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Register a new userIn-plane optical spectral weight transfer in optimally doped Bi$_{2}$Sr$_{2}$Ca$_{2}$Cu$_{3}$O$_{10}$
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Added by
Fabrizio Carbone Drs
Publication date
2006
fields
Physics
and research's language is
English
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We examine the redistribution of the in-plane optical spectral weight in the normal and superconducting state in tri-layer bbb (Bi2223) near optimal doping ($T_c$ = 110 K) on a single crystal via infrared reflectivity and spectroscopic ellipsometry. We report the temperature dependence of the low-frequency integrated spectral weight $W(Omega_c)$ for different values of the cutoff energy $Omega_c$. Two different model-independent analyses consistently show that for $Omega_c$ = 1 eV, which is below the charge transfer gap, $W(Omega_c)$ increases below $T_c$, implying the lowering of the kinetic energy of the holes. This is opposite to the BCS scenario, but it follows the same trend observed in the bi-layer compound bb (Bi2212). The size of this effect is larger in Bi2223 than in Bi2212, approximately scaling with the critical temperature. In the normal state, the temperature dependence of $W(Omega_c)$ is close to $T^2$ up to 300 K.
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Here we report extensive ultrafast time-resolved reflectivity experiments on overdoped Bi$_{2}$Sr$_{2}$Ca$_{1-x}$Y$_x$Cu$_{2}$O$_{8+delta}$ single crystals (T$_C$=78 K) aimed to clarify the nature of the superconducting-to-normal-state photoinduced phase transition. The experimental data show the lack of the quasiparticles decay time divergence at the fluence required to induce this phase transition, in contrast to the thermally-driven phase transition observed at T$_C$ and at variance with recently reported photoinduced charge-density-wave and spin-density-wave to metal phase transitions. Our data demonstrate the non-thermal character of the superconducting-to-normal-state photoinduced phase transition. The data have been analyzed using an ad-hoc developed time-dependent Rothwarf-Taylor model, opening the question on the order of this non-equilibrium phase transition.
We estimated the ratios of $^{63}$Cu hyperfine coupling constants in the double-layer high-$T_mathrm{c}$ superconductor HgBa$_2$CaCu$_2$O$_{6+delta}$ from the anisotropies in Cu nuclear spin-lattice relaxation rates and spin Knight shifts to study the nature of the ultraslow fluctuations causing the $T_2$ anomaly in the Cu nuclear spin-echo decay. The ultraslow fluctuations may come from uniform magnetic fluctuations spread around the wave vector $q$ = 0, otherwise the electric origins.
The quantum condensate of Cooper-pairs forming a superconductor was originally conceived to be translationally invariant. In theory, however, pairs can exist with finite momentum $Q$ and thereby generate states with spatially modulating Cooper-pair density. While never observed directly in any superconductor, such a state has been created in ultra-cold $^{6}$Li gas. It is now widely hypothesized that the cuprate pseudogap phase contains such a pair density wave (PDW) state. Here we use nanometer resolution scanned Josephson tunneling microscopy (SJTM) to image Cooper-pair tunneling from a $d$-wave superconducting STM tip to the condensate of Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+x}$. Condensate visualization capabilities are demonstrated directly using the Cooper-pair density variations surrounding Zn impurity atoms and at the Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+x}$ crystal-supermodulation. Then, by using Fourier analysis of SJTM images, we discover the direct signature of a Cooper-pair density modulation at wavevectors $Q_{p} approx (0.25,0)2pi / a_{0}$;$(0,0.25)2pi / a_{0}$ in Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+x}$. The amplitude of these modulations is ~5% of the homogenous condensate density and their form factor exhibits primarily $s$/$s$-symmetry. This phenomenology is expected within Ginzburg-Landau theory when a charge density wave with $d$-symmetry form factor and wave vector $Q_{c}=Q_{p}$ coexists with a homogeneous $d$-symmetry superconductor ; it is also encompassed by several contemporary microscopic theories for the pseudogap phase.
We present the ab-plane optical conductivity of four single crystals of Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+delta}$ (Bi2212) with different carrier doping levels from the strongly underdoped to the strongly overdoped range with $T_c$=66, 88, 77, and 67 K respectively. We focus on the redistribution of the low frequency optical spectral weight (SW) in the superconducting and normal states. The temperature dependence of the low-frequency spectral weight in the normal state is significantly stronger in the overdoped regime. In agreement with other studies, the superconducting order is marked by an increase of the low frequency SW for low doping, while the SW decreases for the highly overdoped sample. The effect crosses through zero at a doping concentration $delta$=0.19 which is slightly to the right of the maximum of the superconducting dome. This sign change is not reproduced by the BCS model calculations, assuming the electron-momentum dispersion known from published ARPES data. Recent Cluster Dynamical Mean Field Theory (CDMFT) calculations based on the Hubbard and t-J models, agree in several relevant respects with the experimental data.
Anomalously high and sharp peaks in the conductance of intrinsic Josephson junctions in Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+delta}$ (Bi2212) mesas have been universally interpreted as superconducting energy gaps, but here we show they are a result of heating. This interpretation follows from a direct comparison to the equilibrium gap, $mathit Delta$, measured in break junctions on similar Bi2212 crystals. As the dissipated power increases with a greater number of junctions in the mesa, the conductance peak abruptly sharpens and its voltage decreases to well below 2$mathit Delta$. This sharpening, found in our experimental data, defies conventional intuition of heating effects on tunneling spectra, but it can be understood as an instability into a nonequilibrium two-phase coexistent state. The measured peak positions occur accurately within the voltage range that an S-shaped backbending is found in the {it calculated} current-voltage curves for spatially {it uniform} self-heating and that S-shape implies the potential for the uniform state to be unstable.
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