Hasegawa Group
Research
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Introduction

Now it is widely recognized that strong correlation between electrons give rise to a variety of interesting and unique properties in solids, including superconductivity and colossal magnetoresistace. In these phenomemna, electronic “inhomogeneity” of a nanometer scale also plays an important role. For instance, it is well known that quasi-2D nature is essential for the appearance of high temperature superconductivity.

In our research group, consisting of three faculty staffs, two post doc fellows and seven graduate students, strongly correlated systems have been studied from chemical, physical and material view points. We are searching for new functionalities in transition metal oxides by using layer-by-layer thin film technology, which enables us to introduce nano-structures under non-equilibrium conditions. It is also an important subject for us to develop new microscopic probes that can directly map out the structures. Combining both techniques, we investigate the electronic and chemical inhomogeneity that governs the material properties.

In addition, we are very much interested in new phenomena specific to interfaces. For instance, we construct nano-devices by controlling the assemblies of atoms and molecules as desired. These techniques will not only open the door into sub-nano scale processing, but also provide us environments in which new quantum phenomena could emerge.

Our recent research activities are following.


Developments of new magnetic materials

Nowadays, ferromagnets bear closely on the needs of our daily lives. However, they are by no means classical materials. In fact, new ferromagnetic materials have been being developed one after another, resulting in the widening of their application field.

We are searching for new magnetic phenomena, focusing on the interactions between electron spins and charges, or between spins and photons. In our project, we use the "combinatorial technology" that can considerably accelerate the efficiency of the material exploration. Fig. 1 shows the magnetic properties of Co-doped TiO2 films grown by the combinatorial laser MBE apparatus (Fig. 2). The material exhibits the highest Currie temperature > 400 K as a magnetic semiconductor. We are also studying on dielectric, magneto-optical and thermodynamic properties of various functional materials.

Fig. 1: Magnetic properties of Ti1-xCoxO2 thin film

 

Fig. 2: Laser MBE technique

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Search for new photo-induced effects

In a system, in which different interactions compete with each other, some inhomogeneity tends to appear. Its electronic structure could drastically change by applying a perturbation, such as photo-irradiation or electric field, to the system. We are seeking such a "huge" effect originated from electronic inhomogeneity. Fig. 3 shows the photo-induced magnetism discovered in Nd1-xSrxMnO3. The effect is maximized around the Sr composition x ∼ 0.45. The once enhanced magnetization is maintained even after switching off the irradiation. Namely, the present material is capable of memorizing the numbers of irradiated photons as the magnitude of magnetization.

Fig. 3: Photo-induced ferromagnetism in Nd1-xSrxMnO3

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Nano-scale characterizations of high temperature superconductors

High temperature superconductors, in general, contain various kinds of inhomogeneity. We are observing their local electronic properties by means of low temperature STM/STS instruments. Fig. 4 shows the STM/STS results of heavily Pb-doped Bi2212. We found a chemical phase separation of the order of several tens nm as well as an electronic phase separation of nanometer scales. By comparing the STM/STS maps with the distributions of various impurities, we discuss the origins of the chemical/electronic instabilities described above.

These phase separation structures are important factors that determine the critical current density (Jc). We contribute to the development of high Jc materials through microscopic imaging studies

Fig. 4: Electronic and chemical phase separation observed in Pb-doped BiSrCaO superconductor

Fig. 5: Vortices trapped in LaSrCuO superconductor

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Development of new microscopic probes for material charcterizations

In order to elucidate the mechanisms of various interesting phenomena as mentioned above, it is necessary to measure physical properties in a local sense under extreme conditions, such as high field and low temperature. We are developing a variety of microscopes that are suitable to our needs. Fig. 6 show the recent version of UHV-LT STM, which can directly "look" at electronic wave function (|Ψ|2). Fig. 7 is a photo of our scanning microwave microscope. The microscope is able to characterize local electric properties (conductivity and dielectric constant) on a nanometer scale.

Fig. 6: UHV-LT STM instrument

Fig. 7: Scanning microwave microscopies

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Fabrication of nano-scale devices on Si surfaces

Silicon is widely used in electronics such as integrated circuits and silicon devices are fabricated by top down approach using lithography and chemical processes. If it becomes possible to assemble functional units such as molecules, this bottom up approach will realize high integration, molecular recognition, chemical processing and magnetism and so on, which is difficult by solely using silicon. It is necessary to establish the communication between molecules and currently used silicon microcircuits in order to make the molecular devices into application, which is one of our objectives. Hydrogen-terminated Si(111) is an inactive surface on which hydrogen is connected to the dangling bonds on Si(111). It is possible to assemble various molecules as an ultrathin crystal with monolayer thickness. The figure shows an example of nano-scale patterning of the hydrogen terminated Si(111). The x-shaped protrusion is made of silicon oxide that is produced by anodic oxidization using AFM tips. It is further possible to chemically modify the surface structure and to utilize difference in the chemical affinity (including van der Waals type interaction) so that functional molecules are regularly assembled on the patterned area.

Fig. 8: Nano-fabrication of hydrogen-terminated Si(111) surfaces
by AFM anodic oxidization

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Control of surface reactions using pulsed molecular beam

Crystal growth is a complicated process in which atoms and molecules coagulate to make ordered phase. We have found that it is possible to change the dynamics of crystallization processes by temporary modulating the supply of constituent materials. We can control the intermolecular collision kinetics on surfaces using pulsed molecular beams. It enables us to change the nucleation frequency in the thin film growth and suppress the material formation. It is possible to broaden the materials combination for the selective growth, which is used in the nano-scale patterning of molecular materials through affinity control.

Fig. 9: Morphology of organic thin films grown by pulsed molecular beams with different cycle time

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Photoelectron spectroscopy for device characterizations

We are studying fundamental issues of molecular electronics and searching novel phenomena in the device structure fabricated by above-mentioned techniques. We present a result on an unresolved problem of the molecular nature of charge carriers in organic solid-state devices. The figure shows ultraviolet photoelectron spectroscopy of a field effect transistor (FET) made of C60. A new peak is observed at the binding energy of 0.8 eV when gate bias voltage is increase. This peak probably corresponds to the energy level of the field-induced charge carriers. Applying spectroscopic techniques to the organic devices will clarify various issues in molecular electronics.

Fig. 10: Ultraviolet photoelectron spectroscopy of C60 field effect transistors

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