Quantization of spin and orbital angular momenta in standard quantum mechanics is rooted in the fundamental rotational symmetry of space in conjunction with the results of experiments on atomic and subatomic particles conducted with the Stern-Gerlach apparatus. We explain some of the most important features of spin and orbital angular momenta as inescapable consequences of elementary Stern-Gerlach experiments viewed in light of the rotational symmetry of space. The rotation operator is first derived for spin-½ particles, then extended to spin-1 particles by treating the case of a hydrogen atom in its ground state, where the spin of the proton combines with that of the electron to create a spin-1 triplet state. Along the way, we examine the evolution of a particle’s spin state under an applied magnetic field, where the interaction energy arises from the existence of a magnetic dipole moment for each particle in proportion to its spin angular momentum. Whereas a constant magnetic field B₀ causes the particle’s magnetic moment to gyrate around the direction of B₀, it will be seen that the presence of a small oscillating magnetic field perpendicular to B₀ brings about the so-called Rabi oscillations, which involve periodic flippings of the particle’s spin orientation. In the case of a hydrogen atom in its ground state, the interaction between the magnetic dipole moments of the proton and the electron splits the ground state between a lower energy singlet having a net angular momentum of zero (spin-0 state), and a higher energy triplet having a total angular momentum of ħ (spin-1 state). This hyperfine splitting is the source of the well-known 21-cm line observed in the spectrum of the hydrogen atom, which arises from transitions between the atom’s singlet and triplet states. In the final section, we put forward an argument (originally made by Richard Feynman) that ties the rotational symmetry of space to the conservation of angular momentum in quantum mechanics. The argument is, in fact, quite general, enabling one to deduce other conservation laws from various inherent symmetries of quantum mechanical systems.

Motivated by the recent experimental realization of a minimal Kitaev chain in quantum dot systems, we present our theoretical findings on the dynamics and fusion of MZMs at or near the “sweet spot” t_h = Δ (where the fermionic hopping th and superconducting coupling Δ are equal). We investigated the dynamics and fusion of MZMs using time-dependent real-space local density-of-states methods. The movement of Majoranas and the detection of fusion channels are crucial for topological quantum computations. Additionally, we discuss our recent discovery of exotic “multi-site” MZMs in Y-shaped Kitaev wires, which is important for the potential braiding of Majoranas in Y-junctions formed from arrays of quantum dots. Finally, we present results on ”non-trivial” fusion using canonical Kitaev wires at the sweet spot.

We present non-volatile memory (NVM) devices based on valley-spin hall effect (VSHE) in monolayer WSe2 and discuss how we utilize their unique properties for logic-memory integration in two applications: (i) compact and low power crossbar arrays capable of performing matrix-vector multiplications for binary neural networks (computing integrated within a memory macro) and (ii) energy-efficient non-volatile flip-flops (NVM integrated in logic) for energy autonomous systems. We discuss the appealing attributes of the VSHE-devices enabled by coupling WSe₂ with perpendicular magnetic anisotropy (PMA) magnets and show their utility in mitigating the design conflicts of memory devices. We show how the true and complementary bit-storage along with the integrated back gate of VSHE-devices enable the design of compact and low power compute-enabled NVM arrays for binary neural networks. Further, we utilize the VSHE-devices to design non-volatile flip-flops, which feature low power data backup (by virtue of VSHE-based write) and robust restore operation (due to differential data storage in a single VSHE-device).

The detection of optically-injected spin-polarized holes by means of inverse spin Hall effect (ISHE) in a Pt/n-doped semiconductor junction is challenging because of the faster spin relaxation of holes compared to electrons at room temperature. Nevertheless, electric fields at the junction arising from the contact potential or an externally-applied bias voltage can favor the transfer of spin-polarized holes repelling electrons from Pt. Here, we report on photo-induced ISHE measurements where spin-polarized holes are detected using two different configurations, namely, i) at low temperature in a Pt/lightly P-doped Si junction, and ii) at 300K in a non-local architecture leveraging graphene as a spin interconnect between Pt and lightly As-doped Ge. Spin-polarized holes are optically oriented with a confocal microscopy setup in the valence band of the semiconductors illuminated with circularly polarized photons with energy above the band gap. In the first device, at T < 22K in Si the spin-relaxation time of holes increases and the majority of phosphorus dopants are not ionized, hence the built-in electric field originating from the potential difference between the work functions of Si and Pt extends to the whole substrate fostering (hampering) the diffusion of holes (electrons) towards Pt. The combination of these two phenomena allows one to measure spin-polarized holes at low temperatures. In the second device, photo-generated spin-polarized holes are successfully transferred to graphene by applying a bias voltage to the graphene/Ge junction. Since graphene is characterized by a significantly-long spin-relaxation time, holes diffuse with negligible spin losses towards Pt where their spin is revealed by means of photo-induced ISHE.

The fusion of spintronics and photonics technologies is expected to reduce significantly the energy consumption of information systems. Opto-spintronic semiconductors, that can function as spin-photon interfaces, are essential for optical communication of electron spin information. However, the spin polarization of electrons is easily lost in conventional nonmagnetic semiconductors at and above room temperature (RT), at which today’s devices operate. In this work, I have focused on the 0D-2D hybrid nanostructures based on III-V semiconductor quantum dots (QDs) and dilute nitride GaNAs with defect-enabled spin filtering working at RT. I have demonstrated not only the efficient spin-photon conversion at RT by using the InGaAs-based QDs buried in a thin quantum well (QW), but also the generation of nearly fully spin-polarized electrons in the QDs by tunnel injecting spin filtered electrons from the GaNAs QW to InAs QDs. The amplification dynamics of electron spin polarization in the GaNAs/QD tunnel-coupled nanostructures was investigated by a combination of time-resolved circularly polarized photoluminescence and rate equation considering the spin capturing time in the GaNAs defect states. The GaNAs/QD structures have been used as active layers of spin-polarized light-emitting diodes and electric field effect optical spin devices operating at RT. In addition, the spin filtering of GaNAs has been utilized for the spin photodiode to recover the spin polarization of conduction band electrons. These results indicate that 0D QD-2D GaNAs QW hybrid nanostructures are promising spin-photon interfaces operating in practical conditions.

This work presents a novel method for achieving controlled polarization-selective enhancement of the Photonic Spin Hall Effect (PSHE) by integrating waveguiding effects with surface plasmonic behavior. The conventional plasmonic wave generation, limited to TM waves, is expanded by introducing waveguiding effects, enabling resonances for TE waves. This dual resonance mechanism contributes to the enhancement of PSHE for both horizontally (H) and vertically (V) polarized waves. Utilizing thin metal layers (Ag and Al) of a few nanometers in conjunction with waveguiding glass layers under 500nm thickness, significant enhancements of PSHE are demonstrated at the submillimeter scale. This integrated approach offers a promising avenue for tailoring and controlling PSHE with applications in advanced photonic devices.

Terahertz (THz) spintronic emitters is a novel class of multilayers which strongly emit THz radiation. This new type of THz source is not related to semiconductor physics but rather to ultrafast spin dynamics, spin currents and spin Hall effects. The spintronic emitters are usually composed from a ferromagnetic layer and a non-magnetic metallic layer which exhibits high spin-orbit coupling. In this work, we focus on the properties of the non-magnetic layers by studying two non-magnetic materials that are not so commonly in the THz spintronic heterostructures: Tantal (Ta) and Gold (Au). We combine the Ta and Au layers with Fe/Pt bilayers. We show the enhancement of the THz signal in Ta/Fe/Pt trilayers and its sensitivity to the individual thicknesses of the layers. We furthermore modify the growth of the Ta layer and we record how different crystallographic phases can influence the spin-to-charge conversion. We show that a transition from a polycrystalline Ta layer to an epitaxial grown one is able to reconstruct the THz emission and bandwidth. Additionally, we dope the Fe/Pt interface with Au atoms. The doping has a range from a few atoms to only 2-3 monolayers. A large enhancement of the signal is observed for a specific doping concentration. We attribute this behavior to an enlarged spin Hall angle of the Au/Pt interface. Our results show that materials engineering have a great influence in THz spintronic heterostructures and it can be of highly importance topic for the future direction of THz spintronics.

Broadband terahertz spectroscopy is a powerful tool for unraveling the complex and often competing interactions in quantum materials. In this work, I describe the development of a multi-decade terahertz spectroscopy and dilution refrigerator system suitable for the study of frequency scaling in quantum materials.

Magnetometers are not only useful for Earth-based applications, but also very important for planetary science and heliophysics investigations. This is the reason they are flown on almost all missions in space. Heritage fluxgate magnetometers and optically-pumped atomic gas magnetometers are typically large in size, weight, and require watts of power to operate, preventing their infusion onto smaller platforms like CubeSats, drones, landers, and rovers. Here, we report on the development of a 4H silicon carbide (SiC) magnetometer, promising to be a low complexity, lightweight, low power, and inexpensive alternative to these heritage technologies. It measures magnetic field induced changes in spin dependent recombination (SDR) current within a pn junction, both in vector and scalar modes, thereby giving the instrument the ability to self-calibrate in the remoteness of space.

The booming fields of antiferromagnetic spintronics and terahertz (THz) magnonics urge to understand the ultrafast dynamics triggered in antiferromagnets by ultrashort stimuli. The interest in ultrafast magnetism of antiferromagnets has led to new and vastly counter-intuitive findings in experimental and theoretical research. We report on the ultrafast spin and lattice dynamics in a rutile antiferromagnet.

Despite the enormous investigations in the past two decades, the understanding of in-plane current-induced switching of perpendicular magnetization remains elusive. This work presents the recent discoveries of two novel sin-orbit effects for magnetization manipulation, i.e., strong variation of the spin-orbit torque with the relative spin relaxation rate of the magnetic layer and the long-range Dzyaloshinskii-Moriya interaction effect.

We theoretically investigate the transport of magnon orbitals in a honeycomb antiferromagnet. We find that the magnon orbital Berry curvature is finite even without spin-orbit coupling and thus the resultant magnon orbital Hall effect is an intrinsic property of the honeycomb antiferromagnet rooted only in the exchange interaction and the lattice structure. Due to the intrinsic nature of the magnon orbital Hall effect, the magnon orbital Nernst conductivity is estimated to be orders of magnitude larger than the predicted values of the magnon spin Nernst conductivity that requires finite spin-orbit coupling. For the experimental detection of the predicted magnon orbital Hall effect, we invoke the magnetoelectric effect that couples the magnon orbital and the electric polarization, which allows us to detect the magnon orbital accumulation through the local voltage measurement. Our results pave a way for a deeper understanding of the topological transport of the magnon orbitals and also its utilization for low-power magnon-based orbitronics, namely magnon orbitronics.

Atomic core-level spectroscopy is an invaluable metrological tool in a wide array of fields, from quantum and materials science to semiconductor metrology. When applied to dynamical systems, it enables the measurement of element- and layer-specific dynamics. While such spectroscopy has been applied widely in conjunction with optical excitation of samples, its combination with a high-frequency microwave excitation is less common; in principle, however, this combination enables in operando measurements of devices. Toward this goal, we have developed an instrument that uses an RF frequency comb generator to produce high-frequency microwaves (>60GHz) that are synchronized to a tabletop, high-harmonic generation light source with <1.1ps timing jitter. This system can be used to study, with element-specificity, the switching behavior of devices at their operating frequency as well as the resonant behavior of devices or novel materials and systems. For instance, by applying an external magnetic field and tuning the microwave frequency to the ferromagnetic resonance in magnetic films, we can perform high-frequency x-ray or extreme ultraviolet detected ferromagnetic resonance (XFMR) spectroscopy. As a demonstration, we measure XFMR of three sample systems (permalloy, CoFe, and a Fe/TaO_x/Ni multilayer). In the future, we can augment this capability with coherent diffractive imaging to perform high-frequency, resonant spectroscopy with sub-100nm spatial resolution.

Quantum measurements of magnons are crucial for the development of quantum applications based on magnonics. We theoretically analyze the efficacy of Brillouin light scattering (BLS) for quantum tomography of magnons. We consider a finite-length optomagnonic waveguide made of Yttrium Iron Garnet (YIG), and derive the relation between the transmitted photons and the state of the magnons. While the signal-to-noise ratio (SNR) is low due to a small magneto-optical coupling, we show that significant improvement can be achieved by injecting squeezed vacuum of photons. Then, we discuss a protocol of reconstructing the magnon’s density matrix based on the observed statistics of the transmitted photons, using maximum likelihood principle. We find that the classical component of a magnon state, defined as the regions of positive Wigner function, can be reconstructed with a high accuracy. Reconstructing the nonclassical component requires improved SNR or larger datasets, and we explore potential methods to achieve these goals.

For certain applications such as in Artificial Intelligence and neuromorphic computing, modern computing schemes can require prohibitively large circuit- and energy-footprints. Probabilistic computing offers an alternative approach that seeks to exploit its inherently probabilistic nature to act as low-cost natural hardware accelerators for solving a range of complex problems from large-scale combinatorial optimization to Bayesian inference, and invertible Boolean logic. The base unit of probabilistic computing is known as the probabilistic bit, or p-bit, and requires tunable stochasticity; low-barrier Magnetic Tunnel Junctions (MTJs), in which the magnetization of the free layer fluctuates at room-temperature, are a natural spintronics-based solution for such high-quality random number generation and p-bit purposes. In this work, we present the experimental realization of a scaled p-bit core, integrating a stochastic in-plane MTJ with a novel multi-finger 2D-MoS₂ transistor to achieve a compact spintronics-based p-bit platform that displays true randomness and a high degree of voltage-tunable stochasticity.

Domain-wall (DW) logic holds promise for compact and energy-efficient logic circuits. Indeed, fast DW motion driven by spin-orbit torque (SOT) and the Dzyaloshinskii-Moriya interaction (DMI) in magnetic/heavy-metal multilayers led to the experimental demonstration of current-driven DW logic circuits by magnetic imaging. Advancing towards applications, we present DW devices with electrical write/read using a magnetic tunnel junction (MTJ) stack with a hybrid free layer on 300-mm wafers. The first layer provides efficient spin-transfer torque (STT) and high tunneling magnetoresistance (TMR), while the second layer enables fast SOT-driven DW motion. It allows full electrical control of nanoscale DW devices, involving write/read at input/output MTJs and SOT-driven propagation between them. Finally, to alleviate challenges in current-driven DW motion, we present a concept of chirally coupled MTJs through DMI. This enables current-free information processing in compact inverter and minority gates.

The discovery of materials with non-trivial topological properties has led to the realization of novel Josephson junctions with anomalous properties. In particular, it has been proposed that in some conditions such junctions can be in a superconducting topological state. In this work we present results for Josephson junctions based on three different heterostructures: Al/InAs, W/BiSb, and Al/Cd₃As₂. Junctions based on each of these heterostructures are predicted to have unique properties, and can in principle be tuned into a topological state, due to the fact that InAs has a very strong spin-orbit coupling, BiSb is a topological insulator, and Cd₃As₂ is a Dirac semimetal. We show how features of the Shapiro steps of the current-voltage characteristic under microwave radiation can be used, in realistic conditions, to extract detailed information on the microscopic electronic properties of the junctions, such as their topological state, and the presence of Leggett modes in the superconducting leads. We then discuss how in SQUIDs formed by some of the studied Josephson junctions a microwave-tunable diode effect might be present.

We theoretically investigate spin transport in a junction system composed of a ferromagnetic insulator (FI) and a two-dimensional electron gas (2DEG) with both Rashba and Dresselhaus spin-orbit interactions. We briefly present our findings on spin pumping into the 2DEG under microwave irradiation and subsequent current generation via the inverse Rashba-Edelstein effect. We highlight the crucial role of vertex corrections in the theoretical description of these effects.

The integration of ferromagnetic resonance (FMR) with x-ray absorption spectroscopy (XAS) as the underlying detection mechanism marks an important achievement for the exploration of magnetic interactions, as it extends the scope of x-ray magnetic circular dichroism (XMCD) to the dynamic range. This enables the direct study of magnetization dynamics with element, site, and valence state specificity and may even be employed to disentangle spin and orbital contributions to the magnetic excitations.

We have utilized these tools to individually survey the spin dynamics of coherently coupled cations in a ferrimagnetic insulator and to quantify the orbital-to-spin ratio in the magnetization dynamics of Ni in a permalloy/Ho heterostructure. Further, we were able to extend the capabilities of the technique towards time-resolved dynamic x-ray magnetic linear dichroism (XMLD) as a new tool to study GHz spin dynamics in systems beyond ferromagnetic order.

Energy-efficient spintronic devices require the following two criteria: (1) a large spin-orbit torque (SOT) and (2) low damping to excite magnetic precession with low current input. Conventional ferromagnet/nonmagnetic-metal bilayers can obtain sizeable SOTs; however, this comes at the expense of drastically increasing the damping. Because the origin or the transmission of spin is interfacial in nature, the ferromagnetic layer must be restricted to ∼1nm in thickness to see substantial SOTs. Here, we present an alternative approach to producing sizeable SOTs that allows for a thicker ferromagnetic layer maintaining low damping. Instead of relying on a single interface, we continuously break the bulk inversion symmetry with a vertical compositional gradient of two ferromagnetic elements: Fe with low intrinsic damping and Ni with sizable spin-orbit coupling. We find low effective damping parameters of α_eff < 5 × 10⁻³ in the FeNi alloy films, despite the steep compositional gradients. Moreover, we reveal a sizable anti-damping SOT efficiency of θ_AD ≈ 0.05, even without an intentional compositional gradient. Through depth-resolved x-ray diffraction, we identify a lattice strain gradient as crucial symmetry breaking that underpins the SOT. Our findings provide fresh insights into damping and SOTs in single-layer ferromagnets for power-efficient spintronic devices.

In this paper we utilize the Landau-Lifshitz equation and its properties to derive a scalar equation of motion and analyze a chain of spins and model clusters of spins in materials that in principle not are ferromagnetic, such as aluminum and others. By calculating the total energy of the system of all interactions between the spins, the Landau-Lifshitz equation governs the local behavior of spins or a chain of spins. This model is well known and so called Heisenberg model. We used the Heisenberg model to write the Hamiltonian of the system and got there the equation of that govern the spin chain dynamics. However, in some systems the approximation of a single magnetic phase may not reflect observations. For example, the observation of superparamagnetism and spontaneous magnetization reversal can be better explained through the interaction between magnetic phases. Such behavior has been observed in both nanoparticles and thin films, and can result in novel behavior such as inverted hysteresis loops. A revised Heisenberg model is proposed to describe this behavior. We also discuss the description of the pattern of the magnetization scalar field through the use of the Schrödinger-like equation, which may contribute to modeling the behavior of the magnetization field at the interface of hybrid materials.

This study explores the magneto-optical properties of the Weyl semimetal Co₃Sn₂S₂ using the Magneto-Optical Kerr Effect (MOKE) and compares these findings with those obtained from Anomalous Hall Effect (AHE) measurements. By employing both single detector MOKE and Kerr imaging techniques, we track the magnetization behavior across various temperatures, highlighting the influence of topological phases. Our results reveal a distinct Kerr rotation angle peak at 125K, attributed to the enhanced Berry curvature near Weyl points. The study demonstrates that MOKE, especially when combined with pump-probe techniques and advanced imaging, provides a comprehensive understanding of magnetic dynamics and topological contributions. These findings pave the way for future research into the interplay of electric and magnetic fields in topological materials, aiming to optimize the application of MOKE in spintronic devices and fundamental physics exploration.