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Register a new userEvolution of quantum criticality in the system CeNi9Ge4
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The heavy fermion system CeNi9Ge4 exhibits a paramagnetic ground state with remarkable features such as: a record value of the electronic specific heat coefficient in systems with a paramagnetic ground state, gamma = C/T simeq 5.5 J/molK^2 at 80 mK, a temperature-dependent Sommerfeld-Wilson ratio, R=chi/gamma, below 1 K and an approximate single ion scaling of the 4f-magnetic specific heat and susceptibility. These features are related to a rather small Kondo energy scale of a few Kelvin in combination with a quasi-quartet crystal field ground state. Tuning the system towards long range magnetic order is accomplished by replacing a few at.% of Ni by Cu or Co. Specific heat, susceptibility and resistivity studies reveal T_N sim 0.2 K for CeNi8CuGe4 and T_N sim 1 K for CeNi8CoGe4. To gain insight whether the transition from the paramagnetic NFL state to the magnetically ordered ground state is connected with a heavy fermion quantum critical point we performed specific heat and ac susceptibility studies and utilized the mu SR technique and quasi-elastic neutron scattering.
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Experimental bulk susceptibility X(T) and magnetization M(H,T) of the S=1-Haldane chain system doped with nonmagnetic impurities, Y2BaNi1-xZnxO5 (x=0.04,0.06,0.08), are analyzed. A numerical calculation for the low-energy spectrum of non-interacting open segments describes very well experimental data above 4 K. Below 4 K, we observe power-law behaviors, X(T)=T^-alpha and M(H,T)/T^(1-alpha)=f(alpha,(H/T)), with alpha (<1) depending on the doping concentration x.This observation suggests the appearance of a gapless quantum phase due to a broad distribution of effective couplings between the dilution-induced moments.
Crystal structure, specific heat, thermal expansion, magnetic susceptibility and electrical resistivity studies of the heavy fermion system CeNi_{9-x}Cu_xGe_4 (0 <= x <= 1) reveal a continuous tuning of the ground state by Ni/Cu substitution from an effectively fourfold degenerate non-magnetic Kondo ground state of CeNi_9Ge_4 (with pronounced non-Fermi-liquid features) towards a magnetically ordered, effectively twofold degenerate ground state in CeNi_8CuGe_4 with T_N = 175 +- 5 mK. Quantum critical behavior, C/T ~ chi ~ -ln(T), is observed for x about 0.4. Hitherto, CeNi_{9-x}Cu_xGe_4 represents the first system where a substitution-driven quantum phase transition is connected not only with changes of the relative strength of Kondo effect and RKKY interaction, but also with a reduction of the effective crystal field ground state degeneracy.
$alpha$-RuCl$_3$ has attracted enormous attention since it has been proposed as a prime candidate to study fractionalized magnetic excitations akin to Kitaevs honeycomb-lattice spin liquid. We have performed a detailed specific-heat investigation at temperatures down to $0.4$ K in applied magnetic fields up to $9$ T for fields parallel to the $ab$ plane. We find a suppression of the zero-field antiferromagnetic order, together with an increase of the low-temperature specific heat, with increasing field up to $mu_0H_capprox 6.9$ T. Above $H_c$, the magnetic contribution to the low-temperature specific heat is strongly suppressed, implying the opening of a spin-excitation gap. Our data point toward a field-induced quantum critical point (QCP) at $H_c$; this is supported by universal scaling behavior near $H_c$. Remarkably, the data also reveal the existence of a small characteristic energy scale well below $1$~meV above which the excitation spectrum changes qualitatively. We relate the data to theoretical calculations based on a $J_1$--$K_1$--$Gamma_1$--$J_3$ honeycomb model.
New thermoelectric power (TEP) measurements on prototype heavy-fermion compounds close to magnetic quantum criticality are presented. The highly sensitive technique of TEP is an unique tool to reveal Fermi surface instabilities, referred here as Lifshitz transitions. The first focus is on the Ising CeRu2Si2 series. Doping CeRu2Si2 with Rh produces a decoupling between the first order metamagnetic transition and the pseudo-metamagnetism observed in the pure compound. Comparison is made with the case of YbRh2Si2 which is often considered as the archetype of local quantum criticality by contrast to CeRu2Si2, taken as an example of spin-density wave criticality. Up to now for ferromagnetic materials showing ferromagnetic wings, no simple case appears where the Fermi surface is preserved between the ferromagnetic and paramagnetic phases. An open issue is the consequence of Lifshitz transitions on superconductivity in these multiband systems.
The strange electronic state of a class of materials which violates the predictions of conventional Fermi-liquid theory of metals remains enigmatic. Proximity to a quantum critical point is a possible origin of this non-Fermi liquid (NFL) behavior, which is usually accomplished by tuning the ground state with non-thermal control parameters such as chemical composition, magnetic field or pressure. We present the spin dynamics study of a stoichiometric NFL system CeRhBi, using low-energy inelastic neutron scattering (INS) and muon spin relaxation (muSR) measurements. It shows evidence for an energy-temperature (E/T) scaling in the INS dynamic response and a time-field scaling of the muSR asymmetry function indicating a quantum critical behavior in this compound. The E/T scaling reveals a local character of quantum criticality consistent with the power-law divergence of the magnetic susceptibility, logarithmic divergence of the magnetic heat capacity and T-linear resistivity at low temperature. The NFL behavior and local criticality occur over a very wide dynamical range at zero field and ambient pressure without any tuning in this stoichiometric heavy fermion compound is striking, making CeRhBi an exemplary model system amenable to in-depth studies for quantum criticality.
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