G. D. Samolyuk

Iowa State University, Ames, IA, United States

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Publications (71)154.16 Total impact

  • Physical review. B, Condensed matter 08/2012; 86(7). · 3.77 Impact Factor
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    ABSTRACT: We present the growths and detailed thermodynamic and transport measurements on single crystals of the recently discovered binary intermetallic superconductors, SrSn4 and BaSn5. Their superconducting transition temperatures Tc are found to be 4.8 K and 4.4 K respectively. Both materials are strongly-coupled, possibly multi-band superconductors. Hydrostatic pressure causes a decrease in the superconducting transition temperature at the rate of -0.068 K/kbar for SrSn4, and -0.053 K/kbar for BaSn5. Band structure and upper superconducting critical field anisotropy of SrSn4 suggest complex, multi-sheet Fermi surface formed by four bands. De Hass-van Alphen oscillations are observed in BaSn5, which indicates a more complex topology of Fermi surface.
    02/2012;
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    ABSTRACT: The anisotropic physical properties of single crystals of orthorhombic PtSn4 are reported for magnetic fields up to 140 kOe, applied parallel and perpendicular to the crystallographic b axis. The magnetic susceptibility has an approximately temperature-independent behavior and reveals an anisotropy between the ac plane and b axis. Clear de Haas-van Alphen oscillations in fields as low as 5 kOe and at temperatures as high as 30 K were detected in magnetization isotherms. The thermoelectric power and resistivity of PtSn4 show the strong temperature and magnetic field dependencies. A change of the thermoelectric power at H=140 kOe is observed as high as ≃50 μV/K. Single crystals of PtSn4 exhibit very large transverse magnetoresistance of ≃5×105% for the ac plane and of ≃1.4×105% for the b axis resistivity at 1.8 K and 140 kOe, as well as pronounced Shubnikov de Haas oscillations. The magnetoresistance of PtSn4 appears to obey Kohler's rule in the temperature and field range measured. The Hall resistivity shows a linear temperature dependence at high temperatures followed by a sign reversal around 25 K which is consistent with thermoelectric power measurements. The observed quantum oscillations and band structure calculations indicate that PtSn4 has three-dimensional Fermi surfaces.
    Physical review. B, Condensed matter 01/2012; 85(3). · 3.77 Impact Factor
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    ABSTRACT: We present detailed thermodynamic and transport measurements on single crystals of the recently discovered binary intermetallic superconductor, SrSn(4). We find this material to be a slightly anisotropic three-dimensional, strongly coupled, possibly multiband, superconductor. Hydrostatic pressure causes a decrease in the superconducting transition temperature at the rate of ≈ - 0.068 K kbar(-1). Band structure calculations are consistent with experimental data on the Sommerfeld coefficient and upper superconducting critical field anisotropy, and suggest a complex, multi-sheet Fermi surface formed by four bands.
    Journal of Physics Condensed Matter 11/2011; 23(45):455703. · 2.22 Impact Factor
  • N. Ni, S. Jia, G. D. Samolyuk, A. S. Sefat A. Kracher
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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⩽0.122) and YFe2(AlxZn1-x)20 (x⩽0.121) were grown out of Zn-rich solution. Magnetization, heat capacity, and resistivity measurements show that, with Al doping, the ferromagnetic phase transition temperatures of the GdFe2(AlxZn1-x)20 compounds decrease from 86 K (x= 0) to 10 K (x= 0.122); for the nonmagnetic analog, the YFe2(AlxZn1-x)20 series, the Stoner enhancement factor Z decreases from 0.88 (x= 0) to 0.35 (x= 0.121) in a similar manner. Tight-binding linear-muffin-tin orbital atomic-sphere approximation band structure calculations are used to rationalize this trend. These results, together with the earlier studies of the R(Fe1-xCox)2Zn20 (R= Gd and Y) series, clearly highlight the importance of band filling and the applicability of even a simple, rigid-band model to these compounds.
    Phys. Rev. B. 01/2011; 83(5).
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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: We use angle-resolved photoemission spectroscopy (ARPES) to study the electronic properties of CaFe2As2-parent compound of a pnictide superconductor. We find that the structural and magnetic transition is accompanied by a three- to two-dimensional (3D-2D) crossover in the electronic structure. Above the transition temperature (T_{s}) Fermi surfaces around Gamma and X points are cylindrical and quasi 2D. Below T_{s}, the Gamma pocket forms a 3D ellipsoid, while the X pocket remains quasi 2D. This finding strongly suggests that low dimensionality plays an important role in understanding the superconducting mechanism in pnictides.
    Physical Review Letters 05/2009; 102(16):167004. · 7.73 Impact Factor
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    ABSTRACT: The London penetration depth lambda(T) has been measured in single crystals of Ba(Fe0.93Co0.07)2As2. The observed low-temperature variation of lambda(T) follows a power law, Deltalambda(T) approximately T(n) with n approximately 2.4+/-0.1, indicating the existence of normal quasiparticles down to at least 0.02T(c). This is in contrast with previous penetration depth measurements on single crystals of NdFeAsO1-xFx and SmFeAsO1-xFx, which indicate an anisotropic but nodeless gap. We discuss possible explanations of the observed power law behavior.
    Physical Review Letters 04/2009; 102(12):127004. · 7.73 Impact Factor
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    ABSTRACT: CaFe2As2 is a parent compound of a new family of FeAs based high-Tc superconductors. It undergoes a first-order structural transition from low-T orthorhombic to high-T tetragonal phase [Ni et al., Phys. Rev. B 78, 014523]. Moderate pressure lowers the transition temperature, and turns on the superconductivity [Torikachvili et al., Phys. Rev. Lett. 101, 057006]. Study on its electronic properties is of crucial importance for understanding the pairing mechanism of the FeAs based superconductors. Here we present angle-resolved photoemission spectroscopy (ARPES) results on both the orthorhombic and the tetragonal phase of CaFe2As2. In the orthorhombic phase, we find strong kz dispersion on the Fermi surfaces, showing a three dimensional electronic structure. We also find dramatic difference of the Fermi surface structure between the orthorhombic and the tetragonal phase.
    03/2009;
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    ABSTRACT: The anisotropy of electrical resistivity in the iron pnictide superconductor Ba(Fe,Co)2As2 has been studied using the Montgomery technique and direct transport measurements in single crystals cut along principal directions of the conductivity tensor (tetragonal c-axis and a-direction perpendicular to it). A good quantitative agreement is found between two sets of data, with a rhoc/ rhoa anisotropy of 5 ± 1 just above the superconducting transition temperature. This is in very good agreement with expectations based on the anisotropy of the critical fields, suggestive of orbital limiting of superconductivity at Tc.
    03/2009;
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    ABSTRACT: We study the stability of magnetic structures in iron pnictides as a function of doping, external pressure and the amount of defects. Several collinear and non-collinear magnetic structures are found to be stable in all classes of pnictides. This stability however is a result of a fragile competition between several nearest neighbor exchange couplings and depends greatly on doping. We determined that for a relatively small electron doping the non-magnetic instability is developed, while already for a small hole doping the stripe structure is instable in many pnictides and other magnetic structures are stabilized. For a larger hole doping the local magnetic moment phase with ferromagnetic long range order can be stabilized. A transition to non-collinear state at small moments is explained by a competition between the anisotropy of the nearest neighbors exchange couplings and third or forth neighbor couplings. Using very extensive calculations of magnetic stability parameter we build a generic pressure-concentration phase diagram of iron pnictides.
    03/2009;
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    ABSTRACT: Single crystal neutron and high-energy x-ray diffraction have identified the phase lines corresponding to transitions between the ambient-pressure tetragonal (T), the antiferromagnetic orthorhombic (O) and the nonmagnetic collapsed tetragonal (cT) phases of CaFe2As2. We find no evidence of additional structures for pressures up to 2.5 GPa (at 300 K). Both the T-cT and O-cT transitions exhibit significant hysteresis effects and we demonstrate that coexistence of the O and cT phases can occur if a non-hydrostatic component of pressure is present.
    03/2009;
  • G. D. Samolyuk
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    ABSTRACT: The density-functional spin susceptibility has been analyzed in different phases of CaFe2As2 and compared with similar data for pure d metals. The conditions for the “no local-moment” itinerant state with large frustrations are found for the “collapsed” phase. This itineracy determines the instability versus the incommensurate magnetic order for the narrow region of wave vectors. For the ambient pressure phase, the local moments on Fe atoms with much less frustrated antiferromagnetic interactions are stabilized and a magnetic short-range or long-range order is developed. The system is close to the point of magnetic instability and spin fluctuations should be included to describe properties of this system.
    Phys. Rev. B. 02/2009; 79(5).
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    ABSTRACT: We determine the anisotropy of the spin-fluctuation-induced pairing gap on the Fermi surface of the FeAs-based superconductors as function of the exchange and Hund’s coupling JH . We find that for sufficiently large JH , nearly commensurate magnetic fluctuations yield a fully gapped s± -pairing state with small anisotropy of the gap amplitude on each Fermi-surface sheet, but significant variations of the gap amplitude for different sheets of the Fermi surface. In particular, we obtain the large variation of the gap amplitude on different Fermi-surface sheets, as seen in angular resolved photoemission spectroscopy experiments. For smaller values of Hund’s coupling incommensurate magnetic fluctuations yield an s± -pairing state with line nodes. Such a state is also possible once the anisotropy of the material is reduced and three-dimensional effects come into play.
    Physical Review B 02/2009; · 3.66 Impact Factor
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    ABSTRACT: Anisotropies of electrical resistivity, upper critical field, London penetration depth, and critical currents have been measured in single crystals of the optimally doped iron pnictide superconductor Ba(Fe(1-x)Co(x))(2)As(2) (x=0.074 and T(c)similar to 23 K). The normal-state resistivity anisotropy was obtained by employing both the Montgomery technique and direct measurements on samples cut along principal crystallographic directions. The ratio gamma(rho)=rho(c)/rho(a) is about 4 +/- 1 just above T(c) and becomes half of that at room temperature. The anisotropy of the upper critical field, gamma(H)=H(c2,ab)/H(c2,c), as determined from specific-heat measurements close to T(c) is in the range of 2.1-2.6, depending on the criterion used. A comparable low anisotropy of the London penetration depth, gamma(lambda)=lambda(c)/lambda(ab), was recorded from tunnel diode resonator measurements and found to persist deep into the superconducting state. An anisotropy of comparable magnitude was also found in the critical currents, gamma(j)=j(c,ab)/j(c,c), as determined from both direct transport measurements (similar to 1.5) and from the analysis of the magnetization data (similar to 3). Overall, our results show that iron pnictide superconductors manifest anisotropies consistent with essentially three-dimensional intermetallic compounds and bear little resemblance to cuprates.
    Physical review. B, Condensed matter 01/2009; 79(9):094507. · 3.77 Impact Factor
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    ABSTRACT: The anisotropy of electrical resistivity was measured in parent compounds of the iron-arsenic high-temperature superconductors AFe2As2 with alkali earth elements A=Ca , Sr, and Ba. Measurements were performed using both the Montgomery technique and direct resistivity measurements on samples cut along principal crystallographic directions. The anisotropy ratio gammarho=rhoc/rhoa is well below ten for all compounds in the whole temperature range studied (4-300 K), in notable contrast to previous reports. The anisotropy at room temperature increases from about two in Ca to about four in Sr and Ba. In all compounds the resistivity ratio decreases on cooling through the structural/antiferromagnetic transition temperature TSM , with the change mainly coming from stronger variation in rhoa as compared with rhoc . This suggests that the transition affects stronger the two-dimensional parts of the Fermi surface. We compare our experimental observations with band-structure calculations, and find similar trend in the evolution of anisotropy with the size of A ion. Our results show that the electronic structure of the iron pnictides has large contribution from three-dimensional areas of the Fermi surface.
    Physical review. B, Condensed matter 01/2009; 79. · 3.77 Impact Factor
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    ABSTRACT: a b s t r a c t We present an overview of the electronic properties of iron arsenic high temperature superconductors with emphasis on low energy band dispersion, Fermi surface and superconducting gap. ARPES data is compared with full-potential linearized plane wave (FLAPW) calculations. We focus on single layer NdFe-AsO 0.9 F 0.1 (R1111) and two layer Ba 1Àx K x Fe 2 As 2 (B122) compounds. We find general similarities between experimental data and calculations in terms of character of Fermi surface pockets, and overall band dis-persion. We also find a number of differences in details of the shape and size of the Fermi surfaces as well as the exact energy location of the bands, which indicate that magnetic interaction and ordering signif-icantly affects the electronic properties of these materials. The Fermi surface consists of several hole pockets centered at C and electron pockets located in zone corners. The size and shape of the Fermi sur-face changes significantly with doping. Emergence of a coherent peak below the critical temperature T c and diminished spectral weight at the chemical potential above T c closely resembles the spectral charac-teristics of the cuprates, however the nodeless superconducting gap clearly excludes the possibility of d-wave order parameter. Instead it points to s-wave or extended s-wave symmetry of the order parameter.
    Physica C Superconductivity 01/2009; 60. · 0.72 Impact Factor
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    ABSTRACT: Single crystal neutron and high-energy x-ray diffraction have identified the phase lines corresponding to transitions between the ambient-pressure tetragonal (T), the antiferromagnetic orthorhombic (O) and the non-magnetic collapsed tetragonal (cT) phases of CaFe2As2. We find no evidence of additional structures for pressures up to 2.5 GPa (at 300 K). Both the T-cT and O-cT transitions exhibit significant hysteresis effects and we demonstrate that coexistence of the O and cT phases can occur if a non-hydrostatic component of pressure is present. Measurements of the magnetic diffraction peaks show no change in the magnetic structure or ordered moment as a function of pressure in the O phase and we find no evidence of magnetic ordering in the cT phase. Band structure calculations show that the transition results in a strong decrease of the iron 3d density of states at the Fermi energy, consistent with a loss of the magnetic moment.
    Physical review. B, Condensed matter 12/2008; · 3.77 Impact Factor
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    ABSTRACT: Anisotropies of electrical resistivity, upper critical field, London penetration depth and critical currents have been measured in single crystals of the optimally doped iron pnictide superconductor Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$, $x$=0.074 and $T_c \sim$23 K. The normal state resistivity anisotropy was obtained by employing both the Montgomery technique and direct measurements on samples cut along principal crystallographic directions. The ratio $\gamma_{\rho} = \rho_c /\rho_a$ is about 4$\pm$1 just above $T_c$ and becomes half of that at room temperature. The anisotropy of the upper critical field, $\gamma_{H} = H_{c2,ab} /H_{c2,c} $, as determined from specific heat measurements close to $T_c$, is in the range of 2.1 to 2.6, depending on the criterion used. A comparable low anisotropy of the London penetration depth, $\gamma_{\lambda}=\lambda_{c}/\lambda_{ab}$, was recorded from TDR measurements and found to persist deep into the superconducting state. An anisotropy of comparable magnitude was also found in the critical currents, $\gamma_j=j_{c,ab}/j_{c,c}$, as determined from both direct transport measurements ($\sim$1.5) and from the analysis of the magnetization data ($\sim$3). Overall, our results show that iron pnictide superconductors manifest anisotropies consistent with essentially three-dimensional intermetallic compound and bear little resemblance to cuprates.
    12/2008;
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    ABSTRACT: Inelastic neutron scattering measurements of the magnetic excitations in CaFe2As2 indicate that the spin wave velocity in the Fe layers is exceptionally large and similar in magnitude to the cuprates. However, the spin wave velocity perpendicular to the layers is at least half as large that in the layer, so that the magnetism is more appropriately categorized as anisotropic three-dimensional, in contrast to the two-dimensional cuprates. Exchange constants derived from band structure calculations predict spin wave velocities that are consistent with the experimental data.
    Physical Review Letters 12/2008; 101(22):227205. · 7.73 Impact Factor

Publication Stats

839 Citations
154.16 Total Impact Points

Institutions

  • 2003–2012
    • Iowa State University
      • • Department of Physics and Astronomy
      • • Department of Chemistry
      • • Ames Laboratory
      Ames, IA, United States
  • 2005
    • Arizona State University
      • Department of Chemical Engineering
      Mesa, AZ, United States
  • 1998
    • Kurchatov Institute
      Moskva, Moscow, Russia