Topological spintronics and magnetoelectronics

Inducing superconductivity in quantum anomalous Hall regime

Interfacing the quantum anomalous Hall insulator with a conventional superconductor is known to be a promising manner for realizing a topological superconductor, which has been continuously pursued for years. Such a proximity route depends to a great extent on the control of the delicate interfacial coupling of the two constituents. However, a recent experiment reported the failure to reproduce such a topological superconductor, which is ascribed to the negligence of the electrical short by the superconductor in the theoretical proposal. Here, we reproduce this topological superconductor with attention to the interface control. The resulted conductance matrix under a wide magnetic field range agrees with the fingerprint of this topological superconductor. This allows us to develop a phase diagram that unveils three regions parameterized by various coupling limits, which not only supports the feasibility to fabricate the topological superconductor by proximity but also fully explains the origin of the previous debate. The present work provides a comprehensible guide on fabricating the topological superconductor.

Topologically Protected Photovoltaics in Bi Nanoribbons

Photovoltaic efficiency in solar cells is hindered by many unwanted effects. Radiative channels (emission of photons) sometimes mediated by nonradiative ones (emission of phonons) are principally responsible for the decrease in exciton population before charge separation can take place. One such mechanism is electron-hole recombination at surfaces or defects where the in-gap edge states serve as the nonradiative channels. In topological insulators (TIs), which are rarely explored from an optoelectronics standpoint, we show that their characteristic surface states constitute a nonradiative decay channel that can be exploited to generate a protected photovoltaic current. Focusing on two-dimensional TIs, and specifically for illustration purposes on a Bi(111) monolayer, we obtain the transition rates from the bulk excitons to the edge states. By breaking the appropriate symmetries of the system, one can induce an edge charge accumulation and edge currents under illumination, demonstrating the potential of TI nanoribbons for photovoltaics.

Investigation of half metallic properties of Tl2Mo(Cl/Br)6 double perovskites for spintronic devices

The manipulation of electronic device characteristics through electron spin represents a burgeoning frontier in technological advancement. Investigation of magnetic and transport attributes of the Tl2Mo(Cl/Br)6 double perovskite was performed using Wien2k and BoltzTraP code. When the energy states between ferromagnetic and antiferromagnetic conditions are compared, it is evident that the ferromagnetic state exhibits lower energy levels. Overcoming stability challenges within the ferromagnetic state is achieved through the manipulation of negative ΔHf within the cubic state. The analysis of the half metallicity character involves an analysis of band structure (BS) and DOS, elucidating its mechanism through PDOS using double exchange model p-d hybridization. The verification of 100% spin polarization is confirmed through factors such as spin polarization and the integer value of the total magnetic moment. Furthermore, the thermoelectric response, as indicated by the ratios of thermal-electrical conductivity and ZT, underscores the promising applications of these compounds in thermoelectric device applications.

Memristive switching in the surface of a charge-density-wave topological semimetal

Owing to the outstanding properties provided by nontrivial band topology, topological phases of matter are considered as a promising platform towards low-dissipation electronics, efficient spin-charge conversion, and topological quantum computation. Achieving ferroelectricity in topological materials enables the non-volatile control of the quantum states, which could greatly facilitate topological electronic research. However, ferroelectricity is generally incompatible with systems featuring metallicity due to the screening effect of free carriers. In this study, we report the observation of memristive switching based on the ferroelectric surface state of a topological semimetal (TaSe4)2I. We find that the surface state of (TaSe4)2I presents out-of-plane ferroelectric polarization due to surface reconstruction. With the combination of ferroelectric surface and charge-density-wave-gapped bulk states, an electric-switchable barrier height can be achieved in (TaSe4)2I-metal contact. By employing a multi-terminal-grounding design, we manage to construct a prototype ferroelectric memristor based on (TaSe4)2I with on/off ratio up to 103, endurance over 103 cycles, and good retention characteristics. The origin of the ferroelectric surface state is further investigated by first-principles calculations, which reveals an interplay between ferroelectricity and band topology. The emergence of ferroelectricity in (TaSe4)2I not only demonstrates it as a rare but essential case of ferroelectric topological materials, but also opens new routes towards the implementation of topological materials in functional electronic devices.

Holographic Nano-Imaging of Terahertz Dirac Plasmon Polaritons in Topological Insulator Antenna Resonators

Excitation of Dirac plasmon polaritons (DPPs) in bi-dimensional materials have attracted considerable interest in recent years, both from perspectives of understanding their physics and exploring their transformative potential for nanophotonic devices, including ultra-sensitive plasmonic sensors, ultrafast saturable absorbers, modulators, and switches. Topological insulators (TIs) represent an ideal technological platform in this respect because they can support plasmon polaritons formed by Dirac carriers in the topological surface states. Tracing propagation of DPPs is a very challenging task, particularly at terahertz (THz) frequencies, where the DPP wavelength becomes over one order of magnitude shorter than the free space photon wavelength. Furthermore, severe attenuation hinders the comprehensive analysis of their characteristics. Here, the properties of DPPs in real TI-based devices are revealed. Bi2Se3 rectangular antennas can efficiently confine the propagation of DPPs to a single dimension and, as a result, enhance the DPPs visibility despite the strong intrinsic attenuation. The plasmon dispersion and loss properties from plasmon profiles are experimentally determined, along the antennas, obtained using holographic near-field nano-imaging in a wide range of THz frequencies, from 2.05 to 4.3 THz. The detailed investigation of the unveiled DPP properties can guide the design of novel topological quantum devices exploiting their directional propagation.

Enhanced Photocharacteristics by Fermi Level Modulating in Sb2 Te3 /Bi2 Se3 Topological Insulator p-n Junction

Topological insulators have recently received attention in optoelectronic devices because of their high mobility and broadband absorption resulting from their topological surface states. In particular, theoretical and experimental studies have emerged that can improve the spin generation efficiency in a topological insulator-based p-n junction structure called a TPNJ, drawing attention in optospintronics. Recently, research on implementing the TPNJ structure is conducted; however, studies on the device characteristics of the TPNJ structure are still insufficient. In this study, the TPNJ structure is effectively implemented without intermixing by controlling the annealing temperature, and the photocharacteristics appearing in the TPNJ structure are investigated using a cross-pattern that can compare the characteristics in a single device. Enhanced photo characteristics are observed for the TPNJ structure. An optical pump Terahertz probe and a physical property measurement system are used to confirm the cause of improved photoresponsivity. Consequently, the photocharacteristics are improved owing to the change in the absorption mechanism and surface transport channel caused by the Fermi level shift in the TPNJ structure.

Supercurrent mediated by helical edge modes in bilayer graphene

Bilayer graphene encapsulated in tungsten diselenide can host a weak topological phase with pairs of helical edge states. The electrical tunability of this phase makes it an ideal platform to investigate unique topological effects at zero magnetic field, such as topological superconductivity. Here we couple the helical edges of such a heterostructure to a superconductor. The inversion of the bulk gap accompanied by helical states near zero displacement field leads to the suppression of the critical current in a Josephson geometry. Using superconducting quantum interferometry we observe an even-odd effect in the Fraunhofer interference pattern within the inverted gap phase. We show theoretically that this effect is a direct consequence of the emergence of helical modes that connect the two edges of the sample. The absence of such an effect at high displacement field, as well as in bare bilayer graphene junctions, supports this interpretation and demonstrates the topological nature of the inverted gap.

Anomalous Hall effect in Nd-doped Bi1.1Sb0.9STe2 topological insulator single crystals

Topological insulators are emerging materials with insulating bulk and symmetry protected nontrivial surface states. One of the most fascinating transport behaviors in a topological insulator is the quantum anomalous Hall effect, which has been observed in magnetic-topological-insulator-based devices. In this work, we report successful doping of rare-earth element Nd into Bi1.1Sb0.9STe2 bulk-insulating topological insulator single crystals, in which the Nd moments are ferromagnetically ordered at ∼100 K. Benefiting from the in-bulk-gap Fermi level, electronic transport behaviors dominated by the topological surface states are observed in the ferromagnetic region. At low temperatures, strong Shubnikov-de Haas oscillations with a nontrivial Berry phase are observed. The topological insulator with long range magnetic ordering in Nd-doped Bi1.1Sb0.9STe2 single crystals provides a good platform for quantum transport studies and spintronic applications.

How spin relaxes and dephases in bulk halide perovskites

Spintronics in halide perovskites has drawn significant attention in recent years, due to their highly tunable spin-orbit fields and intriguing interplay with lattice symmetry. Here, we perform first-principles calculations to determine the spin relaxation time (T1) and ensemble spin dephasing time ([Formula: see text]) in a prototype halide perovskite, CsPbBr3. To accurately capture spin dephasing in external magnetic fields we determine the Landé g-factor from first principles and take it into account in our calculations. These allow us to predict intrinsic spin lifetimes as an upper bound for experiments, identify the dominant spin relaxation pathways, and evaluate the dependence on temperature, external fields, carrier density, and impurities. We find that the Fröhlich interaction that dominates carrier relaxation contributes negligibly to spin relaxation, consistent with the spin-conserving nature of this interaction. Our theoretical approach may lead to new strategies to optimize spin and carrier transport properties.

Synthesis and Future Electronic Applications of Topological Nanomaterials

Over the last ten years, the discovery of topological materials has opened up new areas in condensed matter physics. These materials are noted for their distinctive electronic properties, unlike conventional insulators and metals. This discovery has not only spurred new research areas but also offered innovative approaches to electronic device design. A key aspect of these materials is now that transforming them into nanostructures enhances the presence of surface or edge states, which are the key components for their unique electronic properties. In this review, we focus on recent synthesis methods, including vapor-liquid-solid (VLS) growth, chemical vapor deposition (CVD), and chemical conversion techniques. Moreover, the scaling down of topological nanomaterials has revealed new electronic and magnetic properties due to quantum confinement. This review covers their synthesis methods and the outcomes of topological nanomaterials and applications, including quantum computing, spintronics, and interconnects. Finally, we address the materials and synthesis challenges that need to be resolved prior to the practical application of topological nanomaterials in advanced electronic devices.

Proximity induced band gap opening in topological-magnetic heterostructure (Ni80Fe20/p-TlBiSe2/p-Si) under ambient condition

The broken time reversal symmetry states may result in the opening of a band gap in TlBiSe2 leading to several interesting phenomena which are potentially relevant for spintronic applications. In this work, the quantum interference and magnetic proximity effects have been studied in Ni80Fe20/p-TlBiSe2/p-Si (Magnetic/TI) heterostructure using physical vapor deposition technique. Raman analysis shows the symmetry breaking with the appearance of A21u mode. The electrical characteristics are investigated under dark and illumination conditions in the absence as well as in the presence of a magnetic field. The outcomes of the examined device reveal excellent photo response in both forward and reverse bias regions. Interestingly, under a magnetic field, the device shows a reduction in electrical conductivity at ambient conditions due to the crossover of weak localization and separation of weak antilocalization, which are experimentally confirmed by magnetoresistance measurement. Further, the photo response has also been assessed by the transient absorption spectroscopy through analysis of charge transfer and carrier relaxation mechanisms. Our results can be beneficial for quantum computation and further study of topological insulator/ferromagnet heterostructure and topological material based spintronic devices due to high spin orbit coupling along with dissipationless conduction channels at the surface states.

Quantum-sized topological insulators/semimetals enable ultrahigh and broadband saturable absorption

Two-dimensional topological insulators/semimetals have recently attracted much attention. However, quantum-sized topological insulators/semimetals with intrinsic characteristics have never been reported before. Herein, we report the high-yield production of topological insulator (i.e., Bi2Se3 and Sb2Te3) and semimetal (i.e., TiS2) quantum sheets (QSs) with monolayer structures and sub-4 nm lateral sizes. Both linear and nonlinear optical performances of the QSs are investigated. The QS dispersions present remarkable photoluminescence with excitation wavelength-, concentration-, and solvent-dependence. The solution-processed QSs-poly(methyl methacrylate) (PMMA) hybrid thin films demonstrate exceptional nonlinear saturation absorption (NSA). Particularly, Bi2Se3 QSs-PMMA enables record-high NSA performance with a broadband feature. Specifically, the (absolute) modulation depths up to 71.6 and 72.4% and saturation intensities down to 1.52 and 0.49 MW cm-2 are achieved at 532 and 800 nm, respectively. Such a phenomenal NSA performance would greatly facilitate their applications in mode-locked lasers and related fields.

Novel germanene-arsenene and germanene-antimonene lateral heterostructures: interline-dependent electronic and magnetic properties

Seamlessly stitching two-dimensional (2D) materials may lead to the emergence of novel properties triggered by the interactions at the interface. In this work, a series of 2D lateral heterostructures (LHSs), namely germanene-arsenene (Ge_m-As_8-m) and germanene-antimonene (Ge_m-Sb_8-m), are investigated using first-principles calculations. The results demonstrate a strong interline-dependence of the electronic and magnetic properties. Specifically, the LHS formation along an armchair line preserves the non-magnetic nature of the original materials. However, this is an efficient approach to open the electronic band gap of the germanene monolayer, where band gaps as large as 0.74 and 0.76 eV are induced for Ge₂-As₆ and Ge₂-Sb₆ LHSs, respectively. Meanwhile, magnetism may appear in the zigzag-LHSs depending on the chemical composition (m = 3, 4, 5, and 6 for germanene-arsenene and m = 2, 3, 4, 5, and 6 for germanene-antimonene), where total magnetic moments between 0.13 and 0.50 μ_B are obtained. Herein, magnetic properties are produced mainly by the spin-up state of Ge atoms at the interface, where a small contribution comes from As(Sb) atoms. Spin-resolved band structures show a multivalley profile in both the valence band and the conduction band with a topological insulator-like behavior, where the interface states are derived mainly from the interface Ge-p_z state. The results introduce new 2D lateral heterostructures with novel electronic and magnetic properties to allow new functionalities, which could be further explored for optoelectronic and spintronic applications.

Gate-Tunable Spin Hall Effect in an All-Light-Element Heterostructure: Graphene with Copper Oxide

Graphene is a light material for long-distance spin transport due to its low spin-orbit coupling, which at the same time is the main drawback for exhibiting a sizable spin Hall effect. Decoration by light atoms has been predicted to enhance the spin Hall angle in graphene while retaining a long spin diffusion length. Here, we combine a light metal oxide (oxidized Cu) with graphene to induce the spin Hall effect. Its efficiency, given by the product of the spin Hall angle and the spin diffusion length, can be tuned with the Fermi level position, exhibiting a maximum (1.8 ± 0.6 nm at 100 K) around the charge neutrality point. This all-light-element heterostructure shows a larger efficiency than conventional spin Hall materials. The gate-tunable spin Hall effect is observed up to room temperature. Our experimental demonstration provides an efficient spin-to-charge conversion system free from heavy metals and compatible with large-scale fabrication.

Intrinsic and extrinsic dopings in epitaxial films MnBi2Te4

The intrinsic antiferromagnetic topological insulator MnBi₂Te₄and members of its family have been the subject of theoretical and experimental research, which has revealed the presence of a variety of defects and disorders that are crucial in determining the topological and magnetic properties. This also brings about challenges in realizing the quantum states like the quantum anomalous Hall and the axion insulator states. Here, utilizing cryogenic magnetoelectric transport and magnetic measurements, we systematically investigate the effects arising from intrinsic doping by antisite defects and extrinsic doping by Sb in MnBi₂Te₄epitaxial films grown by molecular beam epitaxy. We demonstrate that the nonequilibrium condition in epitaxy allows a wide growth window for optimizing the crystalline quality and defect engineering. While the intrinsic antisite defects caused by the intermixing between Bi and Mn can be utilized to tune the Fermi level position as evidenced by a p-to-n conductivity transition, the extrinsic Sb-doping not only compensates for this doping effect but also modifies the magnetism and topology of the film, during which a topological phase transition is developed. Conflicting reports from the theoretical calculations and experimental measurements in bulk crystals versus epitaxial films are addressed, which highlights the intimate correlation between the magnetism and topology as well as the balance between the Fermi-level positioning and defect control. The present study provides an experimental support for the epitaxial growth of the intrinsic topological insulator and underlines that the topology, magnetism, and defect engineering should be revisited for enabling a steady and reliable film production.

Single-Atomic-Layer Stanene on Ferromagnetic Co Nanoislands with Topological Band Structures

Introducing magnetism to two-dimensional topological insulators is a central issue in the pursuit of magnetic topological materials in low dimensionality. By means of low-temperature growth at 80 K, we succeeded in fabricating a monolayer stanene on Co/Cu(111) and resolving ferromagnetic spin contrast by field-dependent spin-polarized scanning tunneling microscopy (SP-STM). Increases of both remanence to saturation magnetization ratio (M_r/M_s) and coercive field (H_c) due to an enhanced perpendicular magnetic anisotropy (PMA) are further identified by out-of-plane magneto-optical Kerr effect (MOKE). In addition to ultraflat stanene fully relaxed on bilayer Co/Cu(111) from density functional theory (DFT), characteristic topological properties including an in-plane s-p band inversion and a spin-orbit coupling (SOC) induced gap about 0.25 eV at the Γ̅ point have also been verified in the Sn-projected band structure. Interfacial coupling of single-atomic-layer stanene with ferromagnetic Co biatomic layers allows topological band features to coexist with ferromagnetism, facilitating a conceptual design of atomically thin magnetic topological heterostructures.

Mobile Kink Solitons in a Van der Waals Charge-Density-Wave Layer

Kinks, point-like geometrical defects along dislocations, domain walls, and DNA, are stable and mobile, as solutions of a sine-Gordon wave equation. While they are widely investigated for crystal deformations and domain wall motions, electronic properties of individual kinks have received little attention. In this work, electronically and topologically distinct kinks are discovered along electronic domain walls in a correlated van der Waals insulator of 1T-TaS₂ . Mobile kinks and antikinks are identified as trapped by pinning defects and imaged in scanning tunneling microscopy. Their atomic structures and in-gap electronic states are unveiled, which are mapped approximately into Su-Schrieffer-Heeger solitons. The twelvefold degeneracy of the domain walls in the present system guarantees an extraordinarily large number of distinct kinks and antikinks to emerge. Such large degeneracy together with the robust geometrical nature may be useful for handling multilevel information in van der Waals materials architectures.

Disordered topological graphs enhancing nonlinear phenomena

Complex networks play a fundamental role in understanding phenomena from the collective behavior of spins, neural networks, and power grids to the spread of diseases. Topological phenomena in such networks have recently been exploited to preserve the response of systems in the presence of disorder. We propose and demonstrate topological structurally disordered systems with a modal structure that enhances nonlinear phenomena in the topological channels by inhibiting the ultrafast leakage of energy from edge modes to bulk modes. We present the construction of the graph and show that its dynamics enhances the topologically protected photon pair generation rate by an order of magnitude. Disordered nonlinear topological graphs will enable advanced quantum interconnects, efficient nonlinear sources, and light-based information processing for artificial intelligence.

Intermixing-Driven Surface and Bulk Ferromagnetism in the Quantum Anomalous Hall Candidate MnBi6 Te10

The recent realizations of the quantum anomalous Hall effect (QAHE) in MnBi₂ Te₄ and MnBi₄ Te₇ benchmark the (MnBi₂ Te₄ )(Bi₂ Te₃ )_n family as a promising hotbed for further QAHE improvements. The family owes its potential to its ferromagnetically (FM) ordered MnBi₂ Te₄ septuple layers (SLs). However, the QAHE realization is complicated in MnBi₂ Te₄ and MnBi₄ Te₇ due to the substantial antiferromagnetic (AFM) coupling between the SLs. An FM state, advantageous for the QAHE, can be stabilized by interlacing the SLs with an increasing number n of Bi₂ Te₃ quintuple layers (QLs). However, the mechanisms driving the FM state and the number of necessary QLs are not understood, and the surface magnetism remains obscure. Here, robust FM properties in MnBi₆ Te₁₀ (n = 2) with T_c ≈ 12 K are demonstrated and their origin is established in the Mn/Bi intermixing phenomenon by a combined experimental and theoretical study. The measurements reveal a magnetically intact surface with a large magnetic moment, and with FM properties similar to the bulk. This investigation thus consolidates the MnBi₆ Te₁₀ system as perspective for the QAHE at elevated temperatures.

Seeding and Emergence of Composite Skyrmions in a van der Waals Magnet

Topological charge plays a significant role in a range of physical systems. In particular, observations of real-space topological objects in magnetic materials have been largely limited to skyrmions - states with a unitary topological charge. Recently, more exotic states with varying topology, such as antiskyrmions, merons, or bimerons and 3D states such as skyrmion strings, chiral bobbers, and hopfions, have been experimentally reported. Along these lines, the realization of states with higher-order topology has the potential to open new avenues of research in topological magnetism and its spintronic applications. Here, real-space imaging of such spin textures, including skyrmion, skyrmionium, skyrmion bag, and skyrmion sack states, observed in exfoliated flakes of the van der Waals magnet Fe_3-x GeTe₂ (FGT) is reported. These composite skyrmions may emerge from seeded, loop-like states condensed into the stripe domain structure, demonstrating the possibility to realize spin textures with arbitrary integer topological charge within exfoliated flakes of 2D magnets. The general nature of the formation mechanism motivates the search for composite skyrmion states in both well-known and new magnetic materials, which may yet reveal an even richer spectrum of higher-order topological objects.

Room-Temperature Magnetoelectric Coupling in Atomically Thin ε-Fe2 O3

2D multiferroics with magnetoelectric coupling combine the magnetic order and electric polarization in a single phase, providing a cornerstone for constructing high-density information storages and low-energy-consumption spintronic devices. The strong interactions between various order parameters are crucial for realizing such multifunctional applications, nevertheless, this criterion is rarely met in classical 2D materials at room-temperature. Here an ingenious space-confined chemical vapor deposition strategy is designed to synthesize atomically thin non-layered ε-Fe₂ O₃ single crystals and disclose the room-temperature long-range ferrimagnetic order. Interestingly, the strong ferroelectricity and its switching behavior are unambiguously discovered in atomically thin ε-Fe₂ O₃ , accompanied with an anomalous thickness-dependent coercive voltage. More significantly, the robust room-temperature magnetoelectric coupling is uncovered by controlling the magnetism with electric field and verifies the multiferroic feature of atomically thin ε-Fe₂ O₃ . This work not only represents a substantial leap in terms of the controllable synthesis of 2D multiferroics with robust magnetoelectric coupling, but also provides a crucial step toward the practical applications in low-energy-consumption electric-writing/magnetic-reading devices.

Investigation of the mechanism of the anomalous Hall effects in Cr2Te3/(BiSb)2(TeSe)3 heterostructure

The interplay between ferromagnetism and the non-trivial topology has unveiled intriguing phases in the transport of charges and spins. For example, it is consistently observed the so-called topological Hall effect (THE) featuring a hump structure in the curve of the Hall resistance (R_xy) vs. a magnetic field (H) of a heterostructure consisting of a ferromagnet (FM) and a topological insulator (TI). The origin of the hump structure is still controversial between the topological Hall effect model and the multi-component anomalous Hall effect (AHE) model. In this work, we have investigated a heterostructure consisting of Bi_xSb_2-xTe_ySe_3-y (BSTS) and Cr₂Te₃ (CT), which are well-known TI and two-dimensional FM, respectively. By using the so-called "minor-loop measurement", we have found that the hump structure observed in the CT/BSTS is more likely to originate from two AHE channels. Moreover, by analyzing the scaling behavior of each amplitude of two AHE with the longitudinal resistivities of CT and BSTS, we have found that one AHE is attributed to the extrinsic contribution of CT while the other is due to the intrinsic contribution of BSTS. It implies that the proximity-induced ferromagnetic layer inside BSTS serves as a source of the intrinsic AHE, resulting in the hump structure explained by the two AHE model.

Bulk Physical Properties of a Magnetic Weyl Semimetal Candidate NdAlGe Grown by a Laser Floating-Zone Method

In this study, we report the successful growth of single crystals of a magnetic Weyl semimetal candidate NdAlGe with the space group I41md. The crystals were grown using a floating-zone technique, which used five laser diodes, with a total power of 2 kW, as the heat source. To ensure that the molten zone was stably formed during the growth, we employed a bell-shaped distribution profile of the vertical irradiation intensity. After the nominal powder, crushed from an arc-melted ingot, was shaped under hydrostatic pressure, we sintered the feed and seed rods in an Ar atmosphere under ultra-low oxygen partial pressure (<10−26 atm) generated by an oxygen pump made of yttria-stabilized zirconia heated at 873 K. Single crystals of NdAlGe were successfully grown to a length of 50 mm. The grown crystals showed magnetic order in bulk at 13.5 K. The fundamental physical properties were characterized by magnetic susceptibility, magnetization, specific heat, thermal expansion, and electrical resistivity measurements. This study demonstrates that the magnetic order induces anisotropic magnetoelasticity, magneto-entropy, and charge transport in NdAlGe.

Surface plasmons in anisotropic 3D gapped topological insulators

Topological insulators (TIs) are materials having conductive surfaces but insulating bulk, which are ideal platforms for plasmonic applications. The most commonly known TIs, such as Bi₂Se₃and Bi₂Te₃, are in fact highly anisotropic. The dielectric constants are largely different parallel and perpendicular to the surface. Here, we have extended the electromagnetic calculations of the surface plasmons in TIs to the anisotropic case. Magnetic field perpendicular to the surface is allowed, which opens a gap among the surface states. We model anisotropic TIs as bulk dielectric materials with different in-plane and out-of-plane permittivities; the surface states caused by the band inversion lead to a two-dimensional conductivity which supports surface plasmons. We have found two rather than one surface modes. Due to such anisotropy, quasi transverse electric (TE) polarized mode may occur near the interband transition threshold. Far below the transition frequency, another mode with both TE and transverse magnetic polarized components dominates, the dispersion relation of which is seriously modified by the Hall conductivity. By taking Bi₂Te₃as an example, we have derived the conductivity tensor with the consideration of the hexagonal warping effect, and solved the above mentioned two surface plasmon modes. In the end, finite element method has been used to calculate the electric field distributions. Our extension of the electromagnetic calculations of surface plasmons including a specific kind of anisotropy might be useful in other surface conductive materials with similar symmetry as well.

Theory of optical axion electrodynamics and application to the Kerr effect in topological antiferromagnets

Emergent axion electrodynamics in magneto-electric media is expected to provide novel ways to detect and control material properties with electromagnetic fields. However, despite being studied intensively for over a decade, its theoretical understanding remains mostly confined to the static limit. Here, we introduce a theory of axion electrodynamics at general frequencies. We define a proper optical axion magneto-electric coupling through its relation to optical surface Hall conductivity and provide ways to calculate it in lattice systems. By employing our formulas, we show that axion electrodynamics can lead to a significant Kerr effect in thin-film antiferromagnets at wavelengths that are seemingly too long to resolve the spatial modulation of magnetism. We identify the wavelength scale above which the Kerr effect is suppressed. Our theory is particularly relevant to materials like MnBi₂Te₄, a topological antiferromagnet whose magneto-electric response is shown here to be dominated by the axion contribution even at optical frequencies.

Topology and Symmetry in Quantum Materials

Interest in topological materials continues to grow unabated in view of their conceptual novelties as well as their potential as platforms for transformational new technologies. Electronic states in a topological material are robust against perturbations and support unconventional electromagnetic responses. The first-principles band-theory paradigm has been a key player in the field by providing successful prediction of many new classes of topological materials. This perspective presents a cross section through the recent work on understanding the role of geometry and topology in generating topological states and their responses to external stimuli, and as a basis for connecting theory and experiment within the band theory framework. In this work, effective strategies for topological materials discovery and impactful directions for future topological materials research are also commented.

Field-Free Magnetization Switching in a Ferromagnetic Single Layer through Multiple Inversion Asymmetry Engineering

A simple, reliable, and self-switchable spin-orbit torque (SOT)-induced magnetization switching in a ferromagnetic single layer is needed for the development of next generation fully electrical controllable spintronic devices. In this work, field-free SOT-induced magnetization switching in a CoPt single layer is realized by broken multiple inversion symmetry through simultaneously introducing both oblique sputtering and a vertical composition gradient. A quantitative analysis indicates that multiple inversion asymmetries can produce dynamical bias fields along both z- and x-axes, leading to the observed field-free deterministic magnetization switching. Our study provides a method to accomplish fully electrical manipulation of magnetization in a ferromagnetic single layer without the external magnetic field and auxiliary heavy metal layer, enabling flexible design for future spin-orbit torque-based memory and logic devices.

Synthesis of the Candidate Topological Compound Ni3Pb2

Spin-orbit coupling enables the realization of topologically nontrivial ground states. As spin-orbit coupling increases with increasing atomic number, compounds featuring heavy elements, such as lead, offer a pathway toward creating new topologically nontrivial materials. By employing a high-pressure flux synthesis method, we synthesized single crystals of Ni₃Pb₂, the first structurally characterized bulk binary phase in the Ni-Pb system. Combining experimental and theoretical techniques, we examined structure and bonding in Ni₃Pb₂, revealing the impact of chemical substitutions on electronic structure features of importance for controlling topological behavior. From these results, we determined that Ni₃Pb₂ completes a series of structurally related transition-metal-heavy main group intermetallic materials that exhibit diverse electronic structures, opening a platform for synthetically tunable topologically nontrivial materials.

Energy density as a probe of band representations in photonic crystals

Topological quantum chemistry (TQC) has recently emerged as an instrumental tool to characterize the topological nature of both fermionic and bosonic band structures. TQC is based on the study of band representations and the localization of maximally localized Wannier functions. In this article, we study various two-dimensional photonic crystal structures analyzing their topological character through a combined study of TQC, their Wilson-loop (WL) spectra and the electromagnetic energy density. Our study demonstrates that the analysis of the spatial localization of the energy density complements the study of the topological properties in terms of the spectrum of the WL operator and TQC.

Ultrafast Thermalization Pathways of Excited Bulk and Surface States in the Ferroelectric Rashba Semiconductor GeTe

A large Rashba effect is essential for future applications in spintronics. Particularly attractive is understanding and controlling nonequilibrium properties of ferroelectric Rashba semiconductors. Here, time- and angle-resolved photoemission is utilized to access the ultrafast dynamics of bulk and surface transient Rashba states after femtosecond optical excitation of GeTe. A complex thermalization pathway is observed, wherein three different timescales can be clearly distinguished: intraband thermalization, interband equilibration, and electronic cooling. These dynamics exhibit an unconventional temperature dependence: while the cooling phase speeds up with increasing sample temperature, the opposite happens for interband thermalization. It is demonstrated how, due to the Rashba effect, an interdependence of these timescales on the relative strength of both electron-electron and electron-phonon interactions is responsible for the counterintuitive temperature dependence, with spin-selection constrained interband electron-electron scatterings found both to dominate dynamics away from the Fermi level, and to weaken with increasing temperature. These findings are supported by theoretical calculations within the Boltzmann approach explicitly showing the opposite behavior of all relevant electron-electron and electron-phonon scattering channels with temperature, thus confirming the microscopic mechanism of the experimental findings. The present results are important for future applications of ferroelectric Rashba semiconductors and their excitations in ultrafast spintronics.

Recent advances in two-dimensional ferromagnetism: strain-, doping-, structural- and electric field-engineering toward spintronic applications

Since the first report on truly two-dimensional (2D) magnetic materials in 2017, a wide variety of merging 2D magnetic materials with unusual physical characteristics have been discovered and thus provide an effective platform for exploring the associated novel 2D spintronic devices, which have been made significant progress in both theoretical and experimental studies. Herein, we make a comprehensive review on the recent scientific endeavors and advances on the various engineering strategies on 2D ferromagnets, such as strain-, doping-, structural- and electric field-engineering, toward practical spintronic applications, including spin tunneling junctions, spin field-effect transistors and spin logic gate, etc. In the last, we discuss on current challenges and future opportunities in this field, which may provide useful guidelines for scientists who are exploring the fundamental physical properties and practical spintronic devices of low-dimensional magnets.

Molecular fluorophores for in vivo bioimaging in the second near-infrared window

PURPOSE

This systematic review aims to summarize the current developments of fluorescence and chemi/bioluminescence imaging based on the molecular fluorophores for in vivo imaging in the second near-infrared window.

METHODS AND RESULTS

By investigating most of the relevant references on the web of science and some journals, this review firstly begins with an overview of the background of fluorescence and chemi/bioluminescence imaging. Secondly, the chemical and optical properties of NIR-II dyes are discussed, such as water solubility, chemostability and photo-stability, and brightness. Thirdly, the bioimaging based on NIR-II fluorescence emission is outlined, including the in vivo imaging of polymethine dyes, donor - acceptor - donor (D - A - D) chromophores, and lanthanide complexes. Fourthly, we demonstrate the chemi/bioluminescence in vivo imaging in the second near-infrared window. Fifthly, the clinical application and translation of near-infrared fluorescence imaging are presented. Finally, the current challenges, feasible strategies and potential prospects of the fluorophores and in vivo bioimaging are discussed.

CONCLUSIONS

Based on the above literature research on the applications of molecular fluorescent and chemi/bioluminescent probes in the second near-infrared window in recent years, this review weighs the advantages and disadvantages of fluorescence and chemi/bioluminescence imaging, and NIR-II fluorophores based on polymethine dyes, D - A - D chromophores, and lanthanide complexes. Besides, this review also provides a very important guidance for expanding the imaging applications of molecular fluorophores in the second near-infrared window.