Le Wang

Two-dimensional superconductors provide a versatile platform to explore fundamental physics and applications where the properties of the two-dimensional samples are characterized by the transport of multiple or single layers. However, spatial inhomogeneities introduced during sample preparation can critically alter transport properties, potentially masking intrinsic physics and producing extrinsic signatures that resemble unconventional phenomena. In this work, we systematically investigated the transport signatures arising from different origins of inhomogeneity in superconducting van der Waals NbSe 2 samples. Two distinct categories of spatial variation were examined: growth-induced inhomogeneity (introduced via non-stoichiometric crystal growth) and fabrication-induced inhomogeneity (achieved by modulating layer thickness and step morphology during mechanical exfoliation). Both types of inhomogeneities resulted in anomalous resistance peaks, with growth-induced inhomogeneity producing more pronounced and configuration-dependent transport signatures. The transport response showed strong geometrical dependence. The observed behavior was consistently described within a framework in which spatially separated regions of the device undergo superconducting transitions at different temperatures, reflecting a distribution of local T c values in the samples. The fitting results further reveal that an inhomogeneous superconducting landscape can generate even more complex anomalous transport signatures, including double-peak structures and sign reversals in resistance. In conclusion, our results demonstrate that transport anomalies should be carefully analyzed and inhomogeneity in sample and measurement geometry should be ruled out before attributing them to intrinsic physical mechanisms.

Information technology has a great demand for magnetoresistance (MR) sensors with high sensitivity and wide-temperature-range operation. It is well known that space charge inhomogeneity in graphene (Gr) leads to finite MR in its pristine form, and can be enhanced by increasing the degree of spatial disorder. However, the enhanced MR usually diminishes drastically as the temperature decreases. Here, by stacking a van der Waals ferromagnet Fe₃GeTe₂ (FGT) on top of graphene to form an FGT/Gr heterostructure, we demonstrate a positive MR of up to ~9400% under a magnetic field of 9 T at room temperature (RT), an order of magnitude larger MR compared to pure graphene. More strikingly, the giant MR of the FGT/Gr heterostructure sustains over a wide temperature range from RT down to 4 K. Both control experiments and DFT calculations show that the enhanced MR originates from spin-dependent orbital coupling between FGT and graphene, which is temperature insensitive. Our results open a new route for realizing high-sensitivity and wide-temperature-range MR sensors.

In ordered magnets, the elementary excitations are spin waves (magnons), which obey Bose–Einstein statistics. Similarly to Cooper pairs in superconductors, magnons can be paired into bound states under attractive interactions. The Zeeman coupling to a magnetic field is able to tune the particle density through a quantum critical point, beyond which a ‘hidden order’ is predicted to exist. Here we report direct observation of the Bose–Einstein condensation of the two-magnon bound state in Na₂BaNi(PO₄)₂. Comprehensive thermodynamic measurements confirmed the two-dimensional Bose–Einstein condensation quantum critical point at the saturation field. Inelastic neutron scattering experiments were performed to establish the microscopic model. An exact solution revealed stable two-magnon bound states that were further confirmed by electron spin resonance and nuclear magnetic resonance experiments, demonstrating that the quantum critical point is due to the pair condensation, and the phase below the saturation field is likely the long-sought-after spin nematic phase.