H. H. Wen

Helmholtz-Zentrum Berlin, Berlín, Berlin, Germany

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Publications (210)368.06 Total impact

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    ABSTRACT: Irradiation with 2.5 MeV electrons at doses up to 5.2 $\times$10$^{19}$ electrons/cm$^2$ was used to introduce point-like defects in single crystals of Ba$_{1-x}$K$_x$Fe$_2$As$_2$ with $x=$ 0.19 ($T_c=$ 14 K), $x=$ 0.26 ($T_c=$ 32 K) and 0.34 ($T_c=$ 39 K) to study the superconducting gap structure by probing the effect of non-magnetic scattering on electrical resistivity, $\rho(T)$, and London penetration depth, $ \lambda (T)$. For all compositions, the irradiation suppressed the superconducting transition temperature, $T_c$ and increased resistivity. The low - temperature behavior of $\lambda (T)$ is best described by the power - law function, $\Delta \lambda (T) = A(T/T_c)^n$. While substantial suppression of $T_c$ supports $s_{\pm}$ pairing mechanism, in samples close to the optimal doping, $x=$ 0.26 and 0.34, the exponent $n$ remained high ($n \geq 3$) indicating robust full superconducting gaps. For the $x=$ 0.19 composition, exhibiting coexistence of superconductivity and long - range magnetism, the suppression of $T_c$ was much more rapid and the exponent $n$ decreased toward dirty limit of $n$ = 2. In this sample, the irradiation also suppressed the temperature of structural/magnetic transition, $T_{sm}$, from 103 K to 98 K consistent with the itinerant nature of the magnetic order. Our results suggest that underdoped compositions, especially in the coexisting regime are most susceptible to non-magnetic scattering and imply that in multi-band Ba$_{1-x}$K$_x$Fe$_2$As$_2$ superconductors, the ratio of the inter-band to intra-band pairing strength, and associated gap anisotropy, increases upon the departure from the optimal doping.
    07/2014;
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    ABSTRACT: We measure the magnetic penetration depth $\Delta\lambda(T)$ for NdO$_{1-x}$F$_{x}$BiS$_{2}$ ($x$ = 0.3 and 0.5) using the tunnel diode oscillator technique. The $\Delta\lambda(T)$ shows an upturn in the low-temperature limit which is attributed to the paramagnetism of Nd ions. After subtracting the paramagnetic contributions, the penetration depth $\Delta\lambda(T)$ follows exponential-type temperature dependence at $T\ll T_c$. Both $\Delta\lambda(T)$ and the corresponding superfluid density $\rho_s(T)$ can be described by the BCS model with an energy gap of $\Delta(0)$ $\approx$ 2.0 $k_BT_c$ for both $x$ = 0.3 and 0.5, suggesting strong-coupling BCS superconductivity in the presence of localized moments for NdO$_{1-x}$F$_{x}$BiS$_{2}$.
    06/2014;
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    ABSTRACT: We investigate the nature of the SDW (Spin Density Wave) transition in the underdoped regime of an iron-based high Tc superconductor Ba(Fe1-xCox)2As2 by 75As NMR, with primary focus on a composition with x = 0.02 (T_SDW = 99 K).We demonstrate that critical slowing down toward the three dimensional SDW transition sets in at the tetragonal to orthorhombic structural phase transition, Ts = 105 K, suggesting strong interplay between structural distortion and spin correlations. In the critical regime between Ts and T_SDW, the dynamical structure factor of electron spins S(q,Wn) measured with the longitudinal NMR relaxation rate 1/T1 exhibits a divergent behavior obeying a power law, 1/T1~S(q, Wn)~(T/T_SDW-1)^a with the critical exponent a ~ 0.33.
    06/2014;
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    ABSTRACT: We have performed systematic angle-resolved photoemission spectroscopy (ARPES) of iron-chalcogenide superconductor FeTe1-xSex to elucidate the electronic states relevant to the superconductivity. While the Fermi-surface shape is nearly independent of x, we found that the ARPES spectral line shape shows prominent x dependence. A broad ARPES spectrum characterized by a small quasiparticle weight at x = 0, indicative of incoherent electronic states, becomes progressively sharper with increasing x, and a well-defined quasiparticle peak appears around x = 0.45 where bulk superconductivity is realized. The present result suggests the evolution from incoherent to coherent electronic states and its close relationship to the emergence of superconductivity.
    05/2014;
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    ABSTRACT: We determine the pressure phase diagram of the 1111 compounds CaFeAsF and SrFeAsF, up to 20 GPa and down to 4 K by electrical resistivity measurements and the change of structure up to 40 GPa at room temperature. The antiferromagnetic transition temperature, as determined by the derivative peak, shows a minimum at ~5 GPa (10 GPa) for the Ca (Sr) compound. For CaFeAsF, superconductivity appears at this minimum, coincident with the development of a previously reported monoclinic phase. For SrFeAsF, where the orthorhombic and the monoclinic phase were reported to coexist, superconductivity exists above P≥1 GPa. Both phase diagrams can be scaled by a shift of ~10 GPa pressure at which the volume of SrFeAsF and that of CaFeAsF at ambient pressure coincide. The difference of our phase diagram with that of electron-doped 1111 samples is accounted for by hole doping under pressure, which we verified through electron band structure calculations.
    Journal of Physics Condensed Matter 03/2014; 26(15):155702. · 2.22 Impact Factor
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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.
    02/2014;
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    ABSTRACT: Using the angle-resolved photoemission spectroscopy (ARPES) with resolution of all three components of electron momentum and electronic states symmetry, we explicate the electronic structure of hole-doped BaFe2 As2, and show that widely discussed nesting and dimensionality of Fermi surface (FS) sheets have no immediate relation to the superconducting pairing in iron-based superconductors. Alternatively a clear correlation between the orbital character of the electronic states and their propensity to superconductivity is observed: The magnitude of the superconducting gap maximizes at 10 meV exclusively for iron 3d xz,yz orbitals, while for others drops to 3 meV. Presented results imply that the relation between superconducting and magnetostructural transitions goes beyond simple competition for FS, and demonstrate importance of orbital physics in iron superconductors.
    Physical Review B 01/2014; 89:064514. · 3.66 Impact Factor
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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.
    Nature Communications 12/2013; 4:3010. · 10.02 Impact Factor
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    ABSTRACT: The optical properties of Ba0.6K0.4Fe2As2 have been determined in the normal state for a number of temperatures over a wide frequency range. Two Drude terms, representing two groups of carriers with different scattering rates (1/τ), well describe the real part of the optical conductivity σ1(ω). A “broad” Drude component results in an incoherent background with a T-independent 1/τb, while a “narrow” Drude component reveals a T-linear 1/τn resulting in a resistivity ρn≡1/σ1n(ω→0) also linear in temperature. An arctan⁡(T) low-frequency spectral weight is also strong evidence for a T-linear 1/τ. A comparison to other materials with similar behavior suggests that the T-linear 1/τn and ρn in Ba0.6K0.4Fe2As2 originate from scattering from spin fluctuations and hence that an antiferromagnetic quantum critical point is likely to exist in the superconducting dome.
    Physical Review Letters 09/2013; 111(11). · 7.73 Impact Factor
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    ABSTRACT: The London penetration depth was measured in optimally doped Ba0.6K0.4Fe2As2 crystals, with and without columnar defects produced by 1.4 GeV 208Pb irradiation. The low temperature behavior of unirradiated samples was consistent with a fully gapped superconducting state with a minimum energy gap Δ min/kBTC≈1. Similar gap values were observed for irradiation levels corresponding to mean column-column separations of 32 and 22 nm. At very high irradiation levels (column-column separation of 10 nm) a T2 power law was observed below TC/3, most likely due to elevated scattering. Neither the location nor the sharpness of the superconducting transition was affected by irradiation. The data provide evidence for an s+− pairing state.
    Physical review. B, Condensed matter 05/2013; 87(18). · 3.77 Impact Factor
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    ABSTRACT: We used high-energy resolution angle-resolved photoemission spectroscopy to extract the momentum dependence of the superconducting gap of Ru-substituted Ba(Fe0.75Ru0.25)2As2 (Tc=15 K). Despite a strong out-of-plane warping of the Fermi surface, the magnitude of the superconducting gap observed experimentally is nearly isotropic and independent of the out-of-plane momentum. More precisely, we respectively observed 5.7 and 4.5 meV superconducting gaps on the inner and outer Γ-centered hole Fermi surface pockets, whereas a 4.8 meV gap is recorded on the M-centered electron Fermi surface pockets. Our results are consistent with the J1-J2 model with a dominant antiferromagnetic exchange interaction between the next-nearest Fe neighbors.
    Physical Review B 03/2013; 87(9). · 3.66 Impact Factor
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    ABSTRACT: We present the results of magnetoresistivity and magnetization measurements performed under pressure (P) on single crystals of Ba(Fe1-xCox)2As2 (x = 0, 0.042, 0.06, 0.08). Our results show that the antiferromagnetic phase macroscopically coexists with the superconducting phase and can be induced by the magnetic field locally nucleated in the vortex core for the x = 0.06 sample. In addition, the diamagnetic signal of the x = 0.06 sample shows a huge jump around P = 0.5 GPa, where the superconducting transition temperature displays a maximum. This suggests that a first order antiferromagnetic quantum phase transition (QPT) is present inside the superconducting dome, and that the superconductivity in this system is closely related to this QPT. A magnetic tricritical point is observed inside the superconducting dome, and no quantum critical point is expected in zero magnetic field.
    03/2013;
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    ABSTRACT: We determined the optical conductivity of Bi2Sr2-xLaxCuO6 at dopings covering the phase diagram from the underdoped to the overdoped regimes. The frequency dependent scattering rate shows a pseudogap extending into the overdoped regime. We found that the effective mass enhancement calculated from the optical conductivity is constant throughout the phase diagram. Conversely, the effective optical charge density varies almost linearly with doping. Our results suggest that the low frequency electrodynamics of Bi2Sr2-xLaxCuO6 is not strongly affected by the long range Mott transition. [Y.M. Dai et al., Phys. Rev. B 85, 092504 (2012)].
    03/2013;
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    ABSTRACT: We investigate the enhancement of vortex pinning by compound defects that are composed of correlated and point defects in Ba0.6K0.4Fe2As2 crystals with Tc 37.5. Initial irradiation by high-energy heavy ions to a dose matching field of B=21T increases vortex pinning via columnar defects with no degradation of the superconducting transition temperature. Subsequent proton irradiations further enhance the critical current Jc(H) by suppressing the motion of vortex kinks between the columnar defects. At a temperature of 5K, we find a critical current density of 5.8 MA/cm^2 that is essentially magnetic field independent in fields up to 7 T. This work supported by the Center for Emergent Superconductivity, an Energy Frontier Research Center funded by the U.S. D.O.E., Office of Science, Office of Basic Energy Sciences and by the D.O.E, Office of Basic Energy Sciences, under Contract No. DE-AC02-06CH11357. The operation of the ATLAS facility was supported by the U.S. D.O.E., Office of Nuclear Physics, under Contract No. DE-AC02-06CH11357. The work in China was supported by the NSF of China, the MOST of China (2011CBA00102 and 2012CB821403) and PAPD.
    03/2013;
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    ABSTRACT: The London penetration depth was measured in optimally doped Ba0.6K0.4Fe2As2 crystals, with and without columnar defects produced by 1.4 GeV 208Pb irradiation. The low temperature behavior of unirradiated samples was consistent with a fully gapped superconducting state with a minimum energy gap delta_min/(k_B T_C) = 1. Similar gap values were observed for irradiation levels corresponding to mean column-column separations of 32 nm and 22 nm. At very high irradiation levels (column-column separation of 10 nm) a T^2 power law was observed below Tc/3, most likely due to elevated scattering. Neither the location nor the sharpness of the superconducting transition was affected by irradiation. The data provides evidence for an s+/- pairing state.
    02/2013;
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    ABSTRACT: The magnetization around the superconducting transition was recently measured in a high-quality Ba1−x Kx Fe2As2 single crystal with magnetic fields applied along and transverse to the crystal Fe-layers [J. Mosqueira et al. in Phys. Rev. B 83:094519, 2011]. Here we extend this study to the finite-field (or Prange) regime, in which the magnetic susceptibility is expected to be strongly dependent on the applied magnetic field. These measurements are analyzed in the framework of the three-dimensional anisotropic Ginzburg–Landau (3D-aGL) approach generalized to the short-wavelength regime through the introduction of a total-energy cutoff in the fluctuation spectrum. The results further confirm the adequacy of GL approaches to describe the fluctuation effects close to the superconducting transition of these materials.
    Journal of Superconductivity and Novel Magnetism 01/2013; 26(4). · 0.70 Impact Factor
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    ABSTRACT: Mixed pinning landscapes in superconductors are emerging as an effective strategy to achieve high critical currents in high, applied magnetic fields. Here, we use heavy-ion and proton irradiation to create correlated and point defects to explore the vortex pinning behavior of each and combined constituent defects in the iron-based superconductor Ba0.6K0.4Fe2As2 and find that the pinning mechanisms are non-additive. The major effect of p-irradiation in mixed pinning landscapes is the generation of field-independent critical currents in very high fields. At 7 T ∥ c and 5 K, the critical current density exceeds 5 MA/cm2.
    Applied Physics Letters 01/2013; 103(20):202601-202601-5. · 3.79 Impact Factor
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    ABSTRACT: We show magnetotransport results on Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$ ($0.0 \leq x \leq 0.13$) single crystals. We identify the low temperature resistance step at 23 K in the parent compound with the onset of filamentary superconductivity (FLSC), which is suppressed by an applied magnetic field in a similar manner to the suppression of bulk superconductivity (SC) in doped samples. FLSC is found to persist across the phase diagram until the long range antiferromagnetic order is completely suppressed. A significant suppression of FLSC occurs for $0.02<x<0.04$, the doping concentration where bulk SC emerges. Based on these results and the recent report of an electronic anisotropy maximum for 0.02 $\leq x \leq$ 0.04 [Science 329, 824 (2010)], we speculate that, besides spin fluctuations, orbital fluctuations may also play an important role in the emergence of SC in iron-based superconductors.
    Physical review. B, Condensed matter 08/2012; 86(6). · 3.77 Impact Factor
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    ABSTRACT: The nature of the pairing state in iron-based superconductors is the subject of much debate. Here we argue that in one material, the stoichiometric iron pnictide KFe2As2, there is overwhelming evidence for a d-wave pairing state, characterized by symmetry-imposed vertical line nodes in the superconducting gap. This evidence is reviewed, with a focus on thermal conductivity and the strong impact of impurity scattering on the critical temperature Tc. We then compare KFe2As2 to Ba0.6K0.4Fe2As2, obtained by Ba substitution, where the pairing symmetry is s-wave and the Tc is ten times higher. The transition from d-wave to s-wave within the same crystal structure provides a rare opportunity to investigate the connection between band structure and pairing mechanism. We also compare KFe2As2 to the nodal iron-based superconductor LaFePO, for which the pairing symmetry is probably not d-wave, but more likely s-wave with accidental line nodes.
    Superconductor Science and Technology 07/2012; 25(2012). · 2.76 Impact Factor
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    ABSTRACT: The surprisingly rapid relaxation of the sustainable current density in the critical state of single-crystalline Ba1−xKxFe2As2 is investigated for magnetic fields oriented parallel to the c axis and to the ab plane, respectively. Due to the inadequacy of standard analysis procedures developed for flux creep in the high-temperature superconducting cuprates, we develop a simple, straightforward data treatment technique that reveals the creep mechanism and the creep exponent μ. At low magnetic fields, below the second magnetization peak, μ varies only slightly as a function of temperature and magnetic flux density B. From the data, we determine the temperature and field dependencies of the effective activation barrier for creep. At low temperatures, the measured current density approaches the zero-temperature critical current density (in the absence of creep) to within a factor 2, thus lending credence to earlier conclusions drawn with respect to the pinning mechanism. The comparable values of the experimental screening current density and the zero-temperature critical current density reveal the limited usefulness of the widely used “interpolation formula.”
    Physical review. B, Condensed matter 07/2012; 86(2). · 3.77 Impact Factor

Publication Stats

1k Citations
368.06 Total Impact Points

Institutions

  • 2014
    • Helmholtz-Zentrum Berlin
      Berlín, Berlin, Germany
  • 2012–2014
    • Nanjing University
      • Department of Physics
      Nan-ching, Jiangsu Sheng, China
  • 1995–2014
    • Chinese Academy of Sciences
      • • Institute of Physics
      • • Laboratory of Theoretical Condensed Matter Physics and Computational Materials Physics
      • • Condensed Matter Physics
      Peping, Beijing, China
  • 2013
    • UPMC
      Pittsburgh, Pennsylvania, United States
  • 2011
    • Chestnut Hill College
      Boston, Massachusetts, United States
  • 2008–2011
    • Technical Institute of Physics and Chemistry
      Peping, Beijing, China
    • Peking University
      • State Key Laboratory for Artificial Microstructure and Mesoscopic Physics
      Beijing, Beijing Shi, China
    • University of Tennessee
      • Department of Physics & Astronomy
      Knoxville, TN, United States
  • 1995–2010
    • Northeast Institute of Geography and Agroecology
      • • Condensed Matter Physics
      • • Institute of Physics
      • • National Laboratory for Superconductivity
      Beijing, Beijing Shi, China
  • 2009
    • Institute of physics china
      Peping, Beijing, China
    • Iowa State University
      • Department of Physics and Astronomy
      Ames, IA, United States
  • 2007
    • Hefei Institute of Physical Sciences, Chinese Academy of Sciences
      Luchow, Anhui Sheng, China
  • 2006
    • Pennsylvania State University
      • Department of Physics
      University Park, MD, United States
  • 2005
    • Shanghai Jiao Tong University
      • Department of Physics
      Shanghai, Shanghai Shi, China
  • 2004–2005
    • Nankai University
      T’ien-ching-shih, Tianjin Shi, China
  • 1997
    • VU University Amsterdam
      • Department of Physics and Astronomy
      Amsterdam, North Holland, Netherlands
    • Ruhr-Universität Bochum
      • Institut für Experimentalphysik II
      Bochum, North Rhine-Westphalia, Germany