15 research outputs found
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
Signatures of valence fluctuations in CeCu2Si2 under high pressure
Simultaneous resistivity and a.c.-specific heat measurements have been
performed under pressure on single crystalline CeCu2Si2 to over 6 GPa in a
hydrostatic helium pressure medium. A series of anomalies were observed around
the pressure coinciding with a maximum in the superconducting critical
temperature, . These anomalies can be linked with an abrupt change
of the Ce valence, and suggest a second quantum critical point at a pressure
GPa, where critical valence fluctuations provide the
superconducting pairing mechanism, as opposed to spin fluctuations at ambient
pressure. Such a valence instability, and associated superconductivity, is
predicted by an extended Anderson lattice model with Coulomb repulsion between
the conduction and f-electrons. We explain the T-linear resistivity found at
in this picture, while other anomalies found around can be
qualitatively understood using the same model.Comment: Submitted to Phys. Rev.
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
Effect of band filling in the Kondo lattice: A mean-field approach
The usual Kondo-lattice, including an antiferromagnetic exchange interaction
between nearest-neighboring localized spins, is treated here in a mean-field
scheme that introduces two mean-field parameters: one associated with the local
Kondo effect, and the other related to the magnetic correlations between
localized spins. Phases with short-range magnetic correlations or coexistence
between those and the Kondo effect are obtained. By varying the number of
electrons in the conduction band, we notice that the Kondo effect tends to be
suppressed away from half filling, while magnetic correlations can survive if
the Heisenberg coupling is strong enough. An enhanced linear coefficient of the
specific heat is obtained at low temperatures in the metallic state.Comment: 7 pages, ReVTeX two-column, 7 figure
Meta-orbital Transition in Heavy-fermion Systems: Analysis by Dynamical Mean Field Theory and Self-consistent Renormalization Theory of Orbital Fluctuations
We investigate a two-orbital Anderson lattice model with Ising orbital
intersite exchange interactions by means of dynamical mean field theory
combined with the static mean field approximation of the intersite orbital
interactions. Focusing on Ce-based heavy-fermion compounds, we examine the
orbital crossover between the two orbital states, when the total f-electron
number per site n_f is n_f ~ 1. We show that a "meta-orbital" transition, at
which the occupancy of the two orbitals changes steeply, occurs when the
hybridization between the ground-state f-electron orbital and conduction
electrons are smaller than that between the excited f-electron orbital and
conduction electrons. Near the meta-orbital critical end point, the orbital
fluctuations are enhanced, and couple with the charge fluctuations. A critical
theory of the meta-orbital fluctuations is also developed by applying the
self-consistent renormalization theory of itinerant electron magnetism to the
orbital fluctuations. The critical end point, first-order transition and
crossover are described within Gaussian approximations of orbital fluctuations.
We discuss the relevance of our results to CeAl2, CeCu2Si2, CeCu2Ge2 and the
related compounds, which all have low-lying crystalline-electric-field excited
states.Comment: 11 pages, 6 figures, J. Phys. Soc. Jpn. 79, (2010) 11471
Fermi-liquid instabilities at magnetic quantum phase transitions
This review discusses instabilities of the Fermi-liquid state of conduction
electrons in metals with particular emphasis on magnetic quantum critical
points. Both the existing theoretical concepts and experimental data on
selected materials are presented; with the aim of assessing the validity of
presently available theory. After briefly recalling the fundamentals of
Fermi-liquid theory, the local Fermi-liquid state in quantum impurity models
and their lattice versions is described. Next, the scaling concepts applicable
to quantum phase transitions are presented. The Hertz-Millis-Moriya theory of
quantum phase transitions is described in detail. The breakdown of the latter
is analyzed in several examples. In the final part experimental data on
heavy-fermion materials and transition-metal alloys are reviewed and confronted
with existing theory.Comment: 62 pages, 29 figs, review article for Rev. Mod. Phys; (v2) discussion
extended, refs added; (v3) shortened; final version as publishe
Mass Enhancement in an Intermediate-Valent Regime of Heavy-Fermion Systems
We study the mechanism of the mass enhancement in an intermediate-valent
regime of heavy-fermion materials. We find that the crossovers between the
Kondo, intermediate valent, and almost empty f-electron regimes become sharp
with the Coulomb interaction between the conduction and f electrons. In the
intermediate-valent regime, we find a substantial mass enhancement, which is
not expected in previous theories. Our theory may be relevant to the observed
nonmonotonic variation in the effective mass under pressure in CeCu2Si2 and the
mass enhancement in the intermediate-valent compounds alpha-YbAlB4 and
beta-YbAlB4.Comment: 4 pages, 4 figure
Gutzwiller Method for an Extended Periodic Anderson Model with the c-f Coulomb Interaction
We study an extended periodic Anderson model with the Coulomb interaction Ucf
between conduction and f electrons by the Gutzwiller method. The crossovers
between the Kondo, intermediate-valence, and almost empty f-electron regimes
become sharper with Ucf, and for a sufficiently large Ucf, become first-order
phase transitions. In the Kondo regime, a large enhancement in the effective
mass occurs as in the ordinary periodic Anderson model without Ucf. In
addition, we find that a large mass enhancement also occurs in the
intermediate-valence regime by the effect of Ucf.Comment: 9 pages, 7 figure
The quantum critical point in CeRhIn_5: a resistivity study
The pressure--temperature phase diagram of CeRhIn_5 has been studied under
high magnetic field by resistivity measurements. Clear signatures of a quantum
critical point has been found at a critical pressure of p_c = 2.5 GPa. The
field induced magnetic state in the superconducting state is stable up to the
highest field. At p_c the antiferromagnetic ground-state under high magnetic
field collapses very rapidly. Clear signatures of p_c are the strong
enhancement of the resistivity in the normal state and of the inelastic
scattering term. No clear T2 temperature dependence could be found for
pressures above T_c. From the analysis of the upper critical field within a
strong coupling model we present the pressure dependence of the coupling
parameter lambda and the gyromagnetic ratio g. No signatures of a spatially
modulated order parameter could be evidenced. A detailed comparison with the
magnetic field--temperature phase diagram of CeCoIn_5 is given. The comparison
between CeRhIn_5 and CeCoIn_5 points out the importance to take into account
the field dependence of the effective mass in the calculation of the
superconducting upper critical field H_c2. It suggests also that when the
magnetic critical field H_(0) becomes lower than H_c2 (0)$, the persistence of
a superconducting pseudo-gap may stick the antiferromagnetism to H_c2 (0).Comment: 15 pages, 20 figures, to be published in J. Phys. Soc. Jp