
RT conductivity sRT was in the range 800 - 1200
Scm-1. The DC resistance measurements were
performed using a standard DC technique or
Keithley 617 electrometer in V-I mode.
d8
References 1 H.P. Werner, J.U. von
Schütz, H.C. Wolf, R. Kremer, M. Gehrke, A.
Aumüller, P. Erk and S. Hünig, Solid State
Commun. 65 (1988) 809. 2 S. Tomic, D.
Jérome, A. Aumüller, P. Erk, S. Hünig and J.U.
von Schütz, J. Phys. C. Solid State Phys. 21
(1988) L203. 3 R. Kato, H. Sawa, S. Aonuma,
M. Tamura, M. Kinoshita and H. Kobayashi, Solid
State Commun. 85 (1993) 831. 4 T. Ogawa
and Y. Suzumura, Phys. Rev. B 53 (1996) 7085. 5
M.Pinteric, T. Vuletic, M. Loncaric, S.Tomic,
J. U. von Schütz, to appear in Eur. Phys. J. B
(2000). 6 D. Gomez, J.U. von Schütz, H.C.
Wolf and S. Hünig, J. Phys. I France 6 (1996)
1655. 7 M. Pinteric, M. Miljak, N. Bikup,
O. Milat, I. Aviani, S. Tomic, D. Schweitzer, W.
Strunz and I. Heinen, Eur. Phys. J. B 11 (1999)
217. 8 S. Tomic, M.Pinteric, T. Vuletic,
J. U. von Schütz, D. Schweitzer, these
Proceedings 9 M. Pinteric, N. Bikup, S.
Tomic, J. U. von Schütz, Synth. Metals 103
(1999) 2185.
CDW -- nonlinear transport
h8/d6
d8
d8
Electric field dependent conductivity
h8/d6
As an illustration, we show the field-dependent
conductivity normalized to its Ohmic value at a
few selected temperatures, for both systems. We
point out that the nonlinear effect is quite
small, but the threshold field ET is clearly
defined. The results given for the h8/d6 system
are for the sample denoted Sample 1 in the next
figure.
Threshold field ET and (s2ET-s0)/s0, nonlinear
effect (non-ohmic conductivity at twice the
threshold field, normalized to its ohmic value),
for . system and
70\30\ system, for which we compare two
different single crystals. For the d8 system ET
decreases below TC1warm, reaching a minimum value
of about 400 mV/cm around 65K, and then increases
again towards low temperatures. The nonlinear
effect is constant in the broad T-range between
55 K and 75 K and starts to increase at lower
temperature. For the h8/d6 70\30\ system there
is equally pronounced increase of ET at low
temperatures. However, the magnitude of this
feature appears to be sample dependent. For this
system we did not observe divergence of ET at
temperatures close to TC1warm. Again, as in d8
system, the magnitude of nonlinear effect
decreases towards TC1warm.
h8 /d6
d8
Ohmic and Non-Ohmic conductivity
We show the ohmic conductivity and non-ohmic
conductivity at twice the threshold field vs.
inverse teperature 1/T. The preferred choice
for the fit (full lines) was Mott's variable
range hopping (VRH) formula, which applies when
the dominant conduction mechanism becomes the
conduction by the carriers localized on
impurities. The collective and single particle
conductivities are closely related since they
both obey the same temperature dependence. This
behavior and the T-independent behavior of the
mean relaxation time 5, are in accordance with
an extremely low free electron density in the
insulating state. Using the observed values of ET
we estimated the CDW characteristic length to be
0.1-1 mm, for both systems. The collective and
single particle conductivities are closely
related since they both obey the same temperature
dependence.
CONCLUSION Electric-field-dependent
measurements have revealed clear, albeit weak in
magnitude, non-linear characteristics above
large threshold fields of the order of 1 V/cm. At
low temperatures the threshold field was found to
increase substantially reaching values between 10
and 100 V/cm, concomitantly with the increase in
the magnitude of nonlinear effect. It should be
noted that this represents a feature not usually
encountered in CDW and SDW. However, we have
already reported a qualitatively same behavior in
commensurate SDW of k-(BEDT-TTF)2CuN(CN)2Cl
7. A common aspect of these two systems is a
domain structure of DW ground state 8. On the
other hand, for a DW in an incommensurate
structure a rise of ET followed by a
disappearance of the nonlinear effect, once the
free-electron screening becomes uneffective,
would be expected. Indeed, we have observed such
a behavior in N4 CDW of DCNQI-Li system 9. We
have suggested that metallic islands persisting
below Peierls-Mott transition act as charged
domain walls in the random domain commensurate
structure characterized with broad dielectric
response. This scenario we have correlated with
lower threshold field and nonlinear effect
observed there. Further, we correlate an
important rise of the threshold field at low
temperatures outside hysteretic region with the
Debye relaxation, observed in the same
temperature range, and we suggest that both are
the manifestation of the N3 CDW long range order
established outside the hysteretic region. To
our knowledge this might be the first opportunity
to study experimentally the theoretically
intriguing issue of the collective DC (and AC
5) CDW dynamics in the random domain
commensurate structure in an organic p -
inorganic d hybrid conductor.
d8
h8/d6