Efficient Peltier refrigeration by a pair of normal metal/ insulator/superconductor junctions

We suggest and demonstrate in experiment that two normal metal /insulator/ superconductor (NIS) tunnel junctions combined in series to form a symmetric SINIS structure can operate as an efficient Peltier refrigerator. Specifically, it is shown that the SINIS structure with normal-state junction resistances 1.0 and 1.1 k$\Omega$ is capable of reaching a temperature of about 100 mK starting from 300 mK. We estimate the corresponding cooling power to be 1.5 pW per total junction area of 0.8 $\mu$m$^2$ at $T= 300$ mK.Comment: 7 pages, revtex, 3 figures by fax/conventional mail upon reques

Introducing Fabric Materiality in architectural fibre composites

Textiles and architecture have long been associated; second and third skins have provided shelter and protection, since early days of men. The presence of textiles in the world of architecture spans across multiple layers, far beyond the mere usage of the fabric itself as architectural structure. The term of ‘Fabric Materiality’ is suggested to represent the unique qualities of textiles, their associated techniques and tools, assets and design paradigms; it is suggested as a design approach, to be integrated in field of architecture. The research presented in this paper explores the integration of Fabric Materiality in the field of architectural fibre-composites; it suggests an alternative design and fabrication approach in architectural FRP (fibre reinforced polymers), based on textile qualities. The main constituent of the composite material is fibres, mostly applied under the form of fabrics. All standard composite forming processes apply the fabric material over a rigid mould, to obtain its final shape; no traces are left to the textile material qualities. This research suggest the integration of Fabric Materiality in the design and fabrication of architectural FRP. Enhancing its inherent capacity for self-organisation and resilient quality, Fabric Materiality suggest the release from the necessity for moulds. In architecture, this opens wider possibilities for free architectural expression: from complex free-form morphology to surface articulation and a high degree of variation. The paper will start by introducing the concept of Fabric Materiality. It will then demonstrate its integration in architectural FRP through a material system of surface elements (panels), based on pleating manipulation. It will review its qualities of as a new porous matter-structure, of structural capacities

Point-contact Andreev reflection spectroscopy of heavy-fermion-metal/superconductor junctions

Our previous point-contact Andreev reflection studies of the heavy-fermion superconductor CeCoIn$_5$ using Au tips have shown two clear features: reduced Andreev signal and asymmetric background conductance [1]. To explore their physical origins, we have extended our measurements to point-contact junctions between single crystalline heavy-fermion metals and superconducting Nb tips. Differential conductance spectra are taken on junctions with three heavy-fermion metals, CeCoIn$_5$, CeRhIn$_5$, and YbAl$_3$, each with different electron mass. In contrast with Au/CeCoIn$_5$ junctions, Andreev signal is not reduced and no dependence on effective mass is observed. A possible explanation based on a two-fluid picture for heavy fermions is proposed. [1] W. K. Park et al., Phys. Rev. B 72 052509 (2005); W. K. Park et al., Proc. SPIE-Int. Soc. Opt. Eng. 5932 59321Q (2005); W. K. Park et al., Physica C (in press) (cond-mat/0606535).Comment: 2 pages, 2 figures, submitted to the SCES conference, Houston, Texas, USA, May 13-18, 200

Spin polarized tunneling in ferromagnet/unconventional superconductor junctions

We study tunneling in ferromagnet/unconventional superconductor (F/S) junctions. We include the effects of spin polarization, interfacial resistance, and Fermi wavevector mismatch (FWM) between the F and S regions. Andreev reflection (AR) at the F/S interface, governing tunneling at low bias voltage, is strongly modified by these parameters. The conductance exhibits a very wide variety of features as a function of applied voltage.Comment: Revision includes new figures with angular averages and correction of minor error

Transaction authorization and alert system

An automated method for alerting a customer that a transaction is being initiated and for authorizing the transaction based on a confirmation/approval by the customer thereto. In accordance with one illustrative embodiment, a request to authorize the transaction is received, wherein the request includes a customer identifier; a determination is made whether to authorize the transaction based on the customer identifier; if the determination is to authorize the transaction, that fact is communicated to the customer; a confirmation that the transaction should, in fact, be authorized is received back from the customer; and the transaction is authorized in response to the customer's confirmation thereof. In accordance with another illustrative embodiment, a transaction initiated by an agent of the customer (i.e., the principal) is authorized by the principal when one or more threshold parameters that may be pre-defined by the principal are exceeded. A preferred method of alerting the customer and receiving a confirmation to authorize the transaction back from the customer is illustratively afforded by conventional two-way pagers.Published versio

Theory of anomalous magnetic interference pattern in mesoscopic SNS Josephson junctions

The magnetic interference pattern in mesoscopic SNS Josephson junctions is sensitive to the scattering in the normal part of the system. In this paper we investigate it, generalizing Ishii's formula for current-phase dependence to the case of normal scattering at NS boundaries in an SNS junction of finite width. The resulting flattening of the first diffraction peak is consistent with experimental data for S-2DEG-S mesoscopic junctions.Comment: 6 pages, 5 figures. Phys. Rev. B 68, 144514 (2003

Symmetry and Temperature dependence of the Order parameter in MgB2 from point contact measurements

We have performed differential conductance versus voltage measurements of Au/MgB2 point contacts. We find that the dominant component in the conductance is due to Andreev reflection. The results are fitted to the theoretical model of BTK for an s-wave symmetry from which we extract the value of the order parameter (Delta) and its temperature dependence. From our results we also obtain a lower experimental bound on the Fermi velocity in MgB2.Comment: 7 pages (Including figure captions) and 4 figure

Josephson current through a long quantum wire

The dc Josephson current through a long SNS junction receives contributions from both Andreev bound states localized in the normal region as well as from scattering states incoming from the superconducting leads. We show that in the limit of a long junction, this current, at low temperatures, can be expressed entirely in terms of properties of the Andreev bound states at the Fermi energy: the normal and Andreev reflection amplitudes at the left-hand and at the right-hand S-N interface. This has important implications for treating interactions in such systems.Comment: 25 pages, 5 figure

Nonlinear Transport through NS Junctions due to Imperfect Andreev Reflection

We investigate a normal metal -- superconductor (point) contact in the limit where the number of conducting channels in the metallic wire is reduced to few channels. As the effective Fermi energy drops below the gap energy, a conducting band with a width twice the Fermi energy is formed. Depending on the mode of operation, the conduction band can be further squeezed, leading to various non-linear effects in the current-voltage characteristics such as current saturation, a N-shaped negative differential resistance, bistability, and hysteresis.Comment: 4 pages, RevTeX, three postscript figure