Observation of shell structure in sodium nanowires

The quantum states of a system of particles in a finite spatial domain in general consist of a set of discrete energy eigenvalues; these are usually grouped into bunches of degenerate or close-lying levels, called shells. In fermionic systems, this gives rise to a local minimum in the total energy when all the states of a given shell are occupied. In particular, the closed-shell electronic configuration of the noble gases produces their exceptional stability. Shell effects have previously been observed for protons and neutrons in nuclei and for clusters of metal atoms. Here we report the observation of shell effects in an open system - a sodium metal nanowire connecting two bulk sodium metal electrodes, which are progressively pulled apart. We measure oscillations in the statistical distribution of conductance values, for contact cross-sections containing up to a hundred atoms or more. The period follows the law expected from shell-closure effects, similar to the abundance peaks at `magic numbers' of atoms in metal clusters.Comment: The argumentation in favour of shell structure owing to the fluctuations in the free energy of the nanowires has been strengthened. Further improvements in the presentation include the plot of the radius of the wires versus shell number in Fig.

Electron-vibration interaction in single-molecule junctions: from contact to tunneling regime

Point contact spectroscopy on a H2O molecule bridging Pt electrodes reveals a clear crossover between enhancement and reduction of the conductance due to electron-vibration interaction. As single channel models predict such a crossover at transmission probability of t=0.5, we used shot noise measurements to analyze the transmission and observed at least two channels across the junction where the dominant channel has t=0.51+/-0.01 transmission probability at the crossover conductance, which is consistent with the predictions for single-channel models.Comment: 4 pages, 1 table, 4 figure

Atomic size oscillations in conductance histograms for gold nanowires and the influence of work hardening

Nanowires of different nature have been shown to self-assemble as a function of stress at the contact between two macroscopic metallic leads. Here we demonstrate for gold wires that the balance between various metastable nanowire configurations is influenced by the microstructure of the starting materials and we discover a new set of periodic structures, which we interpret as due to the atomic discreteness of the contact size for the three principal crystal orientations.Comment: This version corrects an error in attributing the three observed periods, and includes a comparison with recent model calculation

Upaya Peningkatan Prestasi Hasil Belajar Ekonomi Dengan Pembelajaran Kooperatif Tipe TPS (Think Pair Share) Pada Kelas IX A SMP Negeri 1 Jabiren Raya Kabupaten Pulang Pisau

Tujuan Penelitian ini adalah untuk mengetahui apakah penerapan pembelajaran kooperatif tipe Think Pair Share (TPS) dapat meningkatkan hasil belajar IPS/ekonomi pada siswa kelas IX.A SMPN-1 Jabiren Raya Tahun pelajaran 2014/2015. Penelitian ini menggunakan metode Penelitian Tindakan Kelas (PTK) yang dilaksanakan sebanyak dua siklus di mana masing-masing siklus dilalui dengan empat tahapan, yaitu: (1) perencanaan tindakan; (2) pelaksanaan tindakan; (3) observasi tindakan; dan (4) refleksi tindakan. Subjek penelitian adalah seluruh siswa kelas SMPN-1 Jabiren Raya Tahun pelajaran 2014/2015 sebanyak 31 siswa dengan komposisi 15 laki-laki dan 16 perempuan. Penelitian ini dilaksanakan secara kolaborator bersama dengan guru mata pelajaran IPS/Ekonomi. Teknik pengumpulan data menggunakan observasi, teknik evaluasi atau tes, dokumentasi, dan wawancara. Hasil penelitian ini yaitu penerapan pembelajaran kooperatif tipe TPS dapat meningkatkan hasil belajar IPS/Ekonomi pada kelas IX.A SMPN-1 Jabiren Raya Tahun pelajaran 2014/2015. Hal tersebut didukung oleh fakta-fakta sebagai berikut: (1) Keaktifan siswa dalam apersepsi meningkat sebanyak 14%. Hasil tersebut ditunjukkan pada siklus 1 sebesar 58% (21 siswa) dan pada siklus 2 sebesar 72% (26 siswa); (2) Keaktifan siswa dalam mengikuti pembelajaran kooperatif tipe TPS meningkat sebanyak 16%. Hasil tersebut ditunjukkan pada siklus 1 sebesar 61% (22 siswa) dan pada siklus 2 sebesar 77% (28 siswa); (3) Keaktifan siswa dalam diskusi berpasangan/kelompok meningkat sebanyak 20%. Hasil tersebut ditunjukkan pada siklus 1 sebesar 61% (22 siswa) dan pada siklus 2 sebesar 81% (29 siswa); (4) Ketuntasan hasil belajar meningkat sebesar 15%. Hasil tersebut ditunjukkan pada siklus 1 sebesar 68% (23 siswa) dan pada siklus 2 sebesar 83% (29 siswa); (5) Keaktifan siswa dalam diskusi mengalami peningkatan terbesar dibandingkan dengan keaktifan dan ketuntasan hasil belajar siswa lainnya. Hal tersebut disebabkan siswa memberi respon yang positif terhadap tugas yang diberikan guru dengan aktif berdiskusi dalam pasangan/kelompok

Aluminium Nanowires: Influence of Work Hardening on Conductance Histograms

Conductance histograms of work-hardened Al show a series up to 11 equidistant peaks with a period of 1.15 +/- 0.02 of the quantum conductance unit G_0 = 2e^2/h. Assuming the peaks originate from atomic discreteness, this agrees with the value of 1.16 G_0 per atom obtained in numerical calculations by Hasmy et al.Comment: 4 pages, 4 figure

Spin Torques in Point Contacts to Exchange-Biased Ferromagnetic Films

Hysteretic magneto-resistance of point contacts formed between non-magnetic tips and single ferromagnetic films exchange-pinned by antiferromagnetic films is investigated. The analysis of the measured current driven and field driven hysteresis agrees with the recently proposed model of the surface spin-valve, where the spin orientation at the interface can be different from that in the bulk of the film. The switching in magneto-resistance at low fields is observed to depend significantly on the direction of the exchange pinning, which allows identifying this transition as a reversal of interior spins of the pinned ferromagnetic films. The switching at higher fields is thus due to a spin reversal in the point contact core, at the top surface of the ferromagnet, and does not exhibit any clear field offset when the exchange-pinning direction or the magnetic field direction is varied. This magnitude of the switching field of the surface spins varies substantially from contact to contact and sometimes from sweep to sweep, which suggests that the surface coercivity can change under very high current densities and/or due to the particular microstructure of the point contact. In contrast, no changes in the effect of the exchange biasing on the interior spins are observed at high currents, possibly due to the rapid drop in the current density away from nanometer sized point contact cores.Comment: 3 pages, 3 figs, presented on 11th Joint MMM-Intermag Conference, Jan. 18-22, 2010, Washington, US

Directional photoelectric current across the bilayer graphene junction

A directional photon-assisted resonant chiral tunneling through a bilayer graphene barrier is considered. An external electromagnetic field applied to the barrier switches the transparency $T$ in the longitudinal direction from its steady state value T=0 to the ideal T=1 at no energy costs. The switch happens because the a.c. field affects the phase correlation between the electrons and holes inside the graphene barrier changing the whole angular dependence of the chiral tunneling (directional photoelectric effect). The suggested phenomena can be implemented in relevant experiments and in various sub-millimeter and far-infrared optical electronic devices.Comment: 7 pages 5 figure