arXiv subjects

Shilpam Sharma

Publications and source records attributed to Shilpam Sharma.

At least 19 recordsLinked to original sources

arXiv2026

High-Efficiency Broadband Mid-Infrared Absorption in Asymmetrically Matched Metallic Meanders: Development of Ti40V60 Alloy based LEKIDs for Mid-Far IR

Fast, highly sensitive, and broadband detectors operating in the mid-to-far-infrared (MIR-FIR) spectral region are essential for applications ranging from astrophysics to time-resolved spectroscopy using laboratory-based sources and beamlines at Infrared Free-Electron Laser (IR-FEL) facilities. Conventional Lumped Element Kinetic Inductance Detectors (LEKID) achieve high optical efficiency using resonant quarter wavelength ({\lambda}/4) backshort cavities. However, this cavity-based approach is inherently narrowband and becomes optomechanically challenging at the mid to far infrared (MIR-FIR) wavelengths. Here, we present a completely backshort-free LEKID absorber architecture that achieves broadband absorption exceeding 50%. The meander absorbers were fabricated from a superconducting Ti-V alloy and characterized using IR radiation from the IR-FEL at RRCAT, India. By illuminating through the silicon substrate and matching the meander's sheet resistance to the silicon substrate's wave impedance, the front-side reflection is strongly suppressed. Simultaneously, the sub-wavelength periodicity of the dense meander inhibits propagating transmission into free space via evanescent wave confinement. This combined mechanism drives efficient Ohmic dissipation within the metallic meander, yielding an experimental absorption efficiency ranging from 50% to 90% across the 12.5-30 micrometer wavelength range. The cavity-free design greatly simplifies the fabrication of the focal plane array while providing the broadband response required for next-generation detectors. To assess the suitability of the Ti40V60 alloy as an active superconducting detector material, a test LEKID resonator was also designed, fabricated, and experimentally characterized.

physics.ins-det↗

arXiv2026

Terahertz Electrodynamics and Kinetic Inductance of Disordered Titanium-Vanadium Alloy Thin Films

Disordered superconductors represent an important area in modern condensed matter physics, where superconductivity survives even in the presence of strong electron scattering and localization effects. Understanding how disorder modifies the high-frequency electrodynamic response is not only important from physics point of view, but is also essential for developing next-generation quantum detectors and superconducting devices. In this work, we investigate the terahertz electrodynamics of disordered Ti40V60 alloy thin films using terahertz time-domain spectroscopy (THz-TDS) to understand the relationship between disorder, quasiparticle dynamics, and kinetic inductance. By analysing the complex conductivity, penetration depth and superfluid response, we show that structural disorder can be systematically used to tune the inductive response while maintaining a robust superconducting phase. Unlike conventional nitride superconductors that require tightly controlled reactive growth conditions, Ti40V60 alloys provide a simpler and more adaptable route for tuning the superconducting energy scales directly through the deposition conditions. These findings establish Ti40V60 alloys as a promising material for kinetic inductance detectors and provide useful insights into the electrodynamics of strongly disordered superconductors.

cond-mat.supr-con↗

arXiv2025

Effect of Underlayer Induced Charge Carrier Substitution on the Superconductivity of Ti40V60 Alloy Thin Films

The influence of metallic and semiconducting (V, Al, and Si) under-layer induced charge carrier substitution on the superconducting properties of the Ti40V60 alloy thin films are studied and also compared with a pristine reference film without any under-layer. All the films exhibit metallic behavior in the normal state and a superconducting transition at low temperatures, where the superconducting transition temperature is tunable between 4.77 K and 5.73 K. Hall measurements on the films reveal that the under-layer strongly affects the charge carrier type and density, leading to a correlation between increasing carrier concentration and decreasing TC. The Si under-layer introduces the highest disorder, yet yields the highest TC. This indicates that in the Ti40V60 alloys, a moderate amount of disorder suppresses the spin-fluctuations (inherent to the alloy system) induced pair breaking, thereby enhancing the superconductivity. The comparable TC of the film with V under-layer and the film without under-layer, and the much smaller coherence length (~6.2 nm) as compared to the film thickness (25 nm), confirm the absence of any significant proximity effects. These findings demonstrate that under-layer engineering provides an effective route to tune the superconducting properties of Ti-V alloy thin films.

cond-mat.supr-con↗

arXiv2025

Effect of Deposition Pressure on the Superconductivity of Ti40V60 Alloy Thin Films

The growth and characterization of high quality superconducting thin films is essential for fundamental understanding and also for the use of these films in technological applications. In the present study, Ti40V60 alloy thin films have been deposited using DC magnetron co sputtering of Ti and V at ambient temperatures. The effect of deposition pressure on the film morphology, superconducting and normal state properties has been studied. Measurement of electrical resistance as a function of temperature indicates that up to a certain deposition pressure, the 20 nm thick Ti40V60 films exhibit metallic behavior in the normal state and superconductivity at low temperatures. Beyond a threshold pressure, the films show a negative temperature coefficient of resistance with a residual resistance ratio less than one. Electrical transport measurements in the presence of magnetic field were performed to find the current voltage characteristics of the thin films. Analysis of the I V curves indicates that the Ti40V60 alloy thin films have a large transport critical current density (JC) e.g. 1.475E10 A per m2 in zero magnetic field and 2.657E09 A per m2 in 4 T (both at 4 K). Analysis of the field dependence of flux line pinning force density indicates a combined effect of core delta k surface and core delta k point pinning mechanisms (where k is the Ginzburg Landau parameter). Additionally, spatial variations in the superconducting critical temperature (TC ) across the sample contribute to delta TC pinning. In higher magnetic fields, a contribution from delta l pinning (where l is the electron mean free path) also becomes significant. The findings indicate the potential of Ti40V60 alloy thin film for superconducting device applications like cryogenic radiation detectors.

cond-mat.supr-con↗

arXiv2025

Extending the optical absorption in a lumped element meander structure to far-infrared wavelengths

Superconducting radiation detectors typically exhibit detection and single photon sensitivity limited to the mid infrared wavelength range. Extending their detection capabilities into the far infrared range (>10 um) requires careful selection of substrate materials and detector geometries. The overall detection efficiency is linked to absorption and coupling efficiencies. In this study, the resonator geometry and absorption efficiency were estimated using electromagnetic simulations in CST Microwave Studio for a lumped-element meander structure. Simulations were performed for the 12 to 50 um wavelength range, corresponding to the Infrared Free Electron Laser (IR FEL) at RRCAT, Indore. Absorption in the meander inductor was influenced by the substrate material, thickness, and impedance matching between the detector and incident photon medium. The results indicate that SiO2 and diamond substrates are suitable for developing lumped-element kinetic inductance detectors (LEKID) in this range. Optimized meander geometries on diamond substrates demonstrated absorption efficiencies of up to 95% for narrow bandwidths and over 50% for wide bandwidths. A 30-pixel LEKID structure was fabricated using electron beam lithography on a 500 um SiO2 coated Si substrate, with a 20 nm thick Ti40V60 alloy resonator. Experimental absorption efficiency was determined through transmission and reflection measurements. Results show that in the 14 to 26 um IR-FEL range, the LEKID achieved up to 75% absorption efficiency. These studies demonstrate that the LEKID structure is ideal for detecting far infrared wavelengths above 10 um, with high absorption efficiency.

cond-mat.supr-con↗

arXiv2025

Sputtering Current Driven Growth & Transport Characteristics of Superconducting Ti40V60 Alloy Thin Films

The room temperature growth, characterization, and electrical transport properties of magnetron sputtered superconducting Ti40V60 alloy thin films are presented. The films exhibit low surface roughness and tunable transport properties. As the sputtering current increases, the superconducting transition move towards higher temperatures. Rietveld refinement of two dimensional XRD (2D XRD) pattern reveals the presence of stress in the films, which shifts from tensile to compressive as the sputtering current increases. Additionally, the crystallite size of the films increases with higher sputtering currents. The films exhibit a strong preferential orientation, contributing to their texturing. The crystallite size and texturing are found to be correlated with the superconducting transition temperature (TC) of the films. As the crystallite size and texturing increase, the TC of the films also rises.

cond-mat.supr-con↗

arXiv2023

Ambient Temperature Growth and Superconducting Properties of Ti-V Alloy Thin Films

A study on the optimization of ambient temperature growth and superconducting properties of Ti-V alloy thin films grown on SiO2-coated Si substrate is reported here. These films have been synthesized by co-sputtering of Ti and V targets, and films having different Ti concentrations were deposited to get the optimized critical temperature (TC) of thin films close to the bulk value. The maximum TC of 5.2 K has been obtained in the Ti40V60 composition, which is further increased to 6.2 K when a 10 nm thick Ti underlayer is added below the Ti-V film. GIXRD measurements confirm the formation of Ti-V alloys in the desired crystal structure. The upper critical field (HC2) of the thin films has been estimated with the help of magnetotransport measurements. The utility of Ti-V alloy thin films in superconducting radiation detection applications is ascertained.

cond-mat.supr-con↗

arXiv2022

Temperature and terahertz frequency dependence of the dielectric properties of Fe3O4 thin films deposited on Si substrate

The Fe$_3$O$_4$/Si films are considered to be promising materials for THz spintronic applications due to their high temperature magnetic transition and semiconducting properties. In this article, we present the real part of the dielectric constant ($ε_1$) and the optical conductivity ($σ_1$) of Fe$_3$O$_4$ films of different thicknesses deposited on Si substrate (Fe$_3$O$_4$/Si) in the THz range at temperatures 2- 300 K. Although the magnetization of the films with thickness $\geq$ 115 nm shows a clear change at the Verwey transition temperature T$_v$ = 121 K, their optical properties in the THz frequency range are drastically different from each other. We have shown that $σ_1$ is maximum and $ε_1$ is minimum when the Fe$^{+2}$/Fe$^{+3}$ ratio is equal to 0.54 which is the ratio of Fe+2/Fe+3 for pure Fe$_3$O$_4$. The $σ_1$ reduces and $ε_1$ increases at all temperatures when the Fe$^{+2}$/Fe$^{+3}$ ratio deviates from 0.54. We have shown that a slight change in the Fe$^{+2}$/Fe$^{+3}$ ratio can induce large changes in the optical properties which shall have implications in the application of the Fe3O4 films in THz spintronics.

cond-mat.mtrl-sci↗

arXiv2020

Studies on DC transport and terahertz conductivity of granular molybdenum thin films for microwave radiation detector applications

The morphological, transport and terahertz optical properties of DC magnetron sputtered granular molybdenum thin-films with nano-grains embedded in an amorphous matrix have been studied in the normal and superconducting states. The superconducting transition temperatures of these films are much higher than that of bulk molybdenum. The optical properties of these thin-films have been studied using terahertz time-domain spectroscopy. Their properties have been compared with the existing materials used for the development of radiation detectors. The resistivity of the films lies in >100 micro-Ohm-cm range which is ideal for making highly sensitive radiation detectors. The Hall measurements indicate the presence of holes as the dominant carriers with very small mean free path and mobility. In the normal state, the films are disordered bad metal but they have large superfluid density and stiffness in their superconducting state. The normal state and superconducting properties of the films are very promising for their use in cryogenic radiation detectors for microwave, terahertz, and far IR frequency ranges.

cond-mat.supr-con↗

arXiv2018

Negative substrate bias induced modifications of the physical properties of DC sputter deposited nano-crystalline Mo thin films

Negative bias on substrate during DC sputter deposition of nano-crystalline molybdenum thin films has been utilized to tune their properties for solar cells and the cryogenic radiation detector applications. Films have been deposited on Si substrates under different out of plane negative biases aiming to improve their physical properties such as the sheet resistance, superconducting transition temperature, width of transition and surface roughness. Significant modifications in the electrical and surface morphological properties of nano-crystalline Mo thin films have been reported. The superconducting transition temperature and crystallite size of the films does not show much improvement but the surface roughness and electrical resistivity of the film have been improved by the application of substrate bias.

physics.app-ph↗

arXiv2017

Anomalous transport properties in Nb/Bi1.95Sb0.05Se3 hybrid structure

We report the proximity induced anomalous transport behavior in a Nb Bi1.95Sb0.05Se3 heterostructure. Mechanically Exfoliated single crystal of Bi1.95Sb0.05Se3 topological insulator (TI) is partially covered with a 100 nm thick Niobium superconductor using DC magnetron sputtering by shadow masking technique. The magnetotransport (MR) measurements have been performed simultaneously on the TI sample with and without Nb top layer in the temperature,T, range of 3 to 8 K, and a magnetic field B up to 15 T. MR on TI region shows Subnikov de Haas oscillation at fields greater than 5 T. Anomalous linear change in resistance is observed in the field range of negative 4T to positive 4T at which Nb is superconducting. At 0 T field, the temperature dependence of resistance on the Nb covered region revealed a superconducting transition (TC) at 8.2 K, whereas TI area showed similar TC with the absence of zero resistance states due to the additional resistance from superconductor (SC) TI interface. Interestingly below the TC the R vs T measured on TI showed an enhancement in resistance for positive field and prominent fall in resistance for negative field direction. This indicates the directional dependent scattering of the Cooper pairs on the surface of the TI due to the superposition of spin singlet and triplet states in the superconductor and TI respectively.

cond-mat.mtrl-sci↗

arXiv2017

Defect induced magnetism and super spin glass state in reactive ion beam deposited nano structured AlN thin films

Defect induced magnetism is reported in undoped aluminium nitride thin films deposited using reactive ion beam sputtering of aluminium in nitrogen plasma. The films have been deposited on silicon substrate at different temperatures. Existence of the defects in the films have been verified using room temperature photo-luminescence measurements. Owing to nano crystalline nature of the films, super-magnetic ground states have been observed. A cross-over from super paramagnetic to super spin glass state has been observed as the grain density increases. Detailed magnetisation measurements along with AC susceptibility measurements have been used to determine the ground states in these AlN thin films.

cond-mat.str-el↗

arXiv2017

Enhanced superconductivity and superconductor to insulator transition in nano-crystalline molybdenum thin films

Disorder driven superconductor to insulator transition via intermediate metallic regime is reported in nano-crystalline thin films of molybdenum. The nano-structured thin films have been deposited at room temperature using DC magnetron sputtering at different argon pressures. The grain size has been tuned using deposition pressure as the sole control parameter. A variation of particle sizes, room temperature resistivity and superconducting transition has been studied as a function of deposition pressure. The nano-crystalline molybdenum thin films are found to have large carrier concentration but very low mobility and electronic mean free path. Hall and conductivity measurements have been used to understand the effect of disorder on the carrier density and mobilities. Ioffe-Regel parameter is shown to correlate with the continuous metal-insulator transition in our samples.

cond-mat.supr-con↗

arXiv2016

Effect of Sb substitution on the Topological Surface States in Bi_{2}Se_{3} single crystals: a magneto-transport study

Magneto-transport measurements have been carried out on Bi2-xSbxSe3 (x = 0, 0.05, 0.1, 0.3, 0.5) single crystals at 4.2 K temperature in the magnetic field range of -15 T to 15 T. Shubnikov-de Haas (SdH) oscillations of 2D nature were observed in samples with Sb concentration upto x = 0.3. The analyses of SdH oscillations observed in magneto-resistance data using Lifshitz-Kosevich equation reveal a systematic decrease in the Fermi surface area with Sb substitution. The Berry phase obtained from the Landau Level fan diagram suggests the occurrence of 2D oscillations arising from a Topological Surface State (TSS) for Sb concentrations of x = 0, 0.05 and 0.1; while 2D oscillation seen at higher Sb concentration is attributed to surface 2D electron gas consequent to downward band bending.

cond-mat.mtrl-sci↗

arXiv2016

Optical band gap and associated band-tails in nanocrystalline AlN thin films grown by reactive IBSD at different substrate temperatures

AlN thin films have been grown on Si (100) substrates by reactive ion beam sputter deposition (IBSD) at different substrate temperatures varying from room temperature (RT) to 500oC. Substrate temperature induced microstructural transition from amorphous at RT, nanocrystalline at 300oC to microcrystalline at 400oC has been observed by Transmission Electron Microscopy (TEM). Average surface roughness (Ra) and morphology has been explored by using Atomic Force Microscopy (AFM). UV-VIS spectrophotometry has been employed to probe the substrate temperature induced changes in optical band-gap (Eg) of grown thin films in reflectance mode. It was found that Eg was increased from 5.08 to 5.21 eV as substrate temperature was increased from RT to 500oC. Urbach energy tail (Eu) along with weak absorption tail (WAT) energy (Et) have been estimated to account for the optical disorder which was found to decrease with associated increase in Eg.

cond-mat.mtrl-sci↗

arXiv2015

Studies on proximity effect in Mo/Bi1.95Sb0.05Se3 hybrid structure

Proximity effect in a mechanically exfoliated Bi1.95Sb0.05Se3 topological insulator (TI) single crystal partially covered with disordered superconducting (SC) Mo thin film is reported. Magnetotransport measurement was performed simultaneously across three different regions of the sample viz. SC, TI and SC/TI junction. Resistance measured across SC shows a TC at 4.3 K concomitantly the resistance measurement on TI showed a metallic trend with a steep upturn at TC. Magneto-resistance (MR) measurement on TI exhibit a positive MR with Shubnikov-de Haas (SdH) oscillations, whereas on SC a positive MR superimposed with steep cusp close to TC is observed. Across SC/TI junction both SdH oscillation and the cusp were observed. The frequency of SdH oscillation on SC/TI junction is found to be lesser (~ 125 T) as compared to a reference Bi1.95Sb0.05Se3sample (~ 174 T). Upper critical field HC2 deduced from WHH fit was found to be 17.14 T for a reference Mo film whereas Mo film deposited on TI showed a decreased HC2 of 4.05 T. The coherence length for the former was found to be 4.38 nm and for the latter 9.01 nm. The interaction between the spin-less Cooper pairs in SC with the spin-momentum locked carriers on the surface of TI is believed to cause such changes in transport properties.

cond-mat.mtrl-sci↗

arXiv2015

Magneto-transport behaviour of Bi2Se3-xTex: Role of disorder

Magnetoresistance and Hall resistance measurements have been carried out in fastcooled single crystals of Bi2Se3-xTex (x: 0 to 2) in 4 to 300 K temperature range, under magnetic fields up to 15 T. The variation of resistivity with temperature that points to a metallic behaviour in Bi2Se3, shows an upturn at low temperatures in the Te doped samples. Magnetoresistance measurements in Bi2Se3 show clear signatures of Shubnikov de Hass oscillations that gets suppressed in the Te doped samples. In the Bi2SeTe2 sample, the magneto-resistance shows a cusp like positive magneto-resistance at low magnetic fields and low temperatures, a feature associated with weak antilocalisation (WAL), that crosses over to negative magneto-resistance at higher fields. The qualitatively different magnetotransport behaviour seen in Bi2SeTe2 as compared to Bi2Se3 is rationalised in terms of the disorder, through an estimate of the carrier density, carrier mobility and an analysis in terms of the Ioffe Regel criterion with support from Hall Effect measurements.

cond-mat.mtrl-sci↗

arXiv2015

Magnetization and magneto-transport studies on Fe$_2$VAl$_{1-x}$Si$_x$

We report on magnetoresistance, Hall and magnetization measurements of Fe2VAl1-xSix Heusler compounds for x= 0.005, 0.015, 0.02. There is a systematic change in the temperature coefficient of resistance (TCR) from negative to positive as the Si composition is increased. The Hall co-efficient shows that the carriers are electron like and the carrier density increases with Si concentration. Resistance measurements under magnetic field indicate a decreasing behavior under the application of magnetic field at low temperature region (T< 60 K), suggesting the suppression of scattering by magnetic field. Temperature and field dependent magnetization measurements did not show any significant change apart from the fact that the presence of super paramagnetic (SPM) cluster and its ordering at low temperatures. Arrott plot analysis of magnetization versus field also indicates the magnetic ordering with applied field below 60 K.

cond-mat.mtrl-sci↗