arXiv subjects

Vivas Bagwe

Publications and source records attributed to Vivas Bagwe.

At least 19 recordsLinked to original sources

arXiv2026

Absolute measurement of penetration depth of superconducting thin films using microwave stripline resonators

Superconducting microstrip resonators, which leverage kinetic inductance to probe electrodynamics, are sensitive tools for studying superconducting thin films at microwave frequencies. However, extracting the absolute superconducting penetration depth from these measurements remains challenging. In this work, we present a hybrid method to determine the absolute value of penetration depth over a wide temperature range by combining resonator measurements with finite-element electromagnetic simulations in COMSOL Multiphysics. We demonstrate this approach by extracting the penetration depth of NbN films by fabricating resonators from films of various thicknesses. Furthermore, we extend the technique to materials with lower critical temperatures by employing a flip-film geometry. By placing a sample above a NbN resonator, separated by a thin Mylar dielectric, we create a coupled structure where changes in the sample's penetration depth shift the resonant frequency. This non-destructive method provides a reliable, high-sensitivity platform for characterizing the penetration depth of diverse superconducting thin films.

cond-mat.supr-con↗

arXiv2026

Phase diagram of the vortex state in an amorphous Re6Zr thin film exhibiting inverse melting

In Type II superconductors, the vortex lattice can exhibit "inverse melting," transitioning from a liquid to a crystalline solid as temperature increases. While recently observed via scanning tunneling microscopy in a 20 nm thick amorphous Re6Zr thin film, this work investigates the corresponding d.c. transport and low-frequency magnetic screening responses. By identifying distinct signatures of these transitions and integrating scanning tunneling spectroscopy imaging, we construct a comprehensive vortex-state phase diagram in the magnetic field-temperature parameter space. Furthermore, we demonstrate that inverse melting is thickness-dependent: a 5 nm film retains an inhomogeneous liquid state, while a 50 nm film maintains a crystalline solid structure except near the upper critical field.

cond-mat.supr-con↗

arXiv2025

Non-linear vortex dynamics in the mixed state of superconducting a-MoGe and NbN thin films using low-frequency two-coil mutual inductance technique

We use a two-coil mutual inductance technique to investigate the non-linear response of the vortex lattice of two type-II superconducting thin films, namely a very weakly pinned amorphous Molybdenum Germanium (a-MoGe) and a moderate-to-strongly pinned Niobium Nitride (NbN). We observe a strong dependence of the magnetic shielding response of the superconductors on the ac excitation amplitude in the primary coil of the two-coil setup. The sample response is studied through the evolution of the radial profile of the induced current density with increasing drive amplitude, which gets significantly modified by the effect of flux creep. We develop a computation scheme where we iteratively combine the coupled Maxwell-London equations for the geometry of the two coils and the sample involved, with a model developed by Coffey and Clem, to analyze the non-linear ac response. The central result of this analysis is that the effect of flux creep gives rise to a strong nonlinearity in the electrodynamic response in the vortex state of the superconducting thin films, that extends down to very low amplitudes of ac excitation. Our results also show that at subcritical low frequency ac drives, the vortex viscosity is exponentially larger than the Bardeen-Stephen estimate. We present a simple scheme to obtain the intrinsic value of the pinning force constant, which otherwise gets affected due to flux creep even at very low ac drives and point out some outstanding issues that need to be addressed in future theoretical and experimental studies.

cond-mat.supr-con↗

arXiv2023

Origin of superconductivity in tungsten thin films

The most common allotrope of tungsten, α-W, has a superconducting transition at a temperature of ~11 mK. However, two other forms of tungsten have been reported to have superconducting transitions in the temperature range Tc ~ 2-5 K when synthesized as thin films: Crystalline \b{eta}-W and amorphous W (a-W). In this work we carry out a systematic study of W films synthesized using d.c. magnetron sputtering, using transport, low frequency magnetic shielding response, and transmission electron microscopy. Our results show that while a-W is indeed a bulk superconductor, superconductivity in \b{eta}-W probably originates from an amorphous phase that forms along with the \b{eta}-W phase. Our findings reconcile some of the anomalies earlier reported in \b{eta}-W, such the very small superconducting gap and the decrease of Tc with increase in film thickness.

cond-mat.supr-con↗

arXiv2023

Low frequency electrodynamics in the mixed state of superconducting NbN and a-MoGe films using two-coil mutual inductance technique

We investigate the low-frequency electrodynamics in the vortex state of two type-II superconducting films, namely, a moderate-to-strongly pinned Niobium Nitride (NbN) and a very weakly pinned amorphous Molybdenum Germanium (a-MoGe). We employ a two-coil mutual inductance technique to extract the complex penetration depth, $\tilde{\lambda}$. The sample response is studied through the temperature variation of $\tilde{\lambda}$ in the mixed state, where we employ a model developed by Coffey and Clem (CC model) to extract the different vortex lattice (VL) parameters such as the restoring pinning force constant (Labusch parameter), VL drag coefficient and pinning potential barrier. We observe that a consistent description of the inductive and dissipative part of the response is only possible when we take the viscous drag on the vortices to be several orders of magnitudes larger than viscous drag estimated from the Bardeen-Stephen model.

cond-mat.supr-con↗

arXiv2023

Structure and dynamics of a pinned vortex liquid in superconducting a-Re_xZr (x ~ 6) thin film

We report the formation of a pinned vortex liquid spanning a very large region of the magnetic field-temperature parameter space in a 5 nm thick amorphous superconducting Re_xZr (x~6) (a-ReZr) thin film, using a combination of low-temperature scanning tunnelling spectroscopic (STS) imaging and magnetotransport measurements. The nature of the vortex liquid differs significantly from a regular liquid. Analysing series of STS images captured as a function of time, we observe that the interplay of pinning and intervortex interactions produces a very inhomogeneous state, where some vortices remain static, whereas others move forming a percolating network along which vortices are mobile. With increase in temperature or magnetic field this network becomes denser eventually encompassing all vortices. Our results provide key insight on the nature of a pinned vortex liquid and some of the peculiarities in the transport properties of ultrathin superconducting films.

cond-mat.supr-con↗

arXiv2023

Correlated carrier dynamics in a superconducting van der Waals heterostructure

The study of Berezinskii-Kosterlitz-Thouless transitions in clean, layered two-dimensional superconductors promises to provide insight into a host of novel phenomena like re-entrant vortex-dynamics, underlying unconventional metallic phases, and topological superconductivity. In this letter, we report the study of charge carrier dynamics in a novel 2-dimensional superconducting van der Waals heterostructure comprising monolayer MoS2 and few-layer NbSe2 (15 nm). Using low-frequency conductance fluctuation spectroscopy, we show that the superconducting transition in the system is percolative. We present a phenomenological picture of different phases across the transition correlating with the evaluated noise. The analysis of the higher-order statistics of fluctuation reveals non-Gaussian components around the transition indicative of long-range correlation in the system.

cond-mat.supr-con↗

arXiv2022

Study of vortex dynamics in an a-MoGe thin film using low-frequency two-coil mutual inductance measurements

We extract the vortex lattice parameters using low-frequency two-coil mutual inductance measurements in a 20-nm-thick superconducting a-MoGe thin film. We fit the temperature dependence of ac penetration depth in the mixed state using a model developed by Coffey and Clem and demonstrate a procedure for extracting vortex lattice parameters such as pinning constant, vortex lattice drag coefficient, and pinning potential barrier. We show that the extracted parameters follow the magnetic field variation expected for a weakly pinned 2-dimensional vortex lattice.

cond-mat.supr-con↗

arXiv2021

Transition from three- to two-dimensional Ising superconductivity in few-layer NbSe2 by proximity effect from van der Waals heterostacking

We report the experimental observation of Ising superconductivity in 3-dimensional NbSe2 stacked with single-layer MoS2. The angular dependence of the upper critical magnetic field and the temperature dependence of the upper parallel critical field confirm the appearance of two-dimensional Ising superconductivity in the 3-dimensional NbSe2 with single-layer MoS2 overlay. We show that the superconducting phase has strong Ising spin-orbit correlations which make the holes spin non-degenerate. Our observation of Ising superconductivity in heterostructures of few-layer NbSe2 of thickness ~ 15 nm with single-layer MoS2 raises the interesting prospect of observing topological chiral superconductors with nontrivial Chern numbers in a momentum-space spin-split fermionic system.

cond-mat.supr-con↗

arXiv2021

Evidence of quantum vortex fluid in the mixed state of a very weakly pinned a-MoGe thin film

Quantum fluids refer to a class of systems that remain in fluid state down to absolute zero temperature. In this letter, using a combination of magnetotransport and scanning tunneling spectroscopy down to 300 mK, we show that vortices in a very weakly pinned a-MoGe thin film can form a quantum vortex fluid. Under the application of a magnetic field perpendicular to the plane of the film, the vortex state transforms from a vortex solid to a hexatic vortex fluid and eventually to an isotropic vortex liquid. The fact that the two latter states remain fluid down to absolute zero temperature is evidenced from the electrical resistance which saturates to a finite value at low temperatures. Furthermore, scanning tunneling spectroscopy measurements reveal a soft gap at the center of each vortex, which arises from large zero point fluctuation of vortices.

cond-mat.supr-con↗

arXiv2020

Superconductivity in amorphous RexZr (x ~ 6) thin films

We report the growth, characterization and superconducting properties of a new amorphous superconductor, RexZr ( x ~ 6 ), in thin film form. Films were grown by pulsed laser deposition with the substrate kept at room temperature. Films with thickness larger than 40 nm showed a superconducting transition temperature (Tc) of 5.9 K. Superconducting properties were measured for films with varying thickness from 120 to 3 nm. The normal state resistance scales linearly with inverse of thickness. The transition temperature, critical field, coherence length, penetration depth and superconducting energy gap changes marginally with decreasing film thickness down to 8 nm. Scanning tunneling spectroscopy and penetration depth measurements provide evidence for a single gap strong coupling s-wave superconductor. Magneto-transport properties indicate a rich magnetic field-temperature phase diagram with the possibility of vortex liquid phases existing over a large fraction of the mixed state.

cond-mat.supr-con↗

arXiv2020

Hall Effect for Dirac Electrons in Graphene Exposed to an Abrikosov Flux Lattice

The proposals for realizing exotic particles through coupling of quantum Hall effect to superconductivity involve spatially non-uniform magnetic fields. As a step toward that goal, we study, both theoretically and experimentally, a system of Dirac electrons exposed to an Abrikosov flux lattice. We theoretically find that non-uniform magnetic field causes a carrier-density dependent reduction of the Hall conductivity. Our studies show that this reduction originates from a rather subtle effect: a levitation of the Berry curvature within Landau levels broadened by the non-uniform magnetic field. Experimentally, we measure the magneto-transport in a monolayer graphene-hexagonal boron nitride - niobium diselenide (NbSe$_2$) heterostructure, and find a density-dependent reduction of the Hall resistivity of graphene as the temperature is lowered from above the superconducting critical temperature of NbSe$_2$, when the magnetic field is uniform, to below, where the magnetic field bunches into an Abrikosov flux lattice.

cond-mat.mes-hall↗

arXiv2020

Destruction of superconductivity through phase fluctuations in ultrathin a-MoGe films

We investigate the evolution of superconductivity with decreasing film thickness in ultrathin amorphous MoGe (a-MoGe) films using a combination of sub-Kelvin scanning tunneling spectroscopy, magnetic penetration depth measurements and magneto-transport measurements. We observe that superconductivity is strongly affected by quantum and classical phase fluctuations for thickness below 5 nm. The superfluid density is strongly suppressed by quantum phase fluctuations at low temperatures and evolves towards a linear-T dependence at higher temperatures. This is associated with a rapid decrease in the superconducting transition temperature, Tc, and the emergence of a pronounced pseudogap above Tc. These observations suggest that at strong disorder the destruction of superconductivity follows a Bosonic route where the global superconducting state is destroyed by phase fluctuations even though the pairing amplitude remains finite.

cond-mat.supr-con↗

arXiv2019

Extreme sensitivity of the vortex state in a-MoGe films to radio-frequency electromagnetic perturbation

Recently, detailed real space imaging using scanning tunneling spectroscopy of the vortex lattice in a weakly pinned a-MoGe thin film revealed that the vortex lattice melts in two steps with temperature or magnetic field, going first from a vortex solid to a hexatic vortex fluid and then from a hexatic vortex fluid to an isotropic vortex liquid. In this paper, we show that the resistance in the vortex fluid states is extremely sensitive to radio-frequency electromagnetic perturbation. In the presence of very low-amplitude excitation above few kilohertz, the resistance increases by several orders of magnitude. On the other hand when the superconductor is well shielded from external electromagnetic radiation, the dissipation in the sample is very small and the resistance is below our detection limit.

cond-mat.supr-con↗

arXiv2018

Melting of the vortex lattice through intermediate hexatic fluid in a-MoGe thin film

The hexatic fluid refers to a phase in between a solid and a liquid which has short range positional order but quasi-long range orientational order. In the celebrated theory of Berezinskii, Kosterlitz and Thouless and subsequently refined by Halperin, Nelson and Young, it was predicted that a 2-dimensional hexagonal solid can melt in two steps: first, through a transformation from a solid to a hexatic fluid which retains quasi long range orientational order and then from a hexatic fluid to an isotropic liquid. In this paper, using a combination of real space imaging and transport measurements we show that the 2-dimensional vortex lattice in a-MoGe thin film follows this sequence of melting as the magnetic field is increased. Identifying the signatures of various transitions on the bulk transport properties of the superconductor, we construct a vortex phase diagram for a two dimensional superconductor.

cond-mat.supr-con↗

arXiv2018

Quantum phase transition in few-layer NbSe$_2$ probed through quantized conductance fluctuations

We present the first observation of dynamically modulated quantum phase transition (QPT) between two distinct charge density wave (CDW) phases in 2-dimensional 2H-NbSe$_2$. There is recent spectroscopic evidence for the presence of these two quantum phases, but its evidence in bulk measurements remained elusive. We studied suspended, ultra-thin \nbse devices fabricated on piezoelectric substrates - with tunable flakes thickness, disorder level and strain. We find a surprising evolution of the conductance fluctuation spectra across the CDW temperature: the conductance fluctuates between two precise values, separated by a quantum of conductance. These quantized fluctuations disappear for disordered and on-substrate devices. With the help of mean-field calculations, these observations can be explained as to arise from dynamical phase transition between the two CDW states. To affirm this idea, we vary the lateral strain across the device via piezoelectric medium and map out the phase diagram near the quantum critical point (QCP). The results resolve a long-standing mystery of the anomalously large spectroscopic gap in NbSe$_2$.

cond-mat.str-el↗

arXiv2017

Controlling of ZnO nanostructures by solute concentrationand its effect on growth, structural and optical properties

ZnO nanostructured films were prepared by chemical bath deposition method on glass substrates without any assistance of either microwave or high pressure autoclaves. The effect of solute concentration on the pure wurtzite ZnO nanostructure morphologies is studied. The controlling of the solute concentration help to control nano-structure in the form of nano-needles, and rods.

cond-mat.mtrl-sci↗

arXiv2016

Disorder-induced two-step melting of vortex matter in Co-intercalated NbSe$_2$ single crystals

Disorder induced melting, where the increase in positional entropy created by random pinning sites drives the order-disorder transition in a periodic solid, provides an alternate route to the more conventional thermal melting. Here, using real space imaging of the vortex lattice through scanning tunneling spectroscopy, we show that in the presence of weak pinning, the vortex lattice in a type II superconductor disorders through two distinct topological transitions. Across each transition, we separately identify metastable states formed through superheating of the low temperature state or supercooling of the high temperature state. Comparing crystals with different levels of pinning we conclude that the two-step melting is fundamentally associated with the presence of random pinning which generates topological defects in the ordered vortex lattice.

cond-mat.supr-con↗