Heavy-electron State Research Articles

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 .

Sum rules and Ward identities in the Kondo lattice

We derive a generalized Luttinger-Ward expression for the Free energy of a many body system involving a constrained Hilbert space. In the large $N$ limit, we are able to explicity write the entropy as a functional of the Green's functions. Using this method we obtain a Luttinger sum rule for the Kondo lattice. One of the fascinating aspects of the sum rule, is that it contains two components, one describing the heavy electron Fermi surface, the other, a sea of oppositely charged, spinless fermions. In the heavy electron state, this sea of spinless fermions is completely filled and the electron Fermi surface expands by one electron per unit cell to compensate the positively charged background, forming a ``large'' Fermi surface. Arbitrarily weak magnetism causes the spinless Fermi sea to annihilate with part of the Fermi sea of the conduction electrons, leading to a small Fermi surface. Our results thus enable us to show that the Fermi surface volume contracts from a large, to a small volume at a quantum critical point. However, the sum rules also permit the possible formation of a new phase, sandwiched between the antiferromagnet and the heavy electron phase, where the charged spinless fermions develop a true Fermi surface.

Scaling in the emergent behavior of heavy-electron materials

We show that the NMR Knight shift anomaly exhibited by a large number of heavy electron materials can be understood in terms of the different hyperfine couplings of probe nuclei to localized spins and to conduction electrons. The onset of the anomaly is at a temperature ${T}^{*}$, below which an itinerant component of the magnetic susceptibility develops. This second component characterizes the polarization of the conduction electrons by the local moments and is a signature of the emerging heavy electron state. The heavy electron component grows as $\mathrm{log}\phantom{\rule{0.3em}{0ex}}T$ below ${T}^{*}$, and scales universally for all measured $\mathrm{Ce}$, Yb and U based materials. Our results suggest that ${T}^{*}$ is not related to the single ion Kondo temperature, ${T}_{K}$, but rather represents a correlated Kondo temperature that provides a measure of the strength of the intersite coupling between the local moments. Our analysis strongly supports the two-fluid description of heavy electron materials developed by Nakatsuji, Pines and Fisk.

Quadrupolar Ordering in Pr-filled Skutterudites Exhibiting Heavy-Electron State

Quadrupolar Ordering in Pr-filled Skutterudites Exhibiting Heavy-Electron State

Depleted Kondo lattices: Quantum Monte Carlo and mean-field calculations

We consider a two dimensional Kondo lattice model with exchange J and hopping t in which three out of four impurity spins are removed in a regular way. At the particle-hole symmetric point the model may be studied with auxiliary field quantum Monte Carlo methods without sign problems. To achieve the relevant energy scales on finite clusters, we introduce a simple method to reduce size effects by up to an order of magnitude in temperature. In this model, a metallic phase survives up to arbitrarily low temperatures before being disrupted by magnetic fluctuations which open a gap in the charge sector. We study the formation of the heavy-electron state with emphasis on a crossover scale T* defined by the maximum in the resistivity versus temperature curve. The behavior of thermodynamic properties such as specific heat as well as spin and charge uniform susceptibilities are studied as the temperature varies in a wide range across T*. Within our accuracy T* compares well to the Kondo scale of the related single impurity problem. Finally our QMC resuls are compared with mean-field approximations.

Anomalous ordered phase in 4f 2-based heavy electron PrFe 4P 12

Recent study on the anomalous ordered state (ODS) in filled-skutterudite PrFe 4P 12 is presented, where the existence of a rare 4f 2-based heavy electron (HE) state outside ODS has been experimentally confirmed recently. From specific heat measurements, it is found that the HE behavior is largely suppressed in ODS. The Pr nuclear Schottky contribution strongly indicates that ODS is non-magnetic, most probably of quadrupolar origin. We argue that the quadrupolar degree of freedom on the Pr-ions may play an essential role in the heavy quasiparticle band formation in PrFe 4P 12. We also report the La-substitution effect on ODS.

Magnetic and thermal properties of nanoscale heavy-fermion particles

The heavy-fermion state in some rare-earth and actinide compounds arises from a hybridization of extended conduction states with strongly correlated localized f states. The properties of the heavy fermions are those of a Fermi liquid with small Fermi energy of the order of ${T}_{K}.$ In nanosized particles, however, the conduction states have discrete energy levels and the energy spacing leads to an additional energy scale that competes with the Kondo temperature. A small heavy-fermion particle is considered, described by the Anderson model in the $U\ensuremath{\rightarrow}\ensuremath{\infty}$ limit, so that only two electronic configurations---namely, ${f}^{0}$ and ${f}^{1}$---are allowed. A mean-field approximation with one slave boson per f site is used to study the susceptibility, the entropy, and the specific heat at low temperatures. All quantities increase rapidly with T as a consequence of exponential activations due to the discreteness of the energy spectrum until the heavy-electron state is formed. The possibility of ferromagnetic order is investigated using the formulation of Kotliar and Ruckenstein in terms of three auxiliary bosons per site. The mean-field approximation yields several possible magnetic phases for the ground state as a function of the f-level position. In the strongly mixed-valent regime the transition from the paramagnetic to the ferromagnetic phase is signaled as a function of temperature by a diverging susceptibility. It is concluded that the thermal and magnetic properties of very small heavy-fermion particles are quite different from those of bulk heavy-fermion material.

De Haas–van Alphen effect in a heavy fermion superconductor UPd 2Al 3

We measured de Haas–van Alphen oscillation in UPd 2Al 3 in order to reveal heavy electronic states at low temperatures down to 40 mK. For this purpose, we grew a high-quality single crystal using a purified uranium metal. All the branches reported in the previous work were observed in this study. In addition, we observed a new branch ζ with an extremely large cyclotron mass 65 m 0. The dHvA frequency 9.96×10 7 Oe for branch ζ coincides with the band calculations based on the itinerant 5f electron picture.

Observation of heavy electrons in UPd 2Al 3

We measured the de Haas–van Alphen oscillations in order to reveal heavy electronic states of UPd 2Al 3. All the branches reported in the previous work were observed in this study. In addition, we observed new branches. These branches are well explained by the results of fully relativistic spin-polarized LAPW band calculations. The cyclotron mass of each branch is large, indicating strong hybridization of 5f components.

Spin density wave of itinerant heavy electrons in Ce(Ru0.85Rh0.15)2Si2

The magnetic properties of Ce(Ru0.85Rh0.15)2Si2 were studied by neutron scattering and measurements of magnetization, susceptibility, specific heat and thermal expansion as a function of temperature. We observe a crossover from a high temperature localized spin to a low temperature heavy electron state. Spin density wave (SDW) behavior appears in the heavy electron state below TN = 5.5 K and the volume change due to spin quantum fluctuations associated with the SDW and the Kondo screening is reminiscent of moment-volume instabilities of the INVAR and anti-INVAR behavior of 3 d transition metal alloys.

Low-temperature properties of Ce(Ru 0.85Rh 0.15) 2Si 2 single crystals

Specific-heat measurements have been performed on single-crystalline samples of Ce(Ru 0.85Rh 0.15) 2Si 2 in various magnetic fields along the tetragonal c-axis up to 8 T in the tempereture range between 0.1 and 1.2 K. Ce(Ru 0.85Rh 0.15) 2Si 2 shows a weak antiferromagnetic ordering ( T N = 5.5 K) with an incommensurate spin structure. Two transitions ( B C ⋟ 3.5 T, B M ⋟ 5 T ) were observed in the magnetization process below T N. The electronic specific heat coefficient, γ, increases drastically above B C, and shows a subtle enhancement, then decreases gradually above B M. This suggests that a heavy-electron state is restored by magnetic fields, and further undergoes a metamagnetic transition similar to that of CeRu 2Si 2 at still higher fields.

Thermopower and electric resistivity of Ce1 − (La or Y) Ni0.8Pt0.2 Kondo system

Abstract The thermopower S of the Ce 1 − x (La or Y) x Ni 0.8 Pt 0.2 Kondo system was measured from 2 K to room temperature. A prominent change of S behaviour well correlated with the existing data of the electric resistivity ρ was observed. The evolutions of the Ce state from a coherent heavy-electron state to a dilute Kondo regime take place as a consequence of the dilution effect of Ce by Y or La atoms.

Aspects of Magnetism in Heavy-Electron Metals

For some time it was believed that heavy-mass electrons at low temperatures, in special rare-earth and actinide compounds, were due to a particular suppression of magnetic order in these materials. Various experiments reveal, however, that magnetic order may occur from a heavy-electron state and that also a heavy-electron state may develop in the presence of a magnetically-ordered matrix. Even in materials with no sign of a cooperative phase transition, microscopic experimental probes give evidence for strong magnetic correlations, involving only very tiny (< 10 -1 μn) ordered moments, however. All these experimental observations indicate that the heavy-electron state cannot always be regarded as a simple Fermi liquid in the usual sense. Various interactions of similar strength need to be considered. It is suggested that the balance of these interactions may lead to electronic inhomogeneities in k space. Both in magnetically-ordered or superconducting systems, unexpected electronic degrees of freedom appear to exist. In other cases experimental observations suggest magnetic inhomogeneities in real space which are not simply due to chemical inhomogeneities. Different examples of these various possibilities are demonstrated and discussed.

Formation of a coherent heavy electron state in the Anderson lattice model.

We present self-consistent calculations for the self-energy, thermodynamic potential, and magnetic susceptibility of the 2D asymmetric Anderson lattice Hamiltonian, in the fluctuation exchange approximation. For parameters that yield a large local moment, a narrow peak at the Fermi energy grows out of an otherwise smooth density of states as the system is cooled. The height of the peak reaches a maximum and then decreases, signaling the onset of coherence. The susceptibility shows a similar temperature dependence

Low-temperature magnetism in YbBiPt

PHYSICAL REVIE% B UOLUME 46, NUMBER 5 Low-temperature magnetism AUGUST 1992-I in YbBiPt A. Amato Institut fiir Mittelenergiephysik der Eidgenossische Technische Hochschule Ziirich, CH-5232 Villigen PSI, Switzerland P. C. Canfield Los Alamos National Laboratory, Los Alamos, New Mexico 87545 R. Feyerherm der Eidgenossische Technische Hochschule CH-5232 Villigen PSI, Switzerland Institut fu rMit'telenergiephysik Ziirich, Z. Fisk Los Alamos National Laboratory, Los Alamos, New Mexico 87545 F. N. Gygax der Eidgenossische Technisch Hochschule CH-5232 Villigen PSI, Switzerland Institut fiir Mittelenergiephysik Ziirich, R. H. Heffner Los Alamos National Laboratory, Los Alamos, New Mexico 87545 D. E. MacLaughlin* Kamerlingh Onnes Laboratorium, University of Leiden, 2300 RA Leiden, The Netherlands H. R. Ott Laboratorium fu'r FestkorperphysikEidge, nossische Technische Hochschule Honggerberg, CH-8093 Ziirich, Switzerland A. Schenck Institut fiir Mittelenergiephysik der Eidgenossische Technische Hochschule CH-5232 Villigen PSI, Switzerland Ziirich, J. D. Thompson Los Alamos National Laboratory, Los Alamos, New Mexico 87545 (Received 13 April 1992) Positive-muon (p ) spin-relaxation experiments have been carried out in a pressed-powder sample of the low-carrier-density heavy-electron and disordered system YbBiPt. Spatially inhomogeneous static Yb magnetism is observed below 5 K, with a strongly reduced Yb moment of -O. ling over -50% of the sample volume at T=0. 06 K. Substantial p+ spin-lattice relaxation, rarely observed in heavy-electron systems, suggests anomalously slow Yb spin fluctuations. Our data are reminiscent of p+ behavior in spin glasses, and raise the question of whether the large low-temperature specific heat in YbBiPt is due in part to low-lying magnetic excitations. The RBiPt (R =rare earth) series of ternary interme- tallic compounds exhibits a rather unique evolution from insulating to weak metallic behavior with increasing rare- earth atomic number. ' Among these materials YbBiPt is of particular interest. Measurements of the specific heat yield an cp is Aux-grown single crystals of this compound enormous Sommerfeld coefficient T=c~/T=8 Jmol K below 4 K, suggestive of an extremely massive itinerant-electron state. A low characteristic temperature is indicated by an entropy release of Rln2 between zero and l K. A cusp in the ac susceptibility at 0. 4 K indicates some sort of low-temperature magnetic phase which seems to coexist with the heavy-electron state. The specific heat of crushed powders of YbBiPt (Ref. 3) is nearly the same as in unstrained material above 0. 5 K, but below this temperature the cp/T ratio is reduced by ap- proximately 50% and there is no clear evidence for a mag- netic phase transition. These results indicate that YbBiPt is situated on the boundary between magnetic and heavy-electron behavior with respect to the Yb spin degrees of freedom. Questions are raised concerning (1) the stability of the Yb moment, (2) the nature of a heavy-electron state which can evolve from a low-carrier-density metal at high temperatures, and (3) the competition between Kondo singlet formation Yosida interactions. and Rudermann-Kittel-Kasuya- This paper presents the results of positive-muon spin ro- tation (p+SR) experiments which bear on the above @1992 The American Physical Society

New aspects in magnetism and superconductivity of heavy-electron compounds

Several new observations in experimental studies of heavy-electron materials are discussed. These include the sensitivity of the heavy-electron state to small changes in the chemical composition, the phenomenon of small-moment ordering, the formation of the heavy-electron state in magnetically ordered matrices, and new facts related with unconventional superconductivity in these materials.

Variation of ground states in CePt 2-xAu xSi 2 compounds (x = 0, 1, 2)

A comparison is made of some low-temperature properties of CePt 2Si 2, CePtAuSi 2 and CeAu 2Si 2. New experimental data on the specific heat, electrical resistivity and magnetic susceptibility of CePtAuSi 2, together with published results on the other compounds, reveal that the electronic ground state in these materials varies in a very significant way. In-between local-moment behaviour in CeAu 2Si 2 and a Kondo-type ground state in CePt 2Si 2, CePtAuSi 2 forms a heavy-electron state at low temperatures.

Low-temperature properties of U 14Au 51

Low-temperature measurements of the specific heat, the magnetic susceptibility, and the electrical resistivity on U 14Au 51 reveal that this compound orders antiferromagnetically and develops a heavy-electron state much below the Néel temperature T N = 22 K.

Magneto-Acoustic Effect of CeCu6at Low Temperatures

Low temperature properties of the elastic constants C 11 and C 33 in the heavy electron compound CeCu 6 have been investigated. The remarkable amount of softening below the Kondo temperature down to 20 mK reflects the heavy electron state at the Fermi surface. The oscillations of the C 33 mode under a magnetic field along the c axis have been found at low temperature ranges below 500 mK.

Spectroscopic evidence for an unrealized heavy-electron state in CeAl2.

The compound ${\mathrm{CeAl}}_{2}$ has been studied at low temperature (15 K) by high-resolution (l20 meV) photoemission using He i and He ii excitation sources. The spectra analysis within the Anderson single-impurity model for T=0 reveals that the extrapolated low-energy parameter \ensuremath{\delta}=0.2 meV is consistent with the analysis of the specific heat performed within the same formalism. The extreme narrowness of the density of low-lying excitations predicted for T=0 indicates that ${\mathrm{CeAl}}_{2}$ would be a heavy-electron metal if the magnetic ordering below 4 K did not prevent the formation of a coherent Kondo lattice.