二维周期结构薄膜超导转变区输运性质的研究

发布时间：2018-04-28 19:14

  本文选题：超导转变 + 磁通　； 参考：《中国科学院大学(中国科学院物理研究所)》2017年硕士论文

【摘要】：超导体具有抗磁性,当其进入超导转变区时,内部会进入磁通,磁通的运动将是影响其电阻变化的主要因素。对于拥有二维周期结构的超导薄膜,其拓扑结构将是影响磁阻变化的主要因素。输运测量作为研究体系集体响应的研究手段,可以间接的说明磁通在超导体内的运动及排布情况。本论文主要介绍通过微纳米加工手段制备具有不同二维结构的孔阵列,利用电磁输运性质的测量手段研究了不同结构对于转变区磁阻曲线的影响,并据此来推测超导体内磁通的各种结构。论文中研究的实验现象分为两部分,第一是磁阻曲线周期的研究。磁阻曲线的振荡效应称为匹配效应,即在特定的磁场下,磁阻曲线出现极小值,这些磁场称为匹配场。匹配场一般具有周期性,具体表现为匹配场为最小匹配场(第一匹配场)的整数倍。磁阻的振荡效应虽然相似,但是却由不同的机制引起。第一种是将磁通视为粒子的钉扎机制。当钉扎中心间隙较大时,由于磁通量子化效应,超导体中的磁通为磁通量子的整数倍,可以将超导体中的磁通视为粒子。磁通之间相互排斥,在具有钉扎中心(一般为超导体中的杂质或者缺陷)的超导体中又受到钉扎力的影响。在较小磁场的情况下,磁通由于钉扎力的作用和钉扎势的存在,被束缚于钉扎中心。当钉扎中心具有周期性时,束缚于每个钉扎点的磁通为磁通量子的整数倍,磁通之间的相互作用力抵消,形成稳定的结构。加磁场之后,超导体的电阻来源于磁通的运动引起的反向电压。当具有周期钉扎中心的超导体加上输运电流之后,磁通虽然受到洛伦兹力的作用,但是钉扎势的存在使得钉扎力与洛伦兹力方向相反,相互抵消,在匹配场下磁通之间相互作用力抵消,所有力的合力为零,磁通处于稳定状态,超导体的电阻表现为极小值。实验的结果验证了磁阻曲线的匹配效应。而随着磁场的增大,磁阻曲线的周期改变,磁阻曲线出现整体的下降,这些现象为间隙磁通的进入提供了证据。匹配场周期的改变是由于间隙磁通的进入而使得磁通格子的周期发生改变,磁阻曲线的整体下降是由于间隙磁通进入进一步束缚了磁通的运动。第二部分是关于第一匹配场内分数匹配效应的研究。实验中在各种不同的样品的测量中都发现了分数匹配场,分数匹配场与第一匹配场的之比决定于圆孔阵列的结构,磁通的分数匹配现象采用磁通的粒子模型是不能解释的。将周期性的圆孔阵列作为钉扎中心与一般的钉扎中心不同在于,孔之间的最小间隙和超导体相干长度差不多,超导序参量在孔之间并不会发生明显变化,因此要采用全磁通量子化条件代替磁通量化条件。此时将较小的间隙区视为线条,将较大的间隙区视为节点,将圆孔阵列简化为线条状网络结构,利用线性GL方程计算,线条状网络的理论计算结果很好的符合了实验现象。
[Abstract]:The superconductor has diamagnetism, when it enters the superconducting transition region, the internal flux will enter, and the movement of the magnetic flux will be the main factor affecting the change of its resistance. For the superconducting thin films with 2-D periodic structure, the topological structure will be the main factor affecting the magnetoresistive change. Transport measurement as a collective response of the research system can indirectly explain the movement and distribution of magnetic flux in superconducting body. In this paper, we mainly introduce the fabrication of pore arrays with different two-dimensional structures by micro-nano fabrication, and study the effect of different structures on the magnetoresistive curves of the transition region by means of the measurement of electromagnetic transport properties. The structure of magnetic flux in superconductor is inferred. The experimental phenomena in this paper are divided into two parts. The first is the period of magnetoresistive curve. The oscillatory effect of the magnetoresistive curve is called the matching effect, that is, the magnetoresistive curve has a minimum value under a specific magnetic field, and these magnetic fields are called the matching field. The matching field is generally periodic, which is the integer multiple of the minimum matching field (the first matching field). Although the magnetoresistive oscillation effect is similar, it is caused by different mechanisms. The first is to consider the flux as the pinning mechanism of particles. When the gap of pinning center is large, the flux in superconductor can be regarded as a particle because of the flux quantization effect. Magnetic flux repeats each other and is affected by pinning force in superconductors with pinning centers (usually impurities or defects in superconductors). In the case of small magnetic field, the magnetic flux is bound to the pinning center because of the effect of pinning force and the existence of pinning potential. When the pinning center is periodic, the flux bound to each pinning point is an integral fold of the flux, which counteracts the interaction between the flux and forms a stable structure. After a magnetic field, the resistance of the superconductor is derived from the reverse voltage caused by the motion of the flux. When a superconductor with a periodic pinning center and a transport current are added, the magnetic flux is subjected to the Lorentz force, but the existence of the pinning potential makes the pinning force counteract each other in the opposite direction of the Lorentz force. In the matching field, the interaction force between the magnetic flux is counteracted, the resultant force of all the forces is zero, the flux is in a stable state, and the resistance of the superconductor is shown as a minimum. The experimental results verify the matching effect of the magnetoresistive curve. With the increase of magnetic field, the period of magnetoresistive curve changes and the magnetoresistive curve decreases as a whole. These phenomena provide evidence for the entry of gap flux. The period of matching field is changed because of the entrance of gap flux, and the whole decline of magnetoresistive curve is due to the movement of flux is further restrained by the entrance of gap flux. The second part is about the study of the fraction matching effect in the first matching field. The fractional matching field is found in various samples in the experiment. The ratio of the fraction matching field to the first matching field is determined by the structure of the circular hole array. The particle model of flux is not able to explain the fraction matching phenomenon of magnetic flux. The difference between the periodic circular aperture array as pinning center and the normal pinning center is that the minimum gap between the holes and the coherent length of the superconductor are about the same, and the superconducting order parameters do not change obviously between the holes. Therefore, the flux quantization condition should be used instead of the flux condition. At this time, the smaller gap region is regarded as a line, the larger gap area is regarded as a node, and the circular hole array is simplified into a line-shaped network structure. The theoretical calculation results of the line-shaped network are in good agreement with the experimental phenomenon by using the linear GL equation.
【学位授予单位】：中国科学院大学(中国科学院物理研究所)
【学位级别】：硕士
【学位授予年份】：2017
【分类号】：TB383.2;TM26
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本文编号：1816523