Dirac electrons come back to life in magic-angle graphene

Date:
June 22, 2020
Source:
Weizmann Institute of Science
Summary:
A new symmetry-broken parent state has been discovered in twisted
bilayer graphene.



FULL STORY ==========================================================================
In 2018 it was discovered that two layers of graphene twisted one with
respect to the other by a "magic" angle show a variety of interesting
quantum phases, including superconductivity, magnetism and insulating behaviours. Now a team of researchers from the Weizmann Institute
of Science led by Prof. Shahal Ilani of the Condensed Matter Physics Department, in collaboration with Prof. Pablo Jarillo-Herrero's group at
MIT, have discovered that these quantum phases descend from a previously unknown high-energy "parent state," with an unusual breaking of symmetry.


========================================================================== Graphene is a flat crystal of carbon, just one atom thick. When two
sheets of this material are placed on top of each other, misaligned at
small angle, a periodic "moire'" pattern appears. This pattern provides
an artificial lattice for the electrons in the material. In this twisted bilayer system the electrons come in four "flavours": spins "up" or
"down," combined with two "valleys" that originate in the graphene's
hexagonal lattice. As a result, each moire' site can hold up to four
electrons, one of each flavour.

While researchers already knew that the system behaves as a simple
insulator when all the moire' sites are completely full (four electrons
per site), Jarillo-Herrero and his colleagues discovered to their
surprise, in 2018, that at a specific "magic" angle, the twisted system
also becomes insulating at other integer fillings (two or three electrons
per moire' site). This behaviour, exhibited by magic-angle twisted bilayer graphene (MATBG), cannot be explained by single particle physics, and is
often described as a "correlated Mott insulator." Even more surprising was
the discovery of exotic superconductivity close to these fillings. These findings led to a flurry of research activity aiming to answer the big question: what is the nature of the new exotic states discovered in MATBG
and similar twisted systems? Imaging magic-angle graphene electrons with
a carbon nanotube detector The Weizmann team set out to understand how interacting electrons behave in MATBG using a unique type of microscope
that utilizes a carbon nanotube single- electron transistor, positioned
at the edge of a scanning probe cantilever.

This instrument can image, in real space, the electric potential produced
by electrons in a material with extreme sensitivity.

"Using this tool, we could image for the first time the 'compressibility'
of the electrons in this system -- that is, how hard it is to
squeeze additional electrons into a given point in space," explains
Ilani. "Roughly speaking, the compressibility of electrons reflects
the phase they are in: In an insulator, electrons are incompressible,
whereas in a metal they are highly compressible." Compressibility
also reveals the "effective mass" of electrons. For example, in
regular graphene the electrons are extremely "light," and thus behave
like independent particles that practically ignore the presence of
their fellow electrons. In magic-angle graphene, on the other hand,
electrons are believed to be extremely "heavy" and their behaviour is
thus dominated by interactions with other electrons ? a fact that many researchers attribute to the exotic phases found in this material. The
Weizmann team therefore expected the compressibility to show a very
simple pattern as a function of electron filling: interchanging between
a highly-compressible metal with heavy electrons and incompressible Mott insulators that appear at each integer moire' lattice filling.



==========================================================================
To their surprise, they observed a vastly different pattern. Instead
of a symmetric transition from metal to insulator and back to metal,
they observed a sharp, asymmetric jump in the electronic compressibility
near the integer fillings.

"This means that the nature of the carriers before and after this
transition is markedly different," says study lead author Uri
Zondiner. "Before the transition the carriers are extremely heavy,
and after it they seem to be extremely light, reminiscent of the 'Dirac electrons' that are present in graphene." The same behaviour was seen
to repeat near every integer filling, where heavy carriers abruptly gave
way and light Dirac-like electrons re-emerged.

But how can such an abrupt change in the nature of the carriers
be understood? To address this question, the team worked together
with Weizmann theorists Profs. Erez Berg, Yuval Oreg and Ady Stern,
and Dr. Raquel Quiroez; as well as Prof. Felix von-Oppen of Freie
Universita"t Berlin. They constructed a simple model, revealing that
electrons fill the energy bands in MATBG in a highly unusual "Sisyphean" manner: when electrons start filling from the "Dirac point" (the point
at which the valence and conduction bands just touch each other), they
behave normally, being distributed equally among the four possible
flavours. "However, when the filling nears that of an integer number
of electrons per moire' superlattice site, a dramatic phase transition
occurs," explains study lead author Asaf Rozen. "In this transition,
one flavour 'grabs' all the carriers from its peers, 'resetting' them
back to the charge-neutral Dirac point." "Left with no electrons, the
three remaining flavours need to start refilling again from scratch. They
do so until another phase transition occurs, where this time one of the remaining three flavours grabs all the carriers from its peers, pushing
them back to square one. Electrons thus need to climb a mountain like
Sisyphus, being constantly pushed back to the starting point in which
they revert to the behavior of light Dirac electrons," says Rozen. While
this system is in a highly symmetric state at low carrier fillings, in
which all the electronic flavours are equally populated, with further
filling it experiences a cascade of symmetry-breaking phase transitions
that repeatedly reduce its symmetry.



==========================================================================
A "parent state" "What is most surprising is that the phase transitions
and Dirac revivals that we discovered appear at temperatures well above
the onset of the superconducting and correlated insulating states observed
so far," says Ilani.

"This indicates that the broken symmetry state we have seen is, in fact,
the 'parent state' out of which the more fragile superconducting and
correlated insulating ground states emerge." The peculiar way in which
the symmetry is broken has important implications for the nature of the insulating and superconducting states in this twisted system.

"For example, it is well known that stronger superconductivity arises when electrons are heavier. Our experiment, however, demonstrates the exact opposite: superconductivity appears in this magic-angle graphene system
after a phase transition has revived the light Dirac electrons. How this happens, and what it tells us about the nature of superconductivity in
this system compared to other more conventional forms of superconductivity remain interesting open questions," says Zondiner.

A similar cascade of phase transitions was reported in another paper
published in the same Nature issue by Prof. Ali Yazdani and colleagues at Princeton University. "The Princeton team studied MATBG using a completely different experimental technique, based on a highly-sensitive scanning tunneling microscope, so it is very reassuring to see that complementary techniques lead to analogous observations," says Ilani.

The Weizmann and MIT researchers say they will now use their scanning
nanotube single-electron-transistor platform to answer these and other
basic questions about electrons in various twisted-layer systems: What
is the relationship between the compressibility of electrons and their
apparent transport properties? What is the nature of the correlated
states that form in these systems at low temperatures? And what are the fundamental quasiparticles that make up these states?

========================================================================== Story Source: Materials provided by Weizmann_Institute_of_Science. Note: Content may be edited for style and length.


========================================================================== Journal Reference:
1. U. Zondiner, A. Rozen, D. Rodan-Legrain, Y. Cao, R. Queiroz, T.

Taniguchi, K. Watanabe, Y. Oreg, F. von Oppen, Ady Stern,
E. Berg, P.

Jarillo-Herrero, S. Ilani. Cascade of phase transitions and Dirac
revivals in magic-angle graphene. Nature, 2020; 582 (7811): 203
DOI: 10.1038/s41586-020-2373-y ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2020/06/200622095027.htm

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