388 research outputs found
Iterative Perturbation Theory for Strongly Correlated Electron Systems with Orbital Degeneracy
A new scheme of the iterative perturbation theory is proposed for the
strongly correlated electron systems with orbital degeneracy. The method is
based on the modified self-energy of Yeyati, et al. which interpolates between
the weak and the strong correlation limits, but a much simpler scheme is
proposed which is useful in the case of the strong correlation with orbital
degeneracy. It will be also useful in the study of the electronic structures
combined with the band calculations.Comment: 6 pages, 3 Postscript figures, to appear in J. Phys. Cond. Matte
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
Electronic State and Magnetic Susceptibility in Orbitally Degenerate (J=5/2) Periodic Anderson Model
Magnetic susceptibility in a heavy fermion systemis composed of the Pauli
term (\chi_P) and the Van-Vleck term (\chi_V). The latter comes from the
interband excitation, where f-orbital degeneracy is essential. In this work, we
study \chi_P and \chi_V in the orbitally degenerate (J=5/2) periodic Anderson
model for both the metallic and insulating cases. The effect of the correlation
between f-electrons is investigated using the self-consistent second-order
perturbation theory. The main results are as follows. (i) Sixfold degenerate
model: both \chi_P and \chi_V are enhanced by a factor of 1/z (z is the
renormalization constant). (ii) Nondegenerate model: only \chi_P is enhanced by
1/z. Thus, orbital degeneracy is indispensable for enhancement of \chi_V.
Moreover, orbital degeneracy reduces the Wilson ratio and stabilizes a
nonmagnetic Fermi liquid state.Comment: 4 pages, revtex, to be published in J. Phys. Soc. Jpn. (No.8
The dwarf phenotype of the Arabidopsis acl5 mutant is suppressed by a mutation in an upstream ORF of a bHLH gene
Loss-of-function mutants of the Arabidopsis thaliana ACAULIS 5 (ACL5) gene, which encodes spermine synthase, exhibit a severe dwarf phenotype. To elucidate the ACL5-mediated regulatory pathways of stem internocle elongation, we isolated four suppressor of acaulis (sac) mutants that reverse the acl5 dwarf phenotype. Because these mutants do not rescue the dwarfism of known phytohormone-related mutants, the SAC genes appear to act specifically on the ACL5 pathways. We identify the gene responsible for the dominant sac51-d mutant, which almost completely suppresses the acl5 phenotype. sac51-d disrupts a short upstream open reading frame (uORF) of SAC51, which encodes a bHLH-type transcription factor. Our results indicate that premature termination of the uORF in sac51-d results in an increase in its own transcript level, probably as a result of an increased translation of the main ORF. We suggest a model in which ACL5 plays a role in the translational activation of SAC51, which may lead to the expression of a subset of genes required for stem elongation
Specific Heat Study of Non-Fermi Liquid Behavior in CeNi_2Ge_2: Anomalous Peak in Quasi-Particle Density-of-States
To investigate the non-Fermi liquid (NFL) behavior in a nonalloyed system
CeNi_2Ge_2, we have measured the temperature and field dependences of the
specific heat C on a CeNi_2Ge_2 single crystal. The distinctive temperature
dependence of C/T (~a-b*T^(1/2)) is destroyed in almost the same manner for
both field directions of B//c-axis and B//a-axis. The overall behavior of
C(T,B) and the low-temperature upturn in magnetic susceptibility can be
reproduced, assuming an anomalous peak of the quasi-particle-band
density-of-states (DOS) at the Fermi energy possessing (epsilon)^(1/2) energy
dependence. Absence of residual entropy around T=0 K in B~0 T has been
confirmed by the magnetocaloric effect measurements, which are consistent with
the present model. The present model can also be applied to the NFL behavior in
CeCu_{5.9}Au_{0.1} using a ln(epsilon)-dependent peak in the DOS. Possible
origins of the peak in the DOS are discussed.Comment: 4 pages, LaTeX, using jpsj.sty, to be published in J. Phys. Soc. Jpn.
66 No. 10 (1997), 7 figures available at
http://494-475.phys.metro-u.ac.jp/ao/ceni2ge2.htm
Absence of Hybridization Gap in Heavy Electron Systems and Analysis of YbAl3 in terms of Nearly Free Electron Conduction Band
In the analysis of the heavy electron systems, theoretical models with c-f
hybridization gap are often used. We point out that such a gap does not exist
and the simple picture with the hybridization gap is misleading in the metallic
systems, and present a correct picture by explicitly constructing an effective
band model of YbAl_3. Hamiltonian consists of a nearly free electron model for
conduction bands which hybridize with localized f-electrons, and includes only
a few parameters. Density of states, Sommerfeld coefficient, f-electron number
and optical conductivity are calculated and compared with the band calculations
and the experiments.Comment: 9 pages, 9 figures, submitted to J. Phys. Soc. Jp
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
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.
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
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