A detailed $^{31}$P (I=1/2) and $^{75}$As (I=3/2) NMR study on polycrystalline CeFeAs$_{1-x}$P$_{x}$O alloys is presented. The magnetism of CeFeAsO changes drastically upon P substitution on the As site. CeFePO is a heavy fermion system without long range order whereas CeFeAsO exhibits a Fe-3$d$ SDW type of ordering accompanied by a structural transition from tetragonal ($TT$) to orthorhombic ($OT$) structure. Furthermore Ce-4$f^{1}$ orders antiferromagnetically (AFM) at low temperature. At the critical concentration where the Fe-magnetism is diminished the Ce-Ce interaction changes to a ferromagnetic (FM) type of ordering. Three representative samples of the CeFeAs$_{1-x}$P$_{x}$O ($x$=0.05, 0.3 and 0.9) series are systematically investigated. 1) For the $x$=0.05 alloy a drastic change of the line width at 130 K indicates the AFM-SDW type of ordering of Fe and the structural change from $TT$ to $OT$ phase. The line width roughly measures the internal field in the ordered state and the transition is most likely first order. The small and nearly constant shift for $^{31}$P and $^{75}$As suggests the presence of competing hyperfine interactions between the nuclear spins and the 4$f$ and 3$d$ ions of Ce and Fe. 2) For the $x$=0.3 alloy, evolution of the Fe-SDW type order takes place at around 70 K corroborating the results of bulk measurement and $μSR$. Here we found strong evidence for phase separation of paramagnetic and magnetic SDW phases. 3) In contrast to heavy fermion CeFePO for the $x$=0.9 alloy a phase transition is found at 2 K. The field-dependent NMR shift gives strong evidence of FM ordering. Above the ordering the spin lattice relaxation rate $^{31}$(1/T$_{1}$) shows unconventional, non-Korringa like behaviour which indicates a complex interplay of Kondo and FM fluctuations.
We performed a systematic study on the properties of CeFeAs$_{1-x}$P$_{x}$O ($0\leq x\leq 1$) by electrical resistivity, magnetization and specific heat measurements. The c-axis lattice constant decreases significantly with increasing P content, suggesting a remarkable chemical pressure. The Fe-3d electrons show the enhanced metallic behavior upon P-doping and undergo a magnetic quantum phase transition around $x \approx 0.4$. Meanwhile, the Ce-4f electrons develop a ferromagnetic order near the same doping level. The ferromagnetic order is vanishingly small around $x=0.9$. The data suggest a …
A theory for anomalous enhancement of NMR relaxation rate $1/T_{1}T$ of $^{125}$Te toward zero temperature observed in Pb$_{1-x}$Tl$_{x}$Te ($x$=0.01) is presented on the idea of the charge Kondo effect of valence skipping element Tl. It is found that such enhancement in $1/T_{1}T$ is caused through enhancement of the pair-hopping and inter-orbital interactions between 6s electrons localized on Tl site and conduction electrons doped in the hole band the semiconductor PbTe, which is the heart of the charge Kondo effect. It is also found that the Knight shift is not influenced in the temperature…
We report detailed Knight shift measurements of the two indium sites in the heavy fermion compound CeIrIn$_5$ as a function of temperature and field orientation. We find that the Knight shift anomaly is orientation-dependent, with a crossover temperature $T^*$ that varies by 50% as the field is rotated from (001) to (100). This result suggests that the hybridization between the Ce 4f states and the itinerant conduction electrons is anisotropic, a result that reflects its collective origin, and may lead to anisotropic Kondo liquid behavior and unconventional superconductivity.
$^{51}$V nuclear magnetic resonance (NMR) and magnetization studies on CeTi$_{1-x}$V$_x$Ge$_3$ have been performed to explore the evolution from the ferromagnetic ($x = 0.113$) to the antiferromagnetic Kondo lattice state ($x = 1$), with focus on the emergence of a possible ferromagnetic quantum critical point (FMQCP) at $x_c \approx 0.4$. From the temperature dependence of the nuclear spin-lattice relaxation rate, $1/T_1T$, and the Knight shift, \textit{K}, for $x=0.113$ and $x=1$ a considerable competition between ferro- and antiferromagnetic correlations is found. Around the critical concen…
We report 209Bi NMR and NQR measurements on a single crystal of the Kondo insulator U3Bi4Ni3. The 209Bi nuclear spin-lattice relaxation rate ($T_1^{-1}$) shows activated behavior and is well-fit by a spin gap of 220 K. The 209Bi Knight shift (K) exhibits a strong temperature dependence arising from 5f electrons, in which K is negative at high temperatures and increases as the temperature is lowered. Below 50 K, K shows a broad maximum and decreases slightly upon further cooling. Our data provide insight into the evolution of the hyperfine fields in a fully gapped Kondo insulator based on 5f el…
The new Kondo material YbIr$_3$Si$_7$, similar to other Kondo insulators, has been reported to exhibit charge-neutral fermionic excitations through measurements of specific heat and thermal conductivity at low temperatures. We performed $^{29}$Si-NMR on YbIr$_3$Si$_7$ to investigate the magnetic response of charge-neutral fermions from a microscopic perspective. In low magnetic fields parallel to the $c$ axis, a single NMR peak in the paramagnetic state splits into three peaks below $T_{\rm N}$. In contrast, only a slight shift of the single NMR peak was observed in high magnetic fields. This …
7Li NMR measurements were performed in the metallic spinel LiV2O4. The temperature dependencies of the line width, the Knight shift and the spin-lattice relaxation rate were investigated in the temperature range 30 mK < T < 280 K. For temperatures T < 1 K we observe a spin-lattice relaxation rate which slows down exponentially. The NMR results can be explained by a spin-liquid behavior and the opening of a spin gap of the order 0.6 K.
We revisit the magnetic properties of the antiferromagnetic Kondo lattice CeNiAsO by $^{75}$As nuclear magnetic resonance measurements. Our results confirm two successive antiferromagnetic transitions of Ce moments at $T_{N1}=9.0(3)$ K and $T_{N2}=7.0(3)$ K. Incommensurate and commensurate antiferromagnetic orders are suggested for $T_{N2}<T<T_{N1}$ and $T<T_{N2}$ respectively, consistent with previous neutron and muon experiments. A Knight shift anomaly, characterized by the failure of $K(T)-χ(T)$ scaling, is observed below $T^*\sim15$ K, which gives a measure of the onset of coherent $c-f$ c…
Spin susceptibility of stabilized δphase in the Pu-Ga alloy is studied by measuring {69,71}^Ga NMR spectra and nuclear spin-lattice relaxation rate {69}T_{1}^{-1} in the temperature range 5 - 350 K. The shift ({69}^K) of the {69,71}^Ga NMR line and {69}^T_{1}^{-1} are controlled correspondingly by the static and the fluctuating in time parts of local magnetic field arisen at nonmagnetic gallium due to transferred hyperfine coupling with the nearest f electron environment of the more magnetic Pu. The nonmonotonic with a maximum around 150 K behavior of {69}^K(T) χ_{s,5f}(T) is attributed to the…
One of the great achievements performed by Jacques Friedel in France has been to promote experimental activities on the electronic properties in condensed matter physics, which has led him, among others, to be an active promoter of the up-rise of the "Laboratoire de Physique des Solides" (LPS). A selective presentation of some of the work achieved in my Nuclear Magnetic Resonance (NMR) group allows me to underline the importance of the initial choices done by J. Friedel for the LPS. I give here an updated review of the work done on Kondo effect in the 1970's which has allowed me to enlighten t…
We present a theory of magnetic (S=1) Ni and nonmagnetic Zn impurities in underdoped cuprates. Both types of impurities are shown to induce S=1/2 moments on Cu sites in the proximity of the impurity, a process which is intimately related to the spin gap phenomenon in cuprates. Below a characteristic Kondo temperature, the Ni spin is partially screened by the Cu moments, resulting in an effective impurity spin S=1/2. We further analyze the Ruderman-Kittel-Kasiya-Yosida-type response of planar Cu spins to a polarization of the effective impurity moments and derive expressions for the correspondi…
We compare the effects of in-plane non magnetic Li$^{+}$ and Zn$^{2+}$ impurities on the normal state of high-T$_{c}$ cuprates. Y NMR shows that the extra hole introduced by Li is not localized in its vicinity. The Tc depression and induced moments on near neighbour Cu sites of Zn or Li are found identical. These universal effects of spinless impurities establish the major influence of the spin perturbation with respect to the charge defect. The susceptibility of the induced moment measured by Li NMR displays a 1/(T+Theta) behavior. Theta increases with doping up to about 200 K in the overdope…
The combination of low-temperature specific-heat and nuclear-magnetic-resonance (NMR) measurements reveals important information of the ground-state properties of CeRu$_4$Sn$_6$, which has been proposed as a rare example of a tetragonal Kondo-insulator (KI). The NMR spin-latticerelaxation rate $1/T_1$ deviates from the Korringa law below 100 K signaling the onset of an energy gap $ΔE_g1/k_B \simeq 30$K. This gap is stable against magnetic fields up to 10 T. Below 10 K, however, unusual low-energy excitations of in-gap states are observed, which depend strongly on the field H. The specific heat…
We performed $^{59}$Co nuclear magnetic and quadrupole resonance (NMR and NQR) measurements under pressure on a single-crystalline CeCoSi, which undergoes an unresolved phase transition at $T_0$. The NQR spectra clearly showed that the phase transition at $T_0$ is nonmagnetic, but any symmetry lowering at the Co site was not seen irrespective of the feature of second-order phase transition. By contrast, the NMR spectra were split by the induced magnetic field perpendicular to the external magnetic field. These results show that the phase below $T_0$ is not a simple paramagnetic state but is mo…
Detailed $^{195}$Pt- and $^{171}$Yb nuclear magnetic resonance (NMR) studies on the heterogeneous mixed valence system YbPtGe$_2$ are reported. The temperature dependence of the $^{195}$Pt-NMR shift $^{195}K(T)$ indicates the opening of an unusual magnetic gap below 200\,K. $^{195}K(T)$ was analyzed by a thermal activation model which yields an isotropic gap $Δ/k_B \approx 200$\,K. In contrast, the spin-lattice relaxation rate $^{195}$($1/T_1$) does not provide evidence for the gap. Therefore, an intermediate-valence picture is proposed while a Kondo-insulator scenario can be excluded. Moreove…
We have determined the physical ground state properties of the compounds CeRuPO and CeOsPO by means of magnetic susceptibility chi(T), specific heat C(T), electrical resistivity rho(T), and thermopower S(T) measurements. chi(T) reveals a trivalent 4f1 cerium state in both compounds. For CeRuPO a pronounced decrease of rho(T) below 50K indicates the onset of coherent Kondo scattering which is confirmed by enhanced S(T). The temperature and magnetic field dependence of chi(T) and C(T) evidence ferromagnetic (FM) order at TC=15K. Thus, CeRuPO seems to be one of the rare example of a FM Kondo latt…
We present a complete characterization of ferromagnetic system CeIr2B2 using powder x-ray diffraction XRD, magnetic susceptibility chi(T), isothermal magnetization M(H), specific heat C(T), electrical resistivity rho(T,H), and thermoelectric power S(T) measurements. Furthermore 11B NMR study was performed to probe the magnetism on a microscopic scale. The chi(T), C(T) and rho(T) data confirm bulk ferromagnetic ordering with Tc = 5.1 K. Ce ions in CeIr2B2 are in stable trivalent state. Our low-temperature C(T) data measured down to 0.4 K yield Sommerfeld coefficient gamma = 73(4) mJ/molK2 which…
Antiferromagnetic Order and Magnetic Frustration in the Honeycomb Heavy-Fermion System Ce(Pt$_{1-x}$Pd$_{x}$)$_6$Al$_3$: $^{27}$Al and $^{195}$Pt NMR Studies
Sb-NMR/NQR study has revealed a formation of a pseudogap at the Fermi level in the density of states in a valence fluctuating compound CeIrSb. The nuclear spin-lattice relaxation rate divided by temperature, 1/T_1T has a maximum around 300 K and decreases significantly as 1/T_1T ~ T^2, followed by a 1/T_1T = const. relation at low temperature. This temperature dependence of 1/T_1T is well reproduced by assuming a V-shaped energy gap with a residual density of states at the Fermi level. The size of energy gap for CeIrSb is estimated to be about 350 K, which is by one order of magnitude larger t…
CeRhIn$_{5}$ is a Kondo-lattice prototype in which a magnetic field B$\bf{^{\ast}\simeq}$ 30 T induces an abrupt Fermi-surface (FS) reconstruction and pronounced in-plane electrical transport anisotropy all within its antiferromagnetic state. Though the antiferromagnetic order at zero field is well-understood, the origin of an emergent state at B$^{\ast}$ remains unknown due to challenges inherent to probing states microscopically at high fields. Here, we report low-temperature Nuclear Magnetic Resonance (NMR) measurements revealing a discontinuous decrease in the $^{115}$In formal Knight shif…
The heavy electron Kondo liquid is an emergent state of condensed matter that displays universal behavior independent of material details. Properties of the heavy electron liquid are best probed by NMR Knight shift measurements, which provide a direct measure of the behavior of the heavy electron liquid that emerges below the Kondo lattice coherence temperature as the lattice of local moments hybridizes with the background conduction electrons. Because the transfer of spectral weight between the localized and itinerant electronic degrees of freedom is gradual, the Kondo liquid typically coexis…
A central question in a large class of strongly correlated electron systems, including heavy fermion compounds and iron pnictides, is the identification of different phases and their origins. It has been shown that the antiferromagnetic (AFM) phase in some heavy fermion compounds is induced by Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction between localized moments, and that the competition between this interaction and Kondo effect is responsible for quantum criticality. However, conclusive experimental evidence of the RKKY interaction in pnictides is lacking. Here, using high resolution $^{…
$\rm CePt_3Si$ is a novel ternary compound exhibiting antiferromagnetic order at $T_N \approx 2.2$ K and superconductivity (SC) at $T_c \approx 0.75$ K. Large values of $H_{c2}' \approx -8.5$ T/K and $H_{c2}(0) \approx 5$ T indicate Cooper pairs formed out of heavy quasiparticles. The mass enhancement originates from Kondo interaction with a characteristic temperature $T_K \approx 8$ K. NMR and $μ$SR measurements evidence coexistence of SC and long range magnetic order on a microscopic scale. Moreover, $\rm CePt_3Si$ is the first heavy fermion SC without an inversion symmetry. This gives rise …
An attractive proposal for the hidden order (HO) in the heavy electron compound URu2Si2 is an itinerant multipole order of high rank. It is due to the pairing of electrons and holes centered on zone center and boundary, respectively in states that have maximally different total angular momentum components. Due to the pairing with commensurate zone boundary ordering vector the translational symmetry is broken and a HO quasiparticle gap opens below the transition temperature T_HO. Inelastic neutron scattering (INS) has demonstrated that for T<T_HO the collective magnetic response is dominated by…
