Dynamics of heavy fermions: Drude response in and

While the effective mass of heavy fermions governs their thermodynamics, the optical properties are dominated by the characteristic relaxation rate which is expected to scale inversely with the effective mass. At the relaxation rate clear features, the so-called Drude response occur in the real and imaginary parts of the complex conductivity. Conventional optical spectroscopy can only indirectly probe the Drude response; thus we use novel broadband microwave spectroscopy to directly measure the frequency-dependent conductivity of UPd2Al3 and UNi2Al3 in the relevant frequency range and unambiguously observe the full low-energy electrodynamics of the heavy fermions including the Drude respons…

Microwave spectroscopy on heavy-fermion systems: probing the dynamics of charges and magnetic moments

Investigating solids with light gives direct access to charge dynamics, electronic and magnetic excitations. For heavy fermions, one has to adjust the frequency of the probing light to the small characteristic energy scales, leading to spectroscopy with microwaves. We review general concepts of the frequency-dependent conductivity of heavy fermions, including the slow Drude relaxation and the transition to a superconducting state, which we also demonstrate with experimental data taken on UPd2Al3. We discuss the optical response of a Fermi liquid and how it might be observed in heavy fermions. Microwave studies with focus on quantum criticality in heavy fermions concern the charge response, …

Extremely slow Drude relaxation of correlated electrons

The electrical conduction of metals is governed by how freely mobile electrons can move throughout the material. This movement is hampered by scattering with other electrons, as well as with impurities or thermal excitations (phonons). Experimentally, the scattering processes of single electrons are not observed, but rather the overall response of all mobile charge carriers within a sample. The ensemble dynamics can be described by the relaxation rates, which express how fast the system approaches equilibrium after an external perturbation1,2,3. Here we measure the frequency-dependent microwave conductivity of the heavy-fermion metal UPd2Al3 (ref. 4), finding that it is accurately described…

Back Cover: Microwave spectroscopy on heavy-fermion systems: Probing the dynamics of charges and magnetic moments (Phys. Status Solidi B 3/2013)

Fabry-Perot resonances in birefringent YAlO_3 analyzed at terahertz frequencies

The optical conductivity of heavy fermions can reveal fundamental properties of the charge carrier dynamics in these strongly correlated electron systems. Here we extend the conventional techniques of infrared optics on heavy fermions by measuring the transmission and phase shift of THz radiation that passes through a thin film of UNi2Al3, a material with hexagonal crystal structure. We deduce the optical conductivity in a previously not accessible frequency range, and furthermore we resolve the anisotropy of the optical response (parallel and perpendicular to the hexagonal planes). At frequencies around 7cm^-1, we find a strongly temperature-dependent and anisotropic optical conductivity t…

Observing the anisotropic optical response of the heavy-fermion compound UNi2 Al3

The optical conductivity of heavy fermions can reveal fundamental properties of the charge carrier dynamics in these strongly correlated electron systems. Here we extend the conventional techniques of infrared optics on heavy fermions by measuring the transmission and phase shift of THz radiation that passes through a thin film of UNi 2 Al 3 , a material with hexagonal crystal structure. We deduce the optical conductivity in a previously not accessible frequency range, and furthermore we resolve the anisotropy of the optical response (parallel and perpendicular to the hexagonal planes). At frequencies around 7 cm -1 , we find a strongly temperature-dependent and anisotropic optical conducti…

Influence of impurity scattering on Drude response in heavy-fermion UPd2Al3

The frequency-dependent conductivity of heavy-fermion metals can often be described within the picture of the Drude response: the transport relaxation rate is the only relevant frequency scale and, furthermore, reduced by orders of magnitude compared to normal metals. While the relaxation-time enhancement corresponds to the effective-mass enhancement in these materials, i.e. a fundamental material characteristic, the absolute value of the relaxation time depends on the details of the relevant scattering processes. Here we discuss the influence of impurity scattering on the Drude response of the heavy fermions in UPd2Al3 by comparing different thin film samples.

Direct observation of Drude behavior in the heavy-fermion by broadband microwave spectroscopy

Abstract Previous optical studies on the heavy-fermion system UPd 2 Al 3 down to frequencies of about 1 cm - 1 ( = 30 GHz ) revealed a well-pronounced pseudogap at low frequencies (below 3 cm - 1 ) that was attributed to magnetic correlations. Thus, the optical conductivity at even lower frequencies is of notable interest because the Drude roll-off (the high-frequency characteristic of a metal which will give information on the quasiparticle dynamics) remained hidden at extremely low frequencies. Using a novel cryogenic broadband microwave spectrometer employing the Corbino geometry we have studied the complex optical conductivity of UPd 2 Al 3 thin films in the frequency range from 45 MHz …

Low-temperature microwave response of heavy-fermion compounds

The electrodynamic properties of heavy fermions are distinct from those of normal metals due to the reduced transport relaxation rate that goes hand in hand with the enhanced mass. Using broadband microwave spectroscopy on thin-film samples of the heavy-fermion materials UPd2Al3 and UNi2Al3, we find that the frequency-dependent conductivity of these compounds at low temperatures follows a simple Drude prediction. The observed relaxation rates in the GHz frequency range are extremely low for a metal.

Terahertz conductivity of the heavy-fermion compound UNi2Al3

We have studied the optical properties of the heavy-fermion compound UNi${}_{2}$Al${}_{3}$ at frequencies between 100 GHz and 1 THz (3 and 35 cm${}^{\ensuremath{-}1}$), temperatures between 2 and 300 K, and magnetic fields up to 7 T. From the measured transmission and phase shift of radiation passing through a thin film of UNi${}_{2}$Al${}_{3}$, we have directly determined the frequency dependence of the real and imaginary parts of the optical conductivity (or permittivity, respectively). At low temperatures the anisotropy of the optical conductivity along the $a$ and $c$ axes is about 1.5. The frequency dependence of the real part of the optical conductivity shows a maximum at low temperat…

Magnetoresistance and Phase Diagram of Thin-Film UNi2Al3

We study the dc resistivity of UNi2Al3 thin films as a function of temperature and magnetic field. We focus on the temperature range around the antiferromagnetic transition (TN \approx 4 K in zero applied field). From a clear signature of TN in the dc resistance along the crystallographic a-direction, we extract the shape of the magnetic phase diagram. Here we find quantitative differences in comparison to previous studies on bulk crystals.