Fengqi Song

Nonreciprocal charge transport provides an effective approach for probing direction-dependent charge dynamics and spin-momentum-locked electronic states. Here, we report highly gate-tunable nonreciprocal charge transport in a dual-gated BiSbTeSe2 topological insulator device. The second-harmonic longitudinal resistance is linear in both current and magnetic field. The nonreciprocal response reaches its maximum near the charge neutrality point, exhibits sign reversals on either side, and is strongly suppressed once the Fermi level enters the bulk states. We suggest that the nonreciprocal response is attributed to the inhomogeneity of spin-momentum locking in the topological surface states. The nonreciprocal signal near the charge neutrality point is about one order of magnitude larger than that without gate modulation and remains robust up to 300 K, highlighting BiSbTeSe2 as a promising platform for room-temperature nonlinear electronics.

Interlayer potential-energy surfaces (PESs) govern stacking stability, sliding barriers, and tribology in two-dimensional bilayers, but first-principles mapping is expensive. We introduce an atom-pair graph neural network (APGNN) whose nodes are interlayer atom pairs, placing the representation directly on the interface. Pretraining on 89 homobilayers from a 112-material 2DMatPedia benchmark reduces the 20-point mean absolute error on 23 unseen materials from 9.22 to 1.94 meV/Å2 and increases the mean R2 from 0.802 to 0.991. Leave-one-element-out tests further demonstrate transfer to compounds containing an element absent from pretraining, reducing the MAE by 50.5–58.6% relative to that for training from scratch at the same target-data budget. For MoS2, 10 single-point energies per stacking recover the equilibrium spacing, distance-dependent sliding barriers, and two-dimensional PES topology. Dense interlayer PESs thus become accessible at a fraction of the first-principles cost. The method is currently limited to rigid homobilayers; heterobilayers and relaxed moiré structures require further training data.

The intertwined many-body correlation, nontrivial topology, and symmetry can lead to exotic low-energy quasiparticle excitation, such as fractional quantum Hall (FQH) states, Fermi liquids, etc. Some FQH states with even denominators appearing at the half-filling of Landau levels can be interpreted by Moore-Read theory, and the corresponding composite fermions satisfy non-Abelian statistics and have the potential to realize topological quantum computation. Possessing extraordinary tunability that stems from the intricate interplay of spin, valley, and orbital degrees of freedom, few-layer graphene stands out as an ideal platform for studying the FQH effect. Here we report the observation of the ν = 7 / 2 FQH state on the hole side of Bernal-stacked trilayer graphene unilaterally encapsulated with hexagonal boron nitride—the first realization of this state on the hole side of pristine trilayer graphene. In contrast to recently reported electron-side even-denominator states hosted in the monolayer-like N = 0 orbital, the ν = 7 / 2 state emerges in the bilayer-like N B = 1 orbital near its crossing with the N B = 0 level, and exhibits a transport activation gap of Δ tr = 45.6 ± 8.4 K, nearly two orders of magnitude larger than its electron-side counterparts. This value, obtained from Arrhenius analysis of the temperature-dependent residual resistance, is a conservative lower bound, as the analysis is limited by the residual-resistance floor. Our results suggest that few-layer graphene is an ideal platform for studying low-dimensional correlation states.

The kagome material has rapidly established itself as a research frontier in condensed matter physics, owing to its distinctive geometric structure and the rich array of unconventional physical phenomena. In the kagome AV3Sb5 (A = K, Rb, and Cs) family, the charge-ordered state exhibits a remarkable characteristic, i.e., anomalous symmetry breaking, which is tied to the topological nature of the electronic band structure. Here, we report the third-order nonlinear longitudinal and Hall responses that persist stably up to room temperature in the kagome material CsV3Sb5. Notably, the nonlinear responses demonstrate significant transition below the charge density wave (∼77 K) order and anomalous symmetry-breaking (∼39 K) state. The scaling analysis indicates that the third-order nonlinear transport is governed jointly by the quantum geometric contribution and extrinsic scattering. This study advances the understanding of the third-order nonlinear transport and provides a distinct method to detect anomalous symmetry breaking in CsV3Sb5.

Exchange bias from ferromagnet (FM) and antiferromagnet (AFM) interface underpins modern spintronics and magnetic technologies. Van der Waals (vdW) magnets allow arbitrary stacking, offering richer material and device choices. However, challenges arise when integrating FM metals with vdW topological materials. Conductive channels from FMs obscure intrinsic topological transport properties of interest for low-dissipation spintronics. Here we report a device obtained by inserting an insulating h-BN spacer between FM Fe3GaTe2 and the AFM topological insulator MnBi2Te4, departing from the conventional requirement of direct FM/AFM contact. Contrary to expectations that a spacer would preclude coupling, we observe an exchange bias field of ∼0.5 T, among the highest in vdW magnetic systems. We propose that Fe3GaTe2 induces a ferromagnetic component on the MnBi2Te4 surface while its interior remains antiferromagnetic, resulting in exchange bias originating within MnBi2Te4 layers. This large exchange bias without direct contact opens new design avenues for vdW magnetic heterostructures and broadens the scope of low-dimensional magnetism.

Understanding and controlling charge states at the level of individual electrons in molecular assemblies is often hindered by system complexity and the lack of reliable theoretical framework to accurately model the underlying many-body non-equilibrium electron dynamics. To address this challenge, here we construct well-defined molecular trimers and hexamers of tetrabromo-tetraazapyrene molecules by scanning probe manipulation and show that the electron dynamics can be completely rationalized using the Anderson impurity model and master equation approach. Our analysis demonstrates that such a treatment, which goes beyond the standard single-particle picture, is essential for understanding this class of molecular systems. The model not only quantitatively reproduces all features visible in measured differential conductance maps, but also uncovers the mechanism for negative differential conductance, which arises from the non-equilibrium occupancy of collective charge states. The analysis also reveals that the charging rings are not necessarily associated with changes in the total charge of the cluster, but often originate from internal charge rearrangements between the sites. These findings provide insight into fundamental quantum many-body effects in strongly interacting molecular systems and open pathways for engineering electronic phases by controlling cluster topology and the electrostatic environment.

Despite the decisive role of mass and morphology in cluster applications, achieving concurrent control over these parameters remains challenging. In this study, by incrementally increasing the argon flow rate in a magnetron sputtering system, mass-selected Pt2925 clusters with ±4.15% mass precision were successfully prepared with controllable morphological transition from quasi-spherical to multipodal. We attribute this Pt2925 morphological transition to a change in growth mode from atom addition to cluster–cluster collision, which is influenced by the nucleation process regulated by the argon flow rate. The signatures of the ion-beam current and mass spectrum provide a valuable guide for synthesizing mass-selected multipodal clusters in other elemental systems (Ru, Hf, and AuPd alloys). Moreover, the bipodal Pt2925 clusters retained their configuration both under prolonged high-dose electron-beam irradiation on amorphous carbon (∼200 s) and during in situ heating on amorphous SiN at 400 °C. These findings indicate that mass-selected bipodal Pt clusters exhibit good thermal stability and are promising for high-temperature applications.

The nonlinear Hall effect is a new member of the Hall effect family, which attracts intense research interests, and it is closely related to the quantum geometry of quantum materials. The previous studies primarily concentrate on the second-order and third-order nonlinear Hall effect. However, the experimental study of higher-order nonlinear Hall effect is scarce at present. In this work, we report the observations of the higher odd-order (third-, fifth-, seventh-order) nonlinear Hall effect in magnetic topological insulator Mn(Bi_1-xSb_x)₂Te₄ thin flakes. The higher odd-order nonlinear Hall voltage exhibits a twofold angular dependence and exists only below the Néel temperature. It reaches its maximum near the charge neutral point and decays exponentially as the order of the nonlinear Hall effect increases. Furthermore, such higher odd-order nonlinear Hall effect is observed in both odd- and even-layer samples with comparable magnitudes. Theoretical analysis indicates that the higher odd-order nonlinear Hall responses may arise from the Berry curvature multipoles. Our work paves the way for the study of the higher-order nonlinear transport phenomena.

Information units are progressively approaching the fundamental physical limits of integration density, including in terms of extremely small sizes, multistates and probabilistic traversal. However, simultaneously encompassing all of these characteristics in a unit remains elusive. Here, via real-time in situ electrical monitoring, we clearly observed stochastic alterations of multiple conductance states in Sc₂C₂@C₈₈. The true random bit sequence generated exhibited an autocorrelation function whose confidence interval fell within ±0.02, demonstrating high-quality randomness. The alterations of multiple conductance states are controllable, that is, whose probability distributions could traverse from 0 to 1, enabling us to factorize 551 into its prime factors. Furthermore, we proposed a matrix-chain multiplication scheme and experimentally verified the multiplication of two 4 × 4 state-transition matrices with a small maximum error of <0.05. Combined with theoretical calculations, the stochastic but controllable multistates are probably attributed to the rich energy landscape, which could be stepwise changed by the electric field. Our findings reveal extremely small multilevel probabilistic bit for matrix multiplication, which pave the way for ultra-compact intelligent electronic devices.