An outstanding question concerning the underdoped cuprate superconductors [pseudogap state] concerns the true nature of their Fermi surface which appear in STM and ARPES experiments as a set of disconnected arcs. Theoretical models have proposed two distinct possibilities:

(1) each arc is the observable part of a partially-hidden closed pocket, and

(2) each arc is open, truncated at its apparent ends.

We show that measurements of the variation of the interlayer resistance with the direction of a magnetic field parallel to the layers can qualitatively distinguish closed pockets from open arcs.

The first figure below shows the weak angular dependence of the field dependence of the interlayer magnetoresistance for the case of Fermi surface pockets.

The second figure shows the strong angular dependence of the case of arcs.

The difference between the two cases arises because the field can be oriented such that all electrons on arcs encounter a large Lorentz force and resulting magnetoresistance whereas some electrons on pockets escape the effect by moving parallel to the field.

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