铊基3d金属硫族化合物的超导电性、磁性和磁热效应研究

发布时间：2018-06-03 01:23

本文选题：Ni基超导体 + 反铁磁　；参考：《浙江大学》2016年博士论文

【摘要】：自1986年铜氧化物高温超导体发现以来,寻找新型非常规超导体及针对其微观机理的研究,一直是凝聚态物理学科重要的研究方向之一。无论是铜氧化物超导体,还是最近发现的铁基超导体,以及早期发现的重费米子超导体,其母体均为反铁磁体。通过化学掺杂引入载流子、或通过外加压力调控,抑制其反铁磁长程序,系统呈现强烈的量子磁涨落,从而出现超导电性,因此人们认为非常规超导电性起源于这种量子磁涨落。在更多的3d金属化合物中,寻找具有不同长程磁有序的化合物,通过对物理参量(如载流子浓度,压力等)的调控,寻找新型非常规超导体,已成为大家公认的有效方法。在过去的三十年里,人们相继在Cu(3d9), Fe(3d6), Co(3d7),Ni(3d8),Mn(3d5),Cr(3d4)等3d金属化合物中发现了超导电性,但有关其微观机理远未达到共识。本论文在过去研究工作的基础上,针对T1基Fe, Co, Ni, Cu金属硫族化合物的磁性,超导电性和磁热效应开展了系统研究,获得了一系列研究成果。全文共分为五章,其主要内容如下：第一章,我们概述了铁基超导体研究现状及其对寻找新型非常规超导体的启示,同时对磁相变中的临界行为、磁热效应等与本文相关的一些物理问题进行了总结和评述。第二章,在采用自助熔方法成功生长出TlCo2-xNixSe2和TlCo2-xNixS2系列单晶的基础上,我们通过对其晶体和磁结构、磁性及输运性质等进行系统观测。结果发现,对于TlCo2-xNixSe2系统,TlCo2Se2(x=0)样品为沿c轴螺旋式(121。)无公度的反铁磁体,而在x=0.2的样品中,演变为有公度的反铁磁结构。随着Ni替代浓度的进一步增加,系统反铁磁Neel温度(TN)先上升后下降并在约x=1.7处达到零K；随后系统呈现大块超导电性,超导临界温度(TC)随着Ni含量的上升而单调增加,在x=2时达到最大值(3.7 K)。从而获得了TlCo2-xNixSe2体系从无公度的反铁磁体到超导体的演变相图。而对于TlCo2-xNixS2系统,其磁性和超导电性的相图较为复杂。首先TlCo2S2为巡游铁磁体,微量的Ni替代导致系统呈现变磁性并具有一系列有趣的临界行为(下一章内容)；随着Ni替代浓度的进一步增加,系统从巡游铁磁体,经过变磁体、反铁磁体,最后进入超导态。这两个系统的磁性和超导电性相图,为理解Ni基超导电性提供了一个新的视角,我们将在论文中进行详细讨论。第三章, 如前所述,TlCo2S2为巡游铁磁体,微量的Ni替代导致系统呈现变磁性,我们对TICo2S2和TlCo1.9Ni0.1S2两单晶样品的等温磁化曲线M(H)进行详细观测,发现前者的铁磁相变临近三重临界点,其临界指数满足的标度率,与三重临界平均场理论一致。微量Ni元素(x=0.1)的替代导致系统呈现变磁性、较小的磁场可以把系统调控为铁磁体,进一步证实了TlCo2S2巡游铁磁性相变确实在三重临界点附近。这些研究结果说明TlCo2S2为少有的三重临界点磁相变规律的研究提供了一个极佳的实验平台。第四章,针对我们研究组首次在TlNi2Se2中观测到3.7 K具有重费米子行为的超导电性,我们通过Cu对Ni的部分替代,成功合成了一系列TlNi2-xCuxSe2(0≤x≤0.69)单晶样品,并对其输运性质和磁性质开展了系统的研究。结果发现,该体系中Cu对Ni的最大替代浓度x=0.69,这可能与Cu在该类化合物中所呈现的价态(+1)有关；随着Cu替代浓度的增大,超导临界温度单调下降,当x=0.69时,系统不再呈现超导电性。所有样品在正常态均呈现Pauli顺磁。第五章,另外,我们还成功地合成了具有准一维结构的TlFe3Te3单晶,通过对其结构、磁性质和输运性质的系统观测,发现该化合物在220 K发生了顺磁-铁磁一级相变,并呈现微小的热滞和磁滞行为。我们进一步的磁热效应研究发现,对于磁场沿着c轴,在220 K附近,当磁场变化AH=0-1 T和AH=0-2 T时,由Maxwell关系获得磁熵变化最大值高达5.9和7.0J/kgK,明显高于相同温度和磁场范围的大多数其他磁热材料,说明TlFe3Te3具有性能优异的磁制冷效应,我们的实验结果为探索新型磁制冷材料提供了一个新的方向。论文的最后对我的博士论文工作进行了总结。
[Abstract]:Since the discovery of copper oxide superconductors in 1986, the search for new unconventional superconductors and their microscopic mechanisms has been one of the most important research directions in condensed matter physics. The matrix of both copper oxide superconductors, the recently discovered iron superconductors and the early discovery of heavy fermion superconductors Antiferromagnets. Through the introduction of chemical doping to the carrier, or by the control of the pressure, the antiferromagnetic long procedure is suppressed. The system presents a strong quantum magnetic fluctuation, which leads to superconductivity. Therefore, people think that the unconventional superconductivity originates from this quantum magnetic fluctuation. In more 3D metal compounds, it is found to have different long range magnetic properties. In the past thirty years, people have found superconductivity in Cu (3D9), Fe (3d6), Co (3D7), Ni (3d8), Mn (3d5), Cr (3d4), etc., but related to its microcosmic effect in the past thirty years. The mechanism is far from consensus. On the basis of the past research work, the magnetic, superconductivity and Magnetocaloric Effect of T1 based Fe, Co, Ni, Cu metal sulfur compounds have been systematically studied and a series of research achievements have been obtained. The full text is divided into five chapters, the main contents are as follows: Chapter 1, we summarize the research of iron based superconductors In the second chapter, on the basis of the successful growth of TlCo2-xNixSe2 and TlCo2-xNixS2 series of single crystals, we pass on its crystal and magnetic field. The structure, magnetic and transport properties are systematically observed. It is found that for the TlCo2-xNixSe2 system, the TlCo2Se2 (x=0) sample is an uncommensurate antiferromagnet along the c axis (121.), and in the sample of x=0.2, it becomes a common antiferromagnetic structure. With the further increase of the Ni substitution concentration, the system anti ferromagnetic Neel temperature (TN) rises first. Decrease and reach zero K at about x=1.7; then the system presents bulk superconductivity, the superconducting critical temperature (TC) increases monotonously with the increase of Ni content, and reaches the maximum value at x=2 (3.7 K). Thus, the evolution phase diagram of the TlCo2-xNixSe2 system from the antiferromagnet to the superconductor is obtained. For the TlCo2-xNixS2 system, its magnetism and superconductivity The phase diagram of electrical conductivity is more complex. First, TlCo2S2 is a cruising ferromagnet, and a trace of Ni substitution causes the system to be magnetic and has a series of interesting critical behaviors (the next chapter); with the further increase in the Ni substitution concentration, the system is from the cruising ferromagnet, through the magnets, antiferromagnets, and finally into the superconducting state. These two systems The phase diagram of magnetic and superconductivity provides a new perspective for understanding the Ni based superconductivity. We will discuss in detail in the paper. In the third chapter, as mentioned earlier, TlCo2S2 is a cruising ferromagnet, and the micro Ni substitution causes the system to be magnetic. The isothermal magnetization curve M (H) of TICo2S2 and TlCo1.9Ni0.1S2 two single crystal samples is detailed. It is found that the ferromagnetic phase transition of the former is near the three critical point, and the scaling rate of the critical exponent is consistent with the theory of the three critical mean field. The substitution of the trace Ni element (x=0.1) causes the system to be magnetic, and the smaller magnetic field can be controlled as a ferromagnet, and it is confirmed that the ferromagnetic phase transition of the TlCo2S2 cruise is indeed three. Near the critical point. These results show that TlCo2S2 provides an excellent experimental platform for the study of the magnetic phase transition of a rare three critical point. Fourth, for the first time, our research group observed the superconductivity of the heavy fermion behavior of 3.7 K in TlNi2Se2. We have successfully synthesized a series of Cu to Ni. TlNi2-xCuxSe2 (0 < < x < 0.69) single crystal sample and systematic study of its transport properties and magnetic properties. The results show that the maximum substituting concentration of Cu to Ni in this system is x=0.69, which may be related to the valence state (+1) of Cu in this kind of compound; with the increase of Cu substitution concentration, the superconducting critical temperature drops monotonously when x=0.69, The system no longer presents superconductivity. All samples are Pauli paramagnetic in normal state. In addition, we have also successfully synthesized TlFe3Te3 single crystal with quasi one-dimensional structure. By systematic observation of its structure, magnetic properties and transport properties, it is found that the compound has a paramagnetic and ferromagnetic phase transition at 220 K, and presents a small size. In our further magnetothermal effect, we find that for the magnetic field along the c axis and near 220 K, when the magnetic field changes AH=0-1 T and AH=0-2 T, the maximum value of the magnetic entropy change from the Maxwell relationship is up to 5.9 and 7.0J/kgK, which is obviously higher than most other magnetothermal materials at the same temperature and magnetic field range, indicating that TlFe3Te3 is of a property. Excellent magnetic refrigeration effect, our experimental results provide a new direction for exploring new magnetic refrigerating materials. Finally, the thesis summarizes my doctoral thesis work.
【学位授予单位】：浙江大学
【学位级别】：博士
【学位授予年份】：2016
【分类号】：O469

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