K. Houben
1
, S. Couet
2
, T. Peissker
1
, R. Lieten
1
, M. Trekels
2
, M. Bisht
2
, J. Jochum
1 , D. Pérez 2
,
B. Opperdoes 1, 2 , T. Picot 1 , D. G. Merkel 3 , A. Chumakov 3 , R. Rüffer 3 , M. Y. Hu 4 , J. Y. Zhao 4 ,
E. E. Alp
4
, S. Roelants
5
, B. Partoens
5
, F. Peeters
5
, A. Vantomme
2
, K. Temst
2
, M. J. Van Bael
1
1
Laboratory of Solid-State Physics and Magnetism, KU Leuven, Belgium
2
Instituut voor Kern- en Stralingsfysica, KU Leuven, Belgium
3
European Synchrotron Radiation Facility, (ESRF), BP 220, F-38043 Grenoble Cedex, France
4 Advanced Photon Source (APS), Argonne National Laboratory, Argonne, Illinois, 60439, USA
5
Departement Fysica, Universiteit Antwerpen, Groenenborgerlaan 171, B-2020 Antwerpen,
Belgium
Interatomic coupling in crystalline solids gives rise to collective vibrations of the atoms. The behavior of these atomic vibrations, i.e. phonons, influences many material properties, such as thermal and mechanical properties. Furthermore, the interaction of phonons with electrons is of crucial importance in conventional superconductivity. When reducing the system dimensions down to the nanometer scale, deviations in the phonon density of states (PDOS) with respect to the corresponding bulk PDOS are observed. These deviations are the result of phonon confinement effects and the appearance of surface phonon modes
1
.
Tin is known as a superconducting material with a bulk superconducting transition temperature
(T
C
) of 3.72 K. An increase in T
C
of up to 21% has been observed in Sn nanostructures 2 . These changes in T
C
are (partially) ascribed to changes in the phonon spectrum. While the phonon spectrum of bulk systems is well understood, considerably less is known about the vibrational behavior in nanostructures because of the difficulty of experimentally probing atomic vibrations at this scale.
How can atomic vibrations be detected in nanoscale samples? To what extent are phonon effects responsible for the observed phenomena, next to other possible causes such as electron confinement effects?
To measure the phonon spectrum, a special nuclear scattering technique using synchrotron radiation is used which probes specifically the
119
Sn isotope. This way, only the phonon contributions are probed allowing to disentangle phonon confinement effects from electron confinement effects.
Furthermore, Sn is an interesting material because of a structural transition which is little understood. The α- to β-Sn transition is very closely related to the atomic vibrations since it is mediated by the difference in vibrational entropy
3
. The Sn phase transition has been studied during an in situ experiment at the ESRF, which constitutes a very clean method of probing the
PDOS since oxidation and capping layers can be avoided. Measuring the PDOS at different times during the transition allows to obtain a high level of understanding in the processes involved in the phase transition. The PDOS of α-Sn layers, β-Sn islands and cluster-assembled

films have been studied at the ESRF and at the APS.
[1] B. Roldan Cuenya et al.
, Phys. Rev. B 64, 235321 (2001)
[2] B. Abeles et al.
, Phys. Rev. Lett. 17, 632 (1966)
[3] P. Pavone et al.
, Phys. Rev. B 57, 10421 (1998)
Figure 1: A: 7x7 µm AFM image of β-Sn islands on a Si(111) substrate; B: 1x1 µm AFM image of 10 nm thick α-Sn layer on an InSb substrate; C: 1x1 µm AFM image of a 50 nm thick β-Sn cluster-assembled film on a SiO
2
substrate