We introduce an effective theory for quantum critical points (QCPs) in heavy fermion systems, involving a change in carrier density without symmetry breaking. Our new theory captures a strongly coupled metallic QCP, leading to robust marginal Fermi liquid transport phenomenology, and associated linear in temperature ($T$) "strange metal" resistivity, all within a controlled large $N$ limit. In the parameter regime of strong damping of emergent bosonic excitations, the QCP also displays a near-universal "Planckian" transport lifetime, $τ_{\mathrm{tr}}\sim\hbar/(k_BT)$. This is contrasted with the conventional so-called "slave boson" theory of the Kondo breakdown, where the large $N$ limit describes a weak coupling fixed point and non-trivial transport behavior may only be obtained through uncontrolled $1/N$ corrections. We also compute the weak-field Hall coefficient within the effective model as the system is tuned across the transition. We further find that between the two plateaus, reflecting the different carrier densities in the two Fermi liquid phases, the Hall coefficient can develop a peak in the critical crossover regime, like in recent experimental findings, in the parameter regime of weak boson damping.
We discuss quantum phase transition by an exactly solvable model in the dual gravity setup. By considering the effect of the scalar condensation on the fermion spectrum near the quantum critical point(QCP), we find that there is a topologically protected fermion zero mode associated with the metal to insulator transition. We also show that the strange metal phase with T-linear resistivity emerges at high enough temperature as far as the gravity has a horizon. The phase boundaries are calculated according to the density of states, giving insights on structures of the phase diagram near the QCP.
Building upon techniques employed in the construction of the Sachdev-Ye-Kitaev (SYK) model, which is a solvable $0+1$ dimensional model of a non-Fermi liquid, we develop a solvable, infinite-ranged random-hopping model of fermions coupled to fluctuating U(1) gauge fields. In a specific large-$N$ limit, our model realizes a gapless non-Fermi liquid phase, which combines the effects of hopping and interaction terms. We derive the thermodynamic properties of the non-Fermi liquid phase realized by this model, and the charge transport properties of an infinite-dimensional version with spatial struc…
Metallic states coined strange metal (SM), with robust linear-$T$ resistivity, have been widely observed in many quantum materials under strong electron correlation, ranging from high-$T_{c}$ cuprate superconductor, organic superconductor to twisted multilayer graphene and MoTe$_{2}$/WSe$_{2}$ superlattice. Despite decades of intensive studies, the mystery of strange metal still defies any sensible theoretical explanation and has been the key puzzle in modern condensed matter physics. Here, we solve a doped Mott insulator model, which unambiguously exhibits SM phenomena accompanied with quantu…
The mechanism of strange metal (SM) with unconventional charge transport near magnetic phase transitions has become an outstanding open problem in correlated electron systems. Recently, an exotic quantum critical SM phase was observed in paramagnetic frustrated heavy-fermion materials near Kondo breakdown. We establish a controlled theoretical framework to this issue via a dynamical large-N fermionic multichannel approach to the two-dimensional Kondo-Heisenberg lattice model, where KB transition separates a heavy-Fermi liquid from fermionic spin-liquid state. With Kondo fluctuations being full…
The 0+1-d Sachdev-Ye-Kitaev (SYK) fermionic model attracts nowadays a wide spread interest of the Condensed Matter community, as a benchmark toy model for strong electron correlation and non Fermi Liquid behavior. It is exactly solvable in the infrared limit and reproduces the linear dependence of the resistivity on temperature T, in linear response, typical of the strange metal phase of High Temperature Superconducting (HTS) materials. The breaking of its conformal symmetry requires ultraviolet corrections for a faithful description of its pseudo Goldstone Modes. Extension of the model to hig…
We introduce the concept of quasi-metaparticles based on the theory of metaparticles, the zero modes of the metastring. We apply the concept of quasi-metaparticles to the problem of non-Fermi liquids and the properties of strange metals. In particular, we point out that the quasi-metaparticle Green's function interpolates between the canonical quasi-particle Green's function and the result found in the context of the SYK model, which presents an exactly solvable model without quasiparticles. The linear dependence of resistivity with temperature is reproduced in the SYK limit. Also, the Cooper …
Strange metal behavior arises in heavy fermion metals close to antiferromagnetic transitions. An increasing amount of experiments indicates a link of such behavior to a Kondo breakdown quantum critical point. To shed light on this intriguing problem, we study the 2D Kondo-Heisenberg model using a dynamical large-N multichannel Schwinger boson approach. We identify and characterize the quantum phase transition from an antiferromagnetically ordered ground state to a Kondo-dominated paramagnetic state, and attribute a jump in certain phase shift to Kondo breakdown. In addition, we calculate trans…
Strain tuning Sr$_{2}$RuO$_{4}$ through the Lifshitz point, where the Van Hove singularity of the electronic spectrum crosses the Fermi energy, is expected to cause a change in the temperature dependence of the electrical resistivity from its Fermi liquid behavior $ρ\sim T^{2}$ to $ρ\sim T^{2}{\rm log}\left(1/T\right)$, a behavior consistent with experiments by Barber et al. [Phys. Rev. Lett. 120, 076601 (2018)]. This expectation originates from the same multi-band scattering processes with large momentum transfer that were recently shown to account for the linear in $T$ resistivity of the str…
The breakdown of the celebrated Fermi liquid theory in the strange metal phase is the central enigma of correlated quantum matter. Motivated by recent experiments reporting short-lived carriers, along with the ubiquitous observations of modulated excitations in the phase diagram of cuprates, we propose a model for this phase. We introduce bosons emerging from the remnants of a pair density wave as additional current carriers in the strange metal phase. These bosonic excitations are finite momentum Cooper pairs and thus carry twice the electronic charge, and its net spin can either be zero or o…
Strange Metal Hall Effect in Underdoped BaFe$_2$(As$_{1-x}$P$_x$)$_2$
University of California, Berkeley（加州大学伯克利分校）；Lawrence Berkeley National Laboratory（劳伦斯伯克利国家实验室）；Max Planck Institute for the Structure and Dynamics of Matter（马克斯·普朗克物质与结构动态研究所）；National High Magnetic Field Laboratory（国家强磁场实验室）；Los Alamos National Laboratory（洛斯阿拉莫斯国家实验室）；Boston University（波士顿大学）
Recent spectroscopic measurements in a number of strongly correlated metals that exhibit non-Fermi liquid like properties have observed evidence of anomalous frequency and momentum-dependent charge-density fluctuations. Specifically, in the strange metallic regime of the cuprate superconductors, there is a featureless particle-hole continuum exhibiting unusual power-laws, and experiments suggest that the plasmon mode decays into this continuum in a manner that is distinct from the expectations of conventional Fermi liquid theory. Inspired by these new experimental developments, we address the …
Quantum spin systems such as magnetic insulators usually show classical magnetic order, but such classical states can give way to quantum liquids with exotic entanglement through two known mechanisms of frustration: geometric frustration in lattices with triangle motifs, and spin-orbit-coupling frustration in the exactly solvable quantum liquid of Kitaev's honeycomb lattice. Here we present the experimental observation of a new kind of frustrated quantum liquid arising in an unlikely place: the magnetic insulator Ba4Ir3O10 where Ir3O12 trimers form an unfrustrated square lattice. Experimentall…
Significant effort has been devoted to the study of "non-Fermi liquid" (NFL) metals: gapless conducting systems that lack a quasiparticle description. One class of NFL metals involves a finite density of fermions interacting with soft order parameter fluctuations near a quantum critical point. The problem has been extensively studied in a large N limit (N corresponding to the number of fermion flavors) where universal behavior can be obtained by solving a set of coupled saddle-point equations. However a remarkable study by S.-S.~Lee revealed the breakdown of such approximations in two spatial …
Despite much theoretical effort, there is no complete theory of the 'strange' metal state of the high temperature superconductors, and its linear-in-temperature, $T$, resistivity. Recent experiments showing an unexpected linear-in-field, $B$, magnetoresistivity have deepened the puzzle. We propose a simple model of itinerant electrons, interacting via random couplings with electrons localized on a lattice of quantum 'dots' or 'islands'. This model is solvable in a large-$N$ limit, and can reproduce observed behavior. The key feature of our model is that the electrons in each quantum dot are de…
Non-Fermi liquids are strange metals whose physical properties deviate qualitatively from those of conventional metals due to strong quantum fluctuations. In this paper, we report transport measurements on the FeSe$_{1-x}$S$_x$ superconductor, which has a quantum critical point of a nematic order without accompanying antiferromagnetism. We find that in addition to a linear-in-temperature resistivity $ρ_{xx}\propto T$, which is close to the Planckian limit, the Hall angle varies as $\cot θ_{\rm H} \propto T^2$ and the low-field magnetoresistance is well scaled as $Δρ_{xx}/ρ_{xx}\propto \tan^2 θ…
Quasiparticle to local moment crossover in bad metals
Institute of Solid State Physics, TU Wien（维也纳工业大学固体物理研究所）；Institute for Theoretical Physics, University of Cologne（科隆大学理论物理研究所）；Center for Computational Quantum Physics, Flatiron Institute（平顿研究所计算量子物理中心）；Department of Condensed Matter Physics, Faculty of Mathematics and Physics, Charles University（查理大学数学物理学院凝聚态物理系）；Department of Physics, Graduate School of Science, Hokkaido University（北海道大学理学研究科物理学系）；Institute of Physics, Goethe-University Frankfurt（法兰克福歌德大学物理研究所）；Collège de France, PSL University（法兰西公学院，巴黎文理研究大学）；Department of Quantum Matter Physics, University of Geneva（日内瓦大学量子物质物理系）；Centre de Physique Théorique, Ecole Polytechnique（巴黎综合理工学院理论物理中心）
