Research in spin quantum cross structure devices and spin conduction


Spin quantum cross structure (SQCS) device diagram Correlation diagram of the physics of SQCS devices

Objective of research
We are working to propose nano-scale quantum cross structure devices that have contacts with the goal of creating next-generation, super-high-density memory and post-CMOS switching devices and of gaining an understanding of the essential nature of spin conduction and itinerant magnetism.

It is possible to create devices with a new structure with the edges of conductive thin films facing each other in a cross shape.
The conductive films facing each other consist of thin metals or semiconductors deposited on organic substrates.
The above mentioned device is a quantum cross structure device.
When a ferromagnetic thin film is used as the electrode, we call the device "spin quantum cross structure device".

If metal (semiconductor) thin films with a thickness of 1 to 20 nm are used, in principle it is possible to fabricate super-fine junctions with sizes of 1 to 20 nm.
By sandwiching a self-assembled monolayer (SAM), DNA, or other material inside two conductive thin films, it is possible to achieve characterizations of a small number of molecules or single molecules.

If an organic molecular film is sandwiched inside two conductive thin layers, the device can be applied to next-generation, super-high-density memory or post-CMOS switching devices.
Furthermore, new spin devices can be fabricated by using a ferromagnet as the electrode.

From a theoretical standpoint, we are conducting Research in transport in quantum cross structure devices as well as transport that takes into account spin using the Anderson model in an effort to implement quantum cross structure devices that provide new functionality such as this.

From an experimental standpoint, we are fabricating quantum cross structure devices consisting of organic molecules sandwiched between the edges of Ni thin films, and we are working with allied institutes to evaluate these devices' structural and electrical characteristics.
Research content (theory)
Theoretical study of spin quantum cross structure device transport characteristics When a ferromagnetic metal is used as the electrode of a quantum cross structure, the resulting device is known as a spin quantum cross structure device.
We developed a general formula for spin conduction using a non-equilibrium Green's function when electrode magnetization is non-collinear when two-level molecules are sandwiched in this structure
.
Figures 1(a) and 1(b) provide a diagram of a spin quantum cross structure device with non-collinear magnetic properties and the model used in our calculations, respectively.
Figure 1 (a) Figure 1 (b)

In the past, non-collinear calculations have taken the angle &theta into account but not Φ. For the first time, we succeeded in developing a general formula that takes the azimuth angle &theta into account.
As a result, we found that the azimuth angle does not appear overtly in the transport equation and that spin conduction depends solely on the angle &theta, which makes up the inner product when treating both magnetizations (spins) as classical vectors.
However, we also found that the energy levels of the molecules between the electrodes split due to the effects of the azimuth angle &theta if the conduction spin is allowed to flip.
Based on this fact, observance of a split in molecular levels indicates that spin inside the molecule has flipped.
The non-collinear general formula is as follows:


Figure 2 illustrates the current-voltage characteristics as calculated using the above equation when spin inside the molecule has not flipped.
Figure 3 illustrates the current's &theta dependency.
Figure 2 Figure 3


Research content (experimental): Implementation of a spin quantum cross structure device
We attempted to implement a spin quantum cross structure device by using nickel ferromagnetic metal for the electrodes and sandwiching a P3HT:PCBM organic material blend between them.

Figure 4 illustrates the method by which we fabricated this Ni/PCBM:P3HT/Ni spin quantum cross structure device.
First, we deposited a Ni thin film onto a PEN organic film.
We prepared two sets of this Ni thin film/PEN organic film combination and sandwiched each between two strips of PMMA.
Next, we polished the edges of the nickel thin film using a mechanochemical polishing process and then sandwiched [6,6]-phenyl-C61 butyric acid methyl ester (PCBM):poly-3-hexylthiophene (P3HT) between the edges of the Ni thin film.

Figure 5 illustrates the current-voltage characteristics of this Ni/PCBM:P3HT/Ni spin quantum cross structure device.
Since the Ni thin film had a thickness of 16 nm, the junction area was 16 nm X 16 nm.
As shown in Figure 5, the device exhibited ohmic characteristics, and its resistance was 32 Ω.
The fact that this value is a good quantitative match for the result of 26.2 Ω (strong coupling) yielded by a theoretical calculation using the Anderson model suggests that we captured molecular conduction in the nano domain.
Figure 4 Figure 5
                  
[1] K. Kondo, H. Kaiju, and A. Ishibashi:
"Theoretical Investigation of New Quantum-Cross-structure Device as a Candidate beyond CMOS",
Mat. Res. Soc. Symp. Proc., Vol.1067, pp.B03011-B03016 (2008).

[2] K. Kondo, H. Kaiju, and A. Ishibashi:
"Theoretical and Experimental Results of Electronic Transport of Spin Quantum Cross Structure Devices",
J. Appl. Phys., Vol.105, pp.07D522-1 07D522-3 (2009).

[3] K. Kondo:
"Theoretical modeling of spin quantum cross structure devices with noncollinear ferromagnetic electrodes",
J. Appl. Phys., Vol.107, pp.09C709-1 09C709-3 (2010).

[4] H. Kaiju, K. Kondo, A. Ono, N. Kawaguchi, J. H. Won, A. Hirata, M. Ishimaru, Y. Hirotsu and A. Ishibashi:
"The fabrication of Ni quantum cross devices with a 17 nm junction and their current-voltage characteristics",
Nanotechnology, Vol.21 pp. 015301-1-015301-6 (2010).

[5] H. Kaiju, K. Kondo, N. Basheer, N. Kawaguchi, S. White, A. Hirata, M. Ishimaru, Y. Hirotsu, and A. Ishibashi:
“Fabrication and Current-Voltage Characteristics of Ni Spin Quantum Cross Devices with P3HT:PCBM Organic Materials”,
Mater. Res. Soc. Symp. Proc., Vol.1252 pp. J02081-J02086 (2010).

[6] H. Kaiju, K. Kondo, and A. Ishibashi:
“Current-Voltage Characteristics in Nanoscale Tunnel Junctions Utilizing Thin-Film Edges”,
Jpn. J. Appl. Phys., Vol.49 pp. 105203-1-105203-5 (2010).

> Quantum cross structure device transport theory
> Electron correlation theory
> Exact calculation of quasiparticle energy





Research topics
・ Basis for connecting top-down and bottom-up phenomena
・ Electron correlation in two-dimensional electron systems
・ Exact calculation of quasiparticle energy
・ Calculation of electron density of electrons trapped at a user-specified potential
・ Fabrication and evaluation of quantum cross structure devices
・ Quantum cross structure device transport theory
・ Spin quantum cross structure device transport theory
・ Fabrication and evaluation of photoelectric conversion devices
・ Development of high-cleanliness environments


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[Address]
Laboratory of Nanostructure Physics
Research Institute for Electronic Science, Hokkaido University
Kita 20, Nishi 10, Kita-ku, Sapporo-shi 001-0020
Phone/fax: 011-706-9427