Wei-Cheng Lee

Binghampton University
What Happens Inside a Unit Cell Matters – Effects of Umklapp Processes on Correlated Materials

Correlated materials exhibiting a number of exotic electronic states of matters have been one of the most studied subjects in condensed matter physics. A well-accepted scheme is that due to the strong short-range repulsive interactions, the quasi-particle picture, the central dogma of Fermi liquid theory, breaks down. Instead of being sharp in momentum-frequency space, the spectral weight of electrons spreads out in a wide range of frequency, which is believed to be the cause of novel temperature and frequency dependences of physical properties like DC resistivity and optical conductivity. However, a fundamental question is that since all these physical properties require a momentum-relaxing mechanism to have finite values, the interaction that breaks the momentum conservation has to be identified in order to understand the experimental results. In this talk, I will propose that since all the correlated materials are crystalline, Umklapp process is the most natural candidate for the momentum-relaxing mechanism. I will show that with the inclusion of Umklapp processese and long-range Coulomb interaction, new collective excitations leading to novel frequency dependence of optical conductivity can emerge in mid-infrared region, and these collective excitations are sensitive to superconducting phase transition. Implications for new energy scale to enhance superconducting transition temperature Tc will be discussed.

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