Publications (218)404.83 Total impact
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ABSTRACT: In the temperatureconcentration phase diagram of most ironbased superconductors, antiferromagnetic order is gradually suppressed to zero at a critical point, and a dome of superconductivity forms around that point. The nature of the magnetic phase and its fluctuations is of fundamental importance for elucidating the pairing mechanism. In Ba{1x}KxFe2As2 and Ba{1x}NaxFe2As2, it has recently become clear that the usual stripelike magnetic phase, of orthorhombic symmetry, gives way to a second magnetic phase, of tetragonal symmetry, near the critical point, between x = 0.24 and x = 0.28. Here we report measurements of the electrical resistivity of Ba{1x}KxFe2As2 under applied hydrostatic pressures up to 2.75 GPa, for x = 0.22, 0.24 and 0.28. We track the onset of the tetragonal magnetic phase using the sharp anomaly it produces in the resistivity. In the temperatureconcentration phase diagram of Ba{1x}KxFe2As2, we find that pressure greatly expands the tetragonal magnetic phase, while the stripelike phase shrinks. This raises the interesting possibility that the fluctuations of the former phase might be involved in the pairing mechanism responsible for the superconductivity. 
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ABSTRACT: LaO[Formula: see text]F[Formula: see text]BiSe2 is a new layered superconductor discovered recently, which shows the superconducting transition temperature of 3.5 K. With angleresolved photoemission spectroscopy, we study the electronic structure of LaO[Formula: see text]F[Formula: see text]BiSe2 comprehensively. Two electronlike bands are located around the [Formula: see text] point of the Brillouin zone, and the outer pockets connect with each other and form large Fermi surface around [Formula: see text] and [Formula: see text]. These bands show negligible [Formula: see text] dispersion, indicating their twodimensional nature. Based on the Luttinger theorem, the carrier concentration is about 0.53 e[Formula: see text] per unit cell, close to its nominal value. Moreover, the photoemission data and the band structure calculations agree very well, and the renormalization factor is nearly 1.0, indicating the electron correlations in this material are rather weak. Our results suggest that LaO[Formula: see text]F[Formula: see text]BiSe2 is a conventional BCS superconductor without strong electron correlations.  [Show abstract] [Hide abstract]
ABSTRACT: We have conducted extensive investigations on the magnetization and its dynamical relaxation on a Ba0.66K0.32BiO3+δ single crystal. It is found that the magnetizationrelaxation rate is rather weak compared to that in cuprate superconductors, indicating a higher collective vortex pinning potential (or activation energy), although the intrinsic pinning potential Uc is weaker. A detailed analysis leads to the following discoveries: (1) A secondpeak effect on the magnetizationhysteresis loop was observed in a very wide temperature region, ranging from 2 to 24 K. Its general behavior is similar to that in YBa2Cu3O7. (2) Associated with the secondpeak effect, the magnetizationrelaxation rate is inversely related to the transient superconducting current density Js, revealing a quite general and similar mechanism for the secondpeak effect in many hightemperature superconductors. (3) A detailed analysis based on the collective creep model reveals a large glassy exponent μ and a small intrinsic pinning potential Uc. (4) Investigation on the volume pinning force density shows that the data can be scaled to the formula Fp ∝ bp(1b)q, with p=2.79 and q=3.14, where b is the reduced magnetic field to the irreversible magnetic field. The maximum normalized pinning force density appears near b≈0.47. Finally, a vortex phase diagram is drawn to show the phase transitions or crossovers between different vortex phases.  [Show abstract] [Hide abstract]
ABSTRACT: FeSe0.5Te0.5 single crystals with superconducting critical temperature of 13.5 K are investigated by scanning tunneling microscopy/spectroscopy (STM/STS) measurements in detail. STM image on the top surface shows an atomically resolved square lattice consisted by white and dark spots with a constant of about 3.73±0.03 Å which is consistent with the lattice constant 3.78 Å. The Se and Te atoms with a height difference of about 0.35 Å are successfully identified since the sizes of the two kinds of atoms are different. The tunneling spectra show very large zerobias conductance value and asymmetric coherent peaks in the superconducting state. According to the positions of coherence peaks, we determine the superconducting gap 2Δ = 5.5 meV, and the reduced gap 2Δ/kBTc = 4.9 is larger than the value predicted by the weakcoupling BCS theory. The zerobias conductance at 1.7 K only have a decrease of about 40% compared with the normal state conductance, which may originate from some scattering and broadening mechanism in the material. This broadening effect will also make the superconducting gap determined by the distance between the coherence peaks larger than the exact gap value. The asymmetric structure of the tunneling spectra near the superconducting gap is induced by the hump on the background. This hump appears at temperature more than twice the superconducting critical temperature. This kind of hump has also been observed in other iron pnictides and needs further investigation. A possible bosonic mode outside the coherence peak with a mode energy Ω of about 5.5 meV is observed in some tunneling spectra, and the ratio between the mode energy and superconducting transition temperature Ω/kBTc ≈ 4.7 is roughly consistent with the universal ratio 4.3 in ironbased superconductors. The highenergy background of the spectra beyond the superconducting gaps shows a Vshape feature. The slopes of the differential conductance spectra at high energy are very different in the areas of Teatom cluster and Seatom cluster, and the difference extends to the energy of more than 300 meV. The differential conductance mapping has very little information about the quasiparticle interference of the superconducting state, which may result from the other strong scattering mechanism in the sample.  [Show abstract] [Hide abstract]
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 inplane infraredactive FeAs 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 Kdoped compound, at odds with the above explanation. This anomalous behavior in Kdoped 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 Kdoped compound.  [Show abstract] [Hide abstract]
ABSTRACT: Superconducting fluctuations were studied through angulardependent torque measurements on a series of ${\mathrm{Bi}}_{2}{\mathrm{Sr}}_{2$${}x}{\mathrm{La}}_{x}{\mathrm{CuO}}_{6+$\delta${}}$ (BSLCO) single crystals. Two distinct superconducting fluctuation diamagnetisms were observed: one is the superconducting thermal fluctuation, with a boundary close to the superconducting phase boundary, while the other one, up to a temperature as high as about 180 K, showing maximum signal in the sample with hole carrier density $p=0.125$, could be due to preformed pairs. In addition, we observed linearly temperaturedependent paramagnetic torque signals in BSLCO samples, possibly a result of quantum criticality from a quantum critical point at the optimal doping.  [Show abstract] [Hide abstract]
ABSTRACT: Irradiation with 2.5 MeV electrons at doses up to 5.2 $\times$10$^{19}$ electrons/cm$^2$ was used to introduce pointlike defects in single crystals of Ba$_{1x}$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 nonmagnetic 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 nonmagnetic scattering and imply that in multiband Ba$_{1x}$K$_x$Fe$_2$As$_2$ superconductors, the ratio of the interband to intraband pairing strength, and associated gap anisotropy, increases upon the departure from the optimal doping. 
Article: BCSlike superconductivity in NdO$_{1x}$F$_{x}$BiS$_{2}$ ($x$ = 0.3 and 0.5) single crystals
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ABSTRACT: We measure the magnetic penetration depth $\Delta\lambda(T)$ for NdO$_{1x}$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 lowtemperature limit which is attributed to the paramagnetism of Nd ions. After subtracting the paramagnetic contributions, the penetration depth $\Delta\lambda(T)$ follows exponentialtype 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 strongcoupling BCS superconductivity in the presence of localized moments for NdO$_{1x}$F$_{x}$BiS$_{2}$.  [Show abstract] [Hide abstract]
ABSTRACT: We investigate the nature of the SDW (Spin Density Wave) transition in the underdoped regime of an ironbased high Tc superconductor Ba(Fe1xCox)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_SDW1)^a with the critical exponent a ~ 0.33.  [Show abstract] [Hide abstract]
ABSTRACT: We investigate the nature of the SDW (spin density wave) transition in the underdoped regime of an ironbased highTc superconductor Ba(Fe1xCox)(2)As2 by As75 NMR, with primary focus on a composition with x = 0.02 (TSDW = 99 K). We demonstrate that critical slowing down toward the threedimensional 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 TSDW, the dynamical structure factor of electron spins S(q,omega(n)) measured with the longitudinal NMR relaxation rate 1/T1 exhibits a divergent behavior obeying a power law 1/T1 proportional to Sigma Sq(q,omega(n)) similar to (T/TSDW  1)(delta) with the critical exponent delta similar to 0.33.  [Show abstract] [Hide abstract]
ABSTRACT: We have performed systematic angleresolved photoemission spectroscopy (ARPES) of ironchalcogenide superconductor FeTe1xSex to elucidate the electronic states relevant to the superconductivity. While the Fermisurface 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 welldefined 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.  [Show abstract] [Hide abstract]
ABSTRACT: NdO$_{0.5}$F$_{0.5}$BiS$_{2}$ is a new layered superconductor. We have studied the lowlying electronic structure of a single crystalline NdO$_{0.5}$F$_{0.5}$BiS$_{2}$ superconductor, whose superconducting transition temperature is 4.87K, with angleresolved 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 multiband and twodimensional 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 electronphonon coupling.  [Show abstract] [Hide abstract]
ABSTRACT: Using the angleresolved photoemission spectroscopy (ARPES) with resolution of all three components of electron momentum and electronic states symmetry, we explicate the electronic structure of holedoped BaFe2 As2, and show that widely discussed nesting and dimensionality of Fermi surface (FS) sheets have no immediate relation to the superconducting pairing in ironbased 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.  [Show abstract] [Hide abstract]
ABSTRACT: SmB6, a wellknown 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 ingap states could be ascribed to topological surface states, which would make SmB6 the first realization of topological Kondo insulator. Here by performing angleresolved 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 ingap 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 ingap states.  [Show abstract] [Hide abstract]
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 heavyion and proton irradiation to create correlated and point defects to explore the vortex pinning behavior of each and combined constituent defects in the ironbased superconductor Ba0.6K0.4Fe2As2 and find that the pinning mechanisms are nonadditive. The major effect of pirradiation in mixed pinning landscapes is the generation of fieldindependent critical currents in very high fields. At 7 T ∥ c and 5 K, the critical current density exceeds 5 MA/cm2.  [Show abstract] [Hide abstract]
ABSTRACT: We measured the inplane optical conductivity of a nearly optimally doped (Tc = 39.1K) single crystal of Ba0.6K0.4Fe2As2. Upon entering the superconducting state the optical conductivity vanishes below similar to 20meV, indicating a fully gapped system. A quantitative modeling of the optical response of this material requires two different isotropic gaps, the larger of which dominates the London penetration depth. The temperature dependence of these gaps indicate a strong interband interaction, but no impurityscatteringinduced pair breaking is present. This contrasts to the large residual conductivity observed in optimally doped BaFe1.84Co0.08As2 and strongly supports an s +/ gap symmetry for both compounds. Copyright (C) EPLA, 2013  [Show abstract] [Hide abstract]
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 Tindependent 1/τb, while a “narrow” Drude component reveals a Tlinear 1/τn resulting in a resistivity ρn≡1/σ1n(ω→0) also linear in temperature. An arctan(T) lowfrequency spectral weight is also strong evidence for a Tlinear 1/τ. A comparison to other materials with similar behavior suggests that the Tlinear 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.  [Show abstract] [Hide abstract]
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 columncolumn separations of 32 and 22 nm. At very high irradiation levels (columncolumn 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.  [Show abstract] [Hide abstract]
ABSTRACT: The magnetization around the superconducting transition was recently measured in a highquality Ba1−x Kx Fe2As2 single crystal with magnetic fields applied along and transverse to the crystal Felayers [J. Mosqueira et al. in Phys. Rev. B 83:094519, 2011]. Here we extend this study to the finitefield (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 threedimensional anisotropic Ginzburg–Landau (3DaGL) approach generalized to the shortwavelength regime through the introduction of a totalenergy 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.
Publication Stats
2k  Citations  
404.83  Total Impact Points  
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Institutions

2015

Fudan University
Shanghai, Shanghai Shi, China


20112015

Nanjing University
 Department of Physics
Nanching, Jiangsu Sheng, China


2014

HelmholtzZentrum Berlin
Berlín, Berlin, Germany


2012

Canadian Institute For Advanced Research
Toronto, Ontario, Canada


19952012

Chinese Academy of Sciences
 • Institute of Physics
 • National Laboratory for Superconductivity
 • Condensed Matter Physics
Peping, Beijing, China


20082011

Technical Institute of Physics and Chemistry
Peping, Beijing, China


2003

University of Leuven
Louvain, Flemish, Belgium


2000

Ningxia University
Ninghsia, Ningxia Huizu Zizhiqu, China


1997

VU University Amsterdam
 Department of Physics and Astronomy
Amsterdam, North Holland, Netherlands
