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Thermodynamic characterization of superconducting and magnetic materials using nanocalorimetry
Stockholm University, Faculty of Science, Department of Physics.
2019 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Measurement of specific heat is a powerful technique for the investigation of novel materials. Superconducting and magnetic systems, in particular, can be thoroughly characterized by studying their electronic contribution to the specific heat. To investigate their behavior in magnetic fields, single crystals need to be used, since the magnetic properties are dependent on the crystalline orientation. Crystal quality is often enhanced when sizes are reduced down to below the 100 μm scale, which is lower than the limit of conventional calorimeters. Nanocalorimetry allows to detect the weak electronic signature in the specific heat for such small samples with a preserved combination of high resolution and good accuracy. This is achieved by miniaturizing the device using microsystems technology and by a proper optimization of the measurement conditions.

In this thesis, a nanocalorimeter designed for the study of samples with masses from sub-μg  to 100 μg in the temperature range 1-350 K is used for studying three different systems, yielding insights into their physical properties.

In the magnetocaloric compound Fe2P a deep thermodynamic understanding of the first-order magnetic phase transition at the Curie temperature TC ≈ 217 K is lacking. The nanocalorimeter is used to map the magnetic phase diagram for fields applied parallel and perpendicular to the easy axis of magnetization. Two different phase diagrams are obtained depending on the applied field orientation. The first-order magnetic phase transition is characterized by specific and latent heat, providing a textbook example of thermodynamic properties around such a transition. The results are complemented with a combined nanocalorimetry - x-ray diffraction study and by magnetization measurements.

The iron-based high-temperature superconductor BaFe2(As1-xPx)2 shows several anomalous physical properties which have been associated to the presence of a quantum critical point. High-resolution specific heat measurements are an important piece of the puzzle in understanding the behavior of this material. The specific heat is measured as a function of phosphorus doping x in the superoptimally substituted range and several superconducting parameters are extracted. An evolution from a single-gap to a two-gap is seen with doping, as well as a decrease of the London penetration depth close to optimum doping, without signs of divergence.

The superconducting properties are as well investigated in the metastable β phase of gallium. β-Ga is obtained in-situ from the stable α-Ga by increasing the temperature about 10 K above the melting point. This novel method to produce β-Ga allows more reproducible and reliable measurements in comparison to traditional methods. A thorough thermodynamic characterization of the metastable phase is obtained, giving insights into the conditions for a strongly enhanced superconductivity in β-Ga in comparison to α-Ga. β-Ga is found to be a strong-coupling superconductor, with a 2.55 higher density of states at the Fermi energy in comparison to α-Ga.

These measurements demonstrate how several problems in condensed matter physics can be addressed through nanocalorimetry, which allows mapping various phase diagrams and obtaining fundamental thermodynamic properties on high-quality samples in magnetic fields.

 

Place, publisher, year, edition, pages
Stockholm: Department of Physics, Stockholm University , 2019.
Keywords [en]
Nanocalorimetry, Superconductivity, Magnetism, MEMS, Specific heat, Thermodynamics
National Category
Condensed Matter Physics
Research subject
Physics
Identifiers
URN: urn:nbn:se:su:diva-163479ISBN: 978-91-7797-564-9 (print)ISBN: 978-91-7797-565-6 (electronic)OAI: oai:DiVA.org:su-163479DiVA, id: diva2:1275454
Public defence
2019-02-26, Room FA32, AlbaNova University Center, Roslagstullsbacken 21, Stockholm, 09:00 (English)
Opponent
Supervisors
Available from: 2019-02-01 Created: 2019-01-06 Last updated: 2019-01-24Bibliographically approved
List of papers
1. Thermodynamics around the first-order ferromagnetic phase transition of Fe2P single crystals
Open this publication in new window or tab >>Thermodynamics around the first-order ferromagnetic phase transition of Fe2P single crystals
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2014 (English)In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 90, no 14, article id 144432Article in journal (Refereed) Published
Abstract [en]

The specific heat and thermodynamics of Fe2P single crystals around the first-order paramagnetic to ferromagnetic (FM) phase transition at T-C similar or equal to 217 K are empirically investigated. The magnitude and direction of the magnetic field relative to the crystal axes govern the derived H-T phase diagram. Strikingly different phase contours are obtained for fields applied parallel and perpendicular to the c axis of the crystal. In parallel fields, the FM state is stabilized, while in perpendicular fields the phase transition is split into two sections, with an intermediate FM phase where there is no spontaneous magnetization along the c axis. The zero-field transition displays a textbook example of a first-order transition with different phase stability limits on heating and cooling. The results have special significance since Fe2P is the parent material to a family of compounds with outstanding magnetocaloric properties.

National Category
Physical Sciences
Research subject
Physics
Identifiers
urn:nbn:se:su:diva-110189 (URN)10.1103/PhysRevB.90.144432 (DOI)000344029500004 ()
Note

AuthorCount:7;

Available from: 2015-02-10 Created: 2014-12-08 Last updated: 2019-01-09Bibliographically approved
2. Superconducting gap evolution in overdoped BaFe2(As1-xPx)(2) single crystals through nanocalorimetry
Open this publication in new window or tab >>Superconducting gap evolution in overdoped BaFe2(As1-xPx)(2) single crystals through nanocalorimetry
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2015 (English)In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 91, no 24, article id 245142Article in journal (Refereed) Published
Abstract [en]

We report on specific heat measurements on clean overdoped BaFe2(As1-xPx)(2) single crystals performed with a high resolution membrane-based nanocalorimeter. A nonzero residual electronic specific heat coefficient at zero temperature gamma(r) = C/T backslash(T -> 0) is seen for all doping compositions, indicating a considerable fraction of the Fermi surface ungapped or having very deep minima. The remaining superconducting electronic specific heat is analyzed through a two-band s-wave alpha model in order to investigate the gap structure. Close to optimal doping we detect a single zero-temperature gap of Delta(0) similar to 5.3 meV, corresponding to Delta(0)/k(B)T(c) similar to 2.2. Increasing the phosphorus concentration x, the main gap reduces till a value of Delta(0) similar to 1.9 meV for x = 0.55 and a second weaker gap becomes evident. From the magnetic field effect on gamma(r), all samples however show similar behavior [gamma(r)(H) -gamma(r)(H = 0) proportional to H-n, with n between 0.6 and 0.7]. This indicates that, despite a considerable redistribution of the gap weights, the total degree of gap anisotropy does not change drastically with doping.

National Category
Physical Sciences
Research subject
Physics
Identifiers
urn:nbn:se:su:diva-118947 (URN)10.1103/PhysRevB.91.245142 (DOI)000356474500006 ()
Available from: 2015-07-24 Created: 2015-07-21 Last updated: 2019-01-09Bibliographically approved
3. Microscopic parameters from high-resolution specific heat measurements on superoptimally substituted BaFe2(As1-xPx)(2) single crystals
Open this publication in new window or tab >>Microscopic parameters from high-resolution specific heat measurements on superoptimally substituted BaFe2(As1-xPx)(2) single crystals
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2016 (English)In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 93, no 1, article id 014509Article in journal (Refereed) Published
Abstract [en]

We investigate the electronic specific heat of superoptimally substituted BaFe2(As1-x P-x(x))(2) single crystals in the superconducting state using high-resolution nanocalorimetry. From the measurements, we extract the substitution dependence of the condensation energy, superconducting gap Delta, and related microscopic parameters. We find that the anomalous scaling of the specific heat jump Delta C proportional to T-c(3) , found in many iron-based superconductors, in this system originates from a T-c-dependent ratio Delta/k(B)T(c) in combination with a substitution-dependent density of states N(epsilon(F)). A clear enhancement is seen in the effective mass m* as the composition approaches the value that has been associated with a quantum critical point at optimum substitution. However, a simultaneous increase in the superconducting carrier concentration n(s) yields a penetration depth lambda that decreases with increasing T-c without sharp divergence at the quantum critical point. Uemura scaling indicates that T-c is governed by the Fermi temperature T-F for this multiband system.

National Category
Physical Sciences
Research subject
Physics
Identifiers
urn:nbn:se:su:diva-126896 (URN)10.1103/PhysRevB.93.014509 (DOI)000368481300009 ()
Available from: 2016-02-17 Created: 2016-02-16 Last updated: 2019-01-09Bibliographically approved
4. Nanocalorimeter platform for in situ specific heat measurements and x-ray diffraction at low temperature
Open this publication in new window or tab >>Nanocalorimeter platform for in situ specific heat measurements and x-ray diffraction at low temperature
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2017 (English)In: Review of Scientific Instruments, ISSN 0034-6748, E-ISSN 1089-7623, Vol. 88, no 12, article id 125108Article in journal (Refereed) Published
Abstract [en]

Recent advances in electronics and nanofabrication have enabled membrane-based nanocalorimetry for measurements of the specific heat of microgram-sized samples. We have integrated a nanocalorimeter platform into a 4.5 T split-pair vertical-field magnet to allow for the simultaneous measurement of the specific heat and x-ray scattering in magnetic fields and at temperatures as low as 4 K. This multi-modal approach empowers researchers to directly correlate scattering experiments with insights from thermodynamic properties including structural, electronic, orbital, and magnetic phase transitions. The use of a nanocalorimeter sample platform enables numerous technical advantages: precise measurement and control of the sample temperature, quantification of beam heating effects, fast and precise positioning of the sample in the x-ray beam, and fast acquisition of x-ray scans over a wide temperature range without the need for time-consuming re-centering and re-alignment. Furthermore, on an YBa2Cu3O7-delta crystal and a copper foil, we demonstrate a novel approach to x-ray absorption spectroscopy by monitoring the change in sample temperature as a function of incident photon energy. Finally, we illustrate the new insights that can be gained from in situ structural and thermodynamic measurements by investigating the superheated state occurring at the first-order magneto-elastic phase transition of Fe2P, a material that is of interest for magnetocaloric applications.

National Category
Physical Sciences
Research subject
Physics
Identifiers
urn:nbn:se:su:diva-152513 (URN)10.1063/1.5016592 (DOI)000418956500066 ()29289216 (PubMedID)
Available from: 2018-02-07 Created: 2018-02-07 Last updated: 2019-01-09Bibliographically approved
5. Raising the superconducting T-c of gallium: In situ characterization of the transformation of alpha-Ga into beta-Ga
Open this publication in new window or tab >>Raising the superconducting T-c of gallium: In situ characterization of the transformation of alpha-Ga into beta-Ga
2018 (English)In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 97, no 18, article id 184517Article in journal (Refereed) Published
Abstract [en]

Gallium (Ga) displays several metastable phases. Superconductivity is strongly enhanced in the metastable beta-Ga with a critical temperature T-c = 6.04(5) K, while stable alpha-Ga has a much lower T-c < 1.2 K. Here we use a membrane-based nanocalorimeter to initiate the transition from alpha-Ga to beta-Ga on demand, as well as study the specific heat of the two phases on one and the same sample. The in situ transformation is initiated by bringing the temperature to about 10 K above the melting temperature of alpha-Ga. After such treatment, the liquid supercools down to 232 K, where beta-Ga solidifies. We find that beta-Ga is a strong-coupling type-I superconductor with Delta(0)/k(B)T(c) = 2.00(5) and a Sommerfeld coefficient gamma(n) = 1.53(4) mJ/molK(2), 2.55 times higher than that in the alpha phase. The results allow a detailed comparison of fundamental thermodynamic properties between the two phases.

Keywords
Crystal structure, Density of states, Specific heat, Superconducting phase transition, Superconductivity, Thermal properties, Thermodynamics
National Category
Physical Sciences
Research subject
Physics
Identifiers
urn:nbn:se:su:diva-157722 (URN)10.1103/PhysRevB.97.184517 (DOI)000433287200004 ()
Available from: 2018-08-02 Created: 2018-08-02 Last updated: 2019-01-09Bibliographically approved

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