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A. Kreyssig,
M. A. Green,
Y Lee, G. D. Samolyuk,
P. Zajdel,
J. W. Lynn,
S. L. Bud'ko,
M. S. Torikachvili,
N. Ni,
S Nandi,
J. Leao,
S. J. Poulton,
D. N. Argyriou,
B. N. Harmon,
P. C. Canfield,
R. J. McQueeney,
A. I. Goldman
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ABSTRACT: Recent investigations of the superconducting iron-arsenide families have highlighted the role of pressure, be it chemical or mechanical, in fostering superconductivity. Here we report that CaFe2As2 undergoes a pressure-induced transition to a non-magnetic, volume "collapsed" tetragonal phase, which becomes superconducting at lower temperature. Spin-polarized total-energy calculations on the collapsed structure reveal that the magnetic Fe moment itself collapses, consistent with the absence of magnetic order in neutron diffraction.
Physical review. B, Condensed matter 02/2013; 78:184517.
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Chang Liu,
Takeshi Kondo,
Ni Ni,
A D Palczewski,
A Bostwick, G D Samolyuk,
R Khasanov,
M Shi,
E Rotenberg,
S L Bud'ko,
P C Canfield,
A Kaminski
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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. · 7.37 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 γρ=ρc/ρa 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 ρa as compared with ρc. 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.
Phys. Rev. B. 04/2009; 79(13).
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R T Gordon,
N Ni,
C Martin,
M A Tanatar,
M D Vannette,
H Kim, G D Samolyuk,
J Schmalian,
S Nandi,
A Kreyssig,
A I Goldman,
J Q Yan,
S L Bud'ko,
P C Canfield,
R Prozorov
[show abstract]
[hide abstract]
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. · 7.37 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(Fe1−xCox)2As2 (x=0.074 and Tc∼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 γρ=ρc/ρa is about 4±1 just above Tc and becomes half of that at room temperature. The anisotropy of the upper critical field, γH=Hc2,ab/Hc2,c, as determined from specific-heat measurements close to Tc is in the range of 2.1–2.6, depending on the criterion used. A comparable low anisotropy of the London penetration depth, γλ=λc/λab, 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, γj=jc,ab/jc,c, as determined from both direct transport measurements (∼1.5) and from the analysis of the magnetization data (∼3). Overall, our results show that iron pnictide superconductors manifest anisotropies consistent with essentially three-dimensional intermetallic compounds and bear little resemblance to cuprates.
Phys. Rev. B. 03/2009; 79(9).
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[show abstract]
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ABSTRACT: The anisotropy of electrical resistivity was measured in parent compounds of the iron-arsenic high temperature superconductors, AEFe2As2 with Alkali Earth elements AE=Ca,Sr, Ba. Measurements were performed using both the Montgomery technique and direct resistivity measurements on samples cut along principal crystallographic directions. The anisotropy ratio \gamma_\rho=\rho_c/\rho_a is well below 10 for all compounds in the whole temperature range studied (4 to 300 K), in notable contrast to previous reports. The anisotropy at room temperature increases from about 2 in Ca, to about 4 in Sr and Ba. In all compounds the resistivity ratio decreases on cooling through the structural/antiferromagnetic transition temperature T_{SM}, with the change mainly coming from stronger variation in \rho_a as compared with \rho_c. This suggests that the transitions stronger affect 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 AE ion. Our results show that the electronic structure of the iron pnictides has large contribution from three-dimensional areas of the Fermi surface.
03/2009;
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[show abstract]
[hide abstract]
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.
01/2009;
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R. T. Gordon,
N. Ni,
C. Martin,
M. A. Tanatar,
M. D. Vannette,
H. Kim, G. D. Samolyuk,
J. Schmalian,
S. Nandi,
A. Kreyssig,
A. I. Goldman,
J. Q. Yan,
S. L. Bud'ko,
P. C. Canfield,
R. Prozorov
[show abstract]
[hide abstract]
ABSTRACT: The London penetration depth lambda(T) has been measured in single crystals of Ba(Fe(0.93)Co(0.07))(2)As(2). The observed low-temperature variation of lambda(T) follows a power law, Delta lambda(T)similar to T(n) with n approximate to 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 NdFeAsO(1-x)F(x) and SmFeAsO(1-x)F(x), which indicate an anisotropic but nodeless gap. We discuss possible explanations of the observed power law behavior.
Phys. Rev. Lett. 01/2009; 102(12):127004.
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[show abstract]
[hide abstract]
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.
Phys. Rev. B. 01/2009; 79(9):094507.
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R J McQueeney,
S O Diallo,
V P Antropov, G D Samolyuk,
C Broholm,
N Ni,
S Nandi,
M Yethiraj,
J L Zarestky,
J J Pulikkotil,
A Kreyssig,
M D Lumsden,
B N Harmon,
P C Canfield,
A I Goldman
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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. · 7.37 Impact Factor
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Chang Liu, G D Samolyuk,
Y Lee,
Ni Ni,
Takeshi Kondo,
A F Santander-Syro,
S L Bud'ko,
J L McChesney,
E Rotenberg,
T Valla,
A V Fedorov,
P C Canfield,
B N Harmon,
A Kaminski
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[hide abstract]
ABSTRACT: We use angle-resolved photoemission spectroscopy to investigate the electronic properties of the newly discovered iron-arsenic superconductor Ba_(1-x)K_(x)Fe_(2)As_(2) and nonsuperconducting BaFe_(2)As_(2). Our study indicates that the Fermi surface of the undoped, parent compound BaFe_(2)As_(2) consists of hole pocket(s) at Gamma (0,0) and larger electron pocket(s) at X (1,0), in general agreement with full-potential linearized plane wave calculations. Upon doping with potassium, the hole pocket expands and the electron pocket becomes smaller with its bottom approaching the chemical potential. Such an evolution of the Fermi surface is consistent with hole doping within a rigid-band shift model. Our results also indicate that the full-potential linearized plane wave calculation is a reasonable approach for modeling the electronic properties of both undoped and K-doped iron arsenites.
Physical Review Letters 11/2008; 101(17):177005. · 7.37 Impact Factor
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ABSTRACT: The density functional non-interacting 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 (corresponding to superconducting 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 or long range order for all wave vectors is developed.
10/2008;
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C Martin,
R. T. Gordon,
M. A. Tanatar,
M. D. Vannette,
M. E. Tillman,
E. D. Mun,
P. C. Canfield,
V. G. Kogan, G. D. Samolyuk,
J. Schmalian,
R. Prozorov
[show abstract]
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ABSTRACT: This paper is no longer active and will NOT appear in print. For new data and analysis, please see: arXiv:0903.2220
08/2008;
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C Liu,
T Kondo,
M. E. Tillman,
R. Gordon, G. D. Samolyuk,
Y Lee,
C Martin,
J. L. McChesney,
S. Bud'ko,
M. A. Tanatar,
E. Rotenberg,
P. C. Canfield,
R. Prozorov,
B. N. Harmon,
A. Kaminski
[show abstract]
[hide abstract]
ABSTRACT: We use angle-resolved photoemission spectroscopy (ARPES) to investigate the electronic properties of the newly discovered oxypnictide superconductor, NdFeAsO_{1-x}F_x. We find a well-defined Fermi surface that consists of a large hole pocket at the Brillouin zone center and a smaller electron pocket in each corner of the Brillouin zone. The overall location and shape of the Fermi surface agrees reasonably well with calculations. The band dispersion is quite complicated with many flat bands located just below the chemical potential. We observe a superconducting gap of 20 meV, which indicates that this system is in the strong coupling regime. The emergence of a coherent peak below the critical temperature Tc and diminished spectral weight at the chemical potential above Tc closely resembles the spectral characteristics of the cuprates.
07/2008;
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C Liu, G. D. Samolyuk,
Y Lee,
N. Ni,
T Kondo,
A. F. Santander-Syro,
S. L. Bud'ko,
J. L. McChesney,
E. Rotenberg,
T. Valla,
A. V. Fedorov,
P. C. Canfield,
B. N. Harmon,
A. Kaminski
[show abstract]
[hide abstract]
ABSTRACT: We use angle-resolved photoemission spectroscopy (ARPES) to investigate the electronic properties of the newly discovered iron-arsenic superconductor, Ba(1-x)K(x)Fe2As2 and non-supercondcuting BaFe2As2. Our study indicates that the Fermi surface of the undoped, parent compound BaFe$_2$As$_2$ consists of hole pocket(s) at Gamma (0,0) and larger electron pocket(s) at X (1,0), in general agreement with full-potential linearized plane wave (FLAPW) calculations. Upon doping with potassium, the hole pocket expands and the electron pocket becomes smaller with its bottom approaching the chemical potential. Such an evolution of the Fermi surface is consistent with hole doping within a rigid band shift model. Our results also indicate that FLAPW calculation is a reasonable approach for modeling the electronic properties of both undoped and K-doped iron arsenites. Comment: 4 pages, 3 figures
06/2008;
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ABSTRACT: Data on temperature-dependent, anisotropic thermal expansion in pure and doped RAgSb(2) (R = Y, Sm, La) single crystals are presented. Using the Ehrenfest relation and heat capacity measurements, uniaxial pressure derivatives for long range magnetic ordering and charge density wave transition temperatures are evaluated and compared with the results of the direct measurements under hydrostatic pressure. In-plane and c-axis pressure have opposite effects on the phase transitions in these materials, with in-plane effects being significantly weaker. Quantum oscillations in magnetostriction were observed for the three pure compounds, with the possible detection of new frequencies in SmAgSb(2) and LaAgSb(2). The uniaxial (along the c-axis) pressure derivatives of the dominant extreme orbits (β) were evaluated for YAgSb(2) and LaAgSb(2).
Journal of Physics Condensed Matter 03/2008; 20(11):115210. · 2.55 Impact Factor
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ABSTRACT: Magnetization, resistivity and specific heat measurements were performed on the solution-grown, single crystals of six GdT$_2$Zn$_{20}$ (T = Fe, Ru, Os, Co, Rh and Ir) compounds, as well as their Y analogues. For the Gd compounds, the Fe column members manifest a ferromagnetic (FM) ground state (with an enhanced Curie temperature, $T_{\mathrm{C}}$, for T = Fe and Ru), whereas the Co column members manifest an antiferromagnetic (AFM) ground state. Thermodynamic measurements on the YT$_2$Zn$_{20}$ revealed that the enhanced $T_{\mathrm{C}}$ for GdFe$_2$Zn$_{20}$ and GdRu$_2$Zn$_{20}$ can be understood within the framework of Heisenberg moments embedded in a nearly ferromagnetic Fermi liquid. Furthermore, electronic structure calculations indicate that this significant enhancement is due to large, close to the Stoner FM criterion, transition metal partial density of states at Fermi level, whereas the change of FM to AFM ordering is associated with filling of electronic states with two additional electrons per formula unit. The degree of this sensitivity is addressed by the studies of the pseudo-ternary compounds Gd(Fe$_x$Co$_{1-x}$)$_2$Zn$_{20}$ and Y(Fe$_x$Co$_{1-x}$)$_2$Zn$_{20}$ which clearly reveal the effect of 3d band filling on their magnetic properties. Comment: 32 pages, 28 figures
12/2007;
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[show abstract]
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ABSTRACT: Data on temperature-dependent, anisotropic thermal expansion in pure and doped RAgSb2 (R = Y, Sm, La) single crystals are presented. Using the Ehrenfest relation and heat capacity measurements, uniaxial pressure derivatives for long range magnetic ordering and charge density wave transition temperatures are evaluated and compared with the results of the direct measurements under hydrostatic pressure. In-plane and c-axis pressure have opposite effect on the phase transitions in these materials, with in-plane effects being significantly weaker. Quantum oscillations in magnetostriction were observed for the three pure compounds, with the possible detection of new frequencies in SmAgSb2 and LaAgSb2. The uniaxial (along the c-axis) pressure derivatives of the dominant extreme orbits (beta) were evaluated for YAgSb2 and LaAgSb2.
11/2007;
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ABSTRACT: Shubnikov-de Haas oscillations were measured in single crystals of highly metallic antiferromagnetic SmAgSb2 and ferromagnetic CeAgSb2 using a tunnel diode resonator. Resistivity oscillations as a function of applied magnetic field were observed via measurements of skin depth variation. The effective resolution of Delta rho similar or equal to 20 p Omega allows a detailed study of the SdH spectra as a function of temperature. The effects of the Sm long-range magnetic ordering as well as its electronic structure (4f electrons) on the Fermi surface topology is discussed.
Phys. Rev. B. 01/2007; 75(1):014413.
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ABSTRACT: We present anisotropic, zero applied magnetic field, temperature dependent resistivity measurements on hexagonal, non-magnetic, YPtIn and LuAgGe single crystals. For these materials the in-plane resistivity, $\rho_{ab}$, is significantly higher than the $c$ - axis one, $\rho_c$, with $\rho_{ab}/\rho_c \approx 1.4$ for YPtIn and $\approx 4.2 - 4.7$ for LuAgGe. The connection between the electronic structure and the anisotropic transport properties is discussed using density functional calculations that link the observed anisotropy with a specific shape of Fermi surface and anisotropy of the Fermi velocities.
08/2005;
