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The connection between the long-range antiferromagnetic order in cuprates and the high-temperature superconductivity is a scientific problem that has yet to be solved after nearly four decades. The properties and difficulties of describing nickelate superconductors are similar to those of cuprates. Recently, Fowlie et al. [Nat. Phys. 18, 1043 (2022)] aimed to detect the antiferromagnetic order in R1−xSrxNiO2 (R = Nd, Pr, La; x ∼ 0, 0.2) films by using the muon spin rotation (μSR) technique. The research group reported the presence of short-range antiferromagnetic order in every nickelate studied. Here, our goal was to prove that this interaction is present in the nickelate films. We did this by analyzing the temperature dependent resistivity, ρ(T), data from the research group. Global ρ(T) data fits to the advanced Bloch–Grüneisen model showed that each of the R1−xSrxNiO2 compounds can be characterized by a unique power-law exponent, p (where p = 2 for the electron–electron scattering, p = 3 for the electron–magnon scattering, and p = 5 for the electron–phonon scattering), and global characteristic temperature, Tω (which has the meaning of the Debye temperature at p = 5). We found that p = 2.0 in Nd- and Pr-based compounds and p = 1.3 for La-based compounds. The latter value does not have any interpretation within established theoretical models. We also analyzed ρ(T) data for Nd1–xSrxNiO2 (0.125 ≤ x ≤ 0.325) reported by Lee et al. [Nature 619, 288 (2023)]. Our analysis of nickelates led us to conclude that a new theoretical model is needed to describe ρ(T) in materials exhibiting a short-range antiferromagnetic order. [ABSTRACT FROM AUTHOR]

Superconductivity in highly pressurized hydrides has become the primary direction for the exploration of the fundamental upper limit of the superconducting transition temperature, T c , after Drozdov et al. (Nature2015, 525, 73) discovered a superconducting state with T c = 203   K in highly compressed sulfur hydride. To date, several dozen high-temperature superconducting polyhydrides have been discovered and, in addition, it was recently reported that highly compressed titanium and scandium exhibit record-high T c (up to 36 K). This exceeded the T c = 9.2   K value of niobium many times over, which was the record-high   T c ambient pressure metallic superconductor. Here, we analyzed the experimental data for the recently discovered high-pressure superconductors (which exhibit high transition temperatures within their classes): elemental titanium (Zhang et al., Nature Communications 2022; Liu et al., Phys. Rev. B 2022), T a H 3 (He et al., Chinese Phys. Lett. 2023), L a B e H 8 (Song et al., Phys. Rev. Lett. 2023), black phosphorous (Li et al., Proc. Natl. Acad. Sci. 2018; Jin et al., arXiv 2023), and violet (Wu et al., arXiv 2023) phosphorous to reveal the nonadiabaticity strength constant T θ T F (where T θ is the Debye temperature, and T F the Fermi temperature) in these superconductors. The analysis showed that the δ -phase of titanium and black phosphorous exhibits T θ T F scores that are nearly identical to those associated with A15 superconductors, while the studied hydrides and violet phosphorous exhibit constants in the same ballpark as those of H 3 S and L a H 10 . [ABSTRACT FROM AUTHOR]

The pinning force density, F p , is one of the main parameters that characterize the resilience of a superconductor to carrying a dissipative-free transport current in an applied magnetic field. Kramer (1973) and Dew-Hughes (1974) proposed a widely used scaling law for this quantity, where one of the parameters is the pinning force density maximum, F p , m a x , which represents the maximal performance of a given superconductor in an applied magnetic field at a given temperature. Since the late 1970s to the present, several research groups have reported experimental data on the dependence of F p , m a x on the average grain size, d , in Nb3Sn-based conductors. F p , m a x d datasets were analyzed and a scaling law for the dependence F p , m a x d = A × l n 1 / d + B was proposed. Despite the fact that this scaling law is widely accepted, it has several problems; for instance, according to this law, at T = 4.2   K and d ≥ 650   nm , Nb3Sn should lose its superconductivity, which is in striking contrast to experiments. Here, we reanalyzed the full inventory of publicly available F p , m a x d data for Nb3Sn conductors and found that the dependence can be described by the exponential law, in which the characteristic length, δ , varies within a remarkably narrow range of δ = 175 ± 13   nm for samples fabricated using different technologies. The interpretation of this result is based on the idea that the in-field supercurrent flows within a thin surface layer (thickness of δ) near grain boundary surfaces (similar to London's law, where the self-field supercurrent flows within a thin surface layer with a thickness of the London penetration depth, λ , and the surface is a superconductor–vacuum surface). An alternative interpretation is that δ represents the characteristic length of the exponential decay flux pinning potential from the dominant defects in Nb3Sn superconductors, which are grain boundaries. [ABSTRACT FROM AUTHOR]

Nickelate superconductors, R1−xAxNiO2 (where R is a rare earth metal and A = Sr, Ca), experimentally discovered in 2019, exhibit many unexplained mysteries, such as the existence of a superconducting state with Tc (up to 18 K) in thin films and yet absent in bulk materials. Another unexplained mystery of nickelates is their temperature-dependent upper critical field, B c 2 (T) , which can be nicely fitted to two-dimensional (2D) models; however, the deduced film thickness, d s c , G L , exceeds the physical film thickness,   d s c , by a manifold. To address the latter, it should be noted that 2D models assume that d s c is less than the in-plane and out-of-plane ground-state coherence lengths, d s c < ξ a b (0) and d s c < ξ c (0) , respectively, and, in addition, that the inequality ξ c (0) < ξ a b (0) satisfies. Analysis of the reported experimental B c 2 (T) data showed that at least one of these conditions does not satisfy for R1-xAxNiO2 films. This implies that nickelate films are not 2D superconductors, despite the superconducting state being observed only in thin films. Based on this, here we propose an analytical three-dimensional (3D) model for a global data fit of in-plane and out-of-plane B c 2 (T) in nickelates. The model is based on a heuristic expression for temperature-dependent coherence length anisotropy: γ ξ (T) = γ ξ (0) 1 − 1 a × T T c , where a > 1 is a unitless free-fitting parameter. The proposed expression for γ ξ (T) , perhaps, has a much broader application because it has been successfully applied to bulk pnictide and chalcogenide superconductors. [ABSTRACT FROM AUTHOR]

Recently, Pei et al. (National Science Review2023, nwad034, 10.1093/nsr/nwad034) reported that ambient pressure β -MoB2 (space group: R 3 ¯ m ) exhibits a phase transition to α -MoB2 (space group: P 6 / m m m ) at pressure P~70 GPa, which is a high-temperature superconductor exhibiting T c = 32   K at P~110 GPa. Although α -MoB2 has the same crystalline structure as ambient-pressure MgB2 and the superconducting critical temperatures of α -MoB2 and MgB2 are very close, the first-principles calculations show that in α -MoB2, the states near the Fermi level, ε F , are dominated by the d-electrons of Mo atoms, while in MgB2, the p-orbitals of boron atomic sheets dominantly contribute to the states near the ε F . Recently, Hire et al. (Phys. Rev. B2022, 106, 174515) reported that the P 6 / m m m -phase can be stabilized at ambient pressure in Nb1−xMoxB2 solid solutions, and that these ternary alloys exhibit T c ~ 8   K . Additionally, Pei et al. (Sci. China-Phys. Mech. Astron. 2022, 65, 287412) showed that compressed WB2 exhibited T c ~ 15   K at P~121 GPa. Here, we aimed to reveal primary differences/similarities in superconducting state in MgB2 and in its recently discovered diboride counterparts, Nb1−xMoxB2 and highly-compressed WB2. By analyzing experimental data reported for P6/mmm-phases of Nb1−xMoxB2 (x = 0.25; 1.0) and highly compressed WB2, we showed that these three phases exhibit d-wave superconductivity. We deduced 2 Δ m (0) k B T c = 4.1 ± 0.2 for α -MoB2, 2 Δ m (0) k B T c = 5.3 ± 0.1 for Nb0.75Mo0.25B2, and 2 Δ m (0) k B T c = 4.9 ± 0.2 for WB2. We also found that Nb0.75Mo0.25B2 exhibited high strength of nonadiabaticity, which was quantified by the ratio of T θ T F = 3.5 , whereas MgB2, α-MoB2, and WB2 exhibited T θ T F ~ 0.3 , which is similar to the T θ T F in pnictides, A15 alloys, Heusler alloys, Laves phase compounds, cuprates, and highly compressed hydrides. [ABSTRACT FROM AUTHOR]

The Fermi velocity vF is one of the primary characteristics of any conductor, including any superconductor. For conductors at ambient pressure, several experimental techniques have been developed to measure vF, and, for instance, Zhou et al. [Nature 423, 398 (2003)] reported that high-Tc cuprates exhibited a universal nodal Fermi velocity v F , u n i v = 2.7 ± 0.5 × 1 0 5 m/s. However, there have been no measurements of vF in highly compressed near-room-temperature superconductors (NRTS), owing to experimental challenges. Here, to answer the question of the existence of a universal Fermi velocity in NRTS materials, we analyze the full inventory of data on the ground-state upper critical field Bc2(0) for these materials and find that this class of superconductors exhibits a universal Fermi velocity v F , u n i v = 1 / 1.3 × 2 Δ 0 / k B T c × 1 0 5 m/s, where Δ(0) is the ground-state amplitude of the energy gap. The ratio 2 Δ 0 / k B T c varies within a narrow range 3.2 ≤ 2 Δ 0 / k B T c ≤ 5 , and so vF,univ in NRTS materials lies in the range 2.5 × 105 m/s ≤ vF,univ ≤ 3.8 × 105 m/s, which is similar to the range of values found for the high-Tc cuprate counterparts of these materials. [ABSTRACT FROM AUTHOR]

Tin-based nanoparticles, proposed to be barium stannate, have been formed in YBCO films fabricated by metal-organic deposition, through modification of the precursor solution. These randomly-oriented 30-50 nm particles are dispersed throughout the film thickness, providing an enhancement in the isotropic flux pinning relative to undoped YBCO. For a low level, of nanopartide addition of 2 vol%, a flux-pinning force enhancement of up to 32% is achieved. We report field and field-angle dependence of the transport critical current, transmission electron microscopy, and x-ray diffraction of our YBCO films on RABiTS substrates, and compare with similar barium zirconate additions. [ABSTRACT FROM AUTHOR]