Analysis of nonlinear conductivity of point contacts on the base of FeSe in the normal and superconducting state
Nonlinear conductivity of point contacts (PCs) on the base of FeSe single crystals has been investigated. Measured dV/dI dependencies demonstrate the prevailing contribution to the PC conductivity caused by the degraded surface. Superconducting (SC) feature in dV/dI like a sharp zero-bias minimum...
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| Cite this: | Analysis of nonlinear conductivity of point contacts on the base of FeSe in the normal and superconducting state / u.G. Naidyuk, N.V. Gamayunova, O.E. Kvitnitskaya, G. Fuchs, D.A. Chareev, A.N. Vasiliev // Физика низких температур. — 2016. — Т. 42, № 1. — С. 42–48. — Бібліогр.: 26 назв. — англ. |
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| author | Naidyuk, Yu.G. Gamayunova, N.V. Kvitnitskaya, O.E. Fuchs, G. Chareev, D.A. Vasiliev, A.N. |
| author_facet | Naidyuk, Yu.G. Gamayunova, N.V. Kvitnitskaya, O.E. Fuchs, G. Chareev, D.A. Vasiliev, A.N. |
| citation_txt | Analysis of nonlinear conductivity of point contacts on the base of FeSe in the normal and superconducting state / u.G. Naidyuk, N.V. Gamayunova, O.E. Kvitnitskaya, G. Fuchs, D.A. Chareev, A.N. Vasiliev // Физика низких температур. — 2016. — Т. 42, № 1. — С. 42–48. — Бібліогр.: 26 назв. — англ. |
| collection | DSpace DC |
| container_title | Физика низких температур |
| description | Nonlinear conductivity of point contacts (PCs) on the base of FeSe single crystals has been investigated.
Measured dV/dI dependencies demonstrate the prevailing contribution to the PC conductivity caused by the
degraded surface. Superconducting (SC) feature in dV/dI like a sharp zero-bias minimum develops for relatively
low ohmic PCs, where the deep areas of FeSe are involved. Analysis of dV/dI has shown that the origin
of the zero-bias minimum is connected with the Maxwell part of the PC resistance, what masks energy dependent
spectral peculiarities. Even so, we have found the specific features in dV/dI — the sharp side maxima,
which may have connection to the SC gap, since their position follows the BCS temperature dependence.
Exploring the dV/dI spectra of the rare occurrence with Andreev-like structure, the two gaps with Δ = 2.5 and
3.5 meV were identified.
|
| first_indexed | 2025-12-07T15:45:47Z |
| format | Article |
| fulltext |
Low Temperature Physics/Fizika Nizkikh Temperatur, 2016, v. 42, No. 1, pp. 42–48
Analysis of nonlinear conductivity of point contacts on the
base of FeSe in the normal and superconducting state
Yu.G. Naidyuk, N.V. Gamayunova, and O.E. Kvitnitskaya,
B. Verkin Institute for Low Temperature Physics and Engineering of the National Academy of Sciences of Ukraine
47 Lenin Ave., Kharkov 61103, Ukraine
E-mail: Naidyuk@ilt.kharkov.ua
G. Fuchs
Leibniz-Institut für Festkörper- und Werkstoffforschung Dresden e.V., Postfach 270116
Dresden D-01171, Germany
D.A. Chareev
Institute of Experimental Mineralogy, Russian Academy of Sciences, Chernogolovka 142432, Moscow District, Russia
A.N. Vasiliev
Low Temperature Physics and Superconductivity Department, Physics Faculty, M.V. Lomonosov Moscow State University
Moscow 119991, Russia
Theoretical Physics and Applied Mathematics Department, Ural Federal University, Ekaterinburg 620002, Russia
National University of Science and Technology “MISiS”, Moscow 119049, Russia
Received October 7, 2015, published online November 23, 2015
Nonlinear conductivity of point contacts (PCs) on the base of FeSe single crystals has been investigated.
Measured dV/dI dependencies demonstrate the prevailing contribution to the PC conductivity caused by the
degraded surface. Superconducting (SC) feature in dV/dI like a sharp zero-bias minimum develops for rela-
tively low ohmic PCs, where the deep areas of FeSe are involved. Analysis of dV/dI has shown that the origin
of the zero-bias minimum is connected with the Maxwell part of the PC resistance, what masks energy de-
pendent spectral peculiarities. Even so, we have found the specific features in dV/dI — the sharp side maxi-
ma, which may have connection to the SC gap, since their position follows the BCS temperature dependence.
Exploring the dV/dI spectra of the rare occurrence with Andreev-like structure, the two gaps with Δ = 2.5 and
3.5 meV were identified.
PACS: 74.45.+c Proximity effects; Andreev effect, SN and SNS junctions;
74.70.–b Superconducting materials other than cuprates;
74.70.Xa Pnictides and chalcogenides.
Keywords: iron-chalcogenide superconductors, point-contacts, Andreev reflection spectroscopy, energy gap.
Introduction
FeSe compound, belonging to the 11-structure groups of
iron based superconductors, is actively investigated nowa-
days. On one hand, this is due to the fact that FeSe has the
simplest crystal structure among other superconducting iron
chalcogenides and pnictides. Besides, it shows only the struc-
tural phase transition at Ts ~ 100 K, without an accompanying
magnetic phase transition. On the other hand, the supercon-
ducting (SC) transition temperature Tc ~ 9 K [1] in FeSe in-
creases drastically under pressure up to 37 K [2] and Tc reach-
es incredible 100 K in the case of a FeSe monolayer [3].
Observation of Shubnikov–de Haas oscillations demon-
strates the low carrier density (~0.01 carriers/Fe) and the
small Fermi energy (~3.6 meV). The Fermi surface occu-
pies only a small part of the Brillouin zone and contains
probably one electron and one hole thin cylinder [4]. The
© Yu.G. Naidyuk, N.V. Gamayunova, O.E. Kvitnitskaya, G. Fuchs, D.A. Chareev, and A.N. Vasiliev, 2016
Analysis of nonlinear conductivity of point contacts on the base of FeSe in the normal and superconducting state
electronic structure of the low-temperature orthogonal
FeSe-phase is similar to that for almost compensated sem-
imetals with ultrafast electron-like minority carriers having
small density of about 1018 cm–3 [5]. These carriers may
occur during formation of a “Dirac cone” or in the case of
the significant anisotropy of the Fermi surface.
Tunnel dI/dV spectra of FeSe demonstrate a V-shaped
zero-bias minimum with side maxima at +/–2.5 meV and
shoulders at +/–3.5 meV, which were taken as the evi-
dence for the two-gap SC state [6]. Thus, the Fermi ener-
gy EF in FeSe is comparable to the value of the SC gap(s)
Δ: Δ/EF ~ 1 (~ 0.3) for the electron (hole) band [6]. As a
result, the BCS (Bardeen–Cooper–Schriffer)–BEC (Bose–
Einstein condensation) crossover in FeSe can be realized.
All mentioned features make FeSe very attractive for
point-contact (PC) investigations [7]. This work presents
the study of current-voltage I(V) characteristics and their
derivatives dV/dI(V) of PCs based on FeSe single crystals
(Tc = 9 K) [1] in the normal and SC state. PC measure-
ments of nonlinear I(V) curves and their derivatives are
used in the method of Yanson PC spectroscopy [7] to iden-
tify specific bosonic (phononic) excitations and to obtain
information about the SC gap utilizing PC Andreev-
reflection spectroscopy.
Results
The plate-like single crystals of FeSe1–x (x = 0.04 +/– 0.02)
superconductor were grown in evacuated quartz ampoules
using flux technique as described in [1]. The resistivity and
magnetization measurements revealed a SC transition tem-
perature up to Tc = 9.4 K. PCs were established by touch-
ing of a sharpened thin Cu wire (or Ag and W wires) to
cleaved by a scalpel at room temperature flat surface of
FeSe or contacting by the wire an edge of plate-like sam-
ples. Thus, we have measured heterocontacts between
normal metal and the title compound. The differential re-
sistance dV/dI(V) ≡ R(V) of PC were recorded by sweeping
the dc current I on which a small ac current i was superim-
posed using a standard lock-in technique. The measure-
ments were performed in the temperature range from 3 K
to slightly above Tc. No principal difference was found by
“flat” or “edge” PC geometry in dV/dI(V) data, because
dV/dI(V) variate more significantly from one PC to anoth-
er. Several attempts have been made with FeSe surface
prepared by polishing using very soft sand paper or even
office paper, but it was more difficult to obtain the SC fea-
tures in the PC spectra in the latter case.
As shown in Fig. 1, the dV/dI spectra of PCs demon-
strate overall “semiconducting” behavior (the negative
dV/dI curvature) representing a broad maximum centered
at zero-bias voltage, which is more pronounced with in-
creasing of the PC resistance. For decreasing PC re-
sistance, the measured below Tc dV/dI spectra tend to have
a V-shaped sharp zero-bias minimum (see Fig. 1).
Figure 2 shows dV/dI for two PCs demonstrating “sem-
iconducting” and “metallic” behavior with the sharp zero-
bias minimum developing below Tc both for “semicon-
Fig. 1. (Color online) Series of dV/dI curves at decreasing of PC
resistance from about 200 Ω to 2 Ω (from the upper curve to the
bottom one). The curves, except the upper one, are shifted down
for clarity. Pronounced zero-bias minimum develops along with
the transition from “semiconducting” (high resistance) to more
“metallic” (low resistance) behavior of dV/dI. Inset shows dV/dI
for two PCs from the main panel at larger bias.
Fig. 2. (Color online) Typical dV/dI spectra (the main panel and
left inset) of two FeSe–Cu PCs measured well below and just
above Tc. Right inset shows the antisymmetric part dV/dI
as(%) =
= 100[dV/dI(V > 0) − dV/dI(V < 0)]/2dV/dI (V = 0) of dV/dI cal-
culated for both contacts at low temperatures.
Low Temperature Physics/Fizika Nizkikh Temperatur, 2016, v. 42, No. 1 43
Yu.G. Naidyuk, N.V. Gamayunova, O.E. Kvitnitskaya, G. Fuchs, D.A. Chareev, and A.N. Vasiliev
ducting” and “metallic” behavior. Note, that in spite of the
different “semiconducting” and “metallic” shape of dV/dI,
both of them show a similar asymmetry (see right inset of
Fig. 2). Figure 3 displays dV/dI with the “metallic” behav-
ior and a much sharper zero-bias dip compared to those in
Fig. 2. In this case dV/dI above Tc shows a shallow zero-
bias maximum. A more complicated dV/dI shape develops
for PC in Fig. 4, where the zero-bias minimum pattern is
more complex with additional sharp side peaks. It turned
out, that the position of the main side peak follows the
BCS temperature dependence.
A rarely observed dV/dI is shown in Fig. 5. Here, dV/dI
measured at the low temperature of 4.2 K demonstrates the
Andreev-like double minimum structure around zero-bias.
The position of the minima is about +/– 3.5mV, what is
close to the large gap value (3.5 mV) in FeSe measured by
tunneling spectroscopy in [6].
Discussion
“Semiconducting” behavior of dV/dI can be due to the
low concentration of carriers and/or depleted (semicon-
ducting) surface layer. As many investigations show, the
transport properties of FeSe are very sensitive to the stoi-
chiometry and the distribution of Fe vacancies. Thus, Chen
et al. [9] reported about the observation of three different
Fe-vacancy orders and among them one was identified to
be nonsuperconducting and magnetic at low temperature.
Also Chang et al. [10] discussed the amorphous oxide on
the surface of the fresh FeSe nanowires, which becomes
thicker with prolonged air exposure. Greenfield et al. [11]
underlined that “Vacancies in the iron sublattice and the
incorporation of disordered oxygen-containing species
are typical for nonsuperconducting antiferromagnetic
samples, whereas a pristine structure is required to pre-
serve superconductivity. Exposure to ambient atmosphere
resulted in the conversion of superconducting samples to
antiferromagnetic ones”. Therefore, we believe that the
“semiconducting” dV/dI shape is due to the degraded on air
thick surface layer. By decreasing of the PC resistance,
Fig. 3. (Color online) Temperature variation of the dV/dI spectrum
(main panel) of FeSe–Cu PC. Left inset shows the antisymmetric part
dV/dI
as(%) = 100[dV/dI (V > 0) − dV/dI(V < 0)]/2dV/dI (V = 0)
of dV/dI calculated for dV/dI at T = 12 K. Right inset shows the
behavior of thermo-emf in single FeSe crystals according to
Kasahara et al. [6] and Song et al. [8].
Fig. 4. (Color online) Temperature variation of the dV/dI spectrum of FeSe–Cu PC with the pronounced side peaks. Upper inset: dV/dI
for the same contact in magnetic field at T = 3 K. Left inset shows dV/dI at a few temperatures at larger bias. Right inset shows the tem-
perature and magnetic field position of the side peak.
44 Low Temperature Physics/Fizika Nizkikh Temperatur, 2016, v. 42, No. 1
Analysis of nonlinear conductivity of point contacts on the base of FeSe in the normal and superconducting state
we “penetrate” deeper to the bulk material. As a result,
dV/dI becomes more “metallic” and the SC zero-bias min-
imum developes.
Interestingly, in the recent report by Venzmer et al. [12],
they measured similar “semiconducting" type of dI/dV in the
planar tunneling junctions FeSe/AlOx/Ag patterned litho-
graphically into mesastructures. They observed also a me-
tallic like behavior in PC noticing that a tunneling barrier
with pinholes can result in a large variety of structures in
the differential conductivity. Sooth to say, dI/dV character-
istics in [12] resemble a little the tunneling behavior, since
their variation with a bias was less than a factor of two and
for some PCs only a few percent.
The lack of characteristic Andreev reflection features in
the dV/dI spectra below Tc (like double minima structure
instead of sharp zero-bias minimum) can be related to the
realization of the thermal regime [7,13] of the current flow
in PC. This regime develops in materials with high resis-
tivity, where inelastic mean free path becomes smaller that
the PC size (diameter) d. In this case, the resistivity ρ(T)
determines the behavior of I(V) characteristics and their
dV/dI derivatives according to the equation [7,13]:
1
2 1/2
0
( )
( (1 ) )PC
dxI V Vd
T x
=
ρ −∫ , (1)
while the temperature in the PC core TPC increases with a
voltage V according to the relation
2 2 2
0 0/4 ,PCT T V L= + (2)
where T0 is a bath temperature, L0 = 2.45∙10–8 V2/K2 is the
standard Lorentz number. In the case of TPC >> T0, the
temperature in the PC core TPC increases linearly with the
applied voltage TPC = V/2√L0 with the rate 3.2 K/mV.
By fitting Eqs. (1) and (2) to the measured dV/dI(V)
(see Fig. 6), the following parameters were estimated: the
Lorentz number in FeSe L ≈ 9L0, the PC residual resistivi-
ty ρ0 ≈ 0.35 mΩ·cm, the PC diameter d ≈ 0.8 μm for the
PC resistance of about 5 Ω. The obtained large value of
9L0 for the Lorentz number in FeSe correlates with its es-
timation from the thermal conductivity and resistivity data
just above Tc at 10 K in [6]. The rather large significant
value of ρ0 can be attributed to the degraded surface and
other imperfections on the surface in the contact area.
The asymmetry of the dV/dI characteristics in the
thermal regime in the case of heterocontacts is connected
with thermo-emf, so that antisymmetric part of dV/dI is
proportional to the difference between the Seebeck coef-
ficients S(T) of the contacting metals [14,15]. As shown
in the insets in Figs. 2 and 3, dV/dIas looks qualitatively
similar to the temperature dependence of S(T) in FeSe,
indicating that the PCs are in the thermal regime. Note,
that in spite of different “metallic” and ”semiconducting”
shape of dV/dI in Fig. 2, their antisymmetric parts are
similar. That is the antisymmetric part of dV/dI is more
reproducible and reflects rather the massive (bulk) mate-
rial properties (see also Appedix B in [16] for the discus-
sion). Here, it should be mentioned that behavior of S(T)
in FeSe samples measured by authors is different (see,
e.g., the inset in Fig. 3). It is known that the thermo-emf
is the most sensitive transport property of metals: it is
some kind of derivative of conductivity and it depends
strongly on the electronic structure [17]. Because of that,
the Seebeck coefficient is very sensitive to the quality of
FeSe samples, much more than the resistivity.
Fig. 5. (Color online) Temperature variation of the dV/dI spectrum
of FeSe–Ag PC with Andreev-like double minimum at zero bias
and lowest temperature. Inset shows a fit (solid red curve) of the
normalized on the normal state dV/dI at 4.2 K (symbols) using the
two-gap model with the parameters shown in the panel. Here, Δ
and Г are in meV. S is the scaling factor, which reflects the dif-
ference in intensities of experimental and calculated curves. In
the ideal case it must be S = 1. w is the partial contribution of the
larger gap 3.5 meV to the calculated spectra.
Fig. 6. (Color online) Fit (dashed red curve) of the dV/dI spectrum
(solid black curve) of FeSe–W PC above the SC minimum
(> 20 mV) according to Eqs. (1), (2) with the parameters d ≈ 0.8 μm,
ρ0 ≈ 0.35 mΩ⋅cm and L = 9L0. Inset shows the resistivity ρ(T)
of FeTe single crystal adapted from [1] and used in Eq. (1), which
is additionally increased by an amount of the enhanced residual
resistivity ρ0 in PC.
Low Temperature Physics/Fizika Nizkikh Temperatur, 2016, v. 42, No. 1 45
Yu.G. Naidyuk, N.V. Gamayunova, O.E. Kvitnitskaya, G. Fuchs, D.A. Chareev, and A.N. Vasiliev
Let us turn to the discussion of the origin of the sharp
zero-bias minimum. Obviously, it is connected with the SC
state in PC. At the same time, the nature of this SC dip has
to be clarified. Such zero-bias dip in dV/dI (maximum in
dI/dV) is often connected with the Andreev bound states in
the case of unconventional d-wave superconductors. How-
ever, a similar structure is observed regularly in simple
elemental (conventional) superconductors [18]. Especially,
such dip in dV/dI is characteristic for the complex SC
compounds with high residual resistivity like high-Tc ma-
terials [19], heavy-fermion systems [20] and amorphous
alloys [21]. Gloos et al. [20] concluded that such zero-bias
dip is due to the Maxwell’s resistance (see Eq. (3)) being
suppressed in the SC state.
Let us try to estimate parameters of PC from its re-
sistance RPC. The latter is expressed by the well known
Wexler formula, which contains the sum of ballistic
Sharvin and diffusive Maxwell resistance:
216 /3 /2 ,PCR l d d≈ ρ π +ρ (3)
where 2 4 2/3 10/ 1.3 10 3.2 10Fl p ne n− −ρ = ≈ ⋅ ≈ ⋅ Ω⋅cm2,
using the carrier density n ≈ 2.53⋅1020 cm−3 from [22]. The
residual resistivity 0ρ in the PC core is unknown in Eq.
(3). If we suppose that 0ρ ≈ 0.035 mΩ⋅cm just above Tc
like in the bulk FeSe crystal [1], then, according to Eq. (3),
a PC diameter of d ≈ 120 nm and an electron mean free
path of l ≈ 90 nm are estimated for the PC with the re-
sistance of about 5 Ω. That is, d ≈ l and the current regime
in the investigated PC is neither ballistic, nor diffusive. More-
over, such PC is affected by a high current density j ≈ V/Rd
2,
increasing with the rate of about 1.4⋅106A/cm2 per 1 mV.
On the other hand, the corresponding parameters estimated
by fitting of the experimental dV/dI curve with similar re-
sistance in Fig. 6 by Eqs. (1) and (2) are d ≈ 0.8 μm and
0ρ ≈ 0.35 mΩ⋅cm. That is, 0ρ is one order of magnitude
larger than that in the bulk. Correspondingly, l is ten times
smaller and this PC is in the diffusive limit d >> l. This is
due to a degraded surface layer resulting in a higher resis-
tivity compared to the bulk. If we take the last calculated
parameters for that PC and use Eq. (3), then the Maxwell
contribution to the PC resistance estimated from Eq. (3)
exceeds the ballistic Sharvin resistance by more than one
order of magnitude. Also the current density in this case
will be still high, i.e. it increases with the rate about
3⋅104A/cm2 per 1 mV*. Thus, as Gloos et al. concluded
[20], the resistive Maxwell term contributes mainly to the
observed SC sharp minimum. Recovering the Maxwell
resistance, which is zero in the SC state, due to increasing
of the current density and/or temperature in the PC core in
consequence of Joule heating produces a zero-bias mini-
mum. Because of the coherence length in FeSe (equal 1.3
and 5.7 nm for the c and ab directions, respectively [4]) is
also much smaller than the PC size (diameter), the transi-
tion of the PC core in the normal state due to increasing
current density will occur smoothly involving successively
further (deeper) regions.
Let’s consider the sharp side peaks shown in Fig. 4.
Their temperature behavior corresponds well to the BCS
curve. So, it looks like this feature is somehow connected
with the SC order parameter or gap. Sharp peaks in dV/dI
may be connected with the abrupt transition from SC to the
normal state of some region in PC. To result in such sharp
transition, this region must be smaller than the coherence
length, which is less than 5.7 nm [4]. More likely, we have
a multicontact structure in this case with at least one PC
with the size of the order or less than the coherence
length.** For such small PC the suppression of supercon-
ductivity may occur due to reaching of pair-breaking cur-
rent density 2/3/ / 3Fj en p en≈ ∆ ≈ ∆ [25]. Using n ≈
≈ 2.53⋅1020 cm−3 from [22], we get j ≈ 107Δ[mV] A/cm2,
where Δ is in mV units. On the other hand, PC with such
small dimension (below the coherence length) is in the
ballistic limit, where current density depends only on the
applied bias 2 2 2/ / 16 /3 /) ,(shj V R d V l d d V l= = ρ π ≈ ρ so
that j ≈ 3⋅106V[mV] A/cm2, where V is in mV units.
Thereby, current density in such PC reaches the above es-
timated pair-breaking current density for Δ = 2–3 mV at
V = 7–10 mV, what is not far from the side peak position
in Fig. 4 taking into account our rough estimation. In this
way, assuming that the side peaks are due to reaching of
pair-breaking critical current density and therefore that they
are connected to the SC gap value, we can suggest the BCS-
like dependence of the SC gap in FeSe (or some averaged
gap because of the multiband FeSe electronic structure).
Let us return to the spectrum with the Andreev-like dou-
ble-minimum in Fig. 5. This structure transforms at first in a
zero-bias minimum and then vanishes above 8 K, which is
close to Tc of the bulk sample. Such transformation of the
double minimum is due to the movement of broad side maxi-
ma, which position shifts to zero voltage with increasing tem-
perature. So, in our opinion, the conductivity of this PC is
governed by two contributions: Andreev reflection and Max-
well term (resistance), which was discussed above. The fit-
ting*** of the AR structure by the two-gap model [26] results
* Note, that the critical current density measured for thin epitaxial films [23] and single crystals [24] in FeSe is of the order of 104A/cm2.
** Several of sharp side peaks in Fig. 4 testify about a couple of such PCs.
*** As we mentioned in the introduction, the Fermi energy of FeSe is comparable to the value of the SC gap. This put a question about
applicability of BTK and similar existing model(s) for extracting a SC gap. However, due to lack of corresponding theory, we
have applied this model and, as it is seen from Fig. 5 (inset), the BTK fit is almost perfect. Anyway, such situation must be ana-
lyzed theoretically to be sure that, at least, the BTK model can be used, even in the case of EF ~ Δ.
46 Low Temperature Physics/Fizika Nizkikh Temperatur, 2016, v. 42, No. 1
Analysis of nonlinear conductivity of point contacts on the base of FeSe in the normal and superconducting state
in the gap values Δ = 2.5 and 3.5 meV, with the about 90%
contribution to the conductivity coming from the large gap.
These values are the same as the resolved ones from the
tunneling spectra in [6]. It is clear, that some variation of
extracted data using seven fitting parameters is possible,
however the gap(s) value(s) must concentrate around the
minima position of about 3.5 meV in any case. Extracted
gaps values result in large 2Δ/kBTc ratios from 6 to 8, testi-
fying strong coupling superconductivity in FeSe.
Conclusion
We have investigated nonlinear conductivity of PCs on
the base of FeSe single crystals. Degraded surface layer
(due to oxidation, apparently deviation from stoichiometry
and perhaps disturbed through the mechanical PC creation)
vastly contributes to the nonlinear conductivity resulting
regularly in its non-metallic behavior. This prevents large-
ly to get spectroscopic information from more bulky mate-
rial. SC features in dV/dI develop mainly due to resistive
(Maxwell) term in the PC resistance because of failure of
ballistic regime in PC. We estimated some material param-
eters in PC and found that PC has an order of magnitude
larger residual resistivity than the bulk material. Also the
estimated Lorentz number is strongly enhanced, but this is
in line with the results of measurements of thermal and
electronic conductivity of FeSe single crystal. Probably,
creation of the PC “in situ” on a cleaved surface at ultra
high vacuum and low temperatures will help to get rid of
degraded surface layer and receive more detailed spectro-
scopic information. Still, as a practical result, we have
found specific features in dV/dI, which have connection to
the SC gap and allow us to monitor its BCS temperature
dependence. As well as, exploring the dV/dI spectra of the
rare occurrence with Andreev-like structure, the two gaps
with Δ = 2.5 and 3.5 meV were retrieved.
Acknowledgments
Funding by the National Academy of Sciences of
Ukraine under project Ф3-19 is gratefully acknowledged.
Yu.G.N. would like to thank G.E. Grechnev for the stim-
ulating discussion on iron-chalcogenide superconductors,
V. Grinenko and K. Nenkov for technical assistance. Yu.G.N.
acknowledges partial support of Alexander von Humboldt
Foundation in the frame of a research group linkage pro-
gram. A.N.V. acknowledges support of the Ministry of Edu-
cation and Science of the Russian Federation in the frames
of Increase Competitiveness Program of NUST «MISiS»
(No. К2-2014-036) and Russian Foundation for Basic Re-
search (№ 14-02-92002). G.F. acknowledges support of the
German Federal Ministry of Education and Research within
the project ERA.Net RUS Plus: No146-MAGNES financed
by the EU 7th FP, grant No. 609556.
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Introduction
Results
Discussion
Conclusion
Acknowledgments
|
| id | nasplib_isofts_kiev_ua-123456789-128447 |
| institution | Digital Library of Periodicals of National Academy of Sciences of Ukraine |
| issn | 0132-6414 |
| language | English |
| last_indexed | 2025-12-07T15:45:47Z |
| publishDate | 2016 |
| publisher | Фізико-технічний інститут низьких температур ім. Б.І. Вєркіна НАН України |
| record_format | dspace |
| spelling | Naidyuk, Yu.G. Gamayunova, N.V. Kvitnitskaya, O.E. Fuchs, G. Chareev, D.A. Vasiliev, A.N. 2018-01-09T15:38:24Z 2018-01-09T15:38:24Z 2016 Analysis of nonlinear conductivity of point contacts on the base of FeSe in the normal and superconducting state / u.G. Naidyuk, N.V. Gamayunova, O.E. Kvitnitskaya, G. Fuchs, D.A. Chareev, A.N. Vasiliev // Физика низких температур. — 2016. — Т. 42, № 1. — С. 42–48. — Бібліогр.: 26 назв. — англ. 0132-6414 PACS: 74.45.+c, 74.70.–b, 74.70.Xa https://nasplib.isofts.kiev.ua/handle/123456789/128447 Nonlinear conductivity of point contacts (PCs) on the base of FeSe single crystals has been investigated. Measured dV/dI dependencies demonstrate the prevailing contribution to the PC conductivity caused by the degraded surface. Superconducting (SC) feature in dV/dI like a sharp zero-bias minimum develops for relatively low ohmic PCs, where the deep areas of FeSe are involved. Analysis of dV/dI has shown that the origin of the zero-bias minimum is connected with the Maxwell part of the PC resistance, what masks energy dependent spectral peculiarities. Even so, we have found the specific features in dV/dI — the sharp side maxima, which may have connection to the SC gap, since their position follows the BCS temperature dependence. Exploring the dV/dI spectra of the rare occurrence with Andreev-like structure, the two gaps with Δ = 2.5 and 3.5 meV were identified. Funding by the National Academy of Sciences of Ukraine under project Ф3-19 is gratefully acknowledged. Yu.G.N. would like to thank G.E. Grechnev for the stimulating discussion on iron-chalcogenide superconductors, V. Grinenko and K. Nenkov for technical assistance. Yu.G.N. acknowledges partial support of Alexander von Humboldt Foundation in the frame of a research group linkage program. A.N.V. acknowledges support of the Ministry of Education and Science of the Russian Federation in the frames of Increase Competitiveness Program of NUST «MISiS» (No. К2-2014-036) and Russian Foundation for Basic Research (№ 14-02-92002). G.F. acknowledges support of the German Federal Ministry of Education and Research within the project ERA.Net RUS Plus: No146-MAGNES financed by the EU 7th FP, grant No. 609556. en Фізико-технічний інститут низьких температур ім. Б.І. Вєркіна НАН України Физика низких температур Свеpхпpоводимость, в том числе высокотемпеpатуpная Analysis of nonlinear conductivity of point contacts on the base of FeSe in the normal and superconducting state Article published earlier |
| spellingShingle | Analysis of nonlinear conductivity of point contacts on the base of FeSe in the normal and superconducting state Naidyuk, Yu.G. Gamayunova, N.V. Kvitnitskaya, O.E. Fuchs, G. Chareev, D.A. Vasiliev, A.N. Свеpхпpоводимость, в том числе высокотемпеpатуpная |
| title | Analysis of nonlinear conductivity of point contacts on the base of FeSe in the normal and superconducting state |
| title_full | Analysis of nonlinear conductivity of point contacts on the base of FeSe in the normal and superconducting state |
| title_fullStr | Analysis of nonlinear conductivity of point contacts on the base of FeSe in the normal and superconducting state |
| title_full_unstemmed | Analysis of nonlinear conductivity of point contacts on the base of FeSe in the normal and superconducting state |
| title_short | Analysis of nonlinear conductivity of point contacts on the base of FeSe in the normal and superconducting state |
| title_sort | analysis of nonlinear conductivity of point contacts on the base of fese in the normal and superconducting state |
| topic | Свеpхпpоводимость, в том числе высокотемпеpатуpная |
| topic_facet | Свеpхпpоводимость, в том числе высокотемпеpатуpная |
| url | https://nasplib.isofts.kiev.ua/handle/123456789/128447 |
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