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International Journal of Modern Physics B

ANOMALOUS BEHAVIOR OF IDEAL FERMI GAS BELOW 2D: THE "IDEAL QUANTUM DOT" AND THE PAUL EXCLUSION PRINCIPLE

M. Grether, MANUEL DE LLANO, M. Howard Lee

Published Aug 20, 2009• 3 citations
Publisher

Abstract

A physical interpretation is given to a curious "hump" that develops in the chemical potential as a function of absolute temperature in an ideal Fermi gas for any spatial dimensionality d < 2, integer or not, in contrast with the more familiar monotonic decrease for all d ≥ 2. The hump height increases without limit as d decreases to zero. The divergence at d = 0 is shown to be a clear manifestation of the Pauli Exclusion Principle whereby two spinless fermions cannot sit on top of each other in configuration space. The hump itself is thus an obvious precursor of this manifestation, otherwise well understood in momentum space. It also constitutes an "ideal quantum dot" when d = 0.

DOI
10.1142/s0217979209063304

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A physical interpretation is given to a curious "hump" that develops in the chemical potential as a function of absolute temperature in an ideal Fermi gas for any spatial dimensionality d < 2, integer or not, in contrast with the more familiar monotonic decrease for all d ≥ 2. The hump height increases without limit as d decreases to zero. The divergence at d = 0 is shown to be a clear manifestation of the Pauli Exclusion Principle whereby two spinless fermions cannot sit on top of each other in configuration space. The hump itself is thus an obvious precursor of this manifestation, otherwise well understood in momentum space. It also constitutes an "ideal quantum dot" when d = 0.
Normal behavior of the thermodynamic properties of a Fermi gas in d> 2 dimensions, integer or not, means monotonically increasing or decreasing of its specific heat, chemical potential or isothermal sound velocity, all as functions of temperature. However, for 0 < d < 2 dimensions these properties develop a “hump ” (or “trough”) which increases (or deepens) as d → 0. Though not the phase transition signaled by the sharp features (“cusp ” or “jump”) in those properties for the ideal Bose gas in d> 2 (known as the Bose-Einstein condensation), it is nevertheless an intriguing structural anomaly which we exhibit in detail. 1 PACS: 71.10.Pm; 05.30.Fk; 05.70.Ce; 71.10.-w; 71.10.Ca
Thermodynamics of ideal Fermi gas trapped in an external generic power law potential U=∑i=1dci∣xiai∣niU=\sum_{i=1} ^d c_i |\frac{x_i}{a_i}|^{n_i}U=∑i=1d​ci​∣ai​xi​​∣ni​ are investigated systematically from the grand thermodynamic potential in ddd dimensional space. These properties are explored deeply in the degenerate limit (μ>>KBTμ>> K_BTμ>>KB​T), where the thermodynamic properties are greatly dominated by Pauli exclusion principle. Pressure and energy along with the isothermal compressibilty is non zero at T=0KT=0KT=0K, denoting trapped Fermi system is quite live even at absolute zero temperature. The nonzero value of compressibilty denotes zero point pressure is not just a constant but depends on volume.
Pauli's exclusion principle forces fermions to occupy distinct quantum states, creating a filled region of momentum space at low temperature, the Fermi sea, whose topology governs the system's response to perturbations and the nature of its correlation functions. Recent theory predicts that for non-interacting fermions, the Euler characteristic of a DDD-dimensional Fermi sea -- the topological invariant that describes its shape -- is encoded in its (DDD+1)-point density correlations. Here we experimentally demonstrate this connection in a two-dimensional degenerate gas of neutral 6^{6}6Li atoms using single-atom-resolved imaging. By measuring three- and four-point connected density correlations in real space, we directly extract topological invariants of the underlying Fermi sea, including the Euler characteristic. Our results are in remarkable agreement with ideal-gas predictions, despite the presence of sizeable interactions, and establish a new pathway for probing many-body topology through correlation measurements.
Fermions in a Fermi gas obey the Pauli exclusion principle restricting any two fermions from occupying the same quantum state. Strong interactions between fermions can completely change the properties of the Fermi gas. In our theoretical study we find an exotic quantum phase in strongly interacting Fermi gases subject to a certain condition imposed on the Fermi surfaces that we call the Fermi surface resonance. The phase is quantum critical in time and space and can be identified by the power-law dependence of the spectral density in frequency and momentum. The linear-response functions are singular in the static limit and at the Kohn anomalies. We analyze the quantum critical state at finite temperatures TTT and finite size LLL of the Fermi gas and provide a qualitative $L\ensuremath{-}T$ phase diagram. The quantum critical phase can be experimentally found in typical semiconductor heterostructures.
At sufficiently low temperatures, bosons predominantly occupy the ground state with zero energy and momentum. In addition, both superconductivity in metals and superfluidity in liquid helium-3, at sufficiently low temperatures, are associated with pairing interactions of fermions that lead to bosonic behavior. In contrast, fermions in a Fermi gas at low T populate the lowest states up to the Fermi level as a result of the Pauli exclusion principle. Beautiful experiments have been carried out on vapors of alkali atoms such as rubidium-87 that are cooled to very low temperatures by means of sophisticated experimental techniques that involve laser cooling followed by evaporative cooling.