1. Physics Department “E. R. Caianiello” and Interdepartmental Centre NanoMatesUniversity of Salerno via Giovanni Paolo II n. 132 Fisciano 84084 Italy
2. Department of Physical and Chemical ScienceUniversity of L’Aquila and CNR‐SPIN L’Aquila, via Vetoio L’Aquila 67100 Italy
3. CNR‐SPIN Salerno via Giovanni Paolo II n. 132 Fisciano 84084 Italy
4. National Laboratory of Solid State MicrostructuresSchool of PhysicsCollaborative Innovation Center of Advanced MicrostructuresNanjing University Nanjing 210093 China