Session Y21: Bi-based Cuprates -- Experiment

Phase Determination for Intra-unit-cell Fourier Transform STM -- Picometer Registration of Zn Impurity States in Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+\delta}$

By studying the real and imaginary components of the Bragg peaks in Fourier transforms of electronic structure images, distinct types of intra-unit cell (IUC) symmetry breaking can be studied using SI-STM [Lawler \textit{et al.}, Nature \textbf{466} 347 (2010)]. However, establishing the precise r-space symmetry point of each unit cell is crucial in defining the phase for such analysis. Exemplary of this challenge is the high-T$_{c}$ superconductor Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+\delta }$ for which the Bi atoms in the surface BiO layer are observable, while it is the invisible Cu atoms that define the relevant CuO$_{2}$ unit-cell symmetry point. We demonstrate, by imaging with pm precision the electronic impurity states at individual Zn atoms substituted at Cu sites, that the phase established using the Bi lattice produces a $\sim $2{\%}(2$\pi)$ error relative to the Cu lattice. In this case, IUC C$_{4v}$ symmetry breaking in the CuO$_{2}$ plane can be determined reliably using the phase assignment from the BiO layer. Moreover, impurity atom substitution at the relevant symmetry site can be of general utility in phase determination for Bragg-peak Fourier analysis of IUC symmetry.   

Imaging broken symmetry in the electronic structure of Bi-doped cuprates by scanning tunneling microscopy

The relationship between the mysterious pseudogap phase in the cuprate superconductors and the myriad broken symmetries observed by various experimental techniques is a source of much controversy. We use low-temperature scanning tunneling microscopy and spectroscopy to image electronic nematic order in the cuprate superconductor Bi2-xPbxSr2CuO6+$\delta $. We determine the robustness of the nematic order parameter to experimental conditions.   

Investigating structural distortions in Bi based superconductors by scanning tunneling microscopy

We use scanning tunneling microscopy to image symmetry-breaking structural distortions in the bismuth-based high-Tc superconductors, Bi$_{2}$Sr$_{2}$Ca$_{n-1}$Cu$_{n}$O$_{2n+4+x}$ (BSCCO). To elucidate the structure, we have implemented a new algorithm that quantifies atomic distortions with picometer resolution. Using this algorithm, we have observed an in-plane inversion-symmetry-breaking orthorhombic structural distortion in the BiO layer. We have also quantified the lattice distortions resulting from the use of Pb substitution to suppress the supermodulation in BSCCO. Each of these structural distortions break point group symmetries of the CuO$_{2}$ plaquette, which may be generally relevant to understanding the broken symmetries of the mysterious pseudogap state in high-Tc cuprates.   

Exploring the Nature of the Electronic Scatters in Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+\delta}$

The superconducting cuprates have been one of the most studied classes of materials over the last 20 years. While much progress has been made in understanding the strange electronic properties which govern their rich phase diagram, the interplay between coulomb interactions (which are thought to play a key role in their unconventional superconductivity) and the corresponding quasi particle scattering caused by coulomb interactions, are still the subject of debate among the community. We attempt to explore this issue by using spectroscopic imaging scanning tunneling spectroscopy (SI-STS) at variable temperatures which allows us to probe the Local Density of States (LDOS). The LDOS can be fitted with the theorized Dynes' formula allowing for the extraction of the quasi particle scattering rates. Furthermore, we seek to understand the scattering process by simulating the patterns produced by quasi particle interference (QPI) by either a pair potential impurity or a non-magnetic impurity. Extraction of these rates at various dopings and temperatures in the phase diagram will be presented.   

Imaging the impact of single dopant atoms on the electronic order and disorder of Bi$_{2+y}$Sr$_{2-y}$CaCu$_{2}$O$_{8+x}$

High-T$_{c}$ cuprate superconductors display startling nanoscale disorder in essential properties such as critical temperature, pseudogap energy, and even band structure. The underlying cause of this disorder has remained mysterious; theoretical explanations have ranged from chemical inhomogeneity to spontaneous electronic phase separation. We extend the energy range of scanning tunneling spectroscopy, allowing the first-ever direct mapping of both types of interstitial oxygen dopants in Bi$_{2+y}$Sr$_{2-y}$CaCu$_{2}$O$_{8+x}$, as well as vacancies at the apical oxygen site. We show that a subset of these dopants are indeed the direct cause of the nanoscale disorder. We further explain how the spatial variations in electronic orders, such as the pseudogap and the charge order, are governed by the disorder in the dopant concentrations, particularly vacancies in the apical oxygen site.   

Imaging Locally Oriented Charge Modulations in a Cuprate Superconductor

We use scanning tunneling microscopy to image the local orientation of the static charge modulations in Bi$_{2-y}$Pb$_y$Sr$_{2-z}$LazCuO$_{6+x}$, for samples spanning a wide range of doping. For each sample, we compute the size distribution of locally $x$-oriented and locally $y$-oriented clusters. We analyze the size distributions within a random field Ising model to obtain the fractal dimension and other critical exponents. We discuss the utility of scaling collapse to extract the critical doping $x_c$ of the smectic charge order in Bi$_{2-y}$Pb$_y$Sr$_{2-z}$LazCuO$_{6+x}$.   

Structure of intra-unit cell C$_{4v}$ symmetry breaking domains in Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+\delta}$ from SI-STM

Mounting evidence from a number of experimental probes supports the idea that the electronic structure of the cuprate pseudogap phase breaks rotational symmetry. Furthermore, ARPES, neutron scattering and spectroscopic imaging scanning tunneling microscopy (SI-STM) data all point to an intra-unit cell origin. We present new findings for near optimally doped Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+\delta }$(BSCCO) which elucidate the domains associated with intra-unit cell C$_{4v}$ symmetry breaking in the electronic structure. The analysis method will be motivated by the preceding talk, `Phase Determination of Intra-Unit Cell Fourier Transform STM -- Picometer Registration of Zn Impurity States in Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8}$' given by I. A. Firmo.   

Optical conductivity of exfoliated Bi$_2$Sr$_2$CaCu$_2$O$_{8+\delta}$ nanocrystals

We report on infrared spectromicroscopy of mechanically exfoliated under-doped and optimally-doped Bi$_2$Sr$_2$CaCu$_2$O$_{8+\delta}$ thin crystals on SiO$_2$/Si substrates. The infrared reflectance and transmission was measured for samples of various thicknesses and the the optical conductivity extracted in the frequency range 0.15 eV to 1 eV. Trends in the optical conductivity with thickness are discussed. In particular, we observe that the conductivity of thicker ($\sim$ 100 nm) samples is comparable to bulk while that of thinner ($\sim$ 20 nm) samples is markedly suppressed.   

Evidence of two-dimensional quantum critical behavior in the superfluid density of deeply underdoped Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+x}$

Evidence of two-dimensional (2-D) quantum critical fluctuations is observed in the superfluid density n$_{s}$(T) $\propto \quad \lambda ^{-2}$(T) of deeply underdoped Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+x }$(Bi-2212). Quantum critical behavior is indicated by the evolution of the T-dependence of n$_{s}$(T)/n$_{s}$(0), which loses any evidence for thermal critical behavior and becomes quasi-linear when underdoping drops the transition temperature T$_{c}$ below roughly 48K. Two-dimensionality is indicated by the linear scaling of transition temperature T$_{c}$ with n$_{s}$(0). The 2-D behavior contrasts with that of the less anisotropic YBa$_{2}$Cu$_{3}$O$_{7-\delta }$, which sustains 3D quantum critical fluctuations.   

Coherent THz emission from isosceles-triangular mesas of Bi2Sr2CaCu2O8

Using the standard emission patterns predicted for thin, isosceles triangular patch antennae, combined with a spatially uniform ac Josephson current source, we have performed two-parameter least-squares fits to our recently obtained unpublished experimental emission data from isosceles-triangularly-shaped Bi$_2$Sr$_2$CaCu$_2$O$_{8+\delta}$ mesas. Our results support the notion that such irregular patch antennae shapes may be useful in constructing high-powered, tunable continuous-wave coherent sources of light in the sub-terahertz to terahertz regime.   

Tunable sub-terahertz emission from intrinsic Josephson junctions in Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+\delta }$ enhanced by the internal cavity resonance

Intense, continuous and coherent terahertz electromagnetic wave emission from the intrinsic Josephson junction system in cuprate high-$T_{c}$ Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+\delta }$ with remarkably higher intensity [L. Ozyuzer \textit{et al.}, Science \textbf{318}, 1291 (2007).] than previously generated from single or arrayed Josephson junctions has been understood by the enhancement of the output intensity by the internal cavity resonance. However, we have recently observed emission, which seems to have low enhancement due perhaps to the relatively low-$Q$ factor, in a wide frequency range, covering almost all frequencies continuously as long as the ac Josephson effect is satisfied. This broadly tunable behavior is very different from that with the tunability found in the inner current-voltage branch region [M. Tsujimoto \textit{et al.}, to be published in 2011.] and enables us to design useful and tunable sub-terahertz source devises. In the presentation, we will also discuss the possible high-power devices using the high-$Q $electromagnetic cavity.   

Effect of magnetic field on the coherent THz emission from mesas of single crystal Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+\delta }$

Coherent and continuous electromagnetic (EM) waves radiation phenomena with a mesa structure of Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+\delta }$ single crystal have been investigated precisely in magnetic field up to only 200 Oe where the emission intensity decreases sharply expectedly for the field $H$ parallel to the $c$-axis. The emission could not be observed above 20 Oe for $H$//$c$-axis whereas it persisted up to 160 Oe for $H$//\textit{ab} plane [1]. These results indicate that both pancake vortices as well as Josephson vortices suppress the THz emission very strongly. On the other hand, the Josephson plasma resonance phenomena have been observed in both $H$//\textit{ab} and $H$//$c$ even in very high fields ($\sim $Tesla). The emission processes are considered to be the reverse processes of the absorption. It is interesting to pose a question what happens in high fields in the EM waves emission. We show interesting experimental results of THz emission in high magnetic fields including low field region and will argue the mechanism of emission in high magnetic fields. \\[4pt] [1] K. Yamaki \textit{et al}., physica C \textbf{470} (2010) S804-805.   

Quasiparticle dynamics in overdoped Bi$_{1.4}$Pb$_{0.7}$Sr$_{1.9}$CaCu$_{2}$O$_{8+\delta}$: Coexistence of superconducting gap and pseudogap below $T_{c}$

Photoexcited quasiparticle relaxation dynamics in overdoped Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+\delta}$ ($T_{c}$=65~K, hole doping $p$=0.22) single crystal is investigated as a function of temperature. We provide evidence of a $\sim$22~meV pseudogap ($T^{\ast}$$\approx$100~K) at this doping level. Moreover, this pseudogap vanishes at $T^{\ast}$. Our data support the scenario where both the superconducting gap and pseudogap coexist in the superconducting state. Our results also suggest an increased scattering rate between electrons and spin fluctuations as the sample enters the pseudogap phase. Phys. Rev. B \textbf{82, 212503 }(2010)   

THz wave radiation from the triangular mesas in layered high-$T_{c}$ superconductor Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+\delta}$ intrinsic Josephson junctions

Recently, it has been reported that the mesa structures fabricated from single crystalline high-$T$c superconductor Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+\delta }$ intrinsic Josephson junctions generate THz waves [1, 2]. However, most of radiations in the previous studies were observed either below the critical current where the junctions switched to the resistive state or around the retrapping region. We present here three types of emission for isosceles triangular mesas in which THz waves were also additionally detected above the critical current where the external current is increasing in the resistive branch structures.Furthermore, intense, coherent, monochromatic, continuous and tunable THz waves were observed in various triangular mesas fabricated by the focus ion beam (FIB) technique [3]. Moreover, elucidating the oscillating modes inside the mesas by precisely studying the IV characteristics, FT-IR spectrum and angular dependence of the emission intensity will be discussed in detail at meeting. \\[4pt] [1] L. Ozyuzer \textit{et al. }Science \textbf{318,} 1291 (2007). \\[0pt] [2] K. Kadowaki \textit{et al. }Physica C \textbf{468} 634 (2008). \\[0pt] [3] K. Delfanazari \textit{et al. }J. Phys. Conf. Ser. Accepted (2011).   

Particle-hole asymmetric QPI in the pseudogap phase of underdoped Bi-2212

Quasi-Particle Interference (QPI) measured by STM revealed many interesting phenomena in Correlated Electron Systems. Particle-hole (p-h) symmetric QPI observed in the superconducting Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+\delta}$ disappears around the reduced zone boundary [1], while p-h asymmetric QPI's on non-superconducting Sr$_{3}$Ru$_{2}$O$_{7}$ [2] and Ca(Fe$_{x}$Co$_{1-x}$)$_{2}$As$_{2}$ [3] identified dominant band structures, suggesting electronic nature of the nematic phases. In light of these discoveries, here we report the observation of p-h asymmetric QPI of underdoped Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+\delta}$ in the pseudogap phase continuously dispersing through E$_{F}$. The Fermi velocity measured from this dispersion is $\sim$0.12$\times$10$^{6}$ m/s. We will discuss the possible origin and implications by comparing its symmetry and dispersion using theoretical model and currently available experimental data from other probes.\\[4pt] [1] Y. Kohsaka et al., Nature 454, 1072 (2008)\\[0pt] [2] Jinho Lee et al., Nature Physics 5, 800 (2009)\\[0pt] [3] Chuang et al., Science 327, 181 (2010)