Transition Edge Sensors
Superconducting Transition Edge Sensor: A High Resolution Spectrometer
We are in the process of developing microcalorimeters based on Transition Edge Sensors (TES) for Gamma ray spectroscopy. A transition-edge sensor is a sensitive thermometer made from a superconducting film operated near its transition temperature Tc. These sensors are so sensitive that ideally they can detect even a single photon falling upon them. A TES detector operating in the calorimeter mode is characterized by heat capacity C, conductance G and time constantt. The sensor itself is a multilayer structure of alternating Mo and Cu layers deposited on free hanging Silicon Nitride substrates. For better energy resolution and sensitivity these detectors work at milliKelvin temperatures typically at 100mK. Proximity effect of Superconducting and normal metals is used to get the TC of that order and the samples will be cooled using Adiabatic Demagnetisation Refrigerator capable of reaching 50mK. When a photon falls on absorber made up of high purity Sn, the energy deposited appears as rise in temperature of the absorber and TES. Due to its biasing in the middle of superconducting transition, a small rise in temperature of TES makes its resistance to increase very sharply. This resistance change then translates into current change in a series inductor which changes the magnetic flux coupled to a SQUID. Thus the SQUID finally reads the change in flux which is proportional to the rise in temperature of TES and in turn the energy of photon.
For a video how TES works visit: TES
Recently we have finished first set of measurements on the TC characterization of pure Mo and Mo/Cu multilayers and found that the TC of the pure Mo film is around 1.27K which is higher than the bulk TC of 0.92K. This increase in TC of the thin films is well reported in literature. The TC of multilayers was found to decrease systematically with increasing Cu layer thickness and goes down to 200mK. Graph shows the effect of Cu layer thickness on TC of multi layers.
More experiments are being planned to optimize the TC of Mo films close to its bulk value. Studies like variation of TC with sputtering power and Ar pressure in sputtering chamber are also planned in near future.
Superconductivity in single crystals of doped BaFe2As2
Superconducting Crystals of Iron-Pnictide based compounds
In order that a superconductor be useful for applications; it's properties should be isotropic. To evaluate this it is required to measure properties in single crystals.
We have now grown a few batches of single crystalline doped BaFe2As2 samples. The synthesis is done without any flux. The BaFe2−xCoxAs2 (xnominal = 0.10, 0.15 and 0.20) single crystals were prepared using stoichiometric mixtures of Ba chunks and FeAs and CoAs powders. The samples were heated, at 50 °C/hour, held at 1190 °C for 24 hours, and then slow cooled to 800 °C at a rate of 2 °C/hour, which was followed by a fast rate of cooling of 50 °C/hour upto room temperature. Large number of small shiny crystals, with flat plate like morphology were found in the crucibles. Some of the crystals have sizes of upto 7 mm × 3 mm × 0.4 mm. These plate-like single crystals were characterized by X-ray Diffraction (XRD), energy dispersive X-ray (EDX) spectroscopy, DC resistivity and magneto-resistance (MR) measurements.
The upper critical field was measured uto 12 T for three BaFe2−xCoxAs2 single crystals with estimated Co concentrations of x = 0.082, x = 0.117 and x = 0.143. HC2 versus temperature was measured from temperature dependent resistivity, for various applied magnetic fields, H || ab and H || c. The [dHC2/dT]T=Tc, normalized with the corresponding TC, decreases with increasing Co content, for both directions. The anisotropy γ defined as HC2 || ab / HC2 || c shows a distinct increase with Co content, and its temperature dependence shows a peak close to the TC. Magneto transport measurements, in the spin density wave regions, showed significant negative MR for H || ab and positive MR of H || c in the x = 0.082 sample.
We have also measured the anisotropy in the magnetic critical current density in the above crystals. The Magnetisation versus magnetic field was done upto 16 T. Ac susceptibility measurements was used to evaluate the irreversibility line. It turns out that Hc2 and Hirr lines in these crystals are very close to each much like the Low TC superconductors.
In a similar vein we have synthesised superconducting Ru doped BaFe2As2 samples. Magneto resistance measurements have been employed to evaluate the Hc2 behaviour in these compounds. The magnetisation measurements are used to obtain the critical current density with the help of Bean model. AC susceptibility measurements are employed to obtained the irreversibility line. From in-field resistance vs temperature measurements flux pinning potential for these crystals was also estimated and is found to be very large.
Our earlier samples of BaFe2As2 showed pressure induced superconductivity at 1.5 Gpa. The undoped and K doped specimens were used to study magnetoresistance behaviour.
Study of Ni nano particles
Study of Ni nano particles
In order to increase the critical current of a superconductor, pinning of vortices is important. The magnitude of pinning can be tuned by addition of impurities and recent research reveals that magnetic nano particles can act as effective pinning centres. We therefore intend to make superconductor- magnetic nano-particle composites. As a prerequisite an understanding of the magnetic nano-particles is essential.
The dynamical properties of Ni nano particles with sizes 4 nm and 12 nm have been investigated. Zero field magnetisation (ZFC) and Field cooled (FC) magnetization measurements showed an anomaly at low temperatures. Using results of magnetisation studies under magnetic field and frequency dependent ac susceptibility measurements we argue that the peak like anomaly seen in the ZFC and FC curves arise due to a spin glass like transition of the nano-particle conglomerate. Memory effects, rejuvenation and ageing are seen below the freezing temperature, the nature of which helps classify that the present Ni nano-particle system is a super-spin glass.


Effect of Chemical Substitutions on the Magnetic Properties of GdBaCo2O5.5
Systematic investigations of transport and magnetic property changes observed as a consequence of chemical substitutions at Gd site and Co site in GdBaCo2O5.5 have been carried out. The samples are hole and electron doped, either replacing Gd by Ca/Sr or Ce and Co by Ni or Fe. The magnetization measurements were done using a commercial VSM operating in the field and temperature range of 0 to 16 Tesla and 4 and 300 K. Apart from thermo magnetic hysterisis measured in the 2 K to 300 K temperature range; isothermal magnetization as a function of applied magnetic field were also measured at select temperatures in all samples. Magnetoresistance was measured in all samples in a home built set up with a magnetic field capability of 12 Tesla.
Phase Transitions in FeNiTe
Ni Substitution at Fe site in FeTe
We have studied the layered iron telluride series Fe1.3-xNixTe (x=0, 0.02, 0.04, 0.08, 0.12 and 0.4) to establish the influence of Ni doping on low temperature electronic and magnetic transitions of the Fe1+dTesystem. The iron telluride system Fe1+dTe exhibits a paramagnetic to anti-ferromagnetic transition below 75-80 K, which occurs concomitantly with an insulator to metal transition and a structural transition from tetragonal to monoclinic. The electronic, magnetic and structural transitions are closely coupled and show the signature of a single first order transition in specific heat. Ni substitution causes interesting changes in the magnetic correlations, which affects the phase transitions. [read more]