Bulletin of the American Physical Society
APS March Meeting 2011
Volume 56, Number 1
Monday–Friday, March 21–25, 2011; Dallas, Texas
Session T22: Correlated Electrons and Magnetic Phase Transitions |
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Sponsoring Units: DCMP Chair: Harold Baranger, Duke University Room: D163 |
Wednesday, March 23, 2011 2:30PM - 2:42PM |
T22.00001: Quantum spin metal state on a decorated honeycomb lattice Konstantin Tikhonov, Mikhail Feigel'man We present a modification of exactly solvable spin-(1/2) Kitaev model on the decorated honeycomb lattice, with a ground state of ``spin metal'' type. The model is diagonalized in terms of Majorana fermions; the latter form a 2D gapless state with a Fermi-circle those size depends on the ratio of exchange couplings. Low-temperature heat capacity $C(T)$ and dynamic spin susceptibility $\chi(\omega,T)$ are calculated in the case of small Fermi-circle. Whereas $C(T)\sim T$ at low temperatures as it is expected for a Fermi-liquid, spin excitations are gapful and $\chi(\omega,T)$ demonstrate unusual behavior with a power-law peak near the resonance frequency. The corresponding exponent as well as the peak shape are calculated. [Preview Abstract] |
Wednesday, March 23, 2011 2:42PM - 2:54PM |
T22.00002: Quantum Order by Disorder Driven Phase Reconstruction at Itinerant Electron Quantum Critical Points Una Karahasanovic, Andrew Green, Gareth Conduit Phase reconstruction at itinerant electron quantum critical points is driven by quantum fluctuations lowering the energy of certain deformations of the Fermi surface through second and higher order perturbation theory, i.e. quantum order by disorder. This approach was previously shown to predict a fluctuation-driven spatially modulated phase near to the ferromagnet to paramagnet quantum critical point [1]; a phase that had previously been predicted from diagrammatic evaluation of non-analytic corrections to Moriya-Hertz-Millis theory. We extend our analysis to include several other phases which may be stabilized at the ferromagnetic quantum critical point, including nematic order and superconductivity. The itinerant quantum critical point is unstable to the formation of multiple phases. \\[4pt] [1] G. J. Conduit, A. G. Green and B. D. Simons, Phys. Rev. Lett. {\bf 103}, 207201 (2009) [Preview Abstract] |
Wednesday, March 23, 2011 2:54PM - 3:06PM |
T22.00003: Fixed Spin Moment Study of Quantum Critical Fe$_3$Mo$_3$N Brian Neal, Warren E. Pickett Quantum critical behavior and weak magnetism occurs in a handful of intermetallic transition metal compounds, with a recent example being Fe$_3$Mo$_3$N with the geometrically frustrated {\it stella quadrangula} lattice. Neutron scattering reveals antiferromagnetic ordering, but a 14 T magnetic field induces a ferromagnetic state as does substitution of 5\% Co on the Fe site [1]. We present the energetics of a transition between these states with density functional based fixed spin moment studies. Our (mean field) ground state occurs with nearly equal Fe1 and Fe2 moments of 1.8 $\mu_B$. As the total moment is reduced, a crossover occurs until at zero total moment the Fe1 moment is -1.1 $\mu_B$ (antialigned with the strong Fe2 moment). We use these results to construct scenarios for discussing the observations. \\[4pt] [1] T. Waki et al., J. Phys. Soc. Japan {\bf 79}, 043701 (2010). [Preview Abstract] |
Wednesday, March 23, 2011 3:06PM - 3:18PM |
T22.00004: Stability of Quantum Critical Points in the Presence of Competing Orders Jian-Huang She, Jan Zaanen, Alan Bishop, Alexander Balatsky We investigate the stability of Quantum Critical Points (QCPs) in the presence of two competing phases. These phases near QCPs are assumed to be either classical or quantum and assumed to repulsively interact via square- square interaction. We find that for any dynamical exponents and for any dimensionality strong enough interaction renders QCPs unstable, and drive transitions to become first order. We propose that this instability and the onset of first order transition leads to spatially inhomogeneous states in practical materials near putative QCPs. [Preview Abstract] |
Wednesday, March 23, 2011 3:18PM - 3:30PM |
T22.00005: Field-Induced Orbital Antiferromagnetism in Mott Insulators K.A. Al-Hassanieh, C.D. Batista, G. Ortiz, L.N. Bulaevskii We report on a new electromagnetic phenomenon that emerges in Mott insulators. The phenomenon manifests as antiferromagnetic ordering due to orbital electric currents which are spontaneously generated from the coupling between spin currents and an external homogenous magnetic field. This novel spin-charge-current effect provides the mechanism to measure the so-far elusive spin currents by means of unpolarized neutron scattering, nuclear magnetic resonance or muon spectroscopy. We illustrate this mechanism by solving a half-filled Hubbard model on a frustrated ladder. [Preview Abstract] |
Wednesday, March 23, 2011 3:30PM - 3:42PM |
T22.00006: Global phase diagram of heavy fermions and the Kondo destroyed quantum critical points of Anderson models with a transverse field Jedediah Pixley, Stefan Kirchner, Qimiao Si Recent studies in quantum critical heavy fermion metals have pointed towards a global phase diagram [1]. The zero-temperature phase diagram involves a combination of phases, featuring Kondo screening/breakdown and antiferromagnetic order/disorder as the quantum fluctuations of the local moments are tuned relative to their effective interaction with the spins of the conduction electrons. In the case of Ising-anisotropic Kondo lattice systems, the fluctuations among the local moments can be generated by coupling them to a transverse magnetic field. With these effects in mind, we study the Kondo-destroyed quantum critical behavior of the Anderson impurity model in the presence of a bosonic bath or a transverse field. We extend our recent studies of the low-temperature quantum critical behavior [2,3] based on the continuous time quantum Monte Carlo, and obtain the dynamical scaling functions of the local spin susceptibility and single-electron Green's function. \\[4pt] [1] Q. Si and F. Steglich, Science 329, 1161 (2010). \\[0pt] [2] M. T. Glossop, S. Kirchner, J. H. Pixley and Q. Si, arXiv:0912.4521 to be published (2009). \\[0pt] [3] J. H. Pixley, S. Kirchner and Q.Si, arXiv:1010.3024 to be published (2010). [Preview Abstract] |
Wednesday, March 23, 2011 3:42PM - 3:54PM |
T22.00007: Electron paring instabilities in 8-site Betts lattice: exact result Kun Fang, Gayanath Fernando, Armen Kocharian We use numerical methods (exact diagonalization and Lanczos method) to study single-orbital and multi-orbital Hubbard models (off half filling). The whole lattice is divided into identical 8-site square clusters immersed in a thermal bath. The electron pairing instabilities, order parameters and quantum critical points are evaluated by monitoring the charge and spin gaps in a wide range of parameters including the on-site interaction $U$. Calculations show level crossing behaviors at zero and finite temperature. The corresponding pairing instabilities are remarkably similar to electronic inhomogeneities observed in correlated systems such as the high temperature superconductors and Fe pnictides. The next nearest hopping is also introduced. We find that it can shift quantum crossover point and gap magnitude, but for reasonable hopping amplitudes, it will not eliminate characteristics of electron paring instabilities. [Preview Abstract] |
Wednesday, March 23, 2011 3:54PM - 4:06PM |
T22.00008: Functional RG for the Single Impurity Anderson Model Michael Kinza, Carsten Honerkamp, Jutta Ortloff We present a functional Renormalization Group (fRG) approach to the Single Impurity Anderson Model at finite temperatures. Starting with the exact spectral function and interaction vertex of a small system (``core'') containing a correlated site, we switch on the hybridization with a non-interacting bath in the RG-flow and calculate spectra of the correlated site. Different truncations of the RG-flow-equations and choices of the core are compared and discussed. Furthermore we calculate the linear conductance as function of temperature and interaction strength. [Preview Abstract] |
Wednesday, March 23, 2011 4:06PM - 4:18PM |
T22.00009: Mesoscopic Anderson Box: Connecting Weak to Strong Coupling Dong E. Liu, Sebastien Burdin, Harold U. Baranger, Denis Ullmo Both the weakly coupled and strong coupling Anderson impurity problem are characterized by a Fermi-liquid theory with weakly interacting quasiparticles. In an Anderson box, mesoscopic fluctuations of the effective single particle properties will be large. We study how the statistical fluctuations in these two problems are connected. We use random matrix theory and the slave boson mean field approximation (SBMF, at low temperature) to address this question, obtaining the following results. First, for a resonant level model such as results from the SBMF approximation, we find the joint distribution of energy levels with and without the resonant level present. Second, if only energy levels within the Kondo resonance are considered, the distribution of perturbed levels collapse to one universal form for both GOE and GUE for all values of the coupling V. Finally, a purely Fermi liquid method is developed for calculating the perturbed levels within the Kondo resonance. Comparing the levels that result to those of the SBMF, we find remarkable agreement. [Preview Abstract] |
Wednesday, March 23, 2011 4:18PM - 4:30PM |
T22.00010: Superconducting pairing of interacting electrons: implications from the two-impurity Anderson model Lijun Zhu, Jian-Xin Zhu We study the non-local superconducting pairing of two interacting Anderson impurities, which has an instability near the quantum critical point from the competition between the Kondo effect and an antiferromagnetic inter-impurity spin exchange interaction. As revealed by the dynamics over the whole energy range, the superconducting pairing fluctuations acquire considerable strength from an energy scale much higher than the characteristic spin fluctuation scale while the low energy behaviors follow those of the staggered spin susceptibility. We argue that the superconducting pairing might not need the spin fluctuations as the glue, but rather originated from the effective Coulomb interaction. On the other hand, critical spin fluctuations in the vicinity of quantum criticality are also crucial to a superconducting pairing instability, by preventing a Fermi liquid fixed point being reached to keep the superconducting pairing fluctuations finite at low energies. A superconducting order, to reduce the accumulated entropy carried by the critical degrees of freedom, may arise favorably from this instability. [Preview Abstract] |
Wednesday, March 23, 2011 4:30PM - 4:42PM |
T22.00011: Pressure Tuning of the Shastry-Sutherland Quantum Phase Transition S. Haravifard, A. Banerjee, T.F. Rosenbaum, G. Srajer, J.C. Lang, Y. Feng, B.D. Gaulin, H.A. Dabkowska SrCu2(BO3)2 is a quasi-2D quantum spin system known to possess a collective singlet ground state. It serves as an experimental realization of the Shastry-Sutherland model for interacting S=1/2 dimers. The ratio of the intra and inter-dimer exchange in this compound is close to a quantum critical point, where the ground state transforms from a gapped, non-magnetic state to a gapless long-range ordered antiferromagnetic state as a function of the ratio of the strength of the magnetic interactions. We use synchrotron x-ray diffraction in a diamond anvil cell to investigate the pressure-driven quantum phase transition in high-quality single crystals of SrCu2(BO3). We will present the evolution of both the magnetic and structural properties up to pressures of 5 GPa. [Preview Abstract] |
Wednesday, March 23, 2011 4:42PM - 4:54PM |
T22.00012: Quantum phase transitions in generalized J-Q models Arnab Sen, Anders Sandvik The ``J-Q'' model is an extension of the Heisenberg model which contains multi-spin interactions that suppress N\'eel order and lead to a valence-bond-solid (VBS) ground state. It is free from quantum Monte Carlo (QMC) sign problems. There is now good evidence from QMC studies for a continuous N\'eel--VBS transition with non-trivial features like a large anomalous exponent $\eta$ and an emergent U(1) VBS symmetry at the quantum-critical point in this model. We study various generalizations of the J-Q model, with both SU(2) and U(1) symmetric interactions, to further elucidate unusual aspects of the N\'eel-VBS transition. In the SU(2) case, we construct a model which stabilizes a staggered VBS instead of the columnar pattern obtained in previous studies. This type of VBS does not harbor an emergent U(1) symmetry near the transition. We find that the transition is strongly first-order, unlike in the original J-Q model. This illustrates the importance of the emergent U(1) symmetry for the possibly exotic transition in the standard J-Q model. We also investigate a new U(1)-symmetric generalization of the J-Q model to explore such unconventional transitions in the easy plane case. [Preview Abstract] |
Wednesday, March 23, 2011 4:54PM - 5:06PM |
T22.00013: Criticality of compact and noncompact $(1+1)D$ quantum dissipative $Z_4$-models Einar Stiansen, Iver Sperstad, Asle Sudbo We study two versions of a $(1+1)D$ $Z_4$-symmetric model with Ohmic bond dissipation. In one version the phase variable is restricted to the interval $[0,2\pi\rangle$, while the domain is unrestricted in the other. The compact model features a completely ordered phase with a broken $Z_4$-symmetry and a disordered phase, separated by a critical line. The non-compact model features three phases. In addition to the two phases exhibited by the compact model, there is also an intermediate phase, characterized by isotropic power-law phase correlations. We calculate the dynamical critical exponent $z$ along the critical lines of both models to see if the compactness of the variable is relevant to the critical scaling between space and imaginary time. We find $z\approx1$ for the single phase transition in the compact model as well as for both transitions in the non- compact model. [Preview Abstract] |
Wednesday, March 23, 2011 5:06PM - 5:18PM |
T22.00014: Kondo and spin-Peierls phases and Berry phase effects in Heisenberg-Kondo chain Pallab Goswami, Qimiao Si Recent theoretical and experimental results on heavy fermion systems have motivated a global phase diagram, as a function of the Kondo coupling and the strength of quantum fluctuations of the local moments. Correspondingly, there has been growing interest in understanding the phase transition from a small Fermi surface antiferromagnet to large or small Fermi surface paramagnets with or without Kondo screening respectively. Because a perturbative nonlinear sigma model analysis only accesses the small Fermi surface antiferromagnetic phase, the transition into the paramagnetic phases must involve non-perturbative effects. We consider here the effect of the instanton configurations of the nonlinear sigma model and the associated Berry's phase for the Kondo singlet formation, and for concreteness focus on the one dimensional Heisenberg Kondo lattice model. Using semiclassical nonlinear sigma model and bosonization techniques both at and away from half-filling, we demonstrate how the competition between the Kondo singlet and spin Peierls phases are manifested through the effects of such a Berry phase. Based on these results we comment upon similar effects that may be realized in higher dimensional Kondo lattice models. [Preview Abstract] |
Wednesday, March 23, 2011 5:18PM - 5:30PM |
T22.00015: Complex Critical Exponents in Diluted Systems of Quantum Rotors Rafael Fernandes, J\"org Schmalian In this work, we investigate the effects of the Berry phase $2 \pi \rho$ on the critical properties of $XY$ quantum-rotors that undergo a percolation transition. This model describes a variety of randomly-diluted quantum systems, such as interacting bosons coupled to a particle reservoir, quantum planar antiferromagnets under a perpendicular magnetic field, and Josephson-junction arrays with an external bias-voltage. Focusing on the quantum critical point at the percolation threshold, we find that, for rational $\rho$, one recovers the power-law behavior with the same critical exponents as in the case with no Berry phase. However, for irrational $\rho$, the low-energy excitations change completely and are given by emergent spinless fermions with fractal spectrum. As a result, critical properties that cannot be described by the usual Ginzburg-Landau-Wilson theory of phase transitions emerge, such as complex critical exponents, log-periodic oscillations, and dynamically-broken scale invariance. [Preview Abstract] |
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