Lecture 23. Degenerate Fermi Gas & Bose-Einstein condensation (Ch. 7)

Lecture 23. Degenerate Fermi Gas & Bose-Einstein condensation (Ch. 7) • Today’s plan: high density limit n ~ nQ • Fermion: Degenerate Fermi Gas (7.3) • Boson: Bose-Einstein condensation (7.6)

When will Maxwell-Boltzmann break down? For a system with fixed density (n),  is a function of T. T*: the temperature (energy) scale where quantum (exchange) effect becomes pronounced.

Degenerate fermi gas (T=0 limit) At T=0, the total # of fermions in a degenerate fermi gas (spin S) is: The total (internal) energy of a degenerate fermi gas is: - a very appreciable zero-point energy!

Degenerate fermi gas (T=0 limit) (cont.) The large (internal) energy U at 0 K is a consequence of Pauli exclusion principle. Recall in ideal gas, UIG=3/2 PV, we expect similar pressure generated by DFG. Indeed: This pressure is called degeneracy pressure. It is the physical mechanism that prevents white dwarfstars (electron) or neutron stars (neutron) from collapse by gravity. (Pr. 7.23, 7.24) Bulk modules (compressibility-1) of electron in metal:

How stars can support themselves against gravity: Star Evolution • gas and radiation pressure supports stars in which thermonuclear reactions occur • pressure of a degenerate electron gas at high densities supports the objects with no fusion: dead stars (white dwarfs) and the cores of giant planets (Jupiter, Saturn) • pressure of a degenerate neutron gas at high densities support neutron stars Nobel 1983 Chandrasekhar

Degenerate fermi gas (T≠0, but T<<TF) To be quantitative, we need to apply Sommerfeld expansion. But the qualitative behavior can be captured by a back-of-envelope calculation.  T>0 When T<<TF, the # of “excited” fermions is: =EF The extra thermal energy acquired by each fermion: The characteristic behavior of electrons in metals. Slightly more complicate cal.:

Chemical potential of fermi gas (with fixed n) Sommerfeld expansion (T<<TF) Maxwell-Boltzmann (T>>TF):

The Fermi Gas of Nucleons in a Nucleus Let’s apply these results to the system of nucleons in a large nucleus (both protons and neutrons are fermions). In heavy elements, the number of nucleons in the nucleus is large and statistical treatment is a reasonable approximation. We need to estimate the density of protons/neutrons in the nucleus. The radius of the nucleus that contains A nucleons: Thus, the density of nucleons is: For simplicity, we assume that the # of protons = the # of neutrons, hence their density is The Fermi energy EF >>> kBT – the system is strongly degenerate. The nucleons are very “cold” – they are all in their ground state! The average kinetic energy in a degenerate Fermi gas = 0.6 of the Fermi energy - the nucleons are non-relativistic

Nobel 2001 Bose-Einstein Condensation (Ch. 7 ) BEC and related phenomena BEC of photons (lasers) Townes Basov Prokhorov Nobel 1964 BEC in a weakly- interacting system (atomic gases) BEC in a strongly- interacting system (superfluid 4He) Einstein described the phenomenon of condensation in an ideal gas of particles with nonzero mass in 1925. In the 1930’s Fritz London realized that superfluity 4He can be understood on terms of BEC. However, the analysis of superfluity 4He is complicated by the fact that the 4He atoms in liquid strongly interact with each other. Landau Nobel 1962 Kapitsa Nobel 1978 70 years after the Einstein prediction, the BEC in weakly interacting Bose systems has been experimentally demonstrated - by laser cooling of a system of weakly-interacting alkali atoms in a magnetic trap.

Two types of Bosons • Two types of bosons: • Composite particles which contain an even number of fermions. These number of these particles is conserved if the energy does not exceed the dissociation energy (~ MeV in the case of the nucleus). • (b) Particles associated with a field, of which the most important example is the photon. These particles are not conserved: if the total energy of the field changes, particles appear and disappear. The chemical potential of such particles is zero in equilibrium, regardless of density.

Ideal Gas of Conserved Bosons Recall: An educated guess: something (BEC) will happen below T* for bosons with fixed density n. Q: Why bosons are special, i.e. forming BEC at low temperature? A: The quantum nature, i.e. any # of bosons can occupy one quantum state (energy level) . A layman’s definition: Bose-Einstein Condensation (BEC) is a special macrostate with macroscopic # of bosons occupying one quantum state (often, the ground state) of a bosonic system. As the system temperature is cooled below certain temperature TC, BEC spontaneously forms. It is a phase transition purely driven by quantum (exchange) effect.

 (n,T) T TC BEC of Conserved Bosons Let’s consider a simple but very special case: T = 0 K. What is the macrostate of a bosonic system? 4 3 All the bosons (a macroscopic #!) occupy the lowest energy level, i.e. the ground state, so that the system has lowest energy. 2 1 On the other hand, 0 around T* according to Maxwell-Boltzmann distribution. Indeed, =0 right at T=TC and stay at 0 as T further decreases.

Bose-Einstein Condensation (TC) Recall Bose-Einstein distribution The total # of bosons: Using density of state approximation Let’s perform the integration at TC, i.e. =0. Critical temperature of BEC Recall Confirm: T*TC

Bose-Einstein Condensation (T<TC) T>TC, the system can adjust (<0) to satisfy the constraint: What happen at T<TC?  is already 0 at TC. The right hand side decreases as T1.5. Something is wrong! Resolving the paradox: The problem is caused by the behavior of the 3D density of states and our use of the continuum approximation. Because g()=0 at =0, our calculations of n ignored all the particles in the ground (=0) state. At low energies, we have to take into account the discreteness of the quantum states. n() g()  = g()n()    Just excited states!

T < TC g()n()  Bose-Einstein Condensation (T<TC) (cont.) The eq. n(T) with  = 0 still works at T<TC for calculating the number of particles not in the ground state (ignore spin): The density of particles in the ground state: a tremendous number of particles all sitting in the very lowest available energy state n>0 n n0 TC T

 (n,T) T TC We can discuss the ideal Bose gas in the same terms of a phase transition. That is, at a critical value of temperature, TC, (n,T) reaches the limit of  = 0 and stops increasing. Beyond this point, the relation Bose-Einstein Condensation (Summary) is no longer able to keep track of all the particles – we miss the particles in the ground state. Below TC, bosons begin to condense into the ground state. The abrupt accumulation of bosons in the ground state is called Bose-Einstein condensation.

Realization of BEC in a Dilute 87Rb Vapor In principle, the lighter the bosons, the greater TC. For example, the BE condensation of excitons (light-induced electron-hole pairs) in semiconductors has been observed before the BE condensation in dilute gases (electron is a fermion, but an electron-hole pair has an integer spin). First observation of the BEC with weakly-interacting gases was observed with relatively heavy atoms of 87Rb. 10,000 rubidium-87 atoms were confined within a “box” with dimensions ~ 10 m (the density ~ 1019 m-3). The spacing between the energy levels: The transition was observed at ~ 0.1 K. This is in line with the estimate: ! - again, it is worth emphasizing that the BEC occurs at kBT >>  : - the greater the total number of particles in the system, the greater this difference.

Realization of BEC in Dilute Vapor (cont.) The atoms are not very close to each other in the classic sense - in fact, the average density of this condensate is very low—one billionth the density of normal solids or liquids. But at this temperature, the quantum volume becomes comparable to the average volume per atom: At T=0.9TC, the number of atoms in the ground state: degeneracy of the 1st excited state in a cube For comparison, in the first excited state: The ratio N0/N1, which is ~ 5 for N = 104, rapidly increases with N at a fixed T/TC (it becomes ~ 25 for N = 106). n>0 n n0 TC T

Problem • Calculate the critical temperature for BE condensation of diatomic hydrogen H2 if the density of liquid hydrogen is 60 kg/m3. Would you expect superfluidity in liquid hydrogen as well? Hydrogen liquefies around 20K and solidifies at 14K. • Above TC, the pressure in a degenerate Bose gas is proportional to T. Do you expect the temperature dependence of pressure to be stronger or weaker at T <TC ? Explain and draw a qualitative graph of the temperature dependence of pressure over the temperature range 0 <T < 2 TC. (a) Liquid hydrogen: Since hydrogen solidifies at 14K, we do not expect to observe superfluidity in liquid hydrogen. (b) The atoms in the ground state do not contribute to pressure. At T < TC, two factors contribute to the fast increase of P with temperature: (i) an increase of the number of atoms in the excited states, and (b) an increase of the average speed of atoms with temperature. As the result, the rate of the pressure increase with temperature is greater at T < TCthan that atT > TC (in fact , P~T5/2at T < TC) . P(T) TC T

Laser How to cool the gas of Rb atoms down to ~0.1 K? The first stage – laser cooling, the second stage – evaporative cooling. Laser Cooling E2 For photon absorption or emission, the photon energy h must be equal to E2-E1 • works for a dilute gas of neutral atoms • (cannot be applied to cool solids) E1 If the laser frequency is tuned slightly below E2-E1, an atom scatters (absorbs and re-emits) photons only is it moves towards the laser (Doppler effect).Atom at rest or moving in the opposite direction doesn’t scatter. If a resonant photon is absorbed, the atom acquires momentum: The corresponding energy: Na, A=23 An apparent limit on T achieved by laser cooling is reached when an atom’s recoil energy from absorbing or emitting a single photon is comparable to its total K. The single-photon recoil temperature limit (for Na):

Laser Cooling and Trapping By laser cooling, T~ 10 K can be reached. At this temperature, the atom’s speed is a few cm/s. These slow-moving atoms are relatively easy to confine in a non-uniform magnetic trap. The magnetic field has a minimum value in the center of the “magnetic bowl”. An atom with spin parallel to the magnetic field (i.e., atomic magnetic moment anti-parallel to the magnetic field), is attracted to the minimum; for spin anti-parallel to the field, the atom is repelled from the minimum. Laser cooling has been used in the experiments on BEC observation for pre-cooling of the gas of alkali atoms. However, to observe this phenomenon, even lower T are necessary. Further reduction of T by ~3 orders of magnitude (below 0.1 K) is required for the exp. vapor densities ~ 1017 m-3. This is achieved by the evaporation cooling after the lasers are turned off. Magneto-Optical Trap

Evaporative Cooling Radio-frequency forced evaporative cooling. The resonance excitation flips the spins and those atoms are ejected (evaporated). Reducing fr frequency evaporates lower energy atoms.

Metastability is the Key The experiments with Rb vapor were aimed at realization of BEC in a weakly-interacting system. Though the interactions are weak in the vapor of Rb atoms, they are sufficiently strong for the phase transition vapor-solid at ultra-low temperatures. In conditions of thermal equilibrium, one cannot get below the blue line without phase separation. vapor-solid/liquid phase boundary vapor cond. matter liquid He BEC ~ 1010 How to cheat the Nature? The key is metastability. If the process of cooling is slow and “gentle” enough, one can realize a “super-saturated” vapor below the coexistence line without a condensed phase ever forming. For this, not only the interaction with walls must be excluded, but also the three-particle collisions that assist forming molecules and, eventually, condensed-matter phase – hence, very low densities.

Observation of BEC To observe the distribution of velocities of atoms in the system, the magnetic trap is turned off. The atoms find themselves in free space, and, because they have some residual velocity, they just fly apart. After they have flown apart for some time, the cloud is much bigger, and it is easier to take a snapshot of the atomic cloud (to make a snapshot, a laser beam is scattered by the cloud). The picture shows the velocity distribution of atoms in the cloud at the time of its release, instead of the spatial distribution. For T > TC, atoms are distributed among many energy levels of the system, and have a Gaussian distribution of velocities. With cooling of the cloud, a spike appears right in its middle. It corresponds to atoms which are hardly moving at all: for T < TC, the concentration of atoms in the lowest state gives rise to a pronounced peak in the distribution at low velocities (condensation in the momentum space). Each frame corresponds to the distance the atoms have moved in about 1/20 s after turning off the trap. This two-component cloud resembles the situation in superfluid helium, where two components coexist: normal and superfluid .

Thermodynamic Functions of a Degenerate Bose Gas By integrating the heat capacity at constant volume, we can get the entropy:  The Helmholtz free energy ( = 0): The pressure exerted by a degenerate Bose gas: does not depend on volume! This is due to the fact that, when compressing a degenerate Bose gas, we just force more particles to occupy the ground state. The particles in the ground state do not contribute to pressure – except of the zero-motion oscillations, they are at rest.

Lecture 23. Degenerate Fermi Gas & Bose-Einstein condensation (Ch. 7)