Research Projects at CAT, Indore

(April 1997-May2002)

(a) Peak Effect in certain hard type-II superconductors - a consequence of a first order phase transition in Vortex matter:

Critical current density (J_C) of a hard type-II superconductor generally decreases with applied magnetic field (H). However, in certain superconductors, the J_C is found to increase with H just below the upper critical field H_C2, which is manifested as a peak in J_C(H) below H_C2, and is referred to as “peak-effect” (PE). Our reported work is a detailed account of PE in a number of hard type-II superconductors, viz. CeRu₂, Bi-2212, 1at%Fe-Nb, Mo_1-XRe_X, and V₃Si. The significant contributions can be summarized as follows:

Ø It is established that the PE in CeRu₂ (T_C»6.2K) and V₃Si (T_C»16.5K) [Physica C, 353 (2001) 29] is a consequence of a first order phase transition (FOPT), from a usual mixed vortex phase to another vortex phase having a higher J_C, and that the PE in Bi-2212 [Physica B, 280 (2000) 209], 1at%Fe-Nb, Mo_1-XRe_X [Philos. Mag. B, 80 (2000) 1393] results from a second order phase transition. The vortex phase with higher Jc, was conjectured to be a theoretically predicted Flude-Ferrel Larkin Ovchinikov (FFLO) state.

Ø Further, it was clearly established that in case of CeRu₂ and V₃Si, the minor hysteresis loops (MHLs) obtained in different histories of H and T are quite anomalous, and did not conform to the predictions of Bean’s Critical State model.[Phys. Rev. B, 62 (2000) 9191].

The above findings are substantiated on the basis of a series of identical results obtained on same set of samples by (i) employing two different techniques of magnetization measurement, i.e., SQUID and VSM, respectively, and by (ii) the magneto-transport measurements on CeRu₂ and V₃Si single crystals (upto H=10 Tesla).[ Solid State Commun., 114 (2000) 5]

In addition, based on detailed investigations of a range of CeRu₂ samples - single crystal, polycrystalline, pure and 5at%Nd-doped CeRu₂ – we obtained another remarkable finding: the peak-effect anticorrelates with the disorder. That is, the PE was prominently seen in single crystal CeRu₂ sample as compared with the polycrystalline samples. This was found to be in sharp contrast to the published reports on superconducting NbSe₂ displaying PE, wherein the PE was reported to be prominent in samples with higher disorder.[Solid State Commun., 111 (1999) 263]

The conjecture about the formation of Flude-Ferrel Larkin Ovchinikov state in CeRu₂ has very recently been experimentally verified through muon-spin relaxation study (giving a microscopic evidence in favor of FFLO state) reported by other group on CeRu₂ single crystal.

(b) Metamagnetic Antiferro-to-Ferromagnetic Transition in Ce(Fe_1-xR_x)₂ - a first order MAGNETIC phase transition:

Having established that the PE in certain superconductors is a consequence of a FOPT between two vortex phases, we probed, in detail, the field induced FOPT between AFM and FM in a magnetic system, Ce(Fe_1-xR_x)₂. The significant contributions that emerged out of this work are summarized below:

Ø For the first time, we experimentally demonstrated all the generic features associated with a FOPT in a single material system, viz., hysteresis, phase coexistence, supercooling and superheating, presence and limits of the metastable regimes in a H-T phase space. [J. Phys.: Condens. Matter 14 (2002) 4477-4490., J. Phys.: Condens. Matter, 12 (2000) 9645.]

Ø Most spectacular has been the anomalous appearance of the virgin curve lying outside the hysteresis loop in dc-magnetization (M-H) and dc-resistivity (r-H) data. The origin of the appearance of the virgin curves has been proposed to the arrest of the first order transition at low temperature due to kinetic hindrance.[ Phys. Rev. B.65 (2002) 94419, Phys. Rev. B, 64 (2001) 104416.]

Ø The other material investigated for its FOT is SmMn₂Ge₂ [Phys. Rev. B 66 (2002) 14424.]

(c) Magnetocaloric Effect (MCE) in La_1-XSr_XCoO₃ system:

By investigating the MCE (a process employed in Refrigeration technology based on magnetic cooling), we have demonstrated that La_1-XSr_XCoO₃ is a potential competitor of the conventionally utilized Gd in magnetic refrigeration technology [J. Mag. Mag. Mat., 202 (1999) 45].