92 research outputs found

    Angle-resolved photoemission spectra in the cuprates from the d-density wave theory

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    Angle-resolved photoemission spectra present two challenges for the d-density wave (DDW) theory of the pseudogap state of the cuprates: (1) hole pockets near (Ï€/2,Ï€/2)(\pi/2,\pi/2) are not observed, in apparent contradiction with the assumption of translational symmetry breaking, and (2) there are no well-defined quasiparticles at the {\it antinodal} points, in contradiction with the predictions of mean-field theory of this broken symmetry state. Here, we show how these puzzles can be resolved.Comment: 4 pages, 3 eps figures, RevTex

    Approximate tight-binding sum rule for the superconductivity related change of c-axis kinetic energy in multilayer cuprate superconductors

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    We present an extension of the c-axis tight-binding sum rule discussed by Chakravarty, Kee, and Abrahams [Phys. Rev. Lett. 82, 2366 (1999)] that applies to multilayer high-Tc cuprate superconductors (HTCS) and use it to estimate--from available infrared data--the change below Tc of the c-axis kinetic energy, Hc, in YBa2Cu3O(7-delta) (delta=0.45,0.25,0.07), Bi2Sr2CaCu2O8, and Bi2Sr2Ca2Cu3O10. In all these compounds Hc decreases below Tc and except for Bi2Sr2CaCu2O8 the change of Hc is of the same order of magnitude as the condensation energy. This observation supports the hypothesis that in multilayer HTCS superconductivity is considerably amplified by the interlayer tunnelling mechanism.Comment: 6 pages, 2 figure

    Sum rules and energy scales in the high-temperature superconductor YBa2Cu3O6+x

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    The Ferrell-Glover-Tinkham (FGT) sum rule has been applied to the temperature dependence of the in-plane optical conductivity of optimally-doped YBa_2Cu_3O_{6.95} and underdoped YBa_2Cu_3O_{6.60}. Within the accuracy of the experiment, the sum rule is obeyed in both materials. However, the energy scale \omega_c required to recover the full strength of the superfluid \rho_s in the two materials is dramatically different; \omega_c \simeq 800 cm^{-1} in the optimally doped system (close to twice the maximum of the superconducting gap, 2\Delta_0), but \omega_c \gtrsim 5000 cm^{-1} in the underdoped system. In both materials, the normal-state scattering rate close to the critical temperature is small, \Gamma < 2\Delta_0, so that the materials are not in the dirty limit and the relevant energy scale for \rho_s in a BCS material should be twice the energy gap. The FGT sum rule in the optimally-doped material suggests that the majority of the spectral weight of the condensate comes from energies below 2\Delta_0, which is consistent with a BCS material in which the condensate originates from a Fermi liquid normal state. In the underdoped material the larger energy scale may be a result of the non-Fermi liquid nature of the normal state. The dramatically different energy scales suggest that the nature of the normal state creates specific conditions for observing the different aspects of what is presumably a central mechanism for superconductivity in these materials.Comment: RevTeX 4 file, 9 pages with 7 embedded eps figure

    Condensation Energy and High Tc Superconductivity

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    From an analysis of the specific heat of one of the cuprate superconductors it is shown, that even if a large part of the experimental specific heat associated with the superconducting phase transition is due to fluctuations, this part must be counted when one tries to extract the condensation energy from the data. Previous work by Chakravarty, Kee and Abrahams, where the fluctuation part was subtracted, has resulted in an incorrect estimation of the condensation energy.Comment: 4 pages, 5 encapsulated Postscript figures, uses ReVTeX.st

    Electron Dynamics in Nd1.85_{1.85}Ce.15_{.15}CuO4+δ_{4+\delta}: Evidence for the Pseudogap State and Unconventional c-axis Response

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    Infrared reflectance measurements were made with light polarized along the a- and c-axis of both superconducting and antiferromagnetic phases of electron doped Nd1.85_{1.85}Ce.15_{.15}CuO4+δ_{4+\delta}. The results are compared to characteristic features of the electromagnetic response in hole doped cuprates. Within the CuO2_2 planes the frequency dependent scattering rate, 1/τ(ω)\tau(\omega), is depressed below ∼\sim 650 cm−1^{-1}; this behavior is a hallmark of the pseudogap state. While in several hole doped compounds the energy scales associated with the pseudogap and superconducting states are quite close, we are able to show that in Nd1.85_{1.85}Ce.15_{.15}CuO4+δ_{4+\delta} the two scales differ by more than one order of magnitude. Another feature of the in-plane charge response is a peak in the real part of the conductivity, σ1(ω)\sigma_1(\omega), at 50-110 cm−1^{-1} which is in sharp contrast with the Drude-like response where σ1(ω)\sigma_1(\omega) is centered at ω=0\omega=0. This latter effect is similar to what is found in disordered hole doped cuprates and is discussed in the context of carrier localization. Examination of the c-axis conductivity gives evidence for an anomalously broad frequency range from which the interlayer superfluid is accumulated. Compelling evidence for the pseudogap state as well as other characteristics of the charge dynamics in Nd1.85_{1.85}Ce.15_{.15}CuO4+δ_{4+\delta} signal global similarities of the cuprate phase diagram with respect to electron and hole doping.Comment: Submitted to PR

    Searching for the Slater Transition in the Pyrochlore Cd2_{2}Os2_{2}O7_{7} with Infrared Spectroscopy

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    Infrared reflectance measurements were made on the single crystal pyrochlore Cd2_{2}Os2_{2}O7_{7} in order to examine the transformations of the electronic structure and crystal lattice across the boundary of the metal insulator transition at TMIT=226KT_{MIT}=226K. All predicted IR active phonons are observed in the conductivity over all temperatures and the oscillator strength is found to be temperature independent. These results indicate that charge ordering plays only a minor role in the MIT and that the transition is strictly electronic in nature. The conductivity shows the clear opening of a gap with 2Δ=5.2kBTMIT2\Delta=5.2k_{B}T_{MIT}. The gap opens continuously, with a temperature dependence similar to that of BCS superconductors, and the gap edge having a distinct σ(ω)∼ω1/2\sigma(\omega)\thicksim\omega^{1/2} dependence. All of these observables support the suggestion of a Slater transition in Cd2_{2}% Os2_{2}O7_{7}.Comment: 4 pages, 4 figure

    Triplanar Model for the Gap and Penetration Depth in YBCO

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    YBaCuO_7 is a trilayer material with a unit cell consisting of a CuO_2 bilayer with a CuO plane of chains in between. Starting with a model of isolated planes coupled through a transverse matrix element, we consider the possibility of intra as well as interplane pairing within a nearly antiferromagnetic Fermi liquid model. Solutions of a set of three coupled BCS equations for the gap exhibit orthorhombic symmetry with s- as well as d-wave contributions. The temperature dependence and a-b in plane anisotropy of the resulting penetration depth is discussed and compared with experiment.Comment: To appear in Physical Review B1 01Mar97; 12 pages with 10 figures; RevTeX+eps

    Conductivity sum rule, implication for in-plane dynamics and c-axis response

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    Recently observed cc-axis optical sum rule violations indicate non-Fermi liquid in-plane behavior. For coherent cc-axis coupling, the observed flat, nearly frequency independent cc-axis conductivity σ1(ω)\sigma_{1}(\omega) implies a large in-plane scattering rate Γ\Gamma around (0,π)(0,\pi) and therefore any pseudogap that might form at low frequency in the normal state will be smeared. On the other hand incoherent cc-axis coupling places no restriction on the value of Γ\Gamma and gives a more consistent picture of the observed sum rule violation which, we find in some cases, can be less than half.Comment: 3 figures. To appear in PR

    Correlation gap in the heavy-fermion antiferromagnet UPd_2Al_3

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    The optical properties of the heavy-fermion compound UPd2_2Al3_3 have been measured in the frequency range from 0.04 meV to 5 meV (0.3 to 40 cm−1^{-1}) at temperatures 2K<T<3002 {\rm K}<T< 300 K. Below the coherence temperature T∗≈50T^*\approx 50 K, the hybridization gap opens around 10 meV. As the temperature decreases further (T≤20T\leq 20 K), a well pronounced pseudogap of approximately 0.2 meV develops in the optical response; we relate this to the antiferromagnetic ordering which occurs below TN≈14T_N\approx 14 K. The frequency dependent mass and scattering rate give evidence that the enhancement of the effective mass mainly occurs below the energy which is associated to the magnetic correlations between the itinerant and localized 5f electrons. In addition to this correlation gap, we observe a narrow zero-frequency conductivity peak which at 2 K is less than 0.1 meV wide, and which contains only a fraction of the delocalized carriers. The analysis of the spectral weight infers a loss of kinetic energy associated with the superconducting transition.Comment: RevTex, 15 pages, 7 figure
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