506 research outputs found
Ferromagnetic quantum critical fluctuations in YbRh_2(Si_{0.95}Ge_{0.05})_2
The bulk magnetic susceptibility of
YbRh(SiGe) has been investigated %by ac-and
dc-magnetometry at low temperatures and close to the field-induced quantum
critical point at T. For T a Curie-Weiss law with a
negative Weiss temperature is observed at temperatures below 0.3 K. Outside
this region, the susceptibility indicates ferromagnetic quantum critical
fluctuations: above 0.3 K, while at low temperatures
the Pauli susceptibility follows and scales with
the coefficient of the term in the electrical resistivity. The
Sommerfeld-Wilson ratio is highly enhanced and increases up to 30 close to the
critical field.Comment: Physical Review Letters, to be publishe
Field-induced suppression of the heavy-fermion state in YbRh_2Si_2
We report DC magnetization measurements on YbRh_2Si_2 at temperatures down to
0.04K, magnetic fields B<11.5T and under hydrostatic pressure P<1.3GPa. At
ambient pressure a kink at B*=9.9T indicates a new type of field-induced
transition from an itinerant to a localized 4f-state. This transition is
different from the metamagnetic transition observed in other heavy fermion
compounds, as here ferromagnetic rather than antiferromagnetic correlations
dominate below B*. Hydrostatic pressure experiments reveal a clear
correspondence of B* to the characteristic spin fluctuation temperature
determined from specific heat
Quantum criticality in the cubic heavy-fermion system CeIn_{3-x}Sn_x
We report a comprehensive study of CeInSn single crystals close to the antiferromagnetic (AF) quantum critical
point (QCP) at by means of the low-temperature thermal
expansion and Gr\"uneisen parameter. This system represents the first example
for a {\it cubic} heavy fermion (HF) in which can be suppressed
{\it continuously} down to T=0. A characteristic sign change of the Gr\"uneisen
parameter between the AF and paramagnetic state indicates the accumulation of
entropy close to the QCP. The observed quantum critical behavior is compatible
with the predictions of the itinerant theory for three-dimensional critical
spinfluctuations. This has important implications for the role of the
dimensionality in HF QCPs.Comment: Physical Review Letters, to be publishe
Low-temperature thermopower study of YbRh2Si2
The heavy-fermion compound YbRh2Si2 exhibits an antiferromagnetic (AFM) phase
transition at an extremely low temperature of TN = 70 mK. Upon applying a tiny
magnetic field of Bc = 60 mT the AFM ordering is suppressed and the system is
driven toward a field-induced quantum critical point (QCP). Here, we present
low-temperature thermopower S(T) measurements of high-quality YbRh2Si2 single
crystals down to 30 mK. S(T) is found negative with comparably large values in
the paramagnetic state. In zero field no Landau-Fermi-liquid (LFL) like
behavior is observed within the magnetically ordered phase. However, a sign
change from negative to positive appears at lowest temperatures on the magnetic
side of the QCP. For higher fields B > Bc a linear extrapolation of S to zero
clearly evidences the recovery of LFL regime. The crossover temperature is
sharply determined and coincides perfectly with the one derived from
resistivity and specific heat investigations.Comment: LT25 conference proceedings in Journal of Physics: Conference Serie
Spin dynamics of observed by Electron Spin Resonance
Below the Kondo temperature electron spin resonance (ESR) usually
is not observable from the Kondo-ion itself because the characteristic spin
fluctuation energy results in a huge width of the ESR line. The heavy fermion
metal YbRhSi seems to be an exceptional case where definite ESR
spectra show characteristic properties of the Kondo-ion Yb well
\textit{below} . We found that the spin dynamics of
YbRhSi, as determined by its ESR relaxation, is spatially
characterized by an anisotropy of the zero temperature residual relaxation
only.Comment: Presented at NanoRes 2004, Kazan; 4 pages, 3 Figure
Effects of in-chain and off-chain substitutions on spin fluctuations in the spin-Peierls compound CuGeO_3
The effect of in-chain and off-chain substitutions on 1D spin fluctuations in
the spin-Peierls compound CuGeO_3 has been studied using Raman scattering in
order to understand the interplay between defect induced states, enhanced
spin-spin correlations and the ground state of low dimensional systems.
In-chain and off-chain substitutions quench the spin-Peierls state and induce
3D antiferromagnetic order at T\leq 5 K. Consequently a suppression of a 1D
gap-induced mode as well as a constant intensity of a spinon continuum are
observed at low temperatures. A 3D two-magnon density of states now gradually
extends to higher temperatures T\leq 60K compared with pure CuGeO_3. This
effect is more pronounced in the case of off-chain substitutions (Si) for which
a N\'eel state occurs over a larger substitution range, starting at very low
concentrations. Besides, additional low energy excitations are induced. These
effects, i.e. the shift of a dimensional crossover to higher temperatures are
due to an enhancement of the spin-spin correlations induced by a small amount
of substitutions. The results are compared with recent Monte Carlo studies on
substituted spin ladders, pointing to a similar instability of coupled,
dimerized spin chains and spin ladders upon substitution.Comment: 14 pages, 6 eps figures, to be published in PR
Exploring high temperature magnetic order in CeTi_1-xSc_xGe
Most of magnetic transitions related to Ce ordering are found below
T_ord~12K. Among the few cases exceeding that temperature, two types of
behaviors can be distinguished. One of them is related to the rare cases of Ce
binary compounds formed in BCC structures, with a quartet ground state, whose
degeneracy is reduced by undergoing different types of transitions mostly
structural. The other group shows evidences of itinerant character with the
outstanding example of CeRh_3B_2 showing the highest T_ord=115K. The second
highest ordering temperature has been reported for CeScGe with T_ord=47K, but
the nature of this magnetic state has not been investigated very deeply. In
order to shed more light into this unusual high temperature ordering we studied
the structural, magnetic, transport and thermal properties of CeTi_1-xSc_xGe
alloys in the stability range of the CeScSi-type structure 0.25<x<1 This system
presents a rich variety of magnetic behaviors along this concentration range,
with the magnetic ordering growing from ferromagnetic (FM) T_C~7K up to an
antiferromagnetic (AFM) transition at T_N=47K. The different regions show the
following characteristics: i) on the Ti rich side (0.25<x<0.50) it exhibits a
FM ground state (GS) with large saturation magnetization values M_sat up to
~1.15 mu_B. ii) Around x=0.60, the first crystal electric field excited doublet
starts to contribute to the GS magnetic properties. Furthermore an AFM
component with a connected metamagnetic transition appears. iii) At x=0.65 a
clear change in the GS nature is associated to a critical point above which the
GS properties can be described like for an itinerant system (with decreasing
M_sat) and an effective GS degeneracy N_eff=4. iv) For x>0.65, the magnetic
phase boundary splits into two transitions, with an intermediate phase
presenting incommensurate spin density waves features.Comment: 8 pages, 10 figure
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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