  We separate double stranded lambda phage DNA by applying a fixed force at a
constant temperature ranging from 15C to 50C, and measure the minimum force
required to separate the two strands, providing the first experimental
determination of the phase boundary between single stranded DNA and double
stranded DNA in the temperature- force plane. The measurements also offer
information on the free energy of dsDNA at temperatures where dsDNA does not
thermally denature in the absence of force. While parts of the phase diagram
can be explained using existing models and free energy parameters, others
deviate significantly. Possible reasons for the deviations between theory and
experiment are considered.
