Multiferroic BiFeO3-BiMnO3 Nanocheckerboard From First Principles

We present a first principles study of an unusual heterostructure, an atomic-scale checkerboard of BiFeO3-BiMnO3, and compare its properties to the two bulk constituent materials, BiFeO3 and BiMnO3. The "nanocheckerboard" is found to have a multiferroic ground state with the desired properties of each constituent: polar and ferrimagnetic due to BiFeO3 and BiMnO3, respectively. The effect of B-site cation ordering on magnetic ordering in the BiFeO3-BiMnO3 system is studied. The checkerboard geometry is seen to give rise to a a novel magnetostructural effect that is neither present in the bulk constituent materials, nor in the layered BiFeO3-BiMnO3 superlattice.Comment: 15 pages, 14 figure

The Casimir Effect from a Condensed Matter Perspective

The Casimir effect, a key observable realization of vacuum fluctuations, is usually taught in graduate courses on quantum field theory. The growing importance of Casimir forces in microelectromechanical systems motivates this subject as a topic for graduate many-body physics courses. To this end, we revisit the Casimir effect using methods common in condensed matter physics. We recover previously derived results and explore the implications of the analogies implicit in this treatment.Comment: Accepted for Publication in American Journal of Physic

Quantum critical scaling and the Gross-Neveu model in 2+1 dimensions

The quantum critical behavior of the 2+1 dimensional Gross--Neveu model in the vicinity of its zero temperature critical point is considered. The model is known to be renormalisable in the large $N$ limit, which offers the possibility to obtain expressions for various thermodynamic functions in closed form. We have used the concept of finite--size scaling to extract information about the leading temperature behavior of the free energy and the mass term, defined by the fermionic condensate and determined the crossover lines in the coupling (\g) -- temperature ($T$) plane. These are given by T\sim|\g-\g_c|, where \g_c denotes the critical coupling at zero temperature. According to our analysis no spontaneous symmetry breaking survives at finite temperature. We have found that the leading temperature behavior of the fermionic condensate is proportional to the temperature with the critical amplitude $\frac{\sqrt{5}}3\pi$. The scaling function of the singular part of the free energy is found to exhibit a maximum at $\frac{\ln2}{2\pi}$ corresponding to one of the crossover lines. The critical amplitude of the singular part of the free energy is given by the universal number $\frac13[\frac1{2\pi}\zeta(3)-\mathrm{Cl}_2(\frac{\pi}3)]=-0.274543...$, where $\zeta(z)$ and $\mathrm{Cl}_2(z)$ are the Riemann zeta and Clausen's functions, respectively. Interpreted in terms the thermodynamic Casimir effect, this result implies an attractive Casimir "force". This study is expected to be useful in shedding light on a broader class of four fermionic models.Comment: 6 pages, 3 figure