Frustration shapes multi-channel Kondo physics: a star graph perspective.

We study the overscreened multi-channel Kondo (MCK) model using the recently developed unitary renormalisation group technique. Our results display the importance of ground state degeneracy in explaining various important properties like the breakdown of screening and the presence of local non-Fermi liquids (NFLs). The impurity susceptibility of the intermediate coupling fixed point Hamiltonian in the zero-bandwidth (or star graph) limit shows a power-law divergence at low temperature. Despite the absence of inter-channel coupling in the MCK fixed point Hamiltonian, the study of mutual information between any two channels shows non-zero correlation between them. A spectral flow analysis of the star graph reveals that the degenerate ground state manifold possesses topological quantum numbers. Upon disentangling the impurity spin from its partners in the star graph, we find the presence of a local Mott liquid arising from inter-channel scattering processes. The low energy effective Hamiltonian obtained upon adding a finite non-zero conduction bath dispersion to the star graph Hamiltonian for both the two and three-channel cases displays the presence of local NFLs arising from inter-channel quantum fluctuations. Specifically, we confirm the presence of a local marginal Fermi liquid in the two channel case, whose properties show logarithmic scaling at low temperature as expected. Discontinuous behaviour is observed in several measures of ground state entanglement, signalling the underlying orthogonality catastrophe associated with the degenerate ground state manifold. We extend our results to underscreened and perfectly screened MCK models through duality arguments. A study of channel anisotropy under renormalisation flow reveals a series of quantum phase transitions due to the change in ground state degeneracy. Our work thus presents a template for the study of how a degenerate ground state manifold arising from symmetry and duality properties in a multichannel quantum impurity model can lead to novel multicritical phases at intermediate coupling.

Erratum: Random Field Driven Spatial Complexity at the Mott Transition in VO_{2} [Phys. Rev. Lett. 116, 036401 (2016)].

This corrects the article DOI: 10.1103/PhysRevLett.116.036401.

Random Field Driven Spatial Complexity at the Mott Transition in VO(2).

We report the first application of critical cluster techniques to the Mott metal-insulator transition in vanadium dioxide. We show that the geometric universal properties of the metallic and insulating puddles observed by scanning near-field infrared microscopy are consistent with the system passing near criticality of the random field Ising model as temperature is varied. The resulting large barriers to equilibrium may be the source of the unusually robust hysteresis phenomena associated with the metal-insulator transition in this system.

Feasibility of a metamagnetic transition in correlated systems.

The long-standing issue of the competition between the magnetic field and the Kondo effect, favoring, respectively, triplet and singlet ground states, is addressed using a cluster slave-rotor mean-field theory for the Hubbard model and its spin-correlated, spin-frustrated extensions in two dimensions. The metamagnetic jump is established and compared with earlier results of dynamical mean-field theory. This approach also reproduces the emergent super-exchange energy scale in the insulating side. A scaling is found for the critical Zeeman field in terms of the intrinsic coherence scale just below the metal-insulator transition, where the critical spin fluctuations are soft. The conditions required for metamagnetism to appear at a reasonable field are also underlined. Gutzwiller analysis on the two-dimensional Hubbard model and a quantum Monte Carlo calculation on the Heisenberg spin system are performed to check the limiting cases of the cluster slave-rotor results for the Hubbard model. Low-field scaling features for magnetization are discussed.

Site-disorder driven superconductor-insulator transition: a dynamical mean field study.

We investigate the effect of site disorder on the superconducting state in the attractive Hubbard model within the framework of dynamical mean field theory. For a fixed interaction strength (U), the superconducting order parameter decreases monotonically with increasing disorder (x), while the single-particle spectral gap decreases for small x, reaches a minimum and keeps increasing for larger x. Thus, the system remains gapped beyond the destruction of the superconducting state, indicating a disorder-driven superconductor-insulator transition. We investigate this transition in depth considering the effects of weak and strong disorder for a range of interaction strengths. In the clean case, the order parameter is known to increase monotonically with increasing interaction, saturating at a finite value asymptotically for U→∞. The presence of disorder results in destruction of superconductivity at large U, thus drastically modifying the clean case behaviour. A physical understanding of our findings is obtained by invoking particle-hole asymmetry and the probability distributions of the order parameter and spectral gap.

From mixed valence to the Kondo lattice regime.

Many heavy fermion materials are known to cross over from the Kondo lattice regime to the mixed valence regime or vice versa as a function of pressure or doping. We study this crossover theoretically by employing the periodic Anderson model within the framework of the dynamical mean field theory. Changes occurring in the dynamics and transport across this crossover are highlighted. As the valence is decreased (increased) relative to the Kondo lattice regime, the Kondo resonance broadens significantly, while the lower (upper) Hubbard band moves closer to the Fermi level. The resistivity develops a two peak structure in the mixed valence regime: a low temperature coherence peak and a high temperature 'Hubbard band' peak. These two peaks merge, yielding a broad shallow maximum upon decreasing the valence further. The optical conductivity likewise exhibits an unusual absorption feature (shoulder) in the deep mid-infrared region, which grows in intensity with decreasing valence. The involvement of the Hubbard bands in dc transport and of the effective f-level in the optical conductivity are shown to be responsible for the anomalous transport properties. A two-band hybridization-gap model, which neglects incoherent effects due to many-body scattering, commonly employed to understand the optical response in these materials is shown to be inadequate, especially in the mixed valence regime. Comparison of theory with experiment carried out for (a) dc resistivities of CeRhIn(5), Ce(2)Ni(3)Si(5), CeFeGe(3) and YbIr(2)Si(2), (b) pressure dependent resistivity of YbInAu(2) and CeCu(6), and (c) optical conductivity measurements in YbIr(2)Si(2) yields excellent agreement.

Preformed excitonic liquid route to a charge density wave in 2H-TaSe2.

Recent experiments on 2H-TaSe(2) contradict the long-held view of the charge density wave arising from a nested band structure. An intrinsically strong coupling view, involving a charge density wave state arising as a Bose condensation of preformed excitons emerges as an attractive, albeit scantily investigated alternative. Using the local density approximation plus multiorbital dynamic mean field theory, we show that this scenario agrees with a variety of normal state data for 2H-TaSe(2). Based thereupon, the ordered states in a subset of dichalcogenides should be viewed as instabilities of a correlated, preformed excitonic liquid.

Field-dependent dynamics in the metallic regime of the half-filled Hubbard model.

A systematic study of the effect of magnetic field (h) on the Hubbard model has been carried out at half-filling within dynamical mean field theory. In agreement with previous studies, we find a zero temperature itinerant metamagnetic transition, reflected in the discontinuous changes in magnetization as well as in the hysteresis, from a paramagnetic (PM) metallic state to a polarized quasi-ferromagnetic (QFM) state, at intermediate and large interaction strengths (U). The jump in magnetization vanishes smoothly with decreasing interaction strength, and at a critical U, the transition becomes continuous. The region of 'coexistence' of the PM and QFM solutions in the field-U plane obtained in this study agrees quantitatively with recent numerical renormalization group calculations, thus providing an important benchmark. We highlight the changes in dynamics and quasiparticle weight across this transition. The effective mass increases sharply as the transition is approached, exhibiting a cusp-like singularity at the critical field, and decreases with field monotonically beyond the transition. We conjecture that the first order metamagnetic transition is a result of the competition between Kondo screening, that tries to quench the local moments, and Zeeman coupling, which induces polarization and hence promotes local moment formation. A comparison of our theoretical results with experiments on (3)He indicate that a theory of (3)He based on the half-filled Hubbard model places it in a regime of intermediate interaction strength.

Quantum critical point at finite doping in the 2D Hubbard model: a dynamical cluster quantum Monte Carlo study.

We explore the Matsubara quasiparticle fraction and the pseudogap of the two-dimensional Hubbard model with the dynamical cluster quantum Monte Carlo method. The character of the quasiparticle fraction changes from non-Fermi-liquid, to marginal Fermi liquid, to Fermi liquid as a function of doping, indicating the presence of a quantum critical point separating non-Fermi-liquid from Fermi-liquid character. Marginal Fermi-liquid character is found at low temperatures at a very narrow range of doping where the single-particle density of states is also symmetric. At higher doping the character of the quasiparticle fraction is seen to cross over from Fermi liquid to marginal Fermi liquid as the temperature increases.

Magnetoresistance in paramagnetic heavy fermion metals.

A theoretical study of magnetic field (h) effects on single-particle spectra and the transport quantities of heavy fermion metals in the paramagnetic phase is carried out. We have employed a non-perturbative local moment approach (LMA) to the asymmetric periodic Anderson model within the dynamical mean field framework. The lattice coherence scale ω(L), which is proportional within the LMA to the spin-flip energy scale, and has been shown in earlier studies to be the energy scale at which crossover to single-impurity physics occurs, increases monotonically with increasing magnetic field. The many body Kondo resonance in the density of states at the Fermi level splits into two, with the splitting being proportional to the field itself. For h≥0, we demonstrate adiabatic continuity from the strongly interacting case to a corresponding non-interacting limit, thus establishing Fermi liquid behaviour for heavy fermion metals in the presence of a magnetic field. In the Kondo lattice regime, the theoretically computed magnetoresistance is found to be negative in the entire temperature range. We argue that such a result could be understood at [Formula: see text] by field-induced suppression of spin-flip scattering and at [Formula: see text] through lattice coherence. The coherence peak in the heavy fermion resistivity diminishes and moves to higher temperatures with increasing field. Direct comparison of the theoretical results to the field dependent resistivity measurements in CeB(6) yields good agreement.