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

Morphology and magnetic properties of submonolayer Gd films

We have performed conductance measurements with a gold–molecule–gold junction employing the mechanically controlled break-junction technique. The organic sample molecules form a stable covalent bridge between the electrodes. Two molecules, which differ essentially by their spatial symmetry, showed discrete stable conductance patterns (IVs), which reflect the symmetry/asymmetry of the sample molecules. A third molecule with an additional tunnel barrier in the central axis shows a strongly suppressed conductance. The correlation between these obvious properties of the molecules and the observed IVs unambigously shows that the junction is indeed formed of the sample molecules. Vice versa, these observations demonstrate that IV characteristics can be appropriately designed by a proper choice of molecules.

Conductance properties of single-molecule junctions

Magnetization of CeCu6 at low temperatures

The fabrication of metallic single-walled carbon nanotube electrodes separated by gaps of typically 20 nm width by electron-beam-induced oxidation is studied within an active device configuration. The tube conductance is measured continuously during the process. The experiment provides a statistical evaluation of gap sizes as well as the electron dose needed for gap formation. Also, the ability to precisely cut many carbon nanotubes in parallel is demonstrated. To show the practicality of the approach, devices of different scales were fabricated using sorted carbon nanotubes and dielectrophoretic deposition.

Controlled fabrication of single-walled carbon nanotube electrodes by electron-beam-induced oxidation

We report high-resolution measurements of the specific heat of the hexagonal antiferromagnet CsCuCl3 with ferromagnetically coupled Cu chains in magnetic fields parallel to the hexagonal axis. The zero-field data can be described assuming a second-order phase transition with a chiral XY critical exponent a near 0.35, except very close to the Neel temperature where the transition seems to be of weakly first order. This might be an indication for the failure of XY chiral universality. The previously observed first-order transition from an ‘‘umbrella’’ to a coplanar spin structure at high fields merges the transition line to the paramagnet in a multicritical point (Tc510.59 K, Bm55.5 T!. The transition to the new coplanar structure from the saturated paramagnetic phase above Bm seems, rather unexpectedly, also to be describable as a second-order transition with a'0.23. @S0163-1829~96!04246-4#

Hilbert von Löhneysen

Papers

Morphology and magnetic properties of submonolayer Gd films

Conductance properties of single-molecule junctions

Magnetization of CeCu6 at low temperatures

Controlled fabrication of single-walled carbon nanotube electrodes by electron-beam-induced oxidation

Magnetic phases of CsCuCl3: Anomalous critical behavior.