Amar B. Karki

Louisiana State University, Baton Rouge, Louisiana, United States

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Publications (76)145.89 Total impact

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    ABSTRACT: Pulsed ultrasonic experiments performed on the parent compounds of FeAs based superconductors AFe2As2 (A =Ba, Ca) revealed elastic anomalies that agree with a two-step process for the structural/magnetic transitions. Upon cooling, a pronounced velocity softening of longitudinal phonons propagating along the c axis is observed at the structural transition Ts due to a distortion of the lattice. Below a slightly lower temperature TN, a biquadratic coupling between the distortion and an antiferromagnetic order parameter produces a steep velocity stiffening upon further cooling, the stiffening being five times larger in CaFe2As2 than in BaFe2As2. Microwave resistivity measurements at 16.5 GHz suggest that, upon cooling, the lattice distortion at Ts is driven by magnetic fluctuations in BaFe2As2, while it is rather due to lattice fluctuations in CaFe2As2. This suggestion appears consistent with a second-order character of the transitions in BaFe2As2 and a first-order one in CaFe2As2.
    03/2014; 89(15).
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    ABSTRACT: Low energy electron diffraction, scanning tunneling microscopy and spectroscopy, and first-principles spin-dependent density functional theory are utilized to investigate the geometric, electronic, and magnetic structures of the stripe-ordered (1×2) surface of Ca(Fe1-xCox)2As2 (x=0, 0.075). The surface is terminated with a 50% Ca layer. Compared to the bulk, the surface Ca layer has a large inward relaxation (∼0.5 Å), and the underneath As-Fe2-As layer displays a significant buckling. First-principles calculations show that the (1×2) phase is stabilized by the bulk antiferromagnetic spin ordering through the spin-charge-lattice coupling. Strikingly, a superconducting gap (∼7 meV at 7.4 K) is observed to spatially coexist with the (1×2) phase (x=0.075 compound). This implies the coexistence of both superconductivity and AFM ordering at the surface.
    Physical Review Letters 02/2014; 112(7):077205. · 7.73 Impact Factor
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    ABSTRACT: Single crystals of the title compounds are grown from the elements (Ln:Ru:Sn = 12:6:11; evacuated silica tube, 1260 °C, 36 h).
    ChemInform 06/2013; 44(25).
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    ABSTRACT: The attractive/repulsive relationship between superconductivity and magnetic ordering has fascinated the condensed matter physics community for a century. In the early days, magnetic impurities doped into a superconductor were found to quickly suppress superconductivity. Later, a variety of systems, such as cuprates, heavy fermions, and Fe pnictides, showed superconductivity in a narrow region near the border to antiferromagnetism (AFM) as a function of pressure or doping. However, the coexistence of superconductivity and ferromagnetic (FM) or AFM ordering is found in a few compounds [RRh4B4 (R = Nd, Sm, Tm, Er), R'Mo6X8 (R' = Tb, Dy, Er, Ho, and X = S, Se), UMGe (M = Ge, Rh, Co), CeCoIn5, EuFe2(As1-xPx)2, etc.], providing evidence for their compatibility. Here, we present a third situation, where superconductivity coexists with FM and near the border of AFM in Fe1-xPdxTe. The doping of Pd for Fe gradually suppresses the first-order AFM ordering at temperature TN/S, and turns into short-range AFM correlation with a characteristic peak in magnetic susceptibility at T'N. Superconductivity sets in when T'N reaches zero. However, there is a gigantic ferromagnetic dome imposed in the superconducting-AFM (short-range) cross-over regime. Such a system is ideal for studying the interplay between superconductivity and two types of magnetic (FM and AFM) interactions.
    Proceedings of the National Academy of Sciences 05/2013; · 9.81 Impact Factor
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    ABSTRACT: Among Iron-based superconductors, a new family of Ca10(PtnAs8)(Fe2As2)5 with n= 3 (``10-3-8'') or n=4 (``10-4-8'') is unique owing to the existence of PtnAs8 layer. This sets them with different electronic properties than the rest of Iron-based superconductors. By cleaving 10-4-8 single crystals (Tc ˜ 34 K) in the ultra-high vacuum, we are able to observe three surfaces: Ca layer, FeAs layer, and Pt4As8 layer. Scanning tunneling microscope (STM) reveals both the topology and electronic density of individual layers. We discuss the implications of our results with the combination of bulk electronic properties.
    03/2013;
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    ABSTRACT: Low energy electron diffraction (LEED) and density functional theory (DFT) have been utilized to investigate the surface structure for the stripe 1x2 phase of Ca(Fe1-xCox)2As2 iron pnictides, for x = 0 and x = 0.075. Quantitative structural analysis of LEED-I(V) using the fractional spots of the 1x2 phase on both parent and doped samples gives a similar surface structure with a termination layer of half Ca atoms. The surface Ca layer has a large inward relaxation about 0.5 Angstrom and the underneath As-Fe-As layer displays a buckling distortion of about 0.07 Angstrom. DFT calculations show significant charge rearrangements at the surface, which is driven by spin charge coupling, verified by freezing the structure and reducing the magnetic moment to zero. The role of spin-charge coupling in determining the surface reconstruction will be elucidated by self-consistent calculations of the structure as a function of the magnetic moment.
    03/2013;
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    ABSTRACT: Needle-like Ca3LiRuO6 and CaFe4As3 single crystals were grown using the flux method. While the structure of Ca3LiRuO6 is characterized by one-dimensional chains of alternating face-sharing LiO6 trigonal prisms and RuO6 octahedra along the c axis, CaFe4As3 consists of edge-sharing FeAs4 tetrahedra ribbons along the b axis. Despite the one-dimensional nature, magnetization measurements reveal evidence of long-range magnetic ordering: Ca3LiRuO6 orders ferromagnetically below TC = 120 K and CaFe4As3 undergoes two successive antiferromagnetic transitions at TN1 = 90 K and TN2 = 26 K. Possible magnetic interactions will be discussed.
    03/2013;
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    ABSTRACT: We present the structure of Ln(30)Ru(4+x)Sn(31-y) (Ln = Gd-Dy) and the anisotropic resistivity, magnetization, thermopower, and thermal conductivity of single crystal Ln(30)Ru(4+x)Sn(31-y). Gd(30)Ru(4.92)Sn(30.54) crystallizes in a new structure-type with space group Pnnm and dimensions of a = 11.784(1) Å, b = 24.717(1) Å, and c = 11.651(2) Å, and V = 3,394(1) Å(3). Magnetic anisotropy and highly anisotropic electrical transport behavior were observed in the single crystals of Gd(30)Ru(4.92)Sn(30.54) and Tb(30)Ru(6)Sn(29.5). Additionally, the lattice thermal conductivity of Tb(30)Ru(6)Sn(29.5) is quite low, and a comparison is made to other Sn-containing compounds.
    Journal of the American Chemical Society 01/2013; · 10.68 Impact Factor
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    ABSTRACT: We report an ultrasonic study of the magneto-elastic coupling of the spin-density wave antiferromagnet CaFe4As3. Longitudinal waves propagating along the a axis reveal anomalies on the acoustic velocity at both the incommensurate (ICM) (TN1=89.3 K) and commensurate (CM) (TN2=26.3 K) spin-density phases, which are consistent with the magnetic structure established from neutron diffraction experiments. Moreover, at higher temperatures, magnetic fluctuations are likely responsible for a reduced stiffening of the velocity below 150 K. Although the ICM phase appears elastically inhomogeneous below 50 K, a precise magnetic field dependence of the ICM-CM transition at TN2 specifies a preferential orientation of the in-plane easy and hard axes, respectively, parallel and perpendicular to the vector d̂ (â·d̂=cos30∘). Within the CM phase, a magnetic field aligned along the ribbon b axis reveals a new magnetic transition of the spin-flop type near 16 T. For this particular field direction a phase diagram is proposed.
    Physical review. B, Condensed matter 01/2013; 87(1). · 3.77 Impact Factor
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    Dataset: CST 2008
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    ABSTRACT: We report both experimental and theoretical investigations of the physical properties of Ba2Mn2Sb2O single crystals. This material exhibits a hexagonal structure with lattice constants a=4.7029(15) Å and c=19.9401(27) Å, as obtained from powder x-ray diffraction measurements, and in agreement with structural optimization through density functional theory (DFT) calculations. The magnetic susceptibility and specific heat show anomalies at TN=60 K, consistent with antiferromagnetic ordering. However, the magnitude of TN is significantly smaller than the Curie-Weiss temperature (|ΘCW|≈560 K), suggesting a magnetic system of reduced dimensionality. The temperature dependence of both the in-plane and out-of-plane resistivity changes from activated at T>Tx∼200 K to logarithmic at T<Tx. Correspondingly, the magnetic susceptibility displays a bump at Tx. DFT calculations at the DFT+U level support the experimental observation of an antiferromagnetic ground state.
    Physical review. B, Condensed matter 11/2012; 86(19). · 3.77 Impact Factor
  • A B Karki, D A Browne, S Stadler, J Li, R Jin
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    ABSTRACT: We report the electrical transport, magnetic, and thermodynamic properties of polycrystalline PdTe which exhibits superconductivity below 4.5 K. Using the measured values for the lower (H(c1)) and upper (H(c2)) critical fields, and the specific heat C(p), we estimate the thermodynamic critical field H(c)(0), coherence length ξ(0), penetration depth λ(0), and the Ginzburg-Landau parameter κ. Compared with band structure calculations, the density of states at the Fermi level is enhanced due to electron-phonon coupling with λ(ep) = 1.4. Furthermore, the large values of ΔC(p)/γ(n)T(c) and 2Δ(0)/k(B)T(c) suggest that PdTe is a strongly coupled superconductor.
    Journal of Physics Condensed Matter 02/2012; 24(5):055701. · 2.22 Impact Factor
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    ABSTRACT: PdTe is a long-known superconductor but its physical properties are almost unknown. We have recently studied its basic physical properties in both normal and superconducting states. While FeTe forms different crystallographic structure and is known to form spin density wave below TN = 70 K, we have successfully synthesized Pd1-xFexTe with x from 0 to 1. By measuring its electrical and magnetic properties, we establish the phase diagram of Pd1-xFexTe for the first time. With increasing x, we found that Tc is quickly suppressed. Ferromagnetism appears for the samples with x >= 0.02. For 0.25 <= x <= 1.0, the system exhibits antiferromagnetic ordering with TN increasing with x. This is a prototype system for studying the interplay between superconductivity and magnetism.
    02/2012;
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    ABSTRACT: Low energy electron diffraction (LEED) and scanning tunneling microscopy/spectroscopy (STM/S) have been utilized to investigate the geometric structure of the stripe 1x2 surface phase of (Ba, Ca)(Fe1-xCox)2As2 iron pnictides. STM images show that the surface consists of competing ordered and disordered regions. The 1x2 phase appears on the surface of all compounds but coexist with (2 x2)R45^o phase on the surface of Ba122. Quantitative structural analysis of LEED-I(V) using the fractional spots of the 1x2 phase on both parent compounds as well as Ca(Fe0.925Co0.075)2As2 gives a similar surface structure with a termination layer of 50% Ca/Ba atoms. The surface Ca/Ba layer has a large inward relaxation ˜ 0.5 å and the underneath As-Fe-As layer displays a buckling distortion. The Pendry Rp factor (˜ 0.24) obtained in the structural refinement is excellent for all three systems.
    02/2012;
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    ABSTRACT: We have grown high-quality single crystals of BaMn2Sb2, which possesses the ThCr2Si2 structure as determined by X-ray powder diffraction technique. Magnetization measurements indicate that BaMn2Fe2 is ferromagnetic below TC = 580K. On the other hand, the temperature dependence of electrical resistivity shows semiconducting behavior, which can be described by thermally-activated resistivity formula with thermal activation energy about 0.25 eV . While the Hall coefficient has positive sign between 2 and 300 K, the Seebeck Coefficient undergoes sign change from positive at high temperatures to negative at low temperatures, reaching -260 μV/K at 70 K. The implication will be discussed.
    02/2012;
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    ABSTRACT: Arrays of nanowires and nanotubes of antimony-telluride (Sb2Te3) have been fabricated by an electrodeposition technique. Scanning electron microscopy was employed to characterize the morphology and size of the fabricated Sb2Te3nanowires and nanotubes. Wavelength dispersive spectroscopy analysis confirmed the composition of the fabricated nanowires and nanotubes. The composition of the nanowires fabricated at a cathodic current density of 10 mA cm−2 and nanotubes fabricated at a cathodic current density of 5.5 mA cm−2 was found to be 39% Sb and 61% Te (2:3 ratio between Sb and Te). The fabricated Sb2Te3nanowire and nanotube arrays were found to be polycrystalline with no preferred orientation. The average lamellar thickness of the nanowires and nanotube crystallites was determined using the Scherrer equation and found to be 36 nm and 43 nm, respectively. The measured room temperature Seebeck coefficients for the Sb2Te3nanowires and nanotubes were +359 µV K−1 and +332 µV K−1, respectively, confirming that the Sb2Te3nanowires and nanotubes were p-type. The electrical resistance measurements indicated that the resistance of the Sb2Te3nanowires and nanotubes decreased with increasing temperature, consistent with semiconducting behavior.
    Journal of Materials Chemistry 03/2011; 21(12):4098-4107. · 5.97 Impact Factor
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    ABSTRACT: New compound CaFe4As3 crystallizes in an orthorhombic structure with Fe2As2 layers aligned along b direction but a rectangular cross-section in ac plane. The needle-shaped CaFe4As3 single crystals were grown and are found to undergo two successive phase transitions occurring at T1˜ 90 K and T2˜ 27 K, respectively. At T1 the electrical resistivity increases and magnetic susceptibility decreases in both parallel and perpendicular to b directions consistent with the scenario of spin-density-wave formation. At T2, resistivity decreases sharply at T2 with hysteresis while magnetic susceptibility increases along either b direction or ac plane. The underlying physics will be discussed by taking into account other physical properties.
    03/2011;
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    ABSTRACT: We present a systematic High Resolution Electron Energy Loss Spectra (HREELS) study of the surfaces lattice dynamics of the cleaved single crystals of the parent compounds AFe2As2 (A=Ba, Ca) as a function of temperature and sample treatment. The different phonon signatures on the competing surface reconstructions 1x2 phase and (2x2)R45^^o phase are studied. For Ba there are two optical phonon modes are observed at 18 and 29 meV, which can be identified as the A1g and B1g vibrations of the As and Fe atoms, respectively. A detailed discussion is given in terms of the interplay between the spin and lattice in this novel system
    03/2011;
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    ABSTRACT: The new class of iron based superconductors has ignited the materials community. Of all of these new superconductors, the Fe chalcogenide system is probably the simplest at least in structure. One of most interesting questions is how the properties of these systems change as the structure is manipulated. We have studied ultra thin films of FeTe grown on a SrTiO3(001) surface by using Pulsed Laser Deposition (PLD) and characterized with surface techniques. We observe the formation of faceted nanocrystalline islands at the surface of thin films. The structure of the islanded surface, including the faceting angle of nanostructures, has been studied by Low Energy Electron Diffraction (LEED) as well as scanning tunneling microscope (STM). The correlation of such a 3D-type growth with growth conditions and substrate-induced strain in the ultra-thin films are discussed. *Supported by NSF-DMR1005562
    03/2011;
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    ABSTRACT: We utilize Low Energy Electron Diffraction (LEED) to determine the surface structure combined with real-space scanning tunneling microcopy/spectroscopy (STM/STS), to investigate the local geometric and electronic structures at the (001) surface of the compounds of AFe2As2 (A= Ba, Ca). In general two competing surface reconstructions are observed with either a 1x2 or a (2x2)R45^o (tetragonal notation) structure. The (2x2)R45^o structure corresponds to the 1x1 orthorhombic phase. While the (2x2)R45^o phase always present for A=Ba, the 1x2 structure dominates for A=Ca. We will discuss the detailed structural change with Co doping, thermal cycling, contamination, electron beam induced damage, and cleaving temperature. Specifically, 1x2 phase is sensitive to the thermal processing, with indications of a temperature dependence phase transition. *Supported by NSF DMR-1002622
    03/2011;

Publication Stats

281 Citations
145.89 Total Impact Points

Institutions

  • 2006–2014
    • Louisiana State University
      • • Department of Physics & Astronomy
      • • Department of Chemistry
      Baton Rouge, Louisiana, United States
  • 2010
    • University of Liège
      • Department of Physics
      Luik, Walloon Region, Belgium
  • 2009–2010
    • Lamar University
      • Department of Chemistry and Biochemistry
      Beaumont, TX, United States
  • 2007–2008
    • University of California, Los Angeles
      • Department of Mechanical and Aerospace Engineering
      Los Angeles, CA, United States
    • National Institute for Materials Science
      Tsukuba, Ibaraki, Japan