Heat transfer in rough nanofilms and nanowires using full band ab initio Monte Carlo simulation.

The Boltzmann transport equation is one of the most relevant frameworks to study the heat transport at the nanoscale, beyond the diffusive regime and up to the micrometer-scale. In the general case of 3D devices, the particle Monte Carlo approach of phonon transport is particularly powerful and convenient, and requires reasonable computational resources. In this work, we propose an original and versatile particle Monte Carlo approach parametrized by using ab initio data. Both the phonon dispersion and the phonon-phonon scattering rates have been computed by DFT calculation in the entire 3D Brillouin zone. To treat the phonon transport at rough interfaces, a combination of specular and diffuse reflections has been implemented in phase space. Thermal transport has been investigated in nanowires and thin films made of cubic and hexagonal Silicon, including edge roughness, in terms of effective thermal conductivity, phonon band contributions and heat flux orientation. It is shown that the effective thermal conductivity in quasi-ballistic regime obtained from our Monte Carlo simulation cannot be accurately fitted by simple semi-analytical Matthiessen-like models and that spectral approaches are mandatory to get good results. Our full band approach shows that some phonon branches exhibiting a negative group velocity in some parts of the Brillouin zone may contribute negatively to the total thermal flux. Besides, the thermal flux clearly appears to be oriented along directions of high density of states. The resulting anisotropy of the heat flux is discussed together with the influence of rough interfaces.

Valley Filtering and Electronic Optics Using Polycrystalline Graphene.

In this Letter, both the manipulation of valley-polarized currents and the optical-like behaviors of Dirac fermions are theoretically explored in polycrystalline graphene. When strain is applied, the misorientation between two graphene domains separated by a grain boundary can result in a mismatch of their electronic structures. Such a discrepancy manifests itself in a strong breaking of the inversion symmetry, leading to perfect valley polarization in a wide range of transmission directions. In addition, these graphene domains act as different media for electron waves, offering the possibility to modulate and obtain negative refraction indexes.

Transport properties through graphene grain boundaries: strain effects versus lattice symmetry.

As most materials available at the macroscopic scale, graphene samples usually appear in a polycrystalline form and thus contain grain boundaries. In the present work, the effect of uniaxial strain on the electronic transport properties through graphene grain boundaries is investigated using atomistic simulations. A systematic picture of transport properties with respect to the strain and lattice symmetry of graphene domains on both sides of the boundary is provided. In particular, it is shown that strain engineering can be used to open a finite transport gap in all graphene systems where the two domains are arranged in different orientations. This gap value is found to depend on the strain magnitude, on the strain direction and on the lattice symmetry of graphene domains. By choosing appropriately the strain direction, a large transport gap of a few hundred meV can be achieved when applying a small strain of only a few percents. For a specific class of graphene grain boundary systems, strain engineering can also be used to reduce the scattering on defects and thus to significantly enhance the conductance. With a large strain-induced gap, these graphene heterostructures are proposed to be promising candidates for highly sensitive strain sensors, flexible electronic devices and p-n junctions with non-linear I-V characteristics.

Strain-induced conduction gap in vertical devices made of misoriented graphene layers.

We investigate the effects of uniaxial strain on the transport properties of vertical devices made of two misoriented (or twisted) graphene layers, which partially overlap each other. We find that because of the different orientations of the two graphene lattices, their Dirac points can be displaced and separated in the k-space by the effects of strain. Hence, a finite conduction gap as large as a few hundred meV can be obtained in the device with a small strain of only a few percent. The dependence of this conduction gap on the strain magnitude, strain direction, channel orientation and twist angle are clarified and presented. On this basis, the strong modulation of conductance and significant improvement of Seebeck coefficient are shown. The suggested devices therefore may be very promising for improving applications of graphene, e.g., as transistors or strain and thermal sensors.

The interplay between the Aharonov-Bohm interference and parity selective tunneling in graphene nanoribbon rings.

We report on a numerical study of the Aharonov-Bohm (AB) effect and parity selective tunneling in pn junctions based on rectangular graphene rings where the contacts and ring arms are all made of zigzag nanoribbons. We find that when applying a magnetic field to the ring, the AB interference can reverse the parity symmetry of incoming waves and hence can strongly modulate the parity selective transmission through the system. Therefore, the transmission between two states of different parity exhibits the AB oscillations with a π-phase shift, compared to the case of states of the same parity. On this basis, it is shown that interesting effects, such as giant (both positive and negative) magnetoresistance and strong negative differential conductance, can be achieved in this structure. Our study thus presents a new property of the AB interference in graphene nanorings, which could be helpful for further understanding the transport properties of graphene mesoscopic systems.

Improved performance of graphene transistors by strain engineering.

By means of numerical simulation, in this work we study the effects of uniaxial strain on the transport properties of strained graphene heterojunctions and explore the possibility of achieving good performance of graphene transistors using these hetero-channels. It is shown that a finite conduction gap can open in the strain junctions due to strain-induced deformation of the graphene bandstructure. These hetero-channels are then demonstrated to significantly improve the operation of graphene field-effect transistors (FETs). In particular, the ON/OFF current ratio can reach a value of over 10(5). In graphene normal FETs, the transconductance, although reduced compared to the case of unstrained devices, is still high, while good saturation of current can be obtained. This results in a high voltage gain and a high transition frequency of a few hundreds of GHz for a gate length of 80 nm. In graphene tunneling FETs, subthreshold swings lower than 30 mV /dec, strong nonlinear effects such as gate-controllable negative differential conductance, and current rectification are observed.

Physical scales in the Wigner-Boltzmann equation.

The Wigner-Boltzmann equation provides the Wigner single particle theory with interactions with bosonic degrees of freedom associated with harmonic oscillators, such as phonons in solids. Quantum evolution is an interplay of two transport modes, corresponding to the common coherent particle-potential processes, or to the decoherence causing scattering due to the oscillators. Which evolution mode will dominate depends on the scales of the involved physical quantities. A dimensionless formulation of the Wigner-Boltzmann equation is obtained, where these scales appear as dimensionless strength parameters. A notion called scaling theorem is derived, linking the strength parameters to the coupling with the oscillators. It is shown that an increase of this coupling is equivalent to a reduction of both the strength of the electric potential, and the coherence length. Secondly, the existence of classes of physically different, but mathematically equivalent setups of the Wigner-Boltzmann evolution is demonstrated.

Graphene nanomesh-based devices exhibiting a strong negative differential conductance effect.

Using atomistic quantum simulation based on a tight binding model, we have investigated the transport characteristics of graphene nanomesh-based devices and evaluated the possibilities of observing negative differential conductance. It is shown that by taking advantage of bandgap opening in the graphene nanomesh lattice, a strong negative differential conductance effect can be achieved at room temperature in pn junctions and n-doped structures. Remarkably, the effect is improved very significantly (with a peak-to-valley current ratio of a few hundred) and appears to be weakly sensitive to the transition length in graphene nanomesh pn hetero-junctions when inserting a pristine (gapless) graphene section in the transition region between n and p zones. The study therefore suggests new design strategies for graphene electronic devices which may offer strong advantages in terms of performance and processing over the devices studied previously.

The conduction gap in double gate bilayer graphene structures.

Using the nonequilibrium Green function method, the electrical behavior of a double gate bilayer graphene structure is investigated. Due to energy bandgap opening when potential energies in the layers are different, a clear gap of electrical current is observed. The sensitivity of this phenomenon to device parameters (gate length, temperature) has been considered systematically. It appears that the threshold voltage can be controlled by tuning the gate voltages and/or the Fermi energy. Our obtained results may be useful and provide new suggestions for further experimental investigations.

Modeling of metal-graphene coupling and its influence on transport properties in graphene at the charge neutrality point.

In this work, the effect of coupling between metallic electrodes and graphene is discussed. We demonstrate that the transport properties of graphene at the charge neutrality point are very sensitive to this coupling. By introducing a model based on two real parameters, namely the real and the imaginary parts of the self-energy which describes the metal-graphene coupling, the obtained results of charge conductivity versus the Fermi energy reproduce well the essential features of experimental data such as the asymmetry between electrons and holes. Additionally, the possible role of scattering processes in the enhancement of the density of states, and thus of the minimum of conductivity, at the charge neutrality point is also discussed. This work is believed to be helpful for further studies of graphene-based devices wherein metallic electrodes may have a major impact on electrical characteristics.

'Treatise on the Spinal Marrow and its Diseases' (Anatomy, functions and general considerations on its diseases) by Ollivier d' Angers CP (1796-1845).

STUDY DESIGN: Collecting and analyzing all possible documents by internet, and consulting medical libraries in different countries. OBJECTIVE: To focus on the work of Ollivier d'Angers who, in the beginning of the 19th century, spent most of his professional life studying the spinal cord, marrow (SM), or medulla spinalis, and publishing the first comprehensive treatise on the subject in 1824. SETTING: ParaDoc database, Swiss Paraplegic-Centre, 6207 Nottwil, Switzerland, in collaboration with Paul Dollfus, ISCoS/Paradoc, Mulhouse, France. RESULTS: Some of d'Angers's clinical descriptions, observations and also pathologic findings, described in the successive editions of his treatise, were very much in advance of his time. CONCLUSIONS: To our knowledge, this was the first comprehensive treatise, in 1824, at least in France. It gave a clear picture on the matter of the SM and in that period of medical history.

Workshop on spinal cord injuries (SCI) management: the Chiang Mai experience.

OBJECTIVES: To give an opportunity to health workers in South Asia (SEA) to improve their knowledge in the comprehensive management of SCI. SETTING: Chiang Mai, Thailand; 5-7 November 1997. PARTICIPANTS: Sixteen faculty members (from Europe, Australia and Thailand) and 160 participants (94 Thais, 66 from SEA, China, India and Bangledesh). METHODS: Two days of lectures and 1 day of workshops and discussion. Evaluation was performed by questionnaires. Suggestions and comments are reported. RESULTS: Ninety-two per cent of participants said the workshop was really useful. Eighty-eight per cent were very satisfied with the quality of the programmes and 92% with the workshops and discussion on the last day. CONCLUSIONS: A follow-up study should be conducted by the Educational Committee of IMSOP. This will help to plan more efficacious training programmes for the future.

The 'Dejerines': an historical review and homage to two pioneers in the field of neurology and their contribution to the understanding of spinal cord pathology.

Our purpose, in this number of Spinal Cord devoted to the French speaking Society of Paraplegia (AFIGAP), is to render homage to two very distinguished doctors, who by their work at the end of the XIXth and the beginning of our century contributed greatly to our knowledge of the nervous system and in particular the spinal cord (SC). This was at the time a field of considerable interest in France and abroad. Professor Jules Dejerine was from 1911-1917 the holder of the Chair for Nervous System Diseases created for Charcot. Dejerine and his American born wife, Augusta Klumpke, and had very limited means for investigation compared to actual technological advances. They relied mainly on their superb clinical observations and neuropathological examinations. Dejerine was also a pioneer in the growing field of neuroanatomy. In 1895 he published a treatise on the anatomy of the nervous system, which is still considered worldwide to be a masterpiece. Augusta Dejerine-Klumpke, the first woman Intern in Paris Hospitals, was not only a fine clinician, neuroanatomist and pathologist, but also contributed greatly to her husband's work. Amongst other things she is known for the 'Klumpke palsy'. She was also a pioneer in France, during the First World War and subsequent following years, in the treatment and rehabilitation (medical and vocational) of the large number of soldiers afflicted by wounds of the nervous system and especially of the SC. During the same period, many authors contributed to SC pathology, but only a few to the treatment and rehabilitation of these patients. This was brought to our attention, in the sixties, by Professor Pierre Houssa, pioneer in Belgium in the field of comprehensive care of those who have SC lesions. Augusta Dejerine-Klumpke also contributed to our present knowledge of heterotopic ossification following a SC injury, including its pathogenesis. Most of their clinical and pathological findings and discussions are recorded in Dejerine's famous monograph which was published in 1914: La séméiologie des Affections du Système Nerveux (The Semiology of the Diseases of the Nervous System).

Neurological issues.

The case histories of two patients who had had a spinal cord injury (SCI) were selected by the senior author and sent to four experts in the field of SCI. Based on the 1992 American Spinal Injury Association (ASIA) and International Medical Society of Paraplegia (IMSOP) standards, the four participants plus the senior author recorded the motor and sensory scores, the ASIA impairment scale (AIS), the neurological level (NL) and the zone of partial preservation (ZPP). Several minor scoring errors occurred among the participants, especially with motor scores when key muscles could not be tested due to pain, or external immobilization devices. Difficulties with interpretation occurred with the motor levels and the ZPP for the patient with a complete injury. This exercise points to the need for all examiners of SCI patients to thoroughly familiarize themselves with the standards and to use the motor and sensory scores to arrive at a NL and ZPP. They also indicate a need to revise the standards to clarify the determination of sensory levels and how to score muscles whose strength is inhibited by pain.

Rehabilitation following injury to the spinal cord.

There are several key aspects in the rehabilitation of patients with traumatic spinal cord injury, including the delivery of lifelong resources and services. Rehabilitation must commence as early as possible after the injury and encompass secondary and tertiary prevention efforts in order to minimize the personal and social consequences associated with neurotrauma. Recent advances in molecular neurobiology and new information gained from randomized clinical trials may help reduce the consequences of this type of injury. These advances need to be extended and research must be encouraged.

[Neurophysiopathology of spinal lesions, evaluation of the 80s. Post-injury assessment and treatment of the neurogenic bladder in the acute period]. / La neurophysiopathologie de la lésion médullaire, bilan des années 80. Le bilan post-traumatique et le traitement de la neurovessie à la période aiguë.

This introductory presentation deals with the recent pathophysiological aspects of the post-traumatic spinal cord lesion and the population concerned. The neurophysiology of the bladder and its sphincters is summarized so as to try and explain some disturbances occurring during the acute phase following a spinal cord injury; their clinical and urological aspects and the methods of bladder drainage.

The locked-in syndrome: a review and presentation of two chronic cases.

The locked-in syndrome (LIS) is a state of an upper motor neurone quadriplegia involving the cranial nerve pairs with usually a lateral gaze palsy, paralytic mutism, full consciousness and awareness by the patient of his environment. A historical presentation of the LIS is given as well as a short description of the clinicoanatomic lesion causing LIS. The usual cause is vascular and corresponds to a pontine infarction due to an obstruction of the basilar artery but other lesions in the brainstem can also be the cause. Non-vascular aetiologies, especially traumatic, are reviewed. The use of electroencephalography (EEG), brain auditory evoked potentials (BAEP) and somesthesic evoked potentials (SEP) are discussed as well as the use in the acute stage of computed tomography (CT), angiography, and magnetic resonance imagery (MRI). The last method may show well delineated ischaemic lesions some time after the event. The communication disability is probably the most difficult to overcome. Two cases of LIS are presented.

Surgical treatment of the static perineal modifications in spinal cord or cauda equina lesions.

Severe dysuria, due to insufficiency of the perineal floor associated during micturition with a posterior tilting of the prostate-bladder block in lower or in associated upper and lower motor neuron lesions, can be treated surgically by a prostato cytso pexy. Since 1971, eight patients with post-traumatic conus and/or cauda equina lesions have been treated by this intervention. On the follow-up the satisfactory results appear to remain stable. An alternative surgical technique using the abdominal pyramidalis muscle is described so as to fix the prostate, associated with a bladder-pexy. The recurrence of dysuria, after the intervention, has always been caused by an additional lower urinary tract pathology. The comfort of the patients has been greatly improved.