Condensed-Matter Physics & Materials Science Seminar

Unconventional states of metals often emerge from a quantum critical point (QCP) - a zero temperature transition between different ground states of matter. Near QCP the standard Landau's Fermi-liquid theory is violated due to quantum fluctuations. Whether the QCP physics can explain the mystery of unusual pseudogapped and `strange metal' non-Fermi-liquid states of high-Tc superconductors and their connection to the origins of high Tc is a fundamental question still under debate. Experimentally, the putative QCP and its surroundings are masked by superconductivity and remain uncertain. By destroying superconductivity and closing the pseudogap with strong magnetic field in Tl2Ba2CuO6+x, we can map the onsets of both pseudogap (T*) and Fermi liquid (TFL) states using transport measurements in a previously unexplored low-temperature, heavy charge-doping region of the phase diagram. We show that T* and TFL converge in the zero-temperature limit at a unique value of doping which shifts with field concurrently with the suppression of Tc. The effect of magnetic field is the key. In a field, the pseudogap boundary T* has scaling properties, and it develops a thermodynamic divergence in the quantum limit corresponding to a zero entropy jump. We conclude that quantum criticality, 'strange' metal states and superconductivity in cuprates are intimately linked.