The BBC News article Physicists observe 'negative mass' talks about a recent observation of negative effective mass in a BEC gas; in its description of the experiment the article writes
To create the conditions for negative mass, the researchers used lasers to trap the rubidium atoms and to kick them back and forth, changing the way they spin.
Is the 'spin' mentioned in that sentence the same thing as the spin generally known from particles?
The short answer to this question is that the "spin" referred to is quantum spin.
A longer answer involves describing the superfluid Bose-Einstein Condensate experimental technique. One technical paper (from 2011) describing a form of the method is Spin-Orbit-Coupled-Bose-Einstein Condensates.
The relevant aspect to your question is that Rubidium atoms (in a Bose-Einstein Condensate) have two quantum spin states selected (by an experimental technique), say |up> and |down>. The momentum of the components is determined by the particle's mass and by a "spin-orbit coupling". This generates a "pseudo-momentum" and associated "effective mass" - in short the component's (pseudo-)momentum is determined not only by its mass and velocity but also by its spin state. This coupling strength is controllable by lasers, so that pseudo-momentum can vary by manipulating laser strengths.
An added complication is that the behaviour of the BEC is described not directly by the Schrodinger equation, but by a non-linear approximation called the Gross-Pitaevksii Equation.
This non-linear equation can have unusual solutions like solitons.
Overall a synthetic quantum environment has been created (in the superfluid) in which one can apparently simulate: negative mass, Galilean non-covariance, and other unusual dynamical pseudo-properties.
