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Visible states of matter are those which we observe under near equilibrium conditions.  For example, phase transitions between states of condensed matter generally occur under near-ergodic conditions, where temperature, pressure, electromagnetic field, chemical pressure or doping by chemical ion substitution cause a change of state. Ergodicity in this case is very important because it implies that all relevant excitations (phonons, electronic or magnetic excitations etc.) are separately each in equilibrium within each subsystem, and the different subsystems are also in equilibrium amongst themselves. This requirement defines the ergodicity timescale over which the transition takes place is longer than the lifetime of the relevant excitations.
Visible states of matter are those which we observe under near equilibrium conditions.  For example, phase transitions between states of condensed matter generally occur under near-ergodic conditions, where temperature, pressure, electromagnetic field, chemical pressure or doping by chemical ion substitution cause a change of state. Ergodicity in this case is very important because it implies that all relevant excitations (phonons, electronic or magnetic excitations etc.) are separately each in equilibrium within each subsystem, and the different subsystems are also in equilibrium amongst themselves. This requirement defines the ergodicity timescale over which the transition takes place is longer than the lifetime of the relevant excitations.


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“Hidden” in this case means that the state does not exist under thermodynamic conditions at any temperature or pressure, nor can it be reached by varying any of these parameters. Loosely speaking, in the language of field theory, the discovery of a hidden state is analogous to the discovery of a parallel universe or a new physical vacuum of elementary particles. Obviously, a-priori, one cannot know of their existence.
“Hidden” in this case means that the state does not exist under thermodynamic conditions at any temperature or pressure, nor can it be reached by varying any of these parameters. Loosely speaking, in the language of field theory, the discovery of a hidden state is analogous to the discovery of a parallel universe or a new physical vacuum of elementary particles. Obviously, a-priori, one cannot know of their existence.


==Examples==
== Examples ==


There are a few examples of hidden states in condensed matter systems. Such states are usually unstable and decay very rapidly, typically in nanoseconds or less.
There are a few examples of hidden states in condensed matter systems. Such states are usually unstable and decay very rapidly, typically in nanoseconds or less.
The difficulty is in distinguishing a genuine hidden state from one which is simply out of thermal equilibrium. The genuine hidden state should differ from the ground state by symmetry and/or microscopic properties, such as different charge or magnetic macroscopic order. 
The difficulty is in distinguishing a genuine hidden state from one which is simply out of thermal equilibrium. The genuine hidden state should differ from the ground state by symmetry and/or microscopic properties, such as different charge or magnetic macroscopic order. 


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. <ref = "Koshihara">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).</ref><ref name="Nasu">Nasu, K. Photoinduced phase transitions. World Scientific, Singapore (2004).</ref><ref>Okamoto, H. et al. Photoinduced phase transition in tetrathiafulvalene-p-chloranil observed in femtosecond reflection spectroscopy. Phys Rev B 70, 165202 (2004).</ref> 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. <ref name="Nasu" />.
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..<ref = "Koshihara">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).</ref><ref name="Nasu">Nasu, K. Photoinduced phase transitions. World Scientific, Singapore (2004).</ref><ref>Okamoto, H. et al. Photoinduced phase transition in tetrathiafulvalene-p-chloranil observed in femtosecond reflection spectroscopy. Phys Rev B 70, 165202 (2004).</ref> 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. <ref name="Nasu" />
Photoexcitation has been shown to produce persistent states in vanadates <ref>Cavalleri, A. et al. Femtosecond Structural Dynamics in VO_ {2} during an Ultrafast Solid-Solid Phase Transition. Phys Rev Lett 87, 237401 (2001).</ref><ref>Tomimoto, S., Miyasaka, S., Ogasawara, T., Okamoto, H. & Tokura, Y. Ultrafast photoinduced melting of orbital order in LaVO<sub>3</sub>. Phys Rev B 68, 035106 (2003).</ref> and manganite materials <ref>Takubo, N. et al. Persistent and Reversible All-Optical Phase Control in a Manganite Thin Film. Phys Rev Lett 95, 017404 (2005).</ref>, leading to filamentary paths of a modified charge ordered phase which is sustained by a passing current. Transient superconductivity was also reported in cuprates <ref>Yu, G., et al. Transient Photoinduced Conductivity in Single-Crystals of YBa<sub>2</sub>Cu<sub>3</sub>O<sub>7-δ</sub> - Photodoping to the Metallic State. Phys Rev Lett 67, 2581–2584 (1991).</ref><ref>Fausti, D. et al. Light-Induced Superconductivity in a Stripe-Ordered Cuprate. Science 331, 189–191 (2011).</ref>.
Photoexcitation has been shown to produce persistent states in vanadates<ref>Cavalleri, A. et al. Femtosecond Structural Dynamics in VO_ {2} during an Ultrafast Solid-Solid Phase Transition. Phys Rev Lett 87, 237401 (2001).</ref><ref>Tomimoto, S., Miyasaka, S., Ogasawara, T., Okamoto, H. & Tokura, Y. Ultrafast photoinduced melting of orbital order in LaVO<sub>3</sub>. Phys Rev B 68, 035106 (2003).</ref> and manganite materials <ref>Takubo, N. et al. Persistent and Reversible All-Optical Phase Control in a Manganite Thin Film. Phys Rev Lett 95, 017404 (2005).</ref>, leading to filamentary paths of a modified charge ordered phase which is sustained by a passing current. Transient superconductivity was also reported in cuprates <ref>Yu, G., et al. Transient Photoinduced Conductivity in Single-Crystals of YBa<sub>2</sub>Cu<sub>3</sub>O<sub>7-δ</sub> - Photodoping to the Metallic State. Phys Rev Lett 67, 2581–2584 (1991).</ref>.<ref>Fausti, D. et al. Light-Induced Superconductivity in a Stripe-Ordered Cuprate. Science 331, 189–191 (2011).</ref>

== A photoexcited transition to an H state ==
== A photoexcited transition to an H state ==


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== Stability ==
== Stability ==


The examples shown indicate that hidden states are typically metastable, having lifetimes between a few ns to microseconds under normal laboratory conditions, no matter what the temperature. Until now no spatially uniform stable states have been reported.
The examples shown indicate that hidden states are typically metastable, having lifetimes between a few ns to microseconds under normal laboratory conditions, no matter what the temperature. Until now no spatially uniform stable states have been reported.


== Applications ==
== Applications ==
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Although the primary interest in hidden states is in different areas of fundamental physics, they may lead to important applications, if their optical, electronic or magnetic properties are different than those of the ground state. They may then be used as switching elements. In case a stable state can be found, it could be used the construction of nonvolatile memory devices.
Although the primary interest in hidden states is in different areas of fundamental physics, they may lead to important applications, if their optical, electronic or magnetic properties are different than those of the ground state. They may then be used as switching elements. In case a stable state can be found, it could be used the construction of nonvolatile memory devices.


==References==
== References ==
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Revision as of 19:58, 31 March 2014

Visible states of matter are those which we observe under near equilibrium conditions.  For example, phase transitions between states of condensed matter generally occur under near-ergodic conditions, where temperature, pressure, electromagnetic field, chemical pressure or doping by chemical ion substitution cause a change of state. Ergodicity in this case is very important because it implies that all relevant excitations (phonons, electronic or magnetic excitations etc.) are separately each in equilibrium within each subsystem, and the different subsystems are also in equilibrium amongst themselves. This requirement defines the ergodicity timescale over which the transition takes place is longer than the lifetime of the relevant excitations.

However, under circumstances of very energetic and short-lived perturbations, during the Big Bang, or such as may be caused by ultrashort laser pulses impinging on solid state matter for example, 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. Our universe may be indeed in such a state.

“Hidden” in this case means that the state does not exist under thermodynamic conditions at any temperature or pressure, nor can it be reached by varying any of these parameters. Loosely speaking, in the language of field theory, the discovery of a hidden state is analogous to the discovery of a parallel universe or a new physical vacuum of elementary particles. Obviously, a-priori, one cannot know of their existence.

Examples

There are a few examples of hidden states in condensed matter systems. Such states are usually unstable and decay very rapidly, typically in nanoseconds or less. The difficulty is in distinguishing a genuine hidden state from one which is simply out of thermal equilibrium. The genuine hidden state should differ from the ground state by symmetry and/or microscopic properties, such as different charge or magnetic macroscopic order. 

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..[1][2][3] 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. [2] Photoexcitation has been shown to produce persistent states in vanadates[4][5] and manganite materials [6], leading to filamentary paths of a modified charge ordered phase which is sustained by a passing current. Transient superconductivity was also reported in cuprates [7].[8]

A photoexcited transition to an H state

A hypothetical schematic diagram for the transition to an H state by photo excitation: 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 EB prevents state H from reverting to the ground state G. If the barrier is sufficiently large compared to thermal energy kBT, where kB is Boltzmann's 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

Stability

The examples shown indicate that hidden states are typically metastable, having lifetimes between a few ns to microseconds under normal laboratory conditions, no matter what the temperature. Until now no spatially uniform stable states have been reported.

Applications

Although the primary interest in hidden states is in different areas of fundamental physics, they may lead to important applications, if their optical, electronic or magnetic properties are different than those of the ground state. They may then be used as switching elements. In case a stable state can be found, it could be used the construction of nonvolatile memory devices.

References

  1. ^ 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).
  2. ^ a b Nasu, K. Photoinduced phase transitions. World Scientific, Singapore (2004).
  3. ^ Okamoto, H. et al. Photoinduced phase transition in tetrathiafulvalene-p-chloranil observed in femtosecond reflection spectroscopy. Phys Rev B 70, 165202 (2004).
  4. ^ Cavalleri, A. et al. Femtosecond Structural Dynamics in VO_ {2} during an Ultrafast Solid-Solid Phase Transition. Phys Rev Lett 87, 237401 (2001).
  5. ^ Tomimoto, S., Miyasaka, S., Ogasawara, T., Okamoto, H. & Tokura, Y. Ultrafast photoinduced melting of orbital order in LaVO3. Phys Rev B 68, 035106 (2003).
  6. ^ Takubo, N. et al. Persistent and Reversible All-Optical Phase Control in a Manganite Thin Film. Phys Rev Lett 95, 017404 (2005).
  7. ^ Yu, G., et al. Transient Photoinduced Conductivity in Single-Crystals of YBa2Cu3O7-δ - Photodoping to the Metallic State. Phys Rev Lett 67, 2581–2584 (1991).
  8. ^ Fausti, D. et al. Light-Induced Superconductivity in a Stripe-Ordered Cuprate. Science 331, 189–191 (2011).
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