B Shen

Publications (9)14.74 Total impact

• Source

  Available from: A. Gurevich

  Article: Significant enhancement of upper critical fields by doping and strain in Fe-based superconductors

  C. Tarantini, A. Gurevich, J. Jaroszynski, F. Balakirev, E. Bellingeri, I. Pallecchi, C. Ferdeghini, B Shen, H. H. Wen, D. C. Larbalestier
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  ABSTRACT: We report measurements of Hc2(T) up to 85 Tesla on Ba1-xKxAs2Fe2 single crystals and FeSe1-xTex films tuned by doping and strain. We observed an Hc2 enhancement by nearly 25 T at 30 K for the optimally-doped Ba1-xKxAs2Fe2 as compared to the previous results and extraordinarily high slopes dHc2/dT = 250-500 T/K near Tc in FeSe1-xTex indicating an almost complete suppression of the orbital pair-breaking. Theoretical analysis of Hc2(T) in FeSe1-xTex and the optimally doped Ba1-xKxAs2Fe2 predicts an inhomogeneous Fulde-Ferrel-Larkin-Ovchinnikov state for H//ab and T < 3-10 K, and shows that Hc2 in multiband Fe based superconductor can be enhanced by doping and strain much more effectively than by the conventional way of increasing disorder.
  08/2011;
• Source

  Available from: arxiv.org

  Article: Comparative studies of the scanning tunneling spectra in cuprate and iron-arsenide superconductors

  N. -C. Yeh, M. L. Teague, A. D. Beyer, B Shen, H. H. Wen
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  ABSTRACT: We report scanning tunneling spectroscopic studies of cuprate and iron-arsenic superconductors, including YBa_2Cu_3O_{7-\delta} (Y-123, T_c = 93 K), Sr_{0.9}La_{0.1}CuO_2 (La-112, T_c = 43 K), and the "122" compounds Ba(Fe_{1-x}Co_x)_2As_2 (Co-122 with x = 0.06, 0.08, 0.12 for T_c = 14, 24, 20 K). For H > 0, pseudogap (\Delta_{PG}) features are revealed inside the vortices, with \Delta_{PG} = [(\Delta_{eff})^2+(\Delta_{SC})^2]^{1/2} > \Delta_{SC} in Y-123 and \Delta_{PG} < \Delta_{SC} in La-112, suggesting that the physical origin of \Delta_{PG} is a competing order coexisting with superconductivity. Additionally, Fourier transformation (FT) of the Y-123 spectra exhibits two types of spectral peaks, one type is associated with energy (\omega)-dependent quasiparticle interference (QPI) wave-vectors and the other consists of \omega-independent wave-vectors due to competing orders and (\pi,\pi) magnetic resonances. For the multi-band Co-122 compounds, two-gap superconductivity is found for all doping levels. Magnetic resonant modes that follow the temperature dependence of the superconducting gaps are also identified. These findings, together with the \omega- and x-dependent QPI spectra, are consistent with a sign-changing s-wave pairing symmetry in the Co-122 iron arsenides. Our comparative studies suggest that the commonalities among the cuprate and the ferrous superconductors include the proximity to competing orders, antiferromagnetic (AFM) spin fluctuations and magnetic resonances in the superconducting (SC) state, and the unconventional pairing symmetries with sign-changing order parameters on different parts of the Fermi surface.
  07/2011;
• Source

  Available from: ArXiv

  Article: Intrinsic Percolative Superconductivity in KxFe2-ySe2 Single Crystals

  B Shen, B. Zeng, G. F. Chen, J. B. He, D. M. Wang, H Yang, H. H. Wen
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  ABSTRACT: Magnetic field penetration and magnetization hysteresis loops (MHLs) have been measured in KxFe2-ySe2 single crystals. The magnetic field penetration shows a two-step feature with a very small full-magnetic-penetration field (Hp1= 300 Oe at 2 K), and accordingly the MHL exhibits an abnormal vanishing of the central peak near zero field below 13 K. The width of the MHL in KxFe2-ySe2 at the same temperature is in general much smaller than that measured in the relatives Ba0.6K0.4Fe2As2 and Ba(Fe0.92Co0.08)2As2, and the MHLs in the latter two samples show the normal central peak near zero field. All these anomalies found in KxFe2-ySe2 can be understood in the picture that the sample is percolative with weakly coupled superconducting islands.
  04/2011;
• Source

  Available from: Jiangping Hu

  Article: Localized and itinerant dichotomy of electrons in Ba(Fe,Co)2As2

  H. Q. Yuan, L. Jiao, F. F. Balakirev, J. Singleton, C. Setty, J. P. Hu, T. Shang, L J Li, G. H. Cao, Z. A. Xu, B Shen, H. H. Wen
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  ABSTRACT: Variant approaches, either based on the Fermi surface nesting or started from the proximity to a Mott-insulator, were proposed to elucidate the physics in iron pnictides, but no consensus has been reached. A fundamental problem concerns the nature of their 3d electrons. Here we report the magnetoresistivity (\rho_xx) and the Hall resistivity (\rho_xy) of Ba(Fe1-xCox)2As2 (x=0 and 0.05) in a magnetic field of up to 55T. The magnetic transition is extremely robust against magnetic field, giving strong evidence that the magnetic ordering is formed by local moments. The magnetic state is featured with a huge magnetoresistance and a distinguished Hall resistivity, \rho_xy(H), which shows a pronounced parabolic field dependence, while the paramagnetic state shows little magnetoresistance and follows a simple linear magnetic field dependence on the Hall resistivity. Analyses of our data, based on a two-carrier model, demonstrate that the electron carriers in the magnetic state rapidly increase upon applying a magnetic field, partially compensating the loss of electron carriers at T_M. We argue that the 3d-electrons in Ba(Fe1-xCox)2As2 are divided into those who are close to forming localized moments controlling the magnetic transition and the others giving rise to complex transport properties through their interaction with the former.
  02/2011;
• Source

  Available from: ArXiv

  Article: Measurement of a sign-changing two-gap superconducting phase in electron-doped Ba(Fe(1-x)Co(x))2As2 single crystals using scanning tunneling spectroscopy.

  M L Teague, G K Drayna, G P Lockhart, P Cheng, B Shen, H-H Wen, N-C Yeh
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  ABSTRACT: Scanning tunneling spectroscopic studies of Ba(Fe(1-x)Co(x))(2)As(2) (x=0.06, 0.12) single crystals reveal direct evidence for predominantly two-gap superconductivity. These gaps decrease with increasing temperature and vanish above the superconducting transition T(c). The two-gap nature and the slightly doping- and energy-dependent quasiparticle scattering interferences near the wave vectors (±π, 0) and (0, ±π) are consistent with sign-changing s-wave superconductivity. The excess zero-bias conductance and the large gap-to-T(c) ratios suggest dominant unitary impurity scattering.
  Physical Review Letters 02/2011; 106(8):087004. · 7.37 Impact Factor
• Source

  Available from: arxiv.org

  Article: Contrasting spin dynamics between underdoped and overdoped Ba(Fe1-xCox)2As2.

  F L Ning, K Ahilan, T Imai, A S Sefat, M A McGuire, B C Sales, D Mandrus, P Cheng, B Shen, H-H Wen
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  ABSTRACT: We report the first NMR investigation of spin dynamics in the overdoped nonsuperconducting regime of Ba(Fe1-xCox)2As2 up to x=0.26. We demonstrate that the absence of interband transitions with large momentum transfer Q{AF} approximately (pi/a,0) between the hole and electron Fermi surfaces results in complete suppression of antiferromagnetic spin fluctuations for x greater than or approximately 0.15. Our experimental results provide direct evidence for a correlation between T{c} and the strength of Q{AF} antiferromagnetic spin fluctuations.
  Physical Review Letters 01/2010; 104(3):037001. · 7.37 Impact Factor
• Source

  Available from: ArXiv

  Article: Superconductivity Near a Quantum Critical Point in Ba(Fe,Co)2As2

  K. Ahilan, F. L. Ning, T Imai, A. S. Sefat, M. A. McGuire, B. C. Sales, D. Mandrus, P Cheng, B Shen, H. H. Wen
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  ABSTRACT: We will examine the possible link between spin fluctuations and the superconducting mechanism in the iron-based high temperature superconductor Ba(Fe,Co)2As2 based on NMR and high pressure transport measurements. Comment: Invited paper to m2s-IX (2009)
  10/2009;
• Source

  Available from: ArXiv

  Article: Contrasting Spin Dynamics Between Underdoped and Overdoped Ba(Fe[1-x]Co[x])2As2

  F. L. Ning, K. Ahilan, T Imai, A. S. Sefat, M. A. McGuire, B. C. Sales, D. Mandrus, P Cheng, B Shen, H. H. Wen
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  ABSTRACT: We report the first NMR investigation of spin dynamics in the overdoped non-superconducting regime of Ba(Fe[1-x]Co[x])2As2 up to x =0.26. We demonstrate that the absence of inter-band transitions with large momentum transfer Q(AF) ~ (pi, 0) between the hole and electron Fermi surfaces results in complete suppression of antiferromagnetic spin fluctuations for x > 0.15. Our experimental results provide direct evidence for a correlation between Tc and the strength of Q(AF) antiferromagnetic spin fluctuations. Comment: Accepted for publication in Phys. Rev. Lett
  07/2009;
• Article: Nodeless superconductivity of single-crystalline K_ {x} Fe_ {2-y} Se_ {2} revealed by the low-temperature specific heat

  B. Zeng, B. Shen, G. F. Chen, J. B. He, D. M. Wang, C. H. Li, H. H. Wen
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  ABSTRACT: Low-temperature specific heat (SH) has been measured in KxFe2-ySe2 single crystals with Tc=32 K. The SH anomaly associated with the superconducting transition is moderate and sharp, yielding a value of ΔC/T|Tc=11.6±1.0 mJ/mol K2. The residual SH coefficient γ(0) in the superconducting state at T→0 is very small with a value of about 0.39 mJ/mol K2. The magnetic-field-induced enhancement of the low-T SH exhibits a rough linear feature indicating a nodeless gap. This is further supported by the scaling based on the nodeless-gap approach of the low-T data at different magnetic fields. A rough estimate indicates that the normal-state SH coefficient γn is about 6±0.5 mJ/mol K2, leading to ΔC/γnT|Tc=1.93 and placing this superconductor in the strong coupling region.
  Phys. Rev. B. 83(14).