%'e show in Fig. 1

%e report measurements of the upper critical field, 8,2, inclined at various angles with respect to the c axis in the hcavy-fermion superconductor UPt3. The angular anisotropy observed near T, =0.53 K cannot be explained quantitatively by presently available theoretical expressions which consider either isotropic or anisotropic pairing. In addition, we find that the anisotropy apparently disappears at T-200 mK, only to reemerge at lower temperatures with an opposite sense, %e have also studied 0,2 in the basal plane of this hexagonal crystal and find no angular dependence within the limits of our measurements. We plot in Fig. 1 critical fields parallel and perpendicular to the hexagonal c axis, H, zii and H, 2i, respectively. The slopes at T, are the largest yet observed for UPt3 (see Table I), from which we determine a superconducting coherence length ( =110 A. In addition, the positive curvature observed by Chen et a!.for H, 2i is less pronounced in Fig. 1

Using a surface acoustic wave based high resolution gravimetric technique where samples close to a monolayer are measured we observe high weight percentage hydrogen (H(2)) uptake with rapid kinetics at room temperature in transition metal (TM) ethylene (C(2)H(4)) complexes formed by laser ablation. By ablating titanium (Ti) in C(2)H(4) we obtain a complex that exhibits 12 wt % uptake of H(2) with substitution by deuterium providing a doubling. Mass spectroscopic studies during ablation of Ti show presence of a species, with a mass = 78 amu, a likely candidate for the high H(2) uptake.

An enhanced susceptibility is a natural consequence of the "heavy fermion" (HF) state rendering the possibility of measurably large nonlinear susceptibilities. In recent work a universal behavior of the peaks observed in the linear ( 1 ) and the third order ( 3 ) susceptibility in HF metamagnets has been identified. This universality is well accounted for by a single energy scale model considering on-site correlations only. A prediction of this model is a peak in the fifth order susceptibility,  5 , as well. In the first measurements on a HF metamagnet, UPt 3 reported herein, we find that  5 rather than attaining a peak, saturates at low temperatures and is positive. The thermodynamic implications of these towards the stability of the metamagnetic HF state are discussed. PACS: 75.30.Mb, 75.20.Hr 35 1 3 5

We report in this paper, measurements of the nonlinear susceptibility χ 3 (T) in the metamagnetic heavy fermion (HF) compound UPt 3 . At high temperatures, χ 3 (T) < 0 and small. It turns positive for T ≤ 35K, forms a peak at T  10K and then decreases to zero with further decreasing temperature. The peak in χ 3 occurs at a temperature T 3 roughly half of T 1 , the temperature of the maximum in the linear susceptibility. We present results on URu 2 Si 2 and UPd 2 Al 3 to show that this feature is common to other HF materials. A two level model to describe the metamagnetic transition, with separation between the levels being the only energy scale, captures all experimentally observed features. *bss2d@virginia.edu -to whom all correspondence should be addressed.2 Given the myriad ways in which one can arrange atoms to form crystal structures and the possibilities of placing electrons in them the discovery of a common behavioral pattern in the electronic properties often leads to new microscopic insights. In metals, recent discoveries of universal rules such as the Kadowaki-Woods relation 1 which connects the electron effective mass enhancement to the temperature dependence of the resistivity in Fermi liquid systems, the constancy of the Wilson ratio 2 , the Homes plot 3 and other similar scaling laws have assisted in identifying the dominant energy scales, and helped in insights to build microscopic theories.Our purpose in this paper is to show that the magnetic response of many electronic materials that undergo a metamagnetic transition also exhibit universal features in their nonlinear susceptibility. Along with a jump in the magnetization at a critical field, H m , metamagnetism also entails a positive nonlinear susceptibility at low temperatures (it is negative at high temperatures), a peak in the linear susceptibility  1 (T) at a temperature T 1 , a peak in the leading order nonlinear susceptibility  3 (T) >0 at a lower temperature T 3 . These are features widely seen in different materials with diverse lattice structures and electronic properties. Hirose et. al 4 have noted the correlation between H m and the temperature T 1 of the maximum in  1 (T). Goto et. al pointed out earlier that the metamagnetic field H m also correlates with the value of  1 (T) at its maximum 5 . Both of these correlations, found in a number of materials, indicate a dominant presence of one energy scale. In the following we report a new correlation, one between the peak temperatures T 3 for  3 (T) and T 1 namely T 3 =T 1 /2. We also present a new model that captures all of these correlations.For purposes of illustrating this universality we consider the heavy electron class of materials. Within these systems metamagnetic behavior is seen in several uranium compounds 6,7,8,9,10 , cerium compounds 11,12,13,14 , as well as in the ytterbium compounds 15 . Most of these systems possess either hexagonal or tetragonal symmetry. The same behavior is also observed in the skutterudite structure in PrOs 4 Sb 12 a moderately heavy...

Using signatures in the longitudinal sound velocity we have mapped for the first time experimentally the pressure dependence of the phase boundaries in superconducting UPt3 for magnetic field H parallel to the a axis. %ith increasing pressure P, parallel to the c axis, the tetracritical point in the H-T plane is observed to shift to lower H and higher temperature T. At a critical stress, P, -2.6 kbar the tetracritical point reaches the H = 0, P, -T plane, and our measurements prove coexistence of at least four phases in this plane as well.

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In this letter we report the first room temperature gravimetric measurements of hydrogen absorption in nanoscale titanium-benzene complexes formed by laser ablation in a benzene atmosphere in a UHV chamber. We are able to obtain a 6% by weight absorption as predicted by recent density functional theory based calculations under the conditions of low benzene pressure (35 milli-torr) and for sub-monolayer samples. For samples synthesized under higher benzene pressures we find a systematic degradation of the hydrogen absorption.