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Struktur und Dynamik Poster: Mi., 14:00–16:30 M-P165
Opening of the SC Gap observed in Neutron Spectroscopy
Frank Weber 1,2 , Rolf Heid 1 , Andreas Kreyssig 3 , Lothar Pintschovius 1 ,
Dmitry Reznik 1,4 , Wilfried Reichardt 1 , Oliver Stockert 5 , Klaudia Hradil 6
1 Forschungszentrum Karlsruhe, Institut für Festkörperphysik, D-76021 Karlsruhe, Germany
– 2 Physikalisches Institut, Universität Karlsruhe (TH), D-76128 Karlsruhe, Germany
– 3 Institut für Festkörperphysik, TU Dresden, D-01062 Dresden, Germany
(present adress: Ames Laboratory, USA) – 4 Laboratoire Léon Brillouin, C.E. Saclay,
F-99191 Gif-sur-Yvette, France – 5 Max-Planck-Institut für Chem. Physik fester Stoffe,
D-01187 Dresden, Germany – 6 Institut für phys. Chemie, Universität Göttingen,
Aussenstelle FRM II, D-85747 Garching, Germany
It was previously noted that acoustic phonons in YNi2B2C [1] and LuNi2B2C [2] with
wavevectors q ≈ (0.5,0,0) undergo strong changes of their lineshapes upon entering the
superconducting (SC) state. However, no convincing interpretation of the data could
be given.
Here, we present inelastic neutron scattering data on YNi2B2C (Tc = 15 K). We made
a systematic study of the already known phonon anomaly in the (100)-direction as well
as of a so far unexplored anomaly at the zone boundary in the (110)-direction (Mpoint).
Our data unambiguously show that the superconductivity-induced changes of
the spectral function of phonons with a strong electron-phonon coupling can extremely
well be understood in the framework of a theory proposed by Allen et al. [3]. The
analysis yields the temperature dependent SC energy gap with high accuracy. As a
consequence, even deviations from BCS like behavior can be assessed with confidence.
Further, we found that the Sc gap extracted from the phonon data for q=(0.5,0,0)
and q=(0.5,0.5,0), respectively, differs by a factor 1.4. This is a direct proof for the
long discussed anisotropy of the SC energy gap in borocarbides (e.g., Ref. [4]). A
prerequisite for the observation of the gap anisotropy is to select phonons for which
the dominant coupling to the quasiparticles stems from a relatively small part of the
Fermi surface. Our choice was guided by DFT calculations [5] which predict both the
phonon frequencies and linewidths very well.
[1] Kawano et al., PRL. 77, 4628 (1996),
[2] Stassis et al., PRB 55, R8678 (1997),
[3] Allen et al., PRB 56, 5552 (1997),
[4] Izawa et al., PRL 89, 137006 (2002),
[5] Reichardt et al., J. Superconductivity, 10.1007 (2005)