Optical fingerprints of the electronic band reconstruction in van der Waals magnetic materials
Understanding the physical mechanism as well as functionalities of van der Waals (vdW) heterostructures and electronic/spintronic devices is at present a central topic of the ongoing solid-state-physics research activities. In this work, we investigate the temperature (T) dependence of the (in-plane) absorption spectrum over a broad spectral range of both Co-and Mn-intercalated 2H-MxTaS2. The investigated compounds belong to the vast family of transition-metal dichalcogenides (TMDCs). Their several decades long investigation was mostly focused on non-magnetic compounds but got recently boosted by the discovery of long-range magnetism in two-dimensional vdW magnets. While the possibility to intercalate TMDCs with magnetic 3d elements M, exhibiting diverse magnetic properties, is known since the 1980s, they were still not comprehensively scrutinised, particularly from a spectroscopic point of view.
We launch first the survey about the T dependence of the real part (σ1(ω)) of the optical conductivity, shown in Fig. 1.24(a-c) for three selected Mn- and Co-concentrations in the energy interval spanning the far- (FIR), mid- (MIR) and near- (NIR) infrared up to the visible spectral ranges at 5 or 10 and 300 K. The selected compositions are particularly pertinent, since they encompass both the ferromagnetic (FM) and antiferromagnetic (AFM) phase transitions for the Co-intercalation and address a representative Mn compound towards its FM one. In order to focus the discussion on the impact of the magnetic phase transition on the electronic properties, we propose their phenomenological Drude–Lorentz fit, which is singled out for the spectra at 5 or 10 and 300 K in Fig. 1.24(a-c).
The main findings of our work are summarised in Fig. 1.24(d-f). There is an opposite and strong redistribution of the spectral weight (SW) between the Lorentz harmonic oscillators (HOs) L2 and L3 depending on whether a FM or AFM transition takes place. We discover a SW shift from high to low energy scales for FM compositions, while reversely SW is removed from low towards high spectral energies for AFM compounds. The SW distribution and its evolution upon crossing the Curie TC or Neel TN temperature seems to be a common property for both Co or Mn intercalations and is exclusively driven by the targeted, final magnetic state (i.e., independent of the element choice).
The charge dynamics of 2H-MxTaS2 (M = Mn and Co), onto the easily accessible FIR-MIR-NIR spectral range, thus allows to shed light on the relevant energy scales shaping the reconstruction of the electronic band structure upon crossing over from the FM to AFM order with varying intercalation (i.e., element and/or its concentration). Our spectroscopic findings turn out to be consistent with dedicated first-principles calculations upon tuning element-intercalation, pressure and/or magnetic field on similar intercalated materials.