Spin-polarized States Research Articles

Vacancy-Induced Magnetism in Fluorographene: The Effect of Midgap State.

Based on density functional theory, we have systematically investigated the geometric, magnetic, and electronic properties of fluorographene with three types of vacancy defects. With uneven sublattice, the partial defect structures are significantly spin-polarized and present midgap electronic states. The magnetic moment is mainly contributed by the adjacent C atoms of vacancy defects. Furthermore, the strain dependence of the bandgap is analyzed and shows a linear trend with applied strain. This defect-induced tunable narrow bandgap material has great potential in electronic devices and spintronics applications.

The role of spin-lattice coupling for ultrafast changes of the magnetic order in rare earth metals

By comparing femtosecond laser-pulse-induced spin dynamics in the surface state of the rare earth metals Gd and Tb, we show that the spin polarization of valence states in both materials decays with significantly different time constants of 15 ps and 400 fs, respectively. The distinct spin polarization dynamics in Gd and Tb are opposed by similar exchange splitting dynamics in the two materials. The different time scales observed in our experiment can be attributed to weak and strong 4f spin to lattice coupling in Gd and Tb, suggesting an intimate coupling of spin polarization and 4f magnetic moment. While in Gd the lattice mainly acts as a heat sink, it contributes significantly to ultrafast demagnetization of Tb. This helps explain why all optical switching is observed in FeGd—but rarely in FeTb-based compounds.

Point and complex defects in monolayer PdSe2 : Evolution of electronic structure and emergence of magnetism

Layered transition-metal dichalcogenides (TMDs) constitute an emerging class of materials that provide researchers a platform to realize fundamental studies and to design promising optoelectronic devices. While defects are an almost unavoidable part of TMDs, they bring additional interesting properties absent in defect-free layers. Moreover, the controlled introduction of defects in TMDs makes it possible to tailor the electromagnetic properties of the materials. Here we report defect-induced properties of single-layer $\mathrm{Pd}{\mathrm{Se}}_{2}$ and demonstrate the emergence of magnetism at the nanoscale. Our first-principle calculations indicate that Se vacancies create in-gap defect states, which can be responsible for trapping of carriers. The complex square ${V}_{\mathrm{Pd}+4\mathrm{Se}}$ vacancy induces spin-polarized states with a total local magnetic moment of $2\phantom{\rule{0.16em}{0ex}}{\ensuremath{\mu}}_{\mathrm{B}}$ per defect, making it possible to introduce magnetization into $\mathrm{Pd}{\mathrm{Se}}_{2}$ and therefore expand the family of two-dimensional (2D) magnets. The defect formation energies are much lower compared to many other TMD materials that can explain the presence of a large number of Se defects after mechanical exfoliation of $\mathrm{Pd}{\mathrm{Se}}_{2}$ layers, while the central location of the Pd atoms preserves them from exfoliation-induced defect formation. The negatively charged vacancies are prone to form and in many cases demonstrate spin-polarized states. The small diffusion barrier of ${\mathrm{V}}_{\mathrm{Se}}$ in 2D $\mathrm{Pd}{\mathrm{Se}}_{2}$ leads to an easy room-temperature migration, while ${\mathrm{V}}_{\mathrm{Pd}}$ demonstrates a high diffusion barrier preventing its spontaneous migration. As a guide for experimentalists, we simulate and characterize scanning tunneling microscope images in valence and conduction states and estimate the electron-beam energy for a controllable production of various defect patterns. These intriguing findings make $\mathrm{Pd}{\mathrm{Se}}_{2}$ an ideal platform to study defect-induced phenomena.

Theoretical investigation of the effect of native defects on spin polarization and antiferromagnetic state in Co3O4

The effect of the native defects on the spin polarization and the antiferromagnetic state of Co3O4 was investigated using the first-principles calculations. The results reveal that the AFM ground state of Co3O4 is changed to a ferromagnetic state by introducing one OCo antisite defect at the A/B site, while O vacancy can induce the highest spin polarization and most stable AFM state. The interactions between two O vacancies with different distances are all AFM interactions and the strength of AFM interaction is strongest at the distance of 2.85 Å. The theoretical investigation suggests an effective route to improve the spin polarization and stability of AFM state by defect engineering, promoting the applications of Co3O4 in antiferromagnetic spintronics.

Weyl Monoloop Semi-Half-Metal and Tunable Anomalous Hall Effect.

Nodal monoloop, enjoying the cleanest scenario with a single loop, is recognized as the basic building block of intricate linked loops including chains, nets, and knots. Here, we explore the interplay of magnetic ordering and band topology in one system by introducing a brand-new quantum state, referred to as Weyl monoloop semi-half-metal, which is characterized by a single loop at the Fermi level stemming from the same spin channel. Such a nodal line Fermion, yielding 100% spin polarization, is protected by mirror () symmetry. As a prominent example, a realistic rutile-type metal fluorides LiV2F6 achieves the hitherto unmaterialized state, featuring fully spin-polarized ultraflat surface states. More interestingly, LiV2F6 has a "soft" ferromagnetic property, which is one of the desired systems to control the anomalous Hall effect by rotating the magnetization direction. Our findings offer a promising candidate for exploring the topology and magnetism with intriguing effects.

Silicene-Based Spin Filter With High Spin-Polarization

In this article, for the first time, we propose a silicene-based spin filter having a TFET configuration. The device portrays a voltage-dependent spin polarization and achieves a “spin polarization” as high as 98% at a minimal “operating voltage” (0.35 V). The operation of the device is based on the “spin-polarized edge states” in a silicene nanoribbon. The “spin polarization” of the output current can be tuned by modifying the bandgap of the spin-up and spin-down states via an in-plane perpendicular “electric field” generated by two “lateral gates.” The device features high spin polarization apart from its expected integration with Si-based technology. The proposed device has a simple construction, wherein a single layer of Si atoms achieves such a high value of spin polarization at a minimal operating voltage. The operation of the proposed device is demonstrated by using the first-principles quantum transport simulations.

Lattice flexibility in Ca3Ru2O7 : Control of electrical transport via anisotropic magnetostriction

Ca3Ru2O7 is a correlated and spin-orbit-coupled system with an extraordinary anisotropy. It is both interesting and unique largely because this material exhibits conflicting phenomena that are often utterly inconsistent with traditional precedents, particularly, the quantum oscillations in the nonmetallic state and colossal magnetoresistivity achieved by avoiding a fully spin-polarized state. This work focuses on the relationship between the lattice and transport properties along each crystalline axis and reveals that application of magnetic field, H, along different crystalline axes readily stretches or shrinks the lattice in a uniaxial manner, resulting in distinct electronic states. Furthermore, application of modest pressure drastically amplifies the anisotropic magnetoelastic effect, leading to either an occurrence of a robust metallic state at H || hard axis or a reentrance of the nonmetallic state at H || easy axis. Ca3Ru2O7 presents a rare lattice-dependent magnetotransport mechanism, in which the extraordinary lattice flexibility enables an exquisite control of the electronic state via magnetically stretching or shrinking the crystalline axes, and the spin polarization plays an unconventional role unfavorable for maximizing conductivity. At the heart of the intriguing physics is the anisotropic magnetostriction that leads to exotic states.

Diverse Spin-Polarized In-Gap States at Grain Boundaries of Rhenium Dichalcogenides Induced by Unsaturated Re–Re Bonding

Diverse Spin-Polarized In-Gap States at Grain Boundaries of Rhenium Dichalcogenides Induced by Unsaturated Re–Re Bonding

Chirality-Induced Spin Polarization over Macroscopic Distances in Chiral Disilicide Crystals.

A spin-polarized state is examined under charge current at room temperature without magnetic fields in chiral disilicide crystals NbSi_{2} and TaSi_{2}. We found that a long-range spin transport occurs over ten micrometers in these inorganic crystals. A distribution of crystalline grains of different handedness is obtained via location-sensitive electrical transport measurements. The sum rule holds in the conversion coefficient in the current-voltage characteristics. A diamagnetic nature of the crystals supports that the spin polarization is not due to localized electron spins but due to itinerant electron spins. A large difference in the strength of antisymmetric spin-orbit interaction associated with 4d electrons in Nb and 5d ones in Ta is oppositely correlated with that of the spin polarization. A robust protection of the spin polarization occurs over long distances in chiral crystals.

Hybridization-Induced Inversion of Spin Polarization at Rubrene/Ferromagnetic Cobalt Interface

The formation of hybrid interface states plays a crucial role in spin injection from a ferromagnetic metal into organic semiconductors. Here, we investigate the energy-level alignment and spin-polarized hybrid interface states between cobalt and rubrene using photoemission spectroscopy. We reveal that the presence of charge injection from cobalt to the rubrene molecular layer is determined by XPS and UPS measurements. We also find a spin-polarized interface state between cobalt and rubrene at 0.5 eV below the Fermi level, which can be attributed to chemical interaction between rubrene and Co(001). This hybridization state can induce spin polarization inversion at the rubrene/Co(001) interface as measured by spin-polarized UPS. The spin polarization inversion at the rubrene/Co(001) interface can be interpreted by a Zener exchange-type mechanism. On the basis of these observations, we obtain a deeper understanding of the electronic structure of the rubrene/Co(001) spinterface and show the importance for the quantitative description of rubrene/Co(001)-based organic spintronic devices.

Reentrant Bloch ferromagnetism

An interacting electron liquid in two (2D) and three (3D) dimensions may undergo a paramagnetic-to-ferromagnetic quantum spin polarization transition at zero applied magnetic field, driven entirely by exchange interactions, as the system density ($n$) is decreased. This is known as Bloch ferromagnetism. We show theoretically that the application of an external magnetic field ($B$), which directly spin polarizes the system through Zeeman spin splitting, has an interesting effect on Bloch ferromagnetism if the applied field and carrier density are both decreased (from some initial applied high magnetic field at a high carrier density) in a power-law manner, $B\ensuremath{\sim}{n}^{p}$. For $p<{p}_{c}$, with ${p}_{c}=1(2/3)$ in $2(3)\mathrm{D}$, the system remains either fully spin polarized or undergoes a single transition from a partially spin-polarized (with two Fermi surfaces corresponding to spin up and down electrons) to a fully spin-polarized state (with a single Fermi surface of one spin) as the density and field decrease, depending on whether the starting point is partially spin polarized or fully spin polarized. However, for $p>{p}_{c}$, the system may undergo two transitions if starting from the fully spin-polarized state: first, a weak second-order transition at high density and field from the field-induced fully polarized phase to the partially polarized phase; and then, at a lower field and density, a reentrant first-order transition back to the fully spin-polarized phase again with a single Fermi surface.

Optomagnonic Josephson effect in antiferromagnets

Combining advanced technologies of optics and antiferromagnetic spintronics, we present a method to realize ultrafast spin transport. The optical Barnett effect provokes quasiequilibrium Bose-Einstein condensates (BECs) of magnons associated with the fully spin-polarized state in insulating antiferromagnets (AFs). This optomagnonic Barnett effect enables us to exploit coherent magnons of high frequency over the conventional ones of (sub-) terahertz associated with the N\'eel magnetic order. We show that the macroscopic coherence of those optical magnon BECs induces a spin current across the junction interface of weakly coupled two insulating AFs, and this optomagnonic Josephson effect realizes ultrafast spin transport. The period of the optomagnonic Josephson oscillation is much shorter than the conventional one of the order of picoseconds. Thus we propose a way to realize ultrafast spin transport in AFs by means of the macroscopic coherence of optical magnon BECs.

Multichannel Depletion-Type Field-Effect Transistor Based on Ferromagnetic Germanene

Based on ferromagnetic germanene, we theoretically propose a multichannel depletion-type field-effect transistor (FET) operated by a vertical electric field. The electron transport properties of the topological transistor are investigated through the nonequilibrium Green's function method. The results reveal that new edge channels can be formed by removing some $\mathrm{Ge}$ atoms along the $x$ axis of the device. The proposed multichannel ferromagnetic germanene transistor has a twice on-state current and half subthreshold swing than that of traditional germanene FETs. Under the coaction of electric induced staggered potential and staggered exchange field, both the $100\mathrm{%}$ spin-polarized edge states and valleys can be achieved. The ferromagnetic germanene FET can be turned off just by adjusting the electric field. In the low electric field conditions, the switching-off mechanism is due to spin blocking caused by the edge-state mismatch, while for relatively high electric field cases, both the edge-state mismatch and energy-valley mismatch are utilized. With the moderate increase in the exchange field, the switching mechanism is not changed, but the threshold electric field and breakdown voltage of the FET can be, respectively, reduced and enhanced significantly. All the results indicate that the proposed ferromagnetic germanene FET is a promising candidate for ultra-low-power dissipation topological devices.

Controllable spin pairing states in silicene-based superconducting hybrid structures with noncollinear magnetizations

We have theoretically investigated the modulation between the opposite- and equal-spin pairings and the related transport properties in silicene-based ferromagnet/ferromagnet/superconductor hybrid structures with noncollinear magnetizations. Due to the exotic electronic properties of silicene, the exclusive fully spin-polarized equal-spin pairing state appears without any contamination from the opposite-spin pairing state in a perpendicular magnetic configuration. Furthermore, the switch effect between fully spin-polarized opposite- and equal-spin pairings can be realized by tuning the Fermi level. In addition, the fully spin-polarized equal-spin pairing correlation can be enhanced by modulation of the magnitude and orientation of the exchange field in the central region. It is also a significant finding that for the formation of the fully spin-polarized equal-spin pairing state, the length of the central layer can be taken to be large, but not for opposite-spin pairing. Our findings provide an ideal platform to explore fully spin-polarized opposite- and equal-spin pairing states separately.

Magnetism and optical properties of LiNbO3 doped with (Fe,Ni,Ga): First-principles calculations

Based on the first principles of the generalized gradient approximation method, the magnetic and optical properties of Fe-, Ni-doped and (Fe,Ni) co-doped LiNbO3 crystals are studied. The results show that the mono-doped LiNbO3 crystals contain magnetic atoms (Fe,Ni) leading to spin polarization, and (Fe,Ni) co-doped LiNbO3 crystals tend to the ferromagnetic state with △EFM of −143 meV. The total magnetic moment of the (Fe,Ni) co-doped LiNbO3 crystal is 5.97 μB, and the local magnetic moments of Fe and Ni are 2.99 μB and 0.93 μB, respectively. It is interesting that the introduction of the Ga atom makes the ferromagnetic state (△EFM of −285.4 meV) of the (Fe,Ni) co-doped LiNbO3 system more stable than the anti-ferromagnetic state. Although Ga atoms do not contribute to the magnetic moment, the (Fe,Ni,Ga) co-doped LiNbO3 is more conducive to the spin-polarized state. In addition, with the introduction of Fe, Ni, and Ga atoms, the visible region absorption changes significantly. Non-volatile holographic storage can be achieved by (Fe,Ni) co-doped LiNbO3, thus increasing the absorption of the visible light range. In summary, this research provides theoretical guidance for the application of LiNbO3 crystals in spintronic devices.

1D topological systems for next-generation electronics

1D topological systems for next-generation electronics

Bench-Top Cooling of a Microwave Mode Using an Optically Pumped Spin Refrigerator.

We experimentally demonstrate the temporary removal of thermal photons from a microwave mode at 1.45GHz through its interaction with the spin-polarized triplet states of photo-excited pentacene molecules doped within a p-terphenyl crystal at room temperature. The crystal functions electromagnetically as a narrowband cryogenic load, removing photons from the otherwise room-temperature mode via stimulated absorption. The noise temperature of the microwave mode dropped to 50_{-32}^{+18} K (as directly inferred by noise-power measurements), while the metal walls of the cavity enclosing the mode remained at room temperature. Simulations based on the same system's behavior as a maser (which could be characterized more accurately) indicate the possibility of the mode's temperature sinking to ∼10 K (corresponding to ∼140 microwave photons). These observations, when combined with engineering improvements to deepen the cooling, identify the system as a narrowband yet extremely convenient platform-free of cryogenics, vacuum chambers, and strong magnets-for realizing low-noise detectors, quantum memory, and quantum-enhanced machines (such as heat engines) based on strong spin-photon coupling and entanglement at microwave frequencies.

Layer-locked spin states revealed in the centrosymmetric nodal-line semimetal HfSiS

The spin-polarized bulk states in the centrosymmetric nodal-line semimetal HfSiS were observed by spin- and angle-resolved photoelectron spectroscopy. Combining with density functional theory calculations and the tight-binding model, the layer-locked spin states' nature of the observed $\mathrm{\ensuremath{\Lambda}}$-shaped bulk bands was revealed and the tight-binding model shows the importance of the next-nearest-neighbor term of spin-orbit coupling in the spin reversal of the bands along the $\overline{\mathrm{\ensuremath{\Gamma}}}\text{\ensuremath{-}}\overline{\mathit{M}}$ direction. Furthermore, the topological essence of attractive Dirac node arc states was confirmed.

Observation of a giant mass enhancement in the ultrafast electron dynamics of a topological semimetal

Topological phases of matter offer exciting possibilities to realize lossless charge and spin information transport on ultrafast time scales. However, this requires detailed knowledge of their nonequilibrium properties. Here, we employ time-, spin- and angle-resolved photoemission to investigate the ultrafast response of the Sb(111) spin-polarized surface state to femtosecond-laser excitation. The surface state exhibits a giant mass enhancement which is observed as a kink structure in its energy-momentum dispersion above the Fermi level. The kink structure, originating from the direct coupling of the surface state to the bulk continuum, is characterized by an abrupt change in the group velocity by ~70%, in agreement with our GW-based band structure calculations. Our observation of this connectivity in the transiently occupied band structure enables the unambiguous experimental verification of the topological nature of the surface state. The influence of bulk-surface coupling is further confirmed by our measurements of the electron dynamics, which show that bulk and surface states behave as a single thermalizing electronic population with distinct contributions from low-k electron-electron and high-k electron-phonon scatterings. These findings are important for future applications of topological semimetals and their excitations in ultrafast spintronics.

Field-Induced Superconductivity near the Superconducting Critical Pressure in UTe2

We report the magnetoresistance in the novel spin-triplet superconductor UTe2 under pressure close to the critical pressure Pc, where the superconducting phase terminates, for field along the three a, b and c-axes in the orthorhombic structure. The superconducting phase for H // a-axis just below Pc shows a field-reentrant behavior due to the competition with the emergence of magnetic order at low fields. The upper critical field Hc2 for H // c-axis shows a quasi-vertical increase in the H-T phase diagram just below Pc, indicating that superconductivity is reinforced by the strong fluctuations which persist even at high fields above 20T. Increasing pressure leads to the disappearance of superconductivity at zero field with the emergence of magnetic order. Surprisingly, field-induced superconductivity is observed at high fields, where a spin-polarized state is realized due to the suppression of the magnetic ordered phases; the spin-polarized state is favorable for superconductivity, whereas the magnetic ordered phase at low field seems to be unfavorable. The huge Hc2 in the spin-polarized state seems to imply a spin-triplet state. Contrary to the a- and c-axes, no field-reinforcement of superconductivity occurs for magnetic field along the b-axis. We compare the results with the field-reentrant superconductivity above the metamagnetic field, Hm for the field direction tilted by about 30 deg. from b to c-axis at ambient pressure as well as the field-reentrant (-reinforced) superconductivity in ferromagnetic superconductors, URhGe and UCoGe.