Current research into semiconductor clusters is focused on the properties of quantum dots-fragments of semiconductor consisting of hundreds to many thousands of atoms-with the bulk bonding geometry and with surface states eliminated by enclosure in a material that has a larger band gap. Quantum dots exhibit strongly size-dependent optical and electrical properties. The ability to join the dots into complex assemblies creates many opportunities for scientific discovery.

Semiconductor Clusters, Nanocrystals, and Quantum Dots

Fabrication techniques developed for graphene research allow the disassembly of many layered crystals (so-called van der Waals materials) into individual atomic planes and their reassembly into designer heterostructures, which reveal new properties and phenomena. Andre Geim and Irina Grigorieva offer a forward-looking review of the potential of layering two-dimensional materials into novel heterostructures held together by weak van der Waals interactions. Dozens of these one-atom- or one-molecule-thick crystals are known. Graphene has already been well studied but others, such as monolayers of hexagonal boron nitride, MoS2, WSe2, graphane, fluorographene, mica and silicene are attracting increasing interest. There are many other monolayers yet to be examined of course, and the possibility of combining graphene with other crystals adds even further options, offering exciting new opportunities for scientific exploration and technological innovation. Research on graphene and other two-dimensional atomic crystals is intense and is likely to remain one of the leading topics in condensed matter physics and materials science for many years. Looking beyond this field, isolated atomic planes can also be reassembled into designer heterostructures made layer by layer in a precisely chosen sequence. The first, already remarkably complex, such heterostructures (often referred to as ‘van der Waals’) have recently been fabricated and investigated, revealing unusual properties and new phenomena. Here we review this emerging research area and identify possible future directions. With steady improvement in fabrication techniques and using graphene’s springboard, van der Waals heterostructures should develop into a large field of their own.

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Van der Waals heterostructures

Topological quantum computation has emerged as one of the most exciting approaches to constructing a fault-tolerant quantum computer. The proposal relies on the existence of topological states of matter whose quasiparticle excitations are neither bosons nor fermions, but are particles known as non-Abelian anyons, meaning that they obey non-Abelian braiding statistics. Quantum information is stored in states with multiple quasiparticles, which have a topological degeneracy. The unitary gate operations that are necessary for quantum computation are carried out by braiding quasiparticles and then measuring the multiquasiparticle states. The fault tolerance of a topological quantum computer arises from the nonlocal encoding of the quasiparticle states, which makes them immune to errors caused by local perturbations. To date, the only such topological states thought to have been found in nature are fractional quantum Hall states, most prominently the $\ensuremath{\nu}=5∕2$ state, although several other prospective candidates have been proposed in systems as disparate as ultracold atoms in optical lattices and thin-film superconductors. In this review article, current research in this field is described, focusing on the general theoretical concepts of non-Abelian statistics as it relates to topological quantum computation, on understanding non-Abelian quantum Hall states, on proposed experiments to detect non-Abelian anyons, and on proposed architectures for a topological quantum computer. Both the mathematical underpinnings of topological quantum computation and the physics of the subject are addressed, using the $\ensuremath{\nu}=5∕2$ fractional quantum Hall state as the archetype of a non-Abelian topological state enabling fault-tolerant quantum computation.

Non-Abelian Anyons and Topological Quantum Computation

REVIEW Semiconductor devices generally take advantage of the charge of electrons, whereas magnetic materials are used for recording information involving electron spin. To make use of both charge and spin of electrons in semiconductors, a high concentration of magnetic elements can be introduced in nonmagnetic III-V semiconductors currently in use for devices. Low solubility of magnetic elements was overcome by low-temperature nonequilibrium molecular beam epitaxial growth, and ferromagnetic (Ga,Mn)As was realized. Magnetotransport measurements revealed that the magnetic transition temperature can be as high as 110 kelvin. The origin of the ferromagnetic interaction is discussed. Multilayer heterostructures including resonant tunneling diodes (RTDs) have also successfully been fabricated. The magnetic coupling between two ferromagnetic (Ga,Mn)As films separated by a nonmagnetic layer indicated the critical role of the holes in the magnetic coupling. The magnetic coupling in all semiconductor ferromagnetic/nonmagnetic layered structures, together with the possibility of spin filtering in RTDs, shows the potential of the present material system for exploring new physics and for developing new functionality toward future electronics.

https://science.sciencemag.org/content/sci/281/5379/951.full.pdf

Making Nonmagnetic Semiconductors Ferromagnetic

Nanocrystals (NCs) discussed in this Review are tiny crystals of metals, semiconductors, and magnetic material consisting of hundreds to a few thousand atoms each. Their size ranges from 2-3 to about 20 nm. What is special about this size regime that placed NCs among the hottest research topics of the last decades? The quantum mechanical coupling * To whom correspondence should be addressed. E-mail: dvtalapin@uchicago.edu. † The University of Chicago. ‡ Argonne National Lab. Chem. Rev. 2010, 110, 389–458 389

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Prospects of Colloidal Nanocrystals for Electronic and Optoelectronic Applications

We report neutron scattering measurements of the structure and magnetism of stage-4 ${\mathrm{La}}_{2}{\mathrm{CuO}}_{4+y}$ with ${T}_{c}\ensuremath{\simeq}42\mathrm{K}.$ Our diffraction results on a single crystal sample demonstrate that the excess oxygen dopants form a three-dimensional ordered superlattice within the interstitial regions of the crystal. The oxygen superlattice becomes disordered above $T\ensuremath{\simeq}330\mathrm{K},$ and a fast rate of cooling can freeze in the disordered-oxygen state. Hence, by controlling the cooling rate, the degree of dopant disorder in our ${\mathrm{La}}_{2}{\mathrm{CuO}}_{4+y}$ crystal can be varied. We find that a higher degree of quenched disorder reduces ${T}_{c}$ by $\ensuremath{\sim}5\mathrm{K}$ relative to the ordered-oxygen state. At the same time, the quenched disorder enhances the spin-density wave order in a manner analogous to the effects of an applied magnetic field.

Neutron scattering study of the effects of dopant disorder on the superconductivity and magnetic order in stage-4 La 2 CuO 4+y

Measurements are reported of the magnetoresistance of thoroughly characterized single crystals of La{sub 2{minus}{ital x}}Sr{sub {ital x}}CuO{sub 4+{ital y}} with 002{le}{ital x}{le}01 Above the superconducting critical temperature, the conductance of the CuO{sub 2} layers increases logarithmically with temperature at low {ital T} However, the magnetoresistance is isotropic and negative, inconsistent with coherent backscattering or conventional interaction effects, suggesting that spin scattering limits the conductance

Isotropic negative magnetoresistance in La2-xSrxCuO4+y.

Photoluminescence (PL) originating from intrinsic defects in neutron-irradiated crystalline (x-) SiO 2 is observed and compared with that from amorphous (a-) SiO 2 . The PL and photoluminescence excitation (PLE) spectra of x-SiO 2 are qualitatively similar to those of a-SiO 2 . This suggests that PL centers, similar to those found in amorphous systems, are present in ordered systems. In addition, the PL and PLE spectra a-SiO 2 and a-As 2 S 3 scale with energy band gap, suggesting that the radiative processes in a-SiO 2 and semiconducting chalcogenide glasses are alike. The similarity of the PL from x-SiO 2 to that from a-SiO 2 and the scaling of the PL from a-SiO 2 and a-As 2 S 3 with energy band gap strongly suggest that PL centers are unique bonding configurations which primarily depend on the chemistry of the system rather than on the degree of long range order.

Photoluminescence from E band centers in amorphous and crystalline SiO2

Temperature-dependent angle-resolved photoemission experiments were performed on overdoped single layer ${\mathrm{Bi}}_{2}{\mathrm{Sr}}_{2}{\mathrm{CuO}}_{6}$ ${(T}_{c}\ensuremath{\sim}4 \mathrm{K})$ and on the parent compounds ${\mathrm{Sr}}_{2}{\mathrm{CuO}}_{2}{\mathrm{Cl}}_{2}$ and ${\mathrm{Ca}}_{2}{\mathrm{CuO}}_{2}{\mathrm{Cl}}_{2}.$ Contrary to the more conventional results obtained on ${\mathrm{Bi}}_{2}{\mathrm{Sr}}_{2}{\mathrm{CuO}}_{6},$ the parent compounds show strong temperature dependence over a wide energy range that is two orders of magnitude larger than the temperature scale involved. The doping dependence strongly suggests that the anomalous temperature dependence has its origin in the novel many-body physics in the Mott insulators. We consider the observed temperature dependence using a heuristic model with multiple initial and final states in the photoemission process. The nature of the multiple states and broad photoemission line shape in parent compounds is also given.

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Anomalous temperature dependence in the photoemission spectral function of cuprates

An electro-optical device can include a plurality of nanocrystals positioned between a first electrode and a second electrode.

Marc Kastner

Papers

Neutron scattering study of the effects of dopant disorder on the superconductivity and magnetic order in stage-4 La 2 CuO 4+y

Isotropic negative magnetoresistance in La2-xSrxCuO4+y.

Photoluminescence from E band centers in amorphous and crystalline SiO2

Anomalous temperature dependence in the photoemission spectral function of cuprates

Electro-optical device