Thursday, November 03, 2022

VO2 can remember like a brain

The following comments were inspired by a popular article (see this) with the title "Scientists accidentally discover a material that can 'remember' like a brain". These materials can remember the history of its physical stimuli. The findings are described in the article "Electrical control of glass-like dynamics in vanadium dioxide for data storage and processing" published in Nature (see see this).

The team from the Ecole Polytechnique Federale de Lausanne (EPFL) in Switzerland did this discovery while researching insulator-metal phase transitions of vanadium dioxide (VO2), a compound used in electronics.

  1. PhD student Mohammad Samizadeh Nikoo was trying to figure out how long it takes for VO2 to make a phase transition from insulating to conducting phase under "incubation" by a stimulation by a radio frequency pulse of 10 μs duration and voltage amplitude V= 2.1 V. Note that the Wikipedia article talks about semiconductor-metal transition. The voltage pulse indeed acted like a voltage in a semiconductor.
  2. As the current heated the sample it caused a local phase transition to metallic state in VO2. The induced current moved across the material, following a path until it exited on the other side. A conducting filament connecting the ends of the device was generated by a percolation type process.
  3. Once the current had passed, the material exhibited an insulating state but after incubation time tinc, which was tinc∼ .1 μs for the first pulse, it became conducting. This state lasted at least 10,000 seconds.

    After applying a second electrical current during the experiment, it was observed that tinc appeared to be directly related to its history and was shorter than for the first incubation period .1 μs. The VO2 seemed to remember the first phase transition and anticipate the next. One could say that the system learned from experience.

Before trying to understand the finding in the TGD framework, it is good to list some basic facts about vanadium and vanadium-oxide VO2 or Vanadium(IV) oxide (see this).
  1. Vanadium is a transition metal, which has valence shells d3s2. It is known that the valence electrons of transition metals can mysteriously disappear, for instance in heating (see this). The TGD interpretation (see this) would be that heating provides energy making it possible to transform ordinary valence electrons to dark valence electrons with a higher value of heff and higher energy. In the recent case, the voltage pulses could have the same effect.
  2. VO2 forms a solid lattice of V4+ ions. There are two lattice forms: the monoclinic semiconductor below Tc=340 K and the tetragonal metallic form above Tc. In the monoclinic form, the V4+ ions form pairs along the c axis, leading to alternate short and long V-V distances of 2.65 Angström and 3.12 Angström. In the tetragonal form, the V-V distance is 2.96 Angström. Therefore size of the unit cell for the monoclinic form is 2 times larger than for the tetragonal form. At Tc IMT takes place. The optical band gap of VO2 in the low-temperature monoclinic phase is about 0.7 eV.
  3. Remarkably, the metallic VO2 contradicts the Wiedemann Franz law, which states that the ratio of the electronic contribution of the thermal conductivity (κ) to the electrical conductivity (σ) of a metal is proportional to the temperature. The thermal conductivity that could be attributed to electron movement was 10 per cent of the amount predicted by the Wiedemann Franz law. That the conductivity is 10 times higher than expected, suggests that the mechanism of conductivity is not the usual one.

    Semiconductor property below Tc suggests that a local phase transition modifying the lattice structure from monoclinic to tetragonal takes place at the current path in the incubation.

One can try to understand the chemistry and unconventional conductivity of VO2 in the TGD framework.
  1. Vanadium could give 4 valence electrons to O2: 3 electrons d3:sta and one from s2. In the TGD Universe, the second electron from s2 could become dark and go to the bond between V4+ ions in the VO2 lattice and take the role of conduction electron.
  2. This could explain the non-conventional character of conductivity. In the semiconductor phase, an electric voltage pulse or some other perturbation, such as impurity atoms or heating, can provide the energy needed to increase the value of heff. Electric conductivity could be due to the transformation of electrons to dark electrons possibly forming Cooper pairs at the flux tube pairs connecting V4+ ions or their pairs. The current would run along the flux tubes as a dark current.
  3. In a semi-conducting (insulating) state, the flux tube pairs connecting V4+ ions would be relatively short. The voltage pulse inducing a local metallic state could provide the energy needed to increase heff and thus the quantum coherence scale. This would be accompanied by a reconnection of the short flux tube pairs to longer flux tube pairs serving as bridges along which the dark current could run.

    One can also consider U-shaped closed flux tubes associated with V4+ ions or ion pairs, which reconnect in IMT to longer flux tubes. The mechanism would be very similar to that proposed for the transition to high temperature superconductivity (see this, this, and this).

Experimenters suggest a glass type behavior.
  1. Spin glass corresponds to the existence of a very large number of free energy minima in the energy landscape implying breaking of ergodicity. A system consisting of regions with varying direction of magnetization is the basic example of spin glass. In the recent case, decomposition to metallic and insulating regions could define the spin glass.
  2. TGD predicts the possibility of spin glass type behavior and leads to a model for spin glasses (see this). The quantum counterpart of spin glass behavior would be realized in terms of monopole flux tube structures (magnetic bodies) carrying dark phases of the radinary particles such as electrons serving as current carries in the metallic phase.The length of the flux tube pair would be one critical parameter near Tc. Quantum criticality against the change of heff increasing the length of the flux tube pair by reconnection would make the system very sensitive to perturbations.
  3. These phases are highly sensitive to external perturbations and represent in TGD inspired theory of consciousness higher levels with longer quantum coherence scale and number theoretical complexity measured by the dimension n= heff/h0 of the extension having interpretation as a kind of IQ. These phases would receive sensory information from lower levels of the hierarchy with smaller values of n and control them.

    The large number of free energy minima as a correlate for number theoretical complexity would make possible the representation of "sensory" information as "memories".

See the article TGD and Condensed Matter or the chapter with the same title.

For a summary of earlier postings see Latest progress in TGD.

Articles related to TGD.

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