Search

The issue concerning the nature and the role of microstructural inhomogeneities in iron chalcogenide superconducting crystals of FeTe0.65Se0.35 and their correlation with transport properties of this system was addressed. Presented data demonstrate that chemical disorder originating from the kinetics of the crystal growth process significantly influences the superconducting properties of an Fe-Te-Se system. Transport measurements of the transition temperature and critical current density performed for microscopic bridges allow us to deduce the local properties of a superconductor with microstructural inhomogeneities, and significant differences were noted. The variances observed in the local properties were explained as a consequence of weak superconducting links existing in the studied crystals. The results confirm that inhomogeneous spatial distribution of ions and small hexagonal symmetry nanoscale regions with nanoscale phase separation also seem to enhance the superconductivity in this system with respect to the values of the critical current density. Magnetic measurements confirm the conclusions drawn from the transport measurements., Comment: 20 pages, 5 figures more...

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, 2011 more...

The pressure dependence, up to 11.3 kbar, of basic parameters of the superconducting state, such as the critical temperature (Tc), the lower and the upper critical fields, the coherence length, the penetration depth, and their anisotropy, was determined from magnetic measurements performed for two singlecrystalline samples of FeTe0.5Se0.5. We have found pressure-induced enhancement of all of the superconducting state properties, which entails a growth of the density of superconducting carriers. However, we noticed more pronounced increase in superconducting carrier density under pressure than that in the critical temperature what may indicate an appearance of a mechanism limiting the increase of Tc with pressure. We have observed that the critical current density increases under pressure by at least one order of magnitude., Comment: 20 pages, 7 figures, 1 table more...

In this paper, we address a number of outstanding issues concerning the nature and the role of magnetic inhomogenities in the iron chalcogenide system FeTe1-xSex and their correlation with superconductivity in this system. We report morphology of superconducting single crystals of FeTe0.65Se0.35 studied with transmission electron microscopy, high angle annular dark field scanning transmission electron microscopy and their magnetic and superconducting properties characterized with magnetization, specific heat and magnetic resonance spectroscopy. Our data demonstrate a presence of nanometre scale hexagonal regions coexisting with tetragonal host lattice, a chemical disorder demonstrating non homogeneous distribution of host atoms in the crystal lattice, as well as hundreds-of-nanometres-long iron-deficient bands. From magnetic data and ferromagnetic resonance temperature dependence, we attribute magnetic phases in Fe-Te-Se to Fe3O4 inclusions and to hexagonal symmetry nanometre scale regions with structure of Fe7Se8 type. Our results suggest that nonhomogeneous distribution of host atoms might be an intrinsic feature of superconducting Fe-Te-Se chalcogenides and we find a surprising correlation indicating that faster grown crystal of inferior crystallographic properties is a better superconductor., Comment: 16 pages, 8 figures, 2 tables more...

The M\"ossbauer measurements performed on powdered single crystals of FeTe0.5Se0.5 (Tc ~ 14.7 K) reveal minor content of two impurity phases, identified as Fe3O4 and Fe7Se8, among the major tetragonal phase. From the shape of impurity subspectra it follows that Fe7Se8 behaves in superparamagnetic manner, most likely because of randomly distributed Fe vacancies in the lattice structure of Fe Te Se. The magnetite content in the powdered absorber exposed to ambient air conditions remains unchanged during period of 16 months. Ageing effects were observed for the samples stored under argon atmosphere and small increase of the isomer shift of the doublet was detected. Presented temperature dependence of the hyperfine parameters can be explained as due to possible orthorhombic distortion or to temperature behaviour of impurity phases Fe3O4 and Fe7Se8. Strong tendency to formation of crystalline texture of powdered sample is observed., Comment: 13 pages, 8 figures more...

Superconducting single crystals of pure FeTe1 xSex and FeTe0.65Se0.35 doped with Co, Ni, Cu, Mn, Zn, Mo, Cd, In, Pb, Hg, V, Ga, Mg, Al, Ti, Cr, Sr or Nd into Fe ions site have been grown applying Bridgman's method. It has been found that the sharpness of transition to the superconducting state in FeTe1 xSex is evidently inversely correlated with crystallographic quality of the crystals. Among all of the studied dopants only Co, Ni and Cu substitute Fe ions in FeTe0.65Se0.35 crystals. The remaining examined ions do not incorporate into the crystal structure. Nevertheless, they form inclusions together with selenium, tellurium and/or iron, what changes the chemical composition of host matrix and therefore influences Tc value. Small disorder introduced into magnetic sublattice, by partial replacement of Fe ions by slight amount of nonmagnetic ions of Cu (~ 1.5 at%) or by magnetic ions of Ni (~ 2 at%) and Co (~5 at%) with spin value different than that of Fe ion, completely suppresses superconductivity in FeTe1 xSex system. This indicates that even if superconductivity is observed in the system containing magnetic ions it can not survive when the disorder in magnetic ions sublattice is introduced, most likely because of magnetic scattering of Cooper pairs., Comment: 18 pages, 12 figures, 3 tables more...

An instrument has been constructed to measure a large range of magnetostriction and thermal expansion between room temperature and 4 K in a superconductive split-coil magnet, that allows investigation in magnetic fields up to 12 T. The very small bulk samples (up to 1 mm in size) as well as big ones (up to 13 mm) of the irregular form can be measured. The possibility of magnetostriction investigation in thin films is shown. A general account is given of both electrical and the mechanical aspects of the design of capacitance cell and their associated electronic circuitry. A simple lever device is proposed to increase the sensitivity twice. The resulting obtained sensitivity can be 0.5 Angstrom. The performance of the technique is illustrated by some preliminary measurements of the magnetostriction of superconducting MgB2, thermal expansion of (La0.8Ba0.2)0.93MnO3 single crystal and magnetoelastic behavior of the Ni/Si(111) and La0.7Sr0.3CoO3/SAT0.7CAT0.1LA0.2(001) cantilevers., Comment: 6 pages, 6 figures, journal paper more...

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. Letters more...

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. more...

We have studied the magnetotransport properties in the normal state for a series of La1.85Sr0.15Cu(1-y)Zn(y)O4 films with values of y, between 0 and 0.12. A variable degree of compressive or tensile strain results from the lattice mismatch between the substrate and the film, and affects the transport properties differently from the influence of the zinc impurities. In particular, the orbital magnetoresistance (OMR) varies with y but is strain-independent. The relations for the resistivity and the Hall angle and the proportionality between the OMR and tan^2 theta are followed about 70 K. We have been able to separate the strain and impurity effects by rewriting the above relations, where each term is strain-independent and depends on y only. We also find that changes in the lattice constants give rise to closely the same fractional changes in other terms of the equation.The OMR is more strongly supressed by the addition of impurities than tan^2 theta. We conclude that the relaxation ratethat governs Hall effect is not the same as for the magnetoresistance. We also suggest a correspondence between the transport properties and the opening of the pseudogap at a temperature which changes when the La-sr ratio changes, but does not change with the addition of the zinc impurities. more...

Measurements of the resistivity, magnetoresistance and penetration depth were made on films of LaSrCuO with up to 12 at.% of Zn substituted for the Cu. The results show that the quadratic temperature dependence of the inverse square of the penetration depth, indicative of d-wave superconductivity, is not affected by doping. The suppression of superconductivity leads to a metallic nonsuperconducting phase, as expected for a pairing mechanism related to spin fluctuations. The metal-insulator transition occurs in the vicinity of kFl~1, and appears to be disorder-driven, with the carrier concentration unaffected by doping., Comment: 4 pages,4 figures, to appear in PRL Replacement of the original paper incorporates minor revisions suggested by the referee more...

The effect of zinc doping on the anomalous temperature dependence of the magnetoresistance and the Hall effect in the normal state was studied in a series of La{1.85}Sr{0.15}Cu{1-y}Zn{y}O{4} films, with values of y between zero and 0.12. The orbital magnetoresistance at high temperatures is found to be proportional to the square of the tangent of the Hall angle, as predicted by the model of two relaxation rates, for all Zn-doped specimens, including nonsuperconducting films. The proportionality constant is equal to 13.7+/-0.5 independent of doping. This is very different from the behavior observed in underdoped La{2-x}Sr{x}CuO{4} films where a decrease of x destroys the proportionality. In addition, the behavior of the orbital magnetoresistance at low temperatures is found to be different depending on whether x is changed or y. We suggest that these differences reflect a different evolution of the pseudogap in the two cases., Comment: 4 pages, 5 postscript figures more...

Magnetoresistance measurements of highly underdoped superconducting La_{2-x}Sr_xCuO_4 films with $x = 0.051$ and $x = 0.048$, performed in dc magnetic fields up to 20 T and at temperatures down to 40 mK, reveal a magnetic-field induced transition from weak to strong localization in the normal state. The normal-state conductances per CuO_2--plane, measured at different fields in a single specimen, are found to collapse to one curve with the use of a single scaling parameter that is inversely proportional to the localization length. The scaling parameter extrapolates to zero near zero field and possibly at a finite field, suggesting that in the zero-field limit the electronic states may be extended., Comment: 8 pages, 5 figures more...