Enhancement Of Spin Polarization Research Articles

Light narrowing over broad temperature range with paraffin-coated vapor cells

This study reports light narrowing in paraffin-coated vapor cells from room temperature 27 to 59 °C, where spin-exchange relaxation is suppressed. By means of a coating lock and eliminating the reservoir effect, an ultra-narrow magnetic resonance linewidth of 0.36 Hz and an atomic coherence lifetime of T2=0.9 s are achieved. In cells free of buffer gas, the narrow linewidth over this broad temperature range is a result of enhanced spin polarization, which is facilitated by the effective suppression of radiation trapping benefiting from the stability of the vapor density. Using such cells in atomic magnetometers, the photon shot noise limit is estimated as 0.2 fT/Hz1/2 and the spin-projection noise limit is estimated as 1.1 fT/Hz1/2. Also, a magnetometer system with the stable coated cell is identified, which demonstrates the potential for achieving relatively stable magnetometer sensitivity without precisely controlling the cell temperature. The long coherence lifetime and the broad operating temperature range expand the potential applications of quantum memory and other quantum sensors such as atomic clocks.

Enhanced Spin Polarization from Biaxially Strained Colloidal Quantum Dots.

Electron and hole spin polarization is crucial for quantum dots to be used in spin lasers and quantum information processing. However, the degree of spin polarization in II-VI and III-V semiconductor quantum dots is low because of the degenerated valence band. Here, we increase the light and heavy hole degeneracy by introducing biaxial strain into CdSe-based quantum dots, enabling the degree of spin polarization to be increased from 20% to 50% under photoexcitation. The optical gain threshold measurement further reveals that the increase in polarization helps to reduce the gain threshold.

Enhanced spin-polarization and detection limit in a spin-based optoelectrochemical DNA hybridization sensor induced by circularly polarized light

Enhanced spin polarization and L.O.D. in a spin-based electrochemical DNA biosensor using circularly polarized light.

Coherent spin transport in an Fe-S protein

Spin transport in a molecule was recently examined to discuss the chiral-induced spin selectivity (CISS) effect. This effect has been particularly reported in metalloproteins because the presence of transition metals improves spin polarization in a helical peptide structure. Among metalloproteins, iron-sulfur proteins have ubiquitous redox properties in photosynthesis and respiration. Iron can enhance spin polarization. In this study, the coherent spin transport in an Fe-S protein was investigated using first-principles calculations based on the Landauer formula. Proteins containing the [4Fe-4S] cluster were used for the calculations. The results of the spin transmission pathway revealed spin flow via Fe-S clusters.

The Contact Interface Electronic Coupling of Cobalt and Zirconia Enables Stable and Highly Efficient 4e- Oxygen Reduction ReactionCatalysis.

Cobalt (Co) is an efficient oxygen reduction reaction (ORR) catalyst but suffers from issues of easy deactivation and instability. Here, it shows that ZrO2 can stabilize Co through interface electron coupling and enables highly efficient 4e- ORR catalysis. Porous carbon nanofibers loaded with dispersed Co-nanodots (≈10nm, 9.63wt%) and ZrO2 nanoparticles are synthesized as the catalyst. The electron transfer from the metallic Co to ZrO2 causes interface-oriented electron enrichment that promotes the activation and conversion of O2, improving the efficiency of 4e- transfer. Moreover, the simulation results show that ZrO2 acts like an electron reservoir to store electrons from Co and slowly release them to the interface, solving the easy deactivation problem of Co. The catalyst exhibits a high half-wave potential (E1/2) of 0.84V, which only decreases by 3.6mV after 10000 cycles, showing great stability. Particularly, the enhanced spin polarization of Co in a magnetic field reinforces the interface electron coupling that increases the E1/2 to 0.864V and decreases the energy barrier of ORR from 0.81 to 0.63eV, confirming that the proposed strategy is effective for constructing efficient and stable ORR catalysts.

Photo‐SABRE: Nuclear Spin Hyperpolarization of cis‐trans Photoswitchable Molecules by Parahydrogen**

AbstractThis work reports the 23000‐ and 9200‐fold enhancement of the 15N nuclear spin polarization of cis and trans isomers of azobenzene, correspondingly, as compared to the thermal NMR signals at 9.4 T by exploiting signal amplification by reversible exchange (SABRE) and parahydrogen molecules at 400 nT simultaneously with light irradiation. Only cis‐azobenzene gains substantial hyperpolarization of its 15N spins directly from parahydrogen in SABRE through coherent polarization transfer. Consequently cis‐trans photoisomerization at ultralow magnetic field preserves the SABRE‐derived nuclear hyperpolarization of cis‐azobenzene resulting in hyperpolarization of trans‐azobenzene as well, despite its direct coordination with the SABRE Ir‐complex being sterically hindered. Moreover, the proposed approach, which we called photo‐SABRE, allows to hyperpolarize the long‐lived spin order of trans‐isomer 15N spins with a lifetime of about 25 minutes, which greatly exceeds their relaxation times at high (10 seconds) and low (200 seconds) magnetic field. Since this spin order in 15N2‐trans‐azobenzene is collectively formed by nuclei of different kinds, it can be detected by both 15N or 1H NMR spectroscopy.

Optimizing d-orbital occupation on interfacial transition metal sites via heterogeneous interface engineering to accelerate oxygen electrode reaction kinetics in lithium-oxygen batteries

Aprotic lithium-oxygen batteries (LOBs) possess extremely high theoretical energy density of about 3500 Wh kg-1, which is superior to state-of-the-art lithium-ion batteries. However, the poor energy efficiency and cyclability originated from sluggish kinetics of oxygen electrode reactions in LOBs severely restrict their development and practical application. The solution to these challenges is closely related to the construction of electrocatalysts with both high catalytic activity and stability for oxygen redox reactions in LOBs. In this work, Co3O4 @NiFe2O4 composite with modified heterogeneous interface is fabricated and studied as oxygen electrode catalyst for LOBs. It is theoretically predicted that the electron transfer at the heterogeneous interface in Co3O4 @NiFe2O4 is capable of reducing the occupation of eg electrons in the interfacial Ni sites, leading to the rearrangement of electrons with enhanced spin polarization in the d orbital and the upshift of the d-band center toward Fermi level, eventually enabling the smooth electron transfer between Co3O4 @NiFe2O4 and oxygenated species due to the facile formation of Ni-O bond. This endows Co3O4 @NiFe2O4 based LOBs with high specific capacity of 12697 mAh g-1 and extremely low overpotential of 0.68 V at 100 mA g-1, as well as long cycle life of 274 times at a high current density of 500 mA g-1. This work provides a new strategy for adjusting the surface electronic structure of oxygen electrode catalysts to accelerate oxygen redox kinetics in LOBs.

High spin polarization and spin signal enhancement in non-local spin valves with Co–Fe alloy injectors and detectors

For applications such as spin accumulation sensors for next-generation hard disk drive read heads, and for fundamental research, it is desirable to increase the spin signal in metallic non-local spin valves, which are central devices in spintronics. To this end, here, we report on the integration of high-spin-polarization Co–Fe binary alloy ferromagnetic injectors and detectors in Al-based non-local spin valves. Room-temperature deposition on amorphous substrates from an alloy target is shown to generate smooth, polycrystalline (110-textured), solid-solution body-centered-cubic Co75Fe25 films, which we characterize by energy dispersive x-ray spectroscopy, x-ray diffraction, x-ray reflectivity, atomic force microscopy, and electronic transport. Simple integration into transparent-interface Al non-local spin valves is then shown to realize up to a factor of ∼5 enhancement of the spin signal relative to Co, with full quantitative analysis yielding strikingly temperature-independent current spin polarizations exceeding 60%. We make a detailed quantitative comparison of these values with prior literature, concluding that Co–Fe alloys present a remarkably facile route to higher spin polarization and spin signals in non-local spin valves, with minimal barrier to adoption.

Carbon-induced magnetic properties and anomalous Hall effect in Co2Mn2C thin films with L10 -like structures

${\mathrm{Co}}_{2}{\mathrm{Mn}}_{2}\mathrm{C}$ thin films were synthesized via vacuum carburization of the host CoMn alloy films based on a conventional gas-solid reaction to demonstrate the effect of C, a common light element, on the magnetic and spintronic materials. The crystal structure transitioned from the disordered face-centered-cubic CoMn to the $L{1}_{0}$-like ${\mathrm{Co}}_{2}{\mathrm{Mn}}_{2}\mathrm{C}$, for which the lattice constant increased from 0.356 to 0.378 nm. The C $1s$ x-ray photoemission spectra of the ${\mathrm{Co}}_{2}{\mathrm{Mn}}_{2}\mathrm{C}$ film indicated hybridization in C-Co and C-Mn and a homogeneous concentration of C in the film. The enhancement of both the saturation magnetization and the anomalous Hall conductivity (${\ensuremath{\sigma}}_{xy}$) was induced by C, attributing to the magnetic transition to the ferrimagnetic spin order. The surface flatness and high ${\ensuremath{\sigma}}_{xy}$ are promising characteristics for spintronic applications such as the spin-anomalous Hall effect. The atom- and spin-resolved density of states (DOS) via first-principles calculations revealed that face-centered Mn(II) and Co could be significantly influenced by C because of p-d hybridization, resulting in enhanced spin polarization of the DOS at the Fermi level of \ensuremath{\sim}0.82. These results demonstrate that the use of C could be an essential way to boost material properties in the future.

Enhanced chiroptic properties of nanocomposites of achiral plasmonic nanoparticles decorated with chiral dye-loaded micelles

The development of circularly polarized luminescence (CPL)-active materials with both large luminescence dissymmetry factor (glum) and high emission efficiency continues to be a major challenge. Here, we present an approach to improve the overall CPL performance by integrating triplet-triplet annihilation-based photon upconversion (TTA-UC) with localized surface plasmon resonance. Dye-loaded chiral micelles possessing TTA-UC ability are designed and attached on the surface of achiral gold nanorods (AuNRs). The longitudinal and transversal resonance peaks of AuNRs overlap with the absorption and emission of dye-loaded chiral micelles, respectively. Typically, 43-fold amplification of glum value accompanied by 3-fold enhancement of upconversion are obtained simultaneously when Au@Ag nanorods are employed in the composites. More importantly, transient absorption spectra reveal a fast accumulation of spin-polarized triplet excitons in the composites. Therefore, the enhancement of chirality-induced spin polarization should be in charge of the amplification of glum value. Our design strategy suggests that combining plasmonic nanomaterials with chiral organic materials could aid in the development of chiroptical nanomaterials.

Effect of 3d Transition Metal Atom Intercalation Concentration on the Electronic and Magnetic Properties of Graphene/MoS2 Heterostructure: A First-Principles Study.

The electronic and magnetic properties of graphene/MoS2 heterostructures intercalated with 3d transition metal (TM) atoms at different concentrations have been systematically investigated by first principles calculations. The results showed that all the studied systems are thermodynamically stable with large binding energies of about 3.72 eV-6.86 eV. Interestingly, all the TM-intercalated graphene/MoS2 heterostructures are ferromagnetic and their total magnetic moments increase with TM concentration. Furthermore, TM concentration-dependent spin polarization is obtained for the graphene layer and MoS2 layer due to the charge transfer between TM atoms and the layers. A significant band gap is opened for graphene in these TM-intercalated graphene/MoS2 heterostructures (around 0.094 eV-0.37 eV). With the TM concentration increasing, the band gap of graphene is reduced due to the enhanced spin polarization of graphene. Our study suggests a research direction for the manipulation of the properties of 2D materials through control of the intercalation concentration of TM atoms.

Modeling size and edge functionalization of MXene-based quantum dots and their effect on electronic and magnetic properties.

In the last six years, the synthesis of MXene-based quantum dots (MXQDs) has gained widespread attention. Due to the quantum confinement effect, it is possible to significantly improve their properties compared to 2D counterparts, such as higher chemical stability and better electronic and optical properties. However, despite the growing interest in their properties, much remains unexplored. One of the biggest challenges is to study in more detail the structure of quantum dots, in particular, their edge functionalization and its effect on their properties. In this paper, the structural stability and electronic and magnetic properties of Ti2CO2 QDs based on different lateral dimensions and edge functionalization (-O, -F, and -OH) are investigated using density functional theory. The study shows that the energy gap of Ti2CO2-O QDs decreases with increasing lateral size for both nonmagnetic (spin-unpolarized, close shell) and magnetic (spin-polarized, open shell) cases. Furthermore, the magnetic behavior of quantum dots was revealed by shrinking from 2D Ti2CO2 to 0D Ti2CO2 QDs with lateral dimensions below 1.4 nm. The binding energy confirms the stability of all three types of edge functionalization, while the most stable structure was observed under fully saturated edge oxygenation. Moreover, it was also found that the spin density distribution and the energy gap of Ti2CO2-X QDs (X = O, F, and OH) are both dependent on the type of atom saturation. Size and edge confinement modeling has been demonstrated to be an effective tool for tuning the electronic and magnetic properties of MXQDs. Moreover, the observed enhanced spin polarization together with tunable magnetic properties makes the ultrafine Ti2CO2-X QDs promising candidates for spintronic applications.

A in-plane biaxial strain tunable electronic structures and magnetic properties of Fe2C monolayer

This work intends to study the effect of a in-plane biaxial strain (ε) on the electronic structures and magnetic properties of monolayer Fe2C by first-principles calculation. The unstrained and strained Fe2C monolayer is thermally, dynamically, and mechanically stable, and unstrained Fe2C monolayer is ferromagnetic metal with the Curie temperature (TC) of 510 K. Introducing the tensile strain, the magnetic moments, spin polarization and magnetic anisotropy energy (MAE) can be significantly enhanced. Furthermore, a phase transition from ferromagnetic to antiferromagnetic ordering occurs at the ε of 12%, −10%. Especially, the result of Monte Carlo simulations demonstrates that the TC decreases with strain and Néel temperature TN increases with the tensile strain. A tensile strain applied on Fe2C monolayer with the value less than 8% facilitates the enhancement of MAE and spin polarization, and maintaining the TC above room temperature. This study provides a theoretical guide for the application of monolayer Fe2C in spintronic devices, and provides a basis for tuning the physical properties to two-dimensional MXene materials.

Manipulating Spin Polarization of Defected Co3O4 for Highly Efficient Electrocatalysis

Electrocatalytic water splitting is limited by kinetics-sluggish oxygen evolution, in which the activity of catalysts depends on their electronic structure. However, the influence of electron spin polarization on catalytic activity is ambiguous. Herein, we successfully regulate the spin polarization of Co3O4 catalysts by tuning the concentration of cobalt defects from 0.8 to 14.5%. X-ray absorption spectroscopy spectra and density functional theory calculations confirm that the spin polarization of Co3O4 is positively correlated with the concentration of cobalt defects. Importantly, the enhanced spin polarization can increase hydroxyl group absorption to significantly 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 O2, which can obviously increase electrocatalytic OER activity. In specific, Co3O4-50 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.

Coherent spin transport in a multi-heme protein molecule

• Chiral-induced spin selectivity (CISS) • Multi-heme protein. • Coherent spin transport. Recently, the chiral-induced spin selectivity (CISS) characteristics of protein molecules with helical peptide structures have been reported. Furthermore, the interaction between CISS-caused spin and paramagnetic ions has been studied to enhance the magnetoresistance of memory and magnetic sensing devices. In this study, a first-principles approach was used to examine spin transport in a multi-heme protein using the Landauer model. Although the molecular size was so large that the conductance values were very low, spin polarization was observed in the molecule. The presence of heme enhanced spin polarization. However, in some cases, the presence of multiple hemes did not increase the spin polarization as expected because the spin transmission route between the electrodes deviated from the hemes. Therefore, the presence of multiple hemes does not necessarily effectively amplify spin polarization.

Ultrafast chiral peptides purification via surface plasmon enhanced spin selectivity

By D-arginine and L-arginine chiral peptides induced spin selectivity and Au NPs enhanced spin polarization, chiral peptides purification has been effectively simplified and the purification performance has raised from a mixture system. The angular momentums of light are operated by the polarizer and wave plates. Au NPs decorated ZnO nanorods electrodes are utilized to modulate the polarization of spintronic. Seed growth methods are for synthesizing spherical Au NPs. UV light reduction methods are for urchin-liked Au NPs. Au NPs are decorated on ZnO nanorods electrodes for rising photon to electron conversion efficiency and enhancing spin polarization rates by surface plasmon effect. From our results, photon to the electron conversion efficiency of ZnO nanorods electrodes has effectively enhanced by urchin-liked Au NPs decorating. Ultrahigh localized plasmon conversion efficiency as high as 60% was also obtained. Besides, density functional theory (DFT) calculations simulated the force on spintronic. Since the D-arginine and L-arginine are on Au substrate, DFT results demonstrate different angular momentum and spin polarization coupling. Along with urchin-liked Au NPs rising chiral induced spin polarization by surface plasmon resonance, the sensitivity of chiral arginine has been raised around 5000% from bare ZnO nanorods electrodes. The purification and separation time of a specific chiral arginine only needs 5 min.

Bulk-sensitive spin-resolved resonant electron energy-loss spectroscopy (SR-rEELS): Observation of element- and spin-selective bulk plasmons.

We have developed spin-resolved resonant electron energy-loss spectroscopy with the primary energy of 0.3-1.5 keV, which corresponds to the core excitations of 2p-3d absorption of transition metals and 3d-4f absorption of rare-earths, with the energy resolution of about 100 meV using a spin-polarized electron source as a GaAs/GaAsP strained superlattice photocathode. Element- and spin-selective carrier and valence plasmons can be observed using the resonance enhancement of core absorptions and electron spin polarization. Furthermore, bulk-sensitive electron energy-loss spectroscopy spectra can be obtained because the primary energy corresponds to the mean free path of 1-10nm. The methodology is expected to provide us with novel information about elementary excitations by resonant inelastic x-ray scattering and resonant photoelectron spectroscopy.

Atomic arrangements in an amorphous CoFeB ribbon extracted via an analysis of radial distribution functions

We discuss the atomic structure of amorphous ferromagnetic FeCoB alloys, which are used widely in spintronics applications. Specifically, we obtain the pair-distribution functions for various atomic pairs based on high-energy x-ray diffraction data taken from an amorphous Co20Fe61B19 specimen. We start our reverse Monte Carlo cycles to determine the disordered structure with a two-phase model in which a small amount of cobalt is mixed with Fe23B6 as a second phase. The structure of the alloy is found to be heterogeneous, where the boron atoms drive disorder through the random occupation of the atomic network. Our analysis also indicates the presence of small cobalt clusters that are embedded in the iron matrix and percolating the latter throughout the structure. This morphology can explain the enhanced spin polarization observed in amorphous magnetic materials.

Cyclic cooling of quantum systems at the saturation limit

The achievable bounds of cooling quantum systems, and the possibility to violate them is not well-explored experimentally. For example, among the common methods to enhance spin polarization (cooling), one utilizes the low temperature and high-magnetic field condition or employs a resonant exchange with highly polarized spins. The achievable polarization, in such cases, is bounded either by Boltzmann distribution or by energy conservation. Heat-bath algorithmic cooling schemes (HBAC), on the other hand, have shown the possibility to surpass the physical limit set by the energy conservation and achieve a higher saturation limit in spin cooling. Despite, the huge theoretical progress, and few principle demonstrations, neither the existence of the limit nor its application in cooling quantum systems towards the maximum achievable limit have been experimentally verified. Here, we show the experimental saturation of the HBAC limit for single nuclear spins, beyond any available polarization in solid-state spin system, the Nitrogen-Vacancy centers in diamond. We benchmark the performance of our experiment over a range of variable reset polarizations (bath temperatures), and discuss the role of quantum coherence in HBAC.

Enhancing spin polarization using ultrafast angular streaking

Through solution of the multielectron, semi-relativistic, time-dependent Schr\"{o}dinger equation, we show that angular streaking produces strongly spin-polarized electrons in a noble gas. The degree of spin polarization increases with the Keldysh parameter, so that angular streaking -- ordinarily applied to investigate tunneling -- may be repurposed to generate strongly spin-polarized electron bunches. Additionally, we explore modifications of the angular streaking scheme that also enhance spin polarization.