STM/STS Study on 4a×4aElectronic Charge Order of Superconducting Bi₂Sr₂CaCu₂O_8+δ

Abstract

We performed low-bias STM measurements on underdoped Bi2212 crystals, and confirmed that a two-dimensional (2D) superstructure with a periodicity of four lattice constants (4a) is formed within the Cu-O plane at T < Tc. This 4a×4a superstructure, oriented along the Cu-O bonding direction, is nondispersive and more intense in lightly doped samples with a zero temperature pseudogap (ZTPG) than in samples with a d-wave gap. The nondispersive 4a×4a superstructure was clearly observed within the ZTPG or d-wave gap, while it tended to fade out outside the gaps. The present results provide a useful test for various models proposed for an electronic order hidden in the underdoped region of high-Tc cuprates.KEYWORDS: STM/STS, pseudogap, electronic charge ordering, superstructure, cuprateRecently STM/STS studies on the pseudogap state of Bi 2 Sr 2 CaCu 2 O 8+δ (Bi2212) at T > T c have revealed a nondispersive two-dimensional (2D) superstructure with a periodicity of about four lattice constants (∼4a×4a superstructure) in the map of energy-resolved differential tunneling conductance dI/dV , proportional to the local density of states (LDOS).1) The nondispersive 4a×4a structure, electronic in origin, was also reported in the LDOS maps taken on the zero temperature pseudogap (ZTPG) regions of lightly doped Ca 2−x Na x CuO 2 Cl 2 (Na-CCOC) and Bi2212 samples.2,3) A similar spatial structure was first observed around the vortex cores of Bi2212 exhibiting a pseudogap-like V-shaped spectrum with no coherence peak.4) Such 2D spatial structures have attracted much attention because they can be a possible electronic order hidden in the pseudogap state.From the LDOS maps taken for the superconducting (SC) state of Bi2212, Hoffman et al. and McElroy et al. reported a strongly dispersive 2D superstructure. 5,6)This superstructure has been successfully explained in terms of a quasiparticle scattering interference. Furthermore, Howald et al. found a nondispersive ∼4a×4a superstructure with anisotropy in addition to the weakly dispersive one, and claimed that the nondispersive ∼4a×4a superstructure will be due to the so-called stripe order and coexist with superconductivity.7−10) However, the nondispersive ∼4a×4a superstructure was not confirmed in later LDOS measurements on Bi2212 at T < T c . 1,5)This inconsistency mainly originates from the difficulty in distinguishing the nondispersive ∼4a×4a superstructure from the weakly dispersive ∼4a×4a one resulting from quasiparticle scattering interference. Since the dispersive features in the LDOS are reduced by integrating the LDOS over a wide range of energy, it is desirable to investigate the nondispersive features of the LDOS by using a conventional STM technique which actually maps the LDOS integrated between E F and E F +eV s , where V s is the sample bias voltage. To clarify whether the nondispersive 4a×4a electronic superstructure persists in the SC state (T < T c ) will promote our understanding of the physics in the underdoped regionIn the present study, we performed low-bias STM me...

Scanning tunneling microscopy and spectroscopy study of4a×4aelectronic charge order and the inhomogeneous pairing gap in superconductingBi2Hashimoto1, Momono2, Oda3 et al. 2006Phys. Rev. B371320View full textAdd to dashboardCite

“…3,6 Very recently the nondispersive 4a ϫ 4a charge order at T Ͻ T c was found to appear in heavily underdoped SC Bi2212. 11 The charge order is commensurate ͑4a ϫ 4a͒ and has an internal structure with a period of 4a /3ϫ 4a / 3, which is just like the electronic charge order reported by Hanaguri et al for lightly doped Na-CCOC. 1 The observation of almost the same charge order for both cuprates Na-CCOC and Bi2212 provides definite evidence that the nondispersive 4a ϫ 4a charge order develops on the Cu-O layer.…”

“…If the 4a ϫ 4a superstructure is due to some kind of lattice distortion, it is difficult to explain why the superstructure appears within a limited bias ͑energy͒ range, especially, associated with the gap size ⌬ 0 ; V s Շ⌬ 0 / e. Such a limitation of V s in observing the 4a ϫ 4a superstructure means that the superstructure is electronic in origin, that is, due to an electronic charge order, as has been claimed in many preceding studies. [1][2][3][9][10][11] The appearance of the 4a ϫ 4a charge order within the pairing gap implies that quasiparticles of the SC state and/or hole pairs will take part in causing the charge order.…”

“…The 4a ϫ 4a charge order is likely to be dynamical in itself, and pinned down over regions with effective pinning centers. 11 To understand the nature of the nondispersive 4a ϫ 4a charge order, it is desirable to investigate the charge order on crystals with different doping levels and/or pinning centers of different natures.…”

Coexistence of a double-Qspin-density wave and a multi-Qpair-density wave in cuprate oxide superconductors

“…It is also reasonable that almost homogeneous checkerboards appear in under doped cuprates where superconductivity disappears [13,15]; almost homogeneously AF moments presumably exist there. Inhomogeneous checkerboards can appear in under doped cuprates with rather low SC T c 's, when inhomogeneous AF moments are induced by disorder.…”

STM studies on electronic charge order in the PG state of Bi2212

“…Recently, a similar charge order, whose amplitude is strongly sample-dependent, has been found in the SC state [3][4][5]. More interestingly, in samples which exhibit strong ∼ 4a 0 × 4a 0 charge orders at T T c , the spatial dependence of energy gap structure is very inhomogeneous in nanometer scale, and vice vasa.…”