An organic superconductor is a synthetic organic
compound that exhibits superconductivity
at low temperatures.
As of 2007 the highest achieved critical temperature
for an organic superconductor at standard
pressure is 33 kelvin,
observed in the alkali-doped fullerene RbCs2C60.
In 1979 Klaus
Bechgaard synthesized the first organic superconductor (TMTSF)2PF6
(the corresponding material class was named after him later) with a transition
temperature of TC = 1.1 K, at an external pressure of 6.5 kbar.
Many materials may be characterized as organic
superconductors. These include the Bechgaard
salts and Fabre salts which are both quasi-one-dimensional, and
quasi-two-dimensional materials such as k-BEDT-TTF2X
charge-transfer complex, λ-BETS2X
compounds, graphite intercalation compounds
and three-dimensional materials such as the alkali-doped fullerenes.
Organic superconductors are of special interest not only for
scientists, looking for room-temperature superconductivity
and for model systems explaining the origin of superconductivity but also for
daily life issues as organic compounds are mainly built of carbon and hydrogen which
belong to the most common elements on earth
in contrast to copper
or osmium.
One-dimensional Fabre and Bechgaard salts
Fabre-salts are composed of tetramethyltetrathiafulvalene
(TMTTF) and Bechgaard salts of tetramethyltetraselenafulvalene
(TMTSF). These two organic molecules are similar except for the sulfur-atoms of
TMTTF being replaced by selenium-atoms in TMTSF. The molecules are stacked in
columns (with a tendency to dimerization) which are separated by anions. Typical
anions are for example octahedral PF6, AsF6 or
tetrahedral ClO4 or ReO4.
Both material classes are quasi-one-dimensional at
room-temperature only conducting along the molecule stacks and share a very
rich phase
diagram containing antiferromagnetic ordering, charge
order, spin-density wave state, dimensional crossover
and of course superconductivity.
Only one Bechgaard salt was found to be superconducting at
ambient pressure which is (TMTTF)2ClO4 with a transition
temperature of TC = 1.4 K. Several other salts become
superconducting only under external pressure. The external pressure one would
have to apply to drive most Fabre-salts to superconductivity is so high, that
under lab conditions superconductivity was observed only in one compound. A
selection of the transition temperature and corresponding external pressure of
several one-dimensional organic superconductors is shown in the table below.
|
Material
|
TC (K)
|
pext (kbar)
|
|
(TMTSF)2SbF6
|
0.36
|
10.5
|
|
(TMTSF)2PF4
|
1.1
|
6.5
|
|
(TMTSF)2AsF6
|
1.1
|
9.5
|
|
(TMTSF)2ReO4
|
1.2
|
9.5
|
|
(TMTSF)2TaF6
|
1.35
|
11
|
|
(TMTTF)2Br
|
0.8
|
26
|
Two-dimensional (BEDT-TTF)2X
BEDT-TTF is the short form of
bisethylenedithio-tetrathiafulvalene commonly abbreviated with ET. These
molecules form planes which are separated by anions. The pattern of the
molecules in the planes is not unique but there are several different phases
growing, depending on the anion and the growth conditions. Important phases
concerning superconductivity are the α- and θ- phase with the molecules
ordering in a fishbone structure and the β- and especially κ-phase which order
in a checkerboard structure with molecules being dimerized in the κ-phase. This
dimerization makes the κ-phases special as they are not quarter- but
half-filled systems, driving them into superconductivity at higher temperatures
compared to the other phases.
The amount of possible anions separating two sheets of
ET-molecules is nearly infinite. There are simple anions such as I3,
polymeric ones such as the very famous Cu[N(CN)2]Br and anions
containing solvents for example Ag(CF3)4·112DCBE. The
electronic properties of the ET-based crystals are determined by its growing
phase, its anion and by the external pressure applied. The external pressure
needed to drive an ET-salt with insulating ground state to a superconducting
one is much smaller than those needed for Bechgaard
salts. For example κ-(ET)2Cu[N(CN)2]Cl needs only a
pressure of about 300 bar to become superconducting, which can be achieved by
placing a crystal in grease which is freezing below 0 °C and then
providing sufficient stress to induce the superconducting transition.
The crystals are very sensitive (never user tweezers on
them) which can be observed impressively in α-(ET)2I3
lying several hours in the sun (or, more controlled in an oven at 40 °C). After this
treatment one gets αTempered-(ET)2I3 which is
superconducting.
In contrast to the Fabre or Bechgaard salts universal phase
diagrams for all the ET-based salts have only been proposed yet. For sure
such a phase diagram wouldn’t only depend on temperature and pressure (i.e.
bandwidth) but also on electronic correlations. In addition to the
superconducting ground state these materials show charge-order,
antiferromagnetism or remain metallic down
to lowest temperatures. One compound is even predicted to be a spin liquid.
The highest transition temperatures at ambient pressure and
with external pressure are both found in κ-phases with very similar anions.
κ-(ET)2Cu[N(CN)2]Br becomes superconducting at TC
= 11.8 K at ambient pressure, and a pressure of 300 bar drives deuterated
κ-(ET)2Cu[N(CN)2]Cl from an antiferromagnetic
to a superconducting ground state with a transition temperature of TC
= 13.1 K. The following table restricts to only a few exemplary superconductors
of this class. For more superconductors see ref 1.
|
Material
|
TC (K)
|
pext (kbar)
|
|
βH-(ET)2I3
|
1.5
|
0
|
|
θ-(ET)2I3
|
3.6
|
0
|
|
k-(ET)2I3
|
3.6
|
0
|
|
α-(ET)2KHg(SCN)4
|
0.3
|
0
|
|
α-(ET)2KHg(SCN)4
|
1.2
|
1.2
|
|
β’’-(ET)2SF5CH2CF2SO3
|
5.3
|
0
|
|
κ-(ET)2Cu[N(CN)2]Cl
|
12.8
|
0.3
|
|
κ-(ET)2Cu[N(CN)2]Cl deuterated
|
13.1
|
0.3
|
|
κ-(ET)2Cu[N(CN)2]Br deuterated
|
11.2
|
0
|
|
κ-(ET)2Cu(NCS)2
|
10.4
|
0
|
|
κ-(ET)4Hg2.89Cl8
|
1.8
|
12
|
|
κH-(ET)2Cu(CF3)4·TCE
|
9.2
|
0
|
|
κH-(ET)2Ag(CF3)4·TCE
|
11.1
|
0
|
Even more superconductors can be found by changing the
ET-molecules slightly either by replacing the sulfur atoms by selenium
(BEDT-TSF, BETS) or by oxygen (BEDO-TTF, BEDO).
Some two-dimensional organic superconductors of the κ-(ET)2X
and λ(BETS)2X families are candidates for the Fulde-Ferrell-Larkin-Ovchinnikov
(FFLO) phase when superconductivity is suppressed by an external magnetic
field.[3]
Doped Buckminster fullerenes
Superconducting fullerenes
(based on the Buckminster fullerene C60) are fairly different from
other organic superconductors. The building molecules are no longer manipulated
hydrocarbons
but pure carbon
molecules. In addition these molecules are no longer flat but bulky which gives
rise to a three-dimensional, isotropic superconductor. The pure C60
grows in a fcc-lattice and is an insulator. By placing alkali atoms in the
interstitials the crystal becomes metallic and eventually superconducting at
low temperatures.
Unfortunately C60 crystals are not stable at
ambient atmosphere. They are grown and investigated in closed capsules,
limiting the measurement techniques possible. The highest transition
temperature measured so far was TC = 33 K for Cs2RbC60.The
highest measured transition temperature of an organic superconductor was found
in 1995 in Cs3C60 pressurized with 15 kbar to be TC
= 40 K. Under pressure this compound shows a unique behaviour. Usually the
highest TC is achieved with the lowest pressure necessary to drive
the transition. Further increase of the pressure reduces the transition
temperature usually. Different in Cs3C60: Superconductivity
sets in at very low pressures of several 100 bar and the transition temperature
keeps increasing with increasing pressure. This indicates a completely
different mechanism then just broadening of the bandwidth.
|
Material
|
TC (K)
|
pext (mbar)
|
|
K3C60
|
18
|
0
|
|
Rb3C60
|
30.7
|
0
|
|
K2CsC60
|
24
|
0
|
|
K2RbC60
|
21.5
|
0
|
|
K5C60
|
8.4
|
0
|
|
Sr6C60
|
6.8
|
0
|
|
(NH3)4Na2CsC60
|
29.6
|
0
|
|
(NH3)K3C60
|
28
|
14.8
|
More organic superconductors
Next to the three major classes of organic superconductors
(SCs) there are more organic systems becoming superconducting at low
temperatures or under pressure. A few examples shall be presented here.
TTP-based SCs
TMTTF as well as BEDT-TTF are based on the molecule TTF (tetrathiafulvalene). Using TTP
(tetrathiapentalene) as basic molecules one receives a variety of new organic
molecules serving as cations in organic crystals. And some of them are
superconducting. This class of superconductors was only reported recently and
investigations are still under process.
Phenanthrene-type SCs
Instead of using sulfated molecules or the fairly big
Buckminster fullerenes
recently it became possible to synthesize crystals from the hydrocarbon picene and phenanthrene.
Doping the crystal Picene and Phenanthrene with some alkali metals such as potassium or rubidium and
annealing for several days leads to superconductivity with transition temperatures
up to 18 K. For the AxPhenanthrene, the superconductivity is possible
unconventional. Both phenanthrene and picene are called phenanthrene-edge-type polycyclic aromatic hydrocarbon.
The increasing number of benzene rings results in higher Tc.
Graphite intercalation SCs
Putting foreign molecules or atoms between hexagon graphite sheets
leads to ordered structures and to superconductivity even if neither the
foreign molecule or atom nor the graphite layers are metallic. Several stoichiometries
have been synthesized using mainly alkali atoms as anions.
Several TCs for unusual SCs
|
Material
|
TC (K)
|
|
(BDA-TTP)2AsF6
|
5.8
|
|
(DTEDT)3Au(CN)2
|
4
|
|
K3.3Picene
|
18
|
|
Rb3.1Picene
|
6.9
|
|
K3Phenanthrene
|
4.95
|
|
Rb3Phenanthrene
|
4.75
|
|
CaC5
|
11.5
|
|
NaC2
|
5
|
|
KC8
|
0.14
|
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