Executive Summary Final - the Center for Nanoscale Science - an ...

IRG 1: Strain-enabled Multiferroics
concept based on BiFeO3 has been demonstrated. The reversal of ferroelectric polarization also
reverses the plane of easy magnetization, which can then be used in an exchange bias geometry
to switch magnetic domains of a magnetic film on the surface.
FUTURE DIRECTIONS: In addition to the specific future plans outlined above, here are the
broad outlines of what the IRG will be investigation.
Hidden Symmetries and Roto Properties: The structure of materials is described by a combination
of rotations, rotation-inversions and translational symmetries. By recognizing the reversal
of static structural rotations between clockwise and counterclockwise directions as a distinct
symmetry operation, , (see Figure 2) Gopalan and Litvin have shown that there are many
more structural symmetries than are currently recognized in right- or left-handed handed helices,
spirals, and in antidistorted structures composed equally of rotations of both handedness. For example,
although a helix or spiral cannot possess conventional mirror or inversion symmetries,
they can possess them in combination with the rotation reversal symmetry. Similarly, many antidistorted
perovskites possess twice the number of symmetry elements as conventionally identified.
These new symmetries, referred to as “roto” symmetries, predict new forms for roto properties
that relate to static rotations, such as rotoelectricity, piezorotation, and rotomagnetism. These
symmetry insights enable a symmetry-based search for new phenomena, such as multiferroicity
involving a coupling of spins, electric polarization and static rotations. The Spaldin group has
predicted piezorotation in strained LaAlO3 from first principles. Fennie has predicted Ca3Mn2O7
to couple polarization and magnetism through octahedral rotations (arXiv:1007.1003v1
[cond-mat.mtrl-sci]). These new properties will
be experimentally studied. Through seed funding,
Trolier-Mckinstry and Gopalan are initiating
experiments to measure linear rotoelectric
effect in a number of oxides.
Figure 2: a. Gopalan has introduced a new symmetry
operation, , to reverse static axial vectors, and complete
the missing entry (orange box). 1' is time reversal
and is spatial inversion symmetries. If the colors of
a structural motif, as in b, denote static rotations, +Φ
and -Φ, the rotation reversal symmetry operation
switches them to -Φ and +Φ, respectively, as in c.
Ferroic Coupling and the Role of Dimensionality
in Layered Complex Oxides: This
seed effort is based on our preliminary (unpublished)
experimental discovery of the first ferroelectric
material with finite n in a
Ruddlesden-Popper (RP) series. Preliminary
results on strained Srn+1TinO3n+1 are shown at
right, where the well-known perovskite SrTiO3
is the n = ∞ member of this series. Strained
SrTiO3 can be driven ferroelectric with a modest
biaxial strain, yet as conjectured by Fennie
and Schlom, the n = 1 member of the series remains
paraelectric for the same strain state. The
manner in which ferroelectricity emerges as the
number of perovskite blocks n increases is a
fundamentally interesting challenge in its own