15 research outputs found
Quantum Critical Point in the Spin Glass-Kondo Transition in Heavy Fermion Systems
The Kondo-Spin Glass competition is studied in a theoretical model of a Kondo
lattice with an intra-site Kondo type exchange interaction treated within the
mean field approximation, an inter-site quantum Ising exchange interaction with
random couplings among localized spins and an additional transverse field in
the x direction, which represents a simple quantum mechanism of spin flipping.
We obtain two second order transition lines from the spin-glass state to the
paramagnetic one and then to the Kondo state. For a reasonable set of the
different parameters, the two second order transition lines do not intersect
and end in two distinct QCP.Comment: 20 pages; 1 figure; to appear in Physical Review
Calorimetric and transport investigations of CePd_{2+x}Ge_{2-x} (x=0 and 0.02) up to 22 GPa
The influence of pressure on the magnetically ordered CePd_{2.02}Ge_{1.98}
has been investigated by a combined measurement of electrical resistivity,
, and ac-calorimetry, C(T), for temperatures in the range 0.3 K<T<10 K
and pressures, p, up to 22 GPa. Simultaneously CePd_2Ge_2 has been examined by
down to 40 mK. In CePd_{2.02}Ge_{1.98} and CePd_2Ge_2 the magnetic
order is suppressed at a critical pressure p_c=11.0 GPa and p_c=13.8 GPa,
respectively. In the case of CePd_{2.02}Ge_{1.98} not only the temperature
coefficient of , A, indicates the loss of magnetic order but also the
ac-signal recorded at low temperature. The residual
resistivity is extremely pressure sensitive and passes through a maximum and
then a minimum in the vicinity of p_c. The (T,p) phase diagram and the
A(p)-dependence of both compounds can be qualitatively understood in terms of a
pressure-tuned competition between magnetic order and the Kondo effect
according to the Doniach picture. The temperature-volume (T,V) phase diagram of
CePd_2Ge_2 combined with that of CePd_2Si_2 shows that in stoichiometric
compounds mainly the change of interatomic distances influences the exchange
interaction. It will be argued that in contrast to this the much lower
p_c-value of CePd_{2.02}Ge_{1.98} is caused by an enhanced hybridization
between 4f and conduction electrons.Comment: 9 pages, 7 figure
Pressure-induced magnetically ordered Kondo lattice state in YbCu
The effect of pressure on the electrical resistivity ρ(T) of several YbCu2Si2 samples
was investigated up to 25 GPa and for
30 mK < T < 300 K. With increasing pressure the
compound crosses from an intermediate
valence state to a magnetic Kondo lattice state at a
critical pressure
PC ~ 8 GPa. Below
PC, i.e.
in the non-magnetic phase,
ρ = ρ0 + AT2 is
found at very low temperature, indicating
the validity of the Fermi liquid description. On
approaching the magnetic instability, the
A coefficient and the residual resistivity ρ0
increase strongly. Close to
PC, ρ0 shows a pronounced maximum due to scattering by
lattice defects. The pressure variation of the magnetic
resistivity ρmag at high temperature is
interpreted in the terms of a pressure induced change of the crystal field splitting
Bi,Pb(2212) and Bi(2223) formation in the Bi-Pb-Sr-Ca-Cu-O system
A new route has been found for producing the Bi2Sr2CaCu2Oy, phase with Pb substituted for part of the Bi (denoted Bi,Pb(2212)). By this novel route, Bi,Pb(2212) has been prepared in air at 860 degrees C. For comparison, this phase has also been prepared in argon at 740 degrees C by the previously known route. Bi,Pb(2212) transforms from a tetragonal to an orthorhombic structure as the Pb content grows. Once this phase was obtained with a nominal composition of Bi2-xPbxSr2CaCu2Oy, with Pb contents x=0, 0.2, 0.4 and 0.6, the Bi(2223) phase was formed starting from Bi(2212) and Bi,Pb(2212). Single-phase samples were obtained after two heat treatments at 849 degrees C with Bi,Pb(2212) and a proportion of Pb of x=0.4. This proportion corresponds to the one used previously by the authors to obtain high critical current carrying tapes in Ag-clad Bi(2223) tapes