Ac Calorimetry Research Articles

Interplay of structure and magnetism in LuFe4Ge2 tuned by hydrostatic pressure

LuFe$_4$Ge$_2$ crystallizes in the ZrFe$_4$Si$_2$-type structure, hosting chains of Fe-tetrahedra giving rise to geometric frustration and low-dimensionality. The compound orders antiferromagnetically at around 36 K accompanied by a simultaneous structural transition from a tetragonal to an orthorhombic phase. The hydrostatic pressure dependence of the magnetic and structural transitions is investigated using electrical-transport, ac magnetic-susceptibility, ac calorimetry, M$\ddot{\rm o}$ssbauer, muon-spin relaxation ($\mu$SR), and x-ray diffraction measurements. External pressure suppresses the first-order transition to the antiferromagnetic phase (AFM1) around 1.8 GPa. The structural transition is largely unaffected by pressure and remains between 30 to 35 K for pressures up to 2 GPa. A second antiferromagnetic phase (AFM2) is observed at higher pressures. The transition from the paramagnetic to the AFM2 phase is of second-order nature and appears to be connected to the structural transition. The magnetic volume fraction obtained from $\mu$SR and M$\ddot{\rm o}$ssbauer measurements reveal that the entire sample undergoes magnetic ordering in both magnetic phases. In addition, similar low-temperature muon-precession frequencies in AFM1 and AFM2 phases point at similar ordered moments and magnetic structures in both phases. Our results further indicate enhanced magnetic fluctuations in the pressure induced AFM2 phase. The experimental observations together with density functional theory calculations suggest that the magnetic and structural order parameters in LuFe$_4$Ge$_2$ are linked by magnetic frustration, causing the simultaneous magneto-structural transition.

Determination of Cooperativity Length in a Glass-Forming Polymer.

To describe the properties of glass-forming liquids, the concepts of a cooperativity length or the size of cooperatively rearranging regions are widely employed. Their knowledge is of outstanding importance for the understanding of both thermodynamic and kinetic properties of the systems under consideration and the mechanisms of crystallization processes. By this reason, methods of experimental determination of this quantity are of outstanding importance. Proceeding in this direction, we determine the so-called cooperativity number and, based on it, the cooperativity length by experimental measurements utilizing AC calorimetry and quasi-elastic neutron scattering (QENS) at comparable times. The results obtained are different in dependence on whether temperature fluctuations in the considered nanoscale subsystems are either accounted for or neglected in the theoretical treatment. It is still an open question, which of these mutually exclusive approaches is the correct one. As shown in the present paper on the example of poly(ethyl methacrylate) (PEMA), the cooperative length of about 1 nm at 400 K and a characteristic time of ca. 2 μs determined from QENS coincide most consistently with the cooperativity length determined from AC calorimetry measurements if the effect of temperature fluctuations is incorporated in the description. This conclusion indicates that-accounting for temperature fluctuations-the characteristic length can be derived by thermodynamic considerations from the specific parameters of the liquid at the glass transition and that temperature does fluctuate in small subsystems.

The Impact of Sample Insulation on Estimating the Heating Power of Magnetic Nanoparticles by AC Calorimetry

Correct estimation of the heating power of magnetic nanoparticles is important for magnetic hyperthermia treatment. This work investigates the impact of sample insulation in AC calorimetry. We show that temperature increase in the insulation can lead to systematic errors when estimating the heating power by the corrected slope method. The errors arise if the temperature of the sample environment is kept fixed at its initial temperature in the data analysis. To correct for this, we propose the use of a local temperature difference between the sample and the sample environment.

Second harmonic AC calorimetry technique within a diamond anvil cell.

Tuning the energy density of matter at high pressures gives rise to exotic and often unprecedented properties, e.g., structural transitions, insulator-metal transitions, valence fluctuations, topological order, and the emergence of superconductivity. The study of specific heat has long been used to characterize these kinds of transitions, but their application to the diamond anvil cell (DAC) environment has proved challenging. Limited work has been done on the measurement of specific heat within DACs, in part due to the difficult experimental setup. To this end, we have developed a novel method for the measurement of specific heat within a DAC that is independent of the DAC design and is, therefore, readily compatible with any DACs already performing high pressure resistance measurements. As a proof-of-concept, specific heat measurements of the MgB2 superconductor were performed, showing a clear anomaly at the transition temperature (Tc), indicative of bulk superconductivity. This technique allows for specific heat measurements at higher pressures than previously possible.

Spatially inhomogeneous superconductivity in UTe2

Newly-discovered superconductor UTe$_2$ is a strong contender for a topological spin-triplet state wherein a multi-component order parameter arises from two nearly-degenerate superconducting states. A key issue is whether both of these states intrinsically exist at ambient pressure. Through thermal expansion and calorimetry, we show that UTe$_2$ at ambient conditions exhibits two detectable transitions only in some samples, and the size of the thermal expansion jump at each transition varies when the measurement is performed in different regions of the sample. This result indicates that the two transitions arise from two spatially separated regions that are inhomogeneously mixed throughout the volume of the sample, each with a discrete superconducting transition temperature (T$_c$). Notably, samples with higher T$_c$ only show a single transition at ambient pressure. Above 0.3 GPa, however, two transitions are invariably observed in ac calorimetry. Our results not only point to a nearly vertical line in the pressure-temperature phase diagram but also provide a unified scenario for the sample dependence of UTe$_{2}$.

Measurement of thermal conductivity and thermal diffusivity of one-dimensional-system material by scanning electron microscopy and infrared thermography

A novel measurement system consisting of scanning electron microscopy and infrared (IR) thermography has been constructed to measure the thermal conductivity and the thermal diffusivity on a micro-/nanometer scale. In this study, we measured them for a stainless-steel wire with a diameter of 50 μm to demonstrate the validity of our system. The time evolution of temperature distribution on the whole of the stainless-steel wire was recorded by the IR thermograph when the wire was irradiated by the electron beam (EB). A time response of the wire temperature corresponding to the on/off of the EB irradiation was clearly observed, indicating that local EB heating was realized. In addition, the experimental time evolution of temperature at its transient phenomena was in good agreement with the theoretical calculation based on a one-dimensional (1D) equivalent-circuit model of heat flow coupled with electric current, where the thermal parameters, such as thermal conductivity and heat capacitance, were set to bulk values. Therefore, it can be concluded that our system is valid to evaluate the thermal conductivity of the 1D wire sample. For evaluating the thermal diffusivity of the wire sample, we applied a periodic EB irradiation by a rectangular wave with a frequency of 0.25 Hz. The thermal diffusivity was evaluated to be (2.8 ± 0.6) × 10−5 m2 s−1, which was seven times as large as that of bulk stainless steel. Our system can be applied to an AC calorimetry method for a 1D system sample after the analysis procedure is further modified.

Oxidation of wüstite rich iron oxide nanoparticles via post-synthesis annealing

When forming magnetic nanoparticles, the decomposition of organo-metallic precursors causes a reduction of Fe(III) to Fe(II) which leads to the formation of an antiferromagnetic rock salt phase of FeO. The antiferromagnetic phase reduces the nanoparticle magnetization, so a new method of oxidation was developed that can convert FeO rich particles to Fe3O4 particles. Iron oxide nanoparticles with different sizes were synthesized to validate the oxidation method. We demonstrate that iron oxide nanoparticles can be oxidized by post synthesis annealing without addition of oxidizing agents. The oxidized particles were measured with XRD, VSM and AC calorimetry to show the effective oxidation by comparing to the as prepared sample. The resulting 20 nm oxidized particles have a saturation magnetization of 72 Am2/kg at 300 K and a specific absorption rate of 181 W/g under a 212 kHz, 33 mT AC field.

Transport and calorimetry study of 20% La-doped CeIn3

CeIn3, a prototypical antiferromagnet, is an ideal candidate for investigating the relationship between magnetism and superconductivity, as superconductivity is induced as the magnetic transition temperature (T N) is lowered to 0 K by applying pressure. When La is substituted for Ce, T N of CeIn3 decreases to 0 K owing to the Ce dilution effects, thereby providing an alternative route to the zero-temperature quantum phase transition. In this study, we report a combinatorial approach to gain access to the critical point by applying external pressure to 20% La-doped CeIn3. Electrical resistivity measurements of La0.2Ce0.8In3 show that the T N of 8.4 K at 1 bar is gradually suppressed under pressure and can be extrapolated to 0 K at approximately 2.47 GPa, thereby showing a similar pressure dependence of T N as shown by undoped CeIn3. The kink-like feature in resistivity at T N of CeIn3 changed to an obvious jump in the doped compound for pressures higher than 1.64 GPa, indicating depletion in the carrier density due to a gap opening. AC calorimetry measurements under applied pressure show that the size of the specific heat jump at T N decreases with increasing pressure, but any signatures associated with the gap opening are not obvious, suggesting that the pressure-induced kink-to-jump change at T N in the resistivity is not a phase transition, but rather a gradual crossover. The low-temperature specific heat divided by temperature, C/T, does not strongly diverge with decreasing temperature, but is almost saturated near the projected quantum critical point, which can be attributed to a weak enhancement in the effective mass up to 2.6 GPa.

Probing insulators under pressure.

Applying pressure on a material can reveal many physical properties and is a very efficient tool to understand its physics. Resistivity measurements have been the ideal probe to study metals under pressure. However, in the case of insulators, resistivity, or conductivity, it is often not the appropriate quantity characterizing the material. In this work, we present a newly developed in situ pressure tuning system that can be used over a wide temperature range (2 K-300 K) and allows changing the pressure at any temperature. We also present AC calorimetry and capacitance/loss measurements under pressure and demonstrate how this combination can be used to characterize a material that is too insulating for standard resistivity techniques.

Development of frequency tuning AC modulation method for high-pressure heat capacity measurements of molecules-based compounds

The automatic frequency tuning method in the high-pressure ac calorimetry system constructed to measure heat capacity for molecules-based compounds with CuBe[Formula: see text]+[Formula: see text]NiCrAl cramp-type pressure cell is reported. This development is performed for increasing resolution and temperature ranges of the heat capacity measurements under external pressure up to 2.0 GPa. The system can check the appropriate conditions by tracing frequency dependence of [Formula: see text] to determine the oscillation frequency at the center of the plateau region of this value. The experiments using the powder samples of metal complexes clarified that the appropriate frequency changes sensitively depending on the difference of temperature and that of external pressures, especially at low temperature region. It decreases with increasing temperature and this relation was found to be almost linear with temperature in ambient pressure and under pressure conditions. The change of thermal diffusion from the sample part to the heat bath should be treated carefully in order to get enough resolution in high pressure AC heat capacity measurements of molecule-based compounds.

Multiple Superconducting Phases and Unusual Enhancement of the Upper Critical Field in UTe2

We performed AC calorimetry and magnetoresistance measurements under pressure for H || a-axis (easy-magnetization axis) in the novel heavy-fermion superconductor UTe2. Thanks to the thermodynamic information, multiple superconducting phases have been revealed under pressure and magnetic field. The (H,T) phase diagram of superconductivity under pressure displays an abrupt increase of the upper critical field (Hc2) at low temperature and in the high field region, and a strong convex curvature of Hc2 at high temperature. This behavior of Hc2 and the multiple superconducting phases require a state for the superconducting order parameter more complex than a spin-triplet equal spin pairing. Above the superconducting critical pressure, Pc, we find strong indications that the possible magnetic order is closer to antiferromagnetism than to ferromagnetism.

Blue phase stabilization by CoPt-decorated reduced-graphene oxide nanosheets dispersed in a chiral liquid crystal

We report on the effect of reduced-graphene oxide nanosheets decorated by CoPt nanoparticles on the blue phase range of a chiral liquid crystal. By means of high-resolution ac calorimetry and polarizing optical microscopy, it is demonstrated that a small concentration of these nanosheets induces the stabilization of a single blue phase structure in comparison to three blue phases existing in the pure compound. The results are compared with other liquid crystal-dispersed graphene studies, and, moreover, a short theoretical discussion of the stabilization effect is included.

Structural, magnetic, and insulator-to-metal transitions under pressure in the GaV4S8 Mott insulator: A rich phase diagram up to 14.7 GPa

In addition to its promising potential for applications, GaV4S8 shows very interesting physical properties with temperature and magnetic field. These properties can be tuned by applying hydrostatic pressure in order to reveal and understand the physics of these materials. Not only pressure induces an insulator-to-metal transition in GaV4S8 but it also has an interesting effect on the structural and magnetic transitions. Using a combination of AC calorimetry, capacitance, and resistivity measurements under pressure, we determine the evolution of the structural and magnetic transitions with pressure and thus establish the T-P phase diagram of GaV4S8. To detect the insulator-to-metal transition, we use optical conductivity and DC resistivity measurements and we follow the evolution of the Mott gap under pressure. The structural transition temperature increases with pressure and a second transition appears above 6 GPa indicating a possible new phase with a very small gap. Pressure has surprisingly a very weak effect on the ferromagnetic transition that persists even very close to the IMT that occurs at around 14 GPa, implying that the metallic state may also be magnetic.

Calorimetric Study on EuCoGe3 and EuRh2In8 under Pressure

We report here the phase diagrams of EuCoGe3 and EuRh2In8 studied by ac calorimetry under pressure using a diamond anvil cell. EuCoGe3 exhibits two antiferromagnetic transitions at TN = 15 K and \(T'_{\text{N}} = 13.4\) K. With increasing pressure TN and \(T'_{\text{N}}\) increase and the antiferromagnetic orders persist up to high pressure. A semi quantitative analysis of the pressure dependence of the specific heat does not show any large changes, but is compatible with a moderate enhancement of the effective mass for EuCoGe3. The antiferromagnetic transition temperature TN for EuRh2In8 initially increases slightly up to 8.7 GPa, and then increases abruptly between 8.7–11.9 GPa, the sudden increase in TN in this pressure range may be due to a change in the magnetic structure with pressure. Further TN increases linearly above 11.9 GPa up to high pressure. No sign of the expected valence transition is seen for either compound implying that this may only occur at rather high pressure for these systems.

In situ temperature-modulated microscopy observation of two-phase co-existing states of coumarin and coumarin derivative in melting

Coumarin (C9H6O2) and coumarin derivative, coumarin 6H (C15H15NO2) samples were observed with a polarizing microscope at and around the melting points. The states of the two samples are crystalline at room temperature. When the temperature of the Coumarin sample was raised and a part was melted, the crystalline phases coexisted with the liquid phase. The crystal surface coexisting with the liquid phase went through a repetition of melting and crystallization on slightly modulating the sample temperature. The two-phase coexisting state was observed with a digital camera attached to a microscope. The changes in the number of pixels in the crystal part of the photographed image showed Debye relaxation with temperature change. It was found that the relaxation time was 4.9 s when the number of pixels decreased due to surface melting or increased due to surface crystallization. This relaxation time was in good agreement with that of the dynamic-specific heat measured by AC calorimetry. The coumarin 6H microcrystal also coexisted with the melt in a temperature range at and around the melting point. The change in the number of pixels of the coumarin 6H crystal part showed Debye relaxation with respect to the temperature, and the relaxation time was 15 s, which was the same as the relaxation time of the dynamic-specific heat measured in the melting temperature range of coumarin 6H.

Simulation of Temperature Distribution under Periodic Heating for Analysis of Thermal Diffusivity in Nanometer-Scale Thermoelectric Materials

With the aim of characterizing the thermal conductivity for nanometer-scale thermoelectric materials, we have constructed a new measurement system based on ac calorimetry. Analysis of the obtained data requires time-evolution of temperature distribution in nanometerscale material under periodic heating. In this study, we made a simulation using a C#-program for time-dependent temperature distribution, based on 2-dimensional heat-diffusion equation inclu1ding the influence of heat emission from material edges. The simulation was applied to AlN with millimeter-scale dimensions for confirming the validity and accuracy. The simulated thermal diffusivity for 10×75-mm2-area AlN was 1.3×10-4 m2/s, which was larger than the value set in the heat-diffusion equation. This overestimation was also observed in the experiment. Therefore, our simulation can reproduce the unsteady heat conduction and be used for analyzing the ac calorimetry experiment.

Thermodynamic Picture of Dimer-Mott Organic Superconductors Revealed by Heat Capacity Measurements with External and Chemical Pressure Control

This article reviews and discusses the thermodynamic properties of dimer-Mott-type molecular superconductive compounds with (BEDT-TTF)2X composition, where BEDT-TTF is bis(ethylenedithio)tetrathiafulvalene and X denotes counter-anions, respectively. We focus mainly on the features occurring in the κ-type structure in which the d-wave superconductive phase appears depending on the Coulomb repulsion U and the bandwidth W, which is tunable by external and chemical pressures. First, we report the high-pressure ac (alternating current) calorimetry technique and experimental system constructed to measure single-crystal samples of molecule-based compounds to derive low-temperature thermodynamic parameters. Using extremely small resistance chips as a heater and a thermometer allows four-terminal detection of an accurate temperature and its oscillation in the sample part with sufficient sensitivity. From the analyses of the temperature dependence of the ac heat capacity of κ-(BEDT-TTF)2Cu(NCS)2 under external pressures, we discuss the changes in the peak shape of the thermal anomaly at the superconductive transition temperature Tc at various external pressures p. The rather sharp peak in CpT−1 at Tc = 9.1 K with a strong coupling character at ambient pressure is gradually reduced to weaker coupling as the pressure increases to 0.45 GPa concomitant with suppression of the transition temperature. This feature is compared with the systematic argument of the chemical–pressure effect on the basis of thermal anomalies around the superconductive transition of κ-(BEDT-TTF)2X compounds and other previously studied typical dimer-Mott 2:1 compounds. Finally, the discussion is extended to the chemical pressure effect on the normal state electronic heat capacity coefficient γ obtained by applying magnetic fields higher than Hc2 and the residual γ*, which remains in the superconductive state due to the induced electron density of states around the node structure. From the overall arguments with respect to both chemical and external pressures, we suggest that a crossover of the electronic state inside the superconductive phase occurs and the coupling strength of electron pairs varies from the electron correlation region near the metal-insulator boundary to the band picture region.

AC calorimetry of H2O at pressures up to 9 GPa in diamond anvil cells

If successfully developed, calorimetry at tens of GPa of pressure could help characterize phase transitions in materials such as high-pressure minerals, metals, and molecular solids. Here, we extend alternating-current calorimetry to 9 GPa and 300 K in a diamond anvil cell and use it to study phase transitions in H2O. In particular, water is loaded into the sample chambers of diamond-cells, along with thin metal heaters (1 μm-thick platinum or 20 nm-thick gold on a glass substrate) that drive high-frequency temperature oscillations (20 Hz to 600 kHz; 1 to 10 K). The heaters also act as thermometers via the third-harmonic technique, yielding calorimetric data on (1) heat conduction to the diamonds and (2) heat transport into substrate and sample. Using this method during temperature cycles from 300 to 200 K, we document melting, freezing, and proton ordering and disordering transitions of H2O at 0 to 9 GPa, and characterize changes in thermal conductivity and heat capacity across these transitions. The technique and analysis pave the way for calorimetry experiments on any non-metal at pressures up to ∼100 GPa, provided a thin layer (several μm-thick) of thermal insulation supports a metallic thin-film (tens of nm thick) Joule-heater attached to low contact resistance leads inside the sample chamber of a diamond-cell.

Implementation of a programmable electromechanical chopper with adjustable frequency and duty cycle for specific heat measurements

A hard disk mechanism was used as an electromechanical light chopper controlled by software developed in LabVIEW with an accuracy of ±0.01Hz in frequency to realize specific heat measurements. This chopper can be used in applications in experimental techniques in which low-frequency light excitation is required. The software provides an user interface to set the excitation frequency and its duty cycle. The evaluation of the efficiency was measured by installing the device in a fully automated high-resolution AC calorimetry system. The device provides a light cutter signal with a frequency varying from 1mHz to 40Hz. Employing the desired wave function, the excitation mode provided by this design can be used under theoretical models proposed for the specific heat technique, obtaining an increased efficiency in the measurements of the specific heat response for the studied materials. This paper reports the performance characteristics of the technology of an asymmetric CA-chopper, which can optimize efficiency in capturing the measurement of the response of specific heat of any given study material in a high-resolution ac calorimetry system. The chopper consists entirely of the positioning mechanism of a hard disk.

Temperature fluctuations and the thermodynamic determination of the cooperativity length in glass forming liquids.

The aim of this paper is to decide which of the two possible thermodynamic expressions for the cooperativity length in glass forming liquids is the correct one. In the derivation of these two expressions, the occurrence of temperature fluctuations in the considered nanoscale subsystems is either included or neglected. Consequently, our analysis gives also an answer to the widely discussed problem whether temperature fluctuations have to be generally accounted for in thermodynamics or not. To this end, the characteristic length-scales at equal times and temperatures for propylene glycol were determined independently from AC calorimetry in both the above specified ways and from quasielastic neutron scattering (QENS), and compared. The result shows that the cooperative length determined from QENS coincides most consistently with the cooperativity length determined from AC calorimetry measurements for the case that the effect of temperature fluctuations is incorporated in the description. This conclusion indicates that-accounting for temperature fluctuations-the characteristic length can be derived by thermodynamic considerations from the specific parameters of the liquid at glass transition and that temperature does fluctuate in small systems.