B. C. Sales

Soochow University (PRC), Wu-hsien, Jiangsu Sheng, China

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Publications (524)1452.1 Total impact

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    ABSTRACT: Using inelastic neutron scattering, we show that the onset of superconductivity in underdoped Ba(Fe$_{1-x}$Co$_{x}$)$_{2}$As$_{2}$ coincides with a crossover from well-defined spin waves to overdamped and diffusive spin excitations. This crossover occurs despite the presence of long-range stripe antiferromagnetic order for samples in a compositional range from x=0.04-0.055, and is a consequence of the shrinking spin-density wave gap and a corresponding increase in the particle-hole (Landau) damping. The latter effect is captured by a simple itinerant model relating Co doping to changes in the hot spots of the Fermi surface. We argue that the overdamped spin fluctuations provide a pairing mechanism for superconductivity in these materials.
    03/2014;
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    ABSTRACT: The anharmonic lattice dynamics of rock-salt thermoelectric compounds SnTe and PbTe are investigated with inelastic neutron scattering (INS) and first-principles calculations. The experiments show that, surprisingly, although SnTe is closer to the ferroelectric instability, phonon spectra in PbTe exhibit a more anharmonic character. This behavior is reproduced in first-principles calculations of the temperature-dependent phonon self-energy. Our simulations reveal how the nesting of phonon dispersions induces prominent features in the self-energy, which account for the measured INS spectra and their temperature dependence. We establish that the phase-space for three-phonon scattering processes, rather than just the proximity to the lattice instability, is the mechanism determining the complex spectrum of the transverse-optical ferroelectric mode.
    Physical review letters. 12/2013; 112(17).
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    ABSTRACT: Materials with very low thermal conductivity are of great interest for both thermoelectric and optical phase-change applications. Synthetic nanostructuring is most promising for suppressing thermal conductivity arising from scattering phonons, but challenges remain in producing bulk samples. In crystalline AgSbTe2, we show that a spontaneously forming nanostructure leads to a suppression of thermal conductivity to a glass-like level. Our mapping of the phonon mean-free-paths provides a novel bottom-up microscopic account of thermal conductivity and also reveals intrinsic anisotropies associated with the nanostructure. Ground-state degeneracy in AgSbTe2 leads to the natural formation of nanoscale domains with different orderings on the cation sublattice, and correlated atomic displacements, which efficiently scatter phonons. This mechanism is general and suggests a new avenue for the nanoscale engineering of materials to achieve low thermal conductivities for efficient thermoelectric converters and phase-change memory devices.
    The Journal of the Acoustical Society of America 11/2013; 134(5):4100. · 1.65 Impact Factor
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    ABSTRACT: Scanning probe microscopy has emerged as a powerful and flexible tool for atomically resolved imaging of surface structures. However, due to the amount of information extracted, in many cases the interpretation of such data is limited to being qualitative and semi-quantitative in nature. At the same time, much can be learned from local atom parameters, such as distances and angles, that can be analyzed and interpreted as variations of local chemical bonding, or order parameter fields. Here, we demonstrate an iterative algorithm for indexing and determining atomic positions that allows the analysis of inhomogeneous surfaces. This approach is further illustrated by local crystallographic analysis of several real surfaces, including highly ordered pyrolytic graphite and an Fe-based superconductor FeTe0.55Se0.45. This study provides a new pathway to extract and quantify local properties for scanning probe microscopy images.
    Nanotechnology 10/2013; 24(41):415707. · 3.84 Impact Factor
  • Physical Review B 10/2013; 88(15). · 3.77 Impact Factor
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    ABSTRACT: M-doped IrTe_{2} (M = Mn, Fe, Co, Ni) compounds were synthesized by solid-state reaction. Single crystal x-ray diffraction experiments indicate that part of the doped M ions (M = Fe, Co, and Ni) substitute for Ir, and the rest intercalate into the octahedral interstitial sites located in between IrTe_{2} layers. Due to the lattice mismatch between MnTe_{2} and IrTe_{2}, Mn has limited solubility in IrTe_{2} lattice. The trigonal structure is stable in the whole temperature range 1.80 K≤ T ≤ 300 K for all doped compositions. No long-range magnetic order or superconductivity was observed in any doped compositions above 1.80 K. A spin glass behavior below 10 K was observed in Fe-doped IrTe_{2} from the temperature dependence of magnetization, electrical resistivity, and specific heat. The low temperature specific heat data suggest the electron density of states is enhanced in Fe- and Co-doped compositions but reduced in Ni-doped IrTe_{2}. With the 3d transition metal doping the trigonal a-lattice parameter increases but the c-lattice parameter decreases. Detailed analysis of the single crystal x-ray diffraction data shows that interlayer Te-Te distance increases despite a reduced c lattice. The importance of the Te-Te, Te-Ir, and Ir-Ir bonding is discussed.
    Physical Review B 10/2013; 88(13). · 3.77 Impact Factor
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    ABSTRACT: The Jeff=1/2 state is manifested in systems with large cubic crystal field splitting and spin-orbit coupling that are comparable to the on-site Coulomb interaction, U. 5d transition metal oxides host parameters in this regime and strong evidence for this state in Sr2IrO4, and additional iridates, has been presented. All the candidates, however, deviate from the cubic crystal field required to provide an unmixed canonical Jeff=1/2 state, impacting the development of a robust model of this novel insulating and magnetic state. We present results that not only show Ca4IrO6 hosts the state, but furthermore uniquely resides close to the ideal case required for an unmixed Jeff=1/2 state.
    Physical Review B 09/2013; 89:081104. · 3.77 Impact Factor
  • T Keiber, F Bridges, B C Sales
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    ABSTRACT: PbTe is a well-known thermoelectric material. Recent x-ray total scattering studies suggest that Pb moves off center along 100 in PbTe, by ∼0.2 Å at 300 K, producing a split Pb-Te pair distribution. We present an extended x-ray absorption fine structure spectroscopy (EXAFS) study of PbTe (and Tl doped PbTe) to determine if Pb or Te is off center. EXAFS provides sensitive r- or k-space phase information which can differentiate between a split peak for the Pb-Te distribution (indicative of off-center Pb) and a thermally broadened peak. We find no evidence for a split peak for Pb-Te or Te-Pb. At 300 K, the vibration amplitude for Pb-Te (or Te-Pb) is large; this thermally induced disorder is indicative of weak bonds, and the large disorder is consistent with the low thermal conductivity at 300 K. We also find evidence of an anharmonic potential for the nearest Pb-Te bonds, consistent with the overall anharmonicity found for the phonon modes. This effect is modeled by a "skew" factor (C3) which significantly improves the fit of the Pb-Te and Te-Pb peaks for the high temperature EXAFS data; C3 becomes significant above approximately 150-200 K. The consequences of these results will be discussed.
    Physical Review Letters 08/2013; 111(9):095504. · 7.94 Impact Factor
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    ABSTRACT: The thermal conductivity of electron-doped Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$ single crystals is investigated below 200K, with an emphasis on the behavior near the magnetic and superconducting (T_c) transition temperatures. An enhancement of the in-plane thermal conductivity $\kappa_{ab}$ is observed below T_c for all samples, with the greatest enhancement observed near optimal doping. The observed trends are consistent with the scattering of heat carriers by low-energy magnetic excitations. Upon entering the superconducting state, the formation of a spin-gap leads to reduced scattering and an enhancement in $\kappa(T)$. Similarly, an enhancement of $\kappa$ is observed for polycrystalline BaFe2As2 below the magnetic transition, and qualitative differences in $\kappa(T)$ between single crystalline and polycrystalline BaFe2As2 are utilized to discuss anisotropic scattering. This study highlights how measuring $\kappa$ near $T_c$ in novel superconductors can be useful as a means to probe the potential role of spin fluctuations.
    Physical Review B 08/2013; 88(6). · 3.77 Impact Factor
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    ABSTRACT: Measurements of magnetization, specific heat, electrical resistivity, Hall effect, and magnetoresistance on single crystalline samples of LaFeAsO grown in a NaAs flux are reported. While this material is known to be a semimetal, the temperature dependence of the electrical resistivity data presented herein is reminiscent of semiconducting behavior and exhibits distinct features associated with a structural transition and spin density wave (SDW) order. Magnetoresistance and Hall coefficient measurements were performed in magnetic fields up to 9 T applied perpendicular to the basal plane using a van der Pauw configuration. The charge carrier density and mobility indicate that electrons are the majority charge carriers and exhibit features indicative of the structural transition and SDW formation. Low temperature X-ray diffraction measurements have confirmed that the structural transition in these samples occurs near 140 K, compared to a transition temperature of 156 K observed in polycrystalline samples. Isotherms of magnetoresistivity measured as a function of magnetic field can be scaled onto a single curve in which the scaling field is a linear function of temperature between 2.2 K and 180 K.
    Physical Review B 08/2013; 88(13). · 3.77 Impact Factor
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    ABSTRACT: The spin and orbital ordering in Dy1−xTbxVO3 (x=0 and 0.2) was studied by measuring x-ray powder diffraction, magnetization, specific heat, and neutron single-crystal diffraction. The results show that G-OO/C-AF and C-OO/G-AF phases coexist in Dy0.8Tb0.20VO3 in the temperature range 2–60 K, and the volume fraction of each phase is temperature and field dependent. The ordering of Dy moments at T* = 12 K induces a transition from G-OO/C-AF to a C-OO/G-AF phase. Magnetic fields suppress the long-range order of Dy moments and thus the C-OO/G-AF phase below T*. The polarized moments induced at the Dy sublattice by external magnetic fields couple to the V 3d moments, and this coupling favors the G-OO/C-AF state. Also discussed is the effect of the Dy-V magnetic interaction and local structure distortion on the spin and orbital ordering in Dy1−xTbxVO3.
    Physical review. B, Condensed matter 06/2013; 87(22). · 3.77 Impact Factor
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    ABSTRACT: Materials with very low thermal conductivity are of great interest for both thermoelectric and optical phase-change applications. Synthetic nanostructuring is most promising for suppressing thermal conductivity through phonon scattering, but challenges remain in producing bulk samples. In crystalline AgSbTe2 we show that a spontaneously forming nanostructure leads to a suppression of thermal conductivity to a glass-like level. Our mapping of the phonon mean free paths provides a novel bottom-up microscopic account of thermal conductivity and also reveals intrinsic anisotropies associated with the nanostructure. Ground-state degeneracy in AgSbTe2 leads to the natural formation of nanoscale domains with different orderings on the cation sublattice, and correlated atomic displacements, which efficiently scatter phonons. This mechanism is general and suggests a new avenue for the nanoscale engineering of materials to achieve low thermal conductivities for efficient thermoelectric converters and phase-change memory devices.
    Nature Nanotechnology 06/2013; · 31.17 Impact Factor
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    ABSTRACT: The effects of temperature and pressure on phonons in B20 compounds FeSi1−xAlx were measured using inelastic neutron scattering and nuclear-resonant inelastic x-ray scattering. The effect of hole doping through Al substitution is compared to results of alloying with Co (electron doping) in Fe1−xCoxSi. While the temperature dependence of phonons in FeSi is highly anomalous, doping with either type of carriers leads to a recovery of the normal quasiharmonic behavior. Density functional theory (DFT) computations of the electronic band structure and phonons were performed. The anomaly in the temperature dependence of the phonons in undoped FeSi was related to the narrow band gap, and its sensitivity to the effect of thermal disordering by phonons. On the other hand, the pressure dependence of phonons at room temperature in undoped FeSi follows the quasiharmonic behavior and is well reproduced by the DFT calculations.
    Physical review. B, Condensed matter 05/2013; 87(18). · 3.77 Impact Factor
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    ABSTRACT: The Fe1+yTe1-xSex series of materials is one of the prototype families of Fe-based superconductors. To provide further insight into these materials we present systematic inelastic neutron scattering measurements of the low energy spin excitations for x=0.27, 0.36, 0.40, 0.49. These measurements show an evolution of incommensurate spin excitations towards the (1/2 1/2 0) wave vector with doping. Concentrations (x=0.40 and 0.49) which exhibit the most robust superconducting properties have spin excitations closest to (1/2 1/2 0) and also exhibit a strong spin resonance in the spin excitation spectrum below Tc. The resonance signal appears to be closer to (1/2 1/2 0) than the underlying spin excitations. We discuss the possible relationship between superconductivity and spin excitations at the (1/2 1/2 0) wave vector and the role that interstitial Fe may play.
    Physical review. B, Condensed matter 05/2013; 87(22). · 3.77 Impact Factor
  • T. Keiber, F. Bridges, B.C. Sales, H. Wang
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    ABSTRACT: When PbTe, a good thermoelectric material, is doped with a few percent Tl, the figure of merit ZT=TS2/(ρκ) [S is the Seebeck coefficient, ρ the electrical resistivity, and κ the thermal conductivity] is dramatically improved. The maximum value of ZT occurs for approximately 2% Tl, but the factors limiting ZT are as yet poorly understood. From a detailed local structure study of PbTe:Tl using the extended x-ray absorption fine structure (EXAFS) technique, we find that Tl substitutes primarily as Tl(+1) on the Pb site, with no evidence for any solubility issue, any significant fraction of Tl(+3), or any Tl interstitials. However it is not a simple substitution as there is evidence for increasing Te vacancies on neighboring sites with increasing Tl concentration, x. In addition there is also increasing disorder with x—Tl-Te bond length disorder as well as the vacancy defects—that will scatter the hole carriers and begin to increase the electrical resistivity in spite of an increase in hole concentration. This increased disorder is likely an important factor limiting ZT.
    Physical review. B, Condensed matter 04/2013; 87(14). · 3.77 Impact Factor
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    ABSTRACT: We study experimentally and theoretically the electronic and magnetic properties of two insulating double perovskites that show similar atomic and electronic structure, but different magnetic properties. In magnetization measurements, La2ZnIrO6 displays weak ferromagnetic behavior below 7.5 K whereas La2MgIrO6 shows antiferromagnetic behavior (AFM) below TN = 12 K. Electronic structure calculations find that the weak ferromagnetic behavior observed in La2ZnIrO6 is in fact due to canted antiferromagnetism. The calculations also predict canted antiferromagnetic behavior in La2MgIrO6, but intriguingly this was not observed. Neutron diffraction measurements confirm the essentially antiferromagnetic behavior of both systems, but lack the sensitivity to resolve the small (0.22 {\mu}B/Ir) ferromagnetic component in La2ZnIrO6. Overall, the results presented here indicate the crucial role of spin-orbit coupling (SOC) and the on-site Coulomb repulsion on the magnetic, transport, and thermodynamic properties of both compounds. The electronic structure calculations show that both compounds, like Sr2IrO4, are Jeff = 1/2 Mott insulators. Our present findings suggest that La2ZnIrO6 and La2MgIrO6 provide a new playground to study the interplay between SOC and on-site Coulomb repulsion in a 5d transition metal oxide.
    Physical Review B 04/2013; 87:155136. · 3.77 Impact Factor
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    ABSTRACT: The relationship between atomically defined structures and physical properties in functional materials remains a subject of constant interest. We explore the interplay between local crystallographic structure, composition, and local superconductive properties in iron chalcogenide superconductors. Direct structural analysis of scanning tunneling microscopy data allows local lattice distortions and structural defects across an FeTe0.55Se0.45 surface to be explored on a single unit-cell level. Concurrent superconducting gap (SG) mapping reveals suppression of the SG at well-defined structural defects, identified as a local structural distortion. The strong structural distortion causes the vanishing of the superconducting state. This study provides insight into the origins of superconductivity in iron chalcogenides by providing an example of atomic-level studies of the structure-property relationship.
    ACS Nano 03/2013; · 12.06 Impact Factor
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    ABSTRACT: The interplay between spin-orbit coupling, bandwidth and on-site coulomb repulsion in layered 5d transition metal oxides (TMO) acquired much interest recently. In Sr2IrO4, the interplay opens a gap near the Fermi energy and stabilizes a Jeff=1/2 spin-orbital entangled insulating state at low temperatures. However, whether this metal-insulating transition (MIT) is Mott-type (electronic-correlation driven) or slater-type (magnetic order driven) is still under hot debate. In this presentation, we give, for the first time, the atomic resolved structure of Sr2IrO4 surface in real space by using scanning tunneling microscopy. Tunneling spectroscopic results illustrate the gap opening of Sr2IrO4 at low temperatures with the gap size of 250 mV, indicating the metal to insulator transition. More importantly, the pair of peaks around gap in spectra suggests the quasi-particle coherent excitation, implying the Slater-type insulating state. This is further confirmed by temperature dependent measurements and density functional theory calculations.
    03/2013;
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    ABSTRACT: In 5d electron transition metal oxides, interplay between spin-orbit coupling and electronic interactions can lead to novel properties. One example is the Mott state in Sr2IrO4 which is believed to be associated with the formation of a Jeff=1/2 band due to large spin-orbit splitting of the t2g band. We use bulk measurements and resonant magnetic x-ray scattering to explore the effects of substituting Mn for Ir in single crystals of Sr2Ir0.9Mn0.1O4. These measurements indicate that 10% Mn doping is sufficient to suppress the magnetic ordering temperature from 240 K to 155 K. Resonant magnetic x-ray scattering measurements indicate a change in the long-range magnetic order when compared to that of undoped Sr2IrO4. Despite the large change in transition temperature and the altered magnetic structure, we observe a difference in the resonant enhancement between the L2 and L3 edges which is very similar to that seen in the pure material. This suggests that the magnetic structure of Sr2IrO4 can be altered by small perturbations whereas the Jeff=1/2 state is robust.
    03/2013;

Publication Stats

7k Citations
1,452.10 Total Impact Points

Institutions

  • 2013
    • Soochow University (PRC)
      Wu-hsien, Jiangsu Sheng, China
    • University of California, Santa Cruz
      • Department of Physics
      Santa Cruz, CA, United States
  • 1981–2013
    • Oak Ridge National Laboratory
      • • Neutron Scattering Science Division
      • • Materials Science and Technology Division
      • • Solid State Division
      Oak Ridge, Florida, United States
  • 2011
    • Simon Fraser University
      • Department of Physics
      Burnaby, British Columbia, Canada
  • 2010–2011
    • University of Alabama at Birmingham
      • Department of Physics
      Birmingham, AL, United States
    • McMaster University
      • Department of Physics and Astronomy
      Hamilton, Ontario, Canada
    • University of West Georgia
      • Department of Physics
      Carrollton, GA, United States
  • 2009
    • Louisiana State University
      • Department of Physics & Astronomy
      Baton Rouge, LA, United States
  • 2005–2009
    • University of South Florida
      • Department of Physics
      Tampa, FL, United States
  • 2001–2009
    • University of Tennessee
      • • Department of Materials Science and Engineering
      • • Department of Physics & Astronomy
      Knoxville, TN, United States
  • 2008
    • Forschungszentrum Jülich
      • Jülich Centre for Neutron Science (JCNS)
      Düren, North Rhine-Westphalia, Germany
    • Florida State University
      • Applied Superconductivity Center (ASC)
      Tallahassee, Florida, United States
  • 2004–2005
    • Chestnut Hill College
      Boston, Massachusetts, United States
    • Memorial University of Newfoundland
      • Department of Physics and Physical Oceanography
      Saint John, New Brunswick, Canada
  • 2003
    • University of Liège
      • Department of Physics
      Liège, WAL, Belgium
  • 2002
    • The University of Tennessee Medical Center at Knoxville
      Knoxville, Tennessee, United States
  • 1998–2001
    • Iowa State University
      • Department of Materials Science and Engineering
      Ames, Iowa, United States
  • 2000
    • University of California, San Diego
      • Department of Physics
      San Diego, CA, United States
  • 1993
    • Universidad de Talca
      Talca, Maule, Chile