Heavy Electron Systems Research Articles

Properties of model-valence-fluctuation heavy-electron systems in applied and molecular fields.

Self-consistent large-orbital-degeneracy perturbation theory for the one-site Anderson model is used to compute the occupancy and magnetization as a function of field and temperature for both spin (1/2 and spin (5/2. The magnetization curves are in good agreement with exact Bethe-ansatz and high-field perturbation-theory results, although some systematic discrepancies are apparent between the two different approaches. The most novel result for the magnetization is that the ``superlinear'' behavior known for high-degeneracy impurities (and observed in YbCuAl) vanishes at precisely the temperature where the zero-field magnetic susceptibility peaks. The one-impurity results are extended to the lattice via perturbation theory in the intersite interactions, after which simple molecular field theory (excluding the possibilities of charge-density wave and superconducting phases) is applied to obtain magnetic-ordering phase boundaries for the spin-(1/2 and spin-(5/2 models which are in gross qualitative agreement with previous lattice calculations, including the so-called ``Kondo necklace'' and ``resonant level'' models. In particular, the criterion that the intersite coupling exceed the characteristic Kondo-effect energy scale is recovered. It is shown that the superlinear behavior obtained for high-degeneracy leads, within molecular field theory, to novel tricritical behavior without the inclusion of anisotropy terms. It is suggested that ${\mathrm{EuRh}}_{3}$${\mathrm{B}}_{2}$ and YbCuAl are likely candidates for observing this behavior. Detailed appendices indicate the approximations necessary to obtain the molecular field theory and thereby its limitations to finite temperatures away from the coherent regime of the Anderson lattice.

Specific Heat due to Spin Fluctuations in Nearly Ferromagnetic and Heavy Electron Metals

The specific heat due to extended (long wavelength) paramagnons is expressed by using four parameters associated with the band structure and the exchange enhancement factor. The expansion form γT+δT3ln T+βT3 of this expression is shown to be valid only at extremely low temperatures outside the range of existing measurements, while the full expression seems to explain the results of measurements. In the localized paramagnon limit, as may be the case in certain heavy electron systems, We find a T3 ln T-lime behavior in a certain temperature range around a characteristic temperature.

Summary and Survey (Heavy Electron Systems)

Summary and Survey (Heavy Electron Systems)

Experimental Perspectives on Heavy-Electron Metals

We motivate the description of heavy-electron metals in terms of concepts from the Kondo problem. These concepts are used to discuss magnetism and superconductivity in heavy-electron systems. Particular attention is given to what we view as the principal outstanding questions in this field and directions in which the field is developing. This will include consideration of the differences between Ce and U heavy-electron compounds as well as the occurrence of very small ordered magnetic moments.

μSR-Study of CeH2.7

Metallic CeH2.7 shows a sizably increased electronic specific heat of γ=100 mJ/mol·K2, compared to γ=0.04 mJ/mol·K2 in LaH2.7 at low tempratures, which could qualify it as another heavy electron system. μSR Knight shift and relaxation studies were started on a powderous CeH2.7 sample in order to test this possibility further. It is found that the relaxation rate λT depends strongly on the applied field strength. These values indicate that the relaxation must be of electronic origin. Strong irreproducibilities at low temperature point to pronounced hysteretic features, which might be related to structural changes. The Knight shift shows a shallow peak around 150 K but no conspicious increase at low temperatures.

First direct evidence for a magnetic phase transition in the heavy-electron system UCd11

Muon-spin-rotation and relaxation measurements performed on polycrystalline UCd11 give first direct evidence for an antiferromagnetic phase transition at 5 K. As far as probed by the positive muon the behaviour close to the phase transition is clearly contrasted to that in other magnetic heavy-electron materials investigated by μ+SR, e.g. U2Zn17 and UCu5, because in UCd11 longitudinal field measurements reveal dynamic relaxation effects which are attributed to the critical slowing down of the uranium 5f-moments. No signal could be observed in the ordered regime because of a large dipolar field spread.

A comparative study of the magnetic heavy-electron materials U2Zn17 and UCu5 by μ+SR

A systematic μ+SR study of the magnetic heavy-electron systems U2Zn17 and UCu5 in the paramagnetic and in the magnetically ordered state is presented. In both systems the antiferromagnetic nature of the low-temperature phase could be at least partially confirmed, but the muon reveals significant differences with regard to the phase transition itself. UCu5 behaves like a model-antiferromagnet showing a drastic increase of the relaxation rate both below and above TN, two spontaneous frequencies in the ordered phase, and a Knight shift above TN which scales with the bulk susceptibility. In contrast U2Zn17 shows a loss of μ+ asymmetry by 20% below TN, which is independent of the external field but can be quenched in sufficiently strong longitudinal fields. No scaling of the Knight shift and the susceptibility was observed and no critical increase of the relaxation rate λ. Most astonishing is the strong and nonlinear field dependence of λ above and below TN in both compounds. The absence of longitudinal relaxation demonstrates the static origin of λ.

Incoherent Kondo ground state in CeCu 1.54Si 1.46

Measurements of the magnetic susceptibility, magnetization, specific heat and electrical resistivity on a new Kondo lattice compound CeCu 1.54Si 1.46 have revealed an antiferromagnetic phase transition at 6.9 K. The analysis of the specific heat demonstrates that this compound is a moderately heavy electron system with strong spin fluctuations. Based on the resistivity result, we maintain that the coherence between Kondo states at Ce sites is hindered by the disorder in the Cu and Si sublattice in this non-stoichiometric compound.

Fermi Liquid Theory on the Basis of the Periodic Anderson Hamiltonian

T-linear term of specific heat and magnetic susceptibility at zero temperature are derived for the heavy electron systems, and relations among these quantities are discussed on the basis of the Fermi liquid theory. Further, a rigorous expression of the T2-term of resistivity at low temperatures is also obtained on the basis of Kubo formula. It is shown that the coefficient of the T2-term arising from the electron interaction is strongly enhanced though it vanishes because of the momentum conservation for a free electron system possessing no crystal lattice.

Partially gapped Fermi surface in the heavy-electron superconductor URu2Si2.

Transport, thermal, and magnetic data for the heavy electron system ${\mathrm{URu}}_{2}$${\mathrm{Si}}_{2}$ indicate that a charge- or spin-density-wave transition opens an energy gap of \ensuremath{\sim}11 meV over a portion of the Fermi surface below ${\mathrm{T}}_{0}$\ensuremath{\approxeq}17.5 K and demonstrate that bulk superconductivity occurs below ${\mathrm{T}}_{\mathrm{c}}$\ensuremath{\approxeq}1.5 K. The pressure dependences of ${\mathrm{T}}_{0}$ and ${\mathrm{T}}_{\mathrm{c}}$ support this interpretation. The unusually large initial slope of the upper critical magnetic field (9.2 T/K) is consistent with the high values of the electronic-specific-heat coefficient and the electrical resistivity.

Superconductivity and magnetism in heavy-electron U intermetallics (invited)

Recent work intended to investigate a Fermi-liquid-type low-temperature state in uranium intermetallic compounds is reviewed. In form of a comparison of physical properties with those of CeAl3, the prototype heavy-electron system, some details concerning the occurrence of this heavy-electron state are discussed. Special attention is then given to experimental data giving evidence for superconductivity or magnetic order, both involving these heavy electrons. The possibility of unconventional superconductivity in these systems is supported by increasing experimental evidence.

Normal-state properties of heavy-electron systems.

A generalization to finite temperatures of the Brinkman-Rice theory of almost-localized-electron systems is presented. It describes a smooth transition from a Fermi-liquid regime at low temperatures to a set of random spins at higher temperatures. The present theory gives a good description of the low-temperature entropy and the specific heat in the normal state of the heavy-electron superconductor ${\mathrm{UBe}}_{13}$.

Superconductivity and magnetism in heavy-electron systems

In certain intermetallic compounds containing rare-earth (mostly Ce) or actinide (mostly U) ions, the low-temperature thermal- and transport properties are reminiscent of those of a Fermi liquid with heavy-mass quasiparticles. The recent discovery of superconductivity or magnetic order in such substances and involving the heavy-mass electrons has led to a renewed interest as to what kind of interactions trigger these phase transitions and how the superconducting or the magnetically ordered states may be characterized. Recent work and achievements in this field are reviewed.

Magnetic Ground State of a Heavy-Electron System:U2Zn17

Measurements of the magnetic susceptibility, specific heat, and electrical resistivity of ${\mathrm{U}}_{2}$${\mathrm{Zn}}_{17}$ reveal a magnetic phase transition of this compound at 9.70 K. Details of the specific heat indicate that the ordering occurs among electrons whose energy spectrum shows an anomalously large density of states at the Fermi energy. This result is of interest with regard to the occurrence of superconductivity in similar systems.

Impurities in the heavy-fermion superconductor UBe13 (invited)

Small amounts of Sc, Lu, Gd, Np, Ce, Th, La, and Ba have been substituted for uranium in UBe13 to observe their effects on the superconducting and normal state properties. The thorium, which was the most complete study, resulted in an extremely unusual nonmonotonic depression of the transition temperature for a nonmagnetic impurity. This comes from an interplay that exists between the lowest temperature resistivity peak and the transition temperature, as the peak is depressed. These results suggest that heavy Fermion superconductivity is only one of the possible ground states for heavy mass electron systems. All of the impurities tested resulted in a transition temperature depression.