arXiv subjects

Pratap Raychaudhuri

Publications and source records attributed to Pratap Raychaudhuri.

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

Cavity based sensing of antiferromagnetic canting and nonzero-momentum spin waves in a van der Waals cavity-magnon-polariton system

Cavity-magnon-polaritons are hybrid excitations from the interaction between cavity photons and magnons, the quanta of collective spin oscillations. Along with the tunability of the magnon-photon coupling strength, fast information transfer and conversion speed are desired in hybrid devices. This can be achieved utilizing the propagating nature of spin waves with non-zero momentum for their ultra-fast time dynamics and reduced ohmic dissipation. Antiferromagnets are particularly interesting as hosts for magnons since stray-field interactions are minimized, and they support multiple modes with distinctive magnetic-field behavior across the phase diagram. CrCl3 is a van der Waals antiferromagnet having a strong easy-plane anisotropy and a weak in-plane easy-axis anisotropy. Despite some magnetic resonance studies, the impact of magnetic reorientation of spins in CrCl3 on cavity-magnon-polariton interaction strength as a function of magnetic field remains largely unexplored. In this study, we investigate the coupling between magnons in CrCl3 and photons in a coplanar waveguide resonator as a function of magnetic field. In particular, we find that the magnon-photon coupling strength varies nonmonotonically and distinctly with the magnetic field for both acoustic and optical magnons, enabling tuning of the coupling strength with an external magnetic field as a knob. We find the signature of spin-flop transition in two harmonics of the cavity due to a stronger dispersive coupling between optical magnons and cavity photons at lower fields. Additionally, we find standing modes formed by spin waves with nonzero momentum associated with the two hybrid magnons when the external field is applied at an angle with the crystal plane. These modes do not undergo substantial coupling with the cavity mode unlike the antiferromagnetic modes and can be used as low-loss propagation channels in hybrid devices.

cond-mat.mes-hall↗

arXiv2025

Exploring unconventional superconductivity in PdTe via Point Contact Spectroscopy

Palladium Telluride (PdTe), a non-layered intermetallic crystalline compound, has captured attention for its unique superconducting properties and strong spin-orbit coupling. In this work, we investigate the superconducting state of PdTe using point-contact Andreev reflection (PCAR) spectroscopy. The experimental data are analyzed using the Blonder-Tinkham-Klapwijk (BTK) model for s, p and d wave symmetries. Our results reveal clear evidence of unconventional superconductivity. The superconducting gap showing features consistent with either p-wave or d-wave pairing symmetries but cannot be fitted with s-wave symmetry. The observed anisotropic gap structure and deviations from conventional BCS behaviour highlight the complex nature of the pairing interactions in PdTe. These findings provide strong evidence of unconventional pairing symmetry in this material.

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↗

arXiv2024

Linear non-saturating magnetoresistance and superconductivity in epitaxial thin films of YbSb$_{2}$

Rare-earth diantimonides display intriguing ground states often associated with structural order, which can be manipulated in thin film geometries. In this study, we report epitaxial synthesis of one such compound, YbSb$_{2}$, on III-V substrates using molecular-beam epitaxy. The synthesized thin films exhibit large, non-saturating, linear magnetoresistance across a wide magnetic field range. Additionally, they demonstrate superconducting properties, with a critical temperature of $\approx$ 1.025 K and a critical field of $\approx$ 83.85 Oe, consistent with the reports in bulk single crystals. While YbSb$_{2}$ has been classified as a Type-I superconductor in its bulk form, our findings provide evidence of a mixed state in the epitaxial thin films. This work paves the way for controlling the electronic ground state in this class of materials through thin film engineering.

cond-mat.supr-con↗

arXiv2024

A compact inertial nano-positioner operating at cryogenic temperatures

Nano-positioning plays a very important role in applications such as scanning probe microscopy and optics. We report the development of a compact inertial nanopositioner along with fully computer interfaced electronics operating down to 2 K, and its use in our fully automated needle-anvil type Point Contact Andreev Reflection (PCAR) apparatus. We also present the fully automated operational procedures using LabVIEW interface with our home-built electronics. The point contact spectroscopy probe has been successfully used to perform PCAR measurements on elemental superconductors at low temperatures. The small footprint of our nano-positioner makes it ideally suited for incorporation in low temperature scanning probe microscopes and makes this design versatile for various research and industrial purposes.

physics.ins-det↗

arXiv2024

Inverse melting and re-entrant transformations of the vortex lattice in amorphous Re6Zr thin film

Melting of a solid is one of the most ubiquitous phenomena observed in nature. Most solids, when heated, melt from a crystalline state to an isotropic liquid at a characteristic temperature. There are however situations where increase in temperature can induce a transition to a more ordered state. Broadly termed as "inverse melting", experimental realisations of such situations are rare. Here, we report such a phenomenon in the 2-dimensional vortex liquid that forms in a moderately pinned amorphous Re6Zr (a-ReZr) thin film, from direct imaging of the vortex lattice using a scanning tunnelling microscope. At low temperature and magnetic fields, we find that the vortices form a "pinned liquid" , that is characterised by a low mobility of the vortices and vortex density that is spatially inhomogeneous. As the temperature or magnetic field is increased the vortices become more ordered, eventually forming a nearly perfectly ordered vortex lattice. Above this temperature/magnetic field, the ordered vortex lattice melts again into a vortex liquid. This re-entrant transformation from a liquid to solid-like state and then back to a liquid also leaves distinct signature in the magnetotransport properties of the superconductor.

cond-mat.supr-con↗

arXiv2023

Origin of superconductivity in tungsten thin films

The most common allotrope of tungsten, {\alpha}-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

Revealing novel aspects of light-matter coupling in terahertz two-dimensional coherent spectroscopy: the case of the amplitude mode in superconductors

Recently developed terahertz (THz) two-dimensional coherent spectroscopy (2DCS) is a powerful technique to obtain materials information in a fashion qualitatively different from other spectroscopies. Here, we utilized THz 2DCS to investigate the THz nonlinear response of conventional superconductor NbN. Using broad-band THz pulses as light sources, we observed a third-order nonlinear signal whose spectral components are peaked at twice the superconducting gap energy $2\Delta$. With narrow-band THz pulses, a THz nonlinear signal was identified at the driving frequency $\Omega$ and exhibited a resonant enhancement at temperature when $\Omega = 2\Delta$. General theoretical considerations show that such a resonance can only arise from a disorder-activated paramagnetic coupling between the light and the electronic current. This proves that the nonlinear THz response can access processes distinct from the diamagnetic Raman-like density fluctuations, which are believed to dominate the nonlinear response at optical frequencies in metals. Our numerical simulations reveal that even for a small amount of disorder, the $\Omega=2\Delta$ resonance is dominated by the superconducting amplitude mode over the entire investigated disorder range. This is in contrast to other resonances, whose amplitude-mode contribution depends on disorder. Our findings demonstrate the unique ability of THz 2DCS to explore collective excitations inaccessible in other spectroscopies.

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

Why shot noise does not generally detect pairing in mesoscopic superconducting tunnel junctions

The shot noise in tunneling experiments reflects the Poissonian nature of the tunneling process. The shot noise power is proportional to both the magnitude of the current and the effective charge of the carrier. Shot noise spectroscopy thus enables, in principle, to determine the effective charge q of the charge carriers that tunnel. This can be used to detect electron pairing in superconductors: in the normal state, the noise corresponds to single electron tunneling (q = 1e), while in the paired state, the noise corresponds to q = 2e. Here, we use a newly developed amplifier to reveal that in typical mesoscopic superconducting junctions, the shot noise does not reflect the signatures of pairing and instead stays at a level corresponding to q = 1e. We show that transparency can control the shot noise and this q = 1e is due to the large number of tunneling channels with each having very low transparency. Our results indicate that in typical mesoscopic superconducting junctions one should expect q = 1e noise, and lead to design guidelines for junctions that allow the detection of electron pairing.

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↗

arXiv2022

Spin-Polarisation measurement using NbN-Insulator-Ferromagnet Tunnel Junction with oxidized barrier

We report a two-step process for the fabrication of superconductor-insulator-normal metal tunnel junctions using NbN as the superconducting electrode and its surface oxide as the insulating tunnel barrier, and investigate its efficacy in measuring spin-polarisation of ferromagnets using the Meservey-Tedrow technique. We observe that for NbN film thickness below 10 nm, under the application of parallel magnetic field, the superconducting density of states show clear "Zeeman" splitting into spin-up and spin-down sub-bands. Tunnelling measurements on devices where ferromagnetic Co is used as the normal electrode shows that these devices can be used to reliably measure spin polarisation of a ferromagnet at temperatures up to 1.6 K. The simplicity of our fabrication process, and the ability to perform spin-polarisation measurements at 4He temperatures makes NbN a very attractive candidate for spin polarisation measurements.

cond-mat.supr-con↗

arXiv2022

Anomalous high-temperature THz nonlinearity in superconductors near the metal-insulator transition

The interplay of strong disorder and superconductivity is a topic of long-term interest in condensed matter physics. Here we explore the nonlinear THz response of superconducting NbN films close to the 3D metal-insulator transition. For the least disordered samples, the magnitude of the nonlinear $\chi^{(3)}$ response follows the temperature dependence of the superfluid density as expected. In contrast, for high disorder samples near the metal-insulator transition the $\chi^{(3)}$ nonlinearity persists to temperatures as high as even 4 times the $T_c$ of the cleanest sample. We discuss the possible origins of this remarkably large nonlinearity, including the possibility that it arises in an enhancement of the temperature scales of superconductivity close to localization. Our work highlights the importance of finite frequency nonlinear THz experiments in detecting superconducting correlations even into regions where long-range ordered superconductivity does not persist.

cond-mat.supr-con↗

arXiv2021

Transport signatures of fragile-glass dynamics in the melting of the two-dimensional vortex lattice

In two-dimensional (2D) systems, the melting from a solid to an isotropic liquid can occur via an intermediate phase that retains orientational order. However, in 2D superconducting vortex lattices, the effect of orientational correlations on transport, and their interplay with disorder remain open questions. Here we study a 2D weakly pinned vortex system in amorphous MoGe films over an extensive range of temperatures ($\bm{T}$) and perpendicular magnetic fields ($\bm{H}$) using linear and nonlinear transport measurements. We find that, at low fields, the resistivity obeys the Vogel-Fulcher-Tamman (VFT) form, $\bm{\rho(T)\propto\exp[-{W}(H)/(T-T_0(H))]}$, characteristic of fragile glasses. As $\bm{H}$ increases, $\bm{T_0(H)}$ is suppressed to zero, and a standard vortex liquid behavior consistent with a $\bm{T=0}$ superconducting transition is observed. Our findings, supported also by simulations, suggest that the presence of orientational correlations gives rise to a heterogeneous dynamics responsible for the VFT behavior. The effects of quenched disorder become dominant at high $\bm{H}$, where a crossover to a strong-glass behavior is observed. This is a new insight into the dynamics of melting in 2D systems with competing orders.

cond-mat.supr-con↗