• Journal of the Korean Magnetics Society
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• v.22 no.2
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• pp.66-72
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• 2012

The findings of ferromagnetic superconductor have attracted much attention not only for fundamental research to investigate how the antagonistic properties of ferromagnetism and superconductivity coexist peacefully but also for potential technological applications. Firstly, in order to help for understanding the ferromagnetic superconductor, I have explained the orbital and paramagnetic pair-breaking effects of magnetic field, which breaks the superconducting Cooper pairs. In addition to such effects of magnetic field, the singlet Cooper pairs become unstable upon going through the ferromagnetic materials by the proximity effect. The proximity effect occurs at the interface of thin films composing of superconductor and ferromagnet and leads to have very short penetration depth of Cooper pairs. However, a type of odd-frequency triplet in comparison with the singlet could be very stable and has a longer effective depth. It needs to be explored for the innovative spintronic devices. Finally, various ferromagnetic superconductors coexist and the lower-dimensional materials under the Quantum confinement effect have been introduced.

• 한국초전도학회:학술대회논문집
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• v.9
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• pp.222-222
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• 1999

From extensive measurements of the resistance and the dynamic resistance as functions of magnetic field and temperature, we find that the transport in the insulating state beyond the superconductor-insulator (S-I) transition is dominated by bosons(Cooper pairs and/or vortices) and cannot be described by the theory of the fermionic insulating phase. The maximum of the magnetoresistance at B = B$_m$ and the following negative slope in R(B) with increasing field can be explained by the crossover from the "Bose-glass" to the "Fermi-glass" phase as suggested by Paalanen, Hebard, and Ruel. The zero bias peak in dv/dl for biases below the characteristic voltage V$_c$ (or current $I_c$), gives a clue for the assumption of the "dirty boson" model which states that the insulating state above the critical magnetic field is the phase where Cooper pairs are localized due to the Coulomb blockade with a nonvanishing order parameter. The shift to a lower value of the critical magnetic field by overlaying thin Au layer, which is known as a strong spin-orbit scatterer, also supports the bosonic nature of the S-I transition.

• Progress in Superconductivity
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• v.4 no.1
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• pp.1-9
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• 2002

Transferring single flux quanta across a Josephson junction at an exactly determined rate has made highly precise voltage measurements possible. Making use of self-shunted Nb-based SINIS junctions, programmable fast-switching DC voltage standards with output voltages of up to 10 V were produced. This development is now extended from fundamental DC measurements to the precise determination of AC voltages with arbitrary waveforms. Integrated RSFQ circuits will help to replace expensive semiconductor devices for frequency control and signal coding. Easy-to-handle AC and inexpensive quantum voltmeters of fundamental accuracy would be of interest to industry. In analogy to the development in the flux regime, metallic nanocircuits comprising small-area tunnel junctions and providing the coherent transport of single electrons might play an important role in quantum current metrology. By precise counting of single charges these circuits allow prototypes of quantum standards for electric current and capacitance to be realised. Replacing single electron devices by single Cooper pair circuits, the charge transfer rates and thus the quantum currents could be significantly increased. Recently, the principles of the gate-controlled transfer of individual Cooper pairs in superconducting A1 devices in different electromagnetic environments were demonstrated. The characteristics of these quantum coherent circuits can be improved by replacing the small aluminum tunnel Junctions by niobium junctions. Due to the higher value of the superconducting energy gap ($\Delta_{Nb}$ ＝ $7\Delta_{Al}$), the characteristic energy and the frequency scales for Nb devices are substantially extended as compared to A1 devices. Although the fabrication of small Nb junctions presents a real challenge, the Nb-based metrological devices will be faster and more accurate in operation. Moreover, the Nb-based Cooper pair electrometer could be coupled to an Nb single Cooper pair qubit which can be beneficial for both, the stability of the qubit and its readout with a large signal-to-noise ratio..

• Proceedings of the Korean Vacuum Society Conference
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• 2016.02a
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• pp.125.1-125.1
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• 2016

The excitonic insulator (EI), which is one of fundamental insulators, was theoretically proposed in 1967 but its material realization has not been established well. Only a few materials were proposed as EIs but their experimental evidences were indirect such as the renormalization of band dispersions or an anomaly in electrical resistivity. We conducted scanning tunneling microscopy / spectroscopy measurements and found out that $Ta_2$ $NiSe_5$, which was the most recently proposed as an EI, had a metal-insulator phase transition with the energy gap of 700 meV at 78 K. Moreover, the spatially delocalized excitonic energy level was observed within the energy gap, which could be the direct evidence of the EI ground state. Our theoretical model calculation with the order parameter of 150 meV reproduces the spectral function and the excitonic energy gap very well. In addition, experimental data shows that the band character is inverted at the valence and conduction band edges by the exciton formation, indicating that the mechanism of exciton condensation is similar to the Bardeen-Cooper-Schrieffer (BCS) mechanism of cooper pairs in superconductors.

• ETRI Journal
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• v.16 no.2
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• pp.1-13
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• 1994

We have identified two new features related to the coherent transport in the mesoscopic loop structure of aluminum wire, including the autocorrelation of the conductance fluctuations beyond $B_c$ and fine structure in the low-field magnetoresistance curve in the superconducting transition regime, which, to the best of our knowledge, have not been reported in the literature. Since the electrons in Al have a phase coherence length larger than $1\;{\mu}m$ at or below T = 3K, which is comparable to the dimensions of the structure, the wave nature of the electronic transport has been clearly observed: the universal conductance fluctuations, the Aharonov-Bohm oscillations, and the Altshuler-Aronov-Spivak oscillations. Due to the transition of Al to a superconducting state at T = 1.3 K, the coherent phenomena of Cooper pairs, i.e., the Little-Parks oscillations, have also been observed.

• Progress in Superconductivity and Cryogenics
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• v.22 no.4
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• pp.6-9
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• 2020

Since the discovery of superconductivity, the unique and mysterious phenomenon has been observed in various metallic material systems. Now days, the superconductivity becomes ubiquitous because almost every metallic material system shows the superconductivity when it is cooled down enough. This ubiquity of the superconductivity is associated with the fermionic nature and itinerancy of electrons in metallic materials. Because fermions are governed by the Pauli's exclusion principle the total energy of fermions is much larger than that of bosons. Therefore, fermionic itinerant electrons are fundamentally instable. Itinerant electrons are able to find "a way" to lead them to their lowest possible energy state through an available bosonization (or pairing) process and Bose-Einstein condensation. Therefore, the lowest possible energy state of itinerant electrons will be a superconducting state, which is "their ultimate destination". This may explain the reason why the superconductivity is ubiquitous.