Strong electron correlations are at the heart of many physical phenomena of current interest to the condensed matter community. Here we present a survey of the mechanisms underlying such correlations in charge density wave (CDW) systems, including the current theoretical understanding and experimental evidence for CDW transitions. The focus is on emergent phenomena that result as CDWs interact with other charge or spin states, such as magnetism and superconductivity. In addition to reviewing the CDW mechanisms in 1D, 2D, and 3D systems, we pay particular attention to the prevalence of this state in two particular classes of compounds, the high temperature superconductors (cuprates) and the layered transition metal dichalcogenides. The possibilities for quantum criticality resulting from the competition between magnetic fluctuations and electronic instabilities (CDW, unconventional superconductivity) are also discussed.

Sharp magnetization switching and large magnetoresistance (MR) were previously discovered in single crystals of 2H-FexTaS2 and attributed to the Fe superstructure and its defects. We report similar sharp switching and large MR in 1T-FexTiS2 (0.086 ≤ x ≤0.703), while providing a side by side comparison of the only two such ferromagnetic transition metal dichalcogenides. The switching field Hs and MR values are similar in both 1T-FexTiS2 and 2H-FexTaS2, with a larger than expected bowtie ρ(H) and a sharper hysteresis loop for H c in the former. The Curie and Weiss temperatures remain roughly constant below x ∼ 1/3 in the T = Ti single crystals, before monotonically increasing for higher x, while Hs and MR reach maxima where defects in the superstructure exist, or a minimum near superstructures compositions, and remain constant above x ∼ 0.4. Despite previous reports, electron diffraction shows only the √ 3× √ 3 superstructure in 1T-FexTiS2. Glassy behavior is shown to coexist within the ferromagnetic state in 1T-FexTiS2 for compositions between 0.1 and 0.703. A simple theoretical model considering first-, second-and third-neighbor interactions yields a phase diagram which accounts for both spin glass behavior and for different superstructures.

Dilute impurities and growth conditions can drastically affect the transport properties of TiSe2, especially below the charge density wave transition. In this paper, we discuss the effects of cooling rate, annealing time and annealing temperature on the transport properties of TiSe2: slow cooling of polycrystalline TiSe2 post-synthesis drastically increases the low temperature resistivity, which is in contrast to the metallic behavior of single-crystalline TiSe2 due to charge doping from the residual iodine transport agent. A logarithmic increase of resistivity upon cooling and negative magnetoresistance with a sharp cusp around zero field are observed for the first time for the polycrystalline TiSe2 samples, pointing to weak-localization effects due to low dimensionality. Annealing at low temperatures has a similar, but less drastic effect. Furthermore, rapid quenching of the polycrystalline samples from high temperatures freezes in disorder, leading to a decrease in the low temperature resistivity.

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.