Disorder promotes ferromagnetism: rounding of the quantum phase transition in Sr(1-x)Ca(x)RuO3.

The subtle interplay of randomness and quantum fluctuations at low temperatures gives rise to a plethora of unconventional phenomena in systems ranging from quantum magnets and correlated electron materials to ultracold atomic gases. Particularly strong disorder effects have been predicted to occur at zero-temperature quantum phase transitions. Here, we demonstrate that the composition-driven ferromagnetic-to-paramagnetic quantum phase transition in Sr(1-x)Ca(x)RuO3 is completely destroyed by the disorder introduced via the different ionic radii of the randomly distributed Sr and Ca ions. Using a magneto-optical technique, we map the magnetic phase diagram in the composition-temperature space. We find that the ferromagnetic phase is significantly extended by the disorder and develops a pronounced tail over a broad range of the composition x. These findings are explained by a microscopic model of smeared quantum phase transitions in itinerant magnets. Moreover, our theoretical study implies that correlated disorder is even more powerful in promoting ferromagnetism than random disorder.

Scattering Form Factors for Russian Doll Aerosol Droplet Models.

Our molecular dynamics (MD) simulations have shown that nanodroplets containing water and nonane are nonspherical and strongly phase-separated. This "Russian doll" structure may be simply but realistically modeled as a spherical nonane lens that partially wets a spherical water droplet. We call this the lens-on-sphere model. Here, we present an analytical calculation of the particle form factor, P(q), needed to analyze the intensity curves for small-angle neutron and X-ray scattering by aerosol particles with this type of structure. For this model, the particle form factor must be evaluated by numerical integration. In addition, an exact formulation of the particle form factor is presented for cylindrically symmetric droplets with otherwise arbitrary position-dependent scattering length density. This result enables direct calculation of P(q) from numerical MD simulation data. Results using both formulations are compared, and excellent agreement is found between them. Analytical results are also presented for two limiting cases of the general result: the sphere-on-sphere model and the sphere-in-sphere (SIS) model, corresponding to contact angles of 180° and 0°, respectively. Finally, a generalization of the SIS model to ellipsoidal droplets is given.