Unusual Normal State Research Articles

The two-channel Kondo route to non-Fermi-liquid metals

We present a pedagogical and critical overview of the two-channel Kondo model and its possible relevance to a number of non-Fermi-liquid alloys and compounds. We survey the properties of the model, how a magnetic two-channel Kondo effect might obtain for ions in metals, and a quadrupolar Kondo effect for ions in metals. We suggest that the incoherent metal behaviour of the two-channel Kondo-lattice model may be useful in understanding the unusual normal-state resistivity of and speculate that the residual resistivity and entropy of the two-channel lattice paramagnetic phase might be removed by either antiferromagnetic (or antiferroquadrupolar) ordering or by a superconducting transition to an odd-frequency pairing state.

Non-Fermi liquid ground states in strongly correlated f-electron materials (invited) (abstract)

Strongly correlated electron materials, especially the high transition temperature (Tc) cuprate superconductors and heavy fermion rare earth and actinide compounds, have been the focus of intense investigation in recent years. The unusual normal state properties of the high Tc cuprates have been ascribed by some researchers to a non-Fermi liquid (NFL) metallic state, while the anomalous properties of certain heavy fermion U-based superconductors such as UBe13 have been attributed to NFL behavior due to a two-channel quadrupolar Kondo effect. Although NFL behavior has not been established in any of the U-based heavy fermion superconductors, evidence for its occurrence in an f-electron material has been found in the U alloy system Y1−xUxPd3. Subsequently, NFL behavior was observed in a number of other f-electron alloy systems, including Sc1−xUxPd3, UCu3.5Pd1.5, Th0.1U0.9Be13, Th1−xUxRu2Si2 (x≤0.07), Th1−xUxPd2Al3, La0.9Ce0.1Cu2Si2, and CeCu5.9Au0.1. Evidence for NFL behavior in Y1−xUxPd3 and related systems is reviewed and discussed within the context of possible microscopic mechanisms. Some systematics of the NFL low temperature behavior observed in several f-electron materials include a linear temperature dependence of the electrical resistivity ρ∼1−aT with the coefficient a either ≳0 or <0, a logarithmically diverging specific heat C/T∼−ln T, and T1/2 asymptotic behavior of the magnetic susceptibility χ∼1−T1/2.

Charge-spin separation, spin-excitation spectrum, and the normal-state properties of the copper oxide superconductors.

We show that the unusual normal-state properties of the copper oxide superconductors can be derived from the one-band t-J model with the use of a rigorously imposed constraint of single-electron occupancy and the Wigner-Jordan representation of the spin-excitation spectrum. These include the linear resistivity in the ab plane, the 1/T resistivity along the c direction, the anomalous spin-relaxation rate in the Cu site, the excessive absorption in the infrared conductivity over that expected from the Drude theory, the flat electronic Raman background, the broadening of the photoemission spectra, and the linear-bias dependence of the normal-metal--normal-metal tunneling conductance (along the ab plane). The zero-bias tunneling conductance along the ab plane is predicted to have a linear temperature dependence. A particularly interesting prediction is that the ratio of the linear temperature term of the zero-bias conductance to the linear-bias term of the zero-temperature conductance is 2T/\ensuremath{\Vert}eV\ensuremath{\Vert}, independent of all other parameters. The static spin susceptibility at low temperature is predicted to have the form A+B ln(${\mathrm{\ensuremath{\omega}}}_{\mathit{c}}$/T), where ${\mathrm{\ensuremath{\omega}}}_{\mathit{c}}$\ensuremath{\sim}J is a cutoff energy. The doping (x) dependences of various physical quantities are also predicted for the range of x large enough to allow the holons to form a band and the spins to have a disordered (liquid) ground state, but not too large to destroy the antiferromagnetic spin correlation. The imaginary part of the spin susceptibility has a wave-vector-independent component with an x dependence of x(1-x${)}^{2}$. The nuclear relaxation at the Cu site, 1/(${\mathit{TT}}_{1}$), has a 1/T component with an x dependence of x(1-x${)}^{2}$. The zero-bias tunneling conductance along the ab plane at zero temperature increases with ${\mathit{x}}^{2}$. The 1/T resistivity along the c direction at low temperature is inversely proportional to x, and the linear resistivity of the ab plane has a weak x dependence in the doping range mentioned above.

Antiferromagnetic spin fluctuations and high- T C superconductivity

A consistent picture for cuprate oxide superconductors emerges from the view-point of antiferromagnetic spin fluctuations in two-dimension. In particular, anomalous normal state properties are well explained by the self-consistent renormalization theory of spin fluctuations. This theory naturally describes a crossover from fermi-liquid regime at low temperatures to the unusual normal state at higher temperatures. The crossover temperature is small and consistent with experiments. Superconductivity due to the spin fluctuations leads to an anisotropic pairing with B 1 g symmetry. The transition temperature estimated by the strong coupling theory is of the right order of magnitude.

Uranium-based heavy-fermion superconductors: an experimental survey

The uranium-based heavy-fermion superconductors were discovered almost one decade ago. Here, we present an experimental survey of their interesting normal and superconducting state properties. It appears that most of the unusual normal-state properties can be attributed to the proximity of an antiferromagnetic instability and the presence of competing electronic interactions. The discovery of a superconducting instability in these strongly-correlated electron systems came totally unexpected. The parameters describing the superconducting state yield strong deviations from the standard BCS behaviour. Accumulating evidence has been gathered for a nontrivial superconducting pair function ( L ≠ 0). We illustrate recent developments by a number of prime studies, like high-field measurements and alloying experiments, and give special attention to multicomponent superconductivity in UPt 3 and (U, Th)Be 13.

Rigorous lower bounds on the ground-state energy of correlated Fermi systems.

We show that the thermodynamic potential E — pN of an itinerant interacting Fermi system is bounded below by the sum of the thermodynamic potentials of small subsystems into which the infinite system is partitioned. Exact lower bounds for the energy in the thermodynamic limit are therefore easily obtainable by diagonalization of small clusters. In one dimension, these bounds are remarkably close to Bethe-ansatz results for the Hubbard model. We present lower bounds for the two-dimensional Hubbard and t-J Hamiltonians. Such bounds may serve as rigorous tests for approximate treatments of this class of problems. Interest in correlated Fermi systems has been heightened in recent years by the suggestion' that strong local electronic interactions could play an important role in the unusual normal-state properties of the oxide superconductors, and possibly in the pairing mechanism itself. Unfortunately, few rigorous results are available to us for the simplest microscopic models thought to describe such systems: the Hubbard model and its strong-coupling limit, the t-J model. In one spatial dimension the Betheansatz solution of Lieb and Wu provides the simplest information on equilibrium thermodynamic and magnetic properties. This approach has recently been extended by many authors to calculate correlation functions. Anderson has suggested that the solution of the twodimensional (2D) problem may possess many features of the Lieb-Wu result, in particular an algebraic singularity in the momentum distribution function at the Fermi surface (Z =0).

Far-infrared reflectivity ofLa2−xSrxCuO4

We have measured and modeled the far-infrared reflectivity of La/sub 2-//sub x/Sr/sub x/CuO/sub 4/ for x = 0.175 in the normal and superconducting states in the frequency range 20 to 350 cm/sup -1/. We find a much stronger drop of the reflectivity with increasing frequency below 100 cm/sup -1/ than is implied by the dc conductivity. In addition, we observe a resonance of large spectral weight at ..omega..approx. =250 cm/sup -1/ which is absent or much weaker in the insulating x = 0 compound. Below the superconducting temperature of the x = 0.175 sample we observe an additional sharp feature at approx. =80 cm/sup -1/, which can be accounted for in terms of the unusual normal-state properties of the material.