54 research outputs found
Susceptibility inhomogeneity and non-Fermi liquid behavior in UCu_{5-x}Pt_x
Transverse-field muSR shifts and relaxation rates have been measured in the
non-Fermi liquid (NFL) alloy system UCu_{5-x}Pt_x, x = 1.0, 1.5, and 2.5. At
low temperatures the fractional spread in Knight shifts delta K/K approx
deltachi/chi is gtrsim 2 for x = 1, but is only half this value for x = 1.5 and
2.5. In a disorder-driven scenario where the NFL behavior is due to a broadly
distributed (Kondo or Griffiths-phase cluster) characteristic energy E, our
results indicate that delta E/E_{rm av} approx (delta K/K)_{T=0} is similar for
UCu_{5-x}Pd_x (x = 1 and 1.5) and UCu_4Pt, but is reduced for UCu_{5-x}Pt_x, x
= 1.5 and 2.5. This reduction is due to a marked increase of E with increasing
x; the spread delta E is found to be roughly independent of x. Our results
correlate with the observed suppression of other NFL anomalies for x > 1 in
UCu_{5-x}Pt_x but not in UCu_{5-x}Pd_x, and are further evidence for the
importance of disorder in the NFL behavior of both these alloy systems.Comment: 4 pages, 2 figures, submitted to 10th International Conference on
Muon Spin Rotation, Relaxation, and Resonance, Oxford, UK, August 200
Glassy Spin Dynamics in Non-Fermi-Liquid UCu_{5-x}Pd_x, x = 1.0 and 1.5
Local f-electron spin dynamics in the non-Fermi-liquid heavy-fermion alloys
UCu_{5-x}Pd_x, x = 1.0 and 1.5, have been studied using muon spin-lattice
relaxation. The sample-averaged asymmetry function Gbar(t) indicates strongly
inhomogeneous spin fluctuations, and exhibits the scaling Gbar(t,H) =
Gbar(t/H^\gamma) expected from glassy dynamics. At 0.05 K \gamma(x=1.0) = 0.35
\pm 0.1, but \gamma(x=1.5) = 0.7 \pm 0.1. This is in contrast to inelastic
neutron scattering results, which yield \gamma = 0.33 for both concentrations.
There is no sign of static magnetism \gtrsim 10^{-3} \mu_B/U ion in either
material above 0.05 K. Our results strongy suggest that both alloys are quantum
spin glasses.Comment: 4 pages, 4 figures, to be published in Physical Review Letter
Effect of annealing on glassy dynamics and non-Fermi liquid behavior in UCu_4Pd
Longitudinal-field muon spin relaxation (LF-muSR) experiments have been
performed in unannealed and annealed samples of the heavy-fermion compound
UCu_4Pd to study the effect of disorder on non-Fermi liquid behavior in this
material. The muon spin relaxation functions G(t,H) obey the time-field scaling
relation G(t,H) = G(t/H^gamma) previously observed in this compound. The
observed scaling exponent gamma = 0.3 pm 0.1, independent of annealing. Fits of
the stretched-exponential relaxation function G(t) = exp[-(Lambda t)^K] to the
data yielded stretching exponentials K < 1 for all samples. Annealed samples
exhibited a reduction of the relaxation rate at low temperatures, indicating
that annealing shifts fluctuation noise power to higher frequencies. There was
no tendency of the inhomogeneous spread in rates to decrease with annealing,
which modifies but does not eliminate the glassy spin dynamics reported
previously in this compound. The correlation with residual resistivity
previously observed for a number of NFL heavy-electron materials is also found
in the present work.Comment: 4 pages, 3 figures, submitted to 10th International Conference on
Muon Spin Rotation, Relaxation, and Resonance, Oxford, UK, August 200
Susceptibility Inhomogeneity and Non-Fermi-Liquid Behavior in Ce(Ru_{0.5}Rh_{0.5})_2Si_2
Magnetic susceptibility and muon spin rotation (\muSR) experiments have been
carried out to study the effect of structural disorder on the non-Fermi-liquid
(NFL) behavior of the heavy-fermion alloy Ce(Ru_{0.5}Rh_{0.5})_2Si_2. Analysis
of the bulk susceptibility in the framework of disorder-driven Griffiths-phase
and Kondo-disorder models for NFL behavior yields relatively narrow
distributions of characteristic spin fluctuation energies, in agreement with
\muSR linewidths that give the inhomogeneous spread in susceptibility. \muSR
and NMR data both indicate that disorder explains the "nearly NFL" behavior
observed above \sim2 K, but does not dominate the NFL physics found at low
temperatures and low magnetic fields.Comment: 6 pages, 4 figures, REVTeX, submitted to Phys. Rev.
Spin dynamics in a structurally ordered non-Fermi liquid compound: YbRh_2Si_2
Muon spin relaxation (muSR) experiments have been carried out at low
temperatures in the non-Fermi-liquid heavy-fermion compound YbRh_2Si_2. The
longitudinal-field muSR relaxation function is exponential, indicative that the
dynamic spin fluctuations are homogeneous. The relaxation rate 1/T_1 varies
with applied field as H^{-y}, y = 1.0 \pm 0.1, which implies a scaling law of
the form \chi''(\omega) \propto \omega^{-y} f(\omega/T), \lim_{x\to0} f(x) = x
for the dynamic spin susceptibility.Comment: 5 pages, 2 figures. To be published in proceedings of musr2002
(Physica B
NMR Detection of Temperature-Dependent Magnetic Inhomogeneities in URu2Si2
We present 29Si-NMR relaxation and spectral data in URu2Si2. Our echo-decay
experiments detect slowly fluctuating magnetic field gradients. In addition, we
find that the echo-decay shape (time dependence) varies with temperature T and
its rate behaves critically near the Neel temperature TN, indicating a
correlation between the gradient fluctuations and the transition to
small-moment order. T-dependent broadening contributions become visible below
100 Kelvin and saturate somewhat above TN, remaining saturated at lower
temperatures. Together, the line width and shift suggest partial lattice
distortions below TN. We propose an intrinsic minority phase below
and compare our results with one of the current theoretical models.Comment: 2 pages RevTeX, 1 figure, SCES 99-Japan, to appear in Physica
Magnetism and superconductivity in CeRh_{1-x}Ir_xIn_5 heavy fermion materials
We report on zero-field muon spin relaxation studies of cerium based
heavy-fermion materials CeRh_{1-x}Ir_xIn_5. In the superconducting x=0.75 and 1
compositions muon spin relaxation functions were found to be temperature
independent across T_c; no evidence for the presence of electronic magnetic
moments was observed. The x=0.5 material is antiferromagnetic below T_N=3.75 K
and superconducting below T_c=0.8 K. Muon spin realxation spectra show the
gradual onset of damped coherent oscillations characteristic of magnetic order
below T_N. At 1.65 K the total oscillating amplitude accounts for at least 85%
of the sample volume. No change in muon precession frequency or amplitude is
detected on cooling below T_c, indicating the microscopic coexistence of
magnetism and superconductivity in this material.Comment: 6 pages with 3 figures. Revision with corrected axis label (mK) in
Fig.
muSR in Ce_{1-x}La_xAl_3: anisotropic Kondo effect?
Zero-field muSR experiments in the heavy-fermion alloys Ce_{1-x}La_xAl_3, x =
0 and 0.2, examine a recent proposal that the system exhibits a strong
anisotropic Kondo effect. We resolve a damped oscillatory component for both La
concentrations, indicative of disordered antiferromagnetism. For x = 0.2 the
oscillation frequency decreases smoothly with increasing temperature, and
vanishes at the specific heat anomaly temperature T* \approx 2.2 K. Our results
are consistent with the view that T* is due to a magnetic transition rather
than anisotropic Kondo behavior.Comment: 5 pages, 2 figures. To be published in proceedings of musr2002
(Physica B
Mesoscopic phase separation in La2CuO4.02 - a 139La NQR study
In crystals of La2CuO4.02 oxygen diffusion can be limited to such small
length scales, that the resulting phase separation is invisible for neutrons.
Decomposition of the 139La NQR spectra shows the existence of three different
regions, of which one orders antiferromagnetically below 17K concomitantly with
the onset of a weak superconductivity in the crystal. These regions are
compared to the macroscopic phases seen previously in the title compound and
the cluster-glass and striped phases reported for the underdoped Sr-doped
cuprates.Comment: 4 pages, RevTeX, 5 figures, to be published in PR
Critical slowing down in the geometrically frustrated pyrochlore antiferromagnet Gd_2Ti_2O_7
Longitudinal-field muon spin relaxation experiments have been carried out in
the paramagnetic state of single-crystal Gd_2Ti_2O_7 just above the phase
transition at T_m = 1.0 K. At high applied fields the exponential relaxation
time T_1 is proportional to field, whereas T_1 saturates below a crossover
field B_c that is ~2.5 T at 1.5 K and decreases as T_m is approached. At low
fields the relaxation rate increases markedly as the freezing temperature is
approached, as expected for critical slowing down of the spin fluctuations, but
the increase is suppressed by applied field. This behavior is consistent with
the very long autocorrelation function cutoff time implied by the low value of
B_c.Comment: 4 pages, 2 figures, submitted to 10th International Conference on
Muon Spin Rotation, Relaxation, and Resonance, Oxford, UK, August 200
- …