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Heavy-fermion behavior in a pseudobinary system: U(In1-xSnx)3
Abstract
The temperature dependence of the magnetic susceptibility, the low-temperature specific heat, and the field dependence of the high-field magnetization were measured for the pseudobinary system U(In1-xSnx)3 for 0x1. The results show a clear evolution from long-range antiferromagnetism for x<0.45 (In rich) to a heavy-fermion region for 0.45x0.80 and to highly enhanced paramagnetism for x>0.8 (Sn rich). These results represent the first systematic study of the onset of the salient features associated with heavy-fermion behavior in a pseudobinary system.
Summary This document is part of Subvolume F2 ‘Actinide Elements and their Compounds with other Elements. Part 2’ of Volume 19 ‘Magnetic Properties of Metals’ of Landolt-Börnstein - Group III Condensed Matter.
Summary This document is part of Subvolume F2 ‘Actinide Elements and their Compounds with other Elements. Part 2’ of Volume 19 ‘Magnetic Properties of Metals’ of Landolt-Börnstein - Group III Condensed Matter. Substances contained in this document (element systems and chemical formulae) In-Np: NpIn3. In-Th: Th-In. In-U: UIn3.
Summary This document is part of Subvolume F2 ‘Actinide Elements and their Compounds with other Elements. Part 2’ of Volume 19 ‘Magnetic Properties of Metals’ of Landolt-Börnstein - Group III Condensed Matter. Substances contained in this document (element systems and chemical formulae) Al-Ce: CeAl3. Al-Ni: Ni3Al. Al-U: UAl2. Al-U: UAl3. Au-Pt-U: UPt5-xAux. Be-Np: NpBe13. Be-Pu: PuBe13. Cd-U: UCd11. Ce-Cu-Si: CeCu2Si2. Ce-Cu: CeCu6. Ce-Pd: CePd3. Ce-Sn: CeSn3. Co: Co. Ga-U: UGa3. In-Sn-U: U(In1-xSnx)3. In-U: UIn3. Np-Sn: NpSn3. Sn-U: USn3.
Summary This document is part of Subvolume F2 ‘Actinide Elements and their Compounds with other Elements. Part 2’ of Volume 19 ‘Magnetic Properties of Metals’ of Landolt-Börnstein - Group III Condensed Matter. Substances contained in this document (element systems and chemical formulae) Al-U: UAl3. Ga-Sn-U: U(Sn1-xGax)3. In-Sn-U: U(In1-xSnx)3. In-U: UIn3. La-Sn-U: UxLa1-xSn3. Np-Sn: NpSn3. Pb-U: UPb3. Pu-Sn: PuSn3. Si-U: USi3. Sn-Th: Th-Sn. Sn-Th: ThSn3. Sn-U: U3Sn5. Sn-U: USn3.
In the older literature some methods for the preparation of uranium —aluminium alloys are reported but no methods for the preparation of single-phase U — Al compounds are presented (see “Aluminium” A5, 1937, pp. 885/6). In the present volume, the newer preparation and characterization work since 1948 is reported.
The alloying of U with Sn is favored by energy effects as is shown by means of a cellular model, which relates these effects to the atomic cells of the pure constituents [1]. The alloying effects originate from the change in boundary conditions when an atom is transferred from the pure metal to the alloy. The energy effects are determined by two terms. The first one represents the difference in the electronegativity between the two types of atoms in an alloy. The second one reflects the discontinuity in the density of electrons at the boundary between dissimilar Wigner-Seitz atomic cells. The electronegativity is similar to the experimental work functions of pure metallic elements. The heat of solution, \(
\Delta {\bar H_{\mathop M\limits^ \circ }}
\), of U in liquid Sn is calculated using a model [2]. The value of \(
\Delta {\bar H_{\mathop M\limits^ \circ }}
\)
= -13 kcal/g-atom indicates that there is a clear tendency to form alloys in the liquid state. The same authors applied the model to predict the alloying behavior in the solid state [3]. They found that the model predictions of values for the heat of formation agree fairly well with much recent experimental data of intermetallic compounds, including some U compounds [4]. The model is able to predict the thermodynamic stability of several equiatomic compounds of a transition metal and Sn [5]. Tables for a variety of transition and nontransition elements have been established by applying a computer program in Algol 60. The value for the infinitely dilute solution of U in Sn is \(
\Delta {\bar H^ \circ }
\) = -99 kJ/g-atom (solute) and \(
\Delta {\bar H^ \circ }
\) = -84 kJ/g-atom for Sn in liquid U [4, 6].
Summary This document is part of Subvolume D ‘Se - Ti’ of Volume 21 ‘Superconductors: Transition Temperatures and Characterization of Elements, Alloys and Compounds’ of Landolt-Börnstein - Group III Condensed Matter.
A new U-based ternary compound UCr2Al20 was
synthesized. The compound was characterized by X-ray diffraction,
specific heat, magnetization and resistivity measurements. The low
temperature electronic specific heat coefficient γ is estimated to
be γ≃80 mJ/mol\cdotK2. The magnetic susceptibility
is slightly temperature dependent with the value χ≃2×
10-3 emu/mol. The ratio χ/χcal, where
χcal is calculated from γ, is about 2, which
suggests that the compound is a magnetically exchange-enhanced system.
The electronic structure of Rh, Pt, In, and Sn in the mixed-valence systems Eu(Rh1−x
Ptx)2 and U(In1−x
Snx)3 has been studied by the x-ray K line-shift method. It has been found that the occupation of the Rh 4d-shell in Eu(Rh1−x
Ptx)2 is higher than that in the metal, and that it grows with decreasing Eu valence (i.e., with increasing 4f-shell occupation). The electronic structure of Pt, In, and Sn in Eu(Rh1−x
Ptx)2 and U(In1−x
Snx)3 does not depend on the Eu and U valence and is practically the same as in the metals. These features in the electronic structure of Rh, Pt, In, and Sn in Eu(Rh1−x
Ptx)2 and U(In1−x
Snx)3 suggest that the electron released in the f
n
→ f
n
−1+e transitions, rather than transferring to the common conduction band, remains localized at the Eu and U atoms.
For sufficiently strong scattering by randomly-distributed imperfections
the integrated intensity of a wave initiated at some starting point
becomes localized within some finite distance. We discuss how this
Anderson localization can be self-induced, with possible enhancement by
magnetic ordering, in light actinide systems. This mechanism provides an
ab initio based prediction, in close agreement with experiment, of the
variation of the magnetic ordering temperatures and low-temperature
ordered moments in a number of uranium compounds which are driven by
alloying through a phase transition from non-magnetic behaviour to
strong magnetic ordering. This mechanism also explains the phase
transition to the delta face-centred-cubic (fcc) structure at 592 K in
elemental plutonium, which has a low-temperature alpha monoclinic
structure, and the substantial depression of melting temperature of
plutonium and neptunium with respect to neighbouring elements. Both the
strongly magnetically-ordered uranium systems and elemental plutonium in
the fcc delta phase are described by the 5f electronic behaviour of a
random-localized-fluctuating-site (RLFS) solid-solution-like phase. The
physical picture developed here shows that hybridization treated via the
CoqblinSchrieffer resonant-scattering point of view (pertinent in the
weak hybridization regime) provides the physical connection (under
certain conditions described herein, an actual phase transition) between
localized (i.e. coupled magnetic ions) magnetism and strongly-correlated
extremely-narrow band behaviour characterized as heavy fermion behaviour
in solids. Furthermore, the overall physical picture thus provided for
the intermediate delocalized regime of transition-shell electron
behaviour (comprising the two subregimes of RLFS random
solidsolution-like behaviour and extreme-narrow-band heavy fermion
behaviour, respectively) provides the physical connection between
localized (e.g. heavy rare earth) and itinerant (e.g. nickel) magnetism.
We have carried out measurements of the magnetic susceptibility, M{umlt o}ssbauer spectra, and specific heat on Gd(Sn{sub 1-{ital x}}In{sub {ital x}})â. We observe an oscillatory variation of the antiferromagnetic transition temperature {ital T}{sub {ital N}1} across the series from GdSnâ ({ital x}=0.0) to GdInâ ({ital x}=1.0). For the range of In concentrations 0.265â¤{ital x}â¤0.565, we find a second magnetic transition, at a temperature {ital T}{sub {ital N}2} well below {ital T}{sub {ital N}1}. These measurements along with the temperature dependence of ¹¹â¹Sn M{umlt o}ssbauer spectra indicate that the ground-state magnetic structure is a type-I antiferromagnetic ordering for the Sn-rich side (0â¤{ital x}â¤0.25) and is a type-II for the very rich In side ({ital x}â1.0). For the range of In concentrations 0.265â¤{ital x}â¤0.565 the magnetic structure is also of type I in the temperature interval {ital T}{sub {ital N}2}>{ital T}>{ital T}{sub {ital N}1}. The magnetic structure for 0.265â¤{ital x}â¤0.565 with {ital T}>{ital T}{sub {ital N}2} and also for 0.565>{ital x}>1.0 with {ital T}>{ital T}{sub {ital N}1} could be a canted antiferromagnetic ordering state. {copyright} {ital 1996 The American Physical Society.}
119Sn Mössbauer spectroscopy and magnetic susceptibility measurements have been performed on the quasi-binary system USn3-xInx. USn3 exhibits a remarkably highly temperature-dependent asymmetry of the quadrupole doublet, which we attribute to the existence of spin fluctuations. Antiferromagnetic ordering (TN~50 K) in USnIn2 indicates quenching of the fluctuations of the U-moments.
This chapter discusses the intermetallic compounds of actinides, and summarizes the electronic properties of actinide intermetallic compounds with a special emphasis on the magnetic behavior. The sequentially increasing occupation of 5f states dominates the electronic properties in the series of actinide elements. In the heavy actinides, on the other hand, the decrease of the energy due to the formation of a spin polarized (non-bonding) electronic state favors the localization of the 5f states. The specific nature of the 5f states in a particular actinide element has consequences for its magnetic and other electronic properties. The chapter also discusses the properties of binary and ternary compounds, the influence of the incorporation of H and B in intermetallic compounds, morphous and icosahedral compounds, and trans-thorium compounds where the 5f-electrons play a dominant role. Materials involving Th are included mainly in cases where they can serve as suitable "background" compounds with unoccupied 5f states.
The recent discovery of anomalous properties associated with extremely narrow f-band character in the electronic density of states in the vicinity of the Fermi energy has stimulated considerable interest in these systems. Some particularly exciting recent discoveries have emerged from studies of the magnetic and electronic properties of cerium- and uranium-based Cu3Au intermetallic compounds which form with group IIIB and group IVB elements. It has been reported that CePb3 is a heavy fermion antiferromagnetic system with a Néel temperature of 1.1K. In addition, this system displays field-induced superconductivity for magnetic fields greater than 15 T and temperatures below 0.5K. Studies of (Ce,La)Pb3 clearly indicate that the heavy fermion behavior reported for this system is primarily a reflection of single-ion Kondo behavior. In addition, studies of U(In,Sn)3 and Ce(In,Sn)3 explicitly indicate that the heavy fermion and mixed-valent behavior reported for these systems are a result of a similar hybridization-driven mechanism. The CeX3 and UX3 systems where X is a group IIIB or group IVB element display many of the features of current interest to magnetic moment formation and allow a unique opportunity to study these phenomena systematically.
The phase equilibria in the ternary system U-Au-Sn have been established in the isothermal section at 750 degrees C. The existence and magnetic properties of the previously reported ternary compounds have been confirmed: UAuSn (hexagonal CaIn2 structure type, a = 4.729(1) Angstrom, c = 7.219(3) Angstrom, antiferromagnet T-N = 35 K), U3Au3Sn4 (cubic Y3Au3Sb4 structure type, a = 9.819(1) Angstrom, paramagnet), and UAu2Sn which exhibits a polymorphic transition from a high temperature cubic MnCu2Al structure type, a = 6.983(2) Angstrom, paramagnet) to a low temperature (<820 degrees C) hexagonal ZrPt2Al structure type (a = 4.719(2) c = 9.202(2) Angstrom, antiferromagnet, T-N = 13 K). This high temperature form is stabilised in the gold-rich part of the homogeneity region UAu2+xSn1-x so that it is also observable at 750 degrees C between the compositions 0.1 < x < 0.33. The binary compounds of the systems U-Au and U-Sn admit significant solubility of Sn and Au, respectively. For UAu2, this solubility extends up to a composition UAu1.55Sn0.45, and superstructure reflections appear for higher Au substitutions, up to the limiting formula corresponding to the ternary composition U3Au4Sn2 which is considered as being a new phase in this system.
New systematics of U-based metallic compounds is proposed in terms of the radius of the U ion, or equivalently, the relative interatomic distance between U and ligand atoms, evaluated with reference to the corresponding isomorphous rare-earth compounds, and the widely varying physical properties of these compounds are discussed in view of this systematics.
The electronic structure of U and Ge in the solid solutions U(Al1−x
Gex)3 is investigated by measuring x-ray line shifts. It is shown that uranium has the mixed valence U3+ [Rn](5f
3)-U4+ [Rn](5f
2) over the entire composition range (0⩽x⩽1) and that the population of the uranium 5f shell increases by ∼0.28 5f electrons/U atom from UAl3 (x=0) to UGe3 (x=1). The electronic structure of Ge is close to the electronic structure of Ge metal over the entire composition range 0<x⩽1. No variation of the population of the Ge 4p shell is detected to within the experimental error (∼0.1 4p electrons/Ge atom) as the composition varies from x=0.2 to 1. It is established that the delocalization of a U 5f electron occurs as a result of its transition to the s or d band of the same uranium atom.
The temperature dependences of Mössbauer spectra, magnetic susceptibility and specific heat have been measured for U(In1−xSnx)3 and U(Pb1−xSnx)3. The low temperature 119Sn Mössbauer spectra of U(Pb1−x )3for 0.02 ≤ x ≤ 0.85 and U(In1−xSnx)3 for 0.02 ≤ x ≤ 0.6 show the existence of a magnetic hyperfine field at the Sn sites. The results for both systems indicate that the magnetic structures of the Pb-rich and In-rich pseudobinary alloys is incompatible with the magnetic structures previously proposed on the basis of neutron diffraction data for pure UPb3 and Ulp3. An identical magnetic structure is proposed for both systems, having a magnetic unit cell of (2a, 2a, 2a) (where a is the lattice constant of the crystal unit cell) and in which the U magnetic moments alternate between the eight possible [±1, ±1, ±1] directions. The new structure is consistent with both neutron diffraction and Mössbauer results.
By applying perturbed angular-correlation spectroscopy we have investigated spin correlations in the “heavy-fermion” compounds U(In1-xSnx)3, with x=1.0, 0.7, and 0.5, by measuring the induced magnetic hyperfine field Bind at diamagnetic 111Cd probe nuclei as a function of temperature and applied magnetic field. In zero applied field, the absence of any detectable magnetic hyperfine field at Cd reveals the absence of static magnetic correlations down to 4.2 K. However, from the field dependence of Bind we find evidence for the presence of field-induced, short-ranged, and dynamic spin correlation between U f electrons at all compositions. The strength and dynamics of these correlated spins strongly depend on x, temperature, and applied magnetic field. As an important feature, for compositions near x=0.5, classified as a heavy-fermion material with electronic specific heat coefficient γ=500mJ/mol K2, these U spin correlations seem to set in from a relatively high temperature (>~37 K), and become very large on lowering temperature and/or increasing magnetic field, reflected in the measured Knight shift value K of about -32% at 4.2 K and Bapp=7 T. We believe that these short-range spin correlations and their relaxation dynamics are responsible for the low temperature increase in magnetic susceptibility and electronic specific heat, previously considered to be an indication of heavy-fermion behavior in this system.
The volume dependence of the magnetic properties of U(In1-xSnx)3, with x=0.2 and 0.4, has been studied using 119Sn nuclear forward scattering of synchrotron radiation and Mössbauer spectroscopy at high pressures up to 25 GPa. The results show that in U(In0.8Sn0.2)3 the 5f magnetic moment is almost localized. Pressure induces an increase of the Néel temperature, while the transferred magnetic hyperfine field shows no change. Conversely, in U(In0.6Sn0.4)3 the transferred field decreases monotonically with increasing pressure and the Néel temperature goes through a maximum, showing a clear delocalization of the 5f electrons. This is discussed in terms of 5f-ligand hybridization and appears to lead to the formation of a high-pressure state characterized by strong dynamical spin correlations.
The pressure–temperature dependence of the electronic and magnetic properties of the compounds U (In1−xSnx)3 and UNiSn has been investigated by means of high pressure x-ray diffraction, 119Sn nuclear forward scattering (NFS) of synchrotron radiation and Mössbauer spectroscopy (MS) measurements. We show that pressure has different effects on these systems: while U(In0.8Sn0.2)3 shows some of the typical properties of nearly localized 5f systems, pressure induces the delocalization of the U 5f electrons in U(In0.6Sn0.4)3 and UNiSn with a pressure dependent interplay between RKKY exchange interaction and the consequent collapse of the magnetic order. These results are discussed in terms of hybridization between U 5f electrons and conduction band electrons such as U 6d, Sn/In 5sp and Ni 3d electrons.
X-ray powder diffraction, magnetization, electrical resistivity and neutron diffraction experiments were performed on the dense Kondo system U2 (Ni1-x Ptx )2 In. Up to x 0.5 the alloys crystallize in a tetragonal structure with space group P 4/mbm while for x 0.6 they crystallize in space group P 42 /mnm . The observed structural change coincides with the magnetic-nonmagnetic crossover in the system. In the concentration range 0 x 0.5, an antiferromagnetic ordering occurs, characterized by a noncollinear arrangement of the magnetic uranium moments in the tetragonal basal plane. In addition, for x = 0 and 0.2 a commensurate magnetic ordering is formed with a propagation vector k = (0,0,0.5) while for x = 0.5 an incommensurate one with k = (0,0,0.5± ), = 0.06 is found. In the concentration range 0.6 x 1.0 no magnetic ordering down to 1.7 K is observed. The bulk and neutron scattering data exhibit features attributed to the interplay of the Kondo effect and magnetic exchange interactions. For interpretation of the magnetic behaviour of the U2 (Ni1-x Ptx )2 In alloys, a strong modification of the density of states at the Fermi surface has been invoked.
The temperature and concentration dependence of the 119Sn Mössbauer spectrum has been measured for U(In1-xSnx)3. UIn3 has an antiferromagnetic transition at 91.5 K determined using both Mössbauer and specific heat measurements, and USn3 is a highly enhanced paramagnet. As Sn is substituted for In in UIn3, the Néel temperature TN (as determined using the temperature dependence of the magnetic susceptibility) is depressed and appears to go to zero between x = 0.4 and x = 0.5. However, the magnetic hyperfine field determined by Mössbauer spectroscopy persists to higher Sn concentration and appears to go to zero in the vicinity of x = 0.6. The difference in magnetic-nonmagnetic boundary is attributed to the presence of short-range order as the system approaches the magnetic instability. Short-range correlation in the vicinity of x = 0.6 appear to be responsible for the apparent heavy-fermion behavior previously reported for this system. In addition, the results of the 119Sn Mössbauer studies for U(In, Sn)3 are inconsistent with the magnetic structures previously proposed to explain neutron diffraction studies and a new magnetic structure is proposed which is consistent with both the neutron and Mössbauer studies.
Magnetic, thermal and transport measurements on the pseudo-ternary system CeNi1−xCuxSn (0 ≤ x ≤ 0.5) demonstrate that a valence-fluctuating state with a pseudo-gap for x = 0 evolves to an antiferromagnetic Kondo state for x ≥ 0.13 through a heavy-fermion region. The energy gap for x = 0 was found to be smeared out not only by the partial substitution of about 10% of Cu but also by magnetic fields higher than 20 T.
A scheme is presented in order to obtain complete information on atomic short range order in crystalline materials based on
measuring the electric-field gradient on a probe nucleus. Limitations and possible improvements of the method are discussed.
When applied to U(In0.5Sn0.5)3, short range order with In-Sn attraction is found.
UGa 3 is an antiferromagnetic system with a Néel temperature T N =67 K and UGe 3 is a weakly paramagnetic system with a nearly temperature‐independent magnetic susceptibility and a slightly enhanced low‐temperature electronic specific heat coefficient of 20 mJ/mole K<sup>2</sup>. The room‐temperature lattice constant and the temperature dependence of the magnetic susceptibility, electrical resistivity, and low‐temperature specific heat have been measured for the pseudobinary alloy system, U(Ga 1-x Ge x ) 3 . T N is rapidly depressed with increasing x and appears to go to zero at x≂0.18. The magnetic susceptibility remains nearly temperature independent across the series and neither the magnetic susceptibility nor low‐temperature specific heat display any significant enhancement in the paramagnetic regime.
Results of 5d-5f resonant photoemission studies on the related series of uranium intermetallic compounds, U(SnxIn1−x)3 for x = 1.0, 0.5 and 0.0 are presented. Various features seen in the spectra are analyzed in terms of calculated partial densities of states. With the help of difference spectra between on and off resonance spectra obtained at photon energies of 98 and 92 eV, respectively, it is shown that spectral shapes of the U 5f related features do not exhibit any significant change within the series, even though the electronic and magnetic properties of these compounds are drastically different. A brief discussion of the implications of these observations is also presented.
Hybridization of the f electrons with the conduction electrons and f electrons on adjacent ions drives the rich non-magnetic to magnetic behavior observed within the lighter actinides and intermetallic compounds containing these elements. Within and near this transition region, many of the very challenging problems associated with narrow band phenomena occur, e.g. strong spin fluctuations, spin glass behavior, itinerate magnetism, Kondo effect and heavy fermion behavior. Measurements of the temperature dependence of the specific heat, electrical resistivity and magnetic susceptibility are presented for several uranium-based pseudobinary alloy systems which display a substantial variation in f-f overlap and/or f conduction electron hybridization. Such studies clearly display the role hybridization plays in the very interesting and varied features reported for these systems.
The paramagnetic spectral response of the pseudobinary alloys U(Sn3-xInx), 0 less-than-or-equal-to x less-than-or-equal-to 1.0, have been investigated by neutron inelastic scattering using the time-of-flight technique. The response was found to consist of a single broad quasielastic component varying slowly with temperature. Good agreement was obtained between neutron and bulk susceptibilities at 100 K, whereas at 2.5 K the Q-averaged neutron susceptibility does not show the large heavy-fermion enhancement seen in the bulk measurements.
We report on measurements of UX3 compounds. The crystal structure of these compounds is of the cubic AuCu3 type. Compounds with this structure show all relevant metallic behaviours. In our case we compared two groups of mixed crystals.
Both of them move from local‐moment magnetism to paramagnetic behaviour but one group develops a heavy fermion state in between.
The temperature dependences of both the electrical resistivity and magnetic susceptibility were measured for the pseudobinary alloy U(Ga1-xSnx)3. Moreover, the temperature variation of the specific heat was determined for UGa3. The results show a clear evolution in magnetic behavior of the system studied from a weakly temperature-dependent paramagnetism with a long-range antiferromagnetic ordering at 67 K in UGa3 to a strongly temperature-dependent paramagnetism, but without any magnetic order, in USn3. The properties of UGa3 are discussed in terms of itinerant 5f-electron magnetism. In contrast, some arguments are given for local 5f-electrons in USn3, which behaves as a nonmagnetic ``Kondo-lattice'' system with TK~=60 K.
The temperature dependence of the electrical resistivity and the magnetic susceptibility has been investigated for the compounds UAl2, UAl3 and the isostructural compounds UX3 (X=Ga, In, Si, Ge, Sn), and finally for UAl4 and UGa2. The influence of a variable U concentration has been studied for the pseudobinary compounds U1-xYxAl2 and U1-xYxGa2. The results are discussed in terms of a localized-spin-fluctuations model. The differences and the similarities between the electrical properties of these U compounds and Ce compounds, such as CeAl2 and CeAl3, are briefly discussed.
The temperature and magnetic field dependence of the specific heat and the temperature dependence of the resistivity have been measured for the pseudobinary U(In, Sn)3 system. The Néel temperature is depressed to zero by 45% Sn substitution while the specific heat coefficient peaks at heavy fermion values near 60% Sn. Los Alamos National Laboratory, Los Alamos, NM 87545, USA.
Since the discovery by Steglich et al. (1979) of superconductivity in the high-effective-mass (approx.200m/sub e/) electrons in CeCuâSiâ, the search for and characterization of such ''heavy-fermion'' systems has been a rapidly growing field of study. The eight heavy-fermion systems known to date include superconductors (CeCuâSiâ, UBeââ, UPtâ), magnets (NpBeââ, UâZnââ, UCdââ), and materials in which no ordering is observed (CeAlâ, CeCuâ). These f-electron materials have, in comparison to normal metals, enormous specific heat ..gamma.. values (450--1600 mJ/mol K²), large values of the low-temperature magnetic susceptibility chi (8--50 x 10â»Â³ emu/mol G), maxima in the resistivity at low temperatures with large rho/sub max/ values (100--200 ..mu cap omega.. cm), and unusual temperature dependences of their specific heats below 10 K. The three heavy-fermion superconductors show such unusual behavior that the possibility of p-wave pairing of the superconducting electrons, rather than the usual BCS s-wave pairing, cannot be ruled out. This paper reviews the experimental results to date, to serve both as a status report and as a starting point for future research. Several correlations between properties are pointed out, including the observation that a low value of the Wilson ratio (approx.chi/..gamma..) appears to correlate with the occurrence of superconductivity.
Measurements of the specific heat as a function of temperature at varying magnetic fields for the Kondo lattice systems CeCu2Si2 and CeAl3 reveal that, on an energy scale ~0.5 K
The magnetic to nonmagnetic transition in actinide alloys and intermetallic compounds is attributable to the delocalization of the 5f electrons due to an increase in the f-f overlap and/or f-spd hybridization. USnâ is paramagnetic at all temperatures and appears to be very near the border between magnetic and nonmagnetic behavior. In contrast, UPbâ has an antiferromagnetic transition at 31 K and the 5f electrons appear to be highly localized. Both of these systems have sufficiently large U-U separations to expect that f-spd hybridization is the major contributing factor to the delocalization of the 5f electrons. The room temperature lattice constant and the temperature dependence of magnetic susceptibility and electrical resistivity have been measured for the U(Sn/sub 1-x/Pb/sub x/)â system to examine the salient features associated with the onset of long range magnetic behavior in this system. The room temperature lattice constant follows Vegard's Law from USnâ to UPbâ and thus indicates no significant change in the degree of localization of the 5f electrons. Upon substitution of Sn for Pb in UPbâ (i.e., decreasing x), the Neel temperature T/sub N/ initially increases to 38 K at x = 0.7 then decreases and appears to go to zero between x = 0.0 and x = 0.1. Also, the spin fluctuation temperature T/sub sf/ for the USnâ rich alloys (i.e., x < 0.60) decreases with increasing x.
The low temperature specific heat of cubic UX3 intermetallic compounds with X = Al, Ga, In, Si, Ge and Sn have been measured. High values for the coefficient of the electronic specific heat have been found, ranging from 14 to 169 mJ/mol K2.ZusammenfassungVon den kubischen intermetallischen Verbindungen UX3 (X = Al, Ga, In, Si, Ge und Sn) wurden bei tiefen Temperaturen die spezifische Wärme bestimmt. Für den Koeffizienten der elektronischen spezifischen Wärme wurden hohe Werde ermittelt welche sich von 14 bis zu 169 mJ/mol K2 hin erstrecken.
We assume that in UBe13 the 5f5/2-Gamma8 crystal-field level of U is filled and that the Gamma7 (Kramer's doublet) lies exactly at the Fermi surface, so as to be half full. Resonant hybridization of the Gamma7 state with the Be(2s)-U(7s) conduction band leads to a Lorentzian density-of-states peak at EF. Its width is 12 K and its peak height is ~250 times that of the conduction band. This model explains the (normal-state) heat capacity, magnetic susceptibility, and nuclear-spin relaxation versus T. Curie-Weiss behavior is obtained even though no local moments are presumed. We show that the hybridization matrix elements are extremely anisotropic-being zero for conduction electrons having k--> along the cubic axes. It follows that the superconducting energy gap is also anisotropic in k--> space, and that the heat capacity below Tc can be explained with singlet pairing caused by phonon-mediated interactions. Two key experiments are proposed.
A variational wave function for the Kondo-lattice limit of the periodic Anderson model is evaluated with a Gutzwiller approximation. We obtain a characteristic energy from this coherent wave function of the Kondo form but with a different exponent in the case of finite degeneracy. The effective mass and charge and spin susceptibilities are evaluated, and only in the case of large degeneracy and not too small hybridization strength is a heavy-Fermi-liquid state stable against magnetic order.
- T. M. Rice