arXiv subjects

K. Iida

Publications and source records attributed to K. Iida.

62 records · Page 4Linked to original sources

arXiv2010

Scaling behaviour of the critical current in clean epitaxial Ba(Fe1-xCox)2As2 thin films

The angular-dependent critical current density, Jc(theta), and the upper critical field, Hc2(theta), of epitaxial Ba(Fe1-xCox)2As2 thin films have been investigated. No Jc(theta) peaks for H || c were observed regardless of temperatures and magnetic fields. In contrast, Jc(theta) showed a broad maximum at theta=90 degree, which arises from intrinsic pinning. All data except at theta=90 degree can be scaled by the Blatter plot. Hc2(theta) near Tc follows the anisotropic Ginzburg-Landau expression. The mass anisotropy increased from 1.5 to 2 with increasing temperature, which is an evidence for multi-band superconductivity.

cond-mat.supr-con↗

arXiv2009

Direct observation of a nodeless superconducting energy gap in the optical conductivity of iron-pnictides

The temperature-dependent optical reflectivity and complex transmissivity of an epitaxially grown Ba(Fe$_{0.9}$Co$_{0.1}$)$_2$As$_2$ thin film were measured and the optical conductivity and permittivity evaluated over a wide frequency range. The opening of the superconducting gap $2Δ_0 = 3.7$ meV below $T_c\approx 20$ K is {\em directly} observed by a completely vanishing optical conductivity. The temperature and frequency dependent electrodynamic properties of Ba(Fe$_{0.9}$Co$_{0.1}$)$_2$As$_2$ in the superconducting state agree well with the BCS predictions with no nodes in the order parameter. The spectral weight of the condensate $1.94\times 10^7 {\rm cm}^{-2}$ corresponds to a London penetration depth $λ_L=3600$ Å.

cond-mat.supr-con↗

arXiv2009

Strong Tc dependence for strained, epitaxial Ba(Fe1-xCox)2As2 thin films

Ba(Fe1-xCox)2As2 superconducting thin films have been grown on SrTiO3, (La,Sr)(Al,Ta)O3, LaAlO3 and YAlO3 (YAO) single crystal substrates by pulsed laser deposition. All the films, except on YAO, have been grown epitaxially without buffer layers. The films deposited on YAO contained 45 degree in-plane rotated grains and showed a broad superconducting transition. The onset Tc of the films is observed to increase from 16.2 K to 24.5 K with increasing c/a, mainly due to a slight distortion of the AsFe4 tetrahedron. From this correlation, we expect that higher superconducting transition temperatures than 24.5 K in a strained epitaxial film may be possible.

cond-mat.supr-con↗

arXiv2009

Anisotropic itinerant magnetism and spin fluctuations in BaFe2As2: A neutron scattering study

Neutron scattering measurements were performed to investigate magnetic excitations in a single-crystal sample of the ternary iron arsenide BaFe2As2, a parent compound of a recently discovered family of Fe-based superconductors. In the ordered state, we observe low energy spin-wave excitations with a gap energy of 9.8(4) meV. The in-plane spin-wave velocity v_ab and out-of-plane spin-wave velocity v_c measured at 12 meV are 280(150) and 57(7) meV A, respectively. At high energy, we observe anisotropic scattering centered at the antiferromagnetic wave vectors. This scattering indicates two-dimensional spin dynamics, which possibly exist inside the Stoner continuum. At T_N=136(1) K, the gap closes, and quasi-elastic scattering is observed above T_N, indicative of short-range spin fluctuations. In the paramagnetic state, the scattering intensity along the L direction becomes "rodlike," characteristic of uncorrelated out-of-plane spins, attesting to the two-dimensionality of the system.

cond-mat.str-el↗

arXiv2004

Thermal Phase Transitions and Gapless Quark Spectra in Quark Matter at High Density

Thermal color superconducting phase transitions in three-flavor quark matter at high baryon density are investigated in the Ginzburg-Landau (GL) approach. We constructed the GL potential near the boundary with a normal phase by taking into account nonzero quark masses, electric charge neutrality, and color charge neutrality. We found that the density of states averaged over paired quarks plays a crucial role in determining the phases near the boundary. By performing a weak coupling calculation of the parameters characterizing the GL potential terms of second order in the pairing gap, we show that three successive second-order phase transitions take place as the temperature increases: a modified color-flavor locked phase (ud, ds, and us pairings) -> a ``dSC'' phase (ud and ds pairings) -> an isoscalar pairing phase (ud pairing) -> a normal phase (no pairing). The Meissner masses of the gluons and the number of gapless quark modes are also studied analytically in each of these phases.

hep-ph↗

arXiv2004

Melting Pattern of Diquark Condensates in Quark Matter

Thermal color superconducting phase transitions in high density three-flavor quark matter are investigated in the Ginzburg-Landau approach. Effects of nonzero strange quark mass, electric and color charge neutrality, and direct instantons are considered. Weak coupling calculations show that an interplay between the mass and electric neutrality effects near the critical temperature gives rise to three successive second-order phase transitions as the temperature increases: a modified color-flavor locked (mCFL) phase (ud, ds, and us pairings) -> a ``dSC'' phase (ud and ds pairings) -> an isoscalar pairing phase (ud pairing) -> a normal phase (no pairing). The dSC phase is novel in the sense that while all eight gluons are massive as in the mCFL phase, three out of nine quark quasiparticles are gapless.

hep-ph↗

arXiv1999

Optical spectra measured on cleaved surfaces of double-exchange ferromagnet La1-xSrxMnO3

Optical reflectivity spectra were measured on cleaved surfaces of La1-xSrxMnO3 single crystals (x=0.175, T_C=283 K) over a temperature range 10-295 K. The optical conductivity $sigma(ω)$ shows, keeping single-component nature, incoherent-to-coherent crossover with increase of electrical conductivity. The $σ(ω)$ spectrum of low-temperature ferromagnetic-metallic phase (10 K) exhibits a pronounced Drude-like component with large spectral weight, contrary to the previous result. The present result indicates that the optical spectrum of the manganites is sensitive to condition of sample surfaces.

cond-mat.str-el↗

arXiv1998

Precise Measurement of Optical Reflectivity Spectra Using Defect-Free Surface of La(1-x)Sr(x)MnO(3): Evidence Against Extremely Small Drude Weight

Optical reflectivity spectra of La(1-x)Sr(x)MnO(3) (0<=x<=0.30) were measured on cleavage surfaces of single crystals. The optical conductivity $σ(w) of ferromagnetic-metal La(0.70)Sr(0.30)MnO(3) is characterized by a Drude-like - but not simple Drude - component with large spectral weight below 1.6 eV, which yields a large effective carrier number N*_eff consistent with the results of Hall coefficient and specific heat measurements. The present result demonstrates that the previous result of "small Drude weight" originates from the damage of the sample surface. The Sr-substitution effect on the electronic structure and the origin of the unconventional $σ(w) are also discussed.

cond-mat.str-el↗