arXiv subjects

Kartik Senapati

Publications and source records attributed to Kartik Senapati.

13 recordsLinked to original sources

arXiv2026

Supercurrent spin Hall effect enabled nanopillar Josephson diodes

In the recent years it has been possible to achieve diode-like, non-reciprocal current-voltage response in Josephson junctions, despite the intrinsic symmetry of the Josephson effect itself. This is typically achieved by incorporating Rashba spin-orbit coupling into the Josephson junction as a strong inversion symmetry breaking component, and external magnetic field as a tuneable time-reversal symmetry breaking component. However, the efficiencies of the external field tuneable Josephson-diodes have remained limited to less than 10 \%, often measured below 100 mK temperature. In this work we take a new approach where non-reciprocity is induced by intrinsic SOC in a heavy metal Josephson barrier via the predicted supercurrent spin-Hall effect. By measuring a series of Nb-Pt-Nb nanopillar junctions we demonstrated field tuneable Josephson diode efficiencies as high as 17\%, measured above liquid Helium temperature. This was possible by the realization of a net non-equilibrium spin segregation in the Pt barrier, due to the supercurrent spin-Hall effect in the Pt barrier, analogous to the normal spin-Hall effect. As the direction of the induced spin moment is determined by the bias current, an external magnetic field causes the associated phases to add with opposite signs for opposite current directions, resulting in a nonreciprocal supercurrent across the junction.

cond-mat.supr-con↗

arXiv2024

Spin-Phonon interaction in quasi 2D- Cr$_2Te_3$

Spin-phonon interaction plays an important role in 2D magnetic materials and motivates the development of next-generation spin- and charge-dependent microelectronic devices. Understanding the spin-phonon interaction by tuning the growth parameter of single crystal Cr$_2Te_3$, a robust quasi-2D room temperature magnetic material, is crucial for spintronic devices. The synthesis of single crystal 2D Cr$_2Te_3$ flakes on a Si substrate from co-deposited thin film by plasma annealing techniques is a significant achievement. The temperature dependence and polarization-resolved Raman spectroscopy with support of density functional theory classified lattice symmetry operations were used to identify the phonon modes to investigate the spin/electron-phonon interactions in Cr$_2Te_3$. The mean-field theory model in single crystal Cr$_2Te_3$ is employed to quantify the spin-phonon interaction and correlate with in-plane and out-of-plane magnetic behavior. The observation of a positive correlation between phonon mode frequency and spin-phonon interaction strength in single crystal Cr$_2Te_3$ can be a potential candidate for spintronic applications.

cond-mat.mtrl-sci↗

arXiv2024

Sensing magnetic flux of Langmuir-Blodgett films of a molecular magnetic system using superconducting films and nano-SQUID devices

We report a study on the response of superconducitng micro-tracks and quantum interference devices (SQUIDs) to a proximal SMM film. As a test case, Langmuir-Blodgett $Mn_{12}$-ac SMM films have been grown on 2 $μ$m wide Nb tracks and Nb nano-SQUIDs to observe the proximity effect of magnetic moment and magnetization tunneling, respectively. The superconducting critical temperature of thin Nb tracks (thinner than the coherence length of Nb) were found to decrease by the magnetic moment of $Mn_{12}$-ac SMM. Following the thermally activated flux flow (TAFF) model, we found an increase in the vortex unbinding energy of the SMM coated Nb tracks, near critical temperature. More importantly, the random alignment of moments of the $Mn_{12}$-ac molecules at low fields seemed to have the enhancing effect on vortex unbinding energy rather than the saturated state of $Mn_{12}$-ac molecules at high fields. In the fully superconducting state, on the other hand, the vortex pinning effects were found to be more effective in the saturated state of the $Mn_{12}$-ac molecules, as seen from magnetoresistance and field dependent critical current measurements. In a separate experiment, a Langmuir-Blodgett film of SMM was grown on a nano-SQUID to look for local changes in magnetization arising from magnetizatin tunnelling phenomenon in SMMs. Upon magnetizing the SMM (deposited on SQUIDs) at 2 K along the plane of the film and allowing it to relax, we found occasional jumps in the underlying SQUID voltage, unlike bare nano-SQUIDs, which did not show any such jumps over several hours. Therefore, we believe that the jumps in the SQUID voltage are the signatures of random tunneling of magnetization in the SMM layer.

cond-mat.supr-con↗

arXiv2023

Phase biasing of a Josephson junction using Rashba-Edelstein effect

Manifestation of orbital coupling of spin degree of freedom in condensed matter systems has opened up a new dimension for the field of spintronics. The most appealing aspect of the spin-orbit coupling is the apparent Magnus force sensed by a spin system which locks the Fermi momentum with electron spin in a fascinating manner. In the current carrying state, the resulting macroscopic spin polarization becomes directly accessible in the form of spin current or spin density. At a Rashba interface, for example, a charge current shifts the spin-locked Fermi surface, leading to a non-equilibrium spin density at the interface, commonly known as the Rashba-Edelstein effect. Since the Rashba-Edelstein effect is an intrinsically interface property, direct detection of the spin moment is harder to set-up. Here we demonstrate that a simple planar Josephson Junction geometry, realized by placing two closely spaced superconducting electrodes on such a Rashba interface, allows a direct estimation of strength of the non-equilibrium spin moment. Measurements of Fraunhofer patterns of Nb-(Pt/Cu)-Nb planar Josephson junctions in a perpendicular magnetic field showed a shift of the center of the Fraunhofer pattern to a non-zero field value. By performing extensive control measurements, we argue that the screening currents in the junction effectively lock the external field with the spin moment of the Rashba-Edelstein effect induced spin-density, leading to the observed shift in the Fraunhofer patterns. This simple experiment offers a fresh perspective on direct detection of spin polarization induced by various spin-orbit effects. Very interestingly, this device platform also offers the possibility of retaining a controllable phase at zero field in the junction without using any magnetic material, and thereby useful as phase batteries for superconducting quantum circuits.

cond-mat.mes-hall↗

arXiv2023

Bismuth Phase Dependent Growth of Superconducting NiBi3 Nanorods

We report a study on the growth of NiBi3 nanowires and nanorods during the preparation of superconducting NiBi3 films by co-evaporation of Ni and Bi. We find that NiBi3 films grown via co-evaporation of Ni and Bi metals achieve higher transition temperatures (4.4 K) compared even to the single crystal NiBi3. However, in certain parameter space, the film surfaces were spattered with nanoscale features, such as nanowires and nanorods. Ambient temperature deposition resulted in polycrystalline NiBi3 nanorods which were controllable with the evaporation rate of Bi. Deposition at elevated temperatures promoted the emergence of long single crystalline NiBi3 nanorods. High resolution transmission electron microscopy measurements confirmed the crystalline behaviour of the nanorods. We believe that NiBi3 nanowires form in a process analogous to the well known vapor-liquid-solid process, as we observe an amorphous Bi cap on the nanorods. From glancing angle X-ray diffraction measurements we identify that the presence of trigonal Bi with hexagonal primitive cell in the film promotes the nucleation of nanorods. Electrical transport on a single NiBi3 nanowire shows a superconducting transition of 4.3K.

cond-mat.supr-con↗

arXiv2023

Josephson coupling driven magnetoresistance in superconducting NiBi3 nanowires

We present results of magnetoresistance (MR) measurements in granular NiBi3 nanowires in the resistive state below the superconducting transition temperature. MR of 100 nm wide nanowires fabricated by focused Ion beam lithography from granular films of NiBi3 with and without magnetic Ni impurity were compared. The nanowire containing high concentration of Ni impurity showed oscillations in MR and also exhibited a negative MR in certain temperature and field range. None of these effects were observed in the nanowire with no Ni impurities. Therefore, we argue that this effect is a result of the random Josephson couplings realized across superconducting NiBi3 grains via magnetic inter grain regions. Such random couplings can cause local fluctuations in the density and sign of supercurrent, which can lead to negative MR and oscillations in MR, as proposed by Kivelson & Spivak [Kivelson et al. Phy. Rev. B. 45, 10490 (1992)].

cond-mat.supr-con↗

arXiv2021

Unconventional domain wall magnetoresistance of patterned Ni/Nb bilayer structures below superconducting transition temperature of Nb

Scattering of spin-up and spin-down electrons while passing through a ferromagnetic domain wall leads to an additional resistance for transport current, usually observed prominently in constricted magnetic structures. In this report, we use the resistance of the domain wall as a probe to find indirect signatures of the theoretically predicted spin-singlet supercurrent to spin-triplet supercurrent conversion effect of ferromagnetic domain walls. Here we examine the domain wall induced resistance in Ni stripe in a bilayer Ni/Nb geometry in the normal state and in the superconducting state of Nb. By making a 6um wide gap in the top Nb layer we routed the transport current through the Ni layer in the normal state and in the superconducting state of Nb. In the normal state of Nb, in-field transport measurements showed a clear domain wall magneto-resistance (DWMR) peak near the coercive field, where the domain wall density is expected to be maximum. Interestingly, however, below the superconducting transition temperature of Nb, the DWMR peak of the Ni layer showed a sharp drop in the field range where the number of domain walls becomes maximum. This observation may be a possible signature of magnetic domain wall induced spin-triplet correlations in the Ni layer due to the direct injection of spin-singlet Cooper pairs from Nb into the magnetic domain walls.

cond-mat.supr-con↗

arXiv2020

Study of magnetization reversal in Neel and Bloch regime of Ni and Py stripes using Kerr microscopy

We present a systematic study of the magnetization reversal of nickel and permalloy micro-stripes with Neel and Bloch domain walls using Kerr microscopy. Magnetic field driven domain propagation was observed from higher width to lower width stripes for magnetic fields applied along the length of micro-stripes. Stripe like domains were observed with nucleation starting in lower width region followed by their propagation to higher width regions for magnetic fields applied along the width of micro-stripes. The comparison of magnetization reversal in Bloch and Néel domain wall regime showed higher domain wall density in Bloch regime for both nickel and permalloy stripes.

cond-mat.mes-hall↗

arXiv2020

Magnetic field tunable superconducting transition in Nb/Co/Py/Nb exchange spring multilayers

Over the last decade it has been shown that magnetic non-collinearity at a s-wave superconductor/ferromagnet interface is a key ingredient for spin-singlet to spin-triplet pair conversion. This has been verified in several synthetic non-collinear magnetic structures. A magnetically soft and hard ferromagnetic layer combination in a bi-layer structure can function as a field tunable non-collinear magnetic structure which may offer magnetic-field tuneability of singlet-to-triplet pair conversion. From magnetization measurements of Nb/Co/Py/Nb multilayers we demonstrate a reversible enhancement of the superconducting critical temperature of 400 mK by measuring Tc with and without a non-collinear magnetic structure between Co and Py. The sensitivity of Tc in these structures offers the potential for realizing magnetic field tunable Josephson junctions in which pair conversion and Josephson critical currents are controllable using modest magnetic fields.

cond-mat.supr-con↗

arXiv2019

Energy Loss Due to Defect Formation from $^{206}$Pb Recoils in SuperCDMS Germanium Detectors

The Super Cryogenic Dark Matter Search experiment (SuperCDMS) at the Soudan Underground Laboratory studied energy loss associated with Frenkel defect formation in germanium crystals at mK temperatures using in situ $^{210}$Pb sources. We examine the spectrum of $^{206}$Pb nuclear recoils near its expected 103 keV endpoint energy and determine an energy loss of $\left(6.08\pm0.18\right)$ %, which we attribute to defect formation. From this result and using TRIM simulations, we extract the first experimentally determined average displacement threshold energy of $\left(19.7^{+0.6}_{-0.5}\right)$ eV for germanium. This has implications for the analysis thresholds of future germanium-based dark matter searches.

physics.ins-det↗

arXiv2019

Domain wall induced modulation of low field H-T phase diagram in patterned superconductor-ferromagnet stripes

We present a systematic study of the magnetic domain wall induced modulation of superconducting transition temperature (Tc) in Nb/Ni bilayer stripes. By varying the thickness of the Ni layer from 20 nm to 100 nm we have been able to measure the low field Tc-H phase diagram spanning the Neel domain wall and Bloch domain wall range of thicknesses. Micromagnetic simulations and magnetic force microscopy measurements confirmed a stronger out-of-plane stray field in the Bloch domain walls compared to the Neel walls. A suppression in Tc was observed in the magnetization reversal region of the Ni film, the magnitude of which followed linearly to the strength of the out-of-plane stray field due to the domain walls. The magnitude of the stray field was quantified by comparing the Tc of the suppressed region of H-Tc phase diagrams with the unaffected part of the H-Tc curve. With Bloch domain walls a change in Tc of more than 60 mK was observed which is much more compared to the earlier reports. We believe that the narrow stripe geometry of the bilayers and the transverse external field maximized the effect of the domain walls in the Ni layer on the overlying superconducting film, leading to a larger change in Tc. This observation may be useful for domain wall controlled switching devices in superconducting spintronics.

cond-mat.supr-con↗

arXiv2018

A study of electron and thermal transport in layered Titanium disulphide single crystals

We present a detailed study of thermal and electrical transport behavior of single crystal Titanium disulphide flakes, which belongs to the two dimensional, transition metal dichalcogenide class of materials. In-plane Seebeck effect measurements revealed a typical metal-like linear temperature dependence in the range of 85 - 285 K. Electrical transport measurements with in-plane current geometry exhibited a nearly T^2 dependence of resistivity in the range of 10 - 300 K. However, transport measurements along the out-of-plane current geometry showed a transition in temperature dependence of resistivity from T^2 to T^5 beyond 200 K. Interestingly, Au ion-irradiated TiS2 samples showed a similar T 5 dependence of resistivity beyond 200 K, even in the current-in-plane geometry. Micro- Raman measurements were performed to study the phonon modes in both pristine and ion-irradiated TiS2 crystals.

cond-mat.mes-hall↗

arXiv2017

Progressive magnetic softening of ferromagnetic layers in multilayer ferromagnet-nonmagnet systems and the role of granularity

We report a study of the structural and magnetic behavior of the topmost magnetic layer in a ferromagnet-nonmagnet (Co-Au) multilayer system. Glancing angle X-ray diffraction measurements performed on a series of multilayers showed a gradually increasing degree of amorphization of the topmost magnetic layer with increasing number of bilayers. Concurrently, the magnetic hardness and magneto-crystalline anisotropy of the top Co layer were found to decrease, as observed by magneto-optical Kerr effect measurements. This magnetic softening has been discussed in the light of Herzer's random anisotropy model. Micromagnetic simulations of the multilayer system also corroborated these observations.

cond-mat.mtrl-sci↗