11 research outputs found
Anomalous and anisotropic nonlinear susceptibility in the proximate Kitaev magnet α-RuCl3
The leading order nonlinear (NL) susceptibility, χ3, in a paramagnet is negative and diverges as T → 0. This divergence is destroyed when spins correlate and the NL response provides unique insights into magnetic order. Dimensionality, exchange interaction, and preponderance of quantum effects all imprint their signatures in the NL magnetic response. Here, we study the NL susceptibilities in the proximate Kitaev magnet α-RuCl3, which differs from the expected antiferromagnetic behavior. For T  0 implies a broken sublattice symmetry of magnetic order at low temperatures. Classical Monte Carlo (CMC) simulations in the standard K − H − Γ model secure such a quadratic B dependence of M, only for T ≈ Tc with χ2 being zero as T → 0. It is also zero for all temperatures in exact diagonalization calculations. On the other hand, we find an exclusive cubic term (χ3) that describes the high field NL behavior well. χ3 is large and positive both below and above Tc crossing zero only for T > 50 K. In contrast, for B ∥ c-axis, no separate low/high field behaviors are measured and only a much smaller χ3 is apparent
Imaging inter-valley coherent order in magic-angle twisted trilayer graphene
Magic-angle twisted trilayer graphene (MATTG) exhibits a range of strongly
correlated electronic phases that spontaneously break its underlying
symmetries. The microscopic nature of these phases and their residual
symmetries stands as a key outstanding puzzle whose resolution promises to shed
light on the origin of superconductivity in twisted materials. Here we
investigate correlated phases of MATTG using scanning tunneling microscopy and
identify striking signatures of interaction-driven spatial symmetry breaking.
In low-strain samples, over a filling range of about 2-3 electrons or holes per
moir\'e unit cell, we observe atomic-scale reconstruction of the graphene
lattice that accompanies a correlated gap in the tunneling spectrum. This
short-scale restructuring appears as a Kekul\'e supercell -- implying
spontaneous inter-valley coherence between electrons -- and persists in a wide
range of magnetic fields and temperatures that coincide with the development of
the gap. Large-scale maps covering several moir\'e unit cells further reveal a
slow evolution of the Kekul\'e pattern, indicating that atomic-scale
reconstruction coexists with translation symmetry breaking at the much longer
moir\'e scale. We employ auto-correlation and Fourier analyses to extract the
intrinsic periodicity of these phases and find that they are consistent with
the theoretically proposed incommensurate Kekul\'e spiral order. Moreover, we
find that the wavelength characterizing moir\'e-scale modulations monotonically
decreases with hole doping away from half-filling of the bands and depends only
weakly on the magnetic field. Our results provide essential insights into the
nature of MATTG correlated phases in the presence of strain and imply that
superconductivity emerges from an inter-valley coherent parent state.Comment: the main text, extended data figures, and S
Intervalley coherence and intrinsic spin-orbit coupling in rhombohedral trilayer graphene
Rhombohedral graphene multilayers provide a clean and highly reproducible
platform to explore the emergence of superconductivity and magnetism in a
strongly interacting electron system. Here, we use electronic compressibility
and local magnetometry to explore the phase diagram of this material class in
unprecedented detail. We focus on rhombohedral trilayer in the quarter metal
regime, where the electronic ground state is characterized by the occupation of
a single spin and valley isospin flavor. Our measurements reveal a subtle
competition between valley imbalanced (VI) orbital ferromagnets and intervalley
coherent (IVC) states in which electron wave functions in the two momentum
space valleys develop a macroscopically coherent relative phase. Contrasting
the in-plane spin susceptibility of the IVC and VI phases reveals the influence
of graphene's intrinsic spin-orbit coupling, which drives the emergence of a
distinct correlated phase with hybrid VI and IVC character. Spin-orbit also
suppresses the in-plane magnetic susceptibility of the VI phase, which allows
us to extract the spin-orbit coupling strength of eV for
our hexagonal boron nitride-encapsulated graphene system. We discuss the
implications of finite spin-orbit coupling on the spin-triplet superconductors
observed in both rhombohedral and twisted graphene multilayers
• Collaboration with European Universities and Industry
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