Saturday, February 06, 2016

Nematicity and high Tc superconductivity

Waterloo physicists discover new properties of superconductivity is the title of article popurazing the article of David Hawthorn, Canada Research Chair Michel Gingras, doctoral student Andrew Achkar and post-doctoral student Zhihao Hao published in Science.

There is a dose of hype involved. As a matter of fact, it has been known for years that electrons flow along stripes, kind of highways in high Tc superconductors: I know this quite well since I have proposed TGD inspired model explaining this (see this and this )!

The effect is known as nematicity and means that electron orbitals break lattice symmetries and align themselves like a series of rods. Nematicity in long length scales occurs a temperatures below the critical point for super-conductivity. In above mentioned cuprate CuO2 is studied. For non-optimal doping the critical temperature for transition to macroscopic superconductivity is below the maximal critical temperature. Long length scale nematicity is observed in these phases.

In second article it is however reported that nematicity is in fact preserved above critical temperature as a local order -at least up to the upper critical temperature, which is not easy to understand in the BCS theory of superconductivity. One can say that the stripes are short and short-lived so that genuine super-conductivity cannot take place.

These two observations yield further support for TGD inspired model of high Tc superconductivity and bio-superconductivity. It is known that antiferromagnetism is essential for the phase transition to superconductivity but Maxwellian view about electromagnetism and standard quantum theory do not make it easy to understand how. Magnetic flux tube is the first basic new notion provided by TGD. Flux tubes carry dark electrons with scaled up Planck constant heff =n×h: this is second new notion. This implies scaling up of quantal length scales and in this manner makes also super-conductivity possible.

Magnetic flux tubes in antiferromagnetic materials form short loops. At the upper critical point they however reconnect with some probability to form loops with look locally like parallel flux tubes carrying magnetic fields in opposite directions. The probability of reverse phase transition is so large than there is a competion. The members of Cooper pairs are at parallel flux tubes and have opposite spins so that the net spin of pair vanishes: S=0. At the first critical temperature the average length and lifetime of flux tube highways are too short for macroscopic super-conductivity. At lower critical temperature all flux tubes re-connect permantently average length of pathways becomes long enough.

This phase transition is mathematically analogous to percolation in which water seeping through sand layer wets it completely. The competion between the phases between these two temperatures corresponds to quantum criticality in which phase transitions heff/h=n1 ←→n2 take place in both directions (n1 =1 is the most plausible first guess). Earlier I did not fully realize that Zero Energy Ontology provides an elegant description for the situation (see this and this). The reason was that I though that quantum criticality occurs at single critical temperature rather than temperature interval. Nematicity is detected locally below upper critical temperature and in long length scales below lower critical temperature.

During last years it has become clear that condensed matter physicists are discovering with increasing pace the physics predicted by TGD . Same happens in biology. It is a pity that particle physicists have missed the train so badly. They are still trying to cook up something from super string models which have been dead for years. The first reason is essentially sociological: the fight for funding has led to what might be politely called "aggressive competion". Being the best is not enough and there is a temptation to use tricks, which prevent others showing publicly that they have something interesting to say. ArXiv censorship is excellent tool in this respect. Second problem is hopelessly narrow specialization and technicalization: colleague can be defined by telling the algorithms that he is applying. Colleagues do not see physics for particle physics - or even worse, for "physics" or superstrings and branes in 10,11, or 12 dimensions.

For a summary of earlier postings see Links to the latest progress in TGD.

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