non-Fermi Liquid Regime Research Articles

Effects of transverse electron dispersion on photo-emission spectra of quasi-one-dimensional systems

The random phase approximation (RPA) spectral function of the one-dimensional electron band with the three-dimensional long range Coulomb interaction shows a broad feature which is spread on the scale of the plasmon energy and vanishes at the chemical potential. The fact that there are no quasi-particle $\delta$-peaks is the direct consequence of the acoustic nature of the collective plasmon mode. This behaviour of the spectral function is in the qualitative agreement with the angle resolved photo-emission spectra of some Bechgaard salts. In the present work we consider the modifications in the spectral function due to finite transverse electron dispersion. The transverse bandwidth is responsible for the appearance of an optical gap in the long wavelength plasmon mode. The plasmon dispersion of such kind introduces the quasi-particle $\delta$-peak into the spectral function at the chemical potential. The cross-over from the Fermi liquid to the non-Fermi liquid regime by decreasing the transverse bandwidth takes place through the decrease of the quasi-particle weight as the optical gap in the long wavelength plasmon mode is closing.

Magnetic and non-Fermi-liquid phases inU1−xYxPd2Al3

Measurements of the temperature and frequency dependence of the magnetic susceptibility Χ and the temperature and magnetic field dependence of the electrical resistivity p of the U 1 - x Y x Pd 2 Al 3 system were performed for yttrium concentrations in the range 0.1≤x≤0.85. The temperature versus yttrium concentration phase diagram is revised to include a region in which spin-glass freezing is observed for yttrium concentrations 0.25≤x≤0.65, adjacent to the antiferromagnetism previously observed at lower yttrium concentrations. Non-Fermi-liquid (NFL) behavior is found in the electrical resistivity for yttrium concentrations 0.65≤x≤0.85. The temperature dependence of the electrical resistivity in the NFL regime can be described by a power law of the form p(T)=p(0)[1-a(T/T 0 ) n ] with a value of n that changes from n1.5 for x≤0.75 to n1 for x=0.8 and 0.85. It is noteworthy that the abrupt decrease in n occurs at the value of x near the critical concentration x c r at which the spin-glass freezing temperature T S G vanishes, and that the room temperature and residual resistivities also drop sharply at this concentration x. This suggests that a change in the electronic structure is responsible for the suppression of T S G at x c r and the change in n may be determined by the quantum critical point.

Comparison of magnetic and NFL behavior in U1−xMxPd2Al3 (M=La, Y, Th)

The heavy fermion antiferromagnetic superconductor UPd2Al3 displays a number of interesting magnetic and non-Fermi liquid (NFL) ground states when the uranium site is partially occupied by Y, La, or Th. Whereas the magnetic behavior in these systems is determined by valence of the substituent atom, the temperature dependence of the electrical resistivity at low temperatures in the NFL regime correlates with the size of the substituent atom. The phase diagrams, electrical resistivity data, and the lattice parameters of the three systems are compared and contrasted.

Localization-Induced Griffiths Phase of Disordered Anderson Lattices

We demonstrate that local density of states fluctuations in disordered Anderson lattice models universally lead to the emergence of non-Fermi liquid (NFL) behavior. The NFL regime appears at moderate disorder ( W = Wc) and is characterized by power-law anomalies, e.g., C/T approximately 1/T((1-alpha)), where the exponent alpha varies continuously with disorder, as in other Griffiths phases. This Griffiths phase is not associated with the proximity to any magnetic ordering, but reflects the approach to a disorder-driven metal-insulator transition (MIT). Remarkably, the MIT takes place only at much larger disorder W(MIT) approximately 12Wc, resulting in an extraordinarily robust NFL metallic phase.

Non-Fermi liquid regimes in the low-temperature phase diagrams of the systems U 1− xM xPd 2Al 3 (M=Y,Th)

Temperature–composition ( T– x) phase diagrams and non-Fermi liquid (NFL) characteristics in the electrical resistivity, specific heat, and magnetic susceptibility at low temperatures for the U 1− x M x Pd 2Al 3 (M=Y,Th) systems are described. The T– x phase diagrams, the NFL characteristics, and the underlying mechanisms for the NFL behavior exhibit distinct differences for M=Y 3+ and Th 4+, apparently reflecting the difference in valence of the M atom substituents, and suggesting that U is tetravalent in these two systems.

Non-linear susceptibilities of Ce(Ru 1− xRh x) 2Si 2 and CeCu 5.9Au 0.1

We have studied the non-linear susceptibilities χ 3 of Ce(Ru 1− x Rh x ) 2Si 2 (single crystals) and CeCu 5.9Au 0.1 (polycrystals and single crystals). The ground state of Ce(Ru 1− x Rh x ) 2Si 2 varies with its Rh concentration from Fermi liquid (FL) to antiferromagnetic (with well-defined localized spin) through spin density wave (SDW) and non-Fermi liquid (NFL) regimes. The sign of χ 3 for Ce(Ru 1− x Rh x ) 2Si 2 at 1.8 K systematically changes, depending on the magnetic state: negative for x=0 (FL), positive for x=0.15 (SDW), negative for x=0.4 and 0.5 (NFL), and positive for x=0.7 (antiferromagnetic). χ 3 of CeCu 5.9Au 0.1 at 1.8 K is always negative. The temperature dependence of χ 3 for NFL samples, Ce(Ru 1− x Rh x ) 2Si 2 for x=0.4 and 0.5 and CeCu 5.9Au 0.1, seems to diverge at T=0 K.

Non-Fermi liquid behavior in Ni–Pd

Ni–Pd is investigated at the borderline from strongly enhanced paramagnetism to itinerant ferromagnetism. At approximately 2.6 at.% nickel a ferromagnetic quantum-critical point exists and the temperature dependence of the resistivity follows Δ ρ∝ T 5/3. In this paper we focus on the magnetic field dependence of the electrical resistivity close to this zero-temperature phase transition. The electrical resistivity reveals a transition from a non-Fermi liquid regime to a Landau Fermi-liquid, characterized by ρ= ρ 0+ AT 2 in a magnetic field of 2 T. At higher temperature long-range ferromagnetic order is induced.

Pair-kinetic-levels sum and difference in BCS model for generic systems with non-Fermi liquid states-I

The Bardeen–Cooper–Schrieffer (BCS) singlet-pairing model, now with spin-dependent electronic kinetic levels, is analysed in mean-field approximation. Outside the usual equikinetic-pair (EP) superconductivity, new pair-regimes are revealed as in some correlated electron systems (CES), which it now generally models. The essential new feature of this pair scheme is its conformity with Haldane's fractional exclusion principle. Application of the parametrized principle on the model, leads to novel realizations of non-Fermi liquid (NFL) states. The EP superconductivity, and the NFL regimes with non-equikinetic-pair (NEP) states, are governed by non-vanishing symmetry parameters: pair-kinetic-levels sum and difference, respectively. New NEP superconductivity arises from NFL normal state, the later exhibiting magnetism. The obtained pair-states are consistent with those experimentally reported for hole-doped cuprates and, more recently, for CePd 2Si 2, by the Cambridge–Hiroshima collaboration. Green's function analysis of the normal state NFL yields an energy spectrum leading to new semi-empirical formulae for the antiferromagnetic exchange and Neel temperature. Theory is in good qualitative and quantitative agreement with experiment on NFL YBa 2Cu 3O 6+ y and CePd 2(Al 1− x Ga x ) 3.

Unconventional properties of superconducting cuprates

We present an explanation of the unusual peak/dip/hump features observed in photoemission experiments on Bi2212 at T⪡ T c. We argue that the strong spin-fermion interaction combined with the feedback effect on the spin damping due to superconductivity yields a Fermi liquid form of the fermionic spectral function for ω<2 Δ where Δ is the maximum value of the superconducting gap, and a nonFermi liquid form for ω>2 Δ. In the Fermi liquid regime, the spectral function A(k F , ω) displays a quasiparticle peak at ω= Δ; in the nonFermi liquid regime it possesses a broad maximum at ω⪢ Δ. In between the two regimes, the spectral function has a dip at ω∼2 Δ.

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

The non-Fermi-liquid (NFL) behavior observed in the low temperature specific heat $C(T)$ and magnetic susceptibility $\chi(T)$ of f-electron systems is analyzed within the context of a recently developed theory based on Griffiths singularities. Measurements of $C(T)$ and $\chi(T)$ in the systems $Th_{1-x}U_{x}Pd_{2}Al_{3}$, $Y_{1-x}U_{x}Pd_3$, and $UCu_{5-x}M_{x}$ (M = Pd, Pt) are found to be consistent with $C(T)/T \propto \chi(T) \propto T^{-1+\lambda}$ predicted by this model with $\lambda <1$ in the NFL regime. These results suggest that the NFL properties observed in a wide variety of f-electron systems can be described within the context of a common physical picture.

Single-ion scaling of the low-temperature properties of f-electron materials with non-Fermi-liquid groundstates

Certain chemically substituted Ce and U compounds have low-temperature physical properties that exhibit non-Fermi-liquid (NFL) characteristics and apparently constitute a new class of strongly correlated f-electron materials. The NFL behaviour takes the form of weak power law or logarithmic divergences in the temperature dependence of the physical properties that scale with a characteristic temperature , which, in some systems, can be identified with the Kondo temperature . These systems have complex temperature T - chemical substituent composition x phase diagrams, which contain regions displaying the Kondo effect, NFL behaviour, spin glass freezing, magnetic order, quadrupolar order, and, sometimes, even superconductivity. Possible origins of the NFL behaviour include a multichannel Kondo effect and fluctuations of an order parameter in the vicinity of a second-order phase transition at T = 0 K. Recent experiments on the systems and are reviewed. In the and systems, the low-temperature physical properties in the NFL regime scale with the U concentration and , suggesting that single-ion effects are responsible for the NFL behaviour.