Acoustic plasmon

Abstract The superconducting state in the novel family of layered metalochloronitrides is provided by electronic collective excitations, the acoustic plasmons. These plasmons are a characteristic feature of layered conductors. This is the first experimentally observed case of self-supported superconductivity. 

One can show, and this is the subject of our paper, that we are dealing with the remarkable case of self-supported superconductivity. The pairing is provided by the exchange of unusual acoustic plasmons, that is, by collective excitations of the same electronic system, which undergoes the superconducting transition. 

The acoustic branch can contribute to the pairing in a way similar to usual acoustic phonons. Theoretically the pure acoustic plasmon mechanism was studied in [11]. Such a mechanism in conjunction with phonon contribution (phonon-plasmon mechanism) was proposed in [9], The equation for the order... 

Figure 1. Collective excitations in a layered conductor. At ui > vfk there are one optical and Nc — 1 acoustic plasmon branches (Nc is the number of conducting planes).

In summary, it is shown that the superconducting state of the novel family of layered superconductors, the metalochloronitrides, is provided by the plasmon mechanism, or more specifically, by the exchange of acoustic plasmons, the existence of which is specific to layered conductors. To the best of our knowledge, this is the first observed system with self-supported superconductivity, that is, with the pairing provided by collective modes of the same electrons as those undergoing the superconducting transition. 

Bulk and surface imprinting strategies are straightforward tools to generate artificial antibodies. Combined with transducers such as QCM (quartz crystal microbalance), SAW (surface acoustic wave resonator), IDC (interdigital capacitor) or SPR (surface plasmon resonator) they yield powerful chemical sensors for a very broad range of analytes. 

However, the determination of affinity does not necessarily have to rely on labeled ligands. It is also possible with native ligands when using suitable detection methods, as for example nuclear magnetic resonance (NMR), surface plasmon resonance (SPR), acoustic biosensors or calorimetry [48, 49]. A particularly versatile and sensitive detection principle for the investigation of interactions between targets and native ligands is mass spectrometry [50]. 

A number of methods are available for the characterization and examination of SAMs as well as for the observation of the reactions with the immobilized biomolecules. Only some of these methods are mentioned briefly here. These include surface plasmon resonance (SPR) [46], quartz crystal microbalance (QCM) [47,48], ellipsometry [12,49], contact angle measurement [50], infrared spectroscopy (FT-IR) [51,52], Raman spectroscopy [53], scanning tunneling microscopy (STM) [54], atomic force microscopy (AFM) [55,56], sum frequency spectroscopy. X-ray photoelectron spectroscopy (XPS) [57, 58], surface acoustic wave and acoustic plate mode devices, confocal imaging and optical microscopy, low-angle X-ray reflectometry, electrochemical methods [59] and Raster electron microscopy [60]. 

Metal labels have been proposed to resolve problems connected with enzymes. Metal ions [13-16], metal-containing organic compounds [17,18], metal complexes [19-21], metalloproteins or colloidal metal particles [22-28] have served as labels. Spectrophotometric [22,25], acoustic [25], surface plasmon resonance, infrared [24] and Raman spectroscopic [28] methods, etc. were used. A few papers have been dealing with electrochemical detection. However, electrochemical methods of metal label detection may be viewed as very promising taking into account their high sensitivity, low detection limit, selectivity, simplicity, low cost and the availability of portable instruments. 

The transducers most commonly employed in biosensors are (a) Electrochemical amperometric, potentiometric and impedimetric (b) Optical vibrational (IR, Raman), luminescence (fluorescence, chemiluminescence) (c) Integrated optics (surface plasmon resonance (SPR), interferometery) and (d) Mechanical surface acoustic wave (SAW) and quartz crystal microbalance (QCM) [4,12]. 

The piezo-electric effect of deformations of quartz under alternating current (at a frequency in the order of 10 MHz) is used by coating the crystal with a selectively binding substance, e. g. an antibody. When exposed to the antigen, an antibody-antigen complex will be formed on the surface and shift the resonance frequency of the crystal proportionally to the mass increment which is, in turn, proportional to the antigen concentration. A similar approach is used with surface acoustic wave detectors [142] or with the surface plasmon resonance technology (BIAcore, Pharmacia). 

One can show also [10] that the density of states for the plasmon band is peaked at the upper (qz = 0) and the lower (qz = n/dc) branches. As a result, the plasmon band can be modeled as a set of two branches (see Fig. 1). The upper branch is similar to that in the 3D case. The most important factor is the existence of low acoustic branches ( electronic sound). 

The reaction between the analjrte and the bioreceptor produces a physical or chemical output signal normally relayed to a transducer, which then generally converts it into an electrical signal, providing quantitative information of analytical interest. The transducers can be classified based on the technique utilized for measurement, being optical (absorption, luminescence, surface plasmon resonance), electrochemical, calorimetric, or mass sensitive measurements (microbalance, surface acoustic wave), etc. If the molecular recognition system and the physicochemical transducer are in direct spatial contact, the system can be defined as a biosensor [76]. A number of books have been published on this subject and they provide details concerning definitions, properties, and construction of these devices [77-82]. 

Quartz crystal microbalance (QCM) Surface plasmon resonance (SPR) Piezoelectric quartz crystal (PQC) Bulk acoustic wave (BAW) surface acoustic wave (SAW) 4-vinylpyridine (4-VP) Ethylene glycol dimethacrylate (EDMA) Methacrylic acid (MAA), Ethylene glycol dimethacrylate (EDMA) IV-phenylacrylamide (PAM) Diethylamino ethyl methacrylate (DEAEM). 

We have now found that the antierossing regime, whieh is characterized by the huge interlayer electric field and by the fact that the very strong optical-like plasmon shares its amplitude equally with the otherwise weak acoustic-like plasmon, can be accessed... 

Another important area where gold-thiol monolayers might find promising applications is gas- and biosensing. Simple sensors sensitive to certain types of compounds, based on such detection methods as surface plasmon resonance or surface acoustic wave, have been described454,455,531-533. This type of device is usually made of a gold plate coated with a functionalized monolayer. The terminal functional group of such a monolayer is responsible for selective interactions with the analyte, and adsorption of the latter is then detected by the appropriate method. 

Af/f is small whenever rq,2 is close to one. Conversely, since the QCM only works well when the normalized frequency shift Af/ff is small, it makes sense to assume 1. Equation 39 shows that quartz crystals are acoustic re-flectometers. The results of QCM measurements can therefore be easily compared to data obtained with other forms of ultrasonic reflectometry [57,58]. It is well known from optical techniques such as elUpsometry [59] or surface plasmon resonance (SPR) spectroscopy [60] that a film thickness can be inferred from a measurement of the reflectivity. The same applies to acoustics. 

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