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The unusual metallic behavior of heavy-fermion compounds at low temperatures is caused by mobile charge carriers with a large effective mass. This mass enhancement (compared to normal electrons) goes hand in hand with a reduction of the transport scattering time, which can directly be studied with optical spectroscopy: the characteristic Drude roll-off moves to very low frequencies.
Here we combine microwave and THz spectroscopy to study thin films of the heavy-fermion compound UNi2Al3 in a broad frequency range. At frequencies of less than 1 cm-1, a full Drude response indicates the dynamics of the heavy electrons in UNi2Al3. Surprisingly, at considerably higher frequencies (around 10 cm-1) we observe a similar structure that is very reminiscent of Drude behavior. We interpret these two features as the Drude response of - at low frequencies - correlated, slow electrons and - at higher frequencies - uncorrelated, fast electrons. The temperature dependence and anisotropy of these two Drude roll-offs correspond to each other. These results also shed new light on previous studies of the related material UPd2Al3 and of heavy-fermion compounds in general. |
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