Valla Fatemi

Electrically tunable surface-to-bulk coherent coupling in topological insulator thin films

We study coherent transport in density tunable micro-devices patterned from thin films of the topological insulator (TI) Bi2Se3. The devices exhibit pronounced electric field effect, including ambipolar modulation of the resistance with an on/off ratio of 500%. We show that the weak antilocalization (WAL) correction to conductance is sensitive to the number of coherently coupled channels, which in a TI includes the top and bottom surface and the bulk carriers. These are separated into coherently independent channels by the application of gate voltage and at elevated temperatures. Our results are consistent with a model where channel separation is determined by a competition between the coherence time and surface-bulk scattering time.

Unconventional superconductivity in magic-angle graphene superlattices

The behaviour of strongly correlated materials, and in particular unconventional superconductors, has been studied extensively for decades, but is still not well understood. This lack of theoretical understanding has motivated the development of experimental techniques for studying such behaviour, such as using ultracold atom lattices to simulate quantum materials. Here we report the realization of intrinsic unconventional superconductivity—which cannot be explained by weak electron–phonon interactions—in a two-dimensional superlattice created by stacking two sheets of graphene that are twisted relative to each other by a small angle. For twist angles of about 1.1°—the first ‘magic’ angle—the electronic band structure of this ‘twisted bilayer graphene’ exhibits flat bands near zero Fermi energy, resulting in correlated insulating states at half-filling. Upon electrostatic doping of the material away from these correlated insulating states, we observe tunable zero-resistance states with a critical temperature of up to 1.7 kelvin. The temperature–carrier-density phase diagram of twisted bilayer graphene is similar to that of copper oxides (or cuprates), and includes dome-shaped regions that correspond to superconductivity. Moreover, quantum oscillations in the longitudinal resistance of the material indicate the presence of small Fermi surfaces near the correlated insulating states, in analogy with underdoped cuprates. The relatively high superconducting critical temperature of twisted bilayer graphene, given such a small Fermi surface (which corresponds to a carrier density of about 1011 per square centimetre), puts it among the superconductors with the strongest pairing strength between electrons. Twisted bilayer graphene is a precisely tunable, purely carbon-based, two-dimensional superconductor. It is therefore an ideal material for investigations of strongly correlated phenomena, which could lead to insights into the physics of high-critical-temperature superconductors and quantum spin liquids.Yuan Cao, ∗ Valla Fatemi, Shiang Fang, Kenji Watanabe, Takashi Taniguchi, Efthimos Kaxiras, 4 and Pablo Jarillo-Herrero † Department of Physics, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA Department of Physics, Harvard University, Cambridge, Massachusetts 02138, USA National Institute for Materials Science, Namiki 1-1, Tsukuba, Ibaraki 305-0044, Japan John A. Paulson School of Engineering and Applied Sciences, Harvard University, Cambridge, Massachusetts 02138, USA (Dated: May 22, 2018)

Correlated insulator behaviour at half-filling in magic-angle graphene superlattices

A van der Waals heterostructure is a type of metamaterial that consists of vertically stacked two-dimensional building blocks held together by the van der Waals forces between the layers. This design means that the properties of van der Waals heterostructures can be engineered precisely, even more so than those of two-dimensional materials. One such property is the ‘twist’ angle between different layers in the heterostructure. This angle has a crucial role in the electronic properties of van der Waals heterostructures, but does not have a direct analogue in other types of heterostructure, such as semiconductors grown using molecular beam epitaxy. For small twist angles, the moiré pattern that is produced by the lattice misorientation between the two-dimensional layers creates long-range modulation of the stacking order. So far, studies of the effects of the twist angle in van der Waals heterostructures have concentrated mostly on heterostructures consisting of monolayer graphene on top of hexagonal boron nitride, which exhibit relatively weak interlayer interaction owing to the large bandgap in hexagonal boron nitride. Here we study a heterostructure consisting of bilayer graphene, in which the two graphene layers are twisted relative to each other by a certain angle. We show experimentally that, as predicted theoretically, when this angle is close to the ‘magic’ angle the electronic band structure near zero Fermi energy becomes flat, owing to strong interlayer coupling. These flat bands exhibit insulating states at half-filling, which are not expected in the absence of correlations between electrons. We show that these correlated states at half-filling are consistent with Mott-like insulator states, which can arise from electrons being localized in the superlattice that is induced by the moiré pattern. These properties of magic-angle-twisted bilayer graphene heterostructures suggest that these materials could be used to study other exotic many-body quantum phases in two dimensions in the absence of a magnetic field. The accessibility of the flat bands through electrical tunability and the bandwidth tunability through the twist angle could pave the way towards more exotic correlated systems, such as unconventional superconductors and quantum spin liquids.

Observation of the quantum spin Hall effect up to 100 kelvin in a monolayer crystal

Heating up the quantum spin Hall effect Taking practical advantage of the topologically protected conducting edge states of topological insulators (TIs) has proven difficult. Semiconductor systems that have been identified as two-dimensional TIs must be cooled down to near liquid helium temperatures to bring out their topological character. Wu et al. fabricated a heterostructure consisting of a monolayer of WTe2 placed between two layers of hexagonal boron nitride and found that its topological properties persisted up to a relatively high temperature of 100 K. Engineering this so-called quantum spin Hall effect in a van der Waals heterostructure makes it possible to apply many established experimental tools and functionalities. Science, this issue p. 76 Transport measurements show that the monolayer of WTe2 has helical edge modes at elevated temperatures. A variety of monolayer crystals have been proposed to be two-dimensional topological insulators exhibiting the quantum spin Hall effect (QSHE), possibly even at high temperatures. Here we report the observation of the QSHE in monolayer tungsten ditelluride (WTe2) at temperatures up to 100 kelvin. In the short-edge limit, the monolayer exhibits the hallmark transport conductance, ~e2/h per edge, where e is the electron charge and h is Planck’s constant. Moreover, a magnetic field suppresses the conductance, and the observed Zeeman-type gap indicates the existence of a Kramers degenerate point and the importance of time-reversal symmetry for protection from elastic backscattering. Our results establish the QSHE at temperatures much higher than in semiconductor heterostructures and allow for exploring topological phases in atomically thin crystals.

Superlattice-Induced Insulating States and Valley-Protected Orbits in Twisted Bilayer Graphene

Twisted bilayer graphene (TBLG) is one of the simplest van der Waals heterostructures, yet it yields a complex electronic system with intricate interplay between moiré physics and interlayer hybridization effects. We report on electronic transport measurements of high mobility small angle TBLG devices showing clear evidence for insulating states at the superlattice band edges, with thermal activation gaps several times larger than theoretically predicted. Moreover, Shubnikov-de Haas oscillations and tight binding calculations reveal that the band structure consists of two intersecting Fermi contours whose crossing points are effectively unhybridized. We attribute this to exponentially suppressed interlayer hopping amplitudes for momentum transfers larger than the moiré wave vector.

Electrostatic Coupling between Two Surfaces of a Topological Insulator Nanodevice

We report on electronic transport measurements of dual-gated nanodevices of the low-carrier density topological insulator (TI) Bi_{1.5}Sb_{0.5}Te_{1.7}Se_{1.3}. In all devices, the upper and lower surface states are independently tunable to the Dirac point by the top and bottom gate electrodes. In thin devices, electric fields are found to penetrate through the bulk, indicating finite capacitive coupling between the surface states. A charging model allows us to use the penetrating electric field as a measurement of the intersurface capacitance C_{TI} and the surface state energy-density relationship μ(n), which is found to be consistent with independent angle-resolved photoemission spectroscopy measurements. At high magnetic fields, increased field penetration through the surface states is observed, strongly suggestive of the opening of a surface state band gap due to broken time-reversal symmetry.

Magnetoresistance and quantum oscillations of an electrostatically tuned semimetal-to-metal transition in ultrathin WTe 2

We report on electronic transport measurements of electrostatically gated nano-devices of the semimetal WTe\textsubscript{2}. High mobility metallic behavior is achieved in the 2D limit by encapsulating thin flakes in an inert atmosphere. At low temperatures, we find that a large magnetoresistance can be turned on and off by electrostatically doping the system between a semimetallic state and an electron-only metallic state, respectively. We confirm the nature of the two regimes by analyzing the magnetoresistance and Hall effect with a two-carrier model, as well as by analysis of Shubnikov-de Haas oscillations, both of which indicate depletion of hole carriers via the electrostatic gate. This confirms that semiclassical transport of two oppositely charged carriers accurately describes the exceptional magnetoresistance observed in this material. Finally, we also find that the magnetoresistance power law is sub-quadratic and density-independent, suggesting new physics specifically in the semimetallic regime.

Electrically switchable Berry curvature dipole in the monolayer topological insulator WTe 2

Recent experimental evidence for the quantum spin Hall (QSH) state in monolayer WTe2 has linked the fields of two-dimensional materials and topological physics1–7. This two-dimensional topological crystal also displays unconventional spin–torque8 and gate-tunable superconductivity7. Whereas the realization of the QSH has demonstrated the nontrivial topology of the electron wavefunctions of monolayer WTe2, the geometrical properties of the wavefunction, such as the Berry curvature9, remain unstudied. Here we utilize mid-infrared optoelectronic microscopy to investigate the Berry curvature in monolayer WTe2. By optically exciting electrons across the inverted QSH gap, we observe an in-plane circular photogalvanic current even under normal incidence. The application of an out-of-plane displacement field allows further control of the direction and magnitude of the photocurrent. The observed photocurrent reveals a Berry curvature dipole that arises from the nontrivial wavefunctions near the inverted gap edge. The Berry curvature dipole and strong electric field effect are enabled by the inverted band structure and tilted crystal lattice of monolayer WTe2. Such an electrically switchable Berry curvature dipole may facilitate the observation of a wide range of quantum geometrical phenomena such as the quantum nonlinear Hall10,11, orbital-Edelstein12 and chiral polaritonic effects13,14.Optoelectronic experiments show that a monolayer of WTe2 is a material that simultaneously has topological electronic states and electron wavefunctions with a dipole in their Berry curvature.

Electrically tunable low-density superconductivity in a monolayer topological insulator

A monolayer of many talents Superconductors with a topologically nontrivial band structure have been predicted to exhibit exotic properties. However, such materials are few and far between. Now, two groups show that the monolayer of the material tungsten ditelluride (WTe2)—already known to be a two-dimensional topological insulator—can also go superconducting. Fatemi et al. and Sajadi et al. varied the carrier density in the monolayer by applying a gate voltage and observed a transition from a topological to a superconducting phase. The findings may lead to the fabrication of devices in which local gating enables topological and superconducting phases to exist in the same material. Science, this issue p. 926, p. 922 Modulating carrier concentration induces a transition from a topological to a superconducting phase in monolayer WTe2. Turning on superconductivity in a topologically nontrivial insulator may provide a route to search for non-Abelian topological states. However, existing demonstrations of superconductor-insulator switches have involved only topologically trivial systems. Here we report reversible, in situ electrostatic on-off switching of superconductivity in the recently established quantum spin Hall insulator monolayer tungsten ditelluride (WTe2). Fabricated into a van der Waals field-effect transistor, the monolayer’s ground state can be continuously gate-tuned from the topological insulating to the superconducting state, with critical temperatures Tc up to ~1 kelvin. Our results establish monolayer WTe2 as a material platform for engineering nanodevices that combine superconducting and topological phases of matter.

Enhanced Superconductivity and Suppression of Charge-density Wave Order in 2H-TaS 2 in the Two-dimensional Limit

As superconductors are thinned down to the 2D limit, their critical temperature