Bad-Metal Behavior Reveals Mott Quantum Criticality in Doped Hubbard Models

Abstract

Bad-Metal (BM) behavior featuring linear temperature dependence of the resistivity extending to well above the Mott-Ioffe-Regel (MIR) limit is often viewed as one of the key unresolved signatures of strong correlation. Here we associate the BM behavior with the Mott quantum criticality by examining a fully frustrated Hubbard model where all long-range magnetic orders are suppressed, and the Mott problem can be rigorously solved through Dynamical Mean-Field Theory. We show that for the doped Mott insulator regime, the coexistence dome and the associated first-order Mott metal-insulator transition are confined to extremely low temperatures, while clear signatures of Mott quantum criticality emerge across much of the phase diagram. Remarkable scaling behavior is identified for the entire family of resistivity curves, with a quantum critical region covering the entire BM regime, providing not only insight, but also quantitative understanding around the MIR limit, in agreement with the available experiments.PACS numbers: 71.27.+a,71.30.+h Metallic transport inconsistent with Fermi liquid theory has been observed in many different systems; it is often linked to quantum criticality around some ordering phase transition [1, 2]. Such behavior is notable near quantum critical points in good conductors, for example in heavy fermion compounds [3, 4]. In several other classes of materials, however, much more dramatic departures form conventional metallic behavior are clearly observed, where resistivity still rises linearly with temperature, but it reaches paradoxically large values, well past the MIR limit [5, 6]. This "Bad-Metal" (BM) behavior [7], was first identified in the heyday of high-temperature superconductivity, in materials such as La 2−x Sr x CuO 4 [8]. While the specific copper-oxide family and related high-T c materials remain ill-understood and marred with controversy, it soon became clear that BM behavior is a much more general feature [6] of materials close to the Mott metal-insulator transition (MIT) [9]. Indeed, it has been clearly identified also in various oxides [10, 11], organic Mott systems [12][13][14], as well as more recently discovered families of iron pnictides [15]. Despite years of speculation and debate, so far its clear physical interpretation has not been established.To gain reliable insight into the origin of BM behavior, it is useful to examine an exactly solvable model system, where one can suppress all possible effect associated with the approach to some broken symmetry phase, or those specific to low dimensions and a given lattice structure. This can be achieved by focusing on the "maximally frustrated Hubbard model", where an exact solution can be obtained by solving Dynamical Mean-Field Theory (DMFT) equations [16] in the paramagnetic phase. Although various aspects of the DMFT equation have been studied for more than twenty years, only very recent work [17,18] established how to identify the quantum critical (QC) behavior associated with the interaction-driven Mott transition at ...

Orthogonal metal in the Hubbard model with liberated slave spins

“…For D = ∞, or in constrained slave-spin theories, ŝ x i is directly related to the fermion double occupancy n i↑ni↓ , see Eq. (12). The latter is manifestly gauge invariant and provides an experimentally accessible [85] Ising order parameter for the Mott transition also at T > 0 [48,[86][87][88].…”

Non-Fermi-liquid behavior and doping asymmetry in an organic Mott insulator interface

“…Unlike the hole-doped superconductivity, the electrondoped superconductivity shows very abrupt and discontinuous (first-order-like) transition. Although the firstorder transitions are not observed in this study, they may appear at extremely low temperatures because of large U/t as shown in the DMFT calculations [20]. This asymmetry may be attributable to the difference of their noninteracting band structures and resultant single-particle spectral functions in the Mott insulating state, where t exists only for one diagonal (κ-Cl), or for both diagonals (cuprates) of the sites.…”

“…We have found that the BM behavior at high temperatures persists in almost all doping levels, while the pseudogap behavior at low temperatures is quite bandstructure dependent, implying the different energy scales and mechanisms governing these two behaviors. [20], where the temperature is normalized with TMIR, temperature at which the resistivity reaches ρMIR for δ = 20%.…”

Doping-driven metal-insulator transition in correlated electron systems with strong Hund's exchange coupling

“…The second cut in the upper panel at constant U = 3.06 along the µ-axis shows the coexistence region delimited by µ C1 (T ) and µ C2 (T ); a corresponding plot for the one-orbital model can be found in the supplementary material of Ref. 48. All of the results for the twoorbital model with density-density interaction (3) that will be presented in the following were calculated in this plane in parameter space.…”