Electronic structure of the titanium-based oxypnictide superconductorBa0.95Na0.05Ti2Sb2Oand direct observation of its char

Abstract

Ba 0.95 Na 0.05 Ti 2 Sb 2 O is a titanium-based oxypnictide superconductor with possible density wave order. We have performed high resolution angle-resolved photoemission spectroscopy (ARPES) and scanning tunneling microscopy (STM) studies on Ba 0.95 Na 0.05 Ti 2 Sb 2 O. Our STM studies find a charge density wave (CDW) order with the wavevector of (π, π). The electronic structure shows both multi-orbital and three-dimensional nature, consistent with the theoretical calculations. The observed Fermi surfaces are well nested along the (π, π) direction, which might drive the CDW transition. Likely due to the weak strength of the CDW order, we do not observe a leading-edge gap. Instead, we observe an overall spectral weight enhancement with decreasing temperature, but the rate of this enhancement is abruptly reduced or diminished in the CDW state, suggesting the suppression of spectral weight or opening of a partial CDW gap. Our results give a comprehensive picture of the electronic structure and direct observation of the CDW order in Ba 0.95 Na 0.05 Ti 2 Sb 2 O.

Titanium Pnictide Oxide Superconductors

“…To account for the results, intra-unit-cell nematic charge order was proposed, such as that proposed in cuprates [58,59]. Recently, Song et al reported direct observation of the CDW order with the wave vector of (π, π) by STM measurements on Ba0.95Na0.05Ti2Sb2O (Figure 16), which is consistent with that theoretically predicted by Subedi and Nakano et al [46]. They also performed ARPES measurements and revealed the Fermi surface is well nested along the (π, π) direction, which is consistent with STM study, suggesting the electron-phonon mediated superconductivity in Ba0.95Na0.05Ti2Sb2O.…”

“…These predicted FS of BaTi2Pn2O is highly similar to that theoretically predicted in Na2Ti2Pn2O [25,32,45]. Experimentally, Song et al recently performed angle-resolved photoemission spectroscopy (ARPES) studies on Ba0.95Na0.05Ti2Sb2O single crystals and found the multiband electronic structure having electron and hole pockets, consistent with the theoretical studies [46]. …”

Presence of exotic electronic surface states in LaBi and LaSb

“…The perpendicular δ and ε pockets in Figs. 2(c-f) are from two inequivalent X points (X 2 and X 1 , in blue), and appear together due to the poor k z resolution of our experiments using vacuum ultraviolet photons (VUV-ARPES) -the bands from a specific k z plane may have a projection over a wide range of k z 46,47 . Meanwhile, the X 3 point (beige) is projected to the Γ point for the same reason.…”

Phonon dispersions and Fermi surfaces nesting explaining the variety of charge ordering in titanium-oxypnictides superconductors

“…Interestingly, Song et al has very recently reported the existence of (1/2, 1/2) nesting vector by ARPES and STM measurements applied to high-quality single crystals, being consistent with the theoretical predictions. [26] The superlattice structure is hence expected to be detected by further diffraction measurements. The theoretically estimated distortion here is quite small, 0.14 Å (See Supplementary Note 4), being in agreement with the previous calculation by Subedi, [15] so careful detections would be required for experiments.…”

“…Indeed, such shapes have been recently observed by state-of-the-art Angle-Resolved Photo Emission Spectroscopy (ARPES) applied to BaTi 2 As 2 O and BaTi 2 Sb 2 O single crystals. [25,26] Subedi [15] worked on BaTi 2 Sb 2 O using DFT and reported the possible lattice instability toward CDW with √ 2 × √ 2 × 1 superstructure at low temperature. Experimentally, however, no such superlattice peaks were found by neutron and electron diffractions for this compound, [17,27] reviving the argument over if the anoma-lies in ρ(T ) and χ(T ) can be really attributed to the phonondriven CDW or not.…”