Showing 1–50 of 77 results for author: Imai, Y

Three-dimensional zigzag correlations in the van der Waals Kitaev magnet RuBr$_3$

Authors: H. Gretarsson, R. Iwazaki, F. Sato, H. Gotou, S. Francoual, J. Nasu, Y. Imai, K. Ohgushi, J. Chaloupka, B. Keimer, H. Suzuki

Abstract: Ruthenium trihalides Ru$X_3$ ($X$ = Cl, Br, I) provide a tunable platform for Kitaev magnetism in two-dimensional van der Waals materials. Despite their similar crystal structures and zigzag antiferromagnetic order, RuBr$_3$ exhibits a higher Néel temperature ($T_N$) than RuCl$_3$, suggesting their distinct proximity to the Kitaev quantum spin liquid phase. Using Ru $L_3$-edge resonant x-ray scatt… ▽ More Ruthenium trihalides Ru$X_3$ ($X$ = Cl, Br, I) provide a tunable platform for Kitaev magnetism in two-dimensional van der Waals materials. Despite their similar crystal structures and zigzag antiferromagnetic order, RuBr$_3$ exhibits a higher Néel temperature ($T_N$) than RuCl$_3$, suggesting their distinct proximity to the Kitaev quantum spin liquid phase. Using Ru $L_3$-edge resonant x-ray scattering, we show that, while the long-range zigzag order in RuBr$_3$ disappears at $T_N$, the zigzag correlations that persist well above $T_N$ show a pronounced spectral weight redistribution along the interlayer direction. These results suggest that the enhanced interlayer magnetic interactions driven by the extended Br 4$p$ orbitals stabilize three-dimensional zigzag correlations in RuBr$_3$. △ Less

Submitted 6 April, 2026; originally announced April 2026.

Possible Pairing Symmetry of BaPtAs$_{1-x}$Sb$_{x}$ with an Ordered Honeycomb Network

Authors: Tsuyoshi Imazu, Naoya Furutani, Tadashi Adachi, Kazutaka Kudo, Yoshiki Imai, Jun Goryo

Abstract: We investigate the possible pairing symmetry of superconducting $\rm{BaPtAs}_{1-\it{x}}\rm{Sb}_{\it{x}}$ solid solution with an ordered-honeycomb network of Pt and pnictogens. A spontaneous internal magnetic field below the superconducting transition temperature is observed in BaPtSb ($x = 1$) via the muon-spin relaxation measurement. We then pursue a scenario where the pairing symmetry is changed… ▽ More We investigate the possible pairing symmetry of superconducting $\rm{BaPtAs}_{1-\it{x}}\rm{Sb}_{\it{x}}$ solid solution with an ordered-honeycomb network of Pt and pnictogens. A spontaneous internal magnetic field below the superconducting transition temperature is observed in BaPtSb ($x = 1$) via the muon-spin relaxation measurement. We then pursue a scenario where the pairing symmetry is changed from a time-reversal symmetry-breaking (TRSB) state to another one by changing the Sb-concentration utilizing the effective tight-binding model obtained from the first principles calculations for $x = 0$ and $x = 1$, at which we see a significant difference in the shape of the dominant Fermi surfaces. We find that the chiral $d$-wave state with TRSB is most stable at $x = 1$, whereas the nodal $f$-wave or the conventional $s$-wave states without TRSB are competitive at $x = 0$. △ Less

Submitted 12 March, 2026; originally announced March 2026.

Journal ref: J. Phys. Soc. Jpn. 95, 044704 (2026)

Piezomagnetic effect in 5$d$ transition metal oxides Y$_2$Ir$_2$O$_7$ and Cd$_2$Os$_2$O$_7$ with all-in/all-out magnetic order

Authors: Hiroki Nanjo, Yoshinori Imai, Takuya Aoyama, Junichi Yamaura, Kenya Ohgushi

Abstract: We investigated the piezomagnetic effect in pyrochlore-type oxides Y$_2$Ir$_2$O$_7$ and Cd$_2$Os$_2$O$_7$, which show a non-coplanar magnetic structure called the all-in/all-out antiferromagnetic order at low temperatures. The all-in/all-out magnetic order can be viewed as a ferroic order of the $xyz$-type magnetic octupoles. We observed a linear development of magnetization with applying stress f… ▽ More We investigated the piezomagnetic effect in pyrochlore-type oxides Y$_2$Ir$_2$O$_7$ and Cd$_2$Os$_2$O$_7$, which show a non-coplanar magnetic structure called the all-in/all-out antiferromagnetic order at low temperatures. The all-in/all-out magnetic order can be viewed as a ferroic order of the $xyz$-type magnetic octupoles. We observed a linear development of magnetization with applying stress for both materials. We then estimated the powder-averaged piezomagnetic tensor at 50 K to be $Q = 5.74 \times 10^{-6}$ $μ_{\text{B}}$/Ir$\cdot$MPa for Y$_2$Ir$_2$O$_7$, and $Q = 3.49 \times 10^{-7}$ $μ_{\text{B}}$/Os$\cdot$MPa for Cd$_2$Os$_2$O$_7$. We discuss the microscopic mechanism of the piezomagnetic effect in terms of the stress-induced modification of the g-tensor anisotropy and Dzyaloshinskii-Moriya (D-M) interactions. This work paves the way for the further development of piezomagnetic materials using a strategy based on magnetic multipoles. △ Less

Submitted 16 May, 2025; originally announced May 2025.

Gradient-based optimization of spintronic devices

Authors: Yusuke Imai, Shuhong Liu, Nozomi Akashi, Kohei Nakajima

Abstract: The optimization of physical parameters serves various purposes, such as system identification and efficiency in developing devices. Spin-torque oscillators have been applied to neuromorphic computing experimentally and theoretically, but the optimization of their physical parameters has usually been done by grid search. In this paper, we propose a scheme to optimize the parameters of the dynamics… ▽ More The optimization of physical parameters serves various purposes, such as system identification and efficiency in developing devices. Spin-torque oscillators have been applied to neuromorphic computing experimentally and theoretically, but the optimization of their physical parameters has usually been done by grid search. In this paper, we propose a scheme to optimize the parameters of the dynamics of macrospin-type spin-torque oscillators using the gradient descent method with automatic differentiation. First, we prepared numerically created dynamics as teacher data and successfully tuned the parameters to reproduce the dynamics. This can be applied to obtain the correspondence between the simulation and experiment of the spin-torque oscillators. Next, we successfully solved the image recognition task with high accuracy by connecting the coupled system of spin-torque oscillators to the input and output layers and training all of them through gradient descent. This approach allowed us to estimate how to control the experimental setup and design the physical systems so that the task could be solved with a high accuracy using spin-torque oscillators. △ Less

Submitted 13 September, 2024; originally announced September 2024.

Spontaneous magnetic field and disorder effects in BaPtAs_1-x_Sb_x_ with honeycomb network

Authors: T. Adachi, T. Ogawa, Y. Komiyama, T. Sumura, Y. Saito-Tsuboi, T. Takeuchi, K. Mano, K. Manabe, K. Kawabata, T. Imazu, A. Koda, W. Higemoto, H. Okabe, J. G. Nakamura, T. U. Ito, R. Kadono, C. Baines, I. Watanabe, T. Kida, M. Hagiwara, Y. Imai, J. Goryo, M. Nohara, K. Kudo

Abstract: Chiral superconductivity exhibits the formation of novel electron pairs that breaks the time-reversal symmetry and has been actively studied in various quantum materials in recent years. However, despite its potential to provide definitive information, effects of disorder in the crystal structure on the chiral superconductivity has not yet been clarified, and therefore the investigation using a so… ▽ More Chiral superconductivity exhibits the formation of novel electron pairs that breaks the time-reversal symmetry and has been actively studied in various quantum materials in recent years. However, despite its potential to provide definitive information, effects of disorder in the crystal structure on the chiral superconductivity has not yet been clarified, and therefore the investigation using a solid-solution system is desirable. We report muon-spin-relaxation (muSR) results of layered pnictide BaPtAs_1-x_Sb_x_ with a honeycomb network composed of Pt and (As, Sb). We observed an increase of the zero-field muon-spin relaxation rate in the superconducting (SC) state at the Sb end of x=1.0, suggesting the occurrence of spontaneous magnetic field due to the time-reversal symmetry breaking in the SC state. On the other hand, spontaneous magnetic field was almost and completely suppressed for the As-Sb mixed samples of x=0.9 and 0.2, respectively, suggesting that the time-reversal symmetry breaking SC state in x=1.0 is sensitive to disorder. The magnetic penetration depth estimated from transverse-field muSR measurements at x=1.0 and 0.2 behaved like weak-coupling s-wave superconductivity. These seemingly incompatible zero-field and transverse-field muSR results of BaPtAs_1-x_Sb_x_ with x=1.0 could be understood in terms of chiral d-wave superconductivity with point nodes on the three-dimensional Fermi surface. △ Less

Submitted 17 March, 2025; v1 submitted 8 September, 2024; originally announced September 2024.

Comments: 7 pages, 3 figures, 1 table, Phys. Rev. B Letter (in press)

Journal ref: Phys. Rev. B 111, L100508 (2025)

$J=1/2$ Pseudospins and $d$-$p$ Hybridization in the Kitaev Spin Liquid Candidates Ru$X_3$ ($X$ = Cl, Br, I)

Authors: H. Gretarsson, H. Fujihara, F. Sato, H. Gotou, Y. Imai, K. Ohgushi, B. Keimer, H. Suzuki

Abstract: The recent synthesis of ruthenium trihalides Ru$X_3$ ($X$ = Cl, Br, I) has enlarged the set of material candidates for Kitaev spin liquid. The realization of Kitaev model necessitates the formation of $J=1/2$ pseudospins in the octahedral crystal field. We use Ru $L_3$-edge resonant inelastic x-ray scattering to investigate the evolution of multiplet structures in Ru$X_3$. We identified quasi-elas… ▽ More The recent synthesis of ruthenium trihalides Ru$X_3$ ($X$ = Cl, Br, I) has enlarged the set of material candidates for Kitaev spin liquid. The realization of Kitaev model necessitates the formation of $J=1/2$ pseudospins in the octahedral crystal field. We use Ru $L_3$-edge resonant inelastic x-ray scattering to investigate the evolution of multiplet structures in Ru$X_3$. We identified quasi-elastic magnetic correlations and spin-orbit transitions to the $J=3/2$ states without discernible trigonal splitting, thereby validating the $J=1/2$ description of magnetism in the Ru$X_3$ family. Lineshape broadening in RuI$_3$ provides evidence for its bulk metallicity in the vicinity of a bandwidth-controlled metal-insulator transition. Our results highlight the pivotal role of halogen $p$ orbitals in controlling the electronic properties of Ru$X_3$. △ Less

Submitted 28 February, 2024; originally announced February 2024.

Journal ref: Physical Review B 109, L180413 (2024)

Spintronic virtual neural network by a voltage controlled ferromagnet for associative memory

Authors: Tomohiro Taniguchi, Yusuke Imai

Abstract: Recently, an associative memory operation by a virtual oscillator network, consisting of a single spintronic oscillator, was examined to solve issues in conventional, real oscillators-based neural networks such as inhomogeneities between the oscillators. However, the spintronic oscillator still carries issues dissipating large amount of energy because it is driven by electric current. Here, we pro… ▽ More Recently, an associative memory operation by a virtual oscillator network, consisting of a single spintronic oscillator, was examined to solve issues in conventional, real oscillators-based neural networks such as inhomogeneities between the oscillators. However, the spintronic oscillator still carries issues dissipating large amount of energy because it is driven by electric current. Here, we propose to use a single ferromagnet manipulated by voltage-controlled magnetic anisotropy (VCMA) effect as a fundamental element in a virtual neural network, which will contribute to significantly reducing the Joule heating caused by electric current. Instead of the oscillation in oscillator networks, magnetization relaxation dynamics were used for the associative memory operation. The associative memory operation for alphabet patterns is successfully demonstrated by giving correspondences between the colors in a pattern recognition task and the sign of a perpendicular magnetic anisotropy coefficient, which could be either positive or negative via the VCMA effect. △ Less

Submitted 8 April, 2024; v1 submitted 26 January, 2024; originally announced January 2024.

Comments: 18 pages, 5 figures, publication information was added

Journal ref: Sci. Rep. 14, 8188 (2024)

Magnetism in Kitaev Quantum Spin Liquid Candidate RuBr$_3$

Authors: T. Weinhold, C. Wang, F. Seewald, V. Grinenko, Y. Imai, F. Sato, K. Ohgushi, H. -H. Klauss, R. Sarkar

Abstract: The present studies show that long-range magnetic order takes place in RuBr$_3$ at $\approx$ 34 K. The observations of clear oscillations in the muon time spectra demonstrate the presence of well-defined internal fields at the muon sites. The magnetic ordering appears to be very robust and static suggesting a more conventional nature of magnetic ordering in the RuBr$_3$ system at zero field. Prese… ▽ More The present studies show that long-range magnetic order takes place in RuBr$_3$ at $\approx$ 34 K. The observations of clear oscillations in the muon time spectra demonstrate the presence of well-defined internal fields at the muon sites. The magnetic ordering appears to be very robust and static suggesting a more conventional nature of magnetic ordering in the RuBr$_3$ system at zero field. Present investigations prove that in RuBr$_3$ the Kitaev interactions are likely to be weakened at zero field in comparison to the $α$-RuCl$_3$ system. This proves that it is possible to tune the Kitaev interactions by replacing Cl with heavier halogen elements such as Br. △ Less

Submitted 7 November, 2023; originally announced November 2023.

Comments: 5 pages, 5 figures

Journal ref: Phys. Rev. B 109, 014440(2024)

Associative memory by virtual oscillator network based on single spin-torque oscillator

Authors: Yusuke Imai, Tomohiro Taniguchi

Abstract: A coupled oscillator network may be able to perform an energy-efficient associative memory operation. However, its realization has been difficult because inhomogeneities unavoidably arise among the oscillators during fabrication and lead to an unreliable operation. This issue could be resolved if the oscillator network were able to be formed from a single oscillator. Here, we performed numerical s… ▽ More A coupled oscillator network may be able to perform an energy-efficient associative memory operation. However, its realization has been difficult because inhomogeneities unavoidably arise among the oscillators during fabrication and lead to an unreliable operation. This issue could be resolved if the oscillator network were able to be formed from a single oscillator. Here, we performed numerical simulations and theoretical analyses on an associative memory operation that uses a virtual oscillator network based on a spin-torque oscillator. The virtual network combines the concept of coupled oscillators with that of feedforward neural networks. Numerical experiments demonstrate successful associations of $60$-pixel patterns with various memorized patterns. Moreover, the origin of the associative memory is shown to be forced synchronization driven by feedforward input, where phase differences among oscillators are fixed and correspond to the colors of the pixels in the pattern. △ Less

Submitted 29 March, 2024; v1 submitted 22 September, 2023; originally announced September 2023.

Comments: 15 pages, 4 figures (main text), Supplemental Information was added. Trigonometric functions in Eqs. (6), (7), (9), (10), (11), (12), (14), and (15), as well as related terms in main text are corrected, but they do not affect the conclusions

Journal ref: Scientific Reports 13, 15809 (2023)

Study on Susceptibilities of Superconductors BaPtSb and BaPtAs with Honeycomb Structure

Authors: Naoya Furutani, Yoshiki Imai, Tsuyoshi Imazu, Jun Goryo

Abstract: The low-energy electronic properties of the new superconductors BaPtSb and BaPtAs with an ordered honeycomb network are investigated in the normal phase, where the former compound is a candidate for time-reversal symmetry-breaking superconductors. By means of the first-principles calculation, we show that there exist two-dimensional cylinder-like Fermi surfaces around the kz axis and one outer sph… ▽ More The low-energy electronic properties of the new superconductors BaPtSb and BaPtAs with an ordered honeycomb network are investigated in the normal phase, where the former compound is a candidate for time-reversal symmetry-breaking superconductors. By means of the first-principles calculation, we show that there exist two-dimensional cylinder-like Fermi surfaces around the kz axis and one outer spherelike Fermi surface around the K and K' points in both compounds, which are mainly composed of Pt 5d and Sb 5p/As 4p electrons. We construct low-energy effective models, which are well described by using three bands consisting of two Pt 5d orbitals and one Sb 5p/As 4p orbital. By evaluating susceptibilities using effective models, we find that dominant contributions to those susceptibilities result from the outer spherelike Fermi surface. Whereas out-of-plane fluctuations are enhanced in both compounds in the higher-temperature region, in-plane fluctuations become dominant in the very low-temperature region in BaPtSb owing to a better nesting condition in the k_z=0 plane from the outer spherelike Fermi surface. These fluctuations yield instabilities to ordered states, such as superconductivity, and might be associated with the occurrence of the superconducting state with time-reversal symmetry breaking in BaPtSb. △ Less

Submitted 3 September, 2023; originally announced September 2023.

Comments: 7 pages, 9 figures

Journal ref: J. Phys. Soc. Jpn. 92, 094708 (2023)

Input-driven chaotic dynamics in vortex spin-torque oscillator

Authors: Yusuke Imai, Kohei Nakajima, Sumito Tsunegi, Tomohiro Taniguchi

Abstract: A new research topic in spintronics relating to the operation principles of brain-inspired computing is input-driven magnetization dynamics in nanomagnet. In this paper, the magnetization dynamics in a vortex spin-torque oscillator (STO) driven by a series of random magnetic field are studied through a numerical simulation of the Thiele equation. It is found that input-driven synchronization occur… ▽ More A new research topic in spintronics relating to the operation principles of brain-inspired computing is input-driven magnetization dynamics in nanomagnet. In this paper, the magnetization dynamics in a vortex spin-torque oscillator (STO) driven by a series of random magnetic field are studied through a numerical simulation of the Thiele equation. It is found that input-driven synchronization occurs in the weak perturbation limit, as found recently. As well, chaotic behavior is newly found to occur in the vortex core dynamics for a wide range of parameters, where synchronized behavior is disrupted by an intermittency. Ordered and chaotic dynamical phases are examined by evaluating the Lyapunov exponent. The relation between the dynamical phase and the computational capability of physical reservoir computing is also studied. △ Less

Submitted 22 June, 2023; originally announced June 2023.

Comments: 11 pages, 4 figures

Journal ref: Scientific Reports 12, 21651 (2022)

Dzyaloshinskii-Moriya interaction in strongly spin-orbit-coupled systems: General formula and application to topological and Rashba materials

Authors: Yuto Hayakawa, Yusuke Imai, Hiroshi Kohno

Abstract: We theoretically study the Dzyaloshinskii-Moriya interaction (DMI) mediated by band electrons with strong spin-orbit coupling (SOC). We first derive a general formula for the coefficient ${\bm D}_i$ of the DMI in free energy in terms of Green's functions, and examine its variations in relation to physical quantities. In general, the DMI coefficient can vary depending on physical quantities, i.e.,… ▽ More We theoretically study the Dzyaloshinskii-Moriya interaction (DMI) mediated by band electrons with strong spin-orbit coupling (SOC). We first derive a general formula for the coefficient ${\bm D}_i$ of the DMI in free energy in terms of Green's functions, and examine its variations in relation to physical quantities. In general, the DMI coefficient can vary depending on physical quantities, i.e., whether one is looking at equilibrium spin structure (${\bm D}_i$) or spin-wave dispersion (${\bm D}_i^{(2)}$), and the obtained formula helps to elucidate their relations. By explicit evaluations for a magnetic topological insulator and a Rashba ferromagnet with perpendicular magnetization, we observe ${\bm D}_i^{(2)} \ne {\bm D}_i$ in general. In the latter model, or more generally, when the magnetization and the spin-orbit field are mutually orthogonal, ${\bm D}_i$ is exactly related to the equilibrium spin current for arbitrary strength of SOC, generalizing the similar relation for systems with weak SOC. Among various systems with strong SOC, magnetic Weyl semimetals are special in that ${\bm D}_i^{(2)} = {\bm D}_i$, and in fact, the DMI in this system arises as the chiral anomaly. △ Less

Submitted 26 May, 2023; v1 submitted 25 May, 2023; originally announced May 2023.

Comments: 19 pages, 6 figures

Journal ref: Physical Review B 108, 064409 (2023)

Best Thermoelectric Efficiency of Ever-Explored Materials

Authors: Byungki Ryu, Jaywan Chung, Masaya Kumagai, Tomoya Mato, Yuki Ando, Sakiko Gunji, Atsumi Tanaka, Dewi Yana, Masayuki Fujimoto, Yoji Imai, Yukari Katsura, SuDong Park

Abstract: A thermoelectric device is a heat engine that directly converts heat into electricity. Many materials with a high figure of merit ZT have been discovered in anticipation of a high thermoelectric efficiency. However, there has been a lack of investigations on efficiency-based material evaluation, and little is known about the achievable limit of thermoelectric efficiency. Here, we report the highes… ▽ More A thermoelectric device is a heat engine that directly converts heat into electricity. Many materials with a high figure of merit ZT have been discovered in anticipation of a high thermoelectric efficiency. However, there has been a lack of investigations on efficiency-based material evaluation, and little is known about the achievable limit of thermoelectric efficiency. Here, we report the highest thermoelectric efficiency using 12,645 published materials. The 97,841,810 thermoelectric efficiencies are calculated using 808,610 device configurations under various heat-source temperatures (T_h) when the cold-side temperature is 300 K, solving one-dimensional thermoelectric integral equations with temperature-dependent thermoelectric properties. For infinite-cascade devices, a thermoelectric efficiency larger than 33% (~1/3) is achievable when T_h exceeds 1400 K. For single-stage devices, the best efficiency of 17.1% (~1/6) is possible when T_h is 860 K. Leg segmentation can overcome this limit, delivering a very high efficiency of 24% (~1/4) when T_h is 1100 K. △ Less

Submitted 14 March, 2023; v1 submitted 17 October, 2022; originally announced October 2022.

Comments: 32 pages (main+table+figure captions+figures), 7 additional pages for 6 high-resolution-vector-image figures, Supporting Data file is not public yet

Journal ref: iScience 26, 106494 (2023)

Light-induced magnetization driven by interorbital charge motion in a spin-orbit assisted Mott insulator alpha-RuCl3

Authors: T. Amano, Y. Kawakami, H. Itoh, K. Konno, Y. Hasegawa, T. Aoyama, Y. Imai, K. Ohgushi, Y. Takeuchi, Y. Wakabayashi, K. Goto, Y. Nakamura, H. Kishida, K. Yonemitsu, S. Iwai

Abstract: In a honeycomb-lattice spin-orbit assisted Mott insulator α-RuCl3, an ultrafast magnetization is induced by circularly polarized excitation below the Mott gap. Photo-carriers play an important role, which are generated by turning down the synergy of the on-site Coulomb interaction and the spin-orbit interaction realizing the insulator state. An ultrafast 6- fs measurement of photo-carrier dynamics… ▽ More In a honeycomb-lattice spin-orbit assisted Mott insulator α-RuCl3, an ultrafast magnetization is induced by circularly polarized excitation below the Mott gap. Photo-carriers play an important role, which are generated by turning down the synergy of the on-site Coulomb interaction and the spin-orbit interaction realizing the insulator state. An ultrafast 6- fs measurement of photo-carrier dynamics and a quantum mechanical analysis clarify the mechanism, according to which the magnetization emerges from a coherent charge motion between different t2g orbitals (dyz-dxz-dxy) of Ru3+ ions. This ultrafast magnetization is weakened in the antiferromagnetic (AF) phase, which is opposite to the general tendency that the inverse Faraday effect is larger in AF compounds than in paramagnetic ones. This temperature dependence indicates that the interorbital charge motion is affected by pseudo-spin rotational symmetry breaking in the AF phase. △ Less

Submitted 14 September, 2022; v1 submitted 8 July, 2022; originally announced July 2022.

Journal ref: Phys. Rev. Res.4 L032032(2022)

Strongly electron-correlated semimetal RuI$_3$ with a layered honeycomb structure

Authors: Kazuhiro Nawa, Yoshinori Imai, Youhei Yamaji, Hideyuki Fujihara, Wakana Yamada, Ryotaro Takahashi, Takumi Hiraoka, Masato Hagihala, Shuki Torii, Takuya Aoyama, Takamasa Ohashi, Yasuhiro Shimizu, Hirotada Gotou, Masayuki Itoh, Kenya Ohgushi, Taku J Sato

Abstract: A polymorph of RuI$_3$ synthesized under high pressure was found to have a two-layered honeycomb structure. The resistivity of RuI$_3$ exhibits a semimetallic behavior, in contrast to insulating properties in $α$-RuCl$_3$. In addition, Pauli paramagnetic behavior was observed in the temperature dependence of a magnetic susceptibility and a nuclear spin-lattice relaxation rate 1/$T_1$. The band str… ▽ More A polymorph of RuI$_3$ synthesized under high pressure was found to have a two-layered honeycomb structure. The resistivity of RuI$_3$ exhibits a semimetallic behavior, in contrast to insulating properties in $α$-RuCl$_3$. In addition, Pauli paramagnetic behavior was observed in the temperature dependence of a magnetic susceptibility and a nuclear spin-lattice relaxation rate 1/$T_1$. The band structure calculations indicate that contribution of the I 5$p$ components to the low-energy $t_\mathrm{2g}$ bands effectively decreases Coulomb repulsion, leading to semimetallic properties. The physical properties also suggest strong electron correlations in RuI$_3$. △ Less

Submitted 29 September, 2021; v1 submitted 27 September, 2021; originally announced September 2021.

Comments: 4 Figures

Journal ref: J. Phys. Soc. Jpn. 90, 123703 (2021)

Magnetism of Kitaev spin-liquid candidate material RuBr$_3$

Authors: Yoshinori Imai, Kazuhiro Nawa, Yasuhiro Shimizu, Wakana Yamada, Hideyuki Fujihara, Takuya Aoyama, Ryotaro Takahashi, Daisuke Okuyama, Takamasa Ohashi, Masato Hagihala, Shuki Torii, Daisuke Morikawa, Masami Terauchi, Takayuki Kawamata, Masatsune Kato, Hirotada Gotou, Masayuki Itoh, Taku J. Sato, Kenya Ohgushi

Abstract: The ruthenium halide $α$-RuCl$_{3}$ is a promising candidate for a Kitaev spin liquid. However, the microscopic model describing $α$-RuCl$_{3}$ is still debated partly because of a lack of analogue materials for $α$-RuCl$_{3}$, which prevents tracking of electronic properties as functions of controlled interaction parameters. Here, we report a successful synthesis of RuBr$_{3}$. The material RuBr… ▽ More The ruthenium halide $α$-RuCl$_{3}$ is a promising candidate for a Kitaev spin liquid. However, the microscopic model describing $α$-RuCl$_{3}$ is still debated partly because of a lack of analogue materials for $α$-RuCl$_{3}$, which prevents tracking of electronic properties as functions of controlled interaction parameters. Here, we report a successful synthesis of RuBr$_{3}$. The material RuBr$_{3}$~possesses BiI$_3$-type structure (space group: $R\overline{3}$) where Ru$^{3+}$ form an ideal honeycomb lattice. Although RuBr$_{3}$ has a negative Weiss temperature, it undergoes a zigzag antiferromagnetic transition at $T_\mathrm{N}=34$ K, as does $α$-RuCl$_{3}$. Our analyses indicate that the Kitaev and non-Kitaev interactions can be modified in ruthenium trihalides by changing the ligand sites, which provides a new platform for exploring Kitaev spin liquids. △ Less

Submitted 31 August, 2021; originally announced September 2021.

Journal ref: Phys. Rev. B 105, L041112 (2022)

Atomic Manipulation of In-gap States on the $β$-Bi$_2$Pd Superconductor

Authors: Cristina Mier, Jiyoon Hwang, Jinkyung Kim, Yujeong Bae, Fuyuki Nabeshima, Yoshinori Imai, Atsutaka Maeda, Nicolás Lorente, Andreas Heinrich, Deung-Jang Choi

Abstract: Electronic states in the gap of a superconductor inherit intriguing many-body properties from the superconductor. Here, we create these in-gap states by manipulating Cr atomic chains on the $β$-Bi$_2$Pd superconductor. We find that the topological properties of the in-gap states can greatly vary depending on the crafted spin chain. These systems make an ideal platform for non-trivial topological p… ▽ More Electronic states in the gap of a superconductor inherit intriguing many-body properties from the superconductor. Here, we create these in-gap states by manipulating Cr atomic chains on the $β$-Bi$_2$Pd superconductor. We find that the topological properties of the in-gap states can greatly vary depending on the crafted spin chain. These systems make an ideal platform for non-trivial topological phases because of the large atom-superconductor interactions and the existence of a large Rashba coupling at the Bi-terminated surface. We study two spin chains, one with atoms two-lattice-parameter apart and one with square-root-of-two lattice parameters. Of these, only the second one is in a topologically non-trivial phase, in correspondence with the spin interactions for this geometry. △ Less

Submitted 6 May, 2021; v1 submitted 13 April, 2021; originally announced April 2021.

Journal ref: Phys. Rev. B 104, 045406 (2021)

Relationships between Superconductivity and Nematicity in FeSe$_{1-x}$Te$_x$ ($x=0-0.5$) Films Studied by Complex Conductivity Measurements

Authors: H. Kurokawa, S. Nakamura, J. Zhao, N. Shikama, Y. Sakishita, Y. Sun, F. Nabeshima, Y. Imai, H. Kitano, A. Maeda

Abstract: We measured the complex conductivity, $σ$, of FeSe$_{1-x}$Te$_x$ ($x=0-0.5$) films in the superconducting state which show a drastic increase of the superconducting transition temperature, $T_\textrm{c}$, when the nematic order disappears. Since the magnetic penetration depth, $λ$ $(>$ 400 nm), of Fe(Se,Te) is longer than the typical thickness of the film ($\sim$100 nm), we combined the coplanar w… ▽ More We measured the complex conductivity, $σ$, of FeSe$_{1-x}$Te$_x$ ($x=0-0.5$) films in the superconducting state which show a drastic increase of the superconducting transition temperature, $T_\textrm{c}$, when the nematic order disappears. Since the magnetic penetration depth, $λ$ $(>$ 400 nm), of Fe(Se,Te) is longer than the typical thickness of the film ($\sim$100 nm), we combined the coplanar waveguide resonator and cavity perturbation techniques to evaluate both the real and imaginary parts of $σ$. Films with a nematic order showed a qualitatively different temperature dependence in penetration depth and quasiparticle scattering time when compared with those without nematic order, suggesting that nematic order influences the superconducting gap structure. Conversely, the proportionality between superfluid density, $n_\textrm{s}$ ($\proptoλ^{-2}$), and $T_\textrm{c}$ was observed irrespective of the presence or absence of nematic order. This result indicates that the amount of superfluid has a stronger impact on the $T_\textrm{c}$ of Fe(Se,Te) than the presence or absence of nematic order. Combining these results with band dispersions calculated using density functional theory, we propose that the change of the Fermi surface associated with nematicity is the primary factor influencing the change of $T_\textrm{c}$ and the superconducting gap structure in Fe(Se,Te). △ Less

Submitted 22 May, 2021; v1 submitted 31 March, 2021; originally announced March 2021.

Comments: 21 pages, 8 figures

Evolution of charge dynamics in FeSe$_{1-x}$Te$_{x}$: Effects of electronic correlations and nematicity

Authors: M. Nakajima, K. Yanase, M. Kawai, D. Asami, T. Ishikawa, F. Nabeshima, Y. Imai, A. Maeda, S. Tajima

Abstract: We systematically studied in-plane optical conductivity of FeSe$_{1-x}$Te$_{x}$ thin films fabricated on CaF$_{2}$ substrates for $x$ = 0, 0.1, 0.2, and 0.4. This system shows a large enhancement of superconducting transition temperature $T_{\mathrm{c}}$ at $x \sim$ 0.2 and a gentle decrease in $T_{\mathrm{c}}$ with further increasing $x$. The low-energy optical conductivity spectrum is described… ▽ More We systematically studied in-plane optical conductivity of FeSe$_{1-x}$Te$_{x}$ thin films fabricated on CaF$_{2}$ substrates for $x$ = 0, 0.1, 0.2, and 0.4. This system shows a large enhancement of superconducting transition temperature $T_{\mathrm{c}}$ at $x \sim$ 0.2 and a gentle decrease in $T_{\mathrm{c}}$ with further increasing $x$. The low-energy optical conductivity spectrum is described by the sum of narrow and broad Drude components, associated with coherent and incoherent charge dynamics, respectively. With increasing Te content, the spectral weight of the narrow Drude component decreases, while the total weight of the two Drude components increases. As a consequence, the fraction of the narrow Drude weight significantly decreases, indicating that Te substitution leads to stronger electronic correlations. Below the nematic transition temperature, the narrow Drude weight decreases with decreasing temperature. This indicates the reduction of the coherent carrier density, resulting from the Fermi-surface modification induced by the development of the orbital order. The reduction of the narrow Drude weight with temperature stopped at $x \sim$ 0.2, corresponding to the disappearance of the nematic transition. Our result suggests that the increase in the coherent carrier density induced by the suppression of the nematic transition gives rise to the enhancement of $T_{\mathrm{c}}$. The decrease in $T_{\mathrm{c}}$ with further Te substitution likely arises from too strong electronic correlations, which are not favorable for superconductivity. △ Less

Submitted 25 March, 2021; originally announced March 2021.

Comments: 10 pages, 7 figures

Journal ref: Phys. Rev. B 104, 024512 (2021)

Dichotomy Between Orbital and Magnetic Nematic Instabilities in BaFe2S3

Authors: Suguru Hosoi, Takuya Aoyama, Kousuke Ishida, Yuta Mizukami, Kazuki Hashizume, Satoshi Imaizumi, Yoshinori Imai, Kenya Ohgushi, Yusuke Nambu, Motoi Kimata, Shojiro Kimura, Takasada Shibauchi

Abstract: Nematic orders emerge nearly universally in iron-based superconductors, but elucidating their origins is challenging because of intimate couplings between orbital and magnetic fluctuations. The iron-based ladder material BaFe2S3, which superconducts under pressure, exhibits antiferromagnetic order below TN ~ 117K and a weak resistivity anomaly at T* ~ 180K, whose nature remains elusive. Here we re… ▽ More Nematic orders emerge nearly universally in iron-based superconductors, but elucidating their origins is challenging because of intimate couplings between orbital and magnetic fluctuations. The iron-based ladder material BaFe2S3, which superconducts under pressure, exhibits antiferromagnetic order below TN ~ 117K and a weak resistivity anomaly at T* ~ 180K, whose nature remains elusive. Here we report angle-resolved magnetoresistance (MR) and elastoresistance (ER) measurements in BaFe2S3, which reveal distinct changes at T*. We find that MR anisotropy and ER nematic response are both suppressed near T*, implying that an orbital order promoting isotropic electronic states is stabilized at T*. Such an isotropic state below T* competes with the antiferromagnetic order, which is evidenced by the nonmonotonic temperature dependence of nematic fluctuations. In contrast to the cooperative nematic orders in spin and orbital channels in iron pnictides, the present competing orders can provide a new platform to identify the separate roles of orbital and magnetic fluctuations. △ Less

Submitted 17 November, 2020; originally announced November 2020.

Comments: 7 pages 5 figures, to be published in Phys. Rev. Res

Journal ref: Phys. Rev. Research 2, 043293 (2020)

Phenomenology of the chiral $d$-wave state in the hexagonal pnictide superconductor SrPtAs

Authors: Hikaru Ueki, Shoma Inagaki, Ryota Tamura, Jun Goryo, Yoshiki Imai, W. B. Rui, Andreas P. Schnyder, Manfred Sigrist

Abstract: The pairing symmetry of the hexagonal pnictide superconductor SrPtAs is discussed with taking into account its multiband structure. The topological chiral $d$-wave state with time-reversal-symmetry breaking has been anticipated from the spontaneous magnetization observed by the muon-spin-relaxation experiment. We point out in this paper that the recent experimental reports on the nuclear-spin-latt… ▽ More The pairing symmetry of the hexagonal pnictide superconductor SrPtAs is discussed with taking into account its multiband structure. The topological chiral $d$-wave state with time-reversal-symmetry breaking has been anticipated from the spontaneous magnetization observed by the muon-spin-relaxation experiment. We point out in this paper that the recent experimental reports on the nuclear-spin-lattice relaxation rate $T_1^{-1}$ and superfluid density $n_s(T)$, which seemingly support the conventional $s$-wave pairing, are also consistent with the chiral $d$-wave state. The compatibility of the gap and multiband structures is crucial in this argument. We propose that the measurement of the bulk quasiparticle density of states would be useful for the distinction between two pairing states. △ Less

Submitted 5 February, 2020; originally announced February 2020.

Comments: 6 pages, 6 figures. arXiv admin note: substantial text overlap with arXiv:1902.10519

Hydrated lithium intercalation into the Kitaev spin liquid candidate material $α-$RuCl$_3$

Authors: Yoshinori Imai, Katsuya Konno, Yoshinao Hasegawa, Takuya Aoyama, Kenya Ohgushi

Abstract: We study on transport and magnetic properties of hydrated and lithium-intercalated $α$-RuCl$_3$, Li$_x$RuCl$_3 \cdot y$H$_2$O, for investigating the effect on mobile-carrier doping into candidate materials for a realization of a Kitaev model. From thermogravitometoric and one-dimensional electron map analyses, we find two crystal structures of this system, that is, mono-layer hydrated Li$_x$RuCl… ▽ More We study on transport and magnetic properties of hydrated and lithium-intercalated $α$-RuCl$_3$, Li$_x$RuCl$_3 \cdot y$H$_2$O, for investigating the effect on mobile-carrier doping into candidate materials for a realization of a Kitaev model. From thermogravitometoric and one-dimensional electron map analyses, we find two crystal structures of this system, that is, mono-layer hydrated Li$_x$RuCl$_3 \cdot y$H$_2$O~$(x\approx0.56, y\approx1.3)$ and bi-layer hydrated Li$_x$RuCl$_3 \cdot y$H$_2$O~$(x\approx0.56, y\approx3.9)$. The temperature dependence of the electrical resistivity shows a temperature hysteresis at 200-270 K, which is considered to relate with a formation of a charge order. The antiferromagnetic order at 7-13 K in pristine $α$-RuCl$_3$~ is successfully suppressed down to 2 K in bi-layer hydrated Li$_x$RuCl$_3 \cdot y$H$_2$O, which is sensitive to not only an electronic state of Ru but also an interlayer distance between Ru-Cl planes. △ Less

Submitted 23 June, 2019; v1 submitted 23 April, 2019; originally announced April 2019.

Comments: 7 pages, 5 figures

Journal ref: Phys. Rev. B 99, 245141 (2019)

Anomalous thermal Hall effect in the topological antiferromagnetic state

Authors: Kaori Sugii, Yusuke Imai, Masaaki Shimozawa, Muhammad Ikhlas, Naoki Kiyohara, Takahiro Tomita, Michi-To Suzuki, Takashi Koretsune, Ryotaro Arita, Satoru Nakatsuji, Minoru Yamashita

Abstract: The anomalous Hall effect (AHE), a Hall signal occurring without an external magnetic field, is one of the most significant phenomena. However, understanding the AHE mechanism has been challenging and largely restricted to ferromagnetic metals. Here, we investigate the recently discovered AHE in the chiral antiferromagnet Mn3Sn by measuring a thermal analog of the AHE, known as an anomalous therma… ▽ More The anomalous Hall effect (AHE), a Hall signal occurring without an external magnetic field, is one of the most significant phenomena. However, understanding the AHE mechanism has been challenging and largely restricted to ferromagnetic metals. Here, we investigate the recently discovered AHE in the chiral antiferromagnet Mn3Sn by measuring a thermal analog of the AHE, known as an anomalous thermal Hall effect (ATHE). The amplitude of the ATHE scales with the anomalous Hall conductivity of Mn3Sn over a wide temperature range, demonstrating that the AHE of Mn3Sn arises from a dissipationless intrinsic mechanism associated with the Berry curvature. Moreover, we find that the dissipationless AHE is significantly stabilized by shifting the Fermi level toward the magnetic Weyl points. Thus, in Mn3Sn, the Berry curvature emerging from the proposed magnetic Weyl fermion state is a key factor for the observed AHE and ATHE. △ Less

Submitted 18 February, 2019; originally announced February 2019.

Comments: 28 pages, 8 figures

Thermal Hall Conductivity in Superconducting Phase on Kagome Lattice

Authors: Shoma Iimura, Yoshiki Imai

Abstract: Motivated by a previous "$sd^2$-graphene" study, the pairing symmetry in the superconducting state and the thermal Hall conductivity are investigated by a self-consistent Bogoliubov--de Gennes approach on the kagome lattice with intrinsic spin-orbit coupling near van Hove fillings. While the topologically trivial state with broken time-reversal symmetry appears in the absence of spin-orbit couplin… ▽ More Motivated by a previous "$sd^2$-graphene" study, the pairing symmetry in the superconducting state and the thermal Hall conductivity are investigated by a self-consistent Bogoliubov--de Gennes approach on the kagome lattice with intrinsic spin-orbit coupling near van Hove fillings. While the topologically trivial state with broken time-reversal symmetry appears in the absence of spin-orbit coupling, the highest flat band becomes dispersive with a hexagonal symmetry due to spin-orbit coupling, which leads to a topological superconducting state. Since the thermal Hall conductivity in the low-temperature limit is associated with the topological property of time-reversal symmetry breaking superconductors, we study its temperature dependence near van Hove fillings. In particular, the pairing symmetry in the highest flat band is sensitive to the amplitudes of spin-orbit coupling and the attractive interaction, which is reflected remarkably in the thermal Hall conductivity. The obtained result may enable us to investigate the stable superconducting state on the kagome lattice. △ Less

Submitted 16 September, 2018; v1 submitted 6 August, 2018; originally announced August 2018.

Comments: 9 pages, 12 figures, published version

Journal ref: J. Phys. Soc. Jpn., 87, 094715 (2018)

Theory of Cross-correlated Electron-Magnon Transport Phenomena: Case of Magnetic Topological Insulator

Authors: Yusuke Imai, Hiroshi Kohno

Abstract: We study transport phenomena cross-correlated among the heat and electric currents of magnons and Dirac electrons on the surface of ferromagnetic topological insulators. For a perpendicular magnetization, we calculate magnon- (electron-) drag anomalous Nernst/Seebeck (anomalous Ettingshausen/Peltier) effects and magnon-/electron-drag thermal Hall effects. The magnon-drag thermoelectric effects are… ▽ More We study transport phenomena cross-correlated among the heat and electric currents of magnons and Dirac electrons on the surface of ferromagnetic topological insulators. For a perpendicular magnetization, we calculate magnon- (electron-) drag anomalous Nernst/Seebeck (anomalous Ettingshausen/Peltier) effects and magnon-/electron-drag thermal Hall effects. The magnon-drag thermoelectric effects are interpreted to be caused by magnon-induced electromotive force. When the magnetization has in-plane components, there arise thermal/thermoelectric analogs of anisotropic magnetoresistance (AMR). In the insulating state, the thermal AMR is realized as a magnonic analog of AMR. △ Less

Submitted 20 July, 2018; v1 submitted 25 May, 2018; originally announced May 2018.

Journal ref: J. Phys. Soc. Jpn. 87, 073709 (2018)

Superconductivity at 38 K in an electrochemical interface between ionic liquid and Fe(Se0.8Te0.2) on various substrates

Authors: Shunsuke Kouno, Yohei Sato, Yumiko Katayama, Ataru Ichinose, Daisuke Asami, Fuyuki Nabeshima, Yoshinori Imai, Atsutaka Maeda, Kazunori Ueno

Abstract: Superconducting FeSe0.8Te0.2 thin films on SrTiO3, LaAlO3 and CaF2 substrates were electrochemically etched in an ionic liquid DEME-TFSI electrolyte with a gate bias of 5 V. Superconductivity at 38 K was commonly observed on all substrates after etching the films with a thickness above 30 nm, in spite of different Tc of 8 K, 12 K and 19 K before the etching on SrTiO3, LaAlO3 and CaF2 substrates, r… ▽ More Superconducting FeSe0.8Te0.2 thin films on SrTiO3, LaAlO3 and CaF2 substrates were electrochemically etched in an ionic liquid DEME-TFSI electrolyte with a gate bias of 5 V. Superconductivity at 38 K was commonly observed on all substrates after etching the films with a thickness above 30 nm, in spite of different Tc of 8 K, 12 K and 19 K before the etching on SrTiO3, LaAlO3 and CaF2 substrates, respectively. Tc returned to the original value by removing the gate bias. The Tc enhancement on the thick film indicates no relationship between the Tc enhancement and any interface effects between the film and the substrate. The sheet resistance and the Hall coefficient of the surface conducting layer were estimated from the gate bias dependence of the transport properties. The sheet resistance of the surface conducting layer of the films on LaAlO3 and CaF2 showed an identical temperature dependence, and the Hall coefficient is almost temperature independent and -0.05 to -0.2 m2/C, corresponding to 4-17 electrons per one FeSe0.8Te0.2 unit cell area in two dimension. These common transport properties on various substrates suggest that the superconductivity at 38 K appeared in the surface conducting layer produced by electrochemical reaction between the surface of the FeSe0.8Te0.2 thin film and the ionic liquid electrolyte. △ Less

Submitted 14 September, 2018; v1 submitted 7 May, 2018; originally announced May 2018.

Comments: 16 pages, 5 figures, supporting information (3 pages)

Spontaneous thermal Hall effect in three-dimensional chiral superconductors with gap nodes

Authors: Nobuyuki Yoshioka, Yoshiki Imai, Manfred Sigrist

Abstract: Generic chiral superconductors with three-dimensional electronic structure have nodal gaps and are not strictly topological. Nevertheless, they exhibit a spontaneous thermal Hall effect (THE), i.e. a transverse temperature gradient in response to a heat current even in the absence of an external magnetic field. While in some cases this THE can be quantized analogous to the Quantum Hall effect, thi… ▽ More Generic chiral superconductors with three-dimensional electronic structure have nodal gaps and are not strictly topological. Nevertheless, they exhibit a spontaneous thermal Hall effect (THE), i.e. a transverse temperature gradient in response to a heat current even in the absence of an external magnetic field. While in some cases this THE can be quantized analogous to the Quantum Hall effect, this is not the case for nodal superconductors in general. In this study we determine the spontaneous THE for tight binding models with tetragonal and hexagonal crystal symmetry with chiral $p$- and d-wave superconducting phase. At the zero-temperature limit, the thermal Hall conductivity $ κ_{xy} $ provides information on the structure of the gap function on the Fermi surface and the Andreev bound states on the surface. The temperature dependence at very low temperatures is determined by the types of gap nodes, point or line nodes, leading to characteristic power law behaviors in the temperature, as known for other quantities such as specific heat or London penetration depth. The generic behavior is discussed on simple models analytically, while the analysis of the tight-binding models is given numerically. △ Less

Submitted 11 April, 2018; originally announced April 2018.

Comments: 11 pages, 9 figures

Journal ref: J. Phys. Soc. Jpn. 87, 124602 (2018)

Electronic States and Energy Dissipations of Vortex Core in Pure FeSe Single Crystals Investigated by Microwave Surface Impedance Measurements

Authors: Tatsunori Okada, Yoshinori Imai, Takahiro Urata, Yoichi Tanabe, Katsumi Tanigaki, Atsutaka Maeda

Abstract: In order to clarify electronic states and energy dissipations due to a motion of a vortex core in pure FeSe, which is a candidate superconductor possessing a super-clean core, we measured the microwave surface impedance of pure FeSe single crystals under finite magnetic fields. From the magnetic-field dependence of the flux-flow resistivity, we found that a barometer of electronic states inside th… ▽ More In order to clarify electronic states and energy dissipations due to a motion of a vortex core in pure FeSe, which is a candidate superconductor possessing a super-clean core, we measured the microwave surface impedance of pure FeSe single crystals under finite magnetic fields. From the magnetic-field dependence of the flux-flow resistivity, we found that a barometer of electronic states inside the vortex core $ω_{0}τ_{\rm core}$ is $1\pm0.5$, suggesting that the vortex core of pure FeSe is in the moderately clean regime contrary to the expectation of the super-clean core. We also found that the mean-free path inside the vortex core is suppressed at the distance of the order of the core radius. Based on observed results and previous reports, we discussed possible origins of rather small $ω_{0}τ_{\rm core}$ value in terms of the multiple-bands nature of FeSe and additional mechanisms producing extra energy dissipations specific to the vortex core in motion. △ Less

Submitted 25 May, 2021; v1 submitted 31 December, 2017; originally announced January 2018.

Comments: 7 pages, 8 figures (6 in main part and 2 in Appendix)

Journal ref: J. Phys. Soc. Jpn., Vol.90, No.9, (2021)

Surface magnetism in a chiral d-wave superconductor with hexagonal symmetry

Authors: Jun Goryo, Yoshiki Imai, W. B. Rui, Manfred Sigrist, Andreas P. Schnyder

Abstract: Surface properties are examined in a chiral d-wave superconductor with hexagonal symmetry, whose one-body Hamiltonian possesses the intrinsic spin-orbit coupling identical to the one characterizing the topological nature of the Kane-Mele honeycomb insulator. In the normal state spin-orbit coupling gives rise to spontaneous surface spin currents, whereas in the superconducting state there exist bes… ▽ More Surface properties are examined in a chiral d-wave superconductor with hexagonal symmetry, whose one-body Hamiltonian possesses the intrinsic spin-orbit coupling identical to the one characterizing the topological nature of the Kane-Mele honeycomb insulator. In the normal state spin-orbit coupling gives rise to spontaneous surface spin currents, whereas in the superconducting state there exist besides the spin currents also charge surface currents, due to the chiral pairing symmetry. Interestingly, the combination of these two currents results in a surface spin polarization, whose spatial dependence is markedly different on the zigzag and armchair surfaces. We discuss various potential candidate materials, such as SrPtAs, which may exhibit these surface properties. △ Less

Submitted 6 October, 2017; v1 submitted 25 August, 2017; originally announced August 2017.

Comments: 7 pages, 9 figures, the final version has been accepted for publication in Phys. Rev. B

Journal ref: Phys. Rev. B 96, 140502 (2017)

Superconducting fluctuations in FeSe$_{0.5}$Te$_{0.5}$ thin films probed via microwave spectroscopy

Authors: Fuyuki Nabeshima, Kosuke Nagasawa, Yoshinori Imai, Atsutaka Maeda

Abstract: We investigated the microwave conductivity spectrum of FeSe$_{0.5}$Te$_{0.5}$ epitaxial films on CaF$_2$ in the vicinity of the superconducting transition. We observed the critical behavior of the superconducting fluctuations in these films with a dimensional crossover from two-dimensional to three-dimensional as the film thickness increased. From the temperature dependence of the scaling paramete… ▽ More We investigated the microwave conductivity spectrum of FeSe$_{0.5}$Te$_{0.5}$ epitaxial films on CaF$_2$ in the vicinity of the superconducting transition. We observed the critical behavior of the superconducting fluctuations in these films with a dimensional crossover from two-dimensional to three-dimensional as the film thickness increased. From the temperature dependence of the scaling parameters we conclude that the universality class of the superconducting transition in FeSe$_{0.5}$Te$_{0.5}$ is that of the 3D-XY model. The lower limit of the onset temperature of the superconducting fluctuations, Tonset, determined by our measurements was 1.1 Tc, suggesting that the superconducting fluctuations of FeSe$_{0.5}$Te$_{0.5}$ are at least as large as those of optimally- and over-doped cuprates. △ Less

Submitted 6 September, 2017; v1 submitted 20 June, 2017; originally announced June 2017.

Journal ref: Phys. Rev. B 97, 024504 (2018)

Effects of strain on the electronic structure, superconductivity, and nematicity in FeSe studied by angle-resolved photoemission spectroscopy

Authors: G. N. Phan, K. Nakayama, K. Sugawara, T. Sato, T. Urata, Y. Tanabe, K. Tanigaki, F. Nabeshima, Y. Imai, A. Maeda, T. Takahashi

Abstract: One of central issues in iron-based superconductors is the role of structural change to the superconducting transition temperature (T_c). It was found in FeSe that the lattice strain leads to a drastic increase in T_c, accompanied by suppression of nematic order. By angle-resolved photoemission spectroscopy on tensile- or compressive-strained and strain-free FeSe, we experimentally show that the i… ▽ More One of central issues in iron-based superconductors is the role of structural change to the superconducting transition temperature (T_c). It was found in FeSe that the lattice strain leads to a drastic increase in T_c, accompanied by suppression of nematic order. By angle-resolved photoemission spectroscopy on tensile- or compressive-strained and strain-free FeSe, we experimentally show that the in-plane strain causes a marked change in the energy overlap (DeltaE_{h-e}) between the hole and electron pockets in the normal state. The change in DeltaE_{h-e} modifies the Fermi-surface volume, leading to a change in T_c. Furthermore, the strength of nematicity is also found to be characterized by DeltaE_{h-e}. These results suggest that the key to understanding the phase diagram is the fermiology and interactions linked to the semimetallic band overlap. △ Less

Submitted 15 June, 2017; originally announced June 2017.

Comments: 6 pages, 3 figures

Journal ref: Phys. Rev. B 95, 224507 (2017)

Orbital order and fluctuations in the two-leg ladder materials BaFe$_2X_3$ ($X$ = S and Se) and CsFe$_2$Se$_3$

Authors: Kou Takubo, Yuichi Yokoyama, Hiroki Wadati, Shun Iwasaki, Takashi Mizokawa, Teak Boyko, Ronny Sutarto, Feizhou He, Kazuki Hashizume, Satoshi Imaizumi, Takuya Aoyama, Yoshinori Imai, Kenya Ohgushi

Abstract: The electronic structure of BaFe$_2X_3$ ($X$ = S and Se) and CsFe$_2$Se$_3$ in which two-leg ladders are formed by the Fe sites are studied by means of x-ray absorption and resonant inelastic x-ray scattering spectroscopy. The x-ray absorption spectra at the Fe L edges for BaFe$_2X_3$ exhibit two components, indicating that itinerant and localized Fe 3$d$ sites coexist. Substantial x-ray linear di… ▽ More The electronic structure of BaFe$_2X_3$ ($X$ = S and Se) and CsFe$_2$Se$_3$ in which two-leg ladders are formed by the Fe sites are studied by means of x-ray absorption and resonant inelastic x-ray scattering spectroscopy. The x-ray absorption spectra at the Fe L edges for BaFe$_2X_3$ exhibit two components, indicating that itinerant and localized Fe 3$d$ sites coexist. Substantial x-ray linear dichroism (XLD) is observed in polarization dependent spectra, indicating the existence of orbital order or fluctuation in the Fe-ladder even above the Néel temperature $T_N$. Direct exchange interaction along the legs of the Fe-ladder stabilizes the orbital and antiferromagnetic orders in BaFe$_2$S$_3$, while the ferromagnetic molecular orbitals are realized between the rungs in CsFe$_2$Se$_3$. △ Less

Submitted 27 September, 2017; v1 submitted 17 April, 2017; originally announced April 2017.

Comments: 7 pages,5 figures

Journal ref: Phys. Rev. B 96, 115157 (2017)

Quantum-disordered state of magnetic and electric dipoles in a hydrogen-bonded Mott system

Authors: M. Shimozawa, K. Hashimoto, A. Ueda, Y. Suzuki, K. Sugii, S. Yamada, Y. Imai, R. Kobayashi, K. Itoh, S. Iguchi, M. Naka, S. Ishihara, H. Mori, T. Sasaki, M. Yamashita

Abstract: Strongly enhanced quantum fluctuations often lead to a rich variety of quantum-disordered states. A representative case is liquid helium, in which zero-point vibrations of the helium atoms prevent its solidification at low temperatures. A similar behaviour is found for the internal degrees of freedom in electrons. Among the most prominent is a quantum spin liquid (QSL), in which localized spins ar… ▽ More Strongly enhanced quantum fluctuations often lead to a rich variety of quantum-disordered states. A representative case is liquid helium, in which zero-point vibrations of the helium atoms prevent its solidification at low temperatures. A similar behaviour is found for the internal degrees of freedom in electrons. Among the most prominent is a quantum spin liquid (QSL), in which localized spins are highly correlated but fluctuate even at absolute zero. Recently, a coupling of spins with other degrees of freedom has been proposed as an innovative approach to generate even more fascinating QSLs such as orbital--spin liquids. However, such ideas are limited to the internal degrees of freedom in electrons. Here, we demonstrate that a coupling of localized spins with the zero-point motion of hydrogen atoms (proton fluctuations) in a hydrogen-bonded organic Mott insulator provides a new class of QSLs. We find that a divergent dielectric behaviour towards a hydrogen-bond order is suppressed by the quantum proton fluctuations, resulting in a quantum paraelectric (QPE) state. Furthermore, our thermal-transport measurements reveal that a QSL state with gapless spin excitations rapidly emerges upon entering the QPE state. These findings indicate that the quantum proton fluctuations give rise to a novel QSL --- a quantum-disordered state of magnetic and electric dipoles --- through the coupling between the electron and proton degrees of freedom. △ Less

Submitted 1 March, 2017; originally announced March 2017.

Comments: 10 pages, 8 figures

Thermal Hall conductivity in the spin-triplet superconductor with broken time-reversal symmetry

Authors: Yoshiki Imai, Katsunori Wakabayashi, Manfred Sigrist

Abstract: Motivated by the spin-triplet superconductor Sr2RuO4, the thermal Hall conductivity is investigated for several pairing symmetries with broken time-reversal symmetry. In the chiral p-wave phase with a fully opened quasiparticle excitation gap, the temperature dependence of the thermal Hall conductivity has a temperature linear term associated with the topological property directly, and an exponent… ▽ More Motivated by the spin-triplet superconductor Sr2RuO4, the thermal Hall conductivity is investigated for several pairing symmetries with broken time-reversal symmetry. In the chiral p-wave phase with a fully opened quasiparticle excitation gap, the temperature dependence of the thermal Hall conductivity has a temperature linear term associated with the topological property directly, and an exponential term, which shows a drastic change around the Lifshitz transition. Examining f-wave states as alternative candidates with $\bm d=Δ_0\hat{z}(k_x^2-k_y^2)(k_x\pm ik_y)$ and $\bm d=Δ_0\hat{z}k_xk_y(k_x\pm ik_y)$ with gapless quasiparticle excitations, we study the temperature dependence of the thermal Hall conductivity, where for the former state the thermal Hall conductivity has a quadratic dependence on temperature, originating from the linear dispersions, in addition to linear and exponential behavior. The obtained result may enable us to distinguish between the chiral p-wave and f-wave states in Sr2RuO4. △ Less

Submitted 1 February, 2017; originally announced February 2017.

Comments: 8 pages, 8 figures

Journal ref: Phys. Rev. B 95, 024516 (2017)

Investigation of Transport Properties for FeSe$_{1-x}$Te$_x$ Thin Films under Magnetic Fields

Authors: Yuichi Sawada, Fuyuki Nabeshima, Yoshinori Imai, Atsutaka Maeda

Abstract: We investigated the transport properties under magnetic fields of up to 9 T for FeSe$_{1-x}$Te$_x$ thin films on CaF$_2$. Measurements of the temperature dependence of the electrical resistivity revealed that for $x = 0.2 - 0.4$, where $T_{\rm c}$ is the highest, the width of the superconducting transition increased with increasing magnetic field, while the width was almost the same with increasin… ▽ More We investigated the transport properties under magnetic fields of up to 9 T for FeSe$_{1-x}$Te$_x$ thin films on CaF$_2$. Measurements of the temperature dependence of the electrical resistivity revealed that for $x = 0.2 - 0.4$, where $T_{\rm c}$ is the highest, the width of the superconducting transition increased with increasing magnetic field, while the width was almost the same with increasing magnetic field for $x = 0 - 0.1$. In addition, the temperature dependence of the Hall coefficient drastically changed between $x = 0.1$ and $0.2$ at low temperatures. These results indicate that clear differences in the nature of the superconductivity and electronic structure exist between $x=0-0.1$ and $x \ge 0.2$. △ Less

Submitted 3 August, 2016; v1 submitted 3 August, 2016; originally announced August 2016.

Comments: 10 pages, 5 figures

Journal ref: J. Phys. Soc. Jpn. 85 (2016) 073703

Low-temperature-compatible tunneling-current-assisted scanning microwave microscope utilizing a rigid coaxial resonator

Authors: Hideyuki Takahashi, yoshinori imai, Atsutaka Maeda

Abstract: We present a design for a tunneling-current-assisted scanning near-field microwave microscope. For stable operation at cryogenic temperatures, making a small and rigid microwave probe is important. Our coaxial resonator probe has a length of approxomately 30 mm and can fit inside the 2-inch bore of a superconducting magnet. The probe design includes an insulating joint, which separates DC and micr… ▽ More We present a design for a tunneling-current-assisted scanning near-field microwave microscope. For stable operation at cryogenic temperatures, making a small and rigid microwave probe is important. Our coaxial resonator probe has a length of approxomately 30 mm and can fit inside the 2-inch bore of a superconducting magnet. The probe design includes an insulating joint, which separates DC and microwave signals without degrading the quality factor. By applying the SMM to the imaging of an electrically inhomogeneous superconductor, we obtain the spatial distribution of the microwave response with a spatial resolution of approximately 200 nm. Furthermore, we present an analysis of our SMM probe based on a simple lumped-element circuit model along with the near-field microwave measurements of silicon wafers having different conductivities. △ Less

Submitted 9 June, 2016; originally announced June 2016.

Journal ref: Rev. Sci. Instrum. 87, 063706 (2016)

Thermal Hall conductivity and topological transition in a chiral p-wave superconductor for Sr2RuO4

Authors: Yoshiki Imai, Katsunori Wakabayashi, Manfred Sigrist

Abstract: The interplay between the thermal transport property and the topological aspect is investigated in a spin-triplet chiral p-wave superconductor Sr2RuO4 with the strong two-dimensionality. We show the thermal Hall conductivity is well described by the temperature linear term and the exponential term in the low temperature region. While the former term is proportional to the so-called Chern number di… ▽ More The interplay between the thermal transport property and the topological aspect is investigated in a spin-triplet chiral p-wave superconductor Sr2RuO4 with the strong two-dimensionality. We show the thermal Hall conductivity is well described by the temperature linear term and the exponential term in the low temperature region. While the former term is proportional to the so-called Chern number directly, the latter is associated with the superconducting gap amplitude of the gamma band. We also demonstrate that the coefficient of the exponential term changes the sign around Lifshitz transition. Our obtained result may enable us access easily the physical quantities and the topological property of Sr2RuO4 in detail. △ Less

Submitted 13 January, 2016; originally announced January 2016.

Comments: 6 pages, 7 figures

Journal ref: Phys. Rev. B 93, 024510 (2016)

Near-field microwave imaging of inhomogeneous K$_x$Fe$_y$Se$_2$: separation of topographic and electric features

Authors: Hideyuki Takahashi, Yoshinori Imai, Atsutaka Maeda

Abstract: It is important for modern scanning microwave microscopes to overcome the effect of the surface roughness. Here, we report microwave conductivity imaging of the phase-separated iron chalcogenide K$_x$Fe$_y$Se$_2$ ($x=0.8$, $y=1.6$-$2$), in which electric conductivity-induced contrast is distinguished from topography-induced contrast using a combination of a scanning tunneling microscope and a scan… ▽ More It is important for modern scanning microwave microscopes to overcome the effect of the surface roughness. Here, we report microwave conductivity imaging of the phase-separated iron chalcogenide K$_x$Fe$_y$Se$_2$ ($x=0.8$, $y=1.6$-$2$), in which electric conductivity-induced contrast is distinguished from topography-induced contrast using a combination of a scanning tunneling microscope and a scanning microwave microscope (STM-SMM). We observed the characteristic modulation of the local electric property that originates from the mesoscopic phase separation of the metallic and semiconducting phases in two different scanning modes: constant current (CC) mode and constant $Q$ (CQ) mode. In particular, CQ scanning is useful because we obtain a qualitative image in which the topographic contrast is largely eliminated without degradation of the spatial resolution. △ Less

Submitted 12 June, 2015; originally announced June 2015.

Journal ref: Appl. Phys. Lett. 106, 233106 (2015)

Suppression of phase separation and giant enhancement of superconducting transition temperature in FeSe$_{1-x}$Te$_{x}$ thin films

Authors: Yoshinori Imai, Yuichi Sawada, Fuyuki Nabeshima, Atsutaka Maeda

Abstract: We demonstrate the successful fabrication on CaF$_2$ substrates of FeSe$_{1-x}$Te$_{x}$ films with $0 \le x \le 1$, including the region of $0.1 \le x \le 0.4$, which is well known to be the "phase-separation region", via pulsed laser deposition which is a thermodynamically non-equilibrium method. In the resulting films, we observe a giant enhancement of the superconducting transition temperature,… ▽ More We demonstrate the successful fabrication on CaF$_2$ substrates of FeSe$_{1-x}$Te$_{x}$ films with $0 \le x \le 1$, including the region of $0.1 \le x \le 0.4$, which is well known to be the "phase-separation region", via pulsed laser deposition which is a thermodynamically non-equilibrium method. In the resulting films, we observe a giant enhancement of the superconducting transition temperature, $T_\mathrm{c}$, in the region of $0.1 \le x \le 0.4$: the maximum value reaches 23 K, which is approximately 1.5 times as large as the values reported for bulk samples of FeSe$_{1-x}$Te$_{x}$. We present a complete phase diagram of FeSe$_{1-x}$Te$_{x}$ films. Surprisingly, a sudden suppression of $T_\mathrm{c}$ is observed at $0.1<x<0.2$, while $T_\mathrm{c}$ increases with decreasing $x$ for $0.2 \le x < 1$. Namely, there is a clear difference between superconductivity realized in $x=0-0.1$ and in $x \ge 0.2$. To obtain a film of FeSe$_{1-x}$Te$_{x}$ with high $T_\mathrm{c}$, the controls of the Te content $x$ and the in-plane lattice strain are found to be key factors. △ Less

Submitted 3 February, 2015; originally announced February 2015.

Comments: Accepted for publication in Proc. Natl. Acad. Sci. USA

Point-contact Andreev-reflection spectroscopy in Fe(Te,Se) films: multiband superconductivity and electron-boson coupling

Authors: D. Daghero, P. Pecchio, G. A. Ummarino, F. Nabeshima, Y. Imai, A. Maeda, I. Tsukada, S. Komiya, R. S. Gonnelli

Abstract: We report on a study of the superconducting order parameter in Fe(Te$_{1-x}$Se$_{x}$) thin films (with different Se contents: x=0.3, 0.4, 0.5) by means of point-contact Andreev-reflection spectroscopy (PCARS). The PCARS spectra show reproducible evidence of multiple structures, namely two clear conductance maxima associated to a superconducting gap of amplitude $Δ_E \simeq 2.75 k_B T_c$ and additi… ▽ More We report on a study of the superconducting order parameter in Fe(Te$_{1-x}$Se$_{x}$) thin films (with different Se contents: x=0.3, 0.4, 0.5) by means of point-contact Andreev-reflection spectroscopy (PCARS). The PCARS spectra show reproducible evidence of multiple structures, namely two clear conductance maxima associated to a superconducting gap of amplitude $Δ_E \simeq 2.75 k_B T_c$ and additional shoulders at higher energy that, as we show, are the signature of the strong interaction of charge carriers with a bosonic mode whose characteristic energy coincides with the spin-resonance energy. The details of some PCARS spectra at low energy suggest the presence of a smaller and not easily discernible gap of amplitude $Δ_H \simeq 1.75 k_B T_c$. The existence of this gap and its amplitude are confirmed by PCARS measurements in Fe(Te$_{1-x}$Se$_{x}$) single crystals. The values of the two gaps $Δ_E$ and $Δ_H$, once plotted as a function of the local critical temperature $T_c^A$, turn out to be in perfect agreement with the results obtained by various experimental techniques reported in literature. △ Less

Submitted 15 December, 2014; originally announced December 2014.

Comments: 8 pages, 6 figures. This is an author-created, un-copyedited version of an article published in Supercond. Sci. Technol. IOP Publishing Ltd is not responsible for any errors or omissions in this version of the manuscript or any version derived from it. The Version of Record is available online at 10.1088/0953-2048/27/12/124014

Journal ref: Supercond. Sci. Technol. 27, 124014 (2014)

Effect of the RuO6 octahedron rotation at the Sr2RuO4 surface on topological property

Authors: Yoshiki Imai, Katsunori Wakabayashi, Manfred Sigrist

Abstract: We investigate the rotation effect of the RuO$_6$ octahedron around the $c$ axis on the topological and transport properties near the surface of the spin-triplet superconductor Sr$_2$RuO$_4$. While the Fermi level of bulk Sr$_2$RuO$_4$ is near the Lifshitz transition, the RuO$_6$ rotation realized near the surface leads to the change of the Fermi surface topology. The edge current resulting from t… ▽ More We investigate the rotation effect of the RuO$_6$ octahedron around the $c$ axis on the topological and transport properties near the surface of the spin-triplet superconductor Sr$_2$RuO$_4$. While the Fermi level of bulk Sr$_2$RuO$_4$ is near the Lifshitz transition, the RuO$_6$ rotation realized near the surface leads to the change of the Fermi surface topology. The edge current resulting from the time-reversal symmetry breaking in the chiral $p$-wave phase with fully opened excitation gap is less affected around Lifshitz transition. The topological property and the edge state are sensitive to the rotation angle and the amplitude of the nearest neighbor interaction, and the superconducting gap is strongly reduced in the larger next nearest neighbor interaction region. Although the edge state in Sr$_2$RuO$_4$ is topologically protected, it is not robust to the disorder such as impurity or defect. △ Less

Submitted 1 November, 2014; originally announced November 2014.

Comments: 17 pages, 9 figures; to appear in J. Phys. Soc. Jpn

Journal ref: J. Phys. Soc. Jpn. 83, 124712 (2014)

Exceptional Suppression of Flux-Flow Resistivity in FeSe$_{0.4}$Te$_{0.6}$ by Back-Flow from Excess Fe Atoms and Se/Te Substitutions

Authors: Tatsunori Okada, Fuyuki Nabeshima, Hideyuki Takahashi, Yoshinori Imai, Atsutaka Maeda

Abstract: We measured the microwave surface impedance of FeSe$_{0.4}$Te$_{0.6}$ single crystals with- and without external magnetic fields. The superfluid density exhibited a quadratic temperature dependence, indicating a strong pair-breaking effect. The flux-flow resistivity behaved as $ρ_f(B\ll B_{\rm c2})/ρ_n=αB/B_{\rm c2}$. The observed $α$ value of $\approx0.66$ was considerably smaller than that of ot… ▽ More We measured the microwave surface impedance of FeSe$_{0.4}$Te$_{0.6}$ single crystals with- and without external magnetic fields. The superfluid density exhibited a quadratic temperature dependence, indicating a strong pair-breaking effect. The flux-flow resistivity behaved as $ρ_f(B\ll B_{\rm c2})/ρ_n=αB/B_{\rm c2}$. The observed $α$ value of $\approx0.66$ was considerably smaller than that of other Fe-based materials ($α\geq1$) and was attributed to a back-flow of superfluids remarkable in disordered superconductors. This is the first-time observation of the back-flow phenomenon caused by an origin other than the vortex pinning in multiple-band systems. △ Less

Submitted 5 December, 2014; v1 submitted 28 June, 2014; originally announced June 2014.

Comments: 7 pages, 5 figures, and 1 table

Journal ref: Phys. Rev. B 91, 054510 (2015)

Enhancement of the Superconducting Transition Temperature in FeSe Epitaxial Thin Films by Anisotropic Compression

Authors: Fuyuki Nabeshima, Yoshinori Imai, Masafumi Hanawa, Ichiro Tsukada, Atsutaka Maeda

Abstract: In order to investigate the effects of in-plane strain on the superconductivity of FeSe, epitaxial thin films of FeSe were fabricated on CaF$_2$ substrates. The films are compressed along the a-axis and their superconducting transition temperatures $T_{\mathrm c}^{\mathrm {zero}}$ reach 11.4 K, which is approximately 1.5 times higher than that of bulk crystals. The $T_{\mathrm c}$ values are weakl… ▽ More In order to investigate the effects of in-plane strain on the superconductivity of FeSe, epitaxial thin films of FeSe were fabricated on CaF$_2$ substrates. The films are compressed along the a-axis and their superconducting transition temperatures $T_{\mathrm c}^{\mathrm {zero}}$ reach 11.4 K, which is approximately 1.5 times higher than that of bulk crystals. The $T_{\mathrm c}$ values are weakly dependent on the ratio of the lattice constants, $c$ / $a$, compared to that of Fe(Se,Te). Our results indicate that even a binary system FeSe has room for improvement, and will open a new route for the application of Fe-based superconductors. △ Less

Submitted 13 September, 2013; originally announced September 2013.

Comments: 4 pages, 3 figures

Journal ref: Appl. Phys Lett. 103 (2013) 172602

Topological and edge state properties of a three-band model for Sr2RuO4

Authors: Yoshiki Imai, Katsunori Wakabayashi, Manfred Sigrist

Abstract: Modeling the spin-triplet superconductor Sr2RuO4 through a three-orbital tight-binding model we investigate topological properties and edge states assuming chiral p-wave pairing. In concordance with experiments the three Fermi surfaces consist of two electron-like and one hole-like one corresponding to the alpha-, beta- and gamma-band on the level of a two-dimensional system. The quasi-particle sp… ▽ More Modeling the spin-triplet superconductor Sr2RuO4 through a three-orbital tight-binding model we investigate topological properties and edge states assuming chiral p-wave pairing. In concordance with experiments the three Fermi surfaces consist of two electron-like and one hole-like one corresponding to the alpha-, beta- and gamma-band on the level of a two-dimensional system. The quasi-particle spectra and other physical quantities of the superconducting phase are calculated by means of a self-consistent Bogoliubov-de Gennes approach for a ribbon shaped system. While a full quasiparticle excitation gap is realized in the bulk system, at the edges gapless states appear some of which have linear and others nearly flat dispersion around zero energy. This study shows the interplay between spin-orbit coupling induced spin currents, chiral edge currents and correlation driven surface magnetism. The topological nature of the chiral p-wave state manifests itself in the gamma-band characterized by an integer Chern number. As the gamma-band is close to a Lifshitz transition in Sr2RuO4, changing the sign of the Chern number, the topological nature may be rather fragile. △ Less

Submitted 14 October, 2013; v1 submitted 9 July, 2013; originally announced July 2013.

Comments: 10 pages, 16 figures

Journal ref: Phys. Rev. B 88, 144503 (2013)

Reproducible synthetic method for the topological superconductor CuxBi2Se3

Authors: Ryusuke Kondo, Taiki Yoshinaka, Yoshinori Imai, Atsutaka Maeda

Abstract: We report a reproducible synthetic method for superconducting Cu-intercalated Bi2Se3 by an improved melt growth method. Avoiding the production of Cu2Se, which has a higher melting point than that of Bi2Se3, and quenching Cu-Bi-Se mixtures at the liquid phase are keys to obtaining good superconducting samples in a reproducible manner. We report a reproducible synthetic method for superconducting Cu-intercalated Bi2Se3 by an improved melt growth method. Avoiding the production of Cu2Se, which has a higher melting point than that of Bi2Se3, and quenching Cu-Bi-Se mixtures at the liquid phase are keys to obtaining good superconducting samples in a reproducible manner. △ Less

Submitted 15 March, 2013; originally announced March 2013.

Comments: 4 pages, 5 figures

Microscopic analysis of the chemical reaction between Fe(Te,Se) thin films and underlying CaF$_2$

Authors: A. Ichinose, I. Tsukada, M. Hanawa, Seiki Komiya, T. Akiike, F. Nabeshima, Y. Imai, A. Maeda

Abstract: To understand the chemical reaction at the interface of materials, we performed a transmission electron microscopy (TEM) observation in four types of Fe(Te,Se) superconducting thin films prepared on different types of substrates: CaF2 substrate, CaF2 substrate with a CaF2 buffer layer, CaF2 substrate with a FeSe buffer layer, and a LaAlO3 substrate with a CaF2 buffer layer. Based on the energy-dis… ▽ More To understand the chemical reaction at the interface of materials, we performed a transmission electron microscopy (TEM) observation in four types of Fe(Te,Se) superconducting thin films prepared on different types of substrates: CaF2 substrate, CaF2 substrate with a CaF2 buffer layer, CaF2 substrate with a FeSe buffer layer, and a LaAlO3 substrate with a CaF2 buffer layer. Based on the energy-dispersive X-ray spectrometer (EDX) analysis, we found possible interdiffusion between fluorine and selenium that has a strong influence on the superconductivity in Fe(Te,Se) films. The chemical interdiffusion also plays a significant role in the variation of the lattice parameters. The lattice parameters of the Fe(Te,Se) thin films are primarily determined by the chemical substitution of anions, and the lattice mismatch only plays a secondary role. △ Less

Submitted 16 August, 2012; originally announced August 2012.

Comments: 30 pages, 9 figure

Superconductivity at 5.4 K in $β$-Bi$_2$Pd

Authors: Yoshinori Imai, Fuyuki Nabeshima, Taiki Yoshinaka, Kosuke Miyatani, Ryusuke Kondo, Seiki Komiya, Ichiro Tsukada, Atsutaka Maeda

Abstract: We investigate bulk superconductivity in a high-quality single crystal of Bi$_2$Pd ($β$-Bi$_2$Pd, space group; I4/mmm) at temperatures less than 5.4 K by exploring its electrical resistivity, magnetic susceptibility, and specific heat. The temperature dependence of the electrical resistivity shows convex-upward behaviors at temperatures greater than 40-50 K, which can be explained by a parallel-re… ▽ More We investigate bulk superconductivity in a high-quality single crystal of Bi$_2$Pd ($β$-Bi$_2$Pd, space group; I4/mmm) at temperatures less than 5.4 K by exploring its electrical resistivity, magnetic susceptibility, and specific heat. The temperature dependence of the electrical resistivity shows convex-upward behaviors at temperatures greater than 40-50 K, which can be explained by a parallel-resistor model. In addition, we demonstrate that this material is a multiple-band/multiple-gap superconductor based on the temperature dependences of the specific heat and the upper critical field. △ Less

Submitted 28 October, 2012; v1 submitted 25 July, 2012; originally announced July 2012.

Comments: 4 pages, 3 figures

Journal ref: J. Phys. Soc. Jpn., 81 (2012) 113708

Investigation of the Superconducting Gap Structure in SrFe$_2$(As$_{0.7}$P$_{0.3}$)$_2$ by Magnetic Penetration Depth and Flux Flow Resistivity Analysis

Authors: Hideyuki Takahashi, Tatsunori Okada, Yoshinori Imai, Kentaro Kitagawa, Kazuyuki Matsubayashi, Yoshiya Uwatoko, Atsutaka Maeda

Abstract: We measured the microwave surface impedances and obtained the superfluid density and flux flow resistivity in single crystals of a phosphor-doped iron-based superconductor SrFe$_2$(As$_{1-x}$P$_{x}$)$_2$ single crystals ($x=0.30$, $T_c=25 \mathrm{K}$). At low temperatures, the superfluid density, $n_s (T)/n_s(0)$, obeys a power law, $n_s (T)/n_s (0)=1-C(T/T_c)^n$, with a fractional exponent of… ▽ More We measured the microwave surface impedances and obtained the superfluid density and flux flow resistivity in single crystals of a phosphor-doped iron-based superconductor SrFe$_2$(As$_{1-x}$P$_{x}$)$_2$ single crystals ($x=0.30$, $T_c=25 \mathrm{K}$). At low temperatures, the superfluid density, $n_s (T)/n_s(0)$, obeys a power law, $n_s (T)/n_s (0)=1-C(T/T_c)^n$, with a fractional exponent of $n=1.5$-1.6. The flux flow resistivity was significantly enhanced at low magnetic fields. These features are consistent with the presences of both a gap with line nodes and nodeless gaps with a deep minimum. The remarkable difference observed in the superconducting gap structure between SrFe$_2$(As$_{1-x}$P$_{x}$)$_2$ and BaFe$_2$(As$_{1-x}$P$_{x}$)$_2$ in our experiments is important for clarifying the mechanism of iron-based superconductivity. △ Less

Submitted 13 July, 2012; originally announced July 2012.

Journal ref: Phys. Rev. B. 86,144525 (2012)

Properties of edge states in spin-triplet two-band superconductor

Authors: Yoshiki Imai, Katsunori Wakabayashi, Manfred Sigrist

Abstract: Motivated by Sr2RuO4 the magnetic properties of edge states in a two-band spin-triplet superconductor with electron- and hole-like Fermi surfaces are investigated assuming chiral p-wave pairing symmetry. The two bands correspond to the alpha-beta-bands of Sr2RuO4 and are modeled within a tight-binding model including inter-orbital hybridization and spin-orbit coupling effects. Including supercondu… ▽ More Motivated by Sr2RuO4 the magnetic properties of edge states in a two-band spin-triplet superconductor with electron- and hole-like Fermi surfaces are investigated assuming chiral p-wave pairing symmetry. The two bands correspond to the alpha-beta-bands of Sr2RuO4 and are modeled within a tight-binding model including inter-orbital hybridization and spin-orbit coupling effects. Including superconductivity the quasiparticle spectrum is determined by means of a self-consistent Bogolyubov-de Gennes calculation. While a full quasiparticle excitation gap appears in the bulk, gapless states form at the edges which produce spontaneous spin and/or charge currents. The spin current is the result of the specific band structure while the charge current originates from the superconducting condensate. Together they induce a small spin polarization at the edge. Furthermore onsite Coulomb repulsion is included to show that the edge states are unstable against the formation of a Stoner-like spin polarization of the edge states. Through spin-orbit coupling the current- and the correlation-induced magnetism are coupled to the orientation of the chirality of the superconducting condensate. We speculate that this type of phenomenon could yield a compensation of the magnetic fields induced by currents and also explain the negative result in the recent experimental search for chiral edge currents. △ Less

Submitted 8 May, 2012; originally announced May 2012.

Comments: 9 pages, 13 figures, to appear in Physical Review B

Journal ref: Phys. Rev. B 85, 174532 (2012)

Ultralow-dissipative conductivity by Dirac fermions in BaFe$_2$As$_2$

Authors: Yoshinori Imai, Fuyuki Nabeshima, Daisuke Nakamura, Takayoshi Katase, Hidenori Hiramatsu, Hideo Hosono, Atsutaka Maeda

Abstract: We report on the anomalous behavior of the complex conductivity of BaFe$_{2}$As$_{2}$, which is related to the Dirac cone, in the terahertz (THz)-frequency region. Above the spin-density-wave (SDW) transition temperature, the conductivity spectra follow the Drude model. In the SDW state, the imaginary part of the complex conductivity, $σ_2$, is suppressed in comparison to that expected according t… ▽ More We report on the anomalous behavior of the complex conductivity of BaFe$_{2}$As$_{2}$, which is related to the Dirac cone, in the terahertz (THz)-frequency region. Above the spin-density-wave (SDW) transition temperature, the conductivity spectra follow the Drude model. In the SDW state, the imaginary part of the complex conductivity, $σ_2$, is suppressed in comparison to that expected according to the Drude model. The real part, $σ_1$, exhibits nearly Drude-like behavior. This behavior (i.e., almost no changes in $σ_1$ and the depression of $σ_2$) can be regarded as the addition of extra conductivity without any dissipations in the Drude-type conductivity. The origin of this ultralow-dissipative conductivity is found to be due to conductivity contribution from quasiparticles within the Dirac cone. In other words, we are able to observe the dynamics of Dirac fermions through the conductivity spectra of BaFe$_2$As$_2$, clearly and directly. △ Less

Submitted 2 February, 2012; originally announced February 2012.

Comments: 5pages, 3 figures

Journal ref: J. Phys. Soc. Jpn., 82 (2013) 043709