Critical quasiparticle theory applied to heavy fermion metals near an antiferromagnetic quantum phase transition

Abstract

We use the recently developed critical quasiparticle theory to derive the scaling behavior associated with a quantum critical point in a correlated metal. This is applied to the magnetic-field induced quantum critical point observed in YbRh 2 Si 2 , for which we also derive the critical behavior of the specific heat, resistivity, thermopower, magnetization and susceptibility, the Grüneisen coefficient, and the thermal expansion coefficient. The theory accounts very well for the available experimental results.dynamical scaling | non-Fermi-liquid properties | spin excitation spectrum | non-Gaussian fluctuations R ecent advances in low-temperature experimental techniques have stimulated much interest in quantum critical phenomena, which comprise phase transitions at zero temperature ("quantum critical point") and associated effects due to quantum fluctuations at very low temperatures. Ref. 1 gives an introductory review of the subject.These developments have generated a variety of difficult theoretical questions; among them is the issue of how to treat the regime of strongly interacting quantum fluctuations. In a recent paper (2), we developed an extension of the quasiparticle concept of Fermi liquid theory to the non-Fermi-liquid regime near a quantum critical point (QCP). In essence, the theory goes beyond the Gaussian regime of critical fluctuations by introducing interactions among the quantum fluctuations into the correlation function of the fluctuations. Central to the analysis is the concept of critical quasiparticles, which is based on the recognition that the single-particle spectral function can display a quasiparticle peak at nonzero excitation energy or temperature. This is expressed as a nonzero quasiparticle weight ZðωÞ for jωj not too small, although, as in a non-Fermi liquid, at the Fermi surface Zðω ¼ 0Þ ¼ 0.We realized the critical quasiparticle theory for the case of an antiferromagnetic quantum critical point and applied it to several quantities, principally resistivity and specific heat, for successful comparison to experimental results on the heavy-fermion metal YbRh 2 Si 2 (YRS), thereby showing that the theory, which describes a physically transparent scenario, is capable of accounting for experimental results on a quantum critical metal.The implementation of the theory for an antiferromagnetic (AFM) quantum critical point for a heavy-fermion compound is based on the recognition that below a "lattice Kondo temperature" T KL , hybridization between conduction (s, p, d) electrons and local magnetic moments (f orbitals) produces a heavyelectron liquid with an associated mass enhancement due to the originally localized character of the f electrons. However, the f electrons are also responsible for the antiferromagnetism in a region of the phase diagram of the material. Near the AFM critical point, critical spin fluctuations are enhanced and interact with the heavy quasiparticles. This produces further mass enhancement and occurs in two stages. Above a certain temperature T x but below T KL , the...

Magnetic structure of the antiferromagnetic Kondo lattice compounds CeRhAl4Si2 and CeIrAl4Si2

“…Neutrons of wavelength 2.413Å, selected by a vertically focusing a Ge(113) monochromator, were used for the experiment. The calculated 1/e absorption length for this wavelength is 7.3 mm and 2.5 mm for CeRhAl 4 Si 2 and CeIrAl 4 Si 2 , respectively, matching the diameters of the selected aluminum cans. For both compounds, powder diffraction patters were collected for 6 hours for each of the two magnetic phases as well as slightly above the higher temperature magnetic phase.…”

“…In addition, the temperature variation of the intensity of the strongest magnetic peak was also measured to obtain the temperature dependence of the magnetic order parameter. At each temperature, the peak intensity was measured for three and six minutes for CeRhAl 4 Si 2 and CeIrAl 4 Si 2 , respectively. Finally, several NPD patterns were collected while cooling from room temperature (100 K) to the base temperature of 3.2 K in the case of CeRhAl 4 Si 2 (CeIrAl 4 Si 2 ) to check for structural distortions.…”

“…Bulk thermal, transport and magnetic properties reveal two low-temperature antiferromagnetic phases for CeRhAl 4 Si 2 (T N 1 = 14 K and T N 2 = 9 K) and CeIrAl 4 Si 2 (T N 1 = 16 K and T N 2 = 14 K) [19,21]. The effective magnetic moment of both CeRhAl 4 Si 2 and CeIrAl 4 Si 2 obtained in the paramagnetic region are close to Ce 3+ Hunds' Rule value (2.54 µ B ), indicating localized behavior of the Ce 4f moments at high temperature.…”

“…Similarly, there is no long-range magnetic order in two dimensions [2]. In f -electron materials, where magnetic and electronic degrees of freedom are typically strongly coupled, the effects of dimensionality are currently under debate [3,4,5]. Notably, it has been proposed that dimension can be used to tune the type of observed quantum phase transitions, where either the electronic or magnetic degrees of freedom become quantum critical [3,6,7].…”

“…This calls for new classes of f -electron materials in which the dimensionality can be systematically tuned. We recently synthesized CeM Al 4 Si 2 (M = Rh, Ir, Pt) compounds that are promising in this regard [19]. These materials crystallize in KCu 4 S 3 structure type with tetragonal space group P4/mmm (# 123), and exhibit a sequential stacking of a BaAl 4 -type Ce-containing layer separated by a AuAl 2 -type M -containing block along the c-axis (Fig.…”