Giant Hall Effect Research Articles

Interplay between Magnetism and Topology: Large Topological Hall Effect in an Antiferromagnetic Topological Insulator, EuCuAs.

Magnetic interactions in combination with nontrivial band structures can give rise to several exotic physical properties such as a large anomalous Hall effect, the anomalous Nernst effect, and the topological Hall effect (THE). Antiferromagnetic (AFM) materials exhibit the THE due to the presence of nontrivial spin structures. EuCuAs crystallizes in a hexagonal structure with an AFM ground state (Néel temperature ∼ 16 K). In this work, we observe a large topological Hall resistivity of ∼7.4 μΩ-cm at 13 K which is significantly higher than the giant topological Hall effect of Gd2PdSi3 (∼3 μΩ-cm). Neutron diffraction experiments reveal that the spins form a transverse conical structure during the metamagnetic transition, resulting in the large THE. In addition, by controlling the magnetic ordering structure of EuCuAs with an external magnetic field, several fascinating topological states such as Dirac and Weyl semimetals have been revealed. These results suggest the possibility of spintronic devices based on antiferromagnets with tailored noncoplanar spin configurations.

Observation of negative terahertz photoconductivity in strongly correlated electron-doped CaMnO3 thin film

Electron-doped Ca0.96Ce0.04MnO3 (CCMO) possesses a unique band structure and exhibits a giant topological Hall effect contrary to other correlation-driven manganites known for insulator-to-metal transition, magnetoresistance, complex magnetic order, etc. The interaction mechanisms among the fundamental entities and their dynamical evolutions responsible for this unusual topological phase are yet to be understood. Here, we employ time-averaged and sub-picosecond time-resolved terahertz (THz) spectroscopy to explore the low-energy steady-state and ultrafast carrier dynamics, respectively, to unravel the complexity of charge carriers during their transition from a non-equilibrium state to the ground state in CCMO thin film. The THz optical conductivity confirms the presence of dichotomic charge carriers, i.e. heavy and light carriers throughout the temperature range of 15–300 K. A rare observation of both positive and negative photoconductivities along with a sharp crossover between the two resolved to a few picoseconds of illumination confirms the formation of polaron with a lifetime of a few nanoseconds. These optical evidences of dichotomic charge carriers, along with manipulation of the sign of photoconductivity induced by dynamics of related quasiparticles could facilitate a new mechanism for ultrafast optoelectronic switching devices.

Coexistence of multiple stacking charge density waves in kagome superconductor CsV3Sb5

The recently discovered Kagome family ${\mathrm{AV}}_3{\mathrm{Sb}}_5$ (A = K, Rb, Cs) exhibits rich physical phenomena, including non-trivial topological electronic structure, giant anomalous Hall effect, charge density waves (CDW) and superconductivity. Notably, CDW in ${\mathrm{AV}}_3{\mathrm{Sb}}_5$ is evidenced to intertwine with its superconductivity and topology, but its nature remains elusive. Here, we combine x-ray scattering experiments and density-functional theory calculations to investigate the CDWs in ${\mathrm{CsV}}_3{\mathrm{Sb}}_5$ and demonstrate the coexistence of 2 $\times$ 2 $\times$ 2 and 2 $\times$ 2 $\times$ 4 CDW stacking phases. Competition between these CDW phases is revealed by tracking the temperature evolution of CDW intensities, which also manifests in different transition temperatures during warming- and cooling measurements. We also identify a meta-stable quenched state of ${\mathrm{CsV}}_3{\mathrm{Sb}}_5$ after fast-cooling process. Our study demonstrates the coexistence of competing CDW stacking in ${\mathrm{CsV}}_3{\mathrm{Sb}}_5$, offering new insights in understanding the novel properties of this system.

Magnetic-field modulation of topological electronic state and emergent magneto-transport in a magnetic Weyl semimetal

The modulation of topological electronic state by an external magnetic field is highly desired for condensed-matter physics. Schemes to achieve this have been proposed theoretically, but few can be realized experimentally. Here, combining transverse transport, theoretical calculations, and scanning tunneling microscopy/spectroscopy (STM/S) investigations, we provide an observation that the topological electronic state, accompanied by an emergent magneto-transport phenomenon, was modulated by applying magnetic field through induced non-collinear magnetism in the magnetic Weyl semimetal EuB6. A giant unconventional anomalous Hall effect (UAHE) is found during the magnetization re-orientation from easy axes to hard ones in magnetic field, with a UAHE peak around the low field of 5 kOe. Under the reasonable spin-canting effect, the folding of the topological anti-crossing bands occurs, generating a strong Berry curvature that accounts for the observed UAHE. Field-dependent STM/S reveals a highly synchronous evolution of electronic density of states, with a dI/dV peak around the same field of 5 kOe, which provides evidence to the folded bands and excited UAHE by external magnetic fields. This finding elucidates the connection between the real-space non-collinear magnetism and the k-space topological electronic state and establishes a novel manner to engineer the magneto-transport behaviors of correlated electrons for future topological spintronics.

Electrically controlled superconductor-to-failed insulator transition and giant anomalous Hall effect in kagome metal CsV3Sb5 nanoflakes

The electronic correlations (e.g. unconventional superconductivity (SC), chiral charge order and nematic order) and giant anomalous Hall effect (AHE) in topological kagome metals AV3Sb5 (A = K, Rb, and Cs) have attracted great interest. Electrical control of those correlated electronic states and AHE allows us to resolve their own nature and origin and to discover new quantum phenomena. Here, we show that electrically controlled proton intercalation has significant impacts on striking quantum phenomena in CsV3Sb5 nanodevices mainly through inducing disorders in thinner nanoflakes and carrier density modulation in thicker ones. Specifically, in disordered thin nanoflakes (below 25 nm), we achieve a quantum phase transition from a superconductor to a “failed insulator” with a large saturated sheet resistance for T → 0 K. Meanwhile, the carrier density modulation in thicker nanoflakes shifts the Fermi level across the charge density wave (CDW) gap and gives rise to an extrinsic-intrinsic transition of AHE. With the first-principles calculations, the extrinsic skew scattering of holes in the nearly flat bands with finite Berry curvature by multiple impurities would account for the giant AHE. Our work uncovers a distinct disorder-driven bosonic superconductor-insulator transition (SIT), outlines a global picture of the giant AHE and reveals its correlation with the unconventional CDW in the AV3Sb5 family.

Giant Second-Order Nonlinear Hall Effect in Twisted Bilayer Graphene.

In the second-order response regime, the Hall voltage can be nonzero without time-reversal symmetry breaking but inversion symmetry breaking. Multiple mechanisms contribute to the nonlinear Hall effect. The disorder-related contributions can enter the NLHE in the leading role, but experimental investigations are scarce, especially the exploration of the contributions from different disorder sources. Here, we report a giant nonlinear response in twisted bilayer graphene, dominated by disorder-induced skew scattering. The magnitude and direction of the second-order nonlinearity can be effectively tuned by the gate voltage. A peak value of the second-order Hall conductivity reaching 8.76 μm SV^{-1} is observed close to the full filling of the moiré band, four order larger than the intrinsic contribution detected in WTe_{2}. The scaling shows that the giant second-order nonlinear Hall effect in twisted bilayer graphene stems from the collaboration of the static (impurities) and dynamic (phonons) disorders. It is mainly determined by the impurity skew scattering at 1.7K. The phonon skew scattering, however, has a much larger coupling coefficient, and becomes comparable to the impurity contribution as the temperature rises. Our observations provide a comprehensive experimental understanding of the disorder-related mechanisms in the nonlinear Hall effect.

The giant orbital Hall effect in Cr/Au/Co/Ti multilayers

The spin–orbit torques originating from the spin Hall effect of heavy metals are of vital importance for applications in spintronics due to its low consumption of energy. Theoretical calculations have predicted that 3d and 4d light metals can produce a similar amount of torques to heavy metals via the strong orbital Hall effect (OHE). However, few experiments have been conducted since it is technically challenging to directly detect the orbital current from the OHE. Here, we report an effective approach to demonstrate the strong orbital torques in the light metal Cr with the aid of a conversion process from the orbital current to the spin current by introducing an Au interfacial layer in the Cr/ferromagnet structures. A rather large orbital torque efficiency and an increase with the increasing thickness of the Cr-layer are attained in the perpendicularly magnetized Cr/Au/Co/Ti multilayers. Moreover, an energy efficient magnetization switching and the domain wall motion in Cr/Au/Co/Ti multilayers induced by the OHE have also been observed. Our findings confirm the existence of the orbital Hall torques in Cr and provide an effective way to investigate the OHE.

Giant nonlinear Hall effect in twisted bilayer WSe2.

The recently discovered nonlinear Hall effect (NHE) in a few non-interacting systems provides a novel mechanism for generating second-harmonic electrical Hall signals under time-reversal-symmetric conditions. Here, we introduce a new approach to engineering an NHE by using twisted moiré structures. We found that the twisted WSe2 bilayer exhibited an NHE when the Fermi level was tuned to the moiré flat bands. When the first moiré band was half-filled, the nonlinear Hall signal exhibited a sharp peak with a generation efficiency that was at least two orders of magnitude greater than those obtained in previous experiments. We discuss the possible origins of the diverging generation efficiency in twisted WSe2 based on resistivity measurements, such as moiré-interface-induced correlation effects and mass-diverging-type continuous Mott transition. This study demonstrates not only how interaction effects can combine with Berry curvature dipoles to produce novel quantum phenomena, but also the potential of NHE measurements as a new tool for studying quantum criticality.

Magnetic and charge instabilities in vanadium-based topological kagome metals

Vanadium-based topological kagome metals ${\mathrm{AV}}_{3}{\mathrm{Sb}}_{5}$ ($A=\mathrm{K}$, Rb, Cs) have drawn great attention recently due to the discoveries of charge order, nematic phase, time-reversal symmetry breaking, and superconductivity. In this work, we study the Fermi-surface instabilities of topological kagome metals ${\mathrm{AV}}_{3}{\mathrm{Sb}}_{5}$ ($A=\mathrm{K}$, Rb, Cs) in all the charge-orbital-spin channels based on first principles calculations, and provide a first principles theory for the current-loop state in this class of materials. After comprehensive analysis of the interaction-renormalized generalized susceptibility tensor combined with unrestricted Hartree-Fock calculations, we find that the leading instability modes at the Fermi surface are three spin magnetic modes at $\mathrm{\ensuremath{\Gamma}}$ point, giving rise to ferromagnetism within the kagome plane with small magnetization on the V atoms. We further show that counter-propagating current loops within the kagome plane can be generated due to the interplay between spin magnetism and spin-orbit coupling, and the current pattern is consistent with recent muon spin spectroscopy measurements. Moreover, some other puzzling experiments in this system such as the giant anomalous Hall effect and nematicity can also be explained from our results.

Optical anomalous Hall effect enhanced by flat bands in ferromagnetic van der Waals semimetal

Geometrical aspects of electronic states in condensed matter have led to the experimental realization of enhanced electromagnetic phenomena, as exemplified by the giant anomalous Hall effect (AHE) in topological semimetals. However, the guideline to the large AHE is still immature due to lack of profound understanding of the sources of the Berry curvature in actual electronic structures; the main focus has concentrated only on the band crossings near the Fermi level. Here, we show that the band crossings and flat bands cooperatively produce the large intrinsic AHE in ferromagnetic nodal line semimetal candidate Fe3GeTe2. The terahertz and infrared magneto-optical spectroscopy reveals that two explicit resonance structures in the optical Hall conductivity spectra σxy(ω) are closely related to the AHE. The first-principles calculation suggests that both the flat bands having large density of states (DOS) and the band crossings near the Fermi level are the main causes of these Hall resonances. Our findings unveil a mechanism to enhance the AHE based on the flat bands, which gives insights into the topological material design.

Endless Dirac nodal lines in kagome-metal Ni3In2S2

Topological semimetals are a frontier of quantum materials. In multiband electronic systems, topological band crossings can form closed curves, known as nodal lines. In the presence of spin–orbit coupling and/or symmetry-breaking operations, topological nodal lines can break into Dirac/Weyl nodes and give rise to interesting transport properties, such as the chiral anomaly and giant anomalous Hall effect. Recently, the time-reversal symmetry-breaking induced Weyl fermions are observed in a kagome-metal Co3Sn2S2, triggering interests in nodal-line excitations in multiband kagome systems. Here, using first-principles calculations and symmetry-based indicator theories, we find six endless nodal lines along the stacking direction of kagome layers and two nodal rings in the kagome plane in nonmagnetic Ni3In2S2. The linear dipsersive electronic structure, confirmed by angle-resolved photoemission spectroscopy, induces large magnetoresistance up to 2000% at 9 T. Our results establish a diverse topological landscape of multiband kagome metals.

Giant nonlinear Hall effect in strained twisted bilayer graphene

Recent studies have shown that moir\'e flat bands in twisted bilayer graphene (TBG) can acquire nontrivial Berry curvatures when aligned with a hexagonal boron nitride substrate, which can be manifested as a correlated Chern insulator near the 3/4 filling. In this Letter, we show that the large Berry curvatures in the moir\'e bands lead to a strong nonlinear Hall (NLH) effect in strained TBG with general filling factors. Under a weak uniaxial strain $\ensuremath{\sim}0.1%$, the Berry curvature dipole which characterizes the nonlinear Hall response can be as large as $\ensuremath{\sim}200\phantom{\rule{0.28em}{0ex}}\text{\AA{}}$, exceeding the values of previously known nonlinear Hall materials by two orders of magnitude. The dependence of the giant NLH effect as a function of electric gating, strain, and twist angle is further investigated systematically. Importantly, we point out that the giant NLH effect appears generically for a twist angle near the magic angle due to the strong susceptibility of nearly flat moir\'e bands to symmetry breaking induced by strains, which can even induce a topological band inversion. Our results establish TBG as a promising platform for investigating nonlinear effects such as the NLH effect, the nonlinear Nernst effect, and the nonlinear thermal Hall effect due to its giant Berry curvature dipole.

Giant nonlinear anomalous Hall effect induced by spin-dependent band structure evolution

The anomalous Hall effect (AHE) is a key transport signature revealing the topological properties of magnetic compounds. In quantum materials, the classical linear dependence of the AHE on magnetization often breaks down, which is typically ascribed to the presence of topological magnetic or electronic textures. However, the complex electronic structure of these compounds may offer alternative, unexplored mechanisms. Here, we show that a giant nonlinear AHE can originate from a series of magnetic-field-induced Lifshitz transitions in the spin-dependent band structure. In our experiments on ${\mathrm{EuCd}}_{2}{\mathrm{As}}_{2}$ the AHE contributes to 97% of the total Hall response, corresponding to a record anomalous Hall angle of 21%. Our scaling analysis and first-principles calculations demonstrate that the electronic structure is extremely sensitive to spin canting, with the magnetic field causing band crossing and band inversion and introducing a band gap when oriented along specific directions. Our results not only provide an ideal platform for Berry curvature engineering but reveal a general effect that may be applied to other material systems.

Anomalous thermal Hall effect and anomalous Nernst effect of CsV3Sb5

Motived by time-reversal symmetry breaking and giant anomalous Hall effect in kagome superconductor $A{\mathrm{V}}_{3}{\mathrm{Sb}}_{5}$ ($A$ = Cs, K, Rb), we carried out the thermal transport measurements on $\mathrm{Cs}{\mathrm{V}}_{3}{\mathrm{Sb}}_{5}$. In addition to the anomalous Hall effect, the anomalous Nernst effect and the anomalous thermal Hall effect emerge. Interestingly, the longitudinal thermal conductivity ${\ensuremath{\kappa}}_{xx}$ largely deviates from the electronic contribution obtained from the longitudinal conductivity ${\ensuremath{\sigma}}_{xx}$ by the Wiedemann-Franz law. In contrast, the thermal Hall conductivity ${\ensuremath{\kappa}}_{xy}$ is roughly consistent with the Wiedemann-Franz law from electronic contribution. All these results indicate the large phonon contribution in the longitudinal thermal conductivity. Moreover, the thermal Hall conductivity is also slightly greater than the theoretical electronic contribution, indicating other charge neutral contributions. More than that, the Nernst coefficient and Hall resistivity show the multiband behavior with possible additional contribution from Berry curvature at the low fields.

Bipolar semiconductor in two-dimensional covalent organic frameworks

As a tunable spintronic device, the bipolar magnetic semiconductor (BMS) has been intensively studied in recent years, but the candidate materials are limited in the magnetic system. In this work, we report an alternative strategy to realize it in a nonmagnetic system called bipolar semiconductor (BS). Utilizing doping-induced ferromagnetism, the enantiomorphic flat bands in BS are used to switch the spin carriers, achieving the same function as the BMS. Based on the frontier orbital of the molecular building blocks, a universal approach is proposed to search the BS in two-dimensional (2D) covalent organic frameworks (COFs), which is confirmed by first-principles calculations in the experimentally synthesized fluoride-graphdiyne (F-GDY). Moreover, the spin-polarized channels and giant anomalous Hall effect can also be created in F-GDY through the light irradiation. Our results demonstrate a new spintronic application for 2D COFs without the intrinsic magnetism.

Weyl semimetal in the rare-earth hexaboride family supporting a pseudonodal surface and a giant anomalous Hall effect

Rare-earth hexaborides offer a rich tapestry for exploring the interplay of topological orders, magnetism, and electron-correlation effects associated with protected quantum phenomena. Here, using first-principles modeling, we identify in cerium hexaboride ($\mathrm{Ce}{\mathrm{B}}_{6}$) a topological state that harbors a pseudonodal surface (PNS) along with double-Weyl fermions. An analysis of the electronic states in the vicinity of the Fermi level reveals the presence of ${M}_{z}$-mirror-symmetry-protected band crossings that involve itinerant ($d$) and localized ($f$) states of Ce. These band crossings are found to remain nearly gapless as one goes away from the mirror plane, driving the formation of PNS states. We also demonstrate the presence of coexisting double-Weyl fermions with chiral charges of $\ifmmode\pm\else\textpm\fi{}2$ on the ${C}_{4z}$ rotational axis. By analyzing the Berry curvature field associated with the topological states, we predict that $\mathrm{Ce}{\mathrm{B}}_{6}$ supports a giant anomalous Hall conductivity of $\ensuremath{-}1840 {\mathrm{\ensuremath{\Omega}}}^{\ensuremath{-}1}{\text{cm}}^{\ensuremath{-}1}$ in the slightly electron-doped regime, which would be larger than the values reported to date in magnetic Weyl semimetals. Our study thus indicates that $\mathrm{Ce}{\mathrm{B}}_{6}$, which combines ferromagnetic order with topological physics in a heavy-fermion matrix, would provide a promising playground for investigating topological magnetic states with giant anomalous Hall responses.

Tunable and giant valley-selective Hall effect in gapped bilayer graphene.

Berry curvature is analogous to magnetic field but in momentum space and is commonly present in materials with nontrivial quantum geometry. It endows Bloch electrons with transverse anomalous velocities to produce Hall-like currents even in the absence of a magnetic field. We report the direct observation of in situ tunable valley-selective Hall effect (VSHE), where inversion symmetry, and thus the geometric phase of electrons, is controllable by an out-of-plane electric field. We use high-quality bilayer graphene with an intrinsic and tunable bandgap, illuminated by circularly polarized midinfrared light, and confirm that the observed Hall voltage arises from an optically induced valley population. Compared with molybdenum disulfide (MoS2), we find orders of magnitude larger VSHE, attributed to the inverse scaling of the Berry curvature with bandgap. By monitoring the valley-selective Hall conductivity, we study the Berry curvature's evolution with bandgap. This in situ manipulation of VSHE paves the way for topological and quantum geometric optoelectronic devices, such as more robust switches and detectors.

First-principles quantum-computational analysis on the interplay between intermagnetic and intermetallic properties of lead-doped cerium-bismuthides CePbxBi1-x: A new example of heavy-fermionic magnetic conductors

Heavy fermion quantum systems are great testbed both for diverse fundamental quantum prospects and potential promise for quantum technology. This owes to their exotic geometrical, electronic and magnetic properties such as Kondo effect or magnetic frustration, giant Hall effect, anisotropic magnetic order and superconductivity. In this paper, we propose a new example of heavy-fermion conductors on the basis of theoretical quantum-computational analysis of structural, electronic and magnetic properties of the lead-doped cerium bismuthides via CePbxBi1-x (x = 0, 0.25, 0.5, 0.75, 1.0) doping schemes. The results on the quantum computational analysis on the electronic and magnetic properties accomplished by GGA+PBE method predict heavy fermionic, intermetallic and intermagnetic behavior of the doped compounds. We predict a varying metallic nature of the doped compounds subjected to variation in lead-doping concentration, but the conductivity of CeBi enhanced considerably in the phase CePb0.5Bi0.5. Besides, the doped compounds showed anisotropic magnetic order subjected to varying doping concentration of Pb with or without Coulomb repulsion potential U and spin orbit coupling. The exotic quantum properties arise from the quantum correlation of the (Bi+Pb)-p orbitals and Ce-(f+d) orbitals around the Fermi level.

Robustness of the unidirectional stripe order in the kagome superconductor CsV3Sb5

V-based kagome materials AV3Sb5 (A = K, Rb, Cs) have attracted much attention due to their novel properties such as unconventional superconductivity, giant anomalous Hall effect, charge density wave (CDW) and pair density wave. Except for the 2a 0 × 2a 0 CDW (charge density wave with in-plane 2 × 2 superlattice modulation) in AV3Sb5, an additional 1 × 4 (4a 0) unidirectional stripe order has been observed at the Sb surface of RbV3Sb5 and CsV3Sb5. However, the stability and electronic nature of the 4a 0 stripe order remain controversial and unclear. Here, by using low-temperature scanning tunneling microscopy/spectroscopy (STM/S), we systematically study the 4a 0 stripe order on the Sb-terminated surface of CsV3Sb5. We find that the 4a 0 stripe order is visible in a large energy range. The STM images with positive and negative bias show contrast inversion, which is the hallmark for the Peierls-type CDW. In addition, below the critical temperature about 60 K, the 4a 0 stripe order keeps unaffected against the topmost Cs atoms, point defects, step edges and magnetic field up to 8 T. Our results provide experimental evidences on the existence of unidirectional CDW in CsV3Sb5.

A giant topological Hall effect in ferromagnetic compound CeMn2Ge2 with wide temperature range from 70 to 310 k

The transport properties of ferromagnetic compound CeMn2Ge2 have been systematically studied by magnetic resistivity measurements. Strikingly, it has been found that CeMn2Ge2 compound displays a giant topological Hall effect (THE) about 0.8 μΩ·cm with 0.45 T magnetic field under 220 k. THE appears in a wide temperature range between 70 and 310 K in a magnetic field region, and could even appear in a zero magnetic field at room temperature. The magnetoresistance curves present gradual change with kinks which have relation to magnetic transitions. Moreover, these results show that topologically protected spin texture may exist in CeMn2Ge2 ferromagnetic compound.