Low-temperature Electrical Transport Study Of Qusi-1D Doubly Connected Superconducting Cylinder Of Al

The studying of the low dimensional superconducting system with new quantum phenomena has been an important research field of condensed matter physics. In recent years, doubly connected superconducting cylinder has been considered to be a very special system due to the fact that its diameter can be made to be less than its zero-temperature superconducting coherence length, and hence there are the unique physical properties and rich scientific connotation in the doubly connected superconducting cylinders. For example, a destructive regime predicted originally by de Gennes where superconductivity is lost near half-flux quanta even in the zero-temperature limit was found in such systems. Therefore, more and more researchers have devoted themselves to studying the doubly systems. However, there are still several key basic problems for this special system that need to be explored and understood, for example, the quantum phase transition (QPT) mechanism, the nature of the destructive regime and the origin of the quantum phase separation (QPS) phenonmenon and so on. In this thesis, we studied experimently the electrical transport of ultrathin doubly connected cylinders of Al with the diameter matched with its zero-temperature superconducting coherence length.Firstly, ultrathin doubly connected cylinders of Al were fabricated by depositing high-purity A1onto the rotating quartz filament with sub-100nm diameter. The resulted cylinders were characterized with scanning electron microscopy (SEM) and transmission electron microscopy (TEM). As-prepared samples are uniform and typically from100nm to300nm in diameter,30-40nm in film thickness,40±10nm in grain size, and more than500μm in length. Subsequently, we transferred the cylinders of Al onto the surface of N-doped silicon substrate with a300-nm-thick silicon oxide top layer, and then fabricated the4-6electrodes contacted with the cylinders of Al for the following four-probe electrical measurement. In order to have a good electrical contact, each contact point of the electrode lead was fixed by silver epoxy.Secondly, the resistance of Al cylinders was measured at low temperature (less than20mK) as a function of applied magnetic field in a dilution refrigerator equipped with a superconducting magnet where the cylinders were aligned parallel to the magnetic field. In order to obtain the best measurement results, we add shielding and grounding to the whole system as well as the filter board. Here, we mainly measured the resistance dependences on the temperatures at fixed magnetic field and on the magnetic fields at different temperatures, as well as the voltage versus current behaviors at base temperature.Finally, we found that the sample was superconducting for a substantial range of magnetic field below the parallel critical field. However, there are the resistance peaks at Φ=±1/2Φ0which can verify the presence of the destructive regime characterized by the loss of global phase coherence. Based on the experimental results, on the other hand, we have observed the classical regime near the quantum phase transition from the superconducting state to the normal state at both sides of the destructive regime for ultrathin Al cylinders. Furthermore, our data also suggest the possible existence of a quantum regime below about25mK near the onset of this quantum phase transition, where the cylinder resistance increases with decreasing temperatures. However, this novel quantum phenomenon still needs to be studied to fully understand the physical origin.