Session V21: Superconductivity: Mostly Cuprates

Evidence of strong correlations at the Van Hove singularity in the scanning-tunneling spectra of superconducting Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+\delta}$ single crystals

We present realistic multiband calculations of scanning tunneling spectra in Bi$_2$Sr$_2$CaCu$_2$O$_{8+\delta}$ over a wide doping range. Our modeling incorporates effects of a competing pseudogap and pairing gap as well as effects of strong electronic correlations, which are included by introducing self-energy corrections in the one-particle propagators. The calculations provide a good description of the two-gap features seen in experiments at low energies. In particular, the Van Hove singularity (VHS) in the underlying electronic states is found to split into a prominent incoherent feature at high energies and a weaker coherent part near the Fermi level which is strongly involved in gap formation. The progressive hybridization of the localized VHS into the Fermi surface with increasing doping is suggestive of Kondo physics which has been proposed previously for cuprates and heavy fermion compounds.   

Local correlations and deviations of energy scales from SI-STM in Bi-2212

One of the key difficulties in understanding the cuprate superconductors is reconciling the many probes that have been used to investigate them. In particular, in one of the most studied materials BSCCO-2212, this comparison is complicated by the striking atomic scale inhomogeneity that is seen in the electronic structure. We will present spectroscopic imaging scanning tunneling microscopy studies which show that the dominate energy scales are tied together not only as a function of doping but locally. This implies a single parameter that allows much of the low energy structure to be predicted. In addition, local structures which cause deviations in the ability of this single parameter to fit the data correspond to other phenomena previously described.   

Doping dependent study of non-equilibrium quasiparticle dynamics in the superconducting and normal state of La$_{2-x}$Sr$_{x}$CuO$_{4}$ thin films

We report on measurements of non-equilibrium quasiparticle dynamics in La$_{2-x}$Sr$_{x}$CuO$_{4}$ thin films via ultrafast optical pump probe spectroscopy. Quasiparticles are excited into the thin films using ultrashort laser pulses. The transient change in the quasiparticle density causes a corresponding change in the reflectivity of the sample, allowing time resolved measurements of quasiparticle recombination. We describe the doping dependence of these measurements both above and below Tc and discuss their implications on the nature of the electron-boson coupling in the cuprates.   

Doping-dependence of Scattering Interference in Cuprates

Evolution of the k-space electronic structure from the strongly underdoped to overdoped regime upon doping is studied. At the doping level coincident with the disappearance of the pseudogap phenomenology a crossover in the scattering processes is identified [1]. We discuss the significance of such transition in conjunction with other experimental facts. \\[4pt] [1] K. Fujita, M. Hamidian, I, A. Firmo, S. Mukhopadhyay, H. Eisaki, S. Uchida, and J. C. Davis \textit{Submitted} (2011).   

Time resolved reflectivity study of electron and hole doped cuprate superconductors

We study both the hole-doped cuprate HgBa$_{2}$CuO$_{4+\delta}$ (Hg1201) and the electron-doped cuprate Nd$_{2-x}$Ce$_{x}$CuO$_{4+\delta}$ (NCCO) using time-resolved optical reflectivity. In this technique, the sample is excited using an ultrashort 800 nm laser pulse, and the induced change in reflectivity is measured as a function of time after excitation. We focus on the temperature range above the superconducting transition; the fluctuating antiferromagnetic phase in NCCO and the pseudogap phase in Hg1201. In both cases we compare our results directly to neutron experiments, highlighting the relationship of the ultrafast optical response to magnetically ordered phases.   

Magnetic, Structural and Resistivity Characterization of New R$_{3}$Gd$_{3}$Ba$_{2}$Ca$_{2}$Cu$_{7}$O$_{10}$:Li Compounds (R= rare earth)

We report on the synthesis of R$_{3}$Gd$_{3}$Ba$_{2}$Ca$_{2}$Cu$_{7}$O$_{10}$:Li compounds with different rare earth elements (R=~Dy, Ho) by solid state reaction. Reaction products during the synthesis were identified by XRD and their thermal stability measured by Thermo-Gravimetric Analysis (TGA). Rietveld refinement of the XRD spectra enabled us to identify two coupled phases: a tetragonal one and an orthorhombic component. Magnetic measurements were conducted in the range of 5 to 300K to determine their Curie point and evidence for superconducting transitions. We find that in spite of the fact that both the Dy and Ho containing samples have the same crystalline structure, they exhibit different magnetic behavior over the temperature range studied. Evidence for a critical temperature at 15K for the Dy containing sample is observed exhibiting the characteristic Ms vs. T dependence ascribed to superconducting behavior. Detailed structural characterization results and measurements of R vs. T will be presented to confirm that the behavior observed at 15K is evidence for superconducting behavior.   

External vs. ``internal'' pressure effect on the anti-ferromagnetic superexchange energy, $J$, in LnBa$_2$Cu$_3$O$_6$ (Ln=La,Nd,...,Lu)

What causes the difference between the effect of ``internal'' pressure, as caused by ionic substitution, and external pressure on $T_c^{max}$ in the cuprates [1]? Is it the density of states, the pairing boson energy scale ($\omega_B$), condensation energy (which governs fluctuations), or ...? Many models of high temperature superconductivity put the energy scale of $\omega_B$ as the anti-ferromagnetic super-exchange energy, $J$, between adjacent Cu(2) ions in the CuO$_2$ plane. We therefore investigated Raman $B_{1g}$ two-magnon scattering in high quality LnBa$_2$Cu$_3$O$_6$ (Ln123) single crystals, Ln(=La, Nd, Sm, Eu, Gd, Dy, Yb, Lu), at ambient pressure to determine the effect of internal pressure on $J$. Comparing with measurements of $J$ under external pressure reveals that internal and external pressure have \emph{quantitatively} the same effect on $J$. However, and most surprisingly, we find an anticorrelation between $J$ and $T_c^{max}$ when ion size or internal pressure is the implicit variable. Given the opposite effects of internal and external pressure on $T_c^{max}$, this result suggests that some energy scale other than short range anti-ferromagnetic interactions has a more dominant effect on $T_c^{max}$.\\[4pt] [1] e.g. M. Marezio, Physica C, 341-348, 375 (2000)   

Anomalous High-Energy Spin Excitations in the High-$T_c$ Superconductor Parent Antiferromagnet La$_2$CuO$_4$

Inelastic neutron scattering is used to investigate the collective magnetic excitations of the high-temperature superconductor parent antiferromagnet La$_2$CuO$_4$. We find that while the lower energy excitations are well described by spin-wave theory, including one- and two-magnon scattering processes, the high-energy spin waves are strongly damped near the (1/2,0) position in reciprocal space and merge into a momentum dependent continuum. This anomalous damping indicates the decay of spin waves into other excitations, possibly unbound spinon pairs. The spinon dispersion that can explain the observed spin-wave damping in our experiments has the same form as the pseudogap dispersion observed in superconducting cuprates, with a minimum at (1/4,1/4).   

Charge transfer at the interface between ferromagnetic La$_{0.7}$Sr$_{0.3}$MnO$_{3}$ and superconducting EuBa$_{2}$Cu$_{3}$O$_{7}$ probed by STM/STS

La$_{0.7}$Sr$_{0.3}$MnO$_{3}$ (LSMO) is a ferromagnetic half-metallic compound with nearly 100{\%} spin polarization at room temperature, making it an ideal candidate for applications in spintronic devices. However, this useful functionality disappears when the thickness of LSMO film grown on SrTiO$_{3 }$substrate is reduced to below 4 nm, limiting its application in nanoscale devices. Here, we show that metallic and ferromagnetic properties of ultrathin ($<$ 4nm) LSMO film can be restored by interfacing it with a superconductor EuBa$_{2}$Cu$_{3}$O$_{7- \delta }$ (EBCO). We use scanning tunneling microscopy and spectroscopy to probe the evolution of the electronic structure of LSMO film grown on EBCO as functions of LSMO layer thickness and aging of bilayer LSMO/EBCO. Our results reveal that the charge (hole) transfer at LSMO/EBCO interface is responsible for driving LSMO film (of only five-unit-cell thickness) to metallic state. The conductive behavior of aged LSMO/EBCO bilayers varies systematically with the thickness of LSMO layer, allowing us to estimate the charge-transfer depth to be 4$\sim $5 nm on the LSMO side.   

Muon spin rotation investigation of the pressure effect on the magnetic penetration depth in YBa$_2$Cu$_3$O$_x$

The pressure dependence of the magnetic penetration depth $\lambda$ in polycrystalline samples of YBa$_2$Cu$_3$O$_x$ with different oxygen concentrations $x$ = 6.45, 6.6, 6.8, and 6.98 was studied by muon spin rotation ($\mu$SR). The pressure dependence of the superfluid density $\rho_s \propto 1/\lambda^{2}$ as a function of the superconducting transition temperature T$_{\rm c}$ is found to deviate from the usual Uemura line. The ratio $(\partial T_{\rm c}/\partial P)/(\partial\rho_s/\partial P)$ is factor of $\simeq$ 2 smaller than that of the Uemura relation. In underdoped samples, the zero temperature superconducting gap $\Delta_0$ and the BCS ratio $\Delta_0/k_{B}T_{\rm c}$ both increase with increasing external hydrostatic pressure, implying an increase of the coupling strength with pressure. The relation between the pressure effect and the oxygen isotope effect on $\lambda$ is also discussed. In order to analyze reliably the $\mu$SR spectra of samples with strong magnetic moments in a pressure cell, a special model was developed and applied.   

Ultrafast spectroscopy of the stripe phase of underdoped La$_{2-x}$Sr$_{x}$CuO$_{4}$ thin films

We have performed ultrafast measurements on high-T$_c$ thin films of underdoped La$_{2-x}$Sr$_{x}$CuO$_{4}$. In these experiments, femtosecond pulses create photoexcitations derived from both the superconducting and striped phase fraction whose evolution and decay are probed as a function of time. We discuss the separate dynamics of these two electronic components as a function of temperature and excitation density.   

Nonlinear Response of the High-Temperature Superconductor YBa$_{2}$Cu$_{3}$O$_{7-\delta}$ to the Transmission of Intense Terahertz Pulses

High-Power Terahertz Time-Domain Spectroscopy (THz-TDS) was used to examine YBa$_{2}$Cu$_{3}$O$_{7-\delta }$ thin films when transmitted by intense single-cycle THz pulses. This allowed for an investigation of the nonlinear, time-resolved behavior of YBa$_{2}$Cu$_{3}$O$_{7-\delta}$ in the presence of strong THz electric fields for the first time. High field strengths of tens of kV cm$^{-1}$ were achieved by improving the efficiency of optical rectification in LiNbO$_{3}$ through the tilted-pulse-front method and by ensuring a tight focusing of the THz beam. In the case of low field strengths, the behavior of the thin films agrees with previous examinations of YBa$_{2}$Cu$_{3}$O$_{7-\delta }$ by means of conventional, low-power THz-TDS. However, for strong THz electric fields, it was found by analysis with the two-fluid model that the superfluid population decreases dramatically, possibly due to Cooper pair breakup. This was accompanied by a drop in the imaginary part of the conductivity in the investigated frequency range of 0.2 to 0.8 THz. The results further suggest a decrease of the effective mass of the carriers for strong THz fields.   

Carrier mobility of insulating cuprates from time-resolved terahertz photoconductivity measurements

We study the ultrafast transient photoconductivity of three insulating cuprates, Sr$_2$CuO$_2$Cl$_2$, YBa$_2$Cu$_3$O$_6$, and La$_2$CuO$_4$. Terahertz spectroscopy reveals mobile dilute carriers, with a conductivity that appears promptly and decays non-exponentially in picoseconds. The peak photoconductivity is orders of magnitude larger than the static photoconductivity but smaller than that of chemically doped samples, with fast dynamics that depends weakly on material, concentration, and temperature. Decay dynamics indicate hopping transport with a low activation energy, suggesting weak to intermediate polaronic binding.   

Study of the radiation line width and shape from the Bi2212 mesa structure

Continuous electromagnetic waves in terahertz (THz) range have been observed from mesa structures of Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+\delta }$(Bi2212) single crystals$^{1)}$ It has been established that the radiation frequency is determined by both the ac Josephson frequency and the resonance condition of the geometrical cavity$^{2)}$. In order to understand the mechanism of the radiation from the intrinsic Josephson junctions (IJJs) in Bi2212, we studied the radiation line width and shape. These might depend upon the physical parameters of the Bi2212 single crystal such as the number of IJJs the fluctuations of the quasiparticles and pairs, the non-linearity and non-equilibrium conditions, and the stability of the electrical circuit including the IJJs. However, technical difficulties have been delayed the line width measurements in the THz range. Recently, we succeeded in measuring the radiation line width using a frequency mixer. These results will be compared to those from a single junction 1) L. Ozyuzer \textit{et al.}, Science \textbf{318} (2007) 1291., 2) K. Kadowaki \textit{et al.}, J. Phys. Soc. Jpn. \textbf{79} (2010) 023703   

High resolution copper M-Edge resonant inelastic X-ray scattering at MERLIN

The Advanced Light Source MERLIN RIXS spectrometer (beamline 4.0.3) is a new user endstation for resonant inelastic X-ray scattering at the Advanced Light Source. I will discuss new scientific explorations that are expected to be possible with the system, including preliminary data obtained during the commissioning period on cuprate samples. Because very few previous studies have been performed at the M-Edge, the presentation will review how M-Edge scattering data at MERLIN resemble and differ from the more broadly studied transition metal K- and L-edges, including critical parameters of the excitation process such as fitted intermediate state lifetimes, X-ray penetration depth, spin-orbit coupling and scattering intensity for different types of excitation mode.