Lan Luan

Mechanics of Individual, Isolated Vortices in a Cuprate Superconductor

The ability to wiggle and stretch individual superconducting vortices with nanoscale precision enables unprecedented insight into their dynamics and the properties of the superconductor that supports them. Superconductors often contain quantized microscopic whirlpools of electrons, called vortices, that can be modelled as one-dimensional elastic objects1. Vortices are a diverse area of study for condensed matter because of the interplay between thermal fluctuations, vortex–vortex interactions and the interaction of the vortex core with the three-dimensional disorder landscape2,3,4,5. Although vortex matter has been studied extensively1,6,7, the static and dynamic properties of an individual vortex have not. Here, we use magnetic force microscopy (MFM) to image and manipulate individual vortices in a detwinned YBa2Cu3O6.991 single crystal, directly measuring the interaction of a moving vortex with the local disorder potential. We find an unexpected and marked enhancement of the response of a vortex to pulling when we wiggle it transversely. In addition, we find enhanced vortex pinning anisotropy that suggests clustering of oxygen vacancies in our sample and demonstrates the power of MFM to probe vortex structure and microscopic defects that cause pinning.

Local Measurement of the Superfluid Density in the Pnictide Superconductor Ba ( Fe 1 − x Co x ) 2 As 2 across the Superconducting Dome

We measure the penetration depth λab(T) in Ba(Fe(1-x)Co(x))(2)As(2) using local techniques that do not average over the sample. The superfluid density ρs(T) ≡ 1/λab(T)2 has three main features. First, ρs (T = 0) falls sharply on the underdoped side of the dome. Second, λab(T) is flat at low T at optimal doping, indicating fully gapped superconductivity, but varies more strongly in underdoped and overdoped samples, consistent with either a power law or a small second gap. Third, ρs (T) varies steeply near Tc for optimal and underdoping. These observations are consistent with an interplay between magnetic and superconducting phases.

Noise characteristics of 100nm scale GaAs∕AlxGa1−xAs scanning Hall probes

Magnetic imaging techniques are useful for studying systems with magnetic features at microscopic length scales. Magnetic force microscopy offers the highest spatial resolution currently achievable without special sample preparation, around 30 nm. Hall probes offer the prospect of similar resolution but with a nonperturbative probe that directly measures Bz. In this study we demonstrate Hall probes as small as 100 nm, fabricated in a scanning geometry on GaAs/ AlGaAs two-dimensional electron gas 2DEG, and

Moment switching in nanotube magnetic force probes

Magnetic images of high density vertically recorded media using metal-coated carbon nanotube tips exhibit a doubling of the spatial frequency under some conditions (Deng et al 2004 Appl. Phys. Lett. 85 6263). Here we demonstrate that this spatial frequency doubling is due to the switching of the moment direction of the nanotube tip. This results in a signal which is proportional to the absolute value of the signal normally observed in MFM. Our modeling indicates that a significant fraction of the tip volume is involved in the observed switching, and that it should be possible to image high bit densities with nanotube magnetic force sensors.

Meissner response of a bulk superconductor with an embedded sheet of reduced penetration depth

We calculate the change in susceptibility resulting from a thin sheet with reduced penetration depth embedded perpendicular to the surface of an isotropic superconductor, in a geometry applicable to scanning superconducting quantum interference device microscopy, by numerically solving Maxwells and Londons equations using the finite-element method. The predicted stripes in susceptibility agree well in shape with the observations of Kalisky et al. [Phys. Rev. B 81, 184513 (2010)] of enhanced susceptibility above twin planes in the underdoped pnictide superconductor

Magnetic-force-microscope Study of Interlayer _Kinks_ in Individual Vortices in Underdoped Cuprate YBa2Cu3O6 x Superconductor

\text{Ba}{({\text{Fe}}_{1\ensuremath{-}x}{\text{Co}}_{x})}_{2}{\text{As}}_{2}

Spatial resolution of MFM measurements of penetration depth

. By comparing the predicted stripe amplitudes with experiment and using the London relation between penetration depth and superfluid density, we estimate the enhanced Cooper-pair density on the twin planes, and the barrier force for a vortex to cross a twin plane. Fits to the observed temperature dependence of the stripe amplitude suggest that the twin planes have a higher critical temperature than the bulk, although stripes are not observed above the bulk critical temperature.