  The propensity of some materials and multilayers to have a magnetic field
dependent resistance, called magnetoresistance, has found commercial
applications such as giant magnetoresistance harddisk read heads. But
magnetoresistance can also be a powerful probe of electronic and magnetic
interactions in matter. For example, magnetoresistance can be used to analyze
multiband conductivity, conduction inhomogeneity, localized magnetic moments,
and (fractional) Landau level structure. For materials with strong spin-orbit
interaction, magnetoresistance can be used as a probe for weak antilocalization
or a nontrivial Berry phase, such as in topological insulator surface states.
For the three dimensional topological insulators a large and linear
magnetoresistance is often used as indication for underlying non-trivial
topology, although the origin of this effect has not yet been established.
Here, we observe a large magnetoresistance in the conducting bulk state of
Bi$_2$Te$_3$. We show that this type of large magnetoresistance is due to the
competition between helical spin-momentum locking (i.e. spin rotates with
momentum direction) and the unidirectional spin alignment by an applied
magnetic field. Warping effects are found to provide the (quasi) linear
dependence on magnetic field. We provide a quantitative model for the helicity
breaking induced magnetoresistance that can be applied to a vast range of
materials, surfaces or interfaces with weak to strong spin-orbit interactions,
such as the contemporary oxide interfaces, bulk Rashba systems, and topological
insulator surface states.
