Jump to content

Fermionic condensate

From Wikipedia, the free encyclopedia
(Redirected from Chiral condensate)

A fermionic condensate (or Fermi–Dirac condensate) is a superfluid phase formed by fermionic particles at low temperatures. It is closely related to the Bose–Einstein condensate, a superfluid phase formed by bosonic atoms under similar conditions. The earliest recognized fermionic condensate described the state of electrons in a superconductor; the physics of other examples including recent work with fermionic atoms is analogous. The first atomic fermionic condensate was created by a team led by Deborah S. Jin using potassium-40 atoms at the University of Colorado Boulder in 2003.[1][2]

Background

[edit]

Superfluidity

[edit]

Fermionic condensates are attained at lower temperatures than Bose–Einstein condensates. Fermionic condensates are a type of superfluid. As the name suggests, a superfluid possesses fluid properties similar to those possessed by ordinary liquids and gases, such as the lack of a definite shape and the ability to flow in response to applied forces. However, superfluids possess some properties that do not appear in ordinary matter. For instance, they can flow at high velocities without dissipating any energy—i.e. zero viscosity. At lower velocities, energy is dissipated by the formation of quantized vortices, which act as "holes" in the medium where superfluidity breaks down. Superfluidity was originally discovered in liquid helium-4 whose atoms are bosons, not fermions.

Fermionic superfluids

[edit]

It is far more difficult to produce a fermionic superfluid than a bosonic one, because the Pauli exclusion principle prohibits fermions from occupying the same quantum state. However, there is a well-known mechanism by which a superfluid may be formed from fermions: That mechanism is the BCS transition, discovered in 1957 by J. Bardeen, L.N. Cooper, and R. Schrieffer for describing superconductivity. These authors showed that, below a certain temperature, electrons (which are fermions) can pair up to form bound pairs now known as Cooper pairs. As long as collisions with the ionic lattice of the solid do not supply enough energy to break the Cooper pairs, the electron fluid will be able to flow without dissipation. As a result, it becomes a superfluid, and the material through which it flows a superconductor.

The BCS theory was phenomenally successful in describing superconductors. Soon after the publication of the BCS paper, several theorists proposed that a similar phenomenon could occur in fluids made up of fermions other than electrons, such as helium-3 atoms. These speculations were confirmed in 1971, when experiments performed by D.D. Osheroff showed that helium-3 becomes a superfluid below 0.0025 K. It was soon verified that the superfluidity of helium-3 arises from a BCS-like mechanism.[a]

Condensates of fermionic atoms

[edit]

When Eric Cornell and Carl Wieman produced a Bose–Einstein condensate from rubidium atoms in 1995, there naturally arose the prospect of creating a similar sort of condensate made from fermionic atoms, which would form a superfluid by the BCS mechanism. However, early calculations indicated that the temperature required for producing Cooper pairing in atoms would be too cold to achieve. In 2001, Murray Holland at JILA suggested a way of bypassing this difficulty. He speculated that fermionic atoms could be coaxed into pairing up by subjecting them to a strong magnetic field.

In 2003, working on Holland's suggestion, Deborah Jin at JILA, Rudolf Grimm at the University of Innsbruck, and Wolfgang Ketterle at MIT managed to coax fermionic atoms into forming molecular bosons, which then underwent Bose–Einstein condensation. However, this was not a true fermionic condensate. On December 16, 2003, Jin managed to produce a condensate out of fermionic atoms for the first time. The experiment involved 500,000 potassium-40 atoms cooled to a temperature of 5×10−8 K, subjected to a time-varying magnetic field.[2]

Bogoliubov Transformations and Fermion Condensates

[edit]

Bogoliubov transformations are a crucial mathematical tool for understanding and describing fermionic condensates. They provide a way to diagonalize the Hamiltonian of an interacting fermion system in the presence of a condensate, allowing us to identify the elementary excitations, or quasiparticles, of the system.

The Need for Bogoliubov Transformations

[edit]

In a system where fermions can form pairs, the standard approach of filling single-particle energy levels (the Fermi sea) is insufficient. The presence of a condensate implies a coherent superposition of states with different particle numbers, making the usual creation and annihilation operators inadequate. The Hamiltonian of such a system typically contains terms that create or annihilate pairs of fermions, such as:

where and are the creation and annihilation operators for a fermion with momentum , is the single-particle energy, and is the pairing amplitude, which characterizes the strength of the condensate. This Hamiltonian is not diagonal in terms of the original fermion operators, making it difficult to directly interpret the physical properties of the system.

Bogoliubov Transformations: A Change of Basis

[edit]

Bogoliubov transformations provide a solution by introducing a new set of quasiparticle operators, and , which are linear combinations of the original fermion operators:

where and are complex coefficients that satisfy the normalization condition . This transformation mixes particle and hole creation operators, reflecting the fact that the quasiparticles are a superposition of particles and holes due to the pairing interaction. This transformation was first introduced by N. N. Bogoliubov in his seminal work on superfluidity[3].

The coefficients and are chosen such that the Hamiltonian, when expressed in terms of the quasiparticle operators, becomes diagonal:

where is the ground state energy and is the energy of the quasiparticle with momentum . The diagonalization process involves solving the Bogoliubov-de Gennes equations, which are a set of self-consistent equations for the coefficients , , and the pairing amplitude . A detailed discussion of the Bogoliubov-de Gennes equations can be found in de Gennes' book on Superconductivity[4].

Physical Interpretation

[edit]

The Bogoliubov transformation reveals several key features of fermion condensates:

  • Quasiparticles: The elementary excitations of the system are not individual fermions but quasiparticles, which are coherent superpositions of particles and holes. These quasiparticles have a modified energy spectrum , which includes a gap of size at zero momentum. This gap represents the energy required to break a Cooper pair and is a hallmark of superconductivity and other fermionic condensate phenomena.
  • Ground State: The ground state of the system is not simply an empty Fermi sea but a state where all quasiparticle levels are unoccupied, i.e., for all . This state, often called the BCS state in the context of superconductivity, is a coherent superposition of states with different particle numbers and represents the macroscopic condensate.
  • Broken Symmetry: The formation of a fermion condensate is often associated with the spontaneous breaking of a symmetry, such as the U(1) gauge symmetry in superconductors. The Bogoliubov transformation provides a way to describe the system in the broken symmetry phase. The connection between broken symmetry and Bogoliubov transformations is explored in Anderson's work on pseudo-spin and gauge invariance[5].

In summary, Bogoliubov transformations are a powerful and versatile tool for analyzing fermion condensates. They provide a way to diagonalize the Hamiltonian, identify the quasiparticle excitations, and understand the physical properties of these fascinating quantum systems.

Examples

[edit]

Chiral condensate

[edit]

A chiral condensate is an example of a fermionic condensate that appears in theories of massless fermions with chiral symmetry breaking, such as the theory of quarks in Quantum Chromodynamics.

BCS theory

[edit]

The BCS theory of superconductivity has a fermion condensate. A pair of electrons in a metal with opposite spins can form a scalar bound state called a Cooper pair. The bound states themselves then form a condensate. Since the Cooper pair has electric charge, this fermion condensate breaks the electromagnetic gauge symmetry of a superconductor, giving rise to the unusual electromagnetic properties of such states.

QCD

[edit]

In quantum chromodynamics (QCD) the chiral condensate is also called the quark condensate. This property of the QCD vacuum is partly responsible for giving masses to hadrons (along with other condensates like the gluon condensate).

In an approximate version of QCD, which has vanishing quark masses for N quark flavours, there is an exact chiral SU(N) × SU(N) symmetry of the theory. The QCD vacuum breaks this symmetry to SU(N) by forming a quark condensate. The existence of such a fermion condensate was first shown explicitly in the lattice formulation of QCD. The quark condensate is therefore an order parameter of transitions between several phases of quark matter in this limit.

This is very similar to the BCS theory of superconductivity. The Cooper pairs are analogous to the pseudoscalar mesons. However, the vacuum carries no charge. Hence all the gauge symmetries are unbroken. Corrections for the masses of the quarks can be incorporated using chiral perturbation theory.

Helium-3 superfluid

[edit]

A helium-3 atom is a fermion and at very low temperatures, they form two-atom Cooper pairs which are bosonic and condense into a superfluid. These Cooper pairs are substantially larger than the interatomic separation.

See also

[edit]

Footnotes

[edit]
  1. ^ The theory of superfluid helium-3 is a little more complicated than the BCS theory of superconductivity. These complications arise because helium atoms repel each other much more strongly than electrons, but the basic idea is the same.

References

[edit]
  1. ^ DeMarco, Brian; Bohn, John; Cornell, Eric (2006). "Deborah S. Jin 1968–2016". Nature. 538 (7625): 318. doi:10.1038/538318a. ISSN 0028-0836. PMID 27762370.
  2. ^ a b Regal, C.A.; Greiner, M.; Jin, D.S. (28 January 2004). "Observation of resonance condensation of Fermionic atom pairs". Physical Review Letters. 92 (4): 040403. arXiv:cond-mat/0401554. Bibcode:2004PhRvL..92d0403R. doi:10.1103/PhysRevLett.92.040403. PMID 14995356. S2CID 10799388.
  3. ^ Bogoliubov, N. N. (1958). "On a New Method in the Theory of Superconductivity". Il Nuovo Cimento. 7 (6): 794–805.
  4. ^ de Gennes, P. G. (1999). Superconductivity of metals and alloys. Westview press.
  5. ^ Anderson, P. W. (1958). "Random-phase approximation in the theory of superconductivity". Physical Review. 112 (6): 1900.

Sources

[edit]