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Intra-Unit-Cell Nematic Charge Order in the Titanium-Oxypnictide Family of Superconductors

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Intra-Unit-Cell Nematic Charge Order in the Titanium-Oxypnictide Family of Superconductors ARTICLE Received 9 Jun 2014 | Accepted 5 Nov 2014 | Published 8 Dec 2014 DOI: 10.1038/ncomms6761 Intra-unit-cell nematic charge order in the titanium-oxypnictide family of superconductors Benjamin A. Frandsen1,*, Emil S. Bozin2,*, Hefei Hu2,w, Yimei Zhu2, Yasumasa Nozaki3, Hiroshi Kageyama3, Yasutomo J. Uemura1, Wei-Guo Yin2 & Simon J.L. Billinge2,4 Understanding the role played by broken-symmetry states such as charge, spin and orbital orders in the mechanism of emergent properties, such as high-temperature super- conductivity, is a major current topic in materials research. That the order may be within one unit cell, such as nematic, was only recently considered theoretically, but its observation in the iron-pnictide and doped cuprate superconductors places it at the forefront of current research. Here, we show that the recently discovered BaTi2Sb2O superconductor and its parent compound BaTi2As2O form a symmetry-breaking nematic ground state that can be naturally explained as an intra-unit-cell nematic charge order with d-wave symmetry, pointing to the ubiquity of the phenomenon. These findings, together with the key structural features in these materials being intermediate between the cuprate and iron-pnictide high- temperature superconducting materials, render the titanium oxypnictides an important new material system to understand the nature of nematic order and its relationship to superconductivity. 1 Department of Physics, Columbia University, New York, New York 10027, USA. 2 Condensed Matter Physics and Materials Science Department, Brookhaven National Laboratory, Upton, New York 11973, USA. 3 Department of Energy and Hydrocarbon Chemistry, Graduate School of Engineering, Kyoto University, Nishikyo, Kyoto 615-8510, Japan. 4 Department of Applied Physics and Applied Mathematics, Columbia University, New York, New York 10027, USA. * These authors contributed equally to this work. w Present address: Intel Corporation, Folsom, California 95630, USA. Correspondence and requests for materials should be addressed to S.J.L.B. (email: [email protected]). NATURE COMMUNICATIONS | 5:5761 | DOI: 10.1038/ncomms6761 | www.nature.com/naturecommunications 1 & 2014 Macmillan Publishers Limited. All rights reserved. ARTICLE NATURE COMMUNICATIONS | DOI: 10.1038/ncomms6761 ather than being an anomalous side effect in one or two the temperature of the transport anomaly. On the other hand, cuprate systems, broken-symmetry states are now thought high-sensitivity electron diffraction measurements failed to detect Rto be widespread in strongly correlated electron systems any superlattice peaks in the bulk at low temperature, indicating and other complex materials. Extensive study of the manga- that this transition does not break translational symmetry. The nites1,2, cuprates3,4, iron pnictides5 and a variety of other systems low-temperature phase, therefore, constitutes a nematic state. In has made it increasingly evident that local and global symmetry light of the pronounced upturn in resistivity accompanying this breaking in the charge, orbital, lattice and spin degrees of freedom nematic transition, together with the absence of any ordered are associated with the appearance of emergent phenomena, SDW26, we attribute the nematicity to an intra-unit-cell charge such as colossal magnetoresistance and high-temperature order with d-wave symmetry by charge transfer between superconductivity (HTSC), but the exact relationship is not neighbouring Ti sites—similar to that between neighbouring understood. Historically, the study of such broken-symmetry oxygen sites in cuprate superconductors—and find that it states has been very challenging. Taking the cuprates as an naturally explains the temperature dependence of the lattice example, 8 years elapsed from the initial discovery of constants. These results establish this family of materials as superconductivity to the first observation of symmetry-broken another playground for studying symmetry-breaking electronic charge order (stripes) in one system6, another 7 years passed phases and their relationship to superconductivity. before hints were found in others7,8 and only within the last 3 years has charge order begun to emerge as a possibly ubiquitous feature of the cuprates9,10. Results Several possibilities arise when considering the symmetries that Structural and electronic properties of BaTi2Pn2O. The basic can be broken by these states. Most charge-/spin-density waves structural unit of BaTi2Pn2O is a planar square net of titanium (C/SDWs) break the translational symmetry of the lattice, folding and oxygen, in analogy with the cuprates (Fig. 1a,b), with the the Brillouin zone and resulting in superlattice diffraction peaks. crucial difference that the positions of the metal and oxygen ions On the other hand, orbital ordering, where charge transfers are switched between the structures (the complete titanate between orbitals centred at the same site, can break the metric structure is shown as an inset in Fig. 2b). In Fig. 1, the square net rotational symmetry without lowering the translational symme- is shown by solid lines along the nearest neighbour bonds, with try. Examples are charge-nematic11 and loop-current12,13 orders dashed lines showing the net joining second neighbour ions. This in the doped cuprates. In this context, nematic order is defined as second-nearest-neighbour square net connects oxygen ions in the one that breaks the rotational point group symmetry while cuprates, but metal ions in the titanate compounds and also in the preserving the lattice translational symmetry. The fact that iron-based superconductors (Fig. 1c). Thus, in terms of chemistry nematic symmetry-broken states have recently been discovered and structure, the titanate compounds bridge between the ferrous experimentally in both the cuprate11 and iron-based14 and cuprate superconductors. The Ti 3d orbitals are occupied by superconductors raises the importance and relevance of this one electron per Ti atom, which is found to reside in a nominally observation to HTSC. It is, therefore, critically important to 1/4-filled band formed via hybridization of the dxy and understand the role and ubiquity of symmetry breaking, dy2 À z2 =dx2 À z2 orbitals for the Ti(1)/Ti(2) ions (defined in including intra-unit-cell nematicity, to the superconducting Fig. 1b). The local geometry of the Ti(1) site is shown in Fig. 1d phenomenon. and the arrangement of the d-energy levels is shown in Fig. 1e. Standard theoretical treatments of HTSC, such as the effective Furthermore, the phase diagram (Fig. 2b) is highly reminiscent single-band t–J model, have typically ignored the possibility of of the cuprates and iron-based superconductors, with super- intra-unit-cell orders15. When multiple atoms per unit cell are conductivity appearing on doping and transport behaviour that is explicitly included in the theory, qualitatively different ground- strongly suggestive of a competing electronic transition such as state solutions may be found16, underscoring the subtlety and the formation of a CDW or SDW21,24. The transport is metallic at importance of accounting correctly for this phenomenon. Hence, high temperature21, with a positive resistivity slope versus finding related but distinct systems that exhibit this phenomenon temperature (Fig. 2a). However, on cooling, a pronounced is expected to shed new light on this critical question. upturn in the resistivity is found for all x in the solid solution 17–19 Very recently, superconductivity was discovered in BaTi2As1 À xSbxO. The feature occurs at a temperature Ta that titanium-oxypnictide compounds, such as ATi2Pn2O(A ¼ Na2, decreases monotonically from 200 K for x ¼ 0 to 50 K for B Ba, (SrF)2, (SmO)2; Pn ¼ As, Sb, Bi), which are close structural x ¼ 1, with superconductivity appearing below 1 K for the 20–24 and chemical cousins to the cuprates and iron-pnictides .In antimony endmember and increasing to 5 K for BaTi2Bi2O (refs particular, in isovalent BaTi2(Sb1–xBix)2O and aliovalent Ba1– 19,24). Anomalies in the magnetic susceptibility and specific heat 21 xNaxTi2Sb2O (ref. 25), muon spin rotation and heat capacity are also observed at Ta. 26–28 measurements point to fully-gapped s-wave superconductivity . Density functional theory (DFT) calculations for BaTi2Sb2O Interestingly, a number of compounds in this family also show predicted an instability towards a bicollinear SDW formation32,33 strong anomalies in resistivity and/or magnetic susceptibility that or a commensurate CDW ground state driven by an unstable are thought to be signatures of symmetry-breaking charge- or spin- phonon mode that doubles the unit cell by distorting the Ti ordered ground states21,24,25,29,30, suggesting that these materials squares and preserves the tetragonal symmetry34. The possibility are excellent candidates for studying the interplay between broken- of SDW formation in BaTi2(As,Sb)2O, either commensurate or symmetry states and superconductivity. In light of these strong incommensurate, was subsequently ruled out by muon spin transport anomalies, it is then quite surprising that subsequent relaxation and 121/123Sb nuclear resonance measurements, which experiments have failed to uncover any direct evidence for a show conclusively that no magnetic order develops at any conventional spin- or charge-density wave ground state26,27,31, temperature probed26,27,31. On the other hand, a conventional leaving open the question of whether these materials do possess CDW should be evident through an associated structural
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