arXiv subjects

A. Iyo

Publications and source records attributed to A. Iyo.

At least 55 records · Page 3Linked to original sources

arXiv2012

Universal heat conduction in the iron-arsenide superconductor KFe2As2 : Evidence of a d-wave state

The thermal conductivity of the iron-arsenide superconductor KFe2As2 was measured down to 50 mK for a heat current parallel and perpendicular to the tetragonal c-axis. A residual linear term (RLT) at T=0 is observed for both current directions, confirming the presence of nodes in the superconducting gap. Our value of the RLT in the plane is equal to that reported by Dong et al. [Phys. Rev. Lett. 104, 087005 (2010)] for a sample whose residual resistivity was ten times larger. This independence of the RLT on impurity scattering is the signature of universal heat transport, a property of superconducting states with symmetry-imposed line nodes. This argues against an s-wave state with accidental nodes. It favors instead a d-wave state, an assignment consistent with five additional properties: the magnitude of the critical scattering rate for suppressing Tc to zero; the magnitude of the RLT, and its dependence on current direction and on magnetic field; the temperature dependence of the thermal conductivity.

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arXiv2012

Anomalous two peak structure in the Angle-resolved Photoemission Spectra of Ba1-xKxFe2As2

The electronic structure near the Fermi level (EF) of Ba1-xKxFe2As2 (BaK122 ; x = 0.2 - 0.7) is studied using laser ultrahigh-resolution angle-resolved photoemission spectroscopy(ARPES). For the optimally doped case of x = 0.4, we clearly observe two peaks below Tc in the ARPES spectra at a binding energies (BE) of 5 meV and 13meV. The former is assigned to a superconducting (SC) coherence peak since it appears and evolves below the bulk SC transition at Tc (= 36 K), accompanying a gap opening centered at EF. In contrast, the latter peak, which appears below ~ 90 K without any gap formation, is interpreted to be not directly related to a SC coherence peak. This high-BE peak is observed from x = 0.2 to 0.6, reduces in energy with overdoping (x > 0.4) and is absent for x = 0.7. The temperature(T)- and doping-dependent ARPES results suggest that the high-BE peak originates from coupling to a bosonic mode of energy ~ 8 meV.

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arXiv2012

Orbital-dependent electron correlation effect on the two- and three-dimensional Fermi surfaces in KFe2As2 revealed by angle-resolved photoemission spectroscopy

We have performed an angle-resolved photoemission study of the hole-overdoped iron pnictide superconductor KFe2As2, which shows a low Tc of ~4 K. Most of the observed Fermi surfaces show nearly two-dimensional shapes, while a band near the Fermi level shows a strong dispersion along the kz direction and forms a small three-dimensional hole pocket centered at the Z point, as predicted by band-structure calculation. However, hole Fermi surfaces of yz and zx orbital character centered at the Γpoint of the two-dimensional Brillouin zone are smaller than those predicted by the calculation while the other hole Fermi surfaces of xy orbital character is much larger. Clover-shaped hole Fermi surfaces around the corner of the 2D BZ are also larger than those predicted by the calculation. These observations are consistent with the de Haas-van Alphen measurement and indicate orbital-dependent electron correlation effects. The effective masses of the energy bands show moderate to strong enhancement, partly due to electron correlation and partly due to energy shifts from the calculated band structure.

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arXiv2012

Superconducting Fluctuations and Anomalous Phonon Renormalization much above superconducting transition temperature in Ca4Al2O5.7Fe2As2

Raman studies on Ca4Al2O5.7Fe2As2 superconductor in the temperature range of 5 K to 300 K, covering the superconducting transition temperature Tc ~ 28.3 K, reveal that the Raman mode at ~ 230 cm-1 shows a sharp jump in frequency by ~ 2 % and linewidth increases by ~ 175 % at To ~ 60 K. Below To, anomalous softening of the mode frequency and a large decrease by ~ 10 cm-1 in the linewidth is observed. These precursor effects at T0 (~ 2Tc) are attributed to significant superconducting fluctuations, possibly enhanced due to reduced dimensionality arising from weaked coupling between the well separated (~ 15 Å) Fe-As layers in the unit cell. A large blue-shift of the mode frequency between 300 K to 60 K (~7%) indicates strong spin-phonon coupling in this superconductor.

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arXiv2012

Anisotropic Energy Gaps of Iron-based Superconductivity from Intra-band Quasiparticle Interference in LiFeAs

If strong electron-electron interactions between neighboring Fe atoms mediate the Cooper pairing in iron-pnictide superconductors, then specific and distinct anisotropic superconducting energy gaps Δ_i(k) should appear on the different electronic bands i. Here we introduce intra-band Bogoliubov quasiparticle scattering interference (QPI) techniques for determination of Δ_i(k) in such materials, focusing on LiFeAs. We identify the three hole-like bands assigned previously as γ, α_2 and α_1, and we determine the anisotropy, magnitude and relative orientations of their Δ_i(k). These measurements will advance quantitative theoretical analysis of the mechanism of Cooper pairing in iron-based superconductivity.

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arXiv2012

Abrupt change in the energy gap of superconducting Ba1-xKxFe2As2 single crystals with hole doping

We performed a Laser angle-resolved photoemission spectroscopy (ARPES) study on a wide doping range of Ba1-xKxFe2As2 (BaK) and precisely determined the doping evolution of the superconducting (SC) gaps in this compound. The gap size of the outer hole Fermi surface (FS) sheet around the Brillioun zone (BZ) center shows an abrupt drop with overdoping (for x > 0.6) while the inner and middle FS gaps roughly scale with Tc. This is accompanied by the simultaneous disappearance of the electron FS sheet with similar orbital character at the BZ corner. These results browse the different contributions of X2-Y2 and XZ/YZ orbitals to superconductivity in BaK and can be hardly completely reproduced by the available theories on iron-based superconductors.

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arXiv2012

High-Tc Superconductivity and Antiferromagnetism in Multilayered Copper Oxides - A New Paradigm of Superconducting Mechanism -

High-temperature superconductivity (HTSC) in copper oxides emerges on a layered CuO2 plane when an antiferromagnetic Mott insulator is doped with mobile hole carriers. We review extensive studies of multilayered copper oxides by site-selective nuclear magnetic resonance (NMR), which have uncovered the intrinsic phase diagram of antiferromagnetism (AFM) and HTSC for a disorder-free CuO2 plane with hole carriers. We present our experimental findings such as the existence of the AFM metallic state in doped Mott insulators, the uniformly mixed phase of AFM and HTSC, and the emergence of d-wave SC with a maximum Tc just outside a critical carrier density, at which the AFM moment on a CuO2 plane disappears. These results can be accounted for by the Mott physics based on the t-J model. The superexchange interaction J_in among spins plays a vital role as a glue for Cooper pairs or mobile spin-singlet pairs, in contrast to the phonon-mediated attractive interaction among electrons established in the Bardeen-Cooper-Schrieffer (BCS) theory. We remark that the attractive interaction for raising the $T_c$ of HTSC up to temperatures as high as 160 K is the large J_in (~0.12 eV), which binds electrons of opposite spins to be on neighboring sites, and that there are no bosonic glues. It is the Coulomb repulsive interaction U(> 6 eV) among Cu-3d electrons that plays a central role in the physics behind high-Tc phenomena. A new paradigm of the SC mechanism opens to strongly correlated electron matter.

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arXiv2011

Electronic reconstruction through the structural and magnetic transitions in detwinned NaFeAs

We use angle-resolved photoemission spectroscopy to study twinned and detwinned iron pnictide compound NaFeAs. Distinct signatures of electronic reconstruction are observed to occur at the structural (TS) and magnetic (TSDW) transitions. At TS, C4 rotational symmetry is broken in the form of an anisotropic shift of the orthogonal dxz and dyz bands. The magnitude of this orbital anisotropy rapidly develops to near completion upon approaching TSDW, at which temperature band folding occurs via the antiferromagnetic ordering wave vector. Interestingly, the anisotropic band shift onsetting at TS develops in such a way to enhance the nesting conditions in the C2 symmetric state, hence is intimately correlated with the long range collinear AFM order. Furthermore, the similar behaviors of the electronic reconstruction in NaFeAs and Ba(Fe1-xCox)2As2 suggests that this rapid development of large orbital anisotropy between TS and TSDW is likely a general feature of the electronic nematic phase in the iron pnictides, and the associated orbital fluctuations may play an important role in determining the ground state properties.

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arXiv2011

Manifestations of multiple-carrier charge transport in the magnetostructurally ordered phase of undoped BaFe$_2$As$_2$

We investigated the transport properties of BaFe$_2$As$_2$ single crystals before and after annealing with BaAs powder. The annealing remarkably improves transport properties, in particular the magnitude of residual resistivity which decreases by a factor of more than 10. From the resistivity measurement on detwinned crystals we found that the anisotropy of the in-plane resistivity is remarkably diminished after annealing, indicative of dominant contributions to the charge transport from the carriers with isotropic and high mobility below magnetostructural transition temperature $T_{\rm s}$ and the absence of nematic state above $T_{\rm s}$. We found that the Hall resistivity shows strong non-linearity against magnetic field and the magnetoresistance becomes very large at low temperatures. These results give evidence for the manifestation of multiple carriers with distinct characters in the ordered phase below $T_{\rm s}$. By analyzing the magnetic field dependences, we found that at least three carriers equally contribute to the charge transport in the ordered phase, which is in good agreement with the results of recent quantum oscillation measurements.

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arXiv2011

Soft and Isotropic Phonons in PrFeAsO_{1-y}

Phonons in single crystals of PrFeAsO_{1-y} are investigated using high-resolution inelastic x-ray scattering and ab initio pseudopotential calculations. Extensive measurements of several samples at temperatures spanning the magnetic ordering temperature and the superconducting transition temperature show that there are some changes in phonon spectra with temperature and/or doping. We compare our measurements with several ab initio pseudopotential models (nonmagnetic tetragonal, oxygen-deficient O_{7/8} supercell, magnetic orthorhombic, and magnetic tetragonal) and find that the experimentally observed changes are much smaller than the differences between the experimental data and the calculations. Agreement is improved if magnetism is included in the calculations via the local spin density approximation, as the Fe atomic motions parallel to the ferromagnetic ordering direction are softened. However, the antiferromagnetically polarized modes remain hard, and in disagreement with the experimental data. In fact, given the increasing evidence for anisotropy in the iron pnictide materials, the phonon response is surprisingly isotropic. We consider several modifications of the ab initio calculations to improve the agreement with the experimental data. Improved agreement is found by setting the matrix to zero (clipping the bond) between nearest-neighbor antiferromagnetically aligned Fe atoms in the magnetic calculation, or by softening only the in-plane nearest-neighbor Fe-As force constant in the nonmagnetic calculation. We discuss these results in the context of other measurements, especially of phonons, for several FeAs systems. Fluctuating magnetism may be a partial explanation for the failure of the calculations, but seems incomplete in the face of the similarity of the measured phonon response in all the systems investigated here including those known to have static magnetism.

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arXiv2011

Unprecedented anisotropic metallic state in BaFe2As2 revealed by optical spectroscopy

An ordered phase showing remarkable electronic anisotropy in proximity to the superconducting phase is now a hot issue in the field of high-transition-temperature superconductivity. As in the case of copper oxides, superconductivity in iron arsenides competes or coexists with such an ordered phase. Undoped and underdoped iron arsenides have a magnetostructural ordered phase exhibiting stripe-like antiferromagnetic spin order accompanied by an orthorhombic lattice distortion; both the spin order and lattice distortion break the tetragonal symmetry of crystals of these compounds. In this ordered state, anisotropy of in-plane electrical resistivity is anomalous and difficult to attribute simply to the spin order and/or the lattice distortion. Here, we present the anisotropic optical spectra measured on detwinned BaFe2As2 crystals with light polarization parallel to the Fe planes. Pronounced anisotropy is observed in the spectra, persisting up to an unexpectedly high photon energy of about 2 eV. Such anisotropy arises from an anisotropic energy gap opening below and slightly above the onset of the order. Detailed analysis of the optical spectra reveals an unprecedented electronic state in the ordered phase.

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arXiv2011

Antiferromagnetic Spin Fluctuations and Unconventional Nodeless Superconductivity in an Iron-based New Superconductor (Ca_4Al_2O_{6-y})(Fe_2As_2):75As-NQR Study

We report 75As-nuclear quadrupole resonance (NQR) studies on (Ca_4Al_2O_{6-y})(Fe_2As_2) with Tc=27K, which unravel unique normal-state properties and point to unconventional nodeless superconductivity (SC). Measurement of nuclear-spin-relaxation rate 1/T_1 has revealed a significant development of two dimensional (2D) antiferromagnetic (AFM) spin fluctuations down to Tc, in association with the fact that FeAs layers with the smallest As-Fe-As bond angle are well separated by thick perovskite-type blocking layer. Below Tc, the temperature dependence of 1/T_1 without any trace of the coherence peak is well accounted for by an s(+-)-wave multiple gaps model. From the fact that Tc=27K in this compound is comparable to Tc=28K in the optimally-doped LaFeAsO_{1-y} in which AFM spin fluctuations are not dominant, we remark that AFM spin fluctuations are not a unique factor for enhancing Tc among existing Fe-based superconductors, but a condition for optimizing SC should be addressed from the lattice structure point of view.

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arXiv2011

High-T_c Superconductivity with T_c = 52 K under Antiferromagnetic Order in Five-layered Cuprate Ba_2Ca_4Cu_5O_10(F,O)_2 with T_N = 175 K: 19F- and Cu-NMR Studies

We report on the observation of high-T_c superconductivity (SC) emerging with the background of an antiferromagnetic (AFM) order in the five-layered cuprate Ba_2Ca_4Cu_5O_10(F,O)_2 through 19F-NMR and zero-field Cu-NMR studies. The measurements of spectrum and nuclear spin-lattice relaxation rates 19(1/T_1) of 19F-NMR give convincing evidence for the AFM order taking place below T_N = 175 K and for the onset of SC below T_c = 52 K, hence both coexisting. The zero-field Cu-NMR study has revealed that AFM moments at Cu sites are 0.14 mu_B at outer CuO_2 layers and 0.20 mu_B at inner ones. We remark that an intimate coupling exists between the AFM state and the SC order parameter below T_c = 52 K; the spin alignment in the AFM state is presumably changed in the SC-AFM mixed state.

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arXiv2011

Angle-resolved photoemission spectroscopy study of PrFeAsO_{0.7}: Dependence of the electronic structure on the pnictogen height

We have performed an angle-resolved photoemission spectroscopy (ARPES) study of the iron-based superconductor PrFeAsO_{0.7} and examined the Fermi surfaces and band dispersions near the Fermi level. Heavily hole-doped electronic states have been observed due to the polar nature of the cleaved surfaces. Nevertheless, we have found that the ARPES spectra basically agree with band dispersions calculated in the local density approximation (LDA) if the bandwidth is reduced by a factor of ~2.5 and then the chemical potential is lowered by ~70 meV. Comparison with previous ARPES results on LaFePO reveals that the energy positions of the d_{3z^2-r^2}- and d_{yz,zx}-derived bands are considerably different between the two materials, which we attribute to the different pnictogen height as predicted by the LDA calculation.

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arXiv2011

Incommensurate spin fluctuations in hole-overdoped superconductor KFe2As2

A neutron scattering study of heavily hole-overdoped superconducting KFe$_2$As$_2$ revealed a well-defined low-energy incommensurate spin fluctuation at [$π(1\pm2δ$),0] with $δ$ = 0.16. The incommensurate structure differs from the previously observed commensurate peaks in electron-doped $A$Fe$_2$As$_2$ ($A$ = Ba, Ca, or Sr) at low energies. The direction of the peak splitting is perpendicular to that observed in Fe(Te,Se) or in Ba(Fe,Co)$_2$As$_2$ at high energies. A band structure calculation suggests interband scattering between bands around the $Γ$ and X points as an origin of this incommensurate peak. The perpendicular direction of the peak splitting can be understood within the framework of multiorbital band structure. The results suggest that spin fluctuation is more robust in hole-doped than in electron-doped samples, which can be responsible for the appearance of superconductivity in the heavily hole-doped samples.

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arXiv2010

Strong spin resonance on BaFe2(As0.65P0.35)2 with Tc = 30 K

We performed inelastic neutron scattering on powder sample of the P-doped iron-based superconductor BaFe2(As0.65P0.35)2 with Tc = 30K, whose superconducting (SC) order parameter is expected to have line nodes. We have observed spin resonance at Q $\sim$ 1.2Å^{-1} and E=12 meV in the SC state. The resonance enhancement, which can be a measure of the area of sign reversal between the hole and electron Fermi surfaces (FSs), is comparable to those of other iron-based superconductors without line nodes. This fact indicates that the sign reversal between the FSs is still dominant in this system, and the line nodes should create only limited area of sign-reversal on a single FS. Hence the system can hold relatively high-Tc. Comparison with theoretical calculation indicates horizontal line nodes may be a candidate to reproduce the observation.

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arXiv2010

Quasi-Two-Dimensional Fermi Surfaces and Coherent Interlayer Transport in KFe$_2$As$_2$

We report the results of the angular-dependent magnetoresistance oscillations (AMROs), which can determine the shape of bulk Fermi surfaces in quasi-two-dimensional (Q2D) systems, in a highly hole-doped Fe-based superconductor KFe$_2$As$_2$ with $T_c \approx$ 3.7 K. From the AMROs, we determined the two Q2D FSs with rounded-square cross sections, corresponding to 12% and 17% of the first Brillouin zone. The rounded-squared shape of the FS cross section is also confirmed by the analyses of the interlayer transport under in-plane fields. From the obtained FS shape, we infer the character of the 3d orbitals that contribute to the FSs.

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arXiv2010

The pairing state in KFe2As2 studied by measurements of the magnetic vortex lattice

Understanding the mechanism and symmetry of electron pairing in iron-based superconductors represents an important challenge in condensed matter physics [1-3]. The observation of magnetic flux lines - "vortices" - in a superconductor can contribute to this issue, because the spatial variation of magnetic field reflects the pairing. Unlike many other iron pnictides, our KFe2As2 crystals have very weak vortex pinning, allowing small-angle-neutron-scattering (SANS) observations of the intrinsic vortex lattice (VL). We observe nearly isotropic hexagonal packing of vortices, without VL-symmetry transitions up to high fields along the fourfold c-axis of the crystals, indicating rather small anisotropy of the superconducting properties around this axis. This rules out gap nodes parallel to the c-axis, and thus d-wave and also anisotropic s-wave pairing [2, 3]. The strong temperature-dependence of the intensity down to T<<Tc indicates either widely different full gaps on different Fermi surface sheets, or nodal lines perpendicular to the axis.

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