本文選題：超導(dǎo)轉(zhuǎn)變 + 磁通��； 參考：《中國科學(xué)院大學(xué)(中國科學(xué)院物理研究所)》2017年碩士論文

【摘要】：超導(dǎo)體具有抗磁性,當(dāng)其進(jìn)入超導(dǎo)轉(zhuǎn)變區(qū)時,內(nèi)部會進(jìn)入磁通,磁通的運動將是影響其電阻變化的主要因素。對于擁有二維周期結(jié)構(gòu)的超導(dǎo)薄膜,其拓?fù)浣Y(jié)構(gòu)將是影響磁阻變化的主要因素。輸運測量作為研究體系集體響應(yīng)的研究手段,可以間接的說明磁通在超導(dǎo)體內(nèi)的運動及排布情況。本論文主要介紹通過微納米加工手段制備具有不同二維結(jié)構(gòu)的孔陣列,利用電磁輸運性質(zhì)的測量手段研究了不同結(jié)構(gòu)對于轉(zhuǎn)變區(qū)磁阻曲線的影響,并據(jù)此來推測超導(dǎo)體內(nèi)磁通的各種結(jié)構(gòu)。論文中研究的實驗現(xiàn)象分為兩部分,第一是磁阻曲線周期的研究。磁阻曲線的振蕩效應(yīng)稱為匹配效應(yīng),即在特定的磁場下,磁阻曲線出現(xiàn)極小值,這些磁場稱為匹配場。匹配場一般具有周期性,具體表現(xiàn)為匹配場為最小匹配場(第一匹配場)的整數(shù)倍。磁阻的振蕩效應(yīng)雖然相似,但是卻由不同的機制引起。第一種是將磁通視為粒子的釘扎機制。當(dāng)釘扎中心間隙較大時,由于磁通量子化效應(yīng),超導(dǎo)體中的磁通為磁通量子的整數(shù)倍,可以將超導(dǎo)體中的磁通視為粒子。磁通之間相互排斥,在具有釘扎中心(一般為超導(dǎo)體中的雜質(zhì)或者缺陷)的超導(dǎo)體中又受到釘扎力的影響。在較小磁場的情況下,磁通由于釘扎力的作用和釘扎勢的存在,被束縛于釘扎中心。當(dāng)釘扎中心具有周期性時,束縛于每個釘扎點的磁通為磁通量子的整數(shù)倍,磁通之間的相互作用力抵消,形成穩(wěn)定的結(jié)構(gòu)。加磁場之后,超導(dǎo)體的電阻來源于磁通的運動引起的反向電壓。當(dāng)具有周期釘扎中心的超導(dǎo)體加上輸運電流之后,磁通雖然受到洛倫茲力的作用,但是釘扎勢的存在使得釘扎力與洛倫茲力方向相反,相互抵消,在匹配場下磁通之間相互作用力抵消,所有力的合力為零,磁通處于穩(wěn)定狀態(tài),超導(dǎo)體的電阻表現(xiàn)為極小值。實驗的結(jié)果驗證了磁阻曲線的匹配效應(yīng)。而隨著磁場的增大,磁阻曲線的周期改變,磁阻曲線出現(xiàn)整體的下降,這些現(xiàn)象為間隙磁通的進(jìn)入提供了證據(jù)。匹配場周期的改變是由于間隙磁通的進(jìn)入而使得磁通格子的周期發(fā)生改變,磁阻曲線的整體下降是由于間隙磁通進(jìn)入進(jìn)一步束縛了磁通的運動。第二部分是關(guān)于第一匹配場內(nèi)分?jǐn)?shù)匹配效應(yīng)的研究。實驗中在各種不同的樣品的測量中都發(fā)現(xiàn)了分?jǐn)?shù)匹配場,分?jǐn)?shù)匹配場與第一匹配場的之比決定于圓孔陣列的結(jié)構(gòu),磁通的分?jǐn)?shù)匹配現(xiàn)象采用磁通的粒子模型是不能解釋的。將周期性的圓孔陣列作為釘扎中心與一般的釘扎中心不同在于,孔之間的最小間隙和超導(dǎo)體相干長度差不多,超導(dǎo)序參量在孔之間并不會發(fā)生明顯變化,因此要采用全磁通量子化條件代替磁通量化條件。此時將較小的間隙區(qū)視為線條,將較大的間隙區(qū)視為節(jié)點,將圓孔陣列簡化為線條狀網(wǎng)絡(luò)結(jié)構(gòu),利用線性GL方程計算,線條狀網(wǎng)絡(luò)的理論計算結(jié)果很好的符合了實驗現(xiàn)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.
【學(xué)位授予單位】：中國科學(xué)院大學(xué)(中國科學(xué)院物理研究所)
【學(xué)位級別】：碩士
【學(xué)位授予年份】：2017
【分類號】：TB383.2;TM26
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本文編號：1816523