Transition between the Bragg glass and the disordered phase inNb3Sndetected by third harmonics of the ac magnetic susceptibility

Abstract

We report additional experimental evidences about the presence of an universal behavior in the Field-Temperature Phase Diagram of Type II Superconductors. This behavior is characterized by a phase transition in the vortex matter between the disordered and the Bragg Glass phase. The experimental detection of a Peak Effect phenomenon has been proved to be strictly connected to the existence of this phase transition. In this paper, we show the first observation of a Peak Effect in the compound Nb 3 Sn, by using 1 st harmonics of the AC magnetic susceptibility. Peak Effect has been detected at fields between 3T and 13T, whereas it is not observable at higher fields. This seems to be in contrast with the theoretical predictions of such a phase transition at all fields and, therefore, with the predicted universality in the magnetic behavior of the Type II superconductors. Nevertheless, by measuring the 3 rd harmonics of the AC susceptibility, this phase transition has been detected up to our highest available field (19 T), thus demonstrating the necessity of the higher harmonics analysis in studying these topics and moreover proving the validity of the theoretical predictions. Magnetic flux penetrates a type II superconductor in form of vortices, which distribute in a regular lattice in absence of defects [1,2]. Defects affect the lattice ordering, but prevent the dissipations associated to the vortex movement [2]. An universal field-temperature phase diagram has been supposed for all the type II superconductors with point defects. In this phase diagram a transition in the vortex lattice has been predicted, between a disordered phase and the Bragg Glass Phase [3,4]. This latter is characterized by a quasi long range order and a perfect topological order, in which the vortex lattice stills survives despite the presence of the pinning [3], as it has also been experimentally evidenced [5]. The disordered phase has been supposed to be again a glass phase ("multidomain glass"), but with a topological disorder at the largest length scales [4]. The critical current density, J c [6], generally decreasing with increasing temperature and/or magnetic field [2], shows a local maximum, known as Peak Effect [7], when the Disordered/Bragg Phase transition occurs. For this reason, the observation of the Peak Effect has been widely used to detect this phase transition [3]. The Peak Effect has been evidenced in various classes of type II superconductors, e.g. low-T c [8][9][10][11][12], high-T c [13][14][15], boro-carbides [16] and MgB 2 [17]; however, it has not been observed in Nb 3 Sn up to the present day, to the best of our knowledge. Nb 3 Sn crystallizes in the A15 type structure and is actually the most used material in the manufacturing of superconducting magnets at very high fields [18]. In the present work, measurements have been performed on a high quality Nb 3 Sn single crystal, furnished by M. Toyota [19] (Dimensions ≈ 3 x 1.32 x 0.43 mm 3 ). The sample was characterized by T c = 18.2 K, ΔT c = 0.1 K, T m = 38.84 K. A valu...

Thermal fluctuations and vortex melting in theNb3Snsuperconductor from high resolution specific heat measurements

“…This is valuable information for the interpretation of universal features in the phase diagram of type-II superconductors, e.g., the broadening of the H c2 line in fields 25,38 and the peak effect. [13][14][15][16][17][18] …”

“…3 inset͒. 17,18 Such peak effects have been interpreted as being due to an underlying solid-to-liquid transition in the vortex matter. 30 The onset of the fluctuation anomaly in the specific heat when approaching T c ͑H͒ from below may be interpreted as an "idealized" upward step, similar to the second-order vortex melting observed in some YBCO samples.…”

“…Improving sample quality and experimental sensitivity to observe critical fluctuations thus poses a significant challenge to the experimentalist. Nevertheless, some "universal" features in the fieldtemperature phase diagram of classical type-II superconductors, e.g., a broadening of the upper-critical field line ͓H c2 or T c ͑H͔͒ or a peak effect [13][14][15][16][17][18] show analogies with the HTSCs. New high-resolution calorimetric methods which we developed during years of study on the HTSCs motivated us to investigate whether fluctuation effects and vortex melting might also be responsible in this case.…”

Origin of the magnetization peak effect in theNb3Snsuperconductor

“…For T Ͻ 15 K, a small hysteresis loop appears below H c2 due to a pronounced peak effect, also observed in the ac susceptibility curve for the same sample. 14 In the presence of some flux pinning, the experimental curves can be understood as a superposition of a reversible PHYSICAL REVIEW B 75, 094503 ͑2007͒ thermodynamic contribution, which is independent of the direction of the field sweep and an irreversible contribution, which is directly linked to the critical current density J C . The sign of the second component depends on the direction of the field sweep, thus resulting in a hysteresis loop between the ascending and descending branches of the magnetization.…”

Magnetocaloric studies of the peak effect in Nb

“…[8] Subsequently, the disappearance of the peak effect at low fields has also been reported in other systems. [9,10] Thermodynamic considerations [8] suggest that the MCP is likely a bicritical point. Since bicriticality implies the existence of competing types of order in the vortex system the question of which of the two lines, T c2 or H c3 , is relevant to the bicritical point has major importance.…”