Bulletin of the American Physical Society
2009 APS March Meeting
Volume 54, Number 1
Monday–Friday, March 16–20, 2009; Pittsburgh, Pennsylvania
Session Q41: Strongly Correlated Electron Theory |
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Sponsoring Units: DCMP DMP Chair: Andrey Chubukov, University of Wisconsin Room: 413 |
Wednesday, March 18, 2009 11:15AM - 11:27AM |
Q41.00001: Quantum dots: a new tool for studying quantum phase transitions (QPT) N. Roch, S. Florens, V. Bouchiat, W. Wernsdorfer, F. Balestro QPT were studied in many different systems: spin chains, strongly correlated materials, high Tc superconductors, etc. but all the properties (magnetism, superconductivity ...) of these materials can be difficult to control. On the other hand, thanks to microelectronic technologies, it is now possible to obtain taylor-made quantum dots in which all the interactions can be tuned finely. It was then proposed by several theoretic papers [1] to use them as model systems for probing QPT. In this experimental work, we observed a screening/non screening QPT transition in a single-molecule transistor. We will present a full study as a function of magnetic field, bias voltage and temperature [2].\\[3pt] [1] M.Vojta, Philosophical Magazine,86:13,1807 - 1846 (2006)\\[0pt] [2] N.Roch et al. , Nature 453, 633-637 (2008) [Preview Abstract] |
Wednesday, March 18, 2009 11:27AM - 11:39AM |
Q41.00002: Signs of quantum Griffiths effects in the weak itinerant ferromagnet Ni-V Almut Schroeder, Sara Ubaid-Kassis ?Magnetization and ac-susceptibility data of $Ni_{1- x}V_{x}$ alloys are presented in the vicinity of the critical concentration, $x_c \approx 11\%$, where the ferromagnetic transition temperature is expected to vanish. For $x > x_c$ power laws with unusual exponents are observed, which change with further dilution. [Preview Abstract] |
Wednesday, March 18, 2009 11:39AM - 11:51AM |
Q41.00003: Effects of retardation on a system of polarized fermions Ling Yang, Filippos Klironomos, Shan-Wen Tsai When fermion-fermion interactions are frequency dependent, retardation effects may play an important role in determining the phase diagram and critical energy scales of the system. These effects are particularly significant when there is competition between two or more instabilities of the Fermi liquid state. In order to elucidate these effects, we study a simple model of spinless fermions on a square lattice. We employ a functional renormalization-group method to obtain the flows of vertices and correlation functions of this system. The phase diagram, critical scales and sub-dominant types of order can be obtained this way for various types of interactions. We discuss an analytical understanding for simple cases, and possible ways to observe these effects in fermion-boson cold-atom mixtures on optical lattices. [Preview Abstract] |
Wednesday, March 18, 2009 11:51AM - 12:03PM |
Q41.00004: Phase diagram, correlation gap, and critical properties of the Coulomb glass Matteo Palassini, Martin Goethe We investigate the lattice Coulomb glass model in three dimensions via extensive Monte Carlo simulations. 1. No evidence for an equilibrium glass phase is found down to very low temperatures, contrary to mean-field predictions, although the correlation length increases rapidly near $T=0$. 2. The single-particle density of states near the Coulomb gap satisfies the scaling law $g(e,T)=T^\lambda f(e/T)$ with $\lambda\approx 2.2$. 3. A charge-ordered phase exists at low disorder. The phase transition from the fluid to the charge ordered phase is consistent with the Random Field Ising universality class, which shows that the interaction is effectively screened at moderate temperature. Results from nonequilibrium simulations will also be briefly discussed. Reference: M.Goethe and M.Palassini, arXiv:0810.1047 [Preview Abstract] |
Wednesday, March 18, 2009 12:03PM - 12:15PM |
Q41.00005: Fingerprints of intrinsic phase separation in magnetically-doped 2DEG Hanna Terletska, Vladimir Dobrosavljevic We theoretically study the properties of a recently observed [1] inhomogeneous phase preceding the metal-insulator transition in a magnetically-doped two-dimensional electron gas (2DEG). We show that, due to competition between (ferromagnetic) double-exchange and (anti-ferromagnetic) super-exchange, at very low carrier density such a system is unstable toward intrinsic phase separation (PS). Here, ferromagnetic carrier-rich (metallic) ``droplets'' emerge within a magnetically disordered carrier-poor (insulating) matrix. Our calculations indicate that this regime should display very unusual transport, featuring colossal magneto-resistance with exceptionally weak density dependence - in striking agreement with experiments [1] on CdMnTe quantum wells. Such exotic transport properties - we argue - should be considered as ``fingerprints'' for intrinsic phase separation, a behavior very different from situations where phase coexistence is driven by disorder due to extrinsic impurities or defects. [1] J. JaroszyƱski \textit{et al}., Phys. Rev. B \textbf{76}, 045322 (2007). [Preview Abstract] |
Wednesday, March 18, 2009 12:15PM - 12:27PM |
Q41.00006: The Exponential Downfall of the Weak-Coupling SDW State in Chromium Yejun Feng, R. Jaramillo, T.F. Rosenbaum, J.C. Lang, Z. Islam, G. Srajer, P.B. Littlewood Elemental chromium is the archetypical model system for itinerant antiferromagnetism. The incommensurate spin density wave ground state, originating from the nested Fermi surface, is readily observable with direct scattering probes. Through a combination of low temperature cryogenic, diamond anvil cell, and synchrotron x-ray diffraction techniques, we measure directly the spin and charge order in the pure metal as it is driven towards its quantum critical point under pressure. We observe that both the spin and charge order are suppressed exponentially with pressure, well beyond the region where disorder cuts off such a simple evolution, and they maintain a harmonic scaling relationship over decades in scattering intensity. The observed exponential behavior of the order parameter follows a weak-coupling BCS theory for the ground state, even in the presence of strong pairing correlations that survive to surprisingly high temperatures and energies, as observed by inelastic scattering, transport, and thermal expansion measurements. This duality points to the fundamental issue of how mean-field behavior can describe so successfully important aspects of strongly coupled electron systems. [Preview Abstract] |
Wednesday, March 18, 2009 12:27PM - 12:39PM |
Q41.00007: Nonanalytic spin susceptibility of interacting fermions away and near a ferromagnetic quantum phase transition Dmitrii Maslov, Andrey Chubukov We study nonanalytic paramagnetic response of an interacting Fermi system both away and in the vicinity of a ferromagnetic quantum phase transition (QCP). Previous studies found that the spin susceptibility $\chi$ scales linearly with either the temperature $T$ or magnetic field $H$ in the weak-coupling regime and that the interaction in the Cooper channel affects this scaling via logarithmic renormalization of prefactors of the $T$, $|H|$ term. We show that Cooper renormalization becomes effective only at very low energies, which get even smaller near a QCP. We derive the thermodynamic potential as a function of magnetization and show that it contains, in addition to regular terms, a non-analytic $|M|^3$ term, which becomes $M^4/T$ at finite $T$. We consider the vicinity of a ferromagnetic QCP by generalizing the Eliashberg treatment of the spin-fermion model to finite magnetic field, and show that the $|M|^3$ term crosses over to a non-Fermi-liquid form $|M|^{7/2}$ near a QCP. The prefactor of the $|M|^{7/2}$ term is negative, which indicates that the system undergoes a first-order rather than a continuous transition to ferromagnetism. We compare two scenarios of the breakdown of a continuous QCP: a first-order instability and a spiral phase and show that in a model with a long-range interaction in the spin channel first-order transition occurs before the spiral instability. [Preview Abstract] |
Wednesday, March 18, 2009 12:39PM - 12:51PM |
Q41.00008: Fermionic propagators for 2D systems with singular interactions Tigran Sedrakyan, Andrey Chubukov We analyze the form of the fermionic propagator for 2D fermions interacting with massless overdamped bosons. Examples include a nematic and Ising ferromagnetic quantum-critical points, and fermions at a half-filled Landau level. Fermi liquid behavior in these systems is broken at criticality by a singular self-energy, but the Fermi surface remains well defined. These are strong- coupling problems with no expansion parameter other than the number of fermionic species, $N$. The two known limits, $N >>1$ and $N=0$ show qualitatively different behavior of the fermionic propagator $G(\epsilon_k, \omega)$. In the first limit, $G(\epsilon_k, \omega)$ has a pole at some $\epsilon_k$, in the other it is analytic. We analyze the crossover between the two limits. We show that the pole survives for all $N$, with residue $Z = O(1)$, however at small $N$ it only exists in a range $O(N^2)$. At $N=0$, the range collapses and the behavior of $G (\epsilon_k, \omega)$ becomes analytic. [Preview Abstract] |
Wednesday, March 18, 2009 12:51PM - 1:03PM |
Q41.00009: Non-Fermi-liquid behavior of quantum magnetooscillations near a quantum critical point Chungwei Wang, Dmitrii Maslov We study many-body effects in quantum magnetooscillations of a 2D strongly correlated system near ferromagnetic and antiferromagnetic quantum critical points (QCP). The amplitude of magnetooscillations is determined by the electron self-energy $\Sigma(\pi T, \mathbf{k};T)$ averaged over the Fermi surface, at the Matsubara frequency $\omega=\pi T$ for $T>\omega_c$, where $\omega_c$ is the cyclotron frequency. The major contribution of the bosonic propagator to the electron self-energy comes from static spin fluctuations. In the spin-fermion model, the self-energy behaves as $\Sigma \propto \sqrt{T}$ in the ferromagnetic system and as $\Sigma \propto T\ln{T}$ in the antiferromagnetic system. This shows the non-Fermi-liquid temperature dependence of the self-energy near the QCP and the oscillation amplitude $A\propto \exp{\left[-2\pi (\pi T + \Sigma)/\omega_c\right]}$ can be very different from the Lifshitz-Kosevich form. The momentum dependence of the self-energy contribution to the oscillation amplitude is also discussed. [Preview Abstract] |
Wednesday, March 18, 2009 1:03PM - 1:15PM |
Q41.00010: Mechanism of multifractal spectrum termination at the Anderson metal-insulator transition Matthew Foster, Shinsei Ryu, Andreas Ludwig We revisit the problem of wavefunction statistics at the Anderson metal-insulator transition (MIT) of non-interacting electrons in $d > 2$ spatial dimensions. At the transition, the complex spatial structure of the critical wavefunctions is reflected in the non-linear behavior of the multifractal spectrum of generalized inverse participation ratios (IPRs). For sufficiently large moments of the wavefunction intensity, the spectrum obtained from a \textit{typical} wavefunction associated to a particular disorder realization differs markedly from that obtained from the \textit{disorder-averaged} IPRs---the phenomenon known as the termination of the multifractal spectrum. We provide a derivation for the termination of the typical multifractal spectrum, by fusing the non-linear sigma model framework, conventionally used to access the MIT in $d = 2 + \epsilon$ dimensions, with a functional renormalization group (FRG) methodology. The FRG was previously used to demonstrate the termination of the multifractal spectrum in a very special model of 2D Dirac fermions, subject to a particular type of quenched disorder. [D.\ Carpentier and P.\ Le Doussal, Phys.Rev.\ E \textbf{63}, 026110 (2001)]. Our result shows that the FRG framework can be generalized to the much broader context of the delocalization transition of ordinary electrons in higher dimensions. [Preview Abstract] |
Wednesday, March 18, 2009 1:15PM - 1:27PM |
Q41.00011: Effects of Strong Correlations on the Disorder-Induced Zero Bias Anomaly William Atkinson, Yun Song, Sinan Bulut, Rachel Wortis In conventional metals and semiconductors, density of states anomalies result from the interplay between disorder and interactions. Motivated by a number of experiments that find zero bias anomalies (ZBA) in transition metal oxides, we have performed calculations to determine the effect of strong correlations on the ZBA in disordered interacting systems. We use a self-consistent mean-field theory that incorporates strong correlations and treats spatial fluctuations of the disorder potential exactly. We discuss both the Anderson-Hubbard model and the extended Anderson-Hubbard model. We find that, even for a zero-range interaction, nonlocal self-energy corrections lead to the formation of an Altshuler-Aronov-like ZBA. In the extended Anderson-Hubbard model, Efros-Shklovskii-like physics dominates at large disorder. [Preview Abstract] |
Wednesday, March 18, 2009 1:27PM - 1:39PM |
Q41.00012: Detrended Fluctuation Analysis for Dynamics in an Electron Glass Stephen Arnason As a result of the correlations between electrons, electron glasses show enhanced fluctuations in coductance with a $1/f^{\alpha}$ frequency dependence. We are interested in looking at the time dependence of the fluctuation spectra as the system relaxes towards equilibrium after a discontinuous change in chemical potential. Our measurements are taken on FET structures where the conductance channel is fabricated from amorphous Indium Oxide with stoichiometry close to the superconductor to insulator transition. Changing the potential on the gate electrode allows us to change the chemical potential and we measure the resistance of the conductance channel as a function of time. In addition to the fluctuations there is a slow, logarithmic relaxation of the channel conductance. Because of this slow relaxation it is hard to characterize our signal as stationary, calling into question the application of Fourier transform based analysis techniques. One approach to coping with this problem is the subtraction of the slowly varying background before the calculation of the Fourier transforms, so called detrended fluctuation analysis. This paper presents results on simulations of this technique as applied to computer generated signals with characteristics similar to our actual data. The frequency dependence of the fluctuation spectra is imperfectly preserved but can be similar to the actual fluctuation spectra within certain bounds of analysis parameters. [Preview Abstract] |
Wednesday, March 18, 2009 1:39PM - 1:51PM |
Q41.00013: Transport in a Dissipative Luttinger Liquid Zoran Ristivojevic, Thomas Nattermann We study theoretically the transport through a single impurity in a one-channel Luttinger liquid coupled to a dissipative Ohmic bath. For nonzero dissipation, the single impurity is always a relevant perturbation which suppresses transport strongly. At zero temperature, the current voltage relation of the link is $I\sim \exp(-E_0/eV)$, where $E_0\sim\eta/\kappa$ and $\kappa$ denotes the compressibility and $\eta$ the dissipation strength. At nonzero temperature $T$, the linear conductance is proportional to $\exp(-\sqrt{{\mathcal{C}}E_0/k_B T})$. The decay of Friedel oscillation saturates for at distances larger than $L_{\eta}\sim 1/\eta$ from the impurity.\newline [1] Z. Ristivojevic and T. Nattermann, Phys. Rev. Lett. \textbf{101}, 016405 (2008). [Preview Abstract] |
Wednesday, March 18, 2009 1:51PM - 2:03PM |
Q41.00014: Mapping the Driven Interacting Resonant Level Model to an Equilibrium Problem Eduardo Novais, M.R. Plesser, Harold U. Baranger We map the driven Interacting Resonant Level Model (IRLM) to an equivalent statistical mechanical problem. Correlation functions in the nonequilibrium model are given, to all orders in perturbation theory, by thermal averages in the statistical system. This enables us to apply the traditional theoretical techniques for thermal problems, such as the renormalization group and diagrammatic expansions, to a far from equilibrium problem. Using these tools, we study the current as a function of bias, as well as of the interactions in the leads and the resonant level. As a simple example of a strongly interacting system far from equilibrium, the IRLM has played an important role in motivating and evaluating recent theoretical advances. We compare our new strategy to other recent proposals for studying far from equilibrium interacting systems. [Preview Abstract] |
Wednesday, March 18, 2009 2:03PM - 2:15PM |
Q41.00015: Numerical study of relaxation dynamics in photoexcited states of one-dimensional Mott insulators Hiroaki Matsueda, Takami Tohyama, Sadamichi Maekawa We examine relaxation dynamics of one-dimensional Mott insulators after photoirradiation. This study is motivated by ultrafast metal-insulator switching seen in Sr2CuO3, halogen-bridged Ni compounds, and organic materials. In order to examine energy dissipation due to relaxation processes, we take account of strongly correlated electrons as well as environmental degrees of freedom by introducing the Hubbard--Holstein model. Then, we perform density matrix renormalization group calculations. We find quite large number of phonons excited just after irradiation even for very small electron-phonon coupling. We will discuss the nature of the phonon relaxation characteristic of strongly correlated systems, and how the relaxation is associated with other internal degrees of freedom of correlated electrons. [Preview Abstract] |
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