Measuring the Berry phase of graphene from wavefront dislocations in Friedel oscillations.

Electronic band structures dictate the mechanical, optical and electrical properties of crystalline solids. Their experimental determination is therefore crucial for technological applications. Although the spectral distribution in energy bands is routinely measured by various techniques1, it is more difficult to access the topological properties of band structures such as the quantized Berry phase, γ, which is a gauge-invariant geometrical phase accumulated by the wavefunction along an adiabatic cycle2. In graphene, the quantized Berry phase γ = π accumulated by massless relativistic electrons along cyclotron orbits is evidenced by the anomalous quantum Hall effect4,5. It is usually thought that measuring the Berry phase requires the application of external electromagnetic fields to force the charged particles along closed trajectories3. Contradicting this belief, here we demonstrate that the Berry phase of graphene can be measured in the absence of any external magnetic field. We observe edge dislocations in oscillations of the charge density ρ (Friedel oscillations) that are formed at hydrogen atoms chemisorbed on graphene. Following Nye and Berry6 in describing these topological defects as phase singularities of complex fields, we show that the number of additional wavefronts in the dislocation is a real-space measure of the Berry phase of graphene. Because the electronic dispersion relation can also be determined from Friedel oscillations7, our study establishes the charge density as a powerful observable with which to determine both the dispersion relation and topological properties of wavefunctions. This could have profound consequences for the study of the band-structure topology of relativistic and gapped phases in solids.

Induced superconductivity in graphene grown on rhenium.

We report a new way to strongly couple graphene to a superconductor. The graphene monolayer has been grown directly on top of a superconducting Re(0001) thin film and characterized by scanning tunneling microscopy and spectroscopy. We observed a moiré pattern due to the mismatch between Re and graphene lattice parameters that we have simulated with ab initio calculations. The density of states around the Fermi energy appears to be position dependent on this moiré pattern. Tunneling spectroscopy performed at 50 mK shows that the superconducting behavior of graphene on Re is well described by the Bardeen-Cooper-Schrieffer theory and stands for a very good interface between the graphene and its metallic substrate.

Universal classification of twisted, strained and sheared graphene moiré superlattices.

Moiré superlattices in graphene supported on various substrates have opened a new avenue to engineer graphene's electronic properties. Yet, the exact crystallographic structure on which their band structure depends remains highly debated. In this scanning tunneling microscopy and density functional theory study, we have analysed graphene samples grown on multilayer graphene prepared onto SiC and on the close-packed surfaces of Re and Ir with ultra-high precision. We resolve small-angle twists and shears in graphene, and identify large unit cells comprising more than 1,000 carbon atoms and exhibiting non-trivial nanopatterns for moiré superlattices, which are commensurate to the graphene lattice. Finally, a general formalism applicable to any hexagonal moiré is presented to classify all reported structures.

[Second conservative radiosurgical treatment for ipsilateral breast cancer recurrence]. / Second traitement conservateur radiochirurgical dans les récidives locales du cancer du sein.

PURPOSE: Currently, radical mastectomy represents the gold standard for ipsilateral breast cancer recurrence. However, we already showed that a second conservative treatment was feasible combining lumpectomy plus low-dose rate interstitial brachytherapy. In this study, we reported the preliminary results of a second conservative treatment using a high-dose rate brachytherapy. PATIENTS AND METHODS: From June 2005 to July 2009, 42 patients presenting with an ipsilateral breast cancer recurrence underwent a second conservative treatment. Plastic tubes were implanted intra-operatively at the time of the lumpectomy. After a post-implant CT scan, a total dose of 34 Gy in 10 fractions over 5 consecutive days was delivered through an ambulatory procedure. The toxicity evaluation used the Common Terminology Criteria for Adverse Events v3.0. RESULTS: The median follow-up was 21 months (6-50 months), median age at the time of the local recurrence was 65 years (30-85 years). The median delay between the primary and the recurrence was 11 years (1-35 years). The location of the recurrence was in the tumor bed for 22 patients (52.4%), in the same quadrant for 14 patients (33.3%) and unknown for six patients (14.3%). The median tumor size of the recurrence was 12 mm (2-30 mm). The median number of plastic tubes and plans were nine (5-12) and two (1-3) respectively. The median CTV was 68 cm(3) (31.2-146 cm(3)). The rate of second local control was 97%. Twenty-two patients (60%) experienced complications. The most frequent side effect consisted in cutaneous and sub-cutaneous fibrosis (72% of all the observed complications). CONCLUSION: A second conservative treatment for ipsilateral breast cancer recurrence using high-dose rate brachytherapy appears feasible leading to encouraging results in terms of second local control with an acceptable toxicity. Considering that a non-inferiority randomized trial comparing mastectomy versus second conservative treatment could be difficult to perform, what proof level will be necessary to achieve in order to change the medical procedures?

Disorder-induced inhomogeneities of the superconducting state close to the superconductor-insulator transition.

Scanning tunneling spectroscopy at very low temperatures on homogeneously disordered superconducting titanium nitride thin films reveals strong spatial inhomogeneities of the superconducting gap Delta in the density of states. Upon increasing disorder, we observe suppression of the superconducting critical temperature Tc towards zero, enhancement of spatial fluctuations in Delta, and growth of the Delta/Tc ratio. These findings suggest that local superconductivity survives across the disorder-driven superconductor-insulator transition.

Metal-to-insulator transition and superconductivity in boron-doped diamond.

The experimental discovery of superconductivity in boron-doped diamond came as a major surprise to both the diamond and the superconducting materials communities. The main experimental results obtained since then on single-crystal diamond epilayers are reviewed and applied to calculations, and some open questions are identified. The critical doping of the metal-to-insulator transition (MIT) was found to coincide with that necessary for superconductivity to occur. Some of the critical exponents of the MIT were determined and superconducting diamond was found to follow a conventional type II behaviour in the dirty limit, with relatively high critical temperature values quite close to the doping-induced insulator-to-metal transition. This could indicate that on the metallic side both the electron-phonon coupling and the screening parameter depend on the boron concentration. In our view, doped diamond is a potential model system for the study of electronic phase transitions and a stimulating example for other semiconductors such as germanium and silicon.

Tunneling spectroscopy and vortex imaging in boron-doped diamond.

We present the first scanning tunneling spectroscopy study of single-crystalline boron-doped diamond. The measurements were performed below 100 mK with a low temperature scanning tunneling microscope. The tunneling density of states displays a clear superconducting gap. The temperature evolution of the order parameter follows the weak-coupling BCS law with Delta(0)/kBTc approximately 1.74. Vortex imaging at low magnetic field also reveals localized states inside the vortex core that are unexpected for such a dirty superconductor.

Anomalous proximity effect in an inhomogeneous disordered superconductor.

By combining very low temperature scanning tunneling microscopy and spectroscopy on a TiN film we have observed a nonuniform state comprising of superconducting (S) and normal (N) areas. The local density of states displays a spatial dependence between S and N different from the usual proximity effect. We conclude that mesoscopic fluctuations might play a major role in accordance with recent theories describing superconductor-normal-metal quantum transition.