Quasiparticle Spectral Density Research Articles

Interrelations among zero-field critical current density, irreversibility field and pseudogap in hole doped high-Tc cuprates

Critical current density and the irreversibility magnetic fields are two parameters which control most of the applications of high-temperature superconductors. In this brief communications we have investigated the intrinsic effects that determine the magnitudes of these two parameters. The roles of hole concentration in the CuO2 plane (p) and oxygen deficiency (δ) in the CuO1-δ chains on the zero-field low-temperature critical current density, Jc0, were studied for a series of Y1-xCaxBa2Cu3O7-δ (x = 0.00, 0.05, 0.10, and 0.20) thin films. Jc0 above the optimum hole concentration were found to be high. The magnitude of Jc0 were largely determined by the hole concentration, maximizing at p ∼ 0.185 ± 0.005, regardless of the values of x and δ. This implied that oxygen deficiency/disorder plays a secondary role and the intrinsic Jc0 is governed largely by the number of doped holes in the CuO2 planes of Y1-xCaxBa2Cu3O7-δ. Further support in favor of this finding was garnered from the analysis of the in-plane resistive transitions of thin films of YBa2Cu3O7-δ (YBCO) with magnetic fields (H) applied along the crystallographic c-direction. The characteristic magnetic field (H0), linked to the vortex activation energy and the irreversibility field, exhibits similar p-dependence as shown by Jc0(p). We have explained these observations in terms of the doping dependent pseudogap (PG) energy/temperature scale in the low-energy electronic energy density of states observed in high-Tc cuprates. Both the intrinsic critical current density and the irreversibility field depend directly on the superconducting condensation energy, which in turn is primarily determined by the magnitude of the hole concentration dependent PG in the quasiparticle spectral density.

Estimation of Cooper pair density and its relation to the critical current density in Y(Ca)BCO high-Tc cuprate superconductors

Hole concentration (p) in the CuO2 plane largely controls all the electronic properties in the normal and superconducting states of high-Tc cuprates. The critical current density, Jc, is no exception. Previous hole content dependent studies have demonstrated the role of intrinsic depairing current density in determining the observed critical current density in copper oxide superconductors. The intrinsic pinning energy depends directly on the superconducting condensation energy which determines the magnitude of the depairing critical current density. Superconducting condensation energy is proportional to the Cooper pair density (superpair density) and depends strongly on the hole concentration within the CuO2 plane. We have calculated the Cooper pair density, ρs, of YBCO (Y123), a typical hole doped cuprate, as a function of p, in this study. A triangular pseudogap, pinned at the Fermi level, in the quasiparticle spectral density has been considered. The low-temperature zero-field critical current density of a number of high quality Y(Ca)BCO thin films over wide range of compositions and hole concentrations have been explored. The normalized values of the superpair density and the experimental critical current density exhibit a clear correspondence as the in-plane hole content is varied. This systematic behavior provides us with strong evidence that the critical current density of hole doped cuprates is primarily dependent on the superpair density, which in turn depends on the magnitude of the pseudogap energy. The agreement between the estimated p-dependent superpair density and the previously experimentally determined superfluid density of Y(Ca)BCO is quite good.

Pair Breaking, Pseudogap, and Superconducting Tc of Hole-Doped Cuprates: Interrelations and Implications

Irrespective of the class they belong to, all the hole doped high-Tc cuprate superconductors show an anti-correlation between the superconducting transition temperature and the characteristic pseudogap energy in the underdoped region. The doping dependent pseudogap in the quasiparticle spectral density is believed to remove low-energy electronic states and thereby reduce the superconducting condensate. Impurities within the CuO2 plane, on the other hand, break Cooper pairs in the unitarity limit and diminish superfluid density. Both pseudogap in pure cuprates and impurity scattering in disordered cuprates reduces Tc very effectively. In this study we have compared and contrasted the mechanisms of Tc degradation due to pseudogap and impurity scattering in hole doped cuprates. We have suggested a framework where both these factors can be treated on somewhat equal footing. Beside impurity and pseudogap dependent superconducting transition temperature, the proposed scenario has been employed to investigate the disorder and hole content dependent isotope exponent in high-Tc cuprates in this work.

Effect of dynamical screening on single-particle spectral features of uniaxially strained graphene: Tuning the plasmaron ring

Electronic screening renormalizes the linear bands of graphene and in the vicinity of the Dirac point, creates a diamond shaped structure in the quasiparticle spectral density. This is a result of electron-plasmon scattering processes which produce a finite momentum feature referred to as the `plasmaron ring'. In this work we explore the effects of uniaxial strain on these spectral features with the aim of understanding how strain modifies correlations. We derive and calculate the spectral density to the G$_0$W-RPA level which allows us to identify the dispersive behaviour of the diamond geometry, and thus electron-plasmon scattering, for variation in electron-electron coupling strength and magnitude of applied strain. We find that the application of strain changes the geometry (in momentum) of the electron-plasmon scattering and that renormalizations beyond simple geometrical scalings further enhance this effect. These results suggest that the properties of the plasmaron ring can be tuned through the application of uniaxial strain, effectively producing a larger fine structure constant without the need to change the sample substrate.

Nature of the Pseudogap in High-T c Cuprates: Analysis of the Bulk Magnetic Susceptibility of La2−x Sr x Cu1−y Zn y O4

The nature of the pseudogap in the quasiparticle spectral density in high-Tc cuprates is a matter of intense debate. In this study we have investigated the effects of Zn substitution on the uniform (\(\boldsymbol{q}=0\), where \(\boldsymbol{q}\) is the wave vector) magnetic susceptibility, χ(T), of La2−xSrxCu1−yZnyO4 sintered samples with different hole concentrations, p(≡x), over a wide range of Zn contents (y). Non-magnetic Zn suppressed Tc most effectively and enhanced χ(T) systematically at low temperatures. We have extracted the characteristic pseudogap energy scale, eg, from the analysis of χ(T) data. Unlike Tc,eg was found to be fairly insensitive to the level of Zn substitution, even when Tc was completely suppressed by Zn. The Zn-induced Curie-like enhancement of the χ(T) was also found to be closely related to the PG energy scale. We discuss the possible implications of these findings in this paper.

Extraction of the pseudogap energy scale from the static magnetic susceptibility of single and doubleCuO2 planehigh-Tc cuprates

The effect of added holes per CuO2 plane, p, on the temperature-dependent uniform(q = 0, whereq is the wavevector)magnetic susceptibility, χ(T), of double CuO2 plane YBa2Cu3O7−δ and single CuO2 plane La2−xSrxCuO4 compounds was investigated over a wide range of oxygen deficiencies(δ) and Sr contents(x). Systematicvariations in χ(T) werefound with changing p. The characteristic pseudogap energy scale,εg(p), was extracted from theanalysis of the χ(T,p) data.From the evolution of εg(p), we have found quite abrupt signs of the pseudogap in the quasi-particlespectral density appearing first at around a planar hole content ofp = 0.190 ± 0.005.εg(p) rapidly grows (almost linearly) with decreasingp. In the heavilyunderdoped region, εg(p) extrapolates to ∼Jex (the antiferromagnetic exchange energy of the parent compound) as . These characteristic features of the pseudogap energy scale are almost identical forYBa2Cu3O7−δ and La2−xSrxCuO4 even though the superconducting transition temperature,Tc, and anisotropy are widely different for these two compounds. We have scrutinizeddifferent existing scenarios for the pseudogap in the light of our findings.

Quasiparticle spectra in the vicinity of ad-wave vortex

We discuss the evolution of the local quasiparticle spectral density and the related tunneling conductance measurable by the scanning tunneling microscope, as a function of distance r and angle \theta from the vortex core in a d_{x^2-y^2} superconductor. We consider the effects of electronic disorder and of a strongly anisotropic tunneling matrix element, and show that in real materials they will likely obscure the ~1/r asymptotic tail in the zero-bias tunneling conductance expected from the straightforward semiclassical analysis. We also give a prediction for the tunneling conductance anisotropy around the vortex core and establish a connection to the structure of the tunneling matrix element.

Energy dispersions and bandwidth in Pd4dphotoemission spectra

Atomic and electronic structures of a Pd(111) single crystal and a polycrystalline Pd film have been investigated by using low-energy electron diffraction (LEED) and photoemission spectroscopy (PES). LEED patterns of a clean Pd(111) show a threefold symmetry, suggesting no surface reconstruction. Band structures of Pd(111) for ${k}_{\ensuremath{\parallel}}$ along the $\overline{\ensuremath{\Gamma}}\ensuremath{-}{\overline{M}}^{\ensuremath{'}},$ $\overline{\ensuremath{\Gamma}}\ensuremath{-}\overline{K},$ and $\overline{\ensuremath{\Gamma}}\ensuremath{-}\overline{M}$ symmetry lines have been determined experimentally from angle-resolved photoemission spectroscopy measurements and compared to those calculated within the local-density-functional approximation (LDA). The valence band PES spectrum of Pd film is also obtained and compared to the calculated $4d$ angular momentum projected local density of states within LDA. Experimental and LDA band structures of Pd(111) exhibit a reasonably good agreement, whereas experimental bandwidths are narrower than the LDA bulk bandwidths by 0.5--1 eV. Such a discrepancy can be improved theoretically by taking into account more localized Pd surface states. In contrast, the calculated Pd $4d$ quasiparticle spectral density by including the on-site Coulomb correlation cannot resolve the disagreement between PES and LDA, suggesting that the Coulomb correlation effect is not substantial in understanding PES spectra of Pd.

Quasiparticle electronic structure of the degenerate d-band Hubbard model

The quasiparticle electronic band structure of the degenerate d-band Hubbard model is investigated within second-order U-perturbation treatment of the self-energy. The influence of the k -dependent self-energy on the quasiparticle spectral density of states, energy bands and density of states is discussed. A full k -dependent self-energy leads in a non-selfconsistent approach to narrower and sharper satellite-like structure on the quasiparticle density of states. On the other hand, in a selfconsistent approach the corresponding band structure does not reveal any additional (to the main energy band) quasiparticle excitations. It is also shown, that the quasiparticle and Kohn-Sham Fermi surfaces for electrons in a three-dimensional crystal lattice are non-equivalent.

Dynamical corrections to density-functional theory for quasiparticles in ferromagnetic 4f systems. I. T=0 results for EuO.

A theory for the electronic quasiparticle spectrum of ferromagnetic 4f systems is presented and applied to the semiconductor EuO. The starting point is a d-f exchange model, which we solve exactly for T = 0. One of the results is a simple relationship between the spin-up quasiparticle energies and the ''free'' Bloch energies epsilon-c/sub m/(k), which we use to fix the epsilon-c/sub m/(k) in a highly realistic manner by performing a new self-consistent spin-polarized band-structure calculation based on density-functional theory. With the so-determined Bloch energies we investigate the spin-down quasiparticle spectrum, which exhibits even at T = 0 strong many-body effects as a consequence of spin-exchange processes between localized magnetic 4f moments and itinerant conduction electrons. We discuss in detail the spin-down quasiparticle spectral density for the GAMMAL direction, which should be observable in an inverse photoemission experiment. The shape of this function is strongly k dependent, revealing different types of quasiparticles. The prominent quasiparticle peaks in the spin-down quasiparticle spectral density are used to construct a quasiparticle band structure, which shows some striking deviations from the one-particle solution of the density-functional theory. Furthermore, results for the electronic self-energy and the quasiparticle density of states are presented.