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

At interfaces between complex oxides, electronic systems with unusual electronic properties can be generated. We report on superconductivity in the electron gas formed at the interface between two insulating dielectric perovskite oxides, LaAlO3 and SrTiO3. The behavior of the electron gas is that of a two-dimensional superconductor, confined to a thin sheet at the interface. The superconducting transition temperature of ≅ 200 millikelvin provides a strict upper limit to the thickness of the superconducting layer of ≅ 10 nanometers.

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Superconducting Interfaces Between Insulating Oxides

The semiconductor industry has been able to improve the performance of electronic systems for more than four decades by making ever-smaller devices. However, this approach will soon encounter both scientific and technical limits, which is why the industry is exploring a number of alternative device technologies. Here we review the progress that has been made with carbon nanotubes and, more recently, graphene layers and nanoribbons. Field-effect transistors based on semiconductor nanotubes and graphene nanoribbons have already been demonstrated, and metallic nanotubes could be used as high-performance interconnects. Moreover, owing to the excellent optical properties of nanotubes it could be possible to make both electronic and optoelectronic devices from the same material.

Carbon-based electronics.

日本物理学会誌及びJournal of the Physical Society of Japanの月刊について

Quantum-state engineering, i.e., active control over the coherent dynamics of suitable quantum-mechanical systems, has become a fascinating prospect of modern physics. With concepts developed in atomic and molecular physics and in the context of NMR, the field has been stimulated further by the perspectives of quantum computation and communication. Low-capacitance Josephson tunneling junctions offer a promising way to realize quantum bits (qubits) for quantum information processing. The article reviews the properties of these devices and the practical and fundamental obstacles to their use. Two kinds of device have been proposed, based on either charge or phase (flux) degrees of freedom. Single- and two-qubit quantum manipulations can be controlled by gate voltages in one case and by magnetic fields in the other case. Both kinds of device can be fabricated with present technology. In flux qubit devices, an important milestone, the observation of superpositions of different flux states in the system eigenstates, has been achieved. The Josephson charge qubit has even demonstrated coherent superpositions of states readable in the time domain. There are two major problems that must be solved before these devices can be used for quantum information processing. One must have a long phase coherence time, which requires that external sources of dephasing be minimized. The review discusses relevant parameters and provides estimates of the decoherence time. Another problem is in the readout of the final state of the system. This issue is illustrated with a possible realization by a single-electron transistor capacitively coupled to the Josephson device, but general properties of measuring devices are also discussed. Finally, the review describes how the basic physical manipulations on an ideal device can be combined to perform useful operations.

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Quantum-state engineering with Josephson-junction devices

Grain boundary junctions in high-T/sub c/ thin films generally consist of facets with typical dimensions below 100 nm. In combination with a d(x/sup 2/-y/sup 2/) symmetry component of the order parameter, this faceting gives rise to an inhomogeneous critical current density J/sub c/ along the grain boundary. The inhomogeneity is most prominent for asymmetric 45/spl deg/ [001] tilt grain boundaries. For a large fraction of the facets in these boundaries the order parameter orientation gives rise to an additional /spl pi/ phase difference, and therefore to a 'negative' critical current density. This leads to highly anomalous magnetic field dependences of the critical current. Direct imaging with scanning SQUID microscopy provides evidence that magnetic flux is generated spontaneously in these boundaries. These observations have several significant implications, both for understanding the properties of grain boundaries in high-T/sub c/ superconductors and for their applications.

Influence of d(x/sup 2/-y/sup 2/) symmetry on device applications of high-T/sub c/ grain boundary junctions

wave superconductors, which form a contact at a tilt angle. The contact is a weak link or a Josephson junction, with properties determined by the misalignment angle. Complementing existing theories that discuss the influence of dx2-y2 symmetry on the charge transfer across junctions, the ideas presented provide insights into the origin of the junction formed in such configurations. The mechanism discussed is of relevance for the understanding of the electronic properties of grain boundaries in superconductors, including the angular dependence of the critical current density.

Intrinsic weak link originating from tilt in contacts between dx2-y2 wave superconductors

Inspired by the work of Ohtomo and Hwang in 2004, we shed new light on thin films of layered cuprate high-Tc superconductors (HTS). In principle all HTS materials consist of charged perovskite-like layers which in thin films can lead to polar discontinuities at the interfaces of different materials. The resulting charge redistribution has to occur but we expect it to be far more complex than in the LaAlO3/SrTiO3 system since copper can be multivalent. This makes it hard to predict what will happen in terms of transport or even magnetic properties compared to the 'simple' insulator LaAlO3. Nevertheless, we point out that the picture of systems of charged layers is important and necessary to fully understand heterostructures of these complex materials.

https://iopscience.iop.org/article/10.1088/0953-8984/20/26/264007/pdf

High-Tc superconducting thin films with composition control on a sub-unit cell level; the effect of the polar nature of the cuprates

The capacitance of $24\ifmmode^\circ\else\textdegree\fi{}[001]$ tilt calcium doped ${\mathrm{Y}}_{1\ensuremath{-}x}{\mathrm{Ca}}_{x}{\mathrm{Ba}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\ensuremath{-}\ensuremath{\delta}}$ grain boundaries has been measured for thin films with $x$ in the range $0.0--0.3$. The capacitance was determined from the hysteresis in the $I\text{\ensuremath{-}}V$ characteristic. By measuring the capacitance as a function of the voltage across the junctions it was possible to observe the contribution of both parasitic substrate capacitance and heating to the hysteresis. These effects enable the determination of the intrinsic capacitance of the grain boundaries. The effect of thermal noise on the measurement is also assessed, and found to be much less than the observed changes in the capacitance. The capacitance is found to increase as the calcium doping increases: from $0.2\phantom{\rule{0.3em}{0ex}}{\mathrm{Fm}}^{\ensuremath{-}2}$ for $x=0.0$ to a maximum of $1.2\phantom{\rule{0.3em}{0ex}}{\mathrm{Fm}}^{\ensuremath{-}2}$ for $x=0.3$. The changes in the capacitance per unit area are observed to be inversely proportional to the corresponding changes in the resistance area product.

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Capacitance measurements on grain boundaries in Y1-xCaxBa2Cu3O7-δ

The critical-current density of grain boundaries in high-T/sub c/ superconductors was enhanced to values exceeding the previously known limits both at 4.2 K and at 77 K. Noting the importance of space-charge layers and of the d(x/sup 2/-y/sup 2/)-wave pairing symmetry on grain-boundary transport, we have established a model that provides a comprehensive description of the grain boundaries and proposes ways for their improvement, such as overdoping of the grains and of their boundaries. Exploring as example the effects of overdoping of YBa/sub 2/Cu/sub 3/O/sub 7-/spl delta// with Ca, we enhanced significantly the critical current densities and decreased the normal-state resistivities of grain boundaries to unprecedented values. By introducing doping heterostructures to overdope grain boundaries selectively over a few nanometers by benefiting from grain boundary diffusion, the enhancement of the critical-current density is achieved at all temperatures up to T/sub c/. At 77 K, critical current densities are obtained which before had been found only at 4.2 K. This concept is proposed as a practical and cost-effective route to enhance the performance of high-T/sub c/ coated conductors fabricated by ion beam assisted deposition (IBAD) or by the rolling assisted biaxially aligned substrate process (RABITS).

Hans Hilgenkamp

Papers

Influence of d(x/sup 2/-y/sup 2/) symmetry on device applications of high-T/sub c/ grain boundary junctions

Intrinsic weak link originating from tilt in contacts between dx2-y2 wave superconductors

High-Tc superconducting thin films with composition control on a sub-unit cell level; the effect of the polar nature of the cuprates

Capacitance measurements on grain boundaries in Y1-xCaxBa2Cu3O7-δ

Doping-induced enhancement of grain boundary critical currents