Spin-Gapless Semiconductors

Spin Gapless Quantum Materials and Devices

Quantum materials, with nontrivial quantum phenomena and mechanisms, promise efficient quantum technologies with enhanced functionalities. Quantum technology is held back because a gap between fundamental science and its implementation is not fully understood yet. In order to capitalize the quantum advantage, a new perspective is required to figure out and close this gap. In this review, spin gapless quantum materials, featured by fully spin-polarized bands and the electron/hole transport, are discussed from the perspective of fundamental understanding and device applications. Spin gapless quantum materials can be simulated by minimal two-band models and could help to understand band structure engineering in various topological quantum materials discovered so far. It is explicitly highlighted that various types of spin gapless band dispersion are fundamental ingredients to understand quantum anomalous Hall effect. Based on conventional transport in the bulk and topological transport on the boundaries, various spintronic device aspects of spin gapless quantum materials as well as their advantages in different models for topological field effect transistors are reviewed.

Genuine Dirac Half-Metals in Two-Dimensions

When spin-orbit coupling (SOC) is absent, all proposed half-metals with twofold degenerate nodal points at the K (or K') point in 2D materials are classified as "Dirac half-metals" owing to the way graphene is utilized in the earliest studies. Actually, each band crossing point at the K or K' point is described by a 2D Weyl Hamiltonian with definite chirality; hence, it should be a Weyl point. To the best of its knowledge, there have not yet been any reports of a genuine (i.e., fourfold degenerate) 2D Dirac point half-metal. In this work, using first-principles calculations, it proposes for the first time that the 2D d0 -type ferromagnet Mg4 N4 is a genuine 2D Dirac half-metal candidate with a fourfold degenerate Dirac point at the S high-symmetry point, intrinsic magnetism, a high Curie temperature, 100% spin polarization, topology robust under the SOC and uniaxial and biaxial strains, and spin-polarized edge states. This work can serve as a starting point for future predictions of intrinsically magnetic materials with genuine 2D Dirac points, which will aid the frontier of topo-spintronics research in 2D systems.

Tunable electrical properties and multiple-phases of ferromagnetic GdS2, GdSe2 and Janus GdSSe monolayers

With the continuous miniaturization and integration of spintronic devices, the two-dimensional (2D) ferromagnet coupling of ferromagnetic and diverse electrical properties has become increasingly important. Herein, we report three ferromagnetic monolayers: GdS2, GdSe2 and Janus GdSSe. They are bipolar magnetic semiconductors and demonstrate ferroelasticity with a large reversible strain of 73.2%. Three monolayers all hold large magnetic moments of about 8μB f.u.-1 and large spin-flip energy gaps in both the conduction and valence bands, which are highly desirable for applications in bipolar field effect spin filters and spin valves. Our calculations have testified to the feasibility of the experimental achievement of the three monolayers and their stability. Additionally, intrinsic valley polarization occurs in the three monolayers owing to the cooperative interplay between spin-orbit coupling and magnetic exchange interaction. Moreover, we identified square lattices for GdS2 and GdSe2 monolayers. The new and stable square lattices of GdS2 and GdSe2 monolayers show robust ferromagnetism with high Curie temperatures of 648 and 312 K, respectively, and the characteristics of spin-gapless semiconductors. Overall, these findings render GdS2, GdSe2 and Janus GdSSe monolayers promising candidate materials for multifunctional spintronic devices at the nanoscale.

Spin-gapless van der Waals heterostructure for spin gating through magnetic injection devices

Spin-gapless semiconductors (SGSs) are new magnetic zero-bandgap materials whose band structure is extremely sensitive to external influences (pressure or magnetic fields) and have great potential for high-speed and low-energy spintronics applications. The first-principles method was used to systematically study the heterostructures constructed of an asymmetric surface-functionalized Janus MXene material, Cr₂NOF, and a two-dimensional hexagonal lattice (2DH) semiconductor material and to study the effects of the electronic structure, Curie temperature, magnetism, and the design of unusual band structures and magnetic injection in the bilayer to obtain an SGS structure. Through the design and construction of Cr₂NOF/2DH van der Waals heterojunction spintronic devices, the spin-filtering effect of the devices can reach 100%, especially, realizing spin gating through magnetic injection. We report the transport mechanism of the heterojunction spintronic devices to achieve the goal of a controllable optimization of the device functions, which provides a theoretical basis for the design of MXene van der Waals heterojunctions for high-efficiency and low-power-consumption spintronic devices.

Rich magnetic phase transitions and completely dual-spin polarization of zigzag PC3 nanoribbons under uniaxial strain

Among many modulation methods, strain engineering is often chosen for nanomaterials to produce tunable band gaps continuously. Inspired by the recently reported two-dimensional material PC₃, we explore the tuning of strain on the spin-dependent transport properties of PC₃ nanoribbons using the first-principle approach. Surprisingly, strain regulation achieves uninterrupted completely dual-spin polarization over a wide energy range near E_F. Analysis reveals that the peculiar transmission spectra arise from the interesting evolution of the band structure, in which strain induces bands to shift and broaden/flatten. This results in triggering the transition of PC₃NRs from bandgap-tunable bipolar magnetic semiconductors to spin-gapless semiconductors to ferromagnetic metals or half-metal magnets. Their unique performance demonstrates great potential in spintronics, and our study is expected to provide ideas and theoretical support for the design and application of novel PC₃-based spintronic devices in the future.

Janus V2AsP monolayer : a ferromagnetic semiconductor with a narrow band gap, a high Curie temperature and controllable magnetic anisotropy

The search and design of two-dimensional (2D) magnetic semiconductors for spintronics applications are particularly significant. In this work, we investigated the electronic and magnetic properties of Janus structure based on Dirac half-metallic vanadium phosphide (VP) monolayer (ML) by first-principles calculations. Due to the vertical symmetry breaking, Janus V₂AsP ML becomes an intrinsic ferromagnetic semiconductor with a narrow band gap of 0.21 eV. We analyzed the electronic structure and origin of the in-plane easy axis in Janus V₂AsP. The electron effective mass is anisotropic and only 0.129 m₀along thex-direction. The Curie temperatureTcand magnetic anisotropy energy (MAE) of Janus V₂AsP reach 490 K and 178 µeV per V atom, respectively. A uniaxial tensile stainɛ_xof 5% can increase its band gap and MAE to 0.39 eV and 210.6 µeV per V atom while maintaining itsTcbeing above room temperature. Moreover, the direction of the easy axis can be changed between the in-planex- andy-direction by a small uniaxial tensile strainɛ_xof 2%. Our study can motivate further research on the design the magnetic semiconductors in Janus structures based on 2D Dirac half-metals for spintronics applications.

Mini-review of interesting properties in Mn2CoAl bulk and films

Heusler compounds exhibit many interesting properties, such as high thermopower, magnetocaloric properties, and even topological insulator states. Heusler Mn2CoAl alloy has been experimentally and theoretically proposed as a promising spin-gapless semiconductor with novel electronic, magnetic, spintronic, transport, and topological properties. Furthermore, the spin-gapless semiconducting-like behaviors are also predicted in Mn2CoAl films by measuring the transport and magnetic properties. This mini-review systematically summarizes the interesting properties of Mn2CoAl bulk and Mn2CoAl-based films. This mini-review is hoped to guide further experimental investigations and applications in the particular scientific community.

Two-dimensional spin-gapless semiconductors: A mini-review

In the past decade, two-dimensional (2D) materials and spintronic materials have been rapidly developing in recent years. 2D spin-gapless semiconductors (SGSs) are a novel class of ferromagnetic 2D spintronic materials with possible high Curie temperature, 100% spin-polarization, possible one-dimensional or zero-dimensional topological signatures, and other exciting spin transport properties. In this mini-review, we summarize a series of ideal 2D SGSs in the last 3 years, including 2D oxalate-based metal-organic frameworks, 2D single-layer Fe2I2, 2D Cr2X3 (X = S, Se, and Te) monolayer with the honeycomb kagome (HK) lattice, 2D CrGa2Se4 monolayer, 2D HK Mn-cyanogen lattice, 2D MnNF monolayer, and 2D Fe4N2 pentagon crystal. The mini-review also discusses the unique magnetic, electronic, topological, and spin-transport properties and the possible application of these 2D SGSs. The mini-review can be regarded as an improved understanding of the current state of 2D SGSs in recent 3 years.

A review of bipolar magnetic semiconductors from theoretical aspects

Spintronics, which employs electrons' spin degree of freedom in data storage and transmission, acts as a promising candidate for next-generation information technology owing to its improved processing speed and reduced power consumption. To seek and design materials with highly spin polarized carriers and find an efficient way to control the spin polarization direction of carriers are critical and urgent to spintronics applications. In this aspect, the bipolar magnetic semiconductor (BMS) serves as an ideal solution since it can generate currents with 100% spin polarization, and the direction of spin polarization is easily tunable by an external gate voltage. Up to now, there have been lots of BMSs predicted by first-principles calculations, however, most of them are extrinsically induced by chemical or physical modifications, and a generalized scheme for designing BMS materials is still lacking. This paper is aimed to briefly review the existing BMS materials designed by theoretical simulations, analyze the main obstacles to experimental realization, and put forward suggestions for future development.

Obvious Surface States Connecting to the Projected Triple Points in NaCl's Phonon Dispersion

With the development of computer technology and theoretical chemistry, the speed and accuracy of first-principles calculations have significantly improved. Using first-principles calculations to predict new topological materials is a hot research topic in theoretical and computational chemistry. In this work, we focus on a well-known material, sodium chloride (NaCl), and propose that the triple point (TP), quadratic contact triple point (QCTP), linear and quadratic nodal lines can be found in the phonon dispersion of NaCl with Fm3¯ m type structure. More importantly, we propose that the clear surface states connected to the projected TP and QCTP are visible on the (001) surface. It is hoped that further experimental investigation and verification for these properties as mentioned above.

From half-metallic to magnetic semiconducting triazine g-C4N3: computational designs and insight

We have given, for the first time, physicochemical insight into the electronic structure routes from half-metallic to magnetic semiconducting triazine g-C4N3. To this end, three material designs have been proposed using density functional calculations. In one design, this half-metal is first made semiconducting via hydrogenation, then tailored with B and N atomic species, which gives a new prototype of the antiferromagnetic semiconductor monolayer HC4N3BN. In the others, it can be rendered spin gapless semiconducting with H and B or C, followed by F or O tailoring, which eventually leads to the two new bipolar ferromagnetic semiconductors HC4N3BF and HC4N3CO. These monolayers are considered to be novel materials in spintronics.

Transport properties of Heusler compounds and alloys

Heusler compounds are a large group of intermetallic compositions with versatile material properties. In recent times, they are found to be important for their practical applications in the fields of spintronics and shape memory effect. Interestingly, their physical properties can be easily tuned by varying the valence electron concentration through proper doping and substitution. Empirical laws concerning the valence electron concentration, such as Slater-Pauling or Hume-Rothery rules are found to be useful in predicting their electronic, magnetic and structural properties quite accurately. Electrical transport measurements are simple laboratory-based techniques to gather a handful of information on the electronic properties of metals and semiconductors. The present review aimed to provide a comprehensive view of the transport in 3dand 4dtransition metal-based bulk Heusler compositions. The main emphasis is given on resistivity, magnetoresistance, Hall effect, thermopower and spin-dependent transport in spintronics devices. The review primarily focuses on magnetic Heusler compounds and alloys, albeit it also addresses several non-magnetic materials showing superconductivity or large thermopower.

Penta-Hexa-Graphene Nanoribbons: Intrinsic Magnetism and Edge Effect Induce Spin-Gapless Semiconducting and Half-Metallic Properties

Two-dimensional materials with intrinsic long-range ordered magnetic moments have drawn a lot of attention. However, for practical applications, whether or not the magnetism is stable in their nanostructures has not been revealed. Here, based on the recently proposed magnetic penta-hexa-graphene, we study the electronic and magnetic properties of its nanoribbons (named PHGNRs). The results show that the PHGNRs have intrinsic robust magnetic moments that are different from zigzag graphene nanoribbons, where the magnetic moments caused by the edge effect are vulnerable. Moreover, the magnetic ground states, namely, ferromagnetic (FM) or antiferromagnetic (AFM), can be transformed by changing the width of PHGNRs. Most interestingly, under the FM ground state, the spin-polarized electronic properties reveal that the zigzag PHGNRs transform from spin-gapless semiconductors (SGSs) to half-metals, as the width of nanoribbons increases, while all the armchair PHGNRs are magnetic semiconductors. Furthermore, by considering different edge effects caused by the residual carbon atoms on the edges, the PHGNRs can further derive different types of SGSs, as well as half-metals. Our work suggests that the PHGNRs possessing intrinsic robust magnetic moments have potential applications in the field of spintronic devices.