Condensed-Matter Physics seminars: Winter 2008

Wednesdays in the Physics Reading Room

Date & time Speaker & affiliation Talk title & abstract

Jan 9 4:00pm no talk

Jan 16 4:00pm Adam Kaminski (AMES lab & Iowa State University)
From coupled CuO chains to CuO₂ planes: solving cuprate puzzle
Abstract: Copper oxygen planes - the nest of high temperature superconductivity have long resisted theoretical treatment by various analytical and numerical methods. One approach proposed in the past was to start with weakly coupled 1D CuO chains and then increase the coupling to get insights about properties of the planes. In this talk I will present the data from such system and compare it to rigorous theoretical calculations.

Jan 23 4:00pm Martin Greven (Stanford)
Recent neutron scattering results for the high-temperature superconductors Nd_2-xCe_xCuO_4+δ and HgBa₂CuO_4+δ
Abstract: High-temperature superconductivity develops near antiferromagnetic (AF) phases, and it is possible that magnetic excitations contribute to the superconducting (SC) pairing mechanism. In order to assess the role of antiferromagnetism, it is essential to understand the doping and temperature dependence of the two-dimensional AF spin correlations. The phase diagram is asymmetric with respect to electron and hole doping, and for the comparatively less-studied electron-doped materials, the AF phase extends much further with doping and it appears to overlap with the SC phase: the archetypical compound Nd_2-xCe_xCuO_4+δ shows bulk superconductivity above x~0.13, while evidence for AF order has been found up to x~0.17. However, our new inelastic magnetic neutron scattering measurements point to the distinct possibility that superconductivity does not co-exist with genuine long-range antiferromagnetism [1]. We furthermore show evidence that the pseudogap phenomenon in the electron-doped materials arises from a build-up of AF spin correlations. Finally, we present our latest results for the AF spin excitations [2], which force us to conclude that recent claims for the existence of a magnetic resonance in the electron-doped cuprates have to be reinterpreted.
In a separate effort, we have succeeded in growing sizable single crystals of HgBa₂CuO_4+δ, the single-layer hole-doped compound with the highest superconducting transition temperature. Our inelastic neutron scattering results reveal that the magnetic resonance occurs at an unexpectedly high energy [3]. Finally, we discuss very recent polarized neutron diffraction data that clearly reveal the existence of "hidden" magnetic order in the pseudogap phase of HgBa₂CuO_4+δ [4].
[1] E.M. Motoyama et al., Nature 455, 186 (2007).
[2] G. Yu et al., unpublished.
[3] X. Zhao et al., Adv. Materials 18, 3243 (2006); G. Yu et al., unpublished.
[4] Y. Li et al., unpublished.

Jan 30 4:00pm Joel Moore (UC Berkeley & LBNL)
Spin Hall and spin drag effects in two- and three-dimensional materials
Abstract: Recent experimental and theoretical work reveals several ways in which spin transport differs fundamentally from charge transport. This talk first reviews the spin drag effect observed in semiconductor quantum wells as an example of how interactions modify spin transport more strongly than charge transport. We then discuss the "topological insulator" phase of two- and three-dimensional materials, which shows a robust spin Hall effect carried by edge states and may have been observed recently in HgTe structures.

Feb 6 4:00pm no talk

Feb 13 4:00pm no talk

Feb 20 4:00pm Vadim Oganesyan (Yale)
Quantum and classical localization of interacting particles at finite temperature.
Abstract: Ergodicity of many-particle motion is a fundamental assumption that underlies, among other things, the powerful statistical mechanics description of nature. The ergodic hypothesis can break down under some conditions, most notably in the presence of strong random potentials leading the phenomenon of Anderson localization. The theory of Anderson localization assumes no interactions among particles and it is of considerable practical interest to know whether the phenomenon can persist more generally. I review some recent ideas and results on spectral and transport properties of quantum and classical many-body systems. Within limitations of our methods we observe that localized states of classical particles are unstable against non-linearities, while interacting quantum particles can remain insulating.

Feb 27 4:00pm Maria D'Orsogna (Cal. State Northridge)
Patterns, Stability and collapse for two-dimensional biological swarms
Abstract: One of the most fascinating biological phenomena is the self- organization of individual members of a species moving in unison with one another, forming elegant and coherent aggregation patterns. Schools of fish, flocks of birds and swarms of insects arise in response to external stimuli or by direct interaction, and are able to fulfill tasks much more efficiently than single agents. How do these patterns arise? What are their properties? How are individual characteristics linked to collective behaviors? In this talk we discuss various aspects of biological swarming as seen through the eyes of a physicist. In particular, we investigate a non-linear system of self propelled agents that interact via pairwise attractive and repulsive potentials. We are able to predict distinct aggregation morphologies, such as flocks and vortices, and by utilizing statistical mechanics tools, to relate the interaction potential to the collapsing or dispersing behavior of aggregates as the number of constituents increases. We also discuss passage to the continuum and possible applications of this work to the development of artificial swarming teams.

Mar 5 4:00pm no talk

Mar 12 4:00pm no talk

Mar 19 4:00pm Oleg Kogan (Caltech)
Renormalization group method for predicting frequency clusters in a chain of nearest-neighbor phase oscillators.
Abstract: Collective behavior of systems with quenched disorder is an active area of research. In the classical arena, one such problem is synchronization of self-sustaining oscillators with randomly-distributed intrinsic frequencies. These form a class of models for a variety of natural and man-made systems, ranging from neural networks to integrated arrays of nanomechanical oscillators. In this talk I will begin with an overview of some known facts about such models, including the paradigm known as the Kuramoto model and circumstances under which it may exhibit a transition to a state of global frequency "entrainment". It is known that such transition does not occur in a 1-dimensional chain of self-sustaining phase oscillators coupled via nearest-neighbor interaction. However, the system does develop interesting collective structures such as clusters of common frequency. In the second part of the talk I will describe our effort to predict properties of such clusters by means of a renormalization group (RG) method. The method is designed to work in the regime of strong randomness, where the distribution of intrinsic frequencies and couplings has long tails. We implement the RG numerically and obtain statistics of cluster sizes and frequencies. Our results are consistent with those obtained by direct numerical solution of the chain's equation of motion.

Mar 26 4:00pm sping break

Seminars for Spring 2008
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Seminars for Fall 2007
Seminars for Spring 2007
Seminars for Winter 2007
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Seminars for Winter 2006
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Leonid Pryadko <my first name at landau dot ucr dot edu>

Last modified: Sun Mar 16 15:40:34 PDT 2008

Date & time	Speaker & affiliation	Talk title & abstract
Jan 9 4:00pm	no talk
Jan 16 4:00pm	Adam Kaminski (AMES lab & Iowa State University)	From coupled CuO chains to CuO₂ planes: solving cuprate puzzle Abstract: Copper oxygen planes - the nest of high temperature superconductivity have long resisted theoretical treatment by various analytical and numerical methods. One approach proposed in the past was to start with weakly coupled 1D CuO chains and then increase the coupling to get insights about properties of the planes. In this talk I will present the data from such system and compare it to rigorous theoretical calculations.
Jan 23 4:00pm	Martin Greven (Stanford)	Recent neutron scattering results for the high-temperature superconductors Nd_2-xCe_xCuO_4+δ and HgBa₂CuO_4+δ Abstract: High-temperature superconductivity develops near antiferromagnetic (AF) phases, and it is possible that magnetic excitations contribute to the superconducting (SC) pairing mechanism. In order to assess the role of antiferromagnetism, it is essential to understand the doping and temperature dependence of the two-dimensional AF spin correlations. The phase diagram is asymmetric with respect to electron and hole doping, and for the comparatively less-studied electron-doped materials, the AF phase extends much further with doping and it appears to overlap with the SC phase: the archetypical compound Nd_2-xCe_xCuO_4+δ shows bulk superconductivity above x~0.13, while evidence for AF order has been found up to x~0.17. However, our new inelastic magnetic neutron scattering measurements point to the distinct possibility that superconductivity does not co-exist with genuine long-range antiferromagnetism [1]. We furthermore show evidence that the pseudogap phenomenon in the electron-doped materials arises from a build-up of AF spin correlations. Finally, we present our latest results for the AF spin excitations [2], which force us to conclude that recent claims for the existence of a magnetic resonance in the electron-doped cuprates have to be reinterpreted. In a separate effort, we have succeeded in growing sizable single crystals of HgBa₂CuO_4+δ, the single-layer hole-doped compound with the highest superconducting transition temperature. Our inelastic neutron scattering results reveal that the magnetic resonance occurs at an unexpectedly high energy [3]. Finally, we discuss very recent polarized neutron diffraction data that clearly reveal the existence of "hidden" magnetic order in the pseudogap phase of HgBa₂CuO_4+δ [4]. [1] E.M. Motoyama et al., Nature 455, 186 (2007). [2] G. Yu et al., unpublished. [3] X. Zhao et al., Adv. Materials 18, 3243 (2006); G. Yu et al., unpublished. [4] Y. Li et al., unpublished.
Jan 30 4:00pm	Joel Moore (UC Berkeley & LBNL)	Spin Hall and spin drag effects in two- and three-dimensional materials Abstract: Recent experimental and theoretical work reveals several ways in which spin transport differs fundamentally from charge transport. This talk first reviews the spin drag effect observed in semiconductor quantum wells as an example of how interactions modify spin transport more strongly than charge transport. We then discuss the "topological insulator" phase of two- and three-dimensional materials, which shows a robust spin Hall effect carried by edge states and may have been observed recently in HgTe structures.
Feb 6 4:00pm	no talk
Feb 13 4:00pm	no talk
Feb 20 4:00pm	Vadim Oganesyan (Yale)	Quantum and classical localization of interacting particles at finite temperature. Abstract: Ergodicity of many-particle motion is a fundamental assumption that underlies, among other things, the powerful statistical mechanics description of nature. The ergodic hypothesis can break down under some conditions, most notably in the presence of strong random potentials leading the phenomenon of Anderson localization. The theory of Anderson localization assumes no interactions among particles and it is of considerable practical interest to know whether the phenomenon can persist more generally. I review some recent ideas and results on spectral and transport properties of quantum and classical many-body systems. Within limitations of our methods we observe that localized states of classical particles are unstable against non-linearities, while interacting quantum particles can remain insulating.
Feb 27 4:00pm	Maria D'Orsogna (Cal. State Northridge)	Patterns, Stability and collapse for two-dimensional biological swarms Abstract: One of the most fascinating biological phenomena is the self- organization of individual members of a species moving in unison with one another, forming elegant and coherent aggregation patterns. Schools of fish, flocks of birds and swarms of insects arise in response to external stimuli or by direct interaction, and are able to fulfill tasks much more efficiently than single agents. How do these patterns arise? What are their properties? How are individual characteristics linked to collective behaviors? In this talk we discuss various aspects of biological swarming as seen through the eyes of a physicist. In particular, we investigate a non-linear system of self propelled agents that interact via pairwise attractive and repulsive potentials. We are able to predict distinct aggregation morphologies, such as flocks and vortices, and by utilizing statistical mechanics tools, to relate the interaction potential to the collapsing or dispersing behavior of aggregates as the number of constituents increases. We also discuss passage to the continuum and possible applications of this work to the development of artificial swarming teams.
Mar 5 4:00pm	no talk
Mar 12 4:00pm	no talk
Mar 19 4:00pm	Oleg Kogan (Caltech)	Renormalization group method for predicting frequency clusters in a chain of nearest-neighbor phase oscillators. Abstract: Collective behavior of systems with quenched disorder is an active area of research. In the classical arena, one such problem is synchronization of self-sustaining oscillators with randomly-distributed intrinsic frequencies. These form a class of models for a variety of natural and man-made systems, ranging from neural networks to integrated arrays of nanomechanical oscillators. In this talk I will begin with an overview of some known facts about such models, including the paradigm known as the Kuramoto model and circumstances under which it may exhibit a transition to a state of global frequency "entrainment". It is known that such transition does not occur in a 1-dimensional chain of self-sustaining phase oscillators coupled via nearest-neighbor interaction. However, the system does develop interesting collective structures such as clusters of common frequency. In the second part of the talk I will describe our effort to predict properties of such clusters by means of a renormalization group (RG) method. The method is designed to work in the regime of strong randomness, where the distribution of intrinsic frequencies and couplings has long tails. We implement the RG numerically and obtain statistics of cluster sizes and frequencies. Our results are consistent with those obtained by direct numerical solution of the chain's equation of motion.
Mar 26 4:00pm	sping break