Point-contact Andreev reflection spectroscopy of heavy-fermion-metal/superconductor junctions

Park, W.K.; Stalzer, H.; Sarrao, J. L.; Thompson, J. D.; Pham, Long; Fisk, Z.; Frederick, J.; Canfield, P. C.; Greene, L. H.

doi:10.1016/j.physb.2007.10.229

Cited by 5 publications

(5 citation statements)

References 24 publications

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“…3 shows the point contact spectra of CeRhIn 5 and YbAl 3 at 10 K. 23 For CeRhIn 5 , we take the quasiparticle energy ⑀ 0 = 1.5 meV which is approximately T ‫ء‬ = 20 K, known from Hall anomaly and other experimental probes. 3 The best fit with a = 0.5 results in a zerobias resonance width ␥ ϳ 6.8 meV, a reasonable value if we take into account the broadening at a temperature of half T ‫ء‬ .…”

Section: ͑6͒mentioning

confidence: 99%

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Fano effect in the point contact spectroscopy of heavy-electron materials

Yang

2009

Phys. Rev. B

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We show that Fano interference explains how point contact spectroscopy in heavy-electron materials probes the emergence of the Kondo heavy-electron liquid below the same characteristic temperature T ‫ء‬ as that seen in many other experiments and why the resulting measured conductance asymmetry reflects the universal Kondo liquid behavior seen in these. Its physical origin is the opening of a new channel for electron tunneling beyond that available from the background conduction electrons. We derive the Fano formula with a mean-field slave boson approach for the Kondo lattice model and generalize it to finite temperature and realistic situation by introducing empirical parameters. The resulting simple expression for the Fano interference provides a good fit to the experimental results for CeCoIn 5 , CeRhIn 5 , and YbAl 3 , over the entire range of bias voltages, and deduce a lifetime of the heavy quasiparticle excitations that agrees well with recent state-of-the-art numerical calculations.Heavy-electron materials have a rich phase diagram showing a competition between antiferromagnetism and unconventional superconductivity and quantum critical behavior. Although the underlying physics responsible for this competition is still unclear, it appears to be primarily associated with the heavy electron, or Kondo liquid, that emerges from the collective hybridization of light conduction electrons with the local f moments. 1-4 Understanding the nature and consequence of this hybridization is therefore a central task in the field of heavy-electron physics.Point contact spectroscopy ͑PCS͒ ͑Ref. 5͒ probes the lowenergy collective excitations ͑such as phonons͒ and may be expected to provide important information for our understanding of the low-energy physics of heavy-electron materials. For superconductors, PCS provides a quantitative measure of the Andreev reflection and helps determine the superconducting order parameters and the paring mechanism. 6 Especially, a universal asymmetry has been observed in the point contact tunneling experiments of high-T C superconductors, 7 in contrast to what is expected for Bardeen-Cooper-Schrieffer-type superconductors.For decades, a similar conductance asymmetry has also been observed in many heavy-electron materials such as CeCu 6 ͑Ref. 8͒ and URu 2 Si 2 ͑Ref. 9͒ but its origin has not been understood. For example, the fact that the asymmetry is practically independent of the material of the metallic tips suggests that it is intrinsic and excludes previous explanations based on self-heating effects. 10 On the other hand, the usual tunneling model requires an unrealistic background density of states ͑DOS͒ to explain the experimental data. In CeCoIn 5 , the DOS derived from it has a broad maximum below the Fermi energy, 11 in contradiction with the theoretical expectations and numerical calculations that show a sharp quasiparticle peak developing at low temperatures well above the Fermi energy.An essential clue to the underlying physics comes from two recent observations concerning the p...

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Section: ͑6͒mentioning

confidence: 99%

“…CeRhIn 5 and YbAl 3 .-Fig. 3 shows the point contact spectra of CeRhIn 5 and YbAl 3 at 10 K [26]. For CeRhIn 5 , we take the quasiparticle energy ǫ 0 = 1.5 meV which is approximately T*=20 K known from Hall anomaly and other experimental probes [3].…”

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confidence: 99%

Fano effect in the point contact spectroscopy of heavy-electron materials

Yang

2009

Phys. Rev. B

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“…AR in heavy-fermion-metal/conventional superconductor. Despite some success reported in CeCoIn 5 /Nb point contacts, the magnitude of the AR signal in such junctions is not well determined due to the intervening oxide layer on the Nb tip [137]. Further measurements on such junctions but without oxide interface layers will provide important clues to many issues on AR in heavy fermions.…”

Section: Remaining Issues and Future Workmentioning

confidence: 97%

Andreev reflection and order parameter symmetry in heavy-fermion superconductors: the case of CeCoIn₅

Park

Greene

2009

J. Phys.: Condens. Matter

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We review the current status of Andreev reflection spectroscopy on the heavy fermions, mostly focusing on the case of CeCoIn(5), a heavy-fermion superconductor with a critical temperature of 2.3 K. This is a well-established technique to investigate superconducting order parameters via measurements of the differential conductance from nanoscale metallic junctions. Andreev reflection is clearly observed in CeCoIn(5) as in other heavy-fermion superconductors. Considering the large mismatch in Fermi velocities, this observation seemingly appears to disagree with the Blonder-Tinkham-Klapwijk (BTK) theory. The measured Andreev signal is highly reduced to the order of maximum ∼13% compared to the theoretically predicted value (100%). The background conductance exhibits a systematic evolution in its asymmetry over a wide temperature range from above the heavy-fermion coherence temperature down to well below the superconducting transition temperature. Analysis of the conductance spectra using the extended BTK model provides a qualitative measure for the superconducting order parameter symmetry, which is determined to be the d(x(2)-y(2)) wave in CeCoIn(5). It is found that existing models do not quantitatively account for the data, which we attribute to the intrinsic properties of the heavy fermions. A substantial body of experimental data and extensive theoretical analysis point to the existence of two-fluid components in CeCoIn(5) and other heavy-fermion compounds. A phenomenological model is proposed employing a Fano interference effect between two conductance channels in order to explain both the conductance asymmetry and the reduced Andreev signal. This model appears plausible not only because it provides good fits to the data but also because it is highly likely that the electrical conduction occurs via two channels, one into the heavy-electron liquid and the other into the conduction electron continuum. Further experimental and theoretical investigations will shed new light on the mechanism of how the coherent heavy-electron liquid emerges out of the Kondo lattice, a prototypical strongly correlated electron system. Unresolved issues and future directions are also discussed.

show abstract

“…AR in heavy-fermion-metal/conventional superconductor. Despite some success reported in CeCoIn 5 /Nb point contacts, the magnitude of AR signal in such junctions is not well determined due to the intervening oxide layer on the Nb tip [137]. Further measurements on such junctions but without oxide interface layers will provide important clues to many issues on AR in heavy fermions.…”

Section: Remaining Issues and Future Workmentioning

confidence: 97%

Andreev reflection and order parameter symmetry in heavy-fermion superconductors: the case of CeCoIn$_5$

Park,

Greene

2009

Preprint

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We review the current status of Andreev reflection spectroscopy on the heavy fermions, mostly focusing on the case of CeCoIn 5 , a heavy-fermion superconductor with a critical temperature of 2.3 K. This is a well-established technique to investigate superconducting order parameters via measurements of the differential conductance from nanoscale metallic junctions. Andreev reflection is clearly observed in CeCoIn 5 as in other heavy-fermion superconductors. Considering the large mismatch in Fermi velocities, this observation seemingly appears to disagree with the Blonder-Tinkham-Klapwijk (BTK) theory. The measured Andreev signal is highly reduced to the order of maximum ∼ 13% compared to the theoretically predicted value (100%). The background conductance exhibits a systematic evolution in its asymmetry over a wide temperature range from above the heavy fermion coherence temperature down to well below the superconducting transition temperature. Analysis of the conductance spectra using the extended BTK model provides a qualitative measure for the superconducting order parameter symmetry, which is determined to be d x 2 −y 2wave in CeCoIn 5 . It is found that existing models do not quantitatively account for the data, which we attribute to the intrinsic properties of the heavy fermions. A substantial body of experimental data and extensive theoretical analysis point to the existence of two fluid components in CeCoIn 5 and other heavy-fermion compounds. A phenomenological model is proposed employing a Fano interference effect between two conductance channels in order to explain both the conductance asymmetry and the reduced Andreev signal. This model appears plausible not only because it provides good fits to the data but also because it is highly likely that the electrical conduction occurs via two channels, one into the heavy electron liquid and the other into the conduction electron continuum. Further experimental and theoretical investigations will shed new light on the mechanism of how the coherent heavy-electron liquid emerges out of the Kondo lattice, a prototypical strongly correlated electron system. Unresolved issues and future directions are also discussed.

show abstract

Point-contact Andreev reflection spectroscopy of heavy-fermion-metal/superconductor junctions

Cited by 5 publications

References 24 publications

Fano effect in the point contact spectroscopy of heavy-electron materials

Fano effect in the point contact spectroscopy of heavy-electron materials

Andreev reflection and order parameter symmetry in heavy-fermion superconductors: the case of CeCoIn₅

Andreev reflection and order parameter symmetry in heavy-fermion superconductors: the case of CeCoIn$_5$

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Point-contact Andreev reflection spectroscopy of heavy-fermion-metal/superconductor junctions

Cited by 5 publications

References 24 publications

Fano effect in the point contact spectroscopy of heavy-electron materials

Fano effect in the point contact spectroscopy of heavy-electron materials

Andreev reflection and order parameter symmetry in heavy-fermion superconductors: the case of CeCoIn5

Andreev reflection and order parameter symmetry in heavy-fermion superconductors: the case of CeCoIn$_5$

Contact Info

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Andreev reflection and order parameter symmetry in heavy-fermion superconductors: the case of CeCoIn₅