Abstract Body

Fe chalcogenide (FeCh)[1] is mine of interesting phenomena even in Fe based superconductor family, In addition to nematic state without magnetic order, amazing flexibility of increasing  superconducting Tc by various kinds of pressure application and interesting phenomena associated with extremely small Fermi energy comparable to superconducting gap attract much attention.  Particularly interesting is “high-temperature superconductivity” at around 65 K or at higher temperatures reported to take place in ultrathin, such as monolayer, film grown on some sort of oxide substrate grown by MBE technique[2,3].  Other than electron doping in bulk bands, extra carrier doping from the substrate, and/or interaction with phonons of the substrate are considered to play important roles, which reminds us that the so-called interface high-temperature superconductivity proposed by Ginzburg half century ago[4] may be realized.  However, even after 10 years of the report, Tc shown up in resistivity is very low (30-45 K in onset, and 10-30 K in zero resistivity), which I believe is a very serious problem.

In my view, the most characteristic aspect of superconductivity in FeCh is that three different category of superconductivity appear in the same material; (1) 10-40 K class superconductivity with both hole and electron Fermi surface, (2) 40-50 K superconductivity with electron Fermi surface alone, and (3) high-temperature superconductivity realized only in ultrathin films having almost the same electronic structure as that of the 2nd category, as described above.

We have studied superconductivity of FeCh in films grown by PLD technique and investigated superconductivity of all three categories described just above.  For the first category, we succeeded in preparing a series of FeSe1-xTex samples, avoiding phase separation which used to be inevitable in bulk crystals[5,6].  By the investigations of electronic properties both in normal stare and in superconducting state by ARPES[7,8], optical[9,10], THz[11] and microwave conductivity[12,13], dc electromagnetic studies[14], m-SR[15] and DFT[12] calculation, we conclude that a characteristic behavior of Tc as a function of Te content is the consequence of the change in the electronic structure at the Fermi level taking place caused by the disappearance of the novel pure nematic state without lattice distortion[16].  For the second category, the investigation of the isovalent substitution on the electron doped samples prepared by the electron-double-layer-transistor (EDLT) technique confirmed that the mechanism of superconductivity is definitely different from that of the first category[17,18].  During that procedure, we realized zero-resistance of 46 K[18], which is the highest value among all possible data of FeCh, except for a singular report by Ge et al.[19]  For the third category, we succeeded in realizing the interface superconductivity also by the PLD technique[20,21].  However, zero resistivity temperature, nor the onset temperature of superconductivity fluctuation measured by the Nernst effect have not reached 65 K, but stays around 30 K.

In my talk, I will discuss details on the above mentioned properties in detail, together with results obtained by other groups.  I also introduce our recent challenge to raise Tc in resistivity for the interface superconductivity.

References

[1] F. C. Hsu et al.: Proc. Natl. Acad. Sci. USA. 105 (2008) 14262.

[2] Q. Y. Wang et al.: Chin. Phys. Lett. 29 (2013) 037402 .

[3] S. H. He et al.: Nat. Mater. 12 (2013) 605-610, S. Y. Tan et al.: ibid. 634-640.

[4] V. L. Ginzburg: Phys. Lett. 13 (1964) 101.

[5] Y. Imai et al., Proc. Natl. Acad. Sci. USA. 112 (2015) 1937.

[6] Y. Imai et al., Sci. Rep. 7 (2017) 46653.

[7] G. N. Phan et al.: Phys. Rev. B95 (2017) 224507.

[8] K. Nakayama et al.: Phys. Rev. Res. 3 (2021) L012007.

[9] M. Nakajima et al.: Phys. Rev. B95 (2017) 184502.

[10] M. Nakajima et al.: Phys. Rev. B104 (2021) 024512.

[11] N. Yoshikawa et al.: Phys. Rev. B100 (2019) 035110.

[12] H. Kurokawa et al.: Phys. Rev. B104 (2021) 014505.

[13] G. Matsumoto et al.: J. Phys. Conf. Ser. 2776 (2024) 012002, also arXiv. 2402.18082.

[14] F. Nabeshima et al., Phys. Rev. B101 (2020) 184517.

[15] F. Nabeshima et al., Phys. Rev. B103 (2021) 184504.

[16] Y. Kubota et al.: Phys. Rev. B108 (2023) L100501.

[17] N. Shikama et al., Appl. Phys. Express 13 (2020) 083006.

[18] N. Shikama et al.: Phys. Rev. B104 (2021) 094512.

[19] J. F. Ge et al.: Nat. Mater. 14 (2015) 285.

[20] T. Kobayashi et al.: Supercond. Sci.Tech. 35 (2022) 07LT01.

[21] T. Kobayashi et al: J. Phys: Cond. Mater. 35 (2023) 41LT01.

Abstract Body

Superconducting digital systems have inherent advantages as illustrated by the data-center-in-a-shoebox having superlative 3D compute density, interconnect bandwidth, latency, clock rate, and energy efficiency. Demonstration of a high-end, out-of-order CPU is a near-term goal. On the fabrication side, this requires a stackup with 16 superconducting NbTiN wire & via layers, self-shunted a-silicon barrier Josephson Junctions (JJs), and low loss, high-k tunable hafnium–zirconium oxide (HZO) capacitors. Electrical measurements of these three unit process modules are reported: 50 nm interconnects with critical temperature of 13 K and critical current density JC = 120 mΑ/μm2, 210 nm amorphous Si barrier JJs with JC = 0.8 mΑ/μm2 and ICRN = 1.1 mV, and tunable HZO MIM capacitors with specific capacitance of 28 fF/μm2 and k-value of 30. All devices were fabricated on 300 mm wafers with thermal budget of 420 C, compatible with standard CMOS processes. On the design side, this requires Pulse-Conserving Logic (PCL) that reproduces all the functions of a standard CMOS gate library. Wave-pipelined, SFQ-pulse-driven Josephson SRAM (JSRAM) memory operates with the same clock rate and energy efficiency as for the logic. Such circuits implement efficient signal fanout in the array. Design of an associative memory array is reported that constitutes a key, historically difficult building block of the out-of-order CPU.

Abstract Body

In Japan, during the period of high economic growth since the 1960s, the power system including power plants has been strengthened. Based on the expectation that it would be difficult to increase the power system by conventional technology from the 1980s onward, the development of technologies such as superconducting large-capacity generators, superconducting power cable transmission, and SMES technology was actively promoted mainly by national projects. After that, Japan's energy demand peaked around 2000, and the growth in electricity demand stagnated accordingly. In particular, since the Great East Japan Earthquake in March 2011, energy conservation has been thoroughly implemented, and electricity demand has remained flat or slightly declining to this day.

On the other hand, in order to stop global warming, efforts are being made to reduce carbon dioxide emissions per unit of production in electricity supply by the introduction of the FIT system. At the end of 2024, solar power generation has been introduced by about 73 GW, and wind power generation has been introduced by about 5.6 GW. As for wind power generation, it is expected that the introduction of offshore wind power generation will continue in the future, and the FIT certified capacity is 13.9 GW. Already, mainly in spring and autumn, the amount of power generated including solar power generation exceeds the demand of electricity, and it is becoming normal to suppress output. For this reason, the characteristics required of electric power equipment are also changing. There is a growing need for the ability to adjust ΔkW to match power supply and demand, and the ability to supply inertial and synchronization forces when the proportion of inverter power sources that do not have inertial power is the majority. Superconducting power devices can exhibit various characteristics that are difficult to realize with conventional power equipment, and research and development to maximize these characteristics is beginning.

In addition, Japan is considering importing hydrogen produced using renewable energy overseas in order to pursue a low-carbon energy supply. Liquid hydrogen is considered to be one of the most likely transportation mediums in this case, and consideration of a specific import base has begun. In the vicinity of the import base or the satellite base where hydrogen is supplied in liquid form, liquid hydrogen can be used as a refrigerant or a low-temperature heat source for cooling superconducting equipment. Studies on superconducting power devices that utilize the cold heat of such liquid hydrogen have already begun.

In this presentation, I will give an overview of the history of superconducting power equipment development in Japan and introduce the recent state of research and development.

Abstract Body

Nuclear fusion promises to be a practically inexhaustible energy source and research on its exploitation started in the 1940s, unfortunately, only successful for nuclear weapons so far. Attempts for producing energy for the benefit of humankind set in nearly simultaneously but turned out to be much more challenging. Two different approaches to create a plasma hot and dense enough to produce more energy by fusion reactions than required for its generation were followed since the very beginning: The first being inertial confinement that mimics the mechanism in nuclear weapons. The nuclear fuel is heated and compacted extremely fast resulting in a very dense plasma with a high fusion rate limited to the timescale the plasma needs to expand during the resulting explosion. Recently, a net energy gain, where the fusion energy exceeded the energy deposited in the nuclear fuel (Q>1) was achieved for the first time. The second approach relies on a magnetic confinement of the charged particles and can in principle lead to a continuously burning plasma. However, a net energy gain was not realized so far. In both cases, the progress was continuous but slow, since there was no immediate need for fusion power plants. The first fusion device aiming at a useful energy gain (ITER, Q ≈ 10) has been under construction for many years now and it will take more than another decade until experiments with a burning plasma will start. Europe plans a first demonstration reactor (DEMO) producing electricity for the second half of this century.

The situation changed with the broad public awareness of the climate change creating an immediate need for alternative energy sources. Fusion was reconsidered but the existing programs were obviously too slow to face the challenge; thus new, privately funded initiatives took the stage. The vast majority of these projects is based on the idea to increase the magnetic field for confinement, which is only possible by using high temperature superconductors. A higher magnetic field enhances the power density of the burning plasma and enables much compacter designs, which in turn promises a significant cost reduction and commercial viability of fusion power plants.

However, the increased power density leads to other challenges, among them the increased neutron (and heat flux) density. A small fraction of the neutrons will reach the superconducting magnets degrading their performance and hence limiting their lifetime.

I will follow the journey of the neutrons from their birth in the plasma to the magnets and discuss the damage production, the resulting defect structure and the influence on the properties of the insulator, the stabilizer and the superconductor. The change of superconducting properties will be the main focus and demonstrated by results of neutron irradiation experiments on high temperature superconducting tapes performed at the TRIGA reactor in Vienna. The introduced defects have a positive effect on the critical current by improving pinning but the enhanced scattering degrade the transition temperature and the superfluid density because scattering is pair breaking in d-wave superconductors. The competition between improved pinning, which dominates at low neutron fluences, and the degrading scattering leads to a peak in the critical current as shown in Fig. 1.[1] While the degradation can be modelled due to its universal behavior, the change in pinning depends on the pristine defect structure of the tape and the energy of the incident particles.

The implication for the lifetime of conventional and compact fusion reactor concepts will be compared. Finally, possible mitigation strategies will be discussed. 

References

[1] Raphael Unterrainer, Davide Gambino, Florian Semper, Alexander Bodenseher, Daniele Torsello, Francesco Laviano, David X. Fischer, Michael Eisterer, Responsibility of small defects for the low radiation tolerance of coated conductors, accepted for Superconductor Science and Technology 37 (2024)

Acknowledgment

This work has been carried out within the framework of the EUROfusion Consortium, funded by the European Union via the Euratom Research and Training Programme (Grant Agreement No 101052200 — EUROfusion). Views and opinions expressed are however those of the author(s) only and do not necessarily reflect those of the European Union or the European Commission. Neither the European Union nor the European Commission can be held responsible for them.

Figure 1. Relative change of the critical current density at 30 K, 15 T after fast neutron radiation (E>0.1 MeV) as a function of fluence.[1]

Keywords: Nuclear fusion, Radiation damage, Critical currents, Neutron irradiation