Artificially created two-dimensional (2D) interfaces or structures are ideal for seeking exotic phase transitions due to their highly tunable carrier density and interfacially enhanced many-body interactions. Here, we report the discovery of a metal-insulator transition (MIT) and an emergent gapped phase in the metal-semiconductor interface that is created in $2\mathrm{H}\text{\ensuremath{-}}{\mathrm{MoTe}}_{2}$ via alkali-metal deposition. Using angle-resolved photoemission spectroscopy, we found that the electron-phonon coupling is strong at the interface as characterized by a clear observation of replica shake-off bands. Such strong electron-phonon coupling interplays with disorder scattering, leading to an Anderson localization of polarons which could explain the MIT. The domelike emergent gapped phase could then be attributed to a polaron extended state or phonon-mediated superconductivity. Our results demonstrate the capability of alkali-metal deposition as an effective method to enhance the many-body interactions in 2D semiconductors. The surface-doped $2\mathrm{H}\text{\ensuremath{-}}{\mathrm{MoTe}}_{2}$ is a promising candidate for realizing polaronic insulator and high-${T}_{c}$ superconductivity.

Metal-Insulator Transition and Emergent Gapped Phase in the Surface-Doped 2D Semiconductor 2 H -MoTe 2

We performed electron spin-resonance (ESR), magnetic, and optical absorption studies of a model system used to investigate the metal-insulator transition in a deformable lattice. The ESR and magnetic studies revealed that the small polarons have no correlation to the conductivity below or above the mobility edge. The conducting transition is marked by the appearance of an asymmetric ESR signal simultaneously with a significant suppression of the small bipolaron optical absorption band indicating that the extended states result from an abrupt dissociation of the small bipolarons. Immediately above the mobility edge competing phases of extended states, small polarons, and small bipolarons coexist. We propose a phase diagram for the charge and spin dynamics of an interacting gas of small (bi)polarons across the discontinuous conducting transition that has implications for understanding the long-standing conundrum of Mooij correlations.

Evolution of spin dynamics across the discontinuous metal-insulator transition in a deformable lattice

Optical properties of four model systems (Na-, K-, Rb- or Cs-doped quasi-two-dimensional X 1.95Al1.95Si2.05O8.00; X  =  Na, K, Rb, or Cs) used to study the metal-insulator transition (MIT) in a deformable lattice are investigated. The doping evolution of the optical absorption band(s) originating from small bipolarons show strong variations depending on the electron-lattice coupling strength [Formula: see text]. Despite the increasing number density of small (bi)polarons, the Na-system remains a stubborn (bi)polaronic insulator due to strong [Formula: see text], while the other three systems show closing of the respective mobility gaps giving way to conducting phases with differing properties. These interesting evolutions and dynamical properties are compared and discussed. We conjecture that the manifestation of anomalous electronic transport properties and Mooij correlations near the MIT or superconductor-insulator transition in systems with non-negligible electron-lattice coupling effects may be linked to the coexistence of competing polaronic phases and the dynamical intertwining of the deformable lattice and the random electronic potential.

https://iopscience.iop.org/article/10.1088/1361-648X/ab4d84/pdf

Optical investigations of the (dis)continuous metal-insulator transitions in strongly-coupled electron-lattice systems.

We performed impedance spectroscopy and optical investigations below the reported mobility edge of two models systems (${X}_{n}$/${X}_{195\ensuremath{-}\ensuremath{\delta}}{\mathrm{Al}}_{195}{\mathrm{Si}}_{205}{\mathrm{O}}_{800}$; $X=\mathrm{K}$ or Rb, $n$ is the doping density of guest atoms) used to explore the metal-insulator transition in a deformable lattice The bulk ac conductivity and imaginary dielectric permittivity of insulating samples near the mobility edge in both systems show a power-law behavior and variable-range hopping (VRH) conduction that is indeed related to the proposed polaron relaxation and conduction We show that polaron relaxation and VRH occur simultaneously above a particular density of an interacting gas of small (bi)polarons, which is commensurate with the electron-lattice coupling strength and the intersite electron transfer energy Based on our experimental results, we propose a rich phase diagram for the charge dynamics in a deformable lattice from the perspective of a Wigner lattice of small (bi)polarons

Yoshifumi Nishida

Papers

Evolution of spin dynamics across the discontinuous metal-insulator transition in a deformable lattice

Polaron relaxation and charge dynamics near the mobility edge of a possible Wigner lattice of polarons

Optical investigations of the (dis)continuous metal-insulator transitions in strongly-coupled electron-lattice systems.