Tunneling spectra of break junctions involving Nb₃Sn
The electronic gap structure of Nb3Sn was measured by the break-junction (BJ) tunneling technique. The superconducting gap values are estimated to be in the range 2∆ = 4–5.5 meV at T = 4.2 K as follows from the observed distinct conductance peaks. In addition to the superconducting gap structure,...
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Ekino, Toshikazu Sugimoto, Akira Sakai, Yuta Gabovich, A.M. Akimitsu, Jun 2017-06-08T04:31:01Z 2017-06-08T04:31:01Z 2014 Tunneling spectra of break junctions involving Nb₃Sn / Toshikazu Ekino, Akira Sugimoto, Yuta Sakai, A.M. Gabovich, Jun Akimitsu // Физика низких температур. — 2014. — Т. 40, № 10. — С. 1182-1186. — Бібліогр.: 24 назв. — англ. 0132-6414 PACS 74.55.+v, 74.70.Ad, 71.20.–b, 74.81.Fa, 81.30.Kf https://nasplib.isofts.kiev.ua/handle/123456789/119668 The electronic gap structure of Nb3Sn was measured by the break-junction (BJ) tunneling technique. The superconducting gap values are estimated to be in the range 2∆ = 4–5.5 meV at T = 4.2 K as follows from the observed distinct conductance peaks. In addition to the superconducting gap structure, we observed reproducible hump-like structures at the biases of about ± 20 and ± 50 mV. Such a coexistence of gap and hump structures resembles the situation found in the high-Tc copper-oxide superconductors. Above the superconducting critical temperature Tc ~ 18 K, the humps appear as the only gap-like structures. Their possible origin is discussed in connection to the structural phase transition occurring in Nb₃Sn. This work was supported by a Grand-in-Aid for Scientific Research (245403770) of the Japan Society for the Promotion of Science (JSPS). The work was partially supported by the Project No. 8 of the 2012-2014 Scientific Cooperation Agreement between Poland and Ukraine. en Фізико-технічний інститут низьких температур ім. Б.І. Вєркіна НАН України Физика низких температур III Международный семинар по микроконтактной спектроскопии Tunneling spectra of break junctions involving Nb₃Sn Article published earlier |
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Tunneling spectra of break junctions involving Nb₃Sn |
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Tunneling spectra of break junctions involving Nb₃Sn Ekino, Toshikazu Sugimoto, Akira Sakai, Yuta Gabovich, A.M. Akimitsu, Jun III Международный семинар по микроконтактной спектроскопии |
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Tunneling spectra of break junctions involving Nb₃Sn |
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Tunneling spectra of break junctions involving Nb₃Sn |
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Tunneling spectra of break junctions involving Nb₃Sn |
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Tunneling spectra of break junctions involving Nb₃Sn |
| title_sort |
tunneling spectra of break junctions involving nb₃sn |
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Ekino, Toshikazu Sugimoto, Akira Sakai, Yuta Gabovich, A.M. Akimitsu, Jun |
| author_facet |
Ekino, Toshikazu Sugimoto, Akira Sakai, Yuta Gabovich, A.M. Akimitsu, Jun |
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III Международный семинар по микроконтактной спектроскопии |
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III Международный семинар по микроконтактной спектроскопии |
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2014 |
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Физика низких температур |
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Фізико-технічний інститут низьких температур ім. Б.І. Вєркіна НАН України |
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Article |
| description |
The electronic gap structure of Nb3Sn was measured by the break-junction (BJ) tunneling technique. The superconducting
gap values are estimated to be in the range 2∆ = 4–5.5 meV at T = 4.2 K as follows from the observed
distinct conductance peaks. In addition to the superconducting gap structure, we observed reproducible
hump-like structures at the biases of about ± 20 and ± 50 mV. Such a coexistence of gap and hump structures resembles
the situation found in the high-Tc copper-oxide superconductors. Above the superconducting critical
temperature Tc ~ 18 K, the humps appear as the only gap-like structures. Their possible origin is discussed in
connection to the structural phase transition occurring in Nb₃Sn.
|
| issn |
0132-6414 |
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https://nasplib.isofts.kiev.ua/handle/123456789/119668 |
| citation_txt |
Tunneling spectra of break junctions involving Nb₃Sn / Toshikazu Ekino, Akira Sugimoto, Yuta Sakai, A.M. Gabovich, Jun Akimitsu // Физика низких температур. — 2014. — Т. 40, № 10. — С. 1182-1186. — Бібліогр.: 24 назв. — англ. |
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| first_indexed |
2025-11-25T23:52:47Z |
| last_indexed |
2025-11-25T23:52:47Z |
| _version_ |
1850588855724408832 |
| fulltext |
Low Temperature Physics/Fizika Nizkikh Temperatur, 2014, v. 40, No. 10, pp. 1182–1186
Tunneling spectra of break junctions involving Nb3Sn
Toshikazu Ekino, Akira Sugimoto, and Yuta Sakai
Hiroshima University, Graduate school of Integrated Arts and Sciences, Higashihiroshima 739-8521, Japan
E-mail: ekino@hiroshima-u.ac.jp
Alexander M. Gabovich
Institute of Physics, National Academy of Sciences of Ukraine, Kiev 03680, Ukraine
Jun Akimitsu
Department of Physics, Aoyama-Gakuin University, Sagamihara 252-5277, Japan
Received June 2, 2014, published online August 21, 2014
The electronic gap structure of Nb3Sn was measured by the break-junction (BJ) tunneling technique. The su-
perconducting gap values are estimated to be in the range 2∆ = 4–5.5 meV at T = 4.2 K as follows from the ob-
served distinct conductance peaks. In addition to the superconducting gap structure, we observed reproducible
hump-like structures at the biases of about ± 20 and ± 50 mV. Such a coexistence of gap and hump structures re-
sembles the situation found in the high-Tc copper-oxide superconductors. Above the superconducting critical
temperature Tc ~ 18 K, the humps appear as the only gap-like structures. Their possible origin is discussed in
connection to the structural phase transition occurring in Nb3Sn.
PACS: 74.55.+v Tunneling phenomena: single particle tunneling and STM;
74.70.Ad Metals; alloys and binary compounds;
71.20.–b Electron density of states and band structure of crystalline solids;
74.81.Fa Josephson junction arrays and wire networks;
81.30.Kf Martensitic transformations.
Keywords: tunneling, break junction, superconducting energy gap, Nb3Sn, charge density waves.
1. Introduction
Among the inter-metallic superconductors with A-15
cubic crystal structure, Nb3Sn is one of the most popular
compounds because of its relatively high Tc = 18 K and
stable metallurgical characteristics [1]. Its crystal structure
is not the layered one in contrast to the copper-oxide high
T superconductors, but instead Nb atoms form orthogonal
linear chains along each principal cubic direction [2]. Such
Nb atomic chains were considered to be related to high-T
superconductivity of the A-15 structure. It is well known
that the A-15 cubic compounds undergo the tetragonal
distortion due to the electronic instability of the linear
chains of transition metal atoms (like Nb) at the character-
istic temperature Tm, which is higher than Tc. This phase
transition is a martensitic one, and the antagonistic inter-
play between structural instability and superconductivity
has been investigated both experimentally and theoretically
[1,3,4]. To elucidate the character of superconductivity in
A-15 compounds, tunnel junctions and electron tunneling
spectroscopy were intensively applied to measure the su-
perconducting gap 2∆ and the electron–phonon interaction
described by the Eliashberg function α2F(ω) [5–7]. Almost
all the junctions fabricated so far in order to perform spec-
troscopic measurements involved artificial oxide barriers,
which might lead to the emergence of spurious features in
the tunneling spectra. Break junctions of Nb-Sn filaments
[8] was the only exception.
In this paper we present tunneling measurements of
Nb3Sn single crystals using up-graded break-junction (BJ)
technique. The studies were focused on the superconduct-
ing gap characteristics as well as the electronic features
related to the martensitic transition. The state below Tm is
believed to be one with charge-density waves (CDWs)
induced by the Peierls instability [3]. The electronic gap
formation in the density of states is one of the main conse-
quences of the CDW transition, which can be exactly de-
© Toshikazu Ekino, Akira Sugimoto, Yuta Sakai, Alexander M. Gabovich, and Jun Akimitsu, 2014
Tunneling spectra of break junctions involving Nb3Sn
tected by the BJ technique. Actually, recent point-contact
spectroscopic studies of Nb3Sn not involving artificial bar-
riers confirmed such a viewpoint [9].
2. Experimental procedures
The single crystals were grown by the vapor transport
method. To characterize the sample, the temperature, T,
dependence of the electrical resistance, R(T), is shown in
the inset of Fig. 1. The BJ technique was used to form the
appropriate junction interface. The crucial advantage of
this technique is a cryogenic fracture at 4.2 K of the sam-
ple fixed on the flexible substrate with four electrodes by
applying an external bending force, resulting in a fresh and
clean junction interface that can provide the unaffected gap
features [10]. This junction design forms a superconduc-
tor–insulator–superconductor (SIS) symmetric junction
structure.
3. Results and discussion
Figure 1 shows the tunneling conductance G(V) =
= dI/dV(V) at 4.2 K for different BJs. A sharp and intensive
gap-edge structure is inherent to the bottom curve showing
the peak-to-peak voltage interval Vp-p ≈ 8.6 mV (corre-
sponding biases are ± 4.3 mV) and ~ 10% zero-bias leak-
age as compared to the value G(± 20 mV). This is typical
of the Bardeen–Cooper–Schrieffer (BCS) gap structure
with the SIS junction geometry of BJ, thereby Vp-p = 4∆/e.
Here e is the elementary charge. The sub-gap peaks at
± 2.3 mV appear due to the ± ∆/e singularities as a result of
the partial formation of SIN (N stands for a normal metal)
junctions. The gap value 2∆ = 4.6 meV in fact corresponds
to the Sn deficient region in the crystal [6].
In contrast, the gap peak in G(V) for the top curve is
conspicuously broadened showing quite high conductance
leakage as well as the factor of ~ 103 larger high-bias con-
ductance magnitude. The conductance leakage almost
reaches the normal-state high-bias value and also demon-
strates the Josephson (weak-link) zero-bias peak. There is a
substantial quasiparticle scattering at the interface involv-
ing the Andreev reflection. The broad conductance peaks
at ± 5.5 mV corresponding to ± 2∆/e of an SIS junction are
larger than ± 4.3 mV of the bottom curve. In the middle
curve, the observed features include both gap values ap-
propriate to the junctions described above. The rather
broad ± 5.5 mV peaks outside the ± 4.2 mV peaks are con-
sistent with those of the top curve, thereby indicating the
inherent gap of Nb3Sn, although the appearance of the gap
is not as intensive as in the top curve. On the other hand,
the ± 5.5 mV peaks are absent when the zero-bias peak is
suppressed, as is shown in the bottom curve. Since the ze-
ro-bias peak corresponds to the coherent Cooper-pair tun-
neling process and may not manifest itself in the disor-
dered regions, the absence of the feature supports the idea
that the BJ interface is formed along the immature super-
conducting phase in the nonideal Nb–Sn composite region
in the crystal.
Assuming the isotropic BCS gap to Tc ratio 2∆/kBTc ≈
≈ 3.52, where kB is the Boltzmann constant, the gap values
inferred from the experiment correspond to the local critical
temperatures *
cT = 14 K (bottom curve) — 18 K (top curve).
The former value corresponds to the gap of a nonstoichio-
metric phase in spite of the observed sharp gap edge, while
the latter value is the bulk Tc, although G(V) is substantially
distorted in this case. Anyway, the strong-coupling gap value
of 2∆ > 6.5–7 meV, which can be deduced from the widely
accepted gap ratio 2∆/kBTc = 4.3–4.4 and the bulk Tc = 18 K
[6], was not found in our measurements.
In Fig. 2 we demonstrate the temperature evolution of
G(V) obtained for a mechanically stable BJ. The shape of
G(V) at each T is smeared but is quite stable in the whole T
range from 4.2 K up to ~ 17.4 K. The gap-edge peaks at
6.4 K, which occur at the same biases as in Fig. 1, are grad-
ually suppressed and smeared with increasing T. The ob-
served behavior is typical for BCS superconductors. The
inset shows the SIS conductance fitting results using the
Dynes equation with thermal smearing in order to determine
accurately the gap value at 6.4 K [11]. We can recognize
from these results that the fitted gap-peak position corre-
sponding to the gap parameter ∆ = 2–2.2 meV does not
change substantially even if the phenomenological broaden-
ing parameter Γ is drastically varied (Γ = 0.12–0.46 meV).
From the T evolution of the gap characteristics, we deter-
mine the critical temperature Tc at the BJ interface as 17.5
K, which is close to the bulk Tc = 18 K. The low-T gap
combined with the value Tc ≈ 17.5 K yields the gap to Tc
ratio 2∆/kBTc ≈ 2.7–3, which is far from the literature
strong-coupling value 4.4 [6]. Close look at the curve set in
Fig. 1. (Color online) Tunneling conductance G(V) = dI/dV(V)
obtained for different Nb3Sn break junctions in the superconduct-
ing state at 4.2 K. The inset shows the temperature dependence of
the electrical resistance R(T) normalized by R(T = 100 K).
Low Temperature Physics/Fizika Nizkikh Temperatur, 2014, v. 40, No. 10 1183
Toshikazu Ekino, Akira Sugimoto, Yuta Sakai, Alexander M. Gabovich, and Jun Akimitsu
Fig. 2 shows that the gap structure develops below 14–16 K,
which is consistent with the local BCS value *
cT ≈ 14 K
corresponding to 2∆ = 4.2 meV as was evaluated from data
demonstrated in Figs. 1 and 2. This means that the small gap
is induced by the proximity effect at the junction region pre-
sumably due to the BJ fracture of the local superconducting
phase in the crystal.
Looking at larger-bias region of Fig. 1 more carefully,
one can readily see the broad humps around ± 20 mV be-
ing a third structure in the middle curve along with the
double-gap features at V ≈ ± 4.3 mV and ± 5.5 mV. Humps
at ± 20 mV are similar for different junctions. Namely, the
bias positions and the humps magnitudes of about 2–4%
excess above the background are almost the same. Such
conductance structures at ± 20 mV found here have been
found previously in the Nb–Sn BJ [8]. A rather small am-
plitude of 2–4% testifies that structures at ± 20 mV might
be associated with the strong-coupling effect of the pho-
nons, since the electron–phonon mechanism of supercon-
ductivity is expected to dominate in Nb3Sn. However, this
conclusion seems not to be valid in this case, because simi-
lar hump-like structures are also observed at ± 4.3 mV in
the absence of the ± 5.5 mV gap structures, inherent to the
full-developed superconducting state. Furthermore, alt-
hough the strong-coupling effects of the electron–phonon
interaction should be seen in every G(V) with the ideal
BCS-like gap structures, no such feature was found in
G(V), which demonstrated very distinct gap structure
(Fig. 1, bottom curve).
At the same time, a coexistence in the conductance
spectra of the superconducting and smeared hump struc-
tures resembles the features found in tunneling conduct-
ance of the high-T cupper-oxide superconductors at low
temperatures [12]. The similarity between the hump fea-
ture at ± 20 mV and the normal-state gap manifestation in
the high-T superconductor can be inferred from Fig. 3,
where the G(V) from different BJs are presented at tem-
peratures up to well above Tc, if one bears in mind the
difference in energy scales. The G(V) shapes displayed in
Fig. 3 shows that the asymmetry of the gap-edge peaks
with respect to the zero-bias varies for various junctions,
but the peak locations at ± (20–30) mV are approximately
the same.
From the facts discussed above, the large-bias peculiari-
ties observed here as well as those reported previously for
Nb3Sn [5–8,13,14], which were attributed to the electron-
phonon-interaction manifestations, should rather be con-
sidered as the gap-edge structures of a nonsuperconducting
nature. Moreover, the ± (20–30) mV structures in Nb3Sn
survive regularly at temperatures well above Tc similarly to
the transformation of the large-bias tunnel conductance
features into the pseudogap depression in the normal state
observed in cuprates [14,15]. We want to emphasize that in
the 23.9 K curve of Fig. 3 the fairly well defined peaks of
G(V) occur at biases at ≈ ± 40 mV in addition to the inner
– 20 mV peak or + 20 mV hump structures. The outer va-
lue is twice as much as the inner one.
In Fig. 4 the conductance G(V) is displayed for different
BJs in the large bias range |V| ≤ 120 mV at T = 4.2 K and
higher T > Tc. The G(V) in the superconducting state at 4.2 K
demonstrates the coherent superconducting gap-edge peaks
at ± 4.5 mV, which correspond to ± 2∆/e, the Josephson peak
Fig. 2. (Color online) Temperature variation of G(V) raw data for
a Nb3Sn break junction. The inset shows the SIS fitting results for
the 6.4 K curve using the Dynes equation with thermal smearing
[11]. The dashed and dotted curves correspond to the broadening
parameters Γ = 0.12 meV and 0.46 meV, respectively, for the
same gap parameter ∆ = 2 meV.
Fig. 3. G(V) for different Nb3Sn break junctions in the supercon-
ducting and normal states: comparison of the ± 20 mV structures.
1184 Low Temperature Physics/Fizika Nizkikh Temperatur, 2014, v. 40, No. 10
Tunneling spectra of break junctions involving Nb3Sn
at V = 0 and the hump structures at ≈ ± 20 mV. Other pecu-
liar features of G(V) are the kink structures of G(V) centered
at biases ≈ ± 50 mV with a weak ( ≈ 5%) excess over the
background G(V) level. At T ≈ 35–38 K, i.e., well above Tc,
the characteristic reproducible gap-edge structures are also
observed at about ± (50–60) mV, which agrees with the kink
structures of ± 50 mV shown in G(V) at 4.2 K. It means that
the energy gap of some nature appears in the normal state of
Nb3Sn. Note, that the normal-state gap edge positions ≈ ± 50
mV are roughly twice as large as those with ± (20–30) mV
found in the superconducting state and indicated in Fig. 4 as
well as in Figs. 1 and 3. In particular, the above mentioned
double-gap features inherent to the 23.9 K curve shown in
Fig. 3 (at ± 20 mV and ± 40 mV) can be attributed to the
emergent symmetric SIS and nonsymmetric SIN junctions,
respectively. The substantially suppressed sub-gap conduct-
ance features in the normal state G(V) indicate that a certain
quality of the junction interface should be realized to notice
the partial density of states gapping. It is remarkable that
the positions of the observed normal-state gap peculiari-
ties at ≈ ± (20–30) mV and ≈ ± (50–60) mV do not
change although the temperature was raised up to ≈ 40 K.
The asymmetric character of G(V) above Tc in Fig. 3 can
be understood when the phase of the order parameter of the
CDW states is taken into account. This phase substantially
distorts the quasiparticle tunneling characteristics in the par-
tially CDW-gapped and partially normal-metal state formed
in the symmetric set-up of the BJ between CDW metals.
Hence, if the junction remains symmetric, the non-equality
of G(V) branches may be a consequence of the broken
symmetry phenomenon [15,16], when both of the electrodes
are in the same CDW-gapped state but the order parameters
exhibit different signs. In an alternative scenario, this asym-
metry, together with the double-gap features at about inner
values of ± 20 mV and almost twice larger values ± 40 mV
formed at 23.9 K, indicates that the gap structures pos-
sessing the values ± 20 mV are most probably a conse-
quence of the asymmetric junction formation with the CDW
gap Σ and a singularity at V = ± Σ/e. It might happen that an
asymmetric junction is formed after cracking the sample
with one of the electrodes being in a CDW-free metallic
state. Then G(V) is asymmetric [15,16].
The existence of the CDW gap in the tunnel spectra of
Nb3Sn impedes the conventional studies of the electron–
phonon interaction as strong-coupling features, which can
be performed in more conventional superconductors [17].
Once such structures are found as the intrinsic gaps, like in
our case, the difficulties of interpretation should be re-
solved taking into account the gapping by instabilities in
the electron-hole channel. The weak hump structure de-
picted in Figs. 1 and 3 is now understood as the ± Σ/e sin-
gularity emerging due to the partial CDW gapping. The
asymmetric form may be considered as the broken sym-
metry of CDW with the opposite signs of the order param-
eters. On the other hand, as we have indicated above, the
asymmetric G(V) shape for the apparently symmetric junc-
tion may arise due to the actual formation of the asymmet-
ric (one-side-normal metallic) junction. In this case the
CDW-driven current-voltage characteristics are asymmet-
ric for any phase of Σ except π/2 [15,16]. The realization of
the actually asymmetric configuration in the nominally
symmetric junctions was observed in the BJ measurements
of both CDW conductors [18] and superconductors [19].
There is enough evidence to explain the emergence of
the gap feature, which is due to the martensitic structural
phase transition occurring in the A-15 compound like
Nb3Sn [1,4]. Below the corresponding transition tempera-
ture, the cubic crystallographic symmetry is violated and
the tetragonal periodic lattice distortions appear driven by
the Peierls-type instability due to the displacement of Nb
atoms. The concomitant CDW leads to the quasiparticle
gap formation in the parent electronic density of states
N(E). Since its electronic signature was believed to be very
weak, there were previously not so many observations and
discussions concerning the peculiarities of the tunnel con-
ductance spectra of Nb3Sn junctions. Moreover, CDW
distortions in Nb3Sn seem to be spatially inhomogeneous,
which is similar to what is intrinsic to cuprate layered
structures [16,20]. This would obscure the CDW electronic
singularity as compared with the conventional sharp se-
cond-kind phase transitions. Therefore, the CDW gapping
reveals itself in the tunneling G(V) as a weak pseudogap
feature [16,20–22].
The high-T measurements clarified that the gap-edge
value does not decrease as compared with the low tem-
perature data even near 40 K. Generally speaking, the
inhomogeneity of the CDW formation should result in the
crystallographic scattered CDW values. The broad peak
Fig. 4. G(V) for different Nb3Sn break junctions in the super-
conducting and normal states, displaying the bias range up to
± 120 mV.
Low Temperature Physics/Fizika Nizkikh Temperatur, 2014, v. 40, No. 10 1185
Toshikazu Ekino, Akira Sugimoto, Yuta Sakai, Alexander M. Gabovich, and Jun Akimitsu
structures at ± (50–60) mV can be attributed to the scat-
tered CDW gap edges with ±2Σ/e corresponding to the
martensitic transition, which normally occurs at Tm =
= TCDW [1,23]. According to our data, TCDW is assumed
to be in the range ≈ 43 K, so that the gap ratio 2Σ/kBTCDW
can be estimated as ≈ 14 ± 2. Such values are typical for
CDW phase transitions. For instance, 2Σ/kBTCDW is about
15 for the low-dimensional CDW conductor NbSe3 [24].
Finally, the theoretical approach to the interplay be-
tween superconductivity and CDW phenomena, which
started in connection to A-15 compounds [4], was recently
successfully applied to treat the pseudogap phenomena in
copper oxides [15,16,20,21]. The presented studies of
Nb3Sn strongly support the idea that the dip-hump struc-
tures in A-15 and high-Tc superconductors are of a similar
origin.
4. Conclusions
We have measured the Nb3Sn single crystal by the break
junction tunneling spectroscopy. The maximum supercon-
ducting gap was found to be 2∆ ≈ 5.5 meV, which corre-
sponds to the gap ratio 2∆/kBTc ≈ 3.6–3.7. We never ob-
served the strong-coupling ratio 4.3–4.4 reported elsewhere
in the literature. The normal-state tunneling conductance
exhibits the gap-like structure of 2Σ = 50–60 meV at least up
to ≈ 40 K, which can be attributed to the CDW gap appearing
due to the martensitic structural phase transition below Tm.
The gap ratio 2Σ/kBTm = 13–16 if one assumes Tm ≈ 43 K
agrees with that found in the known CDW conductors.
Acknowledgements
This work was supported by a Grand-in-Aid for Scien-
tific Research (245403770) of the Japan Society for the
Promotion of Science (JSPS). The work was partially sup-
ported by the Project No. 8 of the 2012-2014 Scientific
Cooperation Agreement between Poland and Ukraine.
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1186 Low Temperature Physics/Fizika Nizkikh Temperatur, 2014, v. 40, No. 10
1. Introduction
2. Experimental procedures
3. Results and discussion
4. Conclusions
Acknowledgements
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