Olivier Bourgeois

Measurement of the thermal conductance of silicon nanowires at low temperature

We have performed thermal conductance measurements on individual single-crystalline silicon suspended nanowires. The nanowires (130nm thick and 200nm wide) are fabricated by e-beam lithography and suspended between two separated pads on silicon on insulator substrate. We measure the thermal conductance of the phonon waveguide by the 3ω method. The cross section of the nanowire approaches the dominant phonon wavelength in silicon which is of the order of 100nm at 1K. Above 1.3K the conductance behaves as T3, but a deviation is measured at the lowest temperature which can be attributed to the reduced geometry.

Geothermal control on flow patterns in the Last Glacial Maximum ice sheet of Iceland

Because it is located both on the Mid-Atlantic Ridge and on a mantle plume, Iceland is a region of intense tectonics and volcanism. During the last glaciation, the island was covered by an ice sheet approximately 1000 m thick. A reconstruction of the ice flow lines, based on glacial directional features, shows that the ice sheet was partly drained through fast-flowing streams. Fast flow of the ice streams has been recorded in megascale lineations and flutes visible on the currently deglaciated bedrock, and is confirmed by simple mass balance considerations. Locations of the major drainage routes correlate with locations of geothermal anomalies, suggesting that ice stream activity was favoured by lubrication of the bed by meltwater produced in regions of high geothermal heat flux. Similar control of ice flow by geothermal activity is expected in ice sheets currently covering tectonically and volcanically active area such as the West Antarctic ice sheet. Copyright

Pleistocene subglacial volcanism in Iceland: tectonic implications

Abstract At several stages during the last 700 kyr, tectonic and volcanic activity due to lithospheric spreading occurred beneath a 1000–1500 m thick ice cap in Iceland. Magmatic activity has been recorded by the emplacement of subglacial volcanic edifices. Table volcanoes are the subglacial equivalent of aerial shield volcanoes. Hyaloclastite ridges are the subglacial equivalent of aerial eruptive fissures. Some hyaloclastite ridges are located in currently inactive areas, whereas they are nearly absent in some parts of the currently active Neovolcanic Zone. A part of this discrepancy can be attributed to glacial erosion. A manual reconstruction of the flowing pattern of the ice cap, based on glacial landforms, shows that some parts of the Neovolcanic Zone were occupied by fast flowing ice streams. In these areas, most hyaloclastite ridges have been removed as eruptions proceeded: fast ice flow and water/debris flows triggered by volcanic eruptions have transported subglacial volcanic products to the sea. Subglacial volcanic products have been preserved beneath ice divides, where ice motion was slower, and in some table volcanoes, where magma supply was sufficient to counteract removal by ice flow. Once the effect of glacial removal has been subtracted, the arrangement of the subglacial volcanic edifices appears clearly. Similarly to the post-glacial eruptive fissures, the hyaloclastite ridges are gathered in swarms associated with central volcanoes located in the Neovolcanic Zone. However, the area covered by hyaloclastite ridges is wider than the extent of the currently active fissure swarms. This discrepancy suggests either continuous wandering of the volcanic activity from one fissure swarm to another for the last 700 kyr, or narrowing of the active rift zone at the end of the last glaciation.

Attojoule Calorimetry of Mesoscopic Superconducting Loops

We report the first experimental evidence of nontrivial thermal behavior of the simplest mesoscopic system--a superconducting loop. By measuring the specific heat C of an array of 450,000 noninteracting aluminum loops with very high accuracy of approximately 20 fJ/K, we show that the loops go through a periodic sequence of phase transitions (with a period of an integer number of magnetic flux quanta) as the magnetic flux threading each loop is increased. The transitions are well described by the Ginzburg-Landau theory and are accompanied by discontinuities of C of only several thousands of Boltzmann constants kB.

Least squares restoration of Tertiary thrust sheets in map view, Tajik depression, central Asia

The Tajik depression, located west of the Pamirs and south of the Tien Shan, is a compressional intermontane basin, bounded by basement overthrusts and filled with Mesozoic and Cenozoic sediments. The internal structure is typical of a thin-skinned tbld and thrust belt. Kinematic data available in the literature suggest that indentation of the Pamirs into Asia durino the Cenozoic collision of India and Asia has been accommodated in various ways within the depression, including westward extrusion, thickening combined with wrenching along N-S tblds and thrusts, and counterclockwise rotations. These various detbrmation processes can be analyzed and quantified by reconstruction of the predeformed state of the depression. However, the combina- tion of thrusting, wrenching and block rotations implies a nonplane deformation, which cannot be restored properly using balanced cross sections alone. We have therefore developed a numerical method for restoration of stratigraphic surfaces, designed for regions of nonplane compressional tectonics. The deformed region is represented in map view as a mosaic of fault-bounded blocks, overlapping each other along the faults. Blocks are separately unfolded and then numerically packed together by least squares minimization of overlaps, yielding fields of finite horizontal translations and rotations about vertical axes. To analyze the deformation postdating the collision of India and Asia, we have restored a stratigraphic surface at the base of the Cenozoic: First, in order to test the numerical method, we restored a map that had previously been restored by a purely manual method. Restoration of a second map, drawn from newly available subsurface data, leads to geometrical inconsistencies: overlaps and gaps, which cannot be reduced, remain in the restored state. After correction of these inconsistencies, restoration yields a complex mode of deformation for the depression. Individual thrust slices have undergone counterclockwise rotations about verti- cal axes, the magnitudes increasing from west to east, up to a maximum of 40 o near the Pamirs. Horizontal shortening is 150 km (35%) in the center of the depression and 240 km (85%) in the northeastern part, between the Pamirs and the Tien Shan. Strike slip motions, commonly not revealed by balancing cross sections, are associated with thrusting on faults striking N-S. Inde- pendent paleomagnetic data and slip directions on small-scale faults provide positive checks on the results of our restoration.

Liquid nitrogen to room-temperature thermometry using niobium nitride thin films

Niobium nitride thin-film thermometry has been developed for the temperature range of 70 to 300 K. The deposition parameters have been optimized in order to get the best performances, i.e., the highest temperature coefficient of resistance (TCR), up to 300 K. The TCR is found to be largely higher than 1% as the temperature is lowered from 300 K, up to 6% at 77 K. These significant performances are compared to the one of regular platinum thermometer as well as to other resistive thermometer: semiconductor type or amorphous metal to insulator transition materials. It is discussed how the properties of the NbN thin films could be due to a high-temperature Mott transition.

Phonon Heat Conduction in Corrugated Silicon Nanowires Below the Casimir Limit

The thermal conductance of straight and corrugated monocrystalline silicon nanowires has been measured between 0.3 K and 5 K. It is demonstrated that the corrugation strongly reduces the thermal transport by reducing the mean free path of the phonons. The experimental averaged mean free path is remarkably smaller than the smaller diameter of the nanowire, evidencing a phonon thermal transport reduced below the Casimir limit. Monte Carlo simulations highlight that this effect can be attributed to significant multiple scattering of ballistic phonons occurring on the corrugated surfaces. This result suggests an original approach to transforming a monocrystalline material into a phonon glass.

Highly sensitive thermal conductivity measurements of suspended membranes (SiN and diamond) using a 3ω-Völklein method

A suspended system for measuring the thermal properties of membranes is presented. The sensitive thermal measurement is based on the 3ω dynamic method coupled to a Völklein geometry. The device obtained using micro-machining processes allows the measurement of the in-plane thermal conductivity of a membrane with a sensitivity of less than 10 nW/K (+∕-5 × 10(-3) Wm(-1) K(-1) at room temperature) and a very high resolution (ΔK/K = 10(-3)). A transducer (heater/thermometer) centered on the membrane is used to create an oscillation of the heat flux and to measure the temperature oscillation at the third harmonic using a Wheatstone bridge set-up. Power as low as 0.1 nW has been measured at room temperature. The method has been applied to measure thermal properties of low stress silicon nitride and polycrystalline diamond membranes with thickness ranging from 100 nm to 400 nm. The thermal conductivity measured on the polycrystalline diamond membrane support a significant grain size effect on the thermal transport.

Physical kinetics and thermodynamics of phase transitions probed by dynamic nanocalorimetry

A specific ac nanocalorimeter is presented for the study of kinetics in phase transitions. This apparatus designed by means of microfabrication technologies requires a 50‐μm-thick solid or liquid sample. The use of the ac calorimetric method allows an operating frequency range around two decades combined with a very high sensitivity. The ability of this apparatus for thermal spectroscopy is illustrated by measuring the thermal behavior of a homopolymer, the polytetrafluoroethylene, which exhibits two phase transitions at room temperature. The variation of the thermal frequency allows the observation of kinetic effects directly seen on the thermodynamic properties (enthalpy and heat capacity). We demonstrate the possibility of extracting a quantitative value of the kinetic time constant occurring at a first-order phase transition and consequently the capabilities offered by highly sensitive ac calorimetry to investigate direct thermal dynamics in macromolecular samples.

Highly sensitive parylene membrane-based ac-calorimeter for small mass magnetic samples

We report the microfabrication and operation of a highly sensitive ac-calorimeter designed to characterize small mass magnetic systems operating at very low frequencies (from 0.1 to 5 Hz) in a temperature range from 20 to 300 K. The calorimetric cell is built in the center of a 500 nm thick polymeric membrane of parylene C held up by a Cu frame. On both sides of the membrane defining a three layer structure, electrical leads, heater, and thermometer are deposited as thin film layers of NbN(x), with different nitrogen contents, taking benefit of the poor thermal conductance of niobium nitride to thermally isolate the system. This suspended structure ensures very low heat capacity addenda with values in the microJ/K over the 1 mm(2) area of the measurement cell. The structuring of the membrane along with suspending of the sensing part only by the parylene bridges leads to a highly reduced thermal link. The calorimeter has been characterized as a function of frequency, temperature, and magnetic field. The thermal link measured is really small reaching values well below 10(-8) W/K at 50 K. With these characteristics the frequency of adiabaticity is typically around few hertz and energy exchanges as small as 1 pJ can be detected. Measurements have been performed on Co/Au thin films and on the GdAl(2) microcrystal where the ferromagnetic phase transition is clearly evidenced.