Interaction Driven Subgap Spin Exciton in the Kondo Insulator SmB6
W. T. Fuhrman, J. Leiner, P. Nikolić, G. E. Granroth, M. B. Stone, M. D. Lumsden, L. DeBeer-Schmitt, P. A. Alekseev, J. -M. Mignot, S. M. Koohpayeh, P. Cottingh… 展开作者
W. T. Fuhrman, J. Leiner, P. Nikolić, G. E. Granroth, M. B. Stone, M. D. Lumsden, L. DeBeer-Schmitt, P. A. Alekseev, J. -M. Mignot, S. M. Koohpayeh, P. Cottingham, W. Adam Phelan, L. Schoop, T. M. McQueen, C. Broholm
Using inelastic neutron scattering, we map a 14 meV coherent resonant mode in the topological Kondo insulator SmB6 and describe its relation to the low energy insulating band structure. The resonant intensity is confined to the X and R high symmetry points, repeating outside the first Brillouin zone and dispersing less than 2 meV, with a 5d-like magnetic form factor. We present a slave-boson treatment of the Anderson Hamiltonian with a third neighbor dominated hybridized band structure. This approach produces a spin exciton below the charge gap with features that are consistent with the observed neutron scattering. We find that maxima in the wave vector dependence of the inelastic neutron scattering indicate band inversion.
We study the low energy spectrum of a correlated quantum dot embedded between the normal conducting and superconducting reservoirs and hybridized with the topological superconducting nanowire, hosting the Majorana end-modes. We investigate the leaking Majorana quasiparticle and inspect its interplay with the proximity induced on-dot pairing and correlations. In particular, we focus on the subgap Kondo effect near the quantum phase transition/crossover from the spinfull (doublet) to the spinless (BCS-type singlet) configurations. Treating the correlations perturbatively and within the NRG appro…
Microscopic resolution of the interplay of Kondo screening and superconducting pairing: Mn-phthalocyanine molecules adsorbed on superconducting Pb(111)
The quantum chromodynamics (QCD) Kondo effect is a quantum phenomenon in which heavy quarks ($c$, $b$) exist as impurity particles in quark matter composed of light quarks ($u$, $d$, $s$) at extremely high density. This is analogous to the famous Kondo effect in condensed matter physics. In the present paper, we show theoretically the existence of the "QCD Kondo excitons", i.e., the bound states of light quarks and heavy quarks, as the lowest-excitation modes above the ground state of the quark matter governed by the QCD Kondo effect. These are neutral for color and electric charges, similarly…
The true topological nature of the Kondo insulator SmB$_6$ remains to be unveiled. Our previous tunneling study not only found evidence for the existence of surface Dirac fermions, but it also uncovered that they inherently interact with the spin excitons, collective excitations in the bulk. We have extended such a spectroscopic investigation into crystals containing a Sm deficiency. The bulk hybridization gap is found to be insensitive to the deficiency up to 1% studied here, but the surface states in Sm-deficient crystals exhibit quite different temperature evolutions from those in stoichiom…
The underscreened Kondo lattice consisting of a single twofold degenerate conduction band and a CEF split 4f-electron quasi-quartet has non-conventional quasiparticle dispersions obtained from the constrained mean-field theory. An additional genuinely heavy band is found in the main hybridization band gap of the upper and lower hybridzed bands whose heavy effective mass is controlled by the CEF splitting. Its presence should profoundly influence the dynamical optical and magnetic response functions. In the former, the onset of the optical conductivity is not the main hybridisation energy but t…
We study the quasiparticle spectrum in an underscreened Kondo-lattice (KL) model that involves a single spin degenerate conduction band and two crystalline-electric-field (CEF) split Kramers doublets coupled by both orbital-diagonal and non-diagonal exchange interactions. We find the three quasiparticle bands of the model using a constrained fermionic mean field approach. While two bands are similar to the one-orbital model a new genuinely heavy band inside the main hybridization gap appears in the quasiquartet model. Its dispersion is due to effective hybridization with conduction states but …
The Kondo insulator SmB6 is purported to develop into a robust topological insulator at low temperature. Yet there are several puzzling and unexplained physical properties of the insulating bulk. It has been proposed that bulk spin excitons may be the source of these anomalies and may also adversely affect the topologically-protected metallic surface states. Here, we report muon spin rotation measurements of SmB6 that show thermally-activated behavior for the temperature dependences of the transverse-field (TF) relaxation rate below 20 K and muon Knight shift below 5-6 K. Our data are consiste…
Topological surface states interacting with bulk excitations in the Kondo insulator SmB$_6$ revealed via planar tunneling spectroscopy
六硼化钐（SmB $_6$）是一种著名的近藤绝缘体，其绝缘体来自于强电子关联，由于其拓扑性质的证据越来越多，从而具有受保护的表面态，最近引起了人们的极大关注。然而，与包括Bi $_2$ Se $_3$在内的弱相关对应物不同，仍然缺乏确凿的光谱证据。在这里，我们报告的结果，揭示了详细的光谱特性的钐$_6$平面隧道。在（001）和（011）单晶表面上获得的隧穿电导分别揭示了两个和一个狄拉克锥的线性态密度。非常值得注意的是，它被发现，这些拓扑状态没有被完全保护内的散装杂化间隙。一个现象学模型的隧道过程调用的表面态与体激发（自旋激子）的相互作用，如最近的理论预测，提供了一个一致的解释所有观察到的功能。我们的光谱研究支持并解释了在这种拓扑近藤绝缘体钐$_6$的不完全保护的表面状态的建议图片。
We review the cubic rare-earth boride series which displays diverse electronic states like localized 4f electron multiplets split by the crystal electric field (CEF), itinerant heavy-fermion quasiparticle bands of the Kondo lattice as well as gapped Kondo insulator or mixed-valent semiconductor states. Fairly exotic states may appear due to the 'hidden' order of multipoles carried by degenerate CEF multiplets, in addition to magnetic order present in many RB6 (R = rare earth) compounds. Most prominent are CeB6 and its La-diluted alloys which exhibit antiferro-quadrupolar and octupolar ordering…
Despite more than half a century of studies in heavy-fermion compounds, a full understanding of the various possible magnetic ordering phenomena is still far from complete. Some heavy-fermion materials show so-called hidden-order phases, which are invisible to conventional diffraction techniques. The multipolar moments of the $f$-electrons in their specific crystal field environment play a decisive role in the formation of these phases. Such hidden-order phases have been observed in a variety of compounds containing $4f$ and $5f$ elements, like URu$_2$Si$_2$, NpO$_2$, YbRu$_2$Ge$_2$, and CeB$_…
By means of new muon spin relaxation experiments, we disentangle extrinsic and intrinsic sources of low-temperature bulk magnetism in the candidate topological Kondo insulator (TKI) SmB6. Results on Al-flux grown SmB6 single crystals are compared to those on a large floating-zone grown ^{154}Sm^{11}B6 single crystal in which a 14 meV bulk spin exciton has been detected by inelastic neutron scattering. Below ~ 10 K we detect the gradual development of quasi-static magnetism due to rare-earth impurities and Sm vacancies. Our measurements also reveal two additional forms of intrinsic magnetism: 1…
In recent years it has become understood that quantum oscillations of the magnetization as a function of magnetic field, long recognized as phenomena intrinsic to metals, can also manifest in insulating systems. Theory has shown that in certain simple band insulators, quantum oscillations can appear with a frequency set by the area traced by the minimum gap in momentum space, and are suppressed for weak fields by an intrinsic "Dingle damping" factor reflecting the size of the bandgap. Here we examine quantum oscillations of the magnetization in excitonic and Kondo insulators, for which interac…
Recently discovered advanced materials, such as heavy fermions, frequently exhibit a rich phase diagram suggesting the presence of different competing interactions. A unified description of the origin of these multiple interactions, albeit very important for the comprehension of such materials is, in general not available. It would be therefore very useful to have a simple model where the common source of different interactions could be possibly traced back. In this work we consider a system consisting in a set of localized spins on a square lattice with antiferromagnetic nearest neighbors int…
