Spin-polarized Charge Carriers Research Articles

Tautomerization Makes Topological Phase Transition in 2D Organometallic Lattices

AbstractThe investigation into approaches for achieving reversible control of topological phase transitions has attracted substantial research interest. So far, several system‐dependent physical control methods are developed, while a chemical strategy for efficient regulation of topological properties is still lack. Here, this study presents a chemical means to achieve reversible topological control in 2D organometallic lattices by introducing the lactam‐lactim tautomerization modules in the organic linkers. The strategy is validated in the theoretically designed 2D hexagonal framework with planar tricoordinated Co as nodes and 2,2′‐bithiazole derivatives as organic linkers, which transforms from a ferrimagnetic half metal with a local gap featuring Chern number of +1 to a ferromagnetic Chern insulator possessing Chern number of −1 after lactam‐lactim tautomerization. Simultaneously, a flip in the spin polarization of charge carriers between the two lattice tautomers is also observed.

ZnxFe3-xO4–ZnO heterojunction interfaced with poly(L-DOPA) electron transfer mediator layer for enhanced visible light photocatalytic activity

Z-scheme heterojunctions comprising ZnxFe3-xO4@poly(L-DOPA)@ZnO (ZF@PD@ZnO) were synthesized using solvothermal and precipitation techniques. The study aimed to investigate the impact of the poly(L-DOPA) electron transfer mediator on the photocatalytic performance of ZnxFe3-xO4@ZnO (ZF@ZnO) nanocomposites under visible light irradiation. X-ray diffraction (XRD) and HRTEM analysis confirmed the formation of heterostructures, identifying the crystalline phases of both ZnO and zinc ferrite. BFTEM images revealed a polyhedral shape for all samples, with a tendency toward mild agglomeration. The partial inversion of the zinc ferrite cubic spinel structure was evidenced by FT-IR, XPS, and VSM analyses. Zinc ferrite was found to be ferromagnetic, with cation distributions between tetrahedral (A) and octahedral (B) positions as [Zn0.62+Fe0.43+]A[Fe0.42+Fe1.63+]BO4. The samples exhibited notable photocatalytic activity against Rhodamine B, serving as a model contaminant. Furthermore, the stability and reusability of the samples were examined. Additionally, the study elucidated the photocatalytic mechanism produced by the ternary Z-scheme ZF@PD@ZnO, involving spin-polarized charge carriers. Energy band alignment was evaluated by combining ultraviolet photoelectron spectroscopy (UPS) with UV-Vis spectroscopy. The interplay between band alignment and reactive oxygen species (ROS) generation was discussed in correlation with EPR results. Enhanced ROS generation under visible light illumination, observed in the ZF@PD@ZnO nanocomposite, is mainly due to the addition of the conductive poly(L-DOPA) layer, acting as an electron transfer mediator in the Z-scheme heterojunction, ensuring enhanced charge separation.

(Invited) Quest for Fully Spin and Optically Polarized Semiconductor Nanostructures for Room-Temperature Opto-Spintronics

Spintronics represents a new paradigm for future electronics, photonics and information technology, which explores the spin degree of freedom of the electron for information storage, processing and transfer. Since 1990s, we have witnessed great success of metal-based spintronics that has revolutionized the mass data storage industry. There has also been an enormous push for semiconductor spintronics during the past three decades, with the aim to capitalize the past and current success of charge-based semiconductor technology and to make its spin counterpart the backbone of future spintronics just like semiconductors have done in today’s electronics/photonics. An exclusive advantage of semiconductor spintronics is its potential for opto-spintronics that will allow integration of spin-based information processing and storage with photon-based information transfer and communications. Unfortunately, progresses of semiconductor spintronics have so far been severely hampered by the failure to generate nearly fully spin-polarized charge carriers in semiconductors at and above room temperature (RT) at which today’s devices operate.In this work, we succeed to achieve conduction electron spin polarization exceeding 90% at RT in a semiconductor nanostructure, which remains steadily high even up to 110°C [1]. This represents the highest RT electron spin polarization ever reported in any semiconductor by any approach! This breakthrough is accomplished by a conceptually new approach of defect-engineered remote spin filtering and amplification of InAs quantum-dot (QD) electrons via an adjacent tunneling-coupled GaNAs quantum well acting as a spin filter. The extraordinary spin filtering effect in GaNAs is enabled by spin-dependent recombination via spin-polarized defects, i.e. grown-in Ga self-interstitials, which selectively deplete conduction electrons with an opposite spin orientation to that of the defect electron. In sharp contrast to the general trend of deteriorating spin polarization with increasing temperature seen in all other approaches of spin generation, our approach is gifted with an opposite temperature dependence up to RT thanks to a thermally accelerated remote spin-filtering effect as a result of thermally activated recombination via the defects [2]. We further show that the QD electron spin can be remotely manipulated by spin control in the adjacent spin filter, paving the way for remote spin encoding and writing of quantum memory as well as for remote spin control of spin-photon interfaces. This work demonstrates the feasibility to implement opto-spintronic functionality under practical device operation conditions in a semiconductor nanostructure system based on the mature III-V semiconductor technology commonly used for today’s optoelectronics and photonics. It could also pave the way for a range of potential spintronic and opto-spintronic applications exploiting the state-of-the-art GaAs technology platform, such as spin-LEDs, spin lasers, spin-polarized single-photon sources, quantum spin-photon interfaces, spin qubits, etc.

Probing Spin-Dependent Ballistic Charge Transport at Single-Nanometer Length Scales.

The coherent transport of charge and spin is a key requirement of future devices for quantum computing and communication. Scattering at defects or impurities may significantly reduce the coherence of quantum-mechanical states, thereby affecting the device functionality. While numerous methods exist to experimentally assess charge transport, the real-space detection of a material's ballistic spin transport properties with nanometer resolution remains a challenge. Here we report on a novel approach that utilizes a combination of spin-polarized scanning tunneling microscopy (SP-STM) and the recently introduced molecular nanoprobe (MONA) technique. It relies on the local injection of spin-polarized charge carriers from a magnetic STM tip and their detection by a single surface-deposited phthalocyanine molecule via reversible electron-induced tautomerization events. Based on the particular electronic structure of the Rashba alloy BiAg2, which is governed by a spin-momentum-locked surface state, we prove that the current direction inverses upon tip magnetization reversal.

Thickness dependence of magnetic and magnetotransport properties of textured Fe3O4 ferrimagnetic films on SiO2/n-Si(001)

The efficient injection of spin-polarized charge carriers from a ferro- or ferrimagnet into a semiconductor is an urgent problem. Half-metal magnetite (Fe3O4) is an attractive candidate for the role of a spin injector in spintronic devices. Here, the influence of the thickness of magnetite films on their electrical, magnetic, and magnetotransport properties is presented. Polycrystalline magnetite films with (311)-texture and thickness ranging from 15 to 250 nm are grown on the oxidized silicon surface by reactive iron deposition in the molecular oxygen atmosphere. The saturation magnetization is found to increase nonmonotonically with the film thickness and reaches its maximum value at 170 nm, which is close to that of a single crystal (∼480 emu/cm3). It is shown that for films with a thickness of more than 150 nm, there is a rapid decrease in resistivity by almost three times. The critical film thickness at which the coercive force decreases to the lowest value of ∼250 Oe is 100 nm. The magnetoresistance value reaches − 2.5 % already at a thickness of 50 nm and remains almost unchanged with a subsequent increase in the thickness of the textured film. Since, at a small thickness, textured films have higher magnitudes of conductivity and magnetization than conventional polycrystalline films, the textured-F3O4/SiO2/n-Si(001) junction with amorphous SiO2 barrier is promising for Si-based spintronic devices.

Solvent Modulation of Chiral Perovskite Films Enables High Circularly Polarized Luminescence Performance from Chiral Perovskite/Quantum Dot Composites.

Materials with circularly polarized luminescence (CPL) activity are promising in many chiroptoelectronics fields, such as for biological probes, asymmetric photosynthesis, information storage, spintronic devices, and so on. Promoting the value of the dissymmetry factor (glum) for the CPL-active materials based on chiral perovskite draws increasing attention since a higher glum value indicates better CPL. In this work, we find that, after being treated with a facile solvent modulation strategy, the chirality of 2D chiral perovskite films has been enhanced a lot, which we attribute to an increased lattice distortion degree. By forming chiral perovskite/quantum dot (QD) composites, the CPL-active material is successfully obtained. The calculated maximum |glum| of these composites increased over 4 times after solvent modulation treatment (1.53 × 10-3 for the pristine sample of R-DMF and 6.91 × 10-3 for R-NMP) at room temperature. Moreover, the enhancement of the CPL intensity is ascribed to two aspects: one is the generation and transportation of spin-polarized charge carriers from chiral perovskite films to combine in the QD layer, and the other is the solvent modulation strategy to enlarge the lattice distortion of chiral perovskite films. This facile route provides an effective way to construct CPL-active materials. More importantly, this kind of composite material (chiral perovskite film/QD layer) can be easily applied for fabricating circularly polarized light-emitting diode devices for electroluminescence.

Spin-polarized excitons and charge carriers in chiral metal halide semiconductors

Introducing chirality into metal halide semiconductors brings spin-polarized charges and excitons, exotic chiroptical and spintronic properties, as well as application potential in optoelectronics and spintronics devices.

Spin gapless semiconductor and nearly spin semimetal antiferromagnets: The case of the inverse Heusler compounds Mn2LiZ (Z = Al and Ga)

Using first-principles calculations, we report the structural, electronic, magnetic and electronic transport properties of the new Heusler compounds Mn2LiZ with Z is Al and Ga. We show that both compounds are more stable in the inverse Heusler structure than in the regular one. The band structure calculations indicate that the Mn2LiAl compound is a spin gapless semiconductor and Mn2LiGa is a nearly spin-semimetal. In agreement with the Slater-Pauling rule, the two compounds are non-conventional antiferromagnets: they show zero spin moment accompanied by the fully spin-polarized charge carriers. Strong direct interaction between the spin moments is obtained. Consequently, the mean-field approximation reveals a high Curie temperature in the two compounds. Within the Boltzmann transport theory, the electrical conductivity shows non-metallic behavior for the two compounds at elevated temperatures, and the Seebeck coefficient appears to be large. In particular, the Mn2LiAl compound has a value greater than 150µV/K in a wide temperature range.

Room-temperature electron spin polarization exceeding 90% in an opto-spintronic semiconductor nanostructure via remote spin filtering

An exclusive advantage of semiconductor spintronics is its potential for opto-spintronics, which will allow integration of spin-based information processing/storage with photon-based information transfer/communications. Unfortunately, progress has so far been severely hampered by the failure to generate nearly fully spin-polarized charge carriers in semiconductors at room temperature. Here we demonstrate successful generation of conduction electron spin polarization exceeding 90% at room temperature without a magnetic field in a non-magnetic all-semiconductor nanostructure, which remains high even up to 110 °C. This is accomplished by remote spin filtering of InAs quantum-dot electrons via an adjacent tunnelling-coupled GaNAs spin filter. We further show that the quantum-dot electron spin can be remotely manipulated by spin control in the adjacent spin filter, paving the way for remote spin encoding and writing of quantum memory as well as for remote spin control of spin–photon interfaces. This work demonstrates the feasibility to implement opto-spintronic functionality in common semiconductor nanostructures. An electron spin polarization of 90% is achieved in a non-magnetic nanostructure at room temperature without magnetic field. This is accomplished by remote spin filtering of InAs quantum-dot electrons via an adjacent tunnelling-coupled GaNAs spin filter.

Electronic, magnetic and galvanomagnetic properties of Co-based Heusler alloys: Possible states of a half-metallic ferromagnet and spin gapless semiconductor

Parameters of the energy gap and, consequently, electronic, magnetic and galvanomagnetic properties in different X2YZ Heusler alloys can vary quite strongly. In particular, half-metallic ferromagnets (HMFs) and spin gapless semiconductors (SGSs) with almost 100% spin polarization of charge carriers are promising materials for spintronics. The changes in the electrical, magnetic and galvanomagnetic properties of the Co2YSi (Y = Ti, V, Cr, Mn, Fe) and Co2MnZ Heusler alloys (Z = Al, Si, Ga, Ge) in possible HMF- and/or SGS-states were followed and their interconnection was established. Significant changes in the values of the magnetization and residual resistivity were found. At the same time, the correlations between the changes in these electronic and magnetic characteristics depending on the number of valence electrons and spin polarization are observed.

Large pressure-induced magnetoresistance in a hybrid ferromagnet-semiconductor system: Effect of matrix modification on the spin-dependent scattering

Magnetic nanocomposites based on MnAs clusters embedded in a chalcopyrite host usually do not exhibit large magnetoresistance (MR) at room temperature, while pronounced effects are localized at very low temperatures. In the present work, we observed an appearance of large pressure-induced negative and positive MR at room temperature in the Zn0.1Cd0.9GeAs2 hybrid system containing 10% MnAs inclusions. With the applied pressure, a substantial modification of the electron transport from semimetallic to semiconducting type occurs, followed by a subsequent structural transition at P ≈ 3.5 GPa into almost metallic high-pressure phase. This picture is simultaneously supported by temperature-dependent and room temperature high-pressure transport measurements. Using a semiempirical expression, taking into account a spin-dependent scattering of charge carriers due to MnAs nanoclusters, as well as a two-band conductivity model, we have been able to partially describe the observed MR effects. The predominantly weak positive contribution at P = 1 GPa, which is well described in the framework of the proposed approach indicates the presence of spin-polarized charge carriers. Based on the two-band model calculations, a negative spin polarization was found at P ≥ 3 GPa that ascribed to a structural change of the matrix. As our results indicate, an emerging MR in the structural transition region is characterized by a complex behavior. In particular, the negative part of MR demonstrates a magnetic field dependence different than Δρ/ρ0 ∼ H2, suggesting the presence of unusual scattering mechanisms in magnetotransport.

Spin-locked transport in a two-dimensional electron gas

Spin-orbit interactions in solids have inspired innovative physics for spin-based technologies. One such example is the persistent spin helix, where spin-orbit interactions from the semiconductor lattice are balanced with those in asymmetric quantum wells, to create long-lived spin textures. Spin transport in the presence of the momentum-dependent spin-orbit interactions lead to Larmor precession and subsequent dephasing that challenges the design of current spin-based information processing devices. We demonstrate that external magnetic fields can be applied to overcome this issue for spin-polarized charge carriers transported by in-plane electric fields. A frame of reference picture is introduced to describe the emergence and dynamics of the polarization-locked spin-wave packet after optical excitation. Applying well-matched magnetic fields maintains the persistent spin-helix profile regardless of whether the frame of reference is in motion or not. Monte Carlo simulations allow this traveling persistent spin-helix concept to be extended to a proposed spin Hall-effect transistor to ease design requirements.

Non-Heisenberg magnetism in a quaternary spin-gapless semiconductor

Understanding of spin-gapless semiconductors with fully spin-polarized charge carriers is critically important because of their promise for spintronic applications. Here, we report non-collinear spin structures, magnetic ground state, and effective exchange interactions of the spin-gapless semiconductor CoFeCrAl investigated with noncollinear density functional calculations. The ground state of CoFeCrAl is ferrimagnetic and has a spin configuration with ↓ Fe, ↑ Co and ↑ Cr spins. In our constrained calculations, the magnetizations of the Fe, Co, and Cr sublattices are rotated by various angles θ, which give rise to three sets of noncollinear spin structures. For all three elements, the magnetic energy increases with the angle, which reconfirms the ferrimagnetic spin structure. During rotation, the magnitudes of the Co and Cr spins remain almost unchanged, whereas that of Fe strongly decreases as a function of the angle θ. This indicates that the finite-temperature behavior of CoFeCrAl is characterized by a pronounced non-Heisenberg behavior of the ↓ Fe moments, whereas the ↑ Co and ↑ Cr moments are Heisenberg-like. We discuss how this feature affects the finite-temperature behavior of the alloy beyond the commonly considered intersublattice Heisenberg exchange.

Electrical, magnetic and galvanomagnetic properties of Mn-based Heusler alloys

Half-metallic ferromagnets and spin gapless semiconductors are promising materials for spintronic devices since a high degree of the spin polarization of charge carriers can be realized in such materials. Spin gapless semiconductors make it possible to combine the properties of half-metallic ferromagnets with semiconductor characteristics and to perform fine tuning of the energy gap value. The Mn2MeAl (Me = Ti, V, Cr, Mn, Fe, Co, Ni) Heusler alloys can possess such features. We studied the electrical, magnetic and galvanomagnetic properties of the Mn2MeAl (Me = Ti, V, Cr, Mn, Fe, Co, Ni) Heusler alloys from 4.2 K to 900 K and in magnetic fields up to 100 kOe. The features in the electronic and magnetic properties of Mn2MeAl Heusler alloys were observed, which can be a manifestation of the electronic energy spectrum peculiarities with occurrence of the half-metallic ferromagnet and/or spin gapless semiconductor states.

Strong changes in electronic transport and magnetic properties of Co2YSi Heusler alloys at Y-component variation

The Co2YSi (Y = Ti, V, Cr, Mn, Fe, Co, Ni) Heusler alloys can manifest the properties of half-metallic ferromagnets. These compounds are promising materials for spintronics since almost 100 % spin polarization of charge carriers can be realized at room temperature. We measured the electroresistivity, magnetic and galvanomagnetic properties of the Co2YSi (Y = Ti, V, Cr, Mn, Fe, Co, Ni) Heusler alloys from 4.2 K to 300 K and in magnetic fields up to 100 kOe. The type, concentration and mobility of charge carriers were estimated. The Y-component variation in the Co2YSi Heusler alloys is found to affect strongly the number of current carriers and alter the electronic band structure near the Fermi level EF and, consequently, the electronic transport and magnetic properties of the Co2YSi (Y = Ti, V, Cr, Mn, Fe, Co, Ni) Heusler alloys.

Photon helicity driven surface photocurrent in CuSe films

We report excitation of the spin-polarized currents in CuSe nanocrystalline films and demonstrate that the inversion of the excitation photon helicity reverses the direction of the photocurrent propagating perpendicular to the plane of incidence. By performing measurements of the photocurrent propagating both along and perpendicular to the plane of incidence, we show that the observed spin-polarized currents originate from the circular surface photogalvanic effect (C-SPGE). In contrast to the conventional circular photogalvanic effect, which is associated with spin–orbit interaction and can be observed in gyrotropic media, the C-SPGE originates from the scattering of the spin-polarized charge carriers on the surface. We demonstrate that in CuSe films, the C-SPGE gives rise to the photon helicity sensitive photoresponse, making this material attractive for optoelectronics and spintronics applications.

Generalized Spin-Orbit Interaction and Its Manifestation in Two-Dimensional Electron Systems

In frame of Dirac quantum field theory that describes electrons and positrons as elementary excitations of the spinor field, the generalized operator of the spin-orbit interaction is obtained using non-relativistic approximation in the Hamilton operator of the spinor field taking into account the presence of an external potential. This operator is shown to contain a new term in addition to the known ones. By an example of a model potential in the form of a quantum well, it is demonstrated that the Schr¨odinger equation with the generalized spin-orbit interaction operator describes all spin states obtained directly from the Dirac equation. The dependence of the spin-orbit interaction on the spin states in quasi-two-dimensional systems of electrons localized in a quantum well is analyzed. It is demonstrated that the electric current in the quantum well layer induces the spin polarization of charge carriers near the boundary surfaces of the layer, with the polarization of the charge carriers being opposite at the different surfaces. This phenomenon appears due to the spin-orbit interaction and is known as the spin Hall effect, which was observed experimentally in heterostructures with the corresponding geometry.

Long Carbon Nanotubes Functionalized with DNA and Implications for Spintronics.

Helical molecules such as DNA have recently been found to behave as an efficient source and detector of spin-polarized charge carriers. This phenomenon, often dubbed as chirality-induced spin selectivity or CISS, could be used to significantly improve the performance of spintronic devices, which utilize carrier spins (rather than charge) to realize electronic and sensing functions. Recently, it has been reported that carbon nanotubes, helically wrapped with DNA, can also act as an efficient source and detector of spin-polarized carriers, by virtue of spin–orbit coupling originating from the helical potential. It has been postulated that spin polarization should increase with the length of the wrapped tubes. However, in literature, most fabrication processes yield tubes with submicron lengths, which can produce ∼70% spin polarization. In an effort to enhance this effect further, here, we report a fabrication process that can yield DNA-wrapped nanotubes of length ∼1–4 microns. Detailed characterization of these devices, using atomic force microscopy, Raman, UV–vis, and temperature-dependent transport, has been presented. Initial transport measurements indicate the presence of strong magnetoresistance in these tubes, which could be attributed to spin-dependent effects. Systematic fabrication of long DNA-wrapped nanotubes, which has hitherto not been reported, is expected to enable further investigation into the spin-dependent properties of these ultimate one-dimensional nanoscale hybrids and may have a significant impact on nanoscale spintronics.

Influence of a Domain Wall on the Spin Polarization of Electric Current

The spin polarization of charge carriers moving in a ferromagnet through a 180° Bloch domain wall is calculated. It is demonstrated that a magnetization component parallel to the electric current and proportional to its strength emerges after the passage through a wall.

Magnetic and Electrical Properties of Hg1–x–MnxFeyTe1–zSz Crystals

Electrical and magnetic properties of the semimagnetic semiconductor Hg1–x–yMnxFeyTe1–zSz solid solutions were studied in the ranges of temperatures 77–320 K and magnetic fields 0.25–6 kOe. In the crystals under study, the effect of giant magnetoresistance was observed that reached 75% at low temperatures. This is due to the fact that charge carriers that participate in the current transfer interact with a magnetized ferromagnetic cluster subsystem (Fe–Fe–Fe) and become spin-polarized. It is these spin-polarized charge carriers that are strongly scattered on antiferromagnetic Mn–S–Mn–S and Mn–Te–Mn–Te clusters, since the magnetic moments inside the clusters and the resulting magnetic moments of these clusters are randomly oriented.