Robert Markiewicz

Bulk Fermi surface and momentum density in heavily doped La₂₋ₓSrₓCuO₄ using high-resolution Compton scattering and positron annihilation spectroscopies

W. Al-Sawai et al. — Wed, 18 Apr 2012 10:26:09 PDT

We have observed the bulk Fermi surface (FS) in an overdoped (x=0.3) single crystal of La₂₋ₓSrₓCuO₄ by using Compton scattering. A two-dimensional (2D) momentum density reconstruction from measured Compton profiles yields a clear FS signature in the third Brillouin zone along [100]. The quantitative agreement between density functional theory (DFT) calculations and momentum density experiment suggests that Fermi-liquid physics is restored in the overdoped regime. In particular the predicted FS topology is found to be in good accord with the corresponding experimental data. We find similar quantitative agreement between the measured 2D angular correlation of positron annihilation radiation (2D-ACAR) spectra and the DFT-based computations. However, 2D-ACAR does not give such a clear signature of the FS in the extended momentum space in either the theory or the experiment.

Reconstructing the bulk Fermi surface and superconducting gap properties from neutron scattering experiments

Tanmoy Das et al. — Wed, 18 Apr 2012 10:26:05 PDT

We develop an analytical tool to extract bulk electronic properties of unconventional superconductors from inelastic neutron scattering spectra. We show that the upward and downward branches of the spin excitation spectra have distinct origins, with the upper branch representing a gapped spin-wave dispersion and the lower branch associated with Bogoliubov quasiparticle scattering on the Fermi surface. Combined, they produce an "hourglass" dispersion with 45 degrees rotation of the spectrum, as found experimentally. The downward dispersion can be inverted to reveal the Fermi momentum dispersion of the single-particle spectrum as well as the corresponding superconducting (SC) gap function, analogously to the quasiparticle interference effect in scanning tunneling microscopy (STM). Whereas angle-resolved photoemission spectroscopy and STM provide surface-sensitive information, this inversion procedure provides bulk electronic properties. The technique is essentially model independent and can be applied to a wide variety of materials.

Emergence of multiple Fermi surface maps in angle-resolved photoemission from Bi₂Sr₂CaCu₂O₈+δ

M. C. Asensio et al. — Wed, 18 Apr 2012 10:26:00 PDT

We report angle-resolved photoemission spectra (ARPES) for emission from the Fermi energy (E_F) over a large area of the (kx,ky) plane using 21.2 and 32 eV photon energies in two distinct polarizations from an optimally doped single crystal of Bi₂Sr₂CaCu₂O₈₊_δ (Bi2212), together with extensive first-principles simulations of the ARPES intensities. The results display a wide-ranging level of accord between theory and experiment, demonstrating that the ARPES matrix elements can produce a striking variety of Fermi surface maps, especially in the presence of secondary features arising from modulations of the underlying tetragonal system. Our analysis demonstrates how the energy and polarization dependency of the ARPES matrix element can help to disentangle the primary contributions to the spectrum from the secondary features and indicates that secondary features reflect a direct modulation of the CuO₂ planes.

Systematic doping evolution of the underlying Fermi surface of La₂₋ₓSrₓCuO₄

T. Yoshida et al. — Wed, 18 Apr 2012 10:25:55 PDT

We have performed a systematic doping-dependent study of La₂₋ₓSrₓCuO₄ (LSCO) (0.03 <= x <= 0.3) by angle-resolved photoemWe have performed a systematic doping-dependent study of La₂₋ₓSrxCuO₄ (LSCO) (0.03⩽x⩽0.3) by angle-resolved photoemission spectroscopy. Over this entire doping range, the underlying “Fermi surface” determined from the low-energy spectral weight approximately satisfies Luttinger’s theorem, even down to the lightly doped region. This is in strong contrast to the results on Ca₂₋ₓNaₓCuO₂Cl₂ (Na-CCOC), which show a clear deviation from Luttinger’s theorem. We correlate these differences between LSCO and Na-CCOC with differences in the behavior of chemical potential shift and spectral weight transfer induced by hole doping.ission spectroscopy. Over this entire doping range, the underlying "Fermi surface" determined from the low-energy spectral weight approximately satisfies Luttinger's theorem, even down to the lightly doped region. This is in strong contrast to the results on Ca₂₋ₓNaₓCuO₂Cl₂ (Na-CCOC), which show a clear deviation from Luttinger's theorem. We correlate these differences between LSCO and Na-CCOC with differences in the behavior of chemical potential shift and spectral weight transfer induced by hole doping.

Influence of the third dimension of quasi-two-dimensional cuprate superconductors on angle-resolved photoemission spectra

A. Bansil et al. — Wed, 18 Apr 2012 10:25:50 PDT

Angle-resolved photoemission spectroscopy (ARPES) presents significant simplifications in analyzing strictly two-dimensional (2D) materials, but even the most anisotropic physical systems display some residual three-dimensionality. Here we demonstrate how this third dimension manifests itself in ARPES spectra of quasi-2D materials by considering the example of the cuprate Bi₂Sr₂CaCu₂O₈ (Bi2212). The intercell, interlayer hopping, which is responsible for kz dispersion of the bands, is found to induce an irreducible broadening to the ARPES line shapes with a characteristic dependence on the in-plane momentum k‖. Our study suggests that ARPES line shapes can provide a direct spectroscopic window for establishing the existence of coherent c-axis conductivity in a material via the detection of this broadening mechanism, and bears on the understanding of 2D to 3D crossover and pseudogap and stripe physics in novel materials through ARPES experiments.

X-ray absorption near-edge spectra of overdoped La₂₋ₓSrₓCuO₄ high-T(c) superconductors

Towfiq Ahmed et al. — Wed, 18 Apr 2012 10:25:44 PDT

We present results for realistic modeling of the x-ray absorption near edge structure (XANES) of the overdoped high-T_c superconductor La₂₋ₓSrₓCuO₄ in the hole doping range x = 0.20 - 0.30. Our computations are based on a real-space Green's function approach in which strong-correlation effects are taken into account in terms of a doping-dependent self-energy. The predicted O K-edge XANES is found to be in good accord with the corresponding experimental results in this overdoped regime. We find that the low energy spectra are dominated by the contribution of O atoms in the cuprate planes, with little contribution from apical O atoms.

Renormalization of f levels away from the Fermi energy in electron excitation spectroscopies: Density-functional results for Nd2-xCexCuO4

T. Jarlborg et al. — Wed, 18 Apr 2012 10:25:39 PDT

Relaxation energies for photoemission where an occupied electronic state is excited and for inverse photoemission where an empty state is filled are calculated within the density-functional theory with application to Nd₂₋ₓCeₓCuO₄. The associated relaxation energies are obtained by computing differences in total energies between the ground state and an excited state in which one hole or one electron is added into the system. The relaxation energies of f electrons are found to be of the order of several eV’s, indicating that f bands will appear substantially away from the Fermi energy (E_F) in their spectroscopic images, even if these bands lie near E_F. Similar shifts are obtained for the Gd-f states in Gd₂CuO₄. Our analysis explains why it would be difficult to observe f electrons at the E_F even in the absence of strong electronic correlations.

Magnetic mechanism of quasiparticle pairing in hole-doped cuprate superconductors

R. S. Markiewicz et al. — Wed, 18 Apr 2012 10:25:34 PDT

We have computed α²F’s for the hole-doped cuprates within the framework of the one-band Hubbard model, where the full magnetic response of the system is treated properly. The d-wave pairing weight α²F_d is found not only to contain a low-energy peak due to excitations near (π,π) expected from neutron-scattering data but also to display substantial spectral weight at higher energies due to contributions from other parts of the Brillouin zone as well as pair-breaking ferromagnetic excitations at low energies. The resulting solutions of the Eliashberg equations yield transition temperatures and gaps comparable to the experimentally observed values, suggesting that magnetic excitations of both high and low energies play an important role in providing the pairing glue in the cuprates

Evolution of midgap states and residual three dimensionality in La₂₋ₓSrₓCuO₄

S. Sahrakorpi et al. — Wed, 18 Apr 2012 10:25:29 PDT

We carry out extensive first-principles doping-dependent computations of angle-resolved photoemission (ARPES) intensities in La₂₋ₓSrₓCuO₄ over a wide range of binding energies. Intercell hopping and the associated three dimensionality, which is usually neglected in discussing cuprate physics, is shown to play a key role in shaping the ARPES spectra. Despite the obvious importance of strong coupling effects (e.g., the presence of a lower Hubbard band coexisting with midgap states in the doped insulator), a number of salient features of the experimental ARPES spectra are captured to a surprising extent when kz dispersion is properly included in the analysis.

Nodeless d-wave superconducting pairing due to residual antiferromagnetism in underdoped Pr2-xCexCuO4-δ

Tanmoy Das et al. — Wed, 18 Apr 2012 10:25:24 PDT

We investigate the doping dependence of the penetration depth versus temperature in electron-doped Pr₂₋ₓCeₓCuO_4-δ using a model which assumes the uniform coexistence of (mean-field) antiferromagnetism and superconductivity. Despite the presence of a dₓ²₋_y² pairing gap in the underlying spectrum, we find nodeless behavior of the low-T penetration depth in the underdoped case, in accord with experimental results. As doping increases, a linear-in-T behavior of the penetration depth, characteristic of d-wave pairing, emerges as the lower magnetic band crosses the Fermi level and creates a nodal Fermi surface pocket.

Raising Bi-O bands above the Fermi energy level of hole-doped Bi₂Sr₂CaCu₂O₈+δ and other Cuprate superconductors

Hsin Lin et al. — Wed, 18 Apr 2012 10:25:20 PDT

The Fermi surface (FS) of Bi₂Sr₂CaCu₂O₈₊_δ (Bi2212) predicted by band theory displays Bi-related pockets around the (π,0) point, which have never been observed experimentally. We show that when the effects of hole doping either by substituting Pb for Bi or by adding excess O in Bi2212 are included, the Bi-O bands are lifted above the Fermi energy (E_F) and the resulting first-principles FS is in remarkable accord with measurements. With decreasing hole doping the Bi-O bands drop below E_F and the system self-dopes below a critical hole concentration. Computations on other Bi- as well as Tl- and Hg-based compounds indicate that lifting of the cation-derived band with hole doping is a general property of the electronic structures of the cuprates.

Fermi-surface study of Ba1-xKxBiO3

S. Sahrakorpi et al. — Wed, 18 Apr 2012 10:25:14 PDT

We present all-electron computations of the three-dimensional (3D) Fermi surfaces (FS’s) in Ba₁₋ₓKₓBiO₃ for a number of different compositions based on the self-consistent Korringa-Kohn-Rostoker coherent-potential-approximation approach for incorporating the effects of Ba/K substitution. By assuming a simple cubic structure throughout the composition range, the evolution of the nesting and other features of the FS of the underlying pristine phase is correlated with the onset of various structural transitions with K doping. A parametrized scheme for obtaining an accurate 3D map of the FS in Ba₁₋ₓKₓBiO₃ for an arbitrary doping level is developed. We remark on the puzzling differences between the phase diagrams of Ba₁₋ₓKₓBiO₃ and BaPbₓBi₁₋ₓO₃ by comparing aspects of their electronic structures and those of the end compounds BaBiO₃, KBiO₃, and BaPbO₃. Our theoretically predicted FS’s in the cubic phase are relevant for analyzing high-resolution Compton scattering and positron-annihilation experiments sensitive to the electron momentum density, and are thus amenable to substantial experimental verification.

Bilayer splitting and coherence effects in optimal and underdoped Bi₂Sr₂CaCu₂O₈+δ

Y. D. Chuang et al. — Wed, 18 Apr 2012 10:25:09 PDT

We have carried out extensive high-resolution angle-resolved photoemission (ARPES) experiments on Bi₂Sr₂CACu₂O₈₊_δ samples, covering the entire doping range from the overdoped to the optimally and underdoped regimes in the normal state. Our focus is on delineating the doping dependence of the bilayer splitting which is associated with the intracell coupling of electrons between the two CuO₂ planes. We exploit the photon energy of 47 eV, where strong ARPES matrix element effects are found to provide a tremendous enhancement of the antibonding to bonding component of the bilayer split bands near (π,0), in good agreement with the predictions of corresponding first-principles simulations. Our detailed analysis indicates that the size of the bilayer splitting is only weakly dependent on the doping level, implying that electronic excitations continue to maintain some degree of coherence even in the underdoped regime.

Gutzwiller magnetic phase diagram of the cuprates

R. S. Markiewicz et al. — Wed, 18 Apr 2012 10:25:04 PDT

A general constructive procedure is presented for analyzing magnetic instabilities in two-dimensional materials, in terms of (predominantly) double nesting, and applied to Hartree-Fock plus random-phase approximation (HF+RPA) and Gutzwiller approximation plus RPA calculations of the Hubbard model. Applied to the cuprates, it is found that competing magnetic interactions are present only for hole doping, between half filling and the Van Hove singularity. While HF+RPA instabilities are present at all dopings (for sufficiently large Hubbard U), in a Gutzwiller approximation they are restricted to a doping range close to the range of relevance for the physical cuprates. The same model would hold for charge instabilities, except that the interaction is more likely to be q dependent.

Spectral decomposition and matrix element effects in scanning tunneling spectroscopy of Bi₂Sr₂CaCu₂O₈₊δ

Jouko Nieminen et al. — Wed, 18 Apr 2012 10:24:56 PDT

We present a Green’s function-based framework for modeling the scanning tunneling spectrum from the normal as well as the superconducting state of complex materials where the nature of the tunneling process—i.e., the effect of the tunneling “matrix element,” is properly taken into account. The formalism is applied to the case of optimally doped Bi₂Sr₂CaCu2=₂O_8+δ (Bi2212) high-Tc superconductor using a large tight-binding basis set of electron and hole orbitals. The results show clearly that the spectrum is modified strongly by the effects of the tunneling matrix element and that it is not a simple replica of the local density of states of the Cu dₓ²₋_y² orbitals with other orbitals playing a key role in shaping the spectra. We show how the spectrum can be decomposed usefully in terms of tunneling “channels” or paths through which the current flows from various orbitals in the system to the scanning tip. Such an analysis reveals symmetry-forbidden and symmetry-enhanced paths between the tip and the cuprate layers. Significant contributions arise from not only the CuO₂ layer closest to the tip but also from the second CuO₂ layer. The spectrum also contains a longer range background reflecting the nonlocal nature of the underlying Bloch states. In the superconducting state, coherence peaks are found to be dominated by the anomalous components of Green’s function.

Fermi-surface topology and low-lying electronic structure of the iron-based superconductor Ca₁₀(Pt₃As₈)(Fe₂As₂)₅

Madhab Neupane et al. — Wed, 18 Apr 2012 10:24:50 PDT

We report a study of low-energy electronic structure and Fermi surface topology for the recently discovered iron-based superconductor Ca₁₀(Pt₃As₈)(Fe₂As₂)₅(the 10-3-8 phase, with T_c∼8 K), via angle-resolved photoemission spectroscopy (ARPES). Despite its triclinic crystal structure, ARPES results reveal a fourfold symmetric band structure with the absence of Dirac-cone-like Fermi dots (related to magnetism) found around the Brillouin zone corners in other iron-based superconductors. Considering that the triclinic lattice and structural supercell arise from the Pt₃As₈ intermediary layers, these results indicate that those layers couple only weakly to the FeAs layers in this new superconductor at least near the surface, which has implications for the determination of its pairing mechanism.

Nonmonotonic dₓ2₋y2 superconducting gap in electron-doped Pr0.89LaCe0.11CuO4: Evidence of coexisting antiferromagnetism and superconductivity?

Tanmoy Das et al. — Wed, 18 Apr 2012 10:24:45 PDT

Recent experiments on Pr_0.89LaCe_0.11CuO₄ observe an anisotropic spin-correlation gap and a nonmonotonic superconducting (SC) gap, which we analyze within the framework of a t-t′-t″-t‴-tiv-U model with a dₓ²_−y² pairing interaction including a third-harmonic contribution. By introducing a realistic broadening of the quasiparticle spectrum to reflect small-angle scattering, our computations explain the experimental observations, especially the presence of a maximum in the leading-edge gap in the vicinity of the hot spots. Our analysis suggests that the material behaves like a two-band superconductor with the d-wave third harmonic acting as the interband pairing gap, and that the antiferromagnetic and SC orders coexist in a uniform phase.

One-band tight-binding model parametrization of the high-Tc cuprates including the effect of kz dispersion

R. S. Markiewicz et al. — Wed, 18 Apr 2012 10:24:40 PDT

We discuss the effects of interlayer hopping and the resulting kz dispersion in the cuprates within the framework of the one-band tight-binding model Hamiltonian. Specific forms of the dispersion relations in terms of the in-plane hopping parameters t, t′, t″, and t‴ and the effective interlayer hopping tz in La₂₋ₓSrₓCuO₄ (LSCO) and Nd₂₋ₓCeₓCuO₄ (NCCO) and the added intracell hopping tbi between the CuO₂ bilayers in Bi₂Sr₂CaCu₂O₈ (Bi2212) are presented. The values of the “bare” parameters are obtained via fits with the first-principles local-density-approximation- (LDA-) based band structures in LSCO, NCCO, and Bi2212. The corresponding “dressed” parameter sets which account for correlation effects beyond the LDA are derived by fitting experimental Fermi surface (FS) maps and dispersions near the Fermi energy in optimally doped and overdoped systems. The interlayer couplings tz and tbi are found generally to be a substantial fraction of the in-plane hopping t, although the value of tz in NCCO is anomalously small, reflecting absence of apical O atoms in the crystal structure. Our results provide some insight into the issues of the determination of doping from experimental FS maps in Bi2212, the role of intercell coupling in c-axis transport, and the possible correlations between the doping dependences of the binding energies of the Van Hove singularities and various prominent features observed in the angle-resolved photoemission and tunneling spectra of the cuprates.

effect of orbital symmetry of the tip on scanning tunneling spectra of Bi₂Sr₂CaCu₂O₈+δ

Ilpo Suominen et al. — Wed, 18 Apr 2012 10:24:35 PDT

We discuss how variations in the scanning tunneling microscope (STM) tip, whether unintentional or intentional, can lead to changes in topographic images and dI/dV spectra. We consider the possibility of utilizing functionalized tips in order to improve the sensitivity of STM experiments to local irregularities at the surface or hidden below the surface layers. The change in the tip symmetry can radically alter the contrast of the topographic image due to changes in tip-surface overlap. The dI/dV curves change their shape according to which sample bands the tip orbital tends to overlap. In addition, relative phases between competing tunneling channels can be inverted by changing the tip symmetry, which could help reveal the origin of a local irregularity in the tunneling spectrum.

Induced superconductivity in noncuprate layers of the Bi₂Sr₂CaCu₂O₈+δ high-temperature superconductor: Modeling of scanning tunneling spectra

Ilpo Suominen et al. — Wed, 18 Apr 2012 10:24:30 PDT

We analyze how the coherence peaks observed in scanning tunneling spectroscopy (STS) of cuprate high-temperature superconductors are transferred from the cuprate layer to the oxide layers adjacent to the STS microscope tip. For this purpose, we have carried out a realistic multiband calculation for the superconducting state of Bi₂Sr₂CaCu₂O₈₊_δ (Bi2212) assuming a short-range d-wave pairing interaction confined to the nearest-neighbor Cu dₓ²₋_y² orbitals. The resulting anomalous matrix elements of the Green’s function allow us to monitor how pairing is then induced not only within the cuprate bilayer but also within and across other layers and sites. The symmetry properties of the various anomalous matrix elements and the related selection rules are delineated.