Session X40: Superconductivity: Transport Properties

IR and dc magneto-transport and ARPES compared: p-type cuprates

We report IR Hall Effect measurements in the terahertz (THz) spectral region on optimally doped $Bi_2 Sr_2 Ca Cu_2 O_{8+x}$ (BSCCO) single crystals and in $La_{2-x}Sr_{x}CuO_{4}$ (LSCO) films as a function of doping ranging from 7\% to 16.5\%. Critical comparisons between the dc Hall coefficient $R_H$ and Hall angle $\theta_H$, the IR $R_H$ and $\theta_H$, and the values predicted by ARPES analyzed within the relaxation time approximation will be discussed. In optimally doped BSCCO, the temperature power law associated with the off-diagonal conductivity at frequencies up to 10 meV deviates strongly from that of the longitudinal conductivity, and the IR Hall response is significantly larger than that expected based upon ARPES results. Analysis of dc $R_H$ measurements and ARPES results show a significant anomalous Hall effect $\sim$100K even in severely overdoped Tl-2201. In LSCO, a rapid decrease of the Hall mass with underdoping, a hallmark signature of Fermi surface reconstruction, is shown to strongly deviate from that predicted by the ARPES measured Fermi arcs. These results will be discussed in terms of Fermi surface models. [G. S. Jenkins \emph{et al.}, arXiv:0901.1701.]   

Thermo-Electric Study of Fermi Surface Reconstruction in YBa$_2$Cu$_3$O$_y$

The Seebeck and Nernst coefficients $S$ and $\nu$ of the high-$T_c$ superconductor YBa$_2$Cu$_3$O$_y$ (YBCO) were measured in a single crystal with a hole concentration $p = 0.12$ in magnetic fields up to $H = 28$ T. For temperatures down to 9 K, $\nu$ becomes independent of field by $H \simeq 30$ T, showing that by then the Nernst signal due to superconducting fluctuations has become negligible. In this field-induced normal state, $S/T$ and $\nu/T$ are both large and negative in the $T \to 0$ limit. The magnitude of $S/T$ is consistent with the small Fermi surface pocket previously detected via quantum oscillations in YBCO at a similar doping and its negative sign confirms that the pocket is electron-like. The normal-state $S(T)$ of YBCO is remarkably similar to that of La$_{2-x}$Ba$_x$CuO$_4$, La$_{2-x-y}$Nd$_y$Sr$_x$CuO$_4$ and La$_{2-x-y}$Eu$_y$Sr$_x$CuO$_4$ at $p \simeq 1/8$, all four materials showing a change of sign at $T \simeq 50$ K. Given that in the latter three materials this change of sign is clearly due to the onset of spin/charge density wave (`stripe') order, we infer that a similar density-wave mechanism must cause the Fermi surface reconstruction in YBCO.   

Hall coefficient in the ground state of the charge stripe-ordered La$_{2-x}$Ba$_x$CuO$_4$ single crystals

Temperature dependence of the Hall coefficient, $R_{\rm H}$, has been investigated in charge-spin stripe-ordered La-214 high- $T_{\rm c}$ superconductors [1]. Using the simplest stripe-ordered system of La$_{2-x} $Ba$_x$CuO$_4$, it has been clarified for the first time, to our knowledge, that both the behavior of $R_{\rm H}$ and its sign exhibit significant dependences on the hole concentration. That is, $R_{\rm H}$ is zero in the ground state of the charge- spin stripe order at $x=1/8$, while it is negative in the less- stabilized state of the charge stripe for $x<1/8$. These are interpreted as being due to the delicate balance of the contributions of the hole-like Fermi surface and the possible electron pocket, suggested from the recent quantum- oscillation experiments [2], arising from the formation of the charge-spin stripe order. \\[4pt] [1] T. Adachi {\it et al}., arXiv: 0909.3452. \\[0pt] [2] N. Doiron-Leyraud {\it et al}., Nature (London) {\bf 447}, 565 (2007).   

Identifying the quasi-particle contribution to the Nernst effect in La-based cuprates

It has recently been shown that stripe order causes an enhanced normal-state Nernst effect ($\nu = N/B$) in Nd- and Eu-doped La$_{2-x}$Sr$_x$CuO$_4$ (LSCO) [1]. In magnetic fields up to $B=10$~T, the normal-state Nernst effect was tracked down to $\sim30$~K. Here we report on the Nernst effect in Nd-LSCO at $p=0.20$ and Eu-LSCO at $p=0.125$ in fields up to $B=28$~T, where in both cases $T_C<2$~K. In this field-induced normal state, the quasi-particle contribution to the Nernst effect dominates completely over the signal from superconducting fluctuations. We find that $\nu/T$ grows with decreasing temperature to reach a very sizable value as $T\to 0$. \\ \\ {[1]} \quad Cyr-Choini\`ere \textit{et al.}, \textit{Nature}, \textbf{458}, 743--745 (2009).   

Pseudogap temperature from quasiparticle Nernst effect in La-based cuprates

We compare the Nernst effect in stripe ordered Eu-doped La$_{2-x}$Sr$_x$CuO$_4$ (Eu-LSCO) and similarly hole-doped LSCO. We show that at high temperatures they have essentially the same temperature dependence, showing an upturn and sign change starting at a temperature $T_\nu$. Relating $T_\nu$ to the pseudogap temperature $T^\star$, we present a general phase diagram for LSCO showing that $T^\star$ decreases as the doping increases until it is driven to zero within the superconducting dome. We compare the phase diagram and the $T^\star$ line to the similar ones of YBa$_2$Cu$_3$O$_y$ shown in a recent study [1]. \\ \\ {[1]} R. Daou et al., arXiv:0909.4430   

Doping dependence of the normal-state Hall coefficient in YBCO

We report on the doping dependence of the normal-state Hall coefficient $R_{\rm H}(T)$ in the high-Tc superconductor $\rm YBa_2Cu_3O_y$ for $0.08 < p < 0.15$, measured in magnetic fields up to 60 T. $R_{\rm H}(T)$ is seen to drop below a temperature $T_{\rm H}$, to become negative at low temperature, as previously reported for two dopings (1). We attribute this drop to a reconstruction of the Fermi-surface caused by the onset of broken translational symmetry, which produces an electron-like pocket in the Fermi surface of underdoped YBCO. The fact that $T_{\rm H}$ peaks at $p = 1/8$ strongly suggests that Fermi-surface reconstruction is caused by stripe order. (1) D. LeBoeuf et al., Nature 450, 533 (2007).   

Quantum Oscillations in the C-Axis Transport of YBCO

We have measured the c-axis resistivity in single crystal YBCO in pulsed magnetic field up to 60 Tesla. These high quality samples show very large amplitude Shubnikov de Haas once the vortex lattice has melted. Because of the high resistivity of the c-axis as compared to the a-b plane, the signal to noise ratio of these oscillations is much higher than that of previously reported oscillations, and this allows for a detailed analysis using the well known Lifshitz --Kosevich theory. Data has been collected at several temperatures and angles, and fits to this data yield information about the shape and number of pockets that are contributing to the transport.   

Size, shape and carrier type of multiple Fermi surface pockets in YBCO from two-axis angular quantum oscillation measurements

The Fermi surface topology of YBa$_{2}$Cu$_{3}$O$_{6+x}$ is mapped by means of two-axis angular-dependent quantum oscillation measurements on high quality detwinned single crystals. Multiple Fermi surface pockets are probed by means of in-plane and out-of-plane rotation studies, the characteristics of which enable us to identify the carrier type, shape and location of the individual sections. We further use the temperature dependence of the quantum oscillation amplitude measured over an extended temperature range to discern the statistical distribution of constituent quasiparticles.   

The hall effect as a probe of fluctuating superconductivity in disordered two-dimensional films

Magnetotransport effects have long been used to investigate order parameter fluctuations and vortex dynamics in both high-Tc and conventional superconductors. We will describe recent experimental studies of the fluctuation-induced Hall effect in amorphous superconducting films of tantalum nitride and indium oxide. Our measurements are consistent with theoretical analyses of Gaussian fluctuation contributions to the Hall signal. This approach, which has shown success in recent studies of the fluctuation-induced Nernst effect, may yield an independent measure of the superconducting coupling strength.   

Observation of Anomalous Reentrant Superconductivity in Gd added Bi-2223 Superconductor

We report the observation of an anomalous reentrant superconducting-normal resistive transition behaviour in Gd added Bi$_{1.8}$Pb$_{0.35}$Sr$_{1.9}$Ca$_{2.1}$Cu$_{3}$Gd$_{x}$O$_{y}$ superconductor with x=0, 0.1, 0.2, 0.3, 0.4 and 0.5. The resistive transitions of the samples are measured by four probe contact method under different magnetic fields. Zhao et. al. [1] first observed this behaviour in Bi-2212/2223 intergrowth single crystals. This is interpreted using a model where Josephson coupling between the CuO2 trilayers is weakened by thermally activated resistance. Similar observations are made in the literature [2], [3]. Possible physical origins of this anomalous phenomenon are discussed. \\[4pt] [1] Zhao Y. , et. al. , Physical Review \textbf{B} 51, 3134 (1995) \\[0pt] [2] Wang W. , et al. , Physica \textbf{C} 341-348, 1921 (2000) \\[0pt] [3] Kim D. C., et al., Physical Review \textbf{B} 64, 064502 (2001)   

Particle-Hole Mixture in Bi$_2$Sr$_2$CaCu$_2$O$_8$ Revealed by Laser-Based Angle Resolved Photoemission Spectroscopy

Super-high resolution laser-based angle-resolved photoemission measurements have been carried out on Bi$_2$Sr$_2$CaCu$_2$O$_8$ high temperature superconductor. In this talk, we will report on the observation and analysis of particle-hole mixture in Bi$_2$Sr$_2$CaCu$_2$O$_8$.   

Extraction of Absolute Electron Self-Energy from Angle-Resolved Photoemission Measurements on Pb-Bi220

VUV Laser-based angle-resolved photoemission spectroscopy measurements with super-high resolution have been carried out on heavily over-doped Pb-Bi2201 high temperature superconductors with Tc$<$5K. Along nodal direction, a new approach is developed to determine the real bare band which makes it possible to determine the absolute electron self-energy and abstract the real bare band.   

Possibility of the Quantum Phase Transition in Non-Superconducting Ni-Substituted La$_{2-x}$Sr$_x$Cu$_{1-y}$Ni$_y$O$_4$

The possible quantum phase transition has been investigated in non- superconducting Ni-substituted La$_{2-x}$Sr$_x$Cu$_{1- y}$Ni$_y$O$_4$, in which the superconductivity observed in La$_{2- x}$Sr$_x$CuO$_4$ is suppressed by the Ni substitution without disturbing the magnetic state in the CuO$_2$ plane, from the specific heat, muon spin relaxation, electrical resistivity, magnetic susceptibility [1]. It has been concluded that the ground state changes upon hole doping from a magnetically ordered state with strong binding of a hole by Ni in the underdoped regime to a metallic state with the Kondo effect and that the quantum phase transition between two states is modified to be crossover-like due to the phase separation. [1] Y. Tanabe et al., arXiv: 0911.1016.   

``Bad Metal" Conductivity of Hard Core Bosons

Two dimensional hard core bosons suffer strong scattering in the high temperature resistive state at half filling. The dynamical conductivity is calculated using non perturbative tools such as continued fractions, series expansions and exact diagonalization. We find a large temperature range with linearly increasing resistivity and broad dynamical conductivity, signaling a breakdown of Boltzmann-Drude quasiparticle transport theory. At zero temperature, a high frequency peak in the dynamical conductivity appears above a ``Higgs mass'' gap, and corresponds to order parameter magnitude fluctuations. We discuss the apparent similarity between conductivity of hard core bosons and phenomenological characteristics of cuprates, including the universal scaling of Homes et. al. (Nature 430, 539 (2004)).