Conventional superconductivity and charge-density-wave ordering in Ba1−xNaxTi2

Abstract

We have investigated the low temperature physical properties of BaTi 2 Sb 2 O and Ba 1-x Na x Ti 2 Sb 2 O (x = 0. 05, 0.1, 0.15, 0.2, 0.25, 0.3) by means of muon spin rotation (µSR) and SQUID magnetometry. Our measurements reveal the absence of magnetic ordering below TDW = 58 K in the parent compound. Therefore the phase transition at this temperature observed by magnetometry is most likely due to the formation of a charge density wave (CDW). Upon substitution of barium by sodium in Ba 1-x Na x Ti 2 Sb 2 O we find for x = 0.25 superconductivity with a maximum Tc = 5.1 K in the magnetization and a bulk T c,bulk = 4.5 K in the µSR measurements. The temperature dependency of the London penetration depth λ −2 (T ) of the optimally doped compound can be well explained within a conventional weak-coupling scenario in the clean limit.

Intra-unit-cell nematic charge order in the titanium-oxypnictide family of superconductors

“…The possibility of SDW formation in BaTi 2 (As,Sb) 2 O, either commensurate or incommensurate, was subsequently ruled out by muon spin relaxation and 121/123 Sb nuclear resonance measurements, which show conclusively that no magnetic order develops at any temperature probed 26,27,31 . On the other hand, a conventional CDW should be evident through an associated structural distortion.…”

“…Therefore, in common with earlier discussion 18,21 , we propose that the structural transition is a response to an instability of the electronic system. The earlier muon spin relaxation results 26,27 , together with the ED measurements, allow us to rule out the existing proposals of SDW formation 32,33 or a phonon-driven CDW 34 . Instead, an intra-unit-cell charge-nematic electronic symmetry breaking is implicated, similar to that proposed for doped cuprates 39 .…”

“…Interestingly, a number of compounds in this family also show strong anomalies in resistivity and/or magnetic susceptibility that are thought to be signatures of symmetry-breaking charge-or spinordered ground states 21,24,25,29,30 , suggesting that these materials are excellent candidates for studying the interplay between brokensymmetry states and superconductivity. In light of these strong transport anomalies, it is then quite surprising that subsequent experiments have failed to uncover any direct evidence for a conventional spin-or charge-density wave ground state 26,27,31 , leaving open the question of whether these materials do possess symmetry-broken ground states.…”

“…In particular, in isovalent BaTi 2 (Sb 1-x Bi x ) 2 O and aliovalent Ba 1-x Na x Ti 2 Sb 2 O (ref. 25), muon spin rotation and heat capacity measurements point to fully-gapped s-wave superconductivity [26][27][28] . Interestingly, a number of compounds in this family also show strong anomalies in resistivity and/or magnetic susceptibility that are thought to be signatures of symmetry-breaking charge-or spinordered ground states 21,24,25,29,30 , suggesting that these materials are excellent candidates for studying the interplay between brokensymmetry states and superconductivity.…”

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

“…However, another calculation indicates that a CDW instability may * Electronic address: jjiangcindy@gmail.com † Electronic address: dlfeng@fudan.edu.cn dominate, with the superconductivity driven by conventional electron-phonon interactions in this compound [10]. Experimentally, muon spin rotation (µSR) studies have not found any sign of magnetic order, while a nuclear magnetic resonance (NMR) observed broken in-plane four-fold symmetry at the Sb site without the presence of an internal field [11,12]. Therefore, CDW order is more likely although no direct evi- As discussed below, the data taken with 100 eV photons are close to the k z = 6.8 2π/c plane, and those taken with 28 eV photons are close to the k z = 4.5 2π/c plane.…”