Spontaneous formation of stripes in layers of quantum dipoles

Stripes are apparently ubiquitous in nature. They have been observed in a variety of quasi-two-dimensional or layered materials, where they exist as periodic modulations of the electron density in the two-dimensional layers within the material. Such stripes have even been linked with high temperature superconductivity. However, despite their ubiquity, their origins and behaviour are still under debate. Indeed, a central question is whether stripes are driven by distortions of the underlying crystal lattice or by electron-electron repulsion.

Now, Dr Meera Parish, from the London Centre for Nanotechnology, and Dr Francesca Marchetti have been able to gain insight into the problem by investigating a simpler analogue of these electron systems. They consider quantum dipoles in a single two-dimensional layer, without any underlying lattice structure, and they find that modulations in the dipole density (i.e. stripes) can spontaneously form for sufficiently strong dipole-dipole interactions.

In principle, this quantum dipolar system can be realized experimentally using polar molecules, such as KRb or LiCs, that have been cooled down to ultralow temperatures. These polar molecules can be confined to two dimensions using optical trapping techniques, and their dipole-dipole interactions can be controlled by aligning the dipole moments with an external electric field. While there are other theoretical works that have also investigated stripes in the dipolar system, Dr Parish and Dr Marchetti’s particular theoretical approach is able to capture strong correlations between the dipoles. Crucially, they have shown that the stripe phase exists even when the dipoles are aligned perpendicular to the layer and the dipole-dipole interaction is isotropic, thus demonstrating that stripes can form even when the rotational symmetry isn’t explicitly broken to begin with.

A greater understanding of stripe phases could ultimately allow us to engineer them for technological applications such as information storage, e.g., binary information could be encoded in the stripe orientation.

Journal link:  Density instabilities in a two-dimensional dipolar Fermi gas, Physical Review Letters, 108, 145304 (2012)


Figures:  (left) Dipoles are confined in a two-dimensional layer and are all aligned at an angle θ using an electric field.  (right) For small θ and sufficiently strong dipole-dipole interactions, the dipoles arrange themselves to form a density modulation or stripe. This occurs even in the isotropic case (θ=0), where the system must spontaneously break rotational symmetry to choose a direction for the stripe.