R. T. Gordon

Unconventional London Penetration Depth in Single-Crystal Ba(Fe0.93Co0.07)2As2 Superconductors

The London penetration depth lambda(T) has been measured in single crystals of Ba(Fe0.93Co0.07)2As2. The observed low-temperature variation of lambda(T) follows a power law, Deltalambda(T) approximately T(n) with n approximately 2.4+/-0.1, indicating the existence of normal quasiparticles down to at least 0.02T(c). This is in contrast with previous penetration depth measurements on single crystals of NdFeAsO1-xFx and SmFeAsO1-xFx, which indicate an anisotropic but nodeless gap. We discuss possible explanations of the observed power law behavior.

Anisotropy of the iron pnictide superconductor Ba ( Fe 1 − x Co x ) 2 As 2 ( x = 0.074 , T c = 23 K )

Anisotropies of electrical resistivity, upper critical field, London penetration depth and critical currents have been measured in single crystals of the optimally doped iron pnictide superconductor Ba(Fe1−xCox)2As2, x=0.074 and Tc ∼23 K. The normal state resistivity anisotropy was obtained by employing both the Montgomery technique and direct measurements on samples cut along principal crystallographic directions. The ratio γρ = ρc/ρa is about 4±1 just above Tc and becomes half of that at room temperature. The anisotropy of the upper critical field, γH = Hc2,ab/Hc2,c, as determined from specific heat measurements close to Tc, is in the range of 2.1 to 2.6, depending on the criterion used. A comparable low anisotropy of the London penetration depth, γλ = λc/λab, was recorded from TDR measurements and found to persist deep into the superconducting state. An anisotropy of comparable magnitude was also found in the critical currents, γj = jc,ab/jc,c, as determined from both direct transport measurements (∼1.5) and from the analysis of the magnetization data (∼3). Overall, our results show that iron pnictide superconductors manifest anisotropies consistent with essentially three-dimensional intermetallic compound and bear little resemblance to cuprates.

London penetration depth in Ba(Fesub 1-x}T{sub x}){sub 2}As{sub 2} (T = Co, Ni) superconductors irradiated with heavy ions.

Irradiation with Pb ions was used to study the effect of disorder on the in-plane London penetration depth, {Lambda}(T), in single crystals of Ba(Fe{sub 1-x}T{sub x}){sub 2}As{sub 2} (T = Co, Ni). An increase in the irradiation dose results in a monotonic decrease in the superconducting transition temperature, T{sub c}, without affecting much the transition width. In both Co- and Ni-doped systems we find a power-law behavior, {Delta}{lambda}(T){proportional_to} T{sup n} with the exponent n systematically decreasing with the increase in disorder. This observation, at qualitative odds with the response of s- and d-wave superconductors, finds natural explanation in a nodeless s{sup {+-}} state with pairbreaking (interband) impurity scattering. We are able to describe the effect quantitatively assuming the pairbreaking strength intermediate between Born and unitary limits.

Doping evolution of the absolute value of the London penetration depth and superfluid density in single crystals of Ba(Fe1-xCox)2As2

The zero-temperature value of the in-plane London penetration depth, λab (0) , has been measured in single crystals of Ba(Fe1-x Cox)₂ As₂ as a function of the Co concentration, x , across both the underdoped and overdoped superconducting regions of the phase diagram. For x ≳ 0.047 , λab (0) has been found to have values between 120 ± 50 and 300 ± 50 nm . A pronounced increase in λab (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 λab (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.

Two-gap superconductivity seen in penetration-depth measurements of Lu 2 Fe 3 Si 5 single crystals

Single crystal of Lu2Fe3Si5 was studied with the tunnel-diode resonator technique in Meissner and mixed states. Temperature dependence of the superfluid density provides strong evidence for the two-gap superconductivity with almost equal contributions from each gap of magnitudes ∆1/kBTc = 1.86 and ∆1/kBTc = 0.54. In the vortex state, pinning strength shows unusually strong temperature dependence and is non-monotonic with the magnetic field (peak effect). The irreversibility line is sharply defined and is quite distant from the Hc2(T ), which hints on to enhanced vortex fluctuations in this two-gap system. Altogether our finding provide strong electromagnetic measurements support to the two-gap superconductivity in Lu2Fe3Si5 previously suggested from specific heat measurements.

Nonexponential London penetration depth of external magnetic fields in superconducting Ba1-xKxFe2As2 single crystals

C. Martin, R. T. Gordon, M. A. Tanatar, H. Kim, N. Ni, S. L. Bud’ko, P. C. Canfield, H. Luo, H. H. Wen, Z. Wang, A. B. Vorontsov, V. G. Kogan, and R. Prozorov ∗ Ames Laboratory and Department of Physics & Astronomy, Iowa State University, Ames, IA 50011 Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China Department of Physics, Montana State University, P.O. Box 173840 (Dated: 10 February 2009)

Evidence from anisotropic penetration depth for a three-dimensional nodal superconducting gap in single-crystalline Ba ( Fe 1 − x Ni x ) 2 As 2

The London penetration depth, λ, is directly related to the density, ns, of the Cooper pairs (λ ∝ 1/ns) and its variation with temperature provides valuable insight into the pairing mechanism. Here we study the evolution with doping of the temperature dependence of the in-plane (λab) and out-of-plane (λc) penetration depths in single crystals of electron-doped Ba(Fe1−xNix)2As2. As is the case for other pnictides, λ(T ) ∼ T n over the whole doping range and this behavior extends down to at least T = Tc/100, setting a very small upper limit on the gap minimum. Furthermore, in the overdoped regime: 1) the exponent n becomes substantially smaller than 2, which is incompatible with the models that explain power-law behavior to be due to scattering; 2) the exponent n becomes anisotropic, with λc(T ) showing a clear T -linear behavior over a large temperature interval. These findings suggest that in the overdoped regime the superconducting gap in iron-based pnictide superconductors develops nodal structure, which unlike in the cuprates, cannot be understood within a two-dimensional picture.

London penetration depth in single crystals of Ba ( Fe 1 − x Co x ) 2 As 2 spanning underdoped to overdoped compositions

The London penetration depth

Conventional superconductivity and hysteretic Campbell penetration depth in single crystals MgCNi3

\ensuremath{\lambda}(T)

The London penetration depth in BaFe2As2 superconductors at high electron doping level

has been measured in single crystals of