Fundamental Quantum and Relativistic Formulation of Thermal Noise and Linear conductance in a 1D Quasi-particle Ensemble under Ballistic Transport-regime

The impact of electronic correlation in nanoscale junctions, e.g., formed by single molecules, is analyzed using the single-impurity Anderson model. Numerically exact quantum Monte Carlo calculations are performed to map out the orbital filling, linear response conductance, and spectral function as a function of the Coulomb interaction strength and the impurity level position. These numerical results form a benchmark against which approximate but more broadly applicable approaches to include electronic correlation in transport can be compared. As an example, the self-consistent GW approximation has been implemented for the Anderson model and the results have been compared to this benchmark. For weak coupling or for level positions such that the impurity is either nearly empty or nearly full, the GW approximation is found to be accurate. However, for intermediate or strong coupling, the GW approximation does not properly represent the impact of spin or charge fluctuations. Neither the spectral function nor the linear response conductance is accurately given across the Coulomb blockade plateau and well into the mixed valence regimes.

In this work, we report on numerical simulations of electron transport through three-dimensional (3D) quantum structures in both linear and nonlinear response regimes of transport. The structures considered include 3D quantum point contacts, quantum dot structures, quantum dot superlattices, and resonant tunneling structures. The simulations are done using scattering matrix method. For 3D quantum point contacts, the linear response conductance shows quantizations in units of 2e2/h, but conductance steps can be in multiples of 2e2/h. The resonant tunneling peaks are found in the conductance spectra for both the quantum dot structures and resonant tunneling structures. The linear response conductance of 3D quantum dot superlattices shows multiple-fold splittings, reflecting the formation of energy minibands in the systems. By examining the dependence of peak positions on the distance between barriers, the correspondence between resonant tunneling peaks and resonant states is determined. When a small but finite source-drain voltage is applied, the nonlinear conductance is found to be asymmetric for an asymmetric 3D quantum point contact.

We study two double dot systems, one with dots in parallel and one with dots in series, and argue they admit an exact solution via the Bethe ansatz. In the case of parallel dots we exploit the exact solution to extract the behavior of the linear response conductance. The linear response conductance of the parallel dot system possesses multiple Kondo effects, including a Kondo effect enhanced by a nonpertubative antiferromagnetic RKKY interaction, has conductance zeros in the mixed valence regime, and obeys a nontrivial form of the Friedel sum rule.

A quantization of unimodular gravity is described, which results in a quantum effective action which is also unimodular, ie a function of a metric with fixed determinant. A consequence is that contributions to the energy-momentum tensor of the form of ${g}_{ab}C$, where $C$ is a spacetime constant, whether classical or quantum, are not sources of curvature in the equations of motion derived from the quantum effective action. This solves the first cosmological constant problem, which is suppressing the enormous contributions to the cosmological constant coming from quantum corrections. We discuss several forms of unimodular gravity and put two of them, including one proposed by Henneaux and Teitelboim, in constrained Hamiltonian form. The path integral is constructed from the latter. Furthermore, the second cosmological constant problem, which is why the measured value is so small, is also addressed by this theory. We argue that a mechanism first proposed by Ng and van Dam for suppressing the cosmological constant by quantum effects obtains at the semiclassical level.

Thermally induced magnetization fluctuations in the Co86Fe14 free (sense) layer of micron-sized, photolithographically defined giant magetoresistive spin-valve devices are measured electrically, by passing a dc current through the devices and measuring the current-dependent part of the voltage noise power spectrum. Using fluctuation–dissipation relations, the effective Gilbert damping parameter α for 1.2, 1.8, and 2.4 nm thick free layers is estimated from either the low-frequency white-noise tail, or independently from the observed thermally excited ferromagnetic resonance peaks in the noise power spectrum, as a function of applied field. The geometry, field, and frequency dependence of the measured noise are found to be reasonably consistent with fluctuation–dissipation predictions based on a quasianalytical eigenmode model to describe the spatial dependence for the magnetization fluctuations. The extracted effective damping constant α≈0.06 found for the 1.2 nm free layer was close to 3× larger than that measured in either the 1.8 or 2.4 films, which has potentially serious implications for the future scaling down of spin-valve read heads.

Recently a local mean field theory for both eqilibrium and transport properties of the Coulomb glass was proposed [A. Amir et al., Phys. Rev. B 77, 165207 (2008); 80, 245214 (2009)]. We compare the predictions of this theory to the results of dynamic Monte Carlo simulations. In a thermal equilibrium state we compare the density of states and the occupation probabilities. We also study the transition rates between different states and find that the mean field rates underestimate a certain class of important transitions. We propose modified rates to be used in the mean field approach which take into account correlations at the minimal level in the sense that transitions are only to take place from an occupied to an empty site. We show that this modification accounts for most of the difference between the mean field and Monte Carlo rates. The linear response conductance is shown to exhibit the Efros-Shklovskii behaviour in both the mean field and Monte Carlo approaches, but the mean field method strongly underestimates the current at low temperatures. When using the modified rates better agreement is achieved.

Ferromagnetic insulators deposited on graphene can induce ferromagnetic correlations in graphene. We estimate that induced exchange splittings $\ensuremath{\Delta}\ensuremath{\sim}5\phantom{\rule{0.3em}{0ex}}\mathrm{meV}$ can be achieved by, e.g., using the magnetic insulator EuO. We study the effect of the induced spin splittings on the graphene transport properties. The exchange splittings in proximity-induced ferromagnetic graphene can be determined from the transmission resonances in the linear response conductance or, independently, by magnetoresistance measurements in a spin-valve device. The spin polarization of the current near the Dirac point increases with the length of the barrier, so that long systems are required to determine $\ensuremath{\Delta}$ experimentally.

We study resonant tunneling through an interacting quantum dot coupled to normal metallic and superconducting leads. We show that large Coulomb interaction gives rise to novel effects in Andreev transport. Adopting an exact relation for the Green's function, we find that at zero temperature, the linear response conductance is enhanced due to Kondo-Andreev resonance in the Kondo limit, while it is suppressed in the empty site limit. In the Coulomb blockaded region, on the other hand, the conductance is reduced more than the corresponding conductance with normal leads because large charging energy suppresses Andreev reflection.

We predict a transition to metallicity when a sufficient amount of disorder is induced in graphene. Calculations were performed by means of a first principles stochastic quench method. The resulting amorphous graphene can be seen as nanopatches of graphene that are connected by a network of disordered small and large carbon rings. The buckling is minimal and we believe that it is a result of averaging of counteracting random in-plane stress forces. The linear response conductance is obtained by a model theory as function of lattice distortions. Such metallic behaviour is a much desired property for functionalisation of graphene to realize a transparent conductor, e.g. suitable for touch-screen devices.

Silicon quantum point contact with aluminum gate

In this paper, we study the conductance and shot noise in transport through a multisite system in a two terminal configuration. The dependence of the transport on the number of atoms in the atomic wire is investigated using a tight-binding Hamiltonian and the nonequilibrium Green's function method. In addition to reproducing the even-odd behavior in the transmission probability at the Fermi energy or the linear response conductance in the normal-atomic--wire-normal metallic (NAN) junctions, we find the following. (i) The shot noise is larger in the even-numbered atomic wire than in the odd-numbered wire. (ii) The Andreev conductance displays the same even-odd parity effects in the normal-atomic--wire-superconducting (NAS) junctions. In general, the conductance is higher in the odd-numbered atomic wire than in the even-numbered wire. When the number of sites (N) is odd and the atomic wire is mirror symmetric with respect to the center of the atomic wire, the conductance does not depend on the details of the hopping matrices in the atomic wire, but is solely determined by the coupling strength to the two leads. When N is even, the conductance is sensitive to the values of the hopping matrices.

In this work single charge tunneling effects in double island structures have been studied experimentally. The samples were produced using a twofold shadow evaporation technique which allows to choose the parameters of the tunnel junctions independently for the middle and the outer junctions. As the knowledge of the conductance of each junction is essential for comparison with theory, two of the samples were produced with a special layout which allows the direct determination of the junction conductances. The samples were investigated in a 3 He/ 4 He-dilution refrigerator by measuring the linear response conductance through the system in dependence of temperature and the bias- and gate voltages. The objective was to study the influence of higher order tunneling events which are not accounted for in the simple sequential model of the so called orthodox theory. For this purpose samples with junction conductances near the conductance quantum were investigated. At low temperatures significant deviations from the sequential model could be demonstrated. They manifest in a logarithmic decrease of the maximum conductance with decreasing temperature, as it is predicted by second order perturbation theory. The scale for the temperature dependence of the logarithmic correction could be deduced from the measurements and shows up to be the energy parameter which characterizes the coupling between the two islands.

Even-odd conductance oscillation in atomic wires

We investigate theoretically the switching behavior of a dithiolated phenylene-vinylene oligomer sandwiched between Au(111) electrodes using self-interaction corrected density-functional theory combined with the nonequilibrium Green's-function method for quantum transport. The molecule presents a configurational bistability, which can be exploited in constructing molecular memories, switches, and sensors. We find that protonation of the terminating thiol groups is at the origin of the change in conductance. H bonding at the thiol group weakens the S-Au bond and reduces by about one order of magnitude the transmission coefficient at the Fermi level, and thus the linear response conductance. Furthermore, protonation downshifts in energy the position of the highest occupied molecular orbital, so that the current of the protonated species is lower than that of the unprotonated one along the entire bias range investigated, from $\ensuremath{-}1.5$ to 1.5 V. A second protonation at the opposite thiol group has only minor effects and no further drastic reduction in transmission takes place. Our results allow us to re-interpret the experimental data originally attributing the conductance reduction to H dissociation.

There is a longstanding debate about the zero-point term in the Johnson noise voltage of a resistor. This term originates from a quantum-theoretical treatment of the fluctuation-dissipation theorem (FDT). Is the zero-point term really there, or is it only an experimental artifact, due to the uncertainty principle, for phase-sensitive amplifiers? Could it be removed by renormalization of theories? We discuss some historical measurement schemes that do not lead to the effect predicted by the FDT, and we analyse new features that emerge when the consequences of the zero-point term are measured via the mean energy and force in a capacitor shunting the resistor. If these measurements verify the existence of a zero-point term in the noise, then two types of perpetual motion machines can be constructed. Further investigation with the same approach shows that, in the quantum limit, the Johnson–Nyquist formula is also invalid under general conditions even though it is valid for a resistor-antenna system. Therefore we conclude that in a satisfactory quantum theory of the Johnson noise, the FDT must, as a minimum, include also the measurement system used to evaluate the observed quantities. Issues concerning the zero-point term may also have implications for phenomena in advanced nanotechnology.