Europhys. Lett.
Volume 48, Number 2, October 1999
Page(s) 187 - 193
Section Condensed matter: structure, mechanical and thermal properties
Published online 01 September 2002
DOI: 10.1209/epl/i1999-00465-1

Europhys. Lett, 48 (2), pp. 187-193 (1999)

Excited-state vibrational tunnel splitting of hydrogen trapped by nitrogen in niobium

J. J. Rush 1, T. J. Udovic 1, N. F. Berk 1, D. Richter 2 and A. Magerl 3

1NIST Center for Neutron Research, National Institute of Standards and Technology
Gaithersburg, MD 20899, USA
2Institut für Festkörperforschung, Forschungszentrum Jülich GmbH
D-52425 Jülich, Germany
3Lehrstuhl für Kristallographie und Strukturphysik, Universität Erlangen-Nürnberg
Bismarckstrasse 10, D-91054 Erlangen, Germany

(received 25 September 1998; accepted in final form 30 August 1999)

PACS. 66.35${\rm +a}$ - Quantum tunneling of defects.
PACS. 34.20Cf - Interatomic potentials and forces.
PACS. 61.12Ex - Neutron scattering techniques (including small-angle scattering).


High-resolution inelastic-neutron-scattering spectra of hydrogen trapped by interstitial nitrogen impurities in $\rm NbN_{0.007}H_{0.004}$ have been measured to study the vibrational lineshape of the broad $\rm 105$ $\rm meV$ feature previously observed for $\rm NbN_{0.004}H_{0.003}$. The spectrum measured at $\rm 4.2\;K$ exhibits clear evidence of a doublet that disappears at higher temperatures. We interpret this result as the first spectroscopic observation of excited-state tunnel splitting of light-atom (H) vibrations in a solid-state double-well potential. We also show that both the observed excited-state splitting of $\rm 4.0 \pm 0.3\;meV$ and the previously measured ground-state splitting are well represented by a hardened-harmonic-well potential.


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