Spin-flip Processes Research Articles

On the stability of charge and spin order in the spin-one-half Falicov–Kimball model with the Ising-type Hund coupling

Density matrix renormalization group (DMRG) method is used to study effects of various factors on the stability of charge and spin order in the spin-one-half Falicov–Kimball model with the Ising-type Hund coupling Jz. In particular we examine effects of (i) the exchange interaction (J) between d and f electrons, which is responsible for spin-flip processes, (ii) the f-electron hopping (tf) and (iii) the interband d–f hybridization V. It is shown that for all mentioned generalizations there exists some critical value of the exchange interaction (Jc), the f-electron hopping (tfc) and the interband d–f hybridization Vc at which the magnetic order detected within the spin-one-half Falicov–Kimball model with the Ising interaction is broken. Above these critical points new types of spin orderings are generated, and namely, spin density waves commensurate with charge density waves detected at J=0 in the case of exchange interaction and spin separated phases in the case of f-electron hopping (hybridization), which are at sufficiently large tf (V) replaced by homogeneous phases.

Uncovering the Mechanism of Thermally Activated Delayed Fluorescence in Coplanar Emitters Using Potential Energy Surface Analysis

Planarized emitters exhibiting thermally activated delayed fluorescence (TADF) have attracted attention due to their narrow emission spectra, improved photostability, and high quantum yields, but with large singlet-triplet energy gaps (ΔEST) and no heavy atoms, the origin of their TADF remains a subject of debate. Here we prepare two isomeric, coplanar donor-acceptor compounds, with HMAT-2PYM performing dual TADF and room-temperature phosphorescence but with HMAT-4PYM exhibiting only prompt fluorescence. Although conventional TADF design principles suggest that neither isomer should exhibit TADF, we reveal differences in the excited state potential energy surfaces that enable spin-flip processes in only one isomer. We also find that hydrogen bonding is absent between the planar units of these emitters, despite earlier claims of intramolecular hydrogen bonding in similar compounds. Overall, this work demonstrates that potential energy surface analysis is a practical strategy for designing coplanar TADF materials that might otherwise be overlooked by conventional TADF design metrics.

Theory of resonant Raman scattering due to spin flips of resident charge carriers and excitons in perovskite semiconductors

Recently, the double spin-flip Raman scattering with simultaneous spin reversal of nonequilibrium, localized, and long-lived electrons and holes has been observed in semiconductor perovskites. The authors propose as mechanisms responsible for the double spin-flip processes (i) optical excitation of a localized exciton and its exchange interaction with localized particles, (ii) the process involving biexciton as a resonant intermediate state, and (iii) direct excitation of exciton-polaritons and their scattering by localized carriers. The conditions are discussed, under which the efficiency of double and triple spin-flip scattering can be comparable to that of the first-order process.

Single-spin Landau-Zener-Stückelberg-Majorana interferometry of Zeeman-split states with strong spin-orbit interaction in a double quantum dot

Single spin state evolution induced by the Landau-Zener-St\"uckelberg-Majorana (LZSM) interference in a Zeeman-spit four level system in a periodically driven double quantum dot is studied theoretically by the Floquet stroboscopic method. An interplay between spin-conserving and spin-flip tunneling processes with the Electric Dipole Spin Resonance (EDSR) that is induced in an individual dot and enhanced by the LZSM multiple level crossings with the neighboring quantum dot is investigated as a function of the microwave (MW) frequency, driving amplitude, interdot detuning, and magnetic field. A number of special points in the parameter space are identified, out of which where all the three features are merged. Under this triple-crossing resonance condition the interdot tunneling is combined with a fast spin evolution in each dot at the EDSR frequency. Harmonics of the EDSR are revealed in the spin-dependent tunneling maps versus variable magnetic field and MW frequency. The results are applicable for both electron and hole systems with strong spin-orbit interaction and may be useful for developing new time-efficient schemes of the spin control and readout in qubit devices.

Quantum Advantage in a Molecular Spintronic Engine that Harvests Thermal Fluctuation Energy.

Recent theory and experiments have showcased how to harness quantum mechanics to assemble heat/information engines with efficiencies that surpass the classical Carnot limit. So far, this has required atomic engines that are driven by cumbersome external electromagnetic sources. Here, using molecular spintronics, an implementation that is both electronic and autonomous is proposed. The spintronic quantum engine heuristically deploys several known quantum assets by having a chain of spin qubits formed by the paramagnetic Co center of phthalocyanine (Pc) molecules electronically interact with electron-spin-selecting Fe/C60 interfaces. Density functional calculations reveal that transport fluctuations across the interface can stabilize spin coherence on the Co paramagnetic centers, which host spin flip processes. Across vertical molecular nanodevices, enduring dc current generation, output power above room temperature, two quantum thermodynamical signatures of the engine's processes, and a record 89% spin polarization of current across the Fe/C60 interface are measured. It is crucially this electron spin selection that forces, through demonic feedback and control, charge current to flow against the built-in potential barrier. Further research into spintronic quantum engines, insight into the quantum information processes within spintronic technologies, and retooling the spintronic-based information technology chain, can help accelerate the transition to clean energy.

Magnetic Fingerprints in an All-Organic Radical Molecular Break Junction.

Polycyclic aromatic hydrocarbons radicals are organic molecules with a nonzero total magnetic moment. Here, we report on charge-transport experiments with bianthracene-based radicals using a mechanically controlled break junction technique at low temperatures (6 K). The conductance spectra demonstrate that the magnetism of the diradical is preserved in solid-state devices and that it manifests itself either in the form of a Kondo resonance or inelastic electron tunneling spectroscopy signature caused by spin-flip processes. The magnetic fingerprints depend on the exact configuration of the molecule in the junction; this picture is supported by reference measurements on a radical molecule with the same backbone but with one free spin, in which only Kondo anomalies are observed. The results show that the open-shell structures based on the bianthracene core are interesting systems to study spin–spin interactions in solid-state devices, and this may open the way to control them either electrically or by mechanical strain.

Effect of Surface Functionalization on the Magnetization of Fe3O4 Nanoparticles by Hybrid Density Functional Theory Calculations.

Surface functionalization is found to prevent the reduction of saturation magnetization in magnetite nanoparticles, but the underlying mechanism is still to be clarified. Through a wide set of hybrid density functional theory (HSE06) calculations on Fe3O4 nanocubes, we explore the effects of the adsorption of various ligands (containing hydroxyl, carboxylic, phosphonic, catechol, and silanetriol groups), commonly used to anchor surfactants during synthesis or other species during chemical reactions, onto the spin and structural disorder, which contributes to the lowering of the nanoparticle magnetization. The spin-canting is simulated through a spin-flip process at octahedral Fe ions and correlated with the energy separation between O2– 2p and FeOct3+ 3d states. Only multidentate bridging ligands hamper the spin-canting process by establishing additional electronic channels between octahedral Fe ions for an enhanced ferromagnetic superexchange interaction. The presence of anchoring organic acids also interferes with structural disorder, by disfavoring surface reconstruction.

Effect of magnetic impurity scattering on transport in topological insulators

Charge transport in topological insulators is primarily characterised by so-called topologically projected helical edge states, where charge carriers are correlated in spin and momentum. In principle, dissipation-less current can be carried by these edge states as backscattering from impurities and defects is suppressed as long as time-reversal symmetry is not broken. However, applied magnetic fields or underlying nuclear spin-defects in the substrate can break this time reversal symmetry. In particular, magnetic impurities lead to back-scattering by spin-flip processes. We have investigated the effects of point-wise magnetic impurities on the transport properties of helical edge states in the BHZ model using the Non-Equilibrium Green's Function formalism and compared the results to a semi-analytic approach. Using these techniques we study the influence of impurity strength and spin impurity polarization. We observe a secondary effect of defect-defect interaction that depends on the underlying material parameters which introduces a non-monotonic response of the conductance to defect density. This in turn suggests a qualitative difference in magneto-transport signatures in the dilute and high density spin impurity limits.

Strongly correlated electrons in superconducting islands with fluctuating Cooper pairs

We present a particle-number-conserving theory for many-body effects in mesoscopic superconducting islands connected to normal electrodes, which explicitly includes quantum fluctuations of Cooper pairs in the condensate. Beyond previous BCS mean-field descriptions, our theory can precisely treat the pairing and Coulomb interactions over a broad range of parameters by using the numerical renormalization group method. On increasing the ratio of pairing interactions to Coulomb interactions, the low-energy physics of the system evolves from the spin Kondo regime to the mixed-valence regime and eventually reaches an anisotropic charge Kondo phase, while a crossover from $1e$- to $2e$-periodic Coulomb blockade of transport is revealed at high temperatures. For weak pairing, the superconducting condensate is frozen in the local spin-flip processes but fluctuates in the virtual excitations, yielding an enhanced spin Kondo temperature. For strong pairing, massive fluctuations of Cooper pairs are crucial for establishing charge Kondo correlations whose Kondo temperature rapidly decreases with the pairing interaction. Surprisingly, a charge-exchange-induced local field may occur even at the charge degenerate point, thereby destroying the charge Kondo effect. These are demonstrated in the spectral and transport properties of the island.

Prolonging valley polarization lifetime through gate-controlled exciton-to-trion conversion in monolayer molybdenum ditelluride

Monolayer 2D semiconductors provide an attractive option for valleytronics due to valley-addressability. But the short valley-polarization lifetimes for excitons have hindered potential valleytronic applications. In this paper, we demonstrate a strategy for prolonging the valley-polarization lifetime by converting excitons to trions through efficient gate control and exploiting the much longer valley-polarization lifetimes for trions than for excitons. At charge neutrality, the valley lifetime of monolayer MoTe2 increases by a factor of 1000 to the order of nanoseconds from excitons to trions. The exciton-to-trion conversion changes the dominant depolarization mechanism from the fast electron-hole exchange for excitons to the slow spin-flip process for trions. Moreover, the degree of valley polarization increases to 38% for excitons and 33% for trions through electrical manipulation. Our results reveal the depolarization dynamics and the interplay of various depolarization channels for excitons and trions, providing an effective strategy for prolonging the valley polarization.

(Invited) Kinetic Prediction of Reverse Intersystem Crossing in Thermally Activated Delayed Fluorescence Molecules

Reverse intersystem crossing (RISC), the endothermic spin-flip process of producing an emissive singlet exciton from a non-emissive triplet exciton, is of increasing interest for applications in organic electroluminescent materials, photocatalysts, and biomedical probes. Theoretical prediction of the rate constant of RISC (k RISC) may enable an efficient priori screening of new materials in chemical spaces; yet, the understanding of its kinetics in vastly different structures is challenging. Here, we demonstrate a theoretical expression that reproduces experimental k RISC ranging over five orders of magnitude in twenty different molecules. We show that the spin flip occurs across the singlet–triplet crossing seam involving a higher-lying triplet excited state where the semi-classical Marcus parabola is no longer valid.1 The present model also leads to a newly designed TADF molecule with high k RISC of 108 s–1.2 1. N. Aizawa, Y. Harabuchi, S. Maeda, Y.-J. Pu, Nat. Commun. 11, 3909 (2020).2. N. Aizawa, A. Matsumoto, T. Yasuda, Sci. Adv. 7, 5769 (2021). Figure 1

Ultrafast Triplet-Singlet Exciton Interconversion in Narrowband Blue Organoboron Emitters Doped with Heavy Chalcogens.

Narrowband emissive organoboron emitters featuring the multi-resonance (MR) effect have now become a critical material component for constructing high-performance organic light-emitting diodes (OLEDs) with pure emission colors. These MR organoboron emitters are capable of exhibiting high-efficiency narrowband thermally activated delayed fluorescence (TADF) by allowing triplet-to-singlet reverse intersystem crossing (RISC). However, RISC involving spin-flip exciton upconversion is generally the rate-limiting step in the overall TADF; hence, a deeper understanding and precise control of the RISC dynamics are ongoing crucial challenges. Here, we introduce the first MR organoboron emitter (CzBSe) doped with a selenium atom, demonstrating a record-high RISC rate exceeding 108 s-1 , which is even higher than its fluorescence radiation rate. Furthermore, the spin-flip upconversion process in CzBSe can be accelerated by factors of ≈20000 and ≈800, compared to those of its oxygen- and sulfur-doped homologs (CzBO and CzBS), respectively. Unlike CzBO and CzBS, the photophysical rate-limiting step in CzBSe is no longer RISC, but the fluorescence radiation process; this behavior is completely different from the conventional time-delaying TADF limited by the slow RISC. Benefitting from its ultrafast exciton spin conversion ability, OLEDs incorporating CzBSe achieved a maximum external electroluminescence quantum efficiency as high as 23.9 %, accompanied by MR-induced blue narrowband emission and significantly alleviated efficiency roll-off features.

Realizing External Quantum Efficiency over 25% with Low Efficiency Roll-Off in Polymer-Based Light-Emitting Diodes Synergistically Utilizing Intramolecular Sensitization and Bipolar Thermally Activated Delayed Fluorescence Monomer

Realizing External Quantum Efficiency over 25% with Low Efficiency Roll-Off in Polymer-Based Light-Emitting Diodes Synergistically Utilizing Intramolecular Sensitization and Bipolar Thermally Activated Delayed Fluorescence Monomer

Quadrupolar magnetic excitations in an isotropic spin-1 antiferromagnet

The microscopic origins of emergent behaviours in condensed matter systems are encoded in their excitations. In ordinary magnetic materials, single spin-flips give rise to collective dipolar magnetic excitations called magnons. Likewise, multiple spin-flips can give rise to multipolar magnetic excitations in magnetic materials with spin S ≥ 1. Unfortunately, since most experimental probes are governed by dipolar selection rules, collective multipolar excitations have generally remained elusive. For instance, only dipolar magnetic excitations have been observed in isotropic S = 1 Haldane spin systems. Here, we unveil a hidden quadrupolar constituent of the spin dynamics in antiferromagnetic S = 1 Haldane chain material Y2BaNiO5 using Ni L3-edge resonant inelastic x-ray scattering. Our results demonstrate that pure quadrupolar magnetic excitations can be probed without direct interactions with dipolar excitations or anisotropic perturbations. Originating from on-site double spin-flip processes, the quadrupolar magnetic excitations in Y2BaNiO5 show a remarkable dual nature of collective dispersion. While one component propagates as non-interacting entities, the other behaves as a bound quadrupolar magnetic wave. This result highlights the rich and largely unexplored physics of higher-order magnetic excitations.

Amplification of elliptically polarized sub-femtosecond pulses in neon-like X-ray laser modulated by an IR field

Amplification of attosecond pulses produced via high harmonic generation is a formidable problem since none of the amplifiers can support the corresponding PHz bandwidth. Producing the well defined polarization state common for a set of harmonics required for formation of the circularly/elliptically polarized attosecond pulses (which are on demand for dynamical imaging and coherent control of the spin flip processes) is another big challenge. In this work we show how both problems can be tackled simultaneously on the basis of the same platform, namely, the plasma-based X-ray amplifier whose resonant transition frequency is modulated by an infrared field.

Singlet Oxygen Generation from Polyaminoglycerol by Spin-Flip-Based Electron Transfer.

Reactive oxygen species have drawn attention owing to their strong oxidation ability. In particular, the singlet oxygen (1O2) produced by energy transfer is the predominant species for controlling oxidation reactions efficiently. However, conventional 1O2 generators, which rely on enhanced energy transfer, frequently suffer from poor solubility, low stability, and low biocompatibility. Herein, we introduce a hyperbranched aliphatic polyaminoglycerol (hPAG) as a 1O2 generator, which relies on spin-flip-based electron transfer. The coexistence of a lone pair electron on the nitrogen atom and a hydrogen-bonding donor (the protonated form of nitrogen and hydroxyl group) affords proximity between hPAG and O2. Subsequent direct electron transfer after photo-irradiation induces hPAG•+-O2•– formation, and the following spin-flip process generates 1O2. The spin-flip-based electron transfer pathway is analyzed by a series of photophysical, electrochemical, and computational studies. The 1O2 generator, hPAG, is successfully employed in photodynamic therapy and as an antimicrobial reagent.

Two-qubit logic gates based on the ultrafast spin transfer in π-conjugated graphene nanoflakes

Ultrafast optical spin control allows gate operations to be performed within picosecond time scale, orders of magnitude faster than microwave or electrical control. Here, using high-level quantum chemical computations, we suggest several two-qubit logic gates based on the optically induced ultrafast spin-flip and spin-transfer processes on rhombic graphene nanoflakes (Co4-GNF). It is demonstrated that Co atoms chemisorbed on π-conjugated C atoms can significantly influence the spin properties of the system. The spin density of different energy levels is distributed on different Co atoms, thus opening channels for successful spin-transfer processes between the Co atoms. Reversible local spin-flip processes on each Co atom as well as global spin-transfer processes between the Co atoms are realized. By carefully combining the various spin-dynamics processes achieved in our Co4-GNF structure, both classical (OR, AND, NAND) and quantum (CNOT, SWAP) binary logic gates are constructed. All spin manipulation processes finish at the subpicosecond time scale with fidelities above 96%. This theoretical design, which is exclusively based on laser-induced ultrafast spin dynamics, represents a novel implementation of qubit manipulation and can thus pave the way towards the construction of nano magnetic logic circuits and spintronic devices.

Spin precession of polarons in organic ferromagnets

Based on an extended Su-Schrieffer-Heeger model including spin correlation with radicals and spin-orbit interaction, the static and dynamic properties of a polaron in organic ferromagnets are investigated theoretically. The static property shows that the above two interactions are competitive in determining the spatial symmetry of polaron spin density. The nonadiabatic dynamic simulation demonstrates that the spin-orbit interaction induces spin precession of the polaron accompanied with periodic spin flipping, but such spin precession is suppressed when the spin correlation is considered in the presence of spin radicals. Periodic spin decline appears instead of the spin flipping process with the increase of spin correlation strength. The mechanism analysis indicates that the spin splitting energy caused by spin correlation weakens the exchange of electron occupation probability between polaron levels with different spin components. This work reveals the role of spin radicals in spin precession of polarons in organic ferromagnets.

Cross-relaxation interactions in ZnO:Mn2+: The ground state optical pumping

A steady-state population inversion in the ground state of Mn2+ in ZnO was detected by application of continuous microwave and circularly polarized optical pumping in the temperature range of 3–6 K. Multiple spin-flip processes occur in view of a simultaneous saturation in the harmonically related transitions of Mn2+ spins. It is found that an additional relaxation channel arises at 2.7 K due to dynamic polarization of the 55Mn nuclei through the saturation of the first order electron-nuclear forbidden transitions. The transient populations are created between 55Mn nuclear sublevels.

Tailoring of electronic and surface structures boosts exciton-triggering photocatalysis for singlet oxygen generation

Arising from reduced dielectric screening, excitonic effects should be taken into account in ultrathin two-dimensional photocatalysts, and a significant challenge is achieving nontrivial excitonic regulation. However, the effect of structural modification on the regulation of the excitonic aspect is at a comparatively early stage. Herein, we report unusual effects of surface substitutional doping with Pt on electronic and surface characteristics of atomically thin layers of Bi3O4Br, thereby enhancing the propensity to generate 1O2 Electronically, the introduced Pt impurity states with a lower energy level can trap photoinduced singlet excitons, thus reducing the singlet-triplet energy gap by ∼48% and effectively facilitating the intersystem crossing process for efficient triplet excitons yield. Superficially, the chemisorption state of O2 causes the changes in the magnetic moment (i.e., spin state) of O2 through electron-mediated triplet energy transfer, resulting a spontaneous spin-flip process and highly specific 1O2 generation. These traits exemplify the opportunities that the surface engineering provides a unique strategy for excitonic regulation and will stimulate more research on exciton-triggering photocatalysis for solar energy conversion.