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ABSTRACT: We study the GinzburgLandau lattice gauge model that we introduced
recently for ferromagnetic superconductors, i.e., superconductors in
which the pwave superconducting (SC) order and the ferromagnetic (FM)
order may coexist. We report some interesting results obtained by
MonteCarlo 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 spinup electron pairs and
one for spindown pairs show different behaviors because of the Zeeman
coupling. No preview · Article · Dec 2012 · Journal of Physics Conference Series

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ABSTRACT: We study the interplay of the ferromagnetic (FM) state and the pwave
superconducting (SC) state observed in several materials such as UCoGe and
URhGe in a totally nonperturbative manner. To this end, we introduce a lattice
GinzburgLandau model that is a genuine generalization of the phenomenological
GinzburgLandau theory proposed previously in the continuum and also a
counterpart of the lattice gaugeHiggs model for the swave SC transition, and
study it numerically by MonteCarlo 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. Preview · Article · Jun 2011 · Physical review. B, Condensed matter

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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 tJ model, a canonical model of
stronglycorrelated electrons. We employ the slavefermion 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 nearestneighbor spin pairs and the other for
nearestneighbor 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 MonteCarlo simulations, calculating the
specific heat, spin correlation functions, and instanton densities. We obtain a
phase diagram in the hole concentrationtemperature plane, which is in good
agreement with that observed recently for clean and homogeneous underdoped
samples. Preview · Article · Jul 2010 · Physical Review B

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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 lowenergy 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 shortrange spiral order. We investigated the
phase structure of the gauge theories by means of the MonteCarlo simulations,
and found that there exist three phases, phase with a longrange 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. Preview · Article · Sep 2009 · Physical Review B