Topological quantum materials for spintronics

Electronic structures of topological quantum materials studied by ARPES

Research on the quantum nature of matter and light has flourished in recent years, as the field has grown for theorists and experimentalists alike. New research directions, such as the study of novel phases in quantum materials, have laid the ground for new foundational theories and future quantum technologies. In particular, novel topological materials and superconducting hybrid systems are promising for the development of novel superconductors and quantum computers. This Focus Issue aims to highlight some of the latest achievements in the field of quantum matter. It includes 3 Reviews, 1 Perspective, and 3 Research Articles on a range of topics, including single-photon emitters for quantum technologies, topological quantum materials, and quantum sensing and metrology: Rare-Earth Doped Thin Films for Optical Quantum Technologies (Philippe Goldner et al., 10.1002/qute.202500026): A Review article on rare earth-doped thin films—materials that are highly promising for use in on-chip photonic circuits thanks to their excellent coherence properties. The integration of rare-earth thin films into photonic platforms could potentially enable photonic quantum technologies, such as optical quantum memories, which are relevant for communications and processing. Cuprate twistronics for quantum hardware (Nicola Poccia et al., 10.1002/qute.202500203): A topical Review on the emerging field of cuprate twistronics, which involves stacking and twisting ultrathin layers of cuprate superconductors. This technique creates Moiré patterns and new electronic states, uncovering new opportunities in both applied (e.g., quantum hardware) and fundamental physics. Quantum spin Hall effects in van der Waals materials (Qiong Ma et al., 10.1002/qute.202500327): A Review of the Quantum Spin Hall (QSH) effect in two-dimensional van der Waals (vdW) materials. The QSH effect is promising for the development of low-power electronics and topological quantum computing. The article explores the main developments in the field, emerging research directions, as well as experimental challenges. Beyond kagome: p-bands in kagome metals (Alexander Tsirlin et al., 10.1002/qute.202500336): A Perspective arguing that p-bands (electron bands from non-transition metals) play a critical, often overlooked, role in the electronic instabilities, such as charge-density waves and superconductivity, observed in kagome lattice metals. High-purity single-photon emission in the telecom O-band from droplet-epitaxy InAs quantum dots integrated into a GaAs/AlGaAs planar microcavity on vicinal GaAs(111) (Battulga Munkhbat et al., 10.1002/qute.202500159): A Research article on developing a reliable source of single photons in the telecom O-band (a critical wavelength for fibre communication) using InAs quantum dots grown via droplet epitaxy. Single-photon emission from such sources could potentially be useful for building global quantum communication networks. Can Janus monolayer transition-metal dihalides be bright excitonic insulators? (Yuanchang Li et al., 10.1002/qute.202500345): This Research article explores Janus monolayers (asymmetric 2D materials) as potential candidates for bright excitonic insulators (EIs). EIs are a state of matter where electrons and holes spontaneously form excitons that condense, and bright EIs could provide a unique platform for coherent quantum optical devices. Highly sensitive magnetometer with continuous excitation Ramsey protocol by utilizing long dephasing time of millimetre-scale spin ensembles in (111) diamond (Mutsuko Hatano et al., 10.1002/qute.202500056): This Research article presents a method to significantly enhance the sensitivity of magnetometers by utilizing the long dephasing time of nitrogen-vacancy (NV) centre spin ensembles in a specific orientation of diamond. NV-diamond magnetometers are key for high-precision quantum sensing. Through this Focus Issue, we aim to highlight the major advances in quantum matter research and to offer perspectives on key directions for future research. We hope that readers will find the content of this collection both inspiring and useful.

The recently discovered topological quantum materials (TQMs) have electronic structures that can be characterized by certain topological invariants. In these novel materials, the unusual bulk and surface electrons not only give rise to many exotic physical phenomena but also foster potential new technological applications. To characterize the unusual electronic structures of these new materials, investigators have used angle-resolved photoemission spectroscopy (ARPES) as an effective experimental tool to directly visualize the unique bulk and surface electronic structures of TQMs. In this review, we first give a brief introduction of TQMs and ARPES, which is followed by examples of the application of ARPES to different TQMs ranging from topological insulators to Dirac and Weyl semimetals. We conclude with a brief perspective of the current development of ARPES and its potential application in the study of TQMs.

Topological quantum materials, which feature nontrivial band topology, have been one of the most attractive research topics in condensed-matter physics in recent decades. The low-dimensional topologically nontrivial materials are especially appealing due to the rich implications for topological physics and the potential applications in next-generation spintronic devices. Here, we report the crystal growth, anisotropic magnetotransport, Hall effect, and quantum de Haas--van Alphen (dHvA) oscillations of a quasi-one-dimensional ternary telluride ${\mathrm{NbNiTe}}_{5}$. The pronounced dHvA oscillations under $H\ensuremath{\parallel}b$ reveal three major oscillation frequencies ${F}_{\ensuremath{\alpha}}=136.41\phantom{\rule{0.16em}{0ex}}\mathrm{T}, {F}_{\ensuremath{\beta}}=240.34\phantom{\rule{0.16em}{0ex}}\mathrm{T}$, and ${F}_{\ensuremath{\gamma}}=708.03$ T and the associated light effective masses of charge carriers. From the angular dependence of dHvA oscillations, we have revealed the identified frequencies exhibit anisotropic character, all of which arise from the holelike Fermi surface sheets formed by band 1 (${F}_{\ensuremath{\alpha}}$ and ${F}_{\ensuremath{\beta}}$) and band 2 (${F}_{\ensuremath{\gamma}}$) by comparing with the Fermi surface calculations. First-principles calculations demonstrate that ${\mathrm{NbNiTe}}_{5}$ is a candidate of multiple topological material. In addition to the nonsymmorphic symmetry-protected nodal lines and band inversion (anticrossing) induced topological surface states, a ladder of topological gaps with the coexistence of strong and weak topology and a series of induced topological surface states are also identified.

Metal chalcogenide materials: Synthesis, structure and properties

Topological quantum materials with kagome lattice have become the emerging frontier in the context of condensed matter physics. Kagome lattice harbors strong magnetic frustration and topological electronic states generated by the unique geometric configuration. Kagome lattice has the peculiar advantages in the aspects of magnetism, topology as well as strong correlation when the spin, charge, or orbit degrees of free is introduced, and providing a promising platform for investigating the entangled interactions among them. In this paper, we will systematically introduce the research progress on the kagome topological materials and give a perspective in the framework of the potential future development directions in this field.

Single photon detection (SPD) has found increasingly important applications in many forefront areas of fundamental science and advanced engineering applications. The current SPD scheme has good sensitivity for photons in the high frequencies range (e.g., visible light). However, their sensitivity decreases drastically for low-frequency, low energy, microwave photons. As a result, the detection of single photons at this low frequency is highly prone to error from classical noise. In this talk we will present results from our recent studies of microwave response in a topological superconducting quantum interference device (SQUID) realized in Dirac semimetal Cd3As2. It is observed the effective temperature increases with the microwave power. This observation of large microwave response may pave the way for single photon detection at the microwave frequency in topological quantum materials.

Magnetic topological semimetals (TSMs) are topological quantum materials with broken time-reversal symmetry (TRS) and isolated nodal points or lines near the Fermi level. Their topological properties would typically reveal from the bulk-edge correspondence principle as nontrivial surface states such as Fermi arcs or drumhead states, etc. Depending on the degeneracies and distribution of the nodes in the crystal momentum space, TSMs are usually classified into Weyl semimetals (WSMs), Dirac semimetals (DSMs), nodal-line semimetals (NLSMs), triple-point semimetals (TPSMs), etc. In this review article, we present the recent advances of magnetic TSMs from a computational perspective. We first review the early predicted magnetic WSMs such as pyrochlore iridates and HgCr2Se4, as well as the recently proposed Heusler, Kagome layers, and honeycomb lattice WSMs. Then we discuss the recent developments of magnetic DSMs, especially CuMnAs in Type-III and EuCd2As2 in Type-IV magnetic space groups (MSGs). Then we introduce some magnetic NLSMs that are robust against spin–orbit coupling (SOC), namely Fe3GeTe2 and LaCl (LaBr). Finally, we discuss the prospects of magnetic TSMs and the interesting directions for future research.

Nodal electron spectrum is a characteristic feature of many topological quantum materials, still difficult to experimentally determine and probe. When topology protects the nodes, the low-energy nodal spectrum is distinguished from conventional spectra by its relativistic density of states. Here we review the recent progress in observing and measuring the properties of Weyl electrons with Raman scattering, and argue that, despite being a zero-momentum probe, Raman scattering is a sensitive detector of Weyl electrons capable of extracting the Fermi energy, Fermi momentum and even estimating the Weyl electron lifetime shaped by interactions and disorder. The utility of Raman scattering for the detection of quadratic band touching is also discussed.

Parity anomaly is a quantum mechanical effect that the parity symmetry in a two-dimensional classical action is failed to be restored in any regularization of the full quantum theory and is characterized by a half-quantized Hall conductivity. Here we propose a scheme to explore the experimental signature from parity anomaly in the measurement of optical Hall conductivity, in which the optical Hall conductivity is nearly half quantized for a proper range of frequency. The behaviors of optical Hall conductivity are studied for several models, which reveal the appearance of half-quantized Hall conductivity in low or high-frequency regimes. The optical Hall conductivity can be extracted from the measurement of Kerr and Faraday rotations and the absorption rate of the circularly polarized light. This proposal provides a practical method to explore the signature of parity anomaly in topological quantum materials.

Three-dimensional topological insulators have been studied for approximately fifteen years, but those made from group-IV elements, especially Ge and Sn, seem particularly attractive owing to their nontoxicity, sizable intrinsic spin-orbit coupling (SOC) strength and natural compatibility with the current semiconductor industry. However, group-IV elemental TIs have rarely been reported, except for the low temperature phase of alpha-Sn under strain. Here, based on first-principles calculations, we propose new allotropes of Ge and Sn, named T5-Ge(Sn), as desirable TIs. These new allotropes are also highly anisotropic Dirac semimetals if the SOC is turned off. To the best of our knowledge, T5-Ge(Sn) are the first 3D allotropes of Ge(Sn) that possess topological states in their equilibrium state at room temperature. Additionally, their isostructures of C and Si are metastable indirect and direct semiconductors. Our work not only reveals two promising TIs, but more profoundly, we justify the advantages of group-IV elements as topological quantum materials (TQMs) for fundamental research and potential practical applications, and thus reveal a new direction in the search for desirable TQMs.

The racetrack memory technology utilizes magnetic domains along a nanoscopic wire to obtain ultra-high data storage density. The recent success in the planar racetrack nanowire promised its fabrication feasibility and future scalability, bringing more design challenges and opportunities. In this paper, we initialize the optimization of racetrack memory embracing design considerations across multiple layers, including cell design, array structure, architecture organization, and data management. Our evaluation shows that racetrack memory based cache can achieve 6.4x area reduction, 25% performance enhancement, and 62% energy saving, compared to STT-RAM cache design. The benefit over SRAM technology is even more significant.

Engel-Vosko GGA calculations of the structural, electronic and optical properties of LiYO2

Semiconductors in the shape of nanostructures, including quantum dots (0-dimension) and nanoplatelets (2-dimension) demonstrate unique electronic and optoelectronic properties because of the quantum mechanism related to excitons confined in low dimensions. Such confinement can provide intriguing properties like high capability of photo-activated carriers, band gap tunability, splendid photoluminescence (PL) yield, narrow emission bandwidth, high absorption efficiency of light, fast fluorescence lifetime, etc. These properties attract great attention due to their high potential in applications like solar cells, light-emitting diodes (LEDs), lasers, photodetectors, catalysts, etc. Novel semiconductors including MoS2, MXene and perovskite have received increasing research interest in recent years. Unlike traditional semiconductor materials like Si, Ge, II-VI and III-V materials, the novel ones usually have low-dimensional form natively, which have emerged as very promising candidates for advanced electronic and optoelectronic devices. Among the various novel semiconductor materials, this thesis focuses on two types of them, which are MXene Sc2CO2 and cesium lead halide perovskite CsPbBr3. The two materials have some features in common. Both of them are direct band gap materials, which makes them available for optical applications. And they can be synthesized directly into two-dimensional structure, which o ers the opportunity to learn the influence of one-dimensional confinement. To extend the study into another dimension, the quantum dots of CsPbBr3 are also studied. The first part of the thesis focuses on the study of Sc2COx which is a type of semi-conductive MXene. The material is synthesized for the first time by using magnetron sputtering. X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) are performed to determine the composition of the sample. The band gap of Sc2COx sample is determined by ultraviolet-visible (UV-VIS) range absorption and photoluminescence (PL) spectrum. The Tauc plot indicates that Sc2COx is a direct band gap material. The high sensitivity of scandium to oxygen makes the sample contain both scandium carbide and scandium oxide. Thermal annealing introduces more oxidation into the sample. The structure of MXene will be broken when the annealing temperature is higher than 600C. The second part of the thesis focuses on the study of perovskite CsPbBr3 in forms of nanoplatelets and quantum dots. For CsPbBr3 nanoplatelets, the effect of thickness of cesium lead halide perovskite CsPbBr3 nanoplatelets (NPLs) on their electronic structure and optical properties are investigated using an 8-band k · p model which is based on effective-mass envelope function theory with exciton binding energy consideration. We first reported the CsPbBr3 nanoplatelets' band structure and optical gain with exciton effect. As the thickness of NPLs decreases, their band gap increases and the band mixing is more obvious which influences the transition matrix element (TME). The optical gain of CsPbBr3 nanoplatelets is calculated by taking into account TME, Fermi factor, injected carrier density and thickness. A blue shift of peak position in optical gain can be observed as the thickness of NPLs decreases. For any given NPL with a certain thickness, there is a slight blue shift in the peak position of optical gain as the carrier density increases because of band filling effect. The maximum optical gain of thinner NPLs needs higher carrier density to reach saturation. To obtain high differential gain, the carrier density in thick NPLs has to maintain in a small range. Experimental work is carried out and the results agree with our theoretical results very well. For quantum dot, the size effect on the electronic structure and optical properties of cubic CsPbBr3 perovskite quantum dots are investigated by using an 8-band k · p model which is also based on effective-mass envelope function theory with exciton binding energy consideration. Quantum dots with smaller sizes have larger band gaps due to quantum confinement. The transition matrix element (TME) is also influenced by the size. The optical gain of such material is calculated by considering TME, Fermi factor, injected carrier density, quantum dot size, and dephasing rate. A higher density of the injected carrier is needed for smaller quantum dots to get a positive optical gain. The peak of optical gain has a blue shift as the size of a quantum dot decreases. When the carrier density increases for a quantum dot with a certain size, a blue shift in emission peak position can be observed due to the band filling effect. Smaller quantum dots have higher differential optical gain at carrier densities within a certain range. In a word, two direct band gap semiconductor materials, Sc2COx and CsPbBr3 are studied via theoretical and experimental approaches. Their band-structure and optical properties are presented, followed by the analysis of their potential in applications.

Theoretical studies on band structure and optical properties of 3C–SiC by FPLAPW