Glasses and other noncrystalline solids exhibit thermal and acoustic properties at low temperatures anomalously different from those found in crystalline solids, and with a remarkable degree of universality. Below a few kelvin, these universal properties have been successfully interpreted using the tunneling model, which has enjoyed (almost) unanimous recognition for decades. Here we present low-temperature specific-heat measurements of ultrastable glasses of indomethacin that clearly show the disappearance of the ubiquitous linear contribution traditionally ascribed to the existence of tunneling twolevel systems (TLS). When the ultrastable thin-film sample is thermally converted into a conventional glass, the material recovers a typical amount of TLS. This remarkable suppression of the TLS found in ultrastable glasses of indomethacin is argued to be due to their particular anisotropic and layered character, which strongly influences the dynamical network and may hinder isotropic interactions among low-energy defects, rather than to the thermodynamic stabilization itself. G lasses or amorphous solids are well known (1, 2) to exhibit thermal and acoustic properties very different from those of their crystalline counterparts. Even more strikingly, many of these properties are very similar for any glass, irrespective of the type of material, chemical bonding, etc. Hence the low-temperature properties of noncrystalline solids are said to exhibit a universal "glassy behavior." In particular, below 1−2 K the specific heat of glasses depends approximately linearly on temperature, C p ∝T, and the thermal conductivity almost quadratically, κ ∝ T 2 , in clear contrast with the cubic dependences successfully predicted by Debye theory for crystals. In addition, a broad maximum in C p /T 3 [originated from the so-called "boson peak" in the reduced vibrational density of states g(ω)/ω 2 ] is also typically observed in glasses around 3−10 K, as well as a universal plateau in the thermal conductivity κ(T) in the same temperature range (1, 2).Very soon after the seminal paper by Zeller and Pohl (1) in 1971, Phillips (3) and Anderson et al. (4) independently introduced the well-known standard tunneling model (TM). The fundamental idea of the TM is the ubiquitous existence of atoms or small groups of atoms in amorphous solids due to the intrinsic atomic disorder, which can perform quantum tunneling between two configurations of very similar energy, usually named twolevel systems (TLS). This simple model was able to account for the abovementioned thermal and acoustic anomalies of glasses below 1−2 K, and soon acquired unanimous recognition. Only very few authors (5) posed then criticisms against the standard TM, pointing out how improbable it was that a random ensemble of independent tunneling states would produce essentially the same universal constant for the thermal conductivity or the acoustic attenuation in any substance. Indeed, significant discrepancies with the TM below ∼100 mK have also been reported (6-9), in particular ...

The two most prominent and ubiquitous features of glasses at low temperatures, namely the presence of tunneling two-level systems and the so-called boson peak in the reduced vibrational density of states, are shown to persist essentially unchanged in highly stabilized glasses, contrary to what was usually envisaged. Specifically, we have measured the specific heat of 110 million-year-old amber samples from El Soplao (Spain), both at very low temperatures and around the glass transition Tg. In particular, the amount of two-level systems, assessed at the lowest temperatures, was surprisingly found to be exactly the same for the pristine hyperaged amber as for the, subsequently, partially and fully rejuvenated samples.

The layered rare-earth diantimonides RSb 2 are anisotropic metals with generally low electronic densities whose properties can be modified by substituting the rare earth. LaSb 2 is a nonmagnetic metal with a low residual resistivity presenting a low-temperature magnetoresistance that does not saturate with the magnetic field. It has been proposed that the latter can be associated to a charge density wave (CDW), but no CDW has yet been found. Here we find a kink in the resistivity above room temperature in LaSb 2 (at 355 K) and show that the kink becomes much more pronounced with substitution of La by Ce along the La 1−x Ce x Sb 2 series. We find signatures of a CDW in x-ray scattering, specific heat, and scanning tunneling microscopy (STM) experiments in particular for x ≈ 0.5. We observe a distortion of rare-earth-Sb bonds lying in-plane of the tetragonal crystal using x-ray scattering, an anomaly in the specific heat at the same temperature as the kink in resistivity and charge modulations in STM. We conclude that LaSb 2 has a CDW which is stabilized in the La 1−x Ce x Sb 2 series due to substitutional disorder.

We present the application of a fast quasi-adiabatic continuous method to the measurement of specific heat at 4 He temperatures, which can be used for the study of a wide range of materials.The technique can be performed in the same configuration used for the relaxation method, as the typical time constants between calorimetric cell and thermal sink at 4.2 K are chosen to be of the order of 30 s. The accuracy in the absolute values have been tested by comparing them to relaxation-method results obtained in the same samples (performed in situ using the same set-up), with a deviation between the absolute values < 3% in the whole temperature range. This new version of the continuous calorimetric method at low temperatures allows us to completely characterize and measure a sample within a few hours with a high density of data points, whereas when employing other methods we typically need a few days. An exhaustive study has beenperformed for reproducibility to be tested. In the present work, we have applied this method to two different substances: CeSb 2 , which exhibits three magnetic transitions at 15.5 K, 11.7 K and 9.5 K, and graphite, both highly-oriented pyrolytic graphite (HOPG) and natural crystals. Our results on these graphites are discussed in comparison with previous published data on different kinds of graphite samples.

We have measured the specific heat of amber from the Dominican Republic, an ancient geological glass about 20 million years old, in the low-temperature range 0.6 K ≤ T ≤ 26 K, in order to assess the effects of its natural stabilization (hyperageing) process on the low-temperature glassy properties, i.e. boson peak and two-level systems. We have also conducted modulated differential scanning calorimetry experiments to characterize the thermodynamic state of our samples. We found that calorimetric curves exhibit a huge ageing signal ΔH ≈ 5 J g(-1) in the first upscan at the glass transition Tg = 389 K, that completely disappears after heating up (rejuvenating) the sample to T = 395 K for 3 h. To independently evaluate the phonon contribution to the specific heat, Brillouin spectroscopy was performed in the temperature range 80 K ≤ T ≤ 300 K. An expected increase in the Debye level was observed after rejuvenating the Dominican amber. However, no significant change was observed in the low-temperature specific heat of glassy amber after erasing its thermal history: both its boson peak (i.e., the maximum in the Cp/T(3) representation) and the density of tunnelling two-level systems (i.e., the Cp ∼ T contribution at the lowest temperatures) remained essentially the same. Also, a consistent analysis using the soft-potential model of our Cp data and earlier thermal-conductivity data found in the literature further supports our main conclusion, namely, that these glassy 'anomalous' properties at low temperatures remain essentially invariant after strong relaxational processes such as hyperageing.