Metal-metal transition without crystal-symmetry breaking in Rb$_{1-\delta}$V$_2$Te$_2$O

We report the synthesis, crystal structure, physical properties, and first-principles calculations of a vanadium-based oxytelluride Rb$_{1-\delta}$V$_2$Te$_2$O ($\delta\approx0.2$). The crystal structure bears two-dimensional V$_2$O square nets sandwiched by tellurium layers, mimicking the structural motifs of cuprate and iron-based superconductors. The material exhibits metallic conductivity with dominant hole-type charge carriers. A metal-to-metal transition takes place at $\sim$100 K, which is conformably characterized by a kink/hump in the electrical resistivity, jumps in the Hall and Seebeck coefficients, a drop in the magnetic susceptibility, and a peak in the heat capacity. Nevertheless, neither Bragg-peak splitting nor superlattice reflections can be detected from the low-temperature x-ray diffractions, suggesting absence of any charge-nematic or charge-density-wave order. The band-structure calculations show that V-3$d$ orbitals dominate the electronic states at around Fermi energy where a $d_{yz}/d_{xz}$ orbital polarization shows up. There are three Fermi-surface sheets that seem unfavorable for nesting. Our results suggest an orbital or spin-density-wave order for the low-temperature state and, upon suppression of the competing order, emergence of superconductivity could be expected.