Nanocalorimeter for high resolution measurements of low temperature heat capacities of thin films and single crystals

Abstract

An innovative nanocalorimeter has been developed for measuring specific heats of thin films, multilayers (typical thickness: 1000 Å) and single crystals (mass: 1 μg) in the temperature range of 1.5–20 K. The addenda of the device are as small as 3 nJ/K at 4 K (0.5 nJ/K at 1.5 K), thus samples with a heat capacity of the order of nJ/K at 4 K can be measured. Heat capacity differences as a function of temperature or an external magnetic field (5 T) were determined with a resolution of ΔC/C≃10−4. This way we have seen heat capacity variations of less than a pJ/K. We present as an example measurements on very small Mn12O12 acetate single crystals and a measurement of a thin superconducting Pb layer. In the latter measurement we could evidence via specific heat a finite size effect.

Attojoule Calorimetry of Mesoscopic Superconducting Loops

“…The setup is cooled below the critical temperature T c of the superconducting transition by a 3 He cryostat and then its specific heat is measured by ac calorimetry. The technique of ac calorimetry consists in supplying ac power to the heater, thus inducing temperature oscillations of the thermally isolated membrane and thermometer [19]. For the operating frequency in the middle of the 'adiabatic plateau' [19] (in our case, the frequency of temperature modulation is f ≃ 250 Hz), the temperature of the system 'sensor + sample' follows variations of the supplied power in a quasi-adiabatic manner, allowing measurements of the specific heat C with a resolution of δC/C > ∼ 5 × 10 −5 for signal integration times of the order of one minute.…”

“…The technique of ac calorimetry consists in supplying ac power to the heater, thus inducing temperature oscillations of the thermally isolated membrane and thermometer [19]. For the operating frequency in the middle of the 'adiabatic plateau' [19] (in our case, the frequency of temperature modulation is f ≃ 250 Hz), the temperature of the system 'sensor + sample' follows variations of the supplied power in a quasi-adiabatic manner, allowing measurements of the specific heat C with a resolution of δC/C > ∼ 5 × 10 −5 for signal integration times of the order of one minute. This makes it possible to measure variations of the specific heat as small as 10 fJ/K (which corresponds to several thousands of k B per loop), provided that the specific heat of the sensor (silicon membrane, heater, and thermometer) is reduced to about 10-100 pJ/K.…”

“…Our sample is composed of 450 thousands nominally identical, noninteracting [21] aluminum square loops (2 µm in size, w = 230 nm arm width, d = 40 nm thickness, separation of neighboring loops = 2 µm, total mass of the sample m = 80 ng), patterned by electron beam lithography on a suspended sensor composed of a very thin (4 µm thick) silicon membrane and two integrated transducers: a copper heater and a niobium nitride thermometer [19] (see Fig. 1).…”

Searching for thermal signatures of persistent currents in normal-metal rings

“…2 and described in detail in previous articles [26][27][28]. It consists in applying an ac current through the heater; here at a frequency f elec = 30 Hz.…”

“…between the membrane and the heat bath (for more details see [21,26,27,29]). The setup is cooled down to very low temperatures using a dilution fridge, equipped with a superconducting coil supplying a magnetic field H normal to the plane of the rings.…”