Structural transition kinetics and activated behavior in the superconducting vortex lattice

Abstract

Using small-angle neutron scattering, we investigated the behavior of a metastable vortex lattice state in MgB2 as it is driven towards equilibrium by an AC magnetic field. This shows an activated behavior, where the AC field amplitude and cycle count are equivalent to, respectively, an effective "temperature" and "time". The activation barrier increases as the metastable state is suppressed, corresponding to an aging of the vortex lattice. Furthermore, we find a cross-over from a partial to a complete suppression of metastable domains depending on the AC field amplitude, which may empirically be described by a single free parameter. This represents a novel kind of collective vortex behavior, most likely governed by the nucleation and growth of equilibrium vortex lattice domains. arXiv:1812.05970v1 [cond-mat.supr-con]

Structural studies of metastable and equilibrium vortex lattice domains in MgB2

“…Following the damped field oscillation, the ES VL was either cooled to 2.4 or 2.7 K across the F-L phase boundary to obtain a MS F phase ('supercooled') or warmed to 14.2 K to obtain a MS L phase ('superheated'), as indicated by the arrows in figure 1(a). The higher temperature is close to the F-L phase boundary, but has been verified to lie clearly within the F phase [15]. The VL relaxation is not thermal, and therefore not expected to depend on the exact oscillation temperature or the cooling rate [14,15].…”

“…The higher temperature is close to the F-L phase boundary, but has been verified to lie clearly within the F phase [15]. The VL relaxation is not thermal, and therefore not expected to depend on the exact oscillation temperature or the cooling rate [14,15].…”

“…For each rocking curve the evolution of the VL towards the ES is describe by a 'transition coordinate'. In the supercooled case, where the transition to the ES is discontinuous, this coordinate is the remnant metastable volume fraction, f MS [14]. This is obtained from the intensity ratio of the Bragg peaks belonging to the MS F phase and the total scattered intensity from the VL, discussed in more detail in section 3.2.…”

“…This is obtained from the intensity ratio of the Bragg peaks belonging to the MS F phase and the total scattered intensity from the VL, discussed in more detail in section 3.2. In the superheated case the MS VL domains rotating continuously towards the ES orientation [15]. Here the transition coordinate is defined as the azimuthal peak splitting, Δf, of the VL Bragg peaks on the detector, shown in the inset to figure 4(b).…”

“…Domain nucleation and growth governs the behavior of a wide range of physical systems, and it is natural to expect similarities between the VL and for example martensitic phase transitions [9], domain switching in ferroelectrics [10] or the skyrmion lattice where field/temperature history dependent metastability has also been reported in connection with structural transitions [11][12][13]. Recently, we have studied the MgB 2 VL kinetics as it is driven from the MS to the equilibrium state (ES) by an AC magnetic field [14,15]. This showed an activated behavior, where the AC field amplitude and cycle count correspond to an effective 'temperature' and 'time' respectively.…”