Science - USA (2022-02-18)

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SUPERCONDUCTIVITY

Surprising superconductivity of graphene

An ordinary graphene bilayer exhibits extraordinary superconductivity

ByTero T. Heikkilä

G

raphene-based materials are an in-

teresting platform for studying exotic

electronic states ( 1 , 2 ) such as super-

conductivity, where the electrical re-

sistivity of the material becomes zero.

Although superconductivity has been

observed in graphene, earlier observations

were of samples with unstable physical struc-

tures, hampering a consistent explanation for

the phenomenon. On page 774 of this issue,

Zhou et al. ( 3 ) report how a graphene bilayer

becomes superconducting in the presence

of suitably arranged electric and magnetic

fields. Their experiments indicate a rarely

found exotic state for the electrons, where su-

perconductivity is particularly robust against

large magnetic fields that usually destroy

the effect. The discovery of superconductiv-

ity in as relatively simple a structure as a

graphene bilayer opens an avenue for better

understanding the phenomenon. Such un-

derstanding can facilitate the search for su-

perconducting materials with more-practical

operating conditions.

Graphene is a single-atom-thick layer of

carbon atoms arranged in a hexagonal lat-

tice. Ordinary bilayer graphene is composed

of two layers of graphene. One layer is on

top of the other so that the atoms in both

layers find their minimum-energy positions

(see the figure), in contrast to the unstable

twisted bilayer graphene ( 1 ). This discovery

of superconductivity in bilayer graphene

is surprising because bilayer graphene has

already been thoroughly studied for its po-

tential to replace silicon in building smaller

and faster electronic devices. A single layer

of graphene lacks a bandgap, which is a

problem for its use in digital electronics.

Namely, without a gap between the va-

lence and conduction bands for electrons,

the ratio of conductivities in the on and off

states is too small to be practical for logi-

cal operations. One solution to the problem

is to place a bilayer graphene between two

metallic electrodes, as was demonstrated in

2009 ( 4 ). Doing so opens a tunable band-

gap in the bilayer because of the electrical

potential difference between the two elec-

trodes. The combined device—a transis-

tor—can then be controlled by the common

electrode potential. When the common

potential is zero, the transistor is in the off

state. Increasing or decreasing the common

potential injects electrons into the conduc-

tion band or removes them from the va-

lence band. As a result, the transistor goes

to the on state.

Zhou et al. found that near the onset of

the on state, electronic interactions deform

the electronic band structure at low tempera-

tures and break some of its symmetries. To

begin, they studied how the resistance of the

bilayer graphene oscillates as a function of

the magnetic field in its nonsuperconducting

state. Using such a measurement, the authors

uncovered different types of electronic states

with spontaneously broken spin or valley

symmetries as signatures of a phase transi-

tion to a magnetic state. Spin is a fundamen-

tal quantum property inherent to electrons,

whereas the valley describes the shape of the

band structure, which in turn determines the

range of energy that electrons can have when

moving inside a specific material. Then, the

bilayer graphene device was cooled to tem-

peratures below 30 mK, where the material

becomes superconducting for certain values

of the electric and magnetic fields.

The superconducting transition is formally

described as the build-up of coherent pairing

between electrons. In conventional super-

conductors, the electron pairs contain oppo-

site spins. This pairing is known as a spin-

singlet state and is governed by a quantum-

mechanical principle known as Pauli exclu-

sion, which constrains the types of pairs that

can be formed. The singlet state is vulnerable

to a magnetic field in that the field provides

different energies to the electrons of oppo-

site spins and eventually destroys supercon-

ductivity. The other possibility for electron

pairing is the spin-triplet state, where spins

in the pair point in the same direction and

are not affected by magnetic fields. Such

states are interesting because they may be

useful in quantum computing ( 5 ). Before gra-

phene, spin triplets have been observed only

in materials composed of heavy elements.

Therefore, it is particularly notable that the

superconducting graphene of Zhou et al. re-

quired applying a magnetic field. This along

with the resistance remaining at zero up to

very large fields indicates the possibility of a

spin-triplet superconducting state. Here, the

presence of the valleys in graphene may help

to satisfy the Pauli exclusion principle.

The discovery made by Zhou et al. lines

up with the other recent findings of the su-

perconducting state in few-layer graphene

Stable bilayer graphene has systems (^1 ), especially in bi- and trilayer

been heavily studied for realizing

ultrasmall and fast transistors. Its

superconductivity came as a surprise.

Electric

field

Magnetic

Top layer field

Bottom layer

Nanoscience Center and Department of Physics, University

of Jyväskylä, Jyväskylä, Finland. Email: [email protected]

Recipe for superconductivity

in bilayer graphene

Take two layers of graphene and place them at a

staggered fashion into their most stable arrangement.

Apply a perpendicular electric and an in-plane magnetic

field. Measure while cold to find superconductivity.

18 FEBRUARY 2022 • VOL 375 ISSUE 6582 719