Condensed-Matter Physics seminars: Spring 2008

Wednesdays in the Physics Reading Room

Date & time Speaker & affiliation Talk title & abstract

Apr 2 4:00pm Frank Pollmann (UC Berkeley)
Strongly correlated fermions on geometrically frustrated lattices
Abstract: We study a novel class of systems with electronic degrees of freedom on frustrated lattices. It has been shown that such systems can have excitations which carry fractional charges (e/2) in the limit of strong correlations . In order to understand the mechanism and physical properties of these fractionalized charges, we firstly consider a model of spinless fermions on a geometrically frustrated checkerboard lattice. An effective Hamiltonian is derived which describes the low-lying excitations in the limit of strong correlations. The ground state of the latter Hamiltonian is shown to be charged ordered and the fractional charges are linearly confined. Secondly, we consider a model of spinful fermions on a kagome lattice and study the interplay between charge- and spin-degrees of freedom. In particular, we find here a new mechanism yielding ferromagnetism.

Apr 9 4:00pm Ulrich Schollwöck (RWTH Aachen)
Out-of-equilibrium ultracold atoms in optical lattices
Abstract: Ultracold atoms in optical lattices provide a unique playground for the study of coherent quantum dynamics in essentially closed, strongly interacting quantum systems. On the one hand, old questions of quantum many-body physics can finally be accessed experimentally in a controlled fashion; on the other hand, completely new ways of manipulation have arisen. From a theoretical perspective, time- dependent density-matrix renormalization-group methods provide a powerful tool to simulate such experiments. After a brief introduction into the experimental and theoretical background, I want to discuss old many-body phenomena such as spin-charge separation, discuss new ways of cooling ultracold atoms down to the currently inaccessible low temperatures needed for many strong correlation phenomena by adiabatic state transformations, and - to explore the opposite limit - discuss relaxation in closed quantum systems after a sudden quench.

Apr 16 4:00pm TBA

Apr 23 4:00pm Allen Mills (UCR)
A Proposed Center for Low Energy Positron Science
Abstract: A. Scientific thrusts. The frontier of low energy positron science encompasses the areas of dense positronium, precision tests of quantum electrodynamics theory, electronic properties of condensed matter, and medical imaging. The world†¢s largest ever laboratory source of positron antimatter for this science is being installed at UC Riverside Department of Physics and Astronomy where a 5 MeV 1 mA deuteron accelerator will produce up to 20 Ci of the positron emitter 13N by the reaction 12C(d,n)13N. The availability of 100 times more positrons than before presents a unique opportunity for major advances in fundamental physics, materials science and medical techniques. The new experimental directions to be pursued by the center include developing advanced positron sources, studying the properties of Bose-Einstein condensed positronium and using i to conduct precision spectoscopic measurements, perfecting positron microprobes for imaging and spectroscopy of materials and real time tracking of HC13N-labeled red blood cells via positron emission tomography (PET) s tracers for studyng blood flow, diagnosis of e.g. atrial septal defects or patent foramen ovales (small holes in the heart), and guiding catheter for developig minimally invasive medical procedures.
B. Community building. The new center will be a catalyst for bringing togethr researchers from other positron laboratories large and small and from relatedefforts at the National Labs. A rotating sabbatical will enable senior scietists to visit UCR to join in the training and education of undergraduae and graduate students at the new facility. Annual workshops will bring toether positron workers and scientists studying related topics using compleentary techniques. This will make he attendees aware of the availability an capabilities of the different methods and will encourage topic-related cllaboration at facilities where one does not have to be an expert on the methos of positron production and delivery. An oversight committee composedof delegates to the workshops will assess progress, give advice on prioritiesand contribute to a report on the future of positron research (possibly in the form of a proposal for a major positron facility associated with a synchrtron or neutron source) at the end of the project.
C. Educational, outreach and diversity activities. Coupling of the new UCR ositron source to an advanced suite of measurements and activities in fndamental, technological and medical areas will be a wonderful milieu for traiing and educating our undergraduate and graduate students. Fortnightly meeings of the entire group of students and postdoctoral researchers invlved with this project will make them cognizant of the many related activites, thus helping them to gain a broad view of science needed to produce the mos effective educators, researchers and scientific leaders. Summer Schools ill bring about 35 participants including senior scientists and their students o learn about positron techniques and present their latest results. We wil feature minority and women scientists at our weekly colloquia to encourage paticipation of people from under-represented groups. Our training will etend to pre-college students in the form of summer research internships forhigh school teachers and their students, popular lectures, tours, and an acessible web site.
D. Broader impact. Imaging of blood cells could have a major impact on medica diagnosis and non-invasive surgery. Powerful positron microprobes will e beneficial to our understanding of solid matter and in technologies that nee a high throughput and sensitive characterization of defects of varius kinds. In the long term, studies of the Bose-Einstein condensate of positroium will form the basis of work toward the gamma ray laser, which in turn coul lead to a new and possibly more effective option for inertial fusion, with obious implications for energy sufficiency.
E. Other information on proposed PFC activities. The deuteron accelerator hat is the cornerstone of the proposed activities is funded by a series of thre SBIR†¢s awarded by AFOR to First Point Scientific, Inc. The accelerator wil be housed at UCR for the duration of this proposal where its goal is todemonstrate a sizable advance in the rate and efficiency of antimatter prduction and to make progress towards the annihilation gamma ray laser.

Apr 30 4:00pm Rajiv Singh (UC Davis)
Kagome Lattice Antiferromagnets
Abstract: We present a search for the ground state phase of the kagome lattice antiferromagnet using series expansion methods. Our calculations lead to a selection of a Valence Bond Crystal (VBC) phase, which consists of a honeycomb lattice of "perfect hexagons." Extremely small energy differences between different VBC states suggest a very low ordering temperature. Results for the excitation spectra of the system will be presented, which show a large number of singlets below the lowest triplet on finite clusters, in agreement with Exact Diagonalization studies. We also discuss finite temperature thermodynamic properties of kagome lattice antiferromagnets including Dzyaloshinkii-Moria (DM) anisotropies, exchange anisotropies, and dilution. The results are compared with measurements on recently synthesized material ZnCu₃(OH)₆Cl₂. While the higher temperature experimental data are well described by the Heisenberg model with J = 170K, the low temperature data clearly deviate from the Heisenberg model and imply presence of DM interactions and impurities.

May 7 4:00pm TBA

May 14 4:00pm Ziqiang Wang (Boston College)
Strong Correlation and Unconventional Electronic States in Sodium Doped Cobaltates
Abstract: The sodium cobaltates provide a unique low-spin, triangular lattice fermion system with strong electronic correlation and geometrical frustration in the charge, spin and orbital sectors. We review a spectrum of recent experiments and argue that strong correlation plays an important role in the unusual properties and novel electronic phases in this material. We describe the correlation-induced orbital carrier transfer and provide an explanation of the Fermi surface topology and the quasiparticle dispersion observed by ARPES experiments. We show that the interplay between strong correlation and geometrical frustration leads to the emergence of a new class of inhomogeneous charge and spin ordered states and compare to the findings of recent neutron scattering, NMR, and transport experiments. The important role played by the sodium dopants will be discussed as well as the emergence of itinerant and localized magnetism in the Na rich part of the phase diagram. We propose a class of nodal chiral superconductors and discuss their topological properties in connection to the unconventional superconducting state observed in the hydrated cobaltates.

May 21 4:00pm Eduardo Mucciolo (Univ. Central Florida)
Simulations and Numerical Modeling of Electronic Transport in Disordered Graphene
Abstract: In recent years, graphene has become one of the most exciting new materials for advanced electronic device applications. Yet, much is still not well understood about it, especially how transport properties are affected by disorder. In this talk, I will present our recent theoretical results on the dependence of the conductivity and shot noise of graphene sheets on the strength of background potential fluctuations induced by a substrate. We have numerical evidence that the conductivity dependence on the carrier density obeys a scaling form. The shot noise Fano factor, on the other hand, has a much less universal dependence on the carrier density but is very sensitive to the disorder strength. Comparison between our results and recent experiments will be presented. Finally, I will discuss how edge roughness and geometry affect transport in ultra-narrow ribbons. Our approach provides a way for extracting quantitative and qualitative information about microscopic mechanisms of disorder in graphene, with strong implications for device engineering.

May 28 4:00pm Eduardo C. Marino (Princeton University and Universidade Federal do Rio de Janeiro)
A Stable Mean-Field Solution for a Short-Range Interacting Quantum Spin-Glass
Abstract: We present a mean-field solution for a quantum, short-range interacting, disordered, SO(3) Heisenberg spin model, in which the Gaussian distribution of couplings is centered in an AF coupling J>0, and which, for weak disorder, can be treated as a perturbation of the pure AF Heisenberg system. The phase diagram contains, apart from a Néel phase at T=0, spin-glass and paramagnetic phases whose thermodynamic stability is demonstrated by an analysis of the Hessian matrix of the free-energy. The magnetic susceptibilities exhibit the typical cusp of a spin-glass transition.

Jun 4 4:00pm TBA

Jun 11 4:00pm David Cobden (U Washington)
The metal-insulator phase transition in vanadium dioxide nanobeams
Abstract: A number of first-order phase transitions in solids are known during which an abrupt and rapid change in the electronic properties occurs with only a small accompanying distortion of the lattice. A classic example is the metal-insulator transition (MIT) in vanadium dioxide, which occurs at about 67 ºC at zero pressure. Unfortunately, both fundamental studies and applications of such transitions are impeded by the frustration that occurs in macroscopic crystals due to the combination of latent heat and change in unit cell dimensions, which leads to broadening, hysteresis, and mechanical degradation at the transition. We demonstrate that by working with nanobeams of VO₂ smaller than the characteristic domain size, frustration and degradation can be eliminated, allowing the determination a number of new properties of this famous transition. First, the metallic phase can be supercooled by more than 50 °C. Second, the resistivity of the insulator in coexistence with the metal is independent of temperature. Third, the MIT occurs from the intermediate M2 insulating phase, which we detect near the transition by its higher resistivity, but not directly from the low-temperature M1 phase. These results imply that the MIT is triggered by carrier density and therefore involves electron correlations, and suggest that it takes place in the undimerized vanadium chains present in M2 but not in M1. More generally, these studies illustrate the scientific and technological benefits of studying strongly correlated materials in nanoscale form.

Seminars for Spring 2008
Seminars for Winter 2008
Seminars for Fall 2007
Seminars for Spring 2007
Seminars for Winter 2007
Seminars for Fall 2006
Seminars for Spring 2006
Seminars for Winter 2006
Seminars for Fall 2005

Leonid Pryadko <my first name at landau dot ucr dot edu>

Last modified: Tue May 20 18:15:42 PDT 2008

Date & time	Speaker & affiliation	Talk title & abstract
Apr 2 4:00pm	Frank Pollmann (UC Berkeley)	Strongly correlated fermions on geometrically frustrated lattices Abstract: We study a novel class of systems with electronic degrees of freedom on frustrated lattices. It has been shown that such systems can have excitations which carry fractional charges (e/2) in the limit of strong correlations . In order to understand the mechanism and physical properties of these fractionalized charges, we firstly consider a model of spinless fermions on a geometrically frustrated checkerboard lattice. An effective Hamiltonian is derived which describes the low-lying excitations in the limit of strong correlations. The ground state of the latter Hamiltonian is shown to be charged ordered and the fractional charges are linearly confined. Secondly, we consider a model of spinful fermions on a kagome lattice and study the interplay between charge- and spin-degrees of freedom. In particular, we find here a new mechanism yielding ferromagnetism.
Apr 9 4:00pm	Ulrich Schollwöck (RWTH Aachen)	Out-of-equilibrium ultracold atoms in optical lattices Abstract: Ultracold atoms in optical lattices provide a unique playground for the study of coherent quantum dynamics in essentially closed, strongly interacting quantum systems. On the one hand, old questions of quantum many-body physics can finally be accessed experimentally in a controlled fashion; on the other hand, completely new ways of manipulation have arisen. From a theoretical perspective, time- dependent density-matrix renormalization-group methods provide a powerful tool to simulate such experiments. After a brief introduction into the experimental and theoretical background, I want to discuss old many-body phenomena such as spin-charge separation, discuss new ways of cooling ultracold atoms down to the currently inaccessible low temperatures needed for many strong correlation phenomena by adiabatic state transformations, and - to explore the opposite limit - discuss relaxation in closed quantum systems after a sudden quench.
Apr 16 4:00pm	TBA
Apr 23 4:00pm	Allen Mills (UCR)	A Proposed Center for Low Energy Positron Science Abstract: A. Scientific thrusts. The frontier of low energy positron science encompasses the areas of dense positronium, precision tests of quantum electrodynamics theory, electronic properties of condensed matter, and medical imaging. The world†¢s largest ever laboratory source of positron antimatter for this science is being installed at UC Riverside Department of Physics and Astronomy where a 5 MeV 1 mA deuteron accelerator will produce up to 20 Ci of the positron emitter 13N by the reaction 12C(d,n)13N. The availability of 100 times more positrons than before presents a unique opportunity for major advances in fundamental physics, materials science and medical techniques. The new experimental directions to be pursued by the center include developing advanced positron sources, studying the properties of Bose-Einstein condensed positronium and using i to conduct precision spectoscopic measurements, perfecting positron microprobes for imaging and spectroscopy of materials and real time tracking of HC13N-labeled red blood cells via positron emission tomography (PET) s tracers for studyng blood flow, diagnosis of e.g. atrial septal defects or patent foramen ovales (small holes in the heart), and guiding catheter for developig minimally invasive medical procedures. B. Community building. The new center will be a catalyst for bringing togethr researchers from other positron laboratories large and small and from relatedefforts at the National Labs. A rotating sabbatical will enable senior scietists to visit UCR to join in the training and education of undergraduae and graduate students at the new facility. Annual workshops will bring toether positron workers and scientists studying related topics using compleentary techniques. This will make he attendees aware of the availability an capabilities of the different methods and will encourage topic-related cllaboration at facilities where one does not have to be an expert on the methos of positron production and delivery. An oversight committee composedof delegates to the workshops will assess progress, give advice on prioritiesand contribute to a report on the future of positron research (possibly in the form of a proposal for a major positron facility associated with a synchrtron or neutron source) at the end of the project. C. Educational, outreach and diversity activities. Coupling of the new UCR ositron source to an advanced suite of measurements and activities in fndamental, technological and medical areas will be a wonderful milieu for traiing and educating our undergraduate and graduate students. Fortnightly meeings of the entire group of students and postdoctoral researchers invlved with this project will make them cognizant of the many related activites, thus helping them to gain a broad view of science needed to produce the mos effective educators, researchers and scientific leaders. Summer Schools ill bring about 35 participants including senior scientists and their students o learn about positron techniques and present their latest results. We wil feature minority and women scientists at our weekly colloquia to encourage paticipation of people from under-represented groups. Our training will etend to pre-college students in the form of summer research internships forhigh school teachers and their students, popular lectures, tours, and an acessible web site. D. Broader impact. Imaging of blood cells could have a major impact on medica diagnosis and non-invasive surgery. Powerful positron microprobes will e beneficial to our understanding of solid matter and in technologies that nee a high throughput and sensitive characterization of defects of varius kinds. In the long term, studies of the Bose-Einstein condensate of positroium will form the basis of work toward the gamma ray laser, which in turn coul lead to a new and possibly more effective option for inertial fusion, with obious implications for energy sufficiency. E. Other information on proposed PFC activities. The deuteron accelerator hat is the cornerstone of the proposed activities is funded by a series of thre SBIR†¢s awarded by AFOR to First Point Scientific, Inc. The accelerator wil be housed at UCR for the duration of this proposal where its goal is todemonstrate a sizable advance in the rate and efficiency of antimatter prduction and to make progress towards the annihilation gamma ray laser.
Apr 30 4:00pm	Rajiv Singh (UC Davis)	Kagome Lattice Antiferromagnets Abstract: We present a search for the ground state phase of the kagome lattice antiferromagnet using series expansion methods. Our calculations lead to a selection of a Valence Bond Crystal (VBC) phase, which consists of a honeycomb lattice of "perfect hexagons." Extremely small energy differences between different VBC states suggest a very low ordering temperature. Results for the excitation spectra of the system will be presented, which show a large number of singlets below the lowest triplet on finite clusters, in agreement with Exact Diagonalization studies. We also discuss finite temperature thermodynamic properties of kagome lattice antiferromagnets including Dzyaloshinkii-Moria (DM) anisotropies, exchange anisotropies, and dilution. The results are compared with measurements on recently synthesized material ZnCu₃(OH)₆Cl₂. While the higher temperature experimental data are well described by the Heisenberg model with `J` = 170K, the low temperature data clearly deviate from the Heisenberg model and imply presence of DM interactions and impurities.
May 7 4:00pm		TBA
May 14 4:00pm	Ziqiang Wang (Boston College)	Strong Correlation and Unconventional Electronic States in Sodium Doped Cobaltates Abstract: The sodium cobaltates provide a unique low-spin, triangular lattice fermion system with strong electronic correlation and geometrical frustration in the charge, spin and orbital sectors. We review a spectrum of recent experiments and argue that strong correlation plays an important role in the unusual properties and novel electronic phases in this material. We describe the correlation-induced orbital carrier transfer and provide an explanation of the Fermi surface topology and the quasiparticle dispersion observed by ARPES experiments. We show that the interplay between strong correlation and geometrical frustration leads to the emergence of a new class of inhomogeneous charge and spin ordered states and compare to the findings of recent neutron scattering, NMR, and transport experiments. The important role played by the sodium dopants will be discussed as well as the emergence of itinerant and localized magnetism in the Na rich part of the phase diagram. We propose a class of nodal chiral superconductors and discuss their topological properties in connection to the unconventional superconducting state observed in the hydrated cobaltates.
May 21 4:00pm	Eduardo Mucciolo (Univ. Central Florida)	Simulations and Numerical Modeling of Electronic Transport in Disordered Graphene Abstract: In recent years, graphene has become one of the most exciting new materials for advanced electronic device applications. Yet, much is still not well understood about it, especially how transport properties are affected by disorder. In this talk, I will present our recent theoretical results on the dependence of the conductivity and shot noise of graphene sheets on the strength of background potential fluctuations induced by a substrate. We have numerical evidence that the conductivity dependence on the carrier density obeys a scaling form. The shot noise Fano factor, on the other hand, has a much less universal dependence on the carrier density but is very sensitive to the disorder strength. Comparison between our results and recent experiments will be presented. Finally, I will discuss how edge roughness and geometry affect transport in ultra-narrow ribbons. Our approach provides a way for extracting quantitative and qualitative information about microscopic mechanisms of disorder in graphene, with strong implications for device engineering.
May 28 4:00pm	Eduardo C. Marino (Princeton University and Universidade Federal do Rio de Janeiro)	A Stable Mean-Field Solution for a Short-Range Interacting Quantum Spin-Glass Abstract: We present a mean-field solution for a quantum, short-range interacting, disordered, SO(3) Heisenberg spin model, in which the Gaussian distribution of couplings is centered in an AF coupling `J`>0, and which, for weak disorder, can be treated as a perturbation of the pure AF Heisenberg system. The phase diagram contains, apart from a Néel phase at `T`=0, spin-glass and paramagnetic phases whose thermodynamic stability is demonstrated by an analysis of the Hessian matrix of the free-energy. The magnetic susceptibilities exhibit the typical cusp of a spin-glass transition.
Jun 4 4:00pm	TBA
Jun 11 4:00pm	David Cobden (U Washington)	The metal-insulator phase transition in vanadium dioxide nanobeams Abstract: A number of first-order phase transitions in solids are known during which an abrupt and rapid change in the electronic properties occurs with only a small accompanying distortion of the lattice. A classic example is the metal-insulator transition (MIT) in vanadium dioxide, which occurs at about 67 ºC at zero pressure. Unfortunately, both fundamental studies and applications of such transitions are impeded by the frustration that occurs in macroscopic crystals due to the combination of latent heat and change in unit cell dimensions, which leads to broadening, hysteresis, and mechanical degradation at the transition. We demonstrate that by working with nanobeams of VO₂ smaller than the characteristic domain size, frustration and degradation can be eliminated, allowing the determination a number of new properties of this famous transition. First, the metallic phase can be supercooled by more than 50 °C. Second, the resistivity of the insulator in coexistence with the metal is independent of temperature. Third, the MIT occurs from the intermediate M2 insulating phase, which we detect near the transition by its higher resistivity, but not directly from the low-temperature M1 phase. These results imply that the MIT is triggered by carrier density and therefore involves electron correlations, and suggest that it takes place in the undimerized vanadium chains present in M2 but not in M1. More generally, these studies illustrate the scientific and technological benefits of studying strongly correlated materials in nanoscale form.