238 research outputs found
Tuning Heavy Fermion Systems into Quantum Criticality by Magnetic Field
We discuss a series of thermodynamic, magnetic and electrical transport
experiments on the two heavy fermion compounds CeNi2Ge2 and YbRh2Si2 in which
magnetic fields, B, are used to tune the systems from a Non-Fermi liquid (NFL)
into a field-induced FL state. Upon approaching the quantum-critical points
from the FL side by reducing B we analyze the heavy quasiparticle (QP) mass and
QP-QP scattering cross sections. For CeNi2Ge2 the observed behavior agrees well
with the predictions of the spin-density wave (SDW) scenario for
three-dimensional (3D) critical spin-fluctuations. By contrast, the observed
singularity in YbRh2Si2 cannot be explained by the itinerant SDW theory for
neither 3D nor 2D critical spinfluctuations. Furthermore, we investigate the
magnetization M(B) at high magnetic fields. For CeNi2Ge2 a metamagnetic
transition is observed at 43 T, whereas for YbRh2Si2 a kink-like anomaly occurs
at 10 T in M vs B (applied along the easy basal plane) above which the heavy
fermion state is completely suppressed.Comment: 15 pages, 8 figures, submitted to Journal of Low Temperature Physics,
special Series on "High Magnetic Field Facilities
Anisotropic low field behavior and the observation of flux jumps in CeCoIn5
The magnetic behavior of the heavy fermion superconductor CeCoIn5 has been
investigated. The low field magnetization data show flux jumps in the mixed
state of the superconducting phase in a restricted range of temperature. These
flux jumps begin to disappear below 1.7 K, and are completely absent at 1.5 K.
The magnetization loops are asymmetric, suggesting that surface and geometrical
factors dominate the pinning in this system. The lower critical field (Hc1),
obtained from the magnetization data, shows a linear temperature dependence and
is anisotropic. The calculated penetration depth is also anisotropic, which is
consistent with the observation of an anisotropic superconducting gap in
CeCoIn5. The critical currents, determined from the high field isothermal
magnetization loops, are comparatively low (around 4000 A/cm2 at 1.6 K and 5
kOe).Comment: 4 pages 3 figure
Divergence of the heavy quasiparticle mass at the antiferromagnetic quantum critical point in YbRh2Si2
High magnetic field phase diagram of PrOs4Sb12
The magnetic phase diagram of PrOsSb has been investigated by
specific heat measurements between 8 and 32 T. A new Schottky anomaly due to
excitations between two lowest crystalline-electric-field (CEF) singlets, has
been found for both and above the field
where the field-induced ordered phase (FIOP) is suppressed. The constructed
phase diagram shows weak magnetic anisotropy and implies a crossing of
the two CEF levels at about 8 - 9 T for both field directions. These results
provide an unambiguous evidence for the singlet being the CEF ground
state and suggest the level crossing (involving lowest CEF levels) as the
driving mechanism of FIOP.Comment: Submitted to Phys. Rev. Let
Field-induced quantum critical point in CeCoIn_5
The resistivity of CeCoIn_5 was measured down to 20 mK in magnetic fields of
up to 16 T. With increasing field, we observe a suppression of the non-Fermi
liquid behavior, rho~T, and the development of a Fermi liquid state, with its
characteristic rho=rho_0+AT^2 dependence. The field dependence of the T^2
coefficient shows critical behavior with an exponent of ~4/3. This is evidence
for a new field-induced quantum critical point, occuring in this case at a
critical field which coincides with the superconducting upper critical field
H_c2.Comment: 2 pages, 2 figures, submitted to M2S-Rio 2003 Proceeding
Pressure Evolution of the Magnetic Field induced Ferromagnetic Fluctuation through the Pseudo-Metamagnetism of CeRu2Si2
Resistivity measurements performed under pressure in the paramagnetic ground
state of CeRu2Si2 are reported. They demonstrate that the relative change of
effective mass through the pseudo metamagnetic transition is invariant under
pressure. The results are compared with the first order metamagnetic transition
due to the antiferromagnetism of Ce0.9La0.1Ru2Si2 which corresponds to the
"negative" pressure of CeRu2Si2 by volume expansion. Finally, we describe the
link between the spin-depairing of quasiparticles on CeRu2Si2 and that of
Cooper pairs on the unconventional heavy fermion superconductor CeCoIn5.Comment: 5 pages, 6 figures, accepted for publication in J. Phys. Soc. Jp
Non-Collinear Magnetism due to Orbital Degeneracy and Multipolar Interactions
The origin of non-collinear magnetism under quadrupolar ordering is
investigated with CeB6 taken as a target system. The mode-mixing effect among
15 multipoles is analyzed based on the Ginzburg-Landau free energy. Then the
lower magnetic transition temperature and the order parameters are derived
within the mean-field approximation. In the presence of pseudo-dipole-type
interactions for the next-nearest neighbors, the observed pattern of
non-collinear ordering is indeed stabilized for certain set of interaction
parameters. The stability of the phase III' in the magnetic field is also
explained, which points to the importance of the next-nearest-neighbor
octupole-octupole interaction. Concerning the phase IV in CexLa1-xB6 with x ~
0.75, a possibility of pure octupole ordering is discussed based on slight
modifications of the strength of interactions.Comment: 12 pages, 7 figures, 3 tables, to appear in J. Phys. Soc. Jpn. 70 (6)
(2001
Novel features in the flux-flow resistivity of the heavy fermion superconductor PrOsSb
We have investigated the electrical resistivity of the heavy fermion
superconductor PrOsSb in the mixed state. We found unusual double
minima in the flux-flow resistivity as a function of magnetic field below the
upper critical field for the first time, indicating double peaks in the pinning
force density (). Estimated at the peak exhibits
apparent dependence on applied field direction; composed of two-fold and
four-fold symmetries mimicking the reported angular dependence of thermal
conductivity (). The result is discussed in correlation with the double
step superconducting (SC) transition in the specific heat and the multiple
SC-phases inferred from the angular dependence of .Comment: 5 pages, 7 figures, to appear in J. Phys. Soc. Jpn. Vol. 74, No. 6 or
Spin-Dependent Mass Enhancement under Magnetic Field in the Periodic Anderson Model
In order to study the mechanism of the mass enhancement in heavy fermion
compounds in the presence of magnetic field, we study the periodic Anderson
model using the fluctuation exchange approximation. The resulting value of the
mass enhancement factor z^{-1} can become up to 10, which is significantly
larger than that in the single-band Hubbard model. We show that the difference
between the magnitude of the mass enhancement factor of up spin (minority spin)
electrons z^{-1}_up and that of down spin (majority spin) electrons z^{-1}_down
increases by the applied magnetic field B//z, which is consistent with de
Haas-van Alphen measurements for CeCoIn_5, CeRu_2Si_2 and CePd_2Si_2. We
predict that z^{-1}_up >z^{-1}_down in many Ce compounds, whereas z^{-1}_up <
z^{-1}_down in Yb compounds.Comment: 5 pages, 4 figure
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