arXiv subjects

Madhavi Chand

Publications and source records attributed to Madhavi Chand.

22 records · Page 2Linked to original sources

arXiv2010

Penetration depth and tunneling studies in very thin epitaxial NbN films

We investigate evolution of the magnetic penetration depth and superconducting energy gap in epitaxial NbN films using a low frequency mutual inductance technique and tunneling spectroscopy using a low temperature scanning tunneling microscope (STM). The superconducting transition temperature (Tc) for films grown under optimal growth conditions decreases monotonically from 15.87K to 9.16K as the film thickness is decreased from 50nm to 3nm. With decrease in film thickness delta(0) monotonically decreases, whereas lambda(0) monotonically increases. We observe that Tc, lambda(o) and delta(0) are well described by Bardeen-Cooper-Schrieffer (BCS) theory in all films other than the two thinnest ones where we see evidence of the Kosterlitz-Thouless-Berezinski (KTB) transition close to Tc.

cond-mat.supr-con↗

arXiv2009

Temperature dependence of resistivity and Hall-coefficient in a strongly disordered metal: NbN

We report the temperature dependence of resistivity (rho) and Hall coefficient (R_H) in the normal state of homogeneously disordered epitaxial NbN thin films with kFl~3.27-10.12. The superconducting transition temperature (Tc) of these films varies from 8.13K to 16.8K. While our least disordered film displays usual metallic behavior, for all the films with kFl<8.13, both and are negative up to 300K. We observe that R_H(T) varies linearly with rho(T) for all the films. Measurements performed on a 2nm thick Be film shows similar behavior >. This behavior is inconsistent with existing theories of localization and e-e interactions in a disordered metal.

cond-mat.dis-nn↗

arXiv2008

Tunneling studies in a homogeneously disordered s-wave superconductor: NbN

We report the evolution of superconducting properties as a function of disorder in homogeneously disordered epitaxial NbN thin films grown on (100) MgO substrates, studied through a combination of electrical transport, Hall Effect and tunneling measurements. The thickness of all our films are >50nm much larger than the coherence length ~5nm. The effective disorder in different films encompasses a large range, with the Ioffe-Regel parameter varying in the range kFl~1.38-8.77. Tunneling measurements on films with different disorder reveals that for films with large disorder the bulk superconducting transition temperature (Tc) is not associated with a vanishing of the superconducting energy gap, but rather a large broadening of the superconducting density of states. Our results provide strong evidence of the loss of superconductivity via phase-fluctuations in a disordered s-wave superconductor.

cond-mat.supr-con↗

arXiv2008

Superconducting properties and Hall Effect of epitaxial NbN thin films

We have measured the magnetotransport properties and Hall effect of a series of epitaxial NbN films grown on (100) oriented single crystalline MgO substrate under different conditions using reactive magnetron sputtering. Hall effect measurements reveal that the carrier density in NbN thin films is sensitive to the growth condition. The carrier density increases by a factor of 3 between the film with highest normal state resistivity (rho_n~3.83micro-Ohm-m) and lowest transition temperature (Tc~9.99K) and the film with lowest normal state resistivity (rho_n~0.94micro-Ohm-m) and highest transition temperature (Tc~16.11K) while the mobility of carriers does not change significantly except for the most resistive films. Our results show that the Tc of NbN is governed primarily by the carrier density rather than disorder scattering. By varying the carrier concentration during growth we can vary the effective disorder (kF_l) from the moderately clean limit to the dirty limit which makes this system ideal to study the interplay of carrier density and disorder on the superconducting properties of an s-wave superconductor.

cond-mat.supr-con↗