Strong Spin-orbit Coupling Research Articles

Possible metal-insulator transition in a bismuth iridium oxide with the KSbO3-type structure

Iridium-based compounds have attracted a lot of attention because of strong spin-orbit coupling in them and resulting exotic magnetic and electronic properties. We report here a high-pressure, high-temperature synthesis (at 6 GPa and 1550 K) of a bismuth iridium oxide with the ideal composition of Bi3Ir3O11. This compound crystallizes in the KSbO3-type structure with space group Pn−3 and the lattice parameter a = 9.39556 Å (at room temperature). The crystal structure was investigated by synchrotron powder X-ray diffraction between 100 K and 800 K. No structural phase transitions or other changes were found up to 600 K; above 600 K, a partial decomposition takes place with the appearance of a small amount of Ir impurity. The structural analysis and density measurements suggested a cation-deficient composition of Bi3−δIr3−δO11 with δ ≈ 0.11. Nearly temperature-independent paramagnetic behavior was observed between 2 K and 400 K. Resistivity measurements showed a rise in resistivity below about 85 K suggesting a possible metal-insulator transition. No magnetoresistence effects were observed.

Hole-type superconducting gatemon qubit based on Ge/Si core/shell nanowires

We demonstrate that superconducting gatemon qubits based on superconductor-semiconductor-superconductor Josephson junctions can be constructed on hole-type Ge/Si core/shell nanowires. The frequency of the qubit can be set firstly by controlling the diffusion of Al in the nanowire via thermal annealing, which yields a suitable critical supercurrent allowing the qubit frequency to be within the experimentally accessible range, and then by fine tuning of a gate voltage, by which an accurate adjustment of the frequency can be realized. On the resulted qubit, Rabi oscillation with an energy relaxation time {T}_{1} sim 180,{rm{ns}} was observed in the time domain, an average decoherence time {T}_{2}^{* } sim 15,{rm{ns}} was obtained, and the gate voltage dependence of both {T}_{1} and {T}_{2}^{* } was investigated. Such a hole-type superconducting gatemon qubit, based on materials with strong spin-orbit coupling and potentially the absence of hyperfine interaction after isotope purification, could be used for exploring the quantum coherence phenomena of hole-gas and even Majorana physics in Ge-based quantum devices.

Two‐Dimensional Electron Gases at the Amorphous and Crystalline SrTiO3/KTaO3 Heterointerfaces

The 5d two‐dimensional electron gas (2DEG) based on KTaO3 (KTO) exhibits a lot of exotic properties such as stronger spin‐orbit coupling (SOC) and higher superconductivity transition temperature than those of SrTiO3 (STO)‐based 3d 2DEGs, whereas it has much lower electron mobility. Herein, the property of the 5d 2DEGs is investigated including the carrier mobility and Rashba SOC by interfacing both amorphous and crystalline STO with the crystalline KTO. Metallic 2DEGs are achieved at both interfaces of amorphous and crystalline STO‐capped KTO. Notably, the amorphous STO/KTO heterointerface has a larger carrier concentration and higher Hall mobility than the crystalline STO/KTO counterparts at low temperatures, which stems from two kinds of carriers. In contrast, the 5d 2DEG formed at the crystalline interface exhibits much stronger Rashba SOC as revealed through magnetotransport measurement. The deduced maximum strength of Rashba SOC and spin‐splitting energy are ≈8.56 × 10−12 eV m and ≈33.52 meV, respectively. Our results provide new insights on designing on‐demand properties of KTO‐based conducting interfaces.

Pressure-driven evolution of upper critical field and Fermi surface reconstruction in the strong-coupling superconductor Ti4Ir2O

We report pressure-driven evolution of the superconducting transition temperature (${T}_{\mathrm{c}}$) and upper critical field ${B}_{\mathrm{c}2}$ $(0)$ of strong-coupling superconductor ${\mathrm{Ti}}_{4}{\mathrm{Ir}}_{2}\mathrm{O}$ that possesses an unusually large ${B}_{\mathrm{c}2} (0)$ at ambient pressure (AP). Our results reveal an extremely low-pressure coefficients of ${T}_{\mathrm{c}}$, i.e., $d{T}_{c}/dP\ensuremath{\approx}\ensuremath{-}0.047$ K/GPa for $P<15$ GPa and \ensuremath{-}0.017 K/GPa for $15<P<50$ GPa, presumably associated with an inherent large bulk modulus of $\ensuremath{\sim}252$ GPa. Interestingly, we find that its ${B}_{\mathrm{c}2}$ $(0)$ undergoes a smooth crossover at 35.6 GPa from well beyond to less than the Pauli paramagnetic limit ${B}_{\mathrm{p}}^{\mathrm{BCS}}(0)=1.84\phantom{\rule{0.16em}{0ex}}{T}_{c}$; i.e., the ${B}_{\mathrm{c}2}(0)\ensuremath{\approx}18.2 \mathrm{T}=1.7\phantom{\rule{0.16em}{0ex}}{B}_{\mathrm{p}}^{\mathrm{BCS}} (0)$ at AP decreases to $\ensuremath{\sim}5.8 \mathrm{T}=0.88\phantom{\rule{0.16em}{0ex}}{B}_{\mathrm{p}}^{\mathrm{BCS}} (0)$ at 50 GPa. The density-functional calculations predict the possible occurrence of pressure-induced Fermi surface reconstruction for the energy bands near the $K$ point with strong spin-orbit coupling in 31--41 GPa. The present work sheds more light on this intriguing superconductor capable of resisting large external compression and strong magnetic fields.

Magneto-strain effects in 2D ferromagnetic van der Waal material CrGeTe_3

The idea of strain based manipulation of spins in magnetic two-dimensional (2D) van der Waal (vdW) materials leads to the development of new generation spintronic devices. Magneto-strain arises in these materials due to the thermal fluctuations and magnetic interactions which influences both the lattice dynamics and the electronic bands. Here, we report the mechanism of magneto-strain effects in a vdW material CrGeTe_3 across the ferromagnetic (FM) transition. We find an isostructural transition in CrGeTe_3 across the FM ordering with first order type lattice modulation. Larger in-plane lattice contraction than out-of-plane give rise to magnetocrystalline anisotropy. The signature of magneto-strain effects in the electronic structure are shift of the bands away from the Fermi level, band broadening and the twinned bands in the FM phase. We find that the in-plane lattice contraction increases the on-site Coulomb correlation (U_{eff}) between Cr atoms resulting in the band shift. Out-of-plane lattice contraction enhances the d-p hybridization between Cr–Ge and Cr–Te atoms which lead to band broadening and strong spin-orbit coupling (SOC) in FM phase. The interplay between U_{eff} and SOC out-of-plane gives rise to the twinned bands associated with the interlayer interactions while the in-plane interactions gives rise to the 2D spin polarized states in the FM phase.

Spontaneous rotational symmetry breaking in KTaO3 heterointerface superconductors

Broken symmetries play a fundamental role in superconductivity and influence many of its properties in a profound way. Understanding these symmetry breaking states is essential to elucidate the various exotic quantum behaviors in non-trivial superconductors. Here, we report an experimental observation of spontaneous rotational symmetry breaking of superconductivity at the heterointerface of amorphous (a)-YAlO3/KTaO3(111) with a superconducting transition temperature of 1.86 K. Both the magnetoresistance and superconducting critical field in an in-plane field manifest striking twofold symmetric oscillations deep inside the superconducting state, whereas the anisotropy vanishes in the normal state, demonstrating that it is an intrinsic property of the superconducting phase. We attribute this behavior to the mixed-parity superconducting state, which is an admixture of s-wave and p-wave pairing components induced by strong spin-orbit coupling inherent to inversion symmetry breaking at the heterointerface of a-YAlO3/KTaO3. Our work suggests an unconventional nature of the underlying pairing interaction in the KTaO3 heterointerface superconductors, and brings a new broad of perspective on understanding non-trivial superconducting properties at the artificial heterointerfaces.

Transition Metal Dichalcogenides Heterostructures Nanoribbons

The integration of two-dimensional van der Waals (vdW) heterostructures breaks through the constraints of lattice matching and symmetry, offering substantial opportunities in the development of advanced materials. After the width is reduced, one-dimensional vdW heterostructures show rich band structures and strong spin–orbit coupling behaviors, widening the applications in optoelectronics and spintronics. However, the synthesis of one-dimensional vdW heterostructure nanoribbons has rarely been reported yet. Herein, we report a general approach to realizing transition metal dichalcogenide (TMD) heterostructure nanoribbons for the first time by unzipping self-assembled TMD heterostructure nanoscrolls. As demonstrated by MoS2/WS2 nanoribbons, the obtained TMD heterostructure nanoribbons with alternating stacked layers possess flat edge structures and high quality. Meanwhile, this strategy can be extended to create diverse vdW TMD nanoribbons, demonstrating its versatility. Our work thus shows great potential to prepare TMD heterostructure nanoribbons with various compositions and stacking modes, and new structures can be customized with targeted properties.

Pressure-induced topological crystalline insulating phase in TlBiSe2 : Experiments and theory

We report in situ high-pressure studies on three-dimensional topological insulator ${\mathrm{TlBiSe}}_{2}$ using Raman scattering and synchrotron x-ray-diffraction experiments corroborated with the first-principles theoretical calculations. The phonon modes of a rhombohedral phase of ${\mathrm{TlBiSe}}_{2}$ show a systematic increase in frequencies under hydrostatic pressure up to $\ensuremath{\sim}7.0\phantom{\rule{0.16em}{0ex}}\mathrm{GPa}$. Interestingly, the linewidth of the ${A}_{1g}, N$, and ${E}_{g}$ phonon modes show clear anomalies at $\ensuremath{\sim}2.5\phantom{\rule{0.16em}{0ex}}\mathrm{GPa}$ which is indicating the isostructural electronic transition. With the help of calculated electron-phonon coupling constant $\ensuremath{\lambda}$, anomalies in the Raman linewidth of ${E}_{g}$ mode are attributed to electron-phonon coupling changes. Moreover, our theoretical results reveal that the observed phonon anomalies are due to pressure-induced band inversion at the $F$ points of the Brillouin zone which leads to the changes in electronic topology reflected in the mirror Chern number ${n}_{M}$ and ${\mathbb{Z}}_{2}$ topological invariant. Therefore, the phonon anomalies and change in mirror Chern number ${n}_{M}$ confirm the pressure-induced topological crystalline insulator phase in ${\mathrm{TlBiSe}}_{2}$ at $\ensuremath{\sim}2.5\phantom{\rule{0.16em}{0ex}}\mathrm{GPa}$. Further, a reversible structural phase transition has been observed above $\ensuremath{\sim}7.0\phantom{\rule{0.16em}{0ex}}\mathrm{GPa}$ from both synchrotron x-ray-diffraction and Raman-scattering measurements. Finally, our studies suggest the use of hydrostatic pressure as a potential pathway for exploring the topological crystalline insulating phase in strong spin-orbit coupling compounds, such as the thallium-based III-V-$\mathrm{V}{\mathrm{I}}_{2}$ ternary chalcogenide $\mathrm{TlBi}{X}_{2}$ ($X=\mathrm{S}$, Se, Te) family.

Mobility exceeding 100 000 cm2/V s in modulation-doped shallow InAs quantum wells coupled to epitaxial aluminum

The two-dimensional electron gas residing in shallow InAs quantum wells coupled to epitaxial aluminum is a widely utilized platform for exploration of topological superconductivity. Strong spin-orbit coupling, a large effective $g$ factor, and control over proximity-induced superconductivity are important attributes. Disorder in shallow semiconductor structures plays a crucial role for the stability of putative topological phases in hybrid structures. We report on the transport properties of 2DEGs residing 10 nm below the surface in shallow InAs quantum wells in which mobility may exceed 100 000 ${\mathrm{cm}}^{2}/\mathrm{V}$ s at 2DEG density ${n}_{2\mathrm{DEG}}\ensuremath{\le}1\ifmmode\times\else\texttimes\fi{}{10}^{12}\phantom{\rule{4pt}{0ex}}{\mathrm{cm}}^{\ensuremath{-}2}$ at low temperature.

Magnetic Anisotropy in a Verdazyl-Based Complex with Cobalt(II)

A verdazyl-based complex with Co2+ ion, (p-Py-V-p-Br)2[Co(hfac)2], was successfully synthesized. The molecular arrangement indicates that spins on the radical and Co form a one-dimensional (1D) spin chain, where the intermolecular interaction between the radicals and the intramolecular interaction between the radical and Co exhibit threefold periodicity. The strong antiferromagnetic interaction between radical spins forms a nonmagnetic singlet dimer, yielding paramagnetic behavior associated with the residual spins on the Co2+ ion in the low-temperature region. We evaluated the anisotropic g values by analyzing the electron spin resonance powder patterns. Furthermore, we considered the spin–orbit coupling and distorted crystal fields of the Co2+ ion and explained the triaxial anisotropic behavior. We also evaluated the anisotropy of the expected effective exchange interaction between the fictitious spins on the Co2+ ions, indicating the existence of an Ising-like exchange interaction. These results demonstrate that the strong spin–orbit coupling of the Co2+ ion in the present verdazyl-based compound yields an effective spin-1/2 with anisotropic g values and Ising-like exchange interactions.

Spin-induced valley polarization in heterobilayer Janus transition-metal dichalcogenides

Inspired by potential application prospects of spintronics and valleytronics, a novel heterobilayer Janus structure is designed by replacing the chalcogenide atomic layers in the original bilayer MoS2. Based on first-principles calculations, it is found that the SMoS/SeMoS structure exhibits a direct band-gap semiconductor and a typical type-II band alignment with longer carrier lifetime. The transition metal (TM) atom represented by V/Cr/Mn can be stably adsorbed on the heterobilayer Janus SMoS/SeMoS sheet and effectively introduce magnetic moments (m). The calculation results demonstrate that the most stable adsorption site of the TM atom is CX(A), and the TM (V/Cr/Mn) adatom modified SMoS/SeMoS system is converted into metal (V-) or half-metal (Cr/Mn-), respectively. Under the coupling of different indirect exchange interactions, the structure exhibits stable intrinsic anti-ferromagnetic interactions for V-SMoS/SeMoS and ferromagnetic ground state for Cr/Mn-SMoS/SeMoS, respectively, and the magnetic transition temperature (T c) reaches a high temperature or even room temperature. Moreover, the robust out-of-plane magnetocrystalline anisotropy energy ensures stable long-range magnetic order. Specifically, the combination of spin injection and strong spin–orbit coupling interaction effectively breaks the time-reversal symmetry, which leads to valley polarization of the system. Based on this, the biaxial strain can effectively regulate the electronic structure, magnetic properties and valley polarization of TM-SMoS/SeMoS nanosheets with double breaking of spatial-inversion and time-reversal symmetry.

Direct Probing of a Large Spin–Orbit Coupling in the FeSe Superconducting Monolayer on STO

Spin-orbit coupling (SOC) is a fundamental physical interaction, which describes how the electrons' spin couples to their orbital motion. It is the source of a vast variety of fascinating phenomena in nanostructures. Although in most theoretical descriptions of high-temperature superconductivity SOC has been neglected, including this interaction can, in principle, revise the microscopic picture. Here by preforming energy-, momentum-, and spin-resolved spectroscopy experiments we demonstrate that while probing the dynamic charge response of the FeSe monolayer on strontium titanate, a prototype two-dimensional high-temperature superconductor using electrons, the scattering cross-section is spin dependent. We unravel the origin of the observed phenomenon and show that SOC in this two-dimensional superconductor is strong. We anticipate that such a strong SOC can have several consequences on the electronic structures and may compete with other pairing scenarios and be crucial for the mechanism of superconductivity.

Janus monolayer TaNF: A new ferrovalley material with large valley splitting and tunable magnetic properties

Materials with large intrinsic valley splitting and high Curie temperature are a huge advantage for studying valleytronics and practical applications. In this work, using first-principles calculations, a new Janus TaNF monolayer is predicted to exhibit excellent piezoelectric properties and intrinsic valley splitting, resulting from the spontaneous spin polarization, the spatial inversion symmetry breaking and strong spin-orbit coupling (SOC). TaNF is also a potential two-dimensional (2D) magnetic material due to its high Curie temperature and large magnetic anisotropy energy. The effective control of the band gap of TaNF can be achieved by biaxial strain, which can transform TaNF monolayer from semiconductor to semi-metal. The magnitude of valley splitting at the CBM can be effectively tuned by biaxial strain due to the changes of orbital composition at the valleys. The magnetic anisotropy energy (MAE) can be manipulated by changing the energy and occupation (unoccupation) states of d orbital compositions through biaxial strain. In addition, Curie temperature reaches 373 K under only −3% biaxial strain, indicating that Janus TaNF monolayer can be used at high temperatures for spintronic and valleytronic devices.

Reduction of carrier density and enhancement of the bulk Rashba spin-orbit coupling strength in Bi2Te3/GeTe superlattices

Featured with the large Rashba constant (αR ∼ 4.2 eVÅ) coupled with the ferroelectricity, GeTe has attracted much interest, proposing novel energy-efficient spintronic devices. However, those approaches are hampered by the high hole density of GeTe around 1020-1021 cm−3, which deteriorates both the ferroelectricity and the bulk Rashba effect of GeTe. To solve this problem, we have investigated the superlattices composed of Bi2Te3 and GeTe ([BT|GT] SL), the former of which is known as a topological insulator (TI) with typically n-type conduction and strong spin-orbit coupling. We have investigated the magnetotransport properties of 25 [BTx|GTy]z SLs with varying thicknesses (x, y: the thickness in the multiples of the unit cell of the respective layer) of each layer and the repetition number (z) systematically. We have observed a minimum carrier density of 5.7 × 1019 cm−3 in [BT6|GeTe6]6 superlattice sample smaller than that (∼4.4 ×1020 cm−3) of GeTe single film by an order. In addition, we have found that some [BT|GT] SLs with reduced carrier density have a larger Rashba constant compared to that of a single GeTe film. This is explained by the change of the spin polarization which can be modulated by the Fermi level in the Rashba band. These results provide a viable way to realize robust ferroelectricity and strong SOC in GeTe-related material systems simultaneously.

Non-Lifshitz invariants corrections to Dzyaloshinskii-Moriya interaction energy

We study the continuum limit of two-dimensional chiral magnets in which Dzyaloshinskii-Moriya interaction (DMI) is due to the interplay between a smooth magnetic texture and spin-orbit coupling. The resulting free-energy density of the system contains linear terms in the spatial gradient of the magnetic texture, which mark an instability of the system towards the formation of nontrivial magnetic orders such as skyrmions or chiral domain walls. We perform a microscopic analysis of DMI tensors responsible for this contribution to free energy based on a Berry phase formulation in the mixed space of momentum and position, and reveal that they exhibit non-Lifshitz invariants features. In particular, a perturbation theory shows in the case of Rashba spin-orbit interactions the presence of non-Lifshitz invariants to third order in the small spin-orbit interaction and fourth order in the small exchange coupling. The higher-order terms may even lead to an enhancement of DMI interaction at strong spin-orbit coupling due to divergences in the density of states at the bottom of the conduction band. Finally, we also study the DMI free energy generated from Rashba spin-orbit interaction in different symmetry groups.

Multiple resonance thermally activated delayed fluorescence dendrimers containing selenium-doped polycyclic aromatic hydrocarbon emitters for solution-processed narrowband blue OLEDs

Three multiple resonance thermally activated delayed fluorescence dendrimers (BSeN-Cz, BSeN-DCz and BSeN-TCz) are developed by introducing the first-, second- and third-generation carbazole dendrons in periphery of selenium-doped polycyclic aromatic hydrocarbon emitter for solution-processed narrowband blue organic light-emitting diodes (OLEDs). It is found that the dendrimers show strong thermally activated delayed fluorescence effect with rapid reverse intersystem crossing rate constant of 8.5–9.1 × 106 s−1 owing to the strong heavy-atom effect and spin-orbit coupling of selenium atoms. Meanwhile, intermolecular aggregation is effectively suppressed in the dendrimers owing to steric hindrance effect of carbazole dendrons, leading to nearly invariable narrowband emissions with full width at half-maximum (FWHM) kept at 32 nm at doping concentration up to 100 wt% for BSeN-TCz bearing third-generation carbazole dendron. Solution-processed OLEDs based on multiple resonance dendrimer (BSeN-TCz) reveal blue electroluminescence at 478 nm with FWHM of 31 nm and Commission International de 1′Eclairage coordinates of (0.11, 0.21), together with maximum external quantum efficiency of 19.7%, which is promising device efficiency for narrowband blue electroluminescence by solution process.

Enhanced Superconductivity and Rashba Effect in a Buckled Plumbene-Au Kagome Superstructure.

Plumbene, with a structure similar to graphene, is expected to possess a strong spin-orbit coupling and thus enhances its superconducting critical temperature (Tc ). In this work, a buckled plumbene-Au Kagome superstructure grown by depositing Au on Pb(111) is investigated. The superconducting gap monitored by temperature-dependent scanning tunneling microscopy/spectroscopy shows that the buckled plumbene-Au Kagome superstructure not only has an enhanced Tc with respect to that of a monolayer Pb but also possesses a higher value than what owned by a bulk Pb substrate. By combining angle-resolved photoemission spectroscopy with density functional theory, the monolayer Au-intercalated low-buckled plumbene sandwiched between the top Au Kagome layer and the bottom Pb(111) substrate is confirmed and the electron-phonon coupling-enhanced superconductivity is revealed. This work demonstrates that a buckled plumbene-Au Kagome superstructure can enhance superconducting Tc and Rashba effect, effectively triggering the novel properties of a plumbene.

Spin susceptibility of nonunitary spin-triplet superconductors

The spin susceptibility is an important probe to characterize the symmetry of the order parameter in unconventional superconductors. Among them, nonunitary triplet superconductors have attracted a lot of attention recently in the context of the search for topological superconductivity. Here, we derive a general formula for the spin susceptibility of nonunitary triplet superconductors within a single-band model of nonmagnetic, centrosymmetric materials with strong spin-orbit coupling. We use it to critically assess experimental claims of nonunitary triplet superconductivity in some materials.

High-throughput screening giant bulk spin-split materials

Current-induced spin polarization or electric-field-controlled energy splitting shows great potential in the application of spintronic devices, such as spin–orbit torque devices, spin transistors, and so on. These mechanisms are often found in spin-split materials with strong spin-orbital coupling (SOC), which jointly trigger the Rashba, Dresselhaus, or Zeeman effect. The essential criterion to evaluate the strength of SOC is the spin-split energy. Thus, searching for bulk materials with large spin-split energy has attracted great interest. In this work, a high-throughput workflow was designed with the proposed three-point-spin-texture method, and 440 Rashba, 411 Dresselhaus, and 469 Zeeman-type candidate materials with large spin-split energy were screened out. Moreover, the spin-split energy of Rashba material KSnSb, Dresselhaus material TaSi2, and Zeeman-type material PtN2 achieve 0.19 eV, 0.82 eV, and 0.55 eV, respectively. This work significantly expands the materials database for spintronic devices and paves the way for further experimental research.

Sonication-assisted liquid exfoliation and size-dependent properties of magnetic two-dimensional α-RuCl3

Originating from the hexagonal arrangement of magnetic ions in the presence of strong spin orbit coupling, α-RuCl3 is considered as model system for the Kitaev-Heisenberg model. While the magnetic properties of α-RuCl3 have been studied in bulk single crystals or micromechanically-exfoliated nanosheets, little is known about the nanosheets’ properties after exfoliation by techniques suitable for mass production such as liquid phase exfoliation (LPE). Here, we demonstrate sonication-assisted LPE on α-RuCl3 single crystals in an inert atmosphere. Coupled with centrifugation-based size selection techniques, the accessible size- and thickness range is quantified by statistical atomic force microscopy. Individual nanosheets obtained after centrifugation-based size selection are subjected to transmission electron microscopy to confirm their structural integrity after the exfoliation. The results are combined with bulk characterisation methods, including Raman and x-ray photoelectron spectroscopy, and powder diffraction experiments to evaluate the structural integrity of the nanosheets. We report changes of the magnetic properties of the nanomaterial with nanosheet size, as well as photospectroscopic metrics for the material concentration and average layer number. Finally, a quantitative analysis on environmental effects on the nanomaterial integrity is performed based on time and temperature dependent absorbance spectroscopy revealing a relatively slow decay (half-life of ∼2000 h at 20 °C), albeit with low activation energies of 6–20 kJ mol−1.