The role of different negatively charged layers in Ca10(Fe1-xPtxAs)10(Pt3+yAs8) and superconductivity at 30 K in electron-doped (Ca0.8La0.2)10(FeAs)10(Pt3As8)

The recently discovered compounds Ca10(Fe1-xPtxAs)10(Pt3+yAs8) exhibit superconductivity up to 38 K, and contain iron arsenide (FeAs) and platinum arsenide (Pt3+yAs8) layers separated by layers of Ca atoms. We show that high Tc's above 15 K only emerge if the iron-arsenide layers are at most free of platinum-substitution (x \rightarrow 0) in contrast to recent reports. In fact Pt-substitution is detrimental to higher Tc, which increases up to 38 K only by charge doping of pure FeAs layers. We point out, that two different negatively charged layers [(FeAs)10]n- and (Pt3+yAs8)m- compete for the electrons provided by the Ca2+ ions, which is unique in the field of iron-based superconductors. In the parent compound Ca10(FeAs)10(Pt3As8), no excess charge dopes the FeAs-layer, and superconductivity has to be induced by Pt-substitution, albeit below 15 K. In contrast, the additional Pt-atom in the Pt4As8layer shifts the charge balance between the layers equivalent to charge doping by 0.2 electrons per FeAs. Only in this case Tc raises to 38 K, but decreases again if additionally platinum is substituted for iron. This charge doping scenario is supported by our discovery of superconductivity at 30 K in the electron-doped La-1038 compound (Ca0.8La0.2)10(FeAs)10(Pt3As8) without significant Pt-substitution.Comment: 4 pages, 4 figure

Interplay between Nitrogen Dopants and Native Point Defects in Graphene

To understand the interaction between nitrogen dopants and native point defects in graphene, we have studied the energetic stability of N-doped graphene with vacancies and Stone-Wales (SW) defect by performing the density functional theory calculations. Our results show that N substitution energetically prefers to occur at the carbon atoms near the defects, especially for those sites with larger bond shortening, indicating that the defect-induced strain plays an important role in the stability of N dopants in defective graphene. In the presence of monovacancy, the most stable position for N dopant is the pyridinelike configuration, while for other point defects studied (SW defect and divacancies) N prefers a site in the pentagonal ring. The effect of native point defects on N dopants is quite strong: While the N doping is endothermic in defect-free graphene, it becomes exothermic for defective graphene. Our results imply that the native point defect and N dopant attract each other, i.e., cooperative effect, which means that substitutional N dopants would increase the probability of point defect generation and vice versa. Our findings are supported by recent experimental studies on the N doping of graphene. Furthermore we point out possibilities of aggregation of multiple N dopants near native point defects. Finally we make brief comments on the effect of Fe adsorption on the stability of N dopant aggregation.Comment: 10 pages, 5 figures. Figure 4(g) and Figure 5 are corrected. One additional table is added. This is the final version for publicatio

Low thermal conductivity of the layered oxide (Na,Ca)Co_2O_4: Another example of a phonon glass and an electron crystal

The thermal conductivity of polycrystalline samples of (Na,Ca)Co_2O_4 is found to be unusually low, 20 mW/cmK at 280 K. On the assumption of the Wiedemann-Franz law, the lattice thermal conductivity is estimated to be 18 mW/cmK at 280 K, and it does not change appreciably with the substitution of Ca for Na. A quantitative analysis has revealed that the phonon mean free path is comparable with the lattice parameters, where the point-defect scattering plays an important role. Electronically the same samples show a metallic conduction down to 4.2 K, which strongly suggests that NaCo_2O_4 exhibits a glass-like poor thermal conduction together with a metal-like good electrical conduction. The present study further suggests that a strongly correlated system with layered structure can act as a material of a phonon glass and an electron crystal.Comment: 5 pages 3 figures, to be published in Phys. Rev.

Super-heavy fermion material as metallic refrigerant for adiabatic demagnetization cooling

Low-temperature refrigeration is of crucial importance in fundamental research of condensed matter physics, as the investigations of fascinating quantum phenomena, such as superconductivity, superfluidity and quantum criticality, often require refrigeration down to very low temperatures. Currently, cryogenic refrigerators with $^3$He gas are widely used for cooling below 1 Kelvin. However, usage of the gas is being increasingly difficult due to the current world-wide shortage. Therefore, it is important to consider alternative methods of refrigeration. Here, we show that a new type of refrigerant, super-heavy electron metal, YbCo$_2$Zn$_{20}$, can be used for adiabatic demagnetization refrigeration, which does not require 3He gas. A number of advantages includes much better metallic thermal conductivity compared to the conventional insulating refrigerants. We also demonstrate that the cooling performance is optimized in Yb$_{1-x}$Sc$_x$Co$_2$Zn$_{20}$ by partial Sc substitution with $x\sim$0.19. The substitution induces chemical pressure which drives the materials close to a zero-field quantum critical point. This leads to an additional enhancement of the magnetocaloric effect in low fields and low temperatures enabling final temperatures well below 100 mK. Such performance has up to now been restricted to insulators. Since nearly a century the same principle of using local magnetic moments has been applied for adiabatic demagnetization cooling. This study opens new possibilities of using itinerant magnetic moments for the cryogen-free refrigeration