Heat travels in solids thanks to phonons. In clean non-magnetic crystalline insulators, thermal conductivity is governed by Umklapp phonon-phonon collisions and phonons become ballistic at cryogenic temperatures. The thermal conductivity peaks at an intermediate temperature where most phonon-phonon collisions cannot produce entropy. Recently, a phonon thermal Hall signal has been observed in numerous insulators. In all cases, the thermal Hall angle is maximum at this peak temperature, and its maximum amplitude does not exceed an intriguing upper bound independent of the phonon mean-free-path. I will argue that a plausible explanation of this experimental observation is to be found beyond the adiabatic and the harmonic approximations. Combined with anharmonicity, the breakdown of the Born-Oppenheimer approximation in a magnetic field can offer a geometric phase to acoustic phonons. The expected bound to thermal Hall angle is close to what is seen experimentally in black P, Ge and Si.
Monday 1
Session 1
Talk 2
Space 1401
Optical Control of the Thermal Conductivity in ferroelectrics and charge density wave materials
1- Institut de Ciència de Materials de Barcelona, ICMAB–CSIC, 08193 Bellaterra, Spain
2- Universitat Politècnica de Catalunya, Barcelona 08034, Spain
3 - Luxembourg Institute of Science and Technology (LIST), L-4362 Esch/Alzette, Luxembourg
4- Humboldt-Universität zu Berlin, Zum Großen Windkanal 2, 12489 Berlin, Germany
5- Instituto de Nanociencia y Materiales de Aragón (INMA), CSIC-Universidad de Zaragoza, Zaragoza 50009, Spain
Abstract
An effective strategy for the dynamical tuning of k in solids would be critical for developing novel phononic devices able to perform logic operations with phonons, as well as for solid-state refrigeration, energy harvesting, and thermoelectrics. A promising approach consists in taking advantage of field-induced phase transitions, using electric or magnetic fields to manipulate the crystal lattice of polar and magnetic materials, respectively. Such electrophononic and magnetophononic effects can provide fast and dynamical manipulation of the heat carriers, thus yielding à la carte thermal properties for on-demand applications. On the other hand, the possibility of manipulating k with light has received very little attention. Light- driven control of the thermal conductivity could bypass some of the issues posed by the schemes described above (e.g., application of large driving fields) as well as simplify the design of logic devices (i.e., lack of electrical contacts).
Here we discuss this scenario in the archetypal ferroelectrics BaTiO3 and KNbO3, where by means of first-principles calculations, we show that photoinduced charge injection can trigger a ferro-to-paraelectric phase transition, yielding a (potentially ultrafast) reversible change in thermal transport properties [1,2]. Our results reveal a substantial reduction in lattice thermal conductivity, especially at low photoexcited charge densities, as the material undergoes a polar-to-nonpolar transformation. This reduction is primarily due to the suppression of low-frequency phonon modes, which limits heat flow as a result of enhanced phonon-phonon scattering.
We will also discuss the case of TiSe2, a van der Waals 2D material where a photoinduced phase transition can restore a more symmetric crystal phase [3], similar to what we report with ferroelectric oxides. In particular, photoexcited charges or electron/hole doping suppress the charge density wave (CDW, a periodical modulation of the electron density), which is the ground state below ~200 K. Such a CDW melting is accompanied by a sizable reduction in the thermal conductivity, a variation that also in this case almost entirely originate from the changes in the phonon-phonon scattering processes.
These findings underscore a step forward in tunable thermal conductivity, offering new prospects for efficient thermal management in phonon logic, advanced electronics and energy-harvesting applications.
Spectral and spatial limits of phonon coherence in two-dimensional phononic crystals
Roman Anufriev1, Michele Diego2,
Sebastian Volz1. Masahiro Nomura2
1- LIMMS, CNRS
2- IIS, University of Tokyo
Abstract
Phononic crystals are artificial periodic structures engineered to manipulate mechanical vibrations, sound, and heat through phonon interference. However, the spatial and spectral limits at which phonons still sense the periodicity and experience interference remain unclear, as does the nature of their transition into an incoherent state.
In this work, we use Brillouin-Mandelstam light scattering (BMLS) and Raman spectroscopy to experimentally probe phonon dispersion relations in two-dimensional silicon phononic crystals. We measured the dispersion relations of six phononic crystals with periods ranging from 150 to 4000 nm, with 0.7 diameter-to-period ratio. Our findings reveal distinct interference regimes: at nanoscale dimensions, phonons exhibit in-plane interference consistent with elasticity theory up to at least 35 GHz. However, at larger scales and higher frequencies, phonons transition into an out-of-plane interference regime dominated by the out-of-plane phonon confinement, with the dispersion relation similar to that of plain membrane. This transition occurs approximately at the phonon wavelength twice shorter that the period of phononic crystals. Beyond these regimes, interference ceases altogether and phonon transport transitions into incoherent regime, as evidenced by the Raman spectroscopy data.
These results challenge the prevailing assumption that phonon transport transitions directly from a coherent to an incoherent state at a critical frequency or size. Instead, we identify an intermediate out-of-plane interference regime that persists up to much higher frequencies. This refined understanding of phonon transport establishes a framework for the applications of phononic crystals in quantum computing, sensing, and microelectronics.
Acknowledgments:
This work has been partially funded by the CNRS Energy unit (Cellule Energie) through the project SEGMENT.
