First-principles Study of the RKKY Interaction and the Quadrupole Order in the Pr 1-2-20 systems PrT2Al20 (T=Ti, V)

Electronic states and quadrupole orders in the Pr 1-2-20 systems PrT2Al20 (T=Ti, V) are investigated on the basis of the first-principles calculations. The effective 196 orbital model is derived to reproduce the first-principles electronic structures of LaT2Al20 (T=Ti, V) without contribution from the Pr 4f electrons which are considered to be well localized and is employed to calculate the Ruderman-Kittel-Kasuya-Yosida (RKKY) interactions between quadrupole and octupole moments of the Pr ions. Within the random phase approximation for the RKKY Hamiltonian, the most divergent susceptibility is found to be the quadrupole one for the wave vector Q = (0,0,0) in the case of PrTi2Al20 while that for Q = (pi/a,0,pi/a) in the case of PrV2Al20 as consistent with experimental observations in the both cases which exhibit the ferro-quadrupole (FQ) and the antiferro-quadrupole (AFQ) orders, respectively. We also discuss the ordered states using the mean-field approximation and find that, in the case of PrTi2Al20, the 1st-order phase transition to the O20 FQ order with a tiny discontinuity takes place as predicted by the Landau theory. In the case of PrV2Al20, the system exhibits two distinct O22 AFQ orders, AFQ-I and AFQ-II, and shows subsequent two phase transitions, the 2nd-order one from normal to AFQ-I and the 1st-order one from AFQ-I to AFQ-II, that may be responsible for the double transitions observed by specific heat measurements.Comment: 6 pages, 6 figure

Magnetic Field Dependence of the Paramagnetic to the High Temperature Magnetically Ordered Phase Transition in CeB6

We have measured the magnetic field dependence of the paramagnetic to high temperature magnetically ordered phase transition TQ(H) in CeB6 from 2 to 30 T using cantilever magnetometry. It is found that the phase separation temperature continuously increases in field with an increasingly positive slope. In addition, we find that measurements in strong magnetic field gradients have no effect on the phase transition.Comment: 6 total page including 3 figures, submitted to Physical Review B (also available at http://publish.aps.org/eprint/gateway/eplist/aps1999dec08_006) v2: Corrected typos, added 1 reference, minor content changes (deleted 1 sentence in introduction, added 2 sentences in discussion to explain added reference), fixed the "et al"s in the bibliograph

Theory of Anomalous Hall Effect in a Heavy fermion System with a Strong Anisotropic Crystal Field

In a heavy fermion system, there exists the anomalous Hall effect caused by localized $f$-orbital freedom, in addition to the normal Hall effect due to the Lorentz force. In 1994, we found that the Hall coefficient caused by the anomalous Hall effect ($R_H^{AHE}$) is predominant and the relation $R_H^{AHE} \propto \rho^2$ ($\rho$ is the electrical resistivity) holds at low temperatures in many compounds. In this work, we study the system where the magnetic susceptibility is highly anisotropic due to the strong crystalline electric field on $f$-orbitals. Interestingly, we find that $R_H^{AHE}$ is nearly isotropic in general. This tendency is frequently observed experimentally, which has casted suspicion that the anomalous Hall effect may be irrelevant in real materials. Our theory corresponds to corrections and generalizations of the pioneering work on ferromagnetic metals by Karplus and Luttinger.Comment: 4 pages, revtex, to be published in J. Phys. Soc. Jpn. (No.8

In search for the role of thermospermine synthase gene in poplar vascular development

This work is supported by the FCT project PTDC/AGR-GPL/098369/2008 and FCT PhD grant SFRH/BD/30074/2006 (A.M.).Peer Reviewe

Crystalline-Electric-Field Effect on the Resistivity of Ce-based Heavy Fermion Systems

The behavior of the resistivity of Ce-based heavy fermion systems is studied using a 1/$N$-expansion method a la Nagoya, where $N$ is the spin-orbital degeneracy of f-electrons. The 1/$N$-expansion is performed in terms of the auxiliary particles, and a strict requirement of the local constraints is fulfilled for each order of 1/N. The physical quantities can be calculated over the entire temperature range by solving the coupled Dyson equations for the Green functions self-consistently at each temperature. This 1/N-expansion method is known to provide asymptotically exact results for the behavior of physical quantities in both low- and high-energy regions when it is applied to a single orbital periodic Anderson model (PAM). On the basis of a generalized PAM including crystalline-electric-field splitting with a single conduction band, the pressure dependence of the resistivity is calculated by parameterizing the effect of pressure as the variation of the hybridization parameter between the conduction electrons and f-electrons. The main result of the present study is that the double-peak structure of the $T$-dependence of the resistivity is shown to merge into a single-peak structure with increasing pressure.Comment: 37 pages, 22 figure

The effect of uniaxial pressure on the magnetic anomalies of the heavy-fermion metamagnet CeRu2Si2

The effect of uniaxial pressure (P_u) on the magnetic susceptibility (X), magnetization (M), and magnetoresistance (MR) of the heavy-fermion metamagnet CeRu2Si2 has been investigated. For the magnetic field along the tetragonal c axis, it is found that characteristic physical quantities, i.e., the temperature of the susceptibility maximum (T_max), the pagamagnetic Weiss temperature (Q_p), 1/X at 2 K, and the magnetic field of the metamagnetic anomaly (H_M), scale approximately linearly with P_u, indicating that all the quantities are related to the same energy scale, probably of the Kondo temperature. The increase (decrease) of the quantities for P_u || c axis (P_u || a axis) can be attributed to a decrease (increase) in the nearest Ce-Ru distance. Consistently in MR and X, we observed a sign that the anisotropic nature of the hybridization, which is believed to play an important role in the metamagnetic anomaly, can be controlled by applying the uniaxial pressure. PACS numbers: 75.20.Hr, 71.27.+a, 74.62.FjComment: 7 pages, ReVTeX, 6 EPS figures : Will appear in Phys. Rev.

Thermodynamic and Transport Properties of CeMg2Cu9 under Pressure

We report the transport and thermodynamic properties under hydrostatic pressure in the antiferromagnetic Kondo compound CeMg2Cu9 with a two-dimensional arrangement of Ce atoms. Magnetic specific heat Cmag(T) shows a Schottky-type anomaly around 30 K originating from the crystal electric field (CEF) splitting of the 4f state with the first excited level at \Delta_{1}/kB = 58 K and the second excited level at \Delta_{2}/kB = 136 K from the ground state. Electric resistivity shows a two-peaks structure due to the Kondo effect on each CEF level around T_{1}^{max} = 3 K and T_{2}^{max} = 40 K. These peaks merge around 1.9 GPa with compression. With increasing pressure, Neel temperature TN initially increases and then change to decrease. TN finally disappears at the quantum critical point Pc = 2.4 GPa.Comment: 10 pages, 6 figure

Fluctuation Effects on the Quadrupolar Ordering in Magnetic Field

Effects of magnetic field on the quadrupolar ordering are investigated with inclusion of fluctuation of order parameters. For the simplest model with the nearest-neighbor quadrupolar interaction, the transition temperature and the specific heat are derived by the use of the recently proposed effective medium theory. It is shown that magnetic field H has two competing effects on the quadrupolar ordering; one is to encourage the ordering by suppressing the fluctuation among different components of order parameters, and the other is to block the ordering as in antiferromagnets. The former is found to be of order H^2 and the latter of order H^4. Hence the fluctuation is suppressed for weak fields, and the transition temperature increases with magnetic field. The fluctuation effect is so strong that the entropy released at the quadrupolar ordering is only about half of the full value ln 4 even without the Kondo effect.Comment: 10 pages including 3 Postscript figure

Resonant X-Ray Scattering from CeB6_{6}

We calculate the resonant x-ray scattering (RXS) spectra near the Ce $L_{\rm III}$ absorption edge in CeB$_6$, on the basis of a microscopic model that the $4f$ states of Ce are atomic while the $5d$ states form an energy band with a reasonable density of states. In the initial state, we employ an effective Hamiltonian of Shiina {\it et al}. in the antiferro-quadrupole (AFQ) ordering phase, while we construct the wave function consistent with the neutron scattering experiment in the magnetic ground state. In the intermediate state, we take full account of the intra-atomic Coulomb interaction. Without assuming any lattice distortion, we obtain sufficient RXS intensities on the AFQ superlattice spot. We obtain the spectral shape, the temperature and magnetic field dependences in good agreement with the experiment, thus demonstrating the mechanism that the intensity is brought about by the modulation of $5d$ states through the anisotropic term of the $5d$-$4f$ Coulomb interaction. In the magnetic ground state, a small pre-edge peak is found by the $E_2$ process. On the magnetic superlattice spot, we get a finite but considerably small intensity. The magnetic form factor is briefly discussed.Comment: Latex, 10 pages, 12 figures. To be published in J. Phys. Soc. Jpn., Vol.71, No. 7 (2002