Persistence of strong electron coupling to a narrow boson spectrum in overdoped Bi2Sr2CaCu2O(8+delta) tunneling data

A d-wave, Eliashberg analysis of break-junction and STM tunneling spectra on Bi2Sr2CaCu2O(8+delta) (Bi2212) reveals that the spectral dip feature is directly linked to strong electronic coupling to a narrow boson spectrum, evidenced by a large peak in alpha2F(omega). The tunneling dip feature remains robust in the overdoped regime of Bi2212 with bulk T(c) values of 56 K-62 K. This is contrary to recent optical conductivity measurements of the self-energy that suggest the narrow boson spectrum disappears in overdoped Bi2212 and therefore cannot be essential for the pairing mechanism. The discrepancy is resolved by considering the way each technique probes the electron self-energy, in particular, the unique sensitivity of tunneling to the off-diagonal or pairing part of the self-energy.     

Intermediate coupling model of cuprates: Adding fluctuations to a weak coupling model of pseudogap and superconductivity competition

We demonstrate that many features ascribed to strong correlation effects in various spectroscopies of the cuprates are captured by a calculation of the self-energy incorporating effects of spin and charge fluctuations. The self-energy is calculated over the full doping range from half-filling to the overdoped system. In the normal state, the spectral function reveals four subbands: two widely split incoherent bands representing the remnant of the two Hubbard bands, and two additional coherent, spin- and charge-dressed in-gap bands split by a spin-density wave, which collapses in the overdoped regime. The resulting coherent subbands closely resemble our earlier mean-field results. Here we present an overview of the combined results of our mean-field calculations and the newer extensions into the intermediate coupling regime.     

Electron-phonon coupling on the surface of the topological insulator Bi2Se3 determined from surface-phonon dispersion measurements

In this Letter, we report measurements of the coupling between Dirac fermion quasiparticles (DFQs) and phonons on the (001) surface of the strong topological insulator Bi2Se3. While most contemporary investigations of this coupling have involved examining the temperature dependence of the DFQ self-energy via angle-resolved photoemission spectroscopy measurements, we employ inelastic helium-atom scattering to explore, for the first time, this coupling from the phonon perspective. Using a Hilbert transform, we are able to obtain the imaginary part of the phonon self-energy associated with a dispersive surface-phonon branch identified in our previous work [Phys. Rev. Lett. 107, 186102 (2011)] as having strong interactions with the DFQs. From this imaginary part of the self-energy we obtain a branch-specific electron-phonon coupling constant of 0.43, which is stronger than the values reported from the angle-resolved photoemission spectroscopy measurements.     

Anomalous enhancement of the coupling to the magnetic resonance mode in underdoped Pb-Bi2212

High-resolution angle-resolved photoemission with variable excitation energies is used to disentangle bilayer splitting effects and intrinsic (self-energy) effects in the electronic spectral function near the (pi,0) point of differently doped (Pb,Bi)(2)Sr(2)CaCu(2)O(8+delta). In contrast to overdoped samples, where intrinsic effects at the (pi,0) point are virtually absent, we find in underdoped samples intrinsic effects in the superconducting-state (pi,0) spectra of the antibonding band. This intrinsic effect is present only below the critical temperature and weakens considerably with doping. Our results give strong support for models which involve a strong coupling of electronic excitations with the resonance mode seen in inelastic neutron scattering experiments.     

Evidence for a pseudogap in underdoped Bi{2}Sr_{2}CaCu{2}O{8+delta} and YBa2Cu3O6.50 from in-plane optical conductivity measurements

The real part of the in-plane optical self-energy data in underdoped Bi_{2}Sr_{2}CaCu_{2}O_{8+delta} (Bi-2212) and ortho II YBa2Cu3O6.5 contains new and important information on the pseudogap. Using a theoretical model approach, a major new finding is that states lost below the pseudogap Delta_{pg} are accompanied by a pileup of states just above this energy. The pileup along with a sharp mode in the bosonic spectral function leads to an unusually rapid increase in the optical scattering rate as a function of frequency and a characteristically sloped peak in the real part of the optical self-energy. These features are not found in optimally doped and overdoped samples and represent the clearest signature so far in the in-plane optical conductivity of the opening of a pseudogap.     

Quasiparticle liquid in the highly overdoped Bi(2)Sr(2)CaCu(2)O(8+delta)

Results from the study of a highly overdoped (OD) Bi(2)Sr(2)CaCu(2)O(8+delta) with a T(c) = 51 K using angle-resolved photoemission spectroscopy are presented. We observe a sharp peak in the spectra near ( pi,0) that persists well above T(c), a nodal self-energy which approaches that seen for the Mo(110) surface state, and a more k-independent line shape at the Fermi surface than the lower-doped cuprates. This allows for a realistic comparison of the lifetime values to the experimental resistivity measurements. These observations point to the validity of the quasiparticle picture for the OD even in the normal state.     

New Josephson plasma modes in underdoped YBa2Cu3O6.6 induced by a parallel magnetic field

The c-axis reflectivity spectrum of underdoped YBa2Cu3O6.6 (YBCO) is measured below T(c)=59 K in parallel magnetic fields H parallel CuO2 up to 7 T. Upon application of a parallel field, a new peak appears at finite frequency in the optical conductivity at the expense of suppression of c-axis condensate weight. We conclude that the dramatic change originates from different Josephson coupling strengths between bilayers with and without Josephson vortices. We find that the 400 cm(-1) broad conductivity peak in YBCO gains the spectral weight under parallel magnetic field; this indicates that the condensate weight at omega=0 is distributed to this peak as well as to the new optical Josephson mode.     

Observation of out-of-phase bilayer plasmons in YBa(2)Cu(3)O(7-delta)

The temperature dependence of the c-axis optical conductivity sigma(omega) of optimally and overdoped YBa2Cu3Ox ( x = 6.93 and 7) is reported in the far- (FIR) and midinfrared (MIR) range. Below T(c) we observe a transfer of spectral weight from the FIR not only to the condensate at omega = 0, but also to a new peak in the MIR. This peak is naturally explained as a transverse out-of-phase bilayer plasmon by a model for sigma(omega) which takes the layered crystal structure into account. With decreasing doping the plasmon shifts to lower frequencies and can be identified with the surprising and so far not understood FIR feature reported in underdoped bilayer cuprates.     

High-energy scale revival and giant kink in the dispersion of a cuprate superconductor

In the present photoemission study of a cuprate superconductor Bi1.74Pb0.38Sr1.88CuO6+delta, we discovered a large scale dispersion of the lowest band, which unexpectedly follows the band structure calculation very well. Similar behavior observed in blue bronze and the Mott insulator Ca2CuO2Cl2 suggests that the origin of hopping-dominated dispersion in an overdoped cuprate might be quite complicated. A giant kink in the dispersion is observed, and the complete self-energy containing all interaction information is extracted for a doped cuprate. These results recovered significant missing pieces in our current understanding of the electronic structure of cuprates.     

Same superconducting criticality for underdoped and overdoped La2-xSrxCuO4 single crystals

By measuring the superconducting diamagnetic moments for an underdoped and an overdoped La2-xSrxCuO4 single crystal with equal quality and roughly equal transition temperatures, it is found that the underdoped sample has only one transition which corresponds to H(c2), but the overdoped sample has two transitions with the higher one at H(c2). Further investigation reveals the same upper-critical field H(c2) for both samples although the overall charge densities are very different, indicating the possibility of a very direct and detailed equivalence of the superconducting condensation process in the two doping limits. The second transition for the overdoped sample can be understood as the bulk coupling between the superconducting clusters produced by macroscopic phase separation.     

Self-energy analysis of multiple-bosonic mode coupling in Sr2RuO4

We report angle-resolved photoemission spectroscopy studies on Sr₂RuO₄. We observe multiple-bosonic mode coupling in the α and β band dispersions. To extract the self-energy from the data for which the usual fitting methods do not work well, we propose a scheme that exploits the relation between the spectral intensity and self-energy, termed as relative self-energy. The relative self-energy obtained in that way contains important features of the self-energy. We observe not only the features that can be obtained from the band dispersions but also additional features that were not seen.     

Pressure dependence of the irreversibility line in Bi2Sr2CaCu2O(8+delta): role of anisotropy in flux-line formation

One of the important problems of high-temperature superconductivity is to understand and ultimately to control fluxoid motion. Here we present data on the pressure dependence of the irreversibility line measured up to 2.5 GPa. We observe that the application of pressure changes the interplanar coupling by decreasing the c-axis length, without significantly disturbing the intraplanar superconductivity. Our results directly show the relationship between lattice spacing and the irreversibility line in Bi(2)Sr(2)CaCu(2)O(8+delta), and demonstrate the potential for a dramatic reduction in the flux motion.     

Evidence for two separate energy gaps in underdoped high-temperature cuprate superconductors from broadband infrared ellipsometry

We present broadband infrared ellipsometry measurements of the c-axis conductivity of underdoped RBa_{2}Cu_{3}O_{7-delta} (R=Y, Nd, and La) single crystals. Our data show that separate energy scales are underlying the redistributions of spectral weight due to the normal state pseudogap and the superconducting gap. Furthermore, they provide evidence that these gaps do not share the same electronic states and do not merge on the overdoped side. Accordingly, our data are suggestive of a two gap scenario with a pseudogap that is likely extrinsic with respect to superconductivity.     

Apparent electron-phonon interaction in strongly correlated systems

We study the interaction of electrons with phonons in strongly correlated solids, having high-T(c) cuprates in mind. Using sum rules, we show that the apparent strength of this interaction strongly depends on the property studied. If the solid has a small fraction (doping) delta of charge carriers, the influence of the interaction on the phonon self-energy is reduced by a factor delta, while there is no corresponding reduction of the coupling seen in the electron self-energy. This supports the interpretation of recent photoemission experiments, assuming a strong coupling to phonons.     

Temperature dependence of the electron self-energy in a polar-crystal slab

In this paper we investigate temperature dependence of the electron self-energy in the polar-crystal slab using Green-function method. We introduce Q2D free Green's function for the first time. Numerical calculations of the electron self-energy using GaAs as an example are performed. The results show that the electron self-energy is a decreasing function of temperature. In calculation, we consider the effect of the excited states on the electron self-energy and find the ground-state energy be about 11% lower than that of bulk polaron. The results also imply that the high excited states pay a larger contribution to the electron self-energy with increasing temperature.     

Charge-density-wave-induced modifications to the quasiparticle self-energy in 2H- TaSe2

The self-energy of the photohole in 2H-TaSe2 is measured by angle-resolved photoemission spectroscopy as a function of binding energy and temperature. In the charge-density wave (CDW) state, a structure in the self-energy is detected at approximately 65 meV that cannot be explained by electron-phonon scattering. A reduction in the scattering rates below this energy indicates the collapse of a major scattering channel with the formation of the CDW state accompanying the appearance of a bosonic "mode" in the excitation spectrum of the system.     

Electron self-energy and effective mass in a single heterostructure

In this paper, we investigate the electron self-energy and effective mass in a single heterostructure using Greenfunction method. Numerical calculations of the electron self-energy and effective mass for GaAs/A1As heterostructure are performed. The results show that the self-energy (effective mass) of electrons, which incorporate the energy of electron coupling to interface-optical phonons and half of the three-dimensional longitudinal optical phonons, increase (decrease) monotonically from that of interface polaron to that of the 3D bulk polaron with increasing the distance between the positions of the electron and interface.     

Identification of a new form of electron coupling in the Bi2Sr2CaCu2O8 superconductor by laser-based angle-resolved photoemission spectroscopy

Laser-based angle-resolved photoemission measurements with superhigh resolution have been carried out on an optimally doped Bi(2)Sr(2)CaCu(2)O(8) high temperature superconductor. New high energy features at approximately 115 meV and approximately 150 meV, in addition to the prominent approximately 70 meV one, are found to develop in the nodal electron self-energy in the superconducting state. These high energy features, which cannot be attributed to electron coupling with single phonon or magnetic resonance mode, point to the existence of a new form of electron coupling in high temperature superconductors.     

ARPES studies of c-axis intracell coupling in Bi2Sr2CaCu2O8+δ

Using angle-resolved photoemission spectroscopy we have performed a detailed study of bilayer splitting in Bi₂Sr₂CaCu₂O_8+δ as a function of doping level and temperature. In heavily overdoped samples where the splitting is the clearest, we extract an intracell coupling t_⊥∼55 meV. As a function of photon energy the intensity ratio of the bonding and antibonding bands varies, allowing us to detect the bilayer splitting effect in the optimal and underdoped regimes. Surprisingly, with reduced doping the intracell coupling is not measurably reduced. Upon cooling to the superconducting state, a gap Δ opens in both bands yet the magnitude of the splitting remains unchanged.     

Anisotropic field dependence of the magnetic transition in Cu2Te2O5Br2

In this paper, we present the results of measurements of the thermal conductivity of Cu2Te2O5Br2, a compound where tetrahedra of Cu2+ ions carrying S=1/2 spins form chains along the c-axis of the tetragonal crystal structure. The thermal conductivity was measured along both the c- and the a-direction as a function of temperature between 3 and 300 K and in external magnetic fields H up to 69 kOe, oriented both parallel and perpendicular to the c-axis. Distinct features of (T) were observed in the vicinity of TN=11.4 K in zero magnetic field. These features are unaltered in external fields which are parallel to the c-axis, but are more pronounced when a field is applied perpendicularly to the c-axis. The transition temperature increases upon enhancing the external field, but only if the field is oriented along the a-axis.