Heavy Quasiparticle Band Research Articles

Kondo entanglement in the quasi-two-dimensional heavy fermion compound CeSb2

In the heavy fermion system, development of the Kondo coherence usually occurs as a crossover behavior. However, accurate definition of the onset temperature for complete formation of the heavy quasiparticles is to some extent controversial. Here, the electronic structure of the quasi-two-dimensional Kondo lattice ${\mathrm{CeSb}}_{2}$ has been studied by high resolution angle-resolved photoemission spectroscopy (ARPES) and dynamical mean-field theory approach combined with density functional theory. A heavy quasiparticle band, which originates from the hybridization between $f$ electrons and conduction electrons, has been observed directly. Moreover, temperature-dependent electronic structure study reveals a transition from high-temperature local spins to low-temperature itinerant electrons in ${\mathrm{CeSb}}_{2}$ and the onset temperature of the hybridized c-f spectral weight revealed by laser-ARPES is consistent with the crossover temperature from transport and magnetic measurements. Our findings are essential for a microscopic understanding of different energy scales in the heavy fermion system and demonstrate the probability of laser-ARPES in studying the bulk 4f-electron properties of Ce-based compounds.

The 4f-Hybridization Strength in Ce m Mn In3m+2n Heavy-Fermion Compounds Studied by Angle-Resolved Photoemission SpectroscopySupported by the National Natural Science Foundation of China (Grant Nos. 12074436 and 11574402), and the Innovation-Driven Plan in Central South University (Grant No. 2016CXS032). J.R. and P.M.O. acknowledge support through the Swedish Research Council (VR) and the Swedish National Infrastructure for Computing (SNIC), for computing

We systemically investigate the nature of Ce 4f electrons in structurally layered heavy-fermion compounds Ce m M n In3m + 2n (with M = Co, Rh, Ir, and Pt, m = 1, 2, n = 0–2), at low temperature using on-resonance angle-resolved photoemission spectroscopy. Three heavy quasiparticle bands f 0, and , are observed in all compounds, whereas their intensities and energy locations vary greatly with materials. The strong f 0 states imply that the localized electron behavior dominates the Ce 4f states. The Ce 4f electrons are partially hybridized with the conduction electrons, making them have the dual nature of localization and itinerancy. Our quantitative comparison reveals that the –f 0 intensity ratio is more suitable to reflect the 4f-state hybridization strength.

Three-dimensional and temperature-dependent electronic structure of the heavy-fermion compound CePt2In7 studied by angle-resolved photoemission spectroscopy

The three-dimensional electronic structure and Ce 4f electrons of the heavy fermion superconductor CePt2In7 is investigated. Angle-resolved photoemission spectroscopy using variable photon energy establishes the existence of quasi-two and three dimensional Fermi surface topologies. Temperature-dependent 4d-4f on-resonance photoemission spectroscopies reveal that heavy quasiparticle bands begin to form at a temperature well above the characteristic (coherence) temperature T*. T* emergence may be closely related to crystal electric field splitting, particularly the low-lying heavy band formed by crystal electric field splitting.

Revealing the hidden heavy Fermi liquid in CaRuO3

The perovskite ruthenate has attracted considerable interest due to reports of possible non-Fermi-liquid behavior and its proximity to a magnetic quantum critical point, yet its ground state and electronic structure remain enigmatic. Here we report the first measurements of the Fermi surface and quasiparticle dispersion in CaRuO3 through a combination of oxide molecular beam epitaxy and in situ angle-resolved photoemission spectroscopy. Our results reveal a complex and anisotropic Fermi surface consisting of small electron pockets and straight segments, consistent with the bulk orthorhombic crystal structure with large octahedral rotations. We observe a strongly band-dependent mass renormalization, with prominent heavy quasiparticle bands which lie close to the Fermi energy and exhibit strong temperature dependence. These results are consistent with a heavy Fermi liquid with a complex Fermiology and small hybridization gaps near the Fermi energy. Our results provide a unified framework for explaining previous experimental results on CaRuO3, such as its unusual optical conductivity, and demonstrate the importance of octahedral rotations in determining the quasiparticle band structure, and electron correlations in complex transition metal oxides.

Resolving the VO2 controversy: Mott mechanism dominates the insulator-to-metal transition

We consider a minimal model to investigate the metal-insulator transition in VO$_2$. We adopt a Hubbard model with two orbital per unit cell, which captures the competition between Mott and singlet-dimer localization. We solve the model within Dynamical Mean Field Theory, characterizing in detail the metal-insulator transition and finding new features in the electronic states. We compare our results with available experimental data obtaining good agreement in the relevant model parameter range. Crucially, we can account for puzzling optical conductivity data obtained within the hysteresis region, which we associate to a novel metallic state characterized by a split heavy quasiparticle band. Our results show that the thermal-driven insulator-to-metal transition in VO$_2$ is compatible with a Mott electronic mechanism, providing fresh insight to a long standing "chicken-and-egg" debate and calling for further research of "Mottronics" applications of this system.

Origin of Quantum Criticality in Yb–Al–Au Approximant Crystal and Quasicrystal

To get insight into the mechanism of emergence of unconventional quantum criticality observed in quasicrystal Yb$_{15}$Al$_{34}$Au$_{51}$, the approximant crystal Yb$_{14}$Al$_{35}$Au$_{51}$ is analyzed theoretically. By constructing a minimal model for the approximant crystal, the heavy quasiparticle band is shown to emerge near the Fermi level because of strong correlation of 4f electrons at Yb. We find that charge-transfer mode between 4f electron at Yb on the 3rd shell and 3p electron at Al on the 4th shell in Tsai-type cluster is considerably enhanced with almost flat momentum dependence. The mode-coupling theory shows that magnetic as well as valence susceptibility exhibits $\chi\sim T^{-0.5}$ for zero-field limit and is expressed as a single scaling function of the ratio of temperature to magnetic field $T/B$ over four decades even in the approximant crystal when some condition is satisfied by varying parameters, e.g., by applying pressure. The key origin is clarified to be due to the strong locality of the critical Yb-valence fluctuation and small Brillouin zone reflecting the large unit cell, giving rise to the extremely-small characteristic energy scale. This also gives a natural explanation for the quantum criticality in the quasicrystal corresponding to the infinite limit of the unit-cell size.

Field-orientation dependence of low-energy quasiparticle excitations in the heavy-electron superconductor UBe(13).

Low-energy quasiparticle excitations in the superconducting (SC) state of UBe_{13} were studied by means of specific-heat (C) measurements in a rotating field. Quite unexpectedly, the magnetic-field dependence of C(H) is linear in H with no angular dependence at low fields in the SC state, implying that the gap is fully open over the Fermi surfaces, in stark contrast to previous expectations. In addition, a characteristic cubic anisotropy of C(H) was observed above 2T with a maximum (minimum) for H∥[001] ([111]) within the (11[over ¯]0) plane, in the normal as well as in the SC states. This oscillation possibly originates from the anisotropic response of the heavy quasiparticle bands, and might be a key to understand the unusual properties of UBe_{13}.

Formation of heavy d-electron quasiparticles in Sr3Ru2O7

The phase diagram of Sr3Ru2O7 shows hallmarks of strong electron correlations despite the modest Coulomb interaction in the Ru 4d shell. We use angle-resolved photoelectron spectroscopy measurements to provide microscopic insight into the formation of the strongly renormalized heavy d-electron liquid that controls the physics of Sr3Ru2O7. Our data reveal itinerant Ru 4d-states confined over large parts of the Brillouin zone to an energy range of <6 meV, nearly three orders of magnitude lower than the bare band width. We show that this energy scale agrees quantitatively with a characteristic thermodynamic energy scale associated with quantum criticality and illustrate how it arises from a combination of back-folding due to a structural distortion and the hybridization of light and strongly renormalized, heavy quasiparticle bands. The resulting heavy Fermi liquid has a marked k-dependence of the renormalization which we relate to orbital mixing along individual Fermi surface sheets.

Unconventional superconductivity from local spin fluctuations in the Kondo lattice.

The explanation of heavy-fermion superconductivity is a long-standing challenge to theory. It is commonly thought to be connected to nonlocal fluctuations of either spin or charge degrees of freedom and therefore of unconventional type. Here we present results for the Kondo-lattice model, a paradigmatic model to describe heavy-fermion compounds, obtained from dynamical mean-field theory which captures local correlation effects only. Unexpectedly, we find robust s-wave superconductivity in the heavy-fermion state. We argue that this novel type of pairing is tightly connected to the formation of heavy quasiparticle bands and the presence of strong local spin fluctuations.

Electronic structure of U(Ru1−xRhx)2Si2 studied by laser angle-resolved photoemission spectroscopy

We have studied electronic structures of U(Ru1-xRhx)2Si2 employing ultrahigh-resolution laser angle-resolved photoemission spectroscopy to understand the effect of Rh substitution. A hole-like dispersive feature, which presumably has Rh d-band character, was observed in U(Ru1-xRhx)2Si2 for both x = 0 and x = 0.03. However, although a heavy quasiparticle band appears in the hidden-order state of URu2Si2 (x = 0), it was not observed for x = 0.03 over the temperature range studied. In addition, it was found that energy-distribution curves at the Fermi vector of the hole-like band for x = 0.03 behave similarly to the Fermi-Dirac function. We also present the mapping of Fermi surfaces formed by the hole-like band, which stay nearly unaffected by Rh substitution.

Hall effect signatures of electronic structure change near a field induced quantum critical point in [formula omitted

We investigate the Hall coefficient R H , resistivity, and magnetization of URu 2 Si 2 and U ( Ru 0.96 Rh 0.04 ) 2 Si 2 in the high field regime up to 45 T to trace the electronic structure changes in both high and low temperature regimes. We find that at high temperatures above ∽ 7 K , temperature- and field-dependent positions of R H maxima and magnetoresistance maxima coincide with a coherent temperature T coh at each field. These systematically decrease in temperatures with increasing fields and extrapolate to zero at the field-induced putative quantum critical point (QCP). This observation suggests a collapse of a new energy scale, i.e., a coherent temperature of a heavy quasiparticle band, at the putative field-induced QCP. At lower temperatures, we show there are discontinuous changes of the Hall number n H = 1 / R H e at the phase boundaries. This behavior illustrates that the phase formation is caused by first order reconstructions of the Fermi surface, which we interpret to the polarization of Fermi surface pockets seen in de-Haas van-Alphen (dHvA) measurements.

High-resolution photoemission study of ZrZn 2

We have performed high-resolution ultraviolet photoemission spectroscopy on a weak itinerant ferromagnet ZrZn 2 (ferromagnetic transition temperature T Curie = 28 K), whose superconductivity with T c = 0.29 K has been recently reported [Pfleiderer, et al., Nature 412 (2001) 58]. We have successfully determined the density of states (DOS) in the valence-band and near E F region. The mass-enhancement factor estimated from the present experiment is about 5, suggesting a strong coupling of electrons with a spin fluctuation and/or phonons. This implies that heavy quasi-particle bands in the vicinity of E F play an important role for the occurrence of the ferromagnetism and the superconductivity in ZrZn 2. Although the band calculation predicts the exchange splitting being 70–90 meV, we have observed negligible change in the photoemission spectrum near E F even across T Curie within our energy resolution (7 meV). This suggests that the actual exchange splitting is much smaller and/or the size of the exchange splitting is significantly different for different bands.

Interface phenomena in correlated electron systems

Abstract We present theoretical studies of heterostructures comprised of a Mott insulator and a band insulator. A model calculation based on the Hartree–Fock approximation on LaTiO 3 /SrTiO 3 -type heterostructure is performed. It is found that spin and orbital ordering patterns in thin heterostructures differ from those in bulk LaTiO 3 , and that LaTiO 3 /SrTiO 3 interface region, about three unit cell wide, is always metallic. We further discuss strong correlation effects on the physical properties using dynamical-mean-field theory. It is shown that metallic behavior is supported by heavy quasiparticle bands centered at the interfaces.

LiV2O4: frustration induced heavy fermion metal.

We propose a two-stage spin-quenching scenario for the unusual heavy fermion state realized in the mixed valent metal LiV2O4. In this theory, local valence fluctuations are responsible for the formation of partially quenched, spin-1 / 2 moments below room temperature. Frustration of the intersite spin couplings then drives the system to produce the heavy Fermi liquid seen at low temperatures. The anomalous resistivity and the sign change of the Hall constant can be understood naturally within this model, which also predicts a unique symmetry for the heavy quasiparticle bands that may be observed in de Haas-van Alphen experiments.

Anomalous ordered phase in 4f 2-based heavy electron PrFe 4P 12

Recent study on the anomalous ordered state (ODS) in filled-skutterudite PrFe 4P 12 is presented, where the existence of a rare 4f 2-based heavy electron (HE) state outside ODS has been experimentally confirmed recently. From specific heat measurements, it is found that the HE behavior is largely suppressed in ODS. The Pr nuclear Schottky contribution strongly indicates that ODS is non-magnetic, most probably of quadrupolar origin. We argue that the quadrupolar degree of freedom on the Pr-ions may play an essential role in the heavy quasiparticle band formation in PrFe 4P 12. We also report the La-substitution effect on ODS.

Anomalous heavy-fermion and ordered states in the filled skutteruditePrFe4P12

Specific heat and magnetization measurements have been performed on high-quality single crystals of filled-skutterudite PrFe_4P_{12} in order to study the high-field heavy-fermion state (HFS) and low-field ordered state (ODS). From a broad hump observed in C/T vs T in HFS for magnetic fields applied along the <100> direction, the Kondo temperature of ~ 9 K and the existence of ferromagnetic Pr-Pr interactions are deduced. The {141}-Pr nuclear Schottky contribution, which works as a highly-sensitive on-site probe for the Pr magnetic moment, sets an upper bound for the ordered moment as ~ 0.03 \mu_B/Pr-ion. This fact strongly indicates that the primary order parameter in the ODS is nonmagnetic and most probably of quadrupolar origin, combined with other experimental facts. Significantly suppressed heavy-fermion behavior in the ODS suggests a possibility that the quadrupolar degrees of freedom is essential for the heavy quasiparticle band formation in the HFS. Possible crystalline-electric-field level schemes estimated from the anisotropy in the magnetization are consistent with this conjecture.

Optical Conductivity of CeNiSn, CeRhSb, and CeRhAs

The optical reflectivity spectra of the Kondo semiconductors CeNiSn, CeRhSb, and CeRhAs have been measured in a wide photon energy range to obtain the optical conductivity spectra σ(ω). For CeNiSn and CeRhSb, two pronounced peaks in the mid-infrared (IR) region and small pseudogaps in the far-IR region were observed at low temperatures ( T ). The two peaks are attributed to optical transitions related with the two bands arising from the spin–orbit splitting of Ce 4 f . For CeRhAs, σ(ω) exhibits a gap which develops gradually with cooling. The observed (pseudo)gap widths and T dependent variations are largest for CeRhAs and smallest for CeNiSn, well corresponding to their Kondo temperatures. For CeNiSn and CeRhSb, the T dependence of σ(ω) in the far-IR suggests the formation of a narrow Drude peak caused by a c – f mixing. The results may indicate that the pseudogap is opened in a heavy quasiparticle band.

Quasi-Particle Properties in Two-Orbital Correlated Electron Systems

Two-orbital correlated electron systems with the Hund coupling are investigated in a metallic phase close to the Mott insulator. We compute the single-particle spectrum by means of dynamical mean field theory combined with non-crossing approximation. We focus on two specific models, and discuss how the interplay of the Hund coupling and the orbital splitting would result in a heavy quasi-particle band near half filling.

Specific heat of heavy fermion systems in magnetic field—exchange field contribution

Magnetic field not only shifts and modifies the heavy fermion quasiparticle band, but also disturbs the distribution of residual exchange fields. An influence of the latter effect on the specific heat of non-magnetic heavy fermion systems is discussed.

Heavy Quasi-Particles in the Two-Orbital Hubbard Model

The two-orbital Hubbard model with the Hund coupling is investigated in a metallic phase close to the Mott insulator. We calculate the one-particle spectral function and the optical conductivity within dynamical mean field theory, for which the effective impurity problem is solved by using the non-crossing approximation. For a metallic system close to quarter filling, a heavy quasi-particle band is formed by the Hubbard interaction, the effective mass of which is not so sensitive to the orbital splitting and the Hund coupling. In contrast, a heavy quasi-particle band near half filling disappears in the presence of the orbital splitting, but is induced again by the introduction of the Hund coupling, resulting in a different type of heavy quasi-particles.