John Jesudasan - Academia.edu (original) (raw)

Papers by John Jesudasan

Applied Superconductivity, 1998

Microwave surface resistance and critical current density are measured in Ag-doped YBa2Cu3O7− x t... more Microwave surface resistance and critical current density are measured in Ag-doped YBa2Cu3O7− x thin films as a function of thickness of the film. The microwave surface resistance decreases monotonically as the thickness of the film is increased to an ...

Nature Physics, 2015

The concept of mass-generation via the Higgs mechanism was strongly inspired by earlier works on ... more The concept of mass-generation via the Higgs mechanism was strongly inspired by earlier works on the Meissner-Ochsenfeld effect in superconductors. In quantum field theory, the excitations of longitudinal components of the Higgs field manifest as massive Higgs bosons. The analogous Higgs mode in superconductors has not yet been observed due to its rapid decay into particle-hole pairs. Following recent theories, however, the Higgs mode should decrease below the pairing gap 2∆ and become visible in two-dimensional systems close to the superconductor-insulator transition (SIT). For experimental verification, we measured the complex terahertz transmission and tunneling density of states (DOS) of various thin films of superconducting NbN and InO close to criticality. Comparing both techniques reveals a growing discrepancy between the finite 2∆ and the threshold energy for electromagnetic absorption which vanishes critically towards the SIT. We identify the excess absorption below 2∆ as a strong evidence of the Higgs mode in two dimensional quantum critical superconductors.

Solid State Communications, 2005

We report the structural and transport properties of NdNiO 3 thin films prepared via pulsed laser... more We report the structural and transport properties of NdNiO 3 thin films prepared via pulsed laser deposition over various substrates. The films were well textured and c-axis oriented with good crystalline properties. The electrical resistivity of the films undergoes a metal-insulator transition, depending on the deposition process. Well-defined first order metal-insulator phase transition (T MI ) was observed in the best quality films without high pressure processing. Various growth conditions such as substrate temperature, oxygen pressure and thickness were varied to see their influence on T MI . Deposition temperature was found to have a great impact on the electrical and structural properties of these films. Further the films deposited on LaAlO 3 substrate were found to be highly oriented with uniform grain size as observed from X-ray diffraction and atomic force microscopy, whereas those on Si substrate were polycrystalline, dense and randomly oriented. q

Physica C-superconductivity and Its Applications, 2010

We report vortex matching phenomenon in rectangular antidot array fabricated on epitaxial NbN thi... more We report vortex matching phenomenon in rectangular antidot array fabricated on epitaxial NbN thin film. The antidot array was fabricated using Focussed Ion Beam milling technique. The magneto-transport measurements points to a period doubling transition at higher magnetic field for rectangular lattices. The results are discussed within the light of several models including the multi-vortex model, the matched lattice model

We report robust vortex matching effects in antidot arrays fabricated on thin films of NbN in the... more We report robust vortex matching effects in antidot arrays fabricated on thin films of NbN in the moderately clean limit (kFl � 7). The near absence of hysteresis between field sweep directions indicates a negligible residual pinning in the host thin film. Owing to the very small coherence length of NbN thin films (� � 5 nm), the observations suggests

Solid State Communications, 2006

We report the effect of average A-site cation radius on the structural, magnetic and electrical p... more We report the effect of average A-site cation radius on the structural, magnetic and electrical properties of electron doped manganite La0.7Ce0.3MnO3 thin films. A site cation radius 〈rA〉 is varied systematically by partially replacing La+3 ions by smaller Y+3 ions in the parent compound. The carrier doping, i.e. the fraction of tetravalent Ce atoms at the A-site was kept at

Journal of Physics-condensed Matter, 2007

We report the effect of electric current and magnetic field, separately and in conjugation, on th... more We report the effect of electric current and magnetic field, separately and in conjugation, on the transport behaviour of patterned La0.7Ce0.3MnO3 thin films. In the absence of a magnetic field, a significant reduction in peak resistance (Rp) was found with increasing bias current. This effect is also present when a magnetic field is applied, though the magnitude of the electroresistance

We present scanning tunneling spectroscopy, transport and magneto-transport measurements on disor... more We present scanning tunneling spectroscopy, transport and magneto-transport measurements on disordered epitaxial thin films of NbN as a function of disorder. The studies show a spatial variation of the superconducting energy gap and provide evidence for a pseudogap state where the gap persists well above the superconducting transition temperature (TC) up to a characteristic temperature T* in the strong disorder

We present a phase diagram as a function of disorder in three-dimensional NbN thin films, as the ... more We present a phase diagram as a function of disorder in three-dimensional NbN thin films, as the system enters the critical disorder for the destruction of the superconducting state. The superconducting state is investigated using a combination of magnetotransport and tunneling spectroscopy measurements. Our studies reveal 3 different disorder regimes. At low disorder the (k_{F}l~10-4), the system follows the mean

Physical Review Letters, 2010

We explore the role of phase fluctuations in a 3-dimensional s-wave superconductor, NbN, as we ap... more We explore the role of phase fluctuations in a 3-dimensional s-wave superconductor, NbN, as we approach the critical disorder for the destruction of the superconducting state. Scanning tunneling spectroscopy measurements reveal that in the presence of strong disorder, a finite gap in the electronic spectrum continues to persist at temperatures above Tc. With increase in disorder, the superfluid density is

We report the evolution of upper critical field and the Ginzburg‐Landau coherence length with dis... more We report the evolution of upper critical field and the Ginzburg‐Landau coherence length 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 decreases from∼ 16 K to∼ 2 K.

Journal of Physics D-applied Physics, 2006

We report the structural and transport properties of RNiO3 (R: Nd, Pr) thin films prepared by pul... more We report the structural and transport properties of RNiO3 (R: Nd, Pr) thin films prepared by pulsed laser deposition over various substrates without high pressure annealing. An excimer laser KrF with wavelength of 248 nm was used for deposition. Various substrates such as single-crystal SrTiO3 (100), LaAlO3 (100) and Si (100) wafer are used for deposition of films to understand

Superconductor Science and Technology, 2008

The effect of 200 MeV Ag ion irradiation on the superconducting and normal state properties of th... more The effect of 200 MeV Ag ion irradiation on the superconducting and normal state properties of the high-T c superconductor y O Cu YBa  7 3 2 (YBCO) is studied by in-situ temperature dependent resistance measurement. We show that irradiating YBCO thin films (~150 nm) at low temperature result into a softly defected region of about 85 nm radius due to swift heavy ion induced secondary electrons around the highly amorphized latent tracks of ~ 5 nm radius. This leads to decrease of T c at fluences three orders of magnitude less than the threshold fluence, where overlapping of tracks block supercurrent path. Due to their low energy (4.1 keV for 200 MeV Ag ion), the secondary electrons can induce point defects by inelastic process rather than by direct elastic collision.

Physical Review B, 2009

We report the evolution of superconducting properties as a function of disorder in homogeneously ... more 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 ξ 0~5 nm. The effective disorder in different films encompasses a large range, with the Ioffe-Regel parameter varying in the range k F l~1. 38-8.77. Tunneling measurements on films with different disorder reveals that for films with large disorder the bulk superconducting transition temperature (T c ) 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.

Physical Review B, 2009

We report the temperature dependence of resistivity ͑͒ and Hall coefficient ͑R H ͒ in the normal ... more We report the temperature dependence of resistivity ͑͒ and Hall coefficient ͑R H ͒ in the normal state of homogeneously disordered epitaxial NbN thin films with k F l ϳ 1.68-10.12. The superconducting transition temperature ͑T c ͒ of these films varies from 2.7 to 16.8 K. While our least disordered film displays usual metallic behavior, for all the films with k F l Յ 8.13, both d dT and dR H dT are negative up to 285 K. We observe that R H ͑T͒ varies linearly with ͑T͒ for all the films and ͓ R H ͑T͒−R H ͑285 K͒ R H ͑285 K͒ ͔ = ␥͓ ͑T͒−͑285 K͒ ͑285 K͒ ͔, where ␥ = 0.68Ϯ 0.11. Measurements performed on a 2-nm-thick Be film show similar behavior with ␥ = 0.69. This behavior is inconsistent with existing theories of localization and e-e interactions in a disordered metal.

Physical Review B, 2008

We have measured the magnetotransport properties and Hall effect of a series of epitaxial NbN fil... more 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 (ρ n~3 .83µΩ-m) and lowest transition temperature (T c~9 .99K) and the film with lowest normal state resistivity (ρ n~0 .94µΩm) and highest transition temperature (T c~1 6.11K) while the mobility of carriers does not change significantly. Our results show that the T c 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 (k F 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.

Physica C: Superconductivity, 2010

In superconducting thin films, engineered lattice of antidots (holes) act as an array of columnar... more In superconducting thin films, engineered lattice of antidots (holes) act as an array of columnar pinning sites for the vortices and thus lead to vortex matching phenomena at commensurate fields guided by the lattice spacing. The strength and nature of vortex pinning is determined by the geometrical characteristics of the antidot lattice (such as the lattice spacing a 0 , antidot diameter d, lattice symmetry, orientation, etc) along with the characteristic length scales of the superconducting thin films, viz., the coherence length (ξ) and the penetration depth (λ). There are at least two competing scenarios: (i) multiple vortices sit on each of the antidots at a higher matching period, and, (ii) there is nucleation of vortices at the interstitial sites at higher matching periods. Furthermore it is also possible for the nucleated interstitial vortices to reorder under suitable conditions. We present our experimental results on NbN antidot arrays in the light of the above scenarios.

Physica C: Superconductivity, 2010

Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms, 2010

ABSTRACT In situ temperature dependent resistivity, ρ(T) study on c-axis oriented YBa2Cu3O7−y thi... more ABSTRACT In situ temperature dependent resistivity, ρ(T) study on c-axis oriented YBa2Cu3O7−y thin films irradiated with 200 MeV Ag ions at 79 K is shown to induce point defects in addition to amorphous ion tracks. Annealing characteristics of these defects indicate that the point defects are basically oxygen disorder selectively created in the CuO basal planes of YBa2Cu3O7−y structure by secondary electrons emanating from the path of 200 MeV Ag ions. These electrons are shown to create defects by inelastic interaction process. Contrary to the general expectation, we show that the superconducting transition temperature, Tc is suppressed at a rate two orders of magnitude faster at extremely low fluences where ion tracks are far apart from each other than at high fluences where tracks tend to overlap. The transition width on the other hand remains unaffected while resistivity shows a large increase at high fluences. At high fluences, a two-step superconducting transition emerged, which indicate the evolution of two types of superconducting regions with distinctly different Tcs.

Applied Superconductivity, 1998

Nature Physics, 2015

Solid State Communications, 2005

Physica C-superconductivity and Its Applications, 2010

Solid State Communications, 2006

Journal of Physics-condensed Matter, 2007

Physical Review Letters, 2010

Journal of Physics D-applied Physics, 2006

Superconductor Science and Technology, 2008

Physical Review B, 2009

Physical Review B, 2008

Physica C: Superconductivity, 2010

Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms, 2010