Yao Guang

Nonlinear photogalvanic effects in two-dimensional materials, particularly the nonlinear circular photocurrents (NCPs) that belong to the helicity-dependent spin photocurrents, have sparked enormous research interest. Although notable progress has been witnessed, the underling origin of NCPs remains elusive. Here, we present systematic photocurrent characteristics, symmetry analysis and theoretical calculations to uncover the physical origin of NCPs in MoS_{2}, a prototypical 2D semiconductor. Our results show that the NCP responses in 2D semiconductor MoS_{2} result from the circular photon drag effect (CPDE), rather than the generally believed circular photogalvanic effect. Furthermore, we demonstrate that the NCPs are highly tunable with electrostatic doping and increase progressively with MoS_{2} thickness, evidencing the interlayer constructive nature of CPDE responses. Our Letter unravels the critical role of the previously overlooked CPDE contribution to NCPs, revolutionizing previous understanding and thus providing deep insights into further fundamental studies and technological advances in nonlinear photovoltaic and opto-spintronic devices.

Current-driven dynamics of topological spin textures, such as skyrmions and antiskyrmions, have garnered considerable attention in condensed matter physics and spintronics. As compared with skyrmions, the current-driven dynamics of their antiparticles – antiskyrmions − remain less explored due to the increased complexity of antiskyrmions. Here, we design and employ fabricated microdevices of a prototypical antiskyrmion host, (Fe_0.63Ni_0.3Pd_0.07)₃P, to allow in situ current application with Lorentz transmission electron microscopy observations. The experimental results and related micromagnetic simulations demonstrate current-driven antiskyrmion dynamics confined within stripe domains. Under nanosecond-long current pulses, antiskyrmions exhibit directional motion along the stripe regardless of the current direction, while the antiskyrmion velocity is linearly proportional to the current density. Significantly, the antiskyrmion mobility could be enhanced when the current flow is perpendicular to the stripe direction. Our findings provide novel and reliable insights on dynamical antiskyrmions and their potential implications on spintronics.

Element substitutions with magnetic or non-magnetic atoms are known to significantly impact the magnetic structure and related transport properties of magnets. To clarify the change of magnetic structure of B20-type magnets with element doping, we conduct real-space observations of spin textures and their temperature (T)-magnetic field (H) phase diagrams of a helimagnet FeGe with partially substituting Fe and Ge with Co and Si, respectively. The helical period (λ) changes dramatically by the element doping: λ increases by 147% to 103 nm in 30% Co-doped FeGe, whereas it decreases by around 70% to 49 nm in 30% Si-doped FeGe, compared to the λ =70 nm in FeGe. Upon applying the magnetic field normally to (001), (110), and (111) thin plates of both FeSi_0.3Ge_0.7 and Fe_0.7Co_0.3Ge, the hexagonal skyrmion crystal (SkX) state emerges. The magnetic phase diagrams observed through the real-space imaging reveal that (1) the SkX can extend to a larger T-H window by reducing the sample thickness or by cooling the sample under specific magnetic fields from temperatures above the transition temperature (T_C ); (2) the stability of the SkX phase differs between Si-doped and Co-doped FeGe: the SkX phase is most unstable in the (111) FeSi_0.3Ge_0.7, while it remains robust in the (111) Fe_0.7Co_0.3Ge. These differences indicate distinct anisotropic behavior in FeGe with magnetic (Co) and non-magnetic-element (Si) dopants.

The balance between cell quiescence and proliferation is fundamental to tissue physiology and homeostasis. Recent studies have shown that quiescence is not a passive and homogeneous state but actively maintained and heterogeneous. These cellular characteristics associated with quiescence were observed primarily in cultured cells under a static medium. However, cells in vivo face different microenvironmental conditions, particularly, under interstitial fluid flows distributed through extracellular matrices. Interstitial fluid flow exerts shear stress on cells and matrix strain, and results in continuous replacement of extracellular factors. In this study, we analyzed individual cells under varying fluid flow rates in microfluidic devices. We found quiescence characteristics previously identified under conventional static medium, including serum signal-dependant quiescence entry and exit and time-dependant quiescence deepening, are also present under continuous fluid flow. Furthermore, increasing the flow rate drives cells to shallower quiescence and become more likely to reenter the cell cycle upon growth stimulation. This effect is due to flow-induced physical and biochemical cues. Specifically, increasing shear stress or extracellular factor replacement individually, without altering other parameters, results in shallow quiescence. We show our experimental results can be quantitatively explained by a mathematical model connecting extracellular fluid flow to an Rb-E2f bistable switch that regulates the quiescence-to-proliferation transition. Our findings uncover a previously unappreciated mechanism that likely underlies the heterogeneous responses of quiescent cells for tissue repair and regeneration in different physiological tissue microenvironments.

A three-dimensional singular point that consists of two oppositely aligned emergent monopoles is identified in continuous CoTb thin films, as confirmed by complementary techniques of resonant elastic x-ray scattering, Lorentz transmission electron microscopy, and scanning transmission x-ray microscopy. This new type of topological defect can be regarded as a superposition of an emergent magnetic monopole and an antimonopole, around which the source and drain of the magnetic flux overlap in space. We experimentally prove that the observed spin twist seen in Lorentz transmission electron microscopy reveals the cross section of the superimposed three-dimensional structure, providing a straightforward strategy for the observation of magnetic singularities. Such a quasiparticle provides an excellent platform for studying the rich physics of emergent electromagnetism.

A skyrmionic programmable logic device (SPLD) with complete Boolean logic functions is proposed and analyzed by micromagnetic simulations. The SPLD is based on an antiferromagnet-ferromagnet bilayer structure, in which the antiferromagnetic layer supports the interfacial Dzyaloshinskii-Moriya interaction and the out-of-plane exchange-bias field for stabilizing a zero-field skyrmion. By varying the local exchange-bias field, artificial pinning sites are introduced to trap skyrmions. Depending on the input currents and initial position of skyrmions at different pinning sites, different logic functions can be realized. Micromagnetic simulations show that the proposed SPLD has a robust performance, even under thermal fluctuations and inhomogeneity effects. Our work can provide insights for the design of programmable spin-logic devices.

According to the generally accepted nonlinear principles, second-order nonlinear effect (SONE) is strongly inhibited by the crystalline symmetries and thus can manifest only in non-centrosymmetric materials with broken global spatial inversion symmetry. In stark contrast, here we report the observation of direct-current (DC) related SONE, including circular and linear photogalvanic effects, in centrosymmetric bilayer and multilayer MoS2. In conjunction with relativistic first-principles calculations, we uncover that the observed DC-related SONE in inversion-symmetric MoS2 results from the localized electronic states and the locking of spin with the layer and valley pseudospins. Our results provide a new insight into nonlinear physics and would be applicable to other phenomena thus far believed to occur only in non-centrosymmetric systems, such as quantum spin Hall effect, valley Hall effect, piezoelectricity and unconventional Ising superconductivity.