Bulletin of the American Physical Society
APS March Meeting 2020
Volume 65, Number 1
Monday–Friday, March 2–6, 2020; Denver, Colorado
Session J50: Quantum Criticality and Phase Transitions |
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Sponsoring Units: DCMP Chair: Emilian Nica, Arizona State Univ Room: Mile High Ballroom 1C |
Tuesday, March 3, 2020 2:30PM - 2:42PM |
J50.00001: 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 sign-problem-free 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. |
Tuesday, March 3, 2020 2:42PM - 2:54PM |
J50.00002: Critical properties of antiferromagnetic and valence-bond-solid transitions in lattice quantum electrodynamics Nikolai Zerf, Rufus Boyack, Peter Marquard, John A Gracey, Joseph Maciejko Lattice gauge theories underlie the effective low-energy description of many strongly correlated electron systems, such as frustrated magnets and high-temperature superconductors, but can now also be simulated directly in cold atom experiments. Elucidating the phase diagram of lattice gauge theories is thus an important problem with a wide range of applications to condensed matter physics. Recent sign-problem-free quantum Monte Carlo simulations of lattice quantum electrodynamics (QED) with N flavors of fermions on the square lattice have found evidence of continuous quantum phase transitions between a critical deconfined phase at small gauge coupling and confined antiferromagnetic (for N=2) or valence-bond-solid (for N=4,6,8) phases at large gauge coupling. We derive Landau-Ginzburg-Wilson theories of the QED-Gross-Neveu(-Yukawa) type for these transitions, find stable renormalization-group fixed points corresponding to the observed quantum critical points, and compute their critical exponents using epsilon and large-N expansions. |
Tuesday, March 3, 2020 2:54PM - 3:06PM |
J50.00003: Structure of Quantum Entanglement at a Finite Temperature Critical Point Tarun Grover, Tsung-Cheng Lu We propose a scheme to characterize long-range quantum entanglement close to a finite temperature critical point using tripartite entanglement negativity. Across a conventional order-disorder transition, tripartite negativity does not exhibit any singularity in the thermodynamic limit, in contrast to the short-range component of negativity, which is singular. This indicates that the long-distance critical fluctuations are completely classical, allowing one to define a ‘quantum correlation length’ that remains finite at the transition despite a divergent physical correlation length. |
Tuesday, March 3, 2020 3:06PM - 3:18PM |
J50.00004: Universal Prethermal Dynamics in Gross-Neveu-Yukawa Criticality Shaokai Jian, Shuai Yin, Brian Swingle We study the prethermal dynamics of the Gross-Neveu-Yukawa quantum field theory, suddenly quenched in the vicinity of a critical point. We find that the universal prethermal dynamics is controlled by two fixed points depending on the size of the quench. Besides the usual equilibrium chiral Ising fixed point for a shallow quench, a dynamical chiral Ising fixed point is identified for a deep quench. Intriguingly, the latter is a nonthermal fixed point without any equilibrium counterpart due to the participation of gapless fermionic fields. We also find that in the scaling regime controlled by the equilibrium fixed point, the initial slip exponent is rendered negative if there are enough flavors of fermions, thus providing a unique signature of fermionic prethermal dynamics. We then explore the temporal crossover between the universal scaling regimes governed by the two universality classes. Possible experimental realizations are also discussed. |
Tuesday, March 3, 2020 3:18PM - 3:30PM |
J50.00005: Critical behavior near the many-body localization transition in driven open systems Zala Lenarcic, Ori Alberton, Achim Rosch, Ehud Altman In a many-body localized (MBL) system, the coupling to an external bath typically breaks local integrals of motion. Thus the system relaxes to a unique thermal steady state. When the bath is non-thermal or when the system is weakly driven out of equilibrium, local conservation laws can be excited far from any thermal equilibrium value. I will show how this property can be used to study the MBL phase transition in weakly open systems. Here, the strength of the coupling to the non-thermal bath plays a similar role as a finite temperature in a T=0 quantum phase transition. By tuning this parameter, we can detect key features of the MBL transition: the divergence of dynamical exponent due to Griffiths effects and the critical disorder strength. |
Tuesday, March 3, 2020 3:30PM - 3:42PM |
J50.00006: B_{1g} Raman response near an Ising-nematic quantum critical point: properties of a quasielastic peak Xiaoyu Wang, Erez Berg Raman scattering has long been used to probe dynamical electronic correlations in various condensed matter systems. Here we present a theoretical study of the B_{1g} channel Raman response in the vicinity of an Ising-nematic quantum critical point (QCP) in two space dimensions. We show the appearance of a “quasi-elastic peak” as the QCP is approached. The peak frequency is related to the relaxation rate of the deformation of the Fermi surface in the angular momentum l=2 channel, while the peak intensity is directly proportional to the Ising-nematic thermodynamic susceptibility. We discuss our results in the context of iron selenide materials which host such a quantum critical point. |
Tuesday, March 3, 2020 3:42PM - 3:54PM |
J50.00007: Nematic fluctuations in the Hubbard models Tianyi Liu, Edwin Huang, Brian Moritz, Thomas Devereaux An electron nematic is an electronic phase with translation symmetry, but broken rotational symmetry. Signatures of nematic fluctuations have been observed in strongly correlated materials such as cuprates, iron pnictide, and iron chalcogenide superconductors, and may be related with other sorts of broken symmetries such as charge or spin density waves. Here we report calculations of the nematic susceptibilities from determinant quantum Monte Carlo (DQMC) simulations of the Hubbard models, for various model parameters, doping levels, and temperatures. We draw possible connections between these susceptibilities to other phenomena observed in the Hubbard model. |
Tuesday, March 3, 2020 3:54PM - 4:06PM |
J50.00008: Direct Measurement of the Soft Mode Driving a Quantum Phase Transition Matthew Libersky, Ryan D McKenzie, Daniel Silevitch, Philip Stamp, Thomas F Rosenbaum Understanding the excitations, fluctuations, and dynamics at a quantum phase transition is an important research thrust in pure condensed matter and atomic physics. An important question remains of whether critical excitations remain in the presence of an external spin bath, as is the case for many real-world implementations of quantum systems. Addressing this issue is necessary for connecting theory and modeling to observations and device design. Here, we directly measure the low energy excitation modes of a well-known realization of the quantum Ising model in transverse field, LiHoF_{4}, using microwave spectroscopy techniques to probe energies below what is accessible via neutron scattering experiments. Instead of the single excitation expected for a simple quantum Ising system, we find and characterize a remarkable array of modes arising from coupling of the spin-1/2 Ising electronic spins to a bath of spin-7/2 Ho nuclear spins, the lowest of which indeed softens at the quantum critical point. These results suggest that quantum criticality persists in the presence of a spin bath and that similar modes may exist in other quantum Ising systems, including adiabatic quantum computers. |
Tuesday, March 3, 2020 4:06PM - 4:18PM |
J50.00009: Duality Web on a 3D Euclidean Lattice and Manifestation of Hidden Symmetries Jun Ho Son, Jing-Yuan Chen, Srinivas Raghu We generalize our previous lattice construction of the abelian bosonization duality in 2+1 dimensions to the entire web of dualities as well as the Nf=2 self-duality, via the lattice implementation of a set of modular transformations in the theory space. The microscopic construction provides explicit operator mappings, and allows the manifestation of some hidden symmetries. It also exposes certain caveats and implicit assumptions beneath the usual application of the modular transformations to generate the web of dualities. Finally, we make brief comments on the non-relativistic limit of the dualities. |
Tuesday, March 3, 2020 4:18PM - 4:30PM |
J50.00010: London penetration depth in disordered unconventional superconductors with competing interactions Maxim Khodas, Maxim Dzero, Alex Levchenko A topic of an interplay between disorder and competing electronic phases in multiband superconductors have recently got renewed interest in the context of iron-based superconductors. In my talk I will present a theory of disordered unconventional superconductor with competing magnetic order. Our discussion will be based on the results obtained for on a two-band model with quasi-two-dimensional Fermi surfaces, which allows for the coexistence region in the phase diagram between magnetic and superconducting states in the presence of intraband and interband scattering induced by doping. We will demonstrate that disorder has a crucial effect on the magnetic field penetration depth as a function of disorder scattering rates. |
Tuesday, March 3, 2020 4:30PM - 4:42PM |
J50.00011: Magnetic phase transitions in quantum spin-orbital liquids Shi Feng, Niravkumar Patel, Panjin Kim, Jung Hoon Han, Nandini Trivedi We investigate the spin and orbital correlations of a superexchange model with spin S=1 and orbital L=1 relevant for 5d^{4} transition metal Mott insulators, using exact diagonalization and density matrix renormalization group (DMRG). For spin-orbit coupling λ=0, the orbitals are in an entangled state that is decoupled from the spins, leading to emergent spin-orbital separation within spin-orbital interacting system. We find two phases with increasing λ: (I) the S2 phase with two peaks in the structure factor for λ < λ_{c1 } ≈ 0.34 J, where J is the ferromagnetic exchange, and (II) the S1 phase λ_{c1 }< λ < λ_{c2 } ≈ 1.2 J with antiferromagnetic correlations. The λ = 0, S2 and S1 phases are shown to exhibit power law correlations, indicative of a gapless phase. Increasing λ > λ_{c2} leads to a product state of local spin-orbital singlets that exhibits exponential decay of correlations, indicative of a gapped phase. Using mean-field like approximation, we demonstrate that our model can be approximated with the well-known Uimin-Lai-Sutherland (ULS) model with an external field. |
Tuesday, March 3, 2020 4:42PM - 4:54PM |
J50.00012: Universal quantum glass transition on the Bethe lattice Izabella Lovas, Catalin Pascu Moca, Gergely Zarand We study the Coulomb glass behavior emerging from the interplay of interactions and disorder, by examining a model of spinless fermions at half filling on the Bethe lattice [1,2]. We consider the limit of infinite coordination number, where we combine dynamical mean field theory with a Hartree-Fock approximation to investigate the glass transition and the properties of the glassy phase in the presence of full replica symmetry breaking. This approach allows us to study the opening of the Efros-Shklovskii pseudogap in the glassy phase, and grants us access to the spectral function. In particular, we demonstrate the universal scaling collapse of the pseudogap and the spectral function at close to zero temperatures, where the melting of the glass is governed by the quantum fluctuations induced by the hopping of fermions. We show that this quantum scaling function differs from the classical scaling function of the thermal transition of the spin glass limit. Our results should be relevant for the glassy dynamics observed in Si inversion layers, persisting in the metallic phase [3]. |
Tuesday, March 3, 2020 4:54PM - 5:06PM |
J50.00013: Atomic-scale fragmentation and collapse of antiferromagnetic order in a doped Mott insulator He Zhao, Sujit Manna, Zach Porter, Xiang Chen, Andrew Uzdejczyk, Jagadeesh Moodera, Ziqiang Wang, Stephen Wilson, Ilija Zeljkovic Measuring magnetic and electronic properties at atomic length scales would provide crucial insight into the physics of doped antiferromagnetic Mott insulators, but this has been difficult to achieve. We use spin-polarized scanning tunneling microscopy (SP-STM) to visualize the periodic spin-resolved modulations originating from the antiferromagnetic order in a J_{eff} = ½ strongly spin-orbit coupled Mott insulator Sr_{2}IrO_{4}. We discover that near insulator-to-metal transition (IMT), the long-range antiferromagnetic order melts into a spatially fragmented state with short-range correlations. Importantly, we find that the short-range antiferromagnetic order is locally uncorrelated with the observed spectral gap magnitude. This suggests that static short-range antiferromagnetic correlations are unlikely to be the cause of the inhomogeneous closing of the spectral gap and the emergence of pseudogap regions near the IMT in doped Sr2IrO4. Our work establishes SP-STM as a powerful tool for revealing atomic-scale magnetic information in complex oxides. |
Tuesday, March 3, 2020 5:06PM - 5:18PM |
J50.00014: Phase Structure of Isotropic and Anisotropic Ising Models at Complex Couplings Sankhya Basu, Vadim Oganesyan, Chris A Hooley In this work, we study the phase structures and phase transitions that arise in both the Isotropic and the Anisotropic Ising Models by analytically continuing away from real coupling – both temperature and field. In particular, we observe that away from real temperatures, Yang-Lee circle theorem indeed breaks down, even for the Isotropic Ising Model. We also observe symmetry breaking of the global minima of the free energy away from real temperatures in the high temperature phase. We use both analytical as well as numerical techniques based on Tensor Network formulations (Tensor Renormalization Group(TRG), Higher Order Tensor Renormalization Group(HOTRG)) to access these novel regimes in the phase space that is out of reach of standard Monte Carlo techniques. |
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