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The recently discovered kagome superconductors A V 3 Sb 5 ( A = K, Rb, Cs) exhibit unusual charge-density-wave (CDW) orders with time-reversal and rotational symmetry breaking. One of the most crucial unresolved issues is identifying the symmetry of the superconductivity that develops inside the CDW phase. Theory predicts a variety of unconventional superconducting symmetries with sign-changing and chiral order parameters. Experimentally, however, superconducting phase information in A V 3 Sb 5 is still lacking. Here we report the impurity effects in CsV 3 Sb 5 using electron irradiation as a phase-sensitive probe of superconductivity. Our magnetic penetration depth measurements reveal that with increasing impurities, an anisotropic fully-gapped state changes to an isotropic full-gap state without passing through a nodal state. Furthermore, transport measurements under pressure show that the double superconducting dome in the pressure-temperature phase diagram survives against sufficient impurities. These results support that CsV 3 Sb 5 is a non-chiral, anisotropic s -wave superconductor with no sign change both at ambient and under pressure. In the kagome superconductor CsV 3 Sb 5 the symmetry of the superconducting gap is still in question, both at ambient pressure and under high pressure. Here, via controlled introduction of impurities, the authors report evidence for a non-chiral full superconducting gap with no sign change.

The interplay between superconductivity and charge-density wave (CDW) in 2 H -NbSe 2 is not fully understood despite decades of study. Artificially introduced disorder can tip the delicate balance between two competing long-range orders, and reveal the underlying interactions that give rise to them. Here we introduce disorder by electron irradiation and measure in-plane resistivity, Hall resistivity, X-ray scattering, and London penetration depth. With increasing disorder, the superconducting transition temperature, T c , varies non-monotonically, whereas the CDW transition temperature, T CDW , monotonically decreases and becomes unresolvable above a critical irradiation dose where T c drops sharply. Our results imply that the CDW order initially competes with superconductivity, but eventually assists it. We argue that at the transition where the long-range CDW order disappears, the cooperation with superconductivity is dramatically suppressed. X-ray scattering and Hall resistivity measurements reveal that the short-range CDW survives above the transition. Superconductivity persists to much higher dose levels, consistent with fully gapped superconductivity and moderate interband pairing. The interplay between superconductivity and charge density wave (CDW) in 2 H -NbSe 2 is still not fully understood. Here, Cho et al. use controlled disorder to probe the interplay between these two phases in 2 H -NbSe 2 and find that superconductivity initially competes with CDW but eventually long-range CDW order assists superconductivity.

The delafossite metalsPdCoO2,PtCoO2, andPdCrO2are among the highest conductivity materials known, with low-temperature mean free paths of tens of microns in the best as-grown single crystals. A key question is whether these very low resistive scattering rates result from strongly suppressed backscattering due to special features of the electronic structure or are a consequence of highly unusual levels of crystalline perfection. We report the results of experiments in which high-energy electron irradiation was used to introduce point disorder to the Pd and Pt layers in which the conduction occurs. We obtain the cross section for formation of Frenkel pairs in absolute units, and cross-check our analysis with first-principles calculations of the relevant atomic displacement energies. We observe an increase of resistivity that is linear in defect density with a slope consistent with scattering in the unitary limit. Our results enable us to deduce that the as-grown crystals contain extremely low levels of in-plane defects of approximately 0.001%. This confirms that crystalline perfection is the most important factor in realizing the long mean free paths and highlights how unusual these delafossite metals are in comparison with the vast majority of other multicomponent oxides and alloys. We discuss the implications of our findings for future materials research.

In superconductors with unconventional pairing mechanisms, the energy gap in the excitation spectrum often has nodes, which allow quasiparticle excitations at low energies. In many cases, such as in d -wave cuprate superconductors, the position and topology of nodes are imposed by the symmetry, and thus the presence of gapless excitations is protected against disorder. Here we report on the observation of distinct changes in the gap structure of iron–pnictide superconductors with increasing impurity scattering. By the successive introduction of nonmagnetic point defects into BaFe 2 (As 1− x P x ) 2 crystals via electron irradiation, we find from the low-temperature penetration depth measurements that the nodal state changes to a nodeless state with fully gapped excitations. Moreover, under further irradiation the gapped state evolves into another gapless state, providing bulk evidence of unconventional sign-changing s -wave superconductivity. This demonstrates that the topology of the superconducting gap can be controlled by disorder, which is a strikingly unique feature of iron pnictides. The superconducting gap of most unconventional superconductors has nodes that support low-energy excitations. Mizukami et al . report that disorder introduced by electron irradiation in BaFe 2 (As 1− x P x ) 2 induces a sequence of transitions from a nodal to a nodeless gap and then to another gapless state.

In superconductors, electrons with spin s ¼ 1=2 form Cooper pairs whose spin structure is usually singlet (S ¼ 0) or triplet (S ¼ 1). When the electronic structure near the Fermi level is characterized by fermions with angular momentum j ¼ 3=2 due to strong spin-orbit interactions, novel pairing states such as even-parity quintet (J ¼ 2) and odd-parity septet (J ¼ 3) states are allowed. Prime candidates for such exotic states are half-Heusler superconductors, which exhibit unconventional superconducting properties, but their pairing nature remains unsettled. Here, we show that the superconductivity in the noncentrosymmetric half-Heusler LuPdBi can be consistently described by the admixture of isotropic even-parity singlet and anisotropic odd-parity septet pairing, whose ratio can be tuned by electron irradiation. From magnetotransport and penetration depth measurements, we find that carrier concentrations and impurity scattering both increase with irradiation, resulting in a nonmonotonic change of the superconducting gap structure. Our findings shed new light on our fundamental understanding of unconventional superconducting states in topological materials.

A single crystal of isovalently substituted Ba(Fe1−xRux)2As2 (x=0.24 ) is sequentially irradiated with 2.5 MeV electrons up to a maximum dose of 2.1×1019 e−/cm2 . The electrical resistivity is measured in situ at T=22K during the irradiation and ex situ as a function of temperature between subsequent irradiation runs. Upon irradiation, the superconducting transition temperature Tc decreases and the residual resistivity ρ0 increases. We find that electron irradiation leads to the fastest suppression of Tc compared to other types of artificially introduced disorder, probably due to the strong short-range potential of the pointlike irradiation defects. A more detailed analysis within a multiband scenario with variable scattering potential strength shows that the observed Tc versus ρ0 is fully compatible with s± pairing, in contrast to earlier claims that this model leads to a too rapid suppression of Tc with scattering.

The lattice of magnetic flux lines that can permeate a type II superconductor, such as the high-transition-temperature copper oxide materials, melts from a solid-like state to a liquid-like state at a temperature below the superconducting transition temperature. Contrary to the predictions of mean-field theory, this phase transition in Bi 2 Sr 2 CaCu 2 O 8 is found to be first-order. The vortex liquid discontinuously expands on freezing.

Inverse melting is the process in which a crystal reversibly transforms into a liquid or amorphous phase when its temperature is decreased. Such a process is considered to be very rare 1 , and the search for it is often hampered by the formation of non-equilibrium states or intermediate phases 2 . Here we report the discovery of first-order inverse melting of the lattice formed by magnetic flux lines in a high-temperature superconductor. At low temperatures, disorder in the material pins the vortices, preventing the observation of their equilibrium properties and therefore the determination of whether a phase transition occurs. But by using a technique 3 to ‘dither’ the vortices, we were able to equilibrate the lattice, which enabled us to obtain direct thermodynamic evidence of inverse melting of the ordered lattice into a disordered vortex phase as the temperature is decreased. The ordered lattice has larger entropy than the low-temperature disordered phase. The mechanism of the first-order phase transition changes gradually from thermally induced melting at high temperatures to a disorder-induced transition at low temperatures.

We study the geometrical confinement effect in Bi 2 Sr 2 CaCu 2 O 8 + δ mesoscopic vortex matter with edge-to-surface ratio of 7–12 %. Samples have in-plane square and circular edges, 30 μ m widths, and ∼ 2 μ m thickness. Direct vortex imaging reveals the compact planes of the structure align with the sample edge by introducing topological defects. The defect density is larger for circular than for square edges. Molecular dynamics simulations suggest that this density is not an out-of-equilibrium property but rather determined by the geometrical confinement.

We investigated configurational changes in mesoscopic vortex matter with less than thousand vortices during flux penetration in freestanding 50 μm diameter disks of Bi2Sr2CaCu2O8+δ. High-resolution AC and DC local magnetometry data reveal oscillations in the transmittivity echoed in peaks in the third-harmonics magnetic signal fainting on increasing vortex density. By means of extra experimental evidence and a simple geometrical analysis we show that these features fingerprint the discretized entrance of single-shells of vortices having a shape that mimics the sample edge.