Laboratory for Solid State Physics, ETH Zurich, 8093 Zurich, Switzerland.
National High Magnetic Field Laboratory (NHMFL), Tallahassee, Florida 32310, USA.
Phys Rev Lett. 2014 Oct 31;113(18):186402. doi: 10.1103/PhysRevLett.113.186402. Epub 2014 Oct 30.
In layered superconductors the order parameter may be modulated within the unit cell, leading to nontrivial modifications of the vortex core if the interlayer coherence length ξ(c)(T) is comparable to the interlayer spacing. In the iron pnictide SmFeAs(O,F) (T(c)≈50 K) this occurs below a crossover temperature T(⋆)≈41 K, which separates two regimes of vortices: anisotropic Abrikosov-like at high and Josephson-like at low temperatures. Yet in the transition region around T(⋆), hybrid vortices between these two characteristics appear. Only in this region around T(⋆) and for magnetic fields well aligned with the FeAs layers, we observe oscillations of the c-axis critical current j(c)(H) periodic in 1/sqrt[H] due to a delicate balance of intervortex forces and interaction with the layered potential. j(c)(H) shows pronounced maxima when a hexagonal vortex lattice is commensurate with the underlying crystal structure. The narrow temperature window in which oscillations are observed suggests a significant suppression of the order parameter between the superconducting layers in SmFeAs(O,F), despite its low coherence length anisotropy (γ(ξ)≈3-5).
在层状超导体中，有序参数可能在单位胞内被调制，如果层间相干长度 ξ(c)(T)与层间距相当，那么这将导致涡旋核发生重要的改变。在铁磷化物 SmFeAs(O,F)（T(c)≈50 K）中，这种情况发生在交叉温度 T(⋆)≈41 K 以下，它将两个涡旋区隔开：高温下的各向异性 Abrikosov 型和低温下的 Josephson 型。然而，在 T(⋆)附近的过渡区，这两种特性之间出现了混合涡旋。只有在 T(⋆)附近的这个区域，并且磁场与 FeAs 层很好地对齐时，我们才会观察到由于涡旋间力的微妙平衡和与层状势的相互作用，沿 c 轴的临界电流 j(c)(H)以 1/sqrt[H]为周期的振荡，这与 Josephson 电流有关。当六方晶格与底层晶体结构相匹配时，j(c)(H)会出现明显的最大值。尽管 SmFeAs(O,F)的相干长度各向异性（γ(ξ)≈3-5）较低，但观察到的振荡的窄温度窗口表明，在超导层之间的有序参数受到了显著抑制。
