We address the phenomenon of critical Kondo destruction in pseudogap Bose-Fermi Anderson and Kondo quantum impurity models. These models describe a localized level coupled both to a fermionic bath having a density of states that vanishes like |ε|^r at the Fermi energy (ε=0) and, via one component of the impurity spin, to a bosonic bath having a sub-Ohmic spectral density proportional to |ω|^s. Each bath is capable by itself of suppressing the Kondo effect at a continuous quantum phase transition. We study the interplay between these two mechanisms for Kondo destruction using continuous-time quantum Monte Carlo for the pseudogap Bose-Fermi Anderson model with 0<r<1/2 and 1/2<s<1, and applying the numerical renormalization-group to the corresponding Kondo model. At particle-hole symmetry, the models exhibit a quantum critical point between a Kondo (fermionic strong-coupling) phase and a localized (Kondo-destroyed) phase. The two solution methods, which are in good agreement in their domain of overlap, provide access to the many-body spectrum, as well as to correlation functions including, in particular, the single-particle Green's function and the static and dynamical local spin susceptibilities. The quantum-critical regime exhibits the hyperscaling of critical exponents and ω/T scaling in the dynamics that characterize an interacting critical point. The (r,s) plane can be divided into three regions: one each in which the calculated critical properties are dominated by the bosonic bath alone or by the fermionic bath alone, and between these two regions, a third in which the bosonic bath governs the critical spin response but both baths influence the renormalization-group flow near the quantum critical point.
Critical Kondo destruction in a pseudogap Anderson model: scaling and relaxational dynamics
我们研究了赝能隙Anderson模型作为临界近藤破坏的原型系统。我们得到有限温度（T）的标度函数附近的量子临界点，使用连续时间量子蒙特卡罗方法，并考虑到一个动态的大N极限。我们能够确定的标度函数的行为，在典型的难以访问的量子弛豫制度（hbar w < k_BT），并得出结论，自旋和单粒子激发的弛豫率是线性的温度。我们讨论了这些结果的影响，在重费米子金属的量子临界现象。
Quantum criticality in certain heavy-fermion metals is believed to go beyond the Landau framework of order-parameter fluctuations. In particular, there is considerable evidence for Kondo destruction: a disappearance of the static Kondo singlet amplitude that results in a sudden reconstruction of Fermi surface across the quantum critical point and an extra critical energy scale. This effect can be analyzed in terms of a dynamical interplay between the Kondo and RKKY interactions. In the Kondo-destroyed phase, a well-defined Kondo resonance is lost, but Kondo singlet correlations remain at nonze…
Quantum critical points often arise in metals perched at the border of an antiferromagnetic order. The recent observation of singular and dynamically scaling charge conductivity in an antiferromagnetic quantum critical heavy fermion metal implicates beyond-Landau quantum criticality. Here we study the charge and spin dynamics of a Kondo destruction quantum critical point (QCP), as realized in an SU(2)-symmetric Bose-Fermi Kondo model. We find that the critical exponents and scaling functions of the spin and single-particle responses of the QCP in the SU(2) case are essentially the same as thos…
Quantum criticality that goes beyond the Landau framework of order-parameter fluctuations is playing a central role in elucidating the behavior of strange metals. A prominent case appears in Kondo lattice systems, which have been extensively analysed in terms of an effective Bose-Fermi Kondo model. Here, a spin is simultaneously coupled to conduction electron bands and gapless vector bosons that represent magnetic fluctuations. The Bose-Fermi Kondo model features interacting fixed points of Kondo destruction with such properties as dynamical Planckian ($\hbar ω/ k_{\rm B} T$) scaling and loss …
In this work we are interested to studying the Kondo effect present in a system with a $T$-shape ligation between a single-wall carbon nanotube (SWNT) and a magnetic impurity. The system has been studied under hydrostatic pressure and it was observed the opening of the gap in the density of states of the zigzag metallic tube. The pressure can be modeled by the Pierls instability and in this work we consider the out-of-plane distortion. A tight-binding approximation is used to calculate the SWNT Green's functions with hydrostatic pressure applied. We studied the disappearance of the Kondo peak …
The "states-non-conserving" fermion density of states (DOS), deduced from the specific heat of hole-doped cuprates, could arise from a Kondo or heavy fermion-like DOS being suppressed by anti-ferromagnetic spin fluctuations. The large Fermi surface predicted by band theory and observed experimentally, is still expected for zero pseudogap, but with an effective mass corresponding to a Kondo temperature ~ 800 K. A finite pseudogap could divide it into Fermi arcs. Theoretical results for the asymmetric Anderson model can account for the experimental Wilson ratio.
We study the behavior of the entropy of the pseudogap Bose-Fermi Kondo model within a dynamical large-$N$ limit, where $N$ is related to the symmetry group of the model. This model is a general quantum impurity model that describes a localized level coupled to a fermionic bath having a density of states that vanishes in a powerlaw fashion near the Fermi energy and to a bosonic bath possessing a powerlaw spectral density below a cutoff energy. As a function of the couplings to the baths various quantum phase transitions can occur. We study how the impurity entropy changes across these zero-temp…
Metallic quantum criticality is a central theme in a variety of strongly correlated systems. Recent experiments have raised the fundamental question of how the charge response can be singular in cases where the Landau framework of quantum criticality allows singularity only in the spin channel. Motivated by this emerging issue, we study the particle-hole-asymmetric regime of a Bose-Fermi Anderson model with power-law forms for both the bosonic bath spectrum and the fermionic band density of states. We realize a particle-hole-asymmetric quantum-critical state where quasiparticles are lost due t…
Kondo Destruction in Heavy Fermion Quantum Criticality and the Photoemission Spectrum of YbRh$_2$Si$_2$
重费米子金属为研究量子临界性提供了原型。在实验上，量子临界点已经在越来越多的重费米子化合物中被识别和研究。从理论上讲，近藤破坏提供了一种手段来表征一类非常规的量子临界点，超越了朗道框架的序参数波动。这种局域量子临界性的突出证据之一是在YbRh $_2$ Si $_2$中的测量。在有限温度下观察到的霍尔系数和其他运输和热力学量的等温场的依赖性，这指定了一个$T^*(B)$线的温度（$T$）-磁场（$B$）相图中的快速交叉。在这里，我们通过分析交叉宽度与交叉位置的比率来讨论温度升高时会发生什么。在$T \gtrsim 0.5$ K下，该比值接近1，YbRh $_2$ Si $_2$在零磁场下属于量子临界涨落区，其中单粒子谱函数在小费米面和大费米面都具有显著的谱权重。这意味着，在这个温度范围内，任何对费米面敏感的测量也将在大费米面处看到显著的光谱权重。最近报道的YbRh $_2$ Si $_2$在$T>1$ K的角分辨光电子能谱（ARPES）实验与这一预期是一致的，因此支持$T^*(B)$线与近藤破坏物理学的关联。
Nanoelectronic quantum dot devices exploiting the charge-Kondo paradigm have been established as versatile and accurate analog quantum simulators of fundamental quantum impurity models. In particular, hybrid metal-semiconductor dots connected to two metallic leads realize the two-channel Kondo (2CK) model, in which Kondo screening of the dot charge pseudospin is frustrated. Here, we consider theoretically a two-channel charge-Kondo device made instead from graphene components, realizing a pseudogapped version of the 2CK model. We solve the model using Wilson's Numerical Renormalization Group m…
The Anderson model of an $s$-wave single-orbital correlated impurity placed on a noninteracting honeycomb lattice, a simplified model for studying an impurity on graphene, is used to investigate pseudogap Kondo problem. In this model, there are two quantum phases: the phase of free impurity local moment and the Kondo phase where this local moment is fully screened. The transition between these two phases is under investigation. The work focuses mostly on the case where the impurity is placed on top of a lattice site. In this case, the full phase diagram is constructed using three parameters: t…
The appearance of unconventional superconductivity near heavy-fermion quantum critical points (QCPs) motivates investigation of pairing correlations close to a ``beyond Landau'' Kondo-destruction QCP. We focus on a two-Anderson-impurity cluster in which Kondo destruction is induced by a pseudogap in the conduction-electron density of states. Analysis via continuous-time quantum Monte-Carlo and the numerical renormalization group reveals a previously unstudied QCP that both displays the critical-local moment fluctuations characteristic of Kondo destruction and leads to a strongly enhanced singl…
The competition between the indirect exchange interaction (IEC) of magnetic impurities in metals and the Kondo effect gives rise to a rich quantum phase diagram, the Doniach Diagram. In disordered metals, both the Kondo temperature and the IEC are widely distributed due to the scattering of the conduction electrons from the impurity potential. Therefore, it is a question of fundamental importance, how this Doniach diagram is modified by the disorder, and if one can still identify separate phases. Recently, it has been investigated the effect of Ruderman-Kittel-Kasuya-Yosida (RKKY) correlations…
