Z. R. Ye

Fudan University, Shanghai, Shanghai Shi, China

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Publications (35)163.07 Total impact

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    ABSTRACT: Ba0.95Na0.05Ti2Sb2O is a titanium-based oxypnictide superconductor with possible density wave order. We have performed high-resolution angle-resolved photoemission spectroscopy and scanning tunneling microscopy (STM) studies on Ba0.95Na0.05Ti2Sb2O. Our STM studies find a charge density wave (CDW) order with the wave vector of (π, π). The electronic structure shows both multiorbital and three-dimensional nature, consistent with the theoretical calculations. The observed Fermi surfaces are well nested along the (π, π) direction, which might drive the CDW transition. Likely due to the weak strength of the CDW order, we do not observe a leading-edge gap. Instead, we observe an overall spectral weight enhancement with decreasing temperature, but the rate of this enhancement is abruptly reduced or diminished in the CDW state, suggesting the suppression of spectral weight or opening of a partial CDW gap. Our results give a comprehensive picture of the electronic structure and direct observation of the CDW order in Ba0.95Na0.05Ti2Sb2O.
    No preview · Article · Jan 2016
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    ABSTRACT: Temperature and fluence dependence of the 1.55-eV optical transient reflectivity in BaFe2(As1-xPx)2 was measured and analyzed in the low and high excitation density limit. The effective magnitude of the superconducting gap of ∼5 meV obtained from the low-fluence-data bottleneck model fit is consistent with the angle-resolved photoemission spectroscopy results for the γ- and β-hole Fermi surfaces. The superconducting state nonthermal optical destruction energy was determined from the fluence dependent data. The planar optical destruction energy density scales well with Tc2 and is found to be similar in a number of different layered superconductors. © 2015 American Physical Society.
    No preview · Article · Oct 2015 · Physical Review B
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    ABSTRACT: Temperature and fluence dependence of the 1.55-eV optical transient reflectivity in BaFe$_{2}$(As$_{1-x}$P$_{x}$)$_{2}$ was measured and analysed in the low and high excitation density limit. The effective magnitude of the superconducting gap of $\sim 5$ meV obtained from the low-fluence-data bottleneck model fit is consistent with the ARPES results for the $\gamma$-hole Fermi surface. The superconducting-state nonthermal optical destruction energy was determined from the fluence dependent data. The in-plane optical destruction energy scales well with T$_{\mathrm{c}}^{2}$ and is found to be similar in a number of different layered superconductors.
    Full-text · Article · Jul 2015
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    ABSTRACT: Using angle-resolved photoemission spectroscopy, we studied isovalently doped K$_{x}$Fe$_{2-y}$Se$_{2-z}$S$_{z}$, Rb$_{x}$Fe$_{2-y}$Se$_{2-z}$Te$_{z}$ and (Tl,K)$_{x}$Fe$_{2-y}$Se$_{2-z}$S$_{z}$, in which the superconducting transition temperature (T$_{c}$) decreases with either positive or negative chemical pressures. The bandwidth of Fe 3d bands in these materials changes systematically with doping: with the decreasing of bandwidth, the ground state evolves from a metal to a superconductor, and eventually to an insulator. We unambiguously show that these materials can be all fit into a prototypical phase diagram of bandwidth-controlled Mott system. Our results indicate that iron-based superconductors can be understood as a derivative of Mott insulator, and it might be unified with the cuprate high temperature superconductors in a single framework.
    Full-text · Article · Jun 2015
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    ABSTRACT: The electronic structure of Na2Ti2Sb2O, a parent compound of the newly discovered titanium-based oxypnictide superconductors, is studied by photon energy and polarization dependent angle-resolved photoemission spectroscopy (ARPES). The obtained band structure and Fermi surface agree well with the band structure calculation of Na2Ti2Sb2O in the non-magnetic state, which indicating that there is no magnetic order in Na2Ti2Sb2O and the electronic correlation is weak. Polarization dependent ARPES results suggest the multi-band and multi-orbital nature of Na2Ti2Sb2O. Photon energy dependent ARPES results suggest that the electronic structure of Na2Ti2Sb2O is rather two-dimensional. Moreover, we find a density wave energy gap forms below the transition temperature and reaches 65 meV at 7 K, indicating that Na2Ti2Sb2O is likely a weakly correlated CDW material in the strong electron-phonon interaction regime.
    No preview · Article · May 2015 · Scientific Reports
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    ABSTRACT: The effect of K, Co and P dopings on the lattice dynamics in the BaFe$_2$As$_2$ system is studied by infrared spectroscopy. We focus on the phonon at $\sim$ 253 cm$^{-1}$, the highest energy in-plane infrared-active Fe-As mode in BaFe$_2$As$_2$. Our studies show that the Co and P dopings lead to a blue shift of this phonon in frequency, which can be simply interpreted by the change of lattice parameters induced by doping. In sharp contrast, an unusual red shift of the same mode was observed in the K-doped compound, at odds with the above explanation. This anomalous behavior in K-doped BaFe$_2$As$_2$ is more likely associated with the coupling between lattice vibrations and other channels, such as charge or spin. This coupling scenario is also supported by the asymmetric line shape and intensity growth of the phonon in the K-doped compound.
    Full-text · Article · Mar 2015 · Physical Review B
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    ABSTRACT: We have studied the low-lying electronic structure of a new ${\mathrm{ThCr}}_{2}{\mathrm{Si}}_{2}$-type superconductor ${\mathrm{KNi}}_{2}{\mathrm{Se}}_{2}$ with angle-resolved photoemission spectroscopy. Three bands intersect the Fermi level, forming complicated Fermi surface topology, which is sharply different from its isostructural superconductor ${\mathrm{K}}_{x}\mathrm{Fe}{\phantom{\rule{0.16em}{0ex}}}_{2$-${}y}{\mathrm{Se}}_{2}$. The Fermi surface shows weak variation along the ${k}_{z}$ direction, indicating its quasi-two-dimensional nature. Further comparison with the density functional theory calculations demonstrates that there exist relatively weak correlations and substantial hybridization in the low-lying electronic structure. Our results indicate that the large density of states at the Fermi energy leads to the reported mass enhancement based on the specific heat measurements. Moreover, no anomaly is observed in the spectra when entering the fluctuating charge density wave state reported earlier.
    No preview · Article · Mar 2015 · Physical Review B
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    ABSTRACT: Ta4Pd3Te16 is a newly discovered layered superconductor with quasi-one-dimensional (1D) structure. Recent thermal transport measurements show the possible existence of nodes in the superconducting gap. Here we report low-temperature scanning tunneling microscopy/ spectroscopy study on Ta4Pd3Te16 single crystals. We observed stripelike structure composed of atom chains on the cleaved ((1) over bar 03) surface. There exists charge-density-wave (CDW)- like modulations along stripes with commensurate periods. Meanwhile, the tunneling conductance shows an s-wave-like superconducting gap. The magnetic vortex mapped at low field is highly anisotropic with a bound state in the core. At increased field, strong vortex overlapping is directly observed and the bound state is suppressed, indicating the delocalization of the superconducting quasiparticles. Our observations suggest that Ta4Pd3Te16 is of multiband superconductivity with strong 1D characters, which possibly coexist with CDW transition.
    No preview · Article · Mar 2015 · Physical Review B
  • Y. Zhang · Z.R. Ye · D.L. Feng

    No preview · Article · Jan 2015 · Springer Series in Materials Science
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    ABSTRACT: We have studied the low-lying electronic structure of a new ThCr$_2$Si$_2$-type superconductor KNi$_2$Se$_2$ with angle-resolved photoemission spectroscopy. Three bands intersect the Fermi level, forming complicated Fermi surface topology, which is sharply different from its isostructural superconductor K$_x$Fe$_{2-y}$Se$_2$. The Fermi surface shows weak variation along the $k_z$ direction, indicating its quasi-two-dimensional nature. Further comparison with the density functional theory calculations demonstrates that there exist relatively weak correlations and substantial hybridization of the Ni 3$d$ and the Se 4$p$ orbitals in the low-lying electronic structure. Our results indicate that the large density of states at the Fermi energy leads to the reported mass enhancement based on the specific heat measurements. Moreover, no anomaly is observed in the spectra when entering the fluctuating charge density wave state reported earlier.
    Preview · Article · Nov 2014
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    ABSTRACT: We report the electronic structure reconstruction of Ca$_{1-x}$Pr$_x$Fe$_2$As$_2$ ($x$ = 0.1 and 0.15) in the low temperature collapsed tetragonal (CT) phase observed by angle-resolved photoemission spectroscopy. Different from Ca(Fe$_{1-x}$Rh$_x$)$_2$As$_2$ and the annealed CaFe$_2$As$_2$ where all hole Fermi surfaces are absent in their CT phases, the cylindrical hole Fermi surface can still be observed in the CT phase of Ca$_{1-x}$Pr$_x$Fe$_2$As$_2$. Furthermore, we found at least three well separated electron-like bands around the zone corner in the CT phase of Ca$_{1-x}$Pr$_x$Fe$_2$As$_2$, which are more dispersive than the electron-like bands in the high temperature tetragonal phase. Based on these observations, we propose that the weakening of correlations (as indicated by the reduced effective mass), rather than the lack of Fermi surface nesting, might be responsible for the absence of magnetic ordering and superconductivity in the CT phase.
    Preview · Article · Oct 2014 · Physical Review B
  • M Xia · W H Jiao · Z R Ye · Q Q Ge · Y Zhang · J Jiang · R Peng · X P Shen · Q Fan · G H Cao · T Zhang · B P Xie · D L Feng
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    ABSTRACT: Eu(Fe0.79Ru0.21)2As2 is suggested to be a nodeless superconductor based on the empirical correlation between pnictogen height (hPn) and superconducting gap behavior, in contrast to BaFe2(As0.7P0.3)2 and Ba(Fe0.65Ru0.35)2As2. We studied the low-lying electronic structure of Eu(Fe0.79Ru0.21)2As2 with angle-resolved photoemission spectroscopy (ARPES). By photon energy dependence and polarization dependence measurements, we resolved the band structure in the three-dimensional momentum space and determined the orbital character of each band. In particular, we found that the -originated ? band does not contribute spectral weight to the Fermi surface around Z, unlike BaFe2(As0.7P0.3)2 and Ba(Fe0.65Ru0.35)2As2. Since BaFe2(As0.7P0.3)2 and Ba(Fe0.65Ru0.35)2As2 are nodal superconductors and their hPn's are less than 1.33??, while the hPn of Eu(Fe0.79Ru0.21)2As2 is larger than 1.33 ?, our results provide more evidence for a direct relationship between nodes, orbital character and hPn. Our results help to provide an understanding of the nodal superconductivity in iron-based superconductors.
    No preview · Article · Jun 2014 · Journal of Physics Condensed Matter
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    ABSTRACT: The diversities in crystal structures and ways of doping result in extremely diversified phase diagrams for iron-based superconductors. With angle-resolved photoemission spectroscopy (ARPES), we have systematically studied the effects of chemical substitution on the electronic structure of various series of iron-based superconductors. In addition to the control of Fermi surface topology by heterovalent doping, we found two more extraordinary effects of doping: 1. the site and band dependencies of quasiparticle scattering; and more importantly 2. the ubiquitous and significant bandwidth-control by both isovalent and heterovalent dopants in the iron-anion layer. Moreover, we found that the bandwidth-control could be achieved by either applying the chemical pressure or doping electrons, but not by doping holes. Together with other findings provided here, these results complete the microscopic picture of the electronic effects of dopants, which facilitates a unified understanding of the diversified phase diagrams and resolutions to many open issues of various iron-based superconductors.
    Preview · Article · Apr 2014 · Physical Review X
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    ABSTRACT: We report the electronic structure of YbB6, a recently predicted moderately correlated topological insulator, measured by angle-resolved photoemission spectroscopy. We directly observed linearly dispersive bands around the time-reversal invariant momenta {\Gamma} and X with negligible kz dependence, consistent with odd number of surface states crossing the Fermi level in a Z2 topological insulator. Circular dichroism photoemission spectra suggest that these in-gap states possess chirality of orbital angular momentum, which is related to the chiral spin texture, further indicative of their topological nature. The observed insulating gap of YbB6 is about 100 meV, larger than that reported by theoretical calculations. Our results present strong evidence that YbB6 is a correlated topological insulator and provide a foundation for further studies of this promising material.
    Preview · Article · Mar 2014 · Scientific Reports
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    ABSTRACT: NdO$_{0.5}$F$_{0.5}$BiS$_{2}$ is a new layered superconductor. We have studied the low-lying electronic structure of a single crystalline NdO$_{0.5}$F$_{0.5}$BiS$_{2}$ superconductor, whose superconducting transition temperature is 4.87K, with angle-resolved photoemission spectroscopy. The Fermi surface consists of two small electron pockets around the X point and shows little warping along the $k_z$ direction. Our results demonstrate the multi-band and two-dimensional nature of the electronic structure. The good agreement between the photoemission data and the band calculations gives the renormalization factor of 1, indicating the rather weak electron correlations in this material. Moreover, we found that the actual electron doping level and Fermi surface size are much smaller than what are expected from the nominal composition, which could be largely explained by the bismuth dificiency. The small Fermi pocket size and the weak electron correlations found here put strong constraints on theory, and suggest that the BiS$_2$-based superconductors could be conventional BCS superconductors mediated by the electron-phonon coupling.
    Preview · Article · Feb 2014 · Physical Review B
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    J Jiang · S Li · T Zhang · Z Sun · F Chen · Z.R. Ye · M Xu · Q.Q. Ge · S.Y. Tan · X.H. Niu · M Xia · B.P. Xie · Y.F. Li · X.H. Chen · H.H. Wen · D.L. Feng
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    ABSTRACT: SmB6, a well-known Kondo insulator, exhibits a transport anomaly at low temperature. This anomaly is usually attributed to states within the hybridization gap. Recent theoretical work and transport measurements suggest that these in-gap states could be ascribed to topological surface states, which would make SmB6 the first realization of topological Kondo insulator. Here by performing angle-resolved photoemission spectroscopy experiments, we directly observe several dispersive states within the hybridization gap of SmB6. These states show negligible kz dependence, which indicates their surface origin. Furthermore, we perform photoemission circular dichroism experiments, which suggest that the in-gap states possess chirality of the orbital angular momentum. These states vanish simultaneously with the hybridization gap at around 150 K. Together, these observations suggest the possible topological origin of the in-gap states.
    Preview · Article · Dec 2013 · Nature Communications
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    ABSTRACT: With angle-resolved photoemission spectroscopy, we studied the electronic structure of TaFe$_{1.23}$Te$_3$, which is a two-leg spin ladder compound with a novel antiferromagnetic ground state. Quasi-two-dimensional Fermi surface is observed, indicating sizable inter-ladder hopping, which would facilitate the in-plane ferromagnetic ordering through double exchange interactions. Moreover, an energy gap is not observed at the Fermi surface in the antiferromagnetic state. Instead, the shifts of various bands have been observed. Combining these observations with density-functional-theory calculations, we propose that the large scale reconstruction of the electronic structure, caused by the interactions between the coexisting itinerant electrons and local moments, is most likely the driving force behind the magnetic transition. TaFe$_{1.23}$Te$_3$ thus provides a simpler system that contains similar ingredients as the parent compounds of iron-based superconductors, which yet could be readily modeled and understood.
    Full-text · Article · Oct 2013 · Chinese Physics Letters
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    ABSTRACT: We studied the low-lying electronic structure of the newly discovered iron-platinum-arsenide superconductor, Ca10(Pt4As8)(Fe2−xPtxAs2)5 (Tc=22 K) with angle-resolved photoemission spectroscopy. We found that the Pt4As8 layer contributes to a small electronlike Fermi surface, indicative of metallic charge reservoir layers that are rare for iron based superconductors. Moreover, the electronic structure of the FeAs layers resembles those of other prototype iron pnictides to a large extent. However, there is only dxy-orbital originated holelike Fermi surface near the zone center, which is rather strange for an iron pnictide superconductor with relatively high superconducting transition temperature; and the dxz and dyz originated bands are not degenerate at the zone center. Furthermore, all bands near the Fermi energy show negligible kz dependence, indicating the strong two-dimensional nature of this material. Our results indicate this material possesses a rather interesting electronic structure, which enriches our current knowledge of iron based superconductors.
    No preview · Article · Sep 2013 · Physical Review B
  • Juan Jiang · C. He · Y. Zhang · M. Xu · Q. Q. Ge · Z. R. Ye · F. Chen · B. P. Xie · D. L. Feng
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    ABSTRACT: The in-plane resistivity anisotropy has been studied with the Montgomery method on the detwinned parent compound of the iron-based superconductor FeTe. The observed resistivity in the antiferromagnetic (AFM) direction is larger than that in the ferromagnetic (FM) direction, which is different from that observed in BaFe2As2 before. We show that the opposite resistivity anisotropy behavior in FeTe could be attributed to the strong Hund's rule coupling effects, which should be understood in a localized picture: Hund's rule coupling makes hopping along the FM direction easier than along the AFM direction in FeTe, similar to the colossal magnetoresistance observed in some manganites.
    No preview · Article · Sep 2013 · Physical Review B
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    ABSTRACT: We studied the low-lying electronic structure of the newly discovered iron-platinum-arsenide superconductor, Ca10(Pt4As8)(Fe2-xPtxAs2)5 (Tc=22 K) with angle-resolved photoemission spectroscopy. We found that the Pt4As8 layer contributes to a small electron-like Fermi surface, indicative of metallic charge reservoir layers that are rare for iron based superconductors. Moreover, the electronic structure of the FeAs-layers resembles those of other prototype iron pnictides to a large extent. However, there is only dxy-orbital originated hole-like Fermi surface near the zone center, which is rather unique for an iron pnictide superconductor with relatively high superconducting transition temperature; and the dxz and dyz originated bands are not degenerate at the zone center. Furthermore, all bands near the Fermi energy show negligible kz dependence, indicating the strong two-dimensional nature of this material. Our results indicate this material possesses rather unique electronic structure, which enriches our current knowledge of iron based superconductors.
    Preview · Article · Aug 2013