X-ray Probe of 2kF Density-wave Quantum Phase Transition
From two decades ago, the language of quantum phase transition and the associated quantum critical phenomena was formally developed to provide a platform for studying emergent states and exotic phases when a system is fine-tuned away from its normal state. Athermal tuning methods typically include chemical doping, magnetic and electrical fields, and pressure. Among those, pressure is considered a clean technique since it does not break the time reversal symmetry, and also in carefully crafted conditions, limits the increase of disorder and changes of local chemical environment.
Over the last decade, x-ray diffraction under pressure has been developed at several 3rd generation synchrotron radiation sources to directly study fine-tuned correlated electrons. Here I discuss examples in the family of 2kF density wave systems, including spin density waves in Cr [1-3] and GdSi [4-5], and charge density wave in two-dimensional NbSe2 . Driven by Fermi surface instability, these correlated states are dominated by itinerant electrons in the weak-coupling limit and are often good candidates of continuous quantum phase transition under pressure. Our x-ray diffraction measurements provide direct insight into both fluctuations near a quantum critical point and evolution at the Fermi surface. In addition, I will also discuss how x-ray studies of quantum phase transitions can be extended with coherent x-ray techniques in future.
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