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Doping and two distinct phases in strong-coupling kagome superconductors
TL;DR: In this article, the synthesis of Ti-doped CsV3Sb5 single crystals with controllable carrier doping concentration was reported, and the Ti atoms directly substitute for V in the vanadium kagome layers.
Abstract: The vanadium-based kagome superconductor CsV3Sb5 has attracted tremendous attention due to its unconventional anomalous Hall effect (AHE), its charge density waves (CDWs), and a pseudogap pair density wave coexisting with unconventional strong-coupling superconductivity (SC). The origins of time-reversal symmetry breaking (TRSB), unconventional SC, and their correlation with different orders in this kagome system is of great significance, but, so far, is still under debate. Doping by the chemical substitution of V atoms in the kagome layer provides the most direct way to reveal the intrinsic physics that originates from the kagome lattice, but remains unexplored. Here, we report, for the first time, the synthesis of Ti-doped CsV3Sb5 single crystals with controllable carrier doping concentration. The Ti atoms directly substitute for V in the vanadium kagome layers. Remarkably, the Ti-doped CsV3Sb5 SC phase diagram shows two distinct SC phases. The lightly-doped SC phase has a V-shaped gap pairing, coexisting with CDWs, indicating a strong-coupling unconventional SC nature. The other SC phase has a U-shaped gap pairing without CDWs, displaying a conventional SC feature. This is the first observation of the two distinct phases in superconductors, revealed through Ti doping of CsV3Sb5. These findings pave a new way to synthesise doped CsV3Sb5 and represents a new platform for tuning the superconducting pairing and multiple orders in kagome superconductors.
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TL;DR: The Ginzburg number as discussed by the authors was introduced to account for thermal and quantum fluctuations and quenched disorder in high-temperature superconductors, leading to interesting effects such as melting of the vortex lattice, the creation of new vortex-liquid phases, and the appearance of macroscopic quantum phenomena.
Abstract: With the high-temperature superconductors a qualitatively new regime in the phenomenology of type-II superconductivity can be accessed. The key elements governing the statistical mechanics and the dynamics of the vortex system are (dynamic) thermal and quantum fluctuations and (static) quenched disorder. The importance of these three sources of disorder can be quantified by the Ginzburg number $Gi=\frac{{(\frac{{T}_{c}}{{H}_{c}^{2}}\ensuremath{\varepsilon}{\ensuremath{\xi}}^{3})}^{2}}{2}$, the quantum resistance $Qu=(\frac{{e}^{2}}{\ensuremath{\hbar}})(\frac{{\ensuremath{\rho}}_{n}}{\ensuremath{\varepsilon}\ensuremath{\xi}})$, and the critical current-density ratio $\frac{{j}_{c}}{{j}_{o}}$, with ${j}_{c}$ and ${j}_{o}$ denoting the depinning and depairing current densities, respectively (${\ensuremath{\rho}}_{n}$ is the normal-state resistivity and ${\ensuremath{\varepsilon}}^{2}=\frac{m}{M}l1$ denotes the anisotropy parameter). The material parameters of the oxides conspire to produce a large Ginzburg number $\mathrm{Gi}\ensuremath{\sim}{10}^{\ensuremath{-}2}$ and a large quantum resistance $\mathrm{Qu}\ensuremath{\sim}{10}^{\ensuremath{-}1}$, values which are by orders of magnitude larger than in conventional superconductors, leading to interesting effects such as the melting of the vortex lattice, the creation of new vortex-liquid phases, and the appearance of macroscopic quantum phenomena. Introducing quenched disorder into the system turns the Abrikosov lattice into a vortex glass, whereas the vortex liquid remains a liquid. The terms "glass" and "liquid" are defined in a dynamic sense, with a sublinear response $\ensuremath{\rho}={\frac{\ensuremath{\partial}E}{\ensuremath{\partial}j}|}_{j\ensuremath{\rightarrow}0}$ characterizing the truly superconducting vortex glass and a finite resistivity $\ensuremath{\rho}(j\ensuremath{\rightarrow}0)g0$ being the signature of the liquid phase. The smallness of $\frac{{j}_{c}}{{j}_{o}}$ allows one to discuss the influence of quenched disorder in terms of the weak collective pinning theory. Supplementing the traditional theory of weak collective pinning to take into account thermal and quantum fluctuations, as well as the new scaling concepts for elastic media subject to a random potential, this modern version of the weak collective pinning theory consistently accounts for a large number of novel phenomena, such as the broad resistive transition, thermally assisted flux flow, giant and quantum creep, and the glassiness of the solid state. The strong layering of the oxides introduces additional new features into the thermodynamic phase diagram, such as a layer decoupling transition, and modifies the mechanism of pinning and creep in various ways. The presence of strong (correlated) disorder in the form of twin boundaries or columnar defects not only is technologically relevant but also provides the framework for the physical realization of novel thermodynamic phases such as the Bose glass. On a macroscopic scale the vortex system exhibits self-organized criticality, with both the spatial and the temporal scale accessible to experimental investigations.
4,502 citations
TL;DR: A detailed review of the superconductivity of FePnictide and chalcogenide (FePn/Ch) superconductors can be found in this paper.
Abstract: Kamihara and coworkers' report of superconductivity at ${T}_{c}=26\text{ }\text{ }\mathrm{K}$ in fluorine-doped LaFeAsO inspired a worldwide effort to understand the nature of the superconductivity in this new class of compounds. These iron pnictide and chalcogenide (FePn/Ch) superconductors have Fe electrons at the Fermi surface, plus an unusual Fermiology that can change rapidly with doping, which lead to normal and superconducting state properties very different from those in standard electron-phonon coupled ``conventional'' superconductors. Clearly, superconductivity and magnetism or magnetic fluctuations are intimately related in the FePn/Ch, and even coexist in some. Open questions, including the superconducting nodal structure in a number of compounds, abound and are often dependent on improved sample quality for their solution. With ${T}_{c}$ values up to 56 K, the six distinct Fe-containing superconducting structures exhibit complex but often comparable behaviors. The search for correlations and explanations in this fascinating field of research would benefit from an organization of the large, seemingly disparate data set. This review provides an overview, using numerous references, with a focus on the materials and their superconductivity.
1,349 citations
TL;DR: The surprising discovery of high-temperature superconductivity in a material containing a strong magnet (iron) has led to thousands of publications as discussed by the authors, and it becomes clear what we know and where we are headed.
Abstract: The surprising discovery of high-temperature superconductivity in a material containing a strong magnet—iron—has led to thousands of publications. By placing all the data in context, it becomes clear what we know and where we are headed.
1,224 citations
TL;DR: In this paper, it is proposed that spin-fluctuation mediated pairing is the common thread linking a broad class of superconducting materials, including cuprates, the Fe-pnictides/chalcogenides as well as some heavy fermion and actinide materials.
Abstract: The structures, the phase diagrams, and the appearance of a neutron resonance signaling an unconventional superconducting state provide phenomenological evidence relating the cuprates, the Fe-pnictides/chalcogenides as well as some heavy fermion and actinide materials Single- and multi-band Hubbard models have been found to describe a number of the observed properties of these materials so that it is reasonable to examine the origin of the pairing interaction in these models In this review, based on the experimental phenomenology and studies of the pairing interaction for Hubbard-like models, it is proposed that spin-fluctuation mediated pairing is the common thread linking a broad class of superconducting materials
1,089 citations
TL;DR: The use of tunneling microscopy and spectroscopy has played a central role in the experimental verification of the microscopic theory of superconductivity in classical superconductors as discussed by the authors.
Abstract: Tunneling spectroscopy has played a central role in the experimental verification of the microscopic theory of superconductivity in classical superconductors. Initial attempts to apply the same approach to high-temperature superconductors were hampered by various problems related to the complexity of these materials. The use of scanning tunneling microscopy and spectroscopy (STM and STS) on these compounds allowed the main difficulties to be overcome. This success motivated a rapidly growing scientific community to apply this technique to high-temperature superconductors. This paper reviews the experimental highlights obtained over the last decade. The crucial efforts to gain control over the technique and to obtain reproducible results are first recalled. Then a discussion on how the STM and STS techniques have contributed to the study of some of the most unusual and remarkable properties of high-temperature superconductors is presented: the unusually large gap values and the absence of scaling with the critical temperature, the pseudogap and its relation to superconductivity, the unprecedented small size of the vortex cores and its influence on vortex matter, the unexpected electronic properties of the vortex cores, and the combination of atomic resolution and spectroscopy leading to the observation of periodic local density of states modulations in the superconducting and pseudogap states and in the vortex cores.
790 citations