几种含铕化合物中超导电性与磁性研究
发布时间：2019-02-15 23:05
【摘要】：含有稀土元素的化合物由于其呈现出的丰富多彩以及奇异的物理性质,如中间价态、密度近藤(Kondo)效应及重费米子行为,一直是凝聚态物理中的研究热点。在这些化合物中往往存在着Ruderman-Kittel-Kasuya-Yosida (RKKY)相互作用和Kondo效应竞争。在一些含Eu化合物中,元素Eu可以出现不同程度的价态涨落,而价态的不稳定与非费米液体行为和奇异超导(SC)又密切相关。在Eu基体系中,一个很重要的特征是Eu的价态强烈的依赖于温度变化。本文在三种含Eu的层状化合物中重点研究了Eu的价态、磁性与超导三者之间的相互关系。本论文主要对Eu3Bi2(S,Se)4F4、(Eu,Sr)3Bi2S4F4以及EuPt2As2这三种含Eu的化合物进行研究。在前面两种铋硫基化合物中,我们研究了掺杂对Eu的变价的影响以及与超导电性的关系,而对EuPt2As2单晶的测量我们给出了一个复杂的磁性测量。我们通过固相反应法合成了 一系列Se掺杂的Eu3Bi2(S,Se)4F4,研究了该系统中Se掺杂引致的负的化学压力效应。我们观察到,在电阻上,S位的Se掺杂使得一个类电荷密度波(CDW)异常被逐渐抑制到了低温,同时超导转变温度(Tc)提高。对于掺杂含量为50%的样品,其Tc达到了3.35K,超过了母体的两倍。对磁化率以及磁熵的数据结果分析显示,在该系统中,随Se掺杂浓度提高Eu的平均价态也在升高。这就意味着更多的电子可以从Eu转移到BiS2超导层,可能也是掺杂后Tc增加的主要原因。随后的霍尔测量证实了载流子浓度(空穴和电子)在Se掺杂后出现了很大幅度的提高。鉴于等价替代并不会引入更多的电子,我们认为在该系统中所观察到的掺杂效应主要是由Eu的变价导致的。对于Eu3-xSrxBi2S4F4这个体系,通过部分Sr替代Eu元素,我们发现Tc逐渐降低,对x1.0的样品,超导现象在0.3 K以上消失。对于组分为x= 2.0的样品,其正常态的电阻率呈现出类似半导体的行为,而不再是低掺杂下的金属性行为。磁化率和比热的数据分析表明在Sr掺杂下,Eu2+和Eu3+的含量都在下降。Eu3+的减小对应着载流子浓度的降低,这可能是超导被抑制的主要原因。有意思的是,在比热上我们发现电子系数随Sr掺杂浓度的增加有很大程度的提升。我们认为这可能是由Eu2+的磁矩与导电电子之间的Kondo效应引起的。由于Kondo效应与超导是竞争的关系,这可能是该体系中Tc下降较快的另一个原因。此外我们还用自助熔法合成了接近毫米量级的EuPt2As2单晶并仔细研究了其物理性质。低温下的电阻率、磁化率以及比热的测量均表示该样品存在两个反铁磁(AFM)转变TN1=7.1 K和TN2=5.8 K。从磁化率的结果上我们猜测在EuPt2As2中,当TTTN1时,Eu2+磁矩在ab平面内的排列几乎是共线的,而在低于TN2的温度下,自旋向c轴倾斜。其中TN1源于Eu2+磁矩反铁磁长程序的形成,而TN2可能来自于自旋方向的转变。同时,我们又在2K下观测到了由外磁场引起的亚铁磁转变。最后我们得到了EuPt2As2中磁场沿着c方向时两个反铁磁转变随磁场变化的相图。
[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.
【学位授予单位】：浙江大学
【学位级别】：博士
【学位授予年份】：2017
【分类号】：O469
本文编号：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.
【学位授予单位】：浙江大学
【学位级别】：博士
【学位授予年份】：2017
【分类号】：O469
【参考文献】
相关博士学位论文 前1条
1 罗永康;铈基磷族化合物中磁性和近藤效应的研究[D];浙江大学;2013年
，本文编号：2423787
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