Publications (131)354.67 Total impact
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ABSTRACT: We use inelastic neutron scattering to study the temperature and energy dependence of the spin excitation anisotropy in uniaxialstrained electrondoped iron pnictide BaFe$_{1.9}$Ni$_{0.1}$As$_2$ near optimal superconductivity ($T_c=20$ K). Our work has been motivated by the observation of inplane resistivity anisotropy in the paramagnetic tetragonal phase of electronunderdoped iron pnictides under uniaxial pressure, which has been attributed to a spindriven Isingnematic state or orbital ordering. Here we show that the spin excitation anisotropy, a signature of the spindriven Isingnematic phase, exists for energies below $\sim$60 meV in uniaxialstrained BaFe$_{1.9}$Ni$_{0.1}$As$_2$. Since this energy scale is considerably larger than the energy splitting of the $d_{xz}$ and $d_{yz}$ bands of uniaxialstrained Ba(Fe$_{1x}$Co$_x$)$_2$As$_2$ near optimal superconductivity, spin Isingnematic correlations is likely the driving force for the resistivity anisotropy and associated electronic nematic correlations.Physical Review B 11/2015; 92(18). DOI:10.1103/PhysRevB.92.180504 · 3.74 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: The structural and the magnetic properties of CeCu$_{6x}$Ag$_x$ (0 $\leq$ $x$ $\leq$ 0.85) and CeCu$_{6x}$Pd$_x$ (0 $\leq$ $x$ $\leq$ 0.4) have been studied using neutron diffraction, resonant ultrasound spectroscopy (RUS), heat capacity, xray diffraction measurements and first principles calculations. The structural and magnetic phase diagrams of CeCu$_{6x}$Ag$_x$ and CeCu$_{6x}$Pd$_x$ as a function of Ag/Pd composition are reported. The end member, CeCu$_6$, undergoes a structural phase transition from an orthorhombic ($Pnma$) to a monoclinic ($P2_1/c$) phase at 240 K. In CeCu$_{6x}$Ag$_x$, the structural phase transition temperature (${T_{s}}$) decreases linearly with Ag concentration and extrapolates to zero at $x_{S}$ $\approx$ 0.1. The structural transition in CeCu$_{6x}$Pd$_x$ remains unperturbed with Pd substitution within the range of our study. The lattice constant $b$ slightly decreases with Ag/Pd doping, whereas, $a$ and $c$ increase with an overall increase in the unit cell volume. Both systems, CeCu$_{6x}$Ag$_x$ and CeCu$_{6x}$Pd$_x$, exhibit a magnetic quantum critical point (QCP), at $x$ $\approx$ 0.2 and $x$ $\approx$ 0.05 respectively. Near the QCP, long range antiferromagnetic ordering takes place at an incommensurate wave vector ($\delta_1$ 0 $\delta_2$) where $\delta_1 \sim 0.62$, $\delta_2 \sim 0.25$, $x$ = 0.125 for CeCu$_{6x}$Pd$_x$ and $\delta_1 \sim 0.64$, $\delta_2 \sim 0.3$, $x$ = 0.3 for CeCu$_{6x}$Ag$_x$. The magnetic structure consists of an amplitude modulation of the Cemoments which are aligned along the $c$axis of the orthorhombic unit cell.  [Show abstract] [Hide abstract]
ABSTRACT: The complex interdigitated phases have greatly frustrated attempts to document the basic features of the superconductivity in the alkali metal intercalated iron chalcogenides. Here, using elastic neutron scattering, energydispersive xray spectroscopy, and resistivity measurements, we elucidate the relations of these phases in Rb$_{1\delta}$Fe$_y$Se$_{2z}$S$_z$. We find: i) the iron content is crucial in stabilizing the stripe antiferromagnetic (AF) phase with rhombic iron vacancy order ($y\approx1.5$), the block AF phase with $\sqrt{5}\times \sqrt{5}$ iron vacancy order ($y\approx1.6$), and the iron vacancyfree phase ($y\approx2$); ii) the superconducting phase ($z=0$) evolves into a metallic phase ($z>1.5$) with sulfur substitution due to the progressive decrease of the electronic correlation strength. Both the stripe AF phase and the block AF phase are Mott insulators. Our data suggest that there are miscibility gaps between these three phases. The existence of the miscibility gaps in the iron content is the key to understanding the relationship between these complicated phases.  [Show abstract] [Hide abstract]
ABSTRACT: We present singlecrystal neutron scattering measurements of the spin1/2 equilateral triangular lattice antiferromagnet Ba$_3$CoSb$_2$O$_9$. Besides confirming that the Co$^{2+}$ magnetic moments lie in the ab plane for zero magnetic field, we determine all the exchange parameters of the minimal quasi2D spin Hamiltonian, which confirms that Ba$_3$CoSb$_2$O$_9$ is an almost perfect realization of the paradigmatic model of frustrated quantum magnetism. A comparison with linear and nonlinear spinwave theory reveals that quantum fluctuations induce a strong downward renormalization of the magnon dispersion.  [Show abstract] [Hide abstract]
ABSTRACT: The evolution of the electronic properties of electrondoped (Sr{1x}La{x})2IrO4 is experimentally explored as the solubility limit of La is approached. As electrons are introduced, the electronic ground state transitions from a spinorbit Mott phase into an electronically phase separated state, where longrange magnetic order vanishes beyond x = 0.02 and charge transport remains percolative up to the limit of La substitution (x=0.06). In particular, the electronic ground state remains inhomogeneous even beyond the collapse of the parent state's longrange antiferromagnetic order, while persistent shortrange magnetism survives up to the highest Lasubstitution levels. Furthermore, as electrons are doped into Sr2IrO4, we observe the appearance of a low temperature magnetic glasslike state intermediate to the complete suppression of antiferromagnetic order. Universalities in the electrondoped phase diagrams of single layer and bilayer RuddlesdenPopper strontium iridates are discussed.Physical Review B 06/2015; 92(7). DOI:10.1103/PhysRevB.92.075125 · 3.74 Impact Factor 
Article: Naturally structurally detwinned Fe pnictide superconducting family with metallic spacer layers
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ABSTRACT: We report a comprehensive study of single crystals of the newly discovered 112 iron pnictide superconductors (FBS). In Ca$_{0.73}$La$_{0.27}$FeAs$_2$, we unraveled a monoclinic to triclinic phase transition at 58 K, and a paramagnetic to stripe antiferromagnetic (AFM) phase transition at 54 K, below which a distinct magnetic structure appears with the spins ordering 45$^\circ$ away from the stripe direction. Both phase transitions can be suppressed upon Co substitution on Fe sites and bulk superconductivity is stabilized up to 20 K. Furthermore, we demonstrate, as the structural and chemical consequences of the As chains in the spacer layers, this magnetic FBS is naturally structurally detwinned at ambient pressure with the formation of spin rotation walls (Swalls). Finally, in addition to the centralhole and cornerelectron Fermi pockets usually appearing in FBS, angleresolved photoemission (ARPES) measurements resolve a novel Fermiology where an extra electron pocket exists at the Brillouin zone edge with the As chain character. These unique features open a new avenue to clarify the role of electronic nematicity and metallic spacer layer in affecting the superconductivity.  [Show abstract] [Hide abstract]
ABSTRACT: Ironbased superconductivity develops out of a badmetal state, which is believed to originate from a proximity to a Mott transition. Whether an actual Mott insulator can be realized in the phase diagram of the iron pnictides remains an open question. In this work, we use transport, transmission electron microscopy (TEM), and neutron scattering experiments to demonstrate that Cudoped NaFe$_{1x}$Cu$_x$As pnictides near $x\approx 0.5$ exhibit real space Fe and Cu ordering, and are antiferromagnetic (AF) insulators. In NaFe$_{1x}$Cu$_x$As with $x\approx 0.5$, the insulating behavior persists at temperatures above the N\'eel ordering temperature, thus indicating the presence of a Mott insulator phase. Based on studies of a multiorbital Hubbard model within a slavespin approach, we interpret this effect as arising from an effectively enhanced correlation strength associated with a Cusite blockage of the kinetic motion of the Fe $3d$ electrons. Our discovery that a Mott insulating state can be induced near the iron pnictide superconductors highlights the important role of electron correlations in hightransition temperature (high$T_c$) superconductivity.  [Show abstract] [Hide abstract]
ABSTRACT: The high temperature magnetic order in SrRu$_2$O$_6$ was studied by measuring magnetization and neutron powder diffraction with both polarized and unpolarized neutrons. SrRu$_2$O$_6$ crystallizes into the hexagonal lead antimonate (PbSb$_2$O$_6$, space group \textit{P}$\overline{3}$1\textit{m}) structure with layers of edgesharing RuO$_6$ octahedra separated by Sr$^{2+}$ ions. SrRu$_2$O$_6$ orders at $T_N$=565\,K with Ru moments coupled antiferromagnetically both inplane and outofplane. The magnetic moment is 1.30(2) $\mu_\mathrm{B}$/Ru at room temperature and is along the crystallographic \textit{c}axis in the Gtype magnetic structure. We performed density functional calculations with constrained RPA to obtain the electronic structure and effective intra and interorbital interaction parameters. The projected density of states show strong hybridization between Ru 4$d$ and O 2$p$. By downfolding to the target $t_{2g}$ bands we extracted the effective magnetic Hamiltonian. We performed Monte Carlo simulations to determine the transition temperature as a function of inter and intraplane couplings and find weak inter plane coupling, 3\% of the intraplane coupling, permits three dimensional magnetic order at $T_N$. As suggested by the magnetic susceptibility, twodimensional correlations persist above $T_N$ due to the strong intraplane coupling.Physical Review B 04/2015; 92(10). DOI:10.1103/PhysRevB.92.100404 · 3.74 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: Structure and magnetic properties of highquality 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 Ctype 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 Ctype magnetic order, and Ce moments undergo longrange 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 localmoment antiferromagnetic insulator in which the nearestneighbor 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 isostructural oxypnictides. A schematic illustration of magnetic structures in Mn and Ce sublattices in CeMnAsO is proposed.Physical Review B 02/2015; DOI:10.1103/PhysRevB.91.064418 · 3.74 Impact Factor 
Article: Disorder from order among anisotropic nextnearestneighbor Ising spin chains in SrHo 2 O 4
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ABSTRACT: We describe why Ising spin chains with competing interactions in ${\mathrm{SrHo}}_{2}{\mathrm{O}}_{4}$ segregate into ordered and disordered ensembles at low temperatures $(T)$. 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 doubleN\'eel $($\uparrow${}$\uparrow${}$\downarrow${}$\downarrow${})$ ground states, respectively. Below ${T}_{\mathrm{N}}=0.68(2)\phantom{\rule{0.28em}{0ex}}\mathrm{K}$, the N\'eel chains develop threedimensional long range order (LRO), which arrests further thermal equilibration of the doubleN\'eel chains so they remain in a disordered incommensurate state for $T$ below ${T}_{\mathrm{S}}=0.52(2)\phantom{\rule{0.28em}{0ex}}\mathrm{K}$. ${\mathrm{SrHo}}_{2}{\mathrm{O}}_{4}$ distills an important feature of incommensurate low dimensional magnetism: kinetically trapped topological defects in a $\text{quasi}$${}d$${}\text{dimensional}$ spin system can preclude order in $d+1$ dimensions.Physical Review B 02/2015; 91(5). DOI:10.1103/PhysRevB.91.054424 · 3.74 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: We have performed magnetic, electric, thermal, and neutron powder diffraction (NPD) experiments as well as density functional theory (DFT) calculations on Ba3MnNb2O9. All results suggest that Ba3MnNb2O9 is a spin5/2 triangular lattice antiferromagnet (TLAF) with weak easyaxis anisotropy. At zero field, we observed a narrow twostep transition at TN1 = 3.4 K and TN2 = 3.0 K. The neutron diffraction measurement and the DFT calculation indicate a 120◦ spin structure in the ab plane with outofplane canting at low temperatures. With increasing magnetic field, the 120◦ spin structure evolves into upupdown (uud) and oblique phases showing successive magnetic phase transitions, which fits well to the theoretical prediction for the 2D Heisenberg TLAF with classical spins. Multiferroicity is observed when the spins are not collinear but suppressed in the uud and oblique phases.Physical Review B 12/2014; 90(22):224402. DOI:10.1103/PhysRevB.90.224402 · 3.74 Impact Factor  [Show abstract] [Hide abstract]
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 halfinteger and integer indexed magnetic Bragg peaks consistent with a second commensurately ordered antiferromagnetic state.Physical Review B 10/2014; 90(13). DOI:10.1103/PhysRevB.90.134423 · 3.74 Impact Factor  Physical Review B 10/2014; 90(13). DOI:10.1103/PhysRevB.90.139902 · 3.74 Impact Factor
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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 wellstudied $A_{0.8}$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 inplane 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 orderdisorder 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 inplane 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 inplane antiferromagnetic order arises from strong coupling between local moments. Superconductivity, previously observed in the $A_{0.8}$Fe$_{y}$Se$_{2z}$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.Physical Review B 08/2014; 90(12). DOI:10.1103/PhysRevB.90.125148 · 3.74 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: The crossover from localized to itinerantelectron behavior is associated with many intriguing phenomena in condensedmatter physics. In this paper, we investigate the crossover from localized to itinerant regimes in the spinel system Mn$_{1x}$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 spinlattice coupling produces magnetic frustration. Neutron scattering measurements and firstprinciplesguided spin models reveal that the noncollinear state at high Co doping is produced by weakened local anisotropy and enhanced CoV spin interactions.Physical Review B 07/2014; 91(2). DOI:10.1103/PhysRevB.91.020407 · 3.74 Impact Factor  [Show abstract] [Hide abstract]
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 doubleN\'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 doubleN\'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.  [Show abstract] [Hide abstract]
ABSTRACT: Ca10(Pt3As8)(Fe2As2)5 is the parent compound for a class of Febased hightemperature superconductors where superconductivity with transition temperatures up to 30 K can be introduced by partial element substitution. We present a combined highresolution highenergy xray 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 Febased hightemperature superconductors: an orthorhombic distortion of the hightemperature tetragonal FeAs lattice below T_S = 110(2) K followed by stripelike antiferromagnetic ordering of the Fe moments below T_N = 96(2) K. These findings demonstrate that major features of the Febased hightemperature superconductors are very robust against variations in chemical constitution as well as structural imperfection of the layers separating the FeAs layers from each other and confirms that the FeAs layers primarily determine the physics in this class of material.Physical Review B 06/2014; 90(10). DOI:10.1103/PhysRevB.90.100504 · 3.74 Impact Factor 
Article: The evolution of antiferromagnetic susceptibility to uniaxial pressure in Ba(Fe{1x}Co{x})2As2
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ABSTRACT: Neutron diffraction measurements are presented measuring the responses of both magnetic and structural order parameters of parent and lightly Codoped Ba(Fe{1x}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 Codoping is increased and the superconducting phase is approached. Additionally, as uniaxial pressure is increased within parent and lightlydoped Ba(Fe{1x}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.Physical Review B 06/2014; 89(21). DOI:10.1103/PhysRevB.89.214404 · 3.74 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: Neutron diffraction studies have revealed an uncommon commensurateincommensurate magnetic phase transition with decreasing temperature in the (similar to 5%) Fedoped bilayer ruthenate Ca3(Ru,Fe)(2)O7. An incommensurate phase formed of a cycloidal spiral spin structure coexists with a commensurate one below the phase transition at 42 K and persists down to the lowest temperature, accompanied by higherorder magnetic satellite peaks which indicate the formation of a magnetic soliton lattice. We ascribe these findings to the competing magnetic interactions in this system. This study demonstrates an effective approach to tune novel magnetic and electronic properties of ruthenates via 3d magnetic transitionmetal substitution.Physical Review B 06/2014; 89(22). DOI:10.1103/PhysRevB.89.220407 · 3.74 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: We present speciﬁcheat and neutronscattering results for the S = 1/2 quantum antiferromagnet (dimethylammonium)(3,5dimethylpyridinium)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 twoleg spin1/2 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 interactionanisotropy parameter λ = 0.93(2), according to inelastic neutronscattering 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 Neelordered phase without the assistance of a magnetic ﬁeld.Physical Review B 05/2014; 89(17):174432. DOI:10.1103/PhysRevB.89.174432 · 3.74 Impact Factor
Publication Stats
1k  Citations  
354.67  Total Impact Points  
Top Journals
Institutions

20082015

Oak Ridge National Laboratory
 • Quantum Condensed Matter Division
 • Materials Science and Technology Division
Oak Ridge, Florida, United States


20072014

Iowa State University
 • Department of Physics and Astronomy
 • Ames Laboratory
Ames, Iowa, United States


20062007

University of Tennessee
 Department of Physics & Astronomy
Knoxville, Tennessee, United States


20042006

The University of Tennessee Medical Center at Knoxville
Knoxville, Tennessee, United States


19952000

Nanjing University
 Department of Physics
Nanching, Jiangsu Sheng, China
