Heavy Fermion Behavior Research Articles

Crystal Growth, Structure, and Physical Properties of Ln(Cu,Ga)13−x (Ln = La−Nd, Eu; x ≈ 0.2)

Single crystals of Ln(Cu,Ga)13−x (Ln = La−Nd, Eu; x ≈ 0.2) were grown using Ga flux and their structures determined by single-crystal X-ray diffraction. The Ln(Cu,Ga)13−x (Ln = La−Nd, Eu; x ≈ 0.2), adopting NaZn13 structure type, crystallizes in the cubic Fm3̅c (No. 226) space group, with Z = 8 and lattice parameters a ≈ 11.8 Å. Magnetic susceptibility and heat capacity measurements do not show any indication of long-range magnetic ordering down to 2 K for magnetic analogues. Metallic behavior is observed in the range of 2−300 K for each compound. A large positive magnetoresistance up to 154% at a field (μ0H) of 9 T is also observed for Pr(Cu,Ga)12.85(1). Most interestingly, the Pr analogue shows T2 temperature-dependent resistivity and satisfies Kadowaki-Woods relation, which is indicative of heavy-fermion behavior. Here, we present the crystal structures and physical properties of Ln(Cu,Ga)13−x (Ln = La−Nd, Eu; x ≈ 0.2).

Rare earth based bulk metallic glasses

Recently, the rare earth based bulk metallic glasses (REBMGs) have attracted increasing interest due to their unique properties and potential applications as functional glassy materials. These REBMGs display many fascinating properties such as heavy fermion behavior, thermoplastic properties near room temperature, excellent magnetocaloric effect, hard magnetism, and polyamorphism, all of which are of interest not only for basic research but also for metallurgy and technology. These characteristics and properties are ascribed to the unique electronic, magnetic and atomic structures of the REBMGs. In this review paper, the fabrication, glass-forming ability, polyamorphism, elastic, thermal, and physical properties are summarized and discussed. Owing to the unique electronic structure of rare earth elements, the electric and magnetic properties of the REBMGs are especially addressed. The works have implications for seeking novel metallic glasses with controllable properties and for understanding the nature of glass formation. The development of REBMGs as functional materials might promote and extend the commercial applications of metallic glasses.

Heavy fermion behavior inPrRh2B2C: Excitonic mass enhancement

We report magnetic and transport properties of a quaternary borocarbide PrRh2B2C based on magnetization, resistivity, and specific-heat studies. This compound forms in LuNi2B2C-type tetragonal structure (space group I4/mmm) and does not exhibit magnetic ordering or superconductivity down to 0.5 K. The crystal-field analysis of specific-heat data suggests a singlet ground state in this compound. The high value of the Sommerfeld coefficient, gamma approximate to 250 mJ/mole K-2, together with a singlet ground state suggests that the heavy fermion behavior in PrRh2B2C results from the interaction of the conduction electrons with the low-lying crystal-field excitations. No signature of magnetic ordering or superconductivity is observed in PrRh2B2C under the application of pressure up to 23 kbar.

Magnetic properties of the thulium layered compound Tm2AlB116: An AlB2-type analogue

Tm 2 Al B 11 6 with the Y2ReB6-type crystal structure was synthesized, and the magnetic properties, specific heat, and magnetic entropy were investigated. Recent developments in rare earth aluminoboride compounds with two-dimensional boron layers have attracted interest due to the heavy fermion behavior superconductivity discovered in YbAlB4 and the multiple anomalies manifesting below the Néel temperatures in TmAlB4 attributed to be due to intrinsic building defects. Tm2AlB116 has a strong structural analogy to the AlB2-type structure similar to the REAlB4 compounds. Multiple antiferromagnetic transitions were found in Tm2AlB116 at TN1=3.5 K and TN2=2.8 K at zero field. Interestingly, the magnetic field dependence of the two transitions differed sharply. A low field of 4 kOe suppresses the low temperature transition, whereas TN1 initially reduces with applied field but then exhibits a surprising robustness (i.e., steep phase boundary) versus the magnetic field above 20 kOe with the peak temperature hardly shifting up to 70 kOe. This indicates that a high field phase exists with extreme stability. The complex behavior is indicated to be influenced by different factors compared to related systems TmB4 and TmAlB4, e.g., crystal field effects or surprisingly strong magnetic interactions above TN1 indicated by the specific heat behavior.

Heavy-fermion like behavior of amorphous CexMn100−x alloys system studied by 55Mn NMR

In order to investigate a heavy-fermion like behavior on Ce-rich side of amorphous CexMn100−x alloys system, the 55Mn NMR measurements on the powdered sample of amorphous Ce62Y19Mn19, being diluted with non-magnetic Y, have been carried out in the temperature range from 1.8 to 270 K. A broadened resonance spectrum containing five NQR lines is observed in the measuring temperature region. Temperature dependences of Quadrupole coupling constant (e2Qq) and the line width of 55Mn NMR are obtained by means of analysis of each spectrum. Temperature dependence of the line width is expressed by the Curie-Weiss law with Tc = −12.0 K. The 1/(T1T) is rapidly increased in proportion to inverse square root of temperature with decreasing temperature, but it keeps almost constant below about 6 K, which shows the heavy-fermion state in Ce62Y19Mn19.

High-field ESR study of the Kondo lattice system YbRh2Si2

A multi-frequency electron spin resonance (ESR) study of the heavy fermion compound YbRh2Si2 has been performed in fields up to ∼ 8T. In this regime the system is close to the breakdown of the heavy-fermion behavior which is confined to temperatures and fields T <T0 ≈25K and B < B* ≈10T, respectively. A well-defined strongly anisotropic ESR mode is observed. We find striking similarities between the properties of this signal and the behavior of the electronic specific heat and the 29Si NMR data which reflect the crossover between different electronic regimes in the Kondo state. These similarities suggest that the observed ESR mode corresponds to a "heavy electron spin resonance", i.e., a joint resonance excitation of the strongly hybridized f- and conduction electron states.

Effective Hamiltonian of a three-orbital Hubbard model on the pyrochlore lattice: Application toLiV2O4

To investigate heavy fermion behavior in the vanadium spinel ${\text{LiV}}_{2}{\text{O}}_{4}$, we start from a three-orbital Hubbard model on the pyrochlore lattice and derive its low-energy effective Hamiltonian by an approach of real-space renormalization-group type. We first derive the effective Hamiltonian numerically and then succeed in representing the results into an analytic form with physical operators for low-energy degrees of freedom in tetrahedron unit. The effective Hamiltonian is defined on the coarse-grained lattice, i.e., face-centered-cubic (fcc) lattice, and it operates in a restricted Hilbert space defined in terms of a specific molecular orbital ${T}_{2}$ in the unit. One important tetrahedron configuration has a threefold orbital degeneracy and spin $S=1$, and correspondingly, the effective Hamiltonian has spin and orbital exchange interactions of Kugel-Khomskii type as well as correlated electron hoppings. The coupling constants in the effective Hamiltonian are determined from the numerically obtained renormalized Hamiltonian and also by means of perturbation. We calculate and analyze low-energy states of the effective Hamiltonian for the unit of four coupled tetrahedra both analytically and numerically. Effective hopping elements in the effective Hamiltonian are renormalized to about 1/10 of the original hopping integral. It is important that different virtual processes make opposite contributions to the exchange term, and consequently the coupling constant is given by a remaining small value. This is particularly prominent in the spin-spin channel, where ferromagnetic double-exchange processes compete with antiferromagnetic superexchange processes. Another important point is that various spin and orbital exchange processes are competing to each other. Together with geometrical frustration of the effective fcc lattice, these two features result in nearly degenerate three lowest-energy states of different types in the four coupled tetrahedra, and each of the three has a finite degeneracy in spin and/or orbital. We also calculate spatial correlations of spin and orbital and found that short-range spin-spin correlations are strongly entangled with orbital configurations. This indicates that large remaining entropy at low temperature is related to slow coupled fluctuations of spin and orbital. These results suggest the absence of phase transition in spin and orbital spaces down to very low temperatures and their large fluctuations in the low-energy sector, which are key issues for understanding the heavy fermion behavior in ${\text{LiV}}_{2}{\text{O}}_{4}$.

Anomalous Duality of4fElectrons in Filled SkutteruditeCeOs4Sb12

We study the electronic structure of the filled skutterudite CeOs4Sb12 using photoemission spectroscopy (PES). Soft x-ray excited Ce 3d-4f resonant PES confirms the existence of Ce 4f states at the Fermi level (EF). Temperature dependent high-resolution laser-PES spectra reveal a pseudogap formation around EF, which can be explained in terms of the hybridization gap. Simultaneously, a sharp feature is formed just above EF with decreasing temperature. The heavy-fermion-like specific heat is attributed to the occupation of this feature. The results identify the origin of the anomalous coexistence of heavy-fermion and pseudogap behavior in terms of a symmetry dependent hybridization.

Hybridization Phenomena in Nearly-Half-Filledf-Shell Electron Systems: Photoemission Study ofEuNi2P2

The mixed-valent compound EuNi2P2 was studied by photoemission. Observed splittings and dispersions of the Eu 4f;{6} final state close to energy crossings of the Eu 4f and Ni 3d states are explained in terms of hybridization by a momentum and energy dependence of the electron hopping matrix element. These data obtained for a system with more than one 4f electron (hole) show that dispersions and hybridization gaps related to Kondo and heavy-fermion behavior can be found in other rare-earth-metal compounds apart from Ce and Yb-based ones.

Strongly correlated electrons on frustrated lattices

We review our two recent theoretical works on strongly correlated electrons on typical frustrated lattices. The first topic is about a Mott transition in the single-band Hubbard model on anisotropic triangular lattice, and we discuss a reentrant behaviour of metal-insulator transition, consistent with that in a κ-type BEDT-TTF salt. The second topic is about heavy fermion behaviour in the vanadium spinel LiV2O4. We study the 3-orbital t2g Hubbard model on the pyrochlore lattice at quarter filling and derive its low-energy effective model. The correlations of spin and orbital degrees of freedom are discussed.

Deposition of superconducting CeCoIn 5 thin films by co-sputtering and evaporation

Thin superconducting films of CeCoIn 5 were prepared in situ by simultaneous thermal evaporation of indium and dc magnetic field assisted sputtering of planar metallic Ce and Co targets. To achieve an effective sputtering of the magnetic Co target a special geometry with two facing planar targets (Ce and Co) and magnetic field perpendicular to the targets was used. The stoichiometric (0 0 1)-oriented CeCoIn 5 films were grown on r-cut sapphire substrates with a high-rate of 100 nm/min. The temperature dependence of the electrical resistivity revealed the characteristic heavy-fermion behavior and a superconducting transition at about 2 K in agreement with the literature data for CeCoIn 5 bulk material and thin films.

Current understanding of photoelectron spectra in plutonium systems

Analysis of available photoelectron spectra from Pu compounds indicates the presence of the 5 f 5 multiplet, which dominates the energy scale close to the Fermi level. As shown by the LDA+U+Hubbard I calculations, its intensity scales with the admixture of the 5 f 6 manifold in the 5 f 5 ground state, which can lead to an average occupancy substantially exceeding 5.0. This admixture also leads to the suppression of magnetic order and occurrence of heavy fermion behaviour.

XPS and thermomagnetic characterization of the CeNi 4Cr compound

The magnetic, thermodynamic and electronic structure properties are discussed for the CeNi 4Cr compound. The X-ray photoemission spectra (XPS) provide an evidence of a mixed valence behavior with the occupancy of the f states n f=0.89 and their hybridization with the conduction electrons Δ=30 meV. These values reproduce well the magnetic susceptibility χ( T=0), which is enhanced compared to similar CeNi 4M (M=Al, B, Cu) compounds. In combination with a slightly increased electronic specific heat coefficient (up to 100 mJ mol −1 K −2), this compound can be classified as being on the border of the heavy fermion and mixed valence behavior. Using a small magnetic field in the χ( T) measurements reveals a presence of magnetically ordered impurity phase, which is easily damped by higher fields and it is shown that the contribution of this phase is minor. The question of the dependence of the electronic specific heat coefficient on the magnetic field is also addressed and the observations agree well with theoretical predictions based on the Anderson model.

Effect of non-Fermi liquid behavior on the temperature dependence of the amplitude of de Haas-van Alphen oscillations

We calculate the amplitudes of de Haas-van Alphen oscillations for a system of heavy fermions close to a quantum critical point (QCP). A nested Fermi surface, which consists of an electron and a hole pocket, together with the remaining interaction between the carriers after the heavy particles are formed, leads to itinerant antiferromagnetism. The order can be gradually suppressed by increasing the mismatch between the Fermi momenta, and a quantum critical point is obtained as ${T}_{N}\ensuremath{\rightarrow}0$, giving rise to a heavy-fermion non-Fermi liquid behavior in the specific heat, quasiparticle linewidth, and resistivity. The Lifshitz--Kosevich expression for the de Haas-van Alphen oscillations is modified by the quasiparticle self-energy. The amplitudes are considerably reduced by the interactions. The magnetic field and temperature dependence of the amplitudes shows that the effects of the QCP extend over a large $T$ interval.

Synthesis and magnetic properties of the ThMoB4-type modification of ErAlB4

β - Er Al B 4 with the ThMoB4-type crystal structure, which is a polytype to the YCrB4-type α-crystal structure, was synthesized, and the magnetic properties and specific heat were investigated. Recent developments in the REAlB4 compounds, where RE denotes rare earth, have attracted interest due to the heavy fermion behavior observed for β-YbAlB4 and the multiple anomalies manifesting below the Néel temperatures in α-TmAlB4 attributed to be due to intrinsic building defects. Multiple antiferromagnetic transitions were found for β-ErAlB4 at TN1≈18K and TN2=4.3K. The absence of low temperature anomalies in the magnetic properties and specific heat of β-ErAlB4 below the Néel temperatures indicates that the generation of intrinsic building defects is not a general phenomenon for compounds with the YCrB4 and ThMoB4-type crystal structures but is rather due to a special circumstance of TmAlB4 having close energies in the structure formation.

Thermophysical properties of Nd-, Er-, YNi-alloys

Abstract Nd – Fe – B alloys are widely used in many technical applications due to their excellent permanent magnetic properties. YNi2B2C intermetallic compounds receive much attention because of the coexistence of superconductivity and magnetic ordering phenomena. Er2PdSi3 alloys with hexagonal AlB2-type crystallographic structure are of special interest because of their complex magnetic ordering, Kondo and heavy fermion behavior. The thermophysical properties (electrical conductivity, thermoelectric power, thermal conductivity and viscosity) of these liquid multicomponent intermetallic alloys have been measured in order to supply necessary information for crystal growth and casting processes. Good reproducibility of the results has been obtained.

Exploring Characteristic Electronic States in Filled Skutterudites by Ultrasonic Measurements –Multipole Ordered State in SmRu4P12–

Filled skutterudites show various interesting physical properties, such as unconventional superconductivity, anomalous magnetism, metal–insulator transition, heavy fermion behavior and so on. We have investigated the multipole ordering state of SmRu 4 P 12 by ultrasonic measurements under extreme conditions of low temperature, high pressure and high magnetic field by pulsed method, so far. Recently, the angle dependence of the elastic constant has been measured. The elastic constant shows interesting and peculiar angle dependence below the metal–insulator transition temperature in the magnetic field. It will enable us to reveal characteristic multipole ordered state in SmRu 4 P 12 . In this paper, we will report recent status of the multipole ordered state of SmRu 4 P 12 , and discuss the recent results of our elastic constant measurements and the relation to the octupole ordering scenario proposed for this system.

Finite-Temperature Mott Transition in Two-Dimensional Frustrated Hubbard Models

We investigate the Hubbard model on two typical frustrated lattices in two dimensions, the kagome lattice and the anisotropic triangular lattice, by means of the cellular dynamical mean field theory. We show that the metallic phase is stabilized up to fairly large Hubbard interactions under strong geometrical frustration in both cases, which results in heavy fermion behavior and several anomalous properties around the Mott transition point. In particular, for the anisotropic triangular lattice, we find novel reentrant behavior in the Mott transition in the moderately frustrated parameter regime, which is caused by the competition between Fermi-liquid formation and magnetic correlations. It is demonstrated that the reentrant behavior is a generic feature inherent in the Mott transition with intermediate geometrical frustration, and indeed in accordance with recent experimental findings for organic materials.

Understanding the heavy fermion behavior inCeInPt4

The cubic compound $\mathrm{Ce}\mathrm{In}{\mathrm{Pt}}_{4}$ exhibits heavy fermion behavior with a high extrapolated value of the electronic specific heat, $\ensuremath{\gamma}=2.5\phantom{\rule{0.3em}{0ex}}\mathrm{J}∕\text{mole}\phantom{\rule{0.2em}{0ex}}{\mathrm{K}}^{2}$ as $T\ensuremath{\rightarrow}0\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. The specific heat does not exhibit any sign of long range magnetic ordering down to $100\phantom{\rule{0.3em}{0ex}}\mathrm{mK}$. In order to understand the origin of the high value of $\ensuremath{\gamma}$ and the nature of the $4f$ electrons of this compound, we have carried out neutron diffraction, low-field magnetic susceptibility, muon spin relaxation, and inelastic neutron scattering measurements on $\mathrm{Ce}\mathrm{In}{\mathrm{Pt}}_{4}$. Our susceptibility results show that for temperatures between 20 and $1.5\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, a power law behavior is exhibited, $\ensuremath{\chi}(T)\ensuremath{\sim}{T}^{\ensuremath{-}\ensuremath{\beta}}$ with $\ensuremath{\beta}=0.5$. Below $1.5\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, the susceptibility is almost temperature independent, again without any sign of magnetic ordering down to the lowest available temperature $(300\phantom{\rule{0.3em}{0ex}}\mathrm{mK})$. The muon spin relaxation measurements reveal that below $1\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, the electronic relaxation rate strongly increases without any loss of muon initial asymmetry, indicating the presence of low energy spin fluctuations as an explanation for the high value of $\ensuremath{\gamma}$ in $\mathrm{Ce}\mathrm{In}{\mathrm{Pt}}_{4}$. Heat capacity data reveal a $\mathrm{log}(C∕T)\ensuremath{\propto}\mathrm{log}(T)$ behavior, indicating that $\mathrm{Ce}\mathrm{In}{\mathrm{Pt}}_{4}$ may exhibit non-Fermi-liquid behavior close to a $T\ensuremath{\rightarrow}0\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ quantum critical point. Our inelastic neutron scattering results reveal a broad crystal field excitation centered at $25\phantom{\rule{0.3em}{0ex}}\mathrm{meV}$, indicating the presence of strong hybridization between the $4f$ and the conduction electrons, which is consistent with the observed high value of the paramagnetic Curie temperature $({\ensuremath{\theta}}_{p}=\ensuremath{-}255\phantom{\rule{0.3em}{0ex}}\mathrm{K})$.

Neutron diffraction study of magnetic field induced behavior in the heavy Fermion Ce 3Co 4Sn 13

The specific heat of Ce 3Co 4Sn 13 exhibits a crossover from heavy Fermion behavior with antiferromagnetic correlations at low field to single impurity Kondo behavior above 2 T. We have performed neutron diffraction measurements in magnetic fields up to 6 T on single crystal samples. The (0 0 1) position shows a dramatic increase in intensity in field which appears to arise from static polarization of the 4f level and which at 0.14 K also exhibits an anomaly near 2 T reflecting the crossover to single impurity behavior.