JP3532744B2 - Control method of electron spin state on sample surface using scanning tunneling microscope and bit readout method thereof - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for controlling an electron spin state on a sample surface using a scanning tunneling microscope and a bit reading method therefor.

[0002]

2. Description of the Related Art Conventionally, the interaction between spins is a quantum mechanical principle in which the spin state in a molecular orbital formed when an STM tip and a sample approach each other is strictly determined as the spin multiplicity. Are known.

That is, in order to determine the electronic state of the sample surface including the spin state, there is a method of producing a cluster model of the sample surface and introducing a molecular orbital method which is widely used at present.

[0004]

The electronic theory of solids including the atomic structure is represented by the conventional band theory and molecular orbital method, but the calculation scale is proportional to the third to fourth power of the number of electrons. As a result, the amount of data processing must be enormous. In fact, nanoscale devices are expected to contain more atoms than current computational limits, so
It is extremely difficult to make calculations that are reliable enough to predict and control their properties.

[0005] The present invention is, in view of the above circumstances, proceed one step, it focused on the use of spin material surface as the minimum bit memory, the electronic spin state of the sample surface using a scanning tunneling microscope It is an object to provide a control method and a bit reading method thereof.

[0006]

In order to achieve the above object, the present invention provides [ 1 ] a method for controlling an electron spin state of a sample surface using a scanning tunneling microscope, which comprises a tip of a scanning tunneling microscope probe. Adsorb a molecule whose spin state is well known, bring it close to the sample surface structural unit, tunnel electrons to the positively ionized sample surface structural unit from the probe, and by interaction with the adsorbed molecule on the probe, The electrons are put into any orbits on the sample surface structure unit, and the sample surface structure unit is converted into bits by the difference in spin multiplicity.

[ 2 ] In a method of reading a bit of an electron spin state on a sample surface using a scanning tunneling microscope, a molecule having a well-known spin state is adsorbed on the tip of a scanning tunneling microscope probe to form a sample surface structural unit. , Tunneling electrons to the positively ionized sample surface structure unit from the probe, and by interaction with adsorbed molecules on the probe, put the electron into any orbit on the sample surface structure unit, The sample surface structure unit is bit-formed by the difference in spin multiplicity, and the sample surface bit-ized is scanned with a scanning tunneling microscope probe,
Bits are read out depending on the difference in spin states.

According to the present invention, as described above, the multiplicity of sample molecules attached to the sample surface is controlled by exchanging electrons and atoms between the probe of the scanning tunneling microscope and the sample. Once the multiplicity of a molecule is determined, it does not change even when excited and maintains a stable spin state. Here, molecules with different multiplicities have different electronic states, so when other molecules (substances) are brought closer,
The intermolecular potential formed by both changes depending on the multiplicity.

Utilizing this property, when observing a sample substance with a probe of an atomic force microscope, a difference in force applied to the probe occurs due to a difference in multiplicity. It is possible to control the multiplicity on the sample surface by using the change of the sample surface state by the scanning tunneling microscope and the observation by the atomic force microscope.

[0010]

BEST MODE FOR CARRYING OUT THE INVENTION Embodiments of the present invention will be described in detail below with reference to the drawings.

FIG. 1 is a schematic diagram of an STM according to the present invention and a diagram for explaining the operation principle.

In this figure, 1 is a sample (substance) surface, 2 is an STM probe, 5 is an X piezo (X axis drive piezoelectric element), 6 is a Y piezo (Y axis drive piezoelectric element), and 7 is a Z piezo ( Z-axis driving piezoelectric element).

The STM is a method in which a sharp probe is brought close to the material surface, an electric field is applied between the probe and the material surface, and the material surface is scanned so that the current flowing at that time is kept constant. It is a device for observing the structure.

The scanning of the material surface 1 is performed by scanning the STM probe 2 with x,
By using a molecule (a substance having piezoelectric properties) as a support rod that supports from three directions of y and z, and applying an electric field, it is possible to move minutely in each direction. At this time, considering that both the STM probe 2 and the material surface 1 are composed of atoms, the STM probe 2
-In the material surface 1, interaction between electrons on the atomic scale occurs. In other words, observing the structure of the material surface 1 with the STM probe 2 means that the height of a partial portion is detected by the STM probe 2 like tracing Braille, and the entire image is reproduced by a computer.

First, the relationship between the molecular orbital and the electron spin will be described.

Here, for example, the sample (substance) is assumed to be a silicon surface [Si (111)] and hydrogen atoms adsorbed on the surface, and the STM probe 2 is assumed to be aluminum. There is. Aluminum is not an atom, but the probe 2 is sharpened on an atomic scale, and since it behaves molecularly at the tip of the probe, it was used. Note that these combinations can be widely applied to combinations of other elements.

In order to determine the electronic state of the material surface including the spin state, there is a method of preparing a cluster model of the material surface and introducing the molecular orbital method which is widely used at present. For example, when the silicon surface, the STM probe (Al) near the surface, and the surface adsorbed atom (H) are produced as one cluster model, the surface adsorbed atom between the material surface and the STM probe is caused by the spin multiplicity of the system. It has been revealed that the potential energy surfaces of the are significantly different, as shown in FIGS. 4 and 5.

For this reason, regions having different spin multiplicities were formed on the surface of the material, and spin control was added to the atom manipulation method, which conventionally relied only on electric field pulses and contacts, to utilize magnetic fields and light. You will be able to.

Next, the control of the electron spin state on the material surface will be described.

An atomic scale structure can be formed on the surface of a substance by using an STM probe. It is known that the same operation can be performed using an AFM (atomic force microscope) probe even if the material surface is an insulator. It is known that even if the same structure is formed on the surface of a substance and a stable state is revealed, the electronic state of the entire system greatly differs depending on the number of electrons constituting the substance and the spin multiplicity of electrons.

Therefore, if it is possible to control the number of electrons and the spin multiplicity of such a surface structure, it is possible to stably convert the electronic state of the material surface into bits by arranging the surface structure as a unit. is there.

(1) First, the bit formation of the surface structure unit according to the present invention due to the difference in the number of electrons will be described.

The control of the number of electrons in the material surface structural unit is performed by bringing the STM probe close to the material surface structural unit, positively biasing the material surface, and applying an electric field higher than the ionization energy of the material of the STM probe. It is considered that electrons can be emitted from the STM probe to the material surface. Here, when aluminum and probe of a material, the ionization energy is 6.0 eV.

Similarly, by negatively biasing the material surface and applying an electric field higher than the ionization energy of the material surface structural unit, electrons can be emitted from the material surface to the STM probe.

In this way, as shown in FIG. 2 (a), the substance surface structural unit can be negatively charged or positively charged. However, if the STM probe 2 is brought too close, a tunnel current flows between the STM probe 2 and the material surface 1 as shown in FIG. 2B, and the localized electronic state cannot be changed. Further, as shown in FIG. 2C, when the STM probe 2 is too far away, electrons are exchanged between the other structural units in the vicinity and the probe.

That is, as shown in FIG.
If the M probe 2 gets too close to the material surface 1, the STM probe 2
Large electrons flow from the tip. That is, since electrons freely flow, electrons continuously enter and leave the orbits of both the surface of the material 1 and the STM probe 2, and the surface of the material 1
The electronic state of 1 changes with a slight change between the STM probe 2 and the material surface 1.

Further, as shown in FIG. 2 (c), when the STM probe 2 is too far away from the material surface 1, the electrons interact with a wide range of the material surface 1 and only the adsorbed hydrogen atoms interact with the electrons. It doesn't always work.

Further, it is necessary that there is a barrier between the surface of the substance and the structural unit of the substance surface which is equal to or greater than the threshold value of the tunnel current between the probe and the surface of the substance.

(2) Next, bit formation of the material surface structure unit due to the difference in spin multiplicity will be described.

FIG. 3 is an explanatory view showing a case where one electron is given to the positive ionized substance surface structural unit according to the embodiment of the present invention.

The material surface structural unit (material surface) is shown in FIG.
There can be three electronic states in the upper row. 1 electron from here
Remove the pieces and charge the surface positively.

Then, the STM probe is added to the positively ionized surface structural unit having three electronic states in the middle of FIG.
When electrons in the (up) spin state are given, the case in the lower part of FIG. 3 occurs. At this time, the material surface electrically returns to neutral. More specifically, (a) in the upper left electronic state, if an electron with up spin is removed, an electron state like the middle one is obtained, and from this state, an electron with up spin is removed. When given, the electronic state shown in the lower row (the electronic state similar to the upper left row) is obtained. Here, HOMO (Highes
t Occupied Molecular Orbi
The electron spin of (tal: highest occupied molecular orbital) is up, and the spin state of the entire system is low spin Sta.
te (low spin state).

(B) In the electronic state at the upper center, d
When you remove an electron with a down (down) spin,
The electronic state is as shown in the middle section on the left side.

(C) When in the upper right electronic state, d
When an electron with an own spin is removed, it becomes an electronic state as in the middle middle stage, and when an electron with an up spin is given from this state, an electronic state as in the lower stage (an electronic state similar to the upper right side) become. Here, the orbit of HOMO is closed shell, and the spin state of the whole system is low spi
n state.

When in the upper right electronic state, u
When an electron having a spin of p is removed, it becomes an electronic state as in the middle stage, and when an electron having a spin of up is given, the electronic state becomes as in the lower stage.

Here, the atomic spin of HOMO is do
wn, and the spin state of the entire system is high spin.
It becomes a state (high spin state).

The above will be summarized and explained below. STM
Adsorb molecules with a well-known spin state to the tip of the probe,
Electrons are tunneled from the STM probe to the material surface structure unit that has been positively ionized in the above process (a). At this time, the STM determines which orbit the electron enters on the structural unit of the material surface.
Due to the interaction with the adsorbed molecules on the probe.

The interaction process is divided into cases as shown in Table 1 below. However, it is assumed that the molecule adsorbed on the STM probe is selected so that the HOMO easily has electrons having spin in the up state. The initially electrically neutral material surface structural unit has a spin state of lo
w spin state or high spin st
Although it is in one of the states of ate, the spin state has changed due to positive ionization. H at this time
The spin state of OMO is up or d if there is one electron.
If one of the two is taken and there are two electrons, then HOM
O becomes a closed shell. An electron is now given to this HOMO from the STM probe.

[0039]

[Table 1]

The first spin state is low spin s
In the case of tate, the spin state can be changed from low to high by adsorbing a molecule that easily takes the up state to the STM probe and executing the above process. Similarly, the first spin state is high spin
In the case of the state, by adsorbing the molecule that easily takes the down state to the STM probe,
The spin state can be changed to ow. By this operation, the substance surface structural unit becomes electrically neutral.

(3) Further, reading of information from the bit-structured substance surface structural unit of the present invention will be described.

When the arranged substance surface structural units are scanned by the AFM, in the case of the substance surface whose electronic state is changed by the difference in the number of electrons as in the above (a), it is different from when the substance surface structural units are neutral. It is expected that different attractive or repulsive forces will appear. It is considered that a stronger attractive force acts on the charged material surface structural unit.

As described in (b) above, when the material surface structures having different multiplicities are arranged and scanned by the STM probe, hi is
gh spin state and low spin st
Distinguished as the difference in ate.

FIG. 6 is an explanatory view (1) of a method of arranging material surface structures having different multiplicities and scanning with an STM probe to read out a bit, showing an embodiment of the present invention, and FIG. 7 is an embodiment of the present invention. It is explanatory drawing (the 2) of the method of arranging the sample surface structures from which the multiplicity which shows an example differs, and scanning it with an STM probe, and reading a bit.

In these figures, 1 is a sample (substance) surface, 2 is an STM probe, and 3 is a molecule that easily takes an up-spin state.

The portion A of the sample surface 1 has an up spin of H
Since it is in the OMO, it repels the up spin of molecules attached to the tip of the STM probe 2. Therefore, when observed by STM, the distance between the STM probe 2 and the sample surface 1 becomes large, and therefore it looks high on the topography.

On the other hand, since the down spin of the portion B can enter the same orbit as the electron having the up spin on the STM probe 2, the distance between the STM probe and the sample surface 1 becomes small. Since the HOMO of the portion C is a closed shell, the topography is unlikely to depend on the spin state of the STM probe 2. STM using normal STM probe 2
The bright part has an up spin, the dark part has a down spin as compared with the topography, and the part which is not different from the normal part shows that the electronic state of the sample surface 1 is a closed shell.

Comparing the topographs of FIGS. 6 and 7, u
In the part A having p spin, it is expected that the STM probe 2 having up spin shows a position higher than that of a normal topograph due to repulsion of spin of the same kind. On the other hand, in part B,
Down spin of sample surface 1 and up of STM probe 2
Since the spins share the same orbit, the distance between the STM probe 2 and the sample surface 1 is small. In part C,
The interaction between the spins of the orbital electrons of the STM probe 2 and the orbital electrons of the sample surface 1 is smaller than that in the portions A and B.

The present invention is not limited to the above embodiments, and various modifications can be made based on the spirit of the present invention, and these modifications are not excluded from the scope of the present invention.

[0050]

As described in detail above, according to the present invention, the following effects can be achieved.

[0051] (A) a spin material surface, focused on the use as a minimum bit memory, to provide a control method and a bit reading method of the electron spin state of the sample surface using a scanning tunneling microscope You can

(B) In the atomic scale logic element considered as one of the possibilities of the spin element, the state of electron spin and the multiplicity can be introduced as a signal for quantum computing.

[Brief description of drawings]

FIG. 1 is a schematic diagram of an STM according to the present invention and a diagram for explaining the operation principle.

FIG. 2 is a diagram showing a positional relationship between an STM probe and a material surface according to the present invention.

FIG. 3 is an explanatory diagram in the case where one electron is given to a positive ionized substance surface structural unit showing an example of the present invention.

4 is a characteristic diagram of a sample atomic distance and total energy.

FIG. 5 is a diagram showing a chip sample distance.

FIG. 6 is an explanatory view (No. 1) of a method of arranging material surface structures having different multiplicities and scanning with an STM probe to read out bits according to an embodiment of the present invention.

FIG. 7 is an explanatory view (No. 2) of a method of arranging material surface structures having different multiplicities and scanning with an STM probe to read out a bit according to an embodiment of the present invention.

[Explanation of symbols]

1 Sample (substance) surface 2 STM probe Molecules that easily take the state of 3 up spin 5 X piezo (X axis drive piezoelectric element) 6 Y piezo (Y axis drive piezoelectric element) 7 Z Piezo (Z axis drive piezoelectric element)

─────────────────────────────────────────────────── --Continued from the front page (56) Reference JP-A-8-147775 (JP, A) JP-A-6-325579 (JP, A) JP-A-6-36364 (JP, A) JP-A-8- 8442 (JP, A) JP-A-9-121066 (JP, A) JP-A-5-160385 (JP, A) (58) Fields investigated (Int.Cl. ⁷ , DB name) G01N 13 / 10-13 / 24 G12B 21/00-21/24 G11B 9/00-9/14 JISST file (JOIS)

Claims (2)

(57) [Claims] 1. (a) A molecule of which spin state is well known is adsorbed on the tip of a scanning tunneling microscope probe to bring it close to a sample surface structural unit, and an electron is emitted from the probe to the positively ionized sample surface structural unit. Tunneling, (b) causing the electrons to enter any orbit on the sample surface structural unit by interaction with adsorbed molecules on the probe, (c) the sample surface structural unit due to a difference in spin multiplicity A method for controlling the electron spin state on the surface of a sample using a scanning tunneling microscope, which is characterized by performing bit conversion. 2. (a) A molecule of which spin state is well known is adsorbed on the tip of a scanning tunneling microscope probe to bring it close to the sample surface structural unit, and electrons are emitted from the probe to the positively ionized sample surface structural unit. Tunneling, (b) causing the electrons to enter any orbit on the sample surface structural unit by interaction with adsorbed molecules on the probe, (c) the sample surface structural unit due to a difference in spin multiplicity And (d) scanning the surface of the sample that has been converted into a bit with a scanning tunneling microscope probe, and reading the bit according to the difference in spin state.