低维过渡金属反铁磁体的空穴掺杂和K_2Cr_3As_3超导体的发现

发布时间：2018-05-13 04:40

本文选题：反铁磁体 + 非常规超导体　；参考：《浙江大学》2015年博士论文

【摘要】：超导电性是凝聚态物理的重要领域,而探索新超导材料又是超导研究的一个重要方向。Onnes发现超导电性之后,BCS理论在解释基于电声子耦合的常规超导体上获得了成功。但是,每一类非常规超导体的发现,如重费米子、铜氧化物和铁基等非常规超导体,都对该体系的超导理论提出了重大的挑战。在常规超导体中,磁性会破坏超导Cooper对而不利于超导,而在非常规超导体中,超导Cooper对可能跟磁性密切相关。尽管不同的非常规超导体的超导机理可能不尽相同,但是它们都有个相似的相图：利用某种手段将体系的磁性压制之后超导才出现。本论文借鉴非常规超导体相似的相图,设想通过化学掺杂压制化合物的反铁磁序来探索超导电性。本论文结合当前的研究背景,尝试对三个反铁磁体BaFe2Se3,BaMn2As2和BaCr2As2的钾掺杂效应进行研究,所取得的创新成果如下：(1) BaFe2Se3是一个反铁磁绝缘体,而所有钾掺杂的样品都呈现出绝缘体性质。我们对其中一个钾掺杂量的样品Ba0.6K0.4Fe2Se3进行了细致的物理性质表征,包括各向异性电阻和磁化率以及比热。Ba0.6K0.4Fe2Se3的电阻率具有弱的各向异性,低温下呈现可变程跳跃电导。我们发现Ba0.6K0.4Fe2Se3是一个具有各向异性的Heisenberg型自旋玻璃体。(2) BaMn2As2掺钾之后,体系从绝缘体变为金属。目前的钾掺杂量不能完全压制母体的G型反铁磁,暂时还没有在该体系实现超导电性。我们对组分为x＝0.19和x=0.26的Ba1-xKxMn2As2做了电阻和各向异性磁化率的测量,首次发现了30-50 K左右的弱铁磁转变。体系弱铁磁的易磁化轴沿着ab面,而G型反铁磁的磁矩沿着c方向。我们发现x=0.19和x=0.26样品ab面的磁化率符合奇异的居里外斯行为,其中有效磁矩相当于自旋为1/2的顺磁体。我们对母体和x=0.25的样品进行了第一性原理计算,表明钾掺杂之后As的4p空穴占金属导电的主要部分。我们对该体系中出现弱铁磁性进行了两种可能性的讨论：一种是载流子的自旋发生极化而出现弱铁磁,另一种是G型反铁磁的磁矩沿着ab面发生倾斜而出现铁磁分量。(3) BaCr2As2的钾掺杂样品的合成并不成功,但是我们发现了两个新的准一维化合物K2Cr3As3和KCr3As3。它们由(Cr3As3)∞一维链和钾离子填充而成。我们首次发现铬基砷化物K2Cr3As3在常压下具有6.1 K超导电性,是一个极端的Ⅱ类超导体。多晶样品正常态电阻和温度呈现线性依赖关系,暗示非费米液体行为。K2Cr3As3具有较高的上临界场,是Pauli顺磁极限(Hp=11 T)的3-4倍,暗示自旋三重态配对的可能性。它具有较大的正常态电子比热(γ=70-75 mJ K-2)和比热跳变(等=2.2-2.4),表明体系处于较强的电子关联和强耦合的情况。上述结果强烈指向K2Cr3As3具有非常规超导电性。在K2Cr3As3基础上,利用酒精退去其中的一个钾,我们合成了新的化合物KCr3As3。电阻、磁化率和比热测量表明KCr3As3并不超导。KCr3As3电阻低温下呈现半导体行为,但是电阻变化不大。KCr3As3磁化率高温下呈现出居里外斯行为,铬的有效磁矩为0.68 μB/Cr。直流和交流磁化率证实KCr3As3低温下呈现集团自旋玻璃态。尽管KCr3As3并不超导,但是对于它的研究同样有助于理解K2Cr3As3的超导电性。此外,在论文的最后一章提到了后续工作的展望。
[Abstract]:Superconductivity is an important field in condensed matter physics. While exploring new superconducting materials is an important direction in the study of superconductivity,.Onnes finds superconductivity, and BCS theory has been successful in interpreting conventional superconductors based on electroacoustic coupling. However, the discovery of each kind of unconventional superconductor, such as heavy fermion, copper oxide and iron base, has been found. Unconventional superconductors have made great challenges to the superconducting theory of the system. In conventional superconductors, magnetic properties will destroy superconducting Cooper pairs and are not conducive to superconductivity. In unconventional superconductors, superconducting Cooper is likely to be closely related to magnetism. Although the superconducting mechanisms of different unconventional superconductors may vary, they are different There is a similar phase diagram: the magnetic suppression of the system is made by some means. In this paper, we use the similar phase diagram of the unconventional superconductor to explore the superconductivity by the antiferromagnetic order of chemical doping to suppress the compounds. This paper tries to use three antiferromagnets, BaFe2Se3, BaMn2As2, in combination with the current research background. The effects of potassium doping on BaCr2As2 are studied. The results obtained are as follows: (1) BaFe2Se3 is an antiferromagnetic insulator, and all potassium doped samples show insulators. We have detailed characterization of the physical properties of one of the potassium doped samples, Ba0.6K0.4Fe2Se3, including anisotropic resistance and magnetization. The resistivity of the ratio and the specific heat.Ba0.6K0.4Fe2Se3 has a weak anisotropy, and the variable range jump conductance is presented at low temperature. We found that Ba0.6K0.4Fe2Se3 is an anisotropic Heisenberg type spin vitreo. (2) after the BaMn2As2 is doped with potassium, the system becomes metal from the insulator. The current potassium content can not completely suppress the G type of the parent body. Antiferromagnetic, for the time being, we have not yet achieved superconductivity in this system. We have measured the resistance and anisotropic susceptibility to the Ba1-xKxMn2As2 of x = 0.19 and x=0.26, and we first found a weak ferromagnetic transition about 30-50 K. The susceptibility of the weak ferromagnetic system along the AB surface and the G type antiferromagnetic magnetic moment along the c direction. We found x=0.19. The magnetization of the AB surface of the x=0.26 sample and the sample of the x=0.26 conforms to the singular Curie's behavior, in which the effective magnetic moment is equivalent to the paramagnetic of the spin of 1/2. We have carried out the first principle calculation of the samples of the mother and x=0.25, indicating that the 4P vacancy of As is the main part of the metal conduction after the potassium doping. We have carried out two of the weak ferromagnetism in the system. The discussion of the possibility: one is the spin polarization of the carrier and the presence of weak ferromagnetism, the other is that the G type antiferromagnetic moment inclines along the AB surface and appears ferromagnetic component. (3) the synthesis of potassium doped samples from BaCr2As2 is not successful, but we found that two new quasi one-dimensional compounds, K2Cr3As3 and KCr3As3., are (Cr3As3). It is the first time to find that the chromatid arsenide K2Cr3As3 has 6.1 K superconductivity at atmospheric pressure, and is an extreme class II superconductor. The normal resistance and temperature of the polycrystalline samples show a linear dependence, suggesting that the non Fermi liquid behavior.K2Cr3As3 has a higher critical field and is a Pauli paramagnetic limit (Hp=1 1 T) 3-4 times, suggesting the possibility of pairing spin three heavy states. It has a larger normal electronic specific heat (gamma =70-75 mJ K-2) and specific thermal jump (=2.2-2.4), indicating that the system is in strong electron correlation and strong coupling. The above results strongly point to K2Cr3As3 with unconventional superconductivity. On the basis of K2Cr3As3, alcohol is withdrawn. One of the potassium, we synthesized a new compound KCr3As3. resistance, the susceptibility and specific heat measurements showed that KCr3As3 did not exhibit semiconductor behavior at low temperature at low temperature, but the resistance changed little.KCr3As3 magnetic susceptibility at high temperature, showing Curie's behavior, the effective magnetic moment of chromium was 0.68 mu B/Cr. DC and AC susceptibility confirmed. KCr3As3 presents a group spin glass state at low temperatures. Although KCr3As3 is not superconducting, the study of it also helps to understand the superconductivity of K2Cr3As3. In addition, in the last chapter of the paper, the prospect of follow-up work is mentioned.

【学位授予单位】：浙江大学
【学位级别】：博士
【学位授予年份】：2015
【分类号】：O469;O511.3

【参考文献】