Presentation on theme: "Effect of Band Structure on Quantum Interference in Multiwall Carbon Nanotubes Reference Bernhard Stojetz et al. Phys.Rev.Lett. 94, 186802 (2005) Suzuki-Kusakabe."— Presentation transcript:
Effect of Band Structure on Quantum Interference in Multiwall Carbon Nanotubes Reference Bernhard Stojetz et al. Phys.Rev.Lett. 94, (2005) Suzuki-Kusakabe lab Yoshihisa MINAMIGAWA
Various applications of CNT-FET The nanotube FET is hopeful to be used as –Logic circuit, –Single electron transistor (SET), –Spin FET. CNT-FET is Field Effect Transistor using Carbon Nanotube.
Carbon Nanotubes Armchair tube Zigzag tube Single wall carbon nanotube (SWNT) Multi wall carbon nanotube (MWNT) Density of states
Structure of CNT-FET Gate A single nanotube transistor. A semiconducting nanotube is used. A single electron transistor built from a single nanotube.
Electric field effect to CNT CNT Gate Insulator When the gate voltage is positive… Au When the gate voltage is negative…
1D Density of States for free electron systems
DOS of SWNT Zigzag tube Armchair tube DOS (states/unit cell) (14,0) (14,14)
The purpose of the paper This paper reports… Measurement of conductance of a carbon nanotube under Gate voltage and Magnetic field. Determination of the Chirality of carbon nanotube by conductance measurement is expected to be possible. Reference Bernhard Stojetz et al. Phys.Rev.Lett. 94, (2005)
Gate voltage U dependence of conductance G The bottom of the curve at 300K is nearly = -0.2V. 300K 10K 1K 30mK The fine fluctuation at 30mK is due to the Coulomb Blockade. CNP: Charge neutrality point The fluctuation at 10K and 1K is due to the Universal Conductance Fluctuation (UCF) and the band structure.
Conductance G(U) in Magnetic fields perpendicular to the tube axis B T) =const. B=0T T=10K
The deviation from the zero-field conductance G(U,B)-G(U,B=0) U* -0.2V U* The Magnetoconductance disappears at certain gate voltages U*. The Magnetic fields independence of conductance G under the gate voltage U*. These gate voltages U* are grouped symmetrically around U -0.2V.
Density of states of SWNT (140,140) Black line : DOS of SWNT Gray line : The number of excess electrons on the tube ( N) When fermi level overlaps van Hove singularity, We expect big change in magnetoconductance when the Fermi level of the nanotube come across the singularity.
Relation of U* and N* To confirm next assumptions 1. The current mainly flows in the outermost tube, 2. The chirality of the tube is given by (140,140), 3. Charge is induced by the gate voltage, the next relations U* and * was checked. U*=U*+0.2 (V) U*: Singular points in Fig.b N*: The number of excess electrons on the tube. Circles : Present experiment Triangles : Reference data
Theoretical calculation of the conductance G : Phase coherent length of the electrons W : Diameter of nanotube L : Length of nanotube
Theoretical calculation of the conductance G G(U,B)-G(U,B=0) Calculation data Experimental data U*
Conclusion Phase coherent length is very short at the onset of a subband. Theoretical explanation is unknown. It is expected that deviation from zero-field conductance G(U,B)-G(U,B=0) determines van Hove singularities and structure of the tube.