arXiv subjects

Takahiro Mizusaki

Publications and source records attributed to Takahiro Mizusaki.

At least 37 records · Page 2Linked to original sources

arXiv2011

Shell model study for neutron-rich sd-shell nuclei

The microscopic structure of neutron-rich sd-shell nuclei is investigated by using the spherical shell-model in the sd-pf valence space with the extended pairing plus quadrupole-quadrupole forces accompanied by the monopole interaction (EPQQM). The calculation reproduces systematically the known energy levels for even-even and odd-mass nuclei including the recent data for 43S, 46S and 47Ar. In particular, the erosion of the N=28 shell closure in 42Si can be explained. Our EPQQM results are compared with other shell-model calculations with the SDPF-NR and SDPF-U effective interactions.

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arXiv2010

Novel Extrapolation Method in the Monte Carlo Shell Model

We propose an extrapolation method utilizing energy variance in the Monte Carlo shell model in order to estimate the energy eigenvalue and observables accurately. We derive a formula for the energy variance with deformed Slater determinants, which enables us to calculate the energy variance efficiently. The feasibility of the method is demonstrated for the full $pf$-shell calculation of $^{56}$Ni, and the applicability of the method to a system beyond current limit of exact diagonalization is shown for the $pf$+$g_{9/2}$-shell calculation of $^{64}$Ge.

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arXiv2010

Filter diagonalization of shell-model calculations

We present a method of filter diagonalization for shell-model calculations. This method is based on the Sakurai and Sugiura (SS) method, but extended with help of the shifted complex orthogonal conjugate gradient (COCG) method. A salient feature of this method is that it can calculate eigenvalues and eigenstates in a given energy interval. We show that this method can be an alternative to the Lanczos method for calculating ground and excited states, as well as spectral strength functions. With an application to the $M$-scheme shell-model calculations we demonstrate that several inherent problems in the widely-used Lanczos method can be removed or reduced.

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arXiv2009

Shell-model Hamiltonian from self-consistent mean-field model: $N=Z$ nuclei

We propose a procedure to determine the effective nuclear shell-model Hamiltonian in a truncated space from a self-consistent mean-field model, e.g., the Skyrme model. The parameters of pairing plus quadrupole-quadrupole interaction with monopole force are obtained so that the potential energy surface of the Skyrme Hartree-Fock + BCS calculation is reproduced. We test our method for $N=Z$ nuclei in the $fpg$- and $sd$-shell regions. It is shown that the calculated energy spectra with these parameters are in a good agreement with experimental data, in which the importance of the monopole interaction is discussed. This method may represent a practical way of defining the Hamiltonian for general shell-model calculations.

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arXiv2008

Comment on arXiv:0811.1575 entitled "Quantum phase transitions in the Hubbard model on triangular lattice" by T. Yoshioka, A. Koga and N. Kawakami

We show that the phase boundary between the paramagnetic metal and the nonmagnetic Mott insulator for the Hubbard model on a triangular lattice obtained by Yoshioka et al. in arXiv:0811.1575 does not correctly represent that of the thermodynamic limit but is an artifact of the 6 by 6 lattice they rely on. After the system size extrapolation, the phase boundary is located at U/t=5.2 as proposed by Morita et al., J. Phys. Soc. Jpn. 71 (2008) 2109 and in contrast to Yoshioka et al. Here, U is the onsite Coulomb repulsion and t is the nearest-neighbor transfer.

cond-mat.str-el↗

arXiv2006

Gapless quantum spin liquid, stripe and antiferromagnetic phases in frustrated Hubbard models in two dimensions

Unique features of non-magnetic insulator phase are shown, and phase diagram for t-t' Hubbard model on square lattice is presented. Using the path-integral renormalization group method, we find antiferromagnetic phase for small next-nearest neighbor transfer t' and stripe (or collinear) phase for large t' in the Mott insulating region of strong on site interaction U. For intermediate t'/t~ 0.7 at large U/t>7, we find longer-period antiferromagnetic insulator phase with 2*4 structure. In the Mott insulating region, we also find quantum spin liquid (in other words, non-magnetic insulator) phase near the Mott transition to paramagnetic metals. Correlated electrons often crystallize to the Mott insulator usually with some magnetic orders, whereas the "quantum spin liquid" has been a long-sought issue. We report numerical evidences that a nonmagnetic insulating (NMI) phase gets stabilized near the Mott transition with remarkable properties: The 2D Mott insulators on geometrically frustrated lattices contain a phase with gapless spin excitations and degeneracy of the ground state in the whole Brillouin zone of the total momentum. It has an interpretation for an unexplored type of a quantum liquid. The present concept is useful in analyzing a variety of experimental results in frustrated magnets including organic BEDT-TTF compounds and 3He atoms adsorbed on graphite.

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arXiv2004

Anomalous Properties of Quadrupole Collective States in $^{136}$Te and beyond

The ground and low-lying states of neutron-rich exotic Te and Sn isotopes are studied in terms of the nuclear shell model by the same Hamiltonian used for the spherical-deformed shape phase transition of Ba isotopes, without any adjustment. An anomalously small value is obtained for $B(E2;0^+_1\to 2^+_1)$ in $^{136}$Te, consistently with a recent experiment. The levels of $^{136}$Te up to yrast $12^+$ are shown to be in agreement with observed ones. It is pointed out that $^{136}$Te can be an exceptionally suitable case for studying mixed-symmetry 1$^+$, 2$^+$ and 3$^+$ states, and predictions are made for energies, M1 and E2 properties. Systematic trends of structure of heavier and more exotic Sn and Te isotopes beyond $^{136}$Te are studied by Monte Carlo Shell Model, presenting an unusual and very slow evolution of collectivity/deformation.

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arXiv2004

Extrapolation method in shell model calculations with deformed basis

An extrapolation method in shell model calculations with deformed basis is presented, which uses a scaling property of energy and energy variance for a series of systematically approximated wave functions to the true one. Such approximated wave functions are given by variation-after-projection method concerning the full angular momentum projection. This extrapolation needs energy variance, which amounts to the calculation of expectation value of square of Hamiltonian $\hat{H}^2$. We present the method to evaluate this matrix element and show that large reduction of its numerical computation can be done by taking an advantage of time-reversal symmetry. The numerical tests are presented for $fp$ shell calculations with a realistic residual interaction.

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arXiv2004

Onset of intruder ground state in exotic Na isotopes and evolution of the N=20 shell gap

The onset of intruder ground states in Na isotopes is investigated by comparing experimental data and shell-model calculations. This onset is one of the consequences of the disappearance of the N=20 magic structure, and the Na isotopes are shown to play a special role in clarifying the change of this magic structure. Both the electromagnetic moments and the energy levels clearly indicate an onset of ground state intruder configurations at neutron number N=19 already, which arises only with a narrow N=20 shell gap in Na isotopes resulting from the spin-isospin dependence of the nucleon-nucleon interaction (as compared to a wider gap in stable nuclei like 40Ca). It is shown why the previous report based on the mass led to a wrong conclusion.

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arXiv2004

New effective interaction for $pf$-shell nuclei and its implications for the stability of the $N$=$Z$=28 closed core

The effective interaction GXPF1 for shell-model calculations in the full $pf$ shell is tested in detail from various viewpoints such as binding energies, electro-magnetic moments and transitions, and excitation spectra. The semi-magic structure is successfully described for $N$ or Z=28 nuclei, $^{53}$Mn, $^{54}$Fe, $^{55}$Co and $^{56,57,58,59}$Ni, suggesting the existence of significant core-excitations in low-lying non-yrast states as well as in high-spin yrast states. The results of $N=Z$ odd-odd nuclei, $^{54}$Co and $^{58}$Cu, also confirm the reliability of GXPF1 interaction in the isospin dependent properties. Studies of shape coexistence suggest an advantage of Monte Carlo Shell Model over conventional calculations in cases where full-space calculations still remain too large to be practical.

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arXiv2003

Quantum-number projection in the path-integral renormalization group method

We present a quantum-number projection technique which enables us to exactly treat spin, momentum and other symmetries embedded in the Hubbard model. By combining this projection technique, we extend the path-integral renormalization group method to improve the efficiency of numerical computations. By taking numerical calculations for the standard Hubbard model and the Hubbard model with next nearest neighbor transfer, we show that the present extended method can extremely enhance numerical accuracy and that it can handle excited states, in addition to the ground state.

cond-mat.str-el↗

arXiv2003

New Type of Degenerate Quantum Spin Phase

Correlated electrons often crystalize to the Mott insulator usually with some magnetic orders, whereas the "quantum spin liquid" has been a long-sought issue. We report numerical evidences that a nonmagnetic insulating (NMI) phase gets stabilized near the Mott transition with remarkable properties: The 2D Mott insulators on geometrically frustrated lattices contain a phase with gapless spin excitations and degeneracy of the ground state in the whole Brillouin zone of the total momentum. It has an interpretation for an unexplored type of a quantum liquid. The present concept is useful in analyzing a variety of experimental results in frustrated magnets including organic BEDT-TTF compounds.

cond-mat.str-el↗

arXiv2003

Precise estimation of shell model energy by second order extrapolation method

A second order extrapolation method is presented for shell model calculations, where shell model energies of truncated spaces are well described as a function of energy variance by quadratic curves and exact shell model energies can be obtained by the extrapolation. This new extrapolation can give more precise energy than those of first order extrapolation method. It is also clarified that first order extrapolation gives a lower limit of shell model energy. In addition to the energy, we derive the second order extrapolation formula for expectation values of other observables.

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arXiv2002

Quadrupole and octupole softness in the N=Z nucleus 64Ge

Quadrupole and octupole softness in the even-even N=Z nucleus 64Ge is studied on the spherical shell model basis. We carry out the shell model calculation using the pairing plus quadrupole (QQ) plus octupole (OO) interaction with monopole corrections. It is shown that 64Ge is an unstable nucleus with respect to both the quadrupole and octupole deformations, which is consistent with the previous discussions predicting the gamma softness and octupole instability. It is demonstrated that proton-neutron part QpQn of the QQ interaction is important for the gamma softness or triaxiality.

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arXiv2002

An extrapolation method for shell model calculations

We propose a new shell model method, combining the Lanczos digonalization and extrapolation method. This method can give accurate shell model energy from a series of shell model calculations with various truncation spaces, in a well-controlled manner. Its feasibility is demonstrated by taking the fp shell calculations.

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arXiv2001

Magic numbers in exotic nuclei and spin-isospin properties of {\it NN} interaction

The magic numbers in exotic nuclei are discussed, and their novel origin is shown to be the spin-isospin dependent part of the nucleon-nucleon interaction in nuclei. The importance and robustness of this mechanism is shown in terms of meson exchange, G-matrix and QCD theories. In neutron-rich exotic nuclei, magic numbers such as N = 8, 20, etc. can disappear, while N = 6, 16, etc. arise, affecting the structure of lightest exotic nuclei to nucleosynthesis of heavy elements.

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arXiv2001

Effective mass staircase and the Fermi liquid parameters for the fractional quantum Hall composite fermions

Effective mass of the composite fermion in the fractional quantum Hall system, which is of purely interaction originated, is shown, from a numerical study, to exhibit a curious nonmonotonic behavior with a staircase correlated with the number (=2,4,...) of attached flux quanta. This is surprising since the usual composite-fermion picture predicts a smooth behavior. On top of that, significant interactions are shown to exist between composite fermions, where the excitation spectrum is accurately reproduced in terms of Landau's Fermi liquid picture with negative (i.e., Hund's type) orbital and spin exchange interactions.

cond-mat.mes-hall↗

arXiv2000

Isovector multiphonon excitations in near spherical nuclei

The lowest isoscalar and isovector quadrupole and octupole excitations in near spherical nuclei are studied within the the proton-neutron version of the interacting boson model including quadrupole and octupole bosons (sdf-IBM-2). The main decay modes of these states in near spherical nuclei are discussed.

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