Ferromagnetic quantum critical fluctuations in YbRh_2(Si_{0.95}Ge_{0.05})_2

The bulk magnetic susceptibility $\chi(T,B)$ of YbRh$_2$(Si$_{0.95}$Ge$_{0.05}$)$_2$ has been investigated %by ac-and dc-magnetometry at low temperatures and close to the field-induced quantum critical point at $B_c=0.027$ T. For $B\leq 0.05$ 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: $\chi(T)\propto T^{-0.6}$ above 0.3 K, while at low temperatures the Pauli susceptibility follows $\chi_0\propto (B-B_c)^{-0.6}$ and scales with the coefficient of the $T^2$ 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 CeIn$_{3-x}$Sn$_x$ $(0.55 \leq x \leq 0.8)$ single crystals close to the antiferromagnetic (AF) quantum critical point (QCP) at $x_c\approx 0.67$ 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 $T_{\rm N}$ 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 YbRh2Si2Yb Rh_2 Si_2 observed by Electron Spin Resonance

Below the Kondo temperature $T_{\rm K}$ 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 YbRh$_{2}$Si$_{2}$ seems to be an exceptional case where definite ESR spectra show characteristic properties of the Kondo-ion Yb$^{3+}$ well \textit{below} $T_{\rm K}$. We found that the spin dynamics of YbRh$_{2}$Si$_{2}$, 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