Structural Isomeric Effect on Spin Transport in Molecular Semiconductors

Molecular semiconductor (MSC) is a promising candidate for spintronic applications benefiting from its long spin lifetime caused by light elemental-composition essence and thus weak spin-orbit coupling (SOC). According to current knowledge, the SOC effect, normally dominated by the elemental composition, is the main spin-relaxation causation in MSCs, and thus the molecular structure-induced SOC change is one of the most concerned issues. In theoretical study, molecular isomerism, a most prototype phenomenon, has long been considered to possess little difference on spin transport previously, since elemental compositions of isomers are totally the same. However, here in this study, quite different spin-transport performances are demonstrated in ITIC and its structural isomers BDTIC experimentally, for the first time, though the charge transport and molecular stacking of the two films are very similar. By further experiments of electron-paramagnetic resonance and density-functional-theory calculations, it is revealed that noncovalent-conformational locks (NCLs) formed in BDTIC can lead to enhancement of SOC and thus decrease the spin lifetime. Hence, this study suggests the influences from the structural-isomeric effect must be considered for developing highly efficient spin-transport MSCs, which also provides a reliable theoretical basis for solving the great challenge of quantificational measurement of NCLs in films in the future.

The Role of Polaronic States on the Spin Dynamics in Solution-Processed Two-Dimensional Layered Perovskite with Different Layer Thickness

2D lead halide perovskites (LHPs) show strong excitonic and spin-orbit coupling effects, generating a facile spin injection. Besides, they possess a polaron character due to the soft crystal lattice, which can prolong the spin lifetime, making them favorable materials for spintronic applications. Here, the spin dynamics of 2D PEA2 PbI4 (MAPbI3 )n -l thin films with different layers by temperature- and pump fluence-dependent circularly polarization-resolved transient absorption (TA) measurements is studied. These results indicate that the spin depolarization mechanism is gradually converted from the Maialle-Silva-Sham (MSS) mechanism to the polaronic states protection mechanism with the layer number increasing from = 1 to 3, which is determined by the interplay between the strength of Coulomb exchange interaction and the strength of polaronic effect. While for ≥ 4, the Elliot-Yafet (EY) impurities mechanism is proposed, in which the formed polaronic states with free charge carriers no longer play the protective role.

Giant Spin Lifetime Anisotropy and Spin-Valley Locking in Silicene and Germanene from First-Principles Density-Matrix Dynamics

Through first-principles real-time density-matrix (FPDM) dynamics simulations, we investigated spin relaxation due to electron-phonon and electron-impurity scatterings with spin-orbit coupling (SOC) in two-dimensional Dirac materials silicene and germanene at finite temperatures. We discussed the applicability of conventional descriptions of spin relaxation mechanisms by Elliott-Yafet (EY) and D'yakonov-Perel' (DP) compared to the FPDM method, which is determined by a complex interplay of intrinsic SOC, external fields, and scattering strength. For example, the electric field dependence of the spin lifetime by FPDM is close to the DP mechanism for silicene at room temperature but similar to the EY mechanism for germanene. Because of its stronger SOC strength and buckled structure in contrast to graphene, germanene has a giant spin lifetime anisotropy and spin-valley locking effect under nonzero E_z and low temperatures. More importantly, germanene has a long spin lifetime (∼100 ns at 50 K) and an ultrahigh carrier mobility, making it advantageous for spin-valleytronic applications.

Visualizing Tailored Spin Phenomena in a Reduced-Dimensional Topological Superlattice

Emergent topological insulators (TIs) and their design are in high demand for manipulating and transmitting spin information toward ultralow-power-consumption spintronic applications. Here, distinct topological states with tailored spin properties can be achieved in a single reduced-dimensional TI-superlattice, (Bi₂ /Bi₂ Se₃ )-(Bi₂ /Bi₂ Se₃ )_N or (□/Bi₂ Se₃ )-(Bi₂ /Bi₂ Se₃ )_N (N is the repeating unit, □ represents an empty layer) by controlling the termination via molecular beam epitaxy. The Bi₂ -terminated superlattice exhibits a single Dirac cone with a spin momentum splitting ≈0.5 Å^-1 , producing a pronounced inverse Edelstein effect with a coherence length up to 1.26 nm. In contrast, the Bi₂ Se₃ -terminated superlattice is identified as a dual TI protected by coexisting time reversal and mirror symmetries, showing an unexpectedly long spin lifetime up to 1 ns. The work elucidates the key role of dimensionality and dual topological phases in selecting desired spin properties, suggesting a promise route for engineering topological superlattices for high-performance TI-spintronic devices.

Spin States Protected from Intrinsic Electron-Phonon Coupling Reaching 100 ns Lifetime at Room Temperature in MoSe2

We present time-resolved Kerr rotation measurements, showing spin lifetimes of over 100 ns at room temperature in monolayer MoSe₂. These long lifetimes are accompanied by an intriguing temperature-dependence of the Kerr amplitude, which increases with temperature up to 50 K and then abruptly switches sign. Using ab initio simulations, we explain the latter behavior in terms of the intrinsic electron-phonon coupling and the activation of transitions to secondary valleys. The phonon-assisted scattering of the photoexcited electron-hole pairs prepares a valley spin polarization within the first few ps after laser excitation. The sign of the total valley magnetization, and thus the Kerr amplitude, switches as a function of temperature, as conduction and valence band states exhibit different phonon-mediated intervalley scattering rates. However, the electron-phonon scattering on the ps time scale does not provide an explanation for the long spin lifetimes. Hence, we deduce that the initial spin polarization must be transferred into spin states, which are protected from the intrinsic electron-phonon coupling, and are most likely resident charge carriers, which are not part of the itinerant valence or conduction band states.

An Iron-Porphyrin Complex with Large Easy-Axis Magnetic Anisotropy on Metal Substrate

Easy-axis magnetic anisotropy separates two magnetic states with opposite magnetic moments, and single magnetic atoms and molecules with large easy-axis magnetic anisotropy are highly desired for future applications in high-density data storage and quantum computation. By tuning the metalation reaction between tetra-pyridyl-porphyrin molecules and Fe atoms, we have stabilized the so-called initial complex, an intermediate state of the reaction, on Au(111) substrate, and investigated the magnetic property of this complex at a single-molecule level by low-temperature scanning tunneling microscopy and spectroscopy. As revealed by inelastic electron tunneling spectroscopy in magnetic field, this Fe-porphyrin complex has magnetic anisotropy energy of more than 15 meV with its easy-axis perpendicular to the molecular plane. Two magnetic states with opposite spin directions are discriminated by the dependence of spin-flip excitation energy on magnetic field and are found to have long spin lifetimes. Our density functional theory calculations reveal that the Fe atom in this complex, decoupled from Au substrate by a weak ligand field with elongated Fe-N bonds, has a high-spin state S = 2 and a large orbital angular momentum L = 2, which give rise to easy-axis anisotropy perpendicular to the molecular plane and large magnetic anisotropy energy by spin-orbit coupling. Since the Fe atom is protected by the molecular ligand, the complex can be processed at room or even higher temperatures. The reported system may have potential applications in nonvolatile data storage, and our work demonstrates on-surface metalation reactions can be utilized to synthesize organometallic complexes with large magnetic anisotropy.

Manipulating spin-polarized photocurrents in 2D transition metal dichalcogenides

Manipulating spin polarization of electrons in nonmagnetic semiconductors by means of electric fields or optical fields is an essential theme of the conceptual nonmagnetic semiconductor-based spintronics. Here we experimentally demonstrate an electric method of detecting spin polarization in monolayer transition metal dichalcogenides (TMDs) generated by circularly polarized optical pumping. The spin-polarized photocurrent is achieved through the valley-dependent optical selection rules and the spin-valley locking in monolayer WS2, and electrically detected by a lateral spin-valve structure with ferromagnetic contacts. The demonstrated long spin-valley lifetime, the unique valley-contrasted physics, and the spin-valley locking make monolayer WS2 an unprecedented candidate for semiconductor-based spintronics.