Landau theory of phase transitions - Revision history

Carl McBride: Slight tidy.

2011-04-26T13:48:19Z

Slight tidy.

← Older revision		Revision as of 15:48, 26 April 2011
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	In the '''Landau theory of phase transitions''' of [[Lev Davidovich Landau]], the [[thermodynamic potential]] takes the form of a [[power series]] in the [[order parameters \|order parameter]] (<math>\eta</math>) (~~Ref. 3~~ Eq. 143.1)		In the '''Landau theory of phase transitions''' of [[Lev Davidovich Landau]]<ref>Lev Davidovich Landau "", Physikalische Zeitschrift der Sowjetunion '''11''' pp. 26-47 (1937)</ref><ref>Lev Davidovich Landau "", Physikalische Zeitschrift der Sowjetunion '''11''' pp. 545-555 (1937)</ref>, the [[thermodynamic potential]] takes the form of a [[power series]] in the [[order parameters \|order parameter]] (<math>\eta</math>) (Eq. 143.1 of <ref>L. D. Landau and E. M. Lifshitz, "Statistical Physics" (Course of Theoretical Physics, Volume 5) 3rd Edition Part 1, Chapter XIV, Pergamon Press (1980)</ref>)

	:<math>\left.\Phi(p, T, \eta)\right. = \Phi_0 + \alpha(p,T)\eta + A(p,T)\eta^2 + B(p,T)\eta^3 + C(p,T)\eta^4 + ...,</math>		:<math>\left.\Phi(p, T, \eta)\right. = \Phi_0 + \alpha(p,T)\eta + A(p,T)\eta^2 + C(p,T)\eta^3 + B(p,T)\eta^4 + ...,</math>

			where <math>p</math> is the [[pressure]] and <math>T</math> is the [[temperature]]. It is supposed that <math>C \equiv 0</math>, leading to (Eq. 143.3)

			:<math>\left.\Phi(p, T, \eta)\right. = \Phi_0 + \alpha(p,T)\eta + A(p,T)\eta^2 + B(p,T)\eta^4</math>.

	~~where <math>p</math> is the [[pressure]] and <math>T</math> is the [[temperature]].~~
	The the equilibrium state consistent with the external parameters <math>p</math> and <math>T</math> is a minimum of the Landau potential (often also called Landau free energy). This yields a prescription for determining the equilibrium value adopted by <math>\eta</math>		The the equilibrium state consistent with the external parameters <math>p</math> and <math>T</math> is a minimum of the Landau potential (often also called Landau free energy). This yields a prescription for determining the equilibrium value adopted by <math>\eta</math>

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	The Landau theory is phenomenological in nature. Nothing is known a priori about the coefficients of the series. Whereas this is a strong point in order to highlight universal features, it is clear that any useful assumption about the symmetry or vanishing of such coefficients requires a fair understanding of the transition beforehand		The Landau theory is phenomenological in nature. Nothing is known a priori about the coefficients of the series. Whereas this is a strong point in order to highlight universal features, it is clear that any useful assumption about the symmetry or vanishing of such coefficients requires a fair understanding of the transition beforehand
	==References==		==References==
	~~#Lev Davidovich Landau "", Physikalische Zeitschrift der Sowjetunion '''11''' pp. 26-47 (1937)~~		<references/>
	~~#Lev Davidovich Landau "", Physikalische Zeitschrift der Sowjetunion '''11''' pp. 545-555 (1937)~~		;Related reading
	~~#L. D. Landau and E. M. Lifshitz, "Statistical Physics" (Course of Theoretical Physics, Volume 5) 3rd Edition Part 1, Chapter XIV, Pergamon Press (1980)~~		*[http://dx.doi.org/10.1088/0022-3719/9/9/015 A P Cracknell, J Lorenc and J A Przystawa "Landau's theory of second-order phase transitions and its application to ferromagnetism", Journal of Physics C: Solid State Physics '''9''' pp. 1731-1758 (1976)]
	#[http://dx.doi.org/10.1088/0022-3719/9/9/015 A P Cracknell, J Lorenc and J A Przystawa "Landau's theory of second-order phase transitions and its application to ferromagnetism", Journal of Physics C: Solid State Physics '''9''' pp. 1731-1758 (1976)]
	[[category: classical thermodynamics]]		[[category: classical thermodynamics]]

160.39.193.45 at 03:24, 20 April 2011

2011-04-20T03:24:43Z

← Older revision		Revision as of 05:24, 20 April 2011
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	In the '''Landau theory of phase transitions''' of [[Lev Davidovich Landau]], the [[thermodynamic potential]] takes the form of a [[power series]] in the [[order parameters \|order parameter]] (<math>\eta</math>) (Ref. 3 Eq. 143.1)		In the '''Landau theory of phase transitions''' of [[Lev Davidovich Landau]], the [[thermodynamic potential]] takes the form of a [[power series]] in the [[order parameters \|order parameter]] (<math>\eta</math>) (Ref. 3 Eq. 143.1)

	:<math>\left.\Phi(p, T, \eta)\right. = \Phi_0 + \alpha(p,T)\eta + A(p,T)\eta^2 + C(p,T)\eta^3 + B(p,T)\eta^4 + ...,</math>		:<math>\left.\Phi(p, T, \eta)\right. = \Phi_0 + \alpha(p,T)\eta + A(p,T)\eta^2 + B(p,T)\eta^3 + C(p,T)\eta^4 + ...,</math>

	where <math>p</math> is the [[pressure]] and <math>T</math> is the [[temperature]].		where <math>p</math> is the [[pressure]] and <math>T</math> is the [[temperature]].

Nice and Tidy: Slight tidy.

2009-04-01T15:18:52Z

Slight tidy.

← Older revision		Revision as of 17:18, 1 April 2009
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	In the '''Landau theory of phase transitions''' of [[Lev Davidovich Landau]], the [[thermodynamic potential]] takes the form of a [[power series]] in the [[order parameters \|order parameter]] (<math>\eta</math>) (Ref. 3 Eq. 143.1)		In the '''Landau theory of phase transitions''' of [[Lev Davidovich Landau]], the [[thermodynamic potential]] takes the form of a [[power series]] in the [[order parameters \|order parameter]] (<math>\eta</math>) (Ref. 3 Eq. 143.1)

	:<math>\left.\Phi(p, T, \eta)\right. = \Phi_0 + \alpha(p,T)\eta + A(p,T)\eta^2 + C(p,T)\eta^3 + B(p,T)\eta^4 + ...,</math>		:<math>\left.\Phi(p, T, \eta)\right. = \Phi_0 + \alpha(p,T)\eta + A(p,T)\eta^2 + C(p,T)\eta^3 + B(p,T)\eta^4 + ...,</math>
	where ''p'' is the [[pressure]] and ''T'' is the [[temperature]].
			where <math>p</math> is the [[pressure]] and <math>T</math> is the [[temperature]].
	The the equilibrium state consistent with the external parameters <math>p</math> and <math>T</math> is a minimum of the Landau potential (often also called Landau free energy). This yields a prescription for determining the equilibrium value adopted by <math>\eta</math>		The the equilibrium state consistent with the external parameters <math>p</math> and <math>T</math> is a minimum of the Landau potential (often also called Landau free energy). This yields a prescription for determining the equilibrium value adopted by <math>\eta</math>

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	==Illustration for the [[Ising model]]==		==Illustration for the [[Ising model]]==

	For the [[Ising model]] one chooses the magnetization as order parameter. In the absence of external field, the underlying Hamiltonian is invariant to spin exchange. It must then follow that <math>\Phi</math> is also invariant to the sign of the magnetization. Hence, all odd coefficients of <math>\eta</math> must vanish. For qualitative purposes, the free energy may be truncated to fourth order, thus:		For the [[Ising model]] one chooses the magnetization as order parameter. In the absence of external field, the underlying Hamiltonian is invariant to spin exchange. It must then follow that <math>\Phi</math> is also invariant to the sign of the magnetization. Hence, all odd coefficients of <math>\eta</math> must vanish. For qualitative purposes, the free energy may be truncated to fourth order, thus:

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	This provides three possible candidates for <math>\eta_{\rm eq}</math>, mainly <math>\eta_{\rm eq}=0</math> and <math>\eta_{\rm eq} = \pm (-A/2C)^{1/2} </math>. In order to rule out some candidates, we note that there must be only one possible solution above the [[Curie temperature]], <math> T_c </math> (the system is then not magnetized), while two such solutions are expected below the Curie temperature. These conditions are fulfilled only if <math>A</math> is an odd function of temperature, taking negative values below <math>T_c</math> and positive values at high temperature. The simplest qualitative choice satisfying such conditions is <math>A\propto T-T_c</math>. The full behaviour then follows as:		This provides three possible candidates for <math>\eta_{\rm eq}</math>, mainly <math>\eta_{\rm eq}=0</math> and <math>\eta_{\rm eq} = \pm (-A/2C)^{1/2} </math>. In order to rule out some candidates, we note that there must be only one possible solution above the [[Curie temperature]], <math> T_c </math> (the system is then not magnetized), while two such solutions are expected below the Curie temperature. These conditions are fulfilled only if <math>A</math> is an odd function of temperature, taking negative values below <math>T_c</math> and positive values at high temperature. The simplest qualitative choice satisfying such conditions is <math>A\propto T-T_c</math>. The full behaviour then follows as:
	=== Low temperature solution ===		==== Low temperature solution ====
	At low temperature, there are two possible solutions (<math>\eta_{\rm eq}=0</math> is here not a minimum but a maximum) corresponding to finite magnetization of opposite sign:		At low temperature, there are two possible solutions (<math>\eta_{\rm eq}=0</math> is here not a minimum but a maximum) corresponding to finite magnetization of opposite sign:
	:<math> \eta_{\rm eq} = \pm \sqrt{-\frac{A}{2C}} </math>		:<math> \eta_{\rm eq} = \pm \sqrt{-\frac{A}{2C}} </math>
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	:<math> \Delta \eta \propto t^{1/2} </math>		:<math> \Delta \eta \propto t^{1/2} </math>

	Thus, according to the Landau's theory, the approach to criticality is governed by a power law with universal exponent <math>\beta=1/2</math>, regardless of any specific detail of the ferromagnetic fluid. To point out the universal ~~behavior~~ of the exponent <math>\beta</math> is an ~~achievments~~ of Landau's theory. However, experimental studies have shown that the exponent is not quite 1/2, but rather, close to 1/3.		Thus, according to the Landau's theory, the approach to criticality is governed by a power law with universal exponent <math>\beta=1/2</math>, regardless of any specific detail of the ferromagnetic fluid. To point out the universal behaviour of the exponent <math>\beta</math> is an achievement of Landau's theory. However, experimental studies have shown that the exponent is not quite 1/2, but rather, close to 1/3.
			The explanation of this discrepancy is the goal of [[renormalisation group]] theory of second order phase transitions.
	The explanation of this discrepancy is the goal of ~~renormalization~~ group theory of second order phase transitions.		==== High temperature solution ====
	=== High temperature solution ===
	Above the Curie temperature there is only one possible solution corresponding to the disordered phase of zero magnetization,<math>\eta_{\rm eq}=0</math> .		Above the Curie temperature there is only one possible solution corresponding to the disordered phase of zero magnetization,<math>\eta_{\rm eq}=0</math> .

Lgmac at 13:23, 1 April 2009

2009-04-01T13:23:19Z

← Older revision		Revision as of 15:23, 1 April 2009
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	:<math> \eta_{\rm eq} = \pm \sqrt{-\frac{A}{2C}} </math>		:<math> \eta_{\rm eq} = \pm \sqrt{-\frac{A}{2C}} </math>
	The spontaneous emergence of finite magnetization from a Hamiltonian that is invariant to spin exchange is known as [[symmetry breaking]].		The spontaneous emergence of finite magnetization from a Hamiltonian that is invariant to spin exchange is known as [[symmetry breaking]].

			As the critical point is approached, <math>\Delta \eta = \eta_+ - \eta_-</math> may be approximated as a power of the distance away from the critical point, <math>\Delta \eta \propto (T-T_c)^{\beta}</math>. From the solution of the Landau functional for the spontaneous magnetization, we find:

			:<math> \Delta \eta \propto t^{1/2} </math>

			Thus, according to the Landau's theory, the approach to criticality is governed by a power law with universal exponent <math>\beta=1/2</math>, regardless of any specific detail of the ferromagnetic fluid. To point out the universal behavior of the exponent <math>\beta</math> is an achievments of Landau's theory. However, experimental studies have shown that the exponent is not quite 1/2, but rather, close to 1/3.

			The explanation of this discrepancy is the goal of renormalization group theory of second order phase transitions.
	=== High temperature solution ===		=== High temperature solution ===
	Above the Curie temperature there is only one possible solution corresponding to the disordered phase of zero magnetization,<math>\eta_{\rm eq}=0</math> .		Above the Curie temperature there is only one possible solution corresponding to the disordered phase of zero magnetization,<math>\eta_{\rm eq}=0</math> .

Nice and Tidy: Slight tidy.

2009-02-19T16:31:40Z

Slight tidy.

Lgmac: Significance + application to Ising model

2009-02-19T15:41:39Z

Significance + application to Ising model

← Older revision		Revision as of 17:41, 19 February 2009
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	== Criticism ==		== Criticism ==

	The Landau theory is phenomenological in nature. Nothing is known a priori about the coefficients of the series. Whereas this is a strong point in order to highlight universal features, it is clear that any useful assumption about the symmetry or vanishing of such coefficients requires a fair understanding of the transition beforehand.		The Landau theory is phenomenological in nature. Nothing is known a priori about the coefficients of the series. Whereas this is a strong point in order to highlight universal features, it is clear that any useful assumption about the symmetry or vanishing of such coefficients requires a fair understanding of the transition beforehand

Lgmac: Landau theory of second-order phase transitions moved to Landau theory of phase transitions

2009-02-19T15:38:05Z

Landau theory of second-order phase transitions moved to Landau theory of phase transitions

← Older revision	Revision as of 17:38, 19 February 2009
(No difference)

Lgmac at 15:33, 19 February 2009

2009-02-19T15:33:04Z

@@ Line 1: / Line 1: @@
 {{Stub-general}}
-In the '''Landau theory of second order phase transitions''' of [[Lev Davidovich Landau]], the [[thermodynamic potential]] takes the form of a [[power series]] in the [[order parameters |order parameter]] (<math>\eta</math>) (Ref. 3 Eq. 143.1)
+In the '''Landau theory of  phase transitions''' of [[Lev Davidovich Landau]], the [[thermodynamic potential]] takes the form of a [[power series]] in the [[order parameters |order parameter]] (<math>\eta</math>) (Ref. 3 Eq. 143.1)
 :<math>\left.\Phi(p, T, \eta)\right. = \Phi_0 + \alpha(p,T)\eta + A(p,T)\eta^2 + C(p,T)\eta^3 + B(p,T)\eta^4 + ...,</math>
 where ''p'' is the [[pressure]] and ''T'' is the [[temperature]].
 ==References==
 #Lev Davidovich Landau "",  Physikalische Zeitschrift der Sowjetunion  '''11''' pp. 26-47  (1937)

Carl McBride at 11:23, 26 November 2007

2007-11-26T11:23:00Z

← Older revision		Revision as of 13:23, 26 November 2007
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	{{Stub-general}}		{{Stub-general}}
	In the '''Landau theory of second order phase transitions''' of [[Lev Davidovich Landau]], the [[thermodynamic potential]] takes the form of a [[power series]] in the [[order parameters \|order parameter]] (<math>\eta</math>) (Ref. 1 Eq. 143.1)		In the '''Landau theory of second order phase transitions''' of [[Lev Davidovich Landau]], the [[thermodynamic potential]] takes the form of a [[power series]] in the [[order parameters \|order parameter]] (<math>\eta</math>) (Ref. 3 Eq. 143.1)
	:<math>\left.\Phi(P, T, \eta)\right. = \Phi_0 + \alpha(P,T)\eta + A(P,T)\eta^2 + C(P,T)\eta^3 + B(P,T)\eta^4 + ...,</math>		:<math>\left.\Phi(p, T, \eta)\right. = \Phi_0 + \alpha(p,T)\eta + A(p,T)\eta^2 + C(p,T)\eta^3 + B(p,T)\eta^4 + ...,</math>
	where ''P'' is the [[pressure]] and ''T'' is the [[temperature]].		where ''p'' is the [[pressure]] and ''T'' is the [[temperature]].
	==References==		==References==
	#Lev Davidovich Landau "", Physikalische Zeitschrift der Sowjetunion '''11''' pp. 26-47 (1937)		#Lev Davidovich Landau "", Physikalische Zeitschrift der Sowjetunion '''11''' pp. 26-47 (1937)

Carl McBride at 11:39, 19 November 2007

2007-11-19T11:39:17Z

← Older revision		Revision as of 13:39, 19 November 2007
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	{{Stub-general}}		{{Stub-general}}
	In the '''Landau theory of second order phase transitions''' of [[Lev Davidovich Landau]], the thermodynamic potential takes the form of a [[power series]] in the [[order parameters \|order parameter]] (<math>\eta</math>) (Ref. 1 Eq. 143.1)		In the '''Landau theory of second order phase transitions''' of [[Lev Davidovich Landau]], the [[thermodynamic potential]] takes the form of a [[power series]] in the [[order parameters \|order parameter]] (<math>\eta</math>) (Ref. 1 Eq. 143.1)
	:<math>\left.\Phi(P, T, \eta)\right. = \Phi_0 + \alpha(P,T)\eta + A(P,T)\eta^2 + C(P,T)\eta^3 + B(P,T)\eta^4 + ...,</math>		:<math>\left.\Phi(P, T, \eta)\right. = \Phi_0 + \alpha(P,T)\eta + A(P,T)\eta^2 + C(P,T)\eta^3 + B(P,T)\eta^4 + ...,</math>
	where ''P'' is the [[pressure]] and ''T'' is the [[temperature]].		where ''P'' is the [[pressure]] and ''T'' is the [[temperature]].

← Older revision		Revision as of 18:31, 19 February 2009
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	~~{{Stub-general}}~~
	In the '''Landau theory of phase transitions''' of [[Lev Davidovich Landau]], the [[thermodynamic potential]] takes the form of a [[power series]] in the [[order parameters \|order parameter]] (<math>\eta</math>) (Ref. 3 Eq. 143.1)		In the '''Landau theory of phase transitions''' of [[Lev Davidovich Landau]], the [[thermodynamic potential]] takes the form of a [[power series]] in the [[order parameters \|order parameter]] (<math>\eta</math>) (Ref. 3 Eq. 143.1)
	:<math>\left.\Phi(p, T, \eta)\right. = \Phi_0 + \alpha(p,T)\eta + A(p,T)\eta^2 + C(p,T)\eta^3 + B(p,T)\eta^4 + ...,</math>		:<math>\left.\Phi(p, T, \eta)\right. = \Phi_0 + \alpha(p,T)\eta + A(p,T)\eta^2 + C(p,T)\eta^3 + B(p,T)\eta^4 + ...,</math>
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	:<math>\left ( \frac{\partial\Phi}{\partial \eta}\right )_{\eta=\eta_{eq}} = 0.</math>		:<math>\left ( \frac{\partial\Phi}{\partial \eta}\right )_{\eta=\eta_{eq}} = 0.</math>

	Taking into account very general symmetry properties of the underlying [[Hamiltonian]] one can set a priori the qualitative ~~behavior~~ of the coefficients in the series. The value of those coefficients then very generally define the nature of the phase transition, which may be found to be either first or second order.		Taking into account very general symmetry properties of the underlying [[Hamiltonian]] one can set a priori the qualitative behaviour of the coefficients in the series. The value of those coefficients then very generally define the nature of the phase transition, which may be found to be either first or second order.

	The Landau theory was the first ~~succesful~~ attempt to highlight the universal features of phase transitions. However, the theory is known to fail for [[critical transitions]] of several types because of the neglect of fluctuations (i.e., the fact that the order parameter may fluctuate about the minima of <math>\eta</math> and hence tunnel across free energy barriers separating two such minima.		The Landau theory was the first successful attempt to highlight the universal features of phase transitions. However, the theory is known to fail for [[critical transitions]] of several types because of the neglect of fluctuations (i.e., the fact that the order parameter may fluctuate about the minima of <math>\eta</math> and hence tunnel across free energy barriers separating two such minima.

	==Illustration for the [[Ising model]]==		==Illustration for the [[Ising model]]==
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	:<math> \eta \left (2A + 4C\eta^2 \right ) = 0 </math>		:<math> \eta \left (2A + 4C\eta^2 \right ) = 0 </math>

	This provides three possible candidates for <math>\eta_{\rm eq}</math>, mainly <math>\eta_{\rm eq}=0</math> and <math>\eta_{\rm eq} = \pm (-A/2C)^{1/2} </math>. In order to rule out some ~~condidates~~, we note that there must be only one possible solution above the [[Curie temperature]], <math> T_c </math> (the system is then not magnetized), while two such solutions are expected below the Curie temperature. These conditions are fulfilled only if <math>A</math> is an odd function of temperature, taking negative values below <math>T_c</math> and positive values at high temperature. The simplest qualitative choice satisfying such conditions is <math>A\propto T-T_c</math>. The full ~~behavior~~ then follows as:		This provides three possible candidates for <math>\eta_{\rm eq}</math>, mainly <math>\eta_{\rm eq}=0</math> and <math>\eta_{\rm eq} = \pm (-A/2C)^{1/2} </math>. In order to rule out some candidates, we note that there must be only one possible solution above the [[Curie temperature]], <math> T_c </math> (the system is then not magnetized), while two such solutions are expected below the Curie temperature. These conditions are fulfilled only if <math>A</math> is an odd function of temperature, taking negative values below <math>T_c</math> and positive values at high temperature. The simplest qualitative choice satisfying such conditions is <math>A\propto T-T_c</math>. The full behaviour then follows as:

	=== Low temperature solution ===		=== Low temperature solution ===

	At low temperature, there are two possible solutions (<math>\eta_{\rm eq}=0</math> is here not a minimum but a maximum) corresponding to finite magnetization of opposite sign:		At low temperature, there are two possible solutions (<math>\eta_{\rm eq}=0</math> is here not a minimum but a maximum) corresponding to finite magnetization of opposite sign:
	<math> \eta_{\rm eq} = \pm \sqrt{-\frac{A}{2C}} </math>		:<math> \eta_{\rm eq} = \pm \sqrt{-\frac{A}{2C}} </math>

	The spontaneous emergence of finite magnetization from a Hamiltonian that is invariant to spin exchange is known as [[symmetry breaking]].		The spontaneous emergence of finite magnetization from a Hamiltonian that is invariant to spin exchange is known as [[symmetry breaking]].

	=== High temperature solution ===		=== High temperature solution ===

	Above the Curie temperature there is only one possible solution corresponding to the disordered phase of zero magnetization,<math>\eta_{\rm eq}=0</math> .		Above the Curie temperature there is only one possible solution corresponding to the disordered phase of zero magnetization,<math>\eta_{\rm eq}=0</math> .

	The qualitative nature of liquid--~~vapor~~ phase transitions follow the same reasoning, with the order parameter now defined as <math> \eta = (\rho - \rho_c )/\rho_c</math>. The Ising model then usually takes the name of [[Lattice gas]], with spins down standing for empty space, and spins up for molecules (or droplets). Such Lattice gas is a ~~charicature~~ of a compressible fluid along the coexistence line.		The qualitative nature of liquid--vapour phase transitions follow the same reasoning, with the order parameter now defined as <math> \eta = (\rho - \rho_c )/\rho_c</math>. The Ising model then usually takes the name of [[Lattice gas]], with spins down standing for empty space, and spins up for molecules (or droplets). Such Lattice gas is a caricature of a compressible fluid along the coexistence line.
	== Criticism ==		== Criticism ==

	The Landau theory is phenomenological in nature. Nothing is known a priori about the coefficients of the series. Whereas this is a strong point in order to highlight universal features, it is clear that any useful assumption about the symmetry or vanishing of such coefficients requires a fair understanding of the transition beforehand		The Landau theory is phenomenological in nature. Nothing is known a priori about the coefficients of the series. Whereas this is a strong point in order to highlight universal features, it is clear that any useful assumption about the symmetry or vanishing of such coefficients requires a fair understanding of the transition beforehand





	==References==		==References==
	#Lev Davidovich Landau "", Physikalische Zeitschrift der Sowjetunion '''11''' pp. 26-47 (1937)		#Lev Davidovich Landau "", Physikalische Zeitschrift der Sowjetunion '''11''' pp. 26-47 (1937)