Occurrence Of Metal-insulator Transition Research Articles

Gate-Induced Metal-Insulator Transition in 2D van der Waals Layers of Copper Indium Selenide Based Field-Effect Transistors.

The existence of an exquisite phenomenon such as a metal-insulator transition (MIT) in two-dimensional (2D) systems, where completely different electronic functionalities in the same system can emerge simply by regulating parameters such as charge carrier density in them, is noteworthy. Such tunability in material properties can lead to several applications where precise tuning of function specific properties are desirable. Here, we report on our observation on the occurrence of MIT in the 2D material system of copper indium selenide (CuIn7Se11). Clear evidence of the metallic nature of conductivity (σ) under the influence of electrostatic doping via the gate, which crosses over to an insulating phase upon lowering the temperature, was observed by investigating the temperature and gate dependence of σ in CuIn7Se11 field-effect transistor devices. At higher charge carrier densities (n > 1012 cm-1), we found that σ ∼ (n)α with α ∼ 2, which suggests the presence of bare Coulomb impurity scattering within the studied range of temperature (280 K > T > 20 K). Our analysis of the conductivity data following the principles of percolation theory of transition where σ ∼ (n - nC)δ show that the critical percolation exponent δ(T) has average values ∼1.57 ± 0.27 and 1.02 ± 0.35 within the measured temperature range for the two devices and it is close to the 2D percolation exponent value of 1.33. We believe that the 2D MIT seen in our system is due to the charge density inhomogeneity caused by electrostatic doping and unscreened charge impurity scattering that leads to a percolation driven transition. The findings reported here for CuIn7Se11 system provide a different material platform to investigate MIT in 2D and are crucial in order to understand the fundamental basis of electronic interactions and charge-transport phenomenon in other unexplored 2D electron systems.

Thickness dependent electronic structure of La0.6Sr0.4MnO3 layer in SrTiO3/La0.6Sr0.4MnO3/SrTiO3 heterostructures studied by hard x-ray photoemission spectroscopy

The authors have investigated changes in the electronic structures of digitally controlled La0.6Sr0.4MnO3 (LSMO) layers sandwiched between SrTiO3 as a function of LSMO layer thickness in terms of hard x-ray photoemission spectroscopy (HX-PES). The HX-PES spectra show the evolution of Mn 3d derived states near the Fermi level and the occurrence of metal-insulator transition at 8 ML. The detailed analysis for the thickness dependent HX-PES spectra reveals the existence of the less conducting and nonmagnetic transition layer with a film thickness of about 4 ML in the interface region owing to significant interaction through the interface.