Faculty of International Social Sciences

宇田川 将文

Udagawa Masafumi

基本情報

所属
学習院大学 理学部物理学科

研究者番号
80431790
J-GLOBAL ID
201601016041488112
researchmap会員ID
B000265693

受賞

 1

論文

 35
  • Masafumi Udagawa, Ludovic D. C. Jaubert, Claudio Castelnovo, Roderich Moessner
    2016年3月9日  
    We study the interplay of topological bottlenecks and energetic barriers to<br /> equilibration in a Coulomb spin liquid where a short-range energetic coupling<br /> between defects charged under an emergent gauge field supplements their<br /> entropic long-range Coulomb interaction. This work is motivated by the<br /> prevalence of memory effects observed across a wide range of geometrically<br /> frustrated magnetic materials, possibly including the spontaneous Hall effect<br /> observed in Pr2Ir2O7. Our model is canonical spin-ice model on the pyrochlore<br /> lattice, where farther-neighbour spin couplings give rise to a nearest-neighbor<br /> interaction between topological defects which can easily be chosen to be<br /> unnatural or not, i.e. attractive or repulsive between defects of equal gauge<br /> charge. Among the novel features of this model are the following. After<br /> applying a field quench, a rich dynamical approach to equilibrium emerges,<br /> dominated by multi-scale energy barriers responsible for long-lived<br /> magnetization plateaux. These even allow for the metastability of a<br /> &quot;fragmented&quot; spin liquid, an elusive regime where partial order co-exists with<br /> a spin liquid. Perhaps most strikingly, the attraction produces clusters of<br /> defects whose stability is due to a combination of energetic barriers for their<br /> break-up and proximity of opposite charges along with an entropic barrier<br /> generated by the topological requirement of annihilating a defect only together<br /> with an oppositely charged counterpart. These clusters may take the form of a<br /> &quot;jellyfish&quot; spin texture, comprising an arrangement of same-sign gauge-charges,<br /> centered on a hexagonal ring with branches of arbitrary length. The ring<br /> carries a clockwise or counterclockwise circular flow of magnetisation. This<br /> emergent toroidal degrees of freedom provides a possibility for time reversal<br /> symmetry breaking with possible relevance to the spontaneous Hall effect<br /> observed in Pr2Ir2O7.
  • Joji Nasu, Masafumi Udagawa, Yukitoshi Motome
    PHYSICAL REVIEW B 92(11) 2015年9月  
    Finite-temperature (T) properties of a Kitaev model defined on a honeycomb lattice are investigated by a quantum Monte Carlo simulation, from the viewpoint of fractionalization of quantum S = 1/2 spins into two types of Majorana fermions, itinerant and localized. In this system, the entropy is released successively at two well-separated T scales, as a clear indication of the thermal fractionalization. We show that the high-T crossover, which is driven by itinerant Majorana fermions, is closely related with the development of nearest-neighbor spin correlations. On the other hand, the low-T crossover originates in thermal fluctuations of fluxes composed of localized Majorana fermions, by which the spectrum of itinerant Majorana fermions is significantly disturbed. As a consequence, in the intermediate-T range between the two crossovers, the system exhibits T-linear behavior in the specific heat and coherent transport of Majorana fermions, which are unexpected for the Dirac semimetallic spectrum in the low-T limit. We also show that the flux fluctuations tend to open an energy gap in the Majorana spectrum near the gapless-gapped phase boundary. Our results indicate that the fractionalization is experimentally observable in the specific heat, spin correlations, and transport properties.
  • Masafumi Udagawa, Yukitoshi Motome
    JOURNAL OF STATISTICAL MECHANICS-THEORY AND EXPERIMENT 2015年1月  
    We study the entanglement spectrum of the Hubbard model at half filling on a kagome lattice. The entanglement spectrum is defined by the set of eigenvalues of a reduced thermal density matrix, which is naturally obtained in the framework of the dynamical mean-field theory. Adopting the cluster dynamical mean-field theory combined with continuous-time auxiliary-field Monte Carlo method, we calculate the entanglement spectrum for a three-site triangular cluster in the kagome Hubbard model. We find that the results at the three-particle sector well capture the qualitative nature of the system. In particular, the eigenvalue of the reduced density matrix, corresponding to the chiral degrees of freedom, exhibits a characteristic temperature scale T-chiral, below which a metallic state with large quasiparticle mass is stabilized. The entanglement spectra at different particle number sectors also exhibit characteristic changes around T-chiral, implying the development of inter-triangular ferromagnetic correlations in the correlated metallic regime.
  • Joji Nasu, Masafumi Udagawa, Yukitoshi Motome
    INTERNATIONAL CONFERENCE ON STRONGLY CORRELATED ELECTRON SYSTEMS 2014 (SCES2014) 592 2015年  
    A three-dimensional Kitaev model on a hyperhoneycomb lattice is investigated numerically at finite temperature. The Kitaev model is one of the solvable quantum spin models, where the ground state is given by gapped and gapless spin liquids, depending on the anisotropy of the interactions. This model can be rewritten as a free Majorana fermion system coupled with Z2 variables. The density of states of Majorana fermions shows an excitation gap in the gapped region, while it is semimetallic in the gapless region reflecting the Dirac node. Performing the Monte Carlo simulation, we calculate the temperature dependence of the Majorana spectra. We find that the semimetallic dip is filled as temperature increases in the gapless region, but surprisingly, the spectrum develops an excitation gap in the region near the gapless-gapped boundary. Such changes of the low-energy spectrum appear sharply at the transition temperature from the spin liquid to the paramagnetic state. The results indicate that thermal fluctuations of the Z(2) fields significantly influence the low-energy state of Majorana fermions, especially in the spin liquid formation.
  • Ryo Ozawa, Masafumi Udagawa, Yutaka Akagi, Yukitoshi Motome
    INTERNATIONAL CONFERENCE ON STRONGLY CORRELATED ELECTRON SYSTEMS 2014 (SCES2014) 592 2015年  
    Effects of an open surface on a magnetic Chern insulator are investigated in comparison with those of an interface to a capping magnetic layer. In magnets, an open surface often perturbs the magnetic order by a reconstruction of the magnetic moment directions near the surface. On the other hand, in topological insulators, it leads to the formation of topologically protected surface states. These two contrasting effects may coexist in magnetic Chern insulators, which give rise to nontrivial surface reconstruction. For instance, the chiral edge current is largely enhanced by the edge reconstruction in a two-dimensional magnetic Chern insulator realized in a quarter-filled Kondo lattice model on a triangular lattice. We here show that the edge reconstruction can be described semiquantitatively by a simple junction model between the bulk topological magnetic state and a ferromagnetic capping layer. We further clarify how the chiral edge current is affected by the magnetic structure in the capping layer. Our results indicate that the topological edge state can be controlled magnetically through the junctions.

MISC

 50
  • Tomohiro Yoshida, Masafumi Udagawa
    PHYSICAL REVIEW B 94(6) 2016年8月  
    We study the collective behavior of Majorana modes in the vortex state of chiral p-wave superconductors. Away from the isolated vortex limit, the zero-energy Majorana states communicate with each other on a vortex lattice, and form a coherent band structure with a nontrivial topological character. We reveal that the topological nature of Majorana bands changes sensitively via quantum phase transitions in two-dimensional (2D) systems, by sweeping magnetic field or Fermi energy. Through the idea of dimensional reduction, we show the existence of a generic superconducting Weyl phase in a low magnetic field region of quasi-2D chiral superconductors.
  • 寺澤大樹, 大西隆史, 山下卓也, 常盤欣文, 宇田川将文, 木村健太, HALIM Mario, 中辻知, 寺嶋孝仁, 芝内孝禎, 松田祐司
    日本物理学会講演概要集(CD-ROM) 71(1) ROMBUNNO.21PAU-9 2016年3月22日  
  • 吉田智大, 宇田川将文
    日本物理学会講演概要集(CD-ROM) 71(1) ROMBUNNO.21PBA-12 2016年3月22日  
  • 宇田川将文
    日本物理学会講演概要集(CD-ROM) 71(1) ROMBUNNO.19PAU-5 2016年3月22日  
  • 那須譲治, 宇田川将文, 求幸年
    日本物理学会誌 70(10) 776-781 2015年10月5日  
    2006年,A. Kitaevによって興味深い量子多体模型が提案された.キタエフ模型と呼ばれるこの模型は,スピン1/2をもつ量子スピンが2次元蜂の巣格子上でイジング的な相互作用をしたシンプルなものである.しかしその見かけからは想像し難い豊富な物理を含んでいることから,物性物理のみならず,統計基礎論や量子情報など物理学の様々な分野で大きな注目を集めている.この模型の最大の特徴は,基底状態が厳密に求まることである.2次元以上の量子多体模型で可解なものは非常に限られているため,この特質は様々な新しい知見をもたらしてくれる.とりわけ驚くべきことは,その基底状態が,物質中の新しい量子状態のひとつとされる量子スピン液体となっていることである.さらに,相互作用が異方的な極限では,基底状態がトポロジカルな秩序で特徴付けられる.これは,従来の対称性の破れに基づく分類に収まらない新しい秩序状態である.また,この特異な基底状態を反映して,励起状態も興味深い性質を示す.例えば,フェルミ統計にもボース統計にも従わない非可換エニオンが現れる.この特異な粒子は,トポロジカル量子計算の演算要素として有望視されるものである.別の興味深い側面として,キタエフ模型がイリジウム酸化物などのスピン軌道相互作用が強い強相関電子系で実現する可能性が指摘され,実験・理論の両面から精力的な研究が行われていることも挙げられる.こうした多彩で興味深い性質のうち,我々は物性物理の視点からキタエフ模型が示す量子スピン液体状態に着目し,その熱力学的性質を解明した.これまで,量子スピン液体の理論的研究は,主に三角格子などの幾何学的フラストレーションを有する格子上の強相関電子模型に対して行われてきたが,そこでは負符号問題のために従来の量子モンテカルロ法が適用できず,系統的な研究は困難であった.こうした事情は幾何学的なフラストレーションのないキタエフ模型の場合にも現れる.そこで我々は,マヨラナフェルミオン表示に基づいた新しい量子モンテカルロ法を開発し,それを適用することで,一切近似のない数値的な解析を行うことに成功した.本稿では,キタエフ模型を3次元に拡張した模型に対して得られた計算結果を紹介する.この模型は,最近合成された新規イリジウム酸化物において見出されたものと同等な格子構造の上で定義され,2次元の場合と同様に基底状態は量子スピン液体である.我々は,この3次元キタエフ模型において,低温の量子スピン液体から高温の常磁性状態への相転移が有限温度で生じることを見出した.これは,量子スピン系の"気液"相転移に相当する.通常の流体では,同じ対称性を有する気体と液体は,1次転移の終点である臨界点を回り込むことで連続的に接続しうる.しかしここで我々が見出した相転移は,相互作用パラメータの全領域において常に存在する.この結果は,量子スピン液体と常磁性状態が相転移によって常に明確に区別されることを意味しており,この相転移が通常の気液相転移とは異なる全く新しいタイプのものであることを示唆している.また,この相転移は励起状態がもつトポロジカルな性質の変化によって特徴付けられる.こうしたトポロジーの変化は,量子色力学における閉じ込め・非閉じ込め転移などでも議論されており,分野をまたいだ共通概念の存在を期待させるものである.