The quantum canonical ensemble: a projection operator treatment

Date: 14 June 2017

Venue: UAntwerp, Campus Groenenborger, Room241 - Groenenborgerlaan 141 - 2020 Antwerpen (route: UAntwerpen, Campus Groenenborger)

Time: 4:00 PM - 5:00 PM

Short description: Condensed Matter Theory presented by Dr Wim Magnus

Knowing the exact number of particles N, and taking this knowledge into account, the quantum canonical ensemble imposes a constraint on the occupation number operators. The constraint particularly hampers the systematic calculation of the partition function and any relevant thermodynamic expectation value for arbitrary but fixed N. On the other hand, fixing only the average number of particles, one may remove the above constraint and simply factorize the traces in Fock space into traces over single-particle states. As is well known, that would be the strategy of the grand-canonical ensemble which, however, comes with an additional Lagrange multiplier to impose the average number of particles. The appearance of this multiplier can be avoided by invoking a projection operator that enables a constraint-free computation of the partition function and its derived quantities in the canonical ensemble, at the price of an angular or contour integration.

Introduced in the recent past to handle various issues related to particle-number projected statistics, the projection operator approach proves beneficial to a wide variety of problems in condensed matter physics for which the canonical ensemble offers a natural and appropriate environment. In this light, we present a systematic treatment of the canonical ensemble that embeds the projection operator into the formalism of second quantization while explicitly fixing N, the very number of particles rather than the average. Being applicable to both bosonic and fermionic systems in arbitrary dimensions, transparent integral representations are provided for the partition function Z_N and the Helmholtz free energy F_N as well as for two- and four-point correlation functions. The chemical potential is not a Lagrange multiplier regulating the average particle number but can be extracted from F_{N+1} - F_N, as illustrated for a two-dimensional fermion gas.  As a particular application to semiconductor device simulations, we quote the use of the canonical treatment to predict a MOSFET threshold voltage, typically characterizing a regime with very low electron concentrations, for which the grand-canonical description fails.

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