Our colleague Dino Novko, in collaboration with scientists from China and Singapore, has published a paper in Physical Review Letters, in which they propose a novel and unconventional thermophotonic effect in charge-density-wave material TiSe₂. This finding indicates a breakthrough in understanding of electronic orders, and signify a landmark study of CDW and its sophisticated connection with thermal photonics.

Unconventional Thermophotonic Charge Density Wave

Cheng-Long Zhou, Zahra Torbatian, Shui-Hua Yang, Yong Zhang, Hong-Liang Yi, Mauro Antezza, Dino Novko, and Cheng-Wei Qiu, Phys. Rev. Lett. 133, 066902 (2024).

DOI: 10.1103/PhysRevLett.133.066902

Recent years have witnessed a rapid development in the understanding of the physics of cooperative charge density wave (CDW) electronic states, providing an interesting platform for the discovery of unconventional physical phenomenon, such as, high-temperature superconductors, abnormal fermion behavior, topological quantum state and so on. These interesting electron order states constitute the cornerstone for various electronic and spintronic devices, such as ultrafast integrated switches, memory storages, and thermoelectric devices.

Despite these preliminary discoveries, straightly determining and understanding structural and Fermi surface modifications still remains a challenge, due to various limitations of direct microscopic visualization of electronic structure. It is worth contemplating the fact that thermo-photon, being a more readily detectable property, contains rich physical information, potentially serving as a means to identify characteristics of electronic structure and offer valuable insights into these electronic ordered states. However, given the current state of research, it remains elusive whether there is a correlation between thermal photonics and electronic ordered states.

To address the challenges described above, in this work we establish connection between these two rapidly growing areas of physics research (CDWs and thermophotonics) and present a new angle for studying underlying electron order state.

Starting from an ab initio method, we combine the fluctuational electrodynamics to derive explicit analytic expressions connecting underlying electron order state and thermophotonics. It accurately reflects all the details of the thermophotonic fingerprint corresponding to the CDW Fermi surface modifications. Our theory is applicable to other electron order state structures, which can be extended to study other quantum phases in two dimensions, such as Mott insulating and Wigner crystal states.

We observe that during the CDW transition, photonic energy transport produces a significant negative temperature dependence, i.e., H~T^-n, violating the Stefan Boltzmann prediction H~T³. This negative temperature dependence is highly non-trivial and suggests an intimate connection between the CDW order and thermal photonics. We further show that the nontrivial signature naturally originates from the suppression of the interband excitation associated with the annihilation of the CDW electronic band gap. This is the first result that revel the thermophotonic fingerprint of CDW Fermi surface modifications, proposing thermophotonic-CDW transition (tp-CDW transition).

Figure 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 calculated energy transport coefficient H of single-layer TiSe₂ 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. The black dotted line represents Stephen-Boltzmann’s law of black body (H_bb = 4σT³).

The tp-CDW transition also apply to several nonlinear thermal devices including a negative differential thermal conductance device, a temperature regulator, and a thermal diode, all benefiting from the design space enabled by negative temperature dependence of the tp-CDW transition. Importantly, we find that thanks to its second-order transitions features, the tp-CDW transition could highlight a wider temperature manipulation range, thus providing a stronger thermal operating capacity and a larger design freedom for thermal devices.