Hitting the ground running at I05

Since coming online in 2014, the Angle-Resolved Photoemission Spectroscopy (ARPES) beamline, known as I05, has been publishing ground-breaking research in a number of high-impact physics journals. 2015 saw 22 articles published in 13 different journals (including Nature Nanotechnology, Science, and Nature Physics), together scoring an average Impact Factor of 14.241. Here we get a taste of what I05 can do, and with summaries of the big hitters from last year.

• Witnessing the birth of Weyltronics at Diamond
• Negative electron compressibility in WSe₂
• Investigating superconductivity in lightly doped Sr₂IrO₄

I05 is dedicated to the study of electronic structure using the ARPES technique. By using intense monochromatic UV light, photoelectrons around the Fermi surface are expelled from a material and their energy and momentum are measured using a high-resolution multiplexing electron analyser. This information can then be used to calculate the momentum states, Fermi surface, and electronic structure within the sample.

The understanding of electronic bands within materials, and in particular, knowledge of its Fermi surface, is very important in understanding the fundamental nature of materials, and their properties on a micro and macro-scale. As well as furthering our understanding of material physics, research in this area could lead to developments in new materials with improved properties of conductivity and mechanisms for electronics.

As well as ARPES band and Fermi surface mapping, the beamline can be used to study the spectral function of quasiparticles and spatial mapping in Nano-ARPES, and offers extensive sample preparation facilities. Depending on the technique photon energies between 18-240 eV are available and energy resolutions of 2 meV and angular resolutions of 0.1° are possible.

I05 was initially proposed to Diamond by a group of academics led by Professor Felix Baumberger and then the current Principal Beamline Scientist, Dr Moritz Hoesch (pictured), led the design and construction. To achieve the level of results required it was important that all of the aspects of the beamline were carefully designed and optimised. Elements including the temperature control, sample handling system, detector, and beam itself combine to offer very precise and reliable results. The construction of I05 filled a large hole in the portfolio of scientific facilities available in the UK, and the attention to detail has enabled Diamond to compete as one of the top facilities for high-resolution ARPES in the world.

Shekhar et al. Extremely large magnetoresistance and ultrahigh mobility in the topological Weyl semimetal candidate NbP. Nature Physics (2015). DOI:10.1038/nphys3372

Xu et al. Discovery of a Weyl Fermion state with Fermi arcs in niobium arsenide. Nature Physics (2015). DOI:10.1038/nphys3437

Yang et al. Weyl semimetal phase in the non-centrosymmetric compound TaAs. Nature Physics (2015). DOI:10.1038/nphys3425

In 1928, Paul Dirac derived his eponymous equation, describing the physics of fundamental particles called Fermions. It predicted the existence of electrons and positrons, with positrons discovered in 1932. In 1929, mathematician Hermann Weyl derived another solution to the equation, theorising the existence of massless Fermions.

In the intervening years, Weyl Fermions have proved elusive, but early in 2015 it was predicted that they could be detected in rare but relatively simple solid state crystals, Weyl semimetals, such as tantalum arsenide (TaAs). The race was on to demonstrate their existence, and as it turns out, Diamond’s ARPES beamline, I05, is the perfect tool for the job.

Topological electronic structure of NbAs: Weyl nodes and Fermi arcs. First-principles band structure calculated (left) and the ARPES-measured Fermi surface (right) of the (001) Fermi surface of NbAs. The Fermi arcs are clearly resolved in the ARPES measurements in agreement with the theoretical prediction.

Weyl Fermions are exotic particles, displaying unusual behaviour - the key to finding them is to look for Weyl points within the sample, and Fermi arcs on the surface. ARPES allows researchers to look at both the surface and bulk of a sample with synchrotron light.

A trio of scientific papers recorded the results from the first wave of researchers to go looking. In June 2015, Shekhar et al. published the results of their experiments on the I05 beamline at Diamond and the Advanced Light Source (ALS) in California, which suggested that niobium phosphide (NbP) is a Weyl semimetal.

By August, Xu et al. had published results from experiments at Diamond and the Swiss Light Source, proving that niobium arsenide (NbAs) is a Weyl semimetal. In the same journal, Yang et al. published results from experiments at Diamond and the ALS, demonstrating that TaAs is indeed a Weyl semimetal.

Exciting in their own right, these results also usher in an exciting new era in condensed matter physics, beyond graphene and 3D topological insulators. Electrons can be organised into pairs of massless Weyl Fermions, which could move electric charge around faster than current electronics. The absence of inversion symmetry in the crystal structure of TaAs splits 3D-graphene-like bands obeying the Dirac equation into two bands of Weyl Fermions obeying the Weyl equation. Since these Weyl Fermions are immune to scattering backwards, on top of the fact that they act as if they don’t possess mass, they could give rise to ultra-high mobilities and therefore much more efficient transport of electric charge than conventional devices, or even graphene. These ideas are already giving rise to the nascent field of ‘Weyltronics’.

Weyltronics could therefore offer the possibility of faster electronics and energy efficient computing. And because Weyl Fermions are less prone to interacting with their surroundings, Weyltronics should also allow new kinds of quantum computers that are more resistant to disruption.

Riley et al. Negative electronic compressibility and tunable spin splitting in WSe₂. Nature Nanotechnology (2015). DOI: 10.1038/nnano.2015.217

Transition-metal dichalcogenides (TMDs) are a unique class of two-dimensional semiconductors with wide-ranging practical applications. TMD field-effect transistors have already been fabricated, uncovering a range of intriguing properties, such as chiral light emission, weak antilocalisation, and a density-tuned dome of superconductivity.

Negative electron compressibility (NEC) is an effect whereby an electron system lowers its highest energy level and effectively shrinks in size when more electrons are added. Such effects are critical to our understanding of how semiconductor properties evolve with the application of electrical gate voltages - the standard method for field-effect control of semiconductor devices. However, a detailed understanding of what drives the intriguing emergent properties in TMDs has remained elusive.

Figure: Tunable valley spin splitting. ARPES measurements of the dispersion of the 2DEG formed at T as a function of increasing the surface doping. The associated Fermi surface evolution is shown over k_x,y= k_T ± 0.17Å^-1.

Researchers used the ARPES beamline to provide a direct spectroscopic signature of NEC in tungsten diselenide (WSe₂), a very stable prototypical strong spin–orbit TMD semiconductor. They showed that doping electrons at the surface of WSe₂, which is akin to applying a gate voltage in a transistor-type device, induces a counterintuitive lowering of the surface chemical potential.

Their findings established TMDs as strongly interacting systems, opening up the potential to control and eventually exploit their optoelectronic and spintronic properties for a new generation of multifunctional electronic devices. Developing and exploiting this new understanding will open up opportunities for advanced electronic and quantum-logic devices.

De la Torre et al. Collapse of the Mott gap and emergence of a nodal liquid in lightly doped Sr2IrO4. Physics Review Letters (2015). DOI:10.1103/PhysRevLett.115.176402

Mott insulators are a class of materials that should be conductors, according to conventional band theories, but which are insulators - particularly at low temperatures. First described in 1937, the proposed explanation was that this anomaly is due to electronelectron interactions not considered in conventional band theories.

Mott insulators are of interest because some of them, the layered copper oxides, or ‘cuprates’, become superconducting up to high critical temperature when their chemical composition is modified slightly to ‘dope’ them with additional valence electrons (or ‘holes’).

Our understanding of the copper oxide high-temperature superconductors is hindered by a lack of electronic structure data from related Mott insulators such as Sr₂IrO₄, a layered material characterised by a correlation induced excitation gap in a single half-filled band and strong Heisenberg antiferromagnetic coupling of the spin moments, just as the cuprates. Using ARPES on the I05 beamline, researchers have discovered an electronic state with striking similarities to the ‘pseudopgap’ precursor state of hightemperature superconductivity in lightly doped Sr₂IrO₄.

Their conclusion is that anisotropic pseudogaps are a generic property of two-dimensional doped Mott insulators, rather than a unique hallmark of cuprate high-temperature superconductivity. This could help us to understand the famous high-temperature superconductors such as doped La₂CuO₄ or YBa₂Cu₃O_7-x.

Collapse of the Mott gap. (a),(d) Photoemission intensity in the fully gapped parent insulator along the nodal and antinodal direction, respectively. (b),(e) For χ = 0.05, electronlike metallic states appear at (π/2, π/2) while the apex of the hole-like band at (π, 0) moves above the chemical potential resulting in a collapse of the charge gap. (c),(f) Curvature plots of the raw data to extract band positions.