Low-energy optical phonons induce glassy-like vibrational and thermal anomalies in ordered crystals

Abstract

It is widely accepted that structural glasses and disordered crystals exhibit anomalies in the their thermal, mechanical and acoustic properties as manifestations of the breakdown of the long-wavelength approximation in a disordered dissipative environment. However, the same type of glassy-like anomalies (i.e. boson peak in the vibrational density of states (VDOS) above the Debye level, peak in the normalized specific heat at $T\simeq 10K$ etc) have been recently observed also in perfectly ordered crystals, including thermoelectric compounds. Here we present a theory that predicts these surprising effects in perfectly ordered crystals as a result of low-lying (soft) optical phonons. In particular, it is seen that a strong boson peak anomaly (low-energy excess of modes) in the VDOS can be due almost entirely to the presence of low-energy optical phonons, provided that their energy is comparable to that of the acoustic modes at the Brillouin zone boundary. The boson peak is predicted also to occur in the heat capacity at low $T$. In presence of strong damping (which might be due to anharmonicities in the ordered crystals), these optical phonons contribute to the low-$T$ deviation from Debye's $T^3$ law, producing a linear-in-$T$ behavior which is typical of glasses, even though no assumptions of disorder whatsoever are made in the model. These findings are relevant for understanding and tuning thermal transport properties of thermoelectric compounds, and possibly for the enhancement of electron-phonon superconductivity.

References in corpus (18)

• Evidence for superconductivity above 260 K in lanthanum superhydride at megabar pressures
• Phonon-glass electron-crystal thermoelectric clathrates: Experiments and theory
• Fractal free energy landscapes in structural glasses
• Harmonic Vibrational Excitations in Disordered Solids and the "Boson Peak"
• Anharmonicity, vibrational instability and Boson peak in glasses
• Approximate analytical description of the nonaffine response of amorphous solids
• The evolution of vibrational excitations in glassy systems
• Universal origin of boson peak vibrational anomalies in ordered crystals and in amorphous materials
• Local inversion-symmetry breaking controls the boson peak in glasses and crystals
• Growing timescales and lengthscales characterizing vibrations of amorphous solids
• Breakdown of Elasticity in Amorphous Solids
• Light Bipolarons Stabilized by Peierls Electron-Phonon Coupling
• Perspective: Gardner Physics in Amorphous Solids and Beyond
• Emergence of glassy features in halomethane crystals
• Glassy, Gardner-like Phenomenology in Minimally Polydisperse Crystalline Systems
• Glassy anomalies in the low-temperature thermal properties of a minimally disordered crystalline solid
• Atomic theory of viscoelastic response and memory effects in metallic glasses
• Disentangling $\alpha$ and $\beta$ relaxation in orientationally disordered crystals with theory and experiments

Cited by in corpus (9)

• Unified theory of vibrational spectra in amorphous materials
• Theory of the phonon spectrum in host-guest crystalline solids with avoided crossing
• Scale invariant solids
• Stringlet Excitation Model of the Boson Peak
• Black Rubber and the Non-linear Elastic Response of Scale Invariant Solids
• A new paradigm for the low-T glassy-like thermal properties of solids
• Glassy heat capacity from overdamped phasons and a hypothetical phason-induced superconductivity in incommensurate structures
• Secondary relaxation in the terahertz range in 2-adamantanone from theory and experiments
• Quantum field theory at finite temperature for 3D periodic backgrounds