648 research outputs found
Non-Fermi-liquid behavior at anti-ferromagnetic quantum critical point in heavy fermion system Ce(CuCo)Ge
Polycrystalline samples of Ce(CuCo)Ge were investigated
by means of electrical resistivity (), magnetic susceptibility
(), specific heat () and thermo electric power ()
measurements. The long-range antiferromagnetic (AFM) order, which set in at
= 4.1 K in CeCuGe, is suppressed by non-iso-electronic cobalt
(Co) doping at a critical value of the concentration = 0.6, accompanied
by non-Fermi liquid (NFL) behavior inferred from the power law dependence of
heat capacity and susceptibility i.e. ()/ and ()
down to 0.4 K, along with a clear deviation from
behavior of the electrical resistivity. However, we have not seen any
superconducting phase in the quantum critical regime down to 0.4 K.Comment: 8 pages, 11 figure
GdRhSi: An exemplary tetragonal system for antiferromagnetic order with weak in-plane anisotropy
The anisotropy of magnetic properties commonly is introduced in textbooks
using the case of an antiferromagnetic system with Ising type anisotropy. This
model presents huge anisotropic magnetization and a pronounced metamagnetic
transition and is well-known and well-documented both, in experiments and
theory. In contrast, the case of an antiferromagnetic - system with weak
in-plane anisotropy is only poorly documented. We studied the anisotropic
magnetization of the compound GdRhSi and found that it is a perfect
model system for such a weak-anisotropy setting because the Gd ions in
GdRhSi have a pure spin moment of S=7/2 which orders in a simple AFM
structure with . We observed experimentally in a
continuous spin-flop transition and domain effects for field applied along the
- and the -direction, respectively. We applied a mean field model
for the free energy to describe our data and combine it with an Ising chain
model to account for domain effects. Our calculations reproduce the
experimental data very well. In addition, we performed magnetic X-ray
scattering and X-ray magnetic circular dichroism measurements, which confirm
the AFM propagation vector to be and indicate the absence of
polarization on the rhodium atoms
Effect of Ni-doping on magnetism and superconductivity in Eu0.5K0.5Fe2As2
The effect of Ni-doping on the magnetism and superconductivity in
Eu0.5K0.5Fe2As2 has been studied through a systematic investigation of magnetic
and superconducting properties of Eu0.5K0.5(Fe1-xNix)2As2 (x = 0, 0.03, 0.05,
0.08 and 0.12) compounds by means of dc and ac magnetic susceptibilities,
electrical resistivity and specific heat measurements. Eu0.5K0.5Fe2As2 is known
to exhibit superconductivity with superconducting transition temperature Tc as
high as 33 K. The Ni-doping leads to a rapid decrease in Tc; Tc is reduced to
23 K with 3% Ni-doping, and 8% Ni-doping suppresses the superconductivity to
below 1.8 K. In 3% Ni-doped sample Eu0.5K0.5(Fe0.97Ni0.03)2As2
superconductivity coexists with short range ordering of Eu2+ magnetic moments
at Tm ~ 6 K. The suppression of superconductivity with Ni-doping is accompanied
with the emergence of a long range antiferromagnetic ordering with TN = 8.5 K
and 7 K for Eu0.5K0.5(Fe0.92Ni0.08)2As2 and Eu0.5K0.5(Fe0.88Ni0.12)2As2,
respectively. The temperature and field dependent magnetic measurements for x =
0.08 and 0.12 samples reflect the possibility of a helical magnetic ordering of
Eu2 moments. We suspect that the helimagnetism of Eu spins could be responsible
for the destruction of superconductivity as has been observed in Co-doped
EuFe2As2. The most striking feature seen in the resistivity data for x = 0.08
is the reappearance of the anomaly presumably due to spin density wave
transition at around 60 K. This could be attributed to the compensation of
holes (K-doping at Eu-site) by the electrons (Ni-doping at Fe site). The
anomaly associated with spin density wave further shifts to 200 K for x = 0.12
for which the electron doping has almost compensated the holes in the system.Comment: 9 pages, 10 figure
Magnetic and superconducting properties on S-type single-crystal CeCuSi probed by Cu nuclear magnetic resonance and nuclear quadrupole resonance
We have performed Cu nuclear magnetic resonance/nuclear quadrupole
resonance measurements to investigate the magnetic and superconducting (SC)
properties on a "superconductivity dominant" (-type) single crystal of
CeCuSi. Although the development of antiferromagnetic (AFM)
fluctuations down to 1~K indicated that the AFM criticality was close, Korringa
behavior was observed below 0.8~K, and no magnetic anomaly was observed above
0.6 K. These behaviors were expected in -type
CeCuSi. The temperature dependence of the nuclear spin-lattice
relaxation rate at zero field was almost identical to that in the
previous polycrystalline samples down to 130~mK, but the temperature dependence
deviated downward below 120~mK. In fact, in the SC state could be
fitted with the two-gap -wave rather than the two-gap -wave
model down to 90~mK. Under magnetic fields, the spin susceptibility in both
directions clearly decreased below , indicative of the formation of
spin singlet pairing. The residual part of the spin susceptibility was
understood by the field-induced residual density of states evaluated from
, which was ascribed to the effect of the vortex cores. No magnetic
anomaly was observed above the upper critical field , but the
development of AFM fluctuations was observed, indicating that superconductivity
was realized in strong AFM fluctuations.Comment: 10 pages, 8 figure
Similar temperature scale for valence changes in Kondo lattices with different Kondo temperatures
The Kondo model predicts that both the valence at low temperatures and its
temperature dependence scale with the characteristic energy T_K of the Kondo
interaction. Here, we study the evolution of the 4f occupancy with temperature
in a series of Yb Kondo lattices using resonant X-ray emission spectroscopy. In
agreement with simple theoretical models, we observe a scaling between the
valence at low temperature and T_K obtained from thermodynamic measurements. In
contrast, the temperature scale T_v at which the valence increases with
temperature is almost the same in all investigated materials while the Kondo
temperatures differ by almost four orders of magnitude. This observation is in
remarkable contradiction to both naive expectation and precise theoretical
predictions of the Kondo model, asking for further theoretical work in order to
explain our findings. Our data exclude the presence of a quantum critical
valence transition in YbRh2Si2
Scaling of the magnetic entropy and magnetization in YbRh_2(Si_{0.95}Ge_{0.05})_2
The magnetic entropy of YbRh_2(Si_{0.95}Ge_{0.05})_2 is derived from
low-temperature ( mK) specific heat measurements. Upon field-tuning
the system to its antiferromagnetic quantum critical point unique temperature
over magnetic field scaling is observed indicating the disintegration of heavy
quasiparticles. The field dependence of the entropy equals the temperature
dependence of the dc-magnetization as expected from the Maxwell relation. This
proves that the quantum-critical fluctuations affect the thermal and magnetic
properties in a consistent way.Comment: 6 pages, 2 figures, manuscript submitted to SCES2004 conferenc
Pressure-induced change of the pairing symmetry in superconducting CeCu2Si2
Low-temperature (T) heat-capacity measurements under hydrostatic pressure of
up to p=2.1 GPa have been performed on single-crystalline CeCu2Si2. A broad
superconducting (SC) region exists in the T-p phase diagram. In the
low-pressure region antiferromagnetic spin fluctuations and in the
high-pressure region valence fluctuations had previously been proposed to
mediate Cooper pairing. We could identify these two distinct SC regions. We
found different thermodynamic properties of the SC phase in both regions,
supporting the proposal that different mechanisms might be implied in the
formation of superconductivity.Comment: 4 pages, 5 figure
Quasiquartet CEF ground state with possible quadrupolar ordering in the tetragonal compound YbRuGe
e have investigated the magnetic properties of YbRuGe by means of
magnetic susceptibility (T), specific heat C(T) and electrical
resistivity (T) measurements performed on flux grown single crystals. The
Curie-Weiss behavior of (T) along the easy plane, the large magnetic
entropy at low temperatures and the weak Kondo like increase in (T)
proves a stable trivalent Yb state. Anomalies in C(T), (T) and (T)
at T = 10.2 K, T = 6.5 K and T = 5.7 K evidence complex
ordering phenomena, T being larger than the highest Yb magnetic ordering
temperature found up to now. The magnetic entropy just above T amounts to
almost Rln4, indicating that the crystal electric field (CEF) ground state is a
quasiquartet instead of the expected doublet. The behavior at T is rather
unusual and suggest that this transition is related to quadrupolar ordering,
being a consequence of the CEF quasiquartet ground state. The combination of a
quasiquartet CEF ground state, a high ordering temperature, and the relevance
of quadrupolar interactions makes YbRuGe a rather unique system
among Yb based compounds.Comment: 11 pages, 5 figure, submitted to PRB rapi
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
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