No abstract

The infrared absorption from molecular monolayers is enhanced a factor of 20 by thin metal overlayers or underlayers with use of the attenuated-total-reflection technique.The total enhancement, including contributions from the attenuated-total-reflection geometry, is almost 10 . This effect is consistent with an electric field enhancement due to collective electron resonances associated with the island nature of the thin metal films. PACS numbers: 78.30.-j The vibration@ states of molecular monolayers can be studied experimentally with inelastic electron-tunneling spectroscopy,~Raman scattering, 2 ' inelastic electron scattering, ' and infrared absorption. " In the inelastic electron-tunneling experiments it is necessary for the molecules to be sandwiched between two conducting media.Inelastic electr on-scattering experiments must be performed in vacuum and have relatively low energy resolution. Both conventional Raman scattering' and conventional infrared absorption' from monolayers must be done with high surfacearea samples, with consequent difficulties in sample char acterization, substr ate absorption and Quorescence. Baman scattering from surfaces can become quite strong with thin overlayers' or underlayers'~of metal, but so far only Ag and Au with a few molecular monolayers have shown a lar ge enhancement. The attenuatedtotal-reflectance (ATR) geometry in infrared absorption' achieves a large effective surface area without the need for powder samples. In this Letter, we report the first observation of the enhancement of infrared absorption from molecular monolayers due to thin metal overlayers and underlayers in the ATB geometry and show that this technique has application to a number of interesting current problems in surface science.The samples used in this study consisted of molecular monolayers of organic acids deposited on silicon substrates, either followed by or preceded by the evaporation of a thin metal layer. The sample preparation technique has been described by Hansma and Kirtley. ' The molecular monolayers used in this study were: 4-nitrobenzoic acid, benzoic acid, and 4-pyridine-COOH. The metal overlayers and underlayers studied were either Ag or Au and were evaporated at room temperature to an average thickness, d 100 I no Ag 80 I-LLI O 60 Z 0 I-40 0 M CO 20 I

The physics of the superconducting state in two-dimensional (2D) electron systems is relevant to understanding the high-transition-temperature copper oxide superconductors and for the development of future superconductors based on interface electron systems. But it is not yet understood how fundamental superconducting parameters, such as the spectral density of states, change when these superconducting electron systems are depleted of charge carriers. Here we use tunnel spectroscopy with planar junctions to measure the behaviour of the electronic spectral density of states as a function of carrier density, clarifying this issue experimentally. We chose the conducting LaAlO3-SrTiO3 interface as the 2D superconductor, because this electron system can be tuned continuously with an electric gate field. We observed an energy gap of the order of 40 microelectronvolts in the density of states, whose shape is well described by the Bardeen-Cooper-Schrieffer superconducting gap function. In contrast to the dome-shaped dependence of the critical temperature, the gap increases with charge carrier depletion in both the underdoped region and the overdoped region. These results are analogous to the pseudogap behaviour of the high-transition-temperature copper oxide superconductors and imply that the smooth continuation of the superconducting gap into pseudogap-like behaviour could be a general property of 2D superconductivity.

The quantum spin Hall (QSH) state is a state of matter characterized by a non-trivial topology of its band structure, and associated conducting edge channels. The QSH state was predicted and experimentally demonstrated to be realized in HgTe quantum wells. The existence of the edge channels has been inferred from local and non-local transport measurements in sufficiently small devices. Here we directly confirm the existence of the edge channels by imaging the magnetic fields produced by current flowing in large Hall bars made from HgTe quantum wells. These images distinguish between current that passes through each edge and the bulk. On tuning the bulk conductivity by gating or raising the temperature, we observe a regime in which the edge channels clearly coexist with the conducting bulk, providing input to the question of how ballistic transport may be limited in the edge channels. Our results represent a versatile method for characterization of new QSH materials systems.

These authors contributed equally to this work.The ability to control materials properties through interface engineering is demonstrated by the appearance of conductivity at the interface of certain insulators, most famously the {001} interface of the band insulators LaAlO 3 (LAO) and TiO 2 -terminated SrTiO 3 (STO) 1,2 . Transport and other measurements in this system display a plethora of diverse physical phenomena 3-14 . To better understand the interface conductivity, we used scanning superconducting quantum interference device (SQUID) microscopy to image the magnetic field locally generated by current in an interface. At low temperature, we found that the current flowed in highly conductive narrow paths oriented along the crystallographic axes, embedded in a less conductive background. The configuration of these paths changed upon thermal cycling above the STO cubic to tetragonal structural transition temperature, implying that local conductivity is strongly modified by STO tetragonal domain

It is widely believed that the perovskite Sr 2 RuO 4 is an unconventional superconductor with broken timereversal symmetry. It has been predicted that superconductors with broken time-reversal symmetry should have spontaneously generated supercurrents at edges and domain walls. We have done careful imaging of the magnetic fields above Sr 2 RuO 4 single crystals using scanning Hall bar and superconducting quantum interference device microscopies, and see no evidence for such spontaneously generated supercurrents. We use the results from our magnetic imaging to place upper limits on the spontaneously generated supercurrents at edges and domain walls as a function of domain size. For a single domain, this upper limit is below the predicted signal by 2 orders of magnitude. We speculate on the causes and implications of the lack of large spontaneous supercurrents in this very interesting superconducting system.

The phase of the macroscopic electron-pair wavefunction in a superconductor can vary only by multiples of 2pi when going around a closed contour. This results in quantization of magnetic flux, one of the most striking demonstrations of quantum phase coherence in superconductors. By using superconductors with unconventional pairing symmetry, or by incorporating pi-Josephson junctions, a phase shift of pi can be introduced in such loops. Under appropriate conditions, this phase shift results in doubly degenerate time-reversed ground states, which are characterized by the spontaneous generation of half quanta of magnetic flux, with magnitude 1/2 Phi(0)(Phi(0) = h/2e = 2.07 x 10(-15) Wb) (ref. 7). Until now, it has only been possible to generate individual half flux quanta. Here we report the realization of large-scale coupled pi-loop arrays based on YBa2Cu3O7-Au-Nb Josephson contacts. Scanning SQUID (superconducting quantum interference device) microscopy has been used to study the ordering of half flux quanta in these structures. The possibility of manipulating the polarities of individual half flux quanta is also demonstrated. These pi-loop arrays are of interest as model systems for studying magnetic phenomena--including frustration effects--in Ising antiferromagnets. Furthermore, studies of coupled pi-loops can be useful for designing quantum computers based on flux-qubits with viable quantum error correction capabilities.

We present phase-sensitive evidence that the electron-doped cuprates Nd(1.85)Ce(0.15)CuO(4-y) (NCCO) and Pr(1.85)Ce(0.15)CuO(4-y) (PCCO) have d-wave pairing symmetry. This evidence was obtained by observing the half-flux quantum effect, using a scanning SQUID microscope, in c-axis-oriented films of NCCO or PCCO epitaxially grown on tricrystal [100] SrTiO3 substrates designed to be frustrated for a d(x(2)-y(2)) order parameter. Samples with two other configurations, designed to be unfrustrated for a d-wave superconductor, do not show the half-flux quantum effect.