Unconventional London Penetration Depth in Single-CrystalBa(Fe0.93Co0.07)2As2Superconductors (original) (raw)
Related papers
Physical Review B, 2010
The zero temperature value of the in-plane London penetration depth, λ ab (0), has been measured in single crystals of Ba(Fe1−xCox)2As2 as a function of the Co concentration, x, across both the underdoped and overdoped superconducting regions of the phase diagram. For x 0.047, λ ab (0) has been found to have values between 120 ± 50 nm and 300 ± 50 nm. A pronounced increase in λ ab (0), to a value as high as 950 ± 50 nm, has been observed for x 0.047, corresponding to the region of the phase diagram where the itinerant antiferromagnetic and superconducting phases coexist and compete. Direct determination of the doping-dependent λ ab (0) has allowed us to track the evolution of the temperature-dependent superfluid density, from which we infer the development of a pronounced superconducting gap anisotropy at the edges of the superconducting dome.
Probing Local Variations of Superconductivity on the Surface of Ba (Fe1-xCox) 2As2 Single Crystals
2010
The spatially resolved electrical transport properties have been studied on the surface of optimally-doped superconducting Ba(Fe 1-x Co x ) 2 As 2 single crystal by using a four-probe scanning tunneling microscopy. While some non-uniform contrast appears near the edge of the cleaved crystal, the scanning electron microscopy (SEM) reveals mostly uniform contrast. For the regions that showed uniform SEM contrast, a sharp superconducting transition at T C = 22.1 K has been observed with a transition width ΔT C = 0.2 K. In the non-uniform contrast region, T C is found to vary between 19.6 and 22.2 K with ΔT C from 0.3 to 3.2 K. The wavelength dispersive x-ray spectroscopy reveals that Co concentration remains 7.72% in the uniform region, but changes between 7.38% and 7.62% in the non-* Corresponding author. apli@ornl.gov 2 uniform region. Thus the variations of superconductivity are associated with local compositional change.
The absolute value and temperature dependence of the in-plane magnetic penetration depth λ have been measured on a single crystal of Ba(Co 0.074 Fe 0.926) 2 As 2 using low-energy muon-spin rotation and microwave cavity perturbation. The magnetic field profiles in the Meissner state are consistent with a local London model beyond a depth of 15 nm. We determine the gap symmetry through measurements of the temperature dependence of the superfluid density which follows a two-gap s-wave model over the entire temperature range below T c. While the intermediate to high temperature data is well fit by an energy gap model in the BCS-like (weak-coupling) limit, a second smaller gap becomes apparent at low temperatures.
In-plane resistivity anisotropy in underdoped Ba(Fe1-xNix)2As2 and Ba(Fe1-xCux)2As2
2011
Underdoped Fe arsenide superconductors suffer a structural transition that is either coincident with, or precedes the onset of long range antiferromagnetic order. Crystals tend to form a dense array of twins upon cooling through the structural transition, but uniaxial pressure can be used to almost completely detwin samples, enabling measurement of the associated in-plane electronic anisotropy. Initial experiments on detwinned samples of Ba(Fe1- xCox)2As2 revealed a large in-plane resistivity anisotropy which varied non-monotonically with cobalt concentration. Here we present data extending the initial study to include detwinned samples of Ba(Fe1-xNix)2As2 and Ba(Fe1-xCux)2As2. The composition-dependence of the resistivity anisotropy ρb/ρa reveals a striking correlation with that of the Hall coefficient for all three substitution series.
Local measurement of the penetration depth in the pnictide superconductor Ba(Fe0.95Co0.05)2As2
Physical Review B, 2010
We use magnetic force microscopy ͑MFM͒ to measure the local penetration depth in Ba͑Fe 0.95 Co 0.05 ͒ 2 As 2 single crystals and use scanning superconducting quantum interference device susceptometry to measure its temperature variation down to 0.4 K. We observe that superfluid density s over the full temperature range is well described by a clean two-band fully gapped model. We demonstrate that MFM can measure the important and hard-to-determine absolute value of , as well as obtain its temperature dependence and spatial homogeneity. We find s to be uniform on the submicron scale despite the highly disordered vortex pinning.
Superconducting properties in heavily overdoped Ba(Fe0.86Co0.14)2As2 single crystals
Solid State Communications, 2015
In this work we report the influence of intrinsic superconducting parameters on the vortex dynamics in an overdoped Ba(Fe 1-x Co x) 2 As 2 (x=0.14) single crystal. We find a superconducting critical temperature of 13.5 K, magnetic penetration depth λ ab (0) = 660 ± 50 nm, coherence length ξ ab (0) = 5 nm, and the upper critical field anisotropy γ T→Tc ≈ 3.7. In fact, the Ginzburg-Landau model may explain the angular dependent H c2 for this anisotropic three-dimensional superconductor. The vortex phase diagram, in comparison with the optimally doped compound, presents a narrow collective creep regime. In addition, we found no sign of correlated pinning along the c axis. Our results show that vortex core to defect size ratio and λ play an important role in the resulting vortex dynamics in materials with similar intrinsic thermal fluctuations.
Physical Review B, 2011
Low-temperature specific heat (SH) is measured on Ba(Fe1−xCox)2As2 single crystals in a wide doping region under different magnetic fields. For the overdoped sample, we find the clear evidence for the presence of T 2 term in the data, which is absent both for the underdoped and optimal doped samples, suggesting the presence of line nodes in the energy gap of the overdoped samples. Moreover, the field induced electron specific heat coefficient ∆γ(H) increases more quickly with the field for the overdoped sample than the underdoped and optimal doped ones, giving another support to our arguments. Our results suggest that the superconducting gap(s) in the present system may have different structures strongly depending on the doping regions.
Conductance asymmetry in point-contacts on epitaxial thin films of Ba(Fe0.92Co0.08)2As2
Applied Physics Letters, 2010
One of the most common observations in point-contact spectra on the recently discovered ferropnictide superconductors is a large conductance asymmetry with respect to voltage across the point-contact. In this paper we show that the antisymmetric part of the point-contact spectrum between a silver tip and an epitaxial thin film of Ba(Fe0.92Co0.08)2As2 shows certain unique features that have an interesting evolution with increasing temperature up to a temperature far above the critical temperature Tc. We associate this observation with the existence of a gap above Tc that might originate from strong fluctuations of the phase of the superconducting order parameter.
Competitive and cooperative electronic states in Ba(Fe1−xTx)2As2 with T = Co, Ni, Cr
npj Quantum Materials, 2021
The electronic inhomogeneities in Co, Ni, and Cr doped BaFe2As2 single crystals are compared within three bulk property regions: a pure superconducting (SC) dome region, a coexisting SC and antiferromagnetic (AFM) region, and a non-SC region. Machine learning is utilized to categorize the inhomogeneous electronic states: in-gap, L-shape, and S-shape states. Although the relative percentages of the states vary in the three samples, the total volume fraction of the three electronic states is quite similar. This is coincident with the number of electrons (Ni0.04 and Co0.08) and holes (Cr0.04) doped into the compounds. The in-gap state is confirmed as a magnetic impurity state from the Co or Ni dopants, the L-shape state is identified as a spin density wave which competes with the SC phase, and the S-shape state is found to be another form of magnetic order which constructively cooperates with the SC phase, rather than competing with it.