Yukitoshi Motome

Critical Phenomena of Ferromagnetic Transition in Double-Exchange Systems

Critical phenomena of ferromagnetic transition at finite temperatures are studied in double-exchange systems. In order to investigate strong interplay between charge and spin degrees of freedom, the Monte Carlo technique is applied to include fluctuations in a controlled and unbiased manner. By using finite-size scaling analysis, critical exponents and transition temperature are estimated for a model with Ising spin symmetry in two dimensions. The obtained exponents differ significantly from the mean-field values, but are consistent with those of spin models with short-range exchange interactions. The universality class of this transition belongs to that of short-range interaction with the same spin symmetry. We also discuss the case for three dimensions. The results are compared with experimental results in perovskite manganites which show colossal magnetoresistance.

Critical Temperature of Ferromagnetic Transition in Three-Dimensional Double-Exchange Models

Ferromagnetic transition in three-dimensional double-exchange models is studied by the Monte Carlo method. Critical temperature T c is precisely determined by finite-size scaling analysis. Strong spin fluctuations in this itinerant system significantly reduce T c from mean-field estimates. By choosing appropriate parameters, obtained values of T c quantitatively agree with experiments for the ferromagnetic metal regime of (La,Sr)MnO 3 , which is a typical perovskite manganite showing colossal magnetoresistance. This indicates that the double-exchange mechanism alone is sufficient to explain T c in this material. Critical exponents are also discussed.

Imperfect Crystal and Unusual Semiconductor: Boron, a Frustrated Element

All elements, except for helium, appear to solidify into crystalline forms at zero temperature, and it is generally assumed that the introduction of lattice defects results in an increase in internal energy. beta-Rhombohedral boron, a thermodynamically stable form of elemental boron at high temperature, is known to have a large amount of partial occupied sites, seemingly in conflict with our common knowledge. By using lattice Monte Carlo techniques combined with ab initio calculations, we find that the beta-phase is stabilized by a macroscopic amount of intrinsic defects that are responsible not only for entropic effects but also for a reduction in internal energy. These defects enable the conversion of two-center to three-center bonds and are accompanied by the presence of localized, nonconductive electronic states in the optical gap. In addition we find that the ab initio Ising model describing the partial occupancy of beta-boron has macroscopic residual entropy, suggesting that boron is a frustrated system analogous to ice and spin ice.

Competing orders and disorder-induced insulator to metal transition in manganites.

Effects of disorder on the two competing phases, i.e., the ferromagnetic metal and the commensurate charge/lattice ordered insulator, are studied by Monte Carlo simulation. The disorder suppresses the charge/lattice ordering more strongly than the ferromagnetic order, driving the commensurate insulator to the ferromagnetic metal near the phase boundary in the pure case. Above the ferromagnetic transition temperature, on the contrary, the disorder makes the system more insulating, which might cause an enhanced colossal magnetoresistance as observed in the half-doped or Cr-substituted manganites. No indication of the percolation or the cluster formation is found, and there remains the charge/lattice fluctuations instead which are enhanced toward the transition temperature.

Evolution of electronic structure of doped Mott insulators: reconstruction of poles and zeros of Green's function.

We study the evolution of metals from Mott insulators in the carrier-doped 2D Hubbard model using a cluster extension of the dynamical mean-field theory. While the conventional metal is simply characterized by the Fermi surface (pole of the Green function G), interference of the zero surfaces of G with the pole surfaces becomes crucial in the doped Mott insulators. Mutually interfering pole and zero surfaces are dramatically transferred over the Mott gap, when lightly doped holes synergetically loosen the doublon-holon binding. The heart of the Mott physics such as the pseudogap, hole pockets, Fermi arcs, in-gap states, Lifshitz transitions, and non-Fermi liquids appears as natural consequences of this global interference in the frequency space.

Magnetic transition and orbital degrees of freedom in vanadium spinels

We propose a scenario for the two phase transitions in AV 2 O 4 (A=Zn, Mg, Cd), based on an effective spin-orbital model on the pyrochlore lattice. At high temperatures, spin correlations are strongly frustrated due to the lattice structure, and the transition observed at ∼50 K is attributed to an orbital order, supported by Jahn-Teller lattice distortion. This orbital order introduces spatial modulation of spin-exchange couplings depending on the bond direction. This partially releases the frustration and leads to a spin order observed at ∼40 K. We also study the stable spin configuration by taking account of third-neighbor exchange couplings and quantum fluctuations. The result is consistent with the experimental results.

Fermionic response from fractionalization in an insulating two-dimensional magnet

An intriguing state of matter known as a quantum spin liquid has been predicted to host Majorana fermions. A detailed theoretical and numerical analysis re-interprets existing Raman data for α-RuCl3 and uncovers direct evidence of a fermionic response.

Raman-scattering measurements and theory of the energy-momentum spectrum for underdoped Bi2Sr2CaCuO(8+δ) superconductors: evidence of an s-wave structure for the pseudogap.

S. Sakai , S. Blanc, M. Civelli, Y. Gallais, M. Cazayous , M.-A. Méasson, J. S. Wen, Z. J. Xu, G. D. Gu, G. Sangiovanni , Y. Motome, K. Held, A. Sacuto, A. Georges , and M. Imada Centre de Physique Théorique, École Polytechnique, CNRS, 91128 Palaiseau, France, Department of Applied Physics, University of Tokyo, Hongo, Tokyo 113-8656, Japan, JST-CREST, Hongo, Bunkyo-ku, Tokyo, 113-8656, Japan, Laboratoire Matériaux et Phénom̀ enes Quantiques (UMR 7162 CNRS), Université Paris Diderot-Paris 7, Bat. Condorcet, 75205 P aris Cedex 13, France, Laboratoire de Physique des Solides, Université Paris-Su d, CNRS, UMR 8502, F-91405 Orsay Cedex, France, Matter Physics and Materials Science, Brookhaven National Laboratory (BNL), Upton, NY 11973, USA, Institut für Theoretische Physik und Astrophysik, Univer sität Würzburg, Am Hubland, D-97074 Würzburg, Germany, Institute for Solid State Physics, Vienna University of Tec hnology, 1040 Vienna, Austria, Collège de France, 11 place Marcelin Berthelot, 75005 Pari s, France, DPMC, Université de Genève, 24 quai Ernest Ansermet, CH-1 211 Genève, Suisse (Dated: May 2, 2014)

Thermal fractionalization of quantum spins in a Kitaev model: Temperature-linear specific heat and coherent transport of Majorana fermions

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A Monte Carlo Method for Fermion Systems Coupled with Classical Degrees of Freedom

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