Thomas A. Lograsso

Iowa State University, Ames, Iowa, United States

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Publications (178)431.62 Total impact

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    ABSTRACT: Inelastic neutron scattering was employed to investigate the impact of electronic nematic order on the magnetic spectra of LaFeAsO and Ba(Fe$_{0.953}$Co$_{0.047}$)$_{2}$As$_{2}$. These materials are ideal to study the paramagnetic-nematic state, since the nematic order, signaled by the tetragonal-to-orthorhombic transition at $T_{{\rm S}}$, sets in well above the stripe antiferromagnetic ordering at $T_{{\rm N}}$. We find that the temperature-dependent dynamic susceptibility displays an anomaly at $T_{{\rm S}}$ followed by a sharp enhancement in the spin-spin correlation length, revealing a strong feedback effect of nematic order on the low-energy magnetic spectrum. Our findings can be consistently described by a model that attributes the structural/nematic transition to magnetic fluctuations, and unveils the key role played by nematic order in promoting the long-range stripe antiferromagnetic order in iron pnictides.
    10/2014;
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    ABSTRACT: A study of the formation of Gd11M4In9 (M = Ni, Pd, Pt) and R11Ni4In9 (R = rare earth) compounds revealed a unique and peculiar property, which is to naturally crystallize in a bundle of self-assembled fibers when cooled from the melt. The fibers, which are nano- to millimeters in cross-section and ≈1–40 mm long, grow unidirectionally along a temperature gradient. These compounds adopt the orthorhombic Nd11Pd4In9 structure type (oC48-Cmmm). This structure is layered, with slabs of R atoms alternating with slabs of Ni/In atoms along a short c-axis (much shorter than either the a- or b-axis). The growth direction of the fibers is along the crystallographic c-axis, orthogonal to the a–b plane. Two strong and short In–In bonds lie in the a–b plane, which are even shorter than in In metal. Integrated crystal orbital Hamilton population calculations show that the In–In bonds create isolated “R8Ni4In9” rods growing along the c-axis, with the In–In bonds being part of the rods. This appears to be an important factor explaining the microfibrous nature of these phases. Some physical properties have been measured on the Gd11Ni4In9 homolog. The compound orders ferrimagnetically at TC ≈ 88 K, and at lower temperatures (46 and 10 K), two other magnetic anomalies were observed, probably due to spin reorientations. As expected from the bonding features, the mechanical, magnetic and electrical properties are strongly anisotropic.
    Acta Materialia 07/2014; 73:27–36. · 3.94 Impact Factor
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    ABSTRACT: We report on the different roles of two orbital-active Fe$^{2+}$ at the A site and V$^{3+}$ at the B site in the magnetic excitations and on the anomalous spin wave broadening in FeV$_{2}$O$_{4}$. FeV$_{2}$O$_{4}$ exhibits three structural transitions and successive paramagnetic (PM)-collinear ferrimagnetic (CFI)-noncollinear ferrimagnetic (NCFI) transitions. The high-temperature tetragonal/PM -orthorhombic/CFI transition is accompanied by the appearance of an energy gap with a high magnitude in the magnetic excitations due to strong spin-orbit coupling induced anisotropy at the Fe$^{2+}$ site. While there is no measurable increase in the energy gap from the orbital ordering of V$^{3+}$ at the orthorhombic/CFI-tetragonal/NCFI transition, anomalous spin wave broadening is observed in the orthorhombic/CFI state due to V$^{3+}$ spin fluctuations at the B site. The spin wave broadening is also observed at the zone boundary without softening, which is discussed in terms of magnon-phonon coupling.
    Physical review. B, Condensed matter 06/2014; 89,:224416. · 3.66 Impact Factor
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    ABSTRACT: We report in-plane thermoelectric power measurements on single crystals of (Ba$_{1-x}$K$_x$)Fe$_2$As$_2$ (0.44 $\leq x \leq$ 1). We observe a minimum in the S$|_{T=const}$ versus x at x ~ 0.55 that can be associated with the change in the topology of the Fermi surface, a Lifshitz transition, related to the electron pockets at the center of M point crossing the Fermi level. This feature is clearly observable below ~ 75 K. Thermoelectric power also shows a change in the x ~ 0.8 - 0.9 range, where maximum in the thermoelectric power collapses into a plateau. This Lifshitz transition is most likely related to the reconstruction of the Fermi surface associated with the transformation of the hole pockets at the M point into four blades as observed by ARPES measurements.
    05/2014;
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    ABSTRACT: We report on the different roles of two orbital-active Fe$^{2+}$ at the A site and V$^{3+}$ at the B site in the magnetic excitations and on the anomalous spin wave broadening in FeV$_{2}$O$_{4}$. FeV$_{2}$O$_{4}$ exhibits three structural transitions and successive paramagnetic (PM)-collinear ferrimagnetic (CFI)-noncollinear ferrimagnetic (NCFI) transitions. The high-temperature tetragonal/PM -orthorhombic/CFI transition is accompanied by the appearance of an energy gap with a high magnitude in the magnetic excitations due to strong spin-orbit coupling induced anisotropy at the Fe$^{2+}$ site. While there is no measurable increase in the energy gap from the orbital ordering of V$^{3+}$ at the orthorhombic/CFI-tetragonal/NCFI transition, anomalous spin wave broadening is observed in the orthorhombic/CFI state due to V$^{3+}$ spin fluctuations at the B site. The spin wave broadening is also observed at the zone boundary without softening, which is discussed in terms of magnon-phonon coupling.
    04/2014;
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    ABSTRACT: Self-organized molecular films with long-range quasiperiodic order have been grown by using the complex potential energy landscape of quasicrystalline surfaces as templates. The long-range order arises from a specific subset of quasilattice sites acting as preferred adsorption sites for the molecules, thus enforcing a quasiperiodic structure in the film. These adsorption sites exhibit a local five-fold symmetry resulting from the cut by the surface plane through the cluster units identified in the bulk solid. Symmetry matching between the C60 fullerene and the substrate leads to a preferred adsorption configuration of the molecules with a pentagonal face down, a feature unique to quasicrystalline surfaces, enabling efficient chemical bonding at the molecule-substrate interface. This finding offers opportunities to investigate the physical properties of model 2D quasiperiodic systems as the molecules can be functionalized to yield architectures with tailor-made properties.
    ACS Nano 03/2014; · 12.03 Impact Factor
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    ABSTRACT: We report measurements of electrical resistivity under pressure up to 5.8 GPa, magnetization up to 6.7 GPa and ac susceptibility up to 7.1 GPa in KFe2As2. At a pressure pc= 1.8 GPa, there is a change of slope in the pressure dependence of the superconducting transition temperature Tc(p) as previously reported. Above pc, Tc is almost constant up to 7 GPa. The T-p phase diagram is very sensitive to the pressure conditions as a consequence of anisotropic uniaxial pressure dependence of Tc. In addition, we observe a change in the upper critical field behavior across pc and provide a quantitative measure of this change. We show how this can be used to investigate a change of the superconducting gap structure and suggest the appearance of a kz modulation of the superconducting gap as a possible explanation.
    Physical Review B 02/2014; 89(22). · 3.66 Impact Factor
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    ABSTRACT: We measured the Raman spectra of ferromagnetic nearly half metal CoS2 in a broad temperature range. All five Raman active modes Ag, Eg, Tg(1), Tg(2) and Tg(3) were observed. The magnetic ordering is indicated by a change of the temperature dependences of the frequency and the line width of Ag and T g(2) modes at the Curie point. The temperature dependence of the frequencies and linewidths of the Ag, Eg, Tg(1), T g(2) modes in the paramagnetic phase can be described in the framework of the Klemens approach. Hardening of the Tg(2), Tg(1) and A g modes on cooling can be unambiguously seen in the ferromagnetic phase. The linewidths of Tg(2) and Ag modes behave a natural way at low exciting laser power (decrease with decreasing temperature) in the ferromagnetic phase. At high exciting laser power the corresponding linewidths increase at temperature decreasing below the Curie temperature. Then as can be seen the line width of Ag mode reaches a maxima at about 80K. This intriging feature probably signifies a specific channel of the optical phonon decay in the ferromagnetic phase of CoS2. Tentative explanations of some of the observed effects are given, taking into account the nearly half metallic nature of CoS2.
    Journal of Physics Condensed Matter 02/2014; 26(39). · 2.22 Impact Factor
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    ABSTRACT: The low electron concentration region (e/a < 7.75) of the magnetic phase diagram of the off-stoichiometric Ni–Mn–Sn Heusler alloys was investigated in detail by DSC and magnetization measurements of the Ni40+xMn39−xSn21(x = 0, 2, 4, 6 and 8 at.%) alloys. The alloys show a stable austenitic phase without any martensitic transformation down to 5 K even after heat treatment. The Curie temperature exhibits a broad maximum over a large composition range. The evolution of the magnetic moment with the electron concentration fits the data of previous studies and confirms the peak-like dependence in the extended range of e/a values predicted by ab initio calculations. The explored part of the moment versus e/a curve can be explained in terms of a localized magnetic moment model and full atomic order in the alloys.
    Journal of Alloys and Compounds 01/2014; 594:171–174. · 2.73 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: We observe a distinct excitation-fluence-dependent transient carrier relaxation in a strongly correlated colossal magnetoresistive manganite that correlates with photoinduced magnetic and electronic phase transitions characterized by nonlinear photoexcitation behaviors.
    CLEO: QELS_Fundamental Science; 06/2013
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    ABSTRACT: The technological demand to push the gigahertz (10(9) hertz) switching speed limit of today's magnetic memory and logic devices into the terahertz (10(12) hertz) regime underlies the entire field of spin-electronics and integrated multi-functional devices. This challenge is met by all-optical magnetic switching based on coherent spin manipulation. By analogy to femtosecond chemistry and photosynthetic dynamics--in which photoproducts of chemical and biochemical reactions can be influenced by creating suitable superpositions of molecular states--femtosecond-laser-excited coherence between electronic states can switch magnetic order by 'suddenly' breaking the delicate balance between competing phases of correlated materials: for example, manganites exhibiting colossal magneto-resistance suitable for applications. Here we show femtosecond (10(-15) seconds) photo-induced switching from antiferromagnetic to ferromagnetic ordering in Pr0.7Ca0.3MnO3, by observing the establishment (within about 120 femtoseconds) of a huge temperature-dependent magnetization with photo-excitation threshold behaviour absent in the optical reflectivity. The development of ferromagnetic correlations during the femtosecond laser pulse reveals an initial quantum coherent regime of magnetism, distinguished from the picosecond (10(-12) seconds) lattice-heating regime characterized by phase separation without threshold behaviour. Our simulations reproduce the nonlinear femtosecond spin generation and underpin fast quantum spin-flip fluctuations correlated with coherent superpositions of electronic states to initiate local ferromagnetic correlations. These results merge two fields, femtosecond magnetism in metals and band insulators, and non-equilibrium phase transitions of strongly correlated electrons, in which local interactions exceeding the kinetic energy produce a complex balance of competing orders.
    Nature 04/2013; 496(7443):69-73. · 38.60 Impact Factor
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    ABSTRACT: Formation energies of native defects in Bi2(TexSe3−x), with comparison to ideal Bi2Te2S, are calculated in density-functional theory to assess transport properties. Bi2Se3 is found to be n type for both Bi- and Se-rich growth conditions, while Bi2Te3 changes from n to p type going from Te- to Bi-rich conditions, as observed. Bi2Te2Se and Bi2Te2S are generally n type, explaining observed heavily doped n-type behavior in most samples. A (0/−) transition level at 16 meV above valence-band maximum for Bi on Te antisites in Bi2Te2Se is related to the observed thermally active transport gap causing a p-to-n transition at low temperature. Bi2(TexSe3−x) with x>2 are predicted to have high bulk resistivity due to effective carrier compensation when approaching the n-to-p crossover. Predicted behaviors are confirmed from characterization of our grown single crystals.
    Physical review. B, Condensed matter 03/2013; 87(12). · 3.66 Impact Factor
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    Physical Review B. 01/2013;
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    ABSTRACT: Spin valves have revolutionized the field of magnetic recording and memory devices. Spin valves are generally realized in thin film heterostructures, where two ferromagnetic (FM) layers are separated by a nonmagnetic conducting layer. Here, we demonstrate spin-valve-like magnetoresistance at room temperature in a bulk ferrimagnetic material that exhibits a magnetic shape memory effect. The origin of this unexpected behavior in Mn2NiGa has been investigated by neutron diffraction, magnetization, and ab initio theoretical calculations. The refinement of the neutron diffraction pattern shows the presence of antisite disorder where about 13% of the Ga sites are occupied by Mn atoms. On the basis of the magnetic structure obtained from neutron diffraction and theoretical calculations, we establish that these antisite defects cause the formation of FM nanoclusters with parallel alignment of Mn spin moments in a Mn2NiGa bulk lattice that has antiparallel Mn spin moments. The direction of the Mn moments in the soft FM cluster reverses with the external magnetic field. This causes a rotation or tilt in the antiparallel Mn moments at the cluster-lattice interface resulting in the observed asymmetry in magnetoresistance.
    Physical Review Letters 12/2012; 109(50):246601. · 7.73 Impact Factor
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    ABSTRACT: We present heat capacity, c-axis thermal expansion and pressure dependent, low field, temperature dependent magnetization for pressures up to ~ 12 kbar, data for KFe2As2 single crystals. Tc decreases under pressure with dTc/dP ~ -0.10 K/kbar. The inferred uniaxial, c-axis, pressure derivative is positive, dTc/dpc ~ 0.11 K/kbar. The data are analyzed in comparison with those for overdoped Fe-based superconductors. Arguments are presented that superconductivity in KFe2As2 may be different from the other overdoped, Fe-based materials in the 122 family.
    Physical review. B, Condensed matter 12/2012; 86(22). · 3.66 Impact Factor
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    ABSTRACT: We use hard x-ray photoemission to resolve a controversial issue regarding the mechanism for the formation of quasicrystalline solids, i.e., the existence of a pseudogap at the Fermi level. Our data from icosahedral fivefold Al-Pd-Mn and Al-Cu-Fe quasicrystals demonstrate the presence of a pseudogap, which is not observed in surface sensitive low energy photoemission because the spectrum is affected by a metallic phase near the surface. In contrast to Al-Pd-Mn, we find that in Al-Cu-Fe the pseudogap is fully formed; i.e., the density of states reaches zero at E_{F} indicating that it is close to the metal-insulator phase boundary.
    Physical Review Letters 11/2012; 109(21):216403. · 7.73 Impact Factor
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    ABSTRACT: Abstract–$rm Gd\_5({rm Si\_rm xrm Ge\_1-rm x\})_4\$ exhibits a first order phase transition for the compositions $0<rm x<0.575$ leading to a magnetic phase transition as well. It is not possible to measure the second order phase (magnetic) transition temperatures of the individual phases with direct measurements. This is because the first order phase transition occurs before the second order phase transition. With modified Arrott plots we have shown previously that it is possible to estimate the second order phase transition of the $rm Gd\_5rm Si\_4\$ -type orthorhombic phase. In this paper we have estimated the second order phase transition temperature of the $rm Gd\_5rm Si\_2rm Ge\_2\$-type monoclinic phase using a single crystal sample of $rm Gd\_5rm Si\_1.5rm Ge\_2.5\$ (0.375) which falls in the mixed phase region of the sample.
    IEEE Transactions on Magnetics 11/2012; 48(11):4070-4073. · 1.42 Impact Factor
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    ABSTRACT: We use angle-resolved photoemission spectroscopy to study a nontrivial surface state in a pseudobinary Bi2Te2.8Se0.2 topological insulator. We show that, unlike previously studied binaries, this is an intrinsic topological insulator with the conduction bulk band residing well above the chemical potential. Our data indicate that under a good vacuum condition there are no significant aging effects for more than two weeks after cleaving. We also demonstrate that the shift of the Kramers point at low temperature is caused by UV-assisted absorption of molecular hydrogen. Our findings pave the way for applications of these materials in devices and present an easy scheme to tune their properties.
    Physical review. B, Condensed matter 08/2012; 86(8). · 3.66 Impact Factor

Publication Stats

984 Citations
431.62 Total Impact Points

Institutions

  • 1988–2014
    • Iowa State University
      • • Ames Laboratory
      • • Department of Physics and Astronomy
      • • Department of Materials Science and Engineering
      Ames, Iowa, United States
  • 2012
    • UGC-DAE Consortium for Scientific Research
      Indaur, Madhya Pradesh, India
  • 2010
    • Kurchatov Institute
      Moskva, Moscow, Russia
  • 2008
    • Институт физики высоких энергий
      Protvino, Moskovskaya, Russia
  • 2006
    • The Ohio State University
      • Department of Mechanical and Aerospace Engineering
      Columbus, OH, United States
    • U.S. Department of Energy
      • Ames Lab
      Washington, Washington, D.C., United States
  • 2002
    • Freie Universität Berlin
      • Institute of Experimental Physics
      Berlin, Land Berlin, Germany
  • 2001
    • Naval Undersea Warfare Center
      Newport, Rhode Island, United States
  • 1984
    • Michigan Technological University
      Houghton, Michigan, United States