Heavy Fermion Character Research Articles

Rare earth chalcospinels, electronic structure, intermediate-valent, and heavy-fermion character

Rare earth ARE2X4 (A = Cd, Mg; RE = Ho, Er, Tm, Yb, Lu; X = S, Se) spinels were studied with quantum DFT calculations to investigate the electronic structure of these systems, primarily those related to Intermediate-Valence and Heavy-Fermion behavior. The ARE2X4 compounds have narrow 4f peaks; therefore, if these RE:4f peaks are close to the Fermi level, they could exhibit Intermediate-Valent and Heavy-Fermion character. With RE = Ho, Er, Tm, Lu, the compounds resulted in insulators, while RE = Yb presented metallicity, an Intermediate-Valent and Heavy-Fermion characters. When electron-doped, the ATm2X4 and AEr2X4 spinels were found to have a Heavy-Fermion character.

Large tunable anomalous Hall effect in the kagome antiferromagnet U3Ru4Al12

The Berry curvature in magnetic systems is attracting interest due to the potential tunability of topological features via the magnetic structure. $f$-electrons, with their large spin-orbit coupling, abundance of non-collinear magnetic structures and high electronic tunability, are attractive candidates to search for tunable topological properties. In this study, we measure anomalous Hall effect (AHE) in the distorted kagom$\acute{e}$ heavy fermion antiferromagnet U$_3$Ru$_4$Al$_{12}$. A large intrinsic AHE in high fields reveals the presence of a large Berry curvature. Moreover, the fields required to obtain the large Berry curvature are significantly different between $B \parallel a$ and $B \parallel a^*$, providing a mechanism to control the topological response in this system. Theoretical calculations illustrate that this sensitivity may be due to the heavy fermion character of the electronic structure. These results shed light on the Berry curvature of a strongly correlated band structure in magnetically frustrated heavy fermion materials, but also emphasize 5$f$-electrons as an ideal playground for studying field-tuned topological states.

Electronic properties of a heavy-fermion U(Ru0.92Rh0.08)2Si2 single crystal

We report the crystal structure and highly-anisotropic magnetic, transport and thermal properties of an exceptionally good single crystal of U(Ru0.92Rh0.08)2Si2 prepared using a modified Czochralski method. Our study, that also includes neutron diffraction results, shows all the heavy-fermion signatures of pristine URu2Si2 , however, the superconductivity, hidden order and remanent weak antiferromagnetic orders are absent. Instead, the ground state of the doped system can be classified as a spin liquid that preserves the heavy-fermion character. U(Ru0.92Rh0.08)2Si2 exhibits a short-range magnetic order distinguished by reflections of a Lorentzian profile at qIII = (1/2 1/2 1/2) positions that disappear above approx. 15 K. The short-range order seems to be a precursor of a long-range magnetic order that occurs with higher Rh concentration. We indicate that these short-range fluctuations involve, at least partially, inelastic scattering processes.

Heavy-fermion superconductivity in the quadrupole ordered state of PrV2Al20.

PrV2Al20 is a rare example of a heavy-fermion system based on strong hybridization between conduction electrons and nonmagnetic quadrupolar moments of the cubic Γ3 ground doublet. Here, we report that a high-quality single crystal of PrV2Al20 exhibits superconductivity at Tc=50 mK in the antiferroquadrupole-ordered state under ambient pressure. The heavy-fermion character of the superconductivity is evident from the specific heat jump of ΔC/T∼0.3 J/mol K(2) and the effective mass m*/m0∼140 estimated from the temperature dependence of the upper critical field. Furthermore, the high-quality single crystals exhibit double transitions at TQ=0.75 K and T*=0.65 K associated with quadrupole and octupole degrees of freedom of the Γ3 doublet. In the ordered state, the specific heat C/T shows a T(3) dependence, indicating the gapless mode associated with the quadrupole order, the octupole order, or both. The strong sensitivity to impurity of the superconductivity suggests unconventional character due to significant quadrupolar fluctuations.

Low temperature specific heat anomaly in electron doped R2-xCexCuO4 superconductors

The electron doped rare earth copper oxide superconductors R 2-x Ce x CuO 4 exhibit anomalous heavy fermion behavior at low temperature with large Sommerfeld specific heat coefficient which is different from the conventional heavy fermion systems. The system is described by a model Hamiltonian consisting of staggered magnetic field in the two sub-lattices of the copper sites in presence of hybridization between the localized 4f electrons of Nd atom and the conduction electrons as well as the f-electron kinetic energy term. The Hamiltonian is solved by Zubarev's Green's function technique and the sub-lattice magnetization is calculated and solved self-consistently. The entropy and specific heat are calculated from the free energy of the system. The temperature dependent entropy and specific heat are numerically evaluated by successive differentiations of sub-lattice magnetization and temperature dependent entropy. It is observed that when the position of the f-level of Nd atom is of the order of hybridization strength, the sub-lattice magnetization is destroyed drastically at lower temperatures. As a result, the specific heat exhibits a large enhancement at low temperatures suggesting the enhancement of the electron density of states and the effective mass of the itinerant electrons exhibiting the heavy fermion character. Similarly, the specific heat shows anomalously sharp jump near the Néel temperature.

Single-crystal study on the heavy-fermion antiferromagnetUZn12

Millimetre size UZn12 single crystals were grown by the high temperature solution growth methodusing zinc as the solvent. Single-crystal x-ray diffraction data confirm thatthis compound crystallizes in the hexagonal high temperature form ofSmZn12 (S.G.P6/mmm) and points to aU1.01(1)Zn11.7(1) stoichiometry forthe crystals, with ∼ 4% of the U atoms being located at the 2c site due to the partial substitution of 4h Zn pairs.UZn12 ordersantiferromagnetically at TN = 5.0(2) K, and the magnetization and resistivity measurements suggest that the magnetic moments are confinedwithin the a–b plane. The Sommerfeld coefficient, derived from the paramagnetic region by the standard method, isγp≈200 mJ (mol K2) − 1, which definitelyclassifies UZn12 as a moderate heavy-fermion system. The heavy-fermion character ofUZn12 is also manifested in the overall shape of temperature-dependent electrical resistivity that isdominated by a single-ion Kondo effect at high temperatures and coherent Kondoscattering at low temperatures. The paramagnetic magnetoresistivity isotherms can befairly well superimposed onto each other using Schlottmann’s scaling for the single-ionKondo model, as expected for a Kondo system.

Heavy Fermion Behavior in UT2Zn20(T= Fe, Co, Ru, Rh, Ir) Compounds

Single crystals of U T 2 Zn 20 ( T = Fe, Co, Ru, Rh, Ir) compounds were studied by means of X-ray diffraction, magnetic susceptibility, electrical resistivity and heat capacity measurements. The results revealed moderately heavy fermion character of all these materials with paramagnetic ground states due to itinerant 5 f electrons.

CeRuGe and CeRuSi: Heavy fermion systems with some unusual features

We have investigated the physical properties of CeRuSi, its homologue CeRuGe and some doped La alloys. All of them present similar properties: large paramagnetic temperatures θp ≈ −100 K and the level off of χ(T) below ≈ 10 K in coincidence with the maximum decrease of p(T), which starts to drop below 25 K. The respective γ(CeRuSi) ≊ 0.18 and γ(CeRuGe) ≊ 0.15 J/molK2 coefficients reveal a heavy fermion character, with a Kondo temperature TK ≊ 50 K comparable to the crystal field splitting. In both compounds a broad maximum in CP(T)/T is observed around 5 K, whose origin is yet not clear. In contrast to the usual behavior observed in Kondo lattice systems, when Si is replaced by Ge TK ∝ θp and 1/γ practically does not decreases despite the unit cell volume of CeRuGe is about 3% larger than that of CeRuSi. We discuss possible origins for these unusual features.

Heavy-fermion superconductivity in [formula omitted

The compound Ce 2PdIn 8 is a recently discovered novel member of the series Ce nTIn 3n+2, where T=d-electron transition metal, and n = 1 or 2. So far, only the phases with T=Co, Rh and Ir have been intensively studied for their unconventional superconducting behavior at low temperatures. By means of magnetic susceptibility, electrical resistivity and heat capacity measurements we provide evidence that Ce 2PdIn 8 also has a superconducting ground state with a strong heavy-fermion character. The clean-limit superconductivity sets in at T c = 0.7 K at ambient pressure, likely at a verge of a quantum phase transition that manifests itself in a form of distinct non-Fermi liquid features in the bulk normal state characteristics.

Magnetic and electronic properties of heavy fermion compound CeCu 4In and valence fluctuating compound CeNi 4In

The CeCu 4In and CeNi 4In compounds were investigated by means of magnetization, specific heat and electrical resistivity measurements. These compounds are paramagnetic down to 2 K. CeCu 4In follows the Curie–Weiss law with μ eff = 2.40 μ B/f.u. and θ P = −27 K. The experimental value of μ eff is close to the calculated one for a free Ce 3+ ion, thus indicating the presence of well-localized magnetic moments carried by the stable Ce 3+ ions. The determined electronic heat capacity coefficient γ = 235 mJ mol −1 K −2 confirms heavy fermion character of this compound. The temperature dependence of electrical resistivity is characteristic of heavy fermion systems. Similar analysis for CeNi 4In provided μ eff = 0.89 μ B/f.u. and θ P = 0.5 K. This effective paramagnetic moment is lower than the free Ce 3+ value. The temperature dependence of resistivity is typical of the Kondo impurity system, i.e. it shows a minimum at low temperatures. The reduced effective paramagnetic moment and a small electronic specific heat coefficient γ = 16 mJ mol −1 K −2 indicate on a valence fluctuation in this compound.

Electronic and magnetic properties of heavy fermionCeCu4Al

The studies of the magnetic susceptibility, specific heat, electrical resistivity,x-ray photoemission spectra (XPS) and the band structure calculations forCeCu4Al arereported. CeCu4Al is paramagnetic and follows the Curie–Weiss law withμeff = 2.53 μB/f.u. andθp = −10 K. The experimentalvalue of μeff is close to thecalculated one for a free Ce3+ ion (2.54 μB), thus indicating the presence of well-localized magnetic moments carried by the stableCe3+ ions. Below the Fermi energy the total density of states contains mainlythe d states of Cu atoms, which hybridize with the Ce f electronic states.The analysis of the Ce 3d and 4d XPS spectra indicates the localizedcharacter of the Ce f states. The determined electronic specific heat coefficientγ = 210 mJ K−2 mol−1 confirms the heavyfermion character of the CeCu4Al compounds.

Coexistence of Strongly Mixed-Valence and Heavy-Fermion Character inSmOs4Sb12Studied by Soft- and Hard-X-Ray Spectroscopy

Sm-based heavy-fermion compound SmOs4Sb12 has been investigated by soft x-ray (hnu=1070-1600 eV) and hard x-ray (HX; hnu=7932 eV) spectroscopy. The HX photoemission spectroscopy clearly demonstrates that the strongly mixed-valence state and the heavy-fermion state coexist in the bulk. It is found that the Sm valence decreases below 100 K, indicating that the Kondo coherence develops with approaching the proposed Kondo temperature. Our theoretical analyses suggest that the origin of the coexistence in SmOs4Sb12 is the coincidence of two conditions, namely, (i) the energy difference between Sm divalent and trivalent states is very small and (ii) the hybridization between Sm 4f and conduction electrons is weak.

Magnetotransport and Heat Capacity in Ternary Compounds U3M2M′3′, M=Al, Ga; M=Si, Ge

We report detailed studies of magnetization, electrical resistivity, magnetoresistivity, and heat capacity performed on the novel family of intermetallic compounds U3M2M′3, (M=Al, Ga, and M′=Si, Ge). The present measurements support the earlier conclusions about the ferrimagnetic properties of silicides and ferromagnetic properties of germanides. The resistivity for both compounds U3{Al,Ga}2Si3 exhibits below TC a pronounced maximum observed for the first time in an actinoid-ferrimagnet, probably caused by (a) the reduction of the number of effective conduction carriers or (b) a SDW-type of spin-disorder scattering of electrons. Both low-temperature resistivity (except for U3Ga2Si3) and heat capacity may be described by a T-dependence involving a small gap Δ on the order of 30–50 K in the magnon dispersion. The Cp/T values at 2 K are enhanced and point to a medium–heavy fermion character of all these ternaries. Magnetoresistance for ferrimagnetic U3{Al,Ga}2Si3 is rather small but positive in correspondence of antiferromagnetic interactions. In correspondence to the ferromagnetic materials, negative magnetoresistance is encountered for U3{Al,Ga}2Ge3. Specific features in the temperature dependence of magnetoresistivity Δρ/ρ at various fields confirm the sinusoidal modulation of the magnetic structure for U3Al2Ge3 between 40 and 60 K. Also, such data for U3Ga2Ge3 present strong indications for a similar magnetic modulation between 63 and 93 K, yet to be discovered by neutron diffraction experiments. In addition, the transition at 63 K is furthermore well resolved in the specific heat data of U3Ga2Ge3.

Magnetic, thermal, and transport properties of single crystals of antiferromagnetic Kondo-latticeCe2PdSi3

Magnetization, heat capacity, electrical resistivity, thermoelectric power, and Hall effect have been investigated on single-crystalline ${\mathrm{Ce}}_{2}{\mathrm{PdSi}}_{3}.$ This compound is shown to order antiferromagnetically below N\'eel temperature ${(T}_{N})\ensuremath{\sim}3\mathrm{K}.$ The Sommerfeld coefficient far below ${T}_{N}$ is found to be about $110{\mathrm{m}\mathrm{J}/\mathrm{K}}^{2}\mathrm{mol}\mathrm{Ce},$ which indicates the heavy-fermion character of this compound. The transport and magnetic properties exhibit large anisotropy with an interplay between crystalline-electric-field (CEF) and Kondo effects. The sign of thermoelectric power is opposite for different directions at high temperatures and the ordinary Hall coefficient is anisotropic with opposite sign for different geometries, indicating the anisotropic Fermi surface. The CEF analysis from the temperature dependence of magnetic susceptibility suggests that the ground state is $|\ifmmode\pm\else\textpm\fi{}\frac{1}{2}〉.$ The first and the second excited CEF doublet levels are found to be located at about 30 and 130 K, respectively. The Kondo temperature is estimated to be the same order as ${T}_{N},$ indicating the presence of a delicate competition between the Kondo effect and magnetic order.

Magnetoresistance and low-temperature specific heat of the Yb compounds YbRhSn, YbPdBi, and YbPtSn

The antiferromagnetic compounds YbPtSn, YbRhSn, and YbPdBi were studied by specific heat down to 0.35 K. Their paramagnetic states were investigated by magnetoresistance at temperatures between 4 and 50 K and in fields up to 8 T. The magnetoresistance of YbRhSn and YbPdBi exhibits a single impurity Kondo scaling at all temperatures studied. The respective single-ion characteristic temperatures, derived from this scaling, are 2 and 2.5 K. The heavy fermion character of both compounds was confirmed by large values of the electronic specific-heat coefficients in the ordered states. $C/T$ extrapolated to $T=0$ is $1200\ifmmode\pm\else\textpm\fi{}200\mathrm{mJ}/{\mathrm{K}}^{2}\mathrm{mol}$ for YbRhSn and $1200\ifmmode\pm\else\textpm\fi{}100\mathrm{mJ}/{\mathrm{K}}^{2}\mathrm{mol}$ for YbPdBi. The magnetoresistance of YbPtSn implies the presence of strong ferromagnetic correlations. These ferromagnetic correlations are consistent with the previously reported magnetization and magnetic susceptibility. $C/T$ is sublinear in ${T}^{2}$ below 1 K and extrapolated to 0 K is only of order $100\mathrm{mJ}/{\mathrm{K}}^{2}\mathrm{mol}.$

Yb 4As 3 and Yb 4(As 0.71Sb 0.29) 3, two ways to heavy-fermion character

After the observation of heavy-fermion characteristics in the charge-ordered phase of Yb 4As 3, it is surprising that Yb 4(As 0.71Sb 0.29) 3 shows heavy-fermion behavior, though the charge ordering is suppressed. To shed more light onto this enigma, we measured the optical reflectivity of Yb 4(As 0.71Sb 0.29) 3 at several temperatures down to 15 K in the photon energy range from 1 meV to 5.5 eV. The comparison of the optical functions of Yb 4As 3 and Yb 4(As 0.71Sb 0.29) 3 revealed decisive differences in the far-IR region. For the latter material, we found an hybridization gap of 3.3 meV, absent in Yb 4As 3. The effective mass of the conduction carriers takes the temperature-independent value of 2 m e in Yb 4As 3, while m * shows a logarithmic temperature dependency and reaches 80 m e at 15 K in Yb 4(As 0.71Sb 0.29) 3. This proves that two different mechanisms lead to a large γ value in Yb 4As 3 and Yb 4(As 0.71Sb 0.29) 3.

Heavy fermion systems in the presence of CEF: Thermal properties

The effect of CEF on thermal properties of heavy fermion system CeRu 2Si 2 has been studied in the framework of infinite- U degenerate impurity Anderson model using slave-boson approach within 1 N expansion. The calculation of specific heat and thermal expansion coefficient reveal the presence of characteristic peaks in the low temperature regime. In the presence of crystalline electric field, an overall decrease is found in their values alongwith the shift of peaks towards high temperature side, which implies weakening in the heavy fermion character. However, the experimental data towards the low temperature side is found to be closer to the situation with Δ = 0 and that towards high temperature side, close to the case with Δ ≠ 0. The overall qualitative behavior is found to be consistent with the data.

Pressure induced heavy-fermion behaviour of CeAu 2Si 2 near 17 GPa

We report high-pressure resistivity measurements of the antiferromagnet CeAu 2Si 2 at pressure up to 19.5 GPa. At the highest pressure attained the data shows heavy-fermion character. A transition from the magnetic to a non-magnetic ground state takes place at a pressure of about 18 GPa.

Heavy fermion characteristics in amorphous SmCu 6 alloy

The amorphous SmCu 6 alloy produced by sputter-deposition reveals a heavy fermion character: the electronic specific heat coefficient is about 0.3 J mol −1K −2. Broad peaks at around 3.8 K in the specific heat and magnetic susceptibility (χ), and the hysteresis in the χ between zero-field-cooled and field-cooled specimens demonstrate a spin-glass ordering at low temperatures.

Valence fluctuation in nonequilibrium close-packed Ce-Yb alloys produced by sputtering

Using de high-deposition-rate sputtering equipment, we have produced nonequilibrium solid solutions in an immiscible Ce-Yb alloy system. The results of X-ray diffraction and Fourier transform analysis indicate that nonequilibrium close-packed phases are obtained: a randomly-close-packed-layer phase at around the equiatomic concentration and an fcc phase for both the ultimate concentrations. The random atom stacking is also confirmed by the large electrical resistivity. These alloys show rather large electronic specific heat coefficients, indicating a heavy fermion character and contribution of Yb-4f electrons to the density of state at the Fermi level. Magnetic susceptibility obeys the Curie-Weiss law and the effective magnetic moments are larger than the interpolated ones between the trivalent Ce and the divalent Yb metals. The mean valence state of Yb is estimated from the diffraction and magnetization data to be about 2.2.