Detailed temperature dependence of both superconducting gaps was obtained directly by means of SnS-Andreev spectroscopy. The
Δ
σ
,
π
(
T
)
-curves were shown to be deviated from standard BCS-like ...behavior, due to
k
-space proximity effect between
σ
- and
π
-condensates, which could give a key to experimental determination of interband electron–phonon coupling constants. For the first time, an excellent qualitative agreement with theoretical predictions of Nicol and Carbotte, and Moskalenko and Suhl was shown.
d
I
(
V
)
/
d
V
-spectra of SnS-Andreev contacts based on MgB
2 samples (with defects of crystal structure), and Mg
1−
x
Al
x
B
2 polycrystalline samples (with the local critical temperature
T
C
variation
10
K
≤
T
C
≤
37
K
) were studied by means of the “break-junction” technique within the temperature range
4.2
K
≤
T
≤
T
C
.
► SnS-Andreev spectroscopy was used for Mg
1−
x
Al
x
B
2 (
0
≤
x
≤
0.43
) properties analysis. ► Detailed Mg
1−
x
Al
x
B
2 superconducting gaps temperature dependence was investigated. ► The
σ
- and
π
-gap temperature dependences
Δ
σ
,
π
(
T
)
differ from each other. ► The data are in excellent qualitative agreement with theory of Moskalenko and Suhl. ► A
k
-space proximity effect between two superconducting condensates was observed.
Using the “break-junction” technique, we prepared and studied superconductor–constriction–superconductor (ScS) nanocontacts in polycrystalline samples of Fe-based superconductors CeO
0.88
F
0.12
FeAs ...(Ce-1111;
), LaO
0.9
F
0.1
FeAs (La-1111;
), and FeSe (
). We detected two subharmonic gap structures related with multiple Andreev reflections, indicating the presence of two superconducting gaps with the BCS-ratios 2
Δ
L
/
k
B
T
C
=4.2÷5.9 and 2
Δ
S
/
k
B
T
C
∼1≪3.52, respectively. Temperature dependences of the two gaps
Δ
L
,
S
(
T
) in FeSe indicate a
k
-space proximity effect between two superconducting condensates. For the studied iron-based superconductors, we found a linear relation between the gap
Δ
L
and magnetic resonance energy,
E
res
≈2
Δ
L
.
Superconducting point contacts have been used for measuring magnetic polarizations, identifying magnetic impurities, electronic structures, and even the vibrational modes of small molecules. Due to ...intrinsically small energy scale in the subgap structures of the supercurrent determined by the size of the superconducting energy gap, superconductors provide ultrahigh sensitivities for high resolution spectroscopies. The so-called Andreev reflection process between normal metal and superconductor carries complex and rich information which can be utilized as powerful sensor when fully exploited. In this review, we would discuss recent experimental and theoretical developments in the supercurrent transport through superconducting point contacts and their relevance to sensing applications, and we would highlight their current issues and potentials. A true utilization of the method based on Andreev reflection analysis opens up possibilities for a new class of ultrasensitive sensors.
Planar CdB
x
F
2−
x
–
p-CdF
2–CdB
x
F
2−
x
sandwich nanostructures prepared on the surface of the
n-type CdF
2 bulk crystal are studied to register the spin transistor and quantum spin Hall-effects. ...The current–voltage characteristics of the ultra-shallow
p
+–
n junctions verify the CdF
2 gap, 7.8
eV, and the quantum subbands of the 2D holes in the
p-type CdF
2 quantum well confined by the CdB
x
F
2−
x
δ-barriers. The temperature and magnetic field dependencies of the resistance, specific heat and magnetic susceptibility demonstrate the high temperature superconductor properties for the CdB
x
F
2−
x
δ-barriers. The value of the superconductor energy gap, 2
Δ
=
102.06
meV, determined by the tunneling spectroscopy method appears to be in a good agreement with the relationship between the zero-resistance supercurrent in superconductor state and the conductance in normal state,
πΔ/
e, at the energies of the 2D hole subbands. The results obtained are evidence of the important role of the multiple Andreev reflections in the creation of the high spin polarization of the 2D holes in the edged channels of the sandwich device. The high spin hole polarization in the edged channels is shown to identify the mechanism of the spin transistor and quantum spin Hall-effects induced by varying the top gate voltage, which is revealed by the first observation of the Hall quantum conductance staircase.
The break-junction tunneling has been systematically investigated in MgB
2. Two types of the break-junction contacts have been exploited on the same samples, which demonstrated tunnel contact like ...(SIS) and point contact like (SnS) behavior. Both of them have shown the existence of the two distinct energy gaps. We have also observed peculiarities on the
I(
V)- characteristics related to Leggett's collective mode assisted tunneling.
The temperature evolution of current–voltage characteristics (CVCs) of break junctions made from polycrystalline Y
0.75Lu
0.25Ba
2Cu
3O
7 and La
1.85Sr
0.15CuO
4 is investigated. The experimental ...CVCs have hysteretic features that reflect a part of a curve with negative differential resistance. The temperature evolution of the CVCs is discussed within the framework of the Kümmel–Gunsenheimer–Nicolsky theory for superconductor/normal-metal/superconductor junctions considering multiple Andreev reflections. It is shown that the shape of the CVCs of break junctions is determined by the ratio of the number of “short” and “long” intergrain normal regions in the polycrystalline HTSC under investigation.
The spectral density of background noise emitted by symmetric bicrystal YBaCuO Josephson junctions on sapphire substrates have been measured by a low noise cooled HEMT amplifier for bias voltages up ...to
V≈50 mV. At relatively low voltages
V<4 mV a noticeable noise rise has been registered. At large bias voltages
V>30 mV a clear dependence of noise power, exactly coinciding to the asymptote of the Schottky shot noise function, has been observed for the first time. Experimental results are discussed in terms of multiple Andreev reflections which may take place in d-wave superconducting junctions with low transparency
D≪1.
We analyze the effects of electron–electron interactions in a quantum dot coupled to superconducting leads. It is shown that subgap transport is suppressed in the presence of strong interactions. The ...order of suppression is enhanced as the applied voltage decreases. We discuss this effect by arguing that multiple Andreev reflections through a single quantum dot can be mapped onto the problem of a linear chain of quantum dots. We also show that the subgap harmonics in the current–voltage curve become complicated due to the discrete state in the quantum dot.
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