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Using electron spin resonance spectroscopy and SQUID-magnetometry we obtained direct evidence of the occurrence of magnetic domains in the antiferromagnetically ordered state of a $${\text{EuF}}{{{\text{e}}}_{{\text{2}}}}{\text{A}}{{{\text{s}}}_{{\text{2}}}}$$ single crystal. The resonance spectra of europium ions were measured in the temperature range from 4 to 200 K. Using an equation for the resonance field in an antiferromagnet that takes into account the exchange and anisotropy fields, we have performed an analysis of the angular dependence of the spectrum at a temperature of 4.8 K, measured upon the crystal rotation around the c axis. Data analysis showed that $${\text{EuF}}{{{\text{e}}}_{{\text{2}}}}{\text{A}}{{{\text{s}}}_{{\text{2}}}}$$ is the antiferromagnet with easy anisotropy plane. Besides, we found in the $$ab$$-plane the second order axes of easy magnetization for each of the two types of magnetic domains, related to the structural transition and the formation of twins. Magnetic anisotropy caused by the exchange interaction of europium ions with iron ions indicates the occurrence of nematic magnetic ordering in the basal $$ab$$ plane. An estimate of the magnitude of the exchange field and the anisotropy field is obtained from the angular dependence of the resonance fields.

Layered van~der~Waals materials of the family TaTMTe$_4$ (TM=Ir, Rh, Ru) are showing very interesting electronic properties. Here we report the synthesis, crystal growth and structural characterization of TaIrTe$_4$, TaRhTe$_4$, TaIr$_{1-x}$Rh$_{x}$Te$_4$ ($x = 0.06$; 0.14; 0.78; 0.92) and Ta$_{1+x}$Ru$_{1-x}$Te$_4$ single crystals. For Ta$_{1+x}$Ru$_{1-x}$Te$_4$ off-stoichiometry is shown. X-ray powder diffraction confirms that TaRhTe4 is isostructural to TaIrTe4. We show that all these compounds are metallic with diamagnetic behavior. Ta$_{1.26(2)}$Ru$_{0.75(2)}$Te$_{4.000(8)}$ exhibits an upturn in the resistivity at low temperatures which is strongly field dependent. Below $T \approx 4$K we observed signatures of the superconductivity in the TaIr$_{1-x}$Rh$_{x}$Te$_4$ compounds for $x = 0.92$. Magnetotransport measurements on all samples show weak magnetoresistance (MR) field dependence that is typically quadratic-in-field. However, for TaIr$_{1-x}$Rh$_{x}$Te$_4$ with $x\approx 0.78$, the MR has a linear term dominating in low fields that indicates the presence of Dirac cones in the vicinity of the Fermi energy. For TaRhTe$_4$ series the MR is almost isotropic. We have performed electronic structure calculations for isostructural TaIrTe$_4$ and TaRhTe$_4$ together with the projected total density of states. The main difference is appearance of the Rh-band close to the Fermi level.

In the novel stoichiometric iron-based material RbEuFe4As4 superconductivity coexists with a peculiar long-range magnetic order of Eu 4f states; their coexistance is puzzling and represents a challenge for both experiment and theory. Using angle-resolved photoemission spectroscopy, resonant photoemission spectroscopy, Andreev reflection spectroscopy and scanning tunneling spectroscopy we have addressed this puzzle and unambigously shown that Fe- and Eu-derived states are largely decoupled and that superconducting and a long range magnetic orders exist almost independently from each other.

We consider various methods and techniques for measuring electron magnetization and susceptibility, which are used in experimental condensed matter physics. The list of considered methods for macroscopic measurements includes magnetomechanic, electromagnetic, modulation-type, and also thermodynamic methods based on the chemical potential variation. We also consider local methods of magnetic measurements based on the spin Hall effects and NV-centers. Several scanning probe magnetometers-microscopes are considered, such as magnetic resonance force microscope, SQUID-microscope, and Hall microscope. The review focuses on the spin magnetization measurements of electrons in non-magnetic materials and artificial systems, particularly, in low-dimensional electron systems in semiconductors and in nanosystems, which came to the forefront in recent years., Comment: 31 pages, 26 figures. To appear in: Phys.Usp

In pnictide RbEuFe4As4, superconductivity sets in at 36 K and coexists, below 15-19 K, with the long-range magnetic ordering of Eu 4f spins. Here we report scanning tunneling experiments performed on cold-cleaved single crystals of the compound. The data revealed the coexistence of large Rb-Terminated and small Eu-Terminated terraces, both manifesting 1 × 2 and 2×2 reconstructions. On 2×2 surfaces, a hidden electronic order with a period â 5 nm was discovered. A superconducting gap of â 7 meV was seen to be strongly filled with quasiparticle states. The tunneling spectra compared with density functional theory calculations confirmed that flat electronic bands due to Eu 4f orbitals are situated â 1.8 eV below the Fermi level and thus do not contribute directly to Cooper pair formation.

From measurements of the oscillatory magnetoresistance in weak perpendicular magnetic fields, we found that the quantum oscillations in two-dimensional electron systems in Si-MOS structures are observed down to the critical carrier density nc of the transition to the strongly localized state. For such low densities, the oscillations exhibit an anticipated period, phase, and amplitude, even though the conductivity becomes less than e2/h, and, hence, the mean free path becomes less than the Fermi wavelength λF. We believe that this apparent contradiction with the Ioffe-Regel criterion for diffusive transport is caused by emergence of an inhomogeneous state of the 2D system, in which the regions of diffusive and hopping conduction are spatially separated.

The FeSe wires and tapes with Fe (S22 steel) and Cu/Nb sheath were fabricated by various modifications of the powder-in-tube (PIT) method. The superconducting critical parameters (upper critical field, critical current, critical temperature) depending on heat treatment time were investigated using resistive R (T) and transport measurements I (V) in magnetic fields up to 9T. The low-temperature annealing up to 72 h improve superconducting properties of superconducting wire with steel sheath. However, the annealing is the cause of superconducting wires degradation due to deterioration of contacts between the steel shell and the FeSe core. We show that industrial PIT technology of manufacturing $\text{Nb}_3\text{Sn}$ wires and tapes might be adopted for iron-based superconductors. Moreover, industrial PIT is a promising technique for fabricating large scale and high-quality superconducting wire and tapes. In addition, we show that fine grinding of the FeSe bulks leads mainly to a phase transition from the tetragonal to the hexagonal crystal structure that exhibits no superconductivity.

Layered van der Waals materials of the family TaTMTe

In the study of oscillatory electron transport in high-mobility Si MOSFETs at low temperatures we observed two correlated effects in weak in-plane magnetic fields: a steep decrease of the renormalized magnetic susceptibility $\chi^*(H)$ and an increase of the concentration of mobile carriers $n(H)$. We suggest a phenomenological model of the magnetic field driven redistribution between the extended and localized electronic states that qualitatively explains both effects. We argue that the redistribution is mainly caused by magnetization of the large-spin localized states with energies close to the Fermi energy $E_F$, coexisting with the majority Fermi liquid state. Our findings also resolve a long-standing disagreement between the experimental data on $\chi^*$ obtained in weak ($H \sim k_BT/\mu_B$) and strong ($H \sim E_F/g\mu_B$) magnetic fields., Comment: 11 pages, 8 figures

In the novel stoichiometric iron-based material ${\mathrm{RbEuFe}}_{4}{\mathrm{As}}_{4}$, superconductivity coexists with a peculiar long-range magnetic order of Eu 4f states. Using angle-resolved photoemission spectroscopy, we reveal a complex three-dimensional electronic structure and compare it with density functional theory calculations. Multiple superconducting gaps were measured on various sheets of the Fermi surface. High-resolution resonant photoemission spectroscopy reveals magnetic order of the Eu 4f states deep into the superconducting phase. Both the absolute values and the anisotropy of the superconducting gaps are remarkably similar to the sibling compound without Eu, indicating that Eu magnetism does not affect the pairing of electrons. A complete decoupling between Fe- and Eu-derived states was established from their evolution with temperature, thus unambiguously demonstrating that superconducting and a long-range magnetic orders exist independently from each other. The established electronic structure of ${\mathrm{RbEuFe}}_{4}{\mathrm{As}}_{4}$ opens opportunities for the future studies of the highly unorthodox electron pairing and phase competition in this family of iron-based superconductors with doping.

In a superconductor that lacks inversion symmetry, the spatial part of the Cooper pair wave function has a reduced symmetry, allowing for the mixing of spin-singlet and spin-triplet Cooper pairing channels and thus providing a pathway to a non-trivial superconducting state. Materials with a non-centrosymmetric crystal structure and with strong spin-orbit coupling are a platform to realize these possibilities. Here, we report the synthesis and characterisation of high quality crystals of Sn$_4$As$_3$, with non-centrosymmetric unit cell ($R3m$). We have characterised the normal and superconducting state using a range of methods. Angle-resolved photoemission spectroscopy shows a multiband Fermi surface and the presence of two surface states, confirmed by Density-functional theory calculations. Specific heat measurements reveal a superconducting critical temperature of $T_c\sim 1.14$ K and an upper critical magnetic field of $H_c\gtrsim 7$ mT, which are both confirmed by ultra-low temperature scanning tunneling microscopy and spectroscopy. Scanning tunneling spectroscopy shows a fully formed superconducting gap, consistent with conventional $s$-wave superconductivity., 11 pages, 7 figures

We present detailed study of specific heat of FeSe single crystals with critical temperature T c = 8.45 K at 0.4 − 200 K in magnetic fields 0 − 9 T. Analysis of the electronic specific heat at low temperatures shows the coexistence of isotropic s-wave gap and strongly anisotropic extended s-wave gap without nodes. It was found two possibilities of superconducting gap parameters which give equally description of experimental data: (i) two gaps with approximately equal amplitudes and weight contribution to specific heat: isotropic Δ 1 = 1.7 meV ( 2 Δ 1 / k B T c =4.7) and anisotropic gap with the amplitude Δ 2 max = 1.8 meV ( 2 Δ 2 max / k B T c =4.9 and anisotropy parameter m = 0.85 ); (ii) two gaps with substantially different values: isotropic large gap Δ 1 = 1.65 meV ( 2 Δ 1 / k B T c = 4.52 ) and anisotropic small gap Δ 2 max = 0.75 meV ( 2 Δ 2 max / k B T c = 2 ) with anisotropy parameter m = 0.71 . These results are confirmed by the field behavior of the residual electronic specific heat γ r .

The longitudinal magnetoresistance of the array of parallel-oriented bismuth nanowires each 100 nm in diameter grown by electrochemical deposition in nanopores of an Al2O3 membrane has been studied in magnetic fields up to 14 T and at temperatures down to 0.3 K. The resistance increases with the field and reaches a broad maximum in fields about 10 T. An anomalous increase in the resistance in weak fields is qualitatively consistent with the suppression of the antilocalization correction to the resistance, and the maximum is qualitatively associated with the classical size effect. Near the maximum at temperatures below 0.8 K, manifestations of reproducible magneto-oscillations of the resistance, which are periodic in field, have been detected. The period of these oscillations is close to a value corresponding to the passage of the flux quantum hc/e through the section of a nanowire. The Fourier analysis also confirms that the oscillations are periodic. This result is similar to the manifestation the Aharonov–Bohm effect caused by conducting surface states of Dirac fermions occupying L-valleys of bismuth.

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 k x k y -plane of the momentum space, and resolve a substantial in-plane anisotropy of the large gap. The temperature dependences of the gaps indicate a strong coupling within the bands where ΔL develops, a weak coupling in the condensate with the small gap ΔS, and a moderate interband interaction between the two condensates. The own critical temperatures of both condensates have been estimated (under the hypotherical assumption of zero interband interaction).

By using Shubnikov-de Haas oscillations in crossed magnetic fields, we measured the temperature dependence of the renormalized spin susceptibility $\chi _{i}^{*}(T)$ for strongly interacting itinerant 2D electrons in silicon. The weak $\delta \chi _{i}^{*}(T)$ dependence, only a few percent over the range T = (0.1 − 1) K, agrees qualitatively with the predicted interaction corrections. However, in strong in-plane magnetic fields, the χ ∗(T) dependence does not vanish or weaken as expected for the interaction corrections. We found that the susceptibility variations are correlated with the T-dependence of the density of itinerant electrons extracted from the magnetooscillation period. We conclude therefore that the $\chi _{i}^{*}(T)$ dependence is affected by a T-dependent exchange of electrons between the subsystems of itinerant and localized electrons which are in thermodynamic equilibrium.

Materials today 48, 18 - 28 (2021). doi:10.1016/j.mattod.2021.03.025, Here we report the high-pressure synthesis of a series of lanthanum–yttrium ternary hydrides obtained at pressures of 170–196 GPa via the laser heating of P6 3/mmc La–Y alloys with ammonia borane. As a result, we discovered several novel compounds: cubic hexahydride (La,Y)H$_6$ and decahydrides (La,Y) H$_{10}$ with a maximum critical temperature T$_C$ ~ 253 K and an extrapolated upper critical magnetic field B$_{C2}$ (0) of up to 135 T at 183 GPa. The current–voltage measurements show that the critical current density J$_C$ in (La,Y)H10 is 12–27.7 kA/mm$^2$ at 4.2 K, which is comparable with that of commercial superconducting wires such as NbTi and Nb$_3$ Sn. (La,Y)H$_6$ and (La,Y)H$_{10}$ are among the first examples of ternary high-T$_C$ superconducting hydrides. Our experiments show that part of metal atoms in the structures of recently discovered Im$\bar{3}$m-YH$_6$ and Fm$\bar{3}$m-LaH$_{10}$ can be replaced with lanthanum (~ 70%) and yttrium (~ 25%), respectively, with the formation of unique ternary superhydrides containing metal encapsulated cages La@H$_{24}$ and Y@H$_{32}$, which are specific for Im$\bar{3}$m-LaH$_6$ and Fm$\bar{3}$m-YH$_{10}$. This work demonstrates that hydrides, unstable in pure form such as LaH$_6$ and YH$_{10}$ , may nevertheless be stabilized at relatively low pressures in solid solutions with superhydrides having the desired structure., Published by Elsevier Science, Amsterdam [u.a.]

A dilute two-dimensional (2D) system of interacting electrons has tendency to spontaneous spin polarization with increase of interparticle interaction. This instability, anticipated to occur with lowering carrier density and temperature, in fact has never been observed. Here, we report temperature dependence of the spin susceptibility of a 2D electron system in Si-MOS structures measured by two different techniques: (i) χ T ( T ) by direct thermodynamic measurements, which are sensitive to all the electrons, and (ii) χ ∗ ( T ) from quantum oscillations in vector field, sensitive to the itinerant electrons only. In the former case, the susceptibility strongly diverges as 1 / T 2 . In the latter case we found a weak χ ∗ ( T ) dependence, only a few percent over the range T = ( 0.1 ÷ 1 ) K , which seems to agree qualitatively with the predicted interaction corrections. We provide evidences for this difference to stem from the temperature- and magnetic field-dependent exchange with carriers between the localized and itinerant electrons coexisting in the two-phase state.

We report an experimental observation of superconductivity in ${\mathrm{Cd}}_{3}{\mathrm{As}}_{2}$ thin films without application of external pressure. The films under study were synthesized by magnetron sputtering. Surface studies suggest that the observed transport characteristics are related to the polycrystalline continuous part of the investigated films with a homogeneous distribution of elements and the Cd-to-As ratio close to stoichiometric ${\mathrm{Cd}}_{3}{\mathrm{As}}_{2}$. The latter is also supported by Raman spectra of the studied films where two pronounced peaks inherent to ${\mathrm{Cd}}_{3}{\mathrm{As}}_{2}$ were observed. The obtained x-ray diffraction patterns for studied films also correspond to the ${\mathrm{Cd}}_{3}{\mathrm{As}}_{2}$ lattice. The formation of a superconducting phase in the films under study is confirmed by the characteristic behavior of the temperature and the magnetic field dependence of the sample resistivity, as well as by the presence of pronounced zero-resistance plateaus in the $dV/dI$ characteristics. The corresponding ${H}_{c}\text{\ensuremath{-}}{T}_{c}$ plots reveal a clearly pronounced linear behavior within the intermediate temperature range, similar to that observed for bulk ${\mathrm{Cd}}_{3}{\mathrm{As}}_{2}$ and ${\mathrm{Bi}}_{2}{\mathrm{Se}}_{3}$ films under pressure, suggesting the possibility of a nontrivial pairing in the films under investigation. We discuss a possible role of the sample inhomogeneities and crystal strains in the observed phenomena.

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 $2\Delta_L/k_BT_C > 4.1$ evidences for a strong coupling in the $\Delta_L$-bands., Comment: 6 pages, 4 figures. To be published in Proceedings of Superstripes 2016 Conference

Whether the apparent metal-insulator transition in two-dimensional (2D) correlated electron system is a true quantum phase transition or is a crossover phenomena—this question is in the core of ongoing debates. I present here a novel scenario of this phenomenon, based on experimental finding of the two-phase state in the correlated 2D system. The transport features in the suggested picture are the finite temperature phenomena and a consequence of the magnetic phase transition; the latter manifests in the sign change of the spin magnetization-per-electron. Physically, the magnetic transition means changing the tendency of the two-phase system to either paramagnetic Fermi liquid state (high density), or to the disordered ferromagnet (low density).

Strongly interacting two-dimensional (2D) carrier system has a tendency to spontaneous spin magnetization and mass divergence. Numerous experiments aimed to reveal these instabilities were not entirely convincing. In particular, spin susceptibility of itinerant electrons, determined from quantum oscillations, remains finite at the critical density of the 2D metal-insulator transition (MIT), n = n c . In contrast, the susceptibility and effective mass determined from high field magnetotransport were reported to diverge. Later, it became clear that as interactions grow, the homogeneous 2D Fermi liquid breaks into a two phase state which hampers interpretation of the experimental data. The thermodynamic magnetization measurements have revealed spontaneous formation of the spin-polarized collective electron droplets (“nanomagnets”) in the correlated 2D Fermi liquid, while the spin susceptibility of itinerant electrons in the surrounding 2D “Fermi sea” remains finite. Here, we report how the non Fermi-liquid two-phase state (dilute ferromagnet) reveals itself in magnetotransport and zero field transport. We found in the correlated 2D system a novel energy scale T ∗

We report experimental study of the electronic band structure of SnAs superconductor with the NaCl type lattice structure by angular resolved photoelectron spectroscopy (ARPES). The determined band structure, in general, is in a good agreement with the calculated one. However, at odd with the calculated band structure, the experimental data reveals splitting of one of the upper valence bands into three branches along the $$\bar K - \bar \Gamma - \bar K$$ and $$\bar M - \bar \Gamma - \bar M'$$ symmetry directions. We assume this splitting can be caused by the spin orbit coupling of electrons or a mixed valence of Sn atoms in the compound.

We report comprehensive study of physical properties of the binary superconductor compound SnAs. The electronic band structure of SnAs was investigated using both angle-resolved photoemission spectroscopy (ARPES) in a wide binding energy range and density functional theory (DFT) within generalized gradient approximation (GGA). The DFT/GGA calculations were done including spin-orbit coupling for both bulk and (111) slab crystal structures. Comparison of the DFT/GGA band dispersions with ARPES data shows that (111) slab much better describes ARPES data than just bulk bands. Superconducting properties of SnAs were studied experimentally by specific heat, magnetic susceptibility, magnetotransport measurements and Andreev reflection spectroscopy. Temperature dependences of the superconducting gap and of the specific heat were found to be well consistent with those expected for the single band BCS superconductors with an isotropic s-wave order parameter. Despite spin-orbit coupling is present in SnAs, our data shows no signatures of a potential unconventional superconductivity, and the characteristic BCS ratio $2\Delta/T_c = 3.48 - 3.73$ is very close to the BCS value in the weak coupling limit., Comment: 12 pages, 15 figures, 1 table

We report results of systematic measurements of charge transport properties of the 20.5nm wide HgTe-based quantum well in perpendicular magnetic field, performed under hydrostatic pressures up to 15.1 kbar. At ambient pressure transport is well described by the two-band semiclassical model.In contrast, at elevated pressure, we observed non-monotonic pressure dependence of resistivity at CNP. For pressures lower than $\approx9$ kbar, resistivity grows with pressure, in accord with expectations from the band structure calculations and the model incorporating effects of disorder on transport in 2D semimetals with indirect band overlap. For higher pressures, the resistivity saturates and starts decreasing upon further increase of pressure. Above $\approx14$ kbar the resistance and hopping transport character sharply change, which may indicate formation of the excitonic insulator state. The data also reveals strong influence of disorder on transport in 2D electron-hole system with a small band overlap., 11 pages, 13 figures

We report temperature and density dependences of the spin susceptibility of strongly interacting electrons in Si inversion layers. We measured (i) the itinerant electron susceptibility $\chi^*$ from the Shubnikov-de Haas oscillations in crossed magnetic fields and (ii) thermodynamic susceptibility $\chi_{\rm T}$ sensitive to all the electrons in the layer. Both $\chi^*$ and $\chi_{\rm T}$ are strongly enhanced with lowering the electron density in the metallic phase. However, there is no sign of divergency of either quantity at the density of the metal-insulator transition $n_c$. Moreover, the value of $\chi_{\rm T}$, which can be measured across the transition down to very low densities deep in the insulating phase, increases with density at $n, Comment: 16 pages, 13 figures

We report on the Andreev spectroscopy and specific heat of high-quality single crystals of ${\mathrm{BaFe}}_{1.9}{\mathrm{Ni}}_{0.1}{\mathrm{As}}_{2}$. The intrinsic multiple Andreev reflection spectroscopy reveals two anisotropic superconducting gaps ${\mathrm{\ensuremath{\Delta}}}_{L}\ensuremath{\approx}3.2\ensuremath{-}4.5\phantom{\rule{4pt}{0ex}}\mathrm{meV}$, ${\mathrm{\ensuremath{\Delta}}}_{S}\ensuremath{\approx}1.2\ensuremath{-}1.6\phantom{\rule{4pt}{0ex}}\mathrm{meV}$ (the ranges correspond to the minimum and maximum value of the coupling energy in the ${k}_{x}{k}_{y}$ plane). The $25%\ensuremath{-}30%$ anisotropy shows the absence of nodes in the superconducting gaps. Using a two-band model with $s$-wave-like gaps ${\mathrm{\ensuremath{\Delta}}}_{L}\ensuremath{\approx}3.2\phantom{\rule{4pt}{0ex}}\mathrm{meV}$ and ${\mathrm{\ensuremath{\Delta}}}_{S}\ensuremath{\approx}1.6\phantom{\rule{4pt}{0ex}}\mathrm{meV}$, the temperature dependence of the electronic specific heat can be well described. A linear magnetic field dependence of the low-temperature specific heat offers 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.

This paper describes recent developments in experimental techniques for thermodynamic measurements. Particularly, we focus on the derivatives of the chemical potential with respect to magnetic field and temperature. The former enables to determine the spin magnetization per electron and the latter the entropy per electron. We briefly describe recent results obtained with these techniques and their impact on the current understanding of the still challenging problem of the ground state(s) of strongly correlated two-dimensional electron systems.

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.

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, $\Delta_L \approx 3.3 - 3.4$ meV and $\Delta_S \approx 1$ 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.

Using intrinsic multiple Andreev reflections effect spectroscopy, we studied SnS contacts in the layered oxypnictide superconductors ${\mathrm{Sm}}_{1\ensuremath{-}x}{\mathrm{Th}}_{x}\mathrm{OFeAs}$ 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 $2{\mathrm{\ensuremath{\Delta}}}_{L}/{k}_{B}{T}_{C}=5.0--5.7$ and its eigen-BCS ratio (in a hypothetical case of zero interband coupling) $2{\mathrm{\ensuremath{\Delta}}}_{L}/{k}_{B}{T}_{C}^{L}=4.1--4.6$ both exceeding the weak-coupling limit 3.52, and for the small gap $2{\mathrm{\ensuremath{\Delta}}}_{S}/{k}_{B}{T}_{C}=1.2--1.6$, remain nearly constant within all the ${T}_{C}$ range studied. The temperature dependences ${\mathrm{\ensuremath{\Delta}}}_{L,S}(T)$ agree well with a two-band BCS-like Moskalenko and Suhl model. We prove intraband coupling to be stronger than interband coupling, whereas Coulomb repulsion constants ${\ensuremath{\mu}}^{*}$ are finite in Sm-based oxypnictides.

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 $\Delta_{L,S}(T)$ 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 $\Delta_L$ order parameter (20-30 % anisotropy in k-space) and on the absence of nodes for $\Delta_S$. The BCS-ratio $2\Delta_L/k_BT_C \approx 5.2$ 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 $2\Delta_L \approx E_{res}$. 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) $2\Delta_L/k_BT_C^L = 4.2 - 4.8$ and $2\Delta_S/k_BT_C^S = 3.5 - 4.5$ 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 $\Delta_{L,S}$ 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.

The 2D semimetal in a 20 nm (100) HgTe quantum well is characterized by a comparatively low overlap between the conduction and the valence bands induced by lattice mismatch. In the present paper we report the results of transport measurements in this quantum well under hydrostatic pressure of 14.4 kbar. By applying pressure we have further reduced the band overlap, thereby creating favorable conditions for the formation of the excitonic insulator state. As a result, we observed that the metallic-like temperature dependence of the conductivity at lowering temperature sharply changes to the activated behavior, signaling the onset of an excitonic insulator regime.

We study how the non-Fermi-liquid two-phase state reveals itself in transport properties of high-mobility Si-MOSFETs. We have found features in zero-field transport, magnetotransport, and thermodynamic spin magnetization in a 2D correlated electron system that may be directly related with the two-phase state. The features manifest above a density dependent temperature $T^*$ that represents a novel high-energy scale, apart from the Fermi energy. More specifically, in magnetoconductivity, we found a sharp onset of the novel regime $\delta \sigma(B,T) \propto (B/T)^2$ above a density-dependent temperature $T_{\rm kink}(n)$, a high-energy behavior that "mimics" the low-temperature diffusive interaction regime. The zero-field resistivity temperature dependence exhibits an inflection point $T_{\rm infl}(n)$. In thermodynamic magnetization, the weak-field spin susceptibility per electron, $\partial \chi /\partial n$ changes sign at $T_{dM/dn}(n)$. All three notable temperatures, $T_{\rm kink}$, $T_{\rm infl}$, and $T_{d M/ d n}$, behave critically $\propto (n-n_c)$, are close to each other, and are intrinsic to high-mobility samples solely, we therefore associate them with an energy scale $T^*$ caused by interactions in the 2DE system.

The results of infrared reflectivity measurements for the iron-based high-temperature superconductor Ba(Fe0.9Co0.1)2As2 are reported. The reflectivity is found to be close to unity at frequencies w lower than 2Delta/h (2Delta is the superconducting gap and h is Planck's constant). This is evidence for the s+/- or s+/+ symmetry of the superconducting order parameter in the studied compound. The infrared reflectivity spectra of Ba(Fe0.9Co0.1)2As2 manifest opening of several superconducting gaps at temperatures lower than critical Tc., Comment: 4 pages, 2 figures

Since the 1950’s, the Kharkov school of theoretical physics has been a world leader in the theory of metals. In particular, the research by V. G. Peschansky for many years has focused on the relationship between the magnetic field dependence of components of the resistivity and the electron energy spectrum. Peschansky developed an elegant theory of magnetoresistance that took surface scattering of electrons into account. The physics of bulk 3D metals was almost exhausted by the end of 1970’s and Peschansky extended his research to low-dimensional electron systems. Throughout his scientific life, V. G. Peschansky has advocated the idea that magnetoresistance is a powerful tool for exploring the rich physics of electron systems. The many experimental and theoretical studies of magnetoresistance behavior in various systems, from simple to the most complex, have, by now, confirmed the fruitfulness of this idea.

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 {\Delta}L = 10.5 \pm 2 meV and {\Delta}S = 2.3 \pm 0.4 meV, respectively., Comment: 5 pages, 4 figures, submitted to JETP Letters

The quantum oscillation effect was discovered in Leiden in 1930, by W. J. de Haas and P. M. van Alphen when measuring magnetization, and by L. W. Shubnikov and de Haas when measuring magnetoresistance. Studying single crystals of bismuth, they observed oscillatory variations in the magnetization and magnetoresistance with magnetic field. Shoenberg, whose first research in Cambridge had been on bismuth, found that much stronger oscillations are observed when a bismuth sample is cooled to liquid helium temperature rather than liquid hydrogen, which had been used by de Haas. In 1938 Shoenberg went from Cambridge to Moscow to study these oscillations at Kapitza’s Institute where liquid helium was available at that time. In 1947, J. Marcus observed similar oscillations in zinc and that persuaded Schoenberg to return to this research. After that, the dHvA effect became one of his main research topics. In particular, he developed techniques for quantitative measurement of this effect in many metals. A theoretical explanation of quantum oscillations was given by L. Onsager in 1952, and an analytical quantitative theory by I. M. Lifshitz and A. M. Kosevich in 1955. These theoretical advances seemed to provide a comprehensive description of the effect. Since then, quantum oscillations have been widely used as a tool for measuring Fermi surface extremal cross-sections and all-angle electron scattering times. In his pioneering experiments of the 1960’s, Shoenberg revealed the richness and deep essence of the quantum oscillation effect and showed how the beauty of the effect is disclosed under nonlinear conditions imposed by interactions in the system under study. It was quite surprising that “magnetic interaction” conditions could cause the apparently weak quantum oscillation effect to have such strong consequences as breaking the sample into magnetic (now called “Shoenberg”) domains and forming an inhomogeneous magnetic state. With his contributions to the field of quantum oscillations and superconductivity, Shoenberg is undoubtedly one of the 20th century’s foremost scientists. We describe experiments to determine the quantitative parameters of electron–electron interactions in line with Shoenberg’s idea that quasiparticle interaction parameters can be found by analyzing quantum oscillations as modified by interactions.

We report measurements of dm/dn in Si MOSFET, where m is the magnetization of the two-dimensional electron gas and n is its density. We extended the density range of measurements from well in the metallic to deep in the insulating region. The paper discusses in detail the conditions under which this extension is justified, as well as the corrections one should make to extract dm/dn properly. At low temperatures, dm/dn was found to be strongly nonlinear already in weak magnetic fields, on a scale much smaller than the characteristic scales, expected for interacting two-dimensional electron gas. Surprisingly, this nonlinear behavior exists both in the dielectric, and in the metallic region. These observations, we believe, provide evidence for strong coupling of the itinerant and localized electrons in Si-MOSFET., Comment: 5 pages, 3figures

The magnetoresistance in spin-ordered state of quasi-one-dimensional (TMTSF)2PF6 has been studied. Both the frequency and anisotropy of the magnetoresistance oscillations are found to be independent of temperature; this proves the temperature independence of the geometry of two-dimensional closed contours at the Fermi level. At the same time, the oscillation amplitude decreases with the temperature, indicating the depopulation of the closed contours in the T = 0 limit. An explanation for the above seemingly controversial results has been proposed.

Bismuth chalcogenides are the most studied 3D topological insulators. As a rule, at low temperatures thin films of these materials demonstrate positive magnetoresistance due to weak antilocalization. Weak antilocalization should lead to resistivity decrease at low temperatures; in experiments, however, resistivity grows as temperature decreases. From transport measurements for several thin films (with various carrier density, thickness, and carrier mobility), and by using purely phenomenological approach, with no microscopic theory, we show that the low temperature growth of the resistivity is accompanied by growth of the Hall coefficient, in agreement with diffusive electron-electron interaction correction mechanism. Our data reasonably explain the low-temperature resistivity upturn., Comment: 7 pages, 5 figures

Magnetoresistance in (TMTSF)2PF6 has been studied. It have been found that rapid oscillations of the magnetoresistance are absent in the metallic state and are present in the spin-ordered states solely, including both the lowest and higher order FISDW states. The spin-ordered state, which had previously been believed to be insulating, is not totally gapped; at least, at a finite temperature, there remains a vestigial Fermi surface comprising 2D metallic "pockets." Our data agree qualitatively with the theory that considers the coexistence of two spin-density waves with two respective nesting vectors. © 2006 Pleiades Publishing, Inc.

Magnetoresistance is studied in a strongly correlated 2D electron system in Si in the critical regime in the close vicinity of the 2D metal-insulator transition. Our data are self-consistently compared with solutions of two equations of the crossover renormalization group theory, which describes temperature evolutions of the resistivity and interaction parameters for 2D electron systems. Good agreement is found between the ρ(T, B∥) data and the renormalization group theory in a wide range of the in-plane fields, 0–2.1 T. This agreement supports the interpretation of the observed 2D MIT as the true quantum phase transition.

In order to clarify the origin of the "Rapid Oscillation" (RO) in (TMTSF) 2PF 6, we studied the magnetoresistance anisotropy in the Field-Induced Spin Density Wave (FISDW) phase. We have found that in the FISDW insulating state, the Fermi surface is not totally gapped; the remaining 2D metallic pockets are quantized in magnetic field and give rise to the RO. Decreasing temperature does not change the size and orientation of the closed pockets, rather, it causes depopulation of the delocalized states in favor of the localized ones, resulting in the disappearance of the RO. © Springer Science+Business Media, LLC 2006.

We report first thermodynamic measurements of the temperature derivative of chemical potential (d{\mu}/dT) in two-dimensional (2D) electron systems. In order to test the technique we have chosen Schottky gated GaAs/AlGaAs heterojunctions and detected experimentally in this 2D system quantum magnetooscillations of d{\mu}/dT. We also present a Lifshits-Kosevitch type theory for the d{\mu}/dT magnetooscillations in 2D systems and compare the theory with experimental data. The magnetic field dependence of the d{\mu}/dT value appears to be sensitive to the density of states shape of Landau levels. The data in low magnetic field domain demonstrate brilliant agreement with theory for non-interacting Fermi gas with Lorentzian Landau level shape., Comment: 4 pages, 3 figures

The phase diagram for solidification of polyethylsiloxane (PES-1), which is used as a pressure-transmitting medium, was determined for the first time. The results were obtained using two techniques: by analyzing the changes in the medium thermal expansion coefficient and in its viscosity. Both results qualitatively agree with each other and determine the temperature and pressure ranges within which these parameters must change especially slowly in order to avoid damaging the sample placed inside the chamber.