JP2005308651A - Scanning probe microscope and its using method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a scanning probe microscope suitably adapted to the study of the observation of the lattice of lines of magnetic induction or an electron state in a magnetic flux inlusive of the study of a nanoscale non-uniform superconductive state and the observation of the uneven image on the surface of a sample, and its using method. <P>SOLUTION: As the probe 12 of the scanning probe microscope, a pressure inducing superconductive substance is used. As the pressure inducing superconductive substance, for example, La<SB>2-x</SB>Ba<SB>x</SB>CuO<SB>4</SB>(x is in the vicinity of 1/8) is used. When the probe 12 is brought into contact with the surface of a sample, the leading end part of the probe 12 becomes a superconductive state by the pressure applied to the leading end part of the probe and the current-voltage characteristics between the probe 12 and the sample 13 are changed by Andreev reflection. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a scanning probe microscope and a method for using the same, and is particularly suitable for application to surface exploration of various materials including superconducting materials.

The scanning probe microscope (SPM) is an important tool in nanoscience and nanotechnology, and has been applied in various fields such as surface exploration of various substances and material / device development.
On the other hand, in recent years, research on nanoscale inhomogeneous superconducting state (self-organization phenomenon) occurring in high-temperature superconductors and heavy electronic systems has been conducted.

In addition, in the vicinity of x = 1/8 of the copper oxide high-temperature superconductor La _2-x Ba _x CuO ₄ , the spin / charge stripe order is stabilized at a low temperature, and the system becomes insulating, and superconductivity is suppressed. It is known (for example, Non-Patent Document 1). Further, it is known that when a small pressure is applied to this system, the spin / charge stripe order is suppressed and the superconductivity is restored (for example, Non-Patent Documents 2 and 3).
JM Tranquada et al., Nature Vol.357 (1995) 561 N. Yamada and M. Ido, Physica C Vol.203 (1992) 240 M. Ido et al., J. Low Temp. Phys. Vol.105 (1996) 311

  However, the conventional scanning probe microscope could not be a powerful tool for the study of the nanoscale inhomogeneous superconducting state. For this reason, new tools are being sought, but no effective tools have been proposed so far.

  Therefore, the problem to be solved by the present invention is applied to the observation of the magnetic flux line lattice, the electronic state in the magnetic flux, and the observation of the concavo-convex image on the sample surface, including the study of the nanoscale inhomogeneous superconducting state. Thus, it is an object of the present invention to provide a suitable scanning probe microscope and a method for using the same.

  As a result of intensive studies to solve the above-mentioned problems of the prior art, the present inventors have developed a superconducting characteristic sensitive to pressure on the probe of the scanning probe microscope in order to solve the above problems. Thus, the present inventors have found that it is effective to use a pressure-induced superconducting material capable of controlling the superconducting state by applying a small pressure, and have come up with the present invention.

That is, in order to solve the above problem, the first invention
A scanning probe microscope using a probe made of a pressure-induced superconducting material.
Here, various kinds of pressure-induced superconducting materials can be used, not limited to inorganic materials, but may be organic materials, and can be appropriately selected according to the use. The most representative example is La _2-x Ba _x CuO ₄ (abbreviation LBCO), which is a kind of copper oxide high-temperature superconductor, and in particular, the Ba concentration x is around 1/8 (typically, x = 1/8 ± 0.03). In the LBCO in the vicinity of x = 1/8, the spin / charge stripe order is stabilized at a low temperature, the system becomes insulating, and superconductivity is suppressed. On the other hand, when a small pressure is applied to this system, the spin / charge stripe order is suppressed and the superconductivity is restored. As the pressure-induced superconducting material, there is La _2-xy R _y Ba _x CuO ₄ (abbreviated as LRBCO, R is a rare earth element such as Nd) which is a related substance of La _2-x Ba _x CuO ₄ , and in particular x = 1 In the vicinity of / 8, 0 ≦ y <˜0.5 (typically 0 ≦ y <0.5). Other examples of the pressure-induced superconducting material include a low-dimensional electron material NbSe ₃ , a heavy electron material CeTIn ₅ (T = Rh, Ir, Co), and a ferromagnetic metal material Fe.
A probe made of a pressure-induced superconducting material is typically made of needle-like crystals (including tubular crystals such as nanotubes).

  When a probe made of a pressure-induced superconducting material is brought into contact with the sample surface, the tip is brought into a superconducting state due to the pressure applied to the tip of the probe, and the current (I) between the probe and the sample. -Voltage (V) characteristics change. This change in IV characteristics is due to Andreev reflection occurring at the superconductor-normal conductor interface. Here, Andreev reflection refers to a phenomenon in which in-phase holes (electrons) return because electrons (holes) incident from the normal conductor enter the superconductor as a Cooper pair. Various measurements or observations can be performed using the change in the IV characteristic.

Therefore, the second invention is
A method of using a scanning probe microscope using a probe made of a pressure-induced superconducting material,
The probe is scanned along the surface of the sample while supplying a constant current between the probe and the sample.

  Here, when the probe is scanned along the sample surface while passing a constant current between the probe made of a pressure-induced superconducting material and the sample, the tip of the probe is in a normal state and a superconducting state due to surface irregularities. , The voltage between the probe and the sample changes accordingly, and the current-voltage characteristic between the probe and the sample changes. By imaging this, irregularities on the sample surface can be observed. In particular, when the sample is a superconducting sample, a superconducting state region and a non-superconducting state region coexist on the surface of the superconducting sample, such as a state in which a magnetic flux line enters the superconducting sample. The current-voltage characteristics between the probe and the sample when the probe made of pressure-induced superconducting material is on the superconducting region and on the non-superconducting region. Change. Therefore, it is possible to observe the magnetic flux lattice and the superconducting / non-superconducting non-uniform state. Furthermore, it is possible to measure the mobility of carriers in the sample.

The third invention is
A method of using a scanning probe microscope using a probe made of a pressure-induced superconducting material,
The probe is scanned along the surface of the sample while changing the height of the probe so as to keep the voltage between the probe and the sample constant. .
Here, the unevenness of the sample surface can be observed by imaging the change in the height of the probe.
In the second and third inventions, the matters related to the first invention are established as long as they are not contrary to the nature.

  In the present invention configured as described above, a probe made of a pressure-induced superconducting material is scanned, for example, along the surface of the sample while passing a constant current between the probe and the sample. When a nano-scale inhomogeneous superconducting state or a magnetic flux line lattice exists in the conductive sample, the state can be easily observed using Andreev reflection. In addition, the concavo-convex image on the sample surface can be easily observed in the same manner. Furthermore, by using both together, it is possible to remove irregularities on the sample surface and obtain a precise image of the magnetic flux line grating.

  According to the present invention, there is provided a scanning probe microscope that is extremely effective for research on a nanoscale inhomogeneous superconducting state, observation of a magnetic flux line lattice, study of an electronic state in a magnetic flux, and observation of an uneven image on a sample surface. Can do.

Hereinafter, an embodiment of the present invention will be described with reference to the drawings.
FIG. 1 shows an Andreev reflection scanning probe microscope according to an embodiment of the present invention.
As shown in FIG. 1, in the Andreev reflection scanning probe microscope, x 1/8 La _2−x Ba _x CuO ₄ (LBCO) is placed below a piezoelectric control device 11 similar to a general scanning probe microscope. ) An LBCO probe 12 made of needle crystals is attached, and the LBCO probe 12 can be three-dimensionally scanned in the x, y, and z directions on the sample 13 by the piezoelectric controller 11. ing.

Next, a method of using the Andreev reflection scanning probe microscope will be described.
FIG. 2 shows a phase diagram of La _2-x Ba _x CuO ₄ . As shown in FIG. 2, in the vicinity of x = 1/8, the spin / charge stripe order is stabilized at a low temperature, the system becomes insulating, and superconductivity is suppressed. On the other hand, as shown in FIG. 3, when a small pressure is applied to this system, the spin / charge stripe order is suppressed and the superconductivity is restored. In this case, if the superconducting critical temperature is T _c and the applied pressure is P, the pressure coefficient dT _c / dP of T _c is −3 K / kbar, which is 2 to 3 orders of magnitude higher than that of the conventional superconductor. For this reason, for example, the tip portion can be made superconductive only by applying a force of ˜0.1 μg weight to the LBCO probe 12 having a tip diameter of several nanometers.

  Therefore, the LBCO probe 12 having superconducting characteristics sensitive to such pressure is brought into contact with the surface of the sample 13, and the tip of the LBCO probe 12 has a pressure higher than the pressure at which the superconducting state is restored. When applied, the tip portion of the LBCO probe 12 is in a superconducting state that was insulative before contact, and the IV between the LBCO probe 12 and the sample 13 is caused by Andreev reflection. The characteristics change. However, the LBCO probe 12 is cooled to a temperature lower than the superconducting critical temperature when pressure is applied and higher than the superconducting critical temperature when no pressure is applied.

For example, the following measurement can be performed using the change in the IV characteristic.
In the first example, the unevenness of the surface of the sample 13 is imaged using a change in IV characteristics. That is, as shown in FIG. 4, the LBCO probe 12 is scanned along the surface of the sample 13 while a constant current is supplied between the LBCO probe 12 and the sample 13 by the constant current source 14. Then, in the convex part, pressure is applied to the tip of the LBCO probe 12 to be in a superconducting state, and in the concave part, pressure is not applied to the tip of the LBCO probe 12 to be in a normal conducting state. Accordingly, the tip of the LBCO probe 12 changes between the normal conduction state and the superconducting state, and the voltage between the LBCO probe 12 and the sample 13 changes accordingly, and the I− V characteristics change. For example, as shown in FIG. 5, the IV curve changes as A → B → C. Thus, by imaging this, the unevenness of the surface of the sample 13 can be observed. In the first example, the sample 13 is not particularly limited and may be various types.

  In the second example, the LBCO probe 12 is scanned along the surface of the sample 13 while changing the height of the LBCO probe 12 so as to keep the voltage between the LBCO probe 12 and the sample 13 constant. In this case, the unevenness of the surface of the sample 13 can be observed by imaging the change in the height of the LBCO probe 12. Also in the second example, the sample 13 is not particularly limited and may be various types.

  In the third method, the sample 13 is a superconducting sample. When a superconducting region and a non-superconducting region are present on the surface of the sample 13 as in the state in which magnetic flux lines penetrate into the superconducting sample, the LBCO probe 12 is superconducting. The IV characteristic between the LBCO probe 12 and the sample 13 varies depending on whether the region is on the state region or the non-superconducting region. Therefore, the change in the IV characteristic makes it possible to observe the magnetic flux lattice of the sample 13 and the superconducting / non-superconducting inhomogeneous state.

Next, a method for manufacturing the LBCO probe 12 will be described.
One method is a method of fabricating the LBCO probe 12 by processing a bulk LBCO crystal.
Another manufacturing method is as follows.
First, as shown in FIG. 6A, a cone 21 is produced. If the cone 21 has a melting point that is not heated and softened when an LBCO needle crystal is grown by irradiation with an electron beam, which will be described later, and has a melting point of, for example, 800 ° C. or higher, basically, how is this cone 21? Specifically, for example, Si, Si ₃ N ₄ , SiO ₂ , diamond, alumina (sapphire), TaS ₂ , GaAs, Ni, Ta, or the like can be used.

Next, as shown in FIG. 6B, an LBCO crystal material film 22 to be grown is formed on the surface of the cone 21 in a vacuum. As the raw material film 22, for example, a film such as a La film or a La ₂ O ₃ film, a Ba film or a BaO film, a Cu film or a Cu ₂ O film, or the LBCO film itself may be used. The raw material film 22 is formed by any one of film formation methods such as vacuum deposition, sputtering, chemical vapor deposition (CVD), metal organic chemical vapor deposition (MOCVD), and molecular beam epitaxy (MBE). Alternatively, they can be formed by appropriately combining them.

Next, as shown in FIG. 6C, the electron beam 23 is applied to a point P that is a predetermined distance L, for example, about 1 to 3 μm away from the tip of the cone 21 thus formed with the raw material film 22 along the side surface thereof. Irradiate at room temperature. The spot size of the electron beam 23 is, for example, about 50 nm to 1 μm, the acceleration voltage is 25 to 200 kV, the irradiation current amount is 1 × 10 ⁻⁷ μA, and the irradiation time is 30 minutes to 1 hour. The irradiation with the electron beam 23 is performed in a vacuum at a pressure of 3 to 4 × 10 ⁻⁶ Torr, for example. At this time, as shown in FIG. 6D, the LBCO needle-like crystal 24 grows in the vicinity of the tip portion of the cone 21 instead of the irradiation portion of the electron beam 23. In general, when the electron beam 23 is irradiated, a temperature gradient of 10 to 100 ° C./μm exists between the irradiation portion of the electron beam 23 and the growth portion of the LBCO needle crystal 24 with the tip portion as a low temperature side. In this case, the temperature of the irradiation site of the electron beam 23 is higher than the growth temperature of the LBCO needle crystal 24, but the temperature of the growth site of the LBCO needle crystal 24 is lower and becomes the most suitable temperature for growth. . The growth of the LBCO needle crystal 24 is considered to be due to solid phase epitaxial growth. The thickness (diameter) of the LBCO needle crystal 24 is, for example, about 5 nm to 1 μm, the length is, for example, 10 nm to 2 μm, or 10 to 500 nm, and the aspect ratio (length / thickness) is generally 100 or less. It is.

  As described above, according to this embodiment, observation of a concavo-convex image on a sample surface, research on a nanoscale inhomogeneous superconducting state (self-organization phenomenon) occurring in a high-temperature superconductor or a heavy electron system, magnetic flux lines It is possible to realize a scanning probe microscope that is extremely useful for observation of lattices and study of electronic states in magnetic flux.

Although one embodiment of the present invention has been specifically described above, the present invention is not limited to the above-described embodiment, and various modifications based on the technical idea of the present invention are possible.
For example, the numerical values, configurations, materials, raw materials, processes, and the like given in the above-described embodiments are merely examples, and different numerical values, configurations, materials, raw materials, processes, and the like may be used as necessary.

  Further, instead of the electron beam 23, another energy beam such as a laser beam or an ion beam may be used. Alternatively, the source film 22 may not be formed in advance before the irradiation with the electron beam 23, but the growth may be performed by irradiating the electron beam 23 while supplying the growth source. Further, the growth may be performed by irradiating the electron beam 23 while supplying other growth raw materials in a state where a part of the raw material film is formed.

It is a basic diagram which shows the Andreev reflection scanning probe microscope by one Embodiment of this invention. It is a phase diagram of La _2-x Ba _x CuO ₄ . Is a schematic diagram showing the relationship between the La _2-x Ba _x CuO ₄ of Ba concentration x superconducting critical temperature T _c of the applied pressure as a parameter. It is a basic diagram for demonstrating the usage method of the Andreev reflection scanning probe microscope by one Embodiment of this invention. It is a basic diagram which shows the change by the applied pressure of the IV characteristic between a probe and a sample in the Andreev reflection scanning probe microscope by one Embodiment of this invention. It is a basic diagram for demonstrating the manufacturing method of the LBCO probe used in the Andreev reflection scanning probe microscope by one Embodiment of this invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 11 ... Piezoelectric control device, 12 ... LBCO probe, 13 ... Sample, 14 ... Constant current source, 21 ... Conical body, 22 ... Raw material film, 23 ... Electron beam, 24 ... LBCO needle crystal

Claims (5)

  A scanning probe microscope characterized by using a probe made of a pressure-induced superconducting material.   2. The scanning probe microscope according to claim 1, wherein Andreev reflection is used. _2. The scanning probe microscope according to claim 1, wherein the pressure-induced superconducting material is La _2-x Ba _x CuO ₄ (where x is in the vicinity of 1/8). A method of using a scanning probe microscope using a probe made of a pressure-induced superconducting material,
A method of using a scanning probe microscope, wherein the probe is scanned along the surface of the sample while a constant current is passed between the probe and the sample. A method of using a scanning probe microscope using a probe made of a pressure-induced superconducting material,
A scanning probe microscope characterized in that the probe is scanned along the surface of the sample while changing the height of the probe so as to keep the voltage between the probe and the sample constant. How to use.

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