Heavy Fermion Formation Research Articles

Crystallization of heavy fermions via epitaxial strain in spinel LiV2O4 thin film

The mixed-valent spinel LiV2O4 is known as the first oxide heavy-fermion system. There is a general consensus that a subtle interplay of charge, spin, and orbital degrees of freedom of correlated electrons plays a crucial role in the enhancement of quasi-particle mass, but the specific mechanism has remained yet elusive. A charge-ordering (CO) instability of V3+ and V4+ ions that is geometrically frustrated by the V pyrochlore sublattice from forming a long-range CO down to T = 0 K has been proposed as a prime candidate for the mechanism. Here, we uncover the hidden CO instability by applying epitaxial strain on single-crystalline LiV2O4 thin films. We find a crystallization of heavy fermions in a LiV2O4 film on MgO, where a charge-ordered insulator comprising of a stack of V3+ and V4+ layers along [001], the historical Verwey-type ordering, is stabilized by the in-plane tensile and out-of-plane compressive strains from the substrate. Our discovery of the [001] Verwey-type CO, together with previous realizations of a distinct [111] CO, evidence the close proximity of the heavy-fermion state to degenerate CO states mirroring the geometrical frustration of the V pyrochlore lattice, which supports the CO instability scenario for the mechanism behind the heavy-fermion formation.

Electronic structure evolution accompanying heavy fermion formation in CeCu2Si2

The cooper pairs in the heavy-fermion superconductor CeCu2Si2 are formed of heavy fermions. Therefore, the heavy fermions are fundamental to the emergence of unconventional superconductivity and associated non-Fermi-liquid behavior in the normal state. The interplay between localization and itinerancy manifested on the electronic structure is key for understanding the heavy-fermion behavior. Here, via the first-principle density functional theory (DFT) combined with single-site dynamical mean-field theory (DMFT), we investigate the temperature ( T ) evolution of the electronic structure of CeCu2Si2 in the normal state, focusing on the role of the 4 f states in the low energy regime. Two characteristic temperature scales of this evolution, which accompanied the heavy-fermion formation, are established. The coherence onset temperature is around 130 K, whereas the heavy-fermion band formation temperature is between 40 and 80 K; both characteristic temperature scales are higher than the transport coherence temperature. Furthermore, the heavy-fermion formation is confirmed by calculating its effective mass variation with the temperature. Based on the calculated T- dependent evolution of the 4 f orbital occupancy and electronic structure, an explanation on the behavior of the temperature evolution of the correlation strength of CeCu2Si2 is provided. Our results offer a comprehensive microscopic picture of the heavy-fermion formation in CeCu2Si2, which is essential for further understanding the emergent superconducting pairing mechanism.

Fermi surface enlargement on the Kondo lattice

The Kondo lattice model is a paradigmatic model for the description of local moment systems, a class of materials exhibiting a range of strongly correlated phenomena including heavy fermion formation, magnetism, quantum criticality and unconventional superconductivity. Conventional theoretical approaches invoke fractionalization of the local moment spin through large-N and slave particle methods. In this work we develop a new formalism, based instead on non-canonical degrees of freedom. We demonstrate that the graded Lie algebra su(2|2) provides a powerful means of organizing correlations on the Kondo lattice through a splitting of the electronic degree of freedom, in a manner which entwines the conduction electrons with the local moment spins. This offers a novel perspective on heavy fermion formation. Unlike slave-particle methods, non-canonical degrees of freedom generically allow for a violation of the Luttinger sum rule, and we interpret recent angle resolved photoemission experiments on Ce-115 systems in view of this.

Direct observation of how the heavy-fermion state develops in CeCoIn5

Heavy fermion materials gain high electronic masses and expand Fermi surfaces when the high-temperature localized f electrons become itinerant and hybridize with the conduction band at low temperatures. However, despite the common application of this model, direct microscopic verification remains lacking. Here we report high-resolution angle-resolved photoemission spectroscopy measurements on CeCoIn5, a prototypical heavy fermion compound, and reveal the long-sought band hybridization and Fermi surface expansion. Unexpectedly, the localized-to-itinerant transition occurs at surprisingly high temperatures, yet f electrons are still largely localized at the lowest temperature. Moreover, crystal field excitations likely play an important role in the anomalous temperature dependence. Our results paint an comprehensive unanticipated experimental picture of the heavy fermion formation in a periodic multi-level Anderson/Kondo lattice, and set the stage for understanding the emergent properties in related materials.

Mangnetoresistance Of Heavy Fermion-like Compound Ce(Ni1-xCux)2Al3

CeNi2Al3 system is a potential candidate for low temperature thermoelectrics. Substitution studies, especially at the Ni site are considered to be of importance due to the drastic tuning of its physical properties. Resistivity in magnetic fields and thermoelectric power measurements of Cu doped CeNi2Al3 (x=0.0 to 0.4) system is reported in this investigation. This dense Kondo lattice system is investigated with an aim of understanding its basic transport mechanism. Negative magnetoresistance is seen for x=0.3 and 0.4 in the magnetic field up to 14 T. Deviation from the Kondo behavior occurs at temperatures close to 2 K with a down turn in resistivity. The nature of resistivity at low temperatures is investigated in view of the possible evidence for Fermi liquid behavior and also the formation of heavy Fermion in corroboration with specific heat studies. Doping dependence of linear diffusion coefficient and Sommerfeld coefficient of specific heat are analyzed and discussed in connection with the heavy Fermion formation. The results obtained show a promising trend in tuning these materials by way of Kondo route as well as by the substitution especially at the Ni site in the present system.

Spin-orbit fluctuations in frustrated heavy-fermion metal LiV(2)O(4).

Spin fluctuations were studied over a wide momentum (ℏQ) and energy (E) space in the frustrated d-electron heavy-fermion metal LiV_{2}O_{4} by time-of-flight inelastic neutron scattering. We observed the overall Q-E evolutions near the characteristic Q=0.6 Å^{-1} peak and found another weak broad magnetic peak around 2.4 Å^{-1}. The data are described by a simple response function, a partially delocalized magnetic form factor, and antiferromagnetic short-range spatial correlations, indicating that heavy-fermion formation is attributable to spin-orbit fluctuations with orbital hybridization.

Visualizing Heavy Fermion Formation and their Unconventional Superconductivity in f-Electron Materials

In solids containing elements with f-orbitals, the interaction between f-electron spins and those of itinerant electrons leads to the development of low-energy fermionic excitations with a heavy effective mass. These excitations are fundamental to the appearance of unconventional superconductivity observed in actinide- and lanthanide-based compounds. We use spectroscopic mapping with the scanning tunneling microscope to detect the emergence of heavy excitations with lowering of temperature in Ce- and U-based heavy fermion compounds. We demonstrate the sensitivity of the tunneling process to the composite nature of these heavy quasiparticles, which arises from quantum entanglement of itinerant conduction and f-electrons. Scattering and interference of the composite quasiparticles is used in the Ce-based compounds to resolve their energy-momentum structure and to extract their mass enhancement, which develops with decreasing temperature. Finally, by extending these techniques to much lower temperatures, we ...

Specific heat of Ce x La1 − xB6 in the low cerium concentration limit (x ≤ 0.03)

The specific heat of high-quality CexLa1 − xB6 (x = 0, 0.01, 0.03) single crystals is studied in the temperature range 0.4–300 K. LaB6 samples with various boron isotope compositions (10B, 11B, natB) are analyzed to estimate the effect of boron vacancies. The experimental data are used to take into account the electron component correctly under the renormalization of the density of states at T < 8 K, the contribution of the quasi-local vibrational mode of a rare-earth ion with the Einstein temperature ΘE ≈ 152 K, the Debye contribution from the rigid cage of boron atoms with the Debye temperature ΘD ≈ 1160 K, and the low-temperature Schottky contribution related to the presence of 1.5−2.3% boron vacancies in the rare-earth hexaborides. The detected low-temperature anomalies in the specific heat are shown to be interpreted in terms of the formation of two-level systems with an energy ΔE = 92–98 K caused by the displacement of rare-earth ions from their centrosymmetric positions. A scenario of heavy fermion formation that is alternative to the Kondo mechanism is proposed for the systems with a magnetic impurity.

Anisotropic heavy-Fermi-liquid formation in valence-fluctuatingα-YbAlB4

$\alpha$-YbAlB$_4$ is the locally isostructural polymorph of $\beta$-YbAlB$_4$, the first example of an Yb-based heavy fermion superconductor which exhibits pronounced non-Fermi-liquid behavior above $T_{\rm c}$. Interestingly, both $\alpha$-YbAlB$_4$ and $\beta$-YbAlB$_4$ have strongly intermediate valence. Our single crystal study of the specific heat, magnetization and resistivity has confirmed the Fermi liquid ground state of $\alpha$-YbAlB$_4$ ~in contrast with the quantum criticality observed in $\beta$-YbAlB$_4$. Both systems exhibit Kondo lattice behavior with the characteristic temperature scale $T^* \sim$ 8 K in addition to a valence fluctuation scale $\sim 200$ K. Below $T^*$, $\alpha$-YbAlB$_4$ a heavy Fermi liquid state with an electronic specific heat coefficient $\gamma\sim$ 130 mJ/mol K$^2$ and a large Wilson ratio more than 7, which indicates ferromagnetic correlation between Yb moments. A large anisotropy in the resistivity suggests that the hybridization between 4$f$ and conduction electrons is much stronger in the $ab$-plane than along the c-axis. The strongly anisotropic hybridization as well as the large Wilson ratio is the key to understand the unusual Kondo lattice behavior and heavy fermion formation in mixed valent compounds.

T-dependent Nuclear Hyperfine Coupling at the In Site in CeIrIn5

Temperature dependence of Knight shift K at 115 In sites in heavy fermion superconductor CeIrIn 5 has been determined for applied magnetic field parallel to a - and c -axes. The Knight shift versus the static susceptibility χ: ( K -χ) plot is non-linear at low temperatures in this compound, indicating that the hyperfine coupling constant depends on temperature. The T -dependence of hyperfine coupling constant is related with the modification of electronic state due to a heavy fermion formation. The non-linear relation between K and χ is discussed.

Development of the heavy-fermion state inCe2IrIn8and the effects of Ce dilution in(Ce1−xLax)2IrIn8

We report a study of muon Knight shifts to investigate the formation of the heavy-fermion state in single crystals of ${({\text{Ce}}_{1\ensuremath{-}x}{\text{La}}_{x})}_{2}{\text{IrIn}}_{8}$. Two different kinds of Knight-shift anomalies (deviations from a linear relation between the Knight shift and the susceptibility) are found: (1) a high-temperature effect arising from depopulation of crystalline electric field levels with temperature, and (2) a lower-temperature anomaly arising from the onset of the heavy-fermion state below a characteristic temperature ${T}^{\ensuremath{\ast}}$, in agreement with the ``two-fluid'' model of heavy-fermion formation. In ${\text{Ce}}_{2}{\text{IrIn}}_{8}$, we find ${T}_{c}^{\ensuremath{\ast}}=20.0(6)\text{ }\text{K}$ and ${T}_{a}^{\ensuremath{\ast}}=15.2(1.2)\text{ }\text{K}$ for applied field $H\ensuremath{\parallel}c$ axis and $H\ensuremath{\parallel}a$ axis, respectively. For the Ce diluted systems ${({\text{Ce}}_{1\ensuremath{-}x}{\text{La}}_{x})}_{2}{\text{IrIn}}_{8}$, $x=0.1$, 0.25, 0.50, 0.70, and 0.90, ${T}^{\ensuremath{\ast}}$ decreases linearly for $x\ensuremath{\le}0.5$, reaching zero near $x=0.7$, indicating the reduction in intersite $f$-spin correlations with Ce dilution. A comparison with nuclear magnetic-resonance measurements of Knight-shift anomalies in several other heavy-fermion compounds suggests that the observed small anisotropy in ${T}^{\ensuremath{\ast}}$ may be induced by the applied field, and that ${T}^{\ensuremath{\ast}}$ may be inherently isotropic, even in highly anisotropic materials.

Heavy fermions in transition metals and transition-metal oxides

Heavy-fermion formation in transition metals and transition-metal oxides is reviewed and compared to observations in canonical f-derived heavy-fermion systems. The work focuses on the dynamic susceptibilities which reveal a characteristic temperature and frequency dependence and which can be unambiguously determined via nuclear magnetic resonance and electron-spin resonance measurements as well as via quasielastic neutron-scattering studies. Different routes to heavy-fermion behaviour are discussed, amongst them Kondo systems, frustrated magnets, and electronically correlated systems close to a metal-insulator transition. From a theoretical point of view, utilizing dynamical mean-field theory, we show that dynamic susceptibilities as calculated for the Hubbard model and for the periodic Anderson model look qualitatively rather similar. These different theoretical concepts describe an universal behaviour of the temperature dependent dynamic susceptibility.

Magnetic ordering in the heavy-fermion compounds CePtSi 2 and CeNiGe 2

The ground state of three CeTMX 2 compounds with CeNiSi 2 structure type was investigated by electrical-resistivity, magnetic-susceptibility and specific-heat measurements. Whereas CeNiSi 2 is an intermediate-valent system, we observe in CePtSi 2 and CeNiGe 2 magnetic ordering in the presence of heavy fermions. The trends in heavy-fermion formation on going from either the 111 or 122 to the 112 compounds are discussed.