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

Evidence for one-dimensional electron propagation on Si (111) - (2×1) from Coulomb blockade

A detailed study of the low-temperature magnetization M and specific heat C of CePd2In is reported. For B ≈ 0, M and C show maxima at TN≈ 1.25 K indicating an antiferromagnetic phase transition as observed previously. In a magnetic field B the transition seen in M is shifted to lower T while TN as determined from the specific-heat maximum is essentially unaltered up to fields of 6 T. This different behavior is attributed to magnetic anisotropy of the polycrystalline samples. The hyperfine contribution to C is discussed in terms of nuclear quadrupole splitting as well as Zeeman splitting by applied and transferred hyperfine fields. A rough estimate of the Kondo temperature from C sufficiently above TN yields TK ≈ 1.5 K.

Low-temperature magnetic and thermal properties of CePd 2 In in magnetic fields

Anisotropic lattice changes and ferromagnetic order in Sr 1 − x Ca x RuO 3

X-ray diffraction multipole refinements of single-crystalline MgxAl1−xB2 and polarization-dependent near-edge x-ray absorption fine structure at the B 1s edge reveal a strongly anisotropic electronic structure. Comparing the data for superconducting compounds (x = 0.8, 1.0) with those for the non-superconductor (x = 0) gives direct evidence for a rearrangement of the hybridizations of the boron pz bonds and underline the importance of holes in the σ-bonded covalent sp2 states for the superconducting properties of the diborides. The data indicate that Mg is approximately divalent in MgB2 and suggest predominantly ionic bonds between the Mg ions and the two-dimensional covalent B rings. For AlB2 (x = 0), on the other hand, about 1.5 electrons per Al atom are transferred to the B sheets while the residual 1.5 electrons remain at the Al site which suggests significant covalent bonding between the Al ions and the B sheets. This finding together with the static electron deformation density points to almost equi...

Electronic Structure of Single-Crystalline MgxAl1−xB2 Probed by X-ray Diffraction Multipole Refinements and Polarization-Dependent X-ray Absorption Spectroscopy

Point-contact spectra of polycrystalline YBa 2 (Cu 1− x Zn x ) 3 O 7 for x =0.05 and 0.075 were studied at various temperatures. Two kinds of point-contact spectra, i.e. differential resistance d V /d I versus voltage V , for the same T c in the point-contact region were found. One type exhibits the well-known features attributed to the energy gap while the features of the other indicate gapless superconductivity. These findings are interpreted in terms of gap ansisotropy with a vanishing gap parallel to the c direction.

Hilbert von Löhneysen

Papers

Evidence for one-dimensional electron propagation on Si (111) - (2×1) from Coulomb blockade

Low-temperature magnetic and thermal properties of CePd 2 In in magnetic fields

Anisotropic lattice changes and ferromagnetic order in Sr 1 − x Ca x RuO 3

Electronic Structure of Single-Crystalline MgxAl1−xB2 Probed by X-ray Diffraction Multipole Refinements and Polarization-Dependent X-ray Absorption Spectroscopy

Gap anisotropy and gapless superconductivity in Zn-doped YBa2Cu3O7-δ