Anomalous Hall effect and electronic correlation in a spin-reoriented kagome antiferromagnet LuFe_(6)Sn_(6)

The kagome lattice system has been identified as a fertile ground for the emergence of a number of new quantumstates,including superconductivity,quantum spin liquids,and topological electronic states.This has attracted significantinterest within the field of condensed matter physics.Here,we present the observation of an anomalous Hall effect in aniron-based kagome antiferromagnet LuFe_(6)Sn_(6),which implies a non-zero Berry curvature in this compound.By means ofextensive magnetic measurements,a high Neel temperature,T_(N)=552 K,and a spin reorientation behavior were identifiedand a simple temperature-field phase diagram was constructed.Furthermore,this compound was found to exhibit a largeSommerfeld coefficient ofγ=87 mJ·mol^(-1)·K^(-2),suggesting the presence of a strong electronic correlation effect.Ourresearch indicates that LuFe_(6)Sn_(6)is an intriguing compound that may exhibit magnetism,strong correlation,and topologicalstates.

Anomalous valley Hall effect in two-dimensional valleytronic materials

The anomalous valley Hall effect(AVHE)can be used to explore and utilize valley degrees of freedom in materials,which has potential applications in fields such as information storage,quantum computing and optoelectronics.AVHE exists in two-dimensional(2D)materials possessing valley polarization(VP),and such 2D materials usually belong to the hexagonal honeycomb lattice.Therefore,it is necessary to achieve valleytronic materials with VP that are more readily to be synthesized and applicated experimentally.In this topical review,we introduce recent developments on realizing VP as well as AVHE through different methods,i.e.,doping transition metal atoms,building ferrovalley heterostructures and searching for ferrovalley materials.Moreover,2D ferrovalley systems under external modulation are also discussed.2D valleytronic materials with AVHE demonstrate excellent performance and potential applications,which offer the possibility of realizing novel low-energy-consuming devices,facilitating further development of device technology,realizing miniaturization and enhancing functionality of them.

Multi-functional photonic spin Hall effect sensor controlled by phase transition

Photonic spin Hall effect(PSHE), as a novel physical effect in light–matter interaction, provides an effective metrological method for characterizing the tiny variation in refractive index(RI). In this work, we propose a multi-functional PSHE sensor based on VO_(2), a material that can reveal the phase transition behavior. By applying thermal control, the mutual transformation into different phase states of VO_(2) can be realized, which contributes to the flexible switching between multiple RI sensing tasks. When VO_(2) is insulating, the ultrasensitive detection of glucose concentrations in human blood is achieved. When VO_(2) is in a mixed phase, the structure can be designed to distinguish between the normal cells and cancer cells through no-label and real-time monitoring. When VO_(2) is metallic, the proposed PSHE sensor can act as an RI indicator for gas analytes. Compared with other multi-functional sensing devices with the complex structures, our design consists of only one analyte and two VO_(2) layers, which is very simple and elegant. Therefore, the proposed VO_(2)-based PSHE sensor has outstanding advantages such as small size, high sensitivity, no-label, and real-time detection, providing a new approach for investigating tunable multi-functional sensors.

Surface phonon resonance:A new mechanism for enhancing photonic spin Hall effect and refractive index sensor

Metal-based surface plasmon resonance(SPR)plays an important role in enhancing the photonic spin Hall effect(SHE)and developing sensitive optical sensors.However,the very large negative permittivities of metals limit their applications beyond the near-infrared regime.In this work,we theoretically present a new mechanism to enhance the photonic SHE by taking advantage of SiC-supported surface phonon resonance(SPhR)in the mid-infrared regime.The transverse displacement of photonic SHE is very sensitive to the wavelength of incident light and the thickness of SiC layer.Under the optimal parameter setup,the calculated largest transverse displacement of SiC-based SPhR structure reaches up to 163.8 ym,which is much larger than the condition of SPR.Moreover,an NO_(2) gas sensor based on the SPhR-enhanced photonic SHE is theoretically proposed with the superior sensing performance.Both the intensity and angle sensitivity of this sensor can be effectively manipulated by varying the damping rate of SiC.The results may provide a promising paradigm to enhance the photonic SHE in the mid-infrared region and open up new opportunity of highly sensitive refractive index sensors.

Intrinsic valley-polarized quantum anomalous Hall effect in a two-dimensional germanene/MnI_(2) van der Waals heterostructure

Valley-polarized quantum anomalous Hall effect(VQAHE), combined nontrivial band topology with valleytronics,is of importance for both fundamental sciences and emerging applications. However, the experimental realization of this property is challenging. Here, by using first-principles calculations and modal analysis, we predict a mechanism of producing VQAHE in two-dimensional ferromagnetic van der Waals germanene/MnI_(2) heterostructure. This heterostructure exhibits both valley anomalous Hall effect and VQAHE due to the joint effects of magnetic exchange effect and spin–orbital coupling with the aid of anomalous Hall conductance and chiral edge state. Moreover interestingly, through the electrical modulation of ferroelectric polarization state in In_(2)Se_(3), the germanene/Mn I_(2)/In_(2)Se_(3) heterostructure can undergo reversible switching from a semiconductor to a metallic behavior. This work offers a guiding advancement for searching for VQAHE in ferromagnetic van der Waals heterostructures and exploiting energy-efficient devices based on the VQAHE.

Enhanced anomalous Hall effect in kagome magnet YbMn_(6)Sn_(6)with intermediate-valence ytterbium

We report on the magnetization and anomalous Hall effect(AHE)in the high-quality single crystals of the kagome magnet YbMn_(6)Sn_(6),where the spins of the Mn atoms in the kagome lattice order ferromagnetically and the intermediate-valence Yb atoms are nonmagnetic.The intrinsic mechanism plays a crucial role in the AHE,leading to an enhanced anomalous Hall conductivity(AHC)compared with the other rare-earth RMn_(6)Sn_(6)compounds.Our band structure calculation reveals a strong hybridization between the 4f electrons of Yb and conduction electrons.

Evolution of anomalous Hall effect in ferromagnetic Weyl semimetal Nb_(x)Zr_(1-x)Co_(2)Sn

Magnetic topological semimetal can host various topological non-trivial states leading to exotic novel transport properties.Here we report the systematic magneto-transport studies on the Heusler alloy Nb_(x)Zr_(1-x)Co_(2)Sn considered as a ferromagnetic(FM)Weyl semimetal.The cusp anomaly of temperature-dependent resistivity and large isotropic negative magneto-resistivity(MR)emerge around the FM transition consistent with the theoretical half-metallic predictions.The prominent anomalous Hall effect(AHE)has the same behavior with the applied field along various crystal directions.The Nb doping introduces more disorder resulting in the enhancement of the upturn for the temperature-dependent resistivity in low temperatures.With Nb doping,the AHE exhibits systemic evolution with the Fermi level lifted.At the doping level of x=0.25,the AHE mainly originates from the intrinsic contribution related to non-trivial topological Weyl states.

Tailoring-compensated ferrimagnetic state and anomalous Hall effect in quaternary Mn–Ru–V–Ga Heusler compounds

Cubic Mn_(2)Ru_(x)Ga Heusler compound is a typical example of compensated ferrimagnet with attractive potential for high-density,ultrafast,and low-power spintronic applications.In the form of epitaxial thin films,Mn_(2)Ru_(x)Ga exhibits high spin polarization and high tunability of compensation temperature by freely changing the Ru content x in a broad range(0.3<x<1.0).Herein Mn-Ru-Ga-based polycrystalline bulk buttons prepared by arc melting are systematically studied and it is found that in equilibrium bulk form,the cubic structure is unstable when x<0.75.To overcome this limitation,Mn-Ru-Ga is alloyed with a fourth element V.By adjusting the content of V in the By adjusting the content of V in the Mn_(2)Ru_(0.75)V_(y)Ga and Mn_(2.25-y)Ru_(0.75)V_(y)Ga quaternary systems the magnetic compensation temperature is tuned.Compensation is achieved near 300 K which is confirmed by both the magnetic measurement and anomalous Hall effect measurement.The analyses of the anomalous Hall effect scaling in quaternary Mn-Ru-V-Ga alloy reveal the dominant role of skew scattering,notably that contributed caused by the thermally excited phonons,in contrast to the dominant intrinsic mechanism found in many other 3d ferromagnets and Heusler compounds.It is further shown that the Ga antisites and V content can simultaneously control the residual resistivity ratio(RRR)as well as the relative contribution of phonon and defect to the anomalous Hall effect a"/a0'in Mn-Ru-V-Ga,resulting in a scaling relation a"/a0'∝RRR^(1.8).

Spin direction dependent quantum anomalous Hall effect in two-dimensional ferromagnetic materials

We propose a scheme for realizing the spin direction-dependent quantum anomalous Hall effect(QAHE)driven by spin-orbit couplings(SOC)in two-dimensional(2D)materials.Based on the sp^(3)tight-binding(TB)model,we find that these systems can exhibit a QAHE with out-of-plane and in-plane magnetization for the weak and strong SOC,respectively,in which the mechanism of quantum transition is mainly driven by the band inversion of p_(x,y)/p_(z)orbitals.As a concrete example,based on first-principles calculations,we realize a real material of monolayer 1T-SnN_(2)/PbN_(2)exhibiting the QAHE with in-plane/out-of-plane magnetization characterized by the nonzero Chern number C and topological edge states.These findings provide useful guidance for the pursuit of a spin direction-dependent QAHE and hence stimulate immediate experimental interest.

Thermal Hall effect and the Wiedemann–Franz law in Chern insulator

Thermal Hall effect, where a transverse temperature difference is generated by implementing a longitudinal temperature gradient and an external magnetic field in the perpendicular direction to systems, is a useful tool to reveal transport properties of quantum materials. A systematic study of the thermal Hall effect in a Chern insulator is still lacking. Here,using the Landauer–Büttiker formula, we investigated the thermal Hall transport of the Harper–Hofstadter model with flux φ= 1/2 and its generalizations. We demonstrated that the Wiedemann–Franz law, which states that the thermal Hall conductivity is linearly proportional to the quantum Hall conductivity in the low temperature limit, is still valid in this Chern insulator, and that the thermal Hall conductivity can be used to characterize the topological properties of quantum materials.

Single-layer intrinsic 2H-phase LuX_(2)(X=Cl,Br,I)with large valley polarization and anomalous valley Hall effect

Manipulation of the valley degree of freedom provides a new path for quantum information technology,but the real intrinsic large valley-polarization materials are rarely reported up to date.Here,we perform first-principles calculations to predict a class of 2H-phase single layer(SL)materials LuX_(2)(X=Cl,Br,I)to be ideal candidates.SL-Lu X_(2)are ferrovalley materials with a giant valley-polarization of 55 meV–148 meV as a result of its large spin–orbital coupling(SOC)and intrinsic ferromagnetism(FM).The magnetic transition temperatures of SL-LuI_(2)and SL-LuCl2are estimated to be 89 K–124 K,with a sizable magnetic anisotropy at out-of-plane direction.Remarkably,the anomalous valley Hall effect(AVHE)can be controlled in SL-LuX_(2)when an external electric field is applied.Moreover,the intrinsic valleypolarization of SL-LuI_(2)is highly robust for biaxial strain.These findings provide a promising ferrovalley material system for the experimentation of valleytronics and subsequent applications.

Vectorial spin-orbital Hall effect of light upon tight focusing and its experimental observation in azopolymer films

Hall effect of light is a result of symmetry breaking in spin and/or orbital angular momentum(OAM)possessing optical system and is caused by e.g.refractive index gradient/interface between media or focusing of a spatially asymmetrical beam,similar to the electric field breaking the symmetry in spin Hall effect for electrons.The angular momentum(AM)conservation law in the ensuing asymmetric system dictates redistribution of spin and orbital angular momentum,and is manifested in spin-orbit,orbit-orbit,and orbit-spin conversions and reorganization,i.e.spin-orbit and orbit-orbit interaction.This AM restructuring in turn requires shifts of the barycenter of the electric field of light.In the present study we show,both analytically and by numerical simulation,how different electric field components are displaced upon tight focusing of an asymmetric light beam having OAM and spin.The relation between field components shifts and the AM components shifts/redistribution is presented too.Moreover,we experimentally demonstrate,for the first time,to the best of our knowledge,the spin-orbit Hall effect of light upon tight focusing in free space.This is achieved using azopolymers as a media detecting longitudinal or z component of the electrical field of light.These findings elucidate the Hall effect of light and may broaden the spectrum of its applications.

Nonmonotonic anomalous Hall effect and anisotropic magnetoresistance in SrRuO_(3)/PbZr_(0.52)Ti_(0.48)O_(3)heterostructures

We fabricate SrRuO_(3)/PbZr_(0.52)Ti_(0.48)O_(3)heterostructures each with an in-plane tensile-strained SrRuO_(3)layer and investigate the effect of an applied electric field on anomalous Hall effect.The four-fold symmetry of anisotropic magnetoresistance and the nonmonotonic variation of anomalous Hall resistivity are observed.By applying positive electric field or negative electric field,the intersecting hump-like feature is suppressed or enhanced,respectively.The sign and magnitude of the anomalous Hall conductivity can be effectively controlled with an electric field under a high magnetic field.The electric-field-modulated anomalous Hall effect is associated with the magnetization rotation in SrRuO_(3).The experimental results are helpful in modulating the magnetization rotation in spintronic devices based on SrRuO_(3)heterostructures.

Anomalous Hall effect in ferromagnetic LaCo_(2)As_(2) and ferrimagnetic NdCo_(2)As_(2)

We conducted a comparative study of the magnetic and transport properties of single-crystalline LaCo_(2)As_(2) and NdCo_(2)As_(2).LaCo_(2)As_(2) is a soft metallic ferromagnet which exhibits purely intrinsic anomalous Hall effect(AHE) due to Co-3d electrons. With Nd-4f electronic magnetism, ferrimagnetic NdCo_(2)As_(2) manifests pronounced sign reversal and multiple hysteresis loops in temperature-and field-dependent magnetization, Hall resistivity, and magnetoresistance, due to complicated magnetic structural changes. We reveal that the AHE for NdCo_(2)As_(2) is stemming from the Co sub-lattice and deduce its phase diagram which includes magnetic compensation and two meta-magnetic phase transitions. The sensitivity of the Hall effect on the details of the magnetic structures in ferrimagnetic NdCo_(2)As_(2) provides a unique opportunity to explore the magnetic interaction between 4f and 3d electrons and its impact on the electronic structure.

Coexistence of giant Rashba spin splitting and quantum spin Hall effect in H–Pb–F

Rashba spin splitting(RSS)and quantum spin Hall effect(QSHE)have attracted enormous interest due to their great significance in the application of spintronics.In this work,we theoretically proposed a new two-dimensional(2D)material H–Pb–F with coexistence of giant RSS and quantum spin Hall effec by using the ab initio calculations.Our results show that H–Pb–F possesses giant RSS(1.21 eV·A)and the RSS can be tuned up to 4.16 e V·A by in-plane biaxial strain,which is a huge value among 2D materials.Furthermore,we also noticed that H–Pb–F is a 2D topological insulator(TI)duo to the strong spin–orbit coupling(SOC)interaction,and the large topological gap is up to 1.35 e V,which is large enough for for the observation of topological edge states at room temperature.The coexistence of giant RSS and quantum spin Hall effect greatly broadens the potential application of H–Pb–F in the field of spintronic devices.

Recent progress on the planar Hall effect in quantum materials

The planar Hall effect(PHE),which originates from anisotropic magnetoresistance,presents a qualitative and simple approach to characterize electronic structures of quantum materials by applying an in-plane rotating magnetic field to induce identical oscillations in both longitudinal and transverse resistances.In this review,we focus on the recent research on the PHE in various quantum materials,including ferromagnetic materials,topological insulators,Weyl semimetals,and orbital anisotropic matters.Firstly,we briefly introduce the family of Hall effect and give a basic deduction of PHE formula with the second-order resistance tensor,showing the mechanism of the characteristicπ-period oscillation in trigonometric function form with aπ/4 phase delay between the longitudinal and transverse resistances.Then,we will introduce the four main mechanisms to realize PHE in quantum materials.After that,the origin of the anomalous planar Hall effect(APHE)results,of which the curve shapes deviate from that of PHE,will be reviewed and discussed.Finally,the challenges and prospects for this field of study are discussed.

Stability analysis of magnetization in a perpendicular magnetic layer driven by spin Hall effect

We investigate the stability of magnetization in free layer where the spin torque is induced by the spin Hall effect.In terms of the Landau–Liftshitz–Gilbert equation,we find the low-energy and high-energy equilibrium states,as well as the saddle points.The stability region is defined in the phase diagram spanned by the current density and the spin Hall angle.The spin Hall effect makes the previous saddle point into a stable state above a critical current.However,in the presence of magnetic field,the spin Hall effect leads to the opposite changes in the stable regions of the two low-energy states.

Doping tuned anomalous Hall effect in the van der Waals magnetic topological phases Mn(Sb_(1-x)Bi_(x))_(4)Te_(7)

The van der Waals(vdW)MnSb4Te7is a newly synthesized antiferromagnetic(AFM)topological insulator hosting a robust axion insulator state irrelative to the specific spin structure.However,the intrinsic hole doped character of MnSb_4Te_7makes the Fermi level far away from the Dirac point of about 180 meV,which is unfavorable for the exploration of exotic topological properties such as the quantum anomalous Hall effect(QAHE).To shift up the Fermi level close to the Dirac point,the strategy of partially replacing Sb with Bi as Mn(Sb_(1-x)Bi_(x))_(4)Te_(7)was tried and the magnetotransport properties,in particular,the anomalous Hall effect,were measured and analyzed.Through the electron doping,the anomalous Hall conductanceσAH changes from negative to positive between x=0.3 and 0.5,indicative of a possible topological transition.Besides,a charge neutrality point(CNP)also appears between x=0.6 and 0.7.The results would be instructive for further understanding the interplay between nontrivial topological states and the magnetism,as well as for the exploration of exotic topological properties.

Engineered photonic spin Hall effect of Gaussian beam in antisymmetric parity-time metamaterials

A model of the photonic spin Hall effect(PSHE)in antisymmetric parity-time(APT)metamaterials with incidence of Gaussian beams is proposed here.We derive the displacement expression of the PSHE in APT metamaterials based on the transport properties of Gaussian beams in positive and negative refractive index materials.Furthermore,detailed discussions are provided on the APT scattering matrix,eigenstate ratio,and response near exceptional points in the case of loss or gain.In contrast to the unidirectional non-reflection in parity-time(PT)symmetric systems,the transverse shift that arises from both sides of the APT structure is consistent.By effectively adjusting the parameters of APT materials,we achieve giant displacements of the transverse shift.Finally,we present a multi-layer APT structure consisting of alternating left-handed and right-handed materials.By increasing the number of layers,Bragg oscillations can be generated,leading to an increase in resonant peaks in transverse shift.This study presents a new approach to achieving giant transverse shifts in the APT structure.This lays a theoretical foundation for the fabrication of related nano-optical devices.

First-principles prediction of quantum anomalous Hall effect in two-dimensional Co2Te lattice

The quantum anomalous Hall effect(QAHE) has special quantum properties that are ideal for possible future spintronic devices. However, the experimental realization is rather challenging due to its low Curie temperature and small non-trivial bandgap in two-dimensional(2D) materials. In this paper, we demonstrate through first-principles calculations that monolayer Co2Te material is a promising 2D candidate to realize QAHE in practice. Excitingly, through Monte Carlo simulations, it is found that the Curie temperature of single-layer Co2Te can reach 573 K. The band crossing at the Fermi level in monolayer Co2Te is opened when spin–orbit coupling is considered, which leads to QAHE with a sizable bandgap of Eg= 96 me V, characterized by the non-zero Chern number(C = 1) and a chiral edge state. Therefore, our findings not only enrich the study of quantum anomalous Hall effect, but also broaden the horizons of the spintronics and topological nanoelectronics applications.