Kondo Semimetal CeNiSn Research Articles

Strongly anisotropic high-temperature Fermi surface of the Kondo semimetal CeNiSn revealed by angle-resolved photoemission spectroscopy

The semimetallic behavior of the so-called ``failed Kondo insulator'' CeNiSn has been ascribed to a nodal line in the Kondo hybridization derived from a particular symmetry of the Ce $4f$ orbitals ground state. Here we investigate the geometry of the CeNiSn conduction band by combined angle-resolved photoemission spectroscopy (ARPES) in the high-temperature regime and Open core generalized gradient approximation plus spin-orbit coupling calculations, in order to determine how the nodal hybridization takes place. We identify the Fermi sheet involved in the semimetallic regime from its locus and its shape, respectively, in agreement with the expected nodal line and with quantum oscillations. We further extrapolate and discuss the low-temperature Fermi surface in terms of the expected nodal hybridization with a localized $f$-level. The obtained hypothetical low-temperature Fermi surface is compatible with the description from quantum oscillations, and with both the highly anisotropic magnetoresistance and the isotropic Nernst effect. This work offers an overview of the conduction band of CeNiSn before hybridization, and it paves the way to a definitive understanding of its low-temperature state. In addition, this work serves as a basis for more challenging low-temperature ARPES measurements.

Magnetization Process of the Kondo Insulator YbB12 in Ultrahigh Magnetic Fields

The magnetization process of the Kondo insulator YbB12 has been unveiled in ultrahigh magnetic fields of up to 120 T at 4.2 K. We have found a novel metamagnetic transition at Bc2 = 102 T in addition to the previously known transition at Bc1 = 55 T. It has also been observed that the magnetization tends to saturate at around 112 T. Within the rigid band model, the two-energy-gap structure in the density of states (DOS) explains the successive two-step metamagnetism as a result of the Zeeman effect of the DOS. The metamagnetic transition at Bc1 occurs along with an insulator–metal transition and the field-induced phase is expected to be a heavy fermion metallic state. The Kondo effect can weaken at the second transition of Bc2, as theoretically found in the successive two-metamagnetic-transition process in the Kondo semimetal CeNiSn.

Collapse of the Pseudogap in Kondo Semimetal CeNiSn Observed by Magnetization and Magnetoresistance Measurements in High Magnetic Fields up to 65 T

The effect of magnetic field on the pseudogap in the Kondo semimetal CeNiSn has been reinvestigated by the measurement of magnetization and longitudinal magnetoresistance in pulsed high magnetic fields applied along the principal crystallographic directions of the orthorhombic structure. By extending the field range up to 65 T, metamagnetic behaviors at 45 and 60 T are observed for the first time along the easy magnetization direction of a -axis. No anomaly is found, on the other hand, along the b - and c -axes up to 55 T. The magnetoresistance along the a -axis exhibits the minimum at about 30 T and increases above 40 T, which is in accord with the anomalous increase of the magnetization. By contrast, the magnetoresistance along the b - and c -axes shows only a monotonous decrease above 10 T. These results are discussed in terms of a Zeeman splitting of the double-gapped density of states derived from the anisotropic hybridization-gap model.

High-resolution photoemission spectroscopy of d,f electron systems at the Hiroshima Synchrotron Radiation Center

Recently, high energy-resolution and low-temperature photoemission spectroscopies (Δ E=4.5∼20 meV and T=6∼300 K) have been realized successfully on undulator beamlines installed at the compact light source at the Hiroshima Synchrotron Radiation Center. In the case of Kondo semimetal CeNiSn, the temperature dependence of the spectra provides clear evidence of pseudogap formation below the characteristic temperature between 50 and 60 K. Resonant photoemission spectra of the Kondo semiconductor CeRhAs and the Kondo semimetal CeRhSb revealed unusual Ce 4f-derived electronic states. The spectral features of CeRhAs differ considerably from the predictions of a periodic Anderson model, while the features of CeRhSb are well described by the model. The spectra of Y 1− x Ca x TiO 3 ( x=0.39) exhibits a sharp Fermi edge at low temperature, corresponding to the metal–insulator transition at ∼150 K, where the carrier density in the Ti 3d bands plays an important role.

Hydrogen induced antiferromagnetism in the Kondo semimetal CeNiSn

We have used neutron diffraction and magnetic measurements to investigate the changes in magnetic properties of the Kondo semimetal compound CeNiSn after charging with deuterium gas under moderate conditions. Charging with deuterium led to the formation of the monodeuteride CeNiSnD with filled TiNiSi structure. This compound gives rise to long-range antiferromagnetic ordering below T N=5.1 K. The ordered magnetic moment value equals 1.37(3) μ B per Ce atom, the moments pointing along the [001] direction. No significant ordered magnetic moment on the Ni atoms has been observed by powder neutron diffraction.

High-resolution temperature-dependent photoemission study of Kondo semimetal CeNiSn

The temperature dependence of the electronic states of Kondo semimetal CeNiSn has been studied by high-resolution photoemission spectroscopy. The results have been compared with those of LaNiSn. The temperature-dependent spectral weight reduction of CeNiSn is observed below 30 K at binding energies between ∼20 meV and E F, and is similar to that of the Kondo insulator Ce 3Bi 4Pt 3. This behavior is not observed for LaNiSn. The spectral intensity of Ce 4f derived state near the Fermi level ( E F) is enhanced at temperatures above a characteristic temperature, but the spectral intensity at E F is reduced at 7 K.

Uniaxial-stress induced magnetic order in CeNiSn

We report the observation of an unexpected magnetic transition in the Kondo semimetal CeNiSn under uniaxial pressures. For $P\ensuremath{\parallel}a,$ both the specific heat C and magnetic susceptibility \ensuremath{\chi} decrease steadily with increasing P. By contrast, once $P\ensuremath{\parallel}b$ and $P\ensuremath{\parallel}c$ exceed 0.13 GPa, $C(T)$ exhibits a distinct jump at 4 K followed by a peak at 3 K. The concomitant peaking in $\ensuremath{\chi}(T)$ for $P\ensuremath{\parallel}B\ensuremath{\parallel}c$ is indicative of an antiferromagnetic transition.

Low Temperature Specific Heat of the Kondo-Semimetal CeNiSn in Zero and Applied Magnetic Fields

The specific heat of several CeNiSn single crystals of various purity has been measured in the temperature range from 25 mK to 5 K and in magnetic fields from zero to 7 Tesla. At very low temperatures (below ∼200 mK) the specific heat is found to vary linearly with temperature (C=γT), the coefficient γ decreasing with increasing purity. Above 200 mK, the specific heat is well described as the sum of a linear and a quadratic term. An applied magnetic field affects mostly the linear term, which first slightly decreases, then strongly increases with field. In magnetic fields, a nuclear hyperfine specific heat contribution is superimposed, which is due mostly to the bare Zeeman-splitting of the 115 Sn, 117 Sn, 119 Sn nuclei (all with spin I=1/2 and with abundances of 0.35, 7.61, and 8.58% respectively) in the externally applied field. The results on the specific heat at very low temperatures in applied fields fit into the model of an enhanced (heavy-fermion type) density of states which is modified by coherent antiferromagnetic fluctuations into a V-shaped density of states at the Fermi energy.

Disorder-induced heavy fermion behaviour in CeNi 1− xCu xSn-alloys

We have investigated the effect of alloying Cu to the Kondo-semimetal CeNiSn on its specific heat, magnetic susceptibility and μSR relaxation rate. We find a strongly non-monotonic behaviour, in that the electronic specific heat first increases very strongly (at 5% Cu), before decreasing again when magnetic order appears (at 10% and 20% Cu). The μSR data show that no spin freezing to a spin glass state occurs for concentrations less than ≈6% Cu and that the Ce 3+ spins remain in a fluctuating state below this concentration. We interpret this behavior as a consequence of the destruction of the high coherence of the antiferromagnetic spin fluctuations in pure CeNiSn, when Cu is alloyed to the system.

Pressure effect on magnetic excitation in the Kondo semimetal CeNiSn

Effects of pressure and impurities on the magnetic excitation peak at h ̵ w = 4 meV are studied by neutron scattering on CeNiSn. The high pressure remarkably affects the magnetic excitation: at P = 6 kbar the excitation shifts to a higher energy region; at P = 11 kbar the excitation peak disappears below 7 meV. The increase of the peak energy at 6 kbar is consistent with the explanation by the particle-hole excitation over the hybridization gap. The sample quality, metallic or semiconducting, has no effect on the magnetic excitation spectrum.

Anomalous scattering of quasiparticles by Co and Cu impurities in the Kondo semimetal CeNiSn

We present results of the magnetic susceptibility χ( T), electrical resistivity ϱ( T) and Hall coefficient R H( T) for CeNi 1− x T x Sn (T  Co and Cu, 0 ⩽ x ⩽ 0.05). Co impurities decrease χ along all the principal axes, whereas Cu impurities increase. Both impurities at x = 0.01 cause a remarkable increase in ϱ at 1.4 K with 970 μΩcm/at%Cu at maximum. For x = 0.05, ϱ( T) is saturated to a lower value than that for x = 0.01. R H( T) for x = 0.01 Co and Cu becomes negative below about 4 K and the absolute value is smaller than that for x = 0, while R H for x = 0.05 remains positive down to 1.6 K. The nonmonotonic dependence of ϱ( T) and R H( T) on x indicates extremely strong scattering of quasiparticles from nonmagnetic impurities in the presence of pseudogap.

The influence of impurities and alloying in the Kondo semimetal CeNiSn as seen by μSR

The temperature dependence of the muon spin relaxation rate \lambda was measured in two samples of CeNiSn with markedly different impurity content and in the quasi‐ternary compounds CePtxNi1-xSn (x=0.12,\ ,0.2) and CeCuxNi1-xSn (x=0.05,\ ,0.1,\ ,0.2) in order to study the development of magnetic correlations. No significant change in \lambda was found for the two samples of CeNiSn although their low temperature specific heat differs strongly. Hence μSR probes the intrinsic magnetic properties of the spin‐liquid state of CeNiSn. Replacing Ni with up to 20% Pt results only in minor changes of \lambda. In contrast, a strong effect is seen when Ni is replaced by Cu. Already at 10% Cu the system forms a spin‐glass‐like state below 0.9 K. At 20% Cu this behavior is strengthened, but antiferromagnetic order of the type seen in CePtSn or CePdSn is not reached. The results show further that the change in 4f hybridization initiated by volume expansion in the alloyed samples is not the dominating mechanism strengthening the magnetic correlations. Decisive is the increase in d electron density when replacing Ni by Cu.

The effect of high pressure on antiferromagnetic correlations in the Kondo semimetal CeNiSn

Magnetic excitation spectra of the Kondo semimetal CeNiSn have been studied by neutron scattering under hydrostatic pressure up to 11 kbar. The inelastic peak at has been observed at the ambient pressure below and is regarded as a dynamic antiferromagnetic correlation. As pressure is applied, the 4 meV peak shifts to a higher-energy region and becomes weak. Since the pseudogap is known to be suppressed under high pressure, this result strongly suggests that the antiferromagnetic correlation is closely related to pseudogap formation.

The effect of Co doping on antiferromagnetic correlations in the Kondo semi-metal CeNiSn

Single-crystal neutron scattering experiments have been performed to study magnetic excitations in , in which the pseudo-gap of CeNiSn is suppressed by the doping. In CeNiSn there are two inelastic excitation peaks at and 4 meV, which correspond to dynamic antiferromagnetic correlations. In the 2 meV peak is smeared out, whereas the 4 meV peak becomes very weak and broad, but preserves the same quasi-one-dimensional character as that for CeNiSn. These results suggest the strong relation between the antiferromagnetic correlations and the pseudo-gap formation.

Studies on the development of magnetism in the Kondo semimetal CeNiSn caused by doping with La, Cu and Pt

Muon spin-rotation - relaxation experiments were performed at low temperature in transverse, zero and longitudinal field on , , and as an extension of our previous studies of the CeTSn (T = Ni, Pt, Pd) series. In CeNiSn we have proven the formation of weakly dynamic paramagnetic spin correlations, below 1 K which, however, do not lead to long-range magnetic order even at T = 10 mK. For the predicted heavy-hole gas antiferromagnetism is not seen, but rather a reduction of magnetic correlations. Both Pt-doped compounds exhibit no significant enhancement of spin correlations although CePtSn is an antiferromagnet with . The magnetic correlations increase strongly for up to the point of forming a spin - glass-like state at T < 0.9 K. Doping with either 20% Pt or 10% Cu increases the unit cell volume by roughly 1%. Volume expansion in CeTSn is a known cause for a reduction of the hybridization between f electron states and conduction electrons, thereby strengthening magnetic interactions. Yet, the marked differences in magnetic behaviour between replacing Ni in part by either isoelectronic Pt or by Cu, which has two more d electrons, indicates that the dominating effect must be the increase of the d electron density instead.

Crystal Growth and Characterization of the Kondo Semimetal CeNiSn

Several single crystals of CeNiSn and CeNi 0.95 T 0.05 Sn (T=Co and Cu) were grown and characterized by metallographic examination, electron-probe microanalysis and the measurements of resistivity, magnetoresistance, magnetic susceptibility and specific heat. Below 10 K, the semiconductor-like behavior found for dirty or substituted crystals is suppressed with decreasing impurities. For a crystal purified by the solid-state electrotransport technique, the resistivity along the orthorhombic a axis is metallic with the T 2 dependence, whereas those along the b and c axes increase below 3 K and pass through weak maxima at 0.8 K. These results suggest that the ground state of CeNiSn is not semiconducting but semimetallic with a pseudogap which closes along the a axis.