Bulletin of the American Physical Society
APS March Meeting 2010
Volume 55, Number 2
Monday–Friday, March 15–19, 2010; Portland, Oregon
Session Y30: Disordered Magnetic Materials |
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Sponsoring Units: GMAG Chair: Stefanos Papanikolaou, Cornell University Room: D139 |
Friday, March 19, 2010 8:00AM - 8:12AM |
Y30.00001: The Impact of B Site Disorder in the Manganites Kalpataru Pradhan, Pinaki Majumdar One can generate highly inhomogeneous phase coexistent states in the manganites A$_{1-x}$A'$_x$MnO$_3$ by dilute substitution on the Mn site. On suitable choice of the reference manganite state, and the valence and magnetic character of the dopant, a cluster coexistent state of two competing phases can be created. There is a wealth of data on such `B site' substitution on the $x= 0.5 $ charge ordered manganites, as well as the $x \sim 0.33-0.40$ ferromagnetic metal. The results of substitution vary widely, depending on $x$, the bandwidth of the manganite, and the choice of dopant. While some choice of dopants lead to a phase coexistent state, others lead to a nanoscale correlated glassy phase. We provide a minimal model for B site impurities in manganite hosts, and solve this disordered strong coupling problem through a real space Monte Carlo technique. In addition to the detailed numerical results on the spatial organisation we are able to identify the hierarchy of physical effects that control the impact of B dopants on the manganites. [Preview Abstract] |
Friday, March 19, 2010 8:12AM - 8:24AM |
Y30.00002: Disorder effects in ARPES spectral functions Anamitra Mukherjee, Nandini Trivedi, Mohit Randeria The interplay of disorder and interactions are known to cause novel effects in complex oxides such as cuprates and manganites. Understanding the single-particle spectral function, with a goal of separating out the effects of disorder and interactions, is an important step in elucidating the physics of these complex materials. We numerically evaluate the spectral function $A({\bf r},{\bf r}';\omega)$ for model Hamiltonians with disorder. ARPES measures the occupied part of $A$, averaged over the center-of-mass variable $({\bf r}+{\bf r}')/2$ and Fourier transformed in $({\bf r}-{\bf r}')\to {\bf k}$. We analyze the line shape of the energy distribution curves (EDC) and momentum distribution curves (MDC). We compare our results with experiments on manganites, and on intentionally disordered chalcogenides. [Preview Abstract] |
Friday, March 19, 2010 8:24AM - 8:36AM |
Y30.00003: Theoretical study of disordered $Fe$-$Ru$ alloys: a Monte Carlo approach Ian Diaz We study the magnetic properties and critical behaviour of quenched $F\lowercase{e}_{1-\lowercase{x}}R\lowercase{u}_{\lowercase{x}}$ alloys on a bcc lattice, for the following ruthenium concentrations: $x=0\%$, $4\%$, $6\%$ and $8\%$. This study is carried out within a Monte Carlo approach employing multiple histogram reweighting to analyse the data generated in the simulations. By means of a finite-size scaling analysis of several themodynamic quantities, taking into account up to the leading irrelevant scaling field term, we find estimates of the critical exponents $\alpha$, $\beta$, $\gamma$ and $\nu$ and critical temperature of our model. Our results for $x=0\%$ are in excelent agreement with those for the three-dimensional pure Ising model in the literature, as expected. We show that our estimates of critical exponents for $x=4\%$, $6\%$ and $8\%$ are consistent with those reported for the transition line between paramagnetic and ferromagnetic phases of both randomly dilute and $\pm J$ Ising models. We also compare our results for the behaviour of the Curie temperature as a function of ruthenium concentration and magnetization as a function of temperature with experimental and mean-field results reported elsewhere. [Preview Abstract] |
Friday, March 19, 2010 8:36AM - 8:48AM |
Y30.00004: Two-dimensional Potts model with aperiodic interactions: numerical simulation Nilton Branco, Daniel Girardi The uniform two-dimensional Potts model presents first-order transitions for $q$ (number of states) greater than 4. The introduction of aperiodic modulations on its interactions may change the universality class or the nature of the transition. Previous results for the $q=8$ Potts model on the square lattice suggest that the Harris-Luck criterion is satisfied also for first-order transitions [1]. However, for random disorder, the new universality class that may emerge depends on the number of states of the Potts model [2]. In order to test this possibility for aperiodic modulations, we have made extensive numerical simulations on the $q=6$ Potts model on the square lattice, for three aperiodic sequence. Our results show that the Harris-Luck criterion is obeyed and that the new universality class that may emerge is the same as for the $q=8$ Potts model. Therefore, we stablish that, contrarily to random disorder, the introduction of relevant aperiodic modulation leads the system to a new universality class, irrespective number of states of the Potts model.\\[4pt] [1] C. Chatelain, B. Berche, Phys. Rev. Lett. \textbf{80}, 1670 (1998).\\[0pt] [2] J.L. Jacobsen and J. Cardy, Phys. Rev. Lett. \textbf{79}, 4063 (1997). [Preview Abstract] |
Friday, March 19, 2010 8:48AM - 9:00AM |
Y30.00005: Self-consistent generalization of the disordered local moment (DLM) method for magnetically ordered systems at finite temperatures Paul Larson, Kirill Belashchenko The disordered local moment (DLM) approach using the coherent potential approximation (CPA) has been integrated as a self-consistent addition to the tight binding linear muffin-tin orbital (LMTO) method. This Green's function routine coherently averages spin states in a method similar to that used for random alloys. In principle, the DLM method allows one to describe the electronic structure, magnetic thermodynamics, and transport properties of arbitrarily ordered magnetic states and their changes across magnetic phase transitions. However, existing implementations are limited to rotationally invariant paramagnetic states or do not treat the ordered states in a self-consistent fashion. The new version of DLM discussed here includes self-consistency of the angular distribution function and the angular-dependent site potentials in the ferromagnetic (or antiferromagnetic) state. As in the underlying LMTO code, arbitrary space groups and atomic bases are supported in the DLM approach. Preliminary results are given for transition metal systems as a function of temperature. [Preview Abstract] |
Friday, March 19, 2010 9:00AM - 9:12AM |
Y30.00006: Random-singlet phases beyond one spatial dimension Kevin Beach When the linear Heisenberg spin chain is given non-uniform exhange couplings, its ground state becomes frozen in a quasi-static singlet bond pattern. This so-called random-singlet phase has long been understood via a renormalization-group (RG) procedure that decimates the bonds from strongest to weakest; the flow equations indicate that even infinitesimal disorder drives the system toward an infinite-randomness fixed point. We present an non-RG construction, based on the large-$N$ limit of SU($N$), that reproduces the linear-chain result, but which also generalizes to lattices in higher spatial dimension. Beyond a threshold in disorder $D$ and rank $N$, a random-singlet phase exists and exhibits spin correlations that decay algebraically with characteristic exponents. The predictions in two and three dimensions are verified using Quantum Monte Carlo. [Preview Abstract] |
Friday, March 19, 2010 9:12AM - 9:24AM |
Y30.00007: Merging theory with experiment: improving the accuracy of scaling theories Yan-Jiun Chen, Stefanos Papanikolaou, James P. Sethna, Gianfranco Durin, Stefano Zapperi Motivated by the experimental problem of analyzing data of Barkhausen noise collected through a limited field of view, we have developed a flexible software environment, SloppyScaling, which fits multi-variable scaling functions to both experimental and simulation data. We've used this to test our proposed two-variable scaling functions against simulations on interface depinning models, enabling experiments to make better predictions. Importance sampling algorithms allow us to estimate exponents with honest error bars and improved confidence. Furthermore, we've discovered its utility as a theorist's playground: it allows us to easily identify corrections to scaling, add them to our theory, and explore crossovers away from well-understood scaling behavior. [Preview Abstract] |
Friday, March 19, 2010 9:24AM - 9:36AM |
Y30.00008: Scaling crossover for the average avalanche shape Stefanos Papanikolaou, Felipe Bohn, Rubem L. Sommer, Gianfranco Durin, Stefano Zapperi, James P. Sethna Universality and the renormalization group claim to predict all behavior on long length and time scales asymptotically close to critical points. In practice, large simulations and heroic experiments have been needed to unambiguously test and measure the critical exponents and scaling functions. We announce here the measurement and prediction of universal corrections to scaling, applied to the temporal average shape of Barkhausen noise avalanches. We bypass the confounding factors of time-retarded interactions (eddy currents) by measuring thin permalloy films, and bypass thresholding effects and amplifier distortions by applying Wiener deconvolution. We show experimental shapes that are approximately symmetric, and measure the leading corrections to scaling. We solve a mean-field theory for the magnetization dynamics and calculate the relevant demagnetizing-field correction to scaling, showing qualitative agreement with the experiment. In this way, we move toward a quantitative theory useful at smaller time and length scales and farther from the critical point. [Preview Abstract] |
Friday, March 19, 2010 9:36AM - 9:48AM |
Y30.00009: Probing Spin Glass via conductance fluctuations Guillaume Paulin, David Carpentier In this work we study numerically conductance fluctuations in a low temperature Spin Glass nanowire. In the Spin Glass phase, frozen classical spins dephase the electrons diffusing in the sample. The phase of electrons will then keep track of this encountered configuration. We show numerically that a careful study of conductance correlations between these two different frozen configurations of spins S1 and S2 gives access to intrinsic properties of the Spin Glass, such as spin configurations overlap that encodes how different the configurations are. The onsite disorder potential in the system, described by an Anderson model, has two origins: a scalar one due to the presence of impurities without spin and a magnetic one due to the presence of magnetic impurities. Many configurations of scalar disorder are taken into account to perform averages, but only a few spin configurations are created. The numerical method is based on the Landauer formalism of transport to deduce the conductance of a diffusive sample from the scattering matrix, and on a relation due to Fisher and Lee that relates the scattering matrix to the retarded Green's functions of electrons. [Preview Abstract] |
Friday, March 19, 2010 9:48AM - 10:00AM |
Y30.00010: Almeida Thouless Line in Vector Spin Glasses Auditya Sharma, A. P. Young The de Almeida Thouless line in the magnetic field-temperature plane of spin glasses, separates a high-temperature, high-field paramagnetic phase from a spin glass phase. It is a characteristic feature of Ising spin glasses in the mean field (infinite-range) limit. Here we show that an AT line also occurs for infinite-range \textit{vector} spin glasses in a \textit{random} magnetic field, which does not appear to have been appreciated before. We verify this conclusion by numerical simulations. [Preview Abstract] |
Friday, March 19, 2010 10:00AM - 10:12AM |
Y30.00011: Exact Solution of the Spherical Spin Glass Model Beyond Mean Field Theory Shimul Akhanjee, Joseph Rudnick We present an exact solution of a Gaussian spin-glass model with infinite ranged interactions and a global spherical constraint at zero magnetic field. The replicated spin-glass Hamiltonian is mapped onto a Coulomb gas of logarithmically interacting particles confined by a peculiar single particle potential. The precise free energy is obtained by analyzing the Painlev\'e $\tau^{IV} [n]$ function in the $n\to 0$ limit, accounting for neglected fluctuations beyond the semi-circle density utilized in the large $N$ analysis of Kosterlitz, Thouless and Jones\cite{ktjonesPRL}. This is the first known exact solution of a spin-glass model beyond mean-field theory. [Preview Abstract] |
Friday, March 19, 2010 10:12AM - 10:24AM |
Y30.00012: Magnetic Structure, Lattice Distortion and Phase Diagram in fcc Antiferromagnets Yasuyuki Matsuura, Takeo Jo The Mn-rich alloys $\gamma $Mn$_{1-x}$A$_x $ (A$=$Ni, Ga, Au, Rh) is reported to exhibit the first-kind antiferromagnetic ordering. With varying the concentration of the doping element $x$ and the temperature $T$, it has been confirmed experimentally to take four phases; cubic, tetragonal ($c/a<1$), tetragonal ($c/a>1$), and orthorhombic ones in the antiferromagnetic region [1,2]. The purpose of the present work is to explain the phase diagrams. Our Hamiltonian is composed of the polynomial of the variables describing the multiple spin density wave (MSDW), the coupling between the variables and the symmetry strain and the elastic energy. The polynomial is derived from symmetry consideration. By calculating the partition function and the free energy, we show that the phase diagram is reproduced and elucidate the structure of MSDW at each phase and the condition of the appearance of the orthorhombic phase [3]. We also discuss the effect of whether the variables are continuous or discrete on the phase diagram. [1] T. Hori, Y. Tsuchiya, S. Funahashi, Y. Shimojyo, H. Shiraishi, K. Hojyou and Y. Nakagawa: J. Magn. Magn. Mater. \textbf{196}-\textbf{197} (1999) 663. [2] R. Yamauchi, T. Hori, M. Miyakawa and K. Fukamichi: J. Alloys Compd. \textbf{309} (2000) 16 and references therein. [3] Y. Matsuura and T. Jo: to be published in J. Phys. Soc. Jpn. \textbf{78} No. 12 (2009). [Preview Abstract] |
Friday, March 19, 2010 10:24AM - 10:36AM |
Y30.00013: ``Alive'' Berry Curvature and Emergent Maxwell Dynamics: New Pairing Mechanism in Antiferromagnets Ran Cheng, Yizhuang You, Biao Wu, Qian Niu A covariant Berry curvature in Antiferromagnetic (AFM) system is formulated, which is a Wess-Zumino term in analogous to that in ferromagnetic system. Due to band degeneracy, this curvature is in general non-Abelian. However, it reduces to an Abelian field $F_{\mu\nu}$ when the coupling strength is strong. In disordered region with local Neel order, this curvature becomes ``alive'' by obtaining a $F_{\mu\nu}^2$ kinetic term, yielding a full set of Maxwell equations. A significant prediction is given: a pair of electrons with opposite spins are attractive by exchanging the ``photons'' of the Berry curvature in a $1/r$ Coulomb's law in 2-D disordered AFM, thus creating a bound state of the entire system, which provides a new possible paring mechanism of High-Tc superconductivity. [Preview Abstract] |
Friday, March 19, 2010 10:36AM - 10:48AM |
Y30.00014: The magnetisation distribution of the Ising model - a new approach Per Hakan Lundow, Anders Rosengren A completely new approach to the Ising model in 1 to 5 dimensions is developed. We employ a generalisation of the binomial coefficients to describe the magnetisation distributions of the Ising model. For the complete graph this distribution is exact. For simple lattices of dimensions $d=1$ and $d=5$ the magnetisation distributions are remarkably well-fitted by the generalized binomial distributions. For $d=4$ we are only slightly less successful, while for $d=2,3$ we see some deviations (with exceptions!) between the generalized binomial and the Ising distribution. The results speak in favour of the generalized binomial distribution's correctness regarding their general behaviour in comparison to the Ising model. A theoretical analysis of the distribution's moments also lends support their being correct asymptotically, including the logarithmic corrections in $d=4$. The full extent to which they correctly model the Ising distribution, and for which graph families, is not settled though. [Preview Abstract] |
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