On the phase diagram of the superconducting ferromagnet UCoGe and other unconventional superconductors
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Superconductivity has inspired many scientists all around the world in the past hundred years. The mechanism which causes superconductivity was investigated in detail. Nevertheless, up to now, there is no unifying theory which can describe all the different types of superconductors. Therefore, it is important to collect empirical knowledge of special cases of superconductivity that have emerged in exotic materials in recent years. This thesis is dedicated to an experimental investigation of two fascinating classes of superconductors. The member of the first class, UCoGe, belongs to the family of heavy-fermion system. In this compound, superconductivity forms unexpectedly within the ferromagnetic phase. This makes it a unique laboratory tool to study the experimental realization of equal-spin paring superconductivity. The second type of studied superconductors is topological superconductors: SrxBi2Se3 and HoPdBi. These compounds belong to the family of materials which exhibit a non-trivial band inversion. The combination of the topological band structure and superconductivity can potentially be applied in spintronics and quantum computation. The main experimental techniques used in the thesis are electron transport and thermal expansion. An important part of the thesis project was devoted to the design and construction of a dilatometer with very high sensitivity that could be rotated in the magnetic field.Cite
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Superconductors have many unusual properties not observed in normal metals. The superconducting state is attributed to the pairing of electrons. Conventional forms of superconductivity are produced by distortions in the underlying crystal structure of the material. Recently, it has become evident that not all forms of superconductivity can be explained in this way. The way pairing occurs has to be redressed in these materials. Of particular interest is the interplay between magnetism and superconductivity and the consequences this may have on pair formation.
Magnetism
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Ever since its first discovery in 1911, superconductivity, which is an ordered phase of electronic state, has been regarded as one of the most fascinating topics in modern physics. Recently, remarkable advances in sample fabrication have greatly promoted the research of superconductivity, especially in the field of two-dimensional (2D) materials, where 2D Ising superconductors have sparked immense interests for their unique properties, holding promise in engineering topological superconductivity. In this review, we summarize recent works on both experimental and theoretical studies of 2D Ising superconductivity, with particular attention to the origin of Ising superconductivity as well as their novel properties. We conclude with a discussion of how these unconventional 2D Ising superconductors can play a role in the investigation of topological superconductivity, which is of potential in quantum computing.
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'Conventional' superconductivity, as used in this review, refers to electron–phonon-coupled superconducting electron pairs described by BCS theory. Unconventional superconductivity refers to superconductors where the Cooper pairs are not bound together by phonon exchange but instead by exchange of some other kind, e.g. spin fluctuations in a superconductor with magnetic order either coexistent or nearby in the phase diagram. Such unconventional superconductivity has been known experimentally since heavy fermion CeCu2Si2, with its strongly correlated 4f electrons, was discovered to superconduct below 0.6 K in 1979. Since the discovery of unconventional superconductivity in the layered cuprates in 1986, the study of these materials saw Tc jump to 164 K by 1994. Further progress in high-temperature superconductivity would be aided by understanding the cause of such unconventional pairing. This review compares the fundamental properties of 9 unconventional superconducting classes of materials – from 4f-electron heavy fermions to organic superconductors to classes where only three known members exist to the cuprates with over 200 examples – with the hope that common features will emerge to help theory explain (and predict!) these phenomena. In addition, three new emerging classes of superconductors (topological, interfacial – e.g. FeSe on SrTiO3, and H2S under high pressure) are briefly covered, even though their 'conventionality' is not yet fully determined.
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The discovery of superconductivity is looked back, characteristics of superconductivity are summed up, and many applications of superconductivity are introduced in detail in this paper. It is pointed out that the application foreground of superconductivity is very wide.
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Evidence of inhomogeneous superconductivity, in this case superconductivity with a spatially modulated superconducting order parameter, has now been found in many materials and by many measurement methods. Although the evidence is strong, it is circumstantial in the organic superconductors, scant in the pnictides, and complex in the heavy Fermions. However, it is clear some form of exotic superconductivity exists at high fields and low temperatures in many electronically anisotropic superconductors. The evidence is reviewed in this article, and examples of similar measurements are compared across different families of superconductors. An effort is made to find a consistent way to measure the superconducting energy gap across all materials, and use this value to predict the Clogston–Chandrasakhar paramagnetic limit Hp. Methods for predicting the existence of inhomogeneous superconductivity are shown to work for the organic superconductors, and then used to suggest new materials to study.
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$A_2$Cr$_3$As$_3$ ($A$ = K, Rb, Cs) are the unique Cr-based ambient-pressure superconductors to date discovered by serendipity in 2015. The new superconducting family are structurally characterized by quasi one-dimensional [(Cr$_3$As$_3$)$^{2-}$]$_{\infty}$ double-walled subnanotubes, which exhibit peculiar properties that mostly point to unconventional superconductivity. In this conference paper, we first describe how the superconductors were discovered. Then we overview the recent progresses on crystal structure, electronic structures, theoretical models, and various physical properties in $A_2$Cr$_3$As$_3$. Some new experimental results are included. Finally we conclude by addressing the related open questions in this emerging subfield of superconductivity.
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Superconductivity has played a prominent part in condensed matter physics for more than 100 years, but the understanding of this intriguing phenomenon still remains a theoretical challenge. Almost all current theoretical interpretations consider the key element making up the superconducting condensates to be the very formation of Cooper pairs derived from the microscopic Bardeen-Cooper-Schrieffer (BCS) theory in 1957. In this context, superconductors can be classified as conventional (BCS) or unconventional based on the symmetry of the Cooper pairs. Investigation and understanding of the intrinsic properties of unconventional superconductivity are not only crucial for the realization of novel states of quantum matter but could also pave the way to potential device applications. This PhD work is an experimental study of unconventional superconductivity in the superconducting ferromagnet UCoGe and two candidate topological superconductors CuxBi2Se3 and YPtBi. The main techniques applied have been transport, magnetic and μSR measurements to shed further light on the intricate superconducting pairing mechanism in these novel materials.
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