1D Wires Research Articles

Synthesis and patterning of nanostructures of (almost) anything on anything

A necessary and very challenging condition for progress in nanoscience is being able to synthesize the requisite material. While there has been substantial success for some materials, one would like to develop protocols to fabricate structures of a wide variety of materials on an equally wide range of supports, preferably with size selection and the ability to pattern on large areas at low cost. The paper by Kerner and Asscher in this issue [1] is significant because it shows how at least some of those conditions can be satisfied in a novel and creative way. Building on the techniques of surface science, many researchers have focused on physical vapor deposition and the subsequent assembly of 1D wires, 2D islands, and 3D clusters. The problem, of course, is that Nature provides rather stringent rules that prevent the spontaneous assembly of 3D structures for all but a few materials. In particular, 3D growth will occur only if the adatoms interact only weakly with the surface but strongly with one another, with details dictated by surface and interface energies. A technique was developed a few years ago that by-passed these rules, termed buffer-layer-assistedgrowth (BLAG) [2–4], and the paper by Kerner and Asscher combines this technique with laser induced thermal desorption to form patterns in a novel and potentially important way. The idea behind buffer layer assisted growth is remarkably simple, as depicted in Fig. 1. The buffer layer replaces adatom–substrate interactions with very weak adatom–buffer interactions so that balling-up occurs out of contact with the substrate. The buffer must subsequently be removed so that the clusters are delivered in the ultimate of soft landings to the substrate where wetting is frustrated by kinetics. While there

0.7 Structure in quantum wires observed at crossings of spin-polarised 1D subbands

Low-density ballistic 1D wires defined in GaAs/AlGaAs heterostructures show a spontaneous spin-splitting at crossings of Zeeman-split levels induced by an in-plane magnetic field, giving rise to additional unexpected conductance features. From measurements of magnetic field and temperature dependences, we suggest that the mechanism associated with the new features is similar to that of the 0.7 structure observed at zero magnetic field.

Coherent "metallic" resistance and medium localization in a disordered one-dimensional insulator.

It is believed that a disordered one-dimensional (1D) wire with coherent electronic conduction is an insulator with the mean resistance <rho> approximately equal e(2L/xi) and resistance dispersion Delta(rho) approximately equal e(L/xi), where L is the wire length and xi is the electron localization length. Here we show that this 1D insulator undergoes at full coherence the crossover to a 1D "metal," caused by thermal smearing and resonant tunneling. As a result, Delta(rho) is smaller than unity and tends to be L/xi independent, while <rho> grows with L/xi first nearly linearly and then polynomially, manifesting the so-called medium localization.

Interaction effects at crossings of spin-polarized one-dimensional subbands.

We report conductance measurements of ballistic one-dimensional (1D) wires defined in GaAs/AlGaAs heterostructures in an in-plane magnetic field, B. When the Zeeman energy is equal to the 1D subband energy spacing, the spin-split subband N upward arrow intersects (N+1) downward arrow, where N is the index of the spin-degenerate 1D subband. At the crossing of N=1 upward arrow and N=2 downward arrow subbands, there is a spontaneous splitting giving rise to an additional conductance structure evolving from the 1.5(2e(2)/h) plateau. With further increase in B, the structure develops into a plateau and lowers to 2e(2)/h. With increasing temperature and magnetic field the structure shows characteristics of the 0.7 structure. Our results suggest that at low densities a spontaneous spin splitting occurs whenever two 1D subbands of opposite spins cross.

Decay rate and renormalized frequency shift of superradiant exciton in a cylindrical quantum wire

The decay rate and renormalized frequency shift of superradiant exciton in a cylindrical quantum wire are studied. The transition behavior from 1D wire to 2D film is examined through the property of the radiative decay. Similar to the case in a quantum well, the decay rate of the higher mode exciton is larger than that of the lower mode one. Moreover, it is also found the decay rate and frequency shift do not show oscillatory dependence on wire radius because of the conservation of angular momentum.

Phase-slip-like resistivity in underdopedYBa2Cu3O7

We investigated the anomalous peak resistivity below the onset ${T}_{c}$ in underdoped ${\mathrm{YBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7},$ reminiscent of that observed in one-dimensional (1D) wires of conventional superconductors. We performed measurements of the angular dependence of resistivity $\ensuremath{\rho}(\ensuremath{\theta})$ in a magnetic field and the temperature dependence of resistivity $\ensuremath{\rho}(T),$ which exhibit a peak for $\mathbf{B}\ensuremath{\Vert}$ ab planes. This peak in $\ensuremath{\rho}(T)$ disappears for $\mathbf{B}\ensuremath{\Vert}$ c axis. The width of the corresponding maximum in $\ensuremath{\rho}(\ensuremath{\theta})$ at $\ensuremath{\theta}=0\ifmmode^\circ\else\textdegree\fi{}$ $(\mathbf{B}\ensuremath{\Vert}$ ab planes) decreases with the increasing c-axis component of the field $(B\mathrm{sin}\ensuremath{\theta}).$ The maximum in $\ensuremath{\rho}(\ensuremath{\theta})$ and $\ensuremath{\rho}(T)$ decreases with an increasing applied transport current. We analyzed the data using three different models of resistivity based on a 2D resistor array, flux motion, and thermally activated phase slips. Numerical calculations suggest that in a filamentary underdoped system, the phase-slip events could produce an anomalous resistivity close to ${T}_{c}.$

Quasibound states at thresholds in multichannel impurity scattering

We investigate the threshold behavior of transmission resonances and quasibound states in the multichannel scattering problems of a one dimensional (1D) time-dependent impurity potential, and the related problem of a single impurity in a quasi 1D wire. It was claimed before in the literature that a quasibound state disappears when a transmission zero collides with the subband boundary. However, the transmission line shape, the Friedel sum rule, and the delay time show that the quasibound states still survive and affect the physical quantities. We discuss the relation between threshold behavior of transmission resonances, and quasibound states and their boundary conditions in the general context of multichannel scatterings.

Universal spin-polarization fluctuations in one-dimensional wires with magnetic impurities

We study conductance and spin-polarization fluctuations in one-dimensional wires with spin-5/2 magnetic impurities (Mn). Our tight-binding Green function approach goes beyond mean field thus including s-d exchange-induced spin-flip scattering. In a certain parameter range, we find that spin flip suppresses conductance fluctuations while enhancing spin-polarization fluctuations. More importantly, spin-polarization fluctuations attain a universal value 1/3 for large enough spin flip strengths. This intrinsic spin-polarization fluctuation may pose a severe limiting factor to the realization of steady spin-polarized currents in Mn-based 1D wires.

Current-driven plasma instabilities in parallel quantum-wire systems

A theory of current-driven plasma instability in a double-quantum-wire system is presented. It is shown that, if the wires carry steady currents in opposite directions, the quasi-one-dimensional (quasi-1D) plasma waves propagating along the wires become unstable when the electron drift velocity falls between the phase velocities of the acoustic and optical plasma modes of the Coulomb-coupled quantum wires. Such an instability occurs at experimentally achievable drift velocities because of the softening of the plasma waves in 1D wires. The condition for plasma instability in a lateral double-quantum-wire superlattice is also determined.

Observation of large many-body Coulomb interaction effects in a doped quantum wire

We demonstrate strong one-dimensional (1D) many-body interaction effects in photoluminescence (PL) in a GaAs single quantum wire of unprecedented optical quality, where 1D electron plasma densities are controlled via electrical gating. We observed PL of 1D charged excitons with large binding energy of 2.3 meV relative to the neutral excitons, and its evolution to a Fermi-edge singularity at high electron density. Furthermore, we find a strong band-gap renormalization in the 1D wire, or a large red-shift of PL with increased electron plasma density. Such a large PL red-shift is not observed when we create a high density neutral electron–hole plasma in the same wire, due probably to cancellation of the Coulomb interaction energy in the neutral plasma.

One-dimensional molecular wire on hydrogenated Si(001)

An important example of hybrid organic-silicon systems is the fabrication of styrene molecular wires on a H-passivated Si(001) surface. Here we theoretically demonstrate that a styrene molecule which easily adsorbs on a single H-empty site can be further stabilized (with an energy barrier of 0.88 eV) by abstracting an H atom from a neighboring Si dimer. This H-abstraction process creates another H-empty site, setting off a chain reaction that results in the growth of a styrene wire along the Si dimer row. Our simulated scanning tunneling microscope (STM) image for the resulting one-dimensional (1D) molecular wire is in good agreement with STM data. In particular, the antibonding $\ensuremath{\pi}$ orbitals on adjacent styrene molecules overlap, thus providing ``channels'' that permit charge transport along this 1D wire under a voltage bias.

Effect of temperature and magnetic field on the 0.7 structure in a ballistic one-dimensional wire

Low temperature measurements of ballistic one-dimensional (1D) wires show an additional structure at 0.7(2 e 2/ h) in the conductance characteristics (Phys. Rev. Lett. 77 (1996) 135; Phys. Rev. B 62 (2000) 10 950). This 0.7 structure develops into a plateau at e 2/ h, if, either the in-plane magnetic field is increased (Phys. Rev. Lett. 77 (1996) 135) or the carrier density is decreased (Phys. Rev. B 61 (2000) 13 365). A suggested explanation for this effect is based on spin polarisation induced by many-body interactions. As temperature increases, the e 2/ h plateau in a magnetic field shifts to 0.6(2 e 2/ h), whereas other plateaus remain unchanged. We study the effect of temperature on the evolution of the 0.7 structure with increasing in-plane magnetic field.

Imaging electrostatic microconstrictions in long 1D wires

We present scanned gate microscopy (SGM) images of a 4 μm long one-dimensional (1D) wire. The wire was defined by split-gate surface electrodes over a GaAs/AlGaAs heterostructure incorporating a subsurface 2D electron system. To generate SGM images, a charged tip scans a rectangular region over the wire. The wire conductance is recorded to determine the image contrast. A chain of circular features are seen along the length of the wire, caused by electrostatic microconstrictions. The microconstrictions were studied by changing either the tip voltage or the wire lateral position.

Tailoring magnetism in artificially structured materials: the new frontier

The current standard of electronic devices and data storage media has reached a level such that magnetic materials have to be fabricated on a nanometer scale. In particular, the emerging concept of spintronics, which is based on fact that current carriers have not only charge but also spin, requires the assembling of nanometer-sized magnetic structures with desired magnetic properties. It is this background that motivates scientists and engineers to attempt to grow and characterize magnetic objects at smaller and smaller length scales, from 2D films and multilayers to 1D wires and eventually to 0D dots. In this article, some of the most significant progress in recent years in the effort of growing artificially structured magnetic materials are reviewed. The new structural and magnetic properties of these materials are discussed, with an emphasis on the correlation between structure and magnetism, which also serves as guidance for improving their magnetic properties. The emerging emphasis is on converting the existing knowledge into growing and studying low-dimensional complex materials, which promise to have considerably higher “tuning” ability for desired properties.

MBE growth of calcium and cadmium fluoride nanostructures on silicon

Molecular beam epitaxy (MBE) growth of CdF2 and CaF2 layers and nanoscale heterostructures on Si(111) and Si(001) substrates has been investigated. The surface morphology of CaF2 and CdF2 layers on Si(111), studied by atomic force microscopy (AFM) was found to depend strongly on the growth temperature. The high temperature 2D growth mode would produce atomically flat surface. In contrast to that, well-pronounced pyramids indicating the 3D growth mode were observed on the layers grown at low temperature. Short-period CdF2–CaF2 superlattices (SLs) were also studied. Analysis of reflection high energy electron diffraction (RHEED) intensity oscillations showed asymmetry of the “CaF2 on CdF2” and “CdF2 on CaF2” interfaces resulting in SL surface smoothening. The possible mechanisms relevant to this phenomenon have been discussed.Atomic force microscopy (AFM), and RHEED have been used to study formation of CaF2 epitaxial nanostructures on Si(001). Different types of nanostructures have been grown including ultra-thin 2D wetting layer, quasi 1D wires at high growth temperature and well-organized dots at lower temperatures. An important role of the wetting layer in the transformation of the surface morphology at the initial stages of CaF2 growth on Si(001) at high temperatures has been demonstrated.

Direction-resolved transport and possible many-body effects in one-dimensional thermopower

A single-particle theory due to Mott predicts a proportionality between the diffusion thermopower and the energy derivative of the logarithm of the conductance. Measurements of a ballistic 1D wire show that the Mott theory remains valid in the presence of a finite current, and that it leads to a direction-sensitive probe of electron transport. We observe an apparent violation of the Mott model at low electron densities, when there is a nonquantized plateau in the conductance at ${0.7(2e}^{2}/h).$ There is as yet no successful theoretical explanation of this so called 0.7 structure, but the distinctive thermopower signature, which deviates from single-particle predictions, may provide the key to a better understanding.

Transport in cleaved-edge overgrowth wires

We report the results of 3 terminal measurements on a one-dimensional (1D) wire. By utilizing cleaved-edge overgrowth, we were able to vary the coupling between a 1D conductor and an adjacent two-dimensional electron gas (2DEG) strip attached to the central part of the wire. We study the 1D-2DEG coupling by controlling the width of the 2DEG strip. Ballistic transport along the wire persists for interaction regions as long as 6 μm . Thus, with our shortest interaction region (narrowest strip) of 2 μm , the wire remains almost undisturbed by the presence of the nearby 2DEG allowing a true 3 terminal measurement on a clean wire. Our observations also explain the origin of the non-universal conductance quantization of such 1D wires.

2D-1D coupling in cleaved edge overgrowth

We study the scattering properties of an interface between a one-dimensional (1D) wire and a two-dimensional (2D) electron gas. Experiments were conducted in the highly controlled geometry provided by molecular bean epitaxy overgrowth onto the cleaved edge of a high quality GaAs /AlGaAs quantum well. Such structures allow for the creation of variable length 1D-2D coupling sections. We find ballistic 1D electron transport through these interaction regions with a mean free path as long as 6 &mgr;m. Our results explain the origin of the puzzling nonuniversal conductance quantization observed previously in such 1D wires.

Spin properties of low-density one-dimensional wires

We report conductance measurements of a ballistic one-dimensional (1D) wire defined in the lower two-dimensional electron gas of a GaAs/AlGaAs double quantum well. At low temperatures there is an additional structure at $0.7(2e^2/h)$ in the conductance, which tends to $e^2/h$ as the electron density is decreased. We find evidence for complete spin polarization in a weakly disorderd 1D wire at zero magnetic field through the observation of a conductance plateau at $e^2/h$, which strengthens in an in-plane magnetic field and disappears with increasing electron density. In all cases studied, with increasing temperature structure occurs at $0.6(2e^2/h)$. We suggest that the 0.7 structure is a many-body spin state excited out of, either the spin-polarized electron gas at low densities, or the spin-degenerate electron gas at high densities.

Magnetic anticrossing of 1D subbands in ballistic double quantum wires

We study the low-temperature in-plane magnetoconductance of vertically coupled double quantum wires. Using a novel flip-chip technique, the wires are defined by two pairs of mutually aligned split gates on opposite sides of a ≤1 micron thick AlGaAs/GaAs double quantum well heterostructure. We observe quantized conductance steps due to each quantum well and demonstrate independent control of each 1D wire. A broad dip in the magnetoconductance at ∼6 T is observed when a magnetic field is applied perpendicular to both the current and growth directions. This conductance dip is observed only when 1D subbands are populated in both the top and bottom constrictions. This data is consistent with a counting model whereby the number of subbands crossing the Fermi level changes with field due to the formation of an anticrossing in each pair of 1D subbands.