Recent Publications

Here is a sampling of my recent publications:

"Spin State Crossover in the Multiferroic Ca3Co2-xMnxO6", R. Flint, H.-T. Yi, P. Chandra, S.-W. Cheong and V. Kiryukin, Phys. Rev. B 81, 09402 (2010).

                     Ca3Co2-xMnxO6 (x ~ 0.96) is a multiferroic with spin-chains of alternating Co2+ and Mn4+ ions. The spin state of Co2+ remains
          unresolved due to a discrepancy between high-temperature x-ray absorption (S = 3/2) and low-temperature neutron (S = 1/2) measurements. Using
combination of magnetic modelling and crystal-field analysis, we show that the existing low-temperature data cannot be reconciled with a high spin
    scenario by invoking spin-orbit or Jahn-Teller distortions. To unify the experimental results, we propose a spin-state crossover with specific
experimental predictions.

"Effects of Nematic Fluctuations on the Elastic Properties of Iron Arsenide Superconductors," R.M. Fernandez, L.H. VanBebber, S. Bhattacharya, P. Chandra, V. Keppens, D. Mandrus, M.A. McGuire, B.C. Sales, A.S. Safena and J. Schmalian, arXiv:009011.3084, accepted to Phys. Rev. Lett.

Because the structural (TS) and the antiferromagnetic (TN) transitions track each other closely in these materials and TS > TN, it is natural to assume that the lattice effects are primary and that the antiferromagnetic order is a secondary effect. However it has been suggested in the theoretical community that spin fluctuations, related to the observed antiferromagnetic ordering at TN, will at higher temperatures lead to emergent nematic degrees of freedom that couple to the lattice. The experimental identification of these nematic fluctuations is nontrivial since their associated magnetic field fluctuations average to zero. We exploit the magnetoelastic coupling in these materials, demonstrating that the changes in the elastic properties of the FeAs systems, as seen in our resonant ultrasound spectroscopy, can be naturally understood in terms of fluctuations of the emerging nematic degrees of freedom. Both the softening of the lattice in the normal, tetragonal phase as well as its hardening in the superconducting phase are consistently described by our model. Our results confirm the view that structural order is induced by magnetic fluctuations.
"Multiferroic BiFeO3-BiMnO3 Nanoscale Checkerboard from First Principles," L. Palova, P. Chandra and K.M. Rabe, Phys. Rev. B 872, 075432 (2010).

Room-temperature materials with coexisting large polarization, large magnetization and strong magnotoelectric coupling are desirable for next-generation electronic devices that include ultra-high density memories; however they are not easily found in Nature. Here we present a theoretical study of an atomic-scale checkerboard that has desirable properties distinct from those of either of its two bulk constituents. Performing computational searches over a variety of heterostructure geometries and structure types, we find that characteristics of this nanocheckerboard are crucially dependent on its cation ordering. We discuss its possible realization in the laboratory where dramatic progress has been made in the synthesis of artificially structured oxides that include nanopillar patternings. More generally this study of a nanocheckerboard is a proof-of-principle example of "designed" materials that retain and combine the desired properties of each parent compound. We look forward to exploring properties of similar geometries with different components and length-scales in the future.

"Magnetization, Maxwell's Relations and the Local Physics of Th1-xUxRu2Ru2Si2," A. Toth, P. Chandra, P. Coleman, G. Kotliar and H. Amitsuka, arXiv:1008.4796, submitted to Phys. Rev. B.

               The dilute Kondo compound Th1-xUxRu2Si2 displays non-Fermi liquid behavior but no zero-point entropy; it thus appears to elude
   description by known single-ion models. It may also provide a clue to the underlying local degrees of freedom in its dense counterpart, URu2Si2.
   Here we use high-resolution magnetization studies to crosscheck the thermodynamic consistency of previous experiments. Measurement of the
   field-dependence of the temperature scale, TF(H), associated with Fermi liquid behavior probes the nature of the underlying impurity fixed point.
   We find that TF(H) grows linearly with applied field, in contrast to the quadratic form expected for the two-channel Kondo model. We use a scaling
   argument to show that the observed behavior of TF(H) is consistent with the absence of zero-point entropy, suggesting novel impurity behavior in
   this material. More generally, we suggest the magnetization as a probe of single-ion physics and make predictions for its behavior in other actinide
   compounds.

Recent Publications

"Spin State Crossover in the Multiferroic Ca3Co2-xMnxO6", R. Flint, H.-T. Yi, P. Chandra, S.-W. Cheong and V. Kiryukin, Phys. Rev. B 81, 09402 (2010).

"Effects of Nematic Fluctuations on the Elastic Properties of Iron Arsenide Superconductors," R.M. Fernandez, L.H. VanBebber, S. Bhattacharya, P. Chandra, V. Keppens, D. Mandrus, M.A. McGuire, B.C. Sales, A.S. Safena and J. Schmalian, arXiv:009011.3084, accepted to Phys. Rev. Lett.