다중혈관 관상동맥 환자에서 y-문합을 이용하여 양쪽 내흉동맥만을 사용한 우회술의 조기 성적

The generalization of the Local Density Approximation (LDA) method for the systems with strong Coulomb correlations is presented which gives a correct description of the Mott insulators. The LDA+U method is based on the model hamiltonian approach and allows to take into account the non-sphericity of the Coulomb and exchange interactions. parameters. Orbital-dependent LDA+U potential gives correct orbital polarization and corresponding Jahn-Teller distortion. To calculate the spectra of the strongly correlated systems the impurity Anderson model should be solved with a many-electron trial wave function. All parameters of the many-electron hamiltonian are taken from LDA+U calculations. The method was applied to NiO and has shown good agreement with experimental photoemission spectra and with the oxygen Kα X-ray emission spectrum.

http://absimage.aps.org/image/MAR06/MWS_MAR06-2005-007233.pdf

First-principles calculations of electronic structure and spectra of strongly correlated systems: the LDA+U method

Since the discovery of high Tc iron-based superconductors in early 2008, more than 15,000 papers have been published as a result of intensive research. This paper describes the current status of iron-based superconductors (IBSC) covering most up-to-date research progress along with the some background research, focusing on materials (bulk and thin film) and pairing mechanism.

Iron-based superconductors: Current status of materials and pairing mechanism

Uneven Gap Electron pairs that are responsible for the phenomenon of superconductivity can only be broken by investing a finite amount of energy, called the energy gap. The size of the gap may depend on the position on the Fermi surface; in cuprates, the gap completely disappears at certain points. What happens in the pnictide superconductors is still a subject of debate, not least because there appear to be differences between the different pnictide families. Allan et al. (p. 563) used scanning tunneling spectroscopy to study the compound LiFeAs. The gap was mapped on three of the five bands on which the Fermi surface resides and was found to be anisotropic in momentum space. The energy needed to break up electron pairs in a pnictide superconductor depends on position on the Fermi surface. If strong electron-electron interactions between neighboring Fe atoms mediate the Cooper pairing in iron-pnictide superconductors, then specific and distinct anisotropic superconducting energy gaps Δi(k⃗) should appear on the different electronic bands i. Here, we introduce intraband Bogoliubov quasiparticle scattering interference (QPI) techniques for determination of Δik⃗. in such materials, focusing on lithium iron arsenide (LiFeAs). We identify the three hole-like bands assigned previously as γ, α2, and α1, and we determine the anisotropy, magnitude, and relative orientations of their Δik⃗. These measurements will advance quantitative theoretical analysis of the mechanism of Cooper pairing in iron-based superconductivity.

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Anisotropic Energy-Gaps of Iron-based Superconductivity from Intra-band Quasiparticle Interference in LiFeAs, Part I

The superconducting analogue to the semiconducting diode, the Josephson diode, has long been sought with multiple avenues to realization being proposed by theorists1-3. Showing magnetic-field-free, single-directional superconductivity with Josephson coupling, it would serve as the building block for next-generation superconducting circuit technology. Here we realized the Josephson diode by fabricating an inversion symmetry breaking van der Waals heterostructure of NbSe2/Nb3Br8/NbSe2. We demonstrate that even without a magnetic field, the junction can be superconducting with a positive current while being resistive with a negative current. The ΔIc behaviour (the difference between positive and negative critical currents) with magnetic field is symmetric and Josephson coupling is proved through the Fraunhofer pattern. Also, stable half-wave rectification of a square-wave excitation was achieved with a very low switching current density, high rectification ratio and high robustness. This non-reciprocal behaviour strongly violates the known Josephson relations and opens the door to discover new mechanisms and physical phenomena through integration of quantum materials with Josephson junctions, and provides new avenues for superconducting quantum devices. 

/pdf/the-field-free-josephson-diode-in-a-van-der-waals-2cxf6hkt.pdf

The field-free Josephson diode in a van der Waals heterostructure

Nonlinear optical and electrical effects associated with a lack of spatial inversion symmetry allow direction-selective propagation and transport of quantum particles, such as photons1 and electrons2-9. The most common example of such nonreciprocal phenomena is a semiconductor diode with a p-n junction, with a low resistance in one direction and a high resistance in the other. Although the diode effect forms the basis of numerous electronic components, such as rectifiers, alternating-direct-current converters and photodetectors, it introduces an inevitable energy loss due to the finite resistance. Therefore, a worthwhile goal is to realize a superconducting diode that has zero resistance in only one direction. Here we demonstrate a magnetically controllable superconducting diode in an artificial superlattice [Nb/V/Ta]n without a centre of inversion. The nonreciprocal resistance versus current curve at the superconducting-to-normal transition was clearly observed by a direct-current measurement, and the difference of the critical current is considered to be related to the magnetochiral anisotropy caused by breaking of the spatial-inversion and time-reversal symmetries10-13. Owing to the nonreciprocal critical current, the [Nb/V/Ta]n superlattice exhibits zero resistance in only one direction. This superconducting diode effect enables phase-coherent and direction-selective charge transport, paving the way for the construction of non-dissipative electronic circuits.

Observation of superconducting diode effect.

We theoretically study superconductivity in ${\mathrm{UTe}}_{2}$, which is a recently discovered strong candidate for an odd-parity spin-triplet superconductor. Theoretical studies for this compound faced difficulty because first-principles calculations predict an insulating electronic state, incompatible with superconducting instability. To overcome this problem, we take into account electron correlation effects by a $\mathrm{GGA}+U$ method and show the insulator-metal transition by Coulomb interaction. Using Fermi surfaces obtained as a function of $U$, we clarify topological properties of possible superconducting states. Fermi surface formulas for the three-dimensional winding number and three two-dimensional ${\mathbb{Z}}_{2}$ numbers indicate topological superconductivity at an intermediate $U$ for all the odd-parity pairing symmetry in the $Immm$ space group. Symmetry and topology of superconducting gap nodes are analyzed and the gap structure of ${\mathrm{UTe}}_{2}$ is predicted. Topologically protected low-energy excitations are highlighted, and experiments by bulk and surface probes are proposed to link Fermi surfaces and pairing symmetry. Based on the results, we also discuss multiple superconducting phases under magnetic fields, which were implied by recent experiments.

Insulator-Metal Transition and Topological Superconductivity in UTe_{2} from a First-Principles Calculation.

We provide and analyze a periodic Anderson model for studying magnetism and superconductivity in $\mathrm{U}{\mathrm{Te}}_{2}$, a recently discovered candidate for a topological spin-triplet superconductor. The 24-band tight-binding model reproduces the band structure obtained from a $\mathrm{D}\mathrm{F}\mathrm{T}+U$ calculation consistent with an angle-resolved photoemission spectroscopy. The Coulomb interaction of $f$-electrons enhances Ising ferromagnetic fluctuation along the $a$-axis and stabilizes spin-triplet superconductivity of either ${B}_{3u}$ or ${A}_{u}$ symmetry. When effects of pressure are taken into account in hopping integrals, the magnetic fluctuation changes to an antiferromagnetic one, and accordingly spin-singlet superconductivity of ${A}_{g}$ symmetry is stabilized. Based on the results, we propose pressure-temperature and magnetic field-temperature phase diagrams revealing multiple superconducting phases as well as an antiferromagnetic phase. In particular, a mixed-parity superconducting state with spontaneous inversion symmetry breaking is predicted.

Periodic Anderson model for magnetism and superconductivity in UTe 2

The diode effect is fundamental to electronic devices and is widely used in rectifiers and a.c.–d.c. converters. At low temperatures, however, conventional semiconductor diodes possess a high resistivity, which yields energy loss and heating during operation. The superconducting diode effect (SDE)1–8, which relies on broken inversion symmetry in a superconductor, may mitigate this obstacle: in one direction, a zero-resistance supercurrent can flow through the diode, but for the opposite direction of current flow, the device enters the normal state with ohmic resistance. The application of a magnetic field can induce SDE in Nb/V/Ta superlattices with a polar structure1,2, in superconducting devices with asymmetric patterning of pinning centres9 or in superconductor/ferromagnet hybrid devices with induced vortices10,11. The need for an external magnetic field limits their practical application. Recently, a field-free SDE was observed in a NbSe2/Nb3Br8/NbSe2 junction; it originates from asymmetric Josephson tunnelling that is induced by the Nb3Br8 barrier and the associated NbSe2/Nb3Br8 interfaces12. Here, we present another implementation of zero-field SDE using noncentrosymmetric [Nb/V/Co/V/Ta]20 multilayers. The magnetic layers provide the necessary symmetry breaking, and we can tune the SDE by adjusting the structural parameters, such as the constituent elements, film thickness, stacking order and number of repetitions. We control the polarity of the SDE through the magnetization direction of the ferromagnetic layers. Artificially stacked structures13–18, such as the one used in this work, are of particular interest as they are compatible with microfabrication techniques and can be integrated with devices such as Josephson junctions19–22. Energy-loss-free SDEs as presented in this work may therefore enable novel non-volatile memories and logic circuits with ultralow power consumption. Superconducting diodes, which can operate without dissipation losses at low temperature, usually require a magnetic field to function. A well-designed multilayer device now shows a reversible, non-volatile superconducting diode effect. 

/pdf/field-free-superconducting-diode-effect-in-crj3r06i.pdf

Field-free superconducting diode effect in noncentrosymmetric superconductor/ferromagnet multilayers

We report the magnetic susceptibility and the magnetization under pressures up to 1.7 GPa above the critical pressure, Pc ∼ 1.5 GPa, for H || a, b, and c-axes in the novel spin triplet superconduct...

Jun Ishizuka

Papers

Observation of superconducting diode effect.

Insulator-Metal Transition and Topological Superconductivity in UTe_{2} from a First-Principles Calculation.

Periodic Anderson model for magnetism and superconductivity in UTe 2

Field-free superconducting diode effect in noncentrosymmetric superconductor/ferromagnet multilayers

Magnetic Properties under Pressure in Novel Spin-Triplet Superconductor UTe2