Quantum Hall Research Articles

The Quantum Hall Effect under the Influence of Gravity and Inertia: A Unified Approach

The quantum Hall effect under the influence of gravity and inertia is studied in a unified way. We make use of an algebraic approach, as opposed to an analytic approach. We examine how both the integer and the fractional quantum Hall effects behave under a combined influence of gravity and inertia using a unified Hamiltonian. For that purpose, we first re-derive, using the purely algebraic method, the energy spectrum of charged particles moving in a plane perpendicular to a constant and uniform magnetic field either (i) under the influence of a nonlinear gravitational potential or (ii) under the influence of a constant rotation. The general Hamiltonian for describing the combined effect of gravity, rotation and inertia on the electrons of a Hall sample is then built and the eigenstates are obtained. The electrons mutual Coulomb interaction that gives rise to the familiar fractional quantum Hall effect is also discussed within such a combination.

Spectral Asymmetry Induces a Re-Entrant Quantum Hall Effect in a Topological Insulator.

The band inversion of topological materials in three spatial dimensions is intimately connected to the parity anomaly of 2D massless Dirac fermions, known from quantum field theory. At finite magnetic fields, the parity anomaly reveals itself as a non-zero spectral asymmetry, i.e., an imbalance between the number of conduction and valence band Landau levels, due to the unpaired zero Landau level. This work reports the realization of this 2D Dirac physics at a single surface of the 3D topological insulator (Hg,Mn)Te. An unconventional re-entrant sequence of quantized Hall plateaus in the measured Hall resistance can be directly related to the occurrence of spectral asymmetry in a single topological surface state. The effect should be observable in any topological insulator where the transport is dominated by a single Dirac surfacestate.

Localization of Chiral Edge States by the Non-Hermitian Skin Effect

Localization of Chiral Edge States by the Non-Hermitian Skin Effect

On the zero-field quantization of the anomalous quantum Hall effect in two-dimensional moiré layers

On the zero-field quantization of the anomalous quantum Hall effect in two-dimensional moiré layers

Enhanced anomalous Hall effect in kagome magnet YbMn6Sn6 with intermediate-valence ytterbium

We report on the magnetization and anomalous Hall effect (AHE) in the high-quality single crystals of the kagome magnet YbMn6Sn6, 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 RMn6Sn6 compounds. Our band structure calculation reveals a strong hybridization between the 4f electrons of Yb and conduction electrons.

Fractional Quantum Hall States of the A Phase in the Second Landau Level.

A proposal of the existence of an Anomalous phase (A phase) [Das etal., Phys. Rev. Lett. 131, 056202 (2023)PRLTAO0031-900710.1103/PhysRevLett.131.056202] at the experimental range of moderate Landau-level-mixing strength has recently been made for the 5/2 state. We here report that the gapped A phase is generic to the sequence of spin-polarized fractional quantum Hall states with filling fractions ν=n/(nm-1) and ν=1-n/(nm-1), (n≥1,m≥3), that exhausts almost all the observed states and also predicts some states in the second Landau level for GaAs systems. Our proposed trial wave functions for all these states have remarkably high overlaps with the corresponding exact ground states and can support non-Abelian quasiparticle excitations with charge e/[2(nm-1)]. By analyzing edge modes, we predict experimentally verifiable thermal Hall conductance 2.5(π^{2}k_{B}^{2}T/3h) for all the states in these sequences.

Thermodynamic properties of the noncommutative quantum Hall effect with anomalous magnetic moment

In this paper, we study the thermodynamic properties of the noncommutative quantum Hall effect (NCQHE) with anomalous magnetic moment (AMM) for both relativistic and nonrelativistic cases at high temperatures, where the thermodynamic properties are: the Helmholtz free energy, the entropy, the mean energy, and the heat capacity. We also work with the Euler-MacLaurin formula to construct the partition function. Next, we plotted the graphs of properties as a function of temperature for different values of the magnetic field and of the NC parameters. We note that the Helmholtz free energy decreases with the temperature, increases with the NC parameters, and can decrease or increase with the magnetic field, while the entropy increases with the temperature, decreases with the NC parameters, and can decrease or increase with the magnetic field. Besides, the mean energy increases linearly with the temperature and the heat capacity satisfies the Dulong-Petit law, which also verified that the AMM does not influence the thermodynamic properties.

Symmetry and magnetic direction dependent spin/valley splitting and anomalous Hall conductivity of antiferromagnetic monolayer MnPTe3

Due to the time reversal symmetry breaking or spin-valley coupling, there are intrinsic valley splittings in two-dimensional magnetic materials, which are closely related to the symmetry and magnetic order. Based on MnPTe3 monolayers with different symmetries, we present an understanding of direction dependent spin/valley splitting and anomalous Hall conductivity from the perspective of orbital interactions. Unlike T-MnPTe3 with central inversion symmetry, in-plane magnetization leads to further non-degeneracy of valley electrons of H–MnPTe3, and the splitted valley states are spin polarized, which is perpendicular to the in-plane magnetization direction. The model and first-principle calculations consistently indicate that the splitting of the valley states is almost proportional to sinθxz, which originates from the difference in spin-up and spin-down interactions introduced by the breaking of spatial inversion symmetry. Regardless of spin degeneracy, non-zero anomalous Hall conductivity occurs with out-of-plane magnetization, and this direction dependent anomalous Hall effect is caused by mirror symmetry breaking from the neighboring Te atoms and spin-orbit coupling. Magnetization direction is an important means of regulating valley/spin splittings and anomalous Hall effect, which has certain significance for realizing spin or valley polarized Hall effect and the research of spintronic or valleytronic devices.

Epitaxial Growth of Large-Scale 2D CrTe2 Films on Amorphous Silicon Wafers With Low Thermal Budget.

2D van der Waals (vdW) magnets open landmark horizons in the development of innovative spintronic device architectures. However, their fabrication with large scale poses challenges due to high synthesis temperatures (>500 °C) and difficulties in integrating them with standard complementary metal-oxide semiconductor (CMOS) technology on amorphous substrates such as silicon oxide (SiO2) and silicon nitride (SiNx). Here, a seeded growth technique for crystallizing CrTe2 films on amorphous SiNx/Si and SiO2/Si substrates with a low thermal budget is presented. This fabrication process optimizes large-scale, granular atomic layers on amorphous substrates, yielding a substantial coercivity of 11.5 kilo-oersted, attributed to weak intergranular exchange coupling. Field-driven Néel-type stripe domain dynamics explain the amplified coercivity. Moreover, the granular CrTe2 devices on Si wafers display significantly enhanced magnetoresistance, more than doubling that of single-crystalline counterparts. Current-assisted magnetization switching, enabled by a substantial spin-orbit torque with a large spin Hall angle (85) and spin Hall conductivity (1.02 × 107 ℏ/2e Ω⁻¹ m⁻¹), is also demonstrated. These observations underscore the proficiency in manipulating crystallinity within integrated 2D magnetic films on Si wafers, paving the way for large-scale batch manufacturing of practical magnetoelectronic and spintronic devices, heralding a new era of technological innovation.

Spin-fluctuation-induced sign reversal of the spin Hall angle in Pt100−x Co x alloys near the Curie temperature

The spin Hall effect (SHE), typically emerging in non-magnetic metals with strong spin-orbit couplings, has attracted significant attention for its ability to convert a charge current into a spin current, a key feature in power-efficient spintronic devices. Recently, an enhanced SHE has been detected in the magnetic alloys, where the spin Hall conductivity is strongly modified by the dynamical and thermal spin fluctuations. We find that the spin Hall angle ( θSH ) in Pt100−x Co x alloys dramatically changes at the Curie temperature, which is positive in the paramagnetic phase akin to Pt, while negative in the ferromagnetic phase. Such intriguing behavior of θSH stemming from individual and collective fluctuations in the magnetic moments is further substantiated with the full-fledged Monte Carlo simulations. Our work broadens insights into the SHE and highlights the importance of spin fluctuations for the spin-current generation near the ferromagnetic instability point of magnetic alloys.

Weyl points and anomalous transport effects tuned by the Fe doping in Mn3Ge Weyl semimetal

The discovery of a significantly large anomalous Hall effect in the chiral antiferromagnetic system—Mn3Ge—indicates that the Weyl points are widely separated in phase space and positioned near the Fermi surface. In order to examine the effects of Fe substitution in Mn3Ge on the presence and location of the Weyl points, we synthesized (Mn 1−α Fe α)3 Ge ( α=0−0.30 ) compounds. The AHE was observed in compounds up to α = 0.22, but only within the temperature range where the magnetic structure remains the same as the Mn3Ge. Additionally, positive longitudinal magnetoconductance and planar Hall effect (PHE) were detected within the same temperature and doping range. These findings strongly suggest the existence of Weyl points in (Mn 1−α Fe α)3 Ge ( α=0−0.22 ) compounds. Further, we observed that with an increase in Fe doping fraction, there is a significant reduction in the magnitude of anomalous Hall conductivity, PHE, and positive longitudinal magnetoconductance, indicating that the Weyl points move further away from the Fermi surface. Consequently, it can be concluded that suitable dopants in the parent Weyl semimetals have the potential to tune the properties of Weyl points and the resulting anomalous electrical transport effects.

Unusual multiple magnetic transitions and anomalous Hall effect observed in antiferromagnetic Weyl semimetal, Mn2.94Ge (Ge-rich).

We report on the magnetic and Hall effect measurements of the magnetic Weyl semimetal, Mn2.94Ge (Ge-rich) single crystal. From the magnetic properties study, we identify unusual multiple magnetic transitions below the Ne'el temperature of 353 K, such as the spin-reorientation (TSR) and ferromagnetic-like transitions. Consistent with the magnetic properties, the Hall effect study shows unusual behavior around the spin-reorientation transition. Specifically, the anomalous Hall conductivity increases with increasing temperature, reaching a maximum atTSR, which then gradually decreases with increasing temperature. This observation is quite in contrast to the Mn3+δGe (Mn-rich) system, though both compositions share the same hexagonal crystal symmetry. This study unravels the sensitivity of magnetic and topological properties on the Mn concentration.

Large Anomalous Hall Effect at Room Temperature in a Fermi‐Level‐Tuned Kagome Antiferromagnet

AbstractThe recent discoveries of surprisingly large anomalous Hall effect (AHE) in antiferromagnets have attracted much attention due to their promising use in spintronics devices. However, such AHE‐hosting antiferromagnetic materials are rare in nature. Herein, it is demonstrated that Mn2.4Ga, a Fermi‐level‐tuned kagome antiferromagnet, has a large anomalous Hall conductivity of ≈150 Ω−1 cm−1 at room temperature that surpasses the usual high values (i.e., 20–50 Ω−1 cm−1) observed so far in two outstanding kagome antiferromagnets, Mn3Sn and Mn3Ge. Although the triangular spin structure of Mn2.4Ga shows a weak net magnetic moment of ≈0.05 µB per formula unit, it guarantees a nonzero Berry curvature in the kagome plane. Moreover, the anomalous Hall conductivity exhibits a sign reversal with the rotation of a small magnetic field that can be ascribed to the field‐controlled chirality of the spin triangular structure. This theoretical calculations further suggest that the large AHE in Mn2.4Ga originates from a significantly enhanced Berry curvature associated with the tuning of the Fermi level close to the Weyl points. These properties, together with the ability to manipulate moment orientations using a moderate external magnetic field, make Mn2.4Ga extremely exciting for future antiferromagnetic spintronics.

Realization of Jackiw–Rebbi zero-energy modes at photonic crystal domain walls: Emergence of polarization-indiscriminate surface states

The Jackiw–Rebbi model is a relativistic quantum model credited with the theoretical predictions of zero-energy bound states and charge fractionalization prior to the discovery of topological insulators and the fractional quantum Hall effect. In this work, we demonstrate a photonic equivalent of the Jackiw–Rebbi model by resorting to photonic crystal band structure engineering. Specifically, our photonic realization employs two spatial inversion symmetric binary photonic crystals exhibiting complementary signs of differential effective mass parameter (δm) for their second bandgaps. Their concatenation manifests a step discontinuity in the spatial profile of the effective mass parameter, forming a domain wall at the photonic crystal interface. Upon analyzing the reflectance spectra of the concatenated photonic crystal structure, we find a midgap surface state localized at this domain wall. Furthermore, much in agreement with the Jackiw–Rebbi zero-energy solution, the materialized photonic surface state also exhibits a zero-energy character in a differential energy space corresponding to the δm parameter, which has been quantified experimentally. Crucially, the conceived zero-energy mode amounts to the observation of a peculiar surface state with polarization-indiscriminate dispersion that can help realize all-angle polarization neutral optics.

Tunable anomalous Hall effect and Curie temperature in perpendicular magnetic anisotropic lanthanum doped NiCo2O4 film

Perpendicular magnetic anisotropic NiCo2O4 (NCO) films show volatile physical properties due to the complicated ion occupation/valence disproportion, making it feasible to be manipulated. In this work, the tunable anomalous Hall effect is observed in high-quality lanthanum (La) doped NCO films grown by the off-axis RF sputtering technique. It reveals that the longitudinal resistance (Rxx) significantly increases as the growth of La doping level, while the Curie temperature (TC) decreases with the rising of La doping level. A universal scaling law between the longitudinal conductance (σxx) and the anomalous Hall conductance (σxy), including the terms of dirty scaling mechanism, intrinsic contribution, and side jump, is proposed to explain the observed anomalous Hall effect. Additionally, an intrinsic linear relation between σxx and TC is revealed. The evolution of anomalous Hall resistance (RA), TC, and Rxx is closely related to the complex valence state/occupation of Ni ions induced by La doping. This work provides a strategy to manipulate the physical properties of NCO film, making it a potential material for spintronic applications.

Magnons in a two-dimensional Weyl magnet

The discovery of topological states of matter has led to a revolution in condensed-matter science. While a non-trivial band topology in a material is often associated with intriguing transport properties, much less attention has been given to the impact on spin dynamics and non-equilibrium magnetization states. Here, we provide evidence that a chiral asymmetric magnon dispersion in the two-dimensional Weyl magnet Fe/W(110) is related to the presence of Weyl fermions close to the Fermi energy and surface Fermi arcs. We find that the large anomalous Hall conductivity and the Dzyaloshinskii–Moriya interaction are attributed to the non-trivial band topology in the composite momentum-magnetization space. Our results show the direct impact of Weyl fermions on both the charge and spin dynamics in a two-dimensional magnet. Unveiling these principles can promote innovative technologies in magnonics by utilizing topological materials, where magnons and non-trivial topological electronic states can be manipulated through magnetization.

Highly efficient spin-orbit torque generation in bilayer WTe2/Fe3GaTe2 heterostructure

The spin-orbit torque (SOT) phenomenon is crucial for advancing spintronics. Therefore, we explore the SOT efficiency in the WTe2/Fe3GaTe2 heterostructure. The WTe2/Fe3GaTe2 heterostructure has a ferromagnetic ground state with a perpendicular magnetic anisotropy. Through the Metropolis Monte Carlo simulations, we find that the WTe2/Fe3GaTe2 heterostructure has a Curie temperature of 345 K. We also calculate the temperature-dependent coercive field HC(T) via the extended Landau-Lifshitz-Gilbert (LLG) equation, and the estimated coercive field is 0.32 T at 300 K. From the electrical and spin Hall conductivity of the WTe2 layer, we obtain a giant spin Hall angle of ∼4 is obtained at a low chemical potential. Besides, the WTe2/Fe3GaTe2 heterostructure exhibits a critical current density of ∼7.40 × 104 A/cm2 at 300 K. This value is superior to that found in most previously reported materials. Finally, we achieve the switching power consumption of approximately 1.80 × 1015 W/m3 at 300 K, and this is comparable to or even lower than the values found in other bulk type or thick thickness SOT materials. Due to this highly efficient SOT performance, the ultrathin WTe2/Fe3GaTe2 heterostructure may offer a promising avenue for advanced room temperature spintronic applications with implications for both fundamental research and technological advancements.

Spin-flip excitation and negative energy dispersion in rotating Bose atoms

We investigated the excitation spectra for the rotating Bose system in the fractional quantum Hall effect regime considering short-ranged P o¨ schl-Teller and long-ranged Coulomb type interaction. We found the anomalous negative dispersion in the excited spectra for Jain’s second and third series, whereas conventional spin-wave positive dispersion is observed in the case of Jain’s first series. The higher order Jain series display negative curvature with roton minima at lower momenta and exhibit spin-wave excitation similar to conventional ferromagnets at higher momenta. This anomalous spin-flip excitonic state of particle-hole pairs arises due to the merging of two types of spin-reversed modes i.e. spin-wave with no change in Landau level (LL) index and spin-flip with decreasing LL index.

The reverse quantum limit and its implications for unconventional quantum oscillations in YbB12.

The quantum limit in a Fermi liquid, realized when a single Landau level is occupied in strong magnetic fields, gives rise to unconventional states, including the fractional quantum Hall effect and excitonic insulators. Stronger interactions in metals with nearly localized f-electron degrees of freedom increase the likelihood of these unconventional states. However, access to the quantum limit is typically impeded by the tendency of f-electrons to polarize in a strong magnetic field, consequently weakening the interactions. In this study, we propose that the quantum limit in such systems must be approached in reverse, starting from an insulating state at zero magnetic field. In this scenario, Landau levels fill in the reverse order compared to regular metals and are closely linked to a field-induced insulator-to-metal transition. We identify YbB12 as a prime candidate for observing this effect and propose the presence of an excitonic insulator state near this transition.

Edge and Bulk States in Weyl-Orbit Quantum Hall Effect as Studied by Corbino Measurements

Edge and Bulk States in Weyl-Orbit Quantum Hall Effect as Studied by Corbino Measurements