Using superfluid helium-3 to explore new phenomena
15 May 2009 (Volume 4 Issue 5)
There are liquids that can climb up the walls of their container like a life form and down the other side. Superfluid helium is the only liquid that exhibits this mysterious phenomenon. Helium turns into a liquid when cooled to a temperature close to absolute zero, and into a superfluid state when cooled further. Superfluidity is a quantum mechanical phenomenon that appears in accordance with physical principles that govern the microscopic world, and is a state that we can observe on a macroscopic scale. The Low Temperature Physics Laboratory is using unique methods to observe the properties of superfluid helium-3, and their findings are expected to help us understand the mechanism of superconductivity and the birth of the Universe. Furthermore, the Laboratory has started conducting research aimed at building a quantum computer by manipulating individual electrons on the surface of liquid helium (Fig. 1).
Figure 1: Manipulating individual electrons on the surface of liquid helium.
Applying microwaves to individual electrons on the surface of liquid helium allows us to excite and control them.
The world of quantum mechanics observed at ultralow temperatures
A hundred years have passed since the Dutch physicist Heike Kamerlingh Onnes (the 1913 winner of the Nobel Prize in Physics) succeeded in liquefying helium by cooling it to 4.2 K in 1908 (K, or kelvin, is the absolute scale of temperature; 0 K = −273.15 °C). The success in liquefying helium is recognized as the beginning of low-temperature physics, a field of research into physical phenomena that appear at low temperatures.
Kamerlingh Onnes further tried to cool liquid helium to 2.17 K or lower, discovering a phenomenon that is now recognized as superfluidity. However, the term ‘superfluidity’ was invented in 1938 by the Russian physicist Pyotr Kapitsa (one of the 1978 winners of the Nobel Prize in Physics), who demonstrated by experiments that liquid helium loses its viscosity at ultralow temperatures.
Superfluid helium in a container exhibits a mysterious behaviour: it climbs up the walls of the container and down the other side (Fig. 2). Furthermore, superfluid helium continues to flow after being made to flow round a cylindrical container. The flow velocity of a liquid in a normal state (normal flow) gradually decreases and finally comes to a complete stop as it collides with the surrounding walls; that is, when the movement of individual particles in the liquid becomes random. In the superfluid state, however, the liquid continues to flow because individual particles do not move randomly but in a uniform manner. The lack of viscosity is also attributed to the uniform movement of particles.
Figure 2: Mysterious phenomenon exhibited by superfluid helium-4.
Superfluid helium in a container climbs up the walls and down the other side (as the arrow indicates) because of the lack of viscosity in the fluid.
The phenomenon of particles moving in a uniform manner as observed in the superfluid state is called ‘Bose–Einstein condensation’. According to quantum mechanics, the physical principles that apply to the microscopic world, particles can be classified as ‘Bose particles’ or ‘Fermi particles’, and it is only Bose particles that cause Bose–Einstein condensation. Here, Bose particles are those in which the total number of subatomic particles within the atom (the sum of the number of protons and neutrons in the nucleus and the electrons around it) is even, whereas in Fermi particles the total number of subatomic particles is odd. For example, normal helium-4 (4He) consists of two protons, two neutrons, and two electrons, which means that it is a Bose particle because the sum is six—an even number. Thus, when normal helium is cooled to 2.17 K or lower, Bose–Einstein condensation occurs, and the helium is transformed into the superfluid state.
“When a material is cooled to an ultralow temperature, you can sometimes observe in the macroscopic world phenomena related to quantum mechanics, such as superfluidity. Our main goal is to discover and explore the properties of those phenomena,” says Kimitoshi Kono, Chief Scientist.
Using superfluid helium-3 to solve the mystery of superconductivity
There is an isotope of helium called helium-3 (3He) that contains one less neutron. Helium-3 is a Fermi particle because it consists of two protons, one neutron, and two electrons, making a total of five—an odd number. However, helium-3 is also transformed into the superfluid state when it is cooled to 0.001 K or lower. The reason is thought to be that two helium-3 atoms form a ‘Cooper pair’, which has an even total number of 10 (5 + 5 = 10), and the Cooper pair behaves as a Bose particle, causing Bose–Einstein condensation.
Research into the superfluidity of helium-3 can contribute to elucidating the mechanism of superconductivity, a phenomenon in which electrons flow with no resistance at a certain temperature (the transition temperature). This is because superconductivity appears when electrons (Fermi particles) form Cooper pairs, causing Bose–Einstein condensation.
Superconductivity was discovered by Kamerlingh Onnes in 1911. He found that mercury shows zero electrical resistance when cooled to 4 K. But what force serves to make the negative electrons bind together to form Cooper pairs? In 1957, three physicists proposed the ‘BCS theory’ to explain why superconductivity occurs in metallic substances such as mercury. They were awarded the Nobel Prize in Physics in 1972.
Later, scientists discovered two types of new phenomena that were not explainable by the BCS theory alone. One is the superconductivity exhibited by cerium or uranium compounds. This phenomenon was discovered in the late 1970s. The other is high-temperature superconductivity in copper oxides, which was discovered in 1986. Today, the highest transition temperature has reached 160 K (about −113 °C) at high pressure.
Researchers all over the world are actively engaged in basic studies to understand the mechanism behind these new types of superconductivity and to develop superconducting materials that show superconductivity at progressively higher transition temperatures. They are seeking to develop superconducting materials with zero electrical resistance without being cooled below room temperature. These materials are expected to make a substantial contribution to solving the energy crisis because they can be used in electrical transmission lines and electronic devices without generating heat.
Nowadays, new types of superconducting materials are being developed in rapid succsession. In 2001 Jun Akimitsu, a materials scientist at Aoyama Gakuin University, discovered that magnesium diboride exhibits superconductivity. In 2008 Hideo Hosono, a materials scientist at the Tokyo Institute of Technology, also discovered that some iron compounds show superconductivity at a high transition temperature. These studies have received extensive attention from all over the world.
Some compounds heavier than uranium have recently been discovered to be superconducting materials. In 2002 a research group in the US discovered that a plutonium compound is superconductive, and in 2007 scientists at Tohoku University and the Japan Atomic Energy Agency discovered that a neptunium compound shows superconductivity.
“Superfluidity observed in helium-3 will give us an important clue to understanding these new types of superconductive phenomena,” says Kono. Both superconductivity and superfluidity commonly occur as a result of atoms forming Cooper pairs and causing Bose–Einstein condensation. “In a superconducting substance, electrons are significantly affected by the lattice made of the positive ions that produce the substance, whereas in superfluid helium-3, its atoms affect each other to form Cooper pairs, resulting in a superfluid state. The superfluid phenomenon is easier to understand because no other factors can affect the phenomenon. Furthermore, liquid helium contains no impurities because any element other than helium will already have become solid before the temperature at which helium solidifies. We can therefore conduct very precise experiments.” This means that liquid helium offers the best experimental conditions for conducting research into quantum mechanical phenomena.
Probing superfluid helium-3 with a two-dimensional electron system
When electrons are moving close to the surface of liquid helium, they become captured about 10 nm (1 nm = 10−9 m) from the surface because there is simultaneous electrical attraction and repulsion on them. The captured electrons, arranged at regular intervals as a result of electrical repulsion, form a two-dimensional planar layer with the thickness of a single electron. The planar layer is a ‘two-dimensional electron system’ because each electron can only move parallel to the surface (Fig. 1).
The Low Temperature Physics Laboratory has used the two-dimensional electron system in conducting their unique experiments to investigate the properties of helium-3. “Ours is the only laboratory in the world that can conduct such experiments,” says Kono.
The Cooper pairs observed in helium-3 are divided into two types with different properties. Furthermore, in the superfluid state of helium-3, two phases (A and B) appear when the pressure and temperature are changed (Fig. 3). The A-phase state of the superfluid shows a strong anisotropic nature because of the anisotropy of the Cooper pairs that produce the A-phase state in helium-3. In other words, its properties depend on the direction in which the atoms move. For example, the Cooper pairs of helium-3 atoms in liquid helium will break when the temperature is increased. The separated helium-3 atoms then start to move—but in which direction? To find the answer, Kono and his laboratory members conducted an experiment with the two-dimensional electron system.
Figure 3: Phase diagram for helium-3 (3He).
Superfluid A-phase state appears when helium-3 is cooled to 0.002 K.
When a two-dimensional electron system exists adjacent to the surface of liquid helium, the surface becomes uneven: dents form in the portions close to the electrons. When the two-dimensional electron system is moved, the uneven convex and concave portions also move in accordance with the movement of the electron system. Kono explains, “We tried to measure the behavior of the two-dimensional electron system while gradually increasing the temperature of the liquid helium in the A-phase state. As a result, we successfully demonstrated that the movement of the two-dimensional system is prevented because the separated helium-3 atoms in the liquid helium move more easily in a direction perpendicular to the surface of the helium, and they bump into the uneven surface from inside the liquid helium.”
In some new types of superconductor, Cooper pairs of electrons with similar properties to those that result in the A-phase state in superfluid helium may produce a superconductive state. The experimental data by Kono and his laboratory members should contribute to explaining the mechanism behind these new types of superconductivity.
Conducting experiments in an ‘ultralow-temperature miniature early Universe’
Yoichiro Nambu, who was awarded the Nobel Prize in Physics very recently, was inspired by superfluidity and superconductive phenomena and proposed his theory on ‘broken symmetry’.
Broken symmetry is related to the birth of the Universe and the creation of matter. The particles that create matter include antiparticles with the same mass as normal particles but a negative charge. Only radiation existed in the Universe when it was born as a superhot fireball. Particles and antiparticles were created from the radiation as the Universe expanded and decreased in temperature. Most particles and antiparticles collided with each other and returned to radiation, but scientists guess that a small proportion of the particles remained because of the broken symmetry. Kono is seeking the mechanism of the birth of the Universe and the creation of matter by conducting experiments on helium-3. “According to one hypothesis, there is a correspondence between the process in which matter was created because of the broken symmetry when the temperature of the Universe decreased and the process in which helium-3 changes from a normal fluid state to the superfluid A-phase state that exhibits anisotropy when it is cooled to a very low temperature.”
There is no established physical theory that can explain the moment when the Universe was born. One of the powerful candidates is ‘superstring theory’ based on an idea proposed by Nambu. Kono and his laboratory members have established the ‘Study of Matter at the Early Universe through Symmetries and Universality’ research team in the Research Group for Genesis of Matter at the RIKEN Advanced Science Institute and have started joint studies with the Theoretical Physics Laboratory (Nishina Center for Accelerator-Based Science at RIKEN). “In superfluid helium-3, we can produce phenomena that correspond to the events that must have occurred in the early Universe. These events were predicted by superstring theory. We want to conduct various experiments using superfluid helium-3, an ‘ultralow-temperature miniature early Universe’, and contribute to verifying superstring theory and probing the mystery of the birth of the Universe.”
Manipulating electrons on the surface of liquid helium
Kono and his laboratory members have also started studies on how to manipulate individual electrons on the surface of liquid helium. The electrons captured near the surface of liquid helium form a two-dimensional electron system, and these individual electrons are in a similar state to the electrons captured by protons, that is, the state that they are in hydrogen atoms. The electrons captured near the surface of the liquid helium are easier to manipulate because the energy level that binds them together is weaker than that in hydrogen atoms. These electrons enter an excited state when exposed to microwaves, and they move away from the surface of the liquid helium (Fig. 1).
“The original goal of nanotechnology is to manipulate individual electrons, and the surface can be used as the ‘stage’ for that purpose,” says Kono.
Figure 4: Adiabatic nuclear demagnetization refrigerator.
The Low Temperature Physics Laboratory is starting experiments to discover new phenomena by cooling a quantum dot to an ultralow temperature of 0.001 K in an adiabatic nuclear demagnetization refrigerator.
The Low Temperature Physics Laboratory has also developed a microstructure called a ‘quantum dot’ made of gallium arsenide (GaAs), a type of semiconductor, and has started experiments to manipulate individual electrons. “We have recently developed a technique to prepare our own quantum dots. We have started research into the quantum-mechanical phenomena of electrons by manipulating the individual electrons in quantum dots cooled to an ultralow temperature” (Fig. 4).
What applications can we expect beyond these studies on how to manipulate individual electrons? “The final target is to develop a quantum computer,” says Kono. The quantum computer is a new concept in which quantum-mechanical phenomena are applied to the construction of a calculating machine. It would be so fast that a problem requiring many centuries to be processed with a typical present-day computer could be processed in several hours. Today, scientists around the world are moving forward with basic studies to construct a quantum computer, by various methods.
The next 100 years in low-temperature physics will see the emergence of new phenomena in this area. Society will benefit from currently unimaginable devices based on those phenomena.