Hidden states of matter

A hidden state of matter is a state of matter which cannot be reached under ergodic conditions, and is therefore distinct from known thermodynamic phases of the material.,^[1][2] Examples exist in condensed matter systems, and are typically reached by the non-ergodic conditions created through laser photo excitation.^[3][4] Short-lived hidden states of matter have also been reported in crystals using lasers. Recently a persistent hidden state was discovered in a crystal of Tantalum(IV) sulfide (TaS₂), where the state is stable at low temperatures.^[2] A hidden state of matter is not to be confused with hidden order, which exists in equilibrium, but is not immediately apparent or easily observed.

Using ultrashort laser pulses impinging on solid state matter,^[3] the system may be knocked out of equilibrium so that not only are the individual subsystems out of equilibrium with each other but also internally. Under such conditions, new states of matter may be created which are not otherwise reachable under equilibrium, ergodic system evolution. Such states are usually unstable and decay very rapidly, typically in nanoseconds or less.^[4] The difficulty is in distinguishing a genuine hidden state from one which is simply out of thermal equilibrium.^[5]

Probably the first instance of a photoinduced state is described for the organic molecular compound TTF-CA, which turns from neutral to ionic species as a result of excitation by laser pulses.^[4][6][7] However, a similar transformation is also possible by the application of pressure, so strictly speaking the photoinduced transition is not to a hidden state under the definition given in the introductory paragraph. A few further examples are given in ref.^[4] Photoexcitation has been shown to produce persistent states in vanadates^[8][9] and manganite materials,^[10] leading to filamentary paths of a modified charge ordered phase which is sustained by a passing current. Transient superconductivity was also reported in cuprates.^[11][12]

A photoexcited transition to an H state

A hypothetical schematic diagram for the transition to an H state by photo excitation is shown in the Figure (After ^[4]). An absorbed photon causes an electron from the ground state G to an excited state E (red arrow). State E rapidly relaxes via Frank-Condon relaxation to an intermediate locally reordered state I. Through interactions with others of its kind, this state collectively orders to form a macroscopically ordered metastable state H, further lowering its energy as a result. The new state has a broken symmetry with respect to the G or E state, and may also involve further relaxation compared to the I state. The barrier E_B prevents state H from reverting to the ground state G. If the barrier is sufficiently large compared to thermal energy k_BT, where k_B is the Boltzmann constant, the H state can be stable indefinitely.

A photo excited transition from a ground state to a hidden state typically involves two intermediate states

References

1. ↑ Ichikawa, H. et al. Transient photoinduced ‘hidden’ phase in a manganite. Nature Materials 10, 101–105 (2011).
2. ↑ 2.0 2.1 Stojchevska et al., Ultrafast switching to a stable hidden quantum state in an electronic crystal, Science 344, 177, (2014)
3. ↑ 3.0 3.1 Tokura, Y. Photoinduced Phase Transition: A Tool for Generating a Hidden State of Matter. J. Phys. Soc. Jpn 75, 011001 (2006).
4. ↑ 4.0 4.1 4.2 4.3 4.4 Nasu, K. Photoinduced phase transitions. World Scientific, Singapore (2004).
5. ↑ Miyano, K., Tanaka, T., Tomioka, Y. & Tokura, Y. Photoinduced insulator-to-metal transition in a perovskite manganite. Phys Rev Lett 78, 4257–4260 (1997)
6. ↑ Koshihara, S., Tokura, Y., Mitani, T., Saito, G. & Koda, T. Photoinduced valence instability in the organic molecular compound tetrathiafulvalene-p-chloranil (TTF-CA). Phys. Rev. B 42, 6853–6856 (1990).
7. ↑ Okamoto, H. et al. Photoinduced phase transition in tetrathiafulvalene-p-chloranil observed in femtosecond reflection spectroscopy. Phys Rev B 70, 165202 (2004).
8. ↑ Cavalleri, A. et al. Femtosecond Structural Dynamics in VO_ {2} during an Ultrafast Solid-Solid Phase Transition. Phys Rev Lett 87, 237401 (2001).
9. ↑ Tomimoto, S., Miyasaka, S., Ogasawara, T., Okamoto, H. & Tokura, Y. Ultrafast photoinduced melting of orbital order in LaVO₃. Phys Rev B 68, 035106 (2003).
10. ↑ Takubo, N. et al. Persistent and Reversible All-Optical Phase Control in a Manganite Thin Film. Phys Rev Lett 95, 017404 (2005).
11. ↑ Yu, G., et al. Transient Photoinduced Conductivity in Single-Crystals of YBa₂Cu₃O_7-δ - Photodoping to the Metallic State. Phys Rev Lett 67, 2581–2584 (1991).
12. ↑ Fausti, D. et al. Light-Induced Superconductivity in a Stripe-Ordered Cuprate. Science 331, 189–191 (2011).