Gen Yin

Chiral Majorana fermion modes in a quantum anomalous Hall insulator–superconductor structure

A propagating Majorana mode Although Majorana fermions remain elusive as elementary particles, their solid-state analogs have been observed in hybrid semiconductor-superconductor nanowires. In a nanowire setting, the Majorana states are localized at the ends of the wire. He et al. built a two-dimensional heterostructure in which a one-dimensional Majorana mode is predicted to run along the sample edge (see the Perspective by Pribiag). The heterostructure consisted of a quantum anomalous Hall insulator (QAHI) bar contacted by a superconductor. The authors used an external magnetic field as a “knob” to tune into a regime where a Majorana mode was propagating along the edge of the QAHI bar covered by the superconductor. A signature of this propagation—half-quantized conductance—was then observed in transport experiments. Science, this issue p. 294; see also p. 252 Transport experiments showing half-integer quantized conductance indicate a propagating Majorana edge mode. Majorana fermion is a hypothetical particle that is its own antiparticle. We report transport measurements that suggest the existence of one-dimensional chiral Majorana fermion modes in the hybrid system of a quantum anomalous Hall insulator thin film coupled with a superconductor. As the external magnetic field is swept, half-integer quantized conductance plateaus are observed at the locations of magnetization reversals, giving a distinct signature of the Majorana fermion modes. This transport signature is reproducible over many magnetic field sweeps and appears at different temperatures. This finding may open up an avenue to control Majorana fermions for implementing robust topological quantum computing.

Room-Temperature Skyrmion Shift Device for Memory Application

Magnetic skyrmions are intensively explored for potential applications in ultralow-energy data storage and computing. To create practical skyrmionic memory devices, it is necessary to electrically create and manipulate these topologically protected information carriers in thin films, thus realizing both writing and addressing functions. Although room-temperature skyrmions have been previously observed, fully electrically controllable skyrmionic memory devices, integrating both of these functions, have not been developed to date. Here, we demonstrate a room-temperature skyrmion shift memory device, where individual skyrmions are controllably generated and shifted using current-induced spin-orbit torques. Particularly, it is shown that one can select the device operation mode in between (i) writing new single skyrmions or (ii) shifting existing skyrmions by controlling the magnitude and duration of current pulses. Thus, we electrically realize both writing and addressing of a stream of skyrmions in the device. This prototype demonstration brings skyrmions closer to real-world computing applications.

Tailoring exchange couplings in magnetic topological-insulator/antiferromagnet heterostructures

Magnetic topological insulators such as Cr-doped (Bi,Sb)2Te3 provide a platform for the realization of versatile time-reversal symmetry-breaking physics. By constructing heterostructures exhibiting Néel order in an antiferromagnetic CrSb and ferromagnetic order in Cr-doped (Bi,Sb)2Te3, we realize emergent interfacial magnetic phenomena which can be tailored through artificial structural engineering. Through deliberate geometrical design of heterostructures and superlattices, we demonstrate the use of antiferromagnetic exchange coupling in manipulating the magnetic properties of magnetic topological insulators. Proximity effects are shown to induce an interfacial spin texture modulation and establish an effective long-range exchange coupling mediated by antiferromagnetism, which significantly enhances the magnetic ordering temperature in the superlattice. This work provides a new framework on integrating topological insulators with antiferromagnetic materials and unveils new avenues towards dissipationless topological antiferromagnetic spintronics.

Topological spin Hall effect resulting from magnetic skyrmions

The intrinsic spin Hall effect (SHE) originates from the topology of the Bloch bands in momentum space. The duality between real space and momentum space calls for a spin Hall effect induced from a real space topology in analogy to the topological Hall effect (THE) of skyrmions. We theoretically demonstrate the topological spin Hall effect (TSHE) in which a pure transverse spin current is generated from a skyrmion spin texture.

Skyrmion creation and annihilation by spin waves

Single skyrmion creation and annihilation by spin waves in a crossbar geometry are theoretically analyzed. A critical spin-wave frequency is required both for the creation and the annihilation of a skyrmion. The minimum frequencies for creation and annihilation are similar, but the optimum frequency for creation is below the critical frequency for skyrmion annihilation. If a skyrmion already exists in the cross bar region, a spin wave below the critical frequency causes the skyrmion to circulate within the central region. A heat assisted creation process reduces the spin-wave frequency and amplitude required for creating a skyrmion. The effective field resulting from the Dzyaloshinskii-Moriya interaction and the emergent field of the skyrmion acting on the spin wave drive the creation and annihilation processes.

Tunneling spectroscopy of chiral states in ultra-thin topological insulators

The temperature, Fermi-level, and bias dependencies of the inter-surface tunneling current in thin-film topological insulators show unique, identifying signatures of the surface states and their opposite chiralities. The opposite chiralities of the surface states limit the tunneling to the band edges of the gapped Dirac cones. As a result, the tunneling conductance is sensitive to the temperature, the Fermi level, and the surface-surface potential difference. The temperature dependence of the tunneling conductance changes sign as the Fermi level scans through the Dirac point. The tunneling transmission is a minimum when the opposing surface Dirac cones are perfectly aligned in energy. This minimum state of the tunneling channel can result in negative differential resistance (NDR) in the presence of a built-in Rashba-like splitting. The unique thermal response of the tunneling conductance and the existence of NDR suggest a tunneling spectroscopy experiment to demonstrate the opposite chiralities of the opp...

Coulomb impurity scattering in topological insulator thin films

Inter-surface coupling in thin-film topological insulators can reduce the surface state mobility by an order of magnitude in low-temperature transport measurements. The reduction is caused by a reduction in the group velocity and an increased sz component of the surface-state spin which weakens the selection rule against large-angle scattering. An intersurface potential splits the degenerate bands into a Rashba-like bandstructure. This reduces the intersurface coupling, it largely restores the selection rule against large angle scattering, and the ring-shaped valence band further reduces backscattering by requiring, on average, larger momentum transfer for backscattering events. The effects of temperature, Fermi level, and intersurface potential on the Coulomb impurity scattering limited mobility are analyzed and discussed.

Room-Temperature Skyrmions in an Antiferromagnet-Based Heterostructure

Magnetic skyrmions as swirling spin textures with a nontrivial topology have potential applications as magnetic memory and storage devices. Since the initial discovery of skyrmions in non-centrosymmetric B20 materials, the recent effort has focused on exploring room-temperature skyrmions in heavy metal and ferromagnetic heterostructures, a material platform compatible with existing spintronic manufacturing technology. Here, we report the surprising observation that a room-temperature skyrmion phase can be stabilized in an entirely different class of systems based on antiferromagnetic (AFM) metal and ferromagnetic (FM) metal IrMn/CoFeB heterostructures. There are a number of distinct advantages of exploring skyrmions in such heterostructures including zero-field stabilization, tunable antiferromagnetic order, and sizable spin-orbit torque (SOT) for energy-efficient current manipulation. Through direct spatial imaging of individual skyrmions, quantitative evaluation of the interfacial Dzyaloshinskii-Moriya interaction, and demonstration of current-driven skyrmion motion, our findings firmly establish the AFM/FM heterostructures as a promising material platform for exploring skyrmion physics and device applications.

Spin-Josephson effects in exchange coupled antiferromagnetic insulators

Spin Josephson effects in Exchange coupled Antiferromagnetic Insulators Yizhou Liu, 1, 2 Gen Yin, 1, 2 Jiadong Zang, 3 Roger K. Lake, 1, 2, ∗ and Yafis Barlas 2, 4, † Department of Electrical and Computer Engineering, University of California, Riverside, CA 92521, USA Center of Spins and Heat in Nanoscale Electronic Systems, University of California, Riverside, CA 92521, USA Department of Physics and Materials Science Program, University of New Hampshire, Durham, New Hampshire 03824, USA Department of Physics and Astronomy, University of California, Riverside, CA 92521, USA The spin superfluid analogy can be extended to include Josephson-like oscillations of the spin current. In a system of two antiferromagnetic insulators (AFMIs) separated by a thin metallic spacer, a threshold spin chemical potential established perpendicular to the direction of the N´eel vector field drives terahertz oscillations of the spin current. This spin current also has a non- linear, time-averaged component which provides a ‘smoking gun’ signature of spin superfluidity. The time-averaged spin current can be detected via the inverse spin Hall effect in a metallic spacer with large spin-orbit coupling. The physics illustrated here with AFMIs also applies to easy-plane ferromagnetic insulators. These findings may provide a new approach for experimental verification of spin superfluidity and realization of a terahertz spin oscillator. I. INTRODUCTION One main objective in the field of spintronics is the gen- eration and manipulation of pure spin currents in mag- netically ordered systems. Pure spin currents in mag- netic insulators are carried by collective excitations. This can be achieved by combining elements of conventional spintronics with magnetic insulators [1], for example, magnon mediated spin currents can be generated in het- erostructures composed of ferromagnetic(FM) insulators and metals [2, 3]. A more exotic method of transport- ing spin harnesses the ground states of both easy-plane FMs [4–6] and antiferromagnetic insulators (AFMIs) [7]. It has been long appreciated that magnetically ordered systems with spontaneously broken U (1) symmetry [8, 9] support metastable spin spiral states that can transfer spin angular momentum without dissipation [10]. In this regard, heterostructures composed of AFMIs are advan- tageous to those composed of easy-plane FMs, since they are less sensitive to stray fields or dipolar interactions, which can destroy dissipationless spin transport in easy- plane FMs [11]. It is difficult to experimentally distinguish between spin super-currents and magnon mediated spin currents in magnetic insulators, since the spin wave decay length is long due to the small Gilbert damping. Therefore, other signatures of spin superfluidity in magnetically or- dered systems need to be explored. To this end, it is advantageous to investigate the connections between su- perconductivity and magnetism further. One remark- able phenomena is the Josephson effect [12], which oc- curs in coupled superfluids and superconductors because the coupling energy is a periodic function of the rela- tive phase difference. A similar energy dependence can Corresponding author: rlake@ece.ucr.edu Corresponding author: yafisb@ucr.edu HM j c z m 1 y x m 2 NM FIG. 1. Schematic diagram of the proposed heterostruc- ture to detect the Josephson effect in spin superfluids. The heterostructure consists of two antiferromagnetic insulators (AFMIs) separated by a thin non-magnetic metallic (NM) spacer. The magnetization of the AFMIs lies in the xy-plane as indicated in the inset showing the direction of the N´eel vector and phase φ, with a spin canting in the z ˆ -direction. A spin chemical potential of up spins on the left interface of the AFMI can drive an oscillating spin current through the metallic spacer via spin pumping. The spin Hall effect in a heavy metal (HM) can inject a spin current. The spin current flowing through a spin-orbit (SO) coupled metallic spacer can be detected via the inverse spin Hall effect. be anticipated for exchange coupled AFMIs and easy- plane FMs. This insight suggests that it is instructive to analyze the effect of exchange coupling on the spin cur- rents in heterostructures composed of exchange coupled AFMIs and easy-plane FMs. Josephson dynamics were also predictied in dipole coupled nanomagnets [13]. In this article, we propose a lateral spin valve het- erostructure, which consists of two AFMIs separated by a thin metallic spacer [14]. We show that the spin superfluid analogy can be further extended to realize Josephson-like oscillations of the spin currents flowing through exchange coupled antiferromagnetic insulators (AFMIs). This oscillatory spin current can be detected

A Study of Vertical Transport through Graphene toward Control of Quantum Tunneling

Vertical integration of van der Waals (vdW) materials with atomic precision is an intriguing possibility brought forward by these two-dimensional (2D) materials. Essential to the design and analysis of these structures is a fundamental understanding of the vertical transport of charge carriers into and across vdW materials, yet little has been done in this area. In this report, we explore the important roles of single layer graphene in the vertical tunneling process as a tunneling barrier. Although a semimetal in the lateral lattice plane, graphene together with the vdW gap act as a tunneling barrier that is nearly transparent to the vertically tunneling electrons due to its atomic thickness and the transverse momenta mismatch between the injected electrons and the graphene band structure. This is accentuated using electron tunneling spectroscopy (ETS) showing a lack of features corresponding to the Dirac cone band structure. Meanwhile, the graphene acts as a lateral conductor through which the potential and charge distribution across the tunneling barrier can be tuned. These unique properties make graphene an excellent 2D atomic grid, transparent to charge carriers, and yet can control the carrier flux via the electrical potential. A new model on the quantum capacitances effect on vertical tunneling is developed to further elucidate the role of graphene in modulating the tunneling process. This work may serve as a general guideline for the design and analysis of vdW vertical tunneling devices and heterostructures, as well as the study of electron/spin injection through and into vdW materials.