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Postdoctoral Fellows of the Institute for Condensed Matter Theory
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Eytan Grosfeld |
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My research interests span the field of topological order in many-body systems. I am mainly interested in mechanisms that allow formation of topological states and in phase transitions that may occur between such states. I pursue novel applications of conformal field theories to one-dimensional systems and quantum Hall effect edge dynamics. My research is also concerned with experimental manifestations of non-conventional statistics (fractional and non-Abelian), with the aim of catalyzing experiments demonstrating a clear signature of the presence of such statistics. Some of my past and on-going projects: • Methods of detecting non-abelian statistics in the quantum Hall effect, including through its effect on noise, thermal current, Coulomb blockade peaks, and AC conductivity. • Switching Noise as a Probe of Statistics in the Fractional Quantum Hall Effect, E. Grosfeld, S. H. Simon, and A. Stern, Phys. Rev. Lett. 96, 226803 (2006). • Probing the Neutral Edge Modes in Transport across a Point Contact via Thermal Effects in the Read-Rezayi Non-Abelian Quantum Hall States, E. Grosfeld and S. Das, Phys. Rev. Lett. 102, 106403 (2009). • Coulomb blockade as a probe for non-Abelian statistics in Read-Rezayi states, R. Ilan, E. Grosfeld, and A. Stern, Phys. Rev. Lett. 100, 086803 (2008). • Electronic transport in an array of quasiparticles in the ν=5/2 non-Abelian quantum Hall state, E. Grosfeld and A. Stern, Phys. Rev. B 73, 201303(R) (2006). • Signatures of p-wave superfluid phases through the BEC-BCS topological transition. • Predicted signatures of p-wave superfluid phases and Majorana zero modes of fermionic atoms in RF absorption, E. Grosfeld, N. R. Cooper, A. Stern and R. Ilan, Phys. Rev. B 76, 104516 (2007). • Discovering novel non-abelian states: By using a new counting method, we found an edge lying on the interface of two non-abelian quantum Hall effects, but having different topological properties than either one of the parent fluids. • Non-Abelian anyons: when Ising meets Fibonacci, E. Grosfeld and K. Schoutens, arXiv:0810.1955 (2008). • Synchronization phenomenon in Rydberg atoms. Spatial control of a classical electron state in a Rydberg atom by adiabatic synchronization, E. Grosfeld and L. Friedland, Phys. Rev. E 65, 046230 (2002). More recently, I am interested in measurable quantities related to entanglement entropy, in thermal current in the quantum Hall regime, and in p-wave superconductivity/superfluidity.
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Frank Krüger |
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The complete lack in understanding the quantum critical states in strongly correlated electron systems, most prominently the ones in the high-Tc superconductors and in the heavy fermion intermetallics, is tied to one of the most fundamental problems in condensed matter physics. This so called ‘fermion sign problem’ renders the path integral (PI) non- probabilistic not allowing to establish a connection to critical phenomena in classical systems. Recently, in collaboration with Jan Zaanen from Leiden University I demonstrated that within the Ceperley representation of the fermionic PI in which the minus signs are self consistently translated into a geometrical constraint structure the working of Fermi-Dirac quantum statistics and emergent scale invariance can be reconciled. We conjectured that in order for the fermion system to become critical the constraint structure has to turn into a scale-invariant fractal. My main line of research is devoted to the demonstration of this hypothesis and to further establish the connection to the collapse of the Fermi liquid observed experimentally. Further research activities are related to the interplay of strong correlation physics and disorder, frustrated magnetism, complex ordering phenomena as for example the stripes in the cuprates, and to the mutual interplay of spin, charge, and orbital degrees of freedom crucial for understanding the anomalous metallic properties of the novel iron-pnictide superconductors. Selected publications: • F. Krüger, J. Wu, and P. Phillips, Anomalous suppression of the Bose glass at commensurate fillings in the disordered Bose-Hubbard model, subm. to Phys. Rev. Lett. (arXiv:0904.4480) • H. C. Jiang, F. Krüger, J. E. Moore, D. N. Sheng, J. Zaanen, Z. Y. Weng, Phase diagram of frustrated, spatially anisotropic S=1 antiferromagnet on a square lattice, to appear in Phys. Rev. B (arXiv:0901.3141) • F. Krüger, S. Kumar, J. Zaanen, J. van den Brink, Spin-orbital frustration and the anomalous metal state of the iron pnictide superconductors, Phys. Rev. B 79, 054504 (2009) • F. Krüger and J. Zaanen, Fermionic quantum criticality and the fractal nodal surface, Phys. Rev. B 78, 035104 (2008) • F. Krüger, S. D. Wilson, L. Shan, S. Li, Y. Huang, H. H. Wen, S. C. Zhang, Pengcheng Dai, J. Zaanen, Magnetic fluctuations in n-type high-Tc superconductors reveal breakdown of fermiology, Phys. Rev. B 76, 094506 (2007) • F. Krüger and S. Scheidl, Frustrated Heisenberg antiferromagnets: fluctuation induced first order vs deconfined quantum criticality, Europhys. Lett. 74, 896 (2006) • F. Krüger and S. Scheidl, Spin dynamics of stripes, Phys. Rev. B 67, 134512 (2003) • F. Krüger and S. Scheidl, Non-universal ordering of spin and charge in stripe phases, Phys. Rev. Lett. 89, 095701 (2002)
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Sidhartha Lal |
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Research Interests: My interests lie in exploring several different aspects of the physics of various low dimensional condensed matter systems. An example of recent work involves understanding the nature of edge state transport in an inhomogeneous quantum Hall state. This was motivated by several recent experimental observations pointing to the need for a reinvestigation of the present paradigm of edge state transport. Another recent work involved developing a model for a strongly correlated 1/4-filled electronic chain system whose phase diagram contains a quantum critical point associated with frustrating electronic correlations and competing charge orders. Our results proposed a new framework in which to revisit the phenomenology of the organic charge transfer chain systems, pointing towards the key role played by critical quantum fluctuations in pushing the dimensional crossover as well as the insulator to metal transitions observed experimentally in these systems. My current research projects include looking at spin-orbital physics in magnetic insulators, transport in graphene, competing orders in a toy model for stripe systems and consequences of charge fractionalisation in 1D systems. Recent publications: • On transport in quantum Hall systems with constrictions” S. Lal, Europhys. Lett. 80, 17003 (2007) • From frustrated insulators to correlated anisotropic metals: charge ordering and quantum criticality in coupled chain systems” S. Lal and M. S. Laad, J. Phys. (Condens. Matt.) 20, 235213 (2008) • Transport through constricted quantum Hall edge systems: beyond the quantum point contact” S.Lal, Phys. Rev. B 77, 035331 (2008)
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Fei Lin |
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Research Interests: I am interested in an exact determination of physical properties of strongly correlated many-body quantum systems. This is achieved through the most exact Quantum Monte Carlo (QMC) simulations. My current projects include calculating electrical conductivity of high-pressured liquid hydrogen system, and simulating disordered Bose-Hubbard model with and without confining potentials and Fermi-Hubbard model in an optical lattice. For hydrogen project, we have developed a method to calculate AC conductivity directly in real frequency with QMC for the high-pressured liquid hydrogen. It is also promising to extend the method to other systems with QMC simulations. For optical lattice projects, we aim at comparison with experimental results, calculating properties that can not be measured experimentally, and providing some useful parameters for experiments. My past research is related to Fullerene materials. The most famous member of the Fullerene family is the soccer-ball-shaped C60 molecule (Buckyball), which becomes superconducting upon electron doping by alkali metals. However, the electron pairing mechanism is still unknown/under debates. One goal of my past research is to check one of the proposed mechanism, i.e., purely electronic mechanism (without phonons) for superconductivity. Recent publications: [1] Fei Lin, Miguel A. Morales, Kris T. Delaney, Carlo Pierleoni, Richard M. Martin, and D. M. Ceperley, "Electrical conductivity of high-pressure liquid hydrogen by quantum Monte Carlo methods", arXiv: 0909.2248. [2] Fei Lin and Erik Sørensen, "Estimates of effective Hubbard model parameters for C20 isomers", Phys. Rev. B 78, 085435 (2008). [3] Fei Lin, Erik Sørensen, Catherine Kallin and John Berlinsky, "Strong Correlation Effects in the Fullerene C20", Phys. Rev. B 76, 033414 (2007). [4] Fei Lin, Erik Sørensen, Catherine Kallin and John Berlinsky, "Single-particle Excitation Spectra of C60 Molecules and Monolayers", Phys. Rev. B 75, 075112 (2007). [5] Fei Lin, Jurij Smakov, Erik Sørensen, Catherine Kallin and John Berlinsky, "Quantum Monte Carlo calculation of the electronic binding energy in a C60 molecule", Phys. Rev. B 71, 165436 (2005)
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Babak Seradjeh |
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• [1] C. Weeks, G. Rosenberg, B. Seradjeh, M. Franz, Nature Phys. 3, 796 (2007). • [2] G. Rosenberg, B. Seradjeh, C. Weeks, M. Franz, Phys. Rev. B 79, 205102 (2009). • [3] B. Seradjeh and M. Franz, Phys. Rev. Lett. 101, 146401 (2008). • [4] B. Seradjeh, Nucl. Phys. B 805, 182 (2008). • [5] B. Seradjeh, H. Weber, M. Franz, Phys. Rev. Lett. 101, 246404 (2008). • [6] B. Seradjeh, J. Moore, M. Franz, Phys. Rev. Lett. 103, 066402 (2009).
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Bruno Uchoa |
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My general research interest is in low dimensional correlated systems which exhibit interesting quantum critical behavior. This is the case of many layered compounds with exotic Fermi surfaces, such as high-Tc materials, transition metal dichalcogenides, tellurides and graphite. I have been involved in several different problems in graphene, the newly discovered allotropic form of carbon, which can be thought as a giant flat molecule spread as a carbon sheet of hundreds of microns wide and long, but just one atom thick. Because of the honeycomb lattice, the elementary electronic excitations in graphene are chiral massless Dirac fermions that mimic properties known to relativistic fermions in quantum electrodynamics. My current interests spam from new electronic instabilities in graphene related materials, such as superconductivity, local magnetism and spontaneous mass generation, to problems involving disorder and electron-electron interactions in the perturbative and non-perturbative regimes. Recent publications: • 1/N expansion in correlated graphene, V. N. Kotov, B. Uchoa, A. H. Castro Neto, arXiv:0903.2046 (2009) • Localized magnetic states in graphene, B. Uchoa, V. N. Kotov, N. M. R. Peres, A. H. Castro Neto, Phys. Rev. Lett. 101, 026805 (2008) • Superconducting states in pure and doped graphene, B. Uchoa, A. H. Castro Neto, Phys. Rev. Lett. 98, 146801 (2007)
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Fangfu Ye |
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Research interests: I am interested in theoretic description of various soft-matter systems. In the past years, my research focused on topics such as semi-soft elasticity and patter formation of nematic elastomers, hydrodynamics of chiral granular gas, string defects in nematics, and domain size selection in curved lipid membranes. My current interest is to use amorphous solids formed by worm-like polymer chains as a model system to investigate the elastic properties of cellular cytoskeletons. Recent publications: • Phase Diagrams of Semisoft Nematic Elastomers, Fangfu Ye and T. C. Lubensky, J. Phys. Chem. B 113, 3853-3872 (2009) • Semi-soft Nematic Elastomers and Nematics in Crossed Electric and Magnetic Fields, Fangfu Ye, Ranjan Mukhopadhyay, Olaf Stenull, and T. C. Lubensky, Phys. Rev. Lett. 98, 147801/1-4 (2007) • A Chiral Granular Gas, J.–C. Tsai, Fangfu Ye, Juan Rodriguez, J. P. Gollub, and T. C. Lubensky, Phys. Rev. Lett. 94, 214301/1-4 (2005).
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