As spintronic devices continue to shrink toward nanoscale dimensions, interest has grown in the role that heat plays in these systems. Thermal gradients in spintronics can be unintentional, such as those generated by the high current densities often required to switch nanoscale magnets, or intentional, such as when studying materials using thermal probes. Here we focus on how the heat and charge flow in a ferromagnetic metal can be altered when long-lived spin dynamics are present. Using unique micromachined thermal isolation platforms, we demonstrate a unique magnetic-field direction dependence in thermal properties of alloys with exceptionally low magnetic damping.

We theoretically study the thermal Hall and spin Nernst effect induced by the interaction between magnonic and phononic resonances at THz frequency in FePS3, a two-dimensional antiferromagnet (2D AFM) material. We find that a strong coupling between the magnetic excitation (magnon) and elastic excitation (phonon) in FePS3 combined with time-reversal-symmetry-breaking results in a finite Berry curvature concentrating within the anti-crossing point formed between the two distinct excitation bands. More interestingly, a very large spin Berry curvature emerges even in the absence of an external magnetic field due to phonon-mediated magnon-magnon interactions that cause a small energy gap between two magnon states. This results in topological transverse transport of quasi particles and spin momenta when a temperature gradient is applied to the quasi two-dimensional magnon phonon system in FePS3. We investigate the dependence of the thermal Hall and spin Nernst conductivity on the external magnetic field and temperature and find a very large spin Nernst conductivity at zero magnetic field. This results suggests possible experiments to explore the topological transport of a magnon-polaron system at THz frequency in a realistic 2D AFM material.

In spintronic THz emitters, the initial spin current leads to a local charge accumulation with a subsequent backflow. Although this effect strongly modifies the current profile, it is often not taken into account when the emitted THz radiation is used to extract the spin current physics. We demonstrate that the effect can be modeled using a simple equivalent circuit. It is, however, difficult to disentangle the backflow from the initial current. Besides the theory, we demonstrate that sub-wavelength THz emitters can be used to demonstrate the validity of the model by modifying the emission spectrum in a well-controlled way.

Here we explore the potential for driving entanglement between spatially-separated flopping-mode spin qubits dispersively coupled to a common photonic mode of a superconducting resonator. We propose a scheme for synchronizing single-qubit rotations with a cross-resonance drive to realize a spin-based direct-CNOT[1]. This simultaneous evolution yields gate-times within the entanglement time of the cross-resonance gate. The average gate fidelity (>90%) is calculated in the presence of cavity loss, electron-phonon interaction, and general spin-dephasing. We then extend our analysis of the direct-CNOT to discuss opportunities for driving three-qubit entanglement in spin-based platforms. [1] S. R. McMillan et al., arXiv:2207.13588 (2022)

The field of quantum sensing takes advantage of the unique properties of quantum systems and quantum measurement protocols to realize sensing concepts beyond the limits of classical physics. While solid-state sensors are ubiquitous in our daily life, they usually are based on classical sensing concepts. Quantum 1.0 and 2.0 technologies offer intriguing opportunities for improving their sensitivity, noise performance, accuracy, and precision. In this regard, hybrid quantum systems are particularly attractive, since they unite existing solid-state constituents with quantum capabilities. This makes them an ideal test-bed for engineering the transduction of quantum quantities and compare them to established “useful” classical signals. For the tailored transduction between two sub-systems a controlled hybridization of the states participating is desired. In this presentation, I will focus on the solid-state implementation of spin-photon and spin-phonon hybrids which couple spin excitations with gigahertz excitations in superconducting circuits or acoustic resonators. I will discuss both realisations in terms of the emerging dynamics and their sensing capabilities.

The emergent ferroic two-dimensional (2D) van der Waals (vdW) materials provide ideal platforms to enable the atomic-thin magneto-optical and magnetoelectric devices. Recent years have witnessed the new developments of such 2D ferroic devices, but for many cases they remain in the proof-of-concept stage. Here I will present our recent progress on multiple facets of 2D ferroic materials and devices such as the vdW ferroelectric tunnel junctions with record-high electroresistance and small-voltage, non-volatile control of 2D ferromagnets. Our understanding of 2D ferroics and advancement of 2D ferroic devices potentially pave new avenues to the energy-efficient nanoelectronic devices.

Individually, both graphene and PbxSn1-xTe (PST) are widely investigated for spintronic applications due to graphene’s high carrier mobility and PST’s topologically protected surface states. Here, we combine monolayer graphene with PST and demonstrate a hybrid system with enhanced properties relative to the constituent materials. Through a combination of standard magnetotransport and non-local spin measurements, we demonstrate the system contains two distinct spin transport channels: a low temperature channel with significant Rashba spin-orbit coupling and a high-temperature conventional conducting channel with efficient spin transport up to at least 500 K.

In this work we investigate spin-light coupling phenomena in van der Waals heterostructures (vdWs) consisting of two-dimensional Dirac crystals proximitized by a magnetic layer. A strong magnetic proximity effect results in the modification of the 2D electron spectrum with the appearance of different spin splitting terms and electron’s equilibrium spin polarization. Motivated by these factors we have explored the resonant spin generation of proximitized Dirac electrons by terahertz ac-electric field and have revealed the existence of a new scenario for the electric dipole spin resonance (EDSR). We pay special attention to the resonant spin dynamics of electrons in case of magnetically proximitized single graphene layer, and discuss emerging EDSR features unrepresented for systems with parabolic spectrum. The estimations demonstrate the enhancement of EDSR in a graphene layer featured in addition by an anomalous polarization structure of the absorption coefficient. The developed theory allowed us to come up with a proposal for a THz detector, in which the photogenerated nonequilibrium spin polarization in graphene can be read out by means of the Johnson-Silsbee effect due to spin charge tunneling conversion at the interface.

This talk will highlight new opportunities for exploring superconductor-semiconductor hybrid systems, enabled by the revolutionary measurement capabilities of engineered superconducting circuits. Resonant circuits can probe the fragile superconductor-semiconductor interface, revealing behavior that is invisible to conventional probes. A single Josephson junction can be added to realize a semiconductor-based amplifier operating near quantum limits. Scaling up to junction arrays, the stage is set for studying the electrodynamics and radiometry of the anomalous metal.

Superconductivity is the foundation for modern quantum coherent phenomena in solid-state systems, enabling topological quantum bits, Weyl physics, and macroscopic quantized phenomena. The activation of quantum mechanical phases as degrees of freedom in superconducting multi-terminal structures allows for the realization of complex, topological non-trivial states of matter. This includes Berry spectroscopy of Weyl-Andreev nodes in synthetic dimensions, fractional states of matter of Cooper pair networks, and high-dimensional topology like the Second Chern number with a non-Abelian Berry phase. The talk will discuss theoretical and experimental advancements in realizing effects in spectroscopy, critical currents and quantized transconductance.

Over the past decades, artificial intelligence (AI) has made significant technological advances with the prospect of increased computer capabilities (e.g., automation in decision making and data processing) and acquired an increasingly important role in our everyday technological environment (Dall-E, ChatGPT, etc.). The main issue is that the digital silicon-based computing technologies are very energy-intensive while solving cognitive tasks such as speech or image recognition. We propose to tackle this issue by combining condensed matter physics (spintronics) and artificial intelligence to design nanoscale neuromorphic computing hardware to solve machine learning tasks.

A critical challenge towards integrating 3-dimensional (3D) magnetic nanostructures into neuromorphic circuitry is stabilizing different magnetic states that are controllable via external stimuli. Here, we demonstrate quasi-ordered and random interconnected magnetic nanowire networks as potential candidates. In these networks, step-by-step magnetization reversal mediated by domain wall (DW) pinning / depinning at the network intersections is observed. The pinning/depinning of the DWs can be further controlled by the driving current density. These properties are amenable to implement neuromorphic computing elements such as artificial synapses.

As the field of deep learning continues to expand, it has become increasingly important to develop energy-efficient hardware that can adapt to these advances. However, achieving learning on a chip requires the use of algorithms that are compatible with hardware and can be implemented on imperfect devices. One promising training technique is Equilibrium Propagation, which was introduced in 2017 by Yoshua Bengio. This approach provides gradient estimates based on a spatially local learning rule, making it more biologically plausible and better suited for hardware than backpropagation. However, the mathematical equations of this algorithm cannot be directly transposed to a physical system. In this study, Equilibrium Propagation algorithm is adapted to the use of a real physical system, and its potential application to spintronics devices is discussed.

Spintronic oscillators have gained important interest in recent years. However, a both fast and accurate model describing their dynamics is currently lacking. Here we propose an unconventional semi-analytical model capable of predicting the fundamental properties of the oscillators, when subjected to a spin-polarized current. Using data-driven corrections, the steady-state and transient regimes of oscillations can be simulated quantitatively. This data-driven model is more than two billion times faster than micromagnetic simulations. Such method paves the way for high-throughput simulation campaigns in any prospective applications, such as neuromorphic spintronics.

Nonreciprocal critical current in superconducting Josephson junctions occurs when the critical current Ic depends on the direction the current was swept and can be observed by breaking the time reversal and inversion symmetries in the system. Recently, there has been a surge of interest in studying the superconducting diode effect due to its possible application in electronic circuits, sensors, and detectors. In this work, we study epitaxial InAs/Al Josephson junctions with various superconductor width in the presence of in-plane magnetic fields. By applying a magnetic field parallel to the junction, we observe nonreciprocal critical current which is due to the strong Rashba spin-orbit interaction in the system. However, by reducing the width of the superconducting lead, we observe a change in sign of ΔI=I(+)-I(-) as we increase the in-plane magnetic field. We discuss the possible mechanisms and possibility of finite momentum pairing.

In this work we show that critical current nonreciprocity (CCN) is a generic effect of superconductor heterostructures and is related to the formation of diamagnetic currents by an external magnetic field in the presence of phase locking between superconducting layers due to the Josephson coupling. We demonstrate this so-called superconducting diode effect in nanowires fabricated from InAs/Al heterostructures, where superconductivity in InAs is induced via proximity effect from an Al layer. The observed CCN is linear at small in-plane magnetic fields and becomes non-monotonic function of magnetic field and changes sign at high fields. We attribute CCN nonmonotonic behavior to the formation of Josephson vortices. The studied InAs/Al and analogous heterostructures are positioned as the most promising platform to search for non-Abelian excitations, the observed formation of Josephson vortices and corresponding phase twists at high magnetic fields may need to be taken into account in the future device designs.

Currents in diodes are asymmetric with bias direction, a phenomenon known as nonreciprocal transport. Recently, diode behavior was realized in superconducting systems. I will discuss superconducting diode effects observed in Josephson junctions realized in an STM. Junctions of an elemental superconductor exhibit hysteretic behavior, typical of low-dissipation junctions, but no asymmetry. Nonreciprocal supercurrents emerge when inserting a single magnetic atom, with the preferred direction depending on atomic species. After describing the experiment, I will present a theory of Josephson diodes, relating asymmetries in switching and retrapping currents to different microscopic symmetry requirements and providing a microscopic understanding of experiment.

The flow of electron spins, the so-called “spin current”, is a key concept in the recent progress in spintronics [1,2]. In ferromagnetic metals and semiconductors, the spin current interacts with magnetization by the exchange interaction and induces the motion of the magnetization due to the angular momentum conservation, the so-called spin transfer torque. Its inverse effect is called the spin-motive force which is the electric voltage generated by the magnetization dynamics due to the energy conservation between electrical current and magnetization [2,3,4]. The spin motive-force is derived by introducing spin Berry phase in the Faraday’s law of electro-magnetism. Spin current is induced and manipulated by heat [5] and mechanical motion [6,7] as well. The interconversion between various energies through spin current , the so-called “power spintronics”, is discussed.

The characteristic of the Hall effects and allied is that the macroscopic topological properties of the device determines the electric response of the system [1]. This property is related to the low power consumption of spintronics devices in general. In the present study, we explore the power efficiency of Hall-like effects in spintronics devices by calculating a measuring the power that can be injected from the Hall device to a load circuit. Experimentally, with studied the properties of the anomalous-Hall current that is injected from a Hall bar into a lateral load circuit. The anomalous Hall current is generated by a ferrimagnetic Hall bar with lateral contacts lithographied at the two edges. The current, the voltage and the power injected in the lateral circuit are studied as a function of the magnetization states and the load resistance. A sharp maximum of the power efficiency as a function of the load resistance is observed, which corresponds to the condition of resistance matching with the anomalous Hall resistance. The typical profile and amplitude of voltage, current and power are in conformity with recent predictions based on a non-equilibrium variational approach [2-4]. The result corroborates the role of the screening effects, and the presence of surface currents flowing at the edges of the Hall bar over the Debye-Fermi length scale. [1] Madon et al. Corbino magnetoresistance in ferromagnetic layers: Two representative examples Ni81Fe19 and Co83Gd17 Phys. Rev. B 98, 220405(R) (2018) [2] M. Creff et al.,  Surface current in Hall devices, J. Appl. Phys 128,  054501 (2020) [3] F. Faisant et al., The physical properties of the Hall current,  J. Appl. Phys. 129, 144501 (2021) [4] M. Creff, E. Olive, and J.-E. Wegrowe, Screening effect in Spin-Hall Devices, Phys. Rev. B 105, 174419 (2022)

Ferrimagnets have been at the spotlight of spintronics as their properties can be conveniently engineered and they display intriguing dynamic behavior in the vicinity of the magnetic and angular momentum compensation, useful for device applications.1,2 Among ferrimagnets, rare-earth iron garners have attracted a particular attention due to their insulating character, perpendicular magnetic anisotropy, and long magnon diffusion lengths, enabling efficient current-induced magnetization control,3 domain wall and skyrmion motion,3,4 and spin transport in nonlocal devices.5 In this work, we will first discuss the structural, magnetic, and spintronic properties of epitaxial terbium iron garnet (TbIG) films, a ferrimagnetic insulator with compensation temperature (TM) close to room temperature. The samples are deposited by RF magnetron sputtering with crystallinity and epitaxy on various (substituted) GGG substrates. The films display perpendicular magnetic anisotropy induced by compressive strain and tunable structural and magnetic properties with annealing conditions. Studying the temperature evolution of the anomalous Hall effect in Pt/TbIG heterostructures allows us to determine the TM of the ferrimagnetic ordering, that is found to be approximately ~200 K, some 50 K lower than in bulk. This is attributed to the Tb deficiency of the films corroborated by x-ray photoelectron spectroscopy measurements.6 In the second part of the talk, we will show, for the first time, that the insulating TbIG films display electronic spin-valve behavior when combined with a ferrimagnetic metal and a nonmagnetic spacer. We measure a current-in-plane giant magnetoresistance effect arising from the spin-dependent scattering of electrons at the interface between the TbIG and the spacer layer. Although the magnitude of the effect is smaller than in fully metallic stacks, it provides a means to read out the perpendicular magnetization vector of the insulator in a simple two-terminal geometry. This finding constitutes a major breakthrough, as it allows the integration of insulating magnets in spintronics devices with a technologically-viable electrical read out scheme.7 References [1] S. K. Kim, et al., Nature Materials 21, 24–34 (2022) [2] C.O. Avci, et al., Nature Materials 16, 309–314 (2017) [3] C.O. Avci, et al. Nature Nanotechnology 14, 561–566 (2019) [4] S. Vélez, et al., Nature Nanotechnology 17, 834–841 (2022) [5] L. J. Cornellissen, et al., Nature Physics 11, 1022–1026 (2015) [6] S. Damerio and C.O. Avci Journal of Applied Physics 133, 073902 (2023) [7] S. Damerio, S. Zhang and C.O. Avci, in preparation (2023)

We will discuss first-principles calculations of spin transport and spin-orbit torques in disordered films and multilayers within the nonequilibrium Green's function technique. For a nonmagnetic Pt film, the behavior of the spin accumulation and the transverse spin current deviate significantly from the conventional spin-diffusion model. The effective transverse spin-diffusion length is much shorter than the longitudinal spin-diffusion length. For ferromagnetic trilayers, we find dampinglike and fieldlike torques with unconventional spin polarizations. The large rotated fieldlike component in Co/Cu/Co trilayers is inconsistent with diffusive transport in the spacer layer but can be explained by nonlocal interactions between the ferromagnetic layers when the mean-free path is not small compared to the spacer thickness.

The spin Hall effect has been one of the central subjects in the spintronics area. The grand challenge in this area is improving the conversion ratio between electric current and spin current. In this talk, we propose new mechanisms of spin Hall effect based on critical spin fluctuations in itinerant magnetic systems, in which itinerant electrons interact with localized moments that are dynamically fluctuating. We will provide a detailed description of our theory of critical spin fluctuation mechanisms of the SH effect and discuss possible material realizations. The research by S.O. was supported by the U.S. Department of Energy, Office of Science, Basic Energy Sciences, Materials Sciences and Engineering Division.

The bilayer heterostructures composed of an ultrathin ferromagnetic metal (FM) and a nonmagnetic material hosting strong spin-orbit coupling (SOC) are a principal resource for spin-orbit torque (SOT) [1] and spin-to-charge conversion [2] effects in next generation spintronics. The key to understand these effect is current-driven nonequilibrium spin density [3]. For example, it generates SOT when it is noncollinear to the direction of local magnetization and it can arise due to variety of microscopic mechanisms, including the spin Hall effect, spin-orbit proximity effect and different interfacial scattering mechanisms. The recently discovered two-dimensional (2D) magnetic materials offer new avenue for highly efficient and gate- or disorder-tunable SOT in van der Waals (vdW) heterostructures composed of few monolayers of atomically thin materials where the spin Hall effect from the bulk is absent. Using first-principles quantum transport calculations, which combine nonequilibrium Green functions with noncollinear density functional theory [1], we predicted [4] that injecting unpolarized charge current parallel to the interface of bilayer-CrI3/monolayer-TaSe2 vdW heterostructure will induce SOT-driven dynamics of magnetization on the first monolayer of CrI3 that is in direct contact with metallic transition metal dichalcogenide (TMD) TaSe2 and SO-proximitized by it. By combining calculated complex angular dependence of SOT with the Landau-Lifshitz-Gilbert equation for classical dynamics of magnetization, we find that this can reverse the direction of magnetization on the first monolayer to become parallel to that of the second monolayer, thereby converting bilayer CrI3 from antiferromagnet to ferromagnet as the signature of nonequilibrium phase transition. Such transition can be detetected by passing vertical current, and it is of potentially great interest to magnetic memory applications since it does not require any external magnetic field. Another vdW heterostructure exhibiting SOT is doubly proximitized graphene, which is neither magnetic nor hosts SOC in its isolated form, but proximity induced magnetic moments will exhibit SOT in Cr2Ge2Te6/graphene/WS2 vdW heterostructure which can be tuned by two orders of magnitude via the gate voltage [5]. Finally, we predict [6] that SO-proximitized 2D magnets, pushed out of equilibrium by microwave absorption which leads to steadily precessing magnetization, will pump spin and charge currents exhibiting high harmonics of the microwave frequency with cutoff order ~20, in contrast to two decades old “standard model” [7] of spin pumping at a single frequency.

The outstanding optoelectronic properties of hybrid metal halide perovskites, and their strong spin-orbit coupling, enable efficient manipulation of the charge carrier’s angular momentum. In this work, we investigate the dominant spin relaxation mechanism in CH3NH3PbBr3 films with doping levels up to 50% with the transition metal Mn2+. We investigate the spin relaxation times in these paramagnetic hybrid semiconductors with ultrafast circularly polarized broadband transient absorption spectroscopy at cryogenic temperatures. We report extended spin relaxation lifetimes by a factor of three which we explain with motional narrowing effects in the paramagnetic ensemble of Mn2+ impurities. Our work now presents paramagnetic doping of hybrid semiconductors as a promising concept for quantum technologies and spintronic devices.

To develop sensors for sub-nT magnetic field detection, we fabricate micron-sized magnetic tunnel junctions with symmetric response. Their sensitivity is improved by carefully tuning the exchange field of the soft-pinned free-layer. Furthermore, the use of a flux concentrator significantly enhances the sensor sensitivity at low field. The flux concentrator design is optimized to reach high gain values thanks to a particularly narrow air-gap of 10 µm. We demonstrate an amplification gain of a factor 350 with a flux concentrator realized by electro-chemical deposition. This result opens the way to the development of integrated magnetic sensors for ultra-low field detection.

We have developed tunnel magnetoresistance sensor structures that exhibit an even-function resistance-magnetic field response that is symmetric with respect to the sign of the external magnetic field. These devices are promising for encoder sensors for position and rotation detection and also important for ultra-sensitive magnetic field sensors in combination with the AC modulation technique using AC magnetic fields. From the analysis of the noise in these devices under AC modulation magnetic field, we found that although the 1/f noise with electrical origin is effectively suppressed, the magnetic 1/f noise is also transferred to around the frequency of the AC modulation.

Magnetic tunnel junctions (MTJs) based on a CoFeB/MgO/CoFeB trilayer structure exhibit a giant tunnel magnetoresistance (TMR) effect. They can be grown on a commercialized Si wafer and can be combined with various magnetic materials into a multilayered structure. These advantages drive us to apply the CoFeB/MgO/CoFeB-MTJs to magnetic sensor (hereafter called TMR sensor) that can be used at room temperature, can be microfabricated into a nanometer-scale, and can be driven with a small power consumption. Very recently, its magnetic field detectivity has reached a sub-pT level which enables us to detect a human bio-magnetic fields. These outstanding characteristics are desirable for a next-generation smart society. In this talk, we will introduce our recent works on development of multilayered structures for the TMR sensor.

Magneto-ionics has shown promise for energy-efficient nanoelectronics, where ionic migration can be used to achieve atomic scale control of interfaces in magnetic nanostructures, and in turn modulate a wide variety of functionalities. Recently, we have discovered that chemisorbed oxygen and hydrogen on the surface of ferromagnetic films can induce significant Dzyaloshinskii–Moriya interaction (DMI) [1], a handle to introduce topology into nanoscale magnets. This has enabled direct tailoring of skyrmions winding number as well as wall type at room temperature via oxygen chemisorption. We have also demonstrated a sensitive and reversible chirality switching of magnetic domain walls [2] and writing/deleting of skyrmions [3] via hydrogen chemisorption/desorption [3] or changing the thickness of a sub-monolayer Pd capping layer [4]. These effects offer an ideal platform to gain quantitative understanding of magneto-ionics at buried interfaces, where the ionic motion can be further controlled by an electric field, leading to modulation of such functionality as exchange bias [5,6].

The rich physics present in spintronic materials and devices provide a wide range of opportunities for neuromorphic computing systems. This presentation will overview three distinct proposals to efficiently leverage spintronic phenomena to mimic the behavior of the human brain. The presentation will begin with a purely-spintronic neuromorphic system in which the analog states of magnetic domain wall neurons and synapses emulate neurobiological behavior and enable CMOS-free inference and unsupervised online learning. This will be followed by an approach for neuromorphic inference that marks the first experimental demonstration of a neuromorphic network directly implemented with MTJ synapses, as well as a solution for exploiting stochastic MTJ switching for unsupervised learning in concert with CMOS neuron circuits. This presentation will conclude with a reservoir computing system based on the dynamics of irregular arrays of frustrated nanomagnets that increases overall hardware efficiency by a factor of 10,000,000.

Variational inference (VI) is an approximation of statistically valid Bayesian inference which is well-suited for analog accelerators, and stochastic nanomagnetic devices in particular are a strong candidate to implement this feature by exploiting tunable randomness in magnetic thin-films that can be run quickly and with a low power-draw. In this work, we a) discuss how VI can be reliably implemented with a combination of low-noise nanomagnetic synapses and tunable noise generating sources (magnetic tunnel junctions (MTJs) in a single analog design and b) summarize efforts to characterize the state-dependent noise profiles of various MTJ designs for various applications.

Recent studies have revealed that noncentrosymmetric materials show various nonreciprocal phenomena, which are attracting much attention. In particular, noncentrosymmetric superconductors offer a fertile ground to explore exotic physical phenomena arising from the interplay of nonreciprocity and macroscopic quantum coherence of Cooper pairs. In this talk, we discuss the intrinsic nonreciprocity of the charge transport of noncentrosymmetric superconductors including directional resistance, nonlinear Hall effect, and superconducting diode effect, with particular emphasis on the fingerprint of finite-momentum Cooper pairs.

Recent experiments [Nature Physics 18, 1221 (2022)] revealed a superconducting diode effect in twisted trilayer graphene in the absence of external magnetic fields and with very large critical current asymmetry. In this talk, I will discuss possible microscopic theoretical explanations for this observation. We will see that the observed phenomenology crucially constrains the microscopic physics of graphene moiré systems. I will also discuss why a diode effect based on spontaneous time-reversal-symmetry breaking might reach higher critical current asymmetries than a diode effect resulting from breaking time-reversal symmetry explicitly by a magnetic field.

Planar Josephson junctions (JJs) have recently emerged as promising systems for topological superconductivity [1-5]. These platforms have also been shown to exhibit superconducting diode effect [4-7], including early experiments from 2019 [4]. We investigate the effects of anisotropy in planar JJs [6]. We furthermore consider the interplay of topological phase transitions and the superconducting diode effect in planar JJs [4, 8], both experimentally and theoretically. While planar JJs can be a viable platform to demonstrate fusion and non-Abelian statistics of Majorana bound states [5], the role of disorder needs to be determined. We analytically and numerically investigate the effect of disorder in these systems and demonstrate reentrant behavior of topological properties on planar JJs. We conclude that an intermediate disorder may be beneficial to establishing topological phases [9-10]. [1] J. Shabani et al., Two-Dimensional Epitaxial Superconductor-Semiconductor Heterostructures: A Platform for Topological Superconducting Networks, Phys. Rev. B 93, 155402 (2016). [2] F. Pientka, A. Keselman, E. Berg, A. Yacoby, A. Stern, and B. I. Halperin, Topological Superconductivity in a Planar Josephson Junction, Phys. Rev. X 7, 021032 (2017). [3] A. Fornieri et al., Evidence of Topological Superconductivity in Planar Josephson Junctions, Nature 569, 7754 (2019). [4] M. C. Dartiailh, W. Mayer, J. Yuan, K. S. Wickramasinghe, A. Matos-Abiague, I. Žutić, and J. Shabani, Phase Signature of Topological Transition in Josephson Junctions, Phys. Rev. Lett. 126, 036802 (2021); arXiv:1906.01179 (2019). [5] T. Zhou, M. C. Dartiailh, K. Sardashti, J. E. Han, A. Matos-Abiague, J. Shabani, and I. Žutić, Fusion of Majorana Bound States with Mini-Gate Control in Two-Dimensional Systems, Nat Commun 13, 1738 (2022). [6] B. Pekerten, J. D. Pakizer, B. Hawn, and A. Matos-Abiague, Anisotropic Topological Superconductivity in Josephson Junctions, Phys. Rev. B 105, 054504 (2022). [7] C. Baumgartner et al., Supercurrent Rectification and Magnetochiral Effects in Symmetric Josephson Junctions, Nat. Nanotechnol. 17, 1 (2022). [8] N. Lotfizadeh, B. Pekerten, P. Yu, W. Strickland, A. Matos-Abiague, and J. Shabani, Superconducting Diode Effect Sign Change in Epitaxial Al-InAs Josepshon Junctions, arXiv:2303.01902. [9] B. Pekerten, A. Teker, Ö. Bozat, M. Wimmer, and İ. Adagideli, Disorder-Induced Topological Transitions in Multichannel Majorana Wires, Physical Review B 95, (2017). [10] A. Haim and A. Stern, Benefits of Weak Disorder in One-Dimensional Topological Superconductors, Phys. Rev. Lett. 122, 126801 (2019).

Planar Al/InAs Josephson junctions (JJs) provide an attractive platform for topological superconductivity and fault-tolerant quantum computing. Surprisingly, we show that with a gate-tunable spin-orbit coupling (SOC), these JJs also support a much broader range of applications. The time-dependent SOC offers unexplored mechanisms for switching JJs, accompanied by the 2π-phase jumps and voltage pulses corresponding to single-flux-quantum transitions. In a constant applied magnetic field, with SOC, anharmonic current-phase relations, calculated microscopically in these JJs, yield a nonreciprocal transport and time-dependent superconducting diode effect. This supports a novel switching mechanism at no applied current bias and supporting fractional-flux-quantum superconducting circuits.

The control of the superconducting state by gate voltage or electric field is a timely challenge for the development of novel superconducting electronics with low energy dissipation and ultrafast speed. Two-dimensional (2D) superconductors or nanowires are particularly relevant in this context. They offer innovative opportunities to study and tailor superconducting order parameters as they inherently lack inversion symmetry, in the presence of Rashba-type spin-orbit coupling, and allow large electrostatic tuning of charge carrier density and bands occupation. Here, we present the mechanisms of gate controlled supercurrent and the occurrence of anomalous Josephson effects and phase frustration dynamics in metallic nanowires and in 2D oxides.

Ferrimagnetic metals have evident potential for spintronic applications, combining the advantages of ferromagnets and antiferromagnets in one material that exhibits high spin polarization and magnetic compensation. Three versatile families with high ferrimagnetic Néel temperatures are manganese-based Heusler alloys, manganese-based interstitial antiperovskites and rare-earth cobalt amorphous alloys. Ferrimagnetism in Mn-based alloys is related to the separation-dependence of direct nearest-neighbour exchange interactions. The possibility of single-layer spin-orbit torque excitation is investigated in the Mn-based materials, as well as single-pulse all-optical switching and topological Hall effects in all three systems. A scheme associating light with spintronics in new universal memory will be discussed.

Frequency multiplication and frequency conversion are important processes in modern electronics in which harmonics of the input frequency are generated in nonlinear electronic circuits. Devices based on the propagation and interaction of spin waves represent a promising alternative to conventional electronics. However, the characteristic frequency of spin waves is in the gigahertz range, and thus cannot be easily combined with conventional electronics. We investigate the magnetic excitations in magnetic materials using optical methods and show that excitation at frequencies in the megahertz range can cause magnetic switching processes on the micrometer scale, and these lead to coherent spin wave emission in the gigahertz range. Moreover, at high modulation amplitudes, we demonstrate a new class of nonlinear spin waves that oscillate with half-integer harmonics of the excitation. Imaging of these parametrically generated spin waves enables measurement of the wave vectors and determination of the coherence properties. In addition, we demonstrate the existence of two degenerate phase states, each of which can be selected by the phase position of an external signal source. The frequency multiplication process occurring within the magnetic medium covers more than six octaves and opens new perspectives for applications such as fully magnetic mixers or "on-chip" signal generators. The phase-coherent excitation and control of half-integer harmonic spin waves is highly interesting for applications such as amplifiers and phase-encoded spin-wave-based information processing.

Our study provides a step forward to the practical implementation of a quantum transducer, used for converting quantum information between two different physical realizations, one is good for information transmission while the other is good for manipulation and control. We propose using commercial materials with rare-earth elements, conveniently chosen to have useful optical properties for telecommunications. We find that hosting rare-earth elements within a magnet dramatically speeds up the transduction process.

We present theoretical evidence that atomically sharp domain walls in antiferromagnetic materials have qualitatively different properties from their smooth counterparts, in which the reversal of the antiferromagnetic order parameter occurs gradually over many unit cells. A remarkable difference appears when we consider the effect of the domain wall on the propagation of an antiferromagnetic spin wave. Antiferromagnetic spin waves, unlike ferromagnetic ones, have two possible states of circular polarization: left handed and right handed. While a smooth domain wall does not distinguish between the two cases, allowing both types of waves to be transmitted with high probability (tending to 100% in the infinitely smooth limit), an atomically sharp domain wall can act as a spin wave polarizer, i.e., it allows one type of polarization to be almost completely transmitted while the other is almost completely reflected. This remarkable behavior occur in the vicinity of a sharp to smooth transition, which is controlled, within our model, by the ratio of the easy axis anisotropy to the nearest-neighbor exchange constant. The polarization of the transmitted spin wave depends on the orientation of the spins in the sharp domain wall - a property which can be controlled by an external field or spin torque and has no counterpart in a smooth domain wall. Our discovery of the spin-wave polarizing properties of sharp antiferromagnetic domain walls suggests that they could be used as spin polarizers in magnonic circuits.

We have investigated the effects of high frequency strain on domain wall (DW) textures in ferromagnetic thin films with perpendicular anisotropy. Matched pairs of interdigital transducers are lithographically patterned on 128 Y cut LiNbO3. Excitation at the resonance frequency drives surface acoustic waves (SAW). We describe the response of domain walls (DW) and magnetic bubbles. SAW lower the pinning energy barriers of DWs, showing a monotonic decrease in the characteristic depinning time with SAW amplitude. Application of SAW in a different sample rapidly increases the density of bubbles. We measure SAW driven dynamics of bubble domains.

Whilst the effective g-factor can be anisotropic due to the spin-orbit interaction (SOI), its existence in solids cannot be simply asserted from a band structure, which hinders progress on studies from such the viewpoints. The effective g-factor in bismuth (Bi) is largely anisotropic; especially for holes at T-point, the effective g-factor perpendicular to the trigonal axis is negligibly small (< 0.112), whereas the effective g-factor along the trigonal axis is very large (62.7). We clarified in this work that the large anisotropy of effective g-factor gives rise to the large spin conversion anisotropy in Bi from experimental and theoretical approaches. Spin-torque ferromagnetic resonance was applied to estimate the spin conversion efficiency in rhombohedral (110) Bi to be 17-27% [1], which is unlike the negligibly small efficiency in Bi(111) [2,3]. Harmonic Hall measurements supports the large spin conversion efficiency in Bi(110). This is the first observation of large spin conversion anisotropy as the clear manifestation of the anisotropy of the effective g-factor, and in addition, this is the first realization of large spin conversion efficiency that has been expected for Bi as the largest SOI material among non-radioactive elements.

In this talk, we will discuss different Josephson junctions based on semimetals, metals and topological insulators proximitized with s-wave superconductors. We show that thermal response can be more sensitive to Majorana bound states than an electrical response [1,2,3]. Moreover, due to the 4π periodicity of topological Josephson junctions, the thermal engines built on them are more efficient as the ones on the classical Josephson junctions [4]. Furthermore, we predict that the 2D Luttinger materials describing hole state with j=3/2 proximitized by s-wave superconductor are able to host Majorana bound states even in the absence of BIA and Rashba spin-orbit couplings [5]. This originates from the hybridization of the light and heavy hole bands of the j=3/2 states in combination with the superconducting pairing. We dub this effect as an intrinsic emergence of Majorana bound states [5]. We predict that Majorana bound states should be seen in many classes of materials like p-doped GaAs, bulk HgTe, as well as many half-Heusler compounds [5].

Topological states are recent concepts in condensed matter physics that have important implications due to their robust transport properties or in topological quantum computation through the braiding of Majorana bound states (MBS). Here, we propose a novel topological Josephson junction with the normal region being a quantum spin Hall insulator. We show that in a topological phase of the superconductors with MBS at their boundaries, the Fraunhofer pattern changes its period compared to the case with trivial superconductors. We also discuss how the signatures coming from the MBS can be distinguished from the ones originating from trivial Andreev bound states.

Faraday rotation spectroscopy is a powerful experimental technique to measure the magneto-optical response of spintronic materials and devices. However, due to speed limiting components, such as polarization modulators and lock-in amplifiers, measurements become extremely challenging, in particular if (sub)micrometer spatial resolution is required. We present a high-performance broadband Faraday rotation spectroscopy technique for measurements on the micron scale. Spectral acquisition speeds of two orders of magnitude faster than state-of-the-art modulation spectroscopy setups are demonstrated. As an example, we perform Faraday rotation spectroscopy of excitons in hBN-encapsulated atomically thin WS2 and an amorphous magnetic TbFe alloy thin film.

In this work, we report unconventional superconducting properties of high quality Au(111)/Nb heterostructure samples. We carry out tunneling studies in Au(111) using planar tunneling devices with a highly transparent barrier, which enhances the energy resolution of tunneling spectroscopy and allows us to observe and analyze fine in-gap states. We show a locally enhanced Zeeman field due to a large Landé g-factor at the barrier/Au(111) interface, and a broken of Pauli limit at the surface of Au(111) while maintaining bulk superconductivity. We will also show tunneling results along nanowires of magnetic insulators coupled to Au(111). Our work paves the way for searching and confirming robust topological superconductivity in the Au(111) platform.

Odd-frequency pairing should appear in heterostructures made of a conventional s-wave superconductor and a ferromagnet. We will first show how such pairing manifests by a peak structure close to the Fermi level in thin CuNi layer proximitized by Nb [1]. We will then propose a direct way to measure odd-frequency pairing in the simplest hybrid system: a single magnetic impurity immersed in a conventional s-wave superconductor. We show how scanning tunneling spectroscopy on top of magnetic impurities immersed in a superconductor provides a direct evidence of the presence of the odd-frequency pairing component [2].

In bulk transition metals the orbital degree of freedom is normally quenched due to band structure and crystal field effects. However, recent theoretical work indicates that an applied electric field can induce a net flow of orbital angular momentum in transition metals such as Ti, Cr, and Mn. The generation of the orbital current relies on the variable orbital character of the electronic bands that cross the Fermi level and does not require spin-orbit coupling (SOC). This makes it possible to use light metals in spin-orbitronic devices to actively generate orbital and spin currents. In this talk, we will present experimental results evidencing strong orbital torques and orbital Rashba-Edelstein magnetoresistance in light metal/ferromagnet heterostructures. The observation of an orbital torque requires the conversion of the orbital current into a spin current, which occurs due to SOC in the ferromagnetic itself or in nonmagnetic metallic and insulating spacers placed between the light metal and the ferromagnet. Our work shows that the orbital torque can be tuned by choosing orbital-to-spin conversion layers with different SOC, and the magnitude of the orbital torque can be larger than the spin torque. Finally, we will show that orbital currents are also responsible for magnetoresistive effects akin to the spin Hall magnetoresistance.

Recent experimental and theoretical studies have revealed that orbital transport could provide an alternative mechanism for efficient spin manipulation in the form of orbital torque. This study presents experimental studies on charge-to-orbital conversion and orbital torque in CoFe/Cu/Oxide heterostructures. In these systems, the charge-to-orbital conversion mainly occurs at the Cu/Oxide interface via the orbital Rashba-Edelstein effect. Although we observe sizeable orbital torque in all structures, the torque efficiency varies more than twice depending on the Oxide. Moreover, the torque efficiency can be changed by annealing and/or oxygen incorporation by a maximum of three times.

Spin-orbit torque is a current-induced transfer of angular momentum from an atomic lattice to magnetic order. It is a promising mechanism to write magnetic memories and drive spin torque oscillators. Since its inception, the list of spin-orbit torque mechanisms has grown beyond the conventional spin Hall and Rashba-Edelstein mechanisms to include “unconventional” mechanisms, arising from spin and orbital current generation in ferromagnetic layers, nonmagnetic layers, and their interfaces. In this talk, we use micromagnetic, semiclassical, and first principles calculations to show that unconventional spin-orbit torques are potentially important for devices, from causing nonlocal spin torques in ferromagnetic trilayers to enabling large amplitude, easy-plane spin-orbit torque oscillators.

We’re developing novel low-cost low-power rad-hard miniaturized solid-state magnetometers for planetary magnetic field sensing. This technology leverages spin-carrying quantum centers (QC) in wide-bandgap semiconductors such as diamond or silicon carbide. The U.S. Planetary Science Decadal Survey identifies increased need for remote-sensing capabilities such as Magnetometers. We show recent progress in developing our optically pumped solid state quantum magnetometer (OPuS-MAGNM) and electrically detected silicon carbide magnetometer (SiCMAG) instruments, aiming to tackle performance, mass and power requirements of planetary science magnetometry.

A very simple class of measurements based upon interactions of pairs of electrons called near zero field magnetoresistance (NZFMR) spectroscopy has much of the analytical power of much more complex electron paramagnetic techniques in identifying the physical and chemical nature of point defects which play important roles in the operation of a wide variety of solid state devices. We show that this very simple measurement can provide information about hyperfine interactions with dominating defects in technologically important devices.

Magnetometers have applications in many aspects of our lives: from navigation, the detection of submarines, and mobile phones to the exploration of our solar system. Recently, solid-state magnetometers have stimulated interest due to their smaller size, weight, and power compared to existing magnetometers, and their potential to self-calibrate; two features which improve the efficiency of any device carrying a magnetometer. In this work we model optically detected magnetic resonance of the silicon vacancy in 6H-SiC, a budding candidate for solid-state quantum magnetometry. We model photoluminescence with steady-state density matrix populations and Lindblad master equations, and compare to recent experimental results.

Controlling dissipation in electronic devices is one of the major challenges in modern electronics and spintronics. There is also a parallel research direction to understand the thermodynamics of quantum systems, dubbed quantum thermodynamics, focusing on how heat engines or the statistical mechanics concept of Maxwell's demon, would translate into the quantum domain. Implementing a Maxwell's demon is attractive from a spintronics point of view, as it offers the possibility to control and even reverse dissipation in a device as long as another resource, namely demon's memory, is being consumed. In this talk, I will propose and theoretically investigate a novel spintronics device: a Maxwell's demon implementation based on the spin-momentum locking property of topological insulators. Available nuclear spins of the material acts as a memory resource. I will show how in this device, the dynamically polarized nuclear spins can convert ambient heat into electrical energy, thus reversing dissipation. I will then demonstrate the breakthrough property of this device; it can reach the Landauer's limit i.e an energy $k_B Tln2$ per polarized nuclear spin can be extracted in the form of electrical work. Next, I will focus on the high voltage limit and show that the current-voltage characteristics approach that of an ideal memristor. I will conclude by giving an overview of the current experimental progress towards implementation of this device.

Magnons, the collective excitations of spin waves in magnetically ordered systems, can couple with microwave photons via dipolar interaction, demonstrating level repulsion of the hybridized modes. However, the hybridized modes can also coalesce, resulting in level attraction. Efficient control of magnon-photon coupling, for example, by spin torque, is important for creating hybrid magnonic quantum systems. Here, we theoretically examine the role of damping and field-like torques in the magnon-photon coupling process [J. Magn. Magn. Mater 571, 170558 (2023)]. Our analysis suggests that enhancing the coupling strength for level repulsion is possible if a certain magnitude and direction of the dc current density is reached. Research supported by the U.S. Department of Energy, Office of Basic Energy Sciences, Division of Materials Sciences and Engineering under Award DE-SC0020308.

Nonreciprocal transport is important in technology; asymmetry in the current-voltage response in semiconductor junctions has been the basis for diodes and transistors. Nonreciprocal superconductivity, however, proved elusive and only recently was the superconducting diode effect achieved using magnetic fields. Recently, we demonstrated a Josephson diode (JD) using a quantum material Josephson junction without requirng an applied magnetic field. Using an inversion symmetry breaking 2D-heterostructure, half-wave rectification was achieved with low switching current density and high rectification ratio. Recently, a plethora of nonreciprocal QMJJ and other architectures have been investigated and future directions for optimization and novel exploration will be discussed.

Monolayers of the superconducting transition metal dichalcogenides (TMD), eg. NbSe2, exhibit a large anisotropy in the superconducting critical field due to Ising superconductivity. Doping Ising superconductors or engineering heterostructures with magnetic materials provides a unique opportunity to explore the interplay between proximity-induced magnetism, spin-orbit coupling and superconductivity. In this talk I will provide an overview of the physics of Ising superconductivity and how it is impacted by magnetism and point defects. Finally I will show how our comprehensive first-principles calculations provide microscopic insight into a number of novel experiments, which includes Josephson junctions based on Ising superconductors and 2D magnets and non-trivial doping of Ising superconductors using silicon, sulfur and molybdenum. This work was performed in collaboration with Daniel Agterberg, Menashe Haim, Sergii Khmelevskyi, Maxim Khodas, Igor Mazin, Miguel Ugeda and was supported by ONR.

An ability to control spin currents is important for probing many spin related phenomena in the field of spintronics and for designing logic and memory devices with low dissipation. Spin-orbit torque is an important example in which spin current flows across magnetic interface and helps to control magnetization dynamics. In this talk, I will discuss the spin superfluid transport associated with collective modes in magnetic insulators. We observe that in two dimensional systems at finite temperatures spin superfluidity is affected by the presence of topological defects. We further propose to use the Hall response of topological defects, such as merons and antimerons, to spin currents in 2D magnetic insulator with in-plane anisotropy for identification of the Berezinskii-Kosterlitz-Thouless (BKT) transition in a transistor-like geometry. Our numerical results relying on a combination of Monte Carlo and spin dynamics simulations show transition from spin superfluidity to diffusive spin transport, accompanied by the universal jump of the spin stiffness and exponential growth of the transverse vorticity current. We propose a superfluid spin transistor in which the spin and vorticity currents are modulated by tuning the in-plane magnet across BKT transition, e.g., by changing the exchange interaction, magnetic anisotropy, or temperature [1-3].