| Summary: | Symmetry plays a significant role in the study of condensed matter physics. A typical example of broken inversion symmetry is the second harmonic generation of nonlinear optics. Recent research has expanded the nonlinear transport studies to nonreciprocal electrical transport in noncentrosymmetric systems, which reflects properties related to magnetism and superconductivity. The discovery of nonreciprocal electrical transport in inversion-breaking systems marks a milestone, as the resistance exhibits dependence on both current and magnetic field. Despite the ubiquity of nonreciprocal electrical transport in various complex systems, the origin can be simplified into two categories based on microscopic mechanisms, namely, chiral and polar structures. The two mechanisms exhibit distinct directional dependencies on current and magnetic field. Specifically, chiral systems encompass intrinsic chiral structures and twisted heterostructures, where the nonreciprocal electrical transport is maximized when the magnetic field is along the direction of current and chirality. The polar systems extend from interfacial systems to uniform crystals with polarity, where the nonreciprocal transport is maximized when the magnetic field is orthogonal to the current and polarity.
Nonreciprocal electrical transport is regarded as an inherent rectification effect of charge current within systems with broken inversion and time-reversal symmetries, which stands apart from the conventional rectification effect observed in p-n junctions. This unique characteristic positions nonreciprocal transport as a promising avenue for exploring novel physical phenomena and applications in microscopic devices. Its inherent correlation with spatial and electronic structures makes it a sensitive probe for charge and spin transport, as well as band structure. Moreover, this characteristic finds practical applications in areas such as current and microwave rectifications. In two-dimensional heterostructures, the spatial and electronic geometry modulation from interfacial interaction makes it a fascinating platform for nonreciprocal electrical transport. Consequently, my research focuses on enhancing, manipulating, and excavating the micro-mechanisms of nonreciprocal transport in two-dimensional layered structures.
First, I studied the superconducting vortex phase diagram in a misfit layer compound characterized by alternating stacks of superconductor (NbS2) and semiconductor (SnS) sublayers. within this layered structure, two types of vortices, namely, pancake vortices (PV) and Josephson vortices (JV), reside in the superconducting sublayer and non-superconducting sublayer, respectively. The polarity imposed by the monolayer NbS2 results in the ratchet motion of both PV and JV, leading to nonreciprocal transport. This nonreciprocal motion demonstrates a dependence on the magnetic field direction and can be regulated by vortex density. Furthermore, the nonreciprocal motion undergoes reversal at elevated temperatures, attributed to the collective effects of paraconductivity and thermally assisted flux flow (TAFF). I thereby built a phase diagram of vortex motion in superlattices.
Second, I demonstrated a superconducting diode effect in a FeTe0.7Se0.3 (FTS) junction. The FTS nanosheets were synthesized using chemical vapour deposition (CVD), followed by the integration of an FTS junction using the dry-pickup method. While a single FTS nanosheet displayed a symmetrical V-I relationship, the FTS junction exhibited an asymmetric V-I relationship with different critical currents (Ic) for opposite directions. I proceeded to conduct a half-wave rectification on the device, revealing its capability to switch between zero resistance and a resistive state. Through comprehensive angular-dependent magnetotransport analysis, the superconducting diode effect is attributed to the chirality within the FTS junction.
Third, I investigated the electrical control of multilevel magnetic anisotropy and the magnetization dynamics in a magnetic heterostructure. By employing electrochemical gating, the device exhibited a nonvolatile modulation between a ferromagnetic multidomain (FM-MD) state and a superparamagnetic single domain (SP-SD) state, each associated with distinct nonreciprocal responses due to different spin-orbit torque (SOT) efficiency. I then demonstrated multilevel magnetization switching under an external magnetic field and current, respectively, by locally controlling magnetic anisotropy. Through comprehensive elemental and electrical analysis, the magnetic anisotropy modulation was attributed to the variation of magnetic grain dimensionality.
This thesis validates the efficacy of nonreciprocal transport in describing the classical and quantum behaviours in superconductors and ferromagnets with broken inversion and time-reversal symmetries. Moreover, the study highlights the potential for the development of high-performance and energy-efficient next-generation electronics.
|
|---|