Session Q21: Spin-Dependent Phenomena in Semiconductors: Emerging Devices beyond 2D

Topological Superconductivity and Diode Effects in Semiconductor-Based Josephson Junctions

We theoretically investigate the magnetic field dependence of the current-phase relation (CPR) and critical currents in planar Josephson junctions formed in a semiconducting two-dimensional electron gas proximitized by s-wave superconductors. We show how magnetic-field-induced jumps in the ground-state phase difference across the junction can be extracted from the magneto-CPR and provide a simple analytical expression for estimating the Rashba spin-orbit coupling strength from the size of the ground-state phase jumps [1]. Performing numerical simulations we reveal how to use the magneto-CPR to closely map the junction’s phase diagram and the topological gap protecting the Majorana bound states from local perturbations [1]. The ground-state phase behavior together with the phase diagram provides a robust description of the junction transition into the topological superconducting state. We also study the signatures of the superconducting diode effect (SDE) on the magneto-CPR and critical currents. We discuss possible mechanisms leading to the SDE sign changes observed in recent experiments [2,3] as well as the effects of the Rashba spin-orbit strength and junction transparency on the magnetic field dependence of the forward and reverse critical currents [1,2].

 

[1] B. Pekerten, A. C. Prasannan, B. Scharf, and A. Matos-Abiague (unpublished).

[2] N. Lotfizadeh, B. Pekerten, P. Yu, B. H. Elfeky, W. Strickland, A. Matos-Abiague, and J. Shabani, arXiv:2303.01902 (2023).

[3] A. Costa, C. Baumgartner, S. Reinhardt, J. Berger, S. Gronin, G. C. Gardner, T. Lindemann, M. J. Manfra, J. Fabian, D. Kochan, N. Paradiso, and C. Strunk, Nat. Nanotechnol. (2023).   

Epitaxial growth and characterization of superconductor Al / ferromagnetic semiconductor (In,Fe)As heterostructures on InP substrates

              Heterostructures of superconductor (SC) / n-type ferromagnetic semiconductor (FMS) (In,Fe)As are expected to realize topological SCs without external magnetic field because n-type FMS (In,Fe)As exhibits unique properties such as spontaneous spin-splitting [1], gate controllability of ferromagnetism [2] and spin-triplet superconductivity [3]. High-quality Al / (In,Fe)As heterostructures will be promising for superconducting spintronics devices.

              In this work, we have successfully grown Al / FMS (In,Fe)As heterostructures on InP (001) substrates by molecular beam epitaxy. The (In,Fe)As layers were grown on (In_1-y,Al_y)As (y = 0.10, 0.15, 0.19) buffer layers, and thus receive tunable in-plane compressive strain. Unlike the conventional electron-induced ferromagnetism, the (In,Fe)As layers with large strain show enhanced ferromagnetism while being highly resistive at low temperatures, possibly because the density of interstitial Fe atoms increases as the compressive strain increases. On the other hand, the top Al layer with single crystal domains exhibits a superconducting transition at around 1.2 K.

[1] L. D. Anh et al., Nature Comm. 7, 13810 (2016). [2] L. D. Anh et al, PRB 92, 161201(R) (2015). [3] T. Nakamura et al., PRL 122, 107001 (2019).   

Nuclear spin induced scattering between quantum Hall edges with opposite spin polarization

We consider the nuclear spin induced scattering between two quantum Hall edge state channels with opposite spin polarization. Nonequilibrium spin polarization of nuclear spins is calculated, and dependence on temperature, voltage and magnetic field is analyzed. Nuclear spin relaxation due to spin diffusion caused by magnetic dipole-dipole interactions is considered. A renormalization group (RG) analysis of the hyperfine interaction is performed and a RG flow is obtained for the filling factor $ u=1/q$ case, $q$ being an odd integer. The RG flow appears to be different from that for edge states of a 2D topological insulator. Our results are relevant to recent experiments on nuclear spin related transport in the quantum Hall system and may provide a way to obtain dynamic nuclear polarization.   

Unconventional Spin Hall Effect in Low Symmetry Semimetal for Large Spin-Orbit Readout Unit

This study explores the utilization of unconventional spin Hall effects in a new class of 2D materials for advanced spintronic devices beyond CMOS. The crystalline symmetry breaking in few-layer MoTe₂ enables the coexistence of a large spin Hall angle (θ_SH) and extended spin diffusion length (L_S), as well as relaxing the stringent requirement of orthogonality between spin polarization, spin current, and charge current. Our experimental investigation focuses on the characterization of the unconventional spin Hall conductivity ( ) in few-layer MoTe₂, achieved through the deliberate disruption of mirror symmetry in the crystal structure. Our device design, based on the magnetoelectric spin-orbit (MESO) concept, exhibits a large output resistance of . This notable performance is attributed to the long spin diffusion length and high resistivity of the semi-metallic channel, distinguishing it from conventional metallic giant spin Hall effect (GSHE) materials. Furthermore, we observe a linear increase in the device output with increasing temperature and device dimension scaling, leading to a projected value of . This promising result demonstrates a two-order of magnitude improvement over the current MESO device designs with conventional materials.   

Landauer-Buettiker approach for the spin Hall angle in monolayer-Xenes

We propose a model to calculate the spin Hall angle for the elemental monolayers of group IV-Xenes. This model is based on Landauer-Buttiker formalism for quantum transport [1], in the presence of manual defects, manual dislocations, and interface-induced spin-orbit coupling. The spin-Hall angle of these devices also illustrates the mesoscopic fluctuations, which can be a comparative measure for graphene and other metallic devices. To validate our outcomes, we compare our results with experimental data and numerically extract real-space simulation results based on the nearest-neighbor tight binding model. This work will serve to be a template to calculate spin-Hall conductivity for all the elemental monolayers [2], considering the intrinsic scattering mechanism. This work can be further extended to other material combinations [3] also. 

References:

[1] da Silva, Juliana M., et al. "Spin Hall angle in single-layer graphene." Journal of Applied Physics 132.18 (2022).

[2] Farzaneh et al. Spin splitting and spin Hall conductivity in buckled monolayers of group 14: First-principles calculations. Physical Review B, 104(11), 115205.

[3] Safeer, C. K., et al. "Room-temperature spin Hall effect in graphene/MoS2 van der Waals heterostructures." Nano letters 19.2 (2019): 1074-1082.   

Advancing quantum transport theory for 2D-topological electronics: from quantum matter to emerging devices

This talk concerns quantum transport theory aided explorations on translating topological quantum matter into viable emerging device paradigms.  First, we present a realizable device design for an all-electrical robust topological valley filter that utilizes spin-protected topological kink states hosted on monolayer 2D-Xene materials with large intrinsic spin-orbit coupling. We elucidate the role of spin-orbit coupling in achieving an improved valley filter performance with a perfect quantum of conductance attributed to the topologically protected kink states. We further elaborate clearly the right choice of material, device geometry and other factors that need to be considered for such a functionality. Crucially, we elucidate how gating techniques can be utilized toward realizing “on-demand” topological symmetry protection. We then extend these ideas to propose a topological quantum field-effect transistor (TQFET) that can potentially be engineered to enable sub-thermionic transistor operation coupled with dissipationless ON-state conduction. We finally discuss the applications toward understanding many recent experiments in the rapidly emerging field of spin-valley qubits in bilayer graphene quantum dots. 

    

Proposal for a high-speed 2D-Xene-based antiferromagnetic memory cell

We propose a read-write switch using antiferromagnets coupled with a spin-valley locked channel [1] in a 2D Xene material. We couple the Keldysh non-equilibrium Green’s function (NEGF) [2] formalism with the anti-ferromagnetic Landau-Lifshitz-Gilbert (LLG) equation to uncover novel capabilities of our device structure to store and manipulate spin information.

The simulations are based on the coupled NEGF-LLG framework using an implicit Gauss-Seidel Project scheme. We utilize the high magnetoresistance offered in the topological quantum spin valley Hall phase [1,3] to develop a reading mechanism to store bits as orthogonal configurations of the free antiferromagnetic lead. The proposed device induces a non-equilibrium spin density staggered across the sublattices to generate a spin-transfer torque to switch from the low-resistance state to the high-resistance state efficiently. We also demonstrate pico-second order switching in the memory cell with low critical current and energy cost.

[1] K. Jana and B. Muralidharan, npj 2D Mat. and Appl., 6, 19, (2022). 

[2] Datta, S. (2005). Quantum Transport: Atom to Transistor. Cambridge: Cambridge University Press.

[3] Yafang Xu, G. J. Phys. Rev. B 95, 155425, (2017).   

Spin defects and circulating currents in two-dimensional tight-binding models

Single spins associated with point defects in solid-state materials are promising candidates for qubits and novel quantum spintronic devices for communication, sensing, and information processing [1] . To further understand the electronic interactions for qubit gates a microscopic theory of the spin-correlated currents associated with localized spins in semiconductors is needed. These spin-correlated currents are dissipationless and can be computed analytically for simple continuum models of quantum dots [2] as well as numerically for more complex geometries [3]. However similar results are not available for tight-binding descriptions of spin centers in semiconductors.

Here we extend on our previous work [4] and present a tight-binding theory of dissipationless circulating current induced by a spin defect in two-dimensional materials. An effective model for graphene which takes into account the spin-orbit interaction due to the influence of high-energy unoccupied d orbital is used to calculate circulating currents associated with the broken time-reversal symmetry caused by the local magnetic moment of local spin defects. The formalism can be extended to other two-dimensional lattice materials with stronger atomic spin-orbit coupling, such as the recently synthesized group IV two-dimensional germanene and silicene, thus opening up a possibility to be used at higher temperatures. In our previous work [4] we showed that the dissipationless circulating currents are significant and their anisotropic spatial structure can be detectable by current nanoscale magnetometry [5]. The spatial structure of the defect orbital magnetic moment affects its coupling to nearby rapidly oscillating fields from, e.g., nuclear spins, with implications for spin dynamics and coherent control of single spin states.

[1] Wolfowicz, G. et al., Nat. Rev. Mater. 6, 906 (2021)

[2] van Bree, J., Silov, A. Yu, Koenraad, P.M. and Flatté M.E., Phys. Rev. Lett. 112, 187201 (2014)

[3] van Bree, J., et al Phys. Rev. B 93, 035311 (2016)

[4] da Cruz, A.R. and Flatté, M.E., Phys. Rev. Lett. 131, 086301 (2023)

[5] Palm, M. L. , et al., Phys. Rev. Appl. 17, 054008 (2022)   

An First Principles exploration of ferrimagnetic properties from doped and undoped V(TCNE)2 and similar metal organic frameworks

Vanadium tetracyanoethylene, V(TCNE), is a room temperature ferrimagnetic semiconductor with a Tc ~ 600 K, which has very low loss ferromagnetic resonance and spin-wave propagation. [1,2,3] Recently, VTCNE has also been found to exhibit permanent porosity, with the possibility to accept dopant molecules, unveiling more ways VTCNE could be used in new magnetic devices. [4] Here, we explore the electronic structure, optical properties, magnetic dynamics, magnetic anisotropy, and magnetoelastic properties using a plane-wave DFT code VASP and XC functional Heyd-Scuseria-Ernzerhof(HSE06). [5,6,7,8] We also explore the vast possibilities of doped VTCNE and similar materials like vanadium tetracyanobenzene (VTCNB). We also present our analysis of VTCNE's structural properties using molecular dynamics calculations, which reveals mechanisms behind VTCNE's aging and explains some discrepancies we observed between experiments and our DFT model.   

Utilizing 2D spin-gapless semiconductors to achieve low subthreshold slope and non-local giant magnetoresistance in FETs

The fundamental thermionic-current switching limit of conventional CMOS transistors impedes  the performance improvement of CMOS technology. We propose a novel field-effect transistor  (FET) concept that overcomes this limitation by utilizing type-II spin-gapless semiconductors (SGSs) as source and drain electrodes in conjunction with an intrinsic semiconductor channel material [1]. The unique electronic band structure of the SGS source-drain electrodes filters the transmission of high-energy electrons in the sub-threshold region, leading to a sub-60 mV/dec sub-threshold slope (SS) value. Additionally, the SGS electrodes give rise to a non-local giant magnetoresistance effect, enabling the proposed FET to function also as a memory element. Quantum transport simulations of the proposed FET based on two-dimensional SGS VS₂ indicate a very low SS value of 10 mV/dec, a high on/off ratio  of 10⁸, and a non-local giant magnetoresistance effect of 10⁴ at room temperature. Our proposed FET combines traditional transistor functionality with energy efficiency and non-volatile memory capabilities, enabling logic-in-memory computing.