Takehiro Yamazaki

75As NMR Study of Hole-Doped Superconductor Ba1-xKxFe2As2 (Tc≃38 K)

We report the 75 As nuclear magnetic resonance (NMR) measurement of the hole-doped superconductor Ba 1- x K x Fe 2 As 2 with different lattice parameters and different superconducting volume fractions ( T c ≃38 K). 75 As-NMR spectra revealed that the magnetically ordered and superconducting phases are microscopically separated. The spin-lattice relaxation rate 1/ T 1 in the normal state reflects the existence of a large two-dimensional antiferromagnetic spin fluctuation. The 1/ T 1 in the superconducting state down to the lowest measurement temperature T varies close to T 3 . In addition, it exhibits no coherence peak just below T c . This shows a T dependence similar to those of other iron pnictides.

Appearance of pressure-induced superconductivity in BaFe2As2 under hydrostatic conditions and its extremely high sensitivity to uniaxial stress

We have performed electrical resistivity measurements on single crystal BaFe2As2 under high pressure P up to 16 GPa with a cubic anvil apparatus, and up to 3 GPa with a modified Bridgman anvil cell. The samples were obtained from the same batch, which was grown with a self-flux method. A cubic anvil apparatus provides highly hydrostatic pressure, and a modified Bridgman anvil cell, which contains liquid pressure transmitting medium, provides quasi hydrostatic pressure. For highly hydrostatic pressure, the crystal phase and magnetic transition temperature decreases robustly with P and disappears at around 10 GPa. The superconducting phase appears adjacent to magnetic phase in narrow pressure region between 11 and 14 GPa. The tiny difference of hydrostaticity between the cubic anvil apparatus and modified Bridgman anvil cell induces a drastic effect on the phase diagram of BaFe2As2. This result indicates that small uniaxial stress along c-axis strongly suppresses the structural/antiferromagnetic ordering and stabilizes superconductivity at much lower pressure.

Suppression of Magnetic Order by Pressure in BaFe2As2

We performed the dc resistivity and the ZF 75As-NMR measurement of BaFe2As2 under high pressure. The T-P phase diagram of BaFe2As2 determined from resistivity anomalies and the ZF 75As-NMR clearly revealed that the SDW anomaly is quite robust against P.

75As NMR Study of the Ternary Iron Arsenide BaFe2As2

We report 75As-NMR measurements of the ternary iron arsenide BaFe2As2. 75As-NMR spectra clearly revealed that magnetic transition occurs at around 131 K in our samples, which corresponds to the emergence of spin density wave. Temperature dependence of the internal magnetic field suggests that the transition is likely of the first order. The critical-slowing-down phenomenon in the spin-lattice relaxation rate 1/T1 is not pronounced in this material.

Evaluation of Pressure Transmitting Media by 63Cu-NQR of Cu2O

High pressure technique up to about 3GPa is now widely used in studies of condensed matter physics. This is due to the development of hybrid cylinder clamp cells, especially CuBe/NiCrAl and CuBe/MP35N. Further high pressures can be achieved by Bridgman anvil cell (up to about 10GPa), cubic anvil cell (up to about 20GPa) and diamond anvil cell (up to about 400GPa), and so on. Optical measurements and electric resistivity measurement with the use of the diamond anvil cell are now conventional methods for high pressure measurements beyond about 100GPa. However, other major measurements such as neutron diffraction, nuclear magnetic resonance (NMR), nuclear quadrupole resonance (NQR), and muon spin relaxation ( SR) have technical difficulty since large sample space is essentially needed for these methods. Of these methods, NMR/NQR under high pressure is relatively accessible. We indeed reported Cu-NQR measurements of Cu2O up to about 10GPa by using a modified Bridgman anvil cell which is designed for general-purpose pressure cell. In our previous NQR measurements of Cu2O, the spectral broadening was observed and was nearly proportional to pressure P. This was ascribable to the existence of inhomogeneous pressure distribution inside the cell and/or local shear of the crystalline lattice, induced by the uniaxial stress of the inhomogeneous pressure. A reason of the pressure inhomogeneity except for the problem, originating from the Bridgman anvil cell itself, is that the pressure transmitting medium is already solid at high pressures. Generally, the solid pressure transmitting media make the pressure distribution inside the cell inhomogeneous. However, it is difficult to avoid solidification of the pressure media at high pressures. Hence, it is vital to choose the most appreciate medium for high pressure measurements. The NMR/NQR technique has an advantage to investigate the suitability of the pressure media because the pressure inhomogeneity inside pressure cell is strongly related to the broadening of NMR/NQR spectra. In this paper, we report Cu-NQR measurement of Cu2O with the currently representative liquid pressure-transmitting media. As the first step of the investigation, we demonstrated the NQR experiments under pressure up to about 3GPa at low temperature and room temperature. There are some reports on the liquid pressure-transmitting media, but few appeared in refereed journals. Moreover, to the best of our knowledge, there is no report of the evaluation and the comparison of the pressure media from the viewpoint of NMR/NQR technique. Therefore, we evaluated the pressure quality of the representative pressure-transmitting media by NQR. The Cu-NQR measurements of Cu2O were performed using a phase-coherent pulsed NQR spectrometer in the resonance frequency range 26–28MHz. We used commercial Cu2O powder (99.9%). We performed the NQR measurements at 4.2K and room temperature (about 300K). The obtained spin echo data were Fourier transformed to NQR spectra. In most cases, the line shape at a pressure was obtained at an excitation resonance frequency. We used conventional Cu-Be/NiCrAl hybrid piston cylinder cell. Single-layer coil wound about 20 turns was used in our measurements ( 1⁄4 3mm, l 1⁄4 5mm). The diameter of the initial sample space was 4.5mm and its length was 18mm. We used the following four representative liquid pressure-transmitting media: a 1 : 1 mixture of Fluorinert 70 and 77 (Fluorinert), Daphne 7373 oil (Daphne), a 1 : 1 mixture of n-pentane and isopentane (Pentane), and glycerin (Glycerin). We did not try a mixture of methanol and ethanol since this mixture sometimes dissolves the epoxy which is used for the pressure seal of the piston cylinder cell. Note that the configuration of the sample and the sample coil except for the pressure media was always the same inside the cell in order to compare the contribution of only the pressure media to the NQR spectra. The frequency f dependence of Cu-NQR resonance frequency of Cu2O up to about 9GPa can be used for a pressure manometer. In our previous NQR measurements of Cu2O, we revealed that the calibration formula reported by Reyes et al. is basically consistent with our extended calibration formula. Because our calibration formula was obtained only at low temperature, we used the calibration formula reported by Reyes et al. in this study, which covers the temperature range between 4.2 and 350K and the pressure range up to about 2GPa. We determined the pressure beyond 2GPa by the extrapolation of this formula. We clarified its appropriateness in our previous study. The calibration formulae are f ðP; 4:2KÞ 1⁄4 26:82þ 0:402P 1:496 10 P and f ðP; 300KÞ 1⁄4 25:99þ 0:355P 6:738 10 P. Here, f and P are in MHz and in GPa, respectively. With these formulae and the spectral center f of NQR of Cu2O, we derived pressure inside the cell. In Fig. 1, we show the P dependence of the full width at half maximum (FWHM) of Cu-NQR spectra of Cu2O with the various pressure transmitting media, Fluorinert, Daphne, Pentane, and Glycerin. The data are obtained at room temperature [about 300K, Fig. 1(a)] and 4.2K [Fig. 1(b)]. The accuracy of the data is within 5%. In the inset of Fig. 1(b), we show the Cu-NQR spectra of Cu2O obtained at room temperature under the pressures of 2.15GPa (Fluorinert) and 2.29GPa (Pentane). The most obvious feature of the results is the P dependence of the FWHM of the Fluorinert. The FWHM of the Fluorinert linearly increases with increasing P. This is clear in the inset of Fig. 1(b), which shows that the line shape of the Fluorinert is much broader than that of the E-mail: hideto@nmr.s.chiba-u.ac.jp Journal of the Physical Society of Japan Vol. 76, No. 12, December, 2007, 125001 #2007 The Physical Society of Japan

Application of Nuclear Quadrupole Resonance with Mini Cubic Anvil Apparatus

A wide variety of experimental methods under high pressure are now applied to various materials. For nuclear magnetic resonance (NMR) and nuclear quadrupole resonance (NQR) methods, high pressure measurements up to about 3.5GPa are performed conventionally because of the development of hybrid cylinder clamp cells. In this pressure range, we have examined the suitable pressure transmitting media, which can give more hydrostatic pressure to sample, by comparing the Cu-NQR spectral width of Cu2O and its pressure efficiency between room temperature and 4.2K. Further high pressures for the NMR/NQR methods can be achieved by an indenter-type clamp cell (up to about 5GPa), a modified Bridgman anvil cell (up to about 10GPa), and so on. However, these higher pressure measurements are not general at present stage because of their smaller sample space than that of the piston cylinder cell. Typically the diameter and the height of the sample space is ’ 1:7mm and h ’ 1:4mm for the indenter-type clamp cell and ’ 1:0mm and h ’ 1:0mm for the modified Bridgman anvil cell, while that for the piston cylinder cell is ’ 4:0mm and h ’ 20mm or larger. Moreover, it is quite difficult to obtain hydrostatic pressure above about 5GPa. This is because the pressure by the Bridgman anvil at higher pressures has a tendency of uniaxial stress to sample space. Cubic anvil apparatus has an advantage in this point since the cubic anvil apparatus can induce pressure nearly isotropically to sample space. It can also achieve high pressure up to about 20GPa. The line width of Cu-NQR of Cu2O is a good measure of pressure homogeneity since the pressure homogeneity inside pressure cell is strongly related to the broadening of NMR/NQR spectra. Hence, we performed Cu-NQR measurements of Cu2O up to about 7.5GPa with mini cubic anvil apparatus in order to obtain more hydrostatic pressure under 10GPa-class high pressure. Note that the sample space itself is comparable to that of the indenter-type cell and about four times larger than that of the modified Bridgman anvil cell; ’ 2:0mm diameter and h ’ 1:0mm height. This larger sample enables us to obtain strong enough intensity even at room temperature. To the best of our knowledge, this is the first report on the NMR/NQR measurement with the use of the cubic anvil apparatus. The Cu-NQR measurements of Cu2O were performed using a phase-coherent pulsed NQR spectrometer in the resonance frequency range 26–29MHz. We used Cu2O powder (99.9%) and glycerin mixed with the ratio of 20 : 1. Inside the sample coil, this mixture was set up. We consider that a small amount of the glycerin does not behave as pressure transmitting medium. We performed the NQR measurements at room temperature (about 300K). The obtained spin echo data were Fourier transformed to NQR spectra. Below about 3GPa, the line shape at a pressure was obtained at an excitation resonance frequency because the line shape was narrow enough to detect by measuring NQR pulse width. We used mini cubic anvil apparatus (Fig. 1) which is recently developed by Takeshita et al. We use nonmagnetic WC anvil top at present stage, but we can change other nonmagnetic and insulating anvil tops. Its overall volume including of cryostat is about 30 times smaller than that of conventional cubic anvil apparatus for low temperature measurements. Furthermore, the weight of its guide block is approximately 3 kg, which is about 7 times lighter than that of conventional one. However, its sample space is the same as that of the conventional one. The present maximum pressure of this apparatus is about 10GPa with the use of MgO gasket, but we used pyrophyllite gasket in this study, with which the maximum pressure of the apparatus is about 7.5GPa, in order to ensure reproducibility. Single-layer coil wound about 20 turns was used in our measurements ( 1⁄4 1:5mm, h 1⁄4 0:7mm). We used a 1 : 1 mixture of Fluorinert 70 and 77 (Fluorinert). In our previous Cu-NQR measurements of Cu2O with hybrid piston cylinder cell, the pressure homogeneity using Fluorinert medium was the worst among the available pressure media above 1GPa. We also used Fluorinert with the modified Bridgman anvil cell where the large pressure inhomogeneity exists. In this study, we examine the pressure homogeneity of the mini cubic anvil apparatus and compare the pressure homogeneity with various apparatus using the same pressure transmitting medium. We may expect the drastic improvement of the homogeneity with the cubic anvil apparatus even using the Fluorinert. Fig. 1. (Color online) A mini cubic apparatus and its guide block on the palm.

Superconductivity of NdFeAsO1-y under Hydrostatic Pressure

We briefly report pressure dependence of electrical resistivity in fluorine-free, oxygen- deficient, NdFeAsO 1- y (1- y = 0.85, 0.8, and 0.6, nominal compositions) under hydrostatic high pressure up to 18 GPa. Superconducting temperatures, T C decrease in every composition by applying pressure. In NdFeAsO 0.85 , which shows no superconductivity at ambient pressure, steep drop of resistivity appears at around 20 K above 5 GPa as resistivity anomaly at around 150 K is suppressed by applying pressure.

75As-NMR study of the iron pnictide Ba1?xKxFe2As2 under high pressure

We performed resistivity and zero-external-field (ZF) 75As nuclear magnetic resonance (NMR) measurements of Ba1?xKxFe2As2 (x = 0 and 0.4) under high pressure. ZF 75As NMR measurements revealed that the magnetic ordered state is robust against pressure. On the other hand, superconducting transition temperature Tc has a tendency to decrease monotonically under pressure. The spin-lattice relaxation rate 1/T1 in antiferromagnetic and superconducting states suggests that the coexistence of the both states is not microscopic but is phase separated.

Pressure and K doping induced superconductivity in BaFe2As2

We have measured electrical resistivity under pressure P and 75As NMR on single crystal and poly crystalline BaFe2As2 and (Ba1−xKx)Fe2As2 to elucidate mechanism of antiferromagnetic/superconducting phase transition in BaFe2As2. This system is an antiferromagnet, AF, with a transition temperature of 141 K, which shows superconductivity by applying P or by a carrier doping of K for Ba. From high P resistivity measurements, we understood that the transition was highly sensitive to the P homogeneity. Small uniaxial stress strongly suppresses AF order and stabilizes superconductivity at much lower pressure. 75As NMR study in (Ba1−xKx)Fe2As2 shows existence of large AF spin fluctuations in tetragonal phase having a superconducting ground state. The AF spin fluctuations decrease gradually with increasing K concentration.

High-sensitive optical measurement of spin polarization in a quantum Hall system

Spin polarization measurement is important for the study of a variety of spin states in quantum Hall system. Kerr rotation is proportional to spin polarization, so we developed a high sensitive measurement of Kerr rotation by using homodyne detection and a variety of modulation techniques. Furthermore, we developed Kerr rotation spectra measurement system base on the multi-channel homodyne detection, which enables the assignment of the optical transitions and semi-quantitative estimation of spin polarization by integrating the spectra over the specific optical transition. The spin polarization presents a rapid spin depolarization on both sides of ? = 1 due to Skyrmionic excitation. However the top of spin polarization presents a narrow flat region. Furthermore the spin polarization around ? = 3 also shows a rapid spin depolarization which suggest the existence of Skyrmion at higher odd filling factor.