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Quantum-mechanical fluctuations between competing phases at $T=0$ induce exotic finite-temperature collective excitations that are not described by the standard Landau Fermi liquid framework. These excitations exhibit anomalous temperature dependences, or non-Fermi liquid behavior, in the transport and thermodynamic properties in the vicinity of a quantum critical point, and are often intimately linked to the appearance of unconventional Cooper pairing as observed in strongly correlated systems including the high-$T_c$ cuprate and iron pnictide superconductors. The presence of superconductivity, however, precludes direct access to the quantum critical point, and makes it difficult to assess the role of quantum-critical fluctuations in shaping anomalous finite-temperature physical properties. Here we report temperature-field scale invariance of non-Fermi liquid thermodynamic, transport, and Hall quantities in a non-superconducting iron-pnictide, Ba(Fe$_{1/3}$Co$_{1/3}$Ni$_{1/3}$)$_{2}$As$_{2}$, indicative of quantum criticality at zero temperature and zero applied magnetic field. Beyond a linear in temperature resistivity, the hallmark signature of strong quasiparticle scattering, we find the scattering rate that obeys a universal scaling relation between temperature and applied magnetic fields down to the lowest energy scales. Together with the dominance of hole-like carriers close to the zero-temperature and zero-field limits, the scale invariance, isotropic field response, and lack of applied pressure sensitivity suggests a unique quantum critical system that does not drive a pairing instability.
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An enduring question in correlated systems concerns whether superconductivity is favoured at a quantum critical point (QCP) characterised by a divergent quasiparticle effective mass. Despite such a scenario being widely postulated in high Tc cuprates and invoked to explain non-Fermi liquid transport signatures, experimental evidence is lacking for a critical divergence under the superconducting dome. We use ultra-strong magnetic fields to measure quantum oscillations in underdoped YBa2Cu3O6+x, revealing a dramatic doping-dependent upturn in quasiparticle effective mass at a critical metal-insulator transition beneath the superconducting dome. Given the location of this QCP under a plateau in Tc in addition to a postulated QCP at optimal doping, we discuss the intriguing possibility of two intersecting superconducting subdomes, each centred at a critical Fermi surface instability.
We present numerically exact results from sign-problem free quantum Monte Carlo simulations for a spin-fermion model near an $O(3)$ symmetric antiferromagnetic (AFM) quantum critical point. We find a hierarchy of energy scales that emerges near the quantum critical point. At high energy scales, there is a broad regime characterized by Landau-damped order parameter dynamics with dynamical critical exponent $z=2$, while the fermionic excitations remain coherent. The quantum critical magnetic fluctuations are well described by Hertz-Millis theory, except for a $T^{-2}$ divergence of the static AFM susceptibility. This regime persists down to a lower energy scale, where the fermions become overdamped and concomitantly, a transition into a $d-$wave superconducting state occurs. These findings resemble earlier results for a spin-fermion model with easy-plane AFM fluctuations of an $O(2)$ SDW order parameter, despite noticeable differences in the perturbative structure of the two theories. In the $O(3)$ case, perturbative corrections to the spin-fermion vertex are expected to dominate at an additional energy scale, below which the $z=2$ behavior breaks down, leading to a novel $z=1$ fixed point with emergent local nesting at the hot spots [Schlief et al., PRX 7, 021010 (2017)]. Motivated by this prediction, we also consider a variant of the model where the hot spots are nearly locally nested. Within the available temperature range in our study ($Tge E_F/200$), we find substantial deviations from the $z=2$ Hertz-Millis behavior, but no evidence for the predicted $z=1$ criticality.
Using recent insights obtained in heavy fermion physics on the thermodynamic singularity structure associated with quantum phase transitions, we present here an experimental strategy to establish if the zero-temperature transition in the disordered two dimensional gas is a real quantum phase transition. We derive a overcomplete set of scaling laws relating the density and temperature dependence of the chemical potential and the effective mass, which are in principle verifyable by experiment.
Charge density wave, or CDW, is usually associated with Fermi surfaces nesting. We here report a new CDW mechanism discovered in a 2H-structured transition metal dichalcogenide, where the two essential ingredients of CDW are realized in very anomalous ways due to the strong-coupling nature of the electronic structure. Namely, the CDW gap is only partially open, and charge density wavevector match is fulfilled through participation of states of the large Fermi patch, while the straight FS sections have secondary or negligible contributions.
In contrast to the seminal weak localization prediction of a non-critical Hall constant ($R_{H}$) at the Anderson metal-insulator transition (MIT), $R_{H}$ in quite a few real disordered systems exhibits both, a strong $T$-dependence and critical scaling near their MIT. Here, we investigate these issues in detail within a non-perturbative strong localization regime using cluster-dynamical mean field theory (CDMFT). We uncover $(i)$ clear and unconventional quantum-critical scaling of the $gamma$-function, finding that $gamma(g_{xy})simeq$ log$(g_{xy})$ over a wide range spanning the continuous MIT, very similar to that seen for the longitudinal conductivity, $(ii)$ strongly $T$-dependent and clear quantum critical scaling in both transverse conductivity and $R_{H}$ at the MIT. We find that these surprising results are in comprehensive and very good accord with signatures of a novel kind of localization in disordered NbN near the MIT, providing substantial support for our strong localization view.
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