CN111239175A - Three-dimensional topological insulator Bi2Te3Method for distinguishing upper and lower surface state photoinduced anomalous Hall current - Google Patents

Abstract

The invention relates to a method for distinguishing the upper and lower surface states of a three-dimensional topological insulator Bi ₂ Te ₃ by photo-induced anomalous Hall current. The method comprises: preparing a Bi ₂ Te ₃ sample and depositing a point-shaped titanium-gold electrode and a strip-shaped titanium-gold electrode thereon ; Obtain the XPS spectrum of the sample before oxidation; pass the laser through the polarizer, the photoelastic modulator and the Dewar window in sequence, and irradiate the geometric center of the sample in the vacuum dewar vertically; use a DC voltage source to apply an adjustable voltage to provide Longitudinal electric field, the photo-induced anomalous Hall current before oxidation is measured in the direction perpendicular to the electric field; similarly, the XPS spectrum and photo-induced anomalous Hall current of the sample after oxidation are obtained; the XPS spectrum and photo-induced anomalous Hall current of the sample before and after oxidation are compared. , distinguish the photoinduced anomalous Hall currents from the upper and lower surface states of the three-dimensional topological insulator Bi ₂ Te ₃ . This method is beneficial to simply, quickly and effectively distinguish the photoinduced anomalous Hall currents from the upper and lower surface states of the three-dimensional topological insulator Bi ₂ Te ₃ .

Description

Discrimination of photoinduced anomalous Hall currents between upper and lower surface states of a three-dimensional topological insulator Bi2Te3 method

technical field

The invention belongs to the field of spintronics, and in particular relates to a method for distinguishing light-induced anomalous Hall currents from upper and lower surface states of a three-dimensional topological insulator Bi ₂ Te ₃ .

Background technique

Three-dimensional topological insulators are a new type of quantum matter state with spin-momentum-locked Dirac electronic states on the surface, which have many exotic properties. Bi ₂ Te ₃ is a typical three-dimensional topological insulator with great potential for applications in spintronics and quantum computing. Due to the advantages of low energy consumption, high integration, and high data processing speed, spintronic devices have become a current research hotspot. The generation, manipulation and detection of spin current is an important research content in spintronics.

In three-dimensional topological insulators, due to the strong spin-orbit coupling, the spin Hall effect and the inverse spin Hall effect can be realized, and the spin polarization and spin state can be effectively controlled. If circularly polarized light is used to inject spin-polarized carriers in a system with spin-orbit coupling, under the spin Hall effect, a current is measured in a direction perpendicular to the applied electric field, which is called light cause anomalous Hall current. Photoinduced anomalous Hall current is a powerful tool to study the generation, manipulation and detection of spin current. For three-dimensional topological insulators, there are three sources of photoinduced anomalous Hall currents, namely upper surface state, bulk state and lower surface state. However, the signals of these three are often mixed together. To analyze the generation mechanism and regulation mechanism of the photo-induced anomalous Hall current, it is necessary to distinguish the main contribution of the photo-induced anomalous Hall current, that is, to analyze that it mainly comes from the upper surface state, The body is still the lower surface. However, there is currently no way to distinguish the main contribution of the photoinduced anomalous Hall current.

SUMMARY OF THE INVENTION

In view of this, the purpose of the present invention is to provide a method for distinguishing the upper and lower surface states of a three-dimensional topological insulator Bi ₂ Te ₃ by photo-induced anomalous Hall currents, which is conducive to simple, fast and effective distinction between the upper and lower surface states of a three-dimensional topological insulator Bi ₂ Te ₃ Surface state photoinduced anomalous Hall current.

In order to achieve the above object, the technical solution adopted in the present invention is: a method for distinguishing the upper and lower surface states of a three-dimensional topological insulator Bi ₂ Te ₃ photoinduced anomalous Hall current, comprising the following steps:

Step S1: On the high-resistance single crystal silicon of the (111) plane, a Bi ₂ Te ₃ sample is grown by molecular beam epitaxy equipment, and then a pair of point-shaped titanium-gold electrodes and a pair of strip-shaped titanium electrodes are deposited on the surface of the sample by electron beam evaporation. gold electrode;

Step S2: perform X-ray photoelectron spectroscopy analysis and test on the sample, and remember that the XPS spectrum is XPS1 spectrum;

Step S3: place the sample in a vacuum Dewar flask; emit laser light through the polarizer, the photoelastic modulator and the Dewar flask window in sequence, and irradiate vertically on the geometric center of the sample, that is, the center position of the four electrodes;

Step S4: Connect the strip-shaped titanium-gold electrodes on the left and right sides of the sample to the positive and negative output terminals of the DC voltage source, respectively, and control the DC voltage source to output a set of DC voltages from positive to negative, thereby generating a set of DC voltages from positive to negative. Positive to negative longitudinal electric field; photocurrent is generated after the light in step S3 is irradiated on the sample; photocurrent is collected from the point-shaped titanium-gold electrode, and then input to the preamplifier and the lock-in amplifier in turn, the reference frequency of the lock-in amplifier It is the one frequency operating frequency of the photoelastic modulator, and the signal output by the lock-in amplifier is input into the computer through the data acquisition card; The photocurrent signal with the same frequency doubled as I ⁺ ;

Step S5: Connect the strip-shaped titanium-gold electrodes on the right and left sides of the sample to the positive and negative output terminals of the DC voltage source, respectively, that is, compare the wiring of the two-shaped titanium-gold electrodes in step S4, and control the DC voltage through a computer. The source outputs a set of DC voltages from positive to negative, thereby generating a set of longitudinal electric fields from positive to negative; the photocurrent is generated after the light in step S3 is irradiated on the sample; the photocurrent is collected from the point-shaped titanium-gold electrode, and then sequentially Input the preamplifier and the lock-in amplifier, the reference frequency of the lock-in amplifier is the one-fold operating frequency of the photoelastic modulator, and the signal output by the lock-in amplifier is input into the computer through the data acquisition card; The photocurrent, that is, the photocurrent signal with the same frequency as the one-time frequency of the photoelastic modulator is extracted through the lock-in amplifier, denoted as I ^- ;

Step S6: Extract the photo-induced anomalous Hall current I _PAHE by the following formula (1):

I _PAHE = (I ⁺ -I ^- )/2 (1)

The absolute value of the longitudinal electric field corresponding to I ⁺ and the longitudinal electric field corresponding to I ^- are equal, but the directions are opposite; remember that the photoinduced anomalous Hall current I _PAHE changes with the longitudinal electric field E as I _PAHE1 ;

Step S7: perform oxidation treatment on the sample, and then repeat steps S2 to S6 for the oxidized sample, denote the XPS spectrum obtained after oxidation as the XPS2 spectrum, and denote the photoinduced anomalous Hall current I _PAHE obtained after oxidation with the longitudinal electric field E. The change curve is I _PAHE2 ;

Step S8: Denote the intensity ratio of the characteristic peak of the Bi element of the topological insulator and the characteristic peak of the Bi oxide in the XPS1 and XPS2 spectra respectively as f _Bi1 , f _Bi2 , and denote the Te element of the topological insulator in the XPS1 and XPS2 spectra, respectively. The intensity ratios of the characteristic peaks and the characteristic peaks of Te oxides are f _Te1 and f _Te2 respectively, and the slopes of the curves of I _PAHE1 and I _PAHE2 are respectively σ ₁ and σ ₂ ; if the values of f _Bi2 and f _Te2 are the same after oxidation Compared with the values of f _Bi1 and f _Te1 before oxidation, and the slope σ ₂ decreases compared with the slope σ ₁ , it is judged that I _PAHE1 and I _PAHE2 are both dominated by the contribution of the upper surface state; if after oxidation, f Bi2 and f _Bi2 and The value of f _Te2 decreases compared with the values of f _Bi1 and f _Te1 before oxidation, and the slope σ ₂ has an inverse sign compared with the slope σ ₁ , then it is judged that I _PAHE1 is the dominant contribution of the upper surface state, and I _PAHE2 is The contribution of the lower surface state is dominant; if the values of f _Bi2 and f _Te2 after oxidation decrease compared with the values of f _Bi1 and f _Te1 before oxidation, and the slope σ ₂ decreases compared with the slope σ ₁ , then judge I _PAHE1 and _IPAHE2 are both dominated by the contribution of the lower surface states.

Further, the Bi ₂ Te ₃ sample has a rectangular structure, the pair of point-shaped titanium-gold electrodes is deposited on the midline of the inner sides of the two long sides of the rectangle, and the pair of strip-shaped titanium-gold electrodes is along the inner edges of the two short sides of the rectangle. deposition.

Further, in the step S1, the Bi ₂ Te ₃ sample has a single crystal structure, the short side of the sample is ≥ 2 mm, the long side is ≥ 5 mm, and the thickness is 3-5 nanometers.

Further, the point-shaped titanium-gold electrodes are circular point-shaped titanium-gold electrodes with a diameter of 0.25 mm, and the distance between the inner sides of the two circular point-shaped titanium-gold electrodes is ≥1.5 mm; the width of the strip-shaped titanium-gold electrodes is 1.5 mm. mm, the length is adapted to the short side of the sample, and the distance between the inner sides of the two strip-shaped titanium electrodes is ≥3 mm.

Further, in the step S3, the power of the laser is 30-200 mW, the wavelength of the emitted laser is 1064 nm, and the diameter of the spot hit on the sample is smaller than the distance between the two point-shaped titanium electrodes, and is also smaller than the two strips. The working frequency of the photoelastic modulator is 50 KHz, the angle between the main axis direction of the photoelastic modulator and the polarization direction of the polarizer is 45 degrees, and the phase retardation of the photoelastic modulator is 0.25× wavelength.

Further, in the step S4, the value range of the minimum value of the output voltage of the DC voltage source is -10~-1V, the value range of the minimum value is 1~10V, and the computer-controlled DC voltage source changes every 10 s. Primary voltage, step size is 0.1~0.5 V.

Further, in the step S7, the method for oxidizing the sample is: placing the sample in a dust-free air atmosphere, and naturally oxidizing it at room temperature for 1-20 days.

Compared with the prior art, the present invention has the following beneficial effects: the photo-induced anomalous Hall current is measured by a simple and easy method, and the X-ray photoelectron spectrum and the photo-induced anomalous Hall current before and after oxidation are compared to distinguish three-dimensional The photoinduced anomalous Hall current on the upper and lower surface states of the topological insulator Bi ₂ Te ₃ has the advantages of accurate measurement results, simple operation, low cost, strong practicability and broad application prospects.

Description of drawings

FIG. 1 is a flow chart of a method implementation according to an embodiment of the present invention.

FIG. 2 is a schematic diagram of an experimental optical circuit for measuring photo-induced anomalous Hall current in an embodiment of the present invention.

3 is a comparison diagram of the curves of the photoinduced anomalous Hall current as a function of the longitudinal electric field before and after oxidation of a three-dimensional topological insulator Bi ₂ Te ₃ sample with a thickness of 3 nm in an embodiment of the present invention.

FIG. 4 is the XPS spectrum of the three-dimensional topological insulator Bi ₂ Te ₃ sample with a thickness of 3 nm before oxidation corresponding to the Td3 d and Bi 4 f energy levels in the embodiment of the present invention.

5 is the XPS spectrum of the three-dimensional topological insulator Bi ₂ Te ₃ sample with a thickness of 3 nm corresponding to the Td3 d and Bi 4 f energy levels after oxidation in the embodiment of the present invention.

In Figure 2: 1-1064nm laser, 2-attenuator, 3-polarizer, 4-photoelastic modulator, 5-topological insulator Bi ₂ Te ₃ sample, 6- electrode deposited on sample, 7- front Amplifier, 8-lock-in amplifier, 9-DC voltage source, 10-low temperature dewar, 11-computer.

Detailed ways

The present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments.

The present invention provides a method for distinguishing photo-induced anomalous Hall currents from upper and lower surface states of a three-dimensional topological insulator Bi ₂ Te ₃ , as shown in FIG. 1 , including the following steps:

Step S1: On the high-resistance single crystal silicon of the (111) plane, a Bi ₂ Te ₃ sample is grown by molecular beam epitaxy equipment, and then a pair of point-shaped titanium-gold electrodes and a pair of strip-shaped titanium electrodes are deposited on the surface of the sample by electron beam evaporation. Gold electrodes.

In a preferred embodiment of the present invention, the Bi ₂ Te ₃ sample has a rectangular single crystal structure, the short side of the sample is ≥ 2 mm, the long side is ≥ 5 mm, and the thickness is 3 to 5 nanometers (including 3 nanometers and 5 nanometers) . The pair of point-shaped titanium-gold electrodes is deposited on the midline inside the two long sides of the rectangle, and the pair of strip-shaped titanium-gold electrodes is deposited along the inner edges of the two short sides of the rectangle. The point-shaped titanium-gold electrode is a circular point-shaped titanium-gold electrode with a diameter of 0.25 mm, and the distance between the inner sides of the two circular point-shaped titanium-gold electrodes is ≥1.5 mm; the width of the strip-shaped titanium-gold electrode is 1 mm, and the length is 1 mm. Compatible with the short side of the sample, the distance between the inner sides of the two strip titanium electrodes is ≥3 mm.

In this embodiment, the size of the Bi ₂ Te ₃ sample is 2 mm×12 mm, and the thickness is about 3 nanometers. The point-shaped titanium-gold electrode is a circular point-shaped titanium-gold electrode with a diameter of 0.25 mm, and the distance between the inner sides of the two circular point-shaped titanium-gold electrodes is 1.5 mm; the size of the strip-shaped titanium-gold electrode is 1 mm × 2 mm , the distance between the inner sides of the two strip-shaped titanium electrodes is 10 mm.

Step S2: perform X-ray photoelectron spectroscopy (XPS) test on the sample, and remember that the XPS spectrum obtained is the XPS1 spectrum.

Step S3: In order to prevent the oxidation of the sample, the sample is placed in a vacuum Dewar flask with a vacuum degree of about 1 Pa; laser light is emitted by the laser through the polarizer, the photoelastic modulator and the Dewar flask window in turn, and is irradiated vertically on the sample. Geometric center, that is, the center position of the four electrodes.

In this embodiment, the power of the laser is 30-200 mW, the wavelength of the emitted laser is 1064 nm, and the diameter of the spot hit on the sample is smaller than the distance between the two point-shaped titanium-gold electrodes, and also smaller than the two strip-shaped titanium electrodes. Spacing of gold electrodes. The working frequency of the photoelastic modulator is 50 KHz, the included angle between the main axis direction of the photoelastic modulator and the polarization direction of the polarizer is 45 degrees, and the phase retardation of the photoelastic modulator is 0.25×wavelength.

Step S4: Connect the strip-shaped titanium-gold electrodes on the left and right sides of the sample to the positive and negative output terminals of the DC voltage source, respectively, and control the DC voltage source to output a set of DC voltages from positive to negative, thereby generating a set of DC voltages from positive to negative. Positive to negative longitudinal electric field; photocurrent is generated after the light in step S3 is irradiated on the sample; photocurrent is collected from the point-shaped titanium-gold electrode, and then input to the preamplifier and the lock-in amplifier in turn, the reference frequency of the lock-in amplifier It is the one frequency working frequency of the photoelastic modulator, that is, 50 KHz. The signal output by the lock-in amplifier is input into the computer through the data acquisition card; The photocurrent signal of the same frequency as the one-time frequency of the modulator is denoted as I ⁺ .

Among them, the value range of the minimum value of the output voltage of the DC voltage source is -10~-1 V, and the value range of the minimum value is 1~10 V. The computer controls the DC voltage source to change the voltage every 10 s, and the step size is 0.1~0.5V.

Step S5: Connect the strip-shaped titanium-gold electrodes on the right and left sides of the sample to the positive and negative output terminals of the DC voltage source, respectively, that is, compare the wiring of the two-shaped titanium-gold electrodes in step S4, and control the DC voltage through a computer. The source outputs a set of DC voltages from positive to negative, thereby generating a set of longitudinal electric fields from positive to negative; the photocurrent is generated after the light in step S3 is irradiated on the sample; the photocurrent is collected from the point-shaped titanium-gold electrode, and then sequentially Input the preamplifier and the lock-in amplifier. The reference frequency of the lock-in amplifier is one frequency operating frequency of the photoelastic modulator, that is, 50 KHz. The signal output by the lock-in amplifier is input to the computer through the data acquisition card; The photocurrent under the electric field E, that is, the photocurrent signal with the same frequency as the one-time frequency of the photoelastic modulator extracted by the lock-in amplifier, is denoted as I ^- .

Step S6: Extract the photo-induced anomalous Hall current I _PAHE by the following formula (1):

I _PAHE = (I ⁺ -I ^- )/2 (1)

The absolute value of the longitudinal electric field corresponding to I ⁺ and the longitudinal electric field corresponding to I ^- are equal, but the directions are opposite; remember that the photoinduced anomalous Hall current I _PAHE changes with the longitudinal electric field E as I _PAHE1 ;

Step S7: The sample is oxidized, that is, the sample is placed in a dust-free air atmosphere, and it is naturally oxidized at room temperature for 1 to 20 days, and then steps S2 to S6 are repeated for the oxidized sample, and the XPS obtained after oxidation is recorded. The spectrum is the XPS2 spectrum, and the change curve of the photo-induced anomalous Hall current I _PAHE obtained after oxidation with the longitudinal electric field E is recorded as I _PAHE2 .

Step S8: Denote the intensity ratio of the characteristic peak of the Bi element of the topological insulator and the characteristic peak of the Bi oxide in the XPS1 and XPS2 spectra respectively as f _Bi1 , f _Bi2 , and denote the Te element of the topological insulator in the XPS1 and XPS2 spectra, respectively. The intensity ratios of the characteristic peaks and the characteristic peaks of Te oxides are f _Te1 and f _Te2 respectively, and the slopes of the curves of I _PAHE1 and I _PAHE2 are respectively σ ₁ and σ ₂ ; if the values of f _Bi2 and f _Te2 are the same after oxidation Compared with the values of f _Bi1 and f _Te1 before oxidation, and the slope σ ₂ decreases compared with the slope σ ₁ , it is judged that I _PAHE1 and I _PAHE2 are both dominated by the contribution of the upper surface state; if after oxidation, f Bi2 and f _Bi2 and The value of f _Te2 decreases compared with the values of f _Bi1 and f _Te1 before oxidation, and the slope σ ₂ has an inverse sign compared with the slope σ ₁ , then it is judged that I _PAHE1 is the dominant contribution of the upper surface state, and I _PAHE2 is The contribution of the lower surface state is dominant; if the values of f _Bi2 and f _Te2 after oxidation decrease compared with the values of f _Bi1 and f _Te1 before oxidation, and the slope σ ₂ decreases compared with the slope σ ₁ , then judge I _PAHE1 and _IPAHE2 are both dominated by the contribution of the lower surface states.

The generation principle of the photoinduced anomalous Hall current is as follows: Under the action of the longitudinal electric field, the electrons move in a directional motion. Under the irradiation of circularly polarized light, spin-polarized charge carriers are generated in the sample. According to the spin Hall effect, these spin-polarized carriers will be subjected to a force perpendicular to the direction of motion, and the direction of the force on the spin-up and spin-down electrons is opposite. Therefore, spin-up electrons and spin-down electrons will be deflected to either side of the sample, respectively. Since the number of spin up and down carriers generated under the excitation of circularly polarized light is unequal, the number of electrons deflected to both sides is unequal, thereby generating current. This current is the photoinduced anomalous Hall current. Obviously, this current is perpendicular to the direction of the applied electric field, and its strength is proportional to the strength of the spin-orbit coupling.

The upper and lower surface states of 3D topological insulators have opposite spin-orbit couplings, and therefore, the directions of the photoinduced anomalous Hall currents they generate are opposite. The bulk state also produces a photoreverse spin Hall current. Therefore, the photoreversal spin Hall current of the three-dimensional topological insulator measured by the present invention is the sum of the contributions of the upper surface state, the lower surface state and the bulk state. Since the sample used in the present invention is very thin, only about 3 nanometers, considering that the spin-orbit coupling strength of the bulk state is much smaller than that of the upper and lower surface states, the contribution of the bulk state can be ignored.

FIG. 2 is a schematic diagram of an experimental optical circuit for measuring the photo-induced anomalous Hall current in this embodiment. Among them, sample 5 is a Bi ₂ Te ₃ film grown on a (111)-plane insulating silicon substrate by molecular beam epitaxy (MBE). Since Bi ₂ Te ₃ films are prone to oxidative deterioration in air, the samples need to be packed in Dewar flasks, stored under vacuum conditions and measured.

In this embodiment, a solid-state laser with a wavelength of 1064 nm is used, and the intensity of the light spot is Gaussian distributed. The light from the laser passes through the attenuator, polarizer, and photoelastic modulator in sequence, and then hits the center of the four electrodes of the sample vertically. The diameter of the spot hit on the sample is 1 mm, and the laser power is 50 mW. The photocurrent generated by illumination is amplified by the preamplifier and then enters the lock-in amplifier. The reference frequency of the lock-in amplifier is one frequency signal of the photoelastic modulator. The phase retardation of the photoelastic modulator was set to 0.25×wavelength. At this time, the photoelastic modulator is equivalent to a quarter-wave plate. Therefore, the current generated by the circularly polarized light can be extracted by the lock-in amplifier. Because the photoelastic modulator will introduce some background current signals that are independent of the longitudinal electric field. In order to remove this signal, it is processed according to the method of steps S4-S6, that is, the real photo-induced inverse spin Hall current is extracted by formula (1).

The data shown in the solid small square in Fig. 3 is the variation curve of the photoinduced anomalous Hall current with the longitudinal electric field obtained by the steps S4-S6 before the oxidation of the sample and extracted by the formula (1), namely I _PAHE1 . It can be seen that the slope of _IPAHE1 with the longitudinal electric field is positive. The data shown by the solid small circles in Fig. 3 is the variation curve of the photo-induced anomalous Hall current with the longitudinal electric field obtained by the steps S4-S6 after oxidation of the sample, namely I _PAHE2 . It can be seen that the slope of _IPAHE2 with the longitudinal electric field is negative.

FIG. 4 is the XPS spectrum of the three-dimensional topological insulator Bi ₂ Te ₃ sample with a thickness of 3 nanometers before oxidation corresponding to the Td 3 d and Bi 4 f energy levels in this example. The solid line in the figure is the peak-fitted curve. Among them, the peaks near 572.8 and 583.2 eV are the energy level peaks of Te 3d _3/2 and Te 3d _5/2 in Bi ₂ Te ₃ , and the peaks near 576.4 and 586.9 eV are the energy level peaks of Te oxides . The peaks located around 158.0 and 163.3 eV are the energy level peaks of Bi 4f _7/2 and Bi 4f _5/2 in Bi ₂ Te ₃ , while the peaks located around 164.7 and 159.5 eV correspond to the peaks of Bi oxides.

FIG. 5 shows the XPS spectra of the three-dimensional topological insulator Bi ₂ Te ₃ sample with a thickness of 3 nanometers corresponding to the Td 3 d and Bi 4 f energy levels after oxidation. The solid line in the figure is the peak-fitted curve.

Comparing Figure 4 and Figure 5, it can be found that the ratio f _Bi of the characteristic peak of the Bi element of the topological insulator and the characteristic peak of the Bi oxide in the XPS spectrum after oxidation decreases, and the Te element of the topological insulator in the XPS spectrum after oxidation is reduced. The ratio of the intensities of the characteristic peaks of f and the characteristic peaks of _Te oxides, fTe, is also reduced, indicating that the oxidation degree of the upper surface states is aggravated. Therefore, it can be predicted that the photoinduced anomalous Hall current contributed by the upper surface state will become smaller at this time. Comparing the slope of photoinduced anomalous Hall current with longitudinal electric field before and after oxidation, it is found that after oxidation, the slope of photoinduced anomalous Hall current with longitudinal electric field changes from positive to negative before oxidation. Therefore, it can be inferred that the photoinduced anomalous Hall current before oxidation is mainly contributed by the upper surface states. At this time, the contribution of the upper surface states is greater than that of the lower surface states. Since the photoinduced anomalous Hall currents contributed by the upper and lower surface states have opposite signs, the photoinduced Hall currents at this time have the same sign as the current signals of the upper surface states. After oxidation, the oxidation of the upper surface state is more serious, so that the contribution of the upper surface state is reduced. At this time, the contribution of the lower surface state is greater than that of the upper surface state. Since the photo-induced anomalous Hall currents contributed by the upper and lower surface states are reversed, the photoinduced Hall current after oxidation is reversed compared with that before oxidation. The photoinduced anomalous Hall current also has an inverse sign with the slope of the longitudinal electric field. In this way, by comparing the X-ray photoelectron spectrum and photo-induced anomalous Hall current signals before and after oxidation, it is found that the photo-induced anomalous Hall current of the 3-nanometer sample in this example before oxidation is dominated by the upper surface state, while the photo-induced anomalous Hall current after oxidation is dominated by the upper surface state. The anomalous Hall current is dominated by the lower surface state.

In conclusion, the present invention proposes a simple and easy-to-operate method for distinguishing photo-induced anomalous Hall currents from upper and lower surface states of a three-dimensional topological insulator Bi ₂ Te ₃ . In this method, the laser passes through a polarizer and a photoelastic modulator in turn, and is vertically incident on the geometric center of a Bi ₂ Te ₃ sample with an appropriate thickness. A DC voltage source is used to apply a tunable stable voltage to provide a longitudinally stable electric field. Measure the photoinduced anomalous Hall current. By comparing the X-ray photoelectron spectra and photoinduced anomalous Hall currents of the samples before and after oxidation, the photoinduced anomalous Hall currents of the upper and lower surface states of the three-dimensional topological insulator Bi ₂ Te ₃ were distinguished.

The specific embodiments described above describe in detail the purpose, technical solutions and achievements of the present invention. It should be understood that the above are only specific embodiments of the present invention, and are not intended to limit the present invention. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention shall be included within the protection scope of the present invention.

Claims (7)

1. Three-dimensional topological insulator Bi₂Te₃The method for distinguishing the photoinduced abnormal Hall current of the upper surface state and the lower surface state is characterized by comprising the following steps of:

step S1: growing Bi on the (111) plane high-resistance monocrystalline silicon by using molecular beam epitaxy equipment₂Te₃Depositing a pair of point-shaped titanium gold electrodes and a pair of strip-shaped titanium gold electrodes on the surface of the sample through electron beam evaporation;

step S2: carrying out X-ray photoelectron spectroscopy analysis and test on the sample, and recording the obtained XPS spectrum as XPS1 spectrum;

step S3: placing the sample in a vacuum dewar; laser emitted by a laser sequentially passes through a polarizer, a photoelastic modulator and a Dewar flask window and vertically irradiates the geometric center of a sample, namely the central positions of four electrodes;

step S4: respectively connecting strip titanium electrodes on the left side and the right side of the sample with a positive output end and a negative output end of a direct current voltage source, and controlling the direct current voltage source to output a group of positive-to-negative direct current voltages through a computer so as to generate a group of positive-to-negative longitudinal electric fields; generating a photocurrent after the light irradiated on the sample in the step S3; collecting photocurrent from the point titanium electrode, sequentially inputting into a preamplifier and a phase-locked amplifier, wherein the reference frequency of the phase-locked amplifier is a frequency doubling working frequency of the photoelastic modulator, and the phase-locked amplifier outputs a signalInputting the data into a computer through a data acquisition card; measuring the photocurrent of the group of longitudinal electric fields E, namely extracting a photocurrent signal with the same frequency as one frequency multiplication of the photoelastic modulator through a phase-locked amplifier, and marking as I⁺；

Step S5: connecting the strip titanium gold electrodes on the right side and the left side of the sample to the positive output end and the negative output end of a direct current voltage source respectively, namely, compared with the step S4, exchanging the connection of the two strip titanium gold electrodes, and controlling the direct current voltage source to output a group of direct current voltages from positive to negative through a computer, so as to generate a group of longitudinal electric fields from positive to negative; generating a photocurrent after the light irradiated on the sample in the step S3; collecting photocurrent from a point titanium electrode, and then sequentially inputting the photocurrent into a preamplifier and a phase-locked amplifier, wherein the reference frequency of the phase-locked amplifier is a frequency doubling working frequency of the photoelastic modulator, and signals output by the phase-locked amplifier are input into a computer through a data acquisition card; measuring the photocurrent of the group of longitudinal electric fields E, namely extracting a photocurrent signal with the same frequency as one frequency multiplication of the photoelastic modulator through a phase-locked amplifier, and marking as I^-；

Step S6: extracting the photoinduced abnormal Hall current I by the following formula (1)_PAHE：

I_PAHE= (I⁺－I^-)/2 （1）

I⁺Corresponding longitudinal electric field and I^-The absolute values of the corresponding longitudinal electric fields are equal, but the directions are opposite; recording the obtained photoinduced abnormal Hall current I_PAHEThe change curve along the longitudinal electric field E is I_PAHE1；

Step S7: oxidizing the sample, repeating the steps S2-S6 on the oxidized sample, recording the XPS spectrum obtained after oxidation as the XPS2 spectrum, and recording the photoinduced abnormal Hall current I obtained after oxidation_PAHEThe change curve along the longitudinal electric field E is I_PAHE2；

Step S8: the intensity ratios of the characteristic peak of Bi element and the characteristic peak of Bi oxide of the topological insulator in XPS1 and XPS2 spectra are respectively expressed as f_Bi1、f_Bi2The intensity ratios of the characteristic peak of Te element of topological insulator and the characteristic peak of Te oxide in XPS1 and XPS2 spectra were respectively recordedAre respectively f_Te1、f_Te2Respectively record I_PAHE1、I_PAHE2The slopes of the curves are respectively sigma₁、σ₂(ii) a If after oxidation f_Bi2And f_Te2Value of (a) compared to f before oxidation_Bi1And f_Te1Is reduced and the slope σ is₂Compared with the slope σ₁Decreasing, then judge I_PAHE1And I_PAHE2Are all the dominant contributions of the upper surface states; if after oxidation f_Bi2And f_Te2Value of (a) compared to f before oxidation_Bi1And f_Te1Is reduced and the slope σ is₂Compared with the slope σ₁If the opposite sign appears, the judgment is made_PAHE1Is dominated by the contribution of the upper surface state, and I_PAHE2The contribution of the lower surface state is dominant; if after oxidation f_Bi2And f_Te2Value of (a) compared to f before oxidation_Bi1And f_Te1Is reduced and the slope σ is₂Compared with the slope σ₁Decreasing, then judge I_PAHE1And I_PAHE2Are dominated by the contribution of the lower surface state.

2. The three-dimensional topological insulator Bi of claim 1₂Te₃The method for distinguishing the photoinduced abnormal Hall current of the upper surface state and the lower surface state is characterized in that Bi is used₂Te₃The sample is of a rectangular structure, the pair of point-shaped titanium gold electrodes are deposited on the central line of the inner sides of the two long sides of the rectangle, and the pair of strip-shaped titanium gold electrodes are deposited along the inner side edges of the two short sides of the rectangle.

3. The three-dimensional topological insulator Bi of claim 2₂Te₃The method for distinguishing the photoinduced abnormal Hall current of the upper surface state and the lower surface state is characterized in that in the step S1, the Bi₂Te₃The sample is of a single crystal structure, the short edge of the sample is larger than or equal to 2 mm, the long edge of the sample is larger than or equal to 5mm, and the thickness of the sample is 3-5 nanometers.

4. The three-dimensional topological insulator Bi of claim 3₂Te₃The method for distinguishing the photoinduced abnormal Hall current of the upper surface state and the lower surface state is characterized in that the point-like titanium gold is used as a current sourceThe electrode is a round point titanium electrode with the diameter of 0.25 mm, and the distance between the inner sides of the two round point titanium electrodes is more than or equal to 1.5 mm; the width of the strip titanium gold electrode is 1 mm, the length of the strip titanium gold electrode is matched with the short side of the sample, and the distance between the inner sides of the two strip titanium gold electrodes is more than or equal to 3 mm.

5. The three-dimensional topological insulator Bi of claim 1₂Te₃The method for distinguishing the photoinduced anomalous Hall current of the upper surface state and the lower surface state is characterized in that in the step S3, the power of a laser is 30-200 mW, the wavelength of the emitted laser is 1064nm, and the diameter of a light spot on a sample is smaller than the distance between two point-shaped titanium gold electrodes and the distance between two strip-shaped titanium gold electrodes; the working frequency of the photoelastic modulator is 50 KHz, the included angle between the main shaft direction of the photoelastic modulator and the polarization direction of the polarizer is 45 degrees, and the phase delay of the photoelastic modulator is 0.25 multiplied by the wavelength.

6. The three-dimensional topological insulator Bi of claim 1₂Te₃The method for distinguishing the photoinduced anomalous Hall currents of the upper surface state and the lower surface state is characterized in that in the step S4, the minimum value of the output voltage of the direct current voltage source ranges from minus 10 to minus 1V, the minimum value ranges from 1 to 10V, the computer controls the direct current voltage source to change the voltage every 10S, and the step length is 0.1 to 0.5V.

7. The three-dimensional topological insulator Bi of claim 1₂Te₃The method for distinguishing the photoinduced abnormal hall currents of the upper surface state and the lower surface state is characterized in that in the step S7, the method for carrying out the oxidation treatment on the sample comprises the following steps: and placing the sample in a dust-free air atmosphere, and naturally oxidizing the sample at room temperature for 1-20 days.

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