NANO TECHNOLOGY AND PHYSICS

NANO AND PHYSICS Advanced Optical Imaging Research centres on advanced optical imaging techniques and applications to bio-imaging down to the nanoscale. In particular, techniques for ultra-high resolution imaging by use of novel contrast mechanisms.Plasmonics and Ultrafast NanoOptics Research includes fundamental physical research to applications in the biological and medical sectors. The overall research goal is to develop optical and spectroscopic "imaging" methods capable of sensitivity to both primary and secondary structural properties of proteins embedded in biological samples. Computational NanobioResearch includes theoretical and computational nanoscience, biophysics and chemical physics, statistical mechanics and molecular dynamics of biomolecular systems, structural bioinformatics/cheminformatics and multi-scale modeling of biomolecules and complex fluids. Liquid Atomic Force Microscopy The underlying theme is to understand, manipulate, and ultimately utilise the function of molecules at the nanometer scale. The group has particular strengths in the development and application of novel atomic force microscopy techniques, particularly in the liquid environment.ModellingResearch groups at UCD and Imperial College London conduct fundamental research in key areas of computational nanoscience (usually in collaboration with experimental groups). Nano Photonics Research centres on studying processes that occur on the nanoscale, specifically understanding optical processes that exist on the nanometre length scale and developing and applying emerging nano-imaging techniques.Soft Matter Modelling We use mesoscopic computer simulation methods to model dynamics of biomolecules, colloids, and biointerfaces. Topics of current interest include modelling protective function of endothelium and similar biomimetic surfaces and self-propulsion of microorganisms and artificial micro swimmers.Twisting left and right gives logic another turn

Germanium telluride can be thermally switched between a metastable crystalline phase and an amorphous state. In its crystalline form, it is a ferroelectric with spontaneous polarization. This structural transition would seem promising for data storage, through switching the crystalline (and hence ferroelectric) state on and off. However, applications involving germanium telluride have not been realized, primarily due to its high concentration of intrinsic vacancies.In addition to crystal structure, size also affects ferroelectric properties. In view of this, Engin Durgun and colleagues at Université de Liège and at the Université Catholique de Louvain, both in Belgium, have performed first-principle calculations on germanium telluride nanoplatelets. For platelets with a diameter larger than 2.7 nm, they predict the emergence of polarization vortices that should give rise to a ferrotoroidic ground state, with a spontaneous and reversible toroidal moment. The authors predict that the inhomogeneous strain would play a role in stabilizing the spiral. In addition to fundamental interest, this effect, together with the transition from crystalline to amorphous structure, can be used to record three spin states (right-hand spiral, left-hand spiral, and amorphous/off), which could be used for ternary logic.Out of the substrate, an atomic chain
The ability to accurately and reproducibly create nanostructures with a known composition and set of properties is a fundamental goal in nanoscience. In addition to the lithographic techniques usually employed in this pursuit, self-assembly or self-organization provides an attractive route towards creating nanoscale objects.The self-assembly of atoms and molecules deposited on a surface is guided by the competing mechanisms of diffusion, aggregation, and intermixing. However, if intermixing is dominant during the deposition process, the nano-objects that form can be composed of the substrate material instead of the deposited material.Using scanning tunneling microscopy and spectroscopy, D. H. Wei, Chunlei Gao, Kh. Zakeri, and Marek Przybylski at the Max-Planck-Institut für Mikrostrukturphysik in Halle, Germany, report in Physical Review Letters that whether they deposit a submonolayer of chromium, manganese, iron, cobalt, or nickel on a palladium surface, the metal chains that ultimately form are made of palladium. Gao et al. argue that for the particular palladium surface they have chosen to study, intermixing is more energetically feasible than surface diffusion because of the large lattice mismatch between the substrate and the deposited material and they theorize that the deposited atoms simply diffuse into the bulk.This solves the puzzle of why the scanning tunneling spectra of the atomic chains on this surface showed the same features regardless of the deposition material, and emphasize the role of atomic intermixing in self-assembly of nanostructures.Eliminating charge noise.
Superconducting qubits for quantum information applications come in two flavors: island-based, where the electric charge is quantized, or loop-based where the fluxoid (a generalization of flux to mesoscopic superconducting rings) is quantized. In the former case, the so-called Cooper-pair box, a superconducting island is Josephson coupled to a superconducting reservoir at one end and capacitively coupled at the other end to the rest of the circuit. The latter case consists of a Josephson junction shunted with an inductance, i.e., a superconducting loop with persistent current flowing clockwise or counterclockwise. A major source of decoherence for island-based devices is the presence of impurities with fluctuating charge (charge-offset noise), while loop-based devices are often constrained by flux noise.In a paper appearing in Physical Review Letters, Jens Koch, Vladimir Manucharyan, Michel Devoret, and Leonid Glazman from Yale University in the US find a correspondence between the Cooper-pair box and its inductively shunted partner. Despite the obvious change from a discrete to a continuous spectrum of the charge operator, the excitation spectra of the two systems are found to be closely related. In particular, charging effects are observable in the dynamical response function of the inductively shunted system, which exhibits distinct peaks at frequencies governed by the spectrum of the isolated Cooper-pair box. Therefore, addressing the inductance-shunted Cooper-pair box solely by ac voltages, i.e., microwave radiation, solves the problem of realizing a stable, charge-noise-free Cooper-pair-box artificial atom. These ideas form the basis for a new type of Josephson-junction device called the fluxonium (presented by the same authors in an experimental paper, see Ref. [1]) that consists of a small junction shunted with the large inductance from a series array of large-capacitance tunnel junctions.
When magnetism unchains a break junction
A monatomic chain of atoms left suspended across the tips of a broken wire (a break junction) is potentially a model one-dimensional system. These atomically thin wires are important in the study of fundamental magnetism and could eventually play a role in technological applications in spintronics and quantum computing. There are, for example, theoretical predictions that chains of magnetic transition metal atoms will be more magnetic than their bulk forms.Although it has been possible to make long monatomic chains of selected nonmagnetic transition-metal elements and magnetic transition-metal chains on a surface, creating suspended chains of magnetic transition metals across a break junction has proven difficult. To find out why, Alexander Thiess, Yuriy Mokrousov, and Stefan Blügel at Forschungszentrum Jülich and Stefan Heinze at Christian-Albrechts-Universität zu Kiel, both in Germany, report in Physical Review Letters first-principles calculations on the process of how a monatomic chain forms from a break junction. They show that the presence of a local magnetic moment suppresses chain formation in 3d, 4d, and 5d elements because it effectively lowers the hardness of the chain. This explains why gold, silver, iridium, and platinum—all nonmagnetic elements in bulk—can form long chains and why similar efforts to make iron strands only yielded shorter nanocontacts.
Reaching a new resolution standard with electron microscopy
The invention of spherical aberration correctors has been the most significant contribution to the field of transmission electron microscopy since the field emission gun. Chromatic and spherical aberration, well known from optics, also play a role in electron microscopy. Lens aberrations are not unique to the magnetic lenses used in electron microscopes, but rather a fundamental problem of optical systems. Because of spherical aberration, rays entering a round lens system away from the optical axis are refracted more strongly than those entering close to the optical axis (see Fig. 1, top). A similar effect is chromatic aberration, which occurs when rays with different wavelengths enter a round lens, resulting in the rays diffracting differently depending on their wavelength (see Fig. 1, bottom).Transmission electron microscopes (TEM) and scanning transmission electron microscopes (STEM) equipped with such aberration correctors have been shown to resolve interatomic spacings approaching 50 pm [1, 2] and achieve single-atom sensitivity [3]. To compare, the highest resolution in uncorrected STEM imaging is about 140 pm at the same electron wavelength [4, 5]. Using aberration-corrected TEMs, one can now also perform atomic resolution imaging with longer wavelength electrons, which tend to be less damaging to samples. This is of particular importance for low-dimensional nanostructures, such as nanotubes, metal clusters, or single-layer sheets containing light constituent atoms, such as lithium, boron, or carbon.One of these novel low-dimensional nanostructures is single-layer hexagonal boron nitride. The hexagonal lattice of boron nitride contains boron and nitrogen atoms separated by a distance of 1.44 Å. This two-dimensional structure exhibits intriguing magnetic and electronic transport properties different from its monoatomic cousin, graphene. Single-layer boron nitride is also very sensitive to electron beams at energies higher than 80 kV. One could use beams of lower energy, but a side effect of using electron beam energies as low as 80 kV is that the chromatic aberrations of the imaging system become more dominant in determining the spatial resolution limit. Even in spherical-aberration-corrected TEMs, the smallest spatial resolution at 80 kV is limited by the chromatic aberrations of the objective lens [6]. This means that either chromatic aberration correctors or more monochromatic electron sources are needed in order to achieve atomic resolution.Since the emergence of aberration-correctors and monochromators, the TEM community has been looking for an appropriate benchmark sample to test the resolution and sensitivity of new instruments. In this light, monolayer boron nitride would at first glance appear an unlikely candidate for such resolution tests, since hexagonal boron nitride, in particular, monolayer boron nitride, is highly beam sensitive with a knock-on damage threshold of 74 eV and 84 eVfor boron and nitrogen atoms, respectively.In a paper appearing in Physical Review B [7], Nasim Alem, Rolf Erni, Christian Kisielowski, Marta Rossell, Will Gannett, and Alex Zettl at the University of California, Berkeley, and Lawrence Berkeley National Laboratory, both in the US, have demonstrated that their unique setup of using aberration-corrected and monochromated transmission electron microscopy at 80 kV can image the hexagonal lattice structure of boron nitride, and identify the atomic positions of both boron and nitrogen atoms in single-layer boron nitride. Beyond this milepost in imaging resolution, the group shows they can synthesize TEM samples of hexagonal boron nitride with conventional ex situ preparation techniques, like exfoliation and plasma etching.In agreement with previous studies [6, 8], Alem et al. have shown that vacancies in boron nitride are predominately associated with missing boron atoms in single-layer boron nitride, as well as at the edges of boron nitride sheets. Yet this work goes further than any of the previous studies by showing that the combination of aberration-corrected TEM at 80 kV with a monochromated electron source can achieve ultrahigh resolution imaging down to single atoms. This opens a new path to quantifying the image resolution and chemical sensitivity in next generation transmission electron microscopy instrumentation.To explain the significance of this result, let us consider a TEM operating at 200 kV, where you would have high enough resolution to image boron nitride. Although 200 kV electrons have a wavelength of about 2.5 pm, the spatial resolution of conventional electron microscopy using 200 kVelectrons has so far been limited to about 140 pm, suggesting that the electron-optical system of the TEM is the predominant limiting factor rather than the availability of short wavelength electrons. Since the spatial resolution is proportional to λ3/4 [9], using 80 kV instead of 200 kV in an uncorrected TEM could increase the resolution limit from 140 pm to more than 200 pm for the same spherical and chromatic aberrations of the objective lens. Although chromatic and spherical aberrations are the resolution-limiting elements in most optical microscopes, they can often be corrected by designing round lenses with negative aberrations. Unfortunately, it is not possible to design analogous lenses for electron microscopes, which bend electron beams with magnetic fields, creating an effect analogous to refraction [9].Recently, aberration correctors have become available in state-of-the-art TEMs. These consist of a complex arrangement of quadrupole, hexapole, and octupole magnetic lenses [10,11]. Aberration-corrected electron microscopes have demonstrated that imaging and spectroscopy is now possible with resolutions as high as 50 pm using 200 kV or 300 kVelectron sources [1, 2, 12, 13]. Yet even in spherical-aberration-corrected instruments, lowering the electron energy to 80 kV is limited by the chromatic aberration of the objective lens, or the energy spread in the incoming electron beam ΔE. Using single-layer boron nitride, Meyer et al. demonstrated that spherical aberration correction alone cannot achieve atomic resolution at 80 kV [6]. Since chromatic aberration correctors are not yet commercially available, using a monochromated electron source is the only alternative in high-resolution TEM to minimize the effects of chromatic aberration at low kV and increase the spatial resolution limit for 80kV imaging. However, monochromated electron sources often achieve the desired decrease in ΔE by discarding the majority of electrons, resulting in a significant decrease of the image signal. It was therefore not clear that using a monochromator in connection with an aberration corrector would provide an electron beam intense enough for ultrahigh resolution imaging at 80 kV.The findings of Alem et al. [7] go beyond the simple identification of atomic species in single-layer boron nitride, they also provide a roadmap to identifying the chemical sensitivity of high-resolution images as a function of sample thickness. The images clearly distinguish the atomic positions of nitrogen and boron in the hexagonal lattice of monolayer boron nitride. As expected, for an even number of boron nitride sheets, the intensity of the atomic columns in the hexagonal rings of boron nitride is identical since each column now contains an alternate stacking of boron and nitrogen atoms (see Fig. 1 in Ref. [7]). While this appears to be trivial, it is of crucial importance to this study, and to the field of aberration-corrected high-resolution TEM. As demonstrated by this work, the intensity of the atomic fringes in an aberration-corrected HRTEM micrograph depends on the atomic mass and thickness of the atomic column, as well as on the orientation of the lattice with respect to the incoming electron beam.In conclusion, single-layer boron nitride has clearly become the new benchmark test for imaging resolution in aberration-corrected low-energy TEM/STEM [6, 8, 14]. The hexagonal lattice of boron nitride can, however, only be imaged at 80 kV by aberration-corrected TEM in connection with either monochromated or chromatic-aberration-corrected TEM. Alem et al. show that monolayer boron nitride samples can be made from boron nitride powders using conventional TEM sample preparation methods and furthermore demonstrate that monolayer boron nitride, beyond its importance for fundamental physics and nanoelectronic applications, can also be used for low kV aberration-corrected ultrahigh-resolution imaging with single-atom sensitivity.
Quasiparticles do the twist
One of the most memorable lessons of an undergraduate course in quantum mechanics is that identical particles can have two types of “statistics.” An exchange of two identical bosons leaves the many-particle wave function unchanged, while an exchange of two identical fermions introduces a minus sign. The consequences of this minus sign are profound in all areas of physics. Moreover, it has long been known that two-dimensional systems can have statistics that are neither bosonic nor fermionic. Robert Laughlin’s explanation of the fractional quantum Hall effect introduced quasiparticles with fractional charges such as e/3. These are thought to possess fractional statistics; if you exchange two identical quasiparticles, the wave function picks up a factor exp(iϕ). In more complex quantum Hall states, different quasiparticle exchanges, represented by matrices, do not always commute: the quasiparticles obey non-Abelian statistics.In a theoretical paper in Physical Review B [1], Waheb Bishara and coauthors analyze recent experimental evidence [2] for this remarkable property and discuss how further experiments might probe non-Abelian statistics—an essential ingredient for the “topological quantum computer”—in more detail. Interest in such interferometry experiments has exploded in recent years, driven by the possibility of building a quantum computer that would perform operations by manipulating non-Abelian quasiparticles. The action of a quantum computer can be described by a unitary transformation, and in a topological quantum computer, that unitary transformation is built up of the matrices that describe “braiding” of quasiparticles (Fig. 1, left). These quasiparticle manipulations can be performed while keeping the quasiparticles spatially separated and also maintaining the energy gap between the low-energy states, on which the braiding acts, and others. The resulting advantage over other proposals for quantum computation is that unavoidable small errors in the braiding process, which do not change the topology of the braid, do not degrade the computation.The existence of fractional statistics is a basic consequence of the “topological order” that defines quantum Hall states in the same way that symmetry breaking defines more conventional states such as superfluids and magnets. Another consequence illustrates better why this type of order is called topological: in many quantum Hall states, the number of degenerate ground states depends on the topology of the system (whether it is a sphere or torus, for example) but not on its geometry. The essence of the quantum Hall effect is that plateaus in the Hall conductance are observed in a two-dimensional (2D) electron gas at certain densities where electrons form a quantum liquid, as first explained by Laughlin [3]. However, the nature of a few quantum Hall plateaus could not be explained by the first generalizations of the Laughlin wave function. The location of a plateau is typically described by the filling factor, which is the density in units of Landau levels (Landau levels are the highly degenerate eigenstates of a single 2D electron in a magnetic field). One unexplained plateau was at filling factor ν=5/2 (i.e., two filled Landau levels, plus a half-filled one) where there is no plateau at intermediate temperatures. In high-mobility samples at rather low temperatures, a clear plateau in the Hall conductance does appear at ν=5/2. Gregory Moore and Nicholas Read constructed a remarkable quantum Hall state [4] that Martin Greiter, Xiao-Gang Wen, and Frank Wilczek proposed as an explanation of the plateau at ν=5/2 (see Ref. [5]). The Moore-Read state can be viewed as a superconducting state obtained by pair formation from the composite-fermion metal that exists at slightly higher temperatures.The Moore-Read state was initially appealing on aesthetic grounds and received important numerical support a few years later [6]. Indirect experimental evidence has been accumulating [7], and earlier this year an interferometry experiment by Robert Willett and collaborators observed a remarkable property of the Moore-Read state: its quasiparticles obey statistics that are not just fractional, but non-Abelian. Non-Abelian statistics are possible in a system with degenerate ground states. Moving one quasiparticle around another does not simply multiply the ground state by a phase factor, but acts as a matrix on the whole space of ground states, and the matrices of different quasiparticle “braiding” operations (Fig. 1, left) need not commute. (Here we follow convention in using “ground state” to denote one of the degenerate lowest-energy states with a particular quasiparticle configuration, not the absolute ground state with no quasiparticles.)The first part of the paper by Bishara et al. critically reviews alternate explanations of the data of this interferometry experiment. All forms of fractional statistics, even the simpler Abelian variety, have been surprisingly difficult to confirm experimentally. While fractional charge can be probed relatively simply, by noise measurements of edge currents [8], for example, a measurement of statistics requires a nonlocal process in which one quasiparticle moves either around another or around a suitable defect. One experiment of this type is a Fabry-Pérot interferometer that coherently combines two paths of a quasiparticle moving along the edge around a bulk region that itself contains quasiparticles (Fig. 1, right).The actual interferometry experiment is somewhat complicated. As a side gate changes the area of the quantum Hall liquid, one sees oscillations in the total conductance of the two point contacts encircling the liquid. In Abelian quantum Hall states, the oscillations can be understood from the Aharonov-Bohm effect, which now involves the fractional quasiparticle charge, as may have been observed in one of the early quantum Hall interferometry experiments [9]. At ν=5/2, an additional oscillation appears for even values of the number of bulk quasiparticles. This number modifies the point-contact conductance through the braiding effect.One competing explanation for the observed signal in such an interferometer is the Coulomb blockade effect. When electrons are confined, Coulomb repulsion can give rise to oscillatory features in the tunneling conductance that mimic the Aharonov-Bohm effect [10]. The Coulomb blockade effect would not generally show the same magnetic-field dependence as the Aharonov-Bohm effect, but the field-dependent change in the area of the quantum Hall droplet might cause the effects to appear similar. Bishara et al. consider this and several other scenarios and conclude that current data strongly support the non-Abelian statistics explanation. Future experiments are suggested to distinguish between the Moore-Read state and other proposed states at ν=5/2. In particular, two alternatives that are consistent with the existing experiments are the “anti-Pfaffian” [11] state, which is essentially obtained by “subtracting” the Moore-Read state from filled Landau levels, and the SU(2)2 state [12]. Two recent works begin to investigate how interferometry measurements are modified when bulk quasiparticles interact with the edge strongly (i.e., not just through statistics), which may be necessary for a detailed understanding of the experiments [13,14].Bishara et al. also outline which interferometry experiments would address more complex states than those at ν=5/2. Two families of such states were introduced by Read and Rezayi [15] and by Bonderson and Slingerland [16]. There are at least two motivations for looking at these states. First, observation of the Read-Rezayi candidate state at ν=12/5, for example, would be the first step toward a truly “universal” topological quantum computer. Every operation needed for a quantum computer can be encoded as a braiding of quasiparticles in this state, which is not the case for any of the ν=5/2candidates, nor for the Bonderson-Slingerland candidate at ν=12/5. In the Moore-Read state, for example, the topological braiding operations need to be supplemented by one nontopological operation in order to make a universal computer. Second, observation of the Read-Rezayi state would also be important purely on scientific grounds because its structure is more complex than that of the Moore-Read state. Most of the basic properties of the Moore-Read state appear in any weak-coupling two-dimensional superconductor with “p+ip” symmetry of the pair wave function, and the Bogoliubov-de Gennes description of this type of superconductor gives a simple, useful approach, based upon free particles, to the Moore-Read state. The Read-Rezayi state does not seem to have any comparably simple representation.As these rather complex fractional quantum Hall states are being probed, the topological ideas that came to condensed matter physics via the quantum Hall effect are finding wider application, e.g., in the “topological insulator” materials discovered recently in two [17] and three [18] dimensions. While these discoveries mean that topological phases of electrons can now be studied at room temperature in bulk materials, the most profound examples of how electrons are ordered continue to be found in the physics of the two-dimensional electron gas in a strong magnetic field. The experiments proposed by Bishara and collaborators to separate candidate quantum Hall states atν=5/2 and other fractions will either indicate which existing theory is correct or show that, despite the many aspects of the fractional quantum Hall effect that are understood, there remain mysteries in the statistical interactions of quasiparticles.