The zero-bias anomaly in point-contact characteristics of the organic superconductor &#945;-(BEDT-TTF)2I3 is investigated as a function of temperature and magnetic field. It is found that the zero-bias anomaly is insensitive to magnetic fields up to 5 T. In contrast, a structure at 5 meV, conventionally designated as the superconducting gap - but which is 4 times larger than the expected BCS gap - is strongly affected by magnetic fields above 1 T

Ernst, Gabriele

Nowack, Andreas

Schweitzer, Dieter

Weger, Meir

Online Publikationen der Universität Stuttgart

EUROPHYSICS LEITERS 1 February 1994 
Europhys. LetL, 25 (4), pp. 303-309 (1994) 
Zero-Bias Anomalies in Point-Contact Characteristics 
of CXt-(BEDT -TTFh 13 . 
G. ERNST('), A. NOWACK('), M. WEGER(")(§) and D. SCHWEITZER(") 
(') Experimenuuphysik II, Universitiit Kaln 
ZiUpi£her Str. 77, 5000 Kaln 41, Germany 
(") Physikalisches Institut rkr Universitat Stuttgart 
Pfaffenwaldring 57, 7000 Stuttgart 80, Germany 
(received 16 June 1993; accepted in final form 3 December 1993) 
PACS. 74.50 - Proximity effects, tunnelling phenomena, and Josephson effect. 
PACS. 74.70 - SUpel'Conducting materials. 
AbstracL - The ze.ro·bias anomaly in point-contact characteristics of the organic superconductor 
.,·(BEDT-ITF),I, is investigated as a function of temperature and magnetic field. It is found 
that the zero-bias anomaly is insensitive to magnetic fields up to 5 T. In contrast, a structure at 
5 meV, conventionally designated as the superconducting gap-but which is 4 times larger than 
the expected BCS gap-is strongly affected by magnetic fields above 1 T. 
Introduction. - Tunnelling measurements, as pioneered by Giaever [lJ, provide a most 
direct evidence about the existence and the value of the superconducting gap ~. For 
superconductors obeying BCS theory, the tunnelling conductance dI /dV vanishes for 
voltages V less than aje (for normal-insulating-superconducting (n-i-s) structures) or ~Le 
(for s-i-s structures), and has a sharp maximum at V = aje (or ~LeF. The vanishing of the 
conductance indicates that there are no states in the gap. 
An alternative technique for obtaining similar information is point contact spectroscopy 
(PCS), as pioneered by Yanson [2J. In this technique one investigates nos and s-n-s 
structures, utilizing the phenomenon of Andreev reflections, which increases the 
conductivity dI/dVfor voltages less than aje by as much as a factor of2. The detailed theory 
for such junctions was worked out by Blonder, Tinkham and Klapwijk [31, who also studied 
the continuous trsnsition from ideal Andreev reflection to Giaever tunnelling, as the 
impedance of the nos interface increases. Thus, the information obtained by PCS is similar to 
that obtained by tunnelling, yet this technique does not necessitate to produce high-quality 
insulating layers, which is problematic in several «exotic» superconductors. Thus, good 
«break junctions» PCS data were obtained for high-T, cuprate superconductors [4J. 
(I) On leave from: Racah Institute of Physics, Hebrew University, Jerusalem, Israel. 
3M EUROPHYSICS LETrERS 
In addition to the superconducting gap II, tunnelling and PCS measurements sometimes 
indicate structures at voltages much lower than Il/ e, and these are known as -zero-bias 
anomalies» (ZBA). In nonnal-state tunnelling (i.e. n-i-n structures), these have been known 
for quite some time; Shen and Rowell [5] observed maxima in the conductivity at zero bias, 
which they claimed could be explained by the Appelbaum-Anderson mechanism, and large 
resistance anomalies, which were not explained. More recently ZBAs were observed in 
tunnelling characteristics of YBCO [6], NdCeCuO [7] and KBaBiO [8]. These anomalies 
manifest themselves as a sharp maximum in the resistivity at zero bias in s-i-n structures, 
and as a sharp maximum in the conductivity at zero bias in son structures. The origin of the 
ZBA has not yet been established. One suggestion is that it is due to magnetic impurities in 
the boundary layer, giving rise to an Appelbaum-Anderson mechanism, which is a kind of a 
Kondo effect [9]. Another possibility is that it is a spectroscopic feature, due to the presence 
of states in the superconducting gap. When states are present at any energy eV, tunnelling is 
possible to these states, and as a result the tunnelling conductivity dI/dVat voltage V will 
not vanish. When there is a threshold for such states at an energy a (O), the tunnelling 
conductivity will vanish at lower voltages, accounting for the observed minimum. In BCS 
theory, such states do not exist. Therefore, the observation of such states is an indication of 
some deviation from BCS theory. In BCS theory, the gap function Il is a constant (up to the 
high phonon energy), but in some generalizations of BCS theory, Il(t, B) is a function of the 
angle e (of the k-vector of the electronic state) and/or of the energy E. Specifically, in 
«d-wave pairing» 1l(.,O) is proportional to cos B [10], and thus there are states present down 
to zero energy. When Ilk, B) has a sharp maximum at £ = 0 (the Fermi surface) and falls off 
sharply as going away from the Fermi surface, the excitation energy E(.) = V.' + Il(t}' will 
have a minimum at a finite value of " and thus there are states present at energies between 
the minimum and Il(O), the gap at the Fermi surface. A model based on this concept was 
proposed previously [11-13]. 
Thus, the ZBA serves as an indication of essential deviations from BCS theory, and is of 
vital importance for the elucidation of the superconducting mechanism in high-T, cuprates as 
well as in other superconducting systems. In order to discriminate between the various 
possibilities for the origin of the ZBA, measurements of the magnetic-field dependence of the 
tunnelling characteristica are very useful. Such measurements have been carried out for 
YBCO by Lesueur and Greene [14]. 
The present work is concerned with ZBAs in organic superconductors of the 
(BEDT-TTF),X (X = I., lAuI, Cu(NCS)", etc.) family. A «sign-changing» ZBA was obser-
ved on ,8-(BEDT-TTF)"I. (T, = 12K) [15]. At temperatures of about 1 K this anomaly is 
a maximum in the conductivity (minimum in the resistance). At low temperatures, i.e. 
T = 0.08 K, the sign of the anomaly changes, and it becomes a minimum in the con-
ductivity. 
A ZBA was further investigated on ",-(BEDT-TTF).I, (T, = 8 K) [13]. The PCS data 
show very sharp structures in d'V/dI' VB. V curves in three regions: U(O) ~ 0.7 meV, U(l) ~ 
~ 8-10 meV and 26(') ~ 20-25 meV. It is the a(O) structure we here refer to as _ZBA·. 
Describing the poinkontact between two ",-(BEDT-TTF)"I. crystals as two son junctions in 
series, the width of the ZBA rescaled to a single s-n contact measured from the maximum to 
the half-intensity point is about 0.36 meV at 1.5 K. 
A structure simi1r to the a(l) structure was also seen by Hawley et al. [16] by vacuum 
tunnelling into ,8-(BEDT-TTF),IAuI (T, ~ 4 K), with a(1) ~ 2.5-4 meV, and by Bando et 
al. [17] on K-(BEDT-TTF).Cu(NCS)" (T, = 10.4K) using a scanning tunnelling microscope 
(STM), with a(1) = 4.&-6 meV. Bando et al. also observed structures at higher energies at 
about U(I), which may correspond to the a(2) structure. 
In the tunnelling data, the ZBA structures are resistivity maxima, while they appear as 
G. ERNST et at.: ZER()..BIAS ANOMALIES IN POINT..cONTACT CHARACTERISTICS ETC. 305 
conductivity maxima in the pes data. For normal superconductors, a similar change of sign 
may be accounted for by the BTK theory [3), in which a junction with a low tunnelling 
barrier, described by a parameter Z, has a drop in conductivity as the voltage exceeds the 
gap (Andreev reflection), while one with a high Z has an increase in conductivity at eV = d 
(Giaever tunnelling). This sign reversal may help to discriminate «genuine» spectroscopic 
features from «spurious» ones, such as Josephson tunnelling-that can give rise to a 
maximum in conductivity at zero bias-and critical-current effects. Effects where the 
electronic mean free path falls as eV exceeds the phonon frequency, due to spontaneous 
emission of phonons [18), giving rise to an increase in resistivity for point contact 
spectroscopy, but not affecting tunnelling data, are also excluded for the same reason. 
In the present note, we present the magnetic-field dependence of the ZBA (t'o) and the 
t(1) structures in "'t-(BEDT-TTF), 13 • In addition we show a «tunnelling» curve for 
at-(BEDT-TTF>ZI, (dV/dI and dV" /dI' curves) for a heterojunction between an ",,-crystal 
and a tungsten (W) wire. 
Experimenuu and results. - Two ",,-(BEDT-TTF),I, crystals were pressed together in 
such a way that the current was flowing in the (a, b)-plane, and I-V curves were taken at 
various temperatures [13). The second-derivative curves of the characteristics of such point-
contacts at various temperatures are plotted in fig. 1. The three sharp structures referred to 
as t(O), t(1) and t(2) are clearly seen. 
The magnetic-field dependence ofthe dV/dI and the d'V/dI' curves for the homocontacts 
of the a,-(BEDT-TTF>zI, crystals, at a temperature of 1.6 K, and in magnetic fields up to 5 T, 
is shown in fig. 2. The magnetic field is approximately parallel to the current flow (ie. in the 
(a, b)-plane). It is seen that the t(1) structure decreases strongly with magnetic fields of 
1-2 T, the resistance dV/dI is significantly increased by the magnetic field in the low-voltage 
region (V < 5 mY), while the t<O) structure (as seen in the second-<lerivative curve) is almost 
unaffected even in a field of 5 T. 
In fig. 3 we show the tunnelIing characteristic of a n-i-s tunnelling contact; a W needle was 
6.0 
-
5.0 
4.0 7.5 K 
~ 
:i 3.0 6.4 K 
$ 
" 2.0 ~ 
." 
4.2 K 
> 1.0 
"." 
0.0 1.6K 
-1.0 
J2M~ __ ~~~~~~JJ~ 
-80 -40 0 40 80 
V (mV) 
Fig. 1. - Second derivative (d'V/dI') of the characteristics of an a.-(BEDT-TrF), Is point contact (s-n-s 
homocontact) at various temperatures. 
306 EUROPHYSICS LEttERS 
50 
b) 
, . 
, . 
, , 
r.W.~<JJ... _ .' 
:' , ~I 
: \ ', , 
. '. , 
\ ,I 
, , , 
:, ' ''I 
, 
- 12 - 8 -4 o 4 8 12 
V (mV) 
Fig, 2, - a) First derivative (dV/dl) and b) second derivative (d'V/d/') of the characteristics of an 
. ,-(BEDT-TTF)"J, s-n-s point contact in a magnetic field, --- ST, - ' - ' - 4T, " " 3T, - '- ' - ' - 2T, 
-- --IT, - OT; T=1.6K. 
moved against an a,-(BEDT-TrFl.I. crystal using an STM mechanism, until a finite (but 
vcry large) resistance was obtained. dI/dVand d'I/dV' curves are shown (T = 1.5 K)_ A 
structure is clearly seen as a sharp conductivity maximum at 6.2 meV, The t eo) structure was 
not seen in this experiment (just like in the tunnelling measurements of Hawley [16] and 
Bando [17]), The nature of the insulating barrier (i) in this experiment is not known, 
DiscussWn. - The features of the zero-magnetic-field I-V curves have been discussed 
previously [13], Here we point out again that the t") structure at 6 meV is about four times 
higher than the expected BCS gap, this ratio being considerably larger than the 
corresponding ratio in high-T, cuprates. In Raman spectra, phonons are observed at 3,5 meV 
and at 5 meV [19]. Thus, the t(1) structure coincides with one of the phonons, This may, or 
may not, be an accident, as discussed in [13], 
c. ERNST et cU.: ZERO-BIAS ANOMALIES IN POINT-CONTACT CHARACfERISTICS ETC. 307 
80 60 
a) b) 
40 
60 
-
-
d 20 
• ,; ,.. 
-
- • :> 40 :> 0 
'" 
:!2 I :a " .... -20 
." 
20 U -40 
0 - 60 
-6 -4 -2 0 2 4 6 - 6 -4 - 2 0 2 4 6 
V (mV) V (mV) 
Fig. 3. - a) First derivative (dl/dV) and b) second derivative (d'I/d.V' ) of a tunnelling characteristic of 
a W-a,-{IlEDT-TI'FJ,I, contact (T = 42 K). 
The salient feature of the magnetic-field dependence is the strong field dependence of the 
a(1) structure, and the nearly field-independent behaviour of the a(O) structure. The critical 
field H .. for a,-(BEDT-TTFlzI. (H in the (a,b)-plane) is about 11 Tat 1.6K(20), thus the 
field that drastically decreases the all) structure (1 to 2 T) is very low in comparison and thus 
is not expected to affect the superconductivity of this material. Even a field of 5 T has not a 
strong effect on the superconductivity, and thus the small effect on the a(O) structure is not 
altogether mysterious_ 
However, the ZBA is often attrIbuted to an Appelbaum-Anderson effect [5, 14), which is a 
kind of Kondo effect involving scattering on spins in an insulating layer (9). In a strong 
magnetic field, this effect gives rise to a structure at the Zeeman splitting energy e V = 
= gSI'BH (21). Thus, this effect is modified by a magnetic field which polarises the spins. Since 
the energy involved here is about 0.35 meV, it should be affected by a Zeeman spin splitting 
of this order, i.e. a field of about 3 T for 9 = 2. The effect of the magnetic field on the a(Q) 
structure thus seems to be too weak to account for the structure by this mechanism. 
The effect of the magnetic field on the all) structure is strong; the a( l) structure is 
conventionaUy designated as the superconducting gap (however, see (13)). We are not aware 
of a -basic» physical mechanism by which Zeeman spin splitting should remove (or broaden) 
the superconducting gap structure. 
Since we deal with Andreev reflections, orbital magnetic effects may affect the sharpness 
of the structures of the I-V curves. Thus, we may expect the magnetic field to broaden the 
structures when the cyclotron frequency w, is comparable with the widths of the structures. 
For an effective mass of m.". = mo, w, is about 0.35 meV in a field of3.5 T, and about 0.7 meV 
(i.e. the width of the all) structure) in a field of about 7 T. De Haas-van Alphen oscillations in 
several BEDT-TTF salts indicate m", = 3mo (22); this value can account (roughly) for the 
absence of an effect at 5 T in the t<O) structure; however, the large effect on the aO ) structure 
can be accounted for in this way only if m.". is considerably less than mo . 
Recently it was suggested that, due to a very narrow Lindhard peak, the velocity has a 
maximum at the Fermi level (23). If this is indeed the case, the inverse effective mass 
l/m", = h - I dv(k)/dk is zero right at the Fermi level, but becomes very large at some 
distance away from it. Such a very strong k-dependence of the effective mass can, in 
principle, account for these observations; however, such a mechanism is speculative at the 
present stage. 
Orbital magnetic effects should increase the resistivity when w, is of the order of the 
308 EUROPHYSICS LETrERS 
scattering rate ~ - I • This rate is estimated to be about 100 me V at 300 K; the resistance ratio 
P(300)/p(10) in good samples of BEDT-TI'F radical salts is at least 1000 (24), and the 
resistivity continues to decrease below 10 K (25). Thus, a scattering rate below 0.1 meV at 
1.5 K is plausible, with w, ~ ~ - I but: w,« S(O), accounting for the increase in resistivity with 
magnetic field, without a significant effect on the zero bias anomaly. 
Ccmclusian. - The PCS experiments on superconducting radical salts of BEDT-Tl'F 
indicate the presence of states in the gap, and thus of an essential deviation from BCS theory. 
States in the gap may be due to d-wave pairing, extended 8-wave pairing or a very strong 
energy dependence of the gap function at very low energies (26). The present experiments 
help us to discriminate between these possibilities. For d-wave pairing, states extend all the 
way to zero energy, thus there is no ZBA. Therefore we cannot account for our data by 
d-wave pairing in a «simplistic» way. Extended 8-wave pairing can give rise to two gaplike 
structures-a minimum gap min x~ElI 8)) and a maximum one max x~ElI 8)). We may associate 
the minimum with S(O) and the maximum with s(1) • Such a model was proposed for YBCO by 
Tachiki et al. (27). However, the Fermi surface in BEDT-TI'F salts is cylindrical (28) and 
therefore we doubt whether an anisotropy of more than 10: 1 for the superconducting gap is 
plausible. Also, the lower gap suggests a low critical field, while the experiment shows that 
the S(O) structure is nearly field independent. The third possibility, namely a strong energy 
dependence of the gap (13), is in line with the normal-state properties of organic metals, 
which suggests a very strong k-dependence of the mean free path [23,29). A theory that 
predicts such a behaviour was proposed recently (30). This theory is rather complicated, and 
we cannot describe it properly in the present short note, but an essential feature of it is an 
anomalous behaviour of the effective mass m*, with a maximum at the Fermi surface and a 
rather small value a few meV away. This feature would be consistent with the present 
measurement of the field dependence of the S(O) and S( 1) structures. 
In conclusion, the observation of the ZBA in ",-(BEDT-TI'F)"I. and its magnetic-field 
dependence may serve as an important experimental «fingerprint» that can help us to 
establish the mechanism of superconductivity in organic superconductors. 
*** 
We benefitted greatly from discussions with I. It YANSON, It E. GRAY and A. G. M. 
JANSEN. The stay of MW in Germany was supported by the Minerva Foundation. Support of 
this work by the Forschungsschwerpunkt des Landes Baden-Wiirttemberg is gratefully 
acknowledged. 
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Zero bias anomalies in point-contact characteristics of &#945;t-(BEDT-TTF)2I3

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Zero bias anomalies in point-contact characteristics of αt-(BEDT-TTF)2I3

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Zero bias anomalies in point-contact characteristics of &#945;t-(BEDT-TTF)2I3

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Zero bias anomalies in point-contact characteristics of αt-(BEDT-TTF)2I3