Doping effects of Co, Ni, and Cu in FeTe0.65Se0.35 single crystals

The resistivity, magnetoresistance, and magnetic susceptibility are measured in single crystals of FeTe0.65Se0.35 with Cu, Ni, and Co substitutions for Fe. The crystals are grown by Bridgman's method. The resistivity measurements show that superconductivity disappears with the rate which correlates with the nominal valence of the impurity. From magnetoresistance we evaluate doping effect on the basic superconducting parameters, such as upper critical field and coherence length. We find indications that doping leads to two component superconducting behavior, possibly because of local charge depression around impurities.Comment: 4 pages, 4 figures, 1 table, Proceedings of the XV-th National School "Hundred Years of Superconductivity", Kazimierz Dolny, October 9-13, 201

Enhancement of vortex pinning in superconductor/ferromagnet bilayers via angled demagnetization

We use local and global magnetometry measurements to study the influence of magnetic domain width w on the domain-induced vortex pinning in superconducting/ferromagnetic bilayers, built of a Nb film and a ferromagnetic Co/Pt multilayer with perpendicular magnetic anisotropy, with an insulating layer to eliminate proximity effect. The quasi-periodic domain patterns with different and systematically adjustable width w, as acquired by a special demagnetization procedure, exert tunable vortex pinning on a superconducting layer. The largest enhancement of vortex pinning, by a factor of more than 10, occurs when w ~ 310 nm is close to the magnetic penetration depth.Comment: 5 pages, 3 figures, accepted to Phys. Rev. B, Rapid Communication

Localization and Interaction Effects in Strongly Underdoped La2-xSrxCuO4

The in-plane magnetoresistance (MR) in La2-xSrxCuO4 films with 0.03 < x < 0.05 has been studied in the temperature range 1.6 K to 100 K, and in magnetic fields up to 14 T, parallel and perpendicular to the CuO2 planes. The behavior of the MR is consistent with a predominant influence of interaction effects at high temperatures, switching gradually to a regime dominated by spin scattering at low T. Weak localization effects are absent. A positive orbital MR appears close to the boundary between the antiferromagnetic and the spin-glass phase, suggesting the onset of Maki-Thompson superconducting fluctuations deep inside the insulating phase.Comment: 4 pages, 3 figures, to appear in Phys. Rev. Letter

Tuning Vortex Confinement by Magnetic Domains in a Superconductor/Ferromagnet Bilayer

We use a line of miniature Hall sensors to study the effect of magnetic-domain-induced vortex confinement on the flux dynamics in a superconductor/ferromagnet bilayer. A single tunable bilayer is built of a ferromagnetic Co/Pt multilayer with perpendicular magnetic anisotropy and a superconducting Nb layer, with the insulating layer in-between to avoid the proximity effect. The magnetic-domain patterns of various geometries are reversibly predefined in the Co/Pt multilayer using the appropriate magnetization procedure. The magnetic-domain geometry strongly affects vortex dynamics, leading to geometry-dependent trapping of vortices at the sample edge, nonuniform flux penetration, and strongly nonuniform critical current density. With the decreasing temperature, the magnetic pinning increases, but this increase is substantially weaker than that of the intrinsic pinning. The analysis of the initial flux penetration suggests that vortices may form various vortex structures, including disordered Abrikosov lattice or single and double vortex chains, in which minimal vortex-vortex distance is comparable to the magnetic penetration depth

Tuning Vortex Confinement by Magnetic Domains in a Superconductor/Ferromagnet Bilayer

We use a line of miniature Hall sensors to study the effect of magnetic-domain-induced vortex confinement on the flux dynamics in a superconductor/ferromagnet bilayer. A single tunable bilayer is built of a ferromagnetic Co/Pt multilayer with perpendicular magnetic anisotropy and a superconducting Nb layer, with the insulating layer in-between to avoid the proximity effect. The magnetic-domain patterns of various geometries are reversibly predefined in the Co/Pt multilayer using the appropriate magnetization procedure. The magnetic-domain geometry strongly affects vortex dynamics, leading to geometry-dependent trapping of vortices at the sample edge, nonuniform flux penetration, and strongly nonuniform critical current density. With the decreasing temperature, the magnetic pinning increases, but this increase is substantially weaker than that of the intrinsic pinning. The analysis of the initial flux penetration suggests that vortices may form various vortex structures, including disordered Abrikosov lattice or single and double vortex chains, in which minimal vortex-vortex distance is comparable to the magnetic penetration depth

Orbital-selective Mott phase and spin nematicity in Ni-substituted FeTe0.65_{0.65}Se0.35_{0.35} single crystals

The normal state in iron chalcogenides is metallic but highly unusual, with orbital and spin degrees of freedom partially itinerant or localized depending on temperature, leading to many unusual features. In this work, we report on the observations of two of such features, the orbital selective Mott phase (OSMP) and spin nematicity, evidenced in magnetization and magnetotransport [resistivity, Hall effect, angular magnetoresistance (AMR)] of Ni-substituted FeTe$_{0.65}$Se$_{0.35}$ single crystals. Two series of single crystals Fe$_{1+{\delta}-y}$Ni$_y$Te$_{0.65}$Se$_{0.35}$ were prepared, with $0 < y < 0.2$, and $\delta$ either positive (S crystals) or negative (F crystals), depending on the crystallization rate. The S crystals, with single, tetragonal phase exhibit superconducting (SC) properties inferior to F crystals, which contain Fe vacancy-rich monoclinic inclusions. Substitution of Ni dopes both types of crystals with electrons, what eliminates some of the hole pockets from Fermi level, leaving only one, originating from $d_{xy}$ orbital. We show that electron-dominated transport, observed at low $T$ at large $y$, is replaced by hole-dominated transport at $T > 180$ K, suggesting direct link with the appearance of the $d_{z^2}$ hole pockets at X points of the Brillouin zone in the OSMP phase, as recently reported by angular resolved photoemission experiments (Commun. Phys. 5, 29 (2022)). The AMR of S crystals shows the $C_4$ rotational symmetry of in-plane magnetocrystalline anisotropy at small $y$, replaced by $C_2$ symmetry at intermediate $y$, indicating development of Ni doping-induced spin nematicity. The $C_4$ symmetry is preserved in F crystals due to microstructural disorder related to vacancy-rich inclusions. The tendency towards nematicity, induced by Ni doping, appears to be the most important factor producing inferior superconducting properties of S crystals

Minimum spanning trees on random networks

We show that the geometry of minimum spanning trees (MST) on random graphs is universal. Due to this geometric universality, we are able to characterise the energy of MST using a scaling distribution ($P(\epsilon)$) found using uniform disorder. We show that the MST energy for other disorder distributions is simply related to $P(\epsilon)$. We discuss the relationship to invasion percolation (IP), to the directed polymer in a random media (DPRM) and the implications for the broader issue of universality in disordered systems.Comment: 4 pages, 3 figure

Impurity and strain effects on the magnetotransport of La1.85Sr0.15Cu(1-y)Zn(y)O4 films

The influence of zinc doping and strain related effects on the normal state transport properties(the resistivity, the Hall angle and the orbital magneto- resistance(OMR) is studied in a series of La1.85Sr0.15Cu(1-y)Zn(y)O4 films with values of y between 0 and 0.12 and various degrees of strain induced by the mismatch between the films and the substrate. The zinc doping affects only the constant term in the temperature dependence of cotangent theta but the strain affects both the slope and the constant term, while their ratio remains constant.OMR is decreased by zinc doping but is unaffected by strain. The ratio delta rho/(rho*tan^2 theta) is T-independent but decreases with impurity doping. These results put strong constraints on theories of the normal state of high- temperature superconductors