Quantum-metric-induced nonlinear transport in a topological antiferromagnet.

The Berry curvature and quantum metric are the imaginary part and real part, respectively, of the quantum geometric tensor, which characterizes the topology of quantum states1. The Berry curvature is known to generate a number of important transport phenomena, such as the quantum Hall effect and the anomalous Hall effect2,3; however, the consequences of the quantum metric have rarely been probed by transport measurements. Here we report the observation of quantum-metric-induced nonlinear transport, including both a nonlinear anomalous Hall effect and a diode-like non-reciprocal longitudinal response, in thin films of a topological antiferromagnet, MnBi2Te4. Our observations reveal that the transverse and longitudinal nonlinear conductivities reverse signs when reversing the antiferromagnetic order, diminish above the Néel temperature and are insensitive to disorder scattering, thus verifying their origin in the band-structure topology. They also flip signs between electron- and hole-doped regions, in agreement with theoretical calculations. Our work provides a means to probe the quantum metric through nonlinear transport and to design magnetic nonlinear devices.

DC Magnetic Field Sensitivity Optimization of Spin Defects in Hexagonal Boron Nitride.

Spin defects existing in van der Waals materials attract wide attention thanks to their natural advantages for in situ quantum sensing, especially the negatively charged boron vacancy (VB-) centers in hexagonal boron nitride (h-BN). Here we systematically investigate the laser and microwave power broadening in continuous-wave optically detected magnetic resonance (ODMR) of the VB- ensemble in h-BN, by revealing the behaviors of ODMR contrast and line width as a function of the laser and microwave powers. The experimental results are well explained by employing a two-level simplified model of ODMR dynamics. Furthermore, with optimized power, the DC magnetic field sensitivity of VB- ensemble is significantly improved up to 2.87 ± 0.07 µT/Hz. Our results provide important suggestions for further applications of VB- centers in quantum information processing and ODMR-based quantum sensing.

Spin Defects in hBN assisted by Metallic Nanotrenches for Quantum Sensing.

The omnipresence of hexagonal boron nitride (hBN) in devices embedding two-dimensional materials has prompted it as the most sought after platform to implement quantum sensing due to its testing while operating capability. The negatively charged boron vacancy (VB-) in hBN plays a prominent role, as it can be easily generated while its spin population can be initialized and read out by optical means at room-temperature. But the lower quantum yield hinders its widespread use as an integrated quantum sensor. Here, we demonstrate an emission enhancement amounting to 400 by nanotrench arrays compatible with coplanar waveguide (CPW) electrodes employed for spin-state detection. By monitoring the reflectance spectrum of the resonators as additional layers of hBN are transferred, we have optimized the overall hBN/nanotrench optical response, maximizing thereby the luminescence enhancement. Based on these finely tuned heterostructures, we achieved an enhanced DC magnetic field sensitivity as high as 6 × 10-5 T/Hz1/2.

Excited-State Optically Detected Magnetic Resonance of Spin Defects in Hexagonal Boron Nitride.

Negatively charged boron vacancy (V_{B}^{-}) centers in hexagonal boron nitride (h-BN) are promising spin defects in a van der Waals crystal. Understanding the spin properties of the excited state (ES) is critical for realizing dynamic nuclear polarization. Here, we report zero-field splitting in the ES of D_{ES}=2160 MHz and its associated optically detected magnetic resonance (ODMR) contrast of 12% at cryogenic temperature. In contrast to nitrogen vacancy (NV^{-}) centers in diamond, the ODMR contrast of V_{B}^{-} centers is more prominent at cryotemperature than at room temperature. The ES has a g factor similar to the ground state. The ES photodynamics is further elucidated by measuring the level anticrossing of the V_{B}^{-} defects under varying external magnetic fields. Our results provide important information for utilizing the spin defects of h-BN in quantum technology.

Quantum sensing of radio-frequency signal with NV centers in SiC.

Silicon carbide is an emerging platform for quantum technologies that provides wafer scale and low-cost industrial fabrication. The material also hosts high-quality defects with long coherence times that can be used for quantum computation and sensing applications. Using an ensemble of nitrogen-vacancy centers and an XY8-2 correlation spectroscopy approach, we demonstrate a room-temperature quantum sensing of an artificial AC field centered at ~900 kHz with a spectral resolution of 10 kHz. Implementing the synchronized readout technique, we further extend the frequency resolution of our sensor to 0.01 kHz. These results pave the first steps for silicon carbide quantum sensors toward low-cost nuclear magnetic resonance spectrometers with a wide range of practical applications in medical, chemical, and biological analysis.

Carbon-riveted Pt catalyst supported on nanocapsule MWCNTs-Al2O3 with ultrahigh stability for high-temperature proton exchange membrane fuel cells.

Pt catalyst supported on nanocapsule MWCNTs-Al(2)O(3) (multi-walled carbon nanotubes, MWCNTs) catalyst has been prepared by microwave-assisted polyol process (MAPP). The results of electrochemical measurements show that the nanocapsule Pt/MWCNTs-Al(2)O(3) catalyst has higher activity due to more uniform dispersion and smaller size of Pt nanoparticles, and higher stability ascribed to the stronger metal-support interaction (SMSI) between Pt nanoparticles and nanocapsule support than in Pt/MWCNTs. Furthermore, the carbon-riveted nanocapsule Pt/MWCNTs-Al(2)O(3) catalyst has been designed and synthesized on the basis of in situ carbonization of glucose. The physical characteristics such as X-ray diffraction (XRD), energy dispersive analysis of X-ray (EDAX), transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS) have indicated that α-Al(2)O(3) indeed entered into the inside of the MWCNTs and formed a nanocapsule support of MWCNTs with α-Al(2)O(3) as stuffing. The accelerated potential cycling tests (APCT) show that carbon-riveted nanocapsule Pt/MWCNTs-Al(2)O(3) possesses 10 times the stability of Pt/C and has 4.5 times the life-span of carbon-riveted Pt/TiO(2)-C reported in our previous work. The significantly enhanced stability for carbon-riveted nanocapsule Pt/MWCNTs-Al(2)O(3) catalyst is attributed to the reasons as follows: the inherently excellent mechanical resistance and stability of α-Al(2)O(3) and MWCNTs in acidic and oxidative environments; SMSI between Pt nanoparticles and the nanocapsule support; the anchoring effect of the carbon layers formed during the carbon-riveting process (CRP); the increase of Pt(0) composition during CRP.

Carbon riveted Pt/C catalyst with high stability prepared by in situ carbonized glucose.

Carbon riveted Pt/C catalyst was designed and prepared by in situ carbonized glucose in this paper. Characterization results show that carbon riveted Pt/C prevents Pt nanoparticles from coalescence. Its stability is markedly enhanced due to the anchoring effect of the carbon layers formed during the carbon riveting process.