Pristine and intercalated transition metal dichalcogenide superconductors
Pristine and intercalated transition metal dichalcogenide superconductors
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DOI：
10.1016/j.physc.2015.02.023
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发表时间：
2015-07-15
影响因子：
1.7
通讯作者：
Klemm, Richard A.
中科院分区：
文献类型：
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作者：
Klemm, Richard A.
过渡金属二卤化物(TMDs)是一种准二维层状化合物，表现出电荷密度波(CDW)形成和超导电性(SC)的强烈竞争效应。八面体或三角棱柱TMD构建块的六角层之间的弱van der Waals层间结合允许形成许多多型体。在单层IT多型材料中，可以形成一个或多个CDW态，但原始的TMD不是超导的。2H多型具有两个或多个费米面和鞍带，允许双重有序，这可以是共存的CDW和Sc有序，两个Sc带隙，两个CDW带隙，甚至可能在CDW有序的起始TCDW之上的伪带隙。高阶多晶型允许多个CDW带隙和至少一个超导带隙。在T-c值较低时，CDW相变TCDWS通常大大超过超导相变，其轨道序参数(OPS)一般是高度各向异性的，甚至可以包含节点，Sc序参数受它们同时存在的影响很大。在TMD中普遍存在的CDW的性质与在高T-c铜酸盐中看到的赝隙的性质非常相似。例如，在2H-NbSe2中，CDW使其一般的S波Sc-OP轨道对称性高度各向异性，并强烈地降低了它与传统Sc，Pb的约瑟夫森耦合强度(IcRn)。因此，与铅相比，原始的TMD是高度“非常规”的，但比像URhGe这样的铁磁超导体要“常规”得多。外加压力和插层通常抑制TMD CDW，允许增强SC的形成，即使在IT多型材料中也是如此。失配插层化合物(LASE)(1.14)(NbSe2)和许多2H-TMD与有机Lewis碱基分子，如TaS_2(吡啶)(1/2)，具有完全非相干的c轴输运和空间交叉效应，表现为本征约瑟夫森结的堆叠。除了(LASE)(1.14)(NbSe2)的泡利极限的异常大的明显破坏外，这些插层化合物的正常态和超导性质与高T-c超导体Bi2Sr2CaCu2O8+Delta 5和有机层状超导体K-(Et)(2)Cu[N(CN)(2)]Br非常相似，其中Et是双(乙烯二硫)四硫富瓦烯。TMD与水和金属离子的电解插层会产生与钴的性质非常相似的化合物，如NaxCoO2中心点yh(2)O。(C)2015 Elsevier B.V.保留所有权利。
Transition metal dichalcogenides (TMDs) are quasi-two-dimensional layered compounds that exhibit strongly competing effects of charge-density wave (CDW) formation and superconductivity (SC). The weak van der Waals interlayer bonding between hexagonal layers of octahedral or trigonal prismatic TMD building blocks allows many polytypes to form. In the single layer IT polytype materials, one or more CDW states can form, but the pristine TMDs are not superconducting. The 2H polytypes have two or more Fermi surfaces and saddle bands, allowing for dual orderings, which can be coexisting CDW and SC orderings, two SC gaps as in MgB2, two CDW gaps, and possibly even pseudogaps above the onset TCDWS of CDW orderings. Higher order polytypes allow for multiple CDW gaps and at least one superconducting gap. The CDW transitions TCDWS usually greatly exceed the superconducting transitions at their low T-c values, their orbital order parameters (OPs) are generally highly anisotropic and can even contain nodes, and the Sc OPs can be greatly affected by their simultaneous presence. The properties of the CDWs ubiquitously seen in TMDs are remarkably similar to those of the pseudogaps seen in the high-T-c cuprates. In 2H-NbSe2, for example, the CDW renders its general s-wave SC OP orbital symmetry to be highly anisotropic and strongly reduces its Josephson coupling strength (IcRn) with the conventional SC, Pb. Hence, the pristine TMDs are highly "unconventional" in comparison with Pb, but are much more "conventional" than are the ferromagnetic superconductors such as URhGe. Applied pressure and intercalation generally suppress the TMD CDWs, allowing for enhanced Sc formation, even in the IT polytype materials. The misfit intercalation compound (LaSe)(1.14)(NbSe2) and many 2H-TMDs intercalated with organic Lewis base molecules, such as TaS2(pyridine)(1/2), have completely incoherent c-axis transport, dimensional-crossover effects, and behave as stacks of intrinsic Josephson junctions. Except for the anomalously large apparent violation of the Pauli limit of the upper critical field of (LaSe)(1.14)(NbSe2), these normal state and superconducting properties of these intercalation compounds are very similar to those seen in the high-T-c superconductor, Bi2Sr2CaCu2O8+delta,5 and in the organic layered superconductor, K-(ET)(2)Cu[N(CN)(2)]Br, where ET is bis(ethylenedithio) tetrathiafulvalene. Electrolytic intercalation of TMDs with water and metallic ions leads to compounds with very similar properties to cobaltates such as NaxCoO2 center dot yH(2)O. (C) 2015 Elsevier B.V. All rights reserved.