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M. G. Mikheev

Publications and source records attributed to M. G. Mikheev.

9 recordsLinked to original sources
arXiv2025

Leggett's plasma resonances and two-gap structures in the CVCs of MgB$_2$ break junctions $-$ a direct evidence for a two-gap superconductivity in MgB$_2$

In the present investigation (2002) the current-voltage characteristics (CVCs) of break junctions in polycrystalline $MgB_2$ samples have been studied in the temperature range $4.2 K < T < T_c$. An inelastic Cooper pair tunneling accompanied by the emission of Leggett plasmons has been found for the first time in $MgB_2$ Josephson contacts. A fine structure in the CVCs of $MgB_2$ Andreev point contacts caused by a resonant emission of Leggett plasmons has been observed for the first time. An energy of the Leggett plasmon has been estimated: $E_0$ ~ 4 meV. Distinct two-gap structures have been found in the CVCs of $MgB_2$ tunneling contacts and $MgB_2$ Andreev point contacts with 7.5 meV $< Δ_L <$ 11 meV and 1.5 meV $< Δ_S <$ 2.5 meV. An intrinsic tunneling effect (ITE) and intrinsic multiple Andreev reflections effect (IMARE) have been observed due to the layered structure of $MgB_2$. The obtained experimental results give a direct evidence for a two-gap superconductivity in $MgB_2$. ("V. Tarasov's Readings", November 2002, Russia, Moscow, conference proceedings.)

cond-mat.supr-con↗
arXiv2016

Andreev spectroscopy of iron-based superconductors: temperature dependence of the order parameters and scaling of the $Δ_{L,S}$ with $T_C$

We studied iron-based superconductors of various families with critical temperatures covering almost all range $T_C = 9 - 53$ K. In natural arrays of contacts formed in these materials we observed intrinsic multiple Andreev reflections effect (IMARE). By using IMARE spectroscopy, we detected the two-gap superconductivity, determined the value of the large and the small superconducting gaps, and the corresponding BCS-ratios. The temperature dependencies of the large and the small gaps $Δ_{L,S}(T)$ are similar for various families of the Fe-based superconductors and could be well-fitted in the framework of the two-band model by Moskalenko and Suhl. We concluded on the extended s-wave symmetry of the $Δ_L$ order parameter (20-30 % anisotropy in k-space) and on the absence of nodes for $Δ_S$. The BCS-ratio $2Δ_L/k_BT_C \approx 5.2$ is nearly constant within the whole range of $T_C$ (this means that coupling rate is unchanged), reflecting the 20 % reduction of the $T_C^{local}$ in relation to the eigen $T_C^L$, and the large gap roughly corresponds to the energy of magnetic resonance $2Δ_L \approx E_{res}$. This result requires a special theoretical consideration. Our estimation of the relative coupling constants and eigen parameters of each condensate (in a hypothetical case of a zero interband interaction) $2Δ_L/k_BT_C^L = 4.2 - 4.8$ and $2Δ_S/k_BT_C^S = 3.5 - 4.5$ leads to indirect conclusion that namely a strong electron-phonon interaction in each condensate described in the framework of the Eliashberg theory plays the key role in the superconductivity of iron-based oxypnictides. With it, the two condensates interact weakly with each other. The observed scaling of $Δ_{L,S}$ with $T_C$, as was discussed above, is caused mainly by changing of the density of states $N_{L,S}$ in the bands, whereas Ln-O spacers act as charge reservoirs.

cond-mat.supr-con↗
arXiv2014

Multiple Andreev reflections spectroscopy of two-gap 1111- and 11 Fe-based superconductors

Using the "break-junction" technique we prepared and studied superconductor - constriction - superconductor nanocontacts in polycrystalline samples of Fe-based superconductors CeO$_{0.88}$F$_{0.12}$FeAs (Ce-1111; $T_C^{\rm bulk} = 41 \pm 1 K$), LaO$_{0.9}$F$_{0.1}$FeAs (La-1111; $T_C^{\rm bulk} = 28 \pm 1 K$), and FeSe ($T_C^{\rm bulk} = 12 \pm 1 K$). We detected two subharmonic gap structures related with multiple Andreev reflections, indicating the presence of two superconducting gaps with the BCS-ratios $2Δ_L/k_BT_C = 4.2 ÷5.9$ and $2Δ_S/k_BT_C\sim 1 \ll 3.52$, 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, $E_{\rm res} \approx 2Δ_L$.

cond-mat.supr-con↗
arXiv2013

Experimental study of intrinsic multiple Andreev reflections effect in GdO(F)FeAs superconductor array junctions

We report the first observation of the intrinsic multiple Andreev reflections effect (IMARE) in S-n-S-...-S-arrays (S = superconductor, n = normal metal) formed by "break-junction" technique in GdO(F)FeAs superconductor (Tc = 48 - 53 K). We show that superconducting gap peculiarities at dI/dV-spectra sharpen dramatically in the arrays as compared with that in the single-contact spectra; this enables to improve significantly accuracy of the bulk superconducting parameters determination. Using IMARE, we determined the large and the small gap values Δ_L = 11 +- 1.1 meV and Δ_S = 2.6 +- 0.4 meV. The BCS-ratio 2Δ_L/kTc^{local} = 5.0 - 5.9 > 3.52 (Tc^{local} is the contact area critical temperature) evidences for a strong electron-boson coupling. The results obtained agree well with our previous data by Andreev spectroscopy for single SnS-contacts.

cond-mat.supr-con↗
arXiv2012

Multigap Superconductivity in GdFeAsO$_{0.88}$ Evidenced by SnS-Andreev Spectroscopy

Using intrinsic multiple Andreev reflection effect (IMARE) spectroscopy we studied superconducting properties of nearly optimal oxygen-deficient GdFeAsO$_{0.88}$ polycrystalline samples (bulk critical temperatures $T_C^{bulk} = 49 ÷52$\,K). Temperature dependences for two superconducting gaps $Δ_{L,S}(T)$ ($T_C^{local} = 48 ÷50$\,K) have been measured in the range from 4.2 to 50\,K. The $Δ_{L,S}(T)$ dependences were found to deviate from the BCS-like function; this suggests an importance of the $k$-space (internal) proximity effect between the two condensates.

cond-mat.supr-con↗
arXiv2012

Study of the Two-Gap Superconductivity in GdO(F)FeAs by ScS-Andreev Spectroscopy

Current-voltage characteristics and dynamic conductance of the superconductor - constriction - superconductor junctions in GdO(F)FeAs polycrystalline samples with critical temperatures Tc^local = 46 - 53 K were investigated. Two superconducting gaps, the large Delta_L = 10.5 +- 2 meV, and the small one Delta_S = 2.3 +- 0.4 meV were clearly observed at T = 4.2 K. The 2Delta_L/kTc^local = 5.5 +- 1 ratio gives support to the strong coupling mechanism which is responsible for the high Tc value. Temperature dependence of the large gap Delta_L(T) indicates the presence of intrinsic proximity effect (in k-space) between two superconducting condensates.

cond-mat.supr-con↗
arXiv2011

Observation of Multi-Gap Superconductivity in GdO(F)FeAs by Andreev Spectroscopy

We have studied current-voltage characteristics of Andreev contacts in polycrystalline GdO$_{0.88}$F$_{0.12}$FeAs samples with bulk critical temperature ${T_c}$ = (52.5 \pm 1)K using break-junction technique. The data obtained cannot be described within the single-gap approach and suggests the existence of a multi-gap superconductivity in this compound. The large and small superconducting gap values estimated at T = 4.2K are ΔL = 10.5 \pm 2 meV and ΔS = 2.3 \pm 0.4 meV, respectively.

cond-mat.supr-con↗
arXiv2003

Experimental evidence of two-band behavior of MgB2

The break-junction tunneling has been systematically investigated in MgB2. 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 observed also the peculiarities on the I(V)- characteristics related to Leggett's collective mode assisted tunneling. --> Corresponding author address: kuzmichev@mig.phys.msu.ru

cond-mat.supr-con↗