CN112881773A - Method for measuring magnetophotocurrent caused by larmor precession in topological insulator Bi2Te3 - Google Patents
Method for measuring magnetophotocurrent caused by larmor precession in topological insulator Bi2Te3 Download PDFInfo
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- 239000012212 insulator Substances 0.000 title claims abstract description 53
- 229910002899 Bi2Te3 Inorganic materials 0.000 title claims abstract description 36
- 238000000034 method Methods 0.000 title claims abstract description 17
- 230000010287 polarization Effects 0.000 claims abstract description 35
- 230000005284 excitation Effects 0.000 claims abstract description 8
- 239000000758 substrate Substances 0.000 claims description 32
- 230000003287 optical effect Effects 0.000 claims description 23
- 238000012360 testing method Methods 0.000 claims description 16
- 230000000694 effects Effects 0.000 claims description 8
- 230000008859 change Effects 0.000 claims description 4
- 238000001451 molecular beam epitaxy Methods 0.000 claims description 4
- RTAQQCXQSZGOHL-UHFFFAOYSA-N Titanium Chemical compound [Ti] RTAQQCXQSZGOHL-UHFFFAOYSA-N 0.000 claims description 3
- 239000000969 carrier Substances 0.000 claims description 3
- 238000005566 electron beam evaporation Methods 0.000 claims description 3
- 230000004907 flux Effects 0.000 claims description 3
- PCHJSUWPFVWCPO-UHFFFAOYSA-N gold Chemical compound [Au] PCHJSUWPFVWCPO-UHFFFAOYSA-N 0.000 claims description 3
- 239000010931 gold Substances 0.000 claims description 3
- 229910052737 gold Inorganic materials 0.000 claims description 3
- PYZRQGJRPPTADH-UHFFFAOYSA-N lamotrigine Chemical compound NC1=NC(N)=NN=C1C1=CC=CC(Cl)=C1Cl PYZRQGJRPPTADH-UHFFFAOYSA-N 0.000 claims description 3
- 229960001848 lamotrigine Drugs 0.000 claims description 3
- 239000000696 magnetic material Substances 0.000 claims description 3
- 238000001755 magnetron sputter deposition Methods 0.000 claims description 3
- 238000007747 plating Methods 0.000 claims description 3
- 239000000126 substance Substances 0.000 claims description 3
- 239000010936 titanium Substances 0.000 claims description 3
- 229910052719 titanium Inorganic materials 0.000 claims description 3
- 230000005676 thermoelectric effect Effects 0.000 claims description 2
- 239000000463 material Substances 0.000 description 11
- 238000005259 measurement Methods 0.000 description 4
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- 238000005516 engineering process Methods 0.000 description 3
- 239000010408 film Substances 0.000 description 2
- 238000011160 research Methods 0.000 description 2
- 239000010409 thin film Substances 0.000 description 2
- 230000005355 Hall effect Effects 0.000 description 1
- 230000009471 action Effects 0.000 description 1
- 230000009286 beneficial effect Effects 0.000 description 1
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- 239000012535 impurity Substances 0.000 description 1
- 230000001678 irradiating effect Effects 0.000 description 1
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- 229910052751 metal Inorganic materials 0.000 description 1
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Abstract
The invention relates to a method for measuring a topological insulator Bi2Te3A method of magnetophotocurrent induced by mesomeric precession. The method is to a three-dimensional topological insulator Bi2Te3A magnetic field along the x direction in a plane is applied, 1064nm laser is vertically irradiated (along the z direction) on the middle point of the connecting line of the two electrodes on the sample, and the magnetophotocurrent is excited. Photocurrent under different polarization states is obtained by rotating the quarter-wave plate, and then magnetophotocurrent caused by larmor precession is excited by circularly polarized light through formula fitting. The principle is that the circularly polarized light excitation generates spin polarization S in the vertical directionzThe magnetic field in the x-direction polarizes the spin generated in the topological insulator SzProducing lamor precession and thus spin polarization S in the y directiony. Spin-polarization in this direction will cause charge flow in the x-direction, i.e. circular polarization caused by lamor precessionA magneto current.
Description
Technical Field
The invention belongs to the field of spintronics, and particularly relates to a method for measuring magneto-induced photocurrent caused by larmor precession in a topological insulator Bi2Te 3.
Background
The topological insulator is different from a common metal or insulator, has potential application prospect in the fields of spintronics, quantum computing and the like due to unique physics, and is paid much attention in the fields nowadays. Bi2Te3The material is a three-dimensional topological insulator material and is characterized by having a topological protected gapless surface state, the surface state has time reversal symmetry, and electrons in the surface state are dirac electrons with a locked spin momentum direction. The surface state of such properties enables the material to suppress scattering of nonmagnetic impurities to a large extent, so that surface electrons thereof have extremely high electron mobility. The three-dimensional topological insulator material has good application prospect in the fields of quantum computation, novel spin electronic devices and the like.
In general, a circularly polarized light current technology (referred to as CPGE) can be used as an effective means for researching a spin polarization photocurrent signal of a three-dimensional topological insulator, and a photoinduced current can be generated by exciting surface state electron spin polarization through circularly polarized light. The photo-induced reverse spin Hall effect technology (noted as PISHE) can also be used for detecting spin polarization photocurrent signals in the three-dimensional topological insulator, and the surface-state electrons can be excited by circularly polarized light to generate spin polarization photo-induced current.
However, the problem now is that when using CPGE, due to the spin polarization generated by excitation with circularly polarized light, the circularly polarized light current effect is generated and at the same time other photocurrent effects caused by circularly polarized light, such as photon drag effect, are introduced. Three-dimensional topological insulator Bi2Te3In a surface state of C3vPoint group symmetry with posture D3dPoint group symmetry. The bulk state and the surface state of the material can generate photon drag effectIt is desirable to find ways to separate out the contributions from the different effects from the total photocurrent. When the PISHE is used for research, the three-dimensional topological insulator has an upper surface state and a lower surface state, when the PISHE current is generated, the laser can reach the upper surface state and the lower surface state of the sample, so that photocurrent signals with opposite directions are generated, in addition, the PISHE current signals can be introduced into the bulk state, so that the components of the spin polarization photocurrent become complex, and a searching method is needed to extract the spin polarization photocurrent signals of the surface states. Therefore, the two common technologies are still applied to research the spin polarization photocurrent of the surface state of the three-dimensional topological insulator, and the problems need to be overcome.
Disclosure of Invention
The invention aims to provide a method for measuring magneto-induced photocurrent caused by larmor precession in a topological insulator Bi2Te3, which is convenient to realize, low in cost and accurate in measurement result.
In order to achieve the purpose, the technical scheme of the invention is as follows: method for measuring magneto-induced photocurrent caused by larmor precession in topological insulator Bi2Te3, and three-dimensional topological insulator Bi2Te3Growing on a Si substrate; three-dimensional topological insulator Bi2Te3Growing with a molecular beam epitaxy apparatus, the method comprising the steps of:
step S1, obtaining a three-dimensional topological insulator Bi2Te3Sample and in three-dimensional topological insulator Bi2Te3Growing a 10nm titanium electrode on a sample by magnetron sputtering, and plating a 100nm gold electrode by electron beam evaporation, wherein the electrode is a square electrode with the side length of 0.5mm, and the electrode distance is about 2.5 mm;
step S2, using 1064nm laser as excitation light source, making the laser vertically irradiate on the three-dimensional topological insulator Bi through the chopper, polarizer and quarter wave plate2Te3The position of the midpoint of the connecting line of the two electrodes on the sample; the diameter of the light spot is smaller than the distance between the two electrodes;
step S3, three-dimensional topological insulator Bi2Te3The sample is placed between the N and S poles of the permanent magnet with the electrode lines along the x direction and the magnetic field parallel to the sample plane and along the x directionThe direction of the solution is as follows; changing the magnitude of the magnetic field in the x direction by rotating the magnet; setting the included angle between the magnetic field and the x direction as theta; rotating the quarter-wave plate from 0 degree to 360 degrees at each theta angle, taking 5 degrees as a step length, amplifying the photoelectric current at each quarter-wave plate angle by a current amplifier, amplifying the current by a phase-locked amplifier, and then entering a data card for collection;
step S4, rotating the permanent magnet to change the included angle between the magnetic field and the x direction to be theta, wherein the magnetic field B along the x direction and the y direction in the sample planexAnd ByCan be expressed as:
Bx＝B×sinθ,By＝B×cosθ (1)
step S5, fitting the photocurrent at each magnetic field rotation angle by using a magnetophotocurrent formula to extract a circularly polarized magnetophotocurrent JCNamely, the following formula (2) is adopted for fitting:
wherein, JCExpressing the circularly polarized magneto-optical current caused by unit optical power and unit magnetic field strength along the x direction, I is the optical intensity, phi is the rotation angle of a quarter wavelength, j0Is the background photocurrent, J, caused by the thermoelectric and photovoltaic effectsLMPGE1And JLMPGE2Is the magneto-induced photocurrent induced by linearly polarized light excitation;
step S6, the y-direction spin polarization by the lamor precession is:
wherein the content of the first and second substances,
ωLis the frequency of the lamor precession, S0zIs vertically incident circular polarizationThe spin polarization direction of the electrons is induced by light, g is the ratio of the spin magnetic moment and the spin angular momentum of the electrons, μBIs a magnetic flux of a Bohr magnetic material,
is a reduced Planck constant;
τS、τS⊥、τS∥respectively a three-dimensional topological insulator Bi2Te3The total spin relaxation time, the transverse spin relaxation time, the longitudinal spin relaxation time of spin-polarized carriers; from equations (3) and (4), the following direct relationship of the internal spin polarization to the in-plane magnetic field can be obtained:
namely, it is
Wherein, b ═ g muBτS⊥S0z，
SyIs y-direction spin polarization induced by lamor precession; if the measured circularly polarized magneto-optical current J isCIs caused by lamotrigine, the following relationship must be satisfied:
wherein m and n are fitting constants;
step S7, extracting the circularly polarized magneto-optical current J from the experimental data fitting by the formula (2) in the step S5CAs ordinate, magnetic field B in x-directionxPlotting is the abscissa; fitting the data using equation (6) if the coefficient of fit is determined as R2Above 0.75, the fit is better, indicating that the measuredCircularly polarized magneto-optical current JCCaused by lamor precession;
step S8, performing photocurrent test on the Si substrate by using the same experimental setup and test means, and using the obtained data as Si-compliant C2vFitting a magneto-induced photocurrent formula with point group symmetry to obtain a circularly polarized magneto-induced photocurrent of the Si substrate; data of circular polarized magnetophotocurrent of Si substrate and topological insulator Bi2Te3The measured circularly polarized magnetophotocurrent is compared, and if the former is more than 5 times larger than the latter, the measured circularly polarized magnetophotocurrent is indicated to be generated by a three-dimensional topological insulator and not by the Si substrate.
In one embodiment of the present invention, in step S3, the variation range of the included angle θ between the magnetic field and the x-axis is: the rotation is controlled by a stepping motor from 0 to 360 degrees, and the rotation is performed in 15 degrees as one step.
In one embodiment of the present invention, the measured magnetic field strength B at different magnetic field strengths in step S8xPhotocurrent C of lower Si substrate2vFitting a photocurrent formula of point group symmetry, specifically: performing photocurrent test on Si substrate by using the same experimental setup and test means, and using C as the obtained data2vAnd fitting the photocurrent formula of point group symmetry, wherein the fitting formula is as follows:
where I is the optical power of the incident laser light, Bx、ByIs a magnetic field in the x and y directions, S4Is a magnetophotocurrent, S, excited by circularly polarized light in an in-plane magnetic field1、S2And S3Is the photocurrent, j, generated under the magnetic field due to the special symmetry of the sample, i.e. with optical symmetry0Is the background photocurrent caused by the thermoelectric effect and the photovoltaic effect, and the circularly polarized magneto-induced photocurrent S in the Si substrate is obtained by fitting4。
Compared with the prior art, the invention has the following beneficial effects: the method has the advantages of accurate measurement result, simplicity, high efficiency and high feasibility, and is favorable for popularization and application in the future.
Drawings
FIG. 1 is a schematic diagram of experimental optical paths of an embodiment of the present invention.
Fig. 2 is an experimental schematic diagram of the introduction of variable magnitude in-plane magnetic fields by rotatable permanent magnets in an embodiment of the present invention.
FIG. 3 is a schematic diagram of the principle of generation of circularly polarized magnetocurrents caused by lamor precession in an embodiment of the present invention.
Fig. 4 is a graph showing the variation of the x-axis photocurrent with the quarter-wave plate rotation angle obtained from the tests in steps 3 and 4 when the rotation angle θ of the magnetic field is 150 ° in the embodiment of the present invention. Wherein the small circles are experimental data and the solid line is a fitted curve using equation (2). The photocurrent data has been divided by the optical power value, i.e. normalized with the optical power.
FIG. 5 shows three-dimensional topological insulators Bi of thickness 7 and 20nm, respectively, measured in an embodiment of the invention2Te3Magneto-optical current J normalized by optical power in thin filmcI magnetic field B along x directionxAnd the magneto-optical current S measured in the Si substrate normalized by the optical power4I magnetic field B along x directionxThe change curve of (2). Where I is the optical power and the solid line is the fitting result using equation (6).
Detailed Description
The technical scheme of the invention is specifically explained below with reference to the accompanying drawings.
The invention relates to a method for measuring magneto-induced photocurrent caused by larmor precession in a topological insulator Bi2Te3, and a measured three-dimensional topological insulator Bi2Te3Growing on a Si substrate; three-dimensional topological insulator material Bi2Te3Growing by using a Molecular Beam Epitaxy (MBE) device; the method proposes to obtain a topological insulator Bi2Te3The method for producing the circularly polarized magneto-induced photocurrent caused by the lamor precession comprises the following specific steps:
step S1, obtaining topological insulator Bi2Te3Sample and in the topological insulator Bi2Te3Growing a 10nm titanium electrode on a sample by magnetron sputtering, and plating a 100nm gold electrode by electron beam evaporation, wherein the electrode is a square electrode with the side length of 0.5mm, and the electrode distance is about 2.5 mm;
and step S2, using 1064nm laser as excitation light source, making the laser pass through chopper, polarizer and quarter wave plate, and vertically irradiating the laser at the midpoint of the connecting line of the two electrodes on the sample. The diameter of the light spot is smaller than the distance between the two electrodes;
step S3, the sample is placed between the N pole and S pole of the permanent magnet, the electrode is connected along the x direction, and the magnetic field is parallel to the sample plane and along the x direction. The magnitude of the magnetic field in the x-direction is changed by rotating the magnet. Setting the included angle between the magnetic field and the x direction as theta; rotating the quarter-wave plate from 0 degree to 360 degrees at each theta angle, taking 5 degrees as a step length, amplifying the photoelectric current at each quarter-wave plate angle by a current amplifier, amplifying the current by a phase-locked amplifier, and then entering a data card for collection;
step S4, rotating the permanent magnet to change the included angle between the magnetic field and the x direction to be theta, wherein the magnetic field B along the x direction and the y direction in the sample planexAnd ByCan be expressed as:
Bx＝B×sinθ,By＝B×cosθ (1)
step S5: fitting the photoelectric current under each magnetic field rotation angle by using a magneto-induced current formula to extract a circularly polarized magneto-induced current JCNamely, the following formula (2) is adopted for fitting:
wherein, JCExpressing the circularly polarized magneto-optical current caused by unit optical power and unit magnetic field strength along the x direction, I is the optical intensity, phi is the rotation angle of a quarter wavelength, j0Is the background photocurrent, J, caused by the thermoelectric and photovoltaic effectsLMPGE1And JLMPGE2Is the magneto-induced photocurrent induced by linearly polarized light excitation.
Step S6, the y-direction spin polarization by the lamor precession is:
wherein the content of the first and second substances,
ωLis the frequency of the lamor precession, S0zIs the spin polarization direction of electrons caused by perpendicularly incident circularly polarized light, g is the ratio of the electron spin magnetic moment to the spin angular momentum, muBIs a magnetic flux of a Bohr magnetic material,
is a reduced planck constant.
τS、τS⊥、τS∥Respectively being three-dimensional topological insulator material Bi2Te3Total spin relaxation time, transverse spin relaxation time, longitudinal spin relaxation time of spin-polarized carriers. From equations (3) and (4), the following direct relationship of the internal spin polarization to the in-plane magnetic field can be obtained:
namely, it is
Wherein, b ═ g muBτS⊥S0z，
SyIs the y-direction spin polarization caused by lamor precession. If the measured circularly polarized magneto-optical current J isCIs caused by lamotrigine, the following relationship must be satisfied:
where m and n are fitting constants.
Step S7, extracting the circularly polarized magneto-optical current J from the experimental data fitting by the formula (2) in the step S5CAs ordinate, magnetic field B in x-directionxPlotted on the abscissa. Fitting the data using equation (6) if the coefficient of fit is determined as R2Above 0.75, the fit is better, indicating a measured circularly polarized magnetophotocurrent JCCaused by lamor precession.
Step S8, performing photocurrent test on the Si substrate by using the same experimental setup and test means, and using the obtained data as Si-compliant C2vAnd fitting the magneto-induced photocurrent formula with point group symmetry to obtain the circular polarization magneto-induced photocurrent of the Si substrate. Data of circular polarized magnetophotocurrent of Si substrate and topological insulator Bi2Te3The measured circularly polarized magnetophotocurrent is compared, and if the former is more than 5 times larger than the latter, the measured circularly polarized magnetophotocurrent is indicated to be generated by a three-dimensional topological insulator and not by the Si substrate.
Further, in step S3, the variation range of the included angle θ between the magnetic field and the x-axis is: the rotation is controlled by a stepping motor from 0 to 360 degrees, and the rotation is performed in 15 degrees as one step.
Further, the measured magnetic field strength B at different magnetic field strengths in step S8xPhotocurrent C of lower Si substrate2vFitting a photocurrent formula of point group symmetry, wherein the concrete contents are as follows: performing photocurrent test on Si substrate by using the same experimental setup and test means, and using C as the obtained data2vAnd fitting the photocurrent formula of point group symmetry, wherein the fitting formula is as follows:
wherein I is incident laserOptical power of light, Bx、ByIs a magnetic field in the x and y directions, S4Is a magnetophotocurrent, S, excited by circularly polarized light in an in-plane magnetic field1、S2And S3Is due to the special symmetry of the sample (i.e. having optical (gyropic) symmetry) under the magnetic field]Photocurrent j generated0Is the background photocurrent due to the thermoelectric and photovoltaic effects. Fitting to obtain circularly polarized magneto-induced photocurrent S in Si substrate4。
In the embodiment, a rotatable permanent magnet is used for generating an in-plane magnetic field with adjustable size in a film plane, 1064nm laser passes through a polarizer and a quarter-wave plate and then irradiates on a sample, periodically-changed polarized light is generated by rotating the quarter-wave plate, and circular polarization magneto-induced photocurrent generated by circular polarization laser is extracted by fitting the generated magneto-induced photocurrent. Measurement of Bi of different thicknesses2Te3And photocurrent generated by the Si substrate under different magnetic field sizes, extracting circular polarization magneto-induced photocurrent by using a photocurrent fitting formula of different point group symmetry materials, comparing the circular polarization magneto-induced photocurrent of the three-dimensional topological insulator material with the photocurrent generated by the substrate material Si to eliminate the contribution of the photocurrent generated by the substrate, and fitting circular polarization magneto-induced photocurrent data by using a spin polarization formula related to lamor precession to prove that the measured circular polarization magneto-induced photocurrent is really caused by the lamor precession.
In this example, the sample is Bi grown on a Si substrate by Molecular Beam Epitaxy (MBE)2Te3And the film thickness is about 7nm and 20 nm. A1064 nm laser is used, the laser power is 40mw, and the spot is about 1mm directly. The laser sequentially passes through the chopper, the polarizer and the quarter-wave plate and then irradiates the midpoint of a connecting line of the two electrodes of the sample. The frequency of the chopper is 217 Hz. As shown in fig. 1.
Fig. 2 gives an experimental schematic of the introduction of a variable magnitude in-plane magnetic field by a rotatable permanent magnet. The vertical incidence laser causes the surface state electrons of the three-dimensional topological insulator to generate the spin polarization S in the z directionzIn-plane spin polarization component S generated by lamor precession under the action of magnetic fieldyThereby creating parallelismMagneto-photocurrent (MPGE) in the direction of the magnetic field is shown in fig. 3.
Photocurrent J in three-dimensional topological insulatorxConsisting of two parts, one of which is of particular symmetry with the material [ i.e. optical symmetry (gyroplac)]The linearly polarized magnetophotocurrent, denoted J, which is caused and caused by linearly polarized lightLMPGE1And JLMPGE2(ii) a One is the circularly polarized magnetophotocurrent, denoted J, caused by circularly polarized light associated with the spins in the surface statesCMPGE. Fig. 4 is a graph showing the variation of the x-axis photocurrent with the quarter-wave plate rotation angle obtained from the tests in steps 3 and 4 when the rotation angle θ of the magnetic field is 150 ° in the embodiment of the present invention. Wherein the small circles are experimental data and the solid line is a fitted curve using equation (2). The photocurrent data has been divided by the optical power value, i.e. normalized with the optical power.
In order to eliminate the influence of the Si substrate, the Si substrate is subjected to circular polarization magneto-induced photocurrent test by the same experimental setup and test means, and the obtained data is used as C2vMagneto-induced photocurrent formula corresponding to point group [ formula (4) ]]Fitting is carried out, and the obtained determination coefficient R is obtained by fitting2About 0.9, indicating a better fit. By fitting, we obtain different magnetic field strengths BxThe lower circularly polarized magnetophotocurrent is shown by the sign of the triangle in fig. 5 (having been divided by, i.e. normalized by, the optical power). FIG. 5 also shows the three-dimensional topological insulator Bi of the present embodiment with a thickness of 7 and 20nm, respectively2Te3Magneto-optical current J normalized by optical power in thin filmcI magnetic field B along x directionxWherein I is the optical power. The solid line is the fitting result using equation (6). As can be seen from FIG. 5, the circularly polarized magnetophotocurrent of the Si substrate was about Bi2Te35% of the circularly polarized magneto-optical current in the sample indicates that the signal we measured is from the three-dimensional topological insulator Bi2Te3Rather than a Si substrate.
The solid line in FIG. 5 shows the fitting result of equation (6), and it can be seen that the fitting effect is better, the coefficient of determination is about 0.8, indicating that Bi2Te3The circularly polarized magnetophotocurrent in the sample is indeed determined by the pullingCaused by moat precession.
It can be seen from the above embodiments that the implementation of this embodiment is more convenient, and is succinct high-efficient, and the measurement is comparatively accurate. The above description is only a preferred embodiment of the present invention, and all equivalent changes and modifications made in accordance with the claims of the present invention should be covered by the present invention.
Claims (3)
1. A method for measuring magneto-induced photocurrent caused by larmor precession in a topological insulator Bi2Te3 is characterized in that the three-dimensional topological insulator Bi2Te3Growing on a Si substrate; three-dimensional topological insulator Bi2Te3Growing with a molecular beam epitaxy apparatus, the method comprising the steps of:
step S1, obtaining a three-dimensional topological insulator Bi2Te3Sample and in three-dimensional topological insulator Bi2Te3Growing a 10nm titanium electrode on a sample by magnetron sputtering, and plating a 100nm gold electrode by electron beam evaporation, wherein the electrode is a square electrode with the side length of 0.5mm, and the electrode distance is about 2.5 mm;
step S2, using 1064nm laser as excitation light source, making the laser vertically irradiate on the three-dimensional topological insulator Bi through the chopper, polarizer and quarter wave plate2Te3The position of the midpoint of the connecting line of the two electrodes on the sample; the diameter of the light spot is smaller than the distance between the two electrodes;
step S3, three-dimensional topological insulator Bi2Te3The sample is placed between the N pole and the S pole of the permanent magnet, the electrode connecting line is along the x direction, and the magnetic field is parallel to the sample plane and along the x direction; changing the magnitude of the magnetic field in the x direction by rotating the magnet; setting the included angle between the magnetic field and the x direction as theta; rotating the quarter-wave plate from 0 degree to 360 degrees at each theta angle, taking 5 degrees as a step length, amplifying the photoelectric current at each quarter-wave plate angle by a current amplifier, amplifying the current by a phase-locked amplifier, and then entering a data card for collection;
step S4, rotating the permanent magnet to change the included angle between the magnetic field and the x direction to be theta, wherein the magnetic field B along the x direction and the y direction in the sample planexAnd ByCan be expressed as:
Bx＝B×sinθ,By＝B×cosθ (1)
step S5, fitting the photocurrent at each magnetic field rotation angle by using a magnetophotocurrent formula to extract a circularly polarized magnetophotocurrent JCNamely, the following formula (2) is adopted for fitting:
wherein, JCExpressing the circularly polarized magneto-optical current caused by unit optical power and unit magnetic field strength along the x direction, I is the optical intensity, phi is the rotation angle of a quarter wavelength, j0Is the background photocurrent, J, caused by the thermoelectric and photovoltaic effectsLMPGE1And JLMPGE2Is the magneto-induced photocurrent induced by linearly polarized light excitation;
step S6, the y-direction spin polarization by the lamor precession is:
wherein the content of the first and second substances,
ωLis the frequency of the lamor precession, S0zIs the spin polarization direction of electrons caused by perpendicularly incident circularly polarized light, g is the ratio of the electron spin magnetic moment to the spin angular momentum, muBIs a magnetic flux of a Bohr magnetic material,
is a reduced Planck constant;
τS、τS⊥、τS∥respectively a three-dimensional topological insulator Bi2Te3The total spin relaxation time, the transverse spin relaxation time, the longitudinal spin relaxation time of spin-polarized carriers; from equations (3) and (4), the following direct relationship of the internal spin polarization to the in-plane magnetic field can be obtained:
namely, it is
Wherein, b ═ g muBτS⊥S0z，
SyIs y-direction spin polarization induced by lamor precession; if the measured circularly polarized magneto-optical current J isCIs caused by lamotrigine, the following relationship must be satisfied:
wherein m and n are fitting constants;
step S7, extracting the circularly polarized magneto-optical current J from the experimental data fitting by the formula (2) in the step S5CAs ordinate, magnetic field B in x-directionxPlotting is the abscissa; fitting the data using equation (6) if the coefficient of fit is determined as R2Above 0.75, the fit is better, indicating a measured circularly polarized magnetophotocurrent JCCaused by lamor precession;
step S8, performing photocurrent test on the Si substrate by using the same experimental setup and test means, and using the obtained data as Si-compliant C2vFitting a magneto-induced photocurrent formula with point group symmetry to obtain a circularly polarized magneto-induced photocurrent of the Si substrate; data of circularly polarized magnetophotocurrent of Si substrateAnd a topological insulator Bi2Te3The measured circularly polarized magnetophotocurrent is compared, and if the former is more than 5 times larger than the latter, the measured circularly polarized magnetophotocurrent is indicated to be generated by a three-dimensional topological insulator and not by the Si substrate.
2. The method for measuring magnetophotocurrent, caused by larmor precession in a topological insulator Bi2Te3, as recited in claim 1, wherein in step S3, the included angle θ between the magnetic field and the x-axis is: the rotation is controlled by a stepping motor from 0 to 360 degrees, and the rotation is performed in 15 degrees as one step.
3. The method of claim 1, wherein the magnetophotocurrent, caused by lamotrir precession in the topological insulator Bi2Te3, is measured at different magnetic field strengths B measured in step S8xPhotocurrent C of lower Si substrate2vFitting a photocurrent formula of point group symmetry, specifically: performing photocurrent test on Si substrate by using the same experimental setup and test means, and using C as the obtained data2vAnd fitting the photocurrent formula of point group symmetry, wherein the fitting formula is as follows:
where I is the optical power of the incident laser light, Bx、ByIs a magnetic field in the x and y directions, S4Is a magnetophotocurrent, S, excited by circularly polarized light in an in-plane magnetic field1、S2And S3Is the photocurrent, j, generated under the magnetic field due to the special symmetry of the sample, i.e. with optical symmetry0Is the background photocurrent caused by the thermoelectric effect and the photovoltaic effect, and the circularly polarized magneto-induced photocurrent S in the Si substrate is obtained by fitting4。
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