Bulletin of the American Physical Society
APS March Meeting 2016
Volume 61, Number 2
Monday–Friday, March 14–18, 2016; Baltimore, Maryland
Session L27: Quantum Criticality: Theory |
Hide Abstracts |
Sponsoring Units: DCMP Chair: Lucile Savary, Massachusetts Institute of Techology Room: 326 |
Wednesday, March 16, 2016 11:15AM - 11:27AM |
L27.00001: Vestigial nematicity from spin and/or charge order in the cuprates Laimei Nie, Akash Maharaj, Eduardo Fradkin, Steven Kivelson Nematic order (C4 rotation symmetry breaking) has manifested itself in a variety of materials in the cuprates family, yet its origin remains debatable, with possible links to lattice, charge, and spin degrees of freedom across different doping regimes. We propose an effective field theory of a layered system with incommensurate, intertwined spin- and charge-density wave (SDW and CDW) orders, each of which consists of two components related by C4 rotations. Using a variational free energy approach, we study the growth of the associated nematicity from partially melting those density waves by either increasing temperature or adding quenched disorder. Under the assumption that the zero-disorder, zero-interaction SDW transition temperature is higher than CDW at small doping (and vice versa for large doping), we find that for the general case with finite disorder and interactions there is a universal nematic transition across the entire doping range, accompanied by SDW and CDW transitions (or strong fluctuations at large enough disorder) at lower temperatures. We also discuss the issues concerning the difference between La-based materials and the other hole-doped cuprates. [Preview Abstract] |
Wednesday, March 16, 2016 11:27AM - 11:39AM |
L27.00002: Quantum critical dynamics of a magnetic impurity in a semiconducting host Nagamalleswararao Dasari, Swagata Acharya, A Taraphder, Juana Moreno, Mark Jarrell, N. S. Vidhyadhiraja We have investigated the finite temperature dynamics of the singlet to doublet continuous quantum phase transition in the gapped Anderson impurity model using hybridization expansion continuous time quantum Monte Carlo. Using the self-energy and the longitudinal static susceptibility, we obtain a phase diagram in the temperature-gap plane. The separatrix between the low-temperature local moment phase and the high temperature generalized Fermi liquid phase is shown to be the lower bound of the critical scaling region of the zero gap interacting quantum critical point. We have computed the nuclear magnetic spin-lattice relaxation rate, the Knight shift, and the Korringa ratio, which show strong deviations for any non-zero gap from the corresponding quantities in the gapless Kondo screened impurity case. [Preview Abstract] |
Wednesday, March 16, 2016 11:39AM - 11:51AM |
L27.00003: Corner entanglement as a probe of quantum criticality William Witczak-Krempa, Pablo Bueno, Robert C. Myers The entanglement entropy in many gapless quantum systems in 2+1D receives a contribution from corners in the entangling surface. It is characterized by a universal function $a(\theta)$ that depends non-trivially on the corner opening angle $\theta$. Focusing on a large family of quantum critical theories with emergent Lorentz invariance (CFTs), we argue that the smooth limit $a(\theta \approx \pi)$ is entirely determined by the energy-density or stress tensor 2-point function coefficient. This explains recent results obtained via cutting edge simulations on the quantum critical Ising, XY and Heisenberg models. We also show how to extract the full thermal entropy of the quantum critical system using corner entanglement of the groundstate alone. ** Bueno, Myers, WK, Phys. Rev. Lett. (2015) [Preview Abstract] |
Wednesday, March 16, 2016 11:51AM - 12:03PM |
L27.00004: Deconfined quantum criticality beyond designer Hamiltonians Thomas C. Lang, Ribhu K. Kaul The SU(6) symmetric generalization of the Hubbard model on the square lattice provides the simplest microscopic realization of the quantum phase transition from a N\'{e}el to a valence bond solid (VBS) ordered phase. By constructing dimensionless quantities such as ratios of the magnetic structure factor and valence bond correlations we are able to unambiguously determine the existence of weak, but robust antiferromagnetic order in the weak coupling regime and a plaquette VBS in the strong coupling limit. Furthermore these ratios provide a tool to accurately determine the (critical) point from both sides of the phase transition separating the two limits. Preliminary results suggest a direct continuous transition for which we extract estimates for the critical exponents and compare the scaling function with the SU(6) \textit{designer} spin-models to investigate whether this quantum phase transition is compatible with the scenario of deconfined quantum criticality. [Preview Abstract] |
Wednesday, March 16, 2016 12:03PM - 12:15PM |
L27.00005: Fermion-induced quantum critical points: beyond Landau criterion Hong Yao, Zi-Xiang Li, Yi-Fan Jiang, Shao-Kai Jian According to Landau criterion, phase transitions must be first-order when cubic terms of order parameters in the Landau-Ginzburg free energy are allowed by symmetry. Here, from both renormalization group analysis and sign-problem-free quantum Monte Carlo simulations, we show that second-order quantum phase transitions can occur at such putatively-first-order quantum phase transitions in strongly-interacting Dirac semimetals in two spatial dimensions. Such type of Landau-criterion-violating quantum critical points are induced by massless fermionic modes at the quantum phase transitions. We call them ``fermion-induced quantum critical points''. From Majorana-quantum-Monte-Carlo simulations and renormalization analysis, we find that the critical exponentials at the kekule valence-bond-solid transition of the Dirac fermions on the honeycomb lattice are highly-nonclassical. We also discuss experimental signatures of the kekule quantum critical point which may be realized in graphene-like systems. [Preview Abstract] |
Wednesday, March 16, 2016 12:15PM - 12:27PM |
L27.00006: Nonlinear I-V Curve at a Quantum Impurity Quantum Critical Point Harold Baranger, Chung-Hou Chung, Chao-Yun Lin, Gu Zhang, Chung-Ting Ke, Gleb Finkelstein The nonlinear I-V curve at an interacting quantum critical point (QCP) is typically out of reach theoretically. Here, however, we provide a striking example of an analytical calculation of the full nonlinear I-V curve at the QCP. The system that we consider is a quantum dot coupled to resistive leads -- a spinless resonant level interacting with an ohmic EM environment in which a QCP similar to the two-channel Kondo QCP occurs. Recent experiments studied this criticality via transport measurements: the transmission approaches unity at low temperature and applied bias when tuned exactly to the QCP (on resonance and symmetric tunnel barriers) and approaches zero in all other cases. To obtain the current at finite temperature and arbitrary bias, we write the problem as a one-dimensional field theory and transform from electrons in the left/right leads to right-going and left-going channels between which there is weak two-body backscattering. Drawing on dynamical Coulomb blockade theory, we thus obtain an analytical expression for the full I-V curve. The agreement with the experimental result is remarkable. [Preview Abstract] |
Wednesday, March 16, 2016 12:27PM - 12:39PM |
L27.00007: Finite-temperature Dynamics and Quantum Criticality in a Model for Insulating Magnets Jianda Wu, Wang Yang, Congjun Wu, Qimiao Si Theoretical understanding of the finite-temperature dynamics in quantum critical systems is a challenging problem, due to the mixing of thermal and quantum fluctuations. Recently, neutron scattering experiments in the three-dimensional quantum dimmer material TlCuCl3 under pressure tuning have mapped out the magnetic dynamics at finite temperatures in the quantum critical regime [1], thereby providing the opportunity for systematic understandings. In this work, we calculate the spin spectral function of an $O(n)$ symmetric field theory using a field-theory procedure to two loops. We calculate the temperature dependence of the energy and damping rate of the spin excitations in the quantum critical regime, demonstrate a good agreement with the experimental results, and determine the parameter regime of the field theory that is appropriate for TlCuCl3. From our calculations we can also suggest further experimental means to test the applicability of the underlying field theory in this and related systems. [1] P. Merchant, B. Normand2, K.W. Krämer, M. Boehm, D. F. McMorrow and Ch. Rüegg, Nat. Phys. 10, 373 (2014). [Preview Abstract] |
Wednesday, March 16, 2016 12:39PM - 12:51PM |
L27.00008: Competing phases in the single-band Hubbard model on the 1/5-depleted square lattice Michael Mulanix, Ehsan Khatami Using exact diagonalization of small clusters, we study the Hubbard model on the 1/5-depleted square lattice. This geometry, which arises in ordered-vacancy iron selenide superconductors, consists of 2 by 2 plaquettes connected through inter-plaquette bonds. Previous determinantal quantum Monte Carlo simulations have shown that the model at half filling displays multiple quantum phase transitions by tuning the ratio of hoppings for the two types of bonds, or by varying the interaction strength. We extend those results to the region away from half filling and study the magnetic, charge and pairing correlation functions for a wide range of interaction strengths and the hopping ratios. We find an interesting variation of the magnetic ordering wavevector as the density changes, particularly if the hopping ratio is tuned in favor of the intra-plaquatte bond. We also find that, for small interaction strengths and at low densities, unexpected charge or pair density waves dominate. [Preview Abstract] |
Wednesday, March 16, 2016 12:51PM - 1:03PM |
L27.00009: Metallic quantum critical points with finite BCS couplings Shamit Kachru, Srinivas Raghu, Gonzalo Torroba, Huajia Wang We study the fate of superconductivity in the vicinity of a class of metallic quantum critical points obtained by coupling a Fermi surface to a critical boson. In such systems there is a competition between the enhanced pairing tendency due to the presence of long-range attractive interactions near criticality, and the suppression of superconductivity due to the destruction of the Landau quasiparticles. We show that there are regimes in which these two effects offset one another, resulting in a novel non-Fermi liquid fixed point with {\it finite}, scale invariant, BCS coupling. While these interactions lead to substantial superconducting fluctuations, they do not drive the system into a superconducting ground state. The metallic quantum critical fixed points are connected to the superconducting regime by a continuous phase transition. These results are established using a controlled expansion in the deviation from $d=3$ spatial dimensions, as well as in a large number $N$ of internal flavors. We discuss the possible relevance of our findings to the phenomenon of superconducting domes condensing out of a non-Fermi liquid normal state near quantum critical points. [Preview Abstract] |
Wednesday, March 16, 2016 1:03PM - 1:15PM |
L27.00010: Metallic quantum critical ferromagnets: a quantum Monte Carlo calculation of Non-Fermi liquid exponents Sam Ridgway, Chris Hooley We study a lattice field theory describing the quantum ferromagnetic transition of a metal in two spatial dimensions, using a sign-problem free quantum Monte Carlo algorithm. We provide evidence for the continuous nature of the transition, and calculate universal critical exponents that indicate non-Fermi liquid behaviour at the critical point. [Preview Abstract] |
Wednesday, March 16, 2016 1:15PM - 1:27PM |
L27.00011: Violation of hyperscaling at the Ising-nematic quantum critical point in a two-dimensional metal Andreas Eberlein, Ipsita Mandal, Subir Sachdev Spatially isotropic critical quantum states in $d$ spatial dimensions which have the hyperscaling property have an optical conductivity that scales as $\omega^{(d-2)/z}$ for high frequencies $\omega >> T$, where $T$ is the temperature and $z$ the dynamic critical exponent. We examine the Ising-nematic quantum critical point in $d = 2$ using the fixed point theory by Dalidovich and Lee (Phys. Rev. B 88, 245106 (2013)) and compute the optical conductivity in an expansion in $\epsilon = 5/2 - d$. We show that hyperscaling is violated at this quantum critical point and discuss the scaling behaviour of the optical conductivity at $T = 0$. [Preview Abstract] |
Wednesday, March 16, 2016 1:27PM - 1:39PM |
L27.00012: Rescuing a Quantum Phase Transition with Quantum Noise Gu Zhang, Eduardo Novais, Harold Baranger We show that placing a quantum system in contact with an environment can enhance non-Fermi-liquid correlations, rather than destroying quantum effects as is typical. The system consists of two quantum dots in series with two leads; the highly resistive leads couple charge flow through the dots to the electromagnetic environment (noise). The similarity to the two impurity Kondo model suggests that there will be a quantum phase transition between a Kondo phase and a local singlet phase. However, this transition is destabilized by charge tunneling between the two leads. Our main result is that sufficiently strong quantum noise suppresses this charge transfer and leads to stabilization of the quantum phase transition. We present the phase diagram, the ground state degeneracy at the four fixed points, and the leading temperature dependence of the conductance near these points. [Preview Abstract] |
Wednesday, March 16, 2016 1:39PM - 1:51PM |
L27.00013: Anisotropic Non-Fermi Liquids Shouvik Sur, Sung-Sik Lee We study non-Fermi liquids that arise at quantum critical points associated with spin (SDW) and charge density wave (CDW) transitions in metals with twofold rotational symmetry. We use the `codimensional' regularization scheme, where a one-dimensional Fermi surface is embedded in $3-\epsilon$ dimensional momentum space. In three dimensions, quasilocal marginal Fermi liquids arise at the SDW and CDW critical points. Below three dimensions, a perturbative anisotropic non-Fermi liquid state is realized at the SDW critical point, where not only time but also different spatial coordinates develop distinct anomalous dimensions. The stable non-Fermi liquid exhibits an emergent algebraic nesting as the patches of the Fermi surface are deformed into a universal power-law shape near the hot spots. Due to the anisotropic scaling, the energy of spin fluctuations disperse with different power laws in different momentum directions. In contrast, at the CDW critical point, the perturbative expansion breaks down immediately below three dimensions as the interaction renormalizes the speed of charge fluctuations to zero within a finite renormalization group scale. [Preview Abstract] |
Wednesday, March 16, 2016 1:51PM - 2:03PM |
L27.00014: Universal quantum criticality in Hubbard models with massless Dirac dispersion Yuichi Otsuka, Seiji Yunoki, Sandro Sorella We investigate the metal-insulator transition of two-dimensional interacting electrons with massless Dirac-like dispersion, describe by the Hubbard models on two geometrically different lattices: honeycomb and $\pi$-flux square lattices. By performing large-scale quantum Monte Carlo simulations followed by careful finite-size scaling analyses, we find that the transition from semi-metallic to antiferromagnetic insulating phases is continuous and evaluate the critical exponents with a high degree of accuracy for the corresponding universality class, which is described in the continuous limit by the Gross-Neveu model. We furthermore discuss the fate of the quasiparticle weight and the Fermi velocity across this transition. [Preview Abstract] |
Wednesday, March 16, 2016 2:03PM - 2:15PM |
L27.00015: Mott Quantum Criticality in the Anisotropic 2D Hubbard Model Benjamin Lenz, Salvatore R. Manmana, Thomas Pruschke, Fakher F. Assaad, Marcin Raczkowski We present evidence for Mott quantum criticality in an anisotropic two-dimensional system of coupled Hubbard chains at half-filling. In this scenario emerging from variational cluster approximation and cluster dynamical mean-field theory, the interchain hopping $t_{\perp}$ acts as control parameter driving the second-order critical endpoint $T_c$ of the metal-insulator transition down to zero at $t_{\perp}^{c}/t\simeq 0.2$. Below $t_{\perp}^{c}$ the volume of hole and electron Fermi pockets of a compensated metal vanishes continuously at the Mott transition. Above $t_{\perp}^{c}$ the volume reduction of the pockets is cut off by a first-order transition. We discuss the relevance of our findings to a putative quantum critical point in layered organic conductors whose location remains elusive so far. [Preview Abstract] |
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