Synopsis: Keep it local

A new tool has been developed to measure the local London penetration depth in a superconductor.
Synopsis figure
Illustration: L. Luan et al., Phys. Rev. B (2010)

The typical distance that an applied magnetic field penetrates a superconductor—called the penetration depth, λ—is an important measure that can be directly related to the superfluid density. Absolute values of λ are notoriously difficult to ascertain. Most experiments therefore measure the difference, Δλ = λ(T)-λ(0), which has the same temperature dependence as the superfluid density at low temperatures. A power-law dependence in Δλ suggests there are nodes in the superconductivity gap, while an exponential dependence implies a fully open gap.

Recent measurements of Δλ in iron pnictide superconductors of the Ba-122 family show conflicting behavior: when the superconductor is doped with cobalt, there is a power-law dependence, suggesting nodes in the gap, but when doped with potassium, different experiments do not agree on the absence or presence of nodes in the gap. It turns out that to resolve this conflict, it may be necessary to measure the absolute value of λ.

In a Rapid Communication appearing in Physical Review B, Lan Luan and collaborators from Stanford University in the US have found a new way of measuring both the absolute value of the penetration depth and its spatial homogeneity using magnetic force microscopy (MFM) and SQUID susceptometry. Lan Luan et al. observe that for the Ba-122 compound doped with cobalt, the superfluid density has a temperature dependence that is consistent with a fully gapped two-band model, similar to the potassium-doped material. Further, λ is found to be spatially homogeneous at the submicron scale, and absolute values of λ suggest that phase fluctuations are not as important for iron pnictides as for the underdoped cuprates. The ability of the new tool to obtain the absolute values of the penetration depth and to map its spatial variation down to the submicron scale is likely to be extremely useful. – Sarma Kancharla


Announcements

More Announcements »

Subject Areas

Superconductivity

Previous Synopsis

Next Synopsis

Statistical Physics

Good things come in threes

Read More »

Related Articles

Viewpoint: A Tale of Two Domes
Condensed Matter Physics

Viewpoint: A Tale of Two Domes

Iron selenide films peppered with potassium atoms exhibit a high-temperature superconducting phase that emerges separately from a low-temperature superconducting phase. Read More »

Focus: <i>Landmarks</i>—Superconductor Quantizes Magnetic Field
Superconductivity

Focus: Landmarks—Superconductor Quantizes Magnetic Field

In 1961, confirmation that a magnetic field inside a superconducting ring is limited to discrete values demonstrated that superconducting electrons pair up. Read More »

Viewpoint: Wiring Up Superconducting Qubits
Quantum Physics

Viewpoint: Wiring Up Superconducting Qubits

A qubit made of a semiconducting nanowire sandwiched between two superconductors could simplify the design of quantum information processing architectures. Read More »

More Articles