arXiv subjects

Gopi Nath Daptary

Publications and source records attributed to Gopi Nath Daptary.

13 recordsLinked to original sources

arXiv2026

Low-frequency noise as a probe of microscopic disorder in CVD-grown graphene

We report a detailed investigation of low-frequency resistance fluctuations (1/f noise) in chemical vapor deposition (CVD) grown graphene. Systematic measurements reveal that the magnitude of 1/f noise in CVD-grown graphene is significantly higher by several orders of magnitude than that typically observed in exfoliated single-crystal graphene. This enhancement is attributed to structural imperfections such as grain boundaries and defect states within the polycrystalline film. Detailed analysis of the temperature dependence of the noise demonstrates that the resistance fluctuations arise from thermally activated dynamics of localized defects. These results provide key insights into the microscopic mechanism of noise in scalable graphene films and highlight the role of defect engineering in optimizing graphene for large-scale electronic applications. Our findings establish low-frequency noise as a sensitive probe of microscopic disorder in CVD graphene, providing a practical pathway for assessing material quality in scalable electronic technologies.

cond-mat.mes-hall↗

arXiv2026

Conductance Oscillations in a Topological Insulator-Disordered Superconductor Hybrid Interface

We report on the observation on proximity-induced superconductivity in the topological insulator BiSbTeSe2 coupled to a disordered superconductor, amorphous indium oxide (a-InO). Resistance temperature measurements reveal superconducting signatures at low temperatures, even when InO is in an insulating state, indicating the persistence of superconducting correlations. Differential conductance spectra reveal nearly periodic oscillations at higher bias, together with a pronounced zero-bias conductance peak. Both effect disappears at high temperature, marking the critical temperature (T*) of the superconducting islands in InO. These results underscore the influence of topological surface states on proximity-induced superconductivity and highlight the role of superconducting fluctuations in disordered superconductor/topological-insulator hybrid interfaces.

cond-mat.mes-hall↗

arXiv2025

Exploring the Electronic Nature of Spinel Oxides: A Review of Their Electron Interactions and Prospects

This review discusses the multifaceted electronic properties of spinel oxides with a particular focus on Lithium Vanadate (LiV2O4), Lithium Titanate (LiTi2O4), and Magnesium Titanate (MgTi2O4). We selected LiTi2O4, LiV2O4, and MgTi2O4 because they serve as quintessential examples of spinel oxides' diverse and intriguing electronic phenomena. LiV2O4 heavy fermion behaviour challenges traditional theories in d-electron systems, LiTi2O4 being the first oxide superconductor provides critical insights into unconventional superconductivity driven by strong electron-phonon interactions, and MgTi2O4 pronounced orbital ordering and metalinsulator transition offers a clear model for exploring electron-lattice coupling. This shows how the inherent structural versatility of the spinel lattice, characterised by its cubic close-packed oxygen network and variable cation distributions, enables a rich interplay of electron-electron correlations, electron-lattice coupling, and orbital degrees of freedom. In LiV2O4, the combination of mixed-valence vanadium ions and a geometrically frustrated pyrochlore lattice gives rise to heavy fermion behaviour, whereas LiTi2O4 exhibits unconventional superconductivity driven by a high density of states at the Fermi level and strong electronphonon interactions. MgTi2O4 undergoes a pronounced metal-insulator transition, where orbital ordering triggers a Peierls-like distortion that stabilises a low-temperature insulating state through Ti-Ti dimerization. How the composition of these compounds affects their properties, based on both theoretical research and experimental findings, is illustrated by this review. It illustrates the promise of Spinels in practical technologies such as energy storage, electrocatalysis, and high-temperature lubrication.

cond-mat.mtrl-sci↗

arXiv2022

Enhancement of Superconductivity upon reduction of carrier density in proximitized graphene

The superconducting transition temperature (Tc) of a single layer graphene coupled to an Indium oxide (InO) film, a low carrier-density superconductor, is found to increase with decreasing carrier density and is largest close to the average charge neutrality point in graphene. Such an effect is very surprising in conventional BCS superconductors. We study this phenomenon both experimentally and theoretically. Our analysis suggests that the InO film induces random electron and hole-doped puddles in the graphene. The Josephson effect across these regions of opposite polarity enhances the Josephson coupling between the superconducting clusters in InO, along with the overall Tc of the bilayer heterostructure. This enhancement is most effective when the chemical potential of the system is tuned between the charge neutrality points of the electron and hole-doped regions.

cond-mat.supr-con↗

arXiv2020

Superconducting Dirac point in proximetized graphene

Two-dimensional (2D) materials, composed of single atomic layers, have attracted vast research interest since the breakthrough discovery of graphene. One major benefit of such systems is the simple ability to tune the chemical potential by back-gating, in-principle enabling to vary the Fermi level through the charge neutrality point, thus tuning between electron and hole doping. For 2D Superconductors, this means that one may potentially achieve the strongly-coupled superconducting regime described by Bose Einstein Condensation physics of small bosonic tightly bound electron pairs. Furthermore, it should be possible to access both electron and hole based superconductivity in a single system. However, in most 2D materials, an insulating gap opens up around the charge neutrality point, thus preventing approach to this regime. Graphene is unique in this sense since it is a true semi-metal in which the un-gapped Dirac point is protected by the symmetries. In this work we show that single layer graphene, in which superconducting pairing is induced by proximity to regions of a low density superconductor, can be tuned from hole to electron superconductivity through the strong coupling regime. We study, both experimentally and theoretically, the vicinity of this "Superconducting Dirac point" and find an unusual situation where reflections at interfaces between normal and superconducting regions within the graphene, suppress the conductance and, at the same time, Andreev reflections maintain a large phase breaking length. In addition, the Fermi level can be adjusted so that the momentum in the normal and superconducting regimes perfectly match giving rise to ideal Andreev reflection processes.

cond-mat.supr-con↗

arXiv2020

Effect of microstructure on the electronic transport properties of epitaxial CaRuO$_3$ thin films

We have carried out extensive comparative studies of the structural and transport properties of CaRuO$_3$ thin films grown under various oxygen pressure. We find that the preferred orientation and surface roughness of the films are strongly affected by the oxygen partial pressure during growth. This in turn affects the electrical and magnetic properties of the films. Films grown under high oxygen pressure have the least surface roughness and show transport characteristics of a good metal down to the lowest temperature measured. On the other hand, films grown under low oxygen pressures have high degree of surface roughness and show signatures of ferromagnetism. We could verify that the low frequency resistance fluctuations (noise) in these films arise due to thermally activated fluctuations of local defects and that the defect density matches with the level of disorder seen in the films through structural characterizations.

cond-mat.str-el↗

arXiv2020

Continuous transition from weakly localized regime to strong localization regime in Nd_{0.7}La_{0.3}NiO_{3} films

We report an investigation of Metal Insulator Transition (MIT) using conductivity and magnetoconductance (MC) measurements down to 0.3 K in Nd_{0.7}La_{0.3}NiO_{3} films grown on crystalline substrates of LaAlO_{3} (LAO), SrTiO_{3} (STO), and NdGaO_{3}(NGO) by pulsed laser deposition. The film grown on LAO experiences a compressive strain and shows metallic behavior with the onset of a weak resistivity upturn below 2 K which is linked to the onset of weak localization contribution. Films grown on STO and NGO show a crossover from a Positive Temperature Coefficient (PTC) resistance regime to Negative Temperature Coefficient (NTC) resistance regime at definite temperatures. We establish that a cross-over from PTC to NTC on cooling does not necessarily constitute a MIT because the extrapolated conductivity at zero temperature σ_{0} though small (<10 S/cm) is finite, signalling the existence of a bad metallic state and absence of an activated transport. The value of σ_{0} for films grown on NGO is reduced by a factor of 40 compared to that for films grown on STO. We show that a combination of certain physical factors makes substituted nickelate (that are known to exhibit first order Mott type transition), undergo a continuous transition as seen in systems undergoing disorder/composition driven Anderson transition. The MC measurement also support the above observation and show that at low temperature there exists a positive MC that arises from the quantum interference which co-exists with a spin-related negative MC that becomes progressively stronger as the electrons approach a strongly localized state in the film grown on NGO.

cond-mat.str-el↗

arXiv2019

Effect of spin-orbit interaction on the vortex dynamics in LaAlO$_3$/SrTiO$_3$ interfaces near the superconducting transition

Controlling spin-orbit interaction and its effect on superconductivity has been a long-standing problem in two-dimensional inversion symmetry broken superconductors. An open challenge is to understand the role of various energy scales in shaping the complex phase diagram in these systems. From a combined experimental and theoretical study of resistance fluctuations and its higher order statistics, we propose a phase diagram for the superconducting phase in the magnetic-field--spin orbit interaction energy plane for the quasi-two dimensional electron gas at the interface of LaAlO$_3$/SrTiO$_3$ heterostructures. The relative variance of resistance fluctuations increases by few orders of magnitude below the spin-orbit field B$_{SO}$ and a non-Gaussian component to the fluctuations arises for fields below the upper critical field B$_{C2}$. Theoretical calculations show that the non-Gaussian noise predominantly arises due to percolative nature of the superconducting transition. We quantify the strength and the relative importance of the spin-orbit interaction energy, Zeeman energy and the pairing potential. Our work highlights the important role played by the interplay between these energy scales in framing the fascinating phases seen in two-dimensional inversion-symmetry-broken superconductors.

cond-mat.mes-hall↗

arXiv2019

Conductivity noise across temperature driven transitions of rare-earth nickelate heterostructures

The metal-insulator transition (MIT) of bulk rare-earth nickelates is accompanied by a simultaneous charge ordering (CO) transition. We have investigated low-frequency resistance fluctuations (noise) across the MIT and magnetic transition of [EuNiO$_3$/LaNiO$_3$] superlattices, where selective suppression of charge ordering has been achieved by mismatching the superlattice periodicity with the periodicity of charge ordering. We have observed that irrespective of the presence/absence of long-range CO, the noise magnitude is enhanced by several orders with strong non-1/$f$ ($f$ = frequency) component when the system undergoes MIT and magnetic transition. The higher order statistics of resistance fluctuations reveal the presence of strong non-Gaussian components in both cases, further indicating inhomogeneous electrical transport arising from the electronic phase separation. Specifically, we find almost three orders of magnitude smaller noise in the insulating phase of the sample without long-range CO compared to the sample with CO. These findings suggest that digital synthesis can be a potential route to implement electronic transitions of complex oxides for device application.

cond-mat.str-el↗

arXiv2018

Effect of multiband transport on charge carrier density fluctuations at the LaAlO$_3$/SrTiO$_3$ interface

Multiband transport in superconductors is interesting both from an academic as well as an application point of view. It has been postulated that interband scattering can significantly affect the carrier dynamics in these materials. In this article we present a detailed study of the electrical transport properties of the high-mobility two-dimensional electron gas residing at the interface of LaAlO$_3$/SrTiO$_3$, a prototypical multi-band superconductor. We show, through careful measurements of the gate dependence of the magnetoresistance and resistance fluctuations at ultra-low temperatures, that transport in the superconducting regime of this system has contributions from two bands which host carriers of very different characters. We identify a gate-voltage tunable Lifshitz transition in the system and show that the resistance fluctuations have strikingly different features on either side of it. At low carrier densities, resistance noise is dominated by number-density fluctuations arising from trapping-detrapping of charge carriers from defects in the underlying SrTiO$_3$ substrate, characteristic of a single-band semiconductor. Above the Lifshitz transition, the noise presumably originates from inter-band scattering. Our work highlights the importance of inter-band scattering processes in determining the transport properties of low-dimensional systems and projects resistance fluctuation spectroscopy as a viable technique for probing the charge carrier dynamics across a Lifshitz transition.

cond-mat.mes-hall↗

arXiv2017

Observation of transient superconductivity at the LaAlO$_3$/SrTiO$_3$ interface

We report the observation of a magnetic field assisted transient superconducting state in the two dimensional electron gas existing at the interface of LaAlO$_3$/SrTiO$_3$ heterostructures. This metastable state depends critically on the density of charge carriers in the system. It appears concomitantly with a Lifshitz transition as a consequence of the interplay between ferromagnetism and superconductivity and the finite relaxation time of the in-plane magnetization. Our results clearly demonstrate the inherently metastable nature of the superconducting state competing with a magnetic order in these systems. The co-existence of superconductivity and ferromagnetism in the conducting electronic layer formed at the interface of insulating oxides has thrown up several intriguing and as yet unanswered questions. An open question in this field is the energetics of the interplay between these two competing orders and the present observation goes a long way in understanding the underlying mechanism.

cond-mat.str-el↗

arXiv2016

Correlated Non-Gaussian phase fluctuations in LaAlO$_3$/SrTiO$_3$ heterointerface

We probe the existence of large correlated non-Gaussian phase fluctuations in the vicinity of the superconducting phase transition in the conducting layer residing at the interface of LaAlO$_3$/SrTiO$_3$ heterostructures. The non-Gaussian fluctuations appear between the Berezinskii-Kosterlitz-Thouless transition temperature $T_{BKT}$ and the mean field transition temperature $T_C$. Subsequent theoretical analysis reveals that non-Gaussianity arises predominantly due to the percolative transition of a Josephson coupled network of superconductors. Our results confirm that the superconductivity in this system is confined to two-dimensions. Our study of the non-Gaussian resistance fluctuation spectrum provides a novel means to explore the BKT-transition in two-dimensional inhomogeneous superconductors.

cond-mat.supr-con↗

arXiv2014

Probing a spin-glass state in SrRuO3 thin films through higher-order statistics of resistance fluctuations

The complex perovskite oxide SrRuO3 shows intriguing transport properties at low temperatures due to the interplay of spin, charge, and orbital degrees of freedom. One of the open questions in this system is regarding the origin and nature of the low-temperature glassy state. In this paper we report on measurements of higher-order statistics of resistance fluctuations performed in epitaxial thin films of SrRuO3 to probe this issue. We observe large low-frequency non-Gaussian resistance fluctuations over a certain temperature range. Our observations are compatible with that of a spin-glass system with properties described by hierarchical dynamics rather than with that of a simple ferromagnet with a large coercivity.

cond-mat.mes-hall↗