Garima Saraswat

Phase Fluctuations in a Strongly Disordered s -Wave NbN Superconductor Close to the Metal-Insulator Transition

We explore the role of phase fluctuations in a three-dimensional s-wave superconductor, NbN, as we approach the critical disorder for destruction of the superconducting state. Close to critical disorder, we observe a finite gap in the electronic spectrum which persists at temperatures well above T(c). The superfluid density is strongly suppressed at low temperatures and evolves towards a linear-T variation at higher temperatures. These observations provide strong evidence that phase fluctuations play a central role in the formation of a pseudogap state in a disordered s-wave superconductor.

Role of the vortex-core energy on the Berezinskii-Kosterlitz-Thouless transition in thin films of NbN.

We analyze the occurrence of the Berezinskii-Kosterlitz-Thouless (BKT) transition in thin films of NbN at various film thickness, by probing the effect of vortex fluctuations on the temperature dependence of the superfluid density below T(BKT) and of the resistivity above T(BKT). By direct comparison between the experimental data and the theory, we show the crucial role played by the vortex-core energy in determining the characteristic signatures of the BKT physics, and we estimate its dependence on the disorder level. Our work provides a paradigmatic example of BKT physics in a quasi-two-dimensional superconductor.

A 350 mK, 9 T scanning tunneling microscope for the study of superconducting thin films on insulating substrates and single crystals

We report the construction and performance of a low temperature, high field scanning tunneling microscope (STM) operating down to 350 mK and in magnetic fields up to 9 T, with thin film deposition and in situ single crystal cleaving capabilities. The main focus lies on the simple design of STM head and a sample holder design that allows us to get spectroscopic data on superconducting thin films grown in situ on insulating substrates. Other design details on sample transport, sample preparation chamber, and vibration isolation schemes are also described. We demonstrate the capability of our instrument through the atomic resolution imaging and spectroscopy on NbSe2 single crystal and spectroscopic maps obtained on homogeneously disordered NbN thin film.

Phase Diagram and Upper Critical Field of Homogeneously Disordered Epitaxial 3-Dimensional NbN Films

We report the evolution of superconducting properties with disorder, in 3-dimensional homogeneously disordered epitaxial NbN thin films. The effective disorder in NbN is controlled from moderately clean limit down to Anderson metal–insulator transition by changing the deposition conditions. We propose a phase diagram for NbN in temperature-disorder plane. With increasing disorder, we observe that as kFl→1 the superconducting transition temperature (Tc) and normal state conductivity in the limit T→0 (σ0) go to zero. The phase diagram shows that in homogeneously disordered 3-D NbN films, the metal–insulator transition and the superconductor–insulator transition occur at a single quantum critical point, kFl∼1.

Two step disordering of the vortex lattice across the peak effect in a weakly pinned Type II superconductor, Co0.0075NbSe2

The vortex lattice in a Type II superconductor provides a versatile model system to investigate the order-disorder transition in a periodic medium in the presence of random pinning. Here, using scanning tunnelling spectroscopy in a weakly pinned Co0.0075NbSe2 single crystal, we show that at low temperatures, the vortex lattice in a 3-dimensional superconductor disorders in two steps across the peak effect. At the onset of the peak effect, the equilibrium Bragg glass transforms into an orientational glass through the proliferation of dislocations. At a higher field, the dislocations dissociate into isolated disclination giving rise to an amorphous vortex glass. We also show the existence of a variety of additional non-equilibrium metastable states, which can be accessed through different thermomagnetic cycling.The vortex lattice in a Type II superconductor provides a versatile model system to investigate the order-disorder transition in a periodic medium in the presence of random pinning. Here, using scanning tunnelling spectroscopy in a weakly pinned Co0.0075NbSe2 single crystal, we show that the vortex lattice in a 3-dimensional superconductor disorders through successive destruction of positional and orientational order, as the magnetic field is increased across the peak effect. At the onset of the peak effect, the equilibrium quasi-long range ordered state transforms into an orientational glass through the proliferation of dislocations. At a higher field, the dislocations dissociate into isolated disclination giving rise to an amorphous vortex glass. We also show the existence of a variety of additional non-equilibrium metastable states, which can be accessed through different thermomagnetic cycling.

Upper Critical Field and Coherence Length of Homogenously Disordered Epitaxial 3‐Dimensional NbN Films

We report the evolution of upper critical field (Hc2) and the Ginzburg‐Landau coherence length (ξGL) with disorder, in 3 dimensional homogeneously disordered epitaxial NbN thin films. The effective disorder in NbN is controlled from moderately clean limit down to Anderson metal‐insulator transition by changing the deposition conditions. We observe that the Ginzburg‐Landau coherence length increases by a factor of 2 as the superconducting critical temperature (Tc) decreases from ∼16 K to ∼2 K.

Disordering of the vortex lattice through successive destruction of positional and orientational order in a weakly pinned Co0.0075NbSe2 single crystal.

The vortex lattice in a Type II superconductor provides a versatile model system to investigate the order-disorder transition in a periodic medium in the presence of random pinning. Here, using scanning tunnelling spectroscopy in a weakly pinned Co0.0075NbSe2 single crystal, we show that the vortex lattice in a 3-dimensional superconductor disorders through successive destruction of positional and orientational order, as the magnetic field is increased across the peak effect. At the onset of the peak effect, the equilibrium quasi-long range ordered state transforms into an orientational glass through the proliferation of dislocations. At a higher field, the dislocations dissociate into isolated disclination giving rise to an amorphous vortex glass. We also show the existence of a variety of additional non-equilibrium metastable states, which can be accessed through different thermomagnetic cycling.

Highly oriented, free-standing, superconducting NbN films growth on chemical vapor deposited graphene

NbN films are grown on chemical vapor deposited graphene using dc magnetron sputtering. The orientation and transition temperature of the deposited films is studied as a function of substrate temperature. A superconducting transition temperature of 14 K is obtained for highly oriented (111) films grown at substrate temperature of 150 °C, which is comparable to epitaxial films grown on MgO and sapphire substrates. These films show a considerably high upper critical field of ∼33 T. In addition, we demonstrate a process for obtaining flexible, free-standing NbN films by delaminating graphene from the substrate using a simple wet etching technique. These free-standing NbN layers can be transferred to any substrate, potentially enabling a range of novel superconducting thin-film applications.

Evolution of Kosterlitz-Thouless-Berezinskii (BKT) Transition in Ultra-Thin NbN Films

We explore the Berezinskii-Kosterlitz-Thouless (BKT) transition in ultrathin NbN superconducting films. We have measured the super fluid density (ns∝1/λ2), normal carrier density and resistivity (ρ) of a set of NbN thin films. Our results show that while the ground state is well described by BCS theory, at elevated temperatures, ultra-thin films show sudden drop in superfluid density associated with the BKT transition close to superconducting transition temperature (Tc). The sudden drop starts at higher superfluid density than expected from 2D XY model, which can be understood by considering the low vortex core energy and slight inhomogeneity in the system. Resistivity data is explained using effective medium theory (EMT) including both Aslamazov-Larkin and KT fluctuations. This is also in good agreement with the corresponding superfluid density.

Anomalies in the electronic structure of a Pauli paramagnet, La2CoSi3 and a Kondo lattice, Ce2CoSi3

We study the electronic structure of a Pauli paramagnetic compound, La2CoSi3 using photoemission spectroscopy and ab initio band structure calculations. Experimental valence band spectra exhibit signature of electron correlation-induced feature around 2.5 eV —the correlation strength among Co 3d electrons is estimated to be close to 3 eV. The Co 2p core level spectra also exhibit correlation-induced satellite features consistent with the scenario in the valence band spectra suggesting importance of conduction electron correlation in addition to the local moment in Kondo lattice systems. The La2CoSi3 valence band spectra could be utilized to extract Ce 4f related spectral features and thus provide a good reference to study Kondo lattice systems in this class of materials. Temperature evolution of various core level spectra is found to be complex, revealing deviations from a typical Fermi-liquid behavior and emergence of a distinct surface-bulk difference in the electronic structure at finite temperature.