A. Kracher

Iowa State University, Ames, IA, United States

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Publications (29)56.82 Total impact

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    ABSTRACT: Single crystals of Ba(Fe1-x-yCoxTMy)2As2 (TM=Cr, Mn) have been grown and characterized by structural, magnetic and transport measurements, both in the as-grown state (quenched from ˜1000^o C) as well as after post-growth annealing. This phase space has many parameters and is rich and complex, with superconducting transition temperatures depending upon x and y, as well as annealing temperature and time. In this talk, we will present T-x and T-y, as well as T-time and T-T (for annealing) phase diagrams and discuss the implications for future research into these complex materials.
    02/2012;
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    ABSTRACT: Single crystals of Ba(Fe1-xMnx)2As2, 01.85K. Phase diagrams of temperature versus doping level based on electrical transport and magnetization measurements will be presented and compared to those of the Ba(Fe1-xTMx)2As2 (TM=Cr, Co, Ni, Cu) series.
    Physical review. B, Condensed matter 10/2011; 84(14). · 3.77 Impact Factor
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    ABSTRACT: Single crystals of Ba(Fe1-xMnx)2As2, 0<x<0.148, have been grown and characterized by structural, magnetic, electrical transport, and thermopower measurements. Although growths of single crystals of Ba(Fe1-xMnx)2As2 for the full 0⩽x⩽1 range were made, we find evidence for phase separation (associated with some form of immiscibility) starting for x>0.1–0.2. Our measurements show that whereas the structural/magnetic phase transition found in pure BaFe2As2 at 134 K is initially suppressed by Mn substitution, superconductivity is not observed at any substitution level. Although the effect of hydrostatic pressure up to 20 kbar in the parent BaFe2As2 compound is to suppress the structural/magnetic transition at the approximate rate of 0.9 K/kbar, the effects of pressure and Mn substitution in the x=0.102 compound are not cumulative. Phase diagrams of transition temperature versus substitution concentration x based on electrical transport, magnetization, and thermopower measurements have been constructed and compared to those of the Ba(Fe1-xTMx)2As2 (TM= Co and Cr) series.
    Phys. Rev. B. 10/2011; 84(14).
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    ABSTRACT: The high ferromagnetic ordering temperature of the dilute, rare-earth-bearing, intermetallic compound GdFe2Zn20 has been understood as being the consequence of the Gd3+ moment being embedded in a nearly ferromagnetic Fermi liquid. To test this understanding in detail, single crystals of the pseudoternary series GdFe2(AlxZn1-x)20 (x
    Physical review. B, Condensed matter 01/2011; 83. · 3.77 Impact Factor
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    ABSTRACT: Single crystals of Ba(Fe1−xRux)2As2, x<0.37, have been grown and characterized by structural, magnetic, and transport measurements. These measurements show that the structural/magnetic phase transition found in pure BaFe2As2 at 134 K is suppressed monotonically by Ru doping but, unlike doping with TM=Co, Ni, Cu, Rh, or Pd, the coupled transition seen in the parent compound does not detectably split into two separate ones. Superconductivity is stabilized at low temperatures for x>0.2 and continues through the highest doping levels we report. The superconducting region is domelike, with maximum Tc(∼16.5 K) found around x∼0.29. A phase diagram of temperature versus doping, based on electrical transport and magnetization measurements, has been constructed and compared to those of the Ba(Fe1−xTMx)2As2 (TM=Co, Ni, Rh, and Pd) series as well as to the temperature-pressure phase diagram for pure BaFe2As2. Suppression of the structural/magnetic phase transition as well as the appearance of superconductivity is much more gradual in Ru doping, as compared to Co, Ni, Rh, and Pd doping, and appears to have more in common with BaFe2As2 tuned with pressure; by plotting TS/Tm and Tc as a function of changes in unit-cell dimensions, we find that changes in the c/a ratio, rather than changes in c, a, or V, unify the T(p) and T(x) phase diagrams for BaFe2As2 and Ba(Fe1−xRux)2As2, respectively.
    Phys. Rev. B. 06/2010; 82(1).
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    ABSTRACT: Single crystals of NdFe1 − xCoxAsO with x = 0, 0.025, 0.05, 0.075, 0.10 and 0.15 were grown via high pressure synthesis. Structural and elemental analysis showed successful substitution of Co for Fe with no detectable change in the oxygen site occupation. Magnetic, electrical transport and tunnel-diode resonator measurements were used to characterize the NdFe1 − xCoxAsO crystals. These measurements indicated that superconductivity was achieved with x ≥ 0.025 and optimal doping was established with x = 0.05 with the maximum superconducting critical temperature of 25 K. Measurement of temperature dependent resistance in applied magnetic fields as high as 14 T showed a broadening of the superconducting transition and an Hc2(T) curve that was consistent with those of other iron pnictide superconductors.
    Superconductor Science and Technology 04/2010; 23(5):054008. · 2.76 Impact Factor
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    ABSTRACT: Single crystals of Ba(Fe1-xRux)2As2, x0.2 and continues through the highest doping levels we have been able to produce. The superconducting region is dome like, with maximum Tc˜18K found for x ˜0.25. The phase diagram of temperature versus doping based on our measurements will be compared to those of the Ba(Fe1-xTMx)2As2 (TM=Co, Ni, Rh, Pd) series as will the associated changes in unit cell dimension and volume. Since Ru doping does not provide extra electrons, the T-x phase diagram will also be compared with T-P phase diagram of pure BaFe2As2.
    03/2010;
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    ABSTRACT: Microscopic, structural, transport, and thermodynamic measurements of single crystalline Ba(Fe1−xTMx)2As2 (TM=Ni and Cu) series, as well as two mixed TM=Cu/Co series, are reported. In addition, high-magnetic field, anisotropic Hc2(T) data were measured up to 33 T for the optimally Ni-doped BaFe2As2 sample. All the transport and thermodynamic measurements indicate that the structural and magnetic phase transitions at 134 K in pure BaFe2As2 are monotonically suppressed and increasingly separated in a similar manner by these dopants. In the Ba(Fe1−xNix)2As2 (x≤0.072), superconductivity, with Tc up to 19 K, is stabilized for 0.024≤x≤0.072. In the Ba(Fe1−xCux)2As2 (x≤0.356) series, although the structural and magnetic transitions are suppressed, there is only a very limited region of superconductivity: a sharp drop of the resistivity to zero near 2.1 K is found only for the x=0.044 samples. In the Ba(Fe1−x−yCoxCuy)2As2 series, superconductivity, with Tc values up to 12 K (x∼0.022 series) and 20 K (x∼0.047 series), is stabilized. Quantitative analysis of the detailed temperature-dopant concentration (T−x) and temperature-extra electrons (T−e) phase diagrams of these series shows that there exists a limited range of the number of extra electrons added, inside which the superconductivity can be stabilized if the structural and magnetic phase transitions are suppressed enough. Moreover, comparison with pressure-temperature phase diagram data, for samples spanning the whole doping range, further re-enforces the conclusion that suppression of the structural/magnetic phase transition temperature enhances Tc on the underdoped side, but for the overdoped side TCmax is determined by e. Therefore, by choosing the combination of dopants that are used, we can adjust the relative positions of the upper phase lines (structural and magnetic phase transitions) and the superconducting dome to control the occurrence and disappearance of the superconductivity in transition metal, electron-doped BaFe2As2.
    Physical review. B, Condensed matter 01/2010; 82(2). · 3.77 Impact Factor
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    ABSTRACT: We report the discovery of a binary icosahedral phase in a Sc-Zn alloy obtained through solution-growth, producing millimeter-sized, facetted, single grain quasicrystals that exhibit different growth morphologies, pentagonal dodecahedra, and rhombic triacontahedra, under only marginally different growth conditions. These two morphologies manifest different degrees of quasicrystalline order. The discovery of i-Sc12Zn88 suggests that a re-examination of binary phase diagrams at compositions close to crystalline approximant structures may reveal other binary quasicrystalline phases.
    Physical review. B, Condensed matter 01/2010; 81(2). · 3.77 Impact Factor
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    ABSTRACT: Single crystals of Ba(Fe1-xRux)2As2 , x0.2 and continues through the highest doping levels we report. The superconducting region is domelike, with maximum Tc(˜16.5K) found around x˜0.29 . A phase diagram of temperature versus doping, based on electrical transport and magnetization measurements, has been constructed and compared to those of the Ba(Fe1-xTMx)2As2 ( TM=Co , Ni, Rh, and Pd) series as well as to the temperature-pressure phase diagram for pure BaFe2As2 . Suppression of the structural/magnetic phase transition as well as the appearance of superconductivity is much more gradual in Ru doping, as compared to Co, Ni, Rh, and Pd doping, and appears to have more in common with BaFe2As2 tuned with pressure; by plotting TS/Tm and Tc as a function of changes in unit-cell dimensions, we find that changes in the c/a ratio, rather than changes in c , a , or V , unify the T(p) and T(x) phase diagrams for BaFe2As2 and Ba(Fe1-xRux)2As2 , respectively.
    Physical review. B, Condensed matter 01/2010; 82(1). · 3.77 Impact Factor
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    ABSTRACT: We report the discovery of a new binary icosahedral phase in a Sc-Zn alloy obtained through solution-growth, producing millimeter-sized, facetted, single grain, quasicrystals that exhibit different growth morphologies, pentagonal dodecahedra and rhombic triacontahedra, under only marginally different growth conditions. These two morphologies manifest different degrees of quasicrystalline order, or phason strain. The discovery of i-Sc$_12$Zn$_88$ suggests that a reexamination of binary phase diagrams at compositions close to crystalline approximant structures may reveal other, new binary quasicrystalline phases. Comment: Incorrect spelling in author list resolved
    10/2009;
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    ABSTRACT: Millimeter-sized single crystals of LaFeAsO, LaFeAsO1-xFx, and LaFe1-xCoxAsO were grown in NaAs flux at ambient pressure. The detailed growth procedure and crystal characterizations are reported. The as-grown crystals have typical dimensions of 3 * 4 * 0.05-0.3 mm3 with the crystallographic c-axis perpendicular to the plane of the plate-like single crystals. Some crystals manifest linear dimensions as large as 4-5 mm. X-ray and neutron single crystal scattering confirmed that LaFeAsO crystals exhibit a structural phase transition at Ts ~ 154 K and a magnetic phase transition at TSDW ~ 140 K. The transition temperatures agree with those determined by anisotropic magnetization, in-plane electrical resistivity and specific heat measurements and are consistent with previous reports on polycrystalline samples. Co and F were successfully introduced into the lattice leading to superconducting LaFe1-xCoxAsO and LaFeAsO1-xFx single crystals, respectively. This growth protocol has been successfully employed to grow single crystals of NdFeAsO. Thus it is expected to be broadly applicable to grow other RMAsO (R = rare earth, M = transition metal) compounds. These large crystals will facilitate the efforts of unraveling the underlying physics of iron pniticide superconductors.
    Applied Physics Letters 09/2009; · 3.79 Impact Factor
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    ABSTRACT: Study and comparison of over 30 examples of electron-doped BaFe2As2 for transition metal (TM)=Co, Ni, Cu, and (Co/Cu mixtures) have led to an understanding that the suppression of the structural/antiferromagnetic phase transition to low-enough temperature in these compounds is a necessary condition for superconductivity but not a sufficient one. Whereas the structural/antiferromagnetic transitions are suppressed by the number of TM dopant ions (or changes in the c axis) the superconducting dome exists over a limited range of values of the number of valence electrons added by doping (or values of the a/c ratio). By choosing which combination of dopants is used we can change the relative positions of the upper phase lines and the superconducting dome, even to the extreme limit of suppressing the upper structural and magnetic phase transitions without the stabilization of a lower-temperature superconducting dome.
    Phys. Rev. B. 08/2009; 80(6).
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    ABSTRACT: We present thermodynamic, structural and transport measurements on Ba(Fe0.973Cr0.027)2As2 single crystals. All measurements reveal sharp anomalies at ~ 112 K. Single crystal x-ray diffraction identifies the structural transition as a first order, from the high-temperature tetragonal I4/mmm to the low-temperature orthorhombic Fmmm structure, in contrast to an earlier report.
    Physical review. B, Condensed matter 06/2009; · 3.77 Impact Factor
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    ABSTRACT: Single crystalline Ba(Fe(1-x)TMx)2As2 (TM = Rh, Pd) series have been grown and characterized by structural, thermodynamic and transport measurements. These measurements show that the structural/magnetic phase transitions, found in pure BaFe2As2 at 134 K, are suppressed monotonically by the doping and that superconductivity can be stablized over a dome-like region. Temperature-composition (T-x) phase diagrams based on electrical transport and magnetization measurements are constructed and compared to those of the Ba(Fe(1-x)TMx)2As2 (TM = Co, Ni) series. Despite the generic difference between 3d and 4d shells and the specific, conspicuous differences in the changes to the unit cell parameters, the effects of Rh doping are exceptionally similar to the effects of Co doping and the effects of Pd doping are exceptionally similar to the effects of Ni doping. These data show that whereas the structural / antiferromagnetic phase transition temperatures can be parameterized by x and the superconducting transition temperature can be parameterized by some combination of x and e, the number of extra electrons associated with the TM doping, the transition temperatures of 3d- and 4d- doped BaFe2As2 can not be simply parameterized by the changes in the unit cell dimensions or their ratios.
    05/2009;
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    ABSTRACT: Single crystal samples of Ba(Fe1-xCox)2As2 , x< 0.12, have been characterized by microscopic, thermodynamic and transport measurements. With increasing Co concentration, the features of the structural and magnetic transitions are suppressed at a rate of roughly 15K per percent of Co. Superconductivity is stabilized at low temperatures for 0.038
    03/2009;
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    ABSTRACT: Single crystalline Ba(Fe1-xMx)2As2 (M=Rh,Pd) series have been grown and characterized by structural, thermodynamic, and transport measurements. These measurements show that the structural/magnetic phase transitions, found in pure BaFe2As2 at 134 K, are suppressed monotonically by the doping and that superconductivity can be stabilized over a domelike region. Temperature-composition (T-x) phase diagrams based on electrical transport and magnetization measurements are constructed and compared to those of the Ba(Fe1-xMx)2As2 (M=Co,Ni) series. Despite the generic difference between 3d and 4d shells and the specific, conspicuous differences in the changes to the unit cell parameters, the effects of Rh doping are exceptionally similar to the effects of Co doping and the effects of Pd doping are exceptionally similar to the effects of Ni doping. These data show that whereas the structural/antiferromagnetic phase-transition temperatures can be parameterized by x and the superconducting transition temperature can be parameterized by some combination of x and e , the number of extra electrons associated with the M doping, the transition temperatures of 3d - and 4d -doped BaFe2As2 cannot be simply parameterized by the changes in the unit-cell dimensions or their ratios.
    Physical review. B, Condensed matter 01/2009; 80(2). · 3.77 Impact Factor
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    ABSTRACT: Single crystalline samples of Ba(Fe1−xCox)2As2 with x<0.12 have been grown and characterized via microscopic, thermodynamic, and transport measurements. With increasing Co substitution, the thermodynamic and transport signatures of the structural (high-temperature tetragonal to low-temperature orthorhombic) and magnetic (high-temperature nonmagnetic to low-temperature antiferromagnetic) transitions are suppressed at a rate of roughly 15 K/% Co. In addition, for x≥0.038 superconductivity is stabilized, rising to a maximum Tc of approximately 23 K for x≈0.07 and decreasing for higher x values. The T-x phase diagram for Ba(Fe1−xCox)2As2 indicates that either superconductivity can exist in both low-temperature crystallographic phases or that there is a structural phase separation. Anisotropic superconducting upper critical-field data [Hc2(T)] show a significant and clear change in anisotropy between samples that have higher temperature structural phase transitions and those that do not. These data show that the superconductivity is sensitive to the suppression of the higher temperature phase transition.
    Phys. Rev. B. 12/2008; 78(21).
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    ABSTRACT: Single crystalline samples of Ba(Fe1-xCox)2As2 with x<0.12 have been grown and characterized via microscopic, thermodynamic, and transport measurements. With increasing Co substitution, the thermodynamic and transport signatures of the structural (high-temperature tetragonal to low-temperature orthorhombic) and magnetic (high-temperature nonmagnetic to low-temperature antiferromagnetic) transitions are suppressed at a rate of roughly 15 K/% Co. In addition, for x≥0.038 superconductivity is stabilized, rising to a maximum Tc of approximately 23 K for x≈0.07 and decreasing for higher x values. The T-x phase diagram for Ba(Fe1-xCox)2As2 indicates that either superconductivity can exist in both low-temperature crystallographic phases or that there is a structural phase separation. Anisotropic superconducting upper critical-field data [Hc2(T)] show a significant and clear change in anisotropy between samples that have higher temperature structural phase transitions and those that do not. These data show that the superconductivity is sensitive to the suppression of the higher temperature phase transition.
    Physical review. B, Condensed matter 11/2008; 78(21):214515. · 3.77 Impact Factor
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    ABSTRACT: Plate-like single crystals of SrFe2As2 as large as 3x3x0.5 mm3 have been grown out of Sn flux. The SrFe2As2 single crystals show a structural phase transition from a high temperature tetragonal phase to a low temperature orthorhombic phase at To = 198 K, and do not show any sign of superconductivity down to 1.8 K. The structural transition is accompanied by an anomaly in the electrical resistivity, Hall resistivity, specific heat, and the anisotropic magnetic susceptibility. In an intermediate temperature range from 198 K to 160 K, single crystal X-ray diffraction suggests a coexistence of the high-temperature tetragonal and the low-temperature orthorhombic phases.
    Physical review. B, Condensed matter 07/2008; · 3.77 Impact Factor