Rebecca Flint | |
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Contact InformationDepartment of Physics and AstronomyRutgers, The State University of New Jersey 136 Freylinghuysen Rd Piscataway, NJ 08854 USA Office: Serin E281 Telephone: (732)-445-5500 x4665 Email: lastname at physics.rutgers.edu |
| I am a theorist working on strongly correlated electrons, which includes phenomena ranging from heavy fermion superconductors to frustrated magnetism to quantum criticality. Currently, I use large N theories to solve low-energy effective models motivated by real materials. |
| Here, we introduce a new class of large-N expansion that uses symplectic symmetry to protect the odd time-reversal parity of spin and sustain Cooper pairs as well-defined singlets. We show that when a lattice of magnetic ions exchange spin with their metallic environment in two distinct symmetry channels, they can simultaneously satisfy both channels by forming a condensate of composite pairs between local moments and electrons. We then discuss the application of this two channel Kondo model to the heavy fermion superconductors, PuCoGa5 and NpPd5Al2. The inclusion of spin-orbit coupling and the crystal fields predicts a g-wave superconducting order parameter. |
| In this paper, we develop a new large N treatment of the Heisenberg model based on symplectic-N, represent the spins by Schwinger bosons, which allows us study the boundaries between short-range and long-range order. This limit treats ferromagnetic and antiferromagnetic correlations simultaneously, exacting an energy cost for frustrating antiferromagnetic bonds. As an example, we treated the two dimensional J1-J2 model, where the symplectic-N phase diagram improves over previous large N treatments both at zero and finite temperatures. |
| Ca3Co2-xMnxO6(x ~ 0.96) is a multiferroic with spin-chains of alternating Co2+ and Mn4+ ions. The spin state of Co2+ remains unresolved, as there is a discrepancy between high temperature X-ray absorption (S= 3/2) and low temperature neutron (S= 1/2) measurements. Here we study the high-field magnetization using magnetic modeling and confirm the small Co moment. With crystal-field analysis, we show that neither spin orbit coupling nor Jahn-Teller distortions yield a small effective moment with large anisotropy at low temperatures within the high spin (S = 3/2) scenario, while the low spin (S=1/2) can explain both the small moment and large anisotropy. In order to unify the experimental results, we propose a spin-state crossover, and make a number of specific predictions for experiment. |
| In this paper, we extend the symplectic-N treatment of heavy fermion superconductors to the Ce 115 family by treating the two channel Kondo-Heisenberg model. The crystal fields, combined with magnetism, lead to a d-wave composite pair order parameter, which interacts favorably with the d-wave magnetic pairing order parameter, leading a hybrid pairing mechanism with an increased transition temperature. We propose that the two superconducting domes in CeIrIn5 arise by tuning the pairing mechanism from mainly magnetic to mainly composite. |
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Strongly correlated electrons provide a unique challenge to theorists as they sit at the intersection of the kinetic and potential energy scales, where traditional, perturbative many body techniques fail. To make progress, we must develop non-perturbative methods. One method that has had some success here is large N theory, which generalizes the number of components of the electron spin from 2 to N, providing an artificial perturbation expansion about a strongly correlated state which, if chosen properly, captures the essential physics. Large N has been heavily used in both the Kondo lattice and in frustrated magnetism, where SU(2N) is the traditional generalization of the electron spin group, SU(2). In choosing the large N group, we chose which symmetries to preserve and which to discard. Unfortunately, SU(2N) inadvertently loses the time inversion and charge conjugation properties of SU(2); while some generators invert under time reversal like spins, $\vec{S} \rightarrow -\vec{S}$, and remain neutral under charge conjugation, the others behave more like electric dipoles: neutral under time reversal and flipped by charge conjugation. To treat phenomena like frustrated magnetism and superconductivity, which relies on the formation of Cooper pairs, we must restrict ourselves to the subgroup of spin-like generators, SP(2N), a large N limit we call symplectic-N. This limit differs from the SP(2N) limit introduced by Sachdev and Read, which breaks the SU(2N) symmetry of the Hamiltonian down to SP(2N) in that the interaction Hamiltonian is constricted solely from symplectic spins. Symplectic-N has been successfully applied to frustrated magnetism, where it treats ferromagnetic and antiferromagnetic correlations simultaneously, and to the two channel Kondo model, where it treats the Kondo effect and superconductivity simultaneously. We are currently working to develop symplectic-N Hubbard operators to treat the t-J and Anderson models. |