W. Tian

Iowa State University, Ames, Iowa, United States

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Publications (183)572 Total impact

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    ABSTRACT: Structure and magnetic properties of high-quality polycrystlline CeMnAsO, a parent compound of the "1111"-type oxypnictides, have been investigated using neutron powder diffraction and magnetization measurements. We find that CeMnAsO undergoes a C-type antiferromagnetic order with Mn$^{2+}$ ($S=5/2$) moments pointing along the \textit{c}-axis below a relatively high N\'{e}el temperature of $T_{\rm N} = 345$ K. Below $T_{\rm SR} = 35$ K, two instantaneous transitions occur where the Mn moments reorient from the $c$-axis to the \textit{ab}-plane preserving the C-type magnetic order, and Ce moments undergo long-range AFM ordering with antiparallel moments pointing in the \textit{ab}-plane. Another transition to a noncollinear magnetic structure occurs below 7 K. The ordered moments of Mn and Ce at 2 K are 3.32(4) $\mu_{B}$ and 0.81(4)$\mu_{B}$, respectively. We find that CeMnAsO primarily falls into the category of a local-moment antiferromagnetic insulator in which the nearest-neighbor interaction ($J_{1}$) is dominant with $J_{2}<J_{1}/2$ in the context of $J_{1}-J_{2}-J_{c}$ model. The spin reorientation transition driven by the coupling between rare earth Ce and transition metal seems to be common to Mn, Fe and Cr ions, but not to Co and Ni ions in the iso-structural oxypnictides. A schematic illustration of magnetic structures in Mn and Ce sublattices in CeMnAsO is proposed.
    11/2014;
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    ABSTRACT: Neutron diffraction measurements on a single crystal of CeGe1.76 reveal a complex series of magnetic transitions at low temperature. At T_N = 7 K, there is a transition from a paramagnetic state at higher temperature to an incommensurate magnetic structure characterized by a magnetic propagation vector (0 0 tau) with tau approx. 1/4 and the magnetic moment along the a axis of the orthorhombic unit cell. Below T_LI = 5 K, the magnetic structure locks in to a commensurate structure with tau = 1/4 and the magnetic moment remains along the a axis. Below T* = 4 K, we find additional half-integer and integer indexed magnetic Bragg peaks consistent with a second commensurately ordered antiferromagnetic state.
    10/2014;
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    ABSTRACT: We report neutron scattering and transport measurements on semiconducting Rb$_{0.8}$Fe$_{1.5}$S$_2$, a compound isostructural and isoelectronic to the well-studied $A_{0.8}$Fe$_{y}$Se$_2 (A=$ K, Rb, Cs, Tl/K) superconducting systems. Both resistivity and DC susceptibility measurements reveal a magnetic phase transition at $T=275$ K. Neutron diffraction studies show that the 275 K transition originates from a phase with rhombic iron vacancy order which exhibits an in-plane stripe antiferromagnetic ordering below 275 K. In addition, interdigitated mesoscopically with the rhombic phase is an ubiquitous phase with $\sqrt{5}\times\sqrt{5}$ iron vacancy order. This phase has a magnetic transition at $T_N=425$ K and an iron vacancy order-disorder transition at $T_{S}=600$ K. These two different structural phases are closely similar to those observed in the isomorphous Se materials. Based on the close similarities of the in-plane antiferromagnetic structures, moments sizes, and ordering temperatures in semiconducting Rb$_{0.8}$Fe$_{1.5}$S$_2$ and K$_{0.81}$Fe$_{1.58}$Se$_2$, we argue that the in-plane antiferromagnetic order arises from strong coupling between local moments. Superconductivity, previously observed in the $A_{0.8}$Fe$_{y}$Se$_{2-z}$S$_z$ system, is absent in Rb$_{0.8}$Fe$_{1.5}$S$_2$, which has a semiconducting ground state. The implied relationship between stripe/block antiferromagnetism and superconductivity in these materials as well as a strategy for further investigation is discussed in this paper.
    08/2014;
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    ABSTRACT: The crossover from localized- to itinerant-electron behavior is associated with many intriguing phenomena in condensed-matter physics. In this paper, we investigate the crossover from localized to itinerant regimes in the spinel system Mn$_{1-x}$Co$_x$V$_2$O$_4$. At low Co doping, orbital order (OO) of the localized electrons on the V3+ ions suppresses magnetic frustration by triggering a tetragonal distortion. With Co doping, electronic itinerancy melts the OO and suppresses the structural phase transition while the reduced spin-lattice coupling produces magnetic frustration. Neutron scattering measurements and first-principles-guided spin models reveal that the non-collinear state at high Co doping is produced by weakened local anisotropy and enhanced Co-V spin interactions.
    07/2014;
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    ABSTRACT: We describe why Ising spin chains with competing interactions in $\rm SrHo_2O_4$ segregate into ordered and disordered ensembles at low $T$. Using elastic neutron scattering, magnetization, and specific heat measurements, the two distinct spin chains are inferred to have N\'eel ($\uparrow\downarrow\uparrow\downarrow$) and double-N\'eel ($\uparrow\uparrow\downarrow\downarrow$) ground states respectively. Below $T_\mathrm{N}=0.68(2)$ K, the N\'eel chains develop three dimensional (3D) long range order (LRO), which causes the double-N\'eel chains to freeze into a disordered incommensurate state at $T_\mathrm{S}= 0.52(2)$ K. $\rm SrHo_2O_4$ distills an important feature of incommensurate low dimensional magnetism: kinetically trapped topological defects in a quasi$-d-$dimensional spin system can preclude order in $d+1$ dimensions.
    07/2014;
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    ABSTRACT: Ca10(Pt3As8)(Fe2As2)5 is the parent compound for a class of Fe-based high-temperature superconductors where superconductivity with transition temperatures up to 30 K can be introduced by partial element substitution. We present a combined high-resolution high-energy x-ray diffraction and elastic neutron scattering study on a Ca10(Pt3As8)(Fe2As2)5 single crystal. This study reveals the microscopic nature of two distinct and continuous phase transitions to be very similar to other Fe-based high-temperature superconductors: an orthorhombic distortion of the high-temperature tetragonal Fe-As lattice below T_S = 110(2) K followed by stripe-like antiferromagnetic ordering of the Fe moments below T_N = 96(2) K. These findings demonstrate that major features of the Fe-based high-temperature superconductors are very robust against variations in chemical constitution as well as structural imperfection of the layers separating the Fe-As layers from each other and confirms that the Fe-As layers primarily determine the physics in this class of material.
    06/2014;
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    ABSTRACT: Neutron diffraction measurements are presented measuring the responses of both magnetic and structural order parameters of parent and lightly Co-doped Ba(Fe{1-x}Co{x})2As2 under the application of uniaxial pressure. We find that the uniaxial pressure induces a thermal shift in the onset of antiferromagnetic order that grows as a percentage of T_N as Co-doping is increased and the superconducting phase is approached. Additionally, as uniaxial pressure is increased within parent and lightly-doped Ba(Fe{1-x}Co{x})2As2 on the first order side of the tricritical point, we observe a decoupling between the onsets of the orthorhombic structural distortion and antiferromagnetism. Our findings place needed constraints on models exploring the nematic susceptibility of the bilayer pnictides in the tetragonal, paramagnetic regime.
    06/2014;
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    ABSTRACT: We present specific-heat and neutron-scattering results for the S = 1/2 quantum antiferromagnet (dimethylammonium)(3,5-dimethylpyridinium)CuBr4. The material orders magnetically at TN = 1.99(2) K, and magnetic excitations are accompanied by an energy gap of 0.30(2) meV due to spin anisotropy. The system is best described as coupled two-leg spin-1/2 ladders with the leg exchange Jleg = 0.60(2) meV, rung exchange Jrung = 0.64(9) meV, interladder exchange Jint = 0.19(2) meV, and an interaction-anisotropy parameter λ = 0.93(2), according to inelastic neutron-scattering measurements. In contrast to most spin ladders reported to date, the material is a rare example in which the interladder coupling is very near the critical value required to drive the system to a Neel-ordered phase without the assistance of a magnetic field.
    Physical Review B 05/2014; 89:174432. · 3.66 Impact Factor
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    ABSTRACT: Neutron and X-ray diffraction, magnetic susceptibility, and specific heat measurements have been used to investigate the magnetic and structural phase transitions of the spinel system Fe1+xCr2-xO4(0.0<=x<=1.0). The temperature versus Fe concentration (x) phase diagram features two magnetically ordered states and four structural states below 420 K. The complexity of the phase diagram is closely related to the change in the spin and orbital degrees of freedom induced by substitution of Fe ions for Cr ions. The systematic change in the crystal structure is explained by the combined effects of Jahn-Teller distortion, spin-lattice interaction, Fe2+-Fe3+ hopping, and disorder among Fe2+, Fe3+, and Cr3+ ions.
    04/2014; 89(13).
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    ABSTRACT: We use inelastic neutron scattering and dielectric constant measurements to study the doping influence on the spin dynamics and magnetoelectric (ME) effect in hexagonalY0.7Lu0.3MnO3. In undoped YMnO3 and LuMnO3, the Mn trimerization distortion has been suggested to play a key role in determining the magnetic structure and the magnetoelectric effect. In Y0.7Lu0.3MnO3, at the antiferromagnetic zone center, we observed a much smaller Δ12≈0.52 meV gap (which is ̃2.5 meV for both YMnO3 and LuMnO3) that coincides with a weaker in-plane dielectric anomaly at TN; both can be attributed to a weaker Mn trimerization distortion in Y0.7Lu0.3MnO3 compared to YMnO3 and LuMnO3. The results provide strong evidence that the magnitude of ME coupling is linked to the strength of the trimerization distortion, suggesting the Mn trimerization is responsible for the ME effect in Y1-yLuyMnO3.
    03/2014; 89(14).
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    ABSTRACT: Interest in many strongly spin-orbit-coupled 5d-transition metal oxide insulators stems from mapping their electronic structures to a J(eff)=1/2 Mott phase. One of the hopes is to establish their Mott parent states and explore these systems' potential of realizing novel electronic states upon carrier doping. However, once doped, little is understood regarding the role of their reduced Coulomb interaction U relative to their strongly correlated 3d-electron cousins. Here we show that, upon hole-doping a candidate J(eff)=1/2 Mott insulator, carriers remain localized within a nanoscale phase-separated ground state. A percolative metal-insulator transition occurs with interplay between localized and itinerant regions, stabilizing an antiferromagnetic metallic phase beyond the critical region. Our results demonstrate a surprising parallel between doped 5d- and 3d-electron Mott systems and suggest either through the near-degeneracy of nearby electronic phases or direct carrier localization that U is essential to the carrier response of this doped spin-orbit Mott insulator.
    Nature Communications 02/2014; 5:3377. · 10.74 Impact Factor
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    ABSTRACT: Neutron and synchrotron resonant X-ray magnetic scattering (RXMS) complemented by heat capacity and resistivity measurements reveal the evolution of the magnetic structures of Fe and Ce sublattices in single crystal CeFeAsO. The RXMS of magnetic re ections at the Ce LII-edge shows a magnetic transition that is speci�c to the Ce antiferromagnetic long-range ordering at TCe � 4 K with short-range Ce ordering above TCe, whereas neutron di�raction measurements of a few magnetic re ections indicate a transition at T� � 12 K with unusual order parameter. Detailed order parameter measurements on several magnetic re ections by neutrons show a weak anomaly at 4 K which we associate with the Ce ordering. The successive transitions at TCe and T� can also be clearly identi�ed by two anomalies in heat capacity and resistivity measurements. The higher transition temperature at T� � 12 K is mainly ascribed to Fe spin reorientation transition, below which Fe spins rotate uniformly and gradually in the ab plane. The Fe spin reorientation transition and short-range Ce ordering above TCe re ect the strong Fe-Ce couplings prior to long-range ordering of the Ce. The evolution of the intricate magnetic structures in CeFeAsO going through T� and TCe is proposed.
    Physical Review B 11/2013; 88:174517. · 3.66 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. · 1.65 Impact Factor
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    ABSTRACT: Interest in many strongly spin-orbit coupled 5d-transition metal oxides with insulating ground states stems from mapping their electronic structures to a J_eff=1/2 Mott phase. One of the hopes is to establish their Mott parent states and explore these systems potential of realizing novel electronic states upon carrier doping. However, once doped, little is understood regarding the role of their reduced Coulomb interaction U relative to their strongly correlated 3d-electron cousins. Here we show that, upon hole-doping a candidate J_eff=1/2 Mott insulator Sr3Ir2O7, carriers remain localized with an electronically phase separated ground state. A near percolative metal-insulator transition occurs with interplay between localized and itinerant regions, stabilizing an antiferromagnetic metallic phase in the critical region. Our results demonstrate a surprising parallel between the electronic behavior of doped 5d and 3d-electron Mott systems and suggest that U is essential to the in-plane carrier response in a doped spin-orbit Mott insulator.
    11/2013;
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    ABSTRACT: The structural and magnetic properties of BaMn_{2}Si_{2}O_{7} have been investigated. The magnetic susceptibility and specific heat, measured using single crystals, suggest that the quasi-one-dimensional magnetism originating from the loosely coupled Mn^{2+} chain carrying S=5/2 is present at high temperatures, which is similar to the other quasi-one-dimensional barium silicates, BaM_{2}Si_{2}O_{7} (M: Cu and Co). The Néel temperature (T_{N}∼26 K) is high compared to the magnetic interaction along the chain (J=−6 K). Neutron powder diffraction study has revealed that the magnetic structure is long ranged with antiferromagnetic arrangement along the chain (c) direction and ferromagnetic arrangement along the a and b axes. Detailed structural analysis suggests that the interchain interaction via Mn-O-Mn bond along the a axis is relatively large, which makes the system behave more two-dimensionally in the ac plane and enhances T_{N}.
    Physical Review B. 10/2013; 88(14).
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    ABSTRACT: The structural and magnetic properties of BaMn2Si2O7 have been investigated. The magnetic susceptibility and specific heat, measured using single crystals, suggest that the quasi-one-dimensional magnetism originating from the loosely coupled Mn2+ chain carrying S=5/2 is present at high temperatures, which is similar to the other quasi-one-dimensional barium silicates, BaM2Si2O7 (M: Cu and Co). The Neel temperature (TN=26 K) is high compared to the magnetic interaction along the chain (J = -6 K). The neutron powder diffraction study has revealed that the magnetic structure is long-ranged with antiferromagnetic arrangement along the chain (c) direction and ferromagnetic arrangement along the a and b axes. The detailed structural analysis suggests that the interchain interaction via Mn-O-Mn bond along the a axis is relatively large, which makes the system behave more two-dimensionally in the ac plane and enhances TN.
    Physical Review B 09/2013; · 3.66 Impact Factor
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    ABSTRACT: We use inelastic neutron scattering to study spin waves and their correlation with the magnetoelectric effect in Y$_{0.7}$Lu$_{0.3}$MnO$_3$. In the undoped YMnO$_3$ and LuMnO$_3$, the Mn trimerization distortion has been suggested to play a key role in determining the magnetic structure and the magnetoelectric effect. In Y$_{0.7}$Lu$_{0.3}$MnO$_3$, we find a much smaller in-plane (hexagonal $ab$-plane) single ion anisotropy gap that coincides with a weaker in-plane dielectric anomaly at $T_N$. Since both the smaller in-plane anisotropy gap and the weaker in-plane dielectric anomaly are coupled to a weaker Mn trimerization distortion in Y$_{0.7}$Lu$_{0.3}$MnO$_3$ comparing to YMnO$_3$ and LuMnO$_3$, we conclude that the Mn trimerization is responsible for the magnetoelectric effect and multiferroic phenomenon in Y$_{1-y}$Lu$_{y}$MnO$_{3}$.
    08/2013;
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    ABSTRACT: We study the structural and magnetic orders in electron-doped BaFe2-xNixAs2 by high-resolution synchrotron X-ray and neutron scatterings. Upon Ni-doping x, the nearly simultaneous tetragonal-to-orthorhombic structural T_s and antiferromagnetic (T_N) phase transitions in BaFe2As2 are gradually suppressed and separated, resulting in T_s>T_N with increasing x as was previously observed. However, the temperature separation between T_s and T_N decreases with increasing x for x> 0.065$, tending towards a quantum bi-critical point near optimal superconductivity at x=0.1. The zero-temperature transition is preempted by the formation of a secondary incommensurate magnetic phase in the region 0.088< x < 0.104, resulting in a finite value of T_N \approx T_c+10$ K above the superconducting dome around $x\approx 0.1$. Our results imply an avoided quantum critical point, which is expected to strongly influence the properties of both the normal and superconducting states.
    Physical Review Letters 06/2013; 110(25). · 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 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: BaMn2As2 is a local-moment antiferromagnetic insulator with a Néel temperature TN of 625 K and a large ordered moment of 3.9 μB/Mn. Remarkably, this compound can be driven metallic by the substitution of as little as 1.6% K for Ba while retaining essentially the same ordered magnetic moment and Néel temperature, as previously reported. Here, using both powder and single crystal neutron diffraction we show that the local moment antiferromagnetic order in Ba1−xKxMn2As2 remains robust up to x=0.4. The ordered moment is nearly independent of x for 0 ≤x≤ 0.4 and TN decreases to 480 K at x=0.4.
    Physical review. B, Condensed matter 04/2013; 87(14). · 3.66 Impact Factor

Publication Stats

3k Citations
572.00 Total Impact Points

Institutions

  • 2008–2014
    • Iowa State University
      • Department of Physics and Astronomy
      Ames, Iowa, United States
    • Forschungszentrum Jülich
      • Jülich Centre for Neutron Science (JCNS)
      Düren, North Rhine-Westphalia, Germany
  • 2007–2014
    • Oak Ridge National Laboratory
      • • Quantum Condensed Matter Division
      • • Materials Science and Technology Division
      • • Neutron Scattering Science Division
      Oak Ridge, Florida, United States
  • 2004–2013
    • University of Tennessee
      • Department of Physics & Astronomy
      Knoxville, Tennessee, United States
  • 2012
    • Chestnut Hill College
      Boston, Massachusetts, United States
  • 2010
    • Johns Hopkins University
      • Department of Physics and Astronomy
      Baltimore, Maryland, United States
    • Zhejiang University
      • Department of Material Science and Engineering
      Hangzhou, Zhejiang Sheng, China
  • 2008–2010
    • Cornell University
      • Department of Materials Science and Engineering
      Ithaca, New York, United States
    • University of Leuven
      • Department of Physics and Astronomy
      Louvain, Flanders, Belgium
  • 2001–2010
    • Pennsylvania State University
      • • Department of Materials Science and Engineering
      • • Materials Research Institute
      • • Department of Physics
      University Park, Maryland, United States
  • 1998–2009
    • University of Michigan
      • Department of Materials Science and Engineering
      Ann Arbor, Michigan, United States
  • 2005–2007
    • The University of Tennessee Medical Center at Knoxville
      Knoxville, Tennessee, United States
  • 2003
    • University of Wisconsin, Madison
      • Department of Materials Science and Engineering
      Mississippi, United States