Bulletin of the American Physical Society
2009 APS March Meeting
Volume 54, Number 1
Monday–Friday, March 16–20, 2009; Pittsburgh, Pennsylvania
Session W41: Kondo Physics; Dynamical Mean-Field Theory |
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Sponsoring Units: DCMP Chair: Chris Marianetti, Columbia University Room: 413 |
Thursday, March 19, 2009 11:15AM - 11:27AM |
W41.00001: The 2d Kondo effect in p-type Quantum Wells in GaAs Theodore Castner Two groups [Huang et al. [1] and Hamilton et al. [2]] have observed minima in the resistivity $\rho$(T) at very low T well below $\rho$$_{max}$ at T$_{max}$ in metallic samples (p$>$p$_{c}$). Minima in [1] were found at 32 mK for p$\sim$ 2.1 p$_{c}$, 25 mK for p$\sim$ 1.76 p$_{c}$, while no minimum was observed down to 0.5 mK for p$\sim$ 1.24 p$_{c}$. Both groups have interpreted their results in terms of a crossover to insulating (nonmetallic) behavior for T $<$ T$_{m}$ where d$\rho$/dT $<$ 0. An alternative explanation arises from a 2d Kondo effect. Using the Hamann function [3] for the magnetic scattering contribution from localized magnetic moments and a term $\rho$(0)CT/T$_{F}$ from screening (interactions) one obtains an expression for T$_{m}$(p) which is very close to the Kondo temperature T$_{K}$(p) given by (E$_{F}$/k)x exp(-1/N(E$_{F}$absJ). The very strong p-dependence of T$_{m}$(p) and T$_{K}$(p) is dominated by N(E$_{F}$) which is shown to approach zero as p approaches p$_{c}$ because of the pseudogap in the DOS. Good agreement is obtained T$_{m}$(p) for both [1] and [2]. The data in [1] supports metallic behavior down to 0.5 mK. The implications for a true 2d MIT as a QPT are discussed. 1) J. Huang et al. Phys.Rev.Lett.98, 226801 (2007); 2) A. R. Hamilton et al. Phys.Rev.Lett.82, 1542 (1999); D. R. Hamann, Phys.Rev.158, 570 (1967). [Preview Abstract] |
Thursday, March 19, 2009 11:27AM - 11:39AM |
W41.00002: Quantum phase transition in the XY-anisotropic Bose-Fermi Kondo model Mengxing Cheng, Kevin Ingersent Using the numerical renormalization group, we study the quantum phase transition induced by dissipation in the XY-anisotropic Bose-Fermi Kondo model for a spin-one-half magnetic impurity coupled both to the on-site spin of a conduction electron band and, via its x and y spin components, to a bath of vector bosons. We focus on the case of a sub-Ohmic bath characterized by a power-law spectral exponent $s < 1$. Upon increasing the coupling of the impurity to the bosonic bath (at fixed fermionic coupling), the system exhibits a continuous quantum phase transition from a Kondo-screened ground state to a state in which the impurity moment is localized by the dissipation. We probe the quantum-critical behavior in the vicinity of this transition through the calculation of critical exponents describing the static and dynamical response to a local magnetic field both at absolute zero and at finite temperatures. Critical comparisons are made with analytical renormalization-group results obtained previously through expansion around the Ohmic case $s = 1$. [Preview Abstract] |
Thursday, March 19, 2009 11:39AM - 11:51AM |
W41.00003: ABSTRACT WITHDRAWN |
Thursday, March 19, 2009 11:51AM - 12:03PM |
W41.00004: Validity of Equation-of-Motion Approach to Kondo Problem in the Large N limit Yunong Qi, Jian-Xin Zhu, Chin-Sen Ting The Anderson impurity model for Kondo problem is investigated for arbitrary orbit-spin degeneracy $N$ of the magnetic impurity by the equation of motion method (EOM). By employing a new decoupling scheme, a set self-consistent equations for the one-particle Green function are derived and numerically solved in the large-$N$ approximation. For the particle-hole symmetric Anderson model with finite Coulomb interaction $U$, we show that the Kondo resonance at the impurity site exists for all $N \geq 2$. The approach removes the pathology in the standard EOM for $N=2$, and has the same level of applicability as non-crossing approximation. For $N=2$, an exchange field splits the Kondo resonance into only two peaks, consist with the result from more rigorous numerical renormalization group (NRG) method. The temperature dependence of the Kondo resonance peak is also discussed. [Preview Abstract] |
Thursday, March 19, 2009 12:03PM - 12:15PM |
W41.00005: Evolution of a Large Fermi Surface in the Kondo Lattice Junya Otsuki, Hiroaki Kusunose, Yoshio Kuramoto We investigate the Kondo lattice model with use of the continuous-time quantum Monte Carlo method, combined with the dynamical mean-field theory. The antiferromagnetic phase diagram is determined from a divergence of the corresponding susceptibility [1]. In the paramagnetic phase, crossover behavior is traced quantitatively to a heavy Fermi-liquid state from the local-moment state at high temperatures [2]. The momentum distribution in the low-temperature limit acquires a discontinuity at the location that involves the local-spin degrees of freedom. Even without the charge degrees of freedom for local electrons, the excitation spectra exhibit hybridized bands similar to those in the Anderson lattice. Temperature dependence in the zero-energy component of the self-energy is crucial in forming the Fermi-liquid state with the large Fermi surface. [1] J. Otsuki, H. Kusunose and Y. Kuramoto, arXiv:0808.3829; arXiv:0811.1102. [2] J. Otsuki, H. Kusunose and Y. Kuramoto, arXiv:0811.2613 to appear in Phys. Rev. Lett. [Preview Abstract] |
Thursday, March 19, 2009 12:15PM - 12:27PM |
W41.00006: A linear in $\beta$ solver for Cluster Dynamical Mean Field Theory Ehsan Khatami, Mark Jarrell, Che-Rung Lee, Richard Scalettar We develop a Quantum Monte Carlo (QMC) cluster solver for the Dynamical Cluster Approximation (DCA) which scales linearly in the inverse temperature, $\beta$, and has the same minus sign problem as conventional methods. Determinantal QMC (DQMC) used in this method is modified by adding non-interacting bands to mimic the coupling to the host. The DCA hybridization function is fitted to the non-interacting band parameters. We prove that the sign problem has the same statistics as in the Hirsch-Fye (HF) algorithm in the limit of a large number of bath bands ($N_{\alpha}$). Whereas the HFQMC scales as $\beta^3$, this DQMC-based method scales linearly in $\beta$. We demonstrate rapid convergence of the sign to the HF result for different cluster sizes and model parameters as $N_{\alpha}$ increases. We also present results for the convergence of other physical quantities to their HFQMC counterparts. This method can be used to solve other embedded cluster problems including those in Dynamical Mean Field Theory (DMFT), and cellular DMFT. [Preview Abstract] |
Thursday, March 19, 2009 12:27PM - 12:39PM |
W41.00007: The relationship between Hirsch-Fye and weak coupling diagrammatic Quantum Monte Carlo methods Karlis Mikelsons, Alexandru Macridin, Mark Jarrell Two weak coupling Continuous Time Quantum Monte Carlo (CTQMC) methods are shown to be equivalent for Hubbard-like interactions. A relation between these CTQMC methods and the Hirsch-Fye Quantum Monte Carlo (HFQMC) method is established, identifying the latter as an approximation within CTQMC and providing a diagrammatic interpretation of HFQMC. Both HFQMC and CTQMC are shown to be equivalent when the number of time slices in HFQMC becomes infinite, implying the same degree of fermion sign problem in this limit. [Preview Abstract] |
Thursday, March 19, 2009 12:39PM - 12:51PM |
W41.00008: Cluster Dynamical Mean Field Theory of the Mott Transition Hyowon Park, Kristjan Haule, Gabriel Kotliar We address the nature of the Mott transition in the Hubbard model at half-filling using cluster dynamical mean field theory (DMFT). We compare cluster DMFT results with those of single site DMFT. We show that inclusion of the short range correlations on top of the on-site correlations does not change the order of the transition between the paramagnetic metal and the paramagnetic Mott insulator, which remains first order. However, the short range correlations reduce substantially the critical $U$ and modify the shape of the transition lines. Moreover, they lead to very different physical properties of the metallic and insulating phases near the transition point. Approaching the transition from the metallic side, we find an anomalous metallic state with very low coherence scale. The insulating state is characterized by the narrow Mott gap with pronounced peaks at the gap edge. [Preview Abstract] |
Thursday, March 19, 2009 12:51PM - 1:03PM |
W41.00009: Nearly frozen Coulomb liquids Yohanes Pramudya, Sergey Pankov, Efstratios Manousakis, Vladimir Dobrosavljevic We show that very long range repulsive interactions of a generalized Coulomb-like form \textbf{\textit{V(R) $\sim $ R}}$^{-a}$, with \textbf{\textit{a $<$ d}} (\textbf{\textit{d}}-spatial dimensionality), typically introduce very strong frustration, resulting in extreme fragility of any charge-ordered state. An ``almost frozen'' liquid then survives in a broad dynamical range above the (very low) melting temperature \textbf{\textit{T}}$_{c}$. Using a combination of extended dynamical mean-field theory [1] and Monte Carlo simulations we study classical lattice gas models with such long range interaction, focusing on the behavior at \textbf{\textit{T $>$ T}}$_{c}$. We find that a soft, temperature-dependent pseudo-gap emerges in this regime, reflecting strong short-range correlations that persist above the melting temperature. This ``pseudo-gap'' phase is characterized by unusual insulating-like, but very weakly temperature dependent transport, similar to experimental findings [2] in many low carrier density systems. [1] S.Pankov and V. Dobrosavljevic, Phys. Rev. Lett.\textbf{ 94}, 046402 (2005). [2] K. Lai, W. Pan, D. C. Tsui, S. Lyon, M. Muhlberger, and F. Schaffler, Phys. Rev. B \textbf{75}, 033314 (2007). [Preview Abstract] |
Thursday, March 19, 2009 1:03PM - 1:15PM |
W41.00010: Quantum criticality out of equilibrium: Thermopower and shot noise in a ferromagnetic single electron transistor Jedediah Pixley, Stefan Kirchner, Qimiao Si The low-energy properties of a single electron transistor attached to ferromagnetic leads are described by the Bose-Fermi Anderson model. This model can undergo a continuous T=0 dissipative phase transition and it was shown in [1] that the transistor can be tuned through this quantum phase transition. The out-of-equilibrium scaling properties near the quantum critical point and in the adjacent phases of the spin and charge response and their fluctuation-dissipation ratios were recently studied in [2]. In this contribution, we study the thermopower and shot noise, which probe the quantum criticality in a way that goes beyond the current-voltage characteristics. The thermoelectric properties of a quantum dot in the Kondo regime can be directly measured [3]. Bulk thermopower measurements of heavy fermion compounds near their quantum critical point provide valuable information about the Fermi surface [4] and are related to our work through the Extended Dynamical Mean Field Theory that maps the Kondo lattice onto a Bose-Fermi Kondo model augmented by a self-consistency condition. [1] S. Kirchner et al., PNAS 102, 18824 (2005). [2] S. Kirchner and Q. Si, arXiv:0805.3717 (2008). [3] R. Scheibner et al, PRL 95, 176602 (2005). [4] S. Hartmann et al, to be published (2008). [Preview Abstract] |
Thursday, March 19, 2009 1:15PM - 1:27PM |
W41.00011: Mottness scenario for non-Fermi liquid behavior in the periodic Anderson model within dynamical mean-field theory Giovanni Sordi, Adriano Amaricci, Marcelo Rozenberg We study the Mott metal-insulator transition in the periodic Anderson model within dynamical mean-field Theory (DMFT). Near the quantum transition, we find a non-Fermi liquid metallic state down to a vanishing temperature scale. We identify the origin of the non-Fermi liquid behavior as due to magnetic scattering of the doped carriers by the localized moments. The non-Fermi liquid state can be tuned by either doping or external magnetic field. Our results show that the coupling to spatial magnetic fluctuations (absent in DMFT) is not a prerequisite to realize a non-Fermi liquid scenario for heavy fermion systems. Refs: {\sl Phys. Rev. Lett.} {\bf 99}, 196403 (2007); {\sl Phys. Rev. Lett.} {\bf 101}, 146403 (2008). [Preview Abstract] |
Thursday, March 19, 2009 1:27PM - 1:39PM |
W41.00012: Percolation transition in nanowire magnetorheological fluids Josh Karli, Darin Zimmerman, Joseph Filer, Richard Bell, Norman Wereley We measure the yield stress of magnetorheological (MR) fluids that employ cobalt nanowires as the ferromagnetic component and observe a percolation transition in the yield stress at a critical value of the cobalt-nanowire volume fraction, $p_c $. The critical volume fraction depends not only on the particle size and aspect ratio (as expected) but also on the external magnetic field applied to the MR-fluid sample. We fit the yield-stress data using McLachlan's generalized effective medium (GEM) model to determine $p_c $ and the percolation exponents $s$ and $t$ that describe the transition behavior below and above $p_c $, respectively. The phase transition from low- to high-yield stress at low magnetic-particle volume fraction ($<1\% )$ has potential application to the development of precision magnetic sensors and actuators. [Preview Abstract] |
Thursday, March 19, 2009 1:39PM - 1:51PM |
W41.00013: Anderson localization transition in thin films of gadolinium R. Misra, A.F. Hebard, K.A. Muttalib, P. Woelfle \textit{In situ} temperature-dependent transport studies have been performed on a series of gadolinium (Gd) films deposited onto sapphire substrates having sheet resistance $R_{0}$~$\equiv $~$R_{xx}$(5K) varying over the range 4011~$\Omega $ ($\sim $35{\AA}) to 132 K$\Omega $ ($<$ 20 {\AA}). The disorder strength, as measured by $R_{0}$, is sufficiently high so that quantum corrections to the classical Boltzmann conductivity are no longer observed. In this region of moderately strong disorder, we find a temperature-dependent conductivity of the form \textit{$\sigma $}($T)$~ = \textit{A~+BT}$^{p}$ where $A$ and $B$ are disorder-dependent constants and $p$ is a power with value 0.4. We find that $A$ is proportional to (1-$R_{0}/R_{c})^{s}$ where the conductivity exponent $s~$=1 and the critical resistance $R_{c }$= 22.7 k$\Omega $. This change in sign of $A$ with unity exponent at critical disorder describes the critical regime of an Anderson localization transition[1] with the temperature-dependent localization length sufficiently small so that the Gd films can be considered to be in the 3D regime, rather than the 2D regime where metallic behavior does not occur [2]. [1] Lee {\&} Ramakrishnan, RMP 57, 287 (1985); Belitz {\&} Kirkpatrick, RMP 66, 261 (1994) [2] Abrahams, Anderson, Licciardello {\&} Ramakrishnan, PRL 42, 673 (1979) [Preview Abstract] |
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