'Inverse' melting of a vortex lattice.
Inverse melting is the process in which a crystal reversibly transforms into a liquid or amorphous phase when its temperature is decreased. Such a process is considered to be very rare, and the search for it is often hampered by the formation of non-equilibrium states or intermediate phases. Here we report the discovery of first-order inverse melting of the lattice formed by magnetic flux lines in a high-temperature superconductor. At low temperatures, disorder in the material pins the vortices, preventing the observation of their equilibrium properties and therefore the determination of whether a phase transition occurs. But by using a technique to 'dither' the vortices, we were able to equilibrate the lattice, which enabled us to obtain direct thermodynamic evidence of inverse melting of the ordered lattice into a disordered vortex phase as the temperature is decreased. The ordered lattice has larger entropy than the low-temperature disordered phase. The mechanism of the first-order phase transition changes gradually from thermally induced melting at high temperatures to a disorder-induced transition at low temperatures.
Avraham
N
N
Department of Condensed Matter Physics, The Weizmann Institute of Science, Rehovot 76100, Israel. nurit.avraham@weizmann.ac.il
Khaykovich
B
B
Myasoedov
Y
Y
Rappaport
M
M
Shtrikman
H
H
Feldman
D E
DE
Tamegai
T
T
Kes
P H
PH
Li
M
M
Konczykowski
M
M
van der Beek
K
K
Zeldov
E
E