arXiv subjects

Akashdeep Kamra

Publications and source records attributed to Akashdeep Kamra.

At least 55 records · Page 3Linked to original sources

arXiv2022

Magnonic spin Joule heating and rectification effects

Nonlinear devices, such as transistors, enable contemporary computing technologies. We theoretically investigate nonlinear effects, bearing a high fundamental scientific and technical relevance, in magnonics with emphasis on superconductor-ferromagnet hybrids. Accounting for finite magnon chemical potential, we theoretically demonstrate magnonic spin-Joule heating, the spin analogue of conventional electronic Joule heating. Besides suggesting a key contribution to magnonic heat transport in a broad range of devices, it provides insights into the thermal physics of non-conserved bosonic excitations. Considering a spin-split superconductor self-consistently, we demonstrate its interface with a ferromagnetic insulator to harbor large tunability of spin and thermal conductances. We further demonstrate hysteretic rectification I-V characteristics in this hybrid, where the hysteresis results from the superconducting state bistability.

cond-mat.mes-hall↗

arXiv2022

Hybrid magnetization dynamics in $\text{Cu}_\text{2}\text{OSeO}_\text{3}$/$\mathrm{NiFe}$ heterostructures

We investigate the coupled magnetization dynamics in heterostructures of a single crystal of the chiral magnet $\mathrm{Cu_2OSeO_3}$ (CSO) and a polycrystalline ferromagnet $\mathrm{NiFe}$ (Py) thin film using broadband ferromagnetic resonance (FMR) at cryogenic temperatures. We observe the excitation of a hybrid mode (HM) below the helimagnetic transition temperature of CSO. This HM is attributed to the spin dynamics at the CSO/Py interface. We study the HM by measuring its resonance frequencies for in plane rotations of the external magnetic field. We find that the HM exhibits dominantly four-fold anisotropy, in contrast to the FMR of CSO and Py.

cond-mat.mes-hall↗

arXiv2022

Observation of nonreciprocal magnon Hanle effect

The precession of magnon pseudospin about the equilibrium pseudofield, the latter capturing the nature of magnonic eigen-excitations in an antiferromagnet, gives rise to the magnon Hanle effect. Its realization via electrically injected and detected spin transport in an antiferromagnetic insulator demonstrates its high potential for devices and as a convenient probe for magnon eigenmodes and the underlying spin interactions in the antiferromagnet. Here, we observe a nonreciprocity in the Hanle signal measured in hematite using two spatially separated platinum electrodes as spin injector/detector. Interchanging their roles was found to alter the detected magnon spin signal. The recorded difference depends on the applied magnetic field and reverses sign when the signal passes its nominal maximum at the so-called compensation field. We explain these observations in terms of a spin transport direction-dependent pseudofield. The latter leads to a nonreciprocity, which is found to be controllable via the applied magnetic field. The observed nonreciprocal response in the readily available hematite films opens interesting opportunities for realizing exotic physics predicted so far only for antiferromagnets with special crystal structures.

cond-mat.mes-hall↗

arXiv2022

Non-reciprocity of Vortex-limited Critical Current in Conventional Superconducting Micro-bridges

Non-reciprocity in the critical current has been observed in a variety of superconducting systems and has been called the superconducting diode effect. The origin underlying the effect depends on the symmetry breaking mechanisms at play. We investigate superconducting micro bridges of NbN and also NbN/magnetic insulator (MI) hybrids. We observe a large diode efficiency of $\approx$~30\% when an out-of-plane magnetic field as small as 25~mT is applied. In both NbN and NbN/MI hybrid, we find that the diode effect vanishes when the magnetic field is parallel to the sample plane. Our observations are consistent with the critical current being determined by the vortex surface barrier. Unequal barriers on the two edges of the superconductor strip result in the diode effect. Furthermore, the rectification is observed up to a temperature $\sim$10~K, which makes the device potential for diode based applications over larger temperature range than before.

cond-mat.supr-con↗

arXiv2022

Master equation approach to magnon relaxation and dephasing

There has been a recent upsurge of interest in the quantum properties of magnons for quantum information processing. An important issue is to examine the stability of quantum states of magnons against various relaxation and dephasing channels. Since the interaction of magnons in magnetic systems may fall in the ultra-strong and even deep-strong coupling regimes, the relaxation process of magnon states is quite different from the more common quantum optical systems. Here we study the relaxation and dephasing of magnons based on the Lindblad formalism and derive a generalized master equation that describes the quantum dynamics of magnons. Employing this master equation, we identify two distinct dissipation channels for squeezed magnons, i.e., the local dissipation and collective dissipation, which play a role for both ferromagnets and antiferromagnets. The local dissipation is caused by the independent exchange of angular momentum between the magnonic system and the environment, while the collective dissipation is dressed by the parametric interactions of magnons and it enhances the quantumness and thermal stability of squeezed magnons. Further, we show how this formalism can be applied to study the pure dephasing of magnons caused by four-magnon scattering and magnon-phonon interactions. Our results provide the theoretical tools to study the decoherence of magnons within a full quantum-mechanical framework and further benefit the use of quantum states of magnons for information processing.

cond-mat.mes-hall↗

arXiv2022

Theory of drift-enabled control in nonlocal magnon transport

Electrically injected and detected nonlocal magnon transport has emerged as a versatile method for transporting spin as well as probing the spin excitations in a magnetic insulator. We examine the role of drift currents in this phenomenon as a method for controlling the magnon propagation length. Formulating a phenomenological description, we identify the essential requirements for existence of magnon drift. Guided by this insight, we examine magnetic field gradient, asymmetric contribution to dispersion, and temperature gradient as three representative mechanisms underlying a finite magnon drift velocity, finding temperature gradient to be particularly effective.

cond-mat.mes-hall↗

arXiv2022

Quantum magnonics: when magnon spintronics meets quantum information science

Spintronics and quantum information science are two promising candidates for innovating information processing technologies. The combination of these two fields enables us to build solid-state platforms for studying quantum phenomena and for realizing multi-functional quantum tasks. For a long time, however, the intersection of these two fields was limited. This situation has changed significantly over the last few years because of the remarkable progress in coding and processing information using magnons. On the other hand, significant advances in understanding the entanglement of quasi-particles and in designing high-quality qubits and photonic cavities for quantum information processing provide physical platforms to integrate magnons with quantum systems. From these endeavours, the highly interdisciplinary field of quantum magnonics emerges, which combines spintronics, quantum optics and quantum information science.Here, we give an overview of the recent developments concerning the quantum states of magnons and their hybridization with mature quantum platforms. First, we review the basic concepts of magnons and quantum entanglement and discuss the generation and manipulation of quantum states of magnons, such as single-magnon states, squeezed states and quantum many-body states including Bose-Einstein condensation and the resulting spin superfluidity. We discuss how magnonic systems can be integrated and entangled with quantum platforms including cavity photons, superconducting qubits, nitrogen-vacancy centers, and phonons for coherent information transfer and collaborative information processing. The implications of these hybrid quantum systems for non-Hermitian physics and parity-time symmetry are highlighted, together with applications in quantum memories and high-precision measurements. Finally, we present an outlook on the opportunities in quantum magnonics.

quant-ph↗

arXiv2022

Pure dephasing of magnonic quantum states

For a wide range of nonclassical magnonic states that have been proposed and demonstrated recently, a new time scale besides the magnon lifetime - the magnon dephasing time - becomes important, but this time scale is rarely studied. Considering exchange interaction and spin-phonon coupling, we evaluate the pure magnon dephasing time and find it to be smaller than the magnon lifetime at temperatures of a few Kelvins. By examining a magnonic cat state as an example, we show how pure dephasing of magnons destroys and limits the survival of quantum superpositions. Thus it will be critical to perform quantum operations within the pure dephasing time. We further derive the master equation for the density matrix describing such magnonic quantum states taking into account the role of pure dephasing, whose methodology can be generalized to include additional dephasing channels that experiments are likely to encounter in the future. Our findings enable one to design and manipulate robust quantum states of magnons for information processing.

cond-mat.mes-hall↗

arXiv2021

Influence of low-energy magnons on magnon Hanle experiments in easy-plane antiferromagnets

Antiferromagnetic materials host pairs of spin-up and spin-down magnons which can be described in terms of a magnonic pseudospin. The close analogy between this magnonic pseudospin systems and that of electronic charge carriers led to the prediction of fascinating phenomena in antiferromagnets. Recently, the associated dynamics of antiferromagnetic pseudospin has been experimentally demonstrated and, in particular, the first observation of the magnon Hanle effect has been reported. We here expand the magnonic spin transport description by explicitly taking into account contributions of finite-spin low-energy magnons. In our experiments we realize the spin injection and detection process by two Platinum strips and investigate the influence of the Pt-strips on the generation and diffusive transport of magnons in films of the antiferromagnetic insulator hematite. For both a 15 nm and a 100 nm thick film, we find a distinct signal caused by the magnon Hanle effect. However, the magnonic spin signal exhibits clear differences in both films. In contrast to the thin film, for the thicker one, we observe an oscillating behavior in the high magnetic field range as well as an additional offset signal in the low magnetic field regime. We attribute this offset signal to the presence of finite-spin low-energy magnons.

cond-mat.mes-hall↗

arXiv2021

Tunable anisotropic quantum Rabi model via a magnon--spin-qubit ensemble

The ongoing rapid progress towards quantum technologies relies on new hybrid platforms optimized for specific quantum computation and communication tasks, and researchers are striving to achieve such platforms. We study theoretically a spin qubit exchange-coupled to an anisotropic ferromagnet that hosts magnons with a controllable degree of intrinsic squeezing. We find this system to physically realize the quantum Rabi model from the isotropic to the Jaynes-Cummings limit with coupling strengths that can reach the deep-strong regime. We demonstrate that the composite nature of the squeezed magnon enables concurrent excitation of three spin qubits coupled to the same magnet. Thus, three-qubit Greenberger-Horne-Zeilinger and related states needed for implementing Shor's quantum error-correction code can be robustly generated. Our analysis highlights some unique advantages offered by this hybrid platform, and we hope that it will motivate corresponding experimental efforts.

quant-ph↗

arXiv2021

Interfacial control of vortex-limited critical current in type II superconductor films

In a small subset of type II superconductor films, the critical current is determined by a weakened Bean-Livingston barrier posed by the film surfaces to vortex penetration into the sample. A film property thus depends sensitively on the surface or interface to an adjacent material. We theoretically investigate the dependence of vortex barrier and critical current in such films on the Rashba spin-orbit coupling at their interfaces with adjacent materials. Considering an interface with a magnetic insulator, we find the spontaneous supercurrent resulting from the exchange field and interfacial spin-orbit coupling to substantially modify the vortex surface barrier, consistent with a previous prediction. Thus, we show that the critical currents in superconductor-magnet heterostructures can be controlled, and even enhanced, via the interfacial spin-orbit coupling. Since the latter can be controlled via a gate voltage, our analysis predicts a class of heterostructures amenable to gate-voltage modulation of superconducting critical currents. It also sheds light on the recently observed gate-voltage enhancement of critical current in NbN superconducting films.

cond-mat.supr-con↗

arXiv2021

Spin pumping at terahertz nutation resonances

We investigate spin pumping current injected by the nutation resonances of a ferromagnet or an antiferromagnet into an adjacent metal. Comparing the dc spin pumping current between the normal precession and nutation resonances, we find that the ratio of spin pumping current at the nutation resonance to the precession resonance is more pronounced in antiferromagnets. We further show that the spin pumping current injected by the nutation resonance is opposite in sign as compared to the normal precession mode. This could offer a useful experimental signature for identifying such nutation resonances. Analyzing the nature of the nutational eigenmodes, we show that the sign change in spin current is rooted in a reversal of the precession sense for the nutation mode(s). Furthermore, the nutational modes in antiferromagnets are found to be dominated by precession of one of the two sublattices only.

cond-mat.mes-hall↗

arXiv2021

Control of nonlocal magnon spin transport via magnon drift currents

Spin transport via magnon diffusion in magnetic insulators is important for a broad range of spin-based phenomena and devices. However, the absence of the magnon equivalent of an electric force is a bottleneck. In this work, we demonstrate the controlled generation of magnon drift currents in yttrium iron garnet/platinum heterostructures. By performing electrical injection and detection of incoherent magnons, we find magnon drift currents that stem from the interfacial Dzyaloshinskii-Moriya interaction. We can further control the magnon drift by the orientation of the magnetic field. The drift current changes the magnon propagation length by up to $\pm$ 6 % relative to diffusion. We generalize the magnonic spin transport theory to include a finite drift velocity resulting from any inversion asymmetric interaction, and obtain results consistent with our experiments.

cond-mat.mes-hall↗

arXiv2021

Electrically switchable entanglement channel in van der Waals magnets

Two dimensional layered van der Waals (vdW) magnets have demonstrated their potential to study both fundamental and applied physics due to their remarkable electronic properties. However, the connection of vdW magnets to spintronics as well as quantum information science is not clear. In particular, it remains elusive whether there are novel magnetic phenomena only belonging to vdW magnets, but absent in the widely studied crystalline magnets. Here we consider the quantum correlations of magnons in a layered vdW magnet and identify an entanglement channel of magnons across the magnetic layers, which can be effectively tuned and even deterministically switched on and off by both magnetic and electric means. This is a unique feature of vdW magnets in which the underlying physics is well understood in terms of the competing roles of exchange and anisotropy fields that contribute to the magnon excitation. Furthermore, we show that such a tunable entanglement channel can mediate the electrically controllable entanglement of two distant qubits, which also provides a protocol to indirectly measure the entanglement of magnons. Our findings provide a novel avenue to electrically manipulate the qubits and further open up new opportunities to utilize vdW magnets for quantum information science.

cond-mat.mes-hall↗

arXiv2021

Intersublattice entanglement entropy as an extensive property in antiferromagnets

Recent advancements in our understanding of ordered magnets call for a quantification of their entanglement content on an equal footing with classical thermodynamic quantities, such as the total magnetic moment. We evaluate the entanglement entropy (EE) between the two sublattices of a bipartite ordered antiferromagnet finding it to scale with volume. Thus, the EE density becomes an intensive property and is evaluated to be a universal dimensionality-dependent constant when exchange is the dominant interaction. Our analytic results are validated against the DMRG-based analysis of a one-dimensional (1D) system, finding good agreement. Further, our evaluated EE per bond provides a useful shortcut towards obtaining the central-cut EE in 1D, and the area law in higher-dimensional magnets.

cond-mat.mes-hall↗

arXiv2021

Large Enhancement of Critical Current in Superconducting Devices by Gate Voltage

The gate-voltage-induced suppression of critical currents in metallic superconductors observed recently [De Simoni et al., Nat. Nanotechnol. 13, 802 (2018)] has raised crucial questions regarding the nature and mechanism of the electric field effect in these systems. Here, we demonstrate an enhancement of up to 30 % in critical current in the type II superconductor NbN, micro- and nano superconducting bridges, tunable via a back-gate voltage. Our suggested plausible mechanism of this enhancement in critical current based on surface nucleation and pinning of Abrikosov vortices is consistent with expectations and observations for type-II superconductor films with thicknesses comparable to their coherence length. Furthermore we demonstrate infinite electroresistance and a hysteretic resistance dependence on the applied electric field which could lead to logic and memory applications in a superconductors-based low-dissipation digital computing paradigm. Our work thus provides the first demonstration of an electric field enhancement in the superconducting property in metallic superconductors, constituting a crucial step towards understanding of electric field-effects on the fundamental properties of a superconductor and its exploitation for future technologies.

physics.app-ph↗

arXiv2020

Thermally induced spin torque and domain wall motion in superconductor/antiferromagnetic insulator bilayers

We theoretically investigate domain wall motion in an antiferromagnetic insulator layer caused by thermally generated spin currents in an adjacent spin-split superconductor layer. An uncompensated antiferromagnet interface enables the two crucial ingredients underlying the mechanism - spin splitting in the superconductor and absorption of spin currents by the antiferromagnet. Treating the superconductor using the quasiclassical theory and the antiferromagnet via Landau-Lifshitz-Gilbert description, we find domain wall propagation along the thermal gradient with relatively large velocities $\sim 100$ m/s. Our proposal exploits the giant thermal response of spin-split superconductors in achieving large spin torques towards driving domain wall and other spin textures in antiferromagnets.

cond-mat.supr-con↗

arXiv2020

Observation of Antiferromagnetic Magnon Pseudospin Dynamics and the Hanle effect

We report on experiments demonstrating coherent control of magnon spin transport and pseudospin dynamics in a thin film of the antiferromagnetic insulator hematite utilizing two Pt strips for all-electrical magnon injection and detection. The measured magnon spin signal at the detector reveals an oscillation of its polarity as a function of the externally applied magnetic field. We quantitatively explain our experiments in terms of diffusive magnon transport and a coherent precession of the magnon pseudospin caused by the easy-plane anisotropy and the Dzyaloshinskii-Moriya interaction. This experimental observation can be viewed as the magnonic analogue of the electronic Hanle effect and the Datta-Das transistor, unlocking the high potential of antiferromagnetic magnonics towards the realization of rich electronics-inspired phenomena.

cond-mat.mtrl-sci↗