Peculiarities of crystal field effects in CeInCu2 based heavy-fermion compounds

We have studied the evolution of the inelastic neutron magnetic scattering spectra of a compound with cubic symmetry, CeInCu₂, in the temperature range 10–130 K, and also their transformation with variation of the Kondo temperature T _K due to substitution of cerium ions in the system Ce_1−x (La,Y)_xInCu₂ at T=10 K. It turns out that the energy of the transition between the ground state and excited state of the 4 f electrons (Δ_CF) in the crystal electric field in CeInCu₂ increases with growth of the population of the ground state as the temperature is reduced, with a slight change in its intensity. Such behavior is inconsistent with the notion of classical one-ion effects of the crystal electric field. We have found that the scale of the observed variations in the excitation spectra of the 4f electrons depends on the Kondo temperature T _K and is insensitive to disorder in the rare-earth sublattice. Thus, despite the fact that T _K ≪Δ_CF, hybridization with states in the conduction band has a substantial effect on all parameters of the excitation spectrum of the ground multiplet of the 4f electrons at low temperatures. Zh. éksp. Teor. Fiz. 115, 2197–2206 (June 1999) A. A. Baikov Institute of Metallurgy     

Spin liquid in an almost ferromagnetic Kondo lattice

A theory of stabilization of a spin liquid in a Kondo lattice at temperatures close to the temperature of antiferromagnetic instability has been developed. Kondo exchange scattering of conduction electrons leads to emergence of a state of the spin liquid of the resonating valence bonds (RVB) type at T>T _K. Owing to this stabilization, low-energy processes of Kondo scattering with energies below T _K are frozen so that the “singlet” state of the Kondo lattice is not realized; instead a strongly correlated spin liquid with developed antiferromagnetic fluctuations occurs. A new version of the Feynman diagram technique has been developed to describe interaction between spin fluctuations and resonant valence bonds in a self-consistent manner. Emergence of a strongly anisotropic RVB spin liquid is discussed. Zh. éksp. Teor. Fiz. 112, 729–759 (August 1997)     

Resistance maximum across the spin glass to Kondo transition

In a recent theory of the noise model of alloys like AuFe a singular point at zero temperature was found to separate a spin glass phase at high concentrations and a Kondo phase at low concentrations. Despite this there is a resistance maximum in both “phases”, although of different characters. In the present letter a relation is given between the temperature of the maximum, T_m, the noise temperature, Δ_c, and the Kondo temperature, T_K. This extends a previously given expression, that is only valid in the spin glass limit Δ_c >> T_K, across the transition at Δ_c = T_K into the Kondo phase and values of Δ_c less than T_K.     

Calorimetric investigation under high pressure of the heavy fermion superconductor CeCu2Si2

The heavy fermion compound CeCu₂Si₂ is commonly regarded as a Kondo lattice system. Though it has been shown that the heavy mass quasiparticles participate in its superconductivity below ～ 0.7 K, a detailed understanding of the interdependence of the superconducting and the Kondo lattice parameters is still to be developed. The application of pressure is one useful approach to study this problem. In this paper we present results of specific heat measurements between 0.3 K and 2 K under pressures up to 5.9 kbar. While in our sample T_c hardly changes, the normal state specific heat, which is exclusively of electronic origin in the present temperature range, is rapidly decreased in a monotonous way, qualitatively corresponding to the expected rise of the Kondo temperature with pressure. In contrast to this behaviour, a strong nonlinear change of the jump Δc(T_c) passing through a maximum near 3 kbar is observed. We suggest that this reflects changes of the Kondo lattice coherence structure in the quasiparticle density of states near E_F.     

CePd2Ga3: A new ferromagnetic Kondo lattice

Various temperature-, pressure- and field dependent investigations on CePd₂Ga₃ indicate this ternary compound as belonging to the group of ferromagnetically ordered Kondo lattices, with the Curie temperatureT _C =6K and the Kondo temperatureT _K =4K. The first excited crystal field level of this hexagonal compound is about 40 K above the crystal field ground state, while the overall splitting is much larger.     

Field and pressure studies of Ce2Pd2In

Various measurements performed on the new ternary compound Ce₂Pd₂In indicate Kondo type interaction (T _K ≈1K) in the presence of strong crystal field splitting. Both an antiferromagnetic and a ferromagnetic transition were observed (T _N =4.4 K,T _C =3.8 K). Work supported by the Austrian FWF, P10269.     

Numerical renormalization group studies of SU(4) Kondo effect in quantum dots

We theoretically study an enhancement of the Kondo effect in quantum dots with two orbitals and spin . The Kondo temperature and conductance are evaluated as functions of energy difference Δ between the orbitals, using the numerical renormalization group method. The Kondo temperature is maximal around the degeneracy point (Δ=0) and decreases with increasing |Δ| following a power law, T_K(Δ)=T_K(0)(T_K(0)/|Δ|)^γ, which is consistent with the scaling analysis. The conductance at T=0 is almost constant 2e²/h. Both the orbitals contribute to the conductance around Δ=0, whereas the current through the upper orbital is negligibly small when |Δ|T_K(0). These are characteristics of SU(4) Kondo effect.     

重电子金属CeCu5.8M0.2（M=Ni,Zn, Cd）低温物性的比较

研究了不同元素等量掺杂下重电子金属CeCu_5.8M_0.2(M=Ni，Zn，Cd)低温电阻、比热容和磁化率随温度的变化关系.分析讨论了掺杂元素M(M=Ni，Zn，Cd)的磁性、价态及原子尺寸对近藤温度T_K、相干温度（即电阻极大值的温度Ｔ^R_max）、每个Ce离子在较高温度时的平均磁矩μ_Ｂ以及在温度降低时被传导电子自旋屏蔽过程的影响. 关键词： 重电子系统 低温电阻 低温比热容 磁化率     

Influence of unstable valence of cerium ions on the crystal field in ReNi compounds

To examine the effect of hybridization of 4f electrons with conduction electrons on the crystal field potential using neutron spectroscopy, we studied the effects of the crystal electric field (CEF) in intermetallic compounds of the type ReNi, in which chemical substitution is followed by a transition of the cerium ions from an intermediate valence state to the Kondo state. Measurements were performed both on cerium ions in the compounds Ce_1−x La_xNi (x=0.5, 0.8), where they have a whole-number population of the 4f shell, and on the paramagnetic impurity ion Nd in the series of compounds Re_1−x Nd_xNi (Re=Ce, La, Y), in which the cerium ions are found either in an intermediate valence state or in the Kondo state. From the neutron inelastic magnetic scattering spectra on Nd ions, we have reconstructed the crystal field parameters in ReNi compounds and calculated the CEF level diagram of Ce ions in these compounds as functions of the interion distances Re-Ni. The results of our calculations are in good agreement with the experimentally determined splitting diagram of the ground-state multiplet of the Ce ions. We have determined that as the degree of hybridization with the conduction electrons grows the CEF potential varies considerably and the effective splitting of the 4f shell of the cerium ions increases. The estimated energy scale of the splitting of the ground-state multiplet of the Ce³⁺ ions in the ReNi CEF (Δ_CEF∼15 meV) turns out to be commensurate with the Kondo temperature (T _K ;140 K for CeNi. Analysis indicates that the CEF potential has a substantial effect on the formation of the valence-unstable ground state of the f shell in this compound. Zh. éksp. Teor. Fiz. 113, 1731–1747 (May 1998)     

A natural bond orbital analysis of aryl‐substituted polyfluorinated carbanions: negative hyperconjugation

Aryl‐substituted polyfluorinated carbanions, ArCHR_f^? where R_f = CF₃ ( 1 ), C₂F₅ ( 2 ), i‐C₃F₇ ( 3 ), and t‐C₄F₉ ( 4 ), were analyzed by means of the natural bond orbital (NBO) theory at the B3LYP/6‐311+G(d,p) computational level. A lone pair NBO at the formal anionic center carbon (Cα) was not found in the Lewis structure. Instead, significant donor/acceptor NBO interactions between π(Cα‐C1) and σ*(Cβ‐F) or σ*(Cβ‐Cγ) were observed for 1 , 2 , 3a (strong electron‐withdrawing substituent, from p‐CF₃ to p‐NO₂), and 4 . Their second‐order donor/acceptor perturbation interaction energy, E(2), values decreased with the increase of the stability of carbanions. A larger E(2) value corresponds to longer Cβ‐F and Cβ‐Cγ bonds and a shorter Cα‐Cβ bond, indicating that the E(2) values can be associated with the negative hyperconjugation of the Cβ‐F and Cβ‐Cγ bonds. In accordance with this, the E(2) values for π(Cα‐C1) → σ*(Cβ‐F) were linearly correlated with the ΔG^o_β‐F values (an empirical measure of β‐fluorine negative hyperconjugation obtained from an increased acidity). In 3b (weak electron‐withdrawing substituents, from H to m‐NO₂) very large E(2) values for LP(Fβ) → π*(Cα‐Cβ) were obtained. This was attributed to the Cβ‐F bond cleavage and the Cα‐Cβ double bond formation in the Lewis structure that is caused by the extremely strong negative hyperconjugation of the Cβ‐F bond.     

Kondo effect,spin dynamics and magnetism in anomalous rare earth and actinide compounds

The problem of magnetic ordering in Kondo lattices is discussed. Criteria of ferro- and antiferromagnetism in the mean-field approximation for thes-f exchange model are obtained. In the case of a constant bare DOS, the saturated Kondo ferromagnetic solution is energetically stable at small intersite interactionsJ. The role of spin-fluctuation corrections at lowT _K in the building up the low-energy scale and the ground state magnetization is considered. Renormalization of spin-fluctuation frequencies atT>T _K is calculated. A possible picture of the formation of magnetism is presented starting from the high-T limit.     

Magnetoresistivity of the Kondo-system (La,Ce)B6

The electrical resistivity of the Kondo system (La, Ce)B₆ has been measured in longitudinal and transversal magnetic fields up to 6 Tesla in the temperature range 0.04–20K. Corresponding to the strong increase of the resistivity with decreasing temperature the alloys show a very large negative magnetoresistivity with a Kondo temperatureT _K =1.05K and a Kondo magnetic fieldB _K =1.1 Tesla. The observed anisotropy of the resistivity due to the magnetic field direction cannot be explained well by existing theories.     

Half-life measurements of intrinsic states in deformed odd-mass nuclei

The half-lives of the following intrinsic states in deformed odd-mass nuclei has been measured by delayed coincidences with a time-to-amplitude converter:

1. 5/2 5/2⁺[642] at 86.5 keV in¹⁵⁵Gd:T _1/2=6.7±0.3 ns, which results in the determination of theE1,ΔK=1 transition probability to the ground state 3/2 3/2^?[521] and first rotational state 5/2 3/2^?[521], yielding hindrance factors ofF _N ≈5.5 and ≈1.8 (F _W =3.1×10⁴ and 2.3×10⁴) respectively.
2. (3) 5/2 5/2^?[512] at 191.4 keV in¹⁶⁹Yb:T _1/2=3.35±0.15 ns and at 122.39 keV in¹⁷¹Yb:T _1/2=265±20 ns which results in the determination of the transition probabilities of theE1,ΔK=1 transitions to the ground states 7/2 7/2⁺[633], of theK-forbiddenM1 transitions to the 5/2 and 3/2 1/2^?[521] and of theE2 transitions to the 5/2, 3/2 and 1/2 1/2^?[521] states in both nuclei.

TheE1 transition probabilities are compared to the transitions between the same Nilsson states in¹⁷³Yb and¹⁷⁵Hf discussing the influence of the position of the Fermi surface — obtained from recent stripping and pick-up reactions — on these transition probabilities. Additional information on the decay scheme of¹⁷¹Lu→¹⁷¹Yb is obtained by delayed coincidence measurements. For testing the used time-to-amplitude converter the well known half-lives of the 482 keV level in¹⁸¹Ta (T _1/2=10.4±0.3 ns) and of the 279 keV level in²⁰³Tl (T _1/2=0.285 ±0.015 ns) were measured, in good agreement with other measurements.     

Size-dependent energy scales in the ideal fermi gas

Abstract

The simplest model for the electronic properties of small metal particles is an ideal Fermi gas confined to a finite volume. When the confining region of size L has a regular shape such as a sphere or a cube, there are two distinct scales of energy which characterize the spectrum of eigenvalues near the Fermi energy E_F ≡ ?² k ² _f/2m. The inner scale δ ～ E_F /(k_FL)² is the mean spacing between successive energy levels, while the outer energy scale Δ ～ E_F /(k_FL) describes clustering of several levels, or shell structure. Consequences for the behaviour of thermodynamic properties are investigated. There are three regimes of temperature T: normal metallic (T > Δ), shell-metallic (δ < T < Δ) and semiconductor-like (T < δ). Finally, if the shape of a hard-walled container is allowed to vary so as to minimize the energy, it is argued that the optimal shape fluctuates between spherical and distorted as L is changed.     

Infrared Laser Spectroscopy of Jet-Cooled C2F6near 10 μm

The ν₅(A_2u) and ν₇(E_u) C–F stretching fundamentals of hexafluoroethane, C₂F₆, have been recorded in a supersonic jet by diode laser absorption spectroscopy. The parallel 5¹_oband is accompanied by five satellite bands, of which three have been assigned to hot bands. A fourth satellite band arises from¹²CF₃¹³CF₃. Transitions satisfying 0 ≤KΔK≤ 7 of the perpendicular 7¹_oband are unperturbed while those having −10 ≤KΔK≤ 7 can be fitted assuming anR_z-Coriolis interaction with a state lying at ν_p= 1256 cm⁻¹. A second localized perturbation affects lines withKΔK≥ 8. The band origins are 1117.10736 (7) cm⁻¹(5¹_o) and 1252.96950 (17) cm⁻¹(7¹_o, 0 ≤KΔK≤ 7), and the rotational constantB₀= 0.0615759 (27) cm⁻¹.     

Steric effects on the mechanism of reaction of nucleophilic substitution of β‐substituted alkoxyvinyl trifluoromethyl ketones with four secondary amines

The kinetics of the reaction of β‐substituted β‐alkoxyvinyl trifluoromethyl ketones R¹O‐CR²?CH‐COCF₃ ( 1a – e ) [( 1a ), R¹?C₂H₅, R²?H; ( 1b ), R¹?R²?CH₃; ( 1c ), R¹?C₂H₅, R²?C₆H₅; ( 1d ), R¹?C₂H₅, R²?V^?pNO₂C₆H₄; ( 1e ), R¹?C₂H₅, R²?C(CH₃)₃] with four aliphatic amines ( 2a – d ) [( 2a ), (C₂H₅)₂NH; ( 2b ), (i‐C₃H₇)₂NH; ( 2c ), (CH₂)₅NH; ( 2d ), O(CH₂CH₂)₂NH] was studied in two aprotic solvents, hexane and acetonitrile. The least reactive stereoisomeric form of ( 1a – d ) was the most populated ( E‐s‐Z‐o‐Z ) form, whereas in ( 1e ), the more reactive form ( Z‐s‐Z‐o‐Z ) dominated. The reactions studied proceeded via common transition state formation whose decomposition occurred by ‘uncatalyzed’ and/or ‘catalyzed’ route. Shielding of the reaction centre by bulky β‐substituents lowered abruptly both k′ (‘uncatalyzed’ rate constant) and k″ (‘catalyzed’ rate constant) of this reaction. Bulky amines reduced k″ to a greater extent than k′ as a result of an additional steric retardation to the approach of the bulky amine to its ammonium ion in the transition state. An increase in the electron‐withdrawing ability of the β‐substituent increased ‘uncatalyzed’ k′ due to the acceleration of the initial nucleophile attack (k₁) and ‘uncatalyzed’ decomposition of transition state (k₂) via promoting electrophilic assistance (through transition state 8 ). The amine basicity determined the route of the reaction: the higher amine basicity, the higher k₃/k₂ ratio (a measure of the ‘catalyzed’ route contribution as compared to the ‘uncatalyzed’ process) was. ‘Uncatalyzed’ route predominated for all reactions; however in polar acetonitrile the contribution of the ‘catalyzed’ route was significant for amines with high pK_a and small bulk. Copyright © 2007 John Wiley & Sons, Ltd.     

Photoemission study of electronic structure evolution across the metal-insulator transition of heavily B-doped diamond

We studied the electronic structure evolution of heavily B-doped diamond films across the metal-insulator transition (MIT) using ultraviolet photoemission spectroscopy (UPS). From high-temperature UPS, through which electronic states near the Fermi level (E_F) up to ∼5k_BT can be observed (k_B is the Boltzmann constant and T the temperature), we observed the carrier concentration dependence of spectral shapes near E_F. Using another carrier concentration dependent UPS, we found that the change in energy position of sp-band of the diamond valence band, which corresponds to the shift of E_F, can be explained by the degenerate semiconductor model, indicating that the diamond valence band is responsible for the metallic states for samples with concentrations above MIT. We discuss a possible electronic structure evolution across MIT.     

The calculation of valence shell ionization potentials by the ΔSCF method

Valence shell ionization potentials for several small molecules (HF, H₂O, CH₄, CO, H₂S, PH₃, SiH₄, SO₂ and F₂CO) are calculated by means of ab initio ΔSCF calculations on all the valence hole states of each molecule. The results are compared with the experimental low energy photoelectron spectra. In the case of the higher energy ion states, particular attention is paid to finding practical means for obtaining convergence of the ΔSCF iterations, and numerical evidence is presented, indicating that the ΔSCF procedure provides upper bounds to the energy of such states, at least for the cases studied herein. A new procedure, the ‘ combined ΔSCF ’ method, for the determination of a common set of molecular orbitals for use in the construction of wavefunctions for a number of ion states is described.     

Molecular-field theory of moment reduction in Kondo systems with crystal-field effects

Several dense Kondo compounds have a low-temperature ordered phase in which the magnetic moments are reduced with respect to the values expected for the crystal-field (CEF) ground state. In the present work the phenomenon of moment reduction is studied within a molecular-field theory combined with a variational solution of the one-impurity Anderson model with CEF effects. The calculated zero-temperature magnetization and susceptibility agree well with available exact results; the present method is easily applied to systems of any symmetry. We first study the f ¹ configuration in cubic symmetry, for small values of the ratio T _K/Δ between Kondo temperature and CEF splitting. With a Γ _∞ ground state and a field along a 〈100〉 direction, an inflection point occurs in the magnetization curve, which gives rise to a first order transition in the zero-temperature phase diagram. This feature is not found for a field along 〈110〉 or 〈111〉, for which the transition is second order. For a Γ ₇ ground state and small values of T _K/Δ, the magnetic-nonmagnetic transition is second order for all field directions. On increasing T _K/Δ an inflection point in the magnetization curve appears first for a field along 〈111〉. The theory is applied to a study of cubic CeAg, CeAl₂, CePb₃, CeIn₃, CeTe, and hexagonal CePd₂Ga₃. The bare value of the moment is found to be strongly increased by exchange coupling to excited CEF states, and the amount of Kondo reduction is found to be substantial, confirming the importance of the Kondo effect in these compounds.     

Specific heat of a Kondo superconductor: (La, Ce) Al2

The specific heat of LaAl₂ and (La_1-xCe_x)Al₂ (x ? 0.0064) has been measured between 0.3 and 5 K, both in the superconducting and in the normal state. For all samples the same values for the Debye temperature as well as for the electronic specific heat coefficient have been determined. LaAl₂ shows an excellent BCS behavior. A remarkable excess specific heat at low temperatures due to the Kondo effect has been observed for all superconducting as well as for the normal conducting (La_1-xCe_x) Al₂ alloys. The specific heat jump ΔC at T_c depressed rapidly with increasing Ce concentration, allows the Kondo temperature T_K ? 1 K to be determined. ΔC vanishes at finite temperatures.