Rebecca Flint
Contact Information
Department of PhysicsMassachusetts Institute of Technology
77 Massachusetts Avenue
Cambridge, MA 02139 USA
Office: 4-345B
Telephone: 617-253-4407
Email: lastname at mit.edu
Rebecca Flint | |
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Contact InformationDepartment of PhysicsMassachusetts Institute of Technology 77 Massachusetts Avenue Cambridge, MA 02139 USA Office: 4-345B Telephone: 617-253-4407 Email: lastname at mit.edu |
| Strong correlations give rise to emergent phenomena from spin liquids to unconventional supercon- ductivity to quantum criticality, providing both exciting possibilities and unique challenges to theorists as they sit at the intersection of kinetic and potential energy scales, where traditional, perturbative many body techniques fail. My current interests include several heavy fermion phenomena: superconductivity in the 115 materials, hidden order in URu2Si2 and chiral spin liquid physics in Pr2Ir2O7, as well as a variety of problems in frustrated magnetism and unconventional superconductivity. |
| Using a two-channel Anderson model, we develop a theory of composite pairing in the 115 family of heavy fermion superconductors that incorporates the effects of f-electron valence fluctuations. Our calculations introduce “symplectic Hubbard operators”: an extension of the slave boson Hubbard operators that preserves both spin rotation and time-reversal symmetry in a large N expansion, permitting a unified treatment of anisotropic singlet pairing and valence fluctuations. We find that the development of composite pairing in the presence of valence fluctuations manifests itself as a phase-coherent mixing of the empty and doubly occupied configurations of the mixed valent ion. This effect redistributes the f-electron charge within the unit cell. Our theory predicts a sharp superconducting shift in the nuclear quadrupole resonance frequency associated with this redistribution. We calculate the magnitude and sign of the predicted shift expected in CeCoIn5. |
| We consider the internal structure of a d-wave heavy-fermion superconducting condensate, showing that it necessarily contains two components condensed in tandem: pairs of quasiparticles on neighboring sites and composite pairs consisting of two electrons bound to a single local moment. These two components draw upon the antiferromagnetic and Kondo interactions to cooperatively enhance the superconducting transition temperature. This tandem condensate is electrostatically active, with an electric quadrupole moment predicted to lead to a superconducting shift in the nuclear quadrupole resonance frequency. |
| 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. |
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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. |