Tatiana Kuzmicheva

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• PhD
• Senior Researcher at P.N. Lebedev Physical Institute of the Russian Academy of Sciences

In this work, the nature of vortex pinning has been investigated in overdoped NaFe0.935Co0.065As single crystals. We obtain temperature and field dependences of the critical current density from the magnetic hysteresis loops measured for two field orientations B || ab and B || c. The obtained magnetic anisotropy is about gamma ~ 10 near Tc, and dro...

NaFe0.979Co0.021As (т.е. с недостатком замещающего кобальта по сравнению с соединением, обладающим максимальной критической температурой Tc) в сверхпроводящем и нормальном состоянии. На dI(V)/dV-спектрах туннельных контактов при температурах как ниже, так и выше Тс обнаружена воспроизводимая остаточная нелинейность, нехарактерная для классического...

For single crystals of overdoped composition Na 1−δ Fe0.955Co0.045As we present magnetization and magnetotransport probes in two field orientations in combination with microstructure studies using high-resolution transmission electron microscopy. We determine the upper critical field temperature dependence near Tc ≈ 21 K, and estimate zero µ0H ab,c...

Using incoherent multiple Andreev reflection effect spectroscopy, we have studied the properties of the superconducting phase of (K,Na) x Fe 2 – y Se 2 and K z Fe 2 – y (Se,S) 2 iron selenides with critical temperatures T c ≈ 27–31 K. In both compositions, we directly show a presence of a single superconducting order parameter, and directly determi...

We have grown large (8–10 mm in dimension) single crystals of K\(_{0.8}\)Fe\(_{1.7}\)(Se\(_{0.73}\)S\(_{0.27}\))\(_2\) composition with partial isovalent (Se,S) substitution related to the 122-Se family of iron-chalcogenide superconductors. High quality and homogeneity of the superconducting properties below \(T_c \approx 26\) K were confirmed usin...

С помощью спектроскопии эффекта некогерентных многократных андреевских отражений, реализованного в планарных контактах на микротрещине, исследована щелевая структура пниктидов CaKFe4As4 (семейство 1144), переходящих в сверхпроводящее состояние в стехиометрическом составе при критической температуре Tc ≈ 36.1 K. Показано сосуществование трех объемны...

С помощью спектроскопии эффекта некогерентных многократных андреевских отражений исследовано многощелевое сверхпроводящее состояние монокристаллов пниктидов Na(Fe,Co)As недо- и передопированных составов. Напрямую определены величины микроскопических сверхпроводящих параметров порядка, их температурные зависимости и соответствующие характеристически...

Методами туннельной спектроскопии исследованы монокристаллы пниктидов BaFe2−xNixAs2 недодопированного состава (x = 0.08) и передопированных составов (x = 0.12, 0.14) в сверхпроводящем и нормальном состояниях. На полученных I(V )и dI(V )/dV -характеристиках туннельных контактов воспроизводимо наблюдалась сильная нелинейность как ниже, так и выше кри...

Due to multiple-band structure, an interplay between superconducting and magnetic subsystems, the members of the 1144 iron-pnictide family show a number of unique properties not typical to other ferropnictide families. Nonetheless, a quick degradation in open air causes a lack of experimental probes of such extraordinary compounds. In the review, w...

This article reports some results from the 9th International Conference on Superconductivity and Magnetism which was held in Oludeniz, Turkey

Using incoherent multiple Andreev reflection effect spectroscopy, we have studied the multiple-gap superconducting state of underdoped and overdoped Na(Fe, Co)As single crystals. We directly determined the magnitudes of the microscopic superconducting order parameters, their temperature dependences, and characteristic ratios 2Δ(0)/ k B T c . The ob...

We present the study of the superconducting order parameter in single crystals of alkali metal EuCsFe\(_4\)As\(_4\) pnictide belonging to the novel 1144 family, turning to the superconducting state in the stoichiometric composition at critical temperature \(T_c \approx 37\) K, and showing an antiferromagnetic ordering below \(T_c\). In the supercon...

We present a study of the superconducting order parameter in slightly overdoped alkali-1 metal-based pnictides NaF0.955Co0.045As with Tc ≈ 21 K using a combination of incoherent multiple Andreev reflection effect (IMARE) spectroscopy of planar SnS-junctions and specific heat measurements. We observe a multiple-gap superconductivity, directly and lo...

В работе проведено сравнение структуры сверхпроводящего параметра порядка передопированных BaFe1.88 Ni0.12As2 и недодопированных пниктидов BaFe1.92 Ni0.08As2 с близкими Tc ≈ 18.0-18.3 K. С помощью спектроскопии эффекта некогерентных многократных андреевских отражений напрямую определены величины двух микроскопических сверхпроводящих параметров поря...

В данной работе впервые проведены систематические исследования плотности критического тока Jc в двух ориентациях магнитного поля B ab и B c в монокристалле сверхпроводящего пниктида EuCsFe4As4, имеющего магнитный порядок ниже Tc и относящегося к семейству 1144. Определена анизотропия Jc и ее температурная зависимость. Показатели степенной зависимос...

Accepted to Journal of Superconductivity and Novel Magnetism

Статья посвящена воспоминаниям о профессоре кафедры физики низких температур и сверхпроводимости физического факультета Московского государственного университета, заведующем лаборатории туннельной спектроскопии сверхпроводников Ярославе Георгиевиче Пономареве (1938–2015). Авторы попытались охарактеризовать его с разных сторон, как человека и как уч...

In this study, for the first time, we present a systematic study of the critical current density Jc at two magnetic field orientations B//ab and B//c in a single crystal of the superconducting pnictide EuCsFe4As4, which shows a magnetic ordering below Tc and belongs to the 1144 family. The anisotropy of Jc and its temperature dependence are determi...

A comprehensive study of superconducting properties of underdoped NaFe0.979Co0.021As single crystals by a combination of upper critical field measurements and incoherent multiple Andreev reflection effect (IMARE) spectroscopy is presented. The Hc2(T) temperature dependences are measured at magnetic fields up to 16 T with in-plane and out-of-plane f...

Using tunneling spectroscopy, we measured current-voltage characteristics (CVC) and dynamic conductance spectra 𝑑𝐼(𝑉 )/𝑑𝑉 of tunneling contacts in single crystals of slightly overdoped superconducting ferro-pnictide of nominal composition BaFe1.88Ni0.12As2 with bulk critical temperature 𝑇𝑐 = 16−18 K. Besides the Andreev structure, a strong nonlinea...

В работе определена структура сверхпроводящего параметра порядка пниктидов NaFe1-x CoxAs семейства 111 недодопированных составов с Tc ≈ 19-21 K. С помощью спектроскопии эффекта некогерентных многократных андреевских отражений, реализованной в планарных контактах на микротрещине,напрямую определены величины двух микроскопических сверхпроводящих пара...

The structure of the superconducting order parameter of weakly underdoped BaFe 1.92 Ni 0.08 As 2 pnictides with the critical temperature T c ≈ 18.2 K has been determined. Two microscopic superconducting order parameters—the small superconducting gap Δ S (0) and, supposedly, the edges of the large gap with anisotropy in the ab plane $$\Delta _{{\tex...

Выращены монокристаллы пниктида EuCsFe4As4 (семейство 1144) с критической температурой Tc ≈ 36.8 K, обладающего оптимальными сверхпроводящими свойствами в стехиометрическом составе. Ниже Tc с помощью спектроскопии эффекта некогерентных многократных андреевских отражений впервые обнаружена многощелевая сверхпроводимость, определяемая объемными парам...

В работе определена структура сверхпроводящего параметра порядка пниктидов BaFe1.92Ni0.08As2 слабо недодопированного состава с Tc ≈ 18.2K. С помощью спектроскопии эффекта некогерентных многократных андреевских отражений напрямую определены величины двух микроскопических сверхпроводящих параметров порядка – малой сверхпроводящей щели Delta_S(0) и, п...

EuCsFe4As4 pnictide (1144 family) single crystals with a critical temperature of Tc ≈ 36.8 K and optimal superconducting properties in the stoichiometric composition are grown. Multiple-gap superconductivity determined by bulk order parameters $|delta_L^{out}$, $\Delta_L^{in}$, and $\Delta_S$ with characteristic ratios 5.3, 3.2, and 1.3, respective...

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 s...

Quasiparticle excitations provide viable information on the physics of unconventional superconductors. Higgs and Leggett modes are some of the brightest 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...

The structure of the superconducting order parameter of LiFeAs is studied by incoherent multiple Andreev reflections effect (IMARE) spectroscopy. The high transparent superconductor–thin normal metal–superconductor (SnS) contacts are created by a planar “break-junction” technique. Below $$T_c \approx 17.5$$ T c ≈ 17.5 K, the obtained I(V) and dI(V)...

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 characteris...

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 th...

Обзор посвящен одному из самых экзотических семейств железосодержащих сверхпроводников, принадлежащих к структурному типу AFeAs, где A-щелочной металл. Кратко рассмотрены физические и электронные свойства типичных представителей этого семейства - LiFeAs и NaFeAs, обсуждаются предложенные для описания многощелевого сверхпроводящего состояния пниктид...

Using multiple Andreev reflection spectroscopy, we studied superconductor–normal metal–superconductor semiballistic contacts with incoherent transport in overdoped pnictide superconductors BaFe1.86Ni0.14As2 with critical temperature Tc≈12K. We directly determined an anisotropic large gap with the characteristic ratios 2ΔLout(0)/kBTc≈6 and 2ΔLin(0)/...

We present a comprehensive study of vortex matter in the FeSe1−x S x system with various doping degree. The influence of sulphur substitution on peak effect occurrence is studied. Additionally, we study a possible vortex liquid–vortex glass/lattice transition and find an evidence that the vortex liquid–vortex glass phase transition in FeSe1−x S x h...

The current-voltage characteristics and dynamic conductance spectra of the SnS junction in Ba(Fe, Ni)2As2 superconducting pnictide of almost optimal composition have been obtained. A slight slope of the electronic density of states near the Fermi level has been detected. Using multiple Andreev reflection spectroscopy, we have directly determined th...

Using multiple Andreev reflections (MAR) effect spectroscopy of SnS junctions, we directly determine temperature dependences of the superconducting order parameters, excess Andreev current, and zero-bias conductance (ZBC) in polycrystalline samples of superconducting oxypnictides Sm0.7Th0.3OFeAs and NdO0.6H0.36FeA with almost optimal critical tempe...

We present a comprehensive study of vortex matter and pinning evolution in the FeSe$_{1-x}$S$_x$ system with various doping degree. The influence of sulphur substitution on vortex pinning and peak effect occurrence is studied. We show that there is a complex interplay among various pinning contributions in the FeSe$_{1-x}$S$_x$ system. Additionally...

Although several theoretical approaches describing multiple Andreev reflection (MAR) effects in the superconductor–normal metal–superconductor (SNS) junction are elaborated, the problem of the comprehensive and adequate description of MAR is highly actual. In particular, a broadening parameter Γ is still unaccounted at all, whereas a ballistic cond...

Despite several theoretical approaches describing multiple Andreev reflections (MAR) effect in superconductor-normal metal-superconductor (SNS) junction are elaborated, the problem of comprehensive and adequate description of MAR is highly actual. In particular, a broadening parameter $\Gamma$ is still unaccounted at all, whereas a ballistic condit...

Despite the simple crystal structure, LiFeAs having nontrivial band structure and anisotropy of superconducting properties seems the most exotic compound among iron-based superconducting pnictides. Nonetheless, the available experimental studies are not so numerous due to LiFeAs chemical instability even in presence of a trace amount of water vapor...

Physical parameters of condensates in multiple-gap superconductors are determined by coupling both within and between bands. The simplest description of the physics of the system in case of two superconducting order parameters is offered by the Moskalenko-Suhl two-band model, in which strength of coupling is determined by four constants of electron...

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, t...

In multiple-gap superconductors, the physical parameters of the condensates are determined by both, intraband and interband coupling. In case the two superconducting order parameters are in a weak-coupling limit, a model by Moskalenko and Suhl provides the simplest outline of such system, with four electron-boson coupling constants describing the p...

Using intrinsic multiple-Andreev-reflection-effect spectroscopy, we studied ballistic superconductor-normal metal-superconductor (SnS) contacts in layered oxypnictide superconductors NdFeAsO0.6H0.36 with critical temperatures Tc=45−48 K. We directly determined the magnitude of two bulk superconducting order parameters, the large gap ΔL≈10.4 meV, a...

We report on the Andreev spectroscopy and specific heat of high-quality single crystals of BaFe1.9Ni0.1As2. The intrinsic multiple Andreev reflection spectroscopy reveals two anisotropic superconducting gaps ΔL≈3.2−4.5meV, ΔS≈1.2−1.6meV (the ranges correspond to the minimum and maximum value of the coupling energy in the kxky plane). The 25%−30% an...

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 $\Delta_L \approx 3.2 \textendash 4.5$\,meV, $\Delta_S \approx 1.2 \textendash 1.6$\,meV (the ranges correspond to the minimum and...

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...

The superconducting order parameters in optimally doped Ba0.65K0.35Fe2As2 single crystals have been directly measured using multiple Andreev reflection effect spectroscopy of superconductor–normal metal–superconductor break-junctions. We determine two superconducting gaps, which are nodeless in the kx ky -plane of the momentum space, and resolve a...

We performed comprehensive studies of nearly optimal fluorine-substituted GdO1-xFx FeAs oxypnictide superconductors with TC=48-53K. Specific heat measurements revealed a sharp peak at T=3.5K that shifts to lower temperatures with magnetic field increase. This peak corresponds to an antiferromagnetic ordering in Gd3+ ion sublattice and may indicate...

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 and critical temperatures . This structure is unambiguously caused by a multiple boson emission (of the same energy) during the process of multiple Andreev reflections. The direc...

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 a...

We studied nearly optimal single crystals of Ba-122 family by intrinsic multiple Andreev reflections effect (IMARE) spectroscopy ("break-junction" technique [1]). The current-voltage characteristics of SnS-Andreev contacts (S-superconductor, n-a ballistic layer of normal metal) showed a pronounced excess current at low bias, and two subharmonic gap...

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 super...

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, $\Delta_L \app...

The current–voltage characteristics and dynamic conductance spectra of Andreev arrays in nearly optimal GdO(1−x)F(x)FeAs superconductors with the critical temperature Tc = 46–50 K have been studied. A reproducible fine structure that accompanies large gap features and caused by boson emission during the process of multiple Andreev reflections has b...

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...

We present direct measurements of the superconducting order parameter in nearly optimal FeSe0.5Te0.5 single crystals with critical temperature TC ≈ 14K. Using intrinsic multiple Andreev reflection effect (IMARE) spectroscopy and measurements of lower critical field, we directly determined two superconducting gaps, ΔL ≈ 3.3−3.4 meV and ΔS ≈ 1 meV, a...

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 $2\Delt...

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 $2\Delt...

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) spec...

A comprehensive study of superconductor-constriction-superconductor contacts, obtained using the “break junction” technique in layered superconductors. Depending on the constriction transparency, tunneling and SnS-Andreev spectroscopies could be used to directly determine the values of the superconducting gaps, characteristic BCS ratios and tempera...

This paper discusses the synthesis, characterization, and comprehensive study of Ba-122 single crystals with various substitutions and various Tc's. The paper uses éve 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 superco...

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 super...

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 a...

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) spec...

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 $\Delta_{L,S}(T)$ are well-fitted with a two-gap model based on Moskalenko and Suhl system of equations (supplemented with a BCS-integral renormalization)....

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 < Tc, the I(V) cu...

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 th...

key words: neck disrupting Lifshitz transitions; shape resonance between superconducting condensates; weak interband pairing limit, multi-condensates, Cuprates [1-3], diborides [4,5] iron based superconductors [6,7] share the common physics of multi-condensates superconductivity. Multi-condensates superconductivity was considered impossible before...

Iron-based superconductors of various families with critical temperatures covering almost all the range, from 9 K to maximal Tc = 53 K was studied. In natural arrays of contacts formed in these materials, intrinsic multiple Andreev reflections were observed. The spectroscopy of such contacts demonstrated that the multiple Andreev reflections contri...

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...

Using the self-flux technique we grew superconducting (K{1−z}Na{z}){x}Fe{2−y}Se{2} (z = 0.3) single crystals. EDX mapping revealed the uniform elements distribution on the crystal surface while XRD measurements indicate that the crystals are compositionally inhomogeneous on the nanoscale. The physical properties of the as-prepared sample are charac...

Experimental temperature dependences Δσ, π(T) of the energy of superconducting gaps for MgB2 samples with the critical temperatures 22 K < T c < 41 K have been fitted by selecting the renormalized electron-phonon coupling constants λ ij with the use of the Moskalenko-Shul system of equations, the expression for the frequency of collective plasma os...

The superconducting state of LiFeAs single crystals with the maximum critical temperature $T_c\,\approx\,17\,K$ in the 111 family has been studied in detail by multiple Andreev reflections (MAR) spectroscopy implemented by the break-junction technique. The three superconducting gaps, $\Delta_{\Gamma}\,= (5.1-6.5)\,meV$, $\Delta_L\,= (3.8-4.8)\,meV$...

We studied SNS and S-N-S-N-...-S contacts (where S is a superconductor and N is a normal metal) formed by "break-junction" technique in polycrystalline Sm(1-x)Th(x)OFeAs superconductor samples with critical temperatures Tc = 34-45 K. In such contacts (intrinsic) multiple Andreev reflections effects were observed. Using spectroscopies based on these...

We studied SNS- and S-N-S-N-...-S contacts (where S - superconductor, N - normal metal) formed by "break-junction" technique in polycrystalline Sm(1-x)Th(x)OFeAs superconductor samples with critical temperatures $T_c = (34 \div 45)$ K. In such contacts (intrinsic) multiple Andreev reflections effects were observed. Using spectroscopies based on the...

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 w...

The plate-like single crystals of tetragonal (P4/nmm) FeSe(1−x) superconductors were grown using the KCl–AlCl3 flux technique which produced single crystalline tetragonal samples of about 4 × 4 × 0.1 mm^3 dimensions. The energy dispersive X-ray spectroscopy established a ratio of Fe : Se = 1 : 0.96 ± 0.02. The resistivity and magnetization measurem...

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...

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 $T_C^{bulk} = 49 \div 52$\,K). Temperature dependences for two superconducting gaps $\Delta_{L,S}(T)$ ($T_C^{local} = 48 \div 50$\,K) ha...

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...

In our tunneling investigation using Andreev superconductor - normal metal - superconductor contacts on LiFeAs single crystals we observed two reproducible independent subharmonic gap structures at dynamic conductance characteristics. From these results, we can derive the energy of the large superconducting gap $\Delta_L=(2.5 \div 3.4)$ meV and the...

Detailed temperature dependence of both superconducting gaps was obtained directly by means of SnS-Andreev spectroscopy. The Delta(T)-curves were shown to be deviated from standard BCS-like behavior, due to k-space proximity effect between sigma- and pi-condensates, which could give a key to experimental determination of interband electron--phonon...