Elastic constant measurements on iron pnictide Ba(Fe 0:9 Co 0:1 ) 2 As 2 with an optimal superconducting transition temperature of T SC ¼ 23 K have been performed by the ultrasonic pulse echo method. The shear elastic constant of C 66 associated with elastic strain " xy reveals considerable softening of 21% below 300 K down to T SC and becomes increasing in the superconducting phase below T SC , while other shear elastic constants of ðC 11 À C 12 Þ=2 and C 44 show no sign of softening below 80 K down to 4.2 K. The electric quadrupole O v 0 existing in the degenerate d y 0 z and d zx 0 bands participates in the superconductivity of the present iron pnictide system. KEYWORDS: layered iron pnictide, superconductor, Ba(Fe 0:9 Co 0:1 ) 2 As 2 , elastic softeningThe discovery of the superconductor LaFeAs(O 1Àx F x ) with a high transition temperature of T SC ¼ 26 K by Hosono's group triggered thousands of research works pursuing FeAs-based superconductors with various chemical composites and different structures.1) Five crystal structures with a common iron-based layer structure are now known.2)The high transition temperatures of up to 55 K and high critical magnetic fields beyond 50 T have already been realized.3,4) Among these FeAs-based crystals, the tetragonal compound of Ba(Fe 1Àx Co x ) 2 As 2 , in particular, has received much attention, because large single crystals are available for the investigation of physical properties depending on crystal anisotropy. 5)The end-material BaFe 2 As 2 exhibits a simultaneous transition of structural transition from a tetragonal TrCr 2 Si 2 -type structure of space group D orbitals by Co 2þ with 3d 7 orbitals in Ba(Fe 1Àx Co x ) 2 As 2 reduces both the structural transition temperature of T s and the antiferromagnetic one of T N .8) The superconductivity emerges in the systems above x ¼ 0:03 at the temperatures of T SC below the structural point T s and the antiferromagnetic one T N . The quantum critical point, x QCP , where both structural and antiferromagnetic phases die out, exists in the proximity of x ¼ 0:07. The optimum superconducting temperatures of T SC ¼ 23 K are achieved with x ¼ 0:10, which is a slightly excess of the quantum critical concentration x QCP . The superconductivity vanishes in the highly doped region of x ¼ 0:17.The Fermi surfaces of iron pnictides arise from Fe-3d orbitals and have three hole bands at a À-point and two electron bands at an X-point. These bands consist primarily of d y 0 z and d zx 0 with À 5 under tetragonal symmetry and d x 02 Ày 02 with À 3 .9) The increasing of the Fermi level upon electron doping due to the substitution of Fe with Co reduces the three sheets of hole bands around the À point in the endmaterial BaFe 2 As 2 to two sheets of hole bands in the present Ba(Fe 0:9 Co 0:1 ) 2 As 2 . We adopt the electric quadrupole associated with the degenerate d y 0 z and d zx 0 orbital states to describe elastic properties of the compound Ba(Fe 1Àx Co x ) 2 -As 2 . It is noted that the coordinates x 0 and y 0 of the quadrupole are ori...

Results of transport, magnetic, thermal, and 75 As-NMR measurements are presented for superconducting Sr2VFeAsO3 with an alternating stack of FeAs and perovskite-like block layers. Although apparent anomalies in magnetic and thermal properties have been observed at ∼150 K, no anomaly in transport behaviors has been observed at around the same temperature. These results indicate that V ions in the Sr2VO3-block layers have localized magnetic moments and that V-electrons do not contribute to the Fermi surface. The electronic characteristics of Sr2VFeAsO3 are considered to be common to those of other superconducting systems with Fe-pnictogen layers. 2−4) This implies that Sr 2 VFeAsO 3 may give us an opportunity to examine whether Fermi-surface nesting is important for the occurrence of superconductivity of Fe pnictides. On this point, Mazin has reported that, because the FSs constructed by only the Fe orbitals of Sr 2 VFeAsO 3 are similar to those expected in other Fe pnictides, the nesting condition is also satisfied in the present system. 5) However, it seems important to experimentally ensure whether or not the electrons of V ions are itinerant and really contributing to FSs, before studying the relation between Fermi-surface nesting and superconductivity. KEYWORDSWe have carried out transport, magnetic, thermal, and 75 As-NMR measurements, and found that although apparent anomalies in the temperature (T ) dependences of the magnetic and thermal properties of Sr 2 VFeAsO 3 exist at ∼150 K, no anomalies in the transport properties have been observed. On the basis of these results, we argue the electronic state of Sr 2 VFeAsO 3 and answer the question regarding the contribution of V ions to FSs.Polycrystalline samples of Sr 2 VFeAsO 3 were prepared as described in refs. 1 and 6. SrAs powder was first obtained by annealing mixtures of Sr and As in an evacuated quartz tube at 850• C. Mixtures with proper ratios of SrAs, FeAs, SrO, Fe, and V 2 O 3 were pressed into pellets, sealed in an evacuated quartz tube with Ti powder (Ti/Fe = 1/4), which probably acts as the reducing agent, and then fired for 10 h at 900• C and for 30 h 1050• C, successively. The X-ray powder diffraction pattern of one of the * corresponding author(i45323a@cc.nagoya-u.ac.jp) obtained pellets with CuKα radiation at a step of 0.01• of the scattering angle 2θ is shown in Fig.1 (a), where the reflection indices are attached to the corresponding peaks of Sr 2 VFeAsO 3 , and the asterisks indicate the peaks from impurity phases of Sr 2 VO 4 or Sr 3 V 2 O 7 .7,8) The lattice parameters a and c were estimated to be 3.9329(1) and 15.6703(18)Å, respectively.The magnetic moments M were measured at magnetic fields H of 10 Oe and 1 T using a Quantum Design MPMS. The electrical resistivity ρ was measured by the four-terminal method with increasing temperature T at H = 0. The thermoelectric power S was measured by a dc method, where the typical temperature range between two ends of the sample was 0.2−2 K, depending on the temperature region. Details of the me...

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Magnetic excitation spectra of Ba(Fe 0.9 Co 0.1 ) 2 As 2 obtained for aligned crystals with the superconducting transition temperature T c 23 K are presented after an overview of results of our preceding studies on the symmetry of the order parameter . Although many researchers have reported that their experimental results are consistently explained by the so-called S symmetry, and believe that the spin-fluctuation is relevant to the superconducting mechanism of Fe pnictide superconductors. However, the situation is not so simple, because almost all these data can be understood by the S ++ -symmetry, too. Furthermore, as we first pointed out and discussed in detail, the rates of the T c suppression by nonmagnetic impurities can hardly be explained by the S symmetry. Here, we show our own data of the magnetic excitation spectra of Ba(Fe 0.9 Co 0.1 ) 2 As 2 , and discuss which one of the S and S ++ -symmetries can better describe the characteristics of the peak observed there. These processes are important, because the S and S ++ symmetries are connected with the spin-fluctuation mechanism and a newly proposed one related to the orbital degrees of freedom of the systems, respectively.

A low-power, low-noise, and high-bandwidth mag-1 netometer that utilizes the magnetoimpedance (MI) element as a 2 sensor head is presented. The MI element has a high sensitivity, 3 and it can be implemented in the mm-scale through the MEMS 4 process. The analog front-end (AFE) circuit of the magnetometer 5 includes a digital calibration scheme that automatically enhances 6 the loop gain of the system, resulting in high bandwidth and 7 low-noise characteristics. The AFE circuit is designed based on 8 a switched-capacitor (SC) approach, and its dedicated switching 9 scheme can suppress the folded noise of an amplifier. A single-10 coil magnetic negative feedback architecture with correlated 11 double sampling (CDS) enables to achieve a high dynamic range 12 (DR) and stable passband gain in addition to simplifying the 13 structure of the MI element. The AFE chip of the magnetometer 14 is implemented in a 0.18-µm CMOS process, and it achieves an 15 8-pT/ √ Hz noise floor within a 31-kHz bandwidth and the DR of 16 96 dB, where the power consumption is 1.97 mW. 17 Index Terms-Analog front-end (AFE), biomagnetic, digital 18 calibration, Internet of Things (IoT), magnetic feedback, magne-19 toimpedance (MI) element, magnetometer. 20 I. INTRODUCTION 21 A BIOMAGNETIC sensing technique such as magneto-22 myography (MMG) or magnetoencephalography (MEG) 23 is one solution for capturing biological information with a min-24 imum invasive approach. Implantable MMG has the potential 25 to acquire fast neuronal magnetic activity, which corresponds 26 to the action potential of neurons close to skeletal muscle 27 with high spatiotemporal resolution [1], [2], [3], as opposed 28 to an approach with an optically pumped magnetometer that 29 achieves low noise but a relatively large size because of 30 the optical system [4]. Magnetometers for such applications 31 require of low noise less than 100 pT/ √ Hz, high bandwidth 32 over 10 kHz, low power, and small size because they are 33 implanted. Furthermore, a wide input range over 100 μT is 34 desired because there is a need to accept the geomagnetic field 35 and artifact without saturating the signal. A magnetic negative 36