There is, I think, something ethereal about i —the square root of minus one. I remember first hearing about it at school. It seemed an odd beast at that time—an intruder hovering on the edge of reality.

Usually familiarity dulls this sense of the bizarre, but in the case of i it was the reverse: over the years the sense of its surreal nature intensified. It seemed that it was impossible to write mathematics that described the real world in …

I and i

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

The behaviour of strongly correlated materials, and in particular unconventional superconductors, has been studied extensively for decades, but is still not well understood. This lack of theoretical understanding has motivated the development of experimental techniques for studying such behaviour, such as using ultracold atom lattices to simulate quantum materials. Here we report the realization of intrinsic unconventional superconductivity-which cannot be explained by weak electron-phonon interactions-in a two-dimensional superlattice created by stacking two sheets of graphene that are twisted relative to each other by a small angle. For twist angles of about 1.1°-the first 'magic' angle-the electronic band structure of this 'twisted bilayer graphene' exhibits flat bands near zero Fermi energy, resulting in correlated insulating states at half-filling. Upon electrostatic doping of the material away from these correlated insulating states, we observe tunable zero-resistance states with a critical temperature of up to 1.7 kelvin. The temperature-carrier-density phase diagram of twisted bilayer graphene is similar to that of copper oxides (or cuprates), and includes dome-shaped regions that correspond to superconductivity. Moreover, quantum oscillations in the longitudinal resistance of the material indicate the presence of small Fermi surfaces near the correlated insulating states, in analogy with underdoped cuprates. The relatively high superconducting critical temperature of twisted bilayer graphene, given such a small Fermi surface (which corresponds to a carrier density of about 1011 per square centimetre), puts it among the superconductors with the strongest pairing strength between electrons. Twisted bilayer graphene is a precisely tunable, purely carbon-based, two-dimensional superconductor. It is therefore an ideal material for investigations of strongly correlated phenomena, which could lead to insights into the physics of high-critical-temperature superconductors and quantum spin liquids.

Unconventional superconductivity in magic-angle graphene superlattices

This review summarizes recent developments in the theory of spin glasses, as well as pertinent experimental data. The most characteristic properties of spin glass systems are described, and related phenomena in other glassy systems (dielectric and orientational glasses) are mentioned. The Edwards-Anderson model of spin glasses and its treatment within the replica method and mean-field theory are outlined, and concepts such as "frustration," "broken replica symmetry," "broken ergodicity," etc., are discussed. The dynamic approach to describing the spin glass transition is emphasized. Monte Carlo simulations of spin glasses and the insight gained by them are described. Other topics discussed include site-disorder models, phenomenological theories for the frozen phase and its excitations, phase diagrams in which spin glass order and ferromagnetism or antiferromagnetism compete, the Ne\'el model of superparamagnetism and related approaches, and possible connections between spin glasses and other topics in the theory of disordered condensed-matter systems.

Spin glasses: Experimental facts, theoretical concepts, and open questions

1. Introduction 2. Theoretical foundations 3. Propagation and focusing of optical fields 4. Spatial resolution and position accuracy 5. Nanoscale optical microscopy 6. Near-field optical probes 7. Probe-sample distance control 8. Light emission and optical interaction in nanoscale environments 9. Quantum emitters 10. Dipole emission near planar interfaces 11. Photonic crystals and resonators 12. Surface plasmons 13. Forces in confined fields 14. Fluctuation-induced phenomena 15. Theoretical methods in nano-optics Appendices Index.

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Principles of nano-optics

Break-junction experiments have been carried out on the Kondo insulator (Ce1−xLax)3Bi4Pt3. At T=1.5 K and for x=0,  dV/dI versus V curves for point contacts with R0⩽12 k Ω exhibit semiconducting behavior, i.e. decreasing dV/dI with increasing V. The finite dV/dI at V=0 results from a finite density of states inside the band gap. For the La-doped compounds the semiconducting behavior is less pronounced and a step-like gap structure occurs at V≈±Eg/e. With increasing La concentration the step is shifted to lower V in qualitative agreement with resistivity data. The results suggest that La doping has two effects: reduction of the gap and introduction of impurity states in the band gap of Ce3Bi4Pt3.

Point-contact spectroscopy on the La-doped Kondo insulator Ce3Bi4Pt3

The (B, T, p) phase diagram for the unconventional superconductor UPt3 is determined from measurements of the specific heat and the magnetocaloric effect. In zero field above the critical pressure p ∗ ≈3.5 kbar where the two transitions have merged, no re-splitting of the superconducting transition is observed. However, in a magnetic field ( B ¶ c ) there are two superconducting phases: the low-field B phase and the high-field C phase. The results strongly support the concept of a symmetry-breaking field (SBF) due to the weak antiferromagnetism which in zero field and below p ∗ lifts the degeneracy of the two-dimensional order parameter and vanishes at the same critical pressure p ∗ where the two transitions have merged. Above p ∗ the degeneracy of the order parameter is apparently lifted by the magnetic field.

The behavior of the heavy-fermion superconductor UPt3 under hydrostatic pressure and in a magnetic field

Fermi surfaces of the half-Heusler compounds Ce1-xLaxBiPt

The transition temperature and the critical fields of e - -beam evaporated Nb/Gd/Nb trilayers and Nb/Gd multilayers have been determined resistively and by dc-susceptibility. For constant thickness of the Gd layers we observe with decreasing thickness d Nb of Nb layers a decrease of T c and H c 2⊥ down to a critical thickness d c , below which superconductivity is completely destroyed. This is due to pair breaking by the exchange field within the ferromagnetic Gd layers. The parallel critical fields mostly show the square-root like temperature dependence near T c typical for 2D-superconductors. The interplay of orbital pair breaking by the external field and the exchange-field induced pair breaking leads to a nonmonotonic dependence of H c | on d Nb as predicted by recent theories.

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Transition temperature and critical fields of Nb/Gd layers

The (111)-2 × 1 surface of in situ cleaved heavily P- or B-doped Si is investigated by scanning tunnelling microscopy and spectroscopy at room temperature and at low temperature. P atoms have been identified on different sites of the Si(111)-2 × 1 surface by their characteristic voltage-dependent contrast for positive as well as negative buckling of the π-bonded chains. The distributions of dopants per surface area and of nearest-neighbour distances are found to be in agreement with a random arrangement of dopants in Si up to doping levels well above the metal–insulator transition. In addition, P atoms have been identified by their depth-dependent contrast down to the third layer beneath the surface with a volume density in agreement with the bulk doping density. The random electronic disorder supports the view of an Anderson transition driven by disorder close to the critical concentration or critical uniaxial stress.

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Hilbert von Löhneysen

Papers

Point-contact spectroscopy on the La-doped Kondo insulator Ce3Bi4Pt3

The behavior of the heavy-fermion superconductor UPt3 under hydrostatic pressure and in a magnetic field

Fermi surfaces of the half-Heusler compounds Ce1-xLaxBiPt

Transition temperature and critical fields of Nb/Gd layers

Electronic disorder of P- and B-doped Si at the metal–insulator transition investigated by scanning tunnelling microscopy and electronic transport