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ABSTRACT: The optical properties of thin films of the heavy-fermion compound CeCoIn5,
which were deposited by molecular beam epitaxy onto MgF2 substrates, have been
studied at frequencies 7 to 45 cm^{-1} (corresponding to 0.2 to 1.3 THz) and
temperatures 2 to 300 K. We observe an electrodynamic behavior which is typical
for heavy fermions, namely Drude-like conductivity with a relaxation rate at
rather low frequencies. This relaxation rate increases almost linearly with
temperature up to at least 30 K. The coherent heavy-fermion state,
characterized by an increase of the effective mass, continuously evolves upon
cooling and is not fully developed for temperatures as low as 5 K.
04/2013;
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ABSTRACT: Charge doping of iron-pnictide superconductors leads to collective pinning of flux vortices, whereas isovalent doping does not. Moreover, flux pinning in the charge-doped compounds is consistently described by the mean-free path fluctuations introduced by the dopant atoms, allowing for the extraction of the elastic quasiparticle scattering rate. The absence of scattering by dopant atoms in isovalently doped BaFe2(As(1-x)P(x))(2) is consistent with the observation of a linear temperature dependence of the low-temperature penetration depth in this material.
Physical Review Letters 12/2010; 105(26):267002. · 7.37 Impact Factor
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ABSTRACT: Measurements of the superconducting transition temperature, T_c, under hydrostatic pressure via bulk AC susceptibility were carried out on several concentrations of phosphorous substitution in BaFe_2(As_{1-x}P_x)_2. The pressure dependence of unsubstituted BaFe_2As_2, phosphorous concentration dependence of BaFe_2(As_{1-x}P_x)_2, as well as the pressure dependence of BaFe_2(As_{1-x}P_x)_2 all point towards an identical maximum T_c of 31 K. This demonstrates that phosphorous substitution and physical pressure result in similar superconducting phase diagrams, and that phosphorous substitution does not induce substantial impurity scattering. Comment: 5 pages, 4 figures, to be published in Journal of the Physical Society of Japan
09/2010;
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ABSTRACT: Charge doping of iron-pnictide superconductors leads to collective pinning of flux vortices, whereas isovalent doping does not. Moreover, flux pinning in the charge-doped compounds is consistently described by the mean-free path fluctuations introduced by the dopant atoms, allowing for the extraction of the elastic quasiparticle scattering rate. The absence of scattering by dopant atoms in isovalently doped BaFe$_{2}$(As$_{1-x}$P$_{x}$)$_{2}$ is consistent with the observation of a linear temperature dependence of the low-temperature penetration depth in this material. Comment: 4 pages, submitted to Phys. Rev. Lett
05/2010;
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ABSTRACT: 31P NMR was performed on the normal state in the newly discovered iron pnictide superconductor BaFe2(As0.67P0.33)2 with maximum Tc of 30K. The Knight shift is temperature-independent while the nuclear spin–lattice relaxation rate divided by temperature (T1T)-1 exhibits a more than twofold increase on cooling. This contrasting behavior clearly indicates the development of low-lying antiferromagnetic fluctuations in BaFe2(As0.67P0.33)2 as observed in (Ba1−xKx)Fe2As2. The observed low-lying antiferromagnetic fluctuations may be universal in optimally doped “122” superconductors, in contrast to “1111” superconductors.
Physica C-superconductivity and Its Applications - PHYSICA C. 01/2010; 470.
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ABSTRACT: ^{31}P and ^{75}As NMR measurements were performed in superconducting BaFe_2(As_{0.67}P_{0.33})_2 with T_c = 30 K. The nuclear-spin-lattice relaxation rate T_1^{-1} and the Knight shift in the normal state indicate the development of antiferromagnetic fluctuations, and T_1^{-1} in the superconducting (SC) state decreases without a coherence peak just below T_c, as observed in (Ba_{1-x}K_{x})Fe_2As_2. In contrast to other iron arsenide superconductors, the T_1^{-1} \propto T behavior is observed below 4K, indicating the presence of a residual density of states at zero energy. Our results suggest that strikingly different SC gaps appear in BaFe_2(As_{1-x}P_{x})_2 despite a comparable T_c value, an analogous phase diagram, and similar Fermi surfaces to (Ba_{1-x}K_{x})Fe_2As_2. Comment: 4 pages, 5 figures
08/2009;
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ABSTRACT: The transport critical current density Jc of single-crystalline thin films of YBa2Cu3O7-delta was measured as a function of temperature T and magnetic field H applied at varied angles to the crystal axes. Below a characteristic temperature T* which is in the range of 20 K ~ 45 K, the observed value of Jc does not depend on the field component H ab-plane. Furthermore, at T < T* the angular dependence of Jc can be described by the scaling relation of the two-dimentional model.
Physica B Condensed Matter 01/1994; 194:1613-1614. · 1.06 Impact Factor
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ABSTRACT: The transport critical current density Jc of YBa2Cu3O7-delta single-crystalline thin films in the three-dimensional temperature region was measured in magnetic fields H up to 12 T applied at varied angles to the crystals axes. The observed angular dependence of Jc(alpha) and Jc(theta) could be described by the Lorentz-force-dependent critical state model by considering the anisotropy of the volume pinning force and the upper critical field. Here, alpha and theta are the angles between H and transport current I in the ab-plane and in the plane comprising H, I and the c-axis, respectively. Anisotropic pinning mechanisms were discussed based on the intrinsic pinning model proposing step-like flux lines. Jc(theta) had different values from Jc(phi) at theta=phi, where phi is an angle between H and the CuO2 layer under the conditions of I⊥H. We found that the difference between Jc(theta) and Jc(phi) comes from the different contribution of the Lorentz-force to the flux line segments parallel to the CuO2 layers.
Physica C-superconductivity and Its Applications - PHYSICA C. 01/1993; 204:305-314.
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ABSTRACT: Resistice transitions in the presence of magnetic field have been studied on ultrathin YBCO films and YBCO/PrBCO superlattices. We found that: i)The lower critical magnetic field in parallel coniguration (H||ab) is about 10 tesla for 5 unit cell thickness YBCO film, ii)When field is applied perpendicular to ab-plane, correlation length along the flux line is about 10 unit cell length.
Physica C-superconductivity and Its Applications - PHYSICA C. 01/1991; 185:2055-2056.
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ABSTRACT: Static and dynamic magnetic properties of lightly P-substituted BaFe2(As1−xPx)2 were systematically investigated by 31P NMR. The averaged internal magnetic field at the P site in the zero-temperature limit evaluated from the broadening of NMR spectra in the antiferromagnetic (AFM) phase is gradually suppressed toward x∼0.35 with increasing x, which provides definitive evidence for the existence of an AFM quantum critical point (QCP) at x∼0.35. The location of the AFM QCP is consistent with the previous estimation from temperature dependence of spin dynamics in the normal state, and the superconducting transition temperature Tc takes the maximum around the QCP. Our experiments, revealing a signature of a QCP extending up to room temperature, establish BaFe2(As1−xPx)2 as one of the most accessible systems for unraveling the nature of quantum criticality and the relationship between AFM quantum criticality and unconventional superconductivity.
Phys. Rev. B. 85(18).
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ABSTRACT: 2次元空間に「最も重い電子」を実現. 京都大学プレスリリース. 2010-02-19. http://www.kyoto-u.ac.jp/ja/news_data/h/h1/news6/2009/100219_1.htm Condensed-matter systems that are both low-dimensional and strongly interacting often exhibit unusual electronic properties. Strongly correlated electrons with greatly enhanced effective mass are present in heavy fermion compounds, whose electronic structure is essentially three-dimensional. We realized experimentally a two-dimensional heavy fermion system, adjusting the dimensionality in a controllable fashion. Artificial superlattices of the antiferromagnetic heavy fermion compound CeIn3 and the conventional metal LaIn3 were grown epitaxially. By reducing the thickness of the CeIn3 layers, the magnetic order was suppressed and the effective electron mass was further enhanced. Heavy fermions confined to two dimensions display striking deviations from the standard Fermi liquid low-temperature electronic properties, and these are associated with the dimensional tuning of quantum criticality.
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ABSTRACT: 31P and 75As NMR measurements were performed in superconducting BaFe2(As0.67P0.33)2 with Tc=30 K. The nuclear-spin-lattice relaxation rate T 1−1 and the Knight shift in the normal state indicate the development of antiferromagnetic fluctuations, and T 1−1 in the superconducting (SC) state decreases without a coherence peak just below Tc, as observed in (Ba1−xKx)Fe2As2. In contrast to other iron arsenide superconductors, the T 1−1∝T behavior is observed below 4 K, indicating the presence of a residual density of states at zero energy. Our results suggest that strikingly different SC gaps appear in BaFe2(As1−xPx)2 despite a comparable Tc value, an analogous phase diagram, and similar Fermi surfaces to (Ba1−xKx)Fe2As2.
