IHEP scientist Yi Ling and his collaborators have made significant progress in applying holographic gravity to strongly interacting condensed matter systems. Their result, published in Physical Review Letters (Phys. Rev. Lett. 113, 091602(2014)), uses the linear response theory of holographic charge density waves (CDWs) to successfully reproduce the two fundamental features of real CDW materials for the first time, suggestive of a new mechanism for the metal-insulator transition.

The exploration of the microscopic structure and quantum behavior of spacetime can be traced back to the pioneering work on black hole entropy by Bekenstein and Hawking in the 1960s. They found that not only can a black hole be thought of as a thermal object with entropy, but also that its entropy is proportional to its horizon area. This area law is totally different from the volume behavior of entropy for ordinary systems. Motivated by this area law, in the 1990s Gerard 't Hooft and Leonard Susskind in turn proposed the holographic principle for quantum gravity, saying that all the information of quantum gravity in our real world with three spatial dimensions can probably be encoded onto a surface with one less dimension. Nowadays the holographic principle is believed to be a fundamental property for any quantum theory of gravity. Among other theories, the string-inspired gravity/gauge duality (AdS/CFT correspondence) has successfully provided an explicit implementation of the holographic principle. In particular, over the past few years this surprising duality has spurred much effort on the application of general relativity to strongly coupled systems, such as the QCD quark-gluon plasma produced at RHIC and the LHC, and strongly correlated electron systems in condensed matter. Partly because conventional methods do not work for strongly coupled systems, such applied holography has become a front-running and fiercely competitive research field in fundamental physics, where many exciting advances have been made towards our understanding of how strongly coupled systems behave. For instance, the model of holographic superconductors has provided us with a new framework to understand the mechanism for high T_c superconductors.

Charge density waves, a novel ground state characterized by a periodic distribution of charge density due to the electron-phonon interaction, is one of the most interesting cutting-edge topics in condensed matter physics. Prof. Yi Ling, together with his students Chao Niu and Zhuo-Yu Xian, has been pondering over CDW from this holographic perspective for a long time. By collaborating with Associate Professor Jian-Pin Wu from Bohai University and FWO fellow Dr. Hongbao Zhang from Vrije Universiteit Brussel, Ling’s research group successfully constructed a holographic dual model for CDW by numerically solving the highly non-linear bulk equations of motion. The further linear response analysis of this model reproduces the two fundamental features of CDW demonstrated in the real materials, namely the pinned collective modes and gapped single-particle excitation. Not only does this result offer a new mechanism to implement the metal-insulator transition, it also opens a new window for studying  related phenomena in CDW experiments using holography.

This project was supported by the Natural Science Foundation of China (Nos. 11275208, No. 11305018, and No. 11178002), the State Key Laboratory of Theoretical Physics, and the 555 Talent Project of Jiangxi Province.

Figure Caption：Two figures show the real and imaginary parts, respectively, of the optical conductivity for CDW at different temperatures. In the real part of the conductivity, the first peak represents the appearance of pinned collective modes, while the second peak is responsible for the gapped single-particle excitation. The metal to insulator transition is further confirmed by the fact that dc conductivity decreases with the decreased temperature below the critical temperature.