Pabitra Mandal

High sensitivity differential magneto-optical imaging with a compact Faraday-modulator

We present here the design of a sensitive compact Faraday-modulator (CFM) based optical magnetometer for imaging the distribution of weak local magnetic fields inside hysteretic magnetic materials. The system developed has a root-mean-square noise level of 50 mG Hz(-1/2) at a full frame rate of 1 fps (frame per second) with each frame being of size 512 × 512 pixels. By measuring the local magnetic field distribution in different superconducting samples we show that our magnetometer provides an order of magnitude improvement in the signal-to-noise ratio at low fields as compared to ordinary magneto-optical imaging technique. Moreover, it provides the required sensitivity for imaging the weak magnetization response near a superconducting transition where a number of other imaging techniques are practically unviable. An advantage of our CFM design is that it can be scaled in size to fit into situations with tight space constraints.

Visualizing a dilute vortex liquid to solid phase transition in a Bi 2 Sr 2 CaCu 2 O 8 single crystal

Using high-sensitivity magneto-optical imaging, we find evidence for a jump in local vortex density associated with a vortex liquid to vortex solid phase transition just above the lower critical field in a single crystal of Bi2Sr2CaCu2O8. We find that the regions of the sample where the jump in vortex density occurs are associated with low screening currents. In the field–temperature vortex phase diagram, we identify phase boundaries demarcating a dilute vortex liquid phase and the vortex solid phase. The phase diagram also identifies a coexistence regime of the dilute vortex liquid and solid phases and shows the effect of pinning on the vortex liquid to vortex solid phase transition line. We find that the phase boundary lines can be fitted to the theoretically predicted expression for the low-field portion of the phase boundary delineating a dilute vortex solid from a vortex liquid phase. We show that the same theoretical fit can be used to describe the pinning dependence of the low-field phase boundary lines provided that the dependence of the Lindemann number on pinning strength is considered.

Anomalous local magnetic field distribution and strong pinning in CaFe1.94Co0.06As2 single crystals

Magneto-optical imaging of a single crystal of CaFe1.94Co0.06As2, shows anomalous remnant magnetization within Meissner-like regions of the superconductor. The unconventional shape of the local magnetization hysteresis loop suggests admixture of superconducting and magnetic fractions governing the response. Near the superconducting transition temperature the local magnetic field exceeds the applied field resulting in a diamagnetic to positive magnetization transformation. The observed anomalies in the local magnetic field distribution are accompanied with enhanced bulk pinning in the CaFe1.94Co0.06As2 single crystals. We propose that our results suggest a coexistence of superconductivity and magnetic correlations.

A fluctuating state and the critical behavior of the depinning transition in driven vortex matter in superconductors

A status report is presented on explorations in 2H-NbS2 which have revealed a new protocol that generates vortex states corresponding to two limiting ends of critical current density (Jlc(H) and Jhc (H)) for a given vortex density deep in the mixed state. An ordered vortex array set in motion with transport current I (Ilc < I < Ihc) explores its other option of non-moving jammed state of higher end Ihc while steadily enhancing the driving force. Attempts to further unjam the re-pinned state unmask (i) a moving fluctuating state and (ii) the diverging (life) times along with an underlying critical behaviour at the threshold limit of de-pinning the disordered-jammed state, from both below as well as above Ihc.