States in Statistical Mechanics  

In General > s.a. correlations; Macrostates and Microstates; statistical mechanics and models.
* Idea: States in classical statistical mechanics are (possibly t-dependent) functions ρ(q, p) on phase space, also called distribution functions; They can be interpreted in terms of the fraction of time a system spends near each point in phase space (Boltzmann), the probability that the system is found near that point, related to information theory (Jaynes, Katz), or an ensemble of macroscopically indistinguishable systems (Gibbs); The ensemble interpretation has greatly influenced the development and language of statistical mechanics, but is now mainly of historical interest.
@ Reviews: Hillery et al PRP(84).
@ General references: Challa & Hetherington PRL(88), PRA(88) [different ensembles]; Solomon et al qp/04-conf [and combinatorics]; Turko a0711-conf [statistical ensemble equivalence problem]; Werndl a1310 [typicality measures in Boltzmannian statistical mechanics].

Canonical Ensemble or Gibbs State > s.a. statistical mechanics and quantum statistical mechanics [approach to equilibrium, thermalization].
$ Def: The distribution function given by the Boltzmann distribution ρ(q, p) = exp{–βH}/Z, were H(q, p) is the Hamiltonian, and Z the canonical partition function.
$ Partition function: The function Z = ∑states α exp{–βEα} (Z stands for "Zustandsumme"; Q is another common symbol), where for states labeled by continuous variables (e.g., q and p) the sum over states is really a suitably normalized integral over phase space.
$ Boltzmann factor: The factor exp{–βH(q, p} that determines the relative probabilities of different states for a system in a thermal state.
* Interpretation: A canonical ensemble represents a system in thermal equilibrium with a thermal bath (microcanonical ensemble) at temperature T = 1/.
* Properties: The energy fluctuation is σE2 = kT 2CV; This also shows that the heat capacity is always positive.
* Applications: It is very often used, even for systems that are not strictly in thermal equilibrium, because it makes calculations simple.
@ General references: Sorkin IJTP(79); Kalinin TMP(05) [in terms of two-particle partition function]; Parisio & de Aguiar PhyA(07) [semiclassical trace formula]; Reimann PRL(07), comment Brody PRL(08), Plastino & Daffertshofer EPL(08) [from more general state than microcanonical]; Velazquez & Curilef JPA(09) [energy fluctuation relation]; Müller EJP(14) [Boltzmann factor, simplified derivation]; Dai et al PRA(16)-a1508 [joint system-bath state].
@ Relationship with microcanonical: Lukkarinen JPA(99)cm/98; Gurarie AJP(07)aug; Touchette AJP(08)jan [inequivalent]; Zhang et al PRE(10)-a1007 [in coupled-spin systems]; Brandão & Cramer a1502 [equivalence for non-critical quantum systems]; Tasaki a1609 [local equivalence, for quantum spin systems]; Cancrini & Olla a1701 [equivalence, and fluctuations]; > s.a. non-extensive statistical mechanics [in Tsallis statistics]; Potts Model.
@ Generalized: Costeniuc et al JSP(05); Touchette et al PhyA(06); Kakorin AJP(09)jan [finite particle-number correction]; Malyshev RMS(01)gq [on random discrete sets]; Ambruş & Winstanley PLB(14)-a1406 [rotating]; Halpern et al nComm(16)-a1512 [non-Abelian thermal state]; Pachón et al a1401 [deviations from quantum uncertainty relations].
@ Related topics: Chazottes & Hochman CMP(10)-a0907 [example with no zero-temperature limit]; Plastino et al PhyA(14) [partition function and entropy from thermodynamics]; Navrotskaya a1406 [multi-particle inverse problem]; Tatsuta & Shimizu a1703 [conversion into a superposition of macroscopically distinct states]; > s.a. Maxwell-Boltzmann Distribution; modified thermodynamics [relativistic].

Grand Canonical Ensemble > s.a. quantum statistical mechanics.
* Idea: It describes a system in thermal equilibrium at temperature T, with variable energy and particle number,

ρ = exp{–β(EμN)}/Z ,   Z:= tr exp{–β(EμN)} ,   μ = chemical potential .

@ General references: Smii a1001-wd [graphical representation].
@ And canonical ensemble: Herzog & Olshanii PRA(97)at/96 [BEC]; Román et al AJP(99)dec [finite size effects]; Cancrini & Tremoulet JSP(04) [finite volume].
@ Variations: in Beck PhyA(00) [non-extensive]; Klein & Yang MPAG(12)-a1009 [general relativistic, in curved spacetimes].

Microcanonical Ensemble > s.a. Envariance; fluctuations; lyapunov exponents; quantum statistical mechanics; specific heat.
* Idea: The ensemble that correctly describes the equilibrium statistics of Hamiltonian systems; The manifold of all points in phase space with a given total energy.
$ Def: The distribution function ρ = δ(HE) / tr[δ(HE)], for some fixed value E of the energy, where the "tr" stands for integrating over the energy shell.
* Advantages: Phase transitions can be defined for small systems (contrary to canonical ensemble situation).
* With long-range interactions: These systems in general are not additive, which can lead to an inequivalence of the microcanonical and canonical ensembles; The microcanonical ensemble may show richer behavior than the canonical one, including negative specific heats and other non-common behaviors.
@ General references: Rugh JPA(98) [geometric, dynamical approach]; Gross 01, PhyA(04); Bouchet & Barré JSP(05) [long-range interactions]; Kiessling JSP(09)-a0810 [Ruelle's regularized microcanonical measure]; Fine & Hantschel PS(12)-a1010 [alternative]; Chiribella & Scandolo a1608 [purity as a resource, beyond the thermodynamic limit].
@ Examples: Baeten & Naudts Ent(11)-a1009 [monatomic ideal gas, finite N]; Miranda & Bertoldi EJP(13) [small systems]; > s.a. black-hole entropy; Einstein Model.

Other States > s.a. non-equilibrium states.
* Orthodes: (Boltzmann) Statistical ensembles that satisfy the heat theorem.
@ Stationary states: Cohen PhyA(06) [equilibrium and non-equilibrium].
@ Other types: Hanel & Thurner PhyA(05) [derivation of power-law distributions]; Volkov et al JSP(06) [bounded energy and particle number]; Toral PhyA(06); Campisi PhyA(07) [dual orthodes as generalized ensembles interpolating between canonical and microcanonical].

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