  Metal-to-insulator transitions (MIT) can be driven by a number of different
mechanisms, each resulting in a different type of insulator -- Change in
chemical potential can induce a transition from a metal to a band insulator;
strong correlations can drive a metal into a Mott insulator with an energy gap;
an Anderson transition, on the other hand, due to disorder leads to a localized
insulator without a gap in the spectrum. Here we report the discovery of an
alternative route for MIT driven by the creation of a network of narrow
channels. Transport data on Pt substituted for Ti in TiSe$_2$ shows a dramatic
increase of resistivity by five orders of magnitude for few % of Pt
substitution, with a power-law dependence of the temperature-dependent
resistivity $\rho(T)$. Our scanning tunneling microscopy data show that Pt
induces an irregular network of nanometer-thick domain walls (DWs) of charge
density wave (CDW) order, which pull charge carriers out of the bulk and into
the DWs. While the CDW domains are gapped, the charges confined to the narrow
DWs interact strongly, with pseudogap-like suppression in the local density of
states, even when they were weakly interacting in the bulk, and scatter at the
DW network interconnects thereby generating the highly resistive state.
Angle-resolved photoemission spectroscopy spectra exhibit pseudogap behavior
corroborating the spatial coexistence of gapped domains and narrow domain walls
with excess charge carriers.
