CN111239175A - Three-dimensional topological insulator Bi2Te3Method for distinguishing upper and lower surface state photoinduced anomalous Hall current - Google Patents
Three-dimensional topological insulator Bi2Te3Method for distinguishing upper and lower surface state photoinduced anomalous Hall current Download PDFInfo
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Abstract
The invention relates to a three-dimensional topological insulator Bi2Te3A method for distinguishing upper and lower surface state photoinduced anomalous Hall current, which comprises the following steps: preparation of Bi2Te3Depositing a point-shaped titanium gold electrode and a strip-shaped titanium gold electrode on the sample; obtaining an XPS spectrum of a sample before oxidation; laser passes through a polarizer, a photoelastic modulator and a Dewar flask window in sequence and vertically irradiates the geometric center of a sample in a vacuum Dewar flask; applying adjustable voltage by using a direct current voltage source to provide a longitudinal electric field, and measuring the photoinduced abnormal Hall current before oxidation in the direction vertical to the electric field; obtaining an XPS spectrum and a photoinduced abnormal Hall current after the sample is oxidized in the same way; comparing the XPS spectrum of the sample before and after oxidation with the photoinduced abnormal Hall current to distinguish the three-dimensional topological insulator Bi2Te3Upper and lower surface state photoinduced anomalous hall currents. The method is beneficial to simple and rapid,Efficient discrimination of three-dimensional topological insulators Bi2Te3Upper and lower surface state photoinduced anomalous hall currents.
Description
Technical Field
The invention belongs to the field of spintronics, and particularly relates to a three-dimensional topological insulator Bi2Te3A method for distinguishing the photoinduced abnormal Hall current of upper and lower surface states.
Background
The three-dimensional topological insulator is a novel quantum matter state with a spin momentum locking Dirac electron state on the surface, and has a plurality of peculiar properties. Bi2Te3The silicon nitride is a typical three-dimensional topological insulator and has great application potential in the fields of spintronics and quantum computing. Since the spintronics device has the advantages of low energy consumption, high integration, high data processing speed, etc., it is a hot spot in current research. The generation, manipulation and detection of spin currents is an important research item of spintronics.
In the three-dimensional topological insulator, due to the existence of strong spin orbit coupling, the spin Hall effect and the inverse spin Hall effect can be realized, and the spin polarization and the effective control of the spin state can be realized. If spin-polarized carriers are injected with circularly polarized light in a system with spin-orbit coupling, a current is measured perpendicular to the applied electric field under the spin hall effect, which is called a photo-induced abnormal hall current. Photoinduced abnormal hall currents are a powerful tool for studying the generation, manipulation and detection of spin currents. For three-dimensional topological insulators, there are three sources of photo-induced anomalous hall currents, namely the top surface state, the bulk state and the bottom surface state. However, the signals of the three are often mixed together, and to analyze the generation mechanism and the regulation mechanism of the photo-induced abnormal hall current, it is necessary to distinguish the main contribution of the photo-induced abnormal hall current, i.e. whether the main contribution is from the upper surface state, the bulk state or the lower surface state. However, there is currently no method to distinguish the main contributions of photo-induced anomalous hall currents.
Disclosure of Invention
In view of the above, the present invention provides a three-dimensional topological insulator Bi2Te3The method for distinguishing the photoinduced anomalous Hall current of the upper surface state and the lower surface state is favorable for simply, quickly and effectively distinguishing the three-dimensional topological insulator Bi2Te3Upper and lower surface state photoinduced anomalous hall currents.
In order to achieve the purpose, the invention adopts the technical scheme that: three-dimensional topological insulator Bi2Te3The method for distinguishing the photoinduced abnormal Hall current of the upper surface state and the lower surface state comprises the following steps:
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 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 S5: the samples were collected on the right side,The strip titanium electrodes on the left side are respectively connected with the positive output end and the negative output end of the direct current voltage source, namely compared with the step S4 that the connection wires of the two strip titanium electrodes are exchanged, the direct current voltage source is controlled by the computer to output a group of direct current voltages from positive to negative, and therefore a group of longitudinal electric fields from positive to negative are generated; 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：
IPAHE= (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 IPAHEThe change curve along the longitudinal electric field E is IPAHE1；
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 oxidationPAHEThe change curve along the longitudinal electric field E is IPAHE2；
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 fBi1、fBi2The intensity ratios of the characteristic peak of Te element of topological insulator and the characteristic peak of Te oxide in XPS1 and XPS2 spectra are respectively expressed as fTe1、fTe2Respectively record IPAHE1、IPAHE2The slopes of the curves are respectively sigma1、σ2(ii) a If after oxidation fBi2And fTe2Value of (a) compared to f before oxidationBi1And fTe1Is reduced and the slope σ is2Compared with the slope σ1Decreasing, then judge IPAHE1And IPAHE2Are all the dominant contributions of the upper surface states; if after oxidation fBi2And fTe2Value of (a) compared to f before oxidationBi1And fTe1Is reduced and the slope σ is2Compared with the slope σ1If the opposite sign appears, the judgment is madePAHE1Is dominated by the contribution of the upper surface state, and IPAHE2The contribution of the lower surface state is dominant; if after oxidation fBi2And fTe2Value of (a) compared to f before oxidationBi1And fTe1Is reduced and the slope σ is2Compared with the slope σ1Decreasing, then judge IPAHE1And IPAHE2Are dominated by the contribution of the lower surface state.
Further, said Bi2Te3The 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.
Further, 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.
Furthermore, the dot-shaped titanium electrode is a circular dot-shaped titanium electrode with the diameter of 0.25 mm, and the distance between the inner sides of the two circular dot-shaped 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.
Further, in the step S3, the power of the laser is 30-200 mW, the wavelength of the emitted laser is 1064nm, and the diameter of a spot on the sample is smaller than the distance between two point-like ti-au electrodes and smaller than the distance between two strip-like ti-au 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.
Further, in the step S4, the minimum value of the output voltage of the dc voltage source ranges from-10 to-1V, the minimum value ranges from 1V to 10V, and the computer controls the dc voltage source to change the voltage every 10S, with the step length of 0.1V to 0.5V.
Further, in step S7, the method of performing the oxidation treatment on the sample includes: and placing the sample in a dust-free air atmosphere, and naturally oxidizing the sample at room temperature for 1-20 days.
Compared with the prior art, the invention has the following beneficial effects: the photoinduced abnormal Hall current is measured by a simple and easy method, and the three-dimensional topological insulator Bi is distinguished by comparing the X-ray photoelectron spectrum before and after oxidation with the photoinduced abnormal Hall current2Te3The photoinduced anomalous Hall current of the upper surface state and the lower surface state has accurate measurement result, simple and easy operation, low cost, strong practicability and wide application prospect.
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FIG. 1 is a flow chart of a method implementation of an embodiment of the present invention.
Fig. 2 is a schematic diagram of an experimental optical path for measuring a photoinduced abnormal hall current in an embodiment of the invention.
FIG. 3 is a three-dimensional topological insulator Bi of 3 nm thickness according to an embodiment of the present invention2Te3Graph comparing the curves of photo-induced abnormal hall current before and after oxidation of the sample with the curves of longitudinal electric field.
FIG. 4 is a three-dimensional topological insulator Bi of 3 nm thickness according to an embodiment of the present invention2Te3Sample before oxidation corresponds to Td3dAnd Bi 4fXPS spectra of energy levels.
FIG. 5 is a three-dimensional topological insulator Bi of 3 nm thickness according to an embodiment of the present invention2Te3Sample after oxidation corresponds to Td3dAnd Bi 4fXPS spectra of energy levels.
In fig. 2: 1-1064nm laser, 2-attenuation sheet, 3-polarizer, 4-photoelastic modulator, 5-topological insulator Bi2Te3Sample, 6-electrode deposited on sample, 7-preamplifier, 8-phase lock amplifier, 9-DC voltage source, 10-low temperature Dewar flask, 11-computer.
Detailed Description
The invention is described in further detail below with reference to the figures and the embodiments.
The invention provides a three-dimensional topological insulator Bi2Te3The method for distinguishing the upper surface state and the lower surface state photoinduced abnormal Hall current as shown in FIG. 1 comprises the following steps:
step S1: growing Bi on the (111) plane high-resistance monocrystalline silicon by using molecular beam epitaxy equipment2Te3And (3) depositing 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.
In the preferred embodiment of the present invention, said Bi2Te3The sample is of a rectangular 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 (including 3 nanometers and 5 nanometers). The pair of point-shaped titanium gold electrodes are deposited on the center 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. The dot-shaped titanium electrode is a round dot-shaped titanium electrode with the diameter of 0.25 mm, and the distance between the inner sides of the two round dot-shaped 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.
In this embodiment, the Bi2Te3The dimensions of the sample were 2 mm by 12 mm and the thickness was about 3 nm. The dot-shaped titanium electrode is a circular dot-shaped titanium electrode with the diameter of 0.25 mm, and the distance between the inner sides of the two circular dot-shaped titanium electrodes is 1.5 mm; the strip titanium electrode is 1 mm multiplied by 2 mm, and the distance between the inner sides of the two strip titanium electrodes is 10 mm.
Step S2: the sample was subjected to X-ray photoelectron spectroscopy (XPS) and the XPS spectrum was recorded as XPS1 spectrum.
Step S3: to prevent oxidation of the sample, the sample was placed in a vacuum dewar having a vacuum degree of about 1 pa; the laser emitted by the laser sequentially passes through the polarizer, the photoelastic modulator and the Dewar flask window and vertically irradiates the geometric center of the sample, namely the center positions of the four electrodes.
In the embodiment, the power of the laser is 30-200 mW, the wavelength of the emitted laser is 1064nm, and the diameter of a light spot on the sample is smaller than the distance between two point-shaped titanium gold electrodes and smaller than 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.
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 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 the frequency doubling working frequency of the photoelastic modulator, namely 50 KHz, 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+。
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 10 s, and the step length is 0.1 to 0.5V.
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 the frequency doubling working frequency of the photoelastic modulator, namely 50 KHz, and signals output by the phase-locked amplifier are input into a computer through a data acquisition card; measuring photocurrent under the set of longitudinal electric fields E, i.e. extracting one of the photocurrent with the photoelastic modulator by a lock-in amplifierThe frequency-doubled same-frequency photocurrent signal is marked as I-。
Step S6: extracting the photoinduced abnormal Hall current I by the following formula (1)PAHE：
IPAHE= (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 IPAHEThe change curve along the longitudinal electric field E is IPAHE1；
Step S7: oxidizing the sample, namely putting the sample into a dust-free air atmosphere, naturally oxidizing the sample at room temperature for 1-20 days, then 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 oxidationPAHEThe change curve along the longitudinal electric field E is IPAHE2。
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 fBi1、fBi2The intensity ratios of the characteristic peak of Te element of topological insulator and the characteristic peak of Te oxide in XPS1 and XPS2 spectra are respectively expressed as fTe1、fTe2Respectively record IPAHE1、IPAHE2The slopes of the curves are respectively sigma1、σ2(ii) a If after oxidation fBi2And fTe2Value of (a) compared to f before oxidationBi1And fTe1Is reduced and the slope σ is2Compared with the slope σ1Decreasing, then judge IPAHE1And IPAHE2Are all the dominant contributions of the upper surface states; if after oxidation fBi2And fTe2Value of (a) compared to f before oxidationBi1And fTe1Is reduced and the slope σ is2Compared with the slope σ1If the opposite sign appears, the judgment is madePAHE1Is dominated by the contribution of the upper surface state, and IPAHE2The contribution of the lower surface state is dominant; if after oxidation fBi2And fTe2Value of (a) compared to f before oxidationBi1And fTe1Is decreased and is inclinedRate sigma2Compared with the slope σ1Decreasing, then judge IPAHE1And IPAHE2Are dominated by the contribution of the lower surface state.
The principle of generation of photo-induced abnormal hall current is as follows: under the action of the longitudinal electric field, the electrons generate directional motion. Under irradiation of circularly polarized light, spin-polarized 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 received by the spin-up and spin-down electrons is opposite. Thus, the spin-up electrons and the spin-down electrons will be deflected towards the two sides of the sample, respectively. Since the number of generated spin up and spin down carriers is unequal under excitation of circularly polarized light, the number of electrons deflected to both sides is unequal, thereby generating a current. This current is the photo-induced anomalous hall current. It is clear that this current is perpendicular to the direction of the applied electric field and its intensity is proportional to the intensity of the spin-orbit coupling.
The upper and lower surface states of the three-dimensional topological insulator have opposite spin-orbit couplings, and therefore, the directions of the photoinduced abnormal hall currents generated by them are opposite. The bulk state also generates a photo-induced reverse spin hall current. Therefore, the photo-induced inverse spin hall current of the three-dimensional topological insulator measured by the invention is the sum of the contributions of the upper surface state, the lower surface state and the bulk state. Since the samples used in the present invention are very thin, only about 3 nm, the contribution of the bulk states is negligible considering that the spin-orbit coupling strength of the bulk states is much smaller than that of the upper and lower surface states.
Fig. 2 is a schematic diagram of an experimental optical path for measuring the photoinduced abnormal hall current in the present embodiment. Sample 5 therein was Bi grown on a (111) plane silicon-on-insulator substrate by Molecular Beam Epitaxy (MBE) technique2Te3A film. Since Bi2Te3The film is susceptible to oxidative deterioration in air, and therefore it is necessary to load the sample in a dewar, store it under vacuum and perform measurement.
In this embodiment, a 1064nm wavelength solid-state laser is used, and the intensity of the light spot is gaussian distributed. Light from the laser passes through the attenuation sheet, the polarizer and the photoelastic modulator in sequence and then vertically hits the centers of the four electrodes of the sample. The diameter of the light spot on the sample was 1 mm, and the laser power was 50 mW. And the photocurrent generated by illumination enters a phase-locked amplifier after being amplified by a preamplifier. The reference frequency of the phase-locked amplifier is a frequency doubling signal of the photoelastic modulator. The phase retardation of the photoelastic modulator was set to 0.25 × wavelength. In this case, the photoelastic modulator corresponds to a quarter-wave plate. Therefore, the current generated by the circularly polarized light can be extracted by the lock-in amplifier. The photoelastic modulator introduces some background current signal independent of the longitudinal electric field. In order to remove the signal, the processing is performed according to the method of steps S4-S6, i.e. the real photo-induced inverse spin hall current is extracted by formula (1).
The data shown as solid small squares in FIG. 3 are the curves of the photo-induced abnormal Hall current as a function of the longitudinal electric field, i.e., I, processed by steps S4-S6 and extracted by equation (1) before the sample was oxidizedPAHE1. It can be seen that IPAHE1The slope with longitudinal electric field is positive. The data shown as the solid small circles in FIG. 3 are the curves of the photo-induced abnormal Hall current as a function of the longitudinal electric field, i.e., I, processed by steps S4-S6 after the sample is oxidized and extracted by equation (1)PAHE2. It can be seen that IPAHE2The slope with longitudinal electric field is negative.
FIG. 4 shows a three-dimensional topological insulator Bi with a thickness of 3 nm in the present embodiment2Te3Sample before oxidation corresponds to Td3dAnd Bi 4fXPS spectra of energy levels. The solid line in the figure is a curve fitted to the peak. Wherein the peaks near 572.8 and 583.2 eV are Bi2Te3Middle Te 3d3/2And Te 3d5/2And peaks located near 576.4 and 586.9 eV are energy level peaks of an oxide of Te. The peaks near 158.0 and 163.3 eV are Bi2Te3Middle Bi 4f7/2And Bi 4f5/2And peaks located in the vicinity of 164.7 and 159.5 eV correspond to the peaks of the oxide of Bi.
FIG. 5 shows a three-dimensional topological insulator Bi with a thickness of 3 nm in the present embodiment2Te3Sample after oxidationCorresponding to Td3dAnd Bi 4fXPS spectra of energy levels. The solid line in the figure is a curve fitted to the peak.
Comparing fig. 4 and 5, it can be found that the ratio f of the intensity of the characteristic peak of the Bi element of the topological insulator to the characteristic peak of the oxide of Bi in the XPS spectrum after oxidationBiThe ratio f of the intensity of the characteristic peak of Te element of the topological insulator to the characteristic peak of oxide of Te in the XPS spectrum after oxidation is reducedTeAnd also decreased, indicating that the degree of oxidation of the upper surface state is increased. Thus, it is expected that the photo-induced abnormal hall current contributed by the upper surface state will become smaller at this time. Comparing the slope of the photoinduced abnormal Hall current along with the longitudinal electric field before and after oxidation, and finding that the slope of the photoinduced abnormal Hall current along with the longitudinal electric field is changed from positive to negative before oxidation after oxidation. It can therefore be concluded that the photo-induced abnormal hall current before oxidation is mainly contributed by the upper surface states. At this time, the contribution of the upper surface state is larger than that of the lower surface state. The photo-induced hall current at this time is in sign coincidence with the current signal of the top surface state because the photo-induced abnormal hall current contributed by the top and bottom surface states has opposite signs. After oxidation, the upper surface states are oxidized more severely, so that the contribution of the upper surface states is reduced. At this time, the contribution of the lower surface state is larger than that of the upper surface state, and the photoinduced abnormal hall current after oxidation has an opposite sign compared with that before oxidation due to the opposite sign of the photoinduced abnormal hall current contributed by the upper surface state and the lower surface state, and the slope of the photoinduced abnormal hall current along with the longitudinal electric field also has an opposite sign. Thus, by comparing the X-ray photoelectron spectrum and the photo-induced abnormal hall current signals before and after oxidation, it is found that the photo-induced abnormal hall current before oxidation of the 3 nm sample in this example is dominant in the upper surface state, and the photo-induced abnormal hall current after oxidation is dominant in the lower surface state.
In summary, the invention provides a simple and easy-to-operate insulator Bi for distinguishing three-dimensional topology2Te3A method for photoinduced anomalous Hall current of upper and lower surface states. The method comprises the steps of enabling laser to pass through a polarizer and a photoelastic modulator in sequence, and vertically irradiating the laser to Bi with proper thickness2Te3The geometric center of the sample is stabilized longitudinally by applying adjustable stabilizing voltage with DC voltage sourceAnd measuring the photoinduced abnormal Hall current in a direction perpendicular to the electric field. The three-dimensional topological insulator Bi is distinguished by comparing the X-ray photoelectron spectrum and the photoinduced abnormal Hall current of the sample before and after oxidation2Te3Upper and lower surface state photoinduced anomalous hall currents.
The above-mentioned embodiments are intended to illustrate the objects, technical solutions and achievements of the present invention, and it should be understood that the above-mentioned embodiments are only examples of the present invention, and should not be construed as limiting the present invention, and any modifications, equivalent substitutions, improvements, etc. made within the spirit and principle of the present invention should be included in 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：
IPAHE= (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 IPAHEThe change curve along the longitudinal electric field E is IPAHE1；
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 oxidationPAHEThe change curve along the longitudinal electric field E is IPAHE2；
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 fBi1、fBi2The 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 fTe1、fTe2Respectively record IPAHE1、IPAHE2The slopes of the curves are respectively sigma1、σ2(ii) a If after oxidation fBi2And fTe2Value of (a) compared to f before oxidationBi1And fTe1Is reduced and the slope σ is2Compared with the slope σ1Decreasing, then judge IPAHE1And IPAHE2Are all the dominant contributions of the upper surface states; if after oxidation fBi2And fTe2Value of (a) compared to f before oxidationBi1And fTe1Is reduced and the slope σ is2Compared with the slope σ1If the opposite sign appears, the judgment is madePAHE1Is dominated by the contribution of the upper surface state, and IPAHE2The contribution of the lower surface state is dominant; if after oxidation fBi2And fTe2Value of (a) compared to f before oxidationBi1And fTe1Is reduced and the slope σ is2Compared with the slope σ1Decreasing, then judge IPAHE1And IPAHE2Are 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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