Protected Nodes and the Collapse of Fermi Arcs in High-TcCuprate Superconductors

Abstract

Angle resolved photoemission on underdoped Bi 2 Sr 2 CaCu 2 O 8 reveals that the magnitude and d-wave anisotropy of the superconducting state energy gap are independent of temperature all the way up to T c . This lack of T variation of the entire k-dependent gap is in marked contrast to mean field theory. At T c the point nodes of the d-wave gap abruptly expand into finite length ''Fermi arcs.'' This change occurs within the width of the resistive transition, and thus the Fermi arcs are not simply thermally broadened nodes but rather a unique signature of the pseudogap phase. DOI: 10.1103/PhysRevLett.99.157001 PACS numbers: 74.25.Jb, 74.72.Hs, 79.60.Bm We present in this Letter angle resolved photoemission spectroscopy (ARPES) data on the energy gap in the superconducting (SC) and pseudogap phases of the underdoped high T c superconductor Bi 2 Sr 2 CaCu 2 O 8 (Bi2212). Reduced T c samples which lie in between the optimally doped, highest T c material and the undoped Mott insulator are called ''underdoped.'' Such samples exhibit an unusual normal-state pseudogap, whose signature in ARPES [1,2] is a loss of spectral weight in parts of k space, leading to low-energy electronic excitations which live on disconnected ''Fermi arcs '' [3]. Both the SC gap below T c and the pseudogap above T c are anisotropic gaps in the singleparticle excitation spectrum, but their relationship is not well understood. Our goal is to gain insight into how the SC gap, with its d-wave anisotropy [1,2] at low T, changes as a function of temperature and evolves into the anisotropic pseudogap upon heating through T c .Our first result is that the magnitude and anisotropy of the d-wave SC energy gap is essentially T independent for all T T c . This behavior is completely different from a mean field description of a d-wave superconductor. Within mean field theory the anisotropy would be T independent, but the gap magnitude, proportional to the order parameter, would be suppressed with increasing T and vanish at T c . Remarkably, our data show that the k-dependent SC gap in underdoped cuprates does not even know about the scale of T c .Second, there are four point nodes at which the energy gap vanishes for all T < T c , but above T c each point node abruptly expands into a gapless Fermi arc of finite extent. We show that this remarkable change occurs within the width of the resistive transition at T c . The abrupt change from point nodes to gapless arcs is not just thermal smearing, but rather it is closely connected with the loss of superconducting order.We present ARPES data on two underdoped Bi2212 films, one near optimality (T c 80 K) and the other more underdoped (T c 67 K), both of which exhibit a significant pseudogap. The films, prepared by RF sputtering on a SrTiO 3 substrate [4], exhibit only very weak superstructure replicas which simplifies the data analysis. The measurements were carried out at the Synchrotron Radiation Center, Wisconsin, using a Scienta R4000 analyzer with 22 eV photons, and momentum cuts and polarization para...

Self-energy effects in cuprates and the dome-shaped behavior of the superconducting critical temperature

“…However, experiments show a smooth crossover in the temperature behavior of normal state properties instead of an abrupt transition. 51,52,69 Consistent with this observation, ARPES shows that the PG is filling up but not closing with increasing T , giving the impression of a smooth crossover of the spectral properties.…”

Quasiparticle interference in temperature dependent-STM as a probe of superconducting coherence

“…At T ≈ T c , the peaks associated with the dispersing branches q 1 , q 3 , q 5 , and q 7 all vanish (the peaks associated with q 1 and q 7 merge into the background and thus should not be identified as distinctive individual peaks). These insets show that the B-QPI pattern is uniquely associated with superconducting coherence; this strong signature of T c in QPI is correlated with its counterpart in ARPES [1,2] for moderately underdoped systems.…”

“…Owing to this fact, and unlike a BCS superconductor where the order parameter and the excitation gap are identical, there are very few ways to probe directly something as fundamental as the superconducting order parameter. Within the moderately underdoped samples, which we consider throughout this paper, recent angle resolved photoemission spectroscopy (ARPES) experiments have reported [1,2] novel signatures of superconducting order. Fermi arcs around the d-wave nodes above T c rapidly collapse [2] at the transition to form point nodes.…”

“…Within the moderately underdoped samples, which we consider throughout this paper, recent angle resolved photoemission spectroscopy (ARPES) experiments have reported [1,2] novel signatures of superconducting order. Fermi arcs around the d-wave nodes above T c rapidly collapse [2] at the transition to form point nodes. It was argued that, within the superconducting phase, the temperature dependence of the nodal gap has finally provided [1] "a direct and unambiguous observation of a single particle gap tied to the superconducting transition".…”

Spin Polaron-Bipolaron Scenario of Three Gap-Like Energy Scales in Superconducting Cuprates

“…The term pseudogap has been coined to describe this kind of physics. The presence of pseudogap has been established through the measurements of the nuclear magnetic resonance (NMR) [5], the scanning tunneling spectroscopy [6][7][8][9], the analysis of the electronic Raman scattering [10][11][12], through time-resolved optical spectroscopy [13][14][15][16][17][18], and through the ARPES [19][20][21][22][23][24][25]. The observed strong Nernst effect in cuprates has been also attributed to it [26][27][28].…”