Bulletin of the American Physical Society
2007 APS March Meeting
Volume 52, Number 1
Monday–Friday, March 5–9, 2007; Denver, Colorado
Session V10: Critical and Nonequilibrium Phenomena 
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Sponsoring Units: DCMP Chair: Thomas Vojta, University of Missouri  Rolla Room: Colorado Convention Center Korbel 1E 
Thursday, March 8, 2007 11:15AM  11:27AM 
V10.00001: Scaling Functions and Conformal Invariance at the Quantum Critical Point of the SubOhmic BoseFermi Kondo Model Stefan Kirchner, Qimiao Si In one approach to the quantum critical heavy fermion metals, Kondo lattice systems are studied through a selfconsistent BoseFermi Kondo Model (BFKM) within the extended dynamical mean field theory. It has become clear in recent years that the critical behavior of the BFKMs is not captured by the classical critical theory obtained through the standard ``quantumtoclassical mapping'' of the model. In this work, we study the finite temperature scaling functions of the easyaxis BFKM using a clusterupdating Monte Carlo approach, and their counterparts of a spinisotropic BFKM in a dynamical largeN limit. The scaling functions are found to have the form dictated by a boundary conformal field theory even though the underlying Hamiltonian lacks conformal invariance. In the largeN limit, this is established for all multiplespin correlation functions as well. The results raise the possibility that the quantum critical point of the BFKM has an enhanced symmetry, which should be helpful to the understanding of the underlying critical field theory. [Preview Abstract] 
Thursday, March 8, 2007 11:27AM  11:39AM 
V10.00002: Zero Bias Anomaly Out of Thermal Equlibrium Dmitri Gutman, Yuval Gefen, Alexander Mirlin We consider the outofequilibrium tunneling densityofstates for a twodimensional diffusive film. Starting from a Keldysh $sigma$model formalism we have obtained an effective action capable of accounting for both real and virtual processes. The ensuing zero bias anomaly, obtained nonperturbatively in the interaction, exhibits a twodip structure, whose singularity is rounded off by the electronelectron inelastic rate. [Preview Abstract] 

V10.00003: ABSTRACT WITHDRAWN 
Thursday, March 8, 2007 11:51AM  12:03PM 
V10.00004: Numerical renormalization group study of a dissipative quantum dot M.T. Glossop, K. Ingersent We study the quantum phase transition (QPT) induced by dissipation in a quantum dot device at the degeneracy point. We employ a BoseFermi numerical renormalization group approach [1] to study the simplest case of a spinless resonantlevel model that couples the charge density on the dot to a dissipative bosonic bath with density of states $B(\omega)\propto \omega^s$. In anticipation of future experiments [2] and to assess further the validity of theoretical techniques in this rapidly developing area, we take the conductionelectron leads to have a pseudogap density of states: $\rho(\omega) \propto \omega^r$, as considered in a very recent perturbative renormalization group study [3]. We establish the conditions on $r$ and $s$ such that a QPT arises with increasing dissipation strength  from a delocalized phase, where resonant tunneling leads to large charge fluctuations on the dot, to a localized phase where such fluctuations are frozen. We present results for the singleparticle spectrum and the response of the system to a local electric field, extracting critical exponents that depend in general on $r$ and $s$ and obey hyperscaling relations. We make full comparison with results of [3] where appropriate. Supported by NSF Grant DMR0312939. [1] M. T. Glossop and K. Ingersent, PRL {\bf 95}, 067202 (2005); PRB (2006). [2] L. G. G. V. Dias da Silva, N. P. Sandler, K. Ingersent, and S. E. Ulloa, PRL {\bf 97}, 096603 (2006). [3] C.H. Chung, M. Kir\'{c}an, L. Fritz, and M. Vojta (2006). [Preview Abstract] 
Thursday, March 8, 2007 12:03PM  12:15PM 
V10.00005: Anomalous Conductance Quantization in SideGated InAs Quantum Point Contacts Saydur Rahman, Steven Herbert, Richard Newrock, Philippe Debray Conductance measurements as function of Fermi energy of InAs quantum point contacts created by side gating on InAs/InAlAs quantumwell structure exhibit a distinct plateau at e$^{2}$/h and a less distinct one at 2e$^{2}$/h. Applied magnetic field has little or no influence on the conductance quantization pattern. We believe the conductance plateau at e$^{2}$/h indicates a spin contribution of h/e$^{2}$ to the resistance of the onedimensional channel when the electron density is low and the electronelectron interaction is strong. This can be understood in the framework of the recently proposed spinincoherent Luttinger liquid (SILL) state that is considered to exist when \textit{J$<<$T$<<$E}$_{F}$, where $J$ is the spin exchange constant and E$_{F}$ the Fermi energy. In the SILL regime, the spin excitations are reflected back to the leads reducing the conductance of the quantum wire to e$^{2}$/h. At higher Fermi energy, when the electron density is higher, the spin contribution to resistance vanishes and the conductance takes the wellknown quantized value of 2e$^{2}$/h. [Preview Abstract] 
Thursday, March 8, 2007 12:15PM  12:27PM 
V10.00006: Phaseordering dynamics in itinerant quantum ferromagnets R. Saha, D. Belitz, T.R. Kirkpatrick Phase ordering following a rapid quench from the disordered phase to the ordered phase occurs via growth of domains that arise from spontaneous fluctuations. The linear size $L$ of these domains grow as a power law function of time for late times: $L(t) \propto t^{1/z}$, with z a dynamical exponent[1]. Until now this description of phase ordering dynamics has been applied to classical systems only. We apply this theory to describe domain growth in both clean and dirty itinerant quantum ferromagnets. The fluctuation effects that invalidate Hertz's theory of the quantum phase transition[2] also affect the phase ordering. For a quench into the ordered phase a transient regime appears, where the dynamical exponent differs from the classical case, and for asymptotically long times the prefactor of the growth law has an anomalous magnetization dependence[3]. A quench to the quantum critical point results in a growth law which is not a powerlaw function of time.\newline [1] A.J. Bray, Adv. in Phys. {\bf 43}, 357 (1994). [2] D. Belitz, T.R. Kirkpatrick, and T. Vojta, Rev. Mod. Phys. {\bf 77}, 579 (2005). [3] D. Belitz, T. R. Kirkpatrick, and Ronojoy Saha, condmat/0610650. [Preview Abstract] 
Thursday, March 8, 2007 12:27PM  12:39PM 
V10.00007: Universal nonlinear conductivity near to an itinerantelectron quantum critical point Patrick Hogan, Andrew Green Quantum critical systems display universal, powerlaw temperature dependence in their response functions. These universal powerlaws provide an experimental window upon quantum criticality. Concentrating upon conductivity in itinerantelectron systems near to a magnetic quantum critical point, we show that universal powerlaw dependence upon temperature is reflected in a universal nonlinear conductivity; when a strong electric field is applied, the resulting current has a universal powerlaw dependence upon the applied electric field. For a system with thermal equilibrium resistivity proportional to $T^\alpha$, we find a nonlinear resistivity proportional to $E^{\alpha/(1+\alpha)}$. This provides a new experimental handle upon the physics of itinerantelectron quantum critical points. [Preview Abstract] 
Thursday, March 8, 2007 12:39PM  12:51PM 
V10.00008: Quantum critical behavior in itinerant electron systems: Eliashberg theory and instability of a ferromagnetic quantumcritical point Jerome Rech, Catherine Pepin, Andrey Chubukov We consider the problem of fermions interacting with gapless longwavelength collective bosonic modes. The theory describes, among other cases, a ferromagnetic quantumcritical point (QCP). We construct a controllable expansion at the QCP in two steps: we first create a new, non Fermiliquid ``zeroorder'' Eliashbergtype theory, and then demonstrate that the residual interaction effects are small, provided we meet two conditions on the parameters of the system. For an $SU(2)$ symmetric ferromagnetic QCP, we find that the Eliashberg theory itself includes a set of singular renormalizations which can be understood as a consequence of an effective longrange dynamic interaction between quasiparticles, generated by the Landau damping term. These singular renormalizations give rise to a negative nonanalytic $q^{3/2}$ correction to the static spin susceptibility, and destroy a ferromagnetic QCP. We demonstrate that this effect can be understood in the framework of the $\phi^4$ theory of quantumcriticality, and show that it is specific to the $SU(2)$ symmetric case. [Preview Abstract] 
Thursday, March 8, 2007 12:51PM  1:03PM 
V10.00009: Verification of universality in strong critical adsorption with three techniques. Matthew Brown, Bruce Law, Lyle Marschand, Laurence Lurio, Serif Uran Fischer and de Gennes [C. R. Acad. Sci., Ser. B 287, 207 (1978)] suggested that for a mixture which was critical with respect to the demixing phase transition an interfacial adsorption profile would exhibit universal (system independent) scaling behavior. Carpenter et al. [Phys. Rev. E 61, 532 (2000) and references therein] verified this theory with a model which described ellipsometry data previously taken from several critical binary liquid mixtures. Until recently this model had not been verified with other experimental techniques. We describe a successful effort to verify Carpenter's model with data from a neutron reflectometry study of a critical D2O + 3methylpyridine mixture, as well as an analysis of data from both ellipsometry and xray reflectometry from a critical mixture of dodecane + tetrabromoethane. [Preview Abstract] 
Thursday, March 8, 2007 1:03PM  1:15PM 
V10.00010: Critical Behavior Near the Ferromagnetic Transition in Nanostructured Gadolinium Paul Shand, Justin Bohnet, Jared Goertzen, Jeffrey Shield, Geoffrey Shelburne, David Schmitter, Diandra LesliePelecky ac susceptibility and dc magnetization have been measured near the ferromagnetic transition in meltspun nanostructured Gd. Effective critical exponents and the critical temperature were extracted using modified Arrott plots. The values obtained were \textit{$\beta $}$_{eff}$ = 0.415 $\pm $ 0.005, \textit{$\gamma $}$_{ eff}$ = 1.36 $\pm $ 0.04, \textit{$\delta $}$_{ eff}$ = 4.24 $\pm $ 0.02, and $T_{C}$ = 290.0 $\pm $ 0.1 K. These exponent values satisfy the scaling relation \textit{$\beta \delta $ }= (\textit{$\beta $}+\textit{$\gamma $}). The experimental exponent values are also close to those for a shortrange 3D Heisenberg ferromagnet; however, there is a systematic shift toward meanfield values. Such a shift has been previously seen in amorphous ferromagnets and is likely due to the presence of longerranged interactions, especially in Gdbased alloys. The critical exponents for nanostructured Gd are closer to the shortrange 3D Heisenberg exponents than are those for amorphous Gd$_{67}$Co$_{33}$ and Gd$_{80}$Au$_{20}$. This suggests that the dominant GdGd interactions are shorterranged in nanostructured Gd than in the amorphous alloys. [Preview Abstract] 
Thursday, March 8, 2007 1:15PM  1:27PM 
V10.00011: Spinorbit coupling triggered Mott insulator Sr$_{2}$IrO$_{4}$. Bum Joon Kim, Hosub Jin, Soon Jae Moon, JaeYoung Kim, Choon Shik Leem, Jaejun Yu, TaeWon Noh, Changyoung Kim, SeJung Oh, JaeHoon Park, Gang Cao Electronic structures of 5d transitionmetal oxide (TMO) Sr$_{2}$IrO$_{4 }$are investigated by angleresolved photoemission spectroscopy and densityfunctional electronic structure calculations. The insulating nature of this compound and its measured valence band symmetry are correctly accounted for by the calculation only when both spinorbit coupling and electronic correlation effects are included. It is shown that the spinorbit coupling plays a crucial role in stabilizing the Mottinsulating ground state, which is unexpected in a compound with extended 5d valence band. The spinorbit coupling leads to symmetrysplitoff bands near the Fermi level which are narrow enough to be gapped by the Coulomb repulsion of moderate strength. This results in a conduction band as narrow as $\sim $0.5 eV, defying its general character expected for 5$d$ TMO. Our finding marks an establishment of a new type of Mott insulator and suggests possibility of novel Mottderived phenomena in 5$d$ based materials. [Preview Abstract] 
Thursday, March 8, 2007 1:27PM  1:39PM 
V10.00012: Spinorbit coupling in quasiperiodic systems Carlos Wexler, Daijiro Tobe, Mahito Kohmoto Electrons in incommensurate systems (e.g., electrons in a lattice in presence of a perpendicular magnetic field) have a rich behavior exemplified by the beautiful Hofstadter butterfly, a selfsimilar spectrum which is a multifractal Cantor set. We analyze the effect of spinorbit coupling in this system which can be described by a generalization of Harper's equation. We find that the added term significantly modifies the scaling laws and appears to induce a delocalization of the insulator. [Preview Abstract] 
Thursday, March 8, 2007 1:39PM  1:51PM 
V10.00013: Influence of Boundary Conditions on the Critical Scaling of Finite Systems Benjamin VollmayrLee, Erik Luijten We investigate the influence of boundary conditions on the behavior of finite spin systems at criticality; in particular, a comparison is made between periodic boundary conditions and free boundary conditions. Via simulations, we find that the critical magnetization distribution of the shortrange Ising model with free boundary conditions differs significantly from the periodic case. We also consider spin systems with longrange interactions $V\sim 1/r^{(d+\sigma)}$ where for $\sigma \leq 2$ the upper critical dimension is $d_c=2\sigma$, which allows for accurate simulations for $d>d_c$ systems. Using analytic results from Br\'ezin and ZinnJustin for periodic boundary conditions, and generalizing the techniques developed by Rudnick, Gaspari, and Privman for free boundary conditions, we obtain explicit expressions for the scaling of the susceptibility and the shape of the magnetization and energy distributions at criticality. The numerical data exhibit excellent agreement with our analytic results, providing in most cases the first explicit test for these predictions. [Preview Abstract] 
Thursday, March 8, 2007 1:51PM  2:03PM 
V10.00014: Measurement Noise Maximum as a Signature of a Phase Transition Zhi Chen, Clare Yu Noise is ubiquitous and is being increasingly used as an experimental tool to probe condensed matter systems, but unfortunately, when studying phase transitions, the usefulness of the experimental results is diminished by the fact that little is known about what to expect in the noise spectra. We propose that an increase in the measurement noise can be used to signal the onset of a phase transition since noise arises from the fluctuations of microscopic entities which, in turn, play a key role in phase transitions. As an example, we study the noise in the 2D Ising model which undergoes a second order phase transition, and in the 5state Potts model which undergoes a first order phase transition by using Monte Carlo simulations. We monitor these systems as the temperature drops below the critical temperature. At each temperature, after equilibration is established, we obtain the time series of quantities characterizing the properties of the system, i.e., the energy and magnetization per site for different size of systems. We apply different methods, such as the noise power spectrum and the second spectrum of the noise, to analyze the fluctuations in these quantities. We show that fluctuations produce an increase in the low frequency noise and the total noise power as first and second order phase transitions are approached. [Preview Abstract] 
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