Taku J. Sato

Pressure-induced superconductivity in the iron-based ladder material BaFe2S3.

All the iron-based superconductors identified so far share a square lattice composed of Fe atoms as a common feature, despite having different crystal structures. In copper-based materials, the superconducting phase emerges not only in square-lattice structures but also in ladder structures. Yet iron-based superconductors without a square-lattice motif have not been found, despite being actively sought out. Here, we report the discovery of pressure-induced superconductivity in the iron-based spin-ladder material BaFe2S3, a Mott insulator with striped-type magnetic ordering below ∼120 K. On the application of pressure this compound exhibits a metal-insulator transition at about 11 GPa, followed by the appearance of superconductivity below Tc = 14 K, right after the onset of the metallic phase. Our findings indicate that iron-based ladder compounds represent promising material platforms, in particular for studying the fundamentals of iron-based superconductivity.

Nonreciprocal magnons and symmetry-breaking in the noncentrosymmetric antiferromagnet

Inelastic neutron scattering measurements were performed to study spin dynamics in the noncentrosymmetric antiferromagnet α-Cu_{2}V_{2}O_{7}. For the first time, nonreciprocal magnons were experimentally measured in an antiferromagnet. These nonreciprocal magnons are caused by the incompatibility between anisotropic exchange and antisymmetric Dzyaloshinskii-Moriya interactions, which arise from broken symmetry, resulting in a collinear ordered state but helical spin dynamics. The nonreciprocity introduces the difference in the phase velocity of the counterrotating modes, causing the opposite spontaneous magnonic Faraday rotation of the left- and right-propagating spin waves. The breaking of spatial inversion and time reversal symmetry is revealed as a magnetic-field-induced asymmetric energy shift, which provides a test for the detailed balance relation.

Magnon dispersion shift in the induced ferromagnetic phase of noncentrosymmetric MnSi

A small angle neutron inelastic scattering measurement has been performed to study the magnon dispersion relation in the field-induced ferromagnetic phase of the noncentrosymmetric binary compound MnSi. For the magnons propagating parallel or antiparallel to the external magnetic field, we experimentally confirmed that the dispersion relation is asymmetrically shifted along the magnetic field direction. This magnon dispersion shift is attributed to the relativistic Dzyaloshinskii-Moriya interaction, which is finite in noncentrosymmetric magnets, such as MnSi. The shift direction is found to be switchable by reversing the external magnetic field direction.

Spin Fluctuations from Hertz to Terahertz on a Triangular Lattice

The temporal magnetic correlations of the triangular-lattice antiferromagnet NiGa_{2}S_{4} are examined through 13 decades (10^{-13}-1 sec) using ultrahigh-resolution inelastic neutron scattering, muon spin relaxation, and ac and nonlinear susceptibility measurements. Unlike the short-ranged spatial correlations, the temperature dependence of the temporal correlations show distinct anomalies. The spin fluctuation rate decreases precipitously upon cooling towards T^{*}=8.5 K, but fluctuations on the microsecond time scale then persist in an anomalous dynamical regime for 4 K<T≤T^{*}. As this time scale exceeds that of single-site dynamics by 6 orders of magnitude, these fluctuations bear evidence of emergent degrees of freedom within the short-range correlated incommensurate state of NiGa_{2}S_{4}.

Anisotropic inplane spin correlation in the parent and Co-doped BaFe2As2: A neutron scattering study

Abstract Antiferromagnetic spin fluctuations were investigated in the normal states of the parent ( x xa0=xa00), under-doped ( x xa0=xa00.04) and optimally-doped ( x xa0=xa00.06) Ba ( Fe 1 - x Co x ) 2 As 2 single crystals using inelastic neutron scattering technique. For all the doping levels, quasi-two-dimensional antiferromagnetic fluctuations were observed as a broad peak localized at Q = ( 1 / 2 , 1 / 2 , l ) . At lower energies, the peak shows an apparent anisotropy in the h k 0 plane; longitudinal peak widths are considerably smaller than transverse widths. The anisotropy is larger for the higher doping level. These results are consistent with the random phase approximation (RPA) calculations taking account of the orbital character of the electronic bands, confirming that the anisotropic nature of the spin fluctuations in the normal states is mostly dominated by the nesting of Fermi surfaces. On the other hand, the quasi-two-dimensional spin correlations grow much rapidly for decreasing temperature in the x xa0=xa00 parent compound, compared to that expected for nearly antiferromagnetic metals. This may be another sign of the unconventional nature of the antiferromagnetic transition in BaFe 2 As 2 .

Aging, memory, and nonhierarchical energy landscape of spin jam.

Significance Our bulk susceptibility and Monte Carlo simulation study of aging and memory effects in densely populated frustrated magnets (spin jam) and in a dilute magnetic alloy (spin glass) indicates a nonhierarchical landscape with a wide and nearly flat but rough bottom for the spin jam and a hierarchical rugged funnel-type landscape for the spin glass. The notion of complex energy landscape underpins the intriguing dynamical behaviors in many complex systems ranging from polymers, to brain activity, to social networks and glass transitions. The spin glass state found in dilute magnetic alloys has been an exceptionally convenient laboratory frame for studying complex dynamics resulting from a hierarchical energy landscape with rugged funnels. Here, we show, by a bulk susceptibility and Monte Carlo simulation study, that densely populated frustrated magnets in a spin jam state exhibit much weaker memory effects than spin glasses, and the characteristic properties can be reproduced by a nonhierarchical landscape with a wide and nearly flat but rough bottom. Our results illustrate that the memory effects can be used to probe different slow dynamics of glassy materials, hence opening a window to explore their distinct energy landscapes.

Science from the Initial Operation of HRC

Shinichi Itoh1, Tetsuya Yokoo1, Takatsugu Masuda2, Hideki Yoshizawa2, Minoru Soda2, Yoichi Ikeda2, Soshi Ibuka1, Daichi Kawana1, Taku J. Sato3, Yusuke Nambu3, Keitaro Kuwahara4, Shin-ichiro Yano5, Jun Akimitsu5, Yoshio Kaneko6, Yoshinori Tokura6,7, Masaki Fujita8, Masashi Hase9, Kazuaki Iwasa10, Haruhiro Hiraka1, Tatsuo Fukuda11, Kazuhiko Ikeuchi12, Koji Yoshida13 Toshio Yamaguchi13, Kanta Ono1, Yasuo Endoh1,6,8 1Institute of Materials Structure Science, High Energy Accelerator Research Organization, Tsukuba 305-0801, Japan 2The Institute for Solid State Physics, The University of Tokyo, Tokai 319-1106, Japan 3Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, Sendai 980-8577, Japan 4Institute of Applied Beam Science, Ibaraki University, Mito 310-8512, Japan 5Department of Physics and Mathematics, Aoyama Gakuin University, Sagamihara 252-5258, Japan 6RIKEN Center for Emergent Matter of Science, Wako 351-0198, Japan 7Department of Applied Physics, The University of Tokyo, Tokyo 113-8656, Japan 8Institute for Materials Research, Tohoku University, Sendai 980-8577, Japan 9National Institute for Materials Science, Tsukuba 305-0047, Japan 10Department of Physics, Tohoku University, Sendai 980-8578, Japan 11Quantum Beam Science Center, Japan Atomic Energy Agency, Sayo 679-5148, Japan 12Comprehensive Research Organization for Science and Society, Tokai 319-1106, Japan 13Department of Chemistry, Fukuoka University, Fukuoka 814-0180, Japan

Tiny adiabatic-demagnetization refrigerator for a commercial superconducting quantum interference device magnetometer

A tiny adiabatic-demagnetization refrigerator (T-ADR) has been developed for a commercial superconducting quantum interference device magnetometer [Magnetic Property Measurement System (MPMS) from Quantum Design]. The whole T-ADR system is fit in a cylindrical space of diameter 8.5 mm and length 250 mm, and can be inserted into the narrow sample tube of MPMS. A sorption pump is self-contained in T-ADR, and hence no complex gas handling system is necessary. With the single crystalline Gd3Ga5O12 garnet (∼2 g) used as a magnetic refrigerant, the routinely achievable lowest temperature is ∼0.56 K. The lower detection limit for a magnetization anomaly is ∼1 × 10-7 emu, estimated from fluctuation of the measured magnetization. The background level is ∼5 × 10-5 emu below 2 K at H = 100 Oe, which is largely attributable to a contaminating paramagnetic signal from the magnetic refrigerant.

Scaling of Memories and Crossover in Glassy Magnets.

Glassiness is ubiquitous and diverse in characteristics in nature. Understanding their differences and classification remains a major scientific challenge. Here, we show that scaling of magnetic memories with time can be used to classify magnetic glassy materials into two distinct classes. The systems studied are high temperature superconductor-related materials, spin-orbit Mott insulators, frustrated magnets, and dilute magnetic alloys. Our bulk magnetization measurements reveal that most densely populated magnets exhibit similar memory behavior characterized by a relaxation exponent of