Field and pressure response of Yb compounds close to a quantum critical point

YbCu_5−x Al _x provides the possibility to tune ground state properties by a change of the valence due to the Cu/Al substitution, by pressure as well as by the application of a magnetic field. Near to the critical concentration x _cr≈1.5 non-Fermi-liquid properties (NFL) are obvious, obeying hyperscaling. If magnetic order sets in for x>1.5, the application of moderate magnetic fields quenches order and again NFL features become evident. Hyperscaling in this case indicates strongly interacting spin fluctuations.     

Strongly correlated Fermi systems as a new state of matter

The aim of this review paper is to expose a new state of matter exhibited by strongly correlated Fermi systems represented by various heavy-fermion (HF) metals, two-dimensional liquids like ³He, compounds with quantum spin liquids, quasicrystals, and systems with one-dimensional quantum spin liquid. We name these various systems HF compounds, since they exhibit the behavior typical of HF metals. In HF compounds at zero temperature the unique phase transition, dubbed throughout as the fermion condensation quantum phase transition (FCQPT) can occur; this FCQPT creates flat bands which in turn lead to the specific state, known as the fermion condensate. Unlimited increase of the effective mass of quasiparticles signifies FCQPT; these quasiparticles determine the thermodynamic, transport and relaxation properties of HF compounds. Our discussion of numerous salient experimental data within the framework of FCQPT resolves the mystery of the new state of matter. Thus, FCQPT and the fermion condensation can be considered as the universal reason for the non-Fermi liquid behavior observed in various HF compounds. We show analytically and using arguments based completely on the experimental grounds that these systems exhibit universal scaling behavior of their thermodynamic, transport and relaxation properties. Therefore, the quantum physics of different HF compounds is universal, and emerges regardless of the microscopic structure of the compounds. This uniform behavior allows us to view it as the main characteristic of a new state of matter exhibited by HF compounds.     

杂质引起的非费密液体行为

研究含杂质散射的电子系统的非费密液体行为.采用单杂质模型,通过格林函数的计算,证明无论有无束缚态,n(=3,2,1)维电子系统都可出现非费密液体行为. 关键词： 杂质 非费密液体行为     

Scaling behavior of the thermopower of the archetypal heavy-fermion metal YbRh2Si2

We reveal and explain the scaling behavior of the thermopower S/T exhibited by the archetypal heavy-fermion (HF) metal YbRh₂Si₂ under the application of magnetic field B at temperature T. We show that the same scaling is demonstrated by different HF compounds such as β-YbAlB₄ and the strongly correlated layered cobalt oxide [BiBa_0.66K_0.36O₂]CoO₂. Using YbRh₂Si₂ as an example, we demonstrate that the scaling behavior of S/T is violated at the antiferromagnetic phase transition, while both the residual resistivity ρ₀ and the density of states, N, experience jumps at the phase transition, causing the thermopower to make two jumps and change its sign. Our elucidation is based on flattening of the single-particle spectrum that profoundly affects ρ₀ and N. To depict the main features of the S/T behavior, we construct a T –B schematic phase diagram of YbRh₂Si₂. Our calculated S/T for the HF compounds are in good agreement with experimental facts and support our observations.     

A sport and a pastime: Model design and computation in quantum many-body systems

We summarize the recent developments in the model design and computation for a few representative quantum many-body systems, encompassing quantum critical metals beyond the Hertz-Millis-Moriya framework with pseudogap and superconductivity, SYK non-Fermi-liquid with self-tuned quantum criticality and fluctuation induced superconductivity, and the flat-band quantum Moiré lattice models in continuum where the interplay of quantum geometry of flat-band wave function and the long-range Coulomb interactions gives rise to novel insulating phases at integer fillings and superconductivity away from them. Although the narrative choreography seems simple, we show how important the appropriate model design and their tailor-made algorithmic developments - in other words, the scientific imagination inspired by the corresponding fast experimental developments in the aforementioned systems - compel us to invent and discover new knowledge and insights in the sport and pastime of quantum many-body research.