Wydarzenia

Seminarium "Coherence-Correlations-Complexity" (KFT, PWr)

Longitudinal spin excitations in correlated electron systems

Maciej Fidrysiak

Katedra Fizyki Teoretycznej WPPT

Phenomenology of spontaneous breaking of global continuous symmetries involves both transverse and longitudinal excitations of the local order parameter. In magnetically ordered phases, the former are ordinary spin waves whose structure is protected by symmetry and coherence. The latter, on the other hand, are usually more sensitive to the microscopic physics, which makes them a valuable probe of the electronic correlations and the magnitude of quantum spin fluctuations. For this reason, the longitudinal spin excitations (LSEs) have attracted attention in the field of Fe-based superconductivity, where contrasting evidence regarding correlations is provided by various approaches. Moreover, in specific circumstances, the LSEs become interesting on their own right. It happens in the vicinity of magnetic quantum critical points, where (in the effective dimension of four D =3+1) they turn into sharp amplitude (or Higgs) modes with small width to mass ratios. These modes have been recently observed in a class of dimerized quantum antiferromagnets near pressure induced quantum phase transitions, which has triggered a discussion of the stability and visibility of the amplitude modes in condensed matter physics. In the talk I will briefly summarize our recent results concerning the LSEs in magnetic systems. I will focus on two main topics: (1) the relation between the LSEs and microscopic mechanism of magnetic ordering in Fe-pnictides, and (2) phenomenological constraints on the stability of the amplitude mode in D=4 antiferromagnets and their applicability to real materials. A more detailed summary is presented below.