Spin-flip Processes Research Articles

Josephson current through the SYK model

We calculate the equilibrium Josephson current through a disordered interacting quantum dot described by a Sachdev-Ye-Kitaev model fully contacted by two BCS superconductors, such that all modes of the dot contribute to the coupling, which encodes hopping and spin-flip processes. We show that, at zero temperature and at the conformal limit, i.e. in the strong interacting limit, the Josephson current is suppressed by UU, the strength of the interaction, as ln(U)/U and becomes universal, namely it gets independent on the superconducting gap. At finite temperature, instead, it depends on the ratio between the gap and the temperature. A proximity effect exists but the self-energy corrections induced by the coupling with the superconducting leads seem subleading as compared to the interaction self-energy and the tunneling matrix for large number of particles. Finally we compare the results of the original four-fermion model with those obtained considering zero interaction, two-fermions and a generalized qq-fermion model.

Defect-Assisted Domain Nucleation Drives Unique Exchange-Bias Phenomena in MnBi2Te4

The study of the mechanism of exchange-bias phenomena and the achievement of its efficient control are of great importance, as it promotes the revelation of unique exchange interactions and the development of exotic applications. However, it is challenging due to the elusive interface between magnetic phases. In this study, we report an unprecedented exchange-bias phenomenon observed in ultrathin, uncompensated, antiferromagnetic MnBi2Te4. The magnitude and direction of the exchange field can be intentionally controlled by designing a magnetic-field sweep protocol without a field cooling process. The combined experimental and theoretical simulation results indicate that the spin-flip process assisted by the ubiquitous defect-induced pinning domain sites with varying inner exchange interactions might give rise to the emergence and robustness of this peculiar exchange bias. The temperature and thickness dependence of the exchange-bias phenomena are systematically investigated for further study and exploitation of its unique properties. This mechanism allows for highly tunable exchange bias in prevalent magnetic systems by engineering the properties of domain structures and also offers promising avenues for the design of spintronic devices combing its topology based on MnBi2Te4. Published by the American Physical Society 2024

Highly efficient pure-blue organic light-emitting diodes based on rationally designed heterocyclic phenophosphazinine-containing emitters

Multi-resonance thermally activated delayed fluorophores have been actively studied for high-resolution photonic applications due to their exceptional color purity. However, these compounds encounter challenges associated with the inefficient spin-flip process, compromising device performance. Herein, we report two pure-blue emitters based on an organoboron multi-resonance core, incorporating a conformationally flexible donor, 10-phenyl-5H-phenophosphazinine 10-oxide (or sulfide). This design concept selectively modifies the orbital type of high-lying excited states to a charge transfer configuration while simultaneously providing the necessary conformational freedom to enhance the density of excited states without sacrificing color purity. We show that the different embedded phosphorus motifs (phosphine oxide/sulfide) of the donor can finely tune the electronic structure and conformational freedom, resulting in an accelerated spin-flip process through intense spin-vibronic coupling, achieving over a 20-fold increase in the reverse intersystem crossing rate compared to the parent multi-resonance emitter. Utilizing these emitters, we achieve high-performance pure-blue organic light-emitting diodes, showcasing a top-tier external quantum efficiency of 37.6% with reduced efficiency roll-offs. This proposed strategy not only challenges the conventional notion that flexible electron-donors are undesirable for constructing narrowband emitters but also offer a pathway for designing efficient narrow-spectrum blue organic light-emitting diodes.

Spin-flip gluon generalized transverse-momentum-dependent parton distribution F1,2 at small x

Until recently the spin-flip processes in the deep inelastic scatterings are thought to be suppressed in the high energy. We found a positive intercept for the spin-flip generalized transverse momentum-dependent parton distribution (GTMDs) Re(F1,2) as Re(F1,2)∼(1x)α¯s(4 ln2−8/3)(cos3ϕkΔ+cosϕkΔ). This is done by analytically solving the integro-differential evolution equation for Re(F1,2), recently proposed by Hatta and Zhou, in the dilute regime. Interestingly, the surviving solution corresponds to conformal spin n=2 and carries an explicit cos3ϕkΔ+cosϕkΔ azimuthal dependence. As the imaginary part of F1,2, is related to the spin-dependent odderon or gluon Siver function and scales as Im(F1,2)∼x0, the positive intercept for Re(F1,2) implies that it is expected to dominate over the gluon Siver function in the small-x limit and may directly impact the modeling of unpolarized GTMDs and associated spin-flip processes. Published by the American Physical Society 2024

Laser-Induced Ultrafast Spin Injection in All-Semiconductor Magnetic CrI3/WSe2 Heterobilayer.

Spin injection stands out as a crucial method employed for initializing, manipulating, and measuring the spin states of electrons, which are fundamental to the creation of qubits in quantum computing. However, ensuring efficient spin injection while maintaining compatibility with standard semiconductor processing techniques is a significant challenge. Herein, we demonstrate the capability of inducing an ultrafast spin injection into a WSe2 layer from a magnetic CrI3 layer on a femtosecond time scale, achieved through real-time time-dependent density functional theory calculations upon a laser pulse. Following the peak of the magnetic moment in the CrI3 sublayer, the magnetic moment of the WSe2 layer reaches a maximum of 0.89 μB (per unit cell containing 4 WSe2 and 1 CrI3 units). During the spin dynamics, spin-polarized excited electrons transfer from the WSe2 layer to the CrI3 layer via type-II band alignment. The large spin splitting in conduction bands and the difference in the number of spin-polarized local unoccupied states available in the CrI3 layer lead to a net spin in the WSe2 layer. Furthermore, we confirmed that the number of available states, the spin-flip process, and the laser pulse parameters play important roles during the spin injection process. This work highlights the dynamic and rapid nature of spin manipulation in layered all-semiconductor systems, offering significant implications for the development and enhancement of quantum information processing technologies.

XMECP: Reaching State-of-the-Art MECP Optimization in Multiscale Complex Systems.

The Python-based program, XMECP, is developed for realizing robust, efficient, and state-of-the-art minimum energy crossing point (MECP) optimization in multiscale complex systems. This article introduces the basic capabilities of the XMECP program by theoretically investigating the MECP mechanism of several example systems including (1) the photosensitization mechanism of benzophenone, (2) photoinduced proton-coupled electron transfer in the cytosine-guanine base pair in DNA, (3) the spin-flip process in oxygen activation catalyzed by an iron-containing 2-oxoglutarate-dependent oxygenase (Fe/2OGX), and (4) the photochemical pathway of flavoprotein adjusted by the intensity of an external electric field. MECPs related to multistate reaction and multistate reactivity in large-scale complex biochemical systems can be well-treated by workflows suggested by the XMECP program. The branching plane updating the MECP optimization algorithm is strongly recommended as it provides derivative coupling vector (DCV) with explicit calculation and can equivalently evaluate contributions from non-QM residues to DCV, which can be nonadiabatic coupling or spin-orbit coupling in different cases. In the discussed QM/MM examples, we also found that the influence on the QM region by DCV can occur through noncovalent interactions and decay with distance. In the example of DNA base pairs, the nonadiabatic coupling occurs across the π-π stacking structure formed in the double-helix system. In contrast to general intuition, in the example of Fe/2OGX, the central ferrous and oxygen part contribute little to the spin-orbit coupling; however, a nearby arginine residue, which is treated by molecular mechanics in the QM/MM method, contributes significantly via two hydrogen bonds formed with α-ketoglutarate (α-KG). This indicates that the arginine residue plays a significant role in oxygen activation, driving the initial triplet state toward the productive quintet state, which is more than the previous knowledge that the arginine residue can bind α-KG at the reaction site by hydrogen bonds.

Pump-Driven Opto-Magnetic Properties in Semiconducting Transition-Metal Dichalcogenides: An Analytical Model.

Single-layer transition-metal dichalcogenides provide an unique intrinsic entanglement between the spin/valley/orbital degrees of freedom and the polarization of scattered photons. This scenario gives rise to the well-assessed optical dichroism observed by using both steady and time-resolved probes. In this paper, we provide compact analytical modeling of the onset of a finite Faraday/Kerr optical rotation upon shining with circularly polarized light. We identify different optical features displaying optical rotation at different characteristic energies, and we describe in an analytical framework the time-dependence of their intensities as a consequence of the main spin-conserving and spin-flip processes.

Dynamics of electron-hole pairs in interface exciplex OLEDs investigated by magnetic field effects

Interface exciplex organic light-emitting diodes (OLEDs) have attracted intense attention due to the advantages including well-confined recombination regions, barrier-free charge transport and thermally activated delayed fluorescence (TADF) characteristics. To investigate the spin evolutions and dynamics of electron-hole (e-h) pairs, such as polaron pairs (PPs) and charge transfer (CT) excitons, in interface exciplex OLEDs is crucial for a better understanding on their energy gain and loss mechanisms. In this work, microscopic dynamics of e-h pairs in interface exciplex OLEDs were investigated by magneto-electroluminescence (MEL) and magneto-conductivity (MC) responses. The interface exciplex OLEDs were fabricated using 4,4,4-tris(N-3-methylphenyl-N-phenylamino) triphenylamine (m-MTDATA) as the donor and 2,4,6-tris(biphenyl-3-yl)-1,3,5-triazine (T2T) as the acceptor. Hyperfine interaction (HFI)- and different g-factors between electrons and holes (Δg mechanism)-dominated spin flip processes, and recombination and dissociation processes such as triplet-triplet annihilation (TTA) and triplet-charge annihilation (TQA) were identified, and their changes with temperature and current were explored. Our results may provide valuable insights into the evolution of carriers and facilitate the development of interface exciplex-based OLEDs.

Understanding of complex spin up-conversion processes in charge-transfer-type organic molecules

Despite significant progress made over the past decade in thermally activated delayed fluorescence (TADF) molecules as a material paradigm for enhancing the performance of organic light-emitting diodes, the underlying spin-flip mechanism in these charge-transfer (CT)-type molecular systems remains an enigma, even since its initial report in 2012. While the initial and final electronic states involved in spin-flip between the lowest singlet and lowest triplet excited states are well understood, the exact dynamic processes and the role of intermediate high-lying triplet (T) states are still not fully comprehended. In this context, we propose a comprehensive model to describe the spin-flip processes applicable for a typical CT-type molecule, revealing the origin of the high-lying T state in a partial molecular framework in CT-type molecules. This work provides experimental and theoretical insights into the understanding of intersystem crossing for CT-type molecules, facilitating more precise control over spin-flip rates and thus advancing toward developing the next-generation platform for purely organic luminescent candidates.

Using single and double laser pulses on the molecular Ni4@C48H36 system to design integrated nanospintronic units.

The accomplishment of long-distance spin transfer scenarios between several magnetic centers is a big challenge for building and supporting spin-logic units for developing future all-optical magnetic unit operations. Using high-level quantum chemistry theory CCSD and EOM-CCSD, we systematically study the ultrafast laser-induced spin-dynamics process on a carbon-based material, to which four magnetic centers are attached. We show that the CCSD method with the 6-31G basis set calculation is sensitive to the C-Ni bond length. The spin density distribution, which is computed using EOM-CCSD with LanL2DZ+ECP calculations, Mulliken population analysis, including spin-orbit-coupling (SOC) and a magnetic field, fulfills the requirements for achieving spin dynamics processes. Different local spin-flip and spin-transfer processes are accomplished within the subpicosecond regime. The impact of the propagation direction of the laser pulse by switching their polar and the azimuthal angles in spherical coordinates on the spin dynamics processes is analyzed. Double laser pulses with time delay δt ≥ 200 × FWHM yield in a realistic magnetic field gradient selectively a lateral resolution, which corresponds to distances smaller than the CMOS scale (2 nm in 2024) while our system size is comparable to the CMOS scale. Here Λ and V processes with two quasi-degenerate intermediate levels are used. We propose a model of an integrated spin-logic processor created from an array of individual spin-logic blocks, which are realized by four magnetic centers Ni. The findings of this study demonstrate the enormous potential of using laser-induced spin dynamics as the fundamental mechanism for future molecular magnetic technology.

Gadolinium Spin Decoherence Mechanisms at High Magnetic Fields.

Favorable relaxation processes, high-field spectral properties, and biological compatibility have made spin-7/2 Gd3+-based spin labels an increasingly popular choice for protein structure studies using high-field electron paramagnetic resonance. However, high-field relaxation and decoherence in ensembles of half-integer high-spin systems, such as Gd3+, remain poorly understood. We report spin-lattice (T1) and phase memory (TM) relaxation times at 8.6 T (240 GHz), and we present the first comprehensive model of high-field, high-spin decoherence accounting for both the electron spin concentration and temperature. The model includes four principal mechanisms driving decoherence: energy-conserving electron spin flip-flops, direct "T1" spin-lattice relaxation-driven electron spin flip processes, indirect T1-driven flips of nearby electron spins, and nuclear spin flip-flops. Mechanistic insight into decoherence can inform the design of experiments making use of Gd3+ as spin probes or relaxivity agents and can be used to measure local average interspin distances as long as 17 nm.

Ultrafast control of laser-induced spin-dynamics scenarios on two-dimensional Ni3@C63H54 magnetic system.

The concept of building logically functional networks employing spintronics or magnetic heterostructures is becoming more and more popular today. Incorporating logical segments into a circuit needs physical bonds between the magnetic molecules or clusters involved. In this framework, we systematically study ultrafast laser-induced spin-manipulation scenarios on a closed system of three carbon chains to which three Ni atoms are attached. After the inclusion of spin-orbit coupling and an external magnetic field, different ultrafast spin dynamics scenarios involving spin-flip and long-distance spin-transfer processes are achieved by various appropriately well-tailored time-resolved laser pulses within subpicosecond timescales. We additionally study the various effects of an external magnetic field on spin-flip and spin-transfer processes. Moreover, we obtain spin-dynamics processes induced by a double laser pulse, rather than a single one. We suggest enhancing the spatial addressability of spin-flip and spin-transfer processes. The findings presented in this article will improve our knowledge of the magnetic properties of carbon-based magnetic molecular structures. They also support the relevant experimental realization of spin dynamics and their potential applications in future molecular spintronics devices.

Ab Initio Nonadiabatic Molecular Dynamics of Ultrafast Demagnetization in Iron.

Ultrafast demagnetization in magnetic metals is the key to spintronics devices. Taking iron as a prototypical system, we investigate the demagnetization mechanism by simulating the charge and spin dynamics using nonadiabatic molecular dynamics in the presence of explicit spin-orbit coupling (SOC). Strong SOC drives ultrafast spin-flip of electrons and holes, which trigger demagnetization and remagnetization, respectively. Their confrontation decreases the demagnetization ratio and finishes the demagnetization within 167 fs, agreeing with the experimental time scale. The joint spin-flip of electrons and holes correlated with the electron-phonon coupling-induced fast electron-hole recombination further decreases the maximum demagnetization ratio, below 5% of experimental value. Although the Elliott-Yafet electron-phonon scattering model can rationalize the ultrafast spin-flip process, it fails to reproduce the experimental maximum demagnetization ratio. The study suggests the key role of SOC on spin dynamics and emphasizes the interplay between SOC and electron-phonon interactions on the ultrafast demagnetization.

Photo-assisted spin transport in double quantum dots with spin–orbit interaction

We investigate the effect of spin–orbit interaction on the intra- and interdot particle dynamics of a double quantum dot (QD) under ac electric fields. The former is modeled as an effective ac magnetic field that produces electric-dipole spin resonance transitions, while the latter is introduced via spin-flip tunneling amplitudes. We observe the appearance of non-trivial spin-polarized dark states (DSs), arising from an ac-induced interference between photo-assisted spin-conserving and spin-flip tunneling processes. These DSs can be employed to precisely measure the spin–orbit coupling in QD systems. Furthermore, we show that the interplay between photo-assisted transitions and spin-flip tunneling enables the system to operate as a highly tunable spin filter. Finally, we investigate the operation of the system as a resonant flopping-mode qubit for arbitrary ac voltage amplitudes, allowing for high tunability and enhanced qubit control possibilities.

Carrier, spin and optical propagation properties of CsPbBr3 hexagonal nanocrystals

The shape of nanocrystals can effectively adjust their photo-physical properties, which becomes a route to improve the nanocrystal-based semiconductor device. Herein, we further dig the photo-physical properties of CsPbBr3 hexagonal nanocrystals (HNCs), in which CsPbBr3 cubic nanocrystals (CNCs) serves as a reference. It is found that the activity of carrier in CsPbBr3 HNCs is higher than that of CsPbBr3 CNCs, owing to the slow energy dissipation path related to the phonon energy, which slows down the carrier temperature cooling process and accelerate the carrier recombination behavior simultaneously. Moreover, the spin flip process may occur in the CsPbBr3 HNCs, whose lifetime is longer than that of CsPbBr3 CNCs owing to the weakening of phonon scattering of Elliott-Yafet mechanism. In addition, the photoluminescence test of CsPbBr3 HNCs in optical fiber microcavity shows that its light wave-guiding performance is determined by the self-absorption effect and the conventional light wave-guiding effect, which are both super to those of CsPbBr3 CNCs, benefiting from its high PL quantum yield. Our research suggests that the CsPbBr3 HNCs exhibits a huge potential in the field of optoelectronics, spintronics and photo-communication.

Spin-Flip Raman Scattering on Electrons and Holes in Two-Dimensional (PEA)2 PbI4 Perovskites.

The class of Ruddlesden-Popper type (PEA)2 PbI4 perovskites comprises 2D structures whose optical properties are determined by excitons with a large binding energy of about 260meV. It complements the family of other 2D semiconductor materials by having the band structure typical for lead halide perovskites, that can be considered as inverted compared to conventional III-V and II-VI semiconductors. Accordingly, novel spin phenomena can be expected for them. Spin-flip Raman scattering is used here to measure the Zeeman splitting of electrons and holes in a magnetic field up to 10T. From the recorded data, the electron and hole Landé factors (g-factors) are evaluated, their signs are determined, and their anisotropies are measured. The electron g-factor value changes from +2.11 out-of-plane to +2.50 in-plane, while the hole g-factor ranges between -0.13 and -0.51. The spin flips of the resident carriers are arranged via their interaction with photogenerated excitons. Also the double spin-flip process, where a resident electron and a resident hole interact with the same exciton, is observed showing a cumulative Raman shift. Dynamic nuclear spin polarization induced by spin-polarized holes is detected in corresponding changes of the hole Zeeman splitting. An Overhauser field of the polarized nuclei acting on the holes as large as 0.6T can be achieved.

Nonlocal correlation effects due to virtual spin-flip processes in itinerant electron ferromagnets

We present an ab initio method for electronic structure calculations, which accounts for the interaction of electrons and magnons in ferromagnets. While it is based on a many-body perturbation theory we approximate numerically complex quantities with quantities from time-dependent density functional theory. This results in a simple and affordable algorithm which allows us to consider more complex materials than those usually studied in this context ($3d$ ferromagnets) while still being able to account for the nonlocality of the self-energy. Furthermore, our approach allows for a relatively simple way to incorporate self-consistency. Our results are in a good agreement with experimental and theoretical findings for iron and nickel. Especially the experimental exchange splitting of nickel is predicted accurately within our theory. Additionally, we study the half-metallic ferromagnet NiMnSb concerning its nonquasiparticle states appearing in the band gap due to spin-flip excitations.

Strain-Dependent Band Splitting and Spin-Flip Dynamics in Monolayer WS2

Triggered by the expanding demands of semiconductor devices, strain engineering of two-dimensional transition metal dichalcogenides (TMDs) has garnered considerable research interest. Through steady-state measurements, strain has been proved in terms of its modulation of electronic energy bands and optoelectronic properties in TMDs. However, the influence of strain on the spin-orbit coupling as well as its related valley excitonic dynamics remains elusive. Here, we demonstrate the effect of strain on the excitonic dynamics of monolayer WS2 via steady-state fluorescence and transient absorption spectroscopy. Combined with theoretical calculations, we found that tensile strain can reduce the spin-splitting value of the conduction band and lead to transitions between different exciton states via spin-flip mechanism. Our findings suggest that the spin-flip process is strain-dependent, provides a reference for application of valleytronic devices, where tensile strain is usually existing during their design and fabrication.

Influence of defects on the valley polarization properties of monolayer MoS2 grown by chemical vapor deposition

Temperature-dependent polarization-resolved photoluminescence spectroscopy is carried out on as-grown, transferred, and coated 1L-${\mathrm{MoS}}_{2}$ samples grown by the chemical vapor deposition technique to explore the underlying mechanism behind the valley depolarization process. It has been found that the momentum scattering of the excitons due to the sulfur-vacancies-attached-with-air-molecules type of defects has a strong influence in the suppression of valley polarization. Our study reveals that at sufficiently low densities of such defects and temperatures, the long-range electron-hole exchange mediated intervalley transfer of excitons via the Maialle-Silva-Sham (MSS) mechanism, as suggested by a recent theory [Yu and Wu, Phys. Rev. B 89, 205303 (2014)], is indeed the most dominant spin-flip process. In the process, the momentum scattering of the excitons by the defects takes the central stage. Interestingly, the study finds the scattering rate to be proportional to the cube root of the density of the defects. The intervalley transfer process of excitons involving the $\mathrm{\ensuremath{\Gamma}}$ valley also has significance in valley depolarization, especially when the layer is either under a tensile strain or has a high density of ${V}_{S}$ defects, as these perturbations reduces $K$ to $\mathrm{\ensuremath{\Gamma}}$-energy separation. Band-structural calculations carried out within a density functional theory framework validate these findings. The study further suggests that exchange interactions with the physisorbed oxygen molecules can result in the intervalley spin-flip scattering of the excitons and this process gives an important contribution to valley depolarization, especially at the strong scattering regime.

Attosecond magnetization dynamics in non-magnetic materials driven by intense femtosecond lasers

Irradiating solids with ultrashort laser pulses is known to initiate femtosecond timescale magnetization dynamics. However, sub-femtosecond spin dynamics have not yet been observed or predicted. Here, we explore ultrafast light-driven spin dynamics in a highly nonresonant strong-field regime. Through state-of-the-art ab initio calculations, we predict that a nonmagnetic material can transiently transform into a magnetic one via dynamical extremely nonlinear spin-flipping processes, which occur on attosecond timescales and are mediated by cascaded multi-photon and spin–orbit interactions. These are nonperturbative nonresonant analogs to the inverse Faraday effect, allowing the magnetization to evolve in very high harmonics of the laser frequency (e.g. here up to the 42nd, oscillating at ~100 attoseconds), and providing control over the speed of magnetization by tuning the laser power and wavelength. Remarkably, we show that even for linearly polarized driving, where one does not intuitively expect the onset of an induced magnetization, the magnetization transiently oscillates as the system interacts with light. This response is enabled by transverse light-driven currents in the solid, and typically occurs on timescales of ~500 attoseconds (with the slower femtosecond response suppressed). An experimental setup capable of measuring these dynamics through pump–probe transient absorption spectroscopy is simulated. Our results pave the way for attosecond regimes of manipulation of magnetism.