Scale-Invariant transport near quantum critical point in high-temperature superconductors.*

The origin of the anomalous relaxation dynamics and the very nature of charge carriers in the "strange metal" state are among the most pressing open questions in copper and iron based superconductors. In this talk, I will review thermodynamic measurements that indicate the existence of quantum critical point in iron based superconductors, such as enhanced electronic correlations approaching critical doping as well as zero-temperature collapse of a line of phase transitions inside superconducting dome. Then I will discuss high-magnetic-field transport measurements near critical doping which highlight the dynamic consequences of quantum criticality such as scale invariant transport. In both classes of high temperature superconductors these measurements reveal a linear-in-field resistivity at very high magnetic fields (up to 93T) which exhibits a characteristic temperature-field competition as field is lowered. Such scale-invariant behavior of magneto-transport in a broad range of temperatures and magnetic fields is incompatible with quasiparticle transport in the strange metal state.