Bulletin of the American Physical Society
APS March Meeting 2020
Volume 65, Number 1
Monday–Friday, March 2–6, 2020; Denver, Colorado
Session L56: Kondo Models 
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Sponsoring Units: DCMP Chair: Elio Koenig, Rutgers University, New Brunswick Room: Mile High Ballroom 2C 
Wednesday, March 4, 2020 8:00AM  8:12AM 
L56.00001: Topology and the doped Kondo Heisenberg model Julian MayMann, Ryan Levy, Eduardo Fradkin, Bryan Clark, Rodrigo B Soto, Gil Young Cho In previous works, it has been proposed that the doped Kondo Heisenberg model is topological and hosts Majorana zero modes. These Majorana zero modes correspond to solitons of the gapped spin sector of the model. We have investigated this claim on the lattice using DMRG, and found no evidence for these proposed zero modes. In light of this, we revisit the original claims that the model is topological using bosonization. We discuss how this continuum analysis fits with the numeric data and the lattice model. 
Wednesday, March 4, 2020 8:12AM  8:24AM 
L56.00002: Fermionic multichannel dynamical largeN approach to Kondo lattice model Jiangfan Wang, YungYeh Chang, ChungHou Chung We establish a novel theoretical framework to study the quantum phases and phase transitions of a two dimensional Kondo Heisenberg lattice model based on the dynamical largeN approach to the multichannel pseudofermion representation of the model with Sp(N) × SU(K) symmetry with N and K being number of spin flavors and Kondo screening channels, respectively. This approach captures the heavy electron Fermi liquid state when SU(K) symmetry is broken with Kondo hybridization in terms of Bosecondensed holons, and a spinliquid metallic state with gaped fermionic spinons when the antiferromagnetic coupling dominates. A selfconsistent Green’s function method is used. The global phase diagram and the thermodynamic properties of the spin liquid state share close resemblance to CePdAl, a heavyfermion systems with magnetic frustration. 
Wednesday, March 4, 2020 8:24AM  8:36AM 
L56.00003: Kondo effect due to a hydrogen impurity in graphene: A multichannel Kondo problem with diverging hybridization Zheng Shi, Emilian Nica, Ian Affleck We consider the Kondo effect arising from a hydrogen impurity in graphene. Approximating the CH covalent bond as infinitely strong and the Hubbard interaction to be present only on the three nearest neighbors of hydrogen impurity, we obtain a Kondo model with three Z_{3}symmetric impurity spins and three conduction channels, two of which support a diverging local density of states ∝1/[ω ln^{2}(Λ/ω)] near the Dirac point ω→0. When the particlehole (ph) symmetry breaking at the impurity is not too strong, numerical renormalization group shows that the ground state is either a ph symmetric spin1/2 doublet with ferromagnetic impurity spin correlations, or a ph asymmetric spin singlet with antiferromagnetic impurity spin correlations. This behavior is inherited by the Anderson model containing the hydrogen impurity and all four carbon atoms in its vicinity. 
Wednesday, March 4, 2020 8:36AM  8:48AM 
L56.00004: Magnon BoseEinstein Condensation and Superconductivity in a Frustrated Kondo Lattice Jed Pixley, Pavel Volkov, Snir Gazit Magnetically frustrated Kondo lattices are considered to be one of the most promising platforms for the realization of novel types of metallic quantum criticality. Motivated by the recent experiments on the heavy fermion compounds with a nonmagnetic valence bond solid (VBS) ground state of the localized spins, such as YbAl_{3}C_{3}, we study a onedimensional twoleg spin ladder model with a VBS ground state doped with itinerant fermions. Using field theory techniques and precise density matrix renormalization group calculations, we provide a solution of the model as a function of the Kondo coupling and magnetic field. We demonstrate that the magnetic fielddriven transition to an easyplane antiferromagnet (known as magnon BoseEinstein condensation (BEC) in the insulating limit) is stable in the presence of a Fermi sea and its universality class is unchanged, while the critical field decreases as a function of Kondo coupling. We also find that spin fluctuations in the VBS phase can drive unconventional superconducting correlations. Finally, we argue that, depending on the filling of conduction electrons, the magnon BEC transition can remain stable in a metal also in dimensions two and three. 
Wednesday, March 4, 2020 8:48AM  9:00AM 
L56.00005: Magnetic Doublon Bound States in the Kondo Lattice Model Roman Rausch, Michael Potthoff, Norio Kawakami We present a novel pairing mechanism for electrons, mediated by magnons. These paired bound states are termed magnetic doublons. Applying numerically exact techniques (exact diagonalization and the densitymatrix renormalization group, DMRG) to the Kondo lattice model at strong exchange coupling J for different fillings and magnetic configurations, we demonstrate that magnetic doublon excitations exist as composite objects with extremely weak dispersion at excitation energies of the order of 3J/2 above the ground state. They are highly stable, support a novel "inverse'' colossal magnetoresistance and potentially other effects like metastable superconductivity, or cooling via quantum distillation. 
Wednesday, March 4, 2020 9:00AM  9:12AM 
L56.00006: Competing Energy Scales and Quasiparticle Dynamics in a Driven Kondo Impurity System ChenYen Lai, Qimiao Si, JianXin Zhu The ultrafast pumpprobe techniques are used to probe the elementary excitations and its energy scale in materials. Recent study [Nat. Pays. 14 1103 (2018)] shows the collapse and revival of the Kondo correlations from terahertz reflectivity measurement. Here we propose a model study as a proof of principle to first determine the corresponding Kondo energy scale by using the matrix product state to simulate the FermiBoseAnderson impurity model, which is a good starting model for the study of Kondo lattice model. In equilibrium, the model displays a quantum critical point that captures the competition between RKKY interaction and Kondo coupling in the underlying system. As a laser pump pulse is applied to the system, our nonequilibrium simulations show that the driven system exhibits qualitatively different response, which is related to the competing energy scales. Our results benchmark the competing energy scales in a Kondo impurity system and provide new insight on the behavior of the critical quasiparticle in heavy fermion compounds. 
Wednesday, March 4, 2020 9:12AM  9:24AM 
L56.00007: Universality in the ferromagnetic strong coupling regime of the Kondo model out of equilibrium Adrian Culver, Natan Andrei We present a nonperturbative method for calculating the timedependent many body wavefunction following a local quench and use it to find new results in and out of equilibrium in the universal strong ferromagnetic regime of the Kondo model. The method addresses the general problem of calculating e^{i H t }Ψ, where the initial state Ψ is a free Nparticle state (such as a Fermi sea) and H is a many body Hamiltonian which is switched on at t=0. While the method may be of wider applicability, we have so far focused on one dimensional models of quantum dots attached to linearized leads. The method yields the exact many body wavefunction of the two lead Kondo model following the quench. In the strong ferromagnetic regime, we find a new universal scale T_{K} = D^{ }exp[3π^{2} ρ J / 8] and the surprising result that the steady state conductance dI/dV reaches the unitarity limit asymptotically at large voltage or temperature. 
Wednesday, March 4, 2020 9:24AM  9:36AM 
L56.00008: Comparing Different Refermionizations of Multichannel Kondo Hamiltonians Aleksandar Ljepoja, Nayana Shah, C. J. Bolech Following a critical revision of the procedures for bosonization and debosonization of theories with boundaries [1,2] we consider novel alternative schemes to arrive at refermionized Hamiltonians for the standard multichannel Kondo model. We compare them order by order using Greenfunction based perturbative expansions and discuss the results on the basis of symmetries and decoupled dynamical sectors of the original theory. 
Wednesday, March 4, 2020 9:36AM  9:48AM 
L56.00009: Various Fermi liquid phases on the Kondo  Heisenberg model at quarter filling Hee Seung Kim, HyeokJun Yang, SungBin Lee Motivated by exotic phases emerged from Kondo insulators, we study transport properties of electronic system under Kondo and Heisenberg interactions. Particularly, we focus on the Kondo  AF (antiferromagnetic) Heisenberg model on a honeycomb lattice at quarter filling. By using parton meanfield theory, we construct generic meanfield phase diagram and investigate distinct Fermi liquid phases. In strong Kondo coupling limit, due to quarter filling condition, half of localized moments form Kondo singlet and the others develop triangular lattice with AF Heisenberg interaction which yields spin liquid phase. To investigate the lowenergy excitation properties, we evaluate transport coefficients of mobile electrons and fractionalized quasiparticles respectively. Even though mobility of charge carriers is suppressed, Fermi liquid behavior is veried arose from neutral lowenergy excitations. 
Wednesday, March 4, 2020 9:48AM  10:00AM 
L56.00010: Emergent nonFermi liquid phenomena in itinerant electron systems with multipolar local moments Adarsh Patri, Ilia Khait, YongBaek Kim Recent experiments on cubic heavy fermion systems with multipolar local moments offer a new avenue for the discovery of novel spinorbital entangled quantum ground states. In this talk, we theoretically investigate the fate of the multipolar Kondo effect, where the local moment does not carry any dipole moment, but only quadrupolar and octupolar moments. Using perturbative renormalization group methods, we discover a number of nonFermi liquid ground states, which are characterized by an absence of welldefined quasiparticles and singular powerlaw behaviours in physical properties. This work lays a novel ground for the identification of emergent nonFermi liquid phases in many heavy fermion materials. 
Wednesday, March 4, 2020 10:00AM  10:12AM 
L56.00011: Global phase diagram of a Kondo Hund impurity model and the destruction of Fermiliquid theory Yilin Wang, Elias Walter, SeungSup Lee, Katharina M Stadler, Jan Von Delft, Andreas Weichselbaum, Gabriel Kotliar In many correlated materials, notably the Hund metals, the Landau Fermiliquid coherence scale T _{FL }is found to be very small. In this Letter, we interpret its smallness in terms of proximity to quantum critical points (QCPs): We use the numerical renormalization group (NRG) to compute the global phase diagram of the simplest threechannel spinorbital Kondo impurity model capturing the essential physics of Hund metals. When the spin or spinorbital Kondo couplings are tuned into the ferromagnetic regimes, we find quantum phase transitions to a singular Fermiliquid or a novel nonFermiliquid phase, signalling the existence of QCPs, while T _{FL }is suppressed to zero. The new nonFermiliquid phase shows frustrated behavior involving alternating overscreenings in spin and orbital sectors, with universal power laws in the spin (ω^{1/5}), orbital (ω^{1/5}) and spinorbital (ω^{1}) dynamical susceptibilities. These power laws, and the NRG eigenlevel spectra, can be fully understood using conformal field theory arguments, which also clarify the nature of the NFL phase. 
Wednesday, March 4, 2020 10:12AM  10:24AM 
L56.00012: Dynamical Kondo effect and Kondo destruction in effective models for quantumcritical heavy fermion metals Ang Cai, Haoyu Hu, Kevin Ingersent, Silke BuehlerPaschen, Qimiao Si Kondo destruction that results in a sudden reconstruction of Fermi surface at the quantum critical point arises in certain heavyfermion metals [1,2,3,4]. In the Kondodestroyed phase, the STATIC Kondo singlet amplitude vanishes and, thus, a welldefined Kondo resonance is lost, but the Kondo singlet correlations remain at nonzero frequencies. We elucidate the dynamical Kondo effect in BoseFermi Kondo/Anderson models which unambiguously exhibit a Kondodestruction QCP [5]. We demonstrate that the dynamical Kondo effect is important for the stability of Kondodestruction quantum criticality and provides understanding of the enhanced effective mass in the Kondodestroyed phase. 
Wednesday, March 4, 2020 10:24AM  10:36AM 
L56.00013: Multiorbital Hund's coupled impurity in the mixed valence regime Victor DrouinTouchette, Elio Koenig, Yashar Komijani, Piers Coleman Motivated by the relevance of Hund’s coupling in ironbased superconductors, we revisit the problem of a multiorbital Anderson impurity with Hund’s interaction. Using largeN and Schwinger boson techniques, we study the ground state and thermodynamic properties of this system in both integer and mixed valence regimes. The physics is characterized by the interplay of Hund’s coupling, which tends to form large moments by aligning the spins of the impurity, and the Kondo effect, which leads to the lowtemperature screening of the moments. In the integer valence regime, we confirm the formation of large moments, which eventually become screened at an exponentially reduced Kondo temperature due to the socalled Schrieffer effect, and compare to previous renormalization group studies [1]. In the mixed valence regime, the Hund’s coupling gives rise to interesting physics: we explore the presence of a nonFermiliquid ground state and the possibility that the large moments can generate retarded onsite pairing correlations for the conduction electrons. 
Wednesday, March 4, 2020 10:36AM  10:48AM 
L56.00014: Numerical renormalization group method for computing fourpoint correlation functions SeungSup Lee, Fabian Kugler, Jan Von Delft Fourpoint correlation functions commonly appear in various contexts of the theory of strongly correlated systems, including diagrammatic extensions of dynamical meanfield theory (DMFT). Here we develop the numerical renormalization group (NRG) method for computing fourpoint correlation functions in quantum impurity systems. First, we derive the Lehmann representation for general fourpoint functions (i) in imaginary Matsubara frequencies, (ii) on the realfrequency axes at zero temperature, and (iii) on the Keldysh contour. By using the complete basis of energy eigenstates constructed within NRG, fourpoint functions can be computed at arbitrarily low temperatures. We present results for paradigmatic models, including the effective quantum impurity model arising in DMFT treatments of the Hubbard model. 

L56.00015: Fractionalized Fermi liquid in squarelattice KondoHeisenberg model YungYeh Chang, ChungHou Chung, Alexei Tsvelik The KondoHeisenberg model on a (unfrustrated ) square lattice is studied via a controlled largeN (Sp(2N)) approach to demonstrate on a general ground the existence of a peculiar metallic state socalled “fractionalized Fermi liquid (FL*)” with unbroken translational symmetry and a Fermi surface volume not controlled by the total electron density. Close to halffilling, we show that the nesting of Fermi surface favors the Kondo hybridization to form between conduction electrons and local resonantvalencebond spinliquid fermions, which stabilizes FL* state with welldefined quasiparticle and gapped spinon excitations. The system develops an ordered phase at ground state (charge density wave or pair density wave superconducting). Our result generalizes the FL* state previously proposed in Phys. Rev. Lett. 90, 216403 (2003), which is restricted to frustrated Kondo lattice systems. 
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