IUMRS-ICAM2017 Meeting Scene

Prof. X.X. Xi(Temple University, USA) talked about “MgB2 thin films for SRF cavity applications”. Prof. Xi mentioned that clean MgB2 thin films have a low residual resistivity and high Tc of ~40K and are promising materials to RF cavity applications if we can obtain Hc1 values higher than that of Nb. He has been studying MgB2 thin films applying a hybrid-physical-chemical vapor deposition(HPCVD) method. He succeeded in obtaining high Hc1 of MgB2 film, higher than that of Nb by decreasing the film thickness. He also carried out the deposition of MgB2 film on Cu, which is important to cavity application. He observed the formation of Mg-Cu compound layer between Cu substrate and MgB2 film. He also observed the cracks introduced into the MgB2 films after the deposition. He mentioned that these cracks are due to the difference of thermal contraction between Cu and MgB2. He also carried out the coating of RF cavities with MgB2 and investigated the properties.

S.K. Chen(University Putra, Malaysia) reported on “Synthesis and optimization of superconducting properties of MgB2”. Dr. Chen fabricated MgB2 bulks by direct reaction between Mg and B or MgB4 and Mg. He mentioned that with increasing heat treatment temperature a compromise between grain connectivity and defect density becomes crucial in order to optimize Jc. He obtained a few orders of magnitude higher Jc values by the addition of SiC nano-particles. The field dependence of Jc was improved with a slight decrease of Tc by the SiC addition. He also mentioned that addition of excess Mg to MgB4(Mg1.5B2) increased the weight fraction of MgB2 together with unreacted Mg and enhanced Jc values.

P. Badica(National institute of Materials Physics, Romania) reported on “addition, co-additions and raw materials influence on MgB2 obtained by spark plasma sintering”. Dr. Badica mentioned that high density MgB2 bulks can be obtained by spark plasma sintering(SPS). He used MgB2 powder and additives or co-additives as a raw materials. He mentioned that the type of an additive is important, but the key element is not only that. The microstructure of SPS sample seems complicated. He also mentioned that the specific features of the raw powders (morphology, size, purity and so on) and processing conditions(e.g. interaction between co-additions or the mixing level of the powders) can or can’t lead to formation of pinning centers.

M. Muralidhar(Shibaura Institute of Technology, Japan) reported “low cost and high performance bulk MgB2 material for super-magnets”. Prof. Muralidhar fabricated MgB2 bulks by mixing crystalline boron and nano-meter-sized amorphous boron powders encapsulated in carbon to reduce the cost of the bulk materials. He obtained sharp superconducting transitions of with Tc,onset around 38.6-27.2K for the bulks prepared with the mixture of crystallince boron and carbon-coated amorphous boron. He mentioned that the highest Jc values of 470kA/cm2 and 310kA/cm2 in self-field and 1T, respectively, were obtained at 20K for the MgB2 sample with 1.5% of carbon in the carbon-encapsulated boron.

A3-P31-007 S. Kunwar

BCS-BEC (Bose Einstein Condensation) crossover in iron based superconductor was characterized by STM. One of the hottest topics for revealing the mechanism of the superconductivity in these materials.

A3-P31-008 C. M. Palomares-Garcia et al.

Thin film growth of Sr2RuO4 on LSAT substarate by PLD technique. Drop of the resistance was observed at around the bulk Tc.

A3-P31-009 K. Komori et. al.

Fabrication of seamless flux transformer made of a superconducting tape for magnetic measurement. Simple technique but the method is also applicable to seamless superconducting solenoid.

A3-P31-010 T. Maeda et al.

Nb-based 1212 superconductor. Nb was substituted by Cu. Impurity phases were reduced by the substitution.

A3-P31-011 T. Maeda et al.

Substitution of Y-123 superconductor. Substitution of Ba by Sr and Cu by Mo. Superconductivity at relatively higher temperature was observed even in tetragonal phase.

A3-P31-012 Y. Sanogawa

Fabrication of bulk MgB2 superconductor using vapor transport method. The amount of MgO phase can be reduced by this technique.

Keynote talk by prof. Zhao

Photo-Responsive Self-Assembled Organic Nanoparticles for Bioimaging and Therapy

As the keynote talk of our symposium, prof. Yanli ZHAO, Nanyang Technological University, Singapore, presented simple but excellent strategies to apply functional dyes to bioimaging systems on 31th Aug. The prof. Zhao’s research group have developed novel photo-responsive multifunctional nanoparticles for targeted bioimaging and controlled drug delivery in disease therapy. In order to overcome photon scattering and low optical response in biological tissues, they integrated nanoparticles as exogenous contrast agents for enhancing the capability of bioimaging modalities, and also utilized porous nanoparticles to prevent undesired aggregation of dyes. They have synthesized several types of photoresponsive nanoparticles for fluorescent and multi-photon, and photoacoustic bioimaging systems that can work in vivo. These photoresponsive nanoparticles can simultaneously target diseased cells, and enable the location to be imaged by optical methods, and release therapeutic drugs to the diseased cells by commands. The photoresponsive bioimaging and drug delivery strategies developed by the prof. Zhao’s research group could be applied to existing drugs or experimental compounds, and realize longer circulation time or better targeting. These research achievements pave the way for developing next generation of therapeutics.

1. In an invited talk entitled “Low electronic thermal conductivity and violation of the Wiedemann-Franz law in correlated metallic vanadium oxides”, Kedar Hippalgaonkar (Agency for Science, Technology and Research, Singapore) reported about a study moving from the inconsistency between the measured value of VO₂ thermal conductivity and its predicted electronic component based upon the Wiedemann-Franz law. Through independent measurements of the lattice thermal conductivity he showed how the total conductivity would be expected to be much larger than its experimental value. Doping with W recovered the expected electronic contribution, showing how the Wiedemann-Franz “constant” is here a linear function of W content. A sophisticated model explained this anomaly as due to the electronic correlation in the material, where electrons behave as a coherent/incoherent Fermi liquid.
2. Kunpeng Zhao – Cu2S has lower lattice k, Cu2Se has higher PF, what happens if we put them together, nanodomains with 20-8 0nms are formed and a modulated cubic phase is found, which is good for reducing lattice thermal conductivity. Phase transition from 300-700K giving large heat capacity below 700K… Electrical conductivity determines that the PF lies between the two materials. The carrier concentration is increased, so the seebeck drops and conductivity increases. Maximum value comes from those with Cu vacancies. Pisarenko Plot gives an effective mass of 4.2m0, but higher mobility, giving a higher weighted mobility. Low lattice conductivity (~0.3-0.4 W/m-K) with higher PF gives ZT as high as 2.3 at 1000K at 10^20-10^21 cm-3 carrier concentration. A question on stability was asked and the author showed data that the phase transition has hysteresis, but proving that there’s stability.

3.  Lanling Zhao: Investigation of TE properties of Cu2Se(S) – Liquidlike materials should have lower specific heat at higher  temperature. Increased electrical resistivity and Seebeck at higher temperatures and slightly reduced kL (~0.6 W/m-K). zT ~ 1.2 up to 1.8 (single crystals) comparable to polycrystalline bulk. Stability is not an issue for water-quenched CuSe samples. Copper deficient CuS shows increased electrical conductivity, similar kL, but smaller S. But due to larger sigma, zT is higher, but this has poor electrical  and thermal stability. Next, the author talked about Te and I doped samples and found that Te is better for electrical conductivity.  Highest ZT is for CuSe, and the Te/I doping does not help. Then, the author introduced Sulfur doped CuSe, for doping levels <0.16 where the structure remains monoclinic.  For higher doping, the samples are composite multiphase.  TE properties show lower electrical conductivity and increased Seebeck, but the CuSe ZT cannot be improved. Finally, the author introduced composites with CNTs, graphite and hard Carbon – graphite forms nanoclusters with a grain size ~30-60nms. Carbon doping gives higher sigma and comparable Seebeck and reduced kappa, obtaining higher ZT around 2.4 at 970K.
4. Terumasa Tadano – Peierls Boltzmann Theory (phonon gas model) deals with cubic anharmonic scattering. Full anharmonicity is considered in ab initio MD. The author proposed a new method to decrease computational cost, which is between BTE and ab-initio. Usually, DFT considers 2nd and 3rd order force constants, then uses BTE, neglecting 4th order anharmonicity. This breaks down when there’s an imaginary mode underlining that quartic anharmonicity is important, for eg. SnSe (TE materials) ‘soft’ materials causes frequency shift and introduce 4-phonon scattering. The author uses Self-Consistent Phonon Theory to obtain an effective Hamiltonian to obtain a temperature dependent phonon dispersion. Inelastic Nuetron Scattering (INS) experimentally corroborates this. Tested on cubic STO.T-dependent phonon dispersion but PbTe and SnSe show very strong temperature dependence due to more soft modes. Hence, this allows for self-corrected phonons to be added to a BTE calculation, and the key is to use 2nd and 4th order force constants. This calculation was also performed on clathrates (BGG), where a hardening of the rattling mode is observed. These rattling modes scatter acoustic phonons and hence the details are important resulting in an enhancement in lattice thermal conductivity due to quartic anharmonicity. Software is now available at ALAMODE at http://ttadano.github.io/alamode/

Prof. Thimont made an invited talk about their special processing technique for making higher manganese silicide.

Prof. Ikeda, who is the other invited speaker, introduced the technique for making super-saturated solid solutions and nano-sized precipitates in the Si-based materials.

Two of the contributed talks were made by students, Mr. Alinejat and Mr. Takamatsu, and they made nice talk about synthesis technique of Mg2Si and thermoelectric properties of CrSi2 based C40 phase, respectively.

The number of attendee was roughly thirty, and all the talk had many questions.

 

 

High thermoelectric performance in Te-free (Bi, Sb)₂Se₃ (Keynote)

This presentation discussed (Bi,Sb)2Se3 as an affordable material for energy harvesting in the low to mid-range of temperatures. Rhombohedral Bi2Se3 is isostructural with the telluride whereas orthorhombic Sb2Se3 has a chain like structure. Investigation of the pseudo-binary Bi2Se3-Sb2Se3 phase diagram was reported and shown to consist of two single phase R and O regions, separated by a two-phase region. The RO structural change impacts on both electronic and dynamical properties and ZT=0.3 may be attained in O-(Bi,Sb)2Se3. Further improvements in charge carrier concentration through iodine doping raises this to 1.0 at 800 K.

Open die pressing of thermoelectric materials: a solution for material sintering and texture inducing (Invited)

This presentation focused on how to achieve single-crystal–like transport properties in polycrystalline phase through a new processing method, which seeks to exploit structural anisotropy to texture the material. Processing consists of hot-pressing powders contained within a metal sheath for relatively short periods at modest temperatures. (Bi,Sb)Te3 and SnSe were presented as examples. Although an observable degree of texturing was achieved in the latter, single-crystal levels of anisotropy in the properties have not yet been realised. The method was also shown to provide a one-step synthesis and consolidation process for the preparation of nickel-substituted tetrahedrite.

Enhanced thermoelectric performance of p-type MnTe doped with Ag

Maximizing thermoelectric performance of PbSe by combining non-stoichiometric composition optimization and alloying with SnSe

Last two talks were presented from the same group in Tsinghua University, China. They made tellurides, and enhance the thermoelectric properties well by doping. Tellurium is well known one of rare earth materials. They made thermoelectric materials by replacing tellurium to selenium to reduce tellurium consumption. Lead is one of the strong toxic materials. Lead is also replaced to Sn for a practical use of thermoelectric materials. The research group showed us the possibility of atomic substitution for thermoelectric materials without ZT huge reduction.

A packed conference room for Session A-5

Room1 / THIN FILMS, MULTILAYERS, NANOSTRUCTURES I

Prof. Chunlei Wan (Tsinghua University, China) presented high-performance inorganic / organic superlattices for flexible thermoelectric energy harvesting.

The authors showed thermoelectric properties of organic molecule cation intercalated TiS2 (electrochemically), which looks like superlattice (layered structure). They successfully reduced the thermal conductivity while maintain its rather high electrical conductivity. Positively charged organic cations play several important roles such as good donor (strong positive electric field), enhancement of two-dimensionality, and reducing agent of thermal conductivity.

Prof. Kyu Hyoung Lee (Kangwon National University, Korea) presented design and preparation of high performance thermoelectric materials with defect structures.

The authors showed interesting way to enhance thermoelectric figure of merit of several materials such as BiSbTe, BiTeSe, and half-Heusler. They successfully reduced the thermal conductivity by introducing dislocation arrays at around the grain boundaries. They achieved 30-50% enhancement of their zT.

Chairpersons: Akiyasu YAMAMOTO and Valeria BRACCINI

A3-K30-001 Keynote
Vortex Pinning in iron based superconductors
Kees VAN DER BEEK

Jc and Hc2 in IBS look very interesting but way lower than in HTS (in particular in YBCO): the phase 1111 is the better performing in this respect. Flux pinning mechanisms look similar in all IBS, and have been extensively studied and presented in this talk.

Pinning centers can be nm-size inclusions, dislocations or Fe vacancies (in particular in FeSe single crystals, see very recent work by Sprau et al., 357 (2017) 65).

Jc vs field has been extensively investigated, and presents a similar behaviour in various materials. After a plateau where Jc is constant, it decreases proportionally to B^-0.5, then again it smoothest down. At low field it is predominant the contribution from big grains, isolated vortices (strong pinning on a nm scale), at intermediate fields we have collective pinning due to charged dopants / vacancies.

The depairing current density has been investigated: it presents a dome as a function of the doping, as it happens for the Tc vs x.

Vortex pinning has been correlated to the behaviour of .
The multiple band character of superconductivity has a strong influence on Jc and on its anisotropy, which in particular in the ab direction is strongly related to the anisotropy of .

A3-I30-003 Invited
Arsenic Chemistry of Iron-based Superconductors and Strategy for New Superconducting Materials
Minoru NOHARA

Prof. Nohara reported the progress of iron-based superconducting materials with the emphasis on the valence states and chemical bonds of arsenic. They demonstrated that in the novel 112-type CaFeAs2, monovalent arsenic produced As zigzag chains of CaAs intermediary layers, while trivalent arsenic produced FeAs layers. The superconducting transition temperature of this materials was enhanced up to 47 K when La and Sb were simultaneously substituted for Ca and As, respectively. Sb is preferably substituted for the As of the zigzag chains. On the other hand, in the 10-4-8-type Ca10(Pt4As8)(Fe2−xPtxAs2)5 with the highest Tc of 38 K, he emphasized that divalent arsenic produced As2 dimers in Pt4As8 layers. Finally, he discussed the structural phase transition in 122-type CaFe2As2 that is characterized by the formation of As2 dimers between the adjacent FeAs layers, which resulted in the loss of magnetism and disappearance of superconductivity. This transition can be viewed as an As2-/As3- valence transition.

A3-I30-002 Invited
Doping dependent critical current properties in K, Co, and P-doped BaFe2As2 single crystals
Hiroshi EISAKI

Dr. Eisaki reported the in-plane critical current density (Jc) of BaFe2As2-based superconductors, Ba1-xKxFe2As2 (K-Ba122), Ba(Fe1-xCox)2As2 (Co-Ba122), and BaFe2(As1-xPx)2 (P-Ba122) in a wide range of doping concentration (x) by means of magnetization hysteresis loop (MHL) measurements on single crystal samples. Depending on the dopant elements and their concentration, Jc exhibits a variety of magnetic-field (H)- and temperature (T)- dependences. In the case of K-Ba122, the MHL of the under-doped samples (x < 0.33) exhibits the second magnetization peak (SMP), which sustains high Jc at high H and high T, exceeding 105 A/cm2 at T = 25 K and μ0H = 6 T for x = 0.30. On the other hand, the SMP is missing in the optimally- (x ~ 0.36-0.40) and overdoped (x ~ 0.50) samples, and consequently Jc rapidly decreases by more than one order of magnitude, although the change in Tc is within a few K. He pointed out that the T-dependent Jc indicates that the two pinning mechanisms, namely, the spatial variations in Tc (referred to as delta-Tc pinning) and the fluctuations in the mean free path (delta-l pinning), are enhanced for the under-doped samples, which results in the enhancement of Jc. Possible origins for the different pinning mechanism are discussed in connection with the x-dependence of Tc, the residual resistivity, AFO domain boundaries, a possible quantum critical point, etc.

A3-K30-004 Invited
Pressure Effects of FeSe by Novel DAC using metallic Diamond Electrodes
Yoshihiko TAKANO

A deep work published very recently (Matsumoto et al., JJAP 56 (2017)) has been presented, regarding the pressure effects induced in FeSe single crystals by a novel Diamond Anvil Cell.

Superconducting properties of many different materials under high pressure have recently received great attention, especially after the discovery with great surprise of superconductivity in H2S at ∼200 K under 150 GPa which was reported by resistivity measurements using a diamond anvil cell (DAC) in 2015. In fact, if one can measure the resistivity under extremely higher pressure above 300 GPa, superconductivity at room temperature in light elements such as hydrogen would be observed.
The resistivity measurement though is difficult because the sample space is very small (<100 μm) and the electrodes are deformed by compression. A novel diamond anvil cell specialized for resistivity measurements under high pressure has been developed at NIMS, Tsukuba. Once metallic diamond is heavily boron-doped, it shows metallicity and superconductivity at low temperature. Boron-doped metallic diamond electrodes were deposited onto a diamond anvil using a electron beam lithograph: in this way, resistivity measurements can be performed up to 10 GPa.

In particular, results obtained through this technique on FeSe single crystals have been reported. FeSe shows a Tc of 8 K, which can increase up to 37 K with the application of idrostatic pressure (8 GPa). When uniaxial pressure is applied, at NIMS they were able to reach Tc as high as 44 K, similarly to what happens in FeSe phase after K intercalation. This Tc increase depends on the anion height from the Fe layer which can be changed through the intercalation or through the application of the uniaxial pressure.