Multiple phase transformations in weakly pinned vortex matter S. Ramakrishnan and A. K. GroverDepartment of Condensed Matter Physics and Materials ScienceTata Institute of Fundamental Research, Mumbai, IndiaIn collaboration with S. Bhattacharya, K. Ghosh, S. Banerjee, D. Pal, S. Sarkar, A. Tulapurkar, A. Thakur, D. Jaiswal (TIFR), T.V. C. Rao, G. Ravikumar, P.K. Mishra, V.C. Sahni (TPPED, BARC)C.V. Tomy (Department of Physics, IIT Mumbai)McK. Paul, G. Balakrishnan (Univ. Warwick)M. Higgins (NEC, NY)

Motivation: To understand the vortex lattice transformations via the occurrence of anomalous variations in Jc in a variety of low Tc superconductors and compare them with a HTSC.2. Basic premises:Plateau effect: Transformation from individual or single bundle pinning to collective pinning (single domains)Second Magnetization Peak : Transformation from single domain to multidomain vortex systemClassical PE effect: (Multidomain vortex system to maximally disordered /amorphous)

Plan of the talk1. Brief recap on vortex lattice transformations(a) Plateau effect(b) Second magnetization peak (SMP) (c) Classical Peak effect (PE)2. Effect of disorder in 2H-NbSe23. Comparison of SMP and PE in YBCO and Ca3Rh4Sn134. SMP and PE in Borocarbides(LuNi2B2C and YNi2B2C) and 2H-NbX2 (X=Se, S)5. Compare the phase diagram and dynamics in two samples of 2H-NbSe2

(i) 2H-NbSe2 crystals with varying disorder (Tc- 7.1 to 6 K) (Uni. Warwick and NEC, NY)

(ii) 2H-NbS2 crystal (Tc ~ 6.6 K) (J.N.U, India)

(iii) Ca3 Rh4 Sn13 (Tc~8.2 K) , Yb3 Rh4 Sn13 crystals(Tc~7.6 K) (Uni. Warwick)

(iv) LuNi2 B2C (Tc~16 K) (Ames) and YNi2 B2C (Tc~15.4 K) (NIMS, Japan) crystals

(v) CeRu2 Crystal (Tc~6.4 K) (Uni. Osaka,Toyanaka)

(vi) YBCO (few twins, Tc~93 K) (Uni. Wisconsin, Milawauke.)Systems under study using ac and dc magnetization, transport measurements

TplTpOrdered state shrinks as T approaches Tc.Anomalous change in Jc in two crossover regimes . High field is PE and the low field end is Plateau effect.K. Ghosh PRL 76,4600 (1996). Tc~7.17 KS. S. Banerjee et al, PRB 62, 11838 (2000)

Peak Effect and Effect of Increasing disorderOrdered phase is sandwitched between disordered phases. It shrinks with increasing pinning.

Reentrance in PE related to pinning.K. Ghosh et al, PRL, 76,4600 (1996)

S. Banerjee et al, PRB 62, 11838 (2000)

2H-NbSe2 (C)Paltiel et al, PRL,85,3712 (2000)

Observation second magnetization peak and classical peak effect in the same isothermal scan in a low Tc superconductorCa3Rh4Sn13 (Tc =8.18 K)S. Sarkar, D. Pal et al. Phys. Rev. B64, 144510 (2001)From magnetization SMP cannot be distinguished at high temperatures

Observation of the secondmagnetization peak and peak effect in the sameisothermal scan in YBa2Cu3O7- (H || c)YBCO with few twins Tc~93 K

D. Pal et al. Phys. Rev. B63, 132505 (2001)

S. Sarkar,D. Pal et al. Phys. Rev. B64, 144510 (2001)Comparison of SMP and PE in low Tc and high Tc superconductorsSMP and PE are well separated. SMP occupies larger (H,T) space in YBCO as compared to Ca3 Rh4 Sn13

D. Pal et al PRB 65, 965202, (2002)SMP and PE Deligiannis et alPRL, 79, 2121 (1997)

Extensive studiesNishizaki et alPRB 58, 11169 (1998)PRB 53, 82 (1996)PRB 61, 3649 (2000)

Observation of the secondmagnetization peak and peak effect in the sameisothermal scan in YBa2Cu3O7- (H || c)YBCO with few twins Tc~93 K

D. Pal et al. Phys. Rev. B63, 132505 (2001)

Bi2Sr2CaCu2O8Avraham et al Nature, 409, 451 (2001)D. Pal Phys. Rev. B, 64, 144510 (2001), PRB 65, 965202, (2002)D. Pal Phys. Rev. B, 62, 6699 (2000)

Both crystals A and B show similar Tc and exhibit dHvA Oscillations upto 8 K D. Jaiswal (Physica B, 2004 and un published work)Poster P25, P26 IVW 2005

Peak Effect and Effect of Increasing disorderOrdered phase is sandwitched between disordered phases. It shrinks with increasing pinning.

Reentrance in PE related to pinning.S. S. Banerjee et al, PRB 62, 11838 (2000)

Ajay Thakur et al (unpublished.)A. D. Thakur (unpublished)

NO SMP

In crystal A, the PE relaxes faster as compared to disordered crystal C.However, the SMP in C relaxes faster than the PE in the same sample.Caution:SMP is affected by the ripple effect (KuepferIVW 2005)P66 IVW 2005

A. D. Thakur (unpublished)P66 IVW 2005

A.D. Thakur etalP66 IVW 2005Both SMP and PE are present at intermediate fields but at higher fields move closer and A broad peak emerges

Broadening of the PE is maximum inThe FCC response.

First observation of SMP in a temperature dependent scan

Phase diagramTwo 2H-NbSe2 crystalsCrystal AB2Similar to B K. Ghosh et alPRL76, 4600 (1996)Banerjee et alEuro Phys. Lett46, 91 (1998)Crystal CC2 similar to sample C Banerjee et al PRB 11838 (2000)

Anistropy ~68Tc~ 6.6 K, Magnetic response ~2H-NbSe2 crystal CTulapurkar etal , Physica B200A. Thakur et al (2004, unpublished)

A. D. Thakur (unpublished)

To Summarize1. SMP and CPE are two distinct anomalies which can be observed in weakly pinned samples of both LTSC and HTSC systems. They relate to order-disorder transformation in the pinned vortex matter.2. However, both anomalies per se could have different temporal behavior. History and metatstability associated with them depends on times scales involved which are influenced not only by the pinning but also by field (a0). 3. In very weakly pinned sample CPE is sharp but with disorder CPE broadens with emergence of another (SMP) broad peak prior to CPE. Further disorder makes a single broad anomaly commencing well below Hc2.4. The absence of reports on SMP distinct from CPE in 2H-NbSe2 presumably due to the fact that currents associated with SMP decay dies faster than those due to CPE. Commercial Squid magnetometers unable to resolve this.6. Recent results on various samples are in agreement with our premises7. In order to substantiate one needs to go beyond macroscopic measurements (eg. STM (Leiden group) experiments on high quality samples or neutron scattering)

D. Pal et al. Phys. Rev. B63, 132505 (2001)

Like Atomic solids, the Vortex Matter responds like an elastic medium It costs energy to distort a Flux line in FLL. For hexagonal lattice, three independent elastic moduli are adequateat B = 1 T, c66~5x104 dynes/cm2

Vortex Matter Physics belongs to the realm of Soft Condensed MatterWeak elasticity of the vortex matter makes it easily susceptible to disordering effects:Disordering effect of thermal energy (kT) (e.g. melting of atomic solidDisorder produced by sample Inhomogenities (impurites, voids, dislocations), Irradiations

Multiple phase transformations in weakly pinned vortex matter S. Ramakrishnan and A. K. GroverDepartment of Condensed Matter Physics and Materials ScienceTata Institute of Fundamental Research, Mumbai, India

In collaboration with Technical Physics Prototype Engineering Division, Bhabha Atomic Research centre, Mumbai, India

S. Bhattacharya, K. Ghosh, S. Banerjee, D. Pal, S. Sarkar, A. Tulapurkar, A. Thakur, D. Jaiswal (TIFR), T.V. C. Rao, G. Ravikumar, P.K. Mishra, V.C. Sahni (BARC)C.V. Tomy (Department of Physics, IIT Mumbai)McK. Paul, G. Balakrishnan (Univ. Warwick)

Pinning Centers in Vortex MatterFor example: Point Pins

Point pins distort flux line such that it does not remain straight over the entire thickness of the sample. This could lead to loss of long range translational order in FLL. LcProduced by sample Inhomogenities, atomic defects, etc.Random distribution of point defects inside the sample give rise to statistical fluctuations in the defect density, which in turn act as weak pinning agents

Different possible phases of Vortex MatterWeakPinningThermal Fluctuations

Peak Effect Phenomenon in Jc& the Different Phases of Vortex Matter PE first observed in 1961 in NbZr alloy Almost all weakly pinned superconductors show the Peak Effect. 2H-NbSe2 crystals: most studied over years

Current Carrying capacity in a superconductor (Jc) is related to the extent of order in the FLL as wellas strength of pinning Larkin - Ovchinnikov theory (1979)

Reentrant characteristic of Order-Disorder Phase Boundary Flux Line Soft lattice is very soft at low and high feild Reentrance.

(kT)T. Giamarchi and P. Le Doussal (1995); M. Gingras and D. A. Huse (1996).

D. R. Nelson (1998)Temperature and/or pinning could induce re-entrant characteristic