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

We propose an explanation for the recently observed non-Fermi-liquid behavior of metallic alloys ${\mathrm{CeCu}}_{6\ensuremath{-}x}{\mathrm{Au}}_{x}$: Near $x\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}0.1$, the specific heat $C$ is proportional to $T\mathrm{ln}({T}_{0}/T)$, and the resistivity increases linearly with temperature $T$ over a wide range of $T$. These features follow from a model in which three-dimensional conduction electrons are coupled to two-dimensional critical ferromagnetic fluctuations near the quantum critical point ${x}_{c}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}0.1$. This picture is motivated by the neutron scattering data in the ordered phase ( $x\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}0.2$) and is consistent with the observed phase diagram.

Mechanism for the Non-Fermi-Liquid Behavior in CeCu 6-x Au x

We report on nonlocal transport in multiterminal superconductor-ferromagnet structures, which were fabricated by means of $e$-beam lithography and shadow evaporation techniques. In the presence of a significant Zeeman splitting of the quasiparticle states, we find signatures of spin transport over distances of several $\ensuremath{\mu}\mathrm{m}$, exceeding other length scales such as the coherence length, the normal-state spin-diffusion length, and the charge-imbalance length. The relaxation length of the spin signal shows a nearly linear increase with magnetic field, hinting at a freeze-out of relaxation by the Zeeman splitting. We propose that the relaxation length is given by the recombination length of the quasiparticles rather than a renormalized spin-diffusion length.

Long-Range Spin-Polarized Quasiparticle Transport in Mesoscopic Al Superconductors with a Zeeman Splitting

The transition temperature and the critical fields of electron-beam evaporated Nb/Gd/Nb triple layers and Nb/Gd multilayers have been determined by measurements of the electrical resistivity and the dc susceptibility. For constant thickness d(Gd) of the Gd layers, we observe a decrease of T(c) and H(c2 perpendicular-to) with decreasing thickness d(Nb) of Nb layers down to a critical thickness d(c), below which superconductivity is completely destroyed. The parallel critical fields mostly show the square-root temperature dependence near T(c), typical for two-dimensional superconductors. As predicted theoretically, competing pair-breaking mechanisms lead to a nonmonotonic dependence of H(c parallel-to) on d(Nb). We have also studied the dependence T(c)(d(Gd)) with constant d(Nb) and find a decrease of the T(c)(d(Gd)) curve with increasing d(Cd) and a steplike structure at d(Gd) almost-equal-to 20 angstrom. To clarify the nature of this step, the ferromagnetic transition of the Gd films is determined with the transverse magneto-optical Kerr effect. Long-range magnetic order is found only above d(Gd) almost-equal-to 20 angstrom, which is attributed to the formation of a discontinuous film below this thickness. These results indicate a change in the underlying pair-breaking mechanism.

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Superconductivity in layered Nb/Gd films.

The electrical conductivity \ensuremath{\sigma} (extrapolated to T=0) of uncompensated Si:P indicates a crossover as a function of P concentration N at ${\mathit{N}}_{\mathrm{cr}}$ slightly above the metal-insulator transition at ${\mathit{N}}_{\mathit{c}}$. For Ng${\mathit{N}}_{\mathrm{cr}}$ the exponent of \ensuremath{\sigma}\ensuremath{\sim}(N-${\mathit{N}}_{\mathit{c}}$${)}^{\mathrm{\ensuremath{\mu}}}$ is \ensuremath{\mu}\ensuremath{\approxeq}0.64, while \ensuremath{\mu}\ensuremath{\approxeq}1.3 for ${\mathit{N}}_{\mathit{c}}$N${\mathit{N}}_{\mathrm{cr}}$. At ${\mathit{N}}_{\mathrm{cr}}$ d\ensuremath{\sigma}/dT changes sign from negative for Ng${\mathit{N}}_{\mathrm{cr}}$ to positive for N${\mathit{N}}_{\mathrm{cr}}$. $sigma--- in a magnetic field also yields \ensuremath{\mu}\ensuremath{\approxeq}1. The apparent discrepancy between uncompensated and compensated semiconductors is traced back to a difference in the (nonuniversal) width of the critical region.

Possible solution of the conductivity exponent puzzle for the metal-insulator transition in heavily doped uncompensated semiconductors.

The half-Heusler compounds CeBiPt and LaBiPt are semimetals with very low charge-carrier concentrations as evidenced by Shubnikov–de Haas (SdH) and Hall-effect measurements. Neutron-scattering results reveal a simple antiferromagnetic structure in CeBiPt below TN = 1.15 K. The band structure of CeBiPt sensitively depends on temperature, magnetic field and stoichiometry. Above a certain, sample-dependent, threshold field (B>25 T), the SdH signal disappears and the Hall coefficient reduces significantly. These effects are absent in the non-4f compound LaBiPt. Electronic-band-structure calculations can well explain the observed behaviour by a 4f-polarization-induced Fermi-surface modification.

https://iopscience.iop.org/article/10.1088/1367-2630/8/9/174/pdf

Hilbert von Löhneysen

Papers

Mechanism for the Non-Fermi-Liquid Behavior in CeCu 6-x Au x

Long-Range Spin-Polarized Quasiparticle Transport in Mesoscopic Al Superconductors with a Zeeman Splitting

Superconductivity in layered Nb/Gd films.

Possible solution of the conductivity exponent puzzle for the metal-insulator transition in heavily doped uncompensated semiconductors.

Magnetic-field- and temperature-dependent Fermi surface of CeBiPt