964 research outputs found
Measurement of thermal conductance of silicon nanowires at low temperature
We have performed thermal conductance measurements on individual single
crystalline silicon suspended nanowires. The nanowires (130 nm thick and 200 nm
wide) are fabricated by e-beam lithography and suspended between two separated
pads on Silicon On Insulator (SOI) substrate. We measure the thermal
conductance of the phonon wave guide by the 3 method. The cross-section
of the nanowire approaches the dominant phonon wavelength in silicon which is
of the order of 100 nm 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
Thermal signatures of Little-Parks effect in the heat capacity of mesoscopic superconducting rings
We present the first measurements of thermal signatures of the Little-Parks
effect using a highly sensitive nanocalorimeter. Small variations of the heat
capacity of 2.5 millions of non interacting micrometer-sized
superconducting rings threaded by a magnetic flux have been measured by
attojoule calorimetry. This non-invasive method allows the measurement of
thermodynamic properties -- and hence the probing of the energy levels -- of
nanosystems without perturbing them electrically. It is observed that is
strongly influenced by the fluxoid quantization (Little-Parks effect) near the
critical temperature . The jump of at the superconducting phase
transition is an oscillating function of with a period ,
the magnetic flux quantum, which is in agreement with the Ginzburg-Landau
theory of superconductivity.Comment: To be published in Physical Review B, Rapid Communication
Fine frequency shift of sigle vortex entrance and exit in superconducting loops
The heat capacity of an array of independent aluminum rings has been
measured under an external magnetic field using highly sensitive
ac-calorimetry based on a silicon membrane sensor. Each superconducting vortex
entrance induces a phase transition and a heat capacity jump and hence
oscillates with . This oscillatory and non-stationary behaviour
measured versus the magnetic field has been studied using the Wigner-Ville
distribution (a time-frequency representation). It is found that the
periodicity of the heat capacity oscillations varies significantly with the
magnetic field; the evolution of the period also depends on the sweeping
direction of the field. This can be attributed to a different behavior between
expulsion and penetration of vortices into the rings. A variation of more than
15% of the periodicity of the heat capacity jumps is observed as the magnetic
field is varied. A description of this phenomenon is given using an analytical
solution of the Ginzburg-Landau equations of superconductivity
New sensor for thermodynamic measurement of magnetization reversal in magnetic nanomaterials
A sensor for thermal and thermodynamic measurements of small magnetic systems
have been designed and built. It is based on a 5μm-thick suspended polymer
membrane, which has a very low heat capacity (≈ 10-6 J/K at nitrogen
temperature), and on which a heater and a highly sensitive thermometer are
deposited. The sensor properties have been characterized as a function of
temperature and frequency. Energy exchanges as small as 1 picojoule (10-12
Joule) were detected in the 40K- 300K temperature range. Such values correspond
to those required for measuring the thermal signatures occurring during
magnetization reversal in very thin samples (typically 10 nm thick), which
would be deposited on the membrane. It is expected that this method will
constitute a powerful tool in view of analyzing magnetization reversal
processes in magnetic nanosystems, e.g. exhibiting the exchange-spring and
exchange-bias phenomena
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