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title: 2.1_FK_Model
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<ul>
<li><a href="#the-falikov-kimball-model"
id="toc-the-falikov-kimball-model">The Falikov Kimball Model</a>
<ul>
<li><a href="#the-model" id="toc-the-model">The Model</a></li>
<li><a href="#phase-diagrams" id="toc-phase-diagrams">Phase
Diagrams</a></li>
<li><a href="#long-ranged-ising-model"
id="toc-long-ranged-ising-model">Long Ranged Ising model</a></li>
</ul></li>
<li><a href="#bibliography" id="toc-bibliography">Bibliography</a></li>
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<!-- Table of Contents -->
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<ul>
<li><a href="#the-falikov-kimball-model"
id="toc-the-falikov-kimball-model">The Falikov Kimball Model</a>
<ul>
<li><a href="#the-model" id="toc-the-model">The Model</a></li>
<li><a href="#phase-diagrams" id="toc-phase-diagrams">Phase
Diagrams</a></li>
<li><a href="#long-ranged-ising-model"
id="toc-long-ranged-ising-model">Long Ranged Ising model</a></li>
</ul></li>
<li><a href="#bibliography" id="toc-bibliography">Bibliography</a></li>
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<!-- Main Page Body -->
<section id="the-falikov-kimball-model" class="level1">
<h1>The Falikov Kimball Model</h1>
<section id="the-model" class="level2">
<h2>The Model</h2>
<p>The Falikov-Kimball (FK) model is one of the simplest models of the
correlated electron problem. It captures the essence of the interaction
between itinerant and localized electrons. It was originally introduced
to explain the metal-insulator transition in f-electron systems but in
its long history it has been interpreted variously as a model of
electrons and ions, binary alloys or of crystal formation <span
class="citation"
data-cites="hubbardj.ElectronCorrelationsNarrow1963 falicovSimpleModelSemiconductorMetal1969 gruberFalicovKimballModelReview1996 gruberFalicovKimballModel2006"> [<a
href="#ref-hubbardj.ElectronCorrelationsNarrow1963"
role="doc-biblioref">1</a><a href="#ref-gruberFalicovKimballModel2006"
role="doc-biblioref">4</a>]</span>. In terms of immobile fermions <span
class="math inline">\(d_i\)</span> and light fermions <span
class="math inline">\(c_i\)</span> and with chemical potential fixed at
half-filling, the model reads</p>
<p><span class="math display">\[\begin{aligned}
H_{\mathrm{FK}} = &amp; \;U \sum_{i} (d^\dagger_{i}d_{i} -
\tfrac{1}{2})\;(c^\dagger_{i}c_{i} - \tfrac{1}{2}) -\;t \sum_{\langle
i,j\rangle} c^\dagger_{i}c_{j}.\\
\end{aligned}\]</span></p>
<p>The connection to the Hubbard model is that we have relabel the up
and down spin electron states and removed the hopping term for one
species, the equivalent of taking the limit of infinite mass ratio <span
class="citation" data-cites="devriesSimplifiedHubbardModel1993"> [<a
href="#ref-devriesSimplifiedHubbardModel1993"
role="doc-biblioref">5</a>]</span>.</p>
<p>Like other exactly solvable models <span class="citation"
data-cites="smithDisorderFreeLocalization2017"> [<a
href="#ref-smithDisorderFreeLocalization2017"
role="doc-biblioref">6</a>]</span> and the Kitaev Model, the FK model
possesses extensively many conserved degrees of freedom <span
class="math inline">\([d^\dagger_{i}d_{i}, H] = 0\)</span>. The Hilbert
space therefore breaks up into a set of sectors in which these operators
take a definite value. Crucially, this reduces the interaction term
<span class="math inline">\((d^\dagger_{i}d_{i} -
\tfrac{1}{2})\;(c^\dagger_{i}c_{i} - \tfrac{1}{2})\)</span> from being
quartic in fermion operators to quadratic. This is what makes the FK
model exactly solvable, in contrast to the Hubbard model.</p>
<p>Due to Pauli exclusion, maximum filling occurs when each lattice site
is fully occupied, <span class="math inline">\(\langle n_c + n_d \rangle
= 2\)</span>. Here we will focus on the half filled case <span
class="math inline">\(\langle n_c + n_d \rangle = 1\)</span>. Doping the
model away from the half-filled point leads to rich physics including
superconductivity <span class="citation"
data-cites="jedrzejewskiFalicovKimballModels2001"> [<a
href="#ref-jedrzejewskiFalicovKimballModels2001"
role="doc-biblioref">7</a>]</span>.</p>
<p>At half-filling and on bipartite lattices, FK the model is
particle-hole symmetric. That is, the Hamiltonian anticommutes with the
particle hole operator <span
class="math inline">\(\mathcal{P}H\mathcal{P}^{-1} = -H\)</span>. As a
consequence the energy spectrum is symmetric about <span
class="math inline">\(E = 0\)</span> and this is the Fermi energy. The
particle hole operator corresponds to the substitution <span
class="math inline">\(c^\dagger_i \rightarrow \epsilon_i c_i,
d^\dagger_i \rightarrow d_i\)</span> where <span
class="math inline">\(\epsilon_i = +1\)</span> for the A sublattice and
<span class="math inline">\(-1\)</span> for the even sublattice <span
class="citation" data-cites="gruberFalicovKimballModel2005"> [<a
href="#ref-gruberFalicovKimballModel2005"
role="doc-biblioref">8</a>]</span>. The absence of a hopping term for
the heavy electrons means they do not need the factor of <span
class="math inline">\(\epsilon_i\)</span>.</p>
<div id="fig:simple_DOS" class="fignos">
<figure>
<img src="/assets/thesis/background_chapter/simple_DOS.svg"
data-short-caption="Cubic Lattice dispersion with disorder"
style="width:100.0%"
alt="Figure 1: The dispersion (upper row) and density of states (lower row) obtained from a cubic lattice model H = \sum_{i} V_i c^\dagger_{i}c_{i} - t \sum_{\langle i,j\rangle} c^\dagger_{i}c_{j} in one dimension. (a) With not external potential. (b) With a static charge density wave background V_i = (-1)^i (c) A static charge density wave background with 2% binary disorder." />
<figcaption aria-hidden="true"><span>Figure 1:</span> The dispersion
(upper row) and density of states (lower row) obtained from a cubic
lattice model <span class="math inline">\(H = \sum_{i} V_i
c^\dagger_{i}c_{i} - t \sum_{\langle i,j\rangle}
c^\dagger_{i}c_{j}\)</span> in one dimension. (a) With not external
potential. (b) With a static charge density wave background <span
class="math inline">\(V_i = (-1)^i\)</span> (c) A static charge density
wave background with 2% binary disorder.</figcaption>
</figure>
</div>
<p>We will later add a long range interaction between the localised
electrons so we will replace the immobile fermions with a classical
Ising field <span class="math inline">\(S_i = 1 - 2d^\dagger_id_i =
\pm\tfrac{1}{2}\)</span>.</p>
<p><span class="math display">\[\begin{aligned}
H_{\mathrm{FK}} = &amp; \;U \sum_{i} S_i\;(c^\dagger_{i}c_{i} -
\tfrac{1}{2}) -\;t \sum_{\langle i,j\rangle} c^\dagger_{i}c_{j}.\\
\end{aligned}\]</span></p>
<p>The FK model can be solved exaclty with dynamic mean field theory in
the infinite dimensional <span class="citation"
data-cites="antipovCriticalExponentsStrongly2014 ribicNonlocalCorrelationsSpectral2016 freericksExactDynamicalMeanfield2003 herrmannNonequilibriumDynamicalCluster2016"> [<a
href="#ref-antipovCriticalExponentsStrongly2014"
role="doc-biblioref">9</a><a
href="#ref-herrmannNonequilibriumDynamicalCluster2016"
role="doc-biblioref">12</a>]</span>.</p>
<ul>
<li>displays disorder free localisation</li>
</ul>
</section>
<section id="phase-diagrams" class="level2">
<h2>Phase Diagrams</h2>
<div id="fig:fk_phase_diagram" class="fignos">
<figure>
<img src="/assets/thesis/background_chapter/fk_phase_diagram.svg"
data-short-caption="Fermi-Hubbard and Falikov-Kimball Temperatue-Interaction Phase Diagrams"
style="width:100.0%"
alt="Figure 2: Schematic Phase diagrams of the Fermi-Hubbard (left) and Falikov-Kimball model (right) showing temperature (T) and repulsive interaction strength (U). Hubbard model diagram adapted from  [13], Falikov-Kimball model from  [14,15]" />
<figcaption aria-hidden="true"><span>Figure 2:</span> Schematic Phase
diagrams of the Fermi-Hubbard (left) and Falikov-Kimball model (right)
showing temperature (T) and repulsive interaction strength (U). Hubbard
model diagram adapted from <span class="citation"
data-cites="micnasSuperconductivityNarrowbandSystems1990"> [<a
href="#ref-micnasSuperconductivityNarrowbandSystems1990"
role="doc-biblioref">13</a>]</span>, Falikov-Kimball model from <span
class="citation"
data-cites="antipovInteractionTunedAndersonMott2016 antipovCriticalExponentsStrongly2014a"> [<a
href="#ref-antipovInteractionTunedAndersonMott2016"
role="doc-biblioref">14</a>,<a
href="#ref-antipovCriticalExponentsStrongly2014a"
role="doc-biblioref">15</a>]</span></figcaption>
</figure>
</div>
<ul>
<li>rich phase diagram in 2d Despite its simplicity, the FK model has a
rich phase diagram in <span class="math inline">\(D \geq 2\)</span>
dimensions. For example, it shows an interaction-induced gap opening
even at high temperatures, similar to the corresponding Hubbard
Model <span class="citation"
data-cites="brandtThermodynamicsCorrelationFunctions1989"> [<a
href="#ref-brandtThermodynamicsCorrelationFunctions1989"
role="doc-biblioref">16</a>]</span>.</li>
</ul>
<p>At half filling and in dimensions greater than one, the FK model
exhibits a phase transition at some <span
class="math inline">\(U\)</span> dependent critical temperature <span
class="math inline">\(T_c(U)\)</span> to a low temperature charge
density wave state in which the spins order antiferromagnetically. This
corresponds to the heavy electrons occupying one of the two sublattices
A and B <span class="citation"
data-cites="maskaThermodynamicsTwodimensionalFalicovKimball2006"> [<a
href="#ref-maskaThermodynamicsTwodimensionalFalicovKimball2006"
role="doc-biblioref">17</a>]</span>. In the disordered region above
<span class="math inline">\(T_c(U)\)</span> there is a transition
between an Anderson insulator phase at weak interaction and a Mott
insulator phase in the strongly interacting regime <span
class="citation" data-cites="andersonAbsenceDiffusionCertain1958"> [<a
href="#ref-andersonAbsenceDiffusionCertain1958"
role="doc-biblioref">18</a>]</span>.</p>
<ul>
<li>superconductivity when doped</li>
</ul>
<p>In 1D, the ground state phenomenology as the model is doped away from
the half-filled state can be rich <span class="citation"
data-cites="gruberGroundStatesSpinless1990"> [<a
href="#ref-gruberGroundStatesSpinless1990"
role="doc-biblioref">19</a>]</span> but the system is disordered for all
<span class="math inline">\(T &gt; 0\)</span> <span class="citation"
data-cites="kennedyItinerantElectronModel1986"> [<a
href="#ref-kennedyItinerantElectronModel1986"
role="doc-biblioref">20</a>]</span>.</p>
<p>In the one dimensional FK model there is no ordered CDW phase <span
class="citation" data-cites="liebAbsenceMottTransition1968"> [<a
href="#ref-liebAbsenceMottTransition1968"
role="doc-biblioref">21</a>]</span>. The supression of phase transition
is a common phenomena in one dimensional systems. It can be understood
via Peierls argument <span class="citation"
data-cites="peierlsIsingModelFerromagnetism1936 kennedyItinerantElectronModel1986"> [<a
href="#ref-kennedyItinerantElectronModel1986"
role="doc-biblioref">20</a>,<a
href="#ref-peierlsIsingModelFerromagnetism1936"
role="doc-biblioref">22</a>]</span> to be a consequence of the low
energy penalty for domain walls in one dimensional systems.</p>
<p>Following Peierls argument, consider the difference in free energy
<span class="math inline">\(\Delta F = \Delta E - T\Delta S\)</span>
between an ordered state and a state with single domain wall in a
discrete order parameter. Short range interactions produce a constant
energy penalty for such a domain wall that does not scale with system
size. In contrast, the number of such single domain wall states scales
linearly so the entropy is <span class="math inline">\(\propto \ln
L\)</span>. Thus the entropic contribution dominates (eventually) in the
thermodynamic limit and no finite temperature order is possible. In two
dimensions and above, the energy penalty of a domain wall scales like
<span class="math inline">\(L^{d-1}\)</span> so they can support ordered
phases.</p>
</section>
<section id="long-ranged-ising-model" class="level2">
<h2>Long Ranged Ising model</h2>
<p>Our extension to the FK model could now be though of as spinless
fermions coupled to a long range Ising (LRI) model. The LRI model has
been extensively studied and its behaviour may be bear relation to the
behaviour of our modified FK model.</p>
<p><span class="math display">\[H_{\mathrm{LRI}} = \sum_{ij} J(|i-j|)
\tau_i \tau_j = J \sum_{i\neq j} |i - j|^{-\alpha} \tau_i
\tau_j\]</span></p>
<p>Renormalisation group analyses show that the LRI model has an ordered
phase in 1D for $1 &lt; &lt; 2 $ <span class="citation"
data-cites="dysonExistencePhasetransitionOnedimensional1969"> [<a
href="#ref-dysonExistencePhasetransitionOnedimensional1969"
role="doc-biblioref">23</a>]</span>. Peierls argument can be
extended <span class="citation"
data-cites="thoulessLongRangeOrderOneDimensional1969"> [<a
href="#ref-thoulessLongRangeOrderOneDimensional1969"
role="doc-biblioref">24</a>]</span> to long range interactions to
provide intuition for why this is the case. Again considering the energy
difference between the ordered state <span
class="math inline">\(\ket{\ldots\uparrow\uparrow\uparrow\uparrow\ldots}\)</span>
and a domain wall state <span
class="math inline">\(\ket{\ldots\uparrow\uparrow\downarrow\downarrow\ldots}\)</span>.
In the case of the LRI model, careful counting shows that this energy
penalty is: <span class="math display">\[\Delta E \propto
\sum_{n=1}^{\infty} n J(n)\]</span></p>
<p>because each interaction between spins separated across the domain by
a bond length <span class="math inline">\(n\)</span> can be drawn
between <span class="math inline">\(n\)</span> equivalent pairs of
sites. Ruelle proved rigorously for a very general class of 1D systems,
that if <span class="math inline">\(\Delta E\)</span> or its many-body
generalisation converges in the thermodynamic limit then the free energy
is analytic <span class="citation"
data-cites="ruelleStatisticalMechanicsOnedimensional1968"> [<a
href="#ref-ruelleStatisticalMechanicsOnedimensional1968"
role="doc-biblioref">25</a>]</span>. This rules out a finite order phase
transition, though not one of the Kosterlitz-Thouless type. Dyson also
proves this though with a slightly different condition on <span
class="math inline">\(J(n)\)</span> <span class="citation"
data-cites="dysonExistencePhasetransitionOnedimensional1969"> [<a
href="#ref-dysonExistencePhasetransitionOnedimensional1969"
role="doc-biblioref">23</a>]</span>.</p>
<p>With a power law form for <span class="math inline">\(J(n)\)</span>,
there are three cases to consider:</p>
<ol type="1">
<li>$ = 0$ For infinite range interactions the Ising model is exactly
solveable and mean field theory is exact <span class="citation"
data-cites="lipkinValidityManybodyApproximation1965"> [<a
href="#ref-lipkinValidityManybodyApproximation1965"
role="doc-biblioref">26</a>]</span>.</li>
<li>$ $ For slowly decaying interactions <span
class="math inline">\(\sum_n J(n)\)</span> does not converge so the
Hamiltonian is non-extensive, a case which wont be further considered
here.</li>
<li>$ 1 &lt; &lt; 2 $ A phase transition to an ordered state at a finite
temperature.</li>
<li>$ = 2 $ The energy of domain walls diverges logarithmically, and
this turns out to be a Kostelitz-Thouless transition <span
class="citation"
data-cites="thoulessLongRangeOrderOneDimensional1969"> [<a
href="#ref-thoulessLongRangeOrderOneDimensional1969"
role="doc-biblioref">24</a>]</span>.</li>
<li>$ 2 &lt; $ For quickly decaying interactions, domain walls have a
finite energy penalty, hence Peirels argument holds and there is no
phase transition.</li>
</ol>
<div id="fig:alpha_diagram" class="fignos">
<figure>
<img src="/assets/thesis/background_chapter/alpha_diagram.svg"
data-short-caption="Long Range Ising Model Behaviour"
style="width:100.0%" alt="Figure 3: " />
<figcaption aria-hidden="true"><span>Figure 3:</span> </figcaption>
</figure>
</div>
<div class="sourceCode" id="cb1"><pre
class="sourceCode python"><code class="sourceCode python"></code></pre></div>
</section>
</section>
<section id="bibliography" class="level1 unnumbered">
<h1 class="unnumbered">Bibliography</h1>
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