A Study of Heavy-Fermion Superconductors via BCS Equations Incorporating Chemical Potential

Heavy-fermion superconductors (HFSCs) are regarded as outside the purview of BCS theory because it is usually constrained by the inequality , where EF, μ, kB, and θD are, respectively, the Fermi energy, chemical potential, Boltzmann constant, and the Debye temperature. We show that this restriction can be removed by incorporating μ into the equations for Tc and the gap Δ0 at T = 0. Further, when μ kBθD, we curtail the limits of the equations for Tc and Δ0 to avoid complex-valued solutions. The resulting equations are applied to a prominent member of the HFSC family, i.e., CeCoIn5, by appealing to ideas due to Born and Karmann, Suhl et al., and Bianconi et al. Since the equations now contain an additional variable μ, we find that 1) the Tc of the SC can be accounted for by a multitude of values of the (μ, λ) pair, λ being the interaction parameter;2) the λ vs. μ plot has a dome-like structure when μ kBθD;3) the (μ, λ) values obtained in 2) lead to reasonable results for the range of each of the following variables: Δ0, s, and n, where s is the ratio of the mass of a conduction electron and the free electron mass and n is the number density of charge carriers in the SC.

Hybridization Effects Revealed by Angle-Resolved Photoemission Spectroscopy in Heavy-Fermion Ce2IrIn8

We utilize high-resolution resonant angle-resolved photoemission spectroscopy(ARPES)to study the band structure and hybridization effect of the heavy-fermion compound Ce2 IrIn8.We observe a nearly flat band at the binding energy of 7 meV below the coherent temperature Tcoh^40 K,which characterizes the electrical resistance maximum and indicates the onset temperature of hybridization.However,the Fermi vector and the Fermi surface volume have little change around Tcoh,which challenges the widely believed evolution from a hightemperature small Fermi surface to a low-temperature large Fermi surface.Our experimental results of the band structure fit well with the density functional theory plus dynamic mean-field theory calculations.

Uniaxial stress effect on quasi-one-dimensional Kondo lattice CeCo_(2)Ga_(8)

Quantum critical phenomena in the quasi-one-dimensional limit remain an open issue.We report the uniaxial stress effect on the quasi-one-dimensional Kondo lattice CeCo_(2)Ga_(8) by electric transport and AC heat capacity measurements.CeCo_(2)Ga_(8) is speculated to sit in close vicinity but on the quantum-disordered side of a quantum critical point.Upon compressing the c axis,parallel to the Ce-Ce chain,the onset of coherent Kondo effect is enhanced.In contrast,the electronic specific heat diverges more rapidly at low temperature when the intra-chain distance is elongated by compressions along a or b axis.These results suggest that a tensile intra-chain strain(ε_(c)>0)pushes CeCo_(2)Ga_(8) closer to the quantum critical point,while a compressive intra-chain strain(ε_(c)<0)likely causes departure.Our work provides a rare paradigm of manipulation near a quantum critical point in a quasi-1D Kondo lattice by uniaxial stress,and paves the way for further investigations on the unique feature of quantum criticality in the quasi-1D limit.

Lifshitz transition in triangular lattice Kondo-Heisenberg model

Motivated by recent experimental progress on triangular lattice heavy-fermion compounds,we investigate possible Lifshitz transitions and the scanning tunnel microscope(STM)spectra of the Kondo-Heisenberg model on the triangular lattice.In the heavy Fermi liquid state,the introduced Heisenberg antiferromagnetic interaction(JH)results in the twice Lifshitz transition at the case of the nearest-neighbour electron hopping but with next-nearest-neighbour hole hopping and the case of the nearest-neighbour hole hopping but with next-nearest-neighbour electron hopping,respectively.Driven by JH,the Lifshitz transitions on triangular lattice are all continuous in contrast to the case on square lattice.Furthermore,the STM spectra shows rich line-shape which is influenced by the Kondo coupling JK,the Heisenberg antiferromagnetic interaction JH,and the ratio of the tunneling amplitude of f-electron tf versus conduction electron tc.Our work provides a possible scenario to understand the Fermi surface topology and the quantum critical point in heavy-fermion compounds.

Impurity-induced bound states as a signature of pairing symmetry in multiband superconducting CeCu2Si2

The notion of multiband superconductivity with dominant two-gap features has been recently applied to the unconventional superconductor CeCu2Si2 for challenging the previously accepted concept of nodal d-wave pairing. In the proposed study, the realistic multiband Fermi surface topology of CeCu2Si2 was obtained through first-principles calculations, and analysis was conducted with an effective two-band hybridization model including detailed band structure. Within the T-matrix approximation, the obtained calculation results show that different pairing candidates, including fully gapped s-wave, loop-nodal s-wave, and d-wave pairings, could yield qualitatively distinct features characterized by impurity-induced bound states. These features can be verified through high-resolution scanning tunneling microscopy or spectroscopy and provide corroborative justification that would be beneficial for the ongoing research regarding the superconducting gap symmetry of CeCu2Si2 at ambient pressure.

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 heavyfermion 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 4f 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 130K,whereas the heavy-fermion band formation temperature is between 40 and 80K;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.