arXiv subjects

Hamed Vakili

Publications and source records attributed to Hamed Vakili.

18 recordsLinked to original sources

arXiv2026

Supercurrent-Driven Néel Torque in Superconductor/Altermagnet Hybrids

We predict a supercurrent-driven Néel spin-orbit torque in a superconductor/$d$-wave altermagnet heterostructure, associated with the emergence of spin-triplet correlations. The effect can be understood as a consequence of the supercurrent-induced spin polarization, owing to the interplay between spin orbit coupling and momentum-dependent spin splitting, as found, for example, in altermagnets. Remarkably, the supercurrent can be tuned by the Néel-vector direction, and the supercurrent-induced torque can both propel magnetic domain walls and reverse the Néel-vector orientation within a domain wall. These findings establish superconductor/altermagnet heterostructures as a versatile platform for the {supercurrent-driven} control of the Néel vector, with potential applications in racetrack memory, dissipationless superconducting electronics, and unconventional computing.

cond-mat.mes-hall↗

arXiv2026

Gate-Controlled Spin Qubits in Confined Altermagnets

We propose gate-defined spin qubits in electrostatically confined altermagnetic quantum dots. Elliptical confinement of the $d$-wave altermagnetic structure produces a low-energy doublet with opposite spin polarization. For the range of parameters used here, the qubit states energy gap lies in the microwave range while the leakage gap remains in the meV range. Even without spin-orbit coupling, time-dependent simulations show that a phase-controlled quadrupolar gate drive about a fixed bias point implements $X_{π/2}$ and $X_π$ rotations by resonantly modulating the confinement anisotropy. We extend the study to two-qubits using a double quantum dot. We show that the double quantum dot spectrum can be cleanly projected onto isolated quantum dot product states with a nonzero nonlocal Pauli block in the effective logical two-qubit Hamiltonian. Resonant central-barrier modulation then drives the logical two-qubit component close to a maximally entangled state. These calculations show anisotropic altermagnetic quantum dots as a route to locally gate-controlled spin qubits without requiring spin-orbit coupling.

cond-mat.mes-hall↗

arXiv2026

Thermomagnonic Torques in Insulating Altermagnets

We develop a symmetry-controlled theory of anisotropic thermomagnonic torques in insulating altermagnets. We identify a spin-splitter magnonic torque linked to thermally generated, sublattice-odd spin currents and an anisotropic entropic torque dictated by crystal symmetry. These torques produce anisotropic magnetic-texture responses to temperature gradients. In particular, thermally generated spin currents induce domain-wall precession, which reduces domain-wall velocities for selected gradient directions. We also predict an anisotropic skyrmion Hall response, with symmetry-selected directions enabling fast skyrmion motion with strongly suppressed transverse deflection. Our results reveal experimentally testable symmetry fingerprints of insulating altermagnets and extend more broadly to anisotropic magnets with exchange-driven magnon spin splitting.

cond-mat.mes-hall↗

arXiv2025

Switching perpendicular magnets for Processing-in-memory with voltage gated Weyl Semimetals

Processing-in-memory (PIM) reduces data transfer latency by rolling memory and logic elements into one compute location. As an emergent material candidate for such an architecture, we propose a strained Weyl semimetal based spin-orbit-torque random-access memory (SWSM-SOTRAM) device. The spin-orbit torque (SOT) originates from two mechanisms: (1) the inverse spin Galvanic effect (iSGE), which generates nonequilibrium in-plane spin accumulation at interfaces, and (2) a bulk spin Hall effect (SHE), which produces a transverse spin current carrying out-of-plane spin angular momentum. The latter is tunable via an exchange Zeeman field. Both effects are evaluated using the tight-binding model coupled with a nonequilibrium Green's function (TB-NEGF) formalism for quantum transport. Information write is achieved through SOT switching of an out-of-plane free magnet. A piezo attached to a magnetostrictive selector modulates the strain in the latter, leading to the rotation of the magnetization and hence the exchange Zeeman field exerted on the Weyl semimetal. This strain-controlled exchange field enables the symmetry tuning of the Weyl semimetal and modulation of its spin Hall effect. The TB-NEGF calculations of SHE and iSGE, combined with Landau-Lifshitz-Gilbert (LLG) simulations of magnetization dynamics, establish the SOT switching mechanism and demonstrate a pathway toward the SWSM-SOTRAM PIM device.

cond-mat.mes-hall↗

arXiv2025

Effect of Magnetic Anisotropy and Gradient-Induced Dzyaloshinskii-Moriya Interaction on the Formation of Magnetic Skyrmions

Topological spin textures (e.g. skyrmions) can be stabilized by interfacial Dzyaloshinskii-Moriya interaction (DMI) in the magnetic multilayer, which has been intensively studied. Recently, Bloch-type magnetic skyrmions stabilized by composition gradient-induced DMI (g-DMI) have been observed in 10-nm thick CoPt single layer. However, magnetic anisotropy in gradient-composition engineered CoPt (g-CoPt) films is highly sensitive to both the relative Co/Pt composition and the film thickness, leading to a complex interplay with g-DMI. The stability of skyrmions under the combined influence of magnetic anisotropy and g-DMI is crucial yet remains poorly understood. Here, we conduct a systematic study on the characteristics of magnetic skyrmions as a function of gradient polarity and effective gradient (defined as gradient/thickness) in g-CoPt single layers (thickness of 10-30 nm) using magnetic force microscopy (MFM), bulk magnetometry, and topological Hall effect measurements. Brillouin light scattering spectroscopy confirms that both the sign and magnitude of g-DMI depend on the polarity and amplitude of the composition gradient in g-CoPt films. MFM reveals that skyrmion size and density vary with g-CoPt film thickness, gradient polarity, and applied magnetic field. An increased skyrmion density is observed in samples exhibiting higher magnetic anisotropy, in agreement with micromagnetic simulations and energy barrier calculations.

cond-mat.mtrl-sci↗

arXiv2025

Spin-Transfer Torque in Altermagnets with Magnetic Textures

We predict the existence of anisotropic spin-transfer torque effect in textured altermagnets. To this end, we generalize the Zhang-Li torque to incorporate the symmetry associated with prototypical $d$-wave altermagnets and identify the spin-splitter adiabatic and nonadiabatic torques. Applying our results to domain wall dynamics induced by spin-transfer torque, we find that, in certain regimes, the spin-splitter adiabatic torque can induce domain wall precession, significantly slowing down domain wall motion. The response of the domain wall also becomes anisotropic, reflecting the $d$-wave symmetry of the altermagnet. Furthermore, we observe that the spin-splitter adiabatic torque modifies skyrmion dynamics, inducing anisotropic skyrmion Hall effect. The above phenomena can serve as a hallmark of altermagnetism in textured magnets, distinguishing it from the behavior of ordinary antiferromagnets.

cond-mat.mes-hall↗

arXiv2024

Room Temperature Magnetic Skyrmions in Gradient-Composition Engineered CoPt Single Layers

Topologically protected magnetic skyrmions in magnetic materials are stabilized by interfacial or bulk Dzyaloshinskii-Moriya interaction (DMI). Interfacial DMI decays with increase of the magnetic layer thickness in just a few nanometers and bulk DMI typically stabilizes magnetic skyrmions at low temperatures. Consequently, more flexibility in manipulation of DMI is required for utilizing nanoscale skyrmions in energy efficient memory and logic devices at room temperature (RT). Here, we demonstrate the observation of RT skyrmions stabilized by gradient DMI (g-DMI) in composition gradient engineered CoPt single layer films by employing topological Hall effect, magnetic force microscopy, and nitrogen vacancy scanning magnetometry. Skyrmions remain stable at a wide range of applied magnetic fields and are confirmed to be nearly Bloch-type from micromagnetic simulation and analytical magnetization reconstruction. Furthermore, we observe skyrmion pairs which may be explained by skyrmion-antiskyrmion interactions. Our findings expand the family of magnetic materials hosting RT magnetic skyrmions by tuning g-DMI via gradient polarity and choice of magnetic elements.

cond-mat.mes-hall↗

arXiv2024

Strained topological insulator spin-orbit torque random access memory (STI-SOTRAM) bit cell for energy-efficient Processing in Memory

We present a novel design of a strained topological insulator spin-orbit torque random access memory (STI-SOTRAM) bit cell comprising a piezoelectric/magnet (gating)/topological insulator (TI)/magnet (storage) heterostructure that leverages the TI's high charge-to-spin conversion efficiency coupled with the piezo-induced strain-based gating mechanism for low-power in-memory computing. The piezo-induced strain effectively modulates the conductivity of the topological surface state (TSS) by altering the gating magnet's magnetization from out-to-in-plane, facilitating the storage magnet's spin-orbit torque (SOT) switching. Through comprehensive coupled stochastic Landau-Lifshitz-Gilbert (LLG) simulations, we explore the device dynamics, anisotropy-stress phase space for switching, and write conditions and demonstrate a significant reduction in energy dissipation compared to conventional heavy metal (HM)-based SOT switching. Additionally, we project the energy consumption for in-memory Boolean operations (AND and OR). Our findings suggest the promise of the STI-SOTRAM for low-power, high-performance edge computing.

cond-mat.mes-hall↗

arXiv2024

Field-free Josephson diode effect in a d-wave superconductor heterostructure

We study superconductor/normal region/superconductor (S|N|S) Josephson junction formed using superconductors with d, d + id', and d + is superconducting pairings. We show that the quality factor of the Josephson diode effect and its sign can be substantially tuned by the external magnetic field, gate voltage, and the length of the junction for all three types of pairings. We also identify the conditions under which the anomalous Josephson and Josephson diode effects can appear in the junction by analyzing appropriate symmetries. In particular, by breaking a π-rotation symmetry, we show how a large field-free Josephson diode effect can be realized even in the absence of spin-orbit coupling. We also study the role of edge states appearing in the case of chiral superconductor with d + id' pairing. Our results demonstrate that the Josephson diode effect in a planar geometry can be used as a signature of unconventional superconducting pairings.

cond-mat.supr-con↗

arXiv2023

Majorana bound states in d-wave superconductor planar Josephson junction

We study phase-controlled planar Josephson junctions comprising a two-dimensional electron gas with strong spin-orbit coupling and d-wave superconductors, which have an advantage of high critical temperature. We show that a region between the two superconductors can be tuned into a topological state by the in-plane Zeeman field, and can host Majorana bound states. The phase diagram as a function of the Zeeman field, chemical potential, and the phase difference between superconductors exhibits the appearance of Majorana bound states for a wide range of parameters. We further investigate the behavior of the topological gap and its dependence on the type of d-wave pairing, i.e., d, d+is, or d+id', and note the difficulties that can arise due to the presence of gapless excitations in pure d-wave superconductors. On the other hand, the planar Josephson junctions based on superconductors with d+is and d+id' pairings can potentially lead to realizations of Majorana bound states. Our proposal can be realized in cuprate superconductors, e.g., in a twisted bilayer, combined with the layered semiconductor Bi2O2Se.

cond-mat.supr-con↗

arXiv2023

A simplified drift-diffusion model for pandemic propagation

Predicting Pandemic evolution involves complex modeling challenges, often requiring detailed discrete mathematics executed on large volumes of epidemiological data. Differential equations have the advantage of offering smooth, well-behaved solutions that try to capture overall predictive trends and averages. We further simplify one of those equations, the SIR model, by offering quasi-analytical solutions and fitting functions that agree well with the numerics, as well as COVID-19 data across a few countries. The equations provide an elegant way to visualize the evolution, by mapping onto the dynamics of an overdamped classical particle moving in the SIR configuration space, drifting down gradient of a potential whose shape is set by the model and parameters in hand. We discuss potential sources of errors in our analysis and their growth over time, and map those uncertainties into a diffusive jitter that tends to push the particle away from its minimum. The combined physical understanding and analytical expressions offered by such an intuitive drift-diffusion model could be particularly useful in making policy decisions going forward.

physics.bio-ph↗

arXiv2022

Low power In Memory Computation with Reciprocal Ferromagnet/Topological Insulator Heterostructures

The surface state of a 3D topological insulator (3DTI) is a spin-momentum locked conductive state, whose large spin hall angle can be used for the energy-efficient spin orbit torque based switching of an overlying ferromagnet (FM). Conversely, the gated switching of the magnetization of a separate FM in or out of the TI surface plane, can turn on and off the TI surface current. The gate tunability of the TI Dirac cone gap helps reduce its sub-threshold swing. By exploiting this reciprocal behaviour, we can use two FM/3DTI heterostructures to design a 1-Transistor 1-magnetic tunnel junction random access memory unit (1T1MTJ RAM) for an ultra low power Processing-in-Memory (PiM) architecture. Our calculation involves combining the Fokker-Planck equation with the Non-equilibrium Green Function (NEGF) based flow of conduction electrons and Landau-Lifshitz-Gilbert (LLG) based dynamics of magnetization. Our combined approach allows us to connect device performance metrics with underlying material parameters, which can guide proposed experimental and fabrication efforts.

cond-mat.mes-hall↗

arXiv2022

Spin Hall effect of vorticity

Using mapping between topological defects in an easy-plane magnet and electrical charges, we study interplay between vorticity and spin currents. We demonstrate that the flow of vorticity is accompanied by the transverse spin current generation -- an effect which can be termed as the spin Hall effect of vorticity. We study this effect across the BKT transition and establish the role of dissipation and spin non-conservation in the crossover from spin superfluidity to diffusive spin transport. Our results pave the way for low power computing devices relying on vorticity and spin flows.

cond-mat.mes-hall↗

arXiv2021

Positional stability of skyrmions in a racetrack memory with notched geometry

Magnetic skyrmions are chiral spin textures with attractive features, such as ultra-small size, solitonic nature, and easy mobility with small electrical currents that make them promising as information-carrying bits in low-power high-density memory, and logic applications. However, it is essential to guarantee the positional stability of skyrmions for reliable information extraction. Using micromagnetic simulations for the minimum energy path (MEP), we compute the energy barriers associated with stabilizing notches along a racetrack. We vary material parameters, specifically, the strength of the chiral Dzyaloshinskii-Moriya interactions (DMI), the notch geometry, and the thickness of the racetrack to get the optimal barrier height. We find that the reduction of skyrmion size as it squeezes past the notch gives rise to the energy barrier. We find a range of energy barriers up to ~ 45 kBT for a racetrack of 5 nm thickness that can provide years long positional lifetime of skyrmions for long-term memory applications while requiring a moderate amount of current (~ 10^10 A/m2) to move the skyrmions. Furthermore, we derive quasi-analytical equations to estimate the energy barrier. We also explore other pinning mechanisms, such as a local variation of material parameters in a region, and find that notched geometry provides the highest energy barrier. Our results open up possibilities to design practical skyrmion-based racetrack geometries for spintronics applications.

cond-mat.mes-hall↗

arXiv2021

Anatomy of nanomagnetic switching at a 3D Topological Insulator PN junction

A P-N junction engineered within a Dirac cone system acts as a gate tunable angular filter based on Klein tunneling. For a 3D topological insulator with substantial bandgap, such a filter can produce a charge-to-spin conversion due to the dual effects of spin-momentum locking and momentum filtering. We analyze how spins filtered at an in-plane topological insulator PN junction (TIPNJ) interact with a nanomagnet, and argue that the intrinsic charge-to-spin conversion does not translate to an external gain if the nanomagnet also acts as the source contact. Regardless of the nanomagnet's position, the spin torque generated on the TIPNJ is limited by the surface current density, which in turn is limited by bulk band gap. {Using quantum kinetic models, we calculate the spatially varying spin potential and quantify the localization of the current vs applied bias}. Additionally, with the magnetodynamic simulation of a soft nanomagnet, we show that the PN junction can offer a critical gate tunability in the switching probability of the nanomagnet, with potential applications in probabilistic neuromorphic computing.

cond-mat.mes-hall↗

arXiv2021

Computing and Memory Technologies based on Magnetic Skyrmions

Solitonic magnetic excitations such as domain walls and, specifically, skyrmionics enable the possibility of compact, high density, ultrafast,all-electronic, low-energy devices, which is the basis for the emerging area of skyrmionics. The topological winding of skyrmion spins affects their overall lifetime, energetics and dynamical behavior. In this review, we discuss skyrmionics in the context of the present day solid state memory landscape, and show how their size, stability and mobility can be controlled by material engineering, as well as how they can be nucleated and detected. Ferrimagnetsnear their compensation points are important candidates for this application, leading to detailed exploration of amorphous CoGd as well as the study of emergent materials such as Mn$_4$N and Inverse Heusler alloys. Along with material properties, geometrical parameters such as film thickness, defect density and notches can be used to tune skyrmion properties, such as their size and stability. Topology, however, can be a double-edged sword, especially for isolated metastable skyrmions, as it brings stability at the cost of additional damping and deflective Magnus forces compared to domain walls. Skyrmion deformation in response to forces also makes them intrinsically slower than domain walls. We explore potential analog applications of skyrmions, including temporal memory at low density, and decorrelator for stochastic computing at a higher density that capitalizes on their interactions. We summarize the main challenges to achieve a skyrmionics technology, including maintaining positional stability with very high accuracy, electrical readout, especially for small ferrimagnetic skyrmions, deterministic nucleation and annihilation, and overall integration with digital circuits with the associated circuit overhead.

cond-mat.mtrl-sci↗

arXiv2021

Self focusing Hybrid Skyrmions in spatially varying canted ferromagnetic systems

Magnetic skyrmions are quasiparticle configurations in a magnetic film that can act as information carrying bits for ultrasmall, all-electronic nonvolatile memory. The skyrmions can be nucleated and driven by spin-orbit torque from a current driven in a heavy metal underlayer. Along its gyrotropic path, a Magnus force can cause a skyrmion to be annihilated at the boundaries. By combining interfacial and bulk Dzyaloshinskii-Moriya interactions (DMIs), for instance by using a B20 material on top of a heavy metal layer with high spin-orbit coupling, it is possible to engineer a hybrid skyrmion that will travel parallel to the racetrack with zero Magnus force. We show that by using a spatially varying interfacial DMI, a hybrid skyrmion will automatically self-focus onto such a track as its domain angle evolves along the path. Furthermore, using a gate driven voltage controlled magnetic anisotropy, we can control the trajectory of the hybrid skyrmion and its eventual convergence path and lane selection in a racetrack geometry.

cond-mat.mes-hall↗

arXiv2020

Temporal Memory with Magnetic Racetracks

Race logic is a relative timing code that represents information in a wavefront of digital edges on a set of wires in order to accelerate dynamic programming and machine learning algorithms. Skyrmions, bubbles, and domain walls are mobile magnetic configurations (solitons) with applications for Boolean data storage. We propose to use current-induced displacement of these solitons on magnetic racetracks as a native temporal memory for race logic computing. Locally synchronized racetracks can spatially store relative timings of digital edges and provide non-destructive read-out. The linear kinematics of skyrmion motion, the tunability and low-voltage asynchronous operation of the proposed device, and the elimination of any need for constant skyrmion nucleation make these magnetic racetracks a natural memory for low-power, high-throughput race logic applications.

physics.app-ph↗