CN106449739B - Single electron spin filter and single electron spin filtering method based on quantum dots - Google Patents

Abstract

A single electron spin filter and a single electron spin filtering method based on quantum dots. The main component is a single electron transistor. The single electron transistor has a Coulomb island, a source, a drain and a gate. The quantum dot serves as a single electron transistor. Coulomb Island, the Coulomb Island is connected to the source and drain with a tunneling barrier, and the Coulomb Island is capacitively coupled to the gate; the single-electron transistor is placed in a vertical magnetic field, and the drain output of the single-electron transistor is connected to the spin electrons Readout circuit, the spintronic readout circuit is placed in a horizontal non-uniform magnetic field. The present invention adopts an external control method to turn spin-non-polarized electrons into spin-polarized electrons after passing through a single-electron transistor, and can determine and count the spin directions of electrons, which is beneficial to fast data processing and Design and application of spintronic devices with low power consumption and good stability.

Description

Single electron spin filter and single electron spin filtering method based on quantum dots

Technical field

The present invention relates to the fields of nanoelectronic devices, single electron spin and electronic technology, and in particular to a single electron spin filter based on quantum dots and a single electron spin filtering method. Specifically, it relates to a single electron spin filter using a source-drain bias. Device design for spin filtering using the Zeeman splitting energy level of quantum dots in the pressure window.

Background technique

Compared with traditional electronic devices, spintronic devices have the advantages of fast data processing speed, low power consumption, and good stability. Spintronic devices that have been successfully developed include giant magnetoresistance, spin valve, magnetic tunnel junction and magnetic random access memory. However, these spintronic devices based on ferromagnetic metals are difficult to develop spin transistors with amplification functions, and are also difficult to integrate with traditional microelectronic devices.

The spontaneous electron spin polarization rate in semimetals is almost 100%. Therefore, semimetal materials can be used as the emission source of spin polarization to study spin polarization. Common semi-metallic materials include: doped manganese oxide, double perovskite manganese oxide, chromium dioxide, iron oxide and Heussler alloy, etc. However, the Curie temperature of the semimetal is relatively low, and the electron spin polarization rate decreases rapidly as the temperature increases. These shortcomings greatly reduce its practical application value.

In 1988, J. S. Moodera and others first proposed the concept of spin filtering in tunnel junctions. According to the barrier layer material used in the tunnel junction, this type of tunnel junction can be divided into three types: ferromagnetic tunnel junction, ferroelectric tunnel junction and Multi-rail tunnel junctions (including single-phase multi-rail tunnel junctions and composite multi-rail tunnel junctions). Ferromagnetic tunnel junction refers to a tunnel junction that uses ferromagnetic insulating materials or semiconductor materials as the barrier layer. Its spin polarization comes from the spin filtering effect of the ferromagnetic semiconductor barrier. The spin filtering effect of the ferromagnetic tunnel junction originates. Due to the spin correlation of the height of the ferromagnetic barrier, factors such as the magnetism, height and width of the barrier will also have a certain impact on the filtering effect. Ferroelectric tunnel junction refers to a tunnel junction that uses ferroelectric insulating material as a barrier layer. The spontaneous polarization of the ferroelectric barrier causes shielding charges to be generated at the interface close to the barrier in the electrode, thereby forming an electrostatic potential, which distorts the energy band in the electrode, that is, electrons in the two spin-up and spin-down channels pass through the tunnel junction. The tunneling probability is different at different times, so a spin filtering effect is produced. The spin filtering effect of ferroelectric tunnel junctions originates from the fact that the effective width of the barrier layer is spin-dependent. Multiferroic tunnel junctions are made of ferromagnetic-ferroelectric multiferroic materials that are both ferromagnetic and ferroelectric as potential barriers. These two physical mechanisms that produce spin filtering effects are also possible in such multiferroics. Simultaneous action occurs in the tunnel junction.

In nonmagnetic semiconductors and topological insulators, the spin-orbit interaction of particles can produce spin polarization. The structure of silicene material is similar to that of graphene. The edges of its conduction band and valence band appear at the symmetry points of the Brillouin zone of K and K’. However, there is a warped structure in the silicene structure, which makes the spin-orbit coupling intensity in the silicene structure relatively large, resulting in a larger energy gap opened at K and K’. Since the warped structure can be changed by applying a vertical electric field, the size of the energy gap can be controlled externally. When a Zeeman field is applied, the energy band structure of silicene occupies a fully spin-polarized state near the two Dirac points of the Brillouin zone. Therefore, the spin of the current passing through the silicene two-terminal device can be fully polarized. ized, thereby achieving the purpose of controlling the polarization direction of the current. When a non-polarized current passes through a three-terminal device, currents with different spin polarization directions flow out from the other two electrodes respectively, thus achieving spin separation. However, the interface effect between the device and the electrode has a significant impact on the spin polarizability.

One of the research hotspots in condensed matter physics is to explore the characteristics of quantum systems at the nanoscale and seek a new generation of quantum electronic devices. With the development of spintronics, the ability to effectively manipulate the electron spin degree of freedom has become a focus of attention in the physics and materials communities. Quantum dots have artificial controllability, and quantum dot devices manufactured using them have achieved remarkable development, and are a hot topic in the development of nanoelectronic devices today. The spin properties of electrons in quantum dots play an important role. Using the electron spin in quantum dot devices for quantum information processing is considered to be one of the most promising directions for realizing future quantum computers. A lot of research on quantum dots was carried out in the 1990s. By now, quantum dots have become a standard technology for trapping the charge of single electrons. The electrons can be trapped for as long as you like. When an electron tunnels out of a quantum dot, the change in charge can be measured on the order of μs. Controlling individual spins and measuring the spin of individual electrons is very difficult compared to controlling charge, but fortunately, these techniques have been developed. The results show that a quantum dot can confine one or two electrons; the spin of a single electron can be controlled to be placed in a superposition state of up and down states; the two spins can be controlled to interact, and then An entangled state is formed, such as a spin singlet or a spin triplet, and the results of these manipulations can be measured using the spins independently of each other. The ability to fully control the spins of electrons independently of each other allows us to study the single-spin dynamics of fully quantum mechanisms in solid-state environments.

Quantum dots are solid artificial submicron structures that typically contain 10 ³ to 10 ⁹ atoms and a considerable number of electrons. In a semiconductor quantum dot, all but a few free electrons are tightly bound, ranging from zero to several thousand. First, the spin of each electron in a quantum dot is directly affected by the external magnetic field in the form of Zeeman energy. Second, the Pauli exclusion principle prohibits two electrons with the same spin direction from occupying the same orbit. So different electrons go into different orbits, which usually results in different energy states with different energies. Finally, Coulomb interactions lead to energy differences (energy exchange) between different energy states with symmetric and antisymmetric orbital wave functions. Because quantum dots have the characteristics of quantized energy levels, the number of beamed electrons can be controlled, and the energy levels can be controlled, we proposed a method of single electron spin filtering based on quantum dots.

Contents of the invention

The technical problem to be solved by the present invention is to provide a single electron spin filter and a single electron spin filtering method based on quantum dots in view of the problems existing in the above background technology. The quantum dot structure based on a single electron transistor adopts an external The control method makes the spin-non-polarized electrons change into spin-polarized electrons after passing through the single-electron transistor, and the spin direction of the electrons can be determined and counted.

The technical solution adopted by the present invention is: a single electron spin filter based on quantum dots. The main component is a single electron transistor. The single electron transistor has a Coulomb island, a source, a drain and a gate. The quantum dots serve as single electrons. The Coulomb Island of the transistor is connected to the source and drain with a tunneling barrier, and the Coulomb Island is capacitively coupled to the gate; the single-electron transistor is placed in a vertical magnetic field, and the drain output of the single-electron transistor is connected to the spin Electronic readout circuit, the spin electron readout circuit is placed in a horizontal non-uniform magnetic field.

In the above technical solution, the quantum dots are graphene quantum dots.

In the above technical solution, the Coulomb island, source, drain and gate of the single electron transistor are integrated and arranged on the silicon dioxide substrate formed on the surface of the silicon substrate, and the Coulomb island, tunneling barrier, source, drain A protective layer of aluminum oxide is deposited on the electrode and gate.

A single-electron spin filtering method using the above-mentioned single-electron spin filter based on quantum dots. The quantum dots serve as Coulomb islands of single-electron transistors, and their discrete energy levels undergo Zeeman splitting in a vertical magnetic field. The splitting energy levels have self-propelled properties. Spin correlation, with spin filtering effect.

In the above technical solution, the source and drain bias voltages and gate voltages of the single-electron transistor are adjusted so that the Zeeman splitting energy level of a quantum dot with a specific spin direction with spin up or spin down is located at the source and drain bias voltages. The window forms the single electron transport channel of the single electron transistor to complete single electron spin filtering; adjusting the source and drain bias voltages and gate voltages of the single electron transistor can make the quantum dot Zeeman splitting energy level neither located at the source nor the drain. Bias window, which closes the electron channel of a single-electron transistor so that no electrons are spin polarized;

In the above technical solution, the emitted electrons with spin polarization through the single-electron transistor will deflect their movement direction under the action of the horizontal non-uniform magnetic field, thereby achieving separation and detection; the magnetic moments of electrons with opposite spin directions will The orientation is opposite. When moving in a horizontal non-uniform magnetic field, the motion trajectories are separated due to different forces and reach two different detectors.

In the above technical solution, the charging energy of the single-electron transistor is greater than the Zeeman splitting energy, and the Zeeman splitting energy is greater than the thermal energy.

In the above technical solution, the source and drain biases are adjusted so that at most one Zeeman splitting energy level can be accommodated in the bias window; by adjusting the source, drain bias and gate voltage, the free electrons on the Coulomb island can be depleted, and then a single electron is injected to complete single electron spin polarization.

In the above technical solution, the source and drain bias voltages remain unchanged, and only the gate voltage is changed. A certain energy level with a specific spin direction as an electron channel can be adjusted to move up and down to open or close the electron channel.

In the above technical solution, the source and drain bias voltages remain unchanged and only the gate voltage is changed. The energy levels of adjacent quantum dots with different spin directions can be adjusted to appear in the bias window in sequence and become spin-polarized electron channels.

In the above technical solution, the emitted electrons that are spin polarized through the single electron transistor are deflected in the direction of movement under the action of the horizontal non-uniform magnetic field, are separated, and can be detected.

The invention realizes the spin polarization of a single incident electron and the readout of the spin-polarized emitted electron, which is beneficial to the design of spin electronic devices with fast data processing speed, low power consumption and good stability, and is widely used.

Description of drawings

Figure 1 is a single electron transistor model of a single quantum dot; in Figure 1, 1 is the source, 2 is the drain, 3 is the gate, 4 is the Coulomb island, and 11 is the tunneling barrier between the source and the Coulomb island. , 21 is the tunneling barrier between the drain and the Coulomb island, 31 is the coupling capacitance between the gate and the Coulomb island;

Figure 2 is a schematic diagram of the spin filter based on quantum dots; in Figure 2, 51 and 52 are the protection resistors of the single-electron transistor, 52 and 53 constitute a voltage divider for the source-drain input voltage, and the single-electron transistor is placed vertically In the magnetic field B ₁ , the spin electron readout circuit is placed in the horizontal non-uniform magnetic field B ₂ ;

Figure 3 is a schematic diagram of the principle of using few-electron quantum dots as a bipolar spin filter; in Figure 3, 12 is the source Fermi level, 22 is the drain Fermi level, 41 and 42 are the Zeeman of the Coulomb island. Split energy level;

Figure 4 is a schematic diagram of the principle of realizing different spin polarization by adjusting the gate voltage; in Figure 4, (a) is the depletion of free electrons on the quantum dot, (b) is a spin-up energy level in the quantum dot located at the source Drain bias window, (c) is a spin-down energy level in the quantum dot located in the source-drain bias window;

Figure 5 is a schematic diagram of the principle of spin filtering based on quantum dots.

Detailed ways

Referring to the drawings, the main component of the single-electron spin filter based on quantum dots of the present invention is a single-electron transistor. The single-electron transistor has a Coulomb island, a source, a drain and a gate. The quantum dot serves as the Coulomb of the single-electron transistor. The Coulomb island is connected to the source and drain with a tunneling barrier, and the Coulomb island is capacitively coupled to the gate; the single-electron transistor is placed in a vertical magnetic field, and the drain output of the single-electron transistor is connected to the spin electron readout circuit, the spin electron readout circuit is placed in a horizontal non-uniform magnetic field, the quantum dots are graphene quantum dots, and the Coulomb island, source, drain and gate of the single electron transistor are integrated on the surface of the silicon substrate On the formed silicon dioxide substrate, an aluminum oxide protective layer is deposited on the Coulomb island, tunnel barrier, source electrode, drain electrode and gate electrode.

A single-electron spin filtering method using the above-mentioned single-electron spin filter based on quantum dots. The quantum dots serve as Coulomb islands of single-electron transistors, and their discrete energy levels undergo Zeeman splitting in a vertical magnetic field. The splitting energy levels have self-propelled properties. Spin correlation, with spin filtering effect, adjusts the source, drain bias voltage and gate voltage of the single-electron transistor, so that the Zeeman splitting energy level of a quantum dot with a specific spin direction with spin up or spin down is located at The source and drain bias windows form the single electron transport channel of the single electron transistor to complete single electron spin filtering; adjusting the source, drain bias voltage and gate voltage of the single electron transistor can make the quantum dot Zeeman splitting energy levels Not located in the source and drain bias windows, the electron channel of the single-electron transistor is closed, so that no electrons are spin-polarized. The emitted electrons that are spin-polarized through the single-electron transistor move under the action of the horizontal non-uniform magnetic field. The direction will be deflected to achieve separation and detection; electrons with opposite spin directions have opposite magnetic moments. When moving in a horizontal non-uniform magnetic field, their motion trajectories are separated due to different forces and arrive at two different detectors. , the charging energy of a single-electron transistor is greater than the Zeeman splitting energy, and the Zeeman splitting energy is greater than the thermal energy. Adjust the source and drain bias voltages so that at most one Zeeman splitting energy level can be accommodated in the bias window; by adjusting the source, The drain bias and gate voltage can exhaust the free electrons on the Coulomb Island, and then inject a single electron to complete single electron spin polarization. The source and drain biases remain unchanged, and only the gate voltage is changed, which can be adjusted as an electron channel. A certain energy level with a specific spin direction moves up and down to open or close the electron channel. The source and drain biases remain unchanged and only the gate voltage is changed. The energy levels of adjacent quantum dots with different spin directions can be adjusted. , appear in the bias window in turn and become a spin-polarized electron channel.

In the above method, the emitted electrons that are spin-polarized through the single-electron transistor are deflected in the direction of motion under the action of a horizontal non-uniform magnetic field, and are separated and can be detected.

The single-electron spin filter based on quantum dots of the present invention is based on the Pauli exclusion principle and prohibits two electrons with the same spin direction of the quantum dot from occupying the same Zeeman splitting energy level, so the electrons enter a specific self-spin filter. Orbitals in the spin direction are spin polarized.

The above-mentioned spin-polarized electrons will be separated in the horizontal non-uniform magnetic field due to different magnetic moment orientations and reach different detectors to complete the detection of spin states.

To achieve single electron spin filtering, the following three steps can be taken:

(1) Deplete the free electrons on the Coulomb Island of the single-electron transistor;

(2) Inject an electron into the Coulomb Island of the single-electron transistor;

(3) Measure the spin state of the emitted electron.

If the Zeeman splitting exceeds the charging energy, the electron transport through the quantum dot is spin polarized and the quantum dot can be used as a spin filter. As shown in Figure 3, if only a single electron spin-up state energy is in the source-drain bias window, the electron is depleted on the Coulomb island and contains a free electron during the transport process, the electron is spin-up to the pole. ized.

As shown in Figure 4, in the transport process containing one free electron and two free electrons on the Coulomb island, if no excited state is allowed, because the electron channel already has an electron with an upward spin, according to Pauli The exclusion principle allows only one spin-down electron to enter the electron channel, so that the emitted electron is spin-down polarized. Therefore, by tuning the quantum dot to the relevant transport, the polarization of the spin filter can be electrically reversed.

The single electron spin filtering method of the present invention will be further described in detail below through a specific embodiment of the present invention and in conjunction with the accompanying drawings:

(1) The single-electron transistor shown in Figure 1 is used as the core component of the single-electron spin filter. The source and drain are tunnel-coupled with the Coulomb island, and the gate is capacitively coupled with the Coulomb island. As shown in Figure 2, the single-electron transistor completes the spin polarization of a single injected electron, and the spin state of the output electron is detected through the readout circuit.

(2) Adjust the source-drain bias and gate voltage, as shown in Figure 4(a), because the quantum dot spin-up energy level 41 and spin-down energy level 42 are both higher than the electron library Fermi of source 1 Energy level 12, there are no electrons on the quantum dot.

(3) Apply a positive voltage pulse to the gate, as shown in Figure 4(b), to adjust the quantum dot spin-up energy level 41 into the source-drain bias window, while the spin-down energy level 42 is still higher than The electron pool of source 1 has a Fermi level of 12. Therefore, from an energy perspective, an electron is allowed to tunnel above the energy level 41 of the quantum dot, and this electron is spin-polarized upward.

(4) If a large positive voltage pulse is applied to the gate, as shown in Figure 4(c), the quantum dot spin down energy level 42 is adjusted to the source-drain bias window, while the spin up energy level 41 Below the electron pool Fermi level 22 of drain 2. Therefore, from an energy perspective, an electron is allowed to tunnel above the energy level 42 of the quantum dot, and this electron is spin-polarized downward.

(5) After a period of time, the electrons incident on the quantum dot will tunnel to the drain 2. As shown in Figure 5, this electron will receive different forces due to different spin directions under the action of the magnetic field, and will have different characteristics. The motion trajectory is detected by the detector.

Quantum dots are a general-purpose system, and there are many different materials and forms of quantum dots. The above embodiments only illustrate the technical concepts and characteristics of the present invention. Their purpose is to enable those familiar with this technology to understand the content of the present invention and implement it accordingly. They do not limit the scope of protection of the present invention. All other corresponding changes and modifications made based on the technical solutions and technical concepts of the present invention should be covered by the protection scope of the present invention.

Claims (4)

1. A single electron spin filter based on quantum dots, characterized in that: the main component is a single-electron transistor, the single-electron transistor is provided with a coulomb island, a source electrode, a drain electrode and a grid electrode, the quantum dot is used as the coulomb island of the single-electron transistor, the coulomb island is connected with the source electrode and the drain electrode through tunneling potential barriers, and the coulomb island is coupled with the grid electrode in a capacitive mode; the single-electron transistor is arranged in a vertical magnetic field, the drain electrode output end of the single-electron transistor is connected with the spintronic reading circuit, and the spintronic reading circuit is arranged in a horizontal nonuniform magnetic field;

adjusting source and drain bias voltages and gate voltage of the single-electron transistor to enable quantum dot Zeeman splitting energy level of a certain specific spin direction with spin up or spin down to be located in source and drain bias windows, forming a single-electron transport channel of the single-electron transistor, and completing single-electron spin filtration; the source bias voltage, the drain bias voltage and the gate voltage of the single-electron transistor are regulated, so that the quantum dot zeeman splitting energy level is not located in the source bias window and the drain bias window, and the electron channel of the single-electron transistor is closed, and therefore electrons are not polarized by spin.

2. A single electron spin filter method using the quantum dot-based single electron spin filter of claim 1, characterized in that: the quantum dots are used as coulomb islands of single-electron transistors, the discrete energy levels of the coulomb islands are subjected to Zeeman splitting in a vertical magnetic field, and the split energy levels have spin correlation and spin filtering effect;

the outgoing electrons which are subjected to spin polarization through the single-electron transistor deflect in the movement direction under the action of a horizontal nonuniform magnetic field, so that separation and detection are realized; electrons with opposite spin directions have opposite magnetic moment orientations, and when moving in a horizontal nonuniform magnetic field, the movement tracks are separated due to different stresses and reach two different detectors.

3. The single electron spin filtration method of claim 2, wherein: the source bias voltage and the drain bias voltage are regulated so as to only accommodate one Zeeman split energy level at most and be in a bias voltage window; by adjusting the source, drain bias and gate voltage, free electrons on the coulomb island can be depleted and then single electrons can be injected to complete single electron spin polarization.

4. The single electron spin filtration method of claim 2, wherein: the source bias voltage and the drain bias voltage are unchanged, and only the grid voltage is changed, so that a certain energy level which is taken as an electron channel and has a specific spin direction can be adjusted to move up and down, and the electron channel is opened or closed; the source-drain bias voltage is unchanged, only the gate voltage is changed, and the energy levels of adjacent quantum dots with different spin directions can be regulated to sequentially appear in a bias window to become spin polarized electron channels.