Progress in manipulating spin polarization for solar hydrogen production

Photocatalytic and photoelectrochemical water splitting using semiconductor materials are effective approaches for converting solar energy into hydrogen fuel.In the past few years,a series of photocatalysts/photoelectrocatalysts have been developed and optimized to achieve efficient solar hydrogen production.Among various optimization strategies,the regulation of spin polarization can tailor the intrinsic optoelectronic properties for retarding charge recombination and enhancing surface reactions,thus improving the solar-to-hydrogen(STH)efficiency.This review presents recent advances in the regulation of spin polarization to enhance spin polarized-dependent solar hydrogen evolution activity.Specifically,spin polarization manipulation strategies of several typical photocatalysts/photoelectrocatalysts(e.g.,metallic oxides,metallic sulfides,non-metallic semiconductors,ferroelectric materials,and chiral molecules)are described.In the end,the critical challenges and perspectives of spin polarization regulation towards future solar energy conversion are briefly provided.

Switchable hidden spin polarization and negative Poisson's ratio in two-dimensional antiferroelectric wurtzite crystals

Two-dimensional(2D)antiferroelectric materials have raised great research interest over the last decade.Here,we reveal a type of 2D antiferroelectric(AFE)crystal where the AFE polarization direction can be switched by a certain degree in the 2D plane.Such 2D functional materials are realized by stacking the exfoliated wurtzite(wz)monolayers with“self-healable”nature,which host strongly coupled ferroelasticity/antiferroelectricity and benign stability.The AFE candidates,i.e.,Zn X and Cd X(X=S,Se,Te),are all semiconductors with direct bandgap atΓpoint,which harbors switchable antiferroelectricity and ferroelasticity with low transition barriers,hidden spin polarization,as well as giant in-plane negative Poisson's ratio(NPR),enabling the co-tunability of hidden spin characteristics and auxetic magnitudes via AFE switching.The 2D AFE wz crystals provide a platform to probe the interplay of 2D antiferroelectricity,ferroelasticity,NPR,and spin effects,shedding new light on the rich physics and device design in wz semiconductors.

Introducing Spin Polarization into Mixed-Dimensional Van der Waals Heterostructures for High-Efficiency Visible-Light Photocatalysis

The low separation efficiency of the photogenerated carrier and the poor activity of the surface redox reaction are the main barrier to further improvement of photocatalytic materials.To address these issues,introducing spin-polarized electrons in single-component photocatalytic materials emerged as a promising approach.However,the decreased redox ability of photocarriers in these materials becomes a new challenge.Herein,we mitigate this challenge with a carbon nitride sheet(CNs)/graphene nanoribbon(GNR)composite material that has a van der Waals heterostructures(vdWHs)and spin-polarized electron properties.Experimental results and theoretical calculations show that the heterostructure has a strong redox ability,high carrier-separation efficiency,and enhanced surface catalytic reaction.Consequently,the mixed-dimensional CNs/GNR vdWHs exhibit remarkable performance for H_(2)and O_(2)generation as well as CO_(2)production under visible-light irradiation without any cocatalyst.The spin-polarized vdWHs discovered in this study revealed a new type of photocatalytic materials and advanced the development of spintronics and photocatalysis.

Anomalous chiral transports and spin polarization in heavy-ion collisions

Relativistic heavy-ion collisions create hot quark–gluon plasma as well as very strong electromagnetic(EM)and fluid vortical fields.The strong EM field and vorticity can induce intriguing macroscopic quantum phenomena such as chiral magnetic,chiral separation,chiral electric separation,and chiral vortical effects as well as the spin polarization of hadrons.These phenomena provide us with experimentally feasible means to study the nontrivial topological sector of quantum chromodynamics,the possible parity violation of strong interaction at high temperature,and the subatomic spintronics of quark–gluon plasma.These studies,both in theory and in experiments,are strongly connected with other subfields of physics such as condensed matter physics,astrophysics,and cold atomic physics,and thus form an emerging interdisciplinary research area.We give an introduction to the aforementioned phenomena induced by the EM field and vorticity and an overview of the current status of experimental research in heavy-ion collisions.We also briefly discuss spin hydrodynamics as well as chiral and spin kinetic theories.

Recent developments in chiral and spin polarization effects in heavy-ion collisions

We give a brief overview of recent theoretical and experimental results on the chiral magnetic effect and spin polarization effect in heavy-ion collisions.We present updated experimental results for the chiral magnetic effect and related phenomena.The time evolution of the magnetic fields in different models is discussed.The newly developed quantum kinetic theory for massive fermions is reviewed.We present theoretical and experimental results for the polarization of K hyperons and the q00 value of vector mesons.

Spin polarization effect for Fe2 molecule

This paper uses the density functional theory （DFT）（B3p86） of Gaussian03 to optimize the structure of Fe2 molecule. The result shows that the ground state for Fe2 molecule is a 9-multiple state, which shows spin polarization effect of Fe2 molecule of transition metal elements for the first time. Meanwhile, we have not found any spin pollution because the wavefunction of the ground state does not mingle with wavefunctions with higher energy states. So, that the ground state for Fe2 molecule is a 9-multiple state is indicative of the spin polarization effect of Fe2 molecule of transition metal elements. That is, there exist 8 parallel spin electrons. The non-conjugated electron is greatest in number. These electrons occupy different spacious tracks, so that the energy of the Fe2 molecule is minimized. It can be concluded that the effect of parallel spin of the Fe2 molecule is laFger than the effect of the conjugated molecule, which is obviously related to the effect of electron d delocalization. In addition, the Murrell Sorbie potential functions with the parameters for the ground state and other states of Fe2 molecule are derived. Dissociation energy De for the ground state of Fe2 molecule is 2.8586ev, equilibrium bond length Re is 0.2124nm, vibration frequency we is 336.38 cm^-1. Its force constants f2, f3, and f4 are 1.8615aJ.nm^-2, -8.6704aJ.nm^-3, 29.1676aj.nm^-4 respectively. The other spectroscopic data for the ground state of Fe2 molecule weXe, Be, αe are 1.5461 cm^-1, 0.1339cm^-1, 7.3428× 10^-4 cm^-1 respectively.

Spin polarization effect for Mn2 molecule

The density functional theory method （DFT） （b3p86） of Gaussian 03 has been used to optimize the structure of the Mn2 molecule. The result shows that the ground state of the Mn2 molecule is an 11-multiple state, indicating a spin polarization effect in the Mn2 molecule, a transition metal element molecule. Meanwhile, we have not found any spin pollution because the wavefunction of the ground state does not mingle with wavefunctions of higher-energy states. So the ground state for Mn2 molecule being of an 11-multiple state is the indicative of spin polarization effect of the Mn2 molecule among those in the transition metal elements： that is, there are 10 parallel spin electrons in a Mn2 molecule. The number of non-conjugated electrons is the greatest. These electrons occupy different spacious orbitals so that the energy of the Mn2 molecule is minimized. It can be concluded that the effect of parallel spin in the Mn2 molecule is larger than the effect of the conjugated molecule, which is obviously related to the effect of electron d delocalization. In addition, the Murrell-Sorbie potential functions with the parameters for the ground state and other states of the Mn2 molecule are derived. The dissociation energy De for the ground state of the Mn2 molecule is 1.4477 eV, equilibrium bond length Re is 0.2506 nm, vibration frequency ωe is 211.51 cm^-1. Its force constants f2, f3, and f4 are 0.7240 aJ·nm-2, -3.35574 aJ·nm^-3, 11.4813 aJ·nm^-4 respectively. The other spectroscopic data for the ground state of the Mn2 molecule ωeχe, Be, αe are 1.5301 cm^-1, 0.0978 cm^-1, 7.7825×10^-4 cm^-1 respectively.

Spin polarization effect for Co2 molecule

The density functional theory （DFT）（b3p86） of Gaussian 03 has been used to optimize the structure of the Co2 molecule, a transition metal element molecule. The result shows that the ground state for the Co2 molecule is a 7-multiple state, indicating a spin polarization effect in the Co2 molecule. Meanwhile, we have not found any spin pollution because the wavefunction of the ground state is not mingled with wavefunctions of higher-energy states. So for the ground state of Co2 molecule to be a 7-multiple state is the indicative of spin polarization effect of the Co2 molecule, that is, there exist 6 parallel spin electrons in a Co2 molecule. The number of non-conjugated electrons is the greatest. These electrons occupy different spacial orbitals so that the energy of the Co2 molecule is minimized. It can be concluded that the effect of parallel spin in the Co2 molecule is larger than the effect of the conjugated molecule, which is obviously related to the effect of electron d delocalization. In addition, the Murrell-Sorbie potential functions with the parameters for the ground state and the other states of the Co2 molecule are derived. The dissociation energy De for the ground state of Co2 molecule is 4.0489eV, equilibrium bond length Re is 0.2061 nm, and vibration frequency we is 378.13 cm^-1. Its diatomic molecule force constants f2, f3, and f4 are 2.4824 aJ·nm^-2, -7.3451 aJ·nm^-3, and 11.2222 aJ·nm^-4 respectively（1 aJ=10^-18 J）. The other spectroscopic data for the ground state of Co2 molecule ωeХe, Be, and αe are 0.7202 cm^-1, 0.1347 cm^-1, and 2.9120× 10^-1 cm^-1 respectively. And weXe is the non-syntonic part of frequency, Be is the rotational constant, αe is revised constant of rotational constant for non-rigid part of Co2 molecule.

Temperature-controllable spin-polarized current and spin polarization in a Rashba three-terminal double-quantum-dot device

We propose a Rashba three-terminal double-quantum-dot device to generate a spin-polarized current and manipulate the electron spin in each quantum dot by utilizing the temperature gradient instead of the electric bias voltage. This device possesses a nonresonant tunneling channel and two resonant tunneling channels. The Keldysh nonequilibrium Green＇s function techniques are employed to determinate the spin-polarized current flowing from the electrodes and the spin accumulation in each quantum dot. We find that their signs and magnitudes are well controllable by the gate voltage or the temperature gradient. This result is attributed to the change in the slope of the transmission probability at the Fermi levels in the low-temperature region. Importantly, an obviously pure spin current can be injected into or extracted from one of the three electrodes by properly choosing the temperature gradient and the gate voltages. Therefore, the device can be used as an ideal thermal generator to produce a pure spin current and manipulate the electron spin in the quantum dot.

The effects of electric and magnetic fields on the current spin polarization and magnetoresistance in a ferromagnetic/organic semiconductor/ferromagnetic（FM/OSC/FM） system

From experimental results of spin polarized injection and transport in organic semiconductors（OSCs）,we theoretically study the current spin polarization and magnetoresistance under an electric and a magnetic field in a ferromagnetic/organic semiconductor/ferromagnetic（FM/OSC/FM） sandwich structure according to the spin drift-diffusion theory and Ohm＇s law.From the calculations,it is found that the interfacial current spin polarization is enhanced by several orders of magnitude through tuning the magnetic and electric fields by taking into account the specific characteristics of OSC.Furthermore,the effects of the electric and magnetic fields on the magnetoresistance are also discussed in the sandwich structure.

Two types of ground-state bright solitons in a coupled harmonically trapped pseudo-spin polarization Bose-Einstein condensate

We study two types of bright solitons in an attractive Bose-Einstein condensate with a spin-orbit interaction. By solving the coupled nonlinear SchrOdinger equations with the variational method and the imaginary time evolution method, fundamental properties of solitons are carefully investigated in different parameter regimes. It is shown that the detuning between the Raman beam and energy states of the atoms dominates the ground state type and spin polarization strength. The soliton dynamics is also studied for various moving velocities for zero and nonzero detuning cases. We find that the shape of individual component solitons can be maintained when the moving speed of solitons is low and the detuning is small in the coupled harmonically trapped pseudo-spin polarization Bose-Einstein condensate.

Manipulating Spin Polarization of Defected Co_(3)O_(4)for Highly Effi cient Electrocatalysis

Electrocatalytic water splitting is limited by kinetics-sluggish oxygen evolution,in which the activity of catalysts depends on their electronic structure.However,the infl uence of electron spin polarization on catalytic activity is ambiguous.Herein,we successfully regulate the spin polarization of Co_(3)O_(4)catalysts by tuning the concentration of cobalt defects from 0.8 to 14.5%.X-ray absorption spectroscopy spectra and density functional theory calculations confi rm that the spin polarization of Co_(3)O_(4)is positively correlated with the concentration of cobalt defects.Importantly,the enhanced spin polarization can increase hydroxyl group absorption to signifi cantly decrease the Gibbs free energy change value of the OER rate-determining step and regulate the spin polarization of oxygen species through a spin electron-exchange process to easily produce triplet-state O_(2),which can obviously increase electrocatalytic OER activity.In specifi c,Co_(3)O_(4)-50 with 14.5%cobalt defects exhibits the highest spin polarization and shows the best normalized OER activity.This work provides an important strategy to increase the water splitting activity of electrocatalysts via the rational regulation of electron spin polarization.

Effect of Quasi-Fermi Level on the Degree of Electron Spin Polarization in GaAs

With spin-polarized-dependent band gap renormalization effect taken into account, the energy-dependent evolu- tion of electron spin polarization in GaAs is calculated at room temperature and at a low temperature of 1OK. We consider the exciting light with right-handed circular polarization, and the calculation results show that the degree of electron spin polarization is dependent strongly on the quasi-Fermi levels of ｜1/2） and ｜- 1/2） spin conduction bands. At room temperature, the degree of electron spin polarization decreases sharply from 1 near the bottom of the conduction band, and then increases to a stable value above the quasi-Fermi level of the ｜- 1/2） band. The greater the quasi-Fermi level is, the higher the degree of electron spin polarization with excessive en- ergy above the quasi-Fermi level of the ｜- 1/2） band can be achieved. At low temperature, the degree of electron spin polarization decreases from 1 sharply near the bottom of the conduction band, and then increases with the excessive energy, and in particular, up to a maximum of i above the quasi-Fermi level of the ｜1/2） band.

Spin polarization effect for molecule Ta2

Density functional theory （DFT） （B3p86） has been used to optimize the structure of the molecule Ta2. The result shows that the ground state of molecule Ta2 is a 7-multiple state and its electronic configuration is ^7∑u^＋, which shows the spin polarization effect for molecule Ta2 of transition metal elements for the first time. Meanwhile, spin pollution has not been found because the wavefunction of the ground state does not mix with those of higher states. So, the fact that the ground state of molecule Ta2 is a 7-multiple state indicates a spin polarization effect of molecule Ta2 of the transition metal elements, i.e. there exist 6 parallel spin electrons and the non-conjugated electrons are greatest in number. These electrons occupy different space orbitals so that the energy of molecule Ta2 is minimized. It can be concluded that the effect of parallel spin of the molecule Ta2 is larger than the effect of the conjugated molecule, which is obviously related to the effect of d-electron delocalization. In addition, the Murrell-Sorbie potential functions with parameters for the ground state ^7∑u^＋ and other states of the molecule Ta2 are derived. The dissociation energy De, equilibrium bond length Re and vibration frequency we for the ground state of molecule Ta2 are 4.5513eV, 0.2433nm and 173.06cm^-1, respectively. Its force constants f2, f3 and f4 are 1.5965×10^2aJ.nm^-2, -6.4722×10^3aJ·nm^-3 and 29.4851×10^4aJ·nm^-4, respectively. Other spectroscopic data we xe, Be and αe for the ground state of Ta2 are 0.2078cm^-1, 0.0315 cm^-1 and 0.7858×10^-4 cm^-1, respectively.

Spin polarization effect for Os2 molecule

Density functional Theory （DFT） （B3p86） of Gaussian03 has been used to optimize the structure of Os2 molecule. The result shows that the ground state for Os2 molecule is 9-multiple state and its electronic configuration is ^9∑^＋g, which shows spin polarization effect of Os2 molecule of transition metal elements for the first time. Meanwhile, we have not found any spin pollution because the wavefunction of the ground state does not mingle with wavefunctions with higher energy states. So, the fact that the ground state for Os2 molecule is a 9-multiple state is indicative of spin polarization effect of Os2 molecule of transition metal elements. That is, there exist 8 parallel spin electrons. The non-conjugated electron is greatest in number. These electrons occupy different spacious tracks, so that the energy of Os2 molecule is minimized. It can be concluded that the effect of parallel spin of Os2 molecule is larger than the effect of the conjugated molecule, which is obviously related to the effect of electron d delocalization. In addition, the Murrell-Sorbie potential functions with the parameters for the ground state ^9∑^＋g and other states of Os2 molecule are derived. Dissociation energy De for the ground state of Os2 molecule is 3.3971eV, equilibrium bond length Re is 0.2403nm, vibration frequency ωe is 235.32cm^-1. Its force constants f2, f3, and f4 are 3.1032×10^2aJ·nm^-2, -14.3425×10^3aJ·nm^-3 and 50.5792×10^4aJ·nm^-4 respectively. The other spectroscopic data for the ground state of Os2 molecule ωexe, Be and ae are 0.4277cm^- 1, 0.0307cm^- 1 and 0.6491 × 10^-4cm^-1 respectively.

Spin polarization effect of Ni_2 molecule

The density functional theory （DFT） method （b3p86） of Gaussian 03 is used to optimize the structure of the Ni2 molecule. The result shows that the ground state for the Ni2 molecule is a 5-multiple state, symbolizing a spin polarization effect existing in the Ni2 molecule, a transition metal molecule, but no spin pollution is found because the wavefunction of the ground state does not mingle with wavefunctions of higher-energy states. So the ground state for Ni2 molecule, which is a 5-multiple state, is indicative of spin polarization effect of the Ni2 molecule, that is, there exist 4 parallel spin electrons in Ni2 molecule. The number of non-conjugated electrons is greatest. These electrons occupy different spatial orbitals so that the energy of the Ni2 molecule is minimized. It can be concluded that the effect of parallel spin in the Ni2 molecule is larger than that of the conjugated molecule, which is obviously related to the effect of electron d delocalization. In addition, the Murrell-Sorbie potential functions with the parameters of the ground state and other states of the Ni2 molecule are derived. The dissociation energy De for the ground state of the Ni2 molecule is 1.835 eV, equilibrium bond length Re is 0.2243 nm, vibration frequency we is 262.35 cm^-1. Its force constants f2, f3 and f4 are 1.1901 aJ.nm^-2, -5.8723 aJ.nm^-3, and 21.2505 aJ.nm^-4 respectively. The other spectroscopic data for the ground state of the Ni2 molecule ωeχe, Be and αe are 1.6315cm 2, 0.1141 cm^-1, and 8.0145× 10^-4 cm^-1 respectively.

Spin polarization effect for Cr2 molecule

Density functional theory （DFT） （B3P86） of Gaussian 03 has been used to optimize the structure of the Cr2 molecule, a transition metal element molecule. The result shows that the ground state for the Cr2 molecule is a 13- multiple state, indicating that there exists a spin polarization effect in the Cr2 molecule. Meanwhile, we have not found any spin pollution because the wave function of the ground state does not mingle with wave functions of higher-energy states. So the ground state for Cr2 molecule being a 13-multiple state is indicative of spin polarization effect of the Cr2 molecule among transition metal elements, that is, there are 12 parallel spin electrons in the Cr2 molecule. The number of non-conjugated electrons is greatest. These electrons occupy different spatial orbitals so that the energy of the Cr2 molecule is minimized. It can be concluded that the effect of parallel spin in the Cr2 molecule is larger than the effect of the conjugated molecule, which is obviously related to the effect of electron d delocalization. In addition, the Murrell Sorbie potential functions with the parameters for the ground state and other states of the Cr2 molecule are derived. The dissociation energy De for the ground state of the Cr2 molecule is 0.1034eV, equilibrium bond length Re is 0.3396 nm, and vibration frequency we is 73.81cm^-1. Its force constants f2, f3 and f4 are 0.0835, -0.2831 and 0.3535 aJ. nm^-4 respectively. The other spectroscopic data for the ground state of the Cr2 molecule ωeχe, Be and αe are 1.2105, 0.0562 and 7.2938 x 10^-4cm^-1 respectively.

Atomic Spin Polarization Controllability Analysis:A Novel Controllability Determination Method for Spin-Exchange Relaxation-Free Co-Magnetometers

This paper investigates the atomic spin polarization controllability of spin-exchange relaxation-free co-magnetometers(SERFCMs).This is the first work in the field of controllability analysis for the atomic spin ensembles systems,whose dynamic behaviors of spin polarization are described by the Bloch equations.Based on the Bloch equations,a state-space model of the atomic spin polarization for SERFCM is first established,which belongs to a particular class of nonlinear systems.For this class of nonlinear systems,a novel determination method for the global state controllability is proposed and proved.Then,this method is implemented in the process of controllability analysis on the atomic spin polarization of an actual SERFCM.Moreover,a theoretically feasible and reasonable solution of the control input is proposed under some physical constraints,with whose limitation of realistic conditions,the controller design can be accomplished more practically and more exactly.Finally,the simulation results demonstrate the feasibility and validation of the proposed controllability determination method.

Confined states and spin polarization on a topological insulator thin film modulated by an electric potential

We study the electronic structure and spin polarization of the surface states of a three-dimensional topological insulator thin film modulated by an electrical potential well. By routinely solving the low-energy surface Dirac equation for the system, we demonstrate that confined surface states exist, in which the electron density is almost localized inside the well and exponentially decayed outside in real space, and that their subband dispersions are quasilinear with respect to the propagating wavevector. Interestingly, the top and bottom surface confined states with the same density distribution have opposite spin polarizations due to the hybridization between the two surfaces. Along with the mathematical analysis, we provide an intuitive, topological understanding of the effect.

Current spin polarization of a platform molecule with compression effect

Using the first-principles method,the spin-dependent transport properties of a novel platform molecule containing a freestanding molecular wire is investigated by simulating the spin-polarized scanning tunneling microscope experiment with Ni tip and Au substrate electrodes.Transport calculations show that the total current increases as the tip gradually approaches to the substrate,which is consistent with the conductance obtained from previous experiment.More interestingly,the spin polarization(SP)of current modulated by compression effect has the completely opposite trend to the total current.Transmission analyses reveal that the reduction of SP of current with compression process originates from the promotion of spin-down electron channel,which is controlled by deforming the molecule wire.In addition,the density of states shows that the SP of current is directly affected by the organic–ferromagnetic spinterface.The weak orbital hybridization between the Ni tip and propynyl of molecule results in high interfacial SP,whereas the breaking of the C≡C triple of propynyl in favor of the Ni–C–C bond induces the strong orbital hybridization and restrains the interfacial SP.This work proposes a new way to control and design the SP of current through organic–ferromagnetic spinterface using functional molecular platform.