Bulletin of the American Physical Society
APS March Meeting 2019
Volume 64, Number 2
Monday–Friday, March 4–8, 2019; Boston, Massachusetts
Session S06: Quantum Criticality: Theory |
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Sponsoring Units: DCMP Chair: Zhen Bi, Massachusetts Institute of Technology Room: BCEC 109A |
Thursday, March 7, 2019 11:15AM - 11:27AM |
S06.00001: Instabilities of 2d quantum critical metals in the Nf→0 limit Petter Säterskog We study a Fermi surface coupled to fluctuations of one or more critical order parameters in 1+2 dimensions. The limit of vanishing fermion flavor number gives a controlled way of studying this strongly coupled theory. In this talk I show results for charge, spin and pairing susceptibilities and find that the critical fluctuations may induce charge/spin density wave order or pairing depending on the types of critical order parameter fluctuations and their interplay. |
Thursday, March 7, 2019 11:27AM - 11:39AM |
S06.00002: Itinerant Quantum Critical Point with Fermion Pockets and Hot Spots ZiHong Liu, Gaopei Pan, Xiao Yan Xu, Kai Sun, Zi Yang Meng Combining determinantal quantum Monte Carlo (DQMC) and elective momentum ultra-size quantum Monte Carlo (EQMC) methods, we systematically investigated the itinerant quantum critical point on a 2D square lattice with antiferromagnetic spin fluctuations at wavevector Q = (π, π). System sizes of 60×60×320 (L × L × Lτ) are comfortably accessed, and the quantum critical scaling behaviors are revealed with unprecedingly high precision. We found that the antiferromagnetic spin fluctuations introduce effective interactions among fermions and the fermions in return render the bare bosonic critical point into a new universality, different from the bare Ising universality class and the Hertz-Mills-Moriya RPA prediction. At the quantum critical point, a finite anomalous dimension η ∼ 0.125 is observed in the bosonic propagator, and fermions at hot spots evolve into a non-Fermiliquid. In the antiferromagnetically ordered metallic phase, fermion pockets are formed as energy gap opens up at the hot spots. |
Thursday, March 7, 2019 11:39AM - 11:51AM |
S06.00003: Emergent O(4) Symmetry and Conserved Current Continuum at a Deconfined Quantum Critical Point in Shastry-Sutherland Model Jong Yeon Lee, Yizhuang You, Ashvin Vishwanath In this work, we investigate a possibility of deconfined quantum phase transition in the two dimensional Shastry-Sutherland spin model. Using the level-crossing technique for correlation length spectrum in the infinite DMRG simulation, we demonstrate the evidence for a deconfinement and emergent O(4) symmetry at the phase transition between the plaquette valence bond solid and Neel order. Such a phase transition has been observed in the recent experiment, and we propose experimental signatures for this deconfined quantum criticality that can be measured in both phonon and magnon spectra. |
Thursday, March 7, 2019 11:51AM - 12:03PM |
S06.00004: Quantum criticality in Ising chains with random hyperuniform couplings Philip Crowley, Christopher Laumann, Sarang Gopalakrishnan In critical Ising chains, independent random disorder localises almost all excitations, and drives the system to an infinite randomness critical point. However, correlations in the disorder can change the universality, and even make the disorder irrelevant. |
Thursday, March 7, 2019 12:03PM - 12:15PM |
S06.00005: Deconfined quantum criticality from the $\mathrm{QED}_{3}$-Gross-Neveu-Yukawa model: the $1/N$ expansion revisited Rufus Boyack, Ahmed Rayyan, Joseph Maciejko Quantum phase transitions involving dynamical gauge fields are an important class of transitions beyond the standard Landau-Ginzburg-Wilson paradigm. Two subcategories are those where (i) the gauge field deconfines only at the critical point itself, and (ii) the gauge field deconfines in one of the phases separated by the critical point. The latter subcategory is exemplified by the $\mathrm{QED}_{3}$-Gross-Neveu-Yukawa (GNY) model in which there has been great interest recently due to a conjecture relating its critical point to the N\'eel-to-valence-bond-solid (VBS) deconfined critical point in the first subcategory. Motivated by this, we use the $1/N$ expansion to study the $\mathrm{QED}_{3}$-GNY model in fixed three spacetime dimensions, with $N$ flavors of two-component Dirac fermions. We find new contributions to critical exponents arising from Aslamazov-Larkin diagrams missed by previous epsilon- and $1/N$-expansion studies in arbitrary dimensions. For the specific case of $N=2$, when the duality is conjectured to hold, we find that the bosonic anomalous dimension and adjoint fermion bilinear scaling dimension are in reasonable agreement with numerical studies of the N\'eel-to-VBS transition. |
Thursday, March 7, 2019 12:15PM - 12:27PM |
S06.00006: Dynamical spin susceptibility of a Fermi liquid without conservation law Prachi Sharma, Dmitrii Maslov Galilean invariance along with the conservation of charge and total spin guarantees that the corresponding susceptibilities vanish at an infinitely long wavelength (q=0) and finite frequency. But the susceptibility of nematic fluctuations or that of spin fluctuations in the presence of spin-orbit interaction (SOI) is not protected by any conservation laws and hence leads to finite spectral weight outside the particle-hole continuum even at q=0 and finite ω [1,2]. Finite width of the chiral spin modes for a Fermi liquid (FL) with Rashba SOI was studied at q=0 by Maiti and Maslov [3]. Here, we study the effect of residual interaction on the chiral spin modes of a FL with weak Rasbha SOI for a model of dynamic screened Coulomb potential at finite q by going beyond the random phase approximation(RPA). We also study the interplay between the plasmon and chiral spin modes due to their coupling for the non-RPA corrections. |
Thursday, March 7, 2019 12:27PM - 12:39PM |
S06.00007: Critical strange metal from fluctuating gauge fields in a solvable random model Aavishkar Patel, Subir Sachdev Building upon techniques employed in the construction of the Sachdev-Ye-Kitaev 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 structure. We also describe a Higgs transition from this non-Fermi-liquid "strange metal" phase to a weakly-interacting "pseudogap" phase with a relatively reduced low-energy fermion density of states, and gapped gauge field fluctuations. |
Thursday, March 7, 2019 12:39PM - 12:51PM |
S06.00008: Fermion-induced quantum critical points in a generalized SU(N) fermion model Bohai Li, Zixiang Li, Hong Yao The non-Landau quantum criticality dubbed as "fermion-induced quantum critical point” (FIQCP) was proposed in Ref. [1], where it was shown for 2D SU(N) Dirac fermions with N≥2. Here we investigate the nature of quantum phase transition for case of N=1. Our sign-problem-free Majorana quantum Monte Carlo simulations show that, by introducing longer-range interactions, our model exhibits transitions among Dirac semimetals, CDW, and Kekule-VBS phases. This can further provide support for the critical value of N for the occurrence of FIQCP predicted by previous RG studies. |
Thursday, March 7, 2019 12:51PM - 1:03PM |
S06.00009: Incommensurate 2kF charge density wave quantum critical points in two-dimensional metals Matthias Punk, Johannes Halbinger, Dimitri Pimenov We study two-dimensional metals in the vicinity of a quantum critical point, where incommensurate 2kF charge density wave (CDW) order develops. Starting from a model of two antipodal hot spots at the Fermi surface which are connected by a 2kF wavevector, we perform a controlled, perturbative renormalization group analysis in the spirit of earlier work by Dalidovich and Lee [1]. We show that the charge density wave transition is indeed continuous and described by a non-Fermi liquid fixed point with a dynamically nested Fermi surface. Our results are potentially relevant to understand the onset of incommensurate CDW order in CuxTaS2 and NbSe2 at high pressure. |
Thursday, March 7, 2019 1:03PM - 1:15PM |
S06.00010: Quantum criticality of a quantum nonlinear sigma model with Kondo coupling: a renormalization group study Chia-Chuan Liu, Qimiao Si Quantum criticality has been an active research topic in condensed matter physics, with a lot of effort being made into the heavy fermion material in which local moments are coupled with itinerant electrons through Kondo coupling [1]. From a theoretical perspective, the interplay between different kinds of degrees of freedom makes it challenging to develop a unified framework to study the quantum criticality of such systems [2]. Here we approach the problem from the magnetically ordered side, using a quantum non-linear sigma model with an additional coupling to itinerant fermions [3]. By treating the renormalization of the bosonic and fermionic degrees of freedom on an equal footing, we analyze the effect of the Kondo coupling on the criticality. Our results shed new light on the global phase diagram of the heavy fermion systems. |
Thursday, March 7, 2019 1:15PM - 1:27PM |
S06.00011: Incoherent metal in the quantum critical region of SU(2) symmetric model Peter Cha, Olivier Parcollet, Antoine Georges, Eun-Ah Kim Incoherent metals have recently garnered much interest in their relation to quantum criticality, high-temperature superconductivity, quantum chaos, and holography. |
Thursday, March 7, 2019 1:27PM - 1:39PM |
S06.00012: Deconfined Quantum Critical Points in 3+1D Zhen Bi, Senthil Todadri Continuous quantum phase transitions that are beyond the conventional paradigm of fluctuations of a symmetry breaking order parameter are challenging for theory. These phase transitions often involve emergent deconfined gauge fields at the critical points as demonstrated in 2+1D. Examples include phase transitions in quantum magnetism as well as those between Symmetry Protected Topological phases. In this work, we present several examples of Deconfined Quantum Critical Points between Symmetry Protected Topological phases in 3+1D for both bosonic and fermionic systems. These critical theories can be formulated as non-abelian gauge theories either in their Infra-Red free regime, or in the conformal window when they flow to the Banks-Zaks fixed points. We will talk about several interesting quantum critical phenomena. We describe situations in which the same phase transition allows for multiple universality classes controlled by distinct fixed points. We exhibit the possibility - which we dub “unnecessary quantum critical points” - of stable generic continuous phase transitionswithin the same phase. |
Thursday, March 7, 2019 1:39PM - 1:51PM |
S06.00013: Quantum-critical conductivity of the Dirac fluid in graphene TAIRU LYU, Patrick R Gallagher, chanshan yang, Feng Wang Graphene near charge neutrality is expected to behave like a quantum-critical, relativistic plasma—the “Dirac fluid”—in which massless electrons and holes rapidly collide at a rate proportional to temperature. We measure the frequency-dependent optical conductivity of clean micron-scale graphene encapsulated in hexagonal Boron Nitride at electron temperatures between 77 and 300 K using on-chip terahertz spectroscopy. At charge neutrality, we observe the quantum-critical scattering rate characteristic of the Dirac fluid. At higher doping, we uncover two distinct current-carrying modes with zero and nonzero total momenta, a manifestation of relativistic hydrodynamics. Our work reveals the quantum criticality and unusual dynamic excitations near charge neutrality in graphene. |
Thursday, March 7, 2019 1:51PM - 2:03PM |
S06.00014: Emergent Spacetime Supersymmetry at Superconducting Quantum Criticality of a single Dirac Cone Zixiang Li, Abolhassan Vaezi, Christian Mendl, Hong Yao No definitive evidence of spacetime supersymmetry (SUSY) that transmutes fermions into bosons and vice versa has been revealed in nature so far. Moreover, whether spacetime SUSY in 2+1 and higher dimensions can emerge in generic lattice microscopic models remains open. Here, we introduce a lattice realization of a single Dirac fermion in 2+1 dimensions with attractive interactions that preserves both time-reversal and chiral symmetries. By performing sign-problem-free determinant quantum Monte Carlo simulations, we show that the interacting single Dirac fermion in 2+1 dimensions features a superconducting quantum critical point (QCP). More remarkably, we demonstrate that the N=2 spacetime SUSY in 2+1D emerges at the superconducting QCP by showing that the fermions and bosons have identical anomalous dimensions 1/3, a hallmark of the emergent SUSY [1]. We further show some experimental signatures which may be measured to test such emergent SUSY in candidate systems such as the surface Dirac cone of 3D topological insulators. |
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