Local characterization of superconductivity inBaFe2(As1−xPx)<

Abstract

We use magnetic force microscopy (MFM) to characterize superconductivity across the superconducting dome in BaFe 2 (As 1-x P x ) 2 , a pnictide with a peak in the penetration depth ( ab λ )at optimal doping (x opt ), as shown in sample-wide measurements. Our local measurements show a peak at x opt and a C T vs.2 ab λ − dependence similar on both sides of x opt . Near the underdoped edge of the dome ab λ increases sharply suggesting that superconductivity competes with another phase. Indeed MFM vortex imaging shows correlated defects parallel to twin boundaries only in underdoped samples and not for x ≥ x opt .2The origin of superconductivity in the iron-based materials is still under debate although there is mounting evidence for the role of magnetic order and fluctuations [1][2][3][4]. For instance, it is well established that the parent compound for the pnictides is a metal with spin-density-wave (SDW) order and that doping by electrons, holes or isovalently gives rise to superconductivity and suppresses the magnetic order and an associated structural phase transition [5][6][7][8][9]. Moreover, the optimal doping for the superconducting transition temperature ( C T ) is only slightly higher than the maximum doping for which SDW order has been observed. Here we report magnetic force microscopy (MFM) measurements of the local absolute value of the in-plane penetration depth ( ab λ ) in BaFe 2 (As 1-x P x ) 2 . At the location where we measure ab λ we also measure the local C T in order to determine the relationship between these two fundamental superconducting properties. In addition we use MFM to map the location of superconducting vortices, which can become trapped by defects in the material. This allows us to learn about correlated defects that may arise as a result of structural and magnetic phase transitions.Our samples were high-quality single crystals grown by the self-flux method and annealed in affected by a region in the sample only up to a few micrometers in diameter and only a few hundred nanometers deep, on the order of ab λ , our results are less sensitive to inhomogeneity than measurements which average over the whole sample [28,29]. The locality also allows us to check homogeneity by comparing measurements from different areas in each sample.In our setup the magnetic MFM tip [30] is subjected to forces due to the Meissner response from the superconducting sample, the magnetic field from vortices and magnetic fields from other sources, if they are present. We minimize the electrostatic forces between the tip and the sample by compensating for the contact potential difference. We work with frequency modulated MFM in which the forces on the tip shift the resonant frequency of the cantilever holding it:is the cantilever's natural resonance frequency, 0 k is the spring constant, z is the direction normal to the sample surface and C is a constant offset) [31].For ab λ measurements we cool the sample in low magnetic field and find an area without vortices and visible defects. For this we scan...

Diamagnetic vortex barrier stripes in underdopedBaFe2(As1−xPx)2

“…This sample is part of a series spanning the superconducting dome that we reported on previously as part of a study of the dependence of λ ab on doping 49 . The sample was cleaved and analyzed by energy-dispersive x-ray spectroscopy (EDS) to determine the doping x at the actual scanned surface at several different locations using a measurement area of ≈ 50 µm × 50 µm.…”

“…In surveillance mode we retract the tip far enough from the surface for the forces the tip exerts to be too weak to depin vortices in the sample. We then scan the tip in a raster pattern on a plane parallel to the surface and obtain a magnetic image that maps the vortex locations and measures the local diamagnetic interaction between the tip and the sample 42,43,49,60 . This gives local information on the absolute value of λ ab and through it on ρ s ∝ λ −2 ab .…”

“…This is due to the doping being isovalent -unlike electron doped Ba(Fe 1−x Co x ) 2 As 2 and hole doped Ba 1−x K x Fe 2 As 2 , the charge density in BaFe 2 (As 1−x P x ) 2 does not change with x. A second notable property is unconventional behavior near x opt 47 , including a peak in λ ab (x) 48,49 . The origin of this highly unusual and surprising effect is still under debate [50][51][52][53][54] .…”

Single-vortex pinning and penetration depth in superconductingNdFeAsO1−xFx

“…Very disordered vortices were also observed by Bitter decoration in Ba(Fe 1−x Ni x ) 2 As 2 , with a tendency to form vortex stripes in some regions [22]. Finally, MFM imaging of vortices in Ba(As 1−x P x ) 2 Fe 2 showed relatively strong, uniform pinning for overdoped samples, clustered pinning along twin boundaries for slightly underdoped samples, and weak pinning for very underdoped samples [32].…”

“…While scanning tunneling microscopy [10][11][12][13][14][15][16][17][18], Bitter decoration [19][20][21][22][23], and scanning SQUID [24][25][26] techniques can image vortex configurations and offer some insight into pinning, magnetic force microscopy (MFM) is the most powerful technique to directly quantify the single-vortex depinning force F depin and the in-plane penetration depth λ ab . MFM was used to measure vortex lattice correlation length, F depin , and λ ab distributions in doped BaFe 2 As 2 [27][28][29][30][31][32], but these parameters have not been locally quantified in the highest-T c "1111" family of Fe-SCs. More details of previous work are summarized in Appendix A.…”