Spin-polarized States Research Articles

Planar magnetic texture on the surface of a topological insulator

We study the planar magnetic textures coupled to a two-dimensional Dirac surface state, where both the magnetic texture and surface state form in the magnetic topological insulator surface layer. It is shown that the radial vortex with winding number $w=\ifmmode\pm\else\textpm\fi{}1$ leads to the confinement of Dirac states, where a mapping to the Schr\"odinger equation of a two-dimensional hydrogen atom is found. The fully spin-polarized zero-energy bound state forms a flat band that resembles the zeroth Landau level of Dirac electrons in a uniform out-of-plane magnetic field. Remarkably, the number of the zero-energy state is topologically robust. Interestingly, when such a system is proximity coupled to an $s$-wave superconductor, the existence of Majorana zero modes at the Abrikosov vortex depends only on the relative value of the magnetic exchange coupling and the pairing strength. We conclude with a brief discussion on the physical realization with such magnetic textures.

Topological surface states in ultrathin Bi1−xSbx layers

Topological insulator spin-polarized surface states are attractive for spintronic applications, in particular for spin-charge current interconversion, where extremely high conversion efficiencies are predicted. However, the contribution of topologically trivial bulk states is often disregarded although it may play a crucial role in the experimental results and extracted conversion efficiencies. The presence of bulk states at the Fermi level can be avoided by increasing the gap using the confinement effect appearing as the film thickness is reduced. We address this topic by growing ${\mathrm{Bi}}_{1\ensuremath{-}\mathrm{x}}{\mathrm{Sb}}_{x}$ thin films (2.5--15 nm) by molecular beam epitaxy on InSb, ${\mathrm{BaF}}_{2}$, and Si substrates. The surface electronic band structure is studied by angle-resolved photoemission spectroscopy. Two ${\mathrm{Bi}}_{1\ensuremath{-}\mathrm{x}}{\mathrm{Sb}}_{x}$ surface states are observed in the gap for several Sb concentrations and thicknesses, across the topological insulator phase, scanning $x$ between 7% and 30%. Tight-binding calculations of the surface states are in good agreement with the experiments, revealing their polarized nature. Surface states are still present at the \ensuremath{\Gamma} point for the thinnest films (2.5 nm), suggesting highly confined polarized states at the surface.

Spin-polarized hybrid states in epitaxially-aligned and rotated graphene on cobalt

The strong interaction between graphene and elemental ferromagnetic transition metals results in considerable shifts of the graphene π band away from the Fermi level. At the same time, a weakly-dispersing single-spin conical band feature is found in the proximity of the Fermi level at the K̄ point in the surface Brillouin zone of epitaxially-aligned graphene/Co(0001). Here, the robustness of this electronic state against twisting angles at the interface is experimentally and theoretically demonstrated by showing the presence of similar band features also in the case of rotated graphene domains on Co(0001). Spin-resolved reciprocal space maps show that the band feature in rotated graphene has similar Fermi velocity and spin polarization as its counterpart in epitaxially-aligned graphene. Density functional theory simulations carried out for the experimentally observed graphene orientations, reproduce the highly spin-polarized conical band feature at the graphene K̄ point, characterized by a hybrid π-d orbital character. The presence of the conical features in the rotated domains is attributed to the unfolding of the superstructure K̄ point states exclusively to the K̄ point of the graphene primitive cell. The similarities found in the electronic character for different graphene orientations are crucial in understanding the magnetic properties of realistic graphene/Co interfaces, facilitating their implementation in spintronics applications.

Spin-Polarizing Electron Beam Splitter from Crossed Graphene Nanoribbons

Junctions composed of two crossed graphene nanoribbons (GNRs) have been theoretically proposed as electron beam splitters where incoming electron waves in one GNR can be split coherently into propagating waves in two outgoing terminals with nearly equal amplitude and zero back-scattering. Here we scrutinize this effect for devices composed of narrow zigzag GNRs taking explicitly into account the role of Coulomb repulsion that leads to spin-polarized edge states within mean-field theory. We show that the beam-splitting effect survives the opening of the well-known correlation gap and, more strikingly, that a spin-dependent scattering potential emerges which spin polarizes the transmitted electrons in the two outputs. By studying different ribbons and intersection angles we provide evidence that this is a general feature with edge-polarized nanoribbons. A near-perfect polarization can be achieved by joining several junctions in series. Our findings suggest that GNRs are interesting building blocks in spintronics and quantum technologies with applications for interferometry and entanglement.

Boosting photocatalytic activity through tuning electron spin states and external magnetic fields

Photocatalysis is considered as one of the most promising technologies to generate renewable energy and degrade environmental pollutants. Tremendous efforts have been made to improve photocatalytic efficiency. Among these, tuning spin polarization and introducing an external magnetic field are considered two promising strategies to boost photocatalytic performance. Herein this review highlights the recent breakthroughs through manipulating spin states and applying external magnetic fields for enhancing photocatalytic reactions. The relevant characterization techniques and fundamental mechanisms are summarized. Spin polarization states of photocatalysts have received considerable attention due to their unique roles, including inhibiting the recombination of photoexcited carriers owing to spin orientation constraint, enhancing the reaction product selectivity, and reducing the reaction barriers via optimizing the absorption energy and binding strength. As for the effects of external magnetic field on photocatalytic performance, we mainly discuss the separation enhancement of photoinduced carriers under static and time-varying magnetic fields and the magneto-hydrodynamic effect of charged particles. Lastly, the negative magnetoresistance effect is discussed due to the synergistic effects of the electron spin state and an external magnetic field. These discussions in this review may provide new insights into designing new semiconductors for boosting photocatalytic performance in internal and external magnetic fields.

Quantum Spin Hall Edge States and Interlayer Coupling in Twisted Bilayer WTe2.

The quantum spin Hall (QSH) effect, characterized by topologically protected spin-polarized edge states, was recently demonstrated in monolayers of the transition metal dichalcogenide (TMD) WTe2. However, the robustness of this topological protection remains largely unexplored in van der Waals heterostructures containing one or more layers of a QSH insulator. In this work, we use scanning tunneling microscopy and spectroscopy (STM/STS) to explore the topological nature of twisted bilayer (tBL) WTe2. At the tBL edges, we observe the characteristic spectroscopic signatures of the QSH edge states. For small twist angles, a rectangular moiré pattern develops, which results in local modifications of the band structure. Using first-principles calculations, we quantify the interactions in tBL WTe2 and its topological edge states as a function of interlayer distance and conclude that it is possible to engineer the topology of WTe2 bilayers via the twist angle as well as interlayer interactions.

Electronic and magnetic properties of tripentaphene nanoribbons

Tripentaphenes (TPHs) are theoretically proposed two-dimensional nanocarbons resulting from the assembly of acepentalene-like units. They exhibit a metallic behavior, as well as chemical resonance mechanisms governing their structural properties. Here we computationally and theoretically investigate the electronic properties emerging from quantum confinement when TPHs are cut into nanoribbons. We use density functional theory to investigate the influence of edge chirality and width on the electronic properties of tripentaphene nanoribbons that are fully edge passivated with hydrogen. We show that these systems are usually metallic with frontier states spread over the inner region of the structure. We also find a number of semiconducting and half-metallic systems related to the presence of spin-polarized states. These findings are meaningful to understand the relationship between physical properties and details of the atomic structure on TPH nanoribbons that can be tuned for targeted applications in nanoelectronics and spintronics.

Emerging two-dimensional magnetism in nonmagnetic electrides Hf2X (X=S, Se, Te)

Recent experimental discoveries of two-dimensional (2D) magnets have triggered intense research activities to search for atomically thin magnetic systems. Using first-principles calculations, we predict the emergence of 2D magnetism in the monolayers (MLs), few layers, and surfaces of nonmagnetic layered electrides ${\mathrm{Hf}}_{2}X$ $(X=\text{S}, \mathrm{Se}, \mathrm{Te})$ consisting of three-atom-thick $\mathrm{Hf}\text{\ensuremath{-}}X\text{\ensuremath{-}}\mathrm{Hf}$ stacks. It is revealed that each bulk ${\mathrm{Hf}}_{2}X$ hosts a quantum state of Dirac nodal lines with a high density of states arising from $\mathrm{Hf}\text{\ensuremath{-}}5d$ cationic and interlayer anionic electrons around $\ensuremath{-}0.9 \mathrm{eV}$ below the Fermi level ${E}_{F}$. However, for the MLs, few layers, and surfaces of ${\mathrm{Hf}}_{2}X$, such hybridized states are shifted toward ${E}_{F}$ to generate van Hove singularities, leading to a Stoner instability. The resulting surface ferromagnetism gives rise to strongly spin-polarized topological surface states at ${\mathrm{Hf}}_{2}X(001)$, demonstrating that anionic electrons, 2D magnetism, and band topology are entangled with each other. Our findings will open different perspectives for the discovery of 2D magnets via exploiting surface effects in nonmagnetic layered electrides.

Isospin competitions and valley polarized correlated insulators in twisted double bilayer graphene

New phase of matter usually emerges when a given symmetry breaks spontaneously, which can involve charge, spin, and valley degree of freedoms. Here, we report an observation of new correlated insulators evolved from spin-polarized states to valley-polarized states in twisted double bilayer graphene (TDBG) driven by the displacement field (D). At a high field |D | > 0.7 V/nm, we observe valley polarized correlated insulators with a big Zeeman g factor of ~10, both at v = 2 in the moiré conduction band and more surprisingly at v = −2 in the moiré valence band. Moreover, we observe a valley polarized Chern insulator with C = 2 emanating at v = 2 in the electron side and a valley polarized Fermi surface around v = −2 in the hole side. Our results demonstrate a feasible way to realize isospin control and to obtain new phases of matter in TDBG by the displacement field, and might benefit other twisted or non-twisted multilayer systems.

Electronic properties of carbon sheets and nanoribbons based on acepentalene-like building blocks

Two-dimensional allotropes of carbon are the subject of intense research in an effort to fine tune nanocarbon’s physical properties for their insertion into operational nanoscale devices. C57 and C65 lattices are examples of such proposed networks. They are hypothetically formed by the concatenation of acepentalene blocks and are shown to display a metallic behavior. Here, we use density functional theory to investigate the electronic properties of ribbons whose lattices are composed of these two 2D nanocarbons. These systems share the common feature with their parent structures of displaying a metallic behavior. However, they are also found to host spin-polarized states, thereby offering opportunities for their applications in spintronics. Furthermore, one of the 2D parent structures is found to also allow a non-trivial spin distribution, as well as corrugated phases, which was not previously reported for this system. Finally, the structural and electronic properties calculated for the C57 and C65 systems are rationalized in terms of a resonance model.

First principles calculations of the magnetic properties of PbTi1−xMnxO3

Electronic, optical, and magnetic properties of the Mn-doped PbTiO3 compound were investigated by using density functional theory (DFT). We revealed that this compound has an indirect band gap of 3.01 eV. The calculated complex dielectric functions were in good agreement with the available experimental results. Our calculation demonstrated that Mn substitution changes the nonspin-polarized state of pure PbTiO3 to the spin-polarized state. O and Pb vacancies do not exhibit magnetism. However, Ti vacancies cause magnetism in the crystal, and the calculated magnetic moment was 1.68 µB.

Intricate interplay between superconductivity and topological surface states of c axis oriented MoTe2

The interaction between superconductivity and spin-polarized surface states of topological materials provides an exciting platform for the research and development of proximity induced coupling effects, Majorana fermions, spin valves, spintronics, etc. and so on. In this work, the inverse proximity effect observed exactly at the super conducting transition temperature of indium (3.5 K) demonstrates the complex interplay between robust 2D spin-polarized surface states observed in our (002n) oriented MoTe2 nanolayer sheets with that of superconducting states. Interestingly, our phenomenological model based on the Werthamer-Helfand-Hohenberg (WHH) model and Ginzburg–Landau formalism, invoked to validate the experimental observations, indicates a competition between superconductivity and topological order, marked by a close correspondence between the temperature of crossover (Tcr = 2.45 K) of their respective length scales, ξ and Lφ, and the saturation temperature in resistivity.

Molecular rectification assisted by spin-polarized hybrid interfacial states

Based on first-principles method, we construct molecular rectifiers by designing asymmetric hybrid interfacial states. The devices contain a ferromagnetic interface with sp3-d hybrid interfacial states and a nonmagnetic interface without hybrid interfacial states. The transport calculation demonstrates that obvious charge-current rectification and spin-current rectification are realized. The analysis of spin-resolved transmission reveals that the asymmetric shift of hybrid interfacial states plays an important role in determining the rectification ratio as well as the spin polarization orientation of the spin current, and hence the form of spin-current rectification. The effect of molecular length is further discussed. A length-induced increase of rectification ratio is observed for the charge-current rectification, while the largest spin-current rectification ratio appears at a specific length. This work demonstrates the potential of hybrid interfacial states in designing excellent molecular spinterface devices.

Emergence of topological and trivial interface states in VSe2 films coupled to Bi2Se3

Coupling ordinary metals with topological ${\mathrm{Bi}}_{2}{\mathrm{Se}}_{3}$ can instigate the long-range migration of the spin-polarized Dirac states. Instead, for trivial metallic $\mathrm{V}{\mathrm{Se}}_{2}$ films on ${\mathrm{Bi}}_{2}{\mathrm{Se}}_{3}$, topological and trivial Rashba-type interface states emerge, each strongly localized at the $\mathrm{V}{\mathrm{Se}}_{2}/{\mathrm{Bi}}_{2}{\mathrm{Se}}_{3}$ interface. Their rapidly decaying spectral weights are uncovered by thickness-dependent band mappings of $\mathrm{V}{\mathrm{Se}}_{2}$ films and replicated by a phenomenological first-principles model of the spectral function. The results are pertinent to spintronic devices requiring spin transmission across metal/topological insulator interfaces.

Quantum simulator of extended bipartite Hubbard model with broken sublattice symmetry: Magnetism, correlations, and phase transitions

We describe here a quantum simulator of extended bipartite Hubbard model with broken sublattice symmetry. The simulator consists of a structured lateral gate confining two dimensional electrons in a quantum well into artificial minima arranged in a hexagonal lattice. The sublattice symmetry breaking is generated by forming an artificial triangular graphene quantum dot (ATGQD) with zigzag edges. The resulting extended Hubbard model generates tunable ratio of tunneling strength to electron-electron interactions and of sublattice symmetry with control over shape. The validity of the simulator is confirmed for small systems using mean-field and exact diagonalization many-body approaches which show that the ground state changes from a metallic to an antiferromagnetic (AF) phase by varying the distance between sites or depth of the confining potential. The one-electron spectrum of these triangular dots contains a macroscopically degenerate shell at the Fermi level. The shell persists at the mean-field level for weak interactions (metallic phase) but disappears for strong interactions, in the AF phase. We determine the effects of electron-electron interactions on the ground state, the total spin, and the excitation spectrum as a function of filling of the ATGQD. We find that the half-filled charge neutral shell leads to a partially spin polarized state in both metallic and AF regimes in accordance with Liebs theorem. In both regimes a relatively large gap separates the spin polarized ground state to the first excited many-body state at half filling of the degenerate shell. By adding or removing an electron, this gap drops dramatically, and alternate total spin states emerge with energies nearly degenerate to a spin polarized ground state.

Perfect dual spin filtering effect and large magnetoresistance in all-carbon devices based on C18 cyclo molecule from first principles

The carbon-based nanomaterial would push silicon aside and create a post- complementary metal oxide semiconductor world. In this work, we proposed and theoretically investigated an all-carbon spintronic device composed of C18 cyclo molecule and two zigzag graphene nanoribbon (ZGNR) electrodes with two different sturdy connection modes. It is found that both structures have similar excellent spin-dependent properties. Adjusting the magnetization configuration of two electrodes to be anti-parallel, electrons in different spin states are filtered unidirectionally according to the sign of the applied bias voltage. The molecular junction exhibits a perfect dual spin filtering effect. Moreover, a huge magneto-resistance ratio was found in our propose junctions. The matching of symmetry type of the spin-polarized states in two ZGNR electrodes, as well as the states of C18 in central region plays an important role in the occurrence of these effects.

Unveiling the Multiradical Character of the Biphenylene Network and Its Anisotropic Charge Transport.

Recent progress in the on-surface synthesis and characterization of nanomaterials is facilitating the realization of new carbon allotropes, such as nanoporous graphenes, graphynes, and 2D π-conjugated polymers. One of the latest examples is the biphenylene network (BPN), which was recently fabricated on gold and characterized with atomic precision. This gapless 2D organic material presents uncommon metallic conduction, which could help develop innovative carbon-based electronics. Here, using first principles calculations and quantum transport simulations, we provide new insights into some fundamental properties of BPN, which are key for its further technological exploitation. We predict that BPN hosts an unprecedented spin-polarized multiradical ground state, which has important implications for the chemical reactivity of the 2D material under practical use conditions. The associated electronic band gap is highly sensitive to perturbations, as seen in finite temperature (300 K) molecular dynamics simulations, but the multiradical character remains stable. Furthermore, BPN is found to host in-plane anisotropic (spin-polarized) electrical transport, rooted in its intrinsic structural features, which suggests potential device functionality of interest for both nanoelectronics and spintronics.

Scanning tunneling microscopy and spectroscopy of NiTe2

Nickel ditelluride is an unusual member of the transition metal dichalcogenide family which has garnered interest due to potential valley spin-polarized surface states near the Fermi level and the presence of Dirac nodes in its electronic band structure. In this work, exfoliation of bulk nickel ditelluride is performed under ultra-high vacuum in order to generate a clean surface for scanning tunneling microscopy and spectroscopy at 4.8 K. Multiple features in the observed electronic density of states are observed in the vicinity of the Fermi level and compared to calculated band structures to elucidate their origins. Our results are consistent with the presence of the spin-polarized surface states, yet indicate trivial states, which are close to the Fermi level, can interfere with their potential utility in spintronic applications.

Site-dependent spin-polarized tunneling via hybrid interface states on molecule/ferromagnet surface

Based on density functional theory and nonequilibrium Green's function method, we investigate the spin polarization and transport ability of hybrid interface states at benzene/Ni surface by introducing the top probe electrode. The results demonstrate that when the top electrode connects the molecule with different sites, the spin polarization of the projected density of states and the interfacial spin density distribution are disturbed in different ways. A contact site induced inversion of the spin polarization is observed at Fermi energy. The transport calculation indicates that the center contact is better to achieve a large tunneling magnetoresistance at low bias, but the edge and top contacts is useful to get a little larger spin polarization of the current. This work points out that the spin polarization and transport property of the hybrid interface states in a molecular junction depends on the way to introduce the second probe electrode, which can be suitable designed to achieve an excellent performance.

Acoustic-phonon-mediated superconductivity in Bernal bilayer graphene

We present a systematic theory of acoustic-phonon-mediated superconductivity, which incorporates Coulomb repulsion, explaining the recent experiment in Bernal bilayer graphene under a large displacement field. The acoustic-phonon mechanism predicts that $s$-wave spin-singlet and $f$-wave spin-triplet pairings are degenerate and dominant. Assuming a spin-polarized valley-unpolarized normal state, we obtain $f$-wave spin-triplet superconductivity with a $T_c\sim 20$ mK near $n_e=-0.6\times 10^{12}$ cm$^{-2}$ for hole doping, in approximate agreement with the experiment. We further predict the existence of superconductivity for larger doping in both electron-doped and hole-doped regimes. Our results indicate that the observed spin-triplet superconductivity in Bernal bilayer graphene arises from acoustic phonons.