Temperature-Independent Fermi Surface in the Kondo Lattice YbRh 2 Si 2 (original) (raw)
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From mixed valence to the Kondo lattice regime
2011
Many heavy fermion materials are known to cross over from the Kondo lattice regime to the mixed valence regime or vice versa as a function of pressure or doping. We study this crossover theoretically by employing the periodic Anderson model within the framework of the dynamical mean field theory. Changes occurring in the dynamics and transport across this crossover are highlighted. As the valence is decreased (increased) relative to the Kondo lattice regime, the Kondo resonance broadens significantly, while the lower (upper) Hubbard band moves closer to the Fermi level. The resistivity develops a two peak structure in the mixed valence regime: a low temperature coherence peak and a high temperature 'Hubbard band' peak. These two peaks merge, yielding a broad shallow maximum upon decreasing the valence further. The optical conductivity likewise exhibits an unusual absorption feature (shoulder) in the deep mid-infrared region, which grows in intensity with decreasing valence. The involvement of the Hubbard bands in dc transport and of the effective f-level in the optical conductivity are shown to be responsible for the anomalous transport properties. A two-band hybridization-gap model, which neglects incoherent effects due to many-body scattering, commonly employed to understand the optical response in these materials is shown to be inadequate, especially in the mixed valence regime. Comparison of theory with experiment carried out for (a) dc resistivities of CeRhIn 5 , Ce 2 Ni 3 Si 5 , CeFeGe 3 and YbIr 2 Si 2 , (b) pressure dependent resistivity of YbInAu 2 and CeCu 6 , and (c) optical conductivity measurements in YbIr 2 Si 2 yields excellent agreement.
Global Phase Diagram of the Kondo Lattice: From Heavy Fermion Metals to Kondo Insulators
Journal of Low Temperature Physics, 2010
We discuss the general theoretical arguments advanced earlier for the T = 0 global phase diagram of antiferromagnetic Kondo lattice systems, distinguishing between the established and the conjectured. In addition to the wellknown phase of a paramagnetic metal with a "large" Fermi surface (P L), there is also an antiferromagnetic phase with a "small" Fermi surface (AF S). We provide the details of the derivation of a quantum non-linear sigma-model (QNLσ M) representation of the Kondo lattice Hamiltonian, which leads to an effective field theory containing both low-energy fermions in the vicinity of a Fermi surface and low-energy bosons near zero momentum. An asymptotically exact analysis of this effective field theory is made possible through the development of a renormalization group procedure for mixed fermion-boson systems. Considerations on how to connect the AF S and P L phases lead to a global phase diagram, which not only puts into perspective the theory of local quantum criticality for antiferromagnetic heavy fermion metals, but also provides the basis to understand the surprising recent experiments in chemically-doped as well as pressurized YbRh 2 Si 2. We point out that the AF S phase still occurs for the case of an equal number of spin-1/2 local moments and conduction electrons. This observation raises the prospect for a global phase diagram of heavy fermion systems in the Kondo-insulator regime. Finally, we discuss the connection between the Kondo breakdown physics discussed
Effect of Disorder on Fermi Surface in Heavy Electron Systems
Journal of the Physical Society of Japan, 2010
The Kondo lattice model with substitutional disorder is studied with attention to the size of the Fermi surface and the associated Dingle temperature. The model serves for understanding heavy-fermion Ce compounds alloyed with La according to substitution CexLa1−x. The Fermi surface is identified from the steepest change of the momentum distribution of conduction electrons, and is derived at low enough temperature by the dynamical mean-field theory (DMFT) combined with the coherent potential approximation (CPA). The Fermi surface without magnetic field increases in size with decreasing x from x = 1 (Ce end), and disappears at such x that gives the same number of localized spins as that of conduction electrons. From the opposite limit of x = 0 (La end), the Fermi surface broadens quickly as x increases, but stays at the same position as that of the La end. With increasing magnetic field, a metamagnetic transition occurs, and the Fermi surface above the critical field changes continuously across the whole range of x. The Dingle temperature takes a maximum around x = 0.5. Implication of the results to experimental observation is discussed.
Fermi surfaces in Kondo insulators
Journal of physics. Condensed matter : an Institute of Physics journal, 2018
We report magnetic quantum oscillations measured using torque magnetisation in the Kondo insulator YbB12 and discuss the potential origin of the underlying Fermi surface. Observed quantum oscillations as well as complementary quantities such as a finite linear specific heat capacity in YbB12 exhibit similarities with the Kondo insulator SmB6 , yet also crucial differences. Small heavy Fermi sections are observed in YbB12 with similarities to the neighbouring heavy fermion semimetallic Fermi surface, in contrast to large light Fermi surface sections in SmB6 which are more similar to the conduction electron Fermi surface. A rich spectrum of theoretical models is suggested to explain the origin across different Kondo insulating families of a Fermi surface potentially from novel itinerant quasiparticles that couple to magnetic fields, yet do not couple to weak DC electric fields.
Evolution of a Large Fermi Surface in the Kondo Lattice
Physical Review Letters, 2009
Single-particle spectrum of the Kondo lattice model is derived with use of the continuous-time quantum Monte Carlo method, combined with the dynamical mean-field theory. Crossover behavior is traced quantitatively either to a heavy Fermi-liquid state or to a magnetically ordered state from the local-moment state at high temperatures. The momentum distribution in the low-temperature limit acquires a discontinuity at the location that involves the local-spin degrees of freedom. Even without the charge degrees of freedom for local electrons, the excitation spectra exhibit hybridized bands similar to those in the Anderson lattice. Temperature dependence in the zero-energy component of the self-energy is crucial in forming the Fermi-liquid state with the large Fermi surface.
Nature Communications, 2016
The hybridization between localized 4f electrons and itinerant electrons in rare-earth-based materials gives rise to their exotic properties like valence fluctuations, Kondo behaviour, heavy-fermions, or unconventional superconductivity. Here we present an angle-resolved photoemission spectroscopy (ARPES) study of the Kondo lattice antiferromagnet CeRh 2 Si 2 , where the surface and bulk Ce-4f spectral responses were clearly resolved. The pronounced 4f 0 peak seen for the Ce terminated surface gets strongly suppressed in the bulk Ce-4f spectra taken from a Si-terminated crystal due to much larger f-d hybridization. Most interestingly, the bulk Ce-4f spectra reveal a fine structure near the Fermi edge reflecting the crystal electric field splitting of the bulk magnetic 4f 1 5/2 state. This structure presents a clear dispersion upon crossing valence states, providing direct evidence of f-d hybridization. Our findings give precise insight into f-d hybridization penomena and highlight their importance in the antiferromagnetic phases of Kondo lattices.
Insight into the temperature dependent properties of the ferromagnetic Kondo lattice YbNiSn
Physical Review B, 2017
Analyzing temperature dependent photoemission (PE) data of the ferromagnetic Kondo-lattice (KL) system YbNiSn in the light of the Periodic Anderson model (PAM) we show that the KL behavior is not limited to temperatures below a temperature T K , defined empirically from resistivity and specificic heat measurements. As characteristic for weakly hybridized Ce and Yb systems, the PE spectra reveal a 4f-derived Fermi level peak, which reflects contributions from the Kondo resonance and its crystal electric field (CEF) satellites. In YbNiSn this peak has an unusual temperature dependence: With decreasing temperature a steady linear increase of intensity is observed which extends over a large interval ranging from 100 K down to 1 K without showing any peculiarities in the region of T K ∼ TC = 5.6 K. In the light of the single-impurity Anderson model (SIAM) this intensity variation reflects a linear increase of 4f occupancy with decreasing temperature, indicating an onset of Kondo screening at temperatures above 100 K. Within the PAM this phenomenon could be described by a non-Fermi liquid like T-linear damping of the self-energy which accounts phenomenologically for the feedback from the closely spaced CEF-states.
Fermi surface study of LaRu2Si2 and of heavy-fermion CeRu2Si2 above the Kondo temperature
Physical Review B, 2002
The Two-dimensional ͑2D͒ angular correlation of the positron annihilation radiation of the heavy-fermion system CeRu 2 Si 2 was measured above the Kondo temperature T K and compared to that of the reference isostructural non-f -electron system LaRu 2 Si 2 . The k-space densities of the two compounds, obtained via the Lock-Crisp-West folding of the 3D-reconstructed electron-positron momentum densities, were very similar. These results are in reasonable agreement with the band structure calculated for CeRu 2 Si 2 using the localdensity approximation ͑LDA͒. Conversely, in the case of LaRu 2 Si 2 , a clear discrepancy between the LDA calculation and the experiment appears unless the Fermi level (E F ) is raised by ϳ11 mRy. After the E F adjustment the calculated Fermi surfaces are rather similar and in agreement with both experiments. The implications of this similarity on the physics of the heavy fermions are discussed.
Photoemission study of the electronic structure of the Kondo lattices Yb 2 Pt 6 X 15 (X =Al, Ga)
2017
The electronic structure of Yb-based Kondo lattices Yb 2 Pt 6 X 15 (X=Al, Ga) has been investigated by means of photoemission spectroscopy using hard x-ray (hν = 5.95 keV) and vacuum ultraviolet (hν = 182 eV) synchrotron radiation. The Yb 3d spectra of Yb 2 Pt 6 X 15 showed both Yb 2+-and Yb 3+-derived structures, directly indicating valence fluctuation. The Yb valences of Yb 2 Pt 6 Al 15 were estimated to be 2.89 ± 0.01 at 250 K and ∼2.83 at 20 K, while those of Yb 2 Pt 6 Ga 15 were ∼2.34 at 300 K with almost no temperature dependence. With changing X ions from Al to Ga, the Pt 5d and Pt 4f peaks were shifted to shallower binding energies, and the Yb 3+ 4f peaks were shifted to the deeper binding energies. The X dependences of the Yb valence and the Kondo temperature of Yb 2 Pt 6 X 15 are discussed based on the Yb 3+ 4f hole level relative to the Fermi level E F and Pt-derived density of states at E F .