中野 博生

The thermal expansion of low-dimensional itinerant antiferromagnetic metals is discussed along with the recent study on the magneto-volume effect by Takahashi and Nakano [J. Phys.: Condens. Matter 16, 4505 (2004)]. In this study, we find that the thermal volume expansion derived by Takahashi and Nakano shows a stronger enhancement of the T-linear slope of the thermal expansion coefficient proportional to y(-1/2) and ln(1/y), respectively, for one-dimensional and two-dimensional (2D) systems than that of three dimensions as we approach the magnetic instability point (y -> 0), where y is the inverse of the reduced staggered magnetic susceptibility. On the other hand, the same coefficient for the conventional term by Moriya and Usami [Solid State Commun. 34, 95 (1980)] is suppressed proportional to 1/ln(1/y) for 2D systems. The present result suggests that the temperature dependence of low-dimensional weak antiferromagnets is generally dominated by the strongly enhanced T(2)-linear thermal expansion from the magnetic origin. (c) 2007 American Institute of Physics.

The Drude weight of the Hubbard model on the two-dimensional square lattice is studied by the exact diagonalizations applied to clusters up to 20 sites. We carefully examine finite-size effects by consideration of the appropriate shapes of clusters and the appropriate boundary condition beyond the imitation of employing only the simple periodic boundary condition. We successfully capture the behavior of the Drude weight that is proportional to the squared hole doping concentration. Our present result gives a consistent understanding of the transition between the Mott insulator and doped metals. We also find, in the frequency dependence of the optical conductivity, that the mid-gap incoherent part emerges more quickly than the coherent part and rather insensitive to the doping concentration in accordance with the scaling of the Drude weight.

Tight-binding electrons on the honeycomb lattice are studied where nearest-neighbor hoppings in the three directions are t(a), t(b), and t(c), respectively. For the isotropic case-namely, for t(a)=t(b)=t(c)-two zero modes exist where the energy dispersions at the vanishing points are linear in momentum k. Positions of zero modes move in the momentum space as t(a), t(b), and t(c) are varied. It is shown that zero modes exist if parallel to parallel to t(b)/t(a)parallel to-1 parallel to <=parallel to t(c)/t(a)parallel to <=parallel to parallel to t(b)/t(a)parallel to+1 parallel to. The density of states near a zero mode is proportional to parallel to E parallel to but it is propotional to root parallel to E parallel to at the boundary of this condition

The magnetovolume effect of itinerant electron magnets is discussed by the explicit examination of the volume dependence of the spin fluctuation free energy. Magnetic Grüneisen parameters are introduced instead of magnetovolume coupling constants, that enable us to describe all the magnetovolume properties in terms of these parameters. We have particularly found the presence of a new thermal expansion, showing T2-like temperature dependence. It explains the appreciable electronic volume thermal expansions observed experimentally. We also show that the magnetovolume coupling constants for spontaneous and forced magnetostrictions, of different magnitudes, are temperature dependent. Analysis of the pressure effect on the Curie temperature Tc and the spontaneous magnetic moment M shows that the linear relation between them, , is generally violated. © 2006 IOP Publishing Ltd.

Size dependence of the magnetic properties in nanoscale ferrimagnetic rings is investigated by the numerical diagonalization of the Heisenberg model. The field derivative of the magnetization is drastically dependent on size of the ring if the magnetic system has frustration, although the character does not exist in the non-frustrated system. Our numerical data support this tendency that the effect of frustration causes the size dependence of the magnetic properties in the nanoscale ferrimagnetic ring. We also demonstrate that the size dependence caused by frustration is also found in behavior of the translational quantum number of the ground state. © 2005 Elsevier Ltd. All rights reserved.

Magnetization process of (S = 3/2, S = 1) ferrimagnetic spin chain with single-ion anisotropy D is investigated by numerical diagonalization. The magnetization process has a plateau at 1/5 of the saturation moment in the small D region. We find a quantum phase transition that the plateau once vanishes at a critical D and revives in the large D region. The critical value Dc is estimated to be 0.9. In order to clarify the mechanism of this transition, we propose new trial states: a pentamer state in the small D phase and a trimer state in the large D phase. We find that representative ground state is changed from the pentamer state to the trimer one, while D increases across D c. This result suggests that the mechanism of the quantum phase transition is understood as the change of the representative ground state.

We investigate magnetic properties of the nanoscale ferrimagnetic ring [Mn(hfac)2NITPh]6 that is suggested to have a tendency of frustration. The spin densities of the candidate models are discussed. The spin density of the model with frustration shows characteristic behaviour different from those of the other models. Except for the quantum fluctuation which is a little bigger for the frustrated model, the difference in overall behaviour of the spin density is explained by the spin alignment in corresponding classical spin systems. The spin alignment is determined by competition of the Zeeman energy and the Mn-radical exchange interaction, when a canted spin alignment of Mn spins from the axis of the external magnetic field appears in the magnetization process for the other models, while it is absent for our frustrated model.

Based on the spin fluctuation mechanism we have succeeded in deriving formulae for the magnetic entropy and the specific heat of itinerant electron ferromagnets that cover the wide range of the temperature and the external field strength. We show that it is necessary to include an extra term into the free energy for a thermodynamically consistent treatment. We are able to predict several new features on the temperature and the external field dependence, an extra enhancement of the T -linear coefficient at low temperature and the presence of a critical peak anomaly of the specific heat, for instance. They result from terms proportional to the second-order temperature derivative of the spontaneous magnetization. The presence of the terms is connected with the Maxwell relation of thermodynamics.

Magnetic interactions in the nanoscale ferrimagnetic ring [Mn(hfac) 2NITPh]6 abbreviated as Mn6R6 are estimated by comparing experimental data with our numerical data as obtained from the exact diagonalization of a Heisenberg model. Our results suggest that next nearest neighbor radical-radical interactions are essential to reproduce the magnetic structure of Mn6R6 well. The existence of these interactions means that Mn6R6 is a kind of frustrated spin cluster. We predict that the field derivative of magnetization should have two peaks in the range of magnetic field 380-420 T, which is beyond the range of current experimental observations. We also find that the local spin density of the frustrated model shows a different behavior from those of non-frustrated models studied in preceding works. ©2004 The Physical Society of Japan.

The one-dimensional Hubbard model with next-nearest-neighbor hopping is studied by exact-diagonalization calculations. We find that the partially ferromagnetic ground state evidently appears near the half filling. We discuss the behavior of local spin moment when the model shows partial ferromagnetism in the ground state. (C) 2003 Elsevier B.V. All rights reserved.

The external magnetic field dependence of the specific heat of weak itinerant electron ferromagnets is discussed. Based on the entropy expression consistent with the thermodynamic Maxwell relation, we can derive results in good quantitative agreements with observations on Sc3In. (C) 2003 Elsevier B. V. All rights reserved.

Metal-insulator transition in the ground state of the Hubbard chain with the next-nearest-neighbor hopping is studied by exact diagonalization method. This model shows the partially ferromagnetic metal near the Mott insulator for large Coulomb repulsion while the model reveals the non-magnetic metal for small but finite Coulomb repulsion. We find the transition between the partially ferromagnetic metal and the Mott insulator without phase separation. A new critical exponent of the diverging charge compressibility with respect to the hole density is observed when the system approaches the metal-Mott-insulator transition point from the side of the partially ferromagnetic metal. We also find the occurrence of the phase separation between partially ferromagnetic metal and the Mott insulator for even larger Coulomb repulsion. The model proposed here is interesting in view of the magnetism as well as the metal-insulator transition.

We study the one-dimensional Hubbard model with next-nearest-neighbor hopping by means of the numerical diagonalization for finite-size clusters. It is found that the ground state is evidently partially spin-polarized in a wide region of intermediate values of on-site Coulomb repulsion, which provides the first example as a minimal model without orbital degeneracy. Analysis of correlation functions makes it clear that the partially spin-polarized states form a spin polaron. Polaron's size grows with the increasing interaction.

Diffuse panbronchiolitis (DPB) is a pulmonary disease of unknown origin with inflammation in the respiratory bronchioles, bronchiectasis, and recurrent sinusitis. Patients with DPB suffer from chronic airway infections resulting from mucociliary dysfunction. Whereas a high concentration of nasal nitric oxide (NO) has been documented in healthy subjects, only two diseases are known to reduce nasal NO: primary ciliary dyskinesia syndrome and cystic fibrosis. We hypothesized that patients with DPB have abnormal revels of nasal NO. To test our hypothesis, we measured NO with the chemiluminescence technique. Air was sampled directly from the nose in 15 healthy subjects and eight patients with DPB. Nasal NO was 88% tower in DPB patients than in the age-matched control subjects (69 +/- 70 versus 556 +/- 87 nl/min; p < 0.001). Treatment with erythromycin for 2 wk did not alter the nasal NO in four control subjects. DPB is the third pulmonary disease in which nasal NO is low. The reduced nasal NO may well be involved in the pathogenesis of DPB, and NO measurements may serve as a noninvasive test in the diagnosis of DPB.