Electron Spin Orientation Research Articles

A Spin‐Polarized Electron‐Induced MXene‐KBi0.9Co0.1Fe2O5 Photocatalyst for Enhanced Dye Degradation

In photocatalysts aimed to degrade environmental pollutants, photons are generally used as primary stimuli to generate electron–hole pairs to target the chemical bonds to disintegrate. Recently, electron spin has been reported as an essential degree of freedom to improve the performance of photocatalysts. In this work, spin‐polarized electrons and holes are introduced into a brownmillerite KBi0.9Co0.1Fe2O5 semiconductor and a two‐dimensional (2D) MXene composite system by application of an external magnetic field. The spin polarization properties are monitored by measuring the magnetoresistance effect of the MXene‐KBCFO device, which is related to the carrier transfer efficiency. Remarkably, the photocatalytic performance of MXene‐KBCFO is significantly enhanced by the simultaneous application of an external magnetic field and illumination due to the increased number of spin‐polarized photoexcited carriers caused by the synergy effect of the 2D MXene heterostructure together with the ferromagnetic spin ordering. This results in increased reaction hotspots, effective built‐in electric field, extended carrier lifetime, and reduced charge recombination owing to parallel alignment of electron spin orientation. The results show that the efficiency and stability of photocatalytic dye degradation can be effectively enhanced by manipulating the spin‐polarized electrons in the MXene‐based oxide‐perovskite heterostructure photocatalyst.

Spin Texture Sensitive Photodetection by Dion-Jacobson Tin Halide Perovskites.

The organic spacer molecule is known to regulate the optoelectronic properties of two-dimensional (2D) perovskites. We show that the spacer layer thickness determines the nature of optical transitions, direct or indirect, by controlling the structural properties of the inorganic layer. The spin-orbit interactions lead to different electron spin orientations for the states associated with the conduction band minimum (CBM) and the valence band maximum (VBM). This leads to a direct as well as an indirect component of the transitions, despite them being direct in momentum space. The shorter chains have a larger direct component, leading to a better optoelectronic performance. The mixed halide Sn2+ Dion-Jacobson (DJ) perovskite with the shortest 4-C diammonium spacer outshines the photodetection parameters of those having longer (6-C and 8-C) spacers and the corresponding Ruddlesden-Popper (RP) phases. The DJ system with a 4-C spacer and equimolar Br/I embodies an unprecedentedly high responsivity of 78.1 A W-1 under 3 V potential bias at 485 nm wavelength, among the DJ perovskites. Without any potential bias, this phase manifests the self-powered photodetection parameters of 0.085 A W-1 and 9.9 × 1010 jones. The unusual role of electron spin texture in these high-performance photodetectors of the lead-free DJ perovskites provides an avenue to exploit the information coded in spins for semiconductor devices without any ferromagnetic supplement or magnetic field.

Prospects of the nanoelectronic element base of infosystems of autonomous aircraft

The ways of creating a nanoelectronic element base (NEB) of Autonomous Air Vehicle (AАV) with small dimen-sions and weight are analyzed. The autonomy of these devices predetermines the principles of building their in-formation systems using artificial intelligence logic, significant memory resources, as well as functionally special-ized and high-speed information processing methods. And the cost restrictions involve the use of practically de-bugged nano- and microtechnologies for optical devices and integrated circuits. In addition, the small volume and weight of AAV necessitate the miniaturization of their NEB. At the same time, the operation of the AAV in various extreme conditions with increased radiation and bursts of the electromagnetic field (for example, in space) is not ruled out. Under these conditions, the semiconductor elements of their electronics are likely to fail. Therefore, in NEB of AAV, it is expedient to switch to nanovacuum-channel transistors (NVCT), which do not have semiconduc-tors (or their role is auxiliary). In NVKT, the nanochannel of electron transfer between the electrodes is replaced by a vacuum one, and the control is carried out by the voltage on the field electrode - the gate. At the same time, the nanoscale length of the gap between the emitter and the collector provides low voltages on the electrodes of the NVKT. In general, replacing a semiconductor with a vacuum can simplify and reduce the cost of transistor technol-ogy, make it more resistant to radiation and high-energy fields, and will allow to reach THz frequencies. And the creation of new types of NVKT (for example, using the control of the electron spin orientation) will increase the per-formance of AAV information transmission and processing tools.

Floquet-engineered chiral-induced spin selectivity.

The control of electron spin, crucial to the stability of matter, offers new possibilities for manipulating the properties of molecules and materials with potential applications in spintronics and chemical reactions. Recent experiments have demonstrated that electron transmission through chiral molecules depends on the electron spin orientation, a phenomenon known as chiral-induced spin selectivity (CISS). In this study, we show that CISS can be observed in achiral systems driven by an external circularly polarized laser field in the framework of Floquet engineering. By using the Floquet theory for a time-periodically driven system to investigate spin-dependent electron transport in a two-terminal setup, we demonstrate that the spin polarization can approach unity if the light intensity is sufficiently strong, the rate of dephasing is sufficiently low, and the average chemical potential of the two leads is within an appropriate range of values, which is narrow because of the high frequency of the laser field. To obtain a broader range of energies for large spin polarization, a combination of chiral molecules and light-matter interactions is considered, and the spin polarization of electrons transported through a helical molecule driven by a laser field is evaluated.

The origin of magnetization-caused increment in water oxidation

Magnetization promoted activity of magnetic catalysts towards the oxygen evolution reaction (OER) has attracted great attention, but remains a puzzle where the increment comes from. Magnetization of a ferromagnetic material only changes its magnetic domain structure. It does not directly change the spin orientation of unpaired electrons in the material. The confusion is that each magnetic domain is a small magnet and theoretically the spin-polarization promoted OER already occurs on these magnetic domains, and thus the enhancement should have been achieved without magnetization. Here, we demonstrate that the enhancement comes from the disappeared domain wall upon magnetization. Magnetization leads to the evolution of the magnetic domain structure, from a multi-domain one to a single domain one, in which the domain wall disappears. The surface occupied by the domain wall is reformatted into one by a single domain, on which the OER follows the spin-facilitated pathways and thus the overall increment on the electrode occurs. This study fills the missing gap for understanding the spin-polarized OER and it further explains the type of ferromagnetic catalysts which can give increment by magnetization.

BRAND—A detection system for [formula omitted]-decay correlation measurement

The BRAND experiment aims at the search of Beyond Standard Model (BSM) physics via measurement of exotic components of the weak interaction. For this purpose, eleven correlation coefficients of neutron β-decay will be measured simultaneously. The BRAND detection system is oriented for the registration of charged products of β-decay of polarized, free neutrons. With the measurement of the four-momenta of electron and proton, the complete kinematic of the decay will be determined. Moreover, the transverse spin component of the electron, which is the crucial observable to probe BSM exotic components of weak interaction, will be measured via Mott scattering. The electron detection system features both tracking and energy measurement capability. It is also responsible for the determination of the electron spin orientation. A challenging detection of low-energy protons from the β-decay is performed with a system, which involves the acceleration and subsequent conversion of protons into bunches of electrons. To test the feasibility of the proposed experimental techniques, a small-scale prototype setup was installed at the cold neutron beam facility PF1B at the Laue-Langevin Institute (ILL) in Grenoble, France. In this contribution, the preliminary results of the commissioning run are presented with an emphasis on the performance of individual parts of the detection system.

Application of Pyroelectric Sensors Based on PVDF Films for EPR Spectra Detection by Heat Release.

Pyroelectrics are a wide class of materials that change their polarization when the system temperature varies. This effect is utilized for a number of different commercial and industrial applications ranging from simple thermal sensors and laser interferometers to water vapor harvesting. Electron paramagnetic resonance (EPR) spectroscopy is a powerful tool for studying the structure and dynamics of materials with unpaired electrons. Since heating accompanies a resonant change of the orientation of electron spins in an external magnetic field, pyroelectrics can be utilized as versatile detectors for so-called indirect detection of the EPR signal. In this work, we investigated three different types of PVDF (polyvinylidene difluoride) standard pyroelectric films with indium tin oxide, Cu/Ni, and Au coatings to determine their sensitivity for detecting EPR signals. All the films were shown to be able to detect the EPR spectra of about 1 μg of a standard stable free radical by heat release. A comparative study based on the calculation of the noise-equivalent power and specific detectivity from experimental spectra showed that the Au coated PVDF film is the most promising active element for measuring the EPR signal. Using the best achieved sensitivity, estimation is given whether this is sufficient for using a PVDF-based pyrodetector for indirectly detecting EPR spectra by recombination heat release or not.

Excitonic effects on electron spin orientation and relaxation in wurtzite GaN

Excitonic effects on electron spin orientation and relaxation in wurtzite GaN are investigated with photon-energy-dependent time-resolved Kerr rotation spectroscopy at low temperatures. It is observed that the spin orientation generated with circularly polarized light can be manipulated with photon energy upon the excitation energy being resonated with various fine energy levels in the band structure. Three reversals of the spin orientation and a remarkable reduce of spin relaxation time are obtained, which is caused by the photon-energy dependent transitions from the heavy and light hole bands to the exciton levels and the conduction band edge. A long spin relaxation time of 1.7 ns is attributed to the spin polarization of donor-bound electrons as the formation of donor-bound excitons, while the spin relaxation time of the conduction band electrons is shorter and shows a nonmonotonic dependence on the optical excitation power, revealing the dominant role of the D'yakonov-Perel' spin relaxation mechanism in wurtzite GaN.

Human-like stereo sensors using plasmonic antenna embedded MZI with space–time modulation control [Invited

A micro stereo sensor system is proposed for human sensors, where eyes, ears, tongue, nose, body, and brain are applied by six panda rings embedded in a Mach–Zehnder interferometer (MZI). The input power is applied to the upper branch of MZI and propagates within the system. The six antennas (sensors) are formed by the whispering gallery modes of the panda rings. The space–time modulation signal is applied to the MZI lower branch. The modulated stereo signals can be configured as the plasmon (electron) spin orientations, which can be identified and applied for quantum codes and quantum consciousness.

Spin-Dependent Enantioselective Electropolymerization

The electro-oxidative polymerization of an enantiopure chiral 3,4-ethylenedioxythiophene monomer, performed using spin-polarized currents, is shown to depend on the electron spin orientation. The spin-polarized current is shown to influence the initial nucleation rate of the polymerization reaction. This observation is rationalized in the framework of the chiral-induced spin selectivity effect.

Steplike spectral distribution of photoelectrons at the percolation threshold in heavily p -doped GaAs

We study the origin of the step-like shoulder on the high energy side of the low temperature photoluminescence spectrum of heavily $p$-doped GaAs. We show experimentally that it is controlled by the Fermi-Dirac distribution of the holes and by the energy distribution of the photoexcited electrons showing a sharp step-like dependence. This step is attributed to the percolation threshold in the conduction band separating localized from delocalized electron states. A comprehensive set of optical techniques based on spin orientation of electrons, namely the Hanle effect, time- and polarization-resolved photoluminescence, as well as transient pump-probe Faraday rotation are used for these studies. We identify two different electron ensembles with substantially different spin lifetimes of 20 and 280~ps, limited by the lifetime of the electrons. Their spin relaxation times are longer than 2~ns. The relative contribution of short- and long-lived photoexcited electrons to the emission spectrum changes abruptly at the high-energy photoluminescence step-like tail. For energies above the percolation threshold the electron states are empty due to fast energy relaxation, while for lower energies the relaxation is suppressed and the majority of photoelectrons populate these states.

Effect of Chiral Molecules on the Electron's Spin Wavefunction at Interfaces.

Kelvin-probe measurements on ferromagnetic thin film electrodes coated with self-assembled monolayers of chiral molecules reveal that the electron penetration from the metal electrode into the chiral molecules depends on the ferromagnet’s magnetization direction and the molecules’ chirality. Electrostatic potential differences as large as 100 mV are observed. These changes arise from the applied oscillating electric field, which drives spin-dependent charge penetration from the ferromagnetic substrate to the chiral molecules. The enantiospecificity of the response is studied as a function of the magnetization strength, the magnetization direction, and the handedness and length of the chiral molecules. These new phenomena are rationalized in terms of the chiral-induced spin selectivity (CISS) effect, in which one spin orientation of electrons from the ferromagnet penetrates more easily into a chiral molecule than does the other orientation. The large potential changes (>kT at room temperature) manifested here imply that this phenomenon is important for spin transport in chiral spintronic devices and for magneto-electrochemistry of chiral molecules.

Manipulating spin polarization of titanium dioxide for efficient photocatalysis

Photocatalysis has been regarded as a promising strategy for hydrogen production and high-value-added chemicals synthesis, in which the activity of photocatalyst depends significantly on their electronic structures, however the effect of electron spin polarization has been rarely considered. Here we report a controllable method to manipulate its electron spin polarization by tuning the concentration of Ti vacancies. The characterizations confirm the emergence of spatial spin polarization among Ti-defected TiO2, which promotes the efficiency of charge separation and surface reaction via the parallel alignment of electron spin orientation. Specifically, Ti0.936O2, possessing intensive spin polarization, performs 20-fold increased photocatalytic hydrogen evolution and 8-fold increased phenol photodegradation rates, compared with stoichiometric TiO2. Notably, we further observed the positive effect of external magnetic fields on photocatalytic activity of spin-polarized TiO2, attributed to the enhanced electron-spin parallel alignment. This work may create the opportunity for tailoring the spin-dependent electronic structures in metal oxides.

Femtosecond Pump Probe Reflectivity Spectra in CdTe and GaAs Crystals at Room Temperature.

Ultrafast pump probe reflectivity (PPR) signal near band edge is modeled by taking into account band filling (BF) and band gap renormalization (BGR) effects with the carrier density of ~1017/cm3 in GaAs crystal at room temperature. The calculated results indicate that the transient reflectivity ΔR/R is determined by BF and BGR effects. The most interesting feature is that ΔR/R signal experiences a sign change from photo-bleaching (PB) to photo-absorption (PA) due to the competition between BF and BGR effects. We experimentally measured ΔR as a function of photon energy across band edge with carrier density of ~1017/cm3 in GaAs and CdTe crystals, which has a similar trend as that calculated according to our model. In addition, the reflectivity is very sensitive to electron spin orientation, which is well confirmed by the corresponding experiments with 100 fs pump probe reflectivity spectroscopy in bulk CdTe. Our research in this work provides a method to study optoelectronic properties of conventional semiconductors at moderate carrier density excited by ultrafast laser pulse. Importantly, this model can be used for other novel semiconductor materials beyond GaAs and will provide new insights into the underlying spin dependent photophysics properties for new materials.

Chiral molecules and the electron spin

The electron’s spin is essential to the stability of matter, and control over the spin opens up avenues for manipulating the properties of molecules and materials. The Pauli exclusion principle requires that two electrons in a single spatial eigenstate have opposite spins, and this fact dictates basic features of atomic states and chemical bond formation. The energy associated with interacting electron clouds changes with their relative spin orientation, and by manipulating the spin directions, one can guide chemical transformations. However, controlling the relative spin orientation of electrons located on two reactants (atoms, molecules or surfaces) has proved challenging. Recent developments based on the chiral-induced spin selectivity (CISS) effect show that the spin orientation is linked to molecular symmetry and can be controlled in ways not previously imagined. For example, the combination of chiral molecules and electron spin opens up a new approach to (enantio)selective chemistry. This Review describes the theoretical concepts underlying the CISS effect and illustrates its importance by discussing some of its manifestations in chemistry, biology and physics. Specifically, we discuss how the CISS effect allows for efficient long-range electron transfer in chiral molecules and how it affects biorecognition processes. Several applications of the effect are presented, and the importance of controlling relative spin orientations in multi-electron processes, such as electrochemical water splitting, is emphasized. We describe the enantiospecific interaction between ferromagnetic substrates and chiral molecules and how it enables the separation of enantiomers with ferromagnets. Lastly, we discuss the relevance of CISS effects to biological electron transfer, enantioselectivity and CISS-based spintronics applications. Chiral molecules can filter electrons according to their spin. This chiral-induced spin selectivity (CISS) effect can have important applications, such as in spintronics and in enantioseparation. This Review describes the CISS effect, its mechanism and its fascinating applications.

Decay and revival of electron spin polarization in an ensemble of (In,Ga)As quantum dots

The periodic optical orientation of electron spins in (In,Ga)As/GaAs quantum dots leads to the formation of electron spin precession modes about an external magnetic field which are resonant with the pumping periodicity. As the electron spin is localized within a nuclear spin bath, its polarization imprints onto the spin polarization of the bath. The latter acts back on the electron spin polarization. We implement a pulse protocol where a train of laser pulses is followed by a long, dark gap. It allows us to obtain a high-resolution precession mode spectrum from the free evolution of the electron spin polarization. Additionally, we vary the number of pump pulses in a train to investigate the build-up of the precession modes. To separate out nuclear effects, we suppress the nuclear polarization by using a radio-frequency field. We find that a long-living nuclear spin polarization imprinted by the periodic excitation significantly speeds up the buildup of the electron spin polarization and induces the formation of additional electron spin precession modes. To interpret these findings, we extend an established dynamical nuclear polarization model to take into account optically detuned quantum dots for which nuclear spins activate additional electron spin precession modes.

Large effective mass and interaction-enhanced Zeeman splitting of K -valley electrons in MoSe2

We study the magnetotransport of high-mobility electrons in monolayer and bilayer MoSe$_2$, which show Shubnikov-de Haas (SdH) oscillations and quantum Hall states in high magnetic fields. An electron effective mass of 0.8$m_e$ is extracted from the SdH oscillations' temperature dependence; $m_e$ is the bare electron mass. At a fixed electron density the longitudinal resistance shows minima at filling factors (FFs) that are either predominantly odd, or predominantly even, with a parity that changes as the density is tuned. The SdH oscillations are insensitive to an in-plane magnetic field, consistent with an out-of-plane spin orientation of electrons at the $K$-point. We attribute the FFs parity transitions to an interaction enhancement of the Zeeman energy as the density is reduced, resulting in an increased Zeeman-to-cyclotron energy ratio.

Energy levels governed by golden section in O[sbnd]H+...O moiety under proton sharing coupled with spin-orbit interactions

Resonance Raman spectra of protonated meso-tetraphenylporphine (TPP) dimers measured earlier in aqueous solutions have been interpreted in terms of polaronic exciton theory. Consideration of the incident photon interaction with H3O+ producing polaronic exciton shows up spin-orbit interaction depending (βq = 3.741657387) and almost non-depending (β = 1.19100654) on quaternary molecule coordination in the water clusters. Both β parameters that take into consideration proton and electron spin orientations have been involved in major relations derived under the development of polaronic exciton theory. Theoretically estimated energy gap between the levels with different proton spin orientations, i.e. e↓↑p↑e and e↓↓p↑e in the OHδ+...O moiety is found to be 40.766 cm−1. Experimental energy gap 25.5 ± 1.3 cm−1 averaged for several doublets in the Raman spectrum with λex = 441.6 nm is consistent with the theoretical 25.19 cm−1 obtained using golden section as a denominator. It appears that golden section governs the fine structure under the proton sharing in the OH+...O moiety of the water cluster embedded between TPP units of the dimers.

Electrical Initialization of Electron and Nuclear Spins in a Single Quantum Dot at Zero Magnetic Field.

The emission of circularly polarized light from a single quantum dot relies on the injection of carriers with well-defined spin polarization. Here we demonstrate single dot electroluminescence (EL) with a circular polarization degree up to 35% at zero applied magnetic field. The injection of spin-polarized electrons is achieved by combining ultrathin CoFeB electrodes on top of a spin-LED device with p-type InGaAs quantum dots in the active region. We measure an Overhauser shift of several microelectronvolts at zero magnetic field for the positively charged exciton (trion X+) EL emission, which changes sign as we reverse the injected electron spin orientation. This is a signature of dynamic polarization of the nuclear spins in the quantum dot induced by the hyperfine interaction with the electrically injected electron spin. This study paves the way for electrical control of nuclear spin polarization in a single quantum dot without any external magnetic field.

Influence of spin–orbit coupling on the momentum distribution of electron pairs emitted from Au on Ir(111)

We explore by theory and experiment the effects of spin–orbit coupling (SOC) on the pair creation of electrons by spin-polarized primary electrons incident on a pseudomorphic monolayer of Au on Ir(111). An ab-initio calculation of the electronic structure reveals a Rashba-type sp-like surface state in the Au layer, which turns out to lead to a large electron pair creation intensity. The distribution of this intensity over the momenta of the emitted electrons depends strongly on the primary electron spin due to spin–orbit coupling mainly in the incident and the outgoing electron states. For normal incidence, the six-fold rotation symmetry, which would hold without spin–orbit coupling, is broken in a manner depending on the orientation of the primary electron spin.