This week in cond-mat

Super-brief pre-March Meeting blogging. Earlier this week there were five papers that particularly caught my eye on the arxiv. The first three are closely related....

arxiv:0803.0562 - Kiguchi et al., Highly conductive molecular junctions based on direct binding of benzene to Pt electrodes
This paper is from the always impressive van Ruitenbeek group. Here they demonstrate a chemical method of bridging the nanoscale gap between two movable Pt electrodes (in a geometry called a mechanically controllable break junction) by a benzene ring, with direct C-Pt bonds. The result is a junction that has a conductance approaching the conductance quantum, 2e²/h. This is impressive because achieving such strong electronic coupling in single molecule junctions has been challenging in the past without special transport mechanisms (like the Kondo effect). They prove that they have the desired structure by looking at vibrational signatures in the tunneling conductance at finite bias. By comparing regular ¹²C benzene devices and those made with ¹³C, they see a distinct isotopic shift in the vibrational modes - the molecule with heavy carbon has lower vibrational frequencies. They also use sub-gap structure in the tunneling (with the electrodes driven to superconduct by the proximity effect, as in this paper) to show that the conductance comes predominantly from a single, highly transmissive channel. Very nice.

arxiv:0803.0582 - Hybertsen et al., Amine-linked single-molecule circuits: Systematic trends across molecular families
This paper summarizes a large and very pretty body of experimental work done by Latha Venkataraman et al., with complementary theory calculations by Hybertsen and collaborators. Using
the STM equivalent of a mechanical break junction, these folks have made comprehensive studies of single-molecule conductance by compiling histograms of tens of thousands of conductance measurements in various junction configurations. This is a nice review of the work, and is an invited paper that is part of a forthcoming special issue of Journal of Physics: Condensed Matter. Our group also has a contribution to that issue.

arxiv:0803.0710 - Prodan and Car, Tunneling conductance of amine-linked alkyl chains
This is a new theory paper that examines one subset of the devices mentioned in 0803.0582. The neat thing about this is that this work uses a novel approach to density functional theory to do the transport calculations.

Changing the topic,

arxiv:0803.0719 - Marini et al., Fluctuation-dissipation: response theory in statistical physics
This is a long, comprehensive review article about the deep connection between equilibrium fluctuations and nonequilibrium dissipation. The classic example of this is Johnson-Nyquist noise, the voltage noise in a resistor that results from thermal fluctuations of the electron distribution, and its relationship to the actual resistance that determines dissipation when current flows. I need to find the time to read through this in detail - it looks like a real resource.

arxiv:0803.0568 - Wenzler and Mohanty, Measurement of Aharonov-Bohm oscillations in mesoscopic metallic rings in the presence of high-frequency electromagnetic fields
This is another experiment in an area that I continue to find interesting, the challenge of inferring information about the quantum coherence of electrons in solids. As the intro to this paper reminds, there are ambiguities in how various quantum corrections to electronic conduction define the coherence length - the characteristic distance scale that an electron can travel before its quantum mechanical phase becomes ill-defined due to "decoherence mechanisms" (inelastic interactions that somehow change the state of the environment). This paper examines one such correction, the Aharonov-Bohm effect, when electromagnetic radiation is introduced at a frequency related to the coherence length.