Abstract
We study the quantum phase transitions in the nickel pnctides, CeNi
2−
δ
(As
1−
x
P
x
)
2
(
δ
≈ 0.07–0.22) polycrystalline samples. This series displays the distinct heavy fermion behavior ...in the rarely studied parameter regime of dilute carrier limit. We systematically investigate the magnetization, specific heat and electrical transport down to low temperatures. Upon increasing the P-content, the antiferromagnetic order of the Ce-4
f
moment is suppressed continuously and vanishes at
x
c
~ 0.55. At this doping, the temperature dependences of the specific heat and longitudinal resistivity display non-Fermi liquid behavior. Both the residual resistivity
ρ
0
and the Sommerfeld coefficient
γ
0
are sharply peaked around
x
c
. When the P-content reaches close to 100%, we observe a clear low-temperature crossover into the Fermi liquid regime. In contrast to what happens in the parent compound
x
= 0.0 as a function of pressure, we find a surprising result that the non-Fermi liquid behavior persists over a nonzero range of doping concentration,
x
c
<
x
< 0.9. In this doping range, at the lowest measured temperatures, the temperature dependence of the specific-heat coefficient is logarithmically divergent and that of the electrical resistivity is linear. We discuss the properties of CeNi
2−
δ
(As
1−
x
P
x
)
2
in comparison with those of its 1111 counterpart, CeNi(As
1−
x
P
x
)O. Our results indicate a non-Fermi liquid phase in the global phase diagram of heavy fermion metals.
We investigate the competition between the spin-orbit interaction of itinerant electrons and their Kondo coupling with local moments densely distributed on the honeycomb lattice. We find that the ...model at half-filling displays a quantum phase transition between topological and Kondo insulators at a nonzero Kondo coupling. In the Kondo-screened case, tuning the electron concentration can lead to a new topological insulator phase. The results suggest that the heavy-fermion phase diagram contains a new regime with a competition among topological, Kondo-coherent and magnetic states, and that the regime may be especially relevant to Kondo lattice systems with 5d-conduction electrons. Finally, we discuss the implications of our results in the context of the recent experiments on SmB(6) implicating the surface states of a topological insulator, as well as the existing experiments on the phase transitions in SmB(6) under pressure and in CeNiSn under chemical pressure.
A U(1) slave-spin representation is introduced for multiorbital Hubbard models. As with the Z sub(2) form of de'Medici et al. Phys. Rev. B 72, 205124 (2005) ...(http://dx.doi.org/10.1103/PhysRevB.72.205124), this approach represents a physical electron operator as the product of a slave spin and an auxiliary fermion operator. For nondegenerate multiorbital models, our U(1) approach is advantageous in that it captures the noninteracting limit at the mean-field level. For systems with either a single orbital or degenerate multiple orbitals, the U(1) and Z sub(2) slave-spin approaches yield the same results in the slave-spin-condensed phase. In general, the U(1) slave-spin approach contains a U(1) gauge redundancy, and properly describes a Mott insulating phase. We apply the U(1) slave-spin approach to study the metal-to-insulator transition in a five-orbital model for parent iron pnictides. We demonstrate a Mott transition as a function of the interactions in this model. The nature of the Mott insulating state is influenced by the interplay between the Hund's rule coupling and crystal-field splittings. In the metallic phase, when the Hund's rule coupling is beyond a threshold, there is a crossover from a weakly correlated metal to a strongly correlated one, through which the quasiparticle spectral weight rapidly drops. The existence of such a strongly correlated metallic phase supports the incipient Mott picture of the parent iron pnictides. In the parameter regime for this phase and in the vicinity of the Mott transition, we find that an orbital selective Mott state has nearly as competitive a ground-state energy.
Non-volatile phase-change memory devices utilize local heating to toggle between crystalline and amorphous states with distinct electrical properties. Expanding on this kind of switching to two ...topologically distinct phases requires controlled non-volatile switching between two crystalline phases with distinct symmetries. Here, we report the observation of reversible and non-volatile switching between two stable and closely related crystal structures, with remarkably distinct electronic structures, in the near-room-temperature van der Waals ferromagnet Fe
GeTe
. We show that the switching is enabled by the ordering and disordering of Fe site vacancies that results in distinct crystalline symmetries of the two phases, which can be controlled by a thermal annealing and quenching method. The two phases are distinguished by the presence of topological nodal lines due to the preserved global inversion symmetry in the site-disordered phase, flat bands resulting from quantum destructive interference on a bipartite lattice, and broken inversion symmetry in the site-ordered phase.
Abstract
Electronic correlation is of fundamental importance to high temperature superconductivity. While the low energy electronic states in cuprates are dominantly affected by correlation effects ...across the phase diagram, observation of correlation-driven changes in fermiology amongst the iron-based superconductors remains rare. Here we present experimental evidence for a correlation-driven reconstruction of the Fermi surface tuned independently by two orthogonal axes of temperature and Se/Te ratio in the iron chalcogenide family FeTe
1−
x
Se
x
. We demonstrate that this reconstruction is driven by the de-hybridization of a strongly renormalized
d
x
y
orbital with the remaining itinerant iron 3
d
orbitals in the emergence of an orbital-selective Mott phase. Our observations are further supported by our theoretical calculations to be salient spectroscopic signatures of such a non-thermal evolution from a strongly correlated metallic phase into an orbital-selective Mott phase in
d
x
y
as Se concentration is reduced.
Motivated by the global phase diagram of antiferromagnetic heavy-fermion metals, we study the Kondo effect from the perspective of a nonlinear sigma model in the one-dimensional Kondo-Heisenberg ...model away from half-filling. We focus on the effects of the instanton configurations of the sigma-model field and the associated Berry phase. Guided by the results derived using bosonization methods, we demonstrate that the Kondo-singlet formation is accompanied by an emergent Berry phase. This Berry phase also captures the competition between the Kondo-singlet formation and spin-Peierls correlations. Related effects are likely to be realized in Kondo lattice systems in higher dimensions.
SmB6 has been a well-known Kondo insulator for decades, but recently attracts extensive new attention as a candidate topological system. Studying SmB6 under pressure provides an opportunity to ...acquire the much-needed understanding about the effect of electron correlations on both the metallic surface state and bulk insulating state. Here we do so by studying the evolution of two transport gaps （low tempera- ture gap El and high temperature gap Eh） associated with the Kondo effect by measuring the electrical resistivity under high pressure and low temperature （0.3 K） conditions. We associate the gaps with the bulk Kondo hybridization, and from their evolution with pressure we demonstrate an insulator-to- metal transition at ~4 GPa. At the transition pressure, a large change in the Hall number and a divergence tendency of the electron-electron scattering coefficient provide evidence for a destruction of the Kondo entanglement in the ground state. Our results raise the new prospect for studying topological electronic states in quantum critical materials settings.
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