What Is A Superconductor Apex

We synthesized new REO_0.5F_0.fiveBiS_two (RE: rare earth) superconductors with high-entropy-alloy-type (HEA-type) REO blocking layers. The lattice constant a systematically changed in the HEA-blazon samples with the RE concentration and the RE ionic radius. A sharp superconducting transition was observed in the resistivity measurements for all the HEA-type samples, and the transition temperature of the HEA-type samples was higher than that of typical REO_0.fiveF_0.5BiS₂. The sharp superconducting transition and the enhanced superconducting backdrop of the HEA-type samples may signal the effectiveness of the HEA states of the REO blocking layers in the REO_0.vF_0.5BiS₂ system.

Since the discovery of the cuprate superconductors and FeAs-based superconductors,¹ ^, ⁱⁱ ⁾ layered compounds have been extensively studied as a potential organisation in which loftier-transition-temperature (loftier-T _c) superconductivity and unconventional mechanisms of superconductivity emerge. Basically, layered superconductors have a structure based on alternate stacks of electrically conducting layers, such as the CuO_ii layer and FeAs layer, and insulating (blocking) layers, which are typically composed of metal oxides.

In 2012, we discovered new layered superconductors with BiS₂-based conducting layers.³ ^– ⁵ ⁾ Basically, the parent phase of the BiS₂-based superconductors is a semiconductor with a band gap.^{half dozen} ^, ⁷ ⁾ When electron carriers are generated in the BiS₂ layers, superconductivity emerges. In the typical arrangement LaO₁₋ _x F ₁₀ BiS_two, the carrier amount tin can be controlled past the composition of F (x) in the LaO blocking layer.^iv ⁾ Since the discovery in 2012, many BiS₂-based superconductors with different types of blocking layer have been synthesized, for which T _c is largely enhanced past replacing (or partially substituting) elements in the blocking layer.⁵ ⁾ In particular, every bit reported in Refs. eight and 9, the relationship betwixt the superconducting backdrop and the crystal construction has been systematically studied by tuning the chemic pressure consequence in the REO_0.5F_0.fiveBiS₂ organisation, where the average ionic radius of the RE site was systematically changed with the solution of La, Ce, Pr, Nd, or Sm. Based on these investigations on chemical pressure effects, we found that both carrier doping and the optimization of the local structure (specially the suppression of in-airplane disorder of the BiS plane) were essential for the emergence of majority superconductivity in the BiS_two-based superconductors.^viii ^– ¹² ⁾

The chemical pressure consequence in REO_0.5F_0.5BiS_two is basically controlled past the alloying effect at the RE site of the blocking layer. The systematic shrinkage of the blocking layer affects the Bi–S1 (S1 is an in-plane sulfur as depicted in the inset of Fig. 1.) bond distance and induces bulk superconductivity. This fact suggests that the structural properties of the RE(O,F) blocking layer affect the superconducting backdrop in the RE(O,F)BiS_ii system. Furthermore, if the structural stability of the RE(O,F) blocking layer is enhanced, the superconducting properties may increase in RE(O,F)BiS₂. To investigate this possibility, we take studied the crystal structure and the superconducting backdrop of (La,Ce,Pr,Nd,Sm)O_0.5F_0.5BiS₂, where the (La,Ce,Pr,Nd,Sm)O_0.fiveF_0.5 layer was designed with the concept of a high entropy blend (HEA), which has been proposed as a promising strategy for designing novel functional materials.¹³ ⁾

Fig. 1. — **Fig. one.** Pulverisation XRD pattern and Rietveld fitting for B (La_0.2Ce_0.2Pr_0.2Nd_0.2Sm_0.2O_0.vF_0.5BiS_two). The inset shows a schematic epitome of the crystal structure for sample B.

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Typically, an HEA is defined as an alloy containing at to the lowest degree 5 elements with concentrations between 5 and 35 diminutive per centum, according to the newspaper by Yeh et al.^xiv ⁾ Furthermore, Otto et al. suggested that only an alloy that forms a solid solution with no intermetallic phases should be considered equally an HEA, because the formation of an ordered phase decreases the entropy of the system.¹⁵ ⁾ Recently, Koželj et al. discovered superconductivity in Ta₃₄Nb₃₃Hf₈Zr₁₄Ti₁₁ with T _c of vii.3 K.^sixteen ⁾ The observed superconductivity in the HEA was conventional phonon-mediated superconductivity. Notably, high-pressure level measurements revealed that the superconductivity (zero-resistivity country) in the HEA superconductor was robust at pressures up to 190 GPa.¹⁷ ⁾ This fact suggests that the superconducting states of the HEA can be enhanced by the HEA effect under extreme conditions.

On the basis of the observed structural stability in the HEA superconductor nether extremely high pressure, nosotros expected that the local disorder in RE(O,F)BiS₂, which negatively affects the emergence of bulk superconductivity, can be suppressed, and the superconducting properties may exist enhanced by the HEA effect. However, RE(O,F)BiS₂ is a layered arrangement and composed of five dissimilar atomic sites, in which a unproblematic HEA concept cannot be applied. Therefore, nosotros focused on the RE site (come across the inset of Fig. 1) just and designed the HEA-type RE(O,F) blocking layers as a solid solution of RE = La, Ce, Pr, Nd, and Sm. In this letter, four HEA-type REO_0.5F_0.5BiS_two samples were newly synthesized. Powder X-ray diffraction (XRD) and Rietveld refinements revealed that the obtained HEA-type REO_0.5F_0.5BiS₂ is nearly single-phase and structurally homogeneous as in the other typical REO_0.5F_0.5BiS_two (RE = La, Ce, Pr, and Nd), where the RE site consists of ane or two RE elements. For all the HEA-type REO_0.5F_0.5BiS_ii samples, a sharp superconducting transition was observed. Notably, we observed that the superconducting phase diagram of HEA-blazon REO_0.vF_0.5BiS_ii showed a trend clearly different from that of typical REO_0.5F_0.vBiS₂. Although merely the RE site possesses an HEA state in HEA-type REO_0.vF_0.vBiS₂, the superconducting properties seem to be enhanced equally compared with typical REO_0.5F_0.5BiS₂. Hence, the concept of designing layered superconductors with an HEA-type blocking layer volition provide us with a new strategy for synthesizing new layered superconductors with outstanding properties.

We synthesized four REO_0.fiveF_0.vBiS₂ superconductors with nominal compositions of La_0.3Ce_0.3Pr_0.2Nd_0.1Sm_0.1O_0.fiveF_0.vBiS₂, La_0.2Ce_0.iiPr_0.2Nd_0.2Sm_0.2O_0.5F_0.5BiS_ii, La_0.1Ce_0.1Pr_0.3Nd_0.3Sm_0.2O_0.fiveF_0.fiveBiS_two, and La_0.1Ce_0.1Pr_0.2Nd_0.threeSm_0.threeO_0.fiveF_0.5BiS₂, which are labeled every bit A, B, C, and D, respectively. The polycrystalline samples were synthesized past the solid-state-reaction method like to that established for typical REO_0.vF_0.fiveBiS₂.^iv ^, ⁸ ⁾ Powders of La_twoSouthward₃ (99.nine%), Ce_iiS₃ (99.nine%), Pr_twoS₃ (99.9%), Nd₂S₃ (99%), Sm_iiS₃ (99.9%), Bi₂O₃ (99.999%), and BiF_iii (99.ix%) and grains of Bi (99.999%) and S (99.99%) were used as starting materials. The mixture of the starting materials listed above with the nominal composition was mixed well, pelletized, sealed into an evacuated quartz tube, and heated at 700 °C for 20 h. The obtained sample was ground, mixed, pelletized, sealed into an evacuated quartz tube, and heated at 700 °C for 20 h to homogenize the sample.

The phase purity and the crystal structure were examined using powder XRD with Cu Kα radiation past the θ–2θ method. Rietveld refinements were performed to analyze the obtained XRD data. The RIETAN-FP software was used for the Rietveld analysis.¹⁸ ⁾ To visualize the refined crystal structure, the VESTA software was used.¹⁹ ⁾ The actual limerick for the RE site was analyzed using free energy-dispersive X-ray spectrometry (EDX). As listed in Table S1 in the online supplementary data at http://stacks.iop.org/APEX/eleven/053102/mmedia, the analyzed RE concentrations nearly corresponded to the nominal values for all the samples. Therefore, in this paper, we depict the sample names using the nominal values for clarity.

The temperature dependence of the electrical resistivity was measured by the four-terminal method with a current of 1 mA. The temperature dependence of the magnetic susceptibility was measured by a superconducting breakthrough interference device (SQUID) magnetometer subsequently both zero-field cooling (ZFC) and field cooling (FC) with a typical applied field of x Oe.

Figure 1 shows the typical XRD pattern and the Rietveld fitting result for sample B (La_0.2Ce_0.2Pr_0.2Nd_0.2Sm_0.2O_0.vF_0.5BiS_two). Although tiny impurity peaks probably due to RE₂O_twoS were observed, almost all the peaks were refined using the tetragonal P4/nmm model.⁴ ⁾ Run into the online supplementary data at http://stacks.iop.org/Noon/11/053102/mmedia for the Rietveld refinement results for A, C, and D. For sample B, as shown in the inset of Fig. ane, the RE site can be regarded every bit an HEA type with La, Ce, Pr, Nd, and Sm with a concentration of 19–21%, which satisfies the definition of the HEA by Yeh et al.^xiv ⁾ For other samples A, C, and D, loftier purity of the obtained samples was also confirmed by the Rietveld refinements. The obtained crystal construction parameters are listed in Tables S1 and S2 in the online supplementary data at http://stacks.iop.org/APEX/11/053102/mmedia. Irresolute the RE concentration from A to D does not affect the structural model with the P4/nmm space group but systematically decreases the lattice constant a as shown in Fig. 2. This trend is like to that in the case of Ce₁₋ _x Nd _ten O_0.5F_0.5BiS_ii or other related systems.⁸ ^, ⁹ ⁾ Generally, in the REOBiS₂-type compounds, the lattice expansion/shrinkage is directly linked to the hateful ionic radius of the RE site.⁸ ^, ^nine ⁾ However, the lattice constant c for the HEA samples increases from A to D. On the ground of previous works, we have found the tendency that the lattice constant c of the REOBiS₂-type compounds decreases with increasing carrier concentration.⁴ ^, ⁸ ⁾ Therefore, the increase in c for samples C and D may be related to the decrease in the electron carrier concentration. In samples C and D, the Sm concentration is larger than that in the samples A and B. This may signal that the Sm ions are in the mixed-valence state of +2 and +3, as observed in other Sm compounds.^twenty ⁾ In addition, in the plot the lattice abiding a for the HEA-type samples is clearly larger than that for typical REO_0.5F_0.5BiS₂, which also suggests that the RE ionic radius for any RE site is larger than that of RE³⁺. The in-aeroplane Bi–S1 distance decreases with decreasing a. The S1–Bi–S1 angle is virtually 180° and does not show a marked alter betwixt A, B, C, and D. Therefore, with decreasing Bi–S1 distance, in-plane chemical force per unit area is generated without whatsoever modify in the flatness of the BiS1 aeroplane.^nine ⁾

Fig. 2. — **Fig. two.** (a, b) Dependences of lattice constants a and V on mean RE (rare-earth site) ionic radius in typical and high-entropy-alloy-blazon REO_0.fiveF_0.vBiS₂. For, A, B, C, and D, the analyzed RE concentrations were La_0.28Ce_0.31Pr_0.nineteenNd_0.12Sm_0.10, La_0.19Ce_0.21Pr_0.21Nd_0.20Sm_0.nineteen, La_0.09Ce_0.09Pr_0.30Nd_0.32Sm_0.20, and La_0.10Ce_0.11Pr_0.nineteenNd_0.xxxSm_0.30, respectively. The mean RE ionic radius was calculated using the values for RE⁺³ with a coordination number of 8 to exist 116, 114, 113, and 112 pm for La, Ce, Pr, and Nd, respectively. (c) Normalized 110 peaks for A, B, C, D, and RE = La and Nd. (d) FWHM of the 110 meridian for A, B, C, D, and RE = La and Nd.

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Regarding the homogeneity of the HEA-type samples, there is the possibility of local phase separation into grains with different RE ions. However, every bit shown in Fig. 2(c), the typical summit (110 reflection) for the HEA-type samples is not broadened by mixing five RE elements and seems slightly sharper than that of typical systems with RE = La and Nd. In add-on, the full width at half maximum (FWHM) of the 110 peak plotted in Fig. 2(d) shows that the peak sharpness for the HEA-type samples is narrower than that for typical REO_0.5F_0.vBiS₂ (RE = La and Nd). Nosotros consider that the RE ions are homogeneously solved in the grains on the basis of the present powder XRD study. Furthermore, the REO_0.fiveF_0.fiveBiS_ii structure may be stabilized by the HEA upshot.

Figure 3(a) shows the temperature dependences of the electric resistivity for A–D. For all the samples, the resistivity above T _c increases with decreasing temperature, which is a semiconducting-like behavior. In several BiS_ii-based superconductors, a like behavior was observed.⁴ ^, ⁵ ⁾ In typical REO_0.5F_0.5BiS₂, the semiconducting behavior was suppressed with increasing in-plane chemical pressure.¹⁰ ^, ²¹ ⁾ In add-on, in LaO_0.5F_0.5BiS_ii− _x Se _x , which is the best example showing a dramatic in modify the in-plane chemical pressure aamplitude, the increment in the in-plane chemical pressure (Se concentration) induced metallic conductivity and bulk superconductivity.^x ^, ²¹ ⁾ Although the temperature dependences of the resistivity are similar for all the present samples A–D, we discover that the increment in resistivity at low temperatures for samples C and D is weaker than that for A and B in the plot of normalized resistivity [Fig. iii(b)].

**Fig. 3.** (a) Temperature dependences of electrical resistivity for A, B, C, and D. (b) Temperature dependences of resistivity normalized at 200 K for A, B, C, and D. (c) Enlarged figure of (a) below 20 K. (d) Temperature dependences of magnetic susceptibility 4πχ for A, B, C, and D. (due east, f) Dependences of (e) T _c and (f) Δ4πχ as a role of lattice constant of a. Data for REO_0.5F_0.5BiS₂ (RE = La, Ce, Pr, and Nd)¹² ⁾ are plotted for comparison. The cantankerous symbols indicate that a superconducting transition is not observed for the samples higher up 2 Chiliad.

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Effigy 3(c) shows the low-temperature resistivity plots. For all the samples, a sharp superconducting transition was observed. The onset temperature ( $T_{\text{c}}^{\text{onset}}$ ) was defined as the temperature where the resistivity begins to decrease upon cooling. $T_{\text{c}}^{\text{zero}}$ was defined every bit the temperature where the resistivity becomes zero upon cooling. The estimated $T_{\text{c}}^{\text{onset}}$ is iii.4, iv.3, 4.7, and 4.nine Thou for A, B, C, and D, respectively. The estimated $T_{\text{c}}^{\text{zero}}$ is 3.0, 3.viii, 4.two, and 4.5 Thou for A, B, C, and D, respectively. The transition width (ΔT _c) estimated from the difference between $T_{\text{c}}^{\text{onset}}$ and $T_{\text{c}}^{\text{zero}}$ is 0.4, 0.v, 0.five, and 0.four One thousand, for A, B, C, and D, respectively. The sharpness of the transition for the nowadays sample is sharper than that of typical REO_0.vF_0.fiveBiS₂; for instance, ΔT _c of PrO_0.fiveF_0.fiveBiS₂ is ∼0.eight G according to the temperature derivative of the resistivity in Ref. 22. In typical REO_0.5F_0.5BiS₂ systems, the existence of strong superconducting fluctuations has been proposed.²³ ⁾ Furthermore, structural instability, due to the presence of the lone-pair effect of the Bi 6s electrons,¹¹ ⁾ besides exists in the typical REO_0.vF_0.5BiS_two organization,⁴ ^, ⁵ ^, ²⁴ ⁾ which is linked to the emergence of bulk superconductivity. These superconducting fluctuations or the structural instabilities tin can broaden the superconducting transition. On the basis of the discussion in a higher place, we propose that the HEA-type blocking layer may suppress these fluctuations and structural instabilities and induce the abrupt superconducting transition.

Effigy iii(d) shows the temperature dependences of the magnetic susceptibility 4πχ for A, B, C, and D. For all the samples, diamagnetic signals due to the emergence of superconductivity were observed. The transition temperature $T_{\text{c}}^{\text{mag}}$ , estimated as the temperature where the susceptibility begins to drop, is 3.1, 3.8, 4.two, and 4.four Thousand for A, B, C, and D, respectively. $T_{\text{c}}^{\text{mag}}$ almost corresponds to $T_{\text{c}}^{\text{zero}}$ estimated from the resistivity information. For C and D, a large shielding fraction was observed, suggesting the emergence of bulk superconductivity in the HEA-blazon REO_0.5F_0.5BiS₂ samples. The magnetization loop for sample D is shown in the online supplementary data at http://stacks.iop.org/Noon/11/053102/mmedia. We confirmed that the superconducting states in the HEA-type samples are also robust to the magnetic field. In the typical REO_0.fiveF_0.5BiS_ii systems, the shielding volume fraction is enhanced with increasing ship metallicity.⁵ ^, ^x ^, ²¹ ⁾ Therefore, although the suppression is slight, every bit shown in Fig. three(b), the larger shielding fraction in C and D than that of A and B may be related to the suppression of carrier localization.

Here, we discuss the evolution of T _c and shielding fraction [using Δ4πχ calculated by 4πχ(FC) − 4πχ(ZFC) at ii Thou] every bit a function of the lattice constant a. As mentioned above, the flatness of the BiS1 plane is most the same for all the samples including typical REO_0.vF_0.5BiS₂ with RE = La, Ce, Pr, and Nd [plotted in Figs. 3(e) and 3(f) for comparison]. Therefore, the lattice constant a tin can be a skillful indicator to hash out the shrinkage (in-plane chemical pressure) amplitude of the superconducting BiS1 plane. In Figs. 3(e) and 3(f), the dependences of T _c and Δ4πχ as a function of lattice constant a are respectively plotted with the information of REO_0.5F_0.5BiS₂ (RE = La, Ce, Pr, and Nd) and the solid-solution systems with RE = La₁₋ ₁₀ Ce _x and Ce_ane− _x Nd _ten .⁴ ^, ^viii ^, ¹² ⁾ The T _c plot for the HEA-type samples (A, B, C, and D) is slightly higher than that for typical REO_0.5F_0.5BiS₂. Δ4πχ for B, C, and D is close to that for RE = Pr and Nd. Withal, Δ4πχ for A is notably larger than that expected from the dependence of typical REO_0.5F_0.5BiS₂ phases. These results may indicate that the HEA-blazon blocking layer positively affects the superconducting states emerging in the BiS_ii layers. Probably, in-airplane disorder is suppressed by the stabilization of the crystal structure by the HEA effect in REO_0.fiveF_0.5BiS₂. To obtain evidence for the positive link, nosotros volition investigate the local crystal structure using synchrotron XRD and X-ray absorption spectroscopy as carried out on the typical BiS₂-based superconductor organization.⁹ ^– ¹² ⁾

In conclusion, we newly synthesized four superconducting phases, La_0.3Ce_0.3Pr_0.twoNd_0.1Sm_0.1O_0.5F_0.fiveBiS₂, La_0.2Ce_0.2Pr_0.2Nd_0.2Sm_0.2O_0.5F_0.5BiS₂, La_0.1Ce_0.anePr_0.3Nd_0.3Sm_0.2O_0.5F_0.5BiS_two, and La_0.1Ce_0.1Pr_0.twoNd_0.iiiSm_0.3O_0.5F_0.vBiS_two, whose blocking layer has been designed with the HEA concept. The lattice constant a systematically changed in the 4 samples with the RE concentration and the RE ionic radius. For all the samples, a superconducting transition was observed. Notably, the superconducting transition in the resistivity–temperature plot was quite precipitous (sharper than that of typical PrO_0.5F_0.fiveBiS_two). The sharp transition may indicate that the superconductivity fluctuations and/or the structural instability⁴ ^, ¹¹ ^, ²³ ^, ²⁴ ⁾ are suppressed past the HEA effect. The plot of T _c as a part of lattice constant a for the HEA-type samples was higher than those of typical REO_0.5F_0.5BiS₂. Furthermore, for sample A (with a large lattice constant a), the observed shielding volume fraction estimated from the susceptibility data was clearly higher than that expected from the trend in typical REO_0.5F_0.fiveBiS₂. The enhancement of the superconducting properties observed in the HEA-type samples may be related to the HEA effect in the blocking layer. We believe that the introduction of the HEA effects into the blocking layers of layered superconductors will exist a useful strategy to improve the superconducting properties such as T _c, upper critical field, and critical current density.

Nosotros would like to thank O. Miura for his experimental support. This report was partially supported past Grants-in-Aid for Scientific Research (Nos. 15H05886, 16H04493, 16K17944, and 17K19058).

What Is A Superconductor Apex,

Source: https://iopscience.iop.org/article/10.7567/APEX.11.053102

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What Is A Superconductor Apex

What Is A Superconductor Apex,

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