L. Sun’s research while affiliated with Nanjing University and other places

Searching for new materials hosting flat bands is pivotal for exploring strongly correlated effects and designing sensitive quantum devices, but remains challenging. We present a general method for realizing flat bands based on mathematical optimization and symmetry analysis. The method enables the discovery of ∼1000 types of two-dimensional lattices that can host flat bands, in sharp contrast with ∼10 flat-band lattices predicted previously besides the well-known ones. We further verify the method using first-principles calculations. Our approach provides new insights for the design of flat-band lattices, particularly when aiming to create experimentally feasible configurations.

Utilizing spin pumping, we present a comparative study of the spin-charge conversion in RuO2(101) and RuO2(110) films. RuO2(101) shows a robust in-plane crystal-axis dependence, whereas RuO2(110) exhibits an isotropic but stronger one. Symmetry-based analysis and first-principles calculations reveal that the spin-charge conversion in RuO2(110) originates from the inverse spin Hall effect (ISHE) due to nodal lines splitting. In RuO2(101), the ISHE also dominates although the inverse spin splitting effect (ISSE) may coexist. These findings, in sharp contrast to previously attributed ISSE, are further corroborated by the reciprocal relation between the spin pumping and the spin-torque ferromagnetic resonance measurements.

Utilizing a low-temperature scanning-tunneling microscope, we construct Fe kagome-honeycomb lattices on Ag(111) and investigate their lattice parameter dependent electronic properties. The probed spectra exhibit the characteristic lattice peaks, which gradually merge and form a flat-band peak with decreasing the ratio of the high-order and first nearest-neighbor (NN) hopping, in line with the tight-binding calculations. The one-to-one correspondence unambiguously demonstrates a nontrivial band compression effect by minimizing the high-order NN hopping. Our result provides an efficient approach to realizing flat bands and engineering the electronic properties with artificial structures.

In a joint effort of both experiments and first-principles calculations, we resolve a hotly debated controversy and provide a coherent picture on the pure spin transport between Ag/Bi and ferromagnets. We demonstrate a strong inverse Rashba-Edelstein effect (IREE) at the interface in between Ag/Bi with a ferromagnetic metal (FM) but not with a ferromagnetic insulator. This is in sharp contrast to the previously claimed IREE at Ag/Bi interface or inverse spin Hall effect dominated spin transport. A more than one order of magnitude modulation of IREE signal is realized for different Ag/Bi-FM interfaces, casting strong tunability and a new direction for searching efficient spintronics materials.

Spin-torque ferromagnetic resonance (ST-FMR) has been widely used to determine the spin–orbit torque (SOT) efficiency in ferromagnet/heavy-metal bilayer systems. The flow of a radio frequency current through heavy-metal generates an oscillating SOT and Oersted field, resulting in the resonance of the adjacent ferromagnetic layer and subsequent dc voltage due to the rectification effect. The dynamics of the ferromagnet, however, also pumps a spin current back into the heavy-metal. Wherein, an additional contribution to the dc voltage arises from the inverse spin Hall effect (ISHE). The spin pumping-induced ISHE (SP-ISHE) and ST-FMR voltages typically have identical symmetry. In this work, we develop a method to quantitatively obtain the SP-ISHE voltage from the ST-FMR signal in the Py(Ni80Fe20)/Pt bilayer. We find it has the opposite sign to the symmetric component of ST-FMR voltage. After this correction, both the damping-like and field-like-torque efficiency in the Py/Pt bilayer are further estimated through the Py-thickness-dependent measurements.

We present a study of the magnetic configuration due to step-induced magnetic frustration at ferromagnetic/antiferromagnetic (FM/AFM) interfaces. At a substrate monatomic step edge, a 180° domain wall emerges. A physically appealing form for the thickness dependence of the domain wall width is obtained. It follows a universal behaviour in the whole thickness range, from ultrathin film to bulk and in both cases of an AFM domain wall on top of the FM layer and a FM domain wall on top of an AFM substrate. In the ultrathin limit of the capping layer, the domain wall grows linearly with the slope depending only on the ratio of the inter-layer and intra-layer Heisenberg exchange constants, regardless of the presence of magneto-crystalline anisotropy. These findings are in good agreement with previous experimental observations. As the thickness grows beyond the ultrathin regime, the corresponding thickness dependence departs from linearity and tends to its bulk value. The analytical insights are supported by conclusive numerical simulations of two independent varieties, namely, the Monte Carlo method which also includes the growth kinetics and the object oriented micromagnetic framework based micromagnetic simulations. While the quantitative details of the study are naturally dependent on the specific material parameters of the complex magnetic system, the global features of the spin texture in the capping layer are dictated by the topological step-edge defect. The latter in itself is quantifiable by a winding number of ± 1 2 .

Adding conductive one-dimensional carbon nanomaterials to poly(dimethysiloxane) can form bio-compatible composites with significant electromechanical (piezoresistive) response. This effect can be effectively tuned by controlling the carbon nanofiller size, concentration, and distribution. However, to be applied as strain sensors, the composite material has to meet mechanical, sensitivity, temperature stability, and reliability requirements. Here we report on the study of cyclic electromechanical behaviors of poly(dimethysiloxane)/carbon nanofiber composites under different temperatures. Through mechanical training, reproducible and sensitive piezoresistive response suitable for large strain sensing can be obtained.

We present a systematic study of the Kondo effect of single Co adatom placed on one monolayer (ML) Ag covered Cu(111) surface. The 1-ML Ag/Cu(111) shows apparent local density of states (LDOS) modulation which associated with the surface reconstruction. Interestingly, we find the Kondo resonance width of the Co adatom on 1-ML Ag/Cu(111) is position-dependent. When the Co adatom is placed near the triangular dislocation loop (hcp/fcc boundary), the Kondo resonance width is generally wider than the one measured at fcc site. At the fcc region, the measured Kondo resonance width is closely related to the LDOS variation. In conjunction with the tight binding approximation calculation, we identify that the main contribution of the LDOS modulation is the surface-reconstruction-regulated surface states. With a recently developed model, we further determine the contributions of the bulk and surface states to the Kondo resonance. These findings clarify the role of surface reconstruction on the Kondo effect.

The dc spin-to-charge conversions of tantalum (Ta) in Ta/Co40Fe40B20 bilayer structures are investigated utilizing spin pumping and inverse spin Hall effects (ISHE). From Ta thickness (tTa)-dependent resistivity and x-ray diffraction measurements, we found that Ta films, below 30 nm in thickness, are β-phase dominated. The damping enhancement shows a fast increase with tTa when tTa<1nm and reaches a saturation value at ∼1.5nm. The ISHE induced charge voltages have opposite signs for Ta and Pt. From tTa-dependent spin pumping produced ISHE voltage and precession angle measurements, the normalized spin-charge conversion signal is found to increase with tTa and saturate at ∼15nm. Our findings can be understood with a recently developed theory [Phys. Rev. Lett. 114, 126602 (2015)], which includes spin backflow and a spin loss at the interface. With a fitted spin loss factor of 0.02±0.02, we extract the spin Hall angle and spin diffusion length of high resistivity Ta to be θSH=−0.0062±0.001 and λsd=5.1±0.6nm, respectively.