Supersolid
Encyclopedia
A supersolid is a spatially ordered
material with superfluid
properties. Superfluidity is a special quantum
state of matter in which a substance flows with zero viscosity
.
was found, by Peter Kapitza, John F. Allen, and Don Misener
, to exhibit this property when it is cooled below a characteristic transition temperature called the lambda point. Also superfluidity is observed when superconductors are cooled below a critical temperature. However, before the recent observation of supersolid-like behavior in solid helium-4, superfluidity was considered to only be a property of the fluid
state, e.g. superconducting
electron fluids, gases with Bose-Einstein condensates, or unconventional liquid
s such as helium-4
or helium-3
at sufficiently low temperatures.
Superfluid
ity in helium arose from the normal liquid by a second-order phase transition
("lambda transition
").
In a dilute gas of Bose particles it comes about by a phase transition that belongs to the universality class of the
spherical model
. In thin liquid helium films it arises from the normal liquid
by a Kosterlitz-Thouless transition
. In the case of helium-4, it has been conjectured since 1970 that it might be possible to create a supersolid.
In most theories of this state, it is supposed that vacancies, empty sites normally occupied by particles in an ideal crystal, exist even at absolute zero
. These vacancies are caused by zero-point energy
, which also causes them to move from site to site as waves
. Because vacancies are bosons, if such clouds of vacancies can exist at very low temperature then a Bose-Einstein condensation of vacancies could occur at temperatures less than a few tenths of a kelvin. A coherent flow of vacancies is equivalent to a “superflow” (frictionless flow) of particles in the opposite direction. Despite the presence of the gas of vacancies the ordered structure of a crystal is maintained, although with less than one particle on each lattice site on average.
at Pennsylvania State University
saw phenomena which were interpreted as supersolid behavior. Specifically they observed what they later named Non-Classical Rotational Inertia, an unusual decoupling of the solid helium from a container's walls which could not be explained by classical models, but which was consistent with a superfluid-like decoupling of a small percentage of the atoms from the rest of the atoms in the container. If such an interpretation is correct, it would signify the discovery of a new quantum
phase of matter.
The experiment of Kim and Chan looked for superflow by means of a "torsional oscillator". To achieve this, a turntable is attached tightly to a spring-loaded spindle. Instead of rotating at constant speed, the turntable is given an initial motion in one direction. The spring causes the table to oscillate similarly to a balance wheel
. A toroid
filled with solid helium-4 is attached to the table. The rate of oscillation of the turntable and toroid depend on the amount of solid moving with it. If there is frictionless superfluid inside, then the mass moving with the doughnut is less and the oscillation will occur at a faster rate. In this way one can measure the amount of superfluid existing at various temperatures. Kim and Chan found that up to about 2% of the material in the doughnut was superfluid. (Recent experiments have increased the percentage to over 20%). Similar experiments in other laboratories have confirmed these results. A mysterious feature, not in agreement with the old theories, is that the transition continues to occur at high pressures.
High-precision measurements of the melting pressure of helium-4 have not resulted in any observation of a phase transition in the solid.
Prior to 2007
, many theorists performed calculations indicating that vacancies cannot exist at zero temperature in solid helium-4. While there is some debate, it seems more doubtful that what the experiments observed was the supersolid state. Indeed further experimentation, including that by Kim and Chan, has also cast some doubt on the existence of a true supersolid. One experiment found repeated warming followed by slow cooling of the sample causes the effect to disappear. This annealing
process removes flaws in the crystal structure. Further, most samples of helium-4 contain a small amount of helium-3. When some of this is removed, the superfluid transition occurs at a lower temperature, suggesting that the superflow is involved with actual fluid moving along imperfections in the crystal rather than a property of the perfect crystal.
Recently, it has been proposed to realize a supersolid in an optical lattice
.
Starting from a molecular quantum crystal, supersolidity is induced dynamically as an out-of-equilibrium state. When neighboring molecular wave functions overlap, two bosonic species simultaneously exhibit quasicondensation and long-range solid order, which is stabilized by their mass imbalance.
It is demonstrated that this proposal can be realized in present experiments with bosonic mixtures in an optical lattice
that features simple on-site interactions.
Experimental and theoretical work continues in hopes of finally settling the question of the existence of a supersolid.
Solid
Solid is one of the three classical states of matter . It is characterized by structural rigidity and resistance to changes of shape or volume. Unlike a liquid, a solid object does not flow to take on the shape of its container, nor does it expand to fill the entire volume available to it like a...
material with superfluid
Superfluid
Superfluidity is a state of matter in which the matter behaves like a fluid without viscosity and with extremely high thermal conductivity. The substance, which appears to be a normal liquid, will flow without friction past any surface, which allows it to continue to circulate over obstructions and...
properties. Superfluidity is a special quantum
Quantum
In physics, a quantum is the minimum amount of any physical entity involved in an interaction. Behind this, one finds the fundamental notion that a physical property may be "quantized," referred to as "the hypothesis of quantization". This means that the magnitude can take on only certain discrete...
state of matter in which a substance flows with zero viscosity
Viscosity
Viscosity is a measure of the resistance of a fluid which is being deformed by either shear or tensile stress. In everyday terms , viscosity is "thickness" or "internal friction". Thus, water is "thin", having a lower viscosity, while honey is "thick", having a higher viscosity...
.
Background
Liquid helium-4Helium-4
Helium-4 is a non-radioactive isotope of helium. It is by far the most abundant of the two naturally occurring isotopes of helium, making up about 99.99986% of the helium on earth. Its nucleus is the same as an alpha particle, consisting of two protons and two neutrons. Alpha decay of heavy...
was found, by Peter Kapitza, John F. Allen, and Don Misener
Don Misener
Don Misener was a physicist. Along with Pyotr Leonidovich Kapitsa and John F. Allen, Misener discovered the superfluid phase of matter in 1937....
, to exhibit this property when it is cooled below a characteristic transition temperature called the lambda point. Also superfluidity is observed when superconductors are cooled below a critical temperature. However, before the recent observation of supersolid-like behavior in solid helium-4, superfluidity was considered to only be a property of the fluid
Fluid
In physics, a fluid is a substance that continually deforms under an applied shear stress. Fluids are a subset of the phases of matter and include liquids, gases, plasmas and, to some extent, plastic solids....
state, e.g. superconducting
Superconductivity
Superconductivity is a phenomenon of exactly zero electrical resistance occurring in certain materials below a characteristic temperature. It was discovered by Heike Kamerlingh Onnes on April 8, 1911 in Leiden. Like ferromagnetism and atomic spectral lines, superconductivity is a quantum...
electron fluids, gases with Bose-Einstein condensates, or unconventional liquid
Liquid
Liquid is one of the three classical states of matter . Like a gas, a liquid is able to flow and take the shape of a container. Some liquids resist compression, while others can be compressed. Unlike a gas, a liquid does not disperse to fill every space of a container, and maintains a fairly...
s such as helium-4
Helium-4
Helium-4 is a non-radioactive isotope of helium. It is by far the most abundant of the two naturally occurring isotopes of helium, making up about 99.99986% of the helium on earth. Its nucleus is the same as an alpha particle, consisting of two protons and two neutrons. Alpha decay of heavy...
or helium-3
Helium-3
Helium-3 is a light, non-radioactive isotope of helium with two protons and one neutron. It is rare on Earth, and is sought for use in nuclear fusion research...
at sufficiently low temperatures.
Superfluid
Superfluid
Superfluidity is a state of matter in which the matter behaves like a fluid without viscosity and with extremely high thermal conductivity. The substance, which appears to be a normal liquid, will flow without friction past any surface, which allows it to continue to circulate over obstructions and...
ity in helium arose from the normal liquid by a second-order phase transition
Phase transition
A phase transition is the transformation of a thermodynamic system from one phase or state of matter to another.A phase of a thermodynamic system and the states of matter have uniform physical properties....
("lambda transition
Lambda transition
The λ universality class is probably the most important group in condensed matter physics. It regroups several systems possessing strong analogies, namely, superfluids, superconductors and smectics...
").
In a dilute gas of Bose particles it comes about by a phase transition that belongs to the universality class of the
spherical model
Spherical model
The spherical model in statistical mechanics is a model of ferromagnetism similar to the Ising model, which was solved in 1952 by T.H. Berlin and M. Kac...
. In thin liquid helium films it arises from the normal liquid
by a Kosterlitz-Thouless transition
Kosterlitz-Thouless transition
In statistical mechanics, a part of mathematical physics, the Kosterlitz–Thouless transition, or Berezinsky–Kosterlitz–Thouless transition, is a kind of phase transition that appears in the XY model in 2 spatial dimensions. The XY model is a 2-dimensional vector spin model that possesses U or...
. In the case of helium-4, it has been conjectured since 1970 that it might be possible to create a supersolid.
In most theories of this state, it is supposed that vacancies, empty sites normally occupied by particles in an ideal crystal, exist even at absolute zero
Absolute zero
Absolute zero is the theoretical temperature at which entropy reaches its minimum value. The laws of thermodynamics state that absolute zero cannot be reached using only thermodynamic means....
. These vacancies are caused by zero-point energy
Zero-point energy
Zero-point energy is the lowest possible energy that a quantum mechanical physical system may have; it is the energy of its ground state. All quantum mechanical systems undergo fluctuations even in their ground state and have an associated zero-point energy, a consequence of their wave-like nature...
, which also causes them to move from site to site as waves
WAVES
The WAVES were a World War II-era division of the U.S. Navy that consisted entirely of women. The name of this group is an acronym for "Women Accepted for Volunteer Emergency Service" ; the word "emergency" implied that the acceptance of women was due to the unusual circumstances of the war and...
. Because vacancies are bosons, if such clouds of vacancies can exist at very low temperature then a Bose-Einstein condensation of vacancies could occur at temperatures less than a few tenths of a kelvin. A coherent flow of vacancies is equivalent to a “superflow” (frictionless flow) of particles in the opposite direction. Despite the presence of the gas of vacancies the ordered structure of a crystal is maintained, although with less than one particle on each lattice site on average.
Experiments
While several experiments yielded negative results, in the 1980s John Goodkind from UCSD discovered the first 'anomaly' in a solid by using ultrasound. Inspired by his observation, Eun-Seong Kim and Moses ChanMoses H. W. Chan
Moses Hung-Wai Chan is a physics professor at Penn State University, where he holds the rank of Evan Pugh Professor. He is an alumnus of Bridgewater College and Cornell University, where he earned his Ph.D. in 1974. He has been at professor at Penn State's University Park Campus since 1979.Through...
at Pennsylvania State University
Pennsylvania State University
The Pennsylvania State University, commonly referred to as Penn State or PSU, is a public research university with campuses and facilities throughout the state of Pennsylvania, United States. Founded in 1855, the university has a threefold mission of teaching, research, and public service...
saw phenomena which were interpreted as supersolid behavior. Specifically they observed what they later named Non-Classical Rotational Inertia, an unusual decoupling of the solid helium from a container's walls which could not be explained by classical models, but which was consistent with a superfluid-like decoupling of a small percentage of the atoms from the rest of the atoms in the container. If such an interpretation is correct, it would signify the discovery of a new quantum
Quantum
In physics, a quantum is the minimum amount of any physical entity involved in an interaction. Behind this, one finds the fundamental notion that a physical property may be "quantized," referred to as "the hypothesis of quantization". This means that the magnitude can take on only certain discrete...
phase of matter.
The experiment of Kim and Chan looked for superflow by means of a "torsional oscillator". To achieve this, a turntable is attached tightly to a spring-loaded spindle. Instead of rotating at constant speed, the turntable is given an initial motion in one direction. The spring causes the table to oscillate similarly to a balance wheel
Balance wheel
The balance wheel is the timekeeping device used in mechanical watches and some clocks, analogous to the pendulum in a pendulum clock. It is a weighted wheel that rotates back and forth, being returned toward its center position by a spiral spring, the balance spring or hairspring...
. A toroid
Toroid
Toroid may refer to*Toroid , a doughnut-like solid whose surface is a torus.*Toroidal inductors and transformers which have wire windings on circular ring shaped magnetic cores.*Vortex ring, a toroidal flow in fluid mechanics....
filled with solid helium-4 is attached to the table. The rate of oscillation of the turntable and toroid depend on the amount of solid moving with it. If there is frictionless superfluid inside, then the mass moving with the doughnut is less and the oscillation will occur at a faster rate. In this way one can measure the amount of superfluid existing at various temperatures. Kim and Chan found that up to about 2% of the material in the doughnut was superfluid. (Recent experiments have increased the percentage to over 20%). Similar experiments in other laboratories have confirmed these results. A mysterious feature, not in agreement with the old theories, is that the transition continues to occur at high pressures.
High-precision measurements of the melting pressure of helium-4 have not resulted in any observation of a phase transition in the solid.
Prior to 2007
2007 in science
The year 2007 in science and technology involved many significant events.-Astronomy and space exploration:* January 12 - Comet C/2006 P1 reaches perihelion and becomes visible during daylight....
, many theorists performed calculations indicating that vacancies cannot exist at zero temperature in solid helium-4. While there is some debate, it seems more doubtful that what the experiments observed was the supersolid state. Indeed further experimentation, including that by Kim and Chan, has also cast some doubt on the existence of a true supersolid. One experiment found repeated warming followed by slow cooling of the sample causes the effect to disappear. This annealing
Annealing (metallurgy)
Annealing, in metallurgy and materials science, is a heat treatment wherein a material is altered, causing changes in its properties such as strength and hardness. It is a process that produces conditions by heating to above the recrystallization temperature, maintaining a suitable temperature, and...
process removes flaws in the crystal structure. Further, most samples of helium-4 contain a small amount of helium-3. When some of this is removed, the superfluid transition occurs at a lower temperature, suggesting that the superflow is involved with actual fluid moving along imperfections in the crystal rather than a property of the perfect crystal.
Recently, it has been proposed to realize a supersolid in an optical lattice
Optical lattice
An optical lattice is formed by the interference of counter-propagating laser beams, creating a spatially periodic polarization pattern. The resulting periodic potential may trap neutral atoms via the Stark shift. Atoms are cooled and congregate in the locations of potential minima...
.
Starting from a molecular quantum crystal, supersolidity is induced dynamically as an out-of-equilibrium state. When neighboring molecular wave functions overlap, two bosonic species simultaneously exhibit quasicondensation and long-range solid order, which is stabilized by their mass imbalance.
It is demonstrated that this proposal can be realized in present experiments with bosonic mixtures in an optical lattice
Optical lattice
An optical lattice is formed by the interference of counter-propagating laser beams, creating a spatially periodic polarization pattern. The resulting periodic potential may trap neutral atoms via the Stark shift. Atoms are cooled and congregate in the locations of potential minima...
that features simple on-site interactions.
Experimental and theoretical work continues in hopes of finally settling the question of the existence of a supersolid.
See also
- Bose-Einstein condensate
- SuperfluidSuperfluidSuperfluidity is a state of matter in which the matter behaves like a fluid without viscosity and with extremely high thermal conductivity. The substance, which appears to be a normal liquid, will flow without friction past any surface, which allows it to continue to circulate over obstructions and...
- Superfluid filmSuperfluid filmSuperfluidity and superconductivity are macroscopic manifestations of quantum mechanics. There is considerable interest, both theoretical and practical, in these quantum phase transitions. There has been a tremendous amount of work done in the field of phase transitions and critical phenomenon in...
- SuperglassSuperglassA superglass is a phase of matter which is characterized by superfluidity and a frozen amorphous structure.-External links:* **...