Fiber-particle-reinforced silica aerogels are widely applied in thermal insulation. Knowing their effective thermal conductivity (ETC) and radiative characteristics under high temperatures is ...necessary to improve their performance. This article first analyzes the radiation characteristics of silica aerogels doped with opacifier particles and reinforced fibers, and then a universal model is established to predict the ETC. Furthermore, the impacts of different parameters of opacifier particles and reinforced fibers on the thermal insulation performance of silica aerogels are investigated. The results indicate that SiC exhibits comparatively strong absorption characteristics, making it a good alternative for opacifiers to improve thermal insulation performance under high temperatures. For the given type and volume fraction of opacifier particles, there exists an optimal diameter and volume fraction to achieve the best insulation performance of silica aerogel under a certain temperature. Considering that SiO
fibers exhibit a limited extinction capability and higher conductive thermal conductivity under high temperatures, for fiber-particle-reinforced silica aerogels, it is beneficial for their insulation performance to reduce the fiber volume fraction when the required mechanical properties are satisfied.
Spin-fluctuation-mediated unconventional superconductivity can emerge at the border of magnetism, featuring a superconducting order parameter that changes sign in momentum space. Detection of such a ...sign-change is experimentally challenging, since most probes are not phase-sensitive. The observation of a spin resonance mode (SRM) from inelastic neutron scattering is often seen as strong phase-sensitive evidence for a sign-changing superconducting order parameter, by assuming the SRM is a spin-excitonic bound state. Here we show that for the heavy fermion superconductor CeCoIn5, its SRM defies expectations for a spin-excitonic bound state, and is not a manifestation of sign-changing superconductivity. Instead, the SRM in CeCoIn5 likely arises from a reduction of damping to a magnon-like mode in the superconducting state, due to its proximity to magnetic quantum criticality. Our findings emphasize the need for more stringent tests of whether SRMs are spin-excitonic, when using their presence to evidence sign-changing superconductivity.Spin mediated unconventional superconductivity exhibits a sign changing order parameter which is often inferred from the observation of a spin resonance mode. Here, the authors demonstrate that the origin of the spin resonance mode for a heavy fermion superconductor may not lie, as previously thought, in the spin-excitonic bound state and therefore its relation to the ordering parameter needs to be reconsidered.
Spontaneous symmetry breaking—the phenomenon in which an infinitesimal perturbation can cause the system to break the underlying symmetry—is a cornerstone concept in the understanding of interacting ...solid-state systems. In a typical series of temperature-driven phase transitions, higher-temperature phases are more symmetric due to the stabilizing effect of entropy that becomes dominant as the temperature is increased. However, the opposite is rare but possible when there are multiple degrees of freedom in the system. Here, we present such an example of a symmetry-ascending phenomenon upon cooling in a magnetic kagome metal FeGe by utilizing neutron Larmor diffraction and Raman spectroscopy. FeGe has a kagome lattice structure with simple A-type antiferromagnetic order below Néel temperature T_{N}≈400 K and a charge density wave (CDW) transition at T_{CDW}≈110 K, followed by a spin-canting transition at around 60 K. In the paramagnetic state at 460 K, we confirm that the crystal structure is indeed a hexagonal kagome lattice. On cooling to around T_{N}, the crystal structure changes from hexagonal to monoclinic with in-plane lattice distortions on the order of 10^{-4} and the associated splitting of the double-degenerate phonon mode of the pristine kagome lattice. Upon further cooling to T_{CDW}, the kagome lattice shows a small negative thermal expansion, and the crystal structure gradually becomes more symmetric upon further cooling. A tendency of increasing the crystalline symmetry upon cooling is unusual; it originates from an extremely weak structural instability that coexists and competes with the CDW and magnetic orders. These observations are against the expectations for a simple model with a single order parameter and hence can only be explained by a Landau free energy expansion that takes into account multiple lattice, charge, and spin degrees of freedom. Thus, the determination of the crystalline lattice symmetry as well as the unusual spin-lattice coupling is a first step towards understanding the rich electronic and magnetic properties of the system, and it sheds new light on intertwined orders where the lattice degree of freedom is no longer dominant.
Most parent compounds of iron-based superconductors (FeSCs) exhibit a tetragonal-to-orthorhombic lattice distortion below Ts associated with an electronic nematic phase that breaks the four-fold (C4) ...rotational symmetry of the underlying lattice, and then forms collinear antiferromagnetic (AF) order below TN (TN ≤ Ts). Optimal superconductivity emerges upon suppression of the nematic and AF phases. FeSe, which also exhibits a nematic phase transition below Ts but becomes superconducting in the nematic phase without AF order, provides a unique platform to study the interplay amongst the nematic phase and superconductivity. In this review, we focus on the experiments done on uniaxial pressure detwinned single crystals of FeSe compared to other FeSCs and highlight the importance of understanding the electronic and magnetic anisotropy in elucidating the nature of unconventional superconductivity.
In conventional superconductors, lattice vibrations (phonons) mediate the attraction between electrons that is responsible for superconductivity. The high transition temperatures (high-Tc) of the ...copper oxide superconductors has led to collective spin excitations being proposed as the mediating excitations in these materials. The mediating excitations must be strongly coupled to the conduction electrons, have energy greater than the pairing energy, and be present at Tc. The most obvious feature in the magnetic excitations of high-Tc superconductors such as YBa2Cu3O6+x is the so-called 'resonance'. Although the resonance may be strongly coupled to the superconductivity, it is unlikely to be the main cause, because it has not been found in the La2-x(Ba,Sr)xCuO4 family and is not universally present in Bi2Sr2CaCu2O8+δ (ref. 9). Here we use inelastic neutron scattering to characterize possible mediating excitations at higher energies in YBa2Cu3O6.6. We observe a square-shaped continuum of excitations peaked at incommensurate positions. These excitations have energies greater than the superconducting pairing energy, are present at Tc, and have spectral weight far exceeding that of the 'resonance'. The discovery of similar excitations in La2-xBaxCuO4 (ref. 10) suggests that they are a general property of the copper oxides, and a candidate for mediating the electron pairing.
Celotno besedilo
Dostopno za:
DOBA, IJS, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
Since the discovery in 2008 of high-temperature superconductivity in the iron pnictides and chalcogenides, a central issue has been the microscopic origin of the superconducting pairing. In ...particular, it remains unclear whether there is a bosonic mode from the tunnelling spectrum, which has a close and universal relationship with superconductivity as well as with the observed spin excitation. Here, on the basis of measurements of scanning tunnelling spectroscopy, we show clear evidence of a bosonic mode with energy identical to that of the neutron spin resonance in two completely different systems, Ba0.6K0.4Fe2As2 and Na(Fe0.975Co0.025)As, with different superconducting transition temperatures. In both samples, the superconducting coherence peaks and the mode feature vanish simultaneously inside the vortex core or above the transition temperature Tc, indicating a close relationship between superconductivity and the bosonic mode. Our data also demonstrate a universal ratio between the mode energy and superconducting transition temperature, that is Ω/kBTcapproximate4.3, which underlines the unconventional nature of superconductivity in the iron pnictide superconductors. PUBLICATION ABSTRACT
Abstract
The neutron spin resonance is a collective magnetic excitation that appears in the unconventional copper oxide, iron pnictide and heavy fermion superconductors. Although the resonance is ...commonly associated with a spin-exciton due to the
d
(
s
±
)-wave symmetry of the superconducting order parameter, it has also been proposed to be a magnon-like excitation appearing in the superconducting state. Here we use inelastic neutron scattering to demonstrate that the resonance in the heavy fermion superconductor Ce
1−
x
Yb
x
CoIn
5
with
x
=0, 0.05 and 0.3 has a ring-like upward dispersion that is robust against Yb-doping. By comparing our experimental data with a random phase approximation calculation using the electronic structure and the momentum dependence of the "Equation missing"
-wave superconducting gap determined from scanning tunnelling microscopy (STM) for CeCoIn
5
, we conclude that the robust upward-dispersing resonance mode in Ce
1−
x
Yb
x
CoIn
5
is inconsistent with the downward dispersion predicted within the spin-exciton scenario.
We report inelastic neutron scattering studies of magnetic excitations in antiferromagnetically ordered SrFe{sub 2}As{sub 2} (T{sub N}=200-220 K), the parent compound of the FeAs-based ...superconductors. At low temperatures (T=7 K), the magnetic spectrum S(Q,({Dirac_h}/2{pi}){omega}) consists of a Bragg peak at the elastic position (({Dirac_h}/2{pi}){omega}=0 meV), a spin gap ({delta}{<=}6.5 meV), and sharp spin-wave excitations at higher energies. Based on the observed dispersion relation, we estimate the effective magnetic exchange coupling using a Heisenberg model. On warming across T{sub N}, the low-temperature spin gap rapidly closes, with weak critical scattering and spin-spin correlations in the paramagnetic state. The antiferromagnetic order in SrFe{sub 2}As{sub 2} is therefore consistent with a first order phase transition, similar to the structural lattice distortion.
The parent compounds of iron pnictide superconductors are bad metals with a collinear antiferromagnetic structure and Néel temperatures below 220 K. Although alkaline iron selenide Ay Fe1.6+x Se2 ...(A=K, Rb, Cs) superconductors are isostructural with iron pnictides, in the vicinity of the undoped limit they are insulators, forming a block antiferromagnetic order and having Néel temperatures of roughly 500 K. Here we show that the spin waves of the insulating antiferromagnet Rb0.89 Fe1.58 Se2 can be accurately described by a local moment Heisenberg Hamiltonian. A fitting analysis of the spin wave spectra reveals that the next-nearest neighbour couplings in Rb0.89 Fe1.58 Se2 , (Ba,Ca,Sr)Fe2 As2 , and Fe1.05 Te are of similar magnitude. Our results suggest a common origin for the magnetism of all the Fe-based superconductors, despite having different ground states and antiferromagnetic orderings.
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