Antiferromagnetic ordering in the Kondo lattice system Yb2_2Fe3_3Si5_5

Compounds belonging to the R$_2$Fe$_3$Si$_5$ series exhibit unusual superconducting and magnetic properties. Although a number of studies have been made on the first reentrant antiferromagnet superconductor Tm$_2$Fe$_3$Si$_5$, the physical properties of Yb$_2$Fe$_3$Si$_5$ are largely unexplored. In this work, we attempt to provide a comprehensive study of bulk properties such as, resistivity, susceptibility and heat-capacity of a well characterized polycrystalline Yb$_2$Fe$_3$Si$_5$. Our measurements indicate that Yb$^{3+}$ moments order antiferromagnetically below 1.7 K. Moreover, the system behaves as a Kondo lattice with large Sommerfeld coefficient ($\gamma$) of 0.5~J/Yb mol K$^{2}$ at 0.3 K, which is well below T$_N$. The absence of superconductivity in Yb$_2$Fe$_3$Si$_5$ down to 0.3 K at ambient pressure is attributed to the presence of the Kondo effect.Comment: 10 pages, 3 figures, tex document. A fuller version has appeared in PRB. Here we have omitted the figures showing the crystal structure and the fitting of the X-ray pattern. Also the table with the lattice parameters obtained from fitting has been remove

Iron oxidation at low temperature (260–500 C) in air and the effect of water vapor

The oxidation of iron has been studied at low temperatures (between 260 and 500 C) in dry air or air with 2 vol% H2O, in the framework of research on dry corrosion of nuclear waste containers during long-term interim storage. Pure iron is regarded as a model material for low-alloyed steel. Oxidation tests were performed in a thermobalance (up to 250 h) or in a laboratory furnace (up to 1000 h). The oxide scales formed were characterized using SEM-EDX, TEM, XRD, SIMS and EBSD techniques. The parabolic rate constants deduced from microbalance experiments were found to be in good agreement with the few existing values of the literature. The presence of water vapor in air was found to strongly influence the transitory stages of the kinetics. The entire structure of the oxide scale was composed of an internal duplex magnetite scale made of columnar grains and an external hematite scale made of equiaxed grains. 18O tracer experiments performed at 400 C allowed to propose a growth mechanism of the scale

Quantum Criticality in doped CePd_1-xRh_x Ferromagnet

CePd_1-xRh_x alloys exhibit a continuous evolution from ferromagnetism (T_C= 6.5 K) at x = 0 to a mixed valence (MV) state at x = 1. We have performed a detailed investigation on the suppression of the ferromagnetic (F) phase in this alloy using dc-(\chi_dc) and ac-susceptibility (\chi_ac), specific heat (C_m), resistivity (\rho) and thermal expansion (\beta) techniques. Our results show a continuous decrease of T_C (x) with negative curvature down to T_C = 3K at x*= 0.65, where a positive curvature takes over. Beyond x*, a cusp in cac is traced down to T_C* = 25 mK at x = 0.87, locating the critical concentration between x = 0.87 and 0.90. The quantum criticality of this region is recognized by the -log(T/T_0) dependence of C_m/T, which transforms into a T^-q (~0.5) one at x = 0.87. At high temperature, this system shows the onset of valence instability revealed by a deviation from Vegard's law (at x_V~0.75) and increasing hybridization effects on high temperature \chi_dc and \rho. Coincidentally, a Fermi liquid contribution to the specific heat arises from the MV component, which becomes dominant at the CeRh limit. In contrast to antiferromagnetic systems, no C_m/T flattening is observed for x > x_cr rather the mentioned power law divergence, which coincides with a change of sign of \beta. The coexistence of F and MV components and the sudden changes in the T dependencies are discussed in the context of randomly distributed magnetic and Kondo couplings.Comment: 11 pages, 11 figure

Low temperature magnetic phase diagram of the cubic non-Fermi liquid system CeIn_(3-x)Sn_x

In this paper we report a comprehensive study of the magnetic susceptibility (\chi), resistivity (\rho), and specific heat (C_P), down to 0.5 K of the cubic CeIn_(3-x)Sn_x alloy. The ground state of this system evolves from antiferromagnetic (AF) in CeIn_3(T_N=10.2 K) to intermediate-valent in CeSn_3, and represents the first example of a Ce-lattice cubic non-Fermi liquid (NFL) system where T_N(x) can be traced down to T=0 over more than a decade of temperature. Our results indicate that the disappearance of the AF state occurs near x_c ~ 0.7, although already at x ~ 0.4 significant modifications of the magnetic ground state are observed. Between these concentrations, clear NFL signatures are observed, such as \rho(T)\approx \rho_0 + A T^n (with n<1.5) and C_P(T)\propto -T ln(T) dependencies. Within the ordered phase a first order phase transition occurs for 0.25 < x < 0.5. With larger Sn doping, different weak \rho(T) dependencies are observed at low temperatures between x=1 and x=3 while C_P/T shows only a weak temperature dependence.Comment: 7 pages, 7 figures. Accepted in Eur. J. Phys.

Crystal-field effects in the mixed-valence compounds Yb2M3Ga9 (M= Rh, Ir)

Magnetic susceptibility, heat capacity, and electrical resistivity measurements have been carried out on single crystals of the intermediate valence compounds Yb2Rh3Ga9 and Yb2Ir3Ga9. These measurements reveal a large anisotropy due apparently to an interplay between crystalline electric field (CEF) and Kondo effects. The temperature dependence of magnetic susceptibility can be modelled using the Anderson impurity model including CEF within an approach based on the Non-Crossing Approximation.Comment: Accepted to Phys. Rev.

IR reflectivity measurements depending on carbon film thickness

Abstract: In thermonuclear controlled fusion machines using magnetic confinement, carbonisations are realised to prevent metal impurities to enter into the fusion plasma made with hydrogen elements; it consists in helium glows in which methane gas is injected . The methane molecule is broken and the carbon deposits on all inside vessel surfaces : inner walls as well as optic elements like windows and mirrors . We studied the dependence of the reflectivity of infrared thermography stainless steel mirrors with carbon films thickness in the 3-5 m bandwidth . The presented results show a decrease of less than 10% of the temperature announced by the camera . . Magnetic fusion : Thermonuclear controlled fusion Although plasma particles are trapped in non-material magnetic barriers, atomic collisions appear and diffusion, convection,conduction and radiation phenomena occur; this implies heating of contact surfaces . That is why an infrared thermographic system is developped to measure and survey these heatings which can lead to a failure of one of the plasma facing component inside the tokamak [2] . . IR thermographic diagnostic : The Tore Supra IR thermographic diagnostic consists, until now, in three endoscopes [fig 2] situated at 120° one from the other on the top of the tokamak; they are remotely controled so that every element inside the vessel can be observed . Images of objects inside the vessel are captured by a movable stainless steel mirror and returned to an optical system through a sapphire window . This window is necessary to keep http://dx.doi.org/10.21611/qirt.2000.049 ultra high vacuum inside the torus vessel . Inframetrics 3-5 m cameras recuperate images from the optical system which transported them . Plasma inside the tokamak encounters surfaces made of, or covered with graphite . This graphite is sputtered and redeposited during or after plasma is stopped . Furthermore carbonisations are made to cover metal inner walls of the vessel with carbon so that metal impurities rate inside the plasma is lower . Carbonisation, and its erosion, was extensively studied . Experimental apparatus : Our carbonisation system was installed in a special vessel Two 2 polished mirrors, set back to back, are supposed to be hidden from carbon deposition and centimeter by centimeter are exposed to the glow (see . Experimental results : Six carbon films 1 cm x 3 cm wide were obtained : 50 monolayers, 150, 250, 350, 450 and 550 monolayers thick [ The two mirrors present the same visual aspect . A correlation between film thickness and colour was established by J. Winter in his study of carbonisation in Textor tokamak Reflectivity results for both mirrors are presented below [ Reproducibility of our reflectivity measurements is in the order of +/-2 % . Our deposits need special care because of their poor adhesion : we lost part of the thickest deposits by rubbing with a plastic bag . Mirrors surfaces were made very smooth and then it is not so surprising that our films have little adhesion as thick they are . Furthermore, electrostatic interaction may increase this effect . Indeed carbonisation films have quite high electrical resistivity . For example, on Textor samples 1 -10 6 cm was typically measured Measurement of the 550 monolayers zone where the deposit is and where it peeled, was undertaken . Reflectivity of the undamaged part of the film is the lowest . Reflectivity of the peeled film is half between that latter and that of 0 -150 monolayers . If we consider the worst result which means 550 undamaged monolayers deposited, we find that the real temperature is underestimated by less than 10 per cent; this may be dangerous in our application especially at high temperatures : for example, the copper melting point is at 1083°C, with our results the temperature indicated by infrared cameras is around 975°C, but inside the copper 15 bar water circulate to refregirate the plasma facing components which means that with our measurement we wouldn&apos;t understand why there is a water leak in the machine; this leak implies no experiment for three months to mend damaged parts and to restart the machine . . Conclusion : We realised carbonisation with variable thicknessses on two polished stainless steel mirrors used in the infrared 3-5 m band . Their reflectivity before and after carbonisation was measured, on the different thicknesses of the films formed . Below 150 monolayers the reflectivity does not change . Beyond 150 monolayers, a decrease is observed . If deposit peels, reflectivity is only modified but does not come back to the value with few deposit . Over 450 monolayers the deposit shows waves which show poor film adhesion . The two mirrors have the same results even if they were set back to back for carbonisation, one looking at the anode, the other looking at the opposite . We deduced from these reflectivity measurements that in the case of the thickest deposit the real temperature of the tokamak element is under evaluated by less than 10 % . Acknowledgements : The authors want to thank A Grosman for fruitfull discussion and P. Maillet and his team for their advices and help for that work to be done

Mass-Enhanced Fermi Liquid Ground State in Na1.5_{1.5}Co2_2O4_4

Magnetic, transport, and specific heat measurements have been performed on layered metallic oxide Na$_{1.5}$Co$_2$O$_4$ as a function of temperature $T$. Below a characteristic temperature $T^*$=30$-$40 K, electrical resistivity shows a metallic conductivity with a $T^2$ behavior and magnetic susceptibility deviates from the Curie-Weiss behavior showing a broad peak at $\sim$14 K. The electronic specific heat coefficient $\gamma$ is $\sim$60 mJ/molK$^2$ at 2 K. No evidence for magnetic ordering is found. These behaviors suggest the formation of mass-enhanced Fermi liquid ground state analogous to that in $d$-electron heavy fermion compound LiV$_2$O$_4$.Comment: 4 pages, 4 figures, to be published in Phys. Rev. B 69 (2004

Tungsten divertor sources in WEST related to impurity inventory and local plasma conditions

A dedicated series of L-mode deuterium discharges were performed in WEST in which a broad parameter space in terms of divertor plasma temperature (inner divertor at &lt;10 eV, outer divertor up to 50 eV) and impurity flux density was achieved by variation of X point elevation, upstream plasma density and fuelling position and rate. Density steps provided stable reference plasma conditions whilst density scans up to the point of detachment revealed the sputtering threshold behaviour of the dominant sputtering species. Tungsten gross erosion was quantified via the common WI 400.9 nm line and correlated with the residual gas content.</p