Spectroscopic signature of moment-dependent electron-phonon coupling in 2: H -TaS₂

Charge density wave (CDW) order is ubiquitous among low-dimensional electronic systems, as realized in layered materials such as various 1T and 2H polytypes of transition metal dichalcogenides (TMDs). In particular, 2H-TaS₂, a prominent member of the vast family of TMDs, hosts a canonical CDW phase transition whose mechanism remains controversial even after decades of research. Using state-of-the-art angle resolved photoemission spectroscopy (ARPES) measurements, we report, for the first time, pronounced many-body renormalizations in 2H-TaS₂. These renormalizations are manifested by the presence of multiple slope changes, known as "kinks", in the electronic dispersions. The temperature independence of the kink energies and their close correspondence with the phonon frequencies of the system evidence that these renormalizations are caused by electron-phonon interactions. In addition, the electron-phonon coupling parameter is momentum-dependent, and the CDW vector is not compatible with any of the Fermi surface nesting vectors. Collectively, these observations establish that the origin of the CDW transition in 2H-TaS₂ is not the same as that of the Peierls ordering transition, rather this transition is triggered by strong electron-phonon coupling including its momentum anisotropy. The commonality between the ARPES data on 2H-TaS₂ and those on 2H-TaSe₂ and 2H-NbSe₂ rationalize the above interpretation of the CDW transition being generic to incommensurate CDW orders in 2H polytypes of TMDs.