Bulletin of the American Physical Society
APS March Meeting 2014
Volume 59, Number 1
Monday–Friday, March 3–7, 2014; Denver, Colorado
Session F46: Quantum Criticality: Theory and Experiment |
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Sponsoring Units: DCMP Chair: Andriy Nevidomskyy, Rice University Room: Mile High Ballroom 4E |
Tuesday, March 4, 2014 8:00AM - 8:12AM |
F46.00001: Finite-temperature spin dynamics near the quantum critical point of transverse field Ising chain with a small longitudinal field M\'arton Kormos, Jianda Wu, Qimiao Si When the transverse-field Ising chain at its quantum critical point is subjected to a small longitudinal field, the perturbed conformal field theory led to a field theory with an exotic E8 symmetry [1]. Recent neutron scattering experiments have provided evidence for the lightest two particles in this E8 model in the quasi-1D Ising ferromagnet CoNb$_2$O$_6$ [2]. While the zero temperature dynamic of the model is well known, its finite-temperature counterpart has not yet been systematically studied. We study the low-frequency dynamical spin structure factor at finite temperatures using the form-factor method. We show that the dominant contribution to the spin dynamics comes from the channel between two lightest particles, and demonstrate how the spin dynamics differ from a diffusion form. Using these results, we determine the temperature dependence of the NMR relaxation rate. We suggest that, for CoNb$2$O$6$, measurements of the NMR relaxation rate provide a means to further test the applicability of the E8 model. \\[4pt] [1] A. B. Zamolodchikov, Int. J. Mod. Phys. A4, 4235(1989).\\[0pt] [2] R. Coldea, D. A. Tennant, E. M. Wheeler, E. Wawrzynska, D. Prabhakaran, M. Telling, K. Habicht, P. Smeibidl, K. Kiefer2, Science, 327, 177 (2010). [Preview Abstract] |
Tuesday, March 4, 2014 8:12AM - 8:24AM |
F46.00002: Isoelectronically tuned magnetic and Ising quantum phase transitions in iron-based superconductors Jianda Wu, Qimiao Si The bad-metal behavior of the iron arsenides motivated a proximity-to-Mott picture, which led to the theoretical proposal for a quantum critical point (QCP) under iso-electronic phosphorous for arsenic doping in the parent iron arsenides [1]. Here, P doping increases the in-plane electronic kinetic energy and thus the coherent electronic spectral weight, thereby weakening the magnetic order and the associated Ising-nematic spin order. Extensive experimental measurements in the P-doped CeFeAsO and BaFe$_2$As$_2$ [2,3] have provided strong evidence for such a QCP. Here, we explore these phases and their transitions by carrying out a large-N study of an effective low-energy Ginzburg-Landau model for these systems. We determine the parameter range over which second order magnetic and Ising quantum phase transitions arise. [1] J. Dai, Q. Si, J-X Zhu, and E. Abrahams, PNAS, 106, 4118 (2009) [2] C. de la Cruz, et al., Phys Rev Lett, 104, 017204 (2010) [3] S. Kasahara, et al., Phys Rev B, 81, 184519 (2010); K. Hashimoto et al, Science 336, 1554 (2012). [Preview Abstract] |
Tuesday, March 4, 2014 8:24AM - 8:36AM |
F46.00003: Metal-SDW phase transition in $3- \epsilon$ dimensions Shouvik Sur, Sung-Sik Lee The quantum phase transition associated with a spin density wave (SDW) instability in two dimensional metals is relevant to various strongly correlated electron systems including high-$T_c$ superconductors. However, the critical point associated with the phase transition has remained largely inaccessible due to the absence of a small parameter. In this work we use the recently developed dimensional regularization scheme, where the co-dimension of a Fermi surface is extended to general values while its dimension is held fixed, to perturbatively access the quantum critical point in $3-\epsilon$ space dimensions. We derive the beta functions and compute the critical exponents to the one loop order. [Preview Abstract] |
Tuesday, March 4, 2014 8:36AM - 8:48AM |
F46.00004: Optical conductivity of a two-dimensional metal at the onset of spin-density-wave order Dmitrii Maslov, Andrey Chubukov, Vladimir Yudson We consider the optical conductivity of a clean two-dimensional metal at $T=0$ near a spin-density-wave instability. Critical fluctuations destroy fermionic coherence at ``hot spots'' of the Fermi surface but a large part of the Fermi surface is neither ``hot'' or ``cold'' but rather ``lukewarm,'' in a sense that quasiparticles there are strongly renormalized compared to the non-interacting case. We discuss the self-energy of lukewarm fermions and their contribution to the optical conductivity, $\sigma(\Omega)$, due to scattering off composite bosons made of two critical magnetic fluctuations. Recent study [S.A. Hartnoll et al., Phys. Rev. B {\bf 84}, 125115 (2011)] found that composite scattering leads to a singular fermionic self-energy of lukewarm fermions at the quantum critical point. We show that, at the lowest frequencies, the most singular, $\ln^3\Omega/\Omega^{1/3}$ contribution to the conductivity is canceled between the self-energy, vertex-correction, and Aslamazov-Larkin diagrams. However, the cancellation does not extent beyond logarithmic accuracy, and the remaining conductivity still diverges as $1/\Omega^{1/3}$. At larger $\Omega$, $\sigma (\Omega)$ scales in a marginal FL way, as $1/\Omega$. [Preview Abstract] |
Tuesday, March 4, 2014 8:48AM - 9:00AM |
F46.00005: Anomalous Scaling of Magnetic Penetration Depth from Quantum Critical Fluctuations Jian-Huang She, Michael Lawler, Eun-Ah Kim Recently, systematic penetration depth (PD) measurements carried out over several families of unconventional superconductors suggest they are near quantum critical points (QCP). In particular, the temperature dependence of the PD shows anomalous power law scaling. We argue, because the momentum carried by critical fluctuations needs to connect nodal points, this anomalous behavior is not due to AFM ordering. So instead, we focus on instabilities of the d-wave superconducting state associated with developing additional Q=0 order that can alter the scaling behavior of the PD. This additional ordering can be in either the charge channel, the pairing channel or both. We find that fluctuations in the pairing channel leads to scaling exponents smaller than one, while fluctuations in the charge channel leads to scaling exponents larger than one. Based on these results, we argue that the temperature scaling of PD in CeCoIn5 is caused by close a proximity to a QCP associated predominantly with Fermi surface distortions such as a nematic QCP. [Preview Abstract] |
Tuesday, March 4, 2014 9:00AM - 9:12AM |
F46.00006: Unconventional Superconductivity near a Kondo Destroyed Quantum Critical Point Jedediah Pixley, Lili Deng, Kevin Ingersent, Qimiao Si Heavy fermion metals serve as prototypical correlated materials to study antiferromagnetic quantum critical points (QCPs). Theoretical studies have identified a class of unconventional quantum critical points, in which Kondo destruction accompanies the onset of magnetic order. Whether or not such a QCP may promote superconductivity is an open question. Experimentally, there is strong evidence, e.g. from the heavy-fermion material CeRhIn$_{5}$, that such a QCP underlies unconventional superconductivity. With this in mind, we study the superconducting pairing susceptibility in the periodic Anderson model, within a cluster extended dynamical mean field theory (C-EDMFT). We find that the Kondo energy scale is continuously suppressed at the magnetic QCP. In addition, we find the pairing susceptibility to be strongly enhanced when the QCP is approached, both from the paramagnetic Kondo screened side and from the Kondo-destroyed magnetically ordered side. Our results point to a new form of unconventional superconductivity associated with both the magnetic fluctuations and a proximity to electronic localization. [Preview Abstract] |
Tuesday, March 4, 2014 9:12AM - 9:24AM |
F46.00007: Pairing correlations near a Kondo-destruction quantum critical point Lili Deng, Kevin Ingersent, Jedediah Pixley, Qimiao Si Motivated by the unconventional superconductivity observed in heavy-fermion metals, we investigate the pairing susceptibility near a continuous quantum phase transition of the Kondo-destruction type. We first solve two-impurity Bose-Fermi Anderson models with Ising and Heisenberg forms of the inter-impurity exchange interaction using continuous-time Monte-Carlo and numerical renormalization-group methods [1]. For each model, we determine its phase diagram and show a Kondo-destruction quantum critical point separating Kondo-screened and local-moment phases. For antiferromagnetic inter-impurity exchange interactions, singlet pairing is found to be enhanced in the vicinity of the quantum critical point. We then proceed to study the Anderson lattice model based on a cluster extended dynamical mean-field theory (C-EDMFT). We show how the results of the two-impurity models connect to those near the Kondo-destruction quantum critical point of the lattice case, and discuss the implications of our results for superconductivity in quantum-critical heavy fermions.\\[4pt] [1] J. H. Pixley, L. Deng, K. Ingersent, Q. Si, arXiv:1308.0839. [Preview Abstract] |
Tuesday, March 4, 2014 9:24AM - 9:36AM |
F46.00008: Deconfined Quantum Criticality and Conformal Phase Transition Flavio Nogueira, Asle Sudbo We introduce a new perspective on deconfined quantum criticality within a field-theoretic framework. We show that in the allegedly weak first-order transition regime from a N\'eel to a valence-bond solid in $SU(N)$ antiferromagnets, a so-called conformal phase transition leads to a genuine deconfined quantum critical point. In such a transition, the gap vanishes as the critical point is approached, while the spin stiffness at zero temperature has a universal jump at the critical point. We discuss the logarithmic corrections to scaling observed numerically and interpret them in terms of the conformal phase transition. The behavior of the N\'eel and valence-bond solid susceptibilities are discussed at zero and finite temperatures. [Preview Abstract] |
Tuesday, March 4, 2014 9:36AM - 9:48AM |
F46.00009: The dynamics of quantum criticality: Quantum Monte Carlo and holography William Witczak-Krempa, Erik Sorensen, Subir Sachdev Understanding the real time dynamics of systems near quantum critical points at finite temperature constitutes an important yet challenging problem. We present quantum Monte Carlo results for 2 separate realizations of the superfluid-insulator transition of bosons on a lattice: their low-frequency conductivities are found to have the same universal dependence on imaginary frequency and temperature. We use the structure of the real time dynamics of conformal field theories described by the holographic gauge/gravity duality to make progress on the difficult problem of analytically continuing the Monte Carlo data to real time. Our method yields quantitative and experimentally testable results on the frequency-dependent conductivity near the quantum critical point. Connections to other observables and universality classes are discussed, as well as new holographic extensions. [Preview Abstract] |
Tuesday, March 4, 2014 9:48AM - 10:00AM |
F46.00010: Criticality and quenched disorder: rare regions vs. Harris criterion Thomas Vojta, Jose Hoyos We employ scaling arguments and optimal fluctuation theory to establish a general relation between quantum Griffiths singularities and the Harris criterion for quantum phase transitions in disordered systems. If a clean critical point violates the Harris criterion, it is destabilized by weak disorder. At the same time, the Griffiths dynamical exponent $z'$ diverges upon approaching the transition, suggesting unconventional critical behavior. In contrast, if the Harris criterion is fulfilled, power-law Griffiths singularities can coexist with clean critical behavior but $z'$ saturates at a finite value. We present applications of our theory to a variety of systems including quantum spin chains, classical reaction-diffusion systems and metallic magnets; and we discuss modifications for transitions above the upper critical dimension. Based on these results we propose a unified classification of phase transitions in disordered systems. [Preview Abstract] |
Tuesday, March 4, 2014 10:00AM - 10:12AM |
F46.00011: Divergence of the Thermopower Without a Quantum Phase Transition Kridsanaphong Limtragool, Philip Phillips It is generally believed that divergent thermopowers require an underlying quantum phase transition. We show here in two exactly solvable models that this is not the case. We study the quantum XY and Kitaev models and show that the thermopower diverges in a parameter space that has nothing to do with the phase transition. The divergence is tied to a zero of the Onsager coefficient $L_{11}$. The zero of $L_{11}$ is linked to a sign change in the effective charge and a vanishing of the band velocity. At such points, there is no thermodynamic signature only a divergent thermopower. Implications for interpreting divergent thermopowers as phase transitions will be discussed. [Preview Abstract] |
Tuesday, March 4, 2014 10:12AM - 10:24AM |
F46.00012: Nernst and angle-dependent magneto thermopower measurements on Sr3Ru2O7 Chenyi Shen, Hui Xing, Xiaojun Yang, Qian Tao, Mingliang Tian, Zhiqiang Mao, Zhuan Xu, Ying Liu The behavior of the metamagnetic transition in Sr3Ru2O7, the double-layer member of the Ruddlesden-Popper homologous series the Srn$+$1RunO3n$+$1, depends strongly on the direction of the applied magnetic field. With the field applied along the c axis, the end point of the first order metamagnetic transition is located at a very low temperature, pushing the material to be close to a quantum critical point. Exotic phenomena such as an electronic nematic phase was proposed. However, important questions on this phase remains to be unresolved. Recent measurements on the field dependent specific heat appears to suggest a state with excessive entropy away from the quantum critical point. We present our Nernst effect and angle-dependent magneto thermopower measurements on high-quality single crystals of Sr3Ru2O7. The temperature gradient was applied along the a- (or b-) axis and 45-degree from it and the in-plane magnetic field was rotating in the ab plane. Interesting behavior was found in these measurements away from the quantum critical regime. The implications of our data on various issues on Sr3Ru2O7 will be discussed. [Preview Abstract] |
Tuesday, March 4, 2014 10:24AM - 10:36AM |
F46.00013: Infrared spectroscopy of magnetic field-induced quantum criticality in Sr$_3$Ru$_2$O$_7$ Jesse S. Hall, Urmas Nagel, Toomas R{\~o}{\~o}m, J.F. Mercure, Robin S. Perry, Andrew P. Mackenzie, Thomas Timusk Infrared spectroscopy performed on the model quantum critical system Sr$_3$Ru$_2$O$_7$ offers an opportunity to study the frequency dependence of the magnetic field-tuned quantum critical behaviour. The temperature dependence of the resistivity is well characterized in magnetic fields, as is the Fermi surface, so the relevant optical properties can be determined and compared to the theory for non-Fermi liquids. We report infrared reflectance measurements from 1- 12 meV at temperatures below 10 K and magnetic fields up to 17 T. A sharp field-dependent feature appears in the reflectance at 3.8 meV, combined with a broad suppression of the reflectance across the measurement region. The optical scattering rate will be compared to other measurements and the implications for the understanding of quantum criticality will be discussed. [Preview Abstract] |
Tuesday, March 4, 2014 10:36AM - 10:48AM |
F46.00014: Optical Detection of the Electron Nematic Phase in Sr$_3$Ru$_2$O$_7$ Colin Heikes, D. MacNeill, S. Ghosh, R. Perry, J.F. Mercure, E.A. Kim, A. Mackenzie, D.C. Ralph We report the implementation of a fiber-based optical microscope, capable of operating at temperatures below 100 mK and in magnetic fields in excess of 9 Tesla, with sub-micron spatial resolution. This microscope is integrated into the bore of a dilution refrigerator with an optical fiber coupling light to an external optical table. Bench-top optical elements allow for polarization analysis of the reflected light from a surface and thus the detection of magnetic or other polarization-sensitive properties of mater at low temperature and high fields. We are studying the proposed electron nematic phase of the n=2 Ruddlesden-Popper material Sr$_3$Ru$_2$O$_7$, which exhibits a low-temperature phase transition in the form of an in-plane conduction anisotropy. We report recent results from concurrent transport measurements and polarization analysis as well as polarization microscopy with sample temperatures below 150 mK and applied magnetic fields from 0 T to 9 T. [Preview Abstract] |
Tuesday, March 4, 2014 10:48AM - 11:00AM |
F46.00015: Field Tuned Quantum Criticality In YFe2Al10 Liusuo Wu, Monika Gamza, Keeseong Park, Moosung Kim, Manuel Brando, Markus Garst, Meigan Aronson Most studies of quantum criticality have been carried out in $f$-electron based heavy fermions, and the observation and description of the quantum critical behaviors in systems where magnetism comes from d-electrons have been very limited. YFe$_{2}$Al$_{10}$ is a rare d-electron compound that displays pronounced non-Fermi liquid behaviors, including divergencies in the magnetic susceptibility ($\chi $ $\sim$ T$^{-\gamma}$, $\gamma =$1.4) and magnetic specific heat (C$_{\mathrm{M}}$/T $\sim$ -log T). We propose a carried out a scaling analysis of $\chi $(B,T) and C(B,T)/T that indicates YFe2Al10 is located very close to a B$=$0 QCP. We propose a singular free energy and a scaling function that consistently explains the critical exponents as well as the QC-Fermi liquid crossover in terms of a scaling variable T/B$^{0.6}$ Unusually, we find that the spatial dimension d is equal to the dynamical exponent z, and considering the two-dimensional anisotropy of the magnetic susceptibility, we infer that d$=$z$=$2. Hyperscaling is established by the internal consistency of our analysis, and the decidedly non-mean field exponents argue that QC fluctuations are protected since YFe2Al10 is likely a system that is below its upper critical dimension. These experimental observations suggest that YFe$_{2}$Al$_{10}$ is a unique 3d-electron based system that is quantum critical without the need for fine tuning. [Preview Abstract] |
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