Ultrasound Investigations of Orbital Quadrupolar Ordering in UPd_3

For a high-quality single crystal of UPd_3 we present the relevant elastic constants and ultrasonic attenuation data. In addition to the magnetic phase transition at T_2=4.4 +/- 0.1K and the quadrupolar transition at T_1~6.8K, we find orbital ordering at T_0=7.6 +/- 0.1K concomitant with a symmetry change from hexagonal to orthorhombic. A striking feature is the splitting of the phase transition at T_1 into a second-order transition at T_{+1}=6.9 +/- 0.05K and a first-order transition at T_{-1}=6.7 +/- 0.05K. For the four phase transitions, the quadrupolar order parameters and the respective symmetry changes are specified.Comment: 14 pages (RevTex), 3 eps-figures, accepted by PR

Tidal Heating of Extra-Solar Planets

Extra-solar planets close to their host stars have likely undergone significant tidal evolution since the time of their formation. Tides probably dominated their orbital evolution once the dust and gas had cleared away, and as the orbits evolved there was substantial tidal heating within the planets. The tidal heating history of each planet may have contributed significantly to the thermal budget that governed the planet's physical properties, including its radius, which in many cases may be measured by observing transit events. Typically, tidal heating increases as a planet moves inward toward its star and then decreases as its orbit circularizes. Here we compute the plausible heating histories for several planets with measured radii, using the same tidal parameters for the star and planet that had been shown to reconcile the eccentricity distribution of close-in planets with other extra-solar planets. Several planets are discussed, including for example HD 209458 b, which may have undergone substantial tidal heating during the past billion years, perhaps enough to explain its large measured radius. Our models also show that GJ 876 d may have experienced tremendous heating and is probably not a solid, rocky planet. Theoretical models should include the role of tidal heating, which is large, but time-varying.Comment: Accepted for publication to Ap

Uncertainty and stress: Why it causes diseases and how it is mastered by the brain

The term 'stress' - coined in 1936 - has many definitions, but until now has lacked a theoretical foundation. Here we present an information-theoretic approach - based on the 'free energy principle' - defining the essence of stress; namely, uncertainty. We address three questions: What is uncertainty? What does it do to us? What are our resources to master it? Mathematically speaking, uncertainty is entropy or 'expected surprise'. The 'free energy principle' rests upon the fact that self-organizing biological agents resist a tendency to disorder and must therefore minimize the entropy of their sensory states. Applied to our everyday life, this means that we feel uncertain, when we anticipate that outcomes will turn out to be something other than expected - and that we are unable to avoid surprise. As all cognitive systems strive to reduce their uncertainty about future outcomes, they face a critical constraint: Reducing uncertainty requires cerebral energy. The characteristic of the vertebrate brain to prioritize its own high energy is captured by the notion of the 'selfish brain'. Accordingly, in times of uncertainty, the selfish brain demands extra energy from the body. If, despite all this, the brain cannot reduce uncertainty, a persistent cerebral energy crisis may develop, burdening the individual by 'allostatic load' that contributes to systemic and brain malfunction (impaired memory, atherogenesis, diabetes and subsequent cardio- and cerebrovascular events). Based on the basic tenet that stress originates from uncertainty, we discuss the strategies our brain uses to avoid surprise and thereby resolve uncertainty

Role of Fe substitution on the anomalous magnetocaloric and magnetoresistance behavior in Tb(Ni1-xFex)2 compounds

We report the magnetic, magnetocaloric and magnetoresistance results obtained in Tb(Ni1-xFex)2 compounds with x=0, 0.025 and 0.05. Fe substitution leads to an increase in the ordering temperature from 36 K for x=0 to 124 K for x=0.05. Contrary to a single sharp MCE peak seen in TbNi2, the MCE peaks of the Fe substituted compounds are quite broad. We attribute the anomalous MCE behavior to the randomization of the Tb moments brought about by the Fe substitution. Magnetic and magnetoresistance results seem to corroborate this proposition. The present study also shows that the anomalous magnetocaloric and magnetoresistance behavior seen in the present compounds is similar to that of Ho(Ni,Fe)2 compounds

Self-magnetic compensation and Exchange Bias in ferromagnetic Samarium systems

For Sm(3+) ions in a vast majority of metallic systems, the following interesting scenario has been conjured up for long, namely, a magnetic lattice of tiny self (spin-orbital) compensated 4f-moments exchange coupled (and phase reversed) to the polarization in the conduction band. We report here the identification of a self-compensation behavior in a variety of ferromagnetic Sm intermetallics via the fingerprint of a shift in the magnetic hysteresis (M-H) loop from the origin. Such an attribute, designated as exchange bias in the context of ferromagnetic/antiferromagnetic multilayers, accords these compounds a potential for niche applications in spintronics. We also present results on magnetic compensation behavior on small Gd doping (2.5 atomic percent) in one of the Sm ferromagnets (viz. SmCu(4)Pd). The doped system responds like a pseudo-ferrimagnet and it displays a characteristic left-shifted linear M-H plot for an antiferromagnet.Comment: 7 pages and 7 figure

Physical properties of noncentrosymmetric superconductor LaIrSi3: A {\mu}SR study

The results of heat capacity C_p(T, H) and electrical resistivity \rho(T,H) measurements down to 0.35 K as well as muon spin relaxation and rotation (\muSR) measurements on a noncentrosymmetric superconductor LaIrSi3 are presented. Powder neutron diffraction confirmed the reported noncentrosymmetric body-centered tetragonal BaNiSn3-type structure (space group I4\,mm) of LaIrSi3. The bulk superconductivity is observed below T_c = 0.72(1) K. The intrinsic \Delta C_e/\gamma_n T_c = 1.09(3) is significantly smaller than the BCS value of 1.43, and this reduction is accounted by the \alpha-model of BCS superconductivity. The analysis of the superconducting state C_e(T) data by the single-band \alpha-model indicates a moderately anisotropic order parameter with the s-wave gap \Delta(0)/k_B T_c = 1.54(2) which is lower than the BCS value of 1.764. Our estimates of various normal and superconducting state parameters indicate a weakly coupled electron-phonon driven type-I s-wave superconductivity in LaIrSi3. The \muSR results also confirm the conventional type-I superconductivity in LaIrSi3 with a preserved time reversal symmetry and hence a singlet pairing superconducting ground state.Comment: 11 pages, 8 figures, 2 table