361 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
A longitudinal investigation of the relationship between motivation and late second language speech learning in classroom settings
The current study set out to examine the role of learner motivation in second language (L2) speech learning in English-as-a-Foreign-Language classrooms. The motivational orientations of 40 first-year university Japanese students were surveyed via a tailored questionnaire and linked to their spontaneous speech development, elicited via a timed picture description task at the onset and end of one academic semester, in terms of perceived comprehensibility (i.e., ease of understanding) and accentedness (i.e., linguistic nativelikeness). Significant improvement in comprehensibility (but not accentedness) was found among certain individuals. These students likely showed a strong motivation to study English for their future career development as a vague and long-term goal, as well as a high degree of concern for improving comprehensibility, grammatical accuracy and complexity
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
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
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
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
Disorder-induced phonon self-energy of semiconductors with binary isotopic composition
Self-energy effects of Raman phonons in isotopically disordered
semiconductors are deduced by perturbation theory and compared to experimental
data. In contrast to the acoustic frequency region, higher-order terms
contribute significantly to the self-energy at optical phonon frequencies. The
asymmetric dependence of the self-energy of a binary isotope system on the concentration of the heavier isotope mass x can be explained by
taking into account second- and third-order perturbation terms. For elemental
semiconductors, the maximum of the self-energy occurs at concentrations with
, depending on the strength of the third-order term. Reasonable
approximations are imposed that allow us to derive explicit expressions for the
ratio of successive perturbation terms of the real and the imaginary part of
the self-energy. This basic theoretical approach is compatible with Raman
spectroscopic results on diamond and silicon, with calculations based on the
coherent potential approximation, and with theoretical results obtained using
{\it ab initio} electronic theory. The extension of the formalism to binary
compounds, by taking into account the eigenvectors at the individual
sublattices, is straightforward. In this manner, we interpret recent
experimental results on the disorder-induced broadening of the TO (folded)
modes of SiC with a -enriched carbon sublattice.
\cite{Rohmfeld00,Rohmfeld01}Comment: 29 pages, 9 figures, 2 tables, submitted to PR
Magnetization Process in the One-Dimensional Doped Kondo Lattice Model
The magnetization process in the one-dimensional Kondo lattice model for the
doped (n_{c}<1) case is studied by the density matrix renormalization group
(DMRG) method. A rapid increase of the magnetization is caused by the collapse
of the intersite incommensurate correlation of f spins. On the contrary, the
intrasite f-c singlet correlation survives in the larger magnetic field. The
crossover from large to small Fermi surfaces for majority and minority spins is
observed, whereas the Fermi surfaces are always contributed by f spins. A
magnetization plateau appears with the magnitude of 1-n_{c}. Both ends of the
plateau are related to the coherence temperature and the Kondo temperature
which are characteristic energies essential in heavy electron systems.Comment: 4 pages, 3 eps figure
Invariant Form of Hyperfine Interaction with Multipolar Moments - Observation of Octupolar Moments in NpO and CeB by NMR -
The invariant form of the hyperfine interaction between multipolar moments
and the nuclear spin is derived, and applied to discuss possibilities to
identify the antiferro-octupolar (AFO) moments by NMR experiments. The ordered
phase of NpO and the phase IV of CeLaB are studied in
detail. Recent O NMR for polycrystalline samples of NpO are
discussed theoretically from our formulation. The observed feature of the
splitting of O NMR spectrum into a sharp line and a broad line, their
intensity ratio, and the magnetic field dependence of the shift and of the
width can be consistently explained on the basis of the triple \bq AFO
ordering model proposed by Paix\~{a}o {\it et. al.} Thus, the present theory
shows that the O NMR spectrum gives a strong support to the model. The 4
O sites in the fcc NpO become inequivalent due to the secondary triple
\bq ordering of AF-quadrupoles: one cubic and three non-cubic sites. It turns
out that the hyperfine field due to the antiferro-dipole and AFO moments
induced by the magnetic field, and the quadrupolar field at non-cubic sites are
key ingredients to understand the observed spectrum. The controversial problem
of the nature of phase IV in CeLaB is also studied. It is
pointed out that there is a unique feature in the NMR spectra, if the
() AFO ordering is
realized in CeLaB. Namely, the hyperfine splitting of a B
atom pair on the sites crosses zero on the
plane when the magnetic field is rotated around the axis.Comment: 22 pages, 2 figure
Formation Mechanism of Hybridization Gap in Kondo Insulators based on a Realistic Band Model and Application to YbB
A new LDA+U band calculation is performed on the Kondo insulator material
YbB and an energy gap of about 0.001Ryd is obtained. Based on this, a
simple tight-binding model with 5d and 4f orbitals on Yb
atoms and the nearest neighbor -bonds between them is constructed with
a good agreement to the above the LDA+U calculation near the gap. The density
of states is also calculated and the shape is found to be very asymmetric with
respect to the gap. A formation mechanism of the gap is clarified for the first
time in a realistic situation with the orbital degeneracies in both conduction
bands and the f states. This model can be a useful starting point for
incorporating the strong correlation effect, and for understanding all the
thermal, thermoelectric, transport and magnetic properties of YbB.Comment: 15 pages, 15 figures, to appear in J. Phys. Soc. Jpn. Vol. 72 No. 5
(2003
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