Heavy-fermion Compound Ce Research Articles

The 4f-Hybridization Strength in Ce m Mn In3m+2n Heavy-Fermion Compounds Studied by Angle-Resolved Photoemission SpectroscopySupported by the National Natural Science Foundation of China (Grant Nos. 12074436 and 11574402), and the Innovation-Driven Plan in Central South University (Grant No. 2016CXS032). J.R. and P.M.O. acknowledge support through the Swedish Research Council (VR) and the Swedish National Infrastructure for Computing (SNIC), for computing

We systemically investigate the nature of Ce 4f electrons in structurally layered heavy-fermion compounds Ce m M n In3m + 2n (with M = Co, Rh, Ir, and Pt, m = 1, 2, n = 0–2), at low temperature using on-resonance angle-resolved photoemission spectroscopy. Three heavy quasiparticle bands f 0, and , are observed in all compounds, whereas their intensities and energy locations vary greatly with materials. The strong f 0 states imply that the localized electron behavior dominates the Ce 4f states. The Ce 4f electrons are partially hybridized with the conduction electrons, making them have the dual nature of localization and itinerancy. Our quantitative comparison reveals that the –f 0 intensity ratio is more suitable to reflect the 4f-state hybridization strength.

Optical Evidence of Itinerant-Localized Crossover of 4fElectrons in Cerium Compounds

Cerium (Ce)-based heavy-fermion materials have a characteristic double-peak structure (mid-IR peak) in the optical conductivity [$\sigma(\omega)$] spectra originating from the strong conduction ($c$)--$f$ electron hybridization. To clarify the behavior of the mid-IR peak at a low $c$-$f$ hybridization strength, we compared the $\sigma(\omega)$ spectra of the isostructural antiferromagnetic and heavy-fermion Ce compounds with the calculated unoccupied density of states and the spectra obtained from the impurity Anderson model. With decreasing $c$-$f$ hybridization intensity, the mid-IR peak shifts to the low-energy side owing to the renormalization of the unoccupied $4f$ state, but suddenly shifts to the high-energy side owing to the $f$-$f$ on-site Coulomb interaction at a slight localized side from the quantum critical point (QCP). This finding gives us information on the change in the electronic structure across QCP.

Physical properties of theCe2MAl7Ge4heavy-fermion compounds(M=Co,Ir,Ni,Pd)

We report the synthesis, crystal structure and characterization by means of single crystal x-ray diffraction, neutron powder diffraction, magnetic, thermal and transport measurements of the new heavy fermion compounds Ce$_{2}$MAl$_{7}$Ge$_{4}$ (M = Co, Ir, Ni, Pd). These compounds crystallize in a noncentrosymmetic tetragonal space group P\={4}2$_{1}$m, consisting of layers of square nets of Ce atoms separated by Ge-Al and M-Al-Ge blocks. Ce$_{2}$CoAl$_{7}$Ge$_{4}$, Ce$_{2}$IrAl$_{7}$Ge$_{4}$ and Ce$_{2}$NiAl$_{7}$Ge$_{4}$ order magnetically behavior below $T_{M}=$ 1.8, 1.6, and 0.8 K, respectively. There is no evidence of magnetic ordering in Ce$_{2}$PdAl$_{7}$Ge$_{4}$ down to 0.4 K. The small amount of entropy released in the magnetic state of Ce$_{2}$MAl$_{7}$Ge$_{4}$ (M = Co, Ir, Ni) and the reduced specific heat jump at $T_M$ suggest a strong Kondo interaction in these materials. Ce$_{2}$PdAl$_{7}$Ge$_{4}$ shows non-Fermi liquid behavior, possibly due to the presence of a nearby quantum critical point.

Converse effect of pressure on the quadrupolar and magnetic transition inCe3Pd20Si6

The heavy fermion compound Ce$_{3}$Pd$_{20}$Si$_{6}$ displays unconventional quantum criticality as the lower of two consecutive phase transitions is fully suppressed by magnetic field. Here we report on the effects of pressure as additional tuning parameter. Specific heat and electrical resistivity measurements reveal a converse effect of pressure on the two transitions, leading to the merging of both transitions at 6.2 kbar. The field-induced quantum criticality is robust under pressure tuning. We rationalize our findings within an extended version of the global phase diagram for antiferromagnetic heavy fermion quantum criticality.

Quantum critical fluctuations in the heavy fermion compound Ce(Ni0.935Pd0.065)2Ge2

Electric resistivity, specific heat, magnetic susceptibility, and inelastic neutron scattering experiments were performed on a single crystal of the heavy fermion compound Ce(Ni0.935Pd0.065)2Ge2 in order to study the spin fluctuations near an antiferromagnetic (AF) quantum critical point (QCP). The resistivity and the specific heat coefficient for T ⩽ 1 K exhibit the power law behavior expected for a 3D itinerant AF QCP (ρ(T) ∼ T3/2 and γ(T) ∼ γ0 − bT1/2). However, for 2 ⩽ T ⩽ 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.

Thermoelectric properties of CeAl3 prepared by hot-press method

We present data on the heavy fermion compound CeAl3 as a potential p-type thermoelectric material at cryogenic temperatures. Samples of CeAl3 were synthesized by arc-melting, ball milling and hot pressing the nano-powder at different temperatures. Thermal conductivity (κ), Seebeck coefficient (S), electrical resistivity (ρ) and Hall coefficient (RH) of the samples were measured at temperatures ranging from 5 to 330K. The thermoelectric properties of this compound are highly dependent on the hot-pressing (HP) temperature. The thermal conductivity increases with increase in HP temperature which is compensated by increased power factor (S2ρ−1). As a result the samples with higher HP temperatures exhibit higher ZT values. The highest ZT value of 0.016 at 55K was observed for the sample hot-pressed at 800°C. The Hall coefficient data reflects an increase in carrier concentration and a decrease in carrier mobility with decreasing hot pressing temperature.

Description of Quasi Particle Bands of Ce Compounds Using the Wannier Representation

We present a method to describe the quasi-particle bands of heavy-fermion Ce compounds on the basis of the periodic Anderson model taking account of the realistic conduction-band structures using the Wannier functions. We first calculate the conduction-band structure of the corresponding La compound by the pseudo potential method, and represent it by the maximally localized Wannier functions, and then consider the hybridizations between these conduction bands and the 4 f -electron eigen states of the spin–orbit coupling and crystalline electric field by the use of the Slater–Koster integrals. To take into account the correlation effects of the 4 f electrons, we adopt the dynamical mean field theory with the iterative perturbation theory as an impurity solver to obtain the quasi particle bands. We apply this method to describe the quasi particle bands of CeRu 2 Si 2 exhibiting the paramagnetic heavy-fermion states at extremely low temperatures.

Behavior of the Quantum Critical Point and the Fermi-Liquid Domain in the Heavy Fermion Superconductor CeCoIn5 Studied by Resistivity

We report detailed very low temperature resistivity measurements on the heavy fermion compounds Ce_{1-x}La_{x}CoIn5 (x=0 and x=0.01), with current applied in two crystallographic directions [100] (basal plane) and [001] (perpendicular to the basal plane) under magnetic field applied in the [001] or [011] direction. We found a Fermi liquid (\rho \propto T^{2}) ground state, in all cases, for fields above the superconducting upper critical field. We discuss the possible location of a field induced quantum critical point with respect to Hc2(0), and compare our measurements with the previous reports in order to give a clear picture of the experimental status on this long debated issue.

Quantum critical behavior in the heavy Fermion single crystal Ce(Ni0.935Pd0.065)2Ge2

We have performed magnetic susceptibility, specific heat, resistivity, and inelastic neutron scattering measurements on a single crystal of the heavy Fermion compound Ce(Ni0.93BPd0.06B)2Ge2, 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 χ(T) − χ(0) ∝ T−1/6 with χ(0) = 0.032 × 10−6 m3/mole (0.0025 emu/mole). For T < 1 K, the specific heat can be fit to the formula ΔC/T = γ0 − T1/2 with γ0 of order 700 mJ/mole-K2. The resistivity behaves as ρ = ρ0 + AT3/2 for temperatures below 2 K. This low temperature behavior for γ(T) and ρ(T) is in accord with the SCR theory of Moriya and Takimoto[1]. 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 TK = 17 K. In addition, the scattering is enhanced near Q=(1/2, 1/2, 0) with maximum scattering at ΔE = 0.45 meV; we interpret this as scattering from antiferromagnetic fluctuations near the antiferromagnetic QCP.

Effect of Disorder on Fermi Surface in Heavy Electron Systems

The Kondo lattice model with substitutional disorder is studied with attention to the size of the Fermi surface and the associated Dingle temperature. The model serves for understanding heavy-fermion Ce compounds alloyed with La according to substitution Ce{x}La{1-x}. The Fermi surface is identified from the steepest change of the momentum distribution of conduction electrons, and is derived at low enough temperature by the dynamical mean-field theory (DMFT) combined with the coherent potential approximation (CPA). The Fermi surface without magnetic field increases in size with decreasing x from x=1 (Ce end), and disappears at such x that gives the same number of localized spins as that of conduction electrons. From the opposite limit of x=0 (La end), the Fermi surface broadens quickly as x increases, but stays at the same position as that of the La end. With increasing magnetic field, a metamagnetic transition occurs, and the Fermi surface above the critical field changes continuously across the whole range of x. The Dingle temperature takes a maximum around x=0.5. Implication of the results to experimental observation is discussed.

Kondo behavior, ferromagnetic correlations, and crystal fields in the heavy-fermion compoundsCe3X(X=In,Sn)

We report measurements of inelastic neutron scattering, magnetic susceptibility, magnetization, and the magnetic field dependence of the specific heat for the heavy Fermion compounds Ce$_3$In and Ce$_3$Sn. The neutron scattering results show that the excited crystal field levels have energies $E_1$ = 13.2 meV, $E_2$ = 44.8 meV for Ce$_3$In and $E_1$ = 18.5 meV, $E_2$ = 36.1 meV for Ce$_3$Sn. The Kondo temperature deduced from the quasielastic linewidth is 17 K for Ce$_3$In and 40 K for Ce$_3$Sn. The low temperature behavior of the specific heat, magnetization, and susceptibility can not be well-described by J=1/2 Kondo physics alone, but require calculations that include contributions from the Kondo effect, broadened crystal fields, and ferromagnetic correlations, all of which are known to be important in these compounds. We find that in Ce$_3$In the ferromagnetic fluctuation makes a 10-15 % contribution to the ground state doublet entropy and magnetization. The large specific heat coefficient $\gamma$ in this heavy fermion system thus arises more from the ferromagnetic correlations than from the Kondo behavior.

Magnetization measurements under pressure for the heavy-fermion compounds Ce(Ru0.9Rh0.1)2(Si1−yGey)2

Recently, it was reported that a phase transition from the spin density wave (SDW) phase to the localized f-electron antiferromagnetic (LAF) phase in the heavy fermion compounds Ce(Ru0.9Rh0.1)2(Si1−yGey)2 (CRRSG0.1/y) around y = 0.26. It is induced by reduction of the cf-hybridization due to the volume effect with increasing the Ge-concentration, y. In this work, we performed magnetization measurements for several y of CRRSG0.1/y at ambient pressure and those under pressure (P) for CRRSG0.1/0.3 with use of a clamp-type pressure cell for the MPMS magnetometer. Applying pressure for y = 0.30, the TN decreases discontinuously around P ≊ 0.12 GPa, which suggests a first order phase transition from the LAF phase to the SDW phase.

First-Principles Local-Density Approximation Study of Electronic Structure in CeCoSi2

The first-principles band calculation of an itinerant cerium compound CeCoSi 2 is performed using a relativistic LAPW method with exchange and correlation potentials within a local-density approximation (LDA). The c– f hybridized bands with a large dispersion form two-dimensional Fermi surface (FS). The FS formation and the band dispersion including the contribution from each orbital obtained by the LDA calculation are in a qualitatively agreement with Ce 3 d –4 f resonant angle-resolved photoemission spectra. The LDA calculation is a good starting point for the investigation of the electronic structure of the heavy-fermion Ce compounds.

Band structure and Fermi surface of heavy Fermion compounds Ce 2TIn 8[formula omitted] studied by angle-resolved photoemission spectroscopy

We report a systematic study on the electronic structure of Ce-based heavy Fermion materials Ce 2TIn 8 ( T = Rh , Co, Ir) by high-resolution angle-resolved (ARPES) and 4d–4f resonant (RPES) photoemission spectroscopies. We have experimentally determined the band structure and Fermi surface of these three compounds, and found that the Fermi surfaces of three compounds are very similar to each other, having quasi two-dimensional cylindrical Fermi surfaces centered at the M(A) point in the Brillouin zone. RPES results have confirmed the localized picture and, in addition, revealed that Ce 4f electrons in Ce 2RhIn 8 have a relatively stronger localized nature than those in Ce 2CoIn 8. This difference in the localized character explains well the difference in the magnetic property of these compounds.

Observation of field-induced single impurity behavior in the heavy fermion compound [formula omitted

We have performed heat capacity measurements in magnetic fields to 90 kOe on single crystals of the cubic heavy fermion compound Ce 3 Co 4 Sn 13 . In zero field, there are no signs of long-range magnetic order down to 0.35 K. However, C / T increases rapidly below 2 K, reaching a very large maximum value of ∼ 4 J / mol Ce–K around 0.8 K in zero field, and the high-field magnetic entropy approaches R ln 2 at 20 K. Above 25 kOe, the data are consistent with a Kondo impurity with T K = 1.2 K . Short-range magnetic correlations are suppressed by magnetic fields giving way to single impurity behavior above 25 kOe.

Electronic and magnetic properties of CeAl 2 nanoparticles

We presented the X-ray magnetic circular dichroism (XMCD) and X-ray absorption spectroscopy (XAS) studies of heavy fermion compound CeAl 2 bulk and 8 nm nanoparticles, performed at the Ce M 4,5 - and L 3 - absorption edges. XMCD and XAS revealed that Ce in bulk CeAl 2 exhibits localized 4f 1 character with magnetic ordering. The Ce in nanoparticles, on the other hand, shows a small amount delocalized 4f 0 character with non-magnetic Kondo behavior. By applying general sum rules, an estimation of the orbital and spin contribution to those Ce 4f moments can be obtained. Our results also demonstrated that the magnetic behavior in CeAl 2 is very sensitive to the degree of localization of the 4f electrons.

Heavy fermions in CeAl 3

The first heavy-fermion compound—CeAl 3 has been studied by a sample rotation technique applied to the Hall coefficient R H( T, H 0) measurements. The study of the high quality polycrystalline samples of CeAl 3 has been carried out in the temperature range 1.8–300 K. A complicated activation type behavior R H( T, H<5 kOe)∼exp( E a 1,2/ k B T) has been established with activation energies E a 1(50–300 K)/ k B≈125 K and E a 2(10–35 K)/ k B≈3.3 K. Microscopic parameters of charge carriers transport (effective masses, relaxation time of the many-body states at E F) and localization radii a p 1,2* of the heavy fermions ( a p 2*( T<50 K)∼16 Å and a p 1*(T>50 K)∼1.7 Å) were estimated for CeAl 3.

In-NQR study of heavy fermion compounds [formula omitted] ( [formula omitted], Rh)

Ce 2 CoIn 8 is a superconductor with T SC = 0.4 K and Ce 2 RhIn 8 is an antiferromagnet with T N = 2.8 K . Ce 2 RhIn 8 becomes superconducting below 1.4 K at 2.0 GPa. Their crystal structure can be viewed as the successive layers of the heavy fermion compounds Ce T In 5 and CeIn 3 . In this study, we have performed nuclear quadrupole resonance (NQR) measurement on 115In nuclei of Ce 2 T In 8 . Spin–lattice relaxation rate 1 / T 1 of Ce 2 CoIn 8 decreases with decreasing temperature T and is proportional to T just above T SC . This is contrast with the T 1 / 2 dependence of the 1 / T 1 of pressurized Ce 2 RhIn 8 at low temperatures.

Evolution of the heavy fermion state in Ce 2IrIn 8

We report μ SR Knight shift and susceptibility studies for 1 T applied field along the crystalline c- and a-axes of the heavy fermion compound Ce 2IrIn 8. Below a characteristic temperature T * one observes a ‘Knight-shift anomaly’ in which the Knight shift constant K no longer scales linearly with χ . This anomaly is consistent with a scaling law in which the susceptibility χ is composed of a high-temperature component corresponding to non-interacting local moments and a low-temperature component χ c f ∼ ( 1 - T / T * ) ln ( T * / T ) which characterizes the heavy-electron state below T * . We find that T * is anisotropic, with T a * = 59 ( 3 ) K and T c * = 24 ( 1 ) K , and derive the magnitudes of χ c f a , c .

The extent of quantum critical fluctuations into the heavy-fermion phase: A neutron scattering study of Ce(Ru1−xFex)2Ge2

We present inelastic neutron scattering measurements on the heavy-fermion compound Ce(Ru1−xFex)2Ge2 (x=0.87). This composition is close to the quantum critical point (QCP) compound (x=0.76) in which it was observed that the decay of spin fluctuations could no longer be described by a simple exponential in time. Here we show that the relaxation mechanism for spin fluctuations exhibits a qualitatively similar momentum and temperature dependence in both compositions, possibly reflecting the underlying scaling law at the QCP. However, the details of the relaxation mechanism demonstrate that critical fluctuations are strongly damped along the composition axis.