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Mintu Mondal
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ABSTRACT: As a part of my doctoral thesis, I studied the effect of phase fluctuations
in two dimensional (2D) and three dimensional (3D) superconducting films of
conventional s-wave superconductor, NbN, by measuring the electrodynamics
response through magnetic penetration depth at low frequency and the complex
conductivity at microwave frequencies. We show that the phase transition in 2D
or quasi-2D superconductors can be reconciled with the standard
Berezinskii-Kosterlitz-Thouless (BKT) transition when the small vortex core
energy is taken into consideration. In 3D NbN films, we show that in presence
of strong disorder, superconductivity can get destroyed by phase fluctuations
even if Cooper pairs continue to survive in the system. Our study provides a
paradigmatic picture of importance of phase fluctuations in 2D and strongly
disordered 3D superconductors.
03/2013;
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ABSTRACT: The persistence of a soft gap in the density of states above the superconducting transition temperature T, the pseudogap, has long been thought to be a hallmark of unconventional high-temperature superconductors. However, in the last few years this paradigm has been strongly revised by increasing experimental evidence for the emergence of a pseudogap state in strongly-disordered conventional superconductors. Nonetheless, the nature of this state, probed primarily through scanning tunneling spectroscopy (STS) measurements, remains partly elusive. Here we show that the dynamic response above T, obtained from the complex ac conductivity, is highly modified in the pseudogap regime of strongly disordered NbN films. Below the pseudogap temperature, T*, the superfluid stiffness acquires a strong frequency dependence associated with a marked slowing down of critical fluctuations. When translated into the length-scale of fluctuations, our results suggest a scenario of thermal phase fluctuations between superconducting domains in a strongly disordered s-wave superconductor.
Scientific Reports 02/2013; 3:1357.
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ABSTRACT: We measure the frequency dependence of the complex ac conductivity of NbN
films with different levels of disorder in frequency range 0.4-20 GHz. Films
with low disorder exhibit a narrow dynamic fluctuation regime above T_c as
expected for a conventional superconductor. However, for strongly disordered
samples, the fluctuation regime extends well above T_c, with a strongly
frequency-dependent superfluid stiffness which disappears only at a temperature
T* close to the pseudogap temperature obtained from scanning tunneling
measurements. Such a finite-frequency response is associated to a marked
slowing down of the superconducting fluctuations already below T*. The
corresponding large length-scale fluctuations suggest a scenario of thermal
phase fluctuations between superconducting domains in a strongly disordered
s-wave superconductor.
10/2012;
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ABSTRACT: We report directional point contact Andreev reflection (PCAR) measurements on
high-quality single crystals of the non-centrosymmetric superconductor, BiPd.
The PCAR spectra measured on different crystallographic faces of the single
crystal clearly show the presence of multiple superconducting energy gaps. For
point contacts with low resistance, in addition to the superconducting gap
feature, a pronounced zero bias conductance peak is observed. These
observations provide strong evidence of the presence of unconventional order
parameter in this material.
02/2012;
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ABSTRACT: 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.
Physical Review Letters 11/2011; 107(21):217003. · 7.37 Impact Factor
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ABSTRACT: 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 field Bardeen-Cooper-Schrieffer behavior where the
superconducting energy gap vanishes at the temperature where electrical
resistance appears. For stronger disorder (k_{F}l<4) a "pseudogap" state
emerges where a gap in the electronic spectrum persists up to temperatures much
higher than Tc, suggesting that Cooper pairs continue to exist in the system
even after the zero resistance state is destroyed. Finally, very strongly
disordered samples (k_{F}l<1) exhibit a pronounced magnetoresistance peak at
low temperatures, suggesting that localized Cooper pairs continue to survive in
the system even after the global superconducting ground state is completely
destroyed.
07/2011;
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ABSTRACT: 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.
Physical Review Letters 01/2011; 106(4):047001. · 7.37 Impact Factor
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ABSTRACT: 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 strongly suppressed at low temperatures and
evolves towards a linear-T variation at higher temperatures. These observations
provide strong evidence that phase fluctuation leads to the formation of a
pseudogap state in a disordered conventional superconductor.
06/2010;
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Madhavi Chand, Mintu Mondal,
Anand Kamlapure,
Garima Saraswat,
Archana Mishra,
John Jesudasan,
Vivas C. Bagwe,
Sanjeev Kumar,
Vikram Tripathi,
Lara Benfatto,
Pratap Raychaudhuri
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ABSTRACT: We present transport, Hall effect, electronic tunnelling and penetration depth studies in 3D homogeneously disordered epitaxial NbN thin films with disorder ranging from the moderately clean limit (k_Fl~10.12) to the very dirty limit (k_Fl~1.24). The superconducting transition temperature (T_c) decreases from ~17K to less than 350mK with increasing disorder. The T_c and conductivity at the lowest temperature both asymptotically approach zero as k_Fl tends to 1, indicating a coincidence of the metal-insulator transition (MIT) and the superconductor-insulator transition. Close to critical disorder there is spatial inhomogeneity in the superconducting density of states (DOS) and the superconducting state is governed by quantum phase fluctuations. This results in suppression of the superfluid density (n_s) and a pseudogap state where the resistance is no longer zero but the energy gap remains finite Comment: Since the analysis of the data have been refined in a subsequent submission this paper is being withdrawn
06/2010;
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ABSTRACT: 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 k F l→1 the superconducting transition temperature (T c) and normal state conductivity in the limit T0 (σ 0) 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, k F l~1.
Journal of Superconductivity and Novel Magnetism 05/2010; 24(1). · 0.65 Impact Factor
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ABSTRACT: 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. Comment: Modified version with extensive analysis of the KTB transition. One new author added
11/2009;
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Madhavi Chand,
Archana Mishra,
Y. M. Xiong,
Anand Kamlapure,
S. P. Chockalingam,
John Jesudasan,
Vivas Bagwe, Mintu Mondal,
P. W. Adams,
Vikram Tripathi,
Pratap Raychaudhuri
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ABSTRACT: 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. Comment: pdf file with figures
07/2009;
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ABSTRACT: We report directional point contact Andreev reflection (PCAR) measurements on high-quality single crystals of the noncentrosymmetric superconductor BiPd. The PCAR spectra measured on different crystallographic faces of the single crystal clearly show the presence of multiple superconducting energy gaps. For point contacts with low resistance, in addition to the superconducting gap feature, a pronounced zero-bias conductance peak is observed. These observations provide strong evidence for the presence of an unconventional order parameter in this material.
Phys. Rev. B. 86(9).
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Madhavi Chand,
Archana Mishra,
Y. M. Xiong,
Anand Kamlapure,
S. P. Chockalingam,
John Jesudasan,
Vivas Bagwe, Mintu Mondal,
P. W. Adams,
Vikram Tripathi,
Pratap Raychaudhuri
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ABSTRACT: We report the temperature dependence of resistivity (ρ) and Hall coefficient (RH) in the normal state of homogeneously disordered epitaxial NbN thin films with kFl∼1.68–10.12. The superconducting transition temperature (Tc) 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 kFl≤8.13, both dρ/dT and dRH/dT are negative up to 285 K. We observe thatRH(T) varies linearly with ρ(T) for all the films and [RH(T)−RH(285 K)/RH(285 K)]=γ[ρ(T)−ρ(285 K)/ρ(285 K)], where γ=0.68±0.11. Measurements performed on a 2-nm-thick Be film show similar behavior with γ=0.69. This behavior is inconsistent with existing theories of localization and e-e interactions in a disordered metal.
Phys. Rev. B. 80(13).
