Large Spin-orbit Coupling Research Articles

Full-zone persistent spin textures with giant spin splitting in two-dimensional group IV–V compounds

Persistent spin texture (PST), a property of solid-state materials maintaining unidirectional spin polarization in the momentum k-space, offers a route to deliver the necessary long carrier spin lifetimes through the persistent spin helix (PSH) mechanism. However, most of the discovered PST locally occurred in the small part around certain high symmetry k-points or lines in the first Brillouin zone (FBZ), thus limiting the stability of the PSH state. Herein, by symmetry analysis and first-principles calculations, we report the emergence of full-zone PST (FZPST), a phenomenon displaying the PST in the whole FBZ, in the two-dimensional group IV–V (A = Si, Sn, Ge; B = Bi, Sb) compounds. Due to the existence of the in-plane mirror symmetry operation in the wave vector point group symmetry for the arbitrary in the whole FBZ, fully out-of-plane spin polarization is observed in the k-space, thus maintaining the FZPST. Importantly, we observed giant spin splitting in which the PST sustains, supporting large spin–orbit coupling parameters and small wavelengths of the PSH states. Our analysis demonstrated that the FZPST is robust for the non-degenerate bands, which can be effectively controlled by the application of an external electric field, thus offering a promising platform for future spintronic applications.

Multiple-Resonance-Induced Thermally Activated Delayed Fluorescence Materials Based on Indolo[3,2,1-jk]carbazole with an Efficient Narrowband Pure-Green Electroluminescence.

Herein, we report two multiple-resonance thermally activated delayed fluorescence emitters (VTCzBN and TCz-VTCzBN) based on indolo[3,2,1-jk]carbazole unit and boron-nitrogen skeletons, whose emissions peaking at 496 and 521 nm with full width at half maximum of 34 and 29 nm, respectively. Meanwhile, fast rate constants of reverse intersystem crossing of above 106 s-1 are obtained due to small singlet-triplet energy gaps and large spin-orbital coupling values. Notably, planar molecular structures along the transition dipole moment direction endow them with high horizontal emitting dipole ratios of up to 94 %. Consequently, the corresponding organic light-emitting diodes (OLEDs) show the maximum external quantum efficiencies of 31.7 % and 32.2 %, respectively. Particularly, OLED with TCz-VTCzBN display ultra-pure green emission with Commission Internationale de l'Eclairage coordinates of (0.22, 0.71), consistent with the green display standard of the National Television System Committee.

Theoretical study and molecular design of thermally activated delayed fluorescence molecules based on intramolecular-locked strategy

Intramolecular-locked method becomes a new strategy for the design of highly efficient Thermally Activated Delayed Fluorescence (TADF) emitters. In this work, three different intramolecular-locked strategies are used to design a serial of molecules based on triarylboron (TAB)-based compounds. Based on first-principles calculations and decay rates study, intramolecular-locked molecules are compared with free systems. Our study results indicate that locked acceptor (Locked A) strategy can effectively control the geometric change of molecules in electronic transition. Smaller energy gap between the first singlet excited state and the first triplet excited state (ΔEST) value. However, the strategies of locked donor-acceptor (Locked D-A) could induce more local excitation component in transition orbitals, thus large radiative rates are found for the Locked D-A systems. Relatively large spin-orbit coupling values can be obtained although large ΔEST is induced in Locked D-A systems. Consequently, relatively effective upconversion can be realized based on the decay rates calculated. Our calculation results indicate that Locked A&D-A strategy may provide balanced light-emitting properties for TAB based compounds.

Quantum oscillations of the j=3/2 Fermi surface in the topological semimetal YPtBi

The bismuth-based half-Heusler materials host a nontrivial topological band structure, unconventional superconductivity, and large spin-orbit coupling in a system with very low electron density. In particular, the inversion of $p$-orbital-derived bands with an effective angular momentum $j$ of up to 3/2 is thought to play a central role in anomalous Cooper pairing in the cubic half-Heusler semimetal YPtBi, which is thought to be the first ``high-spin'' superconductor. Here, we report an extensive study of the angular dependence of quantum oscillations (QOs) in the electrical conductivity of YPtBi, revealing an anomalous Shubnikov--de Haas effect consistent with the presence of a coherent $j=3/2$ Fermi surface. The QO signal in YPtBi manifests an extreme anisotropy upon rotation of the magnetic field from the [100] to [110] crystallographic direction, where the QO amplitude vanishes. This radical anisotropy for such a highly isotropic system cannot be explained by trivial scenarios involving changes in effective mass or impurity scattering, but rather is naturally explained by the warping feature of the $j=3/2$ Fermi surface of YPtBi, providing direct proof of active high angular momentum quasiparticles in the half-Heusler compounds.

Structural, electronic, and magnetic properties of CrTe2

Two-dimensional chromium ditelluride (${\mathrm{CrTe}}_{2}$) is a promising ferromagnetic layered material that exhibits long-range ferromagnetic ordering in the monolayer limit. The formation energies of the different possible structural phases (1T, 1H, 2H) calculated from density functional theory (DFT) show that the 1T phase is the ground state, and the energetic transition barriers between the phases, calculated by the nudged elastic band method, are large, on the order of 0.5 eV. The self-consistent Hubbard $U$ correction parameters are calculated for all the phases of ${\mathrm{CrTe}}_{2}$. The calculated magnetic moment of 1T-${\mathrm{CrTe}}_{2}$ with $\ensuremath{\ge}2$ layers lies in the plane, whereas the magnetic moment of a monolayer is out-of-plane. Band filling and tensile biaxial strain cause the magnetic moment of a monolayer to switch from out-of-plane to in-plane, and compressive biaxial strain in a bilayer causes the magnetic moment to switch from in-plane to out-of-plane. The magnetic anisotropy is shown to originate from the large spin-orbit coupling (SOC) of the Te atoms and the anisotropy of the exchange coupling constants ${J}_{xy}$ and ${J}_{z}$ in an XXZ type Hamiltonian. Renormalized spin wave theory using experimental values for the magnetic anisotropy energy and Curie temperatures provides a range of values for the nearest neighbor exchange coupling.

Single-layer intrinsic 2H-phase LuX 2 (X = Cl, Br, I) with large valley polarization and anomalous valley Hall effect

Manipulation of the valley degree of freedom provides a new path for quantum information technology, but the real intrinsic large valley-polarization materials are rarely reported up to date. Here, we perform first-principles calculations to predict a class of 2H-phase single layer (SL) materials LuX 2 (X = Cl, Br, I) to be ideal candidates. SL-LuX 2 are ferrovalley materials with a giant valley-polarization of 55 meV–148 meV as a result of its large spin–orbital coupling (SOC) and intrinsic ferromagnetism (FM). The magnetic transition temperatures of SL-LuI2 and SL-LuCl2 are estimated to be 89 K–124 K, with a sizable magnetic anisotropy at out-of-plane direction. Remarkably, the anomalous valley Hall effect (AVHE) can be controlled in SL-LuX 2 when an external electric field is applied. Moreover, the intrinsic valley-polarization of SL-LuI2 is highly robust for biaxial strain. These findings provide a promising ferrovalley material system for the experimentation of valleytronics and subsequent applications.

Spin Pumping through Different Spin-Orbit Coupling Interfaces in β-W/Interlayer/Co2FeAl Heterostructures.

Spin pumping has been considered a powerful tool to manipulate the spin current in a ferromagnetic/nonmagnetic (FM/NM) system, where the NM part exhibits large spin-orbit coupling (SOC). In this work, the spin pumping in β-W/Interlayer (IL)/Co2FeAl (CFA) heterostructures grown on Si(100) is systematically investigated with different ILs in which SOC strength ranges from weak to strong. We first measure the spin pumping through the enhancement of effective damping in CFA by varying the thickness of β-W. The damping enhancement in the bilayer of β-W/CFA (without IL) is found to be ∼50% larger than the Gilbert damping in a single CFA layer with a spin diffusion length and spin mixing conductance of 2.12 ± 0.27 nm and 13.17 ± 0.34 nm-2, respectively. Further, the ILs of different SOC strengths such as Al, Mg, Mo, and Ta were inserted at the β-W/CFA interface to probe their impact on damping in β-W/ILs/CFA. The effective damping reduced to 8% and 20% for Al and Mg, respectively, whereas it increased to 66% and 75% with ILs of Mo and Ta, respectively, compared to the β-W/CFA heterostructure. Thus, in the presence of ILs with weak SOC, the spin pumping at the β-W/CFA interface is suppressed, while for the high SOC ILs effective damping increased significantly from its original value of β-W/CFA bilayer using a thin IL. This is further confirmed by performing inverse spin Hall effect measurements. In summary, the transfer of spin angular momentum can be significantly enhanced by choosing a proper ultrathin interface layer. Our study provides a tool to increase the spin current production by inserting an appropriate thin interlayer which is useful in modifying the heterostructure for efficient performance in spintronics devices.

Topological Pumping in a Floquet-Bloch Band.

Constructing new topological materials is of vital interest for the development of robust quantum applications. However, engineering such materials often causes technological overhead, such as large magnetic fields, spin-orbit coupling, or dynamical superlattice potentials. Simplifying the experimental requirements has been addressed on a conceptual level-by proposing to combine simple lattice structures with Floquet engineering-but there has been no experimental implementation. Here, we demonstrate topological pumping in a Floquet-Bloch band using a plain sinusoidal lattice potential and two-tone driving with frequencies ω and 2ω. We adiabatically prepare a near-insulating Floquet band of ultracold fermions via a frequency chirp, which avoids gap closings en route from trivial to topological bands. Subsequently, we induce topological pumping by slowly cycling the amplitude and the phase of the 2ω drive. Our system is well described by an effective Shockley model, establishing a novel paradigm to engineer topological matter from simple underlying lattice geometries. This approach could enable the application of quantized pumping in metrology, following recent experimental advances on two-frequency driving in real materials.

Quantum Hall effect in a two-dimensional semiconductor with large spin-orbit coupling

We perform magnetotransport studies of atomically thin InSe field-effect transistors with large Rashba spin-orbit coupling (SOC), and extract the Landau level (LL) gaps via thermal activation measurements. Surprisingly, the Landau level gaps at even and odd filling factors are extrapolated to have positive and negative intercepts at $B=0$, respectively, which result from the spin-split Landau spectrum. We also show that for a material with large spin-orbit coupling, its LL gaps may vary nonlinearly and nonmonotonically as a function of magnetic field. Thus, its effective mass and $g$-factor cannot be reliably extracted using conventional expressions, but depend rather sensitively on the SOC's strength and tunability.

Ti2CO2/Ti2CF2 van der Waals heterostructure with robust Schottky barriers under electric fields

Transition metal carbides, nitrides and carbonitrides (MXenes) are considered as multifunctional materials due to their various elemental composition possibilities, surface functional tunability, various magnetic orders, and large spin–orbit coupling. Herein, a novel Ti2CO2/Ti2CF2 heterostructure is designed and its electronic and interfacial properties are investigated based on first-principles calculations. Our results reveal that the band structures of Ti2CO2 and Ti2CF2 layers are well preserved in the heterostructure due to the van der Waals interaction between the two layers, and a n-type Schottky contact forms due to the band bending at the interface. The most striking and much novel property is that the Schottky barrier height is nearly independent of the intensity of external electric field which is kept along the direction perpendicular to the Ti2CO2 or Ti2CF2 plane. The result denotes that a Fermi level pinning behavior occurs upon varying the intensity of electric field, which can change the doping concentration of electrons in the Ti2CO2 layer. This robust Schottky barrier opens the possibility of MXenes device designs for nanoelectronics.

Chirality enhances oxygen reduction

Controlled reduction of oxygen is important for developing clean energy technologies, such as fuel cells, and is vital to the existence of aerobic organisms. The process starts with oxygen in a triplet ground state and ends with products that are all in singlet states. Hence, spin constraints in the oxygen reduction must be considered. Here, we show that the electron transfer efficiency from chiral electrodes to oxygen (oxygen reduction reaction) is enhanced over that from achiral electrodes. We demonstrate lower overpotentials and higher current densities for chiral catalysts versus achiral ones. This finding holds even for electrodes composed of heavy metals with large spin-orbit coupling. The effect results from the spin selectivity conferred on the electron current by the chiral assemblies, the chiral-induced spin selectivity effect.

Facile access to high-performance reverse intersystem crossing OLED materials through an unsymmetrical D-A-D’ molecular scaffold

Organic light-emitting diode (OLED) materials based on a reverse intersystem crossing (RISC) triplet harvesting mechanism have attracted tremendous attention. Currently, many research efforts have been devoted to the construction of D-A or D-A-D RISC-OLED materials whose charge-transfer-featured singlet excited states (1CTD-A) are very close to the local triplet excited states of their D or/and A moieties (3LED or/and 3LEA), so that a small singlet–triplet energy splitting and a large spin–orbit coupling matrix element can be acquired concurrently. However, the chief technical difficulty lies in the accurate prediction and delicate tuning of the 1CTD–A energy levels of these compounds. Herein, by using PCz-TXO2 as an example, we demonstrated that high-performance RISC-OLED materials can be readily achieved by integrating an appropriate D’ subunit into a traditional D-A fluorophore that lacks RISC property, i.e., constructing a D–A–D’ triad whose 1CTD-A is close to its 3LED’. In comparison with its D-A or D-A-D counterpart of P-TXO2 or DP-TXO2 showing satisfactory photoluminescence efficiency, pure blue gamut but poor RISC, the presence of a carbazole-based D’ subunit has negligible influence on the transition feature of its lowest singlet excited state (S1), but triggers significantly enhanced RISC property. Consequently, PCz-TXO2 shows pure blue electroluminescence (EL) inherited from DP-TXO2 [CIE1931: (0.160, 0.089) vs. (0.154, 0.102)], but significantly enhanced external quantum efficiency (EQEmax: 9.5% vs. 6.1%) and exciton utilization efficiency (EUEmax: 93% vs. 42%). Our results present a new method to facilely access RISC materials and can greatly extend the design rationales for high-performance OLED materials.

Strain Tunable Quantum Spin Hall Insulating Phase in Halogen‐Decorated Double‐Layer Stanene

Group IV graphene analogues such as stanene, plumbene, etc. have drawn attention because of their large spin–orbit coupling and thus being promising candidates for topological insulator. However, fabrication of these ultrathin monolayers is a daunting task. To reduce the difficulties of the fabrication of 2D topological insulators, bilayer materials with sizable nontrivial bandgaps can be a viable option. Chemically decorated bilayer stanene is a novel structure which has not been studied before. Herein, the topological characteristics of halogen‐decorated bilayer of stanene are studied and their topological nontriviality is confirmed by calculating the Z2 invariant. As the topological nontriviality can be preserved for these bilayers, these materials will have advantages over their monolayer counterparts in terms of fabrication difficulties. Halogen‐decorated stanene bilayers (except Br) are topologically nontrivial and their nontrivial nature is tunable by external strain. Hybrid functional (HSE06) has been employed to calculate the band structures of the materials at various strains. Also, the band structures of their zigzag nanoribbons have been analyzed. Lastly, the formation energy, quantum molecular dynamics, and phonon dispersion confirm their electronic and structural stability. The proposed material may speed up the fabrication of 2D topological insulators aiming future spintronic and high‐speed electronic devices.

6p valence relativistic effects in 5d photoemission spectrum of Pb atom and bonding properties of Pb-dimer using Dirac–Hartree–Fock formalism including many-body effects

There has been strong recent interest related to the large spin–orbit coupling in Pb monolayers on various properties of graphene and other 2D-materials. The underlying physical/chemical origin of the spin–orbit splitting has been discussed in terms of the valence 6p atomic level of the lead atom. Indeed, the photoelectron spectra of the Pb atom were the subject of investigations about 50 years ago in Dave Shirley’s laboratory at UC Berkeley. In a paper published in 1975, using He-I UV photoelectron spectroscopy, we reported an unexpected relative intensity ratio for the observed atomic Pb peaks (2P1/2 and 2P3/2) after removal of a 6p valence electron and attributed it to the large spin–orbit interaction in that level. In this contribution, we use the Dirac–Hartree–Fock formalism to reanalyze the complex spectral features reported five years later, for the 5d He-II UV photoelectron spectrum of atomic lead, to extract the 6p valence contribution, which turns out to be significant. Furthermore, we calculate the energy levels of the Pb-dimer at the experimental equilibrium geometry of the molecule to also find the significant contribution of the spin–orbit splitting of the atomic 6p levels in the composition of the valence molecular orbitals of the dimer. Such an approach can be extended to larger systems like monolayers containing lead or other heavy atoms, thus helping in designing 2D-materials with controlled and better targeted properties.

High‐Efficiency and Narrowband OLEDs from Blue to Yellow with Ternary Boron/Nitrogen‐Based Polycyclic Heteroaromatic Emitters

AbstractThe development of boron/nitrogen‐based polycyclic heteroaromatic emitters with multiple‐resonance thermally activated delayed fluorescence (MR‐TADF) property can efficiently promote the advancement of high‐efficiency organic light‐emitting diodes (OLEDs) with narrowband emission. Herein, a simple strategy to achieve four ternary boron/nitrogen‐based polycyclic heteroaromatic emitters (SBON, SBSN, DBON, and DBSN) from pure blue (463 nm) to yellow (553 nm) via tuning the coordination between B/N and heteroatom (O or S) is demonstrated, aiming to increase charge transfer delocalization of the polycyclic heteroaromatic emitters and adjust photo‐physical properties. This strategy endows the four emitters with full width at half maximum (FWHM) of 24, 27, 20, and 28 nm, respectively. Additionally, double‐boron type green and yellow narrowband emitters (DBON and DBSN) possess 98% photoluminescence efficiencies in doped films. Besides, considerable rate constants of reverse intersystem crossing (RISC) are achieved because of the small singlet–triplet excited state energy gap and large spin–orbit coupling values. Consequently, the OLEDs covering from blue to yellow based on these emitters show the maximum external quantum efficiency of 13.7%, 17.6%, 26.7%, and 21.8%, respectively, with low‐efficiency roll‐off. These results provide a feasible design strategy to construct boron/nitrogen‐based polycyclic heteroaromatic MR‐TADF emitters for efficient OLEDs with color‐tuning electroluminescence.

Reverse intersystem crossing accelerating assistant dopant for high efficiency and long lifetime in red hyperfluorescence organic light-emitting diodes

A highly efficient deep-red hyperfluorescence device achieving long device lifetime was fabricated by adopting a newly designed 2,5-bis(12H-benzo[4,5]thieno[2,3-a]carbazole-12-yl)-3,6-di(9H-carbazol-9-yl)terephthalonitrile (12BTCzTPN) thermally activated delayed fluorescence (TADF) assistant dopant. The 12BTCzTPN was designed via donor modification of 2,3,5,6-tetra(9H-carbazol-9-yl)terephthalonitrile (4CzTPN) to increase reverse intersystem crossing (RISC) rate for fast up-conversion. The carbazole donor moiety was replaced with 12H-benzo[4,5]thieno[2,3-a]carbazole (12BTCz) donor, which induced a small singlet–triplet energy gap and large spin–orbit coupling via the heavy atom effect of sulfur, resulting in 4.5-times faster RISC rate than the 4CzTPN TADF molecule. The addition of 12BTCzTPN as an assistant dopant to a deep-red hyperfluorescence device resulted in a device with a high external quantum efficiency of 19.9% and a long device lifetime of 1312 h at 3000 cd/m2, which is up to 90% of the initial luminance.

Gas-Phase Reactivity of Ozone with Lanthanide Ions (Sm+, Nd+) and Their Higher Oxides

The kinetics of SmOn+ (n = 0-2) and NdOn+ (n = 0-2) with O3 are measured using a selected-ion flow tube. Reaction of Nd+ to yield NdO+ + O2 occurs rapidly, with a rate constant near the capture-controlled limit of ∼8 × 10-10 cm3 s-1. NdO+ reacts at ∼40% of the capture limit to yield NdO2+ with little temperature dependence from 200 to 400 K. NdO2+ likely reacts very slowly (k ∼ 10-13 cm3 s-1) to yield NdO+ + 2O2, does not react to yield NdO3+, and associates slowly (k ∼ 10-12 cm3 s-1) to yield NdO2+(O3)1-3. Reaction of Sm+ also yields SmO+ at near the capture limit at all temperatures, but a significant fraction (∼50%) of the SmO+ is produced in excited states that are long-lived compared to the millisecond time scale of the experiment. These states are evidently resistant to both radiative and collisional relaxation. The excited-state production is likely due to a spin-conservation constraint on the reaction, despite the large spin-orbit coupling typical for lanthanide-containing species. Ground-state SmO+ reacts inefficiently (k = 2 × 10-11 (T/300)-2.5 cm3 s-1) to yield SmO2+ + O2, while the excited-state SmO+* reacts at the capture limit, with branching to yield Sm+ + 2O2 (ΔHr,0K = 148.7 ± 0.4 kJ mol-1 for ground-state SmO+) approximately 60% of the time, the remainder forming SmO2+, which further reacts with O3 to yield SmO+ at about 1% of the collisional value.

Coherent Hole Transport in Selective Area Grown Ge Nanowire Networks.

Holes in germanium nanowires have emerged as a realistic platform for quantum computing based on spin qubit logic. On top of the large spin–orbit coupling that allows fast qubit operation, nanowire geometry and orientation can be tuned to cancel out charge noise and hyperfine interaction. Here, we demonstrate a scalable approach to synthesize and organize Ge nanowires on silicon (100)-oriented substrates. Germanium nanowire networks are obtained by selectively growing on nanopatterned slits in a metalorganic vapor phase epitaxy system. Low-temperature electronic transport measurements are performed on nanowire Hall bar devices revealing high hole doping of ∼1018 cm–3 and mean free path of ∼10 nm. Quantum diffusive transport phenomena, universal conductance fluctuations, and weak antilocalization are revealed through magneto transport measurements yielding a coherence and a spin–orbit length of the order of 100 and 10 nm, respectively.

Optimizing Intermolecular Interactions and Energy Level Alignments of Red TADF Emitters for High-Performance Organic Light-Emitting Diodes.

Adequately harvesting all excitons in a single molecule and inhibiting exciton losses caused by intermolecular interactions are two important factors for achieving high efficiencies thermally activated delayed fluorescence (TADF). One potential approach for optimizing these is to tune alignment of various excited state energy levels by using different doping concentrations. Unfortunately, emission efficiencies of most TADF emitters decrease rapidly with concentrations which limits the window for energy level tunning. In this work, by introducing a spiro group to increase steric hindrance of a TADF emitter (BPPXZ) with a phenoxazine and a dibenzo[a,c]phenazine, emission efficiency of the resulting molecule (BPSPXZ) is much less affected by concentration increase. This enables exploitation of the concentration effects to tune energy levels of its excited states for obtaining simultaneously small singlet-triplet energy offset and large spin-orbital coupling, leading to high-efficiency reverse intersystem crossing. With these merits, organic light-emitting diodes (OLEDs) using the BPSPXZ emitter from 5 to 60 wt% doping can all deliver EQE of over 20%. More importantly, record-high EQEs of 33.4% and 15.8% are respectively achieved in the optimized and nondoped conditions. This work proposes a strategy for developing red TADF emitters by optimizing the intermolecular interaction and energy level alignments to facilitate exciton utilization over wide doping concentrations.

Orientation-dependent Rashba spin-orbit coupling of two-dimensional hole gases in semiconductor quantum wells: Linear or cubic

Our recent study [Phys. Rev. B 103, 085309 (2021)] verified the existence of $\bf{k}$-linear Rashba spin-orbit coupling (SOC) of two-dimensional hole gases in quantum wells (QWs) which originates from a combination of heavy-hole-light-hole (HH-LH) mixing and direct dipolar coupling to the external electric field. However, the Rashba SOC dependence on QW orientations remains unclear. Here, we explore this dependence on QW orientations and uncover an upper and lower bound to the orientation-dependent $\bf{k}$-linear Rashba SOC along the [110]- and [111]- crystalline directions by performing atomistic pseudopotential calculations associated with theoretical analysis. The intrinsic HH-LH mixing at the Brillouin zone center, maximal in [110]-oriented quantum wells and minimal in [111]- and [001]-oriented QWs, plays an essential role. Remarkably, we find that only $\bf{k}$-cubic Rashba SOC exists in [111]-oriented QWs. These findings help understand the physical mechanism of the Rashba SOC dependence on QW orientations and provide a strategic prediction for experiments to realize the large Rashba SOC.