arXiv subjects

Z. -X. Shen

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

At least 145 records · Page 8Linked to original sources

arXiv2003

Effect of chemical inhomogeneity in the bismuth-based copper oxide superconductors

We examine the effect on the superconducting transition temperature (Tc) of chemical inhomogeneities in Bi2201 and Bi2212 single crystals. Cation disorder at the Sr crystallographic site is inherent in these materials and strongly affects the value of Tc. Partial substitution of Sr by Ln (Ln = La, Pr, Nd, Sm, Eu, Gd, and Bi) in Bi_{2}Sr_{1.6}Ln_{0.4}CuO_{6} results in a monotonic decrease of Tc with increasing ionic radius mismatch. By minimizing Sr site disorder at the expense of Ca site disorder, we demonstrate that the Tc of Bi2212 can be increased to 96 K. Based on these results we discuss the effects of chemical inhomogeneity in other bulk high-temperature superconductors.

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arXiv2003

Effects of next-nearest-neighbor hopping $t^{\prime}$ on the electronic structure of cuprates

Photoemission spectra of underdoped and lightly-doped Bi$_{2-z}$Pb$_z$Sr$_2$Ca$_{1-x}${\it R}$_{x}$Cu$_2$O$_{8+y}$ ($R=$ Pr, Er) (BSCCO) have been measured and compared with those of La$_{2-x}$Sr$_x$CuO$_4$ (LSCO). The lower-Hubbard band of the insulating BSCCO, like Ca$_2$CuO$_2$Cl$_2$, shows a stronger dispersion than La$_2$CuO$_4$ from ${\bf k}\sim$($π/2,π/2$) to $\sim$($π,0$). The flat band at ${\bf k}\sim$($π,0$) is found generally deeper in BSCCO. These observations together with the Fermi-surface shapes and the chemical potential shifts indicate that the next-nearest-neighbor hopping $|t^{\prime}|$ of the single-band model is larger in BSCCO than in LSCO and that $|t^{\prime}|$ rather than the super-exchange $J$ influences the pseudogap energy scale.

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arXiv2003

Fully Gapped Single-Particle Excitations in the Lightly Doped Cuprates

The low-energy excitations of the lightly doped cuprates were studied by angle-resolved photoemission spectroscopy. A finite gap was measured over the entire Brillouin zone, including along the d_{x^2 - y^2} nodal line. This effect was observed to be generic to the normal states of numerous cuprates, including hole-doped La_{2-x}Sr_{x}CuO_{4} and Ca_{2-x}Na_{x}CuO_{2}Cl_{2} and electron-doped Nd_{2-x}Ce_{x}CuO_{4}. In all compounds, the gap appears to close with increasing carrier doping. We consider various scenarios to explain our results, including the possible effects of chemical disorder, electronic inhomogeneity, and a competing phase.

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arXiv2003

An angle-resolved photoemission spectral function analysis of the electron doped cuprate Nd_1.85Ce_0.15CuO_4

Using methods made possible by recent advances in photoemission technology, we perform an indepth line-shape analysis of the angle-resolved photoemission spectra of the electron doped (n-type) cuprate superconductor Nd_1.85Ce_0.15CuO_4. Unlike for the p-type materials, we only observe weak mass renormalizations near 50-70 meV. This may be indicative of smaller electron-phonon coupling or due to the masking effects of other interactions that make the electron-phonon coupling harder to detect. This latter scenario may suggest limitations of the spectral function analysis in extracting electronic self-energies when some of the interactions are highly momentum dependent.

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arXiv2003

Nodal metallic behavior of lightly-doped La$_{2-x}$Sr$_x$CuO$_4$

Lightly-doped La$_{2-x}$Sr$_x$CuO$_4$ in the so-called "insulating" spin-glass/diagonal-stripe phase has been studied by angle-resolved photoemission spectroscopy. A quasi-particle (QP) peak crossing the Fermi level has been observed in the node direction of the d-wave superconducting gap, forming an "arc" of Fermi surface consistent with the recently observed metallic transport behavior at high temperatures of lightly-doped materials. The spectral weight of the nodal QP smoothly increases with hole doping, corresponding to the $n \sim x$ behavior of the carrier number in the underdoped and lightly-doped regions.

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arXiv2003

Evolution of a Metal to Insulator Transition in Ca$_{2-x}$Na$_{x}$CuO$_{2}$Cl$_{2}$, as seen by ARPES

We present angle resolved photoemission (ARPES) data on Na-doped Ca$_2$CuO$_2$Cl$_2$. We demonstrate that the chemical potential shifts upon doping the system across the insulator to metal transition. The resulting low energy spectra reveal a gap structure which appears to deviate from the canonical $d_{x2-y2} ~ |cos(k_x a)-cos(k_y a)|$ form. To reconcile the measured gap structure with d-wave superconductivity one can understand the data in terms of two gaps, a very small one contributing to the nodal region and a very large one dominating the anti-nodal region. The latter is a result of the electronic structure observed in the undoped antiferromagnetic insulator. Furthermore, the low energy electronic structure of the metallic sample contains a two component structure in the nodal direction, and a change in velocity of the dispersion in the nodal direction at roughly 50 meV. We discuss these results in connection with photoemission data on other cuprate systems.

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arXiv2002

Universality of the Electronic Structure from a Half Filled CuO_2 Plane

We present angle resolved photoemission (ARPES) data from insulating A_2CuO_2X_2 (A=Sr, Ca; X=Cl, Br), Sr_2Cu_3O_4Cl_2, and Bi_2Sr_2MCu_2O_8 (M=Er, Dy) single crystals which illustrate that the low energy electronic structure of the half-filled cuprates are independent of the apical atom. By performing a photon energy dependent study on Ca_2CuO_2Cl_2 we are able to distinguish between features which are intrinsic and those which are a result of the photoemission matrix elements. We find that the dispersion is independent of photon energy, while an asymmetry in the electron occupation probability across the antiferromagnetic zone boundary is robust to variations in photon energy. Finally, we show that the d-wave-like dispersion which exists in the insulator along the antiferromagnetic zone boundary does not precisely fit the simple d-wave functional form near the nodal direction.

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arXiv2002

Angle-resolved photoemission spectroscopy of Na-doped Ca2CuO2Cl2 single crystals: Fingerprints of a magnetic insulator in a heavily underdoped superconductor

Electronic evolution from an antiferromagnet to a high-Tc superconductor is revealed by angle-resolved photoemission experiments on tetragonal Ca(1.9)Na(0.1)CuO(2)Cl(2) single crystals, which were successfully grown for the first time under high pressures. In this underdoped superconductor, we found clear fingerprints of the parent insulator: a shadow band and a large pseudo-gap. These observations are most likely described by a "chemical potential shift", which contrasts clearly with the prevailing wisdom of the "pinned chemical potential" learned from the prototype La(2-x)Sr(x)CuO(4), demonstrating that the route to a high-Tc superconductor is not unique.

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arXiv2002

Doping dependence of an n-type cuprate superconductor investigated by ARPES

We present an angle resolved photoemission (ARPES) doping dependence study of the n-type cuprate superconductor Nd_2-xCe_xCuO_4, from the half-filled Mott-insulator to the T_c=24K superconductor. In Nd2CuO4, we reveal the charge-transfer band (CTB) for the first time. As electrons are doped into the system, this feature's intensity decreases with the concomitant formation of near-E_F spectral weight. At low doping, the Fermi surface is an electron-pocket (with volume ~ x) centered at (pi,0). Further doping leads to the creation of a new hole-like Fermi surface (volume ~ 1+x) centered at (pi,pi). These findings shed light on the Mott gap, its doping evolution, as well as the anomalous transport properties of the n-type cuprates.

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arXiv2002

Key Ingredients for Superconductivity in Cuprates

Using high resolution angle-resolved photoemission data in conjunction with that from neutron and other probes, we show that electron-phonon (el-ph) coupling is strong in cuprates superconductors and it plays an important role in pairing. In addition to the strong electron correlation, the inclusion of phonons provides a theoretical framework explaining many important phenomena that cannot be understood by a strongly correlated electronic model alone. Especially it is indispensable to explain the difference among materials. The phonons with the wave number around the (0,qx) and (qx,0) axes create the d-wave pairing while that near (pi,pi) are pair breaking. Therefore the half-breathing mode of the oxygen motions helps d-wave superconductivity.

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arXiv2001

Doping dependent evolution of the electronic structure of La2-xSrxCuO4 in the superconducting and metallic phases

The electronic structure of the La$_{2-x}$Sr$_x$CuO$_4$ (LSCO) system has been studied by angle-resolved photoemission spectroscopy (ARPES). We report on the evolution of the Fermi surface, the superconducting gap and the band dispersion around the extended saddle point $k=(π,0)$ with hole doping in the superconducting and metallic phases. As hole concentration $x$ decreases, the flat band at $(π,0)$ moves from above the Fermi level ($E_F$) for $x>0.2$ to below $E_F$ for $x<0.2$, and is further lowered down to $x=0.05$. From the leading-edge shift of ARPES spectra, the magnitude of the superconducting gap around $(π,0)$ is found to monotonically increase as $x$ decreases from $x=0.30$ down to $x=0.05$ even though $T_c$ decreases in the underdoped region, and the superconducting gap appears to smoothly evolve into the normal-state gap at $x=0.05$. It is shown that the energy scales characterizing these low-energy structures have similar doping dependences. For the heavily overdoped sample ($x=0.30$), the band dispersion and the ARPES spectral lineshape are analyzed using a simple phenomenological self-energy form, and the electronic effective mass enhancement factor $m^*/m_b \simeq 2$ has been found. As the hole concentration decreases, an incoherent component that cannot be described within the simple self-energy analysis grows intense in the high-energy tail of the ARPES peak. Some unusual features of the electronic structure observed for the underdoped region ($x \lesssim 0.10$) are consistent with the numerical works on the stripe model.

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arXiv2001

Electronic structure of MgB$_2$ from angle-resolved photoemission spectroscopy

The first angle-resolved photoemission spectroscopy results from MgB$_2$ single crystals are reported. Close to the $Γ$K and $Γ$M directions, three distinct dispersions are observed approaching the Fermi energy, as can be assigned to the theoretically predicted $σ$ (B $2p_{x,y}$) and $π$ (B $2p_z$) bands. In addition, we also observed a small parabolic-like band centered around $Γ$, and attributed it to a surface-derived state. Good agreement between our results and the band calculations suggests that the electronic structure of MgB$_2$ is of a conventional nature, thus implying that electron correlations are weak, and may be of little importance to the superconductivity in this system.

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arXiv2001

Role of electron-phonon interaction in the strongly correlated cuprates superconductors

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arXiv2001

Electronic structure of the trilayer cuprate superconductor Bi$_2$Sr$_2$Ca$_2$Cu$_3$O$_{10+δ}$

The low-energy electronic structure of the trilayer cuprate superconductor Bi$_2$Sr$_2$Ca$_2$Cu$_3$O$_{10+δ}$ near optimal doping is investigated by angle-resolved photoemission spectroscopy. The normal state quasiparticle dispersion and Fermi surface, and the superconducting d-wave gap and coherence peak are observed and compared with those of single and bilayer systems. We find that both the superconducting gap magnitude and the relative coherence-peak intensity scale linearly with $T_c$ for various optimally doped materials. This suggests that the higher $T_c$ of the trilayer system should be attributed to parameters that simultaneously enhance phase stiffness and pairing strength.

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arXiv2001

Anomalous electronic structure and pseudogap effects in Nd_1.85Ce_0.15CuO_4

We report a high-resolution angle-resolved photoemission spectroscopic (ARPES) study of the electron-doped (n-type) cuprate superconductor Nd_1.85Ce_0.15CuO_4. We observe regions along the Fermi surface where the near-E_F intensity is suppressed and the spectral features are broad in a manner reminiscent of the high-energy ``pseudogap'' in the underdoped p-type (hole doped) cuprates. However, instead of occurring near the (pi, 0) region, as in the p-type materials, this pseudogap falls near the intersection of the underlying Fermi surface with the antiferromagnetic Brillouin zone boundary.

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arXiv2001

Evidence for ubiquitous strong electron-phonon coupling in high-temperature superconductors

Coupling between electrons and phonons (lattice vibrations) drives the formation of the electron pairs responsible for conventional superconductivity. The lack of direct evidence for electron-phonon coupling in the electron dynamics of the high transition temperature superconductors has driven an intensive search for an alternative mechanism. A coupling of an electron with a phonon would result in an abrupt change of its velocity and scattering rate near the phonon energy. Here we use angle resolved photoemission spectroscopy to probe electron dynamics -velocity and scattering rate- for three different families of copper oxide superconductors. We see in all of these materials an abrupt change of electron velocity at 50-80meV, which we cannot explain by any known process other than to invoke coupling with the phonons associated with the movement of the oxygen atoms. This suggests that electron-phonon coupling strongly influences the electron dynamics in the high-temperature superconductors, and must therefore be included in any microscopic theory of superconductivity.

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arXiv2001

Nature of the Electronic Excitations near the Brillouin Zone Boundary of Bi$_2$Sr$_2$CaCu$_2$O$_{8+δ}$

Based on angle resolved photoemission spectra measured on different systems at different dopings, momenta and photon energies, we show that the anomalously large spectral linewidth in the $(π,0)$ region of optimal doped and underdoped Bi$_2$Sr$_2$CaCu$_2$O$_{8+δ}$ has significant contributions from the bilayer splitting, and that the scattering rate in this region is considerably smaller than previously estimated. This new picture of the electronic excitation near $(π,0)$ puts additional experimental constraints on various microscopic theories and data analysis.

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arXiv2001

From Mott insulator to overdoped superconductor: Evolution of the electronic structure of cuprates studied by ARPES

We review our angle-resolved photoemission spectroscopy (ARPES) results on different layered oxide superconductors, and on their insulating parent compounds. The low energy excitations are discussed with emphasis on some of the most recent issues, such as the Fermi surface and remnant Fermi surface, the pseudogap and d-wave-like dispersion, and lastly the signatures in the ARPES spectra of multiple electronic components, many body effects, and the superfluid density. We will focus on systematic changes in the electronic structure which may be relevant to the development of a comprehensive picture for the evolution from Mott insulators to overdoped superconductors.

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