A Raman method for characterizing electron-phonon coupling in type-II Weyl semimetals

Abstract

The present invention provides a Raman method for characterizing electron-phonon coupling in a type-II Weyl semimetal. The method comprises: mechanically exfoliating the type-II Weyl semimetal to obtain a test sample; performing variable-temperature Raman testing on the test sample to obtain a variable-temperature Raman spectrum; obtaining Raman vibration modes based on the variable-temperature Raman spectrum; performing data analysis on the Raman vibration modes to obtain a temperature-peak relationship diagram; and obtaining the electron-phonon coupling relationship based on the temperature-peak relationship diagram. This method characterizes Raman methods in a non-contact, non-destructive manner with high lateral and vertical resolution.

Description

Technical Field

[0001] The present invention relates to the technical field of semimetal characterization, and in particular to a Raman method for characterizing electron-phonon coupling of type II Weyl semimetal. Background Art

[0002] Electron-phonon coupling is a fundamental phenomenon in condensed matter physics, linked to high-temperature superconductivity, electron-phonon fluids, charge density waves, and charge-ordered states. It also influences electron transport in materials, such as resistivity in metals and carrier mobility in semiconductors. Weyl semimetals exhibit large magnetoresistance, high mobility, and anisotropic nonlinear responses. Some type-II Weyl semimetals exhibit superconductivity under conditions such as pressure, and some exhibit surface superconductivity. Electron-phonon coupling contributes significantly to these phenomena, but detection methods are lacking.

[0003] Raman spectroscopy is a molecular structure characterization technique based on the Raman scattering effect. By analyzing the Raman peaks, positions, and intensities obtained by Raman scattering from a material surface, various properties related to the material's structure can be qualitatively or quantitatively characterized. Confocal Raman microscopy is a non-contact, non-destructive detection method with excellent lateral and longitudinal resolution. It can also acquire Raman spectra of samples under external fields (such as polarization, magnetic fields, temperature, and force fields). Variable temperature Raman is a technique that collects Raman spectra of materials under applied temperature conditions. It is achieved by varying the temperature in a gradient and measuring the Raman signals at different temperature points. Variable temperature Raman has been widely used to study phonon modes in semiconductors and semimetals, but its application to type-II Weyl semimetals remains unexplored. Therefore, it is crucial to develop a Raman method to characterize electron-phonon coupling in type-II Weyl semimetals. Summary of the Invention

[0004] The object of the present invention is to provide a Raman method for characterizing electron-phonon coupling in type-II Weyl semimetals in a non-contact, non-destructive, high lateral resolution and high vertical resolution manner.

[0005] To achieve the above object, the present invention provides the following solutions:

[0006] A Raman method for characterizing electron-phonon coupling in a type-II Weyl semimetal comprises the following steps:

[0007] Mechanical exfoliation of the type II Weyl semimetal was performed to obtain a test sample;

[0008] Performing temperature-varying Raman testing on the test sample to obtain a temperature-varying Raman spectrum;

[0009] The Raman vibration mode is obtained according to the temperature-varying Raman spectrum;

[0010] Data analysis of Raman vibration modes was performed to obtain a temperature-peak relationship diagram;

[0011] The electron-phonon coupling relationship was obtained based on the temperature-peak relationship diagram.

[0012] Optionally, the sampling temperature of the variable temperature Raman test includes the temperature at which electron-phonon coupling and phonon-phonon coupling act in opposition to each other.

[0013] Optionally, the Raman vibration mode is obtained according to the variable temperature Raman spectrum, specifically by fitting the variable temperature Raman spectrum with a Lorentz function using the constructed phonon-phonon coupling model to obtain the Raman vibration mode.

[0014] Optionally, the judgment criterion of the Raman vibration mode is: within a preset temperature range, when the temperature decreases, the Raman peak position becomes higher and the peak width becomes smaller, and the current state is determined to be a Raman vibration mode.

[0015] Optionally, the x-axis of the temperature-peak relationship graph is temperature, and the y-axis is peak position or peak width.

[0016] Optionally, the electron-phonon coupling relationship is obtained according to the temperature-peak relationship diagram, specifically by fitting and analyzing the data in the temperature-peak relationship diagram using the first function and the second function to obtain the electron-phonon coupling relationship.

[0017] Optionally, the expression of the first function is: w=w0-A*(1+2 / (exp((h*w0 / (2*k*x))-1)); where w is the phonon frequency, w0 is the bare phonon frequency, h is the reduced Planck constant, k is the Boltzmann constant, x is the temperature, and A is the first variation parameter.

[0018] Optionally, the expression of the second function is: y=y0+B*(1+2 / (exp((h*w0) / (2*k*x))-1)); wherein y is the phonon linewidth, y0 is the absolute zero linewidth produced under the influence of electron-phonon coupling, w0 is the bare phonon frequency, B is the second variation parameter, h is the reduced Planck constant, k is the Boltzmann constant, and x is the temperature.

[0019] According to the specific embodiments provided by the present invention, the present invention discloses the following technical effects: the Raman method for characterizing the electron-phonon coupling of a type-II Weyl semimetal provided by the present invention comprises: mechanically exfoliating the type-II Weyl semimetal to obtain a test sample; performing a variable temperature Raman test on the test sample to obtain a variable temperature Raman spectrum; obtaining a Raman vibration mode based on the variable temperature Raman spectrum; performing data analysis on the Raman vibration mode to obtain a temperature-peak relationship diagram; and obtaining an electron-phonon coupling relationship based on the temperature-peak relationship diagram. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the embodiments. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative work.

[0021] Figure 1 A flow chart of a Raman method for characterizing electron-phonon coupling in a type-II Weyl semimetal according to an embodiment of the present invention;

[0022] Figure 2 This is a temperature-varying Raman spectrum diagram of an embodiment of the present invention;

[0023] Figure 3 This is a temperature-peak position relationship diagram of an embodiment of the present invention;

[0024] Figure 4 This is a temperature-peak width relationship diagram of an embodiment of the present invention;

[0025] Figure 5 This is a temperature-peak position difference relationship diagram of an embodiment of the present invention;

[0026] Figure 6 4 is a temperature-peak width difference relationship diagram of an embodiment of the present invention. DETAILED DESCRIPTION

[0027] The following will clearly and completely describe the technical solutions in the embodiments of the present invention in conjunction with the accompanying drawings. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative efforts are within the scope of protection of the present invention.

[0028] In order to make the above-mentioned objects, features and advantages of the present invention more obvious and easy to understand, the present invention is further described in detail below with reference to the accompanying drawings and specific embodiments.

[0029] like Figure 1 As shown, an embodiment of the present invention provides a Raman method for characterizing electron-phonon coupling in a type-II Weyl semimetal, comprising the following steps:

[0030] Step 100: Mechanically exfoliate the type II Weyl semimetal to obtain a test sample;

[0031] Specifically, the type II Weyl semimetal used in this example is a TaIrTe4 block. The block is placed on scotch tape and repeatedly dissociated until it becomes transparent. It is then transferred to a Si / SiO2 substrate and pressed to eliminate air bubbles. The Si / SiO2 tape is heated on an 80°C hotplate for 20 minutes, and the substrate is removed. An optical microscope is used to roughly determine the thickness of the TaIrTe4 on the substrate, selecting a light blue sample with a size of at least 5 × 5 μm.

[0032] Step 200: Performing a temperature-varying Raman test on the test sample to obtain a temperature-varying Raman spectrum;

[0033] Specifically, the vacuum chamber, displacement stage and Raman spectrometer of the variable temperature Raman test system (this embodiment uses the Horiba LabRAM HR Evolution confocal Raman spectrometer, and adopts the backscattering method) are coupled. The TaIrTe4 test sample is placed on the displacement stage, and the center position of the sample is sampled. The microscope magnification used is 50X, the grating is 1800gr / mm, the laser wavelength is 532nm, the power is 150μW, the integration time is 30s, and the number of integrations is 1. The sampling temperature is between 20K-300K, the temperature change rate is controlled at 1K / min, the sampling interval from 20K-200K is 10K, and the sampling interval from 200K-300K is 20K. Each temperature point is sampled 3-4 times, and the Horiba LabRAM HR Evolution confocal Raman spectrometer is used to obtain the following by backscattering. Figure 2 The Raman spectrum of the TaIrTe4 sample at 172 cm -1 The left and right peak positions change significantly with temperature, and the peak positions become higher as the temperature decreases.

[0034] More specifically, the sampling temperature of the variable temperature Raman measurement includes the temperature at which electron-phonon coupling and phonon-phonon coupling interact with each other.

[0035] Step 300: Obtaining Raman vibration modes according to the temperature-varying Raman spectrum;

[0036] Specifically, a phonon-phonon coupling model was constructed, and the temperature-varying Raman spectrum was fitted using the Lorentz function. The 172 cm-1 spectral region of the TaIrTe4 test sample was collected in the fitting results. -1 The peak position and peak width information of the left and right peaks are obtained to obtain the Raman vibration mode.

[0037] More specifically, the criterion for determining the Raman vibration mode is: within a certain temperature range starting from room temperature, if the Raman peak position becomes higher and the peak width becomes smaller as the temperature decreases, the current state is determined to be a Raman vibration mode.

[0038] Step 400: Perform data analysis on the Raman vibration mode to obtain a temperature-peak relationship diagram; the temperature-peak relationship diagram includes: a temperature-peak position relationship diagram and a temperature-peak width relationship diagram.

[0039] Specifically, the temperature in the Raman vibration mode is the x-axis and the peak position or peak width is the y-axis. Figure 3 The temperature-peak position relationship diagram shown in FIG Figure 4 The temperature-peak width relationship diagram is shown. The existence of electron-phonon coupling is determined based on the temperature-peak position relationship diagram and the temperature-peak width relationship diagram. The judgment criteria are: both the Raman peak position and peak width show relatively obvious monotonic changes, and the temperature points where the changes occur are close.

[0040] More specifically, the temperature-peak position relationship diagram of this embodiment shows a monotonic change in peak position at 60K, and the temperature-peak width relationship diagram shows a monotonic change in peak width at 50K. The temperature points at which the monotonic changes in peak position and peak width occur are similar, confirming the existence of electron-phonon coupling.

[0041] Step 500: Obtaining the electron-phonon coupling relationship according to the temperature-peak relationship diagram.

[0042] Specifically, the peak position and peak width are fitted by the first function and the second function respectively. The peak position is selected in the range of 60K-300K, and the peak width is selected in the range of 50K-300K. The fitting curve of the temperature-peak relationship diagram is extended to 0K temperature to obtain the fitting curve of the phonon-phonon decay model from 0K-300K. Then, the peak position (or peak width) obtained by the variable temperature Raman test is subtracted from the peak position (or peak width) of the corresponding temperature point on the fitting curve of the phonon-phonon decay model to obtain the following: Figure 5 The temperature-peak position difference relationship diagram shown in FIG Figure 6 The temperature-peak width difference relationship diagram is shown. Both the temperature-peak position difference relationship diagram and the temperature-peak width difference relationship diagram use the phonon-phonon decay model fitting curve as the vertical axis zero value, and use temperature as the x-axis and the peak position difference (or peak width difference) corresponding to the temperature as the y-axis to achieve visualization of electron-phonon coupling. Figures 3 to 6 The shaded areas are the areas before monotonic changes occur.

[0043] More specifically, the expression of the first function is: w = w0-A*(1+2 / (exp((h*w0 / (2*k*x))-1)); the expression of the second function is: y = y0+B*(1+2 / (exp((h*w0) / (2*k*x))-1)); wherein w is the phonon frequency, w0 is the bare phonon frequency, y is the phonon linewidth, y0 is the absolute zero linewidth produced under the influence of electron-phonon coupling, h is the reduced Planck constant, k is the Boltzmann constant, x is the temperature, A and B are both variable parameters and are both positive values.

[0044] The beneficial effects of the present invention are as follows:

[0045] 1) The present invention fills the gap in electron-phonon coupling characterization methods and overcomes the influence of the environment on the measurement.

[0046] 2) Raman characterization is achieved in a non-contact, non-destructive manner with high lateral and vertical resolution.

[0047] The various embodiments in this specification are described in a progressive manner, and each embodiment focuses on the differences from other embodiments. The same or similar parts between the various embodiments can be referenced to each other.

[0048] The present invention uses specific examples to illustrate the principles and implementation methods of the present invention. The above examples are only intended to help understand the method and core concept of the present invention. At the same time, those skilled in the art will find that the specific implementation methods and application scopes may vary based on the concept of the present invention. In summary, the contents of this specification should not be construed as limiting the present invention.

Claims

1. A Raman method for characterizing electron-phonon coupling in type II Weyl semimetals, characterized in that: The steps include: Mechanical exfoliation of the type II Weyl semimetal was performed to obtain a test sample; Performing a temperature-varying Raman test on the test sample to obtain a temperature-varying Raman spectrum; Obtaining a Raman vibration mode according to the temperature-varying Raman spectrum; Performing data analysis on the Raman vibration mode to obtain a temperature-peak relationship diagram; The electron-phonon coupling relationship is obtained according to the temperature-peak relationship diagram; specifically: the data in the temperature-peak relationship diagram are fitted and analyzed by the first function and the second function to obtain the electron-phonon coupling relationship; the expression of the first function is: w=w0-A*(1+2 / (exp((h*w0 / (2*k*x))-1)); wherein w is the phonon frequency, w0 is the bare phonon frequency, h is the reduced Planck constant, k is the Boltzmann constant, x is the temperature, and A is the first variation parameter; the expression of the second function is: y=y0+B*(1+2 / (exp((h*w0) / (2*k*x))-1)); wherein y is the phonon linewidth, y0 is the absolute zero linewidth generated under the influence of electron-phonon coupling, w0 is the bare phonon frequency, B is the second variation parameter, h is the reduced Planck constant, k is the Boltzmann constant, and x is the temperature.

2. The Raman method for characterizing electron-phonon coupling in type-II Weyl semimetals according to claim 1, characterized in that: The sampling temperature of the variable temperature Raman test includes the temperature at which electron-phonon coupling and phonon-phonon coupling interact with each other.

3. The Raman method for characterizing electron-phonon coupling in type-II Weyl semimetals according to claim 1, characterized in that: The Raman vibration mode is obtained according to the variable temperature Raman spectrum, specifically by fitting the variable temperature Raman spectrum with a Lorentz function using a constructed phonon-phonon coupling model to obtain the Raman vibration mode.

4. The Raman method for characterizing electron-phonon coupling in type-II Weyl semimetals according to claim 1, characterized in that: The judgment standard of the Raman vibration mode is: within a preset temperature range, when the temperature decreases, the Raman peak position becomes higher and the peak width becomes smaller, and the current state is determined to be the Raman vibration mode.

5. The Raman method for characterizing electron-phonon coupling in type-II Weyl semimetals according to claim 1, characterized in that: The x-axis of the temperature-peak relationship diagram is temperature, and the y-axis is peak position or peak width.

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