arXiv subjects

Shengyuan A. Yang

Publications and source records attributed to Shengyuan A. Yang.

At least 37 records · Page 2Linked to original sources

arXiv2025

Giant field-tunable nonlinear Hall effect by Lorentz skew scattering in a graphene moire superlattice

The nonlinear Hall effect (NHE) can enable rectification and energy harvesting, and its control by external fields, including gate, strain and magnetic field, has been pursued intensively. However, existing tuning pathways rely predominantly on fully quantum mechanical effects and are typically inefficient, resulting in weak NHE signals that limit further progress. In this work, we report the discovery of a distinct type of NHE in a graphene-hBN moire superlattice, which arises from a classical-quantum cooperative effect called Lorentz skew scattering (LSK), induced by a perpendicular magnetic field. This field-driven NHE exhibits a linear dependence on magnetic field and a pronounced unidirectional angular dependence. Remarkably, its magnitude reaches up to 32% of the linear Hall signal. We show that this giant, field-tunable NHE originating from LSK follows a unique quartic scaling law and produces a record-high nonlinear Hall conductivity (36000 μmV-1Ω-1) near van Hove singularities of moire minibands, which is over an order of magnitude larger than all previously reported NHEs. Our findings establish an efficient, magnetic-field-driven route to giant Hall rectification in high-mobility materials, offering a broadly applicable paradigm for modulating the NHE beyond electrostatic gating.

cond-mat.mes-hall↗

arXiv2025

Tunable linear and nonlinear anomalous Hall transport in two-dimensional CrPS$_{4}$

Few-layer CrPS$_{4}$ is a two-dimensional (2D) magnetic material with excellent stability in ambient environment, which attracted significant interest in recent research. Here, via first-principles calculations, we show that 2D CrPS$_{4}$ hosts a variety of anomalous Hall transport phenomena, owing to its layer-dependent magnetism and symmetry character. Monolayer CrPS$_{4}$ can display a sizable linear anomalous Hall effect, while its nonlinear anomalous Hall response is forbidden. In contrast, bilayer CrPS$_{4}$ can produce pronounced intrinsic nonlinear anomalous Hall response from Berry-connection polarizability, in the absence of linear anomalous Hall effect. We clarify that the large peaks in these responses originate from gapped Dirac points in the band structure. Furthermore, we show that linear anomalous Hall effect can be induced and controlled in bilayer CrPS$_{4}$ by gate electric field or in-plane magnetic field, which break the spacetime inversion symmetry. Our findings unveil the interesting layer-dependent Hall transport physics in 2D CrPS$_{4}$ magnets, suggesting its potential in electronic and spintronic device applications.

cond-mat.mes-hall↗

arXiv2025

Projective crystal symmetry and topological phases

Quantum states naturally represent symmetry groups, though often in a projective sense. Intriguingly, the projective nature of crystalline symmetries has remained underexplored until very recently. A series of groundbreaking theoretical and experimental studies have now brought this to light, demonstrating that projective representations of crystal symmetries lead to remarkable consequences in condensed matter physics and various artificial crystals, particularly in their connection to topological phenomena. In this article, we explain the basic ideas and notions underpinning these recent developments and share our perspective on this emerging research area. We specifically highlight that the appearance of momentum-space nonsymmorphic symmetry is a unique feature of projective crystal symmetry representations. This, in turn, has the profound consequence of reducing the fundamental domain of momentum space to all possible flat compact manifolds, which include torus and Klein bottle in 2D and the ten platycosms in 3D, presenting a significantly richer landscape for topological structures than conventional settings. Finally, the ongoing efforts and promising future research directions are discussed.

cond-mat.mes-hall↗

arXiv2025

Intrinsic nonlinear valley Nernst effect

We investigate the intrinsic nonlinear valley Nernst effect, which induces a transverse valley current via a second-order thermoelectric response to a longitudinal temperature gradient. The effect arises from the Berry connection polarizability dipole of valley electrons and is permissible in both inversion-symmetric and inversion-asymmetric materials. We demonstrate that the response tensor is connected to the intrinsic nonlinear valley Hall conductivity through a generalized Mott relation, with the two being directly proportional at low temperatures, scaled by the Lorenz number. We elucidate the symmetry constraints governing this effect and develop a theory for its nonlocal measurement, revealing a nonlocal second-harmonic signal with a distinct $ρ^2$ scaling. This signal comprises two scaling terms, with their ratio corresponding to the square of the thermopower normalized by the Lorenz number. Key characteristics are demonstrated using a tilted Dirac model and first-principles calculations on bilayer WTe$_2$. Possible extrinsic contributions and alternative experimental detection methods, e.g., by valley pumping and by nonreciprocal directional dichroism, are discussed. These findings underscore the significance of band quantum geometry on electron dynamics and establish a theoretical foundation for nonlinear valley caloritronics.

cond-mat.mes-hall↗

arXiv2025

Proper Definition of Intrinsic Nonlinear Current

We show that the three commonly employed approaches that define the same dc (or low-frequency) intrinsic linear anomalous Hall response actually lead to different results for intrinsic nonlinear transport. The difference is due to an intrinsic anomalous distribution (IAD). It originates from a nonlinear field effect during scattering, but its value is completely independent of scattering, because it represents the local equilibration of electron wave packets with field corrected energy. The proper definition of intrinsic current that is detectable in experiment must incorporate the effect of IAD. We also show that IAD is indispensable for consistency with fundamental physical relations. In addition, we predict that under ac driving, the intrinsic reponses in rectified and double-frequency channels exhibit distinct frequency dependence, for which we estimate the signals that can be probed in antiferromagnetic CuMnAs.

cond-mat.mes-hall↗

arXiv2025

Giant magneto-cubic in-plane Hall effect in a nonmagnetic material

In-plane Hall effect (IPHE) triggered by an external magnetic field applied in the transport plane has attracted significant experimental attentions in recent few years 1-6. However, most experiments focus on magnetic materials, where the existence of magnetic ordering may complicate understanding the physics behind, and the relatively small signal magnitudes limit the application of the effect. Here, we report a giant IPHE in a nonmagnetic half-Heusler compound LuAuSn, with a magnitude exceeding all the previously reported values. A -period of IPHE and the consistent cubic dependence on the magnetic field are observed, realizing the long-sought theoretical prediction of magneto-cubic IPHE under threefold rotational symmetry7-9 in an unexpected material. The scaling law analysis and first-principles calculations indicate that extrinsic side jump and skew scattering processes from both impurity and phonon scatterings dominate the observed effect. These findings unravel a new type of magneto-nonlinear IPHE, and its large magnitude and wide-temperature operation may open the door to practical applications of IPHE.

cond-mat.mtrl-sci↗

arXiv2025

Engineering Spin Splitting in Antiferromagnets by Superatoms with Internal Degree of Freedom

Superatoms, stable atomic clusters acting as building blocks for new materials, offer unique opportunities due to their rich properties and potential for 2D material assembly. While extensive research has focused on their similarities to ordinary atoms, the role of their internal degrees of freedom (IDOF) remains largely unexplored. Concurrently, compensated antiferromagnets (AFMs) with intrinsic spin-split band structures have emerged as a promising class of materials for spintronics, yet their experimental realization, particularly in two dimensions, is limited. Here, we bridge these two fields by proposing a novel strategy to achieve spin-split AFMs using superatoms with IDOFs. We establish our core concept using a simple model, demonstrating how superatom IDOFs can be leveraged to engineer system symmetry and induce spin splitting in AFM states. We concretely illustrate this strategy by first-principles calculations on a Mo-decorated carborophene sheets, constructed from closo-carborane superatoms. We show that the distinct IDOFs of carborane isomers (electric-dipole-like and nematic) are critical in determining the symmetry of the resulting 2D superatomic crystal and, consequently, the spin splitting pattern of its AFM states. Our findings underscore the profound significance of superatom IDOFs-a feature absent in ordinary atoms-and introduce a new paradigm for engineering spin splitting in AFM lattices. This work opens novel avenues for the design of advanced spintronic and quantum materials based on superatoms.

cond-mat.mtrl-sci↗

arXiv2025

Room-temperature intrinsic nonlinear planar Hall effect in TaIrTe$_4$

Intrinsic responses are of paramount importance in physics research, as they represent the inherent properties of materials, independent of extrinsic factors that vary from sample to sample, and often reveal the intriguing quantum geometry of the band structure. Here, we report the experimental discovery of a new intrinsic response in charge transport, specifically the intrinsic nonlinear planar Hall effect (NPHE), in the topological semimetal TaIrTe$_4$. This effect is characterized by an induced Hall current that is quadratic in the driving electric field and linear in the in-plane magnetic field. The response coefficient is determined by the susceptibility tensor of Berry-connection polarizability dipole, which is an intrinsic band geometric quantity. Remarkably, the signal persists up to room temperature. Our theoretical calculations show excellent agreement with the experimental results and further elucidate the significance of a previously unknown orbital mechanism in intrinsic NPHE. This finding not only establishes a novel intrinsic material property but also opens a new route toward innovative nonlinear devices capable of operating at room temperature.

cond-mat.mes-hall↗

arXiv2025

Nonlinear Néel Spin-Orbit Torque in Centrosymmetric Antiferromagnets

Electric control of Néel vector is a central task of antiferromagnetic (AFM) spintronics. The major scheme so far relies on the linear Néel torque, which however is restricted to AFMs with broken inversion symmetry. Here, we propose a nonlinear Néel spin-orbit torque, uniquely enabling electric control in the vast class of centrosymmetric AFMs, where the existing scheme fails. Importantly, its intrinsic component, rooted in sublattice-resolved band quantum geometry, offers two additional advantages: It operates also in $\mathcal{PT}$-symmetric AFM insulators, where linear torque is forbidden; and it has anti-damping character, making it more efficient in driving magnetic dynamics. Combined with first-principles calculations, we predict large effect in MnRh and MnBi$_{2}$Te$_{4}$, which can be readily detected in experiment. Our work unveils a new fundamental effect, offers a new strategy of electric control in AFM systems beyond the existing paradigm, and opens the door to the field of nonlinear AFM spintronics.

cond-mat.mtrl-sci↗

arXiv2025

Lorentz Skew Scattering and Giant Nonreciprocal Magneto-Transport

Skew scattering is the well-known dominant mechanism for anomalous Hall transport in highly conductive systems. However, despite extensive research, the primary mechanism governing nonlinear (nonreciprocal) magneto-transport in clean samples remains unknown. This theoretical gap has impeded the development of design principles for efficient nonreciprocal devices. Here, we unveil a hitherto unexplored effect in nonreciprocal magneto-transport from cooperative action of Lorentz force and skew scattering. The significance of this Lorentz skew scattering mechanism lies in that it dominates both longitudinal and transverse responses in highly conductive systems, and it exhibits a scaling behavior distinct from all known mechanisms. At low temperature, it shows a cubic scaling in linear conductivity, whereas the scaling becomes quartic at elevated temperature when phonon scattering kicks in. Applying our developed microscopic theory to surface transport in topological crystalline insulator SnTe and bulk transport in Weyl semimetals leads to significant results, suggesting a new route to achieve giant transport nonreciprocity in high-mobility materials with topological band features.

cond-mat.mes-hall↗

arXiv2025

Orbital enhanced intrinsic nonlinear planar Hall effect for probing topological phase transition in CuTlSe$_{2}$

The intrinsic nonlinear planar Hall effect proposed in recent studies offers a new way to probe intrinsic band geometric properties in a large class of materials. However, the search of material platforms with a large response remains a problem. Here, we suggest that topological Weyl semimetals can host enhanced intrinsic nonlinear planar Hall effect. From a model study, we show that the enhancement is mainly from the orbital contribution, and the response coefficient exhibits a characteristic resonance-like lineshape around the Weyl-point energy. Using first-principles calculations, we confirm these features for the concrete material CuTlSe$_{2}$. Previous studies have reported two different topological states of CuTlSe$_{2}$. We find this difference originates from two slightly different structures with different lattice parameters. We show that the nonlinear planar Hall response is much stronger in the Weyl semimetal state than in the topological insulator state, and the large response is indeed dominated by orbital contribution amplified by Weyl points. Our work reveals a close connection between nonlinear orbital responses and topological band features, and suggests CuTlSe$_{2}$ as a suitable platform for realizing enhanced nonlinear planar Hall effect.

cond-mat.mes-hall↗

arXiv2025

Design of altermagnetic models from spin clusters

Altermagnetism, a new class of collinear compensated magnetic phase, has garnered tremendous interest because of its rich physics and promising applications. Physical models and verified material candidates for altermagnetism remain limited. Here, we propose a general scheme to construct altermagnetic models, which explicitly exhibits the blend of ferromagnetic and antiferromagnetic correlations in real space via the design of spin clusters, echoing the observation that properties of altermagnets resemble a mixture of ferromagnets and antiferromagnets. We show that in some of our models, the desired altermagnetic order can be spontaneously realized by electron-electron interaction in a broad range of the phase diagram. This development facilitates the study of fascinating physics of altermagnetism and sheds light on the discovery of new altermagnetic materials.

cond-mat.str-el↗

arXiv2025

Sign reversal of Berry curvature triple driven by magnetic phase transition in a ferromagnetic polar metal

While the time-reversal-even (T-even) nonlinear Hall effect has been extensively discussed in nonmagnetic materials, the impact of magnetic phase transition on it remains largely overlooked. Here, we report an abrupt enhancement of the T-even nonlinear Hall effect in non-centrosymmetric SrRuO3(111) thin films during the paramagnetic-ferromagnetic transition. Scaling analysis reveals a sign reversal of both the skew scattering and side jump contributions upon the emergence of magnetism, which we attribute to the sign change of Berry curvature triple on the Fermi surface. Density functional theory calculations support this interpretation, ascribing this behavior to distinctive origins of Berry curvature hot spots in paramagnetic and ferromagnetic phases. Our findings unveil the exchange-induced dramatic nonperturbative change of nonlinear Hall effect, and establish SrRuO3(111) thin films as a promising platform for exploring magnetically tunable nonlinear transport effects.

cond-mat.mtrl-sci↗

arXiv2025

Observation of giant nonlinear valley Hall effect

The valley Hall effect (VHE) holds great promise for valleytronic applications by leveraging the valley degree of freedom. To date, research on VHE has focused on its linear response to an applied current, leaving nonlinear valley responses undetected and nonlinear valleytronic devices undeveloped. Here, we report the experimental observation of a nonlinear VHE in a graphene-hBN moire superlattice, evidenced by the generation of second-harmonic nonlocal voltages under AC currents. Remarkably, the nonlinear VHE has magnitude surpassing the linear VHE and is highly tunable via a gate voltage, which exhibits a pair of opposite peaks on the two sides of a Dirac gap. The nonlinear signal shows quadratic scaling with driving current and quartic scaling with local resistance, setting it apart from the linear counterpart. These experimental features are consistent with the theoretical picture of nonlocal transport mediated by nonlinear VHE and linear inverse VHE. We further reveal a nonlinear inverse VHE by observing the third- and fourth-harmonic nonlocal voltages. The nonlinear VHE provides a novel mechanism for valley manipulation and enables a novel valleytronic device, the valley rectifier, that converts AC charge current into DC valley current.

cond-mat.mes-hall↗

arXiv2025

Theory of Nonlocal Transport from Nonlinear Valley Responses

We develop a theory for the nonlocal measurement of nonlinear valley Hall effect. Different from the linear case where the direct and the inverse processes are reciprocal, we unveil that the nonlinear inverse valley Hall effect needed to generate nonlocal voltage signal must have a distinct symmetry character and involve distinct mechanisms compared to the nonlinear valley Hall response it probes. Particularly, it must be valley-even, in contrast to both linear and nonlinear valley Hall effects which are valley-odd. Layer groups that permit such nonlocal valley responses are obtained via symmetry analysis, and formulas for the nonlocal signals are derived. In the presence of both linear and nonlinear valley responses, we show that the different responses can be distinguished by their distinct scaling behaviors in the different harmonic components, under a low-frequency ac driving. Combined with first-principles calculations, we predict sizable nonlocal transport signals from nonlinear valley responses in bilayer $T_{d}$-WTe$_{2}$. Our work lays a foundation for nonlocal transport studies on the emerging nonlinear valleytronics.

cond-mat.mes-hall↗

arXiv2025

Skin-inspired in-sensor encoding of strain vector using tunable quantum geometry

Human skin provides crucial tactile feedback, allowing us to skillfully perceive various objects by sensing and encoding complex deformations through multiple parameters in each tactile receptor. However, replicating this high-dimensional tactile perception with conventional materials' electronic properties remains a daunting challenge. Here, we present a skin-inspired method to encode strain vectors directly within a sensor. This is achieved by leveraging the strain-tunable quantum properties of electronic bands in the van der Waals topological semimetal Td -WTe2. We observe robust and independent responses from the second-order and third-order nonlinear Hall signals in Td -WTe2 when subjected to variations in both the magnitude and direction of strain. Through rigorous temperature-dependent measurements and scaling law analysis, we establish that these strain responses primarily stem from quantum geometry-related phenomena, including the Berry curvature and Berry-connection polarizability tensor. Furthermore, our study demonstrates that the strain-dependent nonlinear Hall signals can efficiently encode high-dimensional strain information using a single device. This capability enables accurate and comprehensive sensing of complex strain patterns in the embossed character "NJU". Our findings highlight the promising application of topological quantum materials in advancing next-generation, bio-inspired flexible electronics.

cond-mat.mes-hall↗

arXiv2024

Scaling Law for Time-Reversal-Odd Nonlinear Transport

Time-reversal-odd ($\mathcal{T}$-odd) nonlinear current response has been theoretically proposed and experimentally confirmed recently. However, the role of disorder scattering in the response, especially whether it contributes to the $σ_{xx}$-independent term, has not been clarified. In this work, we derive a general scaling law for this effect, which accounts for multiple scattering sources. We show that the nonlinear conductivity is generally a quartic function in $σ_{xx}$. Besides intrinsic contribution, extrinsic contributions from scattering also enter the zeroth order term, and their values can be comparable to or even larger than the intrinsic one. Terms beyond zeroth order are all extrinsic. Cubic and quartic terms must involve skew scattering and they signal competition between at least two scattering sources. The behavior of zeroth order extrinsic terms is explicitly demonstrated in a Dirac model. Our finding reveals the significant role of disorder scattering in $\mathcal{T}$-odd nonlinear transport, and establishes a foundation for analyzing experimental result.

cond-mat.mes-hall↗

arXiv2024

Highly anisotropic Drude-weight-reduction and enhanced linear-dichroism in van der Waals Weyl semimetal Td-MoTe2 with coherent interlayer electronic transport

Weyl semimetal (WSM) states can be achieved by breaking spatial-inversion symmetry or time reversal symmetry. However, the anisotropy of the energy reduction contributing to the emergence of WSM states has seldom been investigated by experiments. A van der Waals metal MoTe2 exhibits a type-II WSM phase below the monoclinic-to-orthorhombic-phase-transition temperature Tc ~ 250 K. Here, we report a combined linearly-polarized optical-spectroscopy and electrical-transport study of MoTe2 at different temperatures. The Drude components in the a-axis, b-axis and c-axis optical conductivity spectra, together with the metallic out-of-plane and in-plane electrical resistivities, indicate the coherent inter-layer and in-plane charge transports. Moreover, the Drude weight in σ1a(ω), rather than the Drude weights in σ1b(ω) and σ1c(ω), decreases dramatically below Tc, which exhibits a highly anisotropic decrease in its Drude weight and thus suggests a strongly anisotropic reduction of the electronic kinetic energy in the WSM phase. Furthermore, below Tc, due to the in-plane anisotropic spectral-weight transfer from Drude component to high-energy region, the in-plane inter-band-absorption anisotropy increases remarkably around 770 meV, and has the largest value (~ 0.68) of normalized linear dichroism among the reported type-II WSMs. Our work sheds light on seeking new WSMs and developing novel photonic devices based on WSMs.

cond-mat.mtrl-sci↗