arXiv subjects

A. I. Goldman

Publications and source records attributed to A. I. Goldman.

At least 55 records · Page 3Linked to original sources

arXiv2013

Checkerboard to Stripe Charge Ordering Transition in TbBaFe2O5

A combined neutron and x-ray diffraction study of TbBaFe2O5 reveals a rare checkerboard to charge ordering transition. TbBaFe2O5 is a mixed valent compound where Fe2+/Fe3+ ions are known to arrange into a stripe charge-ordered state below TV = 291 K, that consists of alternating Fe2+/Fe3+ stripes in the basal plane running along the b direction. Our measurements reveal that the stripe charge-ordering is preceded by a checkerboard charge-ordered phase between TV < T < T* = 308 K. The checkerboard ordering is stabilized by inter-site coulomb interactions which give way to a stripe state stabilized by orbital ordering.

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arXiv2012

Effects of transition metal substitutions on the incommensurability and spin fluctuations in BaFe2As2 by elastic and inelastic neutron scattering

The spin fluctuation spectra from nonsuperconducting Cu-substituted, and superconducting Co-substituted, BaFe2As2 are compared quantitatively by inelastic neutron scattering measurements and are found to be indis- tinguishable. Whereas diffraction studies show the appearance of incommensurate spin-density wave order in Co and Ni substituted samples, the magnetic phase diagram for Cu substitution does not display incommensu- rate order, demonstrating that simple electron counting based on rigid-band concepts is invalid. These results, supported by theoretical calculations, suggest that substitutional impurity effects in the Fe plane play a signifi- cant role in controlling magnetism and the appearance of superconductivity, with Cu distinguished by enhanced impurity scattering and split-band behavior.

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arXiv2012

Competition between stripe and checkerboard magnetic instabilities in Mn-doped BaFe2As2

Inelastic neutron scattering measurements on Ba(Fe0.925Mn0.075)2As2 manifest spin fluctuations at two different wavevectors in the Fe square lattice, (1/2,0) and (1/2,1/2), corresponding to the expected stripe spin-density wave order and checkerboard antiferromagnetic order, respectively. Below T_N=80 K, long-range stripe magnetic ordering occurs and sharp spin wave excitations appear at (1/2,0) while broad and diffusive spin fluctuations remain at (1/2,1/2) at all temperatures. Low concentrations of Mn dopants nucleate local moment spin fluctuations at (1/2,1/2) that compete with itinerant spin fluctuations at (1/2,0) and may disrupt the development of superconductivity.

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arXiv2012

Control of magnetic, non-magnetic and superconducting states in annealed Ca(Fe$_{1-x}$Co$_{x}$)$_{2}$As$_{2}$

We have grown single crystal samples of Co substituted CaFe2As2 using an FeAs flux and systematically studied the effects of annealing/quenching temperature on the physical properties of these samples. Whereas the as-grown samples (quenched from 960C) all enter the collapsed tetragonal phase upon cooling, annealing/quenching temperatures between 350C and 800C can be used to tune the system to low temperature antiferromagnetic/orthorhomic or superconducting states as well. The progression of the transition temperature versus annealing/quenching temperature (T-T$_{anneal}$) phase diagrams with increasing Co concentration shows that, by substituting Co, the antiferromagnetic/orthorhombic and the collapsed tetragonal phase lines are separated and bulk superconductivity is revealed. We established a 3D phase diagram with Co concentration and annealing/quenching temperature as two independent control parameters. At ambient pressure, for modest x and T$_{anneal}$ values, the Ca(Fe1-xCox)2As2 system offers ready access to the salient low temperature states associated with Fe-based superconductors: antiferromagnetic/orthorhombic, superconducting, and non-magnetic/collapsed tetragonal.

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arXiv2012

Magnetically polarized Ir dopant atoms in superconducting Ba(Fe$_{1-x}$Ir$_x$)$_2$As$_2$

We investigate the magnetic polarization of the Ir $5d$ dopant states in the pnictide superconductor Ba(Fe$_{1-x}$Ir$_x$)$_2$As$_2$ with $x=0.027(2)$ using Ir $L_3$ edge x-ray resonant magnetic scattering (XRMS). Despite the fact that doping partially suppresses the antiferromagnetic transition, we find that magnetic order survives around the Ir dopant sites. The Ir states are magnetically polarized with commensurate stripe-like antiferromagnetic order and long correlations lengths, $ξ_{\text{mag}}>$ 2800 and $>$850 Å, in the $ab$-plane and along the c-axis, respectively, driven by their interaction with the Fe spins. This Ir magnetic order persists up to the Néel transition of the majority Fe spins at $T_N=74(2)$ K. At 5 K we find that magnetic order co-exists microscopically with superconductivity in Ba(Fe$_{1-x}$Ir$_x$)$_2$As$_2$. The energy dependence of the XRMS through the Ir $L_3$ edge shows a non-Lorentzian lineshape, which we explain in terms of interference between Ir resonant scattering and Fe non-resonant magnetic scattering.

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arXiv2012

Ba{1-x}KxMn2As2: An Antiferromagnetic Local-Moment Metal

The compound BaMn2As2 with the tetragonal ThCr2Si2 structure is a local-moment antiferromagnetic insulator with a Neel temperature TN = 625 K and a large ordered moment mu = 3.9 mu_B/Mn. We demonstrate that this compound can be driven metallic by partial substitution of Ba by K, while retaining the same crystal and antiferromagnetic structures together with nearly the same high TN and large mu. Ba_{1-x}K_xMn2As2 is thus the first metallic ThCr2Si2-type MAs-based system containing local 3d transition metal M magnetic moments, with consequences for the ongoing debate about the local moment versus itinerant pictures of the FeAs-based superconductors and parent compounds. The Ba_{1-x}K_xMn2As2 class of compounds also forms a bridge between the layered iron pnictides and cuprates and may be useful to test theories of high Tc superconductivity.

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arXiv2011

Effect of tensile stress on the in-plane resistivity anisotropy in BaFe2As2

The effect of uniaxial tensile stress and the resultant strain on the structural/magnetic transition in the parent compound of the iron arsenide superconductor, BaFe$_2$As$_2$, is characterized by temperature-dependent electrical resistivity, x-ray diffraction and quantitative polarized light imaging. We show that strain induces a measurable uniaxial structural distortion above the first-order magnetic transition and significantly smears the structural transition. This response is different from that found in another parent compound, SrFe$_2$As$_2$, where the coupled structural and magnetic transitions are strongly first order. This difference in the structural responses explains the in-plain resistivity anisotropy above the transition in BaFe$_2$As$_2$. This conclusion is supported by the Ginzburg-Landau - type phenomenological model for the effect of the uniaxial strain on the resistivity anisotropy.

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arXiv2011

Magnetic order in GdBiPt studied by x-ray resonant magnetic scattering

Rare earth (R) half-Heusler compounds, RBiPt, exhibit a wide spectrum of novel ground states. Recently, GdBiPt has been proposed as a potential antiferromagnetic topological insulator (AFTI). We have employed x-ray resonant magnetic scattering to elucidate the microscopic details of the magnetic structure in GdBiPt below T_N = 8.5 K. Experiments at the Gd L_2 absorption edge show that the Gd moments order in an antiferromagnetic stacking along the cubic diagonal [1 1 1] direction satisfying the requirement for an AFTI, where both time-reversal symmetry and lattice translational symmetry are broken, but their product is conserved.

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arXiv2011

Incommensurate spin-density wave order in electron-doped BaFe2As2 superconductors

Neutron diffraction studies of Ba(Fe[1-x]Co[x])2As2 reveal that commensurate antiferromagnetic order gives way to incommensurate magnetic order for Co compositions between 0.056 < x < 0.06. The incommensurability has the form of a small transverse splitting (0, +-e, 0) from the nominal commensurate antiferromagnetic propagation vector Q[AFM] = (1, 0, 1) (in orthorhombic notation) where e = 0.02-0.03 and is composition dependent. The results are consistent with the formation of a spin-density wave driven by Fermi surface nesting of electron and hole pockets and confirm the itinerant nature of magnetism in the iron arsenide superconductors.

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arXiv2011

Character of the structural and magnetic phase transitions in the parent and electron doped BaFe2As2 compounds

We present a combined high-resolution x-ray diffraction and x-ray resonant magnetic scattering (XRMS) study of as-grown BaFe2As2. The structural/magnetic transitions must be described as a two-step process. At T_S = 134.5 K we observe the onset of a second-order structural transition from the high-temperature paramagnetic tetragonal structure to a paramagnetic orthorhombic phase, followed by a discontinuous step in the structural order parameter that is coincident with a first-order antiferromagnetic (AFM) transition at T_N = 133.75 K. These data, together with detailed high-resolution x-ray studies of the structural transition in lightly doped Ba(Fe{1-x}Co{x})2As2 and Ba(Fe{1-x}Rh{x})2As2 compounds, show that the structural and AFM transitions do, in fact, occur at slightly different temperatures in the parent BaFe2As2 compound, and evolve towards split secondorder transitions as the doping concentration is increased. We estimate the composition for the tricritical point for Co-doping and employ a mean-field approach to show that our measurements can be explained by the inclusion of an anharmonic term in the elastic free energy and magneto-elastic coupling in the form of an emergent Ising-nematic degree of freedom.

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arXiv2011

Stabilization of an ambient pressure, collapsed tetragonal phase in CaFe2As2 and tuning of the orthorhombic / antiferromagnetic transition temperature by over 70 K by control of nano-precipitates

We have found a remarkably large response of the transition temperature of CaFe2As2 single crystals grown out of excess FeAs to annealing / quenching temperature. Whereas crystals that are annealed at 400 C exhibit a first order phase transition from a high temperature tetragonal to a low temperature orthorhombic and antiferromagnetic state near 170 K, crystals that have been quenched from 960 C exhibit a transition from a high temperature tetragonal phase to a low temperature, non-magnetic, collapsed tetragonal phase below 100 K. By use of temperature dependent electrical resistivity, magnetic susceptibility, X-ray diffraction, Mossbauer spectroscopy and nuclear magnetic resonance measurements we have been able to demonstrate that the transition temperature can be reduced in a monotonic fashion by varying the annealing / quenching temperature from 400 to 850 C with the low temperature state remaining antiferromagnetic for transition temperatures larger than 100 K and becoming collapsed tetragonal / non-magnetic for transition temperatures below 90 K. This suppression of the orthorhombic / antiferromagnetic phase transition and its ultimate replacement with the collapsed tetragonal / non-magnetic phase is similar to what has been observed for CaFe2As2 under hydrostatic pressure. Transmission electron microscopy studies indicate that there is a temperature dependent, width of formation of CaFe2As2 with a decreasing amount of excess Fe and As being soluble in the single crystal at lower annealing temperatures. For samples quenched from 960 C there is a fine (of order 10 nm), semi-uniform distribution of precipitate that can be associated with an average strain field whereas for samples annealed at 400 C the excess Fe and As form mesoscopic grains that induce little strain throughout the CaFe2As2 lattice.

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arXiv2010

In-plane anisotropy of electrical resistivity in the strain-detwinned SrFe2As2

Intrinsic, in-plane anisotropy of electrical resistivity was studied on mechanically detwinned single crystals of SrFe$_2$As$_2$ above and below the temperature of the coupled structural/magnetic transition, $T_{\textrm{TO}}$. Resistivity is smaller for electrical current flow along the orthorhombic $a_o$ direction (direction of antiferromagnetically alternating magnetic moments) and is larger for transport along the $b_o$ direction (direction of ferromagnetic chains), which is similar to CaFe$_2$As$_2$ and BaFe$_2$As$_2$ compounds. A strongly first order structural transition in SrFe$_2$As$_2$ was confirmed by high-energy x-ray measurements, with the transition temperature, and character unaffected by moderate strain. For small strain levels, which are just sufficient to detwin the sample, we find a negligible effect on the resistivity above $T_{\textrm{TO}}$. With the increase of strain, the resistivity anisotropy starts to develop above $T_{\textrm{TO}}$, clearly showing the relation of anisotropy to an anomalously strong response to strain. Our study suggests that electronic nematicity cannot be observed in the FeAs based compounds in which the structural transition is strongly first order.

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arXiv2010

Magnetic ordering and structural distortion in Ru doped BaFe2As2 single crystals studied by Neutron and X-ray diffraction

We present a systematic investigation of the antiferromagnetic ordering and structural distortion for the series of Ba(Fe{1-x}Ru{x})2As2 compounds (0 <= x <= 0.246). Neutron and x-ray diffraction measurements demonstrate that, unlike for the electron-doped compounds, the structural and magnetic transitions remain coincident in temperature. Both the magnetic and structural transitions are gradually suppressed with increased Ru concentration and coexist with superconductivity. For samples that are superconducting, we find strong competition between superconductivity, the antiferromagnetic ordering, and the structural distortion.

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arXiv2010

Commensurate antiferromagnetic ordering in Ba(Fe{1-x}Co{x})2As2 determined by x-ray resonant magnetic scattering at the Fe K-edge

We describe x-ray resonant magnetic diffraction measurements at the Fe K-edge of both the parent BaFe2As2 and superconducting Ba(Fe0.953Co0.047)2As2 compounds. From these high-resolution measurements we conclude that the magnetic structure is commensurate for both compositions. The energy spectrum of the resonant scattering is in reasonable agreement with theoretical calculations using the full-potential linear augmented plane wave method with a local density functional.

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arXiv2010

Antiferromagnetic ordering in the absence of a structural distortion in Ba(Fe{1-x}Mn{x})2As2

Neutron and x-ray diffraction studies of Ba(Fe{1-x}Mn{x})2As2 for low doping concentrations (x <= 0.176) reveal that at a critical concentration, 0.102 < x < 0.118, the tetragonal-to-orthorhombic transition abruptly disappears whereas magnetic ordering with a propagation vector of (1/2 1/2 1) persists. Among all of the iron arsenides this observation is unique to Mn-doping, and unexpected because all models for "stripe-like" antiferromagnetic order anticipate an attendant orthorhombic distortion due to magnetoelastic effects. We discuss these observations and their consequences in terms of previous studies of Ba(Fe{1-x}TM{x})2As2 compounds (TM = Transition Metal), and models for magnetic ordering in the iron arsenide compounds.

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arXiv2010

Interplay between Fe and Nd magnetism in NdFeAsO single crystals

The structural and magnetic phase transitions have been studied on NdFeAsO single crystals by neutron and x-ray diffraction complemented by resistivity and specific heat measurements. Two low-temperature phase transitions have been observed in addition to the tetragonal-to-orthorhombic transition at T_S = 142 K and the onset of antiferromagnetic (AFM) Fe order below T_N = 137 K. The Fe moments order AFM in the well-known stripe-like structure in the (ab) plane, but change from AFM to ferromagnetic (FM) arrangement along the c direction below T* = 15 K accompanied by the onset of Nd AFM order below T_Nd = 6 K with this same AFM configuration. The iron magnetic order-order transition in NdFeAsO accentuates the Nd-Fe interaction and the delicate balance of c-axis exchange couplings that results in AFM in LaFeAsO and FM in CeFeAsO and PrFeAsO.

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arXiv2010

Paramagnetic Spin Correlations in CaFe2As2 Single Crystals

Magnetic correlations in the paramagnetic phase of CaFe2As2 (T_N=172 K) have been examined by means of inelastic neutron scattering from 180 K (~ 1.05 T_N) up to 300 K (~1.8 T_N). Despite the first-order nature of the magnetic ordering, strong but short-range antiferromagnetic (AFM) correlations are clearly observed. These correlations, which consist of quasi-elastic scattering centered at the wavevector Q_{AFM} of the low-temperature AFM structure, are observed up to the highest measured temperature of 300 K and at high energy transfer (E> 60 meV). The L dependence of the scattering implies rather weak interlayer coupling in the tetragonal c-direction corresponding to nearly two-dimensional fluctuations in the (ab) plane. The spin correlation lengths within the Fe layer are found to be anisotropic, consistent with underlying fluctuations of the AFM stripe structure. Similar to the cobalt doped superconducting BaFe2As2 compounds, these experimental features can be adequately reproduced by a scattering model that describes short-range anisotropic spin correlations with overdamped spin dynamics.

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arXiv2010

Evidence from neutron diffraction for superconductivity in the stabilized tetragonal phase of CaFe2As2 under uniaxial pressure

CaFe2As2 single crystals under uniaxial pressure applied along the c axis exhibit the coexistence of several structural phases at low temperatures. We show that the room-temperature tetragonal phase is stabilized at low temperatures for pressures above 0.06 GPa, and its weight fraction attains a maximum in the region where superconductivity is observed under applied uniaxial pressure. Simultaneous resistivity measurements strongly suggest that this phase is responsible for the superconductivity in CaFe2As2 found below 10 K in samples subjected to nonhydrostatic pressure conditions.

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