M. A. Ramos

Concentration and temperature dependence of the refractive index of ethanol-water mixtures: Influence of intermolecular interactions

The temperature and concentration dependence of the refractive index, nD(x, T) , in ethanol-water mixtures agrees with previous data in the ethanol-rich concentration range. The refractive index versus concentration x determined at 20 °C shows the expected maximum at about 41 mol% water (22 mass% water). The temperature derivative of the refractive index, dnD/dT, shows anomalies at lower water concentrations at about 10 mol% water but no anomaly at 41 mol% water. Both anomalies are related to intermolecular interactions, the one in nD seems to be due to molecular segregation and cluster formation while the origin of the second one in dnD/dT is still not clear.

Suppression of tunneling two-level systems in ultrastable glasses of indomethacin

Significance Glasses are disordered solids usually obtained by supercooling a liquid bypassing crystallization. A remarkable feature of glasses is that, independently of their nature and composition, they exhibit universal properties in the low-temperature range. Of interest here, the specific heat is characterized by a linear term below 1 K, ascribed to quantum tunneling between two states of similar energy. We have investigated if this ubiquitous behavior also applies to so-called “ultrastable glasses,” directly synthesized from the vapor phase into low-energy positions of the potential-energy landscape. Interestingly, we find a full suppression of the linear term in the specific heat, which questions the current view of the popular tunneling model and sheds light on the microscopic origin of two-level systems in glasses. 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 two-level 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. This explanation may lend support to the criticisms by Leggett and others [Yu CC, Leggett AJ (1988) Comments Condens Matter Phys 14(4):231–251; Leggett AJ, Vural DC (2013) J Phys Chem B 117(42):12966–12971] to the standard tunneling model, although more experiments in different kinds of ultrastable glasses are needed to ascertain this hypothesis.

Two-level systems and boson peak remain stable in 110-million-year-old amber glass.

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.

Thermodynamic study of alkyl-cyclohexanes in liquid, glassy, and crystalline states

The specific heat of some alkyl-cyclohexanes in their liquid, supercooled liquid, crystalline, and (for the first time) glassy states has been measured by quasiadiabatic calorimetry. Thermodynamic properties as well as the glass forming ability have been studied as a function of systematic changes of the molecular structure. Only one stable crystalline phase is observed experimentally for ethylcyclohexane, propylcyclohexane, and butylcyclohexane. In the case of methylcyclohexane, experimental evidence is provided of a crystal-to-crystal transition at temperatures just below the melting.

Thermodynamic and structural properties of the two isomers of solid propanol

Abstract We have measured the specific heat, Cp, of the two chemical isomers of propanol, both at low temperatures (1.5–30 K) and around the glass-transition region (≈100 K ) . Although the two substances present only the stereoisomeric difference, their glass-transition temperatures differ ( T g ≃98 K for 1-propanol and T g ≃115 K for 2-propanol), though with comparable changes in specific heat. At temperatures below 10 K, both isomers in the glassy state have the typical maximum in Cp/T3. Nevertheless, the magnitude of the low-temperature specific heat is much larger in 2-propanol than in 1-propanol, a difference which is also found in the Debye contributions in the corresponding crystalline states. To investigate the structural differences between the two isomers of propanol, and their possible influence on the thermodynamic properties, X-ray scattering experiments have been performed on samples of the glassy and crystalline states. In the glassy state, both isomers present the expected amorphous pattern, with characteristic distances of 0.40±0.06 and 0.42±0.07 nm for 1- and 2-propanol, respectively. Preliminary structural analysis of the crystalline states would assign a triclinic structure for 1-propanol and a monoclinic structure for 2-propanol.

HighT c superconductive materials: Bulk or twinned domain/grain boundary percolative network superconductors?

We have studied the physical properties of Y−Ba−Cu−Oxide superconducting materials by using Levitation, AC-susceptibility, macroscopic conductivity, Scanning Tunneling Microscopy (STM) local conductivity, Scanning Electron Microscope (SEM), X-rays, and Hall effect experimental techniques. Our results tend to indicate systematically that the grains formed in the synthesis do not show bulk superconductivity but rather are superconductors at the domain boundaries of the orthorhombic phase. It seems that a coexistence of semiconductor and metallic regions are formed at the twinned domain boundaries. The pellets become superconductors when the grains form clusters and are in intimate contact. This seems to suggest that the bulk of the grains is semiconducting and that a conducting percolative network of grain and domain boundaries may be responsible for the superconductivity. To understand the observed constant high transition temperature we propose a model of semiconductor-metal-semiconductor boundaries that give rise to superconductivity in a model like that of Little, Ginzburg, and Allender-Bray-Bardeen (1).

Low-temperature specific heat and thermal conductivity of glycerol

We have measured the thermal conductivity of glassy glycerol between 1.5 and 100 K, as well as the specific heat of both glassy and crystalline phases of glycerol between 0.5 and 25 K. We discuss both low-temperature properties of this typical molecular glass in terms of the soft-potential model. Our finding of an excellent agreement between its predictions and experimental data for these two independent measurements constitutes a robust proof of the capabilities of the soft-potential model to account for the low-temperature properties of glasses in a wide temperature range.

Are the high Tc superconducting materials bulk superconductors or grain boundary percolating network superconductors? (abstract)

We have studied the physical properties of high Tc Y‐Ba‐Cu‐Ox superconducting materials with levitation, ac susceptibility, macroscopic resistivity, resistivity as measured by scanning tunneling microscopy (STM) and Hall effect. Levitation experiments show that the powder of the as‐prepared material does not levitate at liquid nitrogen while pellets and powders that have been heated above 450 °C do levitate (are superconducting). These experiments seem to indicate that clustering and intimate contact of fine grains are necessary for levitating. The ac susceptibility experiments show that diamagnetism is extremely sensitive to pellet density. The higher the density and the smaller the field amplitude the less diamagnetic is the system. This is interpreted as evidence for a surface, not a bulk effect. Resistivity at 17 °C measured with macroscopic contacts is of the order of 10 Ω cm. When measured with microscopic STM contacts, a clear semiconducting behavior is observed. This observation does not preclude ...

Are the calorimetric and elastic Debye temperatures of glasses really different

Below 1 K, the specific heat C p of glasses depends approximately linearly on temperature T, in contrast with the cubic dependence observed in crystals, and which is well understood in terms of the Debye theory. That linear contribution has been ascribed to the existence of two-level systems as postulated by the tunnelling model. Therefore, a least-squares linear fit C p = C 1 T +C 3 T 3 has been traditionally used to determine the specific-heat coefficients, although systematically providing calorimetric cubic coefficients exceeding the elastic coefficients obtained from sound-velocity measurements, that is C 3 > C Debye. Nevertheless, C p still deviates from the expected CDebye (T) ∝ T 3 dependence above 1 K, presenting a broad maximum in C p / T 3 which originates from the so-called boson peak, a maximum in the vibrational density of states g(ν)/ν2 at frequencies ν ≈ 1 THz. In this work, it is shown that the apparent contradiction between calorimetric and elastic Debye temperatures long observed in glasses is due to the neglect of the low-energy tail of the boson peak (which contributes as C p ∝ T 5, following the soft-potential model). If one hence makes a quadratic fit C p = C 1 T + C 3 T 3 + C 5 T 5 in the physically meaningful temperature range, an agreement C 3 ≈ CDebye is found within experimental error for several studied glasses.

Superconductivity and magnetism on flux-grown single crystals of NiBi3

We present resistivity, magnetization, and specific-heat measurements on flux-grown single crystals of NiBi