Magnetic Imaging of Pearl Vortices in Artificially Layered(Ba0.9Nd0.1CuO2+x)

Abstract

We have used scanning SQUID magnetometry to image vortices in ultrathin Ba 0:9 Nd 0:1 CuO 2x m = CaCuO 2 n high temperature superconductor samples, with as few as three superconducting CuO 2 planes. The Pearl lengths ( 2 2 L =d, L the London penetration depth, d the superconducting film thickness) in these samples, as determined by fits to the vortex images, agree with those by local susceptibility measurements, and can be as long as 1 mm. The in-plane penetration depths ab inferred from the Pearl lengths are longer than many bulk cuprates with comparable critical temperatures. We speculate on the causes of the long penetration depths, and on the possibility of exploiting the unique properties of these superconductors for basic experiments. DOI: 10.1103/PhysRevLett.92.157006 PACS numbers: 74.72.-h, 73.40.Gk, 73.40.Rw, 74.50.+r Vortices play a central role in many aspects of superconductivity. Not only do the dynamics of vortices determine many of the transport properties of type II superconductors, especially the high critical temperature cuprates [1], but vortices are also of more general interest, since as topological defects they are of great relevance, for instance, to phase transitions [2,3]. The formation of topological defects in phase transitions has even stimulated some analogies between cosmology, gauge theories, and condensed matter physics [4,5]. Vortices in bulk type II superconductors were first predicted by Abrikosov in 1957 [6], and have since been imaged by many different experimental techniques [7]. Vortices in thin superconductors (d L , where d is the superconducting film thicknes and L the London penetration depth, respectively) were first described by Pearl [8] (hence ''Pearl'' vortices). Pearl vortices have several interesting attributes. The field strengths h z perpendicular to the films diverge as 1=r at distances r in Pearl vortices, whereas in Abrikosov vortices the fields diverge as lnr= L [9]. Since in the Pearl vortex much of the vortex energy is associated with the fields outside of the superconductor, the interaction potential V int r between Pearl vortices has a long range component V int =r for r[8], unlike Abrikosov vortices, which have only short range interactions. The interaction between Pearl vortices V int ln=r for r leads to a BerezinskiiKosterlitz-Thouless (BKT) transition which is cut off due to screening on a scale [1]. The logarithmic interaction makes this system very similar to a Coulomb gas and ideal to study screening effects and renormalization in BKT transitions [10]. While superconducting vortices in films with thickness d comparable to the London penetration depth L have been imaged using many techniques, to our knowledge the present work is the first to directly demonstrate experimentally the existence of Pearl vortices for d , and is also the first to use scanning susceptibility measurements to determine penetration depths in superconductors.In the present work, two different types of Ba 0:9 Nd 0:1 CuO 2x m =CaCuO 2 n (CBCO) structures were grown: (a) t...

Imaging Josephson Vortices on the Surface SuperconductorSi(111)−(7×3)Yoshizawa1, Kim2, Kawakami3 et al. 2014Phys. Rev. Lett.753570View full textAdd to dashboardCite

“…As the spectral site approached the center (marked as B), the zero-bias dip and the coherence peaks were both strongly suppressed, indicating breaking of superconductivity. We note that the vortices found here should be called Pearl vortices (PVs) because the present system consists of an atomically thin 2D superconductor [26,27]. [28] For the following images, ZBC is normalized by the dI/dV value at a coherence peak at each pixel point to enhance the signal-to-noise ratio.…”

Sheet-current distribution in a dc SQUID washer probed by vortices

“…We use low-temperature scanning electron microscopy (LTSEM) [2,3,4,5] to image vortices in dc SQUID washers [6,7]. Most techniques for vortex imaging, such as Lorentz microscopy [8], scanning SQUID microscopy [9,10], scanning Hall microscopy [11] or magneto-optics[12] rely on the detection of the stray magnetic field produced in close proximity to a vortex. In contrast, vortex imaging by LTSEM is different from those techniques, as it is based on the electron-beaminduced apparent displacement of a vortex, pinned at position r in the (x, y)-plane of a SQUID washer, which is detected as a change of stray magnetic flux Φ(r) coupled to the SQUID.…”

Time-Correlated Vortex Tunneling in Layered Superconductors

“…As d becomes small the vortices acquire a quasi-2D character and their diameters increase, as predicted by Pearl [35] and directly imaged by Tafuri et al [36]. When ≪ , the size of each vortex is governed by an effective length, Λ, known as the Pearl penetration length [35,36], Λ = 2 ⁄ . Since the relevant length scale becomes Λ = when ≫ , we use the approximation: Λ ≅ + 2 ⁄ .…”