Session H21: Superconductivity: Electronic Structure of BSCCO; ARPES and theory

Extraction of Normal Electron Self-Energy and Pairing Self-Energy in Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8}$ via Laser ARPES

Super-high resolution laser-based angle-resolved photoemission measurements have been performed on a high temperature superconductor Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8}$. The band back-bending characteristic of the Bogoliubov-like quasiparticle dispersion is clearly revealed at low temperature in the superconducting state which gives rise to two peaks in the momentum distribution curves. This makes it possible for the first time to experimentally extract the normal electron self-energy and pairing self-energy in the superconducting state. These information can be used to further determine the Bosonic spectral function that will provide key insight and constraints on the origin of electron pairing in high temperature superconductors.   

Momentum Dependence of the Self-energy and Fluctuation Spectrum of Bi2212 from High Resolution Laser ARPES

We report deduction of the Eliashberg function $\alpha^2 F(\theta,\omega)$ at energy $\omega$ and along momentum cuts at angles $\theta$ normal to the Fermi surface from the high resolution laser angle resolved photoemission spectroscopy on slightly underdoped Bi2212 in the normal and superconducting states. Our principle result is that despite the angle dependence of the extracted single-particle self-energy, the Eliashberg function in the normal state collapses onto a single function of $\omega$ independent of the angle. It has a peak around 0.05 eV, flattens out above 0.1 eV with the angle dependent cut-off. The cut-off energy is given by the intrinsic value of about 0.4 eV or the energy of the band bottom in direction $\theta$, whichever is lower. These results are consistent only with fluctuation spectra which have the correlation length of the lattice constant or shorter. In the superconducting state, the deduced $\alpha^2 F(\theta,\omega)$ exhibits a new peak around 0.015 eV in addition to the 0.05 eV peak and flat spectrum as in the normal state. Both peaks become enhanced as $T$ is lowered or the angle moves away from the nodal direction. The implication of these findings is discussed.   

Mechanisms for Superconductivity in Cuprates compared with results from the Generalized MacMillan-Rowell Analysis of High Resolution Laser- ARPES

The spectra of fluctuations and their coupling to fermions has been deduced from extensive high resolution laser ARPES in several BISCCO samples and quantitatively analyzed. We ask the question whether some of the theories for superconductivity in Cuprates are consistent or inconsistent with the frequency and the momentum dependence of the deductions. We find that any fluctuation spectra, for example that of Antiferromagnetic Fluctuations, whose frequency dependence depends significantly on momentum dependence are excluded. We consider the quantum-critical spectra of the loop-current order observed in under-doped cuprates and its coupling to fermions and find it consistent with the data.   

Comparative study of the MDC and two-dimensional analysis of ARPES intensity for Bi2212 bilayer superconductor

The momentum distribution curve (MDC) analysis is commonly used to analyze the ARPES data. A problem of the MDC analysis, however, is that the matrix elements turn out to be energy dependent. To remedy this we take the two-dimensional (2D) analysis of the ARPES intensity as reported by Meevasana et. al. [1]. We analyze the overdoped Bi2212 superconductor ARPES data by performing both 2D and MDC analysis taking the bilayer splitting into consideration. The deduced self-energy and Eliashberg function from both analysis will be compared and presented. \\[4pt] [1] Meevasana et. al. Phys. Rev. B 77, 104506 (2008).   

Ultra-High Resolution Time- and Angle-Resolved Photoemission Experiments on High Temperature Superconductor Bi$_2$Sr$_2$CaCu$_2$O$_8$

Ultra-high resolution laser-based time- and angle-resolved photoemission measurements have been carried out on various dopings of Bi$_2$Sr$_2$CaCu$_2$O$_8$ high temperature superconductor. In this talk, we will report on the study of the dynamical quasiparticle excitation and recombination of the nodal electronic states in cuprate.   

Momentum-Dependent Ultrafast Quasiparticle Dynamics in Optimally-Doped Bi-2212 Monitored with Time-Resolved ARPES

We have employed the novel technique of time- and angle-resolved photoelectron spectroscopy (t-ARPES) to investigate the quasiparticle dynamics in photoexcited, optimally-doped Bi2Sr2CaCu2O8 superconductor across the superconductor-metal transition. In this talk, we will present and analyze the substantially different ultrafast dynamics, tracked in momentum-dependent fashion, by probing the nodal and antinodal regions of the Brillouin zone, on a 35 femtosecond timescale. The consequences of these findings in terms of reentrant superconductivity will be discussed.   

Tracking Cooper Pair Formation in a Cuprate Superconductor: An Ultrafast ARPES Study

A basic mystery in high temperature superconductivity is the process that drives quasiparticles to form Cooper pairs, the fundamental charge carriers in any superconductor. We use time- and angle-resolved photoemission spectroscopy (TR-ARPES) to measure the relaxation dynamics of low energy excitations in the optimally doped cuprate superconductor Bi-2212. Results are discussed within the context of the Rothwarf-Taylor model of quasiparticle recombination.   

Nodal Gap of Heavily Under-doped Bi2201 Revealed by VUV Laser ARPES

We have carried out VUV laser-based angle-resolved photoemission (ARPES) on heavily under-doped $Bi_2(Sr_{2-x}La_x)CuO_{6+\delta}$ (abbreviated as La-Bi2201) samples with different dopings from antiferromagnetic insulators to superconductors. We find that, along the $(0,0)-(\pi,\pi)$ nodal direction, there is a gap opening that is strongly dependent on the hole doping level. The momentum and temperature dependence of the gap is investigated and the implication of the observations will be discussed.   

Doping Dependence of Pair-breaking and Pair-forming Processes in BSCCO

Angle resolved photo-emission spectroscopy (ARPES) provides a direct measure of electronic scattering processes in materials. However traditional methods (e.g. MDC widths) of determining the scattering processes from ARPES fail to discriminate between pairing and non-pairing interactions. Our new analysis technique, the ARPES tunneling spectrum (ATS), isolates the pairing channel from all other processes. We will report doping, angle and temperature dependence of both the pair-forming strength and the pair-breaking rate in BSCCO.   

Momentum dependence of Fermi surface maps in angle-resolved photoemission spectrum of ${\rm Bi_2Sr_2CuO_6}$ (Bi2201)

We have investigated the effect of ARPES matrix element on photointensity for emission from the Fermi energy as a function of photon energy in Bi2201 using first-principles as well as tight-binding model calculations. Our results show that as the photon energy increases and photoemitted electrons are spread over a larger area of momentum space, the highest spectral intensities generally remain pinned to the largest momenta probed at any given photon energy. The tight-binding calculations, which involve a three band Hubbard Hamiltonian based on Cu $d_{x^2-y^2}$ and O $p_x$, $p_y$ orbitals, give insight into the role of the ARPES matrix element in shaping the photointensities. A relatively simple formula is derived for the matrix element, showing how much of the zone-to-zone variation of the photointensity is controlled by the structure factor associated with the Bloch wavefunction.   

Detecting the minimum gap locus in ARPES spectra of Bi2201

Recent angle-resolved photoemission (ARPES) studies have reported a direct evidence for the competing nature of the pseudogap by showing that the pseudogap dispersion is not tied to Fermi momentum ($k_{F}$)[1,2]. In this study, to get more detailed information on how the competing pseudogap evolves across the pseudogap temperature (T*), we introduce a new analysis method for spectral weight. We found a clear indication that the pseudogap opens at T* with the minimum gap locus deviating from $k_{F}$, which is completely different manner from the gap opening by simple superconductivity, and strongly supports that the pseudogap is another distinct order. [1] M. Hashimoto and R.-H. He {\it et~al.}, Nature Phys. {\bf 6}, 14-418 (2010). [2] R.-H. He and M. Hashimoto {\it et~al.}, Science {\bf 331}, 1579-1583 (2011).   

Evidence for Charge-Density-Wave in Underdoped Bi2201 from ARPES and LEED

While there is mounting evidence for a broken symmetry in the pseudogap state of the high-$T_c$ cuprates, the identification of a specific phase remains elusive. Through the combination of electronic (ARPES) and structural (LEED) probes, we uncover a temperature dependent evolution of the CuO$_2$ plane band dispersion in highly-ordered Bi2201, which is directly associated with a hitherto-undetected evolution of the incommensurate superstructure. The quasilinear, continuous variation of the modulation wavelength $2\pi/Q_2$ from $\sim\!66$ to $43$\AA, below a characteristic $T_{Q_2}\!\simeq\!130$\,K, provides evidence for an electronically-driven charge-density-wave ordering. This points to a remarkable electron-lattice coupling, in which the footprint of the BiO-layer-induced superstructure is found in the modulated electronic structure of the CuO$_2$ plane.   

Electronic Scattering Rates in the High-T$_{c}$ Superconductor Bi$_{2.1}$Sr$_{1.9}$Ca(Cu$_{1-y}$Fe$_{y})_{2}$O$_{x}$

We investigate the effects of Fe impurities in bi-layer BSCCO. It is known that substituting Fe for Cu in this material reduces T$_{c}$, but the mechanism for this decrease is not well understood. We have developed a technique that utilizes ARPES to quantitatively measure the effects of impurities on electronic scattering. Using this technique we investigate the details of how Fe impurities cause an increase in the pair-breaking scattering rate in bi-layer BSCCO.   

Universal 115meV Feature and High Energy Spectral Weight Transfer in Bi$_2$Sr$_2$CaCu$_2$O$_{8+\Delta}$ Superconductor Revealed by Laser ARPES

Quasiparticle dispersion and spectral weight transfer have been investigated by laser-based angle-resolved photoemission spectroscopy in Bi$_2$Sr$_2$CaCu$_2$O$_{8+{\Delta}}$. A universal energy scale near $\sim$115meV can be clearly identified in superconducting state which is insensitive to momentum and doping levels. A concomitant observation with this energy scale is the spectral weight transfer over a large energy range when the sample goes from the normal state to the superconducting state. The origin and implications of these observations will be discussed.   

Towards the Standard Model for Fermi Arcs from a Wilsonian Reduction of the Hubbard Model

Two remarkable features emerge from the exact Wilsonian procedure for integrating out the high-energy scale in the Hubbard model. At low energies, the number of excitations that couple minimally to the electromagnetic gauge is less than the conserved charge, thereby implying a breakdown of Fermi liquid theory. In addition, two charge $e$ excitations emerge in the lower band, the standard projected electron and a composite entity (comprised of a hole and a charge $2e$ bosonic field) which give rise to poles and zeros of the single-particle Green function, respectively. The poles generate spectral weight along an arc centered at $(\pi/2,\pi/2)$ while the zeros kill the spectral intensity on the back-side of the arc. The result is the Fermi arc structure intrinsic to cuprate phenomenology. The presence of composite excitations also produces a broad incoherent pseudogap feature at the $(\pi,0)$ region of the Brillouin zone, thereby providing a mechanism for the nodal/anti-nodal dichotomy seen in the cuprates.