arXiv subjects

C. J. Lambert

Publications and source records attributed to C. J. Lambert.

At least 37 records · Page 2Linked to original sources

arXiv2008

Huge anisotropic magneto-resistance in iridium atomic chains

We analyze in this article the magneto-resistance ratio of finite and infinite iridium and platinum chains. Our calculations, that are based on a combination of non equilibrium Green function techniques and density functional theory, include a fully self-consistent treatment of non-collinear magnetism and of the spin-orbit interaction. They indicate that, in addition to having an extremely large magnetic anisotropy that may overcome the super-paramagnetic limit, infinite and also realistic finite-length iridium chains show sizeable anisotropic magnetoresistance ratios. We therefore propose iridium nanostructures as promising candidates for nanospintronics logic devices.

cond-mat.mes-hall↗

arXiv2008

Oscillating chiral currents in nanotubes: a route to nanoscale magnetic test tubes

With a view to optimising the design of carbon-nanotube (CNT) windmills and to maximising the internal magnetic field generated by chiral currents, we present analytical results for the group velocity components of an electron flux through chiral carbon nanotubes. Chiral currents are shown to exhibit a rich behaviour and can even change sign and oscillate as the energy of the electrons is increased. We find that the transverse velocity and associated angular momentum of electrons is a maximum for non-metallic CNTs with a chiral angle of 18$^o$. Such CNTs are therefore the optimal choice for CNT windmills and also generate the largest internal magnetic field for a given longitudinal current. For a longitudinal current of order $10^{-4}$ amps, this field can be of order $10^{-1}$Teslas, which is sufficient to produce interesting spintronic effects and a significant contribution to the self inductance.

cond-mat.mes-hall↗

arXiv2008

Non-trivial length dependence of the conductance and negative differential resistance in atomic molecular wires

We study the electronic and transport properties of two novel molecular wires made of atomic chains of carbon atoms (polyynes) capped with either, benzene-thiols or pyridines. While both molecules are structurally similar, the electrical conductance of benzene-thiol-capped chains attached to gold electrodes is found to be much higher than that of pyridine-capped chains. We predict that the conductance is almost independent of molecular length, which suggests that these molecules could be ideal molecular wires for sub-10 nm circuitry. Both systems exhibit negative differential resistance (NDR) but its origin and characteristics depend on the type of molecule. We find a novel type of NDR mechanism produced by the movement of the LUMO resonance with bias. We also show that by gating the pyridine-capped molecules it is possible to make the NDR disappear and dramatically modify the $I$-$V$ characteristics and the length dependence.

cond-mat.mes-hall↗

arXiv2008

Generation of EPR pairs and interconversion of static and flying electron spin qubits

We propose a method of generating fully entangled electron spin pairs using an open static quantum dot and a moving quantum dot, realized by the propagation of a surface acoustic wave (SAW) along a quasi-one-dimensional channel in a semiconductor heterostructure. In particular, we consider a static dot (SD) loaded with two interacting electrons in a singlet state and demonstrate a mechanism which enables the moving SAW-dot to capture and carry along one of the electrons, hence yielding a fully entangled static-flying pair. We also show how with the same mechanism we can load the SD with one or two electrons which are initially carried by a SAW-induced dot. The feasibility of realizing these ideas with existing semiconductor technology is demonstrated and extended to yield flying or static pairs that are fully entangled and arbitrary interconversion of static and flying electron spin qubits.

cond-mat.mes-hall↗

arXiv2007

Peculiar Nature of Snake States in Graphene

We study the dynamics of the electrons in a non-uniform magnetic field applied perpendicular to a graphene sheet in the low energy limit when the excitation states can be described by a Dirac type Hamiltonian. We show that as compared to the two-dimensional electron gas (2DEG) snake states in graphene exibit peculiar properties related to the underlying dynamics of the Dirac fermions. The current carried by snake states is locally uncompensated even if the Fermi energy lies between the first non-zero energy Landau levels of the conduction and valence bands. The nature of these states is studied by calculating the current density distribution. It is shown that besides the snake states in finite samples surface states also exist.

cond-mat.mes-hall↗

arXiv2007

Conformation dependence of molecular conductance: chemistry versus geometry

Recent experiments by Venkatamaran {\em et al.} [Nature (London) {\bf 442}, 904 (2006)] on a series of molecular wires with varying chemical compositions, revealed a linear dependence of the conductance on $\mathrm{cos}^2θ$, where $θ$ is the angle of twist between neighboring aromatic rings. To investigate whether or not this dependence has a more general applicability, we present a first principles theoretical study of the transport properties of this family of molecules as a function of the chemical composition, conformation and the contact atom and geometry. If the Fermi energy $E_\mathrm{F}$ lies within the HOMO-LUMO gap, then we reproduce the above experimental results. More generally, however, if $E_\mathrm{F}$ is located within either the LUMO or HOMO states, the presence of resonances destroys the linear dependence of the conductance on $\mathrm{cos}^2θ$ and gives rise to non-monotonic behaviour associated with the level structure of the different molecules. Our results suggest that the above experiments provide a novel method for extracting spectroscopic information about molecules contacted to electrodes.

cond-mat.mtrl-sci↗

arXiv2007

Strongly correlated electron physics in nanotube-encapsulated metallocene chains

The structural, electronic and transport properties of metallocene molecules (MCp$_2$) and isolated or nanotube-encapsulated chains are studied by using a combination of density functional theory and non-equilibrium Green's functions. The analysis first discusses the whole series of isolated MCp$_2$ molecules, where M = V, Cr, Mn, Fe, Co, Ni, Ru, and Os. The series presents a rich range of electronic and magnetic behaviors due to the interplay between the crystal field interaction and Hund's rules, as the occupation of the $d$ shell increases. The article then shows how many of these interesting properties can also be seen when MCp$_2$ molecules are linked together to form periodic chains. It is argued that encapsulation of the chains inside carbon nanotubes, that is exothermic for radii larger than 4.5 Å, provides the missing mechanical stability and electrical isolation. The structural stability, charge transfer, magnetic and electronic behavior of the ensuing chains, as well as the modification of the electrostatic potential in the nanotube wall produced by the metallocenes as thoroughly discussed. We argue that the full devices can be characterized by two doped, strongly correlated Hubbard models whose mutual hybridization is almost negligible. The charge transferred from the chains produces a strong modification of the electrostatic potential in the nanotube walls, which is amplified in case of semiconducting and endothermic nanotubes. The transport properties of isolated metallocenes between semi-infinite nanotubes are also analyzed and shown to lead to important changes in the transmission coefficients of clean nanotubes for high energies.

cond-mat.mtrl-sci↗

arXiv2007

Electronic Properties of Alkali- and Alkaline-Earth-Intercalated Silicon Nanowires

We present a first-principles study of the electronic properties of silicon clathrate nanowires intercalated with various types of alkali or alkaline-earth atoms. We find that the band structure of the nanowires can be tuned by varying the impurity atom within the nanowire. The electronic character of the resulting systems can vary from metallic to semiconducting with direct band gaps. These properties make the nanowires specially suitable for electrical and optoelectronic applications.

cond-mat.mtrl-sci↗

arXiv2006

Singlet-triplet filtering and entanglement in a quantum dot structure

We consider two interacting electrons in a semiconductor quantum dot structure which consists of a small dot within a larger dot, and demonstrate a singlet-triplet filtering mechanism which involves spin-dependent resonances and can generate entanglement. By studying the exact time evolution of singlet and triplet states we show how the degree of both filtering and spin entanglement can be tuned using a time-dependent gate voltage.

cond-mat.mes-hall↗

arXiv2006

Quantum entanglement generation with surface acoustic waves

We propose a scheme to produce spin entangled states for two interacting electrons. One electron is bound in a well in a semiconductor quantum wire and the second electron is transported along the wire, trapped in a surface acoustic wave potential (SAW) minimum. We investigate the conditions for which the Coulomb interaction between the two electrons induces entanglement. Detailed numerical investigation reveals that the two electrons can be fully spin entangled depending on the confinement characteristics of the well and the SAW potential amplitude.

cond-mat.mes-hall↗

arXiv2006

Controlled electron transport through single molecules

Using a first principles approach, we study the electron transport properties of a new class of molecular wires containing fluorenone units, whose features open up new possibilities for controlling transport through a single molecule. We show that the presence of side groups attached to these units leads to Fano resonances close to the Fermi energy. As a consequence electron transport through the molecule can be controlled either by chemically modifying the side group, or by changing the conformation of the side group. This sensitivity, opens up new possibilities for novel single-molecule sensors. We also show that transport can be controlled by tilting a molecule with respect to the electrode surfaces. Our results compare favorably with recent experiments.

cond-mat.mes-hall↗

arXiv2006

Andreev drag effect via magnetic quasiparticle focusing in normal-superconductor nanojunctions

We study a new hybrid normal-superconductor (NS) pi-junction in which the non-local current can be orders of magnitude larger than that in earlier proposed systems. We calculate the electronic transport of this NS hybrid when an external magnetic field is applied. It is shown that the non-local current exhibits oscillations as a function of the magnetic field, making the effect tunable with the field. The underlying classical dynamics is qualitatively discussed.

cond-mat.supr-con↗

arXiv2006

Quantized invariant tori in Andreev billiards of mixed phase space

Comparing the results of exact quantum calculations and those obtained from the EBK-like quantization scheme of Silvestrov et al [Phys. Rev. Lett. 90, 116801 (2003)] we show that the spectrum of Andreev billiards of mixed phase space can basically be decomposed into a regular and an irregular part, similarly to normal billiards. We provide the first numerical confirmation of the validity of this quantization scheme for individual eigenstates and discuss its accuracy.

cond-mat.mes-hall↗

arXiv2006

Tuning the electrical conductivity of nanotube-encapsulated metallocene wires

We analyze a new family of carbon nanotube-based molecular wires, formed by encapsulating metallocene molecules inside the nanotubes. Our simulations, that are based on a combination of non-equilibrium Green function techniques and density functional theory, indicate that these wires can be engineered to exhibit desirable magnetotransport effects for use in spintronics devices. The proposed structures should also be resilient to room-temperature fluctuations, and are expected to have a high yield.

cond-mat.mtrl-sci↗

arXiv2006

Zener quantum dot spin filter in a carbon nanotube

We predict and analyze a novel spin filter in semiconducting carbon nanotubes. By using local electrostatic gates, the conduction and valence bands can be modulated to form a double-barrier structure. The confined region below the valence band defines a Zener quantum dot, which exhibits resonant tunneling. The resonances split in a magnetic field to make a bipolar spin filter for applications in spintronics and quantum information processing. We model this using k*p envelope function theory and show that this is in excellent agreement with a corresponding tight-binding calculation.

cond-mat.mes-hall↗

arXiv2006

Bandgap modulation of narrow-gap carbon nanotubes in a transverse electric field

We propose a method to modulate the bandgaps in quasi-metallic carbon nanotubes using a transverse electric field. Unlike previous investigations, we include curvature effects of the nanotubes by incorporating both $π$- and $σ$-orbitals in our tight-binding calculations. The calculations show that the small curvature-induced bandgaps decrease quadratically with electric field amplitude to zero. As the electric field amplitude continues to increase, the bandgap then expands in a similar manner to that presented in earlier studies. The bandgap dependence is verified by analytical calculations, which also agree with preceding analyses for the limit of no curvature.

cond-mat.mes-hall↗

arXiv2006

Conductance oscillations in zigzag platinum chains

Using first principles simulations we perform a detailed study of the structural, electronic and transport properties of monoatomic platinum chains, sandwiched between platinum electrodes. First we demonstrate that the most stable atomic configuration corresponds to a zigzag arrangement that gradually straightens as the chains are stretched. Secondly, we find that the conductance at equilibrium atomic spacing does not oscillate with the number of atoms $n$ in the chain, but instead decreases almost monotonically with $n$. In contrast, the conductances of chains of fixed $n$ oscillate as the end atoms are pulled apart, due to the gradual closing and opening of conductance channels as the chain straightens.

cond-mat.mtrl-sci↗

arXiv2005

Non-universal behavior of the parity effect in monovalent atomic wires

We propose a mixed analytical-ab-initio method for the accurate calculation of the conductance in monovalent atomic wires. The method relies on the most general formula for ballistic transport through a monovalent wire, whose parameters can be determined from first-principles calculations. Our central result is the demonstration of the highly non-universal behavior of the conductance, which depends on the fine details of the contacts to the leads. We are therefore able to reconcile a large number of the apparently contradictory results that have recently appeared in the literature.

cond-mat.mes-hall↗