arXiv subjects

R. S. Markiewicz

Publications and source records attributed to R. S. Markiewicz.

At least 37 records · Page 2Linked to original sources

arXiv2012

Different doping from apical and planar oxygen vacancies in Ba$_{2}$CuO$_{4-δ}$ and La$_{2}$CuO$_{4-δ}$

First principles band-structure calculations for large supercells of Ba$_{2}$CuO$_{4-δ}$ and La$_{2}$CuO$_{4-δ}$ with different distributions and concentrations of oxygen vacancies show that the effective doping on copper sites strongly depends on where the vacancy is located. A vacancy within the Cu layer produces a weak doping effect while a vacancy located at an apical oxygen site acts as a stronger electron dopant on the copper layers and gradually brings the electronic structure close to that of La$_{2-x}$Sr$_x$CuO$_{4}$. These effects are robust and only depend marginally on lattice distortions. Our results show that deoxygenation can reduce the effect of traditional La/Sr or La/Nd substitutions. Our study clearly identifies location of the dopant in the crystal structure as an important factor in doping of the cuprate planes.

cond-mat.supr-con↗

arXiv2012

Lindhard and RPA susceptibility computations in extended momentum space in electron doped cuprates

We present an approximation for efficient calculation of the Lindhard susceptibility $χ^{L}(q,ω)$ in a periodic system through the use of simple products of real space functions and the fast Fourier transform (FFT). The method is illustrated by providing $χ^{L}(q,ω)$ results for the electron doped cuprate Nd$_{2-x}$Ce$_{x}$CuO$_{4}$ extended over several Brillouin zones. These results are relevant for interpreting inelastic X-ray scattering spectra from cuprates.

cond-mat.str-el↗

arXiv2012

Self Energy and Fluctuation Spectra in Cuprates: Comparing Optical and Photoemission Results

We compare efforts to extract self energies and fluctuation spectra of the cuprates using optical and photoemission techniques. The fluctuations have contributions from both the coherent and incoherent parts of the band, which are spread over the full bare bandwidth of >2eV. Many experimental studies concentrate on the coherent part of the band and hence miss higher energy fluctuations. Our study establishes the universal presence of high energy bosonic fluctuations across various spectroscopies as a key ingredient in the high temperature superconducting cuprates.

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arXiv2012

Short Range Smectic and Long Range Nematic Order in the Pseudogap Phase of Cuprates

We present a model for the combined nematic and `smectic' or stripe-like orders seen in recent scanning tunneling microscopy (STM) experiments in cuprates. We model the stripe order as an electronic charge density wave with associated Peierls distortion -- a `Pomeranchuk wave'. Disorder restricts this primary order to nanoscale domains, while secondary coupling to strain generates nematic order with considerably longer range.

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arXiv2012

Gutzwiller Magnetic Phase Diagram of the Cuprates

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 HF+RPA and Gutzwiller approximation GA+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.

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arXiv2012

Gutzwiller Magnetic Phase Diagram of the Undoped t-t'-U Hubbard Model

We calculate the magnetic phase diagram of the half-filled t-t'-U Hubbard model as a function of t' and U, within the Gutzwiller approximation RPA (GA+RPA). As U increases, the system first crosses over to one of a wide variety of incommensurate phases, whose origin is clarified in terms of double nesting. We evaluate the stability regime of the incommensurate phases by allowing for symmetry-breaking with regard to the formation of spin spirals, and find a crossover to commensurate phases as U increases and a full gap opens. The results are compared with a variety of other recent calculations, and in general good agreement is found. For parameters appropriate to the cuprates, double occupancy should be only mildly suppressed in the absence of magnetic order, inconsistent with a strong coupling scenario.

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arXiv2012

Evidence of strong correlations at the van Hove singularity in the scanning tunneling spectra of superconducting Bi_{2}Sr_{2}CaCu_{2}O_{8+δ} single crystals

We present realistic multiband calculations of scanning tunneling spectra in Bi_{2}Sr_{2}CaCu_{2} O_{8+δ} over a wide doping range. Our modeling incorporates effects of a competing pseudogap and pairing gap as well as effects of strong electronic correlations, which are included by introducing self-energy corrections in the one-particle propagators. The calculations provide a good description of the two-gap features seen in experiments at low energies and the evolution of the Van Hove singularity (VHS) with doping, and suggest a possible quantum critical point near the point where the VHS crosses the Fermi level.

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arXiv2012

Visualizing electron pockets in cuprate superconductors

Fingerprint of the electron-pocket in cuprates has been obtained only in numerous magneto-transport measurements, but its absence in spectroscopic observations pose a long-standing mystery. We develop a theoretical tool to provide ways to detect electron-pockets via numerous spectroscopies including scanning tunneling microscopy (STM) spectra, inelastic neutron scattering (INS), and angle-resolved photoemission spectroscopy (ARPES). We show that the quasiparticle-interference (QPI) pattern, measured by STM, shows additional 7 ${\bm q}$ vectors associated with the scattering on the electron-pocket, than that on the hole-pocket. Furthermore, the Bogolyubov quasiparticle scatterings of the electron pocket may lead to a second magnetic resonance mode in the INS spectra at a higher resonance energy. Finally, we reanalyze some STM, INS, and ARPES experimental data of several cuprate compounds which dictates the direct fingerprints of electron pockets in these systems.

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arXiv2012

Bulk Fermi surface and momentum density in heavily doped La$_{2-x}$Sr$_x$CuO$_4$ using high resolution Compton scattering and positron annihilation spectroscopies

We have observed the bulk Fermi surface (FS) in an overdoped ($x$=0.3) single crystal of La$_{2-x}$Sr$_x$CuO$_4$ 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.

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arXiv2012

Crossover from inelastic magnetic scattering of Cooper pairs to spin-wave dispersion produces low-energy kink in cuprates

We present GW based self-energy calculations for the state of coexisting spin-density wave and d-wave superconductivity in a series of cuprate superconductors. In these systems, the spin resonance spectrum exhibits the typical `hour-glass' form, whose upward and downward dispersion branches come from the gapped spin-wave and magnetic scattering of Cooper pairs, respectively. We show that the crossover between these two different dispersion features leads to an abrupt change in slope in the quasiparticle self-energy, and hence the low-energy kink commences in the single-particle quasiparticle spectrum. The calculated electron-bosonic coupling strength agrees well with experimental data as a function of temperature, doping and material. The results demonstrate that the electronic correlations dominate the quasiparticle spectra of cuprates near the low-energy kink, suggesting a relatively smaller role for phonons in this energy range.

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arXiv2012

Coexisting pseudogap, charge transfer gap, and Mott gap energy scales in the resonant inelastic x-ray scattering spectra of electron-doped cuprates

We present a computation of Cu K-edge resonant inelastic x-ray scattering (RIXS) spectra for electron-doped cuprates which includes coupling to bosonic fluctuations. Comparison with experiment over a wide range of energy and momentum transfers allows us to identify the signatures of three key normal-state energy scales: the pseudogap, charge transfer gap, and Mott gap. The calculations involve a three band Hubbard Hamiltonian based on Cu $d_{x^2-y^2}$ and O $p_x$, $p_y$ orbitals, with a self-energy correction which arises due to spin and charge fluctuations. Our theory reproduces characteristic features e.g., gap collapse, large spectral weight broadening, and spectral weight transfer as a function of doping, as seen in experiments.

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arXiv2011

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

We report a first 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 Tc = 8K), 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 arising from the Pt3As8 intermediary layers, these results indicate that those layers couple only weakly to the FeAs layers in this new superconductor, which has implications for the determination of its potentially novel pairing mechanism.

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arXiv2011

Reconstructing the bulk Fermi surface and the superconducting gap properties from Neutron Scattering experiments

We develop an analytical tool to extract bulk electronic properties of unconventional superconductors through inelastic neutron scattering (INS) spectra. Since the spin excitation spectrum in the superconducting (SC) state originates from Bogoliubov quasiparticle scattering associated with Fermi surface nesting, its energy-momentum relation--the so called `hour-glass' feature--can be inverted to reveal the Fermi momentum dispersion of the single-particle spectrum as well as the corresponding SC gap function. The inversion procedure is analogous to the quasiparticle interference (QPI) effect in scanning tunneling microscopy (STM). Whereas angle-resolved photoemission spectroscopy (ARPES) and STM provide surface sensitive information, our inversion procedure provides bulk electronic properties. The technique is essentially model independent and can be applied to a wide variety of materials.

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arXiv2011

Renormalization of f-levels away from the Fermi energy in electron excitation spectroscopies: Density functional results of Nd$_{2-x}$Ce$_x$CuO$_4$

Relaxation energies for photoemission, when an occupied electronic state is excited, and for inverse photoemission, when an empty state is filled, are calculated within the density functional theory with application to Nd$_{2-x}$Ce$_x$CuO$_4$. 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 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 $E_F$. Our analysis explains why it would be difficult to observe f electrons at the $E_F$ even in the absence of strong electronic correlations.

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arXiv2011

Spectral decomposition and matrix element effects in scanning tunneling spectroscopy of Bi$_2$Sr$_2$CaCu$_2$O$_{8+δ}$

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 Bi$_2$Sr$_2$CaCu$_2$O$_{8+δ}$ (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 (LDOS) of the Cu-$d_{x^2-y^2}$ 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 CuO$_2$ layer closest to the tip, but also from the second CuO$_2$ layer. The spectrum also contains a longer range background reflecting the non-local 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.

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arXiv2011

Effect of orbital symmetry of the tip on Scanning Tunneling Spectra of Bi$_2$Sr$_2$CaCu$_2$O$_{8+δ}$

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 tunneling spectrum.

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arXiv2011

Origin of electron-hole asymmetry in the scanning tunneling spectrum of $Bi_2Sr_2CaCu_2O_{8+δ}$

We have developed a material specific theoretical framework for modelling scanning tunneling spectroscopy (STS) of high temperature superconducting materials in the normal as well as the superconducting state. Results for $Bi_2Sr_2CaCu_2O_{8+δ}$ (Bi2212) show clearly that the tunneling process strongly modifies the STS spectrum from the local density of states (LDOS) of the $d_{x^2-y^2}$ orbital of Cu. The dominant tunneling channel to the surface Bi involves the $d_{x^2-y^2}$ orbitals of the four neighbouring Cu atoms. In accord with experimental observations, the computed spectrum displays a remarkable asymmetry between the processes of electron injection and extraction, which arises from contributions of Cu $d_{z^2}$ and other orbitals to the tunneling current.

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arXiv2011

X-ray absorption near-edge spectra of overdoped La_2-xSr_xCuO_4 high-T_c superconductors

We present results for realistic modeling of the x-ray absorption near edge structure (XANES) of the overdoped high-T_c superconductor La_2-xSr_xCuO_4 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.

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