Ginzburg-Landau Dynamical Simulations On The Nonreciprocal Transport Properties Of Two-dimensional Superconductors

The nonreciprocal charge transport,exhibiting dependence on the polarity of the applied current,such as the diode effect and the rectification effect,is of great importance for both theoretical research and device applications in low-power electronics engineering.To achieve the nonreciprocal transport property in superconductors generally requires to break both the spatial inversion symmetry and the time-reversal symmetry,and therefore touches on the very fundamental issues in superconductivity.Of particular interest is the superconducting diode effect,which exhibits dissipationless superconducting transport for just one direction of the applied current,being the base element of the emergent superconducting electronics and has received extensive attention in recent years as a consequence.In this Ph.D thesis,we report the vortex dynamics coupled with associated heat dissipation by numerically solving time-dependent Ginzburg-Landau equations and heat transfer equation.The nonreciprocal transport properties in three kinds of superconducting systems are studied and strong superconducting diode effects are revealed in two of those systems while while the conventional ratchet behavior is reported for the third.The main results are outlined below.(1)In Chapter 3,we study a superconducting film patterned with a conformal pinning array.We find a giant rectification effect which is consistent with the experimental observation.By simulating vortex dynamics,we can keep tracks of the vortex motion and illuminate the behavior underlying the experimental findings.Within the numerical simulations,we completely explain the mechanism of the rectification effect.The funneling effect emerges due to the unique geometry of the conformal pinning array.In presence of the funneling effect,vortices tend to accumulate in the choke of the funnel,where Joule heating creates hot spots,driving the sample to the normal state.Meanwhile,the density gradient of vortex matter does not match the gradient of pinning sites when the funneling effect happens,leading to a less effective pinning effect.The two mechanisms together lead to the giant rectification effect and the superconducting diode behavior.(2)In Chapter 4,we study the nonreciprocal charge transport in a pinning-free superconducting nano-ring.We systematically calculate the phase diagram of the ratchet signal for applied D.C.current and the response of the ratchet signal to the external magnetic fields,current directions and frequencies in presence of A.C.currents.By analyzing the single vortex potential in a narrow superconducting ring,we find that the nonreciprocal transport property is caused by the asymmetry potential barriers for vortex entry and exit,which originates from the asymmetric sample geometry.(3)In Chapter 5,we study a superconductor/nanoscale-magnetic-dot hybrid structure.In this system we take advantage of the external current to control the nucleation of vortex-antivortex pairs,and design the superconducting diode effect in the absence of applied magnetic fields.Our vortex dynamics simulation details the progress of the excitation and annihilation of vortexantivortex pairs,and the progress of the superconducting-normal phase transition due to the corresponding heat dissipation.Meanwhile we discover that the polarity of the superconducting diode can be switched by simply increasing the magnetization of the magnetic dots.Such a realization of a superconducting diode that does not require external magnetic field is a major step towards facilitated use of superconducting electronics.The nonreciprocal transport properties of the above three systems are all based on the broken symmetry of spatial inversion,which is caused by the anisotropic pinning array,the asymmetric geometry,and the nonuniform distribution of the intrinsic magnetic field,respectively.The mechanisms we discuss in this thesis do not require any special property of the materials and thus can be applied to any conventional superconductors.The present thesis therefore provides a solid theoretical basis for the future design and application of the dissipationless superconducting devices.