B. C. Sales

Oak Ridge National Laboratory, Oak Ridge, Florida, United States

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Publications (536)1607.23 Total impact

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    ABSTRACT: Lattice distortions corresponding to Ba displacements with respect to the FeAs sublattice are revealed to break the room-temperature tetragonal symmetry in Ba(Fe1-x Cox )2 As2. The displacements yield twin domains of the size of ≈10 nm. The domain size correlates with the magnitude of the local Fe magnetic moment and its non-monotonic dependence on Co concentration. © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    Advanced Materials 03/2015; DOI:10.1002/adma.201404079 · 15.41 Impact Factor
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    ABSTRACT: MnFePSi compounds are promising materials for magnetic refrigeration as they exhibit a giant magnetocaloric effect. From first principles calculations and experiments on bulk materials, it has been proposed that this is due to the Mn and Fe atoms preferentially occupying two different sites within the atomic lattice. A recently developed technique was used to deconvolve the obscuring effects of both multiple elastic scattering and thermal diffuse scattering of the probe in an atomic resolution electron energy-loss spectroscopy investigation of a MnFePSi compound. This reveals, unambiguously, that the Mn atoms preferentially occupy the 3g site in a hexagonal crystal structure, confirming the theoretical predictions. After deconvolution, the data exhibit a difference in the Fe L_{2,3} ratio between the 3f and 3g sites consistent with differences in magnetic moments calculated from first principles, which are also not observed in the raw data.
    Physical Review Letters 03/2015; 114(10):106101. · 7.73 Impact Factor
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    ABSTRACT: High quality single crystals of BaFe$_{12}$O$_{19}$ were grown using the floating zone technique in flowing oxygen pressurized to 100 atm. Single crystal neutron diffraction was used to determine the nuclear and magnetic structure of BaFe$_{12}$O$_{19}$ at 4 K and 295 K. At both temperatures, there exist local electric dipoles formed by the off-mirror-plane displacements of magnetic Fe$^{3+}$ ions at the bipyramidal sites. The displacement at 4 K is about half of that at room temperature. The temperature dependence of the specific heat shows no anomaly associated with long range polar ordering in the temperature range from 1.90-300 K. The inverse dielectric permittivity, $1/\varepsilon$, along the c-axis shows a $T^2$ temperature dependence between 10 K and 20 K, with a significantly reduced temperature dependence displayed below 10 K. Moreover, as the sample is cooled below 1.4 K there is an anomalous sharp upturn in $1/\varepsilon$. These features resemble those of classic quantum paraelectrics such as SrTiO$_3$. The presence of the upturn in $1/\varepsilon$ indicates that BaFe$_{12}$O$_{19}$ is a critical quantum paraelectric system with Fe$^{3+}$ ions involved in both magnetic and electric dipole formation.
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    ABSTRACT: We have examined the crystallographic and magnetic properties of single crystals of CrI3, an easily cleavable, layered and insulating ferromagnet with a Curie temperature of 61 K. Our X-ray diffraction studies reveal a first-order crystallographic phase transition occurring near 210-220 K upon warming, with significant thermal hysteresis. The low-temperature structure is rhombohedral (R3̅, BiI3-type) and the high-temperature structure is monoclinic (C2/m, AlCl3-type). We find evidence for coupling between the crystallographic and magnetic degrees of freedom in CrI3, observing an anomaly in the interlayer spacing at the Curie temperature and an anomaly in the magnetic susceptibility at the structural transition. First-principles calculations reveal the importance of proper treatment of the long-ranged interlayer forces, and van der Waals density functional theory does an excellent job of predicting the crystal structures and their relative stability. Calculations also suggest that the ferromagnetic order found in the bulk material may persist into monolayer form, suggesting that CrI3 and other chromium trihalides may be promising materials for spintronic and magnetoelectronic research.
    Chemistry of Materials 01/2015; 27(2):612-620. DOI:10.1021/cm504242t · 8.54 Impact Factor
  • Applied Physics Letters 01/2015; 106(2):021911. DOI:10.1063/1.4905224 · 3.52 Impact Factor
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    ABSTRACT: La$_{0.4}$Na$_{0.6}$Fe$_2$As$_2$ single crystals have been grown out of an NaAs flux in an alumina crucible and characterized by measuring magnetic susceptibility, electrical resistivity, specific heat, as well as single crystal x-ray and neutron diffraction. La$_{0.4}$Na$_{0.6}$Fe$_2$As$_2$ single crystals show a structural phase transition from a high temperature tetragonal phase to a low-temperature orthorhombic phase at T$_s$\,=\,125\,K. This structural transition is accompanied by an anomaly in the temperature dependence of electrical resistivity, anisotropic magnetic susceptibility, and specific heat. Concomitant with the structural phase transition, the Fe moments order along the \emph{a} direction with an ordered moment of 0.7(1)\,$\mu_{\textup{B}}$ at \emph{T}\,=\,5 K. The low temperature stripe antiferromagnetic structure is the same as that in other \emph{A}Fe$_{2}$As$_{2}$ (\emph{A}\,=\,Ca, Sr, Ba) compounds. La$_{0.5-x}$Na$_{0.5+x}$Fe$_2$As$_2$ provides a new material platform for the study of iron-based superconductors where the electron-hole asymmetry could be studied by simply varying La/Na ratio.
    Physical review. B, Condensed matter 01/2015; 91:024501. DOI:10.1103/PhysRevB.91.024501 · 3.66 Impact Factor
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    ABSTRACT: Hexagonal Fe3Sn has many of the desirable properties for a new permanent magnet phase with a Curie temperature of 725 K, a saturation moment of 1.18 MA/m. and anisotropy energy, K1 of 1.8 MJ/m(3). However, contrary to earlier experimental reports, we found both experimentally and theoretically that the easy magnetic axis lies in the hexagonal plane, which is undesirable for a permanent magnet material. One possibility for changing the easy axis direction is through alloying. We used first principles calculations to investigate the effect of elemental substitutions. The calculations showed that substitution on the Sn site has the potential to switch the easy axis direction. However, transition metal substitutions with Co or Mn do not have this effect. We attempted synthesis of a number of these alloys and found results in accord with the theoretical predictions for those that were formed. However, the alloys that could be readily made all showed an in-plane easy axis. The electronic structure of Fe3Sn is reported, as are some are magnetic and structural properties for the Fe3Sn2, and Fe5Sn3 compounds, which could be prepared as mm-sized single crystals.
    Scientific Reports 11/2014; 4:7024. DOI:10.1038/srep07024 · 5.08 Impact Factor
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    ABSTRACT: Structural and physical properties determined by measurements on large single crystals of the anisotropic ferromagnet MnBi are reported. The findings support the importance of magneto-elastic effects in this material. X-ray diffraction reveals a structural phase transition at the spin reorientation temperature $T_{SR}$ = 90 K. The distortion is driven by magneto-elastic coupling, and upon cooling transforms the structure from hexagonal to orthorhombic. Heat capacity measurements show a thermal anomaly at the crystallographic transition, which is suppressed rapidly by applied magnetic fields. Effects on the transport and anisotropic magnetic properties of the single crystals are also presented. Increasing anisotropy of the atomic displacement parameters for Bi with increasing temperature above $T_{SR}$ is revealed by neutron diffraction measurements. It is likely that this is directly related to the anisotropic thermal expansion in MnBi, which plays a key role in the spin reorientation and magnetocrystalline anisotropy. The identification of the true ground state crystal structure reported here may be important for future experimental and theoretical studies of this permanent magnet material, which have to date been performed and interpreted using only the high temperature structure.
    Physical Review B 11/2014; 90(17). DOI:10.1103/PhysRevB.90.174425 · 3.66 Impact Factor
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    ABSTRACT: Neutron scattering is used to probe magnetic interactions as superconductivity develops in optimally doped Fe 1+δ Se x Te 1−x. Applying the first moment sum rule to comprehensive neutron scattering data, we extract the change in magnetic exchange energy [J R−R S R · S R ] in the superconducting state referenced to the normal state. Oscillatory changes are observed for Fe-Fe displacements |R| < ξ, where ξ = 1.3(1) nm is the superconducting coherence length. Dominated by a large reduction in the second nearest neighbor exchange energy [−1.2(2) meV/Fe], the overall reduction in magnetic interaction energy is H mag = −0.31(9) meV/Fe. Comparison to the superconducting condensation energy E SC = −0.013(1) meV/Fe, which we extract from specific heat data, suggests the modified magnetism we probe drives superconductivity in Fe 1+δ Se x Te 1−x .
    Physical Review B 09/2014; 90(10):100501(R). DOI:10.1103/PhysRevB.90.100501 · 3.66 Impact Factor
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    ABSTRACT: The intrinsic Fe local magnetic moment and Fe orbital occupations of iron-based superconductors are unveiled through the local, real-space capability of aberration-corrected scanning transmission electron microscopy/electron energy loss spectroscopy (STEM/EELS). Although the ordering of Fe moments needs to be suppressed for superconductivity to arise, the local, fluctuating Fe magnetic moment is enhanced near optimal superconductivity.
    Advanced Materials 09/2014; 26(35). DOI:10.1002/adma.201401518 · 15.41 Impact Factor
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    ABSTRACT: The crystal structure and physical properties of the layered material EuMnBi2 have been characterized by measurements on single crystals. EuMnBi2 is isostructural with the Dirac material SrMnBi2 based on single crystal x-ray diffraction, crystallizing in the I4/mmm space group (No. 139). Magnetic susceptibility measurements suggest antiferromagnetic (AFM) ordering of moments on divalent Eu ions near T_N=22K. For low fields, the ordered Eu moments are aligned along the c-axis, and a spin-flop is observed near 5.4T at 5K. The moment is not saturated in an applied field of 13T at 5K, which is uncommon for compounds containing Eu^{2+}. The magnetic behavior suggests an anisotropy enhancement via interaction between Eu and the Mn moments that appear to be order antiferromagnetically below approximately 310K. A large increase in the magnetoresistance is observed across the spin-flop, with absolute magnetoresistance reaching approximately 650% at 5K and 12T. Hall effect measurements reveal a decrease in the carrier density below T_N, which implies a manipulation of the Fermi surface by magnetism on the sites surrounding the Bi square nets that lead to Dirac cones in this family of materials.
    Physical Review B 07/2014; 90(7). DOI:10.1103/PhysRevB.90.075109 · 3.66 Impact Factor
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    ABSTRACT: Analysis of neutron diffraction, dc magnetization, ac magnetic susceptibility, heat capacity, and electrical resistivity for DyRuAsO in an applied magnetic field are presented at temperatures near and below those at which the structural distortion (T_S = 25 K) and subsequent magnetic ordering (T_N = 10.5 K) take place. Powder neutron diffraction is used to determine the antiferromagnetic order of Dy moments of magnitude 7.6(1) mu_B in the absence of a magnetic field, and demonstrate the reorientation of the moments into a ferromagnetic configuration upon application of a magnetic field. Dy magnetism is identified as the driving force for the structural distortion. The magnetic structure of analogous TbRuAsO is also reported. Competition between the two magnetically ordered states in DyRuAsO is found to produce unusual physical properties in applied magnetic fields at low temperature. An additional phase transition near T* = 3 K is observed in heat capacity and other properties in fields greater than about 3 T. Magnetic fields of this magnitude also induce spin-glass-like behavior including thermal and magnetic hysteresis, divergence of zero-field-cooled and field-cooled magnetization, frequency dependent anomalies in ac magnetic susceptibility, and slow relaxation of the magnetization. This is remarkable since DyRuAsO is a stoichiometric material with no disorder detected by neutron diffraction, and suggests analogies with spin-ice compounds and related materials with strong geometric frustration.
    Physical Review B 07/2014; 90(1). DOI:10.1103/PhysRevB.90.014425 · 3.66 Impact Factor
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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.
    Physical Review B 03/2014; 89(18). DOI:10.1103/PhysRevB.89.180503 · 3.66 Impact Factor
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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). DOI:10.1103/PhysRevLett.112.175501 · 7.73 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 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. DOI:10.1121/1.4830978 · 1.56 Impact Factor
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    ABSTRACT: Particularly in Sr2IrO4, the interplay between spin-orbit coupling, bandwidth and on-site Coulomb repulsion stabilizes a Jeff = 1/2 spin-orbital entangled insulating state at low temperatures. Whether this insulating phase is Mott- or Slater-type, has been under intense debate. We address this issue via spatially resolved imaging and spectroscopic studies of the Sr2IrO4 surface using scanning tunneling microscopy/spectroscopy (STM/S). STS results clearly illustrate the opening of an insulating gap (150 ~ 250 meV) below the Néel temperature (TN), in qualitative agreement with our density-functional theory (DFT) calculations. More importantly, the temperature dependence of the gap is qualitatively consistent with our DFT + dynamical mean field theory (DMFT) results, both showing a continuous transition from a gapped insulating ground state to a non-gap phase as temperatures approach TN. These results indicate a significant Slater character of gap formation, thus suggesting that Sr2IrO4 is a uniquely correlated system, where Slater and Mott-Hubbard-type behaviors coexist.
    Scientific Reports 10/2013; 3:3073. DOI:10.1038/srep03073 · 5.08 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. DOI:10.1088/0957-4484/24/41/415707 · 3.67 Impact Factor
  • Physical Review B 10/2013; 88(15). DOI:10.1103/PhysRevB.88.159902 · 3.66 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). DOI:10.1103/PhysRevB.88.134502 · 3.66 Impact Factor

Publication Stats

12k Citations
1,607.23 Total Impact Points

Institutions

  • 1981–2015
    • Oak Ridge National Laboratory
      • • Materials Science and Technology Division
      • • Solid State Division
      • • Chemical Sciences Division
      Oak Ridge, Florida, United States
  • 2011
    • Los Alamos National Laboratory
      • Materials Science and Technology Division
      Лос-Аламос, California, United States
  • 2005–2009
    • University of Tennessee
      • • Department of Materials Science and Engineering
      • • Department of Physics & Astronomy
      Knoxville, TN, United States
  • 1996–2007
    • University of Kentucky
      • • Department of Physics & Astronomy
      • • Department of Chemistry
      Lexington, Kentucky, United States
  • 2006
    • American Physical Society
      Maryland, United States
  • 1997–2002
    • The University of Tennessee Medical Center at Knoxville
      Knoxville, Tennessee, United States
  • 2000
    • University of California, San Diego
      • Department of Physics
      San Diego, CA, United States
  • 1998
    • Argonne National Laboratory
      • Division of Materials Science
      Lemont, Illinois, United States
  • 1994
    • Dallas Zoo
      Dallas, Texas, United States
  • 1986
    • Baylor University
      • Department of Physics
      Waco, Texas, United States