Professor Hongliang Yi

Professor Hongliang Yi’s team at the School of Energy Science and Engineering has made significant breakthroughs in the study of radiative heat transfer, discovering an anomalous temperature dependence of thermal radiation in low-dimensional materials. Their findings, titled?Unconventional Thermophotonic Charge Density Wave, have been published in Physical Review Letters.

Strongly correlated quantum systems can exhibit interesting fundamental collective modes characterized by symmetry breaking, such as charge density waves (CDWs), which feature periodic modulation of electron density and corresponding lattice distortions. A key challenge in related research is achieving microscopic visualization of the CDW state using various complementary scattering techniques, as well as directly determining and understanding the corresponding fingerprint spectra of structural and Fermi surface modifications.

FIG. 1. (a) Schematics of the structure and electron dispersion before (after) the CDW transition, including the CDW-induced interband electron excitations. (b) Schematics of thermophotonic CDW transition for the CDW (top) and standard unit cells (bottom). The thermal photons transport between two CDW-bearing materials with vacuum gap d.?(c) The energy transport coefficient H for various vacuum gaps as a function of T. (d) The exponent of the temperature power law as a function of temperature for CDW-bearing TiSe2 and other representative plasmonic materials (such as graphene, Si, and Weyl semi-metal Co3Sn2S2) for d = 50 nm. A positive exponent means that the thermophotonic intensity increases with temperature, while a negative exponent means the opposite.

In response to this challenge, Professor Hongliang Yi’s team has broken through the constraints of existing research approaches by utilizing fluctuation electrodynamics, current-current linear response, and density functional theory to reveal the radiative heat transfer characteristics of CDW materials, referring to this phenomenon as thermophotonic CDW transitions (tp-CDW transitions). The research results indicate that during the tp-CDW transition process, the thermal photon energy transport exhibits significant negative temperature dependence, represented by H∝T^(-n), which contradicts the Stefan-Boltzmann law’s prediction of H∝T^3. This negative temperature dependence is crucial, indicating a close relationship between CDW order and thermal radiation (after adding phonon corrections, the CDW temperature values can be adjusted to better match the actual transition temperatures; however, non-harmonic effects do not impact the optical and thermophotonic characteristics).

FIG. 2.?(a) The CDW and standard crystal structures of TiSe2. (b) The calculated electronic band structure of TiSe2?along highsymmetry points of the Brillouin zone at different temperatures. (c) The interband spectral intensity function finter?at different temperatures for d = 50 nm. The inset gives the spectral intensity ratio ηinter-intra?= fintra=finter?between interband and intraband processes.

Professor Yi’s team further explored the fundamental physical mechanisms behind the non-trivial characteristics during the tp-CDW transition and examined the potential effects of complex mixed modes composed of plasmons and CDW excitations on thermophoton transport. They discovered that this anomalous thermal radiation arises from the suppression of inter-band excitations related to the annihilation of the CDW electronic bandgap. This finding connects two rapidly developing fields in physics research: CDWs and thermal radiation, offering new insights into CDW studies. Additionally, the unusual thermal response of the CDW phase demonstrates its potential applications in thermal management.

Paper link: https://link.aps.org/doi/10.1103/PhysRevLett.133.066902