Spin-polarized Holes Research Articles

Designing Organic Spin-Gapless Semiconductors via Molecular Adsorption on C4N3 Monolayer.

Spin-gapless semiconductor (SGS), a class of zero-gap materials with fully spin-polarized electrons and holes, offers significant potential for high-speed, low-energy consumption applications in spintronics, electronics, and optoelectronics. Our first-principles calculations revealed that the Pca21 C4N3 monolayer exhibits a ferromagnetic ground state. Its band structure displays SGS-like characteristics, with the energy gap between the valence and conduction bands near the Fermi level in the spin-down channel much smaller than the one in the other spin channel. To enhance its SGS properties, we introduced electrons into the Pca21 C4N3 monolayer by adsorbing the CO gas molecule on its surface. Stable gas adsorption (CO@C4N3) effectively narrowed the band gap in the spin-down channel without changing the band gap in the spin-up channel obviously. Moreover, injecting holes into the CO@C4N3 system could increase the net magnetic moments and induce an SGS-to-metallic phase transition, while injecting electrons into the CO@C4N3 system is able to lower the net magnetic moments and cause an SGS-to-half-metallic phase transition. Our findings not only underscore a new promising material for practical metal-free spintronics applications but also illustrate a viable pathway for designing SGSs.

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.

Spin-Flip Raman Scattering on Electrons and Holes in Two-Dimensional (PEA)2 PbI4 Perovskites.

The class of Ruddlesden-Popper type (PEA)2 PbI4 perovskites comprises 2D structures whose optical properties are determined by excitons with a large binding energy of about 260meV. It complements the family of other 2D semiconductor materials by having the band structure typical for lead halide perovskites, that can be considered as inverted compared to conventional III-V and II-VI semiconductors. Accordingly, novel spin phenomena can be expected for them. Spin-flip Raman scattering is used here to measure the Zeeman splitting of electrons and holes in a magnetic field up to 10T. From the recorded data, the electron and hole Landé factors (g-factors) are evaluated, their signs are determined, and their anisotropies are measured. The electron g-factor value changes from +2.11 out-of-plane to +2.50 in-plane, while the hole g-factor ranges between -0.13 and -0.51. The spin flips of the resident carriers are arranged via their interaction with photogenerated excitons. Also the double spin-flip process, where a resident electron and a resident hole interact with the same exciton, is observed showing a cumulative Raman shift. Dynamic nuclear spin polarization induced by spin-polarized holes is detected in corresponding changes of the hole Zeeman splitting. An Overhauser field of the polarized nuclei acting on the holes as large as 0.6T can be achieved.

Influence of the cation distribution, atomic substitution, and atomic vacancies on the physical properties of CoFe2O4 and NiFe2O4 spinel ferrites

${\mathrm{CoFe}}_{2}{\mathrm{O}}_{4}$ and ${\mathrm{NiFe}}_{2}{\mathrm{O}}_{4}$ are well-known insulating and ferrimagnetic spinel ferrites with high Curie temperatures, an important characteristic for electronic and spintronic applications. We used first-principles calculations to investigate how their electronic and magnetic properties can be altered or tuned by the presence of structural point defects. We considered successively the effects of cation distribution in the spinel lattice for stoichiometric compounds and of atom substitutions or vacancies. Our calculations demonstrate that a deviation from the perfectly inverse distribution of cations increases the magnetization and decreases the width of the band gap at the Fermi level. In contrast to cation vacancies, oxygen vacancies are not expected to strongly affect the magnetization. We show that ${\mathrm{NiFe}}_{2}{\mathrm{O}}_{4}$ crystals with an excess of Ni cations can display a spin-polarized hole conductivity. We finally calculated the formation energy of the different defects and we give details on their gap states.

Electron Spin Coherence in CdSe Nanocrystals in a Glass Matrix.

The coherent spin dynamics of electrons in CdSe nanocrystals embedded in a glass matrix with diameters from 3.3 up to 6.1 nm are investigated by time-resolved Faraday ellipticity at room and cryogenic temperatures. Only one Larmor precession frequency is detected, which corresponds to the larger of the two precession frequencies and thus g-factor values found in the typical signal from solution-grown colloidal CdSe nanocrystals. We identify this frequency accordingly as associated with the spin precession of resident electrons localized in the nanocrystals in the vicinity of the surface. We provide a detailed theoretical analysis of the exciton level spin structure in the magnetic field and model the spin dynamics in CdSe nanocrystals of different symmetries. This allows us to exclude the exciton as the origin of the experimentally observed oscillating signal. At a cryogenic temperature of 6 K, an additional nonoscillating component emerges in the spin dynamics. We consider several possible origins of this signal and conclude that it is related to the hole spin polarization.

Electric control of valley polarization in monolayer WSe2 using a van der Waals magnet.

Electrical manipulation of the valley degree of freedom in transition metal dichalcogenides is central to developing valleytronics. Towards this end, ferromagnetic contacts, such as Ga(Mn)As and permalloy, have been exploited to inject spin-polarized carriers into transition metal dichalcogenides to realize valley-dependent polarization. However, these materials require either a high external magnetic field or complicated epitaxial growth steps, limiting their practical applications. Here we report van der Waals heterostructures based on a monolayer WSe2 and an Fe3GeTe2/hexagonal boron nitride ferromagnetic tunnelling contact that under a bias voltage can effectively inject spin-polarized holes into WSe2, leading to a population imbalance between ±K valleys, as confirmed by density functional theory calculations and helicity-dependent electroluminescence measurements. Under an external magnetic field, we observe that the helicity of electroluminescence flips its sign and exhibits a hysteresis loop in agreement with the magnetic hysteresis loop obtained from reflective magnetic circular dichroism characterizations on Fe3GeTe2. Our results could address key challenges of valleytronics and prove promising for van der Waals magnets for magneto-optoelectronics applications.

Spin-Selective Hole-Exciton Coupling in a V-Doped WSe2 Ferromagnetic Semiconductor at Room Temperature.

While valley polarization with strong Zeeman splitting is the most prominent characteristic of two-dimensional (2D) transition metal dichalcogenide (TMD) semiconductors under magnetic fields, enhancement of the Zeeman splitting has been demonstrated by incorporating magnetic dopants into the host materials. Unlike Fe, Mn, and Co, V is a distinctive dopant for ferromagnetic semiconducting properties at room temperature with large Zeeman shifting of band edges. Nevertheless, little known is the excitons interacting with spin-polarized carriers in V-doped TMDs. Here, we report anomalous circularly polarized photoluminescence (CPL) in a V-doped WSe2 monolayer at room temperature. Excitons couple to V-induced spin-polarized holes to generate spin-selective positive trions, leading to differences in the populations of neutral excitons and trions between left and right CPL. Using transient absorption spectroscopy, we elucidate the origin of excitons and trions that are inherently distinct for defect-mediated and impurity-mediated trions. Ferromagnetic characteristics are further confirmed by the significant Zeeman splitting of nanodiamonds deposited on the V-doped WSe2 monolayer.

Pressure effect on spin-resonant tunneling of holes in CdTe/Cd1-xMnxTe double-barrier heterostructure

Light Hole (LH) and Heavy Hole (HH) transmission, spin polarization and dwell time are theoretically studied in CdTe/CdMnTe heterostructures for different pressures. The transfer matrix method is used to find the transport properties taking Dresselhaus spin-orbit coupling into account. Sharp resonance peaks with high transmission probability are obtained for HH. A possibility to manipulate the spin-polarized dwell time of holes in this heterostructure using pressure is inferred from the results.

Role of resident electrons in the manifestation of a spin polarization memory effect in Mn delta-doped GaAs heterostructures

The spin-memory effect in the GaAs / InGaAs heterostructures with $\delta$<Mn> layer in GaAs barrier have been investigated. The effect consists in spin polarization of Mn atoms due to interaction with photogenerated spin-polarized holes. The investigation of the effect was carried out by analyzing the polarization of the probe photoluminescence pulse in the pump-probe technique. It was shown that the circular polarization degree of probe pulse generated photoluminescence is strongly affected by the interaction of hole spins with spins of Mn atoms polarized by the pump pulse. The latter leads to decrease of circular polarization degree as compared with single pulse excitation ($\delta P$ effect). The amplitude of $\delta P$-effect is most strongly affected by the concentration of resident electrons in the quantum well which is believed to be due the specific compliance with selection rules for optical transition with the participation of unpolarized resident electrons. The rest of the sample's parameters including the spatial separation between $\delta$<Mn> layer and InGaAs quantum well ($d_s$) have a minor effect on the $\delta P$ value which leads to a paradoxical situation of decreasing $\delta P$-effect with the decrease of $d_s$. The proposed experimental technique consisting in creating the significant concentration of resident electrons in the QW may serve as a reliable photoluminescence method determining the strength of this effect as well as the Mn spin relaxation time in a particular nanostructure.

Deciphering asymmetric charge transfer at transition metal dichalcogenide-graphene interface by helicity-resolved ultrafast spectroscopy.

Transition metal dichalcogenide (TMD)/graphene (Gr) heterostructures constitute a key component for two-dimensional devices. The operation of TMD/Gr devices relies on interfacial charge/energy transfer processes, which remains unclear and challenging to unravel. Fortunately, the coupled spin and valley index in TMDs adds a new degree of freedom to the charges and, thus, another dimension to spectroscopy. Here, by helicity-resolved ultrafast spectroscopy, we find that photoexcitation in TMDs transfers to graphene by asynchronous charge transfer, with one type of charge transferring in the order of femtoseconds and the other in picoseconds. The rate correlates well with energy offset between TMD and graphene, regardless of compositions and charge species. Spin-polarized hole injection or long-lived polarized hole can be achieved with deliberately designed heterostructures. This study shows helicity-resolved ultrafast spectroscopy as a powerful and facile approach to reveal the fundamental and complex charge/spin dynamics in TMD-based heterostructures, paving the way toward valleytronic and optoelectronic applications.

First-principle study of the effects of biaxial strain on the photocatalytic and magnetic mechanisms of ZnO with Sm doping and point defects (VZn, Hi)

The influence of Sm doping on the physical properties of ZnO has been widely investigated. However, the potential point defects and interstitial H in the Sm-doped ZnO system and the influence of strain on the physical properties of ZnO remain poorly understood. First-principle studies on the effects of biaxial strain on the photocatalysis and magnetic mechanisms of ZnO with Sm doping and point defects (VZn, Hi) are rarely reported. In this work, a method based on first-principle plane wave super soft pseudopotential + U under the framework of density functional theory was adopted. A ZnO system with Sm doping and point defects (VZn, Hi) was constructed, and its formation energy, photocatalysis, and magnetic mechanism were studied within the strain range from −6% to 6%. Results showed that regardless of tensile/compressive strain, the formation energy increased in the doped system compared with that in the unstrained doped system. By contrast, stability was relatively decreased. Under the same strain conditions, the Zn34SmHiO36 system had relatively low formation energy and relatively good stability. In addition, interstitial H can effectively increase the separation speed of electron–hole pairs, thereby prolonging the carrier life. Redshift effect, electric dipole moment, oxidation/reduction reactions, and decisive carrier lifetime are affected by the photocatalytic performance of the actual doping system. Under −4% compressive strain, the Zn34SmHiO36 system acts as an excellent catalyst of light. Regardless of tensile/compressive strain, the Zn34SmO36 and Zn34SmHiO36 systems exhibit magnetic properties, and the interstitial H improves the magnetic properties of the doped system. This finding is consistent with the Ruderman–Kittel–Kasuya–Yosida (RKKY) magnetic theory. Under unstrained conditions, the Zn34SmHiO36 system exhibits a semi-metallic characteristic with 100% hole spin polarization. This research has application value for the design and preparation of novel dilute magnetic semiconductors with spin-hole injection source.

Evidence of itinerant holes for long-range magnetic order in the tungsten diselenide semiconductor with vanadium dopants

One primary concern in diluted magnetic semiconductors (DMSs) is how to establish a long-range magnetic order with low magnetic doping concentration to maintain the gate tunability of the host semiconductor, as well as to increase Curie temperature. Several van der Waals semiconductors have been investigated recently to demonstrate the magnetic order in DMSs. Nonetheless, a comprehensive understanding of the operative mechanism has remained elusive. Here, we demonstrate concrete evidence of the long-range magnetic order through itinerant spin-polarized holes in a monolayer V-doped $\mathrm{W}{\mathrm{Se}}_{2}$ semiconductor. Hybridization between V impurity states and the host valence band has been investigated using scanning tunneling microscopy. Transport characteristics reveal the itinerant holes within the hybridized band and hysteresis in magnetoresistance; this clearly resembles the Zener-type description of the itinerant ferromagnetism as predicted by density functional theory calculations.

Chiral-induced spin selectivity enables a room-temperature spin light-emitting diode.

In traditional optoelectronic approaches, control over spin, charge, and light requires the use of both electrical and magnetic fields. In a spin-polarized light-emitting diode (spin-LED), charges are injected, and circularly polarized light is emitted from spin-polarized carrier pairs. Typically, the injection of carriers occurs with the application of an electric field, whereas spin polarization can be achieved using an applied magnetic field or polarized ferromagnetic contacts. We used chiral-induced spin selectivity (CISS) to produce spin-polarized carriers and demonstrate a spin-LED that operates at room temperature without magnetic fields or ferromagnetic contacts. The CISS layer consists of oriented, self-assembled small chiral molecules within a layered organic-inorganic metal-halide hybrid semiconductor framework. The spin-LED achieves ±2.6% circularly polarized electroluminescence at room temperature.

Spin injection sans magnetism

Optoelectronics Light-emitting diodes (LEDs) that emit circularly polarized light (spin-LEDs) have potential applications in in three-dimensional displays, bioencoding, and tomography. The requisite spin polarization of the charge carriers is usually achieved with ferromagnetic contacts and applied magnetic fields, but Kim et al. report on a room-temperature spin-LED that relies instead on a chiral-induced spin selectivity organic layer. This layer selectively injected spin-polarized holes into metal halide perovskite nanocrystals, where they radiatively recombined with unpolarized electrons with an efficiency of 2.6%. Science , this issue p. [1129][1] [1]: /lookup/doi/10.1126/science.abf5291

Investigation of &lt;i&gt;ab initio&lt;/i&gt; nonadiabatic molecular dynamics of excited carriers in condensed matter systems

The excited state dynamics is always an important and challenging problem in condensed matter physics. The dynamics of excited carriers can have different relaxation channels, in which the complicated interactions between different quasi-particles come into play collectively. To understand such ultrafast processes, the &lt;i&gt;ab initio&lt;/i&gt; investigations are essential. Combining the real-time time-dependent density functional theory with fewest switches surface hopping scheme, we develop time-dependent &lt;i&gt;ab initio&lt;/i&gt; nonadiabatic molecular dynamics (NAMD) code Hefei-NAMD to simulate the excited carrier dynamics in condensed matter systems. Using this method, we investigate the interfacial charge transfer dynamics, the electron–hole recombination dynamics, and the excited spin-polarized hole dynamics in different condensed matter systems. Moreover, we combine &lt;i&gt;ab initio&lt;/i&gt; nonadiabatic molecular dynamics with GW plus real-time Bethe-Salpeter equation for the spin-resolved exciton dynamics. We use it to study the spin-valley exciton dynamics in MoS&lt;sub&gt;2&lt;/sub&gt;. It provides a powerful tool for exciton dynamics in solid systems. The state-of-the-art NAMD studies provide a unique insight into a understanding of the ultrafast dynamics of the excited carriers in different condensed matter systems on an atomic scale.

Long-range ferromagnetic ordering in vanadium-doped WSe2 semiconductor

We report long-range ferromagnetic ordering in a vanadium-doped monolayer WSe2 semiconductor using spin-polarized density functional calculations. We found that the vanadium dopant is located in the fully occupied state inside the valence band, inherent from spin–orbit coupling, leading to the presence of free holes in the valence band. As a consequence, the spin-polarized hole carriers are delocalized not only in the vanadium site but also persistently in the tungsten sites distant from vanadium to facilitate the long-range ferromagnetic ordering in the vanadium-doped monolayer WSe2. Our findings of this study pave the way for the future exploration of carrier-mediated room-temperature two-dimensional ferromagnetic semiconductors via magnetic dopants.

Spin-dependent Dirac electrons and valley polarization in the ferromagnetic stanene/ CrI3 van der Waals heterostructure

Realization and regulation of valley polarization is a core issue in the valleytronics fields. Here, through first-principles calculations, we find that the valley polarization (up to 39.6 meV) can be realized in stanene, by stacking it on a monolayer ferromagnetic insulator $\mathrm{Cr}{\mathrm{I}}_{3}$, forming the ferromagnetic stanene/$\mathrm{Cr}{\mathrm{I}}_{3}$ van der Waals heterostructure. The heterostructure possesses a magneto band-structure effect, whereby the spin orientation of Cr atoms can significantly influence valley polarization. When the spin orientation of Cr atoms is parallel (perpendicular) to the $c$ axis, the largest (smallest) valley polarization is achieved with the value of 71.7 meV (1.7 meV). Additionally, the spin-polarized Dirac electrons and hole doping are induced in stanene due to the magnetic proximity effect, and the Dirac cone of stanene is opened with the value of 187.6 meV. These results indicate that the stanene/$\mathrm{Cr}{\mathrm{I}}_{3}$ heterostructure is very promising to be applied in future spintronics and valleytronics.

Room temperature multiferroicity and magnetoelectric coupling in Na-deficient sodium bismuth titanate

Herein, we report a Na deficient single phase sodium bismuth titanate perovskite exhibiting ferromagnetism with reduced leakage conductivity and a self-biased high magnetoelectric (ME) coupling coefficient at room temperature (27 °C). Observed ferromagnetism is induced by sodium cation vacancy dominated by spin polarization of holes in the valence band. An unexpected high ME coupling coefficient of about 4.18 mV/cm Oe, observed at zero DC magnetic bias field, is relatively large and juxtapose to other single phase multiferroics including BiFeO3. This study opens up an altered strategy in designing multiferroic materials with reinforced ME coupling at room temperature.

Spin-Dependent Ionization of Chiral Molecular Films.

Spin selectivity in photo-emission from ferromagnetic substrates functionalized with chiral organic films was analyzed by ultraviolet photoelectron spectroscopy at room temperature. Using radiation with photon energy greater than the ionization potential of the adsorbed molecules, photoelectrons were collected that originated from both underlying ferromagnetic substrates and the organic films, with kinetic energies in the range of ca. 0-18 eV. We investigated chiral organic films composed of self-assembled monolayers of α-helical peptides and electrostatically adsorbed films of the protein, bovine serum albumin, with different α-helix and β-sheet contents. Ultraviolet photoelectron spectral widths were found to depend on substrate magnetization orientation and polarization, which we attribute to helicity-dependent molecular ionization cross sections arising from photoelectron impact, possibly resulting in spin-polarized holes. These interactions between spin-polarized photoelectrons and chiral molecules are physically manifested as differences in the measured photoionization energies of the chiral molecular films. Substrate magnetization-dependent ionization energies and work function values were deconvoluted using surface charge neutralization techniques, permitting the measurement of relative spin-dependent energy barriers to transmission through chiral organic films.

First-principles studies of a two-dimensional electron gas at the interface of polar/polar LaAlO3/KNbO3 superlattices.

We explored the possibility of producing a two-dimensional electron gas (2DEG) in polar/polar (LaAlO3)m/(KNbO3)n perovskite superlattices that have N type and P type interfaces using the first-principles electronic structure calculations. Two different kinds of LaAlO3/KNbO3 superlattices were constructed, namely stoichiometric NP superlattice (NP-SL) and non-stoichiometric NN superlattice (NN-SL). We discovered that the NP-SL undergoes a transition from an insulating to a metallic state when LaAlO3 has more than 3 unit cells. This reveals the completely spin-polarized two-dimensional hole gas (2DHG), as well as 2DEG with an interfacial charge carrier density of n ∼ 1013 cm-2 and an electron effective mass of 0.240me (for 5 unit cells of LaAlO3). In comparison, the NN-SL is intrinsically metallic, and when LaAlO3 has 4.5 unit cells, the structure shows a 2DEG with a larger density (n ∼ 1014 cm-2) and a smaller electron effective mass (0.185me). In addition, the charge carrier properties are highly sensitive to the number of LaAlO3 unit cells in the NP-SL model, while the size effect of LaAlO3 is negligible for the NN-SL one. Our results demonstrate that electronic reconstruction at the interfaces of the stoichiometric structure can produce both the 2DHG and 2DEG, whereas extra electrons are introduced to form solely the 2DEG at the non-stoichiometric structure interfaces. This research provides fundamental insights into the different interfacial electronic properties and the primary mechanism responsible for the formation of polar/polar heterojunction LaAlO3/KNbO3 superlattices.