STM Imaging of Flux Line Arrangements in the Peak Effect Regime

Abstract

We present the results of a study of vortex arrangements in the peak-effect regime of 2H-NbSe2 by scanning tunneling microscopy. By slowly increasing the temperature in a constant magnetic field, we observed a sharp transition from collective vortex motion to positional fluctuations of individual vortices at the temperature which coincides with the onset of the peak effect in ac-susceptibility. We conclude that the peak effect is a disorder driven transition, with the pinning energy winning from the elastic energy.PACS numbers: 74.60. Ge, 64.60.Cn It is well known that the critical current density j c in weakly disordered type II superconductors shows a sudden increase when the applied magnetic field H approaches the upper critical field H c2 [1]. An explanation for this intriguing phenomenon, known as the peak effect, was suggested by Pippard [2] already in 1969. It is based on the competition between the collective work done by the disorder (the pinning centers) on the vortex lattice (VL) and the elastic energy stored in vortex lattice deformations due to the pinning [3]. The work depends linearly on (H c2 − H) while the elastic energy essentially is proportional to the shear modulus c 66 of the VL, which behaves as (H c2 − H) 2 . Therefore, at some field close to H c2 , the pinning will exceed the elasticity and the VL will accommodate to the random pinning potential, leading to the sudden increase in j c observed experimentally. The discovery of the high temperature superconductors made it relevant to also consider thermal fluctuations as a third energy scale, which lead to melting by suppression of c 66 . This would then yield a peak effect as a precursor of the melting transition [4,5,6].Experimentally, it is hard to probe which mechanism is predominant, because one needs information about structural changes of the vortex lattice positional order as a function of time. Small angle neutron scattering (SANS) yields structural information along the field direction averaged over the sample volume, i.e., about the amount of entanglement of the VL. SANS has been successfully used in the peak-effect regime [7,8,9] but it is resolution limited in the transverse direction. Lorentz microscopy [10], scanning Hall probe (SHP) [11], scanning SQUID [12,13] and, recently, magneto-optics experiments [14] yield positional information on the scale of individual vortices, but sense the magnetic field distribution, and only work in the low flux density regime, usually far below H c2 , where vortices are well separated and the vortex-vortex interaction is very weak. In this Letter we show that information on the time-dependent positional order can be provided by scanning tunneling microscopy (STM), which is uniquely able to access the necessary length scales and flux densities. We present FIG. 1: Phase diagram of NbSe2 obtained from susceptibility measurements on a single crystal, showing Hc2(T ) (solid line / circles), the peak of critical current Hp(T ) (solid line / plusses) and the onset of the peak Hp1(T ) (dashed line...

Imaging superconducting vortex cores and lattices with a scanning tunneling microscope

“…Also, the work of Ref. [52,171] has used the STM in topography mode at the bias voltage where maximum contrast is obtained in the current, which is close to the quasiparticle peak. As mentioned by these authors, when increasing the magnetic field, the contrast is reduced and images are washed out.…”

Electrical transport, magnetic, and structural properties of the vortex lattice ofV3Siin the vicinity of the peak effect

“…This transition has been observed recently in other cubic species, (K,Ba)BiO 3 [6,7] and Nb [9], wherein a quasi-ordered Bragg glass is thought to cross over to a purely disordered state. A more recent confirmation utilized specialized scanning tunneling microscopy [12] which revealed a clear transition from a collective, orderly vortex-lattice to uncorrelated, disorderly motion upon crossing T p (H). These features also confirm the results of a notable numerical simulation [13] modeling the encroachment of disorder as pinning overwhelms lattice elasticity as field and temperature are increased.…”

Exploring metastability via third harmonic measurements in single crystals of2H−NbSe2showing an anomalous peak effect

“…There is a widespread belief that the PE represents a dynamical transition (e.g., from elastic flow to plastic flow) in the driven vortex matter [3,4,5]. Using STM imaging measurements on a very weakly pinned single crystal of the well studied 2H-NbSe 2 system, Troyanovsky et al [6] have shown that the collective motion of neighboring vortices gives way to positional fluctuations of individual vortices just above the onset temperature (T on p ) of the PE. As regards the nature of the phase transition across the PE region, X. S. Ling et al [7] have sought to provide direct structural evidence in favor of the first order nature of the order-disorder transition commencing at T on p in a crystal of Nb.…”