arXiv subjects

Z. -X. Shen

Publications and source records attributed to Z. -X. Shen.

At least 91 records · Page 5Linked to original sources

arXiv2009

Material and doping dependence of the nodal and anti-nodal dispersion renormalizations in single- and multi-layer cuprates

In this paper we present a review of bosonic renormalization effects on electronic carriers observed from angle-resolved photoemission spectra in the cuprates. We specifically discuss the viewpoint that these renormalizations represent coupling of the electrons to the lattice, and review how the wide range of materials dependence, such as the number of CuO$_2$ layers, and the doping dependence can be straightforwardly understood as arising due to novel electron-phonon coupling.

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arXiv2009

Effect of strong correlations on the high energy anomaly in hole- and electron-doped high-Tc superconductors

Recently, angle-resolved photoemission spectroscopy (ARPES) has been used to highlight an anomalously large band renormalization at high binding energies in cuprate superconductors: the high energy 'waterfall' or high energy anomaly (HEA). This paper demonstrates, using a combination of new ARPES measurements and quantum Monte Carlo simulations, that the HEA is not simply the by-product of matrix element effects, but rather represents a cross-over from a quasiparticle band at low binding energies near the Fermi level to valence bands at higher binding energy, assumed to be of strong oxygen character, in both hole- and electron-doped cuprates. While photoemission matrix elements clearly play a role in changing the aesthetic appearance of the band dispersion, i.e. the 'waterfall'-like behavior, they provide an inadequate description for the physics that underlies the strong band renormalization giving rise to the HEA. Model calculations of the single-band Hubbard Hamiltonian showcase the role played by correlations in the formation of the HEA and uncover significant differences in the HEA energy scale for hole- and electron-doped cuprates. In addition, this approach properly captures the transfer of spectral weight accompanying both hole and electron doping in a correlated material and provides a unifying description of the HEA across both sides of the cuprate phase diagram.

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arXiv2009

Unconventional electronic reconstruction in undoped (Ba,Sr)Fe$_2$As$_2$ across the spin density wave transition

Through a systematic high resolution angle-resolved photoemission study of the iron pnictide compounds (Ba,Sr)Fe$_2$As$_2$, we show that the electronic structures of these compounds are significantly reconstructed across the spin density wave ordering, which cannot be described by a simple folding scenario of conventional density wave ordering. Moreover, we find that LDA calculations with an incorporated suppressed magnetic moment of 0.5$μ_{\tiny{\textrm{B}}}$ can match well the details in the reconstructed electronic structure, suggesting that the nature of magnetism in the pnictides is more itinerant than local, while the origin of suppressed magnetic moment remains an important issue for future investigations.

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arXiv2009

Evidence for weak electronic correlations in Fe-pnictides

Using x-ray absorption and resonant inelastic x-ray scattering, charge dynamics at and near the Fe $L$ edges is investigated in Fe pnictide materials, and contrasted to that measured in other Fe compounds. It is shown that the XAS and RIXS spectra for 122 and 1111 Fe pnictides are each qualitatively similar to Fe metal. Cluster diagonalization, multiplet, and density-functional calculations show that Coulomb correlations are much smaller than in the cuprates, highlighting the role of Fe metallicity and strong covalency in these materials. Best agreement with experiment is obtained using Hubbard parameters $U\lesssim 2$eV and $J\approx 0.8$eV.

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arXiv2009

Enhanced superconducting gaps in the tri-layer high-$T_c$ cuprate Bi$_2$Sr$_2$Ca$_2$Cu$_3$O$_{10+δ}$

We have investigated the optimally doped tri-layer cuprate Bi$_2$Sr$_2$Ca$_2$Cu$_3$O$_{10+δ}$ (Bi2223) by angle-resolved photoemission spectroscopy, and observed energy bands and Fermi surfaces originated from the outer and inner CuO$_2$ planes (OP and IP) separately. The OP band is overdoped (hole density $\sim$ 0.26/Cu) with a large d-wave gap of $Δ_0$ $\sim$ 43 meV while the IP is underdoped ($\sim$ 0.06/Cu) with an even large gap of $Δ_0$ $\sim$ 60 meV. We propose that the enhancement of the $Δ_0$ of IP is due to the minimal influence of out-of-plane disorder and that the OP gap is then enhanced through interlayer coupling, most likely a proximity effect from the IP.

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arXiv2009

Effects of out-of-plane disorder on the nodal quasiparticle and superconducting gap in single-layer Bi$_2$Sr$_{1.6}Ln_{0.4}$CuO$_{6+δ}$ ($Ln$ = La, Nd, Gd)

How out-of-plane disorder affects the electronic structure has been investigated for the single-layer cuprates Bi$_2$Sr$_{1.6}$$Ln$$_{0.4}$CuO$_{6+δ}$ ($Ln$ = La, Nd, Gd) by angle-resolved photoemission spectroscopy. We have observed that, with increasing disorder, while the Fermi surface shape and band dispersions are not affected, the quasi-particle width increases, the anti-nodal gap is enhanced and the superconducting gap in the nodal region is depressed. The results indicate that the superconductivity is significantly depressed by out-of-plane disorder through the enhancement of the anti-nodal gap and the depression of the superconducting gap in the nodal region.

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arXiv2009

Underlying Fermi surface of Sr$_{14-x}$Ca$_x$Cu$_{24}$O$_{41}$ in two-dimensional momentum space observed by angle-resolved photoemission spectroscopy

We have performed an angle-resolved photoemission study of the two-leg ladder system Sr$_{14-x}$Ca$_x$Cu$_{24}$O$_{41}$ with $x$= 0 and 11. "Underlying Fermi surfaces" determined from low energy spectral weight mapping indicates the quasi-one dimensional nature of the electronic structure. Energy gap caused by the charge density wave has been observed for $x$=0 and the gap tends to close with Ca substitution. The absence of a quasi-particle peak even in $x$=11 is in contrast to the two-dimensional high-$T_c$ cuprates, implying strong carrier localization related to the hole crystalization.

cond-mat.str-el↗

arXiv2009

Electronic Structure of the BaFe$_2$As$_2$ Family of Iron Pnictides

We use high resolution angle-resolved photoemission spectroscopy to study the band structure and Fermi surface topology of the BaFe$_2$As$_2$ iron pnictides. We observe two electron bands and two hole bands near the X-point, $(π,π)$ of the Brillouin zone, in the paramagnetic state for different doping levels, including electron-doped Ba(Co$_{0.06}$Fe$_{0.94}$)$_2$As$_2$, undoped BaFe$_2$As$_2$, and hole-doped Ba$_{0.6}$K$_{0.4}$Fe$_2$As$_2$. Among these four bands, only the electron bands cross the Fermi level, forming two electron pockets around X, while the hole bands approach but never reach the Fermi level. We show that the band structure of the BaFe$_2$As$_2$ family matches reasonably well with the prediction of LDA calculations after a momentum-dependent shift and renormalization. Our finding resolves a number of inconsistencies regarding the electronic structure of pnictides.

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arXiv2009

ARPES studies of the electronic structure of LaOFe(P,As)

We report a comparison study of LaOFeP and LaOFeAs, two parent compounds of recently discovered iron-pnictide superconductors, using angle-resolved photoemission spectroscopy. Both systems exhibit some common features that are very different from well-studied cuprates. In addition, important differences have also been observed between these two ferrooxypnictides. For LaOFeP, quantitative agreement can be found between our photoemission data and the LDA band structure calculations, suggesting that a weak coupling approach based on an itinerant ground state may be more appropriate for understanding this new superconducting compound. In contrast, the agreement between LDA calculations and experiments in LaOFeAs is relatively poor, as highlighted by the unexpected Fermi surface topology around (pi,pi). Further investigations are required for a comprehensive understanding of the electronic structure of LaOFeAs and related compounds.

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arXiv2008

Universal versus Material-Dependent Two-Gap Behaviors in the High-Tc Cuprates: Angle-Resolved Photoemission Study of La2-xSrxCuO4

We have investigated the doping and temperature dependences of the pseudogap/superconducting gap in the single-layer cuprate La$_{2-x}$Sr$_x$CuO$_4$ by angle-resolved photoemission spectroscopy. The results clearly exhibit two distinct energy and temperature scales, namely, the gap around ($π$,0) of magnitude $Δ^*$ and the gap around the node characterized by the d-wave order parameter $Δ_0$, like the double-layer cuprate Bi2212. In comparison with Bi2212 having higher $T_c$'s, $Δ_0$ is smaller, while $Δ^*$ and $T^*$ are similar. This result suggests that $Δ^*$ and $T^*$ are approximately material-independent properties of a single CuO$_2$ plane, in contrast the material-dependent $Δ_0$, representing the pairing strength.

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arXiv2008

Fermi Surface Evolution Across Multiple Charge Density Wave Transitions in ErTe3

The Fermi surface (FS) of ErTe3 is investigated using angle-resolved photoemission spectroscopy (ARPES). Low temperature measurements reveal two incommensurate charge density wave (CDW) gaps created by perpendicular FS nesting vectors. A large Delta_1 = 175 meV gap arising from a CDW with c* - q_CDW1 ~ 0.70(0) c* is in good agreement with the expected value. A second, smaller Delta_2 = 50 meV gap is due to a second CDW with a* - q_CDW2 ~ 0.68(5) a*. The temperature dependence of the FS, the two gaps and possible interaction between the CDWs are examined.

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arXiv2008

Appearance of Universal Metallic Dispersion in a Doped Mott Insulator

We have investigated the dispersion renormalization $Z_{disp}$ in La$_{2-x}$Sr$_x$CuO$_4$ (LSCO) over the wide doping range of $x=0.03-0.30$, for binding energies extending to several hundred meV's. Strong correlation effects conspire in such a way that the system exhibits an LDA-like dispersion which essentially `undresses' ($Z_{disp}\to 1$) as the Mott insulator is approached. Our finding that the Mott insulator contains `nascent' or `preformed' metallic states with a vanishing spectral weight offers a challenge to existing theoretical scenarios for cuprates.

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arXiv2008

Analysis of the spectral function of Nd1.85Ce0.15CuO4, obtained by angle resolved photoemission spectroscopy

Samples of Nd(2-x)Ce(x)CuO(4), an electron-doped high temperature superconducting cuprate (HTSC), near optimal doping at x = 0.155 were measured via angle resolved photoemission (ARPES). We report a renormalization feature in the self energy ("kink") in the band dispersion at 50 - 60 meV present in nodal and antinodal cuts across the Fermi surface. Specifically, while the kink had previously only been seen in the antinodal region, it is now observed also in the nodal region, reminiscent of what has been observed in hole-doped cuprates.

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arXiv2008

Electronic structure of the iron-based superconductor LaOFeP

The recent discovery of superconductivity in the so-called iron-oxypnictide family of compounds has generated intense interest. The layered crystal structure with transition metal ions in planar square lattice form and the discovery of spin-density-wave order near 130 K seem to hint at a strong similarity with the copper oxide superconductors. A burning current issue is the nature of the ground state of the parent compounds. Two distinct classes of theories have been put forward depending on the underlying band structures: local moment antiferromagnetic ground state for strong coupling approach and itinerant ground state for weak coupling approach. The local moment magnetism approach stresses on-site correlations and proximity to a Mott insulating state and thus the resemblance to cuprates; while the latter approach emphasizes the itinerant electron physics and the interplay between the competing ferromagnetic and antiferromagnetic fluctuations. Such a controversy is partly due to the lack of conclusive experimental information on the electronic structures. Here we report the first angle-resolved photoemission spectroscopy (ARPES) investigation of LaOFeP (Tc = 5.9 K), the first reported iron-based superconductor. Our results favor the itinerant ground state, albeit with band renormalization. In addition, our data reveal important differences between these and copper based superconductors.

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arXiv2008

Fermi surface and van Hove singularities in the itinerant metamagnet Sr3Ru2O7

The low-energy electronic structure of the itinerant metamagnet Sr3Ru2O7 is investigated by angle resolved photoemission and density functional calculations. We find well-defined quasiparticle bands with resolution limited line widths and Fermi velocities up to an order of magnitude lower than in single layer Sr2RuO4. The complete topography, the cyclotron masses and the orbital character of the Fermi surface are determined, in agreement with bulk sensitive de Haas - van Alphen measurements. An analysis of the dxy band dispersion reveals a complex density of states including van Hove singularities (vHs) near the Fermi level; a situation which is favorable for magnetic instabilities.

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arXiv2008

Charge dynamics of doped holes in high Tc cuprates - A clue from optical conductivity

The charge dynamics in weakly hole doped high temperature superconductors is studied in terms of the accurate numerical solution to a model of a single hole interacting with a quantum lattice in an antiferromagnetic background, and accurate far-infrared ellipsometry measurements. The experimentally observed two electronic bands in the infrared spectrum can be identified in terms of the interplay between the electron correlation and electron-phonon interaction resolving the long standing mystery of the mid-infrared band.

cond-mat.str-el↗

arXiv2008

Doping evolution of the electronic structure in the single-layer cuprates Bi$_2$Sr$_{2-x}$La$_x$CuO$_{6+δ}$: Comparison with other single-layer cuprates

We have performed angle-resolved photoemission and core-level x-ray photoemission studies of the single-layer cuprate Bi$_2$Sr$_{2-x}$La$_x$CuO$_{6+δ}$ (Bi2201) and revealed the doping evolution of the electronic structure from the lightly-doped to optimally-doped regions. We have observed the formation of the dispersive quasi-particle band, evolution of the Fermi ``arc'' into the Fermi surface and the shift of the chemical potential with hole doping as in other cuprates. The doping evolution in Bi2201 is similar to that in Ca$_{2-x}$Na$_{x}$CuO$_{2}$Cl$_2$ (Na-CCOC), where a rapid chemical potential shift toward the lower Hubbard band of the parent insulator has been observed, but is quite different from that in La$_{2-x}$Sr$_{x}$CuO$_{4}$ (LSCO), where the chemical potential does not shift, yet the dispersive band and the Fermi arc/surface are formed around the Fermi level already in the lightly-doped region. The (underlying) Fermi surface shape and band dispersions are quantitatively analyzed using tight-binding fit, and the deduced next-nearest-neighbor hopping integral $t'$ also confirm the similarity to Na-CCOC and the difference from LSCO.

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arXiv2008

Extracting the spectral function of the cuprates by a full two-dimensional analysis: Angle-resolved photoemission spectra of Bi2Sr2CuO6

Recently, angle-resolved photoemission spectroscopy (ARPES) has revealed a dispersion anomaly at high binding energy near 0.3-0.5eV in various families of the high-temperature superconductors. For further studies of this anomaly we present a new two-dimensional fitting-scheme and apply it to high-statistics ARPES data of the strongly-overdoped Bi2Sr2CuO6 cuprate superconductor. The procedure allows us to extract theself-energy in an extended energy and momentum range. It is found that the spectral function of Bi2Sr2CuO6 can be parameterized using a small set of tight-binding parameters and a weakly-momentum-dependent self-energy up to 0.7 eV in binding energy and over the entire first Brillouin zone. Moreover the analysis gives an estimate of the momentum dependence of the matrix element, a quantity, which is often neglected in ARPES analyses.

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