The van der Waals liquid orthoterphenyl has long been used as a model system in the study of the glass transition. Motivated by mode-coupling theory, extensive experiments have been undertaken to monitor the onset of structural relaxation on microscopic time and length scales. Using in particular quasielastic neutron scattering, the decay of density and tagged-particle correlations has been measured as a function of temperature, pressure and wave number. A consistent picture is developped in which the mode-coupling singularity appears as a change of transport mechanism in the moderately viscous liquid, at temperatures far above the conventional (caloric) glass transition.

The coherent dynamic structure factor of supercooled orthoterphenyl has been measured by time-of-flight neutron scattering. The data extend previous backscattering and spin-echo data towards shorter times. In the ␤ relaxation regime the two-step decay of density correlations can be described by the same master function as found before for self-correlations. We show that the mode-coupling crossover temperature T c can be determined from ␤ relaxation data alone. The wave-number dependence of the relaxation amplitude follows, to some extent, that of the static structure factor.

The incoherent dynamic structure factor of ortho-terphenyl has been measured by neutron timeof-flight and backscattering technique in the pressure range from 0.1 MPa to 240 MPa for temperatures between 301 K and 335 K. Tagged-particle correlations in the compressed liquid decay in two steps. The α-relaxation lineshape is independent of pressure, and the relaxation time proportional to viscosity. A kink in the amplitude fQ(P ) reveals the onset of β relaxation. The β-relaxation regime can be described by the mode-coupling scaling function; amplitudes and time scales allow a consistent determination of the critical pressure Pc(T ). α and β relaxation depend in the same way on the thermodynamic state; close to the mode-coupling cross-over, this dependence can be parametrised by an effective coupling Γ ∝ nT −1/4 . 64.70. Pf, 62.50.+p, 61.25.Em, The glass transition can be induced by decreasing the temperature or by increasing the pressure. For practical reasons, most experimental investigations concentrate on temperature effects at ambient pressure P 0 = 0.1 MPa, although there is a clear interest in studying supercooled liquid dynamics in the full P, T parameter space. Variable pressure measurements have been decisive in demonstrating that density is not the only driving force behind the glass transition [1][2][3]. In the microscopic approach of mode-coupling theory [4], both pressure and temperature control the dynamics through variations of the static structure factor. Close to the dynamic cross-over, the effect of these variations can be expressed through a single separation parameter which however is not given by density alone, as was confirmed by depolarised light scattering in the fragile liquid cumene [3].Here we report on incoherent neutron scattering in the fragile van der Waals liquid ortho-terphenyl (OTP, T g (P 0 ) = 243 K) at variable pressure. As function of temperature, the microscopic dynamics of OTP has been studied extensively by neutron scattering [5][6][7] [5,10] and from direct observation of fast β relaxation [6-9] a cross-over was located at T c (P 0 ) = 290 ± 5K [16].OTP (Aldrich) was purified by repeated crystallisation from liquid methanol and vacuum destillation. An Al7049.T6 high pressure cell of the Institut LaueLangevin (ILL) with a ratio of outer to inner diameter of 2 allowed us to attain up to 250 MPa at temperatures up to 335 K; helium was used to transmit the pressure.The experiments were performed using backscattering (IN16) and time-of-flight (IN6) spectrometers of the ILL. Incident wavelengths 6.27Å −1 and 5.12Å −1 gave resolutions (fwhm) of about 1 µeV and 80 µeV, respectively.Vanadium was used to calibrate the detectors and to yield the resolution functions. The measured transmission of the empty pressure cell at P 0 was about 84%, that of the sample about 88%. Self shielding and multiple scattering turned out be negligible at large scattering angles. This was demonstrated by comparison of spectra taken with and without the pressure cell at 290 K: the normalised intermediate ...

We present inelastic neutron-scattering results for AC 60 (AϭK,Rb,Cs͒ compounds. The spectra of the high-temperature fcc phases strongly resemble the ones of pristine C 60 in the plastic phase. At equal temperatures we find identical rotational diffusion constants for pristine C 60 and Rb 1 C 60 (D r ϭ2.4 10 10 s Ϫ1 at 400 K͒. The changes taking place in the inelastic part of the spectra on cooling AC 60 indicate the formation of strong intermolecular bonds. The buildup of intensities in the gap region separating internal and external vibrations in pure C 60 is the most prominent signature of this transition. The spectra of the low-temperature phases depend on their thermal history. The differences can be explained by the formation of a polymer phase ͑upon slow cooling from the fcc phase͒ and a dimer phase ͑upon fast cooling͒, respectively. The experimental data are analyzed on the basis of lattice dynamical calculations. The density-of-states are well modeled assuming a ͓2ϩ2͔ bond for the polymer and a single intercage bond for the dimer. Indications for different intercage bonding are also found in the internal mode spectra, which, on the other hand, react only weakly to the charge transfer. The dimer phase is metastable and converts into the polymer phase with a strongly temperaturedependent time constant. The transition from the polymer to the fcc phase is accompanied by inelastic precursor effects which are interpreted as the signature of inhomogeneities arising from plastic monomer regions embedded in the polymer phase. In the polymer phase AC 60 compounds show strong anharmonic behavior in the low-temperature region. The possible connection with the metal-to-insulator transition is discussed.