134 Figure 5.2: (a) Band structure of single-layer graphene in reciprocal space (b) Magnified energy dispersion ratio around one of the Dirac points. 184 Figure 5.30: (a) Problems encountered in DFT simulations when changing the relative orientation of the top layer (b) Moving the top layer towards the bottom layer along an arbitrary direction (c) Result.
Introduction
Ways to open the gap
Potential applications of magnetic TIs
Scope of this thesis
Instrumentation and Methodology
Scanning tunneling microscopy (STM) system
- STM theory
- STM operation
- STM system overview
- STM probe head and upgrade
- A customized STM/AFM system
Several wires connect the STM head to the via on the top of the probe. Conductive wires for various purposes connect the STM head to the tip of the STM probe.
Physical Property Measurement System (PPMS)
- Customization of PPMS to add light to the sample chamber
We can insert the "puck" (Figure 2.14b) into the sample chamber with the puck insertion tool. The puck has 12 sockets on the bottom which connect to the PPMS out tray when the puck is installed.
Glove box and upgrade
- Customized commercial glovebox
Automatic purging – The replacement of the atmosphere inside the glove compartment can be achieved automatically using the purging valves. Glove compartment pressure control – The glove compartment pressure is automatically controlled by the programmable logic controller (PLC).
STM/STS studies of Topological Insulators
Introduction of binary TI 𝐵𝐵𝑖𝑖 2 𝑆𝑆𝑒𝑒 3 and ternary TI (𝐵𝐵𝑖𝑖, 𝑆𝑆𝑆𝑆) 2 𝑇𝑇𝑒𝑒 3
Sample choices and experimental setup
- List of samples investigated in this work
- Sample storage and preparation
- Sample annealing procedure
- Hydrofluoric Acid (HF) sample etching procedure
- Summary of the sample handling before STM experiments
To anneal the TI sample, we turned on the heater and monitored the sample temperature. The sample was taken out of the annealing chamber, transferred to a small stainless steel box, and then the stainless steel box was sealed inside the glove bag. Finally, the sample was taken out of the stainless steel case and loaded onto the STM sample stage, as shown in Figure 3.5.
Sometimes the sample was etched again with HF acid to make additional measurements on the same sample. Furthermore, the surface quality of the sample after cap removal was often questionable because the expected gapless Dirac spectrum did not always appear on the surface at high temperatures.
STM measurement settings
- Compromise between the accuracy and the speed
In addition, the Al-cap has the advantage of preserving the quality of the underlying TI sample for a long period of time. Our cryogenic STM is a wet system, meaning that the cryogen in the Dewars controls the temperature of the STM. For temperature-dependent STS maps, we often obtained such data during the warm-up of the STM.
For most of the STS described in this chapter, we chose the faster scan settings. In these following subsections, detailed studies of the binary TI/MTI system (TI: Bi2Se3, MTI: . Cr-doped Bi2Se3) are presented, which are complemented by some comparative studies of the ternary TI/MTI system (TI: (Bi, Sb)2Te3, MTI: Cr-doped (Bi,Sb)2Te3).
Structural characteristics from surface topographic studies
On the other hand, the Fourier transform (FT) of the surface topography appeared to be dependent on d1. We found that FT of the (1+6)-10% topography showed an expected hexagonal Bragg diffraction pattern for Bi2Se3 plus an additional, weak superlattice structure (Figure 3.6 (c)) attributable to the underlying Cr-doped Bi2Se3 . For example, a periodic substitution of Cr for Bi, as exemplified in Figure 3.6 (f) for a two-dimensional projection of the two Bi layers within a QL, gives a FT pattern (Figure 3.6 (e)) corresponding to it. in Figure 3.6 (c).
Another similar structure with a local Cr concentration of 1/8 (Figure 3.6 (h)) is also feasible within the experimental uncertainties of the superlattice constant and its angle with respect to the Bi lattice (Figure 3.6 (g)). In contrast, FT in Figure 3.6 (d) for the surface topography of a sample revealed only the hexagonal diffraction pattern of pure Bi2Se3 due to the relatively thick d1 layer.
Temperature evolution of the majority spectra in
- Summary and discussion
For (5+6)-10% binary TI/MTI, the temperature evolution of most dI/dV-vs.-V spectra is shown in Figure 3.7 (a), where we selected several representative spectra taken at different temperatures, with each spectrum obtained by averaging a number of similar spectra taken in different regions. For example, in addition to the majority spectrum taken at 114 K as shown in Figure 3.7 (a), there were three other types of minority spectra (Figure 3.7 (b)). The strongly conducting spectra and the absence of a noticeable energy gap until very low temperature from STS studies are also consistent with the temperature evolution of the band structures shown in Figure 4.19.
The majority of the spectra of the (5+6)-10% binary TI/MTI sample revealed similar gapless dI/dV for all temperatures, whereas those for (1+6)-10% and (3+6)-10 % binary TI /MTI samples exhibited gap opening at low temperatures. For ternary TI/MTI samples, the (1+6)-10% ternary TI/MTI also showed a consistent trend of surface gap opening and increase with decreasing temperature, although the spectral evolution with temperature was more monotonous, probably due to the fact The Fermi level of the sample intersects the bulk band, as shown in Figure 4.20.
Evolution of the surface gap with temperature
- Definition of the surface gap
- Summary and discussion of the temperature evolution
However, to study the temperature dependence of the DOS, we need to find a reasonable way to define the gap. The height of the threshold ℎ from the bottom of the curve is necessary to avoid the noise level at the bottom. For extrapolation, we can extrapolate the gap back to the bottom (minimum in the ideal situation) of the curve.
Therefore, the absolute value of the gap should not be taken too seriously and compared with the DOS. However, the gap size did not increase monotonically with decreasing temperature in TI/MTI binary samples.
Magnetic effect on the surface states
These results were not surprising when we compare the STS data with the electrical transport measurement for binary TI/MTI in Chapter 4. It was found that the magnetic field had no effect on the macroscopic transport properties until below 8 K. The measurements were incomplete due to the shutdown and therefore I cannot draw any conclusions.
However, ternary TI/MTI is expected to exhibit novel responses to magnetic fields from electrical transport measurements. Future field-dependent STS measurements at low temperatures on the ternary TI/MTI samples would be useful to better understand the microscopic origin of new field dependence in the electrical transport measurements.
Impurity resonance
- The appearance of the magnetic impurity resonance curve
- Spatial distributions of the magnetic impurity resonance
- The number of impurity resonance vs. temperature
- Summary of impurity resonances
Interestingly, we also observed some single peak magnetic impurities where their peak position was the same as that of one of the double peaks. This extreme localization of magnetic impurity resonances in binary TI/MTI can be attributed to the topological protection of the surface state. In this particular sample, many domain boundaries were present, which were accompanied by the frequent occurrence of impurity resonances.
Future investigations of Al-capped TI/MTI binary samples to Al-capped TI/MTI binary samples may shed light on the importance of Se capping. Magnetic impurity resonances due to isolated spins appeared only in the TI/MTI bilayer binary samples, which can be attributed to the absence of long-range ferromagnetism and the dominance of the surface state for better topological shielding.
Summary and future project
The contrasts between the binary and ternary TI/MTI samples may be mainly due to the differences in their band structures and Fermi level, in particular the persistent presence of bulk band contributions to the tunnel spectra in the ternary TI/MTI system differs significantly from the dominant contributions to the surface state in the TI/MTI binary system. The external magnetic field seemed to have relatively small effects on the gap size of binary TI/MTI at T = 18 K, although the gap homogeneity could be significantly improved. Ternary TI/MTI samples at low temperatures can yield very interesting results as a function of the magnetic field.
Electrical transport measurements in TI/MTI ternary samples have shown the interesting influence of circularly polarized light on the anomalous Hall effect. It will be interesting to investigate how circularly polarized light can affect the tunneling spectra of TI/MTI ternary samples and compare the findings with transport measurements.
Electrical transport studies of Magnetic
Transport studies of bilayer 𝐵𝐵𝑖𝑖 2 𝑆𝑆𝑒𝑒 3 / 𝐶𝐶𝐶𝐶-𝐵𝐵𝑖𝑖 2 𝑆𝑆𝑒𝑒 3
- Sample characteristics and experimental setup
- Summary for transport results of binary TI/MTI samples
Transport studies of bilayer (𝐵𝐵𝑖𝑖 , 𝑆𝑆𝑆𝑆) 2 𝑇𝑇𝑒𝑒 3 / Cr- (𝐵𝐵𝑖𝑖, 𝑆𝑆𝑆𝑆) 2 𝑇𝑇𝑒𝑒 3
- Results from electrical transport measurements on
- Low magnetic field sweep analysis of the
- High magnetic field sweep analysis of the
- Temperature sweep studies of the
- Comparison of the field cooling curves taken on the
Effects of circularly polarized light on the AHE of bilayer
- Effect of circularly polarized light on
However, we may be able to observe changes in the Rxy signal due to its dependence on sample magnetization, which prompted our investigations of electrical transport properties under circularly polarized light excitation. Circularly polarized light appears to increase the magnetization in one direction (B < 0) while having almost no effect in the other direction (B > 0), as shown from the coercive field of the falling field in Fig. 4.18 (a). By measuring T with and without circularly polarized light, we found that Rxy with and without light deviated from each other below 7 K, as shown in Figure 4.17(b), which also confirmed the effect of circularly polarized light on AHE.
Based on these promising preliminary measurements, we expect more interesting effects on the AHE of dual triplet TI/MTI samples from the application of circularly polarized light with different wavelengths and power densities. However, detailed studies of field sweeps at different temperatures both with and without light will be necessary to fully understand the interplay of photoinduced carrier densities and spin stretching by circularly polarized light.
Discussion and Summary
𝑇𝑇𝐶𝐶𝑏𝑏𝑢𝑢𝑣𝑣𝑏𝑏~25𝐾𝐾 where the surface state is gapless, with massless surface electrons being the majority carriers and bulk holes the minor carriers. At = 𝑇𝑇𝑥𝑥2 ~ 7 𝐾𝐾 the bulk holes become the majority carriers, and the surface gap becomes comparable to the indirect bulk gap. For 𝑇𝑇 >𝑇𝑇𝐶𝐶𝑏𝑏𝑢𝑢𝑣𝑣𝑏𝑏 ~ 30 K, the surface gap is closed so that both the surface-state massless Dirac holes and bulk holes contribute to the sample conductance (4.2).
For 𝑇𝑇𝑥𝑥~10 K <𝑇𝑇< 𝑇𝑇𝐶𝐶𝑏𝑏𝑢𝑢𝑣𝑣𝑏 4.21 (b)). Below Tx ~ 10 K, the surface gap becomes large enough so that the dominant carriers are the bulk holes (Figure 4.21(c)).
Graphene and interlayer interaction
Introduction
- The tight-binding model of monolayer graphene
- Tight-binding model of bilayer graphene
- Twist bilayer graphene and the corresponding
If we introduce the momentum 𝒑𝒑=ℏ𝒌𝒌 − ℏ𝑲𝑲± which is measured from the center of the 𝑲𝑲±=𝑲𝑲𝝃𝝃 point becomes the transfer integral matrix. The direct backscattering of opposite momentum is prohibited due to the conservation of pseudospin. For any twist angle except 0 degrees, the twist generates superstructures known as the Moiré patterns (Figure 5.6(a)), which are easily seen in the STM topography.
The size of the superlattice depends on the twist angle, and it can be hundreds of times larger than the lattice constant of graphene at a very small twist angle. Since the size of the reciprocal grid is inversely proportional to the size of the spatial grid, the reciprocal supergrid is small.
Single-step Growth of Graphene by Plasma-enhanced Chemical
- Single-step deposition of high-mobility graphene at
- STM imaging of PECVD grown graphene
- Simulations for the topography and FFT of 2D
- Simulations for graphene on single crystalline
- Graphene on copper (111) surface
- Graphene on copper (100) surface
- Summary of PECVD graphene growth on Cu substrates
As the twist angle θ increases, the size of the Moiré superlattices decreases, while the size of the inner hexagon in the FFT increases and also rotates relative to the outer hexagon. FFT studies are therefore a powerful way to verify the existence of Moiré superlattices. If a similar pattern occurs, it may explain the origin of the Moiré pattern observed in the FFT image of the STM study.
The center of the orange oval for the 25 degree image is also consistent with the real STM image. On the other hand, FFTs of the real space topography images are shown to reveal clear Moiré superlattices in the reciprocal space beyond the reciprocal graphene lattice.
Direct growth of mm-size twisted bilayer graphene by
- Experimental setup and growth conditions
- STM images for PECVD-grown twisted bilayer graphene
- Simulations for the Moiré pattern and FFT of
- Summary
In Figure 5.23 (a), the 4 nm scale corresponds to approximately three lattice constants, and therefore the size of the superlattice is consistent with our calculation from the FFT. For a twist angle near the magic angle, the size of the Moiré superlattices is about 12 to 15 nm. However, closer inspection of the topography of the simulation is not quite the same as the STM image (Figure 5.26, first row).
However, the Bragg spots of the inner hexagon are much larger than those in the FFT of the (100 nm x 100 nm) topography. STM investigation can reveal different Moiré patterns as a function of the twist angle of the bilayer graphene.
Interaction between layers of bilayer graphene by density
- Introduction of the density function theory
- Optimization of graphite and bilayer graphene
- Optimization of bilayer graphene for mixed stacking
- Top layer moving in any direction relative to
- Analysis of the free energy map
- Lock-in effect for motion along the armchair direction
- Summary
Conclusion and future project
Future project
