Anisotropy of the upper critical fields and the paramagnetic Meissner effect in La1.85Sr0.15CuO4 single Crystals

Optimally-doped La1.85Sr0.15CuO4 single crystals have been investigated by dc and ac magnetic measurements. These crystals have rectangular needle-like shapes with the long needle axis parallel to the crystallographic c axis (c-crystal) or parallel to the basal planes (a-crystal). In both crystals, the temperature dependence of the upper critical fields (HC2) and the surface critical field (HC3) were measured. The H-T phase diagram is presented. Close to TC =35 K, for the c-crystal, {\gamma}c = / = 1.80(2), whereas for the a-crystal the {\gamma}a = / =4.0(2) obtained, is much higher than the theoretical value 1.69. At low applied dc fields, positive field-cooled branches known as the "paramagnetic Meissner effect" (PME) are observed, their magnitude is inversely proportional to H. The anisotropic PME is observed in both a- and c-crystals, only when the applied field is along the basal planes. It is speculated that the high {\gamma}a and the PME are connected to each other.Comment: 10 pages, 7 figuer

Evolution from a nodeless gap to d(x2-y2) form in underdoped La(2-x)SrxCuO4

Using angle-resolved photoemission (ARPES), it is revealed that the low-energy electronic excitation spectra of highly underdoped superconducting and non-superconducting La(2-x)SrxCuO4 cuprates are gapped along the entire underlying Fermi surface at low temperatures. We show how the gap function evolves to a d(x2-y2) form as increasing temperature or doping, consistent with the vast majority of ARPES studies of cuprates. Our results provide essential information for uncovering the symmetry of the order parameter(s) in strongly underdoped cuprates, which is a prerequisite for understanding the pairing mechanism and how superconductivity emerges from a Mott insulator.Comment: 5 pages, 4 figure

Effect of Biaxial Strain on the Phase Transitions of Ca(Fe1−xCox)2As2

We study the effect of applied strain as a physical control parameter for the phase transitions of Ca(Fe1−xCox)2As2 using resistivity, magnetization, x-ray diffraction, and 57Fe Mössbauer spectroscopy. Biaxial strain, namely, compression of the basal plane of the tetragonal unit cell, is created through firm bonding of samples to a rigid substrate via differential thermal expansion. This strain is shown to induce a magnetostructural phase transition in originally paramagnetic samples, and superconductivity in previously nonsuperconducting ones. The magnetostructural transition is gradual as a consequence of using strain instead of pressure or stress as a tuning parameter

Origin of the Resistivity Anisotropy in the Nematic Phase of FeSe

The in-plane resistivity anisotropy is studied in strain-detwinned single crystals of FeSe. In contrast to other iron-based superconductors, FeSe does not develop long-range magnetic order below the tetragonal-to-orthorhombic transition at Ts≈90  K. This allows for the disentanglement of the contributions to the resistivity anisotropy due to nematic and magnetic orders. Comparing direct transport and elastoresistivity measurements, we extract the intrinsic resistivity anisotropy of strain-free samples. The anisotropy peaks slightly below Ts and decreases to nearly zero on cooling down to the superconducting transition. This behavior is consistent with a scenario in which the in-plane resistivity anisotropy is dominated by inelastic scattering by anisotropic spin fluctuations

Rotation symmetry breaking in La2−xSrxCuO4{\mathrm{La}}_{2-x}{\mathrm{Sr}}_{x}{\mathrm{CuO}}_{4} revealed by angle-resolved photoemission spectroscopy

Using angle-resolved photoemission spectroscopy it is revealed that in the vicinity of optimal doping the electronic structure of La2−xSrxCuO4 cuprate undergoes an electronic reconstruction associated with a wave vector qa=(π,0). The reconstructed Fermi surface and folded band are distinct to the shadow bands observed in BSCCO cuprates and in underdoped La2−xSrxCuO4 with x≤0.12, which shift the primary band along the zone diagonal direction. Furthermore, the folded bands appear only with qa=(π,0) vector, but not with qb=(0,π). We demonstrate that the absence of qb reconstruction is not due to the matrix-element effects in the photoemission process, which indicates the fourfold symmetry is broken in the system

Relation between cuprate superconductivity and magnetism: A Raman study of (CaLa)1(BaLa)2Cu3 Oy

We present an investigation of charge-compensated antiferromagnetic (CaxLa1.x)(Ba1.75.xLa0.25+x)Cu3Oy single crystals using Raman scattering as well as muon spin rotation. In this system the parameter x controls the Cu-O-Cu superexchange interaction via bond distances and buckling angles. The oxygen content y controls the charge doping. In the absence of doping the two-magnon peak position is directly proportional to the superexchange strength J .We find that both x and y affect the peak position considerably. The N´eel temperature determined from muon spin rotation on the same samples independently confirms the strong dependence of the magnetic interaction on x and y. We find a considerable increase in the maximum superconducting transition temperature T max c with J . This is strong evidence of the importance of orbital overlap to superconductivity in this family of cuprates.1551sciescopu

Effect of Biaxial Strain on the Phase Transitions of Ca(Fe1−xCox)2As2

We study the effect of applied strain as a physical control parameter for the phase transitions of Ca(Fe1−xCox)2As2 using resistivity, magnetization, x-ray diffraction, and 57Fe Mössbauer spectroscopy. Biaxial strain, namely, compression of the basal plane of the tetragonal unit cell, is created through firm bonding of samples to a rigid substrate via differential thermal expansion. This strain is shown to induce a magnetostructural phase transition in originally paramagnetic samples, and superconductivity in previously nonsuperconducting ones. The magnetostructural transition is gradual as a consequence of using strain instead of pressure or stress as a tuning parameter.This article is published as Böhmer, A. E., A. Sapkota, A. Kreyssig, S. L. Bud’ko, G. Drachuck, S. M. Saunders, A. I. Goldman, and P. C. Canfield. "Effect of biaxial strain on the phase transitions of Ca(Fe1−xCox)2As2." Physical Review Letters 118, no. 10 (2017): 107002. DOI: 10.1103/PhysRevLett.118.107002. Posted with permission.</p