30 years of theory versus a second of experiment.
Scientists from the University of California, Los Angeles, have experimentally detected for the first time a rare electronic state that existed for decades only in theoretical models—the liquid phase of a charge density wave. Charge density waves represent a form of electron self-organization in solids, in which particles align themselves into a periodic spatial structure. Such electron configurations can radically alter the properties of materials and, in some cases, are associated with the emergence of superconductivity and other unusual physical effects.
Physicists have long suspected that, at certain temperatures, such ordering would be disrupted through a phase transition similar to the melting of a solid. In other words, the charge density wave would transition from a solid to a liquid state. However, in practice, this process has never been directly observed. Theoretical descriptions existed as early as the early 1990s, but some in the scientific community believed such a phase was impossible due to the rigid bond between the electron structures and the crystal's atomic lattice.
The UCLA team obtained the first direct experimental evidence for the existence of such a state using the layered material 1T-TaS₂, a transition metal dichalcogenide.
Previous scientific papers have mentioned the existence of an intermediate state between solid and liquid—the so-called hexatic phase, in which the spatial structure loses its rigid lattice but retains partial orientational order. The problem was that with conventional heating, the required temperature range was unattainable: the crystalline structure of the material began to deteriorate before the electron transition itself could be observed.
A solution was found using ultrashort laser pulses . The researchers exposed the sample to femtosecond flashes and recorded changes in the electron distribution during those brief moments when the atomic lattice was still stable. This was accomplished using ultrafast electron diffraction, a technique that allows tracking the spatial organization of electron states in near real time.
At the initial stage of the experiment, the particles began to lose their rigid attachment to the structure's nodes, while maintaining the overall direction of order. This pattern corresponded to the hexatic phase described in earlier studies. With further exposure, this orientation also disappeared: the order was completely destroyed, and ring scattering appeared in the diffraction pattern—a characteristic sign of the liquid state of an electron system.
This signal became the first direct experimental confirmation of the existence of a liquid phase charge density wave. Until then, such a state remained the subject of theoretical debate and scientific hypotheses.
The discovery has implications beyond fundamental physics. Charge density waves are closely related to the behavior of electrons in correlated systems , where particles actively influence each other. Such effects play a key role in the physics of high-temperature superconductivity, and the researchers suggest that liquid CDW states may be a key element in the phase diagrams of such materials.
The methods used open up the possibility of studying other difficult-to-observe electronic phases in various substances, not just 1T-TaS₂. The team is currently conducting additional experiments to understand how impurities affect the liquid state of the charge density wave. One of the scenarios being considered involves a transition to a glassy, amorphous form at a certain impurity concentration, but this process remains to be confirmed experimentally.
Scientists from the University of California, Los Angeles, have experimentally detected for the first time a rare electronic state that existed for decades only in theoretical models—the liquid phase of a charge density wave. Charge density waves represent a form of electron self-organization in solids, in which particles align themselves into a periodic spatial structure. Such electron configurations can radically alter the properties of materials and, in some cases, are associated with the emergence of superconductivity and other unusual physical effects.
Physicists have long suspected that, at certain temperatures, such ordering would be disrupted through a phase transition similar to the melting of a solid. In other words, the charge density wave would transition from a solid to a liquid state. However, in practice, this process has never been directly observed. Theoretical descriptions existed as early as the early 1990s, but some in the scientific community believed such a phase was impossible due to the rigid bond between the electron structures and the crystal's atomic lattice.
The UCLA team obtained the first direct experimental evidence for the existence of such a state using the layered material 1T-TaS₂, a transition metal dichalcogenide.
Previous scientific papers have mentioned the existence of an intermediate state between solid and liquid—the so-called hexatic phase, in which the spatial structure loses its rigid lattice but retains partial orientational order. The problem was that with conventional heating, the required temperature range was unattainable: the crystalline structure of the material began to deteriorate before the electron transition itself could be observed.
A solution was found using ultrashort laser pulses . The researchers exposed the sample to femtosecond flashes and recorded changes in the electron distribution during those brief moments when the atomic lattice was still stable. This was accomplished using ultrafast electron diffraction, a technique that allows tracking the spatial organization of electron states in near real time.
At the initial stage of the experiment, the particles began to lose their rigid attachment to the structure's nodes, while maintaining the overall direction of order. This pattern corresponded to the hexatic phase described in earlier studies. With further exposure, this orientation also disappeared: the order was completely destroyed, and ring scattering appeared in the diffraction pattern—a characteristic sign of the liquid state of an electron system.
This signal became the first direct experimental confirmation of the existence of a liquid phase charge density wave. Until then, such a state remained the subject of theoretical debate and scientific hypotheses.
The discovery has implications beyond fundamental physics. Charge density waves are closely related to the behavior of electrons in correlated systems , where particles actively influence each other. Such effects play a key role in the physics of high-temperature superconductivity, and the researchers suggest that liquid CDW states may be a key element in the phase diagrams of such materials.
The methods used open up the possibility of studying other difficult-to-observe electronic phases in various substances, not just 1T-TaS₂. The team is currently conducting additional experiments to understand how impurities affect the liquid state of the charge density wave. One of the scenarios being considered involves a transition to a glassy, amorphous form at a certain impurity concentration, but this process remains to be confirmed experimentally.
