Muon spin relaxation and electron/neutron diffraction studies of BaTi2(As1−xSbx

Abstract

We present the results of muon spin relaxation/rotation, transmission electron microscopy, and neutron diffraction measurements performed on several specimens of BaTi 2 (As 1−x Sb x ) 2 O, which is known to have either charge density or spin density wave ordering at T DW for all x, and superconductivity below T c ≈ 1 K for x = 1. Zero-field muon spin relaxation measurements show no significant increase in relaxation rate at the density wave ordering temperature for any composition, indicating that the density wave is of the charge rather than spin type. The absence of any superstructure peaks in selected area electron and high-resolution neutron diffraction measurements below T DW suggests that the charge density wave does not involve modulation of atomic arrangement. Transverse field muon spin rotation measurements reveal a robust superconducting state below T c ≈ 1 K for x = 1. PACS number(s): 76.75.+i, 74.90.+n, 68.37.Og Layered oxypnictide systems, such as ATi 2 P n 2 O (A = Na 2 , Ba, (SrF) 2 , (SmO) 2 ; P n = As, Sb, Bi), have been found to possess interesting electronic and magnetic properties, including spin/charge density wave (S/CDW) ordering and superconductivity. 1-14 Although the superconducting phase transition in these systems occurs at low temperatures ( 5 K), 7-11 and in some cases not at all, they nevertheless share certain structural and electronic similarities with the cuprate and iron-pnictide compounds exhibiting hightemperature superconductivity. 15,16 These similarities include the presence of planar sheets of Ti 2 O square nets, in analogy to the CuO 2 sheets found in the cuprates, an electron configuration of 3d 1 that is electron-hole symmetric with the 3d 9 configuration of the cuprates, and the close proximity of the S/CDW instability to the superconducting state, as also observed in many cuprates and iron-pnictides. Given these similarities, these oxypnictide systems have the potential to yield important insights into high-temperature superconductivity.Resistivity and magnetic susceptibility measurements recently performed on isovalent systems for BaTi 2 P n 2 O, with P n = As 3− (T c = 0 K), Sb 3− (T c = 1.2 K), Bi 3− (T c = 4.6 K), have revealed an intriguing phase diagram with S/CDW ordering present for BaTi 2 (As 1−x Sb x ) 2 O, which is quickly suppressed and replaced by a superconducting state in BaTi 2 (Sb 1−y Bi y ) 2 O (Fig. 1). 10 Remarkably, the superconducting phase exhibits a two-dome structure as observed in iron pnictide, 17 suggesting the presence of multiple bands at the Fermi surface, in accordance with band calculations revealing contributions of d x 2 −y 2 , d z 2 , and d xy orbitals. 18-20 On the other hand, the hole-doped systems via aliovalent cation and anion substitution, (Ba 1−x Na x )Ti 2 Sb 2 O 8 and BaTi 2 (Sb 1−x Sn x ) 2 O, 11 respectively, display a different phase diagram with a robust S/CDW state even while supporting superconductivity with T c as high as 5.5 K in (Ba 0.67 Na 0.33 )Ti 2 Sb 2 O. In addition, any two-dome structure in the supercondu...

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.…”

“…On the other hand, a conventional CDW should be evident through an associated structural distortion. However, initial electron and neutron diffraction studies on the Sb endmember 26 found no broken symmetry or any signature of superlattice formation at low temperatures, nor was a CDW gap formation observed in angle-resolved photoemission measurements of the nested Fermi surfaces (although a slight depression of the density of states at other momenta was found to correlate in temperature with the resistivity anomaly 35 ). The possibility of an incommensurate CDW was also ruled out by 121/123 Sb nuclear resonance measurements 31 .…”

“…The low-temperature phase, therefore, constitutes a nematic state. In light of the pronounced upturn in resistivity accompanying this nematic transition, together with the absence of any ordered SDW 26 , we attribute the nematicity to an intra-unit-cell charge order with d-wave symmetry by charge transfer between neighbouring Ti sites-similar to that between neighbouring oxygen sites in cuprate superconductors-and find that it naturally explains the temperature dependence of the lattice constants. These results establish this family of materials as another playground for studying symmetry-breaking electronic phases and their relationship to superconductivity.…”

“…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.…”