Low energy electrodynamics of the Kondo-lattice antiferromagnet CeCu2Ge2

Abstract

We present time-domain THz spectroscopy data of a thin film of the Kondo-lattice antiferromagnet CeCu2Ge2. The low frequency complex conductivity has been obtained down to temperatures below the onset of magnetic order. At low temperatures a narrow Drude-like peak forms, which is similar to ones found in other heavy fermion compounds that do not exhibit magnetic order. Using this data in conjunction with DC resistivity measurements, we obtain the frequency dependence of the scattering rate and effective mass through an extended Drude model analysis. The zero frequency limit of this analysis yields evidence for large mass renormalization even in the magnetic state, the scale of which agrees closely with that obtained from thermodynamic measurements.Kondo lattice systems in which localized f moments hybridize with extended conduction band electrons exhibit a rich variety of behavior. At high temperatures these systems behave as an ensemble of weakly interacting conduction electrons with modest masses and free fmoments. At low temperatures, the hybridization of local moments with the conduction electrons results in phenomena such as metallic states with charge carriers whose mass is hundreds of times the free electron mass (so called "heavy-fermion" behavior), superconductivity, and various interesting magnetic states [1][2][3]. Which physics is expressed at low temperature is determined by a delicate balance and competition between the Ruderman-KittelKasuya-Yosida (RKKY) type magnetic interaction and the hybridization of the 4f or 5f electrons with delocalized states [4]. Weak hybridization typically yields the RKKY-type interaction and magnetically ordered ground states. Strong hybridization, on the other hand, leads to the formation of fluctuating valence states, compensation of local moments, and heavy-electron metals. In the limiting case of very strong hybridization, the delocalized band electrons screen the localized f electrons, forming a singlet. Kondo lattice systems typically exhibit their heavy-electron effects below a temperature scale T * at which successive scattering from local moments starts to be in-phase and collective effects can appear.The competition between Kondo screening and the RKKY interaction is exhibited dramatically in the evolution of the electronic structure by Si substitution in the CeCu 2 Ge 2−x Si x Kondo-lattice series or by the application of pressure to the pure compound. Due to its smaller ionic size, Si substitution can be seen as a equivalent to increasing pressure. Pure CeCu 2 Ge 2 (CCG) is a Kondo-lattice system that exhibits an unusual temperature dependence of the resistivity caused by crystal field splitting and magnetic interactions at low temperatures [5][6][7]. An upturn of the resistivity around 25 K indicates the increasing effect upon cooling of scattering of the conduction electrons off the local moments. At approximately a temperature T * ∼ 6 K, the resistivity reaches a maximum, giving evidence for a low Kondo lattice temperature scale, before ordering antiferr...

Field-induced magnetic instability and quantum criticality in the antiferromagnet CeCu2Ge2

“…In all the resistivity curves, two prominent features, which are typical to heavy-fermion systems, have been observed. The broad peak around 100 K is caused by crystal-field splitting, and the other peak at 6 K gives evidence for the Kondo-lattice coherence temperature T * , which corresponds to the formation of heavy quasiparticles20. At lower temperatures below T N  ≈ 4 K, the resistivity drops rapidly.…”

Anomalous frequency and temperature-dependent scattering and Hund's coupling in the almost quantum critical heavy-fermion systemCeFe2Ge2

“…These fits are highly constrained by the use of both THz and dc data. Implicit in this fitting is the reasonable assumption that there are no spectral peaks in σ 1 below the lower end of our measured range [14,27,28]. The fact that we cannot fit our data with a single term Drude model demonstrates that the transport cannot be described by a single energy-independent relaxation time.…”

“…It exhibits a metamagnetic anomaly at 300 kOe [10] and is also believed to be close to a quantum critical point. This behavior is reminiscent of other 4f-electron systems: CeRu 2 Si 2 [11], CeNi 2 Ge 2 [12] and CeCu 6 [13,14]. In addition to metamagnetic anomalies, deviations from ordinary FL behavior occur under certain circumstances [15][16][17] in these materials.…”

CeCu2Ge2: Challenging our understanding of quantum criticality

“…[14,15] According to terahertz spectroscopy, the development of a Kondo lattice in CeCu 2 Ge 2 leads to an enhancement, by one-order of magnitude, in the carrier effective-mass upon cooling from 3 K to values approaching 80 m 0 at ∼ 1 K, where m 0 is the free-electron mass. [16] Although CeCu 2 Si 2 displays a superconducting ground-state, [15,17] claimed to be unconventional and "magnetically mediated" due to its proximity to an antiferromagnetic (AF) QCP (tuned by pressure or composition), [18] CeCu 2 Ge 2 develops long-range incommensurate AF-order below a Néel temperature T N 4.1 K. [19] In the CeCu 2 (Ge x Si 1−x ) 2 series T N is observed to decrease continuously from 4.1 K (for x = 1) to 1 K (for x = 0). [20] In CeCu 2 Si 2 the type, or even the presence of AF-order (or superconductivity), is markedly dependent on the sample synthesis protocol.…”