SCIENCEscience.org 18 FEBRUARY 2022¥VOL 375 ISSUE 6582 775
Fig. 1. Broken isospin symmetry phases in BBG.(A) Lattice structure of BBG.
(B) Band structure calculated within a tight-binding model ( 11 ) near the Brillouin
zone corner. a 0 = 2.46 Å is the graphene lattice constant. (C) Noninteracting
density of states,r, at 10 mK. Different curves are calculated for varying interlayer
potential differences, as marked. Au.c.≈5.24 Å^2 is the graphene unit cell area.
(D)ras a function of carrier density calculated ( 11 ) for fixed interlayer potential
difference of 50 mV and temperature of 10 mK. (Insets) Fermi contours at the
indicated values ofne, plotted on the intervalðÞ 0 : 06 ; 0 : 06 a 0 ^1 for bothkxandky.
(E) Experimentally determined inverse compressibility at zero magnetic field.
(F) Fast Fourier transform (FFT) ofRxxðÞ 1 =B⊥ measured at the (ne,D) points
indicated by the colored symbols in (E). Thexaxis is the frequency normalized to
ne. Peaks reflect fractional areas of the Fermi sea enclosed by a phase-coherent
orbit. For each trace, we show a schematic of the spin-dependent Fermi surfaces in
the two valleys inspired by a rigid-band Stoner model of the type considered in
( 12 ). Here, each quadrant depicts the Fermi surface for a particular spin and valley
isospin flavor. a.u., arbitrary units.Fig. 2. Magnetic fieldÐinduced superconductivity.(A)Rxxmeasured at
fixedB‖= 0 at a nominal temperature of 10 mK. (B) Same as (A) but measured at
B‖= 165 mT. Zero resistance renders as bright cyan in a sash between the
Sym 12 and PIP 2 states forD≳1. (C)nedependence ofRxxmeasured at fixed
D=1.02Vnm−^1 andB‖= 165 mT and variable temperatures. (Inset)R(T)
measured atne=−0.57 × 10^12 cm−^2 andD=1.02Vnm−^1 .(D) Nonlinear transport
in the superconducting state measured atne=−0.57 × 10^12 cm−^2 ,D= 1.02 V nm−^1 ,
andB‖=165 mT. (Inset)V(I) plotted on a log scale. Dotted line corresponds
toV¼I^3 , from which we determineTBKT=26mK.(E)nedependence ofRxx
measured at fixedD=1.02Vnm−^1 andB‖= 165 mT, for variableB⊥.(F)B⊥-
dependent nonlinear transport for fixedne=−0.57 × 10^12 cm−^2 ,D=1.02Vnm−^1 ,
andB‖=165 mT.RESEARCH | REPORTS