Bulletin of the American Physical Society
APS March Meeting 2019
Volume 64, Number 2
Monday–Friday, March 4–8, 2019; Boston, Massachusetts
Session X06: Quantum Phase Stability and Criticality: Theory 
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Sponsoring Units: DCMP Room: BCEC 109A 
Friday, March 8, 2019 8:00AM  8:12AM 
X06.00001: Superconductivity from quantum fluctuations of itinerant quantum critical points Yunchao Hao, Yang Qi, Kai Sun In the vicinity of a quantum critical point, critical fluctuations can often act as glue for the formation of BCS pairs, which is one important path way towards unconventional superconductivity. Recently, thanks to the development in quantum numerical techniques, such as signproblem quantum Monte Carlo simulations, new insights and unbiased numerical understanding about this phenomenon become accessible. In particular, the numerical results suggest superconductivity emerged from these quantum critical points are highly diversified and are highly sensitive microscopic details. In this study, we investigate these systems utilizing analytic approach. By exploring different model systems and compare with numerical results, we aim at understanding universal properties behind these diversified behaviors. 
Friday, March 8, 2019 8:12AM  8:24AM 
X06.00002: Characterizing the quantum critical point between a Dirac spin liquid and an antiferromagnet Eric Dupuis, William WitczakKrempa The spin1/2 kagome Heisenberg antiferromagnet hosts a putative quantum spin liquid phase for which a candidate ground state is the Dirac spin liquid. At low energies, this state is described by quantum electrodynamics in 2+1 dimensions with 2 N_{f} = 4 flavors of twocomponent gapless spinons. We describe a transition to a coplanar antiferromagnetic (AFM) phase by coupling the spinons to a vectorial bosonic order parameter. We find a nontrivial quantum critical point and compute critical exponents using a oneloop d=4ε expansion. The compactness of the U(1) gauge field allows topological configurations named monopoles which must condense to induce the AFM phase. We classify the monopole operators by their symmetries and comment on their gauge invariance. Using the stateoperator correspondence, we compute the scaling dimensions of the monopole operators in the large N_{f} limit and find nontrivial hierarchy. 
Friday, March 8, 2019 8:24AM  8:36AM 
X06.00003: Quantum Criticality in the TwoDimensional Periodic Anderson Model Thomas Schaefer, Andrey A. Katanin, Motoharu Kitatani, Alessandro Toschi, Karsten Held Despite the fascinating phenomena accompanying a quantum critical point, e.g. nonFermi liquid behavior, a general theory for quantum phase transitions is lacking. In this talk, I will present a step forward by analyzing results from the dynamical vertex approximation, a cuttingedge quantum field theoretical method including temporal as well as spatial correlations. Within this framework, I will analyze the fundamental model of strongly correlated heavy fermion compounds, the periodic Anderson model. By varying the hybridization strength of localized felectrons and itinerant delectrons, and a careful analysis of response functions, one can trace the change in the ground state from an antiferromagnet to a paramagnetic Kondo insulating phase, resembling the famous Doniach phase diagram. Eventually, I will show the evolution of the critical exponents of the magnetic susceptibility, which are changing from the one of free spins γ=1 to γ=2 in the quantum critical regime. T. Schäfer, A. Katanin, K. Held, and A. Toschi, PRL 119, 046402 (2017), T. Schäfer, A. Katanin, M. Kitatani, A. Toschi, and K. Held, in preparation. 
Friday, March 8, 2019 8:36AM  8:48AM 
X06.00004: Quantum critical scaling beyond GinzburgLaudauWilson paradigm in heavyfermion metals YungYeh Chang, Stefan Kirchner, ChungHou Chung

Friday, March 8, 2019 8:48AM  9:00AM 
X06.00005: Exact nonlinear IV curve near twochannel KondoLuttinger quantum critical point ChaoYun Lin, YungYeh Chang, Colin Rylands, Natan Andrei, ChungHou Chung It has been known since 1990's that a Kondo impurity coupled to Luttinger liquid wire undergoes an exotic quantum phase transition with decreasing Luttinger parameter K (or increasing electron interactions) from the 1channel to 2channel Kondo ground states at K=1/2. However, the quantum critical properties near this transition is still not known to date due to lack of controlled theoretical tools to examine the physics near the strong coupling 2channel Kondo fixed point. In this paper, we address this longstanding issue via bosonizationrefermionization approach near 2channel Kondo state. We overcome the problem by mapping the system at the Toulouse point onto an effective twolead free fermion model subject to a local tunneling. Remarkably, the nonequilibrium transport of the system is exactly solvable in this limit. An analytic form for the nonlinear differential conductance is obtained. Our results offer an unique example of exactly and analytically accessible nonequilibrium transport near a quantum critical point; they are relevant for the recent experiment in a dissipative Kondo dot. 
Friday, March 8, 2019 9:00AM  9:12AM 
X06.00006: Thermodynamic properties of 3D plaquette antiferromagnet Guang Yu Sun, Nusen Ma, Anders W Sandvik, Zi Yang Meng We design a 3D checkerboard JQ model to capture phase transitions between a plaquettesinglet solid with lattice translational symmetry breaking to a 3D Neel phase with spin rotational symmetry breaking. The system is investigated via unbiased largescale quantum Monte Carlo simulation. We use Binder cumulant to determine the finite temperature phase boundary and measure the specific heat and magnetic susceptibility to reveal the nature of the different phases and their transitions. Our results are closely related with the ongoing experimental effort in pressureinduced magnetic phase transitions in Shastry Sutherland (SS) compound SrCu_{2}（BO_{3}）_{2} 
Friday, March 8, 2019 9:12AM  9:24AM 
X06.00007: Emergence of nonFermi liquid dynamics through nonlocal correlations in an interacting disordered system Sudeshna Sen, N S Vidhyadhiraja, Mark Jarrell We provide strong evidence for a quantum critical point (QCP) associated with the destruction of Kondo screening in the AndersonHubbard model for interacting electrons with quenched disorder. The evidence comprises three elements: (a) the identification of an energy scale, that delineates infrared Landau damping from higher frequency nonFermi liquid (nFL) dynamics; (b) the finding that this crossover scale appears to vanish with increasing disorder; and (c) the concomitant appearance of a finite intercept in a broad distribution of Kondo scales. Our findings indicate a Kondo destruction scenario, albeit distinct from the local QCP picture. The nFL behavior is shown to stem from an interplay of strong electronelectron interactions and the systematic inclusion of shortrange dynamical fluctuations induced by the underlying random potential. The results have been obtained through a computational framework based on the typical medium dynamical cluster approximation. 
Friday, March 8, 2019 9:24AM  9:36AM 
X06.00008: U(N)/U(m)U(Nm) nonlinear sigma model: critical behavior and a physical realization Da Wang The nonlinear sigma model defined on the Grassmann manifold U(N)/U(m)U(Nm) is a direct generalization of the widely studied CP^{N1} model. In space dimension 2<d<4, we use 1/N expansion technique to obtain its critical exponents including twoparticle ones, which are found to be only functions of m/N up to the first order. As a result, larger m effectively reduces N and thus brings stronger fluctuations. Next, we show such a model is a low energy description of the SU(N) Hubbard model which has been realized in cold atom experiments. With determinant quantum Monte Carlo simulations of the halffilled SU(6) Hubbard model on the square lattice, we have found strong evidences of a continuous quantum phase transition from the antiferromagnetic to valence bond solid states as U increases. Such a phase transition is described by the U(6)/U(3)U(3) nonlinear sigma model. 
Friday, March 8, 2019 9:36AM  9:48AM 
X06.00009: Phase transitions in the "easy plane" JQ model Nisheeta Desai, Ribhu Kaul We study the JQ model on the square lattice after adding to it “easy plane” terms that explicitly break the SU(2) symmetry of the original model. The Néel to VBS transition is believed to be continuous for the full SU(2) symmetric model. In the easyplane limit, this model was shown to have a first order transition between the superfluid and the VBS phases. We use Stochastic Series Expansion Quantum Monte Carlo to investigate the nature of this transition for various strengths of this easyplane anisotropy. We find that the first order nature of the transition weakens as we weaken the strength of the easyplane interaction. However, we find no evidence that this transition becomes continuous before the SU(2) symmetric point. 
Friday, March 8, 2019 9:48AM  10:00AM 
X06.00010: Phase diagram of effective model for twisted bilayer graphene at particlehole symmetric point Yuan Da Liao, Xiao Yan Xu, Zi Yang Meng We study an effective extended Hubbard model on the honeycomb lattice with two orbitals per site at the particlehole symmetry filling. The tightbinding part of the model is believed to describe the lowenergy electronic properties of the twisted bilayer graphene. Using largescale projection quantum Monte Carlo method, we map out the ground state phase diagram of the model, the novel properties of electrons in the presence of nonlocal (plaquette) interaction, are revealed with high accuracy. 
Friday, March 8, 2019 10:00AM  10:12AM 
X06.00011: Renormalization group analysis of phase transitions in the two dimensional MajoranaHubbard model Kyle Wamer, Ian Affleck A lattice of interacting Majorana modes can occur in a superconducting film on a topological insulator in a magnetic field. The phase diagram as a function of interaction strength for the square lattice was analyzed recently using a combination of mean field theory and renormalization group methods, and was found to include second order phase transitions. One of these corresponds to spontaneous breaking of an emergent U(1) symmetry, for attractive interactions. Despite the fact that the U(1) symmetry is not exact, this transition was claimed to be in a supersymmetric universality class when time reversal symmetry is present and in the conventional XY universality class otherwise. Another second order transition was predicted for repulsive interactions with time reversal symmetry to be in the same universality class as the transition occurring in the GrossNeveu model, despite the fact that the U(1) symmetry is not exact in the Majorana model. We analyze these phase transitions using a modified εexpansion, confirming the previous conclusions. 
Friday, March 8, 2019 10:12AM  10:24AM 
X06.00012: Imaginaryspinbond order induced by a magnetic field Mengxing Ye, Andrey Chubukov We show that a 2D fermion system with well separated electron and hole pockets may develop a timereversal symmetric directional imaginaryspinbond order. It emerges when some interactions are repulsive and some are attractive. Moreover, we demonstrate that this order is necessarily triggered by a magnetic field, even if the order at zero field is a conventional spindensitywave state. We show that a finite field linearly couples the bond order to the spindensitywave order. We also discuss the ways to probe such an order in experiments. 
Friday, March 8, 2019 10:24AM  10:36AM 
X06.00013: Nonthermal states in nonintegrable magnetic models Neil Robinson, Andrew J James, Giuseppe Brandino, Robert Konik There is a new paradigm emerging for quantum systems: those which are nonintegrable but nevertheless possess nonthermal states embedded throughout their manybody spectrum (whose number is polynomial in the system size). The presence of such states violates a strong version of the eigenstate thermalization hypothesis, and the behaviour expected for generic nonintegrable systems, such as thermalization, can be broken. Why such nonthermal states appear in certain models is currently not understood, although recent works have drawn analogies with the physics of "quantum scars". We show that nonthermal states, extending far into the manybody spectrum, can arise in simple magnetic (continuum and lattice) models that exhibit confinement and the associated formation of "mesonlike" excitations. As a result, certain quenches in these models do not lead to thermalization and the nonequilibrium dynamics of local observables can show persistent, longlived oscillations. 
Friday, March 8, 2019 10:36AM  10:48AM 
X06.00014: Spin echoes in disordered Hubbard models Tasneem Biadsy, Yoav Sagi, Netanel Lindner We study spinecho signals in disordered Hubbard models. We numerically show that the spin echo signal is characterized by a timescale which depends nonmonotonically on disorder strength through the manybody localization transition. Our results yield a tool for detecting the transition which is based on short time evolution, making it efficient numerically and an attractive tool for experimental implementation. 
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