Showing 1–50 of 75 results for author: Fujii, T

Derivation of a non-stoichiometric 1/1 quasicrystal approximant from a stoichiometric 2/1 quasicrystal approximant and maximization of magnetocaloric effect

Authors: Farid Labib, Hiroyuki Takakura, Asuka Ishikawa, Takenori Fujii, Ryuji Tamura

Abstract: The present research introduces a novel strategy for tuning magnetic properties by overcoming the compositional limitation of stoichiometric intermetallic compounds via extension of their stability into a new dimension within valence electron-per-atom (e/a) parameter space. Focusing on approximant crystals (ACs), a "double hetero-valent elemental substitution" is employed in a stoichiometric Ga-Pt… ▽ More The present research introduces a novel strategy for tuning magnetic properties by overcoming the compositional limitation of stoichiometric intermetallic compounds via extension of their stability into a new dimension within valence electron-per-atom (e/a) parameter space. Focusing on approximant crystals (ACs), a "double hetero-valent elemental substitution" is employed in a stoichiometric Ga-Pt-Gd 2/1 AC whereby e/a is lowered from 1.92 to 1.60. Through this approach a new family of stable Ga-based Tsai-type 1/1 ACs with exceptionally wide composition stability within e/a space is derived. Remarkably, magnetic ground state is altered from initially spin-glass to ferromagnetic (FM) with second order phase transition and mean-field-like critical behavior. More importantly, through this strategy, the isothermal magnetic entropy change enhanced significantly and reached a maximum value of -8.7 J/K mol-Gd under a 5 T magnetic field change, even comparable to leading rare-earth magnetocaloric materials including RCo2 phases. These findings demonstrate the high potential of a double hetero-valent elemental substitution for tailoring magnetic properties and magnetocaloric response in stoichiometric compounds, offering a new pathway for designing high-performance magnetic refrigeration materials even beyond the quasicrystals and ACs. △ Less

Submitted 5 June, 2025; originally announced June 2025.

Comments: 5 figures

Proximity-Induced Nodal Metal in an Extremely Underdoped CuO$_2$ Plane in Triple-Layer Cuprates

Authors: Shin-ichiro Ideta, Shintaro Adachi, Takashi Noji, Shunpei Yamaguchi, Nae Sasaki, Shigeyuki Ishida, Shin-ichi Uchida, Takenori Fujii, Takao Watanabe, Wen O. Wang, Brian Moritz, Thomas P. Devereaux, Masashi Arita, Chung-Yu Mou, Teppei Yoshida, Kiyohisa Tanaka, Ting-Kuo Lee, Atsushi Fujimori

Abstract: ARPES studies have established that the high-$T_c$ cuprates with single and double CuO$_2$ layers evolve from the Mott insulator to the pseudogap state with a Fermi arc, on which the superconducting (SC) gap opens. In four- to six-layer cuprates, on the other hand, small hole Fermi pockets are formed in the innermost CuO$_2$ planes, indicating antiferromagnetism. Here, we performed ARPES studies o… ▽ More ARPES studies have established that the high-$T_c$ cuprates with single and double CuO$_2$ layers evolve from the Mott insulator to the pseudogap state with a Fermi arc, on which the superconducting (SC) gap opens. In four- to six-layer cuprates, on the other hand, small hole Fermi pockets are formed in the innermost CuO$_2$ planes, indicating antiferromagnetism. Here, we performed ARPES studies on the triple-layer Bi$_2$Sr$_2$Ca$_2$Cu$_3$O$_{10+δ}$ over a wide doping range, and found that, although the doping level of the inner CuO$_2$ plane was extremely low in underdoped samples, the $d$-wave SC gap was enhanced to the unprecedentedly large value of $Δ_0\sim$100 meV at the antinode and persisted well above $T_{c}$ without the appearance of a Fermi arc, indicating a robust ``nodal metal''. We attribute the nodal metallic behavior to the unique local environment of the inner clean CuO$_2$ plane in the triple-layer cuprates, sandwiched by nearly optimally-doped two outer CuO$_2$ planes and hence subject to strong proximity effect from both sides. In the nodal metal, quasiparticle peaks showed electron-hole symmetry, suggesting $d$-wave pairing fluctuations. Thus the proximity effect on the innermost CuO${_2}$ plane is the strongest in the triple-layer cuprates, which explains why the $T_c$ reaches the maximum at the layer number of three in every multi-layer cuprate family. △ Less

Submitted 21 February, 2025; originally announced February 2025.

Journal ref: Nature Communications 16, 9470 (2025)

Topology meets time-reversal symmetry breaking in FeSe$_{1-x}$Te$_{x}$ superconductor

Authors: M. Roppongi, Y. Cai, K. Ogawa, S. Liu, G. Q. Zhao, M. Oudah, T. Fujii, K. Imamura, S. Fang, K. Ishihara, K. Hashimoto, K. Matsuura, Y. Mizukami, M. Pula, C. Young, I. Markovic, D. A. Bonn, T. Watanabe, A. Yamashita, Y. Mizuguchi, G. M. Luke, K. M. Kojima, Y. J. Uemura, T. Shibauchi

Abstract: Time-reversal symmetry breaking (TRSB) in magnetic topological insulators induces a Dirac gap in the topological surface state (TSS), leading to exotic phenomena such as the quantum anomalous Hall effect. Yet, the interplay between TRSB and topology in superconductors remains underexplored due to limited suitable materials. Here we employ zero-field muon spin relaxation ($μ$SR) as a sensitive prob… ▽ More Time-reversal symmetry breaking (TRSB) in magnetic topological insulators induces a Dirac gap in the topological surface state (TSS), leading to exotic phenomena such as the quantum anomalous Hall effect. Yet, the interplay between TRSB and topology in superconductors remains underexplored due to limited suitable materials. Here we employ zero-field muon spin relaxation ($μ$SR) as a sensitive probe of TRSB to map out the electronic phase diagrams of iron-chalcogenide superconductors FeSe$_{1-x}$Te$_{x}$. For the Te composition $x=0.64$ with the highest superconducting transition temperature $T_{\rm c}=14.5$ K, which is known to host a TSS and Majorana zero modes within vortices, we detect spontaneous magnetic fields below $T_{\rm c}$ distinct from a magnetic order. This signifies a TRSB superconducting state in the bulk, revealing the convergence of unconventional TRSB superconductivity with topologically nontrivial electronic structures in FeSe$_{1-x}$Te$_{x}$. Given the relatively high $T_{\rm c}$ and the tunability of the Fermi level through chemical substitution, iron-chalcogenide superconductors offer an intriguing platform for investigating the synergy between topological superconductivity and TRSB. △ Less

Submitted 6 January, 2025; originally announced January 2025.

Comments: 20 pages, 4 figures

Effects of vortex and antivortex excitations in underdoped Bi$_2$Sr$_2$Ca$_2$Cu$_3$O$_{10+δ}$ bulk single crystals

Authors: Takao Watanabe, Kenta Kosugi, Nae Sasaki, Shunpei Yamaguchi, Takenori Fujii, Ken Hayama, Itsuhiro Kakeya, Toshimitsu Ito

Abstract: The observance of vortex and anti-vortex effects in bulk crystals can prove the existence of phase-disordered superconductivity in the bulk. To gain insights into the mechanisms that govern superconducting transition in copper oxide high-transition temperature ($T_c$) superconductors, this study investigated the transport properties of underdoped Bi$_2$Sr$_2$Ca$_2$Cu$_3$O$_{10+δ}$ (Bi-2223) bulk s… ▽ More The observance of vortex and anti-vortex effects in bulk crystals can prove the existence of phase-disordered superconductivity in the bulk. To gain insights into the mechanisms that govern superconducting transition in copper oxide high-transition temperature ($T_c$) superconductors, this study investigated the transport properties of underdoped Bi$_2$Sr$_2$Ca$_2$Cu$_3$O$_{10+δ}$ (Bi-2223) bulk single crystals.The $I$-$V$ characteristics results and the typical tailing behavior owing to the temperature dependence of in-plane resistivity ($ρ_{ab}$) were consistent with the Kosterlitz-Thouless (KT) transition characteristics. Thus, with increasing temperature, copper oxide high-$T_c$ superconductors transitioned to their normal state owing to destruction of their phase correlations, although a finite Cooper pair density was prevalent at $T_c$. Further, magnetization measurements were performed to determine the temperature dependence of the irreversible magnetic field $B_{irr}$. Consequently, the mechanism governing the KT transition-like superconducting transition in this bulk system was elucidated. These results support the extreme strong-coupling models for high-$T_c$ superconductivity in cuprates. △ Less

Submitted 10 October, 2024; v1 submitted 10 May, 2024; originally announced May 2024.

Comments: 14 pages, 11 figures, revised according to the referees'comments

Journal ref: Phys. Rev. B 110, 134509 (2024)

Experimental nuclear quadrupole resonance and computational study of the structurally refined topological semimetal TaSb$_2$

Authors: T. Fujii, O. Janson, H. Yasuoka, H. Rosner, Yu. Prots, U. Burkhardt, M. Schmidt, M. Baenitz

Abstract: The local electric field gradients and magnetic dynamics of TaSb$_2$ have been studied using $^{121}$Sb, $^{123}$Sb, and $^{181}$Ta nuclear quadrupole resonance (NQR) with density functional theory (DFT) calculations using XRD-determined crystal structures. By measuring all structurally expected thirteen NQR lines, the nuclear quadrupole coupling constant ($ν_Q$) and asymmetric parameter ($η$) for… ▽ More The local electric field gradients and magnetic dynamics of TaSb$_2$ have been studied using $^{121}$Sb, $^{123}$Sb, and $^{181}$Ta nuclear quadrupole resonance (NQR) with density functional theory (DFT) calculations using XRD-determined crystal structures. By measuring all structurally expected thirteen NQR lines, the nuclear quadrupole coupling constant ($ν_Q$) and asymmetric parameter ($η$) for Ta, Sb(1), and Sb(2) sites were obtained. These values are all in good agreement with the presented DFT calculations. Principal axes of the electric field gradients was determined for a single-crystal sample by measuring the angular dependencies of NMR frequency under a weak magnetic field. The unusual temperature dependence of $η$(T) of Sb(2) hints at the suppressed thermal expansion along the $a$-axis. Spin lattice relaxation rate ($1/T_1T$) measurements reveal an activated-type behavior and an upturn below 30 K. Neither the low temperature upturn nor the high temperature activation type behaviors are reproduced by the calculated $1/T_1T$ based on the calculated density of states (DOS). On the other hand, the agreement between the calculated DOS and specific heat measurements indicates that the band renormalization is small. This fact indicates that TaSb$_2$ deviates from the simple semimetal scenario, and magnetic excitations are not captured by Fermi liquid theory. △ Less

Submitted 18 December, 2023; originally announced December 2023.

Comments: 15 pages, 13 figures

Journal ref: Phys. Rev. B 109, 035116 (2024)

Unveiling exotic magnetic phase diagram of a non-Heisenberg quasicrystal approximant

Authors: Farid Labib, Kazuhiro Nawa, Shintaro Suzuki, Hung-Cheng Wu, Asuka Ishikawa, Kazuki Inagaki, Takenori Fujii, Katsuki Kinjo, Taku J. Sato, Ryuji Tamura

Abstract: A magnetic phase diagram of the non-Heisenberg Tsai-type 1/1 Au-Ga-Tb approximant crystal (AC) has been established across a wide electron-per-atom (e/a) range via magnetization and powder neutron diffraction measurements. The diagram revealed exotic ferromagnetic (FM) and antiferromagnetic (AFM) orders that originate from the unique local spin icosahedron common to icosahedral quasicrystals (iQCs… ▽ More A magnetic phase diagram of the non-Heisenberg Tsai-type 1/1 Au-Ga-Tb approximant crystal (AC) has been established across a wide electron-per-atom (e/a) range via magnetization and powder neutron diffraction measurements. The diagram revealed exotic ferromagnetic (FM) and antiferromagnetic (AFM) orders that originate from the unique local spin icosahedron common to icosahedral quasicrystals (iQCs) and ACs; The noncoplanar whirling AFM order is stabilized as the ground state at the e/a of 1.72 or less whereas a noncoplanar whirling FM order was found at the larger e/a of 1.80, with magnetic moments tangential to the Tb icosahedron in both cases. Moreover, the FM/AFM phase selection rule was unveiled in terms of the nearest neighbour (J1) and next nearest neighbour (J2) interactions by numerical calculations on a non-Heisenberg single icosahedron. The present findings will pave the way for understanding the intriguing magnetic orders of not only non-Heisenberg FM/AFM ACs but also non-Heisenberg FM/AFM iQCs, the latter of which are yet to be discovered. △ Less

Submitted 22 October, 2023; originally announced October 2023.

Atomic structure and magnetism of the Au-Ga-Ce 1/1 approximant crystal

Authors: Shintaro Suzuki, Azusa Motouri, Kazuhiko Deguchi, Tsunetomo Yamada, Asuka Ishikawa, Takenori Fujii, Kazuhiro Nawa, Taku J. Sato, Ryuji Tamura

Abstract: We report a new Au-Ga-Ce 1/1 approximant crystal (AC) which possesses a significantly wide single-phase region of 53 - 70 at% Au and 13.6 - 15.1 at% Ce. Single crystal X-ray structural analyses reveal the existence of two types of structural degrees of freedom, i.e., the Au/Ga mixing sites and the fractional Ce occupancy site: the former enables a large variation in the electron concentration and… ▽ More We report a new Au-Ga-Ce 1/1 approximant crystal (AC) which possesses a significantly wide single-phase region of 53 - 70 at% Au and 13.6 - 15.1 at% Ce. Single crystal X-ray structural analyses reveal the existence of two types of structural degrees of freedom, i.e., the Au/Ga mixing sites and the fractional Ce occupancy site: the former enables a large variation in the electron concentration and the latter allows a variation in the occupancy of a magnetic impurity atom at the center of the Tsai-type cluster. Following these findings, the influences of two types of structural modifications on the magnetism are thoroughly investigated by means of magnetic susceptibility and specific heat measurements on the Au-Ga-Ce 1/1 AC. The spin-glass (SG) state is found to be the ground state over the entire single-phase region, showing a robust nature of the SG state against both structural modifications. In addition, a gigantic specific heat (C/T) is commonly observed at low temperatures for all the compositions, which is consistently explained as a consequence of the spin-freezing phenomenon, not of a heavy Fermion behavior as reported elsewhere. Moreover, the origin of the SG state in the 1/1 Au-Ga-Ce AC is attributed to the existence of non-magnetic atom disorder in the Au/Ga mixing sites. Furthermore, a Kondo behavior is observed in the electrical resistivity at low temperatures, which is enhanced by increasing the Ce concentration, verifying that a Ce atom introduced at the cluster center behaves as a Kondo impurity for the first time. △ Less

Submitted 19 August, 2023; originally announced August 2023.

Comments: 8 pages, 4 figures

Comparative $^{181}$Ta-NQR Study of Weyl Monopnictides TaAs and TaP: Relevance of Weyl Fermion Excitations

Authors: Tetsuro Kubo, Hiroshi Yasuoka, Balázs Dóra, Deepa Kasinathan, Yurii Prots, Helge Rosner, Takuto Fujii, Marcus Schmidt, Michael Baenitz

Abstract: Based on our first detailed $^{181}$Ta nuclear quadrupole resonance (NQR) studies from 2017 on the Weyl semimetal TaP, we now extended our NQR studies to another Ta-based monopnictide TaAs. In the present work, we have determined the temperature-dependent $^{181}$Ta-NQR spectra, the spin-lattice relaxation time $T_{1}$, and the spin-spin relaxation time $T_{2}$. We found the following characterist… ▽ More Based on our first detailed $^{181}$Ta nuclear quadrupole resonance (NQR) studies from 2017 on the Weyl semimetal TaP, we now extended our NQR studies to another Ta-based monopnictide TaAs. In the present work, we have determined the temperature-dependent $^{181}$Ta-NQR spectra, the spin-lattice relaxation time $T_{1}$, and the spin-spin relaxation time $T_{2}$. We found the following characteristic features that showed great contrast to what was found in TaP: (1) The quadrupole coupling constant and asymmetry parameter of EFG, extracted from three NQR frequencies, have a strong temperature dependence above $\sim$80 K that cannot be explained by the density functional theory calculation incorporating the thermal expansion of the lattice. (2) The temperature dependence of the spin-lattice relaxation rate, $1/T_{1} T$, shows a $T^{4}$ power law behavior above $\sim$30 K. This is a great contrast with the $1/T_{1} T \propto T^{2}$ behavior found in TaP, which was ascribed to the magnetic excitations at the Weyl nodes with a temperature-dependent orbital hyperfine coupling. (3) Regarding the nuclear spin-spin interaction, we found the spin-echo signal decays with the pulse separation simply by a Lorentzian function in TaAs, but we have observed spin-echo modulations in TaP that is most likely due to the indirect nuclear spin-spin coupling via virtually excited Weyl fermions. From our experimental findings, we conclude that the present NQR results do not show dominant contributions from Weyl fermion excitations in TaAs. △ Less

Submitted 29 July, 2023; originally announced July 2023.

Comments: 8 pages, 10 figures

Journal ref: J. Phys. Soc. Jpn. 92, 094706 (2023)

Emergence of long-range magnetic order from spin-glass state by tuning electron density in a stoichiometric Ga-based quasicrystal approximant

Authors: Farid Labib, Shintaro Suzuki, Asuka Ishikawa, Takenori Fujii, Ryuji Tamura

Abstract: This study reports the first observation of ferromagnetic (FM) order in the non-Au-based approximant crystals (ACs) using a novel approach whereby a total electron-per-atom (e/a) ratio of the spin-glass Ga50Pd36Gd14 1/1 AC is lowered by simultaneously substituting certain ratios of a tri-valent Ga and a zero-valent Pd by a mono-valent Au. The emergence of FM order by this method was confirmed via… ▽ More This study reports the first observation of ferromagnetic (FM) order in the non-Au-based approximant crystals (ACs) using a novel approach whereby a total electron-per-atom (e/a) ratio of the spin-glass Ga50Pd36Gd14 1/1 AC is lowered by simultaneously substituting certain ratios of a tri-valent Ga and a zero-valent Pd by a mono-valent Au. The emergence of FM order by this method was confirmed via magnetic susceptibility, magnetization, and specific heat measurements. The findings of this study open up vast opportunities in developing long-range magnetic orders from stoichiometric ACs, quasicrystals, and even other RKKY compounds with spin-glass ground state. △ Less

Submitted 18 May, 2023; originally announced May 2023.

Journal ref: Physical Review B 106, 174436 (2022)

Analogue black-white hole solitons in travelling wave parametric amplifiers with superconducting nonlinear asymmetric inductive elements

Authors: Haruna Katayama, Noriyuki Hatakenaka, Toshiyuki Fujii, Miles P. Blencowe

Abstract: We show that existing travelling wave parametric amplifier (TWPA) setups, using superconducting nonlinear asymmetric inductive elements (SNAILs), admit soliton solutions that act as analogue event horizons. The SNAIL-TWPA circuit dynamics are described by the Korteweg-de Vries (KdV) or modified Korteweg-de Vries (mKdV) equations in the continuum field approximation, depending on the external magne… ▽ More We show that existing travelling wave parametric amplifier (TWPA) setups, using superconducting nonlinear asymmetric inductive elements (SNAILs), admit soliton solutions that act as analogue event horizons. The SNAIL-TWPA circuit dynamics are described by the Korteweg-de Vries (KdV) or modified Korteweg-de Vries (mKdV) equations in the continuum field approximation, depending on the external magnetic flux bias, and validated numerically. The soliton spatially modulates the velocity for weak probes, resulting in the effective realization of analogue black hole and white hole event horizon pairs. The SNAIL external magnetic flux bias tunability facilitates a three-wave mixing process, which enhances the prospects for observing Hawking photon radiation. △ Less

Submitted 9 December, 2023; v1 submitted 23 December, 2022; originally announced December 2022.

Comments: 5 pages, 4 figures, published version

Journal ref: Phys. Rev. Research 5, L022055(2023)

Electronic phase diagram in Te-annealed superconducting FeTe$_{1-x}$Se$_x$ revealed by magnetic susceptibility

Authors: Takenori Fujii, Yu Uezono, Takumi Otsuka, Shotaro Hagisawa, Takao Watanabe

Abstract: Knowledge of the doping (x)-temperature (T) phase diagram of Fe-based superconductors is crucial in understanding the mechanism of high transition temperature superconductivity. Here, we measured the Se doping dependence of the magnetic susceptibility for Te-annealed FeTe$_{1-x}$Se$_x$. Two characteristic temperatures $T^*_χ$ and $T^{**}_χ$ were observed in the temperature dependence of magnetic s… ▽ More Knowledge of the doping (x)-temperature (T) phase diagram of Fe-based superconductors is crucial in understanding the mechanism of high transition temperature superconductivity. Here, we measured the Se doping dependence of the magnetic susceptibility for Te-annealed FeTe$_{1-x}$Se$_x$. Two characteristic temperatures $T^*_χ$ and $T^{**}_χ$ were observed in the temperature dependence of magnetic susceptibility, where the magnetic susceptibility decreased below T$^{**}_χ$ and tended to increase with further decreasing temperature below T$^*_χ$. The decrease in magnetic susceptibility below T$^{**}_χ$ is considered to be due to the opening of the pseudogap. Whereas the increase in magnetic susceptibility below T$^*_χ$ is attributed to the increase in the carrier concentration of electrons. Furthermore, we found the large superconducting fluctuation, which may be related to BCS-BEC crossover. △ Less

Submitted 10 November, 2022; originally announced November 2022.

Comments: 6 pages, 3 figures. to be published in JPS Conference Proceedings

Onset Temperatures for Superconducting Fluctuations in Te-annealed FeTe$_{1-x}$Se$_x$ Single Crystals: Evidence for the BCS-BEC Crossover

Authors: Yu Uezono, Takumi Otsuka, Shotaro Hagisawa, Haruka Taniguchi, Michiaki Matsukawa, Takenori Fujii, Takao Watanabe

Abstract: Recently, the superconductors' community has witnessed an unsettled debate regarding whether iron-based superconductors, in particular FeSe and FeSe$_{1-x}$S$_x$, are in the Bardeen-Cooper-Shrieffer (BCS) - Bose-Einstein condensation (BEC) crossover regime. Nonetheless, one particular system, FeTe$_{1-x}$Se$_x$, has been less investigated in this regard owing to the screening of its intrinsic supe… ▽ More Recently, the superconductors' community has witnessed an unsettled debate regarding whether iron-based superconductors, in particular FeSe and FeSe$_{1-x}$S$_x$, are in the Bardeen-Cooper-Shrieffer (BCS) - Bose-Einstein condensation (BEC) crossover regime. Nonetheless, one particular system, FeTe$_{1-x}$Se$_x$, has been less investigated in this regard owing to the screening of its intrinsic superconducting properties by the inevitable iron excess. Herein, the onset temperatures for superconducting fluctuations ($T_{scf}$) are investigated by measuring the magnetoresistance (MR) of Te-annealed, high-quality FeTe$_{1-x}$Se$_x$ ($x$ = 0.1, 0.2, 0.3, and 0.4) single crystals. The results reveal very high $T_{scf}$ values for these crystals. Particularly for $x$ = 0.4, $T_{scf}$ reaches approximately 40 K, which is 2.7 times larger than $T_c$. This indicates that the superconductivity of the FeTe$_{1-x}$Se$_x$ system is well within the BCS-BEC crossover regime. △ Less

Submitted 17 September, 2022; originally announced September 2022.

Comments: 6 pages, 3 figures, and 1 table. to be published in JPS Conference Proceedings

Structural 130-K Phase Transition and Emergence of a Two-Ion Kondo State in HT-Ce$_2$Rh$_2$Ga Explored by $^{69,71}$Ga Nuclear Quadrupole Resonance

Authors: Sh. Yamamoto, T. Fujii, S. Luther, H. Yasuoka, H. Sakai, F. Bärtl, K. M. Ranjith, H. Rosner, J. Wosnitza, A. M. Strydom, H. Kühne, M. Baenitz

Abstract: We have studied the microscopic magnetic properties, the nature of the 130-K phase transition, and the ground state in the recently synthesized compound Ce$_2$Rh$_2$Ga by use of $^{69,71}$Ga nuclear quadrupole resonance (NQR). The NQR spectra clearly show an unusual phase transition at $T_t$ $\sim$ 130 K yielding a splitting of the high-temperature single NQR line into two clearly resolved NQR lin… ▽ More We have studied the microscopic magnetic properties, the nature of the 130-K phase transition, and the ground state in the recently synthesized compound Ce$_2$Rh$_2$Ga by use of $^{69,71}$Ga nuclear quadrupole resonance (NQR). The NQR spectra clearly show an unusual phase transition at $T_t$ $\sim$ 130 K yielding a splitting of the high-temperature single NQR line into two clearly resolved NQR lines, providing evidence for two crystallographically inequivalent Ga sites. The NQR frequencies are in good agreement with fully-relativistic calculations of the band structure. Our NQR results indicate the absence of magnetic or charge order down to 0.3 K. The temperature dependence of the spin-lattice relaxation rate, 1/$T_1$, shows three distinct regimes, with onset temperatures at $T_t$ and 2 K. The temperature-independent 1/$T_1$, observed between $T_t$ and 2 K, crosses over to a Korringa process, 1/$T_1$ $\propto$ $T$, below $\sim$ 2 K, which evidences a rare two-ion Kondo scenario: the system goes into a dense Kondo coherent state at 2.0 and 0.8 K for the two different Ga sites. △ Less

Submitted 11 July, 2022; originally announced July 2022.

Comments: 12 pages, 7 figures

Journal ref: Phys. Rev. B 106, 115125 (2022)

NMR evidence for energy gap opening in thiol-capped platinum nanoparticles

Authors: Takuto Fujii, Kaita Iwamoto1 Yusuke Nakai, Taisuke Shiratsu, Hiroshi Yao, Koichi Ueda, Takeshi Mito

Abstract: When the particle size of a metal is reduced, it is expected that an energy gap will open due to the quantum size effect. However, the energy gap in platinum (Pt) metal nanoparticles has not been observed directly by nuclear magnetic resonance (NMR). To investigate the particle size dependence of the electronic state of Pt nanoparticles, we performed 195Pt NMR experiments on thiol-capped Pt nanopa… ▽ More When the particle size of a metal is reduced, it is expected that an energy gap will open due to the quantum size effect. However, the energy gap in platinum (Pt) metal nanoparticles has not been observed directly by nuclear magnetic resonance (NMR). To investigate the particle size dependence of the electronic state of Pt nanoparticles, we performed 195Pt NMR experiments on thiol-capped Pt nanoparticles with three different average diameters of less than 3 nm. For the nanoparticles with a diameter of 2.8 nm, we observed usual metallic behavior with a smaller density of states than that of the bulk Pt. In contrast, the temperature dependence of 1/T1T in nanoparticles less than 2.5 nm in diameter is an activation-energy form above 150 K, which is semiconducting behavior with an energy gap of the order of 2000 K. The significant decrease in 1/T1T by more than two orders of magnitude in the smaller Pt nanoparticles compared to the bulk Pt is attributable to the disappearance of the density of states at the Fermi energy, which is consistent with the opening of an energy gap. These results indicate a metal-insulator transition below 2.5 nm in diameter is present in our thiol-capped Pt nanoparticle samples. The effect of the thiol capping on the electronic structure suggested by the experimental results is also discussed. △ Less

Submitted 7 March, 2022; originally announced March 2022.

Journal ref: Phys. Rev. B 105, L121401 (2022)

Thermoelectric signature of quantum critical phase in a doped spin liquid candidate

Authors: K. Wakamatsu, Y. Suzuki, T. Fujii, K. Miyagawa, H. Taniguchi, K. Kanoda

Abstract: Quantum spin liquid is a nontrivial magnetic state of longstanding interest, in which spins are strongly correlated and entangled but do not order1, 2; further intriguing is its doped version, which possibly hosts strange metal and unconventional superconductivity3. Promising and currently the only candidate of the doped spin liquid is a triangular-lattice organic conductor, kappa-(BEDT-TTF)4Hg2.8… ▽ More Quantum spin liquid is a nontrivial magnetic state of longstanding interest, in which spins are strongly correlated and entangled but do not order1, 2; further intriguing is its doped version, which possibly hosts strange metal and unconventional superconductivity3. Promising and currently the only candidate of the doped spin liquid is a triangular-lattice organic conductor, kappa-(BEDT-TTF)4Hg2.89Br8, recently found to hold metallicity, spin-liquid-like magnetism and BEC-like superconductivity4-6. The nature of the metallic state with the spin-liquid behaviour is awaiting to be further clarified. Here, we report the thermoelectric signature that mobile holes in the spin liquid background is in a quantum critical state and it pertains to the BEC-like superconductivity. The Seebeck coefficient divided by temperature, S/T, is enhanced on cooling with logarithmic divergence indicative of quantum criticality. Furthermore, the logarithmic enhancement is correlated with the superconducting transition temperature under pressure variation, and the temperature and magnetic field profile of S/T upon the superconducting transition change with pressure in a consistent way with the previously suggested BEC-BCS crossover. The present results reveal that the quantum criticality in a doped spin liquid emerges in a phase, not at a point, and is involved in the unconventional BEC-like nature. △ Less

Submitted 25 January, 2022; originally announced January 2022.

Comments: 21 pages, 8 figures, 1 table

Mott-driven BEC-BCS crossover in a doped spin liquid candidate, kappa-(BEDT-TTF)4Hg2.89Br8

Authors: Y. Suzuki, K. Wakamatsu, J. Ibuka, H. Oike, T. Fujii, K. Miyagawa, H. Taniguchi, K. Kanoda

Abstract: The pairing of interacting fermions leading to superfluidity has two limiting regimes: the Bardeen-Cooper-Schrieffer (BCS) scheme for weakly interacting degenerate fermions and the Bose-Einstein condensation (BEC) of bosonic pairs of strongly interacting fermions. While the superconductivity that emerges in most metallic systems is the BCS-like electron pairing, strongly correlated electrons with… ▽ More The pairing of interacting fermions leading to superfluidity has two limiting regimes: the Bardeen-Cooper-Schrieffer (BCS) scheme for weakly interacting degenerate fermions and the Bose-Einstein condensation (BEC) of bosonic pairs of strongly interacting fermions. While the superconductivity that emerges in most metallic systems is the BCS-like electron pairing, strongly correlated electrons with poor Fermi liquidity can condense into the unconventional BEC-like pairs. Quantum spin liquids harbor extraordinary spin correlation free from order and the superconductivity that possibly emerges by carrier doping of the spin liquids is expected to have a peculiar pairing nature. The present study experimentally explores the nature of the pairing condensate in a doped spin-liquid candidate material and under varying pressure, which changes the electron-electron Coulombic interactions across the Mott critical value in the system. The transport measurements reveal that the superconductivity at low pressures is a BEC-like condensate from a non-Fermi liquid and crosses over to a BCS-like condensate from a Fermi liquid at high pressures. The Nernst-effect measurements distinctively illustrate the two regimes of the pairing in terms of its robustness to the magnetic field. The present Mott tuning of the BEC-BCS crossover can be compared to the Feshbach tuning of the BEC-BCS crossover of fermionic cold atoms. △ Less

Submitted 25 January, 2022; originally announced January 2022.

Comments: 25 pages, 10 figures

Journal ref: Physical Review X 12, 011016 (2021)

Revised phase diagram of the high-$T_c$ cuprate superconductor Pb-doped Bi$_2$Sr$_2$CaCu$_2$O$_{8+δ}$ revealed by anisotropic transport measurements

Authors: Keiichi Harada, Yuki Teramoto, Tomohiro Usui, Kenji Itaka, Takenori Fujii, Takashi Noji, Haruka Taniguchi, Michiaki Matsukawa, Hajime Ishikawa, Koichi Kindo, Daniel S. Dessau, Takao Watanabe

Abstract: Although phase diagrams can be leveraged to investigate high transition temperature (high-$T_c$) superconductivity, the issue has not been discussed thoroughly. In this study, we elucidate the phase diagram of the overdoped side of high-$T_c$ cuprates via systematic anisotropic transport measurements for Pb-doped Bi-2212 single crystals. We demonstrate that the characteristic temperatures of the "… ▽ More Although phase diagrams can be leveraged to investigate high transition temperature (high-$T_c$) superconductivity, the issue has not been discussed thoroughly. In this study, we elucidate the phase diagram of the overdoped side of high-$T_c$ cuprates via systematic anisotropic transport measurements for Pb-doped Bi-2212 single crystals. We demonstrate that the characteristic temperatures of the "weak" pseudogap opening and electronic coherence cross each other at a critical doping level, while those of the "strong" pseudogap merges into that of superconducting fluctuations above the critical doping level. Our results indicate the importance of Mottness in high-$T_c$ superconductivity. △ Less

Submitted 5 February, 2022; v1 submitted 24 September, 2021; originally announced September 2021.

Comments: 8 pages, 9 figures; The title is changed

Journal ref: Phys. Rev. B 105, 085131 (2022)

A Sampling Strategy in Efficient Potential Energy Surface Mapping for Predicting Atomic Diffusivity in Crystals by Machine Learning

Authors: Kazuaki Toyoura, Takeo Fujii, Kenta Kanamori, Ichiro Takeuchi

Abstract: We propose a machine-learning-based (ML-based) method for efficiently predicting atomic diffusivity in crystals, in which the potential energy surface (PES) of a diffusion carrier is partially evaluated by first-principles calculations. To preferentially evaluate the region of interest governing the atomic diffusivity, a statistical PES model based on a Gaussian process (GP-PES) is constructed and… ▽ More We propose a machine-learning-based (ML-based) method for efficiently predicting atomic diffusivity in crystals, in which the potential energy surface (PES) of a diffusion carrier is partially evaluated by first-principles calculations. To preferentially evaluate the region of interest governing the atomic diffusivity, a statistical PES model based on a Gaussian process (GP-PES) is constructed and updated iteratively from known information on already-computed potential energies (PEs). In the proposed method, all local energy minima (stable & metastable sites) and elementary processes of atomic diffusion (atomic jumps) are explored on the predictive mean of the GP-PES. The uncertainty of jump frequency in each elementary process is then estimated on the basis of the variance of the GP-PES. The acquisition function determining the next grid point to be computed is designed to reflect the impacts of the uncertainties of jump frequencies on the uncertainty of the macroscopic atomic diffusivity. The numerical solution of the master equation is here employed to readily estimate the atomic diffusivity, which enables us to design the acquisition function reflecting the centrality of each elementary process. △ Less

Submitted 7 May, 2020; v1 submitted 10 March, 2020; originally announced March 2020.

Pressure induced evolution of band structure in black phosphorus studied by $^{31}$P-NMR

Authors: T. Fujii, Y. Nakai, Y. Akahama, K. Ueda, T. Mito

Abstract: Two-dimensional layered semiconductor black phosphorus (BP), a promising pressure induced Dirac system as predicted by band structure calculations, has been studied by $^{31}$P-nuclear magnetic resonance. Band calculations have been also carried out to estimate the density of states $D(E)$. The temperature and pressure dependences of nuclear spin lattice relaxation rate $1/T_1$ in the semiconducti… ▽ More Two-dimensional layered semiconductor black phosphorus (BP), a promising pressure induced Dirac system as predicted by band structure calculations, has been studied by $^{31}$P-nuclear magnetic resonance. Band calculations have been also carried out to estimate the density of states $D(E)$. The temperature and pressure dependences of nuclear spin lattice relaxation rate $1/T_1$ in the semiconducting phase are well reproduced using the derived $D(E)$, and the resultant pressure dependence of semiconducting gap is in good accordance with previous reports, giving a good confirmation that the band calculation on BP is fairly reliable. The present analysis of $1/T_1$ data with the complemental theoretical calculations allows us to extract essential information, such as the pressure dependences of $D(E)$ and chemical potential, as well as to decompose observed $1/T_1$ into intrinsic and extrinsic contributions. An abrupt increase in $1/T_1$ at 1.63GPa indicates that the semiconducting gap closes, resulting in an enhancement of conductivity. △ Less

Submitted 14 May, 2019; originally announced May 2019.

Comments: 11 pages, 4 figures

Journal ref: Phys. Rev. B 101, 161408 (2020)

Evidence for gradual evolution of low-energy fluctuations underlying the first-order structural and valence order in YbPd

Authors: R. Nakanishi, T. Fujii, Y. Nakai, K. Ueda, M. Hirata, K. Oyama, A. Mitsuda, H. Wada, T. Mito

Abstract: The valence orders at $T_a=125$ K and $T_b=105$ K in the cubic compound YbPd have been investigated by $^{105}$Pd-nuclear magnetic resonance (NMR) measurements. Significant decrease in the density of states at the Fermi energy below $T_a$ is evident from the measurement of nuclear spin lattice relaxation rate $1/T_1$, suggesting that the instabilities of Fermi surface are associated with the trans… ▽ More The valence orders at $T_a=125$ K and $T_b=105$ K in the cubic compound YbPd have been investigated by $^{105}$Pd-nuclear magnetic resonance (NMR) measurements. Significant decrease in the density of states at the Fermi energy below $T_a$ is evident from the measurement of nuclear spin lattice relaxation rate $1/T_1$, suggesting that the instabilities of Fermi surface are associated with the transitions. Moreover we observed the unusual evolution of low-energy fluctuations toward the valence transition at $T_a$ behind its drastic first-order nature. The structural transition accompanying the valence order may occur as a result of cooperative effect of Fermi surface and valence instabilities. △ Less

Submitted 17 February, 2019; originally announced February 2019.

Comments: 5 pages, 3 figures

Specific heat, thermal conductivity, and magnetic susceptibility of cyanate ester resins --- An alternative to commonly used epoxy resins

Authors: Sachiko Nakamura, Takenori Fujii, Shoji Matsukawa, Masayuki Katagiri, Hiroshi Fukuyama

Abstract: In low temperature experiments, resins have many applications as glues or thermal and electrical insulators. Cyanate ester resins (CEs) are a high-temperature compatible thermoset resin whose glass-transition temperature $T_g$ is ~300 $^\circ$C. Recently, we found that CEs also withstand low temperatures without microcracking by measuring $^4$He permeability. Here, we measured specific heat C, the… ▽ More In low temperature experiments, resins have many applications as glues or thermal and electrical insulators. Cyanate ester resins (CEs) are a high-temperature compatible thermoset resin whose glass-transition temperature $T_g$ is ~300 $^\circ$C. Recently, we found that CEs also withstand low temperatures without microcracking by measuring $^4$He permeability. Here, we measured specific heat C, thermal conductivity κ, and magnetic susceptibility $χ$ of different kinds of CEs in the wide temperature range from room temperature to 0.5 K for C and 2 K for other two. The thermal properties, C and κ, of different kinds of CEs are surprisingly coincident with each other. We discuss chemical structures and crystallinity of CEs and their blends based on the measured thermal properties. Compared to Stycast 1266, a commonly-used epoxy resin in low temperature experiments, C of CEs is larger by a factor of 3 (<= 30 K), κis lower by a factor of 4 (<= 10 K), indicating the small thermal diffusivity. The χvalues are as small as Stycast 1266, indicative of their high purity. Our results show that cyanate esters are a new option for cryogenic resins with thermal insulative properties in/for low temperature experiments. △ Less

Submitted 19 September, 2018; originally announced September 2018.

Comments: 7 pages, 10 figures

Journal ref: Cryogenics 95, 76--81 (2018)

Frustrated Magnetism of Pharmacosiderite Comprising Tetrahedral Clusters Arranged in the Primitive Cubic Lattice

Authors: Ryutaro Okuma, Takeshi Yajima, Tatsuo Fujii, Mikio Takano, Zenji Hiroi

Abstract: We show that pharmacosiderite is a novel cluster antiferromagnet comprising frustrated regular tetrahedra made of spin-5/2 Fe3+ ions that are arranged in the primitive cubic lattice. The connectivity of the tetrahedra and the inter-cluster interaction of 2.9 K, which is significantly large compared with the intra-cluster interaction of 10.6 K, gives a unique playground for frustration physics. An… ▽ More We show that pharmacosiderite is a novel cluster antiferromagnet comprising frustrated regular tetrahedra made of spin-5/2 Fe3+ ions that are arranged in the primitive cubic lattice. The connectivity of the tetrahedra and the inter-cluster interaction of 2.9 K, which is significantly large compared with the intra-cluster interaction of 10.6 K, gives a unique playground for frustration physics. An unconventional antiferromagnetic order is observed below TN ~ 6 K, which is accompanied by a weak ferromagnetic moment and a large fluctuation as evidenced by Mossbauer spectroscopy. A q = 0 magnetic order with the total S = 0 for the tetrahedral cluster is proposed based on the irreducible representation analysis, which may explain the origin of the weak ferromagnetism and fluctuation. △ Less

Submitted 7 August, 2018; originally announced August 2018.

Comments: 4 pages, 5 figures

Journal ref: Journal of the Physical Society of Japan 87, 093702 (2018)

Bent Electronic Band Structure Induced by Ferroelectric Polarization

Authors: Norihiro Oshime, Jun Kano, Eiji Ikenaga, Shintaro Yasui, Yosuke Hamasaki, Sou Yasuhara, Satoshi Hinokuma, Naoshi Ikeda, Mitsuru Itoh, Takayoshi Yokoya, Tatsuo Fujii, Akira Yasui

Abstract: Bent band structures have been empirically described in ferroelectric materials to explain the functioning of recently developed ferroelectric tunneling junction and photovoltaic devices. This report presents experimental evidence for ferroelectric band bending, which was observed in the depth profiles of atomic orbitals of angle-resolved hard x-ray photoemission spectra of ferroelectric BaTiO3 th… ▽ More Bent band structures have been empirically described in ferroelectric materials to explain the functioning of recently developed ferroelectric tunneling junction and photovoltaic devices. This report presents experimental evidence for ferroelectric band bending, which was observed in the depth profiles of atomic orbitals of angle-resolved hard x-ray photoemission spectra of ferroelectric BaTiO3 thin films. The ferroelectric bent band structure is separated into three depth regions; the shallowest and deepest regions are slightly modulated by the screening effect at surface and interface, respectively, and the intermediate region exhibits the pure ferroelectric effect. In the pure ferroelectric bent band structure, we found that the binding energy of outer shell electrons shows a larger shift than that of inner shell electrons, and that the difference in energy shift is correlated with the atomic configuration of the soft phonon mode. These findings could lead to a simple understanding of the origin of electric polarization. △ Less

Submitted 5 November, 2017; originally announced November 2017.

Comments: 16 pages, 7 figures

A path integral approach to Bayesian inference in Markov processes

Authors: Toshiyuki Fujii, Noriyuki Hatakenaka

Abstract: We formulate Bayesian updates in Markov processes by means of path integral techniques and derive the imaginary-time Schrödinger equation with likelihood to direct the inference incorporated as a potential for the posterior probability distribution We formulate Bayesian updates in Markov processes by means of path integral techniques and derive the imaginary-time Schrödinger equation with likelihood to direct the inference incorporated as a potential for the posterior probability distribution △ Less

Submitted 21 October, 2017; originally announced October 2017.

Comments: 13 pages

Low temperature transport properties of pyrolytic graphite sheet

Authors: Sachiko Nakamura, Daisuke Miyafuji, Takenori Fujii, Tomohiro Matsui, Hiroshi Fukuyama

Abstract: We have made thermal and electrical transport measurements of uncompressed pyrolytic graphite sheet (uPGS), a mass-produced thin graphite sheet with various thicknesses between 10 and 100 μm, at temperatures between 2 and 300 K. Compared to exfoliated graphite sheets like Grafoil, uPGS has much higher conductivities by an order of magnitude because of its high crystallinity confirmed by X-ray diff… ▽ More We have made thermal and electrical transport measurements of uncompressed pyrolytic graphite sheet (uPGS), a mass-produced thin graphite sheet with various thicknesses between 10 and 100 μm, at temperatures between 2 and 300 K. Compared to exfoliated graphite sheets like Grafoil, uPGS has much higher conductivities by an order of magnitude because of its high crystallinity confirmed by X-ray diffraction and Raman spectroscopy. This material is advantageous as a thermal link of light weight in a wide temperature range particularly above 60 K where the thermal conductivity is much higher than common thermal conductors such as copper and aluminum alloys. We also found a general relationship between thermal and electrical conductivities in graphite-based materials which have highly anisotropic conductivities. This would be useful to estimate thermal conductance of a cryogenic part made of these materials from its electrical conductance more easily measurable at low temperature. △ Less

Submitted 2 October, 2017; v1 submitted 11 April, 2017; originally announced April 2017.

Comments: 7 pages, 6 figures

Journal ref: Cryogenics 86, 118-122 (2017)

High-throughput and long-term observation of compartmentalized biochemical oscillators

Authors: Koshi Hasatani, Mathieu Leocmach, Anthony J. Genot, André Estévez-Torres, Teruo Fujii, Yannick Rondelez

Abstract: We report the splitting of an oscillating DNA circuit into $\sim 700$ droplets with picoliter volumes. Upon incubation at constant temperature, the droplets display sustained oscillations that can be observed for more than a day. Superimposed to the bulk behaviour, we find two intriguing new phenomena - slow desynchronization between the compartments and kinematic spatial waves - and investigate t… ▽ More We report the splitting of an oscillating DNA circuit into $\sim 700$ droplets with picoliter volumes. Upon incubation at constant temperature, the droplets display sustained oscillations that can be observed for more than a day. Superimposed to the bulk behaviour, we find two intriguing new phenomena - slow desynchronization between the compartments and kinematic spatial waves - and investigate their possible origin. This approach provides a route to study the influence of small volume effects in biology, and paves the way to technological applications of compartmentalized molecular programs controlling complex dynamics. △ Less

Submitted 4 November, 2015; v1 submitted 1 November, 2015; originally announced November 2015.

Journal ref: Chemical Communications, 49(73), 8090 (2013)

Simultaneous control of thermoelectric properties in p-type and n-type materials by electric double-layer gating : New design for thermoelectric device

Authors: Ryohei Takayanagi, Takenori Fujii, Atsushi Asamitsu

Abstract: We report novel design for thermoelectric device which can control thermoelectric properties of p-type and n-type materials simultaneously by electric double-layer gating. Here, p-type Cu2O and n-type ZnO were used as positive and negative electrodes of the electric double-layer capacitor structure. When the gate voltage was applied between two electrodes, the holes and electrons were accumulated… ▽ More We report novel design for thermoelectric device which can control thermoelectric properties of p-type and n-type materials simultaneously by electric double-layer gating. Here, p-type Cu2O and n-type ZnO were used as positive and negative electrodes of the electric double-layer capacitor structure. When the gate voltage was applied between two electrodes, the holes and electrons were accumulated on the surface of Cu2O and ZnO, respectively. The thermopower was measured by applying thermal gradient along the accumulated layer on the electrodes. We demonstrate here that the accumulated layers are worked as a p-n pair of the thermoelectric device. △ Less

Submitted 19 March, 2015; originally announced March 2015.

Comments: 10 pages, 4 figures. Accepted in Applied physics express

Single-crystal growth of underdoped Bi-2223

Authors: S. Adachi, T. Usui, K. Takahashi, K. Kosugi, T. Watanabe, T. Nishizaki, T. Adachi, S. Kimura, K. Sato, K. M. Suzuki, M. Fujita, K. Yamada, T. Fujii

Abstract: To investigate the origin of the enhanced Tc ({\approx} 110 K) of the trilayer cuprate superconductor Bi2Sr2Ca2Cu3O10+δ (Bi-2223), its underdoped single crystals are a critical requirement. Here, we demonstrate the first successful in-plane resistivity measurements of heavily underdoped Bi-2223 (zero-resistivity temperatures {\approx} 20~35 K). Detailed crystal growth methods, the annealing proces… ▽ More To investigate the origin of the enhanced Tc ({\approx} 110 K) of the trilayer cuprate superconductor Bi2Sr2Ca2Cu3O10+δ (Bi-2223), its underdoped single crystals are a critical requirement. Here, we demonstrate the first successful in-plane resistivity measurements of heavily underdoped Bi-2223 (zero-resistivity temperatures {\approx} 20~35 K). Detailed crystal growth methods, the annealing process, as well as X-ray diffraction (XRD) and magnetic susceptibility measurement results are also reported. △ Less

Submitted 19 March, 2015; originally announced March 2015.

Comments: 4 pages, 4 figures, 27th International Symposium on Superconductivity, ISS 2014, to appear in Physics Procedia

Unscaling Superconducting Parameters with $T_c$ for Bi-2212 and Bi-2223: A Magnetotransport Study in the Superconductive Fluctuation Regime

Authors: Shintaro Adachi, Tomohiro Usui, Hironobu Kudo, Haruki Kushibiki, Kosuke Murata, Takao Watanabe, Kazutaka Kudo, Terukazu Nishizaki, Norio Kobayashi, Shojiro Kimura, Masaki Fujita, Kazuyoshi Yamada, Takashi Noji, Yoji Koike, Takenori Fujii

Abstract: To investigate the origin of the enhanced $T_c$ ($\approx$ 110 K) of the trilayer cuprate superconductor Bi$_{2}$Sr$_{2}$Ca$_{2}$Cu$_{3}$O$_{10+δ}$ (Bi-2223), we have performed systematic magnetoresistance (MR) measurements on this superconductor, as well as on the bilayer superconductor, Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+δ}$ (Bi-2212). The in-plane coherence length, $ξ_{ab}$, and the specific-heat… ▽ More To investigate the origin of the enhanced $T_c$ ($\approx$ 110 K) of the trilayer cuprate superconductor Bi$_{2}$Sr$_{2}$Ca$_{2}$Cu$_{3}$O$_{10+δ}$ (Bi-2223), we have performed systematic magnetoresistance (MR) measurements on this superconductor, as well as on the bilayer superconductor, Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+δ}$ (Bi-2212). The in-plane coherence length, $ξ_{ab}$, and the specific-heat jump, $ΔC$, have been estimated using the theory of renormalized superconductive fluctuations, and the doping dependence of these parameters has been qualitatively explained using the Fermi arc approach. A detailed comparison of the superconducting parameters with $T_c$ for these compounds suggests that an additional superconducting condensation energy exists, due to an increase in the number of stacking CuO$_{2}$ planes in a unit cell. △ Less

Submitted 3 December, 2014; originally announced December 2014.

Comments: 17 pages, 5 figures, to appear in JPSJ

Journal ref: Journal of the Physical Society of Japan 84, 024706 (2015)

Control of Thermoelectric Properties of ZnO using Electric Double Layer

Authors: Ryohei Takayanagi, Takenori Fujii, Atsushi Asamitsu

Abstract: We have successfully controlled thermoelectric properties of ZnO by changing carrier concentration using an electric double layer transistor (EDLT) which is a feld effect transistor gated by electrolyte solution. The resistivity and the thermopower decreased abruptly by applying gate voltage larger than a threshold voltage ( 2V), indicating the increase of carrier concentration on the ZnO surface.… ▽ More We have successfully controlled thermoelectric properties of ZnO by changing carrier concentration using an electric double layer transistor (EDLT) which is a feld effect transistor gated by electrolyte solution. The resistivity and the thermopower decreased abruptly by applying gate voltage larger than a threshold voltage ( 2V), indicating the increase of carrier concentration on the ZnO surface. The temperature dependence of resistivity became metallic, which is characterized by weak temperature dependence of the resistivity, when gate voltage exceeded 2V. Corresponding to the resistivity, the temperature dependence of thermopower changed remarkably. The thickness of the induced metallic layer was estimated to be about 10nm from the critical carrier concentration of metal-insulator transition, and the power factor was calculated to ~8*10-5Wm-1K2. Although the power factor is not as large as bulk ZnO ceramics of optimum doping condition, EDLT is considered to be a useful way to optimize thermoelectric properties by tuning carrier concentration. △ Less

Submitted 23 June, 2014; originally announced June 2014.

Comments: Submit to JJAP

Superconductivity in Ca10(Ir4As8)(Fe2As2)5 with Square-Planar Coordination of Iridium

Authors: Kazutaka Kudo, Daisuke Mitsuoka, Masaya Takasuga, Yuki Sugiyama, Kento Sugawara, Naoyuki Katayama, Hiroshi Sawa, Hiroaki S. Kubo, Kenta Takamori, Masanori Ichioka, Tatsuo Fujii, Takashi Mizokawa, Minoru Nohara

Abstract: We report the unprecedented square-planar coordination of iridium in the iron iridium arsenide Ca10(Ir4As8)(Fe2As2)5. This material experiences superconductivity at 16 K. X-ray photoemission spectroscopy and first- principles band calculation suggest Ir(II) oxidation state, which yields electrically conductive Ir4As8 layers. Such metallic spacer layers are thought to enhance the interlayer couplin… ▽ More We report the unprecedented square-planar coordination of iridium in the iron iridium arsenide Ca10(Ir4As8)(Fe2As2)5. This material experiences superconductivity at 16 K. X-ray photoemission spectroscopy and first- principles band calculation suggest Ir(II) oxidation state, which yields electrically conductive Ir4As8 layers. Such metallic spacer layers are thought to enhance the interlayer coupling of Fe2As2, in which superconductivity emerges, thus offering a way to control the superconducting transition temperature. △ Less

Submitted 5 November, 2013; originally announced November 2013.

Comments: 17 pages, 5 figures, 2 tables

Journal ref: Scientific Reports 3 (2013) 3101

Superconductivity in the noncentrosymmetric half-Heusler compound LuPtBi : A possible topological superconductor

Authors: F. F. Tafti, Takenori Fujii, A. Juneau-Fecteau, S. Rene de Cotret, N. Doiron-Leyraud, Atsushi Asamitsu, Louis Taillefer

Abstract: We report superconductivity in the ternary half-Heusler compound LuPtBi, with Tc = 1.0 K and Hc2 = 1.6 T. The crystal structure of LuPtBi lacks inversion symmetry, hence the material is a noncentrosymmetric superconductor. Magnetotransport data show semimetallic behavior in the normal state, which is evidence for the importance of spin-orbit interaction. Theoretical calculations indicate that the… ▽ More We report superconductivity in the ternary half-Heusler compound LuPtBi, with Tc = 1.0 K and Hc2 = 1.6 T. The crystal structure of LuPtBi lacks inversion symmetry, hence the material is a noncentrosymmetric superconductor. Magnetotransport data show semimetallic behavior in the normal state, which is evidence for the importance of spin-orbit interaction. Theoretical calculations indicate that the strong spin-orbit interaction in LuPtBi should cause strong band inversion, making this material a promising candidate for 3D topological superconductivity. △ Less

Submitted 19 February, 2013; v1 submitted 7 February, 2013; originally announced February 2013.

Comments: 5 pages, 4 figures, Table I revised, discussion of the Pauli limit revised

Journal ref: Phys. Rev. B 87, 184504 (2013)

1/(N-1) expansion based on a perturbation theory in U for the Anderson model with N-fold degeneracy

Authors: Akira Oguri, Rui Sakano, Tatsuya Fujii

Abstract: We study low-energy properties of the N-fold degenerate Anderson model. Using a scaling that takes u=(N-1) U as an independent variable in place of the Coulomb interaction U, the perturbation series in U is reorganized as an expansion in powers of 1/(N-1). We calculate the renormalized parameters, which characterize the Kondo state, to the next leading order in the 1/(N-1) expansion at half-fillin… ▽ More We study low-energy properties of the N-fold degenerate Anderson model. Using a scaling that takes u=(N-1) U as an independent variable in place of the Coulomb interaction U, the perturbation series in U is reorganized as an expansion in powers of 1/(N-1). We calculate the renormalized parameters, which characterize the Kondo state, to the next leading order in the 1/(N-1) expansion at half-filling. The results, especially the Wilson ratio, agree very closely with the exact numerical renormalization group results at N=4. This ensures the applicability of our approach to N > 4, and we present highly reliable results for nonequilibrium Kondo transport through a quantum dot. △ Less

Submitted 11 August, 2011; originally announced August 2011.

Comments: 5 pages, 6 figures

Journal ref: Phys. Rev. B 84, 113301 (2011)

Evolution of the Kondo Effect in a Quantum Dot Probed by the Shot Noise

Authors: Yoshiaki Yamauchi, Koji Sekiguchi, Kensaku Chida, Tomonori Arakawa, Shuji Nakamura, Kensuke Kobayashi, Teruo Ono, Tatsuya Fujii, Rui Sakano

Abstract: We measure the current and shot noise in a quantum dot (QD) in the Kondo regime to address the non-equilibrium properties of the Kondo effect. By systematically tuning the temperature and gate voltages to define the level positions in the QD, we observe an enhancement of the shot noise as temperature decreases below the Kondo temperature, which indicates that the two-particle scattering process gr… ▽ More We measure the current and shot noise in a quantum dot (QD) in the Kondo regime to address the non-equilibrium properties of the Kondo effect. By systematically tuning the temperature and gate voltages to define the level positions in the QD, we observe an enhancement of the shot noise as temperature decreases below the Kondo temperature, which indicates that the two-particle scattering process grows as the Kondo state evolves. Below the Kondo temperature, the Fano factor defined at finite temperature is found to exceed the expected value of unity from the non-interacting model, reaching $1.8 \pm 0.2$. △ Less

Submitted 26 April, 2011; v1 submitted 13 April, 2011; originally announced April 2011.

Comments: 4 pages, 4 figures

Journal ref: Phys. Rev. Lett. 106, 176601 (2011)

Analysis of Shot Noise at Finite Temperatures in Fractional Quantum Hall Edge States

Authors: E. Iyoda, T. Fujii

Abstract: We investigate shot noise at {\it finite temperatures} induced by the quasi-particle tunneling between fractional quantum Hall (FQH) edge states. The resulting Fano factor has the peak structure at a certain bias voltage. Such a structure indicates that quasi-particles are weakly {\it glued} due to thermal fluctuation. We show that the effect makes it possible to probe the difference of statistics… ▽ More We investigate shot noise at {\it finite temperatures} induced by the quasi-particle tunneling between fractional quantum Hall (FQH) edge states. The resulting Fano factor has the peak structure at a certain bias voltage. Such a structure indicates that quasi-particles are weakly {\it glued} due to thermal fluctuation. We show that the effect makes it possible to probe the difference of statistics between $ν=1/5,{}2/5$ FQH states where quasi-particles have the same unit charge.Finally we propose a way to indirectly obtain statistical angle in hierarchical FQH states. △ Less

Submitted 8 April, 2012; v1 submitted 24 January, 2011; originally announced January 2011.

Comments: 5 pages, 3 figures

Journal ref: J. Phys. Soc. Jpn. 80 (2011) 073709

The Kondo crossover in shot noise of a single quantum dot with orbital degeneracy

Authors: Rui Sakano, Tatsuya Fujii, Akira Oguri

Abstract: We investigate out of equilibrium transport through an orbital Kondo system realized in a single quantum dot, described by the multiorbital impurity Anderson model. Shot noise and current are calculated up to the third order in bias voltage in the particle-hole symmetric case, using the renormalized perturbation theory. The derived expressions are asymptotically exact at low energies. The resultin… ▽ More We investigate out of equilibrium transport through an orbital Kondo system realized in a single quantum dot, described by the multiorbital impurity Anderson model. Shot noise and current are calculated up to the third order in bias voltage in the particle-hole symmetric case, using the renormalized perturbation theory. The derived expressions are asymptotically exact at low energies. The resulting Fano factor of the backscattering current $F_b$ is expressed in terms of the Wilson ratio $R$ and the orbital degeneracy $N$ as $F_b =\frac{1 + 9(N-1)(R-1)^2}{1 + 5(N-1)(R-1)^2}$ at zero temperature. Then, for small Coulomb repulsions $U$, we calculate the Fano factor exactly up to terms of order $U^5$, and also carry out the numerical renormalization group calculation for intermediate $U$ in the case of two- and four-fold degeneracy ($N=2,\,4$). As $U$ increases, the charge fluctuation in the dot is suppressed, and the Fano factor varies rapidly from the noninteracting value $F_b=1$ to the value in the Kondo limit $F_b=\frac{N+8}{N+4}$, near the crossover region $U\sim πΓ$, with the energy scale of the hybridization $Γ$. △ Less

Submitted 1 March, 2011; v1 submitted 10 November, 2010; originally announced November 2010.

Comments: 10 pages, 4 figures

Journal ref: Phys. Rev. B 83, 075440 (2011)

Evolution of the electronic structure from the insulator to the superconductor in Bi2Sr2-xLax(Ca,Y)Cu2O8+d

Authors: K. Tanaka, T. Yoshida, K. M. Shen, D. H. Lu, W. S. Lee, H. Yagi, A. Fujimori, Z. -X. Shen, Risdiana, T. Fujii, I. Terasaki

Abstract: La-doped and Y-doped Bi2Sr2CaCu2O8+d (Bi2212) compounds Bi2Sr2-xLax(Ca,Y)Cu2O8+d, which range from the insulator to the deeply underdoped superconductor, have been studied by angle-resolved photoemission spectroscopy. We have observed that the lower Hubbard band (LHB) of the parent insulator is gradually shifted upward with doping without significantly changing the band dispersions, which implie… ▽ More La-doped and Y-doped Bi2Sr2CaCu2O8+d (Bi2212) compounds Bi2Sr2-xLax(Ca,Y)Cu2O8+d, which range from the insulator to the deeply underdoped superconductor, have been studied by angle-resolved photoemission spectroscopy. We have observed that the lower Hubbard band (LHB) of the parent insulator is gradually shifted upward with doping without significantly changing the band dispersions, which implies a downward shift of the chemical potential with hole doping. This behaviour is analogous to Bi2Sr2-xLaxCuO6+d (Bi2201) and Ca2-xNaxCuO2Cl2 (Na-CCOC) but is different from La2-xSrxCuO4 (LSCO), where the LHB stays well below the chemical potential and does not move while its intensity quickly diminishes in the underdoped region. △ Less

Submitted 15 February, 2010; originally announced February 2010.

Comments: Submitted to PRB

Journal ref: Phys. Rev. B 81, 125115 (2010)

Effect of stripe order strength for the Nernst effect in La_{2-x}Sr_xCu_4 single crystals

Authors: Takenori Fujii, Tomonori Matsushima, Toshihide Maruoka, Atsushi Asamitsu

Abstract: We have precisely measured the Nernst effect in Nd-doped La$_{2-x}$Sr$_x$CuO$_4$ single crystals with controlling the strength (stability) of the stripe order. We found that the onset temperature $T_{onset}$, where the Nernst signal starts increasing, does not change conspicuously in spite of Nd-doping. At low temperatures, on the other hand, the absolute value of the Nernst signal is strongly s… ▽ More We have precisely measured the Nernst effect in Nd-doped La$_{2-x}$Sr$_x$CuO$_4$ single crystals with controlling the strength (stability) of the stripe order. We found that the onset temperature $T_{onset}$, where the Nernst signal starts increasing, does not change conspicuously in spite of Nd-doping. At low temperatures, on the other hand, the absolute value of the Nernst signal is strongly suppressed in accordance with the strength of the stripe order. These results imply that the fluctuation of (charge) stripe order enhances the Nernst signal below $T_{onset}$ at high temperatures, and then the stripe order enhanced by Nd-doping suppresses the superconducting fluctuation to reduce the Nernst signal at low temperatures. We also observed an increase of the Nernst signal below the charge order temperature $T_{ch}$ which is observed in diffraction measurement. △ Less

Submitted 1 December, 2009; originally announced December 2009.

Comments: 3pages, 2figures

A momentum-dependent perspective on quasiparticle interference in Bi_{2}Sr_{2}CaCu_{2}O_{8+δ}

Authors: I. M. Vishik, B. Moritz, E. A. Nowadnick, W. S. Lee, K. Tanaka, T. Sasagawa, T. Fujii, T. P. Devereaux, Z. X. Shen

Abstract: Angle Resolved Photoemission Spectroscopy (ARPES) probes the momentum-space electronic structure of materials, and provides invaluable information about the high-temperature superconducting cuprates. Likewise, the cuprate real-space, inhomogeneous electronic structure is elucidated by Scanning Tunneling Spectroscopy (STS). Recently, STS has exploited quasiparticle interference (QPI) - wave-like… ▽ More Angle Resolved Photoemission Spectroscopy (ARPES) probes the momentum-space electronic structure of materials, and provides invaluable information about the high-temperature superconducting cuprates. Likewise, the cuprate real-space, inhomogeneous electronic structure is elucidated by Scanning Tunneling Spectroscopy (STS). Recently, STS has exploited quasiparticle interference (QPI) - wave-like electrons scattering off impurities to produce periodic interference patterns - to infer properties of the QP in momentum-space. Surprisingly, some interference peaks in Bi_{2}Sr_{2}CaCu_{2}O_{8+δ} (Bi-2212) are absent beyond the antiferromagnetic (AF) zone boundary, implying the dominance of particular scattering process. Here, we show that ARPES sees no evidence of quasiparticle (QP) extinction: QP-like peaks are measured everywhere on the Fermi surface, evolving smoothly across the AF zone boundary. This apparent contradiction stems from different natures of single-particle (ARPES) and two-particle (STS) processes underlying these probes. Using a simple model, we demonstrate extinction of QPI without implying the loss of QP beyond the AF zone boundary. △ Less

Submitted 3 September, 2009; originally announced September 2009.

Journal ref: Nature Physics, 5, 718 (2009)

Field cooling memory effect in Bi2212 and Bi2223 single crystals

Authors: D. Shaltiel, H-A. Krug von Nidda, B. Rosenstein, B. Ya. Shapiro, M. Golosovsky., I. Shapiro, A. Loidl, B. Bogoslavsky, T. Fujii, T. Watanabe, T. Tamegai

Abstract: A memory effect in the Josephson vortex system created by magnetic field in the highly anisotropic superconductors Bi2212 and Bi2223 is demonstrated using microwave power absorption. This surprising effect appears despite a very low viscosity of Josephson vortices compared to Abrikosov vortices. The superconductor is field cooled in DC magnetic field H_{m} oriented parallel to the CuO planes thr… ▽ More A memory effect in the Josephson vortex system created by magnetic field in the highly anisotropic superconductors Bi2212 and Bi2223 is demonstrated using microwave power absorption. This surprising effect appears despite a very low viscosity of Josephson vortices compared to Abrikosov vortices. The superconductor is field cooled in DC magnetic field H_{m} oriented parallel to the CuO planes through the critical temperature T_{c} down to 4K, with subsequent reduction of the field to zero and again above H_{m}. Large microwave power absorption signal is observed at a magnetic field just above the cooling field clearly indicating a memory effect. The dependence of the signal on deviation of magnetic field from H_{m} is the same for a wide range of H_{m} from 0.15T to 1.7T. △ Less

Submitted 31 July, 2009; originally announced July 2009.

Quantum Knitting Computer

Authors: Toshiyuki Fujii, Shigemasa Matsuo, Noriyuki Hatakenaka

Abstract: We propose a fluxon-controlled quantum computer incorporated with three-qubit quantum error correction using special gate operations, i.e., joint-phase and SWAP gate operations, inherent in capacitively coupled superconducting flux qubits. The proposed quantum computer acts exactly like a knitting machine at home. We propose a fluxon-controlled quantum computer incorporated with three-qubit quantum error correction using special gate operations, i.e., joint-phase and SWAP gate operations, inherent in capacitively coupled superconducting flux qubits. The proposed quantum computer acts exactly like a knitting machine at home. △ Less

Submitted 14 May, 2009; originally announced May 2009.

Fluxon-based generation of graph states in Josephson qubits

Authors: Toshiyuki Fujii, Masaru Fukunaga, Noriyuki Hatakenaka

Abstract: Graph states are a special kind of multiparticle entangled state with great potential for applications in quantum information technologies, especially in measurement-based quantum computers. These states cause significant reductions of the number of qubits needed for a given computation, leading to shorter execution time. Here we propose a simple scheme for generating such graph states by using… ▽ More Graph states are a special kind of multiparticle entangled state with great potential for applications in quantum information technologies, especially in measurement-based quantum computers. These states cause significant reductions of the number of qubits needed for a given computation, leading to shorter execution time. Here we propose a simple scheme for generating such graph states by using special gate operations, i.e., control-phase and swap gate operations, inherent in superconducting quantum nanocircuits. △ Less

Submitted 13 May, 2009; originally announced May 2009.

Proposal for testing Einstein's moon using three-time correlations

Authors: Toshiyuki Fujii, Munehiro Nishida, Noriyuki Hatakenaka

Abstract: Quantum mechanics has predicted many counterintuitive phenomena in daily life, and has changed our view of the world. Among such predictions, the existence of a macroscopic object in superposition is especially unbelievable. As Einstein asked, "Do you really believe that the moon exists only when you look at it?". However, recent experimental results on a mesoscopic scale will ultimately require… ▽ More Quantum mechanics has predicted many counterintuitive phenomena in daily life, and has changed our view of the world. Among such predictions, the existence of a macroscopic object in superposition is especially unbelievable. As Einstein asked, "Do you really believe that the moon exists only when you look at it?". However, recent experimental results on a mesoscopic scale will ultimately require us to dismiss commonsense so-called macroscopic reality. Leggett and Garg applied the Bell scheme for testing local realism to the time evolution of a macroscopic two-state system, and proposed a temporal version of the Bell inequality (the Leggett-Garg (LG) inequality) for testing macroscopic realism. However, as with the Bell inequality, the statistical approach behind this scheme may be less effective in showing clear incompatibility. Here we propose a temporal version of the Greenberger-Horne-Zeilinger (GHZ) scheme without statistical treatment for testing Einstein's moon using three-time correlations. △ Less

Submitted 13 May, 2009; originally announced May 2009.

Comments: 4 pages, 4 figures

New study of shot noise with the nonequilibrium Kubo formula in mesoscopic systems, application to the Kondo effect at a quantum dot

Authors: Tatsuya Fujii

Abstract: Recently we have developed a theory of Keldysh formalism for mesoscopic systems. The resulting nonequilibrium Kubo formula for differential conductance makes it possible to propose the new formula of shot noise $S_h$, and thus to study shot noise in correlated systems at any temperature and any bias voltage. Employing this new approach, we analyze shot noise in the Kondo regime through a quantum… ▽ More Recently we have developed a theory of Keldysh formalism for mesoscopic systems. The resulting nonequilibrium Kubo formula for differential conductance makes it possible to propose the new formula of shot noise $S_h$, and thus to study shot noise in correlated systems at any temperature and any bias voltage. Employing this new approach, we analyze shot noise in the Kondo regime through a quantum dot for the symmetric case at zero temperature. Using the renormalized perturbation theory, we prove that in the leading order of bias voltage $S_h$ equal to noise power at zero temperature conventionally used as shot noise. With $S_h$, we calculate the Fano factor for a backscattering current $I_b$: $F_b=S_h/2eI_b$. It is shown that the Fano factor takes the universal form of $F_b=1+4(R-1)^2/(1+5(R-1)^2)$ determined by the Wilson ratio $R$ for arbitrary strength of the Coulomb interaction. Using the Wilson ratio R=2, our result coincides with the fractional value of $F_b=5/3$ already derived in the Kondo regime. △ Less

Submitted 30 July, 2008; originally announced July 2008.

Comments: 12 pages, 1 figure, submitted to Phys. Rev. B

Time-dependent DMRG Study on Quantum Dot under a Finite Bias Voltage

Authors: Shunsuke Kirino, Tatsuya Fujii, Jize Zhao, Kazuo Ueda

Abstract: Resonant tunneling through quantum dot under a finite bias voltage at zero temperature is investigated by using the adaptive time-dependent density matrix renormalization group(TdDMRG) method. Quantum dot is modeled by the Anderson Hamiltonian with the 1-D nearest-neighbor tight-binding leads. Initially the ground state wave function is calculated with the usual DMRG method. Then the time evolut… ▽ More Resonant tunneling through quantum dot under a finite bias voltage at zero temperature is investigated by using the adaptive time-dependent density matrix renormalization group(TdDMRG) method. Quantum dot is modeled by the Anderson Hamiltonian with the 1-D nearest-neighbor tight-binding leads. Initially the ground state wave function is calculated with the usual DMRG method. Then the time evolution of the wave function due to the slowly changing bias voltage between the two leads is calculated by using the TdDMRG technique. Even though the system size is finite, the expectation values of current operator show steady-like behavior for a finite time interval, in which the system is expected to resemble the real nonequilibrium steady state of the infinitely long system. We show that from the time intervals one can obtain quantitatively correct results for differential conductance in a wide range of bias voltage. Finally we observe an anomalous behavior in the expectation value of the double occupation operator at the dot $<n_{\uparrow} n_{\downarrow}>$ as a function of bias voltage. △ Less

Submitted 10 July, 2008; v1 submitted 2 May, 2008; originally announced May 2008.

Nonequilibrium Kubo Formula of Finite Conductor Connected to Reservoirs based on Keldysh Formalism

Authors: Tatsuya Fujii

Abstract: We show that the density matrix for a finite conductor attached to reservoirs obtained by Keldysh formalism is of MacLennan-Zubarev form. On the basis of the fact that the density matrix is the invariant part proposed by Zubarev, it is shown that Keldysh formalism may describe the irreversible processes and steady-state feature of the system. An important consequence of the MacLennan-Zubarev for… ▽ More We show that the density matrix for a finite conductor attached to reservoirs obtained by Keldysh formalism is of MacLennan-Zubarev form. On the basis of the fact that the density matrix is the invariant part proposed by Zubarev, it is shown that Keldysh formalism may describe the irreversible processes and steady-state feature of the system. An important consequence of the MacLennan-Zubarev form of the density matrix is a generalization of the Kubo formula in a nonequilibrium case. On the basis of the result, we propose the formula of shot noise and a nonequilibrium identity between differential conductance, noise power and shot noise as a generalized Nyquist-Johnson relation. △ Less

Submitted 17 April, 2008; originally announced April 2008.

Comments: 8 pages, published version

Journal ref: J. Phys. Soc. Jpn. 76, 44709 (2007)

Enhancement of Superconducting Transition Temperature due to the strong Antiferromagnetic Spin Fluctuations in Non-centrosymmetric Heavy-fermion Superconductor CeIrSi3 :A 29Si-NMR Study under Pressure

Authors: H. Mukuda, T. Fujii, T. Ohara, A. Harada, M. Yashima, Y. Kitaoka, Y. Okuda, R. Settai, Y. Onuki

Abstract: We report a 29Si-NMR study on the pressure-induced superconductivity (SC) in an antiferromagnetic (AFM) heavy-fermion compound CeIrSi3 without inversion symmetry. In the SC state at P=2.7-2.8 GPa, the temperature dependence of the nuclear-spin lattice relaxation rate 1/T_1 below Tc exhibits a T^3 behavior without any coherence peak just below Tc, revealing the presence of line nodes in the SC ga… ▽ More We report a 29Si-NMR study on the pressure-induced superconductivity (SC) in an antiferromagnetic (AFM) heavy-fermion compound CeIrSi3 without inversion symmetry. In the SC state at P=2.7-2.8 GPa, the temperature dependence of the nuclear-spin lattice relaxation rate 1/T_1 below Tc exhibits a T^3 behavior without any coherence peak just below Tc, revealing the presence of line nodes in the SC gap. In the normal state, 1/T_1 follows a \sqrt{T}-like behavior, suggesting that the SC emerges under the non-Fermi liquid state dominated by AFM spin fluctuations enhanced around quantum critical point (QCP). The reason why the maximum Tc in CeIrSi3 is relatively high among the Ce-based heavy-fermion superconductors may be the existence of the strong AFM spin fluctuations. We discuss the comparison with the other Ce-based heavy-fermion superconductors. △ Less

Submitted 24 January, 2008; originally announced January 2008.

Comments: 4 pages, 5 figures, To be published in Phys. Rev. Lett

Journal ref: Phys. Rev. Lett., 100, 107003/1-4 (2008)

Time dilation of a bound half-fluxon pair in a long Josephson junction with a ferromagnetic insulator

Authors: Munehiro Nishida, Kyoko Murata, Toshiyuki Fujii, Noriyuki Hatakenaka

Abstract: The fluxon dynamics in a long Josephson junction with a ferromagnetic insulating layer is investigated. It is found that the Josephson phase obeys a double sine-Gordon equation involving a bound pi fluxon solution, and the internal oscillations of the bound pair acting as a clock exhibit Lorentz reductions in their frequencies regarded as a relativistic effect in the time domain, i.e., time dila… ▽ More The fluxon dynamics in a long Josephson junction with a ferromagnetic insulating layer is investigated. It is found that the Josephson phase obeys a double sine-Gordon equation involving a bound pi fluxon solution, and the internal oscillations of the bound pair acting as a clock exhibit Lorentz reductions in their frequencies regarded as a relativistic effect in the time domain, i.e., time dilation. This is the complement to the Lorentz contraction of fluxons with no clock. A possible observation scheme is also discussed. △ Less

Submitted 4 October, 2007; originally announced October 2007.

Comments: 4 pages, 5 figures, to appear in Physical Review Letters

Journal ref: Phys. Rev. Lett. 99, 207004 (2007)

Distinct Fermi-Momentum Dependent Energy Gaps in Deeply Underdoped Bi2212

Authors: Kiyohisa Tanaka, W. S. Lee, D. H. Lu, A. Fujimori, T. Fujii, Risdiana, I. Terasaki, D. J. Scalapino, T. P. Devereaux, Z. Hussain, Z. -X. Shen

Abstract: We use angle-resolved photoemission spectroscopy applied to deeply underdoped cuprate superconductors Bi2Sr2(Ca,Y)Cu2O8 (Bi2212) to reveal the presence of two distinct energy gaps exhibiting different doping dependence. One gap, associated with the antinodal region where no coherent peak is observed, increases with underdoping - a behavior known for more than a decade and considered as the gener… ▽ More We use angle-resolved photoemission spectroscopy applied to deeply underdoped cuprate superconductors Bi2Sr2(Ca,Y)Cu2O8 (Bi2212) to reveal the presence of two distinct energy gaps exhibiting different doping dependence. One gap, associated with the antinodal region where no coherent peak is observed, increases with underdoping - a behavior known for more than a decade and considered as the general gap behavior in the underdoped regime. The other gap, associated with the near nodal regime where a coherent peak in the spectrum can be observed, does not increase with less doping - a behavior not observed in the single particle spectra before. We propose a two-gap scenario in momentum space that is consistent with other experiments and may contain important information on the mechanism of high-Tc superconductivity. △ Less

Submitted 1 December, 2006; originally announced December 2006.

Comments: 12 pages, 3 figures, submitted to Science

Universal Scaling Behavior of Anomalous Hall Effect and Anomalous Nernst Effect in Itinerant Ferromagnets

Authors: T. Miyasato, N. Abe, T. Fujii, A. Asamitsu, S. Onoda, Y. Onose, N. Nagaosa, Y. Tokura

Abstract: Anomalous Hall effect (AHE) and anomalous Nernst effect (ANE) in a variety of ferromagnetic metals including pure metals, oxides, and chalcogenides, are studied to obtain unified understandings of their origins. We show a universal scaling behavior of anomalous Hall conductivity $σ_{xy}$ as a function of longitudinal conductivity $σ_{xx}$ over five orders of magnitude, which is well explained by… ▽ More Anomalous Hall effect (AHE) and anomalous Nernst effect (ANE) in a variety of ferromagnetic metals including pure metals, oxides, and chalcogenides, are studied to obtain unified understandings of their origins. We show a universal scaling behavior of anomalous Hall conductivity $σ_{xy}$ as a function of longitudinal conductivity $σ_{xx}$ over five orders of magnitude, which is well explained by a recent theory of the AHE taking into account both the intrinsic and extrinsic contributions. ANE is closely related with AHE and provides us with further information about the low-temperature electronic state of itinerant ferromagnets. Temperature dependence of transverse Peltier coefficient $α_{xy}$ shows an almost similar behavior among various ferromagnets, and this behavior is in good agreement quantitatively with that expected from the Mott rule. △ Less

Submitted 11 October, 2006; originally announced October 2006.

Comments: 4pages, 4figures, 1table