Browsing by Author "Liu, Chia-Chuan"

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    Quantum criticality enabled by intertwined degrees of freedom
    (National Academy of Sciences, 2023) Liu, Chia-Chuan; Paschen, Silke; Si, Qimiao; Rice Center for Quantum Materials
    Strange metals appear in a wide range of correlated materials. Electronic localization–delocalization and the expected loss of quasiparticles characterize beyond-Landau metallic quantum critical points and the associated strange metals. Typical settings involve local spins. Systems that contain entwined degrees of freedom offer new platforms to realize unusual forms of quantum criticality. Here, we study the fate of an SU(4) spin–orbital Kondo state in a multipolar Bose–Fermi Kondo model, which provides an effective description of a multipolar Kondo lattice, using a renormalization-group method. We show that at zero temperature, a generic trajectory in the model’s parameter space contains two quantum critical points, which are associated with the destruction of Kondo entanglement in the orbital and spin channels, respectively. Our asymptotically exact results reveal an overall phase diagram, provide the theoretical basis to understand puzzling recent experiments of a multipolar heavy fermion metal, and point to a means of designing different forms of quantum criticality and strange metallicity in a variety of strongly correlated systems.
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    Quantum criticality of strongly correlated systems
    (2020-04-22) Liu, Chia-Chuan; Si, Qimiao
    Quantum criticality has been an active research topic in condensed matter physics, with major efforts being devoted to the heavy fermion material in which local moments are coupled with itinerant electrons through Kondo coupling. The competition between Kondo coupling and the antiferromagnetic RKKY coupling between local moments leads to a rich global phase diagram for these systems. It is a fundamentally important but challenging problem to develop a unified scheme to understand such global phase diagram. We approach this issue from the magnetically ordered side by using a quantum non-linear sigma model (QNLS M) to represent the local moments. We firstly study the consequence of skyrmion defects of antiferromagnetism on a honeycomb lattice. We solve the low energy effective Dirac Hamiltonian in the skyrmion background, and then identify the singlet orders through an enhanced correlations in the corresponding channels. In addition, we perform a renormalization group (RG) analysis of the QNLS M with a Kondo coupling by treating both bosonic and fermionic degrees of freedom on an equal footing. These results shed new insight into the global phase diagram of the heavy fermion systems. Recent evidence of two consecutive Kondo destruction quantum critical points(QCPs) in Ce3Pd20Si6 also provides an interesting extension of the global phase diagram. Motivated by this development, we study a spin-orbital coupled Bose-Fermi Kondo model. By performing a Coulomb-gas based RG calculation of this model with Ising anisotropy, we show that a generic trajectory in the parameter space contains two QCPs associated with the destruction of the orbital and spin Kondo effects, respectively. Not only the heavy fermion systems, iron pnictides also provide a platform to study quantum criticality. The new ingredient here is that the quantum critical singularties in the nematic and magnetic channels are concurrent, and their relationship has yet to be clarified. Here we study this problem within an effective Ginzburg-Landau theory for both channels in the presence of a small external uniaxial potential that breaks the lattice C4 symmetry. We establish an identity that connects the spin excitation anisotropy, which is the difference of the dynamical spin susceptibilities at two ordering wave vectors Q1 = ( pi, 0) and Q2 = (0,pi ), with the dynamical magnetic susceptibility and static nematic susceptibility. Using this identity, we introduce a scaling procedure to determine the dynamical nematic susceptibility in the quantum critical regime, and illustrate the procedure in the case of the optimally Ni-doped BaFe2As2.
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    Sequential localization of a complex electron fluid
    (National Academy of Sciences, 2019) Martelli, Valentina; Cai, Ang; Nica, Emilian M.; Taupin, Mathieu; Prokofiev, Andrey; Liu, Chia-Chuan; Lai, Hsin-Hua; Yu, Rong; Ingersent, Kevin; Küchler, Robert; Strydom, André M.; Geiger, Diana; Haenel, Jonathan; Larrea, Julio; Si, Qimiao; Paschen, Silke
    Complex and correlated quantum systems with promise for new functionality often involve entwined electronic degrees of freedom. In such materials, highly unusual properties emerge and could be the result of electron localization. Here, a cubic heavy fermion metal governed by spins and orbitals is chosen as a model system for this physics. Its properties are found to originate from surprisingly simple low-energy behavior, with 2 distinct localization transitions driven by a single degree of freedom at a time. This result is unexpected, but we are able to understand it by advancing the notion of sequential destruction of an SU(4) spin–orbital-coupled Kondo entanglement. Our results implicate electron localization as a unified framework for strongly correlated materials and suggest ways to exploit multiple degrees of freedom for quantum engineering.
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    Skyrmion defects and competing singlet orders in a half-filled antiferromagnetic Kondo-Heisenberg model on the honeycomb lattice
    (American Physical Society, 2017) Liu, Chia-Chuan; Goswami, Pallab; Si, Qimiao
    Due to the interaction between the topological defects of an order parameter and underlying fermions, the defects can possess induced fermion numbers, leading to several exotic phenomena of fundamental importance to both condensed matter and high-energy physics. One of the intriguing outcomes of induced fermion numbers is the presence of fluctuating competing orders inside the core of a topological defect. In this regard, the interaction between fermions and skyrmion excitations of an antiferromagnetic phase can have important consequences for understanding the global phase diagrams of many condensed matter systems where antiferromagnetism and several singlet orders compete. We critically investigate the relation between fluctuating competing orders and skyrmion excitations of the antiferromagnetic insulating phase of a half-filled Kondo-Heisenberg model on a honeycomb lattice. By combining analytical and numerical methods, we obtain the exact eigenstates of underlying Dirac fermions in the presence of a single skyrmion configuration, which are used for computing the induced chiral charge. Additionally, by employing this nonperturbative eigenbasis, we calculate the susceptibilities of different translational symmetry breaking charges, bond and current density wave orders, and translational symmetry preserving Kondo singlet formations. Based on the computed susceptibilities, we establish spin Peierls and Kondo singlets as dominant competing orders of antiferromagnetism. We show favorable agreement between our findings and field theoretic predictions based on the perturbative gradient expansion scheme, which crucially relies on the adiabatic principle and plane-wave eigenstates for Dirac fermions. The methodology developed here can be applied to many other correlated systems supporting competition between spin-triplet and spin-singlet orders in both lower and higher spatial dimensions.