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Thomas A. Kaplan, Professor Emeritus

Thomas KaplanE-mail:
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PhD: 1954, University of Pennsylvania, physics
BS: 1948, University of Pennsylvania, mechanical engineering



Research Focus

My interests have been mainly in understanding magnetism on a microscopic scale. This includes theories of complex magnetic ordering, electronic structure theory behind models of such ordering (and disordering), e.g. Heisenberg models, etc. Also, recently, I have been interested in the connection between such ordering and ferroelectricity--the field of multiferroics.

A recent work, top reference below, on the magnetism of the frustrated Heisenberg model with added biquadratic exchange, was motivated in part by a puzzle in connection with the magnetism observed in a series of multiferroic manganites. This paper obtains the exact ground state phase diagram that accounts for the complex magnetic orderings that occur in the materials RMnO3, R = the rare earths from La to Ho (thus solving the puzzle). The work considers the 2-dimensional lattice appropriate to these materials. The phase diagram is seen in Fig. 1, which considers the actual orthorhombic structure of these materials. ( a and γ measure the strength of the biquadratic terms and the frustrating Heisenberg terms respectively.) We also considered the (classic and much studied) square lattice. The phase diagram is shown in Fig. 2. We found a new type of ordering, which we call a conical vortex lattice (conical VL); it is seen to exist for negative a, 0<<0.5. Fig. 3 illustrates the spin structure.

fgure 1
Figure 1
figure 2
Figure 2

Figure 3

Selected Publications

Frustrated Classical Heisenberg and XY Models in Two Dimensions with Nearest-Neighbor Biquadratic Exchange: Exact Solution for the Ground-State Phase Diagram, L.X. Hayden, T. A. K. and S. D. Mahanti, Phys. Rev. Lett. 105, 047203 (2010)

Frustrated classical Heisenberg model in one dimension with nearest-neighbor biquadratic exchange: Exact solution for the ground-state Phase diagram, T. A. K.. Phys. Rev. B 80, 012407 (2009)

Scanning-probe spectroscopy of semiconductor donor molecules, I. Kuljanishvili, C. Kayis, J. F. Harrison, C. Piermarocchi, T. A.K., S. H. Tessmer, L. N. Pfeiffer, and K. W. West, Nature Physics 4, 227-233 (2008)

Thermally or Magnetically Induced Polarization Reversal in the Multiferroic CoCr2O4, Y. J. Choi, J. Okamoto, K. S. Chao, H. J. Lin, C. T. Chen, M. van Veenendahl, T. A. K and S-W. Cheong, Phys. Rev. Lett 102, 067601 (2009).