For a successful point-contact spectroscopy (PCS) measurement, metallic tips of proper shape and smoothness are essential to ensure the ballistic nature of a point-contact junction. Until recently, the fabrication of Au tips suitable for use in point
-contact spectroscopy has remained more of an art involving a trial and error method rather than an automated scientific process. To address these issues, we have developed a technique with which one can prepare high quality Au tips reproducibly and systematically. It involves an electronic control of the driving voltages used for an electrochemical etching of a gold wire in an HCl-glycerol mixture or an HCl solution. We find that a stopping current, below which the circuit is set to shut off, is a single very important parameter to produce an Au tip of desired shape. We present detailed descriptions for a two-step etching process for Au tips and also test results from PCS measurements using them.
The intermediate valence compound YbAl$_3$ is known to undergo a hybridization process between itinerant and localized electrons. The resulting heavy Fermi liquid remains non-magnetic and non-superconducting. A microscopic understanding of the hybrid
ization process in YbAl$_3$ is still lacking although some characteristic temperature and energy scales have been identified. Here we report results from novel spectroscopic measurements based on quasiparticle scattering. From the conductance spectra taken over a wide temperature range, we deduce that the band renormalization and hybridization process begins around 110 K, causing the conductance enhancement with a Fano background. This temperature, a new scale found in this work, is much higher than the coherence temperature (34 K). Our observation is in agreement with the slow crossover scenario discussed recently in the literature. The indirect hybridization gap appears to open concomitantly with the emergence of a coherent Fermi liquid. Thus, we suggest its measurement as a more rigorous way to define the coherence temperature than just taking the temperature for a resistivity peak.
The nature of the second order phase transition that occurs in URu2Si2 at 17.5 K remains puzzling despite intensive research over the past two and half decades. A key question emerging in the field is whether a hybridization gap between the renormali
zed bands can be identified as the long-sought hidden order parameter. We report on the measurement of a hybridization gap in URu2Si2 employing a spectroscopic technique based on quasiparticle scattering across a ballistic metallic junction. The differential conductance exhibits an asymmetric double-peak structure, a clear signature for a Fano resonance in a Kondo lattice. The extracted hybridization gap opens well above the transition temperature, indicating that it is not the hidden order parameter. Our results put stringent constraints on the origin of the hidden order transition in URu2Si2 and demonstrate that quasiparticle scattering spectroscopy can probe the band renormalizations in a Kondo lattice via detection of a novel type of Fano resonance.
Our previous point-contact Andreev reflection studies of the heavy-fermion superconductor CeCoIn$_5$ using Au tips have shown two clear features: reduced Andreev signal and asymmetric background conductance [1]. To explore their physical origins, we
have extended our measurements to point-contact junctions between single crystalline heavy-fermion metals and superconducting Nb tips. Differential conductance spectra are taken on junctions with three heavy-fermion metals, CeCoIn$_5$, CeRhIn$_5$, and YbAl$_3$, each with different electron mass. In contrast with Au/CeCoIn$_5$ junctions, Andreev signal is not reduced and no dependence on effective mass is observed. A possible explanation based on a two-fluid picture for heavy fermions is proposed. [1] W. K. Park et al., Phys. Rev. B 72 052509 (2005); W. K. Park et al., Proc. SPIE-Int. Soc. Opt. Eng. 5932 59321Q (2005); W. K. Park et al., Physica C (in press) (cond-mat/0606535).
