Universal low-temperature anomalies of amorphous solids

5 problems

• Origin of the boson peak in glasses

  Contested

  Glasses have more low-frequency vibrations than a crystal would, which shows up as a bump in the vibration spectrum near one terahertz called the boson peak. Which structural feature creates this excess is disputed.

• Frequency law of sound damping in glasses below the boson peak

  Contested

  At sound frequencies approaching a terahertz, sound in glasses loses energy by scattering off structural irregularities. Whether the damping grows as the fourth power of frequency, as for small random obstacles, or carries an extra logarithmic factor from long-range correlations of the elastic properties, is disputed.

• What atoms tunnel in the two-level systems of amorphous silica?

  Open

  The tunneling model assumes small groups of atoms that flip between two nearly equal positions. Which atoms move, and how many, has never been identified in any glass.

• Can two-level systems be removed by making glasses more stable?

  Partially resolved

  Glasses grown very slowly, for example by vapor deposition, can be far more stable than ordinary glasses, and some show far fewer tunneling defects or none detectable. Whether oxide glasses can be made free of them, and how fast their number drops with stability, is open.

• Why is sound absorption by tunneling defects the same in all glasses?

  Open

  In every glass tested, sound waves lose nearly the same small fraction of their energy per cycle at low temperature, within a factor of about twenty. The standard tunneling model fits this but gives no reason why the number should be universal.