Rebecca Flint
	Contact Information Department of Physics Massachusetts Institute of Technology 77 Massachusetts Avenue Cambridge, MA 02139 USA Office: 4-345B Telephone: 617-253-4407 Email: lastname at mit.edu

Research Interests

Publications

• Chiral RKKY interaction in Pr₂Ir₂O₇
  Rebecca Flint and T. Senthil
  Phys. Rev. B 87, 125147 (2013)
  (previous version available as arXiv:1301.0815

  Motivated by the potential chiral spin liquid in the metallic spin ice Pr2Ir2O7, we consider how such a chiral state might be selected from the spin ice manifold. We propose that chiral fluctuations of the conducting Ir moments promote ferro-chiral couplings between the local Pr moments, as a chiral analogue of the magnetic RKKY effect. Pr2Ir2O7 provides an ideal setting to explore such a chiral RKKY effect, given the inherent chirality of the spin-ice manifold. We use a slave-rotor calculation on the pyrochlore lattice to estimate the sign and magnitude of the chiral coupling, and find it can easily explain the 1.5K transition to a ferro-chiral state. * Editor's Suggestion

• Hastatic order in URu₂Si₂
  Premala Chandra, Piers Coleman and Rebecca Flint
  Nature 493, 621 (2013)
  (previous version available as arXiv:1207.4828)

  The development of collective long-range order via phase transitions occurs by the spontaneous breaking of fundamental symmetries. Magnetism is a consequence of broken time-reversal symmetry while superfluidity results from broken gauge invariance. The broken symmetry that develops below 17.5K in the heavy fermion compound URu2Si2 has long eluded such identification. Here we show that the recent observation of Ising quasiparticles in URu2Si2 results from a spinor order parameter that breaks double time-reversal symmetry, mixing states of integer and half-integer spin. Such hastatic order hybridizes conduction electrons with Ising 5f2 states of the uranium atoms to produce Ising quasiparticles; it accounts for the large entropy of condensation and the magnetic anomaly observed in torque magnetometry. Hastatic order predicts a tiny transverse moment in the conduction sea, a collosal Ising anisotropy in the nonlinear susceptibility anomaly and a resonant energy-dependent nematicity in the tunneling density of states.

• Basal-Plane Nonlinear susceptibility: A Direct Probe of the Single-Ion Physics in URu₂Si₂
  Rebecca Flint, Premala Chandra and Piers Coleman
  Phys. Rev. B 86, 155155 (2012)
  (also available as arXiv:1207.2433)

  The microscopic nature of the hidden order state in URu2Si2 is dependent on the low-energy configurations of the uranium ions, and there is currently no consensus on whether it is predominantly 5f2 or 5f3. Here we show that measurement of the basal-plane nonlinear susceptibility can resolve this issue; its sign at low-temperatures is a distinguishing factor. We calculate the linear and nonlinear susceptibilities for specific 5f2 and 5f3 crystal-field schemes that are consistent with current experiment. Because of its dual magnetic and orbital character, a Γ5 magnetic non-Kramers doublet ground-state of the U ion can be identified by χ1c(T) ∝ χ3ab(T) where we have determined the constant of proportionality for URu2Si2.

• Origin of the Large Anisotropy in the χ₃ Anomaly in URu₂Si₂
  Premala Chandra, Piers Coleman and Rebecca Flint
  Conference Proceedings for Invited Talk at Materials and Mechanisms of Superconductivity in Washington, D.C. 2012
  arXiv:1210.6425 (2012)

  Motivated by recent quantum oscillations experiments on URu2Si2, we discuss the microscopic origin of the large anisotropy observed many years ago in the anomaly of the nonlinear susceptibility in this same material. We show that the magnitude of this anomaly emerges naturally from hastatic order, a proposal for hidden order that is a two-component spinor arising from the hybridization of a non-Kramers Γ5 doublet with Kramers conduction electrons. A prediction is made for the angular anisotropy of the nonlinear susceptibility anomaly as a test of this proposed order parameter for URu2Si2.

• The symplectic-N t-J model and s_± superconductors
  Rebecca Flint and Piers Coleman
  Phys. Rev. B 86, 184508 (2012)
  also available at arXiv:1204.2302
  

  The possible discovery of s± superconducting gaps in the moderately correlated iron-based superconductors has raised the question of how to properly treat s± gaps in strongly correlated superconductors. Unlike the d-wave cuprates, the Coulomb repulsion does not vanish by symmetry, and a careful treatment is essential. Thus far, only the weak correlation approaches have included this Coulomb pseudopotential, so here we introduce a symplectic N treatment of the t-J model that incorporates the strong Coulomb repulsion through the complete elimination of on-site pairing. Through a proper extension of time-reversal symmetry to the large N limit, symplectic-N is the first superconducting large N solution of the t-J model. For d-wave superconductors, the previous uncontrolled mean field solutions are reproduced, while for s± superconductors, the SU(2) constraint enforcing single occupancy acts as a pair chemical potential adjusting the location of the gap nodes. This adjustment can capture the wide variety of gaps proposed for the iron based superconductors: line and point nodes, as well as two different, but related full gaps on different Fermi surfaces.

• Composite pairing in a mixed-valent two-channel Anderson model
  Rebecca Flint, Andriy Nevidomskyy and Piers Coleman
  Phys. Rev. B 84, 064514 (2011)

  (also available as arxiv 1103.1550)
  
  

  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.

  Tandem Pairing in Heavy Fermion Superconductors
  Rebecca Flint and Piers Coleman
  Phys. Rev. Lett. 105, 246404 (2010)

  (also available as arXiv:0912.2339)
  
  

  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.

• Heavy electrons and the symplectic symmetry of spin
  Rebecca Flint, Maxim Dzero, and Piers Coleman
  Nature Physics 4, 643 - 648 (2008)

  (also available as arxiv 0710.1126 with supplementary material at arxiv 0710.1128).
  
  

  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.

• Symplectic N and time reversal in frustrated magnetism
  Rebecca Flint and Piers Coleman
  Physical Review B 79, 014424(2009)

  (also available as arxiv 0810.5144).
  
  

  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.

• Magnetization and Spin State Crossover in the Multiferroic Ca₃Co_2-xMn_xO₆
  Rebecca Flint, Hee-Taek Yi, Premala Chandra, Sang-Wook Cheong, and Valery Kiryukhin
  Phys. Rev. B 81, 092402 (2010)

  (also available as arXiv 0909.4773).
  
  

  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.

My PhD Thesis