Andreev-spectroscopy study of unconventional superconductivity in MgB₂:(La,Sr)MnO₃ nanocomposite

Unconventional high-temperature superconductivity in MgB₂:La₀:₆₅Sr₀:₃₅MnO₃ (MgB:LSMO) nanocomposite has been found recently [Phys. Rev. B 86, 10502 (2012)]. In this report, the symmetry of the nanocomposite superconducting order parameter and plausible pairing mechanisms have been studied by the point-contact Andreev-reflection (PCAR) spectroscopy. To clarify the experimental results obtained, we consider a model of a ferromagnetic superconductor, which assumes a coexistence of itinerant ferromagnetism and mixed-parity superconductivity. The Balian–Werthamer state, with quasiparticle gap topology of the same form as that of the ordinary s-wave state, fits the experimental data reasonably well. Utilizing the extended Eliashberg formalizm, we calculated the contribution of MgB₂ in the total composite’s conductivity and estimated the magnitude of the electron–phonon effects originated from MgB₂ in I–V characteristics of the composite at above-gap energies. It was found that distinctive features observed in the PC spectra of the MgB:LSMO samples and conventionally attributed to the electron–phonon interaction cannot be related to the MgB₂ phonons. It is argued that the detected singularities may be a manifestation of the electron-spectrum renormalizations due to strong magnetoelastic (magnon–phonon) interaction in LSMO

Point-contact Andreev-reflection spectroscopy of doped manganites: Charge carrier spin-polarization and proximity effects (Review Article)

Materials with spin-polarized charge carriers are the most demanded in the spin-electronics. Particularly requested are the so-called half-metals which have the maximum attainable value of carrier spin polarization. Doped manganites are in the list of compounds with, potentially, half-metallic properties. The point-contact (PC) Andreevreflection (AR) spectroscopy is a robust and direct method to measure the degree of current spin polarization. In this report, advances in PCAR spectroscopy of ferromagnetic manganites are reviewed. The experimental results obtained on “classic” s-wave superconductor — ferromagnetic manganites PCs, as well as related theoretical models applied to deduce the actual value of charge carrier spin-polarization, are discussed. Data obtained on “proximity affected” contacts is also outlined. Systematic and repeatable nature of a number of principal experimental facts detected in the AR spectrum of proximity affected contacts suggests that some new physical phenomena have been documented here. Different models of current flow through a superconductor–half-metal ferromagnet interface, as well as possibility of unconventional superconducting proximity effect, have been discussed

Possibility of local pair existence in optimally doped SmFeAsO₁₋х in pseudogap regime

We report the analysis of pseudogap Δ* derived from resistivity experiments in FeAs-based superconductor SmFeAsO₀.₈₅, having a critical temperature Tc=55 K. Rather specific dependence Δ*(T) with two representative temperatures followed by a minimum at about 120 K was observed. Below Ts ≈ 147 K, corresponding to the structural transition in SmFeAsO, Δ*(T) decreases linearly down to the temperature TAFM ≈ 133 K. This last peculiarity can likely be attributed to the antiferromagnetic (AFM) ordering of Fe spins. It is believed that the found behavior can be explained in terms of Machida, Nokura, and Matsubara theory developed for the AFM superconductors

Unconventional ferromagnetism and transport properties of (In,Mn)Sb dilute magnetic semiconductor

Narrow-gap higher mobility semiconducting alloys In_{1-x}Mn_{x}Sb were synthesized in polycrystalline form and their magnetic and transport properties have been investigated. Ferromagnetic response in In_{0.98}Mn_{0.02}Sb was detected by the observation of clear hysteresis loops up to room temperature in direct magnetization measurements. An unconventional (reentrant) magnetization versus temperature behavior has been found. We explained the observed peculiarities within the frameworks of recent models which suggest that a strong temperature dependence of the carrier density is a crucial parameter determining carrier-mediated ferromagnetism of (III,Mn)V semiconductors. The correlation between magnetic states and transport properties of the sample has been discussed. The contact spectroscopy method is used to investigate a band structure of (InMn)Sb near the Fermi level. Measurements of the degree of charge current spin polarization have been carried out using the point contact Andreev reflection (AR) spectroscopy. The AR data are analyzed by introducing a quasiparticle spectrum broadening, which is likely to be related to magnetic scattering in the contact. The AR spectroscopy data argued that at low temperature the sample is decomposed on metallic ferromagnetic clusters with relatively high spin polarization of charge carriers (up to 65% at 4.2K) within a cluster.Comment: 19 pages, 9 figures, 1 tabl

Magnetoresonance properties of manganite-perovskite nanocomposite with Bi2223-superconductor additives at T = 300 K, 4.2 K

Results of research of electron magnetic resonance absorption in nanocomposites with different ratios of La₀.₆Sr₀.₄MnO₃ manganite and superconductor Bi₁.₇Pb₀.₃Ca₂Sr₂Cu₃O₇₋x (Bi2223) additives in the wide range of frequencies of 22-40 GHz, 68-80 GHz are presented at temperatures T = 340 K and 4.2 K. The manifestation of two peaks in the Electronic Magnetic Resonance spectrum is assigned to appearance of collinear ferromagnetic and paramagnetic phases in the structure under study

ab-plane tunneling and Andreev spectroscopy of superconducting gap and pseudogap in (Bi,Pb)₂Sr₂Ca₂Cu₃O₁₀₊β and Bi₂Sr₂CaCu₂O₈₊δ

We have measured the temperature dependence of gap features revealed by Andreev reflection (Ds) and by tunneling (D) in the ab-plane of optimally and slightly overdoped microcrystals of (BiPb)₂Sr₂Ca₂Cu₃O₁₀₊δ (Bi2223) with critical temperature Tc = 110-115 K, and Bi₂Sr₂CaCu₂O₈₊δ (Bi2212) with Tc = 80-84 K. The tunneling conductance of a Bi2223-insulator-Bi2223 junction shows peaks at the 2D gap voltage, as well as dips and broad humps at other voltages. In Bi2223, similarly to the well-known Bi2212 spectra, the energies corresponding to 2D, to the dip, and to the hump structure are in the ratio 2:3:4. This confirms that the dip and hump features are generic to the high-temperature superconductors, irrespective of the number of CuO₂ layers or the BiO superstructure. On the other hand, in both compounds the D(T) and Ds(T) dependences are completely different, and we conclude that the two entities are of different natures

Pulse and quasi-static remagnetization peculiarities of Nd₀.₅Sr₀.₅MnO₃ single crystal

The hysteretic behavioral features of magnetization and resistance upon remagnetization under quasistatic (up to 9 T) and pulse (up to 14 T) magnetic fields have been investigated. The relaxation processes of magnetization and resistance after the effect of 9 T magnetic field have also been studied. The mechanism of remagnetization of the antiferromagnetic insulating-ferromagnetic metallic (AFM/I–FM/M) phases and the existence of high conductivity state of the sample after removal of the magnetizing field is proposed for low temperatures. The mechanism is caused by structural transition, which is induced by magnetic field (due to magnetostriction), and slow relaxation of the FM-phase (greater volume) to the equilibrium AFM-phase (smaller volume) after the field removal. Remagnetization of Nd₀.₅Sr₀.₅MnO₃ single crystal under pulse field at low temperatures (18 K) has shown that time of the AFM/I→FM/M phase transition was lower than time of the return FM/M→AFM/I phase transition by 6 orders of magnitude