We present the science and technology roadmap for graphene, related two-dimensional crystals, and hybrid systems, targeting an evolution in technology, that might lead to impacts and benefits reaching into most areas of society. This roadmap was developed within the framework of the European Graphene Flagship and outlines the main targets and research areas as best understood at the start of this ambitious project. We provide an overview of the key aspects of graphene and related materials (GRMs), ranging from fundamental research challenges to a variety of applications in a large number of sectors, highlighting the steps necessary to take GRMs from a state of raw potential to a point where they might revolutionize multiple industries. We also define an extensive list of acronyms in an effort to standardize the nomenclature in this emerging field.

/pdf/science-and-technology-roadmap-for-graphene-related-two-4q9m7czlkc.pdf

Science and technology roadmap for graphene, related two-dimensional crystals, and hybrid systems

The magnetoresistance effect in WTe2, a layered semimetal, is extremely large: the electrical resistance can be changed by more than 13 million per cent at very high magnetic fields and low temperatures. Apply a magnetic field to a magnetoresistive material and its electrical resistance changes — a technologically useful phenomenon that is harnessed, for example, in the data-reading sensors of hard drives. Mazhar Ali and colleagues have now identified a material (tungsten ditelluride or WTe2) in which the magnetoresistance effect is unusually large: the electrical resistance can be changed by more than 13 million per cent. Its remarkable magnetoresitance is evident at very high magnetic fields and at extremely low temperatures, so practical applications are not yet in prospect. But this finding suggests new directions in the study of magnetoresistivity that could ultimately lead to new uses of this effect. Magnetoresistance is the change in a material’s electrical resistance in response to an applied magnetic field. Materials with large magnetoresistance have found use as magnetic sensors1, in magnetic memory2, and in hard drives3 at room temperature, and their rarity has motivated many fundamental studies in materials physics at low temperatures4. Here we report the observation of an extremely large positive magnetoresistance at low temperatures in the non-magnetic layered transition-metal dichalcogenide WTe2: 452,700 per cent at 4.5 kelvins in a magnetic field of 14.7 teslas, and 13 million per cent at 0.53 kelvins in a magnetic field of 60 teslas. In contrast with other materials, there is no saturation of the magnetoresistance value even at very high applied fields. Determination of the origin and consequences of this effect, and the fabrication of thin films, nanostructures and devices based on the extremely large positive magnetoresistance of WTe2, will represent a significant new direction in the study of magnetoresistivity.

Large, non-saturating magnetoresistance in WTe2.

The search for the coexistence between superconductivity and other collective electronic states in many instances promoted the discovery of novel states of matter. The manner in which the different types of electronic order combine remains an ongoing puzzle. 1T-TaS(2) is a layered material, and the only transition-metal dichalcogenide (TMD) known to develop the Mott phase. Here, we show the appearance of a series of low-temperature electronic states in 1T-TaS(2) with pressure: the Mott phase melts into a textured charge-density wave (CDW); superconductivity develops within the CDW state, and survives to very high pressures, insensitive to subsequent disappearance of the CDW state and, surprisingly, also the strong changes in the normal state. This is also the first reported case of superconductivity in a pristine 1T-TMD compound. We demonstrate that superconductivity first develops within the state marked by a commensurability-driven, Coulombically frustrated, electronic phase separation.

/pdf/from-mott-state-to-superconductivity-in-1t-tas2-u2ardn6g05.pdf

From Mott state to superconductivity in 1T-TaS2

The occurrence of charge-density waves in three selected layered transition-metal dichalcogenides-1T-TaS(2), 2H-TaSe(2) and 1T-TiSe(2)-is discussed from an experimentalist's point of view with a particular focus on the implications of recent angle-resolved photoelectron spectroscopy results. The basic models behind charge-density-wave formation in low-dimensional solids are recapitulated, the experimental and theoretical results for the three selected compounds are reviewed, and their band structures and spectral weight distributions in the commensurate charge-density-wave phases are calculated using an empirical tight-binding model. It is explored whether the origin of charge-density waves in the layered transition-metal dichalcogenides can be understood in a unified way on the basis of a few measured and calculated parameters characterizing the interacting electron-lattice system. It is found that the predictions of the standard mean-field model agree only semi-quantitatively with the experimental data and that there is not one generally dominant factor driving charge-density-wave formation in this family of layer compounds. The need for further experimental and theoretical scrutiny is emphasized.

https://iopscience.iop.org/article/10.1088/0953-8984/23/21/213001/pdf

On the origin of charge-density waves in select layered transition-metal dichalcogenides.

The ability to tune material properties using gating by electric fields is at the heart of modern electronic technology. It is also a driving force behind recent advances in two-dimensional systems, such as the observation of gate electric-field-induced superconductivity and metal-insulator transitions. Here, we describe an ionic field-effect transistor (termed an iFET), in which gate-controlled Li ion intercalation modulates the material properties of layered crystals of 1T-TaS2. The strong charge doping induced by the tunable ion intercalation alters the energetics of various charge-ordered states in 1T-TaS2 and produces a series of phase transitions in thin-flake samples with reduced dimensionality. We find that the charge-density wave states in 1T-TaS2 collapse in the two-dimensional limit at critical thicknesses. Meanwhile, at low temperatures, the ionic gating induces multiple phase transitions from Mott-insulator to metal in 1T-TaS2 thin flakes, with five orders of magnitude modulation in resistance, and superconductivity emerges in a textured charge-density wave state induced by ionic gating. Our method of gate-controlled intercalation opens up possibilities in searching for novel states of matter in the extreme charge-carrier-concentration limit.

Gate-tunable phase transitions in thin flakes of 1T-TaS2

High-resolution angle-resolved photoemission spectroscopy (ARPES) was used to investigate the transition metal dichalcogenide (TMC) $1T\ensuremath{-}{\mathrm{TiSe}}_{2}$ above and below the phase transition. We find that this system fulfills special conditions such as narrow band width and flat dispersion for bands within $5{\mathrm{k}}_{B}T$ of the Fermi energy. These prerequisites allow ARPES to observe energy dispersion of bands above ${E}_{F}$ without normalization procedures and a leading edge of the Fermi-Dirac distribution cutoff, which is considerably shifted to the unoccupied region with respect to ${E}_{F}.$ As a consequence we conclude that the $\mathrm{Ti}3d$ band is only thermally occupied at room temperature and considerably shifts towards the occupied range upon cooling. When passing the phase transition, the $\mathrm{Se}4p$ bands become backfolded due to new symmetry restrictions. The temperature behavior of the ARPES spectra can, in accordance to transport data, be explained as the occurrence of an excitonic phase suggested by Kohn [Phys. Rev. Lett. 19, 439 (1967)].

/pdf/photoemission-of-bands-above-the-fermi-level-the-excitonic-39g7fqm8kd.pdf

Photoemission of bands above the Fermi level: The excitonic insulator phase transition in 1 T − TiSe 2

https://doc.rero.ch/record/9076/files/Diederich_L._-_Photoelectron_emission_from_nitrogen_20080409.pdf

Photoelectron emission from nitrogen- and boron-doped diamond (100) surfaces

We present a detailed high-resolution angle-resolved photoemission study of the electronic band structure of the room-temperature quasicommensurate charge-density-wave phase of $1T\ensuremath{-}{\mathrm{TaS}}_{2}.$ In particular, we show that no crossings of the Fermi level are visible in the complete Brillouin zone, indicating that an electron-electron correlation-induced pseudogap in the Ta $5d$ derived band exists already above the Mott localization-induced transition at 180 K. Moreover, we find that the electronic structure is governed by at least two quasiparticle peaks, which can be assigned to electrons from starlike shells of Ta atoms within the distorted crystal lattice. These peaks show quasilocalized (dispersionless) behavior in parts of the Brillouin zone where the one-particle band is unoccupied and they follow the one-particle dispersion in the occupied part. In order to address the question of possible Fermi-surface (FS) nesting, we scanned the remaining remnant FS and found regions with a considerable decrease of spectral weight. However, we find no clear evidence for FS nesting.

http://doc.rero.ch/record/8998/files/Pillo_Th._-_Interplay_between_electron-electron_20080327.pdf

Interplay between electron-electron interaction and electron-phonon coupling near the Fermi surface of 1 T − TaS 2

We report high resolution angle-scanned photoemission and Fermi surface (FS) mapping experiments on the layered transition-metal dichalcogenide 1T-TaS2 in the quasicommensurate (metallic) and the commensurate (insulating) charge-density-wave (CDW) phase. Instead of a nesting induced partially removed FS in the CDW phase we find a pseudogap over large portions of the FS. This remnant FS exhibits the symmetry of the one-particle normal state FS. Possibly, this Mott localization induced transition represents the underlying instability responsible fur the pseudogapped FS.

http://arxiv.org/pdf/cond-mat/9902327.pdf

Remnant fermi surface in the presence of an underlying instability in layered 1T-TaS2

https://doc.rero.ch/record/9105/files/Aebi_P._-_On_the_search_for_Fermi_surface_20080410.pdf

Th. Pillo

Papers

Photoemission of bands above the Fermi level: The excitonic insulator phase transition in 1 T − TiSe 2

Photoelectron emission from nitrogen- and boron-doped diamond (100) surfaces

Interplay between electron-electron interaction and electron-phonon coupling near the Fermi surface of 1 T − TaS 2

Remnant fermi surface in the presence of an underlying instability in layered 1T-TaS2

On the search for Fermi surface nesting in quasi-2D materials