Quasiparticle Spectra, Charge-Density Waves, Superconductivity, and Electron-Phonon Coupling in2H−NbSe2

Abstract

High-resolution photoemission has been used to study the electronic structure of the charge density wave and superconducting dichalcogenide, 2H-NbSe2. From the extracted self-energies, important components of the quasiparticle interactions have been identified. In contrast to previously studied TaSe2, the CDW transition does not affect the electronic properties significantly. The electronphonon coupling is identified as a dominant contribution to the QP self-energy and is shown to be very anisotropic (k-dependent) and much stronger than in TaSe2.

Gaps and kinks in the electronic structure of the superconductor2H-NbSe2from angle-resolved photoemission at 1 K

“…This range of kink energies agrees with previous results obtained from less sharp Nb bands with single kinks only. 15 All kink positions are consistent with the energies of optic and acoustic phonon modes at the corresponding wavevectors. 29,30 Figure 3 and Tab.…”

“…1(a) and 2(a), these Nb sheets are typically well resolved, whereas a further, smaller, three-dimensional sheet around the Γ point-the Se 4p zderived "pancake"-generally appears blurred due to k z broadening. 3,[14][15][16][17]19,20,[25][26][27] Since for 2H-NbSe 2 the k z dispersion of the Nb 4d states is weak 3 and the effective k z resolution is in the order of the k z extension of the Brillouin zone, the Nb 4d contours in the Fermi-surface maps may, to a good approximation, be interpreted as k z averages. We obtain the best tight-binding simulation of the two Nb 4d bands in the vicinity of E F for the parameters listed in Tab I.…”

“…15,16 Since the bands crossing E F in 2H-NbSe 2 are steep, we can extract the renormalized ("kinked") band dispersions from appropriate E-versus-k ARPES intensity maps by momentum-distribution-curve fitting, i.e., by fitting Lorentzians (plus a linear background) to each constant-E profile of the maps. The parameters v 0 F and v * F are then determined by fitting lines, which are forced through the Fermi-level crossing, to different portions of the extracted band dispersion: v 0 F is evaluated from an energy interval well below E F , typically ranging from −100 to −90 meV, and v * F from the energy window spanned by the highest kink energy and E F .…”

“…1 At first glance it seems surprising that the above questions are still largely open although many different sorts of experiments have been performed on 2H-NbSe 2 , including specific heat, 6 thermal conductivity, 7 magnetization, 8 and penetration depth 9 measurements, as well as tunneling spectroscopy [10][11][12][13] and angle-resolved photoemission spectroscopy (ARPES). [14][15][16][17] But much of the discord can indeed be traced back either to the lack of k resolution of the experimental probes or to the relatively high temperatures ( > ∼ 0.7 T c ) at which the ARPES measurements [14][15][16][17] were done.…”

Charge density waves in the graphene sheets of the superconductor CaC6

“…1b,c). Although this seems unusual, NbSe 2 , one of the most widely studied CDW systems [18][19][20] , displays a similar phenomenon. Furthermore, analogous effects have been seen in the high-temperature superconducting cuprates around the pseudogap temperature 21 , which some ascribe to a CDW transition 22 .…”