arXiv subjects

Anjan K. Gupta

Publications and source records attributed to Anjan K. Gupta.

At least 19 recordsLinked to original sources

arXiv2025

Superinsulating behavior in granular Pb film on gated few-layer MoS$_2$

We report a super-insulating behavior, in a device having granular Pb film on back-gated few-layer $\mathrm{MoS_2}$, below an onset temperature same as the critical temperature $T_{\rm C}\approx7$ K of bulk Pb. Below $T_{\rm C}$, the current-voltage characteristics exhibit a threshold voltage marking a crossover between the low-bias insulating and the high-bias normal-resistance states, consistent with the known super-insulating state behavior. A temperature dependent critical magnetic field is also found above which the insulating behavior is suppressed. The threshold voltage is found to vary with the gate-voltage but the critical field remains unchanged. With reducing temperature, the sample conductance saturates to a finite value, which depends on magnetic field and gate-voltage. This saturation behavior is found to be inconsistent with the charge-BKT and the thermal activation models but it can be fitted well to a combination of thermal activation and quantum fluctuations.

cond-mat.supr-con↗

arXiv2024

Blocking transition of SrTiO$_3$ surface dipoles in MoS$_2$/SrTiO$_3$ field effect transistors with counterclockwise hysteresis

A counterclockwise hysteresis is observed at room temperature in the transfer characteristics of SrTiO$_3$ (STO) gated MoS$_2$ field effect transistor (FET) and attributed to bistable dipoles on the STO surface. The hysteresis is expectedly found to increase with increasing range, as well as decreasing rate, of the gate-voltage sweep. The hysteresis peaks near 350 K while the transconductance rises with rising temperature above the room temperature. This is attributed to a blocking transition arising from an interplay of thermal energy and an energy-barrier that separates the two dipole states. The dipoles are discussed in terms of the displacement of the puckered oxygen ions at the STO surface. Finally, the blocking enables a control on the threshold gate-voltage of the FET over a wide range at low temperature which demonstrates it as a heat assisted memory device.

cond-mat.mes-hall↗

arXiv2024

Superconductor to metal quantum phase transition with magnetic field in Josephson coupled lead islands on Graphene

Superconductor-to-metal transition with magnetic field and gate-voltage is studied in a Josephson junction array comprising of randomly distributed lead islands on exfoliated single-layer graphene with a back-gate. The low magnetic-field superconductivity onset temperature is fitted to the Werthamer-Helfand-Hohenberg theory to model the temperature dependence of the upper critical field. The magnetoresistance in the intermediate temperature and field regime is described using thermally activated flux flow dictated by field dependent activation barrier. The barrier also depends on the gate voltage which dictates the inter-island Josephson coupling and disorder. The magnetoresistance near the upper critical field at low temperatures shows signatures of a gate dependent continuous quantum phase transition between superconductor and metal. The finite size scaling analysis shows that this transition belongs to the $(2+1)$D-XY universality class without disorder.

cond-mat.supr-con↗

arXiv2024

Enhanced performance of MoS$_2$/SiO$_2$ field-effect transistors by hexamethyldisilazane (HMDS) encapsulation

Scalable methods for improving the performance and stability of a field-effect transistor (FET) based on two-dimensional materials are crucial for its real applications. A scalable method of encapsulating the exfoliated MoS$ _{2} $ on SiO$ _{2} $/Si substrate by hexamethyldisilazane (HMDS) is explored here for reducing the influence of interface traps and ambient contaminants. This leads to twenty-five times reduction in trap density, three times decrease in subthreshold swing, three times increase in the peak field-effect mobility and a drastic reduction in hysteresis. This performance remains nearly the same after several weeks of ambient exposure of the device. This is attributed to the superhydrophobic nature of HMDS and the SiO$_2$ surface hydrophobization by the formation of covalent bonds between the methyl groups of HMDS and silanol groups of SiO$_{2}$.

cond-mat.mtrl-sci↗

arXiv2024

Gate-tunable crossover between vortex-interaction and pinning dominated regimes in Josephson-coupled Lead-islands on graphene

Resistance of a Josephson junction array consisting of randomly distributed lead (Pb) islands on exfoliated single layer graphene shows a broad superconducting transition to zero with an onset temperature close to the transition temperature of bulk Pb. The transition evolves with the back-gate voltage and exhibits two peaks in temperature derivative of resistance. The region above the lower temperature peak is found to be well described by Berezinskii-Kosterlitz-Thouless model of thermal unbinding of vortex anti-vortex pairs while that below this peak fits well with the Ambegaokar- Halperin model of thermally-activated phase slip or vortex motion in Josephson junction arrays. Thus a gate-tunable crossover between interaction and pinning dominated vortices is inferred as the Josephson energy, dictating the pinning potential magnitude, increases with cooling while the effective screening length, dictating the range of inter-vortex interaction, reduces.

cond-mat.supr-con↗

arXiv2023

Blocking transition of interface traps in MoS$_2$-on-SiO$_2$ FETs

Electrical conductivity with gate-sweep in a few layer MoS$_2$-on-SiO$_2$ field-effect-transistor shows an abrupt reduction in hysteresis when cooled. The hysteresis and time dependent conductivity of the MoS$_2$ channel are modeled using the dynamics of interface traps' occupancy. The reduction in hysteresis is found to be steepest at a blocking temperature near 225 K. This is attributed to the interplay between thermal and barrier energies and fitted using a distribution of the latter. Further, the charge stored in the blocked traps is programmed at low temperatures by cooling under suitable gate voltage. Thus the threshold gate-voltage in nearly non-hysteretic devices at 80 K temperature is reversibly controlled over a wide range.

cond-mat.mes-hall↗

arXiv2022

Acoustic analog to multiple avoided-crossings in two coupled acoustic cavities

A cylindrical pipe with closed ends and with a partition in-between exhibits acoustic modes in the two, thus formed, one-dimensional cavities at certain frequencies. A partial transmission through the partition leads to interaction between the two cavities' modes and to multiple avoided crossings between modes' frequencies as a function of the partition position. This is analogous to a quantum system that has two multi-level and interacting sub-systems and thus exhibits multiple avoided crossings. Such an acoustic analog is realized and studied by measuring the sound transmission as a function of frequency through a pipe with a partially transmitting and movable partition. An excellent agreement is obtained between the experimental results and a simple model based on sound wave transmission and reflection at different interfaces.

physics.ed-ph↗

arXiv2022

Magnetization Reversal Across Multiple Serial Barriers in a Single Fe$_3$O$_4$ Nanoparticle

Depinning of nanoscale magnetic textures, such as domain walls, vortices and skyrmions, is of paramount importance for magnetic storage and information processing. We measure time-resolved magnetic switching statistics of an individual, non-single-domain Fe$_3$O$_4$ nanoparticle using a micrometer-scale superconducting quantum interference device. Surprisingly, a strong narrowing of the waiting-time distributions before reaching the final state is observed as compared to the exponential distribution expected for a single barrier. The magnetization reversal across the nanostructure is thus shown to result from multiple serial barriers in the minimum energy pathway.

cond-mat.mes-hall↗

arXiv2021

Stochastic resonance in thermally bistable Josephson weak-links and micro-SQUIDs

Constriction-based Josephson weak-links display a thermal bi-stability between two states exhibiting zero and finite voltages. This manifests in experiments either as hysteresis in weak-links current voltage characteristics or as random telegraphic signal in voltage. In the latter case, a noise-driven amplification of a sinusoidal excitation of the device is observed, at frequencies matching the characteristic switching frequency in telegraphic signal, a phenomenon known as stochastic resonance. The observed behavior is understood using a two-state model of stochastic resonance and is exploited to illustrate an enhanced signal-to-noise-ratio in a micro-SQUID as a magnetic field sensor.

cond-mat.supr-con↗

arXiv2019

Elimination of thermal bistability in superconducting weak links by an inductive shunt

The quantum phase-coherent behavior of superconducting weak links (WL) is often quenched in the finite voltage state, due to the heat dissipation and related thermal hysteresis. The latter can be reduced by improving heat evacuation and/or by lowering the critical current, so that a phase-dynamic regime is obtained, albeit over a narrow bias-current and temperature range. Here we demonstrate that an inductive shunt with well-chosen parameters introduces unexpected nonlinear dynamics that destabilize an otherwise stable fixed point in the dissipative branch. This leads to a nonhysteretic behavior with large voltage oscillations in intrinsically hysteretic WL-based micron-size superconducting quantum interference devices. A dynamic thermal model quantitatively describes our observations and further allows us to elaborate on the optimal shunting conditions.

cond-mat.supr-con↗

arXiv2018

Josephson Coupling in the Dissipative State of a Thermally Hysteretic $μ$-SQUID

Micron-sized superconducting interference devices ($μ$-SQUIDs) based on constrictions optimized for minimizing thermal runaway are shown to exhibit voltage oscillations with applied magnetic flux despite their hysteretic behavior. We explain this remarkable feature by a significant supercurrent contribution surviving deep into the resistive state, due to efficient heat evacuation. A resistively shunted junction model, complemented by a thermal balance determining the amplitude of the critical current, describes well all experimental observations, including the flux modulation of the (dynamic) retrapping current and voltage by introducing a single dimensionless parameter. Thus hysteretic $μ$-SQUIDs can be operated in the voltage read-out mode with a faster response. The quantitative modeling of this regime incorporating both heating and phase dynamics paves the way for further optimization of $μ$-SQUIDs for nano-magnetism.

cond-mat.supr-con↗

arXiv2016

Inhomogeneous screening of gate electric field by interface states in graphene FETs

The electronic states at graphene-SiO$_2$ interface and their inhomogeneity was investigated using the back-gate-voltage dependence of local tunnel spectra acquired with a scanning tunneling microscope. The conductance spectra show two, or occasionally three, minima that evolve along the bias-voltage axis with the back gate voltage. This evolution is modeled using tip-gating and interface states. The energy dependent interface states' density, $D_{it}(E)$, required to model the back-gate evolution of the minima, is found to have significant inhomogeneity in its energy-width. A broad $D_{it}(E)$ leads to an effect similar to a reduction in the Fermi velocity while the narrow $D_{it}(E)$ leads to the pinning of the Fermi energy close to the Dirac point, as observed in some places, due to enhanced screening of the gate electric field by the narrow $D_{it}(E)$

cond-mat.mes-hall↗

arXiv2016

Micron size superconducting quantum interference devices of lead (Pb)

Micron size superconducting quantum interference devices ($μ$-SQUID) of lead (Pb), for probing nano-magnetism, were fabricated and characterized. In order to get continuous Pb films with small grain size, Pb was thermally evaporated on a liquid nitrogen cooled Si substrate. Pb was sandwiched between two thin Cr layers for improved adhesion and protection. The SQUID pattern was made by e-beam lithography with Pb lift-off after deposition. The current-voltage characteristics of these devices show a critical current, which exhibits the expected SQUID oscillations with magnetic field, and two re-trapping currents. As a result these devices have hysteresis at low temperatures, which disappears just below the critical temperature.

cond-mat.supr-con↗

arXiv2015

Controlling hysteresis in superconducting constrictions with a resistive shunt

We demonstrate control of the thermal hysteresis in superconducting constrictions by adding a resistive shunt. In order to prevent thermal relaxation oscillations, the shunt resistor is placed in close vicinity of the constriction, making the inductive current-switching time smaller than the thermal equilibration time. We investigate the current-voltage characteristics of the same constriction with and without the shunt-resistor. The widening of the hysteresis-free temperature range is explained on the basis of a simple model.

cond-mat.supr-con↗

arXiv2015

Reversibility of Superconducting Nb Weak Links Driven by the Proximity Effect in a Quantum Interference Device

We demonstrate the role of proximity effect in the thermal hysteresis of superconducting constrictions. From the analysis of successive thermal instabilities in the transport characteristics of micron-size superconducting quantum interference devices with a well-controlled geometry, we obtain a complete picture of the different thermal regimes. These determine whether the junctions are hysteretic or not. Below the superconductor critical temperature, the critical current switches from a classical weak-link behavior to one driven by the proximity effect. The associated small amplitude of the critical current makes it robust with respect to the heat generation by phase-slips, leading to a non-hysteretic behavior.

cond-mat.supr-con↗

arXiv2015

Weakening of the spin density wave gap at low temperatures in SrFe$_2$As$_2$ single crystals

We report on temperature dependent scanning tunneling microscopy and spectroscopy studies of undoped SrFe$_{2}$As$_{2}$ single crystals from 6 K to 292 K. Resistivity data show spin density wave (SDW) transition at T$_{SDW}$ $\approx$ 205 K and the superconducting transition at $\sim$ 21 K while magnetic susceptibility does not show any superconductivity. Conductance maps and local tunneling spectra show an electronically homogeneous surface at all studied temperatures. Tunnel spectra correlate well with the SDW transition with a depression in the density of states near the Fermi energy below T$_{SDW}$. On further lowering the temperature, the width of this partial energy gap associated with the SDW decreases. Using the anti-correlation of superconducting phase with SDW, we interpret this gap weakening as a precursor to superconductivity. This may give rise to a facile nucleation of superconductivity near defects.

cond-mat.str-el↗

arXiv2014

Electronic inhomogeneities in the superconducting phase of CaFe1.96Ni0.04As2 single crystals

Superconductivity in Iron-Arsenic based pnictides emerges in close proximity to an antiferromagnetic (AFM)ordered parent state and the AFM phase overlaps with superconducting (SC) phase in some pnictides for certain range of doping. CaFe2-xNixAs2 belongs to this category, where both the phases overlap. Here we use scanning tunneling microscopy and spectroscopy to investigate the local electronic properties of uderdoped CaFe1.96Ni0.04As2 single crystals in the vicinity of the boundary of the two phases. Both resistivity and magnetic measurements show that a tiny portion (~ 1.2 %) of this compound becomes superconductor below the SC onset temperature TC ~ 15 K. Topographic images show reasonably flat surface with signatures of atomic resolution. High temperature spectra are spatially homogeneous and show signatures of spin density wave (SDW) gap with a finite density of states near the Fermi energy. Below TC, spectra show significant spatial inhomogeneity with a SDW gap everywhere but at some locations we also see an asymmetric or symmetric depression in ~ +/- 5 meV energy range together with the SDW gap. Inhomogeneity reduces significantly as the temperature goes above TC and disappears completely far above TC. These observations are discussed in terms of an inhomogeneous electronic phase that may exist due to the vicinity of this composition to the SC dome boundary on the underdoped side of the phase diagram.

cond-mat.str-el↗

arXiv2014

Temperature and phase dynamics in superconducting weak-link

A time dependent thermal model for a superconducting constriction based weak-link (WL) is discussed for investigating the deterministic dynamics of its temperature and phase. A new dynamic regime is found where a non-zero voltage exists across the WL and its temperature stabilizes between the bath temperature and superconductor's critical temperature. This regime exists over a limited bias current range and gives rise to a new hysteretic regime in current-voltage characteristics. We also discuss the effect of fluctuations on the current-voltage characteristics and experimental implications of this dynamic regime.

cond-mat.supr-con↗