R. Chau

Evidence for a Common Physical Description of Non-Fermi-Liquid Behavior in Chemically Substituted f -Electron Systems

The non-Fermi-liquid (NFL) behavior observed in the low temperature specific heat C(T) and magnetic susceptibility {chi}(T) of many chemically substituted f -electron systems is analyzed within the context of a recently developed theory based on Griffiths{close_quote} singularities. Measurements of C(T) and {chi}(T) in the systems Th{sub 1{minus}x}U {sub x}Pd{sub 2}Al{sub 3} , Y{sub 1{minus}x}U {sub x}Pd{sub 3} , and UCu{sub 5{minus}x }M{sub x} (M=Pd,thinspPt ) are found to be consistent with C(T)/T{proportional_to}{chi}(T){proportional_to}T{sup {minus}1+{lambda}} predicted by this model with {lambda}{lt}1 in the NFL regime. These results suggest that the NFL properties observed in a wide variety of disordered f -electron systems can be described within the context of a common physical picture. {copyright} {ital 1998} {ital The American Physical Society}

Pd/Cu Site Interchange and Non-Fermi-Liquid Behavior in UCu{sub 4}Pd

X-ray-absorption fine-structure measurements of the local structure in UCu{sub 4}Pd are described which indicate a probable lattice-disorder origin for non-Fermi-liquid behavior in this material. Short Pd-Cu distances are observed, consistent with (24{plus_minus}3){percent} of the Pd atoms occupying nominally Cu sites. A {open_quotes}Kondo disorder{close_quotes} model, based on the effect on the local Kondo temperature T{sub K} of this interchange and some additional bond-length disorder, agrees quantitatively with previous experimental susceptibility data, and therefore also with specific heat and magnetic resonance experiments. {copyright} {ital 1998} {ital The American Physical Society }

Glassy Spin Dynamics in Non-Fermi-Liquid UCu5xPdx, x = 1.0 and 1.5

Local f-electron spin dynamics in the non-Fermi-liquid heavy-fermion alloys UCu5-xPdx, x = 1.0 and 1.5, have been studied using muon spin-lattice relaxation. The sample-averaged asymmetry function G(t) indicates strongly inhomogeneous spin fluctuations and exhibits the scaling G(t,H) = G(t/H(gamma)) expected from glassy dynamics. At 0.05 K gamma(x = 1.0) = 0.35+/-0.1, but gamma(x = 1.5) = 0.7+/-0.1. This is in contrast to inelastic neutron scattering results, which yield gamma = 0.33 for both concentrations. There is no sign of static magnetism approximately greater than 10(-3)(B)/U ion in either material above 0.05 K. Our results strongly suggest that both alloys are quantum spin glasses.

Pressure dependence of the electrical resistivity of the filled skutterudites LnFe4Sb12 (Ln = Ce, Yb)

We report the pressure dependence of the electrical resistivity of two filled skutterudite compounds YbFe4 Sb12 and CeFe4 Sb12 from 1.2 K up to room temperature and under applied pressures up to 17 kbar. Both YbFe4 Sb12 and CeFe4 Sb12 show a rapid decrease of at ~50 K and ~100 K, respectively, which is caused by the onset of coherent scattering of electrons from the rare-earth sublattice. The resistivity curves can be collapsed onto the ambient pressure curve using a single scaling temperature T 0 , indicating that a single characteristic energy dominates the transport properties. For CeFe4 Sb12 , T 0 is found to increase with pressure, while the opposite behaviour is observed for YbFe4 Sb12 . The pressure dependence of as well as the change of T 0 are similar to the findings for a number of other intermediate-valence and heavy-fermion systems.

Non-Fermi-liquid scaling in ( x = 1, 1.5)

We review the compiled measurements of the imaginary part of the dynamical magnetic susceptibility X(ω, T), static susceptibility χ(T), electrical resistivity ρ(T) and specific heat C(T) in the uranium intermetallics UCu 5-x Pd x (x = 1, 1.5). We assess the temperature- and energy-dependences predicted by single-ion and disorder-dominated models and compare these results to experiments. For temperatures T and excitation energies ω in the range 12 K < ω, T < 150 K, our analysis suggests that the dynamics of isolated uranium ions are responsible for the observed temperature and frequency scaling, although inter-ion interactions may become important at lower temperatures and frequencies.

Disorder effects in strongly correlated uranium compounds

Abstract Moderate levels of crystallographic disorder can dramatically affect the ground-state properties of heavy fermion compounds. In particular, the role of disorder close to a quantum critical point has been investigated in detail. However, crystallographic disorder is equally effective in altering the properties of magnetically ordered heavy fermion compounds like URh 2 Ge 2 , where disorder-induced spin-glass behavior has been observed. In this system, moreover, the magnetic ground state can be tuned from a spin-glass to a long-range ordered antiferromagnetic one by means of an annealing treatment. The transformation of the magnetic state is accompanied by a transition in the transport properties from “quasi-insulating” (d ρ /d T 2 Ge 2 will be discussed. Of particular interest is the resistivity of as-grown URh 2 Ge 2 , which resembles the Non-Fermi-liquid system UCu 4 Pd, suggesting that a common mechanism — the crystallographic disorder — controls the transport properties of these materials.

Specific heat and magnetic susceptibility of UCu4Pd at very low temperatures

Abstract UCu 5− x Pd x alloys are heavy-fermion materials with indications of non-Fermi liquid (NFL) behavior for x = 1 and 1.5. We report on the specific heat C of UCu 4 Pd measured between 0.08 and 3.5 K which shows NFL behavior C / T ∼ −1n( T / T 0 ) down to 0.2 K but indicates spin-glass freezing around 0.2 K. This is confirmed by measurements of the DC magnetization and the AC susceptibility.

U 5f spectral weight variation in UPd3-xPtx

Abstract We report a photoemission study of the 5f spectral weight variation in UPd 3− x Pt x . Relative to a previous study the results show both agreement and very significant differences. New spectral detail is resolved.

Fermi- and non-Fermi-liquid ground states in M1−xUxPd3 (M Sc, Y, La, Pr, Zr, Th) systems

Abstract A growing number of chemically substituted intermetallic compounds of Ce and U exhibit non-Fermi-liquid (NFL) behavior in their low temperature physical properties and apparently constitute a new class of strongly correlated f-electron materials. In this paper, we update the experimental situation for the archetypal NFL f-electron system Y1−xUxPd3 and briefly describe recent experiments on the related systems M1−xUxPd3 (M  Sc, La, Pr, Zr, Th).

Disorder and the quantum critical point in UCu5−xPdx

We have measured the magnetization of UCu5−x Pdx (x = 0.7, 1, 1.5, 2) in fields H as large as 30 T and for temperatures between 1.4 K and 150 K. A composition- and temperature- dependent characteristic field H ∗ (x, T ) is identified, which separates two different non-Fermi liquid scaling regimes, present even in zero field. For HH ∗ (x, T ), composition-independent scaling is found, while for HH ∗ (x, T ), different exponents are found for each composition. This second phase is centred in temperature and composition around the x = 1 ,T = 0 critical point, and we argue that it is consistent with the presence of a Griffiths phase. In recent years, it has been discovered that a zero-temperature critical point can be induced in a variety of metallic magnets by using pressure or compositional variation to suppress magnetic ordering temperatures to zero. The properties of this novel phase transition have prompted much experimental and theoretical work (1,2). The fundamental excitations near such quantum critical points lead to unusual temperature and energy dependences in measured quantities, which are said to display non-Fermi liquid (nFL) behaviour. There is mounting evidence that disorder plays an important role in these systems, especially very near the T = 0 critical point. However, there is little consensus about how profoundly disorder and nFL behaviour are interrelated, although a variety of models have been proposed. The Kondo disorder model argues that the nFL behaviour is associated with a distribution of Kondo temperatures TK due to disorder, with no need to invoke long-range magnetic correlations (3, 4). More recently, it has been proposed that nFL behaviour has its roots in the critical phenomena of the T = 0 magnetic transition, with disorder limiting the growth of magnetic correlations, similar in spirit to the case of finite temperature phase transitions (5, 6). Finally, it has been proposed that nFL behaviour is an intrinsic feature of a T = 0 magnet, and for much of the magnetic phase diagram is essentially unaffected by the presence of modest amounts of disorder (7, 8). Experimentally, the resolution of this conflict must lie with delineating parts of the phase diagram over which the magnetic correlations develop freely and those where they are limited by disorder. This requires the formulation of an experimental criterion for such a crossover, which we present here. Given this criterion, it will be possible to revisit the existing scaling studies (8-10) to assess whether the nFL behaviour is limited to the disorder-dominated regime or whether it is an intrinsic feature associated with the T = 0 critical point. We have studied the scaling properties of the magnetization of UCu5−xPdx over a very broad range of fields and temperatures for compositions both near and far from the critical composition. We confirm that