Observation of a novel vortex structure driven by magnetic interactions near a sawtooth twin boundary inYBa2Cu3

Abstract

We have used magnetic decoration to study the pattern of vortices formed near an isolated sawtooth twin boundary in single-crystal YBa2Cu3-O7-5. When the pitch of the sawtooth is comparable to both the intervortex spacing and the penetration depth, we find an unusual vortex structure which has a reduced symmetry relative to the sawtooth itself. This vortex pattern arises from the magnetic interaction between the vortices and the twin boundary which occurs for an asymmetric twin. This is the first observation of a vortex structure caused by the magnetic interaction with a crystalline defect.PACS numbers: 74.60.GeMeasurements of the interactions which pin magnetic vortices to defects in the oxide superconductors are important in the quest to engineer materials with the highest possible critical currents. To date, most such studies have relied on transport and magnetization measurements [1]. In YBa2Cu307-5, the most heavily studied of the high-r^. materials, extended defects appear to be particularly promising as pinning sites. These can either be columnar pins [2] introduced through ion bombardment, screw dislocations [3], or the ubiquitous twin boundaries [4] found in most as-grown single crystals.Pinning of vortices normally arises as a result of the interaction between the normal core, of the size of the coherence length (^, and defects in the superconducting crystal [5]. On the other hand, vortex structures such as the hexagonal lattice [6] and vortex chains [7] are formed as a result of the magnetic interaction between vortices which exists on a length scale given by the penetration depth X. Magnetic interactions with defects are not generally thought to induce significant pinning. However, the oxide superconductors are strongly type II with >.>(^. Therefore the major part of the vortex energy is carried by the persistent currents which circulate around the vortex at an average distance X from the vortex axis. This suggests that purely magnetic interactions with energy of order 0oA^ per unit length of vortex line may play a role in pinning vortices to extended defects in addition to the familiar core pinning which has an energy of the same order,//.^^ 2-0^ AI Although the pinning/orc^ due to the core is still greater by a factor of X/^, the magnetic part of the interaction energy can be important in determining the equilibrium flux-lattice structure in the vicinity of an extended defect and thereby can indirectly affect the pinning of vortices to that defect.In this Letter we report on the vortex patterns seen near a peculiar type of sawtooth twin occasionally found in single-crystal YBa2Cu307-5. We find a pattern in which the vortices populate only one side of the teeth of the sawtooth. By contrast, previous observations of straight twins only showed a uniform line of vortices at the boundary. We argue that the surprisingly regular distortions of the flux-line lattice caused by this boundary can be understood by assuming purely magnetic interactions between the vortices and an asymmetric sawtooth twin. This i...

Behavior of vortices near twin boundaries in underdoped Ba(Fe1−xCox)2As<

“…Our main observation, that twin boundaries are a barrier for vortex motion, provides a mechanism for the enhancement of critical currents. Our observation, that vortices do not pin on the twin boundaries, seems to be in contrast to the pinning that was observed on the twin boundaries in the cuprate superconductors in Bitter decoration experiments [29][30][31][32][33]. Similar to Ref.…”

“…The magnetometry data is shown at 4.8K since the vortices are smaller and their centers are more easily determined at low temperatures. That the vortices are found only between the twin boundaries is in sharp contrast to the behavior observed in YBCO, where vortices decorate twin boundaries [29][30][31][32][33], and ErNi 2 B 2 C, where, in a small magnetic field, all vortices appear to sit on twin boundaries in the antiferromagnetic and superconducting state [40,41].As displayed in Fig. 1d, the vortex distribution appears close to a uniform random distribution, but the vortices appear to avoid twin boundaries.…”

“…A repulsive force is expected from the enhanced superfluid density on the twin boundary. This force is maximal when the vortex approaches the twin boundary and depends on the extent of the superfluid density variation across the twin boundary [43].In YBCO, Bitter decoration experiments observed vortices lining up on the twin boundaries [29][30][31][32][33], which led to classifying twin boundaries as locations with stronger pinning. However, some of the magneto-optics works that followed the penetration of vortices to the sample near twin boundaries reported that the twins act as barriers for vortex motion, and suggested that the vortices accumulate near the twin boundary and not on it [35].…”

Diamagnetic vortex barrier stripes in underdopedBaFe2(As1−xPx)2

“…When this happens the TB acts as a pinning site because of the reduced energetic cost of locating a vortex core on it. Such pinning behavior has been observed in the cuprates 13,14,16,21,25 where TBs also act as channels that are easy for vortices to move along, and hard for them to cross [17][18][19][20][21]25 . Other behavior is also possible.…”

“…TBs are not limited to FeSCs -they occur in many superconductors, including cuprates [13][14][15][16][17][18][19][20][21][22][23][24][25] , and are important for several reasons. In Fe-SCs their properties encode information about the nature of the superconducting phase and its competing orders [26][27][28] .…”