Strongly Correlated van der Waals Oxide: 2 H-NbO2

Abstract

Two-dimensional (2D) materials are attracting attention as an advanced class of materials. However, despite the strongly correlated electron systems exhibiting emergent properties, transition metal oxides are not recognized as 2D materials due to their infrequent van der Waals (vdW) crystal structures. Herein, we present a synthesis route for a vdW 2H-type transition metal dioxide, 2H-NbO₂, via a soft-chemical approach from the only 2H-type layered oxide LiNbO₂. By combining epitaxially stabilized thin films with a nanoscale thickness and solvothermal reaction, we successfully deintercalated Li, exceeding the bulk limit, while maintaining the single-crystal nature. We demonstrate that the synthesized 2H-NbO₂ is a correlated insulator with a half-filled single band, which endows exotic superconductivity to Li_1-xNbO₂, whose quantum critical phase diagram has similarities to those of cuprates and Moiré superlattices. Our findings indicate that "strongly correlated vdW oxides" could be an approach for introducing Mott physics to 2D systems and a key that connects each research field.

Keywords: 2D materials; metal−insulator transition; quantum criticality; strongly correlated oxides; superconductivity; van der Waals compounds.

1

Research concepts and approaches. The crystal structure and known electronic phase diagram of Li_1–x NbO₂. x ∼ 0.5 is the deintercalatable limit of Li, where two-dimensional superconductivity with strange metallic behaviors is emerged. Imaginal x = 1 phase can be considered as 2H-type NbO₂ with the d ¹ two-dimensional triangular lattice and half-filled Nb 4d _z2 single band.

2

Structural characteristics. (A) Out-of-plane X-ray diffraction profiles of as-grown and HNO₃-reacted Li_1–x NbO₂ films. Asterisks indicate reflections coming from MgAl₂O₄ substrates. Filled and open triangles indicate reflections coming from a sample stage and secondary phases, respectively. (B) The reaction temperature dependence of the c-axis length of Li_1–x NbO₂ films. Inset: Magnification of the LiNbO₂ 002 reflections. Broken lines show literature values for LiNbO₂ and Li_0.45NbO₂. (C) The a-axis and (D) c-axis length changes as a function of x in Li_1–x NbO₂ obtained by DFT calculations. GGA-PBE and optB88-vdW functional are used. The value obtained in the HNO₃ reaction at 150 °C and previously known experiment values are also indicated.

3

Composition analysis. (A) Li 1s core-level spectra of as-grown and HNO₃-reacted Li_1–x NbO₂ films. (B–D) Depth profiles of time-of-flight–secondary ion mass spectra for (B) as-grown films with Al₂O₃ capping layer and HNO₃-reacted films at (C) RT and (D) 150 °C. (E) Depth dependence of the Li⁺/NbO⁺ ratio for as-grown and HNO₃-reacted Li_1–x NbO₂ films. The interface of films and substrates is defined as the origin of depth. Inset: The reaction temperature dependence of the Li⁺/NbO⁺ ratio averaged over entire films.

4

Electrical transport properties. (A) Temperature dependence of resistivity for as-grown and HNO₃-reacted Li_1–x NbO₂ films. (B) Resistivity normalized by values obtained at 300 K as a function of the logarithmic temperature. (C) Temperature dependence of the carrier concentration for as-grown and HNO₃-reacted Li_1–x NbO₂ films.

5

Optical properties and band structure. (A) Absorption spectra for as-grown and HNO₃-reacted Li_1–x NbO₂ films. Inset: Optical images. (B) Band gap (E _g ) and (C) absorption coefficient at 0.4 eV (α_0.4 eV) as a function of the averaged Li⁺/NbO⁺ ratio. E _g is defined as the energy where the absorption coefficient reaches 5 × 10^–4 cm^–1. (D) Schematic of band structure changes for Li_1–x NbO_2..

6

Phase diagram and calculation results. (A–C) Temperatures correspond to transition from non-Fermi liquid to Fermi liquid (T _FL), metal–insulator transition (T _min), and superconductivity (T _c) as a function of the Li⁺/NbO⁺ ratio. Background colors correspond to (A) resistivity, (B) magnetoresistance at 5 T, and (C) temperature exponent of resistivity (ρ ∼ T ⁿ ). Broken lines are guide for eyes. (D–F) Projected density of states obtained by DFT calculation for (D) 2H-NbS₂, (E) 2H-NbO₂, and (F) spin-resolved 2H-NbO₂.