Highly spin-polarized carrier dynamics and ultralarge photoinduced magnetization in CH3NH3PbI3 perovskite thin films

Low-temperature solution-processed organic-inorganic halide perovskite CH3NH3PbI3 has demonstrated great potential for photovoltaics and light-emitting devices. Recent discoveries of long ambipolar carrier diffusion lengths and the prediction of the Rashba effect in CH3NH3PbI3, that possesses large spin-orbit coupling, also point to a novel semiconductor system with highly promising properties for spin-based applications. Through circular pump-probe measurements, we demonstrate that highly polarized electrons of total angular momentum (J) with an initial degree of polarization Pini ∼90% (i.e., -30% degree of electron spin polarization) can be photogenerated in perovskites. Time-resolved Faraday rotation measurements reveal photoinduced Faraday rotation as large as 10°/μm at 200 K (at wavelength λ = 750 nm) from an ultrathin 70 nm film. These spin polarized carrier populations generated within the polycrystalline perovskite films, relax via intraband carrier spin-flip through the Elliot-Yafet mechanism. Through a simple two-level model, we elucidate the electron spin relaxation lifetime to be ∼7 ps and that of the hole is ∼1 ps. Our work highlights the potential of CH3NH3PbI3 as a new candidate for ultrafast spin switches in spintronics applications.

Theory of Chiral Induced Spin Selectivity

A general theory of the chiral induced spin selectivity (CISS) effect is presented. It is based on the fact that the spin-orbit (SO) coupling is small, a few meV, for the light atoms, which make up typical chiral molecules in experiments. We present a theorem based on the Onsager reciprocal principle, which states that the CISS effect vanishes when thermally averaging over all electron states. This zero result is avoided by the very nonthermal character of the incoming optically generated electrons in experimental realizations. Despite the small SO-coupling, the presence of accidental degeneracies in the molecular spectrum yields a sizable spin polarization. The CISS effect in the presence of magnetic leads is special. We prove that, in a situation with one magnetic lead, the other lead will become magnetized. This results from the interplay between the spin-orbit coupling in the chiral molecule and the magnetized lead. Numerical calculations for realistic chiral molecules confirm the theory.

Tailoring the d-Orbital Splitting Manner of Single Atomic Sites for Enhanced Oxygen Reduction

Regulating the electronic states of single atomic sites around the Fermi level remains a major concern for boosting the electrocatalytic oxygen reduction reaction (ORR). Herein, a Fe d-orbital splitting manner modulation strategy by constructing axial coordination on FeN₄ sites is presented. Experimental investigations and theoretical calculations reveal that the axial tractions induce the distortion of square-planar field (FeN₄ SP), up to the quasi-octahedral coordination (FeN₄ O₁ OC_quasi ), thus leading to the electron rearrangement with a diluted spin polarization. The declined population of unpaired electrons in d_z ² , d_{x z} and d_yz states engenders a moderate adsorption of ORR intermediates, thereby reinforcing the intrinsic reaction activity. In situ infrared spectroscopy further demonstrates that the reordering of d-orbital splitting and occupation facilitates the desorption of *OH. The FeN₄ O₁ OC_quasi exhibits a dramatic improvement of kinetic current density and turnover frequency, which are fivefold and tenfold higher than those of FeN₄ SP. This work presents a novel understanding on improving the electrocatalytic performance through the orbital-scale manipulation.

Electrical detection of spin-polarized surface states conduction in (Bi(0.53)Sb(0.47))2Te3 topological insulator

Strong spin-orbit interaction and time-reversal symmetry in topological insulators enable the spin-momentum locking for the helical surface states. To date, however, there has been little report of direct electrical spin injection/detection in topological insulator. In this Letter, we report the electrical detection of spin-polarized surface states conduction using a Co/Al2O3 ferromagnetic tunneling contact in which the compound topological insulator (Bi0.53Sb0.47)2Te3 was used to achieve low bulk carrier density. Resistance (voltage) hysteresis with the amplitude up to about 10 Ω was observed when sweeping the magnetic field to change the relative orientation between the Co electrode magnetization and the spin polarization of surface states. The two resistance states were reversible by changing the electric current direction, affirming the spin-momentum locking in the topological surface states. Angle-dependent measurement was also performed to further confirm that the abrupt change in the voltage (resistance) was associated with the magnetization switching of the Co electrode. The spin voltage amplitude was quantitatively analyzed to yield an effective spin polarization of 1.02% for the surface states conduction in (Bi0.53Sb0.47)2Te3. Our results show a direct evidence of spin polarization in the topological surface states conduction. It might open up great opportunities to explore energy-efficient spintronic devices based on topological insulators.

Orbital Occupancy and Spin Polarization: From Mechanistic Study to Rational Design of Transition Metal-Based Electrocatalysts toward Energy Applications

Over the past few decades, development of electrocatalysts for energy applications has extensively transitioned from trial-and-error methodologies to more rational and directed designs at the atomic levels via either nanogeometric optimization or modulating electronic properties of active sites. Regarding the modulation of electronic properties, nonprecious transition metal-based materials have been attracting large interest due to the capability of versatile tuning d-electron configurations expressed through the flexible orbital occupancy and various possible degrees of spin polarization. Herein, recent advances in tailoring electronic properties of the transition-metal atoms for intrinsically enhanced electrocatalytic performances are reviewed. We start with discussions on how orbital occupancy and spin polarization can govern the essential atomic level processes, including the transport of electron charge and spin in bulk, reactive species adsorption on the catalytic surface, and the electron transfer between catalytic centers and adsorbed species as well as reaction mechanisms. Subsequently, different techniques currently adopted in tuning electronic structures are discussed with particular emphasis on theoretical rationale and recent practical achievements. We also highlight the promises of the recently established computational design approaches in developing electrocatalysts for energy applications. Lastly, the discussion is concluded with perspectives on current challenges and future opportunities. We hope this review will present the beauty of the structure-activity relationships in catalysis sciences and contribute to advance the rational development of electrocatalysts for energy conversion applications.

Perspectives of hyperpolarized noble gas MRI beyond 3He

Nuclear Magnetic Resonance (NMR) studies with hyperpolarized (hp) noble gases are at an exciting interface between physics, chemistry, materials science and biomedical sciences. This paper intends to provide a brief overview and outlook of magnetic resonance imaging (MRI) with hp noble gases other than hp (3)He. A particular focus are the many intriguing experiments with (129)Xe, some of which have already matured to useful MRI protocols, while others display high potential for future MRI applications. Quite naturally for MRI applications the major usage so far has been for biomedical research but perspectives for engineering and materials science studies are also provided. In addition, the prospects for surface sensitive contrast with hp (83)Kr MRI is discussed.

The solid-state photo-CIDNP effect

The solid-state photo-CIDNP effect is the occurrence of a non-Boltzmann nuclear spin polarization in rigid samples upon illumination. For solid-state NMR, which can detect this enhanced nuclear polarization as a strong modification of signal intensity, the effect allows for new classes of experiments. Currently, the photo- and spin-chemical machinery of various RCs is studied by photo-CIDNP MAS NMR in detail. Until now, the effect has only been observed at high magnetic fields with (13)C and (15)N MAS NMR and in natural photosynthetic RC preparations in which blocking of the acceptor leads to cyclic electron transfer. In terms of irreversible thermodynamics, the high-order spin structure of the initial radical pair can be considered as a transient order phenomenon emerging under non-equilibrium conditions and as a first manifestation of order in the photosynthetic process. The solid-state photo- CIDNP effect appears to be an intrinsic property of natural RCs. The conditions of its occurrence seem to be conserved in evolution. The effect may be based on the same fundamental principles as the highly optimized electron transfer. Hence, the effect may allow for guiding artificial photosynthesis.

Long-Lived Hole Spin/Valley Polarization Probed by Kerr Rotation in Monolayer WSe2

Time-resolved Kerr rotation and photoluminescence measurements are performed on MOCVD-grown monolayer tungsten diselenide (WSe2). We observe a surprisingly long-lived Kerr rotation signal (∼80 ns) at 10 K, which is attributed to spin/valley polarization of the resident holes. This polarization is robust to transverse magnetic field (up to 0.3 T). Wavelength-dependent measurements reveal that only excitation near the free exciton energy generates this long-lived spin/valley polarization.

Symmetry-Broken 2D Lead-Tin Mixed Chiral Perovskite for High Asymmetry Factor Circularly Polarized Light Detection

Symmetry-broken-induced spin splitting plays a key role for selective circularly polarized light absorption and spin carrier transport. Asymmetrical chiral perovskite is rising as the most promising material for direct semiconductor-based circularly polarized light detection. However, the increase of asymmetry factor and extension of response region remain to be a challenge. Herein, we fabricated a two-dimensional tin-lead mixed chiral perovskite with tunable absorption in the visible region. Theoretical simulation indicates that the mixing of the tin and lead in chiral perovskite breaks the symmetry of the pure ones, resulting in pure spin splitting. We then fabricated a chiral circularly polarized light detector based on this tin-lead mixed perovskite. A high asymmetry factor for the photocurrent of 0.44 is achieved, which is 144% higher than pure lead 2D perovskite, and it is the highest value reported for the pure chiral 2D perovskite-based circularly polarized light detector using a simple device structure.

Unusual Spin Polarization in the Chirality-Induced Spin Selectivity

Chirality-induced spin selectivity (CISS) refers to the fact that electrons get spin polarized after passing through chiral molecules in a nanoscale transport device or in photoemission experiments. In CISS, chiral molecules are commonly believed to be a spin filter through which one favored spin transmits and the opposite spin gets reflected; that is, transmitted and reflected electrons exhibit opposite spin polarization. In this work, we point out that such a spin filter scenario contradicts the principle that equilibrium spin current must vanish. Instead, we find that both transmitted and reflected electrons present the same type of spin polarization, which is actually ubiquitous for a two-terminal device. More accurately, chiral molecules play the role of a spin polarizer rather than a spin filter. The direction of spin polarization is determined by the molecule chirality and the electron incident direction. And the magnitude of spin polarization relies on local spin-orbit coupling in the device. Our work brings a deeper understanding on CISS and interprets recent experiments, for example, the CISS-driven anomalous Hall effect.

Hyperpolarized (131)Xe NMR spectroscopy

Hyperpolarized (hp) (131)Xe with up to 2.2% spin polarization (i.e., 5000-fold signal enhancement at 9.4 T) was obtained after separation from the rubidium vapor of the spin-exchange optical pumping (SEOP) process. The SEOP was applied for several minutes in a stopped-flow mode, and the fast, quadrupolar-driven T(1) relaxation of this spin I = 3/2 noble gas isotope required a rapid subsequent rubidium removal and swift transfer into the high magnetic field region for NMR detection. Because of the xenon density dependent (131)Xe quadrupolar relaxation in the gas phase, the SEOP polarization build-up exhibits an even more pronounced dependence on xenon partial pressure than that observed in (129)Xe SEOP. (131)Xe is the only stable noble gas isotope with a positive gyromagnetic ratio and shows therefore a different relative phase between hp signal and thermal signal compared to all other noble gases. The gas phase (131)Xe NMR spectrum displays a surface and magnetic field dependent quadrupolar splitting that was found to have additional gas pressure and gas composition dependence. The splitting was reduced by the presence of water vapor that presumably influences xenon-surface interactions. The hp (131)Xe spectrum shows differential line broadening, suggesting the presence of strong adsorption sites. Beyond hp (131)Xe NMR spectroscopy studies, a general equation for the high temperature, thermal spin polarization, P, for spin I ≥ 1/2 nuclei is presented.

Observation of current-induced, long-lived persistent spin polarization in a topological insulator: A rechargeable spin battery

Topological insulators (TIs), with their helically spin-momentum-locked topological surface states (TSSs), are considered promising for spintronics applications. Several recent experiments in TIs have demonstrated a current-induced electronic spin polarization that may be used for all-electrical spin generation and injection. We report spin potentiometric measurements in TIs that have revealed a long-lived persistent electron spin polarization even at zero current. Unaffected by a small bias current and persisting for several days at low temperature, the spin polarization can be induced and reversed by a large "writing" current applied for an extended time. Although the exact mechanism responsible for the observed long-lived persistent spin polarization remains to be better understood, we speculate on possible roles played by nuclear spins hyperfine-coupled to TSS electrons and dynamically polarized by the spin-helical writing current. Such an electrically controlled persistent spin polarization with unprecedented long lifetime could enable a rechargeable spin battery and rewritable spin memory for potential applications in spintronics and quantum information.

Multi-Domain versus Single-Domain: A Magnetic Field is Not a Must for Promoting Spin-Polarized Water Oxidation

The reaction kinetics of spin-polarized oxygen evolution reaction (OER) can be enhanced by ferromagnetic catalysts under an external magnetic field. However, applying a magnetic field necessitates additional energy consumption and creates design difficulties for electrolyzers. Here, we prove that a single-domain FM catalyst without external magnetic fields exhibits a similar OER increment to its magnetized multi-domain one. The evidence is given by comparing the pH-dependent increment of OER on multi- and single-domain FM catalysts with or without a magnetic field. The intrinsic activity of a single-domain catalyst is higher than that of a multi-domain counterpart. The latter can be promoted to approach the former by the magnetization effect. Reducing the FM catalyst size into the single-domain region, the spin-polarized OER performance can be achieved without a magnetic field, illustrating an external magnetic field is not a requirement to reap the benefits of magnetic catalysts.

Multistate Switching of Spin Selectivity in Electron Transport through Light-Driven Molecular Motors

It is established that electron transmission through chiral molecules depends on the electron's spin. This phenomenon, termed the chiral-induced spin selectivity (CISS), effect has been observed in chiral molecules, supramolecular structures, polymers, and metal-organic films. Which spin is preferred in the transmission depends on the handedness of the system and the tunneling direction of the electrons. Molecular motors based on overcrowded alkenes show multiple inversions of helical chirality under light irradiation and thermal relaxation. The authors found here multistate switching of spin selectivity in electron transfer through first generation molecular motors based on the four accessible distinct helical configurations, measured by magnetic-conductive atomic force microscopy. It is shown that the helical state dictates the molecular organization on the surface. The efficient spin polarization observed in the photostationary state of the right-handed motor coupled with the modulation of spin selectivity through the controlled sequence of helical states, opens opportunities to tune spin selectivity on-demand with high spatio-temporal precision. An energetic analysis correlates the spin injection barrier with the extent of spin polarization.

Regulating Spin Polarization through Cationic Vacancy Defects in Bi4Ti3O12 for Enhanced Molecular Oxygen Activation

Molecular oxygen (O2) activation technology is of great significance in environmental purification due to its eco-friendly operation and cost-effective nature. However, the activation of O2 is limited by spin-forbidden transitions, and efficient molecular oxygen activation depends on electronic behavior and surface adsorption. Herein, we prepared cationic defect-rich Bi4Ti3O12 (BTO-MV2) catalysts containing Ti vacancies (VTi) for O2 activation in water purification. The VTi on BTO nanosheets can induce electron spin polarization, increasing the number of spin-down photogenerated electrons and reducing the recombination of electron-hole pairs. An active surface VTi is also formed, serving as a center for adsorbing O2 and extracting electrons, effectively generating •OH, O2•- and 1O2. The degradation rate constant of tetracycline achieved by BTO-MV2 is 3.3 times faster than BTO, indicating a satisfactory prospect for practical application. This work provides an efficient pathway to activate molecular oxygen by constructing new active sites through cationic vacancy modification technology.

Spin Polarization Dynamics of Free Charge Carriers in CsPbI3 Nanocrystals

Lead halide perovskites (LHPs) exhibit large spin-orbit coupling (SOC), leading to only twofold-degenerate valence and conduction bands and therefore allowing for efficient optical orientation. This makes them ideal materials to study charge carrier spins. With this study we elucidate the spin dynamics of photoexcited charge carriers and the underlying spin relaxation mechanisms in CsPbI₃ nanocrystals by employing time-resolved differential transmission spectroscopy (DTS). We find that the photoinduced spin polarization significantly diminishes during thermalization and cooling toward the energetically favorable band edge. Temperature-dependent DTS reveals a decay in spin polarization that is more than 1 order of magnitude faster at room temperature (3 ps) than at cryogenic temperatures (32 ps). We propose that spin relaxation of free charge carriers in large-SOC materials like LHPs occurs as a result of carrier-phonon scattering, as described by the Elliott-Yafet mechanism.

Highly spin-polarized materials and devices for spintronics∗

The performance of spintronics depends on the spin polarization of the current. In this study half-metallic Co-based full-Heusler alloys and a spin filtering device (SFD) using a ferromagnetic barrier have been investigated as highly spin-polarized current sources. The multilayers were prepared by magnetron sputtering in an ultrahigh vacuum and microfabricated using photolithography and Ar ion etching. We investigated two systems of Co-based full-Heusler alloys, Co₂Cr_{1 - x} Fe _x Al (CCFA(x)) and Co₂FeSi_{1 - x} Al _x (CFSA(x)) and revealed the structure and magnetic and transport properties. We demonstrated giant tunnel magnetoresistance (TMR) of up to 220% at room temperature and 390% at 5 K for the magnetic tunnel junctions (MTJs) using Co₂FeSi_0.5Al_0.5 (CFSA(0.5)) Heusler alloy electrodes. The 390% TMR corresponds to 0.81 spin polarization for CFSA(0.5) at 5 K. We also investigated the crystalline structure and local structure around Co atoms by x-ray diffraction (XRD) and nuclear magnetic resonance (NMR) analyses, respectively, for CFSA films sputtered on a Cr-buffered MgO (001) substrate followed by post-annealing at various temperatures in an ultrahigh vacuum. The disordered structures in CFSA films were clarified by NMR measurements and the relationship between TMR and the disordered structure was discussed. We clarified that the TMR of the MTJs with CFSA(0.5) electrodes depends on the structure, and is significantly higher for L2₁ than B2 in the crystalline structure. The second part of this paper is devoted to a SFD using a ferromagnetic barrier. The Co ferrite is investigated as a ferromagnetic barrier because of its high Curie temperature and high resistivity. We demonstrate the strong spin filtering effect through an ultrathin insulating ferrimagnetic Co-ferrite barrier at a low temperature. The barrier was prepared by the surface plasma oxidization of a CoFe₂ film deposited on a MgO (001) single crystal substrate, wherein the spinel structure of CoFe₂O₄ (CFO) and an epitaxial relationship of MgO(001)[100]/CoFe₂ (001)]110]/CFO(001)[100] were induced. A SFD consisting of CoFe₂ /CFO/Ta on a MgO (001) substrate exhibits the inverse TMR of - 124% at 10 K when the configuration of the magnetizations of CFO and CoFe₂ changes from parallel to antiparallel. The inverse TMR suggests the negative spin polarization of CFO, which is consistent with the band structure of CFO obtained by first principle calculation. The - 124% TMR corresponds to the spin filtering efficiency of 77% by the CFO barrier.

Half-Metallic Behavior in 2D Transition Metal Dichalcogenides Nanosheets by Dual-Native-Defects Engineering

Two-dimensional transition metal dichalcogenides (TMDs) have been regarded as one of the best nonartificial low-dimensional building blocks for developing spintronic nanodevices. However, the lack of spin polarization in the vicinity of the Fermi surface and local magnetic moment in pristine TMDs has greatly hampered the exploitation of magnetotransport properties. Herein, a half-metallic structure of TMDs is successfully developed by a simple chemical defect-engineering strategy. Dual native defects decorate titanium diselenides with the coexistence of metal-Ti-atom incorporation and Se-anion defects, resulting in a high-spin-polarized current and local magnetic moment of 2D Ti-based TMDs toward half-metallic room-temperature ferromagnetism character. Arising from spin-polarization transport, the as-obtained T-TiSe_1.8 nanosheets exhibit a large negative magnetoresistance phenomenon with a value of -40% (5T, 10 K), representing one of the highest negative magnetoresistance effects among TMDs. It is anticipated that this dual regulation strategy will be a powerful tool for optimizing the intrinsic physical properties of TMD systems.

Insights into the Crucial Role of Electron and Spin Structures in Heteroatom-Doped Covalent Triazine Frameworks for Removing Organic Micropollutants

The water shortage crisis, characterized by organic micropollutants (OMPs), urgently requires new materials and methods to deal with it. Although heteroatom doping has been developed into an effective method to modify carbon nanomaterials for various heterogeneous adsorption and catalytic oxidation systems, the active source regulated by intrinsic electron and spin structures is still obscure. Here, a series of nonmetallic element-doped (such as P, S, and Se) covalent triazine frameworks (CTFs) were constructed and applied to remove organic pollutants using the adsorption-photocatalysis process. The external mass transfer model (EMTM) and the homogeneous surface diffusion model (HSDM) were employed to describe the adsorption process. It was found that sulfur-doped CTF (S-CTF-1) showed a 25.6-fold increase in saturated adsorption capacity (554.7 μmol/g) and a 169.0-fold surge in photocatalytic kinetics (5.07 h^-1), respectively, compared with the pristine CTF-1. A positive correlation between electron accumulation at the active site (N1 atom) and adsorption energy was further demonstrated with experimental results and theoretical calculations. Meanwhile, the photocatalytic degradation rates were greatly enhanced by forming a built-in electric field driven by spin polarization. In addition, S-CTF-1 still maintained a 98.3% removal of 2,2',4,4'-tetrahydroxybenzophenone (BP-2) micropollutants and 97.6% regeneration after six-cycle sequencing batch treatment in real water matrices. This work established a relation between electron and spin structures for adsorption and photocatalysis, paving a new way to design modified carbon nanomaterials to control OMPs.

Cooperative Effect of Electron Spin Polarization in Chiral Molecules Studied with Non-Spin-Polarized Scanning Tunneling Microscopy

Polyalanine molecules (PA) with an α-helix conformation have recently attracted a great deal of interest, as the propagation of electrons through the chiral backbone structure comes along with spin polarization of the transmitted electrons. By means of scanning tunneling microscopy and spectroscopy under ambient conditions, PA molecules adsorbed on surfaces of epitaxial magnetic Al₂O₃/Pt/Au/Co/Au nanostructures with perpendicular anisotropy were studied. Thereby, a correlation between the PA molecules ordering at the surface with the electron tunneling across this hybrid system as a function of the substrate magnetization orientation as well as the coverage density and helicity of the PA molecules was observed. The highest spin polarization values, P, were found for well-ordered self-assembled monolayers and with a defined chemical coupling of the molecules to the magnetic substrate surface, showing that the current-induced spin selectivity is a cooperative effect. Thereby, P deduced from the electron transmission along unoccupied molecular orbitals of the chiral molecules is larger as compared to values derived from the occupied molecular orbitals. Apparently, the larger orbital overlap results in a higher electron mobility, yielding a higher P value. By switching the magnetization direction of the Co layer, it was demonstrated that the non-spin-polarized STM can be used to study chiral molecules with a submolecular resolution, to detect properties of buried magnetic layers and to detect the spin polarization of the molecules from the change in the magnetoresistance of such hybrid structures.

Switching of Perpendicular Magnetization by Spin-Orbit Torque

Magnetic materials with strong perpendicular magnetic anisotropy are of great interest for the development of nonvolatile magnetic memory and computing technologies due to their high stabilities at the nanoscale. However, electrical switching of such perpendicular magnetization in an energy-efficient, deterministic, scalable manner has remained a big challenge. This problem has recently attracted enormous efforts in the field of spintronics. Here, recent advances and challenges in the understanding of the electrical generation of spin currents, the switching mechanisms and the switching strategies of perpendicular magnetization, the switching current density by spin-orbit torque of transverse spins, the choice of perpendicular magnetic materials are reviewed, and the progress in prototype perpendicular SOT memory and logic devices toward the goal of energy-efficient, dense, fast perpendicular spin-orbit torque applications is summarized.

Enantiospecific spin polarization of electrons photoemitted through layers of homochiral organic molecules

Electrons photoemitted through layers of purely organic chiral molecules become strongly spin-polarized even at room temperature and for double-monolayer thicknesses. The substitution of one enantiomer for its mirror image does not revert the sign of the spin polarization, rather its direction in space. These findings might lead to the obtention of highly efficient spin filters for spintronic applications.

Looking Inside the Perchlorinated Trityl Radical/Metal Spinterface through Spectroscopy

We report on a spectroscopic multitechnique approach to study the metal/radical spinterface formed by a perchlorinated trityl radical derivative and either gold or silver. The spectroscopic fingerprint of their paramagnetic properties could be determined by comparison with their diamagnetic precursor and by DFT calculations. Thanks to the presented approach, we could gain unprecedented insight into the radical-metal interaction and how this latter perturbs the spin polarization and consequently the magnetoelectronic properties of the radical adlayer. Knowledge of the factors influencing the spinterface is an essential tool toward the tailoring of the properties of spin-based electronic devices.

Exploring a Proximity-Coupled Co Chain on Pb(110) as a Possible Majorana Platform

Linear chains of magnetic atoms proximity coupled to an s-wave superconductor are predicted to host Majorana zero modes at the chain ends in the presence of strong spin-orbit coupling. Specifically, iron (Fe) chains on Pb(110) have been explored as a possible system to exhibit topological superconductivity and host Majorana zero modes [ Nadj-Perge , S. et al., Science 2014 , 346 , 602 - 607 ]. Here, we study chains of the transition metal cobalt (Co) on Pb(110) and check for topological signatures. Using spin-polarized scanning tunneling spectroscopy, we resolve ferromagnetic order in the d bands of the chains. Interestingly, also the subgap Yu-Shiba-Rusinov (YSR) bands carry a spin polarization as was predicted decades ago. Superconducting tips allow us to resolve further details of the YSR bands and in particular resonances at zero energy. We map the spatial distribution of the zero-energy signal and find it delocalized along the chain. Hence, despite the ferromagnetic coupling within the chains and the strong spin-orbit coupling in the superconductor, we do not find clear evidence of Majorana modes. Simple tight-binding calculations suggest that the spin-orbit-split bands may cross the Fermi level four times which suppresses the zero-energy modes.

Photothermal-Magnetic Synergistic Effects in an Electrocatalyst for Efficient Water Splitting under Optical-Magnetic Fields

The slow oxygen evolution reaction (OER) limits water splitting, and external fields can help improve it. However, the effect of a single external field on the OER is limited and unsatisfactory. Furthermore, the mechanism by which external fields improve the OER is unclear, particularly in the presence of multiple fields. Herein, a strategy is proposed for enhancing the OER activity of a catalyst using the combined effect of an optical-magnetic field, and the mechanism of catalytic activity enhancement is studied. Under the optical-magnetic field, Co3 O4 reduces the resistance by increasing the catalyst temperature. Meanwhile, CoFe2 O4 further reduces the resistance via the negative magnetoresistance effect, thus decreasing the resistance from 16 to 7.0 Ω. Additionally, CoFe2 O4 acts as a spin polarizer, and electron polarization results in a parallel arrangement of oxygen atoms, which increases the kinetics of the OER under the magnetic field. Benefiting from the optical and magnetic response design, Co3 O4 /CoFe2 O4 @Ni foam requires an overpotential of 172.4 mV to reach a current density of 10 mA cm-2 under an optical-magnetic field, which is significantly higher than those of recently reported state-of-the-art transition-metal-based catalysts.