Strong Spin Orbit Research Articles

Ferrimagnetic Skyrmions in Topological Insulator/Ferrimagnet Heterostructures.

Magnetic skyrmions are topologically nontrivial chiral spin textures that have potential applications in next-generation energy-efficient and high-density spintronic devices. In general, the chiral spins of skyrmions are stabilized by the noncollinear Dzyaloshinskii-Moriya interaction (DMI), originating from the inversion symmetry breaking combined with the strong spin-orbit coupling (SOC). Here, the strong SOC from topological insulators (TIs) is utilized to provide a large interfacial DMI in TI/ferrimagnet heterostructures at room temperature, resulting in small-size (radius ≈ 100 nm) skyrmions in the adjacent ferrimagnet. Antiferromagnetically coupled skyrmion sublattices are observed in the ferrimagnet by element-resolved scanning transmission X-ray microscopy, showing the potential of a vanishing skyrmion Hall effect and ultrafast skyrmion dynamics. The line-scan spin profile of the single skyrmion shows a Néel-type domain wall structure and a 120 nm size of the 180° domain wall. This work demonstrates the sizable DMI and small skyrmions in TI-based heterostructures with great promise for low-energy spintronic devices.

First order reversal curve Hall analysis of zero-field skyrmions on Pt/Co/Ta multilayers

Magnetic skyrmions are non-trivial spin textures that resist external disturbances and are promising candidates for use in next generation spintronic devices. However, a major challenge in the realization of devices based on skyrmions is the stabilization of ordered arrangements of these spin textures under ambient temperature and zero applied field conditions. Multilayers of ferromagnetic materials (Co) interspersed with heavy metals with strong spin–orbit coupling (Pt and Ta), exhibit interesting properties, as the Dzyaloshinskii–Moriya interaction, which is an anti-symmetric exchange interaction that tilts the spins of neighboring layers, helps to stabilize skyrmions. In this work we study the formation and stabilization of magnetic skyrmions in Pt/Co/Ta films using a first order reversal curve (FORC) analysis, obtained from Hall effect measurements. The analysis of the FORC Hall diagrams was used to determine the intensity of the magnetic fields that should be applied to nucleate and stabilize skyrmions at remanence. Magnetic force microscopy images and micromagnetic simulations were compared to determine the correlation of the domain textures with the applied field and magnetic parameters of the multilayers.

First principles calculations of the electric field gradient tensors of Ba2NaOsO6, a Mott insulator with strong spin orbit coupling

We present first principles calculations of the electrostatic properties of Ba2NaOsO6 (BNOO), a 5d1 Mott insulator with strong spin orbit coupling (SOC) in its low temperature quantum phases. In light of recent NMR experiments showing that BNOO develops a local octahedral distortion that is accompanied by the emergence of an electric field gradient (EFG) and precedes the formation of long range magnetic order (Lu et al 2017 Nat. Commun. 8 14407, Liu et al 2018 Phys. Rev. B 97 224103; Liu et al 2018 Physica B 536 863), we calculated BNOO’s EFG tensor for several different model distortions. The local orthorhombic distortion that we identified as most strongly agreeing with experiment corresponds to a Q2 distortion mode of the Na–O octahedra, in agreement with conclusions given in (Liu et al 2018 Phys. Rev. B 97 224103). Furthermore, we found that the EFG is insensitive to the type of underlying magnetic order. By combining NMR results with first principles modeling, we have thus forged a more complete understanding of BNOO’s structural and magnetic properties, which could not be achieved based upon experiment or theory alone.

Epitaxial growth of antimony nanofilms on HOPG and thermal desorption to control the film thickness**Project supported by the National Natural Science Foundation of China (Grant Nos. 21622304, 61674045, 11604063, and 61911540074), the National Key Research and Development Program of China (Grant No. 2016YFA0200700), the Strategic Priority Research Program and Key Research Program of Frontier Sciences and Instrument Developing Project (Chinese

Group-V elemental nanofilms were predicted to exhibit interesting physical properties such as nontrivial topological properties due to their strong spin–orbit coupling, the quantum confinement, and surface effect. It was reported that the ultrathin Sb nanofilms can undergo a series of topological transitions as a function of the film thickness h: from a topological semimetal (h > 7.8 nm) to a topological insulator (7.8 nm > h > 2.7 nm), then a quantum spin Hall (QSH) phase (2.7 nm > h > 1.0 nm) and a topological trivial semiconductor (h > 1.0 nm). Here, we report a comprehensive investigation on the epitaxial growth of Sb nanofilms on highly oriented pyrolytic graphite (HOPG) substrate and the controllable thermal desorption to achieve their specific thickness. The morphology, thickness, atomic structure, and thermal-strain effect of the Sb nanofilms were characterized by a combination study of scanning electron microscopy (SEM), atomic force microscopy (AFM), and scanning tunneling microscopy (STM). The realization of Sb nanofilms with specific thickness paves the way for the further exploring their thickness-dependent topological phase transitions and exotic physical properties.

Topological superconductivity in hybrid devices

Topological superconductivity can emerge from the combination of conventional superconductivity in a metal and strong spin–orbit coupling in a semiconductor when they are made into a hybrid device. The most exciting manifestation of topological superconductivity is the Majorana zero modes that are predicted to exist at the ends of the proximatized nanowires. In this Perspective, we review the evidence for the existence of Majorana zero modes that has accumulated in numerous experiments and the remaining uncertainties, and discuss what additional evidence is desirable. One very important factor for future development is the quality of the interface between the superconductor and semiconductor; we sketch out where further progress in the materials science of these interfaces can take us. We then discuss the path towards applying these modes in topologically protected quantum computing and observing more exotic kinds of superconductivity based on the same materials platform, and how to make connections to high-energy physics. Hybrid devices of superconductors and semiconductor nanowires may be topological and host majorana. This Perspective summarizes the current situation of the field, and highlights the developments in materials science required to make progress.

Intra- and inter-band magneto-optical absorption in monolayer WS2

Abstract In this paper, the optical absorption coefficients (OACs) and the refractive index changes (RICs) due to both intra- and inter-band transitions in monolayer WS 2 are theoretically investigated in the presence of a magnetic field. The results showed that both OACs and RICs display the blue-shift behavior with the increase of the magnetic field. The Zeeman fields have no effect on the peak positions but reduce slightly peak intensities. Besides, the strong spin–orbit coupling in monolayer WS 2 causes the significant difference of the peak due to spin up and down. The OACs and RICs due to intra-band transition display only one peak in the THz range, while the inter-band spectra show a series of peaks in the near-infrared to the visible optical range, making monolayer WS 2 to be a promising candidate for novel optoelectronic applications.

Manipulation of room-temperature valley-coherent exciton-polaritons in atomically thin crystals by real and artificial magnetic fields

Strong spin–orbit coupling and inversion symmetry breaking in transition metal dichalcogenide monolayers yield the intriguing effects of valley-dependent optical selection rules. As such, it is possible to substantially polarize valley excitons with chiral light and furthermore create coherent superpositions of K and K’ polarized states. Yet, at ambient conditions dephasing usually becomes too dominant, and valley coherence typically is not observable. Here, we demonstrate that valley coherence is, however, clearly observable for a single monolayer of WSe2, if it is strongly coupled to the optical mode of a high quality factor microcavity. The azimuthal vector, representing the phase of the valley coherent superposition, can be directly manipulated by applying magnetic fields, and furthermore, it sensibly reacts to the polarization anisotropy of the cavity which represents an artificial magnetic field. Our results are in qualitative and quantitative agreement with our model based on pseudospin rate equations, accounting for both effects of real and pseudo-magnetic fields.

Experimental formation of monolayer group-IV monochalcogenides

Monolayer group-IV monochalcogenides (MX, M=Ge, Sn, Pb; X=S, Se, Te) are a family of novel two-dimensional (2D) materials that have atomic structures closely related to that of the staggered black phosphorus lattice. The structure of most monolayer MX materials exhibits a broken inversion symmetry and many of them exhibit ferroelectricity with a reversible in-plane electric polarization. A further consequence of the noncentrosymmetric structure is that when coupled with strong spin–orbit coupling, many MX materials are promising for the future applications in non-linear optics, photovoltaics, spintronics, and valleytronics. Nevertheless, because of the relatively large exfoliation energy, the creation of monolayer MX materials is not easy, which hinders the integration of these materials into the fast-developing field of 2D material heterostructures. In this Perspective, we review recent developments in experimental routes to the creation of the monolayer MX, including molecular beam epitaxy and two-step etching methods. Other approaches that could be used to prepare the monolayer MX are also discussed, such as liquid phase exfoliation and solution-phase synthesis. A quantitative comparison between these different methods is also presented.

Distinctly Diverse PLQY and Inverse Solid‐State Luminescent Properties in Structure‐Similar Diphenyl Sulfone TADF Molecules: A Role of C─C

AbstractThermally activated delayed fluorescence (TADF) materials can greatly increase internal quantum efficiencies (IQEs) of organic light‐emitting diodes (OLEDs) beyond the spin statistical bottleneck of 25%. However, the mechanisms behind TADF and solid‐state luminescence are not clear. Recently, two diphenyl sulfone TADF molecules with quite similar structures, A1‐D1 and A2‐D1, have been observed to demonstrate distinctly diverse photoluminescence quantum yield (PLQY) and inverse solid‐state luminescent properties. In order to understand the origin of this phenomenon, the TADF mechanism of both molecules is studied using density function theory. The results show that strong spin–orbit coupling, low minimum energy cross point, dense triplet energy level distribution, and a mediate locally excited triplet state (3LE) can facilitate the rapidly reverse intersystem crossing (RISC) process in A1‐D1. Moreover, the solid states of both molecules are characterized via the combined quantum mechanics and molecular mechanics (QM/MM) method. It is attributed to the compact intermolecular stacking; the nonradiative consumption of A2‐D1 is suppressed substantially by the restricted intermolecular rotation (RIR) effect. Therefore, A2‐D1 with low PLQY can exhibit an aggregation‐induced emission (AIE) phenomenon in neat films. This work may benefit the rational design of nondoped OLEDs.

Electronic structure and superconductivity of the non-centrosymmetric Sn4As3

In a superconductor that lacks inversion symmetry, the spatial part of the Cooper pair wave function has a reduced symmetry, allowing for the mixing of spin-singlet and spin-triplet Cooper pairing channels and thus providing a pathway to a non-trivial superconducting state. Materials with a non-centrosymmetric crystal structure and with strong spin–orbit coupling are a platform to realize these possibilities. Here, we report the synthesis and characterisation of high quality crystals of Sn4As3, with non-centrosymmetric unit cell (R3m). We have characterised the normal and superconducting states using a range of methods. Angle-resolved photoemission spectroscopy shows a multiband Fermi surface and the presence of two surface states, confirmed by density-functional theory calculations. Specific heat measurements reveal a superconducting critical temperature of Tc ∼ 1.14 K and an upper critical magnetic field of μ0Hc ≳ 7 mT, which are both confirmed by ultra-low temperature scanning tunneling microscopy and spectroscopy. Scanning tunneling spectroscopy shows a fully formed superconducting gap, consistent with conventional s-wave superconductivity.

Zero Field Splitting of Heavy-Hole States in Quantum Dots.

Using inelastic cotunneling spectroscopy we observe a zero field splitting within the spin triplet manifold of Ge hut wire quantum dots. The states with spin ±1 in the confinement direction are energetically favored by up to 55 μeV compared to the spin 0 triplet state because of the strong spin–orbit coupling. The reported effect should be observable in a broad class of strongly confined hole quantum-dot systems and might need to be considered when operating hole spin qubits.

Spin-Hall magnetoresistance in Ta/Cr/YIG trilayers with different Cr thicknesses

We report an investigation of spin-Hall magnetoresistance (SMR) in Ta/Cr/Y3Fe5O12 (Ta/Cr/YIG) trilayers with different thicknesses of the Cr inserted layer. The Ta/Cr/YIG film with a 1-nm-thick Cr layer shows a maximum SMR of 4.2×10-4 at 100 K, which is an order of magnitude larger than that previously reported in the Ta/YIG or Cr/YIG systems. The enhancement of SMR in the Ta/Cr/YIG is ascribed to the strong spin–orbit coupling (SOC) effect of both Ta and Cr and the anti-oxidation protection by the Ta coating. A negative SMR different from the conventional SMR is found for the Ta/Cr/YIG with the Cr thicknesses of 3 and 6 nm at the temperature below 100 and 260 K, respectively, which suggests the occurrence of the spin-flop coupling between the antiferromagnetic Cr and ferromagnetic YIG moments. Both the anisotropic magnetoresistance (AMR) and the anomalous-Hall resistance (AHR) indicate an induced ferromagnetism of Cr which is likely due to the spin-flop transition or the uncompensated magnetic moments. Our results suggest that the 3d transition metal Cr can be used as a spin current generator, and the antiferromagnetic structure of Cr can be employed to regulate the transport of spin current.

Study of Phonon Dispersion of Iridium Oxide Ca5Ir3O12 with Strong Spin–Orbit Interaction

In this study, we report the results of experiments and calculations of phonon dispersion of iridium oxide with a strong spin–orbit interaction (SOI). Using inelastic X-ray scattering (IXS), we measured the IXS spectra of Ca5Ir3O12 along Γ–A, Γ–M, and Γ–K–M directions in the Brillouin zone of a hexagonal lattice at room temperature. We also show ab initio density-functional phonon dispersions based on local density approximation and generalized gradient approximation (GGA) considering SOI. By comparing the experimental and calculated results, we found that the GGA phonon dispersion with SOI is in very good agreement with the experimental results. The phonon calculation was performed for supercells of 1 × 1 × 3 and preliminary 2 × 2 × 1. We found no phonon instability within these supercells. Low-energy phonon properties such as Debye temperature, specific heat, and sound velocity are also discussed.

Characteristics and temperature-field-thickness evolutions of magnetic domain structures in van der Waals magnet Fe3GeTe2 nanolayers

In two-dimensional van der Waals magnets, the presence of magnetic orders, strong spin–orbit coupling, and asymmetry at interfaces is the key ingredient for hosting noncollinear spin textures. Here, we investigate the characteristics and evolution of magnetic domain structures in thin Fe3GeTe2 nanolayers as a function of temperature, applied magnetic field, and specimen thickness using advanced magnetic electron microscopy. Specifically, electron holography analyses reveal the spin configurations of Bloch-type, zero-field-stabilized magnetic bubbles in 20-nm-thick Fe3GeTe2 nanolayers at cryogenic temperature. In situ Lorentz transmission electron microscopy measurements further provide detailed magnetic phase diagrams of noncollinear spin textures, including magnetic spirals and bubbles in Fe3GeTe2 as a function of temperature, applied magnetic field, and specimen thickness. We further estimate the micromagnetic parameters of Fe3GeTe2, such as anisotropy energy density and magnetization at specific specimen temperature using the critical thicknesses measured from Lorentz microscopy measurements. Our experimental results of magnetic domain structures in Fe3GeTe2 nanolayers reveal that due to their intrinsic highly uniaxial magnetocrystalline anisotropy, a very thin film of tens of nanometers of Fe3GeTe2 can support the spontaneous and stable formation of zero-field magnetic bubbles.

Hole spin in tunable Ge hut wire double quantum dot

Holes in germanium (Ge) exhibit strong spin–orbit interaction, which can be exploited for fast and all-electrical manipulation of spin states. Here, we report transport experiments in a tunable Ge hut wire hole double quantum dot. We observe the signatures of Pauli spin blockade (PSB) with a large singlet-triplet energy splitting of ∼1.1 meV and extract the g factor. By analyzing the PSB leakage current, we obtain a spin–orbit length of ∼40–100 nm. Furthermore, we demonstrate the electric dipole spin resonance. These results lay a solid foundation for implementing high quality tunable hole spin–orbit qubits.

Strain healing of spin–orbit coupling:a cause for enhanced magnetic moment in epitaxial SrRuO3 thin films

Enhanced magnetic moment and coercivity in SrRuO3 thin films are significant issues for advanced technological usages and hence are researched extensively in recent times. Most of the previous reports on thin films with enhanced magnetic moment attributed it to the high spin state. Our magnetization results show high magnetic moment of 3.3 μB/Ru ion in the epitaxial thin films grown on LSAT substrate against 1.2 μB/Ru ion observed in bulk compound. Contrary to the previous reports the Ru ions are found to be in low spin state and the orbital moment is shown to be contributing significantly in the enhancement of magnetic moment. We employed x-ray absorption spectroscopy and resonant valance band spectroscopy to probe the spin state and orbital contributions in these films. The existence of strong spin–orbit coupling responsible for the de-quenching of the 4d orbitals is confirmed by the observation of the non-statistical large branching ratio at the Ru M2,3 absorption edges. X-ray magnetic circular dichroism studies performed at the Ru M2,3 edges provided direct evidence of significant contribution of orbital moment in the film grown on LSAT. The relaxation of orbital quenching by strain engineering provides a new tool for enhancing magnetic moment and strain disorder is shown to be an efficient mean to control the spin–orbit coupling.

Validity of magnetotransport detection of skyrmions in epitaxial SrRuO3 heterostructures

A technically simple way of probing the formation of skyrmions is to measure the topological Hall resistivity that should occur in the presence of skyrmions as an additional contribution to the ordinary and anomalous Hall effect. This type of probing, lately intensively used for thin film samples, relies on the assumption that the topological Hall effect contribution can be extracted unambiguously from the measured total Hall resistivity. Ultrathin films and heterostructures of the $4d$ ferromagnet ${\mathrm{SrRuO}}_{3}$ have stirred up a lot of attention after the observation of anomalies in the Hall resistivity, which resembled a topological Hall effect contribution. These anomalies, first reported for bilayers in which the ${\mathrm{SrRuO}}_{3}$ was interfaced with the strong spin-orbit coupled oxide ${\mathrm{SrIrO}}_{3}$, were attributed to the formation of tiny N\'eel-type skyrmions. Here we present the investigation of heterostructures with two magnetically decoupled and electrically parallel connected ${\mathrm{SrRuO}}_{3}$ layers. The two ${\mathrm{SrRuO}}_{3}$ layers deliberately have different thicknesses, which affects the coercive field and ferromagnetic transition temperature of the two layers, and the magnitude and temperature dependence of their anomalous Hall constants. The ${\mathrm{SrRuO}}_{3}$ layers were separated by ultrathin layers of either the strong spin-orbit coupling oxide ${\mathrm{SrIrO}}_{3}$ or of the large band-gap insulator ${\mathrm{SrZrO}}_{3}$. Our magnetic and magnetotransport studies confirm the additivity of the anomalous Hall transverse voltages for the parallel conducting channels originating from the two ferromagnetic ${\mathrm{SrRuO}}_{3}$ layers as well as the possibility to tune the global anomalous Hall resistivity by magnetic field, temperature, or structural modifications at the epitaxial all-oxide interfaces. The Hall voltage loops of these two-layer heterostructures demonstrate the possibility to generate humplike structures in the Hall voltage loops of ${\mathrm{SrRuO}}_{3}$ heterostructures without the formation of skyrmions and emphasize that the detection of skyrmions only by Hall measurements can be misleading.

The effect of Co on the electronic and magnetic properties of InAs graphene-like: a DFT study

In this paper, we perform first-principles calculations of substituting Cobalt impurity into Indium Arsenide (InAs) graphene-like, and investigate the electronic, magnetic and optical properties of the resulting compound compared to the pure InAs graphene-like. The calculations are executed using the full-potential augmented plane wave method in the framework of the density-functional theory using the computational Wien2k. Looking into the band structure and density of states diagrams in the electronic discussion, it is found that the energy gap is reduced with doping Cobalt into InAs graphene-like. Moreover, in magnetic analysis, we conclude that by substituting Indium atoms with Cobalt, the InAs nano-sheets are transformed from non-magnetic semiconductors to semi-metals with a magnetic moment of about 4µB due to a strong spin–orbit effect between the electrons and the nuclei of Co atoms. This compound reaches a 100% spin polarization and turns to a half-metal which can have lots of applications in spintronics industry.

Superconducting Current in Mesa Structures with Interlayer of Strontium Iridate, a Material Having Strong Spin-Orbit Interaction

We report on observation of superconducting current and AC Josephson effect in mesa-heterostructures with the interfaces comprised spin-singlet superconducting electrodes, coupled by a barrier made from strontium iridate, a material with the strong spin-orbit interaction. The superconducting critical current density was jC ≈ 0.3 A/cm2 at T = 4.2 K for mesa-heterostructures with d = 7 nm and jC ≈ 4 A/cm2 for d = 5 nm. The zero-bias conductance peak has been observed due to low energy states originated at Sr2IrO4/YBa2Cu3Ox interface. Under influence of weak magnetic field the critical current IC(H) dependences showed Fraunhofer-like pattern indicating absence of pinholes, supported also by oscillating with microwave power Shapiro steps. Fiske resonance steps with voltage positions deviated from the ordinary ones were registered for mesas with d = 5 nm.

Concomitance of superconducting spin–orbit scattering length and normal state spin diffusion length in W on (Bi,Sb)2Te3

We report the observation of an anomalously large in-plane upper critical field, exceeding at least 2.5 times the Pauli paramagnetic limit, in a thin superconducting W film grown on a topological insulator (Bi,Sb)2Te3. This can be accounted for by setting the spin–orbit scattering length of superconducting W to a value ranging from 1 to 2 nm, which is comparable to the spin diffusion length of normal state W. The coupling between the topological surface states of (Bi,Sb)2Te3 and the wave functions of superconducting W may also contribute to the observed giant critical field. Our results suggest the universality of the spin–orbit scattering formalism for describing the transport involving the diffusive carriers as well as the Cooper pairs in systems with strong spin–orbit coupling.