R. W. Giannetta

A Sharp Peak of the Zero-Temperature Penetration Depth at Optimal Composition in BaFe2(As1–xPx)2

A Spike Inside the Dome The transition temperature Tc of iron-based superconductors has a dome-shaped dependence on chemical doping, and the superconductivity that develops underneath may obscure a potential quantum critical point (QCP) residing at absolute zero. With the aim of detecting signatures of this quantum criticality, Hashimoto et al. (p 1554; see the Perspective by Sachdev) measured the penetration depth of the pnictide series BaFe2(As1−xPx)2 as a function of x. A sharp peak right around the point where Tc has a maximum (x = 0.30) was observed, implying that the superfluid density diminishes sharply where one would expect it to be the most robust. This unusual finding is interpreted as a sign of a QCP at x = 0.30. A quantum critical point may be lurking inside the superconducting dome of a pnictide series. In a superconductor, the ratio of the carrier density, n, to its effective mass, m*, is a fundamental property directly reflecting the length scale of the superfluid flow, the London penetration depth, λL. In two-dimensional systems, this ratio n/m* (~1/λL2) determines the effective Fermi temperature, TF. We report a sharp peak in the x-dependence of λL at zero temperature in clean samples of BaFe2(As1–xPx)2 at the optimum composition x = 0.30, where the superconducting transition temperature Tc reaches a maximum of 30 kelvin. This structure may arise from quantum fluctuations associated with a quantum critical point. The ratio of Tc/TF at x = 0.30 is enhanced, implying a possible crossover toward the Bose-Einstein condensate limit driven by quantum criticality.

LOW-TEMPERATURE PENETRATION DEPTH OF KAPPA -(ET)2CUN(CN)2BR AND KAPPA -(ET)2CU(NCS)2

We present high precision measurements of the penetration depth lambda of single crystals of kappa-(ET)_2Cu[N(CN)2]Br and kappa-(ET)_2Cu(NCS)_2 at temperature down to 0.4 K. We find that, at low temperatures, the in-plane penetration depth varies as a fractional power law, lambdasim T^1.5. W hilst this may be taken as evidence for novel bose excitation processes, we show that the data are also consistent with a quasi-linear variation of the superfluid density, as is expected for a d-wave superconductor with impurities. Our data for the interplane penetration depth show similar features and give a direct measurement of the absolute value, lambda(0)=100pm 20 mu m.

Measurements of the absolute value of the penetration depth in high-Tc superconductors using a low-Tc superconductive coating

A method is presented to measure the absolute value of the London penetration depth, λ(T=0), from the frequency shift of a resonator. The technique involves coating a high-Tc superconductor with film of low-Tc material of known thickness and penetration depth. The method is applied to obtain λ(YBa2Cu3O7−δ)≈1460±150u200aA, λ(Bi2Sr2CaCu2O8+δ)≈2690±150u200aA and λ(Pr1.85Ce0.15CuO4−δ)≈2790±150u200aA. λ(YBa2Cu3O7−δ) from this method is very close to that obtained by several other techniques. For both Bi2Sr2CaCu2O8+δ and Pr1.85Ce0.15CuO4−δ the values exceed those obtained by other methods.

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.

ABSENCE OF NONLINEAR MEISSNER EFFECT IN YBA2CU3O6.95

We present measurements the field and temperature dependence of the penetration depth (lambda) in high purity, untwinned single crystals of YBa2Cu3O6.95 in all three crystallographic directions. The temperature dependence of lambda is linear down to low temperatures, showing that our crystals are extremely clean. Both the magnitude and temperature dependence of the field dependent correction to lambda however, are considerably different from that predicted from the theory of the non-linear Meissner effect for a d-wave superconductor (Yip-Sauls theory). Our results suggest that the Yip-Sauls effect is either absent or is unobservably small in the Meissner state of YBa2Cu3O6.95.

Energy gap and proximity effect in MgB 2 superconducting wires

Measurements of the penetration depth

Field-dependent diamagnetic transition in magnetic superconductor Sm1.85Ce0.15CuO4-y.

ensuremath{lambda}(T,H)

Low-Temperature Penetration Depth of κ-(ET)2Cu[N(CN)2]Br and κ-(ET)2Cu(NCS)2

in the presence of a dc magnetic field were performed in

London Penetration Depth and Superfluid Density of Single-crystalline Fe1+y(Te1−xSex)and Fe1+y(Te1−xSx)

{mathrm{MgB}}_{2}

Nodal superconductivity in isovalently substituted SrFe 2 (As 1 − x P x ) 2 pnictide superconductor at the optimal doping x = 0.35

wires. In as-prepared wires