17 research outputs found
Competing magnetic interactions in CeNi9-xCoxGe4
CeNi9Ge4 exhibits outstanding heavy fermion features with remarkable
non-Fermi- liquid behavior which is mainly driven by single-ion effects. The
substitution of Ni by Cu causes a reduction of both, the RKKY coupling and
Kondo interaction, coming along with a dramatic change of the crystal field
(CF) splitting. Thereby a quasi-quartet ground state observed in CeNi9Ge4
reduces to a two-fold degenerate one in CeNi8CuGe4. This leads to a
modiffcation of the effective spin degeneracy of the Kondo lattice ground state
and to the appearance of antiferromagnetic (AFM) order. To obtain a better
understanding of consequences resulting from a reduction of the effective spin
degeneracy, we stepwise replaced Ni by Co. Thereby an increase of the Kondo and
RKKY interactions through the reduction of the effective d-electron count is
expected. Accordingly, a paramagnetic Fermi liquid ground state should arise.
Our experimental studies, however, reveal AFM order already for small Co
concentrations, which becomes even more pronounced with increasing Co content
x. Thereby the modiffcation of the effective spin degeneracy seems to play a
crucial role in this system
Antiferromagnetic behavior in CeCoGe
We investigate the novel intermetallic ternary compounds
\emph{R}CoGe with \emph{R} = La and Ce by means of -ray
diffraction, susceptibility and specific heat measurements. CeCoGe
crystallizes in the space group 4/ and is characterized by the
coexistence of two different magnetic sublattices. The Ce-based sublattice,
with an effective moment close to the expected value for a Ce-ion,
exhibits a magnetically ordered ground state with K. The
Co-based sublattice, however, exhibits magnetic moments due to itinerant 3
electrons. The magnetic specific heat contribution of the Ce-sublattice is
discussed in terms of a resonance-level model implying the interplay between an
antiferromagnetic phase transition and the Kondo-effect and an underlying
Schottky-anomaly indicating a crystal field level scheme splitting into three
twofold degenerated micro states ( K, K).Comment: 4 pages, 3 figures, conference SCES0
Evolution of Quantum Criticality in CeNi_{9-x}Cu_xGe_4
Crystal structure, specific heat, thermal expansion, magnetic susceptibility
and electrical resistivity studies of the heavy fermion system
CeNi_{9-x}Cu_xGe_4 (0 <= x <= 1) reveal a continuous tuning of the ground state
by Ni/Cu substitution from an effectively fourfold degenerate non-magnetic
Kondo ground state of CeNi_9Ge_4 (with pronounced non-Fermi-liquid features)
towards a magnetically ordered, effectively twofold degenerate ground state in
CeNi_8CuGe_4 with T_N = 175 +- 5 mK. Quantum critical behavior, C/T ~ \chi ~
-ln(T), is observed for x about 0.4. Hitherto, CeNi_{9-x}Cu_xGe_4 represents
the first system where a substitution-driven quantum phase transition is
connected not only with changes of the relative strength of Kondo effect and
RKKY interaction, but also with a reduction of the effective crystal field
ground state degeneracy.Comment: 15 pages, 9 figure
Crystal field studies on the heavy fermion compound CeNiCuGe
Substitution of nickel by copper in the heavy fermion system
CeNiCuGe alters the local crystal field environment of the
Ce-ions. This leads to a quantum phase transition near ,
which is not only driven by the competition between Kondo effect and RKKY
interaction, but also by a reduction of an effectively fourfold to a twofold
degenerate crystal field ground state. To study the consequences of a changing
crystal field in CeNiCuGe on its Kondo properties, inelastic neutron
scattering (INS) experiments were performed. Two well-defined crystal field
transitions were observed in the energy-loss spectra at 4 K. The crystal field
level scheme determined by neutron spectroscopy is compared with results from
specific heat measurements.Comment: 4 pages, 3 figures, conference SCES0
Evolution of quantum criticality in the system CeNi9Ge4
The heavy fermion system CeNi9Ge4 exhibits a paramagnetic ground state with
remarkable features such as: a record value of the electronic specific heat
coefficient in systems with a paramagnetic ground state, \gamma = C/T \simeq
5.5 J/molK^2 at 80 mK, a temperature-dependent Sommerfeld-Wilson ratio,
R=\chi/\gamma, below 1 K and an approximate single ion scaling of the
4f-magnetic specific heat and susceptibility. These features are related to a
rather small Kondo energy scale of a few Kelvin in combination with a
quasi-quartet crystal field ground state. Tuning the system towards long range
magnetic order is accomplished by replacing a few at.% of Ni by Cu or Co.
Specific heat, susceptibility and resistivity studies reveal T_N \sim 0.2 K for
CeNi8CuGe4 and T_N \sim 1 K for CeNi8CoGe4. To gain insight whether the
transition from the paramagnetic NFL state to the magnetically ordered ground
state is connected with a heavy fermion quantum critical point we performed
specific heat and ac susceptibility studies and utilized the \mu SR technique
and quasi-elastic neutron scattering.Comment: 8 pages, 3 figures, will be published in J.Phys.: Conf. Series
(Proceedings of the International & Interdisciplinary Workshop on Novel
Phenomena in Intergrated Comples Sciences: From Living to Non-living Systems,
Japan, held in Kyoto, October 11-14, 2010
Crystal field studies on the heavy fermion compound CeNi₈CuGe₄
Substitution of nickel by copper in the heavy fermion system CeNi₉₋ₓCuₓGe₄ alters the local crystal field environment of the Ce³⁺-ions. This leads to a quantum phase transition near x ≈ 0.4, which is not only driven by the competition between Kondo effect and RKKY interaction, but also by a reduction of an effectively fourfold to a twofold degenerate crystal field ground state. To study the consequences of a changing crystal field in CeNi₈CuGe₄ on its Kondo properties, inelastic neutron scattering (INS) experiments were performed. Two well-defined crystal field transitions were observed in the energy-loss spectra at 4K. The crystal field level scheme determined by neutron spectroscopy is compared with results from specific heat measurements