Self Introduction

• MSc Physics + BE Electronics and Instrumentation from BITS, India
• Cryogenic probe: NISER, Bhubaneswar
• Simulation of a reticle missile and its deflection: DRDO, India
• Simulation of a BMS for EV: Kaynes, India
• Master's Thesis on Josephson Junctions at NISER
• Research Software Engineer at MPSD, Hamburg
• Software / Electronics Hobbyist

Electronics projects

Introduction to Josephson Junctions

• Josephson junctions are weak links between two superconductors.
• They are used to create superconducting qubits, SQUIDs, and other devices.

Josephson Relations - I

• Current through a Josephson junction is given by $$I_s = I_c \sin(\delta)$$
• Here, $I_c$ is the critical current and $\delta$ is the phase difference between the superconductors ($\phi_1 - \phi_2$).

Josephson Relations - II

• Voltage across a Josephson junction is given by $$V = \frac{\Phi_0}{2\pi} \frac{d\delta}{dt}$$
• Here, $\Phi_0$ is the magnetic flux quantum. $$\Phi_0 = \frac{h}{2e} \approx 2.067\mathrm{x}10^{-15} Wb$$

Josephson Junction in a Magnetic Field

• When a magnetic field is applied, the phase difference $\delta$ changes.
• In case of small junctions, the current magnetic field relation is given by $$I_{c}=I_{0}\left|\frac{\sin\left(\pi\frac{\Phi_{J}}{\Phi_{0}}\right)}{\pi\frac{\Phi_{J}}{\Phi_{0}}}\right|$$
• Here, $\Phi_{J}=\mu_{0}HLd$ is the magnetic flux through the junction area.

Josephson Junction in a Magnetic Field

• This is of the standard Fraunhofer pattern form.
$F(x)=I_{0}|\frac{\sin{\pi x}}{\pi x}|$
• A signature of Josephson Junction

Example plot of the JJ Fraunhofer pattern [ E. R. Caianiello et. al. Journal of Modern Physics Vol.06 No.05(2015) ]

Another Josephson Junction signature

• Non linear relation between the current and the voltage across the junction.

Simulating a normal Josephson Junction

• Resistively and capacitively shunted junction model (RCSJ).

$$ \frac{\Phi_0}{2\pi}C \ddot{\delta} + \frac{\Phi_0}{2\pi R} \dot{\delta} = I-I_c\sin{\delta}$$

Rearranging, we get $$ \ddot{\delta}+\frac{1}{RC}\dot{\delta}=(\frac{2\pi}{C\Phi_{0}})(I-I_{c}\sin(\delta)) $$

The RCSJ Model

Rearranging, we get $$ \ddot{\delta}+\frac{1}{RC}\dot{\delta}=(\frac{2\pi}{C\Phi_{0}})(I-I_{c}\sin(\delta)) $$
• dynamics of $\delta$ resembles a damped particle with
• Effective mass $C$
• Coefficient of friction $\frac{1}{R}$
• Potential on the particle $-(\frac{2\pi}{C\Phi_{0}})(I\delta-I_{c}\delta\sin(\delta))$

IV Characteristics

Rashba Edelstein Effect

• Interface effect between Metal and Heavy Metal.
• Generates a spin accumulation perpendicular to the charge current.

[V.M. Edelstein, Solid State Communications,Volume 73,issue 3,1990]

Detection of Rashba-Edelstein Effect

• Interface effect $\implies$ Direct detection is not easy

Reported sofar:

• Detection via inverse Rashba-Edelstein effect [1]
• Interaction of charge current with spin density at the interface via FMR [2]

Phase biasing of a Josephson junction using Rashba-Edelstein effect

• superconducting electrode on a rashba interface.
• moment from Rashba-Edelstein effect induces flux in the junction.
• Detected via $I_cH$ and $VH$.

Part of Master's Thesis (2021) Prof Kartik Senapati, Superconductivity Lab, NISER, India.

Sample preparation

Instrument	UV Mask Aligner	DC Magnetron sputtering	Ga - FIB + SEM (Crossbeam 340)
Stage	Photolithography	Tri-layer deposition	Final Sample

Junction Geometries

Vertical Josephson. Junction	Planar Josephson. Junction

Fabrication of device

SEM Image of the device

(With false colours indicating the layers)

Transport measurement

RT plot of a JJ showing transition of electrode and proximatisation of Cu. Inset shows IV of the same device.

VH plot of the JJ showing the characteristic Fraunhofer Pattern.

Rashba-Edelstein effect in JJs and SQUIDs

Senapati, T., Karnad, A.K. & Senapati, K. Nat Com 14, 7415 (2023)

Simulation to verify $I_cH$ vs $VH$

• Simulated using the ODE from RCSJ model.
• Compared the $I_cH$ and $VH$ patterns (periodicity at $\frac{\Phi_J}{\Phi_0}$).

Normalised Plot of simulated $I_cH$

Normalised Plot of simulated $VH$

Mechanism explanation in JJ

Physical effects in JJ above $T_c$

Physical effects in JJ below $T_c$

Senapati, T., Karnad, A.K. & Senapati, K. Nat Com 14, 7415 (2023)

Publication

Senapati, T., Karnad, A.K. & Senapati, K. Nat Com 14, 7415 (2023)

Conclusion

• Direct detection of Rashba-Edelstein effect is possible using Josephson Junctions.

• Phase difference $\delta$ of the junction can be tuned by the junction geometry.
• Isolate RE from SHE as the system is superconducting

Auxilary work:

Control of scientific measurement with python

Cryostat Probe Design

• Under Prof. Kartik Senapati
• Superconductivity Lab, NISER, India
• Summer 2019

Cryostat arrangement

Existing probe design

CAD Model - I

Sample holder for non magnetic measurements

CAD Model - II

Sample holder for magnetic measurements

RC Filter design (low pass)

RC filter circuit used

Measured Bode plot (strength vs frequency)

PCB Design

PCB Design

Fabricated PCB

Software for data acquisition

Sample measurement

Measured sample

Measured RT

Simulation of IR seeker missiles and its counter measure

• Defense Avionics Research Establishment
• DRDO, Bangalore, India.
• Summer 2019

Design and Simulation of Battery Management System Algorithms for Electric Vehicle Applications

• Kaynes Technology India Pvt Limited, Mysore, India
• Summer 2020

Modeling of an EV Li-ion battery and drive parameters

Work as a Research Software Engineer

Server room at MPCDF PC: Prof. Hans Fangohr

• Prof. Hans Fangohr
• Max Planck Institute MPSD, Hamburg
• 2022 onwards

Postopus : POST processing of OctoPUS data

Post processing of Benzene DFT calculation

Postopus : POST processing of OctoPUS data

Plot forces on Benzene electron density

Postopus : POST processing of OctoPUS data

Slice of Benzene electron density

ML on micromagnetic configuration

The energy density difference between identified equilibrium states and the corresponding ground state. [Beg, M., Carey, R., Wang, W. et al. Sci Rep 5, 17137 (2015).]

Example magnetic configurations PC: Swapneel

My interest in functional metal oxides

HZO Memristive crossbar array for vector multiplication applications

A physical reservoir computing scheme uses a magnetic reservoir or ferroelectric reservoir (here made from domain walls) (center) for clustering. [inspired from Karin Everschor-Sitte et. al. arXiv (2023)]

Thank you!