Switching on and off the spin polarization of the conduction band in antiferromagnetic bilayer transistors.

Antiferromagnetic conductors with suitably broken spatial symmetries host spin-polarized bands, which lead to transport phenomena commonly observed in metallic ferromagnets. In bulk materials, it is the given crystalline structure that determines whether symmetries are broken and spin-polarized bands are present. Here we show that, in the two-dimensional limit, an electric field can control the relevant symmetries.

Spin-Valve Effect in Junctions with a Single Ferromagnet.

Spin valves are essential components in spintronic memory devices whose conductance is modulated by controlling spin-polarized electron tunnelling through the alignment of the magnetization in ferromagnetic elements. Whereas conventional spin valves unavoidably require at least two ferromagnetic elements, here we demonstrate a van der Waals spin valve based on a tunnel junction that comprises only one such ferromagnetic layer. Our devices combine an FeGeTe electrode acting as a spin injector together with a paramagnetic tunnel barrier, formed by a CrBr multilayer operated above its Curie temperature.

Moiré magnetism in CrBr multilayers emerging from differential strain.

Interfaces between twisted 2D materials host a wealth of physical phenomena originating from the long-scale periodicity associated with the resulting moiré structure. Besides twisting, an alternative route to create structures with comparably long-or even longer-periodicities is inducing a differential strain between adjacent layers in a van der Waals (vdW) material. Despite recent theoretical efforts analyzing its benefits, this route has not yet been implemented experimentally.

Magnetism-Induced Band-Edge Shift as the Mechanism for Magnetoconductance in CrPS Transistors.

Transistors realized on the 2D antiferromagnetic semiconductor CrPS exhibit large magnetoconductance due to magnetic-field-induced changes in the magnetic state. The microscopic mechanism coupling the conductance and magnetic state is not understood. We identify it by analyzing the evolution of the parameters determining the transistor behavior─carrier mobility and threshold voltage─with temperature and magnetic field.

Multiple antiferromagnetic phases and magnetic anisotropy in exfoliated CrBr multilayers.

In twisted two-dimensional (2D) magnets, the stacking dependence of the magnetic exchange interaction can lead to regions of ferromagnetic and antiferromagnetic interlayer order, separated by non-collinear, skyrmion-like spin textures. Recent experimental searches for these textures have focused on CrI, known to exhibit either ferromagnetic or antiferromagnetic interlayer order, depending on layer stacking. However, the very strong uniaxial anisotropy of CrI disfavors smooth non-collinear phases in twisted bilayers.

Electronic Structure of Few-Layer Black Phosphorus from μ-ARPES.

Black phosphorus (BP) stands out among two-dimensional (2D) semiconductors because of its high mobility and thickness dependent direct band gap. However, the quasiparticle band structure of ultrathin BP has remained inaccessible to experiment thus far. Here we use a recently developed laser-based microfocus angle resolved photoemission (μ-ARPES) system to establish the electronic structure of 2-9 layer BP from experiment.

Determining spin-orbit coupling in graphene by quasiparticle interference imaging.

Inducing and controlling spin-orbit coupling (SOC) in graphene is key to create topological states of matter, and for the realization of spintronic devices. Placing graphene onto a transition metal dichalcogenide is currently the most successful strategy to achieve this goal, but there is no consensus as to the nature and the magnitude of the induced SOC. Here, we show that the presence of backscattering in graphene-on-WSe heterostructures can be used to probe SOC and to determine its strength quantitatively, by imaging quasiparticle interference with a scanning tunneling microscope.

Gate-Controlled Magnetotransport and Electrostatic Modulation of Magnetism in 2D Magnetic Semiconductor CrPS.

Using field-effect transistors (FETs) to explore atomically thin magnetic semiconductors with transport measurements is difficult, because the very narrow bands of most 2D magnetic semiconductors cause carrier localization, preventing transistor operation. Here, it is shown that exfoliated layers of CrPS -a 2D layered antiferromagnetic semiconductor whose bandwidth approaches 1 eV-allow the realization of FETs that operate properly down to cryogenic temperature. Using these devices, conductance measurements as a function of temperature and magnetic field are performed to determine the full magnetic phase diagram, which includes a spin-flop and a spin-flip phase.

Full Control of Solid-State Electrolytes for Electrostatic Gating.

Ionic gating is a powerful technique to realize field-effect transistors (FETs) enabling experiments not possible otherwise. So far, ionic gating has relied on the use of top electrolyte gates, which pose experimental constraints and make device fabrication complex. Promising results obtained recently in FETs based on solid-state electrolytes remain plagued by spurious phenomena of unknown origin, preventing proper transistor operation, and causing limited control and reproducibility.

Dynamic magnetic crossover at the origin of the hidden-order in van der Waals antiferromagnet CrSBr.

The van-der-Waals material CrSBr stands out as a promising two-dimensional magnet. Here, we report on its detailed magnetic and structural characteristics. We evidence that it undergoes a transition to an A-type antiferromagnetic state below T ≈ 140 K with a pronounced two-dimensional character, preceded by ferromagnetic correlations within the monolayers.

Quenching the bandgap of two-dimensional semiconductors with a perpendicular electric field.

Perpendicular electric fields can tune the electronic band structure of atomically thin semiconductors. In bilayer graphene, which is an intrinsic zero-gap semiconductor, a perpendicular electric field opens a finite bandgap. So far, however, the same principle could not be applied to control the properties of a broader class of 2D materials because the required electric fields are beyond reach in current devices.

Band Gap Opening in Bilayer Graphene-CrCl/CrBr/CrI van der Waals Interfaces.

We report experimental investigations of transport through bilayer graphene (BLG)/chromium trihalide (CrX; X = Cl, Br, I) van der Waals interfaces. In all cases, a large charge transfer from BLG to CrX takes place (reaching densities in excess of 10 cm), and generates an electric field perpendicular to the interface that opens a band gap in BLG. We determine the gap from the activation energy of the conductivity and find excellent agreement with the latest theory accounting for the contribution of the σ bands to the BLG dielectric susceptibility.

Probing Magnetism in Exfoliated VI Layers with Magnetotransport.

We perform magnetotransport experiments on VI multilayers to investigate the relation between ferromagnetism in bulk and in exfoliated layers. The magnetoconductance measured on field-effect transistors and tunnel barriers shows that the Curie temperature of exfoliated multilayers is = 57 K, larger than in bulk ( = 50 K). Below ≈ 40 K, we observe an unusual evolution of the tunneling magnetoconductance, analogous to the phenomenology observed in bulk.

Light sources with bias tunable spectrum based on van der Waals interface transistors.

Light-emitting electronic devices are ubiquitous in key areas of current technology, such as data communications, solid-state lighting, displays, and optical interconnects. Controlling the spectrum of the emitted light electrically, by simply acting on the device bias conditions, is an important goal with potential technological repercussions. However, identifying a material platform enabling broad electrical tuning of the spectrum of electroluminescent devices remains challenging.

Quasi-1D Electronic Transport in a 2D Magnetic Semiconductor.

Electronic transport through exfoliated multilayers of CrSBr, a 2D semiconductor of interest because of its magnetic properties, is investigated. An extremely pronounced anisotropy manifesting itself in qualitative and quantitative differences of all quantities measured along the in-plane a and b crystallographic directions is found. In particular, a qualitatively different dependence of the conductivities σ and σ on temperature and gate voltage, accompanied by orders of magnitude differences in their values (σ /σ  ≈ 3 × 10  to 10 at low temperature and negative gate voltage) are observed, together with a different behavior of the longitudinal magnetoresistance in the two directions and the complete absence of the Hall effect in transverse resistance measurements.

Magnetization dependent tunneling conductance of ferromagnetic barriers.

Recent experiments on van der Waals antiferromagnets have shown that measuring the temperature (T) and magnetic field (H) dependence of the conductance allows their magnetic phase diagram to be mapped. Similarly, experiments on ferromagnetic CrBr barriers enabled the Curie temperature to be determined at H = 0, but a precise interpretation of the magnetoconductance data at H ≠ 0 is conceptually more complex, because at finite H there is no well-defined phase boundary. Here we perform systematic transport measurements on CrBr barriers and show that the tunneling magnetoconductance depends on H and T exclusively through the magnetization M(H, T) over the entire temperature range investigated.

Persistence of Magnetism in Atomically Thin MnPS Crystals.

The magnetic state of atomically thin semiconducting layered antiferromagnets such as CrI and CrCl can be probed by forming tunnel barriers and measuring their resistance as a function of magnetic field () and temperature (). This is possible because the spins within each individual layer are ferromagnetically aligned and the tunneling magnetoresistance depends on the relative orientation of the magnetization in adjacent layers. The situation is different for systems that are antiferromagnetic within the layers in which case it is unclear whether magnetoresistance measurements can provide information about the magnetic state.

Design of van der Waals interfaces for broad-spectrum optoelectronics.

Van der Waals (vdW) interfaces based on 2D materials are promising for optoelectronics, as interlayer transitions between different compounds allow tailoring of the spectral response over a broad range. However, issues such as lattice mismatch or a small misalignment of the constituent layers can drastically suppress electron-photon coupling for these interlayer transitions. Here, we engineered type-II interfaces by assembling atomically thin crystals that have the bottom of the conduction band and the top of the valence band at the Γ point, and thus avoid any momentum mismatch.

Synthetic Semimetals with van der Waals Interfaces.

The assembly of suitably designed van der Waals (vdW) heterostructures represents a new approach to produce artificial systems with engineered electronic properties. Here, we apply this strategy to realize synthetic semimetals based on vdW interfaces formed by two different semiconductors. Guided by existing ab initio calculations, we select WSe and SnSe mono- and multilayers to assemble vdW interfaces and demonstrate the occurrence of semimetallicity by means of different transport experiments.

Determining the phase diagram of atomically thin layered antiferromagnet CrCl.

Changes in the spin configuration of atomically thin, magnetic van der Waals multilayers can cause drastic modifications in their opto-electronic properties. Conversely, the opto-electronic response of these systems provides information about the magnetic state, which is very difficult to obtain otherwise. Here, we show that in CrCl multilayers, the dependence of the tunnelling conductance on applied magnetic field, temperature and number of layers tracks the evolution of the magnetic state, enabling the magnetic phase diagram to be determined experimentally.

Band Filling and Cross Quantum Capacitance in Ion-Gated Semiconducting Transition Metal Dichalcogenide Monolayers.

Ionic liquid gated field-effect transistors (FETs) based on semiconducting transition metal dichalcogenides (TMDs) are used to study a rich variety of extremely interesting physical phenomena, but important aspects of how charge carriers are accumulated in these systems are not understood. We address these issues by means of a systematic experimental study of transport in monolayer MoSe and WSe as a function of magnetic field and gate voltage, exploring accumulated densities of carriers ranging from approximately 10 cm holes in the valence band to 4 × 10 cm electrons in the conduction band. We identify the conditions when the chemical potential enters different valleys in the monolayer band structure (the K and Q valley in the conduction band and the two spin-split K-valleys in the valence band) and find that an independent electron picture describes the occupation of states well.

Microfocus Laser-Angle-Resolved Photoemission on Encapsulated Mono-, Bi-, and Few-Layer 1T'-WTe.

Two-dimensional crystals of semi-metallic van der Waals materials hold much potential for the realization of novel phases, as exemplified by the recent discoveries of a polar metal in few-layer 1T'-WTe and of a quantum spin Hall state in monolayers of the same material. Understanding these phases is particularly challenging because little is known from experiments about the momentum space electronic structure of ultrathin crystals. Here, we report direct electronic structure measurements of exfoliated mono-, bi-, and few-layer 1T'-WTe by laser-based microfocus angle-resolved photoemission.

Scanning Tunneling Microscopy of an Air Sensitive Dichalcogenide Through an Encapsulating Layer.

Many atomically thin exfoliated two-dimensional (2D) materials degrade when exposed to ambient conditions. They can be protected and investigated by means of transport and optical measurements if they are encapsulated between chemically inert single layers in the controlled atmosphere of a glovebox. Here, we demonstrate that the same encapsulation procedure is also compatible with scanning tunneling microscopy (STM) and spectroscopy (STS).