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Register a new userNuclear magnetic resonance investigation of the heavy fermion system Ce$_2$CoAl$_7$Ge$_4$
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We present nuclear magnetic resonance (NMR) and nuclear quadrupole resonance (NQR) measurements performed on single crystalline ccag{}, a member of a recently discovered family of heavy fermion materials Ce$_2M$Al$_7$Ge$_4$ ($M$ = Co, Ir, Ni, or Pd). Previous measurements indicated a strong Kondo interaction as well as magnetic order below $T_M = 1.8$ K. Our NMR spectral measurements show that the Knight shift $K$ is proportional to the bulk magnetic susceptibility $chi$ at high temperatures. A clear Knight shift anomaly ($K otpropto chi$) is observed at coherence temperatures $T^* sim 17.5$ K for $H_0 parallel hat{c}$ and 10 K for $H_0 parallel hat{a}$ at the ${}^{59}$Co site, and $T^* sim 12.5$ K at the ${}^{27}$Al(3) site for $H_0 parallel hat{a}$ characteristic of the heavy fermion nature of this compound. At high temperatures the ${}^{59}$Co NMR spin-lattice relaxation rate $T_1^{-1}$ is dominated by spin fluctuations of the 4$f$ local moments with a weak metallic background. The spin fluctuations probed by ${}^{59}$Co NMR are anisotropic and larger in the basal plane than in the $c$ direction. Furthermore, we find $(T_1TK)^{-1} propto T^{-1/2}$ at the ${}^{59}$Co site as expected for a Kondo system for $T > T^*$ and $T> T_K$. ${}^{59}$Co NQR slrr{} measurements at low temperatures indicate slowing down of spin fluctuations above the magnetic ordering temperature $T_M sim 1.8$ K. A weak ferromagnetic character of fluctuations around $mathbf{q}=0$ is evidenced by an increase of $chi T$ versus $T$ above the magnetic ordering temperature. We also find good agreement between the observed and calculated electric field gradients at all observed sites.
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Electric resistivity, specific heat, magnetic susceptibility, and inelastic neutron scattering experiments were performed on a single crystal of the heavy fermion compound Ce(Ni$_{0.935}$Pd$_{0.065}$)$_2$Ge$_2$ in order to study the spin fluctuations near an antiferromagnetic (AF) quantum critical point (QCP). The resistivity and the specific heat coefficient for $T leq$ 1 K exhibit the power law behavior expected for a 3D itinerant AF QCP ($rho(T) sim T^{3/2}$ and $gamma(T) sim gamma_0 - b T^{1/2}$). However, for 2 $leq T leq$ 10 K, the susceptibility and specific heat vary as $log T$ and the resistivity varies linearly with temperature. Furthermore, despite the fact that the resistivity and specific heat exhibit the non-Fermi liquid behavior expected at a QCP, the correlation length, correlation time, and staggered susceptibility of the spin fluctuations remain finite at low temperature. We suggest that these deviations from the divergent behavior expected for a QCP may result from alloy disorder.
We have performed magnetic susceptibility, specific heat, resistivity, and inelastic neutron scattering measurements on a single crystal of the heavy Fermion compound Ce(Ni$_{0.935}$Pd$_{0.065}$)$_2$Ge$_2$, which is believed to be close to a quantum critical point (QCP) at T = 0. At lowest temperature(1.8-3.5 K), the magnetic susceptibility behaves as $chi(T)-chi (0)$ $propto$ $T^{-1/6}$ with $chi (0) = 0.032 times 10^{-6}$ m$^3$/mole (0.0025 emu/mole). For $T<$ 1 K, the specific heat can be fit to the formula $Delta C/T = gamma_0 - T^{1/2}$ with $gamma_0$ of order 700 mJ/mole-K$^2$. The resistivity behaves as $rho = rho_0 + AT^{3/2}$ for temperatures below 2 K. This low temperature behavior for $gamma (T)$ and $rho (T)$ is in accord with the SCR theory of Moriya and Takimotocite{Moriya}. The inelastic neutron scattering spectra show a broad peak near 1.5 meV that appears to be independent of $Q$; we interpret this as Kondo scattering with $T_K =$ 17 K. In addition, the scattering is enhanced near $Q$=(1/2, 1/2, 0) with maximum scattering at $Delta E$ = 0.45 meV; we interpret this as scattering from antiferromagnetic fluctuations near the antiferromagnetic QCP.
We report the results of a $^{45}$Sc nuclear magnetic resonance (NMR) study on the quasi-one-dimensional compound Cu$_2$Sc$_2$Ge$_4$O$_{13}$ at temperatures between 4 and 300 K. This material has been a subject of current interest due to indications of spin gap behavior. The temperature-dependent NMR shift exhibits a character of low-dimensional magnetism with a negative broad maximum at $T_{max}$ $simeq $ 170 K. Below $% T_{max}$, the NMR shifts and spin lattice relaxation rates clearly indicate activated responses, confirming the existence of a spin gap in Cu$_2$Sc$_2$Ge% $_4$O$_{13}$. The experimental NMR data can be well fitted to the spin dimer model, yielding a spin gap value of about 275 K which is close to the 25 meV peak found in the inelastic neutron scattering measurement. A detailed analysis further points out that the nearly isolated dimer picture is proper for the understanding of spin gap nature in Cu$_2$Sc$_2$Ge$_4$O$_{13}$.
Polycrystalline samples of Ce(Cu$_{1-x}$Co$_x$)$_2$Ge$_2$ were investigated by means of electrical resistivity $rho$($T$), magnetic susceptibility $chi$($T$), specific heat $C$$_p$($T$) and thermo electric power $S$($T$) measurements. The long-range antiferromagnetic (AFM) order, which set in at $T$$_N$ = 4.1 K in CeCu$_2$Ge$_2$, is suppressed by non-iso-electronic cobalt (Co) doping at a critical value of the concentration $x$$_c$ = 0.6, accompanied by non-Fermi liquid (NFL) behavior inferred from the power law dependence of heat capacity and susceptibility i.e. $C$($T$)/$T$ and $chi$($T$) $propto$ $T$$^{-1+lambda}$ down to 0.4 K, along with a clear deviation from $T$$^2$ behavior of the electrical resistivity. However, we have not seen any superconducting phase in the quantum critical regime down to 0.4 K.
Ce$_{2}$PtIn$_{8}$ is a recently discovered heavy-fermion system structurally related to the well-studied superconductor CeCoIn$_{5}$. Here, we report on low-temperature de Haas-van Alphen-effect measurements in high magnetic fields in Ce$_{2}$PtIn$_{8}$ and Pr$_{2}$PtIn$_{8}$. In addition, we performed band-structure calculations for localized and itinerant Ce-$4f$ electrons in Ce$_{2}$PtIn$_{8}$. Comparison with the experimental data of Ce$_{2}$PtIn$_{8}$ and of the $4f$-localized Pr$_{2}$PtIn$_{8}$ suggests the itinerant character of the Ce-$4f$ electrons. This conclusion is further supported by the observation of effective masses in Ce$_{2}$PtIn$_{8}$, which are strongly enhanced with up to 26 bare electron masses.
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