Daniel G. Mazzone (Laboratory for Neutron Scattering and Imaging, Paul Scherrer Institut, Switzerland)
Although quantum mechanics was developed to treat nonrelativistic interacting particles, it has also been widely used to describe collective phenomena in condensed matter physics. A macroscopic quantity of interacting particles (~1023) can give rise to novel quantum phases with emergent magnetic and transport properties that are currently hotly debated among theorists and experimentalists alike. Particularly exciting are quantum liquids, including superconducting condensates, that feature coherence phenomena over macroscopic length scales. Pressure, magnetic field or chemical substitution can induce quantum phase transitions that originate from fluctuations that are governed by Heisenberg's uncertainty principle. These collective quantum fluctuations can trigger the collapse or emergence of an order parameter and, they can even dominate the finite temperature properties. Quantum fluctuations generally separate phases with different organizing principles [1, 2], leading to magnetic phases with different symmetry as the strength of the fluctuations evolve. Our recent neutron diffraction results provide evidence for a field‐induced quantum phase transition, in superconducting Nd0.05Ce0.95CoIn5, that separates two antiferromagnetic phases with identical magnetic symmetry. At zero field, we find a spin-density wave that is suppressed at the critical field μ0H* = 8 T. For H > H* a second spin-density phase appears and shares many properties with the Q‐phase in CeCoIn5 [3, 4]. These results provide evidence that the magnetic instability is not purely driven by magnetic fluctuations and suggest the emergence of a spatially modulated superconducting order parameter at H*.
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