Akihiro Shimizu

Nagoya Institute of Technology, Nagoya, Aichi, Japan

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Publications (4)10.99 Total impact

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    ABSTRACT: We study the Ginzburg-Landau lattice gauge model that we introduced recently for ferromagnetic superconductors, i.e., superconductors in which the p-wave superconducting (SC) order and the ferromagnetic (FM) order may coexist. We report some interesting results obtained by Monte-Carlo simulations. In particular, we have two types of coexisting states distinguished by the transition temperatures of the SC order TSC and the FM order TFM; (i) homogeneous state for TFM/TSC > 1 and (ii) inhomogeneous state for TFM/TSC < 0.7. In (ii) the two orders appear only near the surface of the lattice as observed in ZrZn2. We also study vortex configurations of SC order parameters. Two kinds of vortices, one for spin-up electron pairs and one for spin-down pairs show different behaviors because of the Zeeman coupling.
    Journal of Physics Conference Series 12/2012; 400(2):2095-.
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    ABSTRACT: We study the interplay of the ferromagnetic (FM) state and the p-wave superconducting (SC) state observed in several materials such as UCoGe and URhGe in a totally nonperturbative manner. To this end, we introduce a lattice Ginzburg-Landau model that is a genuine generalization of the phenomenological Ginzburg-Landau theory proposed previously in the continuum and also a counterpart of the lattice gauge-Higgs model for the s-wave SC transition, and study it numerically by Monte-Carlo simulations. The obtained phase diagram has qualitatively the same structure as that of UCoGe in the region where the two transition temperatrures satisfy $T_{\rm FM}>T_{\rm SC}$. For $T_{\rm FM}/T_{\rm SC} < 0.7$, we find that the coexisting region of FM and SC orders appears only near the surface of the lattice, which describes an inhomogeneous FMSC coexisting state.
    Physical review. B, Condensed matter 06/2011; 85(14). · 3.66 Impact Factor
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    ABSTRACT: In the present paper, we study a system of doped antiferromagnet in three dimensions at finite temperatures by using the t-J model, a canonical model of strongly-correlated electrons. We employ the slave-fermion representation of electrons in which an electron is described as a composite of a charged spinless holon and a chargeless spinon. We introduce two kinds of U(1) gauge fields on links as auxiliary fields, one describing resonating valence bonds of antiferromagnetic nearest-neighbor spin pairs and the other for nearest-neighbor hopping amplitudes of holons and spinons in the ferromagnetic channel. In order to perform numerical study of the system, we integrate out the fermionic holon field by using the hopping expansion in powers of the hopping amplitude, which is legitimate for the region in and near the insulating phase. The resultant effective model is described in terms of bosonic spinons and the two U(1) gauge fields, and a collective field for hole pairs. We study this model by means of Monte-Carlo simulations, calculating the specific heat, spin correlation functions, and instanton densities. We obtain a phase diagram in the hole concentration-temperature plane, which is in good agreement with that observed recently for clean and homogeneous underdoped samples.
    Physical Review B 07/2010; 83(6). · 3.66 Impact Factor
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    K. Nakane, A. Shimizu, I. Ichinose
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    ABSTRACT: In this paper, we study phase structure of $Z_2$ lattice gauge theories that appear as an effective field theory describing low-energy properties of frustrated antiferromagnets in two dimensions. Spin operators are expressed in terms of Schwinger bosons, and an emergent U(1) gauge symmetry reduces to a $Z_2$ gauge symmetry as a result of condensation of a bilinear operator of the Schwinger boson describing a short-range spiral order. We investigated the phase structure of the gauge theories by means of the Monte-Carlo simulations, and found that there exist three phases, phase with a long-range spiral order, a dimer state, and a spin liquid with deconfined spinons. Detailed phase structure and properties of phase transitions depend on details of the models.
    Physical Review B 09/2009; 80(22). · 3.66 Impact Factor