D1branes

D1-branes, or D-strings, are one-dimensional dynamical objects in type IIB string theory on which open strings can end. Their worldvolume is two-dimensional, with one temporal and one spatial dimension, so they are string-like objects in spacetime. The D1-brane carries Ramond-Ramond charge associated with the RR 2-form potential C2 and couples to it via a Chern-Simons term. Its tension is T1 = 1/(2π α' g_s). The low-energy dynamics are described by the Dirac-Born-Infeld action for the pullback of the spacetime metric, dilaton, and the B-field, together with a worldvolume U(1) gauge field whose field strength F captures the endpoints of open strings.

Under S-duality of type IIB string theory, the D1-brane is mapped to the fundamental string, so D1-branes participate in the SL(2,Z) family of (p,q)-strings, which are bound states of p fundamental strings and q D-strings. Configurations with multiple D1-branes or with worldvolume flux F can realize different (p,q) charges.

Like other D-branes, a D1 can end on higher-dimensional branes such as D3-branes; the endpoint carries charge in the worldvolume theory and has a geometrical interpretation as a magnetic or electric flux on the host brane, depending on the configuration.

In flat space, a single D1-brane preserves half of the spacetime supersymmetry, i.e., it is a BPS object. D1-branes also play a key role in nonperturbative string dynamics and in holographic contexts, where they model flux tubes and contribute to dual descriptions of gauge theories.