In correlated materials, the metal-insulator transition is often accompanied by a change in crystal symmetry. For NdNiO3, cooling below 180 K drives a transition from a paramagnetic metal with an orthorhombic lattice to an antiferromagnetic insulator with a monoclinic lattice. Whether the link between electronic order and structural symmetry survives ultrafast optical excitation remained an open question. The work sits within a series of studies of photoinduced phases in nickelates, where electronic and structural responses are usually measured in separate experiments, making direct comparison difficult.
A thin film of NdNiO3 about 15 nm thick on an NdGaO3 substrate was used. The sample was excited by laser pulses with a photon energy of 1.55 eV and a duration of about 30 fs. Simultaneously, the linear reflectance of a probe pulse (0.95 eV) and the intensity of the second harmonic (1.9 eV), sensitive to the breaking of inversion symmetry at the surface, were recorded. The polarization dependence of the second harmonic was measured at various temperatures and time delays. A 'monoclinicity' parameter was introduced to quantify the deviation of the polarization dependence from orthorhombic symmetry.
At a high excitation energy density (2.9 mJ/cm²), reflectance drops by 2.3% over a time limited by the setup resolution (45 fs), indicating a fast transition to the metallic state. Monoclinicity changes with a delay of about 40 fs and much more slowly: even after 225 fs it does not reach the level of the high-temperature phase. At an intermediate energy density of 0.64 mJ/cm², reflectance reaches a steady metallic value, whereas the second-harmonic intensity almost fully recovers, indicating that monoclinic symmetry is preserved. The amplitude of the reflectance change saturates at densities above 0.6 mJ/cm², while the second-harmonic amplitude continues to grow linearly, and full recovery of orthorhombic symmetry is achieved only at 2.3 mJ/cm².
The work shows that a metallic state can exist without recovery of the high-temperature lattice symmetry. This means that symmetry breaking alone is not a sufficient condition for the insulating state. Measuring several order parameters simultaneously in a single experiment removes the uncertainty associated with different conditions in separate runs and opens a path to reliable identification of nonequilibrium phases in correlated materials.
Further studies could target the magnetic structure of the optically induced monoclinic metal and the search for similar states in other nickelates and correlated oxides. Time-resolved techniques such as resonant X-ray scattering could be used to directly observe the ordering of charge and spin degrees of freedom.
The results matter for the physics of strongly correlated electron systems and for the development of devices based on metal-insulator transitions, where light-controlled phase switching could be used for ultrafast operation.
Test the stability of the monoclinic metal over longer times and at different temperatures. Investigate the effect of epitaxial strain and chemical doping on the threshold for decoupling of symmetry and conductivity.
In condensed matter physics, the question remains open as to the minimal set of order parameters needed to describe the metal-insulator transition in correlated oxides. This work shows that the electronic and structural subsystems can evolve independently, which calls for a revision of simplified models with rigid coupling of orders.
This section is addressed to the author of the paper and their colleagues. Neighbouring works were found by meaning-based search across our archive, not by word overlap. We do not review: we show the field around the work.
What the work does
This study investigates an optically induced insulator-to-metal transition in a thin film of NdNiO3. By simultaneously recording reflectance and second-harmonic generation, the authors show that the metallic state appears at a lower excitation energy than the recovery of the high-temperature orthorhombic symmetry. A metastable state is found that combines metallic conductivity with the monoclinic symmetry of the insulating phase.
Where its strength is
Measuring several order parameters simultaneously in a single experiment removes the uncertainty that comes from differing conditions in separate runs, and it opens the way to reliably identifying nonequilibrium phases in correlated materials.
Where it could go next
Worth trying
Why this may matter
Decoupling the electronic and structural degrees of freedom could lead to new ways of controlling phase transitions in correlated materials, which matters for applications in optoelectronics and ultrafast electronics.
Nearby works in our archive
The archive is sparse around this work: the closest review is only 0.59, against a usual 0.64. It looks like few have drilled nearby.
The transition temperature of NdNiO3 is about 180 K, only slightly above the boiling point of liquid nitrogen (77 K), so a standard cryostat is enough to observe the effect.
The idea of a material that changes its conductivity while keeping its crystal structure recalls the concept of 'phase transitions without a change of shape' from science fiction, for example in Isaac Asimov's novel The Gods Themselves, where a substance can switch to a state with different properties without visible changes.