Pseudogap in the chain states of YBa2Cu3O6.6

Abstract

As established by scanning tunneling microscopy (STM) cleaved surfaces of the high temperature superconductor YBa 2 Cu 2 O 7−δ develop charge density wave (CDW) modulations in the one-dimensional (1D) CuO chains. At the same time, no signatures of the CDW have been reported in the spectral function of the chain band previously studied by photoemission. We use soft X-ray angle resolved photoemission (SX-ARPES) to detect a chain-derived surface band that had not been detected in previous work. The 2k F for the new surface band is found to be 0.55 Å −1 , which matches the wave vector of the CDW observed in direct space by STM. This reveals the relevance of the Fermi surface nesting for the formation of CDWs in the CuO chains in YBa 2 Cu 2 O 7−δ . In agreement with the short range nature of the CDW order the newly detected surface band exhibits a pseudogap, whose energy scale also corresponds to that observed by STM. PACS numbers: 71.45.Lr , 74.72.Kf, 79.60.Jv The CuO chain structure in YBa 2 Cu 2 O 7−δ (Y-123) provides a physical realization of a quasi-one-dimensional electronic system with non-vanishing coupling to the CuO 2 bilayers. 1,2 One of the reasons why 1D electronic systems remain in the focus of solid state research is that even weak interactions transform the quasi-particles of Fermi liquid theory into collective excitations of density wave type. [3][4][5] In particular, the peculiar topological structure of the chain Fermi surface (FS) makes this electronic system prone to formation of charge density wave via the Peierls instability. 6 Indeed, charge density modulations along the Cu-O chains have been extensively studied probing cleaved surfaces of Y-123 in direct space with scanning tunneling microscopy. [7][8][9][10][11][12][13][14][15] It has been shown that Y-123 crystals cleave between the CuO chains and the BaO layer, so that the chains turn out to be the nearest to the surface building block. 7,9,10,15 Both the earliest 9 and the most recent studies 14 present a consistent picture of a CDW, appearing as corrugations of the electronic density with a short correlation range of about 40 Å and a period between 9 and 14 Å, depending on the sample stoichiometry.In the case of 2D systems, the effect of CDW on the electronic states in the reciprocal space has been examined in great detail with modern angle resolved spectroscopy, which provides both momentum and energy resolution when measuring the one particle spectral function. Occurrences of the pseudogap for an incommensurate (or short range ordered) state, which finally develops into a true CDW band gap below T CDW , are well documented. [16][17][18][19] In the case of 1D systems other than Y-123, modifications to the spectral function with the onset of the CDW state have been detected as well. 20-22 the emergence of a pseudogap-like state is also expected in theory and can be understood as a result of a fluctuating Peierls order parameter. [23][24][25][26][27] At the same time, despite the abundant evidence for charge modulations in direct spa...

dc Josephson Current Between an Isotropic and a d-Wave or Extended s-Wave Partially Gapped Charge Density Wave Superconductor

“…Another viewpoint consists in relating pseudogaps to those competing orderings, but treating them, on the equal footing with superconductivity, as well-developed states that can be made allowance for in the mean field approximation, fluctuation effects being non-crucial. We believe that the available observations support the latter viewpoint (see, e.g., recent experimental evidences of CDW formation in various cuprates [202][203][204][205]). Moreover, although undoped cuprates are antiferromagnetic insulators [206], the CDW seems to be a more suitable candidate responsible for the pseudogap phenomena, which competes with Cooper pairing in doped high-T c oxide samples [123][124][125][126][127], contrary to what is the most probable for iron-based pnictides and chalcogenides [78,207].…”

Competing charge, spin, and superconducting orders in underdopedYBa2Cu3Oy

“…103,109 In agreement with that, a recent soft x-ray angle-resolved photoemission spectroscopy (SX-ARPES) experiment detected a gapped surface chain band whose nesting vector matches the modulation wave vector found by STM. 110 Comparing our bulk sensitive x-ray data results to these surface related observations is not straightforward, since the chain layer at the surface is known to be heavily overdoped. 111 However, both the modulation period (∼9-14Å) and the correlation length (∼ 40Å) reported by STM and SX-ARPES studies 103,109,110 are intriguingly close to our values for δ b (CDW) and ξ b (CDW) in Fig.…”

“…110 Comparing our bulk sensitive x-ray data results to these surface related observations is not straightforward, since the chain layer at the surface is known to be heavily overdoped. 111 However, both the modulation period (∼9-14Å) and the correlation length (∼ 40Å) reported by STM and SX-ARPES studies 103,109,110 are intriguingly close to our values for δ b (CDW) and ξ b (CDW) in Fig. 4.…”

“…4. Common interpretations of the charge modulations on the chains are Friedel oscillations 103,109 caused by chain defects, and a Peierls-like CDW instability 110 . The correlation length of the Friedel oscillations, being a local perturbation, may not depend strongly on hole doping or ξ b (chain).…”

Fermi surface reconstruction from bilayer charge ordering in the underdoped high temperature superconductor YBa2Cu3O6+x

“…We also note that whereas the formation of a CuO 2 plane-centered charge modulation involving the bonding band [23] is mostly decoupled from oxygen ordering within the chains, as indicated by recent x-ray scattering experiments [24], the opposite may be true for a charge modulation involving the antibonding band owing to a greater overlap of its wave function with the chains (the chains themselves being prone to different forms of order [50,51]). Local interactions with the oxygen order could cause the latter to become more susceptible to a lock-in transition or to differences in the strength of the modulation along the a and b lattice directions, potentially revealed in NMR experiments [21].…”