1,527 research outputs found
Electrical Conductance of Molecular Wires
Molecular wires (MW) are the fundamental building blocks for molecular
electronic devices. They consist of a molecular unit connected to two continuum
reservoirs of electrons (usually metallic leads). We rely on Landauer theory as
the basis for studying the conductance properties of MW systems. This relates
the lead to lead current to the transmission probability for an electron to
scatter through the molecule. Two different methods have been developed for the
study of this scattering. One is based on a solution of the Lippmann-Schwinger
equation and the other solves for the {\bf t} matrix using Schroedinger's
equation. We use our methodology to study two problems of current interest. The
first MW system consists of 1,4 benzene-dithiolate (BDT) bonded to two gold
nanocontacts. Our calculations show that the conductance is sensitive to the
chemical bonding between the molecule and the leads. The second system we study
highlights the interesting phenomenon of antiresonances in MW. We derive an
analytic formula predicting at what energies antiresonances should occur in the
transmission spectra of MW. A numerical calculation for a MW consisting of
filter molecules attached to an active molecule shows the existence of an
antiresonance at the energy predicted by our formula.Comment: 14 pages, 5 figure
An assessment of financial sector rescue programmes
We analyse the wide array of rescue programmes adopted in several countries, following Lehman Brothersâ default in September 2008, in order to support banks and other financial institutions. We first provide an overview of the programmes, comparing their characteristics, magnitudes and participation rates across countries. We then consider the effects of the programmes on banksâ risk and valuation, looking at the behaviour of CDS premia and stock prices. We then proceed to analyse the issuance of government guaranteed bonds by banks, examining their impact on banksâ funding and highlighting undesired effects and distortions. Finally, we briefly review the recent evolution of bank lending to the private sector. We draw policy implications, in particular as regards the way of mitigating the distortions implied by such programmes and the need for an exit strategy.bank asset guarantees, capital injection, banks, financial sector, financial crisis, bank consolidation, bank mergers and acquisitions, event studies, government guaranteed bonds, credit crunch, exit strategy
Charging induced asymmetry in molecular conductors
We investigate the origin of asymmetry in various measured current-voltage
(I-V) characteristics of molecules with no inherent spatial asymmetry, with
particular focus on a recent break junction measurement. We argue that such
asymmetry arises due to unequal coupling with the contacts and a consequent
difference in charging effects, which can only be captured in a self-consistent
model for molecular conduction. The direction of the asymmetry depends on the
sign of the majority carriers in the molecule. For conduction through highest
occupied molecular orbitals (i.e. HOMO or p-type conduction), the current is
smaller for positive voltage on the stronger contact, while for conduction
through lowest unoccupied molecular orbitals (i.e. LUMO or n-type conduction),
the sense of the asymmetry is reversed. Within an extended Huckel description
of the molecular chemistry and the contact microstructure (with two adjustable
parameters, the position of the Fermi energy and the sulphur-gold bond length),
an appropriate description of Poisson's equation, and a self-consistently
coupled non-equilibrium Green's function (NEGF) description of transport, we
achieve good agreement between theoretical and experimental I-V
characteristics, both in shape as well as overall magnitude.Comment: length of the paper has been extended (4 pages to 6 pages), two new
figures have been added (3 figures to 5 figures), has been accepted for PR
Skipping Breakfast Leads to Weight Loss but Also Elevated Cholesterol Compared with Consuming Daily Breakfasts of Oat Porridge or Frosted Cornflakes in Overweight Individuals: A Randomised Controlled Trial
Eating breakfast may reduce appetite, body weight and CVD risk factors, but the breakfast type that produces the greatest health benefits remains unclear. We compared the effects of consuming a high-fibre breakfast, a non-fibre breakfast, or no-breakfast control on body weight, CVD risk factors and appetite. A total of thirty-six overweight participants (eighteen men and eighteen women) (mean age 33·9 (SD 7·5) years, mean BMI 32·8 (SD 4·7) kg/m2) were randomly assigned to consume oat porridge (n = 12), frosted cornflakes (n = 12) or a water control (n = 12) breakfast daily for 4 weeks. Appetite ratings were collected on the first day and weekly thereafter. Before and after the intervention, body weight, composition, blood pressure and resting energy expenditure (REE) were measured and a fasting blood sample was collected. Across the 4 weeks, fullness was higher and hunger was lower in the oat porridge group compared with the control group (P \u3c 0·05). Mean weight change over the intervention was significantly different in the control group (â1·18 (SD 1·16) kg) compared with both the cornflakes (â0·12 (SD 1·34) kg) and oat porridge (+0·26 (SD 0·91) kg) groups (P \u3c 0·05). However, the control group also showed elevated total cholesterol concentrations relative to the cornflakes and oat porridge groups (P \u3c 0·05). There were no differences between groups in changes in body composition, blood pressure, REE or other CVD risk factors. In conclusion, although skipping breakfast led to weight loss, it also resulted in increased total cholesterol concentrations compared with eating either oat porridge or frosted cornflakes for breakfast
Tuning the conductance of molecular junctions: transparent versus tunneling regimes
We present a theoretical study of the transport characteristics of molecular
junctions, where first-row diatomic molecules are attached to (001) gold and
platinum electrodes. We find that the conductance of all of these junctions is
of the order of the conductance quantum unit , spelling out that they
belong to the transparent regime. We further find that the transmission
coefficients show wide plateaus as a function of the energy, instead of the
usual sharp resonances that signal the molecular levels in the tunneling
regime. We use Caroli's model to show that this is a rather generic property of
the transparent regime of a junction, which is driven by a strong effective
coupling between the delocalized molecular levels and the conduction channels
at the electrodes. We analyse the transmission coefficients and chemical
bonding of gold/Benzene and gold/Benzene-dithiolate (BDT) junctions to
understand why the later show large resistances, while the former are highly
conductive.Comment: 9 pages, 7 figure
Efficiency Improvements for Encrypt-to-Self
Recent work by Pijnenburg and Poettering (ESORICS'20) explores the novel
cryptographic Encrypt-to-Self primitive that is dedicated to use cases of
symmetric encryption where encryptor and decryptor coincide. The primitive is
envisioned to be useful whenever a memory-bounded computing device is required
to encrypt some data with the aim of temporarily depositing it on an untrusted
storage device. While the new primitive protects the confidentiality of
payloads as much as classic authenticated encryption primitives would do, it
provides considerably better authenticity guarantees: Specifically, while
classic solutions would completely fail in a context involving user
corruptions, if an encrypt-to-self scheme is used to protect the data, all
ciphertexts and messages fully remain unforgeable.
To instantiate their encrypt-to-self primitive, Pijnenburg et al propose a
mode of operation of the compression function of a hash function, with a
carefully designed encoding function playing the central role in the
serialization of the processed message and associated data. In the present work
we revisit the design of this encoding function. Without questioning its
adequacy for securely accomplishing the encrypt-to-self job, we improve on it
from a technical/implementational perspective by proposing modifications that
alleviate certain conditions that would inevitably require implementations to
disrespect memory alignment restrictions imposed by the word-wise operation of
modern CPUs, ultimately leading to performance penalties. Our main
contributions are thus to propose an improved encoding function, to explain why
it offers better performance, and to prove that it provides as much security as
its predecessor. We finally report on our open-source implementation of the
encrypt-to-self primitive based on the new encoding function.Comment: this is the full version of content that appears at CYSARM'2
Antiresonances in Molecular Wires
We present analytic and numerical studies based on Landauer theory of
conductance antiresonances of molecular wires. Our analytic treatment is a
solution of the Lippmann-Schwinger equation for the wire that includes the
effects of the non-orthogonality of the atomic orbitals on different atoms
exactly. The problem of non-orthogonality is treated by solving the transport
problem in a new Hilbert space which is spanned by an orthogonal basis. An
expression is derived for the energies at which antiresonances should occur for
a molecular wire connected to a pair of single-channel 1D leads. From this
expression we identify two distinct mechanisms that give rise to antiresonances
under different circumstances. The exact treatment of non-orthogonality in the
theory is found to be necessary to obtain reliable results. Our numerical
simulations extend this work to multichannel leads and to molecular wires
connected to 3D metallic nanocontacts. They demonstrate that our analytic
results also provide a good description of these more complicated systems
provided that certain well-defined conditions are met. These calculations
suggest that antiresonances should be experimentally observable in the
differential conductance of molecular wires of certain types.Comment: 22 pages, 5 figure
A mesoscopic ring as a XNOR gate: An exact result
We describe XNOR gate response in a mesoscopic ring threaded by a magnetic
flux . The ring is attached symmetrically to two semi-infinite
one-dimensional metallic electrodes and two gate voltages, viz, and
, are applied in one arm of the ring which are treated as the inputs of
the XNOR gate. The calculations are based on the tight-binding model and the
Green's function method, which numerically compute the conductance-energy and
current-voltage characteristics as functions of the ring-to-electrode coupling
strength, magnetic flux and gate voltages. Our theoretical study shows that,
for a particular value of () (, the elementary
flux-quantum), a high output current (1) (in the logical sense) appears if both
the two inputs to the gate are the same, while if one but not both inputs are
high (1), a low output current (0) results. It clearly exhibits the XNOR gate
behavior and this aspect may be utilized in designing an electronic logic gate.Comment: 8 pages, 5 figure
Driving current through single organic molecules
We investigate electronic transport through two types of conjugated
molecules. Mechanically controlled break-junctions are used to couple thiol
endgroups of single molecules to two gold electrodes. Current-voltage
characteristics (IVs) of the metal-molecule-metal system are observed. These
IVs reproduce the spatial symmetry of the molecules with respect to the
direction of current flow. We hereby unambigously detect an intrinsic property
of the molecule, and are able to distinguish the influence of both the molecule
and the contact to the metal electrodes on the transport properties of the
compound system.Comment: 4 pages, 5 figure
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