arXiv subjects

Avradeep Pal

Publications and source records attributed to Avradeep Pal.

9 recordsLinked to original sources

arXiv2026

Non-volatile superconducting tunnelling magnetoresistance memory enabled by exchange-field gap engineering

Scalable, low-dissipation memory operating below 4 K is a critical requirement for superconducting and quantum computing systems. Existing cryogenic memory technologies rely on CMOS derivatives or hybrid architectures that incur leakage, refresh overhead or limited compatibility with superconducting logic. Here we demonstrate a superconducting tunnelling magnetoresistance device that functions as a non-volatile cryogenic memory element across the full superconducting temperature range. By integrating a de Gennes spin valve with a superconducting tunnel junction in a current perpendicular-to-plane geometry, we realise exchange-field control of the superconducting energy gap. This produces two magnetically switchable gap voltages and robust quasiparticle tunnelling magnetoresistance down to 0.25 K.The device operates at millivolt bias with nanowatt-level read power and zero standby dissipation. Its vertical junction architecture and Nb-based materials platform enable compatibility with superconducting logic and scalable cryogenic memory arrays.

cond-mat.supr-con↗

arXiv2025

Superconducting exchange coupling driven bistable and absolute switching

As per de Gennes predictions, a superconducting layer placed between two ferromagnetic insulators can drive an antiferromagnetic exchange coupling between them. Using two ferromagneticinsulating GdN layers having dissimilar switching fields sandwiching a superconducting Vanadium thin film, we demonstrate evidence of such exchange coupling. We demonstrate that such an exchange coupling promotes switching between zero and finite resistance states of Vanadium. Our devices hold either a finite resistance or a zero-resistance state at zero magnetic field, dependent on their magnetic field history. Moreover, we demonstrate the absolute switching effect, thus making such devices suitable for application at the lowest temperatures as non-volatile cryogenic memory useful for futuristic quantum circuits and for several other superconducting spintronic applications.

cond-mat.supr-con↗

arXiv2023

Universal route towards field-free electrically polarity-reversible Josephson diode

The realisation of superconducting diodes that operate without external magnetic fields and allow electrical control of polarity is a key goal for the integration of nonreciprocal elements into cryogenic and quantum technologies. Here, we demonstrate a universal and scalable approach to achieving such field-free and electrically reconfigurable Josephson diode functionality. Our method relies on long Josephson junctions with ferromagnetic barriers and asymmetric current injection - a configuration that inherently breaks both time-reversal and inversion symmetries. We show that the diode polarity is set by an applied bias current and can be reversed using short current pulses, without the need for magnetic fields or thermal cycling. The effect is robust, material-agnostic, and compatible with standard established fabrication processes. Our results provide a practical platform for integrating low-dissipation, programmable diodes into superconducting and quantum electronic circuits

cond-mat.supr-con↗

arXiv2023

Structural transitions in superconducting NbTiN thin films

Superconducting NbTiN thin films have garnered extensive interest due to their use in Superconducting Nanowire Single-Photon Detectors (SNSPDs) and other low-temperature applications for potential use in quantum computing and nanoelectronics. This study examines structural phase transitions observed in NbTiN thin films by analyzing the grazing angle x-ray diffraction patterns of a set of reactive magnetron sputter deposited NbTiN thin films with varying nitrogen partial pressures in the reactive gas mixture. The superconducting transition temperature (T_C) of the NbTiN thin films showed a correlation with the crystal structure, with the highest T_C of 14.26 K obtained for the highly crystalline FCC phase.

cond-mat.supr-con↗

arXiv2023

Superconductivity controlled bulk magnetism

Ferromagnetism's ability to influence superconducting order is well known and well established, but the converse phenomena remains relatively less explored. Theoretical work on the subject includes Anderson and Suhl prediction of a crypto-ferromagnetic state, and De Gennes proposal of two ferromagnetic insulators exchange coupled through a superconductor. In this study, we present compelling evidence of coexistence of both phenomena in a superconducting spin valve system. We demonstrate that superconducting exchange coupling enables reliable bistable states, and the coexistence of SEC and CFM leads to a wide range of reproducible zero field micro-magnetic states in the SSV, which are a function of the strength of the superconducting state. These micromagnetic states can in turn influence the superconducting state, leading to multiple reproducible and non-volatile resistance states; thus paving the way for a novel direction in cryogenic in-memory computing.

cond-mat.supr-con↗

arXiv2023

Large tuneable exchange fields due to purely paramagnetically limited domain wall superconductivity

The ability to locally apply and tune large magnetic fields is a crucial requirement for several devices, most notably for detection and generation of majorana fermions. Such a functionality can be achieved in Superconductor (S) /Ferromagnet (F) bilayers, where superconductivity is strengthened on top of domain walls due to local lowering of the proximity induced effective exchange fields. This is predicted to result in significant superconducting Tc enhancements and possible complete magnetic controlled switching on and off of the superconducting state. By using thin films of superconducting Nb and ferromagnetic insulating (GdN) bilayers, and through detailed magneto-transport measurements, we demonstrate the previously unobserved phenomena of complete switching in and out of the S state in S/F bilayers. In the thinnest of Nb layers, we estimate that the domain wall state induced tunability of proximity induced exchange fields can be as high as 1.3T with application of in plane external fields of only a few mT.

cond-mat.supr-con↗

arXiv2017

Triplet Cooper pairs induced in diffusive s-wave superconductors interfaced with strongly spin-polarized magnetic insulators or half-metallic ferromagnets

Interfacing superconductors with strongly spin-polarized magnetic materials opens the possibility to discover new spintronic devices in which spin-triplet Cooper pairs play a key role. Motivated by the recent derivation of spin-polarized quasiclassical boundary conditions capable of describing such a scenario in the diffusive limit, we consider the emergent physics in hybrid structures comprised of a conventional s-wave superconductor (e.g. Nb, Al) and either strongly spin-polarized ferromagnetic insulators (e.g. EuO, GdN) or halfmetallic ferromagnets (e.g. CrO2, LCMO). In contrast to most previous works, we focus on how the superconductor itself is influenced by the proximity effect, and how the generated triplet Cooper pairs manifest themselves in the self-consistently computed density of states (DOS) and the superconducting critical temperature Tc. We provide a comprehensive treatment of how the superconductor and its properties are affected by the triplet pairs, demonstrating that our theory can reproduce the recent observation of an unusually large zero-energy peak in a superconductor interfaced with a half-metal, which even exceeds the normal-state DOS. We also discuss the recent observation of a large superconducting spin-valve effect with a Tc change ~1K in superconductor/half-metal structures, in which case our results indicate that the experiment cannot be explained fully by a long-ranged triplet proximity effect.

cond-mat.supr-con↗

arXiv2017

Spectroscopic evidence of odd frequency superconducting order

Spin filter superconducting S/I/N tunnel junctions (NbN/GdN/TiN) show a robust and pronounced zero bias conductance peak at low temperatures, the magnitude of which is several times the normal state conductance of the junction. Such a conductance anomaly is representative of unconventional superconductivity and is interpreted as a direct signature of an odd frequency superconducting order.

cond-mat.supr-con↗

arXiv2014

Pure 2nd harmonic current phase relation in Spin filter Josephson junctions

Higher harmonics in current phase relations of Josephson Junctions (JJs) are predicted to be observed when the first harmonic is suppressed. Conventional theoretical models predict higher harmonics to be extremely sensitive to changes in barrier thickness, temperature, etc. In contrast, experiments with JJs incorporating a spin dependent tunnelling barrier reported here reveal a current phase relation for highly spin polarized barriers which is purely 2nd harmonic in nature, and this is insensitive to changes in barrier thickness. This observation is consistent with recent theoretical predictions of a robust 2nd harmonic current phase relation for certain JJs with ferromagnetic barriers and implies that the standard theory of Cooper pair transport through tunnelling barriers is not applicable for spin dependent tunnelling barriers.

cond-mat.supr-con↗