The synthesis and properties of LiFeAs, a high-T c Fe-based superconducting stoichiometric compound, are reported. Single crystal x-ray studies reveal that it crystallizes in the tetragonal PbFCl type ͑P4/nmm͒ with a = 3.7914͑7͒ Å and c = 6.364͑2͒ Å. Unlike the known isoelectronic undoped intrinsic FeAs compounds, LiFeAs does not show any spin-density wave behavior but exhibits superconductivity at ambient pressures without chemical doping. It exhibits a respectable transition temperature of T c = 18 K with electronlike carriers and a very high critical field, H c2 ͑0͒ Ͼ 80 T. LiFeAs appears to be the chemical equivalent of the infinite layered compound of the high-T c cuprates. DOI: 10.1103/PhysRevB.78.060505 PACS number͑s͒: 74.70.Dd, 61.66.Fn, 74.25.Fy Until recently the chemical realm of high-T c superconductivity had been limited mainly to copper oxide-based layered perovskites. The latest search for noncuprate superconductors in strongly correlated electron layered systems has led to the discovery of high-T c superconductivity in doped quaternary rare-earth iron oxypnictides, ROFePn ͑R = rare-earth metal and Pn= pnicogen͒. 1-3 These superconductors generated enormous interest in the materials community due to the high T c 's involved ͑up to 41-55 K͒ as well as the critical presence of a magnetic component, Fe, considered antithetical to conventional s-wave superconductivity. 3,4 High-pressure studies suggest maximum T c in R͑O,F͒FeAs may be about 50 K but higher T c 's ͑Ͼ50 K͒ may yet be discovered in structurally different compounds that are electronically related to R͑O,F͒FeAs. 5 Analogous alkaline-earth iron arsenides, AeFe 2 As 2 ͑Ae= Sr and Ba͒, reportedly having formal ͑Fe 2 As 2 ͒ 2− layers as in ROFFeAs but separated by simple Ae layers as in the cuprates, were found to behave similarly. 6,7 The AeFe 2 As 2 phases become superconducting ͑maximum T c ϳ 37 K͒ with appropriate substitution of Ae atoms with alkali metals. 8,9 It was also found that isostructural compounds KFe 2 As 2 and CsFe 2 As 2 with formal ͑Fe 2 As 2 ͒ 1− layers were superconducting, having much lower T c 's of 3.8 and 2.6 K, respectively. 9 Moreover, the evolution from a superconducting state to a spin-density wave ͑SDW͒ state by chemical substitution was observed in K 1−x Sr x Fe 2 As 2 . 9 Critical to the high-T c FeAs superconductors is the need to introduce sufficient amounts of charge carriers: with electrons ͑n type͒ by F doping ͑15-20 atm %͒ or holes ͑p type͒ by Sr doping ͑4-13 atm %͒ in ROFeAs, and ͑K/Sr͒ substitution ͑40: 60 atm %͒ in AeFe 2 As 2 . These results established the unique role of ͑Fe 2 As 2 ͒ layers in high-T c superconductivity. Since simple elemental K, Cs, ͑K/ Sr͒, or ͑Cs/Sr͒ layers separate the ͑Fe 2 As 2 ͒ layers in the AFe 2 As 2 superconductors, a Li-based analog, LiFeAs, was investigated. Its crystal structure was previously reported to be of the Cu 2 Sb type that features a Fe 2 As 2 substructure similar to the known FeAs superconductors. 10 However, the locations of the Li atoms were problematic....
Superconductivity and phase relationships were explored in the Na-Fe-As system. The PbFCl-type 111 phase is stable only within a Na stoichiometry range of 1.00 to ϳ0.85, and exhibits bulk superconductivity within an even narrower range around 0.90 in Na 0.9 FeAs. In particular, stoichiometric NaFeAs is not a bulk superconductor. The onset of the superconducting transition varies in a totally different way and the highest T c occurs in multiphase samples with a nominal composition of Na: Fe: As= 0.5:1:1, where the superconductive volume-fraction is almost zero. Such doping dependency is rather surprising and in disagreement with most expectations. DOI: 10.1103/PhysRevB.79.184516 PACS number͑s͒: 74.70.Dd, 74.62.Dh, 74.62.Bf The recent discovery of superconductivity in layered transition-metal oxypnictides, La͑O , F͒FeAs, 1 has attracted intense interest in the FeAs-based compounds. Superconductivity up to 55 K has been observed in three classes of FeAsbased compounds, i.e., ͑R ,Ae͒͑O , F͒FeAs, ͑Ae, A͒Fe 2 As 2 and AFeAs, where R, Ae, and A are rare earth, alkaline earth, and alkali elements, respectively.2-7 The FeAs-based superconducting compounds have often been compared with the well-investigated cuprate superconductors. The doping dependency of the superconductivity, however, appears to be rather different in the FeAs-family as it varies significantly from one member to another. 7,8 The main doping effects reported so far in the FeAs family, however, appear still to be a smooth, bell-like T c vs. carrier filling x 0 , where T c is the transition temperature. Competitions with magnetic ordering are often suggested in interpreting the data.9,10 Significant change in the superconducting volume-fraction V S , on the other hand, occurs only near the normal conductorsuperconductor boundary. The V S , it should be pointed out, is actually a convolution of the T c ͑x 0 ͒ and the local x 0 -distribution ͑composition inhomogeneity͒ if x 0 is a sole parameter. A constant V S , therefore, is expected if the superconductive range, ⌬, is much broader than the x 0 -spread, e.g., the full width at half height ͑FWHH͒ of a normal distribution. The effect on T c , in such cases, will be the main focus. At the opposite extreme of ⌬Ӷ, however, the spread would lead to the same T c distribution but a drastic V S change with x 0 , though this is rarely observed or discussed. Herein we report our observations in the superconducting system, Na y FeAs, which possesses a PbFCl-type structure isotypic to that of LiFeAs. This PbFCl-type structure as well as ͑trace͒ superconductivity exist over the whole nominalcomposition range investigated, i.e., with the nominal composition of Na y FeAs, with 0.5Յ y Յ 1.0. The samples are single phase, however, only for y Ն 0.9, and the impurity phase FeAs appears at lower y. A rather unusual doping effect is also observed. On one hand, the samples become bulk superconducting, e.g., with V S Ͼ 10%, only around y = 0.9 with an estimated spread Ӷ0.1. The apparent T c , on the other hand, monotonically...
New high-Tc Fe-based superconducting compounds, AFe2As2 with A = K, Cs, K/Sr and Cs/Sr, were synthesized. The Tc of KFe2As2 and CsFe2As2 is 3.8 and 2.6 K, respectively, which rises with partial substitution of Sr for K and Cs and peaks at 37 K for 50-60% Sr substitution, and the compounds enter a spin-density-wave state (SDW) with increasing electron number (Sr-content). The compounds represent p-type analogs of the n-doped rare-earth oxypnictide superconductors. Their electronic and structural behavior demonstrate the crucial role of the (Fe2As2)-layers in the superconductivity of the Fe-based layered systems, and the special feature of having elemental Alayers provides new avenues to superconductivity at higher Tc.
The high density of heat generated in power electronics and optoelectronic devices is a critical bottleneck in their application. New materials with high thermal conductivity are needed to effectively dissipate heat and thereby enable enhanced performance of power controls, solid-state lighting, communication, and security systems. We report the experimental discovery of high thermal conductivity at room temperature in cubic boron arsenide (BAs) grown through a modified chemical vapor transport technique. The thermal conductivity of BAs, 1000 ± 90 watts per meter per kelvin meter-kelvin, is higher than that of silicon carbide by a factor of 3 and is surpassed only by diamond and the basal-plane value of graphite. This work shows that BAs represents a class of ultrahigh-thermal conductivity materials predicted by a recent theory, and that it may constitute a useful thermal management material for high-power density electronic devices.
Establishing the appropriate theoretical framework for unconventional superconductivity in the iron-based materials requires correct understanding of both the electron correlation strength and the role of Fermi surfaces. This fundamental issue becomes especially relevant with the discovery of the iron chalcogenide superconductors. Here, we use angle-resolved photoemission spectroscopy to measure three representative iron chalcogenides, FeTe0.56Se0.44, monolayer FeSe grown on SrTiO3 and K0.76Fe1.72Se2. We show that these superconductors are all strongly correlated, with an orbital-selective strong renormalization in the dxy bands despite having drastically different Fermi surface topologies. Furthermore, raising temperature brings all three compounds from a metallic state to a phase where the dxy orbital loses all spectral weight while other orbitals remain itinerant. These observations establish that iron chalcogenides display universal orbital-selective strong correlations that are insensitive to the Fermi surface topology, and are close to an orbital-selective Mott phase, hence placing strong constraints for theoretical understanding of iron-based superconductors.
In this work, we study the AxFe2−ySe2 (A=K, Rb) superconductors using angle-resolved photoemission spectroscopy. In the low temperature state, we observe an orbital-dependent renormalization for the bands near the Fermi level in which the dxy bands are heavily renormliazed compared to the dxz/dyz bands. Upon increasing temperature to above 150K, the system evolves into a state in which the dxy bands have diminished spectral weight while the dxz/dyz bands remain metallic. Combined with theoretical calculations, our observations can be consistently understood as a temperature induced crossover from a metallic state at low temperature to an orbital-selective Mott phase (OSMP) at high temperatures. Furthermore, the fact that the superconducting state of AxFe2−ySe2 is near the boundary of such an OSMP constraints the system to have sufficiently strong on-site Coulomb interactions and Hund's coupling, and hence highlight the non-trivial role of electron correlation in this family of iron superconductors.
Thermoelectric power generation is one of the most promising techniques to use the huge amount of waste heat and solar energy. Traditionally, high thermoelectric figure-of-merit, ZT, has been the only parameter pursued for high conversion efficiency. Here, we emphasize that a high power factor (PF) is equivalently important for high power generation, in addition to high efficiency. A new n-type Mg 2 Sn-based material, Mg 2 Sn 0.75 Ge 0.25 , is a good example to meet the dual requirements in efficiency and output power. It was found that Mg 2 Sn 0.75 Ge 0.25 has an average ZT of 0.9 and PF of 52 μW·cm ·K−2 over the temperature range of 25-450°C, a peak ZT of 1.4 at 450°C, and peak PF of 55 μW·cm·K −2 at 350°C. By using the energy balance of one-dimensional heat flow equation, leg efficiency and output power were calculated with T h = 400°C and T c = 50°C to be of 10.5% and 6.6 W·cm −2 under a temperature gradient of, respectively.thermoelectrics | magnesium | tin | power factor | output power T hermoelectric power generation from waste heat is attracting more and more attention. Potential fuel efficiency enhancement by recovering the waste heat is beneficial for automobiles and many other applications (1, 2). In addition, solar thermoelectric generator provides an alternative route to convert solar energy into electrical power besides the photovoltaic conversion (3). Thermoelectric generator (TEG) can be regarded as a heat engine using electrons/holes as the energy carrier. The conversion efficiency of a TEG is related to the Carnot efficiency and the material's average thermoelectric figure of merit ZT (4):where ZT = (S 2 σ/κ)T, and S, σ, κ, and T are Seebeck coefficient, electrical conductivity, thermal conductivity, and absolute temperature, respectively. Pursuing high ZT has been the focus of the entire thermoelectric community by applying various phonon engineering via nanostructuring approaches to reduce the thermal conductivity (5-7), or by exploring new compounds with intrinsically low thermal conductivity, such as compounds having complex crystalline structure, local rattlers, liquid-like sublattice, and highly distorted lattice (8-11). However, for practical applications, efficiency is not the only concern, and high output power density is as important as efficiency when the capacity of the heat source is huge (such as solar heat), or the cost of the heat source is not a big factor (such as waste heat from automobiles, steel industry, etc.). The output power density ω is defined as the output power W divided by the cross-sectional area A of the leg, i.e., ω = W/A, which is related to power factor PF = S 2 σ by the following:Eq. 2 contains two main parts: square of the temperature difference divided by leg length, and material power factor PF = S 2 σ.Clearly, to achieve higher power density for a given heat source, we have to either increase the power factor PF or decrease the leg length. However, decreasing the leg length could cause severe consequences such as increase of large heat flux that will incr...
a b s t r a c tThe newest homologous series of superconducting Fe-pnictides, LiFeAs (Li111) and NaFeAs (Na111) have been synthesized and investigated. Both crystallize with the layered tetragonal anti-PbFCl-type structure in P4/nmm space group. Polycrystalline samples and single crystals of Li111 and Na111 display superconducting transitions at $18 K and 12-25 K, respectively. No magnetic order has been found in either compound, although a weak magnetic background is clearly in evidence. The origin of the carriers and the stoichiometric compositions of Li111 and Na111 were explored.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.