366 research outputs found
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 -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 () is predominant and the relation ( 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 -orbitals. Interestingly, we find that 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/-expansion method a la Nagoya, where is the spin-orbital
degeneracy of f-electrons. The 1/-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 -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 CeB
We calculate the resonant x-ray scattering (RXS) spectra near the Ce absorption edge in CeB, on the basis of a microscopic model that the
states of Ce are atomic while the 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 states
through the anisotropic term of the - Coulomb interaction. In the
magnetic ground state, a small pre-edge peak is found by the 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
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