Room Temperature Superconductors

This piece is a bit heavy to make sense out of butgive it a try.  I separated everysentence to assist the reader and bolded some key points.  The writer is not overly coherent here either.

The upshot isthat we are apparently close to room temperature superconductivity.  We may still be a long way from practical devicesthough.  For now we can take roomtemperature and work toward mastering the theoretical basis for all this.

One can appreciatefrom this why we have been at it for several decades and still do not have itfigured out.

DECEMBER 11, 2010

The possibility to achievethe room temperatures superconductivity has been argued for decades in thesuperconductivity research field.

Because the real mechanism ofsuperconductivity has never been revealed, so the estimates about the upperbound on the superconducting transition temperature are all empirical.

Based on thesuperconductivity mechanism proposed in this paper, clearly, the roomtemperatures superconductivity must lie in the materials in which the threecriteria for superconductivity have to be optimally satisfied.

For the time being, we cannotpredict what the upper bound of the superconducting transition temperatureshould be, but we assert that it isdefinitely higher than the room temperatures.

We believe that the dream toachieve the room temperatures superconductivity will come true in the nearfuture.

The physicalmechanism of superconductivity is proposed on the basis of carrier-induceddynamic strain effect.

By this new model,superconducting state consists of the dynamic bound state of superconductingelectrons, which is formed by the high-energy nonbonding electrons throughdynamic interaction with their surrounding lattice to trap themselves into thethree - dimensional potential wells lying in energy at above the Fermi level ofthe material.

The binding energy of superconductingelectrons dominates the superconducting transition temperature in thecorresponding material.

Under an electric field,superconducting electrons move coherently with lattice distortion wave andperiodically exchange their excitation energy with chain lattice, that is, thesuperconducting electrons transfer periodically between their dynamic boundstate and conducting state.

Thus, the intrinsic feature of superconductivity is to generate anoscillating current under a dc voltage.

The coherence lengths incuprates must have the value equal to an even number times the latticeconstant.

A superconducting materialmust simultaneously satisfy three criteria required by superconductivity.

Almost all of the puzzlingbehavior of the cuprates can be uniquely understood under this new model.

We demonstrate that thefactor 2 in Josephson current equation, in fact, is resulting from 2V, thevoltage drops across the two superconductor sections on both sides of ajunction, not from the Cooper pair, and the magnetic flux is quantized in unitsof h/e, postulated by London, not in units of h/2e.

The central features ofsuperconductivity, such as Josephson effect, the tunneling mechanism inmultijunction systems, and the origin of the superconducting tunnelingphenomena, are all physically reconsidered under this superconductivitymodel. 

A superconducting material must simultaneously satisfy the following threenecessary conditions required by superconductivity.

First, the material must possess the high-energynonbonding electrons with certain concentrations requested by coherencelengths. Following this criterion, it is not surprising that most ofalkaline metal, the covalent and closed-shell compounds, and the excellentconductors, copper, silver and gold do not show superconductivity at normalcondition.

Second, the material must have thethree-dimensional potential wells lying in energy at above the Fermi level ofthe material, and the dynamic bound state of superconducting electrons inpotential wells of a given superconducting chain must have the same bindingenergy and symmetry.

According to the types ofpotential wells in which the superconducting electrons trap themselves to formsuperconducting dynamic bound state, thesuperconductors as a whole can be divided into two groups.

 One of them iscalled as usual as the conventionalsuperconductors in which the potential well are formed by the microstructures of materials, such as crystalgrains, clusters, nanocrystals, superlattice, and the charge inversionlayer in metal surfaces.

We propose that the type 1superconductors are most likely achieved by the last kind of potential wellsabove.

The common feature for thissort of superconductors is that the volume of the potential wells for trappingsuperconducting electrons varies with the techniques using to synthesize thesuperconductors, so that the superconducting transition temperature inconventional superconductors usually shows strongly sample-dependent andirreproducible.

Since the potential wells inconventional superconductors generally have relatively large confined volumeand low potential height, so the conventional superconductors normally haverelatively low transition temperature, but magnesium diboride is an exception.

Another group isreferred to as the high-Tc superconductors in which the potential wells fortrapping superconducting electrons are formed by the latticestructure of material only, such as CuO6 octahedrons and CuO5 pyramidspotential wells for cuprates, BiO6 octahedron for BaKBiO3 compounds, C60 inA3C60 fullerides and FeAs4 tetrahedrons in LaOFeAs compounds.

The small and fixed volume ofpotential wells makes the high-Tc superconductors usually have relatively highand fixed transition temperature.

Finally, in order to enable the normal state of the material being metallic,the band structure of the superconducting material must have a widelydispersive antibonding band, which crosses the Fermi level and runs over theheight of potential wells.

The symmetry of theantibonding band into which the superconducting electrons trap themselves toform a dynamic bound state dominates the types of the superconductingdistortion waves.

The typical example forsuperconductivity derived from this criterion perhaps belongs to transitionmetals and their compounds.

Matthias was the first topropose that the transition temperature in transition metals depends upon thenumber of valence electrons per atom, Ne, and two values Ne = 5e/a for V, Nb,and Ne = 7e/a for Tc and Re are favorable to have high value of Tc.62

The similar phenomenon wasalso found in transition metal compounds. It has been confirmed that thedensity of electronic states for both bcc and hcp transition metals are allresulted from a number of the narrow density peaks derived from the d -orbitals bonding states overlapping with a broad low density of states arisenfrom the s - electron antibonding band.

Based on the rigid bandmodel, the Fermi levels for the transition metal with Ne = 1 to 4 all fall inthe region where the density of states is dominated by the d - electron bondingstates.

The potential wells formed bythe grain boundaries, which normally have a potential height less than 0.1 eV,should also overlap with bonding states of the d - orbitals.

In this case, the dynamicbound state cannot be formed in the potential wells, thus it is not surprisingthat the superconductivity cannot be found in these transition metals.

However, for V and Nb, whichhave five valence electrons, Ne = 5e/a, the Fermi level shifts toward the highenergies at where the density of states is mainly resulted from the selectron-antibonding band.

In this circumstance, theenergy levels at the top of potential wells formed by grain boundaries arederived from the s electron-antibonding band, and so the superconducting statecan be achieved and has a s-symmetry wave.

The similar process isrepeated for the transition metal Tc and Re with Ne = 7 e/a.

On the basis of the mechanism of superconductivity proposed above, the key point to achieve superconductivityis that the superconducting electron must periodically exchange its excitationenergy with chain lattice.

That is, the excitationenergy of the superconducting electrons must be reversibly transferred betweensuperconducting electrons and chain lattice.

It is well known that theinteraction between electrons and atomic magnetic moments is irreversible,which, thus, in any case cannot become the driving force of superconductivity.

However, it can be seen fromthis new model that superconductivity and atomic magnetic moments in principleare not intrinsically exclusive each other.

As long as there exists thesame magnetic moment in every potential well in a given superconducting chain,as in the case of the ferromagnetic materials LaOFeAs, and the three necessaryconditions required for superconductivity are satisfied, the superconductingstate can be formed and the superconducting process will persist withoutdissipating energy.

Since the electromagneticinteraction energy for superconducting electrons with atom magnetic momentmaintains the same in every potential well, thus the binding energy ofsuperconducting electrons in potential wells cannot be affected by the atommagnetic moment, and so the scattering centers for superconducting electronscannot be introduced.

But this conditionessentially cannot be achieved for conventional superconductors, so the atomicmagnetic moments are generally detrimental to superconductivity.

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