Measurement of the vortex-core radius by scanning tunneling microscopy

Abstract

Using a scanning tunneling microscope operated in a spectroscopic mode we imaged flux-line lattices in niobium diselenide at various external magnetic fields. From the evaluation of a large number of tnnnelingcurrent profiles taken across the individual vortices we deduced the dependence of the vortex-core radius on the applied magnetic field. It was found that the core radius shows a pronounced decrease with increasing field, even for H/Hc2 << 1. This behavior is qualitatively well characterized by self-consistent solutions of the Usadel equations.The outstanding analytical potential of scanning tunneling microscopy (STM) is due to its inherent capability of obtaining spectroscopic information at the sub-meV level combined with a lateral resolution of the order of angstroms. This enables one to probe superconductive properties on a length scale smaller than the coherence length. The present investigations have been performed on 2H-NbSe 2 which is a layered type-II superconductor. It is particularly well suited for STM because the freshly cleaved surfaces are relatively inert against contamination.The experiments have been performed with a low-temperature STM which has been described in some detail in Ref. 1. The local surface density of quasiparticle states is probed in the following way [2]: At any point on the surface the STM's feedback loop is interrupted for a short period (< 1 second), while the tunneling voltage V is decreased from beyond the superconductor's gap voltage Vg = Ao/e to a certain value V o < Vg. Upon raster-scanning the normal conducting tip the corresponding tunneling current I(Vo) then continuously varies between an upper limit Ima x right at the center of a vortex and a lower limit Imi n farthest from all adjacent vortices. All present measurements have been performed at Tfr c = 0.6. To change the vortex spacing the external magnetic field perpendicular to the sample surface was varied within the interval 0 _< WHc2_< 0.3. Figure 1 shows a tunneling-current image of the vortex lattice at an external field of H/Hc2 = 0.14. The sub-gap tunneling voltage was Vo/Vg = 0.88 at negative sample bias.We systematically analyzed numerous line scans across individual vortices. The actual vortex profiles were deduced from complete images such as displayed in Fig. 1. A characteristic measure of the respective vortex-core radius is arbitrarily defined by the particular distance p from the vortex center

Anomalous critical fields in quantum critical superconductors

“…This is in fact expected in cases of multiband superconductivity or superconductors with strong gap anisotropy. In MgB 2 [31,32] and also in the anisotropic gap superconductor 2H-NbSe 2 [33] the effective core size has been found to be around 3 times ξ GL , similar to that needed to explain the behaviour here. BaFe 2 (As 1−x P x ) 2 is known to have a nodal gap structure [34] which remains relatively constant across the superconducting dome [9] and so we should expect the core size to be uniformly enhanced for all x.…”

Enhancement of superconductivity under pressure and the magnetic phase diagram of tantalum disulfide single crystals

“…This effect will enlarge the vortex core size, which in turn will suppress the H c 2 . The effective core size has been found to be around three times that of the coherence length in 2 H -NbSe 2 32 . From the behavior of H c2 vs. T for the different field orientations, we have calculated the anisotropy as = ξ ab / ξ c .…”

Vortex-core properties and vortex-lattice transformation in FeSe

“…STM observation of the field dependence of the coherence length has been reported in the single-band superconductor β-Bi 2 Pd 58 and in the two-gap superconductors NbSe 2 59 and CaKFe 4 As 4 60 , whereas a magnetic field independent coherence length has been observed in multigap materials in the dirty limit such as 2H-NbSe 1.8 S 0.2 and 2H-NbS 2 58 .…”