幾種含銪化合物中超導電性與磁性研究
發(fā)布時間：2019-02-15 23:05
【摘要】：含有稀土元素的化合物由于其呈現(xiàn)出的豐富多彩以及奇異的物理性質(zhì),如中間價態(tài)、密度近藤(Kondo)效應及重費米子行為,一直是凝聚態(tài)物理中的研究熱點。在這些化合物中往往存在著Ruderman-Kittel-Kasuya-Yosida (RKKY)相互作用和Kondo效應競爭。在一些含Eu化合物中,元素Eu可以出現(xiàn)不同程度的價態(tài)漲落,而價態(tài)的不穩(wěn)定與非費米液體行為和奇異超導(SC)又密切相關。在Eu基體系中,一個很重要的特征是Eu的價態(tài)強烈的依賴于溫度變化。本文在三種含Eu的層狀化合物中重點研究了Eu的價態(tài)、磁性與超導三者之間的相互關系。本論文主要對Eu3Bi2(S,Se)4F4、(Eu,Sr)3Bi2S4F4以及EuPt2As2這三種含Eu的化合物進行研究。在前面兩種鉍硫基化合物中,我們研究了摻雜對Eu的變價的影響以及與超導電性的關系,而對EuPt2As2單晶的測量我們給出了一個復雜的磁性測量。我們通過固相反應法合成了 一系列Se摻雜的Eu3Bi2(S,Se)4F4,研究了該系統(tǒng)中Se摻雜引致的負的化學壓力效應。我們觀察到,在電阻上,S位的Se摻雜使得一個類電荷密度波(CDW)異常被逐漸抑制到了低溫,同時超導轉(zhuǎn)變溫度(Tc)提高。對于摻雜含量為50%的樣品,其Tc達到了3.35K,超過了母體的兩倍。對磁化率以及磁熵的數(shù)據(jù)結(jié)果分析顯示,在該系統(tǒng)中,隨Se摻雜濃度提高Eu的平均價態(tài)也在升高。這就意味著更多的電子可以從Eu轉(zhuǎn)移到BiS2超導層,可能也是摻雜后Tc增加的主要原因。隨后的霍爾測量證實了載流子濃度(空穴和電子)在Se摻雜后出現(xiàn)了很大幅度的提高。鑒于等價替代并不會引入更多的電子,我們認為在該系統(tǒng)中所觀察到的摻雜效應主要是由Eu的變價導致的。對于Eu3-xSrxBi2S4F4這個體系,通過部分Sr替代Eu元素,我們發(fā)現(xiàn)Tc逐漸降低,對x1.0的樣品,超導現(xiàn)象在0.3 K以上消失。對于組分為x= 2.0的樣品,其正常態(tài)的電阻率呈現(xiàn)出類似半導體的行為,而不再是低摻雜下的金屬性行為。磁化率和比熱的數(shù)據(jù)分析表明在Sr摻雜下,Eu2+和Eu3+的含量都在下降。Eu3+的減小對應著載流子濃度的降低,這可能是超導被抑制的主要原因。有意思的是,在比熱上我們發(fā)現(xiàn)電子系數(shù)隨Sr摻雜濃度的增加有很大程度的提升。我們認為這可能是由Eu2+的磁矩與導電電子之間的Kondo效應引起的。由于Kondo效應與超導是競爭的關系,這可能是該體系中Tc下降較快的另一個原因。此外我們還用自助熔法合成了接近毫米量級的EuPt2As2單晶并仔細研究了其物理性質(zhì)。低溫下的電阻率、磁化率以及比熱的測量均表示該樣品存在兩個反鐵磁(AFM)轉(zhuǎn)變TN1=7.1 K和TN2=5.8 K。從磁化率的結(jié)果上我們猜測在EuPt2As2中,當TTTN1時,Eu2+磁矩在ab平面內(nèi)的排列幾乎是共線的,而在低于TN2的溫度下,自旋向c軸傾斜。其中TN1源于Eu2+磁矩反鐵磁長程序的形成,而TN2可能來自于自旋方向的轉(zhuǎn)變。同時,我們又在2K下觀測到了由外磁場引起的亞鐵磁轉(zhuǎn)變。最后我們得到了EuPt2As2中磁場沿著c方向時兩個反鐵磁轉(zhuǎn)變隨磁場變化的相圖。
[Abstract]:The compound containing rare earth elements has been a hot spot in the condensed matter physics because of its rich and colorful and strange physical properties, such as the intermediate valence state, the density of the Kondo effect and the heavy-fermionic behavior. There are often the Ruderman-Koittel-Kasuya-Yosida (RKKY) interaction and the Kondo effect competition in these compounds. In some Eu-containing compounds, the valence state fluctuation of the element Eu can occur, and the instability of the valence state is closely related to the non-Fermi liquid behavior and the singular superconductivity (SC). In the Eu-based system, a very important feature is that the valence state of Eu is strongly dependent on temperature variations. In the three kinds of Eu-containing layered compounds, the relationship between the valence state, the magnetism and the superconductivity of Eu is studied. This paper mainly studies the three Eu-containing compounds of Eu3Bi2 (S, Se) 4F4, (Eu, Sr) 3Bi2S4F4 and EuPt2As2. In the above two kinds of sulfur-based compounds, we have studied the influence of doping on the valence of Eu and the relation with superconductivity, and we give a complicated magnetic measurement to the measurement of EuPt2As2 single crystal. A series of Se-doped Eu3Bi2 (S, Se) 4F4 was synthesized by solid-phase reaction, and the negative chemical pressure effect induced by Se doping in the system was studied. We have observed that, on the resistance, the Se doping of the S-bit causes a class-like charge density wave (CDW) to be gradually suppressed to a low temperature, while the superconducting transition temperature (Tc) is increased. For samples with a doping content of 50%, Tc reached 3.35K, more than twice the parent. The data results of magnetic susceptibility and magnetic entropy show that, in this system, the average valence state of Eu increases with Se doping concentration. This means that more electrons can be transferred from Eu to the BiS2 superconducting layer, which may also be the main reason for the increased Tc increase. The subsequent Hall measurements confirmed a significant increase in the carrier concentration (holes and electrons) after Se doping. In view of the equivalent substitution and no more electrons are introduced, we believe that the doping effect observed in this system is mainly due to the valence of Eu. For Eu3-xSrxBi2S4F4 system, by replacing the Eu element with part Sr, we find that Tc is gradually decreasing, and the superconducting phenomenon of the sample with X1.0 is disappeared over 0.3K. For a sample with component x = 2.0, the resistivity of its normal state exhibits behavior similar to that of a semiconductor, and no longer a low-doped metal behavior. The data analysis of the magnetic susceptibility and specific heat shows that the content of Eu2 + and Eu3 + is decreasing in Sr doping. The reduction of Eu3 + corresponds to a reduction in the carrier concentration, which may be the main cause of the superconducting being suppressed. It is interesting that, at specific heat, we have found that the increase of the electron coefficient with the concentration of Sr doping has a great degree of improvement. This may be caused by the Kondo effect between the magnetic moment of Eu2 + and the conductive electron. Because the Kondo effect is a competitive relationship with superconductivity, this may be another reason for the faster Tc drop in the system. In addition, EuPt2As2 single crystal on the order of millimeters was synthesized by self-fluxing method and the physical properties of EuPt2As2 were studied. The resistivity, magnetic susceptibility and specific heat measurements at low temperature indicate that the sample has two anti-ferromagnetic (AFM) transitions TN1 = 7.1K and TN2 = 5.8K. From the results of the magnetic susceptibility, we have speculated that in EuPt2As2, the arrangement of the Eu2 + magnetic moment in the ab plane is almost collinear, and at a temperature below TN2, the spin is tilted to the c-axis. where TN1 is derived from the formation of the Eu2 + magnetic moment anti-ferromagnetic long program, and TN2 may come from a transition in the spin direction. At the same time, we observed the sub-ferromagnetic transition caused by the external magnetic field at 2K. In the end, we obtained the phase diagram of the change of the two anti-ferromagnetic transitions with the magnetic field when the magnetic field in EuPt2As2 is in the direction of c.
【學位授予單位】：浙江大學
【學位級別】：博士
【學位授予年份】：2017
【分類號】：O469
本文編號：2423787
[Abstract]:The compound containing rare earth elements has been a hot spot in the condensed matter physics because of its rich and colorful and strange physical properties, such as the intermediate valence state, the density of the Kondo effect and the heavy-fermionic behavior. There are often the Ruderman-Koittel-Kasuya-Yosida (RKKY) interaction and the Kondo effect competition in these compounds. In some Eu-containing compounds, the valence state fluctuation of the element Eu can occur, and the instability of the valence state is closely related to the non-Fermi liquid behavior and the singular superconductivity (SC). In the Eu-based system, a very important feature is that the valence state of Eu is strongly dependent on temperature variations. In the three kinds of Eu-containing layered compounds, the relationship between the valence state, the magnetism and the superconductivity of Eu is studied. This paper mainly studies the three Eu-containing compounds of Eu3Bi2 (S, Se) 4F4, (Eu, Sr) 3Bi2S4F4 and EuPt2As2. In the above two kinds of sulfur-based compounds, we have studied the influence of doping on the valence of Eu and the relation with superconductivity, and we give a complicated magnetic measurement to the measurement of EuPt2As2 single crystal. A series of Se-doped Eu3Bi2 (S, Se) 4F4 was synthesized by solid-phase reaction, and the negative chemical pressure effect induced by Se doping in the system was studied. We have observed that, on the resistance, the Se doping of the S-bit causes a class-like charge density wave (CDW) to be gradually suppressed to a low temperature, while the superconducting transition temperature (Tc) is increased. For samples with a doping content of 50%, Tc reached 3.35K, more than twice the parent. The data results of magnetic susceptibility and magnetic entropy show that, in this system, the average valence state of Eu increases with Se doping concentration. This means that more electrons can be transferred from Eu to the BiS2 superconducting layer, which may also be the main reason for the increased Tc increase. The subsequent Hall measurements confirmed a significant increase in the carrier concentration (holes and electrons) after Se doping. In view of the equivalent substitution and no more electrons are introduced, we believe that the doping effect observed in this system is mainly due to the valence of Eu. For Eu3-xSrxBi2S4F4 system, by replacing the Eu element with part Sr, we find that Tc is gradually decreasing, and the superconducting phenomenon of the sample with X1.0 is disappeared over 0.3K. For a sample with component x = 2.0, the resistivity of its normal state exhibits behavior similar to that of a semiconductor, and no longer a low-doped metal behavior. The data analysis of the magnetic susceptibility and specific heat shows that the content of Eu2 + and Eu3 + is decreasing in Sr doping. The reduction of Eu3 + corresponds to a reduction in the carrier concentration, which may be the main cause of the superconducting being suppressed. It is interesting that, at specific heat, we have found that the increase of the electron coefficient with the concentration of Sr doping has a great degree of improvement. This may be caused by the Kondo effect between the magnetic moment of Eu2 + and the conductive electron. Because the Kondo effect is a competitive relationship with superconductivity, this may be another reason for the faster Tc drop in the system. In addition, EuPt2As2 single crystal on the order of millimeters was synthesized by self-fluxing method and the physical properties of EuPt2As2 were studied. The resistivity, magnetic susceptibility and specific heat measurements at low temperature indicate that the sample has two anti-ferromagnetic (AFM) transitions TN1 = 7.1K and TN2 = 5.8K. From the results of the magnetic susceptibility, we have speculated that in EuPt2As2, the arrangement of the Eu2 + magnetic moment in the ab plane is almost collinear, and at a temperature below TN2, the spin is tilted to the c-axis. where TN1 is derived from the formation of the Eu2 + magnetic moment anti-ferromagnetic long program, and TN2 may come from a transition in the spin direction. At the same time, we observed the sub-ferromagnetic transition caused by the external magnetic field at 2K. In the end, we obtained the phase diagram of the change of the two anti-ferromagnetic transitions with the magnetic field when the magnetic field in EuPt2As2 is in the direction of c.
【學位授予單位】：浙江大學
【學位級別】：博士
【學位授予年份】：2017
【分類號】：O469
【參考文獻】
相關博士學位論文 前1條
1 羅永康;鈰基磷族化合物中磁性和近藤效應的研究[D];浙江大學;2013年
，本文編號：2423787
本文鏈接：http://sikaile.net/shoufeilunwen/jckxbs/2423787.html
最近更新
教材專著
