arXiv subjects

S. A. Kuzmichev

Publications and source records attributed to S. A. Kuzmichev.

At least 19 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.)

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arXiv2022

Direct observation of the spin exciton in Andreev spectroscopy of iron-based superconductors

Quasiparticle excitations provide viable information on the physics of unconventional superconductors. Higgs and Leggett modes are some of the classic examples. Another important bosonic excitation is the spin exciton originating from the sign-changing superconducting gap structure. Here we report a direct observation of the temperature-dependent spin exciton in the Andreev spectra of iron-based superconductors. Combined with the other experimental evidence, our observation confirms the extended $s$-wave ($s_\pm$) order parameter symmetry and indirectly proves the spin-fluctuation mechanism of Cooper pairing.

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arXiv2021

Superconducting Order Parameters in overdoped BaFe$_{1.86}$Ni$_{0.14}$As$_2$ Revealed by Multiple Andreev Reflection Spectroscopy of Planar Break-Junctions

Using multiple Andreev reflection (MAR) spectroscopy, we studied superconductor - normal metal - superconductor (SnS) semiballistic contacts with incoherent transport in overdoped pnictide superconductors BaFe$_{1.86}$Ni$_{0.14}$As$_2$ with critical temperature $T_c \approx 12$ K. We directly determined an anisotropic large gap with the characteristic ratios $2Δ_L^{\rm out}(0)/k_BT_c \approx 6$ and $2Δ_L^{\rm in}(0)/k_BT_c \approx 4$, almost isotropic small gap with $2Δ_S(0)/k_BT_c \approx 2$, and the temperature dependence of the resolved superconducting order parameters. Above the large gap position $2Δ_L$, a fine dI(V)/dV structure fully vanishing at $T_c$ and possibly caused by a resonant coupling with a bosonic mode was observed.

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arXiv2021

Electron and Superconducting Properties of the AFeAs (A= Li, Na) Family Alkali-Metal Pnictides: Current Stage of the Research (mini-review)

The review is focused on one of the most exotic families of iron-based superconductors belonging to the AFeAs structural type, where A is alkali metal. We briefly concern physical and electron properties of the typical members of this family, LiFeAs and NaFeAs, discuss the theoretical models describing the multiple-gap superconducting state, and the experimental data available in literature. As well, we specify the main unsolved problems, that seem crucial for both the AFeAs family and for iron-based superconductors in general.

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arXiv2019

Superconducting Order Parameter and Bosonic Mode in Hydrogen-Substituted NdFeAsO$_{0.6}$H$_{0.36}$ Revealed by Multiple Andreev Reflection Spectroscopy

Using intrinsic multiple Andreev reflections effect (IMARE) spectroscopy, we studied ballistic superconductor - normal metal - superconductor (SnS) contacts in layered oxypnictide superconductors NdFeAsO$_{0.6}$H$_{0.36}$ with critical temperatures $T_c = 45-48$ K. We directly determined the magnitude of two bulk superconducting order parameters, the large gap $Δ_L \approx 10.4$ meV, and a possible small gap $Δ_S \approx 1.8$ meV, and their temperature dependence. Additionally, a resonant coupling with a characteristic bosonic mode was observed. The boson energy at 4.2 K, $\varepsilon_0 = 10.5-11.0$ meV being less than the indirect gap ($Δ_L < \varepsilon_0 < Δ_L +Δ_S$).

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arXiv2018

Superconducting gap symmetry in BaFe$_{1.9}$Ni$_{0.1}$As$_{2}$ superconductor

We report on the Andreev spectroscopy and specific heat of high-quality single crystals BaFe$_{1.9}$Ni$_{0.1}$As$_{2}$. The intrinsic multiple Andreev reflection spectroscopy reveals two anisotropic superconducting gaps $Δ_L \approx 3.2 \textendash 4.5$\,meV, $Δ_S \approx 1.2 \textendash 1.6$\,meV (the ranges correspond to the minimum and maximum value of the coupling energy in the $k_xk_y$-plane). The $25 \textendash 30 \%$ anisotropy shows the absence of nodes in the superconducting gaps. Using a two-band model with s-wave-like gaps $Δ_L \approx 3.2$\,meV and $Δ_S \approx 1.6$\,meV, the temperature dependence of the electronic specific heat can be well described. A linear magnetic field dependence of the low-temperature specific heat offers a further support of s-wave type of the order parameter. We find that a d-wave or single-gap BCS theory under the weak-coupling approach cannot describe our experiments.

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arXiv2018

Evidence of a multiple boson emission in Sm$_{1-x}$Th$_x$OFeAs

We studied a reproducible fine structure observed in dynamic conductance spectra of Andreev arrays in Sm$_{1-x}$Th$_x$OFeAs superconductors with various thorium concentrations ($x = 0.08 - 0.3$) and critical temperatures $T_c = 26-50$\,K. This structure is unambiguously caused by a multiple boson emission (of the same energy) during the process of multiple Andreev reflections. The directly determined energy of the bosonic mode reaches $\varepsilon_0 = 14.8 \pm 2.2$\,meV for optimal compound. Within the studied range of $T_c$, this energy as well as the large $Δ_L$ and the small $Δ_S$ superconducting gaps, nearly scales with critical temperature with the characteristic ratio $\varepsilon_0/k_BT_c \approx 3.2$ (and $2Δ_L/k_BT_c \approx 5.3$, correspondingly) resembling the expected energy $Δ_L + Δ_S$ of spin resonance and spectral density enhancement in $s^{\pm}$ and $s^{++}$ states, respectively.

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arXiv2017

Break-junction technique in application to layered superconductors (Review article)

Here presented a systematic study of superconductor-constriction-superconductor contacts realized by a break-junction technique in layered superconductors. Depending on the constriction transparency the tunneling and SnS-Andreev spectroscopies could be used, for the direct determination of values of superconducting gaps, characteristic BCS-ratios and gap temperature dependences in cuprate superconductors, magnesium diboride, novel pnictides and chalcogenides. Basing on these data, one can estimate the gap anisotropy magnitude as well as values of electron-boson coupling constants. We discuss the advantages and difficulties of the break-junction technique and demonstrate this method is powerful enough for high-resolution investigation of optical phonon modes in high-temperature superconducting cuprates and for creating of the contacts with the selective transparence in Mg$_{1-x}$Al$_x$B$_2$ compounds.

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arXiv2017

On the Structure of Superconducting Order Parameter in High-Temperature Fe-Based Superconductors

This paper discusses the synthesis, characterization, and comprehensive study of Ba-122 single crystals with various substitutions and various $T_c$. The paper uses five complementary techniques to obtain a self-consistent set of data on the superconducting properties of Ba-122. A major conclusion of the authors work is the coexistence of two superconducting condensates differing in the electron-boson coupling strength. The two gaps that develop in distinct Fermi surface sheets are nodeless in the $k_xk_y$-plane and exhibit s-wave symmetry, the two-band model represents a sufficient data description tool. A moderate interband coupling and a considerable Coulomb repulsion in the description of the two-gap superconducting state of barium pnictides favor the $s^{++}$-model.

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arXiv2017

Direct Evidence of Two Superconducting Gaps in FeSe$_{0.5}$Te$_{0.5}$: SnS-Andreev Spectroscopy and Lower Critical Field

We present direct measurements of the superconducting order parameter in nearly optimal FeSe$_{0.5}$Te$_{0.5}$ single crystals with critical temperature $T_C \approx 14$ K. Using intrinsic multiple Andreev reflection effect (IMARE) spectroscopy and measurements of lower critical field, we directly determined two superconducting gaps, $Δ_L \approx 3.3 - 3.4$ meV and $Δ_S \approx 1$ meV, and their temperature dependences. We show that a two-band model fits well the experimental data. The estimated electron-boson coupling constants indicate a strong intraband and a moderate interband interaction.

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arXiv2017

Probing Bulk Superconducting Order Parameter in Ba(K)Fe$_2$As$_2$ by Four Complementary Techniques

Using four different experimental techniques, we performed comprehensive studies of the bulk superconductive properties of single crystals of the nearly optimally doped $Ba_{1-x}K_xFe_2As_2$ ($T_{c} \approx 36\,K$), a typical representative of the 122 family. We investigated temperature dependencies of the (i) specific heat $C_{el}(T)$, (ii) first critical magnetic field $H_{c1}(T)$, (iii) intrinsic multiple Andreev reflection effect (IMARE), and (iv) infrared reflectivity spectra. All data clearly show the presence of (at least) two superconducting nodeless gaps. The quantitative data on the superconducting spectrum obtained by four different techniques are consistent with each other: (a) the small energy gap $Δ_S(0) \approx 1.8 - 2.5\,meV$, and the large gap energy $Δ_L(0) \approx 9.5 - 11.3\,meV$ that demonstrates the signature of an extended s-wave symmetry ($\sim~33 \%$ in-plane anisotropy), (b) the characteristic ratio $2Δ_L/k_BT_C$ noticeably exceeds the BCS value.

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arXiv2016

Evolution of Superconducting Gaps in Th-Substituted Sm$_{1-x}$Th$_x$OFeAs Studied by Multiple Andreev Reflection Spectroscopy

Using intrinsic multiple Andreev reflections effect (IMARE) spectroscopy, we studied SnS contacts in the layered oxypnictide superconductors Sm$_{1-x}$Th$_x$OFeAs with various thorium doping and critical temperatures $T_C = 21-54$ K. We observe a scaling between both superconducting gaps and $T_C$. The determined BCS-ratio for the large gap $2Δ_L/k_BT_C = 5.0-5.7$ and its eigen BCS-ratio (in a hypothetical case of zero interband coupling) $2Δ_L/k_BT_C^L = 4.1-4.6$ both exceeding the weak-coupling limit 3.52, and for the small gap $2Δ_S/k_BT_C = 1.2-1.6$ remain nearly constant within all the $T_C$ range studied. The temperature dependences $Δ_{L,S}(T)$ agree well with a two-band BCS-like Moskalenko and Suhl model. We prove intraband coupling to be stronger than interband coupling, whereas and Coulomb repulsion constants $μ^{\ast}$ are finite in Sm-based oxypnictides.

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arXiv2016

Vortex Structure and Anisotropic Superconducting Gaps in Ba[Fe(Ni)]$_2$As$_2$

We studied nearly optimally Ni-substituted BaFe$_{2-x}$Ni$_x$As$_2$ (BFNA) single crystals with $T_C \approx 18.5$\,K. In irreversible magnetization measurements, we determined the field dependence of the critical-current density and discuss the nature of observed strong bulk pinning. Using intrinsic multiple Andreev reflections effect (IMARE) spectroscopy, we directly determine two distinct superconducting gaps and resolve their moderate anisotropy in the momentum space. The BCS-ratio for the large gap $2Δ_L/k_BT_C > 4.1$ evidences for a strong coupling in the $Δ_L$-bands.

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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.

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arXiv2015

Estimation of intraband and interband relative coupling constants from temperature dependences of the order parameter for two-gap superconductors

We present temperature dependences of the large and the small superconducting gaps measured directly by SnS-Andreev spectroscopy in various Fe-based superconductors and MgB$_2$. The experimental $Δ_{L,S}(T)$ are well-fitted with a two-gap model based on Moskalenko and Suhl system of equations (supplemented with a BCS-integral renormalization). From the the fitting procedure, we estimate the key attribute of superconducting state \textemdash relative electron-boson coupling constants and eigen BCS-ratios for both condensates. Our results evidence for a driving role of a strong intraband coupling in the bands with the large gap, whereas interband coupling is rather weak for all the superconductors under study.

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arXiv2015

Intrinsic Multiple Andreev Reflections in layered Th-doped Sm$_{1-x}$Th$_x$OFeAs

Layered oxypnictide Sm$_{1-x}$Th$_x$OFeAs (Sm-1111) is an ideal candidate to be probed by intrinsic multiple Andreev reflections effect (IMARE) spectroscopy. Using the classical "break-junction" technique, we formed ballistic Andreev arrays of identical S--n--S-contacts, where $S$ is superconductor, and $n$ is a layer of normal metal. For $T < T_C$, the I(V) curve shows an excess-current and a subharmonic gap structure (SGS): a set of sharp dI(V)/dV-dips at positions which depend on the superconducting gap value, the number of junctions in the array, and the natural subharmonic order, thus manifesting the effect of intrinsic multiple Andreev reflections. Here we present the I(V) and dI(V)/dV with up to 4 SGS dips for Andreev arrays formed in optimally doped Sm-1111 with critical temperatures $T_C \approx 49$\,K, as well as in underdoped samples with $T_C \approx 37$\,K. We show that a number of Andreev subharmonics facilitates the determination of the superconducting gap with a better accuracy.

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arXiv2014

Lower critical field and SNS-Andreev spectroscopy of 122-arsenides: Evidence of nodeless superconducting gap

Using two experimental techniques, we studied single crystals of the 122-FeAs family with almost the same critical temperature, Tc. We investigated the temperature dependence of the lower critical field of a single crystal under static magnetic fields parallel to the axis. The temperature dependence of the London penetration depth can be described equally well either by a single anisotropic -wave-like gap or by a two-gap model, while a d-wave approach cannot be used to fit the London penetration depth data. Intrinsic multiple Andreev reflection effect spectroscopy was used to detect bulk gap values in single crystals of the intimate compound, with the same Tc. We estimated the range of the large gap value 6-8 meV (depending on small variation of and its a space anisotropy of about 30%, and the small gap 1.7 meV. This clearly indicates that the gap structure of our investigated systems more likely corresponds to a nodeless s-wave two gaps.

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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, $E_{\rm res} \approx 2Δ_L$.

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