Showing 1–50 of 148 results for author: Iida, K

Phonon spectrum in the spin-Peierls phase of CuGeO$_3$

Authors: L. Spitz, A. Razpopov, S. Biswas, H. Lane, S. E. Nikitin, K. Iida, R. Kajimoto, M. Fujita, M. Arai, M. Mourigal, Ch. Rüegg, R. Valentí, B. Normand

Abstract: CuGeO$_3$ has long been studied as a prototypical example of the spin-Peierls transition in a $S = 1/2$ Heisenberg chain. Despite intensive investigation of this quasi-one-dimensional material, systematic measurements and calculations of the phonon excitations in the dimerized phase have not to date been possible, leaving certain aspects of the spin-Peierls phenomenon unresolved. We perform state-… ▽ More CuGeO$_3$ has long been studied as a prototypical example of the spin-Peierls transition in a $S = 1/2$ Heisenberg chain. Despite intensive investigation of this quasi-one-dimensional material, systematic measurements and calculations of the phonon excitations in the dimerized phase have not to date been possible, leaving certain aspects of the spin-Peierls phenomenon unresolved. We perform state-of-the-art density functional theory (DFT) calculations to compute the electronic structure and phonon dynamics in the low-temperature dimerized phase. We also perform high-resolution neutron spectroscopy to measure the full phonon spectrum over multiple Brillouin zones. We find excellent agreement between our numerical and experimental results that extend to all measurement temperatures. Notable features of our phonon spectra include a number of steeply dispersive modes, nonmonotonic dispersion features, and specific phonon anticrossings, which we relate to the mode eigenvectors. By calculating the magnetic interactions within DFT and studying the effects of different phonon modes on the superexchange paths, we discuss the possibility of observing spin-phonon hybridization effects in experiments performed both in and out of equilibrium. △ Less

Submitted 17 July, 2025; v1 submitted 16 July, 2025; originally announced July 2025.

Comments: Added hyperlink to Reference 76

Journal ref: Phys. Rev. B 112, 184302 (2025)

Reexamination of the charge-ordered dimer pattern in the spinel compound CuIr2S4 using single-crystal synchrotron x-ray diffraction

Authors: T. Ohashi, N. Katayama, K. Kojima, M. Emi, C. Koyama, T. Hara, K. Hashimoto, S. Kitani, H. Kawaji, H. S. Suzuki, S. Nagata, K. Sugimoto, K. Iida, H. Sawa

Abstract: We have re-investigated the crystal structure of a spinel type CuIr2S4 at low temperatures using a single-crystal in a synchrotron radiation x-ray diffraction experiment. The crystal structure of the low-temperature phase of CuIr2S4 has been already studied by diffraction experiments using a powder sample, and it has been reported that the formation of dimer molecules accompanied by charge orderin… ▽ More We have re-investigated the crystal structure of a spinel type CuIr2S4 at low temperatures using a single-crystal in a synchrotron radiation x-ray diffraction experiment. The crystal structure of the low-temperature phase of CuIr2S4 has been already studied by diffraction experiments using a powder sample, and it has been reported that the formation of dimer molecules accompanied by charge ordering of Ir has been achieved. The crystal structure of the low-temperature phase obtained in our reanalysis was the same as the previously reported structure in that it showed the formation of Ir dimers accompanied by charge ordering, but the charge ordering pattern and arrangement of the dimers in the unit cell were different. We will discuss the validity of the structure obtained in this study and provide the structural parameters revealed in the reanalysis. The results of this study should provide a basis for further studies of the physical properties of CuIr2S4, which are still being actively investigated. △ Less

Submitted 30 June, 2025; originally announced June 2025.

Comments: 13 pages, 4 figures, 7 tables

Journal ref: Physical Review B 111, 224114 (2025)

Tunable Fujita-Miyazawa-Type Three-Body Force in Ultracold Atoms

Authors: Hiroyuki Tajima, Eiji Nakano, Kei Iida

Abstract: We show how a Fujita-Miyazawa-type three-body force emerges among three impurity atoms immersed in an atomic Bose-Einstein condensate near an interspecies Feshbach resonance. As a result of thermal average over excitations in the medium and impurities as well as expansion with respect to the impurity-medium and Feshbach resonance couplings, two superfluid phonons and a closed channel resonance pla… ▽ More We show how a Fujita-Miyazawa-type three-body force emerges among three impurity atoms immersed in an atomic Bose-Einstein condensate near an interspecies Feshbach resonance. As a result of thermal average over excitations in the medium and impurities as well as expansion with respect to the impurity-medium and Feshbach resonance couplings, two superfluid phonons and a closed channel resonance play a role in producing an effective three-body force, as in the original three-nucleon case in which two pions and a $Δ$ resonance are involved. The proposed Fujita-Miyazawa-type three-body force can be enhanced by tuning the closed-channel energy level via an external magnetic field, and moreover, its strength can be confirmed experimentally by measuring the impurity equation of state. Our result gives a new insight into an analogy between atomic polarons and nuclear few-body systems. △ Less

Submitted 25 May, 2025; originally announced May 2025.

Comments: 6 pages, 4 figures

Status of Iron Based Superconductors: characteristics and relevant properties for applications

Authors: Kazumasa Iida

Abstract: Since the discovery of iron-based superconductors (IBSs) on LaFePO in 2006, many types of IBSs have been fabricated. IBSs have usually been compared to cuprates and MgB2, and the methodology of research developed by them have been implemented to IBSs. As a result, many similarities between IBSs and cuprates have been revealed, e.g., the parent compounds being antiferromagnets and grain boundaries… ▽ More Since the discovery of iron-based superconductors (IBSs) on LaFePO in 2006, many types of IBSs have been fabricated. IBSs have usually been compared to cuprates and MgB2, and the methodology of research developed by them have been implemented to IBSs. As a result, many similarities between IBSs and cuprates have been revealed, e.g., the parent compounds being antiferromagnets and grain boundaries being weak-links to some extent. On the other hands, the distinct features of IBSs are highlighted as multiband superconductors (i.e., the 5 bands of Fe 3d orbital crossing Fermi level) and extended s-wave symmetry. Additionally, some of the IBSs are topological superconductors that can be possible platforms for quantum computing. In this paper, an overview of IBS research and development in the last 18 years will be reported, involving characteristics of IBSs as well as strategies of increasing the superconducting transition temperature and critical current density. △ Less

Submitted 24 February, 2025; originally announced February 2025.

Comments: ASC2024, Plenary talk

Journal ref: IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, VOL. 35, NO. 5, 7400109, AUGUST 2025

Tripling Fluctuations and Peaked Sound Speed in Fermionic Matter

Authors: Hiroyuki Tajima, Kei Iida, Toru Kojo, Haozhao Liang

Abstract: A crossover involving three-fermion clusters is relevant to the hadron-quark crossover, which, if occurring in a neutron star, could naturally reproduce the dense-matter equation of state recently deduced from simultaneous observations of neutron-star masses and radii. To understand the crossover mechanism, we examine the role of tripling fluctuations induced by the formation of three-fermion clus… ▽ More A crossover involving three-fermion clusters is relevant to the hadron-quark crossover, which, if occurring in a neutron star, could naturally reproduce the dense-matter equation of state recently deduced from simultaneous observations of neutron-star masses and radii. To understand the crossover mechanism, we examine the role of tripling fluctuations induced by the formation of three-fermion clusters. The phase-shift representation of fluctuations manifests an interplay of bound and scattering states, leading to non-monotonic momentum distributions of baryon-like clusters and peaked sound speed at finite densities. We demonstrate them by applying our approach to a nonrelativistic system of one-dimensional three-color fermions analogous to the hadron-quark matter. △ Less

Submitted 6 December, 2024; originally announced December 2024.

Comments: 7 pages, 5 figures

Report number: RIKEN-iTHEMS-Report-24

Journal ref: Phys. Rev. Lett. 135, 042701 (2025)

Spin dynamics of triple-Q magnetic orderings in a triangular lattice: Implications for multi-Q orderings in general two-dimensional lattices

Authors: Pyeongjae Park, Woonghee Cho, Chaebin Kim, Yeochan An, Kazuki Iida, Ryoichi Kajimoto, Sakib Matin, Shang-Shun Zhang, Cristian D. Batista, Je-Geun Park

Abstract: Multi-Q magnetic structures on two-dimensional (2D) lattices provide a key route to realizing topological physics in 2D magnetism. A major experimental challenge is to unambiguously confirm their formation by excluding the possibility of topologically trivial multi-domain single- or double-Q magnetic orders, which cannot be distinguished using conventional diffraction techniques. Here, we propose… ▽ More Multi-Q magnetic structures on two-dimensional (2D) lattices provide a key route to realizing topological physics in 2D magnetism. A major experimental challenge is to unambiguously confirm their formation by excluding the possibility of topologically trivial multi-domain single- or double-Q magnetic orders, which cannot be distinguished using conventional diffraction techniques. Here, we propose that long-wavelength spin dynamics offers a universal diagnostic for triangular lattices: triple-Q orders that preserve rotational symmetry and single- or double-Q orders that break it exhibit qualitatively distinct anisotropies in their Goldstone mode velocities, stemming from fundamental differences in their underlying spin configurations. We validate this concept using the metallic triangular lattice antiferromagnet Co$_{0.325}$TaS$_{2}$, which hosts both a stripe-type single-Q state and a triple-Q tetrahedral ordering at different temperatures. Using inelastic neutron scattering (INS) and spin dynamics simulations, we first refine the spin Hamiltonian by fitting the paramagnetic excitation spectra, allowing us to develop an unbiased model independent of magnetic ordering. We then show that the observed velocity profiles of the Goldstone modes agree with the high-temperature model's predictions: markedly anisotropic for the single-Q phase and near isotropic for the triple-Q phase. Importantly, this contrast persists across various exchange parameters, highlighting its model-independent nature and suggesting potential applicability to other 2D lattice systems. This work provides universal insight into the dynamical properties of topological multi-Q magnetic orderings in 2D lattice structures, offering a broadly applicable diagnostic to distinguishing them from topologically trivial single- or double-Q counterparts. (For the full abstract, please refer to the manuscript) △ Less

Submitted 14 July, 2025; v1 submitted 2 October, 2024; originally announced October 2024.

Comments: 16 pages with 8 figures (plus Appendices), accepted for publication in Phys. Rev. X

Structural analysis and transport properties of [010]-tilt grain boundaries in Fe(Se,Te)

Authors: Kazumasa Iida, Yoshihiro Yamauchi, Takafumi Hatano, Kai Walter, Bernhard Holzapfel, Jens Hänisch, Zimeng Guo, Hongye Gao, Haoshan Shi, Shinnosuke Tokuta, Satoshi Hata, Akiyasu Yamamoto, Hiroshi Ikuta

Abstract: Understanding the nature of grain boundaries is a prerequisite for fabricating high-performance superconducting bulks and wires. For iron-based superconductors [e.g. Ba(Fe,Co)$_2$As$_2$, Fe(Se,Te), and NdFeAs(O,F)], the dependence of the critical current density $J_\mathrm{c}$ on misorientation angle ($θ_\mathrm{GB}$) has been explored on [001]-tilt grain boundaries, but no data for other types of… ▽ More Understanding the nature of grain boundaries is a prerequisite for fabricating high-performance superconducting bulks and wires. For iron-based superconductors [e.g. Ba(Fe,Co)$_2$As$_2$, Fe(Se,Te), and NdFeAs(O,F)], the dependence of the critical current density $J_\mathrm{c}$ on misorientation angle ($θ_\mathrm{GB}$) has been explored on [001]-tilt grain boundaries, but no data for other types of orientations have been reported. Here, we report on the structural and transport properties of Fe(Se,Te) grown on CeO$_2$-buffered symmetric [010]-tilt roof-type SrTiO$_3$ bicrystal substrates by pulsed laser deposition. X-ray diffraction and transmission electron microscopy revealed that $θ_\mathrm{GB}$ of Fe(Se,Te) was smaller whereas $θ_\mathrm{GB}$ of CeO$_2$ was larger than that of the substrate. The difference in $θ_\mathrm{GB}$ between the CeO$_2$ buffer layer and the substrate is getting larger with increasing $θ_\mathrm{GB}$. For $θ_\mathrm{GB}\geq24^\circ$ of the substrates, $θ_\mathrm{GB}$ of Fe(Se,Te) was zero, whereas $θ_\mathrm{GB}$ of CeO$_2$ was continuously increasing. The inclined growth of CeO$_2$ can be explained by the geometrical coherency model. The $c$-axis growth of Fe(Se,Te) for $θ_\mathrm{GB}\geq24^\circ$ of the substrates is due to the domain matching epitaxy on (221) planes of CeO$_2$. Electrical transport measurements confirmed no reduction of inter-grain $J_\mathrm{c}$ for $θ_\mathrm{GB}\leq9^\circ$, indicative of strong coupling between the grains. △ Less

Submitted 19 September, 2024; originally announced September 2024.

Comments: 6 figures, with Supplementary Information

Journal ref: Sci. Technol. Adv. Mater. 25, 2384829 (2024)

Polaronic neutron in dilute alpha matter: A $p$-wave Bose polaron

Authors: Hiroyuki Tajima, Hajime Moriya, Tomoya Naito, Wataru Horiuchi, Eiji Nakano, Kei Iida

Abstract: We theoretically investigate quasiparticle properties of a neutron immersed in an alpha condensate, which is one of the possible states of dilute symmetric nuclear matter. The resonant $p$-wave neutron-alpha scattering, which plays a crucial role in forming halo nuclei, is considered. This system is similar to a Bose polaron near the $p$-wave Feshbach resonance that can be realized in cold-atomic… ▽ More We theoretically investigate quasiparticle properties of a neutron immersed in an alpha condensate, which is one of the possible states of dilute symmetric nuclear matter. The resonant $p$-wave neutron-alpha scattering, which plays a crucial role in forming halo nuclei, is considered. This system is similar to a Bose polaron near the $p$-wave Feshbach resonance that can be realized in cold-atomic experiments. Calculating the self-energy within the field-theoretical approach, we give an analytical formula for the effective mass of a polaronic neutron as a function of alpha condensation density. Moreover, two adjacent neutrons in a medium, each of which behaves like a stable polaron having an enhanced effective mass, can form a bound dineutron, with the help of $^1S_0$ neutron-neutron attraction. This is in contrast to the case of the vacuum, where a dineutron is known to be unbound. Our result would be useful for understanding many-body physics in astrophysical environments as well as the formation of multi-nucleon clusters in neutron-halo nuclei. △ Less

Submitted 5 February, 2025; v1 submitted 27 August, 2024; originally announced August 2024.

Comments: 12 pages, 6 figures. Published version

Report number: RIKEN-iTHEMS-Report-24, NITEP 221

Journal ref: Phys. Rev. C 111, 025802 (2025)

Ferromagnetic inter-layer coupling in FeSe$_{1-x}$S$_{x}$ superconductors revealed by inelastic neutron scattering

Authors: Mingwei Ma, Philippe Bourges, Yvan Sidis, Jinzhao Sun, Guoqing Wang, Kazuki Iida, Kazuya Kamazawa, Jitae T. Park, Frederic Bourdarot, Zhian Ren, Yuan Li

Abstract: FeSe$_{1-x}$S$_{x}$ superconductors are commonly considered layered van der Waals materials with negligible inter-layer coupling. Here, using inelastic neutron scattering to study spin excitations in single-crystal samples, we reveal that the magnetic coupling between adjacent Fe layers is not only significant, as it affects excitations up to \textcolor{black}{15} meV, but also ferromagnetic in na… ▽ More FeSe$_{1-x}$S$_{x}$ superconductors are commonly considered layered van der Waals materials with negligible inter-layer coupling. Here, using inelastic neutron scattering to study spin excitations in single-crystal samples, we reveal that the magnetic coupling between adjacent Fe layers is not only significant, as it affects excitations up to \textcolor{black}{15} meV, but also ferromagnetic in nature, making the system different from most unconventional superconductors including iron pnictides. Our observation provides a new standpoint to understand the absence of magnetic order in FeSe$_{1-x}$S$_{x}$. Since intercalating between the Fe layers is known to enhance superconductivity and suppress the inter-layer coupling, superconductivity appears to be a more robust phenomenon in the two-dimensional limit than antiferromagnetic order. △ Less

Submitted 7 July, 2024; originally announced July 2024.

Journal ref: Phys. Rev. B 110, 174503 (2024)

Universality of giant diffusion in tilted periodic potentials

Authors: Kento Iida, Andreas Dechant, Takuma Akimoto

Abstract: Giant diffusion, where the diffusion coefficient of a Brownian particle in a periodic potential with an external force is significantly enhanced by the external force, is a non-trivial non-equilibrium phenomenon. We propose a simple stochastic model of giant diffusion, which is based on a biased continuous-time random walk (CTRW) with flight time. By introducing a flight time representing traversa… ▽ More Giant diffusion, where the diffusion coefficient of a Brownian particle in a periodic potential with an external force is significantly enhanced by the external force, is a non-trivial non-equilibrium phenomenon. We propose a simple stochastic model of giant diffusion, which is based on a biased continuous-time random walk (CTRW) with flight time. By introducing a flight time representing traversal dynamics, we derive the diffusion coefficient using renewal theory and demonstrate its universal peak behavior under various periodic potentials, especially in low-temperature regimes. Giant diffusion is universally observed in the sense that there is a peak of the diffusion coefficient for any tilted periodic potentials and the degree of the diffusivity is greatly enhanced especially for low-temperature regimes. The biased CTRW models with flight times are applied to diffusion under three tilted periodic potentials. Furthermore, the temperature dependence of the maximum diffusion coefficient and the external force that attains the maximum are presented for diffusion under a tilted sawtooth potential. △ Less

Submitted 16 June, 2025; v1 submitted 19 April, 2024; originally announced April 2024.

Comments: 12 pages, 7 figures

Journal ref: Phys. Rev. E 111, 024113 (2025)

Non-relativistic trace anomaly and equation of state in dense fermionic matter

Authors: Hiroyuki Tajima, Kei Iida, Haozhao Liang

Abstract: We theoretically investigate a non-relativistic trace anomaly and its impact on the low-temperature equation of state in spatially one-dimensional three-component fermionic systems with a three-body interaction, which exhibit a non-trivial three-body crossover from a bound trimer gas to dense fermionic matter with increasing density. By applying the $G$-matrix approach to the three-body interactio… ▽ More We theoretically investigate a non-relativistic trace anomaly and its impact on the low-temperature equation of state in spatially one-dimensional three-component fermionic systems with a three-body interaction, which exhibit a non-trivial three-body crossover from a bound trimer gas to dense fermionic matter with increasing density. By applying the $G$-matrix approach to the three-body interaction, we obtain the analytical expression for the ground-state equation of state relevant to the high-density degenerate regime and thereby address how the three-body contact or, equivalently, the trace anomaly emerges. The analytical results are compared with the recent quantum Monte Carlo data. Our study of the trace anomaly and the sound speed could have some relevance to the physics of hadron-quark crossover in compact stars. △ Less

Submitted 7 February, 2024; originally announced February 2024.

Comments: 7 pages, 4 figures

Report number: RIKEN-iTHEMS-Report-24

Journal ref: Phys. Rev. C 109, 055203 (2024)

Composition dependence of bulk properties in the Co-intercalated transition-metal dichalcogenide Co$_{1/3}$TaS$_2$

Authors: Pyeongjae Park, Woonghee Cho, Chaebin Kim, Yeochan An, Maxim Avdeev, Kazuki Iida, Ryoichi Kajimoto, Je-Geun Park

Abstract: Spontaneous Hall conductivity has recently been reported in the triangular lattice antiferromagnet Co$_{1/3}$TaS$_2$ under a zero magnetic field. This phenomenon originates from the distinctive noncoplanar triple-Q magnetic ground state, possessing uniform real-space Berry curvature characterized by scalar spin chirality. We investigated the physical properties of Co$_{1/3}$TaS$_2$ by judiciously… ▽ More Spontaneous Hall conductivity has recently been reported in the triangular lattice antiferromagnet Co$_{1/3}$TaS$_2$ under a zero magnetic field. This phenomenon originates from the distinctive noncoplanar triple-Q magnetic ground state, possessing uniform real-space Berry curvature characterized by scalar spin chirality. We investigated the physical properties of Co$_{1/3}$TaS$_2$ by judiciously controlling the composition, revealing a drastic change in its bulk properties, even by slight variations in cobalt composition, despite the same crystal structure. For $0.299 < x < 0.325$, Co$_x$TaS$_2$ keeps all the characteristics of the ground state consistent with the previous studies -- two antiferromagnetic phase transitions at $T_{N1}$ and $T_{N2} (< T_{N1})$, a large spontaneous Hall conductivity ($σ_{xy} (H=0)$), and a weak ferromagnetic moment along the c-axis. However, samples with $x > 0.330$ exhibit distinct bulk properties, including the absence of both $σ_{xy} (H=0)$ and the weak ferromagnetic moment. Our neutron diffraction data reveal that Co$_x$TaS$_2$ with $x > 0.330$ develops coplanar helical magnetic order with $q_{m1} = (1/3, 0, 0)$. This is entirely different from what has been seen in $x < 0.325$, explaining the observed composition dependence. △ Less

Submitted 18 January, 2024; originally announced January 2024.

Comments: 15 pages, 4 figures, accepted for publication in Phys. Rev. B

Journal ref: Phys. Rev. B 109, L060403 (2024)

Intersections of ultracold atomic polarons and nuclear clusters: How is a chart of nuclides modified in dilute neutron matter?

Authors: Hiroyuki Tajima, Hajime Moriya, Wataru Horiuchi, Eiji Nakano, Kei Iida

Abstract: Neutron star observations, as well as experiments on neutron-rich nuclei, used to motivate one to look at degenerate nuclear matter from its extreme, namely, pure neutron matter. As an important next step, impurities and clusters in dilute neutron matter have attracted special attention. In this paper, we review in-medium properties of these objects on the basis of the physics of polarons, which h… ▽ More Neutron star observations, as well as experiments on neutron-rich nuclei, used to motivate one to look at degenerate nuclear matter from its extreme, namely, pure neutron matter. As an important next step, impurities and clusters in dilute neutron matter have attracted special attention. In this paper, we review in-medium properties of these objects on the basis of the physics of polarons, which have been recently realized in ultracold atomic experiments. We discuss how such atomic and nuclear systems are related to each other in terms of polarons. In addition to the interdisciplinary understanding of in-medium nuclear clusters, it is shown that the quasiparticle energy of a single proton in neutron matter is associated with the symmetry energy, implying a novel route toward the nuclear equation of state from the neutron-rich side. △ Less

Submitted 30 October, 2023; originally announced October 2023.

Comments: 28 pages, 2 figures

Report number: NITEP 188

Journal ref: AAPPS Bull. 34, 9 (2024)

Easy-plane multi-$\mathbf{q}$ magnetic ground state of Na$_3$Co$_2$SbO$_6$

Authors: Yuchen Gu, Xintong Li, Yue Chen, Kazuki Iida, Akiko Nakao, Koji Munakata, V. Ovidiu Garlea, Yangmu Li, Guochu Deng, I. A. Zaliznyak, J. M. Tranquada, Yuan Li

Abstract: Na$_3$Co$_2$SbO$_6$ is a potential Kitaev magnet with a monoclinic layered crystal structure. Recent investigations of the $C_3$-symmetric sister compound Na$_2$Co$_2$TeO$_6$ have uncovered a unique triple-$\mathbf{q}$ magnetic ground state, as opposed to a single-$\mathbf{q}$ (zigzag) one, prompting us to examine the influence of the reduced structural symmetry of Na$_3$Co$_2$SbO$_6$ on its groun… ▽ More Na$_3$Co$_2$SbO$_6$ is a potential Kitaev magnet with a monoclinic layered crystal structure. Recent investigations of the $C_3$-symmetric sister compound Na$_2$Co$_2$TeO$_6$ have uncovered a unique triple-$\mathbf{q}$ magnetic ground state, as opposed to a single-$\mathbf{q}$ (zigzag) one, prompting us to examine the influence of the reduced structural symmetry of Na$_3$Co$_2$SbO$_6$ on its ground state. Neutron diffraction data obtained on a twin-free crystal reveal that the ground state remains a multi-$\mathbf{q}$ state, despite the system's strong in-plane anisotropy. This robustness of multi-$\mathbf{q}$ orders suggests that they are driven by a common mechanism in the honeycomb cobaltates, such as higher-order magnetic interactions. Spin-polarized neutron diffraction results show that the ordered moments are entirely in-plane, with each staggered component orthogonal to the propagating wave vector. The inferred ground state favors a so-called XXZ easy-plane anisotropic starting point for the microscopic model over a Kitaev one, and features unequal ordered moments reduced by strong quantum fluctuations. △ Less

Submitted 12 June, 2023; originally announced June 2023.

Comments: 4 pages, 4 figures, plus Supplemental Material

Journal ref: Physical Review B 109, L060410 (2024)

Polaronic Proton and Diproton Clustering in Neutron-Rich Matter

Authors: Hiroyuki Tajima, Hajime Moriya, Wataru Horiuchi, Eiji Nakano, Kei Iida

Abstract: We show that strong spin-triplet neutron-proton interaction causes polaronic protons to occur in neutron matter at subnuclear densities and nonzero temperature. As the neutron density increases, proton spectra exhibit a smooth crossover from a bare impurity to a repulsive polaron branch; this branch coexists with an attractive polaron branch. With the neutron density increased further, the attract… ▽ More We show that strong spin-triplet neutron-proton interaction causes polaronic protons to occur in neutron matter at subnuclear densities and nonzero temperature. As the neutron density increases, proton spectra exhibit a smooth crossover from a bare impurity to a repulsive polaron branch; this branch coexists with an attractive polaron branch. With the neutron density increased further, the attractive polarons become stable with respect to deuteron formation. For two adjacent protons, we find that the polaron effects and the neutron-mediated attraction are sufficient to induce a bound diproton, which leads possibly to diproton formation in the surface region of neutron-rich nuclei in laboratories as well as in neutron stars. △ Less

Submitted 2 April, 2023; originally announced April 2023.

Comments: 6 pages, 4 figures (8 pages, 6 figures in the supplement)

Report number: NITEP 170

Journal ref: Phys. Lett. B 851, 138567 (2024)

Tetrahedral triple-Q magnetic ordering and large spontaneous Hall conductivity in the metallic triangular antiferromagnet Co1/3TaS2

Authors: Pyeongjae Park, Woonghee Cho, Chaebin Kim, Yeochan An, Yoon-Gu Kang, Maxim Avdeev, Romain Sibille, Kazuki Iida, Ryoichi Kajimoto, Ki Hoon Lee, Woori Ju, En-Jin Cho, Han-Jin Noh, Myung Joon Han, Shang-Shun Zhang, Cristian D. Batista, Je-Geun Park

Abstract: The triangular lattice antiferromagnet (TLAF) has been the standard paradigm of frustrated magnetism for several decades. The most common magnetic ordering in insulating TLAFs is the 120 structure. However, a new triple-Q chiral ordering can emerge in metallic TLAFs, representing the short wavelength limit of magnetic skyrmion crystals. We report the metallic TLAF Co1/3TaS2 as the first example of… ▽ More The triangular lattice antiferromagnet (TLAF) has been the standard paradigm of frustrated magnetism for several decades. The most common magnetic ordering in insulating TLAFs is the 120 structure. However, a new triple-Q chiral ordering can emerge in metallic TLAFs, representing the short wavelength limit of magnetic skyrmion crystals. We report the metallic TLAF Co1/3TaS2 as the first example of tetrahedral triple-Q magnetic ordering with the associated topological Hall effect (non-zero σ_{xy}(H=0)). We also present a theoretical framework that describes the emergence of this magnetic ground state, which is further supported by the electronic structure measured by angle-resolved photoemission spectroscopy. Additionally, our measurements of the inelastic neutron scattering cross section are consistent with the calculated dynamical structure factor of the tetrahedral triple-Q state. △ Less

Submitted 12 November, 2023; v1 submitted 7 March, 2023; originally announced March 2023.

Comments: 23 pages, 4 figures, SI not included. Accepted for publication in Nature Communications

Journal ref: Nature Communications 14, 8346 (2023)

Density-Induced Hadron-Quark Crossover via the Formation of Cooper Triples

Authors: Hiroyuki Tajima, Shoichiro Tsutsui, Takahiro M. Doi, Kei Iida

Abstract: We discuss the hadron--quark crossover accompanied by the formation of Cooper triples (three-body counterpart of Cooper pairs) by analogy with the Bose--Einstein condensate to Bardeen--Cooper--Schrieffer crossover in two-component fermionic systems. Such a crossover is different from a phase transition, which often involves symmetry breaking. We calculate the in-medium three-body energy from the t… ▽ More We discuss the hadron--quark crossover accompanied by the formation of Cooper triples (three-body counterpart of Cooper pairs) by analogy with the Bose--Einstein condensate to Bardeen--Cooper--Schrieffer crossover in two-component fermionic systems. Such a crossover is different from a phase transition, which often involves symmetry breaking. We calculate the in-medium three-body energy from the three-body $T$-matrix with a phenomenological three-body force characterizing a bound hadronic state in vacuum. With increasing density, the hadronic bound-state pole smoothly undergoes a crossover toward the Cooper triple phase where the in-medium three-body clusters coexist with the quark Fermi sea. The relation to the quarkyonic matter model can also be found in a natural manner. △ Less

Submitted 1 February, 2023; v1 submitted 25 November, 2022; originally announced November 2022.

Comments: 8 pages, 4 figures

Report number: RIKEN-iTHEMS-Report-22

Journal ref: Symmetry 15, 333 (2023)

Resonance-to-bound transition of $^5$He in neutron matter and its analogy with heteronuclear Feshbach molecule

Authors: Hiroyuki Tajima, Hajime Moriya, Wataru Horiuchi, Kei Iida, Eiji Nakano

Abstract: We theoretically investigate the fate of a neutron-alpha $p$-wave resonance in dilute neutron matter, which may be encountered in neutron stars and supernova explosions. While $^5$He is known as a resonant state that decays to a neutron and an alpha particle in vacuum, this unstable state turns into a stable bound state in the neutron Fermi sea because the decay process is forbidden by the Pauli-b… ▽ More We theoretically investigate the fate of a neutron-alpha $p$-wave resonance in dilute neutron matter, which may be encountered in neutron stars and supernova explosions. While $^5$He is known as a resonant state that decays to a neutron and an alpha particle in vacuum, this unstable state turns into a stable bound state in the neutron Fermi sea because the decay process is forbidden by the Pauli-blocking effect of neutrons. Such a resonance-to-bound transition assisted by the Pauli-blocking effect can be realized in cold atomic experiments for a quantum mixture near the heteronuclear Feshbach resonance. △ Less

Submitted 28 July, 2022; originally announced July 2022.

Comments: 7 pages, 4 figures

Report number: NITEP 142

Journal ref: Phys. Rev. C 106, 045807 (2022)

Chern numbers of topological phonon band crossing determined with inelastic neutron scattering

Authors: Zhendong Jin, Biaoyan Hu, Yiran Liu, Yangmu Li, Tiantian Zhang, Kazuki Iida, Kazuya Kamazawa, A. I. Kolesnikov, M. B. Stone, Xiangyu Zhang, Haiyang Chen, Yandong Wang, I. A. Zaliznyak, J. M. Tranquada, Chen Fang, Yuan Li

Abstract: Topological invariants in the band structure, such as Chern numbers, are crucial for the classification of topological matters and dictate the occurrence of exotic properties, yet their direct spectroscopic determination has been largely limited to electronic bands. Here, we use inelastic neutron scattering in conjunction with ab initio calculations to identify a variety of topological phonon band… ▽ More Topological invariants in the band structure, such as Chern numbers, are crucial for the classification of topological matters and dictate the occurrence of exotic properties, yet their direct spectroscopic determination has been largely limited to electronic bands. Here, we use inelastic neutron scattering in conjunction with ab initio calculations to identify a variety of topological phonon band crossings in MnSi and CoSi single crystals. We find a distinct relation between the Chern numbers of a band-crossing node and the scattering intensity modulation in momentum space around the node. Given sufficiently high resolution, our method can be used to determine arbitrarily large Chern numbers of topological phonon band-crossing nodes. △ Less

Submitted 25 July, 2022; originally announced July 2022.

Comments: 14 pages, 9 figures including supplemental material

Journal ref: Phys. Rev. B 106, 224304 (2022)

K-doped Ba122 epitaxial thin film on MgO substrate by buffer engineering

Authors: Dongyi Qin, Kazumasa Iida, Zimeng Guo, Chao Wang, Hikaru Saito, Satoshi Hata, Michio Naito, Akiyasu Yamamoto

Abstract: Molecular beam epitaxy of K-doped Ba122 (Ba$_{1-x}$K$_x$Fe$_\text{2}$As$_\text{2}$) superconductor was realized on a MgO substrate. Microstructural observation revealed that the undoped Ba122 served as a perfect buffer layer for epitaxial growth of the K-doped Ba122. The film exhibited a high critical temperature of 39.8 K and a high critical current density of 3.9 MA/cm$^\text{2}$ at 4 K. The suc… ▽ More Molecular beam epitaxy of K-doped Ba122 (Ba$_{1-x}$K$_x$Fe$_\text{2}$As$_\text{2}$) superconductor was realized on a MgO substrate. Microstructural observation revealed that the undoped Ba122 served as a perfect buffer layer for epitaxial growth of the K-doped Ba122. The film exhibited a high critical temperature of 39.8 K and a high critical current density of 3.9 MA/cm$^\text{2}$ at 4 K. The successful growth of epitaxial thin film will enable artificial single grain boundary on oxide bicrystal substrates and reveal the grain boundary transport nature of K-doped Ba122. △ Less

Submitted 13 July, 2022; originally announced July 2022.

Comments: 5 pages, 4 figures, accepted manuscript Supercond. Sci. Technol 2022

Possible condensation of Cooper triples

Authors: Sora Akagami, Hiroyuki Tajima, Kei Iida

Abstract: We theoretically discuss the possible condensation of Cooper triples, which correspond to a three-body version of Cooper pairs, in three-component Fermi systems with three-body attractive interactions. A macroscopic number of Cooper triples can occupy a zero center-of-mass momentum state in the presence of a Fermi surface of constituent particles, even though the three-body operator exhibits anti-… ▽ More We theoretically discuss the possible condensation of Cooper triples, which correspond to a three-body version of Cooper pairs, in three-component Fermi systems with three-body attractive interactions. A macroscopic number of Cooper triples can occupy a zero center-of-mass momentum state in the presence of a Fermi surface of constituent particles, even though the three-body operator exhibits anti-commutation relation associated with the Fermi-Dirac statistics. Such a condensation with internal degrees of freedom is similar to bosonization in a system of infinite-component fermions. We propose a variational wave function for condensed Cooper triples and show that in the ground state, the condensed state is energetically favored compared to the normal state. Also, we discuss effects of the Fermi-surface distortion in a lattice system described by a three-component Hubbard model. △ Less

Submitted 11 October, 2022; v1 submitted 7 July, 2022; originally announced July 2022.

Comments: 6 pages, 2 figures, proceedings of LT29

Magnetic molecular orbitals in MnSi

Authors: Zhendong Jin, Yangmu Li, Zhigang Hu, Biaoyan Hu, Yiran Liu, Kazuki Iida, Kazuya Kamazawa, M. B. Stone, A. I. Kolesnikov, D. L. Abernathy, Xiangyu Zhang, Haiyang Chen, Yandong Wang, Chen Fang, Biao Wu, I. A. Zaliznyak, J. M. Tranquada, Yuan Li

Abstract: A large body of knowledge about magnetism is attained from models of interacting spins, which usually reside on magnetic ions. Proposals beyond the ionic picture are uncommon and seldom verified by direct observations in conjunction with microscopic theory. Here, using inelastic neutron scattering to study the itinerant near-ferromagnet MnSi, we find that the system's fundamental magnetic units ar… ▽ More A large body of knowledge about magnetism is attained from models of interacting spins, which usually reside on magnetic ions. Proposals beyond the ionic picture are uncommon and seldom verified by direct observations in conjunction with microscopic theory. Here, using inelastic neutron scattering to study the itinerant near-ferromagnet MnSi, we find that the system's fundamental magnetic units are interconnected, extended molecular orbitals consisting of three Mn atoms each, rather than individual Mn atoms. This result is further corroborated by magnetic Wannier orbitals obtained by ab initio calculations. It contrasts the ionic picture with a concrete example, and presents a novel regime of the spin waves where the wavelength is comparable to the spatial extent of the molecular orbitals. Our discovery brings important insights into not only the magnetism of MnSi, but also a broad range of magnetic quantum materials where structural symmetry, electron itinerancy and correlations act in concert. △ Less

Submitted 27 June, 2022; originally announced June 2022.

Comments: 27 pages, 27 figures including supplemental; comments are welcome

Journal ref: Science Advances 9, eadd5239 (2023)

Giant magnetic in-plane anisotropy and competing instabilities in Na3Co2SbO6

Authors: Xintong Li, Yuchen Gu, Yue Chen, V. Ovidiu Garlea, Kazuki Iida, Kazuya Kamazawa, Yangmu Li, Guochu Deng, Qian Xiao, Xiquan Zheng, Zirong Ye, Yingying Peng, I. A. Zaliznyak, J. M. Tranquada, Yuan Li

Abstract: We report magnetometry data obtained on twin-free single crystals of Na3Co2SbO6, which is considered a candidate material for realizing the Kitaev honeycomb model for quantum spin liquids. Contrary to a common belief that such materials can be modeled with the symmetries of an ideal honeycomb lattice, our data reveal a pronounced two-fold symmetry and in-plane anisotropy of over 200%, despite the… ▽ More We report magnetometry data obtained on twin-free single crystals of Na3Co2SbO6, which is considered a candidate material for realizing the Kitaev honeycomb model for quantum spin liquids. Contrary to a common belief that such materials can be modeled with the symmetries of an ideal honeycomb lattice, our data reveal a pronounced two-fold symmetry and in-plane anisotropy of over 200%, despite the honeycomb layer's tiny orthorhombic distortion of less than 0.2%. We further use magnetic neutron diffraction to elucidate a rich variety of field-induced phases observed in the magnetometry. These phases manifest themselves in the paramagnetic state as diffuse scattering signals associated with competing ferro- and antiferromagnetic instabilities, consistent with a theory that also predicts a quantum spin liquid phase nearby. Our results call for theoretical understanding of the observed in-plane anisotropy, and render Na3Co2SbO6 a promising ground for finding exotic quantum phases by targeted external tuning. △ Less

Submitted 15 October, 2022; v1 submitted 9 April, 2022; originally announced April 2022.

Comments: 19 pages, 15 figures, 1 animation; to appear in Phys. Rev. X

Journal ref: Phys. Rev. X 12, 041024 (2022)

Inter- to Intra-Layer Resistivity Anisotropy of NdFeAs(O,H) with Various Hydrogen Concentrations

Authors: M. Chen, K. Iida, K. Kondo, J. Hänisch, T. Hatano, H. Ikuta

Abstract: With molecular beam epitaxy and topotactic chemical reaction, we prepared NdFeAs(O,H) epitaxial thin films with various hydrogen concentrations on 5° vicinal cut MgO substrates. By measuring the resistivities along the longitudinal and transversal directions, the ab plane and the c axis resistivities (\{rho}_ab and \{rho}_c) were obtained. The resistivity anisotropy γ_\{rho}=\{rho}_c \ \{rho}_ab o… ▽ More With molecular beam epitaxy and topotactic chemical reaction, we prepared NdFeAs(O,H) epitaxial thin films with various hydrogen concentrations on 5° vicinal cut MgO substrates. By measuring the resistivities along the longitudinal and transversal directions, the ab plane and the c axis resistivities (\{rho}_ab and \{rho}_c) were obtained. The resistivity anisotropy γ_\{rho}=\{rho}_c \ \{rho}_ab of NdFeAs(O,H) with various hydrogen concentrations was compared with that of NdFeAs(O,F). At the H concentrations which led to superconducting transition temperatures Tc over 40 K, γ_\r{ho} recorded ~100-150 at 50 K. On the other hand, a low γ_\{rho} value of 9 was observed with the mostly doped sample. The exponent \{beta} of the ab plane resistivity obtained by fitting a power law expression \{rho}_{ab}(T)=\{rho}_0+AT^\{beta} to the data was close to unity down to low temperature in the vicinity where the second antiferromagnetic phase locates, which may be related to the quantum critical point discussed at the over-doped side of the phase diagram. △ Less

Submitted 4 April, 2022; originally announced April 2022.

Comments: Appeared in Physical. Rev. Materials

Journal ref: Phys. Rev. Materials 6, 054802 (2022)

Excitations in the ordered and paramagnetic states of honeycomb magnet Na2Co2TeO6

Authors: Weiliang Yao, Kazuki Iida, Kazuya Kamazawa, Yuan Li

Abstract: Na2Co2TeO6 is a proposed approximate Kitaev magnet, yet its actual magnetic interactions are elusive due to a lack of knowledge on the full excitation spectrum. Here, using inelastic neutron scattering and single crystals, we determine the system's temperature-dependent magnetic excitations over the entire Brillouin zone. Without committing to specific models, we unveil a distinct signature of the… ▽ More Na2Co2TeO6 is a proposed approximate Kitaev magnet, yet its actual magnetic interactions are elusive due to a lack of knowledge on the full excitation spectrum. Here, using inelastic neutron scattering and single crystals, we determine the system's temperature-dependent magnetic excitations over the entire Brillouin zone. Without committing to specific models, we unveil a distinct signature of the third-nearest-neighbor coupling in the spin waves, which signifies the associated distance as an emerging "soft link" in the ordered state. The presence of at least six non-overlapping spin-wave branches is at odds with all models proposed to date. Above the ordering temperature, persisting dynamic correlations can be described by equal-time magnetic structure factors of a hexagonal cluster, which reveal the leading instabilities. Our result sets definitive constraint on theoretical models for Na2Co2TeO6 and provides new insight for the materialization of the Kitaev model. △ Less

Submitted 1 March, 2022; originally announced March 2022.

Comments: 4 pages, 4 figures plus Supplemental Material; comments are welcome

Journal ref: Physical Review Letters 129, 147202 (2022)

Evidence for strong correlations at finite temperatures in the dimerized magnet Na$_2$Cu$_2$TeO$_6$

Authors: Yanyan Shangguan, Song Bao, Zhao-Yang Dong, Zhengwei Cai, Wei Wang, Zhentao Huang, Zhen Ma, Junbo Liao, Xiaoxue Zhao, Ryoichi Kajimoto, Kazuki Iida, David Voneshen, Shun-Li Yu, Jian-Xin Li, Jinsheng Wen

Abstract: Dimerized magnets forming alternating Heisenberg chains exhibit quantum coherence and entanglement and thus can find potential applications in quantum information and computation. However, magnetic systems typically undergo thermal decoherence at finite temperatures. Here, we show inelastic neutron scattering results on an alternating antiferromagnetic-ferromagnetic chain compound Na$_2$Cu$_2$TeO… ▽ More Dimerized magnets forming alternating Heisenberg chains exhibit quantum coherence and entanglement and thus can find potential applications in quantum information and computation. However, magnetic systems typically undergo thermal decoherence at finite temperatures. Here, we show inelastic neutron scattering results on an alternating antiferromagnetic-ferromagnetic chain compound Na$_2$Cu$_2$TeO$_6$ that the excited quasiparticles can counter thermal decoherence and maintain strong correlations at elevated temperatures. At low temperatures, we observe clear dispersive singlet-triplet excitations arising from the dimers formed along the crystalline $b$-axis. The excitation gap is of $\sim$18 meV and the bandwidth is about half of the gap. The band top energy has a weak modulation along the [100] direction, indicative of a small interchain coupling. The gap increases while the bandwidth decreases with increasing temperature, leading to a strong reduction in the available phase space for the triplons. As a result, the Lorentzian-type energy broadening becomes highly asymmetric as the temperature is raised. These results are associated with a strongly correlated state resulting from hard-core constraint and quasiparticle interactions. We consider these results to be not only evidence for strong correlations at finite temperatures in Na$_2$Cu$_2$TeO$_6$, but also for the universality of the strongly correlated state in a broad range of quantum magnetic systems. △ Less

Submitted 11 February, 2022; originally announced February 2022.

Comments: Published in PRB, 8 pages, 6 figures

Journal ref: Phys. Rev. B 104, 224430 (2021)

Nanoscale Texture and Microstructure in NdFeAs(O,F)/IBAD-MgO Superconducting Thin Film with Superior Critical Current Properties

Authors: Zimeng Guo, Hongye Gao, Keisuke Kondo, Takafumi Hatano, Kazumasa Iida, Jens Hänisch, Hiroshi Ikuta, Satoshi Hata

Abstract: This paper reports the nanoscale texture and microstructure of a high-performance NdFeAs(O,F) superconducting thin film grown by molecular beam epitaxy on a textured MgO/Y$_2$O$_3$/Hastelloy substrate. The NdFeAs(O,F) film forms a highly textured columnar grain structure by epitaxial growth on the MgO template. Although the film contains stacking faults along the $ab$-plane as well as grain bounda… ▽ More This paper reports the nanoscale texture and microstructure of a high-performance NdFeAs(O,F) superconducting thin film grown by molecular beam epitaxy on a textured MgO/Y$_2$O$_3$/Hastelloy substrate. The NdFeAs(O,F) film forms a highly textured columnar grain structure by epitaxial growth on the MgO template. Although the film contains stacking faults along the $ab$-plane as well as grain boundaries perpendicular to the $ab$-plane, good superconducting properties are measured: a critical temperature, $T _{\rm c}$, of 46 K and a self-field critical current density, $J_{\rm c}$, of $2 \times 10^6 \,{\rm A/cm^2}$ at 4.2 K. Automated crystal orientation mapping by scanning precession electron diffraction in transmission electron microscopy is employed to analyze the misorientation angles between adjacent grains in a large ensemble (247 grains). 99% of the grain boundaries show in-plane misorientation angles ($Δγ$) less than the critical angle $θ_{\rm c}$, which satisfies one of the necessary conditions for the high $J_{\rm c}$. Comparing the columnar grain size distribution with the mean distance of the flux line lattice, the triple junctions of low-angle grain boundaries are found to be effective pinning centers, even at high temperatures ($\ge$35 K) and/or low magnetic fields. △ Less

Submitted 19 November, 2021; originally announced November 2021.

Comments: Main text with Supplement Material, 32 pages, 16 figures

Journal ref: ACS Appl. Electron. Mater., 3, 7 (2021)

Approaching the ultimate superconducting properties of (Ba,K)Fe2As2 by naturally formed low-angle grain boundary networks

Authors: Kazumasa Iida, Dongyi Qin, Chiara Tarantini, Takafumi Hatano, Chao Wang, Zimeng Guo, Hongy Gao, Hikaru Saito, Satoshi Hata, Michio Naito, Akiyasu Yamamoto

Abstract: The most effective way to enhance the dissipation-free supercurrent in presence of magnetic field for type II superconductors is the introduction of defects that acts as artificial pinning centres (APCs) for the vortices. For instance, the in-field critical current density of doped BaFe2As2 (Ba122), one of the most technologically important Fe-based superconductors, has been improved over the last… ▽ More The most effective way to enhance the dissipation-free supercurrent in presence of magnetic field for type II superconductors is the introduction of defects that acts as artificial pinning centres (APCs) for the vortices. For instance, the in-field critical current density of doped BaFe2As2 (Ba122), one of the most technologically important Fe-based superconductors, has been improved over the last decade by APCs created by ion-irradiation. The technique of ion-irradiation has been commonly implemented to determine the ultimate superconducting properties. However, this method is rather complicated and expensive. Here, we report on a surprisingly high critical current density and strong pinning efficiency close to the crystallographic c-axis for a K-doped Ba122 epitaxial thin film without APCs, achieving performance comparable to ion-irradiated K-doped Ba122 single crystals. Microstructural analysis reveals that the film is composed of columnar grains having width around 30-60 nm. The grains are rotated around the b- (or a-) axis by 1.5 degree and around the c-axis by -1 degree, resulting in the formation of low-angle grain boundary networks. This study demonstrates that the upper limit of in-field properties reached in ion-irradiated K-doped Ba122 is achievable by grain boundary engineering, which is a simple and industrially scalable manner. △ Less

Submitted 29 October, 2021; originally announced October 2021.

Comments: Main text with Supplement Material

Journal ref: NPG Asia Materials, 13, 68 (2021)

Superconductivity-driven ferromagnetism and spin manipulation using vortices in the magnetic superconductor EuRbFe4As4

Authors: Shigeyuki Ishida, Daniel Kagerbauer, Sigrid Holleis, Kazuki Iida, Koji Munakata, Akiko Nakao, Akira Iyo, Hiraku Ogino, Kenji Kawashima, Michael Eisterer, Hiroshi Eisaki

Abstract: Magnetic superconductors are specific materials exhibiting two antagonistic phenomena, superconductivity and magnetism, whose mutual interaction induces various emergent phenomena, such as the reentrant superconducting transition associated with the suppression of superconductivity around the magnetic transition temperature (Tm), highlighting the impact of magnetism on superconductivity. In this s… ▽ More Magnetic superconductors are specific materials exhibiting two antagonistic phenomena, superconductivity and magnetism, whose mutual interaction induces various emergent phenomena, such as the reentrant superconducting transition associated with the suppression of superconductivity around the magnetic transition temperature (Tm), highlighting the impact of magnetism on superconductivity. In this study, we report the experimental observation of the ferromagnetic order induced by superconducting vortices in the high-critical-temperature (high-Tc) magnetic superconductor EuRbFe4As4. Although the ground state of the Eu2+ moments in EuRbFe4As4 is helimagnetism below Tm, neutron diffraction and magnetization experiments show a ferromagnetic hysteresis of the Eu2+ spin alignment. We demonstrate that the direction of the Eu2+ moments is dominated by the distribution of pinned vortices based on the critical state model. Moreover, we demonstrate the manipulation of spin texture by controlling the direction of superconducting vortices, which can help realize spin manipulation devices using magnetic superconductors. △ Less

Submitted 18 October, 2021; originally announced October 2021.

Comments: Main text: 16 pages, 6 figures, Supplementary Information: 9 pages, 5 figures, 1 table

Journal ref: Proc. Natl. Acad. Sci. USA, 118 (37) e2101101118 (2021)

Dissipation-relaxation dynamics of a spin-1/2 particle with a Rashba-type spin-orbit coupling in an ohmic heat bath

Authors: Tomohiro Hata, Eiji Nakano, Kei Iida, Hiroyuki Tajima, Junichi Takahashi

Abstract: Spin-orbit coupling (SOC), which is inherent to a Dirac particle that moves under the influence of electromagnetic fields, manifests itself in a variety of physical systems including non-relativistic ones. For instance, it plays an essential role in spintronics developed in the past few decades, particularly by controlling spin current generation and relaxation. In the present work, by using an ex… ▽ More Spin-orbit coupling (SOC), which is inherent to a Dirac particle that moves under the influence of electromagnetic fields, manifests itself in a variety of physical systems including non-relativistic ones. For instance, it plays an essential role in spintronics developed in the past few decades, particularly by controlling spin current generation and relaxation. In the present work, by using an extended Caldeira-Leggett model, we elucidate how the interplay between spin relaxation and momentum dissipation of an open system of a single spin-$1/2$ particle with a Rashba type SOC is induced by the interactions with a spinless, three-dimensional environment. Staring from the path integral formulation for the reduced density matrix of the system, we have derived a set of coupled nonlinear equations that consists of a quasi-classical Langevin equation for the momentum with a frictional term and a spin precession equation. The spin precesses around the effective magnetic field generated by both the SOC and the frictional term. It is found from analytical and numerical solutions to these equations that a spin torque effect included in the effective magnetic field causes a spin relaxation and that the spin and momentum orientations after a long time evolution are largely controlled by the Rashba coupling strength. Such a spin relaxation mechanism is qualitatively different from, e.g., the one encountered in semiconductors where essentially no momentum dissipation occurs due to the Pauli blocking. △ Less

Submitted 14 August, 2021; originally announced August 2021.

Comments: 14page, 10figures

Resonant pair-exchange scattering and BCS-BEC crossover in a system composed of dispersive and heavy incipient bands: a Feshbach analogy

Authors: Kazunari Ochi, Hiroyuki Tajima, Kei Iida, Hideo Aoki

Abstract: We theoretically show that a two-band system with very different masses harbors a resonant pair scattering that leads to novel pairing properties, as highlighted by the Bardeen-Cooper-Schrieffer (BCS) to Bose-Einstein condensation (BEC) crossover. Most importantly, the interband pair-exchange coupling induces an effective intraband attraction in each band, enhancing the superfluidity/superconducti… ▽ More We theoretically show that a two-band system with very different masses harbors a resonant pair scattering that leads to novel pairing properties, as highlighted by the Bardeen-Cooper-Schrieffer (BCS) to Bose-Einstein condensation (BEC) crossover. Most importantly, the interband pair-exchange coupling induces an effective intraband attraction in each band, enhancing the superfluidity/superconductivity. The effect, a kind of Suhl-Kondo mechanism, is specifically enhanced when the second band has a heavy mass and is incipient (lying close to, but just above, the chemical potential, $μ$), which we call a resonant pair scattering. By elucidating the dependence of the effective interactions and gap functions on $μ$, we can draw an analogy between the resonant pair scattering and the Feshbach resonance. △ Less

Submitted 10 November, 2021; v1 submitted 29 July, 2021; originally announced July 2021.

Comments: 11 pages, 8 figures

Journal ref: Phys. Rev. Research 4, 013032 (2022)

Three-body crossover from a Cooper triple to bound trimer state in three-component Fermi gases near a triatomic resonance

Authors: Hiroyuki Tajima, Shoichiro Tsutsui, Takahiro M. Doi, Kei Iida

Abstract: We theoretically investigate ground-state properties of a three-component Fermi gas with pairwise contact interactions between different components near a triatomic resonance where bound trimers are about to appear. Using variational equations for in-medium two- and three-body cluster states in three dimensions, we elucidate the competition of pair and triple formations due to the Fermi surface ef… ▽ More We theoretically investigate ground-state properties of a three-component Fermi gas with pairwise contact interactions between different components near a triatomic resonance where bound trimers are about to appear. Using variational equations for in-medium two- and three-body cluster states in three dimensions, we elucidate the competition of pair and triple formations due to the Fermi surface effects. We present the ground-state phase diagram that exhibits transition from a Cooper pair to Cooper triple state and crossover from a Cooper triple to tightly bound trimer state at negative scattering lengths. This three-body crossover is analogous to the Bardeen-Cooper-Schrieffer to Bose-Einstein condensation crossover observed in a two-component Fermi gas. We predict that the threshold scattering length $a_{-}$ for three-body states can be shifted towards the weak-coupling side due to the emergence of Cooper triples. △ Less

Submitted 28 July, 2021; originally announced July 2021.

Comments: 21 pages, 4 figures

Report number: RIKEN-QHP-500, RIKEN-iTHEMS-Report-21

Journal ref: Phys. Rev. A 104, 053328 (2021)

Unitary $p$-wave Fermi gas in one dimension

Authors: Hiroyuki Tajima, Shoichiro Tsutsui, Takahiro M. Doi, Kei Iida

Abstract: We elucidate universal many-body properties of a one-dimensional, two-component ultracold Fermi gas near the $p$-wave Feshbach resonance. The low-energy scattering in this system can be characterized by two parameters, that is, $p$-wave scattering length and effective range. At the unitarity limit where the $p$-wave scattering length diverges and the effective range is reduced to zero without conf… ▽ More We elucidate universal many-body properties of a one-dimensional, two-component ultracold Fermi gas near the $p$-wave Feshbach resonance. The low-energy scattering in this system can be characterized by two parameters, that is, $p$-wave scattering length and effective range. At the unitarity limit where the $p$-wave scattering length diverges and the effective range is reduced to zero without conflicting with the causality bound, the system obeys universal thermodynamics as observed in a unitary Fermi gas with contact $s$-wave interaction in three dimensions. It is in contrast to a Fermi gas with the $p$-wave resonance in three dimensions in which the effective range is inevitably finite. We present the universal equation of state in this unitary $p$-wave Fermi gas within the many-body $T$-matrix approach as well as the virial expansion method. Moreover, we examine the single-particle spectral function in the high-density regime where the virial expansion is no longer valid. On the basis of the Hartree-like self-energy shift at the divergent scattering length, we conjecture that the equivalence of the Bertsch parameter across spatial dimensions holds even for a one-dimensional unitary $p$-wave Fermi gas. △ Less

Submitted 24 June, 2021; originally announced June 2021.

Comments: 9 pages, 5 figures

Report number: RIKEN-QHP-498, RIKEN-iTHEMS-Report-21

Journal ref: Phys. Rev. A 104, 023319 (2021)

Spin texture induced by non-magnetic doping and spin dynamics in 2D triangular lattice antiferromagnet h-Y(Mn,Al)O3

Authors: Pyeongjae Park, Kisoo Park, Joosung Oh, Ki Hoon Lee, Jonathan C. Leiner, Hasung Sim, Taehun Kim, Jaehong Jeong, Kirrily C. Rule, Kazuya Kamazawa, Kazuki Iida, T. G. Perring, Hyungje Woo, S. -W. Cheong, M. E. Zhitomirsky, A. L. Chernyshev, Je-Geun Park

Abstract: Novel effects induced by nonmagnetic impurities in frustrated magnets and quantum spin liquid represent a highly nontrivial and interesting problem. A theoretical proposal of extended modulated spin structures induced by doping of such magnets, distinct from the well-known skyrmions has attracted significant interest. Here, we demonstrate that nonmagnetic impurities can produce such extended spin… ▽ More Novel effects induced by nonmagnetic impurities in frustrated magnets and quantum spin liquid represent a highly nontrivial and interesting problem. A theoretical proposal of extended modulated spin structures induced by doping of such magnets, distinct from the well-known skyrmions has attracted significant interest. Here, we demonstrate that nonmagnetic impurities can produce such extended spin structures in h-YMnO3, a triangular antiferromagnet with noncollinear magnetic order. Using inelastic neutron scattering (INS), we measured the full dynamical structure factor in Al-doped h-YMnO3 and confirmed the presence of magnon damping with a clear momentum dependence. Our theoretical calculations can reproduce the key features of the INS data, supporting the formation of the proposed spin textures. As such, our study provides the first experimental confirmation of the impurity-induced spin textures. It offers new insights and understanding of the impurity effects in a broad class of noncollinear magnetic systems. △ Less

Submitted 15 March, 2021; v1 submitted 10 March, 2021; originally announced March 2021.

Comments: 18 pages, 4 figures and supplementary Information. Accepted for publication in Nature Communications

Journal ref: Nature Communications 12, 2306 (2021)

High $J_{\rm c}$ and low anisotropy of hydrogen doped NdFeAsO superconducting thin film

Authors: Kazumasa Iida, Jens Hänisch, Keisuke Kondo, Mingyu Chen, Takafumi Hatano, Chao Wang, Hikaru Saito, Satoshi Hata, Hiroshi Ikuta

Abstract: The recent realisations of hydrogen doped $Ln$FeAsO ($Ln$=Nd and Sm) superconducting epitaxial thin films call for further investigation of their structural and electrical transport properties. Here, we report on the microstructure of a NdFeAs(O,H) epitaxial thin film and its temperature, field, and orientation dependencies of the resistivity and the critical current density $J_{\rm c}$. The super… ▽ More The recent realisations of hydrogen doped $Ln$FeAsO ($Ln$=Nd and Sm) superconducting epitaxial thin films call for further investigation of their structural and electrical transport properties. Here, we report on the microstructure of a NdFeAs(O,H) epitaxial thin film and its temperature, field, and orientation dependencies of the resistivity and the critical current density $J_{\rm c}$. The superconducting transition temperature $T_{\rm c}$ is comparable to NdFeAs(O,F). Transmission electron microscopy investigation supported that hydrogen is homogenously substituted for oxygen. A high self-field $J_{\rm c}$ of over 10 MA/cm$^2$ was recorded at 5 K, which is likely to be caused by a short London penetration depth. The anisotropic Ginzburg-Landau scaling for the angle dependence of $J_{\rm c}$ yielded temperature-dependent scaling parameters $γ_{\rm J}$ that decreased from 1.6 at 30 K to 1.3 at 5 K. This is opposite to the behaviour of NdFeAs(O,F). Additionally, $γ_{\rm J}$ of NdFeAs(O,H) is smaller than that of NdFeAs(O,F). Our results indicate that heavily electron doping by means of hydrogen substitution for oxygen in $Ln$FeAsO is highly beneficial for achieving high $J_{\rm c}$ with low anisotropy without compromising $T_{\rm c}$, which is favourable for high-field magnet applications. △ Less

Submitted 26 February, 2021; originally announced February 2021.

Comments: 17 pages, 6 figures with Supplementary Information, accepted in Scientific Reports

Journal ref: Sci. Rep. 11, 5636 (2021)

Condensation of Cooper Triples

Authors: Sora Akagami, Hiroyuki Tajima, Kei Iida

Abstract: The condensation of Cooper pairs, originating from the Fermi-surface instability due to a weakly attractive interaction between two fermions, opened a new frontier for exploring many-body physics in interdisciplinary contexts. In this work, we discuss the possible condensation of Cooper triples, which are three-body counterparts of Cooper pairs for three-component fermions with a three-body attrac… ▽ More The condensation of Cooper pairs, originating from the Fermi-surface instability due to a weakly attractive interaction between two fermions, opened a new frontier for exploring many-body physics in interdisciplinary contexts. In this work, we discuss the possible condensation of Cooper triples, which are three-body counterparts of Cooper pairs for three-component fermions with a three-body attraction. Although each composite trimer-like state obeys the Fermi-Dirac statistics, its aggregate can form a condensate at zero center-of-mass momentum in the presence of the internal degrees of freedom associated with the relative momenta of constituent particles of momenta close to the Fermi surface. Such condensation can be regarded as bosonization in infinite-component fermions. We propose a variational wave function for the condensate of Cooper triples in analogy with the Bardeen-Cooper-Schrieffer ground state and obtain the ground-state energy. △ Less

Submitted 24 November, 2021; v1 submitted 6 February, 2021; originally announced February 2021.

Comments: 7 pages, 2 figure

Journal ref: Phys. Rev. A 104, L041302 (2021)

Polaron Problems in Ultracold Atoms: Role of a Fermi Sea across Different Spatial Dimensions and Quantum Fluctuations of a Bose Medium

Authors: Hiroyuki Tajima, Junichi Takahashi, Simeon I. Mistakidis, Eiji Nakano, Kei Iida

Abstract: The notion of a polaron, originally introduced in the context of electrons in ionic lattices, helps us to understand how a quantum impurity behaves when being immersed in and interacting with a many-body background. We discuss the impact of the impurities on the medium particles by considering feedback effects from polarons that can be realized in ultracold quantum gas experiments. In particular,… ▽ More The notion of a polaron, originally introduced in the context of electrons in ionic lattices, helps us to understand how a quantum impurity behaves when being immersed in and interacting with a many-body background. We discuss the impact of the impurities on the medium particles by considering feedback effects from polarons that can be realized in ultracold quantum gas experiments. In particular, we exemplify the modifications of the medium in the presence of either Fermi or Bose polarons. Regarding Fermi polarons we present a corresponding many-body diagrammatic approach operating at finite temperatures and discuss how mediated two- and three-body interactions are implemented within this framework. Utilizing this approach, we analyze the behavior of the spectral function of Fermi polarons at finite temperature by varying impurity-medium interactions as well as spatial dimensions from three to one. Interestingly, we reveal that the spectral function of the medium atoms could be a useful quantity for analyzing the transition/crossover from attractive polarons to molecules in three-dimensions. As for the Bose polaron, we showcase the depletion of the background Bose-Einstein condensate in the vicinity of the impurity atom. Such spatial modulations would be important for future investigations regarding the quantification of interpolaron correlations in Bose polaron problems. △ Less

Submitted 15 April, 2021; v1 submitted 19 January, 2021; originally announced January 2021.

Comments: 39 pages, 9 figures

Journal ref: Atoms 9, 18 (2021)

Realization of epitaxial thin films of the superconductor K-doped BaFe$_\text{2}$As$_\text{2}$

Authors: Dongyi Qin, Kazumasa Iida, Takafumi Hatano, Hikaru Saito, Yiming Ma, Chao Wang, Satoshi Hata, Michio Naito, Akiyasu Yamamoto

Abstract: The iron-based superconductor Ba$_{1-x}$K$_x$Fe$_\text{2}$As$_\text{2}$ is emerging as a key material for high magnetic field applications owing to the recent developments in superconducting wires and bulk permanent magnets. Epitaxial thin films play important roles in investigating and artificially tuning physical properties; nevertheless, the synthesis of Ba$_{1-x}$K$_x$Fe$_2$As$_2$ epitaxial th… ▽ More The iron-based superconductor Ba$_{1-x}$K$_x$Fe$_\text{2}$As$_\text{2}$ is emerging as a key material for high magnetic field applications owing to the recent developments in superconducting wires and bulk permanent magnets. Epitaxial thin films play important roles in investigating and artificially tuning physical properties; nevertheless, the synthesis of Ba$_{1-x}$K$_x$Fe$_2$As$_2$ epitaxial thin films remained challenging because of the high volatility of K. Herein, we report the successful growth of epitaxial Ba$_{1-x}$K$_x$Fe$_\text{2}$As$_\text{2}$ thin films by molecular-beam epitaxy with employing a combination of fluoride substrates (CaF$_\text{2}$, SrF$_\text{2}$, and BaF$_\text{2}$) and a low growth temperature (350$-$420$^\circ$C). Our epitaxial thin film grown on CaF$_\text{2}$ showed sharp superconducting transition at an onset critical temperature of 36 K, slightly lower than bulk crystals by ~2 K due presumably to the strain effect arising from the lattice and thermal expansion mismatch. Critical current density ($J$$_\text{c}$) determined by the magnetization hysteresis loop is as high as 2.2 MA/cm$^\text{2}$ at 4 K under self-field. In-field $J$$_\text{c}$ characteristics of the film are superior to the bulk crystals. The realization of epitaxial thin films opens opportunities for tuning superconducting properties by epitaxial strain and revealing intrinsic grain boundary transport of Ba$_{1-x}$K$_x$Fe$_\text{2}$As$_\text{2}$. △ Less

Submitted 6 January, 2021; v1 submitted 26 December, 2020; originally announced December 2020.

Comments: 9 pages, 8 figures

Journal ref: Phys. Rev. Materials 5, 014801 (2021)

Spin-orbit phase behaviors of Na2Co2TeO6 at low temperatures

Authors: Wenjie Chen, Xintong Li, Zhenhai Hu, Ze Hu, Li Yue, Ronny Sutarto, Feizhou He, Kazuki Iida, Kazuya Kamazawa, Weiqiang Yu, Xi Lin, Yuan Li

Abstract: We present a comprehensive study of single crystals of Na2Co2TeO6, a putative Kitaev honeycomb magnet, focusing on its low-temperature phase behaviors. A new thermal phase transition is identified at 31.0 K, below which the system develops a two-dimensional (2D) long-range magnetic order. This order precedes the well-known 3D order below 26.7 K, and is likely driven by strongly anisotropic interac… ▽ More We present a comprehensive study of single crystals of Na2Co2TeO6, a putative Kitaev honeycomb magnet, focusing on its low-temperature phase behaviors. A new thermal phase transition is identified at 31.0 K, below which the system develops a two-dimensional (2D) long-range magnetic order. This order precedes the well-known 3D order below 26.7 K, and is likely driven by strongly anisotropic interactions. Surprisingly, excitations from the 3D order do not support the order's commonly accepted "zigzag" nature, and are instead consistent with a "triple-q" description. The 3D order exerts a fundamental feedback on high-energy excitations that likely involve orbital degrees of freedom, and it remains highly frustrated until a much lower temperature is reached. These findings render Na2Co2TeO6 a spin-orbit entangled frustrated magnet that hosts very rich physics. △ Less

Submitted 16 December, 2020; originally announced December 2020.

Comments: 5 pages, 4 figures, and Supplemental Materials

Journal ref: Phys. Rev. B 103, 180404 (2021)

Cooper Triples in Attractive Three-Component Fermions: Implication for Hadron-Quark Crossover

Authors: Hiroyuki Tajima, Shoichiro Tsutsui, Takahiro M. Doi, Kei Iida

Abstract: We investigate many-body properties of equally populated three-component fermions with attractive three-body contact interaction in one dimension. A diagrammatic approach suggests the possible occurrence of Cooper triples at low temperature, which are three-body counterparts of Cooper pairs with a two-body attraction. We develop a minimal framework that bridges the crossover from tightly-bound tri… ▽ More We investigate many-body properties of equally populated three-component fermions with attractive three-body contact interaction in one dimension. A diagrammatic approach suggests the possible occurrence of Cooper triples at low temperature, which are three-body counterparts of Cooper pairs with a two-body attraction. We develop a minimal framework that bridges the crossover from tightly-bound trimers to Cooper triples with increasing chemical potential and show how the formation of Cooper triples occurs in the grand-canonical phase diagram. Moreover, we argue that this non-trivial crossover is similar to the hadron-quark crossover proposed in dense matter. A coexistence of medium-induced triples and the underlying Fermi sea at positive chemical potential is analogous to quarkyonic matter consisting of baryonic excitations and the underlying quark Fermi sea. The comparison with the existing quantum Monte Carlo results implies that the emergence of these kinds of three-body states can be a microscopic origin of the peak of the sound velocity along the crossover. △ Less

Submitted 8 October, 2021; v1 submitted 7 December, 2020; originally announced December 2020.

Comments: 10 pages, 6 figures

Journal ref: Phys. Rev. Research 4, L012021 (2022)

Extracting non-local inter-polaron interactions from collisional dynamics

Authors: Junichi Takahashi, Hiroyuki Tajima, Eiji Nakano, Kei Iida

Abstract: This study develops a novel experimental method of deducing the profile of interaction induced between impurities in a trapped gas of ultracold Fermi/Bose atoms, which are often referred to as Fermi/Bose polarons. In this method, we consider a two-body Fermi/Bose polaron collision experiment in which impurities and atoms interact only weakly. Numerical simulations of the quantum dynamics reveal th… ▽ More This study develops a novel experimental method of deducing the profile of interaction induced between impurities in a trapped gas of ultracold Fermi/Bose atoms, which are often referred to as Fermi/Bose polarons. In this method, we consider a two-body Fermi/Bose polaron collision experiment in which impurities and atoms interact only weakly. Numerical simulations of the quantum dynamics reveal the possibility to obtain information regarding the non-local induced interaction between two polarons from a measured profile of the polaron wave packet at several snapshots. This is because the potential of the induced interaction is well balanced by the quantum potential whenever the WKB approximation for the relevant Schrödinger equation is applicable. △ Less

Submitted 16 November, 2020; originally announced November 2020.

Comments: 6 pages, 4 figures

Journal ref: Phys. Rev. A 103, 043334 (2021)

The ground state of polaron in an ultracold dipolar Fermi gas

Authors: Kazuya Nishimura, Eiji Nakano, Kei Iida, Hiroyuki Tajima, Takahiko Miyakawa, Hiroyuki Yabu

Abstract: An impurity atom immersed in an ultracold atomic Fermi gas can form a quasiparticle, so-called Fermi polaron, due to impurity-fermion interaction. We consider a three-dimensional homogeneous dipolar Fermi gas as a medium, where the interatomic dipole-dipole interaction (DDI) makes the Fermi surface deformed into a spheroidal shape, and, using a Chevy-type variational method, investigate the ground… ▽ More An impurity atom immersed in an ultracold atomic Fermi gas can form a quasiparticle, so-called Fermi polaron, due to impurity-fermion interaction. We consider a three-dimensional homogeneous dipolar Fermi gas as a medium, where the interatomic dipole-dipole interaction (DDI) makes the Fermi surface deformed into a spheroidal shape, and, using a Chevy-type variational method, investigate the ground-state properties of the Fermi polaron: the effective mass, the momentum distribution of a particle-hole (p-h) excitation, the drag parameter, and the medium density modification around the impurity. These quantities are shown to exhibit spatial anisotropies in such a way as to reflect the momentum anisotropy of the background dipolar Fermi gas. We have also given numerical results for the polaron properties at the unitarity limit of the impurity-fermion interaction in the case in which the impurity and fermion masses are equal. It has been found that the transverse effective mass and the transverse momentum drag parameter of the polaron both tend to decrease by $ \sim 10\%$ when the DDI strength is raised from $0$ up to around its critical value, while the longitudinal ones exhibit a very weak dependence on the DDI. △ Less

Submitted 9 April, 2021; v1 submitted 29 October, 2020; originally announced October 2020.

Comments: 27 pages, 6 figures; accepted version

Journal ref: Phys. Rev. A 103, 033324 (2021)

Robust atomic orbital in the cluster magnet LiMoO2

Authors: N. Katayama, H. Takeda, T. Yamaguchi, Y. Yamada, K. Iida, M. Takigawa, Y. Ohta, H. Sawa

Abstract: In this study, we present a rutile-related material, LiMoO2, that becomes a cluster magnet and exhibits a spin singlet formation on a preformed molybdenum dimer upon cooling. Unlike ordinary cluster magnets, the atomic dyz orbital robustly survives despite the formation of molecular orbitals, thereby affecting the magnetic properties of the selected material. Such hybrid cluster magnets with the c… ▽ More In this study, we present a rutile-related material, LiMoO2, that becomes a cluster magnet and exhibits a spin singlet formation on a preformed molybdenum dimer upon cooling. Unlike ordinary cluster magnets, the atomic dyz orbital robustly survives despite the formation of molecular orbitals, thereby affecting the magnetic properties of the selected material. Such hybrid cluster magnets with the characters of molecular and atomic orbitals realize multiple independent spins on an isolated cluster, leading to an ideal platform to study the isolated spin dimers physics. △ Less

Submitted 10 August, 2020; originally announced August 2020.

Comments: 6 pages, 4 figures

Journal ref: Phys. Rev. B 102, (2020) 081106(R)

$\mathbf{q}=\mathbf{0}$ long-range magnetic order in centennialite CaCu$_3$(OD)$_6$Cl$_2$$\cdot$0.6D$_2$O: A spin-1/2 perfect kagome antiferromagnet with $J_1$-$J_2$-$J_d$

Authors: K. Iida, H. K. Yoshida, A. Nakao, H. O. Jeschke, Y. Iqbal, K. Nakajima, S. Ohira-Kawamura, K. Munakata, Y. Inamura, N. Murai, M. Ishikado, R. Kumai, T. Okada, M. Oda, K. Kakurai, M. Matsuda

Abstract: Crystal and magnetic structures of the mineral centennialite CaCu$_3$(OH)$_6$Cl$_2\cdot0.6$H$_2$O are investigated by means of synchrotron x-ray diffraction and neutron diffraction measurements complemented by density functional theory (DFT) and pseudofermion functional renormalization group (PFFRG) calculations. CaCu$_3$(OH)$_6$Cl$_2\cdot0.6$H$_2$O crystallizes in the $P\bar{3}m1$ space group and… ▽ More Crystal and magnetic structures of the mineral centennialite CaCu$_3$(OH)$_6$Cl$_2\cdot0.6$H$_2$O are investigated by means of synchrotron x-ray diffraction and neutron diffraction measurements complemented by density functional theory (DFT) and pseudofermion functional renormalization group (PFFRG) calculations. CaCu$_3$(OH)$_6$Cl$_2\cdot0.6$H$_2$O crystallizes in the $P\bar{3}m1$ space group and Cu$^{2+}$ ions form a geometrically perfect kagome network with antiferromagnetic $J_1$. No intersite disorder between Cu$^{2+}$ and Ca$^{2+}$ ions is detected. CaCu$_3$(OH)$_6$Cl$_2\cdot0.6$H$_2$O enters a magnetic long-range ordered state below $T_\text{N}=7.2$~K, and the $\mathbf{q}=\mathbf{0}$ magnetic structure with negative vector spin chirality is obtained. The ordered moment at 0.3~K is suppressed to $0.58(2)μ_\text{B}$. Our DFT calculations indicate the presence of antiferromagnetic $J_2$ and ferromagnetic $J_d$ superexchange couplings of a strength which places the system at the crossroads of three magnetic orders (at the classical level) and a spin-$\frac{1}{2}$ PFFRG analysis shows a dominance of $\mathbf{q}=\mathbf{0}$ type magnetic correlations, consistent with and indicating proximity to the observed $\mathbf{q}=\mathbf{0}$ spin structure. The results suggest that this material is located close to a quantum critical point and is a good realization of a $J_1$-$J_2$-$J_d$ kagome antiferromagnet. △ Less

Submitted 22 June, 2020; originally announced June 2020.

Comments: 4 figures and supplemental materials

Journal ref: Phys. Rev. B 101, 220408(R) (2020)

Quasiparticle properties of a single alpha particle in cold neutron matter

Authors: Eiji Nakano, Kei Iida, Wataru Horiuchi

Abstract: Light clusters such as alpha particles and deuterons are predicted to occur in hot nuclear matter as encountered in intermediate-energy heavy-ion collisions and protoneutron stars. To examine the in-medium properties of such light clusters, we consider a much simplified system in which like an impurity, a single alpha particle is embedded in a zero-temperature, dilute gas of non-interacting neutro… ▽ More Light clusters such as alpha particles and deuterons are predicted to occur in hot nuclear matter as encountered in intermediate-energy heavy-ion collisions and protoneutron stars. To examine the in-medium properties of such light clusters, we consider a much simplified system in which like an impurity, a single alpha particle is embedded in a zero-temperature, dilute gas of non-interacting neutrons. By adopting a non-selfconsistent ladder approximation for the effective interaction between the impurity and the gas, which is often used for analyses of Fermi polarons in a gas of ultracold atoms, we calculate the quasiparticle properties of the impurity, i.e., the energy shift, effective mass, quasiparticle residue, and damping rate. △ Less

Submitted 16 October, 2020; v1 submitted 27 May, 2020; originally announced May 2020.

Comments: 19 pages, 3 figures, version accepted for PRC: discussions on p-wave contributions and the validity range of the model were added

Journal ref: Phys. Rev. C 102, 055802 (2020)

Non-Hermitian Ferromagnetism in an Ultracold Fermi Gas

Authors: Hiroyuki Tajima, Kei Iida

Abstract: We develop a non-Hermitian effective theory for a repulsively interacting Fermi gas in the excited branch. The on-shell $T$-matrix is employed as a complex-valued interaction term, which describes a repulsive interaction between atoms in the excited branch and a two-body inelastic decay to the attractive branch. To see the feature of this model, we have addressed, in the weak coupling regime, the… ▽ More We develop a non-Hermitian effective theory for a repulsively interacting Fermi gas in the excited branch. The on-shell $T$-matrix is employed as a complex-valued interaction term, which describes a repulsive interaction between atoms in the excited branch and a two-body inelastic decay to the attractive branch. To see the feature of this model, we have addressed, in the weak coupling regime, the excitation properties of a repulsive Fermi polaron as well as the time-dependent number density. The analytic expressions obtained for these quantities qualitatively show a good agreement with recent experiments. By calculating the dynamical transverse spin susceptibility in the random phase approximation, we show that a ferromagnetic system with nonzero polarization undergoes a dynamical instability and tends towards a heterogeneous phase. △ Less

Submitted 12 April, 2020; originally announced April 2020.

Comments: 6 pages, 2 figures

Journal ref: J. Phys. Soc. Jpn. 90, 024004 (2021)

Horizontal Line Nodes in Sr2RuO4 Proved by Spin Resonance

Authors: K. Iida, M. Kofu, K. Suzuki, N. Murai, S. Ohira-Kawamura, R. Kajimoto, Y. Inamura, M. Ishikado, S. Hasegawa, T. Masuda, Y. Yoshida, K. Kakurai, K. Machida, S. -H. Lee

Abstract: We investigated the low-energy incommensurate (IC) magnetic fluctuations in Sr$_2$RuO$_4$ by the high-resolution inelastic neutron scattering measurements and random phase approximation (RPA) calculations. We observed a spin resonance with energy of $\hbarω_\text{res}=0.56$~meV centered at a characteristic wavevector $\mathbf{Q}_\text{res}=(0.3, 0.3, 0.5)$. The resonance energy corresponds well to… ▽ More We investigated the low-energy incommensurate (IC) magnetic fluctuations in Sr$_2$RuO$_4$ by the high-resolution inelastic neutron scattering measurements and random phase approximation (RPA) calculations. We observed a spin resonance with energy of $\hbarω_\text{res}=0.56$~meV centered at a characteristic wavevector $\mathbf{Q}_\text{res}=(0.3, 0.3, 0.5)$. The resonance energy corresponds well to the superconducting gap $2Δ=0.56$~meV estimated by the tunneling spectroscopy. The spin resonance shows the $L$ modulation with a maximum at around $L = 0.5$. The $L$ modulated intensity of the spin resonance and our RPA calculations indicate that the superconducting gaps regarding the quasi-one-dimensional $α$ and $β$ sheets at the Fermi surfaces have the horizontal line nodes. These results may set a strong constraint on the pairing symmetry of Sr$_2$RuO$_4$. We also discuss the implications on possible superconducting order parameters. △ Less

Submitted 7 April, 2020; originally announced April 2020.

Comments: arXiv admin note: substantial text overlap with arXiv:1904.01234

Journal ref: J. Phys. Soc. Jpn. 89, 053702 (2020)

Occurrence control of charged exciton for a single CdSe quantum dot at cryogenic temperatures on an optical nanofiber

Authors: K. Muhammed Shafi, Kazunori Iida, Emi Tsutsumi, Akiharu Miyanaga, Kohzo Hakuta

Abstract: We discuss photo-luminescence characteristics of CdSe core/shell quantum dots at cryogenic temperatures using a hybrid system of a single quantum dot and an optical nanofiber. The key point is to control the emission species of quantum dot to charged excitons, known as trions, which have superior characteristics to neutral excitons. We investigate the photocharging behavior for the quantum dots by… ▽ More We discuss photo-luminescence characteristics of CdSe core/shell quantum dots at cryogenic temperatures using a hybrid system of a single quantum dot and an optical nanofiber. The key point is to control the emission species of quantum dot to charged excitons, known as trions, which have superior characteristics to neutral excitons. We investigate the photocharging behavior for the quantum dots by varying the wavelength and intensity of irradiating laser light, and establish a method to create a permanently charged situation which lasts as long as the cryogenic temperature is maintained. The present photocharging method may open a new route to applying the CdSe quantum dots in quantum photonics, and the hybrid system of photocharged quantum-dot and optical nanofiber may readily be applicable to a fiber-in-line single-photon generator. △ Less

Submitted 23 March, 2020; originally announced March 2020.

Journal ref: Applied Physics B volume 126, Article number: 68 (2020)

Grain boundary characteristics of Fe-based superconductors

Authors: Kazumasa Iida, Jens Hänisch, Akiyasu Yamamoto

Abstract: Understanding the nature of grain boundary (GB) characteristics in combination with creating low-energy GBs by modifying the processing conditions, so-called GB engineering, is of great importance for controlling and reducing the defect density, leading to improved functionalities of polycrystalline metals and ceramics. For superconductors particularly, including both low- and high-temperature sup… ▽ More Understanding the nature of grain boundary (GB) characteristics in combination with creating low-energy GBs by modifying the processing conditions, so-called GB engineering, is of great importance for controlling and reducing the defect density, leading to improved functionalities of polycrystalline metals and ceramics. For superconductors particularly, including both low- and high-temperature superconductors, GB engineering has been developed to improve especially the critical current densities, Jc, across these GBs. The intrinsic physical properties of a given superconductor such as the coherence length, the order parameter symmetry, and their anisotropies would determine the strategy of GB engineering. In this topical review, we present an overview of the GB characteristics and GB engineering of Fe-based superconductors (FBS) in the form of polycrystalline bulks and wires, and thin films with application potential, e.g. for high-field magnet wires. Prior to the FBS, GB engineering of the cuprates and MgB2 are also briefly covered. △ Less

Submitted 20 March, 2020; originally announced March 2020.

Comments: Invited review

Journal ref: Superconductor Science and Technology 33, 043001 (2020)

Anisotropy of the transport properties of NdFeAs(O,F) thin films grown on vicinal substrates

Authors: Kazumasa Iida, Takuya Matsumoto, Keisuke Kondo, Takafumi Hatano, Hiroshi Ikuta

Abstract: NdFeAs(O,F) thin films having different fluorine contents were grown on 5 deg. or 10 deg. vicinal cut MgO and CaF2 single crystalline substrates by molecular beam epitaxy. Structural characterisations by reflection high-energy electron diffraction and x-ray diffraction confirmed the epitaxial growth of NdFeAs(O,F). The resistivities of the ab-plane and along the c-axis were derived from the resist… ▽ More NdFeAs(O,F) thin films having different fluorine contents were grown on 5 deg. or 10 deg. vicinal cut MgO and CaF2 single crystalline substrates by molecular beam epitaxy. Structural characterisations by reflection high-energy electron diffraction and x-ray diffraction confirmed the epitaxial growth of NdFeAs(O,F). The resistivities of the ab-plane and along the c-axis were derived from the resistivity measurements in the longitudinal and transversal directions. The c-axis resistivity was always higher than the ab-plane resistivity, resulting from the anisotropic electronic structure. The resistivity anisotropy at 300 K was almost constant in the range of 50-90 irrespective of the F content. On the other hand, the resistivity anisotropy at 56 K showed a strong fluorine dependence: the resistivity anisotropy was over 200 for the films with optimum F contents (superconducting transition temperature Tc around 50 K), whereas the resistivity anisotropy was around 70 for the films in the under-doped regime (Tc between 35 and 45 K). The mass anisotropy are the effective masses along the c-axis and on the ab-plane) close to Tc derived from the anisotropic Ginzburg-Landau approach using the angular-dependency of the ab-plane resistivity was in the range from 2 to 5. On the assumption that the square of the mass anisotropy is equal to the resistivity anisotropy, those values are small compared to the normal state anisotropy. △ Less

Submitted 20 March, 2020; originally announced March 2020.

Journal ref: Superconductor Science and Technology 33, 044016 (2020)