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Abstract
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 1T-TiSe2 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 ρ(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.
Highlights
Pure 1T-TiSe2, whose crystal structure is shown in Fig. 1a, undergoes a charge density wave (CDW) transition at TCDW = 200 K with a 2 × 2 × 2 charge order[1]
The temperature-dependent resistivity data on single crystals (Fig. 2a) give a high-to-low temperature resistivity ratio ρ(300 K)/ρ (2 K) = 10−α with an exponent α that is ≤0 for dopings x ≤ 0.015, and >0 for x ≥ 0.02, indicative of Metal-to-insulator transitions (MIT) at around x = 0.015–0.02 Pt doping with a remarkable ~5 orders of magnitude increase in the scaled resistivity compared to the x = 0 compound. [x is an average nominal composition, which can vary locally within a crystal.] Around this composition, the signatures of the CDW transition are obscured by the diverging resistivity, consistent with the observation on the polycrystalline samples
Guided by the scanning tunneling microscopy (STM) and angle-resolved photoemission spectroscopy (ARPES) data, we propose a model of narrow metallic channels on the domain walls of CDW, in an otherwise insulating background, which potentially provides explanations for the power-law temperature dependence of the resistivity, and points to an alternative path to obtain an MIT
Summary
A transport study has found finite resistivity in polycrystalline TiSe2 samples at low temperature[12], consistent with metallic behavior. The sensitivity of the low-temperature transport property on the synthesis condition[13] suggests that the observed metallic behavior in the polycrystalline sample is likely due to self-doping (see Fig. 1b).
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