Heavy-electron Compound Research Articles

Transport and thermodynamic properties of

We present measurements of the specific heat, the electrical resistivity, the Hall effect, and the magnetic susceptibility of CeAuAl3, a new heavy-electron compound that crystallizes in an ordered derivative of the tetragonal BaAl4-type structure. For comparison we have also done some of these measurements on the isostructural non-magnetic reference compound LaAuAl3, which appears to be a simple metal. Below T N = 1.32K, CeAuAl3 orders antiferromagnetically and below 1K, we encounter Fermi liquid behaviour with considerably enhanced effective masses, i.e., a quadratic temperature dependence of the resistivity with a large prefactor and a sizable linear-in-T contribution to the specific heat. This linear-in-T contribution increases by more than a factor 50 from its value at T ≫ T N to its value at T ≫ T N. Consequently CeAuAl3 develops a heavy-electron ground state, coexisting with antiferromagnetic order. The small energy scales involved in the problem make CeAuAl3 a good candidate for tuning it, by varying external parameters, towards a quantum critical point. At high temperatures we observe local moment behaviour. From the temperature dependence of the magnetic susceptibility and the specific heat we have derived the crystalline-electric-field-split level scheme of the Ce3+ J = 5/2 multiplet. Distinct features in the electrical resistivity provide additional evidence for this level splitting.

Origin of the frequency branch γ of the de Haas-van Alphen effect in CeSn 3

CeSn 3 is a typical heavy-electron compound which belongs to the valence-fluctuation regime. The major experimental frequency branches of the de Haas-van Alphen effect are explained reasonably well by the large hole sheet and the large electron sheet of the Fermi surface which are derived on the basis of an itinerant 4f-electron model. In particular, it is shown by visualizing the extremal electron orbits that one of the major experimental frequency branches, i.e., the branch γ, can be explained by those orbits which run through the small hollow on the large electron sheet.

The electrodynamic response of heavy-electron materials with magnetic phase transitions

We have investigated the electrodynamic response of the heavy-electron compounds U2Zn17, UPd2Al3, UCu5 and URu2Si2. Particular emphasis has been devoted to the optical evidence of the antiferromagnetic phase transitions at TN = 9.7 K, 14 K, 15 K and 17 K for U2Zn17, UPd2Al3, UCu5 and URu2Si2, respectively. In the excitation spectrum of UCu5 and URu2Si2, we found an absorption in the far-infrared, which develops below TN and is ascribed to the excitation across a spin-density-wave type gap, suggesting that the antiferromagnetic phase transition might be itinerant in nature, and invokes a Fermi surface instability. Since this gap-like feature is absent in U2Zn17 and UPd2Al3, we argue that these latter compounds belong to a characteristically different class of antiferromagnets, representative of the heavy-electron compounds with an ordering of essentially localized magnetic moments. The antiferromagnetic ordering then leads to a suppression of the spin-flip mechanism below TN. At low temperatures, we observe for all compounds the formation of a narrow Drude-like resonance in the optical conductivity, which is ascribed to the electrodynamic response of the heavy-quasiparticles, and is indicative of the progressive development of the manybody coherent Kondo state, coexisting with both types of magnetic ordering. In this review, we also present the evolution of the optical properties due to Ni- and Redoping in UCu5 and URu2Si2, respectively. The optical evidence of the itinerant antiferromagnetic ordering is suppressed in both compounds upon doping and particularly for the URu2Si2 compound this is consistent with a crossover to a ferromagnetic ground state upon Re-doping.

Nuclear magnetic resonance studies of CePd2In and LaPd2In at low temperatures

We report on In nuclear magnetic resonance (NMR) and nuclear quadrupolar resonance (NQR) measurements on the new heavy-electron compound CePdZIn over a wide temperature range, from 45 mK up to 30 K. CePd2In undergoes an antiferromagnetic (AF) phase transition at TN = 1.23 K involving small localised Ce moments of 0.11 μB. In zero field, the spin-lattice relaxation rate T1−1 (T) shows remarkable changes in its temperature dependence. Above 3 K, T1−1 is constant and 850 sec−1. Between TN and 2TN, (T1T)−1 = 330 (Ksec)−1, but rapidly decreases below TN. A Korringatype relaxation, characteristic for simple metals at low temperatures, with (T2T)−1 = 17(Ksec)1 is resumed below 0.6 K. This value is an order of magnitude larger than (T1T)−1 for LaPd2In and therefore is associated with low-energy excitations of the itinerant charge carriers with 4f symmetry. The T1−1 data at various non-zero magnetic fields fall on a single curve when plotted as a function of (T/H) if H exceeds 3.5 T. Thus the AF ordering, the 4f moment fluctuations and the Kondo screening are drastically suppressed by the application of fields H of the order of 3.5 T.

The effect of spin fluctuations on thermal properties in strongly correlated fermion systems

The effect of spin fluctuations on the thermal properties of strongly correlated itinerant fermion systems is discussed based on the self-consistent renormalized (SCR) spin fluctuation model. Specific heat and magnetic susceptibility in the normal phase of liquid-3He and heavy electron compounds at finite temperatures are well described by the unified SCR picture including the mode-mode coupling of spin fluctuations beyond the random phase approximation (RPA).

De Haas-van Alphen effect in Ce 3Pd 20Ge 6

We have first observed the de Haas-van Alphen (dHvA) effect in a new ternary heavy electron compound Ce 3Pd 20Ge 6 and a isostructural compound La 3Pd 20Ge 6 down to 35 mK in the magnetic fields up to 13.5 T. We have measured the angular dependence of the dHvA frequencies of Ce 3Pd 20Ge 6 in (0 01) and (1 1 1) planes. We have found that an effective cyclotron mass, ranging from 0.7 to 4.3, is much smaller than the thermal mass estimated from the specific heat measurement.

Spin glass behaviour in URh 2Ge 2

URh 2Ge 2 occupies an extraordinary position among the 122 heavy-electron compounds. Its physical properties strongly deviate from those of the other members of this materials class by exhibiting some previously unidentified form of magnetic correlations at low temperatures, instead of the usual antiferromagnetic ground state. Here, we present new results of AC-susceptibility and neutron diffraction measurements on single-crystalline as-grown URh 2Ge 2. These data clearly indicate that crystallographic disorder on a local scale produces spin-glass behaviour in the sample. We therefore conclude that URh 2Ge 2 is a random-bond, heavy-fermion spin glass.

Fermi surfaces and local curvature factors of Ce compounds with cubic Laves phase structure

The Fermi surface is calculated for the heavy-electron compounds CeRu 2, CeRh 2 and CeCo 2 with the cubic Laves phase crystal structure by a self-consistent relativistic linear augmented-plane-wave method. The calculated Fermi surface sheets can explain the recent experimental results for the de Haas-van Alphen effect reasonably well. Some of the frequency branches may be missed in the experiments, and the local curvature factor is investigated for these frequency branches in detail as a possible origin of the disappearance.

Fermi Surface of CeRu2within Local-Density Functional Theory

The Fermi surface is investigated for the heavy-electron compound CeRu 2 , which is known as a type-II superconductor exhibiting anomalous magnetic properties, on the basis of the itinerant-electron model for the 4 f electrons. The energy band structure is calculated by a relativistic linear augmented-plane-wave method with the exchange-correlation potential in the local-density approximation. The Fermi surface is found to consist of a set of six small hole pockets, a large multiply-connected hole sheet and a set of three complex electron sheets and a set of three small electron pockets. Recent experimental results for the de Haas-van Alphen effect are explained reasonably well in terms of the large hole sheet and the small electron pockets except for the cyclotron effective mass. It seems that many frequency branches may have been missed in the experiment. A nesting property of the Fermi surface is also investigated.

Magnetic susceptibility of nonmagnetic heavy electron compound CeRu2Si2 below 1 mK

A nonmagnetic heavy electron compound CeRu2Si2 shows a metamagnetic transition from the itinerant paramagnetic state to the spin polarized state at about 8T. To investigate the electronic ground state of CeRu2Si2, the temperature dependence of the magnetic susceptibility was measured below 1 mK. The maximum of the magnetic susceptibility was observed at about 800 mK. The anisotropic magnetic susceptibility was also measured below 1 K.

Absence of a first-order metamagnetic transition in CeRu2Si2.

Metamagnetic transition of the heavy-electron compound ${\mathrm{CeRu}}_{2}$${\mathrm{Si}}_{2}$ has been examined on high-purity single crystals by means of a static magnetization measurement at very low temperatures down to 90 mK. No hysteresis is observed in the magnetization process. The peak value of the differential susceptibility at the metamagnetic field ${\mathit{B}}_{\mathit{M}}$\ensuremath{\sim}7.7 T follows a simple ${\mathit{T}}^{2}$ law below T\ensuremath{\sim}0.5 K, and saturates to a finite value 1.88${\mathrm{\ensuremath{\mu}}}_{\mathit{B}}$/T Ce as T\ensuremath{\rightarrow}0. From these results for two single crystals with slightly different quality, we have confirmed the nonexistence of a first-order phase transition at ${\mathit{B}}_{\mathit{M}}$ in the clean limit of this system. The itinerant nature of the 4f electrons is most likely preserved in the magnetized state at B\ensuremath{\ge}${\mathit{B}}_{\mathit{M}}$.

Low temperature magnetization measurements on heavy-electron Ce compounds

A new Faraday force magnetometer has been developed to perform static magnetization measurements on heavy electron compounds at temperatures down to 100 mK and in fields up to 9 T. The results of the measurements on single crystals of CeB 6, CePt 2Si 2 and CeRu 2Si 2 reveal new features in the low temperature metamagnetic properties of these compounds.

Optical investigation of the antiferromagnetic phase transitions in heavy-electron compounds

We have investigated the electrodynamic response of the heavy-electron compounds UPd 2Al 3 ( T N = 14.5 K) and UCu 5 ( T N = 15 K). The complete excitation spectra were obtained by Kramers-Kronig transformation of the optical reflectivities measured on a fairly broad frequency range (i.e. from 15 up to 10 5 cm −1), combined with the direct measurement of the conductivity at millimeter and microwave frequencies. In UCu 5 we found an absorption in the far infrared below T N, which is ascribed to excitations across a spin-density-wave-type gap and indicates the itinerant nature of the magnetic phase transition. This feature is absent in UPd 2Al 3, in accordance with the local magnetic moment character of the antiferromagnetic order.

Low-Energy Magnetic Excitations in the Antiferromagnetically Ordered State of an Incipient Heavy-Electron Compound UP as Studied by31P NMR

The microscopic magnetic properties of UP with the cubic NaCl-type structure have been investigated by 31 P NMR in the magnetically ordered state. 1/ T 1 is extraordinarily large in the single- Q type-I phase (\(T_{0}\cong 22.5\) K\(<t<t_{\rm N}\cong 122\) K), and shows about two orders of magnitude abrupt decrease through the single- Q ↔double- Q phase transition at T 0 . Below about 4 K in the double- Q type-I phase ( T < T 0 ), 1/ T 1 shows a Korringa-like temperature-dependence, which corresponds qualitatively to a 5 f -derived γ T term in the specific heat. We discuss this behavior as a clear indication of the existence of 5 f -derived very-low-energy magnetic excitations even in the ordered state, and an incipient heavy-electron character of the compound is concluded. We also present a simple qualitative model of a system of exchange-coupled Kondo impurities under internal exchange (or molecular) field in order to discuss the origin of these excitations.

Indication for a delocalized precursor μ+ state in CeAl3

The low temperature (≲ 0.6 K) ZF-μSR signal in the heavy electron compound CeAl3 displays a three component structure with one component revealing the precession ofμ + in a static internal field of 220 G. Surprisingly the phase of this signal is shifted by the order of 90° which indicates the presence of a precursor state. The precursor state manifests itself by a quickly decaying nonoscillating component. A simple two state model with zero average field in the precursor state and 220 G in the final state can describe the results except for the magnitude of the phase shift.

A new nonmagnetic heavy-electron compound, CeFeGe 3

We report a new nonmagnetic Kondo compound, CeFeGe 3, which exhibits typical heavy-electron behavior at low temperatures; large T-independent linear specific heat ( γ= 150 mJ mol K 2 ) and T 2-dependent electrical resistivity. The magnetic susceptibility saturates with a faint maximum around 50 K. The compound did not exhibit any phase transitions above 0.5 K.

Fermi Surface of CeRh2

On the basis of the itinerant-electron model for the 4 f electrons, the energy band structure, especially the Fermi surface, is calculated for the paramagnetic heavy-electron compound CeRh 2 , which has the low-temperature electronic specific heat coefficient of 20 mJ/K 2 mol and the Kondo temperature of 400 K, by a self-consistent symmetrized relativistic APW method with the exchange and correlation potential in a local-density approximation. The Fermi surface consists of two hole spheres and a hollow electron sphere with small windows, all of which are centered at the Γ point in the Brillouin zone. The spherical property of the Fermi surface explains qualitatively well the behavior of the de Haas-van Alphen frequency branches measured recently by Sugawara et al . Candidates for the frequency branches missing in the experiment are suggested.

Phase diagram and superconducting properties of a heavy electron material UPt3

We investigate (1) the superconducting properties of a heavy electron compound UPt3 by identifying the low T phase as a non-unitary pairing state and (2) its phase diagram in H versus T by solving a GL functional numerically. Our study is based on a spatially one-dimensional representation with odd-parity or triplet pairing function in weak spin-orbit coupling. The obtained phase diagram is topologically similar to the observed one, independent of the field orientation. The large “residual” T-linear specific heat at low T is found to be attributable to the gapless excitation branch due to non-unitarity of the identified triplet pairing function.

Single Uranium-Site Properties of the Dilute Heavy Electron System UxTh1-xRu2Si2(x≤0.07)

Specific heat C , electrical resistivity ρ , magnetization M and magnetic susceptibility χ have been measured for the dilute tetragonal heavy-electron compounds U x Th 1- x Ru 2 Si 2 ( x ≤0.07), in the temperature range 100 mK≤ T ≤360 K and in the magnetic fields H up to 55 kOe. Non-Fermi-liquid behavior with unusual lnT dependence of C , ρ and χ has been observed at the low temperatures: as the temperature is lowered below about 10 K, C / T and χ increase logarithmically, and ρ decreases logarithmically. Systematical variations of these quantities with uranium concentration indicate that the observed non-Fermi-liquid behavior is ascribed to the properties of a single U ion embedded in the Th site of the ThRu 2 Si 2 metal. The magnetization curves M vs H / T indicate a strong uniaxial magnetic anisotropy along the tetragonal c -axis as well as a strong reduction of the 5 f moment of U ions. A possible interpretation based on the recent developments of the S =1/2 two-channel Kondo model is given.

Physics of YbBiPt

YbBiPt has a low temperature linear specific heat coefficient of 8 J/mol Yb K 2 and a small specific heat anomaly at 0.4 K. We discuss new experiments on the specific heat of diluted YbBiPt, magnetic field dependent effects and electrical resistivity in pure YbBiPt. We argue that in this material the Kondo and crystal field energy scales are small and of comparable magnitude, placing YbBiPt in the same class as many uranium heavy electron compounds.