SATL

29/06/2018

https://millie.pubpub.org/

The web page is for memorizing the late Professor Mildred S. Dresselhaus. You can find many photos in the history and messages from former collaborators that attended in the special symposium at MIT in the last November, 2017. The page was made by Miss Shoshi Cooper-san who is her grand daughter. We thank her for such a great work.

(R. Saito)

05/06/2018

"Zao meeing on nano-carbon and atomic layer materials" will be held in Zao mountain area, Yamagata from August 1st -2nd, 2018, which is organized by ATI (Advance Technology Institute). All presentation is given by Oral and in English. Because of the limited schedule, the number of participant is selected up to 30! Thus if you wish to join this meeting, visit to the web page and contact to Dr. Hiromichi Kataura! Everyone is welcome.

http://kataura.html.xdomain.jp/ATI-ZAO-2018.html

17/05/2018

http://www.fntg55.net/index_e.php

The next FNTG symposium on graphene and atomic layer materials will be held in September, 2018, in Sendai. Please check the HP of 55th FNTG. Welcome to Sendai!! (R. Saito)

09/04/2018

In Youtube we introduce Faculty of Science, Tohoku University in Japanese. We all hope that many people will be interested in Science and Tohoku University. Since it hits in SNS in Japan last week, the video is accessed more than 90,000 times within 4 days! Less than a week, we got 100,000! on April 11 morning, JST. Thank you for your watching.

https://www.youtube.com/watch?v=IJn6LuaRGJM

p.s. Saito appears as nanotube and SATL reseacher.

東北大学理学部には7つの学科、大学院理学研究科には6つの専攻があります。理学部・理学研究科では、素粒子・原子核をはじめとしたミクロなスケールから、宇宙といったマクロなスケールにいたるまで、自然科学分野全....

31/03/2018

The SATL project is finished the five-year project today, on March 31st 2018.

We have had more than 40,000 accesses on this Facebook. Thank you for your kind access to our pages.

This page is open for a moment but the weekly news will finished in this week. We will send you some irregular news in this page until the new project will start in the near future.

We all hope that the field of science of atomic layer materials will increase significantly by collaborating with you.

Thank you. Bye!

Riichiro Saito, Tohoku University, SATL project leader

30/03/2018

Can Weyl semimetal be a meta-material?

When light propagates from air into water　with an angle , the propagation direction is changed to as is known as refraction phenomenon (Fig. 1). The refraction of light is given by Snell's law that is given by sinθ1/ sinθ2 = n2 / n1, where n1 and n2 are called refractive indices of the air and the water, respectively. The refraction index of conventional materials is a positive value. If the refraction index becomes a negative value, we can get a negative as shown as the purple arrow in Fig. 1. The material that has a negative refractive index is called meta-material. Since this century, many researchers investigate the metamaterial which was successful for making many new optical applications as shown in Fig.2. However, in order to make the negative refraction index, in particular negative permeability, they lined up split-ring resonators (C shaped metallic wire) which makes difficult to make meta-material for a large frequency.
We theoretically predict that Weyl semimetal such as "pyrochlore, Eu2Ir2O7" that is a new atomic-layer material can have a negative refraction index for special frequency region of electro-magnetic (EM) wave below the plasma frequency. In the normal metal, it is known that EM wave cannot (or can) propagate in the metal for smaller (or larger) frequency of the EM the plasma frequency. However, we found that for some frequency region below the plasma frequency, the EM wave can propagate in the Weyl semimetal in which we theoretically found that the Weyl semimetal has a negative refraction index. It means that we can make metamaterials without making split-ring resonators. The corresponding frequency region that was calculated for our special case is from 625THz to 800THz. Even though it is not an easy EM frequency to measure, we hope that some experimentalists will confirm our theoretical prediction.
By. R. Saito (Tohoku University)

Fig.1: Refraction of light that propagates from air to water. The incident angle and the refracted angle obeys Snell's law, that is sinθ1/ sinθ2 = n2 / n1, where n1 and n2 are called refractive indices of the air and the water, respectively. If n2 becomes negative, becomes negative, too, as shown by the purple arrow. Such material that n2

23/03/2018

Visualization with terahertz waves
What is the mechanism for seeing things with humans’ eyes? When the sun light or electric lamp is illuminated onto an object, light reflection or scattering occurs. When the eyes sense it, "color" is recognized. Differences in colors depend on the wavelength of light, thus leading to a colorful world. However, humans can recognize only a narrow range of wavelengths of electromagnetic waves (from about 380 to about 780 nanometers). Currently, in cutting-edge research fields, terahertz waves with a wavelength of about 30 to 3000 micrometers, are attracting much attention. By utilizing the terahertz waves, we can see something invisible in advanced fields of materials, life and space science, as well as industrial and medical practical fields.
Our laboratory conducts research on the development of terahertz imaging technologies and their applications. As one of the important subjects, we are aiming at developing high speed inspection systems for examining, without opening bags, whether prescribed medicines are wrong and whether foreign matters are mistakenly mixed into foods. Figure 1(a) compares an optical photograph and terahertz absorption images of the measured drugs that are packed with a polyethylene film with a thickness of 10 micrometers. Since both drugs are white tablets, it is impossible to distinguish the two tablets from the optical photograph. In contrast, the terahertz images in two different frequency bands enable to determine metoclopramide (left tablet: strong absorption at 1.36 THz) and famotidine (right tablet: strong absorption at 2.28 THz), respectively. This difference in the terahertz images arises because molecules that make up the drugs exhibit absorption spectra specific to the terahertz region (Fig. 1(b)), which are referred to as fingerprint spectra. The above results thus demonstrate that terahertz measurements are useful for nondestructive and noncontact quality inspections of pharmaceuticals. Recently, we have developed a flexible terahertz camera with carbon nanotubes, and we are conducting research aiming at employing it for in-line inspections in factories.

Reference:
D. Suzuki and Y. Kawano, “Terahertz Imaging and Spectroscopy as a Tool for Non-destructive and Non-contact Quality Inspections of Medical Drugs and Polymer Films”, Bunseki Kagaku 66, 893-899 (2017).
D. Suzuki and Y. Kawano, “A flexible terahertz camera with carbon nanotubes and its application to multi-view inspection”, New Diamond 34, 17–21 (2018).

Laboratory for Future Interdisciplinary Research of Science and Technology,
Tokyo Institute of Technology
kawano@ee.e.titech.ac.jp

Fig. 1. (a) Photograph of drugs (left) and terahertz absorption images of the same samples (right). Two drugs that are indistinguishable with the visible light are successfully identified. (b) Terahertz absorption spectra (fingerprint spectra) of the drugs of (a).

16/03/2018

“Enhancement of electrostatic capacitance by molecular design”
Capacitors, which consists of two metal electrodes and a dielectric, are electronic devices that can charge and discharge electricity. Including field effect transistors, which are current switching devices based on the similar concept with capacitors, capacitors are used everywhere in electronic gadgets or computers. When a voltage is applied to capacitors, electrical charges with different sign appear on each electrode. The amount of the charges, which is denoted as Q, is proportional to the voltage V; therefore, Q = CV holds. Here, C is the electrostatic capacitance, which is a constant and depends on the structure of devices. It is widely known that the value of C is inversely proportional to the distance between the two electrodes. This means that the charging ability is enhanced by reducing the thickness of the dielectrics. There are, however, a variety of factors to limit the thickness of solid dielectric materials. Then, how can we realize an ideal capacitor, in other words, an extremely thin dielectric layer?
An approach is to use liquids as dielectrics. Especially, when ionic liquids are used, the length scale for the dielectric layer corresponds to one ion molecule, which is as small as 1 nm (1 nano-meter is one billionth of one meter). So-called electric-double-layers are formed on the surface of the electrodes, where positive ions (or negative ions) align electrostatically. The thickness of the electric-double-layer is typically ~1 nm, so that a value of C is ~10 F/cm2, which is about 1000 times larger than that for oxide-based thin film capacitors. Studies that focus on such a large C value started at the end of the 20 century for further enhanced Q. If Q becomes large enough, some batteries would be replaced by capacitors with ionic liquids.
By designing molecular shapes, we have investigated the molecular structures of ionic liquids that efficiently accumulate charges [1]. Specifically, we synthesized new ionic liquids by two approaches; the installation of repeated structures in the chemical structures of ionic liquids, and the variation of the number of the repetition. We have analyzed the new ionic liquids through electrochemical methods and found that the value of C enhanced from ~10 F/cm2 to ~60 F/cm2 with increasing the number of the repetition. In addition, we applied the new ionic liquids to oxide-based transistors as gate dielectrics. The voltage required for switching electrical current was reduced compared to that for devices with conventional ionic liquids.
　The concept of the electric double layer has been extensively studied for a long time; however, it has never been considered that the molecular geometry, i.e. the repeated structure, is crucial for the enhancement of the charge accumulation ability. As a future perspective, the present study is expected to be applied to various fields such as energy storage devices and basic physics researches with electric double layer transistors [2].

Reference: [1] M. Matsumoto, S. Shimizu, R. Sotoike, M. Watanabe, Y. Iwasa, Y. Itoh, and T. Aida, J. Am. Chem. Soc. 139, 16072-16075 (2017). DOI: 10.1021/jacs.7b09156 http://pubs.acs.org/doi/abs/10.1021/jacs.7b09156
[2] S. Bisri, S. Shimizu, M. Nakano, and Y. Iwasa, Adv. Mater. 29, 1607054 (2017). DOI:10.1002/adma.201607054 http://onlinelibrary.wiley.com/doi/10.1002/adma.201607054/full

Contact: Sunao Shimizu, RIKEN Center for Emergent Mater Science
e-mail: sshimizu@riken.jp

Figure: Schematic of electric double layer consisting of charges in electrode materials and ions. When a voltage is applied to capacitors with ionic liquid as a dielectric, cations (or anions) align on the surface of the electrode. This electric double layer works as a nano-gap capacitor, which typically realizes capacitances as large as ~10 F/cm2.

Exceptionally High Electric Double Layer Capacitances of Oligomeric Ionic Liquids

09/03/2018

"One-step Precise Synthesis of nanographenes through Pd-catalyzed APEX reaction"
Polycyclic aromatic hydrocarbon (PAH) and nanographene are the nano meter-sized atomic-layered compounds that attract significant attentions because of their applicability toward organic photo luminescent diodes (OLED) and organic photo voltaic (OPV). In general, the synthesis of atomic-layered compounds requires step-wise synthetic protocols including halogenaition (i.g. bromination) of small aromatic components, coupling reactions and cyclodehydrogenations. Therefore, the limitation of starting materials as well as annoying step-wise protocols diminishes the synthetic efficiency and limits the scope of nanographene.
Herein, we report the novel palladium-catalyzed reaction, "annulative π-extension (APEX) reaction", for the one-step precise synthesis of PAHs and nanographenes. In the presence of a Pd catalyst, silver pivalate, and trifluoromethane sulfonic acid and diiodobiaryls as π-extending agents, the phenanthrene skeleton, a smallest unit structure of PAH, is directly extended to give nanographenes in a good yield. The futures of this reactions is (1) K-region of PAHs are selectively and directly reacted, and (2) a variety of PAHs and diiodobiaryls are applicable as starting materials and π-extending agents. For example, the reaction of phenanthrene with diiodophenylphenanthrene gives dibenzonaphthopentaphene (figure c), whereas the reaction of chrysene with diiodobiphenyl affords tetrabenzonaphthopentaphene (figure b). The latter nanographene shows blue-green and yellow emission in CH2Cl2 and in the solid state, respectively (figure c and d), which imply the potent applicability toward organic electroluminescense devises.
Newly developed APEX reaction enables direct transformation of PAHs and precise and efficient synthesis of structurally uniform nanographenes. Further π-extensions of synthesized nanographenes as well as the elongation to graphene nanoribbons would be potentially possible by utilizing our APEX methodology.
Reference: W. Matsuoka, H. Ito,* K. Itami* Angew. Chem. Int. Ed. 2017, 56, 12224. (https://goo.gl/iZnBLp)
Contact: Hideto Ito, Institute of Liberal Arts & Sciences, Nagoya University.
E-mail: ito.hideto@g.mbox.nagoya-u.ac.jp

02/03/2018

- Measurement of Berry phase of electrons in atomic layered materials by magnet-

We present a recent research on anomalous Hall effect in atomic-layer materials attached to a magnet. In a magnetic field, the motion of the electron generally strays from the straight path due to transverse Lorenz force and such a curved motion can be detected by the electric voltage perpendicular to the current. This is called Hall effect.

Here, we theoretically showed that, when an atomic-layer of transition-metal dichalcogenide (TMDC) is attached on a magnetic insulator, the motion of electrons in TMDC is also curved and leads to the Hall effect. TMDC itself is a non-magnetic material, and thus there is no Lorenz force to electrons. But the quantum wave of electrons inside TMDC contains a non-trivial phase factor called the Berry phase, which works as an effective magnetic field for the electrons. In the intrinsic TMDC without the mag-net attached, the number of electrons curving to the right and the number of those to the left are exact-ly balanced due to the time-reversal symmetry, and the system does not show the Hall effect. The magnet directly breaks the time-reversal symmetry and leads to the Hall effect. The Hall effect without magnetic field is called anomalous Hall effect. The Berry phase sensitively depends on the energy band of TMDC, and we can show that the sign of the Hall voltage (i.e., the direction of curve) can even be reversed by changing the electron density. The complex variation is different from the con-ventional Hall effect in which the sign of Hall voltage is determined by the sign of charge density and the direction of magnetic field. The present work provides a simple method to detect the intrinsic Ber-ry phase of the atomic-layer materials using the Hall measurement.
Reference: Tetsuro Habe and Mikito Koshino, Phys. Rev. B 96, 085411 (2017)

Correspondence:
Tetsuro Habe, Department of Physics, Osaka University
habe@qp.phys.sci.osaka-u.ac.jp

23/02/2018

Development of tunneling transistor using HfS2,

Data processing in the large-scale integrated circuits (LSI) is a key technology for the information society. Recently, a requirement to suppress the power consumption of LSI has been more important due to the huge spread of mobile devices (e.g., smartphone). For example, the suppression of power consumption of each transistor is important because several billions of transistors are used to construct electrical switches in a smartphone. Therefore, new principles which achieve the robust operation with ultra-low voltage are expected. A tunnel transistor can achieve the requirements using the quantum mechanics.
Recently, the adoption of van der Waals (vdW) heterojunction with two kinds of atomic layered materials has been in focus to improve the performance of tunneling transistors. We are focusing and investigating on vdW heterojunction with hafnium disulfide (HfS2), which is a novel atomic layer material and has the suitable properties to realize the vdW heterojunction devices. We fabricated the vdW heterojunction with HfS2 and molybdenum disulfide (MoS2) by aligned transfer using the sticky tape and organic polymer named PDMS [Fig. (a)]. The transmission electron microscopic image shows that the two different layered crystals are stacked together. After the formation of electrodes on each material, we first observed the transistor operation with HfS2 based vdW heterojunction [Fig. (b)]. Hence, the fabrication process is developed, and we would like to proceed the improvement of performance for low voltage operation.

Reference:
T. Kanazawa, T. Amemiya, A. Ishikawa, V. Upadhyaya, K. Tsuruta, T. Tanaka, and Y. Miyamoto, Scientific Reports, 6, 22277 (2016).
Contact: A03 Application Group, Yasuyuki Miyamoto (Tokyo Institute of Technology)
miya@ee.e.titech.ac.jp
Fig. (a) Fabrication flow and cross–sectional observation of a vdW heterojunction device. (b) Current-Voltage characteristics of the fabricated tunneling transistor.

16/02/2018

“To obtain an ultrashort strobe light”
　In my previous article on March 13, 2017, observation of ultrafast phenomena in the femtosecond (1 femtosecond = 10^15 second) regime was introduced. For such observation, it is required to obtain information only within a short time. An analogous example that we can look around is below. Say, you are a photographer, and you try to capture the finishing moment on a 100-meter final at World Championships in Athletics. At the exact moment, you press the shutter bottom with a short strobe light (light pulse) to shine the runners. Then, you successfully obtain a photo at the moment when the first-prize winner is determined. As the duration of the strobe light decreases, faster phenomena such as a car motion in a Formula One World Championship race can be captured. In this article, a way to obtain a light pulse with the femtosecond duration is introduced.
　The duration of the strobe for camera is about 1 microsecond (= 10^6 second). When we use an electric pulse to control the light propagation, the duration of 1 nanosecond (=10^9 second) can be achieved. To obtain shorter light pulses, we use interference of light.
　The light shows wave properties. When two waves propagate in a medium, superposition of them occurs. In the case where crests (troughs) of the original waves are aligned, the superposition results in a composite waveform with a larger amplitude, i.e., constructive interference. In contrast, when crests of the original wave meettroughs of the other, a composite waveform has a smaller amplitude, i.e., destructive interference. As shown in the figure, in the case of the superposition of sinusoidal waves with various wavelengths (blue curves), the composite waveform (red curve) shows a finite amplitude in a short duration. As the spectral width of the original sinusoidal waves increases, this duration becomes short (for details, see Fourier transformation in some textbooks). Thus, ultrashort light pulses (ultrashort strobe light) can be achieved. An important point in this superposition is that crests (troughs) of all original waves meet each other (in phase) at a certain time. Otherwise, the composite waveform in the figure cannot be obtained.
　Laser lights are highly coherent and suitable for generation of ultrashort pulses. In ultrafast laser systems, several ways to set the phases of light waves (set the phases of longitudinal modes in a cavity; mode-locking) are taken. Mode-locking techniques include active mode locking with an acousto-optic device, passive mode locking with a saturable absorber, and Kerr-lens mode locking using self-focusing effect in a laser medium itself. The generation of ultrashort pulses from optical fiber lasers by using graphene has been introduced in the article on March 16, 2017 by Professor Yamash*ta. It is helpful to read the article for advancing understanding.
Contact: Takeshi Koyama Department of Applied Physics, Nagoya University
e-mail: koyama@nuap.nagoya-u.ac.jp

Figure: Superposition of sinusoidal waves.
The superposition of blue curves (sinusoidal waves) makes the red curve. Numbers at the right sides of the blue curves are weight factors (amplitudes) of them for the superposition.

09/02/2018

Molecular beam epitaxy growth of a two-dimensional metal

Mechanical exfoliation and chemical v***r deposition (CVD) are widely used for preparation of atomic layers. Atomic layers can be obtained through repeated peeling off of thin flakes from a bulk crystal in the mechanical exfoliation and chemical reaction of source materials on a substrate at high temperature in the CVD method. Both methods have been successfully applied for preparation of semiconductor atomic layers, but application of these methods for metallic atomic layers is not straightforward. The reason is probably that formation of multilayers easily occurs in the case of metallic atomic layers due to strong interlayer interaction caused by free electrons.
To overcome this limitation, we have been focusing on molecular beam epitaxy (MBE) method for the preparation of metallic atomic layers. In MBE, source materials are heated under ultra-high vacuum condition, and this leads to v***rization of sources to be supplied as “beams”. One of the most important advantages of MBE is that we can control intensity of the beams precisely, leading to precise control in crystal growth of atomic layers. Using MBE, we have successfully grown metallic atomic layers (monolayer NbSe2, Fig. a), which is difficult to grow by the mechanical exfoliation and CVD method. Fig. b shows an AFM image of the monolayer NbSe2 grown on a hBN substrate. The triangular contrasts in Fig. b correspond to single crystals of NbSe2, whose monolayer structure were confirmed by AFM height analyses and Raman spectroscopy. We are now working on application of the MEB method for preparation of various atomic layers and atomic layer heterostructures.

Reference:
T. Hotta, T. Tokuda, S. Zhao, K. Watanabe, T. Taniguchi, H. Shinohara, and R. Kitaura, Appl. Phys. Lett. 109 , 133101 (2016).
Contact: T., Hotta, R. Kitaura, Dept. of Chem., Nagoya University
Fig. (a)A structure model of monolayer NbSe2 grown on a hBN substrate. (b) an AFM image of monolayer NbSe2

02/02/2018

Huge diamagnetism -- materials to repel magnets

Non-magnetic materials are classified either into paramagnetic or diamagnetic by the response to an external magnetic field. Diamagnetic materials are repelled by a magnet
while paramagnetic materials are attracted by a magnet. The magnetic response is closely related to the electronic band structure of the material. In general, the diamagnetism tends to be large in a linear band in which the energy disperses linearly to momentum [Fig. (a)]. This is exactly the case in graphene, where the diamagnetic susceptibility diverges in zero temperature. In graphite (a 3D stack of graphene layers),
however, the susceptibility per unit mass becomes much lower than graphite, because in more than two-layer stack, the energy band becomes quadratic in momentum. Still, the diamagnetism of graphite is strong enough to levitate a small flake on the permanent magnet.[Figure (b)]

If we could assemble a 3D material keeping the linear energy band, we could possibly have enormous diamagnetism which exceeds graphite. For example, if graphene layers are stacked with random rotational angles, the linear dispersion remains intact because the inter-layer hopping of electrons are strongly suppressed, and then we expect a strong diamagnetism. The similar effect is also expected in a composite multilayer where graphene and h-BN (hexagonal boron nitride) are overlaid alternately. Moreover,　recent studies found various materials having 3D linear band, such as Weyl semimetals and Dirac semimetals. Our recent study theoretically calculated the magnetic response for various 3D materials with linear band, and found the diamagnetism tend to be extremely large in so-called nodal-line semimetals, where the band touching point like Fig. (a) extends on a line in the momentum space. It has been recently found that ZrSiS has the nodal line structure and there are also several theoretically predicted materials. While the diamagnetism has not been measured in these systems, it might be possible to have a 3D material with much greater diamagnetism than graphite's.
References：M. Koshino, I. F. Hizbullah, Phys. Rev. B 93 (4), 045201 (2016).
Contact: Mikito Koshino, Dept. of Physics, Osaka University

26/01/2018

Plasmonic applications in optical sensors
Plasma is a state in which positive and negative charges are moving. Plasma-based techniques have been utilized in our daily lives, and the most familiar device is a fluorescent light. The fluorescent lamp emits white light using the ultraviolet light radiated from the sealed mercury in the plasma state. It has no longer been produced, but it was also used for a plasma TV. As industrial applications, the plasma is also employed for dry etching, one of semiconductor microfabrication technologies. In general one might imagine that the plasma would be generated in a large container. However, there is a powerful technique that enables the plasma to be generated in a small area of a solid-state device and to be confined to one point. This is called "surface plasmon" or "localized plasmon". When light is irradiated onto the surface of solid-state materials, surface plasmons are excited, and in some cases localized plasmons are generated in a tiny area. The localized plasmons can be confined in a region smaller than the wavelength of the light, and their electric field of the light can be largely enhanced.
In this research, we succeeded in generating and controlling the plasmons in the terahertz region by making use of fine processing of semiconductors and metals. The fabricated plasmonic structure (Fig. (a)) has excellent characteristics such as generation of terahertz plasmons in a region (2-20μm) smaller than the wavelength the wavelength of 300μm, selection of a specific frequency band (Fig. (b)), and large terahertz field enhancement. These advantages make it possible to detect a tiny amount of materials and bio-samples. Also, recently, by combining the localized plasmon with graphene photodetectors, the detection sensitivity greatly increased, and the frequency filter function was added. We expect that this integrated device can be effectively used for materials and bio-analysis as "graphene plasmonic chip".

Reference: T. Iguchi, T. Sugaya, and Y. Kawano, Applied Physics Letters 110, 151105-1-4 (2017).
Yukio Kawano
Laboratory for Future Interdisciplinary Research of Science and Technology
Tokyo Institute of Technology
kawano@ee.e.titech.ac.jp

Fig. (a) Photograph of terahertz plasmonic structure (left) and enlarged view of the center of the left image (right). (b) Terahertz transmission spectrum through the central aperture (2-20μm) smaller than the wavelength of 300μm. This result shows selective transmission of only 1-THz waves.

19/01/2018

The ratio of the areas deduced from the magnetoresistance

Single-layer graphene (SLG) – a sheet of carbon atoms arranged in the honeycomb lattice – hosts a two-dimensional electron system (2DES) characterized by exotic properties including linear dispersion relation. Bilayer graphene (BLG), consisting of two SLG layers in Bernal stacked form, also contains 2DES but with the characteristics distinctly different from those of SLG. For instance, BLG possesses quadratic-like dispersion resembling that of a conventional semiconductor 2DES.
In the present study, we measure the magnetoresistance of epitaxial graphene simultaneously containing the two different species of 2DES, SLG and BLG. An epitaxial graphene sample is prepared by heating a SiC substrate. Graphene grows layer by layer on the surface of the substrate following the selective sublimation of silicon atoms. If the heating is stopped before the completion of the second layer, the surface is shared by SLG and BLG domains. The distribution of the domains can be visualized by the phase image of the atomic force microscope (AFM) (see Fig. 2(a)), which is known to be highly sensitive to the number of graphene layers.
In an ordinary one-component 2DES, the longitudinal resistance (Rxx) does not vary with the magnetic field applied perpendicular to the 2DES provided that we can neglect the effects of the Landau quantization and the quantum interference, while the Hall resistance (Rxy) varies linearly with the magnetic field, as schematically depicted in Fig. 1. The behaviors become altered, however, when two or more components are simultaneously present. The traces of the magnetoresistance we measured are shown in Fig. 2(b) with solid lines. Rxx shows positive magnetoresistance, which tends to saturate at high magnetic fields, while Rxy exhibits sublinear lineshape. These lineshapes are reproduced extremely well by the semiclassical two-carrier model, as demonstrated by dashed lines in Fig. 2(b). From the fitting to the model, we can deduce the mobilities and the averaged electron densities of the two types of the carriers.
Noting that the SLG and BLG domains are electrically in contact with each other, we can expect the Fermi level to be aligned between them, as illustrated in the inset of Fig. 2(b). This, along with the known difference  in the Dirac point energy between SLG and BLG epitaxially grown on SiC, provides us with the relation between the local carrier densities within the SLG and BLG domains. The relation, combined with the averaged electron densities (densities deduced from the two-carrier model assuming that the two components are spread all over the surface), let us know the ratio of areas of the two types of the domains. Thus, the ratio of SLG areas to BLG areas can be deduced solely from the magnetoresistance data. The ratio obtained from the data shown in Fig. 2(b) was 0.36, which agreed well with the ratio 0.36 obtained by directly counting the numbers of pixels in the AFM image.

Fig. 1. Schematic diagram of the magnetic-field dependence of the diagonal resistance (Rxx) and the Hall resistance (Rxy) for an ordinary one-component two-dimensional electron system.

Fig. 2.　(a) AFM phase image of epitaxial graphene containing both single-layer graphene (SLG) and bilayer graphene (BLG) domains. (b) Magnetic-field dependence of the diagonal resistance (Rxx, left axis) and the Hall resistance (Rxy, right axis). Solid and dashed lines represent the experimental results and the fitting by the semiclassical two-carrier model, respectively. Inset: dispersion relations of single-layer graphene (SLG) and bilayer graphene (BLG). The Fermi level is represented by the horizontal green line.

References: A. Endo et al., Philos. Mag. 97, 1755 (2017).
Contact: Akira Endo, Institute for Solid State Physics, University of Tokyo
akrendo@issp.u-tokyo.ac.jp